Microwave-Frequency Effects on Microplasma

Capacitively coupled microplasma generation becomes more efficient at microwave frequencies. Helium and argon microplasmas are characterized using excitation frequencies of 450 MHz, 900 MHz, and 1.8 GHz. These microplasmas are tested at both atmospheric pressure and 0.4 torr. We have experimentally determined the microplasma's electrical impedance, which consists of bulk plasma resistance and capacitive sheath reactance. These two parameters were measured by fitting theoretical power reflection coefficients to experimental forward and reflected microwave power as a function of frequency. Microplasma resistance decreases with increasing frequency, showing that the generation of free electrons depends on the driving frequency. In addition, the reactive sheath impedance and the microwave electrode voltage also decrease with an increase of frequency. A 3-D microplasma simulation shows that a narrower sheath width exists for higher frequency microplasma, and this is responsible for reducing the reactive impedance and the peak-to-peak electrode voltage. At higher microwave frequency, the decreased electrode voltage reduces both the plasma potential and the ion kinetic energy losses, thus increasing the electron density.