High-precision alignment for low temperature wafer bonding

A novel wafer-to-wafer alignment method is developed using centrosymmetric moiré gratings. Moiré 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-6 direction simultaneously on wafer scale. In our experiment, the misalignments in the order of ±0.5μm in X-Y direction can be easily identified using a simple infrared (IR) microscope. Moreover, these moiré gratings are suitable for being secondary alignment mark to achieve high-precision alignment as well. And the alignment accuracy assisted by these moiré gratings can be further improved to nanometer range if processing errors for the fabrication of the square centrosymmetric gratings are reduced.