Non-invasive VHF monitoring of low-temperature atmospheric pressure plasma

• Published in: Plasma sources science & technology Vol. 19; no. 3; p. 034008
• Main Authors: Law, V J, Daniels, S, Walsh, J L, Kong, M G, Graham, L M, Gans, T
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Bristol IOP Publishing 01.06.2010; Institute of Physics
• Subjects: Atmospheric pressure; Barometric pressure; Chambers; Communication, education, history, and philosophy; Current density; Electric and magnetic measurements; Electric power generation; Electrodes; Exact sciences and technology; Mathematical models; Optical (ultraviolet, visible, infrared) measurements; Physics; Physics literature and publications; Physics of gases, plasmas and electric discharges; Physics of plasmas and electric discharges; Plasma diagnostic techniques and instrumentation; Reactors; VHF; Oxygen; Atmospheric pressure; Plasma assisted processing; Shielding; Experimental study; Helium; Electrical measurement; Plasma jets; Plasma diagnostics; Reflectometry; Crossed fields; Current density; Low temperature; Parallel plate
• ISSN: 0963-0252; 1361-6595
• DOI: 10.1088/0963-0252/19/3/034008

A real-time VHF swept frequency (20--300 MHz) reflectometry measurement for radio-frequency capacitive-coupled atmospheric pressure plasmas is described. The measurement is scalar, non-invasive and deployed on the main power line of the plasma chamber. The purpose of this VHF signal injection is to remotely interrogate in real-time the frequency reflection properties of plasma. The information obtained is used for remote monitoring of high-value atmospheric plasma processing. Measurements are performed under varying gas feed (helium mixed with 0--2% oxygen) and power conditions (0--40 W) on two contrasting reactors. The first is a classical parallel-plate chamber driven at 16 MHz with well-defined electrical grounding but limited optical access and the second is a cross-field plasma jet driven at 13.56 MHz with open optical access but with poor electrical shielding of the driven electrode. The electrical measurements are modelled using a lumped element electrical circuit to provide an estimate of power dissipated in the plasma as a function of gas and applied power. The performances of both reactors are evaluated against each other. The scalar measurements reveal that 0.1% oxygen admixture in helium plasma can be detected. The equivalent electrical model indicates that the current density between the parallel-plate reactor is of the order of 8--20 mA cm-2. This value is in accord with 0.03 A cm-2 values reported by Park et al (2001 J. Appl. Phys. 89 20--8). The current density of the cross-field plasma jet electrodes is found to be 20 times higher. When the cross-field plasma jet unshielded electrode area is factored into the current density estimation, the resultant current density agrees with the parallel-plate reactor. This indicates that the unshielded reactor radiates electromagnetic energy into free space and so acts as a plasma antenna.