Bond-Integrity Testing of Sapphire Chips Mounted with Eutectic Preforms

The 3000-5000 Å polysilicon film deposited on the back surface of a sapphire wafer during the polysilicon-gate deposition process is shown to provide a satisfactory bonding layer for eutectic mounting of silicon-on-sapphire (SOS) chips to gold-plated packages using Au-Si eutectic preforms, provided that the back-surface polysilicon film is maintained in the undoped state during subsequent wafer processing. N+doped polysilicon films provide less satisfactory bonding surfaces, passing all MIL-STD-883B environmental tests, but failing more severe tests which the undoped polysilicon films are shown to pass. Bare sapphire is shown to provide a surprisingly good bonding interface with Au-Si eutectic preforms giving MIL-STD-883B test results equivalent to the undoped polysilicon films and destructive-test results which were comparable to the n+ polysilicon films.