Effects of RF bias phase on plasma characteristics in an inductively coupled plasma

Abstract Using a two-dimensional axisymmetric direct-implicit particle-in-cell/Monte Carlo collision model, the effects of the radio frequency (RF) phase difference between the voltage waveforms applied to an inductive coil and a capacitively coupled bias electrode, θ , on a low pressure inductively coupled plasma (ICP) are investigated. The simulations reproduce phase-dependent variations in plasma density in the E-mode at low ICP power, P ICP , reported experimentally by Berger et al (2017 Appl. Phys. Lett. 111 201601). As P ICP increases, the dc self-bias at the wafer electrode V bias becomes more positive and the plasma density increases, which leads to thinning sheaths and an increase of the effective capacitance of the discharge. Tuning θ rephases the expansion/collapse timing of the sheaths adjacent to the coil and bias electrode, which stronlgy affects electron transport: at θ = − 180 ∘ , energetic electron beams generated by sheath expansion heating at the coil, reach the RF biased electrode during the local sheath collapse. This leads to a negative current spike at the electrode at this phase, whereas at θ = − 60 ∘ such electron beams reach the RF biased electrode sheath, when it is expanded and the negative current spike is attenuated. The ion energy distribution function at the biased electrode exhibits θ -dependent shifts in peak position and broadening, while the electron energy probability function evolves from a θ -sensitive bi-Maxwellian in E-mode to a near-Maxwellian with weak θ dependence as P ICP increases and the plasma transitions into H-mode. These results elucidate how phase control provides an effective control parameter for process optimization.