Influence of substrate material on the initial thin film growth during ion deposition from a glow discharge

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If the argon is replaced by e.g. CH 4 during rf-sputtering, a carbon film will be grown on the rf-excited electrode. Two major processes take place simultaneously at this electrode: physical sputtering of the electrode surface by ionized energetic methane fragments. Ionized energetic methane fragments stick to the electrode surface, thereby forming a carbon film. Depending on the sticking coefficients and sputtering yield values of the methane fragments onto the exposed substrate surface there will be a certain time interval before the substrate surface is completely covered by a carbon layer. The average carbon film thickness d( t) does not increase linearly with time at the initial stage of growth (i.e. before the carbon film completely covers the substrate—electrode surface). The reason for this effect is that during the initial film growth the substrate surface gradually changes from a 100% substrate material surface to a 100% carbon film surface. Since the sticking coefficients of the methane fragments onto these two materials may differ, a non-linear growth rate will occur. Experimental results are presented that verify that the initial carbon film growth rate depends on the substrate material. By considering the combination of sputter etching and deposition a model that describes the general behaviour of the initial growth has been developed. The growth rate reaches a steady state value at an average thickness d 0 of only 20–50 Å. Since this represents only 10–20 atomic layers, considerable fluctations in film thicknesses at different points may occur due to the statistical nature of the process. To get information about the expected fluctuations a simple Monte Carlo simulation of the process has been carried out. From this simulation we obtain a first order approximation about film roughness, atomic mixing and film formation at the substrate / carbon film interface. These results also confirm the growth mechanism responsible for different growth rates on different substrate surfaces.