Effect of deposition parameters on the stress gradient of CVD and PECVD poly-SiGe for MEMS applications

Abstract

The effect of the deposition parameters and Ge content on the stress gradient in poly-SiGe films was investigated. The films, ranging in thickness from 1.2 to 2.3 μm, were deposited by chemical vapor deposition (CVD) at 450 °C and plasma enhanced chemical vapor deposition (PECVD) at 520 °C. The Ge content was varied between 45 and 64 at%. X-ray diffraction revealed that both PECVD and CVD films were polycrystalline. The stress gradient was determined by measuring the deflection of 1 mm long released cantilevers. The stress gradient was found to decrease with increasing Ge content. A CVD film with 55 at% Ge was thinned using a very low power SF 6 /O 2 plasma. The stress gradient was measured as a function of film thickness. The stress profile was calculated by matching the bending moment of the calculated profile to the bending moment obtained from the measured stress gradient. The largest change in stress occurs right at the thin film/substrate interface. PECVD films were found to possess a lower stress gradient compared to CVD films with similar thickness. This was explained by differences in TEM microstructure: CVD films have more V-shaped grains, while PECVD films have more columnar grains.