Stress analysis in new improved differential vertical comb capacitive micro accelerometer using SOI technology

Abstract

Recently a new improved differential vertical comb type capacitive accelerometer using silicon on insulator (SOI) technology reported having better performance than dissolve wafer process (DWP) technology. Effective figure of merit shown is much higher compared to other comb-based technologies presented in literature. Theoretical formulations developed for residual and working stress which can be utilized for optimizing the performance of microaccelerometer. Maximum reported experimental mean stress (500 MPa) and stress gradient (0.1 MPa/lm) is also studied in detail. A comparison of analytical and simulations for stress induced deflections are in good agreement (within 5.29% and 3.97%) for residual planar and axial stress respectively. Working shear stress in torsional beams at 30 g in proposed new differential vertical SOI comb type accelerometer is lesser by 20.9 MPa compared to DWP technology. For 1000 g shock, the SOI case, stress of 0.55 GPa having a better safety margin by a of factor of 2 compared to fracture limit (FL) of 1.1 GPa of silicon. In contrast to this, DWP has a stress of 1.25 GPa and hence it crosses FL value and has no safety margin. Warping stress induced in restrained torsional beam have been analyzed and compared with simulation results and found to be in good agreement within 1.87% for SOI technology case and 11.25% for DWP technology case. The effect of initial tip deflection (3.4 lm due to 500 MPa stress and 0.1 MPa/lm stress gradient) on sensitivity has marginal effect on milli g range but has moderate influence on limiting high g operational range.