A method and apparatus for forming an optical stack having uniform and accurate layers is provided. A processing tool used to form the optical stack comprises, within an enclosed environment, a first transfer chamber, an on-board metrology unit, and a second transfer chamber. A first plurality of processing chambers is coupled to the first transfer chamber or the second transfer chamber. The on-board metrology unit is disposed between the first transfer chamber and the second transfer chamber. The on-board metrology unit is configured to measure one or more optical properties of the individual layers of the optical stack without exposing the layers to an ambient environment.