F. Bénédic

Spectroscopic diagnostics and modeling of Ar∕H2∕CH4 microwave discharges used for nanocrystalline diamond deposition

In this paper Ar∕H2∕CH4 microwave discharges used for nanocrystalline diamond chemical vapor deposition in a bell-jar cavity reactor were characterized by both experimental and modeling investigations. Discharges containing 1% CH4 and H2 percentages ranging between 2% and 7% were analyzed as a function of the input microwave power under a pressure of 200mbar. Emission spectroscopy and broadband absorption spectroscopy were carried out in the UV-visible spectral range in order to estimate the gas temperature and the C2 density within the plasma. Infrared tunable diode laser absorption spectroscopy was achieved in order to measure the mole fractions of carbon-containing species such as CH4, C2H2, and C2H6. A thermochemical model was developed and used in order to estimate the discharge composition, the gas temperature, and the average electron energy in the frame of a quasihomogeneous plasma assumption. Experiments and calculations yielded consistent results with respect to plasma temperature and compositio...

Formation of soot particles in Ar/H 2 /CH 4 microwave discharges during nanocrystalline diamond deposition: A modeling approach*

Homogenous mechanism of soot formation in moderate-pressure Ar/CH4/H2 microwave discharges was analyzed with the help of kinetics modeling of the thermally nonequilibrium plasmas. Two main reaction mechanisms based on either neutral molecular growth and condensation reaction nucleation process were considered. These mechanisms were incorporated in a numerical model that solves for the plasma species and energy equations under a quasi-uniform plasma assumption. This enabled us to estimate the plasma species density and temperature along with the nucleation rate at different discharge conditions. The results showed that soot particles might form at significant density values by both neutral and ionic mechanisms. Their formation mainly takes place at the discharge edges where the temperature level favors the development of large molecular edifices. Simulations showed that the formation of soot is unlikely to happen in the bulk of the discharge where the gas temperature is high and the large molecular hydrocarbon (HC) cannot form at significant concentrations.

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

Abstract 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 C 2 radicals as well as atomic hydrogen. The chemistry of CH 4 /H 2 discharge is very rich in hydrocarbon radicals. Nevertheless, some works have shown that the CH x radicals are the key species for the diamond deposit whereas C 2 H y 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.

Improvement of nanocrystalline diamond film growth process using pulsed Ar/H2/CH4 microwave discharges

For the first time nanocrystalline diamond (NCD) films were deposited by the pulsed microwave plasma assisted chemical vapour deposition process starting from an Ar/H2/CH4 gas mixture. Comparisons with continuous mode deposition gave evidence for the improvement in film quality when the microwave power was modulated with a pulse repetition rate in the range 50–1000 Hz. A reduction in grain size and surface roughness, especially at low pulse repetition rate, accompanied by a decrease in soot particle formation was observed. A thermo-chemical plasma model, developed for pulsed Ar/H2/CH4 microwave discharges, provides evidence for the fact that the pulsed mode permits the enhancement of the mole fraction of the C2 dimer assumed to be the growth precursor of NCD. This may be responsible for a high secondary nucleation rate improving the nanostructure of the film in pulsed discharges.

Reactive ion etching of diamond using microwave assisted plasmas

Abstract This work reports on a novel plasma etching apparatus in which the plasma is excited at microwave frequencies and allowed to diffuse to the diamond surface. In this apparatus, an electric bias voltage independent of gas pressure and injected power can be applied. Gas mixtures of O 2 in Ar or Kr, in the pressure range of 3–100 mtorr and microwave input power up to 700 W have been investigated. Optical emission spectroscopy (OES) has been used in order to measure the relative concentration of atomic oxygen. It has been found that the addition of krypton allows the production of atomic oxygen at lower microwave powers and to obtain a stable discharge at lower gas pressures. It also results in diffusion of the plasma at longer distances than in the case of O 2 –Ar mixtures. Etch rates up to 4 μm/h have been observed resulting in vertical wall etched features and relatively smooth etched surfaces, with a surface roughness degradation after etching from 20 to less than 30-nm Ra.

Plasma-Assisted Synthesis: Plasma Experimental Diagnostics and Modeling

Publisher Summary This chapter discusses on a thorough analysis and description of Ar/H2/CH4 discharges ignited in microwave cavity systems and employed for nanocrystalline diamond layer deposition. The purpose is to describe the methodology and tools that must be implemented to improve the understanding and the control of the synthesis process. Low-pressure glow discharges are widely used for material surface processing, such as etching, deposition of thin films, or modifications of surface properties. In the field of diamond film synthesis using plasma-enhanced chemical vapor deposition (PECVD) systems, attention should be paid to the absolute density of key species. Optical emission spectroscopy (OES) is the most popular plasma diagnostic technique because it can be easily implemented and is cost effective. The calculation results are analyzed and discussed, especially in terms of C2 production mechanisms and efficiency.

Nanostructure Of Carbon Dust Generated In Plasmas Of Different Parameters

Carbon nanoparticles are produced in supersaturated carbon vapors of sputtering discharges as well as from complex reactions between hydrocarbon radicals in Ar/CH4/H2 microwave discharges. A summary of their formation is reported. Despite different molecular precursors, it is shown that they can exhibit similar carbon concentric microtexture. Correlations are established between the final nanoparticle solid state and the heating mechanisms which are characteristic of each discharge.

P3O-5 High Frequency and High Temperature SAW Devices Based on Nanocrystalline Diamond Film on Langasite Substrate

The new layered structure diamond/langasite is studied for high temperature, high frequency and relatively high electromechanical coupling (K2) SAW devices. Theoretical study was performed in order to calculate the evolution, as a function of diamond film thickness, of phase velocity and K2 of the Rayleigh mode and its higher modes. The calculation results show that phase velocities up to 12000 m/s and up to 9500 m/s are obtained for the mode 2 and mode 1, respectively, while the mode 0 exhibits high K2 values up to 2.9 %. In order to realize the considered layered structure, first experiments were performed to investigate the nanocrystalline diamond (NCD) films deposition on the non conventional substrate, Langasite. Characterization results show that NCD films with good properties (low grain size, low surface roughness, satisfactory purity) can be grown on Langasite. However, in the considered conditions only an optimal NCD thickness estimated around 1 mum permits to avoid film delamination and cracking when returning to room temperature