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Spotlight on Optics

Spotlight on Optics


  • May 2013

Optics InfoBase > Spotlight on Optics > Thin-film thickness profile measurement by three-wavelength interference color analysis

Thin-film thickness profile measurement by three-wavelength interference color analysis

Published in Applied Optics, Vol. 52 Issue 10, pp.1998-2007 (2013)
by Katsuichi Kitagawa

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Spotlight summary: Modern optical and electronic industries would be impossible without the production of thin films of various materials. From classical high-reflective or anti-reflective coatings, high-performance laser mirrors, insulator layers in semiconductor chip production to photovoltaics and organic electronics: thin films can be found in virtually every high-tech device manufactured today. Since the beginnings of thin-films technology, one of the main technological challenges has not changed: the thickness of the thin films and their homogeneity must be measured in a fast and reliable way, if possible in situ. This task alone has generated an entire industry for film thickness measurement devices.

In this Applied Optics paper the author introduces a fast way to measure thicknesses by measuring the normal incidence reflectance at 3 different spectral points which are already widely used in RGB color cameras. This method allows for very fast measurements and is applicable also within an imaging device with a very high resolution. As with many optical thin film analysis methods, the problem is mainly the computational analysis of the measurement. Therefore, the author of this paper pays special attention to the presentation of the algorithms used. The thickness of the sample layer is calculated by two different methods: a global fitting algorithm (GMFT) which is applied in various forms in virtually all optical thickness determination programs (e.g. for ellipsometry) and a direct thickness determination by phase unwrapping (ACOS method). The latter algorithm is only useful when there is already a sensible thickness estimate to start with. By a combination of these algorithms, a fast and reliable thickness determination is achieved. This method is then tested with simulated measurement values as well as with real measurements taken with a home-built camera system on film-thickness standard samples. These tests show the usefulness and accuracy of the method when applied to data relevant to industrial measurement tasks. The method’s usefulness stems mainly from its ease of use and speed of the overall analysis process. It must be noted, though, that this approach is applicable only in cases where the refractive index dispersion of the materials in question is already known, and therefore standard methods like spectral reflectometry and ellipsometry cannot be replaced by this method when materials with an unknown dielectric function are involved.

The concise and clear presentation of the algorithm and the thorough testing of the new method make this paper a highlight of industrial thin-film metrology development. For fast imaging measurements, this three-wavelength approach has a true advantage over the slower methods. With more development on the computational side, this method could become a very important innovation for film thickness measurement. Therefore, this work will surely stimulate innovation in industrial thin film coatings measurements.

--Andreas Hertwig

Technical Division: Optical Design and Instrumentation
ToC Category: Instrumentation, Measurement, and Metrology
OCIS Codes: (120.3180) Instrumentation, measurement, and metrology : Interferometry
(120.3940) Instrumentation, measurement, and metrology : Metrology
(240.0310) Optics at surfaces : Thin films
(310.6860) Thin films : Thin films, optical properties

Posted on May 17, 2013

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