UBC Theses and Dissertations
The effects of mobile sodium contamination on MOS structures Bouthillier, Thomas Michael
As the present trend to larger scales of integration of chips continues an increasing demand on reliability must be met. One source of premature device failure is contamination by mobile sodium ions. This thesis is a study of some of the effects of sodium contamination in silicon dioxide using internal photoemission and self healing breakdown. Lateral inhomogeneities in the barrier height at the Si-Si02 interface were probed by scanning a focussed beam of ultraviolet light along a MOS capacitor. It was found that barrier height was uniform unless highly contaminated samples were subjected both to a high temperature and also to a positive bias on the field plate for a short time at moderate temperatures. Qualitative features of lateral inhomogeneities in the surface potential were characterized by high frequency capacitance-voltage measurements showing similar effects. A model for the photoemission process was proposed. The effect of illumination while measuring photoemission was found to affect the band bending substantially so that a relatively simple model neglecting band bending in the silicon was used. Barrier height lowering caused by sodium contamination was measured on samples having a uniform photocurrent. By measuring samples at different contamination levels an experimental relationship between sodium concentration and barrier height lowering was found. A simple model taking into account Schottky barrier lowering resulting from the dipole field was found to be in good agreement with the results for low levels of surface concentration. Good experimental agreement was found with previous work. Self healing breakdown of the sample was also performed showing a convincing correlation between photoemission peaks and areas of early breakdown.
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