An Investigation of MEMS Anchor Design for Optimal Stiffness and Damping

Micro-resonators for use in accelerometer applications and polysilicon flexures designed to damp unwanted vibrations of a micro-positioner are two examples of Micro-Electro-Mechanical Systems (MEMS) that require optimal anchor stiffness and damping properties. This investigation sought to determine how the geometric shape of the anchor affects the quality of the beam-anchor coupling in MEMS devices. By utilizing 3-D solid finite element modeling in Patran/ABAQUS, several anchor designs were subjected to static loads in transverse, vertical, and axial directions in order to determine their corresponding stiffnesses. The anchor stiffnesses were converted to "effective lengths" and "effective axial spring constants" to rank the effect of inplane shape changes on the anchor behavior. Strain energy and stress results were obtained to approximate the damping propensities of the anchors. Single-ended tuning fork (SETF) anchor stiffnesses were found to depend proportionately on front wall width, and inversely on front wall thickness. Multiple or large anchor connections to the substrate provided substantial increases in SETF stiffnesses. In the case of double-ended tuning fork (DETF) anchors, the front wall width is the most important dimension for maintaining stiffness in all loading conditions.