Diamonds were deposited onto the silicon substrate from the decomposition of a gas mixture of hydrogen($H_2$) and methane($CH_4$) using a hot-filament chemical vapor deposition(HFCVD) method. In order to obtain a fundamental information for the growth process of diamond from the gas phase, the relationship between the change in filament characteristics and diamond growth, the effects of deposition parameters and the growth kinetics of diamond film have been studied. The interface between diamond films and silicon substrate has been also investigated to obtain a better understanding of the characteristics of diamond-substrate interface.
At low filament temperatures and/or high methane concentrations, the carburization of the tungsten filament and the deposition of graphite on the surface of the filament take place, while only carburization of the filament occurs at high filament temperatures and/or low methane concentrations. Under the conditions for graphite deposition on the filament, ball-shaped diamond particles are obtained on the silicon substrate. It is suggested that the deposition of ball-shaped diamond particles is attributed to the etching process of graphite by atomic hydrogen at the filament resulting in severely reduced hydrogen atom concentrations reaching the deposition layer on the substrate. Consequently, it is considered that high quality diamonds can be only obtained under the conditions where no condensed phases are deposited on the filament(i.e., high filament temperatures and/or low methane concentrations) in hot-filament chemical vapor deposition(HFCVD).
Deposition of diamond under CVD conditions is a case of crystal growth far from equilibrium conditions in competition with the more stable graphite which may indeed become incorporated in the growing diamond. Therefore, the deposition conditions for high quality diamond should be determined to study the kinetics of deposition of a pure diamond. In order to determine the deposition conditions for high quality diamond(i.e., the deposition conditions for the kinetics study), the effects of deposition parameters (gas pressure, flow rate, substrate temperature and methane concentration) on diamond growth have been investigated under the conditions where no condensed phases are deposited on the filament. The surface morphologies and preferred orientations of diamond films do not change significantly with deposition conditions. However, from the variations of Raman spectra with deposition parameters, it is observed that diamonds of the best quality are deposited under the conditions of low pressure, high flow rate, low substrate temperature and low methane concentration under these experimental conditions. And the deposition conditions for the kinetics study are determined from the variations of Raman spectra of diamond films with deposition conditions. The growth kinetics of diamond film has been studied under these conditions.
Diamond film thickness is directly proportional to the deposition time. This indicates that diamond films are deposited in the steady state. The variation of growth rate with substrate temperature shows a distinct maximum. The maximum temperature is shifted somewhat to the higher temperature side with increasing methane concentration. It is found that this behavior is due to the decrease of supersaturation with increasing substrate temperature. However, the growth rate of diamond film increase linearly with methane concentrations. Therefore, the growth rate of diamond film can be described by the rate constant for a first-order reaction. Form the kinetic data of diamond deposition, the deposition of diamond is controlled by the surface reaction(activation energy of 11 Kcal/mole) at the lower temperature range, while the deposition is gas phase diffusion controlled(activation energy of 3.8 Kcal/mole) at the higher temperature range.
The interface between diamond films and silicon substrate has been also investigated. In order to investigate the interface morphology, the backside surface of diamond film and the substrate surface have been observed after the deposited film had been separated from the silicon substrate. On the backside surface of the separated film, the vacant space between particles is found and the central part of each particle caves in. Also, many of the hillocks are present on the Si substrate surface after the film is removed. From these observations, it is found that the interfaces between diamond film and silicon substrate is very rough. It is suggested that such interface morphology is attributed to the etching of the silicon substrate which takes place at the early stage of the synthesis. For the energy dispersive spectroscopy experiment of the backside surface of the separated film, more silicon is detected at the central part of each particle. This result implies that strong Si-C bonds are locarized at the center of each particle, which would result in the poor adhesion.
