ИСТИНА

A comprehensive study of microwave (MW) activated CH4/H2/Ar(He) plasmas used for diamond chemical vapor deposition is reported, focusing on plasma-chemical mechanisms, power balance and effects of wide range variations of noble gas fractions in H/C/Ar (or He) mixtures. The experimental data of Bristol University (BU) (OES, CRDS, calorimetric data from water cooling contours) are complemented by extensive 2-D(r, z) modeling of the plasma chemistry, plasma parameters (electron energy distributions and reaction coefficients, spatial distributions of electron and ions concentrations, gas temperature), absorbed power transformations and global power balance1,2. The set of non-stationary conservation equations for mass, momentum, energy and species concentrations are solved numerically by a finite difference method in (r, z) coordinates1. The 2-D model takes into account the changes in plasma parameters and conditions as a result of variations in reactor parameters like pressure p, input power P, the gas mixture composition (mole fractions of the supplied species X0(CH4), X0(H2) and X0(Ar) or X0(He). The base plasma-chemical mechanism for C/H/Ar gas mixtures involves 38 species and >240 reactions1,2. Under H2-rich conditions, >80% of the input MW power absorbed by the electrons is partitioned into vibrational and rotational excitation of (mainly) H2 and hydrocarbons. Other processes of note occurring at the plasma region include elastic collisions of electrons with H2, H, Ar and CxHy, electron impact induced dissociation of H2 and other molecular species, electronic excitation and ionization. Fast V-T and R-T energy transfer from the rovibrationally excited H2 and CxHy through collision with H atoms results in translational excitation of the latter, which then dissipates into gas heating. The input power transferred from the electrons to the gas dissipates further by (i) thermal conduction from the hot plasma to the reactor walls, substrate and substrate holder, base plate and top plate (window), (ii) chemical conversions (H2 dissociation and H-atoms activated gas phase and surface chemistries, e.g. H abstraction H + CH ↔ H2 + C* and addition H + C* ↔ CH at sites C* and CH of growing diamond, the last process provides additional substrate heating) and (iii) radiative losses. Radiative losses have a relatively small affect on the overall plasma power balance, accounting for <7% of the total input power for mixtures with X0(H2)15%. This fraction can reach ~20-30% in the case of ultrananocrystalline diamond3 (UNCD) regimes (e.g. 0.5%CH4/1%H2/98.5%Ar) plasmas as a result of the greatly increased emission from CyHx species (e.g. C3*, C2* and, possibly, C2H*). This increase of radiative losses is indirectly corroborated by the observed drop in the total power abstracted via the circulated water cooling the reactor walls and base plate.