Variable Hold Time in Dynamic Random Access Memories

Abstract : Unstable data retention time has been observed in small populations of bits in certain dynamic random access memory (DRAM) products. This effect has been called variable hold time (VHT), and it represents a potential soft error failure mechanism. This is especially true for devices with poorly controlled refresh time distributions in situations where system refresh time requirements may be aggressive (eg. at either high temperature where Tref is low, or in a low temperature application which assumes high Tref values). Commonly specified device testing procedures do not determine the distribution of bit refresh times or VHT characteristics, and therefore there is no assurance that VHT soft errors will not occur in use. This report describes testing procedures developed to characterize VHT in DRAMs, and includes test results for a limited number of samples of a few device types. Our test results indicate that generally refresh times are specified conservatively enough, and that VHT instabilities are low enough that no soft errors would be expected in these particular devices. However, at least one device type tested was marginal in this respect, and further study is needed to understand lot-to-lot VHT variations with different manufacturers' materials and processing. Dynamic random access memory, DRAM, Variable hold time, VHT, Refresh time, Instability, Reliability, Screen.