Surface energy as a function of self-bias voltage in oxygen plasma wafer bonding

a limitation in the use of wafer bonding has been the necessity for high-te mperature annealing after contacting the wafers at room temperature. In thi s paper, we try to find the highest surface energy as a function of self-bi as voltage in oxygen plasma-activated wafer bonding, in order to achieve a low-temperature bonding process. The bonding was performed in situ the vacu um chamber. It was found that oxygen plasma has a smoothing effect on the s urface roughness, rather independent of the plasma self-bias. However, a mo derate self-bias voltage proved to give the highest surface energy for the bonded wafers, both at room-temperature and after annealing at 200 degrees C. We believe that this is due to the fact that a moderate self-bias is the most efficient in removing surface contaminants, like water and hydrocarbo ns. It was also found that even after annealing at higher temperatures, 480 degrees C and 720 degrees C, the plasma-bonded wafers showed higher surfac e energy values than wafers bonded in ambient air. This investigation was f ocused on low-effect plasmas, < 200 W, keeping the induced plasma damages a t a minimum. (C) 2000 Elsevier Science S.A. All rights reserved.