Various compound films have increasingly used for the development of modern engineering technology, and compound films account for about 70% of all film materials. The compound film can be made by various chemical vapor deposition or physical vapor deposition methods. In the past, most of the compound films were prepared by the CVD method. CVD technology has now developed new processes such as plasma-enhanced CVD and metal organic compound CVD. However, due to the high temperature required by the CVD method, the sources of the materials are limited, and some of them are toxic and corrosive to pollute the environment, which limits the production of the compound film to some extent.
If using the PVD method to produce the dielectric thin film and the compound thin film, in addition to the radio frequency sputtering method, the reactive sputtering method may also be used. That is, in the sputter coating process, some active reactive gases are introduced artificially and react with sputtered target substances and then deposit on the substrate to obtain the film that in different target substances. For example, the oxide is obtained by sputtering in O2, the nitride is obtained in N2 or NH3, the oxynitride is obtained in a mixed gas of O2+N2, the carbide is obtained in C2H2 or CH4, the silicide is obtained in silane and Fluoride is obtained in HF or CF4, etc. At present, from the demand of large-scale production of compound thin films on an industrial scale, reactive magnetron sputtering deposition technology has obvious advantages.
1. The mechanism of reactive sputtering
The reaction sputtering process is shown in Figure 1. Typical reaction gases include oxygen, nitrogen, methane, acetylene, carbon monoxide, etc. During the sputtering process, the reaction process can take place on the substrate or on the cathode (after the reaction, it migrates as a compound onto the substrate) depending on the pressure of the reaction gas. If the pressure of the reaction gas is high, it is possible to react on the cathode sputtering target and then migrate as a compound onto the substrate to form a film. Under normal circumstances, the pressure of the reactive sputtering is relatively low, so the gas phase reaction is not significant and it is mainly performed as a solid-phase reaction on the surface of the substrate. Usually the high current flowing in the plasma can effectively promote the decomposition, excitation and ionization of the reactant gas molecules. During the reactive sputtering process, a strong stream of particles consisting of free energy-laden atoms is generated, and it flows from the cathode target to the substrate the sputtered target atoms, overcoming the activation threshold energy of the thin film diffusion growth on the substrate to produce the compound films.
Fig 1. Principle diagram of reactive sputtering
2. The characteristics of reactive sputtering
Reactive magnetron sputtering refers to supplying reaction gas to react with the sputtered particles during sputtering to form a compound thin film. It can react with a reactive gas while the compound target is sputtered. It can also react with a reaction gas while the metal or alloy target is sputtered to produce a compound with a predetermined chemical composition. Reactive magnetron sputtering of compound thin films is characterized by:
◆ High purity can easily be obtained from target materials (single element targets or multi-element targets) and reactive gases used for reactive magnetron sputtering, and thus it is advantageous for preparing high-purity compound thin films.
◆ In the reactive magnetron sputtering, by adjusting the deposition process parameters, the compound film that has stoichiometric proportions or non-stoichiometric proportions can be produced, so as to achieve the purpose of regulating film properties by adjusting the composition of the film.
◆ During the reactive magnetron sputtering deposition, the substrate temperature is generally not too high. Furthermore, the film formation process does not usually require a very high temperature heating of the substrate, so there is less restriction on the substrate material.
◆ Reactive magnetron sputtering is suitable for the production of large-area uniform thin films, and it can achieve the industrial production of a single machine with an annual output of millions of square meters of coating.