Detailed Structure Of Vacuum Coating Machine

- Mar 02, 2019-

Detailed structure of vacuum coating machine


High vacuum coating machine, coating machine is the most widely used equipment in the production of vacuum conditions. Its related components: mechanical pump, booster pump, oil diffusion pump, condensing pump, vacuum measuring system.

The following I introduce the composition and working principle of each part in detail.


1. Vacuum main body -- vacuum chamber


According to the different requirements of processing products, the size of the vacuum cavity is not the same, the most commonly used at present is 1.3M, 0.9M, 1.5M, 1.8M, etc., the cavity made of stainless steel material, not rust, solid, etc., each part of the vacuum cavity has a connection valve, used to connect the pump.


2. Auxiliary pumping system


The exhaust system is composed of "diffusion pump + mechanical pump + roots pump + low temperature cold trap +polycold"


The exhaust flow is as follows: the mechanical pump first pumps the vacuum chamber to a low vacuum state of less than 2.0* 10-2pa, which provides the premise for the diffusion pump to pump the vacuum. Later, when the diffusion pump pumps the vacuum chamber, the mechanical pump and the oil diffusion pump form a series to complete the pumping action in this way.


The exhaust system is an important part of the vacuum system of the coating machine. It is mainly composed of mechanical pump, booster pump (mainly introduced roots pump) and oil diffusion pump.


Mechanical pump: also known as the front stage pump, mechanical pump is one of the most widely used low vacuum pump, it is oil to maintain the sealing effect and rely on mechanical methods to constantly change the volume of the suction cavity in the pump, so that the volume of the gas in the pumped container is constantly expanding to obtain vacuum.


There are many kinds of mechanical pumps, commonly used have sliding valve type (this is mainly used in large equipment), piston reciprocating, fixed and rotary blade type (this is currently the most widely used, this paper mainly introduced) four types.


Mechanical pump is often used to remove dry air, but can not remove oxygen content is too high, explosive and corrosive gas, mechanical pump is generally used to remove permanent gas, but no good effect on water gas, so it can not be removed water gas. Rotary vane pump plays a major role in the parts of the stator, rotor, shrapnel, etc., the rotor in the stator but different from the stator mandels, like two inscribed circle, rotor groove with two pieces of shrapnel, two pieces of shrapnel with a spring in the middle, to ensure that shrapnel tightly adhered to the stator wall.


Its two shrapnel alternately play two roles, on the one hand from the air intake into the gas, on the other hand has been compressed into the gas, gas discharge pump. Each rotation of the rotor, the pump completed two suction and two exhaust. When the pump continuously clockwise rotation, rotary vane pump continuously through the air intake intake gas, and continuously from the exhaust exhaust outside the pump, to achieve the purpose of the container pumping. In order to improve the limiting vacuum pump, the pump stator soaked in oil, and everywhere in the clearance of harmful space often keep sufficient oil inside, fill the gap, so the oil lubrication, on one hand, on the other hand and seal and plug gap and the harmful effect of the space, prevent gas molecule reverse flow through various channels to the low pressure of space.


The mechanical pump starts from the atmosphere work, its main parameter has the limit vacuum, the pumping rate, this for the design and selects the mechanical pump the important basis. Single stage pump can pump the container from the atmosphere to the limit vacuum of 1.0* 10-1pa, double stage mechanical pump can pump the container from the atmosphere to 6.7*10-2 pa, or even higher.


The extraction rate refers to the volume of gas that can be discharged per unit time when the rotary vane pump operates according to the rated number of revolutions, which can be calculated by the following formula




Fs stands for the sectional area of the space-time cavity at the end of inspiration, L stands for the length of the cavity, coefficient stands for the exhaust process of the rotor twice every rotation cycle, Vs stands for the end of inspiration when the rotor is in the horizontal position, the volume in the space-time cavity is the largest, and the rotation speed is n.


The effect of the mechanical pump exhaust with the speed of the motor and the looseness of the belts have relationship, when the belt of motor is loose, motor speed is slow, mechanical pump exhaust effect will become worse, so often to maintain, the tally, also need to often check mechanical pump oil seal, oil is too little, can not meet the sealing effect, inside the pump leakage, oil too much, the suction hole plugging, inspiration and exhaust, generally, 0.5 cm in the oil level in the offline.


Turbo pump/roots pump: it is to have a pair of synchronous high-speed rotating double leaf shape or lobed rotor mechanical pump, because it works the same as the roots blower, so also can call the roots vacuum pump, the pump in the range of 100-1 mpa pressure extraction speed greatly, it makes up for the mechanical pump exhaust ability insufficient shortcomings in this scope, the pump can't start work from the atmosphere, have no direct discharge of the atmosphere, its role is only to increase the differential pressure between inlet and outlet, the rest of the mechanical pump is needed to complete, therefore, it must match with mechanical pump as the backing pump.

Mechanical pump in the use of the process, must pay attention to the following issues:

1, mechanical pump to be installed in a clean and dry place.

2, the pump itself to keep clean and dry, pump oil has a sealing and lubrication, so to be added in accordance with the specified amount.

3, to regularly replace the pump oil, the replacement should be discharged before the waste oil, cycle for at least three months to six months to replace once.

4. Connect the wires according to the instructions.

5, before the mechanical pump stop work to close the intake valve, then power failure and open the air valve, the air into the pump through the inlet.

6, during the work of the pump, the oil temperature can not exceed 75 degrees Celsius, otherwise the viscosity of the oil is too small and lead to lax seal.

7, or check the mechanical pump belt tightness, motor speed, the speed of the roots pump motor, and sealing ring sealing effect.


Oil diffusion pump: the limit vacuum of mechanical pump is only 10-2 pa, when reaching 10-1 pa, the actual pumping speed is only 1/10 of the theory, if you want to get high vacuum, you must use oil diffusion pump.


Since oil diffusion pump is the first pump used to obtain high vacuum, it is cheap, easy to maintain and widely used, so this paper will focus on discussion.


Oil diffusion pump application pressure range is 10-1 -10-7 pa, it is the use of gas diffusion phenomenon to exhaust, it has a simple structure, easy to operate, pumping speed is large (the highest can reach 10+5 l/s) and other characteristics. The oil diffusion pump is mainly composed of pump shell, nozzle, diversion pipe and heater. The oil diffusion pump (d-704 # in Japan) is mainly added inside. According to the number of nozzles, it can be divided into single-stage pump and multi-stage pump.


At the bottom of the diffusion pump is stored the oil of the diffusion pump. The upper part is the air inlet, and the lower part on the right side is the air outlet. During the operation, the air outlet is provided with the preposition pressure by the mechanical pump, and the mechanical pump ACTS as the preposition pump.


When the oil of the diffusion pump is heated by the electric furnace, the oil vapor produced provides the preloading pressure, and the mechanical pump ACTS as the preloading pump. When the diffusion pump oil is heated by the electric furnace, the oil vapor is emitted down the pipe through the umbrella nozzle. Because there is a 1-10-1 pa vacuum provided by the mechanical pump outside the nozzle, the oil vapor can be ejected for a certain distance, forming a jet flow in the direction of the air outlet. Finally, the jet meets the pump wall cooled by cooling water, condenses into liquid and flows back to the evaporator, i.e., evaporates, jets, condenses, and circulates repeatedly to realize air extraction.


By the gas molecules in the air inlet into the pump, once fell into the steam flow, gets down the momentum of the movement down, because of the jet with high velocity (200 m/SEC), high density of the steam, and the diffusion pump oil with high molecular weight (300-500), it can effectively take gas molecules, so within the interface of jet, the gas molecules can't be stranded for a long time, and the jet flow on both sides of the interface, is the concentration of the smoke gas is very bad, just because of this concentration difference across the interface of action in the smoke gas diffusion into jet, was taken to the exit, and at the exit by the mechanical pump.


The oil vapor pressure of the diffusion pump is an important factor to determine the ultimate vacuum of the pump.


Diffusion pump can not be used for pumping alone, the general requirements of the maximum outlet pressure of the pump is 40 pa. Diffusion pump pumping victory is scheduled for the first level of the nozzle and the pump body of inlet diameter, the size of the circular area between pumping speed is not a constant value, but varies with the air inlet pressure, when pressure in 2 ~ 10-10-3, diffusion pump suction speed is the fastest, when the pressure is less than 5 * 10-4 mpa, diffusion pump suction speed minimum, almost no suction capacity (at this point, the inlet pressure is higher, because of its high density of air, make steam manifold become a high-speed jet to block the air diffusion, so the pumping speed down).


The diffusion pump should be cleaned before installation, and then the diffusion oil can be loaded. Before the oil is heated, the pump must be vacuumed first, and the diffusion pump oil must be cooled to 60 ~ 70 degrees Celsius before the machine is shut down. Then the front stage exhaust gas can be closed, and finally the cooling water can be closed.


Because the oil diffusion pump is unable to put an end to return oil chance, so there is no way to guarantee precision products 100% pure, especially in the semiconductor industry, so there is a "high vacuum condensate pump + low vacuum mechanical pump" oil-free vacuum system, composed of the condensate pump exhaust system composed of not only exhaust efficiency is very high, and effectively guarantee the clean of the vacuum chamber, ensure the quality of the product (to avoid contaminated products, enhance the adhesion between coating and substrate), but its maintenance costs are very high, expensive, so there is no oil diffusion pump widely in penetration.


Low temperature condensing pump: it is a kind of pump that condenses gas molecules on the surface of low temperature to realize pumping.


The working principle of the condensing pump: it is mainly the condensation, cold trapping and physical low temperature adsorption of the gas on the low temperature surface.


Cryogenic condensation: liquid helium or refrigeration cycle helium is used for cooling according to the characteristics of various gases.


Cold trapping: it is the phenomenon that non-condensable gas is trapped by condensable gas. Usually, gases such as carbon dioxide, water vapor, nitrogen and compressed gas first form frost and then form an adsorption layer on the low-temperature surface, so as to achieve the purpose of adsorbing other gases. This is the reason why the effect of cryogenic pump in removing mixed gas is better than that of single gas.


Low temperature adsorption: refers to the adsorption of gas by adsorbent on the surface of low temperature. Due to the strong interaction between adsorbent and gas molecules, the vapor pressure can be lower than the saturation vapor pressure at the surface temperature of condensation. The adsorbent is usually activated carbon.


The pumping rate of the condensing pump and the pumping rate affecting the condensing pump are related to the size of the condensing surface area. According to the data, the pumping rate per unit of the condensing surface area is 11.6 l/s. In addition, the adsorption surface geometry and location of activated carbon, activated carbon particle structure, bonding materials and bonding process, has a great impact on the pumping speed. Secondly, the key is the refrigerating capacity of the refrigerator to be large enough.


Vacuum gauge: vacuum gauge is an important part of the vacuum coating machine, it is an important means of testing the vacuum degree of the coating machine. The vacuum gauge can be divided into absolute vacuum gauge and relative vacuum gauge according to its working principle. The absolute vacuum gauge can directly measure the pressure level, while the relative vacuum gauge can only indirectly measure the vacuum degree.


This paper mainly introduces the following vacuum gauges commonly used in the coating machine:

Resistance vacuum gauge (also known as pirani vacuum gauge) :


It is mainly composed of electric heating wire, shell and bracket, mainly based on the low pressure, the thermal conductivity of the gas is proportional to the pressure of the way to work. The above opening is connected with the vacuum system under test. The hot wire is made of metal wire with high temperature coefficient resistance. The two support leads are connected with the measuring line. When the pressure decreases, the heat lost through gas heat conduction decreases. Therefore, when the heating current of the hot wire is stable, the temperature of the hot wire rises and the resistance of the hot wire increases. The pressure is indirectly measured by measuring the resistance of the hot wire.


This is the working principle of resistance vacuum gauge, the vacuum gauge measuring range is: between the 100-10-1 mpa, is the WP - 02 models used at present.


Magnetically controlled discharge vacuum gauge:

How it works: at the beginning of the discharge, because of the space, free electrons from the anode movement under the effect of orthogonal electromagnetic field, the electron trajectory is not straight line but spiral, and because the anode is frame, so the electronic not met for the first time the anode, but through the anode, and rejection by the cathode, and after return. This can be repeated many times on the anode. As the electron path is greatly lengthened, the number of molecules colliding and ionizing is increased, so that the discharge (also called penning discharge) is maintained at a relatively low pressure (below 10-4 pa).

At present, more models include PKR251 and gi-pary.


Discharge tube vacuum gauge: two metal electrodes are sealed in the glass tube, and a high dc voltage of several thousand volts is added on it. Self-sustaining discharge can be caused within a certain pressure range (1*10-3 ~ 2*10 brackets). The vacuum degree can be determined by the discharge color.


So far this kind of vacuum gauge has been rarely used because of its large error, easy damage, and short life.


3. Evaporation system


Evaporation system mainly refers to the film forming device. There are many film forming devices in the coating machine, including resistance heating, electron gun evaporation, magnetron sputtering, rf sputtering, ion plating, etc. I will introduce the two methods of resistance heating and electron gun evaporation, because I use these two methods more.


According to its structure and working principle, resistance evaporation is by far the most widely used evaporation method and also the one with the longest application time. The way it works is that the tungsten as a ship, and then installed in the middle of the two electrodes, in the middle of the tungsten boat with herbs, again slowly to the electrode current, current through the tungsten, tungsten boat electric heating, the low voltage, high current make tungsten boat to generate heat, high melting point and heat transfer to the coating material, when the quantity of heat of tungsten boat is higher than the melting point of coating material, material sublimation or vaporization, this method because of easy operation, simple structure, low cost, so is a lot of equipment used, but the evaporation of thin film because of the low density, plus a lot of material can't use this way to evaporation, so it has certain limitations. Tungsten boat evaporation coating material, material melting point must be less than the melting point of tungsten boat, otherwise there is no way to carry out.


Electron gun evaporation is the most widely used evaporation method so far. It can evaporate any kind of coating material. Its working mode is: Coating material in the crucible, evaporation source into a filament shape, using a special control cabinet, the filament with a strong current, high voltage, due to the material is tungsten filament, so it will be hot, in the end will launch electronic, and adoption of a certain magnetic field will be gathered into a certain shape, and drawing on the crucible, thus formed a beam, due to the electron temperature is very high, can melt any coating materials, as the coating materials is after electron beam melting (some of the material is directly sublimate), medicinal herbs of molecules (atoms or ions) in a vacuum into linear motion, then meets the base board, and then condense, After this way of growth, forming a film! The most commonly used are making the deflection Angle of the electron beam into 270 degrees, or the e-line electron gun with the trajectory of e or the electron beam with the trajectory of 180 degrees, and the c-type electron gun with the trajectory of c.


The biggest advantage of electron beam evaporation is: the spot of the electron beam can be adjusted at will, the filament can be hidden, avoid pollution, can evaporate any coating material, easy maintenance, evaporation speed can be controlled at will, small material decomposition, high film density. Good mechanical strength.


Sputtering method is to bombard the surface of the target material with high speed positive ions. Through kinetic energy transmission, molecules (atoms) of the target material have enough energy to escape from the surface of the target material, and then coagulate on the surface of the product to form a film.


The film deposited by sputtering method has strong adhesion and high purity, and can sputtering a variety of different materials at the same time. However, it has high requirements for the target material and cannot save resources like the electron gun.


At present, magnetron sputtering is the most commonly used method. Magnetron sputtering is to apply an enhanced electric field parallel to the cathode surface, and restrain the electrons near the cathode target surface to improve ionization efficiency. It is the simplest kind of operation, so it is widely used.


4. Film forming control system


At present, there are many methods of film monitoring, such as visual monitoring, fixed value monitoring, crystal oscillation monitoring, time monitoring and so on. I mainly introduce visual monitoring, fixed (extreme) monitoring and crystal oscillation monitoring three.


Visual monitoring, also known as direct monitoring, is the use of eye monitoring, because the film in the process of growth, due to the interference phenomenon will have color changes, we are based on color changes to control the thickness of the film, this method has a certain error, so it is not very accurate, we need to rely on experience.


Fixed value (extreme value) monitoring: mainly USES the reflection type (through type) optical monitoring. Extreme value monitoring method: when the film thickness increases, its reflectance and penetration rate will follow the change, when the reflectance or penetration rate to the extreme point, we can know that the coating optical thickness of ND is a quarter of the monitoring wavelength (into) integer times. However, the error of the extreme value method is relatively large, because when the reflectivity or transmittance changes very slowly near the extreme value, that is, the film thickness ND increases a lot, R/T changes. The more sensitive position is at one eighth of the wavelength.


Fixed value monitoring method: this method USES the stop plating point not to monitor the quarter-wave level of the wavelength, and then the computer calculates the reflectance (or penetration rate) of the total film thickness at the wavelength, which is the stop coating point.


Crystal oscillation monitoring:


The working principle of crystal oscillation is that the vibration frequency of quartz crystal is inversely proportional to its mass. However, one of the disadvantages of quartz monitoring is that when the film thickness increases to a certain thickness, the vibration frequency is not entirely due to the characteristics of quartz itself, so there is a linear relationship between the thickness and frequency.


Several monitoring methods have their own advantages and disadvantages, but usually the multi-layer coating, optical monitoring will be the main, quartz crystal oscillation as an auxiliary method.


In addition, for some in the coating process need to be filled with gas flow control meter or pressure control meter, these need to use sophisticated valves and photoelectric sensing system to control.


In the coating process also need rotary control system, is to put the main shaft of the umbrella in the bearing, and then use the motor to drive the bearing, so that the umbrella rotary. Then the PLC control its rotary speed.


The crucible rotation is driven by electric motor, the photoelectric induction counting method is adopted, and the shielding plate is rotated by pneumatic switch.


In order to speed up the extraction rate and achieve a certain degree of vacuum, the vacuum chamber also needs to be refrigerated, that is, the air inside the vacuum chamber is frozen to -130 degrees Celsius, and the water air inside the vacuum chamber is frozen and pumped out.


PLC automatic control, electrical control part of the article is the first in the PLC input design program in advance, the processor main circuit connected to the empty system on the operation panel, when press the switch on the operation panel, information transmission to the central processing unit (CPU), and then by the central control system analysis and implement and complete the actions by the branch issued instructions.


Coating machine is a multi-discipline of equipment, IT integrates the industry's most advanced mechanical and electronic technology, control technology, electrical automation, IT technology, refrigeration technology, microcircuit integrated systems, high pressure control system, mechanical technology, processing technology, photoelectric technology, optical technology, pneumatic control technology, photoelectric sensor technology, communication technology, vacuum technology, optical film and coating technology, and so on.


It can be said that the coating machine is a new representative of the industry.


Today, coating machine has been widely used, plating film especially widely, the production of various kinds of thin film was applied to the photoelectric system and optical instruments, such as digital cameras, digital cameras, binoculars, projector, energy control, optical communication, display technology, interferometer, the satellite missiles, semiconductor laser, mems, information industry, the production of laser, all kinds of filter, lighting industry, sensors, architectural glass, automotive industry, decoration, COINS, glasses, etc., coating machine is closely linked with human life.


The cost of an imported coating machine usually reaches 3 million to 10 million RMB, and the cost of domestic equipment is about 1 million RMB.

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