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What is the advantage to an all metal sealed container?

Wafer fabrication for today’s smaller device nodes require that the process chemicals be supplied with very low impurity levels and controlled within tight variability limitations. To maintain low impurity levels, all-welded metal containers are commonly employed to protect the chemical precursors against all sources of contamination from outside the container.

Stainless steel is a preferred container fabrication material because of its high yield strength, impermeability, stability and corrosion resistance, high temperature performance and ease of fabrication. Furthermore, sources of contamination can be easily removed from its internal surfaces prior to service by chemical cleaning and thermal desorption. These attributes of metal surfaces apply similarly to metal gasket seals. When designed and installed properly, metal gasket seals can provide a helium leak tight seal with a leak rate better than 1×10-9 atm cc/sec and with no measurable atmospheric permeation.

Container leak rate, permeation, and outgassing are very important to maintaining a long chemical shelf life. Polymeric O-ring seals are commonly used for the mechanical inspection ports, level sensor ports, and maintenance access fittings of chemical containers. Permeation and outgassing are physical properties of elastomers that will degrade chemical purity and reduce shelf life. Gases and moisture diffuse through the voids in polymer chains by a thermally activated process, and as a result, permeation rates are much larger for those materials than for metal gasket seals. Short term helium leak rate measurements for elastomer O-ring seals can be misleading—the test duration is often terminated in a matter of minutes, which is too short to measure the significantly larger permeation through the seal.

For the increasingly rare applications where atmospheric air components are not detrimental to the integrity of the chemistry, compatible polymeric O-ring seals can be the better seal design selection. Yet, testing the polymeric materials compatibility with the chemistry is critical to ensure precursor purity and personnel safety. If compatibility has not been adequately proven in literature or lab testing, it may be optimal to select a metal gasket material of the same construction as the container.

Versum Materials has designed container seal solutions that meet the requirements of advanced technology nodes through 10 nm and beyond, and welcomes any opportunity to work with IDMs to integrate the most optimal container design into their process.

References: O’Hanklon, J. User’s Guide to Vacuum Technology, 3rd edition, July 2003

Author

Tom Steidl worked in chemical container development for 38 years and is recently retired. He graduated from the University of Akron with a BS in Mechanical Engineering.