A focus monitoring method has been developed by using oblique propagation of a light beam and a binary mask. Oblique propagation of the light intensity was generated by using the main space pattern and asymmetric assist patterns. This method achieves a sensitive focus monitoring with a binary mask instead of a phase shift mask. The Fresnel diffraction image of asymmetric space patterns is formed near the mask and is reconstructed on the wafer with a projection lens at the defocus condition. We can obtain the focal value by measuring the lateral position shift of the Fresnel diffraction image. To measure the lateral pattern shift efficiently, we use bar-in-bar marks, which consist of asymmetric space patterns because the relative displacement of the inner and outer bars can be measured with a commercially available overlay measurement system. (C) 2004 American Vacuum Society.