engleski

beta-W PHASE OCCURENCE AND STABILITY IN SPUTTER-DEPOSITED TUNGSTEN THIN FILMS

Interest for metallic tungsten films in modern technologies has been motivated by its useful properties, such as high electrical conductivity, high melt temperature, high mechanical strength and corrosion resistance. In this work we report the effects of deposition conditions on the occurence of beta-W phase, and correlate it to the amount of strain in thin tungsten films deposited by magnetron sputtering. The oxygen content in beta-W phase is determined, and beta-W thermal stability is examined. Tungsten thin films were deposited onto glass, monocrystalline silicon, and sapphire substrates, in a two-magnetron aputtering device. The effects of Ar working gas pressure (0.7-2.8 Pa), substrate temperature LN2, RT and 2500C, and deposition duration (15, 30 and 60 min) on the film phase composition were examined. The phase and microstructure of the deposited W films were analyzed by X-ray diffraction (XRD). The residual lattice micro strain and grain-size were determined by several methods of the XRD peak line profile analysis. The macrostrain results were obtained by measurements of thin glass substrates deformation caused by the deposited films. The oxygen content was determined by bombardment of the films with 0, 85 MeV D-atoms, and measuring proton production due to the nuclear reaction 16O (D, p) 17O, and corroborated by the RBS for films with higher oxygen content. Thermal stability of the beta-phase containing tungsten films was determined by in-situ monitoring of the film electric resistivity variation during heating up to 700°C in vacuum. In the examined range of deposition parameters, the prepared films generally contain both stable alfa-W phase (bcc) and metastable A15 beta-W phase (W3W). Lower working gas pressure, higher substrate temperature and greater film thickness enhances the a-W fraction, while the opposite conditions favor the formation of beta-W phase. The stress of the prepared films is strongly compressive at 0, 7 Pa Ar, diminishes with the working gas pressure increase, and at 2, 6 Pa Ar turns into tensile stress. The beta-W phase fraction and the macrostrain evolution with the working gas pressure correlate very well. The oxygen content of the films increases with the working gas pressure (from 1 at. % at 0, 7 Pa, to about 13 at. % at 2, 8 Pa Ar). The preliminary results show that beta-W phase extended transformation to a stable alfa-W structure starts at about 300°C.

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