Single event upset mechanism transition and radiation hardening in 28 nm D flip-flops under proton irradiation

• The dominant SEU mechanism in 28 nm DFFs shifts from nuclear reactions to direct ionization for low-energy protons. • Low-energy protons induce an SEU cross-section 1-2 orders of magnitude higher than intermediate-energy protons. • LEAP layout hardening reduces the SEU cross-section by about an order of magnitude. • DICE-based design fully mitigates SEUs, offering robust hardening for 28 nm DFFs. This work investigates the single-event upset (SEU) sensitivity of 28 nm bulk-silicon CMOS D Flip-Flops (DFFs) under proton irradiation. Experimental results from both intermediate-energy (20–100 MeV) and low-energy (≤7 MeV) protons reveal that the SEU cross-section induced by low-energy protons is 1–2 orders of magnitude higher than that by intermediate-energy protons, indicating a shift in the dominant mechanism from nuclear reaction-induced secondary ionization to direct ionization. Meanwhile, the Geant4 simulation result suggests that a small but non-negligible fraction of incident protons may undergo sufficient energy loss to become relevant for direct ionization. Furthermore, several radiation hardening strategies based on DICE and layout enhancement are implemented and verified to effectively mitigate proton-induced SEUs. This study provides critical insights and hardening solutions for developing reliable 28 nm DFFs for space applications.