Comparison of the Melt Casting and Spray Rotating Sputtering targets
The target material is an important raw material for vacuum coating. The purity and density of the target material directly affect the composition and comprehensive performance of the film system. With the rapid development of sputtering coating technology, new target preparation technology is emerging, and the target quality and performance have been improved significantly. Because of the high utilization rate, the rotating target has been widely used in the coating industry. AEM deposition can provide various kinds of rotating sputtering targets, as shown in the table below:
There are two processes for rotating target: Casting and spraying. Next, the author makes a comparative analysis of two kinds of rotating targets from the aspects of preparation technology, target structure and sputtering performance:
Comparison of spraying and casting target technology
The preparation of metal target
by spraying is a process in which the target material is heated to molten or semi molten state by electric arc, atomized by high-speed gas to form small droplets, and sprayed to the surface of liner or lining plate to cool and solidify into metal coating target rapidly. Nonmetallic and ceramic targets are usually made by plasma heating powder materials.
Target casting technology can be divided into three different casting forms: integral casting without liner or liner (such as NiCr, NIV target), direct casting with liner or liner (such as Zn, Sn and its alloy target), segmented casting with sling and then bonding to target (AG, etc.).
Comparison of sputtering target microstructure
Because of the different preparation methods, the rotating target has different properties, especially the microstructure. The microstructure of the thermal spraying rotating target is a round, flat and fine grain with the grain orientation parallel to the rotating axis. There are a lot of micro pores in the tissue, and the overall porosity is as high as 5-20%. The number and shape of pores are strongly dependent on the coating material used, spray technology and gas flow. The porosity is directly related to the environmental humidity, oxygen adsorption and desorption in the target preparation and storage. Therefore, in the process of storage and transportation, the thermal spraying target must be packaged strictly to prevent moisture and pollution. Before sputtering, the surface adsorbed gas must be removed with an isolated front pump.
The microstructure of the target is coarse equiaxed or dendrite, and the grain orientation is perpendicular to the rotation axis. However, the structure is compact and there are no pores, indicating that there is no adsorption or desorption of oxygen and moisture in the preparation process. The disappearance of the pores improves the thermal conductivity of the target, which helps to transfer the heat from the sputtering target surface to the cooling water (or liner) on the inner surface of the rotating target rapidly.
Comparison of sputtering target oxygen content
The porous structure is an important defect of thermal spraying target. In the spraying process, the target surface is in contact with O2 and N2 in the air in a large area. Even the vacuum plasma spraying technology can not avoid the generation of oxides and nitrides in the alloy target. In contrast, it is easy to prevent the oxidation and nitridation of metals by slag reaction, so as to obtain pure metal or alloy targets with very low oxygen content.
Comparison of material types and bonding process
The coating materials of bar, wire and powder particles can be used in thermal spraying (without evaporation before melting), so the target making technology of thermal spraying is suitable for many kinds of materials such as metal, alloy and ceramics. The melting casting target technology is only limited to the metal or alloy materials matching the melting point and casting equipment. Brittle ceramics are not suitable for casting, and special casting equipment is required if the melting point is too high. Moreover, some ceramic materials will sublime or decompose before reaching the melting point.
Thermal spraying rotating target is to spray the coating material directly on the liner without additional bonding process. This is the same as the direct pouring of low melting point metal (< 450 ℃) and the integral strip casting. However, some "soft" metals with higher melting point, such as Ag, must be bonded separately.
Comparison of target sputtering performance
The difference of target structure between the two methods determines the difference of target performance. The target structure of thermal spraying is loose and porous, with high oxygen content, so it presents gray surface and lacks metallic luster; the adsorbed impurities and moisture hinder the rapid acquisition of high vacuum, which is easy to cause discharge in the sputtering process. The high temperature on the target sputtering surface at an instant is easy to cause loose particles to fall, contaminate the surface of the plated parts (glass), and affect the coating quality and the qualification rate of the coated products.
The structure of the casting target is compact, and it is not easy to be invaded by moisture and impurities. The high-purity metal elements are sputtered stably, so as to reduce the failure rate of the equipment and the defective rate of the coating products. Moreover, the high density of the target determines the better heat conduction effect, more sputterable materials and longer service life. It has been proved by experience that the casting target is winning more and more attention of coating manufacturers for its excellent sputtering performance and high cost performance.
Sputtering target cost comparison
Different production processes of thermal spraying and casting target materials determine different cost components. The production cost of thermal spraying target increases with the increase of target thickness. The larger the target thickness is, the longer the spraying time is, the greater the energy and gas consumption is. Moreover, different target sizes can be obtained by adjusting the spraying process.
Relatively speaking, the cost of melting and casting the rotating target changes little with the thickness. The target is first melted and molded, and then machined to meet the size requirements. For different sizes of rotating target, the raw materials and processing costs are basically fixed. In a word, the target shows us high cost performance with high purity, high density and excellent sputtering performance.