Patent ID: 7907269

Claim:
An apparatus for detecting top scattered light from a top surface of a substrate, the apparatus comprising: a light source for directing a beam of light onto a focal position on the top surface of the substrate, where the substrate is at least partially transmissive to the beam of light, the beam of light thereby, (a) specularly reflecting off of the top surface of the substrate to produce a specular beam, (b) scattering up off of the focal position on the top surface of the substrate to produce the top scattered light, and (c) refracting into the substrate and scattering up off of a bottom surface of the substrate to produce bottom scattered light, an objective for receiving the top scattered light and an unblocked portion of the bottom scattered light, the objective having a first focal point focused on the focal position on the top surface of the substrate, and a second focal point focused on a pinhole field stop, the pinhole field stop for receiving the top scattered light and the unblocked portion of the bottom scattered light, and passing the top scattered light that is focused on the pinhole field stop and blocking the unblocked portion of the bottom scattered light, and a sensor disposed opposite the objective across the pinhole field stop for receiving and quantifying the top scattered light.