3D Simulation and Analysis of the Radiation Tolerance of Voltage Scaled Digital Circuits

In recent times, dynamic supply voltage scaling (DVS) has been extensively employed to minimize the power and energy of VLSI systems. Also, sub-threshold circuits are becoming more popular. At the same time, the reliability of VLSI systems has become a major concern under Single Event Upsets (SEUs). SEUs are very problematic even for circuits operating at nominal voltages. With the increasing demand for low power reliable systems, it is therefore necessary to harden DVS and sub-threshold circuits efficiently. In this paper, we perform 3D simulations of radiation particle strikes in an inverter implemented using DVS and sub-threshold design. We analyze the sensitivity of the inverter to radiation particle strikes by varying the inverter size, the inverter load, the supply voltage (VDD) and the energy of the radiation particles. From these 3D simulations, we make several non-intuitive observations which are important to consider during radiation hardening of DVS and sub-threshold circuits. Based on these observations, we propose several guidelines for radiation hardening of DVS and sub-threshold circuit designs. These guidelines suggest that the traditional radiation hardening approaches need to be revisited for DVS and sub-threshold designs. We also propose a charge collection model for DVS circuits. Our model can accurately estimate (with an average error of 6.3%) the charge collected at the output of a gate for different supply voltages and different gate sizes for medium and high energy particle strikes. The parameters of our charge collection model can be included in SPICE model cards of transistors, to improve the accuracy of SPICE based radiation simulations for DVS circuits.