Yasuo Takigawa

SiO bond breaking in SiO2 by vacuum ultraviolet laser radiation

Abstract Serious optical damage is observed in the surface layer of synthetic quartz glass (SiO2) when it has been used for a high power argon excimer laser operating at 126 nm as a cavity mirror. Microscopic surface observations, XUV reflectivity measurements and XPS analysis show that the damage is mainly ascribed to a photochemical effect. The photochemical effect results in the SiO bond breaking and subsequent crystalline Si isolation in the surface layer.

Laser beam profiler in the vacuum ultraviolet spectral range using photostimulable phosphor

We propose and demonstrate the use of a photostimulable phosphor material, BaFBr:Eu2+, for recording two‐dimensional intensity distributions within output beams of vacuum ultraviolet lasers. The sensitivity characteristics were measured not only for nanosecond pulsed radiation from KrF, ArF, and Ar2 excimer lasers but also for quasi‐continuous‐wave synchrotron radiation. We designed and constructed a laser beam profiler to record, read, and erase images in vacuum. The intensity distribution in the output beams from an Ar2 excimer laser was measured with it, and then the threshold gas pressure for obtaining the well collimated beams was found to be 22 kg/cm2. The intensity is not uniform in the beams but stronger in the peripheral part than in the central one.

Super-polished silicon carbide mirror for high-power operation of excimer lasers in a vacuum ultraviolet spectral range

We demonstrate the usefulness of a high‐quality SiC mirror for the high‐power operation of vacuum ultraviolet lasers. A void free SiC film of 400‐μm thickness was deposited onto a high‐purity graphite substrate by chemical vapor deposition. The super‐polished SiC mirror, whose surface roughness was 0.2 nm rms, was used as a cavity reflector of a high‐power Ar excimer laser, which operates at 126 nm. The highest output energy of 400 mJ/cm2 per pulse, which corresponds to the energy density of 1 J/cm2 on the SiC mirror surface, has been obtained without any damage to the SiC mirrors.

Surface alterations of SiO2 optics by 9.8 and 8.5 eV laser photons

Abstract Alterations of SiO 2 mirror surfaces, measured by means of surface profiler, reflectance and transmission spectroscopy, and X-ray photoelectron spectroscopy, are reported. The alterations were formed when the mirrors were used as a cavity reflector for Ar and Kr excimer lasers. It is found that Si is enriched in the surface layers which were exposed to 9.8 eV photons from the Ar excimer laser. The Kr excimer laser photons, the energy of which is 8.5 eV, do not induce such a phenomenon. The Ar excimer laser photons, surmounting the fundamental band gap of SiO 2 , 9 eV, are assumed to create a high density of excitons that induce SiO breaking, resulting in silicon enrichment and oxygen depletion. The Si enrichment is observed for both glassy and crystalline SiO 2 .

X-ray emission spectroscopic studies of silicon precipitation in surface layer of SiO2 induced by argon excimer laser irradiation

Abstract The ultra-soft X-ray emission spectra were taken from surfaces of bulk silica glass and silica glass films exposed to an argon excimer laser ( λ =126 nm) and compared with the spectra taken from the virgin surfaces. The precipitation of crystalline silicon is found to take place in thin surface layers of the irradiated bulk silica glass and 15 nm film. An estimation of concentration of crystalline silicon precipitation with the depth is given on the basis of the measurements of Si L 2,3 X-ray emission spectra obtained at different accelerating voltages of the electron beam on the X-ray tube. Based upon the precipitation conditions for these samples, we discuss the crystalline silicon precipitation mechanism: the electronic excitation induces the bond-breaking between Si and O atoms, although there is a critical density of photons for the bond-breaking and temperature rise enhances the crystalline silicon precipitation.

Growth of silicon microcrystals in thin surface layers of quartz glass with vacuum ultraviolet laser processing

Abstract Silicon precipitates in surface layers as thin as 50 nm of quartz glass plates where 9.8 eV photons from an argon excimer laser are irradiated. The surfaces have many protrusions having a spherical shape with submicrometer diameter. Raman spectra indicates that they are made of crystalline silicon. Such a phenomenon has not been observed by 8.5 eV photons from a krypton excimer laser. VUV laser material processing offers a novel way to produce directly polycrystalline silicon circuits in thin surface layers of quartz glass plates.

Fabrication, characteristics, and performance of diamond mirrors for vacuum ultraviolet excimer lasers

We have deposited crystalline diamond films on sintered Si 3 N 4 substrates with a conventional hot-filament method and further finished the surfaces with a diamond lapping method. The diamond mirrors fabricated are proposed and demonstrated as among the most promising mirrors for vacuum ultraviolet rare-gas excimer lasers. We have obtained pulses with 15-mJ energy, 2.7-MW peak power, and 6-ns duration at 126 nm from an argon excimer laser, and with 70 mJ, 5.5 MW, and 5 ns at 146 nm from a krypton excimer laser without any surface damage. The latter values are the highest so far obtained

Silicon precipitation in a surface layer of SiO2 by high-power vacuum ultraviolet laser irradiation

Abstract The interaction of an intense laser beam in the VUV region with synthetically grown crystalline SiO 2 as well as with synthetic amorphous SiO 2 are extensively studied by X-ray photoelectron spectroscopy. The desorption of oxygen is induced by the bombardment of 9.8 eV photons produced from an Ar excimer laser, leaving behind elemental silicon in a surface layer of 50 nm depth. 8.5 eV photons produced from a krypton excimer laser do not induce such phenomena. The oxygen desorption and silicon precipitation are an intrinsic property of silicon dioxide. The reaction kinetics are also discussed.

Beam profile measurement of vacuum ultraviolet lasers with photostimulable phosphor

The linear response of photostimulable phosphor material (BaFBr:Eu2+, imaging plate: IP) is confirmed for ArF and Ar2 excimer laser pulses whose wavelength and duration are 193 nm and 10 ns, and 126 nm and 5 ns, respectively. We demonstrate two-dimensional image recording on the IP with vacuum ultraviolet laser beams.

Excimer laser irradiation of Si3N4 films deposited on Si

We have investigated the effect of argon excimer laser irradiation of silicon nitride films deposited on silicon substrates. When the film thickness is equal to or greater than 40 nm, the irradiation induces amorphous silicon precipitation near the thin surface layer of the film, which has been clarified by means of x-ray photoelectron and Raman spectroscopy. The formation mechanism of amorphous silicon with the irradiation is discussed. The phase of silicon precipitation depends on the thermal properties of the substrate material.