Detection and diagnosis of scan cell internal defects

1. A method of scan cell internal defect diagnosis, comprising:

identifying a failing scan chain;

determining an upper bound and a lower bound of candidate scan cells in the failing scan chain; and

identifying one or more scan cell internal fault candidates in the candidate scan cells based on a failure log and failing scan test patterns associated with the failure log, wherein the one or more scan cell internal fault candidates are for one or more static scan cell internal defects that exhibit static defect behavior but that do not behave as stuck-at faults and also do not behave as transition faults, wherein the static scan cell internal defect is a resistive short or open, and wherein the static defect behavior is characterized by a faulty output value being consistently output from a scan cell output for a respective combination of input values at scan cell inputs, the faulty output value being 0 for at least one combination of input values and being 1 for at least another combination of input values.

2. The method recited in claim 1 , further comprising:

performing a diagnostic simulation process for the one or more scan cell internal fault candidates.

3. The method recited in claim 2 , further comprising:

ranking the one or more scan cell internal fault candidates based on results of the diagnostic simulation process.

4. The method recited in claim 1 , wherein the identifying a failing scan chain is based on applying one or more scan test patterns, or one or more chain test patterns, or both.

5. The method recited in claim 1 , wherein the identifying one or more scan cell internal fault candidates comprises:

selecting a candidate scan cell from the candidate scan cells;

determining whether the candidate scan cell has a same output value for identical input combinations based on a failure log and failing scan test patterns associated with the failure log.

6. The method recited in claim 5 , wherein the identifying one or more scan cell internal fault candidates further comprises:

repeating the selecting and the determining until all of the candidate scan cells are selected.

7. The method recited in claim 5 , wherein the determining whether the candidate scan cell has a same output value for identical input combinations comprises:

changing scan load values of the candidate scan cell and downstream scan cells to Xs; and

performing a simulation process using one or more of the failing scan test patterns to determine input combinations of the candidate scan cell during various unload shift cycles.

8. A non-transitory storage medium storing processor-executable instructions for causing one or more processors to perform a method of scan cell internal defect diagnosis, the method comprising:

identifying a failing scan chain;

determining an upper bound and a lower bound of candidate scan cells in the failing scan chain; and

identifying one or more scan cell internal fault candidates in the candidate scan cells based on a failure log and failing scan test patterns associated with the failure log, wherein the one or more scan cell internal fault candidates are for one or more static scan cell internal defects that exhibit static defect behavior but that do not behave as stuck-at faults and also do not behave as transition faults, wherein the static scan cell internal defect is a resistive short or open, and wherein the static defect behavior is characterized by a faulty output value being consistently output from a scan cell output for a respective combination of input values at scan cell inputs, the faulty output value being 0 for at least one combination of input values and being 1 for at least another combination of input values.

9. The storage medium recited in claim 8 , wherein the method further comprises:

performing a diagnostic simulation process for the one or more scan cell internal fault candidates.

10. The storage medium recited in claim 9 , wherein the method further comprises:

ranking the one or more scan cell internal fault candidates based on results of the diagnostic simulation process.

11. The storage medium recited in claim 8 , wherein the identifying a failing scan chain is based on applying one or more scan test patterns, or one or more chain test patterns, or both.

12. The storage medium recited in claim 8 , wherein the identifying one or more scan cell internal fault candidates comprises:

selecting a candidate scan cell from the candidate scan cells;

determining whether the candidate scan cell has a same output value for identical input combinations based on a failure log and failing scan test patterns associated with the failure log.

13. The storage medium recited in claim 12 , wherein the identifying one or more scan cell internal fault candidates further comprises:

repeating the selecting and the determining until all of the candidate scan cells are selected.

14. The storage medium recited in claim 12 , wherein the determining whether the candidate scan cell has a same output value for identical input combinations comprises:

changing scan load values of the candidate scan cell and downstream scan cells to Xs; and

performing a simulation process using one or more of the failing scan test patterns to determine input combinations of the candidate scan cell during various unload shift cycles.