C. M. Dozier

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.

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.

Sensitivity of x‐ray film. II. Kodak No‐Screen film in the 1–100‐keV region

The absolute sensitivity of Kodak No‐Screen x‐ray film was measured for 11 photon energies between 1 and 8 keV using filtered fluorescent radiation. The shapes of the density‐vs‐exposure curves were observed to be a function of the photon energy. This calibration was demonstrated to be consistent with two earlier calibrations of No‐Screen response for photon energies between 5 and 100 keV and with a theoretical model of film sensitivity by Brown, Criss, and Birks.

X radiation from high‐energy‐density exploded‐wire discharges

Exploded‐wire discharges of tungsten and titanium driven by a high‐power pulse generator have been used to produce intense x‐ray continuum and line radiation. A calibrated LiF crystal spectrograph recorded the radiation spectrum in the 3‐ to 25‐keV range. More than 20 J of x radiation are emitted in this photon energy band by tungsten plasmas in less than 50 nsec. The source of emission is less than 1 mm in diameter and about 3.5 cm long.

The Use of Low Energy X-Rays for Device Testing - A Comparison with Co-60 Radiation

Low (~10 keV) energy x-ray sources have been proposed as alternatives to Co-60 for device testing. Several effects caused by differences in the photon energies of the two types of sources are evaluated. Quantitative estimates of the magnitude of these effects and other factors which should be considered in setting up test protocols are presented. Several cases are defined where the differences in the effects caused by x-rays and Co-60 are expected to be small. Other cases where the differences may be as great as a factor of five are described. Lateral spreading of the collimated x-ray beam beyond the desired irradiated chip region is also discussed.

Growth and Annealing of Trapped Holes and Interface States Using Time-Dependent Biases

Defect growth and annealing mechanisms in MOS devices have been studied. Biases were changed during irradiations. Significant radiation-induced annealing of trapped holes was observed. Apparent room temperature annealing of interface states was also observed. A consistent explanation of this apparent annealing is presented, i.e., an effect of LNUs on the results of the subthreshold analysis technique. A possible physical mechanism for the creation of LNUs due to inhomogeneous energy deposition is explored.

Evidence for two types of radiation-induced trapped positive charge

New experimental evidence is presented that supports a model that assumes two distinguishable types of positive oxide charge following x-irradiation. Two new experiments have been performed designed to separate the annealing properties of the two types of trapped positive charge. It is found that one type of trapped positive charge can be permanently removed at room temperature using substrate hot electron injection. The second type of trapped positive charge is found to be stable at temperatures up to 160/spl deg/C. >

Effect of bias on the response of metal-oxide-semiconductor devices to low-energy x-ray and cobalt-60 irradiation

The response of metal‐oxide‐semiconductor (MOS) transistors and capacitors to high‐energy Co‐60 gamma and low‐energy x‐ray irradiation is evaluated as a function of gate bias during exposure. It is demonstrated that, in contrast to previous expectations, the relative response of MOS devices to Co‐60 gamma and 10 keV x‐ray irradiation cannot be explained simply in terms of electron‐hole recombination and dose enhancement effects.