Question: Recently a customer asked why indium is typically used to bond sputtering targets to backing plates since it melts at such a low temperature and could result in the target de-bonding at high power densities.
Answer: Indium is the preferred “solder” used in bonding sputtering targets to backing plates due to its unique metallurgical properties. Indium has a tetragonal crystalline structure and can twin, or slip, on the plane. This means that indium can “move” laterally without disjointing in the “Z” or horizontal direction. The advantage of this unique physical property lies in the fact that, if two dissimilar materials are affixed or attached together with an indium “solder”, they can expand and contract at a differential rate upon heating and cooling (based on the individual coefficients of expansion for the corresponding materials) without causing a high stress or strain on the material. Such high energy stresses can cause the materials to de-bond, crack or even break. Indium is also highly conductive, making it ideal for both electrical transfer from the cathode assembly to the sputtering target and thermally conductive to facilitate cooling. Indium also has a low vapor pressure, so it will not outgas in a vacuum environment. The issue here is that indium does melt at 156 degrees Centigrade, rendering it ineffective for higher temperature applications. However, since most magnetically enhanced cathode assemblies utilize some type of rare earth metal in the magnetic array the cathode itself should never exceed around 100 degrees Centigrade. Such rare earth magnets have Currie temperatures slightly above this temperature. Above the Currie temperature the magnets lose their magnetic capability rendering them ineffective for further use in the cathode assembly.
However, indium is not the only material that can be utilized in bonding two dissimilar compositions together. After the proper layers have been sputtered onto the back sides of the materials to be bonded; to facilitate adhesion, a diffusion barrier and wetting requirements, other materials can be utilized in bonding the two materials together. Although it does not slip on the plane, tin can also be used. Tin melts at 232 degrees Centigrade. For bonds above 300 degrees Centigrade I would recommend lead at 327 degrees Centigrade.