CIGS and CdTe Thin Film Solar Cells: New Target Designs for More Material Utilization
Changing the sputtering system from planar to rotary targets increases the target utilization from about 30% to 75%, which saves expensive raw material.
Depositing the molybdenum back contact for CIGS or CdTe solar cells by monolithic molybdenum rotary targets - which are completely made of molybdenum - gives additional benefits:
The bonding of the molybdenum material on a backing tube is not required, which makes more material available to be sputtered and the material utilization is increased. In addition, a higher sputtering power of up to 30 kW/m can be applied. This creates a very high heat load which cannot be accommodated by bonded-type targets: Indium is used as bonding material which melts at 156 °C, thus the risk of de-bonding is eliminated with monolithic targets. A higher sputtering power results in a higher deposition rate and improved thin film properties, e.g. higher electrical conductivity.
In contrast to the high lateral homogeneity of thin films deposited by sputtering, a sputtered target generally has an erosion profile which is not homogeneous – a result of different plasma densities. As a consequence, the targets have to be replaced even if there is sufficient material on most of the target area. To overcome this limitation, targets with varying outer diameter (named “dogbone” for rotary targets) or thickness is a solution to extend the target lifetime and increase the target exchange interval.