A vacuum–UV F2 laser of 157 nm wavelength induced strong oxidation of 10-nm-thick Al thin films, forming transparent Al2O3 on silica glass. The laser-induced modification occurred at the surface of Al thin films; consequently, the thickness of the formed Al2O3 thin films increased linearly with increasing number of F2 laser photons. The formation of equivalent-phase Al2O3 thin films was confirmed by X-ray photoelectron spectroscopy. The oxidation reaction in the laser-induced modification of 10-nm-thick Al thin films was slower than that for 20- and 60-nm-thick Al thin films. Morphological changes leading to the crystallization of the Al2O3 thin films were also observed when the thickness of Al thin films increased from 10 to 20 and 60 nm.
A 157 nm F2 laser induced strong oxidation of an Al thin film surface, allowing it to show chemical resistance to KOH aqueous solution used for selective metallization on silica glass or native oxide Si substrate. The strong oxidation reactions on the surface and in the depth direction were confirmed by X-ray photoelectron spectroscopy. A high adhesion strength of 663 kgf/cm2 between Al and silica glass was also obtained for the F2-laser-irradiated sample, compared with that of the nonirradiated sample, 16 kgf/cm2. The suitable thickness of Al thin films for the F2-laser-irradiated surface and interface modifications was examined to be approximately 20 nm. The mechanism of the F2-laser-induced interface modification was discussed regarding the dependence of substrate material and the analyses of the chemical bonding state of silica glass underneath Al thin films.
Photochemical surface and interface modifications of Al thin films on silica glass were successfully carried out using a 157 nm F2 laser for micropatterning. The surface modification phenomenon was discussed in relation to by changing the laser wavelength using a 193 nm ArF laser or a 266 nm neodymium-doped yttrium aluminum garnet (Nd:YAG) laser. The ArF laser could induce the surface modification of Al thin films to form a protective Al2O3 layer resistant to KOH aqueous solution, similarly to the F2 laser. However, the mechanical hardness of the ArF-laser-irradiated sample was clearly lower than that of the F2-laser-irradiated sample. The origin of the surface modification was examined by irradiating the F2 laser in vacuum. The interface modification phenomenon was analyzed by X-ray photoelectron spectroscopy in the three cases. The adhesion strengths of the samples were also compared. The 266 nm Nd:YAG laser was not effective for the present photochemical modifications.
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