Diamond thin film growth by pulsed microwave plasma at high power density in a CH4–H2 gas mixture

Microwave plasma assisted chemical vapour deposition in hydrogen–methane gas mixture is one of the most widespread methods used for the growth of diamond thin films. It has been proved that at low microwave power (300 W) the modulation of the power can lead to a significant improvement of the growth rate or the quality of diamond films. In this work, we show that this advantage can be validated at an industrial scale in reactors working at higher power (peak power up to 6 kW). The pulsed mode operation shows that whatever the frequency or the duty cycle, the quality of diamond (according to the micro-Raman spectroscopy) can be appreciably improved at the same growth rate. An optimal range of temporal parameters as the pulse and afterglow duration, corresponding to an optimal range of frequency and duty cycle has been determined. The main reactive species observed by optical emission spectroscopy in such plasmas are the CH and C2 radicals as well as atomic hydrogen. The chemistry of CH4/H2 discharge is very rich in hydrocarbon radicals. Nevertheless, some works have shown that the CHx radicals are the key species for the diamond deposit whereas C2Hy is rather responsible for the graphite growth and the atomic hydrogen preferentially for the graphite etching. Time resolved optical emission spectroscopy and double pulse technique have been carried out to investigate the evolution of the active species during the discharge on-time as well as during the afterglow. These plasma studies lead to a better understanding of the advantages in modulating the power and allow establishing some correlations between the temporal evolution of the observed species and the characteristics of the deposited films.