What special design techniques exist for radiation-hardened FPGAs?

Radiation-hardened FPGAs are designed to operate reliably in high-radiation environments, such as space, nuclear reactors, or high-altitude avionics. These FPGAs employ several specialized design techniques to mitigate the effects of ionizing radiation, including Single-Event Effects (SEEs), Total Ionizing Dose (TID), and Displacement Damage (DD). Here are key radiation-hardening techniques:

1. Radiation-Hardened-by-Process (RHBP)

• Silicon-on-Insulator (SOI) or Epitaxial Substrates: Reduces latch-up susceptibility by isolating transistors.

• Hardened Gate Oxides: Thicker or radiation-tolerant oxides to mitigate TID effects.

• Special Doping Profiles: Custom doping to reduce charge trapping and leakage.

2. Radiation-Hardened-by-Design (RHBD)

• Triple Modular Redundancy (TMR): Critical logic is triplicated, and voting circuits correct Single-Event Upsets (SEUs).

• Error-Correcting Codes (ECC): Protects memory (SRAM/Flash) from bit flips.

• Scrubbing: Periodic or real-time memory refresh to correct configuration upsets.

• Hardened Flip-Flops: SEU-resistant storage cells (e.g., Dual-Interlocked Storage Cells - DICE).

• Guard Rings & Layout Techniques: Prevents latch-up and reduces charge collection.

3. Mitigation of Single-Event Effects (SEE)

• SEU-Resistant Configuration Memory: Uses hardened SRAM or antifuse/Flash-based FPGAs (e.g., Microsemi RTG4, Xilinx Virtex-5QV).

• Self-Correcting Circuits: Watchdog timers and auto-recovery mechanisms.

• Charge Cancellation Techniques: Balanced node design to neutralize ion strikes.

4. Total Ionizing Dose (TID) Mitigation

• Radiation-Tolerant Materials: Special oxides and passivation layers.

• Biasing Techniques: Adaptive body biasing to compensate for threshold voltage shifts.

• Leakage Current Management: Guard bands and leakage compensation circuits.

5. Latch-Up Prevention

• SOI Technology: Eliminates parasitic bipolar transistors.

• Deep N-Wells & Guard Rings: Suppresses latch-up paths in bulk CMOS.

• Current Limiting Circuits: Detects and shuts down latch-up events.

6. Redundancy & Fault Tolerance

• Partial Reconfiguration: Allows dynamic repair of damaged logic.

• Diverse Redundancy: Different implementations of the same function to avoid common-mode failures.

7. Testing & Qualification

• Accelerated Radiation Testing: Proton, neutron, and gamma exposure tests.

• Heavy-Ion Testing: Validates SEE susceptibility at facilities like NASA’s SEE test centers.

Examples of Radiation-Hardened FPGAs

• Xilinx: Virtex-5QV (SRAM-based with TMR), Kintex UltraScale (RHBD).

• Microsemi (Microchip): RTG4 (Flash-based, SEU-resistant), RTAX-S/SL (Antifuse).

• Boeing: HX500 (SOI-based, high reliability).

Conclusion

Rad-hard FPGAs combine process hardening, circuit design techniques, and system-level redundancy to ensure reliability in extreme radiation environments. The choice between SRAM-based (with scrubbing), Flash-based, or antifuse FPGAs depends on the mission's requirements for reconfigurability vs. robustness.