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Laser forming or laser bending is a newly developed, flexible technique which modifies the curvature of sheet metal by thermal residual stresses instead of external forces. The process is influenced by many parameters such as laser parameters, material properties, and target dimensions. In this work, a pulsed Nd:YLF laser was used as the energy source. The laser beam was focused into a line shape irradiating on the stainless steel specimen to induce bending. The bending angle was measured at various processing conditions. A finite element analysis was performed with the use of a two-dimensional plane strain model to calculate the thermoelastoplastic deformation process. Experimental measurements and computational results were in good agreement. Numerical sensitivity studies were performed to evaluate the effects of the unavailable material property data at high temperature. It was found that both optical reflectivity and thermal expansion coefficient influenced the bending angle significantly, while other extrapolated material properties at high temperature yielded acceptable results.

The transient pressure generated by the interaction of short‐pulsed laser light with the liquid–solid interface is studied quantitatively. A KrF excimer laser beam of tens of nanoseconds pulse duration irradiates water on a solid surface and induces rapid thermal expansion and explosive vaporization. The pressure pulses launched into water by such processes are detected experimentally by the photoacoustic probe beam deflection method and a broadband piezoelectric transducer. The peak intensities of the traveling pressure wave measured by these two methods are compared with the theoretical thermoelastic predictions. The measurements show that a compressional pressure wave packet is radiated from the water‐solid interface with the peak intensity of the order of 1 MPa at laser fluences up to about 100 mJ/cm2. Simultaneous monitoring of the bubble growth kinetics by the optical specular reflectance probe has been performed. It is observed that the pressure generation is enhanced by the bubble expansion in the superheated water for laser fluences exceeding the bubble nucleation thresholds.

The thermodynamics of the rapid vaporization of a liquid on a solid surface heated by an excimer laser pulse is studied experimentally. The transient temperature field is measured by monitoring the photothermal reflectance of an embedded thin film in nanosecond time resolution. The transient reflectivity is calibrated by considering a temperature gradient across the sample based on the static measurements of the thin film optical properties at elevated temperatures. The dynamics of bubble nucleation, growth, and collapse is detected by probing the optical specular reflectance. The metastability behavior of the liquid and the criterion for the onset of liquid–vapor phase transition in nanosecond time scale are obtained quantitatively for the first time.

The dynamics of liquid-vapor phase-change in the nanosecond time-scale induced by pulsed-laser heating of a liquid on a solid sample is studied by means of optical reflectance and scattering measurements, and the piezoelectric detection technique. The Iiquids studied include water, ethanol, merhanol, IsoproPropyl Alcohol (IPA), and mixtures of water and IPA. The threshold fluence for nucleation is determined with high accuracy using the optical and acoustic signals. Heat diffusion calculations performed for the threshold fluences indicate that the liquids are sufficiently superheated before nucleation sets on. The transient optical reflectance signal is analyzed by an effective-medium theory to provide bubble-growth kinetics, so that the bubble-growth velocity for the test liquids could be estimated. In addition, it is observed that, following the thermally induced nucleation, repetitive acoustic cavitation at the surface of the solid sample occurs, with a time interval related to the speed ofsound in the liquid.

The enhancement of acoustic caviation at a liquid-solid interface following laser-induced bubble formation is studied. The experiment results indicate that metastable ultramicroscopic bubbles formed on the solid surface cause a long-term memory effect on acoustic cavitation. By performing a double-pulse experiment using two excimer lasers, the temporal decay of this memory effect is determined for two different liquids on a chromium surface. An explanation of the observed decay mode by a diffusion model is presented.

The explosion or rapid vaporization of a liquid film on an opaque surface by a pulsed laser is studied experimentally. Using a probe‐beam deflection sensing (PDS) scheme, together with a previously developed transmission monitor, the distortion of the PDS signal due to the generation of shock waves by the exploding liquid is investigated. Various liquids, including alcohols and pure water in contact with substrates such as polyimide, amorphous carbon, and silicon, are studied for a wide range of excimer laser fluences. It is concluded that the present PDS technique is highly sensitive to the explosion threshold.

Optical reflectance and light scattering measurements are used to study bubble nucleation of various liquids and subsequent explosive vaporization at the surface of a thin chromium film heated by a 248-nm KrF excimer laser. Liquids studied include water, ethanol, methanol, isopropanolm, and different mixtures of water and isopropanol. The reflectance and scattering signals show distinct transients when the excimer laser fluence exceeds a certain liquid-dependent threshold. The reflectance signals are analyzed to provide bubble growth kinetics and explosive vaporization thresholds of the liquids.

Laser-induced removal of particles from surfaces was demonstrated. Such “laser cleaning” can be performed on the bare contaminated surface or with additional micronthickness liquid film coverage on the surface. With the latter “steam laser cleaning” we achieved removal of epoxy, alumina, silicon, and gold particles of size 0.1 μm to 10 μm from silicon surfaces. The proposed mechanism of particle ejection is described.

Pulsed-laser-induced removal of particles from surfaces is a new cleaning technique. This laser cleaning can be performed on dry surfaces as well as on wet surfaces with a micron-thick liquid film during pulsed laser irradiation to provide enhanced removal efficiency. Using the latter technique, to be called ‘steam laser cleaning’ here, we are able to remove epoxy, alumina, silicon or gold particles with diameters in the range 0.1–10 μm from silicon and other surfaces.

The transient temperature field development during heating of an amorphous silicon (a-Si) film, deposited on a fused quartz substrate, by pulsed excimer laser irradiation is studied. Static reflectivity and transmissivity measurements are used to obtain the thin film optical properties at elevated temperatures. Experimental in-situ, transient, optical transmission data are compared with heat transfer modeling results. The variation with temperature of the material complex refractive index across the thin film thickness is taken into account. The effects of the film thickness and thermal diffusivity, as well as of the laser pulse shape, are discussed.

The dynamics of the explosion of a liquid film by an ultraviolet excimer pulsed laser is studied experimentally on top of an amorphous silicon (a‐Si) film deposited on fused quartz. In particular, the transient temperature as well as the nucleation of the superheated liquid at the interface for laser fluences below and above the spontaneous nucleation threshold are monitored using an optical transmission technique. Results for different pure and mixed liquids are presented for cases of different thicknesses of the a‐Si film. Details for the calibration of the transmission signal to obtain the interface temperature are provided through the application of the one‐dimensional diffusion model and the characteristic matrix theory using data obtained from previous steady‐state experiments. The calibrated results are in reasonable agreement with the known data for superheated liquids in the literature as well as theoretical calculations. In addition, the delay times for spontaneous nucleation of the liquids are estimated to be ≲1 μs, generally comparable with those observed in droplet explosion experiments.

Flash laser heating using short‐pulsed laser irradiation of a surface is demonstrated to be a promising new approach for effective removal of particulate contaminations of sizes as small as 0.1 μm. This is very useful because micron‐ and submicron‐sized particulates adhere tenaciously onto a solid surface, and conventional cleaning techniques are inadequate for removal. Several varieties of the new laser‐cleaning techniques have been developed by us as well as by others. For example, the pulsed laser irradiation can be used with or without the simultaneous deposition of a thin liquid film on the surface to be laser cleaned. The laser wavelength can also be chosen so that absorption occurs mainly at the sample surface, or in the liquid, or in the particulate, or in a combination of these. In this paper, we discuss and compare examples of these different approaches. We find that laser cleaning with highest efficiency is achieved by choosing a laser wavelength that is strongly absorbed by the surface together with pulse depositing a water film of thickness on the order of microns on the surface momentarily before the pulsed laserirradiation. This permits the effective removal of particles smaller than ∼20 μm, down to as small as 0.1 μm, from a solid surface using a modest ultraviolet laser fluence of ∼0.1 J/cm2.

‘Liquid Film Enhanced Laser Cleaning” is a technique to remove particulate contamination from surfaces. A thin liquid film deposited onto the contaminated surface is flood irradiated with a short laser pulse. The resulting sudden evaporation of the liquid film leads to high transient explosive forces, large enough to expel even submicron particles from the surface. For instance 0.1μm alumina particles could efficiently be removed from a silicon surface with KrF excimer laser irradiation of 16 ns pulse length and 120 mJ/cm 2 energy density.

The temperature dependence of the optical properties for amorphous silicon is studied at wavelengths of 632.8 and 752 nm. Both the refractive index and extinction coefficient increase linearly with temperature for 752 nm, while the refractive index decreases and the extinction coefficient increases for 632.8 nm. The rate of increase of the extinction coefficient at 632.8 nm is twice as much as that for 752 nm.

The traveling thermal lens technique is a new all-optical method for probing concentration and velocity patterns in flowing media. A thermal lens created by absorption of a short laser pulse moves with the flow and can be probed by a cw laser beam further downstream as a deflection signal. This technique permits optical measurements of the flow-velocity component perpendicular to the probe laser at high spatial resolution. Its application requires no seeding with particulates, as is needed in most other optical techniques.

A pulsed nitrogen laser is used to induce a sharp thermal gradient in a thin film, and the infrared thermal radiation from the irradiated region is monitored from the same side as the excitation beam (i.e., single-ended detection). We show that the profile of this pulsed photothermal radiometry signal can be analyzed to provide the thermal diffusivity or thickness of the sample as well as information on subsurface modifications or the degree of thermal contact with a substrate. We present data for several important classes of film, including metal, polymer, and paper (e.g., in currency) and show the important features of the present technique for thin-film characterization, namely, nondestructive, fast, and remote sensing.

We have used a pulsed dye-laser beam at 6010 A, of 1-microsec duration and less, similar-mJ energy, to produce a line plasma in a Cs metal vapor. This produces a line source of large thermal energy. The heat diffusion after the laser pulse can be studied by transmission monitoring of a weak cw He-Ne laser beam that is parallel to but separated from the pulsed laser beam by an adjustable displacement. This is possible because the He-Ne beam is absorbed by the minority Cs(2) molecules but not by the majority Cs atoms; as the heat pulse diffuses through the probe beam, the transient temperature increase causes a corresponding change in the population distribution of the Cs(2) molecules and hence a transient increase in transmission of the probe beam. In other words, the Cs(2) works like a thermometer. Our work demonstrates a new and simple method to measure heat diffusion in a well-defined geometry, and it is especially suitable for use in a hot corrosive system because it is a noncontact method.

Compared to mechanical, thermal, electric, or chemical means of removing plastic wire insulation, laser removal offers precision and speed without the necessity of contacting the material. Utilizing the technique of excimer laser ablation of organic polymers, it is shown that excimer laser removal of polyurethane‐type wire insulation proceeds with much higher precision and cleanliness than does pulsed CO2 laser wirestripping. A reason for this difference is polyurethane’s much higher absorptivity of ultraviolet compared to infrared radiation. Scanning electron microscope photographs of both stripped wires and ablated regions on polyurethane‐coated flat disks clearly show the superior quality of the excimer method. The paper concludes with a discussion of the photophysical parameters and mechanisms responsible for the large difference between excimer and CO2 stripping.

Laser cleaning was demonstrated to be a new, promising approach to efficiently remove particulate contamination of micron and submicron size from wafer surfaces as well as from the surface and trenches of thin silicon membrane stencil masks as used for e-beam projection lithography.

A novel photothermal displacement method has been applied to probe the pulsed laser-induced feature formation of Ni–P hard disk substrates in the laser zone texturing process. The deflection signals of the reflected probing beam show the variation of the feature shape resulting from different pulse energies of the heating laser beam. A laser flash photography system is also developed to visualize the feature growth dynamics. This system has nanosecond time resolution and about one micron spatial resolution. Both techniques show clearly the transient melting and deformation process and the time scale of such deformation.

A photothermal displacement method has been developed to probe the pulsed laser-induced transient melting and surface deformation of Ni–P hard disk substrates. A probing He–Ne laser beam is aligned collinearly with the near-infrared nanosecond pulsed heating beam. The He–Ne beam spot is scanned on the microfeatures formed on the sample surface by the pulsed laser heating. The deflection signals show the variation of the feature shape resulting from different pulse energies of the heating laser beam. The transient deflection signal also reveals that the time scale of the surface motion is in the range of several hundred nanoseconds.

CO/sub 2/-laser produced micro-bumps on glass substrates are not only of interest in tribological applications, e.g. zone texturing of substrates as discussed in this paper, but for other applications such as micro-optics fabrication. These bumps are positive protruding domes with net volume gain. Mass conservation implies a local density decrease inside the bump. We have studied this density variation using nanoindentation to probe local hardness. Our findings indicate a clear and significant decrease in bump hardness (up to 30%) relative to the background glass-consistent with this local change in density. SIMS and electron microprobe analysis indicate no chemical changes between the bump surface, and background regions. Regardless, the glass texture zone displays excellent bump durability and contact start/stop performance. Additionally, the role played by the chemical surface strengthening and its effect on bump height has been investigated. For chemically engineered surface stresses ranging from 100 to 500 MPa, the bump height increases with increasing surface stress at a rate of 0.064 nm/MPa. Extrapolation of the plot to zero stress confirms a non-zero thermal expansion contribution to the bump height-consistent with our theory for bump generation. Reflectivity measurements have shown that the optical constants vary little with changing surface stress, implying that the laser coupling to the surface is essentially unaltered.

Multiple pulse exposure provides a means for making bumps in the 1-100 nm height range on Ni-P substrates. Although the bump height is generally a nonlinear function of the number of pulses, below the fluence threshold for chemicapillary effects, bump height increases linearly with the number of pulses. At low fluence levels, when the peak melt temperature is below 1400/spl deg/K, the bump surface becomes rough. Avoiding the low fluence regime, this technique can be reliably used for making smooth bumps with flexible shapes and sizes which are useful for tribology studies.

Important issues of laser zone texturing in manufacturing are discussed. We report practical strategies in terms of tooling and process control. We found that good bump height uniformity is achieved by operating in the most stable region where the effects of laser pulse energy fluctuation, disk thickness variation, and disk runout are minimized. By selecting optimum process parameters, a bump height standard deviation of less than 1.5% can be achieved.

Infrared laser pulses of microsecond duration derived from a stabilized carbon dioxide laser are used to produce a textured zone on a glass disk substrate. Each laser pulse can produce a microscopically smooth dome-shaped bump that protrudes from the initial disk surface. Typical bump heights of interest are 20-30 nm with bump diameters /spl sim/10 /spl mu/m. The laser-textured zone, composed of /spl sim/10/sup 5/ bumps, provides a dedicated region for contact start/stop. The zone exhibits excellent tribological properties (stiction and durability).

The variable capillary force arising between the surfaces of a slider and a textured magnetic disk is explored. Disk texture is modeled with an ensemble of bumps having geometries characteristic of laser-texture. Lubricant redistribution is modeled dynamically while elastic deformation of bumps is described by Hertzian contact models. The results demonstrate a significant impact of geometry on capillary spreading and on the subsequent net capillary force.

The thermodynamics of the rapid boiling of a liquid on a solid surface heated by an excimer laser pulse is studied experimentally. The dynamics of bubble nucleation, growth, and collapse is detected by probing the optical specular reflectance. The transient temperature field is measured by monitoring the reflectance of a thin film with calibrated optical properties. The metastability behavior of the liquid and the criterion for the liquid‐vapor phase transition in nanosecond time scale are obtained for the pressure from 1 atmosphere to 3.3 MPa.

We provide here extended experimental data on the bump shape formed on Ni-P disk substrate under various laser and substrate surface conditions, and describe a model for the Sombrero-shape bump formation based on competition between thermocapillary and chemicapillary flows. This model rationalizes a large body of experimental data and provides insight into how one might tailor bump shapes to specific requirements. For comparison, laser bump formation on non-Ni-P substrates has also been studied.

This paper reviews damage induced by pulsed excimer lasers. Most previous damage studies have addressed the "time-zero" performance limits of optical materials, i.e., virgin samples exposed to one or a small number of laser pulses. In this "single-shot" damage regime, coatings are almost always the weakest link in the optical system. Considerable progress has been made in identifying optimum coating materials and material combinations for particular wavelengths, and in exploring effects of undercoats and overcoats, non-quarter-wave designs, laser pre-conditioning, and alternate polishing and deposition methods on the measured damage threshold. However, adequate single-shot damage resistance is a necessary but not sufficient condition for many practical applications. >

target is irradiated by a focusedlaser beam passing across the target surface with a certain speed. Heating and cooling cause plastic deformation in the laserheated area, thus change the curvature of the target permanently. The bending mechanisms are determined by the temperaturefield induced by the laser, the thickness and thermophysical properties of the specimens, scanning speed of the laser beam,etc. Three mechanisms have been discussed [1-3]: the temperature gradient mechanism, the buckling mechanism, and theupsetting mechanism. Bending is always toward the laser beam with the temperature gradient mechanism, while the bending

The transient temperature field development during heating of an amorphous silicon (a‐Si) film, deposited on a fused quartz substrate by pulsed excimer laser irradiation is studied. Experimental optical transmission data are compared with heat transfer modeling results. The temperature‐dependence of the material complex refractive index through the thin film thickness is taken into account.

The temperature dependence of the optical constants for amorphous silicon (a‐Si) is studied for two different sample thicknesses at two infrared wavelengths. It is observed that the extinction coefficient of a‐Si can increase significantly with temperature in the strong absorption regime. In addition, using the Mott–Davis formula, results are obtained for the variation of the optical gap energy for a‐Si with temperature, with similar feature observed for both amorphous and crystal silicon.

We show for the first time the use of pulsed photoacoustic detection using a miniature piezoelectric transducer to study and monitor photoablation in air by an ultraviolet KrF excimer laser. At incident laser fluences below the ablation threshold, the photoacoustic signal is produced by the thermal expansion of the air near the irradiated surface (‘‘thermal‐piston signal’’) and propagates at the speed of sound. Above the ablation threshold, the photoacoustic signal contains an additional component due to the ejection of ablated materials (‘‘ablative‐piston signal’’); this propagates initially at supersonic speeds. The amplitude of the ablative‐piston signal is found to be a monotonic function of the etch‐depth per pulse. Hence, by accumulating the amplitude of the ablative acoustic pulse, the total etch depth can be monitored in real time.

The technique of laser micro-processing has recently found several important and widespread applications in the manufacturing of disk-drive components. Examples provided here include the cleaning of surface contaminants, the formation of nano-bumps on disk surfaces for controlled surface texturing or for making glide height standards, and the micro-bending of magnetic head sliders for flight-height controls. Short-pulsed laser irradiation at suitable wavelength, fluence, and incidence direction can be used to clean off particulate and organic-film contaminants from surfaces of critical components, for example, the slider and the disk. Controlled disk texturing is needed to alleviate the problem of stiction which occurs when the disk stop spinning and the super smooth slider comes into stationary contact with the super smooth disk. A compact laser operating at high pulse repetition rate can be used to produce a low-stiction racetrack composed of typically a million nano-bumps. This can be done both for NiP/aluminum disks, or for glass disks. Single isolated bump with a specified height for providing height-standard can also be tailor-made. Very recently, we have developed a 'laser curvature adjust technique' and implemented it into production of magnetic head sliders. Here, microscopic adjustments of the curvature of air bearing surface of sliders can be produced by suitable laser scribing at the back side of the ceramic slider.

Laser cleaning with pulsed ultraviolet and infrared lasers is successfully employed to remove particulate contamination from silicon wafer surfaces and from delicate lithography membrane masks. Particulate material investigated include latex, alumina, silicon, and gold. Gold particles as small as 0.2 μm can be effectively removed. This new and highly efficient laser cleaning is achieved by choosing a pulsed laser with short pulse duration (without causing substrate damage), and a wavelength that is strongly absorbed by the surface; the removal efficiency is further enhanced by depositing a liquid film of thickness on the order of micron on the surface just before the pulsed laser irradiation.

We have studied changes in the absorption, temperature, ultraviolet (UV) spectrum, birefringence, and surface profile of various high‐purity ‘‘UV‐grade’’ fused silica samples during KrF excimer laser irradiation at 400 mJ/cm2 and 300 Hz and up to a total of 60 million pulses. All samples exhibit permanent and irreversible changes in the spectroscopic and physical properties. Some samples show a partial ‘‘self‐annealing’’ behavior during the irradiation in which the absorption first increases to a peak value of ∼10%/cm, and then decreases and levels off at a lower value. This may indicate the formation of a new stable state. The radiation‐induced effects are significantly affected by the laser repetition rate, annealing, and ambient temperature of the sample. For comparison, crystalline quartz is tested under similar conditions, and is found to be essentially unchanged by the radiation.

Laser forming or laser bending is a newly developed, flexible technique which modifies the curvature of sheet metal by thermal residual stresses instead of external forces. The process is influenced by many parameters such as laser parameters, material properties and specimen dimensions. In this work, a pulsed Nd:YLF laser was used as the energy source. The laser beam was focused into a line shape irradiating on the stainless steel specimen to induce bending. The bending angle was measured at various processing conditions. A finite element analysis was performed with the use of a 2-D plane strain model to simulate the temperature field and the thermo-elasto-plastic deformation process. Experimental measurement and computational results were in good agreement. Numerical sensitivity studies were performed to evaluate the effects of the unavailable material property data at high temperature. It was found that both optical reflectivity and thermal expansion coefficient influenced the bending angle significantly, while other extrapolated material properties at high temperatures yielded acceptable results.

This work investigates deformation of stainless steel and ceramic specimens with a precision on the order of submicrometers by use of a pulsed laser beam as the energy source. Such a technique is useful, for example, in a process of removing distortions on magnetic head components for a better contact between the magnetic disk head and the hard disk surface. Experiments are conducted to study the bending behavior of stainless steel and ceramics due to laser irradiation. A pulsed Nd:YLF laser beam is used to scan over the specimen to create out-of-plane deformation. The amount of deformation from each laser scan is correlated with various laser and processing parameters. A theoretical model of the laser deformation process is presented based on thermo-elasticity/plasticity. The laser deformation process is explained as a result of the laser-induced non-uniform distribution of the residual strain. Numerical simulations are carried out to estimate the laser-induced temperature field, the residual stress field, and the amount of deformation of the specimen. These theoretical studies help to understand the complex phenomena involved in the pulsed laser deformation process.

The objective of this work is to develop a novel process by which topographical changes are produced on a micrometer scale using a pulsed Nd:YLF laser, and to investigate the energy transfer and fluid flow phenomena involved in the process. The surface of thin chromium films is altered through the laser- induced phase transformation and fluid flow. Experimental parametric studies were conducted to correlate the laser parameters with the topography of the laser irradiated surfaces. Experimental and analytical work were also performed to study the transport phenomena involved in the process. A numerical finite element analysis was carried out to simulate the transient field variables. A nanosecond-time resolution, fast photography system was constructed to capture the phase change and the fluid flow occurring at the target surface. Comparison between the numerical and the experimental data helps to understand the mechanisms of the process as well as to develop a controlled surface modification technique.

This work investigates the transport phenomena and the mechanisms of droplet formation during a laser micro-machining process. The surface of the target material was altered through a laser-induced material flow process in the molten phase induced by a tightly-focused laser energy flux. As with many other machining techniques, debris (droplets) is often generated during the laser micro-machining process. Understanding the mechanism of droplet formation is essential for developing a debris-free micro machining technique. Experimental parametric studies were carried out. It was found that a narrow range of operation parameters and target conditions existed for ‘clean’ structures to be fabricated on the target. Deviation of the operation parameters from that range generated debris. The transient process of the clean hole formation and the process of debris formation were studied in-situ, using the stop action photography technique. Numerical simulations of the laser-induced surface deformation were also performed to obtain the transient field variables and to track the deforming surface. The comparison between the numerical and experimental work showed that, within the energy intensity ranges investigated in this work, the surface deformation was attributed to the surface tension driven flow, and debris generation was due to the instability of the fluid flow in the laser-induced molten pool.This work investigates the transport phenomena and the mechanisms of droplet formation during a laser micro-machining process. The surface of the target material was altered through a laser-induced material flow process in the molten phase induced by a tightly-focused laser energy flux. As with many other machining techniques, debris (droplets) is often generated during the laser micro-machining process. Understanding the mechanism of droplet formation is essential for developing a debris-free micro machining technique. Experimental parametric studies were carried out. It was found that a narrow range of operation parameters and target conditions existed for ‘clean’ structures to be fabricated on the target. Deviation of the operation parameters from that range generated debris. The transient process of the clean hole formation and the process of debris formation were studied in-situ, using the stop action photography technique. Numerical simulations of the laser-induced surface deformation were also perf...

Traditional "workhorse" lasers for industrial laser processing are carbon dioxide lasers, lamp-pumped Nd:YAG lasers, and to a less extent, argon and krypton ion lasers. These have been widely used in various industries, including welding, drilling, cutting, surface hardening, printing, and entertainment. In the past several years, new laser sources have become mature, reliable, and readily available for new applications in industrial materials processing; particularly noticeable are excimer lasers and high-power diode lasers as well as diode-pumped solid-state lasers. This paper concentrates on some novel applications of these new laser sources in "hi-tech" manufacturing of semiconductor and data-storage devices.

Pulsed-laser irradiation on a solid surface induces a highly efficient surface cleaning of submicron-sized particulates and undesirable organic overcoat films. The nanosecond-pulsed UV laser irradiation, shortly after the deposition of a thin liquid film on the surface, induces rapid vaporization of the liquid film and removal of particulates (`steam cleaning'). The laser beam also causes ablative photodecomposition of organic film contaminants on the surface (`dry cleaning'). A laser cleaning tool is constructed with an aim toward practical use based on an industrial grade KrF excimer laser. The tool includes a computer-controlled laser beam scanning system and a stable liquid film deposition unit, providing a cleaning rate of over 200 cm2/min. The cleaning strategy and the physical mechanisms of the laser cleaning techniques are also studied.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The transient temperature and pressure field development in the excimer laser-induced vaporization of liquids in contact with a solid surface is studied. A thin silicon film, which has temperature-dependent optical properties, is embedded between an absorbing chromium film and a transparent fused quartz substrate. Static reflectivity measurement is performed to determine the thin film optical properties at elevated temperatures. The transient backward reflectance responses from the silicon layer are compared with heat transfer modeling results. The backward reflectance probe is not affected by the creation of bubbles and is successfully employed for the first time to measure non-intrusively the temperature development during the rapid vaporization process. The optical reflectance probes are applied from the front-side and back-side of the sample simultaneously to monitor the dynamic bubble nucleation behavior and transient temperature development, respectively, at various ambient pressures using a high- pressure cell. The investigation on the effect of ambient pressure on the bubble nucleation threshold combined with the surface temperature measurement determines the thermodynamic state of the superheated metastable liquid at the interface and subsequently the explosion pressure.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The formation of bubbles at a liquid-solid interface due to acoustic cavitation depends particularly on the preconditions of the interface. Here, it is shown that following laser- induced bubble formation at the interface the acoustic cavitation efficiency is strongly enhanced. Optical reflectance measurements reveal that this observed enhancement of acoustic cavitation due to preceding laser-induced bubble formation, which could be termed as a memory effect, decays in a few hundred microseconds. By performing a double-pulse experiment using two excimer lasers the influence of process parameters, such as liquid temperature and salt concentration, on the temporal decay of the memory effect has been studied. An analysis of the experimental results by a diffusion model is presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High-purity synthetic fused silica glass is known to undergo changes in optical properties during high repetition rate KrF laser exposures. (Results of studies on six glass types irradiated in air are summarized elsewhere at this symposium.) We have also irradiated several samples in vacuum (248 nm, 300 Hz, (Phi) equals 400 mJ/cm2). Transmission at 248 nm, transmission at 210 nm, fluorescence at 650 nm, and vacuum cell pressure were monitored in real time. Although bulk outgassing is expected to be very slow at room temperature, our results indicate that the vacuum environment does affect the interior of the sample. Furthermore, a curious irradiation effect on the transmission of the CaF2 windows used on the vacuum cell is discussed.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The short wavelengths and high peak powers of rare gas-halide excimer lasers suggest many uses in industry. To exploit these opportunities, lasers must be developed that can operate reliably and economically at high repetition rates and high pulse energies, and optics must be fabricated that can withstand hundreds of millions or billions of laser shots without degradation. On the laser front, rapid progress is being made by several manufacturers. This talk focuses on the optics, in particular, transmissive optical materials for KrF laser applications. A variety of synthetic fused silicas were tested in front of an industrial KrF laser at high repetition rate (300 Hz) and moderate fluence (500 mJ/cm 2 ). All samples develop a characteristic absorption band at 210 nm in the early stages of the exposure. The absorption relaxes gradually after the laser is turned off. In almost every sample, this gradual behavior is followed by a large and sudden increase in the 248 nm absorption (up to 12% per cm). The nature, consequences, and possible causes of this `strong absorption transition' are discussed. Some data on high-purity single-crystal CaF 2 is included for comparison.

Pulsed laser heating of a surface is shown to be a promising new approach for effective cleaning of small particulate contaminations. Various versions of such a technique of laser cleaning is possible, depending on where the laser irradiation is absorbed and whether a thin film is deposited on the surface to enhance the cleaning. We have observed that laser cleaning with the highest efficiency can be achieved by choosing a laser wavelength (typically ultraviolet) that is strongly absorbed by the surface, and by pulse-depositing a water film of thickness on the order of microns on the surface momentarily before the pulse laser irradiation. This permits the effective removal of particles smaller than approximately equals 20 micrometers , down to as small as 0.1 micrometers , from a solid surface, using a modest ultraviolet laser fluence of approximately equals 0.1 J/cm2.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Photothermal spectroscopy has emerged as a popular spectroscopic technique over the past decade because it is convenient and sensitive. It is convenient since a wide range of samples of all phases from highly opaque, light- scattering or reflective to highly transparent materials can be measured to some level of accuracy over a broad spectral range with little or no sample preparation. It is sensitive since, in principle, it is a 'zero background' technique, unlike the tradition extinction technique to measure absorption. However, in practice, various sources of noises become significant when the absorption approaches the part-per-million level or below, and various considerations of noise suppression and signal enhancement are essential to exploit photothermal spectroscopy, in particular, photoacoustic spectroscopy and probe-beam deflection spectroscopy. The authors consider here the physical basis for signal generation and enhancement, as well as noise sources and reduction schemes for pulsed and continuous-modulated excitations. Examples of experimental techniques are given to illustrate the points