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    Plasma Science Monday Sessions
       Session PS-MoP

Paper PS-MoP17
Evaluation of Langmuir Probe Theories via Comparison with Microwave Interferometry and Plasma Oscillation Probe Methods in ICPs

Monday, October 29, 2001, 5:30 pm, Room 134/135

Session: Plasma Diagnostics and Plasma-Surface Interactions Poster Session
Presenter: J.D. Evans, University of California, Los Angeles
Authors: J.D. Evans, University of California, Los Angeles
F.F. Chen, University of California, Los Angeles
W. Zawalski, Hiden Analytical, Ltd., England
Correspondent: Click to Email
Measurements of plasma density (Np) are performed under identical conditions using microwave interferometry (MWI), plasma oscillation probe (POP) and Langmuir probes. Np via MWI is treated as the "known" result to which the other techniques are compared. Data is obtained in a chamber that lies downstream from a PlasmaTherm ICP source, over a wide range of parameters: Prf<1kW, Te=2-5eV, Np=10^10 - 10^12 cm^-3. Ne obtained via POP agrees well with MWI in relatively low-collisional regimes, as expected. I-V curves are acquired using Hiden Analytical's ESPion Langmuir probe, which employs broadband RF compensation via inline chokes and a compensation electrode. A variety of probe tip radii are used, which allows for a broad base of comparison. Conventional probe theories (OML, ABR and BRL) are used, as well as an approximation based on the assumption that the ion current (Ii) is determined mainly by the unipolar sheath expansion, whose thickness follows a modified Child-Langmuir (C-L-mod) law in which Ii is nearly proportional to Vo^(4/3), where Vo = |Vbias-Vplasma|. Different probe theories yield vastly differing values of ion density (Ni), whereas Te is reliably determined in most cases. In most cases the dependence of Ii with Vo agrees very well the C-L-mod law. Excellent agreement between Ni thus obtained and the MWI results is observed throughout the parameter range. However, it is also found that Hiden's ESPsoft algorithm, which employs OML theory that predicts Ii^2 proportional to Vo, yields values for Ne in good agreement with those of MWI and POP. Reasons for the success of the C-L-mod law in light of this apparent paradox are presented.