"A method for improving focusing of an image to be projected onto a semiconductor wafer in a semiconductor manufacturing process"

A method for improving focusing of an image of a reticle onto a surface (3) of a photoresist layer (4) of a wafer (5) comprises preparing a relationship graph (36) of the relationship between the pattern densities of the metal layers (37,38) beneath a metal layer (6) of which the photoresist layer (4) corresponds and corresponding offset distance values between optimum focusing positions of a stepper table (15) of the apparatus (1) and the actual focusing position of the stepper table (15) determined by a detector (33) which detects a reflected beam (30) of a focusing light beam (25) from a reflecting area (29) of the surface (3) for a plurality of wafers (5) of different pattern densities beneath the reflecting area (29). In the design of each reticle (2) from which an image is projected onto the surface (3) of the relevant photoresist layer (4) the pattern density beneath the reflecting area (29) is computed from the computer aided design system in which the wafer circuitry is designed. The offset distance value is read from the relationship graph (36) corresponding to the pattern density beneath the reflecting area (29), and the offset distance value is entered into the apparatus (1). Suitable software corrects the position of the stepper table (15) by the offset distance so that the entire photoresist layer (4) is within the depth of focus A of the apparatus (1).