Laser interference methods with focused probing beams for thickness measurements

Dmitry V. Lyakin; Mickail I. Lobachev; Irina F. Minenkova; Vladimir P. Ryabukho

doi:10.1117/12.484601

29 July 2002 Laser interference methods with focused probing beams for thickness measurements

Dmitry V. Lyakin, Mickail I. Lobachev, Irina F. Minenkova, Vladimir P. Ryabukho

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Proceedings Volume 4900, Seventh International Symposium on Laser Metrology Applied to Science, Industry, and Everyday Life; (2002) https://doi.org/10.1117/12.484601
Event: Seventh International Symposium on Laser Metrology Applied to Science, Industry, and Everyday Life, 2002, Novosibirsk, Russian Federation

Abstract

Two laser interferometric methods with a focused laser probing beam for thickness measurements of transparent films and layers are discussed. In both methods the exact focusing ofprobing beam on the objects surfaces permits one to determine the parameters of random inhomogeneous objects, such as coatings on the rough surfaces. One is the so-called focused oblique laser beam (FOLB) method. In this method at illumination by a focused oblique laser beam of a transparent layer with rough rear surface the reflected field performs superposition of beams with smooth and speckle-modulated wave fronts. A quasiregular interference pattern in the reflected field is observed when the waist of the focused laser beam overlap a rough rear surface of the layer being tested. Knowing the angular period of the observed interference fringes and refractive index of layer medium one can determine the local geometrical thickness of the layer. The latter method is based on the so-called Laser Wave Front Matching Interferometer (LWFMI), which is constructed under scheme of Michelson interferometer with focusing microobjectives in the interferometer branches (Linnik scheme) and a wide integrating detector aperture in the common exit branch. In the envelope of a interference LWFMI signal as a function of object displacement relative to the focal point of an interferometer focusing objective, peaks are observed when the probing beam is focused on the front and rear surfaces of the layer. The distance between the interference signal peaks is proportional to the geometrical thickness of the layer and can be measured by the technique of interference fringe counting. As in the first method, geometrical thickness can be obtained, if the refractive index of the layer medium is known. The results of geometrical thickness measurements of some objects by both methods are compared and discussed.

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Dmitry V. Lyakin, Mickail I. Lobachev, Irina F. Minenkova, and Vladimir P. Ryabukho "Laser interference methods with focused probing beams for thickness measurements", Proc. SPIE 4900, Seventh International Symposium on Laser Metrology Applied to Science, Industry, and Everyday Life, (29 July 2002); https://doi.org/10.1117/12.484601

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