Plasma Structure and Dynamics in Dual-Frequency Discharges : A Multi-Diagnostic Approach from Bulk to Sheath

This dissertation examines the structure and dynamics of low-pressure dual-frequency capacitively coupled plasmas (2f-CCPs) and elucidates the role of the Electrical Asymmetry Effect (EAE). By adjusting the relative phase between two applied RF frequencies, ion energy and ion flux can be tuned largely independently of reactor geometry. The aim is to experimentally determine how this phase control shapes plasma behaviour from the quasi-neutral bulk to the sheath. A multi-diagnostic approach was employed. A custom-built Langmuir probe provided access to electron density, electron temperature, and plasma potential in the bulk. These measurements reveal a pronounced asymmetric dependence of all parameters on the excitation phase, originating from phase-dependent electron heating and resulting ionization dynamics. Complementary optical emission photometry enabled a non-invasive determination of the time-averaged sheath width and exhibited similar trends. A central element of this work is the use of optical tweezers. A single microparticle, stably trapped by counter-propagating laser beams, served as a highly sensitive, non-invasive force probe in the boundary regions. At high laser power, the strong electric field force in the sheath was quantified, whereas at reduced power the enhanced trap sensitivity allowed the first direct measurement of the much weaker ion drag force in the presheath. The resulting force profiles reconstruct the classical discharge structure: a nearly force-free bulk, a presheath shaped by ion drag, and a sheath dominated by the electric field. Across all diagnostics, the measured quantities display a clear asymmetric dependence on the excitation phase. These findings demonstrate how the EAE couples global driving conditions to local plasma behaviour and establish optically trapped microparticles as a powerful diagnostic tool for boundary regions that remain inaccessible to conventional techniques.

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