Single event latchup: hardening strategies, triggering mechanisms, and testing considerations

Abstract

Single event latchup (SEL) is a serious reliability concern for CMOS integrated circuits (ICs), and can be especially problematic in the space radiation environment. It can occur because of a parasitic pnpn circuit inherent in CMOS, which, if activated, introduces a low-impedance path across the power supply lines. This leads to a large current that can cause thermal damage in the IC. This dissertation describes an experimental study that focuses on SEL hardening strategies, triggering mechanisms, and testing considerations. The frontside single-photon absorption laser test method is used extensively. The backside two-photon absorption laser, broadbeam heavy ion, proton, and neutron test methods are also used. The majority of the work is done using custom test structures fabricated in a 180 nm bulk CMOS technology. Data are also presented on a commercial power device controller and 130 nm technology SRAMs.

We evaluate the effectiveness of various SEL hardening strategies, including thick-film silicon-on-insulator (SOI), triple well, and guard rings. Although SOI technology is widely reported to be immune to SEL, conventional pnpn latchup can occur and has been observed in non-dielectrically isolated SOI processes. The introduction of triple well is shown to be an effective zero-area-penalty hardening strategy, although it does not result in the SEL immunity that was achieved when guard rings were introduced. After triggering latchup with the pulsed laser in a given pnpn region, latchup was observed to spread and infect many adjacent pnpn regions. The physical mechanisms of this spreading are discussed, along with the implications for device characterization. Lasers are used in other experiments to map the shapes and positions of SEL sensitive regions, and show that the position of maximum sensitivity is not centered on a pnpn region, but between two neighboring pnpn regions, due to synergistic triggering. Finally, the SEL sensitivity maps demonstrate that laser light reflected from metal lines toward the silicon can contribute to the single event effect response in some cases, for both backside- and frontside-incident laser tests.