Paper PS-ThA7
Measurements of RF Magnetic Fields and Plasma Current in Coupled Low and Very High Dual-Frequency Plasma Sources

Thursday, October 25, 2018, 4:20 pm, Room 104A

Plasma processing systems capacitively driven at very high frequencies (VHF, e.g. 100MHz) have attracted much interest for semiconductor and flat panel display processing. VHF has the advantage of generating plasma with more efficiency as power is coupled more into electrons and less into ions in the sheath. Benefits are seen for processes requiring reduced ion energy, ostensibly to minimize damage, high ion and radical flux to the substrate. The benefits of VHF are accompanied by challenges. The short wavelength associated with VHF power is reduced even further in the presence of high density plasma. The wavelengths are comparable to the RF electrode dimension. As a result, spatial variations in plasma density and sheath voltage can arise and lead to undesired non-uniformities in process parameters. Skin effects associated with high plasma density and plasma-sheath local resonances are other destroyers of plasma uniformity. Previously, we have reported¹ a detailed investigation on the spatial and temporal evolution of RF magnetic field and plasma current in a 100MHz plasma source performed with a magnetic field probe (B-dot loop). The probe translated across the diameter of the VHF plasma measured the magnitude and phase of the fundamental and harmonics of the plasma excitation frequency as a function of radial position. The measured magnetic fields displayed a transition from simple to complex behaviors depending on plasma conditions. The spatiotemporal resolved magnetic field exhibits a series of fast current reversals and subsequent circulation driven by inward wave propagation that are electromagnetic in nature. We showed how the onset, frequency and amplitude of the current reversal and subsequent circulation were strongly related to applied plasma conditions. We also showed that plasma current derived from the magnetic field distribution was closely correlated to the plasma density profile measured by a plasma absorption probe. In order to further understand these fundamental electromagnetic structures in VHF plasma, in the current study, we apply LF on top of the VHF aimed to modify the electromagnetic structures. Performed with B-dot probe, we report the spatial and temporal evolution of VHF magnetic field and plasma current as a function of different LF phases as well as the detailed correlation of VHF electromagnetic structure and the LF modulated plasma sheath variation. We show that the coupling of the VHF electromagnetic field to the plasma strongly depends on the phase of the LF driven source. Measurements are compared against different theories for how the VHF power couples to the plasma.

¹ PS+VT-ThA12, 64^th AVS Symposium, Tampa FL