14 May 2012 Improving the performance of the critical dimension-scanning electron microscope with the contrast transfer function
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Critical dimension scanning electron microscopes (CD-SEMs) are used extensively by the semiconductor industry to perform highly accurate dimensional metrology of patterned features. To ensure optimal feedback for process control, these tools must produce highly reproducible measurements. This means monitoring and minimizing not only day-to-day variations on a given tool, but also tool-to-tool variations whether within the same production facility or at different sites. It has been shown that the contrast transfer function (CTF) can be used to evaluate the imaging performance of SEMs by giving a quantitative measure of the fidelity with which specimen contrast information (i.e., point-to-point variations in emitted signal intensity) is represented in the image data as a function of spatial frequency. Because all imaging defects and artifacts as well as the point spread function impact the shape of the CTF, it is an ideal means with which to monitor deviations from a baseline performance. By using a thoughtfully designed and thoroughly characterized test specimen, the CTF of a given tool can be decoupled from the specimen information, allowing for characterization of the imaging system itself. Fresnel zone plates and pseudorandom arrays of dots are good candidates for such test structures, if they can be fabricated with sufficient resolution to assess the performance of the tool up to its information limit. The feasibility of this approach has been assessed with test structures fabricated using nano-imprint lithography with 22 nm design rules. The advantages of using the CTF of a specific instrument to improve CD-SEM image simulations are also demonstrated.
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Aron J. Cepler, Bradley L Thiel, "Improving the performance of the critical dimension-scanning electron microscope with the contrast transfer function", Proc. SPIE 8378, Scanning Microscopies 2012: Advanced Microscopy Technologies for Defense, Homeland Security, Forensic, Life, Environmental, and Industrial Sciences, 83780T (14 May 2012); doi: 10.1117/12.919293; https://doi.org/10.1117/12.919293

Contrast transfer function

Spatial frequencies

Scanning electron microscopy


Imaging systems

Electron microscopes

Fourier transforms

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