X-ray Photoelectron Spectroscopy of the Interface Between Diamond Films and Tantalum Substrates
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ABSTRACT Diamond films were deposited in a microwave plasma chemical vapor deposition (MPCVD) system on Ta substrates using a mixture of hydrogen and methane gases. The films were grown for varying lengths of time to provide samples with no diamond growth to a continuous diamond film. These films were analyzed using X-ray photoelectron spectroscopy (XPS) in order to understand the time dependent interactions between the substrate and the incoming carbon flux. Photoelectron peaks in the Ta 4f, C ls and 0 ls regions have been analyzed. In the initial stages of growth, a layer of carbide forms on the substrate. As the substrate becomes supersaturated with carbon, graphite starts to form on the surface. A diamond peak begins to appear after about 30 minutes of deposition. INTRODUCTION Diamond, due to its hardness and chemical inertness, is a good candidate for protective hard coatings applications. Diamond coated tool bits are already being produced commercially and several papers related to the diamond deposition for abrasion resistance applications have appeared in the literature.(1 -3) However, diamond deposition on metallic substrates is nontrivial due to (i) little or no interaction between the metal and the carbon, e.g., Cu, Au, Ag, etc., or (ii) the substrate acting as a C sink delaying diamond nucleation, e.g., Fe, Ni, Pt, etc.(4) Metals that readily form carbides are supposed to be easier for the deposition of diamond films due to the formation of a reacted interface but even in this case a large difference in the thermal
expansion coefficients of the metals and diamond film makes the adhesion almost impossible. Further complications might also exist where more than one carbide is formed, each having a different thermal expansion coefficient, as is the case with Ti. Other factors affecting growth are the difference between the lattice constants and surface energies of diamond, the metal and the metal carbide. (5) The film-substrate surface interaction, as stated before, can greatly influence the time needed to nucleate and grow diamond film on the substrate.( 6-7) Transition metals belonging to the TiV, and Cr subgroups are some examples of metals which do form carbides yet there is very little adhesion of diamond films to these metals. Due to a large difference in the thermal expansion coefficients of diamond (0.8 x 10"6/oC) and these metals (4.5 x 10-6/"C for W) at room temperature(8 ), a thicker interface is desired to accommodate the stresses generated during cooling from the process temperatures. Since film growth is a dynamic process, if the film surface moves faster than the interdiffusion of C and the metals at the interface, the reaction to form carbide slows down and, ultimately, stops. This results in a very thin carbide layer that is incapable of accommodating the stresses generated during the cooling and, as a result, the film peels off immediately upon cooling. Slow deposition of the film allows a thicker interface formation and, therefore, better adhesion of the film to the substrate. Although
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