Session JW61: Capacitively Coupled Plasmas II

Student Excellence Award Finalist: Collisional electron momentum loss in low temperature plasmas: On the validity of the classical approximation

Understanding electron power absorption in low temperature plasmas is of great importance due to their numerous industrial applications. One of the main power absorption mechanisms, ubiquitous to all types of plasma, is Ohmic heating. 

The electron momentum loss, fundamental to its calculation, is usually approximated using the collision frequency. In this work, the electron momentum loss obtained from PIC/MCC simulations, and the classical approximation based on the electron-neutral collision frequency, are calculated and compared in low pressure CCPs in various gases. 

We find that the classical approximation, commonly used in fluid models, exaggerates the role of low-energy electrons and can lead to a significantly lower momentum loss compared to the exact value, even if the exact electron distribution function is known. This leads to an underestimation of the Ohmic power absorption and a change in the harmonic content of the momentum loss. For gases with a Ramsauer-Townsend minimum (e.g. argon), the classical approximation is found to be particularly poor due to the decreased number of collisions for low energy electrons. This is confirmed by using a ``fake'' argon gas where the Ramsauer-Townsend minimum is artificially removed. The results are of broad general relevance to low-temperature plasmas, and can be useful for assessing errors in plasma fluid models   

Not Participating

In recent years, capacitively coupled discharges (CCP) operated at very high-frequency (VHF) is of great interest. Plasma properties in collisionless CCP discharges operated at very low pressure ( 2 mTorr), applied potential of kiloVolt range triggered by VHF (60 MHz) is reported here. It is observed that a very high density plasma can be produced in collisionless limit which may be of great interest for various industrial applications. It is also noticed the presence of higher harmonics in the centre of discharge which modifies the bulk plasma properties significantly. This is verified by analyzing at the shape of electron energy distribution function (EEDF) in the bulk plasma. The electron heating mechanism in the discharge is also reported here. We have used electrostatic 1D-3V particle-in-cell simulation technique for argon discharges in the present study.    

The effect of secondary electrons in low-pressure capacitively coupled oxygen plasmas

In this work, the role of plasma-surface interactions in capacitively coupled plasmas (CCPs) operated in oxygen is studied by Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations. The discharges are driven by a single-frequency (13.56 MHz) sinusoidal waveform, and they are operated in the low-pressure regime (< 1 Pa). Two different models are used to describe the interaction of electrons with the electrodes: (i) a simple one assuming only elastic reflection of the electrons with a constant probability of 0.2 and (ii) a realistic one which takes elastic reflection, inelastic reflection and secondary electron emission into account as a function of the energy and angle of incidence of the electrons. Regarding the heavy particles hitting the electrode surface, three different approximations are used: (i) the simplest one considers a constant  secondary electron emission (SEE) coefficient for the O₂⁺ ions; (ii) a more realistic one considers a  coefficient that depends on the energy of the incoming O₂⁺ ions; and (iii) the third one also takes SEE caused by O₂ neutrals into account in an energy-dependent way in addition. When the realistic model is used for electron-induced SEE, complex electron emission and ionization dynamics of ion-induced and electron-induced secondary electrons (γ- and δ-electrons, respectively) are found at low pressures, which are similar to the dynamics recently observed in argon under the same discharge conditions. In oxygen, electron-induced secondary electrons also have a remarkable effect on the O₂ (a¹Δ_g) metastable density and the electronegativity of the discharge: the metastable density increases and the plasma is found to be less electronegative as a result of the emergence of secondary electrons under the same discharge conditions. The model used for heavy particle induced SEE is also critical: at high voltage amplitudes, a transition to electropositive discharge can be observed when ion-induced secondary electron emission is treated realistically.   

The effect of a negative direct-current voltage on striated structures and electrical parameters in a capacitively coupled rf discharge in CF4

The effects of a negative direct-current (dc) voltage on the striated structures and the electrical parameters have been studied by multi-fold experimental diagnostics and particle-in-cell/Monte Carlo collision (PIC/MCC) simulations in a dc superposed rf (radio-frequency, 8 MHz) capacitive discharge in CF₄. The measured electron density, the electron-impact excitation rate and the electrical parameters are compared with the corresponding simulation results. It was found that the plasma bulk is compressed and the electron density decreases slightly at low magnitude of the dc voltage, |V_dc|, while the plasma bulk broadens at high |V_dc| when the ionization induced by secondary electrons adjacent to the powered electrode is enhanced. By increasing |V_dc|, the region of the strong ionization/excitation shrinks towards the edge of the collapsing sheath adjacent to the grounded electrode. And the ionization adjacent to the powered electrode is significantly suppressed during the collapsing phase of the sheath. This is mainly attributed to the fact that the dc component of the bulk electric field and the spatial gradient of the electron density are simultaneously enhanced during the transition from the ‘striation’ mode to ‘striation-γ’ hybrid mode when increasing |V_dc|. Besides, we found that the rf power deposition is somewhat suppressed at low |V_dc|, and is enhanced at high |V_dc|. The magnitude of the dc current exhibits a complex dependence on |V_dc|, due to nonlinear change of the plasma dc resistance.   

Two-Dimensional Particle in Cell Simulation of Dual Frequency Effect in a Torrregime Capacitively Coupled Plasma

Capacitively coupled plasma (CCP) equipment has been widely used in semiconductor and display processing. Due to the strong nonlinearity in high-power CCPs, particle-in-cell (PIC) simulations are generally required to investigate the kinetic effects. However, dual-frequency (DF) CCPs are seldom analyzed with a multi-dimensional PIC simulation because of the heavy computational load. This study investigates the DF-driven CCPs in an actual equipment scale for the deposition process in a Torr-regime using a two-dimensional particle-in-cell simulation parallelized with a graphics processing unit [1]. We observed that the time-averaged plasma potential and the sheath thickness increase with the ion power absorption, increasing primarily with the low-frequency (LF) power. We also observed that the ion flux increases linearly with the LF power. Furthermore, the ion flux on the substrate keeps uniformity in the presence of the LF power while the electron density becomes nonuniform abruptly.

[1] M. Y. Hur, J. S. Kim, I. C. Song, J. P. Verboncoeur, and H. J. Lee, Plasma Res. Express 1, 015016 (2019).