Interferometric detection of forward scattered light from small particles

Abstract

The detection of subtenth micron paticles in processing fluids is a critical and growing need in the semiconductor industry. In this letter, we show that a small dielectric particle in a focused monochromatic light beam produces a scattered wave (Rayleigh scattering) in phase quadrature with the far-field incident beam, therefore causing a phase shift in this beam. Thus, the forward scattered field due to the particle may be detected using a bright field interferometer. This allows detection which is near Shott noise limited even for very small particles, and measures the sign of the scattered field as well, such that particles may be distinguished from bubbles. We describe an appropriate interferometer design based on Nomarski optics, which we have used to verify our calculation, measuring scattering in water from single polystyrene spheres as small as 0.038 μm in diameter.