Cell Aware and stuck-open tests

Abstract

Cell Aware testing (CAT) has received much publicity in recent years, with several reported success stories in screening defects missed by traditional stuck-at and transition delay fault (TDF) testing. For example, at ITC 2012, Hapke et al. reported 885 DPPM test escapes in a 32 nm notebook processor part despite industrial strength stuck-at and 5-detect TDF testing. Significant additional fallout from cell aware tests has also been reported for automotive parts already screened to stringent “zero-defect” standards. Importantly, the vast majority of these test escapes were observed to cause failure in actual system application, pointing to a serious field reliability issue. This raises some important questions: What are these new defects that are being detected by cell aware tests? Why are they missed by traditional stuck-at and TDF testing? Why has this problem only recently been discovered? Can traditional cell unaware test generation be enhanced to detect these faults? This tutorial presents a detailed study of the cell aware test generation methodology to answer these questions. The aim is not only to analyze and understand the defects in modern standard cell libraries missed by traditional tests that are uniquely covered by this new test approach, but also which of these test escapes necessarily need cell layout information for detection, and which can be systematically targeted in a layout unaware manner by enhanced stuck-at and two-pattern test generation. The large majority of the additional fallout from cell aware tests appears to be from the delay patterns that primarily target open defects. However, the structural limitations of scan DFT can limit the detectability of open defects even with CAT. Commonly employed LOC delay tests are not sufficient to screen all open defects capable of generating erroneous circuit outputs. We show that hazard activated open defects are the likely causes of at least some of the failures being observed in system level tests (SLT) even after parts are screened with CAT.