Operation regimes, plasma parameters, and applications of the low-frequency (∼500 kHz) inductively coupled plasma (ICP) sources with a planar external coil are investigated. It is shown that highly uniform, high-density (ne∼9×1012 cm−3) plasmas can be produced in low-pressure argon discharges with moderate rf powers. The low-frequency ICP sources operate in either electrostatic (E) or electromagnetic (H) regimes in a wide pressure range without any Faraday shield or an external multipolar magnetic confinement, and exhibit high power transfer efficiency, and low circuit loss. In the H mode, the ICP features high level of uniformity over large processing areas and volumes, low electron temperatures, and plasma potentials. The low-density, highly uniform over the cross-section, plasmas with high electron temperatures and plasma and sheath potentials are characteristic to the electrostatic regime. Both operation regimes offer great potential for various plasma processing applications. As examples, the efficie...

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Low-frequency, high-density, inductively coupled plasma sources: Operation and applications

Typical inductive discharges, such as are used for plasma processing, exhibit two modes of operation: the true inductive discharge known as the H mode, and a weak capacitive discharge known as the E mode. Experimentally, the transition between these modes as the coil current is increased is clear and is marked by a large increase in discharge power, plasma density and optical emission occurring as the H mode appears. According to simple theory, this transition and the reverse transition occur at a single well defined current. In practice, this is usually not the case. The E-to-H transition occurs at a larger coil current than the H-to-E transition, and a range of currents between these values supports either E or H mode. This effect is called hysteresis. In this paper we show that hysteresis can be understood to arise from nonlinear effects, most notably in the electron power balance equation. We survey various mechanisms that can produce hysteresis and attempt to provide quantitative estimates of their significance.

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Hysteresis and the E-to-H transition in radiofrequency inductive discharges

It has been suggested that plasmonic nanostructures could boost nonlinear optical processes in atoms. However, an incomplete understanding of the complex physics in such systems has hampered attempts to harness this idea in applications. An in-depth study now shows that phenomena such as high-harmonic generation might in fact be limited by the tiny volumes involved at the nanoscale.

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Extreme-ultraviolet light generation in plasmonic nanostructures

In order to characterize a nonequilibrium molecular plasma from the point of view of translational, vibrational and rotational degrees of freedom and their interaction, the characteristic temperatures of such a plasma were measured in an ICP rf reactor. Both pure nitrogen and argon–nitrogen mixture plasmas were examined for this purpose.The experimental results of rotational (Tr), vibrational (Tv) and electron (Te) temperatures are presented. Vibrational and rotational temperatures were measured as a function of nitrogen content for both E and H modes of ICP discharge using a power range of 45–200 W and pressure range of 2.6–13.3 Pa. Additionally, the pressure dependence of electron temperature in a pure nitrogen discharge was studied. Results show that rotational temperature is ≈370 K for E mode and ≈470 K for H mode and almost does not depend on either the applied rf power or the nitrogen content in the discharge. Vibrational temperature groups in the range 5000–12 000 K increase with applied rf power and constantly decay with an increase of nitrogen content. The measured values and behaviour of electron temperature are comparable with those for the positive column of the dc glow discharge. The results also prove that these three temperatures obey the classical inequality Te > Tv > Tr, as well as clarifying the differences in both vibrational and rotational temperature for different modes of the ICP discharge.

https://iopscience.iop.org/article/10.1088/0022-3727/40/4/016/pdf

Determination of the vibrational, rotational and electron temperatures in N2 and Ar?N2 rf discharge

The physics underlying the rotating magnetic field current drive technique is presented. The rotamak is a compact torus configuration having the unique and distinctive feature that the toroidal plasma current is driven in a steady-state, noninductive fashion by means of the application of a rotating magnetic field. In its basic form, the rotamak is operated as a field-reversed configuration (Rotamak-FRC). However, by means of a simple modification, a steady toroidal magnetic field can be added to the basic rotamak apparatus and the configuration then becomes that of a spherical tokamak (Rotamak-ST). The performance of a 50-liter rotamak device, both as an FRC and as an ST, is described. Toroidal currents of over 10.5 kA have been achieved with input powers of 300 kW (at 0.5 MHz). Hydrogen plasmas with ne≈7×1018 m−3 and Te≈35 eV have been obtained. The noteworthy reproducibility of the rotamak discharge has enabled the magnetic field lines of an ST to be directly reconstructed from experimental data for th...

A review of rotating magnetic field current drive and the operation of the rotamak as a field-reversed configuration (Rotamak-FRC) and a spherical tokamak (Rotamak-ST)

Inductively coupled rf discharges typically exhibit two modes of operation, namely, a low-density mode known as the E mode and a higher density mode known as the H mode. The transition between these modes exhibits hysteresis. Experimental observations of these transitions are presented. By means of a mixture of electromagnetic theory and circuit analysis and by invoking the requirement that the power absorbed and lost by the electrons should balance, the possible working points of an inductively coupled rf plasma source are identified in (P, ) space, where P is the power absorbed by the electrons, is the peak rf current in the induction coil and is the electron number density. Once the loci of the operating points have been identified in this manner, it is possible to construct a consistent explanation for all the experimental observations reported in the first part of the paper. In particular, it is possible to present an explanation for the hysteresis-like behaviour manifested by the mode transitions. Basically, the transitions occur when it is no longer possible to balance the power absorbed and lost by electrons.

https://iopscience.iop.org/article/10.1088/0022-3727/31/21/014/pdf

Hysteresis in the E- to H-mode transition in a planar coil, inductively coupled rf argon discharge

An air-cored transformer model is commonly used to analyse macroscopic measurements of the electrical properties of inductively coupled, H-mode RF discharges. In this paper, we explicitly expose, via an electromagnetic theory of the H-mode plasma produced within a planar coil ICP, the manner in which the values of the circuit elements of the transformer representation are related to the values of the electron number density and effective collision frequency of the generated plasma. A comparison of experimental measurements and theoretical predictions lends credence to the view that the transformer model of H-mode discharges has a very useful role to play in the design and understanding of inductively coupled RF plasma sources.

https://iopscience.iop.org/article/10.1088/0963-0252/7/2/012/pdf

The electromagnetic basis of the transformer model for an inductively coupled RF plasma source

This paper describes theoretical and experimental investigations of the electromagnetic field within a planar coil, inductively coupled RF plasma source. Theoretical expressions are derived for the spatial distributions of the RF magnetic field within the reactor chamber, both for the situation in which no plasma is generated and for the situation in which an H-mode discharge exists. Detailed measurements of the RF magnetic field and the electron number density and temperature for one particular set of working conditions are reported. These are analysed, with the aid of the theory, to investigate how well the properties of the plasma can be inferred from the magnetic field data, and vice versa. It is concluded that, to within a factor of about two, useful information concerning these quantities can be deduced.

https://iopscience.iop.org/article/10.1088/0963-0252/7/2/011/pdf

Theoretical and experimental investigations of the electromagnetic field within a planar coil, inductively coupled RF plasma source

A 32 cm diameter, low-frequency (0.56 MHz), planar-coil ICP source is described. The RF generator is coupled to the load via a simple series circuit which allows the coil current to be varied in a straightforward manner. The global electrical properties of argon discharges produced with filling pressures lying in the range 0.4 mTorr - 8 Torr are determined. Measurements of both the amplitude and phase of the secondary current induced in the plasma are reported. All of these measurements are combined in an application of transformer theory to yield values of the plasma resistance and inductance and the coupling constant between the induction coil and the plasma.

https://iopscience.iop.org/article/10.1088/0963-0252/6/2/014/pdf

Measurement of the induced plasma current in a planar coil, low-frequency, RF induction plasma source

By adding a current-carrying central rod to the basic rotamak apparatus, a magnetic configuration has been produced which is that of a spherical tokamak (ST) maintained in steady state by the application of a rotating magnetic field. The noteworthy reproducibility of the rotamak-ST discharges has facilitated the measurement of the time-averaged magnetic field components throughout a poloidal plane. These measurements, together with an assumption of axisymmetry, have enabled the field lines of an ST to be directly reconstructed from experimental data for the first time.

I M El-Fayoumi

Papers

Hysteresis in the E- to H-mode transition in a planar coil, inductively coupled rf argon discharge

The electromagnetic basis of the transformer model for an inductively coupled RF plasma source

Theoretical and experimental investigations of the electromagnetic field within a planar coil, inductively coupled RF plasma source

Measurement of the induced plasma current in a planar coil, low-frequency, RF induction plasma source

Operation of the Rotamak as a Spherical Tokamak: The Flinders Rotamak-ST