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Disposable Germanium Diffusion Source

IP.com Disclosure Number: IPCOM000120588D
Original Publication Date: 1991-May-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 1 page(s) / 58K

Publishing Venue

IBM

Related People

Chu, JO: AUTHOR [+3]

Abstract

Polysilicon has been used as a diffusion source for forming shallow junctions in bipolar transistors, particularly for the emitter. The minimum depth of junctions formed by direct ion implantation is limited by ion channeling and the need to eliminate implant damage (which generally requires a high temperature anneal). These constraints are eliminated by using outdiffusion from polysilicon, because the implant damage and ion channeling are confined to the polysilicon. Polysilicon is a good diffusion source because the solubility and diffusivity of dopants in polysilicon are high, resulting in a high dopant concentration at the interface between the polysilicon and the substrate. A limitation of polysilicon as a diffusion source is that it cannot be removed selectively with respect to the substrate.

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Disposable Germanium Diffusion Source

      Polysilicon has been used as a diffusion source for
forming shallow junctions in bipolar transistors, particularly for
the emitter.  The minimum depth of junctions formed by direct ion
implantation is limited by ion channeling and the need to eliminate
implant damage (which generally requires a high temperature anneal).
These constraints are eliminated by using outdiffusion from
polysilicon, because the implant damage and ion channeling are
confined to the polysilicon. Polysilicon is a good diffusion source
because the solubility and diffusivity of dopants in polysilicon are
high, resulting in a high dopant concentration at the interface
between the polysilicon and the substrate.  A limitation of
polysilicon as a diffusion source is that it cannot be removed
selectively with respect to the substrate. Hence, it has not been
used for source/drain formation in conventional FET structures or for
intrinsic base formation in conventional bipolar structures.

      A potential alternative diffusion source to polysilicon is Ge.
The solubilities of As and P in Ge are high (0.18 and 0.16 at %
respectively, at 800~C), and the diffusivities in single crystal Ge
are over 1000X greater than in single crystal Si.  The solubility of
B in Ge is considerably lower.  Hence, there is little B outdiffusion
from Ge. However, it may be possible to use other p-type dopants,
such as Ga, which has a maximum solubility in Ge of 1 at %. In
princi...