A novel moiré fringe assisted method for nanoprecision alignment in wafer bonding

High precision alignment is essential to 3D integrated circuit and other multilayer structure fabrication. A novel centrosymmetric square moire grating is developed for aligning between the top and bottom wafers. The mismatched moire fringes produced by the two superimposed centrosymmetric square gratings are highly sensitive with the misalignments and misaligned directions without requiring any external reference. Using two pairs of these square moire gratings, misalignments of the bonded wafers are determined in X-Y-thetas direction simultaneously on wafer scale. In our experiment, the misalignments in the order of plusmn 0.5 mum in X-Y direction can be easily identified using a simple infrared (IR) microscope. Moreover, the measurement of misalignments using the moire fringe distances does not depend on the gap between the two wafers changing from tens of micrometers to zero during alignment process. Therefore, these moire gratings are suitable for being secondary alignment mark to achieve high-precision alignment as well. A moire fringe assisted alignment method is proposed for realization of alignment accuracy in nanometer-scale.