Measurement of alignment accuracy for wafer bonding by Moiré method

A centrosymmetric square grating was developed in this paper. Moiré fringes produced by the two centrosymmetric square gratings are highly sensitive with the misalignments and misaligned directions. The distance between the mismatched moiré fringes are expected to be measured directly by an infrared (IR) microscope without requiring any external reference. Using two pair of the moiré square gratings, misalignments of bonded wafers are proposed to be measured in the X-Y-θ axis simultaneously on wafer scale. The measurement accuracy of 0.5 μm in the X-Y axis and 3 × 10 ^-4deg in the θ-axis is presented as an example, which is sufficient for the alignment accuracy of currently available wafer bonding. Moreover, the measurement accuracy by this moiré method can be improved to nanometer level if the fabrication errors and IR microscope resolution are allowed.