Electron energy distribution functions (EEnFs) have been measured in low-pressure capacitive R F discharges over a wide range of well defined (geometrically and electrically) discharge conditions. Measurements have been made in argon and helium ranging in gas pressure between 3mTorr and 3Torr and in discharge current density between 0.1 mAcm-Z and 10mAcm-Z. The measurements show changes in the EEDF due to the occurrence of physical phenomena such as stochastic electron heating and t h e effect of discharge transition into the y mode. Substantial differences in the EEDF in Ramsauer and non-Ramsauer gases are also demonstrated and discussed. To achieve these results a higher level of performance was required from the measurement system than had been attained in previous EEnF measurements in R F discharges. EEnF measurements were made using a probe system specifically designed to remove or reduce the severity of many problems inherent to such measurements in RF discharges. The rationale and considerations in the probe system design, as well as many construction details of the probe system itself, are discussed.

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An electric-probe method for the diagnostics of electron-distribution functions (EDFs) in plasmas is reviewed with emphasis on receiving reliable results while taking into account appropriate probe construction, various measurement errors and the limitations of theories. The starting point is a discussion of the Druyvesteyn method for measurements in weakly ionized, low-pressure and isotropic plasma. This section includes a description of correct probe design, the influence of circuit resistance, ion current and plasma oscillations and probe-surface effects on measurements. At present, the Druyvesteyn method is the most developed, consistent and routine way to measure the EDF. The following section of the review describes an extension of the classical EDF measurements into higher pressures, magnetic fields and anisotropic plasmas. To date, these methods have been used by a very limited number of researchers. Therefore, their verification has not yet been fully completed, and their reliable implementation still requires additional research. Nevertheless, the described methods are complemented by appropriate examples of measurements demonstrating their potential value.

Electron energy distribution functions (EEDFs) have been measured in a cylindrical inductively coupled plasma (ICP) with a planar coil over a wide range of external parameters (argon pressure, discharge power and driving frequency). The measurements were performed under well-defined discharge conditions (discharge geometry, rf power absorbed by plasma, external electrical characteristics and electromagnetic field and rf current density profiles). Problems found in many probe measurements in ICPs were analysed and a rationale for designing probe diagnostics that addresses these problems is presented in this paper. A variety of plasma parameters, such as, plasma density, effective and screen electron temperatures, electron-atom transport collision frequency, effective rf frequency and rates of inelastic processes, have been found as appropriate integrals of the measured EEDFs. The dependence of these ICP parameters over a wide range of argon pressure, rf power and frequency results in experimental scaling laws that are suitable for comparison with ICP models and helpful in ICP design for many applications.

A microwave resonator probe is a resonant structure from which the relative permittivity of the surrounding medium can be determined. Two types of microwave resonator probes (referred to here as hairpin probes) have been designed and built to determine the electron density in a low-pressure gas discharge. One type, a transmission probe, is a functional equivalent of the original microwave resonator probe introduced by R. L. Stenzel [Rev. Sci. Instrum. 47, 603 (1976)], modified to increase coupling to the hairpin structure and to minimize plasma perturbation. The second type, a reflection probe, differs from the transmission probe in that it requires only one coaxial feeder cable. A sheath correction, based on the fluid equations for collisionless ions in a cylindrical electron-free sheath, is presented here to account for the sheath that naturally forms about the hairpin structure immersed in plasma. The sheath correction extends the range of electron density that can be accurately measured with a particular wire separation of the hairpin structure. Experimental measurements using the hairpin probe appear to be highly reproducible. Comparisons with Langmuir probes show that the Langmuir probe determines an electron density that is 20–30% lower than the hairpin. Further comparisons, with both an interferometer and a Langmuir probe, show hairpin measurements to be in good agreement with the interferometer while Langmuir probe measurements again result in a lower electron density.