K. Hassouni

CVD diamond films: from growth to applications

Abstract The present review provides an up-to-date report on the main potential of CVD diamond films for industrial applications as well as on recent basic research which seeks to understand diamond deposition microwave plasma reactors. This review includes firstly an overview of diamond film applications. Elements which explain variations in diamond film characteristics as a function of synthesis conditions are given. Also experimental results are reported which show variations in diamond characteristics (quality, microstructure, growth rate, growth mechanisms) as four plasma variables (pressure, power, percentage of methane, substrate temperature) are systematically changed. In the second part, we discuss the effects of these variables on local parameters such as electron temperature, gas temperature, carbon-containing species and H-atom densities. Finally, based on these results, relationships between key local parameters and diamond characteristics are established and discussed.

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.

Spectroscopic analysis and chemical kinetics modeling of a diamond deposition plasma reactor

Abstract The quality and the growth rate of diamond films produced in a microwave bell jar plasma reactor are strong functions of the plasma characteristics at the plasma-surface interface (temperatures, species concentrations). These local parameters are shown to be functions of the plasma operating conditions (dissociation in volume) and of the boundary conditions at the surface. Spectroscopic measurements of some plasma parameters based on coherent anti-Stokes Raman and optical emission spectroscopy are presented; the deposition results are correlated with these measurements. A zero-dimensional chemical kinetics model is developed for studying the influences of the gas temperature and the electron temperature on the production of atomic hydrogen.

Modeling of microwave discharges of H2 admixed with CH4 for diamond deposition

Microwave discharges of H2 admixed with CH4 in a moderate-pressure quartz bell jar reactor used for diamond deposition are studied numerically. Special attention was devoted to high-power densities which provide the most effective way for producing high-quality diamond films. First, a one-dimensional radial model describing the coupled phenomena of chemistry, energy transfer, as well as species and energy transport along the reactor’s radial coordinate was developed. Species densities predicted with the model were compared with measurements with infrared tunable diode laser spectroscopy, resulting in validation of the model. Second, a one-dimensional axial model was used to describe the plasma flow along the reactor axis in a region between the reactor end wall and the substrate surface. This model was particularly useful for studying the plasma behavior in the vicinity of the substrate surface, where thermal and composition gradients are large. Both the radial and axial transport models are based on the ...

Chemical kinetics and energy transfer in moderate pressure H2 plasmas used in diamond MPACVD processes

A kinetic model for moderate-pressure microwave H2 plasmas obtained in diamond deposition reactors is presented. This model involves three groups of reactions which describe the vibrational kinetics of H2, the chemistry of H2 and H electronically excited states and the groundstate species kinetics, respectively. The set of species kinetic equations resulting from this model, coupled to the electron Boltzmann equation and the total energy equation were solved under a quasi-homogenous plasma assumption. This enables the estimation of the species densities, the electron distribution function and related electron properties, as well as the gas temperature. The results show that the most important ionization channel is that due to the quenching of H (n = 2) excited states by H2. The production of the H-atom is mainly due to electron impact dissociation at low microwave power density and to thermal dissociation at high power density. A simplified physical model which may be used for describing the non-equilibrium H2 plasma flow in diamond deposition microwave plasmas reactors is also proposed.

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.

Non-equilibrium plasma kinetics: a state-to-state approach

State-to-state approaches are used to shed light on (a) thermodynamic and transport properties of LTE plasmas, (b) atomic and molecular plasmas for aerospace applications and (c) RF sustained parallel plate reactors. The efforts made by the group of Bari in the kinetics and dynamics of electrons and molecular species are discussed from the point of view of either the master equation approach or the molecular dynamics of elementary processes. Recent experimental results are finally rationalized with a state-to-state kinetics based on the coupling of vibrational kinetics with the Boltzmann equation for the electron energy distribution function.

Modeling of H2 and H2/CH4 Moderate-Pressure Microwave Plasma Used for Diamond Deposition

One-dimensional transport models of moderate-pressure H2and H2/CH4plasmas obtained in a diamond deposition microwave reactor are presented. These models describe the plasma as a thermochemically nonequilibrium flow with three different energy modes. The solution of the one-dimensional plasma transport equations enabled the estimation of plasma species concentrations and temperatures on the axis of the reactor. As far as pure H2plasmas are concerned, results showed that the model predictions of gas and vibration temperatures are in good agreement with experimental measurements. The model also yields a relatively good qualitative prediction of the variations of H-atom mole fraction with the power density absorbed by the plasma. The results obtained for H2/CH4discharges showed that the model prediction on the variations of H-atom mole fraction with methane percentage in the discharge is in good qualitative agreement with experimental results. They also showed that methane is rapidly converted to acetylene before reaching the discharge zone. The concentrations of neutral hydrocarbon species in the reactor are mainly governed by thermal chemistry. The addition of methane strongly affects the ionization kinetics of the plasma. Three major ions are generally obtained in H2/CH4plasmas: C2H2+, C2H3+, and C2H5+. The relative predominance of these ions depends on the considered plasma region and on the discharge conditions. The ionic species concentrations are also mainly governed by chemistry, except very near the substrate surface. Finally the use of this transport model along with the surface chemistry model of Goodwin(1)enabled us to estimate the diamond growth rate for several discharge conditions.

Interaction between soot particles and NOx during dielectric barrier discharge plasma remediation of simulated diesel exhaust

Plasma remediation is being investigated as a means to remove NOx from combustion effluent and from diesel exhausts in particular. Soot particles are inevitably present in actual exhausts and may, through heterogeneous chemistry, affect the remediation process. In this article, a computational investigation of the effect of soot on the plasma chemistry of NOx removal in a simulated diesel exhaust processed in a dielectric barrier discharge reactor is presented using a zero-dimensional global-kinetics simulation. A surface chemistry model is employed to describe soot oxidation by O and OH radicals, and soot-NOx interactions. The NOx chemistry may be substantially affected by the reactions at the soot surface. In particular, for soot particles having densities of 108 cm−3 and diameters of 100 nm, significant increases of NO are obtained when taking into account NO2→NO conversion on the soot surface. Heterogeneous reaction of NO2 also results in an increase in the gas-phase OH density which results in the in...

Gas temperature measurements by laser spectroscopic techniques and by optical emission spectroscopy

Abstract H-atom temperatures were measured using two-photon allowed transition laser induced fluorescence in a microwave plasmareactor used for diamond deposition. These temperatures are compared with the rotational temperatures of ground state molecular hydrogen, equal to the gas temperature, measured previously by coherent anti-stokes Raman spectroscopy. We observed an excellent agreement between the two temperatures. Rotational temperatures of the G 1 {g} + electronically excited state of molecular hydrogen were also obtained by measuring the relative line intensities in emission of the R -branch of the {itG 1 {g} + B 1 {u} + (0,0) band by optical emission spectroscopy. These measurements yielded lower temperatures than the gas temperature by 550 K, but followed its variation as a function of the power density. We defined then {itT R (G 1 {g} + ) as a spectroscopic parameter useable for optimizing and monitoring the plasma conditions for diamond deposition, within the range of conditions tested here.