Ultrapure water
(UPW)
Introduction
In the production of integrated circuits, semiconductor chips and liquid crystal or plasma displays, water of extreme purity is required in certain processing steps. The production of UPW requires a combination of several purification technologies, including ion exchange (IX), reverse osmosis (RO) and other processes. Integrated circuits contain very fine conductors, spaced by a fraction of a µm. Any minute impurity sitting across two conductors will create a short-circuit and ruin the whole device. The closer the distance between the conductors, the more drastic is the required purity of water used in the manufacturing process.
An example of UPW circuit
The design of an UPW circuit requires a specific knowledge that only specialised water treatment companies can offer. The schema above with fifteen steps is only an example, and many variations exist.
How pure is ultrapure?
As a basic requirement, the treated water must have a resistivity of 18.2 MΩ·cm (conductivity 0.055 µS/cm). Moreover, the content of organic matter must be extremely low. Common specifications call for a TOC (total organic carbon) of only 1 to 3 µg/L (ppb). Individual inorganic species are measured in the ng/L range (ppt). Particulate matter is also kept very low. All this can only be obtained with a combination of technologies, whereby ion exchange is an indispensable critical component. As this site is devoted to ion exchange, we will not comment the other technologies (filtration, RO, degasifier, UV lamps, UF).
Resins used
In the pre-treatment steps, common softening or demineralising resins are used in conventional equipment. In the primary demineralisation section (item 10 in the illustration), a regenerable mixed bed unit is usually installed. Use:
- Amberjet UP1400 (cation exchange)
- Amberjet UP4000 (anion exchange)
One of the most critical elements of the UPW circuit is the polishing MB (item 14 in our example). This unit is not regenerated, because the efficiency of regeneration would not be sufficient to achieve the complete conversion of the ultrapure resin. On the other hand, the operating time of the polishing unit is generally one year or more, because the ionic load to this unit is extremely low. Consequently, the general rule is to install a new charge of resin every year. Recommended mixed resins for the final point-of-use polisher:
- Amberjet UP6150
- Amberjet UP6040
