Fault dictionary-based scan chain failure diagnosis

• US 20080250284A1
• Filed: 06/13/2007
• Published: 10/09/2008
• Est. Priority Date: 04/04/2007
• Status: Active Grant

• Current Assignee: Mentor Graphics Corporation (Siemens AG)
• Original Assignee: Mentor Graphics Corporation (Siemens AG)
• Inventors: Guo, Ruifeng, Huang, Yu, Cheng, Wu-Tung
• Granted Patent: US 8,615,695 B2
• Time in Patent Office: Days
• US Class Current: 714/726
• CPC Class Codes: G01R 31/3177 Testing of logic operation, e.g. by logic analysers
• CPC Class Codes: G01R 31/318525 Test of flip-flops or latches
• CPC Class Codes: G01R 31/318541 Scan latches or cell details

Abstract

A dictionary-based scan chain fault detector includes a dictionary with fault signatures computed for scan cells in the scan chain. Entries in the fault dictionary are compared with failures in the failure log to identify a faulty scan cell. In one embodiment a single fault in a scan chain is identified. In another embodiment, a last fault and a first fault in a scan chain are identified.

Claims

1. A method of identifying faulty scan cells in a scan chain, comprising:

receiving a failure log that indicates failures detected during scan chain testing of an integrated circuit;

reading one or more fault signatures from a fault dictionary that specifies failures that will occur in a scan chain readout if a scan cell in the scan chain has a fault; and

using the fault signatures stored in the fault dictionary and the failure log to identify a faulty scan cell in the scan chain.

2. The method of claim 1, wherein for at least one scan cell, the fault dictionary stores a complete list of the failures that will occur in a scan chain readout if the scan cell has a fault, and wherein the faulty scan cell is identified by finding a match between the complete list of failures for the at least one scan cell and the failures included in the failure log.

3. The method of claim 1, wherein for at least one scan cell, the fault dictionary stores a partial list of the failures that will occur in a scan chain readout if the scan cell has a fault, and wherein the faulty scan cell is identified by constructing a complete list of failures for the at least one scan cell and finding a match between the complete list of failures and the failures included in the failure log.

4. The method of claim 1, wherein for at least one scan cell, the fault dictionary stores a partial list of the failures that will occur in a scan chain readout if the at least one scan cell has a fault and wherein the faulty scan cell is identified by:

determining a list of relative failure differences between failures in the failure log and a complete list of failures stored in the fault dictionary for a base scan cell, and

determining the relative fault signature differences between the partial list of failures stored for the at least one scan cell and the complete list of failures stored for the base scan cell; and

comparing the relative fault signatures for the at least one scan cell and the list of relative failure differences to identify a faulty scan cell.

5. The method of claim 1, further comprising storing an identity of the faulty scan cell.

6. One or more computer-readable media storing an identity of the faulty scan cell identified using the method of claim 1.

7. One or more computer-readable media storing a series of instructions that are executable by a computer to perform any of the method claims 1-5.

8. A method of identifying a last fault in a faulty scan chain having multiple faults, the method comprising:

receiving a failure log indicating failures detected during scan chain testing of an integrated circuit, the failure log indicating failures on the faulty scan chain and failures on scan chains that are free of faults;

reading a fault signature from a fault dictionary that specifies failures that will occur in scan chains of the integrated circuit when a candidate scan cell in the faulty scan chain has a fault, the failures specified including failures in the faulty scan chain and failures on scan chains that are free of faults; and

comparing the failures specified in the fault signature for scan chains that are free of faults with failures in the failure log for scan chains that are free of faults to identify the candidate scan cell as the last fault in the faulty scan chain.

9. The method of claim 8, further comprising storing an identity of the candidate scan cell identified as the last fault.

10. The method of claim 8, wherein the fault signature is a relative signature, the method further comprising deriving the relative fault signature from at least a differential signature and a complete base signature.

11. The method of claim 8, wherein the fault is a stuck-at 0 or stuck-at 1 fault.

12. The method of claim 8, wherein the fault is a timing fault.

13. The method of claim 8, wherein the failure log further indicates failures on primary outputs, wherein the fault signature further specifies failures that will occur at primary outputs when a candidate scan cell in the faulty scan chain has a fault, and wherein the act of comparing includes comparing the failures specified in the fault signature for the primary outputs with failures in the failure log for primary outputs to identify the candidate scan cell as the last fault of the multiple faulty scan cells in the faulty scan chain.

14. The method of claim 8, wherein the candidate scan cell is a last fault candidate scan cell, the method further comprising:

reading a fault signature for a first fault candidate scan cell;

creating a composite fault signature for the first fault candidate scan cell, the composite fault signature including failures defined in the fault signature for the first fault candidate scan cell that are upstream of the first fault candidate scan cell and failures defined in the fault signature for the last fault scan cell candidate that are downstream of the first fault candidate scan cell; and

comparing the failures in the composite fault signature with failures listed in the failure log to identify the first fault candidate scan cell as the first fault in the faulty scan chain.

15. The method of claim 8, further comprising:

determining a first fault scan cell by;

comparing a composite fault signature for a candidate scan cell with failures listed in the failure log to identify the first fault scan cell in the faulty scan chain, wherein the composite fault signature for the candidate scan cell includes the fault signature for the candidate scan cell and its upstream scan cells specifying failures that will be detected in a defect-free scan chain as determined from a previously determined last fault scan cell and a fault signature of scan cells that are downstream of the candidate scan cell that will be detected in the scan chain that includes the first fault candidate scan cell.

16. One or more computer-readable media storing an identity of the faulty scan cell identified using the method of claim 8.

17. One or more computer-readable media storing a series of instructions that are executable by a computer to perform any of the method claims 8-15.

18. A method of identifying a first fault in a faulty scan chain, the method comprising:

reading a fault signature for a candidate scan cell that may be the first fault in the faulty scan chain;

reading a fault signature for a previously determined last fault in the faulty scan chain;

creating a composite fault signature for the candidate scan cell that includes failures defined in the fault signature for the candidate scan cell that are upstream of the candidate scan cell and failures defined in the fault signature for the last fault that are downstream of the candidate scan cell; and

comparing the failures in the composite fault signature with failures listed in a failure log to identify the candidate scan cell as the first fault in the faulty scan chain.

19. The method of claim 18, further comprising storing an identity of the candidate scan cell identified as the first fault.

20. The method of claim 18, wherein at least one of the fault signatures for the candidate scan cell or the fault signature for the previously determined last fault is a relative signature, the method further comprising deriving the at least one of the fault signatures from at least a differential signature and a complete base signature.

21. The method of claim 18, wherein the fault is a stuck-at 0 or stuck-at 1 fault.

22. The method of claim 18, wherein the fault is a timing fault.

23. One or more computer-readable media storing an identity of the faulty scan cell identified using the method of claim 18.

24. A computer storage media including a sequence of instructions that are executable by a computer system to perform the method of any of claims 18-22.

25. A method of generating a fault dictionary to determine faulty scan cells in a scan chain, comprising:

receiving a description of an integrated circuit that includes combinational and sequential logic that can be tested by applying test patterns to the sequential logic circuits and reading scan chain outputs;

for one or more scan cells in the scan chains, determining respective lists of failures that will be observed at the scan chain outputs when each of the one or more scan cells has a defined fault; and

storing the respective lists of failures for each of the one or more scan cells.

26. The method of claim 25, further comprising:

determining a difference between the failures for a first scan cell in a scan chain versus the failures for a second scan cell in the scan chain; and

storing the difference in failures as the fault signature for the first scan cell in the fault dictionary.

27. The method of claim 25, wherein the first and second scan cells are adjacent in the scan chain.

28. The method of claim 25, further comprising storing a complete list of failures as a fault signature of at least one scan cell in the scan chain in the fault dictionary.

29. One or more computer-readable media storing a fault dictionary generated at least in part by the method of claim 25.

30. One or more computer-readable media storing a sequence of instructions that are executable by a computer to perform any of method claims 25-28.