In our most recent BLOG we briefly described the process of sintering ceramic type sputtering target materials via. the powder metallurgical process referred to as sintering. We further described the consolidation steps associated with this manufacturing process referred to as simple Hot Pressing. What was described was how sintering is an effective heat treatment applied to a powdered compositional material in order to compact that material to add both strength and integrity without altering the initial composition of the constituent powders. The narrative of the simple Hot Pressing mechanism, comprised of a carbon die set containing heating elements, illustrated a typical scenario of how such a mechanism can be used to manufacture an array of different metallic composite materials (oxides, nitrides, carbides, sulfides, etc.) for use as sputtering targets. What wasn’t mentioned, however, were certain physical limitations associated with this simplistic model.
In the application model given for Hot Pressing, we mentioned that heat was applied conductively through heaters placed in or around the carbon mold containing the initial powders to be consolidated. When power is applied to the heating elements it slowly heats up the core of the carbon die and eventually heats up the powder within the die cavity. Simultaneously the powder is being mechanically compacted with an hydraulic press through a set of geometrically opposed carbon rams. This means that the heat being generated from the heating elements needs to transfer conductively through the carbon die to the powder being compacted. For circular targets this means that heat will first be transmitted along the outer perimeter of the powder within the die and then migrate toward the center of the material over time. This creates a thermal lag within the material being diffused into a solid form or, more importantly, a temperature differential through the material during the sintering process. The outer perimeter of the material being sintered is exposed to the heat generated from the heating elements sooner, and correspondingly longer, than that within the central region of the material being consolidated. The outer powders diffuse and consolidate before those within the central region. As these outer regions begin to consolidate, the localized density in this material increases and the localized volume decreases. Since the pressure supplied to the material is applied from the carbon rams unidirectionally, across the entire face of the target surface, there is effectively a thermal diffusion gradient taking place over time. The result is a certain degree of non-homogeneity in density within the consolidated target. This can result in differential stresses within the target material as well as a variance in sputter yields during deposition. At any given power density the plasma supplies a fixed amount of energy containing a constant velocity of impinging ions contained within that plasma. Ions with a fixed momentum strike the target surface uniformly. However, if the target material consists of atoms with varying bond energies, a differentially varying sputter yield will occur. More, or less, atoms of target material will break away from the target surface as a function of ion collisions based on location of impact within the target surface. Effectively this could alter the nucleation and growth of the thin film being produced and thus alter the physical properties and performance characteristics of the resultant film.
These affects are subtle and not necessarily well pronounced. Like most process control conditions the affects are very much dependent upon the actual chemical composition and physical properties of the target materials being sputtered as well as the process conditions associated with the deposition. In general, such subtle affects are less of a concern for geometrically smaller sputtering targets where the gradients are less pronounced. In larger targets, where the temperature gradients are larger and the thermodynamics and kinetics of the diffusion process are more varied, higher internal stresses within the target may also cause cracking within the target (again depending on the mechanical properties associated with the composition being consolidated) or even spalling when the hydraulic pressure in the sintering operation is released.
There are variations within Hot Pressing technology that can reduce, or even eliminate, these physically induced inhomogeneities which will be discussed in future BLOGS. It is sufficient here to say that, based on the phenomenon described above, care should always be taken during the deposition process to maximize the cooling efficiency of cathode assembly through to the target material and properly pre-condition all sputtering targets prior to deposition onto a substrate.