Optical thin film preparation related technology - vacuum technology
Vacuum is the basis for the preparation of optical thin films. At present, most of the thin films are prepared under vacuum conditions. This paper briefly introduces the basic vacuum knowledge related to optical thin film preparation.
1. Discovery of vacuum
In 1641, Torricelli, an Italian mathematician, filled a long glass tube with mercury at one end, then slowly inverted it into a container filled with mercury. The maximum height of the mercury column inside the tube was 76 centimeters. If mercury is replaced with water, the maximum height of the column is 10.3 meters.
2. Definition and unit of vacuum
A vacuum is a state of gas in a given space at less than one atmospheric pressure. Vacuum is usually represented by vacuum, and vacuum is measured by pressure (force per unit area). The legal measurement unit of pressure is Pascal (Pascal), which is a unit of meters per kilogram per second. It is the international system of units (SI), or Pa, which is recommended internationally at present. At present, there are several old units still in use in practical engineering technology. The conversion relationship between several old units and PASCAL is as follows:
(1) standard atmosphere (ATM) :
1 ATM = 1.01325 х 105 pa = 760 Torr.
1 ATM Torr = 1/760 = 133.3 Pa.
1 bar= 1 ATM =1000 mbar
1 mbar = 7.5 х 10-1 Torr = 100 pa.
3. The role of vacuum in thin film plating
All the time, the gas molecules are in irregular thermal motion, and they are constantly colliding with each other, as well as with the wall of the container. Under normal conditions, the molecules of the gas density is about 3 х 1019 / cubic centimeter, each air molecules to 1010 times per second. The molecules of a gas move not in a straight line, but in a broken line that changes direction as they collide. If the film is coated in such an environment, the evaporating particles will collide with other molecules frequently and change direction constantly, which will increase the probability of reaction with other molecules, and the evaporation rate and film thickness will not be controlled. To avoid these disadvantages, we need to do it in a vacuum.
The division of the vacuum region. For the convenience of discussion and practical application, vacuum is often divided into four zones: coarse vacuum (> 103Pa), low vacuum (103~ 10-1pa), high vacuum (10-1~ 10-6pa) and ultra-high vacuum (< 10-6pa). The gas space of coarse vacuum is approximate to the atmospheric state, and the thermal motion of molecules is the main characteristic, and the gas characteristics are mainly the collision between gas molecules. The flow of low vacuum gas molecules gradually transfers from viscous flow state to molecular flow state. The gas flow in high vacuum is molecular flow, which is dominated by the collision between gas molecules and vessel wall, and the collision frequency is greatly reduced. The material vaporized in high vacuum will fly in a straight line.
Vacuum degree is the macroscopic expression of the thermal motion of gas molecules, and there is another microscopic parameter "free path" : the distance between two adjacent collisions of gas molecules, whose statistical average is called "average free path".
Set the travel distance of N0 vaporized particles d, and the number of particles not impacted by residual gas is:
Nd= N0e-d/l (1)
Percent of molecules collided:
f= 1-nd/N0= 1-e-d /l (2)
According to equation (2), when the average free path is equal to the distance from the evaporation source to the base, 63% of the evaporated particles collide. If the average free path increases by a factor of 10, the number of particles colliding decreases to 9%. It can be seen that collisions can be effectively reduced only when the average free path is much larger than the distance from the evaporation source to the base.
If the average free path is large enough and satisfies the condition l>>d, then there is
F material d/l (3)
Because l≈0.667/P (P is pressure).
Substitute equation (4) into equation (3) to get: f≈1.5dP (5)
To ensure the quality of the film layer, let f≤10-1. When the distance from the evaporation source to the base d=30cm, P≤2.2× 10-3pa. According to equation (5), the larger the vacuum chamber of the coating machine is, and the longer the distance between the evaporation source and the substrate is, the higher the vacuum degree is required.
Vacuum plays two roles in membrane preparation: one is to reduce the collision between evaporating particles and other gas particles, and the other is to inhibit the reaction between evaporating molecules and other gas molecules.
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