Influence of polonium diffusion at elevated temperature on the alpha emission rate and memory SER performance

Alpha emission rate measurements were performed on solder bump material at room temperature and post bake at high temperature. Results indicate an increase in alpha emission rate after high temperature bake process attributable to Po-210 out-diffusion from such solder bumps. Real-time system-level soft error rate (SER) measurements show >60% higher SER/bit when the tests are conducted at elevated junction temperature. The temperature-induced variations in alpha and neutron SER due to circuit response were characterized through accelerated tests and indicate <;10% increase over similar temperature range. Thus these results indicate that variations in alpha SER caused by material emission rate variations can be a significant factor and must be included when assessing product SER. The physical aspects of the SER impact are discussed based on modeling of the Po-210 out-diffusion within the solder bump and the decay of the out-diffused Po-210. The model suggests that the out-diffusion effect will be more pronounced initially for about a couple of years depending on the temperature of IC operation. Beyond this initial period when secular equilibrium dominates, out-diffusion is expected to continue but in manner that mimics and envelopes the behavior expected primarily from secular equilibrium.