Abstract
Experimental investigation and modeling of pulsed H2/CH4 plasmas used for diamond deposition are presented. Two plasma configurations are studied : a 2.45 GHz microwave cavity configuration and a 915 MHz surface-wave configuration. Time-resolved measurements of the gas temperature determined from the Doppler broadening of the Balmer Ha line, of the H-atom relative density and of the discharge volume (Vpl) are reported. The experimental time-variations of the gas temperature are characterized by a sharp increase at the beginning of the pulse (t 1 ms). The simulations enable us to estimate time-variations of the electron energy distribution function, gas temperature and chemical species densities. The in-pulse steady state temperature obtained from the model is in agreement with the measured one, although a discrepancy is obtained on the shape of the early time-variation. Calculations were carried out in order to study the effects of the in-pulse power, the duty cycle and the off-plasma time on the H-atom and CH3-radical densities. It is seen that, at a constant power density averaged over a period, low duty cycles favor high H-atom and CH3 - radical densities, while too long off-plasma times reduce the H-atom density during the pulse. In addition, the production of H atoms was seen to be governed by thermal dissociation in the 2.45 GHz microwave cavity system, and by electronic impact dissociation in the 915 MHz surface wave system, the latter operating under high gas velocities.
Highlights
The interest in CVD diamond films worldwide is undoubtedly due to the possibility of synthesis of a large variety of them
The results show that, while microwave power is mainly deposited in the near surface region and the electron temperature follows this behavior rather well, the gas temperature is maximal in the plasma volume
This paper was dedicated to the determination of new driving parameters which control the production and loss of the main active species for diamond growth (H atoms and CH3 radicals) in pulsed plasmas relatively to continuous plasmas
Summary
The interest in CVD diamond films worldwide is undoubtedly due to the possibility of synthesis of a large variety of them. Due to the relatively high cost of production in particular for very high quality and thick films, electronic industry remains still noncommittal about the use of this material. A better understanding of plasma reactors would certainly allow reducing the production cost. In continuous plasmas (cw), due to the high temperatures (2500 K to 3500 K) reached for power density greater than 15 Wcm-3, H-atom production is mainly driven by thermal processes rather than by electronic processes[5]. Due to the high gas temperatures reached in the plasma bulk, a significant increase of the reactor wall temperature is generally observed[6].
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