Effect of vacuum packaging on bandwidth of push–pull type capacitive accelerometer structure

Abstract

This paper presents the effect of vacuum packaging on the band-width of push–pull type capacitive accelerometer structure. The accelerometer structure (for ± 30 g application) consists of silicon proof mass (1000 μm × 1000 μm × 30 μm) suspended by four L-shaped beams over 1 μm deep cavity. The fixed electrodes (Au) are on Pyrex glass substrates which are anodically bonded to the Si substrate. The squeeze film damping factor (ς) arising due to the trapped air between the electrodes is found to be ~ 2600 at atmospheric pressure. Introduction of holes in the proof-mass reduces ς to 750. In such a highly damped environment, the operational bandwidth (3 dB) bandwidth is found to be only around 20 Hz which is not suitable for many of the navigation applications. With vacuum packaging (vacuum level 100–760 Torr), there is not much noticeable improvement in the bandwidth due to high level of ς (> 150). In the 1–100 Torr range, the bandwidth is found to be improving linearly (from 20 Hz to 2 kHz) with the improving vacuum level. Further improvement of vacuum level (< 1 Torr) leads to the under-damped (ς < 1) condition and the proof mass start oscillating which also undesirable. Thus, vacuum packaging in the 1–10 Torr range (having ς: 2–15) is found to be suitable for achieving optimum operation bandwidth for the current accelerometer sensor.