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

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 f L increases, n p first decreases at low f L , due to compressed the plasma bulk length by the LF source, and then increases slightly with f L , suggesting that the plasma is dominated by the α mode. When f L exceeds some critical value, n p increases dramatically with f 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 f L . Under various conditions, including the high-frequency voltage f H , the pressure p, the electrode gap L, and the electrode material, different dependences of n p on f L are also discussed. It is found that the discharge turns into the γ mode at a lower f L when f 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 (f 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.

Electron power absorption mode transition in capacitively coupled Ar/CF4 discharges: hybrid modeling investigation

“…There are four main electron power absorption modes, depending on how energy or power is transferred, i.e. the α mode [4][5][6][7][8][9], the γ mode [10][11][12][13][14], the drift-ambipolar (DA) mode [15], and the striation (STR) mode [16]. Since the discharge mode switches under different external parameters, the investigation of mode transition is of significant importance for improving the performance of plasma sources in practical processing.…”

On the role of secondary electron emission in capacitively coupled radio‐frequency plasma sheath: A theoretical ground

“…CCP is commonly generated by applying radio‐frequency (rf) voltage or current to electrodes immersed in plasma, which creates high voltage capacitive sheath between the electrode and plasma bulk. Boundary effects, namely plasma–surface interactions, are of vital significance in understanding capacitively coupled radio‐frequency (CCRF) sheath, among which secondary electron emission (SEE) induced by ion flux is one of the most common features and is worth investigating . The aim of this paper is to establish an ab initial theoretical ground on rf sheath of electron‐emitting surface, aka emissive rf sheath, and facilitate future works regarding CCP discharge in either concept or application.…”

“…Lafleur et al proposed a concise expression to characterize electrical asymmetric effect induced by different secondary emission yield (SEY) , also called emission coefficient . A series of simulations were performed, which incorporated ISEE and illustrated its influences on discharge parameters, showing some differences with simulations where ISEE is not involved …”

Intense boundary emission destroys normal radio-frequency plasma sheath

“…Conduction current prevails displacement current in bulk plasma and field in bulk plasma is weak. But in this letter we will show that the bias can be primarily consumed by bulk plasma and electric field in plasma center needs not to be shielded by sheathes, due to intense boundary emission.Numerous studies have been done regarding boundary emission in CCP, but its influences are mostly assumed to be unessential 4,[8][9][10][11][12][13][14][15] . The steady flux of ion 𝛤 𝑖 produces a surface emission flux 𝛤 𝑒𝑚 = 𝛾 𝑖 𝛤 𝑖 due to ion-induced secondary electron emission (SEE), with 𝛾 𝑖 the emission coefficient.…”

“…Numerous studies have been done regarding boundary emission in CCP, but its influences are mostly assumed to be unessential 4,[8][9][10][11][12][13][14][15] . The steady flux of ion 𝛤 𝑖 produces a surface emission flux 𝛤 𝑒𝑚 = 𝛾 𝑖 𝛤 𝑖 due to ion-induced secondary electron emission (SEE), with 𝛾 𝑖 the emission coefficient.…”