Production testing for complex chips usually involves multiple test methods. Scan-based automatic test pattern generation (ATPG) for the stuck-at defect model has been the standard for many years, but experience as well as a number of theoretical analyses have shown that the stuck-at fault model is incomplete. Many devices pass high coverage stuck-at tests and still fail to operate in system mode.
Analysis of the defective chips often reveals that speed or timing problems are the culprits. At 90nm and smaller processes, the percentage of timing related defects is so high that static testing is no longer considered sufficient. Functional tests have been used to check (cheque for banks) for at-speed operation. But generating functional at-speed test patterns is difficult and running this volume of tests on the automatic test equipment (ATE) is expensive. As an alternative, scan test has been adapted to detect timing-related defects. Like standard stuck-at scan tests, high coverage at-speed scan test vectors can be automatically generated by ATPG tools. Manufacturing testing of deep subµm designs now routinely includes "at-speed" tests along with stuck-at tests.
Little has been done so far to make front end designers aware of at-speed test solutions at the register transfer language (RTL) level of abstraction. This document is intended to present basic concepts and issues for at-speed testing, as well as demonstrate the at-speed coverage estimation and diagnosis capability built-in to the SpyGlass-DFT DSM product for RTL designers and test engineers.
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