Physical Vapor Deposition - Thermal Evaporation
-- In PVD system, theraml evaporation and sputtering technology is popular and widely used. Optics industry usually adapt Thermal Evaporation to produce their products.
Thermal Evaporation :
This method use high temperature to melt or sublimate the target (source material) into vapor state. The atom or molecule of target is speed up by high temperature. Passing through a vacuum space, condensing of a vaporized form to substrate surfaces. The vacuum is required to allow the molecules to evaporate freely in the chamber, and they subsequently condense on substrate surfaces. As the the term "Thermal" indicated, the thermal high temperature is the keyrole of this method. "Thermal High Temperature" is same for all evaporation technologies, only the techniques used to the heat (evaporate) the target differs.Several techniques are used as the thermal high temperature means.
Resistive Heating: Use a big current passing through the resistor (the target material is put or attached on the resisor), the resistor will generate high temperature and melt the target material. Traditionally the resistors are made by W(Tm=3380℃), Ta(Tm=2980℃), Mo(Tm=2630℃) which is with very high melting temperature.
The melting temperature of target is much lower than resistors. When the current pass, only the target melted or vaporized. And "fly" to the substrate surface.
The resistor can be made to different shapes depending on what amount of target you want to evaporate and the uniformity of evaporation. The wire (filament) is fisrt to be used as resistor and attached with target (Al, Ag, Au, Cr...). In some coating machine and target material required, the boat or basket shape resistor also used.
Flash evaporation is used for two or more different targets (material) in one coating procedure. Since different target material need different melting temperature, The resistor do not attach the target before melting temperature. It is empty. Once the temperature reach the working condition, the target will be put to the resistor and evaporated immediately. Then another material target be put to the resistor and evaporated. By this way, different material could be manipulated and evaporated to same substrate. And make a multilayer thin films.
Advantage:
1. It's equipment is relative simple and cheap.
2. The target (source material) could make to different shape depending on your need.
Disadvantage:
1. Since high temperature is generated by resistor and conducted to target, the resistor material will react with the target. And.pollute the evaporation purity. Will harm the thin film quality. (The improvement can add a layer of ceramic, but waste more.electricity)
2. Fit for metal material target, the dielectric material is difficult to melt. The evaporating temperature of oxide is too high to resistorto gererate.
3. The speed of deposition is very slow.
4. Compound target may be decomposed by high temperature. Only part of compound target could use in flash evaporation.
5. the hardness of the film is not good. The density is poor.
Electron Beam Gun Evaporation: Using high speed electron hit the target material, the kinetic energy will transfer to thermal energy and create a high temperature when electron bombard the target material. The electron beam is generated by an electron gun, which uses the thermionic emission of electrons produced by a tantalum cathode. Emitted electrons are accelerated towards an anode with very high speed. (if you add a 10000V Voltage, the electron will be accerated to 60000 km/sec. This velocity can create 5000~6000℃ temperature when bombarding target material) The crucible which put the target acts as the anode. A magnetic field is also applied to bend the electron trajectory. Avoiding the gun blocks the evaporation path.
After 1968, Hanks developed a Electron Beam with 270° bending. This progress allowed the gun installed under the crucible. It hugely improved the target vapor contaminating the cathod tip and can prevente cathod failure.
Via the control of the magnetic mechanism (term "magnetic lens"), the electrons can be focalized and change its bombard poistion. It can obtain a very localized heating on the material to evaporate. It also allows to control the evaporation rate, from low to very high values. Most important of all, It could deposite the materials with high melting point (W, Ta, C, dielectric oxide, etc.). Effective water cooling to crucible can avoid contamination problems which will heating and degasification undesired material in the coater.
Magnetic Lens
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270° Bending Electron Beam
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Advantage:
1. Using the water cooling to the crucible, the crucible material contaminatating target vapor can be eliminated to lowest level.
2. Thin film purity is much higher than resistive heating.
3. Most dilectric
Oxide target material could be deposited by this method.
4. With different targets put in different crucibles which installed in a circle, It can easily deposite different target film and make a mulitlayers deposition.
5. With adequat adjustment of the electron bombard spot size, This method allow maker to increase the uniformity of film thickness.distribution.
Disadvantage:
1. Inappropriate control of electron beam or electron flow may decompose or ionize the target. It may result material aborption or.accumlating electron and arc to damage the quality of film surface.
2. The electron beam gun need large electric energy consumption. With a 10~15K Volt power consumption, it usually need to.sustain several hours. This characteristic make it to consume more energy than other methods.
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Electron Beam Gun Evaporation Technology got its predominance position in Thermal Evaporation field since 1950's. With the progress of computer and automatic control skill, almost all multilayers optical thin film (especially dilectric, oxide target ) are made by this method in this morden era. It is good to control the film thickness in nonometer scale which is necessary for optical purpose film.
From 1950 to now, more additive techniques developed in these decades. The progress is mainly on how to increase the adhesion force of the film.
Activated Reactive Evaporation (ARE): |
The evaporation of target material occurs in the presence of a reactive gas (depending on.target material) and a plasma to deposit compounds and can increase adhesion force and increased deposition rates. |
| Bias Voltage Evaporation(BVE): |
Aid a high voltage plate in front of substrate. Make the target atom electrified when leaving the crucible. It will speed up and evaporate to high voltage substrate. The deposited film will increase the density of target.film. |
| Plasma Assistance: |
Adding a several KV voltage in the space between source and substrate and filling in the Argon..It could make a plasma in the middle
of evaporating path. When evaporation material pass through the plasma. Part of material will be ionizedand and attracted by the upper electric field. This could make the evaporation more powerful flying and condensing on the substrate. . |
| Ion beam Assisted Deposition (IAD): |
This is most effective assistant method to increase the adhesion force amoung all techniques. By adding an ion source beside the target source, The ion will directly bombard the substrate surface which condensing the target evaporation. The target will be direct hit into the substrate surface and form the film layer.
For ion is a heavy particle compare to atom or molecular state of target. This hit is an effective way to increase adhesion..Further more, the ion source is an independent mechanism. It can be operated independent to change the ion bombard angle, diffusion angle, current, voltage, ect. Not necessary to be subjected the source operation status. Many coating.method.(evaporation, sputtering) can add this techniques as assistance. Resistive Heating vaporation, Electron Beam.Gun Evaporation, RF Sputtering, Magnetron Sputtering can adapt it. There is a new modification techniques named "APS" (Advanced Plasma Source). This modification a little.changed parts location. But the main principle is still the same...More discussion on IAD will be in next page. |
Different Assisted Method Accocation in Coater Chamber
Laser Deposition: Using laser beam as a evaporation means. The laser can evaporate the target source by heat or photodissociation.
Without the electric charge accumlating, the film will not harm by electric effect. The laser beam is a focus energy beam, it do not disperse many through long distance. The target source can locate far compare to other means. This made contamination reduced to lowest level. However, laser is an expensive equipment. The high cost of coater make this method not popular to industry producer.
Molecular beam epitaxy (MBE): It is a special method apply to depositing single crystals. It produce the film with crystallization and crystal orientation. Most of optical light pass through the film do not hope get orientation characteristic, It only used in some special purpose products.
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