U.S. patent application number 14/874039 was filed with the patent office on 2017-04-06 for implementing stress in a bipolar junction transistor.
This patent application is currently assigned to GLOBALFOUNDRIES INC.. The applicant listed for this patent is GLOBALFOUNDRIES INC.. Invention is credited to James W. Adkisson, Renata Camillo-Castillo, David L. Harame, Vibhor Jain, Qizhi Liu.
Application Number | 20170098699 14/874039 |
Document ID | / |
Family ID | 58359573 |
Filed Date | 2017-04-06 |
United States Patent
Application |
20170098699 |
Kind Code |
A1 |
Camillo-Castillo; Renata ;
et al. |
April 6, 2017 |
IMPLEMENTING STRESS IN A BIPOLAR JUNCTION TRANSISTOR
Abstract
Device structure and fabrication methods for a bipolar junction
transistor. One or more trench isolation regions are formed in a
substrate to define a device region having a first width. A protect
layer is formed on a top surface of the one or more trench
isolation regions and a top surface of the device region. An
opening is formed in the protect layer. The opening is coincides
with the top surface of the first device region and has a second
width that is less than or equal to the first width of the first
device region. A base layer is formed that has a first section on
the device region inside the first opening and a second section on
the protect layer.
Inventors: |
Camillo-Castillo; Renata;
(Essex Junction, VT) ; Liu; Qizhi; (Essex
Junction, VT) ; Jain; Vibhor; (Essex Junction,
VT) ; Adkisson; James W.; (Jericho, VT) ;
Harame; David L.; (Essex Junction, VT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GLOBALFOUNDRIES INC. |
GRAND CAYMAN |
|
KY |
|
|
Assignee: |
GLOBALFOUNDRIES INC.
Grand Cayman
KY
|
Family ID: |
58359573 |
Appl. No.: |
14/874039 |
Filed: |
October 2, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/762 20130101;
H01L 29/732 20130101; H01L 29/7371 20130101; H01L 29/0821 20130101;
H01L 29/0642 20130101; H01L 29/66242 20130101; H01L 29/66272
20130101 |
International
Class: |
H01L 29/732 20060101
H01L029/732; H01L 29/66 20060101 H01L029/66; H01L 29/06 20060101
H01L029/06; H01L 21/02 20060101 H01L021/02; H01L 29/04 20060101
H01L029/04; H01L 21/762 20060101 H01L021/762; H01L 21/225 20060101
H01L021/225 |
Claims
1. A method of fabricating a device structure, the method
comprising: forming one or more trench isolation regions in a
substrate to define a first device region having a first width;
forming a protect layer on a top surface of the one or more trench
isolation regions; forming a first opening in the protect layer
that coincides with a top surface of the device region and that has
a second width that is less than or equal to the first width of the
first device region; and forming a base layer of a first bipolar
junction transistor having a first section on the first device
region inside the first opening and a second section on the protect
layer, wherein the first section of the base layer is comprised of
single crystal semiconductor material, and the protect layer and
the second section of the base layer are comprised of
polycrystalline semiconductor material.
2. The method of claim 1 wherein the second width of the first
opening in the protect layer is less than the first width of the
first device region.
3. The method of claim 1 wherein the second width of the first
opening in the protect layer is equal to the first width of the
first device region.
4. (canceled)
5. The method of claim 1 wherein an edge of the first opening in
the protect layer is located at or inside an edge of the one or
more trench isolation regions.
6. The method of claim 1 wherein the protection layer is further
formed on a portion of the top surface of the device region, and
the protect layer and the second section of the base layer
partially cover the top surface of the device region.
7. The method of claim 1 further comprising: forming an emitter on
the base layer, wherein the emitter has a third width parallel to
the first width and the second width, and a top surface of the base
layer is contained in a first plane that is raised above a second
plane containing a junction between the emitter and the base
layer.
8. The method of claim 7 wherein the second section of the base
layer is configured to exert a compressive stress on the emitter
and the junction between the emitter and the base layer that
increases dopant diffusion from the emitter into the base
layer.
9. The method of claim 1 wherein the protect layer is located
between the base layer and the first device region, and the protect
layer includes a first dopant concentration that is less than a
second dopant concentration of the base layer.
10. The method of claim 1 further comprising: forming a second
opening in the protect layer that is aligned with the top surface
of a second device region, wherein the second opening has a third
width that is less than the second width of the first opening, and
a second bipolar junction transistor formed using the second device
region has a different peak cutoff frequency or transistor gain
than the first bipolar junction transistor formed using the first
device region.
11. A device structure comprising: one or more trench isolation
regions in a substrate, the one or more trench isolation regions
defining a first device region in the substrate, and the first
device region having a top surface and a first width; a protect
layer on the top surface of the one or more trench isolation
regions and the top surface of the first device region, the protect
layer including a first opening that coincides with the top surface
of the device region, and the first opening having a second width
that is less than or equal to the first width of the first device
region; and a base layer of a first bipolar junction transistor
having a first section on the first device region inside the first
opening and a second section on the protect layer, the first
section of the base layer is comprised of single crystal
semiconductor material, wherein the protect layer and the second
section of the base layer are comprised of polycrystalline
semiconductor material.
12. The device structure of claim 11 wherein the second width of
the first opening in the protect layer is less than the first width
of the first device region.
13. The device structure of claim 11 wherein the second width of
the first opening in the protect layer is equal to the first width
of the first device region.
14. (canceled)
15. The device structure of claim 11 wherein the one or more trench
isolation regions have an edge at an interface with the device
region, and the first opening in the protect layer has an edge that
is located at or inside the edge of the one or more trench
isolation regions.
16. The device structure of claim 11 wherein the protection layer
is further formed on a portion of the top surface of the device
region, and the protect layer and the second section of the base
layer partially cover the top surface of the device region.
17. The device structure of claim 11 further comprising: an emitter
on the base layer, the emitter having a third width parallel to the
first width and the second width, wherein the base layer has a top
surface that is contained in a first plane that is raised above a
second plane containing a junction between the emitter and the base
layer.
18. The device structure of claim 17 wherein the second section of
the base layer is configured to exert a compressive stress on the
emitter and the junction between the emitter and the base layer
that reduces dopant diffusion from the emitter into the base
layer.
19. The device structure of claim 11 wherein the protect layer is
located between the base layer and the first device region, and the
protect layer includes a first dopant concentration that is less
than a second dopant concentration of the base layer.
20. The device structure of claim 11 further comprising: a second
device region having a top surface; and a second opening in the
protect layer that coincides with the top surface of the second
device region, wherein the second opening has a third width that is
less than the second width of the first opening, and a second
bipolar junction transistor formed using the second device region
has a different peak cutoff frequency or transistor gain than the
first bipolar junction transistor formed using the first device
region.
21. The device structure of claim 11 wherein the protect layer is
removed from areas of the substrate outside of the first device
region.
Description
BACKGROUND
[0001] The invention relates generally to semiconductor devices and
integrated circuit fabrication and, in particular, to fabrication
methods and device structures for bipolar junction transistors and
heterojunction bipolar transistors.
[0002] Bipolar junction transistors may be found, among other end
uses, in RF transceivers, multi-gigabit analog-to-digital
converters, optical networks, automotive radar, and high-speed
circuits. Bipolar junction transistors may be combined with
complementary metal-oxide-semiconductor (CMOS) field effect
transistors in bipolar complementary metal-oxide-semiconductor
(BiCMOS) integrated circuits, which take advantage of the favorable
characteristics of both transistor types.
[0003] Bipolar junction transistors are three-terminal electronic
devices that include an emitter, an intrinsic base, and a collector
defined by regions of different semiconductor materials. In the
device structure, the intrinsic base situated between the emitter
and collector. An NPN bipolar junction transistor may include
n-type semiconductor material regions constituting the emitter and
collector, and a region of p-type semiconductor material
constituting the intrinsic base. A PNP bipolar junction transistor
includes p-type semiconductor material regions constituting the
emitter and collector, and a region of n-type semiconductor
material constituting the intrinsic base. In operation, the
base-emitter junction is forward biased and the base-collector
junction is reverse biased. The collector-emitter current may be
controlled by the base-emitter voltage.
[0004] A heterojunction bipolar transistor is a variety of bipolar
junction transistor in which two or more of the terminals--emitter,
intrinsic base, and/or collector--are composed of semiconductor
materials with unequal band gaps, which creates heterojunctions.
For example, the base of a heterojunction bipolar transistor may be
composed of silicon germanium (SiGe), which is characterized by a
narrower band gap than silicon.
[0005] Device performance may be improved by thinning the layers of
semiconductor material used to form the terminals, optimizing the
germanium concentration profile across the thickness of the layer
forming the base, adding carbon to the semiconductor layer forming
the base in order to reduce the diffusion of the
electrically-active dopant, and reducing the number of thermal
cycles to minimize diffusion. Despite these measures, improved
fabrication methods and device structures are needed for bipolar
junction transistors and heterojunction bipolar transistors.
SUMMARY
[0006] In an embodiment of the invention, a method is provided for
fabricating a device structure. One or more trench isolation
regions are formed in a substrate to define a device region having
a first width. A protect layer is formed on a top surface of the
one or more trench isolation regions and a top surface of the
device region. An opening is formed in the protect layer. The
opening coincides with the top surface of the first device region
and has a second width that is less than or equal to the first
width of the first device region. A base layer is formed that has a
first section on the device region inside the first opening and a
second section on the protect layer.
[0007] In an embodiment of the invention, a device structure
includes one or more trench isolation regions in a substrate. The
one or more trench isolation regions define a device region in the
substrate. The device structure includes a protect layer on a top
surface of the one or more trench isolation regions and a top
surface of the device region. The protect layer includes an opening
that coincides with the top surface of the device region. The
opening in the protect layer has a second width that is less than
or equal to a first width of the device region. A base layer
includes a first section on the device region inside the opening
and a second section on the protect layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate various
embodiments of the invention and, together with a general
description of the invention given above and the detailed
description of the embodiments given below, serve to explain the
embodiments of the invention.
[0009] FIG. 1A is a top view of a portion of a substrate at an
initial fabrication stage of a processing method for fabricating a
device structure in accordance with an embodiment of the
invention.
[0010] FIG. 1B is a cross-sectional view taken generally along line
1A-1A in FIG. 1.
[0011] FIG. 2 is a cross-sectional view similar to FIG. 1B at a
subsequent fabrication stage of the processing method.
[0012] FIG. 3 is a graph plotting peak cutoff frequency as a
function of relative pattern dimension for the opening in the
protect layer.
[0013] FIG. 4 is a graph plotting peak threshold frequency as a
function of the relative pattern dimension for the opening in the
protect layer.
[0014] FIG. 5 is a graph plotting collector current as a function
of the relative pattern dimension for the opening in the protect
layer.
[0015] FIG. 6 is a graph plotting lateral strain as a function of
position along a centerline of the device structure for an opening
in the protect layer with a width less than the width of the device
region.
DETAILED DESCRIPTION
[0016] With reference to FIGS. 1A, 1B and in accordance with an
embodiment of the invention, a substrate 10 comprises a
single-crystal semiconductor material usable to form the devices of
an integrated circuit. The semiconductor material constituting the
substrate 10 may include an epitaxial layer at its top surface 10a,
which may contain an amount of an electrically-active dopant that
enhances its electrical properties relative to the remainder of the
substrate 10. For example, the substrate 10 may include an
epitaxial layer of single crystal silicon that is doped with a
concentration of, in a construction for an NPN transistor, an
n-type dopant from Group V of the Periodic Table (e.g., phosphorus
(P), arsenic (As), or antimony (Sb)) in a concentration effective
to impart n-type conductivity.
[0017] Trench isolation regions 12 are located in the semiconductor
material of the substrate 10. The trench isolation regions 12
extend from the top surface 10a of the substrate 10 to a shallow
depth beneath the top surface 10a. The trench isolation regions 12
may be formed by depositing a hardmask, patterning the hardmask and
substrate 10 with lithography and etching processes to define
trenches, depositing an electrical insulator to fill the trenches,
planarizing the electrical insulator relative to the hardmask using
a chemical mechanical polishing (CMP) process, and removing the
hardmask. In one embodiment, the trench isolation regions 12 may be
comprised of silicon dioxide (SiO.sub.2) deposited by chemical
vapor phase deposition (CVD).
[0018] A device region 14 used in fabricating a device structure is
located between the trench isolation regions 12. The device region
14 adjoins the trench isolation regions 12 at interfaces 15, which
may be vertically oriented relative to the top surface 10a. The
device region 14 has a width W1 and a length that are determined by
the arrangement of the trench isolation regions 12 and that may be
measured as distances between the opposite interfaces 15 with the
trench isolation regions 12. The device region 14 is comprised of a
portion of the semiconductor material of the substrate 10. The
device region 14 also has a vertical dimension or height in a
direction normal to a plane containing its length and width that is
established by the depth relative to its top surface 10a that the
trench isolation regions 12 penetrate into the substrate 10.
[0019] A collector 16 may be comprised of a section or all of the
material of the device region 14 located between the trench
isolation regions 12. The collector 16 may contain a concentration
of an n-type dopant in a concentration that is effective to impart
n-type conductivity to its semiconductor material. A subcollector
18 may extend laterally in the substrate 10 beneath the trench
isolation regions 12 in order to couple the collector 16 of the
device region 14 with a collector contact region.
[0020] A protect layer 20 is deposited on a top surface of the
substrate 10, which includes a top surface 12a of the trench
isolation regions 12 and a top surface of the device region 14. The
protect layer 20 may be comprised of a polycrystalline
semiconductor material, such as polycrystalline silicon (i.e.,
polysilicon), deposited by chemical vapor deposition.
Alternatively, the protect layer 20 may be comprised of a
dielectric material, such as silicon dioxide or silicon nitride
deposited by chemical vapor deposition, or a combination of a
dielectric material with a semiconductor material, such as
polysilicon. In a specific embodiment, the protect layer 20 may
include a layer of polysilicon on a layer of silicon nitride. The
protect layer 20 may be used to protect a different device region
on the substrate 10 when the device region 14 is being processed to
fabricate the device structure 25. For example, the protect layer
20 may be used to protect CMOS field effect transistors in other
device regions while the device structure is being fabricated using
device region 14.
[0021] The protect layer 20 is opened by patterning to introduce an
opening 22 that extends through it full thickness to the device
region 14 at the top surface 10a of the substrate 10. The surface
area of the device region 14 opened through the opening 22 may be
used to subsequently form the base and emitter of a device
structure. The opening 22 in the protect layer 20 is laterally
arranged in a plane including the length and width of the device
region 14 to be in alignment with the device region 14. The opening
22 is closed geometrically by edges 24 of the protection layer 20,
and is adjusted in size so that the protect layer 20 completely
covers the trench isolation regions 12 at least in the width
dimension. The edges 24 of the opening 22 are aligned relative to
the nearby edges of the trench isolation regions 12, which are
located at the interfaces 15. The opening 22 has a width W2 between
opposite edges 24 that is less than or equal to the width of the
device region 14 at its interfaces 15 with the trench isolation
regions 12. In the representative embodiment, the width of the
opening 22 in the protect layer 20 is less than or equal to the
width of the device region 14. In an alternative embodiment, the
width of the opening 22 in the protect layer 20 may be equal to the
width of the device region 14.
[0022] The opening 22 also has a length that is transverse to its
width, and that is greater than its width such that the opened area
is rectangular. The length of the opening 22 in the protect layer
20 may be less than or equal to the length of the device region 14.
As a result, the open area of the opening 22 inside the edges 24
may be less than or equal to the surface area of the device region
14 at the top surface 10a of the substrate 10. A bipolar junction
transistor or a heterojunction bipolar transistor is formed within
the opening 22 using the device region 14.
[0023] The opening 22, which coincides with the top surface of the
device region 14, may be formed by applying a mask layer to the top
surface 10a of the substrate 10. The mask layer may comprise, for
example, a photoresist that is applied with a spin coating process,
pre-baked, exposed to a radiation projected through a photomask,
baked after exposure, and developed with a chemical developer to
define a pattern with an opening at the intended location of the
opening 22. An etching process may be used to remove the unmasked
dielectric material of the protect layer 20 to define the opening
22. The etching process may comprise a wet chemical etch or a dry
etch, and may rely on a given etch chemistry that removes the
dielectric material of the protect layer 20 selective to (i.e., at
a higher rate than) the semiconductor material of the device region
14.
[0024] The protect layer 20 may promote the integration of features
of bipolar junction transistors or heterojunction bipolar
transistors into a complementary metal-oxide-semiconductor (CMOS)
process before the manufacture of the field-effect transistors is
completed. The fabrication of the field-effect transistors may be
paused after gate formation and before formation of the source and
drain regions, and the field-effect transistors are covered with
the protect layer 20 during the processing of the device
structures. After the bipolar junction transistors or
heterojunction bipolar transistors are processed, the protect layer
20 is subsequently removed and the fabrication of the field-effect
transistors is completed. In accordance with the embodiments of the
invention, the layout of the protect layer 20 may be utilized to
improve the intrinsic device characteristics for the heterojunction
bipolar transistors instead of merely being removed after serving
to isolate the underlying field-effect transistors on the wafer
while processing the bipolar junction transistors or heterojunction
bipolar transistors.
[0025] With reference to FIG. 2 in which like reference numerals
refer to like features in FIG. 1 and at a subsequent fabrication
stage of the processing method, the remainder of a device structure
25 for a bipolar junction transistor or a heterojunction bipolar
transistor is then fabricated using the device region 14 and the
protect layer 20. An intrinsic base layer 26 of a given thickness
is located on the top surface 10a of substrate 10 in the device
region 14 and on a top surface 20a of the protect layer 20. The
intrinsic base layer 26 may include a single crystal section 28
positioned in vertical alignment with the device region 14 and that
directly contacts the single crystal semiconductor material of the
device region 14. The intrinsic base layer 26 may further include a
polycrystalline section 30 that adjoins the single crystal section
28.
[0026] The intrinsic base layer 26 may be comprised of a different
semiconductor material than the device region 14 and may have an
opposite conductivity type from the collector 16. For example, the
intrinsic base layer 26 may be comprised of a semiconductor
material, such as silicon-germanium (SiGe) in an alloy with a
content of silicon (Si) ranging from 95 atomic percent to 50 atomic
percent and a content of germanium (Ge) ranging from 5 atomic
percent to 50 atomic percent. The germanium content of the
intrinsic base layer 26 may be uniform across the thickness of
intrinsic base layer 26 or graded and/or stepped across the
thickness of intrinsic base layer 26. The semiconductor material of
the intrinsic base layer 26 may comprise a dopant, such as a p-type
dopant selected from Group III of the Periodic Table (e.g., boron)
in a concentration that is effective to impart p-type conductivity
and, optionally, carbon (C) to suppress the diffusion of the p-type
dopant.
[0027] The intrinsic base layer 26 may be formed on a top surface
of the device region 14 inside the opening 22 from a semiconductor
material layer deposited using a low temperature epitaxial (LTE)
growth process, such as vapor phase epitaxy (VPE). Single crystal
semiconductor material (e.g., single crystal silicon and/or single
crystal SiGe) epitaxially grows in the single crystal section 28,
which is disposed on the device region 14. The crystal structure of
the single crystal semiconductor material of the device region 14
serves as a crystalline template for the epitaxial growth of the
crystal structure of the single crystal section 28 of the intrinsic
base layer 26. A polycrystalline section 30 of the intrinsic base
layer 26 is formed on the top surface 20a of the protect layer 20,
which lacks that ability to serve as a crystalline template for
epitaxial growth. The polycrystalline section 30 may be comprised
of polycrystalline semiconductor material, such as polycrystalline
silicon or polycrystalline SiGe.
[0028] The single crystal section 28 of the intrinsic base layer 26
defines an intrinsic base that participates in forming an
emitter-base junction. The single crystal section 28 of the
intrinsic base layer 26 has a width that is equal to the width of
the opening 22. The single crystal section 28 of the intrinsic base
layer 26 also has a length that is transverse to its width, and
that is greater than its width.
[0029] The opening 22 in the protect layer 20 operates to establish
the boundaries for the transition in the crystal structure for the
intrinsic base layer 26 from the single crystal semiconductor
material of the single crystal section 28 to the polycrystalline
semiconductor material of the polycrystalline section 30. The
reduced width of the opening 22 of the protect layer 20 relative to
the width of the device region 14 and in comparison with opening
width in conventional constructions causes the polycrystalline
section 30 of the intrinsic base layer 26 to extend inward to a
location overlying the device region 14. The single crystal section
28 of the intrinsic base layer 26 terminates at a position
nominally coinciding with (i.e., coterminous with) the position of
the edges 24 of the opening 22 in the protect layer 20.
[0030] A dielectric layer is deposited and patterned to form a pad
32 on the single crystal section 28 of the intrinsic base layer 26.
The pad 32 is patterned when an emitter opening is formed to expose
the single crystal section 28 of the intrinsic base layer 26
[0031] An extrinsic base layer 34 is deposited on the intrinsic
base layer 26. The extrinsic base layer 34 may be comprised of
polycrystalline semiconductor material, such as polycrystalline
silicon, formed by chemical vapor deposition. The semiconductor
material of the extrinsic base layer 34 may comprise a dopant, such
as a p-type dopant selected from Group III of the Periodic Table
(e.g., boron or phosphorus) in a concentration that is effective to
impart p-type conductivity and, optionally, carbon (C) to suppress
the diffusion of the p-type dopant. The dielectric layer 36 may be
comprised of an electrical insulator, such as silicon nitride
(Si.sub.3N.sub.4) or silicon dioxide (SiO.sub.2) deposited using
chemical vapor deposition.
[0032] The thickness of the extrinsic base layer 34 on the protect
layer 20 is additive to the thickness of the polycrystalline
section 30 of the intrinsic base layer 26. The opening 22 in the
protect layer 20 operates to establish the boundaries at which the
crystal structure for the intrinsic base layer 26 and the extrinsic
base layer 34 transition from single crystal semiconductor material
to polycrystalline semiconductor material. The reduced dimensions
(width and length) of the opening 22 of the protect layer 20
relative to the dimensions (width and length) of the device region
14 causes the polycrystalline semiconductor material to overlie the
device region 14 in addition to the trench isolation regions 12.
The polycrystalline semiconductor material has a significantly
enhanced thickness due to the presence of the polycrystalline
semiconductor material of the protect layer 20 at the time of
deposition/growth of base layers 26, 34 and in comparison to the
growth of polycrystalline semiconductor material that would occur
if nucleated on the trench isolation regions 12 instead of the
overlying protect layer 20.
[0033] A dielectric layer 36 is deposited on the extrinsic base
layer 34. The dielectric layer 36 may be comprised of an electrical
insulator, such as silicon nitride (Si.sub.3N.sub.4) or silicon
dioxide (SiO.sub.2) deposited using chemical vapor deposition. The
dielectric layer 36 may be patterned with the emitter opening is
formed and/or when the emitter is formed.
[0034] An emitter 40 is located in the emitter opening that extends
through the dielectric layer 36, the extrinsic base layer 34, and
the pad 32 to the intrinsic base layer 26. Non-conductive spacers
42 clad the emitter opening and are formed before the emitter 40 is
formed. The non-conductive spacers 42 may be formed by depositing a
conformal layer comprised of an electrical insulator, such as
Si.sub.3N.sub.4 deposited by CVD, and shaping the conformal layer
with an anisotropic etching process, such as RIE, that
preferentially removes the electrical insulator from horizontal
surfaces. The emitter opening, which is aligned with the single
crystal section 28 of the intrinsic base layer 26, may be formed by
patterning with photolithography and etching processes. The single
crystal section 28 of the intrinsic base layer 26 is located
vertically between the emitter 40 and the collector 16.
[0035] The emitter 40 has a width, W3, that is aligned parallel to
the widths of the opening 22 and the device region 14. The emitter
40 also has a length that is transverse to its width, and that is
greater than its width. The emitter 40 may be formed from a layer
of semiconductor material that is deposited and then itself
patterned using lithography and etching processes. The emitter 40
may be comprised of a heavily-doped semiconductor material that is
deposited and then patterned using lithography and etching
processes. For example, the emitter 40 may be comprised of
polysilicon or polycrystalline silicon-germanium deposited by CVD
or LPCVD and heavily doped with a concentration of a dopant, such
as an impurities species from Group V of the Periodic Table, such
as phosphorus (P) or arsenic (As), effective to impart n-type
conductivity.
[0036] The device structure 25 features a vertical architecture in
which the collector 16, the single crystal section 28 of the
intrinsic base layer 26, and the emitter 40 are vertically
arranged. The conductivity type of the semiconductor material
constituting the intrinsic base layer 26 is opposite to the
conductivity type of the semiconductor materials constituting the
emitter 40 and the collector 16. The device structure 25 may be
characterized as a heterojunction bipolar transistor if two or all
three of the collector 16, the intrinsic base layer 26, and the
emitter 40 are comprised of different semiconductor materials. An
emitter-base junction 48 is defined at the interface between the
emitter 40 and the intrinsic base layer 26. The extrinsic base
layer 34 is raised relative to the emitter-base junction 48 and, in
particular, a top surface 34a of the extrinsic base layer 34 is
contained in a plane that is above a plane containing the
emitter-base junction 48. A base-collector junction is defined at
the interface between the collector 16 and the intrinsic base layer
26. The device structure 25 can be divided into an intrinsic device
region coinciding with the portions of the collector 16, the
intrinsic base layer 26, and the emitter 40 participating in the
junctions and an extrinsic device region outside of the intrinsic
device region.
[0037] The protect layer 20 and any overlying layers may be removed
in the regions surrounding the device structure 25 in order to
complete the fabrication of the CMOS transistors in those regions.
Middle-of-line (MOL) processing and back-end-of-line (BEOL)
processing follows, which includes silicidation, formation of
dielectric layers, via plugs, and wiring for an interconnect
structure coupled by the local interconnect structure with the
bipolar junction transistor, as well as other similar contacts for
additional device structures like device structure 25 and CMOS
transistors included in other circuitry fabricated on the substrate
10.
[0038] The proximity of the protect layer 20 to the intrinsic
device region, and the emitter opening and the emitter 40 in
particular, through the selection of the size and location of the
opening 22 may be effective to influence the performance of the
device structure 25. Locating the edges 24 of the opening 22 at a
location over the top surface of the device region 14 relocates the
polycrystalline semiconductor material of base layers 26, 34 to
have greater proximity to the intrinsic device region. Compressive
stress is transferred from the protect layer 20 to the intrinsic
device region. Among other effects, the transferred stress may be
effective to increase the diffusion of dopant (e.g., n-type dopant)
from the emitter 40 to the underlying intrinsic base of the
intrinsic base layer 26, which has an opposite conductivity
type.
[0039] The magnitude of the compressive stress applied to the
intrinsic device region can be varied for different device
structures by varying the width of the opening 22. The ability to
vary the compressive stress may permit the diffusivity of the
dopant contained in the emitter 40 to be manipulated as a variable.
As a result, the emitter-base junction may be independently
controlled such that peak cutoff frequency and the transistor gain
can be controlled for devices of otherwise the same dimensions
fabricated on the same substrate. For example, device structures 25
characterized by openings 22 of different widths each smaller than
the width of the device region 14 may be fabricated using different
device regions on the same substrate. Due to the variation in the
opening width, the different device structures may have different
performance characteristics, such as peak cutoff frequency and
transistor gain.
[0040] The intervening presence of the protect layer 20 between the
base layers 26, 34 and the device region 14 may also be effective
to reduce the concentration of dopant (e.g., p-type dopant) at the
device periphery available for diffusion from the base layers 26,
34 to the device region 14, which has an opposite conductivity type
from the base layers 26, 34. The semiconductor materials of the
protect layer 20 may be doped with the same dopant as the intrinsic
base layer 26 and the extrinsic base layer 34. However, the dopant
concentration in the protect layer 20 may be lower than the dopant
concentration in either the intrinsic base layer 26 or the
extrinsic base layer 34. Consequently, the protect layer 20
contains less dopant than contributes to diffusion during thermal
cycles, and operates as a buffer zone of semiconductor material of
relative low dopant concentration between semiconductor material
containing a relatively high dopant concentration and the device
region 14.
[0041] Multiple device structures were fabricated and tested in
which the baseline device structure was a 0.1.times.2.0 .mu.m.sup.2
SiGe heterojunction bipolar transistor designed to operate at a
peak cutoff frequency of 300 and a maximum frequency of 360 GHz as
nominal figures of merit. The device construction was similar to
the device constructions for device structures shown and discussed
herein. The size of the opening in the protect layer was varied and
the effects on device performance measured as a function of the
opening dimensions.
[0042] The device AC performance was extracted using two port
S-parameter measurements with standard Load-Reflect-Reflect-Match
(LRRM) calibration and open and short de-embedding to remove
parasitic impedances associated with measurement cables, probes,
wiring and pads.
[0043] FIG. 3 is a graph plotting peak cutoff frequency for the
device structure as a function of relative pattern dimension for
the opening in the protect layer. The peak cutoff frequency
exhibits a sensitivity to the layout of the protect layer. As the
width of the opening in the protect layer is reduced, the peak
cutoff frequency does not exhibit a significant change with
decreasing dimensions of the opening until a minimum dimension at
which the dimensions (length and width) of the opening are equal to
the dimensions (length and width) of the device region. In other
words, the open area of the opening is equal to the surface area of
the device region (i.e., a relative pattern dimension equal to
unity or 1) when the peak cutoff frequency begins to increase in
comparison with the peak cutoff frequency for an open area of the
opening being greater than the surface area of the device region
(e.g., a relative pattern dimension equal to 1.35). The peak cutoff
frequency at relative pattern dimensions increases as the
dimensions (e.g., length and width) of the opening are reduced
(i.e., relative pattern dimensions between 0.65 and 0.8). The
improvement in peak cutoff frequency at relative pattern dimensions
between 0.65 and 0.8 is observed to be on the order of 20 GHz over
that of the nominal device. A relative pattern dimension of 0.65
represents a limitation on the reduction in opening size imposed by
lithography limits.
[0044] FIG. 4 is a graph plotting peak threshold frequency as a
function of collector current for different pattern dimensions for
the opening in the protect layer. Devices with an opening having a
relative pattern dimension in a range between 0.65 and 1 exhibit a
collector current with the base-emitter junction biased at 0.72
volts that is greater than the collector current with the
base-emitter junction biased at 0.72 volts for devices with an
opening having a relative pattern dimension greater than 1. The
increase in peak cutoff frequency is also apparent in FIG. 4 for
devices having a relative pattern dimension less than or equal to
1.
[0045] FIG. 5 is a graph plotting collector current with the
base-emitter junction biased at 0.72 volts as a function of the
relative pattern dimension for the opening in the protect layer.
The collector current is observed to increase with decreasing
relative pattern dimension, and is greatest for relative pattern
dimensions of 1 or less.
[0046] FIG. 6 is a graph plotting lateral strain as a function of
position along a centerline of the device for an opening in the
protect layer with a width less than the width of the device region
(Narrow PPL). For comparison, the lateral strain for an analogous
device structure with an opening in the protect layer having with a
width greater than the width of the device region (Wide PPL) is
also shown. The graph was generated using Technology Computer-Aided
Design (TCAD) computer simulations of devices that were tested.
[0047] For a device having an opening in the protect layer with a
relative pattern dimension of less than 1, compressive stress is
observed in a region immediately below the emitter interface (i.e.,
at -0.31 in the plot) as indicted by curve 100 (Narrow PPL). In
contrast, for a device having an opening in the protect layer with
a relative pattern dimension greater than 1, tensile stress is
observed in the same region as indicted by curve 110 (Wide PPL).
Consequently, the compressive stress observed for comparatively
narrow opening dimensions is opposite to the tensile stress
observed for comparatively wide opening dimensions. The compressive
stress is directed inward toward the intrinsic device and the
emitter of the device structure.
[0048] The methods as described above are used in the fabrication
of integrated circuit chips. The resulting integrated circuit chips
can be distributed by the fabricator in raw wafer form (e.g., as a
single wafer that has multiple unpackaged chips), as a bare die, or
in a packaged form. In the latter case, the chip is mounted in a
single chip package (e.g., a plastic carrier, with leads that are
affixed to a motherboard or other higher level carrier) or in a
multichip package (e.g., a ceramic carrier that has either or both
surface interconnections or buried interconnections). In any case,
the chip is then integrated with other chips, discrete circuit
elements, and/or other signal processing devices as part of either
(a) an intermediate product, such as a motherboard, or (b) an end
product. The end product can be any product that includes
integrated circuit chips, ranging from toys and other low-end
applications to advanced computer products having a display, a
keyboard or other input device, and a central processor.
[0049] A feature may be "connected" or "coupled" to or with another
element may be directly connected or coupled to the other element
or, instead, one or more intervening elements may be present. A
feature may be "directly connected" or "directly coupled" to
another element if intervening elements are absent. A feature may
be "indirectly connected" or "indirectly coupled" to another
element if at least one intervening element is present.
[0050] The descriptions of the various embodiments of the present
invention have been presented for purposes of illustration, but are
not intended to be exhaustive or limited to the embodiments
disclosed. Many modifications and variations will be apparent to
those of ordinary skill in the art without departing from the scope
and spirit of the described embodiments. The terminology used
herein was chosen to best explain the principles of the
embodiments, the practical application or technical improvement
over technologies found in the marketplace, or to enable others of
ordinary skill in the art to understand the embodiments disclosed
herein.
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