In this paper, we investigate a new distributed antenna array PECVD system, with 16 microwave plasma sources arranged in a 2D matrix, which enables the growth of 4-inch diamond films using H 2 /CH 4 /CO 2 gas mixture at low gas pressure, typically below 0.45 mbar, and at substrate temperature of 400 8C. The influence of substrate position with respect to elementary microwave sources is investigated for three sets of gas pressures in order to improve the diamond growth process in this low temperature/large area deposition setup. Results show that the nanocrystalline diamond films are formed of polycrystalline globular aggregates of 50-200 nm in size, depending on growth conditions, composed of diamond grains around 10-20 nm. An optimal deposition condition corresponding to a pressure of 0.45 mbar and a distance between substrate and microwave sources of 65 mm can be found. In these conditions, highest growth rate (60 nm h À1 ) with good nanocrystalline features, i.e. smallest grain size (10 nm) and lowest roughness (around 15 nm), is obtained.

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.