Wolf-Prize 2011 for Aberration-Correction: A Breakthrough in Electron Microscopy

Three famous physicists, Harald H. Rose (Ulm University, SALVE member), Maximillian Haider (CEOS GmbH, SALVE member) and Knut W. Urban (Ruska Centre Jülich) have received the Wolf-prize in physics 2011 for the realization of aberration-corrected electron microscopy. This highly prestigious prize is an award for special achievements to the greatest benefit of mankind and honors the scientists for their novel and creative research.

Harald H. Rose [1] set forth the legendary work of Otto Scherzer [2] on aberration correction by designing a novel corrector formed by hexapole elements and round lenses. This corrector produces a negative 3rd-order spherical aberration which can be adjusted in such a way to compensate for the positive spherical aberration of the whole system. Maximilian Haider has built and successfully incorporated the so-called hexapole corrector in a 200 kV transmission electron microscope forming artifact-free images with atomic resolution. The microscope was then moved from Heidelberg to Jülich, Germany for application in materials science. The impressive results obtained by Knut W. Urban and co-workers are considered as a quantum step in high-resolution electron microscopy (HREM) enabling genuine atomic resolution.

1. Wolf-Prize Winners Reached First Significant Improvements in AC-HR-TEM

The first successful correction of the 3rd-order spherical aberration of round lenses was realized in 1997 [3] by means of the hexapole corrector [1]. The incorporation of multipole elements for eliminating the unavoidable spherical aberrations of round lenses is based on an ingenious idea by Scherzer [2], in 1947. His electrostatic Cs-corrector for compensating the spherical aberration consisted of round lenses, cylinder lenses, and octopoles. The system was built and tested by Seeliger in 1953 [4] and subsequently by Möllenstedt [5]. About 20 years later, it has been demonstrated by Hawkes (1965) [6], Beck (1979) [7], Crewe and Kopf 1980 [8], Crewe 1982 [9], Rose (1981) [10], Shao (1988) [11] and Chen and Mu (1991) [12] that hexapoles are also candidates for Cs-correction because their secondary effects produce rotationally symmetric 3rd-order aberrations. In particular the spherical aberration has opposite sign to that of round lenses. Therefore, if the large primary effects of hexapoles could be nullified, it should be possible to use these elements in a corrector compensating for the unavoidable spherical aberration of round lenses. The problem was solved, in 1990, by Rose [1] who proposed the semi-aplanat consisting of the objective lens, two hexapoles, and two telescopic round-lens transfer doublets. M. Haider immediately conceived this design as being practical. At the end of 1990, Haider, Rose, and Urban (Fig. 1) submitted a joint grant application to the German Volkswagen Foundation to obtain the necessary funds for constructing the corrector. The successful realization of the corrector by Haider in 1997 improved for the first time the resolution of the 200 kV TEM by a factor of about 2 from 0.24 nm to 0.13 nm, thereby achieving genuine atomic-resolution images free of artifacts known as the delocalization of the image contrast [2, 13]. The corrector was made available commercially by the company CEOS. Nowadays, this corrector is incorporated in the corrected electron microscopes of TEM manufacturers around the world (FEI, Hitachi, JEOL, ZEISS).

With these developments [1, 2, 13], the spherical aberration coefficient Cs has become an adjustable parameter which can be set to arbitrary positive or negative values, at least in principle. The direct Cs-correction avoids successfully delocalization of the image contrast provided that the residual aberrations of the imaging system, such as 2-fold astigmatism, 2nd-order coma and 3-fold astigmatism, to name only a few, are corrected or suppressed sufficiently [14 - 16]. If the contrast transfer function is made positive (by adjusting both the spherical aberration and the defocus), the phase contrast is negative. In this case, images of atoms appear as bright spots with enhanced contrast, which is favorable for detecting even light-atom columns close to heavy-atom columns, as shown in Fig. 2 [14 - 17]. In 2008, Takai et al. have proposed a novel a posteriori Cs-correction procedure based on focus modulation [18]. Fig. 3 shows their result for the TEM image of a gold nano-particle together with the image (uncorrected) taken at Scherzer defocus. For a direct reduction of delocalization using the CEOS CETCOR aberration corrector see Fig. 4 taken from Inamoto et al. 2010 [19].

Within the last 5 years since the commercialization of aberration-corrected electron microscopy, more than 200 of these instruments have been ordered by universities, research institutes and industrial research laboratories all over the world [20]. Aberration-corrected electron microscopy has let to new generations of instruments with enormous improvement in imaging resolution, stability and spectroscopic capabilities [2, 15 - 18, 21].

2. Wolf-Prize Winners Enable Next Generation of AC Instrumentation and New Projects

The successful Volkswagen project has triggered further research projects on aberration-correction such as:

1. The German project Sub-Angstrom-Transmission-Electron-Microscope (SATEM) [22] has developed a new ultra stable Cs-corrected 200 kV electron microscope column and achieved sub angstrom resolution of better than 0.09 nm. The SATEM instrument is compensated for spherical aberration and reduces the chromatic aberration disc by means of a monochromator [23].
2. In 2000, the first coordinator Murray Gibson of the U.S. Transmission Electron Achromatic Microscope (TEAM) project invited Harald H. Rose, to design a corrector compensating for spherical aberration (Cs), chromatic aberration (Cc), and off-axial coma. The correction of chromatic aberration was first shown experimentally by Hardy in 1967 [24] by means of a proof of principle and later in the aberration-corrected microscope developed within the frame of the so-called Darmstadt Project. The corrector was designed by Rose in 2002. By imposing symmetry conditions on the multipole fields and on the course of the fundamental paraxial rays, he succeeded to cancel out a large number of aberrations. Using this procedure, a feasible corrector for the TEAM microscope [25, 26] was designed. Rose achieved the largest reduction of aberrations by imposing symmetry conditions on the system as a whole and on each half of it. The final design, construction, and implementation of the corrector were performed by Haider and co-workers at the CEOS company [27]. The novel Cs/Cc multipole corrector is commercially available and incorporated in the TEAM 1.0 microscope [28].
3. The German project Sub-Å Low-Voltage Electron microscopy (SALVE) concerns the development of a Cs/Cc-corrected TEM operating at low accelerating voltages in the range between 20 and 50 kV [29 - 31]. First results at 20 kV with Cs-correction and monochromator are shown on the SALVE website and by Kaiser et al. 2011 [31].

3. Future Trends of Aberration Corrected Electron Microscopy

Wolf-Prize winner Harald H. Rose describes the future trends for achieving sub-Å resolution at low acceleration voltages in the transmission electron microscope as follows: "we need to correct the spherical and chromatic aberrations of the imaging system and we must increase mechanical and electrical stability to reduce the information limit [30]".

If we look back on the last 3 years of research on aberration correction, we see a strong increase of aberration corrected low-voltage electron microscopy, as illustrated in Fig. 5. This figure has been obtained on the basis of available publications using S/TEM and hard-ware aberration correction. Dahmen et al. 2009 says: "Now tunable electron optical components and optimum electron beam voltage [†] can be used. The TEAM instrument is based on the community’s earlier consensus that the voltage range between 80-300 kV covers the important region determined by the balance between usable sample thickness, tolerable radiation damage, and achievable resolution. However, this balance is shifting (because the additional correction of chromatic aberration enables low-voltage microscopy with high resolution [30]). At the same time, many future samples may no longer be foils thinned down from bulk materials, but small nanoparticles or molecules supported on single-atom thick sheets such as graphene or similar compounds. For such samples, aberration-corrected imaging at much lower voltages may become advantageous. In view the greatly improved S/N of the new generation of electron microscopes including new electron-detection strategies, it will be important to re-examine the sensitivity of objects to radiation damage, especially as light elements are becoming accessible [29, 32]."

In this context, three advanced electron microscope manufactures (JEOL, Nion Corp. and Zeiss) have recently started developing low-voltage S/TEM instruments equipped with novel aberration correctors optimized for low voltages [33 - 35]. So far, none of these instruments is commercially available yet. The first performance of a spherical and chromatic aberration-corrected transmission electron microscopy operating at 30 kV was shown at the Microscopy Society of America's annual meeting in Nashville, Tennessee, August 7-11, 2011 by Sasaki et al. [36].

4. Potential New Applications with Fully Corrected Low-Voltage Electron Microscopy

To identify the classes of materials which may benefit the most from low acceleration voltages, the central task is to understand and minimize radiation damage. Two classes of materials might benefit the most from low voltages:

1. Electrically and thermally conducting solid materials, such as metals, nanocarbons and graphene [e. g. 37 - 42].
2. Insulators, and organic materials provided that the increase in ionization damage for a fixed dose is compensated by the increase in contrast and fast electron detection [31, 42].

However, the imaging of very light atoms such as Li and weakly bond structures is a challenge for state-of-the-art electron microscopy techniques. With the actual SALVE technology, disposed by CEOS and Zeiss NTS, we come a step closer to the aim of the investigation of light atoms, a goal that has been first formulated by Jia et al. in 2003, see Fig. 2 [14]. Others have also come to this result [32, 44 - 47].

[†] Note that optimum aberration-corrected instruments cover a voltage range below 100 kV.

[‡] For evaluation we have used literature until the end of 2011, see the "Publication list for AC-(S)TEM for 20-80 kV" and "Publication list for AC-(S)TEM for 100-300 kV".