Investigation of E-Beam Evaporated Silicon Film Properties for MEMS Applications

This paper investigates the crystallinity, microstructure, surface morphology, stress characteristics, and Young's Modulus of ultrahigh vacuum (UHV) electron-beam (E-beam) evaporated silicon films with a low thermal budget. The films are evaporated at various substrate temperatures ranging from 200 °C-625 °C, deposition rates ranging from 50 to 400 nm/min, and annealed at 600 °C for various durations. Some of the preliminary results were reported by Michael and Kwok. The results indicate that the film characteristics of the evaporated silicon films are significantly different from and better suited for microelectromechanical systems (MEMS) applications than low pressure chemical vapor deposition silicon films, commonly used for MEMS devices. The very attractive properties of UHV E-beam deposited silicon films are remarkably low residual stress (both average and gradient), very smooth surface morphology, and thick layers at a low thermal budget with reasonably large deposition rates. Two different mechanisms have been identified as responsible for initiating the formation of crystal grains in these films: 1) kinetic energy of evaporated silicon atoms; and 2) thermal energy from the substrate heating. The first mechanism leads to fine columnar grains responsible for the smooth surface morphology and easily controllable low stress characteristics. The second mechanism results in coarse grain formation with a relatively higher proportion of (111) oriented grains. Cantilever beams of 30-μm thickness have been fabricated from such films with rms roughness of c5 nm, residual stress of 20 MPa, and stress gradient of c1.1 MPa/μm with Young's Modulus of 169 GPa.