Daniel M. Fleetwood

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 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 tr...

Radiation Effects in MOS Oxides

Electronic devices in space environments can contain numerous types of oxides and insulators. Ionizing radiation can induce significant charge buildup in these oxides and insulators leading to device degradation and failure. Electrons and protons in space can lead to radiation-induced total-dose effects. The two primary types of radiation-induced charge are oxide-trapped charge and interface-trap charge. These charges can cause large radiation-induced threshold voltage shifts and increases in leakage currents. Two alternate dielectrics that have been investigated for replacing silicon dioxide are hafnium oxides and reoxidized nitrided oxides (RNO). For advanced technologies, which may employ alternate dielectrics, radiation-induced voltage shifts in these insulators may be negligible. Radiation-induced charge buildup in parasitic field oxides and in SOI buried oxides can also lead to device degradation and failure. Indeed, for advanced commercial technologies, the total-dose hardness of ICs is normally dominated by radiation-induced charge buildup in either parasitic field oxides and/or SOI buried oxides. Heavy ions in space can also degrade the oxides in electronic devices through several different mechanisms including single-event gate rupture, reduction in device lifetime, and large voltage shifts in power MOSFETs.

'Border traps' in MOS devices

The author recommends that the terminology for oxide charges developed in 1979 be updated to include near-interfacial oxide traps that communicate with the underlying Si and that these defects collectively be called border traps. Justification for this nomenclature is presented and defining features of border traps are discussed. Border traps play an important role in determining low-frequency (1/f) noise levels in metal-oxide-semiconductor (MOS) transistors and also appear to have been observed in recent spin-dependent recombination studies on irradiated devices at microwave frequencies. This terminology is intended to add focus to discussions of defect type and location in MOS structures. >

Trends in the total-dose response of modern bipolar transistors

The excess base current in an irradiated BJT increases superlinearly with total dose at low-total-dose levels. In this regime, the excess base current depends on the particular charge-trapping properties of the oxide that covers the emitter-base junction. The device response is dose-rate-, irradiation-bias-, and technology-dependent in this regime. However, once a critical amount of charge has accumulated in the oxide, the excess base current saturates at a value that is independent of how the charge accumulated. This saturated excess base current depends on the device layout, bulk lifetime in the base region, and the measurement bias. In addition to providing important insight into the physics of bipolar-transistor total-dose response, these results have significant circuit-level implications. For example, in some circuits, the transistor gain that corresponds to the saturated excess base current is sufficient to allow reliable circuit operation. For cases in which the saturated value of current gain is acceptable, and where other circuit elements permit such over-testing, this can greatly simplify hardness assurance for space applications. >

Charge separation for bipolar transistors

The effects of the midgap-level interface trap density and net oxide charge on the total-dose gain degradation of a bipolar transistor are separately identified. The superlinear dose dependence of the excess base current is explained.

Border traps: Issues for MOS radiation response and long-term reliability

Abstract We have performed an extensive study of the effects of border traps (near-interfacial oxide traps that can communicate with the underlying Si over a wide range of time scales) on the response of metal-oxide-semiconductor (MOS) devices to ionizing radiation. Estimates of border-trap densities for several types of MOS devices are obtained by capacitance-voltage (CV) hysteresis, 1 f noise, and combined CV/thermally-stimulated-current methods. A new “dual-transistor border-trap” (DTBT) technique is described in detail which combines conventional threshold-voltage and 1-MHz charge-pumping measurements on n- and p- channel MOS transistors to estimate radiation-induced oxide-, interface-, and border-trap charge densities. Estimates of border-trap charge densities obtained via the DTBT technique agree well with trap densities inferred from other techniques. In some devices, border-trap charge densities (which can be greater than 1012 cm−2 following ionizing radiation exposure) can approach or exceed interface-trap charge densities, emphasizing the need to distinguish border-trap effects from interface-trap effects in models of MOS radiation response and long-term reliability. This appears to be especially critical for MOS devices with ultrathin (less than ∼6 nm) oxides, in which border traps and interface traps likely will be the dominant defect types. Effects of border traps on MOS scattering rates, cryogenic applications, and long-term reliability assessment are also discussed.

Charge yield for cobalt-60 and 10-keV X-ray irradiations of MOS devices

The radiation response of MOS devices exposed to /sup 60/Co and low-energy ( approximately 10 keV) X-ray irradiation is evaluated as a function of electric field during exposure. Improved charge yield estimates are obtained for /sup 60/Co irradiations at fields below 1 MV/cm by matching voltage shifts due to oxide-trap and interface-trap charge to an E/sup -0.55/ electric field dependence. Combining these improved charge yield estimates and calculated dose enhancement factors, the relative response of X-ray to /sup 60/Co irradiations is accurately predicted for oxide electric fields from 0.03 MV/cm to 5.0 MV/cm. The ability to predict the relative response to X-ray to /sup 60/Co irradiations-should speed acceptance of X-ray testers as a hardness assurance tool. >

Physical model for enhanced interface-trap formation at low dose rates

We describe a model for enhanced interface-trap formation at low dose rates due to space-charge effects in the base oxides of bipolar devices. The use of analytical models allows one to reduce significantly the number of free parameters of the theory and to elucidate the main physical mechanisms that are responsible for interface-trap and oxide-trap formation processes. We found that the hole trapping in the oxide cannot be responsible for all the enhanced low-dose-rate sensitivity (ELDRS) effects in SiO/sub 2/, and the contribution of protons is also essential. The dynamics of interface-trap formation are defined by the relation between the proton mobility (transport time of the protons across the oxide) and the time required for positive-charge buildup near the interface due to trapped holes. The analytically estimated and numerically calculated interface-trap densities were found to be in very good agreement with available experimental data.

1/f noise and radiation effects in MOS devices

An extensive comparison of the 1/f noise and radiation response of MOS devices is presented. Variations in the room-temperature 1/f noise of unirradiated transistors in the linear regime of device operation correlate strongly with variations in postirradiation threshold-voltage shifts due to oxide trap charge. A simple number fluctuation model has been developed to semi-quantitatively account for this correlation. The 1/f noise of irradiated n-channel MOS transistors increases during irradiation with increasing oxide-trap charge and decreases during postirradiation positive-bias annealing with decreasing oxide-trap charge. No such correlation is found between low-frequency 1/f noise and interface-trap charge. The noise of irradiated p-channel MOS transistors also increases during irradiation, but in contrast to the n-channel response, the p-channel transistor noise magnitude increases during positive-bias annealing with decreasing oxide-trap charge. A qualitative model involving the electrostatic charging and discharging of border traps, as well as accompanying changes in trap energy, is developed to account for this difference in n- and p-channel postirradiation annealing response. The correlation between the low-frequency 1/f noise of unirradiated devices and their postirradiation oxide-trap charge suggests noise measurements can be used as a nondestructive screen of oxide trap charge related failures in discrete MOS devices and for small scale circuits in which critical transistors can be isolated. It also suggests that process techniques developed to reduce radiation-induced-hole trapping in MOS devices can be applied to reduce the low-frequency 1/f noise of MOS circuits and devices. In particular, reducing the number of oxygen vacancies and vacancy complexes in the SiO/sub 2/ can significantly reduce the 1/f noise of MOS devices both in and outside a radiation environment. >

Field dependence of interface-trap buildup in polysilicon and metal gate MOS devices

The electric field dependence of radiation-induced oxide- and interface-trap charge ( Delta V/sub ot/ and Delta V/sub it/) generation for polysilicon- and metal-gate MOS transistors is investigated at electric fields (E/sub ox/) from -4.2 MV/cm to +4.7 MV/cm. If electron-hole recombination effects are taken into account, the absolute value of Delta V/sub ot/ and the saturated value of Delta V/sub it/ for both polysilicon- and metal-gate transistors are shown to follow an approximate E/sup -1/2/ field dependence for E/sub ox/>or=0.4 MV/cm. An E/sup -1/2/ dependence for the saturated value of Delta V/sub it/ was also observed for negative-bias irradiation followed by a constant positive-bias anneal. The E/sup -1/2/ field dependence observed suggests that the total number of interface traps created in these devices may be determined by hole trapping near the Si/SiO/sub 2/ interface for positive-bias irradiation or near the gate/SiO/sub 2/ interface for negative bias irradiation, though H/sup +/ drift remains the likely rate-limiting step in the process. Based on these results, a hole-trapping/hydrogen transport model-involving hole trapping and subsequent near-interfacial H/sup +/ release, transport, and reaction at the interface-is proposed as a possible explanation of Delta V/sub it/ buildup in these polysilicon- and metal-gate transistors. >