An improved deposition chamber (2) includes a housing (4) defining a vacuum chamber (18) which houses a substrate support (14). A set of first nozzles (34) have orifices (38) opening into the vacuum chamber in a circumferential pattern spaced apart from and generally overlying the periphery (40) of the substrate support. One or more seconds nozzle (56, 56a), positioned centrally above the substrate support, inject process gases into the vacuum chamber to improve deposition thickness uniformity. Deposition thickness uniformity is also improved by ensuring that the process gases are supplied to the first nozzles at the same pressure. If needed, enhanced cleaning of the nozzles can be achieved by slowly drawing a cleaning gas from within the vacuum chamber in a reverse flow direction through the nozzles using a vacuum pump (84).