In the chamber of a vacuum coating machine, vacuum measurement refers to the determination of the vacuum level in a specific space using specific instruments and devices. This type of instrument or device is called a vacuum gauge (instrument, gauge). There are many types of vacuum gauges, usually divided into absolute vacuum gauges and relative vacuum gauges according to measurement principles. Any vacuum gauge that directly obtains gas pressure by measuring physical parameters is an absolute vacuum gauge, such as U-shaped pressure gauges, compression type vacuum gauges, etc. The physical parameters measured by these vacuum gauges are independent of the gas composition and the measurement is relatively accurate. However, it is extremely difficult to directly measure at low gas pressures; A vacuum gauge that measures physical quantities related to pressure and compares them with an absolute vacuum gauge to obtain the pressure value is called a relative vacuum gauge, such as a discharge vacuum gauge, a heat conduction vacuum gauge, an ionization vacuum gauge, etc. Its characteristic is that the measurement accuracy is slightly lower and depends on the type of gas. In actual production, except for vacuum calibration, relative vacuum gauges are mostly used.
Resistance vacuum gauge
Resistance vacuum gauge is a type of heat conduction vacuum gauge that indirectly obtains the degree of vacuum by measuring the temperature of a hot wire in a vacuum. The principle is that the heat conduction of gas under low pressure is related to pressure, so how to measure temperature parameters and establish the relationship between resistance and pressure is the problem that the resistance vacuum gauge needs to solve.
The structure of a resistance vacuum gauge is that the heating filament in the gauge is a tungsten or platinum wire with a high temperature coefficient of resistance. The heating filament is connected to a Wheatstone bridge and serves as one arm of the bridge. When heated under low pressure, the heat generated by the filament Q can be expressed as: Q=Q1+Q2
In the formula, Q1 is the heat radiated by the filament, which is related to the temperature of the filament; Q2 is the heat carried away by gas molecules colliding with the filament, and its magnitude is related to the pressure of the gas. When the temperature of the hot wire is constant, Q1 is a constant quantity, that is, the amount of heat radiated by the hot wire remains unchanged. Under a constant heating current condition, when the pressure of the vacuum system decreases, that is, the number of gas molecules in the space decreases, Q2 will decrease accordingly. At this time, the heat generated by the filament will relatively increase, causing the temperature of the filament to rise and the resistance of the filament to increase. The relationship between the pressure of the vacuum chamber and the resistance of the filament is P ↓ → R ↑, so the pressure can be indirectly determined by measuring the resistance value of the filament.
The range for measuring vacuum with a resistance vacuum gauge is 105 to 10-2Pa. As it is a relative vacuum gauge, the measured pressure is highly dependent on the type of gas, and its calibration curve is for dry nitrogen or air. Therefore, if the composition of the measured gas changes significantly, the measurement results should be corrected to some extent. In addition, after long-term use, the hot wire of the resistance vacuum gauge will experience zero drift due to oxidation. Therefore, it is necessary to avoid prolonged exposure to the atmosphere or working under high pressure during use, and often adjust the current to calibrate the zero position.
Thermocouple vacuum gauge
The gauge of a thermocouple vacuum gauge mainly consists of heating filaments C and D (platinum wires) and thermocouples A and B (platinum rhodium or constantan nickel chromium) used to measure the temperature of the heating filaments. The thermocouple is connected to the hot wire at the hot end and to the millivoltmeter in the instrument at the cold end. The electromotive force of the thermocouple can be measured from the millivoltmeter. When measuring, the thermocouple gauge tube is connected to the vacuum system being tested, and the hot wire is passed through with a constant current. Unlike a resistance vacuum gauge, at this time, a portion of the heat Q generated by the filament will be conducted and dissipated between the filament and the thermocouple wire. When the pressure of the gas decreases, the temperature at the junction of the thermocouple will increase with the increase of the hot wire temperature. Similarly, the temperature difference electromotive force at the cold end of the thermocouple will also increase, and there is a relationship between the gas pressure and the electromotive force of the thermocouple: P ↓→ε↑.
The measurement results of thermocouple vacuum gauge for different gases are different due to the different thermal conductivity of various gas molecules. Therefore, certain corrections need to be made when measuring different gases. Table 1-3 provides correction factors for some gases or vapors. The measurement range of a thermocouple vacuum gauge is approximately 102-10-1Pa, and the measurement pressure should not be too low. This is because when the pressure is lower, the amount of heat dissipated by gas molecules through thermal conduction is very small, and the heat loss caused by thermal conduction and radiation of the hot wire and thermocouple wire is the main factor. Therefore, the change in thermocouple electromotive force will not be caused by changes in pressure.
Thermocouple vacuum gauges have thermal inertia, and when the pressure changes, the temperature of the hot wire often lags behind for a period of time, so the reading of data should also lag behind for some time; In addition, like a resistance vacuum gauge, the heating filament of a thermocouple gauge is also made of tungsten or platinum wire. If used for a long time, the heating filament will experience zero drift due to oxidation. Therefore, when using it, the heating current should be adjusted regularly and the heating current value should be recalibrated.
Ionization vacuum gauge
The ionization vacuum gauge is a widely used vacuum measuring instrument that uses the principle of gas molecule ionization to measure vacuum degree. According to different gas ionization sources, it is divided into hot cathode ionization vacuum gauge and cold cathode ionization vacuum gauge. The former is further divided into ordinary hot cathode ionization gauge, ultra-high vacuum hot cathode ionization gauge, and low vacuum hot cathode ionization gauge. It mainly consists of three electrodes: the filament that emits electrons as the emitter A, the spiral acceleration and electron collection gate (also known as the acceleration electrode) B, and the cylindrical ion collection electrode C. The emitter is connected to zero potential, the acceleration electrode is connected to positive potential (several hundred volts), and the collection electrode is connected to negative potential (several tens of volts). There is a repulsive field between B and C. The working principle of an ionization meter is that the hot cathode A emits electrons, which are accelerated by the accelerating electrode, and most of the electrons fly towards the collecting electrode. Under the repulsive field between B-C, the electron velocity decreases. When the velocity decreases to zero, the electrons fly back to the B electrode. When the electron flies towards the B-C space, it is also subjected to the repulsive field. When the velocity decreases to zero, the electron rotates back to the C electrode. The repeated motion of the electron in the B-C space will collide with gas molecules, causing them to gain energy and produce ionization. The electrons are eventually collected by the accelerating electrode, and the positive ions produced by ionization are accepted by the collecting electrode and form an ion flow I+. For a certain tube, when the potential of each electrode is constant, I+and The linear relationship between the emission electron flow Ie and the gas pressure is as follows: I+=kIeP
In the formula, k is a proportionality constant, which means the current value of the ion obtained at unit electron current and unit pressure, with the unit of 1/Pa, and can be determined through experiments. For different gases, the size of k varies, and its range of existence is between 4-40. When the emission current is constant, the ion current is only proportional to the pressure of the gas, so the gas pressure value in the vacuum chamber can be determined based on the size of the ion current.
The measurement range of a regular hot cathode vacuum gauge is 1.33 × 10-1 to 1.33 × 10-5Pa. Regardless of whether it is above or below this measurement limit, it will cause a loss of linear relationship between the ion flow I+and the gas pressure. When the pressure is high, the probability of multiple collisions between electrons and molecules greatly increases. Due to the acceleration potential being much higher than the ionization potential of the gas (several tens of volts), the electrons generated by ionization are sufficient to cause gas ionization. This will cause a sharp increase in the electron flow in the ionization gauge. At the same time, due to the high gas density, the free path of electrons is short, and most collisions are low-energy collisions that cannot cause ionization. Many factors lead to a linear relationship between ion flow and pressure no longer being maintained at higher pressures; When the pressure is low (less than 1.33 × 10-1Pa), high-speed electrons reaching the acceleration electrode will generate soft X-rays, which will then be directed towards the ion collection electrode C. This will cause the collection electrode to produce photoelectric emission, emitting an electron current, causing the original ion flow measurement circuit to superimpose this pressure independent current, resulting in a loss of linear relationship between the ion flow I+and the gas pressure. At this time, the ionization vacuum gauge cannot measure the pressure in the vacuum chamber.
The vacuum coating machine uses an ionization vacuum gauge to quickly and continuously measure the total pressure of the gas to be tested, and the gauge tube has a small volume and is easy to connect. However, the emitter in the gauge tube is made of tungsten wire. When the pressure is higher than 10-1Pa, the gauge tube life will be greatly reduced, and even burned out. It should be avoided to work under high pressure; When the vacuum system is exposed to the atmosphere, the inner surface of the glass envelope and various electrodes of the ionization gauge will adsorb gases, which will affect the accuracy of vacuum measurement. Therefore, when the vacuum system is exposed to the atmosphere for a long time or used for a period of time, regular degassing treatment of the gauge should be carried out.
