Research On Wavefront Aberration Measurement Of The High NA Lithographic Objective

As the mainstream apparatus for manufacturing super large scale integration,deep ultraviolet lithography is urgent to be developed in our country. Current projection lithography objective which is the core part of the lithography has broken through the diffraction limit of Rayleigh criterion, which leads to that wavefront aberration becomes the most direct and effective image quality evaluation criteria.The requirement of wavefront aberration for 22 nm node lithography reaches λ/100.In order to obtain higher resolution, the numerical aperture(NA) of the image space of 193 nm non-immersion objective lens is close to 1. It brings difficulties for wavefront aberration measurement. It is hard to get an appropriate standard component using traditional interferometer. When using shear interferometer, the environment criterion is quite strict, leading to impossibility for testing on line. It is also hard to obtain an ideal spherical wavefront in such high NA by a point diffraction interferometer. The design and manufacturing for a corresponding high NA collimator is difficult using a Hartmann-Shack(H-S) sensor. Meanwhile the accuracy of wavefront measurement of common H-S sensor does not meet the requirement for the lithography objective.The wavefront aberration of NA=0.75, λ=193 nm projection lithography objective reaches 6.5 nm,and the accurate of wavefront aberration measurement is2nm(0.01 λ). This paper proposes a "reverse" test program for high-NA projection lithography objective wavefront aberration measurement using H-S wavefront sensor.It overcomes the need of the corresponding high NA and high-precision collimator system. By analyzing the sources of wavefront aberration measurement error, several specific methods have been put forward: a whole-part adaptive window segmentation algorithm, the two-dimensional interpolation polynomial reconstruction algorithm and a phase-retrieval algorithm based on iterative Fourier transform, so that the measurement accuracy of the wavefront aberration is satisfied.The details are as follows:(1) A whole-part window adaptive segmentation algorithm is presented.Increasing the spot centroid detection accuracy achieves high accuracy of the optical system wavefront aberration measurement. The method uses nonlinear filtering and adaptive window segmentation on the whole spot image globally. Then it combining median filtering, cubic spline interpolation and adaptive Otsu threshold method processes a single spot locally. A known wavefront aberration of the optical system is tested simulately. The PV value of wavefront aberration measurement error is0.0098 λ, and the RMS value reaches 0.0027 λ.(2) To meet the demand of a high NA projection lithography objective, taking modal reconstruction method, the tested wavefront is fitted in Zernike polynomials.In addition, two-dimensional polynomial interpolation polynomials are proposed to alternate Zernike polynomials as the base of reconstruction polynomials for the tested wavefront. Simulation results show the accuracy of the reconstruction of two kinds of polynomials both reach 0.015 λ.(3) A phase-retrieval algorithm based on iterative Fourier transform is proposed to achieve higher measurement accuracy of high-NA projection lithography objective wavefront aberration. This approach is compared with the traditional method of slope measurement. It avoids the mathematical approximate calculation and takes the full use of details in the internal sub-aperture microlens. The accuracy of wavefront reconstruction in the simulated experiment is improved to λ/1000. Andthe experimental wavefront aberration restruction RMS error is 0.0281 λ.Finally, H-S wavefront sensor has been used to measure the wavefront aberration of experimental projection objective relatively and absolutely. The relative measurement has higher accuracy. The PV value of the wavefront aberration measurement is 0.5916 λ, while the RMS value is 0.1328 λ. Compared with the interferometer testing results, the PV value of the measurement error is 0.0274 λ, and the RMS value is 0.0078 λ.