Determination of recombination coefficients for hydrogen, oxygen, and nitrogen gasses via in situ radical probe system

Abstract

The determination of the recombination coefficients of gases on solid surfaces depends on the plasma processing environment including factors such as temperature, surface morphology, impurities, and chamber geometry that play a role in energy transfer mechanisms of association, dissociation, and collisional nature of gases in low pressure plasmas. To determine those recombination coefficients, a zero-dimensional plasma model was created to predict radical and ion densities of hydrogen, oxygen, and nitrogen using experimental data, with electron temperatures and densities as inputs. The model inputs (electron density, electron temperature, and plasma gas temperature) were experimentally obtained by a Langmuir probe and a thermocouple. Each radical density measurement requires two radical probes with different catalytic coatings, which yield different temperatures due to different recombination coefficients of the coatings. The measurements are compared with the radical density obtained from a plasma model in order to determine the value of recombination coefficient. Recombination coefficient of hydrogen radicals on the gold surface is found to be 0.115 ± 0.018. Recombination coefficients of oxygen and nitrogen on copper are found to be 0.31 ± 0.063 and 0.18 ± 0.034, respectively. Ion densities vary from 109 to 1011 cm−3 s, over 10–100 mTorr pressure range and power range between 300 and 900 W. Radical densities are in the order of 1013 cm−3 to 1015 cm−3. Simultaneously with this article, a parallel study is published explaining in situ measurements of the radical probe system for single and mixed gases.