There are a number of different ways to manufacture sputtering targets. Depending on the composition of the target being considered for production it is necessary to choose the proper method of consolidation that best suits the metallurgical criteria required for the finished product. In general there are two major categories of processing techniques to initially consolidate the raw materials that will eventually end up as a sputtering target. These can be broadly classified as either vacuum melting or powder metallurgy.
The first general classification is referred to as vacuum melting. I would like to address this form of fundamental technology here and then deal with powder metallurgical technology in a future Blog. Vacuum melting can be accomplished in a variety of ways. Arc melting. Induction melting. Resistant heating. Levitation. These are all variations of systems that essentially heat the various constituents, either elemental or pre-alloyed, that will compose the final composition of the target. These materials are weighed out according to the required specifications and then heated within the confines of a vacuum chamber to a temperature high enough to melt, or liquefy, the entire lot, or batch, of material being processed. It may be necessary to further process the melt while in the liquid state depending on the ability of the constituents to mix together. For elemental materials this is not necessary but for binary or multi elemental complex alloys the associated constituents may not want to combine or react with one another. This inability to alloy in the liquid state (like oil and water for example) may be due to non-polar molecules trying to mix with polar molecules, differential viscosities, surface tension, etc. In such cases some additional processing is required such as some form of mechanical agitation, a magnetic or electrical field may need to be applied, an inert bubbling gas may be introduced, etc. to further assist the mixing of the various liquid components prior to solidification. In certain cases it may be necessary to implement a rapid solidification technique to freeze in a non-equilibrium state during the cooling through the liquidus. Additional problems may arise if one of the constituents being alloyed has a boiling point below that of the melting point of other constituents in which case the lower temperature material simply boils away before the complete lot is molten. In these situations it is necessary to confine the liquid solution in an enclosed environment within the vacuum chamber.
Once the alloy that is being produced is in the liquid state it is then cooled through the liquidus to form a solid ingot. Depending on the specific composition of the ingot it is then mechanically worked in some fashion to form a plate. For ductile compositions the ingot is rolled and cross rolled to a given gauge or thickness suitable for finish machining to the required geometrical specifications of the target being produced. The ingot may, or may not, require additional annealing (heat treatment) steps between reduction stages depending on the material composition being fabricated. For brittle materials the ingot is hot forged into the suitable plate thickness. Once the material is in plate form it is then finish machined to the final dimensional tolerances via numerically controlled equipment.
