Because of the increase in the capacity of semiconductor devices, the limitations difficulty of manufacturing process techniques using plasma, such as plasma-assisted etching and deposition, have significantly increased [1–7]. As plasma processes become more complicated, their analysis and optimization by evaluating the equipment properties and plasma parameters have become important [8–10]. It is necessary to evaluate the complex interactions between plasma and external parameters and optimize plasma processes, including the tools and equipment conditions [10–14]. Furthermore, to conduct these measurements accurately, the accuracy and reliability of the plasma process monitoring sensor such as the voltage current (VI) sensor, should be improved [15,16].

As a radio frequency (RF) power and plasma process-monitoring sensor, a VI probe has been utilized and developed to monitor and optimize plasma processes in semiconductor fabrication [10,17]. It can be used to detect changes in the RF signal as well as to analyze plasma parameters using the plasma equivalent circuit [12,13,18]. However, in plasma processes, there are a variety of RF power system conditions, such as the VI probe installation position, RF connector type, and RF cable length, which affect the VI probe measurement results. Because the measurement characteristics of the VI probe affect the accuracy of the RF signal and reliability of the plasma process monitoring, depending on the equipment properties, it is important to examine the measurement characteristics based on various conditions of the measuring systems.

In this study, we investigated the characteristics of RF signals with respect to RF power transmission lines using a VI probe. The RF signal was measured using a VI probe for different VI probe installation positions, RF connector types, and RF cable lengths. The delivered power, RF voltage, and RF current were measured using an input power configuration to analyze the RF measurement results for various RF power transmission lines.

Figure 1 shows a schematic diagram and photographs of the experimental setup for analyzing the characteristics of RF power transmission lines using a VI probe. As shown in Fig. 1(a), a capacitively coupled plasma (CCP) source was used. A discharge chamber with an inner diameter of 900 mm and an electrode with a diameter of 300 mm were used to generate the plasma. The RF power was fed to the electrode at a frequency of 12.56 MHz using an RF generator (REX- 600, RFPT) and a matching network. Argon gas was injected into the chamber, and a fixed pressure of 20 mTorr was monitored using a capacitance manometer gauge.

Figure 1. (a) Schematic diagram of the experimental setup. The VI probe installation positions are represented using numbers (1)−(3). Various (b) types of RF connectors and (c) lengths of RF cables for the analysis of the characteristics of RF power transmission lines.

The VI probe incorporates capacitive and inductive components to extract the voltage and current signals of the transmission line. This step enables the calculation of the phase difference between the real and imaginary components of the voltage and current signals, allowing for the derivation of the impedance and delivered power measurements. In this work, we employed the VI probe (Octiv Suite 2.0, Impedans), which was calibrated in accordance with the procedures specified in the ‘Development of the automatic high-power calibration system (KRISS-98-024-IR)’ of the Korea Research Institute of Standards and Science. The characteristics of the RF power transmission lines were analyzed using the VI probe by measuring the delivered power, RF voltage, and RF current at various measurement positions, RF connector types, and RF cable lengths. The installation positions of the VI probes are shown in Fig. 1(a). In the figure, Positions (1), (2), and (3) correspond to specific locations: Position (1) is located behind the RF generator, Position (2) is situated in front of the matcher, and Position (3) is between the electrode and the matcher. These designations provide a clear reference for the different VI probe installation positions within the experimental setup. Figure 1(b) shows the different types of RF connectors, namely N, TNC, BNC, UHF, and HN used for the RF measurements. The RF cables (RG-393, Thermax) have an impedance of 50 Ω, and various lengths of 3.7 and 5.0 m; the RF cables are shown in Fig. 1(c) for analysis based on the RF cable length.

The delivered power was measured at each position to analyze various VI probe measurement positions, as shown in Fig. 1(a). Figure 2 shows the RF power transmission at various positions as a function of the 12.56 MHz frequency input power in the range of 50−300 W. The VI probe was connected to a 3.7 m length RF cable using an N-type RF connector. The RF power transmission is obtained using Eq. (1):

Figure 2. RF power transmission measured for various positions of the VI probe as a function of the input power.

where, P_out and P_in are the delivered power measured using the VI probe and the input power applied using the RF generator, respectively. At a very small RF power loss, the calculated RF power transmission value tended to zero. Given that the delivered power at Position (1) is nearly equivalent to the input power, the RF power transmission values at each Position were normalized by referring to the RF power transmission value at Position (1), as shown in Fig. 2.

In Fig. 2, the RF transmission value for Position (2) is approximately -0.2 dB, indicating that approximately 95 % of the input power was delivered. However, the RF transmission value at Position (3) is approximately -1.7 dB, which is equivalent to approximately 33 % of the input power. This result indicates that power loss occurred within the matcher, as noted in [19].

Figure 3 shows the VI probe measurement results for various RF connector types to compare the characteristics of RF transmission lines comprising different connector types. In these measurements, an input power in the range of 50−300 W at a frequency of 12.56 MHz was fed to the electrode via a 3.7 m RF cable. The VI probe captures the RF signals at Position (2) as shown in Fig. 1(a). The RF generator, matcher, RF cable, and VI probe are equipped with N-type adapters. This allows for interchangeability of the RF connector types through a configuration involving two connectors: an N - x (male) connector and x (female) − N-type connector. In this context, ‘x’ represents various RF connector types such as N, TNC, BNC, UHF, and HN. The switched connector location was positioned between the VI probe and the RF cable attached to the RF generator. These connector variations were applied to the VI probe measurements, to account for the effect of the different RF connector types. In Fig. 3(a), the RF power loss (dB) can be obtained using Eq. (2).

Figure 3. (a) RF power loss, (b) RF voltage, and (c) RF current for various RF connector types as a function of the input power.

According to Eq. (2), the RF power loss by the transmission line, including the RF connector, is low when the delivered power is close to the input power.

The results obtained from the RF measurements considering various RF connector types indicate that the variations between each connector type were determined to be less than -13 dB (accounting for approximately 4 % of the input power). These findings suggest that the discrepancies in the power loss attributed to the different RF connector types are minimal. Consequently, it can be inferred that the differences in the impedance at a frequency of 12.56 MHz are negligible for the range of connector types considered.

To analyze the characteristics of the RF power transmission line length, we plotted the RF power loss data obtained using the VI probe in relation to varying cable lengths, as shown in Fig. 4. To isolate other considerations, we installed the VI probe at Position (2) using an Ntype connector, as shown in Fig. 1. The switched cable location in Fig. 4 is positioned between the VI probe attached to the N-type connector and the RF generator. The RF power loss per unit length is calculated using Eq. (3) as given below:

Figure 4. RF power loss for a unit length with respect to the transmission line length at varying input power.

In the above equation, L represents the length of the RF cable. Higher values of RF power dissipation in the transmission line result in the RF power loss (in dB/m) tending zero, whereas lower RF power loss values indicate enhanced RF power transmission efficiency.

As shown in Fig. 4, the RF cables are uniformly rolled in three turns. Despite the consistent number of turns, the longer RF cables (13 and 15 m) exhibit a larger bending radius compared with those of the shorter ones (3.7 and 5.0 m). The larger bending radius, despite the equal turn count, contributes to a reduction in the RF power loss. However, noteworthy differences arise for cables that are longer than a quarter wavelength of the 12.56 MHz power (approximately 5.9 m) [20]. Specifically, the RF power loss for longer RF cables exceeds that of shorter RF cables, as shown in Fig. 4. Consequently, the RF power loss for shorter cables is less than -5 dB/m (equivalent to approximately 3 % of the input power), whereas that of longer cables is more than -1.1 dB/m (approximately 7 % of the input power).

We analyzed the characteristics of RF signals measured using a VI probe with respect to various RF power transmission lines, including the VI probe installation position, RF connector type, and RF cable length. For the analysis, the VI probe measured the delivered power, RF voltage, and RF current in a low-pressure argon CCP. The analysis was performed at different VI probe installation positions and we found that the delivered power was lower than the input power owing to power loss in the matcher. The VI probe measurement results for various RF connector types were compared, and the result showed that the difference in the power loss for various RF connectors was very small for the 12.56 MHz frequency power. The delivered power was compared using various lengths of RF cables. The power loss was significant when the cable length was longer than the RF power wavelength. Consequently, the analysis of the characteristics of the RF signals measured using a VI probe with respect to RF power transmission lines enhanced the accuracy and reliability of the RF signal. An extended analysis of RF signals in more complicated systems using the VI probe is needed for plasma processing applications.

This research was supported by the Material Innovation Program (Grant No. 2020M3H4A3106004) of the National Research Foundation (NRF) of Korea and was funded by Ministry of Science and ICT and the R&D Convergence Program (Grant No. CRC-20-01-NFRI) of the National Research Council of Science and Technology (NST), Republic of Korea; Korea Evaluation Institute of Industrial Technology (Grant No. 1415181740); and Korea Research Institute of Standards and Science (Grant No. KRISS GP2023-0012-08, GP2023-0012-09).

The authors declare no conflicts of interest.