Proceedings Volume 1279

Laser-Assisted Processing II

Lucien Diego Laude
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Proceedings Volume 1279

Laser-Assisted Processing II

Lucien Diego Laude
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Volume Details

Date Published: 1 August 1990
Contents: 5 Sessions, 23 Papers, 0 Presentations
Conference: The International Congress on Optical Science and Engineering 1990
Volume Number: 1279

Table of Contents

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Table of Contents

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  • Fundamental Processes
  • Ceramics and Polymers
  • Ablation and Plasmas
  • LCVD
  • Microprocessing
Fundamental Processes
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Laser/matter interaction at intensities of 1012 W/cm2 and below
S. Robert Goldman, Ronald S. Dingus, Ronald C. Kirkpatrick, et al.
For single pulsed laser-matter interactions at sufficiently high intensity, the electron density in the ablated vapor is large enough to absorb the laser radiation before it can reach the dense target material. The resulting interaction can be described in terms of energy flows: laser energy is absorbed in the plasma in front of the target and reappears as thermal electron energy and secondary radiation, part of which impinges upon and heats the dense target material at the dense material-vapor interface. This heating in turn drives ablation, thereby providing a selfconsistent mass source for the laser absorption, energy conversion, and transmission. Under typical conditions of laser intensity, pulse width and spot size, the flow patterns can be strongly two-dimensional. We have modified the inertial confinement fusion code LASNEX to simulate gaseous and some dense material aspects for the relatively low intensity, long pulse-length conditions of interest in many laser-related applications. The unique aspect of our treatment consists of an ablation model which defines a dense material-vapor interface and then calculates the mass flow across this interface. The model, at present, treats the dense material as a rigid, two-dimensional simulational mass and heat reservoir, suppressing all hydrodynamical motion in the dense material. The modeling is being developed and refined through simulation of experiments, as well as through the investigation of internal inconsistencies, and some simulation of model problems. The computer simulations and additional post-processors provide a wealth of predictions for possible measurements, including impulse given to the target, pressures at the target interface, electron temperatures and densities, and ion densities in the vapor-plasma plume region, transmission and emission of radiation along chords through the plume, total mass ablation from the target and burn-through of the target material at selected radial locations. We will present an analysis of some relatively well-diagnosed experimental behavior which has been useful in development of our modeling.
Scattering of electronically excited sulphur dioxide from graphite
Franz Kaspar, Karl Ludwig Kompa, Christoph Mueller, et al.
So2 molecules in a He-seeded molecular beam were electronically excited with a XeC1 excimer laser and scattered from the basal plane of graphite in an UHV chamber. Their characteristic scattering features were measured by counting those molecules which radiate after scattering. The survival probability of SO2 scattered from graphite is 2.5 * lO-. This implies that the probability of detecting excited molecules which have collided twice on the surface is already negligible. In this favourable case, the dynamics of scattering can be described with a hard cube model. The parallel component of the momentum is conserved, whereas the perpendicular component in our case is reduced to almost half of its initial value.
Interaction between two light-induced atomic dipoles near a metal surface: the electric field at the atomic sites
Ole Keller, Peter Sonderkaer
The integral equation formalism previously used by the present authors to describe scattering of monochromatic light from a few atomic dipoles on a flat metal surface is used to investigate the properties of the electric field on the sites of the dipoles. By treating the light-induced radiation from an atom in the electric dipole approximation a set of linear equations which allows us to determine the electric field on the atomic sites selfconsistently is established. The contributions to the selfconsistent field from radiative and nonradiative modes and from the excitation of surface polaritons are described. In the final chapter of our paper a number of numerical results for the case where the incident electric field is p-polarized are presented. Using dielectric data for Al a detailed analysis of the frequency dependence of the electric field on the site of one of the dipoles is undertaken.
ESR investigation of structure changes in quartz irradiated by CO2 laser
Anel F. Mukhammedgalieva, Igor M. Swedov
The effect of CO2 laser focised radiation (the wavelength of 1O.6,ii't , power density of io 10 VJ/crnt) quartz by method of electron spin resonance (ESR) has been investigated. ESR spectra of crystallic powder (particle size of iO..iOO,M) befor and after laser irradiation at the room temperature have been registrated. Irradia-.. tion of 1O.6,Mlead to appearance of intensive wide asymmetric line ( '& H - 90 iiiT , g '-' 2 , 5 ) and more slight narrow line on the right side of wide line with g=1.943. Appearance of wide line with g2.5 is connected with activisation of paramagnetic centra on the surface of desorienting powder particles. More narrow line with g=1.943 is due to distru.ction of bonds inside of network in region of laser radiation penetration depht.
Optical characterization of the fog created by the impaction of a low shock wave with a liquid air interface
Norbert M. Fargere
Though subaquatic explosions have been the subject of many studies, the interaction of the subaquatic shock wave with the liquid air interface is not well known, particularly in its earliest effects. In particulary, microscopic and precocious events occur during the acceleration of the free surface of the liquid. Several techniques (fast camera, image convector, schlieren and ombroscopic records,...) have been used to record the main phenomena in water and surface motion. The free surface motion details have been observed by holographic techniques which appeared very suitable for the description of the first phenomena. We used light sheet for the observation of latter events, plume and its composition.
Ceramics and Polymers
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Kinetic analysis of laser-induced reactions in polymer films
Christian Decker
Laser-assisted processing is one of the most efficient methods to induce ultrafast reactions in polymer films. The photopolymerization of acrylic systems is shown to occur almost instantly by simple exposure to a pulsed or CW laser emitting in the UV range. The kinetic profile of the reaction was directly recorded in the millisecond timescale by using the newly developed real-time infrared (RTIR) spectroscopy. The important kinetic parameters have thus been determined, such as the actual rate and quantum yield of the polymerization, the sensitivity and the contrast of the photoresist. Timeresolved JR spectroscopy is particularly well suited to study the kinetics of any polymer system undergoing ultrafast chemical modifications upon laser irradiation. Compared to the other analytical methods used in kinetic analysis, R1'IR spectroscopy proved to be more sensitive and reliable, while providing instantly quantitative information about the actual extent of reactions that develop within a fraction of a second. The strongly crosslinked polymers obtained by laser irradiation exhibit remarkable properties, in particular an outstanding resistance to chemicals, organic solvents, heat and laser radiation. The main sectors of application of this laser technology are expected to be found in microelectronics for the high-speed writing of complex patterns at sub-micronic resolution, in stereolithography for the creation of 3 D solid objects and in optical engineering for the ultrafast coating of glass fibers and the production of laser-resistant optical components.
Simple analytic model including shielding by the plume during excimer laser ablation of polyimide
Klaus Schildbach
For a quantitative description of the ablation of polyimide by excimer laser pulses in the nanosecond range, the concept of a moving surface during the pulse and of the shielding of its trailing edge by the plume is very essential. In a recent publication by Mahan et al.(1989) the concept of a moving interface was solved analytically. The main purpose of this paper is to include here the shielding effect. It will be shown that the resulting analytical formula describes most experimental data with surprising accuracy.
Processing of ceramics by excimer lasers
Isamu Miyamoto, Hiroshi Maruo
Processing of S13N4 arid Zr02 ceramics was rerformed by using ArF , KrF and XeF excimer lasers with mainly mask projection, and characteristics of ceramic processing and surface appearance were analyzed. Focused excimer laser beam from unstable resonator was also used for cutting, and compared with focused c02 and YAG lasers of nonnal pulse. Material removal process of excimer lasers was compared with O2 and YAG lasers of normal pulse on the basis of energy analysis, X-ray diffraction and spectral analysis during ablation of Si3N4 . It was found that in material processing by 002 and YAG lasers, Si3N4 is decomposed into N and liquid Si, which attaches to the surface, producing roughened surface with cracks . On the other hand, in excimer laser processing, Si3N4 is decomposed into Si. (g) and N with accompanying ionized Si by its high peak power , resulting in excellent quality of processing without deposition of decomposed materials . The mechanism of uneven laser processing in Si3N4 and Zr02 ceramics was also discussed.
Synthesis of ultrafine ceramic powders by means of CO2 laser in a flow reactor
Roberta Fantoni, E. Borsella, S. Piccirillo, et al.
High purity, ultrafine Si, SiC and Si N4 powders have been produced from gas phase reactants (SiH, hydrocarbons, NH3) in a C2 laser induced process. Optical on-line diagnostics have been performed to investigate the reaction mechanism in the gas phase. Final product chemical analysis has been employed to check the formation of stoichiometric compounds. Morphologic and crystallographic characterizations of the powders have been performed in order to establish their phase composition, their micro-structure and to understand the growth mechanism which is mostly affected by laser power and residence time.
In-process clad quality monitoring using optical method
L. Li, William M. Steen
Although laser cladding has been accepted by industry as one of the methods for the improvement of material surface properties, it is always difficult to produce consistent products over long periods. The reason for these could be the instability of laser parameters, powder feeding parameters (when powder feeder is used) and the rise of substrate surface temperature during processing. This paper investigates the possibility of using an opto-elecironic sensor for in-process monitoring of clad quality. The experiments have revealed that clad bonding condition, clad porosity, clad uniformity (i.e. clad roughness, thickness variation and overlap consistency), substrate faults and system failure (e.g. nozzle clipping, lens cracking, powder feed failure ) etc. could be recognized by the optical signals from the melt pool radiation picked up by the sensor during processing. The use of the sensor for on-line laser cladding fault diagnostic and quality control is also discussed.
Ablation and Plasmas
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Excimer-laser-produced plasma studies
The interaction of an Excimer laser with an aluminum surface was studied by a characterization of the blowoff plasma plume. The SDIOIMLI Two-Meter Laser, operating at XeF and KrF wavelengths, was used to illuminate mm sized spots on aluminum targets at fluences of 400 - 1250 J/cm2 for 1 .8 ps. The resultant plasma was characterized by numerous techniques. The temperature of the plasma as a function of distance out in the plume was measured by both a modified line reversal technique and spectral methods. The absolute ion density was measured as a function of distance out in the plume by a laser absorption technique. The temperature in the blowoff plasma plume was found to be relatively constant, dropping by approximately 50% in 1 cm. The density, however, fell by an order of magnitude in the same distance. The measured results agree with predictions from the 2-D Hydrocode STREAK.
Plasma formation during excimer laser irradiation of thin selenium films in air
Alain Jadin, Igor V. Filiouguine, Michel Wautelet, et al.
Thin selenium films are irradiated in air by means of an excimer laser beam (wavelength 248 nm). The fluence threshold for plasma formation is measured as a function of the thickness of the film (20 to 230 nm) and the nature of the substrate (glass, quartz, sapphire). The fluence threshold, F(2), varies in the range 0.5-3.5 J.cm2. At low fluences, films are totally ablated after a certain number of pulses. This number decreases with increasing fluence, independently of the initial thickness of the film. Experimental results are compared with numerical simulation of temperature where the role of the film thickness and the nature of the substrate are introduced.
Spectroscopic studies of iron plasmas induced by continuous high-power CO2 laser
D. Bermejo, Remy Fabbro, Lilian Sabatier, et al.
Some basic aspect concerning spectroscopic determination of electronic density and temperature of high power (1KW to 10KW) laser-induced iron plasmas are discussed. Spatial evolution of the temperature into the key hole and above the target has been measured for different shielding gases and different power densities. At power densities of the order of 1.0-2.0 MW/cm2 the evolution of the temperature with the height over the target depends strongly on the shielding gas. Some spectral lines identified as corresponding to Fe(II) transitions have also been observed. From the measured temperatures and relative intensities ratios between lines corresponding to Fe(I) and Fe(II) , electronic densities have been calculated.
Numerical investigation of the Knudsen-layer, appearing in the laser-induced evaporation of metals
B. R. Finke, M. Finke, P. D. Kapadia, et al.
The non-equilibrium evaporation of metals is increasingly important in recent applications of high intensity power sources such as lasers, electron beams and arc heated plasmas. Powerful jets of evaporated metal arise when cw-lasers or pulsed laser beams are used, especially in surface processing, ablation and sublimation cutting. An exact understanding of the physical conditions in this evaporation jet is essential to control the ablation rate and minimize the energy loss due to evaporation by adjustment of the external process parameters. The ablation jet is maintained by an appropriate supply of newly evaporated particles and in effect sets the boundary conditions for the hydrodynamic or plasma regime that arises. Exact knowledge concerning the metallic vapour that constitutes the plasma which arises in a wide variety of material processing techniques with a high intensity beam allows to predict the ignition behaviour of the plasma vapour. This is a particular advantage since the presence of the plasma can totally change the physical behaviour of the process. In high intensity beam welding processes a narrow keyhole appears filled with metallic vapour whose behaviour does not depart too strongly from equilibrium. The plasma which is detected in the keyhole is important for the energy transfer from the incident beam to the workpiece1, so that in this case the physical conditions in the vapour are of special interest, as they determine the development of the plasma. When a metal surface is heated to a temperature close to the boiling point of the material of which it is composed, a jet of evaporated material originates at the metal surface. Depending on the surface temperature and the external pressure, the evaporation process ranges from a steady state of thermodynamic equilibrium which describes a vapour with constant spatial density and temperature, and no significant net motion, to one involving a strong non-equilibrium process with a velocity up to the local speed of sound. In all but the equilibrium case, however, a thin surface layer, known as the Knudsen layer, forms in the vapour. The transition from a non-equilibrium velocity distribution at the metal surface to a local Maxwell-Boltzmann distribution some few mean free paths above the metal surface occurs in this layer. This non-equilibrium regime is described by Boltzmann's equation which is solved here in the BGK approximation. This is achieved by employing an iteration algorithm for the solution of an equivalent integral equation with suitable boundary conditions. The temperature, density and velocity either in or behind the Knudsen layer are derived together with the velocity distribution function everywhere in the layer.
Deposition of thin films by high-energy excimer laser ablation
K. Schmatjko, B. Roas, G. Endres, et al.
The laser deposition of textured thin films was investigated for different degrees of crystal complexity. At the target high laser pulse energy density in a homogeneously illuminated spot is required throughout for stoichiometric material transfer to the substrate. The epitaxial growth is controlled by the substrate terrrperature within a narrow range; the simple MgO structure does not need a crystalline substrate. Fragments volatile in the ablation process may be supplied from the background gas, as oxygen for YBaCuO. Some applications will require large area deposition. This has been accomplished by greater target-to-substrate distance without film deterioration, and by translational motion through the plasma cone.
Large-area silica films deposited on silicon substrates by a CO2 laser
Tamas Szoerenyi, P. Gonzalez, M. Dolores Fernandez, et al.
Conventional (thermal) CVD of Si02 is a well established method widely used e.g. in the microelectronic industry. Recognizing the big success of CO2 laser induced CVD in depositing high quality amorphous silicon (a-Si) films from silane it is quite surprising that -to the best of our knowledge- there has been no report on a-SiQ thin film deposition initiated by a cw CO laser in parallel configuration. In this contribution we report the first results of a systematic study on cw CO2 laser induced CVD of silicon oxide films onto silicon wafers from SiH I N20 I Ar gas mixtures in parallel configuration in comparison with similar experiments for a-Si film formation. Gas mixture absorption behaviour and total pressure rise under laser irradiation demonstrate that the SiO production can be due to homogeneous gas heating similar to the a-Si:H deposition. The films deposited are characterized by IR transmission spectroscopy. The fact that well adhering silicon oxide films can be produced with reasonable growth rates by using a simple and inexpensive laser makes this method attractive for industrial applications.
Surface patterning by pulsed-laser-induced transfer of metals and compounds
Zsolt Toth, Peter Mogyorosi, Tamas Szoerenyi
Besults of a systematic study on Q-switched nthy laser induced rrrn2 area transfer of supported titanium and chranium thin films and Ge/Se multilayer structures are reported. The appearance of the prints is governed by film-support adhesion and source-target spacing. Best quality prints are produced by ablating well adhering ntal films in close proximity ( spacing < 15 pm) to the target to be patterned. Transfer fran stacked elenntaxy layers as a source offers a unique possibility of depositing acinpound films by mixing the constituents and transferring the material onto the target substrate in a single step.
LCVD
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Formation of nickel silicides by excimer laser CVD of Ni(CO)4
E. Borsella, Karl Ludwig Kompa, H. Reiner, et al.
Nickel silicides have been grown on single crystal silicon sub- strates. A XeCl excimer laser was used for all process steps: substrate cleaning, nickel deposition , silicide formation and annealing. The nickel films were grown by photodecoxnposition of Ni(CO)4 adsorbate layers with an excess of CO to prevent homogeneous nucleation and hence the formation of dust. The samples were analysed by X-ray fluorescence, SIMS and RBS. The results indicate that epitaxial silicide layers with a thickness of 50 ma can be obtained after careful choice of laser fluence and Ni film thickness. In an alternative approach we used a molecular beam of Ni(CO)4, part of which is laser excited prior to impinging on the substrate. This allows the combination of CVD and conventional MBE techniques. In this experiment we also investigate the interaction of photofragments with substrate surfaces and other processes responsible for material deposition.
ArF excimer laser photolysis of tetramethyltin Sn(CH3)4 probed by dye-laser-induced resonant multiphoton ionization
Stefano Fontana, E. Borsella, R. Larciprete
Laser-photolysis of tetramethyltin Sn(CH?)4 (TMT) is very interesting being this molecule an attractive precursor for laser photodeposition of Sn thin films. Efficient dissociation and ionization are obtained irradiating the organometallic with the ArF laser beam, being the energy of one photon at 193 nm resonant with the first TMT absorption band. At low laser radiation density, the observation of the Sn(CH3) dominating the mass spectrum shows that photolysis is initiated by the elimination of one methyl group. The relative abundance of lighter SnCH3 and Sn ions is strongly depending on the energy density. Neutral photoproducts of the 1Jv photolysis are sampled by dye-laser-induced resonant Multi-Photon Ionization (MPI); suitable wavelengths are identified through visible MPI in the 370-410 nm range. Furthermore neutral CH3 fragments are detected by ZR+l ionization induced at 333.5 nm. Varying the delay between the UV and the probing laser pulses, informations on the reaction dynamics are achieved. The presence of a strong dissociative channel evidenced by the two-colour ionization experiments demonstrates the suitability Qf TMT as metal precursor for ArF laser induced deposition of Sn and Sn-containing films.
Nitride layer formation by multipulse excimer laser irradiation of solid samples
Emilia D'Anna, Gilberto Leggieri, Armando Luches, et al.
We report a study of the formation of nitride surface layers on semiconductor (Si) and metal (Ti) samples by multipuise (up to 2500) XeC1 excimer laser (A=308 nm) irradiation in N2 and NH atmosphere. After irradiation the samples were examined by optical and electron microscopy (SEM) . and then analyzed by Rutherford backscattering spectroscopy (RBS) , nuclear reaction analysis (NRA) , Auger and X-ray photoelectron spectroscopy (XPS) to positively identify the formed cornpounds. The electrical characteristics of the laser synthesized nitride layers were also measured. The amount of nitride has been observed to depend on the number of subsequent laser pulses and on the nature of the ambient gas.
Microprocessing
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Application to optical data storage of laser-induced synthesis of CuTe
Francoise Hanus, Lucien Diego Laude
Many optical recording media have been investigated since a few years, but none appears to be fully suitable for WORM optical disk. Te and its alloys have been studied. For alloys, contrast is obtained by crystallization of the amorphous initial film. The constrast is low because it is only due to a difference in structure between the irradiated and non-irradiated zone. We propose a sensitive layer made of thin films of Cu and Te, in 1:1 atomic proportion '. As it is well known, laser annealing of thin sandwich films of that kind allows the synthesis of crystalline compounds. Contrast, in this case, is high due to synthesis of the semiconducting compound CuTe, with optical properties (both in transmission and reflection) quite different from the non-irradiated film ones. A study is made of the irradiation conditions (power density), of the reduction to im scale spots and of the optimization of the contrast. The stability of the layer is also studied by ageing procedure. In particular, we demonstrate the feasibility of a WORM glass disk with this coating. The disk is irradiated with a 830 nm solid state laser diode, pulsed at a frequency of .1 MHz, with instant power within the range 7-10mW. Spots with a diameter in the i.tm range were obtained. Long term stability of the virgin layer makes the technology described here a convincing competitor in the WORM disk market.
Active optical systems for laser microprocessing
Mishik A. Kazaryan, Gueorgii G. Petrash, Konstantin I. Zemskov
High gain of active media of pulse metal vapour lasers in addition with their specific properties gives a unique possibility to use the same active elements not only for image aniplification but simultaneously for creating strong laser beams, processing the object to be observed.
Excimer laser treatment of electrical contact materials
Kenneth G. Snowdon, Stephen Rolt
Many organic materials are used in the electrical and electronics industries. However, even thin layers of organics on electrical contacts can lead to high resistance problems. The use of an excimer laser in removing organic contamination has been studied. Excimer laser radiation successfully removed rosin flux and polyimide materials from deliberately contaminated test coupons. Laser cleaning of unknown organics and an inorganic silver sulphide tarnish film from relay contacts was also accomplished. A study of endpoint detection methods for laser cleaning will also be discussed. Excimer laser flash melting of unpiated and thin gold plated connector contacts to reduce porosity will also be reported.