Single-Event Effects Induced by Heavy Ions in SONOS Charge Trapping Memory Arrays

We investigate the sensitivity of silicon-oxide-nitride-silicon-oxide (SONOS) charge trapping memory technology to heavy-ion induced single-event effects. Threshold voltage (V-T) statistics were collected across multiple test chips that contained in total 18 Mb of 40-nm SONOS memory arrays. The arrays were irradiated with Kr and Ar ion beams, and the changes in their V-T distributions were analyzed as a function of linear energy transfer (LET), beam fluence, and operating temperature. We observe that heavy ion irradiation induces a tail of disturbed devices in the "program" state distribution, which has also been seen in the response of floating-gate (FG) flash cells. However, the V-T distribution of SONOS cells lacks a distinct secondary peak, which is generally attributed to direct ion strikes to the gate-stack of FG cells. This property, combined with the observed change in the V-T distribution with LET, suggests that SONOS cells are not particularly sensitive to direct ion strikes but cells in the proximity of an ion's absorption can still experience a V-T shift. These results shed new light on the physical mechanisms underlying the V-T shift induced by a single heavy ion in scaled charge trap memory.