Optimizing diamond growth for an atmospheric oxyacetylene torch
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Optimizing diamond growth for an atmospheric oxyacetylene torch Lua’y A. Zeatoun and Philip W. Morrison, Jr. Chemical Engineering Department, Case Western Reserve University, Cleveland, Ohio 44106 (Received 12 July 1996; accepted 23 January 1997)
Diamond growth conditions for an atmospheric combustion flame have been optimized using statistical experimental design. Films are grown on a molybdenum bolt for 40 min at a distance of 1 mm from the flame cone. The diamond films have been characterized using Raman spectroscopy, x-ray diffraction, and scanning electron microscope. The input process variables are varied over a range of conditions: total gas flow rate Q 2–4 standard literymin, substrate surface temperature Ts 800–1000 ±C, and flow ratio of O2yC2 H2 R 0.93 –0.99. The experimental response outputs are growth rate, full width half maximum (FWHM) of the diamond Raman peak, Raman diamond fraction (b) in the film, ratio of luminescence to diamond peak height (LR), and the relative intensity of the h220j, h311j, h400j, and h331j orientations. The film quality indices FWHM, b, and LR improve by increasing the gas ratio (R), by increasing substrate surface temperature (Ts ), and lowering the growth rate by decreasing total gas flow rate. Diamond film shows a small amount texturing in h220j and h400j orientation at low R and Ts . At high R and low Ts crystals are oriented with the h111j direction normal to the substrate surface. Jet and boundary layer theory have been applied to understand the growth rate, the thickness profile, and the morphological instability of the diamond films. Surface Damk¨ohler calculation shows that the deposition process is marginally controlled by mass transfer. Growth rate of an open flame is higher than for an enclosed flame, while the Raman quality measurements of the enclosed flame are more uniform than open flame over the range of the comparison.
I. INTRODUCTION
Diamond is extreme in its hardness, thermal conductivity, chemical inertness, strength, and the range of its optical transparency. These superior properties make diamond an attractive material for heat sinks, tool coatings, and optics.1–4 Diamond growth using an oxyacetylene torch has become widely recognized as a method for growing diamond films due to its advantages of relatively low setup and operating costs, ease of control, and its exceptionally high growth rate. Counterbalancing these advantages, however, is the limitation of small deposition area,5,6 and a tendency toward morphological instabilities and secondary nucleation as the film grows because of the high arrival rate of species to the film surface.6,7 Although many workers have studied the growth of diamond films using an atmospheric oxyacetylene torch, only a few of these studies have systematically varied more than two torch parameters and measured more than one or two film properties.3,4,8–11 For example, Snail et al.8 have constructed maps of Raman spectra and morphology as functions of gas flow ratio (R
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