The knowledge of deposition velocity for a given particle diameter is important in controlling the particle deposition on a wafer surface. In this study, the average particle deposition velocity toward a horizontal semiconductor wafer surface is measured in a vertical laminar airflow chamber. The number of particles deposited on the wafer surface is estimated from the measurements using a wafer surface scanner (PMS SAS-3600) to shorten the exposure time and hence to improve repeatability. The particle size response of the wafer surface scanner, prior to measure the deposition velocity, is studied using ideal spherical particles of PSL (polystyrene latex) and irregularly shaped, real world particles of Si and $SiO_2$. Experimental study is made for particles in the 0.1 to 1.0㎛ diameter range. By using the standard test wafers, on which monodisperse particles were uniformly deposited, the particle size difference between the manufacturer's calibration and actual size is studied. In addition, the count repeatability of the instrument is studied by performing statistical analysis of the count data collected by the instrument.
The average deposition velocity toward a horizontal wafer surface is measured for PSL spheres of diameters between 0.2 and 1.0㎛. Convection, diffusion, and sedimentation comprise important agents for deposition mechanism. Average deposition velocities are obtained from more than ten data set for each PSL sphere size and the deposition velocity distributions from the measurement data are compared to the theoretical distributions.
To investigate thermophoretic effect on particle deposition, the average deposition velocity toward a horizontal wafer surface is measured keeping the wafer surface temperature different from the surrounding air temperature. In the present measurement, the temperature difference is maintained in the range from -10 to 4 ℃. PSL spheres of diameter between 0.3 and 0.8㎛ are used for the experiment. Experimental data are compared with prediction model results. Since thermophoresis changes greatly the particle deposition velocity, temperature difference necessary to keep the particle deposition velocity under a given value is sought as a means to control the deposition velocity. The minimum temperature difference required to keep the average deposition velocity smaller than $10^{-4}$ cm/sec and $10^{-5}$ cm/sec in a clean room environment are suggested based on the prediction model and are verified by the experimental data.
