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Indexed by:期刊论文

Date of Publication:2021-01-27

Journal:PLASMA SOURCES SCIENCE & TECHNOLOGY

Volume:27

Issue:6

ISSN No.:0963-0252

Key Words:dual-frequency CCRF plasmas; secondary electron emission; hairpin probe; phase resolved optical emission spectroscopy

Abstract:Effects of secondary electron emission (SEE) on the plasma density and electron excitation dynamics in dual-frequency (2 MHz and 14 MHz) capacitively coupled Ar plasmas are investigated. The plasma density n(p) is measured with a hairpin probe, and the spatio-temporal distribution of electron excitation rate (ground state into Ar 2p(1) state) is determined by phase resolved optical emission spectroscopy. It is generally found that as the low-frequency (LF) voltage phi(L), increases, n(p) first decreases at low phi(L), due to compressed the plasma bulk length by the LF source, and then increases slightly with phi(L), suggesting that the plasma is dominated by the a mode. When phi(L) exceeds some critical value, n(p) increases dramatically with phi(L), due to significantly enhanced ionization by secondary electrons, indicating a alpha-gamma mode transition. An excitation pattern caused by SEE at the edge of the completely expanded LF sheath is observed at relatively high phi(L). Under various conditions, including the high-frequency voltage phi(H), the pressure p, the electrode gap L, and the electrode material, different dependences of n(p) on phi(L) are also discussed. It is found that the discharge turns into the gamma mode at a lower phi(L) when phi(H) is higher. As p increases, the density peak moves axially towards the powered electrode, due to reduced sheath thickness, while its distance to the powered electrode is almost independent of other external conditions (phi(H), L, electrode material, etc). A higher p or higher L is favorable for the enhancement of n(p) in gamma mode, because secondary electrons can contribute more to the ionization. Due to the higher SEE coefficient of the aluminum electrode, a more significant increase in n(p) in the gamma mode can be seen than that with a stainless steel or copper electrode. Meanwhile, the spatio-temporal distributions of the electron excitation rate under the same conditions are analyzed to further understand the SEE effects.