The latest development and trend of magnetron sputtering coating technology
The basic process of glow plasma sputtering is that under the action of energy-carrying ions in the glow plasma on the negative electrode, the target atoms are sputtered out from the target and then condensed on the substrate to form a thin film. In this process, secondary electrons are simultaneously emitted from the target surface, which play a key role in maintaining the stable existence of plasma. The appearance and application of sputtering technology have gone through many stages. After more than 30 years of development, magnetron sputtering technology has developed into an irreplaceable method for the preparation of ultra-hard, wear-resistant, low-friction coefficient, corrosion resistant, decorative, optical, electrical and other functional films.Pulsed magnetron sputtering is another important development in this field. It is almost impossible to achieve dense, defect-free insulating films, especially ceramic films, by dc reactive sputtering deposition, because of the low deposition rate, the easy occurrence of arc discharge of target materials and the change of structure, composition and performance. Pulsed magnetron sputtering technology can be used to overcome these shortcomings. The pulsed frequency is 10 ~ 200kHz, which can effectively prevent the target material from arc discharge and stabilize the reactive sputtering deposition process, so as to achieve high-speed deposition of high quality reactive films.The authors mainly discuss the magnetron sputtering technology in unbalanced magnetron sputtering, pulsed magnetron sputtering progress, as well as the magnetic control sputtering deposition, sputtering, high-speed high purity at low pressure membrane preparation and improve the quality of reaction sputtering film technology progress carried on the thorough analysis, the last call for China's petrochemical industry should vigorously the development and application of magnetron sputtering technology.
Non - equilibrium magnetron sputtering technology
Compared with conventional magnetron sputtering, non-equilibrium magnetron sputtering technology has little difference in design, but leads to huge difference in deposition characteristics. FIG. 1 schematic diagram of plasma region characteristics of non-equilibrium magnetron sputtering technology and conventional magnetron sputtering technology
In conventional magnetron sputtering, the plasma is completely confined to the target area, and the typical value is about 6cm on the target surface. Figure 1 c (called invasive) unbalanced magnetron sputtering, the outer magnetic field intensity, magnetic field intensity is higher than the center lines of magnetic force did not form a closed loop between the center and the periphery, part of the outer surface of the magnetic field lines extended to substrate, making part of the secondary electron can reach the surface of the substrate along the lines of magnetic force, plasma is no longer confined to the area of target, but to be able to reach the surface of the substrate, the substrate ion beam density increase, usually can reach more than 5 ma/cm2. In this way, the sputtering source is also an ion source bombarding the substrate. The ion beam density of the substrate is proportional to the current density of the target material. The current density of the target material increases, the deposition rate increases, and the ion beam density of the substrate increases.Figure 1b(called cohesion) is another non-equilibrium magnetic field, which is characterized by a higher central magnetic field intensity than the periphery, magnetic field lines not closed but directed to the wall of the device, and low plasma density on the substrate surface. Due to the low density of substrate ion beam, this method is rarely used. However, studies have shown that this method can obtain films with high specific surface and high activity, and the porosity of the obtained film can be more than 1000 times that of the dense surface, and the porosity can be controlled at the same time. Porous films have important applications as catalysts, ignition devices and blackbodies. Further development of non-equilibrium magnetron sputtering (CFUBMS) is characterized by the use of multiple non-equilibrium magnetron sputtering sources installed in a certain way, which is used to overcome the great difficulty of using a single target to uniformly deposit thin films on the surface of complex substrates.In multi-target system, the relation between two adjacent targets can be placed in parallel or relative. There are also two magnetic field modes in adjacent targets, as shown in FIG. 2. When adjacent magnetic poles are opposite, they are called closed magnetic field modes. When adjacent magnetic poles are the same, it is called mirror magnetic field mode. In the way of closed magnetic field, magnetic field lines between different target material closed, by the wall loss less electronic, the plasma density on the surface of the substrate is high, is the ratio of ions and atoms reach the surface of the substrate mirror magnetic field or single target more than 2 ~ 3 times of unbalanced magnetic field, when the substrate and target spacing increases, the closed magnetic field on the surface of the substrate ion and the influence of the atomic ratio is more significant. In the mirror mode, the magnetic field line is directed to the wall, and the secondary electrons are consumed by the wall along the magnetic field line, resulting in the decrease of plasma density on the substrate surface.
On the basis of non-equilibrium magnetron sputtering technology, variable magnetic field intensity magnetron sputtering technology has recently emerged, which is characterized by adjustable magnetic pole position. By changing the distance between two magnetic poles and the surface of the target, the magnetic field intensity on the surface of the target can be changed. Variable magnetic field design provides a new technical parameters, implementation of sedimentary ions, atoms than fine adjustment, such as sedimentary phase begin to hope to higher ion beam, in order to improve the film adhesion, but further deposit of ion beam can lead to high thin film stress and defect, at any time change the magnetic field can change the ion beam and eliminate this problem. When depositing gradient film and multilayer film, this technique can achieve the best combination of various film properties. This technology can also control the sputtering corrosion characteristics of the target and achieve the uniform sputtering of the target.
Pulsed magnetron sputtering (PMS)
The pulsed magnetron sputtering is formed by replacing the traditional dc power source with the pulsed dc power source. This technology has a series of remarkable advantages, such as lower deposition temperature, high-speed and defect-free ceramic film deposition. For example, when depositing oxide films, metal targets can be traditionally used, reactive sputtering deposition in an appropriately controlled oxygen atmosphere, or rf (generally 13156MHz) sputtering oxide target deposition. However, both methods have limitations. High quality films can be obtained by rf sputtering with very low deposition rate (m/h level) and complex systems. The problem in reactive sputtering is that the target is poisoned. During reactive sputtering, the non-main glow area on the surface of the target is covered by insulating sediments, leading to the accumulation of charge in the insulation and insulating layer of the target until the occurrence of arc discharge.Arc discharge makes the components of the target material evaporate in the form of droplets and causes various film defects when deposited on the substrate surface, such as loose structure of the film, coarse grain, composition or structure segregation, etc., which has a very adverse impact on the performance of the film, especially the optical and corrosion resistance. Pulsed magnetron sputtering technology can effectively inhibit the generation of arc and eliminate the film defects, and greatly improve the deposition rate of sputtering to reach the deposition rate of pure metal, that is, 10 m/h. In the process of pulsed sputtering, the pulse voltage added on the target is the same as that of general magnetron sputtering (400 ~ 500V), and the time of discharge with voltage added on the target is controlled to ensure that the target is not poisoned and arc discharge occurs. Then the target voltage is switched off, even making the target positively charged. Because the electron velocity in the plasma is much higher than the ion velocity, the positive voltage of the transformed target material generally only needs 10% ~ 20% of the negative bias voltage, which can prevent the arc discharge (this kind of power supply is called the asymmetric bipolar dc power supply).Studies have shown that when the pulse frequency is lower than 20kHz, the occurrence of arc discharge cannot be inhibited. When the pulse frequency is higher than 20kHz, the arc discharge can be completely suppressed. At the same time, the pulse width (the ratio of positive and negative voltage to time) plays a key role. The positive voltage has no obvious influence on whether arc discharge is generated or not, but it has a great influence on the deposition rate. When the positive voltage increases from 10% to 20%(as compared with the negative voltage), the deposition rate can be increased by 50%. This effect is thought to be enhanced by a high positive voltage for target cleaning. PMS technology can be used for bipolar magnetron sputtering, and two magnetron sputtering targets are respectively positive and negative poles. In the working process, one target is sputtering while the other is cleaning, and the cycle repeats.This technology has many advantages such as long time (300h) stable operation, and has an important application in the deposition of optical thin films used in construction, automobile and polymer materials. Another recent development is the application of pulse bias to substrates. Pulse bias can greatly improve the ion beam on the substrate. In magnetron sputtering, when the dc negative bias is generally added to -100v, the substrate ion beam will reach saturation. Increasing the negative bias will not increase the substrate ion beam. It is generally considered that the saturation current is an ion beam and electrons cannot get close to the substrate surface. The results show that pulse bias not only increases substrate saturation current, but also increases with negative bias. When the pulse frequency increases, the effect is more significant. The mechanism is still not clear, which may be related to the plasma ionization rate and the higher electron temperature produced by the oscillating electric field. The negative bias of substrate pulse provides a new method to effectively control substrate current density, and this effect can be applied to optimize membrane structure, adhesion, and shorten sputtering cleaning and substrate heating time. With the progress of mechanical, power, control and other related technologies, magnetron sputtering technology will be further developed. For example, recently, due to the application of rare earth permanent magnet, the magnetic field strength on the target surface used to be only 300 ~ 500Gs, but now it has been improved to 1kGs, which further improves the efficiency and capability of magnetron sputtering.
New magnetron sputtering coating technology
From common metal target sputtering, reactive sputtering, bias sputtering, etc., along with the industrial demand and the emergence of new magnetron sputtering technology, new technologies such as low-pressure sputtering, high-speed deposition, self-supporting sputtering deposition, multiple surface engineering and pulse sputtering have become the development trend in this field. The key problem of low-pressure sputtering is that at low pressure (generally <011Pa), the collision probability between electrons and gas atoms is reduced. In conventional magnetron sputtering technology, it is not enough to maintain the glow discharge on the surface of the target material, resulting in the inability of sputtering deposition to continue. By optimizing the magnetic field design, the motion distance of the electron space is prolonged, and non-equilibrium magnetron sputtering technology can realize sputtering deposition in vacuum at the level of 10-2pa.In addition, sputtering deposition at lower pressure and higher intensity can be realized by restraining electron motion with external electromagnetic field. High-speed deposition can greatly improve work efficiency, reduce gas consumption and obtain new film. The main problem to be solved in high-speed deposition is that the current density of the target material is increased without arcing discharge. With the increase of power density, the cooling capacity of target material and substrate needs to be improved. At present, the target power density is over 100W/cm2, and the deposition rate is over 1 m/min. High - speed deposition is an attractive alternative to conventional electroplating. In the process of high-speed deposition, by increasing the ionization rate of sputtering particles, the working gas can be cut off and the discharge deposition can be maintained, that is, self-supporting sputtering deposition can be formed. Self-supported sputtering deposition plays an important role in improving the adhesion between thin film and substrate, eliminating internal defects of thin film and preparing high purity thin film. The combination of magnetron sputtering technology and other surface engineering technology is another main development direction of magnetron sputtering technology.Although magnetron sputtering technology has many advantages, it still occupies a small share in the field of industrial surface engineering, and the traditional surface technology still occupies a dominant position. One of the main reasons affecting its application is that the substrate materials such as low-alloy steel and titanium alloy are too soft to match with the ultra-hard films obtained by sputtering technology. The substrate is too soft to withstand load pressures as opposed to very hard coatings. Conversely, for corrosion resistant applications, pinprick defects can result in coating failure. In order to overcome these problems, multiple surface engineering technologies were developed, that is, several surface engineering technologies were used to modify the materials successively, and the surface modification layers obtained had incomparable advantages over the single surface technology. A typical example of this is n-deposition followed by sputtering deposition, which provides a subsurface of 500 m thickness and a hardness of 10GPa, followed by deposition of 3 ~ 5 m TiN. TiN provides high wear resistance and n-layer provides high load bearing and fatigue resistance.
Domestic development status and application in petrochemical industry
Magnetron sputtering technology has been widely used in the fields of building materials, decoration, optics, corrosion prevention, and grinding tool reinforcement in China. Currently, the preparation of functional films such as photoelectricity, photothermal, magnetism, superconductivity, dielectric, and catalysis by magnetron sputtering technology is a research hotspot. However, as for the non-equilibrium magnetron sputtering technology, especially the new deposition process, few units have been understood and studied in China. After searching, it is found that there are only less than 20 scientific research articles in Chinese so far, and the number of author units is even fewer.Anti-corrosion and high hardness film can play an important role in improving the performance and life of petroleum machinery, low friction coefficient, lubrication, anti-mud bag, catalysis, optics and other functional film when applied to the petrochemical industry is expected to significantly improve the work efficiency, product quality and environmental protection, safety and so on. With the development and application of new magnetron sputtering technology and process, and the increasing demand for improving production efficiency, environmental protection and safety in the petroleum and chemical industry, the importance of magnetron sputtering technology to the petroleum and chemical industry will continue to increase. However, at present, the petrochemical industry in China lacks sufficient understanding and application of magnetron sputtering technology, and there are no specialized institutions engaged in this field. Therefore, the author calls for strengthening the support of magnetron sputtering technology.
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