This post is also available in: Japanese
Backing plates are one of the components of a sputtering cathode. It is shaped to fit the target and attached on the backside of the target.
Its main functions are as an electrode for discharging, and as a cooling plate to suppress the temperature rise of the target. Copper is often used for its suitable quality for such functions.
Backing plate contains a channel for cooling water inside. As the target itself has no mechanism to attach to the device, it is attached to the backing plate using Indium (In) or In alloy as a bonding material.