AVS 52nd International Symposium
    Vacuum Technology Wednesday Sessions
       Session VT-WeM

Paper VT-WeM5
Selective Water Vapor Cryopumping Through Argon

Wednesday, November 2, 2005, 9:40 am, Room 201

Session: Gas Flow and Pump Technology
Presenter: O. Podcherniaev, Helix Polycold Systems, Russia
Authors: A.P. Kryukov, Moscow Power Engineering Institute, Russia
O. Podcherniaev, Helix Polycold Systems, Russia
P.H. Hall, Polycold Systems
D.J. Plumley , Helix Polycold Systems, Russia
V.Yu. Levashov, Moscow Power Engineering Institute, Russia
I.N. Shishkova, Moscow Power Engineering Institute, Russia
Correspondent: Click to Email
A selective cryopumping process for water vapor control takes place in vacuum systems for web coating or plasma operations; such as sputter deposition, etching, etc. Excessive water vapor content will affect quality of the processes and final products. These vacuum systems typically operate at pressures corresponding to transitional or viscous flow regimes, and water vapor cryopumping is highly limited by diffusion of water vapor molecules through a non-condensable process gas (argon, air). An analytical model was created to describe water vapor condensing process through a non-condensable gas diffusion barrier. The model accounts for the collisions of different molecules by means of Boltzmann kinetic equations for two-component rarefied gas. It was assumed that water vapor content is about three orders of magnitude lower than that of the non-condensable gas (argon). Cryopumping process was analyzed for two simplified cases when water vapor source and cryosurface are: parallel plates and coaxial cylinders.  The calculations were conducted for different water vapor outgassing rates and argon pressures ranging from 0.5*10@super -3@to 20.0*10@super -3@ torr.  At certain parameters a strongly non-linear distribution of water vapor pressure and density vs. distance between source and cryosurface was obtained.  At high argon pressures an increase of water vapor pressure was observed nearby an outgassing surface.  The results were used for calculation of water vapor cryopumping rates.