New stocker paradigm

A fab needs storage of WIP. The process is highly recursive, with long factory cycle times, during which process changes and priorities may occur. I.e. long term storage (more then minutes or hours). Just how much of this type of storage is required is process management dependent. The process also needs short term storage or buffering. The quantity of such storage for in-process buffering can be deducted from the process itself. Generally, specifications for storage by buyers of AMHS systems are handed out as a one lump sum number. For example, a capacity to store 5000 carriers. This number including both long term storage and in process buffering. The number is experimentally derived using a known model of discrete vehicle AMHS. With the advent of conveyor networks such numbers do not work at all. A conveyor network is inherently a storage system also, usually capable of supporting the storage of all in process WIP. This cuts down on the static storage requirement for the fab (long term storage remaining).

The in-process storage, or buffer, is required for two reasons. One, because the process is dynamic with fluctuations of incoming volume rate of flow . Two, because the transport and process tool resources are unevenly loaded in time and space. This variation of the flow calls for a system which can absorb the peaks by storing the excess foups and then discharging them later by consumption at the mean design rate of flow. Vehicle technology is based on each vehicle transporting and holding one foup at a time. Its total holding and buffering capability is much less than that of a conveyor network. Even if vehicles could be used for storing excess incoming foups, it would be an expensive proposition. Thus vehicle systems must rely on external storage devices, such as stockers. Stockers are a corollary of discrete device transports. Conveyor networks, on the other hand have the capacity to absorb the load fluctuations by their inherent storing ability. In factories which employ conveyor networks, 60-70 percent of WIP flow does not see stockers, but instead moves direct between tools. Thus, as a first rule of thumb, conveyor based AMHS are offered with 50% less stockers.