D.S. Chao

Formation of tungsten silicide films by ion beam synthesis

Abstract In the present study, a MEVVA ion implanter was employed to implant tungsten ions into silicon wafers at an elevated temperature of 100°C. The acceleration voltage was 40 kV and the charge states of the implanted tungsten ions were 1+ (8%), 2+ (34%), 3+ (36%), 4+ (19%), and 5+ (3%). The ion fluences were 1×1017 and 3×1017 ions/cm2. The as-implanted specimens were furnace annealed in Ar under various temperatures for 30 min. The tungsten silicide film was analyzed by secondary ion mass spectroscopy, X-ray diffraction, cross-sectional transmission electron microscopy, Rutherford backscattering spectroscopy, and a four-point probe. The results indicate that the tungsten silicide film thickness is approximately 30–40 nm, depending on annealing temperature as well as ion fluence. The sheet resistance of the tungsten silicide film closely correlates to the depth profile of ion-implanted tungsten, the Si/W ratio, the crystallographic structure, and the microstructure of the tungsten silicide film. The maximum sheet resistance was obtained at annealing temperatures of 400°C and 550°C for ion fluences of 1×1017 and 3×1017 ions/cm2, respectively, while the minimum sheet resistance was obtained at annealing temperatures above 800°C for both ion fluences. The hexagonal crystallization phase of tungsten silicide, existing at annealing temperatures between 400 and 550°C and leading to smaller mean depth of the synthesized layer, can be clearly observed only at 3×1017 ions/cm2. The tetragonal crystallization phase of tungsten silicide, starting to form at an annealing temperature of 550°C and resulting in larger mean depth of the synthesized layer, is clearly observed at annealing temperatures above 800°C for both ion fluences. The microstructure of the as-implanted tungsten silicide film possesses amorphous and non-continuous properties as well as a rough surface. However, surface roughness can be markedly improved and a continuous and epitaxial layer of tungsten silicide can be obtained when the annealing temperature is increased to 800°C or higher.