Surface blistering of low temperature annealed hydrogen and helium co-implanted germanium and its application to splitting of bonded wafer substrates

Applications involving transfer of germanium layers to silicon-based substrates often require a process involving a restricted thermal budget. The use of relatively low temperatures has a major advantage in reducing stresses when thermal splitting of implanted germanium wafers bonded to silicon-based substrates is used to create germanium-on-oxide (GeOI) layers. The present study investigates the phenomenon of blistering of hydrogen and helium co-implanted germanium over the temperature range 250-400 °C, optical microscopy being used to detect the initial appearance of the blisters. Results showed that plots of Ln(time) vs. blister initiation temperature consisted of several straight-line regions yielding an activation energy for each region. The plots showed similarities to those observed in previous work with silicon co-implanted and annealed under similar conditions. At temperatures below the blister initiation temperature, transmission electron microscopy (TEM), revealed the presence of spherical bubbles at a depth below the surface estimated to be approximately that of the hydrogen implant projected range. GeOI layers were produced by thermal splitting of co-implanted germanium wafers bonded to oxide-coated silicon substrates wafers at a temperature of 300 °C. The RMS roughness of the split germanium surface measured by atomic force microscopy (AFM) was about 11 nm averaged over the wafer surface. In addition there were isolated and randomly distributed regions of 27 nm roughness covering about 20% of the total surface area of the wafer.