Adhesion of thin films of evaporated titanium–copper after electroplating

Abstract

Titanium–palladium–gold is a highly reliable multilayer metallization for thin-film integrated circuits. Efforts to reduce the amount of Au and hence the cost of the circuits have led to development of a Ti–Cu–Ni–Au metallization. This system is fabricated by selectively electroplating Cu–Ni–Au onto evaporated Ti–Cu. During the introduction of the new metallization into manufacture, it was observed that evaporated Cu frequently delaminated from evaporated Ti after Cu–Ni–Au plating. The factors investigated were the nature of the substrate surface, residual gas pressure during evaporation of the Ti and/or Cu, thickness of the evaporated Cu, various combinations of plated films and postplating heat treatment. Adherence was evaluated by a tape test, thermocompression bonding and pull testing of leads, and by a simple substrate break-and-peel test. In-depth profiles of the evaporated TiCu interface were obtained by sputter removal and Auger electron spectroscopy techniques. Oxidation of the evaporated Ti layer at pressures greater than 2.7×10−4 Pa prior to evaporation of the Cu layer was found to be a significant factor leading to subsequent delamination after electroplating. Solutions to the delamination problem include optimization of the evaporation equipment, use of a thin layer (∠150 Å) of Pd between the Ti and Cu, elimination of a distinct interface between the Ti and Cu by coevaporation techniques to produce a graded film, or sputtering of the films rather than evaporation.