Silici Into layer dielectric 1 Silici layer dielectric 1 Silicid Inte layer dielectric 1 Oxide Oxide Oxide Oxide Oxide Oxide Oxide Oxide Oxide Oxide Poly Si Buried oxide N° e well N well P well Poly Si gate ench- Poly Si gate P. spilary p* substrate P substrate p substrate Figure 93. Types of SOI technologies compared to bulk silicon. For transistors processed on a thick silicon (e.g. tsi > 1 um) substrate ("bulk-like technologies"), no effect of parasitic buried transistor with ionizing dose is observed. In this case the hardness of an SOI technology is identical to bulk technology since buried oxide is moved away from Drain/Source [Davi-85, Plat-88, Flam-93, Flam-94]. For technology processed on relatively thin or intermediate-thin silicon (tsi ~ 2000 À), devices operate in partial depletion, and coupling effects with buried transistor are evolving with ionizing dose. This coupling with the effect of field of charge then trapped in the buried oxide obliges us to take into account the hardening of this type of oxide [Lera-85]. Transistors manufactured on very thin silicon (tsi ~ 800 À or less) work in total depletion regime, and this mode is directly controlled by the electric field created by charges which are present in the buried oxide. Thus, the behavior of the buried oxide is of extreme importance, especially for applications where high ionizing dose level is encountered [Ferl-96]. The total dose hardness of an SOI technology depends primarily on the radiation response of the same kinds of oxides as in bulk technologies: gate oxide and lateral isolation. This lateral isolation can be LOCOS or Trench, as in bulk silicon. Another system of lateral isolation specific to SOI is Mesa, which can be view as a Trench without refill. And finally, the ionizing dose response of MOS/SOI transistors can also depends on Buried Oxide, as is descibed in the following subsection. 10.3. PARASITICS IN A THIN-FILM SOI TECHNOLOGY Once a SOI structure is irradiated, charge trapping in the buried oxide gives birth to a parasitic 