Session DT2: Capacitively Coupled Plasmas I

Resonant sheath heating in weakly magnetized capacitively coupled plasmas due to electron-cyclotron motion

An electron heating mechanism based on a resonance between the gyrating electrons and the oscillating sheath is investigated in weakly magnetized capacitively coupled plasmas at low pressure. If the electron gyrofrequency coincides with half of the driving frequency, the gyrating electrons will coherently collide with the expanding sheath, gaining substantial energy. The plasma density is thus to see a pronounced increase in the case of resonance, which is verified experimentally. Particle simulations are carried out to reveal electron kinetics by tracking the trajectory of the resonant electron. It is found that the operating pressure, electrode gap, and driving voltage are important factors characterizing the resonance effect.

    

GEC Student Excellence Award Finalist Presentation - Uniformity control by customized electrode designs in capacitive RF plasmas

Plasma uniformity control in capacitive RF discharges by tailored electrode topologies and materials is investigated in argon at low pressure by 2D Particle-In-Cell/Monte Carlo collision simulations. Embedding ring-shaped trenches into an electrode is found to increase the plasma density locally by enhancing the electron interaction with the sheath at the trench position. By analyzing the trajectories of selected electrons as well as the time evolution of their energy and velocity inside and above such trenches, the electrons are found to gain high energies by bouncing between the sheaths at the trench sidewalls, leading to high ionization rates at the trench orifice. Radial variations of the electrode material characterized by different secondary electron emission/electron reflection coefficients are also found to provide control of the radial profile of the ionization rate due to the significant contribution of the emitted/reflected electrons to the ionization. Both methods of customizing the electrode design are able to remarkably improve the plasma uniformity above the wafer placed on the powered electrode by tailoring only the design of the grounded counter electrode.   

Author not Attending

It is known that in very-high-frequency capacitively coupled plasmas (CCPs), the higher harmonics generated by nonlinear sheath motion can enhance the standing wave effect, leading to a center-peaked plasma density. It is also known that the dual-frequency (DF) CCP is widely used for the independent control of the ion flux and ion bombarding energy. In this work, a nonlinear transmission line model is coupled to a bulk plasma model and a numerical sheath model to study the effect of the low-frequency (LF) voltage on nonlinear standing wave excitation and plasma uniformity in DF asymmetric capacitive argon discharges at 3 Pa. The simulations are conducted with a 2 MHz LF voltage varied from 0 to 700 V and a 60 MHz high-frequency (HF) voltage fixed at 50 V. Simulation results indicate that when the LF voltage is 0 V, the standing wave effect enhanced by nonlinear harmonics leads to a significant center peak of the plasma density. As the LF voltage increases, the surface wavelength increases due to a thicker sheath width, and the amplitudes of the nonlinear harmonics decay dramatically. As a result, the plasma density profile becomes more uniform with increasing the LF voltage.   

Surface effects in a capacitive argon discharge in the intermediate pressure regime

One-dimensional particle-in-cell/Monte Carlo collisional simulations are performed on a capacitive argon discharge in the intermediate pressure regime (1.6 Torr). The excited argon states (metastable levels, resonance levels, and the 4p manifold) are modeled self-consistently with the particle dynamics as spaceand time-varying fluids. Most of the ionization occurs near the plasma-sheath interfaces, with little ionization within the plasma bulk region. When the excited states, and secondary electron emission due to neutral and ion impact on the electrodes are included in the discharge model, the discharge operation transitions from α-mode to γ-mode, in which nearly all the ionization is due to secondary electrons. Secondary electron production due to the bombardment of excited argon atoms is roughly 14.7 times greater than that due to ion bombardment. Electron impact of ground state argon atoms by secondary electrons contributes about 76% of the total ionization; primary electrons, about 11%; metastable Penning ionization, about 13%; and multi-step ionization, about 0.3%. Furthermore, the simulation results are compared to experimental findings at low pressure (100 mTorr) and intermediate pressure (1 Torr).   

The effects of different boundary surface materials on electron power absorption dynamics in capacitive RF plasmas

In technological high frequency plasmas, the boundary surface materials are known to affect process relevant discharge characteristics, such as the production of charged species and radicals. However, the scientific origin of these effects is often unclear. In this work, we experimentally study the effect of different electrode surface materials on the spatio-temporally resolved dynamics of energetic electrons by Phase Resolved Optical Emission Spectroscopy in a symmetric low pressure capacitive RF discharge (13.56 MHz). Measurements are done as a function of pressure and driving voltage amplitude in argon gas with a small admixture of neon. We find that changing the electrode material can induce an ⍺- to ??-mode transition due to the different ion induced secondary electron emission coefficients. Using different surface materials at the powered and grounded electrode can lead to the presence of different modes at each electrode. This breaks the symmetry of the discharge and, thus, causes the generation of a DC self bias. With the obtained results it is shown experimentally that to more precisely model the plasma physics in simulations, material dependent electron emission coefficients need to be included, which is often not done.   

Author not Attending

Magnetically enhanced capacitively coupled radio frequency plasma discharges (MECCPs) are widely used in various industrial applications. In the absence of a magnetic field, the secondary electrons (SEs) emitted from the electrodes can obtain the full sheath potential and exhibit significant nonlocal kinetics at low pressure. Whereas, with the introduction of the transverse magnetic field, SEs can be localized near the sheath and interact with the electric field inside the sheath for multiple times due to the strong magnetic confinement effect. Meanwhile, an interesting phenomena "branches" can be observed in the electron energy probability distributions (EEPFs) and spatiotemporal electron density. In this work, the secondary electrons kinetics and "branches" are investigated by a combination of 1d3v Particle-In-Cell/Monte Carlo Collision simulations and testing particle method, in MECCPs. The analysis of the SEs trajectories indicates that the "branches" observed in EEPFs and spatiotemporal total electron density, are direct results of the collective motion of SEs emitted at different radio frequency phases, and the number of the branches are related to the applied magnetic field strength. *This work was financially supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 11935005, 12105035), the Fundamental Research Funds for the Central Universities (No. DUT21TD104)   

GEC Student Excellence Award Finalist Presentation - Striations in dual-low-frequency (2/10 MHz) driven capacitively coupled CF4 plasma

In electronegative radio-frequency plasmas, striations (STRs) can appear if the bulk plasma is dominated by positive and negative ions. Here, we investigate the behaviors of STRs and plasma parameters in dual-low-frequency (2/10 MHz) capacitively coupled CF₄ plasmas by phase-resolved optical emission spectroscopy and particle-in-cell/Monte Carlo collision simulations combined with analytical model. This choice of the frequencies is made to ensure that the ions can react to both the lower (2 MHz, 'low frequency', LF) and the higher (10 MHz, 'high frequency', HF) components of the excitation waveform. A strong interplay of the two excitation components is revealed. The maximum ion density decreases slightly as a function of LF voltage, φ_LF, while the minimum of the ion density remains almost constant. The spatio-temporal distributions of the excitation and ionization rates are modulated both by the LF and HF with maxima that occur at the first HF period that follows the complete sheath collapse at a given electrode. These maxima are caused by a high local ambipolar electric field. The LF components of the F⁻ ion velocity and of the electric field are much lower than the respective HF components due to the lower LF component of the displacement current in the sheaths.