On design of test structures for lithographic process corner identification

Lithographic process variations, such as changes in focus, exposure, resist thickness introduce distortions to line shapes on a wafer. Large distortions may lead to line open and bridge faults and the locations of such defects vary with lithographic process corner. Based on lithographic simulation, it is easily verified that for a given layout, changing one or more of the process parameters shifts the defect location. Thus, if the lithographic process corner of a die is known, test patterns can be better targeted for both hard and parametric defects. In this paper, we present design of control structures such that preliminary testing of these structures can uniquely identify the manufacturing process corner. If the manufacturing process corner is known, we can easily attain highest possible fault coverage for lithography related defects during manufacturing test. Parametric defects such as delay defects are notorious to test because such defects may affect paths that are subcritical under nominal conditions and not ordinarily targeted for test. Adoption of the proposed approach can easily flag such paths for delay tests.