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

There is a growing need for the inclusion of nonvolatile memory within impl antable medical devices in order to store product identification, operating parameters, calibration information, as well as patient and diagnostic dat a. Due to the critical nature of the application however, the data retentio n reliability is of utmost importance, In the case of nonvolatile memories, a source of concern regards their exposure to ionizing radiation as the re sult of diagnostic or therapeutic procedures performed on the patient. This paper reports on X- and gamma-ray experiments and calculations on a repres entative modern electrically erasable and programmable read only memory (EE PROM) (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 dos e 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 increas ed as a function of total dose from 47 mu A prior to exposure to 385 mu A a t 30 krad(Si), The mean highest no-failure level for 10 unbiased parts expo sed to Co-60 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 rad iation doses suggest that floating-gate EEPROM technology is reliable in th e presence of photon ionizing-radiation exposures typical of medical diagno stic and therapeutic environments.