照射系统(电子枪、高压系统)

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1.Cathode

1-1. Cathode

An electrode, to which a negative potential (voltage) is applied against the facing anode. The "cathode" means the filament of the thermionic-emission electron gun or the emitter (electron source) of the field-emission electron gun.

Related term
anode

1-2. cathode-ray tube

The cathode-ray tube (CRT) displays a two-dimensional image on the tube surface in such a way that an electron beam is accelerated, focused and deflected by electric voltages and magnetic fields to scan the tube phosphor surface.

1-3. cold (cathode) field-emission electron gun

The cold (cathode) field-emission electron gun (CFEG) emits electrons from the tungsten (W) tip emitter by tunneling the potential barrier (~4.5 eV) where the emitter is kept at room temperature in a strong electric field. Since the energy spread of the emitted electrons from the CFEG is narrower (~0.4 eV) than the Schottky type, the CFEG provides a superbly high energy resolution in EELS. Since the size of the virtual source produced is as small as ~10 nm, the electron beam has a high coherence, suitable for electron holography. Its brightness is as high as ~8×108 A/cm2.sr at 200 kV. The CFEG can produce a small-sized probe, but its total emission current is small. Thus, it is suitable for high magnification observations in the TEM mode but the Schottky type gun is more convenient at low magnification observations. Since the emitter surface is likely to be contaminated by residual gases, the emission current is likely to fluctuate. It is necessary to flash the emitter tip in a commercially-available CFEG at intervals of about 8 hours. Recently, the stability of the beam current has greatly been improved by acquiring a better vacuum. Thus, the barrier for EELS, EDS and WDS experiments by using CFEG is lowered.

Related term 
field-emission electron gun, FEG, thermal (thermally assisted) field-emission electron gun, electron holography

2. Wehnelt electrode

A cylindrical electrode with a hole of a diameter of 1 to 2 mm, which is installed in the electron gun. By applying a bias voltage induced through a bias resistance, the "Wehnelt electrode" converges a diverging electron beam emitted from the electron gun.

3. energy spread

"Energy spread" means an energy width of an electron beam. This is determined by fluctuations of the initial speed of electrons emitted from the cathode and by inelastic scattering of electrons in a specimen.

4. virtual source

In the case of the filed-emission electron gun, the action of the electrostatic lens is weak and the crossover is not produced in front of the tip of the electron gun. The "virtual source" is a point behind the tip, where all trajectories of the emitted electrons virtually meet (at one point) by the extrapolation of the electron trajectory.

Related term
crossover

5. acceleration tube (accelerating tube)

The "acceleration tube" consists of acceleration electrodes used for sequentially accelerating an electron beam, which is emitted from the electron gun, up to a required voltage. In a 200 kV TEM, six acceleration electrodes constitute the acceleration tube.

6. accelerating voltage

6-1. accelerating voltage

A voltage to accelerate electrons, which are emitted from the electron gun and illuminate a specimen. This voltage is a voltage applied between the cathode and the final electrode of the acceleration tube.

6-2. accelerating (acceleration) voltage center

When fluctuations are added to accelerating voltage using a high-tension wobbler, a TEM image spirally enlarges and shrinks. The center of this enlargement and shrinkage is called "accelerating (acceleration) voltage center." Alignment of the accelerating voltage center is carried out to bring the accelerating voltage center to the center of the fluorescent screen for viewing the image by the use of a double-deflection coil system. Since the fluctuations of the high voltage are small (<10-6), this alignment is used to minimize the effect of energy spread due to inelastic scattering (plasmon scattering) in a specimen rather than the effect of high-voltage fluctuations. This alignment is required when taking images at a medium magnification lower than 100,000×.

Related term
high-tension wobbler, double-deflection system, objective current center

7. brightness

"Brightness" means the current density per unit solid angle, which is a measure of the quality of an electron source. As the tip of the cathode is smaller, the brightness is higher. The brightness is kept constant at any stage of an optical system if the optical system is free from aberrations.

Related term
crossover, virtual source

8. crossover

8-1. crossover

An electron beam emitted from the cathode is converged by an electrostatic acceleration lens in the electron gun, and then forms the minimum cross section ahead of the cathode. The minimum cross section is termed a "crossover." The crossover is formed in the case of an electron gun fitted with a conventional tungsten emitter or a LaB6 tip. The brightness of the electron gun refers to that of the crossover.

Related term 
virtual source

8-2. crossover point

A point where the cross section of the electron beam becomes minimum when the beam is converged with the electron lens.

9. high-tension cable

A high withstand-voltage cable that connects the high-voltage power supply for accelerating an electron beam with the electron gun in an electron microscope.

10. high-tension (voltage) tank

A container that houses a high-voltage generator to accelerate electrons emitted from the electron gun.

11. high-voltage power supply

A power supply that generates a high negative voltage to accelerate electrons emitted from the electron gun. The stability of the "high-voltage power supply" at present is ~1×10-6.

Related term
high-voltage generator

12. highly accelerated electron

An electron that is accelerated at a high voltage (>100 kV) having a high energy. As the accelerating voltage is higher, the wavelength of the electron is shorter.

Related term
ultra-high voltage electron microscope, UHV-EM

13. Cockcroft-Walton high-voltage circuit(CWC)

A multi-stage circuit that combines rectifiers and capacitors to generate a stable high DC voltage from an AC voltage. The "Cockcroft-Walton high-voltage circuit (CWC)" is used for reducing voltage fluctuations of the high-voltage power supply (high-voltage generator).

14. work function

"Work function" means the energy required for removing an electron from a solid.

Related term
field emission

15. dose

A "dose" is the amount of the degree of (electron-beam) irradiation. The dose is defined by the irradiation beam energy per unit area on the specimen.

Related term
minimum dose system, MDS

16. Schottky-type electron gun

Schottky effect means a phenomenon where the potential barrier of a substance decreases in a strong electric field, resulting in ease of thermoelectron emission. In the Schottky-type electron gun, the tungsten (W) tip emitter is heated at a lower temperature (~1800 K) than the temperature that can effectively emit thermoelectrons, and a strong electric field is applied to the tip, thus decreasing the potential barrier to emit electrons from the emitter. In the actual Schottky-type electron gun, the surface of the tip is covered with a thin layer of zirconium oxide (ZrO) to make electron emission easy by a decrease of the work function of the tip (~2.7 eV). The energy spread of the emitted electrons is ~0.7 eV. Its brightness is as high as 4×108 A/cm2.sr at 200 kV. The size of the virtual source produced is >10nm. The Schottky type gun is broadly used because of its high stability of the emission current. This type of gun is not the field emission type because the tunnel effect is not used.

Related term 
Schottky effect

17. Schottky effect

A phenomenon where the potential barrier decreases when a strong electric field is applied to a substance. The Schottky-type electron gun emits sufficient electrons with the aid of a strong electric field at a lower temperature (~1800 K) than the temperature that can effectively emit thermoelectrons. In the actual Schottky-type electron gun, the surface of the tungsten (W) tip is covered with a thin layer of zirconium oxide (ZrO) to further decrease the potential barrier.

Related term
Schottky-type electron gun, potential barrier

18. multi-stage acceleration electrode

The "multi-stage acceleration electrode" consists of a cascade of acceleration electrodes used for accelerating an electron beam, which is emitted from the electron gun, up to a required voltage. In a 200 kV TEM, a six-stage acceleration electrode is adopted.

19. field extraction

"Field extraction" is a technique to extract electrons in a solid to the outside without heating the solid, by applying a strong electric field (107 V/cm or more) to the surface of the solid.

20. field emission

20-1. field emission

"Field emission" is a technique to emit electrons from the tip of the cathode. A sharpened cathode made of a material (normally, tungsten) that has an appropriate work function is placed in a strong electric field produced by the extraction electrode to emit electrons from the cathode tip.

Related term
field-emission electron gun, FEG

20-2. cold (cathode) field-emission electron gun

The cold (cathode) field-emission electron gun (CFEG) emits electrons from the tungsten (W) tip emitter by tunneling the potential barrier (~4.5 eV) where the emitter is kept at room temperature in a strong electric field. Since the energy spread of the emitted electrons from the CFEG is narrower (~0.4 eV) than the Schottky type, the CFEG provides a superbly high energy resolution in EELS. Since the size of the virtual source produced is as small as ~10 nm, the electron beam has a high coherence, suitable for electron holography. Its brightness is as high as ~8×108 A/cm2.sr at 200 kV. The CFEG can produce a small-sized probe, but its total emission current is small. Thus, it is suitable for high magnification observations in the TEM mode but the Schottky type gun is more convenient at low magnification observations. Since the emitter surface is likely to be contaminated by residual gases, the emission current is likely to fluctuate. It is necessary to flash the emitter tip in a commercially-available CFEG at intervals of about 8 hours. Recently, the stability of the beam current has greatly been improved by acquiring a better vacuum. Thus, the barrier for EELS, EDS and WDS experiments by using CFEG is lowered.

Related term 
field-emission electron gun, FEG, thermal (thermally assisted) field-emission electron gun, electron holography

20-3. field-emission electron gun

There are two types of field-emission electron gun (FEG); the cold cathode type and the thermal (thermally assisted) type. The FEG emits electrons from a sharpened tip of a cathode by applying a strong electric field. Compared with the thermionic-emission electron gun, the beam current of the FEG is small but its brightness is as high as 107 to 108. The FEG produces a small electron probe with a small energy spread. The FEG is essential for electron holography.

Related term
field emission, Schottky effect, thermal (thermally assisted) field-emission electron gun, cold (cathode) field-emission electron gun, probe diameter, energy resolution, electron holography

20-4. thermal (thermally assisted) field-emission electron gun

The thermal (thermally assisted) field-emission electron gun (TFEG) emits electrons from a tungsten (W) tip emitter by tunneling the potential barrier (~4.5 eV) where the emitter is heated at ~1600 K in a strong electric field. Compared with the cold cathode type, its emission current is very stable for a long time because the emitter does not adsorb residual gases due to constant heating. Thus the electron gun is more advantageous for micro-area or nano-area analysis than the cold cathode type. The energy spread of the emitted electrons from the TFEG is ~0.7 eV. Its brightness is as high as <8×108 A/cm2.sr at 200 kV. The size of the virtual source produced is >10 nm. This type of gun is not available commercially but has been replaced by a Schottky type gun.

Related term
field-emission electron gun, FEG, Schottky effect, cold (cathode) field-emission electron gun

21. thermoelectron

Electron(s) emitted by heating a substance. For example, electrons are emitted from a heated tungsten filament or an LaB6 tip.

22. thermionic-emission electron gun

An electron gun, which emits thermoelectrons from the tip of the cathode by heating a tungsten filament or a lanthanum hexaboride (LaB6) tip.

Related term
hairpin filament, lanthanum hexaboride single-crystal tip

23.noise canceller

In a cold-field emission gun, short-time intensity fluctuations of the emission current (called "emission noise (chip noise)" exist. As a result, light-and-dark horizontal line contrast appears in an image of SEM or STEM. A system to reduce this adverse contrast is called "noise canceller." The noise canceller consists of a detector and an operational circuit. The canceller detects a part of the emission current, feeds back the fluctuation signal to the image signal to remove the effect of the fluctuation, and reduces the horizontal line contrast on-line. In this process, the emission current is detected by a dedicated aperture for current measurement or a condenser aperture fitted with a current-detection mechanism.

24. beam-rocking technique

A technique that rocks the incident electron beam at a point on the specimen in the orthogonal (x, y) directions over a certain angular range. Using the double-deflection system, the electron beam is rocked by the first-stage coils and synchronously unrocked to illuminate the same specimen position by the second-stage coils. The technique is used to observe variations of ALCHEMI signals against angular changes in the x and y directions, and to obtain LACBED patterns.

Related term
double-deflection system, ALCHEMI, large-angle convergent-beam electron diffraction, LACBED

25. extraction electrode

An electrode in a field-emission electron gun, to which a positive potential (voltage) is applied for extracting electrons from the emitter. A voltage of 2.5 to 3 kV is applied to the cathode.

Related term
field extraction

26. probe diameter

The diameter of the incident electron beam on the specimen. The minimum probe size (diameter) at present is ~0.2 nm for the field-emission electron gun, whereas ~1 nm for the LaB6 thermionic-emission electron gun. When a Cs corrector is used, the probe diameter less than 0.1 nm is achieved.

Related term
field-emission electron gun, FEG, thermionic-emission electron gun

27. hairpin filament

A filament used for a thermoelectron source. A thin tungsten (W) wire is curved to form a hairpin shape and this filament is directly heated at about 2800 K. Its brightness is 5×105 A/cm2・sr at 200 kV. The size of the crossover is ~20 μm. The energy spread of the emitted electrons from the filament is ~3 eV. Nowadays, most electron microscopes are equipped with a LaB6cathode.

Related term
lanthanum hexaboride, thermionic-emission electron gun

28. Boersch effect

When the current of electrons emitted from the electron gun is increased, Coulomb interactions between the electrons make an increase of the energy spread of the electrons. This phenomenon is termed "Boersch effect," which gives rise to the increase of the chromatic aberration.

Related term
chromatic aberration

29. potential barrier

"Potential barrier" means a barrier expressed in the potential dimension that stops the passing of particles through certain regions.

Related term
Schottky effect

30. anode

An electrode, to which a positive potential (voltage) is applied against the facing cathode (electron source). The "anode" receives the flow of electrons (electron beam) emitted from the cathode, and guides the electron flow downward through the hole at its center. In the case of a six-stage acceleration of 200 kV, the voltage applied to the anode is ~33 kV against the cathode.

Related term
cathode

31. lanthanum hexaboride

31-1. lanthanum hexaboride

A material used for the tip of the thermionic-emission electron gun, instead of tungsten used so far. This tip requires a higher vacuum than the tungsten filament, but its brightness is higher than that of the tungsten filament.

31-2. lanthanum hexaboride single-crystal tip

A tip used as a thermoelectron source. A lanthanum hexaboride (LaB6) single crystal, which is sharpened to a cone shape, is used. The LaB6 tip is indirectly heated at about 1800 K. Its brightness is 5×106 A/cm2.sr at 200 kV. The size of the crossover is ~10 μm. The energy spread of the emitted electrons from the filament is ~2 eV.

Related term
hairpin filament, field-emission electron gun, FEG

32. cold (cathode) field-emission electron gun

The cold (cathode) field-emission electron gun (CFEG) emits electrons from the tungsten (W) tip emitter by tunneling the potential barrier (~4.5 eV) where the emitter is kept at room temperature in a strong electric field. Since the energy spread of the emitted electrons from the CFEG is narrower (~0.4 eV) than the Schottky type, the CFEG provides a superbly high energy resolution in EELS. Since the size of the virtual source produced is as small as ~10 nm, the electron beam has a high coherence, suitable for electron holography. Its brightness is as high as ~8×108 A/cm2.sr at 200 kV. The CFEG can produce a small-sized probe, but its total emission current is small. Thus, it is suitable for high magnification observations in the TEM mode but the Schottky type gun is more convenient at low magnification observations. Since the emitter surface is likely to be contaminated by residual gases, the emission current is likely to fluctuate. It is necessary to flash the emitter tip in a commercially-available CFEG at intervals of about 8 hours. Recently, the stability of the beam current has greatly been improved by acquiring a better vacuum. Thus, the barrier for EELS, EDS and WDS experiments by using CFEG is lowered.

Related term
field-emission electron gun, FEG, thermal (thermally assisted) field-emission electron gun, electron holography

33. flashing

The cold (cathode) field-emission electron gun (CFEG) is operated only under an electric field without heating the emitter, while the Shottky type electron gun is used by heating the emitter. Thus, the emitter surface of the CFEG suffers gas adsorption and ion sputtering, leading to a decrease of the emission current and an unstable emission current. To remove adsorbed gasses and surface roughness due to ion sputtering, the emitter is heated. This procedure is called “flashing.” In the existing CFEG, immediately after flashing, the work function of the emitter becomes large and the emission current decreases due to gas adsorption onto the clean emitter surface. Thus, the CFEG is usually used after the emission current becomes stable with the emitter surface being is covered with a thin absorbed-gas layer, and is used until its emission current is lowered by absorption of many gasses. In recent years, a CFEG has been developed, whose pressure of the residual gases in the vicinity of the emitter surface is low. The newly developed CFEG achieves no waiting time after flashing and a stable emission current with less contamination by maintaining the work function small. It provides a high brightness electron beam for a long operation time.

Related term
cold (cathode) field-emission electron gun, Schottky-type electron gun