Increase in the Heavy-Ion Upset Cross Section of Floating Gate Cells Previously Exposed to TID

Electronic chips working in the space environment are constantly subject to both single event and total ionizing dose effects. To emulate this scenario for Flash memories, we tested under heavy ions floating gate cells, previously irradiated with x-rays, without performing any erase and program operation in between the two exposures. We observed an increase in the heavy-ion upset cross section in the devices that were submitted to TID irradiations, especially at low LETs. This effect is attributed to the combination of the threshold voltage shifts induced by heavy ions and x-rays. Implications for the hardness assurance of Flash memories are discussed.