Dual-frequency Source Research Articles

Hybrid simulation of instabilities in capacitively coupled RF CF4/Ar plasmas driven by a dual frequency source

Instabilities in capacitively coupled Ar/CF4 plasma discharges driven by dual frequency sources are investigated using a one-dimensional fluid/electron Monte Carlo hybrid model. Periodic oscillations of the electron density and temperature on the timescale of multiple low frequency (LF) periods are observed. As the electron density increases, an intense oscillation of the electron temperature within each high frequency (HF) period is initiated. This causes a fluctuation of the electron density and results in a discharge instability. This phenomenon is consistent with the discharge behavior observed in scenarios with single-frequency (SF) sources, as reported by Dong et al (2022 Plasma Sources Sci. Technol. 31 025006). However, unlike the SF case, plasma parameters such as the electron density, electric field, electron power absorption and ionization rate exhibit not only periodic fluctuations but also a spatial asymmetry under the influence of the dual-frequency source. This spatial asymmetry leads to a non-uniform distribution of the electron density between the electrodes, which is related to a spatially asymmetric electric field, electron heating, and ionization around a region of minimum electron density (inside the bulk). This region of minimum electron density is shifted back and forth through the entire plasma bulk from one electrode to the other within multiple LF period. The above phenomena are related to superposition effect between the instabilities and the dual-frequency source. Moreover, the time averaged electric field influences the spatio-temporal evolution of ion fluxes. The ion fluxes at the electrodes, which play an important role in etching processes, are affected by both the high and LF components of the driving voltage waveform as well as the observed instabilities. As the HF increases, the electronegativity and electron temperature are reduced and the electron density increases, resulting in a gradual disappearance of the instabilities.

Numerical Study of Ar/O2 Discharge Properties Influenced by Small Secondary Electron Emission in Dual-Frequency Atmospheric Pressure Discharge

A one-dimensional fluid model was employed to investigate the influence of the small secondary electron emission (SEE) coefficient on the characteristic properties of an Ar/O2 gas discharge at atmospheric pressure driven by a dual frequency source. The study includes basic physical quantities such as particle density, electron temperature, the electron heating mechanism, and energy loss. The research results illustrate that with an increase in the secondary electron emission coefficient, the electron density increases, while electron temperature and electric field decrease. The densities of various particles increase to different degrees, except for the metastable O2* molecule and the O atom. The density of the metastable O2* molecule and the density of the O atom are hardly affected by the SEE coefficient; however, the time required for both to reach steady state decreases. The time required for the electron density to reach steady state increases. Electron heating and energy loss increase to varying degrees when the SEE coefficient changes from 0.001 to 0.01.

Research on characteristics of dual-frequency coupling electroaerodynamic thruster

Breaking through the corona discharge current limit and improving the ionization and acceleration process are beneficial to improve the performance of the electroaerodynamic thruster. In this paper, a dual-frequency source of DC and NSP (nanosecond pulse) are applied to generate ionic wind. Electrical, optical and thrust characteristics are compared for the electroaerodynamic thruster with and without the NSPD (nanosecond pulse discharge). The experimental results indicate that the thrust characteristics are enhanced under the effect of dual-frequency sources. Moreover, the inception DC voltage to generate ionic wind is much lower.

Fluid simulation of the superimposed dual-frequency source effect in inductively coupled discharges

Superimposition of dual frequencies (DFs) is one of the methods used for controlling plasma distribution in an inductively coupled plasma (ICP) source. The effects of a superimposed DF on the argon plasma characteristics have been investigated using a two-dimensional self-consistent fluid model. When both currents are fixed at 6 A, the plasma density drops with decrease in one of the source frequencies due to less efficient heating and the plasma uniformity improves significantly. Moreover, for ICP operated with superimposed DFs (i.e., 4.52 MHz/13.56 MHz and 2.26 MHz/13.56 MHz), the current source exhibits the same period as the low frequency (LF) component, and the plasma density is higher than that obtained at a single frequency (i.e., 4.52 and 2.26 MHz) with the same total current of 12 A. However, at superimposed current frequencies of 6.78 MHz/13.56 MHz, the plasma density is lower than that obtained at a single frequency of 6.78 MHz due to the weaker negative azimuthal electric field between two positive maxima during one period of 6.78 MHz. When the superimposed DF ICP operates at 2.26 and 13.56 MHz, the rapid oscillations of the induced electric field become weaker during one period of 2.26 MHz as the current ratio of 2.26 MHz/13.56 MHz rises from 24 A/7 A to 30 A/1 A, and the plasma density drops with the current ratio due to weakened electron heating. The uniformity of plasma increases due to sufficient diffusion under the low-density condition.

Gap length effect on discharge mode and etching profiles in asymmetric dual frequency capacitive CF<sub>4</sub>/Ar discharges

The capacitive CF<sub>4</sub>/Ar discharges driven by a dual frequency source based on the electrical asymmetry effect (EAE) are studied by using a one-dimensional fluid coupled with Monte-Carlo (MC) model and a two-dimensional trench model. The effects, induced by varying the relative gap distance, on self-bias voltage, electronegativity, ion flux, neutral flux and other plasma characteristics are systematically discussed. In this asymmetric discharge, as the gap distance increases, the absolute value of the self-bias voltage and electronegativity decrease. Meanwhile, the plasma density and absorption power increase accordingly because the effective discharge area expands but the boundary loss is still limited. In addition, both <inline-formula><tex-math id="M72">\begin{document}$ \mathrm{\alpha } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20210546_M72.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20210546_M72.png"/></alternatives></inline-formula> mode and drift-ambipolar (DA) mode can play their important roles in the discharges with different gap distances, though DA mode is weakened in large gap discharge. Owing to the fact that the self-bias is larger and electronegativity is stronger for the case of smaller gap distance, the sheath expansion electric field at the powered electrode and the bulk electric field heat the electrons, leading the ionization rate to greatly increase near the collapse of the sheath at the grounded electrode. Besides, at the larger gap distance, the maximum value of the ionization rate decreases due to the reduction of electrons with relatively high-energy, and the ionization rate near the grounded electrode is reduced evidently. Moreover, with the increase of the gap distance, the maximum ion energy decreases and the ion energy distribution width becomes smaller due to the reduction of the self-bias voltage. Meanwhile, the etching rate increases a lot since the neutral flux increases significantly near the powered electrode. However, as the gap distance increases to 5 cm, the etching rate stops increasing and the trench width at the bottom becomes narrow because the neutral flux increases greatly compared with ion flux, forming a thick layer of polymer. So, besides separately controlling the ion energy and flux, optimizing the synergistic effect of ion flux and neutral group flux to adjust the etching rate and improve the etching morphology is also an interesting topic in the asymmetric CF<sub>4</sub>/Ar discharges.

Investigation of the Growth of Carbon Films by Ar/CH4 Plasma Jet Driven by 100-MHz/100-kHz Dual Frequency Source at Atmospheric Pressure

The effects of radio-frequency (RF) power on the formation and growth of carbon films are investigated in Ar/CH4 plasma jet driven by the 100-MHz/100-kHz dual-frequency at atmospheric pressure. The excited species, morphology, and structure of the samples were evaluated by optical emission spectroscopy (OES), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR), respectively. These results showed that as the RF power increases, the size of the carbon particles becomes smaller, and the deposition area becomes larger. The morphology became more plain and smooth with the increase in the RF power. The 1-D particle-in-cell/Monte Carlo (PIC/MC) method was used to calculate the electron and ion densities and electric field. With the increase in the RF power, both the electron density and the sheath depth increases, resulting in a mode transition.

Heating mechanism in direct current superposed single-frequency and dual-frequency capacitively coupled plasmas

In this work particle-in-cell/Monte Carlo collision simulations are performed to study the heating mechanism and plasma characteristics in direct current (dc) superposed radio-frequency (RF) capacitively coupled plasmas, operated both in single-frequency (SF) and dual-frequency (DF) regimes. An RF (60/2 MHz) source is applied on the bottom electrode to sustain the discharge, and a dc source is fixed on the top electrode. The heating mechanism appears to be very different in dc superposed SF and DF discharges. When only a single source of 60 MHz is applied, the plasma bulk region is reduced by the dc source, thus the ionization rate and hence the electron density decrease with rising dc voltage. However, when a DF source of 60 and 2 MHz is applied, the electron density can increase upon addition of a dc voltage, depending on the gap length and applied dc voltage. This is explained from the spatiotemporal ionization rates in the DF discharge. In fact, a completely different behavior is observed for the ionization rate in the two half-periods of the LF source. In the first LF half-period, the situation resembles the dc superposed SF discharge, and the reduced plasma bulk region due to the negative dc bias results in a very small effective discharge area and a low ionization rate. On the other hand, in the second half-period, the negative dc bias is to some extent counteracted by the LF voltage, and the sheath close to the dc electrode becomes particularly thin. Consequently, the amplitude of the high-frequency sheath oscillations at the top electrode is largely enhanced, while the LF sheath at the bottom electrode is in its expanding phase and can thus well confine the high-energy electrons. Therefore, the ionization rate increases considerably in this second LF half-period. Furthermore, in addition to the comparison between SF and DF discharges and the effect of gap length and dc voltage, the effect of secondary electrons is examined.

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.

Influence of dual-frequency source powers on ion density and electron temperature in capacitively-coupled argon plasma

A complete floating double probe technique is used to measure the ion density and electron temperature in a dual frequency capacitively coupled plasma for argon discharges. It has been shown that there is an obvious modulating effect of the high- and low-frequency powers on the plasma parameters and their spatial distributions. In addition, the influence of the discharge gap on the ion density and electron temperature is also investigated by varying the LF power and fixed the HF power.

Lidar detection using a dual-frequency source

A new technique of dual-frequency Doppler-lidar measurement is investigated. This technique is based on the use of a coherently locked, tunable, dual-frequency laser source and is shown to accurately measure velocities as small as 26 mum/s. It is generated by exploiting the nonlinear dynamics of a semiconductor laser through a proper combination of optical injection and operating conditions.

20-km distributed temperature sensor based on spontaneous Brillouin scattering

We report on a novel experimental setup for distributed measurement of temperature, based on spontaneous Brillouin scattering in optical fiber. We have developed a mode-locked Brillouin fiber ring laser in order to generate the dual frequency source required for a heterodyne detection of the backscattered signal. This relatively simple system enables temperature measurements over 20 km with a spatial resolution of 7 m.

Brillouin based distributed fibre sensor incorporating a mode-locked Brillouin fibre ring laser

A pulsed Brillouin fibre ring laser has been developed and we describe its main features. The pump and the Brillouin laser are shown to form an excellent dual frequency source for distributed sensing. A first application for fire detection is demonstrated.

Analysis of nonlinear harmonic generation for arbitrary dual frequency transducer excitation

An earlier study of finite amplitude axisymmetric sound beams [J. H. Ginsberg, J. Acoust. Soc. Am. Suppl. 1 73, S83 (1983)] considered the case of monochromatic excitation. That work featured a singular perturbation analysis combining asymptotic integration and the renormalization version of the method of strained coordinates. The present paper initiates an extension of those techniques to the case of a dual frequency source. The parametric array, in which the primary beams are at closely spaced frequencies, has already received much attention. The system discussed here permits disparate frequencies. Aside from a restriction to axisymmetry, the excitation at each frequency is arbitrary. The analysis thus far has obtained the first two orders of approximation for the velocity potential. This expression describes the manner in which nonlinear effects accumulate for the various sum and difference frequencies. It is the foundation for a future derivation of an expression for the pressure that is descriptive of the entire field. In addition, the trend for harmonic generation indicated by the analysis suggests that conversion efficiency in the parametric array might be improved by altering the transverse vibration pattern of the individual primary beams. [Work supported by ONR, code 425-UA.]

Asymptotic fourier analysis of a “sawtooth like” wave for dual frequency source excitation

An exact weak shock zone solution was found previously for Burgers' equation for plane waves. An approximate solution for spherical waves also was obtained. Both of these solutions held for dual frequency excitation at the source. In the work reported in the present paper, an asymptotic formula was derived which yields harmonic amplitudes at a point beyond the “sawtooth like” zone. The formula's predicted harmonic amplitudes have been found to agree well with numerical results for a specific dual frequency case. Excellent agreement also was obtained with the well-known solution for a monochromatic source. The new formula predicts that for certain frequency ratios of the primary signals the amplitude of the parametrically generated difference signal will exceed that of a directly projected signal at the same frequency and with the same total input power. This result holds for infinite planar and spherical source geometries. The formula should be useful in estimating beam axis values of the harmonic amplitudes for practical source geometries by applying the plane wave version in the near field and the spherical version in the far field. Such information can be of value to those who are constructing mathematical models of parametric array behavior.