Reactive Sputtering of Nanostructured WC x
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Reactive Sputtering of Nanostructured WCx Abdul K Rumaiz 1, S. Ismat Shah 1,2, C. Ni 2, J. Derek Demaree 3, J.K. Hirvonen3. 1. Department of Physics and Astronomy 2. Department of Materials Science and Engineering University of Delaware, Newark, DE 19716. US Army Research Laboratory, Aberdeen Proving Ground, MD. ABSTRACT: Nanostructured WCx thin films were reactively sputtered in argon-methane plasma. A DC planar magnetron with a pure tungsten target was used. Films were deposited on glass, quartz, silicon and sapphire substrates at temperatures up to 700 oC. The carbon content in the film was varied by changing the partial pressure of methane. The carbon content in the films was analyzed by using X-ray Photon Spectroscopy (XPS) and Rutherford Backscattering Spectroscopy (RBS). The effect of temperature on the grain size was studied using Transmission Electron Microscopy (TEM). The target poisoning behavior was studied by measuring the target current. The critical methane concentration at which the metal – poison mode transition occurs was measured to be around 42% of CH4 in Ar. This transition is characterized by a sharp fall in the target current. The hysteresis in the target current can also be attributed to target poisoning. In this paper we describe the effects of film growth parameters on the film composition. We also discuss an alternate sputtering technique, hollow cathode sputtering, and the advantages of this technique over the conventional planar magnetron technique. INTRODUCTION: Tungsten carbides with their superior electrical, mechanical and chemical properties are of interest for a variety of applications including electronics, fuel cells, catalysis, etc. Their hardness at high temperatures finds application in many mechanical processes [1]. Their good electrical conductivity combined with high temperature stability make them ideal candidate for thin film diffusion barrier materials for microelectronic devices [2]. Their chemical properties make them an ideal replacement for the more expensive Pt group metals for automobile catalytic converters [3]. The current regulation based on 1990 Clean Air Act Amendments (CAAA) require a vehicle’s NOx emission to be less than 0.6g/mile within 100,000 miles duration. However, starting from 2004 a more stringent standard will be enforced which will require the emission of NOx to be reduced to 0.2g/mile [4]. Considering the limited availability of Pt based catalyst, it has become imperative to find an alternative catalyst. The main objective of our work is to synthesize nano structured WCx thin films and characterize the films for the structural and chemical properties. We used the method for reactive sputtering wherein the inert gas (Ar) and the reactive gas (CH4) are introduced into the chamber. The Ar plasma formed the metal target. The sputtered flux, predominantly atomic, reacts with the CH4 to form WCx. Though this method seems promising, it has some disadvantages like hysteresis in all deposition parameters, poor growth rate close to the poison mode
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