First, the pump pumping speed is defined as a certain suction pressure, the unit of time by the pump port was removed gas volume. A complete vacuum system, no matter what the application, should have a vessel or chamber to be evacuated, a vacuum unit, a vacuum pump, connecting pipes, valves, cold traps, etc. . The pipes, valves, cold traps and other components as part of the vacuum system, have a certain impediment to the flow of gas. Conversely, they have a certain ability to guide the flow of gas, which capacity is called conductance. This is a very important concept in gas flow, which is defined as the flow rate per unit of pressure differential. The natural flow of gas always flows from high pressure to low pressure. When the pressures at the two ends are P1 and P2, respectively, and the amount of gas flowing through is Q, the flow guide of the component U = Q / (P1 - P2) The conductance of different vacuum system components can be determined by means of calculations, simulations, measurements, etc., which, in addition to the geometry, are related to the gas flow conditions. The flow of different components can be in series and parallel. The flow of different components of the parallel series vacuum pump is to evacuate the gas inside the vacuum vessel, but often the pump suction port can not be directly connected with the container being drawn due to process requirements or to reduce the pollution of the vacuum unit with oil vapor pollution The degree must pass the cold trap, the valve and the pipeline to be connected with the evacuated container. Since each vacuum component has a definite conductance, it can be said that the pump must pass through a certain conductance to be connected with the evacuated container, as shown in the figure The connection between the pump and the vacuum chamber in the figure can include cold traps and valves. Assuming that the conductance between the pump and the vacuum chamber is U, the pump must pass through the flow guide U to pump the vacuum chamber and its pumping capacity to be limited. In this case, the suction of the container should really be of a vacuum The pumping chamber at the effective pumping speed S0. If the nominal pumping speed of the pump is S, then according to the law of the flow conservation when the gas is flowing steadily, the relationship between S0, S and U can be deduced. The above formula is called the vacuum basic equation which is based on the vacuum system design The basic law. According to the basic vacuum equation, two extreme results can be mathematically obtained. That is, when the conductance U is very large, the effective pumping speed S0 of the vacuum chamber can be approximately equal to the pumping speed S of the pump. When the pumping speed S of the pump is very large , Or when the conductance U is very small, the effective pumping speed S0 of the vacuum chamber is approximately equal to the conductance U. The above results may be more physically understandable, the gas drawn from the suction port of the vacuum chamber must be guided through the flow guide U (pipes, valves, etc.) to be pumped by the vacuum pump, except that the evacuated gas flows from the evacuation port of the vacuum chamber to the pump The process of mouth movement is from high pressure to low pressure flow, and from the pump port is forced flow from low pressure to high pressure based on some kind of suction principle. If the flow guide U is very large, that is, the amount of gas passing through it is not limited, then the pumping capacity of the pump depends on the size of its own pumping speed, which is the same as the pump port directly connected to the vacuum chamber. But if the pump pumping speed is very large, which is relative to the pump pumping speed U is very small, then the pump's actual pumping capacity does not depend on its pumping speed depends on the size of the gas through the guide U Capability, the value of conduction is exactly pump effective pumping speed S0. In order to maximize pump pumping capacity, to maximize the flow guide U is the most effective way, but often difficult to achieve. And blindly increase pumping speed is more impractical. Therefore, the day using a large flow guide and selection of day pumping speed pump is worth the trade-off. From the basic vacuum equation we can see that the effective pumping speed S0 with S or U are monotonically increasing function. The description of the vacuum basic equation is not esoteric, but not as simple as everyone can know, so in many applications, the user often neglects the guide of pumping speed restrictions, which led to the application of the vacuum effect Affected. Second, for a no leak, there is no deflation of the vacuum system, such as the volume of the vacuum chamber V, the effective pumping speed of the vacuum chamber S0, then with the extraction process, the vacuum chamber pressure with time to comply with the following changes in which P0 T = 0 when the pressure, that is, the initial pressure, t = V / S0 is called the time constant. The above law reveals that the pressure in the vacuum chamber is reduced by an order of magnitude after each passage of time. Obviously, the smaller the pressure t is, the faster the pressure drops. When V is constant, the larger the effective pumping speed S0 is, the smaller it is. However, none of the vacuum systems are leak-tight and deflation-free, and deflation is always present even though it is truly airtight. In fact, equation (3) reflects the pressure of the pump during evacuation of the gas in the vacuum chamber The law of change. When the pressure is high, the influence of system leakage and deflation volume on the relative space gas volume is negligible, and it can be considered that the condition of airtightness and deflation is approximately satisfied, that is, the law of (3) can be approximated Set up When the pressure is low, the system leakage and deflation can not be ignored or even become the main gas load, the deviation of the law of (3), manifested in the pressure drop into a slow, the general pressure of this change in 0.5Pa or so, so a typical vacuum system evacuating process pressure drops quickly, to a certain pressure began to slow down. Since a qualified vacuum system has strict requirements on leak rate, deflation is a major factor that affects system pressure reduction. Deflation is a slow process. Even if baking and other strengthening measures are used to achieve a predetermined Pressure, often go through a long time. Any vacuum system wants to minimize the pumping time, which is related to improving efficiency and reducing energy consumption, but not all vacuum applications have the conditions to shorten the pumping time. Different vacuum applications can be divided into two categories: one is the system does not change the amount of deflation, but only the vacuum requirements; the other is required to fully evacuate the vacuum chamber, that is to go to the deflation rate A certain threshold. These two different types of applications require different pump configurations. For the former kind of application, if the degree of vacuum is above 0.5pa, as long as the time constant is small enough, the time of pumping can be shortened day by day. However, if the vacuum requirements below 0.5Pa, it is necessary to consider the impact of deflation on the pressure changes. Deflation changes slowly over time. Especially in the case of no baking. In order to achieve a higher degree of vacuum in a predetermined short period of time, it is necessary to extract a larger purge volume at a higher pumping speed. In other words, if the deflated quantity is Q, the effective pumping speed of the pump is S0, then the equilibrium pressure P = Q / S0 can be achieved. If the equilibrium pressure is determined, the shorter the time to reach, the greater the effective pump speed required. Evaporation coating is a typical example of this type of application. Due to the high speed and short time of vapor deposition, the influence of the amount of deflation (ie the influence of the active gas) is not considered. However, the energy of the evaporated particles is low, requiring most of the particles to be deposited onto the workpiece without collisions to ensure the binding force and reduce the scattering. This requires that the average free path in the vacuum chamber is not less than the distance from the evaporation source to the workpiece. Correspondingly, Pressure is about 1 × 10-2Pa, which is the vacuum evaporation coating requirements. How to achieve this pressure in the shortest possible time, the effective pump pumping speed requirements, the principle is the shorter the time, due to the larger the deflated, the effective pumping speed on the larger requirements. Therefore, the evaporation coating is generally equipped with pumping speed powerful oil diffusion pump unit, the power of dozens of kilowatts, a few minutes to ten minutes can reach the working vacuum, but the system caused by the oil vapor pollution is inevitable, especially Plastic metallized film is easy to yellow. Turbomolecular pump pumping speed can not meet the needs of large-scale evaporation. The pumping speed of the cryogenic pump is the industrial scale coating can not afford. According to the characteristics of the gas load in the gas-drawn space, the use of the molecular booster pump to remove the permanent gas, combined with the low-temperature condensate water trap pump to remove the water vapor, is expected to achieve a new pumping technology for obtaining a clean vacuum with a large pumping speed. When the pressure in the vacuum chamber is above 0.5 Pa, the main gas component is permanent gas, while the main gas component below 0.5 Pa is water vapor (90%). As the molecular booster pump with super-vacuum pumping capacity, from 100Pa to 0.5Pa pumping time is very short, but after the opening of 0.1Pa low-temperature condensate trap pump, the indoor pressure can be reduced in a short time 1 An order of magnitude, to 1 × 10-2Pa. For the 3-5m3 large-scale evaporation coating equipment, it is undoubtedly groundbreaking to dispose 3-4 units of 1000 l / s molecular booster pump and a 5kw low-temperature condensate trap . For the latter application, the change of deflation depends on the temperature and the time, but not the pressure of the vapor space. As long as the pressure is lower than the equilibrium pressure corresponding to the existing adsorption capacity, it is generally satisfied during the evacuation condition. Therefore, with a strong pumping speed even in a very short period of time to a very low pressure in the room, still can not significantly reduce the amount of deflation in the vacuum chamber, and must be equipped with a suitable pumping speed at a reasonable baking temperature, reasonable The amount of deflated to meet the requirements of the level of technology, which generally go through dozens of minutes of time. Such applications are more typical of the titanium industry sputtering coating and ion plating, rare earth permanent magnet material melting. Among them, the excess of active gas will affect the quality of the film and the quality of the material, so the process has a longer period of finishing pumping process. For the coating chamber is about 1m3 sputtering or ion plating equipment, the general configuration of 4000 l / s pumping speed vacuum unit, in order to promote the vacuum chamber and the workpiece more quickly deflated, often baked to 300 ℃ temperature. It is worth emphasizing that in the titanium coating, the pumping speed of the pump size, the characteristics of the pump, pumping process and the deposition pressure required to show a correct relationship between. In a coating cycle, the pumping of the vacuum unit can be divided into three phases, namely the fine pumping stage, the glow bombardment and the sputter deposition stage. The purpose of fine pumping is to reduce the amount of deflation in the vacuum chamber, the result depends mainly on the baking temperature and pumping time, and space pressure has little to do, especially the pressure in the same order of magnitude. Therefore, the main pump pumping speed within the appropriate differences, the effect of fine pumping is the same, the vacuum chamber evacuation rate can be reduced to the same level, although the corresponding limit vacuum. In particular, the 1000 l / s molecular boost pump and the 1500 l / s turbomolecular pump have the same effect at this stage of pumping. Glow boom
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