Enhanced metal-ceramic adhesion by sequential sputter deposition and pulsed laser melting of copper films on sapphire substrates

Abstract

A study is presented of the effect of pulsed XeCl (308 nm) laser treatment on the adhesion between sputter-deposited copper films and sapphire substrates. Laser treatment (LT) of individual 80 nm thick copper films results in adhesion enhancement, relative to the assputtered film for XeCl energy densities >0.35 J cm⁻². Thicker (∼ 1μm), strongly adherent copper films can be built up by alternating discrete and sequential sputter deposition with pulsed laser irradiations carried out in air. This sequential process yields smooth films whose adherence, as measured by the scratch test, is a factor of more than two to three greater than for as-sputtered films. The only way to remove the copper layer after irradiation was by cutting through the sapphire. Although formation of a metal oxide is a common consequence of LT in air, adhesion tests reveal no significant effect of carrying out LT in oxidizing or reducing atmospheres. During the earliest stages of the sequential process, the laser-melted film tends to break into small clusters. It is concluded that this process is driven by a surface energy gradient generated by lateral thermal gradients in the melt. These gradients, in turn, are due to the early establishment of isolated regions of good bonding and thermal contact with the substrate. One of the characteristic features of the sequential process is that this good bonding, once established in a given region, is maintained throughout successive meltings of the region. Adhesion mechanisms under LT are discussed.