Gary M. Swift

Total dose effects in conventional bipolar transistors and linear integrated circuits

Total dose damage is investigated for discrete bipolar transistors and linear integrated circuits that are fabricated with older processing technologies, but are frequently used in space applications. The Kirk effect limits the current density of discrete transistors with high collector breakdown voltage, increasing their sensitivity to ionizing radiation because they must operate low injection levels. Bias conditions during irradiation had different effects on discrete and integrated circuit transistors: discrete devices were strongly dependent on bias conditions, whereas damage in the linear ICs was nearly the same with or without bias. There were also large differences in the response of these devices at low dose rates. None of the discrete transistors exhibited enhanced damage at low dose rates, whereas substantially more damage occurred in the linear devices under low dose rate conditions, particularly for parameters that rely directly on p-n-p transistors. The threshold for dose rate effects in p-n-p transistors was about 0.01 rad(Si)/s, which is approximately two orders of magnitude lower than the corresponding threshold for n-p-n transistors in integrated circuits. >

Radiation effects on advanced flash memories

Radiation tests of advanced flash memories including, multi-level flash technology, are compared with results from previous generations. Total dose failure levels are comparable to or lower than those of older technologies, but are likely still caused by degradation of the internal charge pump. Small numbers of read errors were observed during single event tests of the multi-level devices that appear to be caused by shifts in the sense amplifier detection levels or cell threshold shifts rather than loss of electrons off the floating gate.

Latchup in integrated circuits from energetic protons

Proton latchup was investigated for several CMOS integrated circuits, including a modern microprocessor. The proton latchup cross sections of these devices differed by more than two orders of magnitude. A modeling approach that takes differences in charge collection processes for long- and short-range particles into account was effective in comparing latchup cross sections in heavy-ion and proton environments, as well as explaining why the proton cross sections were so different among the device types.

SEU mitigation testing of Xilinx Virtex II FPGAs

SRAM-based reconfigurable programmable logic is widely used in commercial applications and occasionally used in space flight applications because of susceptibility to single-event upset (SEU). Upset detection and mitigation schemes have been tested on the Xilinx Virtex II X-2V1000 in heavy-ion and proton irradiation to control the accumulation of SEUs and to mitigate their effects on the intended operation. Non-intrusive upset detection and partial reconfiguration in combination with TMR can repair the design to maintain state information. In-beam results on a simple test design demonstrate the effectiveness of these methods when used together.

Breakdown of gate oxides during irradiation with heavy ions

Breakdown of thin gate oxides from heavy ions is investigated using capacitor test structures. Soft breakdown was observed for 45 /spl Aring/ oxides, but not for 75 /spl Aring/ oxides. Lower critical fields were observed when experiments were done with high fluences during each successive step. This implies that oxide defects play an important role in breakdown from heavy ions and that breakdown occurs more readily when an ion strike occurs close to a defect site. Critical fields for 75 /spl Aring/ oxides are low enough to allow gate rupture to occur at normal supply voltages for ions with high LET.

In-flight observations of multiple-bit upset in DRAMs

In interplanetary space, the Cassini solid-state recorder is experiencing the predicted number of upsets, but a very high rate of uncorrectable errors. An experimental investigation of the flight DRAMs susceptibility to multiple-bit upset (MBU) proved enlightening, revealing an unexpectedly high rate of MBUs (even caused by protons). In combination with an architectural flaw in the error correction circuitry, these explain the flight anomaly.

Comparison of Xilinx Virtex-II FPGA SEE sensitivities to protons and heavy ions

A comparison of heavy-ion and proton-induced single event effect sensitivities has been made using the Xilinx Virtex-II field programmable gate array (FPGA). Recently fabricated test samples are selected for observations of single event upset and single event functional interrupt. A complex relationship appears to exist between the heavy ion and proton sensitivities due to effects such as multiple-bit upsets and elastic nuclear scattering.

Radiation response of a MEMS accelerometer: an electrostatic force

Particle irradiation on the mechanical sensor of the ADXL50 microelectromechanical accelerometer shifts the output voltage. An earlier conclusion, that a dielectric below the sensor becomes charged, is extended by quantifying the effect of this charge on device output. It is shown that an electrostatic force is consistent with the observation that the output voltage shift is independent of acceleration. Possible charging mechanisms are suggested. An appendix derives a convenient algorithm for calculating electrostatic forces, which may also be used for other MEMS devices.

Dynamic testing of Xilinx Virtex-II field programmable gate array (FPGA) input/output blocks (IOBs)

Heavy-ion irradiation and fault injection experiments were conducted to evaluate the upset sensitivity of the Xilinx Virtex-II field programmable gate arrays (FPGAs) input/output block (IOB). Full triple module redundancy (TMR) of the IOBs, in combination with regular configuration scrubbing, proved to be a quite effective upset mitigation method.

Analysis of radiation effects on individual DRAM cells

A novel way to measure the radiation characteristics of DRAM memory cells is presented. Radiation exposure tends to drive retention times lower for cells. The change in retention time (the time period required for a cell to upset without refreshing) is used to measure the effect of irradiation on the DRAM cells. Both the radiation response of a single DRAM cell and the response of all cells as a statistical whole are analyzed.