De-Qi Wen

Heating mode transition in capacitively coupled CF4 discharges: comparison of experiments with simulations

The electron heating mode transitions in capacitively coupled CF4 discharges were studied by synergistically using two diagnostic methods in combination with Particle-in-Cell/Monte Carlo collision (PIC/MCC) simulations. Based on the method of phase resolved optical emission spectroscopy of trace rare gas, the spatiotemporal evolutions of energetic electrons were presented. The time-average electron density at the discharge center was measured by using a hairpin probe. All the experimental results were compared with those obtained from PIC/MCC simulations. Two different electron heating modes were observed depending on the discharge conditions: (1) the α mode (or electropositive mode), in which the electron heating maximum occurs near the sheath boundary, dominated by the sheath electric field during its expansion phase, (2) the drift-ambipolar (DA) mode (or electronegative mode), in which the electron heating maxima occur inside the entire bulk plasma and near the collapsing sheath edge, dominated by the drift field inside the bulk and the ambipolar fields near the collapsing sheath edge, respectively. The transitions between the two modes were presented when changing the rf power, working pressure and driving frequency.By increasing the power, the heating mode experiences a transition from DA to α mode. This is ascribed to the fact that at high powers, the sheath heating is enhanced, leading to a drastic decrease in the electronegativity, and consequently the DA electric field is significantly reduced. By increasing the pressure, a heating mode transition from a pure α mode, then a combination of α and DA modes, finally into a DA mode is induced. We found that the mode transition is much more sensitive to the change of working pressure than that of rf power. When increasing the pressure, there is an evident enhancement in the electron attachment, which can generate the negative ions and deplete the electrons, resulting in a higher electronegativity as well as a higher DA field, and therefore the excitation and ionization in the bulk are enhanced. The driving frequency is found to significantly affect the electronegativity, i.e. as the driving frequency increases, the discharge becomes more electropositive, and the sheath heating (α mode) dominates. Furthermore, we conclude that as the driving frequency is increased, the pressure, at which the mode transition occurs, is increased, while the power, at which the mode transition occurs, is decreased.

Characterization of O2/Ar inductively coupled plasma studied by using a Langmuir probe and global model

An O2/Ar inductively coupled plasma is investigated by a Langmuir probe and a global model (volume averaged model). The electron density, electron temperature and electron energy distribution function (EEDF) are measured at different O2 contents, gas pressures and applied powers. At fixed pressure and power, the electron density first drops quickly with the O2 ratio and then tends to saturate in the high O2 ratio range. The effective electron temperature exhibits completely opposite behaviors at low and high pressures. This is caused by the different evolving behaviors of low and high energy electrons of the EEDFs with the O2 ratio. Both the Langmuir probe and the global model predict that the electron density of O2/Ar mixed plasma first increases, peaks and then drops constantly, upon increasing the pressure. An analysis based on the simulation reveals that the non-monotonic variation of electron density with the pressure is due to the non-monotonic variation of the ionizations from both ground state O and metastable O*. Due to the strong ionizations, the electron density increases linearly with the power. The effective electron temperature is unchanged because the EEDF shape that determines the electron temperature is not varied upon increasing the power. The calculated electron density and temperature when varying the power agree better with the experiments at high pressure, i.e. 45 mTorr. The quantitative deviation between the model and the experiment when varying the pressure and the O2 ratio can be explained by two aspects. (1) The electron energy probability function is assumed to have a Maxwellian distribution in the global model while the realistic EEDFs vary significantly with the pressure and/or the O2 ratio, as revealed by the experiment. (2) The power transfer efficiency (i.e. the fraction of the power coupled into plasma) increases with the pressure.

Experimental and numerical investigations of electron density in low-pressure dual-frequency capacitively coupled oxygen discharges

The electron density is measured in low-pressure dual-frequency (2/60 MHz) capacitively coupled oxygen discharges by utilizing a floating hairpin probe. The dependence of electron density at the discharge center on the high frequency (HF) power, low frequency (LF) power, and gas pressure are investigated in detail. A (1D) particle-in-cell/Monte Carlo method is developed to calculate the time-averaged electron density at the discharge center and the simulation results are compared with the experimental ones, and general agreements are achieved. With increasing HF power, the electron density linearly increases. The electron density exhibits different changes with the LF power at different HF powers. At low HF powers (e.g., 30 W in our experiment), the electron density increases with increasing LF power while the electron density decreases with increasing LF power at relatively high HF powers (e.g., 120 W in our experiment). With increasing gas pressure the electron density first increases rapidly to reach a maximum value and then decreases slowly due to the combined effect of the production process by the ionization and the loss processes including the surface and volume losses.

Nonlinear series resonance and standing waves in dual-frequency capacitive discharges

It is well-known that the nonlinear series resonance in a high frequency capacitive discharge enhances the electron power deposition and also creates standing waves which produce radially center-high rf voltage profiles. In this work, the dynamics of series resonance and wave effects are examined in a dual-frequency driven discharge, using an asymmetric radial transmission line model incorporating a Child law sheath. We consider a cylindrical argon discharge with a conducting electrode radius of 15 cm, gap length of 3 cm, with a base case having a 60 MHz high frequency voltage of 250 V and a 10 MHz low frequency voltage of 1000 V, with a high frequency phase shift between the two frequencies. For this phase shift there is only one sheath collapse, and the time-averaged spectral peaks of the normalized current density at the center are mainly centered on harmonic numbers 30 and 50 of the low frequency, corresponding to the first standing wave resonance frequency and the series resonance frequency, respectively. The effects of the waves on the series resonance dynamics near the discharge center give rise to significant enhancements in the electron power deposition, compared to that near the discharge edge. Adjusting the phase shift from π to 0, or decreasing the low frequency from 10 to 2 MHz, results in two or more sheath collapses, respectively, making the dynamics more complex. The sudden excitation of the perturbed series resonance current after the sheath collapse results in a current oscillation amplitude that is estimated from analytical and numerical calculations. Self-consistently determining the dc bias and including the conduction current is found to be important. The subsequent slow time variation of the high frequency oscillation is analyzed using an adiabatic theory.

A hybrid model of radio frequency biased inductively coupled plasma discharges: description of model and experimental validation in argon

A hybrid model, i.e. a global model coupled bidirectionally with a parallel Monte-Carlo collision (MCC) sheath model, is developed to investigate an inductively coupled discharge with a bias source. This hybrid model can self-consistently reveal the interaction between the bulk plasma and the radio frequency (rf) bias sheath. More specifically, the plasma parameters affecting characteristics of rf bias sheath (sheath length and self-bias) are calculated by a global model and the effect of the rf bias sheath on the bulk plasma is determined by the voltage drop of the rf bias sheath. Moreover, specific numbers of ions are tracked in the rf bias sheath and ultimately the ion energy distribution function (IEDF) incident on the bias electrode is obtained. To validate this model, both bulk plasma density and IEDF on the bias electrode in an argon discharge are compared with experimental measurements, and a good agreement is obtained. The advantage of this model is that it can quickly calculate the bulk plasma density and IEDF on the bias electrode, which are of practical interest in industrial plasma processing, and the model could be easily extended to serve for industrial gases.

F-atom kinetics in SF6/Ar inductively coupled plasmas

The F-atom kinetics in SF6 and SF6/Ar inductively coupled plasmas(ICPs) were investigated using a global model. This report particularly focuses on the effects of ICP power and Ar fraction on F-atom density and its main production and loss mechanisms. The results are presented for a relatively wide pressure range of 1–100 mTorr. Very different behaviors were observed for Ar fractions in the low- and high-pressure limits, which can be attributed to different electron kinetics. In addition, the authors found that increasing the Ar fraction in the SF6/Ar plasma has almost the same effects on the F-atom kinetics as increasing the power in the SF6plasma. This is because a high electron density occurs in both cases. Moreover, it was confirmed that, for both sample types, a cycle of F atoms formed in the bulk plasma. The source of these is F2 molecules that are first formed on the chamber wall and then emitted. Finally, the simulations of F-atom kinetics are validated by quantitatively comparing the calculated electron and F-atom densities with identical experimental discharge conditions.

A nonlinear electromagnetics model of an asymmetrically-driven, low pressure capacitive discharge

It is well-known that standing waves having radially center-high voltage profiles exist in high frequency driven capacitive discharges. Capacitive sheaths can also nonlinearly excite driving frequency harmonics near the series resonance that can be spatially near-resonant, and therefore enhance the on-axis power deposition. The powered-electrode/plasma/grounded-electrode sandwich structure of an asymmetrically excited cylindrical discharge forms a three electrode system in which both z-symmetric and z-antisymmetric radially propagating wave modes can exist. We develop a nonlinear electromagnetics model for this system with radially- and time-varying sheath widths, incorporating both symmetric and antisymmetric modes, and the plasma skin effect. Waves generated in the electrostatic wave limit are also treated. The discharge is modeled as a uniform density bulk plasma with either homogeneous or Child law sheaths at the electrodes, incorporating their nonlinear voltage versus charge relations. The model incl...

Experimental and numerical investigations on time-resolved characteristics of pulsed inductively coupled O2/Ar plasmas

The time-resolved characteristics of pulsed inductively coupled O2/Ar plasmas have been investigated in this paper, by means of a Langmuir probe and a global model. The plasma properties, e.g., the electron density, effective electron temperature, and electron energy probability function (EEPF), have been experimentally investigated under various discharge conditions, combined with the comparison with simulated results. It is found that when the power is switched on, the electron density increases rapidly and then it reaches to a steady state with a constant value. When the power is switched off, the electron density exhibits a peak at the initial afterglow period, and then it decays gradually to a very low value. This peak may be caused by the detachment of negative ions. Moreover, it is noted that the effective electron temperature also increases to a peak value at the early afterglow, which can be understood by examining the evolution of EEPFs with time. Indeed, when the power is switched off, more mod...

Effect of a dielectric layer on plasma uniformity in high frequency electronegative capacitive discharges

The authors use a fast 2D axisymmetric fluid-analytical code to study the effect of adding a dielectric layer over the wafer electrode of a high frequency capacitively coupled plasma (CCP) reactor. At higher frequencies and larger areas, the wavelengths of the radially propagating surface waves in the plasma can become significantly shorter than the reactor dimensions, leading to center-high plasma nonuniformities. These wavelengths increase with increasing sheath widths, suggesting that a method to suppress wave effects in a high frequency CCP is to increase the effective sheath width by adding a dielectric layer over the wafer electrode. The authors conducted simulations with and without a dielectric layer and found that the dielectric layer improved plasma uniformity. The authors also studied the effect of adding a thin conducting or resistive silicon wafer above the dielectric layer and found that a conducting silicon wafer shorts out the fields and shields the discharge from the dielectric layer, while the resistive silicon wafer allows the fields to pass through to the dielectric layer.The authors use a fast 2D axisymmetric fluid-analytical code to study the effect of adding a dielectric layer over the wafer electrode of a high frequency capacitively coupled plasma (CCP) reactor. At higher frequencies and larger areas, the wavelengths of the radially propagating surface waves in the plasma can become significantly shorter than the reactor dimensions, leading to center-high plasma nonuniformities. These wavelengths increase with increasing sheath widths, suggesting that a method to suppress wave effects in a high frequency CCP is to increase the effective sheath width by adding a dielectric layer over the wafer electrode. The authors conducted simulations with and without a dielectric layer and found that the dielectric layer improved plasma uniformity. The authors also studied the effect of adding a thin conducting or resistive silicon wafer above the dielectric layer and found that a conducting silicon wafer shorts out the fields and shields the discharge from the dielectric layer, whi...