High-Precision Alignment for Low-Temperature Wafer Bonding

A wafer-to-wafer alignment method is developed using centrosymmetric moire gratings. Moire fringes produced by the two centrosymmetric square gratings are highly sensitive with the misalignments and misaligned directions without requiring any external reference. Using two pairs of these moire square gratings, misalignments of the bonded wafers are determined in X-Y-θ direction simultaneously on wafer scale. In our experiment, the misalignments on the order of ±0.5 μm in X-Y direction can be easily identified using a simple IR microscope. Moreover, these moire gratings are suitable for being the secondary alignment mark to achieve high-precision alignment as well. And the alignment accuracy assisted by these moire gratings can be further improved to nanometer range if processing errors for the fabrication of the square centrosymmetric gratings are reduced.