The vacuum evaporation technology of optical thin films mainly includes sputtering method and thermal evaporation method. Next, we will learn about the coating process and method characteristics of optical thin films in detail:
Sputter method
The sputtering method was applied earlier, which is to ionize the remaining gas under the action of a strong electric field, and ionize the positive ion to move towards the cathode at a high speed under the action of the field. After hitting the cathode surface, it transfers its energy to the sputtering target located on the cathode surface. The target surface atoms {or molecules} are deposited on the substrate to form the required film. The cathode sputtering is rarely used to deposit dielectric materials. This is because the charged ions accumulate on the substrate and the target surface to form a shielding electric field, which affects the continuation of the deposition, resulting in slow deposition rate and poor effect. High-frequency sputtering is designed to overcome this shortcoming of cathode sputtering. Unlike ordinary cathode sputtering, high-frequency sputtering applies high-frequency voltage between the two poles, with a frequency of 5~30Mhz, so that the charge adsorbed on the pole surface in the upper half of the cycle is released in the lower half of the cycle, which will not affect the deposition due to the accumulation of charge. High-frequency sputtering can be used to deposit various dielectric materials. This film has a compact structure and good robustness, The preparation of electrical film is widely used, but its uniformity is not good enough. It is difficult to control the thickness, and it is rarely used in optical thin films. If the above shortcomings are overcome, it is expected to be applied in overcoming the damage of high power laser to thin films.
Thermal evaporation method
At present, thermal evaporation is widely used in the deposition of optical thin films. Its basic principle is to heat the evaporated material to the evaporation temperature, so that it can be volatilized and deposited to the substrate to form the required film layer
Vacuum evaporation coating
Resistance heating evaporation
Most materials, especially compounds, can only be evaporated indirectly, that is, an evaporation source is needed to hold and heat the materials
Three basic requirements for selecting evaporation source: melting point and vapor pressure of evaporation source materials; The reaction between the evaporation source material and the film material and the wettability of the film material
The simple and common method is to use the material with high melting point as the heater, which is equivalent to a resistance. After being energized, the heat will be generated, and the resistivity will increase. When the temperature is 1000 degrees, the resistivity of the evaporation source is 4 to 5 times that of the cooling source; At 2000 ℃, the joule heat generated by the heater is enough to make the molecules or atoms of the vaporized material obtain enough kinetic energy to evaporate when it is increased to 10 times
The commonly used evaporation source materials include graphite, tungsten, tantalum, molybdenum, platinum, and the commonly used ones are molybdenum sheet and tungsten wire. The advantages of resistance heating method are simple equipment, convenient operation and easy to realize the automation of film deposition process, but it can not directly evaporate insoluble metals and high-temperature dielectric materials; It is difficult to avoid the pollution of the evaporation source to the membrane material.
Electronic card evaporation
The principle is that when the metal is in a high temperature state, a part of its internal electric current will select the surface due to obtaining enough energy, which is called hot electron emission. If a certain electric field is applied, the electrons will move towards the anode direction in the electric field, and the higher the electric field voltage, the faster the electrons will move. Under the action of a certain electromagnetic field, the high-speed moving electrons will gather into a fine beam and bombard the surface of the coated material, making the kinetic energy into heat energy, This causes the film material to evaporate.
The structure of electronic gun has many forms. At present, magnetic deflection e-shaped gun is widely used, which basically overcomes the influence of secondary electric shock. The so-called “e-shaped” is named because the electric shock path is e-shaped, also known as 270 ° magnetic deflection gun. It is composed of cathode filament, focusing plate anode, deflection magnet and oxygen-free steel water-cooled crucible.
The hot electrons emitted from the filament are accelerated and focused by the high-voltage electric field between the cathode and the anode, and deflected by the magnetic field to reach the surface of the crucible evaporation material. Because the evaporation material and the anode are separated and separately in the magnetic field, the secondary electrons deflect again due to the effect of the magnetic field, reducing the probability of emission to the substrate, The relative position of the focusing electrode and the electrode. The deflection of the electron beam mainly depends on the size of the high voltage and the magnetic field current. The e-shaped gun can effectively inhibit the secondary electrons, and it is convenient to adjust the beam spot position by changing the size of the magnetic field. In addition, the built-in cathode is used, which not only prevents the ion discharge between the plates, but also avoids the filament pollution.
Laser evaporation
The high-energy laser beam passes through the vacuum chamber window to heat the evaporated material. Through focusing, the laser beam power density can be increased to facilitate material evaporation
The advantages of laser evaporation are that high melting point materials can be evaporated; The non-contact heating source is placed outside the vacuum chamber to reduce pollution and reduce the vacuum chamber. It is very suitable for the preparation of pure films under high vacuum. It also has high evaporation rate but high cost, and the evaporation material is limited
Reactive evaporation
Many compounds will decompose in the process of high-temperature evaporation, such as direct evaporation of Al203. Ti02, which will lead to oxygen loss and increase absorption. Therefore, reactive evaporation should be used, that is, evaporation of metal or low valent compounds in a certain reaction atmosphere, so that chemical reactions occur during the deposition process to generate the required high valence compound film. In order to strengthen the oxidation effect in reactive evaporation, the technology of ionic oxygen reactive evaporation has been paid attention to and applied. The preparation parameters must be strictly controlled, such as background vacuum and gas-filled vacuum or gas-filled flow control
Lon plating
The so-called ion plating is a new technology developed by the combination of vacuum evaporation and sputtering. Its advantages are: strong adhesion of the film, high density of the film, good uniformity, and fast deposition rate. It is mainly used to manufacture mechanical tools with high hardness and wear-resistant solid lubricating film, and to produce durable decorative film on metal and plastic products. At present, the low pressure reactive ion plating technology has been developed, The film with good surface roughness can be obtained
Lon assisted plating
It uses a beam of ion source, While coating, bombard the substrate with certain gas ions {For example, Ar). It improves the properties of the film, improves the microstructure and stress state of the film. Its principle is to transfer the energy of the external beam to the deposited molecules, thus improving the migration rate of the molecules. Because of the large mass of the ions, it is easy to transfer the energy to the deposited molecules or atoms, making the ion bombardment greatly affect the deposition of the film. The application in the coating is mainly manifested in the matrix cleaning to promote the growth of the film, enhance the strength of the film, and improve the film The adhesion of the membrane is improved.
