ArF Laser Light Research Articles

Synthesis of polymers with a norbornane backbone by radical copolymerization of alkyl 2-norbornene-2-carboxylates for photoresist applications

The radical copolymerization of alkyl 2-norbornene-2-carboxylates, 1a–c, with alkyl (meth)acrylates, to produce copolymers with norbornane in the main chain, is described here. γ-Butyrolactone- and hydroxy group-containing norbornene-based monomers (1b and 1c, respectively) were freshly synthesized, and their radical copolymerization behavior with n-butyl acrylate was examined. Methyl 2-norbornene-2-carboxylate, 1a, and the new monomers, 1b and 1c, were employed as comonomers for radical terpolymerization with lactone- and adamantane-containing (meth)acrylates to produce copolymers that could be applied as new, more chemically robust 193-nm photoresist materials. The dissolution rate of the spin-coated copolymer films after exposure to a photoacid generator, a significant characteristic in the photoresist application, was measured for various exposure doses of 193-nm ArF laser light under standard conditions in an aqueous alkaline solution.

Onset of the optical damage in CaF_2 optics caused by deep-UV lasers

The exterior sides of calcium fluoride (CaF(2)) outcoupling mirrors are damaged by ArF laser light irradiation with high pulse-energy densities (80 mJ/cm(2)). The damage is generated by a partial alteration of the CaF(2) substrate to calcite. The CaF(2) decomposition is driven by photochemical processes due to the UV light and the presence of water vapor and is supported by elevated temperatures within the laser beam transmitting area. Small filaments act as starting points for the decomposition process, where kerogenous carbon and calcite can occur.

Recombination luminescence of oxygen-deficient centers in silica

The luminescence of silica glass, prepared by plasma chemical vapor deposition (PCVD) and quartz glass of type IV (trade mark KS-4V) methods, were studied while irradiated with pulses of ArF laser (193nm) light in the range of sample temperatures between 10 and 300K. The samples contain less than 0.1ppm metallic and hydroxyl impurities. The samples synthesized by PCVD were of two kinds. The first one (amorphous) was as-deposited from plasma at a substrate tube temperature of ∼1200°C. The second one (fused) was prepared from the first by the tube collapsing with an external burner. In this process, a section of the substrate tube with the deposited glass was installed in a lathe and processed at a temperature of ∼2100°C during ∼20min until the tube was transformed to a rod. After such processing, the rod was cooled down to room temperature in air at an average rate of about 400°C per min. The only observed luminescence possesses two broad bands, with not well defined position, one at 2.6–2.9eV (a blue band) and another in the range of 4.4eV (an UV band). There is a correspondence in luminescence properties between KS-4V silica and fused PCVD silica. Those bands have been attributed to oxygen deficient centers (ODC). No luminescence is observed in amorphous PCVD silica under irradiation with 193nm laser light. So, formation of the sample by melting at least stimulates formation of ODCs at 193nm. The blue band decays obeys to power law ∼t−1 and is detected in the range of time 10ns to 300μs. The UV band possesses a fast, practically repeating excitation pulse, and a slow component (∼30μs). The obtained new kinetics data are compared with known in literature for lone twofold-coordinated silicon having exponential decay for the blue band equal to 10ms and 4.5ns for the UV band. That shows the blue band of new studied samples under ArF laser possesses decay component faster and the UV band slower than that of the twofold-coordinated silicon center. This corresponds to the recombination process of luminescence excitation by laser. We propose a model of the processes as charge separation under excitation with creation of a nearest self-trapped hole and electron trapped on the twofold-coordinated silicon, modified by its surrounding atoms or ions. This pair is recombining then with luminescence.

VUV-photon Induced Formation of Hydrophilic and Hydrophobic Micro Domains Structure on Intraocular Lens Surface for Blocking after Cataract

ABSTRACTA micro domain structure, which hydrophilic and hydrophobic groups were arrayed alternately, was formed on the surface of an intraocular lens [IOL] by using ultra violet rays [VUV]. With this technique, the IOL that is free from fibrin has been developed. In order to substitute the hydrophilic groups in matrix-form on the surface, an ArF laser light was then irradiated on the hydrophobic surface in the presence of water for the −OH groups, through the 50-micrometer dot-patterned reticle. With this selective photochemical surface modification, the hydrophilic and hydrophobic groups were arrayed alternately on the sample surface. The modified IOL was soaked in 0.1-wt % fibrin solutions, and the fibrin-sticking rate was measured by using an infrared spectroscopy [FT-IR]. It results that the absorption coefficient of amide band reached to 0.0006, decreasing to one-six of non-treatment sample's.

Numerical Analytic Prediction and Experimental Validation of Focused ArF Laser Pulse Absorption Process in Air

In order to understand the single-photon absorption process of focused ArF laser pulse in air, numerical simulations and spectroscopic experiments were carried out. The partial saturation of the laser spectrum and the depopulation of oxygen molecule at high vibrational levels in the ground state during laser emission was taken into consideration in the numerical simulation (relaxation-free model). The analytical results showed that these two effects caused the substantial decrease in the absorption coefficient, and the comparison of absorbance between the simulations and the measurement results revealed that analytical results with relaxation-free model were in better agreement than the results with the thermal equilibrium model, in which the depopulation effect was not considered. However, in the high temperature condition, the difference in the transmitted energy between experimental and analytical results was larger. It was found that the experimental ArF laser spectrum has a self-absorption region, which was not considered in the simulations. The difference in transmission due to these effects is likely to be enhanced at high temperature. The absorption spectra calculated with the relaxation-free model and the thermal equilibrium model were compared with the measured spectra in longer wavelength beyond the region influenced by the self-absorption. Better agreement was found for the relaxation-free model, leading to the conclusion that the spectral partial saturation and the depopulation of oxygen molecule were important to understand the absorption process of pulsed ArF laser light in air.

VUV and IR absorption spectra induced in H2-loaded and UV hyper-sensitized standard germanosilicate preform plates through exposure to ArF laser light

H2-loaded Ge-doped preform plates have been UV hyper-sensitized and subsequently post-exposed by means of UV ArF laser pulses at 193nm. Both Fourier Transform Infrared (FTIR) and Vacuum Ultraviolet (VUV) absorption spectroscopy has been carried out at each step of the sensitization process with a view to get a better understanding of the UV hyper-sensitization process at 193nm. Exposing the H2-loaded samples firstly triggers a partial bleaching of the 5eV absorption band, followed by a new growth of the band for longer exposure time. As it has already been reported for exposure at 248nm, the exposure at 193nm firstly yields an increase in the absorption ascribed to hydroxyl (≈3600cm−1), hydride species (≈2140–2185cm−1) and GeE′ defects (≈6.3eV). The evolution of the absorption related to the GeH2 species is not monotonous but rather the absorption reaches a peak (Npre=2000) and then slightly decreases for further exposure time. Post-exposing the hyper-sensitized samples (Npre=2000) lead to a partial bleaching of the absorption ascribed to hydride species and to increases in the intensity of the GeE′ defect-related absorption and in the absorption ascribed to hydroxyl species. The accurate determination of the species concentration from the absorption spectra proved to be rather tricky due to the problem of accurately measuring the depth of the UV beam penetration at each time of the exposure. Nevertheless, a correlation could be established between the growth of the UV-induced UV excess loss ascribed to GeE′ species and the UV-induced decrease in the intensity of the IR bands related to hydride species. These observations are discussed within the frame-work of the two step model.

Copper Pattern Formation on Fluorocarbon Film by Single Shot of ArF Laser

ABSTRACTCopper nuclei have been photo-chemically patterned on the fluorocarbon film surface in copper sulfate atmosphere via the reticle with just one shot of the ArF laser. Previously, we have reported that it required more than 3, 000 shots of the ArF laser to substitute the Cu atoms on the fluorocarbon surface.Fluorocarbon is regarded as promising as a printed wiring board material in the high- frequency band. However, the fluorocarbon is chemically stable making it difficult to bond to copper foil. Generally, a copper foil is formed on the fluorocarbon surface by electroless plating with a catalyst core. In this method, however, the surface is made rough in pretreatment, which impairs its characteristics. Using the catalysts also causes differences in the dielectric constant, generating a high frequency noise. Thus, we demonstrated the direct formation of copper nuclei on the fluorocarbon surface photo-chemically by using the Xe2 excimer lamp and the ArF excimer laser.The sample surface was first irradiated by Xe2 excimer lamplight to ma ke it hydrophilic. The surface was then irradiated with circuit patterned ArF laser light in the presence of copper-sulfate (CuSO4) aqueous solution to substitute the Cu atom. The modified sample was immersed in the electroless plating solution at 60 degrees Celsius for 15 minutes, and the copper foil was locally formed on the areas exposed to light.

Effect of F 2 laser power on defect formation in high-purity SiO 2 glass

Laser power dependence of defect formation in synthetic SiO 2 glass by F 2 laser irradiation was measured. Concentration of the E ′ center dissociated from the strained Si–O–Si bond was almost proportional to the F 2 laser power less than ∼10 mJ cm −2 pulse −1 (∼0.5 MW cm −2). However, it increased as nearly square of the F 2 laser power above the threshold. Quantum yield of formation of the E ′ center by two-photon absorption of F 2 laser light was ∼3 orders of magnitude larger than that of the KrF or ArF laser light, and we suggest that two-step absorption of F 2 laser photons via real intermediate states dominates the formation of the E ′ center.

Time-resolved photoluminescence study of silica nanoparticles as compared to bulk type-III fused silica

Photoluminescence (PL) from composites of 7- and 15-nm silica (amorphous ${\mathrm{SiO}}_{2})$ nanoparticles and bulk type-III fused silica induced by two-photon band-to-band excitation with 193-nm (6.4-eV) ArF laser light has been measured in time-resolved detection mode. The PL spectra taken for 15-nm particles allow one to identify three PL bands peaked in the red (\ensuremath{\sim}1.9 eV), green (\ensuremath{\sim}2.35 eV), and blue (\ensuremath{\sim}2.85 eV) spectral ranges. The green and blue bands are normally overlapped in conventional scan measurements, giving no way for determining their exact peak positions. Similar red and green PL bands were observed for 7-nm particles, whereas the blue band extends toward the higher-energy range and is peaked at \ensuremath{\sim}3.25 eV. The aforementioned red, green, and blue PL bands are assigned to nonbridging oxygen hole centers, hydrogen-related species, and self-trapped excitons (STE's), respectively. The red and green PL bands for bulk type-III fused silica are peaked at practically the same spectral positions as those for nanoscale silicas, indicating the similarity of light-emitter types. However, the blue band for bulk silica used is peaked at \ensuremath{\sim}2.75 eV, that is, at typical position for STE's in crystalline ${\mathrm{SiO}}_{2}.$ The blueshift of STE (PL) (STEPL) band with decreasing nanoparticle size is consistent with the previously proposed model of phonon-assisted radiative relaxation of STE's [Yu. D. Glinka et al., Phys. Rev. B 64, 085421 (2001)]. As a result of time-resolved measurements, the extremely broad PL band peaked at \ensuremath{\sim}2.35 eV, which is typically observed for bulk silicas and initially assigned to STE's, is found to consist of two bands peaked at \ensuremath{\sim}2.75 and \ensuremath{\sim}2.35 eV. We suggest that these bands are due to the radiative deexcitation of STE's and hydrogen-related centers, respectively. We thus conclude that the STEPL band is peaked at \ensuremath{\sim}2.75 eV for both bulk amorphous and crystalline ${\mathrm{SiO}}_{2}.$

パルスレーザ照射によるMEMS用Si酸化物カンチレバーのスティクション解消

Recovery of stiction-failed micro-cantilevers with excimer laser irradiation has been attempted on silicon oxide cantilevers. Micro-cantilevers with various lengths and widths have been fabricated from thermo-oxidized silicon with micromachining process. ArF laser (λ=193nm) and KrF laser (λ=248nm) were used for experiments. Laser stiction recovery is possible on micro-cantilevers made of silicon oxide. Recovery yield is dependent on laser fluence and wavelength. ArF laser light, which is well absorbed by silicon oxide, can recover stiction-failed cantilevers more effectively. Adhesion and stiction-recovery do not depend on the width of cantilever. The results are compared with previous data of silicon cantilevers. Using the model that considers the sum of elastic force and thermoelastic force as restoring force, the free standing yield after laser irradiation is calculated. The theoretical results exhibit qualitative agreement with the experimental data of both silicon oxide and silicon cantilevers. This indicates that the thermoelastic force plays an important role in stiction recovery, and its temporal profile may have an influence on the recovery yield.

Single Shot of Arf Laser-Induced Photochemical Nucleation of Copper on Polyethylene-Terephthalate Surface

ABSTRACTCopper nuclei was grown on a PET surface in the copper sulfate water solution ambience with only one shot of ArF laser.· General electronic circuit printed boards have been made by sticking the copper foils with adhesive agents after having made the foil surface rough. Consequently, even the printed board of low conductivity might generate a high frequency noise due to the difference between the copper foils and adhesive agents. If atoms composing plastics and metal could be combined by a chemical method instead of the conventional physical methods, a hybrid material made of both plastics and metal would be produced.We have reported for the substitution of copper atoms on an all-aromatic-polyimide surface by a single shot of ArF laser irradiation In this study, with one single shot of ArF laser, we succeeded in growing the Cu nuclei on the PET film whose ultraviolet absorption is lower than all-aromatic-polyimide. The sample surface was photo-oxidized with the Xe2 excimer lamp in pre-treatment. A fused silica glass was placed on the sample surface, and the sulfate water solution was poured into the gap between the glass and the sample to form a thin liquid layer. Then, the sample was vertically irradiated once by the circuit patterned ArF laser light with the laser fluence of 28mJ/cm2. The photo-dissociated copper atom formed the C-O-Cu bond with active oxygen to sub stitute on the PET surface. The modified sample was, then, immersed in the electro less plating solution at 60 degrees Celsius for 15 minutes; the copper thin film of about 24mm was grown on the modified surface.

Size effect in self-trapped exciton photoluminescence fromSiO2-based nanoscale materials

Direct evidence for a size effect in self-trapped exciton (STE) photoluminescence (PL) from silica-based nanoscale materials as compared with bulk type-III fused silica is obtained. Two kinds of mesostructures were tested: (1) silica nanoparticle composites with primary particle size of 7 and 15 nm, (2) ordered and disordered mesoporous silicas with pore size ranging from \ensuremath{\sim}2 to \ensuremath{\sim}6 nm and wall thickness \ensuremath{\sim}1 nm. The PL was induced by the two-photon absorption of focused 6.4 eV ArF laser light with intensity $\ensuremath{\sim}{10}^{6}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and measured in a time-resolved detection mode. Two models are applied to examine the blue shift of the STE PL (STEPL) band with decreasing size of nanometer-sized silica fragments. The first model is based on the quantum confinement effect on Mott-Wannier-type excitons developed for semiconductor nanoscale materials. However, the use of this model leads to a contradiction showing that the model is completely unusable in the case of wide-band-gap nanoscale materials (the band-gap of bulk silica ${E}_{g}\ensuremath{\cong}11\mathrm{eV}).$ In order to explain the experimental data, we propose a model that takes into account the laser heating of Frenkel-type free excitons (FE's). The heating effect is assumed to be due to the FE collisions with the boundary of nanometer-sized silica fragments in the presence of an intense laser field. According to the model, laser heating of FE's up to the temperature in excess of the activation energy required for the self-trapping give rise to the extremely hot STE's. Because the resulting temperature of the STE's is much higher than the lattice temperature, the cooling of STE's is dominated by the emission of lattice phonons. However, if the STE temperature comes into equilibrium with the lattice temperature, the absorption of lattice phonons becomes possible. As a result, the blue shift of the STEPL band is suggested to originate from the activation of hot (phonon-assisted) electronic transitions. Good agreement between experimental data and our FE laser heating model has been obtained.

Laser cleaning of polymer surfaces

We have investigated the removal of small spher- ical particles from polymer surfaces by means of 193-nm ArF and 248-nm KrF laser light. Polystyrene (PS) particles with diameters in the range of 110 nm to 1700 nm and silica particles (SiO2) with sizes of 400 nm and 800 nm are success- fully removed from two different substrates, polyimide (PI) and polymethylmethacrylate (PMMA). Experiments were performed in air (23 ◦ C, relative humidity 24%-28%) and in an environment with a relative humidity (RH) of about 90%.

Photochemical Nucleation of Copper on Polyimide Surface with 10ns Laser Irradiation - Growth of Copper Metal on Copper Nuclei and Deposition of SiO2 film on Copper Metal

AbstractCopper atoms were substituted on the polyimide surface in the presence of CuSO4 solution with only 10ns single shot of ArF laser. Then, we have succeeded in substituting copper atoms with dangling bonds of carbon cross- linking with oxygen atoms. In this study, we placed fused silica glass on the polyimide surface firstly, and poured the sulfate solution into the gap between the silica glass and the film, forming a thin liquid layer. Then, one shot of circuit patterned ArF laser light having 42mJ/cm2 was irradiated vertically onto the sample. The photon is dissociating and exciting the copper sulfate solution and the polyimide surface. The dissociated copper atoms form the C-O-Cu bond with active oxygen on the polyimide surface. However, we were able to grow a copper thin film of about 24μm after immersing the sample into the electro less plating solution for 15 minutes at 60.On the other hand, we have laminated transparent and low dielectric constant SiO2 film at room temperature on polyimide surface by using Xe2 excimer lamp. Then, the copper film deposited on polyimide film and the piece of silicon were placed in the reaction chamber with mixing NF3 and O2 gasses in the ratio of 10:1; total pressure 330 Torr. Then, Xe2 excimer lamp was irradiated for 20 minutes. By the photochemical reaction, SiF4 and NO2 were produced. The adsorbed SiF4 on copper and polyimide surface of one molecule layer and NO2 up react with NO2, and growth SiO2 film on the sample surface. A chain reaction of film formation is brought about by these two chemical reactions of adsorption and oxidization. By this film formation film thickness was not more than 600 after 20 minutes with the Xe2 excimer lamp irradiation in order to lose in SiF4. Then, the silicon was supplied, and insufficient SiF4 was filled by ablating the silicon wafer by focused ArF excimer laser beam.

Photoluminescence Spectroscopy of Silica-Based Mesoporous Materials

The photoluminescence (PL) at 300 and 90 K from the mesoporous silicas (MSs) [ordered (MCM-41) and disordered (DMSs)] of variously sized pores induced either by ArF laser light (λexc = 193 nm) or by Nd:YAG (yttrium−aluminum−garnet) laser light (λexc = 266 nm) was studied. The spectra were measured in both conventional and time-resolved detection modes. It has been shown that PL induced by 266-nm light results from a direct one-photon excitation of hydrogen-related species (the green band) situated on the internal pore surfaces and nonbridging oxygen hole centers (NBOHCs, the red bands) located both on the surface and inside ∼1-nm sized bulk fragments (pore walls). Alternatively, an indirect excitation of the light emitters due to the energy transfer through two-photon-produced free excitons (FEs) is responsible for PL in the case of 193-nm excitation. In addition, the self-trapping of FEs followed by radiative relaxation results in the blue band, which can only be induced by 193-nm light. The PL propertie...

Two-photon-excited luminescence and defect formation in SiO2 nanoparticles induced by 6.4-eV ArF laser light

The photoluminescence (PL) from 7- and 15-nm silica $({\mathrm{SiO}}_{2})$ nanoparticles induced both by ArF laser light $[{\ensuremath{\lambda}}_{\mathrm{exc}}=193\mathrm{nm}(6.4\mathrm{eV}),$ ${\ensuremath{\tau}}_{L}=15\mathrm{ns}]$ and by Nd:YAG (yttrium-aluminum-garnet) laser light $[{\ensuremath{\lambda}}_{\mathrm{exc}}=266\mathrm{nm}(4.66\mathrm{eV}),$ ${\ensuremath{\tau}}_{L}=8\mathrm{ns}]$ was studied. The laser light intensity dependencies of the PL yields reveal the two-photon (TP) process of the PL excitation in the case of ArF laser light. The PL results from the radiative relaxation of self-trapped excitons (STE- the blue band), also from the surface hydrogen-related species (the green band), and the bulk nonbridging oxygen hole centers (NBOHC's- the red band) excited by a radiationless relaxation of TP-produced free excitons (FE's). The main point is focused on the effect of the nanoparticle surface condition on the FE dynamics. The dynamics includes either an elastic scattering or quenching by the nanoparticle boundary, the laser heating of FE's up to energies in excess of the STE barrier, the FE energy transfer to the surface and bulk NBOHC's and hydrogen-related centers, the saturation of the FE density, and the biexciton process in the formation of Frenkel defects with their subsequent transformation into NBOHC's.

High-precision in situ oxygen isotope analysis of quartz using an ArF laser

High precision in situ oxygen isotope analyses of quartz using an ArF laser are described. The high precision is documented by comparing in situ ArF laser data with CO 2 laser and conventional analyses of silica and quartz. In contrast to the CO 2 and Nd:YAG laser, no oxygen isotope fractionation occurs during in situ ablation of quartz with high power ultra-violet ArF laser light pulses. Therefore, high oxygen yields are no prerequisite for accurate and precise δ 18O measurements. No data correction are necessary when analyzing quartz. The high spatial resolution and in situ capabilities of the ArF laser technique are demonstrated by analyzing hydrothermally precipitated quartz, which shows a rhythmic growth history. A model is also presented that explains the behavior of quartz during ablation with ultra-violet laser.

Generation of optical absorption bands in CaF2 single crystals by ArF excimer laser irradiation: Effect of yttrium impurity

Generation of optical absorption bands was observed in CaF2 single crystals with different Y impurity concentrations, excimer grade (yttrium impurity; <1 ppm) and UV grade (5 ppm), by irradiation with ArF excimer laser light at room temperature. The transmittance at the wavelength of 193 nm gradually decreased from 100% down to a saturation level of ∼95% for the UV grade specimen during the ArF excimer laser irradiation. This degradation process was found to obey first order kinetics. On the other hand, no significant decrease in transmittance was observed for the excimer grade specimen. “Four-band structure” which was a characteristic of Y-associated F centers (YFCs) and a band peaking at ∼6.6 eV were observed in the spectrum of the irradiated UV grade CaF2. The 6.6 eV band, which substantially controlled transmission loss at the wavelength of 193 nm and was generated via one photon process, was tentatively attributed to YFCs from growth kinetics on the laser irradiation and decay kinetics upon annealing. Neither intrinsic F centers nor H centers were observed by the ArF laser light irradiation at room temperature. These results demonstrated that a trace amount of Y impurity enhanced the formation of F centers perturbed by a nearest Y impurity and that an absorption band of the resulting YFCs controlled the transmission loss at the wavelength of 193 nm.

Photo-Chemical Etching on Silicon-Carbide by Using Krf Excimer Laser and Xe2* Excimer Lamp

ABSTRACTSilicon-carbide (SiC) has excellent refractivity in the range of soft X-ray and is well-used as a diffraction grating for Synchrotron-radiation (SR) light. This material has a high melting point, hardness and chemical stability. Therefore, etching of the material by chemical or physical methods is very difficult. We reported a photo-chemical etching method in which a SiC surface is placed in NF3 gas atmosphere and irradiated by the Xe2* excimer lamp light parallelly and the grating patterned KrF laser light of 248nm perpendicularly on the sample surface. The Xe2* excimer lamp light are employed for NF3 gas decomposition, and KrF laser light used for excitation on the sample surface. This photochemical etching reaction are detected by XPS, QMS and FTIR measurements. This method achieved 0.18 Å/shot in etching efficiency, and became maximum approximately 7 times as high as ArF laser light for photodecomposition.

Radiation-induced coloring and paramagnetic centers in synthetic SiO2:Al glasses

Optical absorption bands and paramagnetic centers induced with γ-rays or excimer laser lights were examined in high purity synthetic SiO2:Al glasses prepared by VAD method. Gamma-induced absorptions ranging 1.5–6.2 eV were deconvoluted into 5-Gaussian components peaking at 2.3, 3.2, 4.1, 4.9 and 5.8 eV. Two paramagnetic EPR centers associated with an Al ion were observed, Al-OHC and Al E' center and both centers are close to each other. These results indicate that a hole and an electron generated by irradiation are predominantly trapped at an oxygen bridging an Al and a Si and an Al coordinated with only three oxygens, respectively. A close correlation between 5-optical bands except the 5.8-eV band and Al-OHC/Al E′-center was obtained. It was assigned from a consideration of the nature of these Al-related centers that a band peaking at 2.3 eV are due to Al-OHC and Al E' centers give a band peaking at 4.1 eV. Oscillator strength was evaluated to be 0.06 for the 2.3 eV-band or 0.2 for the 4.1 eV-band. Although no coloring and paramagnetic centers were noted for XeCl laser light (304 nm), the results for ArF laser light (193 nm) were almost the same as the case of γ-irradiation. This difference suggests that a pair generation of Al-OHC and Al E'-center occurs via two photon process.