Microwave plasma sources using elongated field applicators that extend over distances large in comparison with the free-space wavelength are in demand for many applications, mainly for the processing of large sulfaces of materials that can be moved transversely with respect to the applicator. Various long applicators have been presented in the literature but no unified description is available to assist the potential user in selecting the one best suited for his or her particular needs. This paper is intended to meet this need.All applicators of this kind provide an electromagnetic wave from which originates the electric field sustaining the discharge. However, they differ in some essential aspects which reflect on their performance.and on the modelling of the resulting plasma. We propose to classify these applicators first into two main categories according to the way the power flows to the plasma: (i) transmissionline applicators: and (ii) antenna applicators. Then, in each category, we need to distinguish two possible wave characters: travelling and standing. This leaves us with four systems to examine. For each of these systems, we briefly approach discharge modelling, mainly to examine the longitudinal distribution of plasma properties and establish guidelines for the methodical design and operation of these plasma sources. Finally, we evaluate the advantages and disadvantages of the four systems.

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We review the literature on cold plasma chemistry, dealing with the plasma deposition of organic (plasma polymers) and inorganic thin films, and with plasma etching, in which the plasma excitation frequency is the principal variable. Based on recent theoretical results by Ferreira and Loureiro, we propose that some observed frequency effects should be interpreted as resulting from different electron energy distribution functions (EEDF), as the shapes of these EEDF depend upon the ratio ν/ω (ν being the electron-neutral collision frequency for momentum transfer and ω the angular frequency of the applied electromagnetic field) of the plasma under consideration. For gas pressures in the range 50–500 mTorr, this leads one to distinguish between low frequency (LF, ≤13.56 MHz, where ν/ω≥1) and microwave plasmas (>300 MHz, the microwave frequency range, for which ν/ω ≪ 1); the microwave plasmas have a comparatively higher population of very energetic electrons. We then present a selection of our own experimental data comparing LF and microwave (2.45 GHz) plasma deposition of organosilicon plasma polymers, and of plasma deposited silicon nitride. Deposition rates in microwave plasmas are found to be about ten times higher than in LF plasmas, and films tend to differ in their structures and physical properties. These results are tentatively attributed to differences in the EEDF, pending confirmation by selected plasma diagnostic experiments.

At gas pressures >1 kPa, tubular rare-gas electrical discharges can contract radially yielding a single axially centred bright filament. When sustaining the discharge with microwave fields of a high enough frequency, instead of a single filament, two or more smaller-diameter and off-centre plasma filaments can be formed. When a specific percentage (<1%) of a rare gas having a lower ionization potential is added to a pure rare-gas atmospheric-pressure discharge, the single-filament contracted discharge fully expands radially or the initially multi-filament discharge becomes homogeneous. Experimental characteristics of this phenomenon and the required operating conditions are reported here for the first time.

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