Figure 38.1: Schematic of thermal evaporation system with Vacuum system.

Figure 38.1 shows the typical evaportation system using resistance based heating sources. Refractory metals like tungsten, molybdenum or tantalum are generally used in the form of wire or strip having different shapes. The choice of a particular refractory metal as a heating source depends on the materials to be evaporated, so that evaporant materials do not react with the refractory metal at high temperature of evaporation. However, the formation of alloy with the source always cannot be avoided.
In addition, since the distribution of the vapor flux has a Cosθ characteristic, the direct condensation on the plane of the large substrate provides films of inhomogeneous thickness. Therefore, a large source-substrate distance is required or the substrates must be moved in a suitable manner. Another disadvantage of thermal evaporators is the different evaporation speed for different materials at the same temperature leading to deviations of the film composition compared with the source material. A possible solution of the problem faced in the thermal evaporation process is the flash evaporation from a very hot crucible or the laser ablation [2].