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In this paper, a method for improving the process window is described by simultaneous source mask optimization (SMO). The method optimizes the source and mask of a critical pattern by optimizing the mask in the frequency domain. The minimum image log slope (ILS) is maximized at fragmentation points in the critical pattern while simultaneously maintaining the printing fidelity. The mask optimized in the frequency domain is then converted into a chromeless phase lithography (CPL) mask. The process window with the optimized source and optimized CPL mask doubles the aerial image contrast in comparison to an attenuating PSM with source optimization only. After optimizing the mask and source for a critical pattern, the remaining parts of the full-chip design are optimized with interference mapping. Another technique for optimizing the source for a full chip is presented in which the source is optimized by using the pitch frequency of the design. From the pitch frequency, the source is optimized by solving an integral equation for the first eigenfunction in which the first eigenfunction is calculated from the sum of coherent system (SOCS) representation of the transfer cross coefficient (TCC).

The optimization of the source topology and mask design [1,2] is vital to future advanced ArF technology node development. In this study, we report the comparison of an iterative optimization method versus a newly developed simultaneous source-mask optimization approach. In the iterative method, the source is first optimized based on normalized image log slopes (NILS), taking into account the ASML scanner's diffractive optical element (DOE) manufacturability constraints. Assist features (AFs) are placed under the optimized source, and then optical proximity correction (OPC) is performed using the already placed AFs, in the last step the source is re-optimized using the OPC-ed layout with the AFs. The source is then optimized using the layout from the previous stage based on a set of user specified cost function. The new approach first co-optimizes a pixelated freeform source and a continuous transmission gray tone mask based on edge placement error (EPE) based cost function. ASML scanner specific constraints are applied to the optimized source, to match ASML's current and future illuminator capabilities. Next, AF "seeds" are identified from the optimized gray tone mask, which are subsequently co-optimized with the main features to meet the process window and mask error factor requirement. The results show that the new method offers significant process window improvement.

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