Microbridge testing of plasma-enhanced chemical-vapor deposited silicon oxide films on silicon wafers

摘要

Plasma-enhanced chemical-vapor deposited (PECVD) silane-based oxides (SiO_x) have been widely used in both microelectronics and microelectromechanical systems (MEMS) to form electrical and/or mechanical components. In this paper, a nanoindentation-based microbridge testing method is developed to measure both the residual stresses and Young's modulus of PECVD SiO_x films on silicon wafers. Theoretically, we considered both the substrate deformation and residual stress in the thin film and derived a closed formula of deflection versus load. The formula fitted the experimental curves almost perfectly, from which the residual stresses and Young's modulus of the film were determined. Experimentally, freestanding microbridges made of PECVD SiO_x films were fabricated using the silicon undercut bulk micromachining technique. Some microbridges were subjected to rapid thermal annealing (RTA) at a temperature of 400℃, 600℃, or 800℃ to simulate the thermal process in the device fabrication. The results showed that the as-deposited PECVD SiO_x films had a residual stress of-155± 17 MPa and a Young's modulus of 74.8±3.3 GPa. After the RTA, Young's modulus remained relatively unchanged at around 75 GPa, however, significant residual stress hysteresis was found in all the films. A microstructure-based mechanism was then applied to explain the experimental results of the residual stress changes in the PECVD SiO_x films after the thermal annealing.