The Study of Stress Effects in GaN Epilayers on Very Thin Sapphire Substrates Using Chemical Mechanical Polishing Technique

The wafer bonding process and chemical mechanical polishing (CMP) technique are used to remove the sapphire substrate for fabricating the thin-GaN light emitting diodes (LEDs). The stress effects in GaN epilayers on very thin sapphire substrates are analyzed by Raman spectroscopy and photoluminescence (PL). By reducing the thickness of the sapphire substrate from 450 µm to the range between 1.1 and 20.3 µm, the compressive stress in GaN epilayers will be almost released. By analyzing the main scattering signal of E2 (high) mode, the sample with thinnest sapphire of 1.1 µm exhibits 248.3 MPa stress relaxation which is obtained from the Raman shift Δω of 1.54 cm-1. By analyzing the variation of the biaxial stress with sapphire thickness, the relationship between the Raman wavenumbers ω (in cm-1) and residual sapphire thicknesses d (in µm) can be expressed by ω=569.51-1.92×exp (-d/5.81) (cm-1). Finally, an almost linear relationship between residual sapphire thickness and energy bandgap of GaN films was also observed during the CMP process.