Broadband alignment scheme for a stepper system using combinations of diffractive and refractive lenses.

By using broadband illumination, a stepper alignment system is less sensitive to asymmetric coverage of photoresist around alignment targets and realizes high accuracy. However, a projection lens causes chromatic aberrations at the wavelengths in the broadband light. To correct the chromatic aberrations, we employed a Schupmann system composed of a projection lens, an achromat lens, and a refractive-diffractive hybrid lens. With the proposed system the spread of image planes and the magnification difference in the spectral range from 550 to 650 nm can be reduced to 50 µm and 0.05%, whereas without the correction system these values are 11.28 mm and 1.18%, respectively.