Alpha-particle tracks in silicon and their effect on dynamic MOS RAM reliability

Recent investigations [1] have found low levels of alpha particles (< 0.1 counts/cm². h) emitted from the immediate chip environment to be responsible for soft errors in 16K MOS dynamic RAM´s. We have investigated this problem in two related studies. First, we review the interaction of alpha particles with the Si lattice and present range-energy and specific ionization data. Experimental values of the actual charge collected at shallow junctions for 4.9-MeV alpha events are presented. The dynamics of the collection process and the influence of the interface electric field in p-p⁺epitaxial material are identified. We further show the dependence of the collected charge on the incident particle energy and angle. Second we review the operation of current memory devices and indicate how soft failures due to alpha particles can be identified. We present soft error rate versus duty cycle data for a number of devices displaying different failure modes. The dependence of the soft error rate on incident particle energy and angle is also shown. Finally, we propose a simple accelerated test to evaluate device susceptibility to this failure mode.