Binary Mask Optimization Method For Aberration Measurement In Lithographic Tools

Wavefront aberrations of projection lens in optical lithographic systems are an important indicator of affecting the stepper's performance. With the gradual increase of NA, the wavefront aberrations of projection lens meet more strict demand to ensure the quality of the image, which requires some further research on the principles and methods of measurement technique for wavefront aberrations in lithographic tools. Due to the advantage of easier producing and lower cost, binary grating mask have been wildly used in aberration metrology techniques. A set of binary grating marks with different pitches and orientations which are sensitive to wavefront aberrations can decrease the propagation of errors in Zernike coefficients, so that the aberration metrology method maintains a high accuracy estimate of wavefront. In a word, the optimization method of binary grating mask is crucial in aerial image based aberration measurement techniques.According to partial coherent illumination theory and the wavefront aberrations concept, methods and the detection processes for in-situ aberration measurement in optical lithographic systems are generalized by combining the imaging models and the inverse problem of Zernike coefficients.Based on error propagation theory and sensitivity analysis, the cost functions of the optimization problem for different conditions are proposed individually by analysis the sensitivity of optic aberrations to TCC (transmission cross coefficient) and aerial image intensity. Taking coherent source illumination condition as an example, a suitable optimization method is selected from comparing the Constraint Optimal Direct Method and Constraint Optimal Indirect Method. And the results for partial coherent illumination are shown in this paper using the selected method.The optimization process is calculated by Matlab programming and the simulator Prolith. Meanwhile, the aberration measurement results for optimized masks and the random masks are compared in each illumination source separately. The accuracy of simulation work has demonstrated that the results obtained by the optimized grating masks are much better than those by the random ones. The results verify the feasibility of the optimization method.