Showing papers on "Contrast transfer function published in 2008"

Quantitative comparison of direct phase retrieval algorithms in in-line phase tomography

Abstract: A well-known problem in x-ray microcomputed tomography is low sensitivity. Phase contrast imaging offers an increase of sensitivity of up to a factor of 10(3) in the hard x-ray region, which makes it possible to image soft tissue and small density variations. If a sufficiently coherent x-ray beam, such as that obtained from a third generation synchrotron, is used, phase contrast can be obtained by simply moving the detector downstream of the imaged object. This setup is known as in-line or propagation based phase contrast imaging. A quantitative relationship exists between the phase shift induced by the object and the recorded intensity and inversion of this relationship is called phase retrieval. Since the phase shift is proportional to projections through the three-dimensional refractive index distribution in the object, once the phase is retrieved, the refractive index can be reconstructed by using the phase as input to a tomographic reconstruction algorithm. A comparison between four phase retrieval algorithms is presented. The algorithms are based on the transport of intensity equation (TIE), transport of intensity equation for weak absorption, the contrast transfer function (CTF), and a mixed approach between the CTF and TIE, respectively. The compared methods all rely on linearization of the relationship between phase shift and recorded intensity to yield fast phase retrieval algorithms. The phase retrieval algorithms are compared using both simulated and experimental data, acquired at the European Synchrotron Radiation Facility third generation synchrotron light source. The algorithms are evaluated in terms of two different reconstruction error metrics. While being slightly less computationally effective, the mixed approach shows the best performance in terms of the chosen criteria.

Development of aberration-corrected electron microscopy.

Abstract: The successful correction of spherical aberration is an exciting and revolutionary development for the whole field of electron microscopy. Image interpretability can be extended out to sub-Angstrom levels, thereby creating many novel opportunities for materials characterization. Correction of lens aberrations involves either direct (online) hardware attachments in fixed-beam or scanning TEM or indirect (off-line) software processing using either off-axis electron holography or focal-series reconstruction. This review traces some of the important steps along the path to realizing aberration correction, including early attempts with hardware correctors, the development of online microscope control, and methods for accurate measurement of aberrations. Recent developments and some initial applications of aberration-corrected electron microscopy using these different approaches are surveyed. Finally, future prospects and problems are briefly discussed.

Diffractive imaging of the dumbbell structure in silicon by spherical-aberration-corrected electron diffraction

Abstract: The dumbbell structure in crystalline silicon as known with the separation of 0.136 nm has been reconstructed clearly by diffractive imaging using an electron beam. The spatial resolution in the result is estimated at about 0.1 nm. By utilizing the selected area diffraction technique in a spherical-aberration-corrected transmission electron microscope, one can reconstruct nanostructures with atomic resolution, even if they are not surrounded by empty space such as localized structures embedded in thin film samples. This means that the present method has a unique potential to expand the versatility of diffractive imaging by electron beams drastically.

Effect of primary spherical aberration on high-numerical-aperture focusing of a Laguerre-Gaussian beam.

Abstract: Effect of primary spherical aberration on the tight focusing of linearly and circularly polarized Laguerre-Gaussian (LG) beams is studied by using the vectorial Debye integral. Results are presented for the intensity distribution and square of the polarization components. In the case of the linearly polarized LG beam with unit and double topological charge, the presence of aberration reduces the residual intensity at the focal point and spreads the sidelobes. If the beam is circularly polarized, the aberration results in an increase in the size of the dark core along with a reduction in the intensity at the periphery of the bright ring. The effect of aberration is also discussed in the context of the fluorescent spot size in the focal plane of a stimulated-emission-depletion microscope.

Image contrast in X-ray reflection interface microscopy: comparison of data with model calculations and simulations.

Abstract: The contrast mechanism for imaging molecular-scale features on solid surfaces is described for X-ray reflection interface microscopy (XRIM) through comparison of experimental images with model calculations and simulated measurements. Images of elementary steps show that image contrast is controlled by changes in the incident angle of the X-ray beam with respect to the sample surface. Systematic changes in the magnitude and sign of image contrast are asymmetric for angular deviations of the sample from the specular reflection condition. No changes in image contrast are observed when defocusing the condenser or objective lenses. These data are explained with model structure-factor calculations that reproduce all of the qualitative features observed in the experimental data. These results provide new insights into the image contrast mechanism, including contrast reversal as a function of incident angle, the sensitivity of image contrast to step direction (i.e. up versus down), and the ability to maximize image contrast at almost any scattering condition defined by the vertical momentum transfer, Q(z). The full surface topography can then, in principle, be recovered by a series of images as a function of incident angle at fixed momentum transfer. Inclusion of relevant experimental details shows that the image contrast magnitude is controlled by the intersection of the reciprocal-space resolution function (i.e. controlled by numerical aperture of the condenser and objective lenses) and the spatially resolved interfacial structure factor of the object being imaged. Together these factors reduce the nominal contrast for a step near the specular reflection condition to a value similar to that observed experimentally. This formalism demonstrates that the XRIM images derive from limited aperture contrast, and explains how non-zero image contrast can be obtained when imaging a pure phase object corresponding to the interfacial topography.

Aberration fields of a combination of plane symmetric systems

Abstract: By generalizing the wave aberration function to include plane symmetric systems, we describe the aberration fields for a combination of plane symmetric systems. The combined system aberration coefficients for the fields of spherical aberration, coma, astigmatism, defocus and distortion depend on the individual aberration coefficients and the orientations of the individual plane symmetric component systems. The aberration coefficients can be used to calculate the locations of the field nodes for the different types of aberration, including coma, astigmatism, defocus and distortion. This work provides an alternate view for combining aberrations in optical systems.

Chapter 8 Aberration-Corrected Imaging in Conventional Transmission Electron Microscopy and Scanning Transmission Electron Microscopy

Abstract: This chapter reviews and compares the operation of aberration corrected instruments in both the conventional transmission electron microscopy (CTEM) and scanning transmission electron microscope (STEM) configurations from a theoretical perspective. In addition to correcting the spherical aberration, electron optical correctors provide control over many other aberrations and may actually introduce certain higher-order aberrations if not optimized. Hence, achieving the highest possible resolution requires careful adjustment of the corrector settings, which are detailed for the CTEM and STEM geometries. Understanding this optimization also requires knowledge of the effects of partial coherence on the imaging process and a comparison of coherence effects in CTEM and STEM is also presented.

Boron Observation in p-Type Silicon Device by Spherical Aberration Corrected Scanning Transmission Electron Microscope

Abstract: The boron detection in p-type metal oxide silicon (MOS) device had long been pursued with electron microscopy based analytical tools, but was not successful because of weak signal and interference from the matrix. With spherical aberration corrected electron microscope with newly designed sample holder, mesh and focused ion beam (FIB) damage removing process, boron in the extension area of the p-type MOS device was detected. This enables us to visualize the dopant distribution in silicon devices, which is indispensable to analyze transistor characteristics.

Chapter 7 Novel Aberration Correction Concepts

Abstract: The motivation for the development projects for spherical aberration (Cs) correction in electron microscopy was the expected improvement in resolution. This improvement could be demonstrated for transmission electron microscopes (TEM) and scanning transmission electron microscopes (STEM). Surprisingly, the more important benefit of Cs correction for many materials science investigations was interpretability. Especially, contrast delocalization effectively prevents structural interpretation at interfaces and defects for uncorrected TEMs. Cs correction can reduce contrast delocalization below the information limit. A similar development can be expected for chromatic aberration (Cc) correction, which is probably more effective in improving resolution than Cs correction because it influences the damping envelope of temporal coherence, which defines the information limit. Cc correction will have a strong impact on a large variety of topics, such as in situ TEM, analytical electron microscopy, and electron microscopy of biological objects.

Using Contrast Transfer Function to Evaluate the Effect of Motion Blur on Microscope Image Quality

Abstract: Scanning of microscope slides is an important part of cytogenetic diagnosis. Metaphase chromosomes arranged in a karyotype reveal the nature and severity of cancer and other diseases. Searching for metaphases spreads is a lengthy and tedious process that can benefit from computer aided systems. When slides are searched by such systems in continuous motion, the image quality is reduced. The motion blur is a function of the scan speed, the camera frame rate and sample time, and the level of magnification. In this study, normalized contrast transfer function (CTF) is used to define the amount of image degradation.

Aberration correction for electron beam inspection, metrology, and lithography

Abstract: This article investigates, with computer simulations, whether electron optical aberration correctors could be used to improve the performance of electron beam equipment for the semiconductor manufacturing industry. The simulations are performed using the differential algebraic method. Three types of aberration corrector are investigated: (1) a quadrupole-octopole corrector for critical dimension scanning electron microscopy for metrology and inspection (it is shown that this type of corrector, which corrects spherical and chromatic aberrations, can provide a smaller probe diameter with a larger numerical aperture, thereby improving resolving power and throughput), (2) a hexapole planator for projection electron beam lithography (it is demonstrated that field curvature, astigmatism, and spherical aberration can be corrected, thereby permitting a larger field size), and (3) a mirror corrector for reflective electron beam lithography (it is shown how field curvature and chromatic aberration in such systems can be corrected by using an electron mirror).

Aberration corrected TEM: current status and future prospects

Abstract: Aberration correction leads to a substantial improvement in the interpretable resolution of Transmission Electron Microscopes. Electron optical correctors based on two strong hexapole elements linked through a round lens transfer doublet enables direct correction of all axial aberrations to third order. Subsequent, indirect computational analysis of a focal or tilt series of images offers the possibility of further compensation of the axial aberrations to fifth order. This paper describes 1st and 2nd generation aberration corrected instrumentation installed in Oxford and also the use of combinations of direct and indirect correction / compensation in a variety of different geometries to achieve specimen exit plane wavefunctions containing directly interpretable structural information significantly below 0.1 nm.

CHAPTER 10 - Spherical Aberration-Corrected Transmission Electron Microscopy for Nanomaterials

Abstract: This chapter presents the application of Spherical Aberration (Cs)-corrected high resolution electron microscopy (HREM) to nano-materials. This chapter presents overview on Cs-corrected transmission electron microscopy (TEM)/ Scanning transmission electron microscopy (STEM) by using hexapole and quadrupole-octupole correctors is provided and their advantages are shown primarily by experimental data obtained by the author and other groups in Japan and elsewhere for observation of nanomaterials. The related electron optics and detailed aberration correction methods in the present state of the art are also described in this chapter. One basic interest in Cs-corrected HREM is determining the limitation of resolution in TEM and STEM by quantum mechanics. The minimum size of an exit wave function for TEM imaging can be calculated as a scattering problem where multiple scattering by atomic columns has to be considered in various dynamical diffraction theories.

CHAPTER 6 - Aberration Correction With the SACTEM-Toulouse: From Imaging to Diffraction

Abstract: This chapter focuses on the aberration correction with the spherical aberration corrected transmission electron microscope (SACTEM)-toulouse. The benefits of aberration correction have been underestimated and largely unexpected. Aberration correction allows strain mapping to be performed on thick and damaged focused ion beam (FIB)-prepared specimens. The least expected benefits have been for electron diffraction experiments. Aberration correction allows large-angle convergent-beam electron diffraction (LACDIF) and convergent-beam electron holography (CHEF) configurations to be used effectively and efficiently. The extra lenses provided by the corrector allow original optical configurations to be explored. Indeed, the stability of the microscope and the computer-assisted alignment has allowed the SACTEM to be used continuously and routinely in a wide range of modes ever since its installation.

Backprojection-based reconstruction and correction for distance-dependent defocus in cryoelectron microscopy

Abstract: We study two methods of micrograph processing in cryo- electron microscopy: the defocus-gradient corrected back- projection algorithm and the correction of micrographs for the contrast transfer function based on the frequency-distance relation. Analyzing integral images produced by the methods we conclude that they are asymptotically equivalent within the framework of stationary phase approximation.

Influence of spherical aberration of the thermal lens on the mode profile of a large-volume TEM 00 -mode resonator

Abstract: The spherical aberration of the thermal lens of the active media is obvious when the solid state lasers are strongly pumped. It can change the mode profile, the single-pass loss, the output power and so on, especially in a large-volume TEM 00 -mode resonator. The self-consistent mode is calculated using Fox-Li diffraction iterative algorithm. There are side lobes with the fundamental mode when the spherical aberration is introduced, hence the beam quality is degraded. The calculation results agree well with the experimental results. The beam filling factor plays an important role in the design of a laser system. The optimized value of the beam filling factor should be determined experimentally according to the spherical aberration.

Optical depth sectioning in the aberration-corrected scanning transmission and scanning confocal electron microscope

Abstract: The use of spherical aberration correctors in the scanning transmission electron microscope (STEM) has the effect of reducing the depth of field of the microscope, making three-dimensional imaging of a specimen possible by optical sectioning. Depth resolution can be improved further by placing aberration correctors and lenses pre and post specimen to achieve an imaging mode known as scanning confocal electron microscopy (SCEM). We present the calculated incoherent point spread functions (PSF) and optical transfer functions (OTF) of a STEM and SCEM. The OTF for a STEM is shown to have a missing cone region which results in severe blurring along the optic axis, which can be especially severe for extended objects. We also present strategies for reconstruction of experimental data, such as three-dimensional deconvolution of the point spread function.

Optical design of Kirkpatrick-Baez microscope for ICF

Abstract: A new flux-resolution optical design method of Kirkpatrick-Baez microscope(KB microscope) is proposed.In X-ray imaging diagnostics of inertial confinement fusion(ICF),spatial resolution and flux are always the key parameters.While the traditional optical design of KB microscope is to correct on-axis spherical aberration and astigmatic aberration,flux-resolution method is based on lateral aberration of full field and astigmatic aberration.Thus the spatial resolution related to field dimension and light flux can be estimated.By the expressions of spatial resolution and actual limits in ICF,rules of how to set original structure and optical design flow are summarized.An instance is presented and it shows that the design has met the original targets and overcome the shortcomings of image characterization in compressed core by traditional spherical aberration correction.