F2 Laser Irradiation Research Articles

Initial stage of laser ablation of LiCaAlF6 single crystal under F2 laser irradiation

Initial stage of F2 laser ablation of LiCaAlF6 single crystal was investigated for clarifying the possibility of applying this wide bandgap fluoride crystal to vacuum ultraviolet (VUV) optical components. The ablation threshold, determined by the appearance of line emission from ablated species, was approximately 2 J cm-2pulse-1, similar to that of VUV grade CaF2 single crystal. The laser-induced damage on the front surface of LiCaAlF6 was faint, though adhesion of aggregated particulates of several microns was observed.

Swelling and modification of silicone surface by F2 laser irradiation

The surface of silicone rubber swelled and was modified by 157-nm F2 laser irradiation at a laser fluence less than the ablation threshold. The irradiated surface swelled to a height of approximately 3 μm. Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy showed that the irradiated surface was modified to SiO2. 193-nm ArF laser irradiation of the silicone rubber induced the surface to swell, but not to modify to SiO2. The IR peaks of end groups of silicone were observed in the FT-IR spectra of the surface. From these results, it is concluded that the main chains (Si-O) of silicone were photodissociated and generation of low molecular weight silicones caused the swelling. In addition, it was observed that methane and carbon dioxide were released from silicone rubber when each laser beam irradiated it. These gases were generated by photodissociation of the side chains (Si-CH3) of silicone. An F2 laser beam can photodissociate the Si-CH3 bonds and the Si-O bonds of silicone and O2 effectively even at a laser fluence less than the ablation threshold, resulting in the modification to SiO2 and the swelling.

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.

SiO2 Humps Formed on Silicone Rubber by F2 Laser Irradiation

An unexpected finding of SiO2 humps on silicone rubber fabricated by a 157 nm F2 laser at a laser fluence less than the ablation threshold was made. The fabrication of the humps which progressed by increasing the number of laser pulses was examined by measuring the height and surface roughness of the humps and by Fourier transform infrared spectroscopy (FT-IR). The FT-IR spectra showed that the position of the Si–O peak of the fabricated hump agreed with that of a synthetic fused silica. The chemical composition of the entire hump was also confirmed to be SiO2 by X-ray photoelectron spectroscopy (XPS). The SiO2 hump could be removed completely by 1 wt% hydrofluoric acid. This finding resulted from swelling of the silicone surface and the subsequent photodissociation of Si–CH3 from the swelled silicone. We could not fabricate the SiO2 humps by a 193 nm ArF laser.

Metrology of pulsed radiation for 157-nm lithography.

In the framework of current development in 157-nm lithography we have investigated the performance of photodetectors with emphasis to their stability and linearity. The measurements were performed in the radiometry laboratories of the Physikalisch-Technische Bundesanstalt at the Berlin electron-storage rings BESSY I and BESSY II with spectrally dispersed synchrotron radiation as well as with highly pulsed F2 laser radiation at 157 nm in combination with a cryogenic radiometer as the primary detector standard. Relative standard uncertainties of as little as 1% were achieved for the calibration of photodetectors in the spectral range of ultraviolet and vacuum-ultraviolet radiation.

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.

Influence of laser fluence and irradiation timing of F 2 laser on ablation properties of fused silica in F 2 -KrF excimer laser multi-wavelength excitation process

A collinear irradiation system of F2 and KrF excimer lasers for high-quality and high-efficiency ablation of hard materials by the F2 and KrF excimer lasers’ multi-wavelength excitation process has been developed. This system achieves well-defined micropatterning of fused silica with little thermal influence and little debris deposition. In addition, the dependence of ablation rate on various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number is examined to investigate the role of the F2 laser in this process. The multi-wavelength excitation effect is strongly affected by the irradiation timing, and an extremely high ablation rate of over 30 nm/pulse is obtained between -10 ns and 10 ns of the delay time of F2 laser irradiation. The KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing F2 laser fluence. Moreover, the ablation rate shows a linear increase with the logarithm of KrF excimer laser fluence when the F2 laser is simultaneously irradiated, while single KrF excimer laser ablation shows a nonlinear increase. The ablation mechanism is discussed based on these results.

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.

Formation and decay of nonbridging oxygen hole centers in SiO2 glasses induced by F2 laser irradiation: In situ observation using a pump and probe technique

Formation and decay of nonbridging oxygen hole centers (NBOHC, an oxygen dangling bond) in SiO2 glasses by F2 excimer laser (7.9 eV) irradiation were in situ analyzed by monitoring 1.9 eV photoluminescence of NBOHC using a pump and probe technique. In wet SiO2, the SiO–H bond was efficiently photolyzed by F2 laser photons to form NBOHC with a quantum yield of ∼0.2. However, the recombination with dissociated hydrogenous species suppressed the buildup of NBOHC. In dry SiO2, in contrast, NBOHC formation by dissociation of strained Si–O–Si bonds was inefficient but NBOHC accumulated with the number of F2 pulses due to a negligibly slow reverse recombination reaction.

Modification of sol–gel-derived amorphous Al2O3 thin films by F2 excimer laser irradiation at ambient temperature

F2 excimer laser (157 nm) irradiation was applied to modify sol–gel-derived amorphous Al2O3 thin films at ambient temperature. The surface morphology and density of the film were significantly altered by the laser irradiation (power: 2 mW/cm2/pulse). The surface properties of the film were also changed from hydrophilic to hydrophobic. These alterations were not observed when using ArF excimer laser (193 nm) irradiation at the same laser power as that of the F2 laser. It was found that the changes induced by F2 laser irradiation mainly arose from the direct photoexcitation of C = O groups in ethylacetoacetate, which was added as a chelating agent of aluminum-alkoxides. Consequently, photochemical reactions of the Norrish-type occur, resulting in the formation of hydrocarbon or olefin and the elimination of carbon monoxide (CO) or decomposition products. The elimination of CO is considered to cause the marked change in structure and surface properties of the film. Patterning of the gel films was successfully performed by using these findings.

Defect formation and structural alternation in modified SiO2 glasses by irradiation with F2 laser or ArF excimer laser

The formation and restoration of defects by F2 laser irradiation with high laser fluence were investigated for modified silica glasses, and the results were compared with those by ArF excimer laser irradiation. F2 laser irradiation induced oxygen deficient centers (ODCs) and E′ centers via one-photon-absorption processes, while ODC and E′ defects are generated by two-photon-absorption processes by an ArF excimer laser. As-doped SiOHs and photoinduced SiOHs enhanced the formation of defects markedly in the case of F2 laser irradiation. F2 laser light transformed isolated SiOH bonds into hydrogen-bonded SiOHs, while such a process did not occur under ArF excimer laser light. These results suggest that silica glass networks were dissociated by two types of processes. The dominant process is the formation of pairs of E′ centers and NBOHCs, followed by conversion to the SiHs and SiOHs as a result of chemical reactions with hydrogen molecules in silica glass at room temperature. The other is the generation of ODC defects accompanied by interstitial oxygen molecules, which are also decomposed partly into E′ centers with the aid of F2 laser light.

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.

Rapid long-period grating formation in hydrogen-loaded fibre with 157 nm F2-laser radiation

Long-period gratings were fabricated in hydrogen-loaded telecommunication fibre using 157 nm F/sub 2/-laser radiation. A >20 dB attenuation peak at 1590 nm was induced with only 5.0 J/cm/sup 2/ accumulated fluence, a value >250-fold smaller in comparison with fluence requirements for 248 nm KrF-laser irradiation.

Ozone production in oxygen by means of F2-laser irradiation at ?=157.6 nm

Ozone is generated in pure oxygen (p≦5 kPa), synthetic air (p≦7 kPa) and oxygen-argon mixtures (p≦3 kPa) by irradiation of these gases with the VUV light of a repetitively pulsed (fL≦15 Hz) F2-laser at λ=157.6 nm with maximum about 4 mJ/pulse. An absorption photometer measurement operating at 253.7 nm (Hg line) determines the ozone concentration as a function of oxygen and/or additive gas pressure, the repetition frequency of the laser and the wall temperature of the reaction chamber. The temporal development of the ozone concentration as a function of these parameters is calculated by means of rate equations for the species O(3P), O2(X3∑g−), O3(1A1), O(1D), O2(a1Δg), O2(b1∑g+) and vibrationally excited O3*(1A1) and the photon distribution. The maximum concentration of O3 in the sealed-off chamber reaches 1.6% in pure O2, 4.1% in air and 1.2% in a 1:5 O2-Ar mixture at 3 kPa. The annihilation of O3 by the wall and temperature dependent volume processes (300 K≦T≦395 K) is studied and the experimental and theoretical results are compared.