Warpage and stress optimization of wafer-level package of MEMS with glass frit bonding

Abstract

Warpage issue of wafer level package (WLP) has caught the attention of WLP industry. This paper aims at the warpage characteristics and optimization of the WLP (consisting of silicon MEMS wafer and silicon cover wafer) with glass frit bonding. Finite-element method (FEM) was used to study the warpage and stress optimization of the Si-Si bonding WLP. Some factors which affect WLP warpage, such as CTE (coefficient of thermal expansion) and Young's modulus of glass frit, ring thickness & width of glass frit and bonding temperature etc. were optimized. The stress of WLP was also calculated and the reliability level of the MEMS device was estimated. It turned out that CTE and Young modulus of glass frit are the key parameters for decreasing WLP warpage and stress and promoting the MEMS WLP. Ring width of glass frit and the thickness of silicon cover plate wafer have little impacts on WLP stress and warpage. With the optimized parameters taken into account, the stress in MEMS WLP falls down. Those optimization results have been put into actual WLP manufacture.