Comparison of two wafer inspection methods for particle monitoring on semiconductor manufacturing

The two methods examined differ in the procedure used to select wafers for inspection. In the first, or fixed-policy, method, the same number of wafers is inspected regardless of defect density. In the second method a variable policy is used. It is found that in a superclean production environment these inspection methods are not equivalent in defect density estimation: estimates obtained by the variable policy may be biased, while those obtained with the fixed policy are always unbiased. Fundamental reasons for such a phenomenon are discussed and recommendations are made