Contrast transfer function

About: Contrast transfer function is a research topic. Over the lifetime, 934 publications have been published within this topic receiving 26533 citations.

Improving the performance of the critical dimension-scanning electron microscope with the contrast transfer function

Abstract: Critical dimension scanning electron microscopes (CD-SEMs) are used extensively by the semiconductor industry to perform highly accurate dimensional metrology of patterned features. To ensure optimal feedback for process control, these tools must produce highly reproducible measurements. This means monitoring and minimizing not only day-to-day variations on a given tool, but also tool-to-tool variations whether within the same production facility or at different sites. It has been shown that the contrast transfer function (CTF) can be used to evaluate the imaging performance of SEMs by giving a quantitative measure of the fidelity with which specimen contrast information (i.e., point-to-point variations in emitted signal intensity) is represented in the image data as a function of spatial frequency. Because all imaging defects and artifacts as well as the point spread function impact the shape of the CTF, it is an ideal means with which to monitor deviations from a baseline performance. By using a thoughtfully designed and thoroughly characterized test specimen, the CTF of a given tool can be decoupled from the specimen information, allowing for characterization of the imaging system itself. Fresnel zone plates and pseudorandom arrays of dots are good candidates for such test structures, if they can be fabricated with sufficient resolution to assess the performance of the tool up to its information limit. The feasibility of this approach has been assessed with test structures fabricated using nano-imprint lithography with 22 nm design rules. The advantages of using the CTF of a specific instrument to improve CD-SEM image simulations are also demonstrated.

Simulation of magnetic sector deflector aberration properties for low-energy electron microscopy

Abstract: In this paper, low-order aberration properties of magnetic sector deflectors are analysed by computer simulation and their use in the low-energy electron microscope (LEEM) and the spectroscopic scanning electron microscope (SPSSEM) is examined. The simulation method is based upon direct ray tracing through field distributions derived by the finite element method and semi-analytical techniques. A variety of beam conditions and geometries have been investigated in order to operate the sector as a round lens while deflecting the primary beam through 90°. Predicted results from this study are also compared to previous simulation work.

Structure of HIV-1 capsid assemblies by cryo-electron microscopy and iterative helical real-space reconstruction.

Abstract: Cryo-electron microscopy (cryo-EM), combined with image processing, is an increasingly powerful tool for structure determination of macromolecular protein complexes and assemblies. In fact, single particle electron microscopy 1 and two-dimensional (2D) electron crystallography 2 have become relatively routine methodologies and a large number of structures have been solved using these methods. At the same time, image processing and three-dimensional (3D) reconstruction of helical objects has rapidly developed, especially, the iterative helical real-space reconstruction (IHRSR) method 3 , which uses single particle analysis tools in conjunction with helical symmetry. Many biological entities function in filamentous or helical forms, including actin filaments 4 , microtubules 5 , amyloid fibers 6 , tobacco mosaic viruses 7 , and bacteria flagella 8 , and, because a 3D density map of a helical entity can be attained from a single projection image, compared to the many images required for 3D reconstruction of a non-helical object, with the IHRSR method, structural analysis of such flexible and disordered helical assemblies is now attainable. In this video article, we provide detailed protocols for obtaining a 3D density map of a helical protein assembly (HIV-1 capsid 9 is our example), including protocols for cryo-EM specimen preparation, low dose data collection by cryo-EM, indexing of helical diffraction patterns, and image processing and 3D reconstruction using IHRSR. Compared to other techniques, cryo-EM offers optimal specimen preservation under near native conditions. Samples are embedded in a thin layer of vitreous ice, by rapid freezing, and imaged in electron microscopes at liquid nitrogen temperature, under low dose conditions to minimize the radiation damage. Sample images are obtained under near native conditions at the expense of low signal and low contrast in the recorded micrographs. Fortunately, the process of helical reconstruction has largely been automated, with the exception of indexing the helical diffraction pattern. Here, we describe an approach to index helical structure and determine helical symmetries (helical parameters) from digitized micrographs, an essential step for 3D helical reconstruction. Briefly, we obtain an initial 3D density map by applying the IHRSR method. This initial map is then iteratively refined by introducing constraints for the alignment parameters of each segment, thus controlling their degrees of freedom. Further improvement is achieved by correcting for the contrast transfer function (CTF) of the electron microscope (amplitude and phase correction) and by optimizing the helical symmetry of the assembly.

The Electron Microscope—A Review

Abstract: Publisher Summary This chapter presents a review of the electron microscope. Discussion of the resolving power of the electron microscope is complicated by the inevitable presence of spherical aberration. In light microscopy, the resolving power is defined as equal to the radius of the central maximum of the Airy diffraction pattern. The resolving power may be improved by reducing the aperture until a region is reached where both diffraction and spherical aberration are effective, and within which a minimum value of resolving power will occur at some optimum value of the aperture angle. Image contrast in the electron microscope is produced not by the differential absorption of the electrons but by differential scattering. All electrons scattered by a local area of specimen outside a certain angle are stopped by the objective aperture, and hence lost to the image. In this chapter, the basics of resolving power, image contrast, and magnetic objective lens related to electron microscope are discussed. Possible methods for the correction of spherical aberration are elaborated in the chapter. The relation between mechanical defects and astigmatism is also explained in the chapter.