Session GT2: Capacitively Coupled Plasmas I

Live

Motivated by High Aspect Ratio etching applications the operation characteristics of argon CCPs in the sub-Pa pressure range operated with a base frequency of 400 kHz and primary voltage amplitude of 10 kV is investigated by means of 1d3v PIC/MCC simulations. An optimization of electron and ion energy distributions (EEDF and IEDF) at the electrodes is performed by varying the complex voltage waveforms including multiple harmonics and pulsed shapes, as well as using sophisticated surface material models for SiO$_2$ and pure Si. A strong influence of the secondary electron emission properties of the electrode surfaces on the charged particle dynamics and the EEDF is found especially in the case of unequal electrode materials.   

Live

Plasma non-uniformity caused by standing wave effect (SWE) has brought about great challenges for material processing. In previous studies of SWE, researchers emphasized the role of the fundamental driving frequency. Nevertheless, nonlinear SWE associated with the higher harmonics generated by plasma series resonance (PSR) have been recently attracting even more attention. In this talk, I will present the experimental observation of nonlinear standing waves excited by PSR-enhanced harmonics in very high frequency (VHF) CCPs. Special emphasis is placed on the role of higher harmonic excitations on plasma non-uniformities. In experiment, we employed a home-made magnetic probe to determine the radial profile of the harmonic magnetic field, in combination with a double Langmuir probe to measure the radial profile of the plasma density. The measured harmonic magnetic fields are well reproduced by an electromagnetics model, which allows us to analyze the underlying physics. The nonlinear SWE is observed to be prominent at a low pressure, in which regime higher harmonics are excited by PSR, which can induce spatial wave resonances, with current/voltage peaked on-axis, resulting in a center-high plasma. The effects of DC (direct current) and LF (low frequency) source parameters on the plasma uniformity have been systematically investigated in experiment. It was found that the plasma uniformity is significantly affected by changing the DC and the LF source, which can be well explained by the changes of measured harmonic magnetic fields with the DC voltage and LF source parameters. Particularly, this study was extended to the case of more complex driving waveforms (i.e., 30 MHz $+$60 MHz), the experimental results show that the harmonic magnetic fields and the plasma uniformity can be well controlled by adjusting their phase angle.   

Live

The standing wave effect, which may lead to center-high density profiles in high frequency capacitive discharges, can be enhanced by nonlinearly excited harmonics. In this work, a nonlinear transmission line model, which solves for electromagnetic fields in the time domain, is coupled to a 2D bulk plasma fluid model to study nonlinear effects in asymmetric cylindrical capacitive discharges. In this model, the bulk plasma fluid model is solved to obtain plasma density and electron temperature distributions. An analytical collisional or collisionless ion sheath model is used to determine the electron sheath heating and the nonlinear dependence of sheath voltage on sheath charge. We first compared the results for collisional and collisionless ion sheath models to address the influence of collisions. Then, the effect of pressure on nonlinear harmonic excitation was investigated. At low pressure, the nonlinear harmonics significantly enhance the standing wave effect and on-axis power deposition. But the nonlinear harmonics gradually damp out as pressure increases, leading to a transition from center-high to edge-high density profiles.   

Live

Results of one-dimensional particle-in-cell simulations are compared systematically with plasma characteristics measured in a geometrically symmetric single frequency (13.56 MHz) capacitively coupled reactor driven by a sinusoidal voltage waveform in argon. Pressures from 1 Pa to 100 Pa and driving voltage amplitudes of 150 V – 350 V are used. We show that the simulations predict the measured spatio-temporal excitation dynamics, the central electron densities, and the ion energy distribution functions at the electrodes well for most of the investigated conditions. Noticeable differences are observed at the lowest pressures, where the experimental results indicate that the plasma density is underestimated in the simulations. The treatment of plasma-surface interactions in the simulation is identified as the reason for this disagreement. We find that increasing the electron reflection coefficient leads to higher densities and a better agreement with the experimental data at low pressures, while the densities at higher pressures remain unchanged.   

Live

Two important questions in low pressure capacitively coupled plasmas are, how do the electrons gain and loss their energy and what is a typical electron temperature? The first issue is frequently studied as ‘electron power absorption’ which can be determined by the product of the electric field and the electron current density. Spatio-temporal results show that most of the power absorption is observed at the plasma sheath edge. The second question deals with the electron temperature which is determined by thermodynamics and is usually between 1 and 5 eV. However, the classical concept of the electron temperature has some limitations, because in such a low pressure regime the electron velocity distribution function is very anisotropic. In this work, a concept of a kinetic temperature is introduced which distinguishes the temperatures in different directions. This concept shows that collision events are essential in order to transfer thermal energy between the different directions. Finally, the temperature shall be linked to the electron power absorption to show that heating and power absorption are two different physical mechanisms. The presented results are obtained by means of 1D3V PIC/MCC simulations. [1] S. Wilczek et al. (2020) Journal of Applied Physics, 127(18), 181101   

Live

Capacitively coupled plasmas (CCP) are widely used in the semiconductor industry for plasma-enhanced chemical vapor deposition (PECVD), atomic layer deposition (PEALD) and plasma etching applications. Spatial uniformity of critical discharge characteristics such as species densities and ion energy distribution is important for achieving optimum processing results. However, the spatial distribution of CCP discharge can be significantly affected by electromagnetic effect (EME) at very high frequency (VHF). Stringent processing conditions also require flexible control of ion fluxes and energies for adequate selectivity. Electrical asymmetric effect (EAE) has been extensively studied for separate control of ion flux and energy by applying a fundamental frequency and its higher harmonics in CCP discharges. In this study, EAE is systematically investigated by tailored-waveform excitations in the VHF regime where EME becomes significant. A fully coupled electromagnetic fluid plasma model is used to simultaneously study both EAE and EME. Argon CCP discharge is excited using VHF source in a geometrically asymmetric reactor. The discharge asymmetry is electrically tuned by tailored waveforms. This study provides fundamental understandings of the interplay between EAE and EME and insights to flexible control of ion fluxes and energies in the VHF CCP discharge.   

Live

In plasma etching equipment, focus rings are widely used to achieve acceptable uniform fluxes near the wafer edge. The ion incident angle, particle, and energy fluxes to the wafer are controlled by adjusting the wafer-focus ring gap, focus ring height, and relative electrical impedance return of the focus ring. The sheath structure and voltage drop across the sheath are also affected by these focus ring properties. Usually, the focus ring is viewed as a capacitor which divides total sheath voltage drop and thereby normalizes voltage to the main electrode values. In this work, the plasma is simplified to an equivalent circuit model and adapted to simulate the effect of focus ring on sheath properties. The simulations are further compared to experimental results measured using hairpin probe, Langmuir probe, VI probe, and a retarding field energy analyzer (Impedans Vertex RFEA). Results show that the focus ring acts as a capacitor only in high voltage cases, and it will increase sheath voltage drops in low voltage cases and does not rigorously follow the capactive voltage divider model typically used.   

On Demand

This work demonstrates the presence of a small number of high-energy ballistic electrons (HEBEs) that originate from secondary electrons in low-pressure radio-frequency (rf) plasmas. The kinetic behaviors of the HEBEs are illustrated through electron energy probability functions from the fully kinetic particle-in-cell simulations, showing two wavy high-energy tails and two bifurcations during one rf cycle. Test-particle simulations and a semi-analytical method associated with nonlocal electron kinetics are performed to characterize the HEBE trajectories, which reveal the ballistic nature of the HEBEs and their typical bouncing features between the rf sheaths. Parameter dependence of the HEBEs on the discharge conditions (e.g., gas pressure, gap distance, and rf frequency) are identified, which is relevant to the plasma collisionality. With a pronounced presence of HEBEs, the overall impacts of the secondary electron emission on discharge parameters, such as electron power absorption and ionization rate, are also illustrated.