Historical notes

Efficiency of process in manufacturing environments is historically achieved in one of two ways. One method provides buffered work flow between process steps (expecting asynchronous tool behavior), while the other balances tool performance to allow synchronization of the process steps. The first of the above allows longer cycles times of the product through the factory, while the other does not. This second method is based on lean WIP. AMHS systems must be capable of dealing with both methods, or the mixture of them.

The first view holds that in the recursive, multi product, semiconductor environment, WIP arrivals at the tool are classified as mathematically chaotic, in which system a general FIFO logic is most efficient. WIP arrives at a tool from multiple destinations, and the first to arrive claims the tool. This, also calls for independence in transport connections. I. e. a tool should not need to wait for a WIP while the transport is delivering WIP to an other tool. Theoretically, there are two ways to accomplish this. One is to stack up enough WIP at process destinations (or near by) in order to avoid tool wait times. The other way is to have an infinite number of delivery vehicles. The first solution increases factory cycle times, while the second can be very costly. Conveyors realize the second solution by providing an “infinite number of vehicles”. (Ideally, a conveyor can accept a new WIP for delivery as soon as a preceding WIP has cleared a WIP footprint for the next one.) Thus, on conveyor networks a large number of WIP may be moved to a large number of destinations in a massively parallel fashion, independent of each other. Mathematically, the variability of arrival rates to destinations is reduced (i.e. the coefficient of the operating characteristic equation of the fab is improved).

Following the synchronous method of manufacturing we find even more emphasis on reducing arrival variability. Ideally, transferring WIP between tools should take no time at all (high utilization and low WIP). And if such transfer takes time then these times should be close to equal. Or at least predictable to allow balancing of the production line. A most difficult task. And sacrifices in tool utilization are made to achieve the shorter fab cycle times. Conveyor transport systems are commonly employed in this type of manufacturing environment because of their contribution to reducing the arrival variability of the WIP. Being always available for a transport move, conveyors remove the variable part of the transport time. I.e. the unpredictable time to wait for a discrete vehicle to become available to execute the transport. The transport time becomes predictable, or deterministic, depending only on invariable travel distance and speed. Conveyors are a simple solution, requiring no scheduling of discrete move devices.

The above arguments have made conveyors a preferred choice in most industries. The cleanliness argument has, however excluded conveyors from semiconductors, and the latter industry chose vehicles instead. In 1984 Middlesex has “cleaned up” the conveyor to be suitable beyond the so called “class 1” environments. This allowed the introduction of conveyor networks into disk drive, and later, in 1993, into open carrier semiconductor manufacturing. These conveyor networks allowed unified, stocker-less, direct moves of WIP from tool to tool much before vehicle systems could be used in automating the bay. But, in spite of these early successes, the conveyor solution has missed the first generation 300 mm installations. An industry, with much aversion to risk, had been worried about the lack of an abundant supply base for the technology. Although the leadership exhibited by Middlesex has spawned new suppliers of conveyors at this early stage, these young companies combined could not overcome the inertia developed by I300I and SEMI towards the use of vehicle and stocker technologies. Today, the industry experience with the vehicle technologies brings back into focus the undisputed higher efficiency of the conveyor network solution. And the supply base is now ready. 300 mm installations have proved the conveyors performance with all 300 mm standards. Control software developments today allow the exploitation of the conveyor network efficiencies. And finally, the reliability of conveyors has been now proven to surpass those of vehicle systems. Found, for over 14 years, with an install base.