Pump-Down Curve Simulation and Software Development for Vacuum Systems

This paper presents a method to simulate pressure versus time pump down curve for vacuum chambers. Related basic theories, including gas laws, conductance for several kinds of flow regimes, equivalent length for pipes, outgassing, diffusion, and permeation etc., are used to simulate the pump-down curve. The simulation program consists of a MFC module and a MATLAB module. The MFC module is used to input necessary parameters, including start and target pressure for desired simulation pressure range, volume and inner surface area of vacuum chambers, configuration of pumping lines, performance of vacuum pumps, and gas loads. The MATLAB module deals with the pump-down curve calculation based on related theories. The governing equation of the conservation of mass in a pumped vacuum chamber is derived from extended Temkin isotherm. After equivalent length for pipes are calculated and the simulation pressure range is separated into several segments by Knudsen’s number and Reynolds’ number, the pump-down curve of each segment is simulated by three steps, including calculating the conductance of pipes for corresponding gas flow regimes, the effective pumping speed of vacuum pumps and finally pump-down time. The simulated results are compared with the experiment of two different vacuum systems and the results simulated by the commercial software, VacTran. The developed program can simulate the pump-down curve with good accuracy in the range from low vacuum pressure to high vacuum pressure.