HIGH PURITY GASES

Gases used in the manufacture of semiconductor materials fall into three principal areas the inert gases, used to shield the manufacturing processes and prevent impurities from entering the source gases, used to supply the molecules and atoms that stay behind and contribute to the final product, and the reactive gases, used to modify the electronic materials without actually contributing atoms or molecules. 

Whereas the underlying separation or purification technology may be straightforward, the purity achieved is often far less than that which the separation processes are capable of producing. More often than not, recontamination by impurities released by the materials of construction used in the purification, storage, and delivery equipment represents the tme limit to the purity that can be achieved in practice. 

Chemical Conversion. In both on-site and merchant air separation plants, special provisions must be made to remove certain impurities. The main impurity of this type is carbon monoxide, CO, which is difficult to separate from nitrogen using distillation alone. The most common approach for CO removal is chemical conversion to C02 using an oxidation catalyst in the feed air to the air separation unit. The additional C02 which results, along with the C02 from the atmosphere, is then removed by a prepurification unit in the air separation unit. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

The first process utilizes a bed of nickel catalyst which has been regenerated with hydrogen to reduce the nickel content to metallic form. The finely divided metal then reacts with impurities and retains them in the bed, probably as nickel oxide in the case of oxygen or as physisorbed compounds for other impurities. Periodically, the bed is regenerated at elevated temperature using hydrogen to restore the metallic content. The nickel process can be used and regenerated indefinitely. 

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The apphcations of high purity gases are primary in the semiconductor industries. From 1991 to 1995, the North American semiconductor bulk gas sales increased from U.S. 214 to 252 million (aimual growth rate of 4.2%) and specialty gas sales increased from U.S. 78 million to 169 million (aimual growth rate of 21.4%). 

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