Characterization of the hot-electron-induced degradation in thin SiO₂ gate oxides

Abstract

With decreasing oxide thickness, some of the established methods to characterize oxide degradation become inapplicable because of limited sensitivity and because of direct tunneling which gives rise to large leakage currents through the oxide. However, new techniques are emerging which could not previously be used on thicker oxides, such as stress-induced leakage current measurements, current noise measurements, hot-electron emission microscopy, ballistic electron emission microscopy and hot-carrier luminescence. Some of these techniques provide unprecedented information on the local current densities with high spatial resolution and can be used to study inhomogeneous degradation in thin oxides at low voltages where homogeneous hot-carrier degradation becomes energetically unfavorable. In Si/SiO2/poly-Si structures, three different, homogeneous, hot-electron induced degradation processes have been identified, with threshold voltages at 12V, 7.5V and about 4V. These are the generation of holes by impact ionization in the oxide, the injection of holes from the anode, and the release of hydrogen mostly from near the anode, respectively. The released hydrogen is very reactive and is responsible for the generation of many stress-induced defects. The existence of energy thresholds for homogeneous defect generation may limit the use of voltage acceleration for reliability evaluations. © 1998 Elsevier Science Ltd.