fundamentals of vacuum technology

- May 10, 2019-

fundamentals of vacuum technology


In 1643, Italian physicist torricelli demonstrated the famous atmospheric pressure experiment, revealing the existence of the physical state of "vacuum" for human beings. In the following centuries, especially in the early 20th century, vacuum technology developed rapidly and was widely used in military and civil fields. Similarly, vacuum technology is also the basis of thin film preparation. Almost all thin film materials are prepared in a vacuum or at low atmospheric pressure. Therefore, in this chapter, we will briefly introduce some basic knowledge of vacuum, vacuum acquisition and vacuum measurement.


First, the basic knowledge of vacuum

One,A unit of vacuum

Human contact with the vacuum can be roughly divided into two kinds: one is the existence of space vacuum, called the "natural vacuum"; The other is the vacuum obtained by pumping gases from a container with a vacuum pump. It is called an "artificial vacuum". A vacuum of any kind is called a vacuum when the pressure of a gas is less than one atmosphere in a given space. The state of space without any gas is often called an absolute vacuum. In the general sense, "vacuum" does not mean "there is nothing". At present, there are hundreds of molecules per cubic centimetre of volume, even at the lowest pressure possible with the most advanced vacuum preparation techniques. Therefore, when we talk about vacuum, we mean relative vacuum state. In vacuum technology, the idiomatic term "vacuum degree" and the physical quantity "pressure" are commonly used to express the degree of vacuum in a certain space, but their physical meanings should be strictly distinguished. A lower pressure in a space means a higher vacuum, whereas a higher pressure space means a lower vacuum.


One of the earliest and most widely used units of pressure, the millimeter mercury (mmHg), measures the vacuum directly by measuring its length. Especially in the use of torricelli pneumatic timing, the millimeter as a pressure measurement is more intuitive. But in 1958, in memory of torricelli, "Torr" was used instead of MMHG. 1 torr is the pressure per unit area of 1 MMHG of MMHG in the standard state, expressed as 1 torr = 1 MMHG. In 1971, international metrology conference formally determined "PASCAL" as the international unit of gas pressure, 1Pa = 1 N/m2 7.5 10-3torr. Table 1-1 shows the units of pressure commonly used in current vacuum technology and their conversion relations.


Table 1-1 conversion relations of several pressure units



Two.The division of vacuum regions

In order to study the vacuum and make it convenient for practical use, the vacuum is often divided into the following areas according to the different physical characteristics of each pressure range:

Rough vacuum: 1 x105 ~ 1 x102 Pa
Low vacuum: 1x 102 ~ 1x 10-1pa
High vacuum: 1x 10-1 ~ 1x 10-6pa
Ultra high vacuum: <1x 10-6pa


The molecular motion properties of gases in different regions of vacuum are different. Under rough vacuum, the gaseous space is approximately atmospheric state, and the molecules are still mainly in thermal motion, and the collisions between molecules are very frequent. Low vacuum is the transition of the flow of gas molecules from viscous flow state to molecular state. When the high vacuum is reached, the flow of gas molecules has become a molecular flow, and the collisions between gas molecules and vessel walls are mainly, and the number of collisions is greatly reduced. At ultra high vacuum, there are fewer molecules in the gas, there are almost no collisions between molecules, and there are fewer chances for molecules to collide with the wall.



Three.Adsorption and desorption of gases by solids

In vacuum technology, a variety of gases are often encountered, and the phenomenon of adsorption and desorption of these gases on the solid surface is very common, which is of great significance for high vacuum technology, especially ultra-high vacuum technology. For example, in order to improve the vacuum degree in the tube, the parts need to be degassed in advance. This process is the process of the gas molecules on the solid surface desorption. With the desorption of the gas, a certain degree of vacuum will be formed in the container. In addition, in vacuum equipment, various adsorption pumps are often made by using the adsorption principle to obtain a high vacuum. Sometimes, the ability of adsorbing a large number of gas molecules on the clean surface is also used to obtain a vacuum.


The so-called gas adsorption is the phenomenon of gas molecules captured on the solid surface, which can be divided into physical adsorption and chemical adsorption. Physical adsorption has no selectivity, and any gas can occur on the solid surface, which is mainly caused by the mutual attraction between molecules. Desorption is easy to occur in the physically adsorbed gas, and the adsorption is only effective at low temperature. Chemical adsorption, on the other hand, occurs at a higher temperature. Similar to chemical reactions, gas is not easy to desorption, but adsorption can only occur when atoms on the surface of gas and solid come into contact with each other to form compounds. Gas desorption is the reverse process of gas adsorption. The process by which the molecules of gas adsorbed on the solid surface are released from the solid surface is usually called gas desorption.


In vacuum technology, the phenomenon of adsorption and desorption of gas on solid surface always exists. Generally, the main factors affecting the adsorption and desorption of gas on solid surface are the pressure of gas, the temperature of solid, the density of gas adsorbed on solid surface and the properties of solid itself, such as the degree of surface smoothness and cleanliness. When the temperature of the solid surface is high, the gas molecules are easy to desorption. In addition to the above effects, in some vacuum pumps and vacuum meters with ionization phenomenon, there are varying degrees of electric absorption and chemical scavenging, which will also accelerate the adsorption of solid to gas. Among them, electrical absorption refers to the formation of positive ions after ionization of gas molecules. Positive ions have stronger chemical activity than neutral gas molecules, so they often form physical or chemical adsorption with solid molecules. Chemical removal occurs when active metals (such as barium, titanium, etc.) are vacuum-vaporized to form compounds with non-inert gas molecules, resulting in chemical adsorption.


Section two vacuum acquisition


The acquisition of vacuum is often referred to as "vacuum pumping", that is, the use of various vacuum pumps will be pumped out of the gas container, so that the pressure of the space is lower than one atmosphere. At present, the commonly used vacuum equipment includes rotary mechanical vacuum pump, oil diffusion pump, compound molecular pump, molecular sieve adsorption pump, titanium sublimation pump, sputtering ion pump and cryogenic pump. The first three vacuum pumps belong to the gas transfer pump, that is, through the continuous inhalation of gas and out of the vacuum pump to achieve the purpose of exhaust; The last four vacuum pumps belong to the gas capture pump, which is a kind of gas suction space will be sucked to achieve the required vacuum degree by taking advantage of the unique suction effect of various getter materials. Because these capture pump work without oil as a medium, it is also called no oil pump. Table 1-2 lists the operating pressure ranges of several commonly used vacuum pumps and the generally available limiting pressures. The ultimate pressure is one of the important parameters to represent the performance of the vacuum pump. It refers to the minimum pressure when the standard container is used as the load and the pump works normally under specified conditions for a period of time and the vacuum degree no longer changes but tends to be stable. The dotted lines in the table show the areas that the vacuum pump can be extended when used in combination with other devices.


Table 1-2 operating pressure range of several common vacuum pumps


As can be seen from the table, the pressure representing the vacuum degree varies in a range of more than ten orders of magnitude. If air is pumped from the atmosphere, it is difficult to achieve ultra-high vacuum degree with only one vacuum pump, that is, no vacuum pump can cover the working range from atmospheric pressure to 10-8pa. Two or three vacuum pumps are often combined to form a composite exhaust system to obtain the high vacuum required. For example, in an oil vacuum system, the pressure of 10-6~ 10-8pa can be obtained by the combination device of oil seal mechanical pump (both poles) and oil diffusion pump. In the oil-free system, the pressure of 10-6~ 10-9pa can be obtained by the adsorption pump + sputtering ion pump + titanium sublimation pump device. Sometimes there will be oil, oil - free system mix, such as the use of mechanical pump + compound molecular pump device can obtain ultra - high vacuum. Mechanical pump and adsorption pump are from a atmospheric pressure to start pumping, so often referred to as the "front pump", and those can only be from the lower pressure to lower pressure vacuum pump known as the "secondary pump". This section will focus on the structure and working principle of mechanical pump, compound molecular pump and cryogenic pump.


One.Rotary vane mechanical vacuum pump

General use of mechanical movement (rotation or sliding) to obtain a vacuum pump, known as mechanical pump. It is a typical vacuum pump that can start from the atmospheric pressure. It can be used alone or used as the front stage pump of high vacuum pump or ultra high vacuum pump. Because this pump is oil to seal, so it belongs to the oil type of vacuum pump. This kind of mechanical pump commonly have rotary vane type, fixed vane type and sliding valve type (also known as plunger type), among which rotary vane type mechanical pump is the most common.



Rotary vane vacuum pump is oil to maintain the sealing between the moving parts, and by mechanical means, so that the volume of the sealing space increased periodically, that is, pumping; Reduce, exhaust namely, achieve the purpose that takes out gas and exhaust continuously thereby. FIG. 1-1 is the structure diagram of single-stage rotary vane pump. The pump body is mainly composed of stator, rotor, rotary vane, inlet pipe and exhaust pipe. The stator ends are sealed to form a sealed pump chamber. Pump chamber, eccentrically equipped with a rotor, the actual equivalent of two inscribed circle. An opening slot is opened along the axis of the rotor, in which there are two pieces of rotor blades. The middle part of the rotor blades is connected with a spring, and the spring makes the rotor blades always slide along the inner wall of the stator when the rotor rotates.


As shown in FIG. 1-1, rotary vane 2 divides the pump chamber into A and B parts. When the rotary vane rotates in the direction given in the figure, since the space pressure behind the rotary vane 1 is less than the pressure at the air inlet, the gas is sucked through the air inlet, as shown in FIG. 1-2(A). Figure 1-2(b) shows the inspiration cutoff. At this point, the pump to the maximum intake, gas began to compress; When the rotor continues to move to the position shown in 1-2(c), the air compression increases the space pressure after the rotor 1. When the pressure is higher than 1 atmosphere, the gas pushes the exhaust valve to discharge the gas. Continuing to move, the rotor is returned to the position shown in FIG. 1-1. The exhaust is over and the next cycle of intake and exhaust is resumed. The ultimate vacuum of single-stage rotary vane pump can reach 1Pa, while that of two-stage rotary vane pump can reach 10-2pa.


FIG. 1-1 structure diagram of rotary vane pump



FIG. 1-2 schematic diagram of rotary vane pump

As a result of pump work, stator, rotor all immersed in the oil, in each inhalation, exhaust cycle will have a small amount of oil into the container, so the requirements of mechanical pump oil to have a low saturation vapor pressure and a certain lubricity, viscosity and higher stability.



Two.Compound molecular pump

Molecular pump is a major development of rotary vane mechanical vacuum pump. Like the mechanical pump, the molecular pump is also a gas transfer pump, but it is an oil-free pump, can be combined with the front pump device, so as to obtain ultra-high vacuum. At present, molecular pump can be divided into traction pump (pressure pump), turbine molecular pump and composite molecular pump three categories. Among them, the traction pump in the structure is more simple, small speed, but the compression ratio is large; Turbo molecular pump can be divided into "open" blade type and overlapping blade type. Former rotate speed is high, pumping speed is bigger also, latter criterion is opposite. The compound molecular pump combines the advantages of high extraction capacity of the turbine molecular pump with the advantages of large compression ratio of the traction molecular pump, and USES the high-speed rotating rotor to carry gas molecules to obtain ultra-high vacuum. Figure 1-3 is its structure diagram.


FIG. 1-3 molecular pump structure diagram


The pump has a speed of 24,000 RPM. The first part is a turbo molecular pump with several stages of open blades, and the second part is a multi-groove traction molecular pump with a pumping speed of 460L/s and a compression ratio of 150 when the speed is zero.


Three. Cryopump

Cryogenic pump is a kind of pump that condenses gas molecules to realize pumping by using the low temperature surface below 20K. It is the pump with the highest limit vacuum at present. It is mainly used in large vacuum systems, such as high-energy physics, preparation of superconducting materials, aerospace space simulation station, etc. Cryogenic pump also known as condensing pump, cryogenic pump. According to its working principle, it can be divided into cryogenic adsorption pump, cryogenic condensation pump, cryogenic machine cryogenic pump. The first two pumps directly use cryogenic liquid (liquid nitrogen, liquid helium, etc.) for cooling, the cost is high, usually only as an auxiliary means of pumping; Cryogenic pump of refrigerator is a pump that USES deep low temperature generated by refrigerator to extract air. Its basic structure is shown in figure 1-4. The cooling head of the first stage of the refrigerator is equipped with a radiation screen and a radiation baffle at a temperature of 50-77k, which is used for condensation and extraction of water vapor and carbon dioxide and other gases. The deep cold plate is installed on the second-level cold head with a temperature of 10-20k. The smooth metal surface on the front of the plate can remove gases such as nitrogen and oxygen, while the activated carbon on the opposite side can absorb gases such as hydrogen, helium and neon. The purpose of removing all kinds of gases can be achieved through the cold head at both poles, so as to obtain the ultra high vacuum state.


Cryogenic pump as a capture pump, can be used to capture a variety of including harmful or flammable and explosive gas, make it condensation on the refrigeration plate, in order to achieve the purpose of pumping. However, after a period of time, the cryogenic pump low temperature exhaust capacity will be reduced, so must be "regenerative" treatment, that is, the removal of low temperature condensation layer. Regeneration must follow the following requirements:


(1) once the regeneration process is started, it must be completely cleared. This is because the local heating will cause a large amount of condensed water vapor on the shield plate to transfer to the internal cryogenic aspirating plate, seriously damaging the pumping capacity of the cryogenic pump.
(2) during regeneration, the condensation layer should be vaporized stably, and the gas pressure in the system must not exceed the allowable value. Otherwise, when hydrogen is removed, such flammable and explosive gas, once it leaks into the air, there will be an explosion risk.
(3) during regeneration, the hydrocarbon from the front stage pump shall be strictly prevented from entering the cryogenic pump to contaminate the suction surface, so the extraction time shall be as short as possible.


FIG. 1-4 schematic diagram of cryogenic pump structure


Section three vacuum measurement


Vacuum measurement refers to the measurement of vacuum height in a specific space with specific instruments and devices. This instrument or device is called a vacuum gauge (instrument, gauge). There are many kinds of vacuum gauge, which can be divided into absolute vacuum gauge and relative vacuum gauge according to measuring principle. All vacuum gauges that directly obtain the gas pressure by measuring physical parameters are absolute vacuum gauges, such as u-type pressure gauge and compression-type vacuum gauge. The physical parameters measured by such vacuum gauges are independent of the gas composition, and the measurement is relatively accurate. By measuring the physical quantity related to the pressure and comparing with the absolute vacuum gauge, the vacuum gauge that obtains the pressure value is called the relative vacuum gauge, such as discharge vacuum gauge, heat conduction vacuum gauge, ionization vacuum gauge, etc., which is characterized by slightly poor measurement accuracy and is related to the type of gas. In actual production, except for vacuum calibration, most use relative vacuum gauge. This section mainly introduces the working principle and measuring range of resistance vacuum gauge, thermocouple vacuum gauge and ionization vacuum gauge.


One.resistance vacuum gauge


Resistance vacuum gauge is a kind of heat conduction vacuum gauge, it is used to measure the temperature of hot wire in the vacuum, so as to obtain the vacuum degree indirectly. The principle is that the heat conduction of gas under low pressure is related to the pressure, so how to measure the temperature parameters and establish the relationship between resistance and pressure is the problem to be solved by the resistance vacuum gauge.


The structure of the resistance vacuum gauge is shown in figure 1-5. The heating filament in the regulation is a tungsten or platinum wire with a high temperature coefficient of resistance. When heated under low pressure and high strength, the heat Q generated by the filament can be expressed as:

Q = Q1 + Q2

Where Q1 is the heat of the filament radiation, which is related to the filament temperature; Q2 is the heat taken away by the molecules of the gas bumping into the filament, depending on the pressure of the gas. When the temperature of hot wire is constant, Q1 is constant, that is, the heat of hot wire radiation does not change. At a constant current of the wire heating conditions, when the pressure of the vacuum system is reduced, the number of molecules of the gas in the space to reduce, Q2 will decreases, the heat generated by the filament at this time will be relatively increased, the filament temperature rise, the filament resistance will increase, the pressure in vacuum chamber and the existence of such a relationship between the filament resistance P left - > R write down, so the electric resistance of the filament can be used to indirectly determine the pressure.


FIG. 1-5 resistance vacuum gauge

The resistance vacuum gauge measures a vacuum in the range of 105 ~ 10-2pa. As it is a relative vacuum gauge, the measured pressure is largely dependent on the type of gas, and its calibration curves are all for dry nitrogen or air. Therefore, if the measured gas composition changes greatly, the measurement results should be modified to some extent. In addition, after a resistance vacuum gauge is used for a long time, the hot wire will drift to zero due to oxidation. Therefore, it is necessary to avoid contact with the atmosphere for a long time or work under high pressure and strength, and it is often necessary to adjust the current to calibrate the zero position.


Two.ThermoCouple vacuum Gauge

Figure 1-6 is the structure diagram of thermocouple vacuum gauge. The thermocouple vacuum gauge is mainly composed of heating filament C and D (platinum wire) and thermocouple A and B (platinum-rhodium or constant-copper-nickel-chromium) used to measure the temperature of hot wire. The thermocouple is connected to the hot wire at the hot end and the millivoltmeter in the instrument at the cold end. The thermocouple electromotive force can be measured from the millivoltmeter. During the measurement, the thermocouple gauge is connected to the vacuum system under test, and the hot wire is connected with a constant current. Different from the resistance vacuum gauge, at this time, part of the heat Q generated by the filament will dissipate in the conduction between the filament and the thermocouple. When the pressure of the gas decreases, the temperature at the junction of the thermocouple increases with the temperature of the hot wire.


Figure 1-6 thermocouple vacuum gauge

The measurement results of thermocouple vacuum gauge for different gases are different, which is due to the different thermal conductivity of various gas molecules. Therefore, when measuring different gases, certain corrections should be made. Table 1-3 shows the correction coefficients for some gases or vapors.


Table 1-3 correction coefficients for common gases and vapors


Of the thermocouple gage measurement range is roughly 102 ~ 10-1 pa, measuring the pressure does not allow too low, this is because when the pressure lower, gas molecules heat conduction heat escape to very few, but by the hot wire, thermocouple wire heat conduction and thermal radiation caused by the heat loss is given priority to, the change of the thermocouple electromotive force will not be caused by the change of pressure.


Thermocouple vacuum gauge has thermal inertia. When pressure changes, the change of hot wire temperature usually lags for some time, so the reading of data should also lag for some time. In addition, like the resistance vacuum gauge, the heating filament of thermocouple meter is also tungsten wire or platinum wire, which will drift due to oxidation if used for a long time. Therefore, the heating current should be adjusted frequently and the heating current value should be recalidated.



Three,ionization vacuum gauge

Ionization vacuum gauge is a widely used vacuum gauge, which is based on the principle of ionization of gas molecules. According to the different sources of gas ionization, it can be divided into hot cathode ionization vacuum gauge and cold cathode ionization vacuum gauge. Figure 1-7 shows the regulatory structure of the general ionization gauge, which is mainly composed of three electrodes: the filament emitting electrons as the emitter A, the gate (also known as the accelerator) B that accelerates and collects electrons spirally, and the cylindrical ion collector C.Where the emitter is connected to zero potential, the accelerating electrode is connected to positive potential (several hundred volts), and the collecting electrode is connected to negative potential (several tens of volts). There is a repulsive field between B and C. Working principle of ionization gauge is hot cathode emission electron, after acceleration pole, most electronic flew to collector, rejects field between B and C, the electronic movement speed is reduced, when speed reduced to zero, the electrons to fly to B, in the electronic fly to B - C space, have also been rejects field effect, the speed is reduced to zero, electronic faces backward fly to C, the repetitive movement of the electrons in the B - C space, to constantly collide with gas molecules, make gas molecules produced by ionization energy, electron was eventually accelerate the collection, and the positive ions are generated by the ionization ion flow collection highly accepted and making the I +, For a certain regulation, when the potential of each electrode is constant, I+ has the following linear relationship with the emission electron flow Ie and the pressure of the gas

I+ = kIeP

Where, k is the proportionality constant, which means the current value of ions obtained under unit electronic current and unit pressure, unit is 1/Pa, which can be determined through experiments. For different gases, the size of k is different, and its existence range is between 4 and 40. When the emission current is constant, the ion flow is only proportional to the pressure of the gas, so the pressure of the gas in the vacuum chamber can be determined according to the size of the ion flow.


FIG. 1-7 ionization vacuum gauge


The measurement range of the common hot-cathode vacuum gauge is 1.33 10-1 -- 1.33 10-5pa, and the linear relationship between ionic flow I+ and the pressure of the gas will lose no matter whether the measurement limit is higher or lower. When the pressure is higher, greatly increase the risk of collision between electron and molecular many times, due to the accelerating potential than gas ionization potential (a few volts) is much higher, so enough to cause gas ionization ionization produced by the electron, so, will make the ionization gauge of electron flow has increased dramatically, due to the high gas density at the same time, the electron free path is short, most of the collision to low-energy collisions, cannot cause ionization, many factors lead to high pressure ion flow and no longer keep the linear relationship between the pressure; When the pressure is low (less than 1.33 X 10-1 pa), high-speed movement of the electrons to accelerometers will produce soft X-ray, soft X-ray and then toward the ion collector, C can cause photoemission collector, emit electrons, so that the original ion flow superposition this has nothing to do with the pressure in the measurement circuit of current, the ion current (I) + and lose the linear relationship between the pressure of the gas, the ionization gauge will not be able to measure the pressure in vacuum chamber.


The ionization vacuum gauge can quickly and continuously measure the total pressure of the gas to be measured, and the regulator is small in size and easy to connect. However, the emitter of the regulator is made of tungsten wire. When the pressure is higher than 10-1pa, the regulator life will be greatly reduced or even burned down. When the vacuum system is exposed to the atmosphere, the inner surface of the glass shell of the gauge and the electrodes will absorb gases, which will affect the accuracy of the vacuum measurement. Therefore, when the vacuum system is exposed to the atmosphere for a long time or used for a period of time, the degassing process of the gauge should be carried out regularly.

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