Dai Zhongling

Nonlinear Plasma Dynamics in Electron Heating of Asymmetric Capacitive Discharges with a Fluid Sheath Model

A fluid radio-frequency (rf) sheath model coupled to an equivalent circuit method is adopted to describe the nonlinear series resonance effects due to nonlinear interaction of plasma bulk and sheath in asymmetric capacitive discharges. With the fluid sheath model, we can determine self-consistently the relationship between the instantaneous potential drop across the rf sheath and the instantaneous sheath thickness. The numerical results demonstrate that the self-excitation of the plasma series resonance significantly enhances both ohmic heating and stochastic heating. Also, we observe that the effects of nonlinear series resonance increases the total power dissipation by factors of 2–5 for low pressure capacitive plasmas. Furthermore, we find that the largest harmonic is about 13 for the plasma current.

Feature Profile Evolution During Etching of SiO2 in Radio-Frequency or Direct-Current Plasmas

We have developed a plasma etching simulator to investigate the evolution of pattern profiles in SiO2 material under different plasma conditions. This model focuses on energy and angular dependent etching yield (physical sputtering in this paper), neutral and ion angular distributions, and reflection of ions or neutrals on the surface of a photoresist or SiO2. The effect of positive charge accumulation on the surface of insulated mask or SiO2 is studied and the charge accumulation contributes to a deflection of ion trajectory. The wafer profile evolution has been simulated using a cellular-automata-like method under radio-frequency (RF) bias and direct-current (DC) bias, respectively. On the basis of the critical role of angular distribution of ions or neutrals, the wafer profile evolution has been simulated for different variances of angles. Observed microtrenching has been well reproduced in the simulator. The ratio of neutrals to ions has been considered and the result shows that because the neutrals are not accelerated by an electric field, their energy is much lower compared with ions, so they are easily reflected on the surface of SiO2, which makes the trench shallower.

Two-Dimensional Self-Consistent Kinetic Model for Solenoidal Inductively Coupled Plasma

A two-dimensional self-consistent kinetic model was developed to study the influence of the various factors on the electron energy distribution function. These factors include gas pressure, the driving frequency, the radius and length of the inductively coupled plasma equipment, the amplitude of the radio-frequency coil current, and the number of turns of rf coils. The spatial profiles of the rf electric field and power density have also been calculated under the same parameters. Numerical results show that the electron energy distribution functions are significantly modified and the spatial profiles of the rf electric field and rf power density are also demonstrated.

Investigation of Dual Radio-Frequency Driven Sheaths and Ion Energy Distributions Bombarding an Insulating Substrate

Dual radio-frequency (rf) sources at widely different frequencies are often simultaneously used to separately optimize the plasma parameters and ion energy distributions (IEDs) incident onto a substrate. Characteristics of collisionless dual rf biased-sheaths and IEDs impinging on an insulating substrate are studied with a self-consistent one-dimensional fluid model. In order to describe the sheath dynamics over a wide range of frequency, the model includes all the time-dependent terms in the ion fluid equation. Meanwhile, an equivalent circuit model is used to self-consistently determine the relationship among the instantaneous voltage on the insulating substrate, the instantaneous sheath thickness, and the dual currents applied to the electrode. The numerical results show that some parameters such as the bias frequency and bias power of the lower frequency source are crucial for determining the parameters of dual rf biased-sheaths and IEDs arriving at the insulating substrate.

Simulation of Dual Frequency Capacitive Sheath over a Concave Electrode in Low Pressure

A self-consistent two-dimensional (2D) collisionless fluid model is developed to simulate the characteristics of a dual frequency capacitive sheath over an electrode with a cylindrical hole. The model consists of 2D time-dependent fluid equations coupled with Poissons equation, in which the low-frequency (LF) and high-frequency current sources are applied to an electrode. Thus, the so-called equivalent circuit model coupling with the fluid equations will be able to self-consistently determine the relationship between the instantaneous voltage on the powered electrode and the sheath thickness. The time-averaged potential, electric field, ion density in the sheath and ion energy distributions at the bottom of the hole are calculated and compared for different LF frequencies. The results show that the LF frequency is crucial for determining the sheath structure. The existence of the cylindrical hole on the electrode obviously affects the sheath profile in the parallel to the electrode and makes the sheath profile tend to adapt the contours of the electrode, which is the plasma molding effect.

A Multi-Scale Study on Silicon-Oxide Etching Processes in C4F8/Ar Plasmas ⁄

A multi-scale numerical method coupled with the reactor, sheath and trench model is constructed to simulate dry etching of SiO2 in inductively coupled C4F8 plasmas. Firstly, ion and neutral particle densities in the reactor are decided using the CFD-ACE+ commercial software. Then, the ion energy and angular distributions (IEDs and IADs) are obtained in the sheath model with the sheath boundary conditions provided with CFD-ACE+. Finally, the trench profile evolution is simulated in the trench model. What we principally focus on is the effects of the discharge parameters on the etching results. It is found that the discharge parameters, including discharge pressure, radio-frequency (rf) power, gas mixture ratios, bias voltage and frequency, have synergistic effects on IEDs and IADs on the etched material surface, thus further affecting the trench profiles evolution.

Simulations of Ion Behaviors in a Photoresist Trench During Plasma Etching Driven by a Radio-Frequency Source

Ions behavior plays an important role in plasma etching processes and is determined by the local electric potential in the etched trenches. In this study, with the trench powered by a radio frequency (rf) source, the Laplace equation is solved to obtain the electric potential. The ion trajectories and the ion energy distribution (IED) at the bottom of the trench are obtained self-consistently by tracking the ions in the trench. The results show that the aspect ratio of depth- to-width of the photoresist trench and the voltage amplitude of the rf source applied to the electrode are important parameters. The larger the aspect ratio and the smaller the amplitude are, the more ions hit the sidewalls, which results in a notching phenomenon. Meanwhile, there are a higher high-energy peak and a lower low-energy peak in the IED with the increase in aspect ratio.

Characteristics of a Collisional Sheath Biased by a Dual Frequency Source

A hybrid model is used to simulate the characteristics of a collisional sheath in a capacitively coupled plasma (CCP) driven by a dual frequency source including a RF and a pulsed current source applied to the same electrode. The hybrid model includes a fluid model used to simulate the characteristics of the collisional sheath, and a Monte-Carlo (MC) method to obtain both ion energy and ion angular distributions (IEDs and IADs) impinging on the substrate. The effects of the low frequency of the pulsed source and the gas pressure on the characteristics of the sheath, as well as the IEDs and IADs, are studied. The results show that the ratio of pulse/RF frequency and the gas pressure are crucial for the characteristics of the sheath and the IEDs. The IADs are significantly more sensitive to the gas pressure.

Dust Particle Properties in a Dual-Frequency Driven Sheath

We study characteristics of a single dust particle in a dual-frequency capacitively coupled plasma sheath, such as charging and suspending processes, using a collisionless self-consistent model. Also, the movement of the dust grain with time is investigated for the various radii and initial velocities. The numerical results show that, after several microseconds, the charging process of the dust particle reaches equilibrium, and the grain obtains its equilibrium position, In addition, it is found that the parameters of the low-frequency source impact on the charging and suspending processes of the dust grain significantly.

Characteristics of single and dual radio-frequency (RF) plasma sheaths

The characteristics of radio-frequency (RF) plasma sheaths have been topics of much scientific study for decades, and have also been of great importance in the manufacture of integrated circuits and fabricating microelectromechanical systems (MEMS), as well as in the study of physical phenomena in dusty plasmas. The sheaths behave special properties under various situations where they can be treated as collisionless or collisional, single-or dual-RF, one-or two-dimensional (1D or 2D) sheaths, etc. This paper reviews our recent progress on the dynamics of RF plasma sheaths using a fluid method that includes the fluid equations and Poission’s equation coupled with an equivalent circuit model and a hybrid method in which the fluid model is combined with the Monte-Carlo (MC) method. The structures of RF sheaths behave differently in various situations and plasma parameters such as the ion density, electron temperature, as well as the external parameters such as the applied frequency, power, gas pressure, magnetic field, are crucial for determining the characteristics of plasma sheaths.