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Novel Selective EPI Base for Double Poly Structures

IP.com Disclosure Number: IPCOM000108929D
Original Publication Date: 1992-Jul-01
Included in the Prior Art Database: 2005-Mar-23
Document File: 3 page(s) / 145K

Publishing Venue

IBM

Related People

Burghartz, JN: AUTHOR [+4]

Abstract

Disclosed is a selective epitaxial (epi) base structure for double-poly self-aligned bipolar transistors to achieve reduced collector-base capacitance. The selective epi base is integrated late in the fabrication process to achieve minimum base width.

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Novel Selective EPI Base for Double Poly Structures

       Disclosed is a selective epitaxial (epi) base structure
for double-poly self-aligned bipolar transistors to achieve reduced
collector-base capacitance.  The selective epi base is integrated
late in the fabrication process to achieve minimum base width.

      Double-poly self-aligned bipolar device technology had been
invented to reduce the collector-base capacitance (CCB) by formation
of a polysilicon base contact over the field oxide.  CCB is therein
limited by the lithographic line width and overlay which require that
the collector window be significantly wider than the emitter window.
An additional reason for this asymmetry is that the extrinsic base
contact diffusion has to link up to the intrinsic base region.   The
lateral spacing between emitter and collector window edge can be
significantly reduced if the intrinsic base is grown by selective
epitaxy instead of being ion implanted.  The modified process flow
for the selective epi base integration into double-poly self-aligned
technology is illustrated in Fig. 1.  After field oxide formation a
thinner oxide is grown inside the collector opening.  A stack of
p-implanted polysilicon, oxide and nitride is then deposited.  The
emitter window is next defined by lithography, and etched into the
stack with endpoint on the thermal oxide.  Then, an emitter sidewall
spacer is formed (Fig. 1a).  The described process steps are similar
to the conventional double-poly self-aligned technology, except the
growth of a thermal oxide within the collector window.  This oxide
avoids overetching into the silicon substrate during emitter etch,
and reduces CCB.

      Next, the exposed oxide is etched in hydrofloric acid off the
emitter window bottom, with sufficient undercut underneath the
emitter spacer sidewall (Fig. 1b).  Then, a sel...