Yuri A. Mankelevich

Understanding the chemical vapor deposition of diamond: recent progress

In this paper we review and provide an overview to the understanding of the chemical vapor deposition (CVD) of diamond materials with a particular focus on the commonly used microwave plasma-activated chemical vapor deposition (MPCVD). The major topics covered are experimental measurements in situ to diamond CVD reactors, and MPCVD in particular, coupled with models of the gas phase chemical and plasma kinetics to provide insight into the distribution of critical chemical species throughout the reactor, followed by a discussion of the surface chemical process involved in diamond growth.

Plasma-chemical processes in microwave plasma-enhanced chemical vapor deposition reactors operating with C/H/Ar gas mixtures

Microwave (MW) plasma-enhanced chemical vapor deposition (PECVD) reactors are widely used for growing diamond films with grain sizes spanning the range from nanometers through microns to millimeters. This paper presents a detailed description of a two-dimensional model of the plasma-chemical activation, transport, and deposition processes occurring in MW activated H/C/Ar mixtures, focusing particularly on the following base conditions: 4.4%CH4/7%Ar/balance H2, pressure p=150 Torr, and input power P=1.5 kW. The model results are verified and compared with a range of complementary experimental data in the companion papers. These comparators include measured (by cavity ring down spectroscopy) C2(a), CH(X), and H(n=2) column densities and C2(a) rotational temperatures, and infrared (quantum cascade laser) measurements of C2H2 and CH4 column densities under a wide range of process conditions. The model allows identification of spatially distinct regions within the reactor that support net CH4→C2H2 and C2H2→CH4...

Effects of NH3 and N2 additions to hot filament activated CH4/H2 gas mixtures

Resonance enhanced multiphoton ionization and cavity ring down spectroscopies have been used to provide spatially resolved measurements of relative H atom and CH3 radical number densities, and NH column densities, in a hot filament (HF) reactor designed for diamond chemical vapor deposition and here operating with a 1% CH4/n/H2 gas mixture—where n represents defined additions of N2 or NH3. Three-dimensional modeling of the H/C/N chemistry prevailing in such HF activated gas mixtures allows the relative number density measurements to be placed on an absolute scale. Experiment and theory both indicate that N2 is largely unreactive under the prevailing experimental conditions, but NH3 additions are shown to have a major effect on the gas phase chemistry and composition. Specifically, NH3 additions introduce an additional series of “H-shift” reactions of the form NHx+H⇌NHx−1+H2 which result in the formation of N atoms with calculated steady state number densities >1013 cm−3 in the case of 1% NH3 additions in ...

Validating optical emission spectroscopy as a diagnostic of microwave activated CH4/Ar/H2 plasmas used for diamond chemical vapor deposition

Spatially resolved optical emission spectroscopy (OES) has been used to investigate the gas phase chemistry and composition in a microwave activated CH4/Ar/H2 plasma operating at moderate power densities (∼30 W cm−3) and pressures (≤175 Torr) during chemical vapor deposition of polycrystalline diamond. Several tracer species are monitored in order to gain information about the plasma. Relative concentrations of ground state H (n=1) atoms have been determined by actinometry, and the validity of this method have been demonstrated for the present experimental conditions. Electronically excited H (n=3 and 4) atoms, Ar (4p) atoms, and C2 and CH radicals have been studied also, by monitoring their emissions as functions of process parameters (Ar and CH4 flow rates, input power, and pressure) and of distance above the substrate. These various species exhibit distinctive behaviors, reflecting their different formation mechanisms. Relative trends identified by OES are found to be in very good agreement with those ...

Unravelling aspects of the gas phase chemistry involved in diamond chemical vapour deposition

We describe laser and mass spectroscopic methods, and related modelling studies, that have been used to unravel details of the gas phase chemistry involved in diamond chemical vapour deposition (CVD) using both H/C (i.e. hydrocarbon/H2) and H/C/O (e.g. CO2/CH4) gas mixtures, and comment on the relative advantages and limitations of the various approaches. In the case of the more extensively studied hydrocarbon/H2 systems we pay particular emphasis to investigations (both experimental, and 2- and 3-dimensional modelling) of transient species like H atoms and CH3 radicals, their spatial distributions within the reactor and the ways in which these distributions vary with process conditions, and the insight provided by such investigations into the chemistry underpinning the diamond CVD process. These analyses serve to highlight the rapid thermochemical cycling amongst the various hydrocarbon species in the reactor, such that the gas phase composition in the vicinity of the growing diamond surface is essentially independent of the particular hydrocarbon source gas used. Such applies even to the case of hot filament activated C2H2/H2 gas mixtures, for which we show that CH3 radical formation (hitherto often presumed to involve heterogeneous hydrogenation steps) can be fully explained in terms of gas phase chemistry. Diamond growth using H/C/O-containing gas mixtures has traditionally been discussed in terms of an empirically derived H–C–O atomic phase composition diagram (P. K. Bachmann, D. Leers, H. Lydtin and D. U. Wiechert, Diamond Relat. Mater., 1991, 1, 1). Detailed studies of microwave activated CO2/CH4 gas mixtures, accompanied by simpler zero-dimensional thermochemical modelling of this and numerous other H/C/O-containing input gas mixtures, provide a consistent rationale for the ‘no growth ’, ‘diamond growth’ and ‘non-diamond growth’ regions within the H–C–O atomic phase composition diagram.

Probing the plasma chemistry in a microwave reactor used for diamond chemical vapor deposition by cavity ring down spectroscopy

Absolute column densities of C2(a) and CH radicals and H(n=2) atoms have been measured in a diamond growing microwave reactor operating with hydrocarbon/Ar/H2 gas mixtures as functions of height (z) above the substrate surface and process conditions. The monitored species are each localized in the hot plasma region, where Tgas∼3000 K, and their respective column densities are each reproduced, quantitatively, by two-dimensional (r,z) modeling of the plasma chemistry. The H(n=2) distribution is seen to peak nearer the substrate, reflecting its sensitivity both to thermal chemistry (which drives the formation of ground state H atoms) and the distributions of electron density (ne) and temperature (Te). All three column densities are found to be sensitively dependent on the C/H ratio in the process gas mixture but insensitive to the particular choice of hydrocarbon (CH4 and C2H2). The excellent agreement between measured and predicted column densities for all three probed species, under all process conditions ...

Quantum cascade laser investigations of CH4 and C2H2 interconversion in hydrocarbon/H2 gas mixtures during microwave plasma enhanced chemical vapor deposition of diamond

CH4 and C2H2 molecules (and their interconversion) in hydrocarbon/rare gas/H2 gas mixtures in a microwave reactor used for plasma enhanced diamond chemical vapor deposition (CVD) have been investigated by line-of-sight infrared absorption spectroscopy in the wavenumber range of 1276.5−1273.1 cm−1 using a quantum cascade laser spectrometer. Parameters explored include process conditions [pressure, input power, source hydrocarbon, rare gas (Ar or Ne), input gas mixing ratio], height (z) above the substrate, and time (t) after addition of hydrocarbon to a pre-existing Ar/H2 plasma. The line integrated absorptions so obtained have been converted to species number densities by reference to the companion two-dimensional (r,z) modeling of the CVD reactor described in Mankelevich et al. [J. Appl. Phys. 104, 113304 (2008)]. The gas temperature distribution within the reactor ensures that the measured absorptions are dominated by CH4 and C2H2 molecules in the cool periphery of the reactor. Nonetheless, the measurem...

Experimental and Theoretical Study of Ion Energy Distribution Function in Single and Dual Frequency RF Discharges

Ion energy distribution functions (IEDFs) at the electrodes in single frequency (SF) and dual frequency (DF) radio-frequency discharges in Ar at pressures of 20 and 45 mtorr are measured and calculated. A numerical simulation of the IEDF on the base of a self-consistent particle-in-cell model with Monte Carlo collisions was performed. In addition, a semianalytical model was developed to calculate the IEDF in collisionless and collisional SF and DF plasmas. The IEDF width for the intermediate frequency case was determined from both experimental and theoretical results. The possibility of frequency decoupling is discussed.

Combined experimental and modeling studies of microwave activated CH4/H2/Ar plasmas for microcrystalline, nanocrystalline, and ultrananocrystalline diamond deposition

A comprehensive study of microwave (MW) activated CH4/H2/Ar plasmas used for diamond chemical vapor deposition is reported, focusing particularly on the effects of gross variations in the H2/Ar ratio in the input gas mixture (from H2/Ar mole fraction ratios of > 10:1, through to ∼1:99). Absolute column densities of C2(a) and CH(X) radicals and of H(n = 2) atoms have been determined by cavity ringdown spectroscopy, as functions of height (z) above a substrate and of process conditions (CH4, H2, and Ar input mole fractions, total pressure, p, and input microwave power, P). Optical emission spectroscopy has also been used to explore the relative densities of electronically excited H atoms, and CH, C2, and C3 radicals, as functions of these same process conditions. These experimental data are complemented by extensive 2D (r, z) modeling of the plasma chemistry, which provides a quantitative rationale for all of the experimental observations. Progressive replacement of H2 by Ar (at constant p and P) leads to a...

Experimental and theoretical study of RF plasma at low and high frequency

A capacitive coupled radio-frequency plasma at argon pressure of 100 mtorr, 13.56 and 81 MHz frequency and high specific input powers has been studied both experimentally and theoretically. The different numerical models were developed and used for simulation. It was shown that the main ionization source in low frequency (LF) discharge at all studied powers is due to secondary electrons emitted from the electrode by ion impact. The high-frequency (HF) discharge at the same powers is operated in alpha-mode. Applicability of fluid and kinetic models for simulation of LF and HF discharges are studied on the base of our experimental data