粒子数基準ウェハの作製手順―ウェハ表面検査装置の校正

Wafer surface scanners (WSS) are evaluated using a calibration wafer which has size standard polystyrene latex spheres (PSLS) on its surface. Aerosolized PSLS are deposited onto a wafer surface to make the calibration wafer, and the methods for making the wafer are standardized. This study introduces a new procedure for making a calibration wafer whose number of deposited particles and a method for evaluating the uncertainty of the particle number on the wafer are known. The important parameter in the procedure is called the particle number conversion coefficient, γw|p, which is the coefficient for calculating the number of particles deposited on a wafer, Nw, from the number of aerosol particles introduced to a wafer, Np, by Nw= γw|p・Np. To accurately evaluate the value of γw|p the PSLS are deposited inside a circular area whose diameter is a few hundred micrometer. Then, the value of Nw was obtained by visually counting all the deposited PSLS using a scanning electron microscope, and the value was compared to a known value of Np. In order to deposit the PSLS within a narrow area on a wafer the PSLS were first grown to micrometer-sized droplets by condensation, and these droplets were deposited onto a wafer by inertial impaction. In this study the value of γw|p was evaluated using 0.81 μm PSLS, and the relative expanded uncertainty of the predicted particle number on a wafer was ±9.6%. The particle number standard wafers were made by depositing PSLS uniformly over a wafer using a XY stage. Then, the wafers were used to evaluate the counting efficiency of a WSS.