Achievement of homogeneous AuSn solder by pulsed laser-assisted deposition

The deposition of AuSn solder at the eutectic composition (80 wt.% Au, 20 wt.% Sn) on a wetted, chemically inert metallic barrier has been studied in relation to its use in optoelectronic packaging. The bonding structure, consisting of a W barrier, the top part of which is doped with Ni (or Ti) to provide wetting by molten AuSn, and the homogeneous 80-20 AuSn soldier several micrometers thick, has been grown by the Pulsed Laser-assisted Deposition (PLD) technique on 2{''} silicon wafers. The composition of the AuSn layer was controlled within better than 1 wt.% as probed by EDX across the wafer diameter. The molten solder exhibited good wetting properties on the W modified layer and the whole structure was found to be chemically stable against thermal cycling at 320 degrees C for over 3 min. The use of molten AuSn targets makes the PLD technique a most competitive one for the achievement of high quality and reliable AuSn solder.