# Mechanism and Equivalence of Single Event Effects Induced by 14 MeV Neutrons in High-Speed QDR SRAM
## 4 Monte-Carlo Simulation
### Simulation Results  
Figure 8 shows the 14 MeV neutron and atmospheric neutron-induced secondary ion species in the device SV. An obvious difference can be seen. The total yield of secondary ions generated by atmospheric neutrons is higher than that generated by 14 MeV neutrons. Additionally, it is important to note the logarithmic scale on the Y-axis in Figure 8; it is very clear that the main reaction products are n, p, Si, etc., of which protons are perfectly capable of leading to some soft errors in the QDR SRAM with a critical charge of 1 fC (shown in Table 3). This reason may account for this phenomenon in Figure 3, which can be described as the 14 MeV neutron-induced SEU cross-section being lower.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**DUT** & **Memory Cell** & **SV (x \(\times\) y)** & **Depth of SV** & **LET\({}_{\text{th}}\)** & **Critical Charge** \\ \hline QDR SRAM & 1 \(\mu\)m \(\times\) 0.5 \(\mu\)m & 0.2 \(\mu\)m \(\times\) 0.19 \(\mu\)m & 0.45 \(\mu\)m & 0.22 MeV-cm\({}^{2}\)/mg [14] & 1 fC [15] \\ \hline \end{tabular}
\end{table}
Table 3: Memory cell size and SV parameters of the QDR SRAM.  
Figure 7: 3D device model in the Monte-Carlo simulation.  
Besides, compared with the 14 MeV neutrons, atmospheric neutrons are able to generate more kinds of secondary ions in the SV within the scope from p to W. Due to this wide energy spectrum, more high-Z secondary ions, such as W, Ta, Hf, etc., are produced by atmospheric neutrons. This reason may account for the phenomenon in Figures 3 and 5 where the MCU cross-section, ratio, and maximum size induced by atmospheric neutrons are higher than those induced by 14 MeV neutrons, since heavier secondary ions with higher LET in Figure 8 are the main cause of MCU events.  
Figure 9 further shows the LET features of the secondary ions caused by 14 MeV and atmospheric neutrons in the SV of devices. First, under atmospheric neutron irradiation, the maximum LET of secondary ions in the device SV can be achieved at 31.5 MeV\(\cdot\)cm\({}^{2}\)/mg. While the maximum LET decreases to 7.8 MeV\(\cdot\)cm\({}^{2}\)/mg under the 14 MeV neutron irradiation. Second, as the LET increases, the overall ratio decreases rapidly under the 14 MeV neutron irradiation. These results suggest that the usage of 14 MeV neutrons may underestimate or even miss SEE types with high LET\({}_{\text{th}}\), such as SEU in hardened devices.  
Figure 10 displays the characteristic of deposited charges, Q\({}_{d}\), in the SV induced via 14 MeV and atmospheric neutrons. In the area of \(Q_{d}\) < 17 FC, the deposited charge probability under the 14 MeV neutron irradiation is slightly higher than that under the atmospheric neutron irradiation. While in the area of \(Q_{d}\) >17 FC, the apparent result of this is that atmospheric neutrons are capable of generating secondary ions with larger LET values, longer ranges, and thus more deposited charges in the device SV, with also higher probability. The secondary ion makes the maximum of the charge deposited reach 118.5 FC.  
Therefore, we can conclude that the equivalence of 14 MeV neutrons for atmospheric neutron-induced SEE evaluation is closely related to the critical charge of the DUT. For the SBU response of the QDR SRAM in this work, with a critical charge of 1 fC, the SBU cross-section induced by 14 MeV neutrons is approximately 22.8% lower than the value in the real atmosphere. It can be predicted that the difference will decrease as the feature size shrinks. However, for MCU and other SEE types with high critical charge, the discrepancy between 14 MeV neutrons and atmospheric neutrons will increase.  
Figure 8: 14 MeV and atmospheric neutron-induced secondary ion species in the device SV.  
Figure 9: LET features of secondary ions generated by 14 MeV and atmospheric neutrons in the device SV.