X- and gamma-ray hardness of floating-gate EEPROM technology as applied to implantable medical devices

There is a growing need for the inclusion of nonvolatile memory within implantable medical devices in order to store product identification, operating parameters, calibration information, as well as patient and diagnostic data. Due to the critical nature of the application however, the data retention reliability is of utmost importance. In the case of nonvolatile memories, a source of concern regards their exposure to ionizing radiation as the result of diagnostic or therapeutic procedures performed on the patient. This paper reports on X- and gamma-ray experiments and calculations on a representative modern electrically erasable and programmable read-only memory (EEPROM) (Atmel 24C64). No transient upsets due to 150 kVp X-rays were observed in 10 unbiased and five biased DTU´s up to the maximum achievable 27 rad(Si)/s for a total dose of 200 rad(Si). Unbiased parts had no failure to an average total-dose of 40.9 krad(Si). The lowest failure level observed for an unbiased part was 30.0 krad(Si). In the biased parts, the read-mode operating current increased as a function of total dose from 47 μA prior to exposure to 385 μA at 30 krad(Si). The mean highest no-failure level for 10 unbiased parts exposed to Co⁶⁰ gamma-rays was 36.9 krad(Si) with a sigma of 2.3. Five biased DUT failures occurred at a mean of 27.84 krad(Si) with a sigma of 2.42. The analysis of these data, in comparison to maximum therapeutic photon radiation doses suggest that floating-gate EEPROM technology is reliable in the presence of photon ionizing-radiation exposures typical of medical diagnostic and therapeutic environments