The effects of plasma pretreatment and storage time on silicon fusion bonding

Abstract

This research investigates the effect of storage time on the bond strength of plasma activated Si wafers by studying the surface characteristics and chemistry of the wafers at each of the various processing levels. Initially pairs of silicon (100) wafers were plasma activated using O2 in a reactive ion etch chamber. The wafers were stored for various time intervals before bonding in a substrate bonder. Different surface characteristics like the growth of the oxide layer, surface roughness, etc. were compared and profiled with respect to storage time, before bonding the wafers. An optimized plasma activation and bonding recipe was proposed. After a final thermal anneal, near infrared imagery was employed in order to optically assess the bond quality of the wafer stack. Conducting tensile tests on dies diced from the wafer stack quantized the bond strength between the wafers. The chemistry involved during the plasma activation, bonding, and thermal annealing were investigated thoroughly. Interesting phenomenon such as the increase in the bond strength of wafers stored for more than a specific amount of time was analyzed. Finally a closer look was taken at the interface layer of the wafer stack through scanning electron microscopy, and the formation of a silicon dioxide layer at the interface was confirmed.