A meta-stable leakage phenomenon in DRAM charge storage —Variable hold time

Abstract

A new leakage phenomenon called variable hold time (VHT) is reported which can compromise the data retention performance of modern DRAMs. Careful observations of retention (hold) time on many devices with planar cells and grounded field plates has uncovered a very small portion of the bit population which exhibits multi-valued and metastable leakage current at room temperature. Stable periods last from seconds to hours and are punctuated by nearly instantaneous transitions. Electrical characterization of VHT bits show increased transition rates with temperature and substrate bias. Physical investigation usually uncovers a silicon material defect present in the offending cell. A mechanism is proposed wherein VHT is the spatially-resolved manifestation of burst noise commonly observed in larger junctions.