Thin-film interference
Reflectometric thin film measurement is a measurement method based on the principle of thin-film interference, and serves to determine the thickness of thin films dielektischen. Also can be obtained from the data of the refractive index of the material. This information is of increasing importance, since thin films are nowadays widely used, for example, in anti-reflective coatings, the coating of medical implants or in the solar industry.
Theory
As a thin layer of material having a thickness is referred to in the nano - to micrometer range. Incident light is on the border material surrounding medium (usually air ) is either reflected or transmitted; the transmitted light, in turn, strikes the boundary surface thin film substrate and is reflected again here. How much light is reflected or transmitted in this case, can be determined using the Fresnel equations. Between the light reflected at the two interfaces share it comes in the wake of constructive or destructive interference. This is dependent on the layer thickness, the refractive index of the material and the angle of incidence of the light. Also there may be a phase shift of 180 ° or π.
The optical path (OPD ) of the incident light can be calculated using the figure as follows:
As well as
After Snell follows
And thus
In thickness reflectometric measurement light from a white light source is directed at an angle of 90 ° to the layer to be measured and measured with a spectrometer in reflection as a function of the wavelength. From the above equation with that for only certain wavelengths, constructive interference occurs and these wavelengths are directly dependent on the layer thickness and the refractive index.
This results in a characteristic oscillation patterns whose frequency depends directly on the layer thickness: The thicker the layer, the higher the frequency. So it can be concluded with a known refractive index of the layer thickness. On the other hand, it is possible to determine the refractive index for a known thickness.