Despite being crucial in an optical lithography process, “dose” has remained a relative concept in the computational lithography regime. It usually takes the form of a percentage deviation from a pre-identified “nominal condition” under the same illumination shape. Dose comparison between different illumination shapes has never been rigorously defined and modeled in numerical simulation to date. On the other hand, the exposure-limited nature of EUV lithography throughput demands the * illumination shape being optimized with the physical dose impact consciously taken into consideration. When the projection pupil is significantly obscured (as in the ASML EXE high NA scanner series), the lack of a proper physical dose constraint may lead to suboptimal energy utilization during exposure. In this paper, we demonstrate a method to accurately model the physical dose in an optical lithography process. The resultant dose concept remains meaningful in the context of a changing illumination pupil, which enables co-optimization of imaging quality and a throughput metric during the Source-Mask Optimization (SMO) phase, known as the Dose-Aware SMO. With a few realistic test cases we demonstrate the capability of Dose-Aware SMO in terms of improving EUV throughput via reducing the effective exposure time, in both regular and obscured projection systems. The physical dose modeling capability in computational lithography not only addresses those immediate challenges emergent from EUV throughput, but also opens the gate towards a broad class of exciting topics that are built upon physical dose, such as optical stochastic phenomena and so on.
With the adoption of extreme ultraviolet (EUV) lithography for high-volume production of advanced nodes, stochastic variability and resulting failures, both post litho and post etch, have drawn increasing attention. There is a strong need for accurate models for stochastic edge placement error (SEPE) with a direct link to the induced stochastic failure probability (FP). Additionally, to prevent stochastic failure from occurring on wafers, a holistic stochastic-aware computational lithography suite of products is needed, such as stochastic-aware mask source optimization (SMO), stochastic-aware optical proximity correction (OPC), stochastic-aware lithography manufacturability check (LMC), and stochastic-aware process optimization and characterization. In this paper, we will present a framework to model both SEPE and FP. This approach allows us to study the correlation between SEPE and FP systematically and paves the way to directly correlate SEPE and FP. Additionally, this paper will demonstrate that such a stochastic model can be used to optimize source and mask to significantly reduce SEPE, minimize FP, and improve stochastic-aware process window. The paper will also propose a flow to integrate the stochastic model in OPC to enhance the stochastic-aware process window and EUV manufacturability.
Inverse lithography is increasingly being used as a viable OPC solution to maximize process window (PW), improve CD uniformity (CDU) and minimize the mask error factor (MEF), especially for memory devices. The device yield is typically limited by the process window of a few critical layers, and the Via layer is identified as one of the process window limiters for advanced 3D-NAND devices. To maximize the on-chip yield, ASML has developed advanced image based Mask-3D (M3D) inverse technology that can optimize freeform mask shapes and enhance design printability throughout the mask optimization flow. Mask rule checks (MRC) and side-lobe printing are optimized simultaneously to deliver the maximum process window.
The advanced image based M3D inverse lithography technology (ILT) is used to perform full chip mask correction on the Via layer of a 3D-NAND device. 3D NAND devices contain highly repetitive cell and page buffer patterns. To ensure the full chip device performance, the consistency of the mask correction is important. Our strategy is to use the computationally intensive mask optimization solution from the new advanced image based M3D inverse technology to generate a freeform mask which gives the best lithography performance. We then use Tachyon’s Pattern Recognition and Optimization (PRO) engine to propagate the freeform mask solution of the repetitive patterns to the full chip. The periphery of the chip is optimized using conventional OPC methods. The simulation results from the advanced image based M3D inverse technology are compared against the baseline flow, which uses a standard inverse solution. The simulation results from both the flows are further validated on wafer. Significant improvement in overlapping process window (OPW) and CD uniformity is observed using the new advanced inverse technology. The simulation data shows a 32% improvement in depth of focus (DOF), a 5% improvement in the image log slope (ILS) and a 25% reduction in best focus shift (BFS) range. The improvement has also been verified at the wafer-level.
Sub-Resolution Assist Features (SRAF) are widely used for Process Window (PW) enhancement in computational
lithography. Rule-Based SRAF (RB-SRAF) methods work well with simple designs and regular repeated patterns, but
require a long development cycle involving Litho, OPC, and design-technology co-optimization (DTCO) engineers.
Furthermore, RB-SRAF is heuristics-based and there is no guarantee that SRAF placement is optimal for complex
patterns. In contrast, the Model-Based SRAF (MB-SRAF) technique to construct SRAFs using the guidance map is
sufficient to provide the required process window for the 32nm node and below. It provides an improved lithography
margin for full chip and removes the challenge of developing manually complex rules to assist 2D structures. The
machine learning assisted SRAF placement technique developed on the ASML Brion Tachyon platform allows us to
push the limits of MB-SRAF even further. A Deep Convolutional Neural Network (DCNN) is trained using a
Continuous Transmission Mask (CTM) that is fully optimized by the Tachyon inverse lithography engine. The neural
network generated SRAF guidance map is then used to assist full-chip SRAF placement. This is different from the
current full-chip MB-SRAF approach which utilizes a guidance map of mask sensitivity to improve the contrast of
optical image at the edge of lithography target patterns. We expect that machine learning assisted SRAF placement can
achieve a superior process window compared to the MB-SRAF method, with a full-chip affordable runtime significantly
faster than inverse lithography. We will describe the current status of machine learning assisted SRAF technique and
demonstrate its application on the full chip mask synthesis and how it can extend the computational lithography
roadmap.
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