Effects of self-bias on plasma density distribution in radio frequency capacitively coupled plasmas

In radio-frequency (RF) capacitively coupled plasma (CCP) systems, a blocking capacitor is essential to prevent direct current (DC) components from entering the RF power supply, ensuring that only alternating current (AC) is applied to the plasma. However, the use of a blocking capacitor inevitably induces a self-bias voltage due to the difference in mobility between electrons and ions. In this study, we investigate the effects of self-bias on plasma characteristics, particularly the plasma density distribution, under low-pressure (20 mTorr) RF CCP conditions. Two RF source cases are considered in this study: a bipolar dual-frequency pulsed waveform (40 MHz and 400 kHz) and a single-frequency sinusoidal waveform (40 MHz). In the pulsed waveform case, when the blocking capacitor is sufficiently large to suppress the formation of self-bias, the peak plasma density appears near the peripheral sidewall region. However, when a 10 nF blocking capacitor is used, DC self-bias is present, and the plasma density peaks shift toward the region above the powered electrode. This finding suggests that self-bias not only influences ion energy but also reshapes the global plasma density profile, which may critically affect etch uniformity in high aspect ratio structures. Our study emphasizes the spatial redistribution of plasma density by the change of dominant electron heating mechanism depending on the presence of DC self-bias.