Method of optical metrology of unresolved pattern arrays

A process for determining critical dimension bias or overlay error in a substrate formed by a lithographic process initially provides an array of elements on a substrate, the 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 define a pitch of the elements. Edges of the elements are aligned along a line forming opposite array edges, with the distance between array edges comprising the array width. An optical metrology tool used for measurement of the array is adjustable for one or more of i) wavelength of the light source, ii) numerical aperture value or iii) partial coherence. The process includes selecting the pitch of the elements, the wavelength of the light source, the numerical aperture and the partial coherence such that the pitch of the elements is less than or about 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 whereby individual elements are not resolved within the array. The edges of the array are resolved with the optical metrology tool and the width of the array is measured to determine bias or overlay error in the substrate. Where the pitch of the array differs in different directions, the optical metrology tool has a non-circular pupil and the numerical aperture value NA of the optical metrology tool in the direction of minimum array pitch is selected to be less than the numerical aperture value NA of the optical metrology tool in a direction of maximum array pitch, such that the array edges are resolved and individual elements are not resolved.