Electron lenses are a result of magnetic fields which are produced by an electromagnet which functions similarly to the function of an optical convex lens. It is not possible to produce electron lenses whose function is similar to concave lens. This leads to a major problem: Lens aberrations cannot be corrected as in conventional optics. To minimize optical aberration a good alignment of the electron beam is essential. The lens system consists of two parts. At first there is the FEG Column, which demagnifies the electron beam of the electron gun and controls the diameter of the beam when it hits the specimen. The second is the the projector system.
Conventional FEG column The conventional FEG column consists of 5 lenses: the gun lens, first condenser lens, second condenser lens, minicondenser lens and objective lens.
The gun lens is positioned directly behind the field emitter and extractor. It is an electrostatic lens controlling the first demagnification. Increasing the gun lens will lead to increasing demagnification and decreasing of the current beam.
The first condenser lens is the next lens. Increasing ‘spot size’ leads to increased demagnification of the source and decreased current in the beam. To estimate increasing the spot leads to a reduction of the beam current by roughly 50%.
The second condenser lens transports the image of the the first condenser lens to the minicondenser lens and the objective lens.
In STEM, this lens is used to focus the beam on the specimen. Therefore, the C2 lens is connected to the focus knob in STEM.
In TEM, the second condenser lens lens is used to defocus the beam on the specimen so in this way the size of the area which the beam hits can be controlled. Therefore, the C2 lens is connected to the intensity knob in TEM.
The minicondenser lens is the following lens. For the minicondenser lens there are two settings the micro-probe and the nano-probe setting. For micro-probe little demganification is set and for nano-probe large demagnification is set. Micro-probe is normally used for TEM applications and nano-probe for STEM applications.
Afterwards the Objective lens is positioned. The specimen is located approximately in the middle of the magnetic field of the objective lens. This field gives lens action in front of the specimen (the condenser-objective lens) and lens action after the specimen (the imaging-objective lens). Therefore the imaging-objective lens collects the exiting beam from the specimen and forms the first image of the intermediate image and diffraction pattern.
The projector lenses magnify the image. The projector system can be switched between intermediate image and diffraction image. In intermediate mode the first projector lens magnifies the intermediate image formed by the objective lens or in other words the first projector lens is focused on the image plane of the objective lens. Diffraction images are formed in the back focal plane of the objective lens. Therefore in diffraction mode the first projector lens is refocused on the back focal plane of the objective lens.