Function of backing plate for a sputter target

A backing plate is usually made by copper or molybdenum (lower thermal expansion coefficient). It is actually not a required part for sputtering process, but a backing plate played an important role in PVD industry and has been a popular parts for a lot of sputter target set. Some of the reasons as follow,

Cooling

Sputtering generate a quite large amount of heat and the temperature could increase a lot for the sputter target. If the temperature goes too high, it is possible to break the ceramic material or simply melt the metal targets with low melting point. Thus, it is very important to cool down the target. A bonded metal backing could be used as cooler and it is a lot easier to be mounted to the water cooling system.

Please notice that, although the backing plate could be a cooler itself, the efficient is low and cooling water or other cooling system on the sputter is still required. Also, without extra cooling, the bonding material (such as indium) could melt and the target set could break apart.

Conductor

In a lot of cases, the sputter target itself is an electric insulator. To enhance the conductivity between the sputter target and electrode, a backing plate is bonded. Usually, indium is used as the bonding agent to connect sputter target material with the backing plate. As In melt at a very low temperature, the process is usually safe for the target material.

Adapter

For ceramic sputter targets, such as carbides, nitrides, fluorides and some alloy, the material is quite fragile and it will be a disaster if the equipment require screw or bolts to mount the target on without a backing plate. Because drill holes on these kind of material is not only hard but also highly possible to break the target.

On the other hand, a large backing plate that holds a few pieces of relatively small sputter target together could form a large sputter target, which could be used in coating a very large substrate, such as the LCD panel.

Also, it is possible that the target material is extremely expensive and soft, such as gold. A backing plate is frequently used to obtain a convenient thickness and strength without cost too much on the material.

There is also cases that a backing plate is not suitable. As an example, the PTFE material, which has quite large coefficient of thermal expansion (even much larger than copper). The bonding easily broken during the sputtering process, due to the different thermal expansion.