Resolution chromatic aberration

Once the primary electron beam is created, it must be demagnified with condenser lenses and then focused onto the sample with objective lenses. These electron lenses are electromagnetic in nature and use electric and magnetic fields to steer the electrons. Such lenses are subject to severe spherical and chromatic aberrations. Therefore, a point primary beam source is blurred into a primary beam disk to an extent dependent on the energy and energy spread of the primary electrons. In addition, these lenses are also subject to astigmatism. AH three of these effects ultimately limit the primary beam spot size and hence, the lateral resolution achievable with sem. 

A limiting factor in electron microscopy is the quality of the electron beam. Aberrations introduced by the optics limit both spatial resolution and analytical capabilities. There is a need to correct for the spherical and chromatic aberrations introduced by the electron optics. This will result in improved coherence of the beam and improved imaging and diffraction. In particular, these advances will permit the analysis of amorphous samples. Smaller beam sizes can also be achieved, allowing for sub-Angstrom resolution chemical analysis of samples. Development of higher-quality electron beams and short pulses of electron beams would broaden and deepen the application of electron microscopy. 

The resolution in TEM is limited by lens aberrations. In contrast to optical microscopy, where by serially ordering concave and convex lenses, aberrations can be compensated and hence the wavelength of the radiation is resolution limiting in TEM lens aberrations cannot be compensated since concave electron lenses are not feasible. The objective lens is the crucial part for image defining the microscope s resolution. The quality of the objective lens is described by the constants of spherical Cs(" 0.5-3mm) and chromatic aberration Cc ( 1-2 mm). Recently, microscopes equipped with complex correctors for the spherical aberration have become available. ... 

The 1 1 photocathode projection scheme uses UV radiation to irradiate a Csl photocathode, which is masked hy a thin metal pattern. Photoelectrons are ejected from the cathode and accelerated to the wafer, which acts as the anode. A uniform magnetic field is used to focus the electrons at the wafer. Chromatic aberrations tend to limit resolution in these systems. Other problems that have been identified to hamper this technique include issues such as the uniformity of the electric and magnetic fields, substrate flatness requirements, and poor cathode life. ... 

The aberration characteristics of the objective lens are determined by the polepiece geometry and magnetic field strength. The diameters of the discs of confusion associated with these aberrations, i.e. the magnitude of the aberration and the associated image resolution limitation, are directly proportional to two lens parameters called the spherical aberration coefficient (Cg) and the chromatic aberration coefficient (C ). 

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