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

Abstract

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 micro-machined 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 (a bout 100MPa) occur in the plane of seal glass in cause of cooling after bonding. The influence of bond pressure is rather small. The residual stresses have to consider 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

Mechanical properties of glass frit bonded micro packages

“…8). In former investigations this superposition was investigated for different sizes and shapes of the glass frit layer (Ebert and Bagdahn 2004).…”

Wafer capping of MEMS with fab-friendly metals

“…It is difficult to determine the absolute value of these stresses, but Ebert calculated their distributions as a function of major design variables. 7 These residual stresses are not low. This raises the possibility that cracks may form in the glass under some conditions, and propagate during the product lifetime.…”

An Experimental and Numerical Study on Glass Frit Wafer-to-Wafer Bonding

“…Since gf mechanical properties are highly dependent on the composition, different values are reported in the literature. For instance, Ebert and Bagdahn [ 27 ] indicated E = 50 GPa and = 0.23 for Young’s modulus and Poisson’s ratio, respectively. Grabham et al [ 28 ] inspected the mechanical properties of gf binder material through experiments on cantilever beams loaded at the free ends and numerical simulations.…”