Trace Metal Ion Removal from Ultrapure Water

Resistivity measurements of 18.2 MOhm-cm were once considered an indication that ultrapure water (UPW) did not harbor any metal ions or other ionic species. With the advent of ever finer line widths and thinner gate oxides, an increase in defectivity levels was observed. Although the causes are myriad, one of the areas of focus is decreasing the levels of trace metal ions such as sodium, calcium, iron and copper. In fact, the 2006 International Technology Roadmap for Semiconductors (ITRS) has specified < 50 ppt for these contaminants. Obviously ion exchange polishing bottles come to mind as the first choice for removing remaining metal ion contamination not rejected by reverse osmosis or captured by the main mixed resin bed. However, in a number of cases, they have been shown to pass trace quantities of cations. These ions can be effectively captured by membrane purifiers, such as the Pall IonKleen™ family of purifier cartridges.
The IonKleen membrane purifiers consist of a microporous ultra-high molecular weight polyethylene substrate with covalently bonded, strong acid cation exchange groups. Configured either into standard cartridges or capsules, they have a very large effective surface area with a small footprint, enabling then to be placed right at the point-of use.
The IonKleen purifiers are highly efficient because they do not rely on slow diffusion into a resin bead to achieve ionic adsorption. Rather, due to the intimate contact of the water with the densely packed ion exchange groups on the membrane, rapid kinetics occur with immediate and spontaneous removal of the trace contaminants. This is schematically depicted in Figure 1.

Figure 1: Ion Adsorption ( Resin vs Membrane )

The efficiency of the IonKleen Purifier is relatively independent of flow rate provided its recommended maximum flow rate is not exceeded. The same cannot be said for ion exchange columns or polishers.
Since it is extremely difficult to directly detect these trace ions, their existence and deleterious effects were demonstrated by the following experiments. A Pall IonKleen ion exchange purifier cartridge was examined after six months of service in UPW. The purifier was extracted with 10% HCl. The acid was then analyzed using inductively coupled plasma mass spectrometry (ICP-MS). A variety of cations were detected. The highest concentrations are shown in

Table I. These represent significant cations removed from the UPW during the service life of the ion exchange purifier/filter.

Cation	?g per 254 mm/10 in. IonKleen Purifier
Na	85.8 ± 2.7
Mg	277.5 ± 8.9
Ca	147.0 ± 4.7
Fe	267.0 ± 8.5
Ni	75.9 ± 2.4

Tests with silicon wafers revealed that significant levels of metals could be detected on wafers rinsed with standard UPW as compared to the low concentrations on control wafers exposed to equal amounts of membrane purified UPW. Pall IonKleen™ Purifiers are being used successfully at point-of-use in a number of UPW systems. Because of its high metal ion capacity, typical service life ranges from 1-2 years.

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