HIGHLY RELIABLE AU-SN EUTECTIC BONDING WITH BACKGROUND GAAS LSI CHIPS

GaAs large-scale integrated circuits (GaAs LSI) chips (5.40 mm by 5.18 mm) with ground back surfaces (Rmax = 0.1 - 1.0-mu-m) have been successfully bonded on alumina substrates using Au-Sn (80-20 wt%) eutectic alloy with well-controlled scrubbing action. The bonded chips which are ground to 450-mu-m thickness by our original technology were proved to be as highly reliable as polished ones through 1000 cycles of thermal shock between -65-degrees-C and + 150-degrees-C. No chip fracture occurred and no induced void was observed with scanning acoustic microscopy. The shear strength of the chips after thermal shocks remained at more than 10 kg, clearly passing the MIL-STD-883C test. Surface flaws due to backgrinding, which would cause chip fracture, were completely eliminated by the slight chemical etching after backgrinding. Scrubbing action has been confirmed to be necessary to obtain void-free bondings consistently in low-cost production. The Sn in the Au-Sn preform easily forms an oxide film at the surface, which tends to prevent wetting at the bonding interface. This tin oxide film (300-400 angstrom) was observed through Auger electron spectroscopy (AES) to be broken down by scrubbing action.