| Radio frequency inductively coupled plasma (RF ICP) source is extensively applied in semiconductor manufacturing and the material processing fields because of its desirable characteristics, such as high plasma density at low pressure, easy control of uniformity, simple configuration and so on. As is well known, two discharge modes exist in typical discharge reactors, i.e., the capacitive E mode at low density and the inductive H mode at high density. Either continuous or abrupt transitions between the two discharge modes can be triggered by varying the discharge conditions (such as coil current, input power, series capacitance, discharge frequency and discharge pressures), accompanied by the sudden change of the plasma parameters (such as plasma density, electron temperature) and the discharge circuit electrical parameters (such as coil current, coil voltage and circuit impedance). By cycling the discharge parameters, the discharge also exhibits hysteresis. The mode transition and hysteresis behaviors of ICP can lead to unstable discharge, so systematical investigation of the mode transition and hysteresis is of great importance for the control and optimization of the plasma processing technology.In this thesis, the mode transition and hysteresis behaviors of a planar H2 ICP reactor are simulated using the fluid model and fluid/Monte Carlo hybrid model, which include equivalent circuit module, electromagnetic module, fluid module and electron MC module. The evolutions of the plasma parameters and the equivalent circuit electrical parameters are investigated during the mode transition and hysteresis process, and the underlying mechanisms are analyzed.In Chapter 1, a brief review is given about the plasma microfabrication technology and the characteristics of several common plasma sources. Then a detailed description of the ICP reactor and its two typical excitation modes are given, together with the background, recent advances, and the existing problems of mode transition and hysteresis. The contents of Chapters 2,3,4 and 5 are presented separately below.In Chapter 2, a detailed description of the two dimensional fluid model with an external circuit used in the simulation is given. The model consists of three modules, i.e., fluid module, equivalent circuit module and electromagnetic module. The circuit module includes a radio-frequency (RF) power source, a ?-type matching box, and capacitive and inductive coupling branches. It is used to calculate the electrical parameters, such as the current flowing through the coil, the voltage drop across the coil and the impedance of the circuit components, etc. The electromagnetic module is used for obtaining the RF electromagnetic fields by solving the Maxwell's equations. The fluid module describes the behaviors of all charged and neutral species and calculates the plasma parameters such as the electron density and the electron temperature. Besides, the boundary conditions and the numerical method are also introduced.In Chapter 3, using the fluid model with an external circuit, the mode transition and hysteresis behaviors of the H2 inductively coupled plasma are systematically studied. First, the effect of the equivalent circuit on the mode transition is investigated by adjusting the series capacitance C,, and the evolutions of plasma parameters, such as electron density and temperature, as well as the electromagnetic fields, are characterized in this process. The results show that as the series capacitance increases, the discharge mode turns from E mode dominated to H mode dominated. The electron density first increases slightly and then it jumps suddenly to a very high value, and meanwhile, the electron density profile also changes after the mode transition. The electron temperature decreases abruptly after the mode transition and the profile is significantly changed. The profiles of the electromagnetic fields are almost unchanged after the transition occurs, however, the magnitudes of the capacitive electric fields are significantly reduced, and the inductive electric fields are significantly increased.In addition, the influence of the external circuit on the hysteresis under different pressures is investigated by adjusting C,. The results show that when the pressure is 100 mTorr, as cycling C,, both the discontinuity and hysteresis appear for the plasma parameters and the transferred impedances of both the inductive and capacitive discharge components, while at 20 mTorr, only the discontinuity is observed.Moreover, the effect of input current on mode transition and hysteresis is investigated by adjusting the input current values under fixed C1. When the input current reaches a certain value, the discharge turns from E mode to H mode. The reverse H to E mode transition occurs when reducing the input current. When the gas pressure is high, a clear hysteresis loop is formed, but the hysteresis is not observed under low pressure.In chapter 4, in order to investigate the effect of nonlocal dynamics of electrons on mode transition and hysteresis in the ICP source, the pure fluid model is expanded to a fluid/Monte Carlo hybrid model. In the hybrid model, the macro behavior of the plasma is determined by the fluid model, while the collisions between electrons and neutral particles are given by MC method. Using the hybrid model, the evolutions of the electron energy distribution function (EEDF) with the mode transition under different pressures are investigated. At high pressure, when the discharge is in the E mode, the ratio of low-energy electrons is relatively small. When the discharge turns to H mode, the ratio of low energy electron increases, and the high-energy electrons are depleted severely. But at low pressure, the striking peak of low energy is not observed, which is due to that H2 is non-Ramsauer effect gas and the low energy electrons can be effectively heated through Ohmic collision. Under high pressure, the EEDF changes obviously after the mode transition, while under low pressure, the difference between the EEDF before and after the mode transition is relatively small. The effects of electron dynamics on mode transition and hysteresis are investigated by comparing the hybrid model results with the fluid model results. Besides, by changing the discharge pressure, the effect of pressure on the mode transition and hysteresis is also investigated, and the simulation results are compared with the experimental data obtained by other researchers. The results show that when the discharge pressure increases, the threshold value of the capacitance at which the E to H mode transition happens first decreases and then increases, and the minimum threshold capacitance occurs when the pressure is about 50 mTorr. When the pressure is low, no hysteresis is observed, but the hysteresis appears at high pressure and the hysteresis loop becomes wider as the pressure increases.In Chapter 5, the main conclusions of this thesis and prospects for future work are given. |
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