High-purity tungsten and
tungsten alloy targets for semiconductors are the basic raw materials for preparing thin-film materials by sputtering. Their performance directly affects the final performance of the sputtered thin film, which affects the purity, density and grain structure of the target. Its uniformity and other aspects have put forward extremely demanding requirements.
Purity
With the increasing integration of semiconductor integrated circuits, the size of transistors and circuits in semiconductor circuits is getting smaller and smaller.
The purity requirements of materials such as tungsten and tungsten alloys are also getting higher and higher. The purity of pure tungsten targets used in the manufacture of large-scale integrated circuits is required to reach 99.999% (5N) or more, even 99.999% (6N), and the purity of tungsten alloy targets such as W-Ti is required to reach 99.9959% (4N5) or more. In addition to the lower the total impurity content, the better, the individual impurity elements in tungsten metal also have an adverse effect on semiconductor integrated circuits. For example, alkali metal ions (K, Na, Li, etc.) tend to form mobile ions in the insulating layer, reducing device performance; radioactive elements (U, Th, etc.) will release rays, causing soft breakdown of components; transitional group Metal (Fe, Ni, Cr, etc.) ions will cause interface leakage and oxygen element increase; halogen (Cl, etc.) ions will corrode the circuit; gas elements (C, O, N, etc.) damage the stability of the film and become the increase in film resistance the reason.
Density
The compactness of the target is extremely important. In order to reduce the pores in the solid of the target and improve the performance of the sputtering film, a theoretical density of 99.5% of the sputtering target is generally required. The density of the target material not only affects the deposition rate during sputtering, the density of sputtered film particles and the discharge phenomenon, but also affects the electrical and optical properties of the sputtered film. The denser the target material, the lower the density of the sputtered film particles, the weaker the discharge phenomenon, and the better the performance of the film. By controlling the powder sintering process, a tungsten target with a theoretical density of 82.59%-99.5% was prepared, and a 1 600-thick film was deposited on the silicon wafer by magnetron sputtering. The results of tissue analysis showed that the higher the density of the target, the higher the erosion rate and film deposition rate of the target, the lower the resistance of the sputtered film, and the better the performance of the film.
Grain size and size distribution
The target material is usually a polycrystalline structure, and the grain size can be from the order of μm to the order of mm. The sputtering rate of the target with the same composition and the small grain target is faster than that of the coarse grain; but the grain size difference is small The thickness distribution of the deposited film is also more uniform. For high-purity tungsten targets for semiconductors, the grain size is generally required to be controlled below 20 μm.
