Current Optics and Photonics

Optical Society of Korea (한국광학회)
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Precise Edge Detection Method Using Sigmoid Function in Blurry and Noisy Image for TFT-LCD 2D Critical Dimension Measurement

  • Lee, Seung Woo (School of Mechanical and Aerospace Engineering, Seoul National University) ;
  • Lee, Sin Yong (School of Mechanical and Aerospace Engineering, Seoul National University) ;
  • Pahk, Heui Jae (School of Mechanical and Aerospace Engineering, Seoul National University)
  • Received : 2017.05.11
  • Accepted : 2017.11.29
  • Published : 2018.02.25
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Abstract

This paper presents a precise edge detection algorithm for the critical dimension (CD) measurement of a Thin-Film Transistor Liquid-Crystal Display (TFT-LCD) pattern. The sigmoid surface function is proposed to model the blurred step edge. This model can simultaneously find the position and geometry of the edge precisely. The nonlinear least squares fitting method (Levenberg-Marquardt method) is used to model the image intensity distribution into the proposed sigmoid blurred edge model. The suggested algorithm is verified by comparing the CD measurement repeatability from high-magnified blurry and noisy TFT-LCD images with those from the previous Laplacian of Gaussian (LoG) based sub-pixel edge detection algorithm and error function fitting method. The proposed fitting-based edge detection algorithm produces more precise results than the previous method. The suggested algorithm can be applied to in-line precision CD measurement for high-resolution display devices.

Keywords

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FIG. 1. Image of 20 μm standard grid sample in 20X, 50X magnification. (a) Image in 20X. (b) Image in 50X.

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FIG. 2. Intensity profile of standard grid sample’s edge in 20X, 50X magnification.

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FIG. 3. Intensity distribution of 50X magnified image in three dimensions.

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FIG. 4. Intensity and derivative of intensity along the edge normal direction of the standard grid sample in 50X magnification.

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FIG. 5. Sigmoid function when m = -0.2, n = 10, K = 200, D = 25.

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FIG. 6. 2D sigmoid function for line geometry when the geometry function is . (a) Top view of the function. (b) Isometric view of the function.

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FIG. 7. 2D sigmoid function for circle geometry when the geometry function is . (a) Top view of the function. (b) Isometric view of the function

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FIG. 8. 2D sigmoid function for ellipse geometry when (a) Top view of the function. (b) Isometric view of the function.

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FIG. 9. General optical microscope system using the Piezo Transducer (PZT).

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FIG. 10. Indium Tin Oxide (ITO), Half-tone photoresist (HT-PR) and hole pattern images. (a) ITO. (b) HT-PR. (c) Hole A. (d) Hole B.

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FIG. 11. Fitting results for line (ITO and HT-PR edge) and circle (Hole A and Hole B edge) pattern. (a) Original intensity of line image. (b) Fitting intensity of line image. (c) Original intensity of circle image. (d) Fitting intensity of circle image.

TABLE 1. Specifications of the measurement system

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TABLE 2. CD measurement results from LoG mask sub-pixel method and sigmoid surface fitting method. (a) Measurement results of line width (ITO, HT-PR). (b) Measurement results of circle diameter (Hole A, B).(a) Line width (unit: μm)

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TABLE 2. CD measurement results from LoG mask sub-pixel method and sigmoid surface fitting method. (a) Measurement results of line width (ITO, HT-PR). (b) Measurement results of circle diameter (Hole A, B) (Continue).(b) Circle diameter (unit: μm)

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