High-purity tantalum sputtering target’s Preparation by smelting ingot method
The smelting and casting method is currently the main method for preparing tantalum sputtering targets. Generally, tantalum raw materials are smelted (electron beam or arc, plasma smelting, etc.) and cast, and the resulting ingots or billets are repeatedly hot forged and annealed, After rolling, annealing, and finishing, the target material is formed. The ingot or billet undergoes hot forging to destroy the casting structure, causing pores or segregation to diffuse and disappear, and then to recrystallize it through annealing, thereby improving the density and strength of the structure.
In order to ensure that the target material can sputter high-quality films, generally there are high purity requirements for tantalum sputtering targets. Generally, the higher the purity of the target material, the better the film quality. For example, when used on integrated circuits, due to the alpha decay of uranium and thorium, alpha particles will cause electrical breakdown of devices in semiconductor materials. The content of uranium and thorium must be below the μg/kg level; alkali metal sodium potassium ions It is easy to diffuse into the insulating layer (SiO2) and act as an electric carrier; iron element may reduce the performance of the device; therefore, there are strict requirements for sodium potassium impurities and iron elements. Excessive gas elements (C, N, O, S, etc.) will reduce the sputtering performance of the target, while other refractory metal elements such as tungsten, molybdenum, niobium and other elements are also required to be reduced as much as possible. The smelting ingot method can prepare high-purity or ultra-pure tantalum sputtering targets, generally up to 4 N or more.
If the prepared tantalum sputtering target contains pores, it will greatly affect the sputtering performance. For example, if the pores containing gas are opened during the sputtering process, gas may be released, making the sputtering process unstable or even The arc phenomenon occurs, making it difficult to ensure the uniformity of the deposited film. Therefore, the tantalum sputtering target should be dense. The tantalum target prepared by the smelting ingot method is usually relatively dense.
The target grain size and the uniformity of the grain size are also one of the important factors affecting the sputtering performance of the target. Generally speaking, the smelted ingot or billet has coarse grains, usually above 50 mm in diameter. After hot forging and recrystallization annealing, grains below 100 μm can be obtained. The electron beam smelting ingot billet with a purity (removal of oxygen and other gas impurities) of 99.998% or more, a height of 200 mm, a diameter of 200 mm, and a grain size of about 55 mm, is repeatedly forged, rolled, and recrystallized Annealing, etc., can obtain a tantalum sputtering target with an average grain size of 110 μm or less, a grain diameter deviation of ±20% or less, a uniform structure and a good coating uniformity. However, the smelting and ingot method for preparing the target material still has the disadvantage that it is difficult to control the target crystal grain size and the uniformity of the crystal grain texture orientation, and it is easy to produce ribbon texture.
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