针对现有离轴检焦技术在浸没式光刻方法中的局限性，提出了一种新的基于干涉的同轴检焦方法。测量光通过光刻物镜入射到硅片表面，在硅片表面反射后再次经过光刻物镜后，测量光和参考光产生干涉条纹，并被CCD接收，从而将硅片的离焦量信息调制在干涉条纹的相位信息中。通过对干涉条纹的相位提取，即可获得硅片的离焦量。仿真结果表明，该方法可以达到λ/25(λ=632.8 nm)的检焦精度，并具有良好的抗噪性，满足浸没式光刻高精度、实时、非接触焦面测量的要求。

A novel coaxial focusing method based on interference is proposed to compensate for the shortage of off-axis focusing methods in immersion lithography. The measurement light is projected on the wafer plane through a lithography objective, and then reflected by the wafer and entries the objective. The resulting image created by interference between the reference light and the measurement light is detected by the detection system, so the defocusing amount of the wafer is modulated in the phase of the interference image. The defocusing amount is obtained by demodulating the phase. The simulated experiment shows that the proposed method can achieve a focusing accuracy of λ/25 (λ=632.8 nm), and has good noise immunity, meeting the demand of high-precision, real-time as well as non-contact measurement in immersion lithography.