Abstract
We developed a closed form analytical solution for thermal stresses on membrane based “released” MEMS devices. As all the layers forming the membrane has thicknesses close to one another, the assumption of having thick substrate fails. We employed stress superposition principles to obtain the intrinsic stresses caused by coefficient of thermal expansion (CTE) mismatch of the membrane layers. We also accounted for the membrane release. We illustrated that for a bilayer strip consisting of a thin film on a thick substrate, our formulation simplifies to the well-known Stoney equation. The results indicate that the thermal stress can be minimized by the appropriate choice of dielectric materials, metal electrode selections and deposition tools. Replacing the silicon nitride (Si3N4) layer deposited by low pressure chemical vapor deposition (LPCVD) with a silicon oxide (SiO2) layer deposited by plasma enhanced chemical vapor deposition (PEVCD) reduces the average thermal stress by 93%.
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