Large-area overdense microwave plasmas

For the processing of large area planar substrates, a flat and nearly two-dimensional plasma has many advantages. There is additional utility for some applications if the plasma has a low sheath potential, which reduces or eliminates substrate damage due to low ion kinetic energy. One route to achieving both criteria is to use an overdense high-frequency plasma. Because the overdense condition depends on local electron density, in any reactor the overdense condition will have a spatial dependence resulting from the details of the plasma source construction. For microwave plasmas, the spatial dependence of the plasma density is challenging because of the short wavelength of the drive frequency. In this talk, we will review previous work on microwave plasma source designs, their characterization, and operating range. We will present a plasma system architecture that uses multiple antennae with independent solid-state power sources to achieve a large area microwave plasma at 2.45 GHz. Simulation, characterization, and imaging will be presented to discuss the evidence of an overdense condition and its effect on plasma shape over a range of operating conditions. Potential applications will be discussed.