Bulletin of the American Physical Society
74th Annual Gaseous Electronics Conference
Volume 66, Number 7
Monday–Friday, October 4–8, 2021;
Virtual: GEC Platform
Time Zone: Central Daylight Time, USA
Session DT21: Capacitively Coupled Plasmas I |
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Chair: Shahid Rauf, Applied Materials Room: Virtual GEC platform |
Tuesday, October 5, 2021 10:15AM - 10:30AM |
DT21.00001: Ion Energy and Density Profile Control by Focus Ring and Associated External Circuit on Capacitively Coupled Plasma Yuhua Xiao, Joel Brandon, Sang Ki Nam, Kiho Bae, Jang-Yeob Lee, Steven Shannon Capacitively coupled plasmas are widely used in semiconductor processes. However, control of the plasma to obtain uniform deposition and etching over a large process space is still an open problem. One promising approach is to utilize focus ring (FR) with a tunable external circuit consisting of a variable capacitor and an inductor to extend the uniformity to the wafer edge. Usually, the geometry of FR allows gross adjustments of the plasma processing uniformity at the wafer edge. The addition of an external circuit coupling the ring to ground may optimize the ion energy and density profile by changing the impedance between the FR and the ground and allow wafer edge tuning over a wide range of operating parameters. In this work, it is found that the adjustable circuit can control the edge ion energy through distributing the voltage and the energy loss between different sheaths. The FR and its external circuit also control the spatial distribution of the electric field and thereby reduce plasma edge effects and improve density uniformity. These results point to possible source designs for engineering the distribution of power dissipation across these sheaths in industrial plasma reactors. |
Tuesday, October 5, 2021 10:30AM - 10:45AM |
DT21.00002: Frequency effects on nonlinear harmonic excitations in an asymmetrically-driven capacitive discharge Jian-Kai Liu, Emi Kawamura, Michael A Lieberman, Allan J Lichtenberg, You-Nian Wang Nonlinearly excited harmonics due to spatial or series resonances can strongly affect plasma uniformity in high frequency capacitive discharges. In our previous work [PSST 30(2021) 045017], we used a nonlinear transmission line (NTL) model to study the effect of nonlinearly excited harmonics on a high frequency argon capacitive discharge at various pressures. In this work, we generalize the NTL model by including the effects of a variable-sized dielectric between the powered and grounded electrodes, the electron-neutral elastic collision frequency on the plasma dielectric constant, and the radial variation of the plasma density. Then, we examine the frequency effects on a low pressure asymmetrically driven argon capacitive discharge. The radially varying density is determined by coupling the NTL model to a 2D bulk plasma fluid model. An analytical collisionless ion sheath model is used to determine the electron sheath heating and the nonlinear relation between sheath voltage and sheath charge. We first study the effects of adding plasma radial variation to the NTL model by assuming different fixed radial profiles. Then, we examine the discharge in a frequency range of 30-100 MHz at fixed electron power of 40 W and pressure of 20 mTorr. An antisymmetric spatial resonance is observed with increasing frequency. |
Tuesday, October 5, 2021 10:45AM - 11:00AM |
DT21.00003: On the Similarity and Scaling Laws in Low-Pressure Capacitive Radio Frequency Discharges Yangyang Fu, Bocong Zheng, Huihui Wang, Peng Zhang, Qi Hua Fan, Xinxin Wang, John P. Verboncoeur In the work, we studied the similarity and scaling characteristics of low-pressure capacitive radio frequency (rf) discharges via particle-in-cell/Monte Carlo collision simulations. The similarities of the rf discharge at different length scales are demonstrated while maintaining the combined parameters, i.e., reduced driving frequency f/p and reduced gap length pd the same, with f being the driving frequency, p the gas pressure, and d the gap distance. The electron kinetic invariance in similar discharges is confirmed and the foundation of the similarity principles is interpreted based on the scaling of the electron Boltzmann equation. We further compared the similarity relation for electron density with the traditional frequency scaling and found that the parameter dependence is rather close within certain parameter regimes, which can be understood from the same scaling relation of the electron power absorption. The discrepancy of the similarity relation and frequency scaling is also observed when the sheath width becomes sufficiently large and thus the stationary bulk plasma region no longer exists. The effects of the nonlinear collisional processes in terms of similarity laws, such as stepwise ionization, on the electron density scaling are also examined. The results offer new insights and a more comprehensive understanding for correlating the rf discharge of different dimensions across a wide range of parameter regimes. |
Tuesday, October 5, 2021 11:00AM - 11:30AM |
DT21.00004: 2D PIC simulations of geometrically asymmetric capacitive RF plasmas driven by tailored voltage waveforms Invited Speaker: Li Wang Two different types of electrical asymmetry effects, i.e. the amplitude and slope asymmetry, have been investigated in geometrically asymmetric capacitive RF discharges (having a smaller powered electrode area with respect to the grounded one) operated in argon at low pressure by 2D Particle-In-Cell/Monte Carlo collision simulations. Significantly different effects of these electrical asymmetries are observed in geometrically asymmetric discharges compared to those observed in geometrically symmetric discharges as a result of a waveform dependent constructive or destructive superposition of the geometrical and electrical asymmetries. This coupling effect strongly changes the plasma density and charged particle distribution functions at boundary surfaces, e.g. a reduced plasma density is observed in the valleys- and sawtooth-down waveform cases as compared to the peaks- and sawtooth-up waveforms. By including realistic energy and material dependent secondary electron emission coefficients in the simulations, the electron induced secondary electrons are found to greatly contribute to the ionization at low pressures, especially in the peaks- and sawtooth-up cases. |
Tuesday, October 5, 2021 11:30AM - 11:45AM |
DT21.00005: Tuning the Ion Energy Distribution by Tailored Low-frequency Voltage Waveforms in CCPs Peter Hartmann, Ihor Korolov, Julian Schulze, Wouter van Gennip, Koen Buskes In a wide range of plasma-assisted surface modification technologies, the tuning of the flux energy distribution of the positive ion species (IFED) is of key importance in order to control the desired process. Low-pressure CCPs, characterized by a collisionless electrode sheath, utilizing multi-frequency Tailored Voltage Waveforms can be used to control the energy of the impacting ions separately from the ion flux. We present experimental IFED and DC bias voltage data, accompanied by detailed 1D and 2D PIC/MCC simulation results of argon discharges driven with tailored, pulsed, low-frequency (100 kHz) voltage waveforms in combination with a 27.1 MHz high-frequency power signal. Detailed insights into the operation, DC self-bias formation, sheath dynamics, electron, and ion acceleration processes are provided. |
Tuesday, October 5, 2021 11:45AM - 12:00PM |
DT21.00006: The effects of surface processes on the plasma parameters in low-pressure multi-frequency capacitive discharges Aranka Derzsi, Benedek Horvath, Zoltan Donko, Julian Schulze Based on Particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations, a systematic investigation of the effects of surface processes on the plasma parameters and control of particle flux-energy distributions at the electrodes is performed in geometrically symmetric capacitively coupled Ar discharges driven by tailored voltage waveforms. The simulations are based on a discharge model in which realistic approaches are implemented for the description of the secondary electron (SE) emission induced by electrons and heavy-particles at the electrodes, as well as for the sputtering of the electrodes by heavy-particles. The driving voltage waveform is composed of multiple consecutive harmonics of the fundamental frequency and is tailored by adjusting the identical phases of the even harmonics. The variation of the energy of heavy-particles at the electrodes by adjusting the phase angle of the driving harmonics affects the surface processes involving these particles, which in turn influences the plasma parameters and the quality of the control of ion properties at the electrodes. For all conditions investigated, electron induced SEs (δ-electrons) induce strong ionization in the α-mode and dominate the ionization dynamics at high voltage amplitudes. |
Tuesday, October 5, 2021 12:00PM - 12:15PM Not Participating |
DT21.00007: Structural and Electrical Methods for Enhancing Homogeneity in Intermediate Pressure Capacitively Coupled Plasma Processing Sources Scott J Doyle, Gregory J Smith, Gregory Daly, Geoff Hassall, James P Dedrick Capacitively coupled plasma manufacturing processes are ubiquitous in semiconductor fabrication pipelines where control of the radial plasma density profile is key to maintaining the resulting etch or deposition quality. Developing a detailed understanding of the physics mechanisms that underpin spatial uniformity control, driven for example by the structure of the electrode profile and the shape of the driving voltage waveform, is critical as substrate sizes increase. In this work, self-consistent 2D fluid/Monte-Carlo simulations are employed to study the effects of introducing structures into the electrodes of a capacitively coupled argon plasma operating at 1 Torr in argon. The improvements to uniformity that are achieved by changing the physical structure of the electrode are compared with the distinct technique of applying 'tailored' waveform driving voltages, formed through the superposition of 5 harmonics of 13.56 MHz with a variable phase offset. Continued enhancement of radial uniformity in plasma-assisted manufacturing reactors enables higher fidelity outputs in existing infrastructure, and presents the capability to maintain current processing quality with increased substrate diameters in next-generation sources. |
Tuesday, October 5, 2021 12:15PM - 12:30PM |
DT21.00008: Electron power absorption in radio frequency driven capacitively coupled chlorine discharge Jon T Gudmundsson, Andrea Proto, Gardar A Skarphedinsson Particle-in-cell Monte Carlo collision simulations and Boltzmann term analysis are applied to study the origination and properties of the electric field and the electron power absorption within the electronegative core of a capacitively coupled discharge in chlorine as the pressure is varied in the range from 1 to 50 Pa. The electronegativity is very high and the electron power absorption is mainly due to drift-ambipolar (DA) electron power absorption as electric field develops within the electronegative core [1]. It is found that the electron power absorption increases and the ion power absorption decreases as the pressure is increased. At 1 Pa the electron power absorption is due to both the pressure and Ohmic terms. At the higher pressures > 10 Pa the Ohmic term dominates and all the other contributions to the electron power absorption become negligible, and the discharge eventually behaves as a resistive load. Tailored voltage waveform, composed of a fundamental frequency and the second harmonic are then applied to the capacitive chlorine discharge [2]. For pressures of 1 and 10 Pa the creation of a dc self-bias and asymmetric response is demonstrated and applied to control the ion bombardment energy while the ion flux on the electrodes remains fixed. However, the available range in mean ion energy is rather limited and the range is significantly narrower than typically observed for electropositive discharges. For operating pressure of 50 Pa it is not possible to control the ion bombardment energy by the phase angle between the fundamental frequency and the second harmonic. |
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