Mechanisms of laser activation of dielectric materials

Abstract

Aluminum Nitride (AlN), Alumina, Sapphire, Magnesium Oxide (MgO), Silica and Polymide can be laser activated. The irradiation promotes near‐surface modifications that catalyze the deposition of copper when the substrates are immersed in an electroless solution. All the ceramics that exhibit laser activation have a very low absorption coefficient or are transparent to ultraviolet radiation. The damage generated during laser irradiation plays a strong role in enhancing the laser‐dielectric interaction in alumina, magnesium oxide and silica. On the other hand, after the first irradiation the interaction between the aluminum nitride and the UV radiation is controlled by the metallic aluminum on the surface. A thin layer of metallic aluminum is produced in alumina and in sapphire by laser irradiation in an Ar‐4% H2 atmosphere. This film is not present if the irradiations are conducted in air. XPS analyses of alumina and sapphire substrates show no significant changes in the positions of the aluminum and oxygen peaks after irradiation. However, the electronic structure of the very near surface region in alumina and sapphire is modified by laser irradiation. These changes are probably due to random and small modifications in the degree of oxidation of Al. The damage detected by RBS in both polished and irradiated sapphire extends to a depth of at least 400 nm. As received, polished specimens are much more heavily damaged than the irradiated ones. The kinetics of electroless copper deposition is considerably faster in the presence of the metallic film produced by laser irradiation than in the cases where this film is absent. Although the damages detected by XPS and by RBS are similar in irradiated and in mechanically polished specimens, no surface activation is observed in the latter.