Spectroscopic Diagnostics of Pulsed Microwave Plasmas used for Nanocrystalline Diamond Growth

Abstract

In 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/H 2 /CH 4 gas mixture. Both the gas temperature and C 2 total density are determined from the C 2 (D 1 S u R S X 1 S g R ) Mulliken system. Time-resolved and timeaveraged 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 C 2 density depend weakly on the pulse repetition rate, with values always in the range 3600-4000 K, and around 1 T 10 14 cm S3, 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 S1 , which is very long compared to conventional H 2 /CH 4 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 C 2 H 2 ! C 2 ! 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 C 2 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.

Deposition of carbon based materials by continuous and pulsed discharges

“…Additionally, the higher ion bombardment can promote the crosslinking and amorphous nature of the films (ii) low DC ((7%) and W average values give a very low radical density, and they polymerise at low deposition rate on the surface. The adsorbed CF 2 11 Equilibrium WCA versus deposition time for CW and PW (DC55%, t P 5320 ms) CF x films: line is only guide for eyes 12 Growth mechanism of CF x films radicals have enough time to diffuse on the surface to lower energy sites, which become the nucleation centres for the formation of nanostructured crystalline Teflon-like ribbons. Under these conditions, the saturation emission intensity profile can be easily explained since a stationary state of CF 2 is rapidly attained due to its low amount…”

“…the time between two successive pulses, and the duty cycle [DC5(t ON / t P )6100]), indicating the percentage of the period in which the discharge is ON. The investigation of these two modulation parameters has been widely documented in the literature by considering various types of discharges (dc, rf and mw) [7][8][9] and different gases employed in material processing, 4,8,[10][11][12][13][14] as well as from theoretical and experimental points of view. 9,15 Generally, the beneficial effects of modulation are observed when the pulse period is shorter than the species residence time in the reactor as well as the lifetime of the reactive species; on the contrary, at pulse periods larger than the residence time and lifetime, the system reaches a state similar to the CW conditions, and the modulation becomes ineffective.…”

Study of polycrystalline diamond deposition by continuous and pulsed discharges

“…Analogously, the papers of refs. [10,12] maintained the same peak power, but without keeping constant the deposition temperature; conversely several papers [3,8,[10][11][12][13][14][15] chose to use the same average mw power in PW and CW regimes.…”

“…The growth of diamond-based materials as single- [9], poly- [10][11][12][13][14] and nano-crystalline diamond films [8,15] by means of PW microwave (mw) plasmas has been and is still a hot research topic. Several explanations of the obtained results have been put forward.…”

“…The investigation of these two modulation parameters has been widely documented in literature by considering various types of discharges (dc, rf and mw) [1][2][3] and different gases employed in material processing, (i.e. material treatment [4], thin film etching [5] and deposition [2,6]) from theoretical and experimental points of view [3,7,8].…”