Conventional Transmission Electron Microscope

Electrons are emitted in the electron gun by thermo-ionic, Schottky, or field emission. The latter are used when high gun brightness and coherence are needed. In a thermo-ionic emission TEM works much like a light bulb. A filament (cathode) is the source of electrons. It is usually a hairpin-shaped tungsten wire. An accelerating voltage (fixed amount of negative high voltage) is applied to the surrounding cathode cap. A small emission current is then applied to the filament to achieve the release of electrons. The point at which the gun achieves good thermal emission as well as an acceptable filament life is called the saturation point. The cathode cap (also called Wehnelt cylinder) must be slightly more negative than the filament. A resister is located in the gun assembly and is controlled by a knob marked "bias". It creates the difference in negative voltage between the filament and the cathode cap. This allows the electrons to collect inside the cap, forming an electron "cloud". An anode located below the gun assembly, is electrically at ground, creating a positive attraction for the negatively charged electrons, which overcome the negative repulsion of the cathode cap and accelerate through the small hole in the anode.

Simulation of the Thermionic Electron Gun (try changing the three parameters which control the electron gun):

Filament current, A: The filament current controls the temperature of the filament and hence the number of electrons emitted or 'beam current'.

Bias: The bias potential controls the size of the region of filament which emits electrons and hence it affects both the source size and the beam current. If the bias is too high no region of the filament will emit and the beam is said to be pinched off.

High Tension, kV: In the TEM, higher energy electrons permit the examination of thicker specimens, but may cause specimen damage. Higher voltage microscopes are also more expensive. Most TEMs have a maximum HT of ~ 200kV. In the SEM, the maximum HT is usually ~ 25kV.

(If the flash movie is not loaded, please open Elgun.swf file with your Web browser)

Glass lenses, of course, would impede electrons; therefore electron microscope (EM) lenses are electromagnetic converging lenses. A tight wound wrapping of copper wire makes up the magnetic field that is the essence of the lens. Surrounding these coils is a shroud made of a metal that will not hold a magnetic charge when the lens is shut off. The electron moves through the center hole in this solenoid. The electron path is further constricted by a brass lining inside this space known as the pole piece. The pole piece has a small gap within it at which point the beam is most influenced by the electromagnetic current. This is appropriately referred to as the pole piece gap.

Electron paths are usually represented by straight lines running through a convex lens. More accurately, however, the electron paths form a tight spiral as they are accelerated through the lenses. The path and trajectory taken by the electrons are influenced by the lens current as they pass though a small opening in the lens.

The electromagnetic lenses simulation: (Click on 'Draw rays' to compare the action of an electromagnetic lens with an optical lens)

(If the flash movie is not loaded, please open Lens_1.swf file with your Web browser)

The image is rotated, to a degree that depends on the strength of the lens. Focal length can be altered by changing the strength of the current.

The condenser lenses in the TEM serve much the same function as that of the condenser in the light microscope. They gather the electrons of the first crossover image and focus them onto the specimen to illuminate only the area being examined. A condenser aperture is used to reduce spherical aberration.

The condenser system simulation:

(If the flash movie is not loaded, please open Lens_2.swf file with your Web browser)

The Objective lens is used primarily to focus and initially magnify the image. The specimen stage is inserted into the objective lens for imaging purposes. A cold finger or anti-contaminator also sits near the objective lens. It consists of a thin copper rod at liquid nitrogen temperatures, so that contaminants are attracted to it. The cold finger reservoir must be must filled with liquid nitrogen before the microscope is used. Contaminants sometimes cause a phenomenon known as drift. Drift is the apparent "movement" of the specimen across the screen. It is caused by poor contact between the grid and the specimen holder causing a buildup of heat and static charges. An objective aperture is used to enhance specimen contrast. Intermediate lenses magnify the image coming from the objective lens. The lens can be focused on:

* Initial image formed by the objective lens, or

* Diffraction pattern formed in the back focal plane of the objective lens.

This determines whether the viewing screen of the microscope shows a diffraction pattern or an image.

(If the flash movie is not loaded, please open Lens_3.swf file with your Web browser)

Finally, projector lenses further magnify the image coming from the intermediate lens and projects it on to the phosphorescent screen. Magnification in the electron microscope can be varied from hundreds to several hundred thousands of times. This is done by varying the strength of the projector and intermediate lenses. Not all lenses will necessarily be used at lower magnifications.

(If the flash movie is not loaded, please open Lens_4.swf file with your Web browser)

The depth of field QUOTE is the range of distance along the optical axis in which the specimen can move without the image apppearing to lose sharpness. This obviously depends on the resolution of the microscope.

The depth of focus QUOTE is the extent of the region around the image plane in which the image will appear to be sharp. This depends on magnification QUOTE .

The simulation of depth of field and depth of focus:

(If the flash movie is not loaded, please open Depth.swf file with your Web browser)

The final image is viewed by projection onto a phosphorescent screen which gives off photons when irradiated by the electron beam. A film camera is located beneath the phosphorescent screen. The screen is raised in order to expose a special photographic film with a thicker emulsion layer than light film. An alternative to photographic film is digital capture with a computer digitizing and archiving (CCD) camera.

As in the light microscope several factors detract from this number. Spherical aberration is also present in the TEM as electrons passing through the periphery of the lens are refracted more than those passing along the axis. All the electrons will therefore not reach a common focal point. To reduce spherical aberration, an aperture is used to eliminate some of the periphery electrons.

One would not normally expect chromatic aberration to be a problem in an electron microscope, but, electromagnetic radiation of different energies converges at different focal planes. This is essentially the same problem as the chromatic aberration observed in the light microscope. To correct for chromatic aberration, increase accelerating voltage, improve the vacuum and/or use a thinner specimen.

An astigmation occurs when a lens field is not symmetrical in strength, but weaker in one plane than another. Astigmation can be caused by imperfect pole-piece boring, non-homogenous blending of pole-piece materials, or by dirt on the pole-pieces, apertures, and/or specimen holders. A stigmator can be used to apply a correcting field of the appropriate strength in the proper direction to counteract the asymmetry. Stigmators are located in the objective and condenser lenses

Although diffraction can be useful, diffraction of electron waves around the aperture openings can interfere with the initial wave front. The results are an unclear or out-of-focused image. It is important to create a balance between reduction of spherical aberration and diffraction by selecting an appropriate sized aperture.