Patent ID: 6130750
Filing Date: 2000-10-10
Classification: G03F

Abstract:
A process for determining bias or overlay error in a substrate formed by a lithographic process comprising the steps of:a) providing a target having an array of elements on a substrate, said array comprising a plurality of spaced, substantially parallel elements having a length and a width, the sum of the width of an element and the spacing of adjacent elements defining a pitch of said elements, edges of said elements being aligned along a line forming opposite array edges, the distance between array edges comprising the array width;b) providing an optical metrology tool comprising a source of light having a wavelength; an object plane adapted to receive said substrate having thereon said target; an objective lens between the light source and said object plane adapted to focus light from said light source onto said target on said substrate in one direction and focus reflected light containing an image of said target in an opposite direction; an image plane adapted to receive light containing an image of said target reflected from said object plane; and an adjustable aperture between said objective lens and said image plane adapted to vary the effective numerical aperture of said optical metrology tool;c) adjusting said numerical aperture such that the pitch of said elements is less than or approximately equal to the ratio of the wavelength of the light source to the numerical aperture value of the optical metrology tool in the direction of the array edges; andd) resolving said edges of said array with said optical metrology tool and measuring the width of said array to determine bias or overlay error in said substrate.