X-ray reflectivity

X-ray reflectivity (XRR) is a nondestructive analytical technique for determining the structural properties of single or multilayered materials. 

For example thickness and density of the individual layers can be deduced from the data which is produced by X-rays reflected by the sample. XRR can be used to help product development and quality control.

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X-ray reflectivity (XRR), also known as X-ray specular reflectivity or X-ray reflectometry, is used for characterizing materials by analysing X-rays which are reflected by the sample. With XRR, it is possible to determine the thickness, density and roughness of films of different materials, such as semiconductor, magnetic and optical materials. Both crystalline (single crystal or polycrystalline) and amorphous matter in very thin layers can be analysed, and materials can have single or multiple layers or coatings. The properties of both the surface of the material and the interfaces of the layers can be determined.

Reflection or the X-rays and the critical angle of the material

In XRR the flat surface of the sample is irradiated with a beam of X-rays in such angles of incidence that they only graze the surface. When the X-rays hit the surface of the sample they are reflected back by the electrons of the atoms in the sample. The total reflection angle of the X-ray beam occurs at or below a critical angle, θ. This critical angle is very small and that is why also the angles of the reflected X-rays are extremely low. The critical angle varies depending on the electron density of the material used. 

The higher the incident angle of the X-ray beam hitting the sample relative to the critical angle of the reflected X-rays, the deeper the X-rays go down into the material. The incident and reflection angles of the X-rays are always equal. Therefore, if the angle of total reflection of X-rays is smaller than the critical angle, X-rays go through the sample only a few nanometers, but when the reflection angle rises above the critical angle the depth of the penetrating X-rays increases rapidly. 

The intensity of the reflected X-rays

The XRR analyzer measures the intensity of the reflected X-rays (reflectivity of the material) in such a direction, where the reflection angle of the X-rays is equal to the grazing incident angle of the original X-rays. When analysing a material whose surface is ideally flat, the reflectivity of the material suddenly decreases when the angle of the reflected X-ray beam rises above the critical angle of the material and the depth of the penetrating X-rays increases. If the surface of the material is rough, the reflectivity decreases more drastically when the reflection angle of the X-rays rises. 

Information deduced from the intensity of the reflected X-rays

In case that the interface between two layers in the material (a coating layer and the substrate) is not perfectly sharp and smooth, the intensity of the reflected X-rays will deviate from the predicted value according to the law of Fresnel reflectivity. The intensity of the reflected X-rays is approximately proportional to the square of modulus of the Fourier transform of the electron density. Therefore the electron density profile of the material can be deduced from the measured intensity pattern of the reflected X-rays with the help of Fourier transform. By analysing the deviations of the reflected X-rays' intensity, the density profile of the interface normal to the surface of the sample can be produced. The individual layer parameters such as thickness and density as well as interface and surface roughness can be determined from the reflectivity curves of the material. Thus the lateral properties, such as roughness and correlation properties of interfaces or lateral layer structure, can be deduced from the measured intensity pattern. 

Infromation deduced from the interference of the reflected X-rays

At every interface of individual films of the material where the electron density changes even a little, only a part of the X-ray beam is reflected. If a rough material is used as a substrate and evenly coated with another material, which has a different electron density, the reflected X-rays from the interface of the substrate and the thin top layer along with the X-rays from the free surface of the top layer will interfere with each other. This happens because the X-rays reflected from different surfaces have travelled different distances and thus they are in different phases. The interference happens either constructively (the rays amplify each other) or destructively (the rays deaden each other). This phenomenon creates an interference induced oscillation pattern which is observed during the analysis. The vertical properties, that is the thicknesses of the layers, can be determined from the periodicity of the oscillation pattern. Information about the surface of the material and possible interface of two materials (e.g. roughness) can be deduced from the angular dependency of the amplitude of the oscillation pattern. 

Suitable sample matrices

  • Samples having multiple films or layers of different materials
  • Samples that consist of one thin film or layer
  • Crystalline materials
  • Amorphous materials
  • Semiconductors
  • Magnetic materials
  • Optical materials

Ideal uses of XRR

  • Determining the thickness, density and roughness of thin materials and individual films in film stacks
  • Examining the properties of the surface and possible interfaces of materials
  • Analyses of the structural properties of very thin single films and coatings
  • Research and product development as well as quality control and optimization of production processes

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Frequently asked questions

What is XRR commonly used for?

XRR is commonly used for examining the structural properties of materials consisting of one single film or a stack of many different films. The parameters, such as thickness and density, of individual layers of the material can be determined. Roughness of both the surface and interfaces of different layers of the material can be discovered as well. Also the correlation properties of interfaces or lateral layer structure can be deduced from the measured data.

This characterization of materials can be a huge help in their research and product development as well as in process optimization and quality control.

What are the limitations of XRR?

The elemental composition of the sample cannot be determined with XRR and so the elements that the material consists of should be found out in advance. In addition to the material, its approximate thickness is also needed to know before the analysis, because only less than 5000 nanometres thick layers can be examined. 

Possible lateral inhomogeneities cannot be detected with XRR, thus a model of the  sample produced with this technique does not take these properties into account. 

The particle level properties, like crystal structure and lattice parameters, of thin films, layers and coatings cannot be analysed with XRR. Instead, grazing incidence X-ray diffraction (GIXRD) is a suitable method for this purpose.

What kind of samples can be analyzed with XRR?

Materials analysed with XRR can be for example semiconductors, magnetic or optical. XRR works best when the sample is smooth, uniform and flat with roughness less than 2 nanometres. The overall thickness of the film examined must be less than 5000 nanometres. 

The analysed samples can consist of very thin single layer or multiple layers and they can have different kinds of coatings. The individual films can consist of either single crystal, polycrystalline or amorphous matter.

What is Measurlabs?

Measurlabs offers a variety of laboratory analyses for product developers and quality managers. We perform some of the analyses in our own lab, but mostly we outsource them to carefully selected partner laboratories. This way we can send each sample to the lab that is best suited for the purpose, and offer high-quality analyses with more than a thousand different methods to our clients.

How does the service work?

When you contact us through our contact form or by email, one of our specialists will take ownership of your case and answer your query. You get an offer with all the necessary details about the analysis, and can send your samples to the indicated address. We will then take care of sending your samples to the correct laboratories and write a clear report on the results for you.

How do I send my samples?

Samples are usually delivered to our laboratory via courier. Contact us for further details before sending samples.

With XRR, the structural properties of both film stacks of different materials and samples consisting of only one layer can be determined with high precision. The parameters of individual layers and the whole material are discovered in the best accredited laboratories carefully selected by the experts of Measur.