Control of the Electron Energy Distribution and Plasma Ignition Delay in a Novel Dual Tandem Inductively Coupled Plasma
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The electron energy distribution function (EEDF) plays an essential role in non-equilibrium low-temperature plasmas. The EEDF governs the rate of electron-impact reactions and determines the plasma gas composition, which in turn, determines the fluxes of radicals, ions, and photons striking the substrate. As such, control of the EEDF is of paramount importance for both fundamental plasma studies and practical applications. A novel dual plasma source was developed to control the EEDF in a main inductively coupled plasma (ICP) separated by a grid from a tandem auxiliary ICP. The auxiliary ICP was continuously powered while the main ICP power was pulsed. Langmuir probe measurement of the EEDFs during the afterglow of the main ICP, suggested that transport of hot electrons from the auxiliary plasma kept the tail of the EEDF and bulk electron temperature elevated. Results from a computer simulation of the trends in the evolution of the EEDFs agreed with experimental measurements. For certain operating conditions, plasma ignition delays were observed in the main ICP. Power to the Faraday-shielded main ICP was pulsed with a frequency of 1 kHz, while the (also Faraday shielded) auxiliary ICP was operated in continuous wave (cw) mode. In chlorine plasmas, ignition delay was observed for duty cycles greater than 60% and, in contrast to expectation, the delay was longer with increasing duty cycle up to ~99.5%. The ignition delay could be varied by changing the auxiliary and/or main ICP power. Langmuir probe measurements provided the temporal evolution of electron temperature, and electron and positive ion densities. These measurements revealed that the plasma that was ignited shortly after the decaying positive ion density (n+) in the afterglow of the main ICP, reached the density (n+,aux) prevailing when only the auxiliary ICP was powered. At that time, the production rate of electrons dominated their loss in the main ICP due to hot electron injection from the auxiliary ICP. As a result, ne increased rapidly and the plasma was ignited. Plasma ignition delay occurred when the afterglow of the pulsed plasma was not long enough for the ion density to reach n+,aux during the afterglow. Besides Cl2, plasma ignition delays were also observed in other electronegative gases (SF6, CF4/O2 and O2) but not in an electropositive gas (Ar).
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