Properties of low-pressure CVD tungsten silicide for MOS VLSI interconnections

Low-pressure chemical vapor deposition of tungsten silicide has been done and the properties of the deposited films have been studied to determine the process compatibility and suitability to form gate electrodes and interconnections in MOS VLSI applications. The silicide was deposited on single-crystal silicon and on oxidized silicon with and without a coating of polycrystalline silicon film. Auger analysis of the As-deposited films showed absence of any contaminants in it. X-ray diffraction and transmission electron microscopy showed that As-deposited films were microcrystalline with grains smaller than 30 Å and upon annealing became polycrystalline WSi
2
with hexagonal structure at 500°C and tetragonal structure at or above 600°C with a corresponding decrease in resistivity from 600-900 µΩ . cm to 35-60 µΩ . cm depending upon anneal temperature and time. No appreciable change in the thickness of the silicide was found during the high-temperature anneals. Silicon-rich silicide films remained stable, smooth, and free of cracks through high-temperature anneals and oxidations, and their adherence to the wafer remained excellent. On the other hand, metal-rich films had overall inferior properties. Thermal oxidation of WSi
2
on polysilicon in dry oxygen in the temperature range of 900 to 1100°C was found to be similar to that of silicon except the linear regime of oxidation was extremely rapid and the entire process could be modeled by a parabolic equation
X^{2) = Bt
with an activation energy of 1.7 eV. MOS capacitors were fabricated with silicide and polycide gate electrodes. Polysilicon thickness variation from 0 to 5000 Å had no adverse effect on the electrical characteristics or mechanical integrity of the devices. In all cases, low values of N
f
(1 × 10
¹⁰
-7 × 10
¹⁰
cm
^-2
) and N
it
(< 10
¹⁰
cm
^-2
. eV
^-1
) and high gate oxide dielectric strength (
\sime 8
MV/cm) were obtained.