There are many factors restricting the development of vacuum plating machines in China, especially the processing of tantalum and parts made of tantalum, the key raw material used for manufacturing evaporation sources of plating machines. Tantalum and tantalum alloys are suitable for manufacturing supporting accessories, heaters, and heat shields in vacuum equipment due to their high melting point, good stability, and low steam pressure at high temperatures. At present, few domestic enterprises engage in the production of tantalum parts for vacuum evaporators, which leads to heavy dependence on imports of tantalum parts for vacuum evaporators, and there are many difficulties in the production and manufacturing of tantalum parts for vacuum evaporators in China.
Resistance heating is often used as the evaporation source for materials with evaporation temperatures of 1 000~2 000 C. Generally, it is required that the melting point of the evaporation source material is about 1000 C higher than the evaporation working temperature, the equilibrium steam pressure is low, the brittleness after high-temperature cooling is small, and it has good chemical stability in a vacuum environment. Therefore, tantalum is a common material for vacuum evaporation.
Tantalum is a light grey metal with slightly blue color and a high density (16.5) × 103 kg/m3), high melting point (2 996 C), low coefficient of linear expansion (6.5 between 0 and 100 C) × 10-6 K-1), ductile, tougher than copper, cold drawn into fine wire or foil. The thermal conductivity of tantalum at 300K is 52.1W (m K) -1 and the elastic modulus is 192× 103 MPa at room temperature.
The tantalum content of Ta1 is more than 90.35%; Ta2 contains more than 79.50% tantalum. However, the purity of tantalum is more than 99.95% for common vacuum evaporation plating and 99.99% for high-end OLED plating machines. Obviously, the basic requirements of tantalum for vacuum plating can not be met by using existing brands and standards. The production of high-purity tantalum has become a key factor restricting the localization of tantalum parts for vacuum plating machines.
The low melting point of other metal elements (such as Fe, Ni, etc.) or high equilibrium vapor pressure (such as W), or low stability (Ti) in tantalum material. When low-purity tantalum material evaporates at high temperatures, other metal elements may decompose and evaporate or react with other molecules in the evaporation chamber, resulting in film components deviating from evaporator material components. Therefore, vacuum evaporation of high-purity tantalum parts can significantly reduce the pollution of evaporation source material to the coating.
