SIZING ACCURACY, COUNTING EFFICIENCY, LOWER DETECTION LIMIT AND REPEATABILITY OF A WAFER SURFACE SCANNER FOR IDEAL AND REAL-WORLD PARTICLES

The performance characteristics of a Tencor Surfscan 4000 wafer surfac e scanner have been evaluated using ideal polystyrene latex (PSL) sphe res and irregularly shaped real-world particles of Si and SiO2. The pa rticles were uniform in size and were deposited on bare silicon wafers and used as standard calibration wafers to study the scanner response . Particles in the 0.1 to 1.0 mum diam range were used. The sizing acc uracy, counting efficiency, lower detection limit, and count repeatabi lity of the scanner were studied systematically. The full Maxwell's el ectromagnetic equations also have been solved numerically on a superco mputer to obtain the light-scattering cross section of the particles o n bare silicon wafers. The calculation compares favorably with the exp erimental results. Because of the high refractive index of silicon, th e wafer surf ace scanner detects Si particles to a considerably smalle r size than PSL. The data suggest that Si particles as small as 0.1 mu m have been detected by the Surfscan 4000 with 90% counting efficiency even though the nominal lower limit of the instrument is 0.3 mum base d on PSL calibration.