The source vapor transport to surface is a line of sight deposition. In a good vacuum environment, long mean free path is achieved and thus collisions in gas can be avoided. For longer source to substrate distance, one needs higher vacuums. Typically, for a separation of 10 to 100 cm, a vacuum of 10^-5 torr and lower are required. The particles have energies (~kT) comparable to evaporation temperature and at 1000°C, it is about 0.2 eV.

The distribution of evaporant depends on geometry of source and on position of target. Point source and planar source (surface Source) are considered in this section. The distribution of evaporant in the case of point source case is given in Fig. 3.23. If the target of area dA_S is tilted of from radial direction by an angle projection of dA_S onto sphere of radius r is dA_Scos as shown.