Focusing system and method

#The present application is a U.S. National Phase application of PCT/GB2004/003969, filed 10 Sep. 2004, the entirety of which is incorporated herein by reference. This application also claims the benefit of British Patent Application NO. 0321918.5 filed 19 Sep. 2003.

#The invention relates to a focusing system and method, in particular to an automatic focusing system and method for focusing on a generally planar object in bright field microscopy, for example as used for examining silicon or other semiconductor wafers for the purpose of process control, and in particular for overlay metrology.

#Automatic focus of an optical system requires the acquisition of information about the relative position of the object and the optical system. In many instances, the object approximates to a plane reflective surface, and the auto-focus will project a light beam onto the object and use the zero-order reflection from the object to determine the object distance.

#Several embodiments of the invention relate to focusing systems on microscopes having a light source, an objective lens or lens system, a light path to direct incident light through the objective lens or lens system to be reflected by the object, an aperture to limit the spatial extent of the incident light and serve as an illumination pupil, a light path to direct at least some of the reflected light to an imaging system, and an imaging system to image the reflected light so directed.

#In accordance with selected embodiments of the invention, a method of automatically focusing such a system comprises directing a beam of light from a light source through an objective of a microscope system to an object whereby light is reflected from the surface thereof; collecting at least some of the light reflected thereby and directing the same to an imaging system, wherein the incident beam of light is limited in spatial extent by imaging an aperture to form an illumination pupil, the centroid of illumination of the illumination pupil is aligned with the incident optical axis of the instrument, and reflected light is projected to the imaging system comprising at least two images, for example at least one pair of images, from eccentric sections of an imaging pupil differentially displaced from the optical axis, and wherein the separation of the images thereby produced is determined to provide an indication of the object distance.

#The differential displacement of the at least two images as the focus changes means that the separation distance also varies as the focus changes. The differential displacement comprises differential movement in extent and/or direction, preferably a pair of images displace in opposite directions.