Test wafer management for semiconductor manufacturing

Test wafers (TWs) are used for equipment qualification purposes in semiconductor manufacturing. TW management is unique because of the possibility of downgrading a TW to test lower class processes. Since the yearly TW costs add up to several million dollars for a typical semiconductor fab, effective TW management can substantially reduce costs by identifying the right quantity of TWs to purchase, to downgrade, and to hold in the inventory. While the current industry practice is to use suboptimal rules to manage TWs, this paper develops a network-based formulation named TW Inventory Network (TWIN) to eliminate this suboptimality. Several special cases are analized here, and a numerical analysis is provided to shed further operational insights on the TW problem.