A Failure Caused by Extended Cross-Defect in DRAM

Abstract

Abstract As memory devices decrease in dimensions, particles are found to be a major source of defects in unexpected DRAM failures due to their relatively large size. Among many DRAM defects, cross-defects account for the majority of system failures for a long time. Hard cross-defects are frequently observed in the first test step. Most of these defective cells are repaired with row or column redundancy resources. However, after high voltage and temperature stress, some of the cross-defects can additionally spread to adjacent rows or columns with reduced resistance. Recently, a new type of bridge cross-defect accompanied by row failures has emerged. This can be detected electrically through current measurements of 2 wordline (WL) under active mode. But, these defects are not obvious even after both high temperature and voltage stress. The bridge causes intermittent failure in the row-direction during DRAM operation. This soft 2-WL bridge is considered a serious fault source that can cause uncorrectable error (UE) at the system level even though on-die error correction code (ODECC) is introduced. Therefore, it is very important to find, improve, and develop control methods on such defects for future DRAM enhancement. In this paper, 2-WL defects or extended cross-defect were intensively analyzed through electrical failure analysis (eFA) and physical FA (pFA). Results reveal fine particles as the cause.