Reactive Radical\nEtching of Quartz by Microwave Activated\nCH<sub>4</sub>/H<sub>2</sub> Plasmas Promotes Gas Phase Nanoparticle\nFormation

An attenuation of visible probe radiation identified\nin earlier\nabsorption studies of microwave plasma-activated CH<sub>4</sub>/H<sub>2</sub>/Ar gas mixtures is shown to arise from nanoparticles in under-pumped\nregions on opposing sides of a reactor used for diamond chemical vapor\ndeposition. The present modeling studies highlight (i) ejection of\nSi-containing species into the gas phase by reactive radical etching\nof the quartz window through which the microwave radiation enters\nthe reactor, enabled by suitably high window temperatures (<i>T</i><sub>SiO2</sub>) and the synergistic action of near-window\nH atoms and C<sub><i>y</i></sub>H<sub><i>x</i></sub> radicals; (ii) subsequent processing of the ejected material,\nsome of which are transported to and accumulate in stagnation regions\nin the entrance to the reactor side arms; and (iii) the importance\nof Si in facilitating homogeneous gas phase nucleation, clustering,\nand nanoparticle growth in these regions. The observed attenuation,\nits probe wavelength dependence, and its variations with changes in\nprocess conditions can all be rationalized by a combination of absorption\nand scattering contributions from Si/C/H containing nanoparticles\nwith diameters <i>d</i> in the range of 50–100 nm.\nPossible implications for Si incorporation in CVD diamond samples\nare discussed.