Slot antenna 2.45 GHz microwave plasma source

A new large-volume microwave plasma source of interest for technological applications has been developed. An annular waveguide resonator with axial slots on its inner side acts as a field applicator to sustain a plasma at 2.45 GHz in a fused silica cylinder of 16 cm in diameter and 49 cm in length. The distance between slots is equal to a waveguide wave length. The slot antenna (SLAN) extends axially for about 9 cm. The plasma fills the tube axially as a result of surface wave propagation at the plasma-fused silica interface. The geometrical properties of the source were optimized by use of a numerical code allowing calculation of electric and magnetic fields inside the slot antenna. The calculated field patterns fit perfectly to those collected by use of a small loop antenna inside the SLAN without a plasma. A long time stable operation of the plasma source for pressure range from 0.01 to 1 mbar and power range from 50 to 1200 W was achieved. Argon plasma was characterized by use of a double Langmuir probe. Typical results are an ion density of 1.2*10¹² cm^-3 and an electron temperature of 1.2 eV*11604 K eV^-1 for a microwave power of 1.2 kW and a gas pressure of 1 mbar. The worst case azimuthal variation of the ion density for high pressure (1 mbar) does not exceed 25%. For low pressure (0.01 mbar) the azimuthal homogeneity is better than 5%.