Thickness, density and roughness of ultra-thin film systems: X-ray reflectometry (XRR)

XRR measurement on ultrathin films.
© Fraunhofer IST, Kirsten Schiffmann
X-ray diffractometer X’Pert MRD Pro from Panalytical for XRR measurement on ultrathin films.

X-ray reflectometry (XRR)

X-ray reflectometry (XRR) works with the grazing reflection of X-rays from the surface and internal interfaces of materials. This permits layer thickness, density and interfacial roughness of ultra-thin layers and layer systems on very smooth substrates to be determined. Due to the roughness, the detectable layer thickness range is around 2 - 200 nm. Substrates should preferably be glass or silicon wafers. Accuracies of ~ 0.2 nm are achieved when determining layer thickness.

Fields of application: Density of thin layers - Roughness of internal interfaces

X-ray reflectometry (XRR) is one of the few methods permitting the determination of density without weighing and can also determine the roughness of “buried“ internal interfaces between layers. Pure density determinations are also possible on thicker layers provided the roughness of the surfaces is low. XRR can be performed on single, double, triple or quadruple layers and also works on periodic layer stacks with any number of layers. 

XRR measurement for determination of precision coating thickness

XRR measurement for precision film thickness determination. Reflectometry curve of a dual-layer Ag-ZnO film on glass.
© Fraunhofer IST

Reflectometry curve of an Ag-ZnO double layer on glass. The density, layer thickness and roughness are determined by assuming a model whose parameters are refined until the calculated reflectometry curve (black) reaches optimum agreement with the experimental curve (yellow).

Layer thickness measurement on 31-layer, non-periodic rugate filter of Nb2O5 and SiO2

Die genaue Schichtdickenbestimmung ist für optischen Kantenfilter eine wichtige Qualitätskontrolle.
© Fraunhofer IST
Die genaue Schichtdickenbestimmung ist für optischen Kantenfilter eine wichtige Qualitätskontrolle.
© Fraunhofer IST

The exact determination of the layer thickness is an important quality check for optical edge filters. A successful fit of the reflectometry data with up to 96 parameters is only possible if very good starting parameters for the layer thicknesses are already available. These were determined here via a SIMS depth profile over the 2.3 µm total layer thickness, correcting for the sputter rates of the different layer materials.