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OPC (optical proximity correction) is a well-known and widely used RET (resolution enhancement technique) in optical lithography, which main purpose is improving pattern fidelity and process window. OPC relies on CD-SEM (critical dimension scanning electron microscope) images as a source of information for EPE (edge placement error) measurement, as input for the OPC modeling flow. However, as the pitch scales, stricter OPC modeling specifications, reduced metrology error budget, and increasing pattern complexities, create challenges for traditional CD-SEM metrology. Also, parameters like materials, landing energy and other scanning conditions, may have a different impact in the final image in terms of shrinkage, charging and distortion. All these effects need to be characterized during the modeling step to optimize OPC. Examples of EPE uniformity and pattern shrinkage are illustrated in Figure 1, for different scan conditions. With the increasing adoption of EUV (extreme ultraviolet) technology in the field of patterning and the upcoming introduction of High-NA (high numerical aperture) EUV, the need for further understanding of the EUV resist material interaction with a CD-SEM electron beam is also increasing. Low landing energy scanning approach is a way to minimize EUV resist damage during image acquisition and provide a more accurate view of EUV lithography effects. Also, to improve the design space coverage, resist (or lithographic) and etch modeling require thousands of measurements over different types of structures. Additionally, massive EPE statistics makes model building more robust. A contour may provide the equivalent of hundreds of different measurements – using the entirety of the contour instead of only relying on the placement of gauges, as in conventional measurement, further increases the sampling and the coverage of the produced model, which make contours the suitable choice. The influence of low landing energy is expected to be seen in the etch modeling step, where it should have higher accuracy and lower etch bias when compared to litho SEM data. However, a drawback of using low landing energy for image acquisition is a lower SNR (Signal-to-Noise ratio) compared to standard imaging conditions. This may represent extra challenges to the contour extraction algorithm which needs to be robust. These challenges can be addressed by using a model-based approach. In this work, different CD-SEM scanning conditions were studied in order to find optimal working points for modeling purposes. Since different patterns have varied outcomes under different e-beam conditions, thus affecting the model residues, more than 600 distinct patterns from EUV lithography – going from 1D to complex 2D – were selected to provide enough sensitivity to when building the litho and the etch models. Contour metrology is used to detect the edges, for EPE analysis, and to provide inputs for the modeling flow. At last, one of the key aspects of this work is the use of low landing energy (150eV) scan approach to minimize the resist shrinkage effect, while accounting for resist charging in the modeling. All these factors will provide further insight and optimization into this new era of metrology.
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