Empirical inference of in situ wafer sagging in Si epitaxy using pocket susceptor

An empirical description of wafer sagging during silicon epitaxy using a dish-shaped susceptor pocket is presented. The proposed method is based on the discovery that the process history can be retrieved from the transfer pattern on the wafer backside. The characteristic tree-ring pattern is due to the contact transfer of the susceptor coating to the sagging wafer. Each ring is uniquely identified as a process step and vice versa. The number of rings is related to the growth schedule. The size and shape of the rings are sensitive to the growth conditions. A full picture of the in situ wafer sagging can be reconstructed by examining the transfer pattern after the wafer is unloaded from the reactor. The proposed method has been used to evaluate the wafer deformation in response to changes in the key process variable such as the growth temperature, susceptor geometry and reactor design.