You-Nian Wang

Control of plasma properties in capacitively coupled oxygen discharges via the electrical asymmetry effect

Using a combined experimental, numerical and analytical approach, we investigate the control of plasma properties via the electrical asymmetry effect (EAE) in a capacitively coupled oxygen discharge. In particular, we present the first experimental investigation of the EAE in electronegative discharges. A dual-frequency voltage source of 13.56 MHz and 27.12 MHz is applied to the powered electrode and the discharge symmetry is controlled by adjusting the phase angle θ between the two harmonics. It is found that the bulk position and density profiles of positive ions, negative ions, and electrons have a clear dependence on θ, while the peak densities and the electronegativity stay rather constant, largely due to the fact that the time-averaged power absorption by electrons is almost independent of θ. This indicates that the ion flux towards the powered electrode remains almost constant. Meanwhile, the dc self-bias and, consequently, the sheath widths and potential profile can be effectively tuned by varying θ. This enables a flexible control of the ion bombarding energy at the electrode. Therefore, our work proves the effectiveness of the EAE to realize separate control of ion flux and ion energy in electronegative discharges. At low pressure, the strength of resonance oscillations, which are found in the current of asymmetric discharges, can be controlled with θ.

Modulating effects of the low-frequency source on ion energy distributions in a dual frequency capacitively coupled plasma

With the energy resolved quadrupole mass spectrometer and hybrid simulation, the influence of low-frequency (LF) source parameters on the ion energy distributions (IEDs) of argon ions impinging on the grounded electrode was studied, both experimentally and numerically, in a dual frequency capacitively coupled plasma. It was shown that for decreasing LF or increasing LF power, the high energy peak in IEDs shifts toward the high energy region significantly. The simulation results were in general agreement with the experimental data.

Heating mechanisms and particle flow balancing of capacitively coupled plasmas driven by combined dc/rf sources

Heating mechanisms and particle flow balancing of capacitively coupled plasmas driven by combined dc/rf sources have been investigated by particle-in-cell/Monte Carlo simulations. At low pressure, Ohmic heating will be suppressed and stochastic heating will be enhanced while increasing dc voltage. But the overall heating power will decrease. No heating mode transitions are observed. At high pressure, bulk plasma density decreases at low dc and rf voltage, and the one-side α‐γ transition will occur while increasing dc voltage. After the transition, the plasma density abruptly increases and average electron energy drops. As the result of that, the plasma is sustained by secondary electrons instead of the Ohmic heating of the bulk electrons. The dc source will reduce, or even eliminate at high voltage, the electron charge flowing into the dc powered electrode. Therefore the ratio of electron-to-ion charge flowing into the rf powered electrode over one period increases from −1.0 to −2.0–−2.3 for low pressure ...

Implicit and electrostatic particle-in-cell/Monte Carlo model in two-dimensional and axisymmetric geometry: I. Analysis of numerical techniques

We developed an implicit particle-in-cell/Monte Carlo model in two-dimensional and axisymmetric geometry for simulations of radio-frequency discharges, by introducing several numerical schemes which include variable weights and a multigrid field solver. Compared with the standard explicit models, we found that the computational efficiency is significantly increased and the accuracy is maintained. Numerical schemes are discussed and benchmark results are presented. The code can be used to simulate practical reactors.

Comparison of electrostatic and electromagnetic simulations for very high frequency plasmas

A two-dimensional self-consistent fluid model combined with the full set of Maxwell equations is developed to investigate an argon capacitively coupled plasma, focusing on the electromagnetic effects on the discharge characteristics at various discharge conditions. The results indicate that there exist distinct differences in plasma characteristics calculated with the so-called electrostatic model (i.e., without taking into account the electromagnetic effects) and the electromagnetic model (which includes the electromagnetic effects), 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 evolves to a very different distribution when the electromagnetic effects are dominant. Furthermore, the dependence of the plasma characteristics on the voltage and pressure is also investigated, at constant frequency. It is observed that whe...

Simulations of dual rf-biased sheaths and ion energy distributions arriving at a dual rf-biased electrode

Spatio-temporal characteristics of collisionless dual rf-biased sheaths and ion energy distributions (IEDs) impinging on a dual rf-biased electrode are studied with a self-consistent one-dimensional hydrodynamic model. The model includes all the time-dependent terms in the ion fluid equations to ensure that it can describe the sheath dynamics over a wide range of frequencies. In addition, an equivalent circuit model is used to self-consistently determine the relationship between the instantaneous sheath thickness and the instantaneous voltage on the dual rf-biased electrode. The numerical results show that, due to dual radiofrequencies being applied to an electrode, the sheath structures and parameters of dual rf-biased sheaths are different from those in the case of single frequency-biased plasma. Multiple peaks appear in the IEDs arriving at the dual rf-biased electrode rather than a bimodal shape as the IEDs are incident onto a single frequency-biased electrode. It is also shown that some parameters su...

Numerical investigation of ion energy distribution and ion angle distribution in a dual-frequency capacitively coupled plasma with a hybrid model

A one-dimensional hybrid model is developed to study the characteristics of energy and angular distributions of the ions and fast neutrals impinging on the rf-biased electrode in a dual-frequency capacitively coupled Ar discharge. The hybrid model consists of a fluid model that determines the spatiotemporal evolution of the discharge, and a Monte-Carlo model that, including the electron-neutral, ion-neutral, and fast neutral-neutral collisions, predicts the energy and angular distributions of the ions and fast neutrals on the rf-biased electrode. The influence of pressure, voltage amplitude, and frequencies of the two rf sources on the energy and angular distributions is discussed. The ion energy distributions (IEDs) appear to have multiple peaks in the dual-frequency capacitively coupled rf discharge rather than bimodal shape in a conventional single-frequency rf discharge. The ion angle distributions (IADs) have a significant peak at a small angle, and most ions strike to the process surface with the an...

Nanostructured fuzz growth on tungsten under low-energy and high-flux He irradiation

We report the formation of wave-like structures and nanostructured fuzzes in the polycrystalline tungsten (W) irradiated with high-flux and low-energy helium (He) ions. From conductive atomic force microscope measurements, we have simultaneously obtained the surface topography and current emission images of the irradiated W materials. Our measurements show that He-enriched and nanostructured strips are formed in W crystal grains when they are exposed to low-energy and high-flux He ions at a temperature of 1400 K. The experimental measurements are confirmed by theoretical calculations, where He atoms in W crystal grains are found to cluster in a close-packed arrangement between {101} planes and form He-enriched strips. The formations of wave-like structures and nanostructured fuzzes on the W surface can be attributed to the surface sputtering and swelling of He-enriched strips, respectively.

Plasmon excitation in metal slab by fast point charge: The role of additional boundary conditions in quantum hydrodynamic model

We study the wake effect in the induced potential and the stopping power due to plasmon excitation in a metal slab by a point charge moving inside the slab. Nonlocal effects in the response of the electron gas in the metal are described by a quantum hydrodynamic model, where the equation of electronic motion contains both a quantum pressure term and a gradient correction from the Bohm quantum potential, resulting in a fourth-order differential equation for the perturbed electron density. Thus, besides using the condition that the normal component of the electron velocity should vanish at the impenetrable boundary of the metal, a consistent inclusion of the gradient correction is shown to introduce two possibilities for an additional boundary condition for the perturbed electron density. We show that using two different sets of boundary conditions only gives rise to differences in the wake potential at large distances behind the charged particle. On the other hand, the gradient correction in the quantum hy...

Effect of bulk electric field reversal on the bounce resonance heating in dual-frequency capacitively coupled electronegative plasmas

The electron bounce resonance heating (BRH) in dual-frequency capacitively coupled plasmas operated in oxygen and argon has been studied by different experimental methods. In comparison with the electropositive argon discharge, the BRH in an electronegative discharge occurs at larger electrode gaps. Kinetic particle simulations reveal that in the oxygen discharge, the bulk electric field becomes quite strong and is out of phase with the sheath field. Therefore, it retards the resonant electrons when traversing the bulk, resulting in a suppressed BRH. This effect becomes more pronounced at lower high-frequency power, when the discharge mode changes from electropositive to electronegative.