There are several sources of contamination in an evaporation scheme. Contamination occurs from source materials, source/substrate heaters and residual gas in chamber. One can have good film purity by using high purity source materials, using low diffusion materials as source/substrate heaters, with better vacuum and higher deposition rate materials . The e-beam process though expensive, is usually "better" than thermal evaporation because of less contamination from the source holder. Only the desired material is heated and sustains high quality vacuum environment. They are especially suitable for higher melting point materials (T~3000°C). Higher deposition rates and better control of deposition rates also makes e-beam an attractive choice. As shown in Fig. 3.27, the step coverage is poor in evaporation methods because of shadow effects. By contrast, the CVD and sputtering offer better step coverage.

Fig. 3.27 : Step coverage in an evaporation and CVD process