The invention relates to a process for depositing under vacuum a multilayers coating stack on a flat glass substrate and to a modular coater for the deposit of thin layers on a flat glass substrate. A gas separation zone disposed between two depositing zones of the modular coater comprises at least one gas injector in the vicinity of the convoying path for the glass substrate which passes through apertures from a depositing zone towards the other depositing zone via the separation zone. The invention allows improvement of the separation factor between the two depositing zones.