Sputtered WSi x for micromechanical structures
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Tungsten silicide (WSix) thin films have been investigated for use as integrated circuit interconnect and self-aligned MESFET (metal-semiconductor field-effect transistor) gates because of their low resistivity and thermal and chemical stability. These same characteristics make them interesting materials for prospective use in micromechanical structures. However, little information on residual stresses, elastic moduli, or other micromechanical properties has been available for refractory metal silicide thin films. This paper presents the morphology and stress characteristics of cosputtered WSix thin films, including crystal structure variations and orientation-dependent stresses, as a function of the deposition pressure. The compositions of WSix thin films were analyzed by Rutherford backscattering spectrometry (RBS). The biaxial elastic modulus and thermal coefficient of expansion were found for the sputtered films. Stress-measurement methods and annealing are discussed. Released diaphragms of different sizes and shapes, having controlled residual stress, have been fabricated.
I. INTRODUCTION The excellent micromechanical properties of silicon, together with semiconductor batch-fabrication methods, have led to a young, but established, technology for three-dimensional micromachining of single crystal and polycrystalline silicon and silicon dielectrics. The work described in this paper was an effort to extend this technology to new materials, motivated by the desire to micromachine conductive, stress-controlled diaphragms for capacitive pressure sensors in a process that would be fully compatible with integrated circuit fabrication. Previous research by our group on sputtered molybdenum1 revealed a serious problem for micromachining applications due to the columnar crystal structure of the films. Any nonplanarity on the substrate surface causes a weak line in the molybdenum film, at which it is prone to break. This observation led us to search for a new material which, like molybdenum, could be deposited at low temperature, and would not require high temperature annealing, but which would yield amorphous films. In this project, we characterized cosputtered WSix films with the focus on residual stress, morphology, and micromechanical properties, and explored their application to three-dimensional micromechanical structures. Deposited under certain sputtering conditions, tungsten silicide thin film is amorphous.2 Tungsten silicide has other physical properties that make it desirable for micromechanical structures: it is electrically conductive a)
Present address: Motorola Inc. Advanced Custom Technologies, Semiconductor Products Sector, P.O. Box 20906, Phoenix, Arizona 85036-0906.
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J. Mater. Res., Vol. 10, No. 7, Jul 1995 Downloaded: 13 Mar 2015
(a trait required in our capacitive pressure-sensor application), and its modulus of elasticity is greater than those of other refractory metal silicides. The well-known ability of WSix to withstand high-temperature processing has been a
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