## 3 Results
### Heavy Ion Irradiation  
Heavy ion experiments were performed at the Texas A&M University's Cyclotron Institute using Ne, Ar, Cu, and Ag ions in a 15 MeV/amu cocktail. Various tests were performed at both a nominal supply voltage and a reduced voltage of 0.6 V. Tests were also performed with incidence angles of 0\({}^{\circ}\) and 60\({}^{\circ}\) from normal. The test chip was tilted such that incoming ions from the beamline travelled parallel to the VDD and VSS lines of the chip. This increased the likelihood of ions striking multiple adjacent transistors simultaneously. Table 2 provides information about the ions that were used, including effective LET values at 60\({}^{\circ}\) angle tests and the ion range in silicon. A vacuum was not used during testing, and the chip was de-lidded. Figure 8 depicts the testing setup.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Ion** & **Angle** & **LET (MeV\(\cdot\)cm\({}^{2}\)/mg)** & **Range (\(\mu\)m)** \\ \hline Ne & 0\({}^{\circ}\) & 2.74 & \\  & 60\({}^{\circ}\) & 5.48 & 292 \\ Ar & 0\({}^{\circ}\) & 8.45 & \\  & 60\({}^{\circ}\) & 16.90 & 212 \\ Cu & 0\({}^{\circ}\) & 19.70 & 155 \\  & 60\({}^{\circ}\) & 39.40 & 155 \\ Ag & 0\({}^{\circ}\) & 46.90 & 130 \\ \hline \end{tabular}
\end{table}
Table 2: Parameters of heavy ions used during testing.  
Figure 8: Heavy ion experimental setup at the TAMU Texas A&M University. The incident angle was set at 60\({}^{\circ}\) from normal.  
SEU cross sections were calculated for each respective LET value by using Equation (1).  
\[\sigma=\frac{N_{Errors}}{\Phi\times N_{bits}} \tag{1}\]  
where \(N_{Errors}\) is the number of recorded errors during testing, and \(\Phi\) is the effective fluence during testing in ions/cm\({}^{2}\). The inclusion of the \(N_{bits}\) term in the denominator normalizes the cross-sectional value to that of one flip-flop. Calculated cross-sectional values were fitted to Weibull curves, as shown in Figures 9 and 10, and error bars were calculated with 95% certainty. When no errors were observed during testing, the cross-sectional value at each respective LET was determined by assuming an error number of one to provide a maximum cross section value. These are indicated in the figures by the inclusion of a grey arrow pointing downwards.  
Figure 10: Cross–sectional values for heavy ion testing at a 60\({}^{\circ}\) incidence angle from normal. (**a**) Core voltage of 0.8 V, (**b**) core voltage of 0.6 V.  
Figure 9: Cross–sectional values for heavy ion testing at a 0\({}^{\circ}\) incidence angle from normal, (**a**) core voltage of 0.8 V, (**b**) core voltage of 0.6 V.  
All presented DICE designs performed remarkably well in comparison to the reference design. As expected, the cross-sectional values for the reference chain were larger in the tests when the core voltage was reduced to 0.6 V, as the critical charge for each cell will decreased with a decrease in the supplied voltage. Of all three presented DICE designs, only one design showed errors during testing. This was the CnRx DICE design, as indicated in Figure 10b. The chain experienced a total of five upsets at an LET of 93.8 when the core voltage was reduced to 0.6 V. These errors occurred during testing also when the incidence angle of the particles striking the board was 60\({}^{\circ}\) from normal.