A while back (January 2016) we discussed a brief but fundamental description of what the basic processes are associated with the production of elemental and metal alloy sputtering targets. At that time we promised to address the analogous process of how ceramic, or complex composite material, sputtering targets are prepared. That time has now come.
Unlike metallic combinations that can be readily melted, quenched and then shaped into various sizes, ceramic targets must be consolidated in such a way that the initial stoichiometry or original composition is not chemically altered from that of the preliminary starting material. There are many ways of establishing this consolidation process but, unfortunately, none of them are particularly easy. In general this involves some form of powder metallurgical processing, i.e. the sintering of fine granules of powder into a solid form. Here we will begin by describing the least complicated form of this sintering process, that of simple Hot Pressing.
Hot Pressing is basically a high pressure low strain rate powder metallurgical process for forming powders, or powder compounds, at elevated temperatures and pressures high enough to induce sintering and creep. Sintering involves the atomic diffusion under energetic conditions supplied by the application of heat and/or pressure across the geometrical boundaries of the constituent particles. These fused particles are then held together by ionic and covalent bonds between the associated atoms or molecules. This diffusion eventually fuses the atomic particles by compacting them together, thus creating a solid composite mass of material. This is accomplished at a temperature below the liquidus temperature of the lowest melting point constituent, i.e. the material is held below the temperature of liquefaction.
There are several types of Hot Pressing technologies (induction heating, field assisted sintering technique, indirect resistance heating, etc. – just to name a few) but the most common method of facilitating this sintering process is to place the powders that are to be consolidated into a simple Hot Press. The most frequently utilized form of such a Hot Press generally consists of a mechanical, or hydraulic, press used to apply pressure on two geometrically opposed rams, usually composed of solid carbon (graphite) rods with an outer diameter somewhat larger than the diameter of the solid mass of the target to be fabricated. These rams fit into a solid carbon (graphite) ring or slab with an inner diameter just slightly larger than that of the outer diameter of the mating rams. Along the outer diameter of the carbon die, or placed within the carbon slab itself positioned near the circumference of the inner diameter, heating rods are placed in a circular pattern evenly spaced apart.
In operation, the bottom ram is generally in a fixed position about half way up from the bottom portion of the solid carbon block. The mixed and weighed powders are then poured into the inner diameter of the top portion of the carbon block on top of the lower ram. Then the top ram is placed into the mold and pushed in downward motion up against the powders that are to be consolidated.
Once the rams are in place, a slight pressure is exerted on the top ram via the hydraulic press while a current is simultaneously applied to the heating elements from an associated power supply. Based on the metallurgical properties of the material constituents to be consolidated, both the pressure and the heat are slowly increased to a point where the pressure being applied begins to drop off. This can be noted on the pressure gauge associated with the hydraulic press. The significance of this pressure drop is the result of the individual particles starting to diffuse together and thus reducing the overall volume of the materials being consolidated. After this reduced pressure reaches an equilibrium, an additional pressure is then applied and held for a given length of time to allow for the diffusion process to reach a kinetic equilibrium. Of course each individual material requires a specific formula of rise and soak times in applying temperature and heat cycles to provide optimal metallurgical properties of homogeneity, density, phase purity, grain size, etc. but experience and practice can generally produce a well qualified product.
Since this is a diffusion process, whereby the constituents are not actually melted, complex compositions such as metallic oxides, nitrides, carbides, borides, sulfides, etc. can all be sintered into solid shapes at, or near, their theoretical densities without decomposing into the elemental forms of their original constituents. This is the beauty of powder metallurgy, there is little or no decomposition. High temperature brittle materials such as refractory metals (tungsten, molybdenum, etc.) are also typically sintered rather than cast to avoid cracking during the cooling stages associated with the melting process.
When the powders have been sufficiently consolidated, the current is removed from the heating elements and the pressure slowly released. Once everything has cooled back down to room temperature, the solid mass can be pressed out through the bottom of the carbon mold by removing the bottom ram and again applying a slight pressure to the top ram with the hydraulic press.
The associated mass is essentially a near-net-shape in the size of the target that is to be produced. This material can then be finish machined, usually through diamond grinding with numerically controlled equipment, to the finial dimensional tolerances associated with the customer specifications.