Generation and Langmuir probe analysis of a DC glow discharge and the deposition of chromium from a carbonyl precursor

Abstract

Experimental investigations detailed in this thesis fall into two distinct areas: Diagnostics of a nitrogen DC discharge with Langmuir probes and spectroscopy, and experiments designed to deposit chromium metal from a chromium hexacarbonyl precursor (Cr(C0)6). New diagnostic equipment was designed, tested and implemented to record Langmuir probe data with high resolution and accuracy. A monochromator was employed to record the light emission from the plasma. This identified which ionized species were present as well as the ion temperature from the resulting band structures present. The monochromator analysis was necessary to validate the assumptions of the Langmuir probe theory and to allow a choice of which theories applied. The Langmuir probe analysis of the recorded data for many plasmas have been well covered in the literature but few papers give a comprehensive coverage of the main probe theories using the one set of data and fewer still make use of double as well as single probes. The majority simply assume the particular theory used is relevant to the plasma conditions and the resulting temperatures and densities are correct. During the experiments and subsequent analysis it was found, similar to other researchers, that the ion densities were consistently higher than the electron densities even though the theoretical analysis applied in the saturation regimes appeared to be valid. Rather than ignoring this issue, and assuming ion collisions or secondary electron emission were the cause, new methods were developed to analyse the data. By determining the gradient of the saturation regimes and comparing them to those expected experimentally, it was found that the ion saturation regime neither followed the orbital nor radial motion theories investigated. However, the electron current did follow the orbital motion theory and it is demonstrated that the densities obtained from the ion currents, using these theories, were incorrect. This had been previously shown to be true only with non-probe methods. Using the same discharge configuration, impurity-free chromium films were deposited onto stainless steel substrates from a chromium hexacarbonyl precursor. This had been attempted by previous researchers, using a variety of discharge configurations, who invariably deposited similar metallic films with significant levels of oxygen and carbon impurities. The combination of the carbonyl precursor and the carrier gases (nitrogen and argon) with the discharge configuration resulted in these impurity-free films even when a negative bias of up to 50 V was applied to the sample.