1-Mbit 3-D DRAM Using a Monolithically Stacked Structure of a Si CMOS and Heterogeneous IGZO FETs

Abstract

We present a three-dimensional (3D) DRAM prototype, which is formed using oxide semiconductor FETs (OSFETs) monolithically stacked on a Si CMOS. The OSFETs are composed of a one-layer planar FET and two-layer vertical FETs (VFETs). The 1T1C memory cells in the VFET layers and a primary sense amplifier in the planar FET layer, which are formed using heterogeneous OSFETs, provide various circuit functions in the DRAM. The operation of the 3D DRAM in a 1-Mbit memory array is demonstrated for the first time. The results show that the proposed DRAM operates with read and write times of 60 ns and 50 ns, respectively. The leakage current of the memory cell is extremely low (comparable to an $2.2\times 10^{-19}$ A/cell at 85°C), indicating that over 99% of the data are retained in the memory array after one hour at 85°C without refresh.

Keywords

• Extremely low off-state current
• heterogeneous OSFETs
• monolithic stacking
• oxide semiconductor (OS)
• planar FET
• vertical FET