Method of fabricating semiconductor integrated circuit
The lateral pnp transistor encompasses a p-type semiconductor substrate, an n-type first buried region disposed on the semiconductor substrate, an n-type uniform base region disposed on the first buried region, an n-type first plug region disposed in the uniform base region, a p-type first emitter region and a first collector region disposed in and at the top surface of the uniform base region, a graded base region disposed in the uniform base region and a first base contact region disposed in the first plug region. The graded base region encloses the bottom and the side of the first main electrode region. The doping profile in the graded base region intervening between the first emitter region and the first collector region is such that the impurity concentration is gradually decreases towards the second main electrode region from the first main electrode region.
1. A method of fabricating a semiconductor integrated circuit comprising: selectively forming a first diffusion region of the second conductivity type on a semiconductor substrate of a first conductivity type;
growing an epitaxial layer of the second conductivity type on said first diffusion region so as to make said first diffusion region a first buried region;
selectively diffusing impurity atoms of the second conductivity type so as to form a first plug region of the second conductivity type, from a top surface of said epitaxial layer such that a bottom of said first plug region reaches to said first buried region;
selectively diffusing impurity atoms of the second conductivity type so as to form a graded base region of the second conductivity type, from the top surface of said epitaxial layer, a lateral position of the graded base region is separated from the lateral position of said first plug region and a vertical position of the graded base region is separated from the vertical position of said first buried region, the graded base region has a doping profile such that impurity concentration decreases towards peripheral region from central region of the graded base region;
forming a first main electrode region of the first conductivity type in and at the top surface of said graded base region; and
forming a said second main electrode region of the first conductivity type, so as to sandwich said graded base region between said first and second main electrode regions, in and at the top surface of said epitaxial layer.
2. The method of claim 1 , further comprising selectively forming a second diffusion region of the second conductivity type on said semiconductor substrate before said growing the epitaxial layer, a lateral position of the second diffusion region is separated from the lateral position of said first diffusion region, wherein said growing the epitaxial layer makes said second diffusion region a second buried region, the second buried region serving as a part of a third main electrode region.
3. The method of claim 2 , further comprising forming a second base region of the first conductivity type above said second buried region in the epitaxial layer.
4. The method of claim 3 , further comprising forming a fourth main electrode region of the second conductivity type in and at a top surface of the second base region.
5. The method of claim 1 , further comprising forming a silicon oxide film on the top surface of said epitaxial layer, wherein impurity atoms for forming said graded base region and said first main electrode region are doped through the same diffusion window formed in said silicon oxide film.
6. The method of claim 1 , wherein said selectively diffusing impurity atoms so as to form the graded base region and said forming the first main electrode region in and at the top surface of the graded base region comprises;
said forming the silicon oxide film on the top surface of the epitaxial layer;
forming the diffusion window in the silicon oxide film;
implanting impurity ions of the impurity atoms of the second conductivity type through the diffusion window; base region and said drift region.