Metrology – Page 6

Helium Ion Microscopy

Field ion microscopy has a long history, going back to Mueller’s 1951 paper in Z. Phys.. A major advance leading to practical sub-nanometer application came in 2006 with the appearance of the ALIS paper in JVST B. This paper revealed the tri-atomic ionization source, which permits high brightness and very small source size.

Hlawacek et al. at Twente posted a review of helium ion microscopy to the arXiv yesterday. Despite its brevity (16 pages) this review covers many theoretical and practical aspects. With 136 references, it provides a jumping-off point for anyone wanting a serious introduction to this technology.

Happy reading!

E. W. Müller, “Das Feldionenmikroskop,” Zeitschrift für Phys. 131, 136–142 (1951)

B. W. Ward, J. A. Notte, and N. P. Economou, “Helium ion microscope: A new tool for nanoscale microscopy and metrology,” J. Vac. Sci. Technol. B 24, 2871 (2006).

G Hlawacek, V Veligura, R van Gastel, and B. Poelsema, “Helium Ion Microscopy”, arXiv:1311.1711v1

Protecting two-dimensional materials during TEM

It is by now routine to achieve sub-atomic resolution (a few 10’s of pm) by aberration-corrected TEM and by STEM. However, reducing the signal to noise ratio sufficient for the highest resolution requires exposing the sample to fairly high electron dose. The electron irradiation causes sample damage while the image is being acquired. Several mechanisms may contribute: electron knock-on (in which an imaging electron transfers enough momentum to an atom to displace the atom from its original position), radiolysis (in which chemical bonds are rearranged, leading the target material to change into something else), heating (including sample melting), and Coulomb explosion (due to sample charging).

In viewing two-dimensional materials, one cannot average the imaging over depth; a two-dimensional material is by definition all surface. So any damage immediately and irreversibly degrades the image.

Two groups, one at Manchester, the other at Ulm, have independently discovered that it is possible to stablize MoS2 during (S)TEM imaging. This is done making a sandwich of graphene/MoS2/graphene. The Ulm group imaged with aberration-corrected TEM, while at Manchester, they used STEM. Additions to the image from the graphene layers, being periodic, can be subtracted out, leaving only the image of the study material, MoS2. It is an interesting coincidence that the two groups compared identical material stacks.

References:
“The pristine atomic structure of MoS2 monolayer protected from electron radiation
damage by graphene”, G Algara-Siller, et al. [arXiv:1310.2411]
“Control of Radiation Damage in MoS2 by Graphene Encapsulation”, R Zan et al. [ACS Nano (DOI: 10.1021/nn4044035), see also arXiv:1310.4012]

TEM Resolution Limit

Careful experiments from CEOS Corrected Electron Optical Systems GmbH in Heidelberg have shown that stochastic magnetic field noise limits the ultimate resolution of transmission electron microscopes. This magnetic field noise is due to unavoidable thermal currents in the conductors of which the TEM is constructed. Best summarized in the authors’ words, “[E]xperiments have revealed a hitherto unknown fundamental performance limitation for electron microscopy due to the stochastic beam deflection caused by the noise fields.” The authors estimate that observed image blur of 15-25 pm can be explained by this phenomenon.

Reference: Stephan Uhlemann, Heiko Müller, Peter Hartel, Joachim Zach, and Max. Haider, “Thermal Magnetic Field Noise Limits Resolution in Transmission Electron Microscopy”, Phys. Rev. Lett. 111, 046101 (2013)

Stability limits in electron microscopy

In a paper coming out of Kamerlingh Onnes Laboratorium, Schramm et al. remind us that instability is inherent in aberration corrected electron microscopes. They propose that the system designer use a stability budget to trade off stability and resolution. The stability budget is somewhat orthogonal to the usual error budget. They further point out that real-time control is required to further advance TEM (and SEM, LEEM, etc. for that matter), but that suitable measurement parameters from which to derive feedback are presently unknown.

The paper is on the arXiv. It would appear that there’s an entrepreneurial opportunity here.