A four-level storage 4-Gb DRAM

A 4-Gb DRAM with multilevel-storage memory cells has been developed, This large memory capacity is achieved by storing data at four levels, each corresponding to two-bit-data storage in a single memory cell, The four-level storage reduces the effective cell size by 50%, A sense amplifier using charge coupling and charge sharing was developed for the four-level sensing and restoring, The sense amplifier uses a hierarchical bit-line scheme and operates in a time-sharing mode, thus reducing the sense amplifier area, A 4-Gb DRAM fabricated using 0.15-mu m CMOS technology measures 986 mm(2). The memory cell is 0.23 mu m(2). Its capacitance of 60 fF is achieved by using a high-dielectric-constant material BST.