Diamond Deposition by a Nonequilibrium Plasma Jet
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DIAMOND DEPOSITION BY A NONEQUILIBRIUM PLASMA JET D.G. KEIL, H.F. CALCOTE, AND W. FELDER AeroChem Research Laboratories, Inc., P.O. Box 12, Princeton, NJ 08542 ABSTRACT A nonequilibrium plasma jet has been used to deposit diamond films on a number of substrates, including silicon, silicon nitride, alumina, and molybdenum. Hydrogen is passed through a glow discharge and expanded through a supersonic nozzle to produce a highly nonequilibrium jet. Methane is added downstream of the nozzle, where it mixes and reacts with the nonequilibrium concentration of hydrogen atoms. The resulting supersonic jet strikes the substrate surface producing a high quality (determined by laser Raman spectrometry) adherent diamond film. Because of the low jet temperature, substrate cooling is unnecessary. Diamond deposition rates have exceeded 2 mg/kWh at andsubstrate I um/h averaged over below 16 cm2600 area; prepared temperatures K. good quality films have been INTRODUCTION The ability to produce CVD diamond films at practical rates and thicknesses blossomed with the use of "activated" hydrogen along with a carbon source gas [1,2]. This early work spawned the development of a wide variety of techniques to activate hydrogen (and carbon species) to deposit diamond films. Filamentassisted CVD (FACVD), microwave plasma-assisted CVD (MWCVD), thermal arc jet CVD (arc jet), and combustion gas CVD (torch) techniques have been successfully used to produce quality polycrystalline diamond films. Differences in the characteristic hydrogen activation processes of each technique define an operating "envelope" of deposition rate and diamond film quality (area, uniformity, morphology, adhesion, etc). Encouraging attempts have been made to connect them within a global description of diamond deposition[3]. However, even if similar chemistry is occuring in these CVD processes, there are practical criteria which differentiate them. The operating envelopes can be translated into advantages and disadvantages for the processes. Based on the current understanding of the CVD diamond mechanism and the characteristics of other CVD techniques, we have developed a new technique designed to optimize the diamond deposition environment. It utilizes efficient, low temperature activation of hydrogen in a nonequilibrium plasma, and expansion of the plasma into a supersonic jet which, when admixed with a carbon species, rapidly transports diamond precursors to the growing diamond film surface. AEROCHEM NONEQUILIBRIUM PLASMA JET The nonequilibrium plasma jet (NEPJ) was originally developed at AeroChem over 30 years ago[4]. Its utility as an efficient source of atomic hydrogen for synthesis was demonstrated in a successful program to deposit photovoltaic grade amorphous silicon. We have since demonstrated and patented[5] its application to diamond film technology, and we anticipate its utility in many CVD processes. Apparatus The basic apparatus, as illustrated in Figure 1, is divided into two sections separated by a nozzle across which supersonic flow develops. The high pr
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