An Advanced Finite Element Model for BiCMOS Process Oriented Ultra-Thin Wafer Deformation

摘要

A process-oriented wafer-scale finite element model is developed and validated. The model is used to study the relationship between the in-plane residual stress and the deformation of state-of-the-art 0.13-mu m SiGe BiCMOS fully processed 8-inch wafers. Based on the in-situ wafer bow measurement results, the residual stress values are extracted regarding each deposited material per process step. The extracted material residual stress values are integrated into the in-plane stresses of each back-end redistribution layer by knowing the material densities, greatly reducing the computational effort. An advanced finite element model composed of these integrated redistribution layers is therefore developed by exploiting the first order shear deformation theory. The model is validated using analytical solutions and is used to characterize the wafer thickness-deflection non-linear relationship. As a comparison, 8 fully processed BiCMOS wafers from the same lot are thinned to different thicknesses ranging from 50 mu m to 600 mu m for bow measurement. After taking the gravity-induced deflection and grinding effect into consideration, the wafer bow predicted by the finite element model deviates less than 20% from the measurement results for all the thickness values.