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 × 10 17 and 3 × 10 17 ions/cm 2 . 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 × 10 17 and 3 × 10 17 ions/cm 2 , 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 × 10 17 ions/cm 2 . 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. © 2001 Elsevier Science B.V. All rights reserved.