With decreasing feature sizes, transistors are being added to ICs in consistently greater numbers, which is leading to dramatic increases in power consumption. Changing process parameters and redesigning circuits are complicated and expensive solutions to lower power dissipation. A simple and cost effective approach is to operate standard cell libraries at ultra-low power (ULP) supply voltages. By lowering the supply voltage, the transistor current drives decrease by orders of magnitude, resulting in dramatically lower power consumption. However, small transistor drive currents also result in slow operating frequencies, so a trade-off must be made between power and performance.
In this work, the use of ring oscillators as a single-event test structure is introduced for the first time. Ring oscillators allow for single-event characterization of technologies by finding the minimum energy at which harmonic oscillation occurs. By finding the minimum laser pulse energy at which the ring oscillator enters a state of sustained harmonic oscillation for a range of power supply voltages, the single-event susceptibility of a technology can be determined. Experimental two-photon, backside laser irradiation results show that ULP circuit operation is most susceptible to single-events, because the threshold laser pulse energy is the lowest in this region.
Additionally, the effect of supply voltage on single-event transient pulsewidth is seen both in 3D TCAD simulation and experimentally through the use of single-photon, topside laser irradiations. 3D TCAD simulations show that transients created by ion strikes on a device operating in the subthreshold region have longer pulsewidths than those from a device operating at nominal voltages due to the decreased currents of the pull-up and pull-down devices connected to the struck nodes. Additionally, when the devices are operating at ultra-low voltages, the single-event transient pulsewidths generated from strikes on PMOS transistors are smaller than those generated from strikes on NMOS transistors. Normally, at the nominal supply voltage, the opposite is true, but when operating ULP circuits, parasitic bipolar amplification does not occur.
The increase in pulsewidths as a function of power supply voltage also is shown experimentally, as well as the independence of pulsewidth on laser pulse energy. At voltages outside of the ULP region, the standard, expected response of increasing single-event transient pulsewidth with increasing laser energy is seen. These trends were seen after strikes on both NMOS and PMOS transistors.