Letter to the Editor
-- Semiconductor International, 9/1/2000
I much appreciate the attention you gave to front-end material transports via conveyors in July's SI. Technically speaking, this methodology is driven by a shift in fab design priorities. It is recognized that material logistics have a first-order effect on factory performance and thus are included in fab design at the start. When you think that way, conveyors are obvious. In the past, emphasis was inevitably on achieving control over fickle semiconductor manufacturing technology and thus assuring good yield. Other efficiency issues were given few thoughts. Manual or other discrete transports were scrutinized only if they affected yield or had too little capacity.Conveyor transport in the front-end fab is still not understood, and the Asyst article ("The Case for Continuous Flow Transport of Wafer Carriers," pg. 299) contributed little to an intuitive grasp of its rationale. People keep asking: why conveyors? The significance of it is great, however; thus the subject matter should be treated more fundamentally. I find the Asyst article commercial and inaccurate. I also regret its lack of perspective. It seems they came away from Middlesex with only a partial understanding of these fundamentals, though they do seem to understand the technology has great commercial potential. To jump at this without the underlying science will repeat the mistakes of the past. Asyst was a systems integrator for the Middlesex conveyor invention for four years and sold several conveyor installations that failed as integrated systems. This resulted in great setback for the technology. Commercial zeal can mask the engineering fundamentals embedded in the product.
It is wrong to assume motivation for conveyors is the 300 mm transition. Middlesex has very successful 200 mm installations several years in service. It also is wrong to assume pods are an enabling technology for conveyors. Conveyors are manufactured for better then class 1 operation and work very well with open carriers. It is predicted that today's technology is good to below 0.1 µm feature constructions, and future improvements are expected to extend the conveyor working range with open carriers. Conveyors are extensively used inside tool environments.
Historically, pods were used in manual work environments and achieved great success in foundries. With operators around, it was difficult to achieve clean environments. Also, clean storage systems or stockers were a yield-enhancement feature of the old fabs. Automated transport of lots became necessary when stockers needed to be connected in order for the bay operators to request lots that may be in neighboring bay stockers. Hence the requirement that monorail vehicles move fast. Inherently, this traditional methodology also required that transport lot sizes be large (25 wafers). Conveyors bypass all these difficulties and thus require a paradigm shift in fab design. How to design with them must be explained clearly. The rewards are great, however, because their use increases invested capital efficiency and allows the reduction of committed capital that is so much of today's topic.
I hope your publications can continue to contribute to the understanding of this new technology.
George W. HornApplications ManagerThe Middlesex Cos. •
