Today, the editor of Huicheng Vacuum will introduce the basic knowledge of vacuum coating machine in detail. Whether it is a new staff member or an experienced engineer, it is essential to understand the basic knowledge of vacuum coating machine. Learning vacuum coating technology is just like Building a house is the same, you must first build the foundation before you can learn more in-depth professional knowledge and skills step by step.

The definition of vacuum The meaning of vacuum refers to the state of gas below an atmospheric pressure in a given space and is a physical phenomenon. In the "Void", sound cannot be transmitted because there is no medium, but the transmission of electromagnetic waves is not affected by the vacuum. In fact, in vacuum technology, vacuum refers to the atmosphere, and part of the material inside a specific space is expelled to make its pressure less than a standard atmospheric pressure, then we generally call this space a vacuum or a vacuum state. Vacuum commonly used Pascal (Pascal) or Torr (Torr) as the unit of pressure. In the natural environment, only outer space can be called the space closest to a vacuum. In 1641, the Italian mathematician Torricelli filled a long pipe with mercury, and then slowly inverted the nozzle into a basin filled with mercury. The end of the mercury column in the pipe was 76 centimeters high. At this time, the mercury-free zone at the top of the glass tube is in a vacuum state. This experiment is called "Torricelli experiment", and the glass tube that completes the experiment is called "Torricelli tube". When Einstein used field theory to study the phenomenon of gravitation,
Having recognized that the idea of a vacuum of nothingness is problematic, he had proposed the idea that vacuum is a special state of a gravitational field. It was P.A.M. Dirac who first gave the vacuum a new physical content. In 1930, in order to get rid of the dilemma of the negative energy solution of the Dirac equation, Dirac proposed that the vacuum is an electron sea full of negative energy states.
 
        Vacuum classification In vacuum technology, according to the level of pressure, we can be divided into:
        Rough Vacuum 760 ~ 10 Torr
        Medium Vacuum 10 ~ Torr
        High Vacuum ~ Torr
        Ultra-High Vacuum Torr or less
 
        The flow of gas in a vacuum, once the vacuum system is started, there is a directional gas flow in the system pipeline, that is, the flow of rarefied gas along the pipeline. The flow state of the gas in the pipeline is different, and the conductance of the pipeline is also different. That is to say, the influence of the pipeline on the flow of the gas depends not only on the geometry and size of the pipeline, but also on the type, temperature and gas flow in the pipeline. pressure related. Therefore, when calculating the conductance of the pipeline to the gas, it is first necessary to determine which flow state of the gas flow in the pipeline is. The flow of rarefied gas along the pipeline is different and related to normal pressure gas. Studying the flow of rarefied gas along the pipeline is one of the important aspects of vacuum science.
 
         Classification of gas flow states There are five flow states of airflow in the vacuum system pipeline: turbulent flow (also known as turbulent flow, eddy current); turbulent-viscous flow; viscous flow (also known as laminar flow, viscous flow, Poji leaf) flow); viscous-molecular flow; molecular flow (also known as free molecular flow, Knudsen flow). Turbulent-viscous flow is a transition state between turbulent and viscous flow. Viscous-Molecular flow is a transition state between viscous and molecular flow. Turbulence flow: The pressure and flow velocity of the gas in the pipeline are high, the streamlines are irregular and there are vortices, the vortices appear and disappear from time to time, the flow is in an unstable state, the velocity of the particle changes sharply, and the acceleration is large, so the inertial force affects the flow. dominate. It usually exists in the initial stage of vacuum system operation.
 
         Viscous flow: When the pressure and flow velocity are gradually reduced, the streamline has a regular pattern, which changes with the change of the shape of the pipe, and the flow becomes a flow layer with different velocities in each part. Internal friction dominates the flow. Molecular flow: When the gas pressure is further reduced, when the mean free path of the gas molecules is equal to the diameter of the pipe, a new flow state begins to appear. The collisions between the gas molecules are few or even negligible, and the gas molecules rely on their own The thermal motion collides with the pipe wall frequently, and is driven by the molecular density gradient in the pipe, and flows from the high pressure end to the low pressure end. This airflow is the combined effect of the individual movements of each molecule. The phenomenon of "diffuse reflection" is an important physical basis for the special motion law of molecular flow.
         When there are multiple gas components in the container at the same time, the total gas pressure in the container is equal to the sum of the partial pressures of each gas. The total pressure measurement method can be divided into two types: direct method and indirect method. The direct method uses the principles of liquid column difference and mechanical deformation to directly measure pressure, including liquid level pressure gauges, compression vacuum gauges, and elastic element vacuum gauges. According to the physical quantities measured by the first two instruments, the pressure value can be calculated, which belongs to the absolute vacuum gauge. The indirect method uses some physical properties of gas (such as heat conduction, viscosity, ionization and light scattering effects, etc.) to measure pressure, including heat conduction vacuum gauge, viscous vacuum gauge and ionization vacuum gauge. The vast majority of vacuum gauges used in vacuum technology use the indirect method, and these gauges must be calibrated with absolute vacuum gauges or other methods. For the vacuum gauge measured by the indirect method, due to the different physical properties of different types of gases, even under the same pressure, the pressure readings vary with the gas, so it should be calibrated with the corresponding gas. When the gas being measured is not a single component, the meaning of these vacuum gauge readings is more complicated. Since the gas used in general vacuum gauge calibration is pure nitrogen, the readings of these vacuum gauges are collectively referred to as equivalent nitrogen pressure before being corrected by the gas type. When the measured space contains a variety of gas components, only the partial pressure measurement can accurately reflect the vacuum state and total pressure in the container. The common vacuum gauge heat conduction vacuum gauge uses the principle that the heat conduction capacity of the gas changes at different pressures to measure the gas. pressure. In this type of vacuum gauge,
          A certain heating current is passed through the gauge head equipped with a hot wire, and the temperature of the hot wire is determined by the balance between heating and heat dissipation. The heat dissipation capacity is a function of the gas pressure, so the temperature of the hot wire varies with the pressure. When measuring low pressure (P<100Pa), the reference chamber is evacuated to a high vacuum, and its pressure is approximately zero. When the pressure in the measuring chamber is different, the process of membrane deformation
The degree is also different. There is a stationary electrode in the measuring chamber, which forms a capacitor with the membrane. When the film is deformed, the capacitance value changes accordingly, and the change in capacitance can be measured through the capacitance bridge to determine the corresponding pressure value. In order to prevent the film from creeping, the zero-position method is usually used, that is, the fixed electrode and the
A DC voltage is applied between the films, and the electrostatic force is used to compensate the stress generated by the pressure difference of the film, so that the diaphragm remains at zero position. Capacitance film vacuum gauge can directly measure the pressure of gas or vapor. The measured value has nothing to do with the type of gas, the structure is firm, and it can withstand baking. If different gauge heads are used for different pressure ranges, higher accuracy can be obtained. Capacitance film vacuum gauges can be used for high-purity gas monitoring, low-vacuum precision measurement and pressure control, and can also be used as a secondary standard for low-vacuum measurement.
       Ionization vacuum gauge, referred to as ionization gauge, uses the principle of gas ionization to measure pressure. Ionization vacuum gauges are divided into two categories: hot cathode and cold cathode. There are usually three electrodes in the gauge head of hot cathode ionization vacuum gauge, namely cathode, anode and collector, which play the role of emitting electrons, accelerating electrons and collecting ions respectively. The electrons ionize the gas in the process of moving from the cathode to the anode. If the secondary ionization effect is ignored (meaning that the new electrons generated in the ionization process are accelerated by the electric field and gain ionization ability and cause new ionization), each electron emitted from the cathode is ionized. The number of positive ions produced is proportional to the density of the gas in the space, and therefore proportional to the pressure at a certain temperature. Therefore, the ion current Ii=SIeP received by the collector, Ie is the cathode electron emission current, and S is the proportionality constant, which is called the ionometer coefficient. After verifying the ionometer coefficient with a standard vacuum gauge at a certain temperature, the pressure can be determined according to the size of the ion current. The main type of hot cathode ionization gauge head The cathode is generally made of tungsten wire, and the anode can be made into a grid, so that the electrons can
The side travels back and forth to increase the travel of electrons, so it is also called the gate.
       The collector of the triode ionometer is cylindrical and placed outside the grid, and its pressure measurement range is 10-1 to 10-5 Pa. When the working pressure is higher than 10-1 Pa, the life of the tungsten wire is shortened, and the relationship between the ion current and the pressure begins to deviate from the linearity due to the secondary ionization effect. Iridium filament cathodes coated with thorium oxide or yttrium oxide can work at pressures up to 100 Pa and have a fairly long life, and the filament will not be damaged even when heated in the atmosphere. If the ionization gauge head adopts this filament, and the anode and the collector are made into special forms, the distance between the electrodes is shortened, the anode voltage is reduced, and the gas ionization probability is reduced (that is, the ionization gauge coefficient is reduced), then this ionization gauge can be Measuring the pressure of 10-3 to 100 Pa is called a high pressure ionization meter. The lower limit of the low-voltage intensity measured by the triode ionometer is determined by the photocurrent of the collector, that is, due to the soft X-rays generated by electrons hitting the anode irradiating the collector to cause photoemission, the photocurrent constitutes the background of the collector current. When the photocurrent accounts for 10% of the ion current, the lower measurement limit of the ionometer is reached. The ionization gauge head collector is made into a filament and placed on the grid axis. The filament is located outside the grid. At this time, the sensitivity of the ionization gauge changes little, and due to the small area of the collector, the X-ray intercepted by it is smaller than Three orders of magnitude less triode type, this ionometer can measure pressures down to 10-8 Pa. It was proposed by Bayard and Albert in 1950, so it is called BA. In order to measure the pressure of 10-9 Pa or lower, a modulated BA meter, a pole ionization meter, a curved column ionization meter or a hot cathode magnetron ionization meter can be used. These ionometers also exclude to a certain extent the influence of gate electron-induced desorbed ions on pressure measurements.
       If you want to become a senior professional coating technical engineer, you must first lay down the basic knowledge of vacuum coating machine coating to prepare for becoming a senior engineer in the later stage.