Components

The Gun:

Electrons are emitted from a wire that has been superheated by an electric current until enough energy (~2500K) is produced to overcome the work function of the metal, usually tungsten or lanthanum hexaboride (LaB6). The emmitted electrons have a Boltzman energy distribution and need to be collated into a tight beam and sent down the microscope column, across the sample, lenses, and apertures. Thermionic and field emission guns (FEG) are the two most common electron guns used.

The Lenses:

Electron lenses are magnetic coils that have been tuned to focus and direct a passing electron beam. Because the lenses are tuned for one particular electron wavelength, they will bend electrons of other wavelengths less evenly. Therefore, there is a splitting of the electron beam as it passes through the lens. This chromatic aberration effect is not unlike the splitting of sunlight into a rainbow by a prism. In the case of a prism, light of different wavelengths are refracted to different angles as they pass through the prism. Lenses can also rotate the electron beam if they are not tuned correctly. When this happens, the image can rotationally jump as different lesnes are inserted.

Three primary lenses are used to form and magnify an image. The objective lens is the topmost lens and does the first step of image magnification and focusing. It also works in conjunction with the objective aperture to generate amplitude contrast. The next lens is the intermediate lens and its positioning and strength controls the magnification of the image or the diffraction pattern. The last lens is the projector lens used to focus and project the image onto the imaging surface.

The Apertures:

Apertures are holes along the microscope column that can limit the size of the electron beam that passes through it. Depending on it's location in the column an aperture can have different uses. The first aperture, the condenser aperture, for example, is located near the top of the column and as the name implies, is used to condense and mantain the coherence of the electron beam. The second aperture, the objective aperture, is located below the sample just after the objective lens. The objective aperture is used primarily to control contrast in the image. See below for a more thorough description of amplitude contrast.

The External Stuff:

The outside of the microscope is just as important as the inner machinery. A typical user console consists of an informational screen and various control surfaces. These knobs, buttons, and switches can be used to control sample movement, beam brightness, beam current, alignment, focusing, and magnification, among other things. The screen generally displays pertinent information on the functioning of the microscope, such as the current magnification, beam strength, defocus value, beam voltage, etc. Older microscopes use CRTs, but newer models are now interfacing with PCs to allow greater flexibility and control.

The sample grid is placed in a metal wand that is part of the goniometer or compustage. In Figure 1, to the left of the compustage are the three control knobs for the three apertures. The tall white cylinder to the right and a bit behind the column is a cryo dewar that holds liquid nitrogen, used to cool down the column for easier removal of water vapor. The two control pads, used to move the sample and beam are seen at the far bottom of the image. To the right edge of the image is the computer monitor that displays the microscope control software. In older microscopes, these controls were often mechanical knobs and buttons. Newer microscopes use a computer interface to directly access microscope functions.