BIMOS transistor devices having bipolar and MOS transistors formed in substrate thereof and process for the fabrication of the same

A process for the fabrication of a BIMOS device with at least one bipolar transistor and at least two MOS transistors formed on a major surface of a semiconductor substrate of a first conductivity type, which comprises the following steps:

• (a) forming a heavily-doped, buried layer of a second conductivity type in a first region of the semiconductor substrate, said first region including a part of the major surface and being located adjacent to said part of said major surface;
• (b) forming an epitaxial layer of the second conductivity type on the semiconductor substrate, including causing the buried layer of the second conductivity type to extend close to the major surface of the semiconductor substrate inside the epitaxial layer of the second conductivity type;
• (c) oxidizing the epitaxial layer of the second conductivity type at a surface thereof and in a portion adjacent to the surface while using the oxidation resistant layer as a mask;
• (d) stripping the thus-oxidized epitaxial layer of the second conductivity type to expose the epitaxial layer of the second conductivity type;
• (e) forming an epitaxial layer of a first conductivity type on the thus-exposed epitaxial layer of the second conductivity type so that a surface of the epitaxial layer of the first conductivity type lies in the same plane as an interface between the oxidation resistant layer and the epitaxial layer of the second conductivity type;
• (f) stripping the oxidation resistant layer;
• (g) forming said at least one bipolar transistor in a second region of the epitaxial layer of the second conductivity type right above the first region, said second region including a part of the surface of the epitaxial layer of the second conductivity type and being located adjacent to said part of said surface of the epitaxial layer of the second conductivity type;
• (h) forming one of said at least two MOS transistors in a second region of the epitaxial layer of the first conductivity type, said second region of the epitaxial layer of the first conductivity type including a part of said surface of the epitaxial layer of the first conductivity type and being located adjacent to said part of said surface of the epitaxial layer of the first conductivity type;
• and
• (i) forming the other one of said at least two MOS transistors in a third region of the epitaxial layer of the first conductivity type, said third region including another part of said surface of the epitaxial layer of the first conductivity type and being located adjacent to said another part of said surface of the epitaxial layer of the first conductivity type.