Showing papers in "IEEE Transactions on Nuclear Science in 1987"

Recent Measurements on Scintillator-Photodetector Systems

Abstract: The pulse height distributions from eleven kinds of scintillators coupled with a photomultiplier(PMT) and with a silicon photodiode were measured for gamma-ray energies between 14 keV and 1836 keV and compared. The numbers of photoelectrons from the PMT photocathode and those of electron-hole pairs in the photodiode were obtained. The intrinsic pulse height broadening was clearly demonstrated in NaI(Tl), CsI(Na), CsI(Tl) and CsI(pure) scintillators.

Correlation of Particle-Induced Displacement Damage in Silicon

Abstract: Correlation is made between the effects of displacement damage caused in several types of silicon bipolar transistors by protons, deuterons, helium ions, and by 1 MeV equivalent neutrons. These measurements are compared to calculations of the nonionizing energy deposition in silicon as a function of particle type and energy. Measurements were made of displacement damage factors for 2N2222A and 2N2907A switching transistors, and for 2N3055, 2N6678, and 2N6547 power transistors, as a function of collector current using 3.7 - 175 MeV protons, 4.3 - 37 MeV deuterons, and 16.8 - 65 MeV helium ions. Long term ionization effects on the value of the displacement damage factors were taken into account. In calculating the energy dependence of the nonionizing energy deposition, Rutherford, nuclear elastic, and nuclear inelastic interactions, and Lindhard energy partition were considered. The main conclusions of the work are as follows: 1) The ratio of the displacement damage factors for a given charged particle to the 1 MeV equivalent neutron damage factor, as a function of energy, falls on a common curve which is independent of collector current. 2) Deuterons of a given energy are about twice as damaging as protons and helium ions are about eighteen times as damaging as protons.

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

Abstract: 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.

Heavy-Ion-Induced, Gate-Rupture in Power MOSFETs

Abstract: A new, heavy-ion-induced, burnout mechanism has been experimentally observed in power metal-oxide-semiconductor field-effect transistors (MOSFETs). This mechanism occurs when a heavy, charged particle passes through the gate oxide region of n- or p-channel devices having sufficient gate-to-source or gate-to-drain bias. The gate-rupture leads to significant permanent degradation of the device. A proposed failure mechanism is discussed and experimentally verified. In addition, the absolute immunity of p-channel devices to heavy-ion-induced, semiconductor burnout is demonstrated and discussed along with new, non-destructive, burnout testing methods.

Radiation Response of MOS Capacitors Containing Fluorinated Oxides

Abstract: By introducing small amounts of fluorine into the gate oxide, we have been able to significantly alter the radiation response of Metal/SiO2/Si (MOS) capacitors, and their subsequent time dependent behavior. Experimentally we have observed that compared with their control capacitors, which have no fluorine introduced into the oxide, the fluorinated samples exhibit the following major differences: (1) the densities of radiation-induced oxide charge and interface traps are drastically reduced, (2) the gate-size dependence of the radiation-induced interface traps is greatly suppressed, and (3) the overall density of the radiation-induced interface traps continues to decrease with time for many hours after irradiation before a turn-around trend is observed. Possible mechanisms involving the roles that fluorine may play in relieving the oxide strain near the SiO2/Si interface and in the post-irradiation defect-reaction chemistry are discussed.

On Heavy Ion Induced Hard-Errors in Dielectric Structures

Abstract: Heavy ion induced failures in SiO2 and SiO2/Si3N4 composite capacitors were studied for ion linear energy transfers (LETs) from 15 to 85 MeV/(mg/cm2). Key findings of this study were the following: 1) hard errors are caused by a combination of energy deposited by the ion and from electrical conduction through the plasma channel formed by the ion strike, 2) there is an inverse linear relationship between ion energy deposition and the bias voltage required for composite device failure and 3) the angular dependence of the failure threshold voltage closely follows an inverse cosine relationship. Empirical equations are presented that allow the critical failure thresholds to be calculated from the ion LET for both silicon dioxide and metal-nitride-oxide-semiconductor (MNOS) composite devices. A failure model is proposed based on the energy deposition in the dielectric and rapid thermal diffusion of gate material through the dielectric.

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

Abstract: 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.

A Reevaluation of Worst-Case Postirradiation Response for Hardened MOS Transistors

Abstract: The "worst-case" postirradiation response of Sandia hardened n-channel transistors following Co-60 exposure to total dose levels of system interest is demonstrated to occur for zero-volt bias during radiation, and positive bias during a subsequent anneal. This observation is explained in terms of oxide-trapped and interface-state charge buildup and anneal. Additional results are presented which suggest that, for future technologies with very thin gate oxides, worst-case device leakage during irradiation may well occur for zero-volt irradiations. These results highlight the importance of periodically reevaluating the response of MOS devices during and after irradiation to determine worst-case test conditions, particularly as technologies advance and gate insulators become thinner.

Post-Irradiation Effects in Field-Oxide Isolation Structures

Abstract: We have studied experimentally the time dependence of leakage currents in six CMOS (complementary metaloxide semiconductor) processes using LOCOS (local oxidation of silicon) isolation structures. These six process lines represent six different U. S. semiconductor companies. In their radiation response, these processes range from very hard to very soft. In the softer processes, the radiation-induced leakage currents are due to the turning on of a leakage path either under the thick field-oxide or along the transistor edge (bird's beak) region. In the hardest process, the field-oxide did not turn on, and the leakage was entirely due to subthreshold current in the gate region. These different mechanisms have qualitatively different time dependences, which we describe and discuss. We also discuss the implications of our results for hardness assurance testing.

First Nondestructive Measurements of Power MOSFET Single Event Burnout Cross Sections

Abstract: A new technique to nondestructively measure single event burnout cross sections for N-channel power MOSFETs is presented. Previous measurements of power MOSFET burnout susceptibility have been destructive and thus not conducive to providing statistically meaningful burnout probabilities. The nondestructive technique and data for various device types taken at several accelerators, including the LBL Bevalac, are documented. Several new phenomena are observed.

Analytical Model for Single Event Burnout of Power MOSFETs

Abstract: The processes causing single event burnout in power MOSFETs are modeled analytically, describing the evolution of the plasma-filament from an ion traversing the structure and the processes constituting the triggering mechanism of second breakdown. Analytically tractable models are achieved by employing simplifying approximations in common use in established semiconductor device theory, and by using initial conditions and parameters typical for simulations of single event upset phenomena. Comparative simplicity and tractability is favored over accuracy to gain lucid relationships between pertinent parameters, which can guide device design and optimization, aid the interpretation of results from simulation and experiment, and help in the development of simulation software.

Problems in Signal-to-Noise Ratio for Attenuation Correction in High Resolution PET

Abstract: In high resolution PET, the number of events required for a high signal-to-noise ratio in the measured attenuation correction is an order of magnitude higlher (50-100 million) than that required for the emission image. This is due to the large number of elements in the attenuation correction sinogram (e.g. 50,000) and the large attenuation factors through the thicker sections of the body (e.g. 50). Another major contribution to noise in high resolution PET is contamination of true coincidences by accidentals and their subtraction from the prompt coincidences. In this work three methods which reduce noise in the measured attenuation are evaluated. The methods are: (1) optimization of energy threshold, (2) processing of random sinogram before subtraction and (3) spatial averaging of the tranismission sinogram before performing attenuation correction.

Applied Field and Total Dose Dependence of Trapped Charge Buildup in MOS Devices

Abstract: A rate equation for charge buildup which includes carrier sweep out, geminate recombination, hole/ electron trapping, and effects of internal fields is developed. The first moment of the resulting charge distribution is calculated to yield the midgap voltage shift as a function of irradiation time. The initial midgap voltage shift per dose and the maximum midgap voltage shift are derived. The field dependence of these quantities is shown to be a consequence of the field dependence of the hole/electron capture cross sections and geminate recombination escape probability The results of this formulation show that the E-1/2 decrease in the midgap shift per dose with field described in the literature is due to the decrease of the hole capture cross section with increasing applied field. The theory is validated by comparison with experimental results obtained on 225 A thermal oxide on p-type silicon test capacitors irradiated under bias at room temperature.

Laser Simulation of Single Event Upsets

Abstract: A pulsed picosecond laser was used to produce upsets in both a commercial bipolar logic circuit and a specially designed CMOS SRAM test structure. Comparing the laser energy necessary for producing upsets in transistors that have different upset sensitivities with the single event upset (SEU) level predicted from circuit analysis showed that a picosecond laser could measure circuit sensitivity to SEUs. The technique makes it possible not only to test circuits rapidly for upset sensitivity but also, because the beam can be focussed down to a small spot size, to identify sensitive transistors.

Total-Dose Failure Mechanisms of Integrated Circuits in Laboratory and Space Environments

Abstract: Total-dose failure mechanisms are identified over a wide range of dose rate for 2K and 16K SRAMs from hardened and commercial CMOS technologies. Failure was defined "parametrically," such that an IC fails if one of its parameters, e.g., static power supply current or timing, exceeds a preset specification following irradiation. These studies demonstrate that the dominant failure mechanisms of SRAMs in space radiation environments are often quite different than those observed at considerably higher laboratory dose rates specified by DoD test guidelines, i.e., 100 to 300 rad(Si)/s. In addition, the total-dose hardness of SRAMs varied significantly between laboratory and space dose rates. Several approaches are discussed for predicting total-dose hardness of ICs in space from laboratory measurements. Results are also presented for transistors irradiated over the same range of dose rate (200 to 0.02 rad(Si)/s) and at all biases, i.e., N-on/off and P-on/off. These transistor measurements are used to characterize the physical mechanisms that govern the radiation response of the more complex SRAMs. In a surprising result, a dose-rate dependence for the buildup of radiation-induced interface states was observed. At dose rates from 200 to 0.02 rad(Si)/s, the number of interface states at a given total dose increased as the dose rate decreased.

An SEU Tolerant Memory Cell Derived from Fundamental Studies of SEU Mechanisms in SRAM

Abstract: A new single event upset (SEU) hardening concept, an LRAM cell, is demonstrated theoretically and experimentally. Decoupling resistors in the LRAM are used only to protect against the short n-channel transient; longer persisting pulses are reduced in magnitude by a voltage divider, a basically new concept for SEU protection. In such a design, smaller resistors provide SEU tolerance, allowing higher performance, hardened memories. As basis for the LRAM idea, techniques were developed to measure time constants for ion induced voltage transients in conventional static random access memories, SRAM. Time constants of 0.8 and 6.3 nsec were measured for transients following strikes at the n- and p-channel drains, respectively--primary areas of SEU sensitivity. These data are the first transient time measurements on full memory chips and the large difference is fundamental to the LRAM concept. Test structures of the new design exhibit equivalent SEU tolerance with resistors 5-to-10 times smaller than currently used in SRAM. Our advanced transport-plus-circuit numerical simulations of the SEU process predicted this result and account for the LRAM experiments, as well as a variety of experiments on conventional SRAM.

The Size Effect of Ion Charge Tracks on Single Event Multiple-Bit Upset

Abstract: Double-bit upset rates in satellite memory cells as high as several percent of total upsets have recently been reported. This significant fraction may be explained by cosmic ion track sizes which are larger than previously postulated. Generation and transport of high-energy secondary electrons along heavy-ion tracks have been analyzed using the Monte Carlo (MC) code TRIPOS-E. Indications are that initial track radii are significantly larger than previously thought. In this paper, an evaluation of the probability of double-bit upsets as a function of track size and hypothetical device dimensions is presented.

New Low-Altitude Dose Measurements

Abstract: The Defense Meteorological Satellite Program (DMSP) F7 satellite, launched in November, 1983, carries a dosimeter that measures radiation dose behind four hemispherical aluminum domes of different thicknesses and distinguishes low (electron) and high (proton) thresholds of energy deposition. The dosimeter also returns accurate, high-time-resolution dose measurements. Short-term measurements of dose from three sources, inner radiation belt protons, outer radiation belt electrons and solar flares, are presented for periods in 1984 and 1985. Empirical models of dose rate are constructed for the 840 km altitude of the DMSP orbit and compared to predictions of the NASA models. The NASA model values for proton dose in the South Atlantic Anomaly (SAA) are approximately 50% higher than the DMSP average values. The NASA outer zone electron model prediction values are too high by an average factor of 6, and are not reliable for short-term predictions. Included in the analysis are two of the largest solar proton events of 1984 and 1985 that occurred on 16 February, 1984, and on 26 April, 1984. The February event was relatively short-lived and produced a hard energy spectrum. The April event was much softer but gave a total dose behind .55 gm/cm2 of aluminum shielding in excess of 25 rad(Si) for the first three days of the event.

Energy Dependence of Proton-Induced Displacement Damage in Gallium Arsenide

Abstract: Nonionizing energy deposition in gallium arsenide has been calculated for protons with energies ranging from 1 to 1000 MeV. The calculations are compared with new experimental results for ion implanted gallium arsenide resistors and Hall samples irradiated with protons in the energy range 1 to 60 MeV. Results are also compared with recent studies of proton induced displacement damage in silicon.

Generation of Interface States by Ionizing Radiation at 80K Measured by Charge Pumping and Subthreshold Slope Techniques

Abstract: Generation of fast interface states D11 by ionizing radiation has been measured in MOS transistors at 80K and 295K using charge pumping and subthreshold slope techniques. Using charge pumping, the more sensitive and reliable technique, we find that D11 are not formed by radiation at 80K. In contrast, subthreshold slope measurements appear to show an increase in D11 in MOSFETs irradiated at 80K. This is shown to be an artifact caused by lateral non-uniformities (LNU) in the radiation-induced fixed charge.

A Model of Lattice Defects in Sapphire

Abstract: A model is proposed to account for the changes under ultraviolet illumination of several of the prominent optical absorption bands in neutron irradiated crystalline sapphire. This model, based on both new and previously observed photobleaching and predicted optical anisotropic properties of single and paired anion vacancies, assigns absorption and luminescent bands to specific charge states of anion divacancies.

Comparison of Neutron, Proton and Gamma Ray Effects in Semiconductor Devices

Abstract: Interest in proton radiation effects has intensified in recent years. A prime focus is the relationship between proton displacement and ionization effects and the separate consideration of neutron-induced displacement and gamma-ionization effects in TREE characterization. Recent definitive work on proton and neutron displacement damage in silicon in terms of nonionizing energy loss has laid the groundwork for comparison of proton effects with the TREE data base. We initiate this comparison with a summary of device radiation susceptibilities in neutron and gamma environments. Proton interactions in silicon devices are then presented in terms of dose deposition and nonionizing energy loss. This leads to a neutron-proton damage equivalence factor and enables the development of simple correspondence. The device susceptibility charts are then combined so both displacement damage and ionization-damage can be schematically examined relative to proton dose. These susceptibility charts demonstrate the dominance of ionization effects for damage in a proton environment for modern silicon microcircuit technologies. This approach is presented as a convenient means of interpreting effects for both proton exposures and TREE simulators. It is concluded that TREE characterization can be used as a good first-order estimate of proton damage effects.

A Hybrid Maximum Likelihood Position Computer for Scintillation Cameras

Abstract: Maximum likelihood (ML) estimators offer advantages of improved spatial resolution and linearity over traditional position estimates in position sensitive detectors. We have constructed a two board, multibus based hybrid position computer capable of performing the ML estimate at SPECT countrates. In addition, the board can implement any estimate linear in the photomultiplier tube outputs.

Models for Total Dose Degradation of Linear Integrated Circuits

Abstract: Mechanisms for total dose degradation of linear circuits are discussed, including bulk effects, oxide charge buildup and recombination at the Si-SiO2 interface. The dependence of damage on bias, dose, particle type and energy is used in conjunction with two-dimensional modeling to identify the failure mechanism in a specific linear device type. The importance of surface recombination is demonstrated along with the absence of bias dependence. Bulk damage is shown to be important for high energy electron irradiation because of wide-base pnp transistors. This causes substantial differences in device failure between electron and cobalt-60 environments that need to be taken into account for test standards and data bases that include commercial bipolar integrated circuits. Valid test methodologies for linear devices must consider the energy and particle type present in the actual environment.

Initial Results from the Donner 600 Crystal Positron Tomograph

Abstract: We describe a positron tomograph using a single ring of 600 close-packed 3 mm wide bismuth germanate (BGO) crystals coupled to 14 mm phototubes. The phototube preamplifier circuit derives a timing pulse from the first photoelectron, and sends it to address and coincidence circuits only if the integrated pulse height is within a pre-set window. The timing delays and pulse height windows for all 600 detectors and the coincidence timing windows are computer adjustable. An orbiting positron source is used for transmission measurements and a look-up table is used to reject scattered and random coincidences that do not pass through the source. Data can be acquired using a stationary mode for 1.57 mm lateral sampling or the two-position clam sampling mode for 0.79 mm lateral sampling. High maximum data rates are provided by 45 parallel coincidence circuits and 4 parallel histogram memory units. With two-position sampling and 1.57 mm bins, the reconstructed point spread function (PSF) of a 0.35 mm diam 22Na wire source at the center of the tomograph is circular with 2.9 mm full-width at half-maximum (fwhm) and the PSF at a distance of 8 cm from the center is elliptical with a radial fwhm of 4.0 mm and tangential fwhm of 3.0 mm.

Measurement of SEU Thresholds and Cross Sections at Fixed Incidence Angles

Abstract: Current SEU testing and analysis techniques have as basic assumptions that the charge deposited at a junction depends linearly on the linear energy transfer (LET) of the ion and the pathlength of the ion through an imagined parallelepiped that represents the depletion region. This study tests these assumptions for two bipolar parts, AMD 27LSOO and Fairchild 93L422, by irradiating at fixed angles while varying the LET of two ion species. It was found that the 27LSOO shows a pronounced ion species dependence, and may show a deviation of deposited charge from the usual inverse-cosine times a fixed depletion depth, while the 93L422 exhibited the expected inverse-cosine dependence and no ion species dependence.

Comparison of Proton and Neutron Carrier Removal Rates

Abstract: Displacement damage induced carrier removal rates for proton irradiations in the energy range 10-175 MeV were compared to 1 MeV equivalent neutrons using power MOSFETs as a test vehicle. The results showed that, within experimental error, the degradation mechanisms were qualitatively similar and the ratio of proton to neutron carrier removal rates as a function of proton energy correlate with a calculation based on nonionization energy loss in silicon. For exposures under junction bias, p-type silicon was found to have a smaller carrier removal rate for both proton and neutron irradiations, whereas, for n-type silicon, junction bias had little effect on the carrier removal rate.

A Comparison of Positive Charge Generation in High Field Stressing and Ionizing Radiation on MOS Structures

Abstract: We compare the effects of ionizing radiation and high field stressing on Metal-Oxide-Silicon oxides. Using electron spin resonance, we compare the point defects responsible for the positive charge generated by ionizing radiation and high field stressing. We find the two processes to be different in that the positive charge generated by ionizing radiation is almost entirely due to E? centers in the oxide; however, less than half the positive charge generated by high field stressing can be accounted for by E? centers.

Simulation of Heavy Charged Particle Tracks Using Focused Laser Beams

Abstract: A laboratory system utilizing a Q-switched Nd-doped YAG laser is used to simulate the ionization track produced as energetic heavy ions traverse a semiconductor device (resulting in single-event upset effects). Details of the optical design for producing the precisely focused spot and the requirements for fast pulses are described. The advantages and disadvantages of the use of this laboratory simulation are discussed. Laser tests on PIN diodes, p-n junctions, bipolar memories, and power MOSFETs are described and compared to high energy particle tests results.

The Nature of the Deep Hole Trap in MOS Oxides

Abstract: We have investigated hole and electron trapping events at E' deep hole traps in metal-oxide-semiconductor oxides. Using a sequence of ultraviolet irradiations, electron spin resonance measurements, and capacitance versus voltage measurements, we have obtained results which are completely consistent with a simple oxygen vacancy model for the hole trap. However, our results are inconsistent with the bond strain gradient model proposed by Grunthaner et al.