1. Introduction

The inductively coupled plasma (ICP) source is one of the most important low-temperature plasma sources that find widespread applications in many fields [1], such as plasma photonic crystals, synthesis of nanomaterials and nanostructured materials, atomic layer processing, agriculture and innovative food cycles, medicines, environments, plasma-assisted combustion and chemical conversion and aerospace application (propulsion and flow control) and so on. Driven within the domains of radio frequency electromagnetic and rather low-pressure

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

(mTorrs) ranges, the ICP sources present several advantages, such as high-plasma density, high anisotropy in the sheath, independent control of incident flux density and energy and simple low-cost reactor configuration (unwanted for the static magnetic devices) over some other plasma sources, such as capacitively coupled plasma and electron cyclotron resonance reactor [2]. As compared to the atmospheric discharges, this sort of low-pressure radio frequency plasma sources are known for their non-equilibrium properties, that is, Te ≫ Ti > Tn, where Te, Ti and Tn are temperatures of electrons, ions and neutrals, respectively [3], due to the low-temperature peculiarity of this type of plasma source. Another essential feature is its weak ionization degree that ensures the abundance of collisions and reactions between charged species and neutrals, which is quite different with the high-temperature fully ionized plasmas where only the Coulomb interactions between charged species are important [4]. Of great importance is the diversity in the mutual interactions among charged and neutral species, which are classified into elastic and inelastic collisions with respect to the principle of kinetic energy balance. Regarding species specialty and colliding outcomes, the inelastic collisions can be described as type (1) ionization, dissociation, electronic, rotational, vibrational excitation, attachment, detachment and de–excitation, which mainly occur between electrons and neutrals; type (2) recombination, associations, charge exchange, excitation transfer and penning ionizations, which mainly happen among heavy species (meant to all species except for electrons); and type (3) the spontaneous radiation from excited state species (without a trigger) [5]. The elastic scattering to some extent determines plasma transport process and hence spatial characteristics of plasma via the parameter of momentum transfer collision frequency, while the inelastic collisions that sustain the weakly ionized plasma mainly determine the energy loss mechanism and give steady-state plasma components optical emission. Finally, all low-temperature plasma sources are generated in chambers with their respective fixed configurations and more importantly with limit space dimension. This means that all the plasmas are bounded plasmas, as compared with the space plasma; therefore, the sheath, produced on all bound surfaces, forms one important constituent of low-temperature plasma physics [6]. In a word, non-equilibrium, weak ionization and plasma bounds characterize the low-pressure radio frequency plasma source as complicated and multi-disciplinary.

power source applied to the coil is temporally varied in the range of radio frequency. At low-coil power, the ICP source is maintained at E mode, where the plasma density and optical emissions are weak, and the glow area of discharge is more localized under the coil. As we increase the coil power, the discharge transfers abruptly or smoothly toward H mode, where the plasma density and current are significantly increased and the optical emission is strengthened. Moreover, the discharge is more uniform. Interestingly, at certain circumstances, when cycling the power source, the trajectories of plasma parameters versus upward and downward powers don't coincide; hence, hysteresis is formed and the ICP source is therefore famous for its other feature, that is, the existence of two stable states at one fixed power value. In labs of academia or enterprise, the ICP sources are triggered from the E mode at the beginning and then transferred to H mode. Most of the plasma processing techniques prefer to be conducted in the H mode due to its better plasma properties. Therefore, understanding the E–H mode transition and hysteresis

Mode Transition and Hysteresis in Inductively Coupled Plasma Sources

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This chapter is outlined as follows. In Section 2, the major achievements of the author on this topic are presented. Three subtopics and the used methodology are discussed and described, aimed at demonstrating to readers the characteristics of plasma parameters, electron kinetics and neutral species during mode transition and excitation of discontinuous mode transition and hysteresis by the external circuit. Finally, the conclusion and further remarks are given in

2. Theoretical and experimental investigations of mode transitions and

In this part, the characteristics of electron parameters, density, temperature and energy distribution function and plasma potential at two modes are presented via the two-dimensional hybrid model [14]. The hybrid model consists of three parts, that is, fluid module, electron Monte Carlo module and electromagnetic module. Species density and momentum, together with the electrostatic field generated by net charge density (analogous to ambipolar diffusion field), are given by the fluid module. Electron transport and collision coefficients, and the effective electron temperature, are calculated through the Monte Carlo method and then transferred to the fluid module. The electromagnetic module calculates the electromagnetic field generated via the coil current and voltage through the Maxwellian's equations, based on the electron conductivity from the fluid module. Both the electrostatic and electromagnetic fields are sent to the Monte Carlo module to push the electrons via Newton's law. The interactions of three modules are illustrated by the model flowchart in Figure 1. The three modules are iterated with each other until a final steady state is achieved. In this chapter, a cylindrical inductively coupled plasma reactor with planar coil is used, as shown in Figure 2. In Figures 3 and 4, the calculated electron density and temperature profiles versus coil current at the pressure of 20 mTorr are presented. In Figure 3, at low-coil current, 10 A, the density

is very meaningful to the related industry.

hysteresis: An overview

2.1. Characteristics of basic plasma parameters

Section 3.

Even with the above complexity, rich and fruitful interesting physics phenomena and mechanisms are already revealed in these low-pressure and radio frequency plasma sources via present efforts. In particular, in the ICP sources, pulsed radio frequency power source [7], standing wave effects [8], nonlinear harmonics [9], double coil discharges [10], anomalous skin effects [11], nonlocal electron kinetics [12], mode transition and hysteresis [13] and so on are still or have been hot research frontiers that draw attention. In this chapter, the mode transitions and hysteresis topic is focused upon. This topic has been historically studied well and continually occupies people's attention due to its complexity of the multi-factor interactions and potential application in achieving stable plasma sources for the processing technique. The ICP source is famous for its capacity of operating at two different modes, that is, capacitive and inductive modes. The capacitive mode is sustained by radial and axial electromagnetic fields, analogous to conventional capacitively coupled plasma source that is excited by the electrostatic field and hence is abbreviated as the E mode. The inductive mode is sustained by the azimuthal electromagnetic field caused by coil current and is abbreviated as H mode. Remember that the power source applied to the coil is temporally varied in the range of radio frequency. At low-coil power, the ICP source is maintained at E mode, where the plasma density and optical emissions are weak, and the glow area of discharge is more localized under the coil. As we increase the coil power, the discharge transfers abruptly or smoothly toward H mode, where the plasma density and current are significantly increased and the optical emission is strengthened. Moreover, the discharge is more uniform. Interestingly, at certain circumstances, when cycling the power source, the trajectories of plasma parameters versus upward and downward powers don't coincide; hence, hysteresis is formed and the ICP source is therefore famous for its other feature, that is, the existence of two stable states at one fixed power value. In labs of academia or enterprise, the ICP sources are triggered from the E mode at the beginning and then transferred to H mode. Most of the plasma processing techniques prefer to be conducted in the H mode due to its better plasma properties. Therefore, understanding the E–H mode transition and hysteresis is very meaningful to the related industry.

This chapter is outlined as follows. In Section 2, the major achievements of the author on this topic are presented. Three subtopics and the used methodology are discussed and described, aimed at demonstrating to readers the characteristics of plasma parameters, electron kinetics and neutral species during mode transition and excitation of discontinuous mode transition and hysteresis by the external circuit. Finally, the conclusion and further remarks are given in Section 3.
