Proceedings Volume 6281

22nd European Mask and Lithography Conference

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Proceedings Volume 6281

22nd European Mask and Lithography Conference

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Volume Details

Date Published: 22 June 2006
Contents: 11 Sessions, 35 Papers, 0 Presentations
Conference: 22nd European Mask and Lithography Conference 2006
Volume Number: 6281

Table of Contents

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Table of Contents

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  • Plenary Session
  • Invited original paper by the authors of the 2005 Bacus Best Poster Paper
  • EUV
  • Advanced Lithography and Simulation
  • Metrology
  • DFM & MDP
  • Mask Making, Mask Repair and Mask Cleaning
  • Nano Imprint Lithography, NIL
  • Immersion Lithography 1
  • Immersion Lithography 2
  • Poster Session
Plenary Session
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Mask costs: a new look
Over the last decade SEMATECH has provided significant guidance in predicting mask costs and their potential effects on the cost of manufacturing semiconductors. Additionally, these projections have been used to appropriately fund activities that could have the most impact on reducing mask costs, improving quality and cycle time. The most recent cost projections provide a comprehensive look at the impact of improvements to the mask fabrication process. We will provide projections that clearly indicate that appropriately funded mask technologies can have a significant impact on manufacturing yields and hence, cost and cycle time. While historical mask cost projections were realistic, the new projections represent the best estimates for mask costs over the next several years based on the current mask technology and processes1. These projections are significantly more optimistic than previous estimates. These changes are due primarily to the introduction of new mask repair technologies, improvements in focused ion beam (FIB), nano-machining and femto-second laser repair.
The status of LEEPL: Can it be an alternative solution?
The concept of LEEPL is based on the discovery that there is a narrow energy space around 2KV of e-beam proximity lithography where a potential solution for NGL exists. [1] LEEPL Corporation and LEEPL Technology Consortium consisting of 32 companies have been formed in 2000 and 2001 respectively for the purpose of developing LEEPL technology. The development has been carried out by 1) building α-tool, 2×β-tools, a pre-production model (LEEPL-3000) and 2) by building the infrastructure supporting LEEPL technology such as development and supply of LEEPL masks, resist materials, mask inspection and repair tools. Here LEEPL's overall status and future prospect will be presented for the first time on the base of the past 5 years' intensive effort. The content of the presentation include LEEPL Development Overview, Exposure and Alignment Systems, Performance of Resolution, Resist Process, Throughput, and Cost of Ownership Comparison, and LEEPL Mask and Related Technologies. In conclusion the potential of LEEPL as an alternative solution for future semiconductor lithography beyond 45 nm node device application will be discussed.
Mask industry assessment trend analysis
Microelectronics industry leaders routinely name mask cost and cycle time as top issues of concern. In 2002, a survey was created with support from SEMATECH and administered by SEMI North America to gather information about the mask industry as an objective assessment of its overall condition. The survey is designed with the input of mask technologists from semiconductor manufacturers, merchant mask suppliers, and makers of mask equipment. The 2005 survey was the fourth in the current series of annual surveys. The survey data can be used as a baseline for the mask industry and the microelectronics industry to gain a perspective on the technical and business status of the mask industry. The results may be used to guide future investments on critical path issues. Questions are grouped into categories: general business profile information, data processing, yields and yield loss mechanisms, delivery times, returns and services, operating cost factors, and equipment utilization. Because the questions covering operating cost factors and equipment utilization were just added to the survey, no trend analysis is possible. Within each category are many questions that together create a detailed profile of both the business and technical status of the mask industry. The assessment participation has changed from year to year. The 2005 survey, for example, includes inputs from eight major global merchant and captive mask manufacturers whose revenue represents approximately 85% of the global mask market.
Invited original paper by the authors of the 2005 Bacus Best Poster Paper
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Multi-project reticle design and wafer dicing under uncertain demand
Andrew B. Kahng, Ion Măndoiu, Xu Xu, et al.
The pervasive use of advanced reticle enhancement technologies demanded by VLSI technology scaling leads to dramatic increases in mask costs. In response to this trend, multiple project wafers (MPW) have been proposed as an effective technique for sharing the cost of mask tooling among up to tens of prototype and low volume designs. Previous works on MPW reticle design and dicing have focused on the simple scenario in which production volumes are known a priori. However, this scenario does not apply for low- and medium-volume production, in which mask manufacturing is typically started when only rough estimates of future customer demands are available. In this paper we initiate the study of MPW use for production under demand uncertainty and propose efficient algorithms for two main optimizations that arise in this context: reticle design under demand uncertainty and on-demand wafer dicing. Preliminary experiments on simulated data show that our methods help reducing the cost overheads incurred by demand uncertainty, yielding solutions with a cost close to that achievable when a priori knowledge of production volumes is available.
EUV
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The first full-field EUV masks ready for printing
Uwe Mickan, Rogier Groeneveld, Marcel Demarteau, et al.
ASML's first EUV alpha demo tool (ADT) is ready for lithographic set up, driving the need for qualified and fully compliant EUV masks. EUV reflection masks are different in blank and mask processes compared to current technologies e.g. masks for 193nm. Although in recent years individual EUV mask parameters have been demonstrated, it is only with the fabrication on the ADT mask set that fully compliant masks have been made. In this paper we discuss the typical requirements of a EUV full-field mask, and show first results from achieving the important milestone of fabricating EUV masks.
Iso-sciatic point: novel approach to distinguish shadowing 3-D mask effects from scanner aberrations in extreme ultraviolet lithography
Leonardus H. A. Leunissen, Roel Gronheid, Weimin Gao
Extreme ultraviolet lithography (EUVL) uses a reflective mask with a multilayer coating. Therefore, the illumination is an off-axis ring field system that is non-telecentric on the mask side. This non-zero angle of incidence combined with the three-dimensional mask topography results in the so-called "shadowing effect". The shadowing causes the printed CD to depend on the orientation as well as on the position in the slit and it will significantly influence the image formation [1,2]. In addition, simulations show that the Bossung curves are asymmetrical due to 3-D mask effects and their best focus depends on the shadowing angle [3]. Such tilts in the Bossung curves are usually associated with aberrations in the optical system. In this paper, we describe an approach in which both properties can be disentangled. Bossung curve simulations with varying effective angles of incidence (between 0 and 6 degrees) show that at discrete defocus offsets, the printed linewidth is independent of the incident angle (and thus independent of the shadowing effect), the so-called iso-sciatic (constant shadowing) point. For an ideal optical system this means that the size of a printed feature with a given mask-CD and orientation does not change through slit. With a suitable test structure it is possible to use this effect to distinguish between mask topography and imaging effects from aberrations through slit. Simulations for the following aberrations tested the approach: spherical, coma and astigmatism.
Characterizing the response of an EUV reticle during electrostatic chucking
Roxann L. Engelstad, Edward G. Lovell, Andrew R. Mikkelson, et al.
Extreme Ultraviolet Lithography (EUVL) is one of the leading candidates for Next-Generation Lithography in the sub-45-nm regime. One of the key components in the development of EUVL is understanding and characterizing the response of the mask when it is electrostatically chucked in the exposure tool. In this study, finite element (FE) models have been developed to simulate the reticle / chuck system under typical exposure conditions. FE simulations are used to illustrate (a) the effects of the nonflatness of the reticle and chuck, (b) the image placement errors induced by back-side particulates, (c) the influence of the coefficient of friction between the reticle and chuck during exposure scanning, and (d) the effects of contact conductance on the thermomechanical response of the reticle. The focus of this paper is to illustrate that mechanical modeling and simulation has now become a fundamental tool in the design of electrostatic pin chucks for the EUVL technology.
Advanced Lithography and Simulation
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Printability study with polarization based AIMS fab 193i to investigate mask polarization effects
Immersion lithography offers the semiconductor industry the chance to extend the current ArF processes to smaller nodes before switching to a shorter wavelength. The move to immersion will require increased attention to the photomask along with new effects influencing the aerial image formation as the numerical apertures (NA) of scanners move up to at least 0.93 and even higher. Feature sizes on the photomask become comparable or even smaller than the wavelength and hence act more like wire grid polaris ers which lead to polarisation effects. As of today AIMSTM fab tools are in operation worldwide. The novel AIMSTM fab 193i tool with a maximum NA of 0.93 is the latest aerial image measurement system for ArF-lithography emulation down to the 65nm node. Common adjustments are numerical aperture, illumination type and partial illumination coherence to match the conditions in 193nm scanners. In addition to non-polarized illumination, the AIMSTM fab 193i allows the user to select linear x and y polarised light for different settings and types, e.g. off-axis annular, quadrupole or dipole illumination. In this paper the polarisation effects of different photomask features are explored by comparing measurement results using linear polarised illumination parallel and perpendicular to line and space patterns and non-polarised illumination. Different MoSiON embedded phase shift masks have been investigated at the highest possible NA=0.93 and for different half-pitches from 500nm to 260nm, the latter corresponding to the 65nm node at the wafer level.
Polarization effects: EAPSM vs. TT EAPSM
Today novel RET solutions are gaining more and more attention from the lithography community that is facing new challenges in attempting to meet the new requirement of the SIA roadmap. Immersion, high NA, polarization, and mask topography, are becoming common place terminology as lithographers continue to explore these areas. Here with, we compare a traditional 6% MoSi based EAPSM reticle and a high transmission solution made of a SiON/Cr film stack. Insights into the manufacturability of high transmission material are provided. Test patterns have been analyzed to determine the overall impact of imaging performance when used with immersion scanners and polarized light. Some wafer results provide reliability of simulations, which are used to make further investigation on polarization and immersion effects.
Rigorous mask modeling beyond the Hopkins approach
J. Schermer, P. Evanschitzky, A. Erdmann
This paper demonstrates the implementation of an imaging algorithm that extends the Hopkins approach in order to cope with the demanding requirements of state of the art lithography. Rigorous mask simulation for oblique angles of incidence is integrated into our model of a lithographic projection system. In order to show the capabilites, Chromeless Phase Lithography (CPL) masks are studied with the new implementation. Immersion conditions are applied and a benchmark against the Hopkins approximation is carried out. An impact on aerial-image formation and on lithographic-process simulation is shown. The approach helps to consider off-axis illumination-effects in mask optimization.
Consequences of plasmonic effects in photomasks
F. M. Schellenberg, K. Adam, J. Matteo, et al.
For 45nm lithography and beyond, polarization and other electromagnetic effects such as surface plasmons may begin to affect the transmission through a photomask. Such phenomena are highly polarization sensitive, and may amplify the effects of line-edge roughness (LER) and variations in mask composition. A reduction in the mask material conductivity can mitigate the impact of these effects, but more accurate simulation is required to predict these effects well.
Calibration of test masks used for lithography lens systems
M. Arnz, W. Häβler-Grohne, B. Bodermann, et al.
We report on the calibration approach and results as well as the application of special test masks for the qualification process of projection lens systems for advanced wafer steppers and scanners. We concentrate here on two different sets of test masks. One test mask set was designed for aerial image based contrast metrology while the other set of qualification masks was applied to provide an absolute reference for the magnification of the lens system. On the contrast test masks we measured variations of CD ratios of about 10% for differently oriented structures. Additionally we compared the lithography lens magnification obtained by using both grating test masks calibrated by the PTB with a pitch ratio uncertainty of 5 x 10-7 with that obtained by employing alternative external scale calibration standards.
Metrology
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Systematic investigation of CD metrology tool response to sidewall profile variation on a COG test mask
F. Gans, R. Liebe, Th. Heins, et al.
Different type of CD metrology instrumentation is in use today for production control of photomasks, namely SEM, AFM as well as optical microscopy and optical scatterometry is emerging, too. One of the challenges in CD metrology is to develop a system of cross calibration which allows a meaningful comparison of the measurement results of the different systems operated within a production environment. Here it is of special importance to understand and also to be able to simulate the response of different metrology instrumentation to variations in sidewall profile of features on photomasks. We will report on the preparation of a special COG test mask with an intended variation of sidewall features and the subsequent metrological characterization of this mask in different type of CD instrumentation. The discussion of the measurement results will be accompanied by a discussion of the simulation of instrument response to feature sidewall variation.
A new life for a 10-year old MueTec2010 CD measurement system: the ultimate precision upgrade with additional film thickness measurement capability
Gian Luca Cassol, Giovanni Bianucci, Shiaki Murai, et al.
A 10-year old MueTec2010, white light CD measurement system, installed at DNP Photomask Europe and previously owned by STMicroelectronics, has been upgraded to fulfill the high-end optical CD measurement requirements, and to add the film thickness measurement capability. That is the ultimate upgrade, consisting of two new computers with WINDOWS 2000 operating system, a new 150X measurement objective, a new 16-bit CCD digital camera, a new tube lens for the old Leica Ergoplan microscope, and the NanoStar software with the pattern recognition option. The upgrade yielded an average 45% repeatability improvement for isolated and dense lines and spaces, with 1.2nm average repeatability in a 0.3-10μm CD nominal range. Contact holes report an average 50% repeatability improvement, with 2.5nm average repeatability. The improved precision allows a ±2-nm CD calibration and correlation down to 0.4μm CD nominal. Overall, the upgraded MueTec2010 shows same or better performance than the already installed Leica LWM250UV CD measurement system, despite the longer illumination wavelength of the former. The improved short and long term repeatability reduced the Gauge RandR figure from 24% to 11% at ±20nm tolerance, which qualifies the system for high-end binary mask down to 0.5μm CD nominal. The feasibility to calibrate the system for 248nm Molybdenum Silicide Phase Shifting Masks is currently being investigated. In addition to that, the new measurement algorithms, the capability to take multiple measurements within the FOV, and the pattern recognition capability included in the NanoStar software gave a 75% throughput boost to the fully automated macros for the weekly calibration tests of the laser writing tools, compared to the LWM250UV run time. With little additional hardware and software, the system has also been upgraded to include the film thickness measurement capability for the PSM resist coating process (2nd exposure), without the need for a dedicated, more expensive system. Two years ago, this 10-year old MueTec2010 system was about to be deinstalled. Today, thanks to creative thinking at DNP Photomask Europe and to the enthusiastic and cooperative MueTec approach, it is ready for another 10 years of honored service with up-to-date performance and with the additional film thickness measurement capability. This upgrade has by far exceeded the technical and return-on-investment expectations.
Spot sensor enabled reticle uniformity measurements for 65nm CDU analysis with scatterometry
Spot sensor Enabled Reticle Uniformity Measurement (SERUM) is a fast and accurate technique for measuring the CD fingerprint of a reticle. It is an alternative for the traditional SEM or optical based reticle line width measurements. The reticle fingerprint is obtained by scanning a spot sensor located on the wafer stage of the ASML Step & Scan system, and measuring the reticle transmission on actinic wavelength. Since the reticle is the main contributor to the 65nm CDU budget, Step & Scan system qualification is only possible when correcting for reticle errors. Especially for scatterometry based measurement an extensive reticle qualification is required. With the traditional methods it involves measuring multiple positions within the gratings to average-out the intra-grating variation. This results in an expensive and time consuming process (5 to 10 hours). SERUM reticle metrology has several applications. ASML specific Step & Scan system qualification can be improved in correcting for reticle errors. The reticle can be inspected on actinic wavelength for changes due to pellicle non-uniformity, transmission losses and contamination. SERUM data can also be used as input for DoseMapperTM to correct for global reticle line width non-uniformity. This metrology allows reticle measurement on 182 locations within 70 seconds, with a repeatability of 3σ < 0.35nm (reticle level).
DFM & MDP
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Data prep: the bottleneck of future applications?
Juergen Gramss, Hans Eichhorn, Melchior Lempke, et al.
There is no doubt that shaped beam systems have been well established in the mask write community since the introduction of the 130nm technology node. Moreover, they are successfully advancing to conquer also the wafer direct write market. To be able to handle today and in the near future the tremendous data volumes with their characteristic complexity as well as to make use of such indispensable methods like PEC and Fogging corrections, new, sophisticated solutions are necessary to master the challenging 45nm technology node. However, we are aware that the 45nm node presents only an intermediate step, because, according to the international roadmap, we soon will be confronted with the hardware and software requirements of the next, the 32nm technology node. In this context it becomes more and more important to consider potential showstoppers, in our case the data preparation process To investigate this complex subject a Linux cluster computer featuring 3.6GHz clock rate CPUs, and a software package supporting distributed computing with a 64Bit version and address units down to 0.1nm were used. The work was focused on the performance of pattern samples down to the 45nm node. Both mask and wafer data as well as NIL template manufacturing were considered, data prep times and CPU loads were analysed. Furthermore, the user-friendly Leica Interface for Data Preparations (LINDA) was applied. In addition, an outlook to future hardware/software configurations for mastering the challenges of the 32nm node will be given. The results presented in this paper prove that data preparation is not the bottleneck of current and future applications.
Mask data volume: historical perspective and future requirements
Mask data file sizes are increasing as we move from technology generation to generation. The historical 30% linear shrink every 2-3 years that has been called Moore's Law, has driven a doubling of the transistor budget and hence feature count. The transition from steppers to step-and-scan tools has increased the area of the mask that needs to be patterned. At the 130nm node and below, Optical Proximity Correction (OPC) has become prevalent, and the edge fragmentation required to implement OPC leads to an increase in the number of polygons required to define the layout. Furthermore, Resolution Enhancement Techniques (RETs) such as Sub-Resolution Assist Features (SRAFs) or tri-tone Phase Shift Masks (PSM) require additional features to be defined on the mask which do not resolve on the wafer, further increasing masks volumes. In this paper we review historical data on mask file sizes for microprocessor, DRAM and Flash memory designs. We consider the consequences of this increase in file size on Mask Data Prep (MDP) activities, both within the Integrated Device Manufacturer (IDM) and Mask Shop, namely: computer resources, storage and networks (for file transfer). The impact of larger file sizes on mask writing times is also reviewed. Finally we consider, based on the trends that have been observed over the last 5 technology nodes, what will be required to maintain reasonable MDP and mask manufacturing cycle times.
ORC and LfD as first steps towards DfM
The role the Optical Rule Check (ORC) in the design flow and future directions are discussed, the benefit of the model-based methodology is illustrated by using realistic layout situations. Concepts for implementation of Litho-friendly Design (LfD), i.e., of layout optimization and lithography simulations in the pre-tapeout design flow are developed.
Mask Making, Mask Repair and Mask Cleaning
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A new generation of progressive mask defects on the pattern side of advanced photomasks
The progressive mask defect problem is an industry-wide mask reliability issue. Even if masks are determined to be clean upon arrival from the mask supplier, some of these masks can show catastrophic defect growth over the course of production usage in the fab. The categories of defects that cause reticle-quality degradation over time are defined as progressive defects, commonly known as crystal growth, haze, fungus or precipitate. This progressive defect problem has been around for more than a decade and was observed at almost every lithographic wavelength. This problem is especially severe at 193nm lithography. Triggering the increased severity are shorter wavelength lithography - where the photons are highly energized - and the concurrent transition to 300 mm wafers, which require photomasks to endure more prolonged exposure as compared to 200 mm wafers. Both embedded phase shift masks (EPSMs) and chrome-on-glass masks are affected by progressive defects. These defects are generally found on the patterned surface underneath the pellicle (on clear, half-tone or chrome patterns), as well as on the backside surface of the masks. Past cases have indicated that this problem mainly starts on the scribes and borders, with emerging semi-transmissive contamination of ~100nm. These defects then propagate into the die area while growing in both size and opaqueness. Compositional analysis has shown that the majority of these defects are ammonium sulfate. However, since significant effort focused on the elimination of ammonium sulfate a new trend has emerged. Current studies show severe defect growth consists of organic contaminants (ammonium oxalate, cyanuric acid etc.) on half-tone edges and on chromium edges. The sources for progressive defect mechanisms are under investigation, though several candidates have been considered: maskmaking materials and process residues (mainly ammonium or sulfate ions), the fab environment, or the stepper environment. Controlling or balancing these sources may help to reduce the frequency at which these defects occur, but thus far has been unable to eliminate the problem. With each successive device shrink, the resultant changes in lithographic wavelength and processing within the mask fabrication facility and IC fab disrupt the fine balance among the above suspected defect sources, resulting in the return of catastrophic progressive defect growth. Due to this uncertainty, strict mask quality monitoring in the fab is essential. The ideal reticle quality control goal in a fab should be to detect any nascent progressive defects before they become yield limiting. Hence, the masks should be monitored on an established frequency that allows problem masks to be removed from production and sent for rework prior to impacting device performance and fab yield.
Advanced processes for photomask damage-free cleaning and photoresist removal
James S. Papanu, Roman Gouk, Han-Wen Chen, et al.
Photon induced haze resulting from sulfur residues that remain after cleaning and photoresist stripping is a key challenge for 193 nm photomasks. In previously reported work, sulfur-free processes for cleaning and photoresist removal on mask blanks were shown. Additional characterization and development of the cleaning and strip/clean processes are presented here. For cleaning the particle adder stability, ammonia chemistry residue levels, and chrome oxide anti-reflection coating (ARC) layer integrity were characterized. It was found that process modification was needed to provide acceptable post-clean ammonia levels and reflectivity change per clean. A strip/clean process with acceptable window for complete resist removal without ARC layer damage was found to be challenging and dependent on the mask photoresist/ARC stack. Dry strip, wet strip, and combined dry/wet stripping approaches (all followed by wet clean) were investigated. Oxidizing dry strip chemistry, while easily removing the bulk photoresist layer, gave unacceptable ARC attack. For FEP photoresist an all-wet process was demonstrated, and for iP and NEB resists, promising results were achieved with less oxidizing dry strip chemistry.
The performance of positive and negative CAR mask exposed by Leica 20 KeV writing system
Chen-Rui Tseng, Eng-Ann Gan, David Lee, et al.
Analyses of the effects of positive and negative chemically amplified resist (CAR) exposed by Leica ZBA31H+ 20 KeV shaped e-beam lithography tool are investigated using CD-SEM. The characterization data will present improved resolution, global CD uniformity and CD linearity. During last few years, several resists were used for masks making. The e-beam resists as ZEP and PBS resists were used on Leica low acceleration voltage e-beam system previous years. In the present investigation, the CA resist exposed by Leica ZBA31H+ 20 KeV writing system has been investigated. The objective of the present work is directed toward that the CAR process improves higher throughput than ZEP resist and promotes better performances than both ZEP and PBS resists. For the high exposed loading, to minimize writing time and fogging effect, and to control mean CD and improve global CD uniformity, the negative CA resist performs better than positive CA resist. TMC provides the actual measurement data obtained on CD-SEM for negative CA resist exposed by Leica 20 KeV writing system. In this paper, we will also provide the applicable profile results obtained on CD-SEM to confirm the feasibility for CAR mask be exposed by Leica 20 KeV system.
Recent application results from the novel e-beam-based mask repair system MeRiT MG
Christian Ehrlich, Klaus Edinger, Thorsten Hofmann, et al.
With the continuing decrease of feature sizes in conjunction with both the enormous costs for current masks and projections for future generations the area of mask repair has often been highlighted. Clearly, a viable repair methodology going forward has the potential to significantly influence and reduce production costs for the complete mask set. Carl Zeiss SMS had, in a concerted development effort with other Zeiss daughter companies, succeeded to develop and deploy a novel mask repair tool capable of repairing specifically all types of advanced masks, such as quartz binary masks, phase shift masks, EUV masks and S-FIL imprint templates. In addition to the pure technical capability of the e-beam based approach a strong emphasis has been made towards the user friendliness and automation features of the repair process as such.
Nano Imprint Lithography, NIL
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Nanoimprint lithography techniques: an introduction
H.-C. Scheer
The main nanoimprint lithography techniques are highlighted in this paper. In addition to details concerning the materials used and the basic processing techniques, some of the major recent developments are summarized. Critical issues are addressed and some ideas for improvement are given. The paper aims to give an introduction to the techniques and a survey of the actual status without going into the details of the specific techniques.
Plasma deposited and evaporated thin resists for template fabrication
Eric Lavallée, Jacques Beauvais, Bertrand Takam Mangoua, et al.
In this paper, we demonstrate electron beam lithography at energies ranging from 2 keV to 100 keV, on sub-100 nm thick resists. Such uniform and thin electron beam sensitive films can be deposited by evaporation or by plasma deposition. Two examples of such resists are studied, QSR-5 which is a negative sterol based evaporated resist and QPR-P50 which is a positive PECVD deposited fluoropolymer resist. AFM measurements demonstrate surface roughness smaller then 3 nm for QSR-5 and 0.7 nm for QPR-P50. In both cases, 50 nm features or better are patterned using electron beam lithography on a template blank consisting of a glass substrate, coated with a 10 nm thick Cr transfer layer and a resist layer. LER (3σ) is measured to be less then 8% for 50 nm wide lines in QSR-5 and of approx. 10% for 50 nm wide lines in QPR-P50.
Immersion Lithography 1
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How refractive microoptics enable lossless hyper-NA illumination systems for immersion lithography
H. Ganser, M. Darscht, Y. Miklyaev, et al.
Uniform illumination of the mask plays an important role in current exposure tools and will be even more challenging for hyper-NA systems. Arrays of refractive microoptics are the ideal solution for high transmission homogenising elements since these arrays can provide very steep intensity profiles (top hat and other profiles), and do not suffer from zero order losses like diffractive elements. This paper discusses monolithic elements based on crossed cylindrical lenses with a fill factor close to 100%. The Simulations and measurements here presented prove that microoptic arrays can be produced which provide a uniformity of the homogenized laser light in the 1% P-V range at numerical apertures above 0.3. Refractive microoptic arrays do not change the polarization state of the transmitted light which is an important prerequisite in immersion exposure tools. LIMO homogenizer sets are manufactured from fused silica and Calcium fluoride and thus suitable for all DUV wavelengths at highest laser fluxes. LIMO produces free form surfaces on monolithic arrays larger than 200 mm with high precision and reproducibility. Surface test methods and the final UV-tests are presented, guaranteeing the performance for the applications. Data gained with these tests are shown with regard to meeting the design parameters, reproducibility over one wafer and reproducibility in large lots. New concepts of "ready to use" pre-aligned homogenizer modules are shown and finally this paper illustrates results of new refractive elements which provide additional functionality like e.g. hexagonal fields or poles. Applications are for example pre-homogenization and pupil filling in illumination systems.
A correlation for predicting film-pulling velocity in immersion lithography
Scott Schuetter, Timothy Shedd, Keith Doxtator, et al.
Immersion lithography seeks to extend the resolution of optical lithography by filling the gap between the final optical element and the wafer with a liquid characterized by a high index of refraction. Several engineering obstacles are associated with the insertion of the immersion fluid. One issue that has recently been identified is the deposition of the immersion liquid onto the wafer from the receding contact line during the scanning process; any residual liquid left on the wafer represents a potential source of defects. The process of residual liquid deposition is strongly dependent on the behavior of the receding three-phase contact line. This paper focuses on an experimental investigation of this behavior under conditions that are relevant to immersion lithography. Specifically, the static and dynamic contact angle and the critical velocity for liquid deposition are presented together with a semi-empirical correlation developed from these measurements. The correlation allows the film-pulling velocity to be predicted for a given resist-coated surface using only a measurement of the static receding contact angle and knowledge of the fluid properties. This correlation represents a useful tool that can serve to approximately guide the development of resists for immersion systems as well as to evaluate alternative immersion fluid candidates to minimize film pulling and defects while maximizing throughput.
The magic of 4X mask reduction
Although changing the mask reduction factor from 4X to a larger value offers several technical advantages, previous attempts to enact this change have not identified enough clear technical advantages to overcome the impact to productivity. Improvements in mask manufacturing, mask polarization effects, and optics cost have not been thought to be sufficient reason to accept a reduced throughput and field size. This paper summarizes the latest workshop and discussion revisiting the mask reduction factor for 32nm half-pitch lithography with hyper-numerical aperture (NA) optical or extreme ultraviolet lithography (EUVL). The workshop consensus was strongly in favor of maintaining the current magnification ratio and field size as long as mask costs can be contained.
Immersion Lithography 2
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Through-pitch and through-focus characterization of AAPSM for ArF immersion lithography
Toshio Konishi, Yosuke Kojima, Yoshimitsu Okuda, et al.
Alternating Aperture Phase Shift Mask (AAPSM) is one of the most effective approaches to improve the resolution of logic gate structures for ArF lithography of the 65nm half-pitch node and beyond because AAPSM shows good performance due to the high image contrast and the small mask error enhancement factor (MEEF). For AAPSM, the issue of intensity imbalance between pi-space and zero-space is well known. In order to solve this issue, several kinds of AAPSM, such as single trench with undercut, single trench with bias are used in production application. The fabrication of single trench with bias AAPSM requires that the quartz dry etch satisfies many conditions. The etched quartz features must not only show excellent depth uniformity but also good etch depth linearity across a wide range of feature sizes. However, in defocus conditions, the through-pitch image placement error becomes worse even with good quartz etch depth linearity. The reason is that the phase error caused by mask topography is different depending on the pitch. In this work, we minimize the phase error through-pitch and through-focus by rigorous 3D mask simulations. Based on the results, we have fabricated two masks with opposite quartz depth linearity signatures to estimate the imaging impact of phase errors and used them for exposures on an ASML XT:1250Di immersion scanner. We discuss the feasibility of this method by comparison of through-focus and through-pitch image placement errors between wafer printing, AIMS, and simulation.
Manufacturability and printability of AAPSM with transparent etch stop layer
Michael Cangemi, Vicky Philipsen, Rudi De Ruyter, et al.
Phase-shift mask (PSM) technology in combination with 193nm illumination remains a viable option for high contrast imaging towards 45nm half-pitch applications. The advent of hyper NA (immersion) lithography increases the imaging sensitivity towards the photomask properties, such as mask-induced polarization. In addition, the use of PSM technology implies taking into account the inherent photomask topography effects for a balanced through pitch imaging. A good quartz etch depth control of +/-1o through pitch is required for optimized wafer imaging [1]. Therefore, a new PSM material stack was proposed based on a transparent etch stop layer (TESL) in order to meet the stringent phase depth requirements beyond 65nm half-pitch [2]. This extra layer allows over-etching of the quartz, resulting in a good etch depth linearity and uniformity. This study examines the manufacturability and printability of TESL-based masks. We examine the effect of an improved quartz etch depth linearity on the through-pitch process windows for a TESL-based alternating aperture (AA)PSM. Moreover, due to the different stack of photomask material compared to a classical photomask blank, the impact on printability is investigated by simulations, AIMS and wafer imaging. The image imbalance compensation by trench biasing needs to be optimized for through-pitch process windows. The actual depth and line width of the structures is systematically probed within the photomask field. Based on photomask metrology data, rigorous electro-magnetic field simulations are compared to wafer prints, obtained on an ASML XT1250Di ArF immersion scanner working with a 0.85NA projection lens and to AIMS results from Zeiss AIMS fab 193i. Furthermore, feature sizes on the order of the lithography wavelength induce photomask polarization effects in the imaging path [3]. The degree of polarization is compared to the polarization behavior of a conventional PSM. In summary, this study assesses the capability of TESL PSM towards the 65nm node through-pitch imaging.
Analysis method to determine and characterize the mask mean-to-target and uniformity specification
Sung-Woo Lee, Leonardus H. A. Leunissen, Jeroen Van de Kerkhove, et al.
The specification of the mask mean-to-target (MTT) and uniformity is related to functions as: mask error enhancement factor, dose sensitivity and critical dimension (CD) tolerances. The mask MTT shows a trade-off relationship with the uniformity. Simulations for the mask MTT and uniformity (M-U) are performed for LOGIC devices of 45 and 37 nm nodes according to mask type, illumination condition and illuminator polarization state. CD tolerances and after develop inspection (ADI) target CD's in the simulation are taken from the 2004 ITRS roadmap. The simulation results allow for much smaller tolerances in the uniformity and larger offsets in the MTT than the values as given in the ITRS table. Using the parameters in the ITRS table, the mask uniformity contributes to nearly 95% of total CDU budget for the 45 nm node, and is even larger than the CDU specification of the ITRS for the 37 nm node. We also compared the simulation requirements with the current mask making capabilities. The current mask manufacturing status of the mask uniformity is barely acceptable for the 45 nm node, but requires process improvements towards future nodes. In particular, for the 37 nm node, polarized illumination is necessary to meet the ITRS requirements. The current mask linearity deviates for pitches smaller than 300 nm, which is not acceptable even for the 45 nm node. More efforts on the proximity correction method are required to improve the linearity behavior.
Mask absorber material dependence of 2D OPC in ArF high NA lithography
Rigorous optical proximity correction (OPC) for 3D reticle effects is critical to the success of 193nm wavelength immersion lithography implementation. The impact of 2D and 3D mask polarization and shadowing effects to 2D imaging in ultra high Numerical Aperture (NA) low-k1 imaging is assessed by simulation. An end-to-end (ETE) dense line 2D feature of various embedded (attenuated) phase shift mask (ePSM) with various material of film stack is studied. Line-end pullback is shown correlated with mask shadowing under TE-polarized OAI. Polarized OAI phase calibrated thinner mask absorber provides less shadowing, better 2D imaging window, and enables further scaling of mask feature patterning.
Poster Session
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A technique to determine a capability to detect adjacent defects during the die-to-database inspection of reticle patterns
Syarhei Avakaw, Aliaksandr Korneliuk, Alena Tsitko
The paper analyses the factors which influence minimal features of detected adjacent defects during the die-to-database inspection of reticles. The analysis of influence of a set of factors, describing an instrumental error of the automatic reticle inspection system, and of a set of factors, describing a reticle patterning process, on various types of adjacent defects is made. Some relations are cited, describing interrelation of the size of the minimal adjacent defect and the pixel size of the automatic reticle inspection system. A concept of the optimum and preset sizes of the minimal detected defect is introduced. The analysis of dependence of the number of false defects on the size of the preset minimal detected adjacent defect is made, as well as a criterion to choose an optimum capability of detection of adjacent defects is given. In conclusion, parameters of automatic reticle inspection systems developed at Planar Concern are given, specifying the adjacent defects detection capability. Also the parameters of the systems designed for 0.35 μm, 0.18 μm and 65 nm processes are described.
Assist feature placement analysis using focus sensitivity models
Sub-Resolution Assist Features (SRAFs) are placed into patterns to enhance the through process imaging performance of critical features. SRAFs are typically placed using complex rules to achieve optimal configurations for a pattern. However, as manufacturing process nodes are growing increasingly complex, the SRAF placement rules will most likely be unable to produce optimal performance on some critical features. A primary impediment to resolving these problems is identifying poorly performing features in an efficient manner. A new process model form referred to as a Focus Sensitivity Model (FSM) is capable of rapidly analyzing SRAF placement for through process pattern performance. This study will demonstrate that an FSM is capable of finding suboptimal SRAF placements as well as missing SRAFs. In addition, the study suggests that the FSM does not need to comprehend the entire photolithography process to analyze SRAF placement. This results in simpler models that can be generated before a manufacturing process enters its development phase.
Design for manufacturing validation tool: fast and reliable conversion of SEM images to GDS images
D. Ronning, D. Ducharme, R. Selzer, et al.
The ability to convert high resolution images from a Scanning Electron Microscope (SEM) of a printed lithographic pattern to a GDS image file which can be input into modeling software (such as litho-simulation, etc.) for rigorous analysis is a powerful tool. Its use can be expanded through the simplication of the SEM2GDS conversion procedure by automation of the tasks. In this paper, we describe our SEM2GDS and SCAN INTERFACE UNIT, which automates both SEM image collection and SEM2GDS conversion.
CD and profile metrology of embedded phase shift masks using scatterometry
Kyung man Lee, Sanjay Yedur, Dave Hetzer, et al.
Linewidth and etch depth control on the photomask is rapidly becoming a major concern in mask processing. In this paper, we report on a Scatterometry based metrology system that provides line width and etch profile measurements on Embedded PSMs on Intel's 65nm and 45nm node test masks. Measurements were made with Nanometric's Atlas-M reticle measurement system. Spectrum data obtained from plates were analyzed using Timbre Technologies' ODP analysis software. We characterized the CD uniformity, linearity, sidewall angle and thickness uniformity. Significant reduction in time per measurement is achieved when compared to CD-SEM. ODP Scatterometry reported a 2x reduction in the CD Uniformity compared to that reported from the SEM. This reduction is typically due to outliers reported by the CD-SEM that is averaged out in ODP Scatterometry. Good correlation to top-down CD-SEM and cross-sectional SEM is reported. R-squared correlation of >0.99 (ODP scatterometry to top down CD-SEM) is reported. Profile measurements from ODP show excellent match to cross-section SEM. The data show that Scatterometry provides a nondestructive way to monitor basic etch profile combined with relatively little CD metrology lag.