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IEEE Transactions on Nuclear Science, Vol. NS-27, No. 6, December 1980 A STUDY OF SINGLE EVENT UPSETS IN STATIC RAMS'S By W. E. Price, D. K. Nichols and K. A. Soliman California Institute of Technology Jet Propulsion Laboratory 4800 Oak Grove Dr. Pasadena, California 91103 Introduction Tests of nine different manufacturing device types of CMOS static random access memories (RAM's) were per- formed at the Naval Research Laboratory (NRL) cyclotron. The devices were exposed to a 50 MeV proton beam and also to a spectrum of neutrons whose energy ranged be- tween 5 to 30 MeV. This work was undertaken in order to establish whether the CMOS static devices were sus- ceptible to single event upsets and/or latchup such as might occur in earth orbit or interplanetary space. As such, this study complements work reported by Guenzer et all who used a similar radiation environment to investigate single event upsets in dynamic 16K RAM's. Guenzer's work on dynamic RAM's showed that upsets oc- curred in all fifteen of the dynamic RAM's that were tested at fluences of 108 to 109 particles/cm2. Our work is also a logical extension of irradiations with heavy ions of static RAM's conducted at the Berkeley 2 cyclotron . In those tests, 150 MeV Krypton ions had sufficient ionizing density to cause bit flips in some (but not all) of the devices under test. Our recent tests showed that none of the CM^OS static RAM's was susceptible to single event upsets for neutron fluences >1011 n/cm2 or proton fluences of 10 protons/cm2. Experimental Procedure In all experiments, the CMOS devices were exposed to the particle radiation while biased in their normal operating mode (5 volts). They formed part of the mem- ory space for a 9900 portable minicomputer located be- hind shielding within the cave. The devices under test were connected by 5 ft. of ribbon cable to a personal- ity board which in turn was connected to the minicom- puter. The computer was connected to a RS-232 tele- typewriter terminal outside the cave which was used to write the devices and record failure addresses and the post-radiation bit states (see Fig. 1). For each test, the device was filled with a check- erboard pattern and then the memory space was checked to ensure that the writing had been correctly done and that the chip was not spontaneously upsetting (no up- sets from voltage transients were ever observed). Then the chip was irradiated to %1011 neutrons/cm2 (first half of experiment) or 4109 protons/cm2 (second half of the experiment), delivered in ten minute periods. After the irradiation was completed, the memory was reinterrogated. Since no bit flips were observed, a few runs of higher fluence were made at the same flux rate. For neutrons, the maximum delivered fluence was 1012 neutrons/cm2 for one device type; for protons, there were several runs at 2 x 10 p/cm2 and one at 3.5 x 10 p/cm2. Also a few runs at lower operating voltage (4.5 volts) were made. However, limitations on the total equivalent dose that should be delivered to the device and the radioactivity of the chamber itself jus- tified only a few such high fluence runs. The neutron beam was obtained by the reaction of 35 MeV deuterons on a thick Beryllium target, yielding the spectrum shown in Fig. 2. The fluence was monitored by pre-test measurements using densitometer readings of exposed film provided by NRL. The proton beam was ex- tracted from a high flux primary beam by scattering the main beam through a 16 mil lead foil. The 50 MeV Rutherford-scattered proton flux obtained at about 300 off axis was %2 x 106 p/cm2-sec. The total fluence was determined by pre-test calibration of a NaI crystal used at the device exposure location against integrated current readings from a Faraday cup located on the pri- mary beam axis. The proton beam energy was determined by a calibra- tion of the beam bending magnet current. Experimental Results None of the tested devices listed in Table I showed bit flips for the 50 MeV proton irradiation or for the neutron fluence shown in Fig. 2. The fact that no bit flips were observed in this test cannot be attributed to an inadequate delivered dose, since the scattered beam was present everywhere in the chamber. A few special TLD readings were taken during the course of the day at the sample location to verify that the pre-experimental calibration between the sample site and the Faraday cup located at the end of the primary beam were in agreement. The adequacy of the instrumentation system was proved out at the heavy ion tests conducted at Berkeley by Kolasinski et a12. Most of the devices tested here were also tested at Berkeley using this same instrumen- tation and the devices did indeed show many flips at that test. Discussion The static RAM's listed in Table I have been shown to be insensitive to the neutron and proton irradia- tions described. This result is consistent with the results of Guenzer et all because it is known that the threshold charge deposition (critical charge) required to cause a bit-flip in the static devices is usually several times larger than for the dynamic RAM's; in part because of the greater capacitance (larger cell size) of the static devices and in part because of their larger tolerance to changes in voltage. The prediction of hardness for the static CMOS de- vices was also indicated by the data taken by Kolasin- ski2 at the Berkeley cyclotron using R,150 MeV Krypton ions. These tests showed that several (but not all) of the devices were susceptible to bit-flips from the 150 MeV Krypton ions which have a rate of4energy deposition or stopping power of dE/dx = 4.5 x 10 MeV-cm2/gm in Silox. However, the same tests showed that the thres- holds for bit-flips were not much lower than this value. 0018-9499/80/1200-1506$00.75 (1980 IEEE 1506
