Resistive Memory Devices for Radiation Resistant Non-Volatile Memory

As space programs increase in number and scope, there is an increasing need for radiation-hardened electronic devices and circuits. In particular, missions to high-radiation environments, such as Europa, would greatly benefit from improved radiation hardness in electronic devices. In pursuit of this goal, resistive memory (RRAM) devices were fabricated at SUNY Polytechnic Institute and evaluated for radiation hardness. Our objectives were to produce RRAM devices resistant to high levels of radiation damage and to demonstrate that these devices would improve mission lifetime in high-radiation environments. Furthermore, the underlying mechanisms of radiation were investigated to provide recommendations for radiation-hardening RRAM devices, which could be applied to any candidate RRAM devices being considered for space applications. Devices were fabricated using several fabrication approaches, including patterning by shadow mask, photolithography-based etching, and photolithography-based liftoff. In each of these cases, total ionizing dose (TID) effects and displacement damage dose (DDD) effects were measured. TID effects from exposure to a 60Co gamma source were not observed to cause changes in device resistance or switching parameters in any experiments, with each device tested to at least 20 Mrad(Si). DDD was measured as radiation-generated oxygen vacancies per cm3 since oxygen vacancies are generally considered to be the active species involved in switching these devices. The lowest DDD level that caused a device to change resistance state was 1021 vacancies per cm3, and most devices failed at 1022 vacancies per cm3. This is an extremely high DDD level, even for RRAM devices, which have been reported to fail in the range of 1017-1020 vacancies per cm3. For comparison, an example flash memory device failed at 1015 vacancies per cm3. Vendor-fabricated devices with a similar composition to our own were also tested against TID and DDD. The vendor-fabricated devices did not exhibit changes due to TID, up to the tested level of 30 Mrad(Si). Meanwhile, vendor devices exhibited resistance state changes at 1021 vacancies per cm3, similar to our own devices. These results indicate that TaOx-based RRAM devices may be particularly resilient to both TID and DDD effects. The very high tolerance to radiation effects is most likely due to the high intrinsic concentration of oxygen vacancies within our devices. Based on X-ray photoelectron spectroscopy (XPS) measurements, there are approximately 1022 oxygen vacancies per cm3 in our devices as deposited. Most devices failed when the radiation-induced vacancies reached this level, indicating suggesting that a high intrinsic vacancy concentration protects against lower levels of displacement damage. High vacancy concentration likely also protects against TID by facilitating leakage of trapped charge out of the oxide. The use of a thin switching oxide (25 nm TaOx for our devices) is also expected to improve radiation hardness, as there is less room for charge trapping. Therefore, those wishing to produce very radiation-tolerant RRAM devices can probably achieve this by using a thin oxide that contains a high intrinsic concentration of oxygen vacancies. Our devices appear to be very tolerant of radiation effects, and would greatly increase the expected lifetime of a mission to Europa or another high-radiation target compared to flash memory devices. The similar radiation performance of vendor-fabricated devices is promising for adoption of RRAM devices as radiation-hardened memory devices for use in space. With continued commercial development of these devices, RRAM devices are strong candidates for next-generation memories that are inherently rad-hard. Note: This research was partly carried out at the Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by a NASA Space Technology Research Fellowship and the National Aeronautics and Space Administration. These experiments were carried out in collaboration with the NASA Jet Propulsion Laboratory and Sandia National Laboratory. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.