High Purity Tungsten Material Preparation Improvement

In the microelectronics industry, chips are developing towards high integration, high frequency and ultra I/O terminals. It is urgent to increase the packaging density. The three-dimensional wiring technology of large-scale integrated circuits arises at the historic moment. Tungsten is widely used in gate materials and wiring materials in integrated circuits with good electrical conductivity, escape work, close to silicon, and excellent thermal stability, and good bonding with silicon.

The tungsten materials for such applications require very high purity, called high purity tungsten. There are two main methods in preparing high purity tungsten materials in China: first, tungsten strips are used as raw materials to purify tungsten metal by arc melting, electron beam smelting and regional melting. Although the purity of the products is high, it has high dependence on equipment, small batch, low rate of finished product and high cost, and it is difficult to form large-scale production. The two is made of tungsten powder metallurgy. In the process of pressing, the binder with carbon containing organic matter is added. Although the high temperature treatment can be used in the subsequent sintering process, the great department binder can be removed. However, this process increases the pollution of carbon content, and the removal of impurities in the production process is general, iron (Fe), oxygen (O), carbon (C) and other impurities are high, so it is difficult to meet the quality requirements of tungsten materials for large scale integrated circuits.

In view of the defects in the pressing and sintering of tungsten powder, the original traditional technology has been improved and the preparation method can effectively reduce the impurity content of iron, oxygen and carbon in tungsten material, at the same time, the tungsten content is increased, and the tungsten density is high and the use performance of the product is stable, in order to meet the ultra large scale integrated circuit use. The process requires seven processes.

(1) With hydrogen reduction, the blue tungsten oxide was passed through the five zone temperature reduction furnace, at the reduction temperature of 600~950°C, the hydrogen flow rate was 4 ~ 6.0m3/h, the hydrogen dew point was less than ‑60℃, the pushing speed was 20 ~ 25min. The loading rate was 280±5g, 330±5g, 380±5g, 430±5g and 480±5g. Tungsten powder with 2±0.2 μm, 2.5±0.2 μm, 3±0.2 μm, 3.5±0.2 μm, 4±0.2μm.

(2) With the removal of impurities, the above particle size powder was washed with 30 ~ 35min using the deionized water of >1.0 x 105Ω·cm respectively. After the stirring and clarification, the supernatant was extracted, the concentration of 4 ~ 5% hydrochloric acid was added, then mixing and washing 45 ~ 50min, the mixing and clarification, the supernatant were removed, and the resistance rate >1.0 x 105Ω·cm deionized water was cleaned to the pH value greater than 3, tungsten powder was filtered by vacuum and then dried in a vacuum drying oven at 70~90°C for 24 to 30h and screened by 160~200 mesh sieves.

(3) According to the order from small to large, the size of tungsten powder after the removal of the acid washing is from 8 to 10:18 to 20:36 ~ 40:18 ~ 20:8 to 10, and the tungsten powder of 2.9 to 3.2μm is synthesized and mixed in the mixer from 25 to 30min.

(4) The obtained powder is molded by conventional technology at 15 ~ 25MPa pressure, and pure tungsten billet is produced.

(5) The pure tungsten bar is placed in the molybdenum boat and sintered 30 to 50min at 1200~1400°C at a hydrogen dew point of less than 70°C.

(6) After pre sintering, the pure tungsten strip is placed in the vertical melt cover and sintered at high temperature under the protection of hydrogen under the dew point of less than ‑70℃. The sintering system is two sections of heating and two sections of heat preservation. The current / time parameters are heating up: (0-3200A) /15min, 3200A/15min, heating up: (3200-4150A) /2min, thermal insulation: 4150A/20min.

(7) The pure tungsten strip obtained by high temperature sintering was cut into 20 ~ 50mm and loaded into the molybdenum boat. At the temperature of 1600±50°C, the high purity tungsten material was obtained at the degassing time of 3 ~ 5h under the hydrogen dew point of less than ‑70℃.

The advantages of the high purity process are as follows: Without the need for re purchase of equipment, the conventional equipment can process the process. The forming process does not add any binder, the product is high purity, low consumption, suitable for large-scale production. 

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