| AVS 61st International Symposium & Exhibition | |
| Plasma Science and Technology | Wednesday Sessions |
| Session PS-WeA |
| Session: | Plasma Diagnostics, Sensors, and Control |
| Presenter: | Valery Godyak, RF Plasma Consulting |
| Authors: | V.A. Godyak, RF Plasma Consulting B.M. Alexandrovich, Plasma Sensors |
| Correspondent: | Click to Email |
Electrostatic (Langmuir) probes are powerful instruments for diagnostics of non-equilibrium plasmas in experimental and industrial plasma reactors. There are three levels of the probe diagnostics comprising different equipment complexity and having different accuracy of obtained the plasma parameters. These three approaches are based on inferring the plasma parameters from: i – the ion part of the probe characteristic, ii – the electron part of the probe characteristic (classical Langmuir probe method), and iii - by differentiation of the probe characteristic to obtain an Electron Energy Distribution Function, EEDF. Then, the electron temperature Te and plasma density N, as well as, the rates of collisional processes and transport coefficients are found as corresponding integrals of the measured EEDF. Methods i and ii assume Maxwellian EEDF which is not valid for most cases of non-equilibrium plasmas. This and many others questionable assumptions in methods i and ii make actual EEDF measurements the only reliable contemporary probe diagnostics. Langmuir probes in plasma processing reactors are often subjected to the probe surface contamination and high level of rf and low frequency noise. Reliable EEDF measurements require the probe system capable of efficient mitigation of these environmental distortions. The presentation main subject is comparison of EEDF measurement results obtained with different commercial probe systems. It is shown that the measurement accuracy of the plasma parameters in many commercial probe systems is compromised by heavily distorted EEDF in low and high energy region. Low energy distortions make impossible to detect low energy electrons (comprising the majority of electron population), thus leading to underestimation of the plasma density, while high energy distortions make impossible detection of fast electrons producing excitation and ionization. Therefore, some commercial probe systems yielding distorted EEDFs are unable to reveal any additional valid information to that obtained with classical Langmuir procedure.