SALVE III microscope

The SALVE III microscope has been built in Phase III (Production Phase) of the SALVE project. The centerpiece of the SALVE III microscope is a new quadrupole-octupole Cc-Cs-corrector by CEOS which has been developed in SALVE phase 2 (SALVE II) and advanced further in SALVE III. The production phase has been started in March 2015 when FEI company joined the SALVE project.

With the completion of the SALVE III microscope by the company FEI in a very strict and short time line, most requirements defined in the SALVE project research and instrument development work package were met. In particular the central requirement with respect to resolution were fulfilled and even exceeded, such that a new world record with respect to the resolution/wavelength ratio was achieved. This demonstrates beside the excellent performance of the SALVE III corrector the very stable platform of the TITAN-Themis column [1]. In particular, the goal was to develop a microscope that accepts opening angles > 50 mrad. The achieved ratio of experimental information limit to wavelength is 16.2 at 20 kV, 16.5 at 30 kV, 15 at 40 kV, 17.1 at 60 kV, and 18.2 at 80 kV, and thus corresponds to aperture opening angles between 67 mrad and 55 mrad. 15 is the lowest ratio of experimental information limit to wavelength ever attempted in a transmission electron microscope.

In order to "use" the transferred information in a proper way, the phase plate, i.e. the aberration function, has been well-controlled beyond the 50 mrad-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. 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. Fig. 1 clearly shows information transfer beyond 50 mrad at 20 kV.

Diffractogram of a tungsten sample

Figure 1: The Young's fringes resolution test for 20 kV (in two directions) on a purely amorphous 2 nm tungsten sample at an accelerating voltage of 20 kV indicates information transfer beyond 50 mrad.

The performance of the SALVE III microscope thus enables atomic-resolution imaging and high-resolution energy-filtered (EF)-TEM with large energy windows even at 20 kV accelerating voltage [1, 2]. It is capable to image a 20 eV window with defocus changes of only 2 nm. [2].

A major part of SALVE III concerned the incorporation of the corrector in the FEI Titan Themis TEM. The microscope column and power meets the stringent requirements of the operation at low voltages. It has been further optimized in many components to tailor the system for the low voltage microscopy application. This does not only include the Cc corrector but involves also the other components such as an improved vacuum system, cryo shields and new stable specimen holders. It exceed the performance of the Titan Themis with special features for the low voltage applications. An overview this is given in Fig. 2.


specs of the SALVE III microscope listed in a table

Figure 2: Specs of the SALVE III microscope, inherit performance of the FEI Titan Themis TEM and features optimized for SALVE operation as well as possible upgrades in the future. (click on the image for a PDF document.)

As shown in Fig. 3 and 4, even at very low accelerating voltage atomic resolution imaging becomes reality. Fig. 4 demonstrates good agreement of experimental and calculated image [3].

image of Au at 20 kV recorded with SALVE III

Figure 3: Atomic resolution image of Au clusters at 20 kV.

image of graphene at 30 kV recorded with SALVE III

Figure 4: (a,b) Atomic resolution image (a) and image simulation (b) of graphene at 30 kV. (c) Comparison of intensity profiles at the position indicated by the horizontal line in (a,b).

Summary

A TEM equipped with a CC/CS-corrector can be used for atomic imaging down to 20 kV. Aberrations are sufficiently small to allow for phase contrast imaging between 55 and 67 mrad. With this microscope SALVE III has now further advanced the technique in aberration-corrected low-voltage transmission electron microscopy.

References
  1. Linck, M., Hartel, P., Uhlemann, S., Kahl, F., Müller, H., Zach, J., Haider, M., Niestadt, M., Bischoff, M., Biskupek, J., Lee, Z., Lehnert, T., Börrnert, F., Rose, H. H. & Kaiser, U. A. (2016). Chromatic Aberration Correction for Atomic Resolution TEM Imaging from 20 to 80 kV. Physical Review Letters, 117: 076101, doi: 10.1103/PhysRevLett.117.076101

  2. Linck, M., Hartel, P., Uhlemann, S., Kahl, F., Müller, H., Zach, J., Biskupek, J., Niestadt, M., Kaiser, U., & Haider, M. (2016) Status of the SALVE-microscope: CC-correction for atomic-resolution TEM imaging at 20kV. The 16th European Microscopy Congress, Lyon, France, 4572, doi: 10.1002/9783527808465.EMC2016.4572

  3. Börrnert, F., Biskupek, J., Lee, Z., Linck, M., Hartel, P., Müller, H., ... & Kaiser, U. A. (2016) Engineering the Contrast Transfer through the Cc/Cs Corrected 20-80 kV SALVE Microscope. Microscopy and Microanalysis, 22: 880-881, doi: 10.1017/S1431927616005249