In photolithography, wafer alignment marks are used to align the current masking layer to the previous layer. Conventional marks are usually gratings with equal width of lines and spaces. In order to achieve more reliable alignment signal, higher order of the reflected beam is required to define the edges precisely. However, the strength of reflected beam for conventional marks decreases as the order number increases. In this article, enhanced order marks, which are able to give better diffraction efficiency at higher reflected order, are studied. A model based on rigorous coupled wave analysis is adopted and developed to analyze different types of step grating alignment marks. As the period-to-wavelength and depth-to-period ratios for alignment mark are acceptable, this model is adequate to generate reliable results. When small periodic gratings of 5.33, 3.20, and 2.29 mum are incorporated into 16 mum periodic marks, the reflected diffraction efficiency is enhanced at third, fifth, and seventh orders, respectively. Forbidden mark depth, which gives almost zero diffraction efficiency for higher reflected order, is also identified for both 633 and 532 nm alignment wavelength. The results also show that, irregardless of the type of marks and alignment wavelength, whenever the zeroth order reflected diffraction efficiency drops to minimum, it gives the strongest higher order reflected diffraction efficiency. When transverse magnetic polarization is considered, it is able to give higher reflected diffraction efficiency if compared to transverse electric polarization. The experimental results demonstrated that overlay as low as 10 nm can be achieved when fifth order enhanced mark was used for 90 nm and 0.11 mum technology node process. (C) 2005 American Vacuum Society.