Abstract
This paper proposes a technique for in situ measurement of lens aberrations up to the 37th Zernike coefficient in lithographic tools under partial coherent illumination. The technique requires the acquisition and analysis of aerial image intensities of a set of 36 binary gratings with different pitches and orientations. By simplifying the theoretical derivation of the optical imaging under partial coherent illumination, two linear models are proposed in a compact expression with two matrixes, which can be easily obtained in advance by numerical calculation instead of by lithographic simulators, and then used to determine the Zernike coefficients of odd aberration and even aberration respectively. The simulation work conducted by PROLITH has validated the theoretical derivation and confirms that such a technique yields a superior quality of wavefront estimate with an accuracy of Zernike coefficients on the order of 0.1 m lambdas (lambda = 193 nm) and an accuracy of wavefronts on the order of m lambdas, due to further considering the influence of the partial coherence factor on pupil sampling. It is fully expected that this technique will simple to implement and will provide a useful practical means for the in-line monitoring of imaging quality of lithographic tools under partial coherent illumination.
Highlights
As the limit of optical lithography is pushed and feature densities continue to increase, lens aberration has become one of the most important factors to evaluate the imaging quality of lithographic tools [1,2,3,4]
By simplifying the theoretical derivation of the optical imaging under partial coherent illumination, two linear models are proposed in a compact expression with two matrixes, which can be obtained in advance by numerical calculation instead of by lithographic simulators and are used to determine the Zernike coefficients of odd aberration and even aberration respectively
To verify the necessity of considering the influence of partial coherence on pupil sampling for the aberration measurement under partial coherent illumination, a comparison was performed with Z37 AIS technique by using the Input Aberration 1 shown in Fig. 5 as an input for the simulation
Summary
As the limit of optical lithography is pushed and feature densities continue to increase, lens aberration has become one of the most important factors to evaluate the imaging quality of lithographic tools [1,2,3,4]. Nikon Corporation has proposed a Z37 AIS (Aerial Image Sensor) technique which is able to measure aberrations up to the 37th Zernike coefficient by introducing a set of 36 binary grating marks with different pitches and orientations [17,18]. As these gratings are corresponding to 72 pupil sampling points, the wavefront aberration at each sampling point over the pupil plane can be obtained by the spectrum of the aerial image intensity. The overall performance of the proposed technique was subsequently simulated by the lithographic simulator PROLITH in order to demonstrate the validity and accuracy of the proposed technique for measuring aberrations up to the 37th Zernike coefficient
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