Microwave plasmas are often used in deposition processes because they have a broad operating range and a relatively high energy efficiency. The aim of this work was firstly to obtain spatial characteristics of argon microwave plasmas and secondly to investigate the effect of pulsed plasma operation on time-average species densities. For that, a combination of both active (Thomson scattering) and passive emission spectroscopy is used. The direct correlation that is still sometimes assumed to exist between electron density and excited species densities is proven to be false. Moreover, it is shown that different excited states can have different radial density profiles. Both effects are related to radial inhomogeneity of the electron temperature. The introduction of molecular nitrogen in otherwise pure argon plasmas was found to strongly decrease the length of the plasma column. This decrease can be attributed to the absorption of energy in rotational and vibrational excited states. It was also found that excited state density profiles at the end of an Ar+N2 plasma column are hollow. It is not completely clear why the hollow profiles form, but the effect may be related to the finite penetration depth of the surface wave. Regarding power interruption it is found that pulsed plasma operation can lead to a local gain in time-averaged excited species densities, but that this gain is most likely lost when integrated over the whole plasma column because of the finite propagation speed of the ionization front. Moreover, the total amount of plasma species is reduced because the column length decreases at higher repetition frequencies.
|Date of Award||31 May 2013|
|Supervisor||E.A.D. Carbone (Supervisor 1), S. Hübner (Supervisor 2), Sander Nijdam (Supervisor 2) & Gerrit M.W. Kroesen (Supervisor 2)|