Silicide Formation with Tungsten Deposited from W-LPCVD; The Role of Back Surface Conditions

Tungsten (W) films were deposited on the front surface of float-zone (FZ) Si wafers, from tungsten hexafluoride (WF6) by low pressure chemical vapor deposition (LPCVD). The back surface conditions of the Si wafers was the major concern of this study. Various back surface coatings were investigated, tungsten, thermal SiO2 and LPCVD-Si3N4. Isothermal heat treatments were performed in an argon flow furnace at 760°C for 8 to 30 min. The silicide formation was monitored by Rutherford backscattering spectrometry (RBS). No difference in the silicidation rate was found on the wafers with a back surface oxide layer as compared to that of the reference wafers with no back surface coating. However, for wafers with the back surface covered with Si3N4 or W, a retarded silicidation rate was observed. This phenomenon appeared to be insensitive to the presence of a cap layer (PECVD-SiO2) on the W films before annealing. A model including the behavior of point defects is proposed to provide an explanation to this observation.