UV/Vis/NIR Spectrometry for the Film Thickness Determination of Transparent Films and Layers

Michael Rother, Dr. Doris Albus, Steffen Piecha
tec5 AG, In der Au 25, D-61440 Oberursel, Germany

Within nearly all branches of industry we find products, which are coated with thin transparent layers to improve the surface features: Window sheet glasses obtain changes in reflection and absorption behavior, ceramics are treated with coatings to lower water adhesion, coatings on metals create corrosion resistance or better sliding factors. Transparent coatings play also a major role in the semiconductor business. One of the important features to be determined and controlled is the thickness of the layers to balance performance and material input during development and production.In the plastics industry there are many manufacturers of transparent foils and films, who also need to know the exact thickness of their products. For both product categories (foils and layers) the film thickness determination using white light interference is a well known and established principle with a significant benefit: the sample will not be destroyed or even touched during the measurement.

Fig. 1: Measurement principle.

Measurement Principle

White-light interference results from the interaction of light partially reflected by optical boundaries depending on the refractive index changes of the materials. The difference in pathlength is a function of the wavelength, the refractive index and the geometrical thickness, causing interference in case that the layers are transparent, smooth, and parallel. The reflected intensity varies with wavelength, creating an interference spectrum. This spectrum can be analyzed by a so-called Fourier-Transform algorithm allowing fast measurements of the thickness during the running production process - without destroying, changing or even touching the surface. If the thickness of two layers are not too similar the thickness of both can be measured simultaneously.

Fig. 2: Evaluation Line (tec5 AG, Germany): compact systems for film thickness measurements in laboratory.

System Solutions

Modern high end spectrometer systems based on diode array and fiber optic technology are well fitted instruments for film thickness measurements of transparent foils and layers. Single and dual layers in the range of 0.1 to a couple of 100µm geometrical thickness can be determined contactlessly. Due to their extremely small and compact design, diode array spectrometers are ideal instruments for film thickness determination during running production processes. With a newly developed high speed electronics (e. g. FEE 1M, tec5 AG Germany) high frequency measurements are also possible. For this reason they are a proper solution for the 3-dimensional scanning of foils and layers during the production process using linear motion bridges.

Fig. 3: Interference spectra of foils with thickness d=10μm, 20μm, 30μm (refractive index n=1.49) in the measurement range of 900nm to 1700nm.

Measurement Range

In Fig. 3 the interference spectra of three foils with refractive index n=1.49 and of thickness 10μm, 20μm and 30μm are shown. The 10μm-foil shows a good evaluable signal in the visible range, whereas the thicknesses of the 20μm- and the 30μm-foils can only be determined in the NIR range beginning at 1000nm. The choice of the measurement range will be determined by the thickness and the refractive index of the layer. Usually, thick films will be evaluated using the NIR-spectra and for thin films UV/Vis-spectra will be analyzed. An example for a thin film interference spectrum of a layer with a thickness ca. 1μm is shown in Fig. 4.

Fig. 4: Interference spectrum of a layer on a wafer (thickness ca. 1μm), measurement range 400nm to 900nm.

Evaluation

The analysis of the interference spectra starts with a Fourier-transformation. Together with the knowledge of the refractive index the film thickness of foils and layers can be calculated. In Fig. 5 the interference spectrum, the Fourier transformed spectrum and the film thickness of a layer calculated are shown.

 

 

Fig. 5: Film thickness calculation with Fourier transformation (software: TFPro-lite UV/Vis, Ingenieurbüro Thomas Fuchs, Germany)

The most important point for proper calculation is the accuracy of the wavelength calibration. Outstanding results can be measured with the precalibrated spectrometer modules (e.g. MMS, MCS and PGS NIR of the Carl Zeiss GmbH, Germany). Compact and robust design without any moving parts allows for a very high wavelength stability without any needs of further calibrations.

Fig. 6: Quality assurance during running production processes with the MultiSpec-series, (tec5 AG, Germany)

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