Source and mask optimization (SMO) technology is an increasingly important resolution enhancement technology (RET) that can optimize the source and mask. Various SMO methods have made great progress in terms of computational efficiency and pattern fidelity. Besides, process window (PW) is also an important indicator to evaluate the performance of lithography imaging. PW consists of exposure latitude (EL) and depth of focus (DOF). However, currently, there are few SMO methods that can directly improve EL. In this paper, we propose an EL aware SMO (ELASMO) method by innovating a new penalty function for improving the exposure latitude. Compared to the conventional SMO, the proposed ELASMO can significantly enhance aerial image contrast and enlarge the exposure latitude from 5% to 11% under the premise of ensuring imaging fidelity. ELASMO achieves high-fidelity lithography in a larger process window.
With the increasing requirement of lithographic resolution, the degradation of 3D mask effect on imaging cannot be ignored. The researches of its polarization properties and effect on imaging are of great significance to the development of imaging-based aberration measurement techniques and computational lithography. In this paper, a novel method for comprehensive and quantitative characterization of 3D mask effect is proposed. By comparing the far-field spectrum of Kirchhoff model and 3D mask model, the 3D mask effect is comprehensively and quantitatively characterized as the form of polarization aberration. Pupil-spectrum comprehensive analysis method and background glitch noise culling method are proposed to improve the systematicness and accuracy of 3D mask characterization. The simulation comprehensively analyzes the effect of mask line width and absorber thickness on all polarization properties of the 3D mask effect, showing that this method can provide a more comprehensive analysis of the 3D mask effect compared with the previous methods.
Source optimization (SO) is an extensively used resolution enhancement technology which can improve the imaging performance of optical lithography. To improve the computational efficiency of traditional SO, compressive sensing (CS) has been involved. In the CS-SO theory, the source pattern needs to be presentation as sparsely as possible by sparse basis, because the sparsity of source pattern can significantly improve the recovery performance of CS-SO. Therefore, the selection of the sparse basis can affect the performance of CS-SO. Discrete Fourier transform (DFT) basis, especially its variant discrete cosine transform (DCT) basis has been widely used in CS. Furthermore, some overcomplete bases have also been used in many fields. In this paper we present a comparison of sparse-based full chip SO with spatial basis, DCT basis, DFT basis, overcomplete DCT (ODCT) basis, overcomplete DFT (ODFT) basis and haar wavelet basis. The full chip SO problem is formulated as a cost function of multi-objective adaptive optimization, and then a soft threshold iterative (IST) algorithm is used to obtain the optimized source pattern. The simulation results show that the sparse-based method can effectively improve the imaging performance. Exactly, in terms of imaging fidelity, spatial, DCT, DFT, ODCT, and haar wavelet bases are similar, and better than the ODFT basis. However, in terms of optimizing speed, the spatial and DCT basis can converge to an acceptable SO solution at a faster speed than other bases.
As lithographic technology continues to advance, the size of nodes has continually been decreased while the control of defocus has become stringent in the actual lithography process. Defocus is always uncertain in the practical exposure process due to multi-factor impact, which is supposed to be considered as an important element of the aerial imaging model. It’s necessary to analyze the influence of defocus on the aerial image. In this paper, aerial image approximates to a second-order polynomial for different defocus through Taylor series expansion. Then the respective and the joint impacts of the first-order defocus term and the second-order defocus term on aerial image for various conditions have been studied by simulation. Simulation shows that annulus illumination source can reduce the impact of the first-order defocus term and the second-order defocus term is more valuable to be studied and controlled to improve lithographic resolution and process robust
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