Improvement of PECVD Silicon-Germanium crystallization for CMOS compatible MEMS applications

This paper investigates the influence of the electrode spacing, chamber pressure, total gas flow, and H-2 dilution on the crystallinity, resistivity, uniformity, and stress of polycrystalline silicon-germanium (poly-SiGe) films grown by plasma-enhanced chemical vapor deposition (PECVD). Boron-doped PECVD SiGe films of 1.6 mu m thick are deposited on 400 nm chemical vapor deposition layers from SiH4, GeH4, and B2H6 precursors. The microstructure is verified by transmission electron microscopy and by X-ray diffraction. It was discovered that for constant temperature and deposition rate, the PECVD SiGe microstructure changes from completely amorphous to polycrystalline by increasing the electrode spacing and pressure due to reduced ion bombardment. A process window of an electrode spacing and pressure for the PECVD poly-SiGe deposition is thus identified based on a sheet resistance mapping method. Increasing the total gas flow dramatically improves the within-wafer crystallinity variation and further reduces the resistivity. Increasing the H-2 flow during PECVD shifts the stress from -51 to 17 MPa and further reduces the crystallinity variation over the wafer. In addition, the effect of changing the SiH4 to GeH4 ratio and the in situ boron doping by adding B2H6 is also investigated. The findings in this paper are expected to facilitate the use of poly-SiGe in the above complementary metal oxide semiconductor (CMOS) microelectromechanical system (MEMS) applications. / status: published