by Robert P. Donovan
Reusing spent rinse water is one option for reducing the demand placed by a semiconductor manufacturing plant on a regional water supply. Water reuse is thus one strategy for expanding production at a site without obtaining an increased water allotment from the local community (see “Recycling spent rinse water,” CleanRooms, February 2000, p. 8).
Water recycling and water reclaim are subsets of rinse water reuse. In the nomenclature promulgated by Sematech, “recycle” means returning spent rinse water, with or without post discharge treatment, back into the plant ultrapure water (UPW) system at some node; “reclaim” is the reuse of spent rinse water as feed water in an application other than UPW production, such as feed water for a plant cooling tower. At present reclaiming water is more common in the semiconductor industry than recycling it.
Despite the generally high quality of spent rinse water, recycling it back into the plant UPW system poses a risk in that many of the chemicals used in semiconductor manufacturing are not found in municipal water and trace concentrations of these chemicals can appear in the spent rinse waters being recycled. Worse yet, some of these chemicals may degrade the membranes and ion exchange resins used to produce UPW, reducing the quality of the UPW fed to a fab and, in the extreme, forcing a temporary fab shutdown! This fear has made rinse water recycling a hard sell at many fabs.
The flip side is that, for the overwhelming majority of the time, recycled water constitutes higher-quality feed water to the UPW system than municipal water, which in turn results in higher-quality UPW being delivered to the fab at lower cost when at least a portion of the feed water to the UPW system is recycled rinse water. In fact, at one location, fab shutdowns that otherwise would be necessitated by unannounced wide swings in the properties of the feed water supplied by the municipality have been avoided because that facility practices rinse water recycling! While this example may represent operation with a marginally designed or an overextended UPW system that is more vulnerable than the typical UPW installation to changes in the quality of the feed water, nonetheless, here rinse water recycling rescues the fab operation rather than threatens it.
The important message is that recycling of spent rinse water can be rewarding but is not a trivial undertaking. The payoffs in higher-quality UPW and lower costs are appealing but performance miscues or slipups can be even more damaging and costly. Just one fab shutdown attributable to a recycling system error can more than wipe out the cost savings achieved by that recycling operation over the course of a year. So you'd better know what you're doing when you opt to recycle spent rinse water. The brighter side is that the technology and understanding now exist to make recycling an attractive, safe option that is being implemented by a growing number of US semiconductor manufacturers. Recycling systems require extensive monitoring and control systems that can divert any unsuitable water stream to industrial waste long before that particular stream joins the plant UPW system. Overseas semiconductor manufacturers have been successfully recycling rinse waters for years; US semiconductor manufacturers are now beginning to catch up.
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Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM. His Sandia project work is developing technology for recycling spent rinse waters from semiconductor wet benches.