|AVS 55th International Symposium & Exhibition|
|Plasma Science and Technology||Thursday Sessions|
|Session:||Atmospheric Plasma Processing and Micro Plasmas|
|Presenter:||J. Xue, Tufts University|
|Authors:||J. Xue, Tufts University
J. Hopwood, Tufts University
|Correspondent:||Click to Email|
In the microwave band, higher excitation frequency is found to enhance microplasma generation. The microplasma is formed by a split-ring resonator (SRR) consisting of a half-wavelength microstrip transmission line formed into a ring with a micromachined discharge gap.1 The SRR plasma can be operated from 0.1 to 760 Torr with less than 0.5 W of power in He and Ar. Typically, microplasmas have been generated with DC, AC, RF, and microwave power. One unanswered fundamental question, however, concerns the effect of frequency on microplasma generation. The excitation frequency of capacitively coupled plasma has been discussed by Surendra and Graves.2 This early work suggests that plasma density scales as the square of the applied excitation frequency. That work focused on large-scale plasma at low pressure and the excitation frequency was limited to less than 120 MHz. This paper presents plasma impedance analysis of three microplasmas operating at excitation frequencies of 450 MHz, 900 MHz, and 1.8 GHz. The electron density and sheath capacitance of the microdischarges are extracted from the plasma impedance. Experimentally, these three SRR’s are fabricated on microwave laminate (Rogers, RT/Duroid 6010LM) with identical microstrip widths (1 mm) and discharge gaps (200 μm). The radii of the rings are scaled by 1/f and the smallest radius is 5mm at 1.8 GHz. To determine the plasma impedance, the microwave reflection coefficient is measured as a function of frequency while maintaining a constant microwave power absorbed by the plasma. Using the method in Ref. 1, the microplasma impedance is found by fitting the theoretical microwave reflection coefficient to the measured reflection coefficient. The results show that microplasmas generated by higher frequency resonators have a lower plasma resistance. The extracted electron densities in argon microplasma at 760 Torr are estimated as 2.4, 6.0, and 8x1013 cm-3 for the 450 MHz, 900 MHz, and 1.8 GHz SRR, respectively. The imaginary part of the plasma impedance provides a model of the plasma sheath capacitance. This data shows a diminishing sheath impedance at high frequency which is responsible for improved electron density.
1 F. Iza and J. Hopwood, Plasma Sources Sci. Technol. 14, 397 (2005).
2 M. Surendra and D. B. Graves, Appl. Phys. Lett. 59, 2091 (1991).