The Role of Heavy Hydrocarbons in CVD Diamond Growth
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THE ROLE OF HEAVY HYDROCARBONS IN CVD DIAMOND GROWTH CHING-HSONG WU, T. J. POTTER, AND M. A. TAMOR Ford Research Laboratory, P.O. Box 2053, MD 3028, Dearborn, MI 48121-2053 ABSTRACT A mass spectrometric analysis of heavy hydrocarbons (HHCs) during hot-filament CVD diamond growth was performed together with in situ monitoring of the growth rate. Many HHCs were detected and tentatively identified. Of all HHCs studied, only diacetylene shows good correlation with the diamond growth rate under various deposition conditions. Its possible role is discussed. INTRODUCTION The mechanism of chemical vapor deposition (CVD) of diamond has received considerable attention in the last several years. Quantum mechanical calculation, 2 3 chemical kinetic modelling, 4 and experimental investigations 5s 7 have been performed to determine the monomers for diamond growth. The major focus has been on two species: methyl radical (CH 3) and acetylene (C2 H2 ). In contrast, relatively little is known about the existence and roles of other heavy hydrocarbons (HHCs) containing three or more carbons. Nucleation of diamond crystals from adamantane and saturated ring compounds has been proposed by Matsumoto and Matsui8 and Angus et al. 9 Frencklach1 0 suggested that polycyclic aromatic hydrocarbons (PAHs) contribute to the co-deposition of graphitic components that inhibit the diamond growth. However, the existence of such species and other HHCs under CVD diamond growth conditions has not been experimentally confirmed due to their low concentrations and the difficulty of detecting them. We have performed a study to determine the presence and possible roles of HHCs in diamond growth. A quadrupole mass spectrometer (QMS) equipped with a differentially-pumped molecular beam (DPMB) and a low-pressure quartz probe (LPQP) sampling system was used to analyze the chemical species present during CVD diamond growth. A newly developed laser reflection interferometric (LRI) technique ' was used for simultaneous in situ measurement of the diamond growth rate. This paper presents preliminary results of the detection of HHCs with mass numbers up to 100 in CVD diamond environment under a range of deposition conditions. Details of mass spectrometric analysis, the correlation of some HHCs with the diamond growth rate, and their possible roles in diamond CVD are discussed. EXPERIMENTAL SECTION Two hot-filament reactors were used for this study. A large 30-liter microwave reactor (35 cm ODx3O cm), already equipped with a QMS coupled to the DPMB sampling system, was converted to a hot-filament reactor. A filament assembly was installed beneath the microwave cavity. To facilitate the gas circulation near the reaction zone, a short quartz tube (10.2 ODx 15.2 cm) was positioned around the filament assembly with gas entering from the top end and with the DPMB skimmer orifice at the bottom. The orifice size of the first skimmer was varied from 0.020 to 0.005 cm ID depending on the reactor pressure of 20 to 90 Torr, and the second orifice size was fixed at 0.040 cm ID. Th
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