Molecular Flow Module

Molecular Flow Module

Software for Modeling Low-Pressure Gas Flow in Vacuum Systems

Image made using the COMSOL Multiphysics® software and is provided courtesy of COMSOL.

In an ion implanter, the average number density of outgassing molecules along the beam path is used as a figure of merit to evaluate the design. It must be computed as a function of wafer angle, with rotation about one axis.

Molecular Flow Module

Understanding and Predicting Free Molecular Flows

Vacuum engineers and scientists use the Molecular Flow Module to design vacuum systems and to understand and predict low-pressure gas flows. The use of simulation tools in the design cycle has become more widespread as these tools improve understanding, reduce prototyping costs, and speed up development. Vacuum systems are usually expensive to prototype. Therefore, an increased use of simulation in the design process can result in substantial cost savings. The gas flows that occur inside vacuum systems are described by different physics than conventional fluid flow problems. At low pressures, the mean free path of the gas molecules becomes comparable to the size of the system and gas rarefaction becomes important. Flow regimes are categorized quantitatively via the Knudsen number (Kn), which represents the ratio of the molecular mean free path to the flow geometry size for gases:

Flow type Knudsen Number
Continuum flow Kn<0.01
Slip flow 0.01<Kn<0.1
Transitional flow 0.1<Kn<10
Free molecular flow Kn>10


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