23 Apr Does your high purity gas system need electropolished (EP) stainless steel components?
For fab managers in semi-conductor and photovoltaic (PV) manfacturing, this is a common question.
Going back to the basics, to convey a “high purity” gas, the gas system must be free from contaminants and leaks. At one time (1970’s) copper tubing was sufficient, then chemically cleaned stainless steel was sufficient (1980’s), but finally electropolishing followed by a hot deionized water final clean was established as the best method to achieve a passive, corrosion resistant, and uncontaminated tubing inner surface.
High purity is a relative term – in the semi-conductor electronics industry the specification may allow ppb (part per billion) or ppt (part per trillion) levels of contaminants whereas in the photovoltaic industry the specification may allow ppm levels in many instances. Typical gas specifications for both industries may be obtained from SEMI (www.semi.org).
Once the gas specification requirement is understood or determined, material can be specified, pragmatically. The choices are electropolished/very smooth (~ 5 – 10 u-inch RA), electropolished/moderately smooth (~ 15 – 25 u-inch RA), anodically cleaned ( >30 u-inch RA), or mechanically cleaned/non-EP (180 grit). A further choice is between 316L and 304L tubing and fittings.
For a ppb or ppt gas spec (or better than 99.9999% purity), electropolished tubing with an internal roughness of 5 – 15 uinch RA surface finish is adequate with the smoother finish reserved for smaller diameter tubes.
For a ppm gas spec, anodically cleaned or mechanically polished tubing is adequate. Some suppliers offer 304L stainless tube and fittings at a discount to 316L that may be attractive for photovoltaic “high purity” gases (that are actually ppm specified gases) and semi-conductor ppm specified gases.
Some users also prefer to classify by gas use: “touches the wafer” or doesn’t “touch the wafer” where only those gases touching the wafer require electropolished tubing. For gases such as purified, compressed air used within lithographic “steppers”, electropolished tubing is being specified because of the critical application.
More targeted specification recommendations are available from Ralph Cohen (author of this post).
Ralph Cohen, P.E., who provided this information, is an engineering consultant to customers for gas systems components and HVAC, specializing in semi-conductor and other industrial manufacturing facilities. He has over 30 years in the field, and continues problem solving for clients through his consultancy. More information on Ralph’s project work, publications, etc. can be found here RalphCohenConsultancy or call 971-227-8989.