Preparation technology of refractory metal target
The preparation methods of refractory metal sputtering targets are mainly divided into powder metallurgy method and smelting method. Among them, W and Mo are mostly produced by powder metallurgy, while Ta and Nb are mostly produced by smelting method. The specific process flow is shown in Figure 3.
1. Powder metallurgy
Powder metallurgy is a traditional method suitable for making refractory metal sputtering targets. The melting point of refractory metal is very high. This method adopts solid/liquid phase sintering, so it is formed at a temperature far below its melting point; when producing alloy targets, two or more alloy powders are passed through the mixer for a long time. It is heated and pressed after mixing evenly over time, which effectively eliminates the segregation of alloy components. Another major advantage is that the grains of the targets prepared by the powder metallurgy method are relatively fine, which can reach below 100 μm. Generally, the sputtering targets prepared by powder metallurgy method adopt three methods: cold isostatic pressing plus sintering, hot pressing sintering and hot isostatic pressing. Among them, the density obtained by hot isostatic pressing is the highest, and the density of the target obtained by the first two methods is relatively low. Therefore, improving the density of targets prepared by powder metallurgy sintering is one of the key technologies that must be solved. The problem of low density of powder metallurgy targets can be effectively improved by press working. Wei Xiuyu[7] studied the effect of rolling deformation on the density of powder metallurgy tungsten targets. With the increase of deformation, the density increases, and the highest can reach more than 99.5%. Zhu Qi [22] studied the effect of extrusion on the microstructure, properties and density of the molybdenum tube target for powder metallurgy, and the density of the molybdenum tube target increased from 9.8 g/cm3 of sintered blank to 10.15 g/cm3, reaching a high density. Require.
Most of the tungsten and molybdenum sputtering targets are prepared by powder metallurgy. Due to the limited purification capacity of powder metallurgy, the raw materials must be high-purity powders. In addition, the mixing of impurity elements must be strictly controlled during the preparation process. At present, there is a big gap between domestic high-purity metal purification technology and industrial developed countries. Guo Rangmin [23] re-neutralized ammonium paratungstate by ammonia solution, purified by activated carbon adsorption crystallization, and prepared high-purity W powder after reduction.
2. Smelting method
Smelting is another important method for preparing refractory metal targets, because refractory metals have high melting points, and electron beam and arc smelting are mostly used. Electron beam melting has the advantages of high temperature, high heating rate, high vacuum, etc., and is suitable for purifying and refining various refractory metals. The metal ingot obtained by electron beam smelting is dense, and the internal structure is free of pores and pores, which is very close to the theoretical density. However, electron beam smelting has two major disadvantages: first, the ingot grains are coarse; Arc melting is suitable for melting alloys, and its ingot density is also very high, but the purification effect is not as good as electron beam melting. Due to the coarse grains of the target obtained by the smelting method, the grains can be made finer by press working and heat treatment, and a certain grain orientation can be obtained. Liu Ning [14] studied the sputtering rate relationship of {110}, {100} and {111} textures in the tantalum target, and proposed to use the hot forging strong plastic deformation process to process the Ta target to make it more uniform. texture components.
Yinan [13] optimized and adjusted key processes such as electron beam melting, forging, rolling, and heat treatment, and obtained a tantalum target with a {111} texture as the main grain size and an excellent performance of less than 100 μm. Wang Guodong[17] studied the Nb target prepared by commutation rolling after heating high-purity niobium ingot smelted by electron beam at 1 100 °C, and the grain orientation was uniform and the grain size distribution was uniform. The author also conducted a series of experiments on pure Nb plate targets. The coarse-grained pure Nb ingots smelted by electron beam are forged, annealed, rolled with 60% deformation, and then finished annealing at about 1200 °C to obtain a 7.5-level grain size (the average number of grains is 1400 pcs/mm2), pure Nb target with uniform structure and excellent performance.