Alignment measurement method for imprint lithography using moire fringe pattern

A simple and high-accuracy alignment measurement method based on a moire fringe pattern is proposed. It involves relative rotation positioning and relative linear displacement measurement. Taking full advantage of the magnification effect of moire fringe in angular and linear displacement, the relative rotation between the template and the wafer is determined first by measuring the inclination of the moire fringe, and then the relative linear displacement between them is acquired by evaluating the spatial phase shift of two matched moire fringes. The frequency components in the orthogonal directions of the fringe image obtained by a fast Fourier transform (FFT) and zooming process are used to measure the inclination of the moire fringe. By selecting different orthogonal directions, a moire fringe with any inclination can be measured accurately. When gratings are adjusted to parallel, a frequency-domain analysis is also used to extract the spatial phase of fringes at a given frequency. According to the relationship between spatial phase and linear displacement, the misalignment is detected. In experiments, the repeatability for the misalignment measurement has reached 4.8 nm (3 sigma). (C) 2008 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3028350]