Investigation of hole formation on Cr/Al, Si/Al, and C/Al bilayer films by laser beam irradiation

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Abstract
Transmission, replica, and scanning electron microscopy studies of the holes formed by laser irradiations on Cr/Al, Si/Al, and C/Al bilayer films were carried out. For Cr/Al and C/Al films, well-shaped circular holes were formed independently of the thickness ratio. In the case of Si/Al film, circular holes were formed only when the thickness ratio of Si was small. The peripheral regions of holes had several types of swell. In Cr/Al film there existed the steep swell at the hole edge and successively the gentle swell. The swells of C/Al film were rough in shape with thickness irregularities. Many thinned portions appeared in the swell of Si/Al having large thickness ratio of Si. The threshold power to form a hole was much lowered by coating the Al film with Cr, Si, or C, and the threshold value of 0.4 W for 60-nm-thick Al was reduced to about 0.04 W for 10 nm Cr/50 nm Al and 0.08 W for 20 nm Si/40 nm Al. The numerical analysis for temperature rise of the bilayer films was made using the finite difference method considering the phase transitions of the materials. The mechanisms of hole and swell formations in the bilayer films are also discussed.