HIGH BOND-ENERGY AND THERMOMECHANICAL STRESS IN SILICON-ON-SAPPHIRE WAFER BONDING

Silicon on sapphire wafer pairs are formed by direct wafer bonding of 3-in. silicon and sapphire wafers. Subsequent annealing commonly used to increase the bond energy imposes serious thermomechanical strain. T he corresponding bending, recorded in situ as a function of temperatur e, reveals relaxations by de- and rebonding until the silicon wafer cr acks into small fragments that mostly remain bonded. After further ann ealing up to 800 degrees C and cooling to room temperature, a strong c urvature of the fragments indicates a frozen-in high temperature bond state with elastic energies around 100 J/m(2). Cross-sectional transmi ssion electron microscopy of the interface reveals an amorphous interm ediate layer the thickness of which considerably increases with increa sing the oxygen partial pressure during annealing. (C) 1997 American I nstitute of Physics.