In order to reduce the number of targets used in the sputtering system, one target is supposed to sputter and deposit alloy films that meet the composition and performance requirements. So alloy targets, composite inlay targets, and multi-target sputtering can be used in this case.
Generally speaking, in the steady state of discharge, according to the composition of the target, various constituent atoms are respectively subjected to sputtering. One advantage of sputtering coating compared with vacuum evaporation and ion plating is that the difference between composition of the film layer and the target is small and the coating composition is more stable. However, in some cases, due to selection sputtering phenomenon of different composition elements, different reverse sputtering rate and adhesion force of film, the composition of the film layer and the target may be greatly different. When using this kind of alloy target, in order to obtain the film of certain components, the temperature of the substrate should be reduced as much as possible to reduce the difference in adhesion rate in addition to formulating the specific target according to the experiment and minimizing the temperature of the target. Also, the appropriate process conditions will reduce the reverse sputtering effect on the film.
In some cases, it is hard to prepare a large-area uniform alloy target or a compound target. So, the composite mosaic target composed of single elements can be used. The surface composition of the target is shown in Fig.1. Among them, the fan-shaped mosaic structure (d) is the most effective, it is easy to control the composition of the film, and the repeatability is also good. In principle, not only binary alloys, but also ternary, quaternary alloy films can be made by this method.
Fig.1. Composite targets in different structure
(a) Square Mosaic Target (b) Round Mosaic Target (c) Small Round Mosaic Target (d) Fan-shaped Mosaic Target
The structure of multi-target sputtering is shown in Figure 2. The substrate is rotated above the two or more targets, and the deposition thickness of each film is controlled to be one or several atomic layers, and the film takes turns to be deposited, so that a compound film can be obtained. For example, the In1-xGax Sb single crystal film was prepared by InSb and GaSb targets. Although this device is complicated,but any component film can be obtained by controlling the rotation speed of the substrate and changing the voltage applied to each target. These parameters can be controlled according to the coating time, the composition of the film is changed in the direction of film thickness and the superlattice structure can be obtained.
Fig.2. Schematic diagram of multi-target sputtering structure
Auxiliary cathode method is generally used when the difference between the components of the film is large. The main cathode target is made of the main component of the alloy, and the auxiliary cathode target is made of the additive component of the alloy. Each target is sputtered simultaneously to form the alloy film. By adjusting the current of the auxiliary cathode target, the amount of added components in the alloy film can be arbitrarily changed.