High-sensitivity surface micromachined structures for internal stress and stress gradient evaluation

The internal stress and stress gradient of thick (10 mu m) and thin (2 mu m) polysilicon films were evaluated with surface micromachined test structures. The structure that measured internal stress consisted of actuator beams rotating an indicator through an angle corresponding to the stress. The indicator deflection was measured in an SEM. Finite element analysis (FEA) was used both to optimize the design and to calibrate the structure. A folded beam design was used to minimize the total area the structure occupied so that it could be incorporated in the wafer layout of other surface micromachined details, and used for online process diagnostics. The indicator was provided with a Vernier scale to facilitate quick evaluation in an optimal microscope. The stress gradient was measured from the deflection of long (1000 mu m) cantilever beams. The deflection was measured in an optical microscope and the output was calibrated with FEA calculations.