A simple and high-accuracy alignment measurement method based on a moiré fringe pattern is proposed. It involves relative rotation positioning and relative linear displacement measurement. Taking full advantage of the magnification effect of moiré fringe in angular and linear displacement, the relative rotation between the template and the wafer is determined first by measuring the inclination of the moiré fringe, and then the relative linear displacement between them is acquired by evaluating the spatial phase shift of two matched moiré 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 moiré fringe. By selecting different orthogonal directions, a moiré 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).
A novel nano-scale alignment technique based on Moire fringe for room-temperature imprint lithography in the submicron realm is proposed. A pair of special slant gratings is used as alignment marks on the wafer and template respectively. Moire signals generated by alignment marks are projected onto a photo-detector array, then the detected signals are used to estimate the alignment errors in x and y directions respectively. Test results indicate that complex differential Moire signal is more sensitive to relative displacement of the pair of marks than each single Moire signal, and the alignment resolutions obtained in x and y directions are ±20nm (3σ) and ±25nm(3σ) respectively. They can meet the requirement of alignment accuracy for submicron imprint lithography.