Patent ID: 7058869

Claim:
A method for selectively masking off undesirable states in selected scan cells, which cause test failures, from being compacted in selected pattern compactors for debug, diagnosis, and/or yield improvement of a scan-based integrated circuit in a selected scan-test mode or selected self-test mode, the scan-based integrated circuit containing a plurality of scan chains, a plurality of pattern generators, a plurality of pattern compactors, an output-mask controller, and an output-mask network, each scan chain comprising multiple scan cells coupled in series, the output-mask controller including a combinational output controller connected to the output-mask network, the combinational output controller comprising one or more selected combinational logic networks other than a complete network of AND gates; said method comprising: (a) generating and shifting in a stimulus through said pattern generators to all said scan cells in said scan-based integrated circuit during a shift-in operation; said generating and shifting in a stimulus through said pattern generators to all said scan cells further comprises generating a compressed stimulus, decompressing said compressed stimulus as said stimulus through said pattern generators, and shifting in said stimulus to all said scan cells in said selected scan-test mode during said shift-in operation; wherein said compressed stimulus is selectively generated internally or supplied externally from an ATE (automatic test equipment); (b) capturing a test response to all said scan cells during a selected capture operation; (c) shifting out said test response or said stimulus to said pattern compactors for compaction by selectively masking off said undesirable states in said selected scan cells from being compacted in said selected pattern compactors using said output-mask controller and said output-mask network, while shifting in a new stimulus to all said scan cells, during a shift-out operation; and (d) repeating steps (b) to (c) until a predetermined limiting criteria is reached.