Proceedings Volume 4426

Second International Symposium on Laser Precision Microfabrication

Isamu Miyamoto, Yong Feng Lu, Koji Sugioka, et al.
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Proceedings Volume 4426

Second International Symposium on Laser Precision Microfabrication

Isamu Miyamoto, Yong Feng Lu, Koji Sugioka, et al.
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 25 February 2002
Contents: 12 Sessions, 106 Papers, 0 Presentations
Conference: Second International Symposium on Laser Precision Micromachining 2001
Volume Number: 4426

Table of Contents

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Table of Contents

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  • Microfabrication and Microstructuring
  • Ultrafast Laser Processing
  • Industrial Applications
  • Modification
  • Microfabrication and Microstructuring
  • Industrial Applications
  • Microfabrication and Microstructuring
  • Fundamental Aspects
  • Pulsed Laser Deposition
  • Microsystems and Microelements
  • Industrial Applications
  • Pulsed Laser Deposition
  • Fundamental Aspects
  • Systems and Optics
  • Cleaning
  • Pulsed Laser Deposition
  • Microfabrication and Microstructuring
  • Industrial Applications
  • Cleaning
  • Microsystems and Microelements
  • Ultrafast Laser Processing
  • Industrial Applications
  • Modification
  • Fundamental Aspects
  • Microfabrication and Microstructuring
  • Fundamental Aspects
  • Systems and Optics
  • Industrial Applications
  • Fundamental Aspects
  • Optics Fabrication
  • Ultrafast Laser Processing
  • Systems and Optics
  • Industrial Applications
  • Microsystems and Microelements
  • Systems and Optics
  • Pulsed Laser Deposition
  • Advanced Microfabrication Technology
  • Ultrafast Laser Processing
  • Pulsed Laser Deposition
  • Optics Fabrication
  • Overview
  • Cleaning
  • Optics Fabrication
  • Industrial Applications
  • Ultrafast Laser Processing
  • Modification
  • Fundamental Aspects
  • Advanced Microfabrication Technology
  • Overview
  • Ultrafast Laser Processing
  • Microfabrication and Microstructuring
  • Advanced Microfabrication Technology
  • Modification
  • Ultrafast Laser Processing
  • Systems and Optics
  • Advanced Microfabrication Technology
  • Industrial Applications
  • Systems and Optics
  • Optics Fabrication
  • Overview
  • Ultrafast Laser Processing
  • Microsystems and Microelements
  • Industrial Applications
  • Cleaning
  • Systems and Optics
  • Overview
  • Fundamental Aspects
  • Systems and Optics
  • Industrial Applications
  • Fundamental Aspects
  • Microfabrication and Microstructuring
  • Industrial Applications
  • Cleaning
  • Microfabrication and Microstructuring
  • Cleaning
  • Pulsed Laser Deposition
  • Systems and Optics
  • Microfabrication and Microstructuring
Microfabrication and Microstructuring
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Microstructure and properties of Fe-base alloy fabricated using selective laser melting
Manlong Sun, Li Lu, Jerry Ying Hsi Fuh
This paper studies recent advances in rapid tooling (RT) of high performance components via selective laser melting (SLM). The present study of SLM focuses on the development of materials used in injection molding. In the process, the pre-alloyed iron-base alloy powder was used to fabricate parts using a 3KW CO2 laser machine. Iron components with nearly full density have been directly produced under suitable laser parameters. The result of the SLM processed parts showed very fine dendrite microstructure. The surface hardness measured is 35-40 HRC, which is comparable to that of the AISI P20 steel (30-35 HRC). The advantages of this technique include elimination of a secondary infiltration process used by former selective laser sintering technology, reduced pre-processing time, and reduction in post- processing steps. The SLM process has a great potential in the field of RT.
Ultrafast Laser Processing
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Optimum energy absorption of a short-pulse laser in a doped dielectric slab
Lay-Kee Ang
A model is used to calculate energy absorption efficiency when a short-pulse laser impinges on a dielectric slab doped with an impurity for which the electrons have a resonant line at the laser wavelength. The amount of the energy resonant absorption is due to the overlapping between laser spectrum and resonance spectrum. The energy absorption efficiency can be maximized for a certain degree of doping concentration (at a given pulselength) and also for a certain pulselength (at a given doping concentration). For a modest amount of impurity, the resonant absorption may increase the fraction of energy absorption up to tens of percent of laser energy at 100s optical cycles when the laser wavelength is tuned with 1% of the resonant line. Dimensionless parameters are constructed so that the scaling to various parameters: laser wavelength, laser pulselength, dielectric constant, slab thickness, impurity concentration, resonant linewidth, and separation between the laser wavelength and the line resonance, could easily be obtained.
Industrial Applications
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Laser cladding of copper-based materials for building electrical discharge machining electrode
Y. Tang, Li Lu, Jerry Ying Hsi Fuh, et al.
The paper investigates the feasibility of laser cladding of copper-based materials. As an application aspect of rapid prototyping (RP), the process could be used to build electrodes for electrical discharge machining (EDM). Two types of material system-Cu/W (with and without nickel) and Cu/B4C (with and without nickel) in certain compositions were studied. The laser cladding was conducted in a 3000W CO2 laser machine using various parameters. The microstructures of laser cladding samples were checked and analysed by SEM. The performance of electrodes made from these materials was examined in the terms of machining rate and wear rate.
Modification
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High-resolution laser flash photography for probing the solidification of the new double laser recrystallization process
Ming-Hong Lee, Seung-Jae Moon, Mutsuko Hatano, et al.
A new double laser recrystallization technique that can produce lateral grains of tens of micrometers is presented. A nanosecond laser (excimer or Nd:YLF laser) and a pulse modulated ArPLU laser are used in the experiment. The effect of different parameters on lateral grain growth is investigated. These parameters include the time delay between the two lasers, the excimer laser fluence, the ArPLU laser power and the pulse duration. This process has wide process window and is insensitive to both the excimer laser fluence and the ArPLU laser power fluctuations. Preheating and melting of the a-Si film with the ArPLU laser before firing the excimer laser is found to be necessary for inducting lateral grain growth. The transient excimer laser irradiation is believed to generate nucleation sites for initiating the subsequent lateral grain growth. The solidification dynamics of the process is probed by high spatial and temporal resolution laser flash photography. A lateral solidification velocity of about 10m/s observed.
Microfabrication and Microstructuring
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Particle-enhanced near-field optical effect and laser writing for nanostructure fabrication
Yongfeng Lu, L. Zhang, Wen Dong Song, et al.
In this paper, we present a novel method of nanostructure fabrication using particle-enhanced near-field optical effect and lasers. For this purpose, spherical silica particles were deposited on a silicon surface. After laser illumination, hillocks with size of about 150 nm were obtained at the original position of the particles. The mechanism can be explained as the enhancement of light intensity near the contact area. Since the characteristic distance between particles and substrate is smaller than the radiation wavelength and the particle size is of the order of a wavelength, particle does not simply play the role of microfocusing lens as in far-field, but possess optical resonance effect in near-field. In our work, the light intensity on the surface under the spherical particle was calculated by solving the electromagnetic boundary problem 'particle on surface.'
Investigation of carbon nanoparticles converted from a polymer by UV-pulsed laser irradiation
Diamond-like polymer, poly(phenylcarbyne), films were irradiated by pulsed UV laser ((lambda) equals 248 nm) at an atmospheric pressure of nitrogen. The morphologies of the resulting samples were examined by scanning electron microscopy (SEM). The structures of the resulting films were investigated by Raman spectra and X-ray diffraction (XRD). The electron field emission properties of the films were investigated. The results indicate that the polymer is converted into nano-particle carbon films by pulsed laser irradiation. The converted carbon film shows good field emission properties, such as low turn-on threshold emission field, high emission current density and high emission light spot density. Field emission images from the converted carbon films have been demonstrated. The mechanisms of both carbon cluster conversion from the polymer and field emission of the converted carbon film have been discussed.
Processes in semiconductor materials after laser cutting
Andrei R. Novoselov, Anatoly G. Klimenko
The results of laser - semiconductor interaction are presented. The characteristic zones are investigated after laser affects the semiconductor substrates by method of electro-physical (dynamic and static) performances of diagnostic structures. It has been found that: 1. Near of area of interaction of a laser radiation with a material of the target, the hot electrons generated by laser radiation, causes changes of properties of a material. It causes increase of reverse currents of the diodes. The magnification of reverse currents of the diode is observed which depends on supply voltage and distances between laser cut and diode. The distance between edge of the diode and edge of the laser cut with which begins increase of reverse currents of the diodes in Si: for supply voltage was 8,2 V - 5 micrometers , 20 V - 26 micrometers ; in HgCdTe: for working voltage (0,1 V) the distance was 18 micrometers . 2. The laser radiation causes temporary increase of reverse currents of the diodes on distances from 2 micrometers and more (measurement of a reverse currents of the diodes on distances 36 and 78 micrometers have shown reduction of a reverse currents, in time about 400 minutes). 3. Our experimental data allow us to develop criteria of definition of parameters of laser radiation for laser cutting of semiconductor materials on distances in some microns from elements of the integrated circuits. The basic criteria of a choice of laser radiation: 1. Repetition frequency of laser pulses; 2. Volume of destruction of a material for one pulse. The laser source for these experiments was an UV laser at 0,34 micrometers wavelength with 7 ns pulses, laser fluency was more than 1,1 J/cm2 that corresponding to minimum energy density required to forming pits. The diagnostic structures included p-n junctions (Si, HgCdTe) or source of MOSFETs (Si). Our experimental data showing that the powerful high- speed laser tools for cutting of materials are of limited usefulness for semiconductor materials.
Industrial Applications
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Laser marking on IC package by micro-encapsulated tapes
Kaidong D. Ye, Chengwu An, Yuan Yuan, et al.
In this paper, we are reporting a new way to do marking on IC package. In this way, white ink is wrapped in microcapsules that are coated on a transparent tape. Laser is irradiated on the tape surface, the microcapsules are broken and the ink is released onto the IC package surface. After an UV light treatment, the ink will stick on the IC surface, forming a high contrast marking. It is found that the quality of the marking depends on tape configuration, tape-IC distance, laser peak power, scan speed of laser irradiation and other laser parameters.
Microfabrication and Microstructuring
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Hole drilling of glass-foam substrates with laser
Yoshikazu Yoshida, Hiroyoshi Yajima, Yuji Hashidate, et al.
Using 10 - 100 msec pulses and a 10.6 micrometers wavelength CO2 laser, focused to a spot size 200 micrometers , we have produced holes into glass-foam substrates. We examine the effect of pulse width on the hole structure and the pile-up around the hole in single-pulse drilling. The height of the pile-up increases 7 - 30 micrometers with increasing the pulse width. The residual melting layer is under 0.5 (mu) m in the hole walls. It seems that the glass-foam substrate is suitable for laser beam machining.
Fundamental Aspects
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Selective photon-breaking of C-S bonds in polythiophene to form quasi-one-dimensional carbon chains in cubic nanocrystals
ZhongMin Ren, Yongfeng Lu, H. W. Choy, et al.
A solid polythiophene pellet was ablated by a KrF excimer laser beam to deposit thin films on silicon substrates. The laser-ablated plasma was studied by optical emission spectroscopy to identify the photon-breaking of C-S bonds in the ablated heterocycles. The selective photon-induced bond breaking was also supported by Raman and X-ray photoelectron spectroscopy measurements of the deposited thin films. The thin films appeared to be composed of cubic nanocrystals with an uniform size of 240 nm. X-ray diffraction measurement determined the cubic crystal structures with a lattice constant of aequals 3.38angstrom and suggested a quasi-one-dimensional carbon chain structure along the body diagonal of the cube.
Pulsed Laser Deposition
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Pulsed-laser deposition of carbon nitride materials
ZhongMin Ren, Yongfeng Lu, Z. F. He
Synthesis of carbon nitride has been an important topic in materials science since 1993. Ion-assisted pulsed laser deposition is proven to be a good method to deposit carbon nitride thin films. Both amorphous and crystal (beta) -C3N4 layers can be deposited on many substrates. A standard experimental set-up comprises a pulsed KrF excimer laser (wavelength 248nm, duration approximately 30ns) that is used to ablate the graphite target and a nitrogen ion beam bombarding simultaneously on the substrate. A variety of experimental derivatives have been developed based on pulsed laser deposition. The deposited thin films have been characterized by Auger Electron Spectroscope (AES), X-ray Photoelectron Spectroscopy (XPS), Mass Time of Flight spectrum (TOF), Optical Emission Spectrum (OES), Rutherford Backscattering (RBS), High Energy Backscattering (HEBS), Raman spectroscopy, Fourier Transform Infra-red Spectroscopy (FTIR), Ellipsometry, Electron Diffraction, Scanning Tunneling Microscope (STM) and Atomic force microscope (AFM). Investigations are carried out to identify the binding structure, nitrogen content, electronic properties, optical properties and crystal structures of the deposited thin films.
Microsystems and Microelements
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Laser beam bending of metallic foils
Manfred Geiger, Frank Meyer-Pittroff
The increasing miniaturization, especially in mass production of electronic and mechatronic devices demands for new technologies for forming, handling and assembly of micro components. Contactless laser beam forming without application of any exterior forces may be such a means. Potential applications for laser forming of micro parts can be found where the introduction of exterior forces or bending moments into the component causes a problem due to its small geometric dimensions, where further handling after the forming process may damage the component or, where a forming step is not required until after the assembly. Contactless laser forming may serve as a solution for high precision manipulation of functional electronic or optical devices or for tuning forces as in relays-springs. Desired changes in position may be in the sub-micrometer range. Due to its extremely short pulse duration, the excimer laser is suited for applying a temperature gradient over the cross section of even very thin metals plates, thus leading to their bending. However, beside thermal mechanisms also non- thermal mechanical effects are responsible for laser beam bending of very thin metal plates by excimer laser irradiation, when irradiating with fluences above the ablation threshold.
Industrial Applications
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Laser microbumping and contamination-free tagging on magnetic hard disk surfaces: techniques and applications
DaMing Liu, Teng Soon Wee, Yuan Yuan, et al.
Laser micro-bumping and contamination-free tagging magnetic hard disk surfaces had been intensively investigated. Diode- pumped Nd:YVO4 laser with fundamental output was used for microbumping on NiP surface and contamination-free tagging on finished disk surface. Atomic force microscopy and Auger electron spectroscopy were employed to analyse processed samples. Processing results versus laser parameters were studied and optimized in detail. Based on the research, the techniques were commercialized subsequently. In this paper, we present our experimental results and introduce their applications.
Laser reflow plastic ball grid array
DaMing Liu, Tao Chen, Yuan Yuan, et al.
A modified fast-modulated CW Nd:YAG is used to reflow plastic ball grid array (PBGA). Sn-Pb eutectic solder balls with a diameter of 760 micrometers and Au-Ni-Cu solder pads are used in the study. Shear strength tests are preformed to find out the optimal reflow parameters. An energy equilibrium model is proposed to estimate the average temperatures of solder joint under the given experimental conditions. Based on the experimental and theoretical results, a parameter range for laser reflow of the 760 micrometers Sn-Pb eutectic solder balls is proposed.
Pulsed Laser Deposition
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Optical and surface properties of ZnO thin films by PLD
Yongfeng Lu, H. Q. Ni, ZhongMin Ren
ZnO films have been grown on silicon (100) and sapphire (0001) substrates by pulsed laser deposition (PLD). The influences of substrate temperature and laser fluence on the properties of the films were studied. The effects of annealing on the ZnO films were investigated by X-ray photoelectron spectroscopy (XPS). The surface properties of ZnO were studied by scanning tunneling spectroscopy (STS). The band edge emission properties of the ZnO films have been studied by the photoluminescence spectroscopy (PL).
Fundamental Aspects
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Laser ablation of Si in water and ambient air
Sha Zhu, Minghui Hong, M. L. Koh, et al.
Laser ablation of Si in ambient air and under water is investigated. It is found that the laser ablation rate of Si varies with the thickness of the water layer above the Si substrates. The laser ablation rate is the most highly enhanced with the water layer of 1.1 mm. It is assumed that the plasma generated in water confinement regime (WCR) with an optimal water layer thickness induces the strongest pressure. This high-pressure, high-temperature plasma results in the high ablation rate. It is found that the first peak-to-peak amplitude of the acoustic wave is the strongest when the water layer thickness is 1.1 mm above the substrate. By proper calibration, acoustic wave detection can be used as a real-time monitoring of the laser ablation.
Systems and Optics
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What engineers should know about lasers
Manfred Witt
Laser training for engineering students, newly hired technical personnel or business managers with partly technical functions can be organized in two levels: 1. Survey Level which avoids expert jargon, complete physical explanations and details, but provides safe guidelines to select the best laser for a given task, evaluate alternatives, organize the implementation and understand the economically decisive trends. 2. Expert Level for specialized manufacturing applications, design of systems, pushing the limits of application technology and in-depth strategic planning. Examples of survey level for Laser Precision Microfabrication are presented, with emphasis on local Singaporean conditions.
Cleaning
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Laser cleaning technology and its application
Wen Dong Song, Minghui Hong, L. Zhang, et al.
Laser cleaning technology has been demonstrated to be an effective cleaning technique for removing particulate and film-type contaminants from solid surfaces in this paper. Dry and steam laser cleaning mechanisms and applications in semiconductor and disk drive industry will be addressed.
Pulsed Laser Deposition
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Growth of (001)-oriented PZT thin films on amorphous SiO2 by pulsed laser deposition
Jing Zhao, Li Lu, Carl V. Thompson, et al.
Using pulsed laser deposition (PLD), (001)-textured Nb-doped Pb (Zr0.52Ti0.48)O3 (PZT) films with highly restricted in-plane crystallographic alignments were successfully deposited on Si(001) wafers covered with amorphous SiO2. MgO and SrTiO3 (STO) films were used as buffer and seed layers, respectively, for growth of YBA2Cu3O7 (YBCO) and PZT layers. The YBCO layer can serve as a bottom electrode for activation of PZT films. Variations in the deposition conditions for the MgO films were found to play a key role in the subsequent deposition of the oriented PZT thin films. MgO films deposited at an O2 partial pressure of 200 mtorr, have a strong (100) texture, leading to subsequent deposition of PZT films with both a strong (100) texture and highly restricted in-plane orientations. Although the MgO and STO films show very strong crystallographic textures, their in-plane orientations are random. The restricted in-plane orientations were only observed in the YBCO and PZT films.
Fabrication of TiNi thin films by pulsed-laser deposition
Xiaoyu Chen, Yongfeng Lu, ZhongMin Ren, et al.
TiNi shape memory alloy (SMA) thin films have been fabricated by pulsed-laser deposition (PLD) at different substrate temperatures. The stoichiometry, surface morphology and crystallinity of the films were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM) and X-ray diffraction (XRD). The transformation behavior and crystallization temperatures were investigated by differential scanning calorimetry (DSC). It is found that the Ni content of the deposited films ranges from 46.7 to 52.09 at.%. The films deposited at low temperature are amorphous. The crystallization temperature of the Ti-51.5 at.% Ni thin film is 449 degree(s)C. The martensitic transformation temperature of the film is -20.8 degree(s)C.
Composition and structure of BCN films prepared by ion beam-assisted pulsed laser deposition
Hao Ling, Zhifeng Ying, JiaDa Wu, et al.
We report on the preparation of boron carbon nitride (BCN) thin films by pulsed laser ablation of a sintered B4C target and the compositional and structural characterization of the films. The film preparation was performed with assistance of nitrogen ion beam from a Kaufman source. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used for compositional analysis and structural characterization. The results showed that the prepared films contain several chemical bonds such as B-N and C-N. The BCN films were found to show good adhesion to the substrates and have a high transparency from visible to near-infrared region. WE also grew films in nitrogen background without ion beam assistance and in vacuum. The atomic ratio of boron, carbon and nitrogen and the chemical bonds are strongly dependent on the deposition conditions. The ion beam assistance, i.e. the reactive nitrogen environment and the bombardment of the growing films by the energetic species in the nitrogen ion beam, is beneficial to the incorporation of nitrogen.
Gallium nitride films synthesized by reactive pulsed laser deposition from a GaAs target
Jian Sun, JiaDa Wu, Hao Ling, et al.
We demonstrated the feasibility of the growth of GaN thin films from polycrystalline GaAs using reactive pulsed laser deposition. The films were grown on Si (100) substrates at temperatures lower than 80 degree(s)C. A bulk of polycrystalline GaAs was used as a target. Reactive nitrogen plasma was provided by electron cyclotron resonance (ECR) microwave discharge in pure nitrogen gas to assist the film growth. Composition analysis showed that the grown films are slightly N-rich, and arsenic can hardly be detected. A strong absorption peak corresponding to Ga-N stretching vibration in the hexagonal-type GaN crystals is clearly resolved from the IR absorption spectrum. The films exhibit transparency in the visible and near-IR regions. The band gap of the films was determined to be about 3.4eV. when excited by 325-nm light at 10.2 K, the grown films luminesce in the blue region.
Electron cyclotron resonance plasma-assisted pulsed laser deposition of carbon nitride thin films
Wei Shi, JiaDa Wu, Jian Sun, et al.
We have prepared carbon nitride thin films by using plasma assisted pulsed laser deposition. In this method, a graphite target was ablated by laser pulses in the environment of a nitrogen plasma generated from electron cyclotron resonance microwave discharge in pure nitrogen gas, while the growing film was simultaneously bombarded by the plasma stream. The deposited films were characterized by Rutherford backscattering spectroscopy (RBS), Fourier transform infrared (FTIR) spectroscopy, and Raman Spectroscopy. Films consisting purely of carbon and nitrogen with nitrogen content over at.50% were obtained on Si (100) substrates at low deposition temperatures (<80 degree(s)C). N atoms in the as-prepared films were found to be bound to C atoms through hybridized sp2 and sp3 configurations. A strong influence of substrate bias voltage on the composition and bonding configuration in the films as well as on the deposition rate was observed.
Influence of laser fluence and laser repetition rate on the synthesis of aluminium nitride thin films by nitrogen-ion-assisted pulsed laser deposition
Yeow Whatt Goh, Yongfeng Lu, ZhongMin Ren, et al.
Aluminum nitride (AIN) thin films have been grown on Si(111) and Al2O3(001) substrates by pulsed KrF excimer laser (wavelength 248nm, duration 30ns) ablation of AIN target with assistance of nitrogen ion beam bombardment. The influence of process parameters such as laser fluence and laser repetition rate has been investigated to obtain high quality AIN films. The XRD spectra of AIN films on Si(111) and Al2O3(001) substrates yield full-width-half- maximum (FWHM) values of approximately 2.1-1.7 degree(s). The chemical composition of the films is characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the films is measured by atomic force microscope (AFM). Al2O3(0001) substrate shows better matching with the AIN films since highest crystallite size can be achieved among the different type of substrates evaluated. Better quality AIN films (with bigger crystallite size) can be achieved with higher laser fluence of 6J/cm2 and an optimum laser repetition rate 7Hz.
Pulsed laser deposition of nanocrystalline ZnSe:N thin films
Ning Xu, Yuancheng Du, Fuming Li, et al.
We have grown nanocrystalline ZnSe thin films on GaAs(200) substrates by pulsed laser deposition (PLD) using KrF excimer laser. Atomic force microscopy (AFM) shows that the ZnSe thin films grown on GaAs(100) at 10 torr are flat and dense and composed of crystallites with particle size of less than 100nm. X-ray diffraction (XRD) results indicated that this ZnSe thin film are of (100) crystalline orientation and the average size of crystallites is about 25nm. X-ray photoelectron spectroscopy (XPS) indicates that the as-deposited thin films contain 7%[N] and below 3% [O], and nitrogen with N-Zn, N-N or N-O bondings are respectively 55% and 45% of total contained nitrogen. Photoluminescence measurements show an donor-to acceptor pair (DAP) recombination emission with a blue shift with respect to bulk ZnSe, which reveals the activation of [N] atoms as shallow acceptors in nanocrystalline ZnSe.
Microfabrication and Microstructuring
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Progressive growth of tungsten microcones using nanosecond pulsed-Nd:YAG laser irradiation in various atmospheres
Yuji Kawakami, Eiichi Ozawa
We have discovered coherent arrays of ultrafine particles around a laser-irradiated mark. We tried to apply these arrays for electron emission guns. However, it was not successful due to their low aspect ratio. We observed microcones, which were made in the laser-irradiated mark on the initial surface of the target material. They seem to have a higher aspect ratio than the coherent arrays because heavy irradiation was needed to obtain cones with a high aspect ratio. Therefore, we examined the tungsten microcone growth in various gas atmospheres. The experimental results indicated that tungsten microcone growth did not depend on the impurities in the gases. In this paper we experimentally show the morphologies of the microcone in order to understand the microcone formation mechanism in various ambient atmospheres such as helium, SF6, and air. The difference in the microcone formation mechanism among the tungsten and Si substrate is discussed as related to the VLS (vapor-liquid-solid) mechanism.
Industrial Applications
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Cutting glass by laser
Hyoung-Shik Kang, Soon-Kug Hong, Seok-Chang Oh, et al.
In FPD (Flat Panel Display) devices, the diamond wheel has been used to scribe glass by means of mechanical contact which needs grinding and cleaning processes to remove particles, glass chips, surface cracks and sharp edges. In recent years, laser glass technology that is different from the conventional method of cutting glass by melting, has been researched and utilizes cutting glass by thermal shock. Laser glass cutting by thermal shock can produce cracks in glass by surface cooling after laser heating on glass by means of stress slope on glass surface. When this technology is applied in FPD manufacturing devices, it has several advantages compared to conventional methods as follows: a) non-contact glass cutting: almost no glass chip occurs. b) according to circumstances, grinding and cleaning can be omitted. c) system maintenance can be simplified.
Laser singulation of IC packages
Chengwu An, Kaidong D. Ye, Yuan Yuan, et al.
The separation of IC packages from a BGA board is realized by means of laser multi-scan method. The laser used in the study is a double frequency Nd-YAG laser with wavelength of 532 nm. The big problem in the laser processing approach mainly arises from the multi-layer materials of BGA board with copper, polyethylene and epoxy glass fiber, because of their different absorption coefficient to the laser beam and their different absorption coefficient to the laser beam and their different heat conductivity. In the experiment approach, the effects of laser parameters, such as wavelength, on the dicing efficiency has been investigated for choosing laser. The influence of sample side for laser incidence on cut profile and, the influence of the focused extent of laser beam on singulation speed are discussed. The experimental results show that laser singulation of IC packages is efficient and reliable.
Cleaning
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Particle on the surface: basic physical problems related to laser cleaning
In the present paper, we discuss some basic physical problems, related on the surface, i.e. optical resonance and near-field effects in laser cleaning. Another problem is the dynamics of the particle in adhesion potential. It is shown that the particle performs oscillations, which can be in principal resonantly enhanced. The analysis of thermal deformation shows that ID thermal expansion of the surface can not explain the experimental results and the 3D effects are of great importance. Some new effects arise due to the thermal contact of the particle with the surface.
Microsystems and Microelements
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Effects of interpulse and intrapulse shaping during laser percussion drilling
David K. Y. Low, Lin Li
An advent advocate of laser percussion drilling is the aerospace industry, where billions of holes have been laser drilled in turbine components, for the provision of effusion cooling holes, which consequently allowed higher combustion temperatures and engine efficiencies. However, in order for other industries to capitalize on the benefits of laser drilling, greater flexibility is required in producing the desired hole geometries. In this study, the effects of intra-pulse and inter-pulse shaping are investigated in terms of hole geometry control. While intra-pulse shaping involves temporally shaping within an individual laser pulse and repeatedly using the shaped pulse throughout the pulse train, inter-pulse shaping is carried out by temporal pulse shaping the overall delivery pattern of a pulse train required to percussion drill a hole. Experiments were performed using a fiber-optic delivered Nd:YAG laser for the drilling of through-holes in stainless steel 304. The fundamental effects of intra- and inter-pulse shaping are also compared with percussion drilling with unshaped rectangular pulses and the normal delivery pattern (NDP). The work has shown that inter-pulse shaping is effective in reducing the hole taper during laser percussion drilling. This was chiefly attributed to the optimized laser pulse distribution as the beam propagates into the hole.
Ultrafast Laser Processing
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Ultrafast laser ablation of dielectrics employing temporally shaped femtosecond pulses
Dielectric materials exposed to ultrashort laser radiation have evidenced individualized paths to deposit the energy into the lattice. Electronic and thermal mechanisms competing in the process of material removal depend on the efficiency of the electrostatic energy accumulation on the surface due to photoionization, as well as on the lattice heating which follows the electron-phonon coupling. The electrostatic surface break-up is a fast, sub-picosecond process, while thermal mechanisms start to dominate on a longer, picosecond time scale given by the electron-lattice equilibration and phase transformation time. The Coulomb- explosion induced ion ejection due to surplus charge accumulated on the surface during the photoionization process is significant only in dielectrics while in semiconductors and metals an efficient neutralization occurs. The significance of the different channels in dielectric materials can be reduced or enhanced by using laser pulses which are modulated on a time scale characteristic for the above mentioned mechanisms. Thus, amplified temporally-shaped pulses, double peaks, or pulse trains with a separation below 1 ps can have a significant effect on the quality of micromachining of transparent materials. The energy deposition can be modulated in such a way that the first pulse of properly chosen energy leads to a softening of the material associated with the onset of heating, thus changing the coupling conditions for the next pulses. This leads to less residual stress accumulation, cleaner structures, and opens the way for a material dependent optimization process.
Industrial Applications
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mu-BGA (ball grid array) singulation with a pulsed Nd:YAG laser
Kwang-Yeol Baek, Cheon Lee
In a microelectronics industry, the (mu) -BGA singulation system has been studied by mechanical type. However, this has burr and device damage by mechanical stress. So we need to research other systems to solve many problems. There are only a few types of lasers - excimer, CO2, Nd:YAG, copper vapor lasers in use for polymers and metals. In this paper, we have studied minimization of the line-width and surface burning which occur after the singulation process with a pulsed Nd:YAG((lambda) equals532 nm, repetition rate equals10 Hz) laser. The thermal energy of a pulsed Nd:YAG laser is used to cut the copper layer. Especially, the sacrifice layer and N2 blowing are effective in minimizing surface burning. The N2 blowing reduces a laser energy loss by debris and suppresses a surface oxidation. The SEM (scanning electron microscopes), non- contact 3D inspector and high-resolution microscope are used to measure cutting line-width and surface state. The (mu) - BGA threshold energy is 234.1 kJ/cm2 at 50micrometers /s scan speed.
Modification
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Characterization of modified surface of indium tin oxide film during process of laser patterning
JiNan Zeng, Hwee Lin Koh, ZhongMin Ren, et al.
Effect of excimer laser irradiation at different fluences on Indium-Tin-Oxide films has been studied with the use of optical transmission, XRD, Mirco-Raman spectra and XPS. Surface modification at low fluence of 154mJ/cm2 is observed to cause the increase of optical transmittance at initial several pulses of UV laser. At moderate fluence of 239mJ/cm2, UV laser irradiation results in apparent coloration and chemical compositional change on the ITO surface. XRD results show the grain size of ITO tends to decrease after irradiation. Novel features appear in Raman spectra, which involve the change of surface crystallinity and composition induced by UV laser irradiation. XPS analysis indicates peak shape of O bond is modified post irradiation and Sn/In ratio presents maximum corresponding to dark coloration. ITO films have also been patterned at high fluence of 1-2J/cm2 using simple masks. The ablation rate on laser pulse demonstrates linear change. An alternative method for laser patterning is proposed by the combination of excimer laser coloration and visible laser patterning.
Fundamental Aspects
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Temperature effect for exciton dynamics in ZnCdSe/ZnSe QWs
JiNan Zeng, Yongfeng Lu, Yasuo Oka
Temperature dependence of photoluminescence (PL) from Zn1-xCdxSe/ZnSe Asymmetric Double Quantum Wells (ADQWs) has been studied by using time-resolved spectroscopy. With raising temperature, the different transient behaviors of exciton emission are observed. In the regime of T<40K, the linear increase of lifetimes in both QWs with temperature is interpreted according to J. Feldmann's theory. On the other hand, the nonradiative recombination process leads to decreases of lifetimes and intensities of the NW and WW excitons in the regime of T>40K. The faster decrease of lifetime of the NW exciton is assigned to the thermal-tunneling processes comparing to that of WW exciton. The thermal-tunneling process results from the electron-hole tunneling process due to the enhancement of ionization of exciton with raising temperature. The rate equations including thermal tunneling process are used to explain the experimental data.
Microfabrication and Microstructuring
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Laser micromachining for Ag ion exchanged glasses
Tadashi Koyama, Keiji Tsunetomo, Kenji Kamada, et al.
Machining properties of Ag containing glasses (ACG) using KrF excimer laser and femtosecond laser are reported. Ag ions in the ACG are effective to prevent cracking and chipping during the laser machining. A 4x4 hole array fabricated by KrF excimer laser onto an ACG has good uniformity of hole diameter, d(ave) equals 129.9um and (sigma) equals 0.8um. V- and U- grooves are fabricated on ACG using a Ti:Al2O3 laser irradiation. The cross- sectional shape is controlled by Ag ion concentration in ACG.
Microstructures formed on silicon wafer by CO2 laser irradiation
Weijie Wang, Yongfeng Lu, Chengwu An, et al.
Laser bumps have been formed on the silicon surface with anisotropic patterns induced by the pulsed CO2 laser under the backside effect conditions. The microstructures are formed with periodic patterns, and vary with the laser parameters. The parallel narrow fringes have period around 2 micrometers , and the fringe orientation is in parallel with the laser polarization direction. The circular fringes with spacing around 1 micrometers appear when the laser energy increase. The fringe patterns were found to be independent on the laser pulses, therefore are re-writable. The bump formation mechanism is considered to be thermal capillary wave effect during the material melting and resolidification processes under laser irradiation. Whereas, the laser absorption may be induced by the hot electrons, or the thermal energy resulting from the laser interaction with the backside coating material.
Fundamental Aspects
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Steam-assisted laser ablation of metal substrate
M. L. Koh, Ming Hui Hong, Sha Zhu, et al.
Excimer laser irradiation has been shown to produce novel effects when used to process metallic surfaces. The effect of a thin film of steam on copper specimens has been investigated, and the results are a vast improvement in comparison to ablation in ambient air or liquid. This can be observed from the acoustic waveform detection during the ablation process. A pulsed excimer laser was used to ablate copper samples with fluence levels ranging from (2.5 J/cm2) to (3.8 J/cm2). The peak-to-peak signals detected in steam are about twice the amplitude of those obtained in air. Ablated specimens have been characterized using alpha-step scans for ablation profiles. Results show that the ablation rates with steam are higher than those done in ambient air or liquid. Discussions will focus on the assessment of factors influencing the production of such features, including laser fluence, laser pulse counts, and also the ambient processing environment.
Diagnostics and real-time monitoring of pulsed laser ablation
Signal generation, diagnostics and real-time monitoring during pulsed laser ablation of solid materials (silicon, metal and IC molding component) are investigated. It is found that there is rich signal emission in the laser ablation. Wide band microphone, ultrafast phototube and tiny metal probe are applied to detect acoustic, optical and electric signals generated during the laser ablation. Optical emission spectrum analyzer and high-speed ICCD photography are used to study laser-ablation-induced plasma dynamics. Signal variation with laser processing conditions during the laser ablation is analyzed. Formation of the signals is also discussed and modeled. Based on the signal diagnostics and analyses, relations between signal characteristic parameters and the laser processing conditions can be established for different applications of laser ablation in industry.
Observation of temporary degeneration of material transmittance under strong laser irradiation
Chengwu An, Kok Leong Ler, Yuan Yuan, et al.
Laser induced temporary degeneration of transmissivity in optical materials has been observed under strong laser irradiation. In the experiment, a green light laser beam was focused onto a transparent sample, an ultra fast photo-tube was used to detect the laser signals behind the sample, an oscilloscope was used to record the amplitude change of the signals. When the laser power density on the sample increases to some levels, the detected signal starts to decrease relatively, which means the transmissivity of the sample commences to degenerate. In this case, if the laser power density is reduced back a little bit, the transmittance of the sample can restore. This means that the degeneration of the transmittance is temporary. When the laser power density increases further more after the degeneration has commenced, damage to the material will happen. Therefore, there is a transition course in material before it is damaged. The transition course is from the beginning of transmittance degeneration to damage rising. The influence of laser repetition rate on the damage formation has been also investigated.
Systems and Optics
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Advancement in very-short-wavelength emission in solid state lasers
Solid state lasers emitting at very short wavelengths (>10nm) are very attractive for several applications in material proce3ssing and holography, but not yet demonstrated. Physical limitations prevent using classic laser schemes. We present a semi-empirical and rigorous demonstration of the feasability of a solid state x-ray laser and find some relations of the suggested scheme with EIT (Electro-Magnetic Induced Transparency) and Lasing Without Inversion (LWI).
Industrial Applications
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Laser thinning of the thickness of a-GeS2 films
Shuichi Kawabata, Yoshihisa Murakami, Yasunori Ozawa, et al.
The amorphous GeS2 film is one of the chalcogenide glass and also shows the phenomenon of Ag-photodoping under the illumination of the high-pressure mercury lamp. We have been carrying out the in-situ measurements of the photodoping process of Ag/a-GeS2 films system with a laser probe ellipsometer. In the process of the thickness measurements, we have observed curious changes of ellipsometric parameters (delta) and (Psi) of the GeS2 film sample during the illumination of the laser probe. The results seem to be suggesting that the illumination of the laser beam might engrave the GeS2 film. The power of the laser is less than 1mW and the beam diameter is 1mm. The idea such as an ablation of the GeS2 film seems to be unlikely to interpret this phenomenon.
Fundamental Aspects
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Orientation dependence of kinetics of laser-induced liquid-solid phase transitions in A3B5 single crystals
The phase transitions in A3B5 single crystals have been studied. GaAs, GaSb and InSb wafers oriented in (100) and (111) plains were irradiated by ruby (80 ns FWHM) and ArF excimer (10 ns, (lambda) equals193 nm) lasers. Dependence of melt duration (tau) on the irradiation energy density W was measured. The values of (tau) were determined from the time-resolved reflectivity measurements (TRR) at (lambda) equals1.06 (ruby laser) and 0.63 micrometers (ArF). The average discrepancies between (tau) 100 and (tau) 111 established for each semiconductor are related to somewhat slower velocity of crystal growth in <111> direction epitaxial process in this direction is characterized by lower crystallization temperature Tcr and by larger undercooling of the liquid phase. The values of Tcr(100) and Tcr(100) for InSb can differ by almost 10%. Increase in difference between (tau) 111 and (tau) 100 values going from GaAs to GaSb and to InSb correlates with the decrease in interface temperature and with the increase of average atomic mass, i.e., with the reduction of mobility and diffusion of atoms (ions) in binary melt.
Optics Fabrication
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Fabrication of microlenses and microlens arrays using local melting of glass plate by CO2 laser
Moriaki Wakaki, Yuichi Komachi, Genichi Kanai, et al.
Microlenses and microlens arrays were formed directly on a surface of a glass plate by use of a CO2 laser. By heating the surface of a glass plate locally to a working point of the glass material using CO2 laser beam, the surface became a hyperboloid owing to surface tension, which resulted in microlens. This method has the merit of complete dry processing. Several types of glass materials were tested. It is important to choose the type of glass with thermostable property with small expansion coefficient. It was found possible to make easily microlens and arrays with different focus lengths by controlling a laser power and a irradiated position. Typically, a CO2 laser power of 1.5W and an irradiation time of 1.1s were used to fabricate a microlens with a 170 micrometers diameter and 181 micrometers focal length with good focusing property. Microlenses fabricated using various beam profiles of CO2 laser were also discussed.
Ultrafast Laser Processing
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Subpicosecond ion emission from transparent dielectrics
Juergen Reif, Florenta Costache, Matthias Henyk
Upon irradiation of barium fluoride (111) crystals with 100- fs pulses at 800nm, explosive emission of singly charged positive ions (Ba+, F+, and larger molecules and clusters) is observed with a kinetic energy of about 100 eV and an energy distribution corresponding to a temperature of only 1 eV, independent of the ion mass. Pump-probe experiments demonstrate that ion emission is always the consequence of preceding multiphoton surface ionization, resonantly enhanced by defect states within the band gap. Yet, the ionization process appears not to be noticeably slowed down by the resonances. Electron and light microscopic investigation of the desorption crater revealed frozen surface waves with the periodicity on the order of some microns. Superimposed, we found a ripple-like periodic fine structure, with a periodicity of 100-400 nm depending on the incident laser intensity. We suppose that this should be the consequence of self-organized relaxation of the surface, rather than the consequence of an interference effect as in the classical model for ripples formation.
Systems and Optics
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All-optical full logic unit with femtosecond switching
Reiner P. Schmid, Juergen Reif
We report on the realization of an all-optical full logic unit switching with a response time on the order of one femtosecond. The device is based on instantaneous transient gratings induced in a thin slide of barium fluoride by up to three femtosecond laser beams. The interference pattern from every tow of the non-collinear beams is, via optical Kerr effect, translated into a transient index grating which reacts within one optical cycle on the input since the applied light frequency is far from resonance in the material. As one consequence of the grating two strong beams of self phase matched third harmonic are generated, emerging in the middle between the two transmitted fundamentals. By introducing a third femtosecond beam we are able to emulate a demultiplexer, a decoder and simple adder. Since the output consists of the third harmonic of the input beams, the system works with uniquely high contrast.
Industrial Applications
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Polymer welding with lasers: chances and hurdles
Polymer welding with lasers has been demonstrated already several years ago, using preferably CO2 and Nd:YAG lasers; however, it did not become a big market so far, which is commonly accepted to be due to cost reasons and the size of the laser systems. With the appearance of the efficient, reliable, extremely compact and easy to integrate high power diode lasers, this technology undergoes a renaissance and first application in industrial manufacturing environment have been reported. However, our experience has also shown, that technological hurdles still exclude certain applications and limit the technology to certain configurations. This is, since additionally to the material parameters of the polymer for conventional welding, e.g. melting resp. glass temperature, decomposition temperature, thermal expansion coefficient etc. for the laser weld of course optical parameters play a key role.
Microsystems and Microelements
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Fabrication of micropumps with Q-switched Nd:YAG-lasers
Microfluidics has become one of the most intense research fields in MEMS technology. In this paper, the use of a 1064nm Q-switched solid state laser to fabricate micro-pump cavities in copper is presented. The focusing technique is employed fore directly structuring the micro-pumps and mixers. In this case, a laser beam with a focal spot of 50 micrometers is canned over the surface and the substrate material is ablated track by track and layer by layer. Machining results such as surface finishing and dimensional resolution are discussed. The dependence of the ablation depth, ablation rate and surface roughness on the process parameters and on the scan overlap are investigated. The laser micromachined structures are free of cracks and without any deposition of debris on the surface. The assembly and first characterization results of the pumps are reviewed. The capability as well as the potential of laser micromachining are also discussed.
Systems and Optics
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Development of a high-performance UV-DPSS laser drilling system for PWB
Daisuke Yokohagi, Yasuhiro Mizutani, Koki Ichihashi, et al.
A high-performance UV-DPSS (Diode Pumped Solid State) laser drilling system (wavelength 355nm) for PWB's, which can drill a less than 50micrometers diameter via-hole, has been developed. This system has two main features as follows: 1. A vua-hole without smear is possible to be drilled for PWB's with the optimized irradiation conditions of UV laser. 2. High throughput was realized by using a high power UV-DPSS laser and a developed high-speed control method of galvanometric scanning. This paper describes the outline of the developed UV-DPSS laser drilling system. This system has the advantages which are required small via-holes formation less than 50micrometers in diameter, compares with a conventional CO2 laser system currently.
Pulsed Laser Deposition
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Film growth on single MgO and SiO2 covered Si substrate by pulsed laser deposition
Ying Su, Jing Zhao, Li Lu, et al.
STO and subsequent YBCO thin films with different orientations have been grown on a single crystal MgO (100) substrate as well as a MgO on a 400nm amorphous SiO2 covered Si (100) substrate using pulsed laser deposition (PLD) technique. Cross-sectional transmission electron microscopy (TEM) showed an epitaxial growth of highly c-axis oriented YBCO(001)/STO(100) on a single crystal MgO(100). TEM investigation on MgO/SiO2 and MgO indicating interfacial reaction between SiO2 and MgO. Chemical reaction may be possible at the interface when films are hold at high temperature based on the calculation of Gibbs free energy. Auger electron spectroscopy (AES) showed that an uphill diffusion of Ba from YBCO occurred during deposition and post thermal treatment processes.
Advanced Microfabrication Technology
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Laser ablation patterning of dielectric layer stacks for 193-nm mask fabricaton
Juergen Ihlemann, Katharina Rubahn, Roland Thielsch
Fabrication of dielectric optical masks by ablation patterning using a 193 nm laser is demonstrated. Ablation of layer systems that are highly reflective at 193 nm (HR 193nm) is accomplished by using an absorbing layer to initiate ablation. This absorbing layer is deposited on the substrate under the HR system. Thus it is possible to induce spatially defined ablation of the whole stack by irradiating the absorbing layer through the substrate (rear side ablation). Using the system as a mask, i.e. irradiating it from the front side, the reflective layer system prevents penetration of high fluence radiation to the absorbing layer, so that the function of the reflective mask is not restricted. A HR 193 nm system, composed of 42 layers, alternating Al2)3 and SiO2 was prepared. The substrate was fused silica. For the absorbing layer (a) HfO2 and (b) SiOx (x<2) of about 30 nm thickness were used. In the case of HfO2, residual fragments of the absorbing layer diminish the transmission in the ablated areas. In the case of SiOx, which is absorbing at 193 nm for x<2, these residuals can be oxidized to SiO2 has high transmission at 193 nm improving the mask performance. For the ablation process single pulse irradiation with a fluence of about 800 mJ/cm2 is sufficient. The mask has a damage threshold of >1 J/cm2.
Ultrafast Laser Processing
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Morphological characterization of various kinds of materials in femtosecond laser micromachining
Kosuke Kawahara, Yasunobu Kurogi, Naoyuki Matsuo, et al.
We are developing applications of femtosecond-laser ablation to the practical material processing such as cutting, drilling, dicing and turning. In this work, we have observed the morphology of ablation traces formed on the surfaces of various materials to investigate the influence of physical properties of work materials on the machining preciseness. Ti:sapphire laser pulses ((lambda) equals 790 nm, (tau) equals 160fs, f equals 10 Hz, E equals 0.6 ~ 1.2 mJ/pulse) were focused on the samples, insulators (silica glass and PMMA), semiconductor (Si), and metals (Cu and Al), fixed on a PC- controlled X-Y stage and moved one-dimensionally at a scan rate of 5.5 ~111 micrometers /sec. Grooves with well-defined edge were observed on the insulators which have low thermal conductivity. In the case of the metallic materials having relatively high thermal conductivity, molten walls were observed near the edge of the grooves. It is considered that mainly thermal conductivity plays an important role in characteristics of the ablation morphology.
Development of laser turning using femtosecond laser ablation
Atsushi Yokotani, Kosuke Kawahara, Yasunobu Kurogi, et al.
Ablation using femtosecond-lasers has taken much attention for micromachining with the advantages of efficient ultra-thin layer peeling without undesirable thermal effects for both opaque and transparent materials. In this work, we have proposed the femtosecond-laser turning based on the fact that femtosecond-laser ablation is a promising technique for excellent surface finishing techniques. The effect of the machining conditions such as focusing condition, energy of laser pulse and plural scanning, on the thickness of peeling layer for various kinds of materials (PMMA, silica glass, silicon, copper and aluminum) have been investigated. As a result, the important information for the laser turning as a practical application of the femtosecond lasers were obtained.
Pulsed Laser Deposition
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Pulsed laser deposition of ceramics and diamond-like carbon on PMMA for tribological applications
Georg Schlaghecken, Yibran Perera, Ernst-Wolfgang Kreutz, et al.
The thin films (thickness 400 nm-3micrometers ) were deposited by Pulsed Laser Deposition (KrF-Excimer laser, (lambda) equals248nm, (tau) equals25ns) with variation of the processing parameters gas pressure, target-to-substrate- distance and laser fluence on the target. Polymethyl methacrylate (PMMA) was chosen as substrate material and the selected coating materials are alumina (formula available in paper) and diamond-like-carbon (DLC). The focus in this work is on the adherence of the coatings. Therefore, different pretreatments of the PMMA substrates like plasma processing, deposition of titanium buffer layer, mechanical and chemical surface modification were performed. The hardness and the elastic modulus of the films are measured by nano- indentation. The thickness of the ceramic deposited films are determined by ellipsometry. X-ray photoemission spectroscopy, optical and electron microscopy are used for the determination of the structural and chemical characteristics of the films.
Fabrication of Ce:YIG film with different composition by pulsed-laser deposition
Yoshiki Nakata, Yuko Tashiro, Tatsuo Okada, et al.
Ce-doped yttrium iron garnet (Ce:YIG) with different compositions of Y2CeFe5O12, Y2CeFe4.25Ga0.75O12, Y1.6Gd0.4CeFe5O12 were deposited by PLD on GGG(111) SUBSTRATE. The crystallinity, transmittance and surface morphology were observed for the films deposited at different depositions conditions of argon or oxygen gas pressures and substrate temperatures.
Optics Fabrication
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Laser-ablation-based technique for flexible fabrication of microlenses in polymer materials
Laser ablation is a versatile technique for fabricating microstructures on polymer surfaces. Due to the nature of the process, the fabrication of the microstruture can take place in a very late stage of the heterogeneous assembly. This makes laser ablation very attractive for fabricating micro-optical components on opto-electronic assemblies in comparison to other fabrication techniques like injection molding and embossing. In this paper we will report on the first experimental results of microlens fabrication with excimer laser ablation techniques. By scanning the polymer surface along multiple circular paths with a circular beam of a pulsed excimer laser, one is able to obtain a lens shape with arbitrary focal distance and diameter. Important issues such as the choice of ablation parameters, the selection of scanning path and the performance of the resulting laser ablated lens will be discussed.
Overview
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Recent progress of laser precision microfabrication in China
Zhaogu Cheng, Guoliang Xu, Quanzhong Zhao, et al.
Review on recent progress of laser precision microfabrication in China is given in this paper. The universal fields of the LPM mostly using short pulse and short wavelength lasers in China are as follows: new materials, advanced manufacturing technology, information technology, biological technology, medical treatment and so on, which are given priority to the development in high-tech fields in China.
Cleaning
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Dry laser cleaning threshold fluence: How can it be measured accurately?
Alanna Fernandes, Deb M. Kane
Dry/damp laser cleaning studies are reported that used a XeCl excimer laser to remove alumina particles from glass. These have compared the laser cleaning of samples with medium and low particle densities. Earlier work at Macquarie University used an UV-copper vapor laser (wavelength 255 nm, pulse length 35ns) to investigate single pulse, dry/damp laser cleaning of alumina particles from microscope slides and fused silica surfaces. Medium densities of particles were used. A threshold fluence for dry laser cleaning of 90mJ/cm2 was measured. Subsequent work using a XeCl excimer laser (wavelength 308 nm, pulse length 8 ns) is reported. The threshold fluence for single pulse laser cleaning of medium density particles increases by a factor of apprixomately 4 at this longer wavelength. Using a sample preparation technique involving laser ablation to prepare lower particle density samples, with reduced particle agglomeration, the results appear qualitatively and quantitatively different than for samples with a medium particle density on the surface. However, careful analysis shows that the dry laser cleaning threshold is the same in each case. This supports the use of medium particle densities as an experimentally reliable method of determining the laser cleaning threshold fluence.
Optics Fabrication
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Laser shaping of diamond for IR diffractive optical elements
V. V. Kononenko, Vitali I. Konov, Sergej M. Pimenov, et al.
Recently a new technique for laser-induced generation of phase relief to manufacture diamond diffractive lenses for the mid IR range has been proposed. In the present paper the realization of more complicated diamond diffractive optical elements (DOEs) is considered, able to transform a CO2 laser beam into arbitrary pre-given focal domains. Two DOEs for completely different tasks of laser beam focusing have been manufactured and finally investigated by means of various optical techniques. Measured intensity distributions in the DOEs focal planes as well as diffraction efficiencies have been compared with related results of computer simulation, and have been found to be in a good mutual concordance. The obtained results indicate that laser ablation technique can be effectively used to manufacture high quality diamond DOEs for laser beam focusing. Special attention is paid to the diamond surface graphitization in the process of laser ablation. Main parameters of excimer laser ablation are investigated and density of laser-induced graphite-like layer is defined. It was demonstrated experimentally that graphitized layer formed at different regimes of irradiation remains almost constant in thickness, but has different crystal structure.
Industrial Applications
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High-speed microvia formation with UV solid state lasers
Laser drilling has emerged in the last five years as the most widely accepted method of creating microvias in high- density electronic inter connect and chip packaging devices. Most commercially available laser drilling tools are currently based on one of two laser types: far-IR CO2 lasers and UV solid state lasers at 355 nm. While CO2 lasers are recognized for their high average power and drilling throughput, UV lasers are known for high precision material removal and their ability to drill the smallest vias, with diameters down to about 25-30 micrometers now achievable in production. This paper presents a historical overview of techniques for drilling microvias with UV solid state lasers. Blind and through via formation by percussion drilling, trepanning, spiralling, and image projection with a shaped beam are discussed. Advantages and range of applicability of each technique are summarized. Drivers of throughput scaling over the last five years are outlined and representative current-generation performance is presented.
Excimer laser crystallized poly-Si TFTs on plastic substrates
Dharam Pal Gosain
Excimer laser crystallized poly-Si TFTs, integrated both for drive circuits and pixels, on a glass substrate for use in various applications, such as, flat panel display are now available in the market. These poly-Si TFTs on glass are generally prepared by excimer laser crystallization of plasma enhanced chemical vapor deposited amorphous silicon films, which, are generally deposited in the substrate temperature range of 300 degree(s)C-400 degree(s)C. Changing the TFT backplate from glass to plastic will reduce the wight, but transparent plastic substrates are generally only temperature-resistant up to 125 degree(s)C. We have focused therefore, on reducing the maximum processing temperature to 110 degree(s)C. Our approach utilizes sputtered a- Si films for crystallization, because they contain no hydrogen and can be crystallized by an excimer laser even if deposited at room temperature. We report high-quality poly-Si obtained by pulse laser crystallization on a plastic substrate coated with a buffer layer. Transmission electron microscope was used to confirm the crystallinity of the films. A doping technique for source and drain compatible with a low temperature wide area substrate, based on excimer laser annealing, has been developed. Self-aligned top-gate TFTs fabricated on a plastic substrate at a substrate temperature of 100 degree(s)C are reported. Transistor field effect mobility of 250 cm2/V.s and s sub-threshold swing of 0.16 V/decade were measured in these devices.
High-resolution 157-nm imaging for lithography and micromachining applications
Malcolm C. Gower, Julian S. Cashmore, Michael D. Whitfield, et al.
A Microstepper exposure tool for high-resolution imaging applications incorporating the F2 157nm laser is described. Details are presented of the optical architecture including beams shaping, homogenization and imaging objectives. Results from the high-resolution metrology, workpiece positioning and gas purging subsystems used in the tool are discussed. Use of the Microstepper for 70nm-node deep-iv lithography and submicron micromachining applications is presented.
Comparison of machining strategies for ceramics using frequency-converted Nd:YAG and excimer lasers
Hans Kurt Toenshoff, Andreas Ostendorf, Klaus Koerber, et al.
This paper compares the machining strategies for ceramics using a frequency converted (3(omega) ) Nd:YAG laser and a KrF excimer laser. In respect to the substantially different beam characteristics of these lasers, it is also a comparison of laser structuring methods using a focusing or an imaging optical system. The presentation concentrates on the quality of the machined structures and on the processing time of micro cutting and drilling with these two laser systems. An innovative approach for the manufacturing of micro holes, based on a newly developed mask technology, is presented for the first time.
Ultrafast Laser Processing
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Focal point images of phase-shifted laser beam using femtosecond optical polarigraphy
Aritsune Nagamura, Hiroki Shibata, Koichi Toyoda
Intensity distribution of high intensity femtosecond laser pulse has been measured using time-resolved optical polarigraphy. Ultrashort laser pulses from Ti:sapphire laser propagating in CS2 are studied and compared the difference between gaussian and phaseshifted beams.
Ablation deposition using phase-shifted ultrashort laser pulse
Masanori Miyazawa, Atsushi Masagaki, Yasutaka Hanada, et al.
The specific deposition and atomic redistribution with use of the phase-shifted laser were examined. Phase-shifting is a unique technique to realize the abrupt intensity change in a very narrow region of focused laser beam. This technique is applied to an ultra-short laser pulse to study the interaction between particles removed from very closely separated ablation spots in focal area. Using the phase- shifted focused laser beam, atomically nonuniform deposition was observed in a femtosecond laser ablation of Cu:Zn alloy target. Atomic concentration ratio of Cu and Zn was obtained by the measurement of energy dispersive X-ray analyzer.
Ultrafast laser-induced crystallization of amorphous silicon films
Taeyul Choi, David J. Hwang, Constantine P. Grigoropoulos
Ultra-short pulsed laser irradiation was used to crystallize an a-Si film with a laser fluence of 0.12 J/cm2. The crystallization process was observed by time-resolved pump- and-probe imaging in the range of 0.2 ps to 100 ns. Reflectivity data along with post-processed SEM pictures provide evidence for a non-thermal ultra-fast solid-phase transition. A crystallization sequence based on atomic self- diffusion is suggested.
Modification
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Synthesis of pure C40 TiSi2 for Si wafer fabrication
S. Y. Chen, Zexiang Shen, S. Y. Xu, et al.
A simple and novel salicidation process applying pulsed laser annealing as the first annealing step was used to induce TiSi2 formation. Both Raman spectroscopy and transmission electron microscope results confirm the formation of a new phase of Ti disilicide, the pure C40 TiSi2 after laser irradiation. Direct C54 phase growth on the basis of C40 template bypassing the C49 phase is accomplished at the second annealing temperature as low as 600 degree(s)C. Line width independent formation of the C54 phase was observed on patterned wafers using this salicidation process and fine line effect is thus eliminated.
Fundamental Aspects
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Dynamic observation of laser-induced damage made inside of transparent materials
Yoshiro Ito, Satoshi Kiyoku, Tsutomu Ogura, et al.
When we focus high power laser pulses on a surface or inside of transparent materials, non-linear absorption of the laser radiation occurs and some laser-induced damages are introduced. This process is used in laser marking of transparent materials. We have made dynamical observations of the process by the stroboscopic imaging system which has a few nanoseconds time resolution. Output of fundamental radiation (1064nm) from a Q-switched Nd:YAG laser was focused at the surface or inside of PMMA and soda glass blocks. Second harmonic radiation (532nm) form the same laser was used as illuminating light and images were taken by a CCD camera with a band-pass filter of 532 nm. Series of images were taken at different intervals between fundamental and second harmonic light, which was controlled by optical delay line. In observation at longer intervals than 50ns, another laser was used as illuminating score. Non-linear absorption in PMMA occurs at several points simultaneously and propagation of shock waves started from these points is clearly observed. Non-linear absorptions in soda glass occurs from inside outward along the laser beam. Laser- induced damages (cracks) continue to develop until some hundreds milliseconds after laser pulse. Effects of pulse energy and roughness of the target surface were examined.
Advanced Microfabrication Technology
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Characterization of fused silica ablation by F2-KrF excimer laser multiwavelength excitation process
Kotaro Obata, Koji Sugioka, Toshimitsu Akane, et al.
The novel technique for high-quality ablation and high- efficiency refractive index modification of fused silica by VUV-UV multiwavelength excitation process has been developed, in which both of the commercially available F2 and KrF excimer lasers are simultaneously irradiated to the sample. The high-quality ablation is ascribed to absorption of KrF excimer laser by excited states formed by F2 laser (excited-state absorption mechanism). Dependences of ablation rate on KrF excimer laser fluence at three deferent F2 laser fluences and irradiation timing of each laser beam are investigated. The multiwavelength excitation process is also applied for effective refractive index change. The multiwavelength excitation process achieves twice of diffraction efficiency compared with single wavelength irradiation of F2 laser at same number of incident total photon number. This is caused by the resonance photoionization-like process through the excited state.
Overview
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Femtosecond pulses for medicine and production technology: overview of a German national project
With the beginning of the new century the German federal government started the funding of a program intended to exploit the potential of femtosecond technology. In a foregoing competition, five research consortia had been successful and have started their investigations in the following fields. - micro-machining of technical materials for microstructure and drilling - medical therapy in : ophthalmology, dentistry, neurology and ear surgery - metrology - laser safety. Lasers, systems and technologies required in these potential fields of applications will be investigated. The program aims at industrial success and is dominated by industrial partners, therefore. The more fundamental research is done in university institutes and research centers.
Ultrafast Laser Processing
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Precision drilling of metals and ceramics with short- and ultrashort-pulsed solid state lasers
Christian Foehl, Detlef Breitling, Knut Jasper, et al.
At the end of 1999 a German National Project called PRIMUS was established, the most important aim of which is to analyze the potential advantages of ultrashort pulses in combination with different drilling strategies and to obtain a better understanding of the ablation and drilling processes. This contribution will present the first results of this project. The advantages of short and ultrashort pulses in view of quality and efficiency will be discussed. It will be shown, that the use of suitable drilling technologies, such as e.g. helical drilling, and a specifically designed trepanning optic can significantly increase the quality of holes as well as expand the possible range of applications.
Microfabrication and Microstructuring
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Laser microfabrication: present to future applications
Gnian Cher Lim, Tuan-Anh Mai
Product miniaturization and component intricacies require advanced processing technologies to satisfy functionality and quality requirements. Various micro-machining technologies have been under extensive development so as to be able to fill this niche. In particular, the development of MEMS devices utilizes machining methods that include silicon fabrication and LIGA technologies. Lasers present an alternative or supplementary tool for micro-fabrication. Present advances in laser system design are bringing about great improvements in beam quality, increase in high peak power, increase in high pulse energy, and shorten pulse width. This enables direct machining of a wide variety of materials with feature sizes down to the order of micrometers. This paper presents some of the recent developments in utilizing laser to micro-machine or micro-fabricate micro- features on devices. Many of these processes are being used in production, or are expected to be commercially feasible.
Advanced Microfabrication Technology
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GaN micromachining by short wavelength pulsed laser irradiation
Toshimitsu Akane, Koji Sugioka, Kotaro Obata, et al.
GaN ablative etching using coaxial irradiation of KrF excimer laser and F2 laser has been explored. The etching process is consisted of pulsed laser ablation and successive acid treatment. Single pulse ablation using KrF excimer planarizes etched surface, however, multiple KrF irradiation roughens etched surface significantly. Small fraction of F2 laser simultaneously irradiated with KrF excimer laser improves surface roughness caused in the case of single-KrF irradiation. Irradiating delay time between both lasers is varied in order to investigate excitation mechanism as well as find optimum irradiation condition.
Modification
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Application of photosensitive GeO2-B2O3-SiO2 thin films to optical waveguide
Hiroaki Nishiyama, Isamu Miyamoto, Kenji Kintaka, et al.
GeO2-B2-O3-SiO2 thin films were fabricated by plasma enhanced chemical vapor deposition method. Boron codoping into a GeO2-SiO2 thin film induced large absorption in the vicinity of 240nm, and OH absorption decreased compared to GeO2-SiO2 films. These films of 5 micrometers in thickness exhibited large positive refractive index change without hydrogen loading by irradiation with ArF (193nm) excimer laser pulses. Induced refractive index change was approximately 0.002 which was measured by the prism coupling method. A waveguide was written in this high photosensitive glass film by UV irradiation. The guided mode of the waveguide seems to be single and estimated refractive index change was approximately from 0.003 to 0.004. Three unique phenomena were found in 0.2micrometers thick films on Si substrate. First, these films exhibited large negative refractive index and positive thickness changes by irradiation with ArF laser pulses. Induced negative index change was larger than 0.02 and thickness change was more than 1%. Silica films doped only boron or germanium didn't exhibit such negative index changes. Second, the annealing before laser irradiation decreased the photosensitivity of these films remarkably. Third, these induce refractive index and thickness changes were decreased with time rapidly. These mechanisms were under investigation.
Ultrafast Laser Processing
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Hole formation process in laser deep drilling with short and ultrashort pulses
Taras V. Kononenko, Sergei M. Klimentov, Serge V. Garnov, et al.
The drilling process in different materials (diamond, steel, ceramics and PMMA) was studied for a large range of pulse lengths from about 100 fs to 10 ns using different approaches. In transparent materials the penetration process was visualized with high-speed video analysis and microscopy. The drilling rate as well as the relation between processing energy density and ablation threshold were determined in situ. The penetration of the laser beam inside the channel and the influence of laser-ignited plasma were investigated by transmission measurements. Mechanisms of energy coupling and heat losses were examined by applying simple analytical calculations. Proposals for the basic understanding of the drilling process are presented.
Systems and Optics
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Highly reliable high-power 266-nm all-solid-state UV laser
Tetsuo Kojima, Susumu Konno, Shuichi Fujikawa, et al.
We obtained the 266nm UV power of 23W by fourth-harmonic generation using a high-brightness high-power all-solid- state green laser and a high-quality CLBO crystal. This value is, to our knowledge, the highest UV power below 300nm wavelength. Moreover, the UV-power variation was negligibly small by using high-crystal CLBO crystals.
High-power high-brightness diode-pumped solid state laser for precise laser processing
Shuichi Fujikawa, Keisuke Furuta, Masaki Seguchi, et al.
We have proposed a highly efficient and high-brightness quasi-cw Nd:YAG rod laser with a novel-side-pumping design using micro-lens free diode-stacks. We achieved 320W output power with 28-% electrical-to-optical efficiency, which is, to our knowledge, the highest efficiency reported for diode- pumped solid-state lasers. We generated the high quality beam of M2equals4 with the output power of 500W while maintaining the electrical-to-optical efficiency of 20%. We also demonstrated through-hole formation of 1mm thick copper plate using the high brightness laser beam and successfully obtained round holes with the diameter of less than 40 micrometers .
In-situ measurement of VUV optical materials under F2 laser irradiation
Yasuo Itakura, Akira Sumitani, Fumika Yoshida, et al.
The 157nm molecular fluorine laser is regarded as the next generation light source for semiconductor exposure technology in the vacuum ultraviolet (VUV) region. Research for high performance F2 laser optical materials is therefore indispensable. In this paper, we describe methods and results of evaluating optical materials used in the 157nm region. In order to evaluate F2 laser optical materials, we have developed in-situ system, which measures the real-time transmittance at 157nm during laser irradiation and the transmittance in the vacuum ultraviolet (VUV) region directly after laser irradiation to avoid airborne contamination. The system is purged with high purity nitrogen gas during irradiation to reduce laser light absorption and to keep contamination at a minimum. Due to F2 laser irradiation cleaning, the transmittance of uncoated calcium fluoride (CaF2) samples initially rapidly then gradually increased during 50 million pulses (Mpls). Thereafter the transmittance remained constant. In addition, durability test results of CaF2 substrates and coatings are also presented. Especially coating quality varied enormously between suppliers.
Advanced Microfabrication Technology
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Precision machining of innovative materials using 157-nm excimer laser radiation
The F2 excimer laser emitting at 157 nm have been developed to reliable industrial machines applicable for micro-fabrication. Fluorine lasers are available at various power ratings, with energies between 1 mJ and more than 50 mJ and repetition rates up to 2 kHz. The applications of the F2 lasers presented here are mainly related to the micro-machining of glasses, with a special focus on fused silica. Depending on the applied fluence, fused silica can be processed with 157 nm by material ablation as well as by modifying the optical properties, when the energy density is beyond the ablation threshold. Both techniques qualify the 157 nm for manufacturing optical elements like planar wave guides or grating structures. Furthermore, the wavelength has been tested for micro-fluidic and micro-mechanical structures.
Industrial Applications
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Developments in laser joining and welding of plastics using high-power laser diodes
Tony Hoult, Raymond Ong
Diode lasers are now being employed in industry for a range of applications, in particular they are starting to be used as alternatives to conventional techniques for thermal joining of plastics. This is being assisted by the use of improved reliability aluminum-free diodes and diode laser systems, partly due to a better understanding of failure mechanisms. The laser welding and related techniques are dependent on transmission of part of an infra-red beam through the upper layer of a joint and semi-quantitative assessment of this is required for specific applications. The technique is applicable not only to high average powers, but also to very low average power, in this regime delicate thin-walled components may be joined. Recent developments using derivatives of this technique have shown that a wide range of similar and dissimilar material combinations may be joined.
Systems and Optics
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Uniformity initialization optical system for phase-change optical disk
Masaaki Takita, Yoshihiro Okino, Sumio Nakahara
The Initializer which has a mechanism that allows for the rotating of the optical disk and scanning the laser head to radial direction, and Initializing the phase-change optical disk with spiral pattern, has basic problems which are caused by this mechanism. In this mechanism, the ruggedness of the laser diode is transcribed on the recording material of the optical disk. This report about the method of reflecting the laser light by the parallel millers that are installed in both sides of the light beam after collimating lens. The reflected lasers light by the parallel millers are focusing on the recording material of the optical disk with the manner of folding. If the reflected time of the lasers light is only one, it is making the 180 degreed rotated images from un-reflected image. And the multiple times reflected laser light make the image at a slightly moved place. So the folded focus image of laser diode is more uniformed compared with the original one and reduces the quality problem, which is caused by un-uniformity of initializing of Phase-Change Optical Disk.
Optics Fabrication
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Holographic optical element for laser soldering
Sumio Nakahara, Tatsuya Hayashi, Noriyuki Sudou, et al.
Experimental studies on the characteristics of holographic lens were carried out, aiming at the simultaneous soldering of multi-spots in electronic assembly by the use of YAG laser. Holograms were recorded on the commercial available photographic plates, and converted into transparent phase type holographic lens by chemical processing. The dependencies of the diffraction efficiency on the recording conditions and two chemical treatment methods of silver halide sensitized gelatin and rehalogenating bleaching were examined in CW YAG laser system ((lambda) equals 1.06 micrometers ).
Overview
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Laser microprocessing in data storage industry
Laser technologies have been applied to material processing for data storage industry in a number of areas: laser cleaning of magnetic heads, laser cleaning of magnetic media, laser microtexturing of magnetic media, laser microbumping and disk tagging, laser-induced periodic structures and laser nano-etching and nanolithography by tip-enhanced laser irradiation.
Ultrafast Laser Processing
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Laser-induced expansion and ablation mechanisms of organic materials
Yoichiroh Hosokawa, Takashi Mito, Takuji Tada, et al.
Time-resolved interferometry, surface light scattering imaging, and optical microscopic imaging have been developed and applied to amorphous and multicrystalline films upon intense pulsed excitation. Nanosecond interferometry of neat polystyrene film gives interesting expansion dynamics followed by complete recovery. Femtosecond surface light scattering imaging reveals the roughing processes of Copper phthalocyanine (CuPc) films. Femtosecond optical microscopic imaging of a single anthracene microcrystal shows dynamics of its laser-induced fracture and destruction. All the morphological behaviors have been directly measured in the time domain by these newly developed pump-probe methods. Ablation of CuPc crystalline and amorphous films show novel dependence of etch profile on excitation pulse width; fs etch depth increases stepwise with laser fluence, while ns etch depth becomes gradually deep as fluence does. The results are discussed in view of how electronic excitation energy evolves to morphological change.
Microsystems and Microelements
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Laser processes for MEMS manufacture
This paper discusses the use of excimer lasers in the manufacture of microelectromechanical devices and systems, with emphasis on two application areas: laser micromachining of polymer masters for replication in metal by electroplating (Laser-LIGA), and laser-assisted manipulation of microparts for hybrid assembly. As a master fabrication method, laser micromachining offers advantages over conventional UV lithography in terms of materials flexibility and 3-dimensional capability. However, these advantages are offset by higher cost and lower throughput. We have been using a combination of laser micromachining an UV lithography to produce relatively complex multi-level fluidic devices, with laser micromachining being used only for layers requiring greater structural height and/or 3D profiling. Process details and examples of prototype devices are presented. Laser-assisted assembly is a new technique based on release and transport of parts by ablation of a sacrificial layer, using light incident through the substrate. We have been using this approach to assemble microelectromechanical devices from parts fabricated on separate substrates. Fundamental aspects of the process are discussed, and results are presented for hybrid electrostatic micromotors assembled by laser-assisted transfer of nickel parts.
Industrial Applications
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High-accuracy laser mask repair technology using ps UV solid state laser
Yukio Morishige
Laser Mask Repair System named LM700A has been developed to satisfy the increasing demands for higher accuracy with thigh throughput photomask repair system in recent semiconductor industry requirements. This paper introduces the basic configuration of the system, evaluated results for Chromium (Cr) binary masks and Molybdenum Silicide (MoSi) HalfTone (HT) phaseshift masks repairing performances, and the features of removing process using mid range (250ps) pulse duration ablation mechanism.
Cleaning
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Modeling of chemical and mechanical aspects in laser restoration of artworks
Costas Fotakis, Athanassia Athanassiou, Efi Andreou, et al.
In this paper, investigation of photochemical and photomechanical effects induced in polymer substrates under pulsed UV ablation is presented. The examined laser parameters are the wavelength at 248 nm and 193 nm in the nanosecond regime, and the fluence below and above the ablation threshold. The two polymeric substrates used are PMMA and blends of PMMA and PS.
Systems and Optics
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Novel ultrashort-pulse fiber lasers and their applications
Heinrich Endert, Almantas Galvanauskes, Gregg Douglas Sucha, et al.
The development of ultrashort pulse laser technology will have a strong impact on the advancement of laser machining. Ultrashort laser pulses can reduce the heat-affected zone and the shock-affected zone, resulting in much cleaner cuts, and therefore higher precision. Also, ultrashort laser pulses have shown remarkable opportunity for processing transparent materials such as glass, fused silica, and sapphire. However, acceptance of ultrafast technology is hindered by the size, cost, and complexity of ultrafast lasers. In this paper, we describe recent progress in fiber- based ultrafast laser technology which promises to be sufficiently compact, rugged, and potentially low-cost.
Overview
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History and future prospects of excimer lasers
Dirk Basting, Klaus D. Pippert, Uwe Stamm
We give a review about the historical development and various technological achievements in the field of excimer lasers since their discovery in 1975. The first commercial excimer laser model in the world, the EMG 500 from Lambda Physik was operating at maximum repetition rates of only 20 Hz, whereas today multi kHz excimer lasers as the NovaLine lasers are available. In 2000, Lambda Physik introduced the most powerful commercial excimer laser, the Lambda Steel with 300w output power having extremely high stability. Over the years, various technological achievements like the NovaTube and DuraTube laser chamber technology, the NovaPowerSwitch pulsed power modules and the HaloSafe fluorine and chlorine generators could be made which make the use of excimer lasers simple and user friendly. Recently, the development of Lambda Physik's DuraTube technology gave a strong push towards the development of high power 157 nm laser technology for microlithography and laser based micromachining of difficult materials as fused silica or teflon. Current and future technological developments are discussed which will strengthen the position of excimer lasers as user friendly work horses in all industrial areas.
Fundamental Aspects
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Scattering of evaporation particles in ultrashort-pulse laser ablation
Etsuji Ohmura, Ichirou Fukumoto, Isamu Miyamoto
Ablation phenomena when Gaussian beam of the fourth harmonics of Nd:YAG laser is irradiated to an aluminum substrate in picosecond were simulated using the modified molecule dynamics method that Ohmura and Fukumoto have developed. Scattering velocity of evaporation particles were displayed by vectors, and both molten pool and edge dislocations in the material were visualized. At the same time, the size and velocity distributions of the scattering particles, changes with time of the number of both evaporation atoms and particles, and angle distribution of them were examined quantitatively. Authors have already clarified that there are two types in ablation form. One is explosive ablation and the other is relatively calm ablation. The simulation results showed that the size and velocity of scattering particles and changes with time of the number of particles and atoms, and standard deviation of scattering angle and its transition depend on the ablation form. Even if ablation forms are different, it is common phenomena that ablation particles are divided during scattering process, therefore the number of particles increases gradually, and the scattering angle has a normal distribution.
Systems and Optics
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Temperature dependence of SHG conversion efficiency of nonlinear optical crystal
Etsuji Ohmura, Tatsuya Taniguchi, Isamu Miyamoto
In recent years, applications of solid-state lasers with conversion technique of the wavelength to precision microfabrication progress rapidly. When nonlinear optical crystal used for wavelength conversion absorbs laser beam, temperature of the crystal rises, and the conversion efficiency decreases. In this study, DKP crystal whose physical properties are well known was supposed for a nonlinear optical crystal. Temperature dependence of conversion was examined theoretically by numerical computations of heat conduction equation and wave equations. The temperature change of the crystal by Nd:YAG laser absorption, the local change of refractive index, and the consequential decreasing of SHG conversion efficiency were analysed. As a result, the temperature dependence of wavelength conversion became clear quantitatively. Difference of refractive indexes of the fundamental and the second harmonic becomes large when temperature of crystal rises due to laser irradiation, and conversion efficiency decreases.
Industrial Applications
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Ultra-line-narrowed F2 laser for microlithography
The Association of Super-Advanced Electronics Technologies (ASET) started The F2 Laser Lithography Development Project in March 2000, to clarify solutions of base F2 lithography technologies. In this project, we are developing an ultra line-narrowed F2 laser light source for exposure tools tat are employing dioptric projection optics. We have developed an intermediate engineering injection- locking laser system that has an oscillator laser and an amplifier to study the feasibility of an ultra line-narrowed F2 laser. A spectral bandwidth of <0.2pm (FWHM) at a repetition rate of 1000Hz and an average power of 14W has been achieved with this laser system. The laser output performance dependence on the relative delay between oscillator laser and amplifier is also measured.
Fundamental Aspects
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Laser-induced rear ablation of metal thin films
Tomokazu Sano, Hirokazu Yamada, Takayuki Nakayama, et al.
The purpose of this study is to investigate the correlation between Laser Induced Forward Transfer (LIFT) process observed experimentally and the deposited structure, especially the size accuracy. Ablated plume and shadowgraph of transferring materials were observed using image intensified CCD camera. The intensity of reflected He-Ne laser from the front and rear side of thin films, respectively, were measured using photodiode to investigate the behavior of thin film during laser pulse. Metal thin films (Au and Ni), with several tens - hundreds of nanometer in thickness deposited on quartz substrate using ion sputtering deposition method, were irradiated by KrF excimer laser (wavelength 248 nm, pulse width 30 ns). The measurement of the reflected He-ne laser shows that the film removal finishes during incident laser pulse. Ablated plume images and shadowgraphs of transferring materials show that the velocity of both the top of ablated plume and the transferring materials become faster in increase fo fluence, and that the transferring materials precede the plume. Optimum fluence exists at each film thickness to achieve high size accuracy of deposited structure. At lower fluence, the deposited structure shows bad feature due to incomplete removal from the support substrate. At higher fluence, the big shock causes the wide range of spread of deposited structure when the transferring particles have a collision with the acceptor substrate. At optimum fluence, the high size accuracy of deposited structure is achieved as the film-substrate distance is made as short as possible.
Microfabrication and Microstructuring
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Generation of periodic microstructures with femtosecond laser pulses
Hans Kurt Toenshoff, Andreas Ostendorf, Frank Korte, et al.
Femtosecond lasers have been proven as excellent tools for micromachining of solid targets. In contrast to other existing technologies, this method of laser processing allows structuring with highest precision by minimal damage to the adjacent material. The possibility of structuring nearly any kind of material gives access to new and innovative approaches in the field of optics. Periodic structures with dimensions on a micrometer scale are used for many photonic applications. Conventional ways of producing micrometers -scaled periodic patterns show the drawback of being limited by specific material properties, e.g. hardness, brittleness, which reduce the variety of machinable materials. However, femtosecond laser pulses offer great possibilities for the generation of periodic microstructures independent of the machined material. This includes the ablation of metals, dielectrics as well as the laser induced polymerization of photosensitive resins. Within this paper results on the generation of periodic microstructures by using femtosecond lasers are presented. Results of machining surfaces for applications like anodes and acceleration grids for streak camera tubes are presented, demonstrating a high potential for fs-laser micromachining in the field of optics.
Industrial Applications
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Precise laser-assisted 3D microstructuring of metals and alloys
Gerd Sepold, Andreas Stephen, Thorsten Lilienkamp, et al.
In the following, three different laser-assisted processes are described and herewith produced components in the fields of micromechanics are presented. First, excimer laser projection technology is modified in order to produce large- area microstructures. It enables the transfers of structures having dimensions much larger than the laser beam section by synchronized scanning of the mask and the substrate. This technology is used to manufacture master structures in polymers for injection molding inserts. The insert tool itself is produced by electroforming of the master leading to metallic copy of the master. In this way, a metallic insert tool is performed for the production of microfluidic components. Its structured area is 2 cm2 at a resolution better than 3 micrometers . Second, laser-induce wet chemical etching using a sw-Nd:YAG laser is described. The principle of this micromachining method is based on a local thermal activation of chemical etching reactions on the surface of the material. The direct processing of the workpiece resulted in high accuracy microstructuring with smooth surfaces and without any debris or thermal influence on the material properties. Among others, one example in the field of application in micromechanics is the fabrication of superelastic micro-grippers prepared by cutting of temperature sensitive shape memory alloys. The achieved sidewall angle is about 3 degrees and the surface roughness less than 0.4 micrometers for machined 200 micrometers thick foils. Third, a combination of the two afore mentioned processes leads to complex shaped microstructures in metallic parts. Thereby, additional microstructures of specific shape, e.g. V-shaped grooves, are machined by laser-induced wet chemical etching into metallic inserts produced by electroforming of excimer laser-induced wet chemical etching into metallic inserts produce by electroforming of excimer laser machined masters. They are used for hot embossing tools enabling the production of special housings which can be hermetically sealed by ultrasonic welding.
Cleaning
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Wavelength dependence of laser cleaning for field emitter arrays
Mikio Takai, Naoki Suzuki, Oguz Yavas, et al.
Different harmonics of the Nd:YLF laser, ranging from infrared to ultraviolet wavelength region ((lambda) equals1047, 523, and 349 nm), were used for cleaning of Nb-gated silicon field emitter tips to improve the emission efficiency. An increase of the emission current by a factor of 5 after laser irradiation was observed only in case of ultraviolet laser irradiation at (lambda) equals349 nm, indicating that contaminants were successfully removed from the tip surface and the cleaning process was strongly wavelength dependent.
Laser cleaning of silicon wafers: mechanisms and efficiencies
Mario Mosbacher, M. Bertsch, H.-J. Muenzer, et al.
We report on experiments on the underlying physical mechanisms in the Dry-(DLC) and Steam Laser Cleaning (SLC) process. Using a frequency doubled, Q-switched Nd:YAG laser (FWHMequals8 ns), we removed polystyrene (PS) particles with diameters from 110-2000 nm from industrial silicon wafers by the DLC process. The experiments have been carried out both in ambient conditions as well as in high vacuum (10-6mbar) and the cleaned areas have been characterized by atomic force microscopy for damage inspection. Besides the determining the cleaning thresholds in laser fluence for a large interval of particle sizes we could show that particle removal in DLC is due to a combination of at least three effects: thermal substrate expansion, local substrate ablation due to field enhancement at the particle and explosive evaporation of absorbed humidity from the air. Which effect dominates the process is subject to the boundary conditions. For our laser parameters no damage free DLC was possible, i.e. whenever a particle was removed by DLC we damaged the substrate by local field enhancement. In our SLC experiments we determined the amount of superheating of a liquid layer adjacent to surfaces with controlled roughness that is necessary, in good agreement with theoretical predictions. Rough surfaces exhibited only a much smaller superheating.
Liquid-assisted pulsed laser cleaning with near-infrared and ultraviolet pulsed lasers
Liquid-assisted steam nanosecond pulsed laser irradiation is shown to be effective for cleaning contaminant particles as small as 0.3 micrometers in diameter from metallic substrates. The cleaning threshold and efficiency are investigated for the fundamental and frequency-tripled Nd:YAG laser harmonics (wavelengths (lambda) equals1064 and 355 nm). The rapid phase- change and thin liquid film ablation processes are examined in order to elucidate the cleaning mechanism. The pressure enhancement accompanying the explosive-vaporization process and the momentum supplied by the ablation plume are the main sources of the augmented cleaning efficiency at moderate laser energy densities.
Peculiarities of steam laser cleaning
Vadim P. Veiko, Elena A. Shakhno, Boris V. Volkovyski
The bubble nucleation in liquid laser cleaning is considered. The action of the thermal distribution in the liquid layer at the bubble size, vapor pressure in the bubble and liquid surface tension is investigated. The threshold value of the substrate surface temperature and its dependencies on physical properties of the liquid and surface roughness are defined.
Model for discoloration effect in pigments in laser cleaning of artworks by laser ablation
Boris S. Luk'yanchuk, Vassilis Zafiropulos
The physical mechanism and the theoretical model for pigment discoloration at laser ablation are discussed. The model includes two subsequent steps: 1) examination of ablation in subthreshold (Arrhenius tail) region and 2) examination of the nucleation process and structural modification within the volume of material close to ablation front. The results of calculations show that in the wide range of parameters exists the strong sharpening effect, when the thickness of modified layer (a few nanometers) is significantly smaller than the heat penetration depth or depth of optical absorption. These results are in good agreement with experimental data on discoloration effect.
Laser cleaning of particles from surfaces: issues relating to sample preparation
Deb M. Kane, Alanna Fernandes
Laser cleaning results depend on the particles (form and material), and the method used to deposit them on the surface. Studies using three different pulsed lasers (255 nm and 248 nm) and different particle deposition techniques are contrasted fro removing alumina particles from glass surfaces.
Microfabrication and Microstructuring
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Optical near-field effects in surface nanostructuring and laser cleaning
H.-J. Muenzer, Mario Mosbacher, M. Bertsch, et al.
We present a method for directly imaging the undisturbed near field of a particle resting on a surface. A comparison with numerical computations shows good agreement with the results of our experiments. These results have important consequences for laser-assisted particle removal where field enhancement may cause local surface damage and is one of the physical key processes in this cleaning method. On the other hand, the application of near fields at particles allows structuring of surfaces with structure dimensions in the order of 100 nm and even below.
Cleaning
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Dynamic particle removal by nanosecond dry laser cleaning: theory
Nikita Arnold, G. Schrems, T. Muehlberger, et al.
A model for ns dry laser cleaning that treats the substrate and particle expansion on a unified basis is suggested. Formulas for the time-dependent thermal expansion of the substrate, valid for temperature-dependent parameters are derived. Van der Waals adhesion, the elasticity of the substrate and particle, as well as particle inertia is taken into account for an arbitrary temporal profile of the laser pulse. Time scale related to the size of the particles and the adhesion/elastic constants is revealed. Cleaning proceeds in different regimes if the duration of the laser pulse is much shorter/longer than this characteristic time. Expressions for cleaning thresholds are provided and compared with experiments on the cleaning of Si surfaces from spherical SiO2 particles with radii between 200 and 2585 nm in vacuum with 248 nm KrF excimer laser and 532 nm frequency doubled Nd-YAG laser. Large discrepancies between the experimental data and theoretical results for KrF laser suggest that ns dry laser cleaning cannot be explained on the basis of thermal expansion mechanism alone.
Laser-induced surface cleaning from diamond particles
Vitali I. Konov, V. V. Kononenko, A. M. Lomonosov, et al.
Kr-laser removal of micro and nano sized diamond particles deposited on Si substrates from alcohol solution in an ultrasonic bath has been investigated. The process control was performed ex situ by optical microscopy and in situ by measurements of the acoustic signals in the substrate and in the surrounding air. Optimal cleaning conditions as well as substrate surface damage threshold were found. The mechanism of diamond particles laser removal process was studied.
Pulsed Laser Deposition
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Comparison of ITO ablation characteristics using KrF excimer laser and Nd:YAG laser
KyoungCheol Lee, Cheon Lee
We studied to develop ITO (Indium Tin Oxide) thin films ablation with a pulsed type KrF excimer ((lambda) equals248 nm) and Nd:YAG laser ((lambda) equals262 and 532 nm) required for the electrode patterning application in flat panel display into small geometry on a large substrate area. The threshold fluence for ablating ITO on glass substrate using KrF excimer laser is about 0.1 J/cm2. In this case, through the optical microscope measurement the surface color of the ablated ITO is changed into dark brown due to increase of surface roughness and transformation of chemical composition by the laser light. And the XPS analysis showed that the relative surface concentration of Sn and In were essentially unchanged (In:Snequals5:1) after irradiating the KrF excimer laser. It has been reported that ripple like structure was by 2nd harmonic Nd:YLF laser (equals523nm). But in case of using 2nd harmonic Nd:YAG laser, in our study, the ablated films have well patterned and the shape is almost clean even though by-product exists in the vicinity of the etched surface region. But the films were damaged at the same laser fluence using 4th harmonic Nd:YAG laser. We will discuss this cause in terms of photon energy and the film thickness, used in this experiment, repetition rate and beam scan speed of each laser.
Systems and Optics
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Inazuma: 12 watts of UV at 355 nm
Dafydd T. Thomas, Mark S. Keirstead, Norman Hodgson
With the trend in portable electronics being continuously driven towards smaller and lighter products, the demand for laser-drilled microvias is increasing exponentially. With over 1,500 laser drilling tools currently being used within the printed circuit board industry, and a forecast of over 2,500 installed units before the end of 2001, Spectra- Physics has developed a high powered Q-switched Nd:YVO4 laser that can generate 12 watts of average power at 355nm. This power is generated with an optical-optical conversion efficiency of 11.5%(808nm to 355nm), which to our knowledge, is the highest reported to date for an end-pumped solid- state laser.
Microfabrication and Microstructuring
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Laser microfabrication by ultrashort UV pulses
I. Zergioti, D. G. Papazoglou, Nikos A. Vainos, et al.
Recent advances of direct printing or elemental and compound microstructures by Laser Induced Forward Technique (LIFT) using a femtosecond UV laser will be presented. LIFT Is a technique enabling the direct controlled transfer of thin film materials between substrates. For example an ultrashort UV laser has been used to transfer molecular material (E.g. InOx) onto glass substrates in order to form optical diffractive structures. The LIFT process enables reproduction of the metal and oxide's structural and physical properties. Subsequent ultraviolet illumination by a 325nm HeCd laser has induced dynamic refractive index changes of the InOx grating. This optical behavior is similar to the holographic recording behavior of the InOx thin films. Similar experiments with metals and other molecular materials will be presented and discussed. The use of laser-based methods, in the fabrication of optically activated microstructures, opens new applications possibilities in the area of optoelectronics.