J.S. Browning

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

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.

Single event upset in irradiated 16 K CMOS SRAMs

The single-event-upset (SEU) characteristics of a CMOS SRAM cell irradiated under conditions that simulate the total-dose degradation anticipated in space applications were experimentally and theoretically investigated. Simulations of SEU sensitivity utilizing a 2-D circuit/device simulator, with measured transistor threshold-voltage shifts and mobility degradations as inputs, are shown to be in good agreement with experimental data at high total dose. Both simulation and experiment show a strong SRAM cell SEU imbalance resulting in a more SEU-tolerant preferred state and a less tolerant nonpreferred state. The resulting cell imbalance causes an overall degradation in SEU immunity, which increases with increasing total dose and should be taken into account in SEU testing and part characterization. >

Dose-Rate Upset Patterns in a 16K CMOS SRAM

Dose-rate LINAC tests have been performed on the Sandia National Laboratories SA3240 16k CMOS SRAM and transient radiation induced upset patterns are presented. These patterns indicate a progression of upsets across the memory array with increasing doserate, as predicted by computer simulations of the rail span collapse effect. The upset bitmap patterns and simulation results show that VDD power supply bussing is not critical, or even necessary, for radiation hardened CMOS epitaxial parts if local VDD taps to a powered substrate are used; the critical factor is the efficiency of the VSS bussing scheme. The effects of initial storage patterns and total ionizing dose on the upset patterns are also presented.

Processing Enhanced SEU Tolerance in High Density SRAMs

We report theoretical calculations and experimental verification of an increase in memory cell SEU tolerance when Sandias 2¿m-technology 16K SRAMs are fabricated with a radiation-hardened 1-¿m CMOS process. An advanced 2D transient transport-plus-circuit simulator has been employed to calculate the differential contributions from each of the vertical dimensional changes in the transition from the 2-¿m process to the 1-¿m process. Error cross-section data, performed at the Berkeley cyclotron, on the first such device lot indicate that total improvement in threshold LET is a factor of 2 or better. A saturation phenomenon associated with the high-LET events is described and physical mechanisms responsible for the saturation are discussed.

Single-event correlation between heavy ions and 252Cf fission fragments

Abstract The results of a correlation study show that 252Cf sources can replace accelerators for single-event tests, provided that the depth of penetration and the linear energy transfer required of the heavy ions fall within the spectrum of the 252Cf fission fragments.

Total dose characterization of a CMOS technology at high dose rates and temperatures

Radiation-hardened MOS ICs and transistors were irradiated using several high-dose-rate sources. A strong dependence of the dose-to-failure level is observed for different circuit designs, the dose per pulse, package temperature, and the average dose rate of the radiation source at ambient and elevated temperatures. The results indicate that extrapolating very high dose rate IC and transistor properties from Co-60 or X-ray source data could be extremely difficult and poses a serious problem for any hardness assurance program. For extrapolating to strategic applications, and intimate knowledge of the circuit design and performance and a knowledge of the transistor performance in the environment of interest are necessary. >

An assessment of the radiation tolerance of large satellite memories in low Earth orbits

A procedure is described that assures the reliable operation in space radiation environments of memory systems that are approximately ten times larger and can be built at about one tenth of the cost of recent, comparable satellite projects. The procedure accounts for combined radiation effects, permitting the radiation tolerance of the memory system to be accurately estimated. Using the procedure a 1-gigabit memory with error detection and correction capability has been designed for a miniature satellite applications. The memory system is constructed entirely out of commercial grade microelectronics.<<ETX>>

Comparison of photocurrent enhancement and upset enhancement in CMOS devices in a medium-energy X-ray environment

Radiation-induced upset levels in SA3001 static random access memories (SRAMs) and SA3246 clock integrated circuits (ICs) have been measured in a medium-energy flash X-ray environment (average photon energy approximately 100 keV), where dose-enhancing effects are very important. By comparing device responses using a non-dose-enhancing ceramic package lid and a dose-enhancing Kovar/gold lid, dose-enhancement factors for photocurrent and upset were generated. The observed upset enhancement factors of 3.0+or-0.5 (SRAM) and 2.2+or-0.2 (clock IC) are in excellent agreement with measurements of photocurrent enhancement factors (2.5+or-0.5) in diodes processed with the same diffusions as the complementary metal-oxide-semiconductor (CMOS) ICs irradiated in a steady-state X-ray environment. These results indicate that upset is dominated by the radiation-induced transient supply current in these ICs, and that steady-state diode photocurrent measurements are a good predictor of both photocurrent and upset enhancement for ICs made with this technology. >

Simulation fidelity issues when using gamma-ray simulators for TREE testing CMOS SRAM

Factors that influence the fidelity of gamma-ray TREE testing are investigated. Specifically, package-induced dose enhancement in 256K CMOS static-random-access-memories (SRAMs) and dose enhancement from finite-range electrons produced (by gamma-ray interactions) in materials external to the SRAM packages are studied. Two gamma-ray simulators with significantly different spectra are used in the studies. The spectral differences produced less change in SRAM upset levels than did surrounding materials of equal mass density but differing atomic number. The implication for gamma-ray simulation testing is that individual devices within electronic systems may respond quite differently in gamma-ray TREE testing because of the structural materials within the system than when tests are performed on these individual devices without the system present. >

Particle LET spectra from microelectronics packaging materials subjected to neutron and proton irradiation

Cumulative fractions for linear energy transfer spectra were measured for particles ejected from microelectronics packaging materials subjected to neutron and proton irradiations. The measurements for the neutron irradiations compare well with Monte Carlo theoretical calculations. The spectra can be used to access microelectronics vulnerabilities in strategic-nuclear-weapon, space-trapped, and neutral-beam directed-energy particle environments. >