KEYWORDS: Metrology, Signal detection, Scanning electron microscopy, Model-based design, Reliability, Signal to noise ratio, Semiconductors, Semiconductor manufacturing, Process control, Optical proximity correction
In the semiconductor manufacturing process, performing metrology over 2D complex features is mandatory for advanced technology nodes. Top-down critical dimension scanning electron microscopes (CD-SEMs) are widely used for many applications and are key enablers for metrology and process control. Nevertheless, one of the limitations one may observe when using CD-SEMs is the contrast variation (or lack of contrast) in areas where the pattern edges are parallel to the CD-SEM acquisition scan direction, which may lead to accuracy and precision loss. Model-based contour extraction solutions are often able to address this reduction in contrast while maintaining the accuracy and precision required by the different usages. However, for particularly low contrast/low SNR images, this may lead to a reduction in accuracy of the metrology process.
This work explores the concept of hybrid contours generation, combining contours – coming from different scan directions – with pattern fidelity metrics, in order to overcome the issues related to edge detection accuracy and precision, especially in 2D shapes. It brings the advantage of combining relevant information from each image, weighted based on their reliability, which is estimated by a combination of the scan direction and the fidelity of the edge profile observed on the patterns. This method of combining contours improve 2D measurements and is also the most reliable way of building contours for OPC modeling, among other uses.
Among metrology tools in the semi-conductor manufacturing, critical dimension scanning electron microscopes (CD-SEM) are the most broadly used, especially due to their high resolution, low destructivity, and high throughput. Contour metrology on CD-SEM images has become essential for characterization, modelling, and control of advanced lithography processes. In particular, OPC model’s accuracy can be highly improved using contours metrology. One of the issues when dealing with CD-SEM metrology is that the results are noise sensitive. Moreover, diminishing noise in CD-SEM acquisition leads to resist shrinkage due to exposure time increase. In addition, post-treatment of these shrinkage effects requires compensation algorithms such as artificial intelligence (AI)- driven algorithms, that are another contributor to the error budget of metrology systems. There is thus a need for an accurate, robust to noise, and purely deterministic edge detection algorithm. In this article, we evaluate the benefits of relying on a model-based contour extraction approach for performing measurements. This approach is applied onto both synthetic and experimental CD-SEM images with various patterns (mostly 2D) and noise levels to assess the influence of image integration (frame number) on the contour detection and CD measurement. We demonstrate that a model-based contour extraction algorithm is able to precisely characterize SEM-induced 2D resist shrinkage. We observe that this model-based approach is more robust to noise than standard algorithms by 21% on synthetic data and by 36% on experimental data. Another way of seeing it is, while keeping the same precision, a model-based contour extraction approach can significantly reduce the requested image frame number. The benefits of adopting this approach range from reducing the shrinkage effects to improving SEM image acquisition time. Eventually, no step of shrinkage modelling calibration nor AI-driven image post processing are needed which implies a gain on simplicity and avoids modelling errors.
Densification and reduction of lithographic features sizes keeping low defectivity is one of the biggest challenges in the patterning area. In order to extend 193 immersion capabilities and meet advanced applications needs, multi exposure image mode is a promising option for non-high volume manufacturing. It allows from a unique pattern with a fixed critical dimension (CD) and pitch, to obtain more dense patterns in a large surface without any process loop of standard flow, a huge benefit compared to litho-etch-litho-etch (LELE) approach. The study carried out explores this method with a specific design of pillars array printed using Negative Tone Development (NTD). The multi-image option relies on exposing multiple times the same initial pattern with a low image-to-image overlay. Based on intrinsic scanner performances, imageto-image placement error should be less than two nm. In this paper, many functionalities are explored to customize patterns from a single and unique mask design. One stake is to transfer (into silicon) a 2 mm * 2 mm pillar array design with a pitch divided by two, covering a wide surface on a 300 mm wafer and answering overlay and stitching requirements. Final results give well defined pillars which intra-wafer CD uniformity (3σ) satisfies application process requests. By using a flexible multi-image mode, mask constraints (cost and quality) can be relaxed, i.e. with a larger pitch structure on the reticle than the targeted one, final feature can be achieved. This development can be extended to hybrid lithography such as NanoImprint Lithography (NIL) or specific applications such as optics.
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