Positive and Negative Ion Motion in Thermal Oxide on Silicon by Radiochemical and Mos Analysis

Failure of conventional Si planar devices by surface potential shift due to oxide impurities has been studied by combined MOS and radiochemical analysis. Radioactive NaBr contaminant was used to study both anion and cation distributions and kinetics. Experimental evidence is presented that Na in the oxide near the Si interface acts to accumulate electrons at the surface of n-type Si. Less well-known is the role of the anion in determining failure in this mode. At temperatures above 800°C, Br is found to diffuse into the oxide more slowly than Na and partly to neutralize Na+. Conventional hot-bias drift experiments at 200°C have been extended to include impurity distribution as a function of time within the oxide. The kinetics of Na flow in the oxide to and away from the Si interface compared to Si surface potential shifts suggest the presence of a rapidly-equilibrating negative species which can neutralize Na+ but not deplete an n-type Si surface.