AVS 61st International Symposium & Exhibition
    Vacuum Technology Tuesday Sessions
       Session VT-TuM

Paper VT-TuM6
Numerical Modeling of Particle Transport in Rarefied Flow

Tuesday, November 11, 2014, 9:40 am, Room 303

Session: Gas Dynamics, Modeling, and Pumping Systems
Presenter: Andreas Mack, TNO Delft, the Netherlands
Authors: Mack, TNO Delft, the Netherlands
Van der Donck, TNO Delft, the Netherlands
O. Kievit, TNO Delft, the Netherlands
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Within wafer handling devices, environments from ambient pressure to ultra-high vacuum are present. The wafers are moved by robots between the compartments which are separated by load locks. With closed load lock valves, the pressure is reduced by pumping-down such that the pressure level of the next compartment is roughly matched. Since the pumping speed is approximately constant, the pumping time to very low pressures would take long or require additional pumps such that usually the target pressure is only matched by two orders of magnitude. The final pressure is then achieved by opening the load lock valve such that the pressure in both compartments reach equilibrium. This process includes a strong expansion of the flow such that locally very high flow velocities can be reached up to supersonic speed. Since the flow is in the rarefied regime, the forces on surfaces such as the wafer are small but particles released during the wafer handling process can be dispersed downstream due to drag or gravitational forces. Since there is only sparse information available about the coupling between contaminating particles and the rarefied flow, the present paper focusses on the numerical modelling of particle transport in rarefied flow. A DSMC (Direct Simulation Monte Carlo) code is applied to typical domains of wafer handling such as load lock valves and coupled with a particle tracer. Both codes are available within the open source software package OpenFoam and have been validated in the relevant regimes by either generic numerical experiments or, where available, experimental or other numerical data. By this, particle contamination in low pressure environments can be investigated. On the one hand, possible particle contamination regions and active or passive measures to reduce particle contamination can be identified. On the other hand, the global dispersion behavior of different particle classes is investigated such that conclusions over the generic movement of particles within a low pressure environment can be drawn. By this, the dispersion of certain particles can be excluded due to geometrical or physical constraints which is valuable information for particle contamination measurement. Beside the modelling of particle transport for the generic valve opening between compartments the present paper includes also the venting-up of a representative load lock to ambient pressure whereas the results of pseudo-3d and a full 3D modelling are discussed with respect to flow topology and particle contamination.

Keywords: DSMC, rarefied flow, particle contamination, load lock