Surface passivation for low-temperature, UHV wafer-bonding of GaAs substrates

The effects of different surface preparations on the bonding of GaAs (001) in Ultra-High Vacuum (UHV) have been studied. The samples used were GaAs wafers with buffer layers grown by Molecular Beam Epitaxy (MBE) and subsequently As capped. As capped GaAs wafers had good bonding characteristics at temperatures as low as 350 degreesC in UI-IV whereas typical air exposed oxide terminated GaAs did not exhibit bonding below 550 degreesC. The bonding process was followed with a Residual Gas Analyzer (RGA) which was placed in line-of-sight to the bonding fixture in the UHV system and was used to detect desorbed species. Previously published results have shown that subsequent to the desorption of tile As cap an atomically clean surface or interface is formed. This suggests that the lower bond temperatures are achieved due to the atomically clean surfaces that are formed. This RGA information also allowed optimization of the time needed to achieve clean GaAs surfaces during bonding. Ultrasonic C-Scan was used to analyze bonded samples generated both internally and externally. The C-Scan imagery was used to determine gross bonding effects such as voids and other non-uniformities. The reflection amplitude at the bond interface was used to determine the quality of the bond. As capped samples were found to have a comparable reflection amplitude to externally generated samples.