Image Number 3 for United States Patent #6723660.
A thin-film forming apparatus of the present invention is capable of reducing variation of film formation rate and forming thin films of a stable thickness. The thin-film forming apparatus can prevent decrease of the film formation rate due to raise of temperatures of an RF electrode and an inner wall of a reaction chamber, by supplying a pressure control gas of a predetermined pressure into the reaction chamber also in non-film formation time to keep a gas pressure in the reaction chamber constant. Thereby, thickness of a film grown on a substrate can be controlled to a constant thickness. Further, by heating the pressure control gas to raise its temperature to a value approximately equal to a temperature of a material gas, variation of the pressure of the gas in the reaction chamber is controlled and the temperatures of the inner wall of the reaction chamber and the RF electrode are kept constant. Thereby, since the film formation rate on the inner wall of the reaction chamber and a surface of the RF electrode are kept constant, the quantity of the material gas substantially supplied to the surface of the substrate is kept constant. Thereby, the film formation rate on the surface of the substrate can be kept constant.