Why would I want to use a copy mask?
If the mask is used in a Stepper, Scanner or Projection Aligner, then it does not make physical contact with the wafer. The mask should, therefore, last a lifetime if it is kept free from physical damage caused by handling or static electricity.
If the mask is used to contact print onto the wafer or other substrate, it will become worn over time, eventually reaching a point where it needs to be replaced.
If a Master Mask was used, then the cost of the replacement mask would be the same as making a new mask. If the dimensions are large enough (>=2μm) and the specifications for tolerance, registration and defectivity are less critical, then a copy of the mask can be made using a contact print process, which is considerably cheaper than making a new master mask.
A copy mask produced by contact printing is not an identical copy; the reading sense of the mask is reversed as part of the contact printing, which is also known as a mirror image copy.