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We performed underground real-time tests to obtain alpha particle-induced soft error rates (alpha-SER) with high accuracies for SRAMs with 180 nm - 90 nm technologies and studied the scaling trend of alpha-SERs. In order to estimate the maximum permissive rate of alpha emission from package resin, the alpha-SER was compared to the neutron-induced soft error rate (n-SER) obtained from accelerated tests. We found that as devices are scaled down, the alpha-SER increased while the n-SER slightly decreased, and that the alpha-SER could be greater than the n-SER in 90 nm technology even when the ultra-low-alpha (ULA) grade, with the alpha emission rate < 1 times 10-3 cm-2h-1, was used for package resin. We also performed computer simulations to estimate scaling trends of both alpha-SER and n-SERup to 45 nm technologies, and noticed that the alpha-SER decreased from 65 nm technology while the n-SER increased from 45 nm technology due to direct ionization from the protons generated in the n + Si nuclear reaction.