A Thermal Desorption Model for the Outgassing Rate versus Time Characteristics in the Pump-Down of an Unbaked Stainless Steel Chamber

概要

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In order to explain theoretically the outgassing rate versus time characteristics in the pump-down of an unbaked vacuum system, a thermal desorption model has been proposed. In this model, four parameters are introduced, the surface coverage θ, the mean residence time τ, the sticking probability s and the pumping parameter a/A (a is the effective area of the pumping orifice and A the adsorbing area), and it is assumed for the pump-down of the vacuum chamber that the initial surface coverage changes by less than one monolayer and adsorption is fully reversible. In addition the activation energy of desorption is represented with the variation of the heat of adsorption in the Temkin isotherm. Two conservation of mass equations are derived for gas molecules at the surface and in the pump-down of the vacuum chamber, by combining these two mass balance equations a differential equation of the outgassing rate is derived, in which it is expressed with a variable separation form of F (θ) (-dθ/dt) = constant (F (θ) is a function of coverage). The discussion shows that it is possible to solve the differential equation analytically, since F (θ) can be approximated with a simple function exp (-bθ) in the range of 0.05<θ<1. As a result, it is suggested that the differential equation gives an outgassing rate which depends not only on pumping time but also on pumping speed.
日本真空協会の論文
1996-12-20