Reactive Radical Etching of Quartz by Microwave Activated CH₄/H₂ Plasmas Promotes Gas Phase Nanoparticle Formation

An attenuation of visible probe radiation identified in earlier absorption studies of microwave plasma-activated CH₄/H₂/Ar gas mixtures is shown to arise from nanoparticles in under-pumped regions on opposing sides of a reactor used for diamond chemical vapor deposition. The present modeling studies highlight (i) ejection of Si-containing species into the gas phase by reactive radical etching of the quartz window through which the microwave radiation enters the reactor, enabled by suitably high window temperatures (T_SiO2) and the synergistic action of near-window H atoms and CH radicals; (ii) subsequent processing of the ejected material, some of which are transported to and accumulate in stagnation regions in the entrance to the reactor side arms; and (iii) the importance of Si in facilitating homogeneous gas phase nucleation, clustering, and nanoparticle growth in these regions. The observed attenuation, its probe wavelength dependence, and its variations with changes in process conditions can all be rationalized by a combination of absorption and scattering contributions from Si/C/H containing nanoparticles with diameters d in the range of 50-100 nm. Possible implications for Si incorporation in CVD diamond samples are discussed.