Spectroscopic Diagnostics of Pulsed Microwave Plasmas used for Nanocrystalline Diamond Growth

Abstract

AbstractIn this paper, we report emission and broadband absorption spectroscopy measurements carried out in order to characterize the pulsed process used for nanocrystalline diamond (NCD) film deposition using an Ar/H2/CH4 gas mixture. Both the gas temperature and C2 total density are determined from the C2 (D1Σu+ − X1Σg+) Mulliken system. Time‐resolved and time‐averaged measurements are performed in order to probe the influence of the pulse repetition rate for a duty cycle of 50%, keeping a constant time‐averaged microwave power. Time‐resolved fast imaging enables the two‐dimensional examination of plasma ignition and expansion during the microwave pulses. The time‐averaged gas temperature and C2 density depend weakly on the pulse repetition rate, with values always in the range 3600–4000 K, and around 1 × 1014 cm−3, respectively. These values are close to those measured for a continuous discharge, which suggests that the changes of the NCD growth process should be linked to the time evolution of the gas temperature and key‐species density. The time evolution of the gas temperature is characteristic of a heating rate of approximately 100 K ms−1, which is very long compared to conventional H2/CH4 microwave discharges. Variation of 1000 K is thus obtained during the microwave pulse at 50 Hz, which implies important changes in the plasma composition during the microwave period. Also, the time‐resolved fast imaging of the plasma discharge for a 50 Hz pulse repetition rate shows that the plasma volume and intensity evolve almost all over the microwave pulse. A considerable time evolution of some plasma characteristics such as absorbed microwave power or species density and gas temperature radial profiles is then expected. Finally, since the C2H2 → C2 → C conversion channels are mainly driven by the gas temperature, the increase of the gas temperature during the microwave pulse seems to lead first to the production of the C2 molecule, while C atoms are produced at the end of the microwave pulse when the gas temperature is high enough. Such time evolution of the plasma chemistry should influence the NCD deposition process in pulsed mode.