The production of semiconductors for applications in microelectronics is operated through photo-lithographic and chemical processes whereof Chemical Vapor Deposition (CVD) technology is one of the most used. CVD processes need to operate in furnaces under controlled atmosphere and, every time the furnace is accessed from the external environment, a purging time is needed to restore the prescribed atmosphere. The optimisation of the utilisation of the furnaces depends on the sequencing of loading and unloading operations entailing the need to disturb the controlled atmosphere. We propose the use of a disaggregated time formulation based on step variables to model a scheduling problem aiming at identifying the optimal sequence of operations and supporting the definition of optimal dispatching policies. © 2019, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.