Invited Paper VT-TuA1
The Evolution of Cryopumps
Tuesday, October 20, 2015, 2:20 pm, Room 230B
Cryopumping is a widely used technique to produce vacuum in an enclosed space via removal of residual gases by cooling them to the point that they are condensed or adsorbed on an appropriate cryogenic surface. Depending upon the application and the gas species, cryopumps operate at different temperatures between 4K and 150K. To attain these temperatures, different thermodynamic cycles are employed ranging from helium liquefaction to mixed gas auto-cascaded systems. Of these various thermodynamic cycles the Gifford-McMahon (GM) cycle has emerged as the dominant cycle for economically delivering the appropriate cooling power at the appropriate temperatures required for cryopumping. Cryopumps based on GM cryocoolers have been commercially adapted to a variety of vacuum processes; notably semiconductor substrate processing equipment, flat panel display fabrication, thin film coatings, analytical instruments and space simulation systems. Built around a dual stage cryocooler, GM cryopumps consists of two internal cryocondensation regions (arrays) that operate at different temperatures. The warmer (first) stage is generally operated at temperatures between 65K and 100K, condensing mainly residual water vapor and other type I gases such as hydrocarbons, carbon dioxide etc. The colder (second) stage is kept at 8K to 20K, which allows condensing of type II gases (such as nitrogen, argon, oxygen) and adsorbing type III gases (hydrogen, helium, neon). To adapt a cryogenic pump to a specific application the geometry and temperatures of the condensing arrays can be modified or tuned to suit the user’s application.
By far the dominant application of cryopumping is for the semiconductor fabrication processes of physical vapor deposition (PVD) and ion implantation. There are significant differences in the vacuum environments, namely the gas species and gas densities in which these two processes are conducted. Over time, the vacuum requirements of these processes have become more stringent as line widths decreased in keeping with Moore’s Law. In response to the changing vacuum requirements for these processes, the designs of closed cycle cryopumps have evolved. The evolutionary performance improvements of GM cryopump that has taken place in the past few decades will be discussed in this work. Extensive developments have led to significant increase of storage capacity (up to factor 5), pumping speed, pressure dynamics, functionality and energy efficiency. The safety aspects of cryopumping of explosive gases such as oxygen (ozone) and hydrogen will also be discussed.