Showing papers on "Contrast transfer function published in 2013"

Monochromated STEM with a 30 meV-wide, atom-sized electron probe

Abstract: The origins and the recent accomplishments of aberration correction in scanning transmission electron microscopy (STEM) are reviewed. It is remembered that the successful correction of imaging aberrations of round lenses owes much to the successful correction of spectrum aberrations achieved in electron energy loss spectrometers 2-3 decades earlier. Two noteworthy examples of the types of STEM investigation that aberration correction has made possible are shown: imaging of single-atom impurities in graphene and analyzing atomic bonding of single atoms by electron energy loss spectroscopy (EELS). Looking towards the future, a new all-magnetic monochromator is described. The monochromator uses several of the principles pioneered in round lens aberration correction, and it employs stabilization schemes that make it immune to variations in the high voltage of the microscope and in the monochromator main prism current. Tests of the monochromator carried out at 60 keV have demonstrated energy resolution as good as 12 meV and monochromated probe size of ∼1.2 A. These results were obtained in separate experiments, but they indicate that the instrument can perform imaging and EELS with an atom-sized probe <30 meV wide in energy, and that an improvement in energy resolution to 10 meV and beyond should be possible in the future.

Direct quantification of ordering at a solid-liquid interface using aberration corrected transmission electron microscopy.

Abstract: We have used aberration corrected in situ transmission electron microscopy to study the interface between liquid Al and different sapphire facet planes, including quantitative analysis of the degree of residual contrast delocalization, ensuring that the experimental contrast perturbations can be associated with density perturbations in the liquid. The results confirm that the liquid is ordered at the interface, and the degree of ordering varies as a function of the sapphire facet planes, with a decreasing degree of order according to ${0006}g{1\overline{2}10}g{10\overline{1}2}\ensuremath{\ge}{\overline{1}014}$.

Multilayer thin-film phantoms for axial contrast transfer function measurement in optical coherence tomography

Abstract: In optical coherence tomography (OCT), axial resolution is one of the most critical parameters impacting image quality. It is commonly measured by determining the point spread function (PSF) based on a specular surface reflection. The contrast transfer function (CTF) provides more insights into an imaging system’s resolving characteristics and can be readily generated in a system-independent manner, without consideration for image pixel size. In this study, we developed a test method for determination of CTF based on multi-layer, thin-film phantoms, evaluated using spectral- and time-domain OCT platforms with different axial resolution values. Phantoms representing six spatial frequencies were fabricated and imaged. The fabrication process involved spin coating silicone films with precise thicknesses in the 8-40 μm range. Alternating layers were doped with a specified concentration of scattering particles. Validation of layer optical properties and thicknesses were achieved with spectrophotometry and stylus profilometry, respectively. OCT B-scans were used to calculate CTFs and results were compared with convetional PSF measurements based on specular reflections. Testing of these phantoms indicated that our approach can provide direct access to axial resolution characteristics highly relevant to image quality. Furthermore, tissue phantoms based on our thin-film fabrication approach may have a wide range of additional applications in optical imaging and spectroscopy.

Characterization and correction of spherical aberration due to glass substrate in the design and fabrication of Fresnel zone lenses

Abstract: As with a conventional lens, a Fresnel zone lens (FZL) can be used to image objects at infinity or nearby In the latter case, the FZL converts a diverging spherical wavefront into a converging spherical wavefront The glass substrate on which the FZL is fabricated introduces spherical aberration resulting in a shift of the image plane and blurring of the image Two novel schemes for correction of this spherical aberration are proposed and studied in this paper To demonstrate them, FZLs are designed with and without aberration correction They are fabricated using electron beam direct writing The devices are evaluated and the accuracy of the proposed aberration correction schemes is validated

High-resolution environmental transmission electron microscopy: modeling and experimental verification.

Abstract: Quantitative modeling for high-resolution (phase contrast) gas cell environmental transmission electron microscopy (ETEM) imaging is presented in this paper. Concepts of pre-specimen scattering object (PreSO) and post-specimen scattering object (PoSO) are introduced to explain electron scattering caused by gas and window membranes associated with the gas environmental cell (E-cell). PreSO preserves the structural phase information and the effect can be evaluated by averaging the contrast transfer functions (CTFs) over random electron scattering. PoSO is treated as information loss and the unscattered electrons play a major role in determining the ETEM image quality. The theoretical model is compared and matched well with our systematic gas ETEM experimental results under various gas pressures. Extension of our approach to the aberration-corrected ETEM is discussed.

Phase Plate for a TEM

Abstract: A phase plate, specifically a Zernike type phase plate, for use in an electron microscope comprises a central hole, and a thin film causing a phase shift of the electrons passing through said film. This phase shift causes the Contrast Transfer Function (CTF) to change from a sine-like function to a cosine-like function. The phase plate is equipped with a film in the form of an annulus, carried by a much thinner film. As a result only in a small spatial frequency range (for low frequencies) the phase is changed (and thus the CTF), and for other spatial frequencies the phase shift is negligible, and thus the CTF remains unchanged. Due to the much smaller thickness of the carrier film the scattering of electrons is negligible as well.

High-resolution imaging of zeolite with aberration-corrected transmission electron microscopy

Abstract: We demonstrated high-resolution imaging of atomic columns in zeolite frameworks with spherical aberration-corrected transmission electron microscopy (AC-HRTEM). An MFI-type zeolite was observed by AC-HRTEM with optimized optical setup. Compared with the conventional imaging mode based on a positive spherical aberration, the negative spherical aberration imaging (NCSI) mode sharpened image contrasts at atomic column positions. The projected atomic columns of zeolite are so complex that sharp image contrast can help to distinguish each atomic column position.

Contrast curves of five different intraoral X-ray sensors: a technical note

Abstract: Objectives The aim of this article is to compare the contrast and resolution properties of five different sensors using a known technical target. Methods Stripe patterns with defined amounts of line pairs per mm (2.5-20 LP/mm) were recorded using five commercial digital sensors. Image data were analyzed using ImageJ and MatLab to calculate different contrast curves using logistic regression. Results The Dexis Platinum Sensor reached a calculated 10% contrast at 29.52 LP/mm. The Duerr VistaRay 6 Sensor reached a 10% contrast at 9.9 LP/mm. The 10% contrast was found at 18.8 LP/mm for the Duerr VistaRay 7. The Sirona Xios+ Sensor reached a calculated 10% contrast at 13.9 LP/mm. The Sirona Fullsize charge-coupled device (CCD) Sensor exhibited 10% contrast at 10.3 LP/mm. Conclusions The contrast transfer function assessment used in the study confirmed that the spatial frequency at 10% contrast was much lower than the theoretical resolution computed from the pixel size.

Solutions with precise prediction for thermal aberration error in low-k1 immersion lithography

Abstract: Thermal aberration becomes a serious problem in the production of semiconductors for which low-k1 immersion lithography with a strong off-axis illumination, such as dipole setting, is used. The illumination setting localizes energy of the light in the projection lens, bringing about localized temperature rise. The temperature change varies lens refractive index and thus generates aberrations. The phenomenon is called thermal aberration. For realizing manufacturability of fine patterns with high productivity, thermal aberration control is important. Since heating areas in the projection lens are determined by source shape and distribution of diffracted light by a mask, the diffracted pupilgram convolving illumination source shape with diffraction distribution can be calculated using mask layout data for the thermal aberration prediction. Thermal aberration is calculated as a function of accumulated irradiation power. We have evaluated the thermal aberration computational prediction and control technology “Thermal Aberration Optimizer” (ThAO) on a Nikon immersion system. The thermal aberration prediction consists of two steps. The first step is prediction of the diffraction map on the projection pupil. The second step is computing thermal aberration from the diffraction map using a lens thermal model and an aberration correction function. We performed a verification test for ThAO using a mask of 1x-nm memory and strong off-axis illumination. We clarified the current performance of thermal aberration prediction, and also confirmed that the impacts of thermal aberration of NSR-S621D on CD and overlay for our 1x-nm memory pattern are very small. Accurate thermal aberration prediction with ThAO will enable thermal aberration risk-free lithography for semiconductor chip production.

Scanning confocal electron energy-loss microscopy using valence-loss signals.

Abstract: Finding a faster alternative to tilt-series electron tomography is critical for rapidly evolving fields such as the semiconductor industry, where failure analysis could greatly benefit from higher throughput. We present a theoretical and experimental evaluation of scanning confocal electron energy-loss microscopy (SCEELM) using valence-loss signals, which is a promising technique for the reliable reconstruction of materials with sub-10-nm resolution. Such a confocal geometry transfers information from the focused portion of the electron beam and enables rapid three-dimensional (3D) reconstruction by depth sectioning. SCEELM can minimize or eliminate the missing-information cone and the elongation problem that are associated with other depth-sectioning image techniques in a transmission electron microscope. Valence-loss SCEELM data acquisition is an order of magnitude faster and requires little postprocessing compared with tilt-series electron tomography. With postspecimen chromatic aberration (C c) correction, SCEELM signals can be acquired in parallel in the direction of energy dispersion with the aid of a physical pinhole. This increases the efficiency by 10×-100×, and can provide 3D resolved chemical information for multiple core-loss signals simultaneously.

Modeling of Image Formation in Cryo-Electron Microscopy

Abstract: Knowledge of the structure of biological specimens is crucial for understanding life. Cryo-electron microscopy (cryo-EM) permits structural studies of biological specimen at their near-native state. The research performed in this thesis represents one of two subprojects of the FOM industrial partnership program with FEI Company. The common aim is to obtain higher resolution in cryo-EM of biological specimens. Currently, the resolution is limited by: i) the noise and blurring of the detector; ii) the oscillatory and dampening character of the contrast transfer function (CTF) originating from defocusing which is employed to produce contrast; and iii) the radiation damage which limits the integrated electron flux that can be used, resulting in images with poor signal-to-noise ratio. Simulation of image formation (forward modeling) provides possibility to easily and cost-effectively investigate the influence of a certain physical parameter on the final image. The main goal of this thesis is to improve our understanding of the relevant physical processes that govern image formation and to develop a quantitative forward model. Such a model is essential for optimizing the acquisition strategy, assisting the regularization (introduction of prior information) in the 3D reconstruction, improving image interpretation, and achieving a resolution beyond the limits imposed by the oscillatory CTF. This thesis addresses the following challenges: i) construction of the electron-specimen interaction potential based on elastic and inelastic electron scattering properties and adequate description of the electron propagation through the specimen; ii) accurate estimation of the CTF parameters, in particular defocus and astigmatism and their uncertainties, iii) characterization of the detector including all relevant statistics; iv) better understanding of certain aspects of radiation damage such as specimen heating, dose-rate effects, and beam-induced movements. The validation of forward model is based on a systematic comparison between simulated and experimental images under various experimental conditions. All parameters are based on physical principles. Defocus and astigmatism as well as detector parameters are accurately estimated from independent measurements using the methods developed in this thesis. Software tools for image simulations, accurate defocus and astigmatism estimation and detector characterization have been developed and are freely available for non-commercial use. The theory and methods presented in this thesis form the essence of an expert system that would optimize the data collection strategy. Furthermore, the influence of new hardware components could be inexpensively and efficiently investigated.

High-Resolution TEM Imaging

Abstract: Spatial resolution is important for any microscopy. This chapter presents the theory, technique, and examples of achieving the ultimate resolution of a transmission electron microscope with the method of “high-resolution transmission electron microscopy.” Recall (Sect. 2.3.3) that the HRTEM image is an interference pattern between the forward-scattered and diffracted electron waves from the specimen. Interference patterns require close attention to the phases of the waves. While the ray optics approach is useful for a few geometrical arguments, the most important issues in HRTEM are best understood in terms of the phase of the electron wavefront and how this phase is altered by the specimen and by the objective lens. The specimen itself is approximated as an object that provides phase shifts to the electron wavefront, sometimes in proportion to its scattering potential. The method of HRTEM also demands close attention to the performance of the objective lens and other characteristics of the microscope.

An improved phase retrieval algorithm for X-ray in-line phase-contrast imaging

Abstract: Phase contrast imaging technique has been improved promptly in recent years. Among these techniques in-line phase contrast imaging is widely used. Various algorithms for in-line phase retrieval have been proposed so far such as TIE (transport of intensity equation), CTF (contrast transfer function), first Born-approximations, GSF (Gerchberg-Saxton-Fienup) and etc. Bronnikov’s algorithm (BA) is a type of linear algorithm that is simple and efficient. But it can only be used for no absorption situations. In this paper an improved algorithm based on BA is presented. The approach adds a Δϕ(x,y) to the phase map ϕ b (x,y) retrieved by BA to make the reconstructed phase map more precise. Further, the approach is evaluated on simulated images and confirmed to be accurate at higher absorption rates.

Critical importance of the correction of contrast transfer function for transmission electron microscopy-mediated structural biology

Abstract: Transmission electron microscopy (TEM) is an excellent tool for studying detailed biological structures. High-resolution structure determination is now routinely performed using advanced sample preparation techniques and image processing software. In particular, correction for contrast transfer function (CTF) is crucial for extracting high-resolution information from TEM image that is convoluted by imperfect imaging condition. Accurate determination of defocus, one of the major elements constituting the CTF, is mandatory for CTF correction. To investigate the effect of correct estimation of image defocus and subsequent CTF correction, we tested arbitrary CTF imposition onto the images of two-dimensional crystals of Rous sarcoma virus capsid protein. The morphology of the crystal in calculated projection maps from incorrect CTF imposition was utterly distorted in comparison to an appropriately CTF-corrected image. This result demonstrates critical importance of CTF correction for producing true representation of the specimen at high resolution.

Image acquisition method and transmission electron microscope

Abstract: PROBLEM TO BE SOLVED: To provide an image acquisition method that enables acquisition of information on a wide frequency band.SOLUTION: An image acquisition method relating to the present invention, which is used for a transmission electron microscope, includes the steps of: suppressing attenuation of a contrast transfer function based on an envelope function, by setting at least one of a spherical aberration coefficient and a chromatic aberration coefficient of an imaging system of the transmission electron microscope; and acquiring an image by the imaging system under a defocus condition.

Simultaneous Dual-Color Fluorescence Microscope: A Characterization Study

Abstract: Background: High spatial resolution and geometric accuracy is crucial for chromosomal analysis of clinical cytogenetic applications. High resolution and rapid simultaneous acquisition of multiple fluorescent wavelengths can be achieved by utilizing concurrent imaging with multiple detectors. However, such class of microscopic systems functions differently from traditional fluorescence microscopes. Objective: To develop a practical characterization framework to assess and optimize the performance of a high resolution and dual-color fluorescence microscope designed for clinical chromosomal analysis. Methods: A dual-band microscopic imaging system utilizes a dichroic mirror, two sets of specially selected optical filters, and two detectors to simultaneously acquire two fluorescent wavelengths. The system’s geometric distortion, linearity, the modulation transfer function, and the dual detectors’ alignment were characterized. Results: Experiment results show that the geometric distortion at lens periphery is less than 1%. Both fluorescent channels show linear signal responses, but there exists discrepancy between the two due to the detectors’ non-uniform response ratio to different wavelengths. In terms of the spatial resolution, the two contrast transfer function curves trend agreeably with the spatial frequency. The alignment measurement allows quantitatively assessing the cameras' alignment. A result image of adjusted alignment is demonstrated to show the reduced discrepancy by using the alignment measurement method. Conclusions: In this paper, we present a system characterization study and its methods for a specially designed imaging system for clinical cytogenetic applications. The presented characterization methods are not only unique to this dual-color imaging system but also applicable to evaluation and optimization of other similar multi-color microscopic image systems for improving their clinical utilities for future cytogenetic applications.

Imaging with aberration-corrected low energy electron microscopy

Abstract: In this thesis we discuss imaging with aberration-corrected Low Energy Electron Microscopy in theory as well as in the experiment.

Analysis of the effect of central screening of the pupil on the contrast transfer function when there is residual wave aberration in a point image

Abstract: Numerical modelling methods are used to show that, when there is residual wave aberration that exceeds three-fourths of the wavelength (W≥0.75λ), it is possible to enhance the image contrast by screening the central zone of the pupil with a screening factor of ks≈0.5.

Experimental and Simulated Electron Microscopy in the Study of Metal Nanostructures

Abstract: The technology supporting the development of electron microscopy systems has reached such a high level that it is currently possible to investigate in a direct way the structure and composition of nanostructures at a sub-Angstrom resolution. Specifically, with the use of ultra-high resolution high angular annular dark field scanning transmission electron microscopy (HAADF-STEM) in the study of metal nanoalloys, it is now possible to describe in detail how two or more metals arrange themselves at atomistic level to form nanoparticles. In this chapter, we describe how the use of HAADF-STEM in conjunction with numerical simulations of the STEM process can be used as a direct way of analyzing chemical composition and structure at the nanoscale, and how the information obtained by this approach can be used later to investigate how local composition, atomistic arrangement, and shape correlate with the physicochemical properties of the nanostructure.