Standing wave effect on plasma distribution in an inductively coupled plasma source with a short antenna

The effect of a standing wave on plasma density is observed in an inductively coupled plasma (ICP) with a single turn antenna whose circumferential length is much shorter than the wavelength of the driving frequency. The experiment is performed in an argon plasma operated at 5 m Torr with 13.56 MHz power applied to the single turn antenna with diameter of 300 mm. Measured plasma density near the ground termination of the antenna is higher than near the power input, which degrades the uniformity in the ICP source. It is analysed with the standing wave effect of the current and voltage on the antenna, which is carried out with a transmission line model based on the transformer circuit model of the ICP. The wavelength and the distribution of the inductively and capacitively coupled powers along the antenna are obtained from the model with the measured impedance of the antenna. The expected density distribution agrees well with the measured one, revealing that the wavelength is shortened and the nonuniformity of the plasma density becomes severe with increase of the input power.