Institute Silicon Photovoltaics
Ellipsometry is an optical analytical method used for the determination of film thickness and dielectric properties (n and k) of thin films or bulk materials. It exploits the change of polarization of an incident light beam with defined polarization upon interaction with an interface of two materials with different dielectric properties. This interaction is described by the Fresnel equations. As the technique measures the intensity ratio and the phase difference between s- and p-polarized components, the technique does not require a reference measurement. Ellipsometry is very sensitive to small changes in thickness or dielectric properties of thin films and an ideal tool to investigate thin films.
The comparison of the measurement to a model system allows the analysis of single or a stack of multiple films on a substrate. In ideal cases a measure for the roughness of surfaces and interfaces can be obtained in addition to the thickness and dielectric properties of each film. The dielectric properties can be further be analyzed to obtain information on composition, crystalline nature or conductivity of the analyzed layers. Ellipsometric measurements can be easily complemented by spectrophotometric measurements like transmission or reflection.
In order to obtain reliable results ellipsometry usually requires the examined films with little roughness and a homogeneous thickness within the measurement spot. Vertical inhomogeneity, for instance caused by the growth procedure, can severely complicate data analysis.
Sentech SE850 DUV
In our institute we use a Sentech SE850 DUV variable angle spectroscopic ellipsometer. A spectral range from 190 nm to 2500 nm can be used. The goniometer is fully automated. Microspots allow the measurement area to be reduced to less than 1 mm.
Our main application areas of ellipsometry are:
- Process control and homogeneity investigations for a-Si:H or dielectric films deposited in-house
- Investigation of metal oxides and other contact materials
- Determination of dielectric properties of various materials as input for optical device simulations
Depicted are ellipsometric measurements at 50°, 60° and 70° with corresponding fit for a 15 nm thick NiOx layer on silicon