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Determination of residual stress in glass frit bonded MEMS by finite element analysis

Glass frit bonding is an universal technique with intermediate glass layer for the packaging of micro-electro-mechanical systems (MEMS). The joining of two wafers is required to protect surfaced micromachined structures during fabrication and use. The object of this paper is to investigate numerically the influence of the bonding process on the generated residual stress in the glass frit layer of a typical chip. The residual stresses are caused by different coefficients of thermal expansion of the frit glass and the silicon wafer. A determination of the stress level is required, since the residual stress can influence the properties of the capped micro-machined structures and also the long-term reliability of the bond frame. A parameterized finite element (FE) model of a typical chip structure was developed which consists of a silicon device wafer, the glass frit layer and a silicon cap wafer with a cavity. Frame width and thickness of the glass, also called seal glass, were varied. It was shown that significant stresses (about 100 MPa) occur in the plane of seal glass because of cooling after bonding. The influence of bond pressure is rather small. The residual stresses have be to considered if further mechanical stresses are investigated. So a tension test normal to the seal glass plane was superposed with the residual stresses. The location of maximum stresses could be shown and it was in agreement with experimental results.