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ETH Zürich - D-PHYS - Solid State Physics - Microstructure Research - Projects - Near Field-Emission SEM

Near Field-Emission SEM

Selected Article

Nat. Comm 7, 13611 (2016)
Critical exponents and scaling invariance in the absence of a critical point

Proc. R. Soc. A 472, 2195 (2016)
Thirty per cent contrast in secondary-electron imaging by scanning field-emission microscopy
Phys. Rev. B 89, 014429 (2014)
Domain-wall free energy in Heisenberg ferromagnets

Phys. Rev. B 87, 115436 (2013)
Scale invariance of a diodelike tunnel junction
L.G. De Pietro, H. Cabrera, D.A. Zanin, G. Bertolini, T. Bähler, U. Ramsperger, and D. Pescia
(last update: 26.03.2013) - Urs Ramsperger

Near Field-Emission Scanning Electron Microscopy (NFESEM)

Left: In NFESEM the electron source is a sharp tip placed only a few 10 nm away from the sample surface to be imaged. The primary electron beam (red ) is emitted from the tip via electric field assisted tunneling. The intensity of the backscattered electrons (light blue ) is measured with a suitable detector. The focusing is produced by the “nearness” between sample and source – no extreme focusing technology is needed as in conventional electron microscopy. A previous version was proposed in the seventies by R. Young et al. and was called “the topografiner”. The invention by Young is correctly referred to in the literature as a precursor of the Scanning Tunneling Microscopy (STM), invented by Binnig and Rohrer few years later at IBM Rüschlikon (and awarded the Nobel Prize in 1986). The topografiner was later abandoned – probably in view of the enormous success of STM and of its fantastic spatial resolution. Right: The tip is positioned between 5 and 20 nm distance from the target and scanned parallel to the surface using STM derived piezo -controlled technology. We can perform both, STM and NFESEM consecutively at the same location and compare the images.
Further reading:

T. L. Kirk. Near Field Emission Scanning Electron Microscopy, Appl. Electron Microscopy, 9 (2010).

D.A. Zanin, H. Cabrera, L.G. De Pietro, M. Pikulski, M. Goldmann, U. Ramsperger, D. Pescia, and
John P. Xanthakis, Advances in Imaging and Electron Physics 170, 227 (2012).

NFESEM Tunnel Junction

Experimental current (I) vs voltage (V) characteristics for different distances d between tip and surface covering six orders of magnitude (left) - i.e. for d varying from few nanometers to few millimeters - collapse onto one single graph (right) when the voltage is rescaled by a a suitable scaling factor R which only depends on d. Thus, we have proved a remarkable analogy between NFESEM tunneling and critical phenomena (where scaling is a more standard phenomenon).
We find the origin of the scaling behavior within the electrostatics of the singularity provided by the sharp tip.
Further reading:

H. Cabrera, D. A. Zanin, L. G. De Pietro, Th. Michaels, P. Thalmann, U. Ramsperger, A. Vindigni, and D. Pescia, Phys. Rev. B 87, 115436 (2013).

Topographic Imaging

Top left: STM image of a nanostructured W(110) surface, with accumulation of matter along the surface steps (running along the diagonal of the image).

Top center: The same surface spot imaged by recording the intensity of the backscattered electrons.

Right: The same surface spot imaged by recording simultaneously with the middle image the field emission current I while scanning the tip at fixed tip-surface distance of 40 nm and at fixed voltage.

Bottom left: Line scan through the STM image: plotted is the height of the surface structures along the black line indicated in top.

Bottom center: Line scan through the NFESEM image: plotted is the detector signal along the black line. The recording of both STM and NFESEM images can be used to calibrate the NFESEM height. A vertical spatial resolution of less than 10-1 nm has been observed at distances of about 20 nanometers.

Chemical Imaging

0.34 atomic layers of Fe on stepped W(110)

Left: STM image, showing atomic thick Fe-patches (bright) residing on the terraces and decorating the steps (originally the steps are sharp).

Right: the same surface spot recorded in NFESEM mode. Allthough the Fe-patches are on top of the W-substrate they appear darker - both the patches on the terraces and along the steps. This might be taken as indication of chemical contrast (paper in preparation). The lateral size of the patches in NFESEM mode is comparable to the size in STM mode, showing that the parallel spatial resolution of NFESEM is similar to STM, despite the distance in NFESEM being more than an order of magnitude bigger than in STM.

Energy Resolved NFESEM

Left: sketch of the setup used to perform energy analysis of the backscattered electrons in NFESEM. Center: the NFESEM head in front of the entrance of the analyzer.

Right: The energy spectrum of the electrons backscattered from a GaAs(110) surface shows that both elastic and secondary electrons are actually able to escape the probed zone. Typical energy losses are also recorded. Taking into account the spatial resolution intrinsic to NFESEM we anticipate the technical feasibility of electron spectroscopy with a few nanometers spatial resolution and within a particularly simple instrument. In addition, the presence of a sizeable amount of truly secondary electrons opens the possibility of spin polarized NFESEM.
Further reading:

D.A. Zanin, M. Erbudak, L.G. De Pietro, H. Cabrera, A. Redmann, A. Fognini, T. Michlmayr,Y.M. Acremann, D. Pescia, and U. Ramsper, Proceeding of the 26th International Vacuum Nanoelectronics Conference, IEEE (2012).

Outlook: Spin Polarized NFESEM technology

Left: We are currently developing NFESEM technology for detection of the spin polarization of the backscattered electrons. This quantity is related univocally to the local magnetization. In principle, this technology could allow measuring the local magnetization with sub-nanometric spatial resolution. Therefore, we should be able to resolve, e.g., magnetic domain walls.

Right: Currently spin-polarized Scanning Electron Microscopy (Spin-SEM: Hitachi, IBM, ETH,etc.) is performed using a primary electron beam originating within a standard scanning electron microscope column. The stripe (s) → bubble (b) → uniform (u) domain transition observed in perpendicularly magnetized Fe films on Cu(100) at a given applied magnetic field is an example of the capability of our standard spin-SEM.
Further reading:

R. Allenspach, Spin-polarized scanning electron microscopy, IBM J. Res. Develop. 44, 553-570 (2000)

N. Saratz, A. Lichtenberger, O. Portmann, U. Ramsperger, A. Vindigni, and D. Pescia, Phys. Rev. Lett. 104, 077203 (2010).
We thank Thomas Bähler for technical assistance and the Swiss National Science Foundation and ETH Zurich for financial support.
© 2018 ETH Zürich | Impressum | 2009-09-16 10:05:24