Rapid and precise focusing method for image-based overlay metrology

Swift and accurate measurement of overlay errors has long been imperative for ensuring throughput and yield in integrated circuit (IC) manufacturing. At present, image-based overlay (IBO) remains the predominant method for overlay metrology, relying on Linnik scanning white light interferometry (LSWLI) to guarantee rapid and precise focus assessment. Nevertheless, the focal plane position determined by LSWLI often does not align with the optimal contrast focal plane for imaging in the IBO system. This paper proposes a method to meticulously calibrate the systematic error in focus measurement. Initially, the Fourier transform method is employed to analyze the acquired LSWLI interference curve and extract the coherence envelope, from which the center of gravity is computed to ascertain the LSWLI focal plane position. Subsequently, the gradient RMS means of the images near the LSWLI focal plane are calculated and a weighted polynomial is fitted to these values to obtain the focal plane position imaged by the IBO system. Finally, by repeating these steps and averaging the results of multiple measurements, the inherent system focus offset (SFO) is obtained. This calibration can be conducted during the equipment test and calibration stage, ensuring that even in challenging working conditions, the IBO system can swiftly and accurately determine the final imaging focal plane position by solely completing the LSWLI focus measurement and supplementing it with the SFO. This calibration method is an important reference for the practical engineering application of LSWLI in IBO focus measurement system.