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Browse Prior Art Database

Method to Achieve Controllable Arsenic Doping in Low Temperature CVD Tools

IP.com Disclosure Number: IPCOM000106065D
Original Publication Date: 1993-Sep-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 2 page(s) / 101K

Publishing Venue

IBM

Related People

Agnello, PD: AUTHOR [+4]

Abstract

Devised is a technique to reduce the memory effect of As in CVD tools which makes the downward modulation of doping difficult in a deposited Si device structure. The memory effect is due primarily to out diffusion of As from poly Si deposits on the susceptor which causes autodoping in the growing Si layer. This invention offers several alternatives to prevent this autodoping effect which include: minimizing poly Si deposits surrounding the wafer, or out diffusing As from the susceptor when the wafer is not present on the susceptor.

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Method to Achieve Controllable Arsenic Doping in Low Temperature CVD Tools

      Devised is a technique to reduce the memory effect of As in CVD
tools which makes the downward modulation of doping difficult in a
deposited Si device structure.  The memory effect is due primarily to
out diffusion of As from poly Si deposits on the susceptor which
causes autodoping in the growing Si layer.  This invention offers
several alternatives to prevent this autodoping effect which include:
minimizing poly Si deposits surrounding the wafer, or out diffusing
As from the susceptor when the wafer is not present on the susceptor.

      Arsenic doping from arsine is a significant problem in silane
based Si epitaxial systems because it significantly slows the growth
rate and leads to unstable solid phase concentrations with a constant
input gas phase concentration.  However, arsine only moderately
affects the growth rate in the ultra-clean APCVD system at low
temperatures, 550-850ºC which uses dichlorosilane (DCS).[*] In
the ASM Episilon One APCVD system it is possible to control the
doping concentration of deposited epi over the range <>10 sup 17 cm
sup -3 to <>10 sup 21 cm sup -3 by changing the input arsine
concentration from 0.05 to 13 ppm, respectively.  Hence it is
possible to modulate the doping upward by simply increasing the input
concentration of arsine in the reactant gas.  However, even in the
APCVD system using DCS the doping exhibits a large memory effect so
that modulation of the concentration downward, for example, so as to
produce a doping "spike" as required in the narrow base of an PNP
bipolar base is not possible.  These attempts to modulate (decrease
downward) the doping in a conventionally configured and operated ASM
tool are limited to changes less than 2x.

      It is believed that the As memory effect is largely related to
the area of exposed Si extraneous to the wafer on which As doped poly
Si grows during the normal epitaxial growth on the wafer.  This poly
Si area out diffuses As which reenters the gas stream and autodopes
the growing Si epi layer.  It is furthermore believed that the
polycrystalline Si on the susceptor and susceptor ring (ASM tool) is
a particularly high capacity As source with rapid out diffusion
capability because large amounts of As are expected to be in the
grain boundaries and rapid diffusion along grain boundaries is
expected even at the low temperatures used here.  Rapid out diffusion
of As from the growing single crystal epitaxial wafer surface
(conventional autodoping) is not expected at these low temperatures.

The As memory effect can be reduced or eliminated by:

1.  eliminating  or minimizing by mechanical design the extraneous
    area for poly Si deposition;

2.  eliminating or minimizing by chemical means the deposition of
    heavily As doped material on the susceptor and ring; or

3.  by changing the normal deposition procedure so the wafer is
    physically moved...