Magnetron sputtering coating belongs to physical vapor deposition. The ions bombard the cathode target at a high speed under the acceleration of the electric field. The atoms of the target material are sputtered and deposited on the surface of the coated body to form a thin film. It is a main method for preparing thin film materials. [1]. Its basic principle is: after the electric field accelerates ions in a vacuum environment, an ion beam with high kinetic energy is formed, which collides with the solid surface. The atoms on the solid surface are sputtered and leave the solid to deposit on the surface of the coated body. The source of the thin film is called the sputtering target [2]. Sputtering is an advanced thin film material preparation technology, which has two characteristics of high speed and low temperature [3]. Since the 1980s, information storage, integrated circuits, laser memory, liquid crystal displays, electronic controllers and other industries have entered a period of rapid development, and magnetron sputtering technology has really entered industrial scale production from the laboratory [4]. my country has gradually become a country with a large demand for thin film targets. Driven by the global market demand, there are many scientific research institutes and enterprises in the field of materials in my country, which have carried out the research and development and production of sputtering targets, and have made great progress.
Refractory metals, generally including tungsten, tantalum, molybdenum, niobium, hafnium, zirconium and titanium, all have melting points above 1 600 °C. Alloy materials such as tungsten and molybdenum have good high temperature strength and creep performance, and are widely used in microelectronics, lighting sources, weapon systems, atomic energy and other industries. Tantalum-niobium alloys have low vapor pressure, low thermal expansion coefficient, and excellent corrosion resistance, and are widely used in aerospace, chemical equipment, integrated circuits, and nuclear energy departments [5]. Making refractory metals into targets can take advantage of their excellent properties in the form of thin films. Table 1 shows the application fields of several refractory metal targets.
1. Types and applications of refractory metal targets
There are several classification methods for sputtering targets: for example, targets can be divided into metal targets, polymer ceramic non-metallic targets and composite targets according to the material. For example, according to the external size, it can be divided into cylindrical, rectangular, square plate targets and tube targets.
Because the common square targets and round targets are generally solid, in the coating operation, the magnetic field generated by the annular permanent magnet on the surface of the target is annular, which will cause uneven erosion, and the thickness uniformity of the sputtered film is not good. , the use efficiency of the target is only about 20% to 30%. The currently popularized hollow tube target can rotate around the fixed bar magnet assembly for a certain period, and the 360-degree target surface can be uniformly etched, which has obvious advantages and increases the utilization rate to 80% [6].
1.1 Tungsten target
Tungsten has the highest melting point of refractory metals, and has many advantages such as stable high temperature characteristics, resistance to electron migration and high electron emission coefficient. Tungsten and tungsten alloy targets are widely used in microelectronics, integrated circuits and other industries. Al, Cu, and Ag are currently the most used interconnect materials for integrated circuit manufacturing. Generally speaking, the dielectric layer is Si or SiO2. Al, Cu, and Ag will diffuse into the dielectric to form silicide, thereby making the metal interconnection The current intensity is weakened sharply, and the function of the entire wiring system may collapse due to this. The best solution is to shield the diffusion layer between the wiring and the dielectric. The barrier metal is WTi.
A large number of experiments have proved that WTi alloy (10% ~ 30% Ti) has been successfully applied to Al, Cu and Ag wiring technology as a barrier layer. Due to the low atomic diffusivity of metal W in other metals, it can block diffusion. Ti can effectively prevent the diffusion of grain boundaries, and on the other hand, it also improves the adhesion and corrosion resistance of the barrier layer [7-8].
Tungsten targets are also used in the decorative coating industry, such as watches, glasses, sanitary ware, hardware parts and other products, not only can beautify the appearance, but also have anti-wear, corrosion and other functions. In recent years, the demand for decorative coating targets has been increasing [9]. The main domestic research and development units of W targets are Shanghai Iron and Steel Research Institute, Beijing Antai Technology, Northwest Nonferrous Metals Research Institute, Zhuzhou Cemented Carbide Group, etc.
1.2 Molybdenum target
Molybdenum has a high melting point, low specific impedance, high electrical conductivity, and good corrosion resistance, and is widely used in LCD displays, wiring and electrodes in photovoltaic cells. There are also barrier materials for integrated circuits.
Metal Cr used to be the material of choice for LCD display wiring, but now super-large, high-precision LCD displays are developing rapidly, which puts forward higher requirements on the specific impedance of the material. In addition, environmental protection must also be taken into account. The specific impedance of the film stress of metal Mo is only half of that of chromium, and it does not pollute the environment. Many advantages make metal Mo one of the best materials for sputtering targets for LCD displays [10].
Copper indium gallium selenide ("CIGS" for short) thin film solar cell is one of the most promising thin film solar cells, with many advantages such as high photoelectric conversion efficiency, no recession, stable performance and low cost. In the field of photovoltaics, scholars at home and abroad have paid great attention to CIGS. The fifth layer of CIGS thin film solar cells is the back electrode, and the performance of the cell is directly affected by the back electrode material. Mo sputtered films have good thermal stability, low resistivity, and can form good ohmic contact with the CIGS layer. At the same time, the metal Mo thin film also has the characteristics of thermal expansion coefficient similar to that of the upper and lower glass layers and CIGS, and has become the necessary material for the back electrode of thin film solar cells [11]. Figure 2 shows the location of Mo in thin-film solar cells [5]. In recent years, the global demand for solar cells has surged, increasing by more than 40% every year. According to reports, the total annual power generation of thin-film solar cells in the world is currently about 660 MW [10]. The main domestic research and development units of Mo target are Jinduicheng Molybdenum Industry, Beijing Antai Technology, Luoyang Hi-tech Sifeng and so on. Antai Technology Co., Ltd. has prepared a large number of molybdenum and its alloy targets by pressing-sintering-hot isostatic pressing method, and the relative density is 99%[12].
1.3 Tantalum target
When large-scale integrated circuits enter the deep sub-micron era, the resistance of Al wires to stress migration and electromigration is relatively weak, which will cause wiring voids and lead to complete failure of the circuit system. Therefore, metal Cu wiring will become mainstream. Cu has higher resistance to electromigration and lower resistivity than Al, which means smaller, denser wires can carry stronger currents. Low resistance increases chip speed. The global 130 nm, 90 nm and below device manufacturers have adopted
In Cu interconnection process, Ta becomes the barrier layer of Cu interconnection. At present, VLSI has gradually developed into Cu/Ta system [13-14]. Because of the high chemical activity of Cu and Si, the diffusion rate is fast, and it is easy to form copper-silicon alloy (Cu-Si). Ta and Ta compounds have high thermal stability, high electrical conductivity and blocking effect on foreign atoms.
Cu and Ta and Cu and N do not react and do not diffuse to form compounds, so Ta and Ta-based films become barrier layers that can effectively prevent the diffusion of copper [15].
my country is rich in Ta reserves, but in the past, we lacked the most basic understanding of semiconductor sputtering targets, which limited the technological development of high-purity Ta targets. For a long time, the high-purity Ta raw materials used in the production of sputtering targets in my country mainly relied on imports. Through years of research and development, Ningxia Dongfang Tantalum has mastered the production process of high-purity Ta sputtering target raw materials, filling the domestic gap. Ningbo Jiangfeng Electronic Materials Co., Ltd. also produced 300 mm high-purity Ta sputtering targets [14]. Xi'an Noble Rare and Precious Metals Company also masters the production process of high-purity Ta target [16].
1.4 Niobium target
In recent years, with the rapid development of optoelectronic technology, Nb thin film materials have been widely used in the manufacture of LCD, TFT plasma display screens, camera lens coatings, optical lens coatings, automotive and architectural glass, which are closely related to people's modern life [17] ]. Niobium targets are also used in surface engineering materials, such as chemical corrosion resistance, shipbuilding, heat resistance, electronic imaging, information storage, high conductivity and other coating industries [18]. Due to the high utilization rate, the rotating hollow tube magnetron sputtering target has been widely promoted in the industry. The niobium tube target is mainly used in flat-panel displays, advanced touch screens and surface coating of energy-saving glass and other industries, and has anti-reflection on glass screens role [19].
The main units that develop Nb targets in my country are Ningxia Dongfang Tantalum Industry and Northwest Nonferrous Metals Research Institute. According to the author's understanding, Baoji Jiajun Company produced a large target with an outer diameter of 152mm, an inner diameter of 125mm and a length of 3900mm through smelting and extrusion.
2. Technical requirements for the target material
In order to ensure the quality of the deposited film and improve the sputtering efficiency, the quality of the target material becomes a key factor. After a large number of studies at home and abroad, it is concluded that the factors that have the greatest impact on the quality of sputtering targets are: purity, density, dimensional accuracy, grain size, texture, etc.
2.1 Purity
The purity of the sputtering target is the primary factor affecting the coating effect. Impurities in the target material and oxygen and moisture in the pores are the main sources of contamination for the deposited films. To improve the performance of sputtered films, the impurity content in the target should be reduced as much as possible, the purity should be improved, the source of pollution should be reduced, and the uniformity of the deposited film should be improved [2]. Targets in different application fields have different requirements for purity, and the target for ordinary coating requires a purity of more than 99% to meet the requirements. Targets with more stringent requirements on purity are used in microelectronics, displays and other fields. At least 40 impurity elements need to be analyzed, and the purity should be above 99.95% (3N5) [20].
2.2 Density
If the inside of the sputtering target is not very dense or gas is accumulated, then the gas release during the sputtering process will generate particles that directly affect the coating quality. At the same time, the pores will cause abnormal discharge during sputtering, resulting in the generation of impurity particles. In order to ensure the quality and performance of the thin film, the density of the sputtering target must reach a high level. The higher the density of the target material, the better the electrical conductivity and thermal conductivity, and the higher the strength. High-density target coating has many advantages: long target life, low sputtering power, high film formation rate, thin film not easy to crack, and high light transmittance.
2.3 Microstructure
The sputtering speed is directly affected by the grain size. The sputtering rate of targets with coarse grains is much slower than that of targets with fine grains, the grain size becomes smaller, and the film deposition rate increases. However, the overall difference in grain size of the same target material is small, and the thickness distribution of the deposited film is more uniform. The average grain size of molybdenum targets developed by Jinduicheng Molybdenum Industry reaches 50 μm, which is the leading level in China [21]. In general, due to the influence of deformation and other factors, the grain size of the flat target is easier to be finer and more uniform than the tubular target.
When the target is sputtered, its atoms will preferentially sputter along the hexagonal close-packed surface. Therefore, in order to improve the sputtering coating rate, the structure of the target can be adjusted as much as possible to have a certain crystal orientation. Within the appropriate range of grain size, the more uniform the grain orientation, the better. The grain orientation of the target will also have a great influence on the thickness and uniformity of the film.
In order to make the microstructure of the target material have a certain crystallographic orientation, it is necessary to adopt different pressure processing methods according to the microstructure characteristics of the target metal, and then adjust and control it with appropriate heat treatment. Many domestic units have done a lot of research on the effect of different processing methods on crystallographic orientation.
2.4 Dimensional accuracy
The sputtering target needs to undergo a series of mechanical processing before post-assembly, and its processing quality and precision (flatness, straightness, roughness) will also affect the film performance. Before sputtering, the target must be connected with the aluminum or oxygen-free copper chassis (backplane), and the close coordination can make the target and the backplane conduct electricity and heat better. Ultrasonic inspection should be used after assembly, if the void area between the two is less than 2% of the total contact surface, so that it can be used in high-power sputtering [10]. Under the same dimensional accuracy requirements, the machining difficulty of tubular targets is greater than that of flat targets, because large-scale tubular targets are generally extruded, and the inner hole has a deep extrusion groove, which causes a greater impact on machining. difficulty. In the industry, high-precision CNC deep hole drilling and boring machines are generally used to process inner holes.