Ellipsometry and Scanning Force Microscopy

Ellipsometry and Scanning Force Microscopy

The polarisation state of light changes when it is reflected from a surface. This change allows a detailed analysis of the surface properties. The surface being investigated is illuminated with linearly polarised light. The reflected light is then generally polarised elliptically, hence the name “ellipsometry”. This polarisation state is determined and established for a specific wavelength range of the analysing light. Then, the optical properties of the surface layer system are calculated from this information. By means of physical models, the underlying parameters of the layer system can be determined: layer thickness, homogeneity, chemical and physical composition, stoichiometry, micro-roughness, density,…

The immense benefit of this method is that the measurements are performed optically and therefore without any contact making them non-destructive. Under optimum conditions, layer thickness changes can be measured in the sub-nanometre range like, for example, polymer coatings on metals, oxides on silicon wafers.

The semiconductor industry uses this method quite often. Other numerous new applications arise for example during the production of hard material layers (TiN,…) by means of PACVD, for the analysis of polymer coatings on glass or for checking glue layers.

Scanning probe microscopy:  Microscopic Analysis of Surface properties

Scanning probe microscopy makes it possible to microscopically identify surface properties. In the process, not only the topography of the surface can be analysed, but magnetic properties, elasticity or friction as well. A lateral resolution in the range of 1 nm also makes it possible to analyse nanostructures. Scanning probe microscopy is therefore an indispensible tool of nanotechnology.

At the heart of a scanning probe microscope is a tiny tip that is mounted on the end of a bar.  This tip responds to interaction forces that arise when scanning the sample laterally, vertically or in both directions. These forces cause the bar to deflect, which can be detected using optical methods. Here, a laser beam is projected onto the back of the bar and the reflected signal is investigated by means of segmented photodiodes. The movements of the bar are then calculated from the different values that arise from the photodiode sectors. A great variety of surface parameters can be measured depending on controlling the feedback on the bar based on the calculated signals. Scanning probe microscopy is an indispensible aid in the analysis of nanostructures. This possibility of easily identifying the topography of the surface is a valuable addition to scanning electron microscopy.