真空系统
目录
- 1 1. (oil) rotary pump
- 2 2. (oil) diffusion pump
- 3 3. rough pumping
- 4 4. ion pump (sputter ion pump)
- 5 5. liquid-nitrogen trap
- 6 6. outgas
- 7 7. back-stream
- 8 8. differential pumping
- 9 9. sublimation pump
- 10 10. isolation valve
- 11 11. starting pressure
- 12 12. evacuation system
- 13 13. scroll pump
- 14 14. turbo-molecular pump
- 15 15. Venting
- 16 16. Degas
- 17 17. ultra-high vacuum
- 18 18. electromagnetic (solenoid) valve
- 19 19. ionization (vacuum) gauge
- 20 20. ultimate pressure
- 21 21. dry-pumping system
- 22 22. backing pressure
- 23 23. hydrocarbon
- 24 24. B-A (vacuum) gauge
- 25 25. Pirani (vacuum) gauge
- 26 26. bakeout device
- 27 27. Penning (vacuum) gauge
- 28 28. main pumping
- 29 29. baking
- 30 30. pre-evacuation
- 31 31. liner tube
- 32 32. critical backing pressure
1. (oil) rotary pump
The (oil) rotary pump rotates a rotor in a case at the hermetic state maintained using oil to aspirate gas molecules into the inlet, and the gas molecules are compressed to open a valve at the outlet, then gasses are ejected to the air. Since the rotary pump operates from atmospheric pressure (105 Pa), the pump is used for rough pumping of a TEM and for evacuation of the back side of the (oil) diffusion pump and the turbo-molecular pump. The working pressure is 1055 to 1 Pa. If a long-time evacuation is continued at the evacuation limit, reflux of the oil vapor of the pump to the vacuum chamber occurs. Thus, the long-time evacuation by the pump should be avoided. When an oil-free pump is required, the scroll pump is used.
Related term scroll pump
2. (oil) diffusion pump
The (oil) diffusion pump heats the oil to a high temperature. The oil vapor is sprayed out at a high speed from the nozzle. The residual gas molecules are carried with the aid of the jet flow of the vapor. The back side of the pump is evacuated with a rotary pump. The pump mechanism is simple and its price is low. Since the diffusion pump operates from 10-1 Pa and its evacuation speed is high, the pump is used for evacuation of the TEM camera chamber with a large evacuation volume and large gas emission. The working pressure is from 10-1 to 10-8 Pa. When oil-free, high vacuum is required, the pump cannot be used. The pump is not suitable for pumping H2O.
3. rough pumping
"Rough pumping" means pumping from atmospheric pressure to the pressure for switching to the main high-vacuum pumping. In the rotary-diffusion pump system, the switching pressure is between 1 Pa and 0.1 Pa.
Related term main pumping
4. ion pump (sputter ion pump)
A dry vacuum pump that does not use oil. In the sputter ion pump, residual gasses are ionized utilizing electric and magnetic fields. The produced ions strike a titanium cathode to sputter titanium atoms. Then, the sputtered titanium atoms form a fresh getter film (film subjected to chemical adsorption). As a result, active gasses (hydrogen, oxygen, carbon monoxide, etc.) are adsorbed to the getter film, and inert gasses (helium, etc.) are ionized and adsorbed to the cathode. The pump is used for evacuation of the electron gun chamber and the column of TEM, which require high vacuum. The working pressure is 10-4 to 10-9 Pa.
5. liquid-nitrogen trap
A device to condense gas molecules onto a metallic surface exposed to vacuum by cooling the surface with liquid nitrogen. In a TEM, the "liquid-nitrogen trap" is utilized to maintain high vacuum in the microscope column to prevent contamination.
Related term contamination
6. outgas
"Outgas" means gas desorption from an object. In the case of a TEM, outgas means that deposited substances are emitted as gas from surfaces exposed to vacuum in a TEM column.
Related term degas
7. back-stream
"Back-stream" is a phenomenon where the oil mist from the rotary pump or the diffusion pump flows to the high-vacuum side. The back-stream must be prevented.
Related term (oil) rotary pump, (oil) diffusion pump
8. differential pumping
"Differential pumping" means that two chambers connected through an orifice or a fine tube with high evacuation resistance are evacuated by separate pumps. Each chamber reaches the pressure determined by the performance of the pumps, without influenced by each other. The camera chamber and the column achieve differential pumping because a fixed aperture (0.5 to 2 mm in diameter) or an orifice is inserted between them. Differential pumping is also executed between the specimen chamber and the electron gun chamber.
9. sublimation pump
In the sublimation pump, titanium is heated and sublimated under a high vacuum. A fresh titanium-deposited film is formed on the inner wall of the pump. The getter action (chemical adsorption of gas molecules) of the fresh film surface is efficient for pumping in the high-vacuum region. The working pressure is 10-5 Pa or better. Since the pump needs heating, its long-time use is inadequate to maintain a high vacuum. To overcome this problem, the sublimation pump is combined with the ion pump, where the ion pump is constantly operated and the sublimation pump is operated when needed. The pump is not used so much to date.
10. isolation valve
A valve to isolate a certain part from the other part. Each "isolation valve" is placed between the electron gun chamber and the column, the specimen chamber and the column, and the column and the camera chamber, so that the vacuum in a part is broken without any effect on the vacuum in the other parts.
11. starting pressure
When the evacuation pump can start without damage and also can perform a normal evacuation action at a certain pressure, the pressure at the start of the pump is called the "starting pressure."
12. evacuation system
The evacuation system of a TEM is composed of the electron gun chamber, the column, the specimen chamber and the camera chamber, and various pumps, valves, and exhaust tubes. In the normal TEM, the electron gun chamber and the column are evacuated by the (oil) rotary pump-the (oil) diffusion pump-the ion pump system, whereas the camera chamber is evacuated by the (oil) rotary pump-the (oil) diffusion pump system. When the electron gun is of the field-emission type, an independent ion pump is used for the electron gun chamber to obtain a high vacuum. Recently, due to increasing requirements for oil-free evacuation system, a system using scroll pump-turbo molecular pump is becoming popular for the evacuation of the camera chamber and the column.
13. scroll pump
A dry vacuum pump that does not use oil. By rotating the scroll blade of the scroll pump, residual gasses are carried to the central part of the pump, are compressed and evacuated. Since oil is not used except a grease for the scroll shaft, the pump is oil-free and oil mist is not exhausted, which occurs for the rotary pump. The pump features low vibration and no noise. The working pressure is from 105 Pa to 1 Pa. Recently, requirements for an oil-free pump evacuation system are increasing even for the camera chamber. For such needs, a combination of the turbo-molecular pump and the scroll pump is adopted, instead of the diffusion pump.
14. turbo-molecular pump
A mechanical, dry vacuum pump that does not use oil. In the turbo-molecular pump, a high-speed rotor provides momenta to residual gas molecules in the pumping direction and the gas molecules are pumped. A rotary pump or the other pump is required to evacuate the backside of the turbo-molecular pump. The pump operates at low to high vacuum. The working pressure is 10-1 to 10-8 Pa. Recently, problems of sound noises and mechanical vibrations have been solved. Any gas can be pumped by this pump. Unlike the diffusion pump, the pump is suitable for pumping water H2O. Thus, the pump is sometimes used for a biological TEM. The price of the pump is higher than the ion pump. Care has to be paid for mechanical failure.
15. Venting
"Venting" is to make the pressure in the vacuum chamber to atmospheric pressure. In venting, air or nitrogen gas is used. (The latter case is used to prevent the entering of moistures in the chamber.)
16. Degas
"Degas" is to emit gasses from an object by artificial operation. In the case of a TEM, degas is carried out by evacuation of the electron-gun and the column and by heating these parts to accelerate gas emission. On the other hand, natural gas desorption is termed "outgas."
17. ultra-high vacuum
A high vacuum of 10-5 Pa or better. To achieve an "ultra-high vacuum," high-end technology is required, which includes selection of appropriate materials of pumps, containers, etc.
18. electromagnetic (solenoid) valve
A valve that operates by an electromagnet (solenoid). The "electromagnetic (solenoid) valve" is used to control the flow of gas or liquid through a tube, and to open or close a vacuum valve.
19. ionization (vacuum) gauge
A vacuum gage that measures the gas pressure by ionization mechanism. In the ionization (vacuum) gauge, residual gasses are ionized, and produced ions or electrons are captured, then the gas pressure is measured using the produced current. Two types of ionization gauge are available: a thermal-cathode ionization gauge that heats a filament and ionizes gasses using thermoelectrons (ex. B-A gauge) and a cold-cathode ionization gauge that ionizes gasses using plasma (ex. Penning gauge).
Related term Penning (vacuum) gauge, B-A (vacuum) gauge
20. ultimate pressure
In a vacuum pump system, the lowest pressure that can be reached within a realistic evacuation time is called the "ultimate pressure."
21. dry-pumping system
An evacuation system that does not use oil or water. The "dry-pumping system" includes the scroll pump, the sputter ion pump and the turbo-molecular pump.
Related term scroll pump, sputter ion pump, turbo-molecular pump
22. backing pressure
22-1. backing pressure
(When gasses are exhausted to a space with a pressure equal to or below atmospheric pressure (105 Pa),) the pressure at the exhaust port (outlet) is called the "backing pressure."
22-2. critical backing pressure
The maximum backing pressure at which the diffusion pump can operate correctly is called the "critical backing pressure."
Related term (oil) diffusion pump
23. hydrocarbon
"Hydrocarbon(s)" deposited and adsorbed on the components of a TEM are emitted in the microscope column and are deposited on the surface of a specimen. If this specimen is irradiated with a high-energy electron beam, the hydrocarbons are polymerized, resulting in contamination.
Related term contamination
24. B-A (vacuum) gauge
A thermal-cathode ionization gauge. In the B-A (vacuum) gauge, a heated filament emits electrons and the emitted electrons are accelerated, and these electrons ionize residual gasses, then the produced ion current is measured. The B-A gauge measures the pressure of the middle to high vacuum region. The measurable range is 0.1 to 10-5Pa. The gauge can measure lower pressures than the Penning gauge can. Since the gauge is made of a glass tube, it is likely to be damaged. In addition, the gauge has a gas emission problem. Thus, the B-A gauge adopts, in many cases, a design where the gauge head is directly exposed to the vacuum. This is called a nude gauge (measurable pressure: ～10-9Pa). The output current is proportional to the pressure. For a TEM, the gauge is mainly used to measure the pressure in the ultra-high vacuum specimen chamber.
25. Pirani (vacuum) gauge
A thermal conductivity (vacuum) gauge that utilizes a phenomenon where electrical resistance of a heated element changes with temperature. In the Pirani (vacuum) gauge, a metal (tungsten) wire placed inside a tube is heated by electric current. Thermal conductivity of the metal wire changes with the degree of vacuum through change of the temperature of the wire. The measurable range is 0.1 to 103Pa. For a TEM, the gauge is used to detect the switching pressure from the rotary pumping (RP) to the diffusion pumping (DP).
Related term Penning (vacuum) gauge
26. bakeout device
A device that bakes out vacuum chambers, such as the FEG chamber, the inside walls of the microscope column and the specimen stage, to obtain a high vacuum in a TEM.
Related term baking
27. Penning (vacuum) gauge
A cold-cathode ionization gauge. In the Penning (vacuum) gauge, an annular electrode (anode) is placed between a pair of parallel plate electrodes (cathode), and a magnetic field is applied parallel to their axes to lengthen the traveling distance of electrons. As a result, constant discharge occurs even at a low pressure, thus the Penning gauge can measure the pressure of the middle to high vacuum region or 1 to 10-4Pa. The produced current is proportional to the pressure of the vacuum. For a TEM, the gauge is used for switching of the evacuation system from the diffusion pumping (DP) to the sputter ion pumping (SIP) because the gauge is robust and easy to use.
Related term Pirani (vacuum) gauge, B-A (vacuum) gauge
28. main pumping
"Main pumping" means pumping for high-vacuum after rough pumping is completed.
Related term rough pumping
29. baking
"Baking" is to heat vacuum chambers under a vacuum state, such as the FEG chamber, the inside walls of the microscope column and the specimen stage, to obtain a high vacuum in a TEM. Baking accelerates degas from the vacuum chambers and decreases subsequent outgas. When a TEM equipped with an FEG is used, baking of the FEG chamber is performed for more than 40 h at a high temperature of ～300 ℃ so that stable operation and long lifetime (operating time) of the FEG are achieved and also discharge damage to the FEG is decreased. When the column and stage which use O-rings are baked, the baking is carried out for about 3 days at ～60 ℃ because these parts cannot be baked at a high temperature.
30. pre-evacuation
"Pre-evacuation" is to evacuate the pre-evacuation chamber for the specimen holder from atmospheric pressure to a certain low pressure. In the pre-evacuation process, first the specimen pre-evacuation chamber is vented and a specimen is loaded on the specimen holder, and next the chamber is pre-evacuated, and finally the isolation valve is opened and the specimen is transferred to the specimen chamber.
31. liner tube
An aluminum pipe with a diameter of 6 to 7 mm, which is inserted into the illumination and imaging systems in the microscope column, to prevent contamination in the column and specimen chamber due to gasses from lenses and other components.
32. critical backing pressure
The maximum backing pressure at which the diffusion pump can operate correctly is called the "critical backing pressure."
Related term (oil) diffusion pump
