Quite often we are asked, “Does this target require a backing plate?”. Unfortunately, sometimes we are not asked, even when we should be. So let’s address the issue here, even though the answer is not quite so straight forward. Certain sputtering targets do require metallic bonding to a backing plate, for a variety of reasons, and certain sputtering targets do not.
Highly conductive, ductile, malleable metals with a low modulus of elasticity (Young’s Modulus), such as gold, aluminum, silver, copper, iron, etc., typically do not require bonding to a backing plate for use in sputtering applications, unless bonding becomes necessary for mechanical clamping requirements associated with mounting of the target into a particular cathode assembly. However, metals that tend to be more brittle, such as chromium, antimony, bismuth, iridium, cobalt, etc., should be bonded.
Additionally, most inorganic, non-metallic ceramics; as well as most oxides, nitrides, carbides, glasses, etc. ; should almost always be bonded to a metallic backing plate prior to use.
The reasons why certain materials necessitate metallic bonding to a backing plate also vary. In certain cases, for example brittle materials (be they metal or ceramic in origin) contain intrinsic internal stresses. These stresses may be the result of mechanical working during the fabrication process, which cannot be annealed out, or simply inherent to the material properties of the individual target composition. In either case, it is important that such internal stresses do not attain levels above the mechanical strength holding, or binding, the atoms/molecules together for any given material composition.
Added to any internal stress inherently present in the target, additional stress is added to the material during the deposition process. During sputtering, charged particles (ions)in the plasma bombard the target surface with enough energy to break the atomic bonds holding the material together. This exothermic momentum transfer results in a significant temperature increase at the target surface. On an atomic level this temperature rise may exceed 1,000,000 degrees C. Such heat must somehow be dissipated during the process. To facilitate this, most cathode assemblies employ some form of a water cooling mechanism to dissipate this unwanted thermal energy. In most cases this is facilitated through a copper cooling channel built into the cathode assembly itself, most often where the cathode cavity holds the target. By definition, since the heat is being generated on the top surface of the target and alleviated from the bottom, the heat must pass through the thickness of the target.
For highly conductive ductile materials, this is generally not a problem. For less conductive brittle materials, a thermal gradient is formed through the target thickness, from top (hot) to bottom (cool). This thermal gradient causes additional stresses within the target material. Added to any residual stresses that may already be present within the target, as described above, the target may crack or explode during the plasma deposition process. By metallically bonding such a target to a highly conductive ductile backing plate, such as oxygen free electronic grade (OFE) copper, which fits directly into the water cooled cavity of the cathode assembly, this heat can be much more readily dissipated.
Care is taken in the bonding process to use only materials that are highly conductive (electrically and thermally), have a low vapor pressure and provide a high mechanical strength. By increasing the overall cooling efficiency of the sputtering target/cathode assembly, the internal stresses are greatly reduced and, under normal operating conditions, avoids the risk of the target cracking or falling apart.