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Method and device for measuring refractive index and layer thickness with ellipsometry


The invention relates to a method for measuring characteristic parameters of ultrathin surfaces and surface layers.


Ellipsometry is an established optical measurement method for surfaces and layers close to the surface. This method aims to determine the refractive index along with the layer thickness, which reveals significant characteristic parameters. Those parameters themselves can be used to gather information like the chemical composition of the material.

Known methods use two steps to measure those parameters. First, the ellipsometic angles Δ and ψ of the coated surface are measured directly. After this, a computer is used to calculate the refractive index and the layer thickness.


With this method, precisely determining these parameters is only possible if the coating thickness is large enough. This is because the influences of the refractive index and the layer thickness on the reflection are not independent of each other if measured on extremely thin layers. This problem occurs if the layer is thinner than 15nm. The determination of the layer thickness in this dimension is only possible if one of both parameters is known before the measurement. This is not the case in most cases


The invention describes a method for Ellipsometry in which the determination of layer thickness is divided into two separate measurements (Measurement A and B) measuring two different pairs of Δ and ψ. Because the testing body is not moved, the angle of incidence remains constant. Both measurements are only different from each other in terms of the thickness of the measured layer. The layer thickness in measurement A is hereby larger than in B. If not it is possible that measurement B is done at the not coated part. In both cases the layer thickness may be smaller than 125nm.

There are several ways to archive the described difference in layer thickness between A and B. One example is first measuring a surface without a coating and comparing this to the surface thickness when it is coated.

A mathematical equation deduced along with this invention makes it possible to calculate the refractive index n out of the different ellipsometric angles. It is now possible to determinate the layer thickness with normal ellipsometric methods.


  • Determination of the refractive index with ellipsometry even on layer thickness below 15nm
  • Usable on semiconductors, wave plates or at blooming and anti reflective coatings

Scope of application

  • Measurement technology
  • Material analysis


  • Disposition
  • Concession agreement
  • Development cooperation

Christian Tholen
+49 381 497474-38
Gerhart-Hauptmann-Straße 23
18055 Rostock

Development status


Patent situation

  • DE 10 2015 105 418 pending
  • EP 2016 057 405 pending


refractive index, layer thickness, ellipsometry, semiconductors, waveplates, coatings

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