Lasers as Tools for Manufacturing of Durable Goods and Microelectronics

Proposed high-power UV industrial demonstration laser at CEBAF

Stephen V. Benson, Joseph J. Bisognano, Courtlandt L. Bohn, et al.

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The Laser Processing Consortium, a collaboration of industries, universities, and the Continuous Electron Accelerator Facility in Newport News, Virginia, has proposed building a demonstration industrial processing laser for surface treatment and micro-machining The laser is a free-electron laser with average power output exceeding 1 kW in the ultraviolet. The design calls for a novel driver accelerator that recovers most of the energy of the exhaust electron beam to produce laser light with good wall-lug efficiency. The laser and accelerator design use technologies that are scalable to much higher power. We will describe the critical design issues in the laser such as the stability, power handling, and losses of the optical resonator, and the quality, power, and reliability of the electron beam. We will also describe the calculated laser performance. Finally progress to date on accelerator development and resonator modeling will be reported.

UV FEL light source for industrial processing

Michael J. Kelley, H. Frederick Dylla, George R. Neil, et al.

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Short-wavelength UV light is strongly absorbed by most materials, creating the opportunity to drive near-surface thermal or chemical processes. The resulting modifications have a wide range of prospective applications, but few have been developed because of the low capacity and high unit cost of light from present sources. We analyze the light source requirements for large-scale applications to polymers and metals. We describe meeting them with free electron laser whose design is described in a companion paper in this session. This machine will deliver 1.0 to 2.5 kW between 190 nm and 350 nm with options in the visible and IR, and serve to further develop FEL technology for much higher powered machines. We gratefully acknowledge support for this work from the Commonwealth of Virginia Center for Innovative Technologies and The U.S. Department of Energy.

Automated beam manipulation in UV laser systems

Warren O. Jackson, Jay A. Guerette

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The unique challenges of UV laser micromachining require an advanced approach to beam shaping and steering to maximize beam utilization and efficiency. As UV laser systems used in manufacturing proliferate, it is also important to streamline optical setup and operation, and move towards automation. We will discuss the use of motor and actuator control in beam delivery, and how these tools, such as motorized imaging or turning optics, and mask or shaping devices, enable UV laser systems to become more powerful and flexible, yet remain simple to use.

Practical consequences of matching real laser sources to target illumination requirements

Glenn Ogura, Rod Andrew, Ronald D. Schaeffer

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UV laser processing as an industrial production step generally requires the beam illumination of target or mask to be of well-defined resolution, intensity and uniformity, and above all reproducible over long periods of time. Real excimer laser sources, despite major improvements over recent years, still deliver beams of non-uniform intensity profile with different beam divergence in each axis. This paper deals with practical approaches to excimer laser energy control, beam shaping and beam homogenization, and the matching of beam illumination optics to projection imaging optics.

Development of a stable external resonant ring-doubler for single-mode cw laser systems: optical design and product application

Ekhard Zanger, Ralf Mueller, Michael Koetteritzsch, et al.

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Second harmonic generation of continuous wave (cw) single-mode laser radiation in a resonant passive ring cavity, the WAVETRAIN, is discussed and experimental results with a recently launched commercial version of a self-developed resonant doubler are presented. Our company, LAS GmbH, is producing the most efficient commercially available frequency doubler unit for nearly the entire wavelength range accessible for today's single-mode cw laser.

UV laser beam shaping by multifaceted beam integrators: fundamental principles and advanced design concepts

Thomas F. E. Henning, Lars Unnebrink, Marcus Scholl

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Many applications of micromachining with UV excimer lasers requires homogeneous illumination of a mask or a workpiece or even illumination by a process adapted beam profile. In this paper the performance of multifaceted beam integrators for UV wavelength is analyzed. A simple model for the coherence function of excimer laser radiation is set up and checked experimentally. The model is used to examine the performance of the two different integrator concepts: the non-imaging multifaceted beam integrator and the imaging integrator. The theoretical analysis shows that the homogenization of laser radiation with a low spatial coherence, which is typical for UV lasers, is performed best by the imaging integrator. However, the non-imaging integrator is shown to offer the possibility for process adapted beam shaping by beam integration with beamlet shaping. Arbitrary process adapted beam profiles are generated by specific structuring of the facets. Beam integration ensures independence of the incoming beam. The success of this concept is demonstrated in an experiment. The known problems by beam transformation with diffractive optical elements are overcome by this new concept.

Durable optical coatings for energetic UV laser sources

Ronald L. J. Cowell

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It is now possible to manufacture repeatable, high laser damage threshold coatings for use at 193 nm and 248 nm. A variety of fluoride based and oxide multilayer coatings are available on standard or custom substrates.

Designing and fabricating of UV laser splitter

Shuying Wang

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The UV lasers-splitter can split a beam of UV laser into several ones with identical intensity and wide application prospects have been found in many situations that need to split a beam of laser, such as image multipliers and optical information process. The UV lasers-splitters is a binary rectangular phase grating. In this paper, the intensity formulae of series diffracting waves of the binary phase grating are obtained from the general complex amplitude transmissions formulae through Fourier integral. This approach simplifies the process of calculation and provides a optimal operation of the design.

Acoustic monitoring of modulated laser beam processing of metals

Hongping Gu, Robert E. Mueller, Walter W. Duley

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We observe sharp resonant emission at discrete acoustic frequencies during the laser processing of metals with pulsed and CW laser radiation. This emission can be dramatically enhanced when modulated or pulsed beam excitation is used. We discuss the origin of this effect in terms of a model which involves the excitation of normal modes of vibration for the laser keyhole, or liquid layer on the surface of the metal being processed. Eigenfrequencies associated with these modes form a band of allowed acoustic frequencies. When a harmonic of the beam modulation frequency falls within such a band, the acoustic signal is resonantly enhanced. The response of the system contains both driven and noise signals. Monitoring of discrete components may yield important new diagnostic information of value in determining optimized process conditions.

Development of the plasma detecting system in CO2 laser welding

Yong Wang, Wuzhu Chen, Xudong Zhang, et al.

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An advanced plasma detecting system of CO₂ laser welding has been developed. The system consists of three sensors, signal processing, A/D data converting and photo-electric coupling units connected with a rapid personal computer. The Photocell Sensor (PS) detects the intensity of the blue light irradiated by the plasma. The Plasma Charge Sensor (PCS) detects the electric density of the plasma plume. The Microphone Sensor detects the sound pressure coming from the rapidly expanding vapor in the keyhole. All of the sensors can exactly distinguish three kinds of welding processes--heat conduction welding, deep penetration welding, unstable mode welding. When the welding parameters are given, the PCS signal depend on the distance between the welding nozzle and the workpiece, the PS signals are correlated closely to the focal point position. Three sensors can be used to control the focal point position (penetration depth) under given laser power and welding speed. In addition, the relation between detecting signals and penetration depth is given. The sensors of the system have features of simple structure, low cost and high sensitivity, which are especially suitable for on-line plasma detecting, quality controlling and off-line plasma analyzing of CO₂ laser welding.

Auto-oscillating ODMV plasma upon 500-W cw CO2 laser radiation exposure and influence on metal surface treatment

Andrey Valentinov Levin, Alexandre E. Zaikin, Alexei L. Petrov

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Auto oscillating surface optical-discharge plasma was formed in a metal vapor under normal conditions by steady-state irradiation with a cw CO₂ laser delivering radiation of moderate (2 - 4.5 MW/cm²) intensity. A strong screening of the metal surface, which caused even completely stop evaporation of the metal, was observed. The auto oscillations were also the reason of vibrational process in the metal pool. Influence of the phenomena on metal surface treatment was discussed.

Holographic imaging of laser penetration processing in transparent media: visualizing the temperature field in real time

M. Olfert, Walter W. Duley

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Many of the phenomena which occur in penetration processing of metals are mirrored in non- metallic systems. Imaging of the penetration processing transverse to the beam incidence has proved to yield insights into the dynamic behavior of the laser induced cavity (keyhole). In this paper we report on the use of an imaging technique known as real time differential holographic interferometry which effectively allows direct visualization of the thermal variation within a transparent medium undergoing laser processing. Differences in heated fluid flow patterns induced by laser drilling of a glycerol-water mixture under varying gravity conditions are presented to demonstrate an immediate application of the holographic technique. By imaging processes which possess cylindrical symmetry, the holographic imaging data can be reduced to temperature field data in a straightforward manner, which proves to be valuable for comparison with thermal modeling of the process. Experiments involving drilling of fused quartz will be discussed and the experimental temperature field data will be compared with a simple numerical model.

Excimer laser: innovation in industrial material processing

Dirk Basting, Heinrich Endert, Rainer Paetzel, et al.

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Excimer lasers offer unique benefits for a wide range of applications, including industrial materials processing, scientific research and medicine. The benefits of the excimer laser stem from its high peak power output delivered in short pulses at a variety of UV wavelengths. In recent years, technical developments by laser manufacturers have lead to remarkable improvements in excimer performance, reliability and utility, as well as lower cost of ownership. As a result, the market for excimer lasers continues to grow and diversify. In this paper we examine some of the more recent advances, look at various industrial applications that are enabled by excimer lasers, and catch a glimpse of the future direction of this technology.

Field testing industrial and medical excimer laser systems

Gary T. Forrest

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Testing of ultraviolet lasers with field portable notebook PC's is accomplished with UV SensorCards responding to wavelengths from 450 nm to 8 angstroms. Beam sizes from over 5 inches to less than one micron have been measured in applications including laser annealing of flat panel displays, multichip module via hole drilling, ink jet cartridge manufacturing, semiconductor lithography, and medical eye surgery. Both instant UV SensorCard and direct camera observation have been employed. The former offers submicron resolution over large areas. The latter offers real time (30 Hz) display and capture of images. UV diffractive beam splitters have been employed to allow simultaneous measurement of absolute laser power and beam profile; including absolute fluence measurement. Among the common problems in laser system operation revealed are beam misalignment, shot to shot spatial energy variation, deterioration of optics, and streaming due to gas impurities.

Laser processing quality control by laser-excited SAW with IR detection (LSAW-IRD)

Alexander A. Karabutov, Alexander P. Kubyshkin, Vladislav Ya. Panchenko

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The new method of nondestructive remote control of laser processing quality is proposed. The subjects of control are structure, material properties and defects in near surface layers, modified by laser action just after laser processing (welding, heat treatment etc.). The proposed technique includes the laser excitation of surface acoustic waves (SAW) and their scattering in the processing area. For the remote registration of SAW the novel method is proposed. It's based on the detection of IR radiation arising due to temperature variation in SAW. Theoretical foundation of this technique is developed. The estimates show the reliability of LSAW-IRD. The advantage of scheme proposed is the ability to investigate rough surfaces in remote manner, as well as it possible compatibility with laser processing units.

Laser welding melt efficiency comparison: cw and Q-switched Nd:YAG

T. C. Webber

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Investigators use melt efficiency to gain understanding of a very dynamic process. The results can be quite useful in the quest to understand the mechanisms of laser welding as well as providing a basis unto which a prediction can be made as to the outcome of a particular parameter set. This analysis utilizes the physical constants of the material to be studied, the weld section area, and other pertinent parameters. It is widely assumed that laser welding is an average power process, in other words, that the physical constants of metal integrate peak energies to produce a net result. To test this assumption, a study was conducted to compare the melt efficiency of continuous wave Nd:YAG laser to a similarly powered Q-Switched Nd:YAG laser. Conditions of the weld test were kept virtually identical. A range of weld speeds were examined. The operating parameters of the Q-Switched laser were varied over several frequency and peak energy values. The results of the study contradicts the assumption of the average power process: melt efficiency increased with increased peak energies.

Laser welding of Zn-coated sheet steels

Marianne P. Graham, Hugh W. Kerr, David C. Weckman

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Compared to other welding processes, laser welding of sheet steels coated with various zinc- rich layers (galvanized, electrogalvanized, galvannealed, etc.) can permit weight reduction of automobiles plus increase in productivity. Some instances of laser welding of such coated steels have been reported. However, wider applications of lasers for this purpose are hampered by the low boiling temperature of zinc compared to the melting temperature of steel. During laser welding in the lap-joint configuration, the presence of vaporized zinc between the steel sheets often leads to expulsion of the weld metal or considerably weld porosity. Attempts to overcome this problem using Nd:YAG laser welding are reviewed. For the lap-joint configuration, techniques examined include provision of a gap between the sheets, use of geometrical solutions such as concave or convex surfaces, and pulsing or modulating the laser waveform. The effects on weld quality of power density, pulse time and pulse shaping (for pulsed welding), the coating type and weight, the location of the beam axis and beam focus with respect to the sheet surface(s) and the joint geometry (lap and edge) have been examined. The results provide insight into the weldability of coated sheet steels by the laser welding process as well as better assessment of viable approaches to this problem.

Lasers in the automobile industry

David M. Roessler, Nasin Uddin

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The use of lasers for automotive materials processing is reviewed both from an historical perspective and in terms of current trends. The initial lead gained in North America has subsequently given way to the remarkable growth in the use of lasers in the Japanese automotive industry. The latter's dominance has resulted in cutting being the most common laser machining application on a global basis, even though welding predominates in the US. About 98% of all automotive laser materials processing employs either CO₂ or Nd:YAG lasers, although there are special applications where the excimer or other lasers can be found. This paper discusses two of the processes currently receiving most attention. Laser technology is not stagnant and the automotive industry continues to benefit from the continuing developments. However, even more striking growth can be expected as the whole process of automotive manufacture is being re-examined in response to the demands for more fuel- efficient and environmentally friendly, but still affordable and satisfying, vehicles.

High-speed high-efficiency 500-W cw CO2 laser hermetization of metal frames of microelectronics devices

Andrey Valentinov Levin

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High-speed, efficient method of laser surface treatment has been developed using (500 W) cw CO₂ laser. The principal advantages of CO₂ laser surface treatment in comparison with solid state lasers are the basis of the method. It has been affirmed that high efficiency of welding was a consequence of the fundamental properties of metal-IR-radiation (10,6 mkm) interaction. CO₂ laser hermetization of metal frames of microelectronic devices is described as an example of the proposed method application.

Combustion control using a multiplexed diode-laser sensor system

Douglas S. Baer, E. R. Furlong, Ronald K. Hanson

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A multiplexed diode-laser sensor system, comprised of two InGaAsP diode lasers and fiber- optic components, has been developed to non-intrusively measure temperature and species concentration over a single path for closed-loop process control using laser absorption spectroscopy techniques. The system was applied to measure and control the gas temperature in the post-flame gases 6 mm above the surface of a Hencken burner (multiple CH₄-air diffusion flames). The wavelengths of the lasers were independently current-tuned across H₂O transitions near 1343 nm (v₁ + v₃ band) and 1392 nm (2v₁, v₁ + v₃ bands), and temperature was determined from the ratio of measured peak absorbances. H₂O concentration was determined from the measured peak absorbance of one transition set at the measured temperature. The temperatures recorded using the sensor compared well with those measured by thermocouples. A computer-controlled closed-loop feedback circuit that actuated the fuel flow in response to the difference between the measured and desired gas temperature was used to control the flame temperature in the probed region. The results obtained with this first generation system demonstrate the potential of multiplexed diode lasers for rapid, continuous, in situ measurements and control of gasdynamic parameters in combustion environments.

Novel photorefractive sensing device for laser surface inspection

Akira Shiratori, Minoru Obara

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We have developed a novel photorefractive sensing device using the hologram storage characteristic of a photorefractive barium titanate crystal. This device is capable of observing surface structures of precisely machined work, and we demonstrated surface sensing and inspection using samples. The angular resolution of this device depended on the angular aperture of the Bragg diffraction of the gratings inside the crystal, and resolution as high as submilliradian is expected.

Laser multisensor system for 3D free-form surface noncontact measurement

Bao Hua Zhuang, Jing Jiang, Wenwei Zhang, et al.

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In this paper a laser diode multisensor system for 3D free form surface non-contact measurement is described. The basic measuring principle is the optical triangulation method. The influence caused by the measured surface inclination is discussed first. Two prototypes of this system have been developed. One is the laser star multisensor system and the other is the linear polarized laser sensor with four PSDs. There are suitable to measure the machined 3D free form surface as well as the 3D free form surface made of `soft' materials such as clay and wood. The detected displacement is +/- 2 mm and the surface inclination could be up to +/- 90 degrees without a rotation servomechanism to track the surface normal. The computer simulation and experimental results are given.

Laser crystallization of a-Si:H/a-SiNx:H multilayers

Xu Guang Huang, Wing-Kee Lee, Da Liu, et al.

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Visible photoluminescence from crystallized a-Si:H/a-SiN_x:H multilayers has ben observed at room temperature. Analyses using transmission optical microscopy, scanning electron microscopy, and x-ray diffraction show that the laser crystallization of multilayer samples take place in the solid-phase crystallization region and the average crystalline grain size is limited by the thickness of the a-Si:H well-layer. The photoluminescence peak position increases with the decrease of well thickness, while the spectral width decreases. Photoluminescence decay time decreases with the decrease of the well thickness. The experimental results are in agreement with those obtained by calculations based on the quantum confinement model.

Excimer laser-induced heating, melting, and mass diffusion in crystal silicon in nanosecond and nanometer scale

X. Zhang, J. R. Ho, Constantine P. Grigoropoulos

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Heat and mass transfer at the nanosecond time scale and the nanometer length scale in pulsed laser fabrication of ultra-shallow p+-junction is studied in this work. A technique is developed to fabricate the ultra-shallow p+-junctions with pulsed laser doping of crystalline silicon with a solid spin-on-glass (SOG) dopant, through the nanosecond pulsed laser heating, melting, and boron mass diffusion in the 100 nm thin silicon layer close to the surface. High boron concentration of 10²⁰ atoms/cc and the `box-like' junction profile are achieved. The key mechanism determining the `box-like' junction shape is found to be the melt-solid interface limited diffusion. The ultra-shallow p+-junctions with the depth from 30 nm to 400 nm are successfully made by the excimer laser. The optimal laser fluence condition for SOG doping is found about 0.6 - 0.8 J/cm² by studying the ultra-shallow p+-junction boron profiles measured by the secondary ion mass spectroscopy versus the laser fluence and the pulse number. The 1D numerical analysis agrees reasonably with the experiment, within the available physical picture. Possible mechanisms such as boron diffusivity dependence on the dopant concentration in the molten silicon are proposed.

Beam optimization for dry cleaning processes

Todd E. Lizotte, Orest P. Ohar, Thomas J. Baker, et al.

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Alternative cleaning and surface preparation methods have been gaining interest recently because of the restrictions on the use of chlorofluorocarbons, their inability to clean contaminants less than 0.5 micron in size, water and chemical consumption and the hazardous waste disposal. These factors have also led to economic hardships to US manufacturers who have to compete on a global scale with countries not currently imposing environmental restrictions. Work on an Excimer Laser based cleaning process has been looked at for some time. In 1991, Allen and Tam1 used this DUV laser in conjunction with water or isopropyl alcohol to create a hydrostatic shock, formed by extreme heating of this liquid. This shock would scatter the contaminant along the surface. Although useful, it had an inherent drawback in that it lacked preferential directionality and could re-contaminate cleaned areas. Engelsberg has demonstrated that using DUV photons in combination with an inert laminar flowing gas provides the directionality needed for the removal of surface contaminants. This is known as the Radiance Process^SM. The process does not cause underlying molecular structure damage of the bulk material and it is a competitive cleaning process from both a pricing and speed perspective. The actual mechanism behind the Radiance Process^SM is unclear. A contaminant is held to the surface by a number of bonds which include covalent, ionic, hydrogen, electrostatic, dipole-dipole or Van der Waals.

Oxide-assisted laser surfacing of aluminum

E. E. Hoepp, Hugh W. Kerr

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CO₂ laser processing has been carried out on pure aluminum substrates for travel speeds from 0.3 to 6.1 mm/s, using laser powers of about 100 W or 300 W, with various preplaced single or mixed powders including CoO, NiO, SiO₂, Fe₂O₃ or TiO₂ usually combined with enough aluminum powder to permit complete reduction of the oxides. The 100 W laser experiments included low, normal and high gravity experiments. The resulting tracks were tested qualitatively for scratch resistance, and examined metallographically. Two types of surfacing were observed; continuous oxide layers produced by melting and an oxidation- reduction reaction of the original oxides with aluminum, and alloying of the substrate by elements reduced by the reaction. Low gravity experiments produced more uniform thicknesses and generally less cracking in the continuous oxides than normal or high gravity experiments. Alloying of the substrate ranged from almost 100% intermetallic layers at low laser powers and low travel speeds to complex mixtures and bands of different phases, depending on the temporal stability of the process, the powder composition and thickness, the laser power and travel speed. Optimization of the process could provide useful wear resistant coatings in a space environment.

Laser planarization of chemical vapor deposited diamond film

Ronald D. Schaeffer, Li Chen, Wen Ho

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Diamond films are grown by depositing carbon from gaseous species on prepared substrates. This process does not give surface uniformity, especially on the growth side where crystalline formations create very rough surfaces. In addition, there is usually an associated bow of the disc caused by strains in the material after removal from substrate. Since diamond is the hardest material, traditional grinding techniques, while fairly low cost, take an extremely long time. An extension of using lasers for cutting the material into usable shapes is to use lasers for the initial `flattening' procedure. This method has a significantly higher cost per hour, but usually requires much less time for equivalent volume removal. The use of lasers to remove bulk surface volume from CVDD and a few other materials will be discussed. Also, an extension of the fundamental concepts to automated manufacturing will be given.

Chemical-free cleaning using excimer lasers

Todd E. Lizotte, Terence R. O'Keeffe

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A critical requirement in many industrial processes is the cleaning of oils and grease, oxides, solvent residues, particles, thin films and other contaminants from surfaces. There is a particularly acute need in the electronics industry for cleaning semiconductor wafers and computer chips and in the metals industry for removing oxides and other contaminants. Cleaning traditionally is done by various wet chemical processes, almost all consuming large amounts of water and producing large amounts of hazardous wastes. To further complicate this, some of these cleaning agents and vast water consumption are undergoing stringent restrictions. The Radiance Process^SM is a novel, patented Excimer Laser approach to dry surface cleaning. The process has removed particles from 80 microns to submicron sizes, paints, inks, oxides, fingerprints, hazes, parts of molecules and metallic ions in fingerprints. The process does not ablate, melt or damage the underlying surface. Micro-roughening on some Silicon and Gallium Arsenide is on the order of 1A or less. This paper will discuss the various applications with this process and the latest results from a beta wafer cleaning prototype test bed system that is being built under an EPA grant and joint partnership between Radiance Services Company, Neuman Micro Technologies, Inc. and the Microelectronics Research Laboratory.

Kinetics and microstructure of laser chemical vapor deposition of titanium nitride

Kieth M. Egland, Jyotirmoy Mazumder

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Titanium nitride (TiN) has been deposited by laser chemical vapor deposition (LCVD) using a CO₂ laser and N₂, H₂, and TiCl₄ reactant gases. Multi-wavelength pyrometry has been used to determine deposition temperatures. Growth rates based on film height are typically 3000 - 10000 angstroms/second. Film profiles are Gaussian with a coarsened polyhedral morphology that show a marked size dependence on deposition temperature. Auger analyses reveal a substoichiometric composition (N/Ti < 1), regardless of reactant gas composition or deposition temperature. Previous work on TiN LCVD suggested a two-regime mechanism, depending on gas composition. By eliminating temperature variation, this work suggests a one-mechanism field over the whole range of gas compositions.

Excimer surface treatment to enhance bonding in coated steels

Robert E. Mueller, M. Olfert, Walter W. Duley, et al.

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Zinc coated sheet steel in the form of temper rolled galvanize and galvanneal are used extensively in the automotive industry. Through a process of excimer laser surface treatment, we have succeeded in significantly enhancing the adhesion characteristics of these coated steels. The laser treatment is performed by scanning focused excimer laser radiation in a raster pattern over the surface to be bonded. Adhesion tests have been carried out in the form of T peel tests, using either a hot melt nylon resin or an epoxy as the adhesive. An increase in bond strength was observed over a substantial range of surface treatment conditions. The largest improvement observed was more than a factor of three greater than for untreated surfaces. With the improved surface condition, the bond strength became limited by the cohesive strength of the adhesive. The physical structure and chemical composition of the parent and excimer treated surfaces have been examined using scanning electron microscopy and X-ray photoelectron spectroscopy to determine the nature and extent of the changes caused by the surface treatment. The effects of the observed changes on the bonding performance will be discussed. Surfaces have been processed under an inert atmosphere to isolate the effects of physical surface modification and surface oxidation. An attempt will be made to correlate the surface changes with the bonding characteristics and thereby indicate which changes are most beneficial. The ultimate goal is to optimize the surface condition for bonding and maximize the process rate.

Laser beam active brazing of metal ceramic joints

Heinz Haferkamp, Friedrich W. Bach, Ferdinand von Alvensleben, et al.

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The use of engineering ceramics is becoming more and more important. Reasons for this are the specific properties of these materials, such as high strength, corrosion resistance and wear resistance. To apply the advantages of ceramics, joining techniques of metal ceramic parts are required. In this paper, joining of metal ceramic joints by laser beam brazing is presented. This joining technique is characterized by local heat input, and the minimal thermal stress of the brazed components. During the investigations, an Nd:YAG laser and a vacuum chamber were applied. The advantages of Nd:YAG lasers are the simple mechanical construction, and laser beam guidance via quartz glass fibers, which leads to high handling flexibility. In addition, most of the materials show a high absorption rate for this kind of radiation. As materials, ceramic Al₂O₃ with a purity of 99.4% and metals such as X5CrNi189 and Fe54Ni29Co17 were used. As a filler material, commercially available silver and silver- copper brazes with chemically active elements like titanium were employed. During this study, the brazing wetting behavior and the formation of diffusion layers in dependence on processing parameters were investigated. The results have shown that high brazing qualities can be achieved by means of the laser beam brazing process. Crack-free joining of metal ceramic parts is currently only possible by the use of metals such as Fe54Ni29Co17 because of its low thermal expansion coefficient, which reduces thermal stresses within the joining zone.

Laser beam soldering of fine-pitch technology packages with solid solder deposits

Hans-Joerg Pucher, Mathias Glasmacher, Manfred Geiger

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Micro electronics is a key technology attracting the attention of information, communication, automation and data processing technologies. Ongoing miniaturization combined with an increasing number of I/Os has inevitably lead to ever finer lead geometries. Therefore the demands put upon the surface mount technology are increasing continuously. Processing of high lead count fine pitch packages, for example those which are applied in high-capacity computers, has not increased the demands put upon the assembly process only, but also on the connecting techniques. By reflow soldering with laser beam radiation the benefits from the tool `laser beam' are used extensively, for example contact and force free processing, strictly localized heating and the good controllability thereof, formation of fine crystalline and homogeneous structures, etc. Within the scope of this paper the fundamentals of laser beam soldering are discussed for fine pitch lead frames (pitch 300 micrometers ) for plastic packages, made by a modified CuFe2P alloy with a 5 micrometers Sn90Pb plating, on solid solder depths (Sn63Pb) performed by the so called High-Pad process. These investigations are unique in the field of laser beam soldering and are carried out by means of a Nd:YAG-laser. A pyrometer is used for detection of the emission of the temperature radiation of the joining area for process control. The additional use of a high-speed camera gives a detailed description of the melting and wetting process. The influence of laser beam parameters and the volume of the solid solder deposits on the joining result are presented.

Laser-induced surface doping of semiconductors

Koji Sugioka, Koichi Toyoda, Masayuki Jyumonji, et al.

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Formation of shallow junction with extremely high carrier concentration is presented by KrF excimer laser doping of GaAs using an ambient SiH₄ gas. The generation mechanism and thermal stability of the high carrier concentration in GaAs formed by the laser doping are also investigated. In addition, submicron patterned doping of Si into GaAs is demonstrated using a projection system. The projection-patterned doping is applied to self-aligned microfabrication of nonalloyed ohmic contacts with a low contact resistance with combination of a following plating process. Furthermore, junction deeper than 1 micrometers is formed by laser-driven diffusion of As⁺-implanted Si. The double-pulse irradiation method using two KrF excimer lasers drastically improves surface morphology and crystalline quality of the deep junction.

New mass transfer mechanism under cw CO2 laser effect and perspectives for applications

Andrey Valentinov Levin

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New mass transfer mechanism was discovered under laser effect on absorbing liquids. It is supposed that the mechanism is universal for different wavelength of laser radiation and different absorbing liquids. Opportunity and perspectives of the mechanism implementation are discussed.

Research on laser melting/alloying combined strengthening of the camshaft of air-cooled diesel engine

Wenjin Liu, Minlin Zhong, Haiyun Zhao, et al.

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This paper reported the research results on 3 kw cw CO₂ laser melting-alloying combined strengthening of the camshaft of air-cooled diesel engine used in the desert oil field. The 45 steel camshaft was pretreated with the conventional quenching and high temperature tempering. A focused laser beam with power density 1.5 - 1.7 X 10⁴ w/cm² was used to alloy the cam lobe area, while the other area of the cam was treated by laser melting using a focused 12 X 1.5 mm rectangular beam (power density 1.1 X 10⁴ w/cm²) produced by a newly developed binary optics. The microstructure of the laser alloyed region is fine Fe-Cr-Si-B multi-element hypereutectic structure with hardness HRC 63 - 64. The melted layer consists of fine needle-shaped martensite and residual austenite structure with hardness HRC 58 - 61. The strengthened layer is 1.0 - 1.3 mm in thickness with pore-free and crack-free and good surface quality. Under the same condition, the Ring-block (SiN ceramic) wear test proves that the wear of the laser alloyed 45 steel ring is only 29 percent of that of induction quenching 45 steel ring. And a 500 hours test engine experiment demonstrates that the average wear of the laser alloyed cam is only 20 percent of that of induction quenched one.

Measurement of diameters of electrical wires using laser with microprocessor

D. Sastikumar, A. Arokiasamy, K. S. Ganesh, et al.

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A simple technique is described to measure the diameters of thin electrical wires using laser with microprocessor. A wire is moved across the laser beam. This results in change in the photodetector output for a finite time duration which is determined using a microprocessor. The product of the time duration and the velocity with which the wire is moved gives the diameter of the wire. This technique can be used to measure the size of the large objects. This technique is also a noncontact method and can be used in hostile environments.

Effects of processing parameters on mode and stability of laser welding

Xudong Zhang, Wuzhu Chen, Jialie Ren, et al.

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The authors have found a third process--unstable-mode welding (UMW) under a certain condition, besides already known stable deep penetration welding (DPW) and stable heat conduction welding (HCW) during high-power CO₂ laser welding. UMW has basic feature that the two welding modes (DPW and HCW) appear intermittently, with the penetration depth and weld width jumping between large and small grades. In this paper, the physical phenomena, especially the signal of plasma during welding and the weld-forming of three kinds of process have been investigated. Effects of welding parameters (focal position, laser power and travel speed) on laser welding mode and weld-forming have been comprehensively studied. Double-U curves of laser welding mode transition have been obtained, which indicate the parameter ranges of the three monitored welding process.

Laser-assisted 3D-LOM systems: analysis and synthesis

Vadim P. Veiko, Sergei A. Rodionov, Boris P. Timofeev, et al.

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Analysis of laser-assisted 3D-LOM systems is given. Parameters of lasers and laser beam delivery systems as scanners and plotters are estimated. Criteria of optimization of them are suggested. Characteristics of two designed LOM systems are given. Optoacoustical device OASCAN for complicated 3D-objects images input is described.

Outdoor laser-tracking system

Katuo Seta, Ichiro Fujima, Iwao Ishikawa

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A laser tracking system has been developed to trace a moving target for outdoor measurements of 3D position. Beam direction of a 670 nm laser diode is controlled with two mirrors mounted on Galvano scanners, for horizontal ((phi) ) and vertical ((theta) ) directions, respectively, in order to trace a target made of reflection sheet. The beam reflected by a the target is detected at a quadrant photodiode to estimate the position of the target. The distance D between the apparatus and the target was measured with a range finder of 830 nm LED, the light beam from which is superposed on the beam for tracking by a beam combiner. Finally 3D position of the target is estimated from two azimuth angles of the 2 scanners and the measured distance. The whole system was set on a crane to measure the position of a weight suspended by the crane's arm. Up to about 100 m, this system traced the target and detected the distance. The 3D position of the target is measured with a resolution of 10 mm.

Pulsed laser deposition of thick films of electronic ceramics (Review Paper)

Douglas B. Chrisey, Paul C. Dorsey, James S. Horwitz, et al.

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Pulsed laser deposition is a superior technique for the growth of high quality thin films (<EQ 1 micrometers ) of electronic ceramics and has satisfied many applications. To meet developing applications, there is a need for thick films (>= 1 micrometers ) of electronic ceramics. Two film qualities principally control the growth of thick films: the film surface morphology and film stress. The deposition parameters which affect these qualities include: film deposition rate, film-substrate lattice mismatch, film-substrate thermal coefficient of expansion mismatch, and film growth kinetics. Our results suggest that it will be difficult to fabricate thick ceramic films of suitable electronic quality by conventional physical vapor deposition techniques.

193-nm lithography (Review Paper)

Mordechai Rothschild, Anthony R. Forte, Mark W. Horn, et al.

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The trend in microelectronics toward printing features 0.25 micrometers and below has motivated the development of lithography at the 193-nm wavelength of argon fluoride excimer lasers. This technology is in its early stages, but a picture is emerging of its strengths and limitations. The change in wavelength from 248 to 193 nm will require parallel progress in projection systems, optical materials, and photoresist chemistries and processes. This paper reviews the current status of these various topics, as they have been engineered under a multi-year program at MIT Lincoln Laboratory.

Photoresist-free microstructuring of III-V semiconductors with laser-assisted dry-etching ablation

Jan J. Dubowski, M. Bielawski, B. Mason

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The progress in manufacturing of integrated microelectronic and optoelectronic devices requires new technologies which would make possible printing of nanometer-size features and/or which would offer cost effective solutions in the fabrication of micrometer-size devices. Laser-induced direct (photoresist-free) patterning of materials has been recently investigated as a method that has some potential in that area. We have applied laser-assisted dry etching ablation for contact, proximity and projection mask lithography of III-V semiconductor films, quantum wells and superlattices. It has been shown that micrometer-size structures of those materials can be directly fabricated following the exposure to an excimer laser radiation in an atmosphere of chlorine diluted in helium. The results indicate that the process has the potential for the fabrication of high-quality quantum wire and quantum dot structures.

Improvement of the reliability of laser beam microwelding as interconnection technique

Mathias Glasmacher, Hans-Joerg Pucher, Manfred Geiger

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The requirements of actual trends for joining within modern electronics production can be met with the technique of laser beam micro welding, which is the topic of this paper. Thereby component leads are welded directly to the conducting tracks of the circuit board. This technique is not limited to electronics, because fine mechanical parts can be joined with the same equipment, too. The advantages as high temperature strength, reduced manufacturing time and simplified material separation at the end of the life cycle are noted. Furthermore the drawbacks of laser beam micro welding as a competitive joining technique to soldering are discussed. The reasons for the unstable process behavior of different welding scenarios can be understood by taking the changes of some process parameters into account. Since the process reliability can be improved by a proper process design as well as by closed-loop-control, results of finite element calculations of the temperature field as well as experimental setup for the determination of the melting point are presented. Future work is stated to spread the applicability of this joining technique as well as to develop an on-line control for high performance welding of locally restricted structures.

High-speed micromachining with UV-copper vapor lasers

Elizabeth K. Illy, Alison C. J. Glover, Michael J. Withford, et al.

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Ablation rate characteristics (etch rates) are presented for micro-machining of polyimide (kapton) and PETG using a frequency doubled Copper Vapor Laser (uv-CVL) at 255 nm and a frequency quadrupled Nd:YLF (4*Nd:YLF) laser at 261 nm. A comparison is made of the etch rates obtained by continuous ablation at 4.25 kHz with the uv-CVL with rates obtained for bursts of pulses with 2 second intervals between bursts. These results suggest that the observed decrease in ablation depth per pulse after a number of pulses for fluences above 0.6 J/cm² is due to attenuation of succeeding laser pulses by the plume of previously-ejected material. Preliminary results of the effect of sample temperature on ablation rates are also presented.

Laser processing of ceramics and metals by high-intensity picosecond and nanosecond laser pulses in UV, visible, and IR range of spectrum

Serge V. Garnov, Sergei M. Klimentov, Taras V. Kononenko, et al.

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Experimental investigations of ablation processes of AlN, Si₃N₄(MgO), Si₃N₄(YAI) ceramics, steels and aluminum have been conducted using first, second and fourth harmonics of picosecond and nanosecond Nd:YAP laser (1078 nm basic wavelength). The measurements have been carried out in a wide range of incident power density values: 10¹⁰ - 10¹⁴ W/cm². The passive-active mode-locked/Q-switched laser with a high quality spatial and temporal distribution of output radiation operated at repetition rates of 1 - 10 Hz was used. The ablation rate dependencies obtained for multipulse irradiation regimes are presented and analyzed. The physical mechanisms responsible for ablation processes in pico- and nanosecond range of laser pulse duration are discussed. Special attention is paid to the role of plasma formation and modification of chemical composition of ceramic surface layers during the laser-matter interaction process. The scanning electron microscope pictures of ablated holes and cuts produced at different irradiation conditions are demonstrated. It is shown that at the relevant irradiation conditions the high quality regular microstructures consisted of holes, cuts, etc., with a typical size varied from tens to several microns could be produced using high peak power ultra short laser pulses.

Nanostructuring of laser-deposited thin films

Wolfgang Pompe, Andre A. Gorbunov

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Pulsed laser deposition (PLD) is an advantageous technique for preparation of metastable thin films for application as a material for nanofabrication technology. Crossed beam pulsed laser deposition has the advantage of a droplet-free plasma with a filtered energy distribution of the plasma particles. Different kinds of nanostructuring of thin films deposited by PLD have been investigated. By localized laser annealing of thin films deposited as a supersaturated solid Fe- C solution a disperse structure of quasi periodically arranged Fe-rich ferromagnetic particles of 500 nm diameter can be produced. By scanning tunneling microprocessing periodic rows of hillocks with 20 - 30 nm in lateral size can be grown on metastable Ni-C multilayers. The optical near-field enhancement of laser radiation near a silver tip of a scanning tunneling microscope has been used for nanostructuring of thin gold films with lateral resolution below the diffraction limit. On the example of localized oxidation of ultrathin titanium films it has been shown that by using the nonlinearity of laserchemical reactions nanostructured metal oxides can be grown by laser scanning with lateral size below the diffraction limit.

Potential role of high-power laser diode in manufacturing

Tariq Manzur, Anthony J. DeMaria, Weiquin Chen, et al.

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We present examples of manufacturing applications like solid freeform fabrication by sintering metal/ceramic powders, and drilling using semiconductor laser diodes. From the stand point of energy requirements, these demonstration indicates that many other laser manufacturing functions like cutting, soldering, marking, printing, etc. can be carried out by currently available commercial laser diodes. Most of these applications are currently being done by the inefficient and bulky lasers like CO₂, Nd:YAG and Excimer. These are 30 years matured technologies and still costs over $DOL150/Watt. We believe the price of semiconductor laser diode chips, like computer chips, will come down rapidly, as volume market opens up, from $DOL300/Watts today to less than $DOL10/Watt in 5 years. The paper begins with some market survey indicating economic opportunities for high power diode companies.

Lasers and optics in stereolithography

Jouni P. Partanen

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Stereolithography is a Rapid Prototyping and Manufacturing (RP&M) technique which can be used to produce 3D plastic parts directly out computer files generated by CAD. Stereolithography systems use ultraviolet lasers to solidify liquid resin into the desired form as defined by the CAD file. Solidified plastic parts are built completely automatically (i.e. unattended) a layer at the time (typical laser thickness is 150 micrometers ) on the surface of the liquid resin. A slice cross section of the part is solidified by directing the laser beam onto the photopolymer surface using two (x and y direction) high speed vector scanning mirrors. This talk describes recent advances involving laser and optics technology applied to stereolithography and RP&M. These include: (1) improved reliability and power from ultraviolet gas lasers (HeCd and Argon Ion), (2) laser beam conditioning control to achieve accurate realization of small features, (3) advanced use of a fast AOM (acousto-optic modulator) shutter in the beam steering, and (4) improved performance of fast scanning mirrors. The application of emerging all solid state ultraviolet laser sources for stereolithography will also be discussed.

Effect of oxygen deposition pressure and temperature on the structure and properties of pulsed laser-deposited La0.67Ca0.33MnOd films

James S. Horwitz, Paul C. Dorsey, N. C. Koon, et al.

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The effect of substrate temperature and oxygen deposition pressure on the structure and properties of thin films of La_xCa_1-xMnO_(delta ) has been investigated. Thin films (approximately 1000 angstroms) of La_0.67Ca_0.33MnO_(delta ) were deposited onto LaAlO₃ (100) substrates by pulsed laser deposition at a substrate temperature of 600 and 700 degree(s)C. A series of films were grown on different oxygen pressures, between 15 and 400 mTorr, which systematically changed the oxygen concentrations in the films. As-deposited films exhibited an oriented orthorhombic structure. At low oxygen deposition pressures films were preferentially (202) oriented. At high pressures deposited films had a (040) preferred orientation. A 900 degree(s)C anneal in flowing oxygen of a film deposited at low oxygen pressure resulted in a decrease in the a lattice parameter and a change in the preferred orientation from (202) to (040). Vacuum annealing at 550 degree(s)C resulted in an increase in the a lattice parameter. The resistivity as a function of temperature showed a significant variation as a function of growth conditions. The peak in the resistivity curve (T_m) varied between 73 and 150 K depending upon the growth conditions. The activation energy associated with the semiconducting phase was approximately the same for all films (approximately 100 meV).

Increase of electrical conductivity of solid C60 films by excimer laser radiation

Qihong Lou, Dong Ning, Jinxing Dong, et al.

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After being irradiated in air by XeCl (308 nm) excimer laser, the electrical conductivity of solid thin-film C₆₀ has been improved by more than six orders of magnitudes. The products resulting from laser irradiation of C₆₀ films have been investigated by Raman scattering and the onset of conductivity can be attributed to laser-induced oxygenation and disintegration of fullerene. Irradiated by approximately 40 ns laser pulses with different fluence, the products with different microstructure were observed. At lower fluence, the Raman features of microcrystalline graphite and fullerene polymer were observed. At a fluence just below the ablation threshold (36 mJ/cm²), the fullerene molecules in film were disintegrated completely and transformed to amorphous graphite.

Local laser-induced film transfer: theory and applications

Vadim P. Veiko, Elena A. Shakhno, B. M. Yurkevich, et al.

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The technique of the local laser-induced film transfer (LIFT) is considered. The peculiarities of the front LIFT and the back one are investigated. The main fields of the LIFT applications are considered. Some particulars of the front and back LIFT applications--laser trimming of the coordinate characteristic of the position sensitive photoreceiver and deposition of the quartz glass local cover on to the silicic substrate--are considered in detail.

Recoil momentum at a solid surface and screening properties of pulsed erosion torch

Lev I. Kuznetsov

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The recoil momentum from a laser light pulse in the intensity range 10⁵ - 10⁷ W/cm² is experimentally investigated for dielectric and metallic targets as a function of the pressure of the surrounding medium and angle of illumination. An equation with empirical coefficients is obtained for the recoil momentum of illuminated targets. Effects of the screening properties of the erosion jet and the back pressure on the recoil momentum are analyzed as the external pressure is varied. The influence of attenuation of the laser radiation during a plum has been solved on the measurements of the reflection coefficient from an irradiated surface.

Maskless fabrication of submicrometer nonharmonic relief gratings for single-frequency DFB semiconductor lasers by three-beam holographic method

Alexander I. Khudobenko, Vladislav Ya. Panchenko, Vladimir N. Seminogov

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Three-beam maskless holographic method for fabricating submicron nonharmonic relief structures with phase shift (pi) , which are capable of providing stable single-frequency oscillation at Bragg wavelength in semiconductor DFB lasers, is developed. To demonstrate the effectivity of this method the biharmonic structure with period d approximately equals 0.5 micrometers and quasisinusoidally modulated amplitude, possessing the phase shift (pi) , is fabricated on n-InP surface in the process of liquid-phase laser photoelectrochemical etching.

Laser maskless formation of submicrometer periodic relief gratings on AIIIBV semiconductor wafers by combination of holographic and surface electromagnetic wave generation methods

Vladislav Ya. Panchenko, Alexander I. Khudobenko, Vladimir N. Seminogov

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The review of experimental investigations on maskless formation of submicron periodic relief structures on A^IIIB^V semiconductor surfaces (period d equals 230 - 800 nm), carried out in the Scientific Research Center for Technological Lasers of Russian Academy of Sciences during the last two years, is presented.

Monolithic flexible hollow silver waveguide for high-power industrial CO2 laser applications

Clifford E. Morrow, Gordon Gu

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A high efficiency, monolithic, flexible, hollow waveguide called Fiberlase^TM has been developed capable of delivering a wide range of wavelengths at very high power for surgical and industrial applications. Operational wavelengths of 3 microns through 10.6 microns have been demonstrated and core diameters of 400 microns and 1500 microns are now in production. Practical lengths of up to 3 meters with a 1000 micron core are being produced. Fiberlase is now replacing the articulated arm delivery systems of CO₂ lasers. Unique to this waveguide is its monolithic substrate structure, avoiding multi-layers, simplifying the manufacturing process, maximizing the heat dissipating properties, and allowing very low manufacturing costs. The power handling, bend performance and beam characteristics for high power industrial CO₂ laser applications will be discussed.

Materials inspection and process control using compensated laser ultrasound evaluation (CLUE): demonstration of a low-cost laser ultrasonic sensor

David M. Pepper, Gilmore J. Dunning, Phillip V. Mitchell, et al.

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We demonstrate the use of a nonsteady-state photo-induced-emf adaptive photodetector as a robust, low-cost laser ultrasonic sensor. This class of sensor enables high-fractional bandwidth ultrasound detection and, in addition, all-optical compensation of adverse in-factory noise, including vibration, speckle, relative platform motion, and optical fiber modal dispersion. Reference-beam and fiber-based time-delay interferometric configurations were demonstrated, as well as the use of a diode laser as a compact optical probe.

UV photosensitivity in PECVD-grown germanosilicate waveguides

David J. Moss, Mike V. Bazylenko, Mark Gross, et al.

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We demonstrate all optically (UV) written waveguide grating structures in germanosilicate trilayers grown by conventional PECVD, as well as novel results with a new hollow cathode PECVD growth technique which is capable of producing films having either positive or negative UV photosensitivity.

Laser hole drilling in thick polypropylene sheets for alignment sensors

D. E. Bergen, Glenn H. Chapman, Taranjit Singh Samra, et al.

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Laser micromachining of polypropylene for transducer applications has the advantage of creating small (< 100 micron) structures through very thick materials (> 400 micron). Normally translucent polypropylene formed using carbon as a dye is an excellent laser machining material having a high optical absorption, and a low thermal conductivity. For an optical alignment system a matrix of high aspect ratio holes of < 130 microns diameter with < 300 micron spacing was needed through thick (> 400 micron) sheets. This alignment sensor is to be used on the end of a robot arm and will aid in the manipulation of the arm. Using an argon ion laser focused through a 50 mm lens (5.2 micron R_1/e2 spot, 55.2 micron focal depth), holes as small as 30 microns on 150 micron spacing were achieved in 400 - 500 micron thick black polypropylene sheets with consistent results. Best results currently are achieved with a laser power of only 0.3 W, using 10 - 100 pulse stream of 10 - 100 microsec pulses, and duty cycles of < 10%. Shorter duty cycles require more power, as do shorter pulse durations, and both result in larger holes at wider spacings. Minimum repeatable hole separation is controlled by the lip of material formed around the hole. These settings have achieved 41 X 29 (1189 hole) arrays on a sample, with a computer driven submicron XYZ positioning system. Commercially available opaque white polypropylene required 17 times the power, and achieved holes of only 127 microns, with 500 microns spacing in 500 micron thick material. Thicker (1 mm) black polypropylene produces 144 micron holes on 500 micron spacings due to the lip material, and required a 100 mm lens.

Optical properties of films, quantum wells, and superlattices grown by pulsed laser deposition

Jerzy M. Wrobel, Jan J. Dubowski

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We discuss optical properties of semiconductor thin films, quantum wells and superlattices grown by pulsed laser deposition. A comparison with semiconductor structures grown by other techniques, such as MBE and CVD, is carried out to evaluate the applicability of laser ablation in the preparation of high quality materials. The discussion is focused on II-VI semiconductor compounds, however other electronic materials, such as CuInSe₂ and SiGe, are also briefly mentioned. The emphasis is placed on photoluminescence as the characterization technique.

Lasers as Tools for Manufacturing of Durable Goods and Microelectronics

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Table of Contents