LSI Product Quality and Fault Coverage

At present, the relationship between fault coverage of LSI circuit tests and the tested product quality is not satisfactorily understood. Reported work on integrated circuits predicts, for an acceptable field reject rate, a fault coverage that is too high (99 percent or higher). This fault coverage is difficult to achieve for LSI circuits. This paper proposes a model of fault distribution for a chip. The number of faults on a defective chip is assumed to have a Poisson density for which the average value is determined through experiment on actual chips. The procedure, which relates the model to the chip being studied, is simple; one or more fabricated chip lots must be tested by a few preliminary test patterns. Once the model is characterized, the required value of fault coverage can be easily determined for any given field reject rate. The main advantage of such a model is that it adapts itself to the various characteristics of the chip (technology, feature size, manufacturing environment, etc.) and the fault model (e.g., stuck-type faults). As an example, the technique was applied to an LSI circuit; realistic results were obtained.