Transient-reflectivity study of pulsed UV-laser-induced oxidation of Sb

Laser-induced oxidation of single-crystalline antimony and polycrystalline Sb films is studied. The samples are irradiated in an oxygen atmosphere (1.2 bar) by means of UltraViolet (UV) laser pulses while reflectivity measurements are used to monitor the oxidation process in real time. It is shown that there is an optical coupling between the growing oxide layer and the bulk material underneath which produces a dynamical change of the optical properties and leads to a non-constant growth rate. Depending on the laser energy density used, there is a critical oxide thickness above which a material-loss process starts, limiting the ultimate growth of the oxide layer.     

Modification of local magnetic properties of steel surfaces by a Q-switched Nd:YAG laser beam

Using a Q-switched Nd:YAG laser, we have successfully obtained laser-induced ripple patterns on various ferric metal surfaces. Changes of magnetic properties associated with the rippled areas were then investigated using a simple magnetic detecting system, which consisted of an audio recorder head, an audio amplifier and an oscilloscope. Significant magnetic changes associated with the formation of the ripples were found on a steel substrate pre-coated with a layer of chromium. This technique can be used as the basis of a machine readable laser marking system on steel surfaces, or even other surfaces that could be cotated with a steel/chromium film.     

ESCA studies of Ni-Cr alloys

ESCA examination on Ni-Cr alloys has shown that a thin passive film was formed after 24 h immersion in 0.1 M NaCl. The film contained only chromium oxide in the form of Cr₂O₃. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate of the alloys. Both Ni-10 wt. % Cr and Ni-20 wt. % Cr alloys showed a slightly higher corrosion rate than the Ni-40 wt. % Cr alloy.The present ESCA study of the Ni-Cr system is part of our programme which involves an examination of the four binary alloy systems Fe-Si, Cr-Co, Ni-Cr, and Mo-Ni [1]. The aim is to correlate the structure and composition of the passive films formed in 0.1 M NaCl to the corrosion behaviour in the same solution.     

ESCA studies of Fe-Ti alloys

Fe-Ti alloys containing 5 to 47% Ti have been studied by ESCA. The alloys were exposed to 0.1 M NaCl for 24 h under open-circuit potential (OCP) during which passive films were formed. The passive film consisted of FeO and TiO₂ in the inner layer while Fe₂O₃, water, and hydroxyl groups were present in the outermost monolayers, irrespective of composition. The thickness of the passive layer was reduced from 4.4 nm to 1.0 nm with increasing Ti content. The amount of iron oxide in the passive layer also decreased with increasing Ti. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate of the alloys. Alloys with 5–28% Ti showed a relatively high corrosion rate but that with 47 wt.% Ti had a much lower corrosion rates.     

Simultaneous formation of contacts and diffusion barriers for VLSI by rapid thermal silicidation of TiW

The formation of (Ti_xW_1–x)Si₂/(Ti_xW_1–x)N, by rapid thermal processing of Ti_xW_1–x on Si in an N₂ ambient is investigated. An activation energy of 1.7 eV is obtained for silicide formation. A distinct snow-ploughing of As atoms is observed during silicide formation whereas the interfacial B concentration decreases with increasing silicide formation temperature. The diffusion barrier properties of the (Ti_xW_1–x)Si₂/(Ti_xWi_1–x)N stack in contact with Al is investigated upon post-metal annealing. No interaction between the layers is found for temperatures as high as 475°C after 60 min. The improved thermal stability of the (Ti_xW_1–x)N layer in contact with Al is attributed to nitrogen blocking of the grain boundaries.     

In-situ diagnostics for plasma surface processing

A survey is given of in-situ diagnostics of plasma and surface for application in plasma etching and deposition. Especially those diagnostics that increase the fundamental understanding of the elementary processes occurring both within the plasma and at the surface are highlighted. In general, diagnostics are performed to determine the value of a physical parameter. This value is fed into models of plasma or surface, and in that way the understanding of the process is enhanced. In the paper first the most interesting physical parameters are defined. Subsequently the diagnostic techniques currently available to determine those parameters are reviewed.     

Dissociative adsorption of alkanes on clean and sulfur-modified nickel surfaces

Recent studies of the dissociative adsorption of methane on clean Ni(111), Ni(100), Ni(110), and sulfur-modified Ni(100), as well as ethane, propane, and n-butane on Ni(100) have been carried out under the high incident flux conditions of 1.00 Torr methane, 0.10 Torr ethane, 0.01 Torr propane, and 0.001 Torr n-butane, respectively. It has been found that the activation energies for these processes range from 3.1±1.0 to 13.3±1.5 kcal mol^–1. A comparison with the results of corresponding molecular beam studies suggests that the effects of vibrational energy on sticking probabilities must be accounted for and the sticking probabilities of molecules with very low normal kinetic energies must be accurately known when attempting to model high pressure processes using molecular beam techniques. While dissociation of ethane, propane, and n-butane on Ni(100) is believed to proceed primarily via a trapped molecular precursor, the results on sulfur-modified Ni(100) surface indicate that the direct channel to methane dissociation likely dominates and the contribution from the trapped molecular precursor mechanism is likely relatively small, with the sulfur atoms poisoning this reaction by a simple site blocking mechanism.     

XeCl laser ablation of yttria stabilized zirconia

XeCl laser ablation of yttria stabilized zirconia (YSZ) in air and in vacuum (1.3×10^–4 Pa) is studied by means of etch depth measurements, scanning electron microscopy, and X-ray photoelectron spectroscopy of ablated surfaces. Results on ablation rate, surface morphology, and surface chemical composition are discussed in terms of the influence of ambient atmosphere on the ablation process, rapid melting and solidification of ablated surfaces, and preferential removal of oxygen atoms from the YSZ surface.     

Pulse-width influence on the laser-induced structuring of CaF2 (111)

We have investigated the morphology of CaF₂ (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs-8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps.     

Negative-ion emission during laser ablation of multicomponent materials

The time of flight technique coupled with an electrostatic energy filter has been used for composition and energy-distribution analysis of the ion species emitted during laser ablation of multicomponent materials Y-Ba-Cu-O, Pb-Sn-Te. The negative-ion output and kinetic-energy distribution as a function of the laser wavelength and the laser fluence on the target were measured. A high output of the negative ions of matrix elements comparable with the positive-ion emission was detected. The barium negative-ion formation was observed in spite of the negative electron affinity of alkaline earth elements in the ground electronic state. The mechanism of negative-ion formation based on the ternary collisional recombination in the laser plasma is analyzed.     

Observation of magnetic-field-enhanced excitation and ionization in the plume of KrF-laser-ablated magnesium

The influence of a magnetic field on the plume produced by KrF-laser ablation of magnesium in vacuum has been investigated using time-integrated photography, streak photography, spectroscopy and charge probes. Line emission spectra in the 200–600 nm interval and effective stream velocities for the plume obtained from the spatiotemporal emission from specific neutral and ion lines are reported. Time of flight velocities are also deduced from measurements using simple charge-collector probes. Changes in the plume structure and dynamics, and enhanced emission and ionization are observed in the presence of the field. A qualitative explanation of the results is given in terms of a magnetohydrodynamic model.     

Direct writing and in-situ material processing by a laser-micromachining projection microscope

The new laser micromachining projection microscope enables a surface to be viewed at high magnification with a built-in facility for micromachining of the same surface according to a predetermined pattern. A XeCl-excimer laser forms part of the device and provides bright viewing illumination in amplified spontaneous emission as well as a small laser spot of sufficient fluence to mark the sample surface. The basic characteristics of image enhancement in the excimer amplifier are presented. The advantage of such a technique for both pattern generation, surface cleaning and mask correction is demonstrated on Al, Cu and W specimens with a spatial resolution better than 3 m.     

Model of laser-induced chemical etching of silicon in chlorine atmosphere

A mathematical model for the chemical etching of silicon in a chlorine atmosphere induced by laser irradiation is described. The model takes into account: dissociation of molecules having absorbed radiant energy into chlorine atoms and their diffusion onto the substrate surface, generation of photocarriers in the silicon substrate, kinetics of chlorine atom chemisorption on the silicon surface, chemical reaction of chemisorbed chlorine atoms with silicon atoms, and desorption of reaction products. The obtained results are compared with experimental data.     

Thermal configurations of oxygen in silicon

The central position and the infrared absorption coefficient of the 9 m band of Si samples were measured with Fourier transform infrared spectroscopy (FTIR) at temperatures from T=77 K to 775 K. The infrared absorption coefficients were corrected by considering background absorption and free carrier absorption calculated from the increased free carrier concentration and from the resistivity determined from Hall effect measurements. We found the central position of the 9 m band to shift to longer wavelengths with increasing temperature. The concentration [O_i] of interstitial oxygen is almost constant for T<600 K, but decreased rapidly for T>600 K. These results verified there are two types of thermal configurations of oxygen in silicon: The bonded Si₂O configuration with a binding energy E _b0.8 to 1.0 eV at T77 K to 600 K, and the Si₂O configuration coexists with a quasi-free interstitial oxygen (QFIO) state for T>600 K. The lattice potential barrier E _L, which retards QFIO atoms from migrating in the lattice, is estimated to be 1.5 to 1.6 eV. From these configurations the anomalous diffusivity of oxygen in silicon can be explained quite well.     

Effect of substrate temperature on the growth of ITO thin films

Indium tin oxide (ITO) thin films were deposited onto glass substrates by rf magnetron sputtering of ITO target and the influence of substrate temperature on the properties of the films were investigated. The structural characteristics showed a dependence on the oxygen partial pressure during sputtering. Oxygen deficient films showed (4 0 0) plane texturing while oxygen-incorporated films were preferentially oriented in the [1 1 1] direction. ITO films with low resistivity of 2.05 × 10⁻³ Ω cm were deposited at relatively low substrate temperature (150 °C) which shows highest figure of merit of 2.84 × 10⁻³ square/Ω⋅     

Laser nitriding of iron: Influence of the laser parameters on the nitriding efficiency

Laser nitriding of Armco iron in nitrogen was studied for KrF-excimer-laser irradiation. The influence of the energy density and number of pulses on the nitrogen take-up and the nitride phases formed was investigated using Resonant Nuclear Reaction Analysis (RNRA) and Mössbauer spectroscopy. Besides the original a-iron, austenite-Fe(N), martensite-Fe(N),-Fe₂₊N, and-Fe₁₆N₂ were identified. The fraction of the e-phase was found to increase with the number of pulses and the energy density. A threshold energy density of 1.8(2) J/cm² for the laser nitriding process was found.     

Ar ion laser assisted chemical etching of via holes in tungsten sheets in air

Ar ion laser assisted chemical etching of 150 m thick annealed tungsten sheets in air is reported. The material removal mechanism involves local heating by the laser to temperatures in the range of 1000–1500 °C that causes rapid oxidation of the W to WO₃ which volatilizes readily. Holes with straight walls and slightly enlarged entrances near the surface were drilled with etch rates as high as 11.5 m/s at 13.8 W, and a minimum hole diameter of 21 m at 8.1 W. The diameters of the holes and the etch rates were measured and found to increase as a function of the laser power. It was found that by increasing the laser power above 11–12 W, no change was observed in the hole diameters which remained constant at about 31 m, whereas the etch rates continued to increase even faster than at low powers. Distinct adjacent holes of 25 m diameter could be drilled with their centers separated by as little as 60 m. This is therefore also the etching resolution in the present study.     

Laser nitriding of iron: Nitrogen profiles and phases

Armco iron samples were surface nitrided by irradiating them with pulses of an excimer laser in a nitrogen atmosphere. The resulting nitrogen depth profiles measured by Resonant Nuclear Reaction Analysis (RNRA) and the phase formation determined by Conversion Electron Mössbauer Spectroscopy (CEMS) were investigated as functions of energy density and the number of pulses. The nitrogen content of the samples was found to be independent of the number of pulses in a layer of 50 nm from the surface and to increase in depths exceeding 150 nm. The phase composition did not change with the number of pulses. The nitrogen content can be related to an enhanced nitrogen solubility based on high temperatures and high pressures due to the laser-induced plasma above the sample. With increasing pulse energy density, the phase composition changes towards phases with higher nitrogen contents. Nitrogen diffusion seems to be the limiting factor for the nitriding process.     

Laser photoelectron microspectroscopy for imaging impurity ions,color centers,and small-size irregularities in dielectric media

New photoelectron microscopy application fields are considered for imaging the arrangement of color centers, small-size defects, and impurity ions in dielectric matrices. The techniques proposed are based on the first experimental results obtained in studying the possibilities of observing stepwise laser photoelectric effect in ZrO₂:Nd³⁺, where photoelectron emission results from the stepwise absorption of light by the impurity ions Nd³⁺. In that case, the electrons being emitted originate in immediate proximity to the impurity ions, which offers possibilities to image the above structures by photoelectron microscopy techniques.