Observation of the standing wave effect in large-area, very-high-frequency capacitively coupled plasmas by using a fiber Bragg grating sensor

Abstract

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.