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 actinom etry may be critical. For these specific conditions, corresponding to an upper limit of electron temperature of 20 000 K and a lower limit o f K-atom mole fraction of 2%-4%, spatial distributions of the ground s tate H-atom relative densities provided either by two photon allowed t ransition or by optical emission spectroscopy (OES) were compared and seen to be proportional This proves that the H atoms excited in the le vel of quantum number it = 3 (level used for OES experiments) are prod uced directly from the ground electronic state during collisions with electrons. Actinometry can then be applied under these experimental co nditions. Second, the emission intensity ratio of two lines issued fro m 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' e nergy as a function of the operating conditions. Thus, experiments (wh ich have been confirmed by calculations) showed that the electron ener gy decreases as a function of the microwave power density and remains constant as a function of the methane percentage introduced in the fee d gas at least up to 6%. The consequence is that the domain of diamond deposition discharge conditions for which actinometry is valid is qui te 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 fractio ns as a function of the operating conditions implies the use of correc ting factors, which are discussed. They are mainly due to the large in fluence 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 in volved in the correcting factors, is presented. Finally, relative vari ations of H-atom mole fraction in space and as a function of the metha ne percentage are shown. (C) 1998 American Institute of Physics.