Relaxation of Residual Stress in Bent GaN Film on Sapphire Substrate by Laser Treatment With an Optimized Surface Structure Design

The serious wafer bending and residual stress are formed during the growth of an epi-GaN layer on sapphire substrate due to the difference of thermal expansion coefficients (TECs) in these two different materials. By using the theoretical analysis and a simulation model with the finite-element method to describe the realistic shape for wafer bending of epi-GaN wafers, we examine the influence of different thicknesses and TECs in the top epi-GaN layer for wafer bending reduction. Furthermore, the wafer bending is also found to be worse when process temperature and wafer size become higher and larger, respectively. Although the commercial patterned sapphire substrate can partially solve this issue, the quality of the epi-GaN layer, grown on this patterned substrate, will be impacted. In this paper, the new process to reduce the wafer bending and relax residual stress is proposed. With an additional laser treatment on the sample surface after the growth of the top epi-GaN layer on sapphire substrate, drilling hole can provide extra space for relaxation of residual stress and will not influence the GaN quality. The wafer bending can be reduced to ~ 37 μm from the original ~ 45 μm in 2-in wafer with an optimized surface structure design by this treatment.