โ Scribed by O'Hanlon, John F.
No coin nor oath required. For personal study only.
Treatises on vacuum technology traditionally include a section entitled "ultrahigh vacuum." For those of us whose interest in vacuum dates to soon after the middle of the last century, that phrase connoted specific construction techniques, components, materials, and procedures, including a 450ยฐC overnight bake. The goal was the achievement of an ultrahigh vacuum environment in which fundamental studies could be performed. Today, such pressures are required for myriad applications ranging from high-energy particle research to the manufacture of magnetic thin-film read-write heads, semiconductor, and opto-electronic devices. Systems have grown in size and complexity. In parallel, technical restrictions have limited the time available to reach ultimate pressure as well as the maximum allowed baking temperatures. Some systems can be baked only to 75-1 50"C, whereas others, by virtue of their size, cannot be baked at all. All these systems require base pressures in the low 1 0-8-to 1 O-9-Pa (1 0-l'to lO-"-Torr) range. Base pressure requirements will continue to decrease to meet advanced processing needs. Additionally, contamination caused by environmental or process-generated particles as small as 10 nm may affect product yield or the ability to do research.
The term "ultrahigh vacuum" is no longer adequate to describe the requirements demanded by the highest technology applications. Not only must the process or experimental vessel reach the classical ultrahigh vacuum pressure range, but also it be ultraclean, and it must allow translational and rotational motion, electrical power, cooling fluids, and electrical and optical instrumentation signals through its walls without adding contamination. Metrology is not simple. UHV pressure gauges and residual gas analyzers may provide incorrect information, should they be mounted or operated incorrectly; misunderstanding of electron-stimulated desorption can result in inaccurate pressure measurements. The need for distributed pumping to evacuate extremely long, narrow accelerator or beam chambers is well understood; however, the designers of complex production systems with shielded interiors do not yet appreciate that they have a similar problem. 403
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