A.R. Knudson

Charge collection in silicon for ions of different energy but same linear energy transfer (LET)

Charge collection measurements in thin silicon structures have indicated that more charge is collected for higher energy ions than for the lower energy ions for incident ions with the same LET. The observed differences are larger than can be explained by uncertainties in energy-loss calculations. A possible explanation is in differences in initial track structure. The higher energy track is more diffuse and might yield more charge to be collected because there is less initial electron-hole pair recombination. >

Pulsed laser-induced single event upset and charge collection measurements as a function of optical penetration depth

We use picosecond laser pulses to investigate single event upsets and related fundamental charge collection mechanisms in semiconductor microelectronic devices and circuits. By varying the laser wavelength the incident laser pulses deposit charge tracks of variable length, which form an approximation to the charge tracks resulting from high energy space particle strikes. We show how variation of the charge track length deposited by laser pulses allows the mechanisms of charge collection in semiconductor devices to be probed in a sensitive manner. With the aid of computer simulations, new insight into charge collection mechanisms for metal–semiconductor field effect transistor (MESFET) devices and heterojunction bipolar transistor devices is found. In the case of the MESFET we point out the correlation between charge collection in the device and the ensuing single event upset in the composite circuit. In favorable cases, we show how probing circuits with tunable laser pulses can estimate a charge collectio...

The effects of radiation on MEMS accelerometers

Exposing just the mechanical part (sensor) of MEMS accelerometers to protons and heavy ions caused large changes in outputs representing the measured acceleration for the ADXL50 and very small changes for the ADXL04. The large voltage shift measured for the ADXL50 is attributed to charge generated by the ions and trapped in dielectric layers below the moveable mass. The trapped charge alters the electric field distribution which, in turn, changes the output voltage. The construction of the ADXL04 differs from that of the ADXL50 in that the dielectric layers are covered with a conducting polycrystalline silicon layer that effectively screens out the trapped charge, leaving the output voltage unchanged.

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.

Ion Track Shunt Effects in Multi-Junction Structures

Charge collection processes are discussed for heavy ion hits across multiple p-n junctions in bipolar transistor or CMOS structures. The concept of a resistive-like ion track shunt bridging two like conductivity regions is introduced and a first-order-model developed for the charge transported along the ion track shunt. This model is shown to be consistent with charge collection measurements on multi-junction CMOS-like structures. It is found that the charge collection at a given p-n junction is influenced and can even be changed in sign by voltages present at a second p-n junction when the ion track penetrates both junctions. This has important consequences for the design of radiation hard integrated circuits and such ion track shunt effects become more important as device dimensions are scaled to smaller values.

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. >

Numerical simulation of heavy ion charge generation and collection dynamics

Describes a complete simulation approach to investigating the physics of heavy-ion charge generation and collection during a single event transient in a p-n diode. The simulations explore the effects of different ion track models, applied biases, background dopings and LET (linear energy transfer) on the transient responses of a p-n diode. The simulation results show that ion track structure and charge collection via diffusion-dominated processes play important roles in determining device transient responses. The simulations show no evidence of rapid charge collection in excess of that deposited in the device depletion region in typical funneling time frames. Further, the simulations clearly show that the device transient responses are not simple functions of the ions incident LET. The simulation results imply that future studies should consider the effects of ion track structure and extend transient charge collection times to insure that reported charge collection efficiencies include diffusion-dominated collection processes. >

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. >

Charge Transport by the Ion Shunt Effect

Information on the quantity of charge transported between two junctions by the ion shunt effect is presented as a function of bias voltages and ionization densities.

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. >