Hydrogen post-etching effect on CVD diamond film

Abstract

In recent years, diamond films have been successfully synthesized from the gaseous phase by various chemical vapour deposition (CVD) methods [1-5]. Most of the methods use hydrogen-diluted hydrocarbon gas as the carbon source and the hydrogen is regarded as playing an important role in the diamond synthesis. In the low pressure and low temperature region, the diamond is thermodynamically unstable relative to the graphite. Chemical kinetics is believed to be important in the formation of diamond phase under CVD conditions. During the deposition, it is proposed that atomic hydrogen etches away the deposited nondiamond phases such as graphite and amorphous carbon [1]. The etching rate of the non-diamond phases is believed to be chemical-kinetically faster than that of the diamond phase. Angus et al. [6] mention the selective cleaning effect of high temperature and high pressure H2 (1 033°C, 50atm) on the graphite and diamond seeds. However, there is no direct evidence which proves the etching of nondiamond phases by hydrogen activated by the thermofilament during synthesis. In CVD diamond synthesis, one of the final objectives of the study is the clarification of the reaction mechanism. In this study, hydrogen post-etching was performed on a diamond film which was synthesized by the simple thermofilament CVD method. The results give a better understanding of the functions of the hydrogen. A schematic illustration of the experimental apparatus is shown in Fig. 1. The source gas was a 2.5 vol % methane (CH4) and 97.5 vol % hydrogen (H2) mixture. A single-crystal silicon wafer with (l 1 1) plane on the surface was used as a substrate. Synthetic conditions were as follows: substrate temperature 800 ° C, tungsten filament temperature 2200°C, total gas pressure 20 torr, gas flow rate 400 sccm and reaction time 12h. The substrate temperature and filament temperature were measured with a Pt-13RhPt thermocouple and an optical pyrometer, respectively. Deposited film was characterized by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD) and Raman spectroscopy. For post-etching, the film-deposited specimen was set on a sample holder in the reaction chamber. The post-etching conditions were as follows: substrate temperature 800°C, filament temperature 2200°C, hydrogen pressure 20torr, flow rate 400 sccm and post-etching time 30min. These were the same as the above synthetic conditions except for the absence of methane and the reaction time. The post-etched film was characterized with SEM, XRD and Raman spectroscopy. Figs 2a and b show SEM micrographs of the asdeposited and hydrogen post-etched film surfaces, respectively. Polycrystalline film was obtained by the