rface is supposed to be of good quality. On the other hand, the interface separating the two insulating layers contains a much larger defect density, which is often the case for deposited or bonded oxides. III-41 Thickness Applied electric field Figure 27. Schematic representation of the field dependence of the measured voltage shift, in the case of charge trapping at the interfaces of a multilayer oxide (left). The corresponding trap distribution (NTp=cst) and charge distributions (x) in each case (E>0 and E<0) are also shown (right). During irradiation under a positive bias, the hole current is maximum at the Si/SiO2 interface (see fig.27), and the holes get trapped mainly near the Si/SiO2 interface, which contains few defects. Many fewer holes get trapped at the interface between the two oxides. Under a negative bias on the other hand, holes are attracted by the gate, and mostly get trapped at the interface between the two oxides. The defect density being much more important there, the image charge induced in the silicon substrate will be more important in this case. It is therefore possible for a negative bias to produce a voltage shift that is larger than that produced under a positive bias (as illustrated in fig. 27), and this is exactly the opposite of what would be found for a pure thermal oxide [an example is given e.g., in Hugh-71]. 3.12.6. BEHAVIOR IN THE HIGH DOSE REGIME In the high dose regime, the electric field in the oxide can no longer be considered as equal to the applied field. The presence of a large trapped charge can strongly disturb the internal field, especially if the charge is distributed throughout the volume. In case of thick oxide, the internal field can be reduced to almost zero in a large part of the oxide. This "Field Collapse Effect" has been conceptually described very early [Grov-66], [Mitc-67] (pp. 63-67). This was experimentally studied by Hughes [Hugh-85] in the case of thermal oxides irradiated at low temperature where significant hole trapping occurs, and by Boesch [Boes-91