The outgassing of volatile organic compounds during photoresist exposure at short wavelengths (<200 nm) has in recent years become a cause for concern as a source for contamination of lithographic optics and has triggered a significant investment of resources to understand and address the problem. In this paper, we report on Lincoln Laboratory’s contribution to this industry-wide effort with results from two types of outgassing measurements aimed at providing a better understanding of the risk posed by this phenomenon. The first method is a quantitative measurement based on a gas chromatograph-mass spectrometer and measures the outgassing by collecting the vapor sample over several minutes in order to enhance sensitivity. This first method cannot determine the outgassing time dependence over the duration of the exposure (~seconds). Our second method, based on laser desorption mass spectrometry, has been performed under UHV conditions to determine the time-dependence of photoresist outgassing and has shown that, for both 193- and 157-nm exposures, a majority of the total outgassed vapor is desorbed during the exposure time. The time dependence of the remaining amount that outgasses after exposure can be fit to a double exponential with characteristic time constants of ~0.5 and ~3 seconds, indicating that even in the limiting case of very short exposures (<<1 second), most material (>50%) would outgas within seconds. The implications of these findings are twofold. First, analytical methods used to measure outgassing that rely on long (>10 minute) sample collection and preconcentration steps must now assume that the measured product is liberated over a few-second time frame when converting the data to outgassing rates. This means that the peak transient outgassing rates for the few seconds during and immediately after exposure derived using this corrected method could be hundreds of times higher than previously reported, with values approaching as much as 1014 molecules/cm2-sec leading to transient concentrations well into the parts per million range. Second, given these higher rates, a review of lens purging designs should be done to ensure protection against these outgassing fluxes. In this paper we also discuss potentially new and unique issues caused by outgassing as they pertain to immersion and 157-nm lithographies that are currently under development.
The lithography prognosticator of the early 1980’s declared the end of optics for sub-0.5μm imaging. However, significant improvements in optics, photoresist and mask technology continued through the mercury lamp lines (436, 405 & 365nm) and into laser bands of 248nm and to 193nm. As each wavelength matured, innovative optical solutions and further improvements in photoresist technology have demonstrated that extending imaging resolution is possible thus further reducing k1. Several authors have recently discussed manufacturing imaging solutions for sub-0.3k1 and the integration challenges. Our industry will continue to focus on the most cost effective solution. What continues to motivate lithographers to discover new and innovative lithography solutions? Recent publications have demonstrated sub 0.30 k1 imaging. The answer is cost. The development of new tooling, masks and even photoresist platforms impacts cost. The switch from KrF to ArF imaging materials has a significant impact on process integration. The requirements stated in the ITRS roadmap for current and future technology nodes are very aggressive. Therefore, it is likely that high NA in combination with enhancement techniques will continue further for aggressive imaging solutions.
We address in this report a set of key questions tied to the implementation of liquid immersion lithography, from the perspective of the resist materials. We discuss the broad question of whether chemically amplified resists are capable of achieving the spatial resolution that ultimately will be required for the most advanced immersion scenario. Initial studies undertaken using model 193 nm resist materials provide some insight into how an aqueous liquid immersion process can affect the resist material.
Immersion lithography has been proposed as a technique to print sub-100nm features using 193nm lithography. The process involves filling the space between the lens fixture of an exposure tool and the photoresist-coated silicon wafer with a liquid. In the case of immersion 193nm lithography, water can serve as that liquid. The immersion option raises questions about how photoresists and water interact. Components of the photoresist could be leached into the water, thus modifying the refractive index of the medium, depositing material on the lens, or altering the solubility switching process of the photoresist. Several phenomena could affect the optical properties of the resist and water and, ultimately, the resolution of the process. To better understand the impact that immersion lithography would have on photoresist performance, a study has been undertaken to measure the amount of resist components that are leached by water from model 193nm photoresists. The components studied were residual casting solvent (propylene glycol methyl ether acetate), the photoacid generator (triphenylsulfonium nonaflate), and the base quencher (triethanolamine). Since it was expected that only a small amount of each material would be leached into the water, 14C-labeled samples of each resist component were synthesized and added to the 193nm resists. Films of the labeled resists were coated onto a silicon wafer and immersed in water. The water was collected and the film was dissolved in casting solvent and collected. The amount of material leached into the water was determined by radiochemical analysis. Spectroscopic ellipsometry was also used to quantify changes in the optical constants of the resists and the water.
We have investigated the interaction of resists with water and perfluoropolyether (PFPE) as immersion fluids. We found that some unique behaviors occurred in immersion lithography. An acetal protected poly(p-hydroxystyrene) type resist in water immersion showed decreased resist thickness after exposure. The deprotection reaction during exposure appeared to be accelerated by water. A COMA (cycloolefine-mareic anhydride alt-copolymer) type resist in water immersion showed an increased dissolution rate. FT-IR measurements indicated that the hydrolysis of maleic anhydride occurred during exposure and post-exposure baking. A reduction in the dissolution rate was observed in the immersion lithography of most resists. In water immersion, the formation of a surface insoluble layer and swelling was observed. We confirmed that a photochemical acid generator (PAG) or generated acid eluted into the water by TOF-SIMS. In PFPE immersion, we think that PFPE penetrating across the resist film blocks the penetration of the alkaline aqueous developer.
Neutron and x-ray reflectivity measurements quantify the non-uniform distribution of water within poly(4-tert-butoxycarbonyloxystyrene) (PBOCSt) and poly(4-hydroxystyrene) (PHOSt) thin films on silicon wafer substrates. Two contrasting surface treatments were examined, silicon oxide, representing a hydrophilic interface and a trimethylsilane primed surface, representing a hydrophobic interface. The distribution of water in the films was sensitive to the surface preparation and photoresist relative hydrophilicity. Depending upon the water contact angle of the substrate in comparison to the polymer film, an excess of water near the interface occurs when the substrate is more hydrophilic than the photoresist. Likewise, interfacial depletion results when the photoresist is more hydrophilic than the substrate. These non-uniform water distributions occurs within (50 ± 10)Å of the photoresist/substrate interface. The water concentration in this interfacial region appears to be independent of the photoresist properties, but is strongly dependent upon the substrate surface energy.
To accomplish minimizing feature size to sub 70nm, new light sources for photolithography are emerging, such as F2(157nm), and EUV(13nm). However there are many problems that should be solved for real device production. So extension of ArF(193nm) is necessary until the maturity of new lithography technique will be prepared. In this paper, we tested the feasibility of quencher gradient resist process (QGRP) to low k process. To compare with normal patterning process, QGRP needs additional step, over-coating. But this over-coating material differs from the normal over-coating materials in that over-coating material of QGRP has acid quencher sources. After the exposure, these quencher materials diffuse into the photoresist and quench excess acid that causes a sloped resist profile. As a result, vertical profile pattern can be obtained with QGRP. Using this QGRP, 70nm process, of which k value is 0.27, is possible with 0.75NA ArF scanner. For contact hole pattern, we could get direct 70nm C/H with QGRP. The exposure latitude of 70nm contact hole was improved more than 50% in case of QGRP compared with normal process. In addition, QGRP is applicable for immersion lithography.
Development of 193-nm negative resists that meet the stringent performance requirements of sub-100 nm resolution with conventional 0.26 N TMAH developer has proven to be a significant challenge. Most of the systems that are currently under development are based on cross-linking mechanisms. They commonly suffer from image distortion caused by micro-bridging. An alternative approach is to look at polarity switch mechanisms. We have investigated the acid-catalyzed elimination of polar molecules as one such mechanism which may provide a pathway to develop negative resists that do not suffer from micro-bridging.
The present report investigates the chemical/physical parameters that influence chemically amplified photoresist deprotection chemistry. It was observed that the efficiency of the acid catalyzed deprotection chemistry used in chemically amplified photoresists is highly dependent on chemical environment. In the first part, a solution IR
spectroscopic investigation was conducted on the deprotection kinetics of a model probe molecule, t-butyl phenyl carbonate, in a variety of solvents. The solvent used in this study where chosen to mimic the chemical composition of typical photoresist polymers used KrF and ArF lithography. It was observed that the activation energy of the deprotection of the probe molecule was strongly dependent upon the polarity and chemical nature of the model solvent systems investigated. In the second part, the probe molecule approach was modified so it can be used to study deprotection kinetics in actual resist films. Therefore, a novel, in-situ technique for monitoring acid-catalyzed deprotection in real-time via absorption spectroscopy was developed. Coumarin 338, which exhibits a significant change in the absorption spectrum upon loss of its t-butyl group, was used as probe molecule. This new methodology offers a number of advantages over existing techniques. The results obtained with this method show a similar dependence of the deprotection kinetics on the polarity of the matrix as those in solution.
It has been previously proposed that negative-tone resist process would have an intrinsic advantage for printing narrow trench geometry. To demonstrate this for 193nm lithography, a negative resist with performance comparable to a leading positive resist is required. In this paper we report the joint development of a hexafluoroalcohol containing, 193nm, negative-tone, chemically amplified resist based on the crosslinking approach. Lithographic performance is presented which includes the ability of the negative-tone resist to print 90nm line/space and isolated trenches with standard resist processing. The impact of the fluorinated polymer on etch performance is also quantified. Finally, key resist characteristics and their influence on performance and limiting factors such as microbridging are discussed.
Recently we have developed a novel non-ionic PAG, which generates a strong acid (perfluorobutanesulfonic acid) by light irradiation, and is applicable for chemically amplified photoresists. Application-relevant properties such as solubility in common organic solvents and water, thermal stability, storage stability in neat form and solution, UV absorption, and sensitivity in model formulations were evaluated. The compound showed good solubility in organic solvents and no solubility in water. Good storage stability was observed in solution, even in the presence of amine, where conventional non-ionic PAGs were not found to be stable. From the evaluation in the model formulation with ArF laser exposure, it was found that this new compound has high transparency at 193 nm and superior photo-efficiency to triphenylsulfonium perfluorobutanesulfonate (TPSPB). In addition, the new compound exhibited significant sensitivities at DUV (254 nm) and i-line (365 nm) wavelengths.
Three modes of scanning electrochemical microscopy (SECM) - voltammetry, pH, and conductivity - have been used to better understand the chemistry at, and diffusion through, the solid/liquid interface formed between a resist film and water in 193 nm immersion lithography. Emphasis has been placed on investigating the photoacid generator (PAG), triphenylsulfonium perfluorobutanesulfonate, and the corresponding photoacid. The reduction of triphenylsulfonium at a hemispherical Hg microelectrode was monitored using square wave voltammetry to detect trace amounts of the PAG leaching from the surface. pH measurements at a 100 μm diameter Sb microelectrode show the formation of acid in the water layer above a resist upon exposure with UV irradiation. Bipolar conductance measurements at a 100 μm Pt tip positioned 100 μm from the surface indicate that the conductivity of the solution during illumination is dependent upon the percentage of PAG in the film. Liquid chromatography mass spectrometric analysis of water samples in contact with resist films has been used to quantify the amounts (< 10 ng/cm2) of PAG leaching from the film in the dark which occurs within the first 30 seconds of contact time. Washing the film removes approximately 80% of the total leachable PAG.
There are numerous methods being explored by lithographers to achieve the patterning of sub-90nm contact hole features. Regarding optical impact on contact imaging, various optical extension techniques such as assist features, focus drilling, phase shift masks, and off-axis illumination are being employed to improve the aerial image. One possible option for improving of the process window in contact hole patterning is resist reflow. We have already reported the resist using a ring opened polymer of maleic anhydride unit(ROMA) during the past two years in this conference. It has several good properties such as UV transmittance, PED stability, solubility and storage stability. The resist using ROMA polymer as a matrix resin showed a good lithographic performance at C/H pattern and one of the best characteristics in a ROMA polymer is the property of thermal shrinkage. It has a specific glass transition temperature(Tg) each polymers, so they made a applying of resist reflow technique to print sub-90nm C/H possible. Recently, we have researched about advanced ROMA polymer(ROMA II), which is composed of cycloolefine derivatives with existing ROMA type polymer(ROMA I), for dry etch resistance increasing, high resolution, and good thermal shrinkage property. In this paper, we will present the structure, thermal shrinkage properties, Tg control, material properties for ROMA II polymer and will show characteristics, the lithographic performance for iso and dense C/H applications of the resist using ROMA II polymer. In addition, we will discuss resist reflow data gained at C/H profile of sub-90nm sizes, which has good process window.
This paper is part of our continuing work on a new generation of more transparent, 157 nm resist platforms, which are based upon capping of fluoroalcohol-substituted, transparent perfluorinated resins (TFR) with a tert-butoxycarbonylmethyl (BOCME) moiety. Recent results indicate that by optimizing both resin structure and loading of photoacid generator and base additive a good compromise can be achieved between resolution power, dark erosion resistance, sensitivity and transparency at 157 nm. Specifically, it was found that a decrease in PAG (50% nominal loading) and base loading (75% nominal loading), coupled with optimization of the TFR resins to achieve higher transparency, gives the best compromise of properties. In this manner, resist systems with a transparency as low as 0.87 AU/micron were designed capable of resolving 60 nm 1:1 features, at a dose of 92 mJ/cm2 (non corrected for sigma), using a strong phase shift mask, and a sigma of 0.3 on a Exitech 157 nm small field mini-stepper. This type of resist material has also been imaged with a larger field tool (DUV30 Micrascan VII) to give 80 nm 1.1.5 L/S features at a dose of 135 mJ/cm2 employing using a Binary mask (σ=0.85). Finally, it was found that our BOCME-TFR based resist system can be used to transfer a 120 nm L/S pattern (imaged by 193 nm lithography) into a hardmask stack on top of silicon.
We earlier developed a series of fluoropolymers (FPRs) for use as first-generation 157-nm photoresist polymers. These FPRs have a partially fluorinated monocyclic structure and provide excellent transparency. However, their etching resistance is low (half that of conventional KrF resists) and an insufficient dissolution rate in tetramethylammonium hydroxide (TMAH) solution. To improve the characteristics of these polymers, while retaining high transparency, we had to redesign the main chain fluoropolymer structure. In this paper, we describe a new monocyclic fluoropolymer structure for a second-generation 157-nm photoresist polymer. This structure also has a fluorine atom in the polymer main chain, as well as a fluoro-containing acidic alcohol group. We synthesized two types of fluoropolymers, ASF-1 and ASF-2. We found that ASF-1 had transparency of 0.18 μm-1, better than that of the FPRs, and the etching resistance was improved. Unfortunately, the dissolution rate was poor. On the other hand, ASF-2 showed even better transparency of 0.1 μm-1, improved etching resistance, and a dissolution rate of more than 600 nm/s, which is sufficient for use as a resist. The introduction of a protecting group (e.g., the methoxymethyl or adamantylmethoxymethyl group) to the hydroxyl group of ASF-2 can be done after the polymerization reaction. Using partially protected ASF-2 with an appropriate protecting group, we were able to fabricate a sub-60-nm line-and-space pattern.
Fluoropolymers are key materials in the single-layer resists used in 157-nm lithography. We have been studying fluoropolymers to determine their potential use as base resins. These polymers are main-chain fluorinated polymers synthesized by co-polymerizing tetrafluoroethylene (TFE) and functional norbornene. We developed a new polymer that is highly transparent and has high dry-etching resistance by attaching a PG-F protecting group, which has high dry-etching resistance, to a TFE/norbornene-based fluorinated polymer. The dry-etching rate for the 15 % blocked polymer was 1.50 times that of a KrF resist and its absorption coefficient at a 157-nm-exposure wavelength was 1.06 /μm. We introduced various photoacid generators (PAGs) to the polymer, and compared lithographic performance. As a result, we found polymer with a triphenylsulfonium-salts-based PAG had a good pattern profile, and polymer with a high-acidity PAG resolved a fine pattern. In particular, polymer with a triphenylsulfonium perfluorooctane sulfonate PAG was able to resolve a 60-nm line and space pattern. We then added various quenchers to the polymer and the PAG, and compared pattern profiles. We found that the use of a high-basicity quencher improved the resolution of the resist and line edge roughness. Consequently, that the polymer with the triphenylsulfonium perfluorooctane sulfonate PAG and tributylamine quencher could resolve a 55-nm line and space pattern. These results provided guidelines for choosing the PAG and quencher for this polymer.
Main-chain-fluorinated base-resins, using the copolymer of tetrafluoroethylene and functional (hexafluoroisopropanol (HFA) group) norbornene, were synthesized. Partial protection of its hydroxyl group as ethoxymethyl group was achieved by two methods, by copolymerization (Method A) or by polymer reaction (Method B). The partial protection by copolymerization was conducted by copolymerizing TFE with the mixture of protected and unprotected monomers (Method A, copolymerization). The partial protection was also carried out by reacting hydroxyl group of the polymer, which is composed of TFE and unprotected monomers with ethoxymethyl chloride in the presence of an amine (Method B). In the polymer reaction, only exo position of the norbornene unit was protected. Their fundamental properties, such as transparency at 157 nm and solubility in a standard alkaline developer, were characterized and studied. A high transparency, i.e., absorbance of less than 0.4 μm-1, was achieved in both methods. However, the polymer prepared by the polymer reaction (Method B) was deprotected more quickly. And this polymer had a higher dissolution rate and development contrast than the polymer prepared by copolymerization (Method A). The Positive-working resists based on this fluororesins were developed and 55 nm dense lines could be delineated by the exposure at 157 nm wavelength with alternating phase shift mask on a 0.9 NA 157 nm exposure tool.
Effect of protecting groups to outgassing characteristics of F2 resists was investigated by using in-situ quadrupole mass spectrometry. The base polymers employed were protected fluorocyclic polymers (FCPs). The protecting groups employed were methoxymethyl (MOM), t-butoxycarbonyl (t-BOC), methoxyethoxymethyl (MOEOM), n-hexanoxymethyl (HOM), and neopentanoxymethyl (NOM) groups. Mass spectra of outgassed species were measured by quadrupole mass spectrometer in the exposure of FCPs with and without photoacidgenerators (PAGs). Kinetic traces of mass spectral intensity were also measured with regard to some noteworthy outgassed species related to deblocking reactions of protecting groups. In the exposure of FCP-HOM with PAGs and FCP-NOM with PAGs, one of the outgassed species related to deblocking reactions are mainly produced only during exposure although that kind of outgassed species originate from irradiated FCP-MOM with PAGs, FCP-MOEOM with PAGs, and FCP-t-BOC with PAGs not only during exposure but also after exposure. Some outgassed species related to the deblocking reactions after exposure are depressed by selecting appropriate protecting groups.
Fluorinated polymers are key materials for single-layer resists used in 157-nm lithography. We have been studying fluorinated polymers to determine their potential for use as the base resin and have developed a new monocyclic fluorinated polymer that has high transmittance (an absorption coefficient of 0.1 μm-1) at a 157-nm exposure wavelength and high dry-etching resistance (a dry-etching rate of 1.86 times that of a KrF resist) under hard mask dry-etching conditions. Moreover, it has a high dissolution rate in standard aqueous tetramethylammoniumhydroxide developer (a dissolution rate of more than 600 nm/s). Using this polymer with adamanthylmethoxymethyl as a protecting group, we were able to resolve a 60-nm line-and-space pattern using a 0.90 numerical aperture 157-nm laser micro-stepper along with a resolution-enhancement alternating phase-shift mask technique. This polymer has enabled both high dry-etching resistance (a dry-etching rate equal to 1.43 times that of a KrF resist) and good imaging performance.
As the required contact holes dimension (CD) reaches to the physical limit of the conventional lithography, the image quality formed in a photoresist film is degraded seriously. To overcome this obstacle, several process-based techniques for ArF lithography have been suggested and some of them are reported to show excellent feasibilities. In this article, three primary techniques for sub-80nm contact holes patterning are examined. They are ArF thermal flow, ArF SAFIER (Shrink Assist Film for Enhanced Resolution) and ArF RELACS (Resolution Enhancement Lithography Assisted by Chemical Shrink). These techniques are originated from different reaction mechanisms and result in distinguished shrink behaviors. Contact holes CDs of different patterns diverge one another depending on the adapted shrink process even though the initial CDs are identical. This is so called a bulk effect and is compensated by the optical proximity correction (OPC) procedure. The relationship of pattern CDs between mask and wafer is used to extract the correction factor. For the shrink process, it is divided to an optical factor and a process factor, that is, the shrink behavior is analyzed in terms of mask error factor (MEF) and process error factor (PEF). The PEF is calculated from the proportionality of post-shrink CD to initial CD of photoresist patterns. Using the PEF, it is possible to characterize each shrink process in the view of CD controllability. Consequently, we classify the shrink processes for the production of 65nm node devices considering the shrink properties and the cost of ownership.
We investigated dependence of ArF resist on Exposed Area Ratio (EAR). Because it can be one of the CD variation factor and it is difficult to correct by OPC. Acrylate polymer based resist showed dependence on EAR. At low EAR, resist showed T-top profile and its CD became large. It could be considered that the profile change was caused by acid evaporation and re-sticking. Resist profile simulation indicated that CD variation appeared at only low EAR. To decreasing the effect of acid evaporation and re-sticking, we tried to increase the amount of acid evaporation by increasing PAB temperature. CD variation by EAR was decreased with increasing PAB temperature.
Recently, we have shown that process effects induced by extending the post-exposure bake time ("overbake") in the process flow of chemically amplified photoresists can lead to significant improvements in Depth-of-Focus (DOF) and small line printing capability. Due to improved acid dose contrasts and a balanced optimization of acid diffusion in the presence of quencher, overbaking has enabled the printing of sub-50nm lines with a large DOF, using binary masks and 193nm lithography. In this paper, the results and findings of a full patterning process in a device flow, using "overbake" as a process enhancement, are presented. The objective is a sub-40nm gate patterning demonstration, using 0.75 NA ArF lithography with phase shift masks as well as with binary masks. Lithographic process latitudes, proximity behaviour, CD linearity, line end shortening, line edge roughness and resist profiles of an overbake process and a standard process are evaluated. Then, the gate patterning capabilities as well as the CD uniformities of ultra-narrow gates obtained by an overbake process are investigated and compared to a standard resist process.
We have evaluated the outgassing products and the in-situ transmittance of a contaminated CaF2 substrate for monocyclic fluoropolymers with four protecting groups: methoxymethyl (MOM), tert-butoxycarbonyl (t-BOC), menthoxymethyl (MM), and 2-cyclohexylcyclohexyloxymethyl (CCOM). We have also evaluated the same type of fluoropolymer with seven kinds of photo-acid generators (PAGs) added to a base fluoropolymer solution. We found little correlation between the total amount of outgassing from the polymer and the decreasing rate of the CaF2 substrate transmittance caused by outgassing adhesion. Although the MOM protecting group generated the largest amount of outgassing products, the most substantial decrease in the transmittance was observed for the t-BOC protecting group. Also, the outgassing products due to use of a PAG did not greatly reduce the absorption coefficient of a CaF2 substrate regardless of the kind of PAG. Therefore, the absorption coefficient of the outgassing-contaminated CaF2 substrate appears to be more sensitive to the type of protecting group, especially the t-BOC protecting group including a t-butyl unit, rather than the type of fluoropolymer or PAG. We analyzed the substrate surface contaminant due to the t-butyl unit by x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and time-of-flight secondary ion mass spectrometry (TOF-SIMS), and found that increasing amounts of organic compounds, containing mainly C- and H-atoms, were adhered to and deposited on the substrate surface with an increasing irradiation dose. We speculate that the contaminants on a CaF2 surface with or without an anti-reflective coating were formed not only through mere physical adsorption, but also through certain chemical combinations. We conclude that in terms of material design of the fluoropolymer resist for 157-nm lithography, we need to pay attention to the protecting group of polymers, especially the t-BOC or t-butyl protecting group, which generates isobutene product during 157-nm irradiation.
Two fundamentally different approaches for chemical ArF resist shrinkage are evaluated and integrated into process flows for 90 nm technology node. The chemical shrink and the corresponding gain in process window is studied in detail for different resist types with respect to CD uniformity through pitch, linearity and resist profiles. For both, SAFIER and RELACS material, the sensitivity of the shrink process with respect to the baking temperature is characterized by a temperature matrix to check process stability, and optimized conditions are found offering an acceptable amount of
shrinkage at contact and trench levels. For the SAFIER material, thermal flow contributes to the chemical shrink which is a function of the photoresist chemistry and its hydrodynamic properties depending on the resists’ glass transition temperature (Tg) and the baking temperature: at baking temperatures close to Tg, a proximity and pattern dependent shrink is observed. For a given resist, line-space patterns and contact holes shrink differently, and their resist profiles are affected significantly. Additionally, the chemical shrinkage depends on the size of contact holes and resist profile prior to the application of the SAFIER process. At baking temperatures below Tg some resists exhibit no shrink at all. The
RELACS technique offers a constant shrink for contacts at various pitches and sizes. This shrink can be moderately adjusted and controlled by varying the mixing bake temperature which is generally and preferably below the glass transistion temperature of the resist, therefore no resist profile degradation is observed. A manufacturable process with a shrink of 20nm using RELACS at the contact layer is demonstrated. Utilizing an increased reticle bias in combination
with an increased CD target prior to the chemical shrink, the common lithography process window at contact layer was increased by 0.15um. The results also indicate a possibility for an extension of the shrink to greater than 50nm for more advanced processes.
Fluorinated polymers are key materials for single-layer resists used in 157-nm lithography. We have evaluated the potential of fluorinated polymer-based resists from the viewpoint of critical dimension (CD) control, using a 0.90 numerical aperture (NA) 157-nm micro-stepper with an alternating phase shift mask (alt-PSM). A resolution limit of 55-nm line-and-space patterns was obtained and the bake temperature dependence of the CD was found to be less than 2 nm/°C. We further evaluated these resists using a 0.80-NA FPA-5800FS1 157-nm scanner for full-field imaging with an alt-PSM. With these resists, 60-nm line-and-space patterns were resolved, and a depth of focus (DOF) of more than 400 nm for 100- and 80-nm line-and-space patterns was confirmed. The CD variation across the wafer for a 100-nm 1:1 dense line pattern was 3.3 nm (3σ). Although there is still a need to improve line edge roughness and dry etching resistance, in terms of CD control the fluorinated polymer-based resists have demonstrated sufficient potential for mass-production of 65-nm-node semiconductor devices and beyond.
ArF lithography has been successfully implemented for the development of sub-90nm DRAM devices. Line width control for the ArF lithography is becoming increasingly challenging as design rules shrink. Many works have been performed on the mask, exposure tools, and tracks to obtain better critical dimension (CD) uniformity, however in-field uniformity, in-wafer uniformity, and wafer-to-wafer uniformity from resist itself was not considered thoroughly. In this experiment, resist parameters that contributes to line width variation were considered in resist perspective. For the in-field uniformity, mask CD uniformity is very important. However, the mask error enhancement factor (MEEF) was different ranging from 3.27 to 5.12 depending on the resists in the k1 0.35 processes even though the screened resists met all the required resolution, depth of focus (DOF), exposure latitude (EL), line edge roughness (LER), and profile. For the resists having good MEEF, the in-filed uniformities of the critical layers were highly improved. The PEB sensitivities of the screened resists were evaluated again in terms of post exposure bake (PEB) sensitivity, which were quite higher than those of KrF resists. They ranged from 4.0 to 11.3 nm/°C. In-wafer uniformity was evaluated and compared using the resists having different PEB sensitivity. The resist with better PEB sensitivity showed better result in in-wafer uniformity. Finally, the wafer-to-wafer uniformities of the resists were evaluated. There was different delay after exposure depending on the sequence of the loaded wafers because it was not easy to control the delay time at the interface of a scanner and a track. The CD increased depending on the sequence, and it coincided well with the delay time of the wafers after exposure. The wafer-to-wafer CD variations were improved using the resists having strong resistance to the delay.
Norbornene monomers with fluorinated substituents are often used in copolymers targeted for photoresist applications at 157 nm. Homopolymers of these norbornene monomers typically exhibit an absorption coefficient greater than 1.5 μm-1. Comonomers, which are often perfluoroolefins, are needed to meet the transparency requirement for 157 nm lithography, namely an absorption coefficient less than 1.0 μm-1. Clearly, a norbornene monomer that gives a homopolymer with an optical density less than 1.0 μm-1 would require less, if any, perfluoroolefin comonomer, providing a distinct advantage in the production of the base resin. Research in Air Products and Chemicals’ labs has led to the discovery that fluorinated hydroxyalkyl ether derivatives of norbornene ring systems with suitable substitution patterns can give homopolymers with absorption coefficients of less than 1 μm-1. The monomers are produced via a novel reaction pathway involving quadricyclane. This pathway provides a versatile and rich synthetic chemistry, and the potential for eliminating, or at least substantially decreasing, perfluoroolefin incorporation into 157 nm photoresists. Specific examples of these reactions are discussed here, along with VUV-VASE and etch resistance data for a series of polymers derived from quadricyclane reactions.
Author(s): Jeff J. Meute; Georgia Rich; Karen Turnquest; Kim Dean; Shashikant Patel; Victoria L. Graffenberg; Michael P. Rodriguez
International SEMATECH(ISMT) has operated a 157nm Resist Test center since June of 2000. During this time, we have processed hundreds of 157nm photoresist samples from major resist suppliers and research organizations. Almost all of these of these early 157nm resists have demonstrated unusual susceptibility to airborne molecular contamination (AMC). Tests were completed at ISMT comparing post exposure delay stability of 157nm fluoropolymer resists to production level 193 and 248 nm resists. The 157nm resist samples were approximately 5X more sensitive to AMC. We have implemented extensive AMC control measures including the introduction of mini-environments and filtered wafer carriers to mitigate the AMC sensitivity of these resists. The effectiveness of the control measures was measured using a real time airborne base monitor, grab samples, and resist delay studies. 157nm resists were severely affected by relatively low airborne base concentrations of approximately 1 ppbv NH3. Hold time stability was significantly improved when the airborne base concentrations was reduced to < 0.5 ppbv using carbon filtration. Small variations in of +/- 0.2 ppbv NH3 appear to be influencing resist profiles and delay response.
So far, there are still many unknown phenomena on the interface of RELACS/resist during mixing bake (MB) processing. Knowing the precise quantitative interaction of these phenomena is significantly important to understand RELACS coating in order to attain much finer contacts as well as spaces with conventional optical lithography. Furthermore, more clear understanding of acid diffusion about RELACS/resist provides us more explicit design concept to increase the shrinkage of RELACS coating for 193nm lithography. In this study, we studied the differences of acid diffusion characteristics between 248nm and 193nm chemically amplified resists with various thermal acid generators (TAGs) in aqueous polymer coating. The diffusion phenomenon from resist to aqueous polymer coating is strongly correlated to the intrinsic diffusion characteristics of both resists. This study also revealed that the quantitative structure properties of organosulfonic acids generated from TAGs affects on the diffusion phenomena from resist to RELACS coating.
Typically resist performance has lagged behind exposure tools as new, shorter wavelengths are introduced in the never-ending industry quest to print smaller features. Over time, however, the performance improves until it matches or exceeds that of the resists used in the previous wavelength node; 193 nm resists have not been the exception. Their resolution and stability has improved but one issue that remains is linewidth slimming. This phenomenon consists in a reduction of resist features when they are exposed to an electron beam in an scanning electron microscope during linewidth metrology. Although this phenomenon has been well described and reasonably well understood, no solution exists to eliminate this problem. In this paper we show linewidth slimming can be significantly reduced by fluorinating the resist after the relief image has been developed, keeping the lithographic dimensions unchanged.
The performance of chemically amplified photoresists, including next generation thin film 157 nm fluorinated copolymers and blends, is affected by such phenomena as polymer/substrate and polymer/air interfacial (surface energy) effects, blend miscibility, small molecule diffusion in thin films, permeability of airborne contaminants, and interactions with products from the deprotection reaction. Using near edge x-ray absorption fine structure (NEXAFS) spectroscopy, it is possible to simultaneously probe the surface and bulk chemistry of chemically amplified photoresists to determine possible causes of pattern degradation, including post exposure delay induced material failure, blend component and small molecule diffusion/segregation to the photoresist surface, and interactions between components of the photoresist formulation and developer. The surface and bulk chemistry of model photoresists were analyzed in the NEXAFS vacuum chamber, equipped with in situ processing capabilities for exposure, controlled dosing of a model contaminant gas (NMP or water vapor), and heating, to quantify component segregation and identify surface phenomena that may be responsible for pattern degradation. For model 157 nm blend films, it was found that there is segregation of one component to the surface of the photoresist film, in excess of the composition of that component in the blend. For polymer blends the more hydrophobic or lower surface tension species will typically wet the film surface when heated in air. Segregation of photo-acid generator has also been demonstrated and the effect of reducing film thickness investigated. As photoresist film thickness continually decreases and the photoresists become increasingly sensitive to environmental contaminants, the interfacial and surface regions dominate the behavior of the material and it is crucial to understand both their physical and chemical nature.
A new strategy for LER simulation is proposed in this paper, which applies a discrete model at 1~2nm scale for exposure, continuous reaction-diffusion model at 7nm scale for post-exposure bake (PEB) and a newly developed continuous statistical lateral dissolution model (SLDM) at 1nm scale for development. Without the computational
complexity of a molecular LER simulator, this new LER modeling is able to simulate LER induced by exposure statistics by incorporating impacts of PEB and dissolution. This LER simulator has been used to investigate factors that impact LER generation, including non-Fickean diffusion, shot noise and resist contrast. SLDM has also been
applied to analyze large unlikely roughness event (LURE) that can lead to chip failure. By finding an equivalent lateral dissolution path for LURE, an analytical estimation of LURE is obtained.
The resist models in PROLITH are designed to be a mechanistic description of the resist chemistry and physics of optical lithography. This is especially true for the expose and post-exposure bake processes, where the resist chemistry can be mapped almost directly to the input parameters in the PROLITH models. In this study, we review the models in PROLITH and show how different chemistry parameters, such as the quantum yield and the reaction kinetics during PEB, can be translated into resist model parameters. With this “chemist to simulator” translator, we show how the models can be used to better understand how resist formulation impacts resist response. Specifically, we will show how quencher loading, and acid and quencher diffusivities impact depth of focus for isolated and dense features.
A variety of experimental evidence suggests that positive-tone chemically amplified photoresists have an intrinsic bias that might limit resolution during high-volume lithographic processing. If this is true, the implications for the semiconductor industry require careful consideration. The design concept of chemical amplification is based on generation of a chemically stable catalytic species in exposed regions of the photoresist film. The catalytic action of the photoproducts on the photoresist polymer causes a change in the dissolution rate in the irradiated regions of the film. Formation of a stable catalyst species is required for chemical amplification, but it has long been recognized that catalyst migration can produce a difference between the initial distribution of exposure energy and the final distribution of photoproducts. This difference, known as diffusion bias, depends on the photoresist chemistry and processing conditions. Diffusion bias is insensitive to exposure conditions, but it is possible to reduce catalyst migration through changes to resist formulation such as increasing the size of the catalyst molecule or processing conditions such as reducing the post exposure bake temperature. Another common approach to limiting diffusion bias is to incorporate base additives into the photoresist formulation to scavenge diffusing acid catalyst. All of these approaches to reducing catalyst migration generally reduce the catalytic efficiency of each photoproduct and therefore increase the total exposure dose required to pattern the film. Increases in required exposure dosage reduce the throughput of the exposure tools and can reduce the profitability of the manufacturing process. In this paper we present experimental results that are suggestive of an intrinsic photoresist bias. This diffusion bias sets a minimum resolution limit for chemically amplified resist systems that can be improved at the cost of reduced throughput and productivity.
Resist technologies that will enable next-generation lithography (NGL) such as extreme ultraviolet lithography (EUV) will require tighter control of critical dimension (CD) with appropriate reduction of line edge roughness (LER) of resist features to levels that seem unrealizable today. Given the delicate balance existing between LER, resolution and sensitivity that is associated with photoresist patterning, alternative processing methodologies that can address such parameters individually are required. In this work a post-processing method designed to control LER is proposed based on the ability of an additive-containing rinse to condition the surface of photoresist patterns. Organic salts added to the final rinse used to quench the development process are found to be particularly effective towards this end. LER reduction up to 15% was observed for a broad range of 193 nm resist systems, while preserving the integrity of the pattern profiles. The dependence of LER reduction on additive concentration was investigated and the limited improvement observed was explained based on the tendency of the additive to self-aggregate. Finally, the advantage of including an additive in the rinse step instead of using an additive-containing developer is discussed in terms of critical dimension bias and overall image integrity control.
A wide range of phenomenological nonlinear dissolution behaviors has often been noted for real resist systems but no comprehensive framework has yet been proposed that explains the underlying physical phenomena and connects them to resist composition. We present a taxonomy of dissolution processes in representative resist copolymers measured using quartz crystal microbalance frequency-resistance analysis and optical reflectance measurements. The underlying physical and chemical processes leading to the observed behaviors are identified using a detailed chemical kinetics implementation of the critical ionization model for resist polymer dissolution.
Current semiconductor manufacturing utilizes exposure wavelengths from 365 nm to 193 nm, and current research is centered on photoresist development for 157 nm. Our research group discovered the strong inhibition response in the fluorocarbon resins designed for use at 157 nm. We have been investigating dissolution inhibitors (DIs), some of which also serve as photoacid generators (PAGs), that strongly inhibit the dissolution of poly(2-(3,3,3-trifluoro-2-trifuoromethyl-2-hydroxypropyl) bicyclo[2.2.1]heptane-5-ene)(PNBHFA) (1) and the Asahi glass RS001 polymer (2). These inhibiting PAGs, in particular, result in the creation of 2-component resist systems consisting only of the resin polymer and the PAG-DI. This design enables greater ease of formulation, reduces the number of variables present in resist development, and offers improvements in sensitivity and line edge roughness. The synthetic approach has been to design transparent, inhibiting compounds for use at 157 nm. However, during our investigation of these compounds, we found that there is an inherent “backwards compatibility” for these PAGs and DIs at 193 nm, 248 nm and 365 nm. This has created the ability to effectively design dissolution inhibitors, photoactive or otherwise, that span virtually all of the wavelengths used in photolithographic processes today. Here we will present the design, development and imaging of modern dissolution inhibitors suitable for use in a wide range of photolithography technologies.
The influence of film thickness and molecular weight on the diffusion coefficients of water, benzene, and trifluoroacetic acid in two photoresist polymers, poly(p-hydroxystyrene) and bis-trifluoromethyl carbinol substituted poly(norbornene), has been studied using quartz crystal microbalance (QCM) methods. Diffusion coefficients for films as thin as approximately 50 nm were determined. It was observed that the diffusion coefficient was a strong function of film thickness, and that the diffusion coefficient decreases drastically as film thickness is reduced below a critical value. This critical thickness value is found to be a function of both polymer structure and molecular weight. In addition, the effect of film thickness on the equilibrium uptake of the various penetrants was also determined. In particular, the equilibrium water uptake was shown to depend strongly on film thickness, polymer structure, and polymer molecular weight.
Model based OPC is critical for mask design employing current design rules. Models based on the aerial image assume resist response will generally follow behavior predicted by diffractive optics, however, some classes of resist introduce non-optical resist response. It is critical to understand the proximity behavior of these resists in order to accurately manufacture lithographic masks. In this paper, we present modeling and experimental results for a class of resist systems exhibiting a strong non-optical resist response; reversed bias in nested versus isolated space pattern dimension. The behavior is ascribed to a secondary source of proximity originating from heat absorbed during PEB within the exposed region, which produces a non-uniform, pattern dependent, effective PEB temperature. A continuum PEB model employing a combined mass and energy balance is developed as well as experimental methods to determine the parameters in the model. The resulting calibrated model reproduces the degree of proximity bias measured with SEM for a variety of process conditions. Both proximity correction and characterization of k1 performance for the resist system are discussed.
It is confirmed that strong inhibitors must not only interact with the polymer but also are highly hydrophobic. A proportional relationship was shown between the inhibition and logP of different additives. In general, PAGs also follow this relationship, however triphenylsulfonium PAGs appear to be less effective inhibitors than bis-t-butylphenyliodonium PAGs having the same anions.
The ability of interdigitated electrodes to serve as novel chemically amplified resist characterization tools has recently been demonstrated through their ability to measure the Dill C kinetic rate constant for photoacid generation. The work presented in this paper attempts to further extend the capabilities of the interdigitated electrode (IDE) sensors by investigating their potential use as a measurement tool for photoacid diffusion coefficients. Impedance spectroscopy of chemically amplified photoresist coated interdigitated electrodes is used to calculate the bulk ionic conductivity of the resist film. The ionic conductivity is subsequently utilized in the Nernst-Einstein equation to calculate the diffusion coefficient of the photoacid, assuming that it is the major charge carrying species in the film. A detailed description of the measurement and data analysis processes required to calculate the diffusion coefficient of triphenylsulfonium triflate in poly(p-hydroxystyrene) is provided. In addition, the effect of varying the relative humidity of the measurement environment upon the impedance data collected has been examined. It has been observed that the presence of water within the resist film, typically as a result of absorption of water from the humid ambient environment, dramatically changes the conductivity of the resist coated IDE. This change is apparently the result of changes in the proton conduction mechanism within the resist as a function of film water content. A discussion of several possible causes of this phenomena and its impact on the interpretation of the electrical data and the calculation and meaning of an acid diffusion coefficient are presented.
LER is found to correlate strongest with the background, or flare, portion of the lithographic aerial image contrast, when compared with the image slope and “standard” (max - min)/(max + min) contrast. A large pool of collected data and rigorous statistical analysis of variance conclude with far greater than 99% confidence that as any of these measures of aerial image profile degrade, LER increases. However, the relation between background exposure and LER is by far the most significant.
A technique was developed to investigate the role of aerial image contrast and image-log-slope (ILS) on the resulting magnitude of line edge roughness (LER) in resist with the goal of determining if the minimization of LER in current state-of-the-art, chemically amplified resist materials was limited by the quality of the projected aerial image or the materials and processing of the resist. The process of image fading was employed as the vehicle for controlled aerial image degradation. By reducing the quality of the aerial image through fading, the image contrast and ILS were decreased in a well-controlled and predictable manner, resulting in increased magnitude of LER. The link between experiment and simulation was made possible by the identification of the iso-fading condition, which in analogy to the iso-focal dose, results in a unique exposure dose for which the critical dimension (CD) of a resist feature does not change with increasing levels of fading. At the iso-fading condition, experimentally measured values for LER were analyzed as a function of the contrast and ILS of the aerial image used for patterning. It was determined that contrast was a poor predictor of the magnitude of LER though variations in feature type or illumination. The change in LER as a function of the ILS, however, produced a common basis for the comparison of LER through variations in line width, pitch, fading, increased background level of light, and illumination conditions. To include the effects of exposure dose on the resulting LER of resist features, the experimentally measured analog of the ILS, the resist edge-log-slope (RELS), was also used to produce a common curve for the evaluation of resist LER. Although overexposure can be used to further increase the ILS of the aerial image at the edge of the printed feature, the magnitude of 3σ LER in PAR735 resist appeared to be limited to a value of approximately 5.0nm in the limit of infinite RELS. This suggested that while the aerial image plays a strong role on determining the magnitude of LER during resist printing, there also exists a fundamental limitation to LER from the resist materials that cannot be improved by further increase in the quality of the aerial image.
ArF lithography is essential to develop a sub-100 nm device, however, line edge roughness (LER) and line width roughness (LWR) is playing a critical role due to the immaturity of photoresist and the lack of etch resistance. Researchers are trying to improve LER/LWR properties by optimizing photoresist materials and process conditions. In this paper, experiment results are presented to study the impact of LER/LWR to device performance so that the reasonable control range of LER/LWR can be defined. To implement the experiment, 80 nm node of single NMOS transistors were fabricated, which had various range of gate length, width, and LER/LWR. The amount of LER/LWR could be successfully controlled by applying different resist materials, defocus, and over etch time. Experimental results show that leakage current is significantly increased when LWR is greater than 10 nm. In addition, it is observed that both threshold voltage and on-off current variation get increased exponentially as gate width decreases.
EUV photoresists must be developed that meet the stringent patterning requirements for the next-generation of microprocessors (32nm node and beyond). In this paper we will address the ability of EUV photoresists to meet the material targets specs (MTS), such as CD resolution, line width roughness (LWR), photo sensitivity, and absorbance. The challenges of meeting CD resolution and line width roughness specs are not restricted to EUV lithography, but also need to be met by other technologies (193nm, 157nm, and 193 immersion technologies). However, EUV photoresists encounter the unique challenge of meeting these MTS with higher photospeeds than any other lithographic technology due to EUV source requirements. The design of EUV resists that meet all of the MTS and have sufficiently high photospeeds is very challenging. In this paper, we will present experimental results of EUV photoresists patterning results from the 10X tool at Sandia National Lab, and the F2X at Lawrence Berkeley National Lab. Data on resolution, LWR, photo sensitivity, and absorbance are included. Finally we address the capabilities of current EUV resists to meet the patterning requirements, and highlight areas where acceleration is required to meet the Intel roadmap.
Organic polar solvent (1-butanol) versus aqueous base (tetramethylammonium hydroxide, (TMAH)) development quality are distinguished by neutral versus charged polymer (polyelectrolyte) dissolution behavior of photoresist bilayers on silicon substrates comprising poly(4-hydroxystyrene) and poly(4-tert-butoxycarbonyloxystyrene), PHOSt and PBOCSt, respectively. This model line-edge was broadened by photoacid catalyzed deprotection to a final interfacial width of 35.7 Å and subjected to different developers. 1-butanol develops with an increased penetration depth than aqueous base development consistent with an increased solubility of the protected containing component in the organic solvent. This enhanced dissolution with the polar solvent results in an increased surface roughness of 73 Å, whereas the development with TMAH at concentrations between (0.1 to 1.1) M1 leads to surface roughness between (4.5 to 14.4) Å, as measured by atomic force microscopy. These measurements suggest that the elimination of resist swelling, in the presence of a protection gradient, is a viable strategy to reduce roughness and control critical dimensions. The influence of added salt to developer solutions was also examined by developing the model bilayer. A decrease in surface roughness from (10 to 6.5) Å was observed between (0 to 0.70) M KCl in 0.26 M TMAH.
The IC industry is moving toward 90nm node and below. The CD size of implant layers has shrunk to 220nm. To achieve better CD uniformity, dyed KrF resist and top anti-reflective coating (TARC) are commonly used in advanced photo process of implant layers. It’s well known that bottom anti-reflective coating (BARC) has better reflection control over TARC. However, dry etching process is required if typical organic BARC is applied to photo process of implant layers. It is undesirable for two reasons. The first reason is the substrate damage caused by plasma etching could affect the device performance. The second reason is higher cost due to additional processing steps. In order to overcome those two shortcomings, developable BARC (DBARC) is introduced. It is a new type of BARC, which is soluble to developer, TMAH solution, in the resist development step. There are some reports on the developer-soluble KrF BARC. Most of them are polyamic acid and their solubility to alkline could be adjusted by changing bake condition. However, its development is isotropic, which make it difficult to get a vertical profile. Therefore, we have developed a photosensitive developer-soluble BARC (DBARC) which is anisotropic after exposure and thus results in a nice vertical profile. The photosensitive DBARC utilizes the same concept as chemically amplified resist. It has acid-cleavable groups in the resin and PAGs in the formulation. The photosensitive DBARC turns soluble to TMAH developer after exposure and resist PEB. The solubility difference caused by exposure makes developing process anisotropic and thus improves profile control. In this article, we will report the evaluation results of various combinations of KrF resists and DBARC for implant layers. Since both the resist and DBARC are photosensitive, matching of the photo speeds of them is essential. The amount and type of PAG in both the resist and the DBARC play a very import role. Finally, the optimized combination showed acceptable lithography process window and good CD uniformity over topography.
As integrated circuit manufacturing moves towards smaller feature sizes, ion implant photo levels are becoming critical layers with lithography demands as tight as 180 nm line/space patterning capability. Advanced materials are required for junction levels to improve the critical dimension (CD) control and resolution. Dyed KrF resists are reaching the limit in their ability to control CD variation due to parasitic light reflections from the underlayer. The use of a bottom anti-reflective coating (BARC) under KrF resists reduces the reflective effect from the oxide substrate, leading to better CD control. Unfortunately, a standard organic BARC that requires plasma etch before implantation can cause silicon substrate oxidation damage as well as increased wafer cost due to additional process steps. The use of a new developer-soluble organic BARC shows an advantage in optics without degrading the underlying substrate before implantation. The advantage of using an ESCAP resist in combination with a wet-developable BARC over the single resist layer scheme has been clearly demonstrated and the system is well adapted to ion implant layers for 65 nm technology.
Satellite spot defects are a class of defects widely observed in photoresist processing in 248 nm and 193 nm lithography. These defects become more and more significant as the feature sizes shrink and can potentially become “killer” defects, leading to bridging between lines and/or blocking vias. Traditional potential solutions (i.e., optimization of development rinse step) have yielded improvements in the past but did not eliminate the problem. The use of water-soluble topcoat layers was shown to eliminate these defects but it imposes limitations on throughput and cost and it is incompatible with 157 nm lithography and 193 nm immersion schemes. In this work, we report the use of aqueous surfactant solutions for the suppression of defects in 248 nm and 193 nm lithography, with emphasis on satellite spot defects. Suppression of total defects by up to ~99% and practically complete elimination of satellite spot defects were achieved by use of aqueous surfactant solutions for various resists. A handful of materials that can be incorporated into rinse solution for the successful elimination of blob defects in a variety of resists were identified. It was determined that the two most important factors that enable successful defect elimination are the surfactant concentration and the extent of surfactant adsorption to specific resist systems.
A rapid supercritical drying process for resist patterns has been developed that features three novel techniques: the removal of rinse water based on a difference in specific gravity, rapid heating with a hot wafer holder, and the rapid release of supercritical fluid using helium. These techniques are efficient and cause no damage to resist patterns because no harmful chemicals, such as surfactants, are used. In addition, the effectiveness of these techniques has been demonstrated on several types of resists: polyhydroxystyrene-based, methacrylate-based, and fluoropolymer-based resists, which are used in 248-nm (KrF), 193-nm (ArF), and 157-nm (F2) lithography, respectively. The release time for helium is less than 30 seconds. The short time needed for this supercritical drying process makes it very practical for both current and next-generation lithography.
The electron beam sensitivity of hydrogen silsesquioxane (HSQ) has been enhanced by including sensitizers that decompose to generate nucleophiles which catalyze the conversion of the silicon hydride (Si-H) moieties in HSQ into the insoluble siloxane (Si-O-Si) network. In this study, the consequences of including triphenylsulfonium hydroxide (TPS-OH) and 2-nitrobenzyl N-cyclohexylcarbamate (NBC) as a photodecomposable base (PDB) and photobase generator (PBG) were investigated, respectively. It was found that using 5 wt% loadings of TPS-OH or NBC in HSQ in conjunction with a post-exposure bake process enhanced the sensitivity of large features exposed at 25 keV accelerating voltage by approximately 50 and 40 %, respectively. Similarly, the electron beam doses required to print single pixel wide lines exposed at an accelerating voltage of 25 keV were enhanced by 70 and 50%, for 5 wt% loaded TPS-OH or NBC films, respectively. It was also found that the basicity and nucleophilic strength of the sensitizer affects the rate of the undesired hydrolysis reaction of HSQ which occurs in solution. For the sensitizers used in this study, the sterically hindered TPS-OH is a poor nucleophile which stabilized the solution against condensation and formation of a siloxane network, while the moderately nucleophillic NBC slightly decreased the stability of the solution. Also, it was found that thermal baking alone could be utilized to enhance the sensitivity of HSQ, but a drastic loss in contrast was observed. The combination of either TPS-OH or NBC and a post-exposure bake produced superior results, as compared to thermal baking alone, in terms of increasing the sensitivity of HSQ while maintaining good contrast.
Due to the involvement of organic solvents and strong bases in the pattern development process, conventional lithography, a technique that has been well-developed and widely used in the semiconductor industry, is not suitable for direct cell and protein patterning. In order to address this issue, we recently developed a biocompatible chemically amplified photoresist, BIORESIST, with which patterns can be generated without involving any harsh chemical treatment. Such a BIORESIST contains tert-butoxycarbonyl (t-BOC) protecting groups. In vitro cell culture study has shown that the t-BOC protected BIORESIST and its carboxyl-substituted counter-part interact very differently with cells. The former is non-cell adhesive, while the latter not only keeps cell attached, but also supports cell proliferation. This unique property prompted us to generate patterns (25 μm L/S) with this BIORESIST with no wet development involved. Rat fibroblast cells were cultured on the patterned surfaces. The results demonstrated that cells were strongly aligned along the patterns and attached exclusively to the adhesive region as opposed to a random appearance on the plain control surface after 24 hr of incubation. With this BIORESIST, the scalability aspect of conventional lithography could be well applied for cell patterning.
EUV lithography is to date the most promising NGL technology for the sub-50nm technology node. In this work, we have designed and synthesized several types of organoelement resists with low oxygen content for high transparency. Boron was incorporated in the resist structures to improve both etch resistance and transparency. Both negative-tone and positive-tone resists were made containing the carborane group. In a preliminary study, these polymers were imaged using a 248nm stepper to produce images with good resolution. The incorporation of a carborane structure provides these polymers with excellent oxygen etch resistance. Hall effect measurements were performed and no contamination was found in the substrate after applying boron-containing polymers.
Author(s): Brian R. Harkness; Geoff B. Gardner; James S. Alger; Michelle R. Cummings; Jennifer Princing; Yeong Lee; Herman Meynen; Mario Gonzales; Bart Vandevelde; Mathieu Vanden Bulcke; Christophe Winters; Eric Beyne
A growing need for low stress high temperature thick film materials has prompted the development of new spin-coatable photopatternable silicones (Dow Corning WL-5000 series) to assist manufactures in building the next generation of electronic devices. These new negative-tone materials can be easily coated onto electronic substrates and patterned using standard i-line and broadband lithographic processes. Films ranging from 6 to 50 μm have been demonstrated with patterned features resolved to an aspect ratio of less than 1.3. The etched regions provide a sloped sidewall and curved surfaces to facilitate metallization processes. The films are cured at low temperatures (150 to 250°C) to provide low modulus values in the range of 150 to 500 MPa, are inherently hydrophobic, and are based on cure chemistry that is acid free and delivers thermally stable cross-links. As a result, the films show very little shrinkage during thermal cure (~2%), do not require extended high temperature processing, and provide a very low residual stress (<8 MPa). They also show excellent thermal stability and mechanical integrity when exposed to high temperatures. A simple wet process has been developed to facilitate film rework and allow for sacrificial layer applications.
For 157-nm single-layer resists, dry etching resistance is an important issue because of the difficulty of striking a balance between 157-nm transparency and an acceptable level of dry etching resistance. To achieve an acceptable trade-off, the fluorine atom can be introduced into the resist polymer structure to obtain higher transparency, despite the fluorine atom’s high reactivity in the plasma etching process. We recently proposed a model for estimating dry-etching-resistance (the KI-model) and have shown that it can be effectively applied to the design of new fluoropolymer structures. Through simulation based on the KI-model, we were able to develop a new fluoropolymer with good dry etching resistance and high transparency. We found that a new protective group, 2-cyclohexylcyclohexanoxymethyl (CCOM), improved the characteristics of our novel fluoropolymer, compared with use of a MOM group, when used in the base resin of the resist. In this paper, we report on the usefulness of the KI-model for developing new fluorinated protective groups and new base polymers. Moreover, we have developed a new base fluoropolymer which has higher transparency and a similar degree of dry etching resistance as a monocyclic fluoropolymer with a CCOM protective group.
The development of a process for contact hole tends to be overdue although priority is generally given to the development of line and space patterns. The size of a contact hole pattern in 45 nm node will be about 60 nm. 157-nm lithography will be applied to 45 nm node. The depth of focus is critical issue. Then we evaluated three types of contact hole shrinkage techniques such as Thermal Flow, SAFIER and RELACS for 157-nm lithography. A resist which was consisted of a polymer with fluoride on a side chain was used. The shrinkage temperature was optimized by applying hole shrinkage processes to patterns formed by 193-nm lithography. It was made clear that Thermal Flow process was not practical in the view point of extremely high shrinkage temperature. On the other hand, hole size was reduced by about 20 nm in the case of SAFIER and RELACS at controllable temperature. It is an advantage of SAFIER process that hole size is changed with shrinkage bake temperature. In RELACS process, small mixing temperature dependence of critical dimension (CD) is advantageous. Also in 157-nm lithography, it was shown that the shrinkage techniques such as SAFIER and RELACS are possible, and the about 75nm hole pattern was formed.
An in-situ quartz crystal microbalance (QCM) method was applied to quantitatively measure the outgassing from 157-nm resists, i.e., fluorinated cyclopolymer (FCP) and its derivatives blocked with alkoxymethyl ether units, in real time. The frequency change of quartz crystal coated with resist films was monitored during exposure and the mass desorbed from the resist films was calculated as amounts of outgassing. The sensitivity of the present QCM system was about 1 ng. The outgassing rate from FCP was much lower than FCP blocked with alkoxymethyl ether moiety, suggesting that the outgassing was mainly caused from the blocking units. Acidic components in outgassing were quantitatively measured by in-situ QCM technique using the quartz crystal coated with poly(4-vinylpyridine) (PVP) or a copolymer (DMEST) of 2-(dimethylamino)ethyl methacrylate and styrene. The acidic materials adsorbed on quart crystal were monitored during exposure and the mass adsorbed was calculated. The amount of acidic compounds in outgassing was dependent on fluorine content of the resist polymers.
Norbornene-based monomers were synthesized to include fluorinated moieties and/or chemical amplification switching groups. Fluorinated homopolymers to be used as the basis of resist materials were synthesized from these monomers by addition polymerization using allylpalladium chloride dimmer. Monomers and polymers have been identified and partially characterized for important lithographic properties. A monomer containing a silsesquioxane group has been incorporated into this platform to enhance etch resistance. A monomer containing an acid-cleavable group has also been synthesized and will be incorporated into this platform.
Electromagnetic radiation in the vacuum-ultraviolet (VUV) region is needed for imaging of very fine features at the 65 nm and 45 nm nodes. Photolithography using 157-nm radiation, emitted from an F2 excimer laser, is a candidate for next generation lithography. Only chemically amplified resists containing fluorinated hydrocarbons and siloxanes have the required transparency at this wavelength. We have identified hexafluoroisopropanol units as a building block for our 157-nm resist polymers. This paper reports our progress on the most recent research development for this platform. The hexafluoroisopropanol functionality, which has a pKa similar to phenol, has been used to increase the transparency of 157-nm single-layer acrylate-based resists. Our recent effort has been focused on the syntheses of new acrylate monomers with highly transparent building blocks based on trifluoroacetone. The first example, a homopolymer derived from trifluoroacetone bearing a fluorinated hemiacetal unit, has moderate transparency at 157 nm (A = 1.9 μm-1). We have also introduced a new acrylate monomer containing a trimer based on trifluoroacetone, where the 6-hydroxy group in the hemiacetal unit is substituted by a fluorine atom, with an acceptable transparency at 157 nm (A = 2.1 μm-1). Copolymers of the former monomer, derived from trifluoroacetone, and tert-butyl α-fluoroacrylate have also been prepared and showed good 248-nm lithographic performance suggesting suitability for 157-nm lithography. This paper will discuss the transparency, etch resistance and chemical properties of several fluorinated acrylate-based resists, synthesized from groups containing pendent hexafluoroisopropanol units and trimers derived from trifluoroacetone.
Recently, there are lots of interest in using chemical amplification (CA) on electron beam lithography for application to photo mask fabrication, direct writing, and projection printing. E-beam resists introducing chemically amplification concepts provide superior lithographic performance in comparison with traditional non CA E-Beam resist in particular high resolution and sensitivity. In first approach, we applied CA concepts to acetyl polymer based E-beam resist (resist thickness: 4,000Å), which can print fine images (<100nm), meet sensitivity (10μC/cm2), and have stability against post exposure delay (PED)(>10hrs) using 50KeV E-beam exposure tool. But, there is vacuum delay problem (40nm CD shrinkage/5hrs) due to thermally unstable blocking group in polymer. To prevent this vacuum delay problem due to polymer-inherent thermal instability in low-activation-energy-acetal polymer, we newly designed various poly(hydroxystyrene-acrylate) copolymer derivatives that contained thermally stable (acrylate) acid-blocking group. In this presentation, first we will discuss the chemistry of newly designed copolymer derivatives, and second, vacuum delay effects and other lithographic performances (resolution, sensitivity, line edge roughness) of these resist systems.
The shrinkage of resist pattern during in-line SEM measurement has been argued and studied as one of the problems unsettled for manufacturing with ArF photolithography. Many of attempts to solve this problem have focused their attentions on the improvement of resist and inspection equipment. We bring up BARC (bottom anti-reflective coating) as a new impact factor on SEM shrinkage of resist. Practically, although the same resist was employed, our shrinkage tests gave the results depending on the kind of BARC. Feature size and depth of focus also affect SEM shrinkage of resist. Effect of reflectivity on SEM shrinkage was evaluated by changing thickness of BARCs and resultantly was somewhat significant. In this paper, the BARC-dependent results of SEM shrinkage are analyzed and discussed to provide a possibility that BARC may have another function of reducing SEM shrinkage.
The ArF resist has been evaluated focusing on resin character such as molecular weight, monomer composition and polydispersity (Pd). The resin properties were investigated to elucidate that which parameter was affected to the line edge roughness (LER). The Pd was correlated with LER. As the Pd was large, the LER was small. The resin molecular weight and monomer composition were affected to their vertical profile. Low molecular weight portion rich resin resulted in round and t-top profile, whilst high molecular weight rich resin resulted in square profile. The amount of lower molecular weight fraction was changed by purification method. The lower molecular weight resin caused severe tapered profile. It was concluded that 1) shift of Mw to smaller and 2) higher content of low molecular size fraction lead to rounded and tapered pattern profile. Lot-to-lot stable good pattern profile has achieved by controlling polymer molecular weight and content of low molecular size fraction in small variation range.
Design and development of novel monomers and copolymers for 193-nm lithography are described. At the present time, 193-nm lithography is required for 65-nm node and below. Novel monomers and copolymers are considered to be candidates for the development of higher performance resist materials. We focused our attention on pattern profile and line edge roughness. In design of novel monomers, molecular orbital calculation was adopted. It was revealed that CN-group has a higher potential than other polar groups. Novel monomers that contain CN-group were designed, synthesized and co-polymerized with traditional acrylate monomers. It is expected that these copolymers could be higher performance resist materials that could be used in 65-nm node and below.
Sensitivity and resolution capability of photoresist depend on various parameters, such as efficiency of photoacid generation, base strength, types and concentration of protection groups on a polymer, as well as lithographic process condition. We have prepared polymers containing different protecting groups and investigated their effects on the sensitivity, and eventually, on ArF resist photolithographic behavior. Also, several different photoacid generators (PAGs) and bases were employed to study the influence of them on the resist sensitivity. We have changed process condition, especially, bake condition to discuss the role of bake temperature on the photochemical efficiency of the resist. It was found that the diffusion of the photogenerated acid and bases is the most significant factor to determine resist sensitivity than others. The detailed results will be discussed in this paper.
A few kinds of N-hydroxy maleopimarimide sulfonate derivatives were synthesized. These compounds have high solubility in commonly used solvents and high thermal stability. The UV absorption properties of these compounds in polyethylene glycol film indicate that they have suitable absorption and transparency at 193 nm wavelength. The photolysis and lithography of these compounds under radiation of low pressure Hg lamp were studied.
Novel water-developable negative resists were designed to induce both cross-linking and polarity change upon exposure and bake. The matrix polymers were synthesized by copolymerization of glyceryl methacrylate and methacrolein. The acid-catalyzed acetalization of the polymer induced cross-linking, polarity change, and increase in dry-etch resistance. The resist formulated with this polymer and cast in a water-ethanol mixture, showed 0.7 μm line and space patterns using a mercury-xenone lamp in a contact printing mode and pure water as a developer.
Post exposure bake temperature sensitivity (PEB sensitivity) is getting important for below 100nm device. There are several factors affecting the PEB sensitivity including acidity and diffusion of photogenerated acid, stiffness and free volume of base polymer, and so on. Among them, the activation energy for deprotection reaction is regarded as the most critical factor. We have investigated the influence of protection group with various activation energies as well as Tg of polymer. Several different protection groups were incorporated into the polymer chain to modify activation energy of the resist. Also, we have investigated the influence of acid diffusion and quencher diffusion ability on PEB sensitivity. Three photoresists were formulated with different concentration of acid diffusion controller to asses the influence of acid diffusion on CD variation. And to evaluate the effect of quencher diffusivity on CD change, photoresist was formulated by adding amines having various different molecular size. Detailed results and new resist with reduced the PEB sensitivity will be reported in this paper.
The via-first process is unique by the fact that a material is needed to fill the vias to some arbitrary value, with little or no isolated-dense via bias so that the underlying layer underneath the via is protected from the trench etch step. Secondly, this material may have to coat over the surface of the wafer with some chosen thickness again with minimum or no bias to maximize the trench photolithography process window. Finally, the material must be easily removed from the via after the trench etch with no residue, crowning, or fencing. The ideal via fill material would be able to perform all the above listed parameters, but no perfect solution exists yet. The etchback process that is discussed herein, called the solvent etchback (SOLVE) process bypasses these lengthy modules, will fit within today’s manufacturing processes and will have little impact on throughput of the photobay coating tools. The process utilizes industry standard photoresists solvents such as PGMEA, Ethyl Lactate, PGME and existing solvent prewet dispense nozzles in the BARC coater module. Also, this process only requires one material that can both fill the via and act as a BARC during the trench photo step with a user defined thickness on top the wafer that will minimize light reflections coming from the substrate. The process flow for the SOLVE process is: 1. Coat a wafer with a thick BARC to planarize the wafer and minimize isolated-dense bias. 2. Bake the BARC so that it is partially crosslinked.
3. Apply a solvent to the wafer and etchback the BARC to a thickness that suits the trench photo step. 4. Bake the BARC to fully crosslink the BARC. Process variables that can have an affect on the SOLVE process are the softbake temperature and time to modify the BARC thickness on the wafer. Dispense parameters that will modify the post-etch uniformity of the wafer include the dispense time, dispense spin speed and the IDI M450 dispense pressure. The repeatability of the process can be modified by changing the solvent spin off speed and acceleration.
This paper discusses a novel approach of using a developer-soluble gap fill material, wherein the gap fill material is coated in a layer thick enough to planarize all the topography and is then recessed using a standard 0.26N TMAH developer. The material recess process takes place in the same coater track where it is coated and therefore simplifies the process and increases wafer throughput. Performance and properties of two types of developer-soluble gap fill materials (EXP03049 and NCA2528) based on two different polymer platforms will be discussed in detail.
Bottom anti-reflective coatings (BARCs) are essential for achieving the 65-nm node resolution target by minimizing the substrate reflectivity to less than 1% and by planarizing substrates. We believe that the developments in 157-nm BARC products are on track to make them available for timely application in 157-nm lithography. We have made some significant improvements in resist compatibility and etch selectivity in relation to the latest available 157-nm resists.
Two chromophores having desired high light absorbance at the 157-nm wavelength have been identified. The prototype BARC formulations basically meet the critical requirements for workable 157-nm BARCs, including optical properties, thermal stability, photo-stability, etch rate and selectivity, and compatibility with photoresists. The BARCs also show good coating quality and stripping resistance. Another essential feature of the BARCs is that they are formulated in industry-accepted safe solvents. The lithographic profiles of a benchmarked 157-nm photoresist on our prototype BARC LH157B show straight 60-nm L/S patterns. LH157B also exhibited excellent lithography performance as an ArF BARC. Optimization of the BARC formulations is in progress.
Dyed photo resist and/or Top Anti-Reflective Coatings (TARC) has been used as mold in ion doping layer. However, in sub 0.25-micron pattern, this system is difficult to apply due to their poor CD control ability. As the chip size is shrunk to sub 0.25-micron in ion doping layer, the use of Organic Bottom Anti-Reflective Coatings (BARC) is strongly required. On the other hand, current Organic KrF BARCs does not dissolve in alkaline developer, and the dry etching process is indispensable in order to remove Organic BARCs. For this reason, it is difficult to apply the current Dry type Organic BARCs to the ion doping layer. To solve this problem, Wet Developable Organic BARCs to be applied to sub 0.25-micron, which have alkaline solubility have been required. NCA800 series that are the suitable alkaline developable KrF Organic BARCs, has been developed and achieved. Alkaline solubility of these BARCs was controllable by baking temperature. NCA800 series show the excellent litho performance and coating property.
Multi-layer lithography processes have been introduced to fabricate very fine structures over a topographic surface for advanced semiconductor device production. The first layer formed on the topographic surface is the planarization layer to provide surface planarity for additional thin layer(s) of material. Such materials could be a photoresist, a hardmask, or both with uniform film thickness for the lithography step to image the structures. However, the large size and distribution variation of the topography structures across the substrate surface have a major impact on the performance of the lithography processes. A new planarization process, contact planarization (CP), has been introduced to improve thickness uniformity and to provide global surface planarity for multi-layer lithography applications. This study focuses on planarizing an experimental organic 193-nm BARC layer on via wafers to minimize iso-dense film thickness bias and provide improved global surface planarity for the bilayer photolithography process. In addition, minimum thickness bias improves control of downstream processes such as plasma etching. This paper will discuss this unique planarization process and its performance with various thicknesses of the experimental 193-nm BARC on via wafers. The photolithography performance of the material and process will be discussed.
Author(s): Jun Kyu Ahn; Seon Ho Choi; Young Keun Kim; Ki Yeop Park; Jae Sung Choi; Eun Suk Hong; Kang Sup Shin; Si Bum Kim; Kyeong Keun Choi; Sung Bo Hwang; Jeong Gun Lee
As via first scheme is employed for dual damascene patterning, via filling process has been posed many challenges to the patterning process. For organic BARC assisted dual damascene patterning, differences in etch selectivity between the organic BARC and ILD material generate fence defect problems. It is highly improbable that organic BARC film remains thick enough to protect the via bottom. To reduce the negative impact on the substrate, the BARC material requires to fill small vias. In addition, anti-reflective behavior for KrF lithography, comparable dry-etching and high wet-etching selectivity to the ILD, and compatibility with photoresist processing are necessary for a successful dual damascene patterning. A sacrificial, spin-on 248nm UV absorbing organosiloxane based inorganic BARC has been developed to meet these needs. Inorganic BARC is a material that fills the vias and reduces iso-dense bias for both fill and top coverage and hole-free substrate at trench lithography.
In this paper, the comparison of the performance of inorganic BARC and organic BARC assisted dual damascene patterning with low-k dielectric was conducted. We evaluated the performance of inorganic BARC in terms of the via fill capability, depth of focus, exposure latitude, etch selectivity and etch profile results. The reduction of iso-dense bias from via filling with inorganic BARC instead of organic BARC is discussed.
Author(s): Charles J. Neef; Vandana Krishnamurthy; Mariya I. Nagatkina; Evan Bryant; Michelle Windsor; Cheryl Nesbit
New fast-etching bottom anti-reflective coatings have been prepared at Brewer Science, Inc., for 193-nm lithography. These materials, EXP03087B and EXP03066, were targeted for first and second reflectivity minima thickness, respectively. The optical constants (n and k) of these materials were 1.70 and 0.50, respectively, for EXP03087B and 1.71 and 0.31, respectively, for EXP03066. After thermosetting, these materials were immiscible with photoresists and were not affected by base developer. Profiles utilizing these BARCs with JSR's AR1221J photoresist have shown 90-nm (l:l line space) dense lines and 100-nm lines with FFA’s GAR8105G1 resist.
A frequent problem encountered by photoresists during the manufacturing of semiconductor device is that activating radiation is reflected back into the photoresist by the substrate. So, it is necessary that the light reflection is reduced from the substrate. One approach to reduce the light reflection is the use of bottom anti-reflective coating (BARC) applied to the substrate beneath the photoresist layer. The BARC technology has been utilized for a few years to minimize the reflectivity. As the chip size is reduced to sub 0.13 micron, the photoresist thickness has to decrease with the aspect ratio being less than 3.0. Therefore, new Organic BARC is strongly required which has the minimum reflectivity with thinner BARC thickness and higher etch selectivity toward resists. Nissan Chemical Industries, Ltd. and Brewer Science, Inc. have developed the advanced Organic BARC for achieving the above purpose. As a result, the suitable high performance NCA3000 series 248nm Organic BARCs were developed. Using CF4 gas as etchant, the plasma etch rate of NCA3000 series is about 1.4-1.6 times higher than that of conventional 248nm resists and 1.1-1.2 times higher than that of the existing product. The NCA3000 series can minimize the substrate reflectivity at below 45nm BARC thickness, shows excellent litho performance and coating properties.
As the semiconductor industry sails into the 100nm node and beyond, enabled by the integration of ArF lithography, new Bottom Antireflective Coatings (B.A.R.C.s) are required to address challenges associated with this new technology. Of these challenges, higher etch rates and better compatibility with the over coated resist are of central importance. New polymer platforms and additives in B.A.R.C. formulations will be required to overcome these challenges. The intent of this publication is to introduce our newly developed B.A.R.C.s designed to addresses the challenges of ArF lithography. All are currently available for integration into mass production of sub 100nm integrated circuit devices.
There are many considerations to the design of BARC materials. Among those many properties, one important property that can effect lithographic performance is BARC coating uniformity. In general, the basic coating property (conformal or planar) depends on basic characteristics of polymer (Mw, chemistry, etc). But another major factor to control the coating uniformity is the choice of solvent system in the formulation of the BARC. According to our experimental results, two major factors that can affect the coating uniformity of one BARC are the vapor pressure and the hydrophilicity of solvents. If any solvent has too high vapor pressure and high hydrophilicity relatively, polymer segregation occurs in BARC surface area in case of high humidity condition, resulted in bad coating uniformity.
In this paper, we will show basic evaluation results including the morphology change of BARC surface with several solvents which can be used in BARC formulation according to various humidity and temperature conditions. And also we will show the solution to overcome this problem in device manufacturing.
With the introduction of copper as the interconnect metal, the Dual Damascene (DD) process has been integrated into integrated circuit (IC) device fabrication. The DD process utilizes organic bottom anti-reflective coatings (BARCs) not only to eliminate the thin film interference effects but also to act as via fill materials. However, three serious processing problems are encountered with organic BARCs. One is the formation of voids, which are trapped gas bubbles (evaporating solvent, byproduct of the curing reaction and air) inside the vias. Another problem is non-uniform BARC layer thickness in different via pitch areas. The third problem is the formation of fences during plasma etch. Fences are formed from materials that are removed by plasma and subsequently deposited on the sidewall surrounding the via openings during the etching process. Voids can cause variations in BARC top thickness, optical properties, via fill percentage, and plasma etch rate. This study focuses on the factors that influence the formation of voids and addresses the ways to eliminate them by optimizing the compositions of formulations and the processing conditions. Effects of molecular weight of the polymer, nature of the crosslinker, additives, and bake temperature were examined. The molecular weight of the polymer is one of the important factors that needs to be controlled carefully. Polymers with high molecular weights tend to trap voids inside the vias. Low molecular weight polymers have low Tg and low viscosity, which enables good thermal flow so that the BARC can fill vias easily without voids. Several kinds of crosslinkers were investigated in this study. When used with the same polymer system, formulations with different crosslinkers show varying results that affect planar fill, sidewall coverage, and, in some cases, voids. Additives also can change via fill behavior dramatically, and choosing the right additive will improve the via fill property. Processing conditions such as bake temperature also greatly affect via fill. Depending on the polymer thermal property and crosslinking reaction, varying the bake temperature can change the via fill behavior of the BARC. By understanding the nature of the polymer, the crosslinking reaction, and the processing conditions, we are able to design BARCs with better flow property to provide planar topography without voids inside the vias.
As the pattern size decreases, the thickness of resist also should be decreased owing to the pattern collapse problem. So the using of surfactant containing rinse material, instead of DI water, can be a solution to the collapse problem. The developing of Bottom Anti Reflective Coating (BARC) that has high etch rate will be helpful to the collapse issue because it enables low thickness resist process and pattern collapse will be decrease. In this paper, Polyacetal, polyacrylate and polyesters BARCs were evaluated. Polyacetal type BARC shows best coating property. Regardless of the topology, polyacetal type BARC shows good conformality. However, polyacrylate and polyesters show coating fail on the topology wafer. In terms of pattern collapse, polyacetal type BARC also shows best results. Among the three types of BARC, ArF BARC that is made by polyester resin shows highest etch rate after 2000ÅBRAC etch. However, when the etching target is 60nm, all BARCs have same etch rate. For the matching with line and space resist, all these three BARCs show good profile. However, polyester type BARC does not match with contact hole resist and could not define contact hole pattern.
The new thin BARC has been developed for the application of small size patterning below 100nm by the optimized simulation and the evaluations on each substrate condition of silicon nitride and silicon oxide. The optical parameters of thin BARC of Exp225 are 1.81 and 0.58 for n and k values, respectively. They are obtained by the simulation for the lower reflectivity at the conditions of silicon nitride and silicon oxide. The optimized BARC thickness of Exp225 are 320Å and 460Å for silicon nitride and oxide substrate, respectively, at the condition of reflectivity. These thickness are much lower than those of commercial BARC of DUV44 for the same substrate conditions. The pattern profile and process margin are compared between the inorganic SiON and organic BARC. The dense L/S pattern profile of 100nm size on SiON shows the severe standing wave and undercutting. However, the pattern on Exp225 is much stable and gives wider depth of focus margin than that of SiON condition. The 85nm dense L/S pattern with feasible process margin is obtained by the application of Exp225 at the thickness of 320Å. The baking temperature is also investigated for the application of mass production. The most optimized baking temperature ranges of Exp225 are between 205°C and 225°C. From the experimental results, it is confirmed that the application of thin BARC is much effective for the small size patterning of 80nm node device. And it is thought that 80nm node device by KrF lithography is possible under the conditions of thin BARC, high contrast resist and high NA exposure tool.
RRC (Reducing Resist Consumption) process is widely used in the Semiconductor industry to decrease the cost of photo resist per wafer. However, the process is often accompanied with various coating defects that make it difficult to improve final yield and further reduce resist cost per wafer. Deep ultraviolet (DUV) bottom anti-reflective coating is critical to critical dimension (CD) control and is used for most critical layers like Poly, STI and Contact layers. This paper will present a novel double pre-wet RRC process to reduce both coating defects and resist consumption of BARC. The relationship between resist consumption per coating and thickness uniformity (mean and range) of the new process was evaluated. The stability test results of the new process shows acceptable manufacturing results. Coating defects from double pre-wet RRC process were analyzed and compared with the normal coating process without RRC. The effect of DUV BARC coating performed by the double pre-wet RRC process on critical dimension (CD) performance of Island, Poly and Contact layer was also reported.
To fulfill industry requirements for EUV resists, the development of entirely new polymer platforms is needed. In order to address transparency issues, we have been studying low absorbance materials, specifically silicon based resist platforms. In this approach, we have synthesized and studied resist materials based on polysilanes, polycarbosilane, and polysilsesquiazanes. Poly(methylphenylsilane) was chemically modified to incorporate polar groups to enhance solubility in polar solvents and developer solution. Copolymerization of the modified polysilane with an acid sensitive monomer has been used to produce chemically amplified copolymers. Preliminary studies have shown promising behavior. Polysilsesquiazanes-based resist were synthesized and tested using a 248 nm stepper. They showed excellent lithographic performance but some issues, including long term stability, are presently unknown. Our strategy to produce silicon-based resist together with outgassing and lithography issues will be discussed.
The high absorption of extreme ultraviolet (EUV) radiation by all materials necessitates the use of thin photoresist films with thicknesses less than 200 nm for EUV lithography to ensure good imaging. Thinning the resist thickness below 150 nm or even 100 nm may produce benefits such as increased sensitivity, larger process latitude, and increased resolution. However, these potential benefits as well as the required need for thin resists come at the expense of reduced etch resistance. EUV lithography will require the use of some type of thin imaging technique such as top-surface imaging, bilayer resists, or single layer resists with hardmasks in order to achieve the necessary etch resistance. In this paper, we discuss results that demonstrate the feasibility of using thin resist approaches for fabricating working devices. We have successfully fabricated working 130-nm-node SRAMs using a single layer 248 nm ultrathin resist (< 150-nm-thick) with a hardmask for both gate and contact layers on the same wafer. This result represents the first demonstration of working devices fabricated using ultrathin resists on multiple device layers. We also present initial patterning experiments using a 193 nm bilayer resist for brightfield applications such as the gate layer, and compare imaging performance to that of a 193 nm single layer resist. The advantages and disadvantages of the single layer and bilayer approaches are discussed.
Resists for the next generation of lithography must be able to meet stringent line width roughness (LWR) targets. The LWR requirements, governed by device performance, are the same regardless of the lithographic technology that is chosen. Unfortunately no resist platform for any technology (EUV, 157 nm, 193 nm) is on track to meet the targets for the 45 nm and the 32 nm technology nodes. In order to understand the fundamental sources of LWR, we designed an experiment to statistically vary resist parameters for EUV resists. The results of this study show methods to improve LWR and shed light on the sources of LWR.
Outgassing of photoresists needs to be minimized to avoid contamination of optics. A new challenge for EUV photoresists - that was not encountered for previous lithography technologies - is that exposures will occur in a vacuum environment. In order to design resists that meet the outgassing requirements for EUV lithography, current EUV photoresists need to be tested to determine if there are any performance gaps. In this paper we will describe the outgassing set-up for the outgassing chamber at the University of Wisconsin, and document Intel’s best known method for collecting and analyzing EUV outgassing data. In addition we will present preliminary outgassing results to benchmark the performance of Intel’s outgassing procedure.
A fast 2D/3D resist dissolution algorithm is used to quantify line-edge roughness and determine its relation to resist material parameters, such as the polymerization length distribution, the end-to-end distance and the radius of gyration, along with the effects of acid-diffusion. The same relation between surface roughness and exposure dose known to hold experimentally is also shown to be valid for line-edge roughness. Increasing average polymerization length results in increased values of line-edge roughness, radius of gyration and end-to-end distance establishing an immediate relation between material properties and measured line-edge roughness (LER). The effects of the edge depth and length of measurement and of the free volume on LER vs. are also investigated.
Photolithography is the driving technology and key enabler for the fabrication of integrated circuits with continuously decreasing feature sizes. Currently, state-of-the-art photolithography materials and processes can fabricate sub-100nm features, but significant technical hurdles remain in making sub-100nm features. These challenges include the understanding of LER (Line Edge Roughness) that will have a broad industrial impact. The 193nm resist has a thin gel layer at the interface of the developed resist and the developer, and resist patterns are formed by random detachment of this gel layer during development in the developer. Since the detachment of gel layer occurs randomly within the gel zone, LER increases in the case of higher gel layer thickness. This gel layer thickness can be determined by gel layer development model which consider two simultaneous reactions at the front and back of gel layer during dissolution of gel layer in the developer. This study attempts to explain LER using the concept of gel layer of which thickness is determined by hydrophilic and hydrophobic balance depending on the formulations of chemically amplified photoresists. LER can be minimized if we control the hydrophilic and hydrophobic balance by tuning the structure of polymer backbone in chemically amplified photoresists and minimize the gel layer thickness.
LER of an acetal-type photoresist (PR) and an annealing-type PR was measured by Atomic Force Microscopy, with which LER is more quantitatively measurable than using SEM. The annealing-type PR showed smaller LER than acetal-type did. Acid diffusion length measurement of these two types of KrF photoresists with a practical method that is a measurement of the thickness loss in a resist film after development which follows placement of exposed resist powder on the surface and applying PEB was also executed. The annealing-type PR has been found to show longer acid diffusion length than that of acetal-type PR. Considering deblocking temperature, acetal group is cleaved right upon exposure before PEB due to its relatively low activation energy. This means that there would be more hydroxystyrene units in acetal-type PR at the beginning of PEB than in annealing-type one. Tg of photoresist samples before and after deblocking reaction was also measured by DSC. After deblocking reaction, it was found that Tg of acetal-type PR is much higher than that of annealing-type PR. This relatively high Tg will make acetal-type PR to have shorter acid diffusion length in conjunction with relatively low PEB temperature comparing with annealing-type in general. The absolute Tg value and Tg change with deblocking reaction depending on types of PRs were correlated to explain the inherent difference in LER performance in different types of PRs.
In this study, surface conditioning solutions were used during post-develop process to enhance the 193 nm lithography performance. These solutions were applied to the wafer surface in a surface treatment step between the DI water rinse and drying steps. Compared to the standard develop process, the formulated surface conditioning solution enabled a 24% reduction in line width roughness, particularly in the high frequency roughness components. The solution also improved the pattern collapse performance by enlarging the non-collapse window and extending the minimum CD feature size by 10 nm. Additional benefits provided by the formulated surface conditioner solution were minimal changes to CD and resist profile.
Recently, there has been a growing interest in using surface conditioning solutions to solve the pattern collapse challenge. In this study, we investigated both pattern collapse and defect performance of surface conditioning solutions on multiple 193 nm resist systems. While the surface conditioning solutions were able to reduce the pattern collapse with good defect control with a majority of resist systems, it can increase the defect level on certain resist. Shortening the surface treatment step and optimizing the formulation can reduce the defect counts to the control level without compromising pattern collapse performance. This study also demonstrated that the surface conditioning solution is compatible with 248 nm processing, enabling the patterning of 90 nm 1:1.2 pitch lines.
In-house rinse, HR31 has a strong point in terms of lithographic performance, defect, bubble, and metal impurity. The collapse behavior was quantified in terms of SMCD (Standing Minimum CD) in 80nm dense L/S ArF resist patterns. It contributed to enlarging process window by improving collapse (SMCD: 84→72nm), CD uniformity (12.3→9.3nm), and lithographic margin [EL (11.7→12.8%), and DOF (0.20→0.25µm)].
The device design rule is continuously shrinking toward optical resolution limit where k1 factor is below 0.3. The requirement for 193 nm photoresist below 90 nm node is quite challenging at the manufacturing phase. Using DI water rinse after development gives a significant amount of line collapse when the aspect ratio is over 3. To avoid line collapse, we co-developed special rinse solution for FIRM process with Tokyo Electron Ltd. Utilizing FIRM process, 90 nm dense line collapse was measured by CD SEM using focus-exposure matrices. The line collapse property has been observed using experimental 193 nm positive tone resist by varying monomer ratio of the polymer and process conditions. The surface property of the resist was also studied to investigate the interaction with rinse solution at the de-protected polymer region. However, a high surfactant concentration in the DI water rinse leads the swelling of the resist pattern profile. The resist component is the key to determine adequate surfactant concentration in rinse solution to minimize line collapse and pattern deformation
Exposure wavelength is being reduced significantly, along with design rule reductions. The sub-100-nm node process is currently underway with 193-nm lithography. The problems that need to be solved for the shift in wavelength from 248-nm to 193-nm lithography are those attributed to resist materials, such as plasma resistance, SEM (scanning electron microscope) shrink, and problems attributed to processes, such as pattern collapse and deposition defects (Fig. 1). Although thin films are preferable to improve resist resolution limits, pattern collapse is more likely to occur in 193-nm and 157-nm processing due to DIW (deionized water) rinse surface tension during the drying step after development. This is because of the increased A/R (aspect ratio) of the resist used to improve etching durability and lower the rigidity of 193-nm resist compared to the 248-nm resist. We had focused on controlling the capillary effect between the resist pattern and the rinse solution to avoid swelling. We evaluated the method with the use of DIW with additives rinse, and named its process “FIRM (Fishing-up by improved rinse materials)”. In this paper, we report the effectiveness of the FIRM treatment for each resist by using a dispenser of track system. We had confirmed the pattern collapse within the wafer, the process margin, CD (critical dimension) variation, CDU (CD uniformity), Defect test and, the effectiveness of the FIRM treatment in the etching process. Results indicated that the FIRM process could be used in mass production. Additionally, we had investigated application of this method to the sub-65-nm node process. We created a 55-nm line (Pitch 200-nm), with A/R = 4.47 by overdosing and performed the FIRM treatment. We were able to confirm that the FIRM treatment improved the results while all patterns had collapsed after a standard development. We believe that the FIRM treatment will be applicable to the 65-nm node.
In this study we investigated a production relevant process to reduce pattern collapse by adding a low concentration of surfactant to the final rinse liquid in the resist development process. X-ray lithography was used to print test structures with critical dimensions as small as 70 nm in an experimental EUV photoresist, XP-1449-L-400, generously supplied by Shipley. By controlling the dimensions of the test structures, the in-plane capillary forces that act to deform the resist structures during drying were well-defined and easily varied. Commercially available fluorosurfactants (Zonyl FSK and FSO) from DuPont Chemicals were used at concentrations of 0.1% and 0.01% in water. Using surfactants, the capillary forces that act on the structures were reduced and the critical aspect ratio of collapse (CARC) of the structures was increased by an average of 20 to 30%, from 2.5 to 3.2, allowing industry to meet the SIA roadmap requirements. The use of surfactants in the rinse in conjunction with the test structures provided insight into the fundamental chemical physics of pattern collapse. Using independently measured receding contact angles (θ) of the rinse liquid on the resist and the surface tensions (γ) of the rinsing solutions, the collapse data could be generalized in terms of the magnitude of the capillary forces that were estimated using classical thermodynamics. The principle conclusion of this study is that the criteria for choosing the optimum rinse liquid to reduce resist collapse is to minimize the magnitude of 2γcosθ.
In the rapidly growing field of microfluidics, there is a tremendous need for alternative fabrication processes and for simple methods to integrate higher levels of functionality into microfluidic systems (i.e., fully-integrated, multi-level fluidic systems with functional valves, pumping systems, and other MEMS components). A fabrication technique recently developed at Georgia Tech involving thermally sacrificial polymeric materials allows for these innovations. In this method, which is completely compatible with traditional IC fabrication processes, thermally sacrificial polymers are coated onto a substrate and patterned into the shape of the desired channels and devices. These polymeric structures are then overcoated with a permanent structural material such as an inorganic glass or polymer. These steps can be repeated to produce complex, three-dimensional systems. Finally, the completed device structure is heated to the decomposition temperature of the sacrificial polymer which volatilizes to leave behind the desired open-channeled structures. These same materials and techniques can also be applied to the fabrication of a variety of microelectromechanical system components, including suspended membrane structures and cantilevers, that are integrated directly with IC devices on a common substrate. This process was first developed using functionalized polynorbornenes that decompose at temperatures near 425°C. In order to make this approach compatible with a wider range of substrates and structural materials, polymers with lower decomposition temperatures were desired, and polycarbonates were identified as a class of polymers with decomposition temperatures in the desired range (200-300°C). In addition, utilizing a polymer that can be patterned directly by conventional lithography greatly simplifies the fabrication process. By exploiting the acid-catalyzed thermolysis of polycarbonates, low decomposition temperature, photosensitive sacrificial polymers can be developed using mixtures of photoacid generators [PAG] and polycarbonates. Preliminary studies of several different polycarbonates, both photosensitive and non-photosensitive, have shown promising results, but optimization of these materials will be required to realize their full potential as sacrificial materials for use in microsystems manufacturing. The imaging characteristics of these polycarbonates vary greatly with the differing polymer thermal properties and polymer crystallinity, which are directly related to the polymer structure. A comparison of several new secondary and tertiary co-polycarbonates and their ability to maintain feature integrity during photolithography are presented.
This experiment is concerned with the development of mist deposition technology as an alternative to spin-on method of photoresist deposition in microelectronic manufacturing. A commercial 200 mm mist deposition tool is used in this study. The results obtained demonstrate effectiveness of mist deposition in resist processing. Basic parameters of resist mist deposition are determined. Deposition rate can be controlled within 10 to 50 nm/min range. Using a stepper and UV-5 photoresist 250 nm patterns were readily defined in 120 nm thick mist deposited resist. It is postulated that mist deposition offers advantages over spin-on process in the case of very thin resist technology as well as in the case of resist deposition on large, non-circular substrates.
Bimetallic thin films containing indium and with low eutectic points, such as Bi/In, have been found to form highly sensitive thermal resists. They can be exposed by lasers with a wide range of wavelengths and be developed by diluted RCA2 solutions. The exposed bimetallic resist Bi/In can work as an etch masking layer for alkaline-based (KOH, TMAH and EDP) “wet” Si anisotropic etching. Current research shows that it can also act as a patterning and masking layer for Si and SiO2 plasma “dry” etch using CF4/CHF3. The profile of etched structures can be tuned by adding CHF3 and other gases such as Ar, and by changing the CF4/CHF3 ratio. Depending on the fluorocarbon plasma etching recipe the etch rate of laser exposed Bi/In can be as low as 0.1nm/min, 500 times lower than organic photoresists. O2 plasma ashing has little etching effect on exposed Bi/In, indicating that laser exposure is an oxidation process. Experiment result shows that single metal Indium film and bilayer Sn/In exhibit thermal resist characteristics but at higher exposure levels. They can be developed in diluted RCA2 solution and used as etch mask layers for Si anisotropic etch and plasma etch.
Critical dimension control is becoming more and more critical in the mask making industry as the exposure wavelength goes down. For laser pattern generators, the move from traditional DNQ/Novolak based towards DUV chemically amplified resist processing was initially troublesome. The relative long total exposure time of pattern generators in contrast to wafer steppers, in combination with thick quartz substrates with relatively low heat capacity, may result in
reduced lithographic performance due to excessive diffusion of photogenerated acid. The photoresist polymer architecture play a large role in determining the acid diffusion characteristics and thereby also the image fidelity and resolution. In the Sigma7300 laser pattern generator the image is created by the spatial light modulator, which acts as a reflective computer-controlled reticle. By adopting a proper writing strategy, the negative effects of acid diffusion could be reduced. One component in the Sigma writing strategy is to expose the pattern in several passes that allows for dose compensation as well as averaging schemes to reduce CD errors. By adjusting the dose per pass and by keeping track of the delay times between each shot as well as the exposure path, a better control of the linewidth may be achieved for certain photoresist chemistry. In this study we present results from investigations of AZ DX 1100P and FEP-171 resists using different writing strategies.
There is a key problem in the fabricating sub-micron mask with lithography, which is how to improve accuracy of the mask by reducing the proximity effects. Backscattering electrons is the main factor of causing proximity effects. In this paper, we use random probability number to determine whether there is an elastic scattering happening between resist and Cr plate in the EB lithographing process. The distribution of the scattering electrons’ energy deposition in PMIMA resist is simulated. The graphs of the forward scattering energy deposition and backscattering energy deposition have been given. This is useful to amend the proximity effects.
A novel and effective approach to nano-fabrication lithography is the vapour deposition of the negative tone electron beam resists QSR-5 and QSR-15 (Quantiscript’s sterol based resist) onto a substrate. Vapour deposition is especially conducive for patterning thin delicate membranes (e.g. advanced masks for X-ray lithography - XRL, and Low Energy Electron Proximity Projection Lithography - LEEPL), that are susceptible to breakage during the spin coating process. With the capability for depositing highly uniform thin layers (<50nm) and a demonstrated resolution better than 60nm, QSR-5 and QSR-15 have potential for the fabrication of next generation lithography masks. Optimized for low energy electron exposure where proximity effects become negligible and thus well suited for 1X lithography mask patterning, QSR-5 and QSR-15 have shown exposure doses as low as 100μC/cm2 at 3KeV. In addition to this type of application, the versatility of QSR-5 and QSR-15 have also been demonstrated by the fabrication of a Fresnel zone plate lens on the tip of an optical fibre with the goal of improving the coupling of diode laser emission into the fiber. This application clearly shows the capabilities of this process for producing nano-scale patterns on very small area surfaces that are completely unsuitable for spin-coating of the resist. A second demonstration of the resist's capabilities is the patterning of optical diffractive elements directly on the facet of a semiconductor laser. This opens the way to direct patterning on laser diode facets in order to control the emission profile from the device. It has also proven capabilities in the manufacture of delicate photo masks. In their natural state, QSR-5 and QSR-15 are solids at room temperature and are sterol based heterocyclic compounds, with unsaturated bonding capable of cross linking. On their own merit, QSR-5 and QSR-15 are capable of cross linking under electron beam exposure and are comparable in certain properties to conventional spin-coated resists such as PMMA. When cross linked, their heterocyclic structure gives it excellent selective resistance to solvent based developers (such as alcohols and ketones) for pattern formation. They have also been shown to be highly resistant to etching solutions, even when working with thin high resolution layers on the order of 30 nm. They are highly stable and have a relatively long shelf life, greater than one year. Compared to conventional resists utilizing complex, toxic, and expensive resin systems, QSR-5 and QSR-15 are non-toxic and significantly cost effective. Before evaporation, the resists are in a powder state that allows for direct evaporation and sublimation onto a target substrate that contributes to film uniformity and capabilities for a very thin film; the powder state allows for a wide degree of adjustment in temperature of the vapour chamber, as a means to achieving the desired film thickness and uniformity.
There is an increasing need for highly viscous and easy to process thick and ultra thick photoresists for the production of Micro-Electro-Mechanical-Systems (MEMS) and advanced packaging. Here we present results with some novel positive tone photoresists formulated for this purpose. For that we transfered the concept of chemically amplified resists (CARs), originally designed to meet the IC-industry demands for miniaturization and higher resolution, to highly viscous resists. Various polymeric materials have been tested regarding their use in thick CARs. Appropriate resist formulations were developed and their lithographic performance was investigated in a thickness range of 50-150 microns. The CARs are sensitive to UV400.
A new negative resist performance was demonstrated for elevated temperature, thick metal sputter process and lift-off application. This resist, AZ EXP nLOF 7000, can endure temperatures in excess of 220°C without pattern profile slope change. The applied resist thickness is ~ 3 μm and is processed to produce sufficient side wall slope adequate for up to 2μm sputter deposited metal thickness and lift-off process.
The performance requirements for ultra-thick photoresists are rapidly increasing with the dramatic growth in new lithographic applications that require electroplating processes. Two of the main applications for ultra-thick photoresists are nanotechnology (MEMS) and advanced packaging. Flipchip packaging has become widely adopted to address electrical device performance and chip form factor considerations. The growth in the nanotechnology market is driven by a wide range of products, which include accelerometers, ink jet print heads, biomedical sensors and optical switches. Electroplating levels for these applications require a photosensitive polymer material capable of coating, exposing and plating with conventional semiconductor equipment and standard ancillary process chemicals. A single coat step to achieve the final photoresist thickness is critical to minimize the number of process steps and cycle time. For this thick layer the sidewall profile, aspect ratio, electroplating durability and subsequent stripability are all important. This study characterized a novel positive photoresist (Shin-Etsu SIPR) for use in a 100 micron thick coat for electroplating on copper. The lithographic performance of the ultra-thick positive photoresist was optimized using a broad band, 1X KARL SUSS MA150 aligner used in the proximity mode. All proximity gaps were hard set at 100 microns. Cross sectional SEM analysis, process linearity, and optimized proximity gaps were used to establish the lithographic capabilities. High aspect ratio structures were then electroplated using the optimized photoresist process to demonstrate photoresist durability and stripability. A recommended process flow is described for this photoresist and exposure tool.
It is vital to control Critical Dimension (CD) within a wafer and pattern profile in photolithography process. We have previously reported our evaluation results with chemically amplified resists that one of the causes of pattern profile fluctuation is a change in resist film composition before exposure such as non-uniform distributions of additives (photo-acid generator (PAG), quencher) concentration and casting solvent etc.; and thus resist film property control is essential to suppress these factors . This is also true for thin resist film in finer line process and with top surface imaging. In the same paper , we have reported that a straightforward method to understand a change in film property is to check the amount of thickness loss by developing unexposed film after post apply bake (PAB). This method can be applied to thin film as well. We created unexposed films with the thickness range of 50-900nm by changing the total solid content (TSC) in resist and spin speed at resist coating. The amount of thickness loss significantly increased with sub-200nm thickness; the film property of which was quite different from that of 200nm-or-over thickness. Moreover, pattern line edge roughness (LER) as well as pattern surface roughness was prominent with 100nm-thick film even when we used a resist which has an ability to create good patterns on film with a thickness of 400nm. This is because the film quality diminishes in bulk below a certain thickness, while the property on the surface or interface layer predominates. Then we studied Scan coating to control thin film property. In Spin coating, chemical liquid dispensed on static wafer is spread by spinning the wafer and solvent is evaporated to form a film. On the other hand, in Scan coating, wafer remains static even after chemical liquid is dispensed and the wafer is dried under reduced pressure -; which means the thinner evaporation rate is slower than that in Spin coating and film property control may be easier. We, therefore, expect that Scan coating is a possible method to control CD and pattern profile. In this study, we compared the process performance and film property of KrF resist films by Scan and Spin coatings and examined the film composition control.
A polysilazane was investigated as a precursor to a spin-on glass (SOG) used for a middle-layer in a tri-level resist system. Higher film density is required for the middle-layer in order to obtain higher etch resistance during the under-resist etching and prevent the acids in the resist from diffusing to the SOG, which induces deteriorating of resist patterns. High film density of the SOG was achieved by spin-coating the polysilazane solution. The compositions of the polysilazane baked at 200 °C and 300 °C are Si42O34C4N20 and Si29O65C1N5, respectively. The polysilazane is converted to silicon-oxide likely structure by baking at 300 °C. The film density of the SOG made from the polysilazane (SGPZ) is 2.07 g/cm3, which was higher than 1.87 g/cm3 of the conventional SOG made from a polysiloxsane. The etch resistance of the SGPZ baked at 300 °C which is expected not to volatilize the under-resist is improved by 90% compared with that of the SOG made from the polysiloxsane baked at 300 °C due to the increased film density of the SGPZ. The refractive indices of the films are n=1.56, k=0.01 (ArF) and n=1.68, k=0.02 (F2). Without the stacked films of SGPZ/under-resist, reflectivities to the resist are 56.2% (ArF) and 44.2% (F2). By optimizing the SGPZ thickness, the reflectivity is reduced to less than 0.7% (ArF) and 0.4% (F2). In conclusions, the polysilazane can be as the superior material for the SOG used for the middle-layer in the tri-level resist system.
Understanding the proximity effect is crucial to fabricating repeatable sub-100 nm features for magnetic recording devices. Top down CD-SEM measurements have been used to measure the proximity effect parameters in negative and positive resists at dimensions below 100 nm. The goal of this work is to experimentally determine the values of the parameters α, β and η and what they depend on.
Conventional lift-off process uses dual-layer resists for transferring image into the substrate (top layer) and releasing the deposit (bottom layer). However, as the critical dimension of top layer approaches sub-100 nm, the undercut of bottom layer for subsequent lift-off process becomes very difficult to control. An alternative approach is to use single-layer resist to do lift-off. Such a process requires resist patterns with a negative-slope sidewall angle, which is not easily achieved by the optical lithographic tools. In this communication, we presented a lift-off method using a tilted electron beam and further development to produce sub-100 nm features with a negative-slope sidewall angle. This process was demonstrated in a negative-tone chemically amplified resist (NEB22A2) by using an exposure electron-beam system (Hitachi-900D). The computations, based on Monte Carlo simulations, were found to be in good agreement with the experimental results. Two extensive applications for recording heads were also presented.
The PR(Photoresist) flow process after the development step has been used for patterning of sub-200nm contact holes as the design rule decreases rapidly. To optimize the layout design and process parameters, we develop the new viscous PR flow model which is verified for various PRs by experimental results. Using the model and simulation, we demonstrate the close agreement with VSEM(vertical scanning electron microscope) of the top corner rounding profile of PR and investigate the effect of the dominant variables such as the contact size, surrounding bulk density, and temperature. This model is also integrated with lithography simulator. The layout design and process condition of patterns with various contact sizes are optimized by using our new methodology. The viscous flow model linked to the lithography simulator can be effectively used in predicting the contact patterning process and optimizing the layout as well as analyzing defects.
This paper describes different simplified simulation models which characterize the behavior of the photoresist during lithography processes. The effectiveness of these models is compared with the results of more physics and chemistry containing simulators. The strengths and weaknesses of the simplified models are demonstrated for practical applications. Simplified resist model parameters are calibrated for 193nm chemically amplified resists (CAR). The results are compared with calibration of full simulation models. The validity of the simulation models under different process conditions is investigated.
We have recently developed a technique that utilizes capacitance data from resist coated interdigitated electrodes to measure the kinetic rate constant of photoacid generation (commonly referred to as the Dill C parameter) for photoacid generators in chemically amplified resists. The work presented in this paper focuses on a recently improved version of the IDE Dill C measurement technique. The original version of the technique required coating several IDEs with resist films containing different loadings of photoacid generator and then using the capacitance data from these IDEs to calculate linear mixing relationships between IDE capacitance and the content of PAG or photoproducts within the resist film. The improved version of the technique reported here totally eliminates the need for this “calibration process” through the use of normalized capacitance data. Elimination of the need to measure linear mixing relationships independently for each PAG and polymer combination gives the improved technique many advantages over the prior version. These include improved curve fitting and accuracy of Dill C calculations; fewer raw materials, IDEs, and experimental time; and most importantly, the potential to measure the Dill C for a resist from a single IDE with no prior knowledge of the resist’s photoacid generator type or loading. A detailed derivation of the normalization scheme is presented in this paper, along with evidence of the dramatic improvement in model curve fit that can be achieved using this technique. In addition, Dill C parameters measured for five different photoacid generators with both the original and normalized version of the IDE technique are presented to demonstrate that both techniques measure the same Dill C parameter and hence are describing the same physical phenomena.
Fabrication of future nanoscale electronic devices will likely require the use of ultra-thin resist films. It has been observed that film thickness, molecular weight, and substrate interactions can significantly affect the thermophysical properties of polymer thin films such as the glass transition temperature and coefficient of thermal expansion. Recently, film thickness has been reported to have a dramatic impact on the diffusion coefficient of small molecules in polymer ultra-thin films. Both of these factors, changes in either the polymer film thermophysical properties or diffusion behavior, can potentially have an impact of the lithographic performance of polymer thin film photoresists. As an extension to this previous work, it is desirable to understand the influence of film thickness on a variety of other lithographically important polymer properties. Dissolution rate is one such important physical property for photoresist polymer thin films that is of particular importance to the microelectronics industry. Simulation of lithographic processes relies to a great extent on knowledge of the dissolution or development behavior of photoresist thin films. Resist contrast is also known to be strongly affected by the dissolution behavior of the resist matrix polymer. So far, the possibility of film thickness significantly affecting the dissolution behavior of thin photoresist films has generally been ignored. This paper reports on work focused on determining the effect of film thickness on the dissolution behavior of a variety of resist polymers including novolac, polyhydroxystyrene (PHOST), and bis-trifluoromethyl carbinol substitute polynorbornene (HFAPNB). In the present work, Quartz Crystal Microbalance (QCM) methods were used to determine the dissolution rate of polymer thin films for thicknesses ranging from approximately 1 μm to 100 nm. It was observed that both poly(hydroxystyrene) (PHOST) and bis-trifluoromethyl carbinol substituted polynorbornene (HFAPNB) exhibit strong surface acceleration behavior as compared to the classic surface inhibition that has been extensively studied in novolac polymers. Most importantly, the dissolution rate of PHOST and HFAPNB thin films was found to depend strongly on the thickness of the polymer film for film thicknesses below a critical thickness value.
This paper describes a systematic and rigorously controlled set of experiments showing the effectiveness of the Electric Field Enhanced Post Exposure Bake (EFE-PEB) with 248nm KrF(ASML) exposures on Apex-E photoresist (IBM) where feature sizes ranged from 0.3 um to 0.5 um. Experimental results showed a significant improvement in process latitude and resist sensitivity for features with a k1 technology factor of 0.68 and below. The experiments were executed using a 248nm KrF stepper (ASML), NA 0.5, and Apex-E photoresist (IBM), which has a relatively high acid diffusivity. An improved experimental setup rigorously controls PEB time, PEB temperature, development time, focus drift, and other environmental variables. Cross sectional SEMs of five line arrays with L=S ranging from 337nm to 500 um show deeper trenches when the electric-field treatment was used. Exposures that were intentionally out of focus cleared 600nm deep in 1um thick photoresist in the control group, but did clear the full 1um with the application of the Electric Field during the PEB. A dose matrix experiment showed an 8% decrease in the dose to clear and two-fold increase in focus latitude. This comprehensive study demonstrated an increase in anisotropic acid diffusivity with the application of the electric field and confirmed that the EFE-PEB offers a relatively inexpensive and simple method for improving photoresist performance.
A new resist threshold model based on image behaviors on directions parallel as well as normal to feature edges has been developed for predicting critical dimensions (CD) of two-dimension patterns. In this new model (2D-RTM), resist threshold is assumed as a second-order polynomial function of five image parameters that consist of image intensity and slope. Extensive verifications of 2D-RTM have been done by using both rigorous resist models and experimental measurements. 2D-RTM is found to be a good approximation of rigorous model within certain range of dose and defocus variation. For 130nm technology in LSI Logic, 2D-RTM improves CD prediction accuracy for typical 2D patterns to a maximum error of 3.1nm and average of 1.21nm, which gives an improvement of a factor of two compared with conventional resist threshold model.
This paper focuses on a novel methodology for a fast and efficient resist model calibration. One of the most crucial parts when calibrating a resist model is the fitting of experimental data where up to 20 resist model parameters are varied. Although general optimization approaches such as simplex algorithms or genetic algorithms have proven suitable for many applications, they do not exploit specific properties of resist models. Therefore, we have developed a new strategy based on Design of Experiment methods which makes use of these specific characteristics. This algorithm will be outlined and then be demonstrated by applying it to the calibration of a Solid-C resist model for one ArF line/space resist. As characterizing dataset we chose: a) a Focus Exposure Matrix (FEM) for the dense array, b) linearity, c) OPE (optical proximity) curve and e) the MEEF (mask error enhancement factor) for a dense array. It turned out that a simultaneous fit of the complete data set was not possible by varying resist parameters only. Considering the optical parameters appeared to be crucial as well. Therefore the influence of the optical settings (illumination, projection, 3D mask effects) on the lithography process will be discussed at this point. Finally we obtained an excellent matching of model predictions and experimental results.
This report describes the results of a study on resist profile simulation in proximity printing, using light intensity distribution and actually measured dissolution rate values, a method that takes the gap effect into consideration (the effect of the distance between mask and wafer on the aerial image and resist profiles). We calculate the light intensity distribution with the gap effect based on the Van Cittert-Zernike theory and on the Hopkins equation as a model of light intensity distribution of proximity printing in resist film. Dissolution rate values are obtained using an apparatus to measure resist film thickness during development. The resist profile simulation is carried out using the combined data thus obtained. To verify the validity of this simulation, we use an SEM to observe resist profiles obtained from a diazonaphthoquinone (DNQ)-novolak resin positive-type resist for thick films, varying the proximity gaps using the mask aligner, which uses light in the broadband wavelengths of 350 mm to 450 mm, and compare the results with the simulation. The results of simulation and those of the SEM observation are in agreement, proving the validity of our method.
Alicyclic polymers, such as substituted polynorbornenes, are one potential material solution for providing photoresist polymer resins with high transparency backbones for photolithography at 193 nm and 157 nm wavelengths. In addition, the bis-trifluoromethyl carbinol functional group has been identified as a highly transparent base soluble group that can be used for producing photoresist resins from polynorbornene materials for 157 nm lithography. In this work, the interactions between commercial photoacid generators (PAGs) and bis-trifluoromethyl carbinol substituted polynorbornene (HFAPNB) are examined. It was found that photoacid generators can act as strong dissolution inhibitors for bis-trifluoromethyl carbinol substituted polynorbornene homopolymers. More importantly, it was found that a variety of photoacid generators can act as photoswitchable dissolution inhibitors for these materials, with exposure of the photoacid generator resulting in a reduction in the dissolution inhibition (i.e. increased dissolution rate) of the functionalized polynorbornene. The complete inhibition of unexposed HFAPNB polymers by iodonium photoacid generators allows for the formulation of photodefinable materials using a simple two component system consisting only of PAG and the HFAPNB polymer.
The bilayer process we developed for 157-nm lithography uses a fluorine-containing silsesquioxane-type resist (F-SSQ). Gate fabrication is done by using a F-SSQ(90 nm)/organic film(200 nm)/poly-Si(150 nm)/SiO2(10 nm)/Si structure. The organic film works well as an anti-reflecting layer. Using a microstepper with a numerical aperture of 0.90 and optimizing the resist thickness, we made a 50-nm 1:1 line-and-space (L/S) pattern by using an alternative phase-shifting mask and made a 45-nm SRAM by using a chromeless phase lithography mask. Neither resist pattern footing nor undercutting was observed on the organic film. The reactive ion etching (RIE) selectivity between the F-SSQ and the organic film was sufficient (about 7), the resist pattern was transferred to the underlayer, and both 50-nm 1:1 L/S and 45-nm SRAM gate patterns were made using the organic film as an etching mask. Contact hole (C/H) fabrication is done by using a F-SSQ(105 nm)/organic film(400 nm)/tetraethyl orthosilicate (TEOS)-SiO2(1200 nm)/Si structure, and we made a 75-nm 1:1 C/H pattern by using the microstepper with a binary mask. The RIE selectivity was sufficient (about 15) for making high-aspect-ratio contact holes, and we made a 75-nm 1:1 C/H pattern in 1200-nm-thick TEOS. This bilayer process is thus promising for making 65-nm-node semiconductor devices.
The ammonia durability of the 157-nm lithography resists is still unclear due to the smaller target dimensions, thinner resist films, and variations in base polymer compared to those of 193-nm and 248-nm resists. It has not been determined what ammonia concentrations must be achieved in order to successfully process 157-nm resists. Until now, the ammonia durability of initial 157-nm resists during post exposure delay (PED) and during post coating delay (PCD) was compared to those of 193-nm and 248-nm resists. It was confirmed that all initial 157-nm resists had low ammonia durability. In this paper, the ammonia durability of newly developed 157-nm resists, that have improved transmittance and resolution, was evaluated during PED and PCD. Then, we found that the ammonia durability of these resists were not enough and that the ammonia concentration from exposure to development should be kept under 0.1 ppb. Thermal desorption spectroscopy results showed that resists with lower ammonia durability tended to have more amount of adsorbed ammonia than other resists. Furthermore, the ammonia durability of 157-nm resist couldn’t be improved to the level of that of 193- and 248-nm resist by the adjustment amount of resist additives. Due to the low ammonia durability, it will be necessary to control the ammonia concentration below 0.1 ppb in processing equipment used in 157-nm lithography.
In optical lithography, small space patterning is the most difficult task. The direct small-space patterning is not good enough with resolution enhancement technique (RET) in sub-80 nm level. Two sequential processes normally achieve the small space. Once the pattern is forming a larger pattern normally, and then makes them shrink to fit to the designed size by additional process. Usually resist thermal flow process has been used to obtain small space as additional process, which has several process issues such as flow amount control of isolated and dense small contacts, uniformity degradation and bowing profile. In order to solve these issues, we introduced the resolution enhancement lithography assisted by chemical shrink (RELACS) and shrink assist film for enhancement resolution (SAFIER) process in ArF lithography. In this paper, the RELACS and SAFIER process are compared with the resist thermal flow process for sub-80 nm space using ArF exposure tool. With the application of this process, we confirmed the improvement of in-wafer uniformity and the successful implementation of sub-80nm small space patterning regardless of pitch size and pattern arrangement.
The continuous shrinkage of critical dimensions on 300 mm wafers has driven ArF lithography to resolve very small features for the next generation node. But the depth of focus (DOF) for 100 nm contact holes with a low NA of 0.75 is not adequate. Some resolution enhancement techniques (RETs), such as high transmission attenuated phase shifting masks, increase isolated contact hole DOF. Annular or quadrapole illumination improves dense hole resolution. However, they still cannot meet the requirement of logic circuit fabrication. To delineate 100-nm contact holes at 200-nm pitch, the resist process for 193 nm light was studied for the feasibility of a robust manufacturing process. In this paper we will discuss how to improve the process conditions of the thermal flow technique, as well as optimizing the illumination settings, prebake / post exposure bake temperatures, the mask dimensions and thermal flow temperature. Moreover, we will show the process window after the thermal flow process with optical proximity correction.
Introduction of ArF lithography has opened the era of sub-90nm patterning. Though the definable feature size was much reduced, poor etch durability and pattern collapse of ArF photoresist make it difficult to extend current patterning process into sub-80nm based on single layer resist scheme. To overcome these obstacles, some alternatives have been proposed, which are composed of thin imaging layer and mask layer with high etch resistance. One of high potential candidates is bi-layer resist (BLR) process, in which Si containing imaging layer is oxidized as a hard mask during dry development. In our previous reports, comparison of several types of bi-layer resists was discussed. Silsesquioxane (SSQ)-based bi-layer resist proved to be the most promising one with no detectable Si outgassing and high Si content. In our experiment, novel SSQ type BLR showed comparable lithographic performance to single layer resist (SLR) in terms of depth of focus (DOF), process window and critical dimension (CD) uniformity for 80-nm node line and space (L/S) patterning. 65nm 1:1 L/S pattern was also resolvable with 0.85 NA ArF scanner in bilayer resist scheme. High selectivity more than 5 was accomplished with vertical profile in the dry development and sub-70nm line patterning could be achieved with trimming technique.
Transparency of the resist film at exposure wavelength affects lithographic performances, such as sensitivity, profile and resolution. Not only binder polymer, but also photo acid generator (PAG) itself has a significant impact on transparency of the formulated resist. Triphenylsulfonium salt (TPS) or Diphenyliodonium salt (DPI) have been widely used as PAGs in DUV chemically amplified (CA) resists, however, aromatic groups there have strong absorption at 193nm and thereby these PAGs have to suffer from low transparency. In this paper, we will report a novel class of transparent enone sulfonium salt PAGs(ENS-PAG), which we believe useful for 193nm resist. The ENS-PAGs do not have any aromatic groups but have an α,β-unsaturated ketone structure for the absorbing moiety in the backbone. These PAGs showed excellent transparency, thermal stability, and demonstrated an advantage in the line edge roughness (LER).
The development process is very important in determining the resolution of the lithography process. As the shrinkage of design rule approaches to sub-100nm, understanding of dissolution mechanism and optimization of development process are needed to control the critical dimension (CD) and to obtain the best lithography performance. It was expected that more diluted developer solution could be used and appropriate for the 193nm lithography. But it has a serious issue in terms of changing and handling the concentration of developer solution in a factory, so application of diluted developer solution to lithography process is expected to be difficult. With this problem in mind, we focus on shortening process time during development stage instead of application of diluted solution, which has a demerit in mass production. Process margins of short time process were evaluated and compared with those of normal development process in 193nm lithography. This short time process was proved to be applicable to 193nm lithography and confirmed in view of line edge roughness (LER), in-wafer CD uniformity, mask error enhancement factor (MEEF), a bias between isolated and dense pattern (ID bias) and so on. This also could make it possible to improve the track throughput, which amount of increment may be 24 wafers per hour in 4 development modules.
It was found that the structure of a matrix polymer has strong influence on the PEB sensitivity of 193nm photoresists. As reported, photoresists containing CO polymers exhibited superior property in terms of PEB sensitivity to photoresists formulated with more popular 193 nm photoresist polymers such as VEMA, COMA and methacrylates. In addition, CO polymers exhibited little variation (< 1 nm/°C) in PEB sensitivity when formulated with different PAGs and/or bases. VEMA polymers exhibited PEB sensitivity in the range of 4 ~ 6 nm/°C. VEMA polymers with less leaving group (or lower blocking ratio) exhibited lower PEB sensitivity, but the nature of a leaving group (i.e., lower or higher temperature leaving groups) had little effect on PEB sensitivity. The most pronounced effect was found with functional monomers. For example, VEMA polymers prepared with novel functional monomers exhibited PEB sensitivity in the range of 3 ~ 4 nm/°C. Photoresists formulated with methacrylates exhibited significant variation in PEB sensitivity ranging from 4 ~ 15 nm/°C depending on the backbone chemistry and composition. For instance, with lower blocking ratio as well as lower temperature leaving group, PEB sensitivity of methacrylates were significantly improved by 40~45%. Again, the most pronounced effect was found with functional monomers with methacrylates and PEB sensitivity of methacrylates with novel monomers resulted in the range of 3 ~ 5 nm/°C.
Keeping post exposure bake (PEB) sensitivity low has become one of the most crucial factors for implementing the 193nm resist process into mass production. In a previous report, we have demonstrated that the nature of the photo acid generator (PAG) has a strong effect on the PEB sensitivity of 193 resists. Based on our findings, we decided to extend our studies to the other important resist components, such as polymers prepared with various monomer compositions, and casting solvents. Also, in an effort to investigate whether PEB sensitivity can be reduced by process optimization, the influence of soft bake and post exposure bake conditions was studied. This paper describes our new findings on some of the important factors that affect the PEB sensitivity of 193 resists.
This paper presents our progress in developing spin-on, thermosetting hardmasks and bottom antireflective coatings (BARCs) for 193-nm trilayer usage. Binder materials that were used in preparing the silicon-containing hardmasks include polymers with pendant alkylsilane function and various polyhedral oligomeric silsesquioxane (POSS) substances, with the hardmasks being very transparent at both 193 and 248 nm. The second generation hardmasks (POSS-containing) offer significant improvements over earlier materials in oxygen (O2) plasma etching resistance. The etching selectivity (O2 plasma) for a trilayer BARC relative to the best-case hardmask is about 31.5:1 (15-second etch), with the selectivity numbers being much higher for longer etching times. The preferred hardmask is both spin-bowl and solution compatible. The new trilayer BARCs use binders that are rich in aromatic content for halogen plasma etching resistance, but the antireflective products also feature optical parameters that allow low reflectivity into the photoresist. The BARCs are very spin-bowl compatible. At about 500-nm film thickness, selected BARCs have provided 80-95% planarity over 200-nm topography. Combining the two thermosetting products (hardmask and BARC) with a thin 193-nm photoresist in a trilayer configuration has given excellent 80-nm L/S (1:1) after exposure and wet-development. A conventional resist has provided 100-nm L/S (1:1.4).
During the wafer coating process, photoresist is spun-coated to desired thickness based on the process requirements. The residual resist is spun out of a wafer and partially deposited on the sidewall of the coater cup. The resist will dry out and become small particles. Those small resist particles may deposit on top of the resist film of the next processing wafer. The small particles act as micro-lens and produce distorted or unwanted patterns. In this work, the effect of those dried resist particles on resist patterning has been studied for both binary and PSM masks.
In this work, the effect of exhaust condition during Post Exposure Bake (PEB) on the critical dimension (CD) performance has been investigated for different DUV resists. Two kinds of PEB chamber cover designs have been tested to see the influence of exhaust flow on CD. It has been found that cleaning and adjusting of PEB exhaust flow will change the resist CD. To confirm the impact of PEB exhaust condition on production, resists for both lines and contact holes are verified by scanning electron microscope (SEM).
A new processing tool, the Thermal Gradient Plate (TGP) is described. The TGP allows for a range of temperatures to be expressed on a wafer or other substrate in a controlled and reproducible manner. Materials coated on a substrate can be baked across a range of temperatures, allowing multiple experiments to be run in parallel. Combining orthogonal TGP bakes creates a matrix of temperature pairs on a single wafer, allowing two bake step processes to be studied in a single experimental run. Experimental results demonstrating a variety of resist studies are described, including resist CD temperature dependence, underlayer cure temperature determination, and two bake step interactions.
As the semiconductor industry moves to sub 193 nm imaging wavelengths the switch to thin film resists with marginal transparency can result in a significant degradation of resist profiles. Tremendous progress has been made recently in lowering the absorbance of 157 nm polymer systems, however etch resistant single layer resists are likely to have absorbance values of 1-2/mm. Current generation EUV resists have even higher absorbance values of 2-3/mm. While the use of thin films mitigates (but does not eliminate) the effect of high resist absorbance the impact of resist footing, notching, and standing waves are exacerbated. Surface effects such as top rounding or surface contamination are also likely to be a significant concern.
One way to minimize the effect of non vertical profiles due to high absorbance and/or resist footing is through the use of optimized underlayer/ photoacid generator (PAG) systems. The PAG’s are selected so as to have higher diffusiveness but lower reactivities than the PAGs used in the resist itself in order to have a greater effect on the resist profile. Characterization data will be presented using a high absorbance resist (> 3.0/mm). Lithographic data on this system will be described and imaging data presented showing features down to 90 nm in a resist 120 nm thick.
Many BEOL semiconductor applications require vertical wall patterns to produce thick metallic structures. To achieve these plated or etched topographies, the resist must endure severe chemical and thermal exposures. Negative-tone resists of the acrylic and acrylic-styrene resin varieties are common choices. One spin-on applied product includes Shipley BPR 100 photoresist, manufactured by Rohm and Haas Electronic Materials, L.L.C. Successful integration requires an aggressive stripper to rapidly dissolve the resin, yet protect the metal. GenSolve 475, produced by General Chemical, achieves these goals, cycle after cycle, in a closed-loop spray system that filters and delivers the stripper back onto the wafer. The resist is dissolved in minutes, even at moderate temperatures, as demonstrated in Semitool’s spray solvent platform, Scepter. Using GenSolve 475, the Scepter dissolves away cured Shipley BPR 100 resist from >50um in-via or mushroom copper studs, water rinses, and spin-dries wafers in a nitrogen environment. The Semitool platform can process 300mm wafers with a total dry-to-dry process time of <30min, corresponding to >100wph throughput in single or >200wph with dual chambers. Metal safety is proven by SEM, profilometry, and ESCA, by observing Cu etch rates of <30 Å/min and conversion of surface Cu(II) to Cu(I).
The performance requirements for ultra-thick photoresists are rapidly increasing due to the dramatic growth of applications such as nanotechnology (MEMS) and advanced packaging. Commercial products such as accelerometers, ink jet print heads, biomedical sensors and optical switches are driving growth in the nanotechnology market. Advanced packaging techniques such as flip chip in package, flip chip in board and wafer level chip scale packaging have become widely adopted to address electrical device performance and chip form factor considerations. The common lithography requirement for these applications is formation of high aspect ratio structures with sufficient process latitude to allow devices to be manufactured in production volumes. The use of a contrast enhancement material (CEM) has been shown to be effective in improving lithographic performance and process latitude for thin photoresist applications. However, CEM technology can also be used for the thick photoresist materials in MEMS and advanced packaging applications. The lithographic performance of three representative thick photoresists was characterized with and without a top CEM. The first two materials are ultra-thick positive photoresists that are widely used for electroplated bump bonding structures. The third material is a thick negative photoresist widely used for electrical redistribution levels. All lithography was performed using a low numerical aperture, 1X stepper to control critical dimensions (CD), sidewall angles and aspect ratios. Cross sectional SEM analysis was used to establish the lithographic capabilities of the three photoresists with and without top CEM. The recommended process flow for each photoresist with top CEM is described. The advantages and disadvantages of using CEM for thick photoresist applications are also discussed.
As the minimum feature size of electronic devices shrinks to less than 0.25 μm, it is critically important that we reduce the defects that occur in lithography processes. Moreover, as the defects to be controlled become ever smaller, this makes them increasingly difficult to detect by conventional fault detection equipment. In order to detect these minute defects in the context of shrinking device geometries, it is essential that we develop a clear understanding of the behavior of micro defects in developer. In principle, there are three ways in which these defects might be dealt with: (1) defects can be prevented from occurring in the first place, (2) defects can be prevented from adhering to the device, or (3) defects can be eliminated after they occur. Our recent work has mainly been concerned with the first and most effective approach of preventing defects from occurring in the first place, and this motivated the present study to investigate the mechanisms by which defects occur. We believe that defects occur in chemically amplified (CA) resists that are insufficiently unprotected at boundary regions between unexposed and exposed areas or in unexposed areas, so that the de-protection reaction in the resist suns to different degrees of completion due to varying exposure doses. In this study we investigate the number of defects in various developers, and determine the size distribution of the defects. Based on analysis of the behavior of defects from their size distribution in develop we conclude that: (1) the size of defects increases when the exposure dose is reduced by appropriate Eops, (2) defects originate in the boundary area between unexposed and exposed areas, and (3) a portion of CA resist polymer that is insufficiently deprotected is dispersed in the developer, coalesces and is deposited in a form that is not very soluble, and is manifested as relatively large particle defects.
The implementation of 193 nm lithography into production has been complicated by high defectivity issues. Many companies have been struggling with high defect densities, forcing process and lithography engineers to focus their efforts on chemical filtration instead of process development. After-etch defects have complicated the effort to reduce this problem. In particular it has been determined that chemical filtration at the 90 nm node and below is a crucial item which current industry standard pump recipes and material choices are not able to address. LSI Logic and Pall Corporation have been working together exploring alternative materials and resist pump process parameters to address these issues. These changes will free up process development time by reducing these high defect density issues. This paper provides a fundamental understanding of how 20nm filtration combined with optimized resist pump set-up and dispense can significantly reduce defects in 193nm lithography. The purpose of this study is to examine the effectiveness of 20 nanometer rated filters to reduce various defects observed in bottom anti reflective coating materials. Multiple filter types were installed on a Tokyo Electron Limited Clean Track ACT8 tool utilizing two-stage resist pumps. Lithographic performance of the filtered resist and defect analysis of patterned and non-patterned wafers were performed. Optimized pump start-up and dispense recipes also were evaluated to determine their effect on defect improvements. The track system used in this experiment was a standard production tool and was not modified from its original specifications.
As the minimum feature size of electronic devices continues to shrink, the industry is moving from 248-nm wavelength KrF excimer laser sources to shorter wavelength 193-nm ArF excimer and 157-nm F2 excimer sources to achieve the higher resolution lithographic processes that are required. This requires optimum control over CD(critical dimension), but also the ability to minimize and reduce device defects is critically important. Satellite defects and various other kinds of defects were found to occur in the development process when chemically amplified resists are used in 248-nm lithography, and these defects clearly have an adverse effect on yields. Now that the industry is moving from KrF (248- nm) to ArF (193- nm) exposure systems, this means the requirements to control and reduce these micro defects are more exacting than ever before. In this paper we describe the generation behavior of defects caused by bottom anti-reflective coating (BARC) and the adherence behavior of defects onto the BARC. In this work we show that the generation behavior of defects is clearly affected by the thickness of the BARC, and the adherence behavior of defects is well explained by potential analysis measurements. With the transition toward shorter wavelength exposure systems, varying the thickness of the BARC is likely to have a major impact on the CD in lithography processes, but controlling the thickness of BARC layers is also extremely important from the standpoint of controlling defects. While we certainly must continue in our efforts to develop better resists that minimum defects, our results suggest that we must also focus attention on optimizing and closely controlling the entire lithographic process.
This paper presents a novel wafer baking system that uses hot air streams as heating media and achieves good temperature uniformity across the entire wafer surfaces during the baking process. Wind tunnel experiments have been carried out to verify the concept of using hot air streams for wafer baking. A simple prototyping wafer baking system has been designed and fabricated, and experiments of the baking process have been conducted. Good temperature uniformity across the wafer surface has been achieved. The experimental results match well with the computer fluid dynamics (CFD) simulation results. It is observed that the velocity of the airflow has significant influence on the temperature transient responses. Further optimization of the parameters of the baking system and analytical modelling studies are currently under way.
A short develop time process was investigated and assessed in terms of various pattern features of a resist. Process latitude for a positive DUV resist was evaluated for various pitches of line-and-space patterns and contact hole patterns for different develop times. It was found that the process latitude, depth of focus (DOF) and exposure latitude (EL) were improved by shortening develop time for various pattern features. The characteristics of CD variation to develop time for each pattern feature agree with the suggestion in our previous paper that expanding resist process latitude was strongly correlated with the resist develop rate and that terminating the develop reaction while the resist develop rate remained large was the key to expanding the process latitude. The short develop time process contributed to the larger γ characteristics of the resist, a smaller thickness loss and also a lesser degree of surface roughness in the resist pattern, which led to an appropriate resist pattern for the semiconductor process. A novel develop application system was developed by considering the loci of movements of Dainippon Screen’s (DNS) slit-scan develop nozzle and a rinse nozzle on the wafer. It was found that the novel develop application system achieved highly accurate CD controllability while realizing the benefits of the short develop time process.
Some form of edge bead removal (EBR) is one of the standard requirements for a lithographic process. Without any intervention, resist may accumulate at the edge of the wafer at up to several times the nominal thickness of the resist. In addition to this edge bead, the resist is likely to wrap around the wafer contaminating the backside of the wafer as well. It’s needless to say that such a condition would present a significant contamination risk not only for the resist track and the exposure tool but for process equipment outside of lithography as well. Two not necessarily exclusive strategies have been used in the past for edge bead removal. One is topside chemical EBR where solvent is dispensed on the edge of the wafer as the wafer is rotated immediately after coating, and the other method is where a ring of exposed resist is formed by subjecting the resist on the outer edges of the wafer to a broadband exposure; also know as wafer-edge exposure (WEE). The advantage of the chemical method is that it will remove the photo resist but also the organic anti-reflective coating (ARC), which is not photosensitive. The disadvantage of this method is obvious as any latitude in tool tolerances or imperfections on the wafer will result in solvent dispense to the undesirable areas of the wafer. While the optical method is much cleaner, its main disadvantage is that it will not remove ARC. As the feature size and die size shrink, there is less and less repairable redundancy on modern semiconductor chips. An observed effect in our manufacturing facility has been an increased sensitivity to tool imperfections and a quantifiable level of yield loss due to solvent splashing for the 140nm generation. Accounting for the fact that the ARC layer is generally an order of magnitude thinner than the resist layer, yield-maximizing setup of edge bead removal for one lithographic layer and complete removal of topside chemical EBR is discussed in detail in this paper as well as the extension of the same principle to maximize yield at other layers.
193nm(ArF) photoresist used for 90nm to 65nm nodes has shown many significant characteristics. Especially, higher sensitivity to PEB (Post Exposure Bake) temperatures compared to 248nm(KrF) photoresist is critical in CD control. We classified CD budget of each process in coater/developer regarding 193nm photoresist to examine each factor’s influence. As a result, it’s found that PEB makes up about 70% of the track-related CD factors. This fact indicates the importance of PEB in 193nm process. We made improvements to inter and intra wafer for enhancing CD control in the 193nm process. Controlling chamber temperature in PEB process made 68.9% of improvement in CD variation of inter wafer. As for the intra wafer, the CD variation was improved 28.6% by modifying thermal history that has a great influence on PEB process. However, we assume that there are cases that don’t apply this budget since there are influences of the warped wafer and of flare in the exposure tool. In these cases, using a divided heater-type hotplate that we have been working on the development enables to make adjustment and results in 38.3% of improvement in intra wafer.
In recent years, the worldwide semiconductor market has changed drastically, and it is expected that the digital device market will continue to expand towards general consumer electronics and away from the personal computers that have been the core of the market. To accommodate this shift, the new devices will be diversified with improved productivity, higher process yield, and higher precision. Clean Track (LITHIUS) design also has been changed drastically to maintain equal productivity with new high throughput exposure equipment. Design changes include increasing the number of processing chambers by stacking reduced size modules in order to meet high throughput and small footprint requirements. However, this design change concept raises concerns about increased wafer-to-wafer
difference (WtW) and module-to-module different (MtM). These variations can result in lower process yield and have a negative effect on design rule shrinkage. The primary causes of WtW difference and MtM difference stem from minute module hardware variations, module height differences, and module parameter adjustment differences during the installation of the tool. Previous Clean Track development focused mainly on reduction of module hardware difference as an approach to reduce WtW variation. However, to further improve lot level uniformity, it is necessary to reduce module height difference factors within the system and module adjustment disparities such as plate temperature calibrations. Highly temperature sensitive ArF processes have necessitated precise manual PEB temperature adjustments. These calibrations are labor intensive and require many field hours to ensure optimal CD uniformity. Therefore, an auto temperature measurement and adjustment tool is
developed to eliminate the human error due to manual adjustment and minimize adjustment time. In order to meet demands for design rules shrinkage and increased process uniformity we minimized the WtW and MtM difference by using thermal history adjustment and transfer time control. This method is also used to improve within wafer CD control technology resulting in a more stable process. In this report, we introduce improved features to reduce WtW and MtM variation and their effect on CD uniformity with 193nm (ArF) resist and 248nm (KrF) resist.
As critical dimensions in microlithography become ever smaller and the importance of line edge roughness becomes
more pronounced, it is becoming increasingly important to gain a fundamental understanding of how the chemical
composition of modern photoresists influences resist performance. Modern resists contain four basic components:
polymer, photoacid generator, dissolution inhibitor, and base quencher. Of these four components, the one that is least
understood is the base quencher. This paper examines the influence of base additives on line edge roughness, contrast,
photospeed, and isofocal critical dimension (CD). A mathematical model describing the tradeoff between contrast and
photospeed is developed, line edge roughness values for different base types and loadings are reported, and isofocal CD
is shown for various photoacid types as well as for different base types and loadings.