F. N. Coppage

Current Induced Avalanche in Epitaxial Structures

A correlation is made between observed photoionization induced avalanche breakdown in epitaxial structures and the analysis of high-current effects in these devices using Poissons equation. The analysis shows that a photocurrent-stimulated conductivity modulation mechanism can lead to avalanche at the epitaxial-substrate junction at bias levels far below the usual breakdown voltages for the structures. Experimental data are presented for both VDMOS power-FET devices and bipolar npn epitaxial transistors which show junction avalanche at low bias levels.

Gamma-Ray and Neutron-Induced Conductivity in Insulating Materials

Excess conductivities induced by steady-state sources of gamma rays and by pulsed sources of neutrons and gamma rays in polyethylene, polystyrene, polypropylene, Nylon, polyisobutylene (impregnated paper), mylar, Teflon, diallylphthalate, H-film, cellulose acetate, reconstituded mica, tantalum oxide, and an epoxy formulation have been measured. The measurements were made at steady-state gamma-ray dose rates within the interval from 1.0 × 10-3 rads(H2O)/sec to 1.0 × 104 rads(H2O)/sec, and at combined pulsedneutron and gamma-ray dose rates less than 2.0 × 108 rads(H2O)/sec. All measurements were made at controlled temperatures between 25°C and 71°C. With steady-state gamma-ray irradiation, an excess conductivity is induced which has distinct features in three time intervals denoted as A, B, and C. In interval A, induced conductivity (? - ?o) is responding to a step increase in gamma-ray dose rate. The conductivity response is exponential (? - ?o) = A(1 - e-t/?o), with the time constant (?o) decreasing with increased gamma-ray dose rare (?). The change in time constant as a function of gamma-ray dose rate at a fixed temperature is approximated by ?o = ko?-? where ko and ? are empirical constants. In interval B the induced conductivity has arrived at an equilibrium value whose magnitude as a function of gamma-ray dose rate at a fixed temperature is characterized to a good approximation by (? - ?o) = A???, where A? and ? are empirical constants. In interval C the conductivity is recovering upon removal of the sample from the radiation environment.

The Influence of Dosimetry on Earlier Damage Equivalence Ratios

Since it was first noted that the experimental damage equivalence ratios for different neutron spectra obtained from change in transistor parameter behavior did not agree with the calculated ratios based on damage to silicon, an explanation has been sought for this apparent discrepancy. Investigations have included device processing techniques device geometry, and finally dosimetry. Careful dosimetry is of the utmost importance to these ratio measurements. Recently extensive studies of the White Sands Fast Burst Reactor (FBR), which is identical to the Sandia Pulsed Reactor (SPR-II), yielded information which suggested that a significant error was possible in the SPR-II dosimetry utilized in earlier studies. A test group at SAl726 (2N4251) transistors were used to make a comparison of the spectrum from the FBR and the SPR-II. The results of the device study showed a difference of 25-30 percent in fluence measured by Sandia dosimetry and by White Sands dosimetry for a given FBR exposure. This difference was for a 10 KeV energy threshold. These findings prompted an investigation of the Sandia dosimetry techniques used at SPR-II. This investigation showed, on the basis of dosimetry measurements excluding fission foils, that the 10 KeV/3 MeV ratio for SPR-II was 7.5 instead of 10 as presently used. The correction in dosimetry from SPR-II when applied to earlier damage equivalence ratios changes the 14 MeV-to-SPR-II damage ratio from 4.2 to approximately 2.9. This value (2.9) is in good agreement with other experimenters and calculations based on displacement damage in silicon.

SAND83-1541C Seeing through the Latch-Up Window

The observation of radiation-induced latch-up windows has caused doubt in the validity of accepted latch-up screens. Models giving a plausible explanation of latch-up windows are presented which show that they are only a special case of latch-up as explained by many investigators and can be treated accordingly. Furthermore, they are eliminated when latch-up is prevented by either neutron irradiation of the devices or utilization of epitaxial substrates.

Field Oxide Inversion Effects in Irradiated CMOS Devices

The primary failure mechanism of CMOS devices in an ionizing radiation environment is a threshold voltage shift in both the p-channel and the n-channel devices due to a buildup of radiation-induced fixed charge in the silicon dioxide and the creation of surface states at the silicon-silicon dioxide interface. It has been observed that many CMOS device types exhibit milliampere range post-rad n-channel leakage for doses between 104 - 105 rad (Si) even though the gate oxide threshold shift is much less than that required for inversion in the channel region. This is caused by field oxide inversion under the gate metal due to improper or a lack of guardbanding of the n-channel device for a radiation environment. The solution to this problem in hardening CMOS devices involves guardbanding the n-channel devices and extending the gate oxide under the gate metallization over the guardband. Experimental results on devices which utilize this technique has shown it to be an effective solution to the problem. Data on several types of CMOS devices are presented to illustrate the n-channel leakage problem and the solution. Special guardbanding problems encountered with silicon gate on bulk Silicon devices are also described and possible solutions to these problems are discussed.

Dependence of ionizing radiation induced h FE degradation on emitter periphery

Devices which differ only iin emitter periphery were subjected to ionizing radiation to investigate the influence of emitter periphery on hFE degradation. The data indicate that, within reasonable limits, the increase in base current from ionizing radiation is directly proportional to the length of the emitter periphery. However, differences in fabrication techniques, passivation materials and processing, and silicon crystal orientation may very well have a greater influence on the degradation from ionizing radiation than the emitter periphery.

Device Degrdation from the Effects of Nuclear Radiation on Passivation Materials

Devices that utilized three different commercial passivation processes were studied in a nuclear radiation environment to determine if the passivation process influences the radiation tolerance of the device. Comparisons were made between devices obtained from the same manufacturer. The passivation processes studied featured silicon dixoide, silicon nitride and aluminum oxide. Irradiations were performed with the devices in both a biased and unbiased mode. The results show that the device with silicon dioxide passivation degraded the most. The nitride devices having beam leads showed no saturation of the induced degradation while the other processes did for a dose of 1 × 107 (Si). All devices studied showed a small bias dependence. The addition of an aluminum oxide or silicon nitride passivation layer (over the silicon dioxide surface) for an increased reliability definitely induces no increased radiation degradation.

Photoconductivity Processes in Low Mobility Organic Materials

Recent results obtained on anthracene crystals make it possible to calculate the number of carriers created in polymers when they are subjected to high energy radiation. These results are described and applied to some recently reported measurements on polystyrene. Reasonable values are found for such quantities as carrier mobility and trap densities.

MOS-Transistor Radiation Detectors and X-Ray Dose-Enhancement Effects

Sandia National Laboratory (SNL) CMOS IC dose detectors and 3N161 MOS Transistors were evaluated as pulsed X-radiation dosimeters and used as monitors to measure doseenhancement effects. Measurements were made in the photon environments from the HydraMITE II, SPR III, MBS and PITHON radiation sources. The dosimeter evaluation data suggest that the 3N161 MOS transistors are useful dosimeters for measuring flash X-ray-induced doses in the oxide layers of modern metal-oxide-semiconductor (MOS) integrated circuits. However, doseenhancement calculations indicate that Monte Carlo codes, using 1-D geometries and calculated source spectra, consistently overpredict measured dose-enhancement ratios by factors as large as two.

Characteristics of Destruction from Latch-Up in CMOS

It is well known that CMOS small scale integrated (SSI) circuits will experience latch-up, when subjected to a high dose rate of ionizing radiation. However, the time required for latch-up and the characteristics of the subsequent behavior causing destruction of the device are not well known. The objectives of this study were (1) to characterize the latch-up time dependence, (2) to determine the sequence of events leading to destruction of the device, and (3) to characterize the destruction of the device. In keeping with these objectives, it was found that (1) the device does not actually latch for > 100 nanoseconds at the minimum dose rate necessary for latch-up, (2) the input protection diodes are usually involved as part of the parasitic four-layer equivalent of silicon-controlled rectifier (SCR), (3) the metallization of the VSS line (lowest potential of power source) is most orten destroyed, (4) contact window resistance increase is a prelude to metallization destruction, and (5) metallization destruction requires hundreds of microseconds.