GEORGE W HORN, Middlesex Industries
The sum of individual events defines the fab. Innumerable number of parameters in each of scheduling, dispatch, and physical processes. Controlled by APC (Advanced Process Control) systems.  Conflicts and interaction between the individual variables and then the conflict between cycle time and throughput. Management of manufacturing works with the acceptance of this complexity. And now, on the one hand there is big data handling, AI/ML (Artificial Intelligence and Machine Learning) [2, 3] which, on a fab wide scale may be like weather forecasting with supercomputers (deterministic dependence on practically infinite numbers of parameters) advancing towards dominance in managing the fab. And, on the other hand, there are the fundamentals in direct manufacturing: the physical capabilities of process equipment and material handling systems quietly making advances. Even though, it is evident that self-optimizing AI, or ML and APC have their limits set by equipment competence.
First, the manufacturing of transistors on silicon substrates became a multi-layered process on costly equipment sets. Then these costs demanded the reentrant use of that equipment set. Replacing the linear sequencing of value adds through a line of machines. Ideally, synchronizing the process steps would yield the maximum throughput and lowest cycle time, erasing the conflict between them. To achieve this in the linear case is relatively easy by balancing the line, while highlighting the very important aspect of a balanced line: the reduction of inter process idling of WIP. Doing the same balancing with reentrant manufacturing greatly multiplies the difficulties (makes it impossible). But we do want the advantages of reduced inter process WIP.
In short, equipment cost forced the reentrant manufacturing, which then prohibits balancing and thus synchronization. Still, we would like to emulate linear manufacturing. Accordingly, what we seek is some fundamental truth to help in achieving greater synchronization of process steps in reentrant manufacturing.