DEPOSITION MECHANISM AND ELECTRICAL-PROPERTIES OF LOW-PRESSURE CHEMICALLY VAPOR-DEPOSITED W AS A GATE ELECTRODE

We have investigated the feasibility that low pressure chemically vapo r deposited W (LPCVD-W) could be used as a gate electrode material of metal-oxide semiconductor (MOS) field effect transistors. Smooth W thi n films with good adhesion to SiO2 were deposited in a relatively shor t time by the pulsing injection method, in which the source gas WF6 wa s periodically injected into the low pressure chemical vapor depositio n (LPCVD) reactor. Right after the WF6 flow was stopped, instantly the ratio of SiH4 to WF6 increased rapidly, enhancing the additional nucl eation between the initially formed W islands. This additional nucleat ion presumably made it possible to form a continuous W thin film on th e oxide surface at the initial stage of deposition. The phase of depos ited W films changed from alpha-W to W5Si3 at the SiH4/WF6 ratio of 2- 3. We have optimized the process conditions in order to deposit a reli able W gate electrode for the application of MOS capacitors. MOS struc ture capacitors were fabricated using a LPCVD-W gate electrode and the ir electrical properties were characterized. It was found that the 20- Angstrom-thick Si3N4 layer protected SiO2 from a chemical attack by WF 6. The composite gate dielectric of SiO2 and Si3N4 layers showed low l eakage current and good dielectric breakdown strength distribution. Th e deposition was carried out at the temperature range of 300-750 degre es C and various SiH4/WF6 ratios of 0.6-14. Transmission electron micr oscopy and scanning electron microscopy were used to analyze the phase and the film morphology of LPCVD-W on the SiO2 substrate, respectivel y. (C) 1996 American Vacuum Society.