Nucleation and Growth of Diamond Thin Films on (100) Silicon with Various Surface Preparations

Diamond crystallites and continuous films were deposited on (100) silicon by microwave plasma assisted chemical vapor deposition, MPACVD, at times varying from 2 minutes to 1600 minutes. Various surface treatments were used: untreated, polished with 1um diamond, scratched with 350 mesh SiC, scratched with 1um alumina, wiped with 350 mesh graphite powder, spin coated with PMMA, and wiped with 10% Pd on alumina and 10% Pd on graphite. Two concentrations of methane in hydrogen were used: 0.5% and 2.9%. The diamond crystallites were counted and measured systematically from SEM micrographs. In each experiment, except those where the Pd catalysts were applied to the silicon, only a small range of crystallite sizes were detected, the average diameter increased linearly with time, and the density of crystallites was essentially constant regardless of the time of the experiment. Thus nucleation of the diamond occurs within a short time interval early in the deposition process and after the nucleation event only growth occurs. With untreated silicon substrates and deposition conditions; 0.5% CH4 , pressure 27 torr, and substrate at 830°C, the median crystallite density was 129/mm2. Applying the Volmer-Weber model, an activation energy for nucleation of diamond on silicon was calculated, 52 kcal/mole. Of the surface treatments used, only diamond polishing and the Pd catalysts effected the crystallite density compared to the untreated substrate. When the silicon substrate was diamond polished, and the deposition conditions were 0.5% CH4 and pressure 27 torr, the crystal density clustered near the median of 5.5x105/mm2, again, regardless of deposition time. When the silicon substrate was treated with the Pd catalysts, the crystallite density increased and a different distribution of diamond crystal sizes was associated locally with the catalyst on the surface. Furthermore, the other surface treatments had no effect on growth rates or morphology. Growth rates were determined by least squares fits to average diameter vs. time for crystals and average thickness vs. time for continuous films. For these trials when the methane concentration was 2.9%, pressure 27 torr, and substrate temperature nominally 820°C, the films grew at 0.88um/hr and when the methane concentration was 0.5%, the growth was 0.48um/hr. As the diamond crystallites grow, they eventually impinge upon each other and form a continuous film with columnar grains. Only the diamond polished substrates had sufficiently high crystal densities to form diamond films within the time of these experiments. The two methane concentrations resulted in two distinct morphologies of the crystallites and surface textures of the continuous films. When the methane concentration was 0.5%, the crystallites were faceted hemispheres with predominantly triangular faces and the films were coarse textured with similar faceting on the surface. When the methane concentration was 2.9%, the crystallites were smooth hemispheres and the films were also relatively smooth