Inspection of masks for EUV lithography using "standing-wave PEEM"

J. Maul, A. Oelsner, G. Schönhense, Institut für Physik, Johannes Gutenberg-Universität  Mainz
Cooperation partners: J. Lin, U. Kleineberg, Fakultät für Physik, Ludwig-Maximilian-Universität München
N. Weber, M. Escher, M. Merkel, Focus GmbH,  Hünstetten/Taunus

EUV lithography

Extreme ultraviolet lithography (EUVL) is considered to be most promising for the pro­duction of next generation semiconductor chips. Both superior in attainable structural sizes and in the time needed for their manufacture, EUVL exceeds the capacity of related tech­niques like electron or ion beam lithography which are unsuited for mass production. For photolithography, the achievable resolution for structuring is given by the Abbe relation  where λ is the wavelength and NA is the numerical aperture of the focusing optics. This resolution limit also governs the structural sizes on semiconductor elements.
A significant resolution enhancement is expected by the use of EUV sources (i.e., spectral range between 1nm and 50nm). Good candidates are plasma sources for future applications in the laboratory. Here, the wavelength of 13.5nm is particularly interesting because of the availability of efficient reflective multilayer optics. Synchrotron radiation can also be used to achieve the ultimate resolution goal, despite being too expensive for routine operation. However, synchrotron based techniques are well suited to investigate the performances of EUVL at a relevant photon energy.

EUV-PEEM for mask inspection

In this work, we exploited synchrotron based EUV photoemission electron microscopy (EUV-PEEM) in a standing-wave mode for an "at wavelength" inspection of buried defects in multilayer mask blanks. We investigated both programmed phase defects and "real" (i.e. unintended) defects found at the surface of small test samples and of an industrial six inches mask blank prototype.
The EUV-PEEM was designed for a sample illumination near normal incidence which resembles the geometry in an EUVL stepper. For this, a toroidal broadband Mo/Si multireflector (spectral range between 80eV and 100eV) was installed in an angle of 4° to the electron axis, near the diffraction plane of the microscope (see Fig.1a). The Focus PEEM allows a field of view between 2.3 µm and 1mm. For EUV illumination of and photoemission from the multilayer samples, synchrotron radiation from BESSY II was taken in an energy range between 89eV and 97eV.
It was shown that defects are clearly detectable down to a size of 30nm from a "standing-wave contrast" in the multilayer [1,2] (see also Figs. 1b and c). Furthermore, a tomographic imaging was recently established for a 3D characterization of buried defects in multilayer masks [3]. Together, these results demonstrate that EUV-PEEM has potential for an inspection tool for reflective masks used in EUV lithography.

J. Maul, J. Lin, A. Oelsner et al., Surf. Science 601, 4758-4763 (2007).
J. Lin, N. Weber, J. Maul et al., Opt. Lett. 32, 1875-1877 (2007).
J. Lin et al., Opt. Express, submitted (2008).

(a) Schematic setup of the EUV-PEEM with irradiation of the sample by EUV light in near-normal incidence. (b) and (c):  EUV-PEEM images showing stripe defects buried underneath a Mo/Si multilayer.

Figure 1. (a) Schematic setup of the EUV-PEEM with irradiation of the sample by EUV light in near-normal incidence. (b) and (c): EUV-PEEM images showing stripe defects buried underneath a Mo/Si multilayer. The PEEM images were recorded near the inspection wavelength of 13.5nm (~91.5eV), showing contrast inversion by tuning from 88.5eV to 91.8eV photon energy. The smallest defect is 35 nm wide and 4 nm high. This work was supported by the 6th Framework program of the European Union within the project ‘Exploring new limits to Moore’s law – More Moore’.




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