
Citation: | Haiyun TAN, Tianyuan HUANG, Peiyu JI, Lanjian ZHUGE, Xuemei WU. Simulation study on electron heating characteristics in magnetic enhancement capacitively coupled plasmas with a longitudinal magnetic field[J]. Plasma Science and Technology, 2022, 24(10): 105403. DOI: 10.1088/2058-6272/ac7385 |
The electron heating characteristics of magnetic enhancement capacitively coupled argon plasmas in presence of both longitudinal and transverse uniform magnetic field have been explored through both theoretical and numerical calculations. It is found that the longitudinal magnetic field can affect the heating by changing the level of the pressure heating along the longitudinal direction and that of the Ohmic heating along the direction which is perpendicular to both driving electric field and the applied transverse magnetic field, and a continuously increased longitudinal magnetic field can induce pressure heating to become dominant. Moreover, the electron temperature as well as proportion of some low energy electrons will increase if a small longitudinal magnetic field is introduced, which is attributed to the increased average electron energy. We believe that the research will provide guidance for optimizing the magnetic field configuration of some discharge systems having both transverse and longitudinal magnetic field.
The parameters of capacitively coupled plasmas (CCPs), such as plasma density and electron energy distribution function (EEDF), etc, play important roles in material etching or thin film deposition, and they usually depend on how electrons gain energy from driving electric fields, which is traditionally known as electron heating [1].
Ohmic heating is an important mechanism in electron heating, which origins from the collision between electrons and neutral particles [2]. In this case, the synchronous relationship between the motion of the colliding electrons and the driving electric fields is destroyed, and therefore the electrons obtain net energy. In some cases, however, the strong power deposition in CCPs cannot be explained well by Ohmic heating, and there must be some additional 'collisionless' mechanisms to heat electrons, as pointed out by Godyak et al [3, 4]. A classical model described collisionless heating is known as hard-wall model, which proposed by Lieberman [5]. In this model, the electric filed in plasma zone is ignored, and the collisionless heating is considered as a result of the random interactions between the moving electrons and the oscillating sheath electric filed. Some further researches show that the strong electric filed reversals in the region between the sheath and plasma zone can also introduce additional collisionless heating [6, 7], which is known as ambipolar electron heating [8]. In addition, the electrostatic waves originated from the energetic electron beam propagating from the sheath edge toward plasma zone can also contribute to the collisionless heating in some cases [9]. Another classical model proposed to understand the collisionless heating is known as the pressure heating model. In this model, the collisionless heating is considered to be related to the pressure action in electronic fluid [10, 11].
To analyze the dependence of collisionless heating on discharge parameters, some analytical models have been proposed to describe the level of collisionless heating [6, 12]. These models, however, often show some errors when compare their results with that from particle-in-cell (PIC) simulations [13], and cannot describe all the physical mechanisms of collisionless heating completely. To determine the level of the heating accurately, Surendra and Dalvie proposed a quantitative method, in which a dynamic fluid model is used to analyze the heating, and parameters of this model are taken from simulations [14]. Through this method, heating components from different physical mechanisms can be distinguished, and thereby which components are important can be determined. They further revealed that the pressure tensor of electrons is the physical origin of collisionless heating, and time-varying electron temperature is very important for this heating. Based on such a method, people have a clearer understanding of electronic heating in CCPs. For example, Lafleur et al found that the total heating in their most CCPs tested cases consists mainly of Ohmic heating and pressure heating, with the additional collisionless heating associated with the electron inertia being negligible [15]. Liu et al found that the thick dielectric sidewall in CCPs can enhance the level of the axial Ohmic heating and result in the increase of axial electron density [16]. Schulze et al found that the strong bipolar electric fields are very important for pressure heating in electropositive CCPs [17], and Proto et al further expanded this conclusion to electronegative CCPs recently [18, 19].
If one introduces a transverse magnetic field which oriented parallel to the electrodes into CCPs, then the electron heating as well as plasma characteristics of the CCPs may be modulated [20]. For example, transverse magnetic field can enhance the level of Ohmic heating significantly, and induce the Ohmic heating to become dominant in total heating [21, 22]. In addition, the electrons near the magnetized plasma sheath edge can undergo periodic cyclotron motion, and in some cases the couple between the oscillating sheath and the cyclotron electron can enhance the electron heating, which is known as electron bounce-cyclotron resonance heating [23–25]. On the other hand, some quantitative studies about the influences of transverse magnetic field on electron heating in CCPs have also been reported. For example, Wang et al found that with the increase of transverse magnetic field, the dominant heating mechanism in magnetized oxygen CCP will translate from electronegative drift-ambipolar heating to electropositive stochastic electron heating [26]. Zheng et al revealed that the transverse magnetic field induced Hall currents play important roles in enhancing the level of Ohmic heating in CCPs [27].
In summary, a series of accurate quantitative researches have discussed the effects of transverse magnetic fields on the electron heating in CCPs, few studies, however, report the effects of longitudinal magnetic fields on that in CCPs. In this work, the electron heating characteristics in a magnetic enhancement CCP with a longitudinal uniform magnetic field are explored through both theoretical and numerical calculations. This paper is structured as follows. In section 2, a dynamic fluid model used for analyzing the electron heating characteristics is reviewed. The simulation model used to perform numerical calculations and its parameter setting is described in section 3. The effects of longitudinal magnetic fields on electron heating characteristics are stated in section 4. The last part is a summary of the full text.
For a magnetized plasma, the first momentum Boltzmann equation for electrons is the momentum balance equation, read as [28]
mene∂ue∂t+me(Γe·∇)ue=−ene(E+ue×B)−∇·Pe⃡ | (1) |
where
(2) |
where the subscript
(3) |
or the heating in global space excluding magnetic field term,
(4) |
represent the level of inertia heating, pressure heating and Ohmic heating, respectively. In the following studies,
In this work, an implicit 1d3v PIC/MCC code is used to perform all the simulations, and the code has been benchmarked with the results from Zheng et al [24] and the results from Yang et al [29]. We consider a magnetic enhancement CCP discharge system operated in argon, as shown in figure 1. In this model, we assume the working pressure and gas temperature are 100 mTorr and 300 K, respectively. The transverse magnetic field
In all simulations, we consider electron–neutral collisions and argon ion–neutral collisions, where the collision types and the collision cross sections are shown in figure 2. The cross sections of electron–neutral collisions are taken from [30], in which for elastic electron–neutral collision the momentum transfer cross-section is used, and all scattering events are treated as isotropic, as referred as [17]. The cross-sections of argon ion–neutral collisions are taken from [31], and elastic ion–neutral collision is isotropic, while the ion after the charge exchange collision (backscattering collision) has a velocity equal to the velocity of the neutral atom that collides with the ion. The velocities of particles after collision events are calculated by the method in [32].
The initial electrons and ions have Maxwell distributions with temperatures of 2 eV and 0.026 eV, respectively, and have a uniform profile in the gap. When the simulations reach a steady state, there are about 60 000–80 000 super-electrons (the number of super-ions is similar). We use a double-time-scale algorithm in all simulations, that is, the time step of electrons and that of ions are
Evolutions of
Figure 4 shows evolutions of the total heating profile
The phenomenon of
Figure 5 shows the spatio-temporal evolutions of the total heating as well as that of different heating components at different
To further analyze the influences of
On the other hand, figure 6 also shows how
The influences of
Meanwhile, the global EEDF shows a similar change. When a small
To understand the dramatic change in both
In summary, the electron heating characteristics of a magnetic enhancement CCP in presence of both longitudinal and transverse uniform magnetic field have been explored by using a combination of theoretical and numerical calculations. It is found that the electron density as well as level of the total heating decreases gradually and tends to be stable with a continuously increased longitudinal magnetic field, which results from the confinement of electrons in the direction which is perpendicular to both the driving electric field and the applied transverse magnetic field (a direction referred to as 'E × B' direction in this work). Further analysis shows that longitudinal magnetic field affects the heating mainly by changing pressure heating along the longitudinal direction and by changing Ohmic heating along the 'E × B' direction. As longitudinal magnetic field increases continuously, the level of the Ohmic heating decreases gradually, and the pressure heating tends to be the dominant heating mechanism. Finally, it is worth mentioning that when a small longitudinal magnetic field is introduced, the electron temperature as well as the proportion of some low energy electrons in the EEDF increases. This is because the small longitudinal magnetic field causes a significant decrease in electron density but remains a relatively high level of the electron heating, which results in more energy per electron. Although the analyses in this paper are based on 1D simulations, these research conclusions, however, are still expected to be effective in 2D cases [34], and can provide guidance for optimizing the magnetic field configuration of some discharge systems with both transverse and longitudinal magnetic fields introduced.
This work is supported by National Natural Science Foundation of China (Nos. 11975163 and 12175160), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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