Fabricating semiconductor device with polysilicon protection layer during processing

Claim 1

A process for fabricating a semiconductor device having a plurality of elements isolated in P - -type silicon substrate by steps of forming N + -type buried layer; implanting arsenic ions into the surface of said silicon substrate and diffusing it at 1200° C.; growing N + -type epitaxial layer having a thickness of 1.6 μm by doping phosphorus ions on said burried layer; etching a portion of the surface of said epitaxial layer in depth of 5500 Å with a mask; and implanting P + -type impurity ions through said mask at 925° C. by wet oxidation method to form an oxide layer of 10KÅ in thickness, said process comprising further the consecutive steps of forming a base region by implanting boron impurity ions; depositing a polysilicon layer of 3000 Å in thickness throughout the entire surface of said silicon substrate by LPCVD; converting the conductivity of said polysilicon layer into N + -type conductivity by implanting arsenic impurity ions therein; depositing the surface of said polysilicon layer consecutively with primary oxide layer having a thickness of 2000 Å and primary nitride layer having a thickness of 2000 Å by LPCVD; defining an emitter portion and a collector portion to be formed in said N + -type polysilicon by photolithography and then etching unnecessary portions there of by dry etching, said N + -type polysilicon layer being overetched underneath said oxide layer so that the emitter width is formed smaller than 2 μm; depositing a secondary oxide layer on the whole surface of said defined and overetched substrate by LPCVD and then etching completely the entire upper part of said deposited secondary oxide layer except the polysilicon side wall portions thereof; depositing a secondary nitride layer of 2000 Å to 3000 Å in thickness thereon and then removing the entire upper part of said secondary nitride layer by plasma etching; etching the silicon surface around said polysilicon layer to the extent of 1500 Å by dry etching and then growing oxide layer of 500 Å in thickness on the surface said etched portion; depositing a tertiary nitride layer having thickness of 700 Å thereon by LPCVD and then removing the entire upper part of said tertiary nitride layer by plasma etching; growing another oxide layer having a thickness of 2500 Å thereon and then removing said primary, secondary and tertiary nitride layers by wet etching; depositing polysilicon layer having a thickness of 3000 Å on said removed portion and then converting said portion into P + -type conductivity by doping boron ions by thermal diffusion; and defining a portion of said converted polysilicon layer by photolithography and dry etching method and then forming P + -type extrinsic base region by thermal diffusion.