Thermogravimetric analysis of the oxidation of CVD diamond films
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I. INTRODUCTION
Diamond synthesis by chemical vapor deposition (CVD) has opened the door to a wide range of applications that take advantage of the exceptional mechanical, thermal, optical, electrical, and chemical properties of diamond.1'2 Some of these potential applications involve elevated temperatures where one must be concerned with possible oxidation of the diamond. The oxidation rate of natural diamond becomes appreciable near 600 °C.3 In order to determine the suitability of CVD diamond as a high temperature material in atmospheric environments, thermogravimetric analyses were carried out. Kinetic data were collected to determine the activation energy of the oxidation, and partially oxidized films were analyzed by scanning electron microscopy (SEM) and Raman and Auger electron spectroscopies. Accounts of our preliminary thermogravimetric analysis (TGA) studies have appeared previously.4'5 II. EXPERIMENTAL
Films were grown by microwave plasma assisted CVD following the procedure from AT&T Bell Laboratories,6 as reported previously.4 Briefly, films were deposited on diamond-powder-abraded Si wafers generally using a gas mixture of 2.1% CH4 and 1.0% O2 in H 2 at 2.8 kPa (21 Torr).6 The substrate temperature was not measured but is estimated to be 700-900 °C, based on the red glow observed from the Si surface facing away from the plasma. The residual diamond seed mass was well below 1% of the film mass. The Si substrate was removed by dissolution in a concentrated HNO 3 /HF (about 1:5) aqueous solution. The films usually contained both faceted and nonfaceted regions which were separated by breaking up the film with a razor blade. Parts of the film that grew on the edge of the substrate were trimmed off. Average film thicknesses were calcu2320
lated from the area of the film assuming the density of pure diamond, 3.5 g/cm3. The calculated result was in agreement (within about 20%) with the average of film edge thicknesses observed under a microscope and with thicknesses calculated from interference fringe spacings in IR transmission spectra. Edge thicknesses varied by as much as a factor of two with a typical ratio of maximum to minimum thickness of about 1.5. The thickness of a sample deposited from 4% CH4 (17 ^u,m) was determined from IR spectra. TGA experiments were conducted on a DuPont 1090 Thermal Analyzer and a DuPont 951 Thermal Gravimetric Analyzer. Samples (1.5-3.3 mg) were placed into a Pt boat on the TGA balance beam and slid into a quartz tube furnace. The quartz tube had an internal volume of about 100 ml and was open to the atmosphere through a 4 mm diameter opening at one end, while the other end opened to an unheated chamber of about 300 ml which was open to the air through a 2 mm stopcock. The experiments were generally conducted under a static air atmosphere (local atmospheric pressure 92.7-94.0 kPa, relative humidity 30-40%). Initial TGA runs involved heating from room temperature to 1000 °C at 10 or 20 °C/min. For constant temperature runs at 600 to 750 °C, the sample was heated initially a
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