The conventional capacitor-under-bitline (CUB) DRAM structure faces problems of high photoresist developing aspect ratio and step-height. The present invention discloses a DRAM with planar upper-plate structure and the upper-plate forms an opening broader than the bitline contacts at the top of the lower-plate neighboring the bitline contacts to isolate from the bitline contacts, and the step height at the interface between the peripheral circuit and cell arrays almost does not exist. Furthermore, conventional problems could be solved because of an oxide plug during producing bitline contacts and the thick oxide deposited on the peripheral circuit. A lightly doped polysilicon is deposited between the lower-plate and the silicon wafer substrate to avoid current leakage of the lower-plate.