Diamond films were deposited by hot filament chemical vapor deposition (HFCVD) using tantalum filaments and acetone (C3H6O) as a carbon source. Diamond films, which were deposited with various processing parameters, were characterized and examined for their deposition behavior.
First, the effect of the filament temperature on the diamond deposition behavior was studied. The temperature distribution inside the system was numerically calculated using the commercial computational fluent dynamics software, ANSYS-FLUENT, for the powers of 12, 14, 16 and 18 kW, which were applied to the filaments.
The power of 16 kW was needed to achieve the optimum temperature, at which the film growth rate is maximal with a minimum amount of the non-diamond phase at a reactor pressure of 4000 Pa and a gas mixture of 400 standard cubic centimeter per minute (sccm) H2 and 90 sccm C3H6O with a working distance of 10 mm between the filament and the susceptor.
To account for radiative heat-transfer in the HFCVD reactor, the discrete ordinate (DO) model was used in the computer simulation. At filament powers of 12, 14, 16 and 18 k, the temperature of the filament surface was calculated respectively to be 2512, 2619, 2715 and 2802 K and that of the susceptor was calculated respectively to be 1076, 1121, 1161 and 1198 K. The temperatures calculated for the filament surface were in good agreement with measured temperatures using a 2-color pyrometer. Second, the effect of the working distance on the diamond deposition behavior was studied. The distance between the filament and the susceptor was varied as 6, 10, 14, 18, 22, and 26 mm. Observation by field-emission scanning electron microscopy (FESEM) showed that at the working distance of 10 mm, the average particle size was the largest with the maximum thickness of the deposited film and the best diamond quality.
Third, the effect of the acetone flux on diamond deposition behavior was studied. For acetone fluxes of 80, 90, 130 and 170 sccm and the respective hydrogen fluxes of 420, 410, 370, and 330 sccm, the film thickness increased with increasing acetone and relatively high quality diamonds were deposited with well-defined facets of (111) and (100). For acetone fluxes of 210 and 250 sccm and the respective hydrogen fluxes of 290 and 250 sccm, however, the diamond quality was degraded with the cauliflower-shape structure being evolved and the film thickness decreased with increasing acetone. The degradation of diamond quality was confirmed by Raman spectra and XRD. Many diamond crystals grown at acetone fluxes of 80, 90, 130 and 170 sccm consist of five (111) facets, indicating an icosahedral structure. At the corner where five (111) facets meet, there were pentagonal dimples, which implies that diamond crystals must have been etched. The decrease in the film thickness at high acetone fluxes of 210 and 250 sccm also implies that the deposited film must have been etched. These results indicate that two irreversible processes of deposition and etching occur simultaneously, which would violate the second law of thermodynamics from the classical concept of crystal growth by an individual atom. These puzzling results could be successfully explained by non-classical crystallization, where the building block for diamond films is nanoparticles formed in the gas phase. At the same time, boron doped diamond (BDD) films were deposited by using trimethyl borate (TMB) as the boron doping source. As the acetone flux was varied, the TMB flux was also varied so that the B/C ratio was fixed at 11400 ppm. As the flux of acetone was increased from 90 to 170 sccm, the grain size and the film thickness of BDD increased. When the acetone fluxes were 90, 130 and 170 sccm, the films showed well-defined (111) facets, indicating that the high quality diamond was deposited. When the acetone flux was increased to 210 sccm, the grain size decreased abruptly and the film thickness, which represents the deposition rate, decreased also. When the acetone flux was increased to 250 sccm, the grain size further decreased, producing a cauliflower structure and the film thickness further decreased. The potential window, which is measured as electrochemical properties of BDD, increased as the acetone flux increased from 90 to 170 sccm and did not change much between 170 and 250 sccm. From these experiments, it was confirmed that varying the flux of acetone could control the morphology, the growth rate and the electrochemical properties of the BDD film.