K. Kang

Laser Simulation of Single Event Upsets

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.

Charge collection from focussed picosecond laser pulses

The magnitude of the charge collected in test structures following the passage of a heavy energetic ion was measured and compared with that generated by a picosecond pulse of laser light to determine whether pulsed lasers can substitute in some cases for the more expensive and time-consuming accelerators currently used for single-event-upset (SEU) testing of circuits. Two phenomena that are known to play a significant role in determining the magnitude of the collected charge generated by an ion beam-funneling and the shunt effect-were also observed for irradiation by a pulsed laser beam. The results show that the collected charge from laser irradiation was proportional to the 4/3 power of the laser energy for the case of funneling and to the 1.683 power of the laser energy for the shunt effect, in full agreement with results previously obtained for ion-beam irradiation. The proportionality constant for these CMOS devices was smaller by a factor of 6.5 for the laser-induced shunt experiments due to the much lower track charge density produced by the laser light. By correcting the laser data to take the lower charge density in the track into account, a good correlation between the ion and laser data was found, suggesting that the pulsed laser can be used to measure SEU sensitivity of integrated circuits. >

Ion induced charge collection in GaAs MESFETs

Charge-collection measurements on GaAs MESFET test structures demonstrate that more charge can be collected at the gate than is deposited in the active layer and more charge can be collected at the drain than the total amount of charge produced by the ion. Enhanced charge collection at the gate edge has also been observed. The current transients produced by the energetic ions have been measured directly with about 20-ps resolution. The significance of this work is that it shows charge-collection phenomena in GaAs MESFETs to be very complex with important implications for modeling SEU (single-event upset) phenomena and developing techniques to mitigate SEU effects. >

Pulsed laser-induced SEU in integrated circuits: a practical method for hardness assurance testing

A pulsed picosecond laser was used to measure the threshold for single event upset (SEU) and single event latchup (SEL) for a detailed study of a CMOS SRAM and a bipolar flip-flop. Comparing the ion and laser upset data for two such vastly different technologies gives a good measure of how versatile the technique is. The technique provided both consistent and repeatable results that agreed with published ion upset data for both types of circuits. However, measurements of the absolute threshold linear energy transfer (LET) using infrared laser light do not agree with those of the ions, being about 50% too high for the SRAMs, and about 20% too high for the bipolar flip-flops. The consistency of the results, together with the advantages of using a laser system, suggests that the pulsed laser can be used for SEU/SEL hardness assurance of integrated circuits. >

Dependence of the SEU window of vulnerability of a logic circuit on magnitude of deposited charge

A pulsed picosecond laser was used to measure the time during which gates in a GaAs logic circuit were sensitive to single event upset (SEU). Circuit analysis showed that those gates would be most sensitive if the laser light arrived just prior to the clock signal going from low to high voltage. By delaying the clock signal with respect to the arrival time of the laser pulse, it was possible to measure a window of vulnerability, which is the time interval prior to the arrival of the clock signal during which the gate is sensitive to upsets. The width of that window was found to depend on the energy of the light pulse. Similar behavior is expected when the circuit is exposed to ions. These results suggest that, at high frequencies and in the presence of ions and with large LETs (linear energy transfers) gates in logic circuits may be sensitive to upsets during a large fraction of their duty cycle. The technique described provides an in situ way of measuring charge collection times at individual transistors and signal propagation times between logic gates using the circuit itself as the detector. >

Laser confirmation of SEU experiments in GaAs MESFET combinational logic (for space application)

Majority vote and self-scrubbing circuitry were utilized to harden the registers of a GaAs logic circuit to single event upsets (SEUs). Ion beam testing of the hardened part at low scrub frequencies showed fewer system upsets than that of an unhardened part. At high frequencies, the upset rate increased with frequency, indicating a clock-dependent SEU sensitive node. A pulsed laser was used to identify an input node in the self-scrubbing circuitry as being the source of the upsets at high frequencies. Because the node was only sensitive for a short duration prior to the rising clock edge, more clock-edge-dependent SEUs could occur at higher frequencies. The relative SEU thresholds of the registers and the input node measured with the laser showed excellent agreement with ion data. All single-point SEU failure nodes were identified and their upset thresholds measured. With this information it was possible to redesign the circuit to reduce its sensitivity to SEU at high scrub frequencies. >

Pulsed laser-induced charge collection in GaAs MESFETs

Pulsed picosecond lasers with variable wavelength were used to investigate the details of charge collection in GaAs MESFETs. In short gate-length devices, charge collection at the drain may be much larger than at the gate and greater than the charge produced by the laser pulses. The results show that a pulsed laser is very useful in studies of charge collection. Two particularly useful features are the absence of the radiation damage which accompanies ion measurements and the ability to observe visually the point at which charge is being produced in the device. >

Charge collection in GaAs MESFETs and MODFETs

A comparison of the amount of charge collected at the drains of GaAs MESFETs irradiated with pulsed laser light and ions and having different gate lengths shows orders of magnitude more charge collected for 0.1 mu m MESFETs than for 1.2 mu m MESFETs manufactured using different processes. Analyses of the dependence of the photocurrent pulses on gate and drain voltages, temperature, and light intensity suggest that the enhanced charge collection is primarily due to modulation of the channel width by positive charge trapped in the vicinity of the channel. Enhanced charge collection via channel modulation also occurs in pseudomorphic MODFETs. Pulses with characteristics similar to those produced by laser light, i.e. large amplitudes and long decay times, were obtained when 0.1 mu m MESFETs were irradiated with He and Si ions. These results reveal the important role played by traps in determining SEU sensitivity in GaAs MESFETs with short gate lengths. >

Spatial and temporal dependence of SEU in a 64 K SRAM

A pulsed picosecond laser was used to measure the spatial and temporal dependence of single event upsets (SEUs) in a 64 K SRAM (static random access memory) (HM65642C) that was not hardened to SEU. Consistent and repeatable upset thresholds and latchup were measured when the light was focused on the drains of the sensitive transistors. The SRAMs sensitivity of SEU was found to depend on the arrival time of the laser pulse relative to the time when the cell was addressed. Striking results were found when the light was focused outside the sensitive drains. Whether an upset occurred depended on both the position of the laser spot and the information stored in the cell. Under certain conditions an upset window was observed. A single pulse of light incident on a particular cell produced upsets in surrounding cells depending on the data stored in those cells. >

SEU characterization and design dependence of the SA3300 microprocessor

A detailed characterization is presented of the single-event upset (SEU) sensitivity of the SA3300 microprocessor focusing specifically on the internal general-purpose registers. SEU response is explored as a function of temperature and logic state of the registers. The effects of two different design variations on SEU vulnerability are outlined. Microprobe measurements using a pulsed Nd:YAG laser suggest that the observed pattern dependence for both design revisions is due to bipolar photocurrent in a vertical n/sup +/pn transistor. A slight temperature dependence was observed in both design revisions. This is consistent with the use of oversized restoring transistors to minimize SEU vulnerability rather than polysilicon feedback resistors. More recent data show threshold above 120 MeV-cm/sup 2//mg with 80-k Omega feedback resistors. >