This work deals with the characterization of the Microwave Plasma Assisted Chemical Vapour Deposition (MPACVD) process suitable for nanocrystalline diamond growth. The microwave system used in this study was a Bell Jar reactor and the discharge was ignited in an Ar/H2/CH4 gas mixture. We were especially interested in the investigation of the effect of the simultaneous increase of the microwave power and H2 concentration. Diamond films were characterized by appropriate surface diagnostic techniques. The plasma was analysed through modelling and spectroscopic diagnostics. Results showed in particular that the increase of H2 amount and microwave power strongly influences the diamond film nanostructure. Besides, the high values measured for gas temperature were in good agreement with those predicted by the model. Eventually, the model showed that the relatively high gas temperature in Ar/H2/CH4 discharges was responsible of a strong thermal dissociation of H2.

Abstract 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.

The thermal and chemical compatibility of silicon nitride ceramics (Si3N4) to diamond ensures an adequate adhesion for tribological applications, such as mechanical seals, biomaterials or cutting tools for hard machining operations. Most of the recent research efforts on these components focused on nanocrystalline diamond (NCD) due to its small grain size and surface smoothness.

Pyrometric interferometry technique is carried out using a bichromatic infrared pyrometer in order to in-situ investigate the influence of the surface temperature variations on the properties of nanocrystalline diamond films, elaborated in Ar/H2/CH4 microwave discharges for different gas compositions. The analysis of the film/substrate system apparent temperature measured by the pyrometer during the synthesis process indicates that the optical absorption of the growing film increases as the deposition temperature is raised. In particular, this trend is underlined by the increase of the film extinction coefficient derived from the experimental curves that ranges from 0.1–0.2 at low and moderate temperatures to 0.6–0.7 at high temperatures. This suggests that the formation of non-diamond absorbent phases within the film, especially sp2-hybridized compounds, is enhanced at high temperatures. This phenomenon is confirmed by further ex-situ characterizations giving evidence for a significant increase with surface temperature of the graphitic contents in the films. This demonstrates that the pyrometric interferometry is a powerful technique suitable for in-situ controlling and investigating the nanocrystalline diamond growth process.

Due to its high-acoustic velocity, diamond, when combined with piezoelectric material, has a promising potentiality for high frequency surface acoustic wave (SAW) devices. In the present work, low-surface roughness nanocrystalline diamond (NCD) were deposited on silicon substrate by microwave plasma assisted chemical vapour deposition process and piezoelectric aluminium nitride films were grown by DC reactive magnetron sputtering. For the first time, SAW devices based on AlN/NCD/Silicon layered structure were successfully performed. The influence of inter-digital transducers periodicity and NCD film thickness on the acoustic velocity of the layered structure was studied. The high frequency characterisation showed that NCD presents a high-surface acoustic velocity similar to that of polycrystalline CVD diamond. Thus, taking advantage of the remarkable surface smoothness of as-grown NCD films, the polishing step of diamond substrates may be no more required prior to SAW device achievement.

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 composition. A relatively high gas temperature ranging between 3000 and 4000K is found for the investigated discharge conditions. The C2 density estimated from both experiments and modeling are quite high compared with what is generally reported in the literature for the same kind of plasma system. It ranges between 1013 and 1014cm−3 in the investigated power range. Infrared absorption measurements and model predictions indicate quite low densities of methane and acetylene, while the atomic carbon density calculated by the model ranges between 1013 and 1015cm−3. The methane and hydrogen introduced in the feed gas are subject to a strong dissociation, which results in a surprisingly high H-atom population with mole fraction ranging between 0.04 and 0.16. Result analysis shows that the power coupling efficiency would range between 70% and 90%, which may at least explain the relatively high values obtained, as compared with those reported in the literature for similar discharges, for gas temperature and C2 population. The high H-atom densities obtained in this work would indicate that growing nanocrystalline diamond films would experience a very high etching. Simulation results also confirm that sp species would play a key role in the surface chemistry that governs the diamond growth.

The spectroscopic characterization of Ar/H2/CH4 discharges suitable for the synthesis of nanocrystalline diamond using the microwave plasma assisted chemical vapour deposition process is reported. The experiments are realized in a moderate-pressure bell jar reactor, where discharges are ignited using a microwave cavity coupling system. The concentration of CH4 is maintained at 1% and the coupled set of hydrogen concentration/microwave power (MWP) ranges from 2%/500 W to 7%/800 W at a pressure of 200 mbar. Emission spectroscopy and broadband absorption spectroscopy studies are carried out on the Mulliken system and the C2(d 3Πg–a 3Πu) Swan system in order to determine the gas temperature and the C2 absolute density within the plasma. For this purpose, and since the Swan system is quite well-known, much importance is devoted to the achievement of a detailed simulation of the Mulliken system, which allows the determination of both the rotational temperature and the density of the ground state, as well as the rotational temperature of the state, from experimental data. All the experimental values are compared to those predicted by a thermochemical model developed to describe Ar/H2/CH4 microwave discharges under quasi-homogeneous plasma assumption. This comparison shows a reasonable agreement between the values measured from the C2 Mulliken system, those measured from the C2 Swan system and that calculated from plasma modelling, especially at low hydrogen concentration/MWP. These consistent results show that the use of the Mulliken system leads to fairly good estimates of the gas temperature and of the C2 absolute density. The relatively high gas temperatures found for the conditions investigated, typically between 3000 K and 4000 K, are attributed to the low thermal conductivity of argon that may limit thermal losses to the substrate surface and reactor wall. The measured C2 absolute densities range from 1013 to 1014 cm−3 depending on the experimental conditions. These high values may result from an enhanced thermal conversion of hydrocarbon species due to the high gas temperature.

For the first time nanocrystalline diamond (NCD) films were deposited by the pulsed microwave plasma assisted chemical vapour deposition process starting from an Ar/H-2/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/H-2/CH4 microwave discharges, provides evidence for the fact that the pulsed mode permits the enhancement of the mole fraction of the C-2 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.

The spectroscopic characterization of Ar/H-2/CH4 discharges suitable for the synthesis of nanocrystalline diamond using the microwave plasma assisted chemical vapour deposition process is reported. The experiments are realized in a moderate-pressure bell jar reactor, where discharges are ignited using a microwave cavity coupling system. The concentration of CH4 is maintained at 1% and the coupled set of hydrogen concentration/microwave power (MWP) ranges from 2%/500 W to 7%/800 W at a pressure of 200 mbar. Emission spectroscopy and broadband absorption spectroscopy studies are carried out on the C-2 (D(1)Sigma(u)(+)-(XEg+)-E-1) Mulliken system and the C-2(d(3)Pi(g)-a(3)Pi(u)) Swan system in order to determine the gas temperature and the C-2 absolute density within the plasma. For this purpose, and since the Swan system is quite well-known, much importance is devoted to the achievement of a detailed simulation of the Mulliken system, which allows the determination of both the rotational temperature and the density of the X(1)Sigma(g)(+) ground state, as well as the rotational temperature of the D(1)Sigma(u)(+) state, from experimental data. All the experimental values are compared to those predicted by a thermochemical model developed to describe Ar/H-2/CH4 microwave discharges under quasi-homogeneous plasma assumption. This comparison shows a reasonable agreement between the values measured from the C-2 Mulliken system, those measured from the C-2 Swan system and that calculated from plasma modelling, especially at low hydrogen concentration/MWP. These consistent results show that the use of the Mulliken system leads to fairly good estimates of the gas temperature and of the C-2 absolute density. The relatively high gas temperatures found for the conditions investigated, typically between 3000 K and 4000 K, are attributed to the low thermal conductivity of argon that may limit thermal losses to the substrate surface and reactor wall. The measured C-2 absolute densities range from 1013 to 10(14) cm(-3) depending on the experimental conditions. These high values may result from an enhanced thermal conversion of hydrocarbon species due to the high gas temperature.

Since its feasibility has been demonstrated (1), th e deposition process of Nano-Crystalline Diamond (NCD ) films has been subject of different investigations. I deed, besides some physical properties similar to those o f polycrystalline diamond (2), NCD films possess a ve ry low surface roughness appropriate for some tribolog ical and electronic applications. According to previous studies (3), NCD films can be achieved using MPACVD processes in Ar/H 2/CH4 discharges characterized by low H2 and CH4 concentrations. The optimization of NCD deposition for specific applications requires then the understanding of the main phenomena that govern the MPACVD process. In this study, we investigated the effects of some experimental parameters, particular ly the H2-concentration in the feed gas (%H 2) and the input Micro-Wave Power (MWP), on the plasma characteristics.