Surface layer evolution in glow discharge optical emission spectroscopy.

Glow discharge optical emission spectroscopy (GDOES) is a modem analytical technique for the analysis of the chemical composition of bulk materials and the depth profiling of multi-layer structures. Most research in the use of GDOES has concentrated on developing accurate methodologies for quantitative analysis and depth profiling. However, this thesis presents a study on various aspects of surface layer evolution under argon ion etching in GDOES. The GDOES technique relies on the ion bombardment of sample surfaces which removes material from the surface, layer by layer, on the atomic scale. During the surface layer evolution, the ion bombardment causes different surface micro-textures and preferential sputtering in individual crystallites, which can cause degradation of depth resolution in GDOES depth profiling. Experimental results using pure iron specimens in this study show a correlation between textures induced by GDOES sputtering and the sputtering rate, and a difference in the sputtering rate for crystallites with different crystal orientations. In studying ion bombardment by GDOES in semiconductors, a novel pitting morphology on the surface of a carbon-coated silicon wafer was observed and characterised in detail. This may have a potential application in the fabrication of micro-lens arrays. The generation and development of the pits were investigated, which are believed to be dependent upon the different sputtering rates between the film and the substrate. Geometric features of the pits were obtained using atomic force microscope (AFM) and the sphere-like surface of the pit was confirmed. The experimental work in this study also shows that the Grimm source in GDOES is a powerful etching tool. Eroded surfaces of metal specimens with little damage to the crystallites and phase structures were obtained by GDOES etching. The method was found to be an ideal process for specimen preparation for electron back-scattered diffraction (EBSD). The GDOES-etched surface of single crystal copper showed that the damaged layer formed by mechanical polishing using 6 micron diamond paste was about 1-2 pm and was removed after only a few tens of seconds of GDOES etching. GDOES etching was also applied to an investigation of internal oxides in carburised steels. The eroded surfaces provided plan views of the morphologies of internal oxides of carburised steels by scanning electron microscopy (SEM) images. Results of energy dispersive spectrometer (EDS)/SEM elemental mappings of different layers of the steels were in good agreement with GDOES depth profiles, which revealed that the elements Cr, Mn and Si were involved in the oxides. The last section of the thesis is about hydrogen detection in GDOES. The study includes a detailed analysis of: hydrogen contamination in GDOES, the hydrogen detection status of GDOES, the sample matrix effects on hydrogen detection and hydrogen effects on elemental concentrations in GDOES measurements. The experiments have confirmed that water vapour is the main source of the hydrogen contamination. When the GDOES system has stabilised, GDOES could be employed to differentiate specimens containing different concentrations of hydrogen. The experiments also showed that different hydrogen intensities could have resulted from different matrices even when the specimens were believed to contain no hydrogen. A possible explanation could be that variations of the y-electron ejection from different matrices and different sputtered atoms in the glow discharge, which altered the plasma and the energy distribution in the glow region, resulted in the variation of the excitation of the hydrogen atoms in the source. However, there are still some results in the matrix effects which could not be explained. The experiments concerning the consequence of hydrogen effects on apparent elemental concentrations in GDOES measurements were also undertaken using two steel standards. The results indicated that the hydrogen in the source has a negative effect on the signal from most of the metal elements in the specimens, and a positive effect on the non-metal and semiconductor elements.