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

To investigate positive and negative thermophoretic effects for polyst yrene latex (PSL) spheres of diameter between 0.3 and 0.8 mu m, the av erage deposition velocity toward a horizontal wafer surface in vertica l airflow is measured keeping the wafer surface temperature different from the surrounding air temperature. The temperature difference range s from -10 degrees to 4 degrees C. The number of particles deposited o n a wafer surface is obtained from the measurements by using a wafer s urface scanner (PMS SAS-3600). Experimental data of particle depositio n velocity are compared with those given by the prediction model to va lidate the model. Since thermophoresis changes greatly the particle de position velocity, temperature difference necessary for the particle d eposition velocity to remain under a given value is sought as a means to control the deposition velocity. The minimum temperature difference s 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.