Contamination Control in Lead Bonding to Thick and Thin Films

Degraded wax residues from processing steps have been found to be a major source of contamination on beam lead devices for hybrid microcircuits. Two avenues of approach were used to solve bonding problems traced to this contamination: 1) a search was initiated for a more thermoxidatively stable and more easily removed wax, and 2) an effort was made to determine the level of cleanliness necessary for successful bonding and the most efficient method of attaining this level. A series of halocarbon waxes (polytrifluorochlorocthylenes) was found to be stable both chemically and thermally. The carbon contamination levels were decreased after improved solvent cleaning techniques and measured ~2 rim. This level compares with values as high as 9 nm after typical solvent cleaning. Plasma and ultraviolet cleaning both reduced carbon residues to 0.2-0.8 nm. Ball bonds made with weld forces of 1.0-1.5 N were successful on ultraviolet (UV) and plasma cleaned surfaces near 200°C and near 327°C for surfaces cleaned by improved solvent techniques. The beeswaxes presently used had adequate and better bondability than the more stable halocarbon waxes when only solvent cleaning was used and temperatures did not exceed 125°C to avoid charring.