A New Stress Isolation Method in the Packaging of Resonant Pressure Micro Sensors

This paper presents a new stress isolation method in a MEMS (microelectronic-mechanical systems) based resonant pressure sensor to minimize thermal stresses arising from device packaging due to thermal mismatches between the silicon sensor body and its housing materials (e.g., kovar and copper). In this study, the resonant sensor was separated from the metal substrate by sandwiching an intermediate silicon based spacing layer, resulting in a non-direct contact between these two layers and therefore a decrease in the stress build-up at higher environmental temperatures. The effects of the geometry of the spacing layer on stress isolation and the reliability of the packaged sensor were investigated using finite element analysis (FEA) and verified by experimental measurements. Numerical simulations showed that at 100 °C uniform temperature load a thermal stress of 1.6 MPa was generated on the resonator due to thermal expansion coefficient (TEC) mismatches between silicon and metal while a thermal stress of 0.04 MPa was calculated from the devices with the optimized stress isolation component. Experimental results indicated a frequency drift less than 0.05% F.S/°C in a temperature range from –40 °C to 70 °C for devices with stress isolation components, which was one order lower than the devices by mounting resonators on top of metal substrates directly (0.8% F.S/°C ∼ 1% F.S/°C). This stress isolation concept by using an intermediate spacing layer may provide new insights in the field of MEMS packaging where the packaging stress is a concern.