What are the material requirements for evaporation source in vacuum coating?

Generally, the material and shape of the evaporation source should be considered. The general requirements of the evaporation source material are:  

The high melting point 

Because the evaporation temperature of the evaporating material (the temperature of the equilibrium vapor pressure of 1.3Pa) is mostly between 1000~2000℃, therefore, the melting point of the evaporating source material should be higher than this temperature.  

Balance steam pressure 

The main purpose is to prevent or reduce the evaporation source material under high temperature evaporation materials and become impurities, into the evaporation coating layer. Only in evaporation source materials when the balance of the air pressure is low enough to ensure that has the smallest self evaporation during evaporation, not affect system vacuum and pollution membrane layer, in order to make the amount of evaporation source materials are very little, evaporation temperature, evaporation source material the choice, should make the material of the evaporation temperature is lower than the evaporation source materials, the balance pressure of 1.3 x 10-6 pa  The temperature corresponding to 1.3×10-3Pa can be used to prepare high quality films.  

Stable chemical properties 

Chemical reaction with evaporating material should not occur at high temperature. At high temperature, some evaporating source materials react and diffuse with evaporating materials to form compounds and alloys. Especially the low eutectic alloy evaporation source is easy to burn off. For example, tantalum and gold will form alloys at high temperature, and aluminum, iron, nickel, cobalt will also form alloys with tungsten, molybdenum, tantalum and other evaporative materials. Tungsten can also react with water or oxygen to form volatile oxides such as WO, WO2 or WO3. Molybdenum can also react with water or oxygen to form volatile MoO3, etc.  Therefore, should choose the material that will not react with the coating material or form the alloy to do the evaporation source material of the material.  

The materials used as evaporation sources must have high melting point, low volatilization and low brittleness after high temperature cooling. The commonly used linear evaporation source should be able to be wetted with the evaporation metal phase, the melting point is far greater than the evaporation temperature of the gold-plated, and does not react with the gold-plated alloy.