ANSYS Academic products were used to model several MEMS sensors and to study residual stress in thin films. A piezoelectric model was built to study the behavior of a commercially available speaker buzzer used as an ice-detection sensor. This was a 3-D model with symmetry boundary conditions in the circumfrential direction. It contained three layers of materials: the piezoelectric layer, an aluminum layer, and an ice layer. The resonant frequency as a function of ice thickness was obtained and compared with experimental data.
A structural model of a pressure sensor fabricated by bulk etching a silicon substrate with a silicon carbide diaphragm was used to determine residual stress and a deflection versus pressure curve. This was an axisymmetric model. A 3-D model was built but was too large to run under this version of ANSYS University Research (5.3). The residual stress in the diaphragm is due to the thermal coefficient of expansion mismatch between the silicon substrate and silicon carbide film, which is grown on the substrate in an APCVD reactor at 1600 K. As the film cools to room temperature, a residual tensile stress occurs. This stress is important since it affects the deflection of the diaphragm under pressure loads. The results were obtained in a three-step process: cooling from reactor to room temperature, etching of the silicon using the element birth and death feature, and application of the pressure load. As a final step, a modal analysis on the silicon carbide diaphragms was done to determine the resonant frequency as a function of the residual stress. These results were compared to an analytical model.
ANSYS University Research was used to model two types of folded-beam, lateral resonating devices. These are built using surface micromachining of silicon carbide films grown on a silicon substrate. The thermal coefficient of expansion of the silicon carbide device is greater than the silicon substrate above 500 K. This induces tensile and compressive stresses in the beams and changes the resonant frequency of the device. Temperature dependent properties for Young's modulus and expansion coefficient were used. A uniform temperature was applied to the whole model in step one. The coordinates of the model were updated in step two using the UPCOORD feature. Finally, a modal analysis using "prestress" ON and nonlinear geometry ON was done in step three. The resonant frequency from 300 to 1300 K was obtained in this manner and compared to experimental data.
Results of research work using ANSYS software was published in the following journal and conference papers: