Investigation on high performance 1 transistor capacitorless DRAM with PiFET and ONO structure for embedded memory

Abstract

In this thesis, we have suggested and demonstrated the alternative structure of 1T DRAM as a candidate of embedded memory. Although various structures are suggested for high performance 1T DRAM, two critical issues are remaining unsolved. One is the good retention property with simple process. Especially it is focused on better ‘0’ retention and good heat immune property by using a bulk wafer. The other is multi-functional 1T DRAM which has high process compatibility with other device. For the better heat immune 1T DRAM, a body tied-PiFET structure is proposed. It has large process compatibility with bulk wafer and large windows of improving body capacitance. The degradation mechanism of retention characteristics is suggested and analyzed carefully by simulation. Quantity of retention degradation could be evaluated with numerical value which called retention factor $(R_F)$. Improved ‘0’-state retention is achieved by partially connected heat dissipation paths and a lowered electric field in the PiFET. ‘1’-state retention characteristics also can be improved by changing the PiOX thickness and selecting an appropriate material. For the multi-functional 1T DRAM, URAM is proposed. As it uses a simple O/N/O flash memory’s process on SOI wafer, large process compatibility and few reliability issue is expected. Due to the bias condition of URAM, both 1T DRAM operation and NVM operation can be feasible. It also shows sufficient retention margin and programming efficiency. By using a multi-functional property, new types of cell size scaling method is introduced at this paper. Ultimate cell size scaling to sub-1F2 is demonstrated by combining the MLC/MBC property of NVM and combining the DRAM/NVM property. In this thesis, possibility of multi-functional memory by using a simple O/N/O type flash memory with 1T DRAM is suggested for the first time. Other types of flash memory which use a NFGM (Nano-Floating Gate Memory) or high-k material with 1T DRAM are also possibl...