Microwave Plasma Excitation Using Cylindrical Cavity with Dual Injection

Large high-density plasmas have been generated by injecting magnetron-based microwaves radiated from slots cut on a wall of a rectangular or coaxial waveguide. However, a standing structural microwave in the waveguide often causes non-uniformity of plasma density. To minimize such inhomogeneity excited by the conventional waveguide, we adopt a resonant cylindrical cavity combined with a solid-state microwave amplifier. Microwave is injected into the cavity from two ports azimuthally apart by 90 degrees to each other (\textit{dual injection}). FDTD simulations are performed for a \textit{TE}$_{111}$ mode resonant cavity excited by \textit{single} or \textit{dual} microwave injection. In the case of the dual injection with a phase difference of $\pi $/2, the wave field azimuthally rotates in the cavity, and hence the slots cut on a cavity bottom wall launch travelling waves, thus minimizing the azimuthal inhomogeneity of the resultant plasma. 40-cm-diameter plasmas are experimentally generated in argon at 0.1 $\sim$ 5 Torr with microwaves of 2.4--2.5GHz and 400W. Threshold powers for plasma ignition are much less in \textit{dual} injection than those in \textit{single }injection. Optical emission images of the cylindrical plasmas show that the plasma uniformity is considerably improved in \textit{dual} injection, particularly at high-pressure and low-power.