Introduction to Scanning Electron Microscopy:

The scanning electron microscope can be subdivided into four systems.

1. The illuminating/imaging system: consist of an electron source and a series of lenses that generate the electron beam and focus it onto the specimen.

2. The information system: comprises the specimen and data signals released during irradiation as well as a series of detectors that discriminate among and analyzes the data.

3. The display system: is simply a cathode ray tube (CRT) synchronized with the electron detectors such that the image can be observes and recorded on film.

4. The vacuum system: removes gases that would otherwise interfere with operation of the scanning electron microscope column.

1. The

illuminating/Imaging System:

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1. A column which generates a beam of electrons.

2. A specimen chamber where the electron beam interacts with the sample.

3. Detectors to monitor the different signals that result from the electron beam/sample interaction.

4. A viewing system that builds an image from the detector signal.

2. Information System:2

 Electron Signals:

Various data signals are simultaneously released by an exposed specimen, and in the presence of appropriate detectors, the signals can be analyzed.

1. Auger electrons: This type of signal is used to characterize the elemental composition of the specimen surface.

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2. Secondary electrons (SE) The SE emission is a function of the surface topography. Other application (e.g. morphology analysis, particles size studies, fracture analyze and structure uniformity determination of coating thickness).

3. X Rays: Two different processes can form this signal: continuous radiation and inner-shell ionization process. The first one is nonspecific and must be analyzed as a background. The second leads the emission of characteristic X rays. This can be used to identify the elemental composition.

4. Backscattered electrons These electrons are result of elastic collisions at a depth of between 300 and 400 Ao. Their energy is high, almost the same as the incident electron beam and usually the sample releases more backscattered electrons than SE .The image formed using this signal has smaller resolution than the SE image.

5. Elastically scattered electrons are used in the transmission electron microscopy.

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3. Vacuum System of Electron Microscope:

Why do we need to operate under vacuum?

1. Produce a coherent beam - The mean free path of electrons at atmospheric pressure is only 1 cm. At 10-6 Torr they can travel several meters (about 6.5 m) and eliminate electron scattering

2. Insulator - no interaction of beam and gas molecules. Eliminate electrical discharges, particularly between anode and cathode and in area around field emitters

3. Increase Filament life - elimination of oxygen prevents “burning out” of filament4. Reduce interaction- between gas molecules, e-beam, and sample that leads to

contamination OR:…………1. To produce a mean-free path for electrons greater than the length of the

electron column. This corresponds to a vacuum of better than about 10-4 torr (<0.1 Pa);

2. To avoid arcing between the cathode (filament) and the anode plate. There is very high voltage between these two components and stray air or gas molecules can cause electrical arcing between them. The dielectric strength of air depends strongly on pressure. To maintain a voltage of 20 keV between the Wehnelt and anode plate at a pressure of 10-4 torr, requires a gap of about 2 mm; higher voltages require better vacuums.

3. To avoid collisions between electrons of the beam and stray molecules. These collisions can result in spreading or diffusing of the electron beam or, more seriously, can result in volatilization event if the molecule is organic in nature (for example, vacuum oil). Volatilizations can severely contaminate the microscope column, especially apertures, and degrade the image;

4. To avoid damaging the filament. The volume around the electron gun must be kept free of gas molecules especially oxygen, which will greatly shorten the filament life.

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5. To prevent absorption of X-rays produced from the sample by air molecules. At high vacuum, even soft X-rays (such as B-Kα) are transmitted without loss.

PUMP-DOWN SEQUENCE:

Initially, a mechanical pump is used to evacuate the column and sample chamber. Pressure is monitored using a thermocouple gauge (10 -3 torr) .

Once the vacuum is sufficiently good, the "gate" valve that isolates an oil diffusion pump from the column is opened. This evacuate the column further. There is a water-cooled baffle located above the pump to prevent oil vapor from entering the electron column. A cold-cathode (ion) vacuum gauge (10 -6

torr), is used to monitor the pressure at higher vacuums.

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Vacuum Pumps in SEM:

For Roughing vacuum = rotary vane pump (Mechanical Pump)Further High vacuum = Oil Diffusion Pump, Turbomolecular Pump, Ion-Getter Pump and Cryo Pump.

Capture pumps, such as ion-getter and cryo, operate by sequestering air molecules onto a surface where they are held it either temporarily (cryo) or permanently (ion-getter).

Momentum transfer pumps, which include oil diffusion, and Positive Displacement Pump which includes turbomolecular pumps, move gas by compressing it using some form of mechanical impact, and exhausting it at a higher pressure into another volume at lower pressure. These pumps require initial pumping (“roughing”) to reach operating vacuum and continued pumping to maintain a low exhaust pressure (“backing”).

Oil Diffusion Pump

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Turbomolecular Pump

Turbo molecular pump works on the principal of positive displacement. In this pump a series of inclined blades are mounted on a shaft which rotates at very high speed. These are called Rotors. Fixed blades or stators are fixed in such a way that Rotors and Stators

alternate. Because of rotation of shaft and incline rotors push air down to next row of Stators. Stators too are inclined in same direction and push this air to next row of Rotors and so on to exit, where a Rotary Pump draws away the air. The blades as thin as possible and slightly bent for max compression. Turbo pumps can reach 10-7 to 10-10 torr. Unfortunately, because of the high rotation speeds (10-20,000 rpm), turbomolecular pumps have shorter life spans than oil diffusion or sealed-oil mechanical pumps.

Sputter Ion Pump

Sputter ion pumps are a type of capture pump, wherein air molecules are removed from the chamber by plating (gettering) them onto a surface.

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The gettering can be accomplished by making the pump wall very cold (cryopumps) or by ionizing gas within a magnetically confined cold cathode discharge (ion pumps).

The ultimate pressure achieved by an ion pump is generally in the region of 2 x 10-11 Torr. Ion pumps require an initial starting pressure is 5 x 10-3 Torr or lower. Operating an ion pump at high pressure for extended periods shortens the pump life.

Principal:

Ion pumps consist of short stainless steel cylinders (anodes) sandwiched between two metal (Ti, or Ti and Ta) plates (cathodes), all situated within a strong magnetic field aligned parallel to the cylinder axes. A high voltage is applied between the anodes and cathodes, and the electrons produced from the cathodes move in long helical trajectories through the anode tubes. The long electron paths increase the probability of collision with and ionization of gas molecules. In the main pumping mechanism, the ionized molecules are accelerated toward one of the cathodes where they are buried by Ti atoms. Ion impacts also sputter titanium from the cathode and the resulting Ti atoms acts as a getter for reactive gases (O, N, CO and H) and are deposited as stable oxides, carbides, nitrides and hydrides elsewhere in the pump.

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Ion pump. (left) Diode ion pump, which has two cathodes. (right) Mechanism of operation of an ion pump. Image source: ??

Capture pumps can effectively remove gas from a chamber at lowpressure. They do so by freezing molecules on a wall (cryogenic pump),chemically reacting with the molecules (getter pump), or accelerating themolecules to a high velocity and burying them in a metal wall (ion pump).

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