Basic knowledge of vacuum

- Dec 12, 2018-

Basic knowledge of vacuum

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Physical vapor deposition (PVD) techniques, such as evaporation, sputtering, and ion deposition, can only be realized under vacuum conditions.

The preparation of modern thin film materials, whether physical vapor deposition technology (PVD) or chemical vapor deposition technology (CVD), involves gas phase generation, transportation, reaction, condensation, deposition and other processes under vacuum conditions. Therefore, the basic knowledge of vacuum involved in the preparation of thin films is briefly introduced in this paper.

Basic knowledge of vacuum

Using external force to remove the gas molecules in a certain closed space, so that the pressure in the space is less than one atmosphere pressure, then the physical state of the gas in the space is called vacuum.

In 1643, Torricelli's famous atmospheric pressure experiment revealed for the first time the existence of vacuum, a low-pressure, thin gas physical state, and obtained the definition of atmospheric pressure (the pressure generated by a 76mm mercury column is defined as 1atm) and the basis for vacuum measurement.

The vacuum degree is represented by the pressure of the gas, and the initial unit of vacuum degree is mmHg (1atm=760mmHg).

In 1958, in memory of Torricelli, the first four letters of his name torr were used to replace mmHg as the unit of vacuum degree (1 torr = 1 mmHg).

The centimeter-gram-second (CGS) system was also adopted, with bar as the unit (1bar=1 x 105Pa), and more commonly mbar (1mbar=100Pa).

At present, with the progress of standardization, the international system of units (SI system, namely MKS system) is gradually prevailing, and the vacuum degree takes Pa as the unit (1atm=1.013 * 105Pa).

Remember that the vacuum in brackets is usually converted into units, and mom won't have to worry about me getting confused by the different units in the literature.

Vacuum acquisition

For example, when drinking a drink through a straw, the principle is that we suck away the air in the straw and create a vacuum inside the straw (the pressure inside the straw is less than the external atmospheric pressure). Under the action of pressure difference, we press the drink inside the can into our mouth through the straw.

Similarly, when preparing modern thin film materials, the vacuum required can also be "sucked" away from the air deposited in the room by a device we call a vacuum pump.

According to the working principle of the vacuum pump, it can be divided into two categories: gas transport pump (gas is constantly inhaled and discharged out of the vacuum pump to achieve the purpose of exhaust) and gas capture pump (using activated carbon and other inspiratory materials and cold source device to be sucked away by gas molecules in the pumping space). According to the vacuum pump operating pressure range, can be divided into the first stage of the pump (high starting pressure) and after the stage of the pump (low starting pressure).

The appearance and internal structure of rotary vane mechanical pump is shown in figure 1. It is a kind of gas transport pump, which can work directly from atmospheric pressure. It is a commonly used front-stage pump.

Figure 1 mechanical pump appearance and internal structure diagram

The working principle of the mechanical pump is to use the rotation of the rotor of the mechanical moving parts on the eccentric wheel to achieve the purpose of inspiration-compression-exhaust, as shown in figure 2 (the gray dots in the figure represent the air).

FIG. 2 schematic diagram of working principle of mechanical pump

Turbomolecular pump is a kind of high-altitude pump generated by modern vacuum technology for the requirements of oil-free and high-vacuum environment. It is a kind of gas transport pump. However, its initial working pressure is required to be less than 1Pa. Its appearance and internal structure are illustrated in figure 3.

FIG. 3 appearance and internal structure of turbine molecular pump

In the molecular pump of the turbine, multi-stage rotors and stators with different rotors and stators are interlaced, and the rotor blade speed is up to 20000~60000k r/min. The gas molecules transported from the upper blade will be further compressed to the lower one under the action of the lower blade, that is, kinetic energy is continuously transferred to the gas molecules through collision, and the gas molecules will be compressed and discharged step by step after being endowed with kinetic energy, as shown in FIG. 4.

FIG. 4 working principle of turbine molecular pump

It is worth mentioning that, in the process of preparing the film, do not directly run the molecular pump, because in the low vacuum (more atmospheric molecules) conditions molecular pump blade is easy to be damaged, was found by the boss criticized small, in case the molecular pump burst damage to their own can not be cost-effective. So it is important to remember to open the mechanical pump and other pre-stage pump, to obtain a certain degree of vacuum before the operation of molecular pump.

Vacuum measurement

In order to understand the vacuum degree (air pressure) in the deposition chamber in real time, a vacuum gauge (vacuum gauge) is needed in the preparation of the film.

According to the principle of vacuum degree measurement, it can be divided into absolute vacuum meter (directly determine the pressure value in a certain space) and relative vacuum meter (first measure other physical quantities related to the pressure, after conversion to obtain the pressure value). Because vacuum gauge is easy to measure, it is often used to measure the vacuum degree of film deposition.

As mentioned above, the vacuum pump has strict requirements of the vacuum operating range, similarly, different degrees of vacuum, need to use different vacuum meters to measure.

Pirani vacuum gauge is often used for low vacuum measurement, which is an improved form of thermocouple vacuum gauge. FIG. 5 is a schematic diagram of the working principle. There are two sets of filaments in the tube. When the two groups of filament are energized and heated, the heat dissipation speed on the filament is also different due to the difference in the ambient air thinness. Therefore, the resistance of the two groups of filament will be different due to the difference in temperature, and the current flowing through the filament will also change accordingly. Due to the fixed air pressure at the reference end, the filament temperature, resistance and current on the reference section remain unchanged, so the vacuum degree in the cavity to be measured can be obtained by comparison.

FIG. 5 schematic diagram of pirani vacuum gauge's working principle

The measurement field of high vacuum adopts ionization vacuum meter, which needs to be used together with pirani and other low vacuum meters. The ionization vacuum gauge is mainly composed of three electrodes: cathode (filament), anode and ion collector. Its working principle is shown in figure 6. Electrons emitted from the hot cathode accelerate to the grid, collide with and ionize molecules of the gas in their path. When electrons reciprocate to speed up and decelerate, they will eventually be trapped by pole deletion. In the process of electron reciprocation oscillation, gas molecules will be ionized continuously, and gas ions will fly to ion collection poles to form loop current. In the case of fixed cathode emission current and fixed gas type, the ion current intensity will only depend on the pressure of ionized gas, and the vacuum degree in the deposition room can be converted by the ion current intensity.

FIG. 6 schematic diagram of working principle of ionization vacuum gauge

Through the introduction of this article, we believe that we have the most basic understanding of the definition of vacuum, conversion of different units, and the acquisition and measurement of vacuum.