In the last couple of writings we described how powder metallurgy can be used to fabricate sputtering targets by sintering the individual particles of a specified composition into a solid shape. Additionally we explained some of the short comings associated with the simple Hot Pressing of certain types of materials and geometries. To eliminate some of the issues associated with applying heat and pressure under static conditions it is possible to provide the necessary energy to sinter the granular particles “Isostatically” (equalized heat and pressure effectively applied in all directions). This is referred to as Hot Isostatic Pressing or HIP’ing. Here, in the final episode of this trilogy on powder metallurgical processing of metal matrix composite materials to produce sputtering targets, the method of HIP’ing will be described.
HIP’ing supplies the necessary energy to promote diffusion of the individual atoms to be sintered under Isostatic conditions. HIP’ing is the simultaneous application of high temperature and pressure to powdered metal matrix composite materials under a specified cycle of rise and soak times to alter the physical properties of the materials being fabricated. Under these controlled conditions the individual particles fuse together to reduce porosity and increase the density, thus improving the mechanical stability and usability of the material as a sputtering target.
In operation, the powdered material to be consolidated is typically “canned” or placed into some form of malleable or ductile sheet material which has been previously formed into a near-net-shape of the finished dimensions associated with the sputtering targets being produced.
This canned material is then placed into a high pressure containment vessel. To provide an isostatic pressure on the materials being consolidated, a media is pressurized pneumatically with high powered pumps. Argon is often the media of choice in HIP’ing due to its neutral or chemically inert nature. Argon can be both heated and pressurized simultaneously and isostatically to provide the energy necessary to facilitate the diffusion of the atoms in the particles being consolidated. Although the actual HIP’ing cycle parameters vary as a function the materials being fabricated they are generally in the range from 20,000 to 100,000 PSI pressure and 500C to 2,000C in temperature. Care must be taken to apply these various pressure and thermal cycles slowly to allow for equilibrium conditions to exist enabling sufficient time for plastic deformation to take place through diffusion, creep and migration at the individual particle interfaces. HIP’ing applies the pressure and heat to the materials being processed uniformly across the entire surface of the canned materials being sintered. It is the application of this energy uniformly across the entire surface of the material being consolidated that provides the homogeneity of density and composition with low internal stress of the finished consolidated sputtering target.
After consolidation, the encapsulated solid matter can be removed from the canister by either peeling off the outer shell or machining it off mechanically leaving a near-net-shape part that can then be machined to the final dimensional tolerances with numerically controlled equipment. The final product is a fully homogeneous, stress free composition ready for physical vapor deposition. This process is ideally suited for larger sputtering targets , targets which may consist of multi piece “Tiles” which are bonded to a backing plate, complex geometries and rotatable composite targets which may, or may not be, bonded to metal backing tubes depending on the overall dimensions.