[논문]Thickness and Surface Measurement of Transparent Thin-Film Layers using White Light Scanning Interferometry Combined with Reflectometry

Jo, Taeyong; Kim, KwangRak; Kim, SeongRyong; Pahk, HeuiJae

doi:10.3807/josk.2014.18.3.236

Thickness and Surface Measurement of Transparent Thin-Film Layers using White Light Scanning Interferometry Combined with Reflectometry 원문보기

Journal of the Optical Society of Korea, v.18 no.3, 2014년, pp.236 - 243

Jo, Taeyong (School of Mechanical and Aerospace Engineering, Seoul National University) , Kim, KwangRak (School of Mechanical and Aerospace Engineering, Seoul National University) , Kim, SeongRyong (School of Mechanical and Aerospace Engineering, Seoul National University) , Pahk, HeuiJae (School of Mechanical and Aerospace Engineering, Seoul National University)

Abstract ▼ AI-Helper

Surface profiling and film thickness measurement play an important role for inspection. White light interferometry is widely used for engineering surfaces profiling, but its applications are limited primarily to opaque surfaces with relatively simple optical reflection behavior. The conventional bucket algorithm had given inaccurate surface profiles because of the phase error that occurs when a thin-film exists on the top of the surface. Recently, reflectometry and white light scanning interferometry were combined to measure the film thickness and surface profile. These techniques, however, have found that many local minima exist, so it is necessary to make proper initial guesses to reach the global minimum quickly. In this paper we propose combing reflectometry and white light scanning interferometry to measure the thin-film thickness and surface profile. The key idea is to divide the measurement into two states; reflectometry mode and interferometry mode to obtain the thickness and profile separately. Interferogram modeling, which considers transparent thin-film, was proposed to determine parameters such as height and thickness. With the proposed method, the ambiguity in determining the thickness and the surface has been eliminated. Standard thickness specimens were measured using the proposed method. Multi-layered film measurement results were compared with AFM measurement results. The comparison showed that surface profile and thin-film thickness can be measured successfully through the proposed method.

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제안 방법

By implementing a normal objective lens and 50 µm optical fiber (Ocean Optics) between the beam splitter and the spectrometer, a 2.5 µm spot for reflectometry measurement was implemented.
A regression method like Levenberg-Marquardt nonlinear fitting algorithm is used for searching for the best-matching theoretical reflectance. The aim of the fitting process is finding the thickness of thin film which minimizes the difference between measured reflectance and theoretically calculated reflectance. The procedure of finding the best matching model is shown in Fig.
The aim of the fitting process is to find the thin film thickness that minimizes the difference between the measured reflectance and the theoretically calculated reflectance.
In this paper we combine the white light scanning interferometry and reflectometry for determination of film thickness and surface profile. The key idea is to divide the measurement into two states, reflectometry mode and interferometry mode, to get the two unknowns of thickness and surface profile separately. The experiment was performed in two steps.
The final thin-film surface profile information is measured by obtaining the best matching model using the non-linear least square fitting. The proposed method gave the proper initial guess for the accurate measurement and it saves calculation time in finding the most adequate interferogram. It took less than 3 seconds to analyze target area of 100*100 pixels.
The thin-film-transistor (TFT) supporting structure, which is a very important structure in the Liquid Crystal Display panel illustrated in Fig. 8, was measured by using the proposed method for industrial application. The sample has two layers from the bottom.
When we applied the proposed method, it took less than 3 seconds to analyze the target area of 100*100 pixels including nonlinear least square fitting.

대상 데이터

For the measurement of reflectometry mode, the spectrometer (Ocean Optics QE65000; 1024 pixels), which has a detecting range from 200 nm to 1100 nm, was used. The spectrometer is aligned at the center of the camera so that the light reflected from the center of the specimen can enter the spectrometer.

이론/모형

In reflectometry mode, after the data acquisition process is completed, a regression method, such as the Levenberg-Marquardt non-linear fitting algorithm, is used for searching the best-matching theoretical reflectance. The aim of the fitting process is to find the thin film thickness that minimizes the difference between the measured reflectance and the theoretically calculated reflectance.

참고문헌 (20)

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Y. M. Hwang, S. W. Yoon, J. H. Kim, S. Kim, and H. Pahk, "Thin-film thickness profile measurement using wavelet transform in wavelength-scanning interferometry," Opt. Lasers Eng. 46, 179-184 (2008).

상세보기
S. W. Kim and G. H. Kim, "Thickness-profile measurement of transparent thin-film layers by white-light scanning interoferometry," Appl. Opt. 38, 5968-5973 (1999).

상세보기
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상세보기
S. K. Debnath, J. You, and S. W. Kim, "Determination of film thickness and surface profile using reflectometry and spectrally resolved phase shifting interferometry," Int. J. Precis. Eng. Manuf. 10, 5-10 (2009).

원문보기 상세보기
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상세보기
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상세보기
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원문보기 상세보기
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상세보기
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상세보기
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상세보기
Y. S. Ghim, A. Suratkar, and A. Davies, "Reflectometrybased wavelength scanning interferometry for thickness measurements of very thin wafers," Opt. Express 18, 6522-6529 (2010).

상세보기
K. G. Larkin, "Efficient nonlinear algorithm for envelope detection in white light interferometry," J. Opt. Soc. Am. A 13, 832-843 (1996).

상세보기
S. Kim, J. Kim, and H. Pahk, "Fringe-order determination method in white-light phase-shifting interferometry for the compensation of the phase delay and the suppression of excessive phase unwrapping," J. Opt. Soc. Korea 17, 415-422 (2013).

원문보기 상세보기
D. Yoon, T. Kim, M. Kim, and H. Pahk, "Unambiguous 3D surface measurement method for a micro-Fresnel lensshaped lenticular lens based on a transmissive interferometer," J. Opt. Soc. Korea 18, 37-44 (2014).

원문보기 상세보기
H. Kang, J. Lim, P. Peranantham, and C. HwangBo, "Determination of optical constants of thin films in extreme ultraviolet wavelength region by an indirect optical method," J. Opt. Soc. Korea 17, 38-43 (2013).

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DOI : 10.3807/JOSK.2014.18.3.236 [무료]
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