Anomalous charge loss of reference cell in MLC flash memory due to process-induced mobile ion

This paper reports an anomalous charge loss observed from the reference cell of MLC flash memory induced by the cobalt salicide process. The reference cell is shown to exhibit unique retention behavior different from that of the normal array cell. Both the source and mechanism of the enhanced charge loss therein are investigated through various bake tests, mobile ion analysis, and structural analysis. A simple yet effective way to protect the reference cell from the process-induced mobile ions is proposed.