Recognizing nanostructures
Small-angle X-ray scattering on subminiature lithographic measuring fields
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)