Viscous effect on neutral radical spatial distribution in microwave plasma chemical vapor deposition

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Abstract Microwave Plasma Chemical Vapor Deposition (MPCVD) is the most promising method for producing single-crystal diamond (SCD), which is acclaimed as the “ultimate semiconductor” material. However, high-quality large-size SCD is still lacking. Prior research asserts the dominance of the electric field and ignores the influence of the flow. In our present work, analysis of orders of magnitude is conducted on the fluid description of the plasma to obtain simplified governing equations based on the typical working parameters of the MPCVD. Our theoretical derivation concludes that the spatial distribution of the neutral radical is determined not only by the density gradient but also by the viscous interaction with the neutral gas flow. For verification, the set of governing equations, encompassing Maxwell's equations, electron and neutral radical number density equations, and Navier-Stokes equations, is numerically solved. The simulation results reveal that the non-uniformity of the spatial distribution of the neutral radical increases with the difference of the inlet velocity, corroborating our theoretical analyses. This finding provides a novel regulatory approach for producing the high-quality large-size SCD and can be possibly extended to other CVD processes for controlling product quality.