Effects of secondary electron emission on plasma density and electron excitation dynamics in dual-frequency asymmetric capacitively coupled argon plasmas

Abstract

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 2 p 1 state) is determined by phase resolved optical emission spectroscopy. It is generally found that as the low-frequency (LF) voltage ϕ L increases, n p first decreases at low ϕ L , due to compressed the plasma bulk length by the LF source, and then increases slightly with ϕ L , suggesting that the plasma is dominated by the α mode. When ϕ L exceeds some critical value, n p increases dramatically with ϕ L , due to significantly enhanced ionization by secondary electrons, indicating a α – γ mode transition. An excitation pattern caused by SEE at the edge of the completely expanded LF sheath is observed at relatively high ϕ L . Under various conditions, including the high-frequency voltage ϕ H , the pressure p , the electrode gap L , and the electrode material, different dependences of n p on ϕ L are also discussed. It is found that the discharge turns into the γ mode at a lower ϕ L when ϕ 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 ( ϕ H , L , electrode material, etc). A higher p or higher L is favorable for the enhancement of n p in γ 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 γ 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.