The other day a student inquired whether it would be better to vacuum cast a Ruthenium sputtering target or sinter it?
There are significant differences between the physical properties of materials produced from Vacuum Cast and Powder Metallurgical processing. Vacuum Cast materials can typically be close to 100% of theoretical density if they are ductile and can be mechanically compressed in a rolling mill or similar form of mechanical working operation. Unfortunately though, ruthenium is not ductile. It is quite brittle by nature. This means that we would have to provide an “As Cast” material that may contain some voids, have large non-equiaxed grains, Griffith micro cracks, slip planes, dislocations and quench cavities.
Sputtering Targets produced from powder metallurgical techniques are consolidated in a Hot Isostatic Press (HIP) from high purity sub-micron powers that are homogeneous and fine grained. Depending on the materials being sintered, the finished products can be close to theoretical density as well. However ruthenium is quite brittle and tends to crack or spall when sintered under extreme elevated temperatures and pressures. This limits the amount of energy that can be applied to the ruthenium powder and places an upper limit on the density attainable. Typical densities for ruthenium are in the 85% area. A normal HIP cycle for ruthenium would produce a near net shape form that could then be diamond ground to finished dimensions. Any residual material being ground away would typically be lost in the grinding procedure, thus increasing the cost of production.
Depending on what specific requirements may be required based on process control in the deposition process Plasmaterials would generally recommend utilizing the powder metallurgy process to produce a sintered ruthenium target and then bond it to a copper backing plate to provide mechanical stability in the cathode assembly as well as better electrical and thermal conductivity.