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LOW TEMPERATURE SYNTHESIS OF POLYCRYSTALLINE DIAMOND BY MICROWAVE PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITION R. RAMESHAM, C. ELLIS, T. ROPPEL, D.A. JAWORSKEt, AND W. BAUGH ELECTRICAL ENGINEERING DEPARTMENT ALABAMA MICROELECTRONICS SCIENCE AND TECHNOLOGY CENTER AUBURN UNIVERSITY, AL 36849-5201 t LEWIS RESEARCH CENTER NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CLEVELAND, OH 44135-3127 ABSTRACT Polycrystalline diamond thin films have been deposited on single crystal silicon substrates at low temperatures ( < 950' C) using a mixture of methane and hydrogen gases by high pressure microwave plasma assisted chemical vapor deposition. Low temperature deposition has been achieved by cooling the substrate holder with nitrogen gas. For deposition at reduced substrate temperature, it has been found that nucleation of diamond will not occur unless methane/hydrogen ratio is increased significantly from its value at higher substrate temperature. Selective deposition of polycrystalline diamond thin films has been achieved at 600* C using our technique previously used at 930* C. Decrease in the diamond particle size and growth rate, and an increase in surface smoothness have been observed with decreasing substrate temperature during the growth of thin films. The morphology is analyzed by scanning electron microscopy and the as-deposited films are identified by Raman spectroscopy.

INTRODUCTION Diamond is one of the known allotropes of carbon and is considered to date an excellent material for many applications based on its unique physical and chemical properties such as high chemical resistance, high band gap, high thermal conductivity, high electrical resistivity, high optical transparency, high Young's modulus, extreme hardness, high breakdown strength, etc. These features are technologically significant, but they have not been fully exploited so far. There are many potential applications especially in microelectronics, optics, corrosion resistance, mechanical, etc., for polycrystalline diamond thin films that will require minimum stress in the thin films, good adhesion strength to the substrate, very high smoothness of the film and related properties. Figure (1) shows an optical micrograph of polycrystalline diamond cantilever beams of various sizes (one side supported) on a single crystal silicon substrate. Fabrication details of diamond cantilever beams on a silicon substrate are reported elsewhere [1]. It is interesting to note from the figure that the free-standing cantilever beams curl upward slightly as soon as the silicon substrate is etched selectively. This is an indication that residual stress (compressive) exists in the polycrystalline diamond thin films. These films are grown at a substrate temperature of 930* C. The observed stress in a thin film could be due to mismatches in thermal expansion coefficients of silicon substrate and diamond thin film and other factors such as nucleation and growth phenomena. Stress in thin films may be minimized by reducing the diamond growth temperature. This may reflect on reducin