Foundations of measurement of electrons, ions and species fluxes toward surfaces in low-temperature plasmas

This foundation article discusses the diagnostics of electrons and ions in plasmas and fluxes of charged and neutral species toward plasma-facing surfaces by non-optical methods. The focus is laid on the fundamentals of the most common methods and their application to non-equilibrium low-pressure electropositive plasmas and ion beams. The key topics are Langmuir probes (LPs), Faraday cups (FCs) and energy analyzers, mass spectrometry, as well as novel calorimetric and force probes (FPs). For each method, the concepts are introduced, hints at best practice are given, and a survey of the recently published literature is included. Starting with the sheath-plasma boundary, the principles of plane and cylindrical LPs and the measurement of electron temperature and density are discussed. Beyond standard textbooks, double, triple and emissive probes are described and modulation techniques for obtaining the electron energy distribution function are presented. More technical issues related to probe compensation in radio-frequency plasmas, probe contamination, and the influence of a magnetic field are also included. The presentation is completed by modern radio-frequency techniques, such as impedance probes, multipole resonance probes, and self-excited electron resonance spectroscopy. FCs, retarding field analyzers (RFAs) as well as novel calorimetric and FPs are useful tools for the measurement of overall, not species resolved, ions and neutral species fluxes toward surfaces. RFAs provide overall ion energy distribution functions, whereas calorimetric and FPs can deliver information about fluxes of fast neutrals. The issues related to secondary electron emission, absolute signal calibration, and analysis of plasmas with complex chemistry are discussed. Mass spectrometry diagnostics is capable of mass and energy-resolved detection of ions and neutral species at the plasma-facing surfaces. Detection, identification and absolute density calibration of neutral stable and radical species are treated in detail and peculiarities of ion mass spectrometry, such as detection of negative ions, ion measurements in front of powered electrodes, or chromatic aberration and acceptance angle limitations in energy spectrum measurements are introduced.