Investigation on the Effect of Storage Time on Bond Strength Between Plasma Activated Silicon Wafers

This paper investigates the effect of storage time on the bond strength of plasma-activated silicon (Si) wafers. Plasma activation was carried out in a reactive ion etch chamber using O₂ gas. The activated wafers were stored in a clean room environment for specific time intervals before pre-bonding them (in a high vacuum environment) using a substrate bonder. Steps involved in wet chemical activation and pre-bonding of the wafers were discussed in detail. The pre-bonded wafers were thermally annealed. The bond quality, in addition to the bond strength, of in-situ and ex-situ thermal annealed wafer pairs were evaluated. The bond quality was determined using near-infrared imagery, and tensile tests were conducted on dies diced out from the bonded pair. The chemistry involved during activation, storage, pre-bonding, and thermal annealing was investigated thoroughly. The bond quality and bond strengths of wafers for corresponding storage times were compared using the near-infrared imagery and tensile tests respectively. Finally interesting phenomenon, like the increase in bond strength after a particular storage time, was studied and explained.