Lpcvd furnace tube with double structure

During epitaxial process, thermal expension coefficient of silica tube is diffrent from that of silicon or other epitaxial materials so that the epitacial silica tube is easily destroied in higher temp. and lower pressure. This invention is characterized by using a furmace tube with double structure, i.e, the outer tube is made from refractory material having better sealing property and the inner tube is made from refractory material without serious influence on characteristics of semiconductor epitaxial wafer. The LPCVD film technology may be used for epitaxial process so as to raise the productivity, to lower the cost and to reniform the thickness of epitaxial layer and the distribution of dopant in the epitaxial layer.