Fab facility design: When a slowdown is good

by Debra Vogler, senior technical editor, Solid State Technology

May 20, 2008 – The rapid rise in the cost of advanced processing equipment has outpaced the cost of building fabs in the last few years. While facility costs have started to increase again, those increases are in line with the consumer price index, according to ConFab presenter Rick Whitney, COO of US operations at M+W Zander.

Whitney told SST that two factors point to the relative decreasing costs of building factories: 1) less of a need to build sub-class 1 clean rooms, and 2) a slowdown in the evolutionary design of facilities. Additionally, as the industry is moving from 300mm to “300mm Prime” and 450mm wafer manufacturing, more effort has been going into designing energy efficient facilities.

The phrase “slow down” seldom evokes a positive response when applied to business, however, when it comes to the evolution of wafer fab facility design, it’s a good thing. Whitney pointed out that in the 1980s and 1990s, facility designs progressed very rapidly from one type of facility to another, from single level, to two levels, and then three levels. Now, however, fab designs are going back to being two-level facilities.

“In the mid-90s, we went to three levels because support equipment was larger than tools and there was a desire to get hi-vacuum pump systems out of the cleanroom,” said Whitney. The main reason for the scaling back to two levels is that the size of the support equipment has not increased as fast as the size of the mainframe tools being supported. As an example, Whitney noted that the support equipment underneath an EUV tool married up to a track is much smaller in footprint than the main equipment (the EUV + track) taken together. “And we’ve gotten more creative in how we use vertical space in the sub-fab,” said Whitney. “We can stack systems.”

While fab designs were scaling the walls in the mid-90s, the class of cleanrooms being built went from class 10, to class 1, to sub-class 1. “Fab design continued to be driven by cleanliness requirements sometimes referred to as the Ohmi era, which was fanatical about cleanliness — everything had to be stainless or have a skin,” explained Whitney. “Now, with closed process chambers, FOUPs and automation, the product is not open to the environment,” so fabs can now work with a less-stringent class 100 turbulent environment. Moreover, what used to be 100% filter coverage is now down to 20%-25% filter coverage, Whitney said. “This is a significant cost reduction since it ripples through the design from air movement to chilled water capacity to electrical equipment and power consumption.”

Another change in fab design is that filter coverage for heat removal and air circulation are as important a consideration as filter coverage for cleanliness. “It used to be that filter coverage for cleanliness was the primary driver to achieve sub-class 1, and it was automatically designed in at 100%,” said Whitney. “Heat removal/air circulation was a secondary effect of that design. Today, we have to balance the requirements for cleanliness and filter coverage with the calculations required to move enough air for heat removal.” Whitney also told SST that in the past, filter coverage at 100% was substantially more expensive than the current 20%-25% levels. “This is a first cost savings for FFUs (fan filter units) and/or air handlers, chiller, cooling towers, pumps, piping and electrical equipment,” he said. “Then it is also a savings from an operational perspective, since you are not powering all those systems just to clean the air.” — D.V.


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