1 April 2010 Comparison of different algorithms to determine areas from SEM images
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We present a comparison of different methods to extract area information from images. Two different physical-based algorithms were tested which determine the areas of arbitrarily shaped 3D nano-structures on wafers or photo-masks (e.g. contact holes) using secondary electron images of scanning electron microscopy (SEM). One of these algorithms, called NANOAREA, was developed by the PTB. The other one is the software package MaskEXPRESS, which was developed by Toppan Printing Co., Ltd. In addition to real SEM images we used Monte Carlo generated SEM images of contact holes of different shapes and sizes. For this, the Monte Carlo simulation program MCSEM, developed at PTB, was applied. MCSEM simulates the electron diffusion and secondary electron generation and transport in solid state material and provides simulated SEM images of arbitrary 3D specimen structures. NANOAREA uses basic image processing routines to estimate the edge position of a structure. Then, one-dimensional profiles which intersect the structure boundary perpendicularly are extracted. A one-dimensional edge detection algorithm determines the edge position on each profile. Finally these detected edge positions are used to calculate the polygon area using the triangle method. NANOAREA showed a very small underestimation of the area of about 0.3 % with regard to the Monte Carlo simulations (i.e. sub-pixel deviation). MaskEXPRESS has a similar approach, however employs a different edge detection algorithm. For quadratic contact holes a very high correlation coefficient r larger than 0.99 of the CDs was seen with an offset of about 0.3 nm for the two tested programs. Here the critical dimension (CD) is defined as the square root of the area. The deviations from the mean offset were smaller than 1 nm over the whole investigated range. For analysis of arbitrarily shaped features we used a double T-shaped structure. Also here almost perfect correlation was found (r = 0.98). The observed mean offset in this case was also about 0.3 nm. The offsets depend on the length of the edge and can vary with the shape of the structure, too. Here we report the excellent correlation of the investigated algorithms and programs to determine area parameters from SEM images. The results found are an important prerequisite for harmonized area measurement based on independent algorithms and pave the way to a standardized approach to area determination and reporting of photomask structures.
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K.-P. Johnsen, C. G. Frase, H. Bosse, I. Yonekura, M. Higuchi, J. Richter, "Comparison of different algorithms to determine areas from SEM images", Proc. SPIE 7638, Metrology, Inspection, and Process Control for Microlithography XXIV, 76382Q (1 April 2010); doi: 10.1117/12.846536; https://doi.org/10.1117/12.846536

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