3-D Modeling Study of Remote Microwave NH3/N2 Plasma for Wafer Native Oxide Cleaning Process

A plasma native oxide cleaning process is used in semiconductor manufacturing to clean oxide impurities on Si wafers. A remote microwave (MW) excited plasma in NH₃-N₂ feedgas is used to produce N_xH_y radicals that facilitate the oxide removal. We perform a 3D model of a typical industrial MW remote plasma source, consisting of a rectangular waveguide intersecting a quartz tube, through which the feed gas flows.

We discuss the nature of 2.45 GHz microwave power deposition into the plasma generated in the tube. The wave interaction is found to be highly three dimensional, with significant wave heating occurring on the side of the tube that directly faces the incident wave. Consequently, the electron density and radical production shows a 3D structure. As the flow carries the species, their profiles become increasingly axisymmetric. The dominant radicals that exit the tube are H₂ and NH₂, with densities indicating nearly complete conversion of the feed gases to product species. The process is accompanied by a gas temperature rise that increases with wave power, but it is not consequential to the overall plasma conversion. The parametric study with changing pressure and feed gas composition illustrates the role of specific chemical reactions in the overall remote plasma process.