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.     

Pulsed 532 nm laser wirestripping: Removal of dye-doped polyurethane insulation

Removal of rhodamine 6G doped polyurethane insulation coated onto 50 m diameter wire is shown to proceed efficiently and cleanly by irradiation with 532 nm Q-switched pulses from a Nd:YAG laser. The stripping action produced by this method is similar in quality to excimer laser wirestripping. Several experimental parameters were explored including fluence, pulse duration, dye concentration, and the number of incident pulses. Acceptable stripping conditions were obtained for a 3–5 s exposure at 10 Hz, using a dye concentration of 10% by weight, and 12 n pulses at 650 mJ/cm². Nearly 0.5 m/pulse is removed at this fluence, which exceeds the threshold fluence of 600 mJ/cm² by only 50 mJ/cm². The measured 532 nm absorption coefficient of the 10% dye-doped polyurethane was 4×10⁴ cm^–1. Lower fluences and/or dye concentrations produced inadequate stripping, while shorter duration pulses caused unacceptable melting of the thin gold layer which covered the copper core of the wire. Pulse-by-pulse photographs of the stripping action clearly show melting of the dye/polymer insulation, and thermal rollback of the insulation near the stripped end. Regardless, excellent edge definition is obtained by this method.     

Porphyrin-sensitized laser swelling and ablation of polymer films

Laser-induced morphological changes of poly(methyl methacrylate), poly(N-vinylcarbazole), and gelatin films doped with porphyrins have been studied by etch depth measurement and scanning electron microscopy. An irreversible swelling of the irradiated surface was observed for all films in the case of low laser fluence. The swelling was replaced by ablation when the fluence was increased. The etch depth depends on the irradiation fluence and the dye concentration in the polymer. The observation of the irradiated surfaces suggests that the thermal effect is predominant both for swelling and ablation. The surface temperature at which swelling or ablation is initiated was estimated, assuming that these morphological changes take place at a certain temperature for any dye concentration in each polymer film.     

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.     

Interaction of atomic hydrogen with the graphite single-crystal surface

The reaction of atomic hydrogen (or atomic deuterium) with highly orientated pyrolytic graphite surfaces has been studied by means of thermal desorption spectroscopy. In some cases atomic deuterium instead of atomic hydrogen, was used solely to assign the desorbed masses unambiguously to the different hydrocarbons. The desorption of D₂ and fourteen hydrocarbons was observed. D₂ desorbed at higher temperatures than the CH-(CD) compounds, the desorption spectra of the hydrocarbons contained two peaks. The dependence of the desorption spectra of several hydrocarbons on the heating rate, the atomic hydrogen exposure and the composition of the desorption products was investigated in detail. The kinetic parameters of the desorption process were determined for CH, C₂H₂, and CD₄. The spectra showed that there must be a first order desorption process for all the hydrocarbons, the values for the activation energy and the frequency factor were the same within experimental errors. The results were discussed by means of a simple model.     

In situ monitoring of laser-induced silicon oxidation

A low cost, computer-controlled, in situ monitor of laser-induced oxidation of c-Si is described. Both oxide thickness and processing temperature can be determined, simultaneously and with high spatial resolution, from the sample reflectivity and a knowledge of the temperature dependence of the optical constants of silicon and silicon dioxide (available in the literature).     

Electrochromic device based on tungsten oxide and on Nnfion-H as polymeric electrolyte

We describe the preparation, electrical and optical characterization of a prototype of a nearly solid-state electrochromic device. It is fabricated with Nafion-H, a polymeric electrolyte, and involves cathodically coloring tungsten trioxide doped with molybdenum. The device switches rapidly with the applied potential ranging from –2.4 up to 0.8V, between blue and transparent state. The corresponding integrated photopic (T _p) and solar (T _s transmittance is T _p (bleached) = 70.7%, T _s (bleached) = 59.6%, T _p (colored) = 15.5%, and T _s (colored) = 12.2%.     

A thermal model for the ablation of polymers by lasers and high intensity ion pulses

The results for the ablation of polymers by High-Intensity Pulsed Ion Beams (HIPIB) as well as by laser pulses of different wavelengths and pulse widths are discussed. A thermal model is proposed that reproduces all available experimental data. An Arrhenius type relation is assumed for the ablation velocityw(T) =w ₀ exp (–T ₁/T). Once the two parametersw ₀ andT ₁ are known for a certain polymer the model allows one to predict the ablation rate as a function of laser wavelength, fluence, and pulse width and for HIPIB pulses.     

Laser chemical processing of magnetic materials for recording-head application

A laser-induced thermochemical reaction has been used to etch single crystalline magnetic ferrite for industrial application to recording-head processing. Scanning focused Ar-ion laser irradiation to ferrite in flowing aqueous solutions of KOH or H₃PO₄ resulted in etch rates of 350–400 m/s with aspect ratios of up to 40. Metal-In-Gap (MIG) head structures with Sendust (FeAlSi) have been successfully fabricated using laser chemical processing. The MIG head fabricated by laser chemical processing showed better performance than that by conventional mechanical machining.Presented at LASERION '93, June 21–23, 1993 München (Germany)     

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.     

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.     

Spectroscopic properties of Tm ions in chalcogenide Ge-As-S glass containing minute amount of Ga and CsBr

We have experimentally confirmed that the absorption and emission properties for intra-4f-configurational transitions of Tm³⁺ ions doped in Ge-As-S glass, one of representative chalcogenide glasses, are modified dramatically upon the introduction of minute amount of Ga and CsBr, tantamount to a low doping level. This compositional adjustment makes local chemical environments of Tm³⁺ being rearranged spontaneously without any further thermal treatment applied. The hypersensitive ³H₆ ↔ ³F₄ transition, in particular, turns out to reflect the modified chemical environments more significantly than other transitions. Redistribution of the stark levels of ³F₄ manifold is mainly responsible for the significant changes in emission spectra for ³H₄ → ³F₄ and ³F₄ → ³H₆ transitions. Since the addition of small amount of the group III elements and alkali halides alters only the optical properties of rare-earth ions, while keeping thermal stability of the chalcogenide glass hosts unchanged, our compositional adjustment method would be quite useful for practical applications of rare-earth activated chalcogenide glasses.     

Surface cleaning of metals by pulsed-laser irradiation in air

The surface-cleaning effect of metals was investigated using KrF-excimer-laser irradiation of metal surfaces in air. The laser-induced cleaning of copper, stainless steel and aluminum surfaces was studied. It is found that laser cleaning is an effective cleaning process for metals even if the metal surfaces are heavily contaminated. It is also found that short wavelength and pulse duration are necessary for laser surface-cleaning. The energy density of the laser pulse is an important parameter in the cleaning process. Low energy density results in a cleaner surface but a larger pulse number is required, whereas high energy density can achieve higher cleaning efficiency but the temperature rise can cause surface oxidation and secondary contamination. In contrast to the KrF-excimer-laser, the pulsed CO₂ laser is not effective in surface-cleaning. The mechanisms of laser cleaning may include laser photodecomposition, laser ablation and surface vibration due to the impact of the laser pulse. Laser cleaning provides a new dry process to clean different substrate surfaces and can replace the conventional wet cleaning processes such as ultrasonic cleaning with CFC and other organic solvents.     

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.     

A study of blast waveforms detected simultaneously by a microphone and a laser probe during laser ablation

We examine blast waves generated in air during irradiation of absorbing samples with Nd: YAG laser pulses of fluences exceeding the ablation threshold. Blast waves were detected simultaneously by a wideband microphone and a laser beam deflection probe. By a comparative analysis of both signals in the time and frequency domain we investigate characteristic features of their nonlinear waveform evolution. To explain the observed phenomena we employ the weak shock solution of the point explosion model.     

Incubation/ablation patterning of polymer surfaces with sub-μm edge definition for optical storage devices

By exposure to low fluence UV laser radiation, the optical absorption coefficient of subsurface polymer material can be increased (incubation) with spatial control, using a suitable contact mask, proper imaging of the mask, or laser direct writing. Spatially selective ablation of polymethylmethacrylate (PMMA) is then achieved with large area XeCl excimer laser pulses at 308 nm. In this way, the transfer of spatial information to the material can be decoupled from the high laser fluence removal (ablation) step. The advantages are: The mask is exposed to only low fluence laser radiation — damage is avoided. Since the mask can be removed before the ablation step, mask contamination by the ablated plume cannot occur. Using this incubation/ablation method, PMMA surfaces can be patterned (248 nm/308 nm) with submicrometer spatial control and edge contrasts better than 0.2 m. This has impact on optical storage technology and laser surface processing techniques in general. The smallest single structure obtained was somewhat smaller than 0.5 m in diameter up to now, given by the mask.Presented at Laserion '91, June 12–14, 1991     

Thermoluminescence in ZrO2 with impurity of ZnO induced by UV radiation

ZrO₂ pellets doped by ZnO after 302 nm UV irradiation have been studied for ThermoLuminescent (TL) glow. The TL peak at 90°C for the 1100°C sintered ZrO₂ pellet shifted to 85°C with intensity increased three times for the 1% ZnO doped ZrO₂ sintered at 1100°C. The peak intensity at 210°C for the doped one is only one tenth of the undoped one. The emission spectra of thermoluminescence for undoped and ZnO-doped ZrO₂ revealed that the effect of ZnO doping is to increase the number of luminescent centers. The trapping center associated to the 90°C TL peak is explained by the similar model as that of Kirsh et al. For the case of 210°C TL peak, we have proposed two different models of trapping centers; one is the Zr⁴⁺ in an asymmetrical oxygen arrangement, and the other is the defect complex formed from an oxygen vacancy and an anion.     

Intensity-induced nonlinear effects in UV window materials

Intensity-induced nonlinear effects in optical window materials have been investigated at 308 nm. The absolute two-photon absorption coefficients for fused silica, CaF₂, BaF₂, Al₂O₃ and ADP crystals have been measured by using a single 120 ps, transform-limited pulse from the second harmonic of a distributed feedback (DFB) dye laser. The nonlinear refractive index coefficient has been obtained from measurements of far-field intensity distributions.     

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.     

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.