Abstract: A quartz tube adapted for thermal processing of semiconductor substrates comprising a double-layered central tube having an external sintered crystalline quartz layer fused with a quartz layer of different quality, i.e. an internal transparent fused quartz layer. Both ends of the central tube are fused with the ends of two transparent fused quartz end tubes. The double-layered tube portion can be produced by compacting quartz particulates by means of centrifugal force within a mould and heating from the center with or without placing a fused quartz tube on its inner surface. Semiconductor substrates are processed using this quartz tube, thereby semiconductor devices having stabilized high qualities are produced at an increased yield as well as at a low cost.