The purity of the film and the purity of the evaporation depend on the following three aspects: one is the purity of the source material; the other is the pollution of the heating device, the evaporation boat and the support material; the third is the residual gas in the vacuum system. During the precipitation process, the evaporates, including atoms and molecules, together with the residual gas simultaneously and independently hit the substrate surface.
Table 1-1 shows the interaction between the precipitation rate and the residual gas pressure in affecting the oxygen content in the film. According to the viscosity coefficient of oxygen (probably in the order of 0.1 or less), the oxygen density can be roughly estimated, but these results are of great significance. In order to deposit very pure thin films, high deposition rates and low gas (H20, CO2, CO, O2, N2) residual pressures are required. These conditions are not difficult for vacuum evaporation, for example, the evaporation rate can reach 1000 Å / s under a pressure of 10-6 torr.
On the other hand, in the sputtering process, the deposition rate is generally 2 orders of magnitude lower than evaporation, and the pressure is 4 orders of magnitude higher than evaporation, so the deposited film contains higher oxygen. It is for this reason that sputtering is not considered as a clean film preparation method like evaporation. However, in the past 20 years, with the commercial development of high precipitation rate magnetron sputtering technology, a large amount of progress has been made in sputtering technology working under low pressure and strong pressure in a clean vacuum system. In the preparation of AI films, the purity of the films obtained by the two methods is roughly equivalent. Finally, the surface of Table 1-1 deposits a thin film under the residual gas pressure of 10-3 torr, and a large amount of oxygen permeates into the thin film. In the process of reactive precipitation of metal oxides, this can be used to introduce oxygen to promote the reaction with the metal.
In the preparation of pure metal thin films, the obvious influence of the presence of oxygen and nitrogen impurities is to reduce the conductivity, reflectivity and other characteristics, such as hardness.