Chemical vapor deposition reactor and process chamber for said reactor

A chemical vapor desposition reactor having a process chamber (101) accomodating a substrate holder (4a) for wafers (5), a first inlet (1A, 1B) providing a first gas flow (Φ1) of reactive gases to process the wafers and a crown-shaped gas collector surrounding the substrate-holder, said gas collector forming a hollow anular space and comprises apertures (12) to let the reactive gases exhaust from the process chamber, wherein said reactor further comprises:

• a base plate (30) and a cover plate (20) disposed respectively beneath and above the subtrate-holder,
• an outer ring (10) surrounding the gas-collector (16) and touching both the base plate (30) and the cover-plate (20),
• wherein the process chamber defines a first space (101), which first space is limited by the base and cover plates and the outer ring and an inlet (39) for a second flow (Φ2) of non-reactive gases,
• characterised in that the first space (101) is surrounded by second and third spaces limited by a substantially cylindrical peripheral wall (19C), a lower horizontal wall (19A) and a cover (28),
• wherein one wall comprises the inlet for the second flow (Φ2) and that said second flow acting as a counter-flow (Φ2) for preventing the first reactive gas flow (Φ1) to exit from the process chamber but through the gas-collector.