The length of the target tube is determined by the substrate length and the coating uniformity requirements. SCI refers to the overall length of the backing tube as the basis for calculation as it will define the system geometry. The actual length of the material to be sputtered will be shorter than the backing tube length.
The standard calculation that we use is the following:
Backing Tube Length = Substrate Length + 4 * Target-to-Substrate (TTS) distance
This equation is typically used for TTS distances between 75 to 150 mm. If the application has uniformity requirements higher than +/-2%, we suggest to add additional length to the target in order to reduce the initial uniformity tuning required for rotary-type sputtering cathodes.