Electron density and temperature of dual-frequency capacitively coupled argon plasma in two-dimensional distribution obtained and studied in experiment

Abstract

Behaviors of plasma in spatial distributions are vital for understanding physical mechanisms of low pressure capacitively coupled discharges. However, due to the lack of simple and reliable diagnostic techniques, few experimental studies have focused on two-dimensional distributions of plasma. Recently, we have developed a new method for rapid two-dimensional plasma diagnosis. With this method, the dual-frequency capacitively coupled plasma, an important plasma commonly used in etching, is systematically studied. Three circumstances are presented here: In case I, the plasma discharge was driven by a radio-frequency source; in case II, two radio-frequency sources were applied to one electrode; and in case III, such sources were applied to two separate electrodes. Results show that both the emission intensity and electron density in case III are higher than those in case II, and because the absolute value of the DC self-bias voltage is smaller, the structure of axial discharge is more symmetric in case III. For the asymmetrically capacitive discharges (cases I and II), the position of the maximum axial electron density is close to the powered electrode due to the presence of the DC self-bias voltage. As the low-frequency power increases or gas pressure decreases, the position of the maximum axial electron density moves gradually toward the grounded electrode because of the enlarged thickness of the sheath. However, high-frequency power has a limited influence on the position of the maximum axial electron density. The radial distribution of plasma is more uniform when plasma density is relatively low. For a higher plasma density situation, the center-peaked density profile results from the higher power deposition at the electrode center and/or more local charged particle dynamics. In addition, under the same discharge conditions, a lower electrode gap is favorable for the occurrence of the α-γ mode transition.