Studies on structural, electrical, compositional, and mechanical properties of WSix thin films produced by low-pressure chemical vapor deposition

Abstract

Tungsten silicide (WSix) thin films have been deposited by low-pressure chemical vapor deposition using silane (SiH4) and tungsten hexafluoride (WF6) at different fluxes ratio ranging from 44 to 745 onto planar Si(100) wafers covered by 130 Å of SiO2 and 2000 Å of phosphorus-doped polysilicon layers to reproduce the control gate structure of a FLASH-EEPROM. The deposited films have been annealed in oxygen ambient at 900 °C. The composition of as deposited films as well as annealed films has been studied by x-ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, and secondary ion mass spectroscopy. Cross section scanning electron microscopy and atomic force microscopy have been employed to study the thickness and the surface morphology of the deposited layers. The resistivity and the mechanical properties of the films have been also investigated. The as-deposited films have shown, growing the SiH4/WF6 ratio, an increase of the Si/W stoichiometric ratio (until 3), a decrease of the film thickness and stress while there is an increase of the sheet resistance. In the case of annealed films, which pass from amorphous state to crystalline structure, as detected by x-ray diffraction, there is a strong decrease of the sheet resistance values as well as of the silicide film thickness depending on the SiH4/WF6 ratio, while the presence of a silicon oxide layer with different thickness has been also detected on top of the silicide layer.