A Case for Point-of-Use RO Ultrapure Water Systems


'Pure water' is a relative term. Practically speaking, no water, naturally occurring or treated by man, consists solely of H2O molecules. Forever in pursuit of achieving equilibrium with its surroundings, water has unique characteristics that result in its ability to dissolve most matter. This property of water to dissolve and transport materials is employed by the electronics industry for the critical rinsing steps essential to the fabrication of integrated microcircuitry. This same property, however, provides a challenge to those engaged in the purification and handling of water for ultrapure applications. Removal of the wide variety of contaminates in water supplies and consistent maintenance of that level of purity to the point-of-use (POU) is essential for the manufacture of integrated circuitry.

Reverse osmosis (RO) as final filtration is currently being evaluated by many of the semiconductor industry's production engineers as well as power industry engineers as the next step in ultrapure water treatment However, one large U.S. integrated circuitry (IC) manufacturer recognized the benefits of RO crossflow polishing filtration more than seven years ago. Since 1979, RO has been used near point-of-use (POU) as a final filtration at their most critical IC fabrication clean rooms in the U.S.A. Seven ultrapure water systems incorporating RO at the point-of-use have been engineered and manufactured by Osmonics®, Inc. of Minnetonka, Minnesota, U.S.A. for this large IC manufacturer. Combined, these systems have the capacity to produce greater than 2000 gpm (454 m3/hr) of ultra-high purity '18 megohm' water. This paper evaluates the evolution of these systems, their benefits to the end user as POU treatment, and suggests water quality criteria for the most critical clean room operations.

Table #1 from Balazs Analytical Laboratories, California, U.S.A. presents water quality criteria for IC fabrication. The authors agree with Balazs Labs and believe that Table #1 is a reasonable guideline for the fabrication of moderate density IC's. Low molecular weight cutoff ultrafiltration (UF) at the POU can meet most moderate density IC fabrication requirements but will not be sufficient for future IC geometry requirements. Crossflow membrane filtration, specifically reverse osmosis is capable of 'hyperfiltration' to 5 Angstroms (0.0005 microns) or nearly 2000 times smaller than current 1-2 micron line widths common in today's IC's. RO is the only technology capable of economical production of ultrapurified water for the manufacture of the most sophisticated IC's.

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