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Optimized surface structure by laser treatment for the relaxation of residual stress in bent GaN film
Serious wafer bending and residual stress are formed during the growth of the epi-GaN layer on a sapphire substrate due to the difference of the thermal expansion coefficients (TECs) in these two different materials. Using the theoretical analysis and a simulation model with the finite element method to describe the realistic shape for the 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, wafer bending is also found to be worse when the process temperature and the wafer size become higher and larger. Although 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 work, the new process to reduce the wafer bending and relax the residual stress is proposed. With an additional laser treatment on the sample surface after the growth of the top epi-GaN layer on the sapphire substrate, a slight crack can provide the extra space for the relaxation of the residual stress and will not influence the GaN quality. The wafer bending can be reduced to ~37 µm from the original ~45 µm in the 2 inch wafer with optimized surface structure design by this treatment.