Ultrapure Water Filtration for Semiconductor Manufacturing
A schematic of a DI water system is depicted below. The locations of the filters are representative of those found in a typical water system.
|Diagram Number||Application||Filter Type||Removal Ratings|
|1||Pre-RO||Microza1 Hollow Fiber UNA Modules||0.1 μm|
|Ultipleat® High Flow Depth Filters||4.5 μm|
|2||Resin Trap||Fluorodyne® Filters||0.2 μm|
|Varafine™ VFSE Series Filters||0.45 μm|
|Ultipleat® UP Filter||3 μm|
|3||UF Prefilter||Posidyne® UP Filter||0.1 μm|
|Varafine VFSE Series Filters||0.1 μm|
|4||Central UF||Microza UF Modules, OLT Series||6,000 daltons|
|5||POU (Cold DI)||IonKleen™ AQ Series||Trace metal ions >90%|
|IonKleen CAF Series||10 nm, Trace metal ions >90%|
|Asymmetric P-Nylon Filter||20 nm|
|Posidyne® UP Filter||40 nm|
|Varafine VFSE Series Filters||20 nm|
|Fluorodyne Filters||40 nm|
|Microza UF Modules: OLT Series||6,000 daltons|
|6||Hot DI Water||Microza UF Modules: OLT Series||6,000 daltons|
|7||POU (Hot DI)||Varafine T Series VTSE Filters||20 nm|
|Fluorodyne Filters||40 nm|
|8||Tank Vent||Emflon® Filter||3 nm (in gas)|
|9||Cooling Water||Ultipleat High Flow Depth Filters||20 μm; 40 μm|
1 Microza is a trademark of Asahi Kasei Corporation
Microza UNA SystemPretreatment is required to operate RO units effectively and economically. The pretreatment will vary depending on the source of the incoming feed water.
With surface water as the feed, conventional pretreatment may include such unit operations as coagulation, flocculation, settling, granular media filtration, and cartridge filtration. Even municipal feeds may have elevated SDI levels due to the presence of fine colloids and bacteria. These colloids may be too fine to be effectively removed by typical multimedia filters, leading to rapid blockage of cartridge filters and/or premature RO membrane fouling.
An alternate to conventional pretreatment for RO units is a microfiltration (MF) system with Pall’s Microza UNA modules.
The Microza MF system provides a consistent, high-quality effluent not generally achievable by conventional processes, regardless of incoming feed water quality. This may permit RO units to be operated at lower pressures for longer periods because of reduced membrane fouling, thus decreasing electrical power costs. The frequency of chemical cleanings may also be significantly decreased, saving on labor and chemical costs and extending RO membrane life.
Cartridge FiltrationFor conventional pretreatment systems, Pall recommends the absolute rated Ultipleat® High Flow Profile filterwith a 99.98% removal at 4.5 μm. Finer ratings are also available.
Pall Microza UF modules with their unique, uniform double-skinned, hollow fiber membrane feature:
- High mechanical strength for long term reliability
- Stable long-term flux
- Uniform smooth outer membrane surfaces minimize fouling by contaminants
- Double filtration ensures high removal efficiency.
Hollow fiber ultrafiltration medium shown at 300X magnification.
RatingWhat data are available to substantiate the claimed efficacy? Bacteria, particle or latex bead challenge tests are helpful in characterizing the performance of a filter cartridge.
Membrane Zeta PotentialMost materials when immersed in water exhibit a zeta potential. The majority of DI water contaminants, including most colloids, particles, bacteria, and pyrogens (bacterial fragments), are negatively charged. Filter media can be chemically modified to give them a positive zeta potential.
Positive and negative zeta potential filters are given the same pore size, i.e., micron ratings.
Positive zeta potential filters thus provide removal by interception equal to that of negative zeta potential filters.
However, Pall positive zeta potential elements provide an important advantage: they additionally remove very fine negatively charged organisms and particles, well below their micron rating. The removal mechanism is electrostatic attraction, and is effective in water over the typical DI water pH range (pH 5-8).
As the active sites become occupied by the collected particles, the removal efficiency by electrostatic attraction decreases. However, actual efficiency will not drop below the removal rating of the filter. Because of the highly porous nature of the membrane, actual total membrane area containing positive zeta potential sites is several orders of magnitude greater than the effective filtration area. Thus, the capacity for electrostatic adsorption of fine particles is very large.
Asymmetric MembranesTo meet the need for particle removal ratings down to 20 nm while maintaining relatively low differential pressures, Pall has developed asymmetric membranes. These membranes have a tapered pore geometry with an open upstream section and a fine downstream structure.
Membrane TypesNylon, PVDF, polysulfone microfiltration and hollow fiber microfiltration/ultrafiltration media are recommended for use in DI water applications. Photomicrographs of these media are shown above.
Integrity TestableAll central and final filters intended for use in a UPW system should be integrity tested by the manufacturer prior to shipment, and should be easily integrity testable by the end user. Such a test will verify that the filter cartridges have not been damaged and have been properly installed in their housings.
TraceabilityFilters should be serialized by the manufacturer for lot control and traceability of components.
PVDF Membrane shown at 3000X magnification.
Highly Asymetric Polysulfone Membrane at 400X magnification.
Asymmetric nylon membrane at 5000x magnification
IonKleen™ Purifiers are targeted for final rinse water to remove trace metallic contaminants that may deposit onto wafers. Their high capacity for and rapid removal of metallic ions allows for economical installations that are typically changed out once a year in UPW systems.
Hot DI Water FiltersPall Corporation offers both UF and MF solutions. Microza OLT modules can operate continuously in 80°C water. The Varafine T filters with their highly asymmetric, polysulfone membranes and polysulfone/ECTFE hardware can readily tolerate hot (95 °C) water service.
The information presented in the table below is for general guidance only. It is not to be considered for long term or continuous exposure to the listed chemicals. Because so many factors can affect the chemical resistance of a given product, it should be tested under the same conditions that it will see in service.
All applicable safety practices should be observed, such as those provided in the Material Safety Data Sheet for each chemical. Please contact Pall Microelectronics for specific recommendations or to ask any questions.
R = Recommended NR = Not Recommended
(< 90 °C)
(< 3% H202)
(20-30 ppb ozone)
|Ultipleat® High Flow||NR||R||NR|
|Posidyne® / Asymmetric P-Nylon||NR||R||NR|
|Microza UF, OLT Series||R||R||NR|
|316L Stainless Steel Electropolished||R||R||R|
|Megaplast™ In-Line PFA||R||R||R|
|FEP / Fluoroelastomer||R||R||R|