MOSFET-based 3D polycrystalline silicon stacked capacitorless DRAM with superior grain boundary influence immunity
When the GB is located in the channel of the n-type MOSFET, the acceptor type trap forms an energy barrier when electrons are trapped as shown in Fig. 3. The GB-induced potential barrier directly suppresses current flow. Consequently, it causes the Ve ascend5. Thus, it is shown that the variations of the conduction characteristics are caused mainly by the number of WBCs located in the channel3. The Ve is defined by the constant current method, which determines the Ve as the Vg the value for ID = 10−seven A/µm15.
Statistical analysis of transfer characteristics
Figure 4 shows the drain current (ID) as a function of the gate voltage (Vg) curves of 4000 samples each ADG 1T-DRAM single layer and ADG 1T-DRAM 3D stacked with a gCut of 30 nm under drain voltage = 0.5 V Histograms of (a) the Ves (b) the sub-threshold oscillates (SSs) (c) the on-currents (Ions), and (d) off currents (Istoppeds) obtained from simulations for 4000 samples with the proposed GB model, for single-layer 1T-DRAM ADGs and proposed 3D stacked 1T-DRAM ADGs are taken from Fig. 4.
In Fig. 5a, the averages of the extracts Ves of single-layer 1T-DRAM ADGs and stacked 3D 1T-DRAM ADGs are 1.002 V and 0.956 V, respectively. The SDs for the Ves are 15.9 mV and 6.5 mV respectively. The RSDs are 1.58% and 0.68% respectively. The RSD is the SD divided by the mean and multiplied by 100. In other words, a large RSD indicates that the SD is large compared to the mean and has a large dispersion. Since the RSD is expressed as a percentage, it is a merit coefficient that can be used to compare the distribution of data sets with different units of measurement. The ID are higher in stacked 3D 1T-DRAM ADGs due to multi-layer structure, resulting in lower Ve. The RSDs of Ves are 1.58% for single-layer 1T-DRAM ADGs and 0.68% for stacked 3D 1T-DRAM ADGs. Since the RSD of single-layer 1T-DRAM ADGs is about twice the RSD of stacked 3D 1T-DRAM ADGs. Therefore, the 3-D structure has better reliability for the Ve variations due to GB.
Figure 5b shows the distribution of SSs. The average of SSs of single-layer 1T-DRAM ADGs and stacked 3D 1T-DRAM ADGs have similar values, 115.0 mv/dec and 114.2 mv/dec, respectively. Single-layer 1T-DRAM ADGs have SS ranging from 110 mv/dec to 140 mv/dec, as can be seen. However, in the case of 3D stacked 1T ADG DRAMs, it can be seen that most SSs are distributed between 112 mv/dec and 117 mv/dec, respectively. This phenomenon occurs because they have 3D stacked structures that complement the fluctuation in current characteristics caused by GBs. This phenomenon is called the averaging effect. In the case of single-layer 1T-DRAM ADG, it is greatly affected by the GBs of a channel region. However, even if one layer is significantly affected by GBs, the current and memory performance variation due to GBs is small in stacked 3D 1T-DRAM ADG because the performance degradation is shared with the other two layers. . This is a phenomenon similar to the decrease in the variation of the work function when the granularity of the metal grid decreases16.17. Therefore, the RSD of SSs of stacked 3D 1T-DRAM ADGs is more than 9 times smaller than single-layer 1T-DRAM ADGs, which can be considered excellent in the reliability of transfer characteristics.
Figure 5c shows the distribution of Ions. The average Ions single-layer 1T-DRAM ADGs and stacked 3D 1T-DRAM ADGs are 1.87×10–4 A/µm and 5.74×10−4 A/µm, respectively. The average Ions of 3D-stacked 1T-DRAM ADGs, which are multi-channel structures, is about three times larger than single-layer 1T-DRAM ADGs. A stocking Ion is a long-standing weakness of poly-Si transistors, but it can be solved by using a 3D stacked structure. Moreover, the DS of Ions are 1.45×10−5 A/µm and 2.49×10−5 A/µm, respectively. For SD, the dispersion of stacked 3D 1T-DRAM ADGs seems larger, but for RSD, which reflects the actual dispersion, the RSD of single-layer 1T-DRAM ADGs is 7.75%, and the Le RSD of stacked 3D 1T-DRAM ADGs is 4.34%, respectively. In conclusion, 3D stacked 1T-DRAM has excellent transfer characteristics, such as greater Ion and more minor variation.
Figure 5d shows the distribution of Istopped s. The average of Istopped s of stacked 3D 1T-DRAM ADGs is 1.6 times larger than single-layer 1T-DRAM ADGs. The Istopped s is the parameter with the most pronounced averaging effect. As mentioned above, thanks to the averaging effect, the RSD of the Istopped of 3-D stacked ADG 1T-DRAM is 3% due to 3-D structure. On the other hand, the transfer curves of single-layer 1T-DRAM ADGs exhibit large variances, which results in an RSD of 130.7%, as shown in Fig. 4. The factor of merit (FOM) of transfer characteristics and memory performance are summarized in Table 4.
Statistical analysis of memory performance
The histograms of (a) detection margins (SM) and (b) RTs from simulations of 4000 samples with the proposed GB model, for the proposed single-layer 1T-DRAM ADGs and stacked 1T-DRAM ADGs in 3D, are shown in Figure 6a. In Figure 6a, the averages of the SMs of single-layer ADG 1T-DRAM and 3D stacked ADG 1T-DRAM are 5.72 µA/µm and 17.4 µA/µm, SDs are 1.44 µA/µm and 2.54 µA/µm, and RSDs are 25.2% and 14.6%, respectively. The SD of SMs in stacked 3D 1T-DRAM ADGs is larger than that of monolayers, but the RSD is the opposite. Also, in the case of single-layer 1T-DRAM ADGs, depending on the GB distribution, some samples get poor SMs below 3 µA/µm which is the reference value18. On the other hand, all samples of the 3D stacked 1T ADG DRAMs have an SM greater than 3 µA/µm.
As shown in Fig. 6b, the average RTs of single-layer 1T-DRAM ADGs and stacked 3D 1T-DRAM ADGs are 212 ms and 200 ms, respectively, which show similar values. However, as shown in Figure 6b, the SDs of the RTs are 116 ms and 82 ms and the RSDs of the RTs are 54.7% and 41%. This indicates that 3D stacked 1T-DRAM ADGs have smaller RT gaps. Additionally, in single-layer 1T-DRAM ADGs, 6.2% of samples are insufficient for RT to meet 64ms, which is the International Roadmap for Devices and Systems (IRDS) memory criteria (> 64ms)19. However, in the case of 3D-stacked 1T-DRAM ADGs, all samples have an RT greater than 60 ms, as shown in Fig. 6b. In terms of RT, stacked 3D 1T-DRAM ADGs are more reliable than single-layer 1T-DRAM ADGs. The memory performance FOM is summarized in Table 4.
Therefore, when considering the SDs and RSDs of transfer characteristics and memory performance, the stacked 3D 1T-DRAM ADGs showed excellent performance not only of transfer characteristics but also of memory performance. . In addition, they also show strong immunity to the impact of GB, especially on retention in the context of memory performance. Thus, our offered stacked 3D 1T-DRAM ADGs can be excellent devices in terms of reliability. The performance comparison of conventional 1T-1C DRAMs, capacitorless DRAMs, and 3D stacked asymmetric double-gate 1T-DRAM is summarized in more detail in the Supplementary Information.