Release policy governor (RPG) for re-entrant semiconductor fabrication lines

In this paper, a method is presented for governing the release of wafers (starts) into a re-entrant semiconductor fabrication line. A flow model for a five-machine six-step re-entrant mini-fab is used to present all developments. Stabilizing clear-a-fraction (CAF) state feedback policies are developed for the mini-fab. A systematic procedure is presented for regulating the flow of starts so that critical system constraints (e.g. machine availability and buffer level constraints, etc.) are never violated, however aggressive the release policy. The regulation is achieved via nonlinear optimization based on state information from the flow model. The resulting algorithm, called a release policy governor (RPG), is shown to produce a closed loop stable system