Vacuum measurement in the cavity of a pvd coating machine refers to the use of specific instruments and devices to measure the vacuum level in a specific space. This instrument or device is called a vacuum gauge (instrument, gauge). There are many types of vacuum gauges, which can usually be divided into absolute vacuum gauges and relative vacuum gauges according to the measurement principle. All vacuum gauges that directly obtain gas pressure by measuring physical parameters are absolute vacuum gauges, such as U-shaped pressure gauges, compression vacuum gauges, etc. The physical parameters measured by this type of vacuum gauge have nothing to do with the gas composition, and the measurement is relatively accurate, but in When the gas pressure is very low, it is extremely difficult to measure directly; a vacuum gauge that obtains the pressure value by measuring physical quantities related to pressure and comparing it with an absolute vacuum gauge is called a relative vacuum gauge, such as a discharge vacuum gauge. Thermal conduction vacuum gauge, ionization vacuum gauge, etc. are characterized by slightly poor measurement accuracy, which is related to the type of gas. In actual production, in addition to vacuum calibration, relative vacuum gauges are mostly used. This section mainly introduces the working principles and measurement ranges of resistance vacuum gauges, thermocouple vacuum gauges, and ionization vacuum gauges.
1. Resistance vacuum gauge
The resistance vacuum gauge is a type of heat conduction vacuum gauge. It measures the temperature of the hot wire in the vacuum to indirectly obtain the degree of 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 has to solve.
The structure of the resistance vacuum gauge is that the heating filament in the gauge tube is a tungsten wire or platinum wire with a large resistance temperature coefficient. The heating wire resistor is connected to the Wheatstone bridge and serves as an arm of the bridge. When heating under low pressure, the heat Q generated by the filament 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 taken away by the collision of gas molecules with the filament, and its size is related to the pressure of the gas. When the temperature of the hot wire is constant, Q1 is a constant, that is, the heat radiated by the hot wire does not change. Under a certain constant heating wire current condition, when the pressure of the vacuum system decreases, that is, when the number of gas molecules in the space decreases, Q2 will decrease accordingly. At this time, the heat generated by the filament will increase relatively, and the temperature of the filament will increase. As the temperature rises, the resistance of the filament will increase. There is such a relationship P↓→ R↑ between the pressure of the vacuum chamber and the resistance of the filament, so the pressure can be determined indirectly by measuring the resistance of the filament.
The resistance vacuum gauge measures vacuum in a range of 105 to 10-2Pa. Since it is a relative vacuum gauge, the measured pressure is highly dependent on the type of gas. Its calibration curves are all for dry nitrogen or air. Therefore, if the measured gas composition changes greatly, the measurement results should be corrected to a certain extent. In addition, after long-term use of the resistance vacuum gauge, the hot wire will drift due to oxidation, so when using it, it is necessary to avoid long-term exposure to the atmosphere or working under high pressure, and it is often necessary to adjust the current to calibrate the zero point position.
2. Thermocouple vacuum gauge
Schematic diagram of the structure of a thermocouple vacuum gauge. The regulation of the 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 hot wire. The hot end of the thermocouple is connected to the hot wire, and the cold end is connected to the millivolt meter in the instrument. The thermocouple electromotive force can be measured from the millivolt meter. During measurement, the thermocouple gauge is connected to the vacuum system under test, and a constant current flows through the hot wire. Unlike the resistance vacuum gauge, part of the heat Q generated by the filament will be between the filament and the thermocouple wire. The conduction disperses. When the pressure of the gas decreases, the temperature at the thermocouple junction will increase as the temperature of the hot wire increases. Similarly, the temperature difference electromotive force at the cold end of the thermocouple will also increase, and there is such a relationship between the gas pressure and the electromotive force of the thermocouple. Relationship: P↓→ ε↑.
The measurement results of the thermocouple vacuum gauge for different gases are different. This is due to the different thermal conductivity properties of various gas molecules. Therefore, certain corrections are required when measuring different gases. Table 1-3 gives some correction factors for gases or vapors. The measuring range of the thermocouple vacuum gauge is roughly 102 ~ 10-1Pa, and the measuring pressure is not allowed to be too low. This is because when the pressure is lower, the gas molecules lose very little heat through thermal conduction, and the heat of the hot wire and the thermocouple wire is The heat loss caused by thermal conduction and thermal radiation is dominant, so the change in the thermocouple electromotive force will not be caused by the change in pressure.
The thermocouple vacuum gauge has thermal inertia. When the pressure changes, the change in the temperature of the hot filament often lags for a period of time, so the reading of the data should also lag for some time. In addition, like the resistance vacuum gauge, the heating filament of the thermocouple gauge also If tungsten wire or platinum wire is used for a long time, the zero point of the hot wire will drift due to oxidation. Therefore, when using it, the heating current should be adjusted frequently and the heating current value should be recalibrated.
3. Ionization vacuum gauge
Ionization vacuum gauge is a widely used vacuum measuring gauge. It uses the principle of ionization of gas molecules to measure vacuum degree. According to the different gas ionization sources, they are divided into hot cathode ionization vacuum gauges and cold cathode ionization vacuum gauges. The former is further divided into ordinary hot cathode ionization gauges, ultra-high vacuum hot cathode ionization gauges and low vacuum hot cathode ionization gauges. Figure 1-7 shows the structure of an ordinary ionization meter gauge. It mainly has three electrodes: a filament that emits electrons as the emitter A, a spiral grid that accelerates and collects electrons (also called an accelerating electrode) B, and a cylindrical type The ion collector C is composed of three parts. The emitter is connected to zero potential, the accelerating electrode is connected to positive potential (several hundred volts), and the collector is connected to negative potential (tens of volts). There is a repulsion field between B and C. The working principle of the ionization meter is that the hot cathode A emits electrons. After being accelerated by the accelerating pole, most of the electrons fly to the collector. Under the action of the repulsion field between B and C, the speed of the electrons decreases. When the speed reduces to zero, the electrons return to the collector. Flying to the B pole again, when the electron flies to the B-C space, it is also affected by the repulsion field. When the speed is reduced to zero, the electron turns back and flies to the C pole. The repeated movement of the electron in the B-C space will interact with the gas molecules. Continuous collisions cause the gas molecules to gain energy and generate ionization. The electrons are eventually collected by the accelerating electrode, and the positive ions generated by the ionization are accepted by the collecting electrode and form an ion flow I+. For a certain regulation, when the potential of each electrode is constant, I+ has the following linear relationship with the emitted electron flow Ie and the pressure of the gas
In the formula, k is a proportional constant, which means the current value of ions obtained under unit electron current and unit pressure. The unit is 1/Pa, which can be determined experimentally. For different gases, the size of k is different, and its existence range is between 4-40. When the emission current is constant, the ion flow 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 flow.
The measurement range of the ordinary hot cathode vacuum gauge is 1.33×10-1-1.33×10-5Pa. Whether it is above or below this measurement limit, the linear relationship between the ion flow I+ and the gas pressure will be lost. When the pressure is high, the probability of multiple collisions between electrons and molecules is greatly increased. Since the acceleration potential is much higher than the ionization potential of the gas (tens of volts), the electrons generated by ionization are enough to cause gas ionization, which will make the ionization regulation The electron flow in the gas increases sharply. At the same time, due to the high density of the gas, the free path of electrons is very short. Most collisions are low-energy collisions and cannot cause ionization. Many factors cause the linear relationship between ion flow and pressure to no longer be maintained at higher pressures. ; When the pressure is low (less than 1.33×10-1Pa), the high-speed moving electrons will produce soft X-rays when they reach the accelerating pole. The soft X-rays will then be emitted to the ion collector C, causing the collector to produce photoelectric emission. The electron flow is emitted, so that the pressure-independent current is superimposed in the original ion flow measurement circuit, causing the linear relationship between the ion flow I+ and the pressure of the gas to lose. At this time, the ionization vacuum gauge cannot measure the pressure in the vacuum chamber. .
The pvd coating machine
uses an ionization vacuum gauge to quickly and continuously measure the total pressure of the gas to be measured, and the gauge tube is small and easy to connect. However, the emitter in the gauge tube is made of tungsten wire. When the pressure is higher than 10 -1Pa, the life of the gauge will be greatly reduced or even burnt, and work under high pressure should be avoided; when the vacuum system is exposed to the atmosphere, the inner surface of the glass bulb of the ionization meter gauge and each electrode will adsorb gases, and these gases will affect the vacuum. Therefore, when the vacuum system is exposed to the atmosphere for a long time or used for a period of time, regulated degassing should be performed regularly.