P. V. Dressendorfer

Hole traps and trivalent silicon centers in metal/oxide/silicon devices

We report electron spin resonance (ESR) measurements of E′‐center (a ‘‘trivalent silicon’’ center in SiO2) density as well as capacitance versus voltage (C‐V) measurements on γ‐irradiated metal/oxide/silicon (MOS) structures. We also report a considerable refinement of earlier ESR measurements of the dependence of radiation‐induced Pb ‐center (a ‘‘trivalent silicon’’ center at the Si/SiO2 interface) occupation as a function of the Fermi level at the Si/SiO2 interface. These measurements indicate that the Pb centers are neutral when the Fermi level is at mid‐gap. Since the Pb centers are largely responsible for the radiation‐induced interface states, one may take ΔVmg Cox/e (where ΔVmg is the ‘‘mid‐gap’’ C‐V shift, Cox is the oxide capacitance, and e is the electronic charge) as the density of holes trapped in the oxide. We find that radiation‐induced E′ density equals ΔVmg Cox/e in oxides grown in both stream and dry oxygen. Etch‐back experiments demonstrate that the E′ centers are concentrated very near ...

Correlating the Radiation Response of MOS Capacitors and Transistors

A new technique is presented for separating the threshold-voltage shift of an MOS transistor into shifts due to interface states and trapped-oxide charge. Using this technique, the radiation responses of MOS capacitors and transistors fabricated on the same wafer are compared. A good correlation is observed between p-substrate capacitors and n-channel transistors irradiated at 10 V, as well as between n-substrate capacitors and p-channel transistors irradiated at 0 V. These correlations were verified for samples having large variations in the amount of radiation-induced trapped holes and interface states. An excellent correlation is also observed between n-channel capacitors and n-substrate transistors irradiated under positive bias. The use of capacitors separately fabricated on control wafers for potential use in process development or monitoring is clearly demonstrated.

Physical Mechanisms Contributing to Device "Rebound"

The physical mechanisms that produce rebound have been identified. The positive increase in threshold voltage during a bias anneal is due to annealing of oxide trapped charge. Rebound can be predicted by measuring the contribution to the threshold voltage from radiation-induced interface states immediately after irradiation.

Device modeling of ferroelectric capacitors

A physically based methodology is developed for modeling the behavior of electrical circuits containing nonideal ferroelectric capacitors. The methodology is illustrated by modeling the discrete ferroelectric capacitor as a stacked dielectric structure, with switching ferroelectric and nonswitching dielectric layers. Electrical properties of a modified Sawyer–Tower circuit are predicted by the model. Distortions of hysteresis loops due to resistive losses as a function of input signal frequency are accurately predicted by the model. The effect of signal amplitude variations predicted by the model also agree with experimental data. The model is used as a diagnostic tool to demonstrate that cycling degradation, at least for the sample investigated, cannot be modeled by the formation of nonswitching dielectric layer(s) or the formation of conductive regions near the electrodes, but is consistent with a spatially uniform reduction in the number of switching dipoles.

An electron spin resonance study of radiation‐induced electrically active paramagnetic centers at the Si/SiO2 interface

We have compared the generation of radiation‐induced Pb (‘‘trivalent silicon’’) centers at the Si/SiO2 interface with the radiation‐induced buildup of interface states. We observe a strong correlation between the density of Pb centers and radiation‐induced interface state density (Dit) and similar annealing behavior of radiation‐induced Pb and Dit. Furthermore, the Pb resonance intensity is strongly bias dependent; this indicates that the charge state of the Pb defect is bias dependent. We conclude that Pb defects account for a very large portion of radiation‐induced interface states.

Effect of bias on radiation‐induced paramagnetic defects at the silicon‐silicon dioxide interface

Electron spin resonance measurements have been made on gamma‐irradiated (111) Si/SiO2 structures as a function of bias across the oxide. We observe a large change in the density of radiation‐induced paramagnetic Pb centers with bais. We conclude that Pb defects (trivalent silicons at the Si/SiO2 interface) account for a very large portion of the radiation‐induced interface states.

The Role of Hydrogen in Radiation-Induced Defect Formation in Polysilicon Gate MOS Devices

The role of hydrogen in the generation of radiation-induced interface-trap and oxide-trapped charge in MOS polysilicon gate capacitors has been investigated. The concentration of radiation-induced interface-trap and oxide-trapped charge measured both immediately after irradiation and after postirradiation anneal increases if high temperature anneals are performed in hydrogen. We have analyzed these results in the context of several models of interface-trap and oxide-trapped charge formation. The mutual increase in the concentration of oxide-trapped charge and the early-time (1 msec to 10 sec) component of interface-trap charge with the amount of hydrogen used during processing suggests that the breaking of Si-H or Si-OH bonds may be responsible for much of the defect formation at or near the silicon/silicon dioxide interface.

Total-Dose Radiation and Annealing Studies: Implications for Hardness Assurance Testing

A series of experiments covering a wide range of dose rate, bias, and annealing conditions has been performed on CMOS test transistors and 2K SRAMs. These experiments, on both hardened and commercial technologies, were designed to address hardness assurance issues associated with total-dose laboratory testing. It is demonstrated that failure dose can be a complicated function of dose rate, and that a peak in the failure-dose versus dose-rate curve generally results when there is a change in failure mode. If only one failure mode exists, then the failure-dose versus doserate curve is monotonic. Implications of proposed changes in MIL-STD-883C, Method 1019.2 are examined in light of their impact on hardness assurance. Our findings support the proposed changes of (1) keeping the time between irradiation and test less than 1 hour and (2) of a more restricted range of dose rate, i.e., 100 to 300 rad(Si)/ s. In addition, it is recommended that zero volt bias be maintained on CMOS devices between irradiation and test. Finally, techniques are presented for relating total-dose hardness as measured in the laboratory to total-dose hardness in real-world space and weapon environments.

Paramagnetic trivalent silicon centers in gamma irradiated metal‐oxide‐silicon structures

We find that two paramagnetic ‘‘trivalent silicon’’ centers appear to be primarily responsible for radiation damage in metal‐oxide‐silicon structures.

Error Analysis and Prevention of Cosmic Ion-Induced Soft Errors in Static CMOS RAMs

Cosmic ray interactions with memory cells are known to cause temporary, random, bit errors in some designs. The sensitivity of polysilicon gate CMOS static RAM designs to logic upset by impinging ions has been studied using computer simulations and experimental heavy ion bombardment. Results of the simulations are confirmed by experimental upset cross-section data. Analytical models have been extended to determine and evaluate design modifications which reduce memory cell sensitivity to cosmic ions. A simple design modification, the addition of decoupling resistance in the feedback path, is shown to produce static RAMs immune to cosmic ray-induced bit errors.