SALVE Cs/Cc-correction realized

August 09, 2016 - By pushing the usable aperture to the highest value ever reached in phase-contrast TEM imaging, the SALVE project completes the list of atomic-resolution TEMs. The SALVE III TEM operates in the range between 20 to 80 kV obtaining a resolution limit as small as 15 wavelenghts which is more than six times smaller than that of uncorrected TEMs.

Since the beginning of aberration-corrector technology, transmission electron microscopy (TEM) provides sub-Angstrom instrumental resolution provided that the specimen is not damaged by the electon beam [1, 2]. In order to avoid atom displacement by knock-on processes, the energy of the electron most stay below a characteristic threshold energy which is proportional to the mass of the atom [3]. Specimens consisting of light elements such as biological objects, polymers, and ceramics are in addition damaged by ionization processes even if the TEM operates at energies below the knock-on threshold [4 - 6].

In order to avoid knock-on damage at atomic resolution an aberration-corrected electron microscope is required which must operate at acceleration voltages as low as 80kV for many materials. Such a TEM was first completed in 2005 [7]. As a result, the exposure time could be increased significantly enabling atomic resolution for a variety of objects.

However, experiments have shown that this voltage must be reduced even further for resolving the atomic structure of interfaces and defects in materials composed of light elements. In order to reach atomic resolution for these materials, the SALVE project [8] was launched in 2009. The SALVE microscope operates at low voltages in the range between 20 and 80 kV below the knock-on threshold of the object. Unfortunately, for achieving atomic resolution at these voltages, the chromatic (Cc) aberration must be eliminated in addition to the spherical (Cs) aberration (see blue boxes at the end of the news article for an explanation). Without Cc correction, the minimum achievable resolution limit is about 0.2nm at these low voltages [9].

In the journal Physical Review Letters, the scientists of the SALVE project now report the successful implementation of the novel aberration corrector which provides an instrumental resolution limt of 0.76 Å - 1.40 Å in the range 80 – 20 kV (Table 1) [†].

"We were able to achieve the new resolution record due to our latest understanding of resolution-limiting factors in electron microscopy and by applying the improved understanding for the optimization of the corrector", said Prof. Max Haider, director of CEOS GmbH which is the world leader in corrector-development. The SALVE corrector is based on an early design of Prof. Harald Rose and his student Mathias Kuhn in 1992 [10]. Prof. Rose, who has a consultative role in the SALVE project, had initially suggested this corrector for realizing an electron optical aplanat which is free of primary chromatic, spherical, and off-axial aberrations, as required by the SALVE microscope. During the evaluation phase of the SALVE project (SALVE I, 2009 - 2011), the researchers concluded that this design is optimal for the SALVE corrector, because it provided by far the best performance of all investigated corrector designs. "I had completely forgotten this design over the years because the simultaneous correction of these aberrations was not necessary at the standard voltages above 80 kV used in TEMs so far" said Prof. Harald Rose. In the following phase "SALVE II" from 2011 - 2014, the corrector was developed, and in Phase III from 2015 - 2016 it was optimized and integrated into the microscope. "That the third phase could be so successfully completed is also due to the enormous commitment of SALVE III partner FEI Company which supplied and developed the microscope column", said Professor Ute Kaiser, principal investigator of the SALVE project.

Improved understanding of resolution limits in the TEM

Important progress has been made in the understanding of resolution limiting processes and parameters in the TEM in the last years, primarily by the members of the company CEOS. In the new publication the scientists explain in detail the most important parameters which affect the information limit. They are the spatial coherence of the illumination system, focus spread in the imaging optics caused by instabilities of the electromagnetic fields, and image spread resulting from Johnson noise [11]:

1. The information-dampening effect due to partial spatial coherence is eliminated by the correction of the spherical aberration. It has first been experimentally demonstrated in 1998 [1] that Cs-correction improves the resolution of an electron microscope.
2. The chromatic aberration transforms the energy spread of the electron source and the instabilities of the accelerating voltage and the lens fields into a focus spread. This focus spread is the main limitation for low-voltage TEM imaging regardless if the spherical aberration is corrected or not [12]. Apart from the primary chromatic aberration, the secondary chromatic aberration must also be sufficiently reduced at low voltages, as has been demonstrated in 2010 [13].
3. After Cc correction, an additional focus spread—caused by instabilities of the focusing elements due to noise from the electronics’ supplies—remains, which must be reduced by using ultra-high quality electronic voltage and current sources [14] and ultra-low environmental noise, which must provide by a mechanically and electromagnetically shielded room.
4. Stochastic beam deflections that are generated by every conductor (both magnetic and nonmagnetic, "Johnson noise" [11]) result in a resolution-limiting image spread.

Figure 1. Foto and path of rays of the SALVE III microscope.

Improved resolution due to improved understanding of resolution limits

To avoid the deleterious effects resulting from the instabilities of the electromagnetic fields, all currents and voltages must be kept extremely stable. To reduce the Johnson noise, the electrons’ ray path and the shape of the metal parts close to the beam are optimized [11]. A trade-off between the bore of the multipole elements, the lens gaps, and the liner tube diameter has been made, that enables the required correction strength and focusing power and, at the same time, prevents the electron beam from suffering too much image spread. These measures are at the edge of technological feasibility in that, for example, the magnetic materials in the multipols were excited close to saturation. Fig. 1 shows a photograph of the corrector and the schematic path of the paraxial rays. Details about the corrector and the microscope are available at the end of this news article.

Figure 2. Resolution of the SALVE III microscope.

Achievements

The SALVE corrector has been designed in such a way that the round aberrations, i.e., defocus C₁ and spherical aberrations of third-order C₃ and fifth-order C₅ are tuned to specific values to establish a uniform phase contrast transfer function (PCTF). Moreover, all non-circular aberrations are reduced to such an extent that they do not prevent a sufficiently uniform phase surface [15]. The measured 20 kV phase plate in Fig. 2(f) indicates that this challenging task has been accomplished with the SALVE corrector. Moreover, off-axial aberrations including off-axial coma are tuned sufficiently small to provide a large field of view such that each object point is imaged in the same way regardless of its position in the object plane.

Measures to quantify the resolution are the Young’s fringe resolution test (Fig. 2) or the paraboloid method [16 – 19]. Both tests match and demonstrate that the SALVE goal of a usable aperture of 50 mrad has been reached and even exceeded for the whole voltage range (Table 1). The results will also be beneficial for the improvement of C_C-correctors developed at a time before the resolution-limiting effect of Johnson noise was discovered.

Figure 3. Experimental and calculated images.

A big step foreward for all sciences that use sensitive materials

With the new corrector, the resolution for graphene and MoS2 at 30 kV is comparable to the current state of the art at 80 kV. This is due to the large effective aperture of the instrument as well as due to increased scattering of the electrons at lower energies. The SALVE III microscope delivers for the first time images of an edge configuration in graphene because it remains fixed when imaged at 30 kV, whereas it is unstable under an 80 kV electron beam [20, 21].

Furthermore, images of free-standing monolayers of graphene and MoS₂ at an accelerating voltage of 30 kV show, in good agreement with simulations, a strongly improved single-atom contrast (Figure 3). In the raw experimental images (1 s exposure time, 2048 × 2048 pixels frame size) the accumulated electron doses are 2.5 * 10⁶ e⁻/nm² and 3 * 10⁵ e⁻/nm² for graphene and MoS₂, respectively. The diffractogram of graphene, which is a measure for the resolution, reveals reflections up to third order, corresponding to 107 pm. For MoS₂, even the fourth-order reflections, i.e., 102 pm, can be detected in the diffractogram. It is possible to identify the single carbon atoms in the hexagonal lattice even under those dose-limited conditions. Moreover, molybdenum can clearly be distinguished from the two sulfur atoms, which are located underneath each other in this crystal projection, thus low voltages of 30 kV will not only allow for detecting single atoms and point-defect structures but also to determine their chemical nature.

The scientists from CEOS GmbH, FEI Company, and Ulm University successfully demonstrated the functionality of a TEM corrector for geometric and chromatic aberration correction for voltages from 20 to 80 kV. The achieved resolution at 40 kV corresponds to only 15 times the electron wavelength. This is a new record in phase-contrast TEM imaging. Furthermore, the correction of chromatic aberration opens up new opportunities for analytic imaging because large energy windows are almost equally well focused at the image plane. The demonstrated information limit of 90 pm at 40 kV corresponds to a maximum aperture angle of 67 mrad. Between 40 and 80 kV, sub angstrom resolution has been obtained. At 30 kV, the stability and resolution of the complete system (microscope and corrector) is more than sufficient to resolve single atoms in raw TEM images. At the current lowest-possible voltage of 20 kV, a resolution better than 0.14 nm can be achieved allowing even for atomic or near-atomic imaging. This is a huge step forward towards materials analysis on delicate, beam-sensitive objects; the SALVE microscope now combines both the desired resolution and the beam energy below knock-on threshold at the same time.

Another object of the SALVE project is to determine the optimum voltage for different materials and scientific objectives. With decreasing voltage ionization damage - known as radiolysis - increases. Therefore, another aim of the SALVE project concerns the reduction of ionization by improved sample preparation [22, 23]. The best choice of the electron energy depends on each individual sample’s compromise between ionization and knock-on damage [24]. "With the new SALVE III microscope scientists can now examine for the first time the sample with atomic resolution at electron energies in the range between 20 and 80 keV. This possibility is a great step for electron microscopists and all sciences which are using electron microscopy," says Prof. Ute Kaiser.

More details about Rose's contributions to hardware aberration correction including other pioneers works can be found here.

Highlighted Topics

Why do low voltages need C_C correction?

The resolving power of the transmission electron microscope is determined by the geometrical and chromatic aberrations of the imaging system and by the stability of the instrumental setup [12, 14]. At low acceleration voltages, the information limit of the phase-contrast image is dominated by the chromatic focus spread. Therefore, low-voltage TEM requires either very special imaging technique (such as tilt-series reconstruction [25], optimized ultrahigh resolution lenses, and cold FEG emitters [26]) or monochromation of the primary electrons [27]. However, these measures do not prevent inelastically scattered electrons from being strongly defocused at the image plane. The resulting blurring of the image contrast can only be avoided by correction of the chromatic aberration.

The parasitic focus spread σ(C₁) caused by the limited stability of the focusing elements has been estimated using the SALVE III microscope by comparing the information transfer under axial and tilted illumination. In the case of a tilted illumination, the focus spread in an uncorrected microscope would dramatically reduce the information transfer in the tilt direction [13]. In the SALVE microscope, however, as seen in Fig. 4(b), even a beam tilt of 3° (52.4 mrad) does not affect the information transfer noticeably. As a consequence, it is very difficult to quantify the focus spread in detail; only an upper limit of σ(C₁) ≤ 1 nm at 20 kV can be given. This value represents the statistic deviation of the information limit in terms of a focus spread as evaluated from series of images. This value is negligible compared to the focus spread of σ(C₁) ≈ 25 nm for a non-monochromated, C_C-uncorrected microscope at 20 kV [C_C = 1.45 mm, σ(E) ≈ 0.35 eV]. It follows, that the corrector’s supplies satisfy the very strict requirements for C_C correction. To achieve the same focus spread in an equivalent C_C-uncorrected TEM by means of a monochromator, the energy width of the gun would have to be limited to σ(E) ≈ 14 meV even without taking into account the instabilities of accelerating voltage and objective lens current. For this case, the monochromator would remove about 96% of the available beam current. Moreover, as already mentioned, inelastic scattered electrons would be defocused depending on their energy loss by the chromatic aberration of the objective lens resulting in a strong loss of contrast and resolution.

Figure 4. Estimation of focus spread: The information transfer under axial illumination (solid profile) does not show any deterioration at all if the beam is tilted to 3° (52.4 mrad) (red line in profile).

Details about the corrector and the microscope

The Rose-Kuhn corrector is a quadrupole-octupole corrector which consists of eight multipole elements. Two of them are dodecapoles, each enabling the excitation of superimposed magnetic and electrostatic quadrupole fields forming a first-order Wien filter. One Wien filter compensates for the x-component of the chromatic aberration, the other for the y-component. The spherical aberration is compensated by a specific combination of magnetostatic octupole fields, as originally proposed by Otto Scherzer in 1947 [28]. At the top, two transfer lenses are used to adapt the corrector to the objective lens; at the bottom, two adapter lenses couple the corrector to the projective system. Fig. 1 shows the schematic path of the paraxial rays. The corrector has 104 channels in total, which — if controlled properly — allow for correcting axial aberrations up to the fifth order inclusively and off-axial aberrations up to the third order inclusively. The coefficient C₅ of the intrinsic fifth-order spherical aberration is positive and its value can be adjusted in the range between 2 and 5 mm. This possibility enables the adjustment of optimum conditions for bright-atom phase contrast [29] and the correction of the off-axial coma allows atomic-resolution imaging with large fields of view. The corrector is incorporated into a FEI Titan Themis³ microscope equipped with a standard SuperTwin objective lens and a high-brightness Schottky-type electron source (FEI X-FEG).

Resource: 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, [PDF], see also the supporting information.