With the goal to enable atomic resolution TEM observations on beam sensitive materials the SALVE project had been initiated to develop a dedicated low-voltage TEM that is corrected for both, spherical and chromatic aberration [1,2].

The centerpiece of the SALVE III microscope is a new quadrupole-octupole Cc-Cs-corrector by CEOS that is based on the so-called Rose-Kuhn-Design [3,4]. The corrector is incorporated into a cubed FEI Titan Themis TEM and has been aligned for five accelerating voltages in the range from 20 to 80kV.

During design of the corrector special care had to be taken to prevent the resolution-limiting effect of thermal magnetic field noise (Johnson-Nyquist noise) that causes an image spread and limits the information transfer in Cc-corrected electron microscopes [5]. As seen in Figure 1(a), these measures have been very successful in that for all desired high-tensions the finally achieved resolutions exceed the 50mrad milestone. Figure 1(b) exemplarily demonstrates the achieved information limit on a purely amorphous 2nm tungsten sample at an accelerating voltage of 20kV. For the two shown perpendicular directions the Young’s fringes significantly surpass the 50mrad aperture. In order to “use” the transferred information in a proper way, the phase plate, i.e. the aberration function, has to be well-controlled beyond the 50mrad-angle. This requires accurate control over axial aberrations up to including 5th order, and -for a considerable field of view- access to off-axial aberrations. The aberration measurement in Figure 1(c) demonstrates that all unround axial aberrations as well as the off-axial aberrations can be tuned sufficiently small. At the same time, the round aberrations can be adjusted for a suitable phase contrast transfer function (indicated in green color). Consequently, as shown in Figure 1(d) even at 20kV atomic resolution imaging becomes reality.

The chromatic aberration causes inelastically scattered electrons, i.e. electrons of lower energy, to be focused much stronger. This effect is compensated in the Cc-corrected instrument. Figure 1(e) compares the energy-dependent defocus effect of a Cc-uncorrected TEM (red line, Cc=1.45mm) and the SALVE microscope (measurements: black dots; 3rd order fit: dashed line) at 20kV. While the gradient of the Cc-uncorrected case is too steep to be distinguished from the axis of the ordinate in the magnified area, the SALVE instrument is capable to image a 20eV window with defocus changes of only 2nm. This will enable new imaging modes such as high-resolution EFTEM at very low voltages. [6]

References:

[1] U. Kaiser et al., Ultramicroscopy 111, Issue 8 (2011), 1239-1246.
[2] http://www.salve-project.de/
[3] H. Rose, Proc. 10th Eur. Congr. El. Micr. (Granada, Spain) (1992), 47.
[4] H. Rose, Patent Application DE 42 04 512 A 1 (1992).
[5] S. Uhlemann et al., Physical Review Letters 111(4) (2013), 046101.
[6] The authors acknowledge funding from the German Research Foundation (DFG) and the Ministry of Science, Research and the Arts (MWK) of the federal state Baden-Württemberg, Germany.

Figures:

Figure 1: (a) Resulting information limit of the SALVE microscope for different high-tensions. (b) The Young's fringes resolution test for 20kV (in two directions) indicates information transfer beyond 50mrad. (c) For this aperture, the aberrations can be tuned for uniform phase contrast (passband: green color, measured coefficients tabulated). (d) Atomic resolution image of Au clusters at 20kV. (e) Achromaticity: Without Cc-correction (red line), inelastically scattered electrons suffer a gigantic defocus. In the SALVE microscope the energy-dependent defocus within a 20eV window is only 2nm dominated by higher order chromatic aberrations (dashed line).

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