Coaxial helical resonators are used in the field of high frequency engineering for narrow band selection of signals from about 10 MHz to 500 MHz (VHF and low UHF range) [1, 2]. Their favorable geometric construction suggests their utilization as plasma source for etching and deposition of a-Si:H at frequencies between 30 and 200 MHz - an additional, but still unusual application.
The resonator consists of a cylindric outer conductor, which is closed on one side by a bottom plate, and a rotationally symmetrical inner conductor of spiral form (helix) Measuring γ/4. One end of the helix is grounded to the bottom plate (short circuit), the other end is open (open circuit). Power supply is feasible either galvanically by connecting to the helix or inductively by means of a coupling coil. Applying a suitable coupling factor allows operation at resonance without matchbox. The plasma is generated within the resonator and above its open end, which can be adjusted to special conditions of application with the aid of several grids. In contrast to the source commonly used, the coaxial helix plasma source has proved advantageous with regard to simple design, easy mounting, Minimal outer electrical equipment, no dependence of generated plasma on substrate measures, good breakdown and discharge behavior, low supply voltage (some tens of volts), high power density, and, due to a discharge occurring without filament, also to life durability and no thermally generated particle emission. The application of very high frequencies leads to a low ion bombardment intensity of a-Si:H, and thus additional magnetic fields are not necessary.