Pulsed UV Excimer Laser Research Articles

Ellipsometric analysis of concentration gradients induced in semiconductor crystals by pulsed laser induced epitaxy

The authors report a novel approach to analyze the behavior of pulsed-laser-induced epitaxy on group-IV semiconductors by spectroscopic ellipsometry measurements and finite-element modeling of the thermo- and hydrodynamic behavior. Gradient-composition epitaxial crystals are obtained from previously atomically sharp heteroepitaxial samples through optically monitored processing with UV excimer laser pulses. Spectroscopic ellipsometry is employed to determine the composition gradient of the semiconductor alloy, and the results are correlated with energy-dispersive x-ray measurements. A finite-element modeling approach is developed based on the experimentally monitored melting dynamics and composition gradient results for understanding and prediction of the dynamics.

Pure, single crystal Ge nanodots formed using a sandwich structure via pulsed UV excimer laser annealing

In this paper, a sandwich structure comprising a SiO2 capping layer, amorphous Germanium (a-Ge) nanodots (NDs), and a pit-patterned Silicon (Si) substrate is developed, which is then annealed by utilizing a pulsed ultraviolet excimer laser in order to fabricate an array of pure, single crystal Ge NDs at room temperature. A wide bandgap SiO2 capping layer is used as a transparent thermally isolated layer to prevent thermal loss and Si–Ge intermixing. The two-dimensional pit-patterned Si substrate is designed to confine the absorbed laser energy, reduce the melting point, and block the surface migration of the Ge. After optimizing the laser radiation parameters such that the laser energy density is 200 mJ cm−2, the laser annealing period is 10 s, and the number of laser shots is 10, pure, single crystal Ge NDs that have both a regular arrangement and a uniform size distribution are obtained in the pits of the Si substrates. The Raman spectrum shows a highly symmetric Ge transversal optical peak with a full width at half maximum of 4.2 cm−1 at 300.7 cm−1, which is close to that of the original Ge wafer. In addition, the high-resolution transmission electron microscopy image for the Ge NDs and the corresponding selected area electron diffraction pattern shows a clear single crystalline structure without any impurities.

Excimer laser technology trends

State-of-the art pulsed UV excimer laser technology has a proven track record of stable performance under three-shift operation conditions in various industrial thin-film applications at stabilized pulse energy levels ranging from 100 to 1000 mJ per pulse. For the last 25 years excimer laser technology has formed the backbone of pulsed laser deposition advances. The available UV average power level nowadays exceeds 1 kW and novel excimer laser beam utilization schemes in combination with sophisticated substrate–target geometries are ready to utilize the full laser upscaling potential and thereby reduce unit costs.

Optical emission spectroscopy and time-of-flight investigations of plasmas generated from AlN targets in cases of pulsed laser deposition with sub-ps and ns ultraviolet laser pulses

We performed a comparative study of the plasma generated from AlN targets under sub-ps vs ns UV (λ=248 nm) excimer laser pulses. Optical emission and time-of-flight spectra recorded in cases of samples irradiated with ns laser pulses showed the presence of Al lines, which became prevalent after the first laser pulse was incident on the target. These observations are congruent with the metallization of AlN targets inside each crater under multipulse ns laser action at laser fluences above the ablation threshold, observed by visual inspection and optical microscopy. Metallization was not observed when working with sub-ps laser pulses. Moreover, our studies confirmed the predominant presence of AlN positive molecular ions in the plasma generated in front of AlN targets submitted to sub-ps multipulse laser irradiation. The optical emission data are in good agreement with time-of-flight mass analysis. We emphasize that all investigations support the experimental evidence reported by György et al. [E. György et al., J. Appl. Phys. 90, 456 (2001)], according to which thin films obtained by pulsed laser deposition with ns laser pulses contain a significant amount of metallic Al, while only AlN is detected in films obtained with sub-ps laser pulses. Measurements of the velocity and kinetic energy distributions of AlN+ indicate that in the case of ns-laser ablation the ions are emitted with thermal energy, while in the case of sub-ps-laser ablation a bimodal distribution exists and has thermal (1 eV) and hyperthermal (10 eV) energy components. This points to different plasma formation mechanisms for the two cases.

Rapid microfluidic mixing.

A preformed T-microchannel imprinted in polycarbonate was postmodified with a pulsed UV excimer laser (KrF, 248 nm) to create a series of slanted wells at the junction. The presence of the wells leads to a high degree of lateral transport within the channel and rapid mixing of two confluent streams undergoing electroosmotic flow. Several mixer designs were fabricated and investigated. All designs were relatively successful at low flow rates (0.06 cm/s, > or = 75% mixing), but had varying degrees of success at high flow rates (0.81 cm/s, 45-80% mixing). For example, one design operating at high flow rates was able to split an incoming fluorescent stream into two streams of varying concentrations depending on the number of slanted wells present. The final mixer design was able to overcome stream splitting at high flow rates, and it was shown that the two incoming streams were 80% mixed within 443 microm of the T-junction for a flow rate of 0.81 cm/s. Without the presence of the mixer and at the same high flow rate, a channel length of 2.3 cm would be required to achieve the same extent of mixing when relying upon molecular diffusion entirely, while 6.9 cm would be required for 99% mixing.

Laser modification of preformed polymer microchannels: application to reduce band broadening around turns subject to electrokinetic flow.

A pulsed UV excimer laser (KrF, 248 nm) was used to modify the surface charge on the side wall of hot-embossed microchannels fabricated in a poly(methyl methacrylate) substrate. Subablation level fluences, less than 2,385 mJ/cm2, were used to prevent any changes in the physical morphology of the surface. It is shown that the electroosmotic mobility, induced by an electric field applied along the length of the channel, increases by an average of 4% in the regions that have been exposed to UV laser pulses compared to nonexposed regions. Furthermore, application of UV modification to electroosmotic flow around a 90 degrees turn results in a decrease in band broadening, as measured by the average decrease in the plate height of 40% compared to flow around a nonmodified turn. The ability to modify the surface charge on specific surfaces within a preformed plastic microchannel allows for fine control, adjustment, and modulation of the electroosmotic flow without using wall coatings or changing the geometry of the channel to achieve the desired flow profile.

Photoacoustic study of the electron–hole transport in a piezoelectric semiconductor

In a piezoelectric semiconductor acoustic waves can be excited via the inverse piezoelectric effect. This excitation process is the result of a screening of the applied electric field by the expanding space distribution of electrons and holes which are generated by pulsed laser radiation. The time and space characteristics of the electron–hole transport in this process are contained in the corresponding acoustic signals. The piezoelectric direct-gap semiconductors cadmium sulfur selenide and gallium arsenide are investigated experimentally at room temperature. A pulsed nanosecond UV excimer laser is used to excite electrons and holes on these samples. The generated longitudinal acoustic waves are detected by a laser interferometer. The obtained acoustic pulses of cadmium sulfur selenide show that both the expansion of the screened region in space and the electron–hole plasma expansion are supersonic at the time scale of laser action. The observations in this optoacoustic experiment indicate that the fast carrier transport is governed by photon recycling, i.e., the reabsorption of photons released by radiative recombination of electron–hole pairs. Experiments for gallium arsenide are in progress and will also be reported.

Foil‐clips for Power Module Interconnects

A novel technology for the interconnection of bare chips in power modules is presented which circumvents the drawbacks of multiple heavy‐wire bonding as well as of reflow soldering of many single copper clips inserted into solder preforms. A piece of structured copper‐clad polyimide foil, here called a foil‐clip, is used to provide all required connections, i.e., die‐to‐die and die‐to‐substrate. Hot‐bar soldering or glueing is used for the final joining after adjustment of the foil‐clip to its land areas. Depending on the complexity of the module only a few touch‐downs of the heated tool are required for full assembly. In contrast to the well‐known TAB technique, in foil‐clip technology a copper layer is the top layer (typical thickness: 60 µm). The polyimide (PI) foil (e.g., 25 µm) is used simultaneously as insulation and solder stop layer on the die side. The contact of the photo‐lithographically patterned copper layer with the pads on top of the die and the substrate is achieved by grooving holes into the PI layer which are then filled in a subsequent step with solder or screen‐printed conductive adhesive. Via‐hole formation is achieved by light‐induced ablation. In the first stage of process development a scanning line beam of a pulsed UV excimer laser is moved across the surface of a metal mask containing the pattern to be eroded in the polyimide layer. The present paper describes the optimisation of process parameters in terms of wavelength, pulse rate and fluence variation as well as speed of the substrate displacement during illumination. In future, the use of a novel UV excimer lamp is envisaged which enables large‐area exposure and selective photoetching by using contact metal masks. Furthermore, measurements of the current load capability of the copper interconnects produced by foil‐clips are presented. With an infrared imaging system, the hot‐spot temperature was determined for increasing load current on tracks of different widths and lengths on their way down from high current rectifier diodes to DCB‐module substrates. It is shown that tracks with many squares (N = length/width => ∞) can be loaded with at least I = 22 A. On the other hand, due to cooling through the solder joints, the shortest tracks (N = 1) carry up to 128 A.

DCXS and RHEED characterization of effectiveness of annealing implanted Si crystals by using pulsed UV excimer laser and sample scanning technique

AbstractIn the experiment (111) oriented silicon plates were implanted with 80 keV Ge ions (dose 5 × 1015 cm−2) and with 150 keV Kr ions (dose 5 × 1015 cm−2). Then the samples were annealed with the pulsed excimer laser by using the sample scanning technique. The effectiveness of the annealing has been analyzed by means of double crystal X‐ray spectrometry (DCXS) and reflection high energy electron diffraction (RHEED). The results of the experiment show that the applied technique of annealing considerably improves the perfection of the structure of the implanted silicon crystals.

Laser Surface Processing and Diagnostics

AbstractLaser processing has become a powerful tool for surface modification and is applied successfully in many areas. In this article we focus on laser ablation as a very attractive way of surface patterning. We report both applications and mechanistic studies. In the first part of this paper we describe a two‐step method for the patterning of technical polymer surfaces by laser ablation. With this process, micron or even submicron structures can be obtained in a simple way. By doping polymers with photochromic compounds the feasibility of this process is improved drastically. In the second part we report the ablation of lithium niobate (LiNbO3), polyimide (PI), poly(tetrafluoroethylene) (PTFE, teflon) and poly(methylmethacrylate) (PMMA) by 0.5 ps UV excimer laser pulses at 248 nm. Time‐resolved measurements were carried out with pulse pairs of variable delay in the range — 200 ps to +200 ps. The ablation rate is very sensitive to the time delay between the two pulses and contains information about the dynamics of the process. For LiNbO3, efficient shielding is observed within a few picoseconds. For polyimide, excited state absorption is found to decrease the ablation rate for delay times below 30 ps. For PTFE and PMMA, and the total fluence just above threshold, the ablation rate versus time delay shows an autocorrelation type behavior with a full width at half maximum below 400 fs, since two‐photon absorption dominates the ablation process. A simple ablation model based on the assumption, that the absorbed volumetric energy density has to exceed a certain threshold value, will be presented. It describes the observed ablation rates for PTFE precisely.

Time resolved dynamics of subpicosecond laser ablation

The ablation of lithium niobate (LiNbO3), poly(tetrafluoroethylene) (PTFE, teflon), poly(methylmethacrylate) (PMMA) and polyimide (PI) by 500 fs UV excimer laser pulses at 248 nm is reported. Time-resolved measurements were carried out with pulse pairs of variable delay in the range from −200 to +200 ps. The ablation rate is very sensitive to the time delay between the two pulses, and —depending on the material and fluence—can increase or decrease for very short time delays. For LiNbO3, efficient shielding is observed within a few picoseconds. For PTFE and PMMA, and the total fluence just above threshold, the ablation rate versus time delay shows an autocorrelation type behavior with a full width at half-maximum below 400 fs, since two-photon absorption dominates the ablation process. For polyimide, excited state absorption is found to decrease the ablation rate for delay times below 30 ps.

Surface morphology of photo-assisted chemical beam epitaxial growth of gallium arsenide

The effect of pulsed UV excimer laser irradiation on the growth rate and surface morphology of GaAs layers grown by chemical beam epitaxy (CBE) has been investigated. Electron and optical microscopy reveals the surface of the laser assisted growths to be comprised of micron sized undulations and ripples which can be detected in real time from dynamic optical reflectivity.

Surface Modification of Polytetrafluoroethylene and the Deposition of Copper Films

AbstractEnhancement of the adhesion of thin copper films on polytetrafluoroethylene substrates was found when the substrate surface was irradiated with a pulsed UV excimer laser prior to metal deposition. The interaction between the laser and the polymer was examined by characterizing the neutral and charged species emitted from the surface during irradiation. The nature of the species emitted indicates that significant chemical modification of the polymer surface occurs. In addition to chemical modification, the interaction with the laser also alters the surface morphology of the polymer. Irradiation at fluences of ∼ 0.6 J/cm2 results in an overall planarization of the surface, whilst irradiation at higher fluences results in the formation and enlargement of voids and localized melting.

Pulsed excimer laser deposition of ferroelectric thin films

Abstract Pulsed UV excimer laser ablation was employed to deposit multi-axial, bi-axial and uni-axial ferroelectric compositions of PZT, bismuth titanate and lead germanate respectively. In general, a fluence lower than 2 J/cm2 caused a preferential evaporation of volatile components, resulting in stoichiometric imbalance. However, the fluences beyond 2 J/cm2 enabled the deposition of stoichiometric thin films of multi-component oxide systems. The intrinsic bombardment due to the energetic ablated species during the thin film deposition seemed to influence the composition, structure, orientation and the electrical properties. The electrical characterization of ferroelectric films indicated a dielectric constant of 800–1000, a Pr of 32μC/cm2 and Ec of 130KV/cm for polycrystalline PZT films and the corresponding quantities were measured to be 150, 7 μC/cm2 and 20 KV/cm for in-situ crystallized c-axis preferred oriented bismuth titanate films. Lead germanate thin films oriented along c-axis (003) showed a di...

HIGH Tc SUPERCONDUCTING THIN FILM DEPOSITION BY LASER INDUCED FORWARD TRANSFER

ABSTRACT We review in this paper the basic mechanisms and potential applications of the Laser Induced Forward Transfer (LIFT) for the rapid deposition and patterning in a clean environment, of high Tc superconducting thin films. A stoichiometric oxide superconductor compound is initially deposited, in a thin layer, on an optically transparent support. By irradiating, under vacuum or in air, this precoated layer with a strongly absorbed single laser pulse through the transparent support, the film is removed from its support to be transferred onto a selected target substrate, held in contact or close to the original film. The mechanisms for transferring YBaCuO and BiSrCaCuO thin films, with a pulsed UV excimer laser are described using a thermal melting model based on the resolution of the heat flow equation. The various possibilities given by the LIFT technique for patterning high Tc films (mask and direct patterning) are also examined.

Generation of three-dimensional free-standing metal micro-objects by Laser Chemical processing

Three-dimensional free-standing metal micro-objects were created using Laser Chemical Vapor Deposition (LCVD) of aluminum from aluminum-trihydride-trimethylamine. A dense grid of thin lines (about 10 µm diameter) was deposited on a pre-formed polycarbonate substrate. The substrate was removed by dissolving it in a suitable solvent after ablating an unsoluble by-product by UV excimer laser pulses. The result is a free-standing three-dimensional aluminum grid micro-structure.

GENERATION OF THREE-DIMENSIONAL FREE-STANDING MICRO-OBJECTS BY LASER CHEMICAL PROCESSING

Three-dimensional free-standing metal micro-objects were created using Laser Chemical Vapor Deposition (LCVD) of aluminum from aluminum-trihydride-trimethylamine. A dense grid of thin lines (about 10 µm diameter) was deposited on a pre-formed polycarbonate substrate. The substrate was removed by dissolving it in a suitable solvent after ablating an unsoluble by-product by UV excimer laser pulses. The result is a free-standing three-dimensional aluminum grid micro-structure.

Kinetics and thermochemistry of the equilibria CH 2Cl+O 2⇔CH 2ClO 2 and CHCl 2+O 2⇔CHCl 2O 2

The equilibria, CH 2 Cl+O 2 ⇔CH 2 ClO 2 and CHCl 2 +O 2 ⇔CHCl 2 O 2 , were studied between 562 and 664 K and 498 and 562 K respectively. Equilibrium constants were measured as a function of temperature. The chlorinated methyl radicals were produced by pulsed UV excimer laser photolysis. The decay of the radicals to their equilibrium concentrations in the presence of O 2 was monitored in time-resolved experiments using photoionization mass spectrometry. The measured equilibrium constants were combined with calculated entropy and heat capacity changes in Third Law calculations to obtain the enthalpy changes (ΔH 298 o ) for the two reactions: −121(±11) kJ mol −1 for the CH 2 Cl+O 2 →CH 2 ClO 2 reaction and −106(±6) kJ mol −1 for the CHCl 2 +O 2 →CHCl 2 O 2 reaction. The results indicate a considerable weakening of the C−O bond in the methylperoxy radical (which is 136 kJ mol −1 ) due to chlorine substitution in the CH 3 group. This effect of chlorine substitution on bond strengths is compared with that found for other compounds of the type CH 3 -X. The implications of weaker R-O 2 bond strengths caused by chlorine substitution in the oxidation chemistry of chlorinated hydrocarbons is briefly discussed. Heats of formation of CH 2 ClO 2 (ΔH f .298 o =−8(±12) kJ mol −1 ) and CHCl 2 O 2 (ΔH f .298 o =−21(±10) kJ mol −1 ) were also obtained using the experimental and theoretical results.

Picosecond UV Excimer Laser Ablation of LiNbO3

ABSTRACTUltrashort UV excimer laser ablation of materials shows several advantages over conventional UV excimer laser ablation. We describe new experimental results on lithium niobate, LiNbO3, using 308 nm pulses of 1 ps duration. The fluence threshold for material removal is reduced by one order of magnitude when compared with 16 ns pulses at 308 nm. In addition the etch rate is up to three times higher for picosecond pulses and it is independent of the diameter of the ablation area. We discuss a simple model describing the etching and ablation behaviour of materials upon interaction with UV excimer laser pulses of different pulse duration.

Comparison between microscopical aspects of a-Si films crystallized by pulsed UV excimer laser and calculated temperature profiles

This paper presents some aspects of the crystallization by ArF excimer laser pulses (λ = 193 nm, τ = 20 ns) of various energy densities (in the range 90–400 mJ cm-2) of a-Si: H films deposited by chemical vapour deposition on amorphous SiO2 substrates. The classical division of the crystallized material into a large grain layer and a fine grain layer is found with these experimental conditions but also we show that a layer with bubbles appears between the two crystallized zones. A comparison with the calculation of temperature leads us to think that crystallization cannot occur if the temperature reached at a given depth is below a temperature Tmin. This temperature may be related to the nucleation rate in a-Si.