The Effect of Thermal Shocks on the Stresses in a Sapphire Wafer

Sapphire wafers can experience temperature variations during processing in a furnace, which in turn can cause large deformation and stresses in the wafers. This paper aims to reveal the mechanism of stress development and evolution in sapphire wafers during thermal shocks, as well as the dependence of the stresses on some process parameters. Finite-element stress analysis was conducted on a single sapphire wafer subjected to thermal shocks. The results show that the thermal gradient in the radial direction induces high stresses even in mechanically unrestrained wafers. The largest stress components occur at the wafer edge as the largest normal stresses are circumferential; whereas the maximum tensile stress is realized upon cooling, the highest value of the maximum shear stress and the minimum compressive stress eventuate in the heating-up phase. The normal stresses have a parabolic distribution in the radial direction. It was found that holding the furnace temperature leads to a more uniform temperature distribution across the wafer but brings about higher tensile stresses in the cooling phase