Back in July I talked a bit about the sputtering of metallic alloys, or composite materials, and the relationship between the starting material compositions and that of the resultant films. Here I would like to do a follow up piece and discuss an alternative method of producing complex alloy, or complex composite, resultant films without the necessity of purchasing countless individual sputtering targets of varying compositions to map out the thin film characteristics of interest.
With the proper equipment it is possible to co-deposit multi component materials utilizing independent cathode assemblies depositing simultaneously. More and more of our customers are now utilizing variances of this technique to develop a wide range of compositions in a minimum amount of time and at a minimum amount of cost.
To perform any series of simultaneous depositions it is first necessary to have the proper equipment. Typically this entails purchasing, constructing or reconfiguring a deposition tool that not only has multiple cathode assemblies but also requires that the plasma deposition processes can be operated both independently and simultaneously. To facilitate this, the cathodes must each have an independent power supply and control (or be equipped with a power splitter with independent controls) as well as some means of depositing onto the substrate, or substrates, at the same time. The most common method of accomplishing this, at least for the initial stages of materials research or development, is to have two or more independent cathodes fixtured within the deposition system in a geometrical configuration that allows for all the cathodes to face a single substrate carrier that is statically configured in a fixed position. This physical alignment is best facilitated by positioning all the cathodes in the system (in a Top Plate or Bottom Plate for example) at specific angles in such a manner so as to enable each cathode assembly to be approximately 100mm from substrate to source distance and pointed normal to the substrate carrier. Each cathode assembly would then be independently connected to a separate power supply that can each be controlled independently from one another. This can be done with as many cathode assemblies as required to produce the stoichiometries and alloys of interest, i.e. two cathodes for a binary alloy system, three cathodes for a ternary alloy system, etc.
Each cathode assembly may contain a single elemental sputtering target, an alloy or a multi constituent composite material. It really doesn’t matter what the composition of the individual targets are as long as the starting compositions are known and documented. As discussed in the July Blog, any alloy or composite material will quickly reach equilibrium conditions of deposition whereby the resultant film species will be identical to that of the starting target stoichiometry.
During processing it then becomes a simple matter of adjusting the power density to each individual cathode assembly under equilibrium conditions to establish a constant deposition rates for each given material. The resultant films will then correspondingly be composed of species made up of a ratio of the impinging atoms or molecules from each constituent material. Film thicknesses will be the result of time and power calculations which will have to be measured after the fact or in situ with a quartz crystal oscillator or similar monitoring device. In all cases the thickness will be consistent from run-to-run as long as the same deposition parameters are held constant and adhered to. Time is linear so it is a simple matter to plot out a few data points for different deposition lengths and film thicknesses that are produced at the same power densities to establish a chart that will allow for the predetermination of thicknesses for a given resultant film composition.
Similarly it is also possible to establish a graph of composition. By making a compositional analysis of the resultant films, to establish the specific chemical composition for a given set of deposition parameters, it is possible to plot a compositional diagram of the various resultant film compositions as a function of varying power densities to each cathode assembly. By simply varying the input power densities to each of the associated power supplies and cathode assemblies it is possible to vary the resultant film stoichiometries accordingly. The resultant films can be examined analytically via various methods, depending on the specific compositions. It is then possible to map out the input power densities vs. stoichiometry to form a rough plot of compositional analysis from which the various properties of concern can then be calculated as a function of thin film composition.