FOCUSING DEVICE, FOCUSING METHOD AND A PATTERN INSPECTING APPARATUS

摘要

A focusing device comprises a first imaging optical system, a second imaging optical system which splits the optical image in the direction of an AF sensor and further splits the optical image so that a front focus image in which the point that is in focus is in front of the optical image on the inspecting sensor and a back focus image in which the point that is in focus is behind the optical image on the inspecting sensor are formed on the AF sensor, a focus detecting circuit which detects an optimum focus position on the basis of a high-frequency component of a front sensor image in a front focus position and a high-frequency component of a back sensor image in a back focus position, and a focus control circuit which controls the focusing of the first imaging optical system on the basis of the focus position.