x neutron beam [297-299]. IBM has taken a similar approach in the development of their advanced soft-error modeling system (SEMM) [300-302]. This code models both terrestrial cosmic rays and on-chip alpha- particle sources, and includes a nuclear spallation simulator to model particle-silicon recoil spectra from first principles [230,303]. In addition to a nuclear spallation model, SEMM includes a Monte Carlo model for diffusion of the incident charge based on the earlier work of Sai-Halasz [249-251], fitting to transient device simulation to determine the temporal shape of the charge collected at the various junctions, circuit simulation to determine critical charges, and can also p-channel transistors n-channel transistors Figure 5.5. Illustration of the simulation of an entire SRAM cell in the three-dimensional device simulation domain. II-54 incorporate statistical data from process variations. This is very computationally intensive, SO SEMM makes extensive use of history files to reduce the amount of computation necessary for any one run. Good agreement has been obtained between SEMM results and field experiments with no parameter adjustments, indicating the predictive nature of the model. Figure 5.6 shows the simulated components of the soft-error rate in a bipolar chip [304]. Components are computed due to both alpha-particle sources and the cosmic ray spectrum. Alpha-particle sources include three levels of metal, Pb-Sn solder pads, and a ceramic layer. Note that for events with small critical charge, alpha particles are dominant and mostly come from the Pb-Sn solder and ceramic materials. For critical charges above 250 fC, only cosmic ray events are capable of inducing upset in this chip. IBM has marketed a soft-error simulation service to outside customers based on these capabilities [305]. Pb-Sn solder Total (alpha) 2nd M Ceramic Cosmic 1st M 3rd M Critical charge (fC) Figure 5.6. Simulations of relative soft-error rate in a bipolar memory cell due to terrestrial cosmic rays and various on-chip and pac