D. B. Brown

Interface trap formation via the two-stage H/sup +/ process

The time dependence and oxide-field dependence of interface trap (N/sub it/) formation in MOSFETs have been studied following pulsed ionizing radiation. Results are compared with the so-called two-stage model for N/sub it/ formation involving slow drift of radiation-induced H/sup +/ ions in the SiO/sub 2/. Detailed data on the gate-oxide-field dependence during each individual stage are presented and discussed. A model is developed for the production of H/sup +/ throughout the oxide. Calculations based on this model correctly predict the complete time-dependent N/sub it/ formation curves. It is also shown that N/sub it/ formation is at a maximum near zero first-stage gate bias. This unexpected behavior apparently arises from the oxide-field dependence of the H/sup +/ production during the first stage. >

Time dependence of radiation-induced interface trap formation in metal-oxide-semiconductor devices as a function of oxide thickness and applied field

This work is a study of the formation mechanisms of interface traps (Nit) in metal‐oxide‐semiconductor devices. The time‐dependence of the Nit formation has been measured as a function of oxide thickness following a short radiation pulse. The Nit formation time is found to increase as t2.6ox when the gate bias is negative during irradiation and positive afterward. This result is in excellent agreement with predictions of a hydrogen transport model where drift of hydrogen ions (H+) is the rate‐limiting step. When the gate bias during irradiation is positive, interpretation of the correlation between data and model suggests that the hydrogen ions are preferentially created near the Si‐SiO2 interface. Finally, the Nit formation time is found to decrease with increasing oxide field as E−1.73ox. This result is compatible with the hydrogen transport model if the average displacement per hop is assumed to be proportional to Em.

Time dependence of interface trap formation in MOSFETs following pulsed irradiation

The time dependence of interference trap (N/sub it/) formation in MOSFETs was studied as a function of gate oxide thickness, oxide growth type, substrate orientation, temperature, and gate bias. Two different N/sub it/ formation mechanisms are observed. Most (typically 90%) of the formation, called the late process, occurs slowly at long times (1-10000 s) after the radiation pulse. From a variety of experimental data, it is concluded that the rate of the late process is limited by drift of a radiation-induced positive ion, probably H/sup +/, through the gate oxide to the Si-SiO/sub 2/ interface where the N/sub it/ are formed. A relatively fast, or early, process is responsible for a small percentage of the total N/sub it/ formation. The time constant for this process appears to be consistent with hole drift through the oxide. >

An Evaluation of Low-Energy X-Ray and Cobalt-60 Irradiations of MOS Transistors

An evaluation of methodologies for irradiating MOS transistors with low-energy x-ray and Co-60 sources has been performed. We find that comparisons of voltage shifts produced by bulk trapped charge and interface states in MOS transistors irradiated using two different low energy x-ray sources (an ARACOR 10 keV W source and an 8 keV Cu source) agree to within better than 30 percent. This quality of agreement is similar in magnitude to that between MOS devices irradiated by different Co-60 sources. In contrast, the measurements indicate that interlaboratory comparisons of ratios of shifts produced by x-ray and Co-60 sources can lead to differences in ratios as large as a factor of ~1.7. Improved electron-hole recombination data for oxides is presented. This recombination correction, in conjunction with a correction for interface dose enhancement, is used to predict the ratios of shifts produced by x-ray and Co-60 sources. However, the results show that corrections for electron-hole recombination and interface dose enhancement do not, by themselves, adequately predict the field dependent behavior of these transistors.

Study of low-dose-rate radiation effects on commercial linear bipolar ICs

The results of a detailed study of the degradation of commercial linear bipolar ICs due to irradiation at four dose rates are presented. The time dependence of the degradation rate at the different dose rates is shown to be consistent with a model that describes a mechanism for defect generation in the devices used in this study. Based on this model, an accelerated test procedure for bipolar devices is proposed.

A proposed hardness assurance test methodology for bipolar linear circuits and devices in a space ionizing radiation environment

A hardness assurance test approach has been developed for bipolar linear circuits and devices in space. It consists of an initial test for dose rate sensitivity and a characterization test method to develop the conditions for a lot acceptance test at high dose rate. For parts with adequate design margin and/or well behaved parts a generic elevated temperature irradiation test is proposed.

Observation of H/sup +/ motion during interface trap formation

The time dependence of changes in the oxide trapped charge during interface trap formation is investigated. Changes in MOSFET threshold voltage V/sub th/ and number of interface traps N/sub it/ are measured in the same sample as a function of time following pulsed irradiation. When the gate bias during irradiation V/sub gl/ is positive, the initial mod Delta V/sub th/ mod is large due to trapping of radiation-induced holes at the Si-SiO/sub 2/ interface and the postirradiation time dependence of Delta V/sub th/ is dominated by hole detrapping, as expected. When V/sub gl/ is negative, interfacial hole trapping is minimized. In this case, an unusual peak in the Delta V/sub th/ vs. time curve provides evidence of the involvement of H/sup +/ ions in the N/sub it/ formation process. >

Comparison of enhanced device response and predicted X-ray dose enhancement effects on MOS oxides

The response of MOS capacitors to low- and medium-energy X-ray irradiation is investigated as a function of gate material (TaSi or Al), oxide thickness, and electric field. The measured device response is compared to the predicted response. In comparisons of 10-keV X-ray and Co-60 irradiations of Al-gate MOS capacitors at an oxide electric field of 1 MV/cm, predictions and experiments agree to within better than 20% for oxide thicknesses ranging from 35 to 1060 nm. For capacitors with TaSi/Al gates, they agree to within better than 30% at 1 MV/cm, with the largest differences occurring for 35-nm gate oxides. At other electric fields, the disagreement between experiment and prediction increases significantly for both Al- and TaSi/Al-gate capacitors. For medium-energy ( approximately 100-keV average photon energy) X-irradiations, the enhanced device response exhibits a much stronger dependence on endpoint bremsstrahlung energy than expected from TIGERP or CEPXS/ONETRAN simulations. Implications for hardness assurance testing are discussed. >

Photon Energy Dependence of Radiation Effects in MOS Structures

MOS capacitors with oxide thicknesses of 750Å, 3500Å and 6000Å were irradiated using a Co60 source and a Cu target x-ray tube. At low fields across the oxides (¿1MV/cm), shifts in the flatband voltages observed with Co60 were twice those measured with the Cu tube at the same oxide dose. At higher fields (>1MV/cm) the differences disappear. The observations are interpreted to be due to differences in the electron-hole recombination dynamics for the two radiation energies. Additionally, it was observed that the Si-SiO2 interface states trap holes with an efficiency that decreases as the square root of the electric field across the oxide.

Effect of Photon Energy on the Response of MOS Devices

Radiation-produced flatband voltage shifts in MOS capacitors have been measured as a function of incident photon energy and applied electric fields with UV, x- and gamma-ray sources spanning the energy range 70 eV to 1.25 MeV. Special interest was directed to the energies below 20 keV where the greatest effects on the flatband voltages were expected. At 70 eV the shifts are almost as great as those observed at 1.25 MeV (60Co). For 1.49 keV incident photons the voltage shifts are less than 1/3 those observed for 60Co photons.