The effects of electron surface interactions in geometrically symmetric capacitive RF plasmas in the presence of different electrode surface materials

Abstract

Particle-in-cell/Monte Carlo collision (PIC/MCC) simulations are performed to investigate the asymmetric secondary electron emission (SEE) effects when electrons strike two different material electrodes in low pressure capacitively coupled plasmas (CCPs). To describe the electron-surface interactions, a realistic model, considering the primary electron impact energy and angle, as well as the corresponding surface property-dependent secondary electron yields, is employed in PIC/MCC simulations. In this model, three kinds of electrons emitted from the surface are considered: (i) elastically reflected electrons, (ii) inelastically backscattered electrons, and (iii) electron induced secondary electrons (SEs, i.e., δ-electrons). Here, we examined the effects of electron-surface interactions on the ionization dynamics and plasma characteristics of an argon discharge. The discharge is driven by a voltage source of 13.56 MHz with amplitudes in the range of 200–2000 V. The grounded electrode material is copper (Cu) for all cases, while the powered electrode material is either Cu or silicon dioxide (SiO2). The simulations reveal that the electron impact-induced SEE is an essential process at low pressures, especially at high voltages. Different electrode materials result in an asymmetric response of SEE. Depending on the instantaneous local sheath potential and the phase of the SEE, these SEs either are reflected by the opposite sheath or strike the electrode surface, where they can induce δ-electrons upon their residual energies. It is shown that highly energetic δ-electrons contribute significantly to the ionization rate and a self-bias forms when the powered electrode material is assumed to be made of SiO2. Complex dynamics is observed due to the multiple electron-surface interaction processes and asymmetric yields of SEs in CCPs.

Diagnostics of a microhollow cathode discharge at atmospheric pressure

“…Plasma generated at atmospheric pressure is also important. It can have a high density and a small spatial range, making it useful for the precise surface treatment of materials [4][5][6]. For example, such plasmas can reduce costs and improve efficiency in chip manufacturing.…”

Control of electron velocity distributions at the wafer by tailored voltage waveforms in capacitively coupled plasmas to compensate surface charging in high-aspect ratio etch features

“…The realistic implementation of plasma-surface interactions include energy and material dependent ion induced secondary electron emission (γ-electrons), electron induced secondary electron emission (δ-electrons), elastic electron reflection as well as inelastic backscattering of electrons. Previous studies demonstrated the importance of including such processes realistically [59][60][61][62][63][64][65][66] and also showed that using two different boundary surface materials at both electrodes can induce a plasma asymmetry [67][68][69][70]. Such surface material asymmetries often occur in actual plasma processes and are also studied in this work.…”

On the electron sheath theory and its applications in plasma–surface interactions

“…The ion-induced SEE is usually counted if the electron temperature is low and the wall material is metallic, which has a lower electron-induced SEE coefficient. Taking the example of plasma processing using capacitively coupled plasma, it is usually the ion-induced SEE that is counted since electrodes are typically metallic and the electron temperature is low [44], whereas the electron-induced SEE was shown to be influential if the neutral pressure is low and the electrodes are made of a more emissive material such as SiO 2 [45].…”