MEASUREMENTS AND CONTROL OF PARTICLE DEPOSITION VELOCITY ON A HORIZONTAL WAFER WITH THERMOPHORETIC EFFECT

To investigate positive and negative thermophoretic effects for polystyrene latex (PSL) spheres of diameter between 0.3 and 0.8 mu m, the average deposition velocity toward a horizontal wafer surface in vertical airflow is measured keeping the wafer surface temperature different from the surrounding air temperature. The temperature difference ranges from -10 degrees to 4 degrees C. The number of particles deposited on a wafer surface is obtained from the measurements by using a wafer surface scanner (PMS SAS-3600). Experimental data of particle deposition velocity are compared with those given by the prediction model to validate the model. Since thermophoresis changes greatly the particle deposition velocity, temperature difference necessary for the particle deposition velocity to remain under a given value is sought as a means to control the deposition velocity. The minimum temperature differences required to keep the average deposition velocity smaller than 10(-4) cm/s and 10(-5) cm/s in a clean room environment are suggested based both on the prediction model and the present experimental data.