Tungsten Multilayers

Different phases of tungsten are produced by magnetron sputtering under different preparation conditions: stable alpha-W phase (b.c.c. structure) is obtained at low working gas pressure, metastable beta-W phase (A15 struc-ture) is obtained at increased working gas pressure, and amorphous-like tung-sten (a-W) is obtained at even higher pressure. The occurence of various phases is related to the oxygen content and built-in stresses in the prepared thin films. We employed these facts to produce tungsten films composed of stacked layers of different tungsten phases. Tungsten multilayers were prepared by sequential deposition of tungsten sputtered under different con-ditions in a cylindrical magnetron device. Working gas pressure was varied in the range of 0, 7 - 3, 5 Pa, and circular substrates (monocrystalline silicon, borosilicate glass) were cooled in contact with the LN2 container. The aver-age current density at the target was kept constant for all depositions, and the particular/single layer thickness was determined by the deposition time. Various combinations of alpha-W layer with beta-W phase or amorphous-like tungsten layer were prepared: simple bilayers, several times repeated bilayers, and inversely-sequenced multilayers. The thickness of single-phase layers was varied, as well. The deposition rate was about 0, 17 nm/s, and the total film thickness range was 50-200 nm. Phase composition of the films was examined by the XRD: The prepared films generally contain stable alpha-W phase layers prepared at low argon pressure, alternating with the layers of metastable beta-W phase or amorphous-like tungsten prepared at 3.5 Pa argon pressure. From the results on single-phased films it is expected that the stress of the layers prepared at 0, 7 Pa is strongly compressive, while the alternating layer is in tensile stress. The stratification of the films was examined by the X-ray re ectancy and GISAXS measurements, using the X-ray beam energy of 8 keV. From the results both total thickness of the prepared films and the thicknesses of the separate layers in a periodic multi-layers were determined. It was found that the tungsten deposition rate at 3, 5 Pa argon pressure is about 15The overall stress in prepared tungsten films results from the opposite stresses of individual layers. However, the effective stress cannot be simply derived from data for single-phase films, probably due to the variation of stress with the layer thickness and the contribution of the interfaces. Profilometric measurements show that by combining several