G. Lombardi

High quality MPACVD diamond single crystal growth: high microwave power density regime

The growth of monocrystalline diamond films of electronic quality and large thickness (>few hundreds of microns) is an important issue in particular for high-power electronics. In this paper, we will describe the different key parameters necessary to reach this objective. First, we will examine the deposition process and establish that only microwave assisted diamond deposition plasma reactors can achieve the optimal growth conditions for the efficient generation of the precursor species to diamond growth. Next, we will consider the influence of the monocrystalline diamond substrate orientation and quality on the growth of the epitaxial layer, especially when the deposited material thickness exceeds 100 µm. The need to use a specific pre-treatment procedure of the substrate before the growth and its impact will also be discussed. Finally we will look at the growth conditions themselves and assess the influence of the process parameters, such as the substrate temperature, the methane concentration, the microwave power density and the eventual presence of nitrogen in the gas phase, on both the morphology and quality of the films on the one hand and the growth rate on the other hand. For this, we will introduce the concept of supersaturation and comment on its evolution as a function of the process parameters.

Application of mid-infrared tuneable diode laser absorption spectroscopy to plasma diagnostics: a review

Within the last decade mid-infrared absorption spectroscopy over a region from 3 to 17?m and based on tuneable lead salt diode lasers, often called tuneable diode laser absorption spectroscopy or TDLAS, has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry in molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, organo-silicon and boron compounds has led to further applications of TDLAS because most of these compounds and their decomposition products are infrared active. TDLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetic phenomena. Information about gas temperature and population densities can also be derived from TDLAS measurements. A variety of free radicals and molecular ions have been detected by TDLAS. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of quantum cascade lasers (QCLs) offers an attractive new option for the monitoring and control of industrial plasma processes. The aim of the present paper is threefold: (i) to review recent achievements in our understanding of molecular phenomena in plasmas, (ii) to report on selected studies of the spectroscopic properties and kinetic behaviour of radicals and (iii) to describe the current status of advanced instrumentation for TDLAS in the mid-infrared.

Determination of gas temperature and C2 absolute density in Ar/H2/CH4 microwave discharges used for nanocrystalline diamond deposition from the C2 Mulliken system

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.

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...

Study of an H2/CH4 moderate pressure microwave plasma used for diamond deposition: modelling and IR tuneable diode laser diagnostic

Infra-red tuneable diode laser spectroscopy (IR TDLAS) has been used to detect and quantify the methyl radical and three stable carbon-containing species (CH4, C2H2 and C2H6) in a moderate pressure microwave (f = 2.45 GHz) bell-jar reactor used for diamond films deposition. A wide range of experimental conditions was investigated, with typical pressure/power required to perform diamond deposition, i.e. pressure from 2500 to 12 000 Pa and power from 600 W to 2 kW, which means gas temperatures ranging from 2200 to 3200 K, when the power density increases from 9 to 30 W cm−3. Since TDLAS is a line of sight averaged technique, the analysis of the experimental data required the use of a one-dimensional non-equilibrium transport model that provides species density and gas temperature variations along the optical beam. This model describes the plasma in terms of 28 species/131 reactions reactive flow. The thermal non-equilibrium is described by distinguishing a first energy mode for the electron and a second one for the heavy species. Parametric studies as a function of power density and methane percentage in the gas mixture are presented. The good agreement obtained between measurement and one-dimensional radial calculations allows a validation of the thermo-chemical model, which can be used as a tool to enlighten the chemistry in the spatially non-uniform H2/CH4 microwave discharge used for diamond deposition. This is especially of interest for high power density discharge conditions that remain poorly understood.

Quantitative detection of methyl radicals in non-equilibrium plasmas: a comparative study

Tuneable infrared diode laser absorption spectroscopy at 16.5??m and broadband ultraviolet absorption spectroscopy at 216?nm have both been used to measure the ground state concentrations of the methyl radical in two different types of non-equilibrium microwave plasmas (f = 2.45?GHz): (i) H2?Ar plasmas of a planar reactor with small admixtures of methane or methanol, at a pressure of 1.5?mbar, and (ii) H2?CH4 plasmas of a bell jar reactor, at pressures of 25 and 32?mbar under flowing conditions. For the first time, two different optical techniques have been directly compared to verify the available data about absorption cross sections and line strengths of the methyl radical. It was found that application of the CH3 absorption cross section of the transition at 216?nm, reported by Davidson et al (1995 J. Quant. Spectrosc. Radiat. Transfer 53 581) and of the line strength of the Q(8,8) line of the ?2 fundamental band near 16.44??m, given by Wormhoudt et al (1989 Chem. Phys. Lett. 156 47), leads to satisfactory agreement.

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.

Overview of the different aspects in modelling moderate pressure H2 and H2/CH4 microwave discharges

This paper deals with the modelling of moderate pressure H2 and H2/CH4 microwave plasmas used for diamond deposition. The first part of the paper reports on the development of a detailed collisional radiative model for moderate pressure pure H2 plasma. Several effects related to electron kinetics, H2 vibrational kinetics and H2 and H excited states kinetics and their effect on the overall dissociation and ionization kinetics are analysed with the model developed. This allows us to propose a reduced model which can be used for the self-consistent modelling of H2 discharges or as the modelling of the more complex H2/CH4 plasmas. The second part of the paper deals with the two-dimensional self consistent modelling of H2 plasmas obtained in a microwave cavity coupling system. The procedure used for coupling the plasma transport equations to the electromagnetic field equations is first discussed and some self-consistent simulation results with respect to the power deposition and plasma temperature and density distributions in the investigated plasma device are given. The last part of the work addresses the one-dimensional transport modelling of H2/CH4 plasmas used for diamond deposition. The spatial variations of neutral and charged hydrocarbon species densities on the reactor axis are presented and the main phenomena and processes that govern these variations are discussed.

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.

Modelling of dust grain formation in a low-temperature plasma reactor used for simulating parasitic discharges expected under tokamak divertor domes

The presence of nanostructured dust particles has been reported in thermonuclear fusion reactors with carbon plasma-facing components. These particles contribute to tritium retention and pollution of the edge plasma. Understanding the way these particles can grow in the plasma phase is necessary for designing engineering solutions that avoid or at least limit their formation. As a first step towards this goal, this paper presents a numerical study of the formation of dust in a simple model laboratory electrical discharge: a dc discharge generated in argon with a graphite electrode. The aim here is to investigate whether carbon sputtering through ion and fast neutral bombardment of the cathode and subsequent molecular growth of carbon clusters and particle nucleation and development can explain dust formation in this model discharge. Results show that field reversal effects and negative cluster formation and trapping can fully explain dust formation in such a dc discharge.