Spectroscopic study of low pressure, low temperature H₂–CH₄–CO₂microwave plasmas used for large area deposition of nanocrystalline diamond films. Part I: on temperature determination and energetic aspects

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 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 , and of five stable molecules, CH 4 , CO 2 , CO, C 2 H 2 and C 2 H 6 , was monitored in the plasma processes by in situ infrared laser absorption spectroscopy using lead salt diode lasers (TDL) and external-cavity quantum cascade lasers (EC-QCL) as radiation sources. OES was applied simultaneously to obtain complementary information about the degree of dissociation of the H 2 precursor gas and of its gas temperature. The experimental results are presented in two separate parts. In Part I, the present paper, 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. To achieve reliable values for the gas temperature inside and outside the plasma bulk as well as for the rotational and vibrational temperatures in the plasma hot zones, which are of great importance for calculation of species concentrations, five different methods based on the emission and absorption spectroscopy data of H 2 , CH 4 , CH 3 and CO have been used. In these, line profile analysis has been combined with Boltzmann plot methods. Based on the wide tuning range of the EC-QCL, a variety of CO lines in the ground and three excited states was measured enabling extensive temperature analysis providing new insight into the energetic aspects of this multi-component plasma. Depending on the different plasma zones the gas temperature was found to range between about 360 and 1000 K inside the DAA reactor. In Part II, based on detailed concentration measurements general plasma chemical aspects will be analyzed and discussed.

Applying Quantum Cascade Laser Spectroscopy in Plasma Diagnostics

“…A 4ˆ4 distributed antenna array (DAA) microwave plasma reactor has successfully been employed to deposit uniform NCD films on a 4-inches wafer at substrate temperatures below 300˝C using H 2 with admixtures of few per cent of CH 4 and CO 2 as feed gas. The admixture of even a low amount of CO 2 provides additional etching species and reduces the activation energy needed for diamond growth, which allows the deposition of NCD films at very low substrate temperatures [66].…”

“…The experimental arrangement of the distributed antenna array (DAA) chemical vapor deposition (CVD) reactor for depositing NCD films together with the optical diagnostics systems is shown in Figure 33 [66]. For IRLAS, a compact and mobile infrared component acquisition (IRMA) system has been utilized based on traditional tunable diode lead salt lasers as radiation sources together with an EC-QCL.…”

“…Hence, 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 and the respective temperatures, gas, rotational, and vibrational, of all these species was monitored in the plasma processes. Information about the gas temperature and the degree of dissociation of the hydrogen molecules of the microwave plasma has been derived by the OES method [66].…”

“…[66]. For IRLAS, a compact and mobile infrared component acquisition (IRMA) system has been utilized based on traditional tunable diode lead salt lasers as radiation sources together with an EC-QCL.…”

“…Hence, the evolution of the concentration of the methyl radical, CH3, of five stable molecules, CH4, CO2, CO, C2H2 and C2H6, and of vibrationally excited CO in the first and second hot band and the respective temperatures, gas, rotational, and vibrational, of all these species was monitored in the plasma processes. Information about the gas temperature and the degree of dissociation of the hydrogen molecules of the microwave plasma has been derived by the OES method [66]. Using this complex spectrometer arrangement, gas composition, rotational and vibrational temperatures were determined depending on power and pressure of the discharge, which are of great importance for calculation of species concentrations [66].…”