In-line Electron Holography Research Articles

Low-Dose Strain Mapping by Dark-Field Inline Electron Holography

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

A nondamaging electron microscopy approach to map In distribution in InGaN light-emitting diodes

Dark-field inline electron holography and, for comparison, high-resolution transmission electron microscopy are used to investigate the distribution of indium in GaN-based commercial high-efficiency green light-emitting diodes consisting of InGaN multiquantum wells (QWs). Owing to the low electron doses used in inline holography measurements; this technique allows to map the indium distribution without introducing any noticeable electron beam-induced damage which is hardly avoidable in other quantitative transmission electron microscopy methods. Combining the large field of view with a spatial resolution better than 1 nm, we show that the InGaN QWs exhibit random alloy nature without any evidence of nanometer scale gross indium clustering in the whole active region.

Quantitative Analysis of Crystal Defects: Towards 3-Dimensional Imaging of Charge Densities and Atomic Structure by Inline Electron Holography

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Strain Mapping of 45 nm MOSFET by Dark-Field Inline Electron Holography

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

An efficient, simple, and precise way to map strain with nanometer resolution in semiconductor devices

We report on the development of the dark-field inline electron holography technique and its application to map strain in technologically relevant structures, using as an example the strain-engineered gate channel in a 45 nm metal-oxide semiconductor field-effect transistor structure. We show that this technique combines a large field of view of several micrometers with high precision (better than 0.01%), high spatial resolution (better than 1 nm), and very loose experimental requirements not possible with any other technique currently available.

Determination of grain boundary potentials in ceramics: Combining impedance spectroscopy and inline electron holography

Abstract The electrostatic potential arising from charge bound at grain boundary cores in ceramics and the accumulation of space charge in their vicinity is in many cases made responsible for the ion blocking or conducting behavior of electroceramics. While interpretation of impedance spectra of nominally undoped and acceptor doped SrTiO3 ceramics and bicrystals implies that grain boundaries are positively charged and accompanied by fairly wide regions of negative space charge on both sides, a critical analysis of electron holography data available in the literature yields very narrow potential profiles of the opposite sign. This paper attempts to reconcile this apparent discrepancy by showing that within the Mott–Schottky model, impedance data which implies space charge dominated grain boundary resistivity must be interpreted in terms of even wider space charge regions than generally assumed. A review of electron holography results from grain boundaries in SrTiO3 is extended by new results which were obt...

Determination of grain boundary potentials in ceramics: Combining impedance spectroscopy and inline electron holography

Abstract The electrostatic potential arising from charge bound at grain boundary cores in ceramics and the accumulation of space charge in their vicinity is in many cases made responsible for the ion blocking or conducting behavior of electroceramics. While interpretation of impedance spectra of nominally undoped and acceptor doped SrTiO3 ceramics and bicrystals implies that grain boundaries are positively charged and accompanied by fairly wide regions of negative space charge on both sides, a critical analysis of electron holography data available in the literature yields very narrow potential profiles of the opposite sign. This paper attempts to reconcile this apparent discrepancy by showing that within the Mott–Schottky model, impedance data which implies space charge dominated grain boundary resistivity must be interpreted in terms of even wider space charge regions than generally assumed. A review of electron holography results from grain boundaries in SrTiO3 is extended by new results which were obtained by a novel inline electron holography reconstruction algorithm at a near Σ13 grain boundary in nominally undoped SrTiO3. This work is dedicated to the memory of Prof. Rowland M. Cannon.

Off-axis and inline electron holography: Experimental comparison

Electron holography is a very powerful technique for mapping static electric and magnetic potentials down to atomic resolution. While electron holography is commonly considered synonymous with its off-axis variant in the high energy electron microscopy community, inline electron holography is widely applied in low-energy electron microscopy, where the realization of the off-axis setup is still an experimental challenge. This paper demonstrates that both inline and off-axis holography may be used to recover amplitude and phase shift of the very same object, in our example latex spheres of 90 and 200 nm in diameter, producing very similar results, provided the object does not charge under the electron beam.

Off-axis and inline electron holography: A quantitative comparison

Capable of quantitatively imaging static magnetic and electric potentials and even strain electron holography is a very versatile and powerful TEM technique. In this paper we compare off-axis electron holography with a recently developed focal series reconstruction algorithm and phase retrieval based on the transport of intensity equation. Based on theoretical considerations and simulations we compare the different techniques with respect to parameters such as the coherence requirements, field of view, resolution, noise properties, and other required experimental conditions.

Mapping Grain Boundary Potentials by Inline Electron Holography

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

Comparison of off-axis and in-line electron holography as quantitative dopant-profiling techniques

Many different dopant-profiling techniques are available for semiconductor device characterization. However, with length scales shrinking rapidly, only transmission electron microscopy (TEM) techniques promise to fulfil the spatial resolution required for the characterization of future device generations. Here, we use three advanced TEM techniques, off-axis electron holography, Fresnel imaging (in-line electron holography) and Foucault imaging, to examine a focused ion beam-prepared silicon p–n junction device. Experiments are carried out on electrically unbiased samples and with an electrical bias applied in situ in the TEM. Simulations are matched to experimental data to allow quantitative conclusions to be drawn about the underlying electrostatic potential distributions. The off-axis electron holography and Fresnel results are compared to assess whether the techniques are consistent, and whether they can be used to provide complementary information about dopant potentials in semiconductor devices.

The measurement and interpretation of electrostatic potential profiles across grain boundaries in strontium titanate

Results from a systematic Fresnel contrast (in-line electron holography) study of a large number of grain boundary potentials from undoped, donor-doped and acceptor-doped polycrystalline strontium titanate samples are summarised. An assessment is made of whether the measured variations in potential can be used to provide information about space charge distributions at the boundaries, or whether they are instead dominated by local changes in composition, density and specimen thickness. The effects on the measured contrast of both specimen preparation for transmission electron microscopy and irradiation by the high-energy electron beam are also discussed.

Quantitative electron holography of biased semiconductor devices

Off-axis and in-line electron holography have been used to determine the electricalproperties of a silicon p–n junction. Specimens were prepared for transmission electronmicroscopy (TEM) by focused ion beam (FIB) thinning and examined in a biasing holder,with holograms recorded as a function of specimen thickness and applied reverse bias. Thedata revealed the important role played by the surfaces of the thin TEM sample whichaffect the electrostatic potential distribution within the specimen. The presence ofelectrically inactive layers, corresponding to the FIB-damaged amorphous surfaceand an additional crystalline layer, was inferred from the experimental results.Such effects must be taken into account if electron holography is to be used in areproducible and quantitative way for the examination of semiconductor devices.

In-line electron holography with an atomic focuser source

The resolution that may be achieved in an image formed by reconstruction from an in-line electron hologram, without aberration correction, is governed by the source size. Computer simulations show that ultrahigh resolution, of better than 0.05 nm for 100 keV electrons, may be possible if the electron source is a crossover formed at the exit face of a thin atomic-focuser crystal when electrons from a beam focused in a scanning transmission electron microscope (STEM) instrument are channeled along a line of atoms through the crystal. Simulations also show that, because of the channeling effect, the resolution of the reconstructed image is not degraded by translations or oscillations of the STEM beam by 0.1 nm or more. We suggest a scheme for removing the unwanted background and conjugate image from the reconstructed image by combining the in-line holography with an off-axis mode.

Twin Image Suppression and Field Inhomogeneity in In-Line Electron Holography.

Several methods have been suggested for elliminating the twin image in in-line holography, including Franhofer holography as used in our previous work, integration over focus, integration over wavelength, integration over lateral source position and recursion methods such as Gerchberg-Saxton or Fienup (see for all refs.). Fraunhofer holography requires the Fresnel number N =, with d the object size. The integration methods depend on the fact that the real and virtual images have different dependencies on some parameter such as focus, and have proved powerful in photoemission holographic diffraction. Here we apply focus integration and recursion to low voltage, point-projection images of carbon films, and discuss the effects of inhomogeneities in the electric field which arise when the tip is very close to a fibre-shaped sample such as DNA. by embedding biomolecules in solid Xenon at 55K, and using a beam energy of about 9 volts (less than the Xenon bandgap minus the electron affinity), radiation damage to the molecule may be minimized, while the molecule is gently supported by an almost electron transparent medium.

Phase-contrast visualization of an undecagold cluster by in-line electron holography

This paper reports the use of Fraunhofer in-line electron holography to visualize an undecagold cluster, which has been virtually invisible by conventional bright-field transmission electron microscopy (TEM), by taking advantage of the coherent electron beam of high brightness from a 200 kV cold field-emission gun and thus making the full use of phase contrast and avoiding radiation damage. Theoretical and experimental investigations of contrast of an in-line hologram show that phase contrast can be maximized by using an appropriate defocusing distance according to the wavelength of illuminating electrons and the size of the object. A parameter termed Fresnel number is found to be important in considering the optimum experimental conditions, especially when in-line electron holography is applied to weak-phase objects.

Fraunhofer in-line electron holography of frozen-hydrated DNA superhelices

We have visualized DNA molecules in solution by Fraunhofer in-line electron holography. So far, applications of the technique at high resolution have been limited to gold particles. This report is the first application of Fraunhofer in-line electron holography to a nanometer-sized biological specimen.The phase shift of an electron beam due to DNA is so small, estimated as only 2π/33 for 100-keV electrons, that it has been only possible to visualize them with either heavy-atom staining or metal shadowing techniques. However, these methods give only low-resolution images of the molecules, and worst of all, preservation of specimens is poor. The frozen-hydrated specimen preparation improved the preservation of specimen, and images of naturally supercoiled DNA in solution have been visualized using underfocusing of 1-2 μm. Such a defocusing is large enough for such a small object as DNA (diameter ∽ 2 nm) to be in a Fresnel condition;z≈ a2/λ, wherezis the defocusing, a is the diameter of an object, andλis the wavelength.

A high-resolution fraunhofer in-line electron holography

In-line holography is advantageous in terms of contrast in imaging weak phase objects. However, most of the recent articles on electron holography had an off-axis configuration employing an electron biprism and were applied successfully only to objects with a rather strong phase shift such as magnetic flux and thin crystals. When it comes to the observation of considerably weak phase object, the off-axis method fails. We applied Fraunhofer in-line electron holography to observe an undecagold cluster, the core of which has a diameter of 0.82 nm, and obtained a high resolution image with high contrast that has been obtained neither by off-axis electron holography nor by conventional transmission electron microscopy.A Hitachi HF-2000 electron microscope equipped with a 200-kV cold field emission gun was used during the experiment. The specimen, Monoamino-Undecagold (NanoProbes, Inc., Stony Brook, N.Y.), was prepared in a highly dispersive form on a thin carbon film supported by a thick holey carbon film.

In-Line Electron Holography in a Stem Instrument

It was suggested earlier that the idea of in-line electron holography, as proposed by Gabor in 1949 could now be implemented by use of a STEM instrument having a cold field emission gun. Reconstruction from holograms by computer methods, in place of the optical techniques suggested by Gabor, was shown to be effective for one-dimensional theoretically derived holograms. We have now applied the method of computerized reconstruction to digitized shadow image holograms recorded in our HB5 STEM instrument which has been modified to facilitate the process.The shadow image is formed on a phosphor screen in the detector plane. The light produced is amplified by means of an electrostatic image intensifier and viewed with a low-light-level TV camera. The output is digitized and stored by use of a TV-rate frame store and a PDP11/34 computer.

A New Detector System For The HB5 Stem Instrument

The HB5 STEM instrument at ASU has been modified previously to include an efficient two-dimensional detector incorporating an optical analyser device and also a digital system for the recording of multiple images. The detector system was built to explore a wide range of possibilities including in-line electron holography, the observation and recording of diffraction patterns from very small specimen regions (having diameters as small as 3Å) and the formation of both bright field and dark field images by detection of various portions of the diffraction pattern. Experience in the use of this system has shown that sane of its capabilities are unique and valuable. For other purposes it appears that, while the principles of the operational modes may be verified, the practical applications are limited by the details of the initial design.