Abstract: An apparatus for depositing thin films on a semiconductor wafer. The thin film deposition apparatus includes a reactor block receiving a semiconductor wafer; a shower head plate covering the reactor block to maintain the internal pressure of the reactor block at a predetermined level; a reaction gas supply connected to the shower head plate, for supplying reaction gases; an inert gas supply connected to the shower head plate, for supplying an inert gas; an exhaust portion connected to the reactor block, for exhausting the gases out of the reactor block; and a diffusion plate installed in the shower head plate, having a plurality of passages connected to the source of inert gas supply, a plurality of nozzles connected to the passages, the inert gas sprayed through the nozzles lowering a wall of inert gas along the inner wall of the reactor block, and a plurality of spray holes, the reaction gases spread over the wafer through the spray holes.