ltage shifts, measured on MOS capacitors and transistors using SIMOX oxide irradiated at a dose of 10 krad(SiO2), are plotted versus the gate voltage applied during irradiation [Pail- Figure 26 shows an example of the experimentally measured voltage shifts induced by the oxide charge trapped in a SIMOX oxide, as a function of the gate bias applied during irradiation. In this example, both MOS capacitors and NMOS transistors have been irradiated to a dose of 10 krad(SiO2)). 3.12.5. CASE OF A MULTILAYER INSULATOR Multilayer insulators possess by construction several interfaces, which are potential sources of defects, and which therefore act as preferential trapping locations during irradiation. Historically, multilayer oxides have been ised in the 60s for better reliability at that time [Hugh-71]. These multilayer structures are used for example to manufacture field oxides. The purpose in this case is to replace the thick thermal LOCOS oxide by a doped oxide usually less sensitive to radiation. The doped oxide is placed between a thermal oxide (which ensures a good Si/SiO2 interface) and an undoped oxide, in order to limit the diffusion of dopants. Some nitrided oxides (ONO structure) can also be considered as multilayer structures, the silicon/oxide interface being fabricated from a pure thermal. MOS devices with Ta2O5 dielectric can also be considered as multilayer, because of the presence of silicon oxide as transition layer between Si and tantalum pentoxide [Autr-95]. Even if they are made up of thermal oxides, buried oxides obtained by wafer bonding (BESOI, e.g. UnibondTM must also be regarded as multilayers, because of the presence of the bonding interface [Pail-95a], [Lera-97]. These various structures being generally made up of oxides, which exhibit little bulk charge trapping, the traps may be considered as being mostly located in the interfacial regions. The expected trap distribution profiles are illustrated in fig. 27 (right). The structure represented here is a two-layer one, and the oxide/silicon inte