ang Zhang

Monte Carlo predictions of proton SEE cross-sections from heavy ion test data *

The limits of previous methods prompt us to design a new approach(named PRESTAGE) to predict proton single event effect(SEE) cross-sections using heavy-ion test data.To more realistically simulate the SEE mechanisms,we adopt Geant4 and a location-dependent strategy to describe the physics processes and the sensitivity of the device.Cross-sections predicted by PRESTAGE for over twenty devices are compared with the measured data.Evidence shows that PRESTAGE can calculate not only single event upsets induced by indirect proton ionization,but also direct ionization effects and single event latch-ups.Most of the PRESTAGE calculated results agree with the experimental data within a factor of 2-3.The limits of previous methods prompt us to design a new approach(named PRESTAGE) to predict proton single event effect(SEE) cross-sections using heavy-ion test data.To more realistically simulate the SEE mechanisms,we adopt Geant4 and a location-dependent strategy to describe the physics processes and the sensitivity of the device.Cross-sections predicted by PRESTAGE for over twenty devices are compared with the measured data.Evidence shows that PRESTAGE can calculate not only single event upsets induced by indirect proton ionization,but also direct ionization effects and single event latch-ups.Most of the PRESTAGE calculated results agree with the experimental data within a factor of 2-3.

Investigation of Threshold Ion Range for Accurate Single Event Upset Measurements in Both SOI and Bulk Technologies

Experimental evidences are presented showing obvious differences in threshold ion range for silicon-on-insulator (SOI) and bulk static random access memories (SRAMs). Single event upset (SEU) cross sections of SOI SRAMs start to decline off the Weibull curve at ion ranges of 20.7 μm to 40.6 μm, depending on the ion species and also the thickness of metallization layers. Whereas for the bulk SRAMs, threshold range of Bismuth beam is unexpectedly larger than 60.4 μm. Underlying mechanisms are further revealed by Monte Carlo simulations and in-depth analysis. The relative location of ions Bragg peak to the sensitive region and also the position of ion LET in the σ-LET curve of test device turn out to be two key parameters in determining the threshold ion range which can explain the experimental results. Significant discrepancies are observed in the deposited energy spectrums in sensitive regions of bulk SRAM by ions at different sides of the Bragg peak, but with almost the same LET at die surface (all with ion range larger than 30 μm). Energy straggling of incident ions at the die surface is considered by Monte Carlo calculations. Implications for hardness assurance testing are also discussed. A formula is proposed for calculating the “worst case” threshold ion range.

Large energy-loss straggling of swift heavy ions in ultra-thin active silicon layers

Monte Carlo simulations reveal considerable straggling of energy loss by the same ions with the same energy in fully-depleted silicon-on-insulator (FDSOI) devices with ultra-thin sensitive silicon layers down to 2.5 nm. The absolute straggling of deposited energy decreases with decreasing thickness of the active silicon layer. While the relative straggling increases gradually with decreasing thickness of silicon films and exhibits a sharp rise as the thickness of the silicon film descends below a threshold value of 50 nm, with the dispersion of deposited energy ascending above ±10%. Ion species and energy dependence of the energy-loss straggling are also investigated. For a given beam, the dispersion of deposited energy results in large uncertainty on the actual linear energy transfer (LET) of incident ions, and thus single event effect (SEE) responses, which pose great challenges for traditional error rate prediction methods.

Supply voltage dependence of single event upset sensitivity in diverse SRAM devices

Experimental evidences are presented showing the variety of supply voltage dependence of single event upset (SEU) sensitivity in diverse SRAM devices. Devices under test (DUTs) from Alliance Memory, ISSI and IDT companies with different technologies were irradiated by several kinds of heavy ions at Heavy ion Research Facility in Lanzhou (HIRFL) cyclotrons. For the Alliance 256 kb SRAM device, SEU cross section increases by more than one order of magnitude as supply voltage decreases from 5.0 V to 3.0 V. SEU data of Alliance 64 kb SRAM also exhibits significant supply voltage dependence. The reduction of critical charge is the predominant factor worsening the device performance. While for the Alliance 8 Mb, ISSI 2 Mb and IDT 256 kb SRAM devices, no obvious trend was observed, which is attributed to the negligible net contribution of competing mechanisms. Those results suggest that the worst-case supply voltage for evaluation of SEU sensitivity depends on the test devices.

Influence of deposited energy in sensitive volume on temperature dependence of SEU sensitivity in SRAM devices

The effects of temperature on the single-event upset (SEU) response of commercial and radiation-hardened SRAMs are investigated by experiment. The results showed that the SEU sensitivity relied on the temperature during the testing. However, it was also observed the amplitude of energy deposition of ions in sensitive volume of SRAM devices is able to affect the SEU sensitivity. When the deposited energy in sensitive volume is far larger than the critical value of inducing a SEU occurrence, the SEU sensitivity both in Bulk and SOI technologies displays a less temperature dependency. This result is attributed to the impact of energy deposition on the ion-induced transient pulse shape. The deposited energy in sensitive volume by different ions was estimated by Monte Carlo simulation and the role of energy deposition in transient pulse shape was discussed. The conclusion of detailed analysis is agreement with the results obtained in experiments.

Angular dependence of multiple-bit upset response in static random access memories under heavy ion irradiation

Experimental evidence is presented relevant to the angular dependences of multiple-bit upset (MBU) rates and patterns in static random access memories (SRAMs) under heavy ion irradiation. The single event upset (SEU) cross sections under tilted ion strikes are overestimated by 23.9%–84.6%, compared with under normally incident ion with the equivalent linear energy transfer (LET) value of ~ 41 MeV/(mg/cm2), which can be partially explained by the fact that the MBU rate for tilted ions of 30° is 8.5%–9.8% higher than for normally incident ions. While at a lower LET of ~ 9.5 MeV/(mg/cm2), no clear discrepancy is observed. Moreover, since the ion trajectories at normal and tilted incidences are different, the predominant double-bit upset (DBU) patterns measured are different in both conditions. Those differences depend on the LET values of heavy ions and devices under test. Thus, effective LET method should be used carefully in ground-based testing of single event effects (SEE) sensitivity, especially in MBU-sensitive devices.

Mechanisms of atmospheric neutron-induced single event upsets in nanometric SOI and bulk SRAM devices based on experiment-verified simulation tool

In this paper, a simulation tool named the neutron-induced single event effect predictive platform (NSEEP2) is proposed to reveal the mechanism of atmospheric neutron-induced single event effect (SEE) in an electronic device, based on heavy-ion data and Monte-Carlo neutron transport simulation. The detailed metallization architecture and sensitive volume topology of a nanometric static random access memory (SRAM) device can be considered to calculate the real-time soft error rate (RTSER) in the applied environment accurately. The validity of this tool is verified by real-time experimental results. In addition, based on the NSEEP2, RTSERs of 90 nm–32 nm silicon on insulator (SOI) and bulk SRAM device under various ambient conditions are predicted and analyzed to evaluate the neutron SEE sensitivity and reveal the underlying mechanism. It is found that as the feature size shrinks, the change trends of neutron SEE sensitivity of bulk and SOI technologies are opposite, which can be attributed to the different MBU performances. The RTSER of bulk technology is always 2.8–64 times higher than that of SOI technology, depending on the technology node, solar activity, and flight height.

Monte-Carlo prediction of single-event characteristics of 65 nm CMOS SRAM under hundreds of MeV/n heavy-ions in space

Single-event performance of 65 nm CMOS SRAM technology was revealed by Monte-Carlo methods, with heavy ions energy ranging from 20 MeV/n to 2 GeV/n. By comparing the energy-deposition spectrums in sensitive volumes (SV) and single event upset (SEU) cross sections, it was found that secondary electrons and nuclear reactions of the 200 MeV/n incident ions have significant impact on the device response. Secondary electrons excited by the energetic space ions can spread beyond the SV of the 65 nm bulk CMOS SRAM and result into partial collection of the ion track. This mechanism causes the constant increase of SEU cross section with ion LET, even deviating from the surface area of the SV by 3.3×. Nuclear reaction can lead to unexpectedly large energy-deposition events in the SV by 10x than the initial LET, which create SEU cross sections in the sub-LETth region, although with lower probability than direct-ionization process by 104×∼105×. Higher ion energy seems to enhance the influence of secondary electrons and nuclear reaction. Moreover, soft error rates of the 65 nm bulk CMOS SRAM technology on GEO were predicted and discussed.

Implementation and verification of different ECC mitigation designs for BRAMs in flash-based FPGAs

Embedded RAM blocks(BRAMs) in field programmable gate arrays(FPGAs) are susceptible to single event effects(SEEs) induced by environmental factors such as cosmic rays, heavy ions, alpha particles and so on. As technology scales, the issue will be more serious. In order to tackle this issue, two different error correcting codes(ECCs), the shortened Hamming codes and shortened BCH codes, are investigated in this paper. The concrete design methods of the codes are presented. Also, the codes are both implemented in flash-based FPGAs. Finally, the synthesis report and simulation results are presented in the paper. Moreover, heavy-ion experiments are performed,and the experimental results indicate that the error cross-section of the device using the shortened Hamming codes can be reduced by two orders of magnitude compared with the device without mitigation, and no errors are discovered in the experiments for the device using the shortened BCH codes.Embedded RAM blocks(BRAMs) in field programmable gate arrays(FPGAs) are susceptible to single event effects(SEEs) induced by environmental factors such as cosmic rays, heavy ions, alpha particles and so on. As technology scales, the issue will be more serious. In order to tackle this issue, two different error correcting codes(ECCs), the shortened Hamming codes and shortened BCH codes, are investigated in this paper. The concrete design methods of the codes are presented. Also, the codes are both implemented in flash-based FPGAs. Finally, the synthesis report and simulation results are presented in the paper. Moreover, heavy-ion experiments are performed,and the experimental results indicate that the error cross-section of the device using the shortened Hamming codes can be reduced by two orders of magnitude compared with the device without mitigation, and no errors are discovered in the experiments for the device using the shortened BCH codes.

A compact positron annihilation lifetime spectrometer

Using LYSO scintillator coupled on HAMAMATSU R9800 (a fast photomultiplier) to form the small size gamma-ray detectors, a compact lifetime spectrometer has been built for the positron annihilation experiments. The system time resolution FWHM=193 ps and the coincidence counting rate similar to 8 cps/mu Ci were achieved. A lifetime value of 219 +/- 1 ps of positron annihilation in well annealed Si was tested, which is in agreement with the typical values published in the previous lectures.