U.S. patent application number 14/592412 was filed with the patent office on 2015-05-07 for band engineered semiconductor device and method for manufacturing thereof.
This patent application is currently assigned to GLOBALFOUNDRIES INC.. The applicant listed for this patent is GLOBALFOUNDRIES INC., IMEC. Invention is credited to David Paul Brunco, Geert Hellings, Benjamin Vincent.
Application Number | 20150126010 14/592412 |
Document ID | / |
Family ID | 49080794 |
Filed Date | 2015-05-07 |
United States Patent
Application |
20150126010 |
Kind Code |
A1 |
Vincent; Benjamin ; et
al. |
May 7, 2015 |
Band Engineered Semiconductor Device and Method for Manufacturing
Thereof
Abstract
The disclosure is related to a band engineered semiconductor
device comprising a substrate and a protruding structure that is
formed in a recess in the substrate. The protruding structure
extends above the recess and has a buried portion and an extended
portion. At least the extended portion comprises a semiconductor
material having an inverted `V` band gap profile with a band gap
value increasing gradually from a first value at lateral edges of
the structure to a second value, higher than the first value, in a
center of the structure. The disclosure is also related to the
method of manufacturing of such a band engineered semiconductor
device.
Inventors: |
Vincent; Benjamin; (San
Francisco, CA) ; Hellings; Geert; (Heverlee, BE)
; Brunco; David Paul; (Tervuren, BE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
IMEC
GLOBALFOUNDRIES INC. |
Leuven
Grand Cayman |
|
BE
KY |
|
|
Assignee: |
GLOBALFOUNDRIES INC.
Grand Cayman
KY
IMEC
Leuven
BE
|
Family ID: |
49080794 |
Appl. No.: |
14/592412 |
Filed: |
January 8, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14024820 |
Sep 12, 2013 |
8963225 |
|
|
14592412 |
|
|
|
|
61701452 |
Sep 14, 2012 |
|
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Current U.S.
Class: |
438/283 |
Current CPC
Class: |
H01L 21/0257 20130101;
H01L 29/16 20130101; H01L 29/1054 20130101; H01L 29/66795 20130101;
H01L 29/7856 20130101; H01L 29/161 20130101; H01L 21/02532
20130101; H01L 29/785 20130101 |
Class at
Publication: |
438/283 |
International
Class: |
H01L 29/66 20060101
H01L029/66; H01L 21/02 20060101 H01L021/02; H01L 29/10 20060101
H01L029/10; H01L 29/161 20060101 H01L029/161 |
Claims
1. A method for manufacturing a semiconductor device comprising:
providing a patterned substrate comprising a dielectric region and
a semiconductor region; forming a recess in the semiconductor
region having a bottom area exposing the substrate and lateral
walls comprising a dielectric material; forming protruding
structures in the recess by growing epitaxially a semiconductor
material in the recess; and recessing back the dielectric material
in the dielectric region revealing an extended portion of the
protruding structures, wherein at least the extended portion of the
protruding structures comprises the semiconductor material having
an inverted `V` band gap profile with a band gap value increasing
gradually from a first value at lateral edges of the structure to a
second value, higher than the first value, in a center of the
structure.
2. The method of claim 1, wherein the semiconductor material is
silicon germanium (SiGe) and the band gap profile is created by a
gradient in Ge concentration, with the highest Ge concentration at
the lateral edges of the protruding structure.
3. The method of claim 1, wherein during the epitaxial growth of
SiGe a pressure and a growth temperature are chosen such that a
growth front perpendicular to the planes is favored, forming facets
at the bottom area of the recess, and wherein the facets propagate
but do not completely merge in order to keep a top plane during the
epitaxial growth, thereby achieving a gradient in Ge
concentration.
4. The method of claim 3, wherein the growth temperature is between
450.degree. C. and 700.degree. C. and the pressure is between 5
Torr and 1 atm.
5. The method of claim 1, further comprising forming a capping
layer conformal around the extended portion of the protruding
structure.
6. The method of claim 1, wherein the semiconductor material
further comprises dopants that are introduced by in-situ doping
during the epitaxial growth or by implantation after the epitaxial
growth.
7. The method of claim 1, wherein the semiconductor material of the
protruding structures comprises two or more layers, and wherein the
layers are stacked vertically.
8. The method of claim 1, further comprising forming a gate
dielectric and gate electrode substantially conformal around the
extended portion of the protruding structures.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional of U.S. patent
application Ser. No. 14/024,820 filed on Sep. 12, 2013, which
claims priority to U.S. Provisional Patent Application Ser. No.
61/701,452 filed Sep. 14, 2012, the entire contents of each of
which are incorporated herein by reference.
FIELD OF THE DISCLOSURE
[0002] The present invention is related to a band engineered
semiconductor device, more specifically with a FinFET device and
the method of manufacturing thereof.
BACKGROUND OF THE DISCLOSURE
[0003] Continued scaling of planar MOSFET transistors give rise to
increased short channel effects. This has sparked interest in
alternative transistor architectures, such as Silicon-On-Insulator
FETs, Quantum Well (QW)-based FETs (such as e.g. the implant free
quantum well IFQW pFET) and multi-gate FETs (MUGFETs or
FinFETs).
[0004] However there is further need to improve the performance of
the new transistor architectures with respect to the short channel
control in order to reduce the OFF-state leakage current.
SUMMARY OF THE DISCLOSURE
[0005] In a first aspect, the disclosure is related to a
semiconductor device comprising a substrate and a protruding
structure which is formed in a recess in the substrate and is
extending above the recess having a buried portion and an extended
portion, and wherein at least the extended portion comprises a
semiconductor material having an inverted `V` band gap profile with
a band gap value increasing gradually from a first value at lateral
edges of the structure to a second value, higher than the first
value, in a center of the structure.
[0006] In embodiments of the disclosure, both the extended portion
and the buried portion comprise a semiconductor material having an
inverted `V` band gap profile.
[0007] In embodiments of the disclosure, the recess has a bottom
area which exposes the substrate and lateral walls comprising a
dielectric material.
[0008] In some embodiments of the disclosure, the semiconductor
material of the protruding structure comprises silicon germanium
(SiGe) and the band gap profile is created by a gradient in Ge
concentration, with the highest Ge concentration at the lateral
edges of the protruding structure.
[0009] In embodiments of the disclosure, Ge concentration at the
lateral edges is between 60 at % and 100 at %.
[0010] Further, in embodiments of the disclosure, Ge concentration
in the center is between 40 at % and 60 at %.
[0011] In embodiments of the disclosure, the substrate is silicon
and the dielectric material is silicon oxide.
[0012] In embodiments of the disclosure, the semiconductor material
may further comprise dopants.
[0013] Furthermore, the protruding structure may comprise two or
more layers, the layers being stacked vertically and each layer may
comprise a semiconductor material having an inverted `V` band gap
profile with a band gap value increasing gradually from a first
value at the lateral edges of the structure to a second value
higher than the first value in the center of the structure.
[0014] In various embodiments of the disclosure, the protruding
structure forms a fin.
[0015] In particular embodiments of the disclosure, the fin has a
tapered shape having a smaller width at a top of the extended
portion.
[0016] In specific embodiments, the fin may further comprise a
capping layer formed conformal around the extended portion of the
protruding structure.
[0017] In particular embodiments, the capping layer is made of SiGe
or Ge, wherein SiGe has a Ge concentration higher than 60 at %.
[0018] Further, the device of the disclosure may comprise a
multiple gate formed conformal around the fin.
[0019] In a second aspect, the disclosure is related to a method
for manufacturing a semiconductor device comprising a number of
steps. The method comprises providing a patterned substrate
comprising dielectric region and a semiconductor region. The method
also comprises forming a recess in the semiconductor region having
a bottom area exposing the substrate and lateral walls comprising a
dielectric material. The method further comprises forming
protruding structures in the recess by growing epitaxial a
semiconductor material in the recess filling and overgrowing the
recess. The method even further comprises recessing back the
dielectric material in the dielectric region revealing an extended
portion of the protruding structures, wherein at least the extended
portion of the protruding structures comprises a semiconductor
material having an inverted `V` band gap profile with a band gap
value increasing gradually from a first value at lateral edges of
the structure to a second value higher than the first value in a
center of the structure.
[0020] The method further comprises performing chemical mechanical
polishing to remove any overgrown semiconductor material before
recessing back the dielectric material in the dielectric
region.
[0021] In different embodiments of the disclosure, the
semiconductor material is silicon germanium (SiGe) and the band gap
profile is created by a gradient in Ge concentration, with the
highest Ge concentration at the lateral edges of the protruding
structure.
[0022] In embodiments of the disclosure, the pressure and the
growth temperature during the epitaxial growth of the SiGe are
chosen such that a growth front perpendicular to the (111) planes
is favored, thereby forming facets at the bottom area of the recess
and wherein the facets propagate, but do not completely merge in
order to keep a (100) top plane during the epitaxial growth thereby
achieving a gradient in Ge concentration.
[0023] In specific embodiments, the growth temperature is between
about 450.degree. C. and about 700.degree. C. and the pressure is
between about 5 Torr to about 1 atm.
[0024] In particular embodiments, the method may further comprise
forming a capping layer conformal around the extended portion of
the protruding structure. Preferably, the capping layer is made of
Ge or SiGe with a Ge concentration higher than 60 at %.
[0025] In embodiments of the disclosure, the semiconductor material
further comprises dopants which are introduced by in-situ doping
during the epitaxial growth or by implantation after the epitaxial
growth.
[0026] The method of the disclosure may further comprise forming a
gate dielectric and a gate electrode substantially conformal around
the extended portion of the protruding structures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] All drawings are intended to illustrate some aspects and
embodiments of the present disclosure. The drawings described are
only schematic and are non-limiting.
[0028] FIG. 1 shows schematically a cross-section through a fin
structure according to the disclosure. The band gap has an inverted
`V` profile, i.e., gradually increasing from the edges of the fin
to the center of the fin (top picture). The valence band (Ev)
energy offset is shown (center plot). A double-gate transistor
formed around a fin structure of the disclosure (bottom picture) is
also shown.
[0029] FIG. 2 shows a Transmission Electron Microscopy (TEM) image
and Scanning Transmission Electron Microscopy (STEM) image of a
structure (fin) grown according to the method of the
disclosure.
[0030] FIG. 3 shows a TEM-image of the structure of the disclosure
(right) and the variation of the Ge concentration percentage across
the fin width as measured by Energy Dispersive Spectrometry (EDS)
(left).
[0031] FIG. 4 shows schematically the evolution of the growth
plane's orientation during the epitaxial growth of the
semiconductor material according to the method of the
disclosure.
[0032] FIG. 5 shows schematically a fabrication process according
to an embodiment of the disclosure.
[0033] FIG. 6 shows schematically a fabrication process according
to another embodiment of the disclosure.
[0034] FIG. 7 shows schematically a fabrication process according
to yet another embodiment of the disclosure.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0035] The present disclosure relates to a semiconductor device
having a band engineered structure. Further the disclosure relates
to a FinFET device comprising a band engineered structure and a
reduced OFF-state leakage current.
[0036] Furthermore, the disclosure relates to a method for
manufacturing an electronic device with a band engineered structure
obtained by epitaxial growth.
[0037] The present disclosure will be described with respect to
particular embodiments and with reference to certain drawings, but
the disclosure is not limited thereto, but only by the claims. The
drawings described are only schematic and are non-limiting. In the
drawings, the size of some of the elements may be exaggerated and
not drawn on scale for illustrative purposes. The dimensions and
the relative dimensions do not correspond to actual reductions to
practice of the disclosure.
[0038] Furthermore, the terms first, second and the like in the
description and in the claims are used for distinguishing between
similar elements and not necessarily for describing a sequence,
either temporally, spatially, in ranking or in any other manner. It
is to be understood that the terms so used are interchangeable
under appropriate circumstances and that the embodiments of the
disclosure described herein are capable of operation in other
sequences than described or illustrated herein.
[0039] Moreover, the terms top, under, and the like in the
description and the claims are used for descriptive purposes and
not necessarily for describing relative positions. It is to be
understood that the terms so used are interchangeable under
appropriate circumstances and that the embodiments of the
disclosure described herein are capable of operation in other
orientations than described or illustrated herein.
[0040] It is to be noticed that the term "comprising," used in the
claims, should not be interpreted as being restricted to the means
listed thereafter; it does not exclude other elements or steps. It
is thus to be interpreted as specifying the presence of the stated
features, integers, steps or components as referred to, but does
not preclude the presence or addition of one or more other
features, integers, steps or components, or groups thereof. Thus,
the scope of the expression "a device comprising means A and B"
should not be limited to devices consisting only of components A
and B. It means that with respect to the present disclosure, the
only relevant components of the device are A and B.
[0041] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present disclosure.
Thus, appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment, but may.
Furthermore, the particular features, structures or characteristics
may be combined in any suitable manner, as would be apparent to one
of ordinary skill in the art from this disclosure, in one or more
embodiments.
[0042] Similarly it should be appreciated that in the description
of exemplary embodiments of the disclosure, various features of the
disclosure are sometimes grouped together in a single embodiment,
figure, or description thereof for the purpose of streamlining the
disclosure and aiding in the understanding of one or more of the
various inventive aspects. This method of disclosure, however, is
not to be interpreted as reflecting an intention that the claimed
disclosure requires more features than are expressly recited in
each claim. Rather, as the following claims reflect, inventive
aspects lie in less than all features of a single foregoing
disclosed embodiment. Thus, the claims following the detailed
description are hereby expressly incorporated into this detailed
description, with each claim standing on its own as a separate
embodiment of this disclosure.
[0043] Furthermore, while some embodiments described herein include
some but not other features included in other embodiments,
combinations of features of different embodiments are meant to be
within the scope of the disclosure, and form different embodiments,
as would be understood by those in the art. For example, in the
following claims, any of the claimed embodiments can be used in any
combination.
[0044] In the description provided herein, numerous specific
details are set forth. However, it is understood that embodiments
of the disclosure may be practiced without these specific details.
In other instances, well-known methods, structures and techniques
have not been shown in detail in order not to obscure an
understanding of this description.
[0045] A MUGFET is a widely used abbreviation for a multi-gate
fin-type field effect transistor. An example of MUGFET is the
FinFET, which is characterized by the fact that the conducting
channel is wrapped by a thin silicon "fin," which forms the body of
the device. The thickness of the fin (measured in the direction
from source to drain) determines the effective channel length of
the device.
[0046] In the technical literature, FinFET is used somewhat
generically to describe any fin-based, multigate transistor
architecture regardless of number of gates.
[0047] In OFF-state, the current in FinFETs devices flows in the
center of the fin. Unfortunately, it is exactly the center of the
fin where the gate control is worse. The ON-current in a FinFET
flows in a different location, along the fin sidewalls and top,
thus much closer to the gate.
[0048] Therefore, in a possible solution to lower the OFF-state
leakage of a fin-based device is to make the center of a fin less
conductive and/or to reduce the local carrier concentration in the
center of the fin.
[0049] One way to reduce the charge density in the middle of the
fin is by introducing a different material in the center of the
fin. The center material has a larger band gap than the "cladding"
material or, more precisely, the center material has a band-offset
towards the cladding material. In this way a tri-gate device can be
built up around the "cladded" fin. However, from a manufacturing
perspective, this is not an easy structure to make; one has to make
a fin of the center material first, and then start growing the
second material on top of this which put a very important challenge
to the deposition/growth process.
[0050] The present disclosure shows that there is an advantageous
alternative for growing "cladding material" on a center material.
This alternative is a one-step process, whereby a fin is formed in
one growth step, with a gradually changing band gap (band offset).
The dual-gate transistor can be formed around the fin structure
grown with the method of the disclosure. Such a transistor has all
the benefits of a device build around a "cladding" structure and,
in addition, other advantages as will be shown herein below. With
some additional manufacturing steps the fin structure can be
adapted to form a basis for a tri-gate transistor as well.
[0051] In order to grow the fin with a gradually changing band gap
the epitaxial growth process was adapted such that a semiconductor
material (e.g., SiGe) is grown in a narrow recess (e.g. STI
trench), with laterally (horizontally) varying concentration. The
structure (fin) obtained by such a one-step growth process has a
center (middle), which is Si-rich and edges which are Ge-rich. As a
result, holes (h+) will be present preferentially in the Ge-rich
areas, thus, at the edges and not in the center.
[0052] Before forming the gate, the fin obtained with the method of
the disclosure has a buried portion and an extended portion. At
least the extended portion has a gradually changing band gap, which
increases from a first (lower) value at the lateral edges of the
fin to a second (higher) value in the center of the fin.
Advantageously, both the buried portion and the extended portion of
the fin obtained with the method of the disclosure have the
gradually changing band gap which increases from a first (lower)
value at the lateral edges of the fin to a second (higher) value in
the center of the fin.
[0053] In a first aspect of the disclosure, a semiconductor device
is disclosed comprising a substrate and a protruding structure on
the substrate, wherein the protruding structure comprises a
semiconductor material having an inverted `V` band gap profile with
a band gap value increasing gradually from a first value at lateral
edges of the structure to a second value, higher than the first
value, in a center of the structure.
[0054] Further, in the first aspect the protruding structure is
formed in a recess (trench) which has a bottom area exposing the
substrate and sidewalls made of an insulating material
(dielectric).
[0055] After removing the overgrown semiconductor material and
recessing back part of the adjacent (insulating) material, the
protruding structure is extending above the remaining recess
thereby having a buried portion and an extended portion.
[0056] FIG. 1 shows schematically a cross-section through a fin
structure of the disclosure. The band gap of the grown
semiconductor material has an inverted `V` profile, i.e., gradually
increasing from the edges of the fin to the center of the fin. The
valence band (Ev) energy offset is also shown. In the bottom
picture of FIG. 1 a double-gate transistor formed around a fin
structure of the disclosure is schematically represented. With the
label "small E.sub.G" it is meant to indicate a material with a
conduction/valence band offset for electrons/holes, such that
electrons/holes preferentially occupy said material.
[0057] In embodiments of the disclosure, the bottom part of the
trench (or recess) which exposes the substrate can have either a
flat surface or a concave surface as result of the recess forming
(etching) process. As soon as facets are formed during the
epitaxial growth a horizontal (lateral) gradient in composition is
observed. The horizontal gradient in composition corresponds to a
gradient in the band gap of the semiconductor material.
[0058] Embodiments of the disclosure describe a device wherein at
least the extended portion of the protruding structure comprises a
semiconductor material having an inverted `V` band gap profile.
[0059] Other embodiments of the disclosure describe a device
wherein both the extended portion and the buried portion comprise a
semiconductor material having an inverted `V` band gap profile.
[0060] In different embodiments of the first and second aspect of
the disclosure, the substrate is silicon (e.g., a silicon wafer).
Further in these embodiments, the dielectric material is silicon
oxide. Hereinafter, the silicon oxide is also referred to as
Shallow Trench Isolation (STI) oxide (2), since it is originating
from a previous STI process sequence performed on a bare Si
substrate.
[0061] In embodiments of the disclosure the semiconductor material
of the protruding structure comprises silicon germanium and the
band gap profile is created by a gradient in Ge concentration, with
the highest Ge concentration at the lateral edges of the protruding
structure.
[0062] FIG. 2 shows a Transmission Electron Microscopy (TEM) image
and Scanning Transmission Electron Microscopy (STEM) image of a
protruding structure (fin) grown according to the method of the
disclosure. The lighter portion in the middle of the structure made
of SiGe indicates a locally higher Si concentration than at the
lateral edges of the fin. The dotted white rectangle on the left
picture indicates the portion around which the dual-gate or
tri-gate transistor will be formed in the subsequent manufacturing
steps.
[0063] FIG. 3 shows a TEM-image of the protruding structure of the
disclosure (right) and the variation of the Ge concentration
percentage across the fin width as measured by Energy Dispersive
Spectrometry (EDS) (left). From the contrast in the right picture
it can be noticed that as soon as facets are formed, Ge lateral
gradient is observed. The absolute difference in Ge concentration
between the center of the fin structure and the lateral edges
amounts about 40 at %.
[0064] In specific embodiments of both aspects of the disclosure,
Ge concentration at the lateral edges is comprised between 60 at %
and 100 at %,
[0065] Further, in particular embodiments of both aspects of the
disclosure, Ge concentration in the center of the structure is
comprised between 40 at % and 60 at %.
[0066] In particular embodiments of the disclosure, the
semiconductor material further may comprise dopants (5). The
dopants (5) can be localized in the protruding structure as shown
in FIG. 6, this configuration bringing additional advantages as
shown below.
[0067] Further in embodiments of the disclosure, the semiconductor
material of the protruding structure may comprise two or more
layers, the layers being stacked vertically as shown in FIG. 7.
Each layer comprises a semiconductor material having a horizontal
gradient in band gap, increasing from a first value at the edges to
a second (higher) value in the center of the structure.
[0068] In different embodiments, the protruding structure of the
disclosure forms a fin (1) in a multi-gate field effect transistor.
Such a MUGFET further comprises a gate oxide (3) and a gate
electrode (4) formed around the fin (1).
[0069] In particular embodiments, the fin has a tapered shape
having a smaller width at a top of the extended portion.
[0070] In embodiments of the disclosure, the fin may further
comprise a capping layer formed conformal around the extended
portion of the protruding structure. In particular examples, the
capping layer comprises SiGe or Ge, wherein SiGe has a Ge
concentration higher than 60 at %.
[0071] In a second aspect of the disclosure, a method for
manufacturing a semiconductor device is disclosed comprising the
following steps. First, providing a patterned substrate comprising
dielectric region and a semiconductor region. Next, forming a
recess in the semiconductor region having a bottom area exposing
the substrate and lateral walls (sidewalls) comprising a dielectric
material. Then, forming protruding structures in the recess wherein
the structures comprise a semiconductor material having an inverted
`V` band gap profile with a band gap value increasing gradually
from a first value at lateral edges of the structure to a second
value in a center of the structure; the formation of the protruding
structures is achieved by growing epitaxial a semiconductor
material in the recess filling and overgrowing the recess. The next
step in manufacturing a semiconductor device is performing chemical
mechanical polishing to remove any overgrown semiconductor
material. A further step in manufacturing a semiconductor device is
recessing (etching) back the dielectric material in the dielectric
region revealing an extended portion of the protruding
structures.
[0072] In state of the art epitaxial growth of SiGe it is known how
to vary the concentration of Ge in a vertical direction, e.g., when
filling up from bottom to top a trench with increasing Ge
concentration. However, such a technique will create a gradient in
Ge concentration only in the vertical direction (a direction
parallel to the sidewalls of the trench).
[0073] Referring to the epitaxial growth in the method of the
disclosure, the semiconductor material (in an example SiGe) is
grown such that the Ge concentration is varied on a horizontal
direction (perpendicular on the sidewalls of the trench). The
gradient in the horizontal direction is the consequence of the
lateral growth from a vertical Si seed surface. In order to achieve
this, the epitaxial process has to be especially adapted as
described below.
[0074] More specifically, the pressure and the temperature of the
epitaxial growth process of SiGe are chosen such that a growth
front perpendicular to the (111) planes is favored, thereby,
forming facets at the bottom area of the recess (trench).
Subsequently, the facets formed at the bottom area of the recess
propagate but do not completely merge such that a (100) top plane
is kept during the epitaxial growth, thereby achieving a gradient
in Ge concentration in the horizontal direction. It is noted that a
horizontal Ge gradient is obtained both in the trench and also on
top of the trenches where (100) planes are formed again after the
lateral overgrowth planes merge.
[0075] FIG. 4 shows schematically the evolution of the growth
planes orientation during the epitaxial growth of SiGe according to
the method of the disclosure.
[0076] In embodiments of the first and second aspect of the
disclosure, the semiconductor material comprises at least two group
IV elements such as Si, Ge, C, or Sn. Preferably, the semiconductor
material comprises Si and Ge. More preferably, the semiconductor
material is silicon germanium (SiGe).
[0077] Further in other embodiments of the first and second aspect
of the disclosure the semiconductor material comprises an III-V
compound. Preferably, the III-V compound is a ternary III-V
compound comprising two group III-elements such as In, Ga, or Al
and one group V element such as As or P. More preferably, the III-V
compound comprises/consist of InGaAs.
[0078] In different embodiments of the second aspect, the growth
temperature is between 450.degree. C. and 700.degree. C. while the
total pressure in the epitaxial reactor is between 5 Torr and 1
atm.
[0079] In the specific and non-limitative example shown in FIG. 4,
dichlorosilane (H.sub.2SiCl.sub.2, DCS) was used as Si precursor
and germane (GeH.sub.4) as Ge precursor while H.sub.2 was used as
carrier gas. The growth temperature was 550.degree. C., total
pressure in the reactor 20 Torr and the flows of the precursors and
carrier gas as following: 20 sccm DCS, 200 sccm GeH.sub.4 and 20
slm H.sub.2. The process conditions (temperature and total pressure
in the epitaxial reactor, flows or partial pressures of precursors
and carrier gas) suitable to grow the semiconductor material such
that the growth front perpendicular to the (111) planes is favored
and facets are formed at the bottom area of the recess (trench) can
be easily identified by those skilled in the art for a particular
epitaxial reactor based on the present disclosure. Facets formation
is especially favored during the epitaxial growth at low pressure
and/or high temperature in the above mentioned ranges.
[0080] In various embodiments, the method of the disclosure may
further comprise the process steps of forming a gate dielectric and
gate electrode substantially conformal around the extended portion
of the protruding structures.
[0081] FIG. 5 shows schematically a fabrication process according
to an embodiment of the disclosure. The step depicted in (101)
shows epitaxial growth of a semiconductor material having a
horizontal gradient in composition, e.g., Si-rich SiGe in the
center of the structure ("A") and Ge-rich SiGe at the lateral edges
of the structure ("B"). The step illustrated in (102) depicts
chemical mechanical processing (CMP) of the overgrown semiconductor
material. The step (103) illustrates the STI oxide (2) etch back
(recess) to expose the fin (1), the exposed portion of the fin is
referred herein also as the extended portion of the fin. The step
(104) shows the formation of gate oxide (3) and gate electrode (4)
around the fin (1) in a tri-gate architecture. The step (104') is
an alternative to the step (104), which shows the formation of gate
oxide (3) and gate electrode (4) in a dual-gate architecture.
[0082] In reference to gate oxide (3), any high-k dielectric
material known in the art to be suitable to act as gate dielectric
can be implemented in the method of the disclosure. Any metal gate
material known in the art to be suitable to act as gate electrode
(4) can be implemented in the method of the disclosure. Both the
gate dielectric and the gate electrode can be formed by a conformal
deposition technique such as Atomic Layer Deposition (ALD) with or
without vacuum break in between.
[0083] In FIG. 5 the center of the structure having a larger band
gap is labeled with "A," while the lateral edges are labeled with
"B." It is important to note that this is a schematic
representation. In reality, there is no abrupt transition between
the band gap value in region/zone "A" of the semiconductor material
and the band gap value in region/zone "B" of the same semiconductor
material. The band gap variation is gradual and continuous,
increasing in a horizontal direction from the edge to the center of
the structure and in this way achieving an inverted `V"
profile.
[0084] In embodiments of the second aspect, the semiconductor
material further comprises dopants (5), which are introduced by
in-situ doping during the epitaxial growth. In alternative
embodiments the dopants are introduced by implantation after the
epitaxial growth.
[0085] FIG. 6 shows schematically a fabrication process according
to an embodiment of the disclosure, wherein dopants are introduced
during epitaxial growth. The step depicted in (201) shows epitaxial
growth of a semiconductor material with a horizontal gradient in
composition, region (5) represents doping introduced during
epitaxial growth. The doping has an abrupt profile intended for s/d
isolation (analogue with the functionality of the halo implants in
planar configurations). Preferentially, the doping is localized at
the bottom of the extended portion of the protruding structure as
shown in FIG. 6. The step (202) depicts CMP of the overgrown
semiconductor material. The step (203) illustrates the STI oxide
(2) etch back (recess) to expose the fin (1). The step (204) shows
formation of gate oxide (3) and gate electrode (4) around the fin
in a tri-gate architecture. The step (204') is an alternative to
step (204), which shows the formation of gate oxide and gate
electrode in a dual-gate architecture.
[0086] The semiconductor material of the protruding structure may
further comprise two or more layers, the layers being stacked
vertically. This embodiment is illustrated in FIG. 7.
[0087] FIG. 7 shows schematically a fabrication process according
to an embodiment of the disclosure wherein the fin comprises
multiple layers (stacked up vertically) having different
compositions, at least one of the layers having a horizontal
gradient in composition. Preferably, all the layers (e.g., in this
example both layers) have a horizontal gradient in composition.
Preferably, the layers are formed by epitaxial growth, in one
process step, without vacuum break, by adapting the type and/or the
amount of precursors present in the epitaxial reactor.
[0088] The following process steps are represented in FIG. 7. The
step (301) shows epitaxial growth of a first semiconductor material
having a horizontal gradient in composition, followed by the
epitaxial growth of second semiconductor material having a
horizontal gradient in composition. The horizontal gradient in
composition (and band gap) is illustrated by the regions "A" and
"B" of the first semiconductor material and, respectively, the
region "C" and "D" of the second semiconductor material. As in case
of FIGS. 5 and 6, the transition between the two regions is not
abrupt, instead the variation in composition and band gap is
gradual in a horizontal direction.
[0089] A CMP step (not shown) of the overgrown second semiconductor
material is performed. Further, STI oxide (3) etch back (recess)
(303) is performed to expose the fin (1), the exposed portion of
the fin is in this case made of the second semiconductor material,
which is having a horizontal gradient in composition. Subsequently,
formation (304) of gate oxide (3) and gate electrode (4) around the
fin in a tri-gate architecture is shown. Alternatively to (304),
formation of gate oxide and gate electrode in a dual-gate
architecture (304').
* * * * *