Defect Tolerance for Yield Enhancement of FPGA Interconnect Using Fine-grain and Coarse-grain Redundancy Anthony J. YuGuy G.F. Lemieux September 15, 2005

2 Outline Introduction and motivation Introduction and motivation Previous works Previous works New architectures New architectures Coarse-grain redundancy (CGR) Coarse-grain redundancy (CGR) Fine-grain redundancy (FGR) Fine-grain redundancy (FGR) Experimentation Results Experimentation Results Conclusions Conclusions

3 Introduction and Motivation Scaling introduces new types of defects Scaling introduces new types of defects Smaller feature sizes susceptible to smaller defects Smaller feature sizes susceptible to smaller defects Expected results Expected results Defects per chip increases Defects per chip increases Chip yield declines Chip yield declines FPGAs are mostly interconnect FPGAs are mostly interconnect FPGAs must tolerate multiple interconnect defects to improve yield (and $$$) FPGAs must tolerate multiple interconnect defects to improve yield (and $$$)

4 General Defect Tolerant Techniques Defect-tolerant techniques minimize impact (cost) of manufacturing defects Defect-tolerant techniques minimize impact (cost) of manufacturing defects FPGA defect-tolerance can be loosely categorized into three classes: FPGA defect-tolerance can be loosely categorized into three classes: Software Redundancy – use CAD tools to map around the defects Software Redundancy – use CAD tools to map around the defects Hardware Redundancy – incorporate spare resources to assist in defect correction (eg. Spare row/column) Hardware Redundancy – incorporate spare resources to assist in defect correction (eg. Spare row/column) Run-time Redundancy – protection against transient faults such as SEUs (eg. TMR) Run-time Redundancy – protection against transient faults such as SEUs (eg. TMR)

5 Previous work – 1 – Xilinx Xilinx’s Defect-Tolerant Approach Xilinx’s Defect-Tolerant Approach Customer (knowingly) purchases “less that perfect” parts Customer (knowingly) purchases “less that perfect” parts Customer gives Xilinx configuration bitstream Customer gives Xilinx configuration bitstream Xilinx tests FPGA devices against bitstream Xilinx tests FPGA devices against bitstream Sells FPGA parts that “appear” perfect Sells FPGA parts that “appear” perfect Defects avoid the bitstream Defects avoid the bitstream Limitation: Limitation: Chips work only with given bitstream – no changes! Chips work only with given bitstream – no changes!

6 Previous work – 2 – Altera Altera’s Defect-Tolerant Approach Altera’s Defect-Tolerant Approach Customer purchases “seemingly perfect” parts Customer purchases “seemingly perfect” parts Make defective resources inaccessible to user Make defective resources inaccessible to user Coarse-grain architecture Coarse-grain architecture Spare row and column in array (like memories) Spare row and column in array (like memories) Defective row/column must be bypassed Defective row/column must be bypassed Use the spare row/column instead Use the spare row/column instead Limitation: Limitation: Does not scale well (multiple defects) Does not scale well (multiple defects)

7 Objective Problem Problem FPGA yield is on decline because of aggressive technology scaling FPGA yield is on decline because of aggressive technology scaling Proposed Solutions Proposed Solutions Defect-tolerance through redundancy Defect-tolerance through redundancy Important Objectives Important Objectives Interconnect defects important (dominates area) Interconnect defects important (dominates area) Tolerate multiple defects (future trend) Tolerate multiple defects (future trend) Preserve timing (no timing re-verification) Preserve timing (no timing re-verification) Fast correction time (production use) Fast correction time (production use)

Background

9 Island-style FPGA

10 Directional Switch Block

11 Directional Switch Block

Course-grain Redundancy (CGR)

13 Coarse-grain Redundancy (CGR)

14 So…what’s wrong with it?

15 Improving yield for CGR – Adding Multiple Global Spares Add multiple global spare to traditional CGR Add multiple global spare to traditional CGR Global spares can be used to repair any defective row/column in the array Global spares can be used to repair any defective row/column in the array Wire extensions are now longer Wire extensions are now longer

16 Yield Impact of Multiple Global Spares

17 Increasing Area+Delay Overhead 1 GLOBAL SPARE 2 GLOBAL SPARES 4 GLOBAL SPARES MAY BE IMPRACTICAL !!! NO SPARES MORE SPARES  MORE MUX OVERHEAD IN EVERY SWITCH ELEMENT

18 Improving yield for CGR – Adding Multiple Local Spares Divide FPGA into subdivisions Divide FPGA into subdivisions Each subdivision has local spare(s) Each subdivision has local spare(s) Distributes spares across chip Distributes spares across chip Reduces mux area overhead (of Global scheme) Reduces mux area overhead (of Global scheme) Limitation: Limitation: Spare(s) can only repair defect within the subdivision Spare(s) can only repair defect within the subdivision

19 Yield Impact of Multiple Local Spares (not as good as Global with same # spares)

Fine-grain Redundancy (FGR)

21 Our Proposed Solution Fine-grain Redundancy (FGR) – Defect Avoidance by Shifting

22 Defect-tolerant Switch Block

23 HSPICE Schematic

24 Switch Implementation Options Several detailed implementations are possible Trade off area / delay / yield(repairability)

25 Defect Avoidance – Switch Implementation Option 1 Can avoid contention by pre-shifting the red signal… OR… [ lower area overhead, lower yield improvement ]

26 Defect Avoidance – Switch Implementation Option 2 …OR … can avoid contention by embedding the IMUX [ higher area overhead, best yield ]

27 Single-length Defects

28 Double-length Defects

29 Minimum Fault-free Radius (MFFR)

30 Experimentation Results Area Area Delay Delay Area Delay Product Area Delay Product Yield Yield Summary Summary

31 Estimated Area overhead at equal yield (80%) * CGR-G1 can only tolerate 1-2 defects

32 Area Overhead for Varying Wire Length

33 Area Results

34 Delay Results

35 Area-Delay Product

36 Yield – 1 Switch Implementation Affects Yield * Assumes all bridging defects

37 Comparison between FGR and CGR – FGR Tolerates Tens of Defects

38 Yield for Varying Wire Length

39 Limitations of Study & Architectures FGR FGR Does not tolerate defects in the logic Does not tolerate defects in the logic Cannot tolerate clustered defects Cannot tolerate clustered defects Requires a detailed fault map Requires a detailed fault map CGR CGR Assumes that all defects can be corrected with a single row/column Assumes that all defects can be corrected with a single row/column Bypass circuitry is approximated Bypass circuitry is approximated

40 Summary of FGR

41 Conclusions CGR effective for 1 or 2 defects CGR is effective for 1 or 2 defects FGR meets desired objectives: FGR meets desired objectives: Tolerates multiple randomly distributed defects Tolerates multiple randomly distributed defects Defect correction does not perturb timing Defect correction does not perturb timing Tolerates an increasing number of defects as array size increases Tolerates an increasing number of defects as array size increases Correction can be applied quickly Correction can be applied quickly FGR potentially capable of correcting crosstalk faults, but has not been explored FGR potentially capable of correcting crosstalk faults, but has not been explored

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