Vacuum system is an indispensable part of vacuum coating equipment. The commonly-used structure of high vacuum system is usually the combination of main pump, forepump, valves, pipes and vacuum measuring instruments. According to the selected main pumps, the vacuum system can be oil vacuum system or oil-free vacuum system.
1. Oil vacuum system
The main pump of the oil high vacuum system is generally the oil diffusion pump. The forepump is mechanical pump or mechanical pump + roots pump.
It is well known that the operating pressure range of the oil diffusion pump is 10-1 to 10-6 Pa. The high pumping rate can up to one hundred thousand liters per second from several liters per second. In the range of 10-2 to 10-4 Pa, the oil diffusion pump has a stable pumping speed without mechanical transmission, friction, vibration, or noise. It is also characterizes by simple structure, reliable performance, long life, easy maintenance and low investment cost. The main disadvantage is that the oil vapor is easy to flow back into the vacuum chamber and then become the pollution sources. The rate of oil return can be reduced by using oil retaining cap, baffle, cold trap and other measures, but it cannot be eradicated. There is still a part of the oil vapor reflux, which reduces the pump pumping rate. In addition, the oil diffusion pump, especially large size pump has high energy consumption and high operating cost as well as the disadvantages of slow start and long stopping time, especially large size pump.
2. Oil-free vacuum system
Nowadays, the oil free vacuum system in which the main pump is molecular pump and cryogenic pump is widely used in vacuum coating equipment.
A molecular pump is a vacuum pump that the pumping is carried out by the surface of high speed moving rigid body carrying the gas molecule. It can continuously pump large quantity of gas to obtain clean and oil-free high vacuum or ultrahigh vacuum. Besides, it has advantages of fast start and short stop. But the pumping speed of heavy gas is faster than light gas, and the pumping speed of steam is very slow.Turbo molecular pump enjoys high efficiency in the field of molecular flow, but when the gas largely flows through the pump, the pumping speed will decrease. In the pressure range above 1Pa, the pumping speed is pretty low, which is unfavorable for using. So a spiral groove traction molecular pump is equipped on the high pressure side of the turbo molecular pump to make the pumping unit of the two pumps form a united whole, which is a wide field type compound molecular pump. The pumping speed of this compound molecular pump keeps constant when the pressure is below 10Pa. The high pressure range with a large pumping speed makes up for the shortage of the turbo molecular pump.
2.2 Characteristics of a vacuum system with a cryogenic pump as the main pump
Cryogenic pump is a gas trap pump, which is a device that uses low temperature surface to condense, adsorb and trap gas and then obtain and maintain vacuum. The forepump is not needed during normal operation, and cryogenic pump has high initial pressure and wide working pressure range (1 ~ 10-8) Pa. It can pump all kinds of gas, but continuous pumping can be achieved. Compared with other pumps with the same caliber, cryogenic pump has high pumping speed especially for water vapor, which is equivalent to four times of the molecular pump with same caliber. So, the partial pressure of water vapor in the pumped container is low, so as to obtain a clean hydrocarbon-free high vacuum and ultrahigh vacuum. Without moving parts, vibration or noise, the cryogenic pump can be installed anywhere. With Small footprint and easy operation, the operating cost of it is lower than the oil diffusion pump.
Therefore, the choice of the main pump of vacuum coating equipment, first of all, is according to the requirement of cleanliness, vacuum and pumping time.
(1) When using the forepump as dry pump, the cryogenic pump as main pump, an oil-free vacuum system without oil vapor or hydrocarbon pollution is available. Because of high pumping rate, the cryogenic pumps are widely used in vacuum coating equipment, especially suitable for equipment whose volume is 0.5 ~ 1.6m3.
(2) An oil-free vacuum system can also be realized when using molecular pump as main pump. Compared with the cryogenic pump, the molecular pump can pump continuously without regenerating, but it is inability to pump water vapor. Compared with the oil diffusion pump, the system of the molecular pump is clean with short starting and stopping time, which helps save the auxiliary time. However, as the molecular pump is high-speed running equipment, its development of specifications is limited. So the molecular pump is suitable for medium and small coating equipment as the main pump in research institutes and Collegial Laboratory. For the large volume vacuum coating equipment, multiple sets of molecular pump systems are needed to pump parallelly, which leads to complex structure.
(3) The oil diffusion pump using low-pressure and high-speed oil vapor flow as operating fluid, the pumped gas are evacuated to the vapor jets and then carried to the pump outlet to exclude in the extraction process. The oil diffusion pump has a wide range of pumping rate. Although it is inefficient, but the price is pretty low, so the oil diffusion pump is still widely used in vacuum coating equipment which demand lower pumping rate.