Conveyor network and vehicle system equivalence

Conveyor networks can do everything that discrete vehicle systems can (the opposite is not true). In a conveyor network, WIP can be delivered in a fab, point to point, without transitioning through stockers. The network inherently also serves as a buffer, accumulating WIP in dynamic move rates. The total WIP content of the network can widely oscillate, depending on delivery demands. Indeed, the accommodating capacity of a conveyor network is so great that most applications do not call for external stockers for in-production WIP. In engineering terms, the conveyor is a wide band system, capable of passing wide oscillations in WIP input rates without external storage devices. All these things cannot be accomplished with discrete vehicle move systems. Illustrating this would be the common occurrence of wait time for a vehicle. When a transport request is made for a FOUP the conveyor is immediately available, and in that instant will hold an increased amount of WIP in its domain. While the vehicle will need to deposit its WIP into a stocker, to be made available for the requested next transport. Some percentage of time, vehicles are empty, transiting to a pickup destination, and thus need their own specific dispatch system to maintain utilization. A difficult additional task on which production depends.

Yet, the two technologies are equivalent in performance when low threshold-capacity deliveries of WIP are concerned. Average vehicle speeds can be matched by conveyor systems. Average delivery times in a fab are equivalent for the two systems, even if a conveyor speed of ½ is effected. The composition of these times in the two systems are different, but the net results are the same. Both systems comply with the same SEMI standards for interfacing transport to tools, stockers, and other robotic handling equipment. Both systems meet vibration limits. And both perform to cleanliness requirements. Traditional cost of ownership is similar (if active performance of the conveyor is neglected). Both systems meet reliability requirements. Although the reliability of the basic conveyor is much higher then that of the vehicle systems, due to simplicity. There are elements in conveyor applications where reliability is restricted to similar numbers to those of vehicles. For example if floor stockers are employed, or if floor to floor elevator systems are included.

After horizontal delivery at ceiling level to a buffer, associated with a tool, or to a bay, both transport systems can effect a vertical delivery to a tool port via hoist mechanism. In the case of the vehicle system the vertical hoisting capability is included in the vehicle. In case of a conveyor, this capability is provided by a separate device, limited to service only a few tools, in a restricted reach. This concept allows free movement of WIP within the bay, divorced from the vertical tool delivery function. In spite of this advantage, both solutions employ a reciprocating motion, which, in itself, is limited in cycle time.

Conveyor networks offer an alternate delivery to the tool ports. This solution provides an in line conveyor, connecting all tool ports at floor level (tool ports are provided with a SEMI exclusion zone for conveyor interfacing). In case of a synchronized line this solution is most favoured. In this case, elevators connect the in-bay floor level conveyor with the inter bay ceiling level conveyor network. This solution allows much faster cycling between in-line tools, and is envisioned as the proposed first choice in further automating WIP delivery in a cell.