A 4-LEVEL STORAGE 4-GB DRAM

A 4-Gb DRAM with multilevel-storage memory cells has been developed, T his large memory capacity is achieved by storing data at four levels, each corresponding to two-bit-data storage in a single memory cell, Th e four-level storage reduces the effective cell size by 50%, A sense a mplifier using charge coupling and charge sharing was developed for th e four-level sensing and restoring, The sense amplifier uses a hierarc hical bit-line scheme and operates in a time-sharing mode, thus reduci ng the sense amplifier area, A 4-Gb DRAM fabricated using 0.15-mu m CM OS 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.