Copula Models of Correlation: A DRAM Case Study

Variable bit retention time observed in a 65-nm dynamic random access memory (DRAM) case study will cause miscorrelation between retention times occurring in Test and Use. Conventional multivariate normal statistics cannot adequately model this miscorrelation. A more general copula-based modeling approach, widely used in financial and actuarial modeling, solves this problem. The DRAM case study shows by example how to use copula models in test applications. The method includes acquiring data using a test vehicle, fitting the data to a copula-based statistical model, and then using the model to compute producer- and customer-oriented figures of merit of a product, different from the test vehicle. Different array sizes, fault tolerance schemes, test coverage, end-use (datasheet), and test condition specifications of the product are modeled.