Matsumoto and Tanaka (1994) describe a method for removing "impurity ions that may affect integrated circuit (IC) processing speed." Their study "confirmed that not only dust but also impurity ions could be removed by filtering as a result of measuring the concentration rate of impurity ions in the air conditioning system of clean rooms. The effect of removing impurity ions by a washer was also confirmed." Their procedure and its background are described as follows:
External air processing systems are to take in air for pressurization from outside to control temperatures and humidity in a clean room and to remove dust. It is therefore necessary to take appropriate actions for adverse effects on ICs brought by contamination materials in the external air. If the plant location is close to sea, removal of salt is important and if the location is in a freeze area in winter, removal of anti-freezers and snow-removers is important. In addition, if the degree of air contamination by SOx and NOx is very severe, strong acid needs to be removed. Consequently, a method to remove contamination materials tailored for each case must be devised in external air processing systems. Filtering is a preferred method to remove contamination materials of floating particles and chemical filtering combined with a washer is the best for those materials that cannot be cleaned by absorption through regular filters. Most of the chemical filters use synthetic zeolite soaked in chemical solution and contamination materials are removed by chemical reaction and chemical absorption. It is necessary to select the optimum combination of chemical filter depending on the type and concentration of contamination materials. Potassium permanganate, potassium carbonate and phosphoric acid are used as chemicals. As it is difficult to predict the life of chemical filters, periodic exchange is necessary.
A removal method by combining a filter and a washer that does not cause re-contamination is desirable to remove impurity ions in external air processing systems. The following have been examined and used ...:
1.Using pure water as washer water
... it was found that impurities in water for humidification were introduced in the air during the washer operation to increase humidity in the winter season. For this reason, pure water can be used as washer water for humidification to reduce the concentration rate of impurity ions.
2.Two-stage operation of humidifier and washer
... impurity ions can be removed by the washer operation ... when humidity is high ... when humidity is low, a two-stage operation can be carried out in which humidity is increased by a humidifier in the first stage and impurity ions can be removed by a washer operation in the second stage.
The above methods of (1) and (2) were verified by using pure water for humidification. When outside humidity is low on sunny days, pure water is added frequently so that the purity rate of pure water is maintained. When humidification is not needed such as on rainy days, impurities in the air are absorbed in pure water and the impurity removal effect is even more efficient.
3.Using HEPA filter at the last stage of filtering
In this method, a three-stage filtering is adopted that consists of a pre-filter (for crude dust), a middle filter (middle capability) and a final filter (high efficiency particulate air filter, abbreviated as an HEPA filter thereafter). By using an HEPA filter at the last stage, introduction of impurity ions in the external air can be minimized. It was confirmed that the concentration rate of impurity ions at downstream of the HEPA filter was reduced to less than a few mg/m3.