So far we have discussed two important physical vapor deposition techniques, evaporation and sputtering and the important features of these two are compared in Table 6. In the next section we will study Chemical Vapour deposition (CVD) process that involves depositing a solid material from a gaseous phase. It is similar in some respects to PVD. PVD differs in that the sources are solid, with the material to be deposited being vaporized from a solid target and deposited onto the substrate. The formerly discussed sputtering or chemical vapor deposition systems can be combined with ion assisted and reactive ions deposition schemes. In case of ion assisted deposition, the surface is bombarded with relatively low voltages (50 - 300 eV) ions not necessarily same type as in film. The goal is not typically to incorporate ions in film but for physical rearrangement and local heating. It can change film properties. Similarly in Reactive Sputter deposition, reactive gases like oxygen, nitrogen etc is added to chamber during deposition to facilitate chemical reaction on substrate and target.