V. CHARACTERIZATION OF FFS SEES FOR ADVANCED TECHNOLOGY
5.3 SE Analysis of Sense-Amplifier Based FF Design
5.3.5 Experimental Results & Analyses
Test with alpha particles and heavy-ions have showed very little differences between input at logic level 0 and 1. Therefore, in this section only input at logic level 0 experimental results are presented. Also, due to the symmetric design of the SAFF, input at logic level 1 and 0 will have similar SEU cross-section.
5.3.5.1 Heavy-Ion Experimental Results
Fig. V.12 shows the heavy-ion results for conventional DFF and SAFF as a function of particle LET. For low LET particles (<20 MeV-cm2/mg), the SAFF shows about better performance compared to conventional DFF design. The decrease in SAFF cross-section is approximately 1.22×100¡¢ cm2/FF. The differences in SEU cross-sections between SAFF and DFF for low-LET particles are similar to what was observed for alpha particle exposures.
For high LET particles (>20 MeV-cm2/mg), SAFF and DFF cross-sections are comparable.
The decrease in SAFF SEU cross-section is approximately 4×100¡¤ cm2/FF. It must be kept in mind that percent difference between SAFF and DFF SEU cross-sections are higher for low-LET particles compared to high-LET particles. The observed experimental results can be explained by the critical charge of conventional DFF and SAFF as mentioned in Section 5.3.3. Qcritical of conventional DFF is 0.807 fC at 0.95V. It is lower than both second phase (1.227 fC) and third phase of SAFF (2.591 fC), but higher than the Qcritical of SAFF first
operation phase (0.159 fC). Even though the Qcritical of conventional DFF is higher than that of the first operation phase of SAFF, DFF design will be vulnerable to LET particles throughout the clock period of 400 ns compare to the first operation phase of SAFF will be vulnerable only for a few 10’s of pico-seconds when circuit operating at 2.5 MHz clock.
Therefore, conventional DFF has higher SEU cross-section than SAFF design. From the heavy-ion experimental results, both circuits design has similar low threshold LETs. Even though, SAFF design second and third phase has higher critical charge than DFF design, SAFF design still has first phase critical charge less than conventional DFF.
5.3.5.2 Alpha Particle Experimental Results
For low LET particles, the SAFF design showed cross-section values that are much smaller than those for the conventional DFF design. Even though SAFF design has higher
Figure V.12: SEU cross-section for heavy-ions as a function of particle LET for conventional DFF and SAFF at 0.95 V.
a result, the upset cross-section for SAFF (1.57×100¡¦ cm2/FF) is orders of magnitude lower than that for conventional DFF (1.42×100¡¢ cm2/FF) design when operating at 2.5 MHz clock. Previously published results have shown that the charge collected at the output node of a 16-nm bulk FinFET inverter for 1 MeV-cm2/mg (particle LET) is 0.7 fC at 0.9 V [112].
Based on the 3D TCAD simulation results, it would imply that the alpha particles don’t produce enough charge collection to exceed the critical charge during second and third operation phase (1.227 fC and 2.591 fC) in the SAFF, but yet may have just enough charge collection to exceed the critical charge in the conventional DFF (0.8 fC). Therefore, DFF design will be vulnerable to alpha particles throughout the clock period of 400 ns, but SAFF will be vulnerable only for a few 10’s of pico-seconds. Fig. V.13 shows the cross-section for both the designs over the temperature range tested for alpha particle exposures. The error bars for all experimental results are calculated as the standard error of measurement. Some error bars are smaller than symbol. As expected, both the FF designs showed very little change in cross-section over this temperature range.
With the SAFF expected to operate at very high frequencies, it is important to evaluate their performance over frequency. Previously, it has been shown that the transparent stage of a FF acts like a logic circuit and ion hits on this stage will increase the overall SEU error rate as a function of frequency [68]. Fig. V.14 shows the SEU performance of conventional DFF and SAFF designs up to the operating frequency of 1 GHz for alpha particle exposures. Results show the SEU cross-section percentage ((1 GHz SEU cross- section – 2.5 MHz SEU cross-section) / 1 GHz SEU cross-section) increase of the SAFF is
~97% (up to 1 GHz) and that for the DFF is ~45% (up to 1 GHz) over static operation. The primary reason for this is that with increasing clock frequency, the operation time for the
Figure V.13: SE cross-section for alpha particle exposures as a function of temperature for conventional DFF and SAFF at 0.95 V.
Figure V.14: SE cross-section for alpha particle exposures as a function of frequency for conventional DFF and SAFF at 1.1 V.
not change with clock frequency. Therefore, with increasing clock frequency, the operation time for the first phase will become larger percentage of time per clock cycle. Additionally, with higher number of sensitive transistors in the SAFF design compared to that for DFF design results in increased vulnerability as operating frequency is increased. If the trend in operating frequency were to continue, the SAFF design will show higher SEU cross-section than DFF designs at 1.6 GHz and beyond. At such high operating frequencies, designers will have to carefully evaluate power and SEU trade offs to determine the best FF design for a given application. The trend in SEU cross-section shown in Fig. V.13 is for alpha particles.