Description
Boundary regions of low-pressure capacitively coupled plasmas (CCPs) – the presheath and sheath – govern ion acceleration, momentum transfer, and plasma–surface interactions, yet remain challenging to probe experimentally. In dual-frequency (2f) CCPs, the Electrical Asymmetry Effect (EAE) allows global tuning of plasma parameters via the relative phase between driving frequencies, but its local impact on boundary-layer forces has remained largely unexplored.
In this work, optically trapped microparticles are employed as spatially
resolved, minimally invasive force probes to investigate boundary-layer dynamics in dual-frequency CCPs. A single SiO2 microparticle (7 μm diameter) is confined in a three-dimensional optical tweezer and scanned vertically through the discharge. By tuning the trap stiffness, strong electrostatic forces in the sheath and weaker ion drag forces in the presheath are selectively resolved (Fig. 1).
The measured force profiles reveal a clear spatial separation of dominant forces: ion drag emerges in the presheath, reaches a maximum near the sheath edge, and is overtaken by the rapidly increasing electric field force inside the sheath. Under dual-frequency excitation, both force magnitude and spatial distribution exhibit pronounced asymmetry with respect to the RF phase, reflecting phase-dependent modifications of
electron heating and plasma density.
Particle charges derived from the sheath force profiles are consistent with orbital-motion-limited theory and literature estimates.
This approach establishes optically trapped microparticles as highly sensitive, force-resolved probes of boundary-layer physics, directly linking bulk plasma behavior with sheath dynamics.
It provides quantitative insight into local force balances, ion acceleration, and momentum transfer in phase-controlled 2f CCPs, and opens new experimental pathways toward force-resolved studies of plasma–surface interaction in technologically relevant RF discharges.