Experimentally Matched Finite Element Modeling of Thermally Actuated SOI MEMS Micro-Grippers Using COMSOL Multiphysics

Engineering MEMS devices using finite element analysis can be very rewarding, provided the analysis is valid, i.e. it accurately simulates reality throughout the desired range of input and resultant imposition upon dependent physical properties and behavior. With respect to FEM’s use in MEMS, it is can be very useful. Doped single crystal silicon is a substance whose material and physical properties have been documented only sporadically, considering the infinitely many possible permutations of: 1) doping type, 2) concentration, and 3) doping profile; on top of these dramatic effects, two identical silicon devices, each at a different 4) temperature, can appear like two completely different materials: these four variables are what make using silicon for thermoelectric applications very difficult. Here, to ensure future utility of FEM, we have compared experimental displacement data of two hot-arm-cold-arm microactuators of different sizes but very similar proportions to simulated parametric sweeps in order to extract the temperature coefficient of resistivity (TCR) and the surface-to-air heat transfer coefficient, commonly referred to as the convective heat transfer coefficient (CHTC). It is expected that 1) the temperature coefficient of resistance, would be the same or very close in both devices—since they are microfabricated by the same process on the same SOIMEMS wafers— and that 2) the heat transfer coefficient quantifying the surface-to-fluid heat loss would rise with increasing area, considering that the Grashof Number, the ratio of buoyancy force to viscosity forces, increases as the area normal to such forces increases. The experiment shows that parametric sweeps of such temperamental material yield somewhat problematic results. Of the two microactuators studied, the magnitude of the convective heat transfer coefficients were opposite than that expected: CHTC – large microgripper was 170/85 W/m∙°C (bottom and sides/top) and of the smaller one CHTC was 1200/600 W/m ∙°C. TCRs of the devices were within 50 % of one another, at least establishing a likely range for refinement of experimentation.