Overlay Measurement Algorithm for Moiré Targets Using Frequency Analysis

The miniaturization of semiconductor chips creates discrepancies between the designed node size and physical values. It has resulted in a tightened on-product overlay (OPO) budget and increased the demand for improved measurement noise reduction and accuracy in optical systems. A solution utilizing moiré targets can address such challenges by enabling the amplification of small misalignments that cannot be achieved with conventional overlay targets using an image-based overlay (IBO) estimator. However, moiré patterns formed within a layer introduce noise sources and problems owing to interference from the reflected light, adversely affecting the precision of overlay measurements and limiting the effective utilization of moiré patterns. We investigate the problems associated with moiré patterns in the IBO measurement method and propose a novel overlay measurement algorithm to mitigate the problems. The proposed algorithm increases the accuracy of the filtering method in the spatial frequency domain and improves the overlay precision by approximately 2% compared with conventional measurement algorithms. The proposed low-frequency selection algorithm and signal indexing algorithm effectively address the challenges posed by high-frequency problems and signal strength degradation in moiré patterns. The proposed practical solution achieves more accurate overlay measurements in semiconductor manufacturing, enabling better control and optimization of chip fabrication processes.