Diamond films are multifunctional materials with a wide range of extraordinary properties and a high application potential in various fields. In this work, we investigate the low temperature (< 500 °C) diamond films chemical vapor deposition on fused silica substrates realized in two different focused microwave plasma systems, i.e. a multimode clamshell cavity reactor (MCC) and a rotational ellipsoid cavity (REC) reactor. During the experiments, the methane to hydrogen ratio, in the hydrogen-rich process gas mixture, varied from 1 % to 15 % for MCC and from 1 % to 9 % for the REC reactor. For both deposition systems, the methane increase led to an increase in the diamond growth rate, e.g. it raised at least by a factor of 2.6 for the MCC reactor. Furthermore, the MCC reactor also allowed the growth of well-shaped diamond crystals at a methane to hydrogen ratio as high as 15 %. This finding is in good agreement with the Raman shift measurements, which also revealed a low content of non-diamond phases in the films making the MCC reactor more preferential for industrial uses. Moreover, the enhanced diamond growth at low temperatures is further advantageous for overcoating fused silica as well as thermally sensitive substrates which altogether opens new vistas for diamond application in optically related fields (optical elements, photonic crystals, sensors).Keywords: Well-faceted diamonds, low temperature deposition, high growth rate, low non-diamond content, focused plasma
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