We have identified several features of the 1/f noise and radiation response of metal-oxide-semiconductor (MOS) devices that are difficult to explain with standard defect models. To address this issue, and in response to ambiguities in the literature, we have developed a revised nomenclature for defects in MOS devices that clearly distinguishes the language used to describe the physical location of defects from that used to describe their electrical response. In this nomenclature, ‘‘oxide 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 underlying Si. Electrically, ‘‘fixed states’’ are defined as trap levels that do not communicate with the Si on the time scale of the measurements, but ‘‘switching states’’ can exchange charge with the Si. Fixed states presumably are oxide traps in most types of measurements, but switching 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 density 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 charge in non-radiation-hardened MOS transistors, and to changes in the 1/f noise of MOS devices through irradiation and elevated-temperature annealing. Border-trap densities of ∼1010–1011 cm−2 are inferred from changes in switching-state density during postirradiation 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 annealing in MOS capacitors with radiation-hardened oxides through steady-state and switched-bias postirradiation annealing. Trapped-hole, trapped-electron, and switching-state densities are inferred via thermally stimulated current and capacitance-voltage measurements. A lower bound of ∼3×1011 cm−2 is estimated for the effective density of border traps that contribute to the electrical response of the irradiated devices. This is roughly 20% of the observed switching-state density for these devices and irradiation conditions. To our knowledge, this represents the first quantitative separation of measured switching-state densities into border-trap and interface-trap components. Possible physical models of border traps are discussed. E′ centers in SiO2 (trivalent Si centers associated with oxygen vacancies) may serve as border traps in many irradiated MOS devices.
Chemical downstream etching of silicon nitride (Si3N4) requires the addition of nitrogen to the discharge for obtaining efficient etch rates. A 10% addition of N2 to a CF4/O2 discharge (CF4/O2 = 1.2, 0.525 Torr) causes a factor of 6 increase in the Si3N4 etch rate and a 8% decrease in the silicon dioxide etch rate. The result is selectivities approaching 9:1. Importantly, the conversion of CF4 to F and F-containing reactive species by the discharge decreases or remains constant as nitrogen is added to the discharge mix, indicating that the etching reaction is not limited by delivery of these species to the substrate. By measuring the amount of NO and NO2 in the etch chamber, it is found that the NO concentration increases by a factor of 6 as N2 is added, while the amount of NO2 remains small and constant. The NO signal is significantly reduced during nitride etching compared to the signal observed during a discharge with an empty etch chamber, implying that the increase in Si3N4 etch rate is related to the formation of NO in the discharge. This view is consistent with the observation that an NF3 plasma in a quartz discharge tube results in a nitride etch rate which is a factor of 2 higher than for the same discharge in a sapphire tube. The conclusion is that the oxygen liberated by erosion of the quartz tube allows the formation of NO. That NO is a key Si3N4 etch reactant was confirmed by performing a series of experiments in which N2, NO, NO2, and N2O were injected into the discharge and then downstream in the reaction chamber during a CF4/O2 discharge. Nitride etch rates increased significantly upon injection of NO into both discharge and etch chamber as compared to injection of the other NxOy species.
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