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Characterisation of coatings

A two-beam measurement method makes it possible to obtain the thickness of optically transparent coatings as thin as 10-120 nm. The technique can be applied with small refractive index differences to the substrate (|Dn|~0.05) and guarantees a measurement with nanometer accuracy. A laser beam with TM polarisation is directed at the surface of the coated sample with an incident angle near the Brewster angle of the substrate and the coating. The laser beam is reflected from the coated surface (fore reflex) and from the rear surface of the substrate (rear reflex). The reflectivities of both these reflexes are measured in a small angle range around the Brewster angle. Using three angles of the reflectivity curves of the fore and rear reflexes measured (two minimum angles and one intersecting angle in the Brewster angular range), the refraction indexes of coating and substrate can be directly calculated and the coating thickness determined by means of a simulation process based on the four-beam interference configuration. Due to a synchronous measurement of the reflectivity curves of fore and rear reflex, the method is not susceptible to external perturbation.

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Mr. Dr. A. Hertzsch
phone 03641 282589

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