Contamination particle traps due to a cone, cube and disc

This paper studies asymmetric particle clouds confined within electrostatic particle traps in the presence of a molecular drag force. The drag force. due to flow of neutral Argas around a particle, is proportional to the gas flow, which can be controlled by a mass flow controller. Our system has a component of the drag force parallel to the wafer electrode surface. The particle clouds associated with a cube and a disc are seen to be highly asymmetric in the direction of the drag force, as would be expected. The asymmetry increases as gas flow increases. In order to visualize the clouds quantitatively, we have inroduced a technique called spot scanning in which we move the laser beam slowly with a serpentine path through the particle cloud. The forward scattered light is recorded by a charge-coupled device camera onto videotape. The image of the scattered light on a frame of the videotape is a nearly circular red spot. Using commercially available computer software, the red spots can be used to determine the boundary of the cloud as well as to find contours of constant power density scattered from the cloud.