EFFECTS OF OXIDE TRAPS, INTERFACE TRAPS, AND BORDER TRAPS ON METAL-OXIDE-SEMICONDUCTOR DEVICES

We have identified several features of the 1/f noise and radiation res ponse of metal-oxide-semiconductor (MOS) devices that are difficult to explain with standard defect models. To address this issue, and in re sponse to ambiguities in the literature, we have developed a revised n omenclature for defects in MOS devices that clearly distinguishes the language used to describe the physical location of defects from that u sed to describe their electrical response. In this nomenclature, ''oxi de traps'' are simply defects in the SiO2 layer of the MOS structure, and ''interface traps'' are defects at the Si/SiO2 interface. Nothing is presumed about how either type of defect communicates with the unde rlying Si. Electrically, ''fixed states'' are defined as trap levels t hat do not communicate with the Si on the time scale of the measuremen ts, but ''switching states'' can exchange charge with the Si. Fixed st ates presumably are oxide traps in most types of measurements, but swi tching states can either be interface traps or near-interfacial oxide traps that can communicate with the Si, i.e., ''border traps'' [D. M. Fleetwood, IEEE Trans. Nucl. Sci. NS-39, 269 (1992)]. The effective de nsity of border traps depends on the time scale and bias conditions of the measurements. We show the revised nomenclature can provide focus to discussions of the buildup and annealing of radiation-induced charg e in non-radiation-hardened MOS transistors, and to changes in the 1/f noise of MOS devices through irradiation and elevated-temperature ann ealing. Border-trap densities of approximately 10(10)-10(11) cm-2 are inferred from changes in switching-state density during postirradiatio n annealing, and from a simple trapping model of the 1/f noise in MOS devices. We also present a detailed study of charge buildup and anneal ing in MOS capacitors with radiation-hardened oxides through steady-st ate and switched-bias postirradiation annealing. Trapped-hole, trapped -electron, and switching-state densities are inferred via thermally st imulated current and capacitance-voltage measurements. A lower bound o f approximately 3 X 10(11) cm-2 is estimated for the effective density of border traps that contribute to the electrical response of the irr adiated devices. This is roughly 20% of the observed switching-state d ensity for these devices and irradiation conditions. To our knowledge, this represents the first quantitative separation of measured switchi ng-state densities into border-trap and interface-trap components. Pos sible physical models of border traps are discussed. E' centers in SiO 2 (trivalent Si centers associated with oxygen vacancies) may serve as border traps in many irradiated MOS devices.