nsideration for advanced DRAM and SRAM development in the last decade. A particular area of recent concern is flip-chip packaging technologies that place a source of alpha particles (Pb-Sn solder bumps) right on the die itself, where they cannot be shielded by coating layers [299]. Even in the absence of on-chip sources of radiation, recent studies have conclusively proved that terrestrial cosmic rays (primarily neutrons) are a significant source of soft errors in both DRAMs and SRAMs [315-317]. Upsets have been observed both at ground level and in aircraft and have been convincingly correlated to the altitude and latitude variation of the neutron flux, as shown in Figure 6.7 [307,315,317]. Lage, et al. have shown that even without alpha particles, a baseline of cosmic-ray upsets still exists for high-density SRAMs [316]. This result is shown in Figure 6.8, which plots the correlation between the measured accelerated soft error rate (ASER, measured using an intense alpha-particle source placed over the SRAM) and the system soft error rate (SSER, the real application error rate measured in many SRAMs operating over a long period of time). If the system SEU rate were due entirely to alpha particles, there would be a simple direct correlation between the SSER and the ASER, but instead the data show a baseline SER due to neutron upsets even in the absence of alpha-particle upsets [316]. O'Gorman has shown that neutron upsets disappear for DRAMs placed 200 meters underground in a salt mine, while they increase dramatically for systems operated above 10,000 feet in Leadville, CO [315]. In addition to SEU observed in memories used in large computer systems and aircraft, upsets have been observed in SRAMs used in implantable medical devices such as cardiac defibrillators Atmos neutron flux In-flight upset rate Latitude (degrees) Figure 6.7. Excellent correlation is observed between upsets recorded in avionics and the atmospheric neutron flux [307]. II-62 100000 SSER events caused by package alphas plus cosmic ray point b