KrF Excimer Laser Irradiation Research Articles

Analysis of the plasma produced by pulsed reactive crossed-beam laser ablation of La 0.6Ca 0.4CoO 3

Pulsed laser deposition (PLD) in conjunction with a synchronized pulsed gas source provides an extra degree of freedom in processing the ablation plume. This technique is called pulsed reactive crossed-beam laser ablation (PRCLA). The ablation plasma of La 0.6Ca 0.4CoO 3 (LCCO) produced by KrF excimer laser (248 nm) irradiation is analyzed by spatially and temporal resolved optical emission spectroscopy in the range of 300–800 nm in vacuum, with oxygen background and with an additional N 2O as a gas pulse. The spectra reveal higher intensities for all the peaks when the gas pulse is applied. No new peaks assigned to the interaction with the gas pulse, i.e. oxygen, nitrogen or diatomic species are observed. The maximum intensities of the plasma with the gas pulse are observed later in time and further away from the target.

Rapid fabrication of microcomponents - UV-laser assisted prototyping, laser micro-machining of mold inserts and replication via photomolding

The rapid fabrication of microcomponents made from polymers or composites will be presented. The whole fabrication process is divided into three main steps: first, direct patterning of polymers with excimer laser radiation enables the fabrication of first prototypes. Second, laser assisted micromachining using Nd:YAG or KrF-Excimer laser radiation allows a rapid manufacturing of microstructured mold inserts made of steel or polymer. Third, the application of UV-light-induced reaction injection molding (UV-RIM, photomolding) process using reactive monomer/polymer resins gives the access to the replication of the previously fabricated mold insert. For mold insert fabrication, the development of different process strategies is in progress in order to meet the best fit for three dimensional shapes (e.g., steep walls or slowly inclined walls). The development of mold inserts made of polymers is performed via UV laser ablation. This is a new promising method for the rapid manufacturing of microcomponents.

Emission Spectra and Relaxation Mechanism of b State O2 in Low Temperature Ar Solids

Abstract The b → X emission of electronically excited O2 isolated in Ar solids was observed under the pulsed KrF excimer laser irradiation at 248 nm. The relaxation proceeds along the b → a → b interstate transition. The time evolution of each b → X (v′, v′′) band was fitted by multi-exponential curves. The nonradiative rate is limited at the b → a transition due to the energy gap, which is larger than that of the a → b transition. The relaxation rate constants of individual vibrational levels (v ≤ 8) of the b state were determined. The vibrational quantum number and temperature dependence of the nonradiative rate constants could be explained based on a multiphonon relaxation theory.

Thermally stabilized photoinduced Bragg gratings

Bragg gratings were printed in Ge–SiO2 and Ge–B–SiO2 thin glass films by KrF excimer laser irradiation through a phase mask. The diffraction efficiency of the grating printed in the Ge–B–SiO2 film was approximately one order of magnitude greater than that of the Ge–SiO2 film. Although the gratings were almost erased upon annealing at temperatures lower than 500 °C, a grating with much higher diffraction efficiency than before annealing was formed exclusively in the Ge–B–SiO2 film after annealing at 600 °C. The diffraction efficiency of the grating was unchanged upon repeated heating between room temperature and 600 °C.

Pattern-induced ripple structures at silicon-oxide/silicon interface by excimer laser irradiation

Ripple structures by KrF excimer laser irradiation have been observed on a silicon surface capped with a thin layer of patterned silicon oxide. The ripples are highly dependent on the patterns of the silicon oxide. They are centered and enhanced at the boundaries of the opened windows, forming a radial-wavelike structure. The formation of the ripples is attributed to the combined effect of surface stress, surface scattered wave and boundary effects.

Surface Modification of Glassy Carbon by Pulsed Laser Irradiation with Several Wavelengths

Glassy carbon is one of several types of amorphous carbon and has a complicated network structure made up of multilayers consisting of sp2 hybridized carbon. We briefly reported the phenomenon of the hardening of glassy carbon induced by KrF excimer laser (λ=248 nm) irradiation. In this study, ArF, KrF, XeCl, fundamental generation and 2nd harmonic generation waves of Nd:YAG lasers were applied on the glassy carbon target directly under the atmospheric pressure of argon to confirm the appropriate conditions for the hardening of glassy carbon. Surface hardening was observed with variations of the surface morphology and the structure of the modified layer.

Laser crystallisation during pulsed laser deposition of barium titanate thin films at low temperatures

Using a high dielectric material as substitute for SiO x N y in dielectric film capacitors of dynamic memories (DRAM) allows a significantly higher integration density and a reduction of the die size, even with planar capacitors. BaTiO 3 is such a material. A dielectric constant of ε r>1000 has been achieved in thin films, made by pulsed laser deposition (PLD). For applications in microelectronic memories it is necessary to produce crystalline, defect-free and oriented BaTiO 3 thin films at substrate temperatures, T S<450 °C. Sintered targets of BaTiO 3 are ablated by KrF excimer laser radiation. The processing gas atmosphere consists of O 2 at pressures of 0.1–50 Pa. The substrate is resitively heated to 360–440 °C and annealed after or during PLD on Pt/Ti/Si multilayer substrates using KrF excimer laser radiation with fluences up to 120 mJ/cm 2. The temperature distribution in the BaTiO 3/Pt/Ti/Si multilayers during laser annealing is dynamically modelled and related to the resulting crystal quality and the dielectric properties of the films. With PLD a minimum substrate temperature of 500 °C is necessary to deposit crystalline BaTiO 3 films. Using in situ laser crystallisation crystalline BaTiO 3 films can be deposited at substrate temperatures of T S=360–440 °C showing a dielectric constant of up to ε r=1200. The ferroelectric and dielectric properties of the films are determined by C– V and P– V impedance measurements and correlated to the chemical and structural properties, as determined by X-ray photoemission spectroscopy, X-ray diffraction, micro Raman spectroscopy and scanning electron microscopy.

The onset of optical breakdown in KrF-laser-irradiated silica glass surfaces

We have investigated the onset of surface optical breakdown on a polished, synthetic silica glass under KrF excimer laser irradiation. With increasing laser fluence, a transient, localized luminescence is often observed. If the fluence is increased further, the luminescence reappears and grows gradually more intense with continued exposure. Subsequent examination of damaged surfaces show small craters, about 1 μm in diameter, in the damaged region. Surfaces damaged by single laser pulses at still higher fluences show similar morphologies, suggesting that the same defects or flaws are responsible for damage under multiple pulse and single pulse conditions.

High-quality polycrystalline Si TFTs fabricated on stainless-steel foils by using sputtered Si films

We have developed a low-temperature fabrication process (/spl les/ 200/spl deg/C for high-quality polycrystalline Si thin-film transistors (poly-Si TFTs) on flexible stainless-steel foils. The fabrication processes is realized through sputter deposition of thin films, including active-Si and gate-SiO/sub 2/ films, crystallization of Si films by KrF excimer laser irradiation, and inductively coupled plasma hydrogenation. High-quality n- and p-channel poly-Si TFTs are successfully fabricated without suffering from problems of substrate bending, film ablation, or cracking in films. The resulting n- and p-channel poly-Si TFTs showed mobilities of 106 and 122 cm/sup 2//V/spl middot/s, respectively. This paper describes the deposition and properties of the sputtered Si films and the fabrication process and electrical characteristics of the poly-SiTFTs.

Photo-Chemical Pattern Etching of Silicon-Carbide by Using Excimer Laser and Hydrogen Peroxide Solution

A circuit pattern etching of a Silicon-carbide (SiC) surface was conducted with KrF excimer laser irradiation in the presence of HF and H2O2 mixed solutions. SiC have excellent properties of a high hardness, high melting point, wide band gap, high resistance to radiation and chemical stability. This material has come to attract attention as an integrated circuit material for high resistance to environment. However, the material is very difficult in minute processing by the photo-lithography because of its chemical stability. Thus, we developed the new etching method in which a SiC surface was photo-oxidized with H2O or H2O2 by using excimer laser irradiation and was etched by HF water solution. In this experiment, the mixed solution was poured into the thin gap between an Al2O3 glass and the SiC surface with capillary phenomenon. A patterned excimer laser light was, then, irradiated on the SiC surface. The H2O or H2O2 in the reaction solution was photo-dissociated, and the photo-dissociated active oxygen reacted with the SiC. CO2 and SiO2 were formed only on the part exposed by the pattered light forcibly, and an oxidized layer was formed. In this chemical reaction, the CO2 evaporated, and the SiO2 remained on the sample surface. The SiO2 layer was then dissolved by the HF water solution. Thus, etching was conducted by the repetition of the forced oxidization of the SiC and the dissolving of the oxidized layer. In this experiment, the most effective conditions were 20% of H2O2 water solution, 15% HF water solution and 256mJ/cm2 of KrF excimer laser. The etching depth was 80 Å at the laser shot number of 10000. It significantly improved compared with that of using an ArF excimer laser (256mJ/cm2, 193nm), 50 Å.

Low-temperature conduction of a system of tunnel-coupled quantum dots in YBaCuO and LaSrMnO dielectric films

This paper reports on new results of experimental investigations into the nature of the electrical resistivity ρ(T) ≅ const at T<Tcrit for YBaCuO and LaSrMnO dielectric films. The films are prepared by pulsed laser deposition and contain nanocrystalline clusters with metallic conductivity. Dependences of the electrical resistivity ρ(T) ≅ const are observed for the epitaxial films YBaCuO (Tcrit=10 K) with a tetragonal structure after exposure to KrF excimer laser radiation and for the as-prepared amorphous films LaSrMnO (Tcrit≅160 K). The effect of interest (ρ(T) ≅ const) manifests itself in the case when the optical spectra of the studied samples contain portions attributed to absorption by free charge carriers. The inference is made that this effect can be associated with tunneling conduction in a system of quantum dots.

X-ray photoelectron spectroscopic study of KrF excimer laser nitrided InP surface

Nitridation of InP is performed by KrF excimer laser irradiation in an NH3 ambient. The N–In and N–P bonds are formed in the irradiated area in proportion to the number of laser pulses. The x-ray photoelectron spectroscopy spectra of the nitrided samples contain a small concentration of oxide components after aging in an air atmosphere than nonirradiated samples

Activation of Mg Acceptor in GaN : Mg with Pulsed KrF (248 nm) Excimer Laser Irradiation

We report on the activation of Mg acceptors in Mg-doped GaN films, grown by metalorganic chemical vapor deposition, via the use of a pulsed KrF (248 nm) excimer laser irradiation. The as-grown GaN : Mg, which was irradiated by the KrF excimer laser at a laser energy density of 590 mJ/cm2 in a N2 ambient showed a hole concentration of 4.42 × 1017 cm—3. Furthermore the hole concentration in GaN : Mg, which was activated by a conventional rapid thermal annealing, was increased from 4.3 × 1017 to 9.42 × 1017 cm—3 as the result of subsequent laser irradiation. These results suggest that a pulsed KrF excimer laser irradiation can dramatically enhance the p-type conductivity of GaN : Mg by efficiently dissociating the Mg–H complexes.

Activation of Mg Acceptor in GaN : Mg with Pulsed KrF (248 nm) Excimer Laser Irradiation

Activation of Mg Acceptor in GaN : Mg with Pulsed KrF (248 nm) Excimer Laser Irradiation

Spectroscopic investigation of the technique of plasma assisted pulsed laser deposition of titanium dioxide

The characterization of plasma assisted pulsed laser deposition (PA-PLD) of titanium dioxide with biased substrate is discussed. Both the stage of plasma expansion and deposition have been studied. Optical emission spectroscopy was employed to estimate laser-induced plasma parameters, while different techniques [optical microscopy, scanning electron microscopy (SEM), spectrophotometry, X-ray photoelectron spectrometry (XPS)] were used to characterize the film properties. It is shown that PA-PLD prevents contamination of the deposited films by particles ejected during the interaction of the KrF excimer laser radiation with the titanium dioxide targets. Investigation made on the film deposited by conventional PLD and PA-PLD, has shown that the PA-PLD technique allows to improve the quality of the deposited films for what concerns their stoichiometry, morphology and deposition rate.

Silicon grain arrays prepared using a pattern crystallization technique of pulsed-excimer laser irradiation

Silicon grain arrays were prepared using a pattern crystallization technique of pulsed KrF excimer laser irradiation. The precursor material was hydrogenated amorphous silicon (a-Si:H) thin films deposited on single crystal Si wafers by plasma-enhanced chemical vapor deposition. It was shown that Si grains with a uniform size and a well-defined periodicity embedded in the a-Si:H matrix were obtained by this simple technique. The grain size was less than 2 μm. Relativly strong photo-luminescence with two peaks at 720 and 750 nm was observed at room temperature. We expect to reduce Si grain sizes by optimizing the growth conditions of a-Si:H thin films and controlling the temperature distribution in the film during laser irradiation.

F2 laser etching of GaN

F2 laser-induced direct etching of GaN has been demonstrated. Well resolved geometric structure is found on edge of etched area. No acceleration of ablation rate at higher laser intensity was confirmed, while the rate acceleration is obvious in case of KrF excimer laser irradiation. The ablation is thought to proceed with direct photoionization initiated by single 7.9eV photon absorption.

Laser coloration and bleaching of amorphous WO3 thin film

Laser-induced chromism of amorphous WO3 thin films prepared by pulsed laser deposition (PLD) has been investigated. The original films could be colored from light brown to purple by a single pulse of KrF excimer laser irradiation at 248 nm and subsequently bleached to brown by a single pulse of Nd–yttrium–aluminum–garnet laser at 1.06 μm in air. Spectroscopic investigations were applied to the films at three different states: original, colored, and bleached. The measurements by ellipsometry spectroscopy showed an increase in the refractive index (n) and decrease in extinction coefficient (k) in the luminous range of films from the colored state to the bleached state. Scanning tunneling microscopy and Raman spectroscopy showed slight crystallization in the films after coloration, with both the grain dimension and the surface roughness around tens of nanometers. The films were very stable to maintain the same color after long-time exposure in air, or in oxygen. X-ray photoelectron spectroscopy was used to study the chromism mechanisms. While some W5+ states were introduced in the original films by PLD, W4+ states were produced when the films were colored by the KrF excimer laser, along with the decrease of W5+ states and the increase of W6+ states. In contrast, laser bleaching was accompanied with increasing W4+ states in the films. Therefore, it is believed that the purple color is due to the polaron transition between the W4+ and W5+ states. Photochemical activation and photothermal oxidation are ascribed to the coloring and the bleaching processes, respectively.

Dual-beam ablation of fused silica by multiwavelength excitation process using KrF excimer and F2 lasers

A new technique of dual-beam laser ablation of fused silica by multiwavelength excitation process using a 248-nm KrF excimer laser (ablation beam) coupled with a 157-nm F2 laser (excitation beam) in dry nitrogen atmosphere is reported. The dual-beam laser ablation greatly reduced debris deposition and, thus, significantly improved the ablation quality compared with single-beam ablation of the KrF laser. High-quality ablation can be achieved at the delay times of KrF excimer laser irradiation shorter than 10 ns due to a large excited-state absorption. The ablation rate can reach up to 80 nm/pulse at the fluence of 4.0 J/cm2 for the 248-nm laser and 60 mJ/cm2 for the F2 laser. The ablation threshold and effective absorption coefficient of KrF excimer laser are estimated to be 1.4 J/cm2 and 1.2×105 cm-1, respectively.

Novel method to measure bulk absorption in optically transparent materials

A novel method to measure low absorption of pulsed laser radiation by materials with high optical transparency is demonstrated. The absorbed energy generates a temperature gradient and thus a refractive index profile. A probe laser beam is deflected by the refractive index profile and its deflection is a measure of pure volume absorption without interference from surface effects. Numerical calculations were carried out to estimate the sensitivity of the discussed laser induced deflection (LID) arrangement and to optimize the path of the probe beam in relation to the irradiated field within the sample. The experimental results agree sufficiently with numerical calculations by finite element method combined with raytracing procedures. Because the transparency is not measured directly a calibration is necessary. To provide this, a sample with known bulk absorption coefficient and thermal properties was used. In order to compensate probe beam fluctuations in air outside the sample, a double beam arrangement was designed. The LID method is applied to investigate fused silica with high ultraviolet (UV) transparency under KrF excimer laser irradiation (λ=248 nm). A He/Ne laser serves as probe beam source. Using sample dimensions of 20×20×10 mm3 and UV beam cross sections of 5×5 mm2 absorbed power down to 1 mW is detected. At typical irradiation conditions (50 s−1, 300 mJ/cm2), this value corresponds with an absorption coefficient of around 2.5×10−4 cm−1. The limitation of the absorption measurement depends on the input power of the pump laser.