U.S. patent application number 13/953551 was filed with the patent office on 2015-01-29 for method to induce strain in 3-d microfabricated structures.
This patent application is currently assigned to STMicroelectronics, Inc.. The applicant listed for this patent is STMicroelectronics, Inc.. Invention is credited to Nicolas Loubet, Pierre Morin.
Application Number | 20150028349 13/953551 |
Document ID | / |
Family ID | 52389744 |
Filed Date | 2015-01-29 |
United States Patent
Application |
20150028349 |
Kind Code |
A1 |
Loubet; Nicolas ; et
al. |
January 29, 2015 |
METHOD TO INDUCE STRAIN IN 3-D MICROFABRICATED STRUCTURES
Abstract
Methods and structures for forming strained-channel finFETs are
described. Fin structures for finFETs may be formed in two
epitaxial layers that are grown over a bulk substrate. A first thin
epitaxial layer may be cut and used to impart strain to an adjacent
channel region of the finFET via elastic relaxation. The structures
exhibit a preferred design range for increasing induced strain and
uniformity of the strain over the fin height.
Inventors: |
Loubet; Nicolas;
(Guilderland, NY) ; Morin; Pierre; (Albany,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
STMicroelectronics, Inc. |
Coppell |
TX |
US |
|
|
Assignee: |
STMicroelectronics, Inc.
Coppell
TX
|
Family ID: |
52389744 |
Appl. No.: |
13/953551 |
Filed: |
July 29, 2013 |
Current U.S.
Class: |
257/77 ; 257/190;
438/283 |
Current CPC
Class: |
H01L 29/7848 20130101;
H01L 29/1608 20130101; H01L 29/7851 20130101; H01L 29/165 20130101;
H01L 29/1054 20130101; H01L 29/161 20130101; H01L 29/785 20130101;
H01L 29/66795 20130101; H01L 29/16 20130101; H01L 29/155
20130101 |
Class at
Publication: |
257/77 ; 438/283;
257/190 |
International
Class: |
H01L 29/78 20060101
H01L029/78; H01L 29/66 20060101 H01L029/66 |
Claims
1. A method for making a strained three-dimensional feature
patterned on a substrate, the method comprising: forming a first
semiconductor layer in a strained state at a surface of a
substrate; forming a second semiconductor layer adjacent the first
semiconductor layer; patterning the three-dimensional feature in at
least the second semiconductor layer; and cutting the first
semiconductor layer in the vicinity of the patterned
three-dimensional feature to relieve strain in the first
semiconductor layer and induce strain in the patterned
three-dimensional feature.
2. The method of claim 1, wherein the thickness of the first
semiconductor layer is between approximately 10 nm and
approximately 60 nm.
3. The method of claim 2, wherein the thickness of the second
semiconductor layer is between approximately 10 nm and
approximately 60 nm.
4. The method of claim 1, wherein the first semiconductor layer is
SiGe or SiC.
5. The method of claim 4, further comprising forming the first
semiconductor layer with a gradient in Ge or C content in a
direction perpendicular to the surface of the substrate.
6. The method of claim 4, wherein the second semiconductor layer is
Si.
7. The method of claim 1, wherein the three-dimensional feature
comprises a fin for a finFET device.
8. The method of claim 7, wherein the cutting comprises etching a
pattern for the fin through the first semiconductor layer.
9. The method of claim 8, wherein 2.55.ltoreq.R.sub.sc.ltoreq.3.12
and R.sub.sc is defined as: R sc = L T si + T f ##EQU00002## where
L represents the length of the fin, T.sub.si represents the
thickness of the first semiconductor layer, and T.sub.f represents
the thickness of the second semiconductor layer.
10. The method of claim 9, wherein T.sub.si is a value between
approximately 10 nm and approximately 60 nm.
11. The method of claim 9, wherein the first semiconductor layer is
SiGe or SiC and the second semiconductor layer is Si.
12. The method of claim 9, further comprising forming a gate
structure for a finFET at a center of the fin in the second
semiconductor layer.
13. The method of claim 9, wherein forming the first semiconductor
layer comprises epitaxially growing the first semiconductor
layer.
14. The method of claim 9, wherein forming the second semiconductor
layer comprises epitaxially growing the second semiconductor
layer.
15. The method of claim 9, wherein patterning the three-dimensional
feature and cutting the first semiconductor layer comprise at least
one etching process.
16. The method of claim 9, wherein the fin is formed by a sidewall
image transfer process.
17. The method of claim 9, wherein the fin has a width between
approximately 5 nm and approximately 30 nm.
18. A strained-channel finFET structure formed on a substrate
comprising: a strain-inducing feature adjacent to a fin of the
finFET structure, wherein the strain-inducing feature is formed
from a first semiconductor material having a first strain; and the
fin formed from a second semiconductor material that is strained by
the strain-inducing feature.
19. The finFET structure of claim 18, wherein the first
semiconductor material is SiGe or SiC.
20. The finFET structure of claim 19, wherein a Ge or C content of
the SiGe or SiC is between approximately 10% and approximately
25%.
21. The finFET structure of claim 19, wherein a Ge or C content of
the SiGe or SiC is between approximately 25% and approximately
40%.
22. The finFET structure of claim 19, wherein the first
semiconductor material has a gradient in Ge or C concentration in a
direction perpendicular to an interfacial surface between the
strain-inducing feature and the fin.
23. The finFET structure of claim 18, wherein the second
semiconductor material is Si.
24. The finFET structure of claim 18, wherein the thickness of the
first semiconductor layer is between approximately 10 nm and
approximately 60 nm.
25. The finFET structure of claim 18, wherein the thickness of the
second semiconductor layer is between approximately 10 nm and
approximately 60 nm.
26. The finFET structure of claim 18, wherein the fin has a width
between approximately 5 nm and approximately 30 nm.
27. The finFET structure of claim 18, wherein
2.55.ltoreq.R.sub.sc.ltoreq.3.12 and R.sub.sc is defined as: R sc =
L T si + T f ##EQU00003## where L represents the length of the fin,
T.sub.si represents the thickness of the strain-inducing feature,
and T.sub.f represents the thickness of the fin.
28. The finFET structure of claim 27, further comprising a gate
structure formed at a center region of the fin.
29. The finFET structure of claim 28 disposed in a smart phone,
computer, tablet computer, PDA, or video display.
Description
BACKGROUND
[0001] 1. Technical Field
[0002] The technology relates to methods to induce strain in
three-dimensional microfabricated structures. As an example, a type
of strain (compressive or tensile) and amount of strain can be
selectively induced in finFET channel structures.
[0003] 2. Discussion of the Related Art
[0004] Transistors are fundamental device elements of modern
digital processors and memory devices, and have found numerous
applications in various areas of electronics, including power
electronics. Currently, there are a variety of transistor designs
or types that may be used for different applications. Various
transistor types include, for example, bipolar junction transistors
(BJT), junction field-effect transistors (JFET),
metal-oxide-semiconductor field-effect transistors (MOSFET),
vertical channel or trench field-effect transistors, and
superjunction or multi-drain transistors. One type of transistor
that has emerged within the MOSFET family of transistors is a fin
field-effect transistor (finFET).
[0005] An example of a finFET 100 is depicted in the perspective
view of FIG. 1A. A finFET may be fabricated on a bulk semiconductor
substrate 110, e.g., a silicon substrate, and comprise a fin-like
structure 115 that runs in a length direction along a surface of
the substrate and extends in a height direction normal to the
substrate surface. The fin 115 may have a narrow width, e.g., less
than 250 nanometers. There may be an insulating layer 105, e.g., an
oxide layer, on a surface of the substrate. The fin may pass
through the insulating layer 105, but be attached to the
semiconducting substrate 110 at a lower region of the fin. A gate
structure comprising a conductive gate material 130 (e.g.,
polysilicon) and gate insulator 135 (e.g., an oxide) may be formed
over a region of the fin. Upper portions of the fin may be doped on
either side of the gate structure to form a source region 120 and
drain region 140 adjacent to the gate.
[0006] FinFETs have favorable electrostatic properties for
complimentary MOS scaling to smaller sizes. Because the fin is a
three-dimensional structure, the transistor's channel can be formed
on three surfaces of the fin, so that the finFET can exhibit a high
current switching capability for a given surface area occupied on
substrate. Since the channel and device can be raised from the
substrate surface, there can be reduced electric field coupling
between adjacent devices as compared to conventional planer
MOSFETs.
SUMMARY
[0007] The described technology relates to methods for making
strained microstructures, such as strained-channel finFETs, and to
related structures. According to some embodiments, a first
strain-inducing layer comprising a first material may be deposited
on a substrate in a strained state. The strain-inducing layer may
comprise SiGe deposited on a Si substrate, in some embodiments. A
second layer of material (e.g., Si) may be deposited adjacent the
strain-inducing layer, and a feature (e.g., a fin of a finFET) may
be patterned in the second layer. The adjacent second layer may be
in direct physical contact with the strain-inducing layer in some
embodiments, or may be separated from the strain-inducing layer by
a thin layer of material in some embodiments. The strain-inducing
layer may be cut or etched in the vicinity of the patterned feature
to relieve strain in the strain-inducing layer, and to impart
strain to the adjacent second layer and its patterned feature. With
regard to a finFET structure, the strain-inducing layer may be thin
(e.g., between approximately 10 nm and 60 nm in some embodiments)
such that strain in the layer is relieved by elastic relaxation
rather than plastic relaxation, so that defects are not generated
at unacceptable levels.
[0008] According to some embodiments, a method for making a
strained three-dimensional feature on a substrate comprises forming
a first semiconductor layer in a strained state at a surface of a
substrate, and forming a second semiconductor layer adjacent the
first semiconductor layer. The method may further include
patterning the three-dimensional feature in at least the second
semiconductor layer, and cutting the first semiconductor layer in
the vicinity of the patterned three-dimensional feature to relieve
strain in the first semiconductor layer and induce strain in the
patterned three-dimensional feature. The cutting may comprise
etching the first semiconductor layer, and the patterned feature
may be a fin of a finFET.
[0009] Structures related to the methods are also contemplated. In
some embodiments, a strained-channel finFET structure formed on a
substrate using methods described herein comprises a
strain-inducing feature adjacent to a fin of the finFET structure,
wherein the strain-inducing feature is formed from a first
semiconductor material having a first strain. The strained-channel
finFET may further comprise the fin formed from a second
semiconductor material that is strained by the strain-inducing
feature. In some embodiments, the strain-inducing feature is in the
shape of the fin, and abuts the fin.
[0010] The foregoing and other aspects, embodiments, and features
of the present teachings can be more fully understood from the
following description in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The skilled artisan will understand that the figures,
described herein, are for illustration purposes only. It is to be
understood that in some instances various aspects of the
embodiments may be shown exaggerated or enlarged to facilitate an
understanding of the embodiments. In the drawings, like reference
characters generally refer to like features, functionally similar
and/or structurally similar elements throughout the various
figures. The drawings are not necessarily to scale, emphasis
instead being placed upon illustrating the principles of the
teachings. Where the drawings relate to microfabrication of
integrated devices, only one device may be shown of a large
plurality of devices that may be fabricated in parallel. The
drawings are not intended to limit the scope of the present
teachings in any way.
[0012] FIG. 1A is a perspective view of a finFET formed on a bulk
substrate;
[0013] FIG. 1B is a perspective view of a strained-channel finFET,
according to some embodiments;
[0014] FIGS. 2A-2H depict process steps that may be used to form
strained fin structures, e.g., via a sidewall image transfer
process, according to some embodiments;
[0015] FIG. 3 is a perspective view of a strained fin structure,
according to some embodiments;
[0016] FIG. 4A shows plots of longitudinal stress computed along
the length of a strained fin structure, starting from the fin's
center, for eight fin lengths, wherein the strain-inducing layer
has a thickness of 40 nm, and the fin layer has a thickness of 20
nm;
[0017] FIGS. 4B-4C show plots of longitudinal stress computed along
the height of a strained fin structure, at its center, for the
eight fin lengths and conditions of FIG. 4A;
[0018] FIG. 4D shows plots of longitudinal stress computed along
the length of a strained fin structure, starting from the fin's
center, for nine fin lengths wherein the strain-inducing layer has
a thickness of 40 nm, and the fin layer has a thickness of 30 nm;
and
[0019] FIGS. 4E-4F show plots of longitudinal stress computed along
the height of a strained fin structure, at its center, for the nine
fin lengths and conditions of FIG. 4D.
[0020] The features and advantages of the embodiments will become
more apparent from the detailed description set forth below when
taken in conjunction with the drawings.
DETAILED DESCRIPTION
[0021] As noted above, finFETs exhibit favorable current-to-size
switching capabilities for integrated circuits, and finFETs like
those shown in FIG. 1A have been fabricated in high density on bulk
silicon (Si) substrates. FinFETs also exhibit favorable
electrostatic properties for scaling to high-density, low-power,
integrated circuits. Because the fin and channel are raised from
the substrate, the devices can exhibit reduced cross-coupling
between proximal devices.
[0022] In some cases, speed, junction leakage current, and/or
breakdown voltage considerations may create a need for
semiconductor material other than silicon. For example, SiGe can
exhibit higher mobilities for electrons and holes, higher device
speed, and lower junction leakage than bulk Si. As a result, some
devices may be fabricated from SiGe that is epitaxially grown on a
silicon substrate. However, conventional epitaxial growth of SiGe
for forming integrated devices has some attributes that may not be
favorable for certain applications. For example, because of a
lattice constant mismatch between SiGe and Si, strain is induced in
the SiGe as it is epitaxially grown. In some cases, the strain
causes dislocations or defects in the SiGe during its growth, which
can adversely affect device performance to unacceptable levels. To
mitigate effects of strain, a thick epitaxial layer of SiGe may be
grown so that the strain is relieved over an appreciable distance.
Depending upon the dopant concentration, SiGe epitaxial layers 1-10
microns thick may be necessary, and plastic relaxation may occur in
the material causing defects. Such an approach may require long and
complex epitaxy steps (e.g., it may be necessary to vary dopant
concentration during the epitaxial growth), and may further require
a thermal annealing step and chemical-mechanical polishing step to
planarize a surface of the SiGe after its growth. The annealing may
reduce some defects generated during epitaxial growth of SiGe, but
typically the defects may not be reduced below 10.sup.5
defects/cm.sup.2, a level not suitable for many industrial
applications.
[0023] Straining of silicon can be used to improve some of its
electrical properties. For example compressive straining of silicon
can improve the hole mobility within silicon. Tensile straining of
Si can improve electron mobility. The inventors have recognized and
appreciated that inducing strain in silicon can be used to
fabricate integrated electronic devices with performance that is
competitive with SiGe devices. For smaller devices, e.g., sub-20-nm
channel-width FETs such as finFETs, the avoidance of thick (>1
micron) SiGe epitaxial layers and increased device performance from
strain may be important factors in the manufacturability of the
strained devices. As an example of inducing strain in an
integrated, three-dimensional device, described below are methods
for making strained-Si-channel finFET devices. Although the
embodiments are directed to a strained-channel Si finFET, the
method of inducing strain may be extended to other devices or
structures, in which other materials may be used. A
strained-channel finFET 102 may appear as depicted in FIG. 1B. The
structure may be similar to that shown in FIG. 1A except that a
portion of the fin structure 115 includes a strain-inducing layer
220. The strain inducing layer may comprise a material or alloy
different than the material or alloy of the device layer 210 where
the device's active region is located. For example, in a Si-based
device, the device layer 210 may be formed from bulk and/or
epitaxial silicon. The strain inducing layer 220 may be formed from
SiGe in some embodiments (e.g., to induce tensile stress in the fin
and improve electron mobility in the device 102), or SiC in other
embodiments (e.g., to induce compressive stress in the fin and
improve hole mobility in the device. As may be appreciated, other
materials exhibiting a lattice mismatch with the substrate and
device layer may be used instead of SiGe or SiC, and different
material systems may be used in other implementations.
[0024] FIGS. 2A-2G depict process steps that may be used to
fabricate strained-channel finFET devices beginning with a bulk
semiconductor substrate. In overview, the active regions of the
fins 115 may be formed from epitaxially-grown semiconductor of high
quality (e.g., epitaxial Si in the example). The fins may be
epitaxially grown on a thin strain-inducing layer 220 of a second
material type, which may be epitaxially grown on a substrate 110.
The strain-inducing layer 220, because of a lattice mismatch with
the substrate, will epitaxially grow in a strained state. When the
fin and underlying straining material are cut via etching, the
strain-inducing layer can locally and elastically relax to relieve
its strain. This elastic relaxation can impart strain to the
adjacent fin. Additionally, since the formation of the
strain-inducing layer and relaxation of that layer can be purely
elastic, there may be no appreciable defects generated in the
strain-inducing layer and/or the device layer, as would be
generated from thicker, plastic SiGe layers that may suffer from
plastic deformation and relaxation, for example. Further details
regarding device fabrication are described below.
[0025] According to some embodiments, a process for forming a
strained-channel finFET may begin with a substrate 110 of a first
semiconductor material upon which an insulator 105 may be formed,
as depicted in FIG. 2A. The insulator may be a pad oxide in some
embodiments, and may be formed by a thermal oxidation process or a
physical or chemical deposition process. The substrate may be a
semiconducting substrate (e.g., a bulk Si substrate), though other
semiconducting materials may be used in other embodiments. The
insulator may comprise SiO.sub.2 or Si.sub.3N.sub.4, though any
suitable insulator may be used. The thickness of the insulator may
be any value between approximately 5 nanometers (nm) and
approximately 2 microns (.mu.m). The terms "approximately" and
"about" may be used to mean within .+-.20% of a target dimension in
some embodiments, within .+-.10% of a target dimension in some
embodiments, within .+-.5% of a target dimension in some
embodiments, and yet within .+-.2% of a target dimension in some
embodiments. The terms "approximately" and "about" may include the
target dimension.
[0026] With reference to FIG. 2B, at a region where fins for
finFETs are to be formed, the insulator 105 may be removed from the
substrate surface (e.g., by a photolithography and etching process)
to open wells in the insulator. In some embodiments, the insulator
105 may be removed from the entire surface of the substrate 110. In
other embodiments, instead of depositing an insulator 105 and
opening wells, a pad or field oxide may be formed around regions
that include finFET transistors.
[0027] Where the substrate surface is exposed, a second
semiconductor material (e.g., SiGe or SiC) may be epitaxially grown
over the surface to form a strain-inducing layer 220. Subsequently,
a device layer 210 of the first semiconductor material may be
epitaxially grown over the second semiconductor material. Because
of the epitaxial growth of the strain-inducing layer 220 and the
device layer 210, the crystalline quality at the device layer 210
is high. For example, it may have a defect density less than
10.sup.5 defects/cm.sup.2in some embodiments, less than 10.sup.4
defects/cm.sup.2 in some embodiments, less than 10.sup.3
defects/cm.sup.2 in some embodiments, and yet less than less than
10.sup.2 defects/cm.sup.2 in some embodiments. According to some
embodiments, the device layer 210 forms a layer in which fins for
strained-channel finFETs may be formed. In some embodiments, the
semiconductor material of the device layer 210 may be different
than the semiconductor material of the substrate.
[0028] The thickness of the device layer 210 may be between 5 nm
and 100 nm in some embodiments, between 10 nm and 60 nm in some
embodiments, and in some embodiments may between about 15 nm and 30
nm. The thickness of the strain-inducing layer 220 may be between 5
nm and 100 nm in some embodiments, between 10 nm and 60 nm in some
embodiments, and in some embodiments may be between 30 nm and 50
nm.
[0029] The fins may be patterned by a sidewall image transfer (SIT)
process that is depicted by steps illustrated in FIGS. 2C-2F. For
this process, additional layers may be deposited over the epitaxial
layers, as depicted in FIG. 2C. A hard mask layer 230 may be
deposited over the device layer 210. The hard mask layer may
comprise silicon nitride (e.g., Si.sub.3N.sub.4) in some
embodiments, though any suitable hard mask material may be used
that exhibits etch selectivity over the underlying semiconductor
materials. The thickness of the hard mask material may be between
10 nm and 100 nm in some embodiments, between 20 nm and 50 nm in
some embodiments, and in some embodiments may be about 40 nm.
[0030] On the hard mask layer 230 an insulating layer 240 may be
formed. The insulating layer may comprise undoped silicate glass
(USG), and its thickness may be between 10 nm and 100 nm in some
embodiments, between 20 nm and 50 nm in some embodiments, and in
some embodiments may be about 30 nm. In some embodiments, the
insulating layer 240 may be applied by any suitable means, e.g.,
via physical deposition, a plasma deposition process, or a spin-on
and anneal process.
[0031] A patterning layer 250 may be deposited over the insulating
layer 240. In some embodiments, the patterning layer 250 may
comprise amorphous silicon that is deposited by a plasma deposition
process, though any suitable material may be used. The patterning
layer 250 may be between 10 nm and 100 nm in some embodiments,
between 20 nm and 50 nm in some embodiments, and in some
embodiments may be about 40 nm.
[0032] Referring to FIG. 2D, bar-like structures 252 may be
patterned in the patterning layer 250 using any suitable method,
e.g., photolithography and etching. The photolithography may
require forming a photoresist layer over the patterning layer 250,
exposing and developing the photoresist, and etching the patterning
layer. In some embodiments, the bar-like structures may be
patterned using a mandrel lithography process. In some
implementations, the bar-like structures may be patterned using
interferometric lithography techniques. The bar-like structures 252
may be patterned to extend for a length L (into the page) that is a
desired length for a fin of a finFET transistor. The width W and
spacing S of the bar-like structures may be chosen to provide
desired spacings between multiple fins of a finFET device or
between multiple finFET devices.
[0033] A blanket masking layer (not shown) may be deposited over
the bar-like structures 252 and insulating layer 240. In some
embodiments, the blanket masking layer may comprise silicon nitride
that is deposited by a plasma deposition process. The thickness of
the masking layer may be between 50 nm and 100 nm in some
embodiments, between 5 nm and 50 nm in some embodiments, and in
some embodiments may be between about 5 nm and about 20 nm. The
blanket masking layer may be etched away to form spacer structures
232, as depicted in FIG. 2D. A series of etching steps may then be
used to pattern fins in the first semiconductor layer 210, where
the spacer structures 232 define the pattern of the fins.
[0034] For example, a first selective, anisotropic etch may be
performed to remove the bar-like structures 252. The same etch
recipe, or a different etch recipe may be used to remove most of
the insulating layer 240, thereby transferring the pattern of the
spacer structures 232 to the insulating layer. The resulting
structure may appear as depicted in FIG. 2E. A second selective,
anisotropic etch may be performed to remove portions of the hard
mask layer 230, thereby transferring the pattern from the
insulating layer to the hard mask layer 230. If the hard mask layer
230 and spacer structures 232 are formed of the same material, most
or all of the spacer structures may be removed during the etch, and
the patterned insulator serves as an etch mask for the hard mask
layer 230. Additional selective, anisotropic etches may be
performed to transfer the pattern from the hard mask layer to the
underlying device layer 210 and strain-inducing layer 220. The
resulting structure may be as depicted in FIG. 2F. In some
embodiments, the additional etching may proceed into the substrate
110 between 5 nm and 50 nm. In some implementations, the additional
etching may stop at approximately the surface of the substrate 110.
The upper insulating material 240 may be removed from the substrate
(e.g., using a wet or dry etch) leaving exposed hard mask features
234, as depicted in FIG. 2G. The hard mask features 234 may then be
removed, e.g., by a wet or dry etch, yielding a structure as
depicted in FIG. 2H.
[0035] As noted above, the strain-inducing layer 220 forms in a
strained state during its epitaxial growth due to a lattice
mismatch between the material used for the strain-inducing layer
and the substrate. As an example, a SiGe strain-inducing layer will
form with compressive strain when grown on a bulk Si substrate. The
amount of strain in the SiGe layer can be controlled to some extent
by controlling the Si:Ge ratio. The combination of etching the fin
structures 115 through the strain-inducing layer and removing the
hard mask feature 234 allows the strain-inducing material to relax
locally at each fin and relieve some of its strain. In doing so,
the strain-inducing layer imparts strain (tensile in this case) to
the adjacent fin formed in the device layer 210 where the channel
of a finFET will be formed. Because of the narrow and long fin
structures, the resulting strain in the device fins will be
substantially uniaxial, longitudinal strain along the length of the
fin. According to some embodiments, relaxation of the
strain-inducing layer may be purely elastic, such that no
appreciable defects are generated.
[0036] To investigate the induced strain imparted to the fin,
numerical simulations based on finite element analysis were carried
out. These computations show that the amount of strain in the
device fin and its uniformity can be controlled through several
device design parameters. Among the controlling parameters are
lattice mismatch between the strain-inducing layer and substrate
(controllable through choice of materials and/or dopant
concentrations), thickness or height of the strain-inducing layer,
thickness or height of the fin, and length of the fin.
[0037] FIG. 3 shows a perspective view of a fin structure in which
longitudinal stress was numerically analyzed. The longitudinal
stress S.sub.yy was computed along a length L of the fin from its
center, and along a height H at the fin center. According to some
embodiments, an active region or channel of a finFET would be
located at the center of the fin structure in the upper device
layer 210. Two sets of simulations were run. In both sets, the
thickness or height of the strain-inducing layer 220 was selected
to be 40 nm, and the concentration of Ge in the SiGe
strain-inducing layer was about 25%. In the first set of
simulations, the thickness of the device layer and active fin was
selected to be 20 nm. In the second set of simulations, the
thickness of the active fin was selected to be 30 nm. Eight fin
lengths were trialed in the first set of simulations: 80 nm, 120
nm, 140 nm, 160 nm, 180 nm, 200 nm, 220 nm, and 240 nm. Nine fin
lengths were trialed in the second set of simulations: 100 nm, 120
nm, 160 nm, 180 nm, 200 nm, 220 nm, 240 nm, 260 nm, and 300 nm.
Results from the simulations are shown in FIGS. 4A-4F.
[0038] FIG. 4A plots longitudinal stress values S.sub.yy (in
gigaPascals) as computed along a length of the active fin (starting
from the center (L=0 nm) and top (H=60 nm) of the fin structure)
for the eight different fin lengths. The results show a tensile
stress at the fin center that falls to zero. The stress reverses to
compressive stress, and then falls to zero again within about 20 nm
from the end of the fins. For the materials (Si/SiGe), dopant
concentration (25% Ge), strain-inducing layer thickness (40 nm),
device layer thickness (20 nm) and channel lengths trialed, the
maximal stress was found to be about (600 .+-.50) MPa for fin
lengths between about 120 nm and about 220 nm. These results
indicate that there is a preferred length range of the fin for
obtaining a high value of stress in the channel region. Increasing
the fin length beyond this point, even though adding more strained
material to the fin structure, can actually reduce the induced
stress at the center of the fin.
[0039] FIGS. 4B-4C plot longitudinal stress values S.sub.yy (in
gigaPascals) as computed along a height of the fin structure
(starting at the center (L=0 nm) from a lower surface (H=0 nm) of
the strain-inducing layer) for the eight different fin lengths.
FIG. 4B shows the stress variations with height from 10 nm into the
substrate 110, through the strain-inducing layer 220, and through
the device layer 210. This plot indicates that compressive stress
in the strain-inducing (SiGe) layer induces tensile stress in the
adjacent materials. FIG. 4C is a replot of the longitudinal stress
in the active fin portion of FIG. 4B (from H=40 nm to H=60 nm). The
results of FIG. 4C indicate that the longitudinal stress is
substantially uniform along the active fin height for a range of
fin length values. Interestingly, the range of length values for
uniformity of stress as a function of height substantially
coincides with a range of fin length values where the induced
longitudinal stress is maximized at the center of the fin. (Compare
with FIG. 4A.) According to some embodiments, the observed
uniformity of stress values as a function of height substantially
coincides with a range of fin length values where the induced
longitudinal stress is improved at the center of the fin. Such
results are beneficial for a finFET where the gate may extend from
the bottom of the fin to its top. The results of FIGS. 4A-4C
suggest that for the selected parameters, a fin length in a range
of about 160 nm to about 180 nm provides increased stress in the
finFET's channel region and high uniformity of stress as a function
of height in the channel region.
[0040] FIGS. 4D-4F show results for the second set of simulations
in which the active fin height was set at 30 nm. This second set of
results suggest that for the selected parameters, a fin length in a
range of about 180 nm to about 220 nm provides increased stress in
the finFET's channel region and high uniformity of stress as a
function of height in the channel region.
[0041] Based upon the results of FIGS. 4A-4F, a dimensionless
design parameter R.sub.sc can be defined for strained-channel
finFETs fabricated according to embodiments described above:
R sc = L T si + T f ( 1 ) ##EQU00001##
where L represents the fin length, T.sub.si represents the
thickness of the strain-inducing layer, and T.sub.f represents the
thickness of the device layer. According to some embodiments, a
design parameter R.sub.sc for strained-channel finFETs may be
within a range given approximately by
2.55.ltoreq.R.sub.sc.ltoreq.3.12. According to some embodiments, a
design parameter R.sub.sc for strained-channel finFETs may be
within a range given approximately by
2.65.ltoreq.R.sub.sc.ltoreq.3.03.
[0042] The results shown in FIGS. 4A-4F were obtained from samples
with uniform doping (25% Ge) in the strain-inducing layer 220 of 40
nm thickness. In some embodiments, it may be possible to increase
the Ge content (or C content for a SiC strain inducing layer)
and/or increase the thickness of the strain-inducing layer before
creating an unacceptable defect density. In some embodiments, a
gradient in Ge (or C) content may be used to increase the thickness
of the strain-inducing layer without creating an unacceptable
defect density. The gradient in Ge (or C) may be in the direction
of epitaxial growth, e.g., in a direction perpendicular to an
interfacial surface between the strain-inducing layer 220 and the
strained or device layer 210. In some embodiments, the Ge (or C)
content may be between about 10% and about 25% within the
strain-inducing layer. In some embodiments, the Ge (or C) content
may be between about 25% and about 40% within the strain-inducing
layer.
[0043] The fins shown in the drawings may be spaced laterally from
each other on one or more regular spacing intervals. For example,
there may be a uniform lateral spacing d.sub.1 between all fins.
Alternatively, there may be two uniform lateral spacings d.sub.1,
d.sub.2 alternating between successive fins. The fins may have a
width between approximately 5 nm and approximately 30 nm. The fins
may be spaced apart between approximately 10 nm and approximately
50 nm, in some embodiments. There may be one or more fins per
finFET device. A gate structure, like that shown in FIG. 1B may be
formed over one or more strained fins to form a finFET. For
example, there may be a common gate shared by multiple strained
fins.
[0044] A finFET device fabricated according to the present
teachings may be formed in an integrated circuit in large numbers
and/or at high densities. The circuits may be used for various
low-power applications, including but not limited to, circuits for
operating smart phones, computers, tablets, PDA's, video displays,
and other consumer electronics. For example, a plurality of finFETs
fabricated in accordance with the disclosed embodiments may be
incorporated in processor or control circuitry used to operate one
of the aforementioned devices.
[0045] The discussion above is directed primarily to a SiGe
strain-inducing layer that imparts tensile stress to a fin of a
finFET device. Accordingly, for a Si finFET, the use of SiGe for
the strain-inducing layer may improve the electron mobility for
n-channel finFETs. For p-channel finFETs, SiC may be used as the
strain-inducing layer. SiC can impart compressive stress to a fin.
In alternative embodiments for which an active fin and channel may
be formed in SiGe, the materials may be reversed. For example, Si
may be epitaxially grown on a SiGe substrate or base layer to form
a strain-inducing layer of Si. The device layer may then be
SiGe.
[0046] In alternative embodiments, the cutting of the
strain-inducing layer need not be at the same time and/or of the
same shape as the patterning of the adjacent device layer. In the
above example of a strained-channel finFET, a device may be
patterned in the device layer 210 in a first step. At later time,
the strain-inducing layer 220 may be etched in the vicinity of the
device fin, so as to cut the strain-inducing layer and induce
strain in the device fin. The etched pattern in the strain-inducing
layer may be different from the pattern used to etch the device
fin.
[0047] The techniques may be applied to other types of finFETs,
e.g., fully insulated finFETs, and other microfabricated devices
and structures such as MEMs devices. In some embodiments, the
techniques may be used in LEDs or laser diodes to strain the device
and adjust emission wavelength. According to some embodiments, a
strain-inducing layer may be used to impart stress to any
three-dimensional structure patterned into a device layer that has
been formed adjacent the strain-inducing layer. For example, a
three-dimensional device or structure may be patterned in the
device layer, and the strain-inducing layer may be cut (e.g.,
etched) in the vicinity of the patterned device or structure to
relieve strain in the strain-inducing layer and impart strain to
the adjacent patterned feature.
[0048] In some embodiments, the strain-inducing layer and/or
adjacent device layer may comprise a material other than
semiconductor material, e.g., crystalline insulator, an oxide, a
ceramic, etc. In some embodiments, the strain-inducing layer and/or
device layer may be formed by methods other than epitaxial growth,
e.g., plasma deposition, plasma deposition and annealing,
sputtering, etc. According to some embodiments, a thin insulating
layer may be formed between the strain-inducing layer and the
device layer where the fin is formed. In some embodiments, at least
a portion of the strain-inducing layer may comprise an active
region or portion of the formed device.
[0049] The technology described herein may be embodied as a method,
of which at least one example has been provided. The acts performed
as part of the method may be ordered in any suitable way.
Accordingly, embodiments may be constructed in which acts are
performed in an order different than illustrated, which may include
performing some acts simultaneously, even though shown as
sequential acts in illustrative embodiments. Additionally, a method
may include more acts than those illustrated, in some embodiments,
and fewer acts than those illustrated in other embodiments.
[0050] Having thus described at least one illustrative embodiment
of the invention, various alterations, modifications, and
improvements will readily occur to those skilled in the art. Such
alterations, modifications, and improvements are intended to be
within the spirit and scope of the invention. Accordingly, the
foregoing description is by way of example only and is not intended
as limiting. The invention is limited only as defined in the
following claims and the equivalents thereto.
* * * * *