Steven C. Witczak

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.

Hardness assurance testing of bipolar junction transistors at elevated irradiation temperatures

The effect of dose rate on radiation-induced current gain degradation was quantified for radiation-hardened poly-Si emitter n-p-n bipolar transistors over the range of 0.005 to 294 rad(Si)/s. Degradation increases sharply with decreasing dose rate and saturates near 0.005 rad(Si)/s. The amount of degradation enhancement at low dose rates decreases monotonically with total dose. In addition, the effect of ambient temperature on radiation-induced gain degradation at 294 rad(Si)/s was investigated over the range of 25 to 240/spl deg/C. Degradation is enhanced with increasing temperature while simultaneously being moderated by in situ annealing, such that, for a given total dose, an optimum irradiation temperature for maximum degradation results. The optimum irradiation temperature decreases logarithmically with total dose and, for a given dose, is smaller than optimum temperatures reported previously for p-n-p devices. High dose rate irradiation at elevated temperatures is less effective at simulating low dose rate degradation for the n-p-n transistor than for the p-n-p transistors. However, additional degradation of the n-p-n device at elevated temperatures is easily obtained using overtest. Differences in the radiation responses of the device types are attributed to the relative effects of oxide trapped charge on gain degradation. High dose rate irradiation near 125/spl deg/C is found to be suitable for the hardness assurance testing of these devices provided a design margin of at least two is employed.

Space charge limited degradation of bipolar oxides at low electric fields

P-type MOS capacitors fabricated in two bipolar processes were examined for ionizing radiation-induced threshold voltage shifts as a function of total dose, dose rate, temperature and bias. Hydrogen passivation of acceptor impurities near the Si surface was observed through decreases in the Si capacitance. The reduction in net electrically active dopants shifts the threshold voltage negative with total dose. The relative contribution of dopant passivation to the radiation-induced threshold voltage shift is most significant for irradiations performed under zero bias above 100/spl deg/C. For zero bias, dopant passivation and densities of radiation-induced interface traps and net positive oxide trapped charge all exhibit true dose rate and time dependent effects. A positive gate bias during irradiation eliminates the dose rate dependence. High dose rate irradiation at elevated temperatures enhances oxide degradation while simultaneously accelerating the annealing of damage. The enhancement in interface trap formation is greater than that of net positive oxide trapped charge and occurs over a greater range of temperatures. The temperature dependence of dopant passivation indicates that hydrogen transport through the oxides is accelerated with temperature. These results strongly suggest that metastably trapped charge in the oxide bulk reduces high dose rate degradation at room temperature by inhibiting the transport of holes and H/sup +/ ions.

Mechanisms for radiation dose-rate sensitivity of bipolar transistors

Mechanisms for enhanced low-dose-rate sensitivity are described. In these mechanisms, bimolecular reactions dominate the kinetics at high dose rates thereby causing a sub-linear dependence on total dose, and this leads to a dose-rate dependence. These bimolecular mechanisms include electron-hole recombination, hydrogen recapture at hydrogen source sites, and hydrogen dimerization to form hydrogen molecules. The essence of each of these mechanisms is the dominance of the bimolecular reactions over the radiolysis reaction at high dose rates. However, at low dose rates, the radiolysis reaction dominates leading to a maximum effect of the radiation.

Moderated degradation enhancement of lateral pnp transistors due to measurement bias

Enhanced low-dose-rate gain degradation of ADI RF25 lateral pnp transistors is examined as a function of the bias at which the gain is measured. Degradation enhancement at low dose rates diminishes rapidly with increasing measurement bias between the emitter and the base. Device simulations reveal that interface trap charging, field effects from oxide trapped charge and emitter metallization, base series resistance and high-level carrier injection all contribute to this behavior. As a practical consequence, accelerated hardness assurance tests of this device require higher irradiation temperatures or larger design margins for low power applications.

Enhanced low-dose-rate sensitivity of a low-dropout voltage regulator

Ionization-induced degradation of the 29372 low-dropout voltage regulator is most severe at low-dose-rate (/spl sim/10 mrad(SiO/sub 2/)/s) and zero load current. The most sensitive parameter is the maximum output drive current, which is a function of the gain of the large lateral pnp output transistor. Significant degradation of this parameter occurs at 5-10 krad(SiO/sub 2/) at low-dose-rate. A moderate load current (/spl sim/250 mA) during irradiation significantly mitigates the damage. The mitigation of the damage is proportional to irradiation load current and is not a strong function of irradiation temperature or input voltage. The mechanism for the mitigation of damage appears to be current density dependent passivation of interface and/or border traps by mobile hydrogen-related species. The worst-case space system application is in unbiased spares.

Impact of passivation layers on enhanced low-dose-rate sensitivity and pre-irradiation elevated-temperature stress effects in bipolar linear ICs

Final chip passivation layers are shown to have a major impact on the total dose hardness of bipolar linear technologies. It is found that devices fabricated without passivation layers do not exhibit enhanced low-dose-rate sensitivity (ELDRS) or pre-irradiation elevated-temperature stress (PETS) sensitivity, whereas devices from the same production lot fabricated with either oxide/nitride or doped-glass passivation layers are ELDRS and PETS sensitive. In addition, removing the passivation layers after fabrication can mitigate ELDRS and PETS effects. ELDRS and PETS effects do not appear to be inherently related to circuit design or layout, but are related to mechanical stress effects, hydrogen in the device, or a combination of the two. These results suggest that proper engineering of the final chip passivation layer might eliminate ELDRS and PETS effects in bipolar integrated circuits.

Radiation Hardness of

Semiconducting TiO2 displays non-volatile multi-state, hysteretic behavior in its I-V characteristics that can be exploited as a memory material in a memristive device. We exposed memristive TiO2 devices in the on and off resistance states to 45 Mrad(Si) of ~1-MeV gamma radiation and 23 Mrad(Si) of 941-MeV Bi-ions under zero bias conditions and none of the devices were degraded. These results suggest that TiO2 memristive devices are good candidates for radiation hard electronics for aerospace.

{\rm TiO}_{2}

Large differences in charge buildup in SOI buried oxides are observed for X-ray and Co-60 irradiations of SIMOX and Unibond transistors. The Co-60 response is typically worse than the X-ray response. These results are consistent with expectations derived from previous work on the relative charge yield versus field in thick oxides. The effects of bias configuration and substrate type on charge buildup and hardness assurance issues are explored via experiments and simulation. The worst-case bias condition is found to be either the off-state or transmission gate configuration. Simulations of the buried oxide electric field in the various bias configurations are used to illustrate the factors that affect charge transport and trapping in the buried oxides. Hardness assurance implications are discussed.

Memristive Junctions

A physical model is developed to quantify the contribution of oxide-trapped charge to enhanced low-dose-rate gain degradation in bipolar junction transistors. Multiple-trapping simulations show that space charge limited transport is partially responsible for low-dose-rate enhancement. At low dose rates, more holes are trapped near the silicon-oxide interface than at high dose rates, resulting in larger midgap voltage shifts. The additional trapped charge near the interface causes an exponential increase in excess base current and a resultant decrease in current gain for some NPN bipolar technologies. Space charge effects also may be responsible for differences in interface trap formation at low and high dose rates.