In the current work, the radial plasma density has been measured by utilizing a floating double probe in single and dual frequency capacitively coupled argon discharges operated in a cylindrical reactor, aiming at a better understanding of electromagnetic effects and exploring a method of improving the radial uniformity. The experimental results indicate that for single-frequency plasma sustained at low pressure, the plasma density radial profile exhibits a parabolic distribution at 90 MHz, whereas at 180 MHz, the profile evolves into a bimodal distribution, and both cases indicate poor uniformities. With increasing the pressure, the plasma radial uniformity becomes better for both driving frequency cases. By contrast, when discharges are excited by two frequencies (i.e., 90 + 180 MHz), the plasma radial profile is simultaneously influenced by both sources. It is found that by adjusting the low-frequency to high-frequency voltage amplitude ratio β, the radial profile of plasma density could be controlled and optimized for a wide pressure range. To gain a better plasma uniformity, it is necessary to consider the balance between the standing wave effect, which leads to a maximum plasma density at the reactor center, and the edge field effect, which is responsible for a maximum density near the radial electrode edge. This balance can be controlled either by selecting a proper gas pressure or by adjusting the ratio β.

The large-area capacitive discharges driven at very high frequencies have been attracting much attention due to their wide applications in material etching and thin film deposition. However, in the regime, the standing wave effect (SWE) becomes a major limitation for plasma material processing uniformity. In this work, a fiber Bragg grating sensor was utilized for the observation of the SWE in a large-area capacitive discharge reactor by measuring the radial distribution of the neutral gas temperature Tg. The influences of the RF power and the working pressure on the radial profiles of Tg were studied. At a higher frequency (100 MHz) and a lower pressure (5 Pa), Tg presents a center-peaked radial distribution, indicating a significant SWE. As the RF power increases, the central peak of Tg becomes more evident due to the enhanced SWE. By contrast, at 100 MHz and a higher pressure (40 Pa), the radial distribution of Tg shows an evident peak at the electrode edge and Tg decays dramatically towards the discharge center because the electromagnetic waves are strongly damped as they are propagating from the edge to the center. At a lower frequency (27 MHz), only edge-high profiles of Tg are observed for various pressures. For the sake of a comparison, a hairpin resonance probe was used to measure the radial distributions of the plasma density np under the same condition. The radial profiles of Tg are found to generally resemble those of np under various conditions. Based on the experimental results, the neutral gas heating mechanism was analyzed.

Neutral temperature Tg in capacitively coupled argon plasmas was measured by using a fiber Bragg grating sensor. The measurement of Tg is based on the thermal equilibrium process between the sensor and neutral gases, which is found to become fast upon increasing pressure, due to enhanced heat conduction. Additionally, Tg was found to increase with increasing high frequency power due to enhancive collisions with charged particles. It is also observed that Tg exhibits a significant gradient in space, ranging from 10 to 120 °C higher than room temperature for the conditions investigated. In addition, the spatial profiles of Tg at different pressures generally resemble those of the Ar+ density ni, measured with a floating double probe. The neutral gas is mainly heated via elastic collisions with ions in the sheath region followed by heat conduction among neutrals.

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