The adhesion of metallic films to laser-irradiated alumina

Abstract

The adhesion strength of copper and nickel films deposited onto polycrystalline alumina (Al2O3) substrates is greatly increased by pulsed ultraviolet excimer (XeCl, 308 nm) laser irradiation of the alumina prior to metal-film deposition. Adhesion enhancement occurs for pulsed-laser energy densities, Ed, both below and above alumina’s melting threshold (Edth∼0.7 J/cm2). Cross-section transmission electron micrographs of alumina irradiated at Ed≳ Edth reveal an outer amorphous layer 40–60 nm thick that is formed during the rapid solidification process that follows pulsed-laser melting. Our results for gold, copper, and nickel films demonstrate that several factors contribute to metal-alumina bonding: (1) the chemical nature of the metal, as indicated by quite different adhesion strengths of sputter-deposited metals on unirradiated alumina substrates: 0.1 MPa for Au, 13 MPa for Cu, and 32 MPa for Ni; (2) the type and extent of laser-generated disorder/damage, since for gold significant bonding enhancement is obtained only if pulsed-laser melting of the alumina occurs; and (3) the irradiation atmosphere, as the adhesion strength of these metals to alumina is greatest if pulsed-laser irradiation is performed in an oxidizing atmosphere.