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 60-300 platform [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 shows atomic resolution in raw micrographs of graphene with adatoms and edges at 20 kV.

image of graphene at the edge and with adatoms at 20 kV with SALVE III

Figure 1: Transmission electron micrographs of a hole in graphene with a heavier atom attached taken with a primary electron energy of 20 keV. The images shown are raw micrographs taken with a dose of 8.5 × 106 e/nm2 in 4-s exposure time. A different defocus chosen for each image results in (a) positive and (b) negative contrast of the carbon lattice. Note that the contrast of the heavier atom remains positive. [2]

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. The reduced axial brightness of the FEI X-FEG Schottky type with a (switched off) monochromator was determined to be larger than 1.8 x 108 A/(m2 sr V). The illumination semi-angle distribution. For the different dose rate conditions, we get a dose rate of 3,500 e/(Å2 s) with an illumination semi-angle distribution of 70 μrad at 20 kV and 110 μrad at 80 kV. Under low dose rate conditions of 34 e/(Å2 s) which is a value typically used in imaging biological samples, the illumination semi-angle distribution is 7 μrad at 20 kV. [3]

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 Figs. 1 and 3, even at very low accelerating voltage atomic resolution imaging becomes reality. Fig. 3 demonstrates good agreement of experimental and calculated image. [4]

image of graphene at 30 kV recorded with SALVE III

Figure 3: (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). [4]

As shown in Fig. 4 the resolution obtained with the Cc/Cs corrected SALVE III microscope at 30 kV for 2D MoS2 is higher by about a factor of 2 than the resolution obtained with Cs-corrected microscopes at 80 kV earlier [5].

image of graphene at 30 kV recorded with SALVE III

Figure 4: Direct comparison of raw data high-resolution TEM images of a MoS2 monolayer recorded with (left) Cc/Cs – corrected SALVE/FEI machine at 30 kV and (right) Cs – corrected FEI Titan 80-300 at 80 kV. The images were recorded with bright atom contrast. In (a) single atoms are resolved so that the hexagonal structure of MoS2 can be observed while in (right) the structure can still be resolved but the single atom contrast is very poor (cf. magnificated areas in the lower left parts). Correspondingly, the FFT of (left) (right upper insert) shows stronger reflections and higher frequencies transfer up to 102 pm than the FFT of (right). The scale bar corresponds to 4 nm. [5]

As shown in Fig. 5 by image calculation we explored the feasibility of the annular differential phase contrast (ADPC) and compare to the incoherent bright-field (IBF) and high-angle annular dark-field (HAADF) Cc/C3 and Cc/C5 STEM mode. [6]


Figure 5: (left) Normalized central value I(0) of the point-spread function (PSF) of the ADPC mode for the Cc/C5-corrected STEM and the Cc/C3-corrected STEM operating at 30 kV. Positive and negative values of I(0) represent bright and dark contrast of the image of the point scatterer, respectively. (right) Depth of field as a function of the accelerating voltage for the ADPC mode and the IBF and HAADF modes of a Cc/Cn-corrected STEM (n = 3, 5). For the Cc/C5-corrected STEM, C7=4 mm; and for the Cc/C3-corrected STEM, C5=4 mm. [6]

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. Börrnert, F., & Kaiser, U. (2018). Chromatic-and geometric-aberration-corrected TEM imaging at 80 kV and 20 kV. Physical Review A, 98: 023861, doi: 10.1103/PhysRevA.98.023861

  3. Börrnert, F., Renner, J., & Kaiser, U. (2018). Electron Source Brightness and Illumination Semi-Angle Distribution Measurement in a Transmission Electron Microscope. Microscopy and Microanalysis, 24: 249-255, doi: 10.1017/S1431927618000223

  4. Kaiser, U. (2016). Properties of low-dimensional electron-beam-sensitive objects by spherical and chromatic aberration-corrected low-voltage high-resolution transmission electron microscopy and spectroscopy. In: The 15th European Microscopy Congress (John Wiley & Sons, Germany, 2016): 8679, doi: 10.1002/9783527808465.EMC2016.8679

  5. Lehnert, T., Biskupek, J., Köster, J., Linck, M., & Kaiser, U. (2016). Quantitative low-voltage spherical and chromatic aberration-corrected high-resolution TEM analysis of beam-specimen interactions in single-layer MoS2 and MoS2/graphene heterostructures. In: The 15th European Microscopy Congress (John Wiley & Sons, Germany, 2016): 5742, doi: 10.1002/9783527808465.EMC2016.5738

  6. Lee, Z., Kaiser, U., & Rose, H. (2019). Prospects of annular differential phase contrast applied for optical sectioning in STEM. Ultramicroscopy, 196: 58-66., doi: 10.1016/j.ultramic.2018.09.012