The proposed 3-D stacked ADG 1 T-DRAM helps implement a high-reliability single-cell memory device. ![]() Recently, due to the complexity of the conventional DRAM’s capacitor fabrication, the 1T-DRAM has attracted great attention as a replacement for the conventional DRAM. The 1T-DRAM operates without the use of external capacitors, instead of relying on the floating body effect. The 1T-DRAM has the advantages of being easy to manufacture and having excellent logic device compatibility 1, 2, 3, 4, 5. However, the smaller sizes of these devices tend to limit their retention characteristics due to the stronger electric field between the body and the source/drain junction. The stronger electric fields accelerate the recombination/generation process of the excess holes, resulting in shorter RTs in scaling of the 1T-DRAM. Various groups have conducted many studies, and the ADG structure can be a great solution to overcome the RT of 1T-DRAMs 5. In addition, the 3-D stacked 1T-DRAM is made of polycrystalline silicon (poly-Si), so high-density 3-D memory arrays can be feasible. Based on their superior advantages in terms of the integrated fabrication technology, the poly-Si-based transistors have been widely used in 3-D memory technology in the past 6, 7, 8. The thermal budget was one of the significant challenges in implementing 3-D stacked transistors. The second layer and beyond of the device require high-temperature processing, which can threaten existing metallization materials or cause dopant diffusion in the lower layers of the device 9. These difficulties in the fabrication can be overcome by using excimer laser crystallization (ELC) and it can implement the 3-D stacked 1T-DRAM. Fabrication steps are outlined in references 4,10 and 11. The key fabrication steps for the proposed 3-D stacked 1T-DRAM are summarized in more detail in the Supplementary Information. ELC can solve the thermal budget issues, however, it cannot solve the random distribution of the GBs. Because they randomly varied depending on the laser irradiation energy density 10. The GBs are important in transistors made of poly-Si because they directly affect the transistor’s electrical performances. An analysis of the effect of GBs in a single channel was carried out in reference 5. However, a statistical study on the effect of the GBs in 3-D stacked ADG 1T-DRAM based on MOSFET with poly-Si has not been reported yet. In this work, the 3-D stacked ADG poly-Si MOSFET based 1T-DRAM with various average grain sizes ( G sizes) cells are investigated. ![]() A TCAD simulation is used to demonstrate the superior reliability of 3-D stacked ADG 1T-DRAM to the effects of the GBs 11. The proposed 3-D stacked ADG 1T-DRAM cells’ transfer characteristics, as well as memory performances, are analyzed and compared with the single-layer ADG 1T-DRAM. ![]() It has been proven to have excellent reliability to the effects of GBs.įigure 1 shows the cross-sectional view of the 3-D stacked ADG poly-Si-MOSFET-based 1T-DRAM cell with an ADG structure to implement high-reliability GB-independent electrical characteristics and memory performances. The main gates are used to control the conventional MOSFET operation during the read period and the tunneling operation during the program period. The control gates located below the channel regions are used to operate the tunneling phenomenon during program operation. Additionally, they sweep out holes during the erase period, helping to maintain the stored holes during the hold operation.
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