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Characterization of interfaces by means of X-ray scattering

X-rays scattered at the interfaces of multilayers carry characteristic information about the layer system: The morphological structure of the interfaces between the layers can be determined from the scattered radiation. Within the framework of a project to investigate interfaces PTB has set up a measurement station to analyse multilayers of just a few nanometres in size to up to approximately 100 nm. The results of this work are of principle interest in the area of surface coating, thin film deposition and fabrication of nano-structured systems.

Scattered intensity of a GaAs/A1As multilayer in reciprocal space. The intensity is colour coded and plotted over the x- and z-components of the scattering vector q. The banana-shaped intensity maxima indicate a strong correlation of the interface roughness.

Two issues are of special interest in examining multilayers: first, the – primarily technologically oriented – measurement of layer thicknesses and second, the morphological parameters of the interfaces such as roughness and correlation lengths.

Layer thicknesses can be determined by analysing the interferences of the specular-reflected partial beams from the interfaces. For this purpose, computer-aided simulation models for the samples are indispensable. The achievable uncertainties are in the subnanometre range depending on layer thickness and sample material.

The morphological structure of the interfaces between the layers is investigated by measuring and analysing the amount of diffusely scattered radiation. The characteristic parameters (roughness, vertical and lateral correlation length, Hurst exponent) are extracted by means of a complex computer model of the scattering process. In analysing the diffusely scattered components, the main focus is on fundamental questions concerning growth models and interface formation in solids.

The most crucial part in the experimental set-up is the high-power rotating anode. The anode, in particular, provides the great dynamic range of the scattered radiation with which the high accuracy for the thickness measurements is achieved. To measure the diffusely scattered components with sufficient resolution a high quality beam of x-rays is required, i. e. highly parallel and monochromatic radiation. This is achieved by means of an optical system connected to the rotating anode which transforms the originally divergent beam. A goniometer is used to monitor the angles of incidence and of reflection independently, with an accuracy of a hundredth degrees, in a range of up to 5 degrees in relation to the sample surface.

Within the framework of the project, a new method for certified layer thickness measurements by X-ray reflectometry is in development. In addition new impetus is created for new methods to measure nanoscopic roughnesses.