Parameter Dependences of Ion Energy Distribution and Ion Flux in Dual-Frequency Capacitively Coupled Argon Discharges

Abstract

Low-pressure dual-frequency capacitively coupled plasmas (DF CCPs) are routinely used in high-aspect-ratio (HAR) dielectric etching for 3-D memory fabrication, due to their capability for independent control of ion flux and ion energy. To prevent unwanted etching profiles caused by the charging effect inside the HAR trenches, a much lower excitation frequency, in the kilohertz range, is necessary to generate higher ion energy. In this work, we investigate the effects of discharge parameters on the ion energy distribution function (IEDF) and the ion flux in a DF argon discharge excited by a high-frequency (HF) source at 27.2 MHz and a low-frequency (LF) source at 0.04/0.4/1.36/2.72/5.44/6.8 MHz, utilizing a retarding field energy analyzer (RFEA). With increasing the LF voltage amplitude ( ${V}_{\text {L}}$ ), the mean ion energy increases, while the ion flux decreases. The decrease in the ion flux versus ${V}_{\text {L}}$ is attributed to a weakened HF electron heating at a higher ${V}_{\text {L}}$ . The IEDF evolves from a broad peak at ${V}_{\text {L}} =50$ V into a bimodal structure at ${V}_{\text {L}} =100$ V. With further increasing ${V}_{\text {L}}$ , the high-energy peak shifts toward higher energies, and the energy width between the high- and low-energy peaks broadens. By contrast, increasing the HF voltage amplitude ( ${V}_{\text {H}}$ ) leads to a rise in both the mean ion energy and ion flux. By increasing the gas pressure (p), both the high- and low-energy peaks in the IEDF become less evident, due to enhanced momentum transfer and/or charge exchange collision between ions and neutrals at a higher p. As the LF frequency ( ${f}_{\text {L}}$ ) increases, the IEDF transitions from a broad bimodal structure to an almost single-peak structure. Notably, with decreasing ${f}_{\text {L}}$ , the ion flux decreases initially, followed by an increase, reaching a maximum at 400 kHz.