U.S. patent application number 14/197746 was filed with the patent office on 2015-09-10 for fabrication process for mitigating external resistance and interface state density in a multigate device.
This patent application is currently assigned to International Business Machines Corporation. The applicant listed for this patent is International Business Machines Corporation. Invention is credited to ANIRBAN BASU, Guy Cohen, Amlan Majumdar.
Application Number | 20150255568 14/197746 |
Document ID | / |
Family ID | 54018203 |
Filed Date | 2015-09-10 |
United States Patent
Application |
20150255568 |
Kind Code |
A1 |
BASU; ANIRBAN ; et
al. |
September 10, 2015 |
FABRICATION PROCESS FOR MITIGATING EXTERNAL RESISTANCE AND
INTERFACE STATE DENSITY IN A MULTIGATE DEVICE
Abstract
A method for fabricating a multigate device includes forming a
fin on a substrate of the multigate device, the fin being formed of
a semiconductor material, growing a first conformal epitaxial layer
directly on the fin and substrate, wherein the first conformal
epitaxial layer is undoped or lightly doped, growing a second
conformal epitaxial layer directly on the first conformal epitaxial
layer, wherein the second conformal epitaxial layer is highly
doped, selectively removing a portion of the second epitaxial layer
to expose a portion of the first conformal epitaxial layer and
thereby form a trench, and forming a gate within the trench.
Inventors: |
BASU; ANIRBAN; (Elmsford,
NY) ; Cohen; Guy; (Mohegan Lake, NY) ;
Majumdar; Amlan; (White Plains, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
54018203 |
Appl. No.: |
14/197746 |
Filed: |
March 5, 2014 |
Current U.S.
Class: |
257/192 ;
438/283 |
Current CPC
Class: |
H01L 29/66795 20130101;
H01L 29/785 20130101; H01L 29/205 20130101 |
International
Class: |
H01L 29/66 20060101
H01L029/66; H01L 29/205 20060101 H01L029/205; H01L 29/78 20060101
H01L029/78 |
Claims
1. A method for fabricating a multigate device, the method
comprising: forming a fin on a substrate of the multigate device,
the fin comprising a semiconductor material; growing a first
conformal epitaxial layer directly on the fin and substrate,
wherein the first conformal epitaxial layer is undoped or lightly
doped; growing a second conformal epitaxial layer directly on the
first conformal epitaxial layer, wherein the second conformal
epitaxial layer is highly doped; selectively removing a portion of
the second epitaxial layer to expose a portion of the first
conformal epitaxial layer and thereby form a trench; and forming a
gate within the trench.
2. The method of claim 1, wherein the semiconductor material is a
III-V compound.
3. The method of claim 2, wherein the III-V compound is indium
gallium arsenide.
4. The method of claim 1, wherein the first conformal epitaxial
layer is a cap layer.
5. The method of claim 4, wherein the first conformal epitaxial
layer has a thickness of approximately two to five nanometers.
6. The method of claim 4, wherein the first conformal epitaxial
layer comprises indium phosphide.
7. The method of claim 1, wherein the second conformal epitaxial
layer has a thickness of at least approximately twenty
nanometers.
8. The method of claim 1, wherein the second conformal epitaxial
layer comprises indium gallium arsenide.
9. The method of claim 1, wherein the second conformal epitaxial
layer is at least four times thicker than the first conformal
epitaxial layer.
10. The method of claim 1, further comprising: depositing an
insulator layer directly on the second conformal epitaxial layer,
prior to the selectively removing, wherein at least a portion of
the insulator layer is removed with the portion of the second
epitaxial layer.
11. The method of claim 1, wherein the trench is formed along
approximately a center axis of the multigate device and extends
from a first end of the multigate device to a second end of the
multigate device.
12. The method of claim 1, wherein the forming a gate comprises:
depositing a dielectric layer on the multigate device; forming a
dielectric spacer within the trench; depositing a gate metal layer
directly on the dielectric layer and the dielectric spacer,
including within the trench; and forming a source contact and a
drain contact directly on the portion of the second conformal
epitaxial layer.
13. A method for fabricating a multigate device, the method
comprising: forming a fin on a substrate of the multigate device,
the fin comprising a III-V compound; growing a first conformal
epitaxial layer comprising a first semiconductor material directly
on the fin and substrate, wherein the first conformal epitaxial
layer is undoped or lightly doped; growing a second conformal
epitaxial layer comprising a second semiconductor material directly
on the first conformal epitaxial layer, wherein the second
conformal epitaxial layer is doped more highly than the first
conformal epitaxial layer and is thicker than the first conformal
epitaxial layer; selectively removing a portion of the second
epitaxial layer to expose a portion of the first conformal
epitaxial layer and thereby form a trench; and forming a gate
within the trench.
14. The method of claim 13, wherein the first semiconductor
material comprises indium phosphide.
15. The method of claim 13, wherein the second semiconductor
material comprises indium gallium arsenide.
16. The method of claim 13, wherein the second conformal epitaxial
layer is at least four times thicker than the first conformal
epitaxial layer.
17. The method of claim 13, wherein the trench is formed along
approximately a center axis of the multigate device and extends
from a first end of the multigate device to a second end of the
multigate device.
18. A multigate device, comprising: a fin formed on a substrate,
the fin comprising a semiconductor material; a first conformal
epitaxial layer grown directly on the fin and substrate, wherein
the first conformal epitaxial layer is undoped or lightly doped; a
second conformal epitaxial layer grown directly on a portion of the
first conformal epitaxial layer, wherein the second conformal
epitaxial layer is highly doped; a trench formed in the second
conformal epitaxial layer; and a gate formed within the trench.
19. The multigate device of claim 18, wherein the semiconductor
material is a III-V compound.
20. The multigate device of claim 18, wherein the second conformal
epitaxial layer is at least four times thicker than the first
conformal epitaxial layer.
Description
FIELD OF THE DISCLOSURE
[0001] The present disclosure relates generally to multigate
devices and relates more specifically to fabrication processes for
lowering interface states of multigate devices.
BACKGROUND OF THE DISCLOSURE
[0002] A multigate device or multiple gate field effect transistor
(MuGFET) is a metal-oxide-semiconductor field effect transistor
(MOSFET) that incorporates more than one gate into a single
device.
[0003] One particular type of multigate device is the finFET, which
refers to a nonplanar, multi-gate transistor built on a
silicon-on-insulator (SOI) substrate and based on the earlier DELTA
(single-gate) transistor design. A distinguishing characteristic of
the finFET is a conducting channel in a thin silicon "fin," which
forms the body of the device.
[0004] Another type of multigate device is the tri-gate or
three-dimensional (3D) transistor (not to be confused with a 3D
microchip) fabrication used for the nonplanar transistor
architecture used in certain processors (e.g., processors based on
the 22 nanometer manufacturing process). Tri-gate transistors
employ a single gate stacked on top of two vertical gates, creating
additional surface area for carriers to travel.
[0005] The performance of finFET and tri-gate devices is severely
limited by high external resistance (R.sub.ext) and interface state
density (D.sub.it). R.sub.ext is difficult to solve in III-V types
of finFET and tri-gate devices, owing to the difficulty in forming
high-quality contacts due to limited thermal budget of processing.
D.sub.it is also difficult to solve in III-V types of finFET and
tri-gate devices, owing to a high concentration of interface states
at the oxide/semiconductor interface.
SUMMARY OF THE DISCLOSURE
[0006] A method for fabricating a multigate device includes forming
a fin on a substrate of the multigate device, the fin being formed
of a semiconductor material, growing a first conformal epitaxial
layer directly on the fin and substrate, wherein the first
conformal epitaxial layer is undoped, growing a second conformal
epitaxial layer directly on the first conformal epitaxial layer,
wherein the second conformal epitaxial layer is highly doped,
selectively removing a portion of the second epitaxial layer to
expose a portion of the first conformal epitaxial layer and thereby
form a trench, and forming a gate within the trench.
[0007] In another embodiment, a method for fabricating a multigate
device includes forming a fin on a substrate of the multigate
device, the fin comprising a III-V compound, growing a first
conformal epitaxial layer comprising a first semiconductor material
directly on the fin and substrate, wherein the first conformal
epitaxial layer is undoped, growing a second conformal epitaxial
layer comprising a second semiconductor material directly on the
first conformal epitaxial layer, wherein the second conformal
epitaxial layer is doped more highly than the first conformal
epitaxial layer and is thicker than the first conformal epitaxial
layer, selectively removing a portion of the second epitaxial layer
to expose a portion of the first conformal epitaxial layer and
thereby form a trench, and forming a gate within the trench.
[0008] In another embodiment, a multigate device includes a fin
formed on a substrate, the fin comprising a semiconductor material,
a first conformal epitaxial layer grown directly on the fin and
substrate, wherein the first conformal epitaxial layer is undoped,
a second conformal epitaxial layer grown directly on a portion of
the first conformal epitaxial layer, wherein the second conformal
epitaxial layer is highly doped, a trench formed in the second
conformal epitaxial layer, and a gate formed within the trench.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The teachings of the present disclosure can be readily
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0010] FIGS. 1A-1H illustrate top views of various steps for
fabricating a multigate device;
[0011] FIGS. 2A-2G illustrate cross sectional views corresponding
to the top views illustrated in FIGS. 1A-1H; and
[0012] FIGS. 3A-3H illustrate cross sectional views corresponding
to the top views illustrated in FIGS. 1A-1H.
[0013] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the Figures.
DETAILED DESCRIPTION
[0014] In one embodiment, the present invention is a method and
apparatus for fabricating a multigate device having a relatively
low external resistance (R.sub.ext) and relatively low interface
state density (D.sub.it). Embodiments of the invention grow two
different conformal epitaxial layers on the fins of a multigate
device (e.g., a finFET or trigate device). One of the epitaxial
layers is a highly doped layer, while the other epitaxial layer is
a thin cap layer grown to mitigate interface state density.
Subsequent processing steps leave the cap layer, but selectively
remove the highly doped layer from the device channel areas.
Portions of the highly doped layer remain in the source and drain
regions of the device.
[0015] FIGS. 1A-1H, 2A-2G, and 3A-3H are schematic diagrams
illustrating various steps for fabricating a multigate device 100,
according to the present invention. In particular, FIGS. 1A-1H
illustrate top views of the various steps for fabricating the
multigate device 100, while FIGS. 2A-2G, and 3A-3H illustrate cross
sectional views corresponding to the top views illustrated in FIGS.
1A-1H. Collectively, FIGS. 1A-1H, 2A-2G, and 3A-3H serve as a flow
diagram illustrating one embodiment of a fabrication process
according to the present invention.
[0016] As illustrated in FIGS. 1A, 2A, and 3A, the multigate device
100 includes a substrate 102 and a plurality of fins
104.sub.1-104.sub.n (hereinafter collectively referred to as "fins
104") deposited on the substrate 102. In one embodiment, the fins
104 comprise a semiconductor material (e.g., a III-V compound such
as indium gallium arsenide or the like). The fins 104 are spaced
apart from each other along the length of the substrate 102 and may
be defined by reactive ion etching (RIE) or a similar process. In
one embodiment, the substrate 102 is a semi-insulating
semiconductor such as indium phosphide (SI InP). In another
embodiment, the substrate 102 is merely used for support and has a
surface layer made of an insulator such as silicon dioxide
(SiO.sub.2). A single-crystal layer of InGaAs, from which the fins
104 are patterned, may be formed over the SiO.sub.2 layer by wafer
bonding and layer transfer.
[0017] As illustrated in FIGS. 1B, 2B, and 3B, two conformal
epitaxial layers are grown on the multigate device 100. In one
embodiment (e.g., where the substrate 102 comprises a
single-crystal semiconductor), the first epitaxial layer 106 is a
cap grown directly on the substrate 102 and fins 104. However, in
an alternative embodiment (e.g., where the substrate 102 comprises
an insulator top layer such as SiO.sub.2), depending on the
deposition method employed, the layer 106 will deposit only on
exposed semiconductor surfaces and will not be deposited on the
insulator top layer. Such selective deposition is typical of
deposition methods such as metal-organic chemical vapor deposition
(MOCVD) and metal-organic molecular beam epitaxy (MOMBE). In one
embodiment, the first epitaxial layer 106 comprises a semiconductor
material (e.g., indium phosphide (InP), indium aluminum arsenide
(InAlAs), or the like) and has a thickness of approximately two to
five nanometers. The second epitaxial layer 108 is grown directly
on the first epitaxial layer 106. In one embodiment, the second
epitaxial layer 108 comprises a semiconductor material (e.g.,
indium gallium arsenide or the like) and has a thickness of at
least approximately twenty nanometers in one embodiment. Thus, in
one embodiment, the second epitaxial layer 108 is at least four
times thicker than the first epitaxial layer 106. The first
epitaxial layer 106 is either undoped or lightly doped (e.g. less
than 10.sup.18 cm.sup.-3 doping density). The second epitaxial
layer 108 is highly doped (e.g., n doped with a doping density
exceeding 10.sup.19 cm.sup.-3). In the embodiment in which the fins
104 are formed over a surface layer of an insulator (such as
SiO.sub.2), the first epitaxial layer 106 and the second epitaxial
layer 108 are formed only over the fins 104; no deposition of these
layers occurs on the top surface of the substrate 102. As discussed
above, such selective deposition (i.e., where the semiconductor
material epitaxially deposits over only the semiconductor surfaces,
but not over the insulator surfaces) is typical of MOCVD and MOMBE
processes, for instance.
[0018] As illustrated in FIGS. 1C, 2C, and 3C, the multigate device
100 is isolated from adjacent devices (not shown in FIGS. 1C, 2C,
and 3C) by mesa isolation. This involves photolithography to cover
region 108 with photoresist and removal of the second epitaxial
layer 108 and the first epitaxial layer 106 to expose the substrate
102 in the regions not covered by photoresist.
[0019] As illustrated in FIGS. 1D, 2D, and 3D, an insulator layer
110 is next deposited on the multigate device 100, directly over
the second epitaxial layer 108 and the substrate 102. The insulator
layer 110 may comprise silicon dioxide, silicon nitride, aluminum
oxide (Al.sub.2O.sub.3), or the like. The insulator layer 110 may
be further planarized, for example using a process such as chemical
mechanical polishing (CMP). A trench 112 is then opened in the
insulator layer 110. The trench 112 removes portions of the second
epitaxial layer 108 and exposes portions of the first epitaxial
layer 106 and the substrate 102. As a result, the fins 104 are
conformally covered by the first epitaxial layer 106 on all
surfaces except for the interface between the fins 104 and the
substrate 102. In particular, the first epitaxial layer 106
passivates the fin surfaces to reduce interface state density. In
one embodiment, the trench 112 is formed along approximately a
center axis (T-T' in FIG. 1D) of the multigate device 100 and
extends from one end of the multigate device 100 to the other end
of the multigate device 100.
[0020] As illustrated in FIGS. 1E, 2E, and 3E a dielectric layer
114 is next deposited on the multigate device 100. Therefore, the
dielectric layer 114 is deposited inside the trench 112 and atop
the insulator layer 110. Inside the trench 112, the dielectric
layer 114 is deposited on the first epitaxial layer 106 and the
substrate 102. The dielectric layer 114 and the first epitaxial
layer 106 form the gate dielectric on the fins 104 of the multigate
device 100. The dielectric layer 114 may comprise a high-k
dielectric such as hafnium oxide (HfO.sub.2), aluminum oxide
(Al.sub.2O.sub.3), or the like. In one embodiment, a large bandgap
material that is epitaxially deposited over the first epitaxial
layer 106 may substitute for the dielectric layer 114. For
instance, where the substrate 102 and fins 104 are lattice matched
to InP, a large bandgap material such as zinc cadmium selenide
(ZnCdSe) or zinc cadmium magnesium selenide
(Zn.sub.xCd.sub.yMg.sub.1-x-ySe) may be used.
[0021] As illustrated in FIGS. 1F, 2F, and 3F, a nitride (e.g.,
silicon nitride) is next deposited in the trench 112 to form a
plurality of spacers 116. In particular, the nitride is deposited
over the surfaces of the fins 104 that are encapsulated with the
first epitaxial layer 106 and over the insulator layer 110. Some of
the nitride is next removed from the sidewalls of the fins 104. The
nitride may be removed using an etch process, such as reactive ion
etching. Residual nitride is left on the walls of the trench 112,
taking caution to ensure that the height of the insulator layer 110
remains greater than the height of the fins 104. The gate will be
self-aligned within the trench 112 as discussed in greater detail
below. The need for the spacers 116 will become clear below.
[0022] As illustrated in FIGS. 1G, 2G, and 3G, a metal gate 118 is
then formed on the multigate device 100 (e.g., directly on the
dielectric layer 114), including within the trench 112. In one
embodiment, the metal gate 118 can be formed by one of three
methods: (1) depositing a gate metal layer, patterning the shape of
the metal gate 118 using photolithography (wherein the shape of the
metal gate 118 is covered by photoresist), and then etching the
gate metal from areas not covered by photoresist (this would
typically result in a "T"-shaped gate); (2) patterning the shape of
the metal gate 118 using photolithography (wherein the shape of the
metal gate 118 is a window in photoresist), depositing a gate metal
layer, and then lifting off the gate metal from areas covered by
photoresist; or (3) depositing a gate metal layer, and using
chemical mechanical polishing (CMP) to removed the excess gate
material outside the trench 112 (this would be helpful when a tight
gate pitch is needed, since the metal gate 118 is made self-aligned
to the trench 112). As illustrated in FIG. 3G, the metal gate 118
is separated from the heavily-doped second epitaxial layer 108 by
the gate dielectric 114 and the spacers 116. Thus, the spacers 116
reduce the overlap capacitance between the metal gate 118 and the
heavily-doped second epitaxial layer 108, which leads to improved
high-speed device performance.
[0023] As illustrated in FIGS. 1H and 3H, source and drain (S/D)
contacts 120 are next formed on the highly doped epitaxial regions
of the multigate device 100. In one embodiment, the S/D contacts
120 are formed by patterning the shape of the S/D contacts 120
using photolithography (wherein the shape of S/D contacts 120) is a
window in photoresist, etching the dielectric layer 114 and the
insulator layer 110 to expose the heavily-doped second epitaxial
layer 108, depositing a S/D metal layer, and then lifting off the
S/D metal from areas covered by photoresist. In another embodiment,
source and drain (S/D) contacts 120 are made by first depositing a
blanket dielectric layer (not illustrated) over the wafer, making
an opening in the blanket dielectric layer and the insulator layer
110 to expose the second epitaxial layer 108, depositing a contact
metal 120 over the wafer, and using CMP to remove the excess metal
over the blanket dielectric layer.
[0024] The steps illustrated in 1A-1H, 2A-2G, and 3A-3H result in a
conformal, highly doped epitaxial growth on the fins 104 of the
multigate device 100. This lowers the contact resistance of the
multigate device. Thus, the disclosed fabrication process,
including the deposition and selective removal of at least one
epitaxial layer, lowers the external resistance and interface state
density of multigate devices with minimal processing
complexity.
[0025] Although various embodiments which incorporate the teachings
of the present invention have been shown and described in detail
herein, those skilled in the art can readily devise many other
varied embodiments that still incorporate these teachings.
* * * * *