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 vapor deposited W (LPCVD-W) could be used as a gate electrode material of metal-oxide semiconductor (MOS) field effect transistors. Smooth W thin films with good adhesion to SiO2 were deposited in a relatively short time by the pulsing injection method, in which the source gas WF6 was periodically injected into the low pressure chemical vapor deposition (LPCVD) reactor. Right after the WF6 flow was stopped, instantly the ratio of SiH4 to WF6 increased rapidly, enhancing the additional nucleation between the initially formed W islands. This additional nucleation presumably made it possible to form a continuous W thin film on the oxide surface at the initial stage of deposition. The phase of deposited 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 reliable W gate electrode for the application of MOS capacitors. MOS structure capacitors were fabricated using a LPCVD-W gate electrode and their electrical properties were characterized. It was found that the 20-Angstrom-thick Si3N4 layer protected SiO2 from a chemical attack by WF6. The composite gate dielectric of SiO2 and Si3N4 layers showed low leakage current and good dielectric breakdown strength distribution. The deposition was carried out at the temperature range of 300-750 degrees C and various SiH4/WF6 ratios of 0.6-14. Transmission electron microscopy and scanning electron microscopy were used to analyze the phase and the film morphology of LPCVD-W on the SiO2 substrate, respectively. (C) 1996 American Vacuum Society.