Low-energy Electron Microscopy - Experimental Setup

Experimental Setup

A typical LEEM setup consists of (see figure):
1. Electron gun, used to generate electrons by way of thermionic or field emission from a source tip. In thermionic emission, electrons escape a source tip (usually made of LaB₆) by resistive heating and application of an electric field to effectively lower the energy needed for electrons to escape the surface. Once sufficient thermal vibrational energy is attained electrons may overcome this electrostatic energy barrier, allowing them to travel into vacuum and accelerate down the lens column to the gun potential (because the lenses are at ground). In field emission, rather than heating the tip to vibrationally excite electrons from the surface, the source tip (usually tungsten) is sharpened to a small point such that when large electric fields are applied, they concentrate at the tip, lowering the barrier to escape the surface as well as making tunneling of electrons from the tip to vacuum level more feasible.
2. Condenser/illumination optics, used to focus electrons leaving the electron gun and manipulate and/or translate the illumination electron beam. Electromagnetic quadrupole electron lenses are used, the number of which depends on how much resolution and focusing flexibility the designer wishes. However, the ultimate resolution of LEEM is usually determined by that of the objective lens.
3. Illumination beam aperture allows researchers to control the area of the specimen which is illuminated (LEEM’s version of electron microscopy’s “selected area diffraction”, termed microdiffraction) and is located in the beam separator on the illumination side.
4. Magnetic beam separator, needed to resolve the illuminating and imaging beam (while in turn spatially separating the optics for each). There has been much development on the technology of electron beam separators; the early separators introduced distortion in either the image or diffraction plane. However, IBM recently developed a hybrid prism array/nested quadratic field design, focusing the electron beams both in and out of the plane of the beampath, allowing for deflection and transfer of the image and diffraction planes without distortion or energy dispersion.
5. Electrostatic immersion objective lens, used to form a real image of the sample by way of a 2/3-magnification virtual image behind the sample. The uniformity of the electrostatic field between the objective lens and specimen, limited by spherical and chromatic aberrations larger than those of any other lenses, ultimately determines the overall performance of the instrument.
6. Contrast aperture, located in the center on the projector lens side of the beam separator. In most electron microscopies, the contrast aperture is introduced into the back focal plan of the objective lens (where the actual diffraction plane lies). However, this is not true in the LEEM, because dark-field imaging (imaging of nonspecular beams) would not be possible because the aperture has to move laterally and would intercept the incident beam for large shifts. Therefore, researchers adjust the excitation of the objective lens so as to produce an image of the diffraction pattern in the middle of the beam separator and choose the desired spot intensity to image using a contrast aperture inserted there. This aperture allows scientists to image diffraction intensities that may be of particular interest (dark field).
7. Illumination optics are employed to magnify the image or diffraction pattern and project it onto the imaging plate or screen.
8. Imaging plate or screen, used to image the electron intensity so that we can see it. This can be done many different ways including, phosphorescent screens, imaging plates, CCDs, among others.