Ohmic heating dominated capacitively coupled plasma: with a sheath inverted

Low-pressure radio frequency capacitively coupled plasma (RF-CCP) typically features Bohm presheath and the capacitive sheath, which is known as Deby sheath. According to previous research, when the boundary emission is significant enough, so that it surpasses the plasma electron flux, space-charge limited sheath should be formed near the boundary [1]. However, if the charge exchange in sheath is considered, i.e. if cold ions are produced in SCL sheath, the sheath apparently becomes unstable. In this case, the SCL sheath will be replaced with an inverse sheath, with a wall potential well above the plasma bulk. The inverse sheath has been proved to exist in various plasmas[2], [3], but its properties, such as plasma impedance, sheath structure, heating mechanisms, are still unclear.

Therefore, in this work, a 1D3V PIC-MCC model is employed, a low-pressure RF CCP with strong boundary emission is simulated. The pressure is 4 – 20 Pa, powered by a rf source with 13.56 MHz. A constant electron emission from both electrodes is adopted, corresponding to the thermionic emission. Asymmetric electron emission is also considered.

It was found that an inverse sheath structure is formed near the wall, with no presheath. With such sheath structure, the ions are confined in the plasma bulk. And the particle balance is mainly achieved by the balance between plasma electrons and boundary emitted electrons. Meanwhile, the plasma is no longer capacitive with the disappearance of capacitive sheath structure. Most importantly, the Ohmic heating dominates in such plasmas and stochastic heating disappears without presheath.