Thermal microactuator performance as a function of mechanical stress

We report on the effects of mechanical stress on thermal microactuator performance. Packaging processes such as die attach and lid sealing usually result in stresses on the die containing microsystems devices. While this phenomenon is known, quantifying the effects systematically is difficult due to challenges in controlling the resultant stress resulting from packaging. In this study, we use a four-point bending stage to apply loads of 12 lbf (53.4 N) in tension and compression to 11.5 mm by 2.9 mm samples. Thermal microactuators and stress gauges were fabricated using the Sandia 5-layer SUMMiT surface micromaching process and diced to fit in the bending stage. At each stress level, the vernier scales on the thermal microactuator were imaged in order to determine the displacements. Thermal microactuator displacements are reported as a function of applied current up to 35 mA at varying stress levels. Increasing tensile stress decreases the initial displacement and flattens the thermal microactuator displacement versus applied current curve. Raman spectroscopy and stress gauge measurements indicate that the stress range for the fourpoint bending stage experiments extends from 200 MPa tensile to -250 MPa compressive. Numerical model predictions of thermal microactuator displacement versus current are in qualitative agreement with the experimental results. Quantitative information on the reduction in thermal microactuator performance as a function of stress provides validation data for MEMS models and can guide future designs so that they will be more robust to stresses resulting from packaging processes.