Synthetic SiO2 Glass Research Articles

Luminescence and Raman Detection of Molecular Cl2 and ClClO Molecules in Amorphous SiO2 Matrix

Chlorine is a common undesirable impurity in synthetic SiO2 glass for ultraviolet optics and optical fibers. It is usually incorporated into glass as bound Si–Cl groups or interstitial Cl2 molecules. We report a high-sensitivity detection of Cl2 in amorphous SiO2 (a-SiO2) by photoluminescence (PL) and also by Raman spectroscopy. The Cl2 PL emission band at 1.22 eV (1016 nm) appears at T < 160 K and shows a characteristic vibronic progression with separations ≈(520–540) cm–1 and an average lifetime of ≈5 ms at 13 K. Its excitation spectrum coincides with the shape of the 3.78 eV (328 nm) optical absorption band of Cl2 in a-SiO2, corresponding to the X → A 1Πu transition to repulsive excited state. Direct X → a singlet-to-triplet excitation was also observed at 2.33 eV (532 nm). Cl2 PL may serve as a sensitive and selective tool for monitoring Cl impurities and their reactions in a-SiO2. A Raman band of Cl2 is found at 546 cm–1. Cl2 photodissociation at energies up to 4.66 eV (266 nm) was not detected, poin...

Abrasive-Free Polishing of Single-Crystal 4H-SiC with Silica Glass Plates

In this study, abrasive-free polishing of a single-crystal 4H-SiC (0001) substrate was investigated. Two different types of polishing plates, namely a synthetic SiO2 glass (quartz) plate and a soda-lime SiO2 glass plate, were used to polish the SiC substrate under atmospheric conditions at room temperature. The surface roughness after polishing was evaluated by differential interference contrast microscopy, phase-shift interferometric microscopy, and atomic force microscopy. In addition, the chemical bonding states of the SiC surface before and after polishing were analyzed by X-ray photoelectron spectroscopy. The experimental results showed that an oxide layer was formed on the SiC surface as a result of the chemical reaction between the interfaces of the synthetic SiO2 glass plate and the SiC substrate. This generated oxide layer was effectively removed by polishing with the soda-lime SiO2 glass plate, resulting in an atomically smooth SiC surface with a root mean square roughness of less than 0.1 nm for 1.5 h. Obtained experimental results indicate that the component materials, temperature and water adsorptive property of the soda-lime SiO2 glass play an important role in the removal of the tribochemically generated layer on the SiC surface during this polishing.

Modification of vacuum-ultraviolet absorption of SiOH groups in SiO2 glass with temperature, F2 laser irradiation, and H–D isotope exchange

Variations of vacuum-ultraviolet (VUV) absorption of silanol (SiOH) groups in synthetic wet SiO2 glass with temperature, exposure to F2 laser light, and with H–D isotope exchange were examined at photon energies below 8eV. The intensity of the VUV absorption band decreases with cooling or with exposure to F2 laser light. The spectral changes in both cases are qualitatively similar and are attributed to an alternation of the bonding configuration of SiOH groups from isolated into hydrogen-bonded states. However, the resultant states are distinctly different: the hydrogen-bonded state formed on cooling is restored reversibly to the isolated state on heating, while that induced by F2 laser light is metastable and does not decay below 300°C. The VUV absorption is also reduced by replacing SiOH groups with SiOD groups due to the decrease of the zero-point energy of the ground state associated with OH groups.

Correlation between oxygen-deficient center formation and volume compaction in synthetic SiO2 glass upon ArF or F2 excimer-laser irradiation.

Correlations between the refractive-index change and the concentration of an oxygen-deficient center (ODC) induced by thermal treatments and laser irradiation are examined to clarify the origin of laser-induced volume compaction in synthetic SiO2 glasses. A linear correlation between them was clearly observed for thermally induced ODC, whereas no correlation was found for ArF or F2 laser irradiation. The results demonstrate that the dominant origin of laser-induced compaction is not ODC formation. Furthermore, we found that the presence of H2 in SiO2 glass had no influence on volume compaction but enhanced crack formation upon laser irradiation, a phenomenon most likely due to stress corrosion.

Power dependence of defect formation in SiO2 glass by F2 laser irradiation

The dependence of defect formation in a high-purity synthetic SiO2 glass on F2 laser power was studied. Above the threshold value of ∼10 mJ cm−2 pulse−1 (∼0.5 MW cm−2), the concentration of the laser-induced E′ center created by the dissociation of Si–O–Si bond increased as a function of the F2 laser power squared. The quantum yield of the E′ center formed by the high-power F2 laser irradiation was ∼3 orders of magnitude larger than that formed via two-photon absorption processes of KrF or ArF laser pulses. This strongly suggests that irradiating with the high-power F2 laser creates the E′ center via two-step absorption processes.

Effects of H2 impregnation on excimer-laser-induced oxygen-deficient center formation in synthetic SiO2 glass

We examined the effect of H2 impregnation on defect formation upon F2 laser (7.9 eV) and ArF excimer laser (6.4 eV) irradiation. It was revealed that H2 impregnation enhanced the formation of oxygen-deficient center (Si–Si bond) as well as suppressed the formation of E′ center and nonbridging oxygen hole center. A Si–Si bond gives an intense absorption band peaking at 7.6 eV, which contributes the absorption at the wavelength of F2 laser light. These results indicate that H2-free SiO2 glass, which is clearly inappropriate for KrF and ArF excimer laser optics, is more suitable for F2 laser optics than H2-impregnated glass.

Interaction of F2 excimer laser pulses with hydroxy groups in SiO2 glass: Hydrogen bond formation and bleaching of vacuum ultraviolet absorption edge

Excitation of vacuum ultraviolet absorption band of SiOH groups by F2 excimer laser photons (7.9 eV) was performed for synthetic SiO2 glasses. Irradiation induced two concurrent optical changes; red-shift of SiOH infrared absorption band and bleaching of vacuum ultraviolet absorption edge which is primarily controlled by SiOH absorption. These optical changes have the same origin, that is, the structural alternation of SiOH from isolated to metastable hydrogen-bonded states. The F2-induced hydrogen-bonded state was thermally stable to ∼300 °C and the kinetic barrier for the thermal relaxation was ∼1.7 eV. The formation of hydrogen-bonded state proceeded via one-photon absorption processes for F2 laser irradiation, but did not occur for ArF laser (6.4 eV) irradiation. This F2-induced hydrogen-bonded state is distinguished from laser-induced compacted state because the latter occurs for ArF or F2 laser irradiations via two-photon absorption processes.

Velocity angular distribution of ground level atomic silicon in the plume of laser ablated silica

Polished silica (synthetic SiO2 glass) samples were ablated to vacuum by 266 nm laser pulses at a flounce of 110 J/cm2. The concentration of ground level atomic silicon in the plume was measured as a function of time, by laser-induced fluorescence (LIF), at distances of 5–7 mm from the ablated point. The angular velocity distribution was found to be highly asymmetric with respect to surface normal, as the LIF signal dropped abruptly at angles approaching the laser direction. The symmetric morphology of the craters that were formed by ablation indicates that the above asymmetry is not likely to be attributed to the interaction of the laser or the ablated plume with the surface. Therefore, it is suggested that the silicon atoms, ejected in the direction of the incident laser beam, were preferentially ionized via a multiphoton off-resonance absorption process. The velocity of the silicon atoms was detected in the 1–13 km/s range. Since the velocity distribution for a thermal ablation process is expected to extend down to zero, the photochemical channel is suggested to be dominant in this case. Doppler broadening, calculated from the lateral velocity distribution, was found to be consistent with the broadening of the silicon absorption spectrum.

Interaction of F2 excimer laser with SiO2 glasses: Towards the third generation of synthetic SiO2 glasses

Changes in optical absorption spectra and defect formation of different types of synthetic SiO2 glasses were examined by irradiation with F2 excimer laser pulses (157 nm). Fluorine-doped, OH-free SiO2 glasses exhibit high optical transmittance (∼80%) at 157 nm in an as-delivered state and the intensity of F2-laser-induced absorption is much less than that in wet or dry F-free samples. The effect of F-doping on the blue shift of the absorption edge and suppression of color center formation was conspicuous up to 1 mol% but was slight upon further doping. It is suggested that elimination of strained Si–O–Si bonds upon F-doping is the primary reason of the improvement of resistance of SiO2 glasses to F2-laser light. Novel optical phenomena by F2-laser irradiation, bleaching of the vaccum UV (VUV) absorption edge and changes in the SiOH infrared absorption, were found in H2-impregnated, or wet SiO2 glasses. These results lead to the conclusion that F-doping to 1 mol% is an effective and practical method to obtain synthetic SiO2 glasses for F2 excimer laser optics as a photomask in optical lithography.

Fluorine-doped SiO_2 glasses for F_2 excimer laser optics:?fluorine content and color-center formation

Color-center formation in F-doped, OH-free synthetic SiO(2) glasses by irradiation with F(2) excimer lasers (157 nm) was examined as a function of the F content. The concentration of photoinduced E(') centers was reduced to approximately 1/20 by 1 mol.% F(2) doping and remained almost constant on further doping to 7.3 mol. %. The absorption edge was considerably shifted to a lower wavelength (157.4 nm -->153 nm for a 5-mm-thick sample) by 1-mol. % doping and decreased only slightly on further doping. The intensities of the Raman bands that are due to three- and four-membered ring structures were significantly reduced by 1-mol. % F doping. These results strongly suggest that elimination of strained Si-O-Si bonds by F doping plays a central role in the improvement of radiation resistance of SiO(2) glasses to F(2) laser light.

F-doped and H2-impregnated synthetic SiO2 glasses for 157 nm optics

Optical transmission change and point defect formation by irradiation with F2 excimer laser light (∼10 mJ/cm2/pulse×3.6×105 pulses, λ=157 nm) were examined for four types of synthetic SiO2 glasses: wet (OH content: 1.7×1019 cm−3), dry (OH content: &amp;lt;1×1016 cm−3), F-doped (F content: 2.2×1020–4.2×1020 cm−3, OH content: &amp;lt;1×1016 cm−3), and H2-impregnated (OH content; 1.9×1018 cm−3, H2 content; 1×1018 cm−3) SiO2 glasses. Intense optical absorption bands peaking at 4.8 and 5.8 eV, which are attributed to nonbridging oxygen hole center and E′ center, respectively, and a shoulder at ∼7 eV were induced in the wet and dry specimens after the irradiation. On the other hand, the intensities of absorption bands induced in the F-doped and H2-impregnated specimens were smaller by an order of magnitude than those in other specimens. The internal (reflection-corrected) transmittance at the wavelength of 157 nm for 5 mm thick samples after the irradiation was 87% for the F-doped, 82% for the H2-impregnated, 14% for the wet, and 2.4% for the dry silica glasses. The present results show that F doping or H2 impregnation is an effective way to improve resistance of SiO2 glasses to F2 laser damage.

Effects of fluorine dimer excimer laser radiation on the optical transmission and defect formation of various types of synthetic SiO2 glasses

Changes in the optical absorption and the formation of point defects in three types of synthetic SiO2 glasses, wet (OH content; 120 ppm), dry (OH content &amp;lt;1 ppm), and fluorine (F)-doped (∼1 mol %) SiO2 glasses, by irradiation with fluorine dimer (F2) excimer laser light pulses (∼8 mJ/cm2/pulse×3.6×105 pulses) were examined by various spectroscopic methods. Intense optical absorptions were induced in the wet and dry silicas in the range of 4–8 eV, whereas the intensity of absorptions induced in the F-doped silica was smaller by an order of magnitude than that in the F-free glasses. The optical transmission at the wavelength of 157 nm after the irradiation was F-doped silica≫wet silica&amp;gt;dry silica. The dominant electron spin resonance-active defect in the irradiated specimens was the nonbridging oxygen-hole center (NBOHC) for the wet silica, or the E′ center in the dry silica. The concentration of NBOHCs or E′ centers in the F-doped silica was lower by an order of magnitude than that in the wet or dry silica. The present results suggest the possibility of using F-doped silica glasses as photomask materials for F2 laser lithography.