Spectroscopic study of low pressure, low temperature H₂–CH₄–CO₂microwave plasmas used for large area deposition of nanocrystalline diamond films. Part II: on plasma chemical processes

Abstract

In a distributed antenna array (DAA) reactor, microwave H 2 plasmas with admixtures of 2.5% CH 4 and 1% CO 2 used for the deposition of nanocrystalline diamond films have been studied by infrared laser absorption and optical emission spectroscopy (OES) techniques. The experiments were carried out in order to analyze the dependence of plasma chemical phenomena on power and pressure at relatively low pressures, up to 0.55 mbar, and power values, up to 3 kW. The evolution of the concentration of the methyl radical, CH 3 , of five stable molecules, CH 4 , CO 2 , CO, C 2 H 2 and C 2 H 6 , and of vibrationally excited CO in the first and second hot band was monitored in the plasma processes by in situ infrared laser absorption spectroscopy using tunable lead salt diode lasers (TDL) and an external-cavity quantum cascade laser (EC-QCL) as radiation sources. OES was applied simultaneously to obtain complementary information about the degree of dissociation of the H 2 precursor and of its gas temperature. The experimental results are presented in two separate parts. In Part I, the first paper in a two-part series, the measurement of the gas (T gas ), rotational (T rot ) and vibrational (T vib ) temperatures of the various species in the complex plasma was the main focus of interest. Depending on the different plasma zones the gas temperature was found to range between about 360 and 1000 K inside the DAA reactor (Nave et al 2016 Plasma Sources Sci. Technol. 25 065002). In Part II, the present paper, taking into account the temperatures determined in the first paper, the concentrations of the various species, which were found to be in a range between 10 11 and 10 15 cm −3 , are the focus of interest. The influence of the discharge parameters power and pressure on the molecular concentrations has been studied. To achieve further insight into general plasma chemical aspects the dissociation of the carbon precursor gases including their fragmentation and conversion to the reaction products has been analyzed in detail.

Effect of the admixture of N2to low pressure, low temperature H2-CH4-CO2microwave plasmas used for large area deposition of nanocrystalline diamond films

“…Further information can be found in the HITRAN database [40]. For the plasma investigated here, an equilibrium between the translational and the rotational temperature was assumed [41]. The gas temperature was determined for every individual species from the Doppler broadening of the absorption lines.…”

Spatially Resolved Optical Emission and Modeling Studies of Microwave-Activated Hydrogen Plasmas Operating under Conditions Relevant for Diamond Chemical Vapor Deposition

“…H 2 shows a wealth of rovibrational structures in the visible and near-infrared (IR) spectral regions associated with transitions between bound excited electronic states. − Most prior experimental studies of MW-activated hydrogen-containing plasmas have focused on the d 3 Π u –a 3 Σ g + (Fulcher) system, − ,− though lines within the G 1 Σ g + –B 1 Σ u + origin band − and a 3 Σ g + –b 3 Σ u + continuum emission , have been used for plasma diagnostics, and limited studies have been undertaken using the e 3 Σ u + –a 3 Σ g + system . As Figure shows, the structured emissions are from excited states that lie at energies > 13 eV above the ground (X 1 Σ g + ) state of H 2 .…”

Qualification of uniform large area multidipolar ECR hydrogen plasma

“…a 3 P g þ ) molecular hydrogen band. 20 We especially focused on the Q-branch 0-0 transition between 601 and 606.5 nm and 1-1 transition between 612 and 616.5 nm. 21 A typical rotational spectrum is shown in Fig.…”