Microwave discharges at low pressures and peculiarities of the processes in strongly non-uniform plasma

Abstract

Microwave discharges (MD) are widely used as a source of non-equilibrium low pressure plasma for different applications. This paper reviews the methods of microwave plasma generation at pressures from 10 −2 approximately to 30 kPa with centimeter-millimeter wavelength microwaves on the basis of scientific publications since 1950 up to the present. The review consists of 16 sections. A general look at MDs and their application is given in the introduction, together with a description of a typical block-schema of the microwave plasma generator, classification of MD, and attractive features of MD. Sections 2-12 describe the different methods of microwave plasma generators on the basis of cavity and waveguide discharges, surface and slow wave discharges, discharges with distributed energy input, initiated and surface discharges, discharges in wave beams, discharges with stochastically jumping phases of microwaves, discharges in an external magnetic field and discharges with a combination of microwave field and dc and RF fields. These methods provide the possibility of producing nonequilibriun high density plasma in small and large chambers for many applications. Plasma chemical activity of nonequilibrium microwave plasma is analyzed in section 13. A short consideration of the history and status of the problem is given. The main areas of microwave plasma application are briefly described in section 14. Non-uniformity is the inherent property of the majority of electrical discharges and MDs are no exception. Peculiarities of physical-chemical processes in strongly non-uniform MDs are demonstrated placing high emphasis on the influence of small noble gas additions to the main plasma gas (section 15). The review is illustrated by 80 figures. The list of references contains 350 scientific publications.

Measurement of C2 and CH temperatures in argon‐acetylene low‐pressure microwave‐induced plasma

“…The BPs of C 2 (0,0), C 2 (0,1), and C 2 (1,0) band R 2 branch lines, recorded from Ar + 20% C 2 H 2 MIP, are shown in Figure 10. The BPs of CH(0-0, B 2 Σ − -X 2 Π) band Q 1 branch lines and CH(0-0, A 2 Δ-X 2 Π) band R 2 branch lines obtained for the same FIGURE 10 The Boltzmann plots (BPs) of C 2 (0,0), C 2 (0,1), and C 2 (1,0) band R 2 branch lines, recorded from Ar + 20% and He+20% C 2 H 2 microwave-induced plasma (MIP)…”

“…Electron impact is the main channel of heavy particle excitation and ionization near the electrode, while the secondary processes with the participation of excited particles may contribute as well in the plasma ball region. [8][9][10] In addition, kinetic modelling of nitrogen plasma showed that all observed bands are excited by electron impact owing to a high electric field near the antenna. [9] Previously, the MIP source was employed for diamond growth on heated silicon substrates in the H 2 /CH 4 /O 2 mixture at 20 mbar [11] as well as for T g and T vib measurements using Swan bands emitted from C7-C16 alkane (C n H 2n+2 ) discharges in the liquid phase.…”

Measurements of nitrogen atom density in a microwave‐excited plasma jet produced under moderate pressures

“…In general, microwave discharges can stably produce low-temperature plasmas even at high pressure compared to RF and VHF discharges. A microwave-excited plasma (MWP) has some advantages over other types of plasma sources as it has a wide range of operating pressures from 10 −2 Pa up to pressures exceeding the atmospheric pressure, allows a wide range of plasma absorbed power, and eliminates the possibility of contamination of the plasma and treated surfaces through electrode erosion because of the electrode less discharge [30]. However, previous MWP sources providing nitrogen radicals such as electron cyclotron resonance plasmas [9,11,21,23], tubular-microwave-reactor plasmas [12,13,17,21,23], and resonant-cavity-downstream plasmas [18] impose limitations on enlarging the processing area and reducing the microwave power at high pressure because of a lack in structural flexibility and expandability of rectangular waveguides, external magnetic field controllers, tubular reactors, and resonance cavities.…”

Design and optical study of a microwave plasma torch in nitrogen used for the evaporation of aluminium wires

“…The design of MWDs has considerable influence on its operation range, for example, available pressure ranges, types of useable feed gases, power dissipation processes, excited discharge volumes, and also vacuum compatibility, and might strongly influence the discharge parameters, including the plasma composition, gas temperature, and electron energy distribution function. The main constructional and operational principles of MWDs developed for reduced pressures are reviewed by Lebedev . During the last decade, much progress has been achieved, in particular, in the generation and sustenance of atmospheric‐pressure MWDs.…”