Thick polysilicon surface-micromachined optically sensed accelerometer

Abstract

An accelerometer based on optical intensity modulation has been designed and fabricated using thick-polysilicon (12μm) surface-micromachining technology. A layer of polysilicon is surface micromachined to form a grating connected to springs, which are anchored on the silicon substrate. A layer of metal is evaporated on the top of the entire device making the suspended grating opaque as well as the regions on the substrate that are exposed by the grating. This leaves the areas directly beneath the grating shadowed from incident light. In the unperturbed state, the light that shines on the device is completely reflected by the metal on top of the entire device. When the grating moves under the influence of acceleration, the areas which are not covered by the metal are then exposed and the amount of light passing through the substrate increases. The optical intensity variation translates to the acceleration experienced by the grating proof mass. 12μm thick polysilicon surface micromachining was developed to improve the performance of the device. The film's mechanical qualities, internal stain, stress gradient and surface roughness have been characterized. This optical accelerometer has a sensitivity of 70 milli-g with 0.5-micron movement.