Semiconductor device and method for manufacturing the same

Abstract

A semiconductor device is provided without degrading the performance of an internal circuit, which has an SOI structure coexistingly having an SOI static electricity protection circuit to prevent an internal circuit from being damaged due to static electricity through an input/output pad. To achieve this, a structure is made by comprising a silicon substrate of a first conductivity type, a buried oxide film formed on the silicon substrate, a first silicon layer of the first conductivity type formed on the buried oxide film, a second silicon layer of the first conductivity type formed on the buried oxide film and having a thickness smaller than the first silicon layer of the first conductivity type, and an SOI static electricity protection circuit provided between an input/output pad and an internal circuit. The internal circuit is formed in the second silicon layer of the first conductivity type, the SOI static electricity protection circuit being formed in the first silicon layer of the first conductivity type.

What is claimed is:  
     
         1 . A semiconductor device comprising: 
 a silicon substrate;    a buried oxide film formed on the silicon substrate;    a first silicon layer of a first conductivity type formed on the buried oxide film;    a second silicon layer of the first conductivity type formed on the buried oxide film and having a thickness smaller than the first silicon layer;    an SOI static electricity protection circuit provided between an input/output pad and an internal circuit; and    wherein the internal circuit is formed in the second silicon layer, and the SOI static electricity protection circuit is formed in the first silicon layer.    
     
     
         2 . A method for manufacturing a semiconductor device comprising: 
 a step of forming a buried oxide film in a silicon substrate of a first conductivity type;    a step of selectively oxidizing a surface of the first-conductivity-type silicon layer on the buried oxide film and selectively forming an oxide film on the surface of the first-conductivity-type silicon layer;    a step of stripping away the oxide film and thereby exposing a surface of a second first-conductivity-type silicon layer smaller in thickness than the first-conductivity-type silicon layer; and    a step of forming an internal circuit in the second first-conductivity-type silicon layer and forming an SOI static electricity protection circuit for protecting the internal circuit in the first-conductivity-type silicon layer.    
     
     
         3 . A method for manufacturing a semiconductor device comprising: 
 a step of dividing a silicon substrate of a first conductivity type with a first silicon layer of the first conductivity type and a second silicon layer of the first conductivity type, forming a first buried oxide film underneath a surface of the first silicon layer of the first conductivity type, and forming a second buried oxide film in a region shallower than the first buried oxide film underneath a surface of the second silicon layer of the first conductivity type; and    a step of forming an internal circuit in the second first-conductivity-type silicon layer and forming an SOI static electricity protection circuit for protecting the internal circuit against static electricity in the first first-conductivity-type silicon layer.