Experimental demonstration of multifrequency impedance matching for tailored voltage waveform plasmas

Driving radiofrequency capacitively coupled plasmas by multiharmonic tailored voltage waveforms (TVWs) has been shown to allow considerable control over various plasma properties for surface processing applications. However, industrial adoption of this technology would benefit from more efficient solutions to the challenge of impedance matching the radiofrequency power source to the load simultaneously at multiple harmonic frequencies. The authors report on the design and demonstration of a simple, practical multifrequency matchbox (MFMB) based on a network of LC resonant circuits. The performance of the matchbox was quantified in terms of a range of matchable impedances (when matching a single frequency at a time), as well as for the independence of each match to changes at adjacent harmonics. The effectiveness of the MFMB was demonstrated experimentally on an Ar plasma excited by a three-frequency TVW with a fundamental frequency of 13.56 MHz. Under the plasma conditions studied, the power coupling efficiency (at the generator output) was increased from less than 40% (without impedance matching) to between 80% and 99% for the different exciting frequencies.