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Evaluation of a thin film by the X-ray reflectivity method


Because the refractive index of materials for X-rays is slightly less than 1, when X-rays enter the surface of a flat substance at a glazing angle, the X-rays cause a total reflection. By measuring the total reflection intensity (reflectivity) as a function of the incident angle with respect to the thin film surface, a profile such as the one shown in Figure 1 can be obtained, and structural parameters such as density, thickness and roughness of each layer of the thin film can be evaluated non-destructively.

Measuring the total reflection intensity
Figure 1: Relation between the profile of the X-ray reflectivity and the structure parameters



Figure 2 shows the measured profile of the X-ray reflectivity (Rigaku's SmartLab multipurpose diffractometer) of a sample where an oxide film is formed on a silicon (Si) substrate and a TiN film is formed on the oxide film using chemical vapor deposition, and the calculated profile using the optimized structure parameters.

Measured profile of the X-ray reflectivity
Figure 2: Measured and calculated profiles of the X-ray reflectivity


Analysis of the reflectivity results are also shown. The observed small period oscillation is caused by the 127.7 nm thick SiO2 film, and the large period oscillation is caused by the TiN film. However, because the TiN film is not optimized with the single layer model, it is analyzed by dividing it into two layers. Both TiN layers have considerably lower densities than those of crystalline substances, so they are considered to have amorphous phases without progressing crystallization, and to have also different degrees of crystallization. As mentioned above, it is a great feature of the reflectivity measurement that it can evaluate the property of thin films of multilayer structures irrespective of whether the substance is crystalline or amorphous.