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Recognizing nanostructures

Small-angle X-ray scattering on subminiature lithographic measuring fields

PTB-News 1.2018
12.01.2018
Especially interesting for

nanometrology

the semiconductor industry

Photolithography is used in industry to manufacture semiconductor elements. Hereby, the nanostructures of the logic gates of a photomask are imaged optically on the semiconductor wafer. For process control, measuring fields consisting of nanostructures in a periodical arrangement are distributed at different points over the entire photomask. The space and funds available being limited, these measuring fields are mostly no larger than 50 μm × 50 μm. Their lithographically processed image on the wafer can then be assessed using a microscope. Working towards quality control applications during the production process, PTB researchers at PTB's laboratory at the BESSY II electron storage ring in Berlin-Adlershof have succeeded in characterizing such measuring fields for the first time by means of small-angle Xray scattering in a reflection geometry.

SEM image of a lithographic test structure: the periodical lines of the measuring field with dimensions of 4 μm × 4 μm (top right) are rotated by 10° with reference to the symmetry axis of the surrounding irregular nanostructures to be able to spatially separate the respective X-ray signals from each other.

Since it is fast and provides high resolution, small-angle X-ray scattering (SAXS) is, in principle, an ideal measurement method to reconstruct periodically arranged nanostructures. It is thus considered when it comes to process- related metrology in the semiconductor industry. Due to X-ray absorption, measurements in a transmission geometry in which the incident X-ray beam penetrates the wafer are only possible with very thin wafers. It is possible to bypass this problem using SAXS in a reflection geometry under grazing incidence (GISAXS). However, due to the very small angles of incidence presupposed by this method, the projection of the incident Xray beam onto the sample is prolonged in such a way that the scattering signals of the tiny measuring fields to be checked overlap with those from the surrounding nanostructured arrays. This is the reason why GISAXS has so far been rejected as an alternative.

However, the problem of the overlapping signals has now been bypassed by means of a simple work-around, namely by slightly rotating the symmetry axis of the measuring field with reference to the surrounding nanostructures. The scattering signals then occur in different solid angle ranges and can easily be distinguished spatially when identified by means of an area detector. In this way, it has been possible to measure fields with edge lengths of a few micrometers without overlapping interfering signals from their environment.

Corresponding test structures for the new procedure have been developed within the scope of a cooperation project with the Helmholtz-Zentrum Berlin and manufactured using electron-beam lithography. These structures have been successfully characterized at PTB by means of GISAXS. The new method’s potential for industrial applications to photomask metrology in the manufacturing of semiconductors has also been demonstrated by scattering experiments with extreme ultraviolet radiation (EUV scatterometry), so that a patent for this procedure is pending.

Contact

Mika Pflüger
Department 7.1
Radiometry with Synchrotron Radiation
Phone: +49 30 3481-7118
E-mail: mika.pflueger(at)ptb.de

Scientific publication

M. Pflüger, V. Soltwisch, J. Probst, F. Scholze, M. Krumrey: Grazing-incidence small-angle X-ray scattering (GISAXS) on small periodic targets using large beams. IUCrJ 4, 431–438 (2017)