Influence of plasma treatment and cleaning on vacuum wafer bonding

Direct wafer bonding was performed in vacuum. We compared two kinds of bonding, Si to Si and Si to SiO₂, in three different circumstances: Bonded in vacuum only, bonded in vacuum after plasma treatment and bonded in vacuum after plasma treatment and RCA1 cleaning. From the comparison of the bonding strength, we found that in both cases, Si-Si and Si-SiO₂, plasma treatment in vacuum bonding degrades the bonding quality if there is no RCA cleaning processed after it, even when RCA cleaning is processed after plasma treatment, the bonding strength is almost the same as the one without plasma treatment. But in traditional plasma activation wafer bonding in air, plasma treatment can improve the bond strength evidently. The reason for this discrepancy is supposed to be that the "vacuum" plays a main role in vacuum wafer bonding, rather than the plasma treatment. On the other hand, the plasma also induces contamination and damage to the surfaces, which increases the unbonded area as shown by means of the scanning acoustic microscope (SAM).