Hannes Lichte

Electron holography approaching atomic resolution

Abstract Image plane off-axis holograms are recorded at atomic resolution by means of an electron biprism; numerical and light-optical reconstruction schemes are compared, and experimental results are presented including the correction of spherical aberration.

Optimum focus for taking electron holograms

Abstract Conventional electron microscopy is heavily restricted by the fact that there are no wave optical techniques for the analysis of the electron object wave available other than defocusing; the optimum phase contrast is realized at Scherzer focus. Electron holography offers a way around the problems in that it divides the imaging process in two separate steps: the first is to collect the maximum amount of object information in a hologram taken in the electron microscope, the second is to retrieve all the collected information by means of a rather unrestricted wave optical analysis performed in a computer. The main question is now, at which focus do we collect the maximum amount of information in the hologram?

Electron holography: I. Can electron holography reach 0.1 nm resolution?

Abstract The idea of solving many of the basic problems in electron microscopy by means of electron holography is straightforward. However, with the progress in experimental realization of electron holography at high resolution, severe difficulties show up. Presently, the major obstacle in reaching 0.1 nm resolution seems to be the imprecise knowledge about the aberrations of the microscope.

Parameters for high-resolution electron holography

Abstract Theoretically, the idea of electron holography is straightforward: the electron image wave is recorded in a hologram and reconstructed by subsequent processing according to the laws of wave optics. However, in particular at high resolution, the experimental realization is restricted, e.g. by the limited coherence, i.e. the brightness of electron beams, and by the limited pixel number available with todays image-processing systems. By careful consideration of the role of the parameters during taking and reconstructing a high-resolution hologram, nevertheless, a resolution limit of 0.1 nm seems to come within reach with available technology.

Electron holography: II. First steps of high resolution electron holography into materials science

Abstract By means of electron holography, the complex electron wave is transferred from the electron microscope to a computer. Consequently, all desirable wave-optical procedures can be numerically applied in a very flexible way to extract and to analyze quantitatively the amplitude and the phase of the object exit wave.

Electron holograms for subångström point resolution

Abstract Using the Mollenstedt-type electron biprism [Mollenstedt and Duker, Z. Phys. 145 (1956) 377] in a Philips EM 420/ST with field emission gun we succeed in taking image holograms with a fringe spacing of 0.032 nm. This is the finest fringe spacing hitherto reported allowing the transfer of periods smaller than 0.1 nm by means of electron holography hence giving a point resolution below 0.1 nm after correction of aberration.

0.1 nm information limit with the CM30FEG-special Tübingen

The Philips CM30FEG electron microscope specially developed for the needs of atomic resolution electron holography was installed in Tubingen. A source brightness of 5×108A⧸(cm2sr) was determined by evaluation of the fringe visibility. From the calculated phase contrast transfer function for an optimum focus of 7.18 (Csλ)12 an information limit of 0.08 nm was estimated, 0.1 nm was measured by means of diffractometry. The experimental tests so far reveal that the requirements for reaching 0.1 nm by holography are met.

High-resolution electron holography with the CM30FEG-special Tübingen

Abstract During the BRITE/EURAM project, a Philips CM30FEG microscope with special features for high-resolution electron holography has been developed and tested [D. Van Dyck, in: Proc. 49th Annual EMSA Meeting, p. 448; H. Lichte et al., Ultramicroscopy 52 (1993) 575]. The presented first experimental results with Si [110] reveal that information far beyond the point resolution limit of the microscope can be retrieved using the holographic off-axis reconstruction technique.

Density correction of photographic material for further image processing in electron microscopy

Abstract Digitizing images recorded on photographic material yields images different from those recorded using a CCD camera directly on the electron microscope. This is due to the non-linear density curve of the photographic material, the “caller” effect, and the non-linear relation between optical density and transmitted intensity. Using a coherent electron source and a Mollenstedt biprism in the microscope, it is possible to compensate those non-linear effects after digitization if one additional ”reference” inage is taken. All photographically recorded images must be developed in one and the same process. Measuring carefully the illumination intensity in the digitizer allows to compensate for the non-linear transmitted intensity. Evaluation of the reference image then allows to compensate the effect of the optical density curve of the photographic material if the reference image has been digitized under the same conditions as the image of interest. The efficiency of the procedure is demonstrated for off-axis electron holography. With respect to linearity the images thus obtained are close to the images obtained from a CCD camera on the microscope.

Electron holography with a superconducting objective lens

Abstract The design of a transmission electron microscope with a superconducting objective lens and a field-emission gun operating at 80–100 kV is described. The microscope, equipped with an electrostatic biprism is designed for the holographic analysis of objects at liquid helium temperatures, e.g. to make use of the cryoprotection of biological objects from radiation damage. Special care was taken to achieve a high electrical and mechanical stability of the microscope. Fundamental phase resolution limits are discussed as well as the construction and performance of the instrument.