Limited yield estimation for visual defect sources

Although kill rate, kill ratio, and limited yield for visual defects are useful concepts in yield management, the formal definitions of these concepts, how to estimate them, especially in the presence of inspection error, and assumptions necessary for their application, are lacking in the literature. The concept of limited yield as the effect of a visual defect source on overall yield of a process is formally derived and the product of the individual limited yields for the visual defect sources is shown to equal the overall yield of the process. As a result of a more rigorous definition of limited yield, a major simplification in the calculation of limited yield over other methods is obtained. Basic to the notion of limited yield are the concepts of kill rate and kill ratio. The kill rate expresses how likely it is that a die with a certain visual defect will be rejected at probe. The kill ratio is shown to be the increased chance, relative to the baseline yield, of a die being rejected when a particular visual defect type is present. The limited yield concept is discussed and illustrated with a practical example using semiconductor visual defect data.