Phase Analysis in PACVD Diamond Films via Oxidation Kinetics
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PHASE ANALYSIS IN PACVD DIAMOND FILMS VIA OXIDATION KINETICS WILLIAM E. BROWER, JR., AND ROBERT BAUER AND NICK M. SBROCKEY *Marquette University, Dept. of Mechanical and Industrial Engineering, Milwaukee, WI 53233 **Eaton Corporation, Milwaukee,
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ABSTRACT The detection and quantification of the diamond cubic, graphite, and amorphous carbon phases in plasma assisted chemical vapor deposition of diamond films is difficult by diffraction techniques. We have employed thermogravimetric analysis (TGA) to analyze these phases by comparison of the oxidation rates, both in the as deposited films and in physical mixtures of powders of the phases. The oxidation rates of the three allotropes of carbon differ enough to allow a separation of peaks in the differential of the weight loss versus time/temperature plots. TGA scans were run between 2.5 and 100 degrees per minute to achieve optimum separation of the oxidation rates as indicated by the weight loss curves. The peak of the differential of the weight loss versus temperature curves indicates the temperature of maximum oxidation rate. The appearance of multiple peaks appears to be a sensitive indicator of the presence of mixed phases in the PACVD deposites on silicon. INTRODUCTION The field of diamond films has been reviewed by August et al. [1], and Spitsyn et al. [2]. These reviews show an extensive literature on a technique which has produced exciting early results on the formation of metastable diamond by an array of techniques. All of the techniques achieve a successful diamond film to a degree; that is, the diamond phase is detected by diffraction techniques and by Raman Spectroscopy. There is much evidence of the co-deposition of amorphous carbon and graphite with the diamond phases. Williams and Glass [3] used a microwave enhanced plasma CVD system and reacted methane and hydrogen, silimal to the current prosystem and procedure at Eaton. Freeman et al. [4] reported a 200% difference in oxidation kinetics between graphite and diamond via TGA. Johnson et al. [5], also using TGA, reported a 100'C difference between the oxidation kinetics of graphite and a diamond film. An even larger difference of 300%C was noted between diamond powder and graphite. Unfortunately, the two studies do not agree as to the order of stability of graphite and diamond. EXPERIMENTAL PROCEDURE PACVD diamond films and physical mixtures of diamond powder and graphite powder were analyzed in a Perkin Elmer TGA7 thermogravimetric analyzer equipped with a high temperature furnace. Temperature scans were made in a pure oxygen atmosphere to compare the oxidation rates and temperatures for the diamond, graphite, and amorphous carbon phases. Weight loss versus temperature plots, as well as the differential of the weight loss curves are reported here. An onset temperature for oxidation was determined from the weight loss curve and maximum weight loss rate peak temperatures were determined from the differential curve. Phase analysis on the PACVD films was carried out by standard XRD diffractometer tech
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