Chip on Board development for a novel MEMS accelerometer for seismic imaging

HP is developing a MEMS accelerometer that offers high sensitivity and low noise level for seismic application. During the packaging process, it was found that the thermal mechanical stress resulted from the CTE mismatch between the silicon and the substrate can distort the MEMS structure and alter the device performance. A finite element model was established to understand the effect of substrate material, die attach material and package geometry on the scale factor and bias offset of the MEMS. The model predicted that ceramic substrate introduced less variation of the sensor performance over the operating temperature range (-40 to 70°C) than the FR4 substrate. In order to use the low cost FR4 substrate, one can use die attach adhesive with low modulus and/or small die attach area to minimize the gap and displacement. Ceramic packages and Chip on Board (COB) packages on FR4 substrate were built using various die attach adhesives to validate the model. The performance of the MEMS was measured from -40 to 70°C. Results showed that using a low modulus adhesive with full coverage and moderate thickness provides minimum temperature distortion. On the other hand, a high modulus adhesive can be employed if the die attach area is reduced. The model also indicated that by modifying the structure of the substrate, the stress on MEMS can be reduced. MEMS on pillar packages were built and evaluated to confirm the modeling result.