Applied Science and Convergence Technology 2023; 32(5): 127-133
Published online September 30, 2023
Copyright © The Korean Vacuum Society.
aApplied Physics lab for PLasma Engineering (APPLE), Department of Physics, Chungnam National University, Daejeon 34134, Republic of Korea
bInstitute of Quantum Systems (IQS), Chungnam National University, Daejeon 34134, Republic of Korea
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Plasma diagnostics, especially electron density measurements, have attracted much attention as it promotes an understanding of plasma and its applications. In this study, a parallel double-curling probe, which consists of two individual curling slot antennae and one microwave input/output port, is demonstrated for multisite electron density measurements using three-dimensional electromagnetic wave simulations. We verify its step-by-step operation, including the demonstration of a single curling probe, double-curling probe, and parallel double-curling probe. For the single curling probe, we introduce a curling probe operation, while for the double-curling probe, we verify the operation of two curling probes with a single microwave input/output port. With respect to the parallel double-curling probe, simulation results indicate that it can measure the multi-site electron density above its individual curling-slot antenna. This study is applicable for the parallelization and multiplication of curling probes in the development of plasma uniformity sensors.
Keywords: Plasma diagnostics, Electron density measurement, Multi-site electron density measurement, Curling probe, Double-curling probe, Parallel double-curling probe, three-dimensional electromagnetic wave simulations
Plasma, which is also called the fourth state of matter, contains physically energetic ions and chemically reactive radicals, making it essential for various applications, such as semiconductor fabrication [1,2], biomedicine [3,4], and agriculture . Among the constituents of plasma, electrons play a crucial role in the generation of ions and radicals , making the electron density measurement a subject of significant interest in plasma diagnostics. Various techniques have been developed for this purpose, including electrical probes [7,8], optical emission/absorption methods , laser-scattering techniques , and microwave probes . In particular, microwave probes have attracted much attention owing to their simple design and analysis, such as cutoff probes [12,13], multipole resonance probes [14,15], and hairpin probes . Recently, non-invasive microwave probes have been developed for real-time plasma status monitoring while minimizing plasma perturbation [17–20]. Kim
In particular, the curling probe has attracted significant interest owing to its simple structure and capacity for the precise measurements of electron density. The curling probe consists of two antennae: i) an excitation monopole antenna at which the microwave radiates and ii) a coupled-slot antenna on which standing wave resonance occurs. The curling probe measures the shift in the resonance frequency caused by the plasma, and can infer the electron density from the shift.
Simulation results reported by Liang
In addition, Ogawa
In addition, the curling probe concept can also be applied as a simple-and-compact sensor for plasma uniformity measurements via parallelization of the curling probes because its slot antenna is compact and it has a highly distinct peak (high
The remainder of this paper is organized as follows. In section 2, we introduce the single-curling probe and demonstrate its operation for electron density measurements. In section 3, we establish a doublecurling probe and analyze
We employed a 3D-EM wave simulation software, CST Microwave Studio Suite, which is a commercial software tool that is widely used to investigate microwave probes [12,22,30]. In this section, we describe two cases to characterize individual curling probes, i.e., case A and case B. Figure 1(a) shows the appearance of the curling probes, especially for case A. It consists of four parts: a monopole antenna, the dielectric, the curling slot-antenna, and the case. The curling slot antenna has a thickness of 0.5 mm, slot width of 1.0 mm, three turns, an average curl length of 120 mm, and an aperture slot with a diameter of 6.0 mm to the center, at which the monopole antenna with a diameter of 2.0 mm is located. Figure 1(b) shows a cross-sectional image of the curling probe. The curling slot antenna was connected to a case with a diameter of 34 mm and height of 17 mm. The monopole antenna is connected to the core of a coaxial cable. The microwave input/output port was located at the end of the coaxial cable. A dielectric with a diameter of 30 mm and thickness of 0.5 mm is located beneath the curling slot antenna. For the curling probe in case B, the number of curling slot antenna turns increased to four. We summarize all the dimensions of the probe parts for cases A and B in Table I.
As previously mentioned, the curling slot antenna is a resonator for which the resonance frequency is described by Eq. (3); this resonance was clearly detected in
In this simulation, plasma is established as the Drude model, and the relative dielectric constant (
where ω is the angular microwave frequency (ω = 2π
Figure 2(a) exhibits the
To verify the operation of the single-curling probe established in this simulation, we inserted cylindrical plasma at a distance of 1.0 mm from the surface of the curling slot antenna, as shown in Fig. 3(a). Figure 3(b) exhibits
Figure 4 represents the calculated
Figure 5(a) shows the configuration of the two curling probes connected to the coaxial cables. The 3-turn curling probe was located on the right side, and the 4-turn curling probe was located on the left side. The specifications of each curling slot antenna are listed in Table I. The microwave power entered the bottom coaxial cable and was channeled towards the curling probes. The divided microwaves were then returned from each curling probe to the microwave input/output port. It should be noted that the two curling probes share the same microwave input/output port, and a single
Figure 5(b) shows the
The simulation results indicate the possibility of using a doublecurling probe. However, this is impractical because it is connected via a coaxial cable. The use of a coaxial cable connection is undesirable because it creates a complicated structure and increases the sensor size. In the next section, we propose a parallel double-curling probe without a coaxial cable connection, and demonstrate its operation.
We established a parallel double-curling probe by slightly modifying the double-curling probe shown in Fig. 5(a). To verify its operation, we compared the resonance peaks of an individual curling probe with those of a parallel double-curling probe as follows. Figures 6(a) and 6(b) represent the 3-turn and 4-turn curling probe configurations, respectively, with a large antenna plate of diameter 100 mm. The coaxial cable was placed at the center of the case, whereas the curling slot antenna was located 25 mm from the center of the large antenna plate. The monopole antenna was curved at the middle of the cavity to be placed at the center of the curling slot antenna. Figure 6(c) shows the configuration of the parallel double-curling probe, which is a combination of a 3-turn and 4-turn curling slot antenna (case C + case D). In this configuration, the distance between the centers of each curling slot antenna was 50 mm. Two curling slot antennas were located opposite to each other at the same distance from the center of the large antenna plate, and their monopole antenna was placed at its center with the same length and diameter.
The configuration in Fig. 6(c) is a simplified configuration for the preliminary demonstration of the parallel double-curling probe; however, it is not practical because of the complicated monopole antenna configuration inside the case: separating the core of the coaxial cable and connecting the center of the curling slot antenna. To address this problem, we employed an activator instead of a curved monopole antenna, as shown in Fig. 6(d) (case E). The activator, which was a large thin-plate with a thickness of 0.5 mm, is located at the mid-point of the case in the vertical direction. The cylindrical rods, which act as individual monopole antennas, are located at the center of each curling slot antenna.
Figure 6(e) exhibits
Figure 6(f) shows the
To verify the operation of the parallel double-curling probe (case E), we inserted a cylindrical single plasma, which had a uniform density
Figure 7(d) shows
Based on geometrical factors, we can verify the operation of the parallel double-curling probe. Figure 8(a) shows a cross-sectional view of the parallel double-curling probe and cylindrical double plasmas, which have individual electron densities. Above the surface of the 4- turn curling slot antenna with a distance of 1.0 mm, the low-density plasma (LDP) with a diameter of 40 mm was aligned at the center of the antenna above the surface of the 4-turn curling slot antenna. In contrast, the high-density plasma (HDP) is aligned at the center of the 3-turn curling slot antenna at the same distance. The input electron density of HDP (
By performing 3D-EM wave simulations, we verified the step-bystep operation of the parallel double-curling probe. The single-curling probe showed good linearity of its fundamental resonance peak with
Based on the simulation results, the multiplication and parallelization of curling probes for plasma uniformity sensors are feasible. Because the curling probe uses standing wave resonance on its curling slot antenna, its size determines the spatial resolution of the electron density measurement. This means that miniaturization of the curling slot antenna is crucial for enhancing spatial resolution. Furthermore, the resonance frequencies at each curling slot antenna do not overlap and should be separated with sufficient differences in the frequency domain. In addition, each curling slot antenna should exhibit a high
This research was supported by a research fund from the Chungnam National University, Republic of Korea.
The authors declare no conflicts of interest.