Numerical Investigation Of The Electromagnetic Effects In Very High Frequency Capacitively Coupled Plasmas

Capacitively coupled plasmas (CCP) are widely applied in the semiconductor industry, for instance, for deposition of thin films, etching of materials and surface treatment. It is well known that a higher frequency produces higher-density plasmas with lower-energy ions. Thus, special attention has been paid to very high frequency (VHF) plasma sources due to their higher ion flux and lower ion bombarding energy. However, at high frequency (i.e., tens of MHz to hundreds of MHz) in large-area reactors, the so-called electromagnetic effects (i.e. standing-wave effect and skin effect) have an important influence on the capacitive discharge, which may limit the plasma spatial uniformity. Indeed, when the excitation wavelength becomes comparable to the electrode dimension, the standing-wave effect becomes dominant, and it results in a substantial power deposition at the reactor center. On the other hand, when the skin depth is not large compared with the plasma thickness due to the high plasma density, the skin effect has a significant influence, and it yields a pronounced power deposition at the reactor edge. These effects are important for plasma processing applications, as they affect the uniformity of the etch and deposition processes. Therefore, it is important to understand the electromagnetic effects, and to suppress the nonuniformity, in order to control the discharge process and to improve the application.In this dissertation, we first briefly review the background, recent advances, and challenges of VHF-CCP, and also the problems we face in Chapter1. The contents of Chapter2to Chapter6are presented as follows.The two dimensional fluid model used in the dissertation is described in Chapter2. In this model, the continuity equations are used to give information on the density evolution for all species. The drift diffusion approximation is assumed for electrons; the momentum balance equation based on the cold fluid approximation is adopted for ions. Because the ions and the neutral species are assumed at room temperature, only the electron energy balance equation is needed. In order to take the electromagnetic effects into account, the full set of Maxwell equations is included instead of solving the electric field by Poisson equation directly. Besides, boundary conditions are also specified in order to complete the problem.The electromagnetic effects at various discharge conditions have been investigated in Ar plasmas by comparing the plasma characteristics obtained from the so-called electrostatic model (i.e., without taking into account the electromagnetic effects) and the electromagnetic model (which includes the electromagnetic effects) in Chapter3. The results indicate that the electromagnetic effects have an important influence on the plasma properties, especially at very high frequencies. Indeed, when the excitation source is in the high frequency regime and the electromagnetic effects are taken into account, the plasma density increases significantly and meanwhile the ionization rate profile evolves to a very different distribution. Furthermore, we also investigated the dependence of the plasma characteristics on the voltage and pressure, at constant frequency. It is observed that when the voltage is low, the difference between these two models becomes more obvious than at higher voltages. As the pressure increases, the plasma density profiles obtained from the electromagnetic model shift smoothly from edge-peaked over uniform to a broad maximum in the center. In addition, the edge effect becomes less pronounced with increasing frequency and pressure, and the skin effect instead of the standing-wave effect becomes dominant when the voltage is high.In Chapter4, the phase-shift effect on the transient behaviour of electrodynamics and power deposition, as well as the influence on the radial uniformity of several plasma characteristics in a hydrogen capacitively coupled plasma has been investigated. It is shown that the spatiotemporal distributions of the plasma characteristics obtained for various phase shift cases are obviously different both in shape and especially in absolute values. At the frequency of13.56MHz, the radial electron flux moves towards the chamber wall first and then is forced in the opposite direction, whereas it exhibits two peaks within one period at the reverse-phase case. In the very high frequency discharge, i.e.,100MHz, the radial electron flux is alternately positive and negative with four peaks during one period, and the ionization mainly occurs in the sheath region at a phase difference equal to π. Furthermore, the phase shift has different influences on the plasma radial uniformity at various frequencies.Chapter5shows the phase-shift effect on the plasma radial uniformity and the plasma composition at various frequencies and gas mixture ratios in Ar/CF4capacitively coupled plasmas. At low concentration of CF4(10%), Ar+are the major positive ions in the entire range of frequencies, and the phase-shift control shows different effects on the plasma uniformity at various frequencies. When the frequency is fixed at100MHz, the phase-shift control shows a different behavior at high concentration of CF4. For instance, the CF3+density profiles shift from edge-high over uniform to center-high at the reverse phase case, as the CF4content increases from10%to90%, which indicates that the skin effect is suppressed by the high electronegativity of the Ar/CF4=0.1/0.9mixture. Besides, the ratio of the total negative ion density to electron density decreases with increasing frequency, and it increases with CF4content. In chapter6, a2D hybrid model, called HPEM (Hybrid Plasma Equipment Model), incorporating a full-wave solution of Maxwell’s equations, is employed to investigate the electromagnetic effects on the plasma characteristics, as well as the power effect on the etch rate in CF4/O2discharges. It is shown that the electromagnetic effects have an important influence on the plasma density distribution. When the electromagnetic effects are taken into account, the plasma density becomes higher, and exhibits different shapes. At the frequency of60MHz, the etch rate has a center-high profile. When adding a low frequency power into the discharge, the etch rate becomes higher and more uniform. As the low frequency power increases from300W to1000W, the etch rate at the reactor center increases faster than at the edge, and therefore the uniformity of the etch rate becomes worse. When the low frequency power is fixed at300W, the etch rate becomes nonuniform with increasing high frequency power, and it becomes higher due to the higher plasma density.