While photolithography is developing fast to meet the ever-growing demand for making integrated circuits with shrinking feature sizes, faster and more accurate lithography metrology systems are highly desired, in order to evaluate many wafer parameters (e.g., overlay, critical dimension uniformity, and focus) in a fast and accurate manner, a growing number of sensors have been integrated into metrology systems. Typically, sensors of a metrology system operate separately (or independently) and each sensor requires multiple optical elements to correct aberrations so as to achieve good image quality. Such an arrangement makes the metrology system cumbersome and expensive. Improvements can be obtained by parallelizing these metrology season in a way that those optical elements, typically used in conjunction with metrology sensors, can be replaced with optimally designed (simplified) lenses. As a result, the total number of optical elements is reduced and hence the optical system is simplified. Furthermore, parallelization of metrology sensors also helps improve the speed of the data collection, leading to a higher throughput. However, with such simplified optical system, image aberrations will not he fully compensated and thus image reconstruction will be needed in order to improve image quality.
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