High-purity tungsten is a basic material for manufacturing integrated circuits. The shape of the material can be foil, sheet, target, filament, insulated wire, straight rod, round tube, powder, single crystal, etc. It is widely used as gate electrode material, wiring material and shielding metal material for semiconductor large-scale integrated circuits.
The improvement of the integration of large-scale integrated circuits in microelectronics technology puts forward higher requirements for materials, and traditional Si-based devices are no longer applicable. High-purity tungsten or ultra-pure tungsten (5N or 6N) is widely used as gate electrode material, wiring material and shielding metal material for semiconductor large-scale integrated circuits due to its high electron migration resistance, high temperature stability and very high electron emission coefficient. . High-purity metal tungsten target is one of the basic materials for manufacturing integrated circuits, and its market prospects are closely related to the development of integrated circuits; if the purity of the tungsten target is not high, it will reduce the operating reliability of large-scale integrated circuits and even cause leakage. The use of high-purity tungsten can reduce or even eliminate the influence of harmful impurities and improve the performance of the end product
The purity of high-purity tungsten reaches 99.999% and 99.9999%, denoted as 5N and 6N pure tungsten. The content of various impurity elements should be between (0.1～1000)×10-12, and there are special requirements for the content of certain impurity elements, such as radioactive elements, alkali metal elements, heavy metal elements and gas elements.
The preparation methods of high-purity tungsten can be divided into powder metallurgy, smelting and chemical vapor deposition.
Powder metallurgy refers to the process in which tungsten powder is heated to a certain temperature below its melting point after being formed, and the densification is completed through material migration, and finally tungsten blanks or some simple tungsten products can be obtained.
The smelting method refers to the process of heating tungsten raw materials above its melting point to form a liquid phase, removing impurities and then cooling and solidifying to achieve densification. Depending on the method used, the specific methods include vacuum consumable arc smelting and electron beam smelting And plasma beam melting method, etc.
Chemical Vapor Deposition
Chemical vapor deposition refers to the use of tungsten compound gas (usually WF6) as the tungsten source, which is reduced by H2 at a certain temperature, and the generated tungsten is deposited on a specific substrate. After the deposition is completed, the substrate material is removed to obtain a compact tungsten blank (or product) )the process of.
High-purity tungsten and its silicides are used in ultra-large-scale integrated circuits as resistance layers, diffusion barriers, etc., as well as gate materials and connection materials in metal oxide semiconductor transistors, etc.
Detection of trace impurities in high-purity tungsten
The determination of trace elements in high-purity tungsten is an important criterion for evaluating the performance of high-purity tungsten. Reducing the lower limit of the determination of trace elements and increasing the number of determinations of trace elements, improving the accuracy and sensitivity of the measurement are the main points of the high-purity tungsten detection process. Research direction. Commonly used methods for the detection of trace elements in high-purity metals include spectral analysis, mass spectrometry, neutron analysis, and so on. With the improvement of material purity, traditional spectroscopic analysis and neutron analysis can not meet the analysis requirements of 6N tungsten powder. However, mass spectrometry has played an increasingly important role in the detection of trace impurities in high-purity tungsten. An increasingly important role.
Inductively coupled plasma mass spectrometry (ICP-MS) determination method is a new generation of trace analysis technology, which has the advantages of low detection limit and simultaneous analysis of multiple elements, but still has the interference problem of matrix effect.
Beijing Research Institute of Nonferrous Metals adopts pre-separation and enrichment technology, and uses ion chromatography to separate the matrix to reduce the interference of the matrix on the detection of impurities. The use of membrane desolvation device to purge the solvent effectively reduces the yield of oxides in the ICP-MS detection process. The impurities in 6N tungsten powder are accurately measured.
Compared with the ICP-MS method, the glow discharge mass spectrometry (GDMS) has the advantages of direct measurement of solid samples and simple sample preparation. At the same time, it avoids the loss of insoluble elements and the introduction of pollution during the sample dissolution process of the ICP-MS method. It can be more effectively applied to the comprehensive analysis and testing of 6N and above high-purity tungsten. With the further improvement of instrument sensitivity, resolution, stability, etc., GD-MS as a mass spectrometry method for direct analysis of ultrapure samples will be more widely used
Research And Outlook
With the development of microelectronics industry and optoelectronic technology, the use of high-purity tungsten is increasing. Among them, the annual consumption of 5N and above high-purity tungsten is 500t. It is expected that its consumption will increase to 1200t by 2020, and the demand is increasing. The purity of W will also put forward higher requirements. Since the development of high-purity and new functionalization of W products cannot keep up with the needs of electronic components, the quality of high-purity tungsten and related products still has a certain gap compared with similar foreign products. Many related technologies are still in the laboratory stage and have not been formed. Scale production capacity. Many companies still use traditional processing technology to produce and rely on price to win on behalf of processing enterprises. The preparation process of high-purity tungsten is tedious, and various purification methods have great differences in the effect of removing different impurities. In the domestic refining process of high-purity tungsten, only simple vacuum degassing treatment is used to remove interstitial impurities. Purity is subject to certain restrictions. The smelting method has great advantages in purifying refractory metals due to the characteristics of high temperature, no pollution, and good purification effect; but its raw material purity requirements are high, the cost is high, and the process is time-consuming. Therefore, how to successfully prepare high-purity tungsten by organically combining various processes has become a research hotspot and difficulty. my country is a big country with W resources and should give full play to its industrial advantages. The development of microelectronics and nuclear technology is unstoppable, and high-purity tungsten is an important raw material for its application; how to improve the purification process to prepare high-purity tungsten and related products with better performance and realize its industrialization is the future development direction