AVS 46th International Symposium
    Vacuum Technology Division Wednesday Sessions
       Session VT-WeM

Invited Paper VT-WeM3
Review of Pumping by Thermal Molecular Pressure

Wednesday, October 27, 1999, 9:00 am, Room 610

Session: Vacuum Pumping Systems
Presenter: J.P. Hobson, National Vacuum Technologies Inc., Canada
Authors: J.P. Hobson, National Vacuum Technologies Inc., Canada
D.B. Salzman, Polychip Inc.
Correspondent: Click to Email
Pumping energy is supplied by temperature changes only. A general feature of such pumps is that the upper pressure limit is reached when the mean free path becomes small relative to the physical dimensions of the pump in the region of the temperature transition. Thus the upper pressure limit of these pumps has been determined by the microfabrication technology of the day; they have operated at relatively low pressures, with low throughputs, and have not become main-line pumps. In recent years, however, MEMS (Micro-Electronic-Mechanical Systems) has introduced a whole new level of miniaturization to devices in general, including vacuum devices, and hence has raised the upper pressure limits, and thus the throughputs, of thermal molecular pumps to near atmospheric levels.@footnote 1@ The purpose of this paper is to review various physical manifestations of pumps using thermal molecular pressure, which have been realized over the years. Emphasis is placed on pumps which have actually been constructed and tested. The general pumping phenomenon has had various names: the Knudsen compressor, thermal transpiration, thermal creep, thermodynamic, thermomolecular, thermal molecular, and accommodation pumping. This multiplicity of names can cause some confusion and it is one of the objectives of this review to simplify this situation. We have chosen to title the paper "Review of Pumping by Thermal Molecular Pressure", following the terminology used by Knudsen.@footnote 2@ It is found that, broadly speaking, these pumps divide into two classes: (a) those using no explicit surface treatment; and (b) those using specially prepared surfaces. It is further found that pumps in class (a) have both an upper and lower bound in pressure, while pumps in class (b) have only an upper bound in pressure. A Table is assembled comparing experimental results of pumps which have actually been built and tested. However, scaling rules for multiple stage pumps, based on results obtained for single stage pumps are presented. Despite their diversity thermal molecular pumps all have the compelling advantage that there are no moving parts, nor any fluids, in the vacuum. @FootnoteText@ @footnote 1@ S.E.Vargo, E.P.Muntz, G.R.Shifflet, and W.C.Tang, J. Vac. Technol. A, (in press). Paper presented at the 45th International Symposium of the American Vacuum Society, Baltimore, 1998. @footnote 2@ M.Knudsen, "The Kinetic Theory of Gases", Methuen’s Monographs on Physical Subjects, John Wiley and Sons Inc., New York, 1934.