Low-energy electron microscope of novel design

The pioneering work of Bauer and Telieps stimulated us to construct a low-energy electron microscope intended mainly for imaging solid surfaces with reflected and diffracted electrons in the 10 to 100 eV range, as well as with photoelectrons. Several novel design features were incorporated.Figure 1 shows the layout of the instrument. The primary-beam column consists essentially of the electron gun, the primary-beam lens, the energy-focusing deflector and the magnet. al lenses are electrostatic. The electron gun comprises the cathode and two acceleration electrodes, the first of which is shaped semi-spherical about the cathode tip and is connected to a variable voltage supply, and the second is the tubular vacuum housing. This constitutes an immersion lens with variable focal length. The cathode is operated in the diode mode. Its virtual diameter is assumed to be dv = dtip √2.7 kT/eVa because in thermionic emission the energy distribution drops to half the peak value at 2.7 kT. The final acceleration voltage is Va = 15 KV. The variable immersion lens together with the primary-beam lens provides a magnification range of about 1 to about 70 without a crossover from the cathode to the backfocal plane of the emission lens. For example, a pointed W cathode with a tip diameter of 1μm imaged with 15× magnification yields a lμm diameter image.

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Performance Evaluation of a Novel Type of Low-Energy Electron Microscope

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100180525 ◽

1990 ◽

Vol 48 (1) ◽

pp. 354-355

Author(s):

Bertholdand Senftinger ◽

Helmut Liebl

Keyword(s):

Electron Microscope ◽

High Resolution ◽

Field Strength ◽

Numerical Aperture ◽

Low Energy ◽

Energy Electron ◽

High Resolution Imaging ◽

Lens System ◽

Resolution Imaging ◽

Low Energy Electron

During the last few years the investigation of clean and adsorbate-covered solid surfaces as well as thin-film growth and molecular dynamics have given rise to a constant demand for high-resolution imaging microscopy with reflected and diffracted low energy electrons as well as photo-electrons. A recent successful implementation of a UHV low-energy electron microscope by Bauer and Telieps encouraged us to construct such a low energy electron microscope (LEEM) for high-resolution imaging incorporating several novel design features, which is described more detailed elsewhere.The constraint of high field strength at the surface required to keep the aberrations caused by the accelerating field small and high UV photon intensity to get an improved signal-to-noise ratio for photoemission led to the design of a tetrode emission lens system capable of also focusing the UV light at the surface through an integrated Schwarzschild-type objective. Fig. 1 shows an axial section of the emission lens in the LEEM with sample (28) and part of the sample holder (29). The integrated mirror objective (50a, 50b) is used for visual in situ microscopic observation of the sample as well as for UV illumination. The electron optical components and the sample with accelerating field followed by an einzel lens form a tetrode system. In order to keep the field strength high, the sample is separated from the first element of the einzel lens by only 1.6 mm. With a numerical aperture of 0.5 for the Schwarzschild objective the orifice in the first element of the einzel lens has to be about 3.0 mm in diameter. Considering the much smaller distance to the sample one can expect intense distortions of the accelerating field in front of the sample. Because the achievable lateral resolution depends mainly on the quality of the first imaging step, careful investigation of the aberrations caused by the emission lens system had to be done in order to avoid sacrificing high lateral resolution for larger numerical aperture.

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