Indexed by:期刊论文

Date of Publication:2021-01-27

Journal:PLASMA SOURCES SCIENCE & TECHNOLOGY

Volume:27

Issue:5

ISSN No.:0963-0252

Key Words:capacitively coupled plasmas; dual-frequency; standing wave effect; uniformity; PROES

Abstract:It is well known that the plasma non-uniformity caused by the standing wave effect has brought about great challenges for plasma material processing. To improve the plasma uniformity, a low-frequency (LF) power source is introduced into a 100 MHz very-high-frequency (VHF) capacitively coupled argon plasma reactor. The effect of the LF parameters (LF voltage amplitude phi(L) and LF source f(L)) on the radial profile of plasma density has been investigated by utilizing a hairpin probe. The result at a low pressure (1 Pa) is compared to the one obtained by a 2D fluid-analytical capacitively coupled plasma model, showing good agreement in the plasma density radial profile. The experimental results show that the plasma density profile exhibits different dependences on phi(L) and f(L) at different gas pressures/electrode driven types (i.e., the two rf sources are applied on one electrode (case I) and separate electrodes (case II)). At low pressures (e.g., 8 Pa), the pronounced standing wave effect revealed in a VHF discharge can be suppressed at a relatively high phi(L) or a low f(L) in case I, because the HF sheath heating is largely weakened due to strong modulation by the LF source. By contrast, phi(L) and f(L) play insignificant roles in suppressing the standing wave effect in case II. At high pressures (e.g., 20 Pa), the opposite is true. The plasma density radial profile is more sensitive to phi(L) and f(L) in case II than in case I. In case II, the standing wave effect is surprisingly enhanced with increasing phi(L) at higher pressures; however, the center-high density profile caused by the standing wave effect can be compensated by increasing f(L) due to the enhanced electrostatic edge effect dominated by the LF source. In contrast, the density radial profile shows a much weaker dependence on phi(L) and f(L) in case I at higher pressures. To understand the different roles of phi(L) and f(L), the electron excitation dynamics in each case are analyzed based on the measured spatio-temporal distributions of the electron-impact excitation rate by phase resolved optical emission spectroscopy.