Nucleation and growth behaviors of diamond have been investigated with applied voltages between d.c. electrodes installed in a hot filament chemical vapor deposition (HF-CVD) system.
Firstly (in chapter 1), to verify the role of electrical bias in the bias methods, where the substrate holder is positively or negatively based against a counter electrode, various configurations of d.c. electrodes have been used with or without biasing the substrate(pre-scratched or virgin Si, pre-scratched Mo). The effects of applied voltage appear only when a glow discharge occurs between d.c. electrodes with any electrical configuration. Nucleation and growth of diamond always enhances around the negative glow of d.c. discharge, though the electron or ion bombardment dose not occur on the substrate. No positive effect of electron or ion bombardment on diamond deposition has been found. On the contrary, the electron bombardment on a substrate itself suppresses diamond deposition when its density is high (above a current density of 30 mA/cm2). And, the ion bombardment on a growing surface itself results in deposition of amorphous carbon at relatively a low negative bias voltage, below 170V, and etching of diamond at a high voltage, above 170V in our experimental conditions. These results come to the following conclusions: the role of applied voltage in the bias methods is generating the d.c. discharge around the substrate independent of the bias sign of the substrate holder; the negative glow of d.c. discharge enhances the nucleation and growth of diamond; the gas activation in the negative glow region might be truly responsible for the enhanced deposition of diamond in the bias methods.
Secondarily (in chapter 2), to know the cause of disturbance of homo-epitaxial growth of diamond during BEN(bias enhanced nucleation) process, the growth of diamond film under negative glow of d.c. discharge has been investigated. On a diamond film directly biased with -100V (against a hot filament), amorphous carbon with nano-crystalline diamond forms. Under normal deposition condition after negative bias, a number of secondary diamond grains grow on a primary one. Meanwhile, under negative glow generated with a separate diamond pre-coated cathode at the same voltge, the diamond film grows homo-epitaxialy. These results indicate that the ion bombardment causes the disturbance of homo-epitaxial growth of primary diamond during BEN. The nucleation of diamond can be enhanced even without ion bombardment when a negative glow is sufficiently approached to a substrate. In this case, the disturbance of homo-epitaxial growth does not occur. Hence, the better quality of diamond nuclei can be achieved if the BEN process is modified on this principle.
Lastly (in chapter 3), the etching of diamond by negative bias has been investigated with 10 mm thick diamond films (pre-deposited on Si). Under normal deposition atmosphere (1 vol % $CH_4$, 20 Torr), the etching rate of diamond film under negative glow is about 2 mm/min at a bias voltage of 250 V. This rate is ten times higher than that (200 nm/min) of previous results by the other methods. The preferential etching occurs at the exposed convex corner or edgy of each diamond grain, and thus planarizes the film surface. When a part of diamond film is masked with gold film, the masked portion is prevented for etching. These results show that diamond film can be planarized or selectively etched (for patterning) in HF-CVD system. The increased ion concentration in the negative glow and the accelerated ion bombardment owing to sheath potential might enhance the diamond etching under the negative glow.
