Materials, Methods and Instrumentations
2.4. Instruments
2.4.5. Field emission scanning electron microscopy
Field emission scanning electron microscopy (FESEM) provides topographical and elemental information at different magnifications ranging up to 5,00,000x depending on the conductivity of the sample. FESEM primarily consists of an electron gun as a source of electrons, a column with lenses, apertures and an object chamber where the samples with conductive layer can be mounted on a special holder. The sigma model of FESEM instrument was used for recording images manufactured by Zeiss.
Light Source Sample
Time to Amplitude Converter (TAC)
TAC Start
Analogue to Digital Converter (ADC)
Multichannel Pulse Height Analyzer
Computer
Filter or Monochromator
Detector PMT/MCP
Stop Discriminator TAC Stop
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Electron gun: The field emission electron gun is a cold cathode field emitter. An extremely thin and sharp tungsten needle with tip diameter <100 nm functions as a cathode in front of a primary and secondary anode. The emission of electrons are collected by placing the filament in a huge electrical potential gradient of magnitude of 0.5 to 30 KV. The electron beam produced by the field emission source is about 1000 times smaller than in a standard microscope which implies markedly better image quality. Field emission sources provide enhanced performance, reliability and lifetime.
The column chamber contains the electro-magnetic lenses such as condenser lens, scan coils, stigmator coils and objective lens to focus the electron beam. These electromagnet ic lenses consist of a coil of wire which generates a magnetic field across the lens gap between the pole pieces.
Condenser lens: It controls the amount of demagnification. The current in the condenser determines the diameter of the beam. A low current results in a small diameter and higher current results in a higher diameter. A narrow beam has the advantage of better resolutio n.
But the worse signal to noise ratio is a major disadvantage.
Stigmator coils: The stigmator coils are utilized to correct irregularities in the x and y deflection of the beam and thus to obtain a perfectly round-shaped beam. When the beam is not circular, but ellipsoidal, the image looks blurred and stretched.
Objective lens: It focuses the electron beam onto the specimen. It is the lowest lens in the column. At a short working distance the objective lens needs to apply a greater force to deflect the electron beam. The shortest working distance produces the smallest beam diameter, the best resolution, but also the poorest depth of field.
Scan coils: It consist of upper and lower coils which deflect the electron beam over the object according to a zig-zag pattern. The formation of the image on the monitor occurs in synchrony with this scan movement. The scan velocity determines the refreshing rate on the screen and the amount of noise in the image. Rapid scan resulted in rapid refreshing which always yield low signal with more noise.
Object chamber: After the object has been covered by a conductive layer, it is mounted on a special holder. The object is inserted through an exchange chamber into the high vacuum part of the microscope and anchored on a moveable stage. The object can be repositioned in the chamber by means of a joy stick that steers in left-right axis, or forward and backward.
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In addition, the object can be tilted, rotated and moved in Z direction. The “secondary electron emission” detector is located at the rear of the object holder in the chamber.
A focused beam of electrons is used to generate an image or to analyze the specimen.
For operation, the gun head, the column and specimen chamber have to be evacuated. The pre vacuum pump and turbo pump evacuate the specimen chamber. Vacuum in the specimen chamber is measured by penning gauge. Colum chamber valve remained closed until the detected pressure is not ready for operation. After vent command, column chamber valve closes and N2 gas flows into the specimen chamber through vent valve.
Figure 2.4: Schematic diagram of FESEM
Electron Gun emits electrons. The beam of electrons passes through anode aperture.
Electron beam passes through a 30 μm multi-hole aperture. Other aperture sizes can be selected by using the gun/aperture alignment system. Condenser lens controls the amount of demagnification. Stigmator makes sure the beam is rotationally symmetrical. Anode and linear tube are connected to form the beam booster. Beam booster provides better protection against external stray fields. Objective lens focuses the electron beam onto the specimen.
Objective lens consists of electromagnetic and electrostatic lens. Deflection system consists of a set of scan coils to move the electron beam in a point‐to‐point scan process. Two imaging modes are used for analysing the samples. In‐lens image mode is located on the beam path. Secondary electrons having low energy are used in this mode. The energy is usually less than 50 eV. This is located on the wall of the specimen chamber. In backscattered electron image mode higher energy electrons (>50 eV) are used. This is located below final lens.
1. Schottky field emitter 2. Anode
3. Multi-hole aperture
4. Gun/aperture alignment system 5. Condenser lens
6. Sigmator 7. Linear tube 8. In-lens detector
9. Scan coils and sigmator 10. Objective cap
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