Permanent Stiffness Reduction by Thermal Oxidation of Silicon

Abstract

Stiffness in compliant micro mechanisms can negatively affect performance. Current methods for stiffness reduction in micro electro mechanical systems (MEMS) consume power, have a large footprint or are relatively complex to manufacture. In this paper stiffness is reduced by static balancing. A building block commonly used for stiffness reduction in large scale compliant mechanisms is made compatible with MEMS. Preloading required to create negative stiffness is obtained from residual film stress by thermal oxidation of silicon. Instead of buckling a plate spring by moving its end points, a SiO2 film 1900 nm to 2500 nm thick will stretch micro-beams 24 $\mu $ m wide, while the end points are fixed. To show efficacy of our method, the building block is coupled with a simple linear stage. However, the building block can readily be combined with other compliant micro mechanisms to reduce their stiffness. Statically balanced MEMS will enable novel designs in low-frequency sensor technology, low-frequency energy harvesting and pave the way to autonomous micro-robotics. We show a stiffness reduction of a factor 9 to 46. The balancing effect remained after SiO2 removal, due to plastic deformation of the beams. [2019-0023]