Disclosed is a modular semiconductor substrate processing system (1), including a plurality of independently operable substrate processing units (100). Each unit (100) comprises a reactor module (104) and a substrate transfer module (102). Within the system (1), the substrate transfer modules (102) of the different units (100) are serially interconnected such that substrates (116) may be exchanged between them. Exchange of substrates (116) between neighboring processing units (100) is facilitated by a shared substrate hand-off station (130) that is associated with each pair of neighboring processing units. The actual transfer of substrates is performed by a substrate handling robot (122), which may preferably be of the SCARA-type.