The effects of simultaneous electrophoresis and thermophoresis on particulate contamination of an inverted EUVL photomask surface in parallel airflow

Abstract

The combined influences of electrophoresis and thermophoresis on particle deposition on the inverted critical surface of a flat plate in parallel airflow were investigated by employing the statistical Lagrangian particle tracking approach in an effort to assess the degree of particulate contamination of EUVL photomasks during horizontal transport in cleanroom environments. The numerical method was validated through the comparison with the experimental data, found in the literature, about particle deposition velocity onto a wafer in vertical airflow with and without electrophoresis or thermophoresis. In addition, the validation of the present model was performed via the comparison with the theoretical prediction of particle deposition velocity onto a flat plate under no phoretic forces in parallel airflow. Then, the particle deposition velocity onto the face-down surface of a flat plate in parallel airflow was obtained by varying the temperature of the inverted critical surface in different strengths of uniform electric fields. Injected particles were assumed to be charged with -1, 0, or +1 elementary unit of charge, in order to consider attractive or repulsive electric force. The degree of particulate contamination of the inverted critical surface was found to be significantly influenced by the combination of electrophoretic and thermophoretic effects.