Semiconductor device manufacturing: process – Forming bipolar transistor by formation or alteration of... – Having heterojunction
Reexamination Certificate
1999-05-03
2002-02-12
Wilczewski, Mary (Department: 2822)
Semiconductor device manufacturing: process
Forming bipolar transistor by formation or alteration of...
Having heterojunction
C257S019000, C257S197000
Reexamination Certificate
active
06346452
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to bipolar transistor manufacturing methods and, in particular, to methods for controlling an N-type dopant concentration into the subsequent P-type base layer grown epitaxially right after the N-layer growth.
2. Description of the Related Art
Silicon (Si) and Silicon/Silicon-Germanium (Si/SiGe) epitaxial-base layers are commonly employed in bipolar junction transistors (BJTs) and heterojunction bipolar transistors (HBT's), respectively. When these transistors are to be used in high frequency performance wireless, radio-frequency (RF) and communication devices, their small-signal unity gain frequency (f
T
) and maximum oscillation frequency (f
max
) performance can be in excess of 40 GHz. See, for example, D. L. Harame, et al.,
Si/SiGe Epitaxial
-
Base Transistors
-
Part I: Materials, Physics, and Circuits,
in
IEEE Transactions on Electron Devices,
Vol. 42, No. 3, pp. 455-486 (March, 1995) and D. L. Harame, et al.,
Si/SiGe Epitaxial
-
Base Transistors
-
Part II: Process Integration and Analog Applications, in IEEE Transactions on Electron Devices,
Vol. 42, No. 3, pp. 469-482 (1995), both of which are hereby incorporated by reference, for a further discussion of the science and technology of epitaxial-base BJT's and HBT's.
Attaining high frequency performance in BJT's and HBT's requires optimization and control of the dopant concentration depth profile in both the base region and the underlying collector region. For example, in the base/collector structure of an NPN BJT or HBT, it is desirable to control the n-type phosphorus (or arsenic) dopant concentration depth profile in regions immediately underlying a p-type base layer.
Unfortunately, the use of dopant ion implantation and thermal drive-in processes for the doping of bipolar transistor epitaxial layers can result in undesirable implant channeling and unacceptably wide and non-abrupt dopant concentration depth profiles in the base region. Furthermore, when phosphorus in-situ doped epitaxial layers are used as a portion of the collector region in high-frequency epitaxial base layer BJT's or HBT's, phosphorus is observed to unintentionally accumulate in overlying p-type layers (i.e. boron-doped silicon and boron-doped SiGe layers), thereby increasing the n-type dopant concentration in the p-type layers to an undesirable level.
The abruptness of arsenic and phosphorus dopant concentration during the onset of in situ doped silicon epitaxy has been reported in the literature. See T. I. Kamins and D. Lefforge, “Control of n-Type Dopant Transitions in Low-Temperature Silicon Epitaxy”
J. Electrochemical
Soc., Vol 144, No. 2, pp. 674-678 (Feb. 1997), which is hereby fully incorporated by reference. Methods for controlling the n-type dopant concentration depth profile in undoped or p-type doped epitaxial layers formed subsequent to an n-type in-situ doped epitaxial layer are, however, not known in the art.
Still needed is a method for the formation of NPN bipolar transistor epitaxial layers, including undoped or p-type doped epitaxial layers formed subsequent to an n-type in-situ doped epitaxial layer, with a controlled n-type dopant concentration depth profile.
SUMMARY OF THE INVENTION
FIG. 1
is a cross-sectional representation of a portion of an NPN BJT
100
that includes a semiconductor substrate
102
, an n-type collector precursor region
104
formed on the surface of the silicon substrate, and silicon dioxide (SiO
2
) isolation layers
106
formed (for example, by a LOCal Oxidation of Silicon [LOCOS] technique) on the surface of the semiconductor substrate
102
. Overlying the n-type collector precursor region
104
are an n-type in-situ doped epitaxial layer
108
(which will become part of a final NPN BJT collector region) and a p-type in-situ doped epitaxial base layer
110
. Polysilicon layers
112
and
114
, formed during deposition of the n-type in-situ doped epitaxial layer
108
and the P-type in-situ doped epitaxial base layer
110
, respectively, over the SiO
2
isolation layers. An idealized dopant concentration depth profile for n-type in-situ doped epitaxial layer
108
and P-type in-situ doped epitaxial base layer
110
is illustrated in FIG.
2
.
FIG. 3
is a cross-sectional representation of a portion of an NPN HBT
200
that includes a semiconductor substrate
202
, an n-type collector precursor region
204
formed on the surface of the semiconductor substrate
202
, and silicon dioxide (SiO
2
) isolation layers
206
formed on the surface of the semiconductor substrate
202
. Overlying the n-type collector precursor region
204
are an n-type in-situ doped epitaxial layer
208
(which will become part of the NPN HBT's collector region) and an epitaxially grown SiGe layer (doped or undoped) with Boron doped cap layer
210
. Polysilicon layers
212
and
214
, formed during the epitaxial deposition of layers
208
and
210
, respectively, over the SiO
2
isolation layers. An idealized dopant concentration depth profile for n-type in-situ doped epitaxial layer
208
and p-type in-situ doped epitaxial base layer
210
is illustrated in FIG.
4
. Structure
200
is similar to structure
100
, however, it further includes a graded Si
1−x
Ge
x
region starting at the end of the layer
208
(N-type epi).
FIG. 5
is a Secondary Ion Mass Spectroscopy (SIMS) dopant (boron and phosphorus) concentration depth profile of an actual structure with a n-type (i.e. phosphorous) in-situ doped epitaxial layer and a p-type (i.e. boron) in-situ doped epitaxial base layer. The arrow at a depth of approximately 1580 angstroms indicates the location where the formation of the n-type (i.e. phosphorous) in-situ doped epitaxial layer began. The arrow at a depth of approximately 575 angstroms indicates the point where the phosphorus dopant source was turned off and the formation of the p-type (i.e. boron) in-situ doped epitaxial base layer began. These actual dopant concentration depth profiles in
FIG. 5
correspond to the idealized dopant concentration depth profiles of FIG.
2
. As shown in
FIG. 5
, the phosphorous dopant concentration depth profile does not decrease to zero (as in the idealized dopant concentration depth profile of
FIG. 2
) even though the phosphorus dopant source was turned off at the start of the formation of the p-type in-situ doped epitaxial base layer. Instead, it tends to increase due to n-type dopant (i.e. phosphorus) accumulation in the overlying p-type layer. Similar n-type dopant accumulation behavior has been observed when a graded Si
1−x
Ge
x
region is present at the epitaxial base layer. As is explained immediately below, this uncontrolled accumulation of n-type dopants in epitaxial layers formed subsequent to (and, therefore, overlying) a P-type in-situ doped epitaxial layer is undesirable for high performance bipolar transistor applications.
It is desirable, with respect to increasing the frequency performance of bipolar transistors, to minimize the collector-base junction capacitance (C
cb
) by reducing the dopant concentration at the collector. This is desirable since minimizing C
cb
increases the maximum oscillation frequency (f
max
). Also to increase F
t
of transistors, the base transit time can greatly be reduced by increasing the base doping. High base doping also increases C
cb
cap and lowers the gain of the transistor. To optimize this situation, it is desirable to have a low collector doping and less compensation of the base P-type dopant by the N-type collector dopant. To minimize C
cb
in NPN bipolar transistors where epitaxial layers (i.e. either a p-type in-situ doped epitaxial base layer in the case of an BJT or an undoped Si
1−x
Ge
x
epitaxial layer in the case of a HBT) are deposited subsequent to formation of n-type in-situ doped epitaxial layer that will become part of the transistor's collector region, the n-type dopant concentration depth profile in these subsequently formed epitaxial layers should be less th
Bashir Rashid
Kabir Abul Ehsanul
National Semiconductor Corporation
Stallman & Pollock LLP
Wilczewski Mary
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