| 摘要 |
PURPOSE:To suppress the generation of crystal defect in the formation of a twin well by a method wherein the depth of a first well, which is formed in the whole surface of a semiconductor substrate and has an impurity concentration higher than that of this substrate, is formed two times or deeper than the depth of a second well formed in a desired region in this first well. CONSTITUTION:First and second wells 6 and 9 are formed in a P-type silicon substrate and an oxide film (an SiO2 film) 5 is formed on the surface of the substrate. The depth of the first well 6 is about 10mum and the depth of the second well 9 is about 5mum. The depth of the well 6 is set at about two times the depth of the well 9. In order to form the well 6, thermal diffusion is performed in an oxygen-containing atmosphere. Then, the whole surface other than the well region is covered with a photosensitive resin 3 by photoetching process. The, an oxide film 2 on the N-type well region is removed by means of an ammon fluoride liquid using the resin 3 as a mask. Then, a thermal diffusion is performed for activating the impurity and for obtaining a desired diffusion depth. Thereby, the second well 9 is formed and a twin well constitution can be formed.
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