Thermomechanical design and modeling of porous alumina-based thin film packages for MEMS

Thin film 0-level or wafer-level MEMS packages exhibit relatively low flexural strength and yet they are required to reliably protect the enclosed MEMS devices under extreme processing and operational conditions. In this paper, we present a thermomechanical study of porous alumina-based thin film MEMS packages by making use of finite element modeling (FEM) techniques. We developed a 2D axisymmetric FEM that includes a porosity-dependent orthotropic representation of the porous alumina layer. The results of the FEM for a typical thin film package show around 15% enhancement over an analytical circular plate model in terms of the accuracy of calculating the maximum cap deflection under 105Pa differential pressure. The simulated package performance illustrates the significance of several parameters such as the package geometry, external hydrostatic pressure, residual stresses in the thin film, and the ambient temperature. Simulations further show that a circular package of 180µm diameter, featuring an 8µm thick cap and a central supporting pillar of 20µm diameter can withstand hydrostatic pressures up to 9MPa, which could occur during the process of plastic packaging with an epoxy molding compound. Furthermore, the thermal expansion mismatch between the different materials composing the thin film package poses a challenge to fulfill the reliability characteristics of these packages. It is however possible, based on FEM simulation results, to achieve reliable operation in the temperature range between −55°C and +125°C for a circular package of 6µm cap thickness and 250µm diameter without a supporting pillar.