Validation of actinometry for estimating relative hydrogen atom densities and electron energy evolution in plasma assisted diamond deposition reactors

The validity of the actinometry method applied to H-atom mole fraction measurements has been analyzed. First, a theoretical approach allowed us to determine boundary conditions for which the validity of actinometry may be critical. For these specific conditions, corresponding to an upper limit of electron temperature of 20 000 K and a lower limit of K-atom mole fraction of 2%-4%, spatial distributions of the ground state H-atom relative densities provided either by two photon allowed transition or by optical emission spectroscopy (OES) were compared and seen to be proportional This proves that the H atoms excited in the level of quantum number it = 3 (level used for OES experiments) are produced directly from the ground electronic state during collisions with electrons. Actinometry can then be applied under these experimental conditions. Second, the emission intensity ratio of two lines issued from excited states of argon was demonstrated to be indirectly related to the ''electron temperature'' of the hot electrons of the plasma. This allowed us to predict the way of evolution of the plasma electrons' energy as a function of the operating conditions. Thus, experiments (which have been confirmed by calculations) showed that the electron energy decreases as a function of the microwave power density and remains constant as a function of the methane percentage introduced in the feed gas at least up to 6%. The consequence is that the domain of diamond deposition discharge conditions for which actinometry is valid is quite wide. Once the microwave volumetric power density is more than 9 W cm(-3), and the percentage of methane less than 6%, actinometry can be applied. However, the estimation of variations of H-atom mole fractions as a function of the operating conditions implies the use of correcting factors, which are discussed. They are mainly due to the large influence of the quenching processes under these experimental conditions. An experimental estimate of the quenching cross section of the H(n = 3) atoms by ground state molecular hydrogen, which was unknown and involved in the correcting factors, is presented. Finally, relative variations of H-atom mole fraction in space and as a function of the methane percentage are shown. (C) 1998 American Institute of Physics.