Through-the-mask (TTM) optical alignment for high volume manufacturing nanoimprint lithography systems

As the most aggressive features in advanced memory designs continue to shrink, so does the overlay budget. The number of layer stacks also creates unwanted topography, and the alignment robustness of lithography tools becomes much more important for on-product overly. Canon developed a through-the-mask moire alignment system for the FPA-1200NZ2C nanoimprint lithography (NIL) system allowing high-speed measurement of several alignment marks within each imprint field and alignment compensation to be completed during the imprinting sequence. To provide increased process flexibility and overlay accuracy while maintaining high-productivity, we have developed a new low-noise and high-resolution moire diffraction alignment system based on spatial phase interferometry. In this paper, we report on the TTM detection system used in FPA-1200NZ2C. In particular, the principle of moire detection and the improvement of the detection method will be described. The measurement error of moire is analyzed by a simplified model calculation and we confirmed the relationship between process change and alignment error. Results of analyses proved that selection of the wavelength are key factors for optimizing alignment accuracy. Based on these results we applied the following improvement items: 1) Dual Dipole illumination, 2) Optimization of the alignment wavelength. We evaluated the new alignment system measurement error by comparing the moire measurement value with the measured overlay values for 24 wafers and confirmed that new TTM alignment system can reduce to the measurement error more than 40%. The data shows that our moire measurement system can provide process robustness and can support mass-production of leading-edge memory products.