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Really small is no problem

27.10.2017

SEM image of a test structure: the metrological target structure (periodical lines) has been positioned at an angle to the surrounding chaotic structure in order to allow the scattering signals in small-angle X-ray scattering to be separated.

At PTB's laboratory at the BESSY II electron storage ring in Berlin-Adlershof, proof has been furnished that even measuring fields with a size of a few micrometers can be characterized by means of small-angle X-ray scattering in a reflection geometry under grazing incidence. The signal overlap problem, which is due to the fact that the incident beam also illuminates the area surrounding the measuring fields, can be bypassed by means of a simple trick. The idea consists in minimally turning the measured fields relative to the surrounding nanostructures.

In industry, test fields from periodically arranged structures are used to check the quality of nanostructured surfaces. In lithographic applications, among others, these fields are distributed throughout a photomask in order to check the quality of the surrounding wafer structure; after processing, this structure images logic circuits. In order to reduce costs, the surface of these control fields is usually limited to less than (50 × 50) µm². X-rays are perfectly suited to characterize these extremely small structures. The very short wavelength of monochromatic X-rays combined with the periodical arrangements of the nanostructures allows the geometrical shape to be determined very precisely by diffraction of the incident radiation.

This procedure is already frequently used in what is called "transmission geometry", i.e. when the radiation can penetrate the sample. One of the natural limitations of this method lies in the absorption of the X-rays: this limits the thickness of the samples that can be investigated. Grazing-incidence small-angle X-ray scattering (GISAXS) allows this problem to be bypassed. However, due to the very small angles of incidence presupposed by this method, the projection of the incident beam onto the sample is prolonged in such a way that the signal from the test fields overlaps with the signal from the surrounding structure. This is the reason why GISAXS has so far been rejected as an alternative in metrological applications.

Test structures developed in cooperation with Helmholtz-Zentrum Berlin by means of the new procedure and manufactured using electron-beam lithography have now been successfully characterized at PTB thanks to GISAXS, thus demonstrating the potential of this new method. A patent application has recently been filed for this procedure.

 

Contact:

M. Pflüger, 7.11, E-Mail: Opens window for sending emailMika.Pflüger(at)ptb.de

V. Soltwisch, 7.12, E-Mail: Opens window for sending emailVictor.Soltwisch(at)ptb.de