U.S. patent application number 11/416423 was filed with the patent office on 2006-09-07 for methods for forming structures including strained-semiconductor-on-insulator devices.
This patent application is currently assigned to AmberWave Systems Corporation. Invention is credited to Glyn Braithwaite, Matthew T. Currie, Eugene A. Fitzgerald, Richard Hammond, Thomas A. Langdo, Anthony J. Lochtefeld.
Application Number | 20060197126 11/416423 |
Document ID | / |
Family ID | 31721737 |
Filed Date | 2006-09-07 |
United States Patent
Application |
20060197126 |
Kind Code |
A1 |
Lochtefeld; Anthony J. ; et
al. |
September 7, 2006 |
Methods for forming structures including
strained-semiconductor-on-insulator devices
Abstract
The benefits of strained semiconductors are combined with
silicon-on-insulator approaches to substrate and device
fabrication.
Inventors: |
Lochtefeld; Anthony J.;
(Somerville, MA) ; Langdo; Thomas A.; (Cambridge,
MA) ; Hammond; Richard; (Harriseahead, GB) ;
Currie; Matthew T.; (Brookline, MA) ; Braithwaite;
Glyn; (Hooksett, NH) ; Fitzgerald; Eugene A.;
(Windham, NH) |
Correspondence
Address: |
GOODWIN PROCTER LLP;PATENT ADMINISTRATOR
EXCHANGE PLACE
BOSTON
MA
02109-2881
US
|
Assignee: |
AmberWave Systems
Corporation
Salem
NH
|
Family ID: |
31721737 |
Appl. No.: |
11/416423 |
Filed: |
May 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10456708 |
Jun 6, 2003 |
7074623 |
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11416423 |
May 2, 2006 |
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60386968 |
Jun 7, 2002 |
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60404058 |
Aug 15, 2002 |
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60416000 |
Oct 4, 2002 |
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Current U.S.
Class: |
257/288 ;
257/E21.415; 257/E21.448; 257/E21.564; 257/E21.568; 257/E21.57;
257/E21.703; 257/E27.112; 257/E29.295; 257/E29.297; 257/E29.298;
438/197 |
Current CPC
Class: |
H01L 29/66795 20130101;
H01L 29/78603 20130101; H01L 21/84 20130101; H01L 29/7842 20130101;
H01L 29/78684 20130101; H01L 29/66772 20130101; H01L 21/76259
20130101; H01L 21/76264 20130101; H01L 29/78687 20130101; H01L
29/785 20130101; H01L 29/66916 20130101; H01L 21/76275 20130101;
H01L 27/1203 20130101; H01L 21/76254 20130101 |
Class at
Publication: |
257/288 ;
438/197 |
International
Class: |
H01L 21/8234 20060101
H01L021/8234; H01L 29/76 20060101 H01L029/76; H01L 31/00 20060101
H01L031/00 |
Claims
1-58. (canceled)
59. A method for forming a structure, the method comprising:
providing a substrate having a strained semiconductor layer
disposed over a substrate dielectric layer; forming a transistor in
the strained layer by forming a gate dielectric layer above a
portion of the strained semiconductor layer, forming a gate contact
above the gate dielectric layer, and forming a source region and a
drain region in a portion of the strained semiconductor layer,
proximate the gate dielectric layer; removing a portion of the
strained layer and the substrate dielectric layer to expose a
portion of the substrate; defining a collector in the exposed
portion of the substrate; forming a base over the collector; and
forming an emitter over the base.
60. The method of claim 59, wherein the strained semiconductor
layer is tensilely strained.
61. The method of claim 60, wherein the strained semiconductor
layer comprises tensilely strained silicon.
62. The method of claim 59, wherein the strained semiconductor
layer is compressively strained.
63. The method of claim 62, wherein the strained semiconductor
layer comprises compressively strained germanium.
64. The method of claim 59, further comprising defining a bipolar
transistor, the bipolar transistor comprising the collector, the
base, and the emitter.
65. The method of claim 64, further comprising interconnecting the
bipolar transistor and the transistor.
66. The method of claim 59, wherein the base is disposed in contact
with the collector and over the exposed portion of the
substrate.
67. The method of claim 59, wherein the strained semiconductor
layer is in contact with the substrate dielectric layer.
68. The method of claim 59, wherein the strained semiconductor
layer comprises at least one of a group II, a group III, a group
IV, a group V, and a group VI element.
69. The method of claim 67, wherein the strained semiconductor
layer comprises at least one of gallium arsenide, indium phosphide,
and zinc selenide.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application 60/386,968 filed Jun. 7, 2002, U.S. Provisional
Application 60/404,058 filed Aug. 15, 2002, and U.S. Provisional
Application 60/416,000 filed Oct. 4, 2002; the entire disclosures
of these three provisional applications are hereby incorporated by
reference.
FIELD OF THE INVENTION
[0002] This invention relates to devices and structures comprising
strained semiconductor layers and insulator layers.
BACKGROUND
[0003] Strained silicon-on-insulator structures for semiconductor
devices combine the benefits of two advanced approaches to
performance enhancement: silicon-on-insulator (SOI) technology and
strained silicon (Si) technology. The strained silicon-on-insulator
configuration offers various advantages associated with the
insulating substrate, such as reduced parasitic capacitances and
improved isolation. Strained Si provides improved carrier
mobilities. Devices such as strained Si metal-oxide-semiconductor
field-effect transistors (MOSFETs) combine enhanced carrier
mobilities with the advantages of insulating substrates.
[0004] Strained-silicon-on-insulator substrates are typically
fabricated as follows. First, a relaxed silicon-germanium (SiGe)
layer is formed on an insulator by one of several techniques such
as separation by implantation of oxygen (SIMOX), wafer bonding and
etch back; wafer bonding and hydrogen exfoliation layer transfer;
or recrystallization of amorphous material. Then, a strained Si
layer is epitaxially grown to form a strained-silicon-on-insulator
structure, with strained Si disposed over SiGe. The
relaxed-SiGe-on-insulator layer serves as the template for inducing
strain in the Si layer. This induced strain is typically greater
than 10-3.
[0005] This structure has limitations. It is not conducive to the
production of fully-depleted strained-semiconductor-on-insulator
devices in which the layer over the insulating material must be
thin enough [<300 angstroms (.ANG.)] to allow for full depletion
of the layer during device operation. Fully depleted transistors
may be the favored version of SOI for MOSFET technologies beyond
the 90 nm technology node. The relaxed SiGe layer adds to the total
thickness of this layer and thus makes it difficult to achieve the
thicknesses required for fully depleted silicon-on-insulator device
fabrication. The relaxed SiGe layer is not required if a strained
Si layer can be produced directly on the insulating material. Thus,
there is a need for a method to produce strained silicon - or other
semiconductor--layers directly on insulating substrates.
Double-Gate MOSFETs
[0006] Double gate MOSFETs have the potential for superior
performance in comparison to standard single-gate bulk or
single-gate SOI MOSFET devices. This is due to the fact that two
gates (one above and one below the channel) allow much greater
control of channel charge then a single gate. This configuration
has the potential to translate to higher drive current and lower
stand-by leakage current.
finFETs
[0007] Fin-field-effect transistors (finFETs), like double-gate
MOSFETs, typically have two gates (one on either side of the
channel, where the channel is here oriented vertically) allowing
much greater control of channel charge than in a single gate
device. This configuration also has the potential to translate to
higher drive current and lower stand-by leakage current. Devices
related to the finFET, such as the wrap-around gate FET (gate on
both sides of as well as above the channel) allow even more channel
charge control and hence even more potential for improved drive
current and leakage current performance.
Bipolar-CMOS
[0008] The bipolar-CMOS (BiCMOS) process is a combination of both
the bipolar transistor and MOSFET/CMOS processes. Individually, the
CMOS process allows low power dissipation, high packing density and
the ability to integrate complexity with high-speed yields. A major
contribution to power dissipation in CMOS circuits originates from
driving the load capacitance that is usually the gate of
sequentially linked logic cells. The size of these gates may be
kept sufficiently small, but when driving higher loads (such as
input/output buffers or data buses) the load or capacitance of such
devices is substantially larger and therefore requires greater gate
width (hence area) of transistor, which inevitably drives down the
switching speed of the MOSFET.
[0009] The bipolar transistor has significant advantages in terms
of the drive current per unit active area and reduced noise signal.
Additionally, the switching speed is enhanced due to the
effectively exponential output current swing with respect to input
signal. This means that the transconductance of a bipolar
transistor is significantly higher than that of a MOS transistor
when the same current is passed. Higher transconductance enables
the charging process to take place approximately ten times more
quickly in emitter coupled logic circuits, or high fan out/load
capacitance.
[0010] Pure bipolar technology has not replaced the high packing
density microprocessor CMOS process for a number of reasons,
including issues of yield and the increased area required for
device isolation. However, integration of bipolar and CMOS may
provide the best aspects of the composite devices.
[0011] The advantages of BiCMOS process may be summarized as
follows:
[0012] 1. Improved speed performance of highly integrated
functionality of CMOS technology;
[0013] 2. Lower power dissipation than bipolar technology;
[0014] 3. Lower sensitivity to fan out and capacitive load;
[0015] 4. Increased flexibility of input/output interface;
[0016] 5. Reduced clock skew;
[0017] 6. Improved internal gate delay; and
[0018] 7. Reduced need for aggressive scaling because a 1-2 .mu.m
BiCMOS process offers circuit speed equivalent to that of
sub-micron CMOS.
SUMMARY
[0019] The present invention includes a
strained-semiconductor-on-insulator (SSOI) substrate structure and
methods for fabricating the substrate structure. MOSFETs fabricated
on this substrate will have the benefits of SOI MOSFETs as well as
the benefits of strained Si mobility enhancement. For example, the
formation of BiCMOS structures on SSOI substrates provides the
combined benefits of BiCMOS design platforms and enhanced carrier
mobilities. SSOI substrates also enable enhanced carrier
mobilities, process simplicity, and better device isolation for
double-gate MOSFETs and finFETs.
[0020] By eliminating the SiGe relaxed layer traditionally found
beneath the strained Si layer, the use of SSOI technology is
simplified. For example, issues such as the diffusion of Ge into
the strained Si layer during high temperature processes are
avoided.
[0021] This approach enables the fabrication of well-controlled,
epitaxially-defined, thin strained semiconductor layers directly on
an insulator layer. Tensile strain levels of .about.10.sup.-3 or
greater are possible in these structures, and are not diminished
after thermal anneal cycles. In some embodiments, the
strain-inducing relaxed layer is not present in the final
structure, eliminating some of the key problems inherent to current
strained Si-on-insulator solutions. This fabrication process is
suitable for the production of enhanced-mobility substrates
applicable to partially or fully depleted SSOI technology.
[0022] In an aspect, the invention features a structure including a
substrate having a dielectric layer disposed thereon and a
fin-field-effect transistor disposed over the substrate. The
fin-field-effect-transistor includes a source region and a drain
region disposed in contact with the dielectric layer, the source
and the drain regions including a strained semiconductor material.
The fin-field-effect-transistor also includes at least one fin
extending between the source and the drain regions, the fin
including a strained semiconductor material. A gate is disposed
above the strained semiconductor layer, extending over at least one
fin and between the source and the drain regions. A gate dielectric
layer is disposed between the gate and the fin.
[0023] One or more of the following features may be included. The
fin may include at least one of a group II, a group III, a group
IV, a group V, of a group VI element. The strained semiconductor
layer may be tensilely strained and may include, e.g., tensilely
strained silicon. The strained semiconductor layer may be
compressively strained and may include, e.g., compressively
strained germanium.
[0024] In another aspect, the invention features a method for
forming a structure, the method including providing a substrate
having a dielectric layer disposed thereon, and a first strained
semiconductor layer disposed in contact with the dielectric layer.
A fin-field-effect transistor is formed on the substrate by
patterning the first strained semiconductor layer to define a
source region, a drain region, and at least one fin disposed
between the source and the drain regions. A dielectric layer is
formed, at least a portion of the dielectric layer being disposed
over the fin, and a gate is formed over the dielectric layer
portion disposed over the fin.
[0025] One or more of the following features may be included. The
first strained semiconductor layer may include at least one of a
group II, a group III, a group IV, a group V, or a group VI
element. The strained semiconductor layer may be tensilely strained
and may include, e.g., tensilely strained silicon. The strained
semiconductor layer may be compressively strained and may include,
e.g., compressively strained germanium.
[0026] In another aspect, the invention features a structure
including a dielectric layer disposed over a substrate; and a
transistor formed over the dielectric layer. The transistor
includes a first gate electrode in contact with the dielectric
layer, a strained semiconductor layer disposed over the first gate
electrode; and a second gate electrode disposed over the strained
semiconductor layer.
[0027] One or more of the following features may be included. The
strained semiconductor layer may include at least one of a group
II, a group III, a group IV, a group V, and a group VI
elements.
[0028] The strained semiconductor layer may be tensilely strained
and may include, e.g., tensilely strained silicon. The strained
semiconductor layer may be compressively strained and may include,
e.g., compressively strained germanium. The strained semiconductor
layer may have a strain level greater than 10.sup.-3.
[0029] A first gate insulator layer may be disposed between the
first gate electrode and the strained semiconductor layer. A second
gate insulator layer may be disposed between the strained
semiconductor layer and the second gate electrode. The strained
semiconductor layer may include a source. The strained
semiconductor layer may include a drain. A sidewall spacer may be
disposed proximate the second gate electrode. The sidewall spacer
may include a dielectric or a conductive material.
[0030] In another aspect, the invention features a method for
forming a structure, the method including forming a substrate
having a first gate electrode layer disposed over a substrate
insulator layer, a first gate insulator layer disposed over the
first gate electrode layer, and a strained semiconductor layer
disposed over the first gate insulator layer. A second gate
insulator layer is formed over the strained semiconductor layer,
and a second gate electrode layer is formed over the second gate
insulator layer. A second gate electrode is defined by removing a
portion of the second gate insulator layer. A dielectric sidewall
spacer is formed proximate the second gate electrode. A portion of
the strained semiconductor layer, a portion of the first gate
insulator layer, and a portion of the first gate electrode layer
are removed to define a vertical structure disposed over the
substrate insulator layer, the vertical structure including a
strained layer region, a first gate insulator region, and-a first
gate electrode layer region disposed under the second gate
electrode. A first gate electrode is defined by laterally shrinking
the first gate electrode layer region.
[0031] One or more of the following features may be included. The
strained semiconductor layer may be tensilely strained and may
include, e.g., tensilely strained silicon. The strained
semiconductor layer may be compressively strained and may include
compressively strained germanium. A conductive sidewall spacer may
be formed proximate the dielectric sidewall spacer. A source and/or
a drain may be defined in the strained semiconductor layer.
[0032] In another aspect, the invention features a structure
including a strained semiconductor layer disposed over a dielectric
layer and a bipolar transistor. The bipolar transistor includes a
collector disposed in a portion of the strained semiconductor
layer, a base disposed over the collector, and an emitter disposed
over the base.
[0033] One or more of the following features may be included. The
strained layer may be tensilely strained and may include, e.g.,
tensilely strained silicon. The strained layer may be compressively
strained.
[0034] In another aspect, the invention features a relaxed
substrate including a bulk material, a strained layer disposed in
contact with the relaxed substrate; and a bipolar transistor. The
bipolar transistor includes a collector disposed in a portion of
the strained layer, a base disposed over the collector, and an
emitter disposed over the base. The strain of the strained layer is
not induced by the underlying substrate.
[0035] One or more of the following features may be included. The
strained layer may be tensilely strained and may include, e.g.,
tensilely strained silicon. The strained layer may be compressively
strained.
[0036] In another aspect, the invention features a structure
including a relaxed substrate including a bulk material, a strained
layer disposed in contact with the relaxed substrate; and a bipolar
transistor including. The bipolar transistor includes a collector
disposed in a portion of the strained layer, a base disposed over
the collector, and an emitter disposed over the base. The strain of
the strained layer is independent of a lattice mismatch between the
strained layer and the relaxed substrate.
[0037] One or more of the following features may be included. The
strained layer may be tensilely strained and may include, e.g.,
tensilely strained silicon. The strained layer may be compressively
strained.
[0038] In another aspect, the invention includes a method for
forming a structure, the method including providing a substrate
having a strained semiconductor layer disposed over a dielectric
layer, defining a collector in a portion of the strained
semiconductor layer; forming a base over the collector; and forming
an emitter over the base.
[0039] One or more of the following features may be included. The
strained layer may be tensilely strained and may include, e.g.,
tensilely strained silicon. The strained layer may be compressively
strained.
[0040] In another aspect, the invention includes method for forming
a structure, the method including providing a first substrate
having a strained layer disposed thereon, the strained layer
including a first semiconductor material. The strained layer is
bonded to a second substrate, the second substrate including a bulk
material. The first substrate is removed from the strained layer,
with the strained layer remaining bonded to the bulk semiconductor
material. A collector is defined in a portion of the strained
layer. A base is formed over the collector; and an emitter is
formed over the base. The strain of the strained layer is not
induced by the second substrate and the strain is independent of
lattice mismatch between the strained layer and the second
substrate.
[0041] One or more of the following features may be included. The
strained layer may be tensilely strained and may include, e.g.,
tensilely strained silicon. The strained layer may be compressively
strained.
[0042] In another aspect, the invention features a method for
forming a structure, the method including providing a relaxed
substrate comprising a bulk material and a strained layer disposed
in contact with the relaxed substrate, the strain of the strained
layer not being induced by the underlying substrate and the strain
being independent of a lattice mismatch between the strained layer
and the relaxed substrate. A collector is defined in a portion of
the strained layer. A base is formed over the collector, and an
emitter is formed over the base.
[0043] One or more of the following features may be included. The
strained layer may be tensilely strained and may include, e.g.,
tensilely strained silicon. The strained layer may be compressively
strained.
[0044] In another aspect, the invention features a method for
forming a structure, the method includes providing a substrate
having a strained semiconductor layer disposed over a substrate
dielectric layer and forming a transistor in the strained layer.
Forming the transistor includes forming a gate dielectric layer
above a portion of the strained semiconductor layer, forming a gate
contact above the gate dielectric layer, and forming a source
region and a drain region in a portion of the strained
semiconductor layer, proximate the gate dielectric layer. A portion
of the strained layer and the substrate dielectric layer are
removed to expose a portion of the substrate. A collector is
defined in the exposed portion of the substrate. A base is formed
over the collector; and an emitter is formed over the base.
[0045] One or more of the following features may be included. The
strained layer may be tensilely strained and may include, e.g.,
tensilely strained silicon. The strained layer may be compressively
strained.
BRIEF DESCRIPTION OF DRAWINGS
[0046] FIGS. 1A-6 are schematic cross-sectional views of substrates
illustrating a method for fabricating an SSOI substrate;
[0047] FIG. 7 is a schematic cross-sectional view illustrating an
alternative method for fabricating the SSOI substrate illustrated
in FIG. 6;
[0048] FIG. 8 is a schematic cross-sectional view of a transistor
formed on the SSOI substrate illustrated in FIG. 6;
[0049] FIGS. 9-10 are schematic cross-sectional views of
substrate(s) illustrating a method for fabricating an alternative
SSOI substrate;
[0050] FIG. 11 is a schematic cross-sectional view of a substrate
having several layers formed thereon;
[0051] FIGS. 12-13 are schematic cross-sectional views of
substrates illustrating a method for fabricating an alternative
strained semiconductor substrate;
[0052] FIG. 14 is a schematic cross-sectional view of the SSOI
substrate illustrated in FIG. 6 after additional processing;
[0053] FIGS. 15-21B are cross-sectional and top views of substrates
illustrating a method for fabricating a fin-field-effect transistor
(finFET) on an SSOI substrate;
[0054] FIGS. 22-35 are cross-sectional views of substrates
illustrating a method for fabricating a dual-gate transistor on an
SSOI substrate;
[0055] FIGS. 36-39 are cross-sectional views of substrates
illustrating a method for fabricating a bipolar transistor on an
SSOI substrate; and
[0056] FIGS. 40A-41D are schematic cross-sectional views of
substrates illustrating alternative methods for fabricating an SSOI
substrate.
[0057] Like-referenced features represent common features in
corresponding drawings.
DETAILED DESCRIPTION
[0058] An SSOI structure may be formed by wafer bonding followed by
cleaving. FIGS. 1A-2B illustrate formation of a suitable strained
layer on a wafer for bonding, as further described below.
[0059] Referring to FIG. 1A, an epitaxial wafer 8 has a plurality
of layers 10 disposed over a substrate 12. Substrate 12 may be
formed of a semiconductor, such as Si, Ge, or SiGe. The plurality
of layers 10 includes a graded buffer layer 14, which may be formed
of Si.sub.1-yGe.sub.y, with a maximum Ge content of, e.g., 10-80%
(i.e., y=0.1-0.8) and a thickness T.sub.1 of, for example, 1-8
micrometers (.mu.m).
[0060] A relaxed layer 16 is disposed over graded buffer layer 14.
Relaxed layer 16 may be formed of uniform Si.sub.1-xGe.sub.x having
a Ge content of, for example, 10-80% (i.e., x=0.1-0.8), and a
thickness T.sub.2 of, for example, 0.2-2 .mu.m. In some
embodiments, Si.sub.1-xGe.sub.x may include Si.sub.0.70Ge.sub.0.30
and T.sub.2 may be approximately 1.5 .mu.m. Relaxed layer 16 may be
fully relaxed, as determined by triple axis X-ray diffraction, and
may have a threading dislocation density of <1.times.10.sup.6
dislocations/cm.sup.2, as determined by etch pit density (EPD)
analysis. Because threading dislocations are linear defects
disposed within a volume of crystalline material, threading
dislocation density may be measured as either the number of
dislocations intersecting a unit area within a unit volume or the
line length of dislocation per unit volume. Threading dislocation
density, therefore, may be expressed in either units of
dislocations/cm.sup.2 or cm/cm.sup.3. Relaxed layer 16 may have a
surface particle density of, e.g., less than about 0.3
particles/cm.sup.2. Further, relaxed layer 16 produced in
accordance with the present invention may have a localized
light-scattering defect level of less than about 0.3
defects/cm.sup.2 for particle defects having a size (diameter)
greater than 0.13 microns, a defect level of about 0.2
defects/cm.sup.2 for particle defects having a size greater than
0.16 microns, a defect level of about 0.1 defects/cm.sup.2 for
particle defects having a size greater than 0.2 microns, and a
defect level of about 0.03 defects/cm.sup.2 for defects having a
size greater than 1 micron. Process optimization may enable
reduction of the localized light-scattering defect levels to about
0.09 defects/cm.sup.2 for particle defects having a size greater
than 0.09 microns and to 0.05 defects/cm.sup.2 for particle defects
having a size greater than 0.12 microns.
[0061] Substrate 12, graded layer 14, and relaxed layer 16 may be
formed from various materials systems, including various
combinations of group II, group III, group IV, group V, and group
VI elements. For example, each of substrate 12, graded layer 14,
and relaxed layer 16 may include a III-V compound. Substrate 12 may
include gallium arsenide (GaAs), graded layer 14 and relaxed layer
16 may include indium gallium arsenide (InGaAs) or aluminum gallium
arsenide (AlGaAs). These examples are merely illustrative, and many
other material systems are suitable.
[0062] A strained semiconductor layer 18 is disposed over relaxed
layer 16. Strained layer 18 may include a semiconductor such as at
least one of a group II, a group 111, a group IV, a group V, and a
group VI element. Strained semiconductor layer 18 may include, for
example, Si, Ge, SiGe, GaAs, indium phosphide (InP), and/or zinc
selenide (ZnSe). In some embodiments, strained semiconductor layer
18 may include approximately 100% Ge, and may be compressively
strained. Strained semiconductor layer 18 comprising 100% Ge may be
formed over, e.g., relaxed layer 16 containing uniform
Si.sub.1-xGe.sub.x having a Ge content of, for example, 50-80%
(i.e., x=0.5-0.8), preferably 70% (x=0.7). Strained layer 18 has a
thickness T.sub.3 of, for example, 50-1000 .ANG.. In an embodiment,
T.sub.3 may be approximately 200-500 .ANG..
[0063] Strained layer 18 may be formed by epitaxy, such as by
atmospheric-pressure CVD (APCVD), low- (or reduced-) pressure CVD
(LPCVD), ultra-high-vacuum CVD (UHVCVD), by molecular beam epitaxy
(MBE), or by atomic layer deposition (ALD). Strained layer 18
containing Si may be formed by CVD with precursors such as silane,
disilane, or trisilane. Strained layer 18 containing Ge may be
formed by CVD with precursors such as germane or digermane. The
epitaxial growth system may be a single-wafer or multiple-wafer
batch reactor. The growth system may also utilize a low-energy
plasma to enhance layer growth kinetics. Strained layer 18 may be
formed at a relatively low temperature, e.g., less than 700.degree.
C., to facilitate the definition of an abrupt interface 17 between
strained layer 18 and relaxed layer 16. This abrupt interface 17
may enhance the subsequent separation of strained layer 18 from
relaxed layer 16, as discussed below with reference to FIGS. 4 and
5. Abrupt interface 17 is characterized by the transition of Si or
Ge content (in this example) proceeding in at least 1 decade (order
of magnitude in atomic concentration) per nanometer of depth into
the sample. In an embodiment, this abruptness may be better than 2
decades per nanometer.
[0064] In an embodiment in which strained layer 18 contains
substantially 100% Si, strained layer 18 may be formed in a
dedicated chamber of a deposition tool that is not exposed to Ge
source gases, thereby avoiding cross-contamination and improving
the quality of the interface between strained layer 18 and relaxed
layer 16. Furthermore, strained layer 18 may be formed from an
isotopically pure silicon precursor(s). Isotopically pure Si has
better thermal conductivity than conventional Si. Higher thermal
conductivity may help dissipate heat from devices subsequently
formed on strained layer 18, thereby maintaining the enhanced
carrier mobilities provided by strained layer 18.
[0065] After formation, strained layer 18 has an initial misfit
dislocation density, of, for example, 0-10.sup.5 cm/cm.sup.2. In an
embodiment, strained layer 18 has an initial misfit dislocation
density of approximately 0 cm/cm.sup.2. Because misfit dislocations
are linear defects generally lying within a plane between two
crystals within an area, they may be measured in terms of total
line length per unit area. Misfit dislocation density, therefore,
may be expressed in units of dislocations/cm or cm/cm.sup.2. In one
embodiment, strained layer 18 is tensilely strained, e.g., Si
formed over SiGe. In another embodiment, strained layer 18 is
compressively strained, e.g., Ge formed over SiGe.
[0066] Strained layer 18 may have a surface particle density of,
e.g., less than about 0.3 particles/cm.sup.2. As used herein,
"surface particle density" includes not only surface particles but
also light-scattering defects, and crystal-originated pits (COPs),
and other defects incorporated into strained layer 18. Further,
strained layer 18 produced in accordance with the present invention
may have a localized light-scattering defect level of less than
about 0.3 defects/cm.sup.2 for particle defects having a size
(diameter) greater than 0.13 microns, a defect level of about 0.2
defects/cm.sup.2 for particle defects having a size greater than
0.16 microns, a defect level of about 0.1 defects/cm.sup.2 for
particle defects having a size greater than 0.2 microns, and a
defect level of about 0.03 defects/cm.sup.2 for defects having a
size greater than 1 micron. Process optimization may enable
reduction of the localized light-scattering defect levels to about
0.09 defects/cm.sup.2 for particle defects having a size greater
than 0.09 microns and to 0.05 defects/cm.sup.2 for particle defects
having a size greater than 0.12 microns. These surface particles
may be incorporated in strained layer 18 during the formation of
strained layer 18, or they may result from the propagation of
surface defects from an underlying layer, such as relaxed layer
16.
[0067] In alternative embodiments, graded layer 14 may be absent
from the structure. Relaxed layer 16 may be formed in various ways,
and the invention is not limited to embodiments having graded layer
14. In other embodiments, strained layer 18 may be formed directly
on substrate 12. In this case, the strain in layer 18 may be
induced by lattice mismatch between layer 18 and substrate 12,
induced mechanically, e.g., by the deposition of overlayers, such
as Si.sub.3N.sub.4, or induced by thermal mismatch between layer 18
and a subsequently grown layer, such as a SiGe layer. In some
embodiments, a uniform semiconductor layer (not shown), having a
thickness of approximately 0.5 .mu.m and comprising the same
semiconductor material as substrate 12, is disposed between graded
buffer layer 14 and substrate 12. This uniform semiconductor layer
may be grown to improve the material quality of layers subsequently
grown on substrate 12, such as graded buffer layer 14, by providing
a clean, contaminant-free surface for epitaxial growth. In certain
embodiments, relaxed layer 16 may be planarized prior to growth of
strained layer 18 to eliminate the crosshatched surface roughness
induced by graded buffer layer 14. (See, e.g., M. T. Currie, et
al., Appl. Phys. Lett., 72 (14) p. 1718 (1998), incorporated herein
by reference.) The planarization may be performed by a method such
as chemical mechanical polishing (CMP), and may improve the quality
of a subsequent bonding process (see below) because it minimizes
the wafer surface roughness and increases wafer flatness, thus
providing a greater surface area for bonding.
[0068] Referring to FIG. 1B, after planarization of relaxed layer
16, a relaxed semiconductor regrowth layer 19 including a
semiconductor such as SiGe may be grown on relaxed layer 16, thus
improving the quality of subsequent strained layer 18 growth by
ensuring a clean surface for the growth of strained layer 18.
Growing on this clean surface may be preferable to growing strained
material, e.g., silicon, on a surface that is possibly contaminated
by oxygen and carbon from the planarization process. The conditions
for epitaxy of the relaxed semiconductor regrowth layer 19 on the
planarized relaxed layer 16 should be chosen such that surface
roughness of the resulting structure, including layers formed over
regrowth layer 19, is minimized to ensure a surface suitable for
subsequent high quality bonding. High quality bonding may be
defined as the existence of a bond between two wafers that is
substantially free of bubbles or voids at the interface. Measures
that may help ensure a smooth surface for strained layer 18 growth,
thereby facilitating bonding, include substantially matching a
lattice of the semiconductor regrowth layer 19 to that of the
underlying relaxed layer 16, by keeping the regrowth thickness
below approximately 1 .mu.m, and/or by keeping the growth
temperature below approximately 850.degree. C. for at least a
portion of the semiconductor layer 19 growth. It may also be
advantageous for relaxed layer 16 to be substantially free of
particles or areas with high threading dislocation densities (i.e.,
threading dislocation pile-ups) which could induce non-planarity in
the regrowth and decrease the quality of the subsequent bond.
[0069] Referring to FIG. 2A, in an embodiment, hydrogen ions are
implanted into relaxed layer 16 to define a cleave plane 20. This
implantation is similar to the SMARTCUT process that has been
demonstrated in silicon by, e.g., SOITEC, based in Grenoble,
France. Implantation parameters may include implantation of
hydrogen (H.sub.2.sup.+) to a dose of 2.5-5.times.10.sup.16
ions/cm.sup.2 at an energy of, e.g., 50-100 keV. For example,
H.sub.2.sup.+ may be implanted at an energy of 75 keV and a dose of
4.times.10.sup.16 ions/cm.sup.2 through strained layer 18 into
relaxed layer 16. In alternative embodiments, it may be favorable
to implant at energies less than 50 keV to decrease the depth of
cleave plane 20 and decrease the amount of material subsequently
removed during the cleaving process (see discussion below with
reference to FIG. 4). In an alternative embodiment, other implanted
species may be used, such as H.sup.+ or He.sup.+, with the dose and
energy being adjusted accordingly. The implantation may also be
performed prior to the formation of strained layer 18. Then, the
subsequent growth of strained layer 18 is preferably performed at a
temperature low enough to prevent premature cleaving along cleave
plane 20, i.e., prior to the wafer bonding process. This cleaving
temperature is a complex function of the implanted species,
implanted dose, and implanted material. Typically, premature
cleaving may be avoided by maintaining a growth temperature below
approximately 500.degree. C.
[0070] In some embodiments, such as when strained layer 18
comprises nearly 100% Ge, a thin layer 21 of another material, such
as Si, may be formed over strained layer 18 prior to bonding (see
discussion with respect to FIG. 3). This thin layer 21 may be
formed to enhance subsequent bonding of strained layer 18 to an
insulator, such as an oxide. Thin layer 21 may have a thickness
T.sub.21 of, for example, 0.5-5 nm.
[0071] In some embodiments, strained layer 18 may be planarized by,
e.g., CMP, to improve the quality of the subsequent bond. Strained
layer 18 may have a low surface roughness, e.g., less than 0.5 nm
root mean square (RMS). Referring to FIG. 2B, in some embodiments,
a dielectric layer 22 may be formed over strained layer 18 prior to
ion implantation into relaxed layer 16 to improve the quality of
the subsequent bond. Dielectric layer 22 may be, e.g., silicon
dioxide (SiO.sub.2) deposited by, for example, LPCVD or by high
density plasma (HDP). An LPCVD deposited SiO.sub.2 layer may be
subjected to a densification step at elevated temperature. Suitable
conditions for this densification step may be, for example, a 10
minute anneal at 800.degree. C. in a nitrogen ambient.
Alternatively, dielectric layer 22 may include low-temperature
oxide (LTO), which may be subsequently densified at elevated
temperature in nitrogen or oxygen ambients. Suitable conditions for
this densification step can be a 10 minute anneal at 800.degree. C.
in an oxygen ambient. Dielectric layer 22 may be planarized by,
e.g., CMP to improve the quality of the subsequent bond. In an
alternative embodiment, it may be advantageous for dielectric layer
22 to be formed from thermally grown SiO.sub.2 in order to provide
a high quality semiconductor/dielectric interface in the final
structure. In an embodiment, strained layer 18 comprises
approximately 100% Ge and dielectric layer 22 comprises, for
example, germanium dioxide (GeO.sub.2); germanium oxynitride
(GeON); a high-k insulator having a higher dielectric constant than
that of Si such as hafnium oxide (HfO.sub.2) or hafnium silicate
(HfSiON, HfSiO.sub.4); or a multilayer structure including
GeO.sub.2 and SiO.sub.2. Ge has an oxidation behavior different
from that of Si, and the deposition methods may be altered
accordingly.
[0072] Referring to FIG. 3, epitaxial wafer 8 is bonded to a handle
wafer 50. Either handle wafer 50, epitaxial wafer 8, or both have a
top dielectric layer (see, e.g., dielectric layer 22 in FIG. 2B) to
facilitate the bonding process and to serve as an insulator layer
in the final substrate structure. Handle wafer 50 may have a
dielectric layer 52 disposed over a semiconductor substrate 54.
Dielectric layer 52 may include, for example, SiO.sub.2. In an
embodiment, dielectric layer 52 includes a material having a
melting point (T.sub.m) higher than a T.sub.m of pure SiO.sub.2,
i.e., higher than 1700 .degree. C. Examples of such materials are
silicon nitride (Si.sub.3N.sub.4), aluminum oxide, magnesium oxide,
etc. Using dielectric layer 52 with a high T.sub.m helps prevents
possible relaxation of the transferred strained semiconductor layer
18 that may occur during subsequent processing, due to softening of
the underlying dielectric layer 52 at temperatures typically used
during device fabrication (approximately 1000-1200.degree. C.). In
other embodiments, handle wafer 50 may include a combination of a
bulk semiconductor material and a dielectric layer, such as a
silicon on insulator substrate. Semiconductor substrate 54 includes
a semiconductor material such as, for example, Si, Ge, or SiGe.
[0073] Handle wafer 50 and epitaxial wafer 8 are cleaned by a wet
chemical cleaning procedure to facilitate bonding, such as by a
hydrophilic surface preparation process to assist the bonding of a
semiconductor material, e.g., strained layer 18, to a dielectric
material, e.g., dielectric layer 52. For example, a suitable
prebonding surface preparation cleaning procedure could include a
modified megasonic RCA SC1 clean containing ammonium hydroxide,
hydrogen peroxide, and water (NH.sub.4OH:H.sub.2O.sub.2:H.sub.2O)
at a ratio of 1:4:20 at 60.degree. C. for 10 minutes, followed by a
deionized (DI) water rinse and spin dry. The wafer bonding energy
should be strong enough to sustain the subsequent layer transfer
(see FIG. 4). In some embodiments, top surfaces 60, 62 of handle
wafer 50 and epitaxial wafer 8, including a top surface 63 of
strained semiconductor layer 18, may be subjected to a plasma
activation, either before, after, or instead of a wet clean, to
increase the bond strength. The plasma environment may include at
least one of the following species: oxygen, ammonia, argon,
nitrogen, diborane, and phosphine. After an appropriate cleaning
step, handle wafer 50 and epitaxial wafer 8 are bonded together by
bringing top surfaces 60, 62 in contact with each other at room
temperature. The bond strength may be greater than 1000 mJ/m.sup.2,
achieved at a low temperature, such as less than 600.degree. C.
[0074] Referring to FIG. 4 as well as to FIG. 3, a split is induced
at cleave plane 20 by annealing handle wafer 50 and epitaxial wafer
8 after they are bonded together. This split may be induced by an
anneal at 300-700.degree. C., e.g., 550.degree. C., inducing
hydrogen exfoliation layer transfer (i.e., along cleave plane 20)
and resulting in the formation of two separate wafers 70, 72. One
of these wafers (70) has a first portion 80 of relaxed layer 16
(see FIG. 1A) disposed over strained layer 18. Strained layer 18 is
in contact with dielectric layer 52 on semiconductor substrate 54.
The other of these wafers (72) includes substrate 12, graded layer
14, and a remaining portion 82 of relaxed layer 16. In some
embodiments, wafer splitting may be induced by mechanical force in
addition to or instead of annealing. If necessary, wafer 70 with
strained layer 18 may be annealed further at 600-900.degree. C.,
e.g., at a temperature greater than 800.degree. C., to strengthen
the bond between the strained layer 18 and dielectric layer 52. In
some embodiments, this anneal is limited to an upper temperature of
about 900.degree. C. to avoid the destruction of a strained
Si/relaxed SiGe heterojunction by diffusion. Wafer 72 may be
planarized, and used as starting substrate 8 for growth of another
strained layer 18. In this manner, wafer 72 may be "recycled" and
the process illustrated in FIGS. 1A-5 may be repeated. An
alternative "recyling" method may include providing relaxed layer
16 that is several microns thick and repeating the process
illustrated in FIGS. 1A-5, starting with the formation of strained
layer 18. Because the formation of this thick relaxed layer 16 may
lead to bowing of substrate 12, a layer including, e.g., oxide or
nitride, may be formed on the backside of substrate 12 to
counteract the bowing. Alternatively substrate 12 may be pre-bowed
when cut and polished, in anticipation of the bow being removed by
the formation of thick relaxed layer 16.
[0075] Referring to FIG. 4 as well as to FIG. 5, relaxed layer
portion 80 is removed from strained layer 18. In an embodiment,
removal of relaxed layer portion 80, containing, e.g., SiGe,
includes oxidizing the relaxed layer portion 80 by wet (steam)
oxidation. For example, at temperatures below approximately
800.degree. C., such as temperatures between 600-750.degree. C.,
wet oxidation will oxidize SiGe much more rapidly then Si, such
that the oxidation front will effectively stop when it reaches the
strained layer 18, in embodiments in which strained layer 18
includes Si. The difference between wet oxidation rates of SiGe and
Si may be even greater at lower temperatures, such as approximately
400.degree. C. -600.degree. C. Good oxidation selectivity is
provided by this difference in oxidation rates, i.e., SiGe may be
efficiently removed at low temperatures with oxidation stopping
when strained layer 18 is reached. This wet oxidation results in
the transformation of SiGe to a thermal insulator 90, e.g.,
Si.sub.xGe.sub.yO.sub.z. The thermal insulator 90 resulting from
this oxidation is removed in a selective wet or dry etch, e.g., wet
hydrofluoric acid. In some embodiments, it may be more economical
to oxidize and strip several times, instead of just once.
[0076] In certain embodiments, wet oxidation may not completely
remove the relaxed layer portion 80. Here, a localized rejection of
Ge may occur during oxidation, resulting in the presence of a
residual Ge-rich SiGe region at the oxidation front, on the order
of, for example, several nanometers in lateral extent. A surface
clean may be performed to remove this residual Ge. For example, the
residual Ge may be removed by a dry oxidation at, e.g., 600.degree.
C., after the wet oxidation and strip described above. Another wet
clean may be performed in conjunction with--or instead of--the dry
oxidation. Examples of possible wet etches for removing residual Ge
include a Piranha etch, i.e., a wet etch that is a mixture of
sulfuric acid and hydrogen peroxide
(H.sub.2SO.sub.4:H.sub.2O.sub.2) at a ratio of, for example, 3:1.
An HF dip may be performed after the Piranha etch. Alternatively,
an RCA SC1 clean may be used to remove the residual Ge. The process
of Piranha or RCA SC1 etching and HF removal of resulting oxide may
be repeated more than once. In an embodiment, relaxed layer portion
including, e.g., SiGe, is removed by etching and annealing under a
hydrochloric acid (HCl) ambient.
[0077] In the case of a strained Si layer, the surface Ge
concentration of the final strained Si surface is preferably less
than about 1.times.10.sup.12 atoms/cm.sup.2 when measured by a
technique such as total reflection x-ray fluorescence (TXRF) or the
combination of vapor phase decomposition (VPD) with a spectroscopy
technique such as graphite furnace atomic absorption spectroscopy
(GFAAS) or inductively-coupled plasma mass spectroscopy (ICP-MS).
In some embodiments, after cleaving, a planarization step or a wet
oxidation step may be performed to remove a portion of the damaged
relaxed layer portion 80 as well as to increase the smoothness of
its surface. A smoother surface may improve the uniformity of
subsequent complete removal of a remainder of relaxed layer portion
80 by, e.g., wet chemical etching. After removal of relaxed layer
portion 80, strained layer 18 may be planarized. Planarization of
strained layer 18 may be performed by, e.g., CMP; an anneal at a
temperature greater than, for example, 800.degree. C., in a
hydrogen (H.sub.2) or hydrochloric acid (HCl) containing ambient;
or cluster ion beam smoothing.
[0078] Referring to FIG. 6, a SSOI substrate 100 has strained layer
18 disposed over an insulator, such as dielectric layer 52 formed
on semiconductor substrate 54. Strained layer 18 has a thickness
T.sub.4 selected from a range of, for example, 50-1000 .ANG., with
a thickness uniformity of better than approximately .+-.5% and a
surface roughness of less than approximately 20 .ANG.. Dielectric
layer 52 has a thickness T.sub.52 selected from a range of, for
example, 500-3000 .ANG.. In an embodiment, strained layer 18
includes approximately 100% Si or 100% Ge having one or more of the
following material characteristics: misfit dislocation density of,
e.g., 0-10.sup.5 cm/cm.sup.2; a threading dislocation density of
about 10.sup.1-10.sup.7 dislocations/cm.sup.2; a surface roughness
of approximately 0.0.1-1 nm RMS; and a thickness uniformity across
SSOI substrate 100 of better than approximately .+-.10% of a mean
desired thickness; and a thickness T.sub.4 of less than
approximately 200 .ANG.. In an embodiment, SSOI substrate 100 has a
thickness uniformity of better than approximately .+-.5% of a mean
desired thickness.
[0079] In an embodiment, dielectric layer 52 has a T.sub.m greater
than that of SiO.sub.2. During subsequent processing, e.g., MOSFET
formation, SSOI substrate 100 may be subjected to high
temperatures, i.e., up to 1100.degree. C. High temperatures may
result in the relaxation of strained layer 18 at an interface
between strained layer 18 and dielectric layer 52. The use of
dielectric layer with a T.sub.m greater than 1700.degree. C. may
help keep strained layer 18 from relaxing at the interface between
strained layer 18 and dielectric layer 52 when SSOI substrate is
subjected to high temperatures.
[0080] In an embodiment, the misfit dislocation density of strained
layer 18 may be lower than its initial dislocation density. The
initial dislocation density may be lowered by, for example,
performing an etch of a top surface 92 of strained layer 18. This
etch may be a wet etch, such as a standard microelectronics clean
step such as an RCA SC1, i.e., hydrogen peroxide, ammonium
hydroxide, and water (H.sub.2O.sub.2+NH.sub.4OH+H.sub.2O), which
at, e.g., 80.degree. C. may remove silicon.
[0081] The presence of surface particles on strained layer 18, as
described above with reference to FIG. 1A, may result in the
formation of bonding voids at an interface 102 between strained
layer 18 and dielectric layer 52. These bonding voids may have a
density equivalent to the density of surface particles formed on
strained layer 18, e.g., less than about 0.3 voids/cm.sup.2.
[0082] In some embodiments, strained semiconductor layer 18
includes Si and is substantially free of Ge; further, any other
layer disposed in contact with strained semiconductor layer 18
prior to device processing, e.g., dielectric layer 52, is also
substantially free of Ge.
[0083] Referring to FIG. 7, in an alternative embodiment, relaxed
layer portion 80 may be removed by a selective wet etch that stops
at the strained layer 18 to obtain SSOI substrate 100 (see FIG. 6).
In embodiments in which relaxed layer portion 80 contains SiGe, a
suitable selective SiGe wet etch may be a solution containing
nitric acid (HNO.sub.3) and dilute HF at a ratio of 3:1 or a
solution containing H.sub.2O.sub.2, HF, and acetic acid
(CH.sub.3COOH) at a ratio of 2:1:3. Alternatively, relaxed layer
portion 80 may be removed by a dry etch that stops at strained
layer 18. In some embodiments, relaxed layer portion 80 may be
removed completely or in part by a chemical-mechanical polishing
step or by mechanical grinding.
[0084] Strained semiconductor-on-insulator substrate 100 may be
further processed by CMOS SOI MOSFET fabrication methods. For
example, referring to FIG. 8A, a transistor 200 may be formed on
SSOI substrate 100. Forming transistor 200 includes forming a gate
dielectric layer 210 above strained layer 18 by, for example,
growing an SiO.sub.2 layer by thermal oxidation. Alternatively,
gate dielectric layer 210 may include a high-k material with a
dielectric constant higher than that of SiO.sub.2, such as
HfO.sub.2, HfSiON, or HfSiO.sub.4. In some embodiments, gate
dielectric layer 210 may be a stacked structure, e.g., a thin
SiO.sub.2 layer capped with a high-k material. A gate 212 is formed
over gate dielectric layer 210. Gate 212 may be formed of a
conductive material, such as doped semiconductor, e.g.,
polycrystalline Si or polycrystalline SiGe; a metal, e.g., titanium
(Ti), tungsten (W), molybdenum (Mo), tantalum (Ta), nickel (Ni), or
iridium (Ir); or metal compounds, e.g., titanium nitride (TiN),
titanium silicon nitride (TiSiN), tungsten nitride (WN), tantalum
nitride (TaN), tantalum silicide (TaSi), nickel silicide (NiSi), or
iridium oxide (IrO.sub.2), that provide an appropriate
workfunction. A source region 214 and a drain region 216 are formed
in a portion 218 of strained semiconductor layer 18, proximate gate
dielectric layer 210. Source and drain regions 214, 216 may be
formed by, e.g., ion implantation of either n-type or p-type
dopants.
[0085] In some embodiments, strained semiconductor layer 18 may be
compressively strained when, for example, layer 18 includes
strained Ge. Compressively strained layers may be prone to
undulation when subjected to large temperature changes. The risk of
such undulation may be reduced by reducing the thermal budget of a
process for fabricating devices, such as transistor 200. The
thermal budget may reduced by, for example, using atomic layer
deposition (ALD) to deposit gate dielectric layer 210. Furthermore,
a maximum temperature for forming gate 212 may be limited to, e.g.,
600.degree. C. by, for example, the use of materials comprising
metal or metal compounds, rather than polysilicon or other gate
materials that may require higher formation and/or dopant
activation temperatures.
[0086] Referring to FIG. 8B, a transistor 250 formed on SSOI
substrate 100 may have an elevated source region and an elevated
drain region proximate a first and a second sidewall spacer 252,
254. These elevated regions may be formed as follows. A
semiconductor layer 256a -256c is formed selectively on exposed
silicon surfaces, i.e., on top surface 258 of a gate 259 containing
silicon, a top surface 260 of a source 262 defined in strained
layer 18, and top surface 264 of a drain 266 defined in strained
layer 18. In an embodiment, semiconductor layer 256a-256c is an
epitaxial layer, such as epitaxial silicon, epitaxial germanium, or
epitaxial silicon-germanium. No semiconductor layer is formed on
non-silicon features, such as sidewall spacers 252, 254 and
dielectric isolation regions 268, 270. Semiconductor layer
256a-256c has a thickness T.sub.256 of, for example, approximately
100-500 .ANG..
[0087] Semiconductor layer 256a-256c has a low resistivity of,
e.g., 0.001 ohm-cm, that facilitates the formation of
low-resistance contacts. To achieve this low resistivity,
semiconductor layer 256a-256c is, for example, epitaxial silicon
doped with, for example, arsenic to a concentration of
1.times.10.sup.20 atoms/cm.sup.3. Semiconductor layer 256a-256c may
be doped in situ, during deposition. In alternative embodiments,
semiconductor layer 256a-256c may be doped after deposition by ion
implantation or by gas-, plasma- or solid-source diffusion. In some
embodiments, the doping of semiconductor layer 256a-256c and the
formation of source 262 and drain 266 are performed simultaneously.
Portions of semiconductor layer 256a, 256c disposed over source 262
and drain 266 may have top surfaces substantially free of facets.
In an embodiment, portions of source 262, drain 266, and/or gate
259 may be etched away to define recess prior to deposition of
semiconductor layer 256a-256c, and semiconductor layer 256a-256c
may then be deposited in the recesses thus formed.
[0088] Referring to FIG. 8C, a metal layer 272 is formed over
transistor 250. Metal layer 272 is formed by, for example, sputter
deposition. Metal layer 272 has a thickness T.sub.272 of, e.g.,
50-200 .ANG. and includes a metal such as cobalt, titanium,
tungsten, nickel, or platinum. The metal is selected to react with
semiconductor layer 256a-256c to form a low-resistance
metal-semiconductor alloy when exposed to heat, as described below.
The metal is also selected such that the metal-semiconductor alloy
remains stable at temperatures typically required to complete
transistor 250 fabrication, e.g., 400-700.degree. C.
[0089] Referring also to FIG. 8D, subsequent to deposition of metal
layer 272, a first rapid thermal anneal is performed, e.g., at
550.degree. C. for 60 seconds. This heating step initiates a
reaction between metal layer 272 and semiconductor layers
256a-256c, forming a high resistivity phase of a
metal-semiconductor alloy, e.g., cobalt silicide (CoSi). Portions
of metal layer 272 are removed by a wet etch, such as sulfuric acid
and hydrogen peroxide. In an alternative embodiment, the wet etch
may be ammonium hydroxide, peroxide, and water. This wet etch
removes portions of metal layer 272 disposed over dielectric
material, such as over first and second sidewall spacers 252, 254
and isolation regions 268, 270. Portions 274 of metal layer 272
disposed over semiconductor layer 256a-256c that have reacted to
form the metal-semiconductor alloy remain in place after the anneal
and wet etch.
[0090] Referring to FIG. 8E, SSOI substrate 100, including
transistor 250, is subjected to a second heat treatment. For
example, in an embodiment in which metal layer 272 includes cobalt,
SSOI substrate 100 undergoes a rapid thermal anneal at 800.degree.
C. for 60 seconds in a nitrogen ambient. This heating step
initiates a reaction in the metal-semiconductor alloy layer which
substantially lowers its resistivity, to form a substantially
homogeneous contact layer 276a-276c. Contact layer 276a-276c
includes a metal-semiconductor alloy, e.g., a metal silicide such
as a low resistivity phase of cobalt silicide (CoSi.sub.2). Contact
layer 276a-276c has a thickness T.sub.276 of, for example, 400
.ANG.. Contact layer 276a-276c has a low sheet resistance, e.g.,
less than about 10 .OMEGA./.quadrature., and enables a good quality
contact to be made to source 262 and drain 266, as well as to gate
259.
[0091] In some embodiments, during formation, contact layer
276a-276c may consume substantially all of semiconductor layer
256a-256c. A bottommost boundary 278a of contact layer 276a,
therefore, shares an interface 280a with strained layer 18 in
source 262, and a bottommost boundary 278c of contact layer 276c,
therefore, shares an interface 280c with strained layer 18 in drain
266. A bottonunost boundary 278b of contact layer 276b shares an
interface 280b with gate 259.
[0092] In other embodiments, contact layer portions 276a, 276c,
disposed over source 262 and drain 266, may extend into strained
layer 18. Interfaces 280a, 280c between contact layer 276a, 276c
and strained layer 18 are then disposed within source 262 and drain
266, respectively, above bottommost boundaries 282a, 282c of
strained layer 18. Interfaces 280a, 280c have a low contact
resistivity, e.g., less than approximately 5.times.10.sup.-7
.OMEGA.-cm.sup.2. In certain other embodiments, during formation,
contact layer 276a-276c may not consume all of semiconductor layer
256a-256c (see FIG. 8D). A bottommost boundary 278a of contact
layer 276a, therefore, shares an interface with semiconductor layer
256a over source 262, and a bottommost boundary 278c of contact
layer 276c, therefore, shares an interface with semiconductor layer
256c over drain 266.
[0093] Because strained layer 18 includes a strained material,
carrier mobilities in strained layer 18 are enhanced, facilitating
lower sheet resistances. This strain also results in a reduced
energy bandgap, thereby lowering the contact resistivity between
the metal-semiconductor alloy and the strained layer.
[0094] In alternative embodiments, an SSOI structure may include,
instead of a single strained layer, a plurality of semiconductor
layers disposed on an insulator layer. For example, referring to
FIG. 9, epitaxial wafer 300 includes strained layer 18, relaxed
layer 16, graded layer 14, and substrate 12. In addition, a
semiconductor layer 310 is disposed over strained layer 18.
Strained layer 18 may be tensilely strained and semiconductor layer
310 may be compressively strained. In an alternative embodiment,
strained layer 18 may be compressively strained and semiconductor
layer 310 may be tensilely strained. Strain may be induced by
lattice mismatch with respect to an adjacent layer, as described
above, or mechanically. For example, strain may be induced by the
deposition of overlayers, such as Si.sub.3N.sub.4. In another
embodiment, semiconductor layer 310 is relaxed. Semiconductor layer
310 includes a semiconductor material, such as at least one of a
group II, a group III, a group IV, a group V, and a group VI
element. Epitaxial wafer 300 is processed in a manner analogous to
the processing of epitaxial wafer 8, as described with reference to
FIGS. 1-7.
[0095] Referring also to FIG. 10, processing of epitaxial wafer 300
results in the formation of SSOI substrate 350, having strained
layer 18 disposed over semiconductor layer 310. Semiconductor layer
310 is bonded to dielectric layer 52, disposed over substrate 54.
As noted above with reference to FIG. 9, strained layer 18 may be
tensilely strained and semiconductor layer 310 may be compressively
strained. Alternatively, strained layer 18 may be compressively
strained and semiconductor layer 310 may be tensilely strained. In
some embodiments, semiconductor layer 310 may be relaxed.
[0096] Referring to FIG. 11, in some embodiments, a thin strained
layer 84 may be grown between strained layer 18 and relaxed layer
16 to act as an etch stop during etching, such as wet etching. In
an embodiment in which strained layer 18 includes. Si and relaxed
layer 16 includes Si.sub.1-yGe.sub.y, thin strained layer 84 may
include Si.sub.1-xGe.sub.x, with a higher Ge content (x) than the
Ge content (y) of relaxed layer 16, and hence be compressively
strained. For example, if the composition of the relaxed layer 16
is 20% Ge (Si.sub.0.80Ge.sub.0.20), thin strained layer 84 may
contain 40% Ge (Si.sub.0.60Ge.sub.0.40) to provide a more robust
etch stop. In other embodiments, a second strained layer, such as
thin strained layer 84 with higher Ge content than relaxed layer
16, may act as a preferential cleave plane in the hydrogen
exfoliation/cleaving procedure described above.
[0097] In an alternative embodiment, thin strained layer 84 may
contain Si.sub.1-xGe.sub.x with lower Ge content than relaxed layer
16. In this embodiment, thin strained layer 84 may act as a
diffusion barrier during the wet oxidation process. For example, if
the composition of relaxed layer 16 is 20% Ge
(Si.sub.0.80Ge.sub.0.20), thin strained layer 84 may contain 10% Ge
(Si.sub.0.90Ge.sub.0.10) to provide a barrier to Ge diffusion from
the higher Ge content relaxed layer 16 during the oxidation
process. In another embodiment, thin strained layer 84 may be
replaced with a thin graded Si.sub.1-zGe.sub.z layer in which the
Ge composition (z) of the graded layer is decreased from relaxed
layer 16 to the strained layer 18.
[0098] Referring again to FIG. 7, in some embodiments, a small
amount, e.g., approximately 20-100 .ANG., of strained layer 18 may
be removed at an interface 105 between strained layer 18 and
relaxed layer portion 80. This may be achieved by overetching after
relaxed layer portion 80 is removed. Alternatively, this removal of
strained layer 18 may be performed by a standard microelectronics
clean step such as an RCA SC1, i.e., hydrogen peroxide, ammonium
hydroxide, and water (H.sub.2O.sub.2+NH.sub.4OH+H.sub.2O), which
at, e.g., 80.degree. C. may remove silicon. This silicon removal
may remove any misfit dislocations that formed at the original
strained layer 18/relaxed layer 80 interface 105 if strained layer
18 was grown above the critical thickness. The critical thickness
may be defined as the thickness of strained layer 18 beyond which
it becomes energetically favorable for the strain in the layer to
partially relax via the introduction of misfit dislocations at
interface 105 between strained layer 18 and relaxed layer 16. Thus,
the method illustrated in FIGS. 1-7 provides a technique for
obtaining strained layers above a critical thickness without misfit
dislocations that may compromise the performance of deeply scaled
MOSFET devices.
[0099] Referring to FIG. 12, in some embodiments, handle wafer 50
may have a structure other than a dielectric layer 52 disposed over
a semiconductor substrate 54. For example, a bulk relaxed substrate
400 may comprise a bulk material 410 such as a semiconductor
material, e.g., bulk silicon. Alternatively, bulk material 410 may
be a bulk dielectric material, such as Al.sub.2 O.sub.3 (e.g.,
alumina or sapphire) or SiO.sub.2 (e.g., quartz). Epitaxial wafer 8
may then be bonded to handle wafer 400 (as described above with
reference to FIGS. 1-6), with strained layer 18 being bonded to the
bulk material 410 comprising handle wafer 400. In embodiments in
which bulk material 410 is a semiconductor, to facilitate this
semiconductor-semiconductor bond, a hydrophobic clean may be
performed, such as an HF dip after an RCA SC1 clean.
[0100] Referring to FIG. 13, after bonding and further processing
(as described above), a strained-semiconductor-on-semiconductor
(SSOS) substrate 420 is formed, having strained layer 18 disposed
in contact with relaxed substrate 400. The strain of strained layer
18 is not induced by underlying relaxed substrate 400, and is
independent of any lattice mismatch between strained layer 18 and
relaxed substrate 400. In an embodiment, strained layer 18 and
relaxed substrate 400 include the same semiconductor material,
e.g., silicon. Relaxed substrate 400 may have a lattice constant
equal to a lattice constant of strained layer 18 in the absence of
strain. Strained layer 18 may have a strain greater than
approximately 1.times.10.sup.-3. Strained layer 18 may have been
formed by epitaxy, and may have a thickness T.sub.5 of between
approximately 20 .ANG.-1000 .ANG., with a thickness uniformity of
better than approximately .+-.10%. In an embodiment, strained layer
18 may have a thickness uniformity of better than approximately
.+-.5%. Surface 92 of strained layer 18 may have a surface
roughness of less than 20 .ANG..
[0101] Referring to FIG. 14, in an embodiment, after fabrication of
the SSOI structure 100 including semiconductor substrate 54 and
dielectric layer 52, it may be favorable to selectively relax the
strain in at least a portion of strained layer 18. This could be
accomplished by introducing a plurality of ions by, e.g., ion
implantation after a photolithography step in which at least a
portion of the structure is masked by, for example, a photoresist
feature 500. Ion implantation parameters may be, for example, an
implant of Si ions at a dose of 1.times.10.sup.15-1.times.10.sup.17
ions/cm.sup.2, at an energy of 5-75 keV. After ion implantation, a
relaxed portion 502 of strained layer 18 is relaxed, while a
strained portion 504 of strained layer 18 remains strained.
Devices
[0102] In addition to the transistors described above with
reference to FIGS. 8A-8E, various other transistors may be formed
on SSOI substrate 100 fabricated by the methods described above All
of these transistors may also be formed on SSOI substrate 100
fabricated with the use of a porous semiconductor substrate, as
described below with reference to FIGS. 40A-41D.
finFET
[0103] A finFET (or any variant of the basic finFET structure such
as the wrap-around gate FET, tri-gate FET, or omega FET) may be
fabricated on SSOI substrate 100 as described below. The finFET and
related devices include two gates located on either side of a FET
channel region. Unlike in a traditional planar FET, this channel
region is raised above the wafer surface: the channel (or portions
of the channel) falls in a plane perpendicular to the wafer
surface. There may in addition be gates above and/or below the
channel region, such as in the wrap-around gate FET.
[0104] Referring to FIG. 15, SSOI substrate 100 includes strained
layer 18 and dielectric layer 52 disposed over substrate 54. In an
embodiment, strained layer 18 includes Si and has thickness T.sub.6
of, e.g., 200-1000 .ANG.. Dielectric layer 52 may be formed from
SiO.sub.2, with thickness T.sub.7 selected from the range of, e.g.,
500-3000 .ANG.. Substrate 54 may be formed from, e.g., Si.
[0105] Referring to FIGS. 16A and 16B, strained layer 18 is
patterned to define a plurality of fins 600. Fins 600 are defined
by the formation of a photolithographic mask (not shown) over
strained layer 18, followed by anisotropic reactive ion etching
(RIE) of strained layer 18. Fins 600 have a width W.sub.1 of, e.g.,
50-300 .ANG.. The photomask/RIE steps also define source mesa
region 602 and drain mesa region 604. Fins 600, source mesa region
602, and source mesa region 604 include portions of strained layer
18 not removed by RIE. The photolithographic mask is removed after
the RIE of strained layer 18.
[0106] Referring to FIG. 17, a gate insulator layer 610 is formed
over SSOI substrate 100. Gate insulator layer 610 is conformally
formed over fins 600, as well as over source and drain mesa regions
602, 604. Gate insulator layer 610 may include, e.g., thermally
grown SiO.sub.2, or a high-k dielectric like HfO.sub.2 or HfSiON,
and have a thickness T.sub.8 of, e.g., 10-100 .ANG.. In some
embodiments, gate insulator layer 610 is grown, and is therefore
formed only over exposed silicon surfaces, i.e., over fins 600 and
source and drain mesa regions 602, 604. In other embodiments, gate
insulator layer 610 is deposited, and is therefore formed over an
entire top surface of SSOI substrate 100.
[0107] Referring to FIGS. 18A and 18B, a gate electrode material
620 is conformally formed over gate insulator layer 610, including
over fins 600. Gate electrode material 620 may be, e.g.,
polycrystalline silicon ("polysilicon"), deposited by CVD, such as
by UHVCVD, APCVD, LPCVD, or PECVD, having a thickness T.sub.62
selected from the range of, e.g., 100-2000 .ANG.. A
photolithographic mask (not shown) is formed over gate electrode
material 620. Portions of gate electrode material 620 are
selectively removed by, e.g., RIE to define a gate 622 crossing
over fins 600, and terminating in a gate contact area 624. Portions
of gate insulator layer 610 are exposed (or even removed) by the
RIE of gate electrode material 620.
[0108] Referring to FIGS. 19A and 19B, a plurality of dopants are
introduced into source and drain mesa regions 602, 604 to define
source 630 and drain 632. To form an n-type finFET, dopants such as
arsenic or phosphorus may be implanted into mesa regions 602, 604.
Possible implantation parameters may be, for example, arsenic with
a dose of 2.times.10.sup.15 atoms/cm.sup.2 implanted at an energy
of 10-50 kilo-electron volts (keV). To form a p-type finFET,
dopants such as boron may be implanted into mesa regions 602, 604.
Possible implantation parameters may be, for example, boron, with a
dose of 2.times.10.sup.15 atoms/cm.sup.2 at an energy of 3-15 keV.
For the formation of a CMOS device, NMOS regions may be protected
by a mask during the implantation of p-type dopants into PMOS
regions. Similarly, PMOS regions may be protected by a mask during
the implantation of n-type dopants into NMOS regions. A suitable
mask for both types of implantation may be, e.g., photoresist.
[0109] During the introduction of dopants into source and drain
mesa regions 602, 604, a plurality of gate dopants 634 are also
introduced into gate 622 and gate contact area 624. Gate dopants
634 serve to increase a conductivity of gate electrode material
620. Gate dopants 630 may be, for example, implanted arsenic or
phosphorous ions for an n-type finFET.
[0110] Dopants for both n-type and p-type finFETs may be implanted
at an angle of 20-50.degree., with zero degrees being normal to
SSOI substrate 100. Implanting at an angle may be desired in order
to implant ions into a side of exposed fins 600 and also into a
side of the vertical surfaces of gate electrode material 620.
[0111] Referring to FIGS. 20A and 20B, a blanket layer of spacer
insulator material is formed over SSOI substrate 100, including
over gate 622, gate contact 624, source 630, and drain 632. Spacer
insulator material may be, for example, SiO.sub.2 or
Si.sub.3N.sub.4 deposited by CVD and have a thickness T.sub.9 of,
for example, 100-1000 .ANG.. Subsequently, portions of spacer
insulator material are removed by an anisotropic RIE to define a
plurality of sidewall spacers 642 proximate vertical surfaces, such
as fins 600, gate 622, and gate contact area 624. Horizontal
surfaces, such as top surfaces of fins 600, are substantially free
of the spacer insulator material.
[0112] After the RIE definition of sidewall spacers 642, the
portions of gate insulator layer 610 exposed by the RIE of gate
electrode material 620 may be removed from top surfaces of source
630, and drain 632 by, e.g., a dip in hydrofluoric acid (HF), such
as for 5-30 seconds in a solution containing, e.g., 0.5-5% HF.
Alternately, this removal may be via RIE, with an etchant species
such as, e.g., CHF.sub.3.
[0113] Referring to FIGS. 21A and 21B, a self-aligned silicide
("salicide") is formed over SSOI substrate 100 to provide low
resistance contacts as follows. A conductive layer is formed over
SSOI substrate 100. For example, a metal such as cobalt or nickel
is deposited by, e.g., CVD or sputtering, with the conductive layer
having a thickness of, e.g., 50-200 .ANG.. An anneal is performed
to react the conductive layer with the underlying semiconductor,
e.g., exposed portions of gate 622 and gate contact area 624, to
form salicide 650 including, e.g., cobalt silicide or nickel
silicide. Anneal parameters may be, for example, 400-800.degree. C.
for 10-120 seconds. Unreacted portions of the conductive layer
disposed directly over insulator material, such as exposed portions
of dielectric layer 52 and sidewall spacers 642, are removed by a
chemical strip. A suitable chemical strip may be a solution
including H.sub.2SO.sub.4:H.sub.2O.sub.2 at a ratio of 3:1. A
second anneal may be performed to further lower resistivity of
salicide 650. The second anneal parameters may be, for example,
600-900.degree. C. for 10-120 seconds A finFET 655 includes fins
600, gate insulator 610, source 630, drain 632, and gate 622. A
finFET 655 having three fins 600 is illustrated in FIG. 21B. The
three fins 600 share a common source 630 and a common drain 632. A
single transistor may have multiple fins to increase current drive
in comparison to a transistor with a single fin.
[0114] In an alternative embodiment, gate dielectric material may
be removed from the top surfaces of the source and drain mesa
regions immediately after the RIE of the gate electrode. In some
embodiments, raised source and drain regions may be formed, as
described above with reference to FIGS. 8B-8D.
Double Gate MOSFETs
[0115] Referring to FIG. 22 as well as to FIG. 1A, epitaxial wafer
8 has layers 10 disposed over substrate 12. Substrate 12 may be
formed of a semiconductor, such as Si, Ge, or SiGe. The plurality
of layers 10 includes graded buffer layer 14, formed of
Si.sub.1-yGe.sub.y, with a maximum Ge content of, e.g., 10-80%
(i.e., y=0.1-0.8). Relaxed layer 16 is disposed over graded buffer
layer 14. Relaxed layer 16 may be formed of uniform
Si.sub.1-xGe.sub.x having a Ge content of, for example, 10-80%
(i.e., x=0.1-0.8). Strained semiconductor layer 18 is disposed over
relaxed layer 16. Strained layer 18 comprises at least one of a
group II, a group III, a group IV, a group V, and a group VI
element. Strained layer 18 may include, for example, Si and may be
tensilely strained.
[0116] A first gate insulator layer 700 is formed over strained
layer 18. First gate insulator layer 700 may include SiO.sub.2 or a
high-k dielectric like HfO.sub.2 or HfSiON, and may be grown or
deposited. First gate insulator layer 700 may have a thickness
T.sub.11 of, e.g., 10-100 .ANG.. A first gate electrode layer 702
is formed over first gate insulator layer 700. First gate electrode
layer 702 may include a conductive material, for example, doped
polycrystalline silicon or tungsten, and may have a thickness
T.sub.12 of, for example, 500-2000 .ANG..
[0117] Referring to FIG. 23, ions 704 are introduced to define
cleave plane 20 in relaxed layer 16, in the manner described above
with reference to FIG. 2A.
[0118] Referring to FIG. 24, epitaxial wafer 8 is bonded to handle
wafer 50, in the manner described above with reference to FIG. 3.
Handle wafer 50 includes dielectric layer 52 disposed over
semiconductor substrate 54.
[0119] Referring to FIG. 25 as well as to FIG. 24, the bond between
epitaxial wafer 8 and handle wafer 50 may be strengthened by an
anneal at a relatively low temperature such as, e.g.,
200-300.degree. C. Epitaxial wafer 8 is separated from handle wafer
50 by inducing a split along cleave plane 20 with an anneal at,
e.g., 300-700.degree.C. After cleaving, a SSOI substrate 710
includes strained layer 18 disposed over first gate insulator 700,
first gate electrode layer 702, insulator 52, and substrate 54.
Residual portion 80 of relaxed layer 16 is disposed over strained
layer 18. Relaxed layer portion 80 is selectively removed by, e.g.,
thermal oxidation and HF strip in the manner discussed above with
reference to FIGS. 4 and 5.
[0120] Referring to FIG. 26, a second gate insulator layer 720 is
formed over strained layer 18. Second gate insulator layer 720 may
include SiO.sub.2 or a high-k dielectric like HfO.sub.2 or HfSiON,
and may be grown or deposited. First gate insulator layer 720 may
have a thickness T.sub.13 of, e.g., 10-100 .ANG.. A second gate
electrode layer 722 is formed over second gate insulator layer 720.
Second gate electrode layer 722 may include a conductive material
such as, for example, doped polycrystalline silicon, and may have a
thickness T.sub.14 of, for example, 500-2000 .ANG..
[0121] Referring to FIG. 27 as well as to FIG. 26, second gate
electrode layer 722 is patterned by photolithography and RIE to
define a second gate electrode 730. A source 732 and a drain 734
are formed in strained layer 18 by, e.g., implanting dopants, such
as n-type or p-type dopants, into strained layer 18. A spacer
dielectric layer is deposited and etched back to define dielectric
sidewall spacers 736 proximate second gate electrode 730.
[0122] Referring to FIG. 28, a conductive spacer layer 740 is
deposited over strained layer 18, second gate electrode 730, and
dielectric sidewall spacers 736. Conductive spacer layer 740
includes a conductive material, such as doped polycrystalline
silicon or a metal. Conductive spacer layer 740 has a thickness
T.sub.15 of, e.g., 500-2000 .ANG..
[0123] Referring to FIG. 29 as well as to FIG. 28, conductive
spacer layer 740 is anisotropically etched to form conductive
sidewall spacers 742, proximate dielectric sidewall spacers
736.
[0124] Referring to FIG. 30 as well as to FIG. 29, an RIE is
performed to remove portions of strained layer 18, first gate
insulator layer 700, and first gate electrode layer 702 not
disposed directly below second gate electrode 730, dielectric
sidewall spacers 736, and conductive sidewall spacers 742. After
this RIE, a vertical structure 744 includes strained layer 18,
first gate insulator layer 700, and first gate electrode layer 702
regions disposed under second gate electrode 730 and sidewall
spacers 736, 742. Vertical structure 744 has a width W.sub.2 of,
e.g., 1000-5000 .ANG.
[0125] Referring to FIG. 31, an isotropic etch is performed to
laterally shrink first gate electrode layer 702 region disposed
under second gate electrode 730, thus defining first gate electrode
750. This isotropic etch may be a wet etch, such as hydrogen
peroxide (in an embodiment in which first gate electrode layer 702
includes tungsten) or an isotropic dry etch. The width of first
gate electrode layer 702 may be reduced such that both the first
gate electrode 750 and the second gate electrode 730 have
approximately the same width W.sub.3 that is less than W.sub.2,
e.g., 100-2000 .ANG..
[0126] Referring to FIG. 32, a thick insulator layer 760 is
deposited over insulator layer 52 and vertical structure 744, i.e.,
over second gate electrode 730 and conductive sidewall spacers 742,
as well as proximate strained layer 18, first gate insulator layer
700, and first gate electrode 750. Thick insulator layer 760 has an
initial thickness T.sub.16 over insulator 52 of, e.g., 5000 .ANG..
Thick insulator layer 760 is then planarized by, e.g., CMP.
[0127] Referring to FIGS. 33-35, contact holes 770 are formed
through thick insulator layer 760 to conductive sidewall spacers
742 and second gate electrode 730. Contact holes 770 may be defined
by the use of photolithography and RIE. Contact holes 770 are
filled with a conductive material such as, e.g., a metal such as
titanium or tungsten. The conductive material is patterned by
photolithography and etch to define contacts 780 to source 732,
drain 734, first gate electrode 750 at a first gate electrode 793,
and second gate electrode 730 at a second gate electrode 795.
Double gate transistor 790 includes first gate electrode 750,
second gate electrode 730, first gate insulator layer 700, second
gate insulator layer 722, source 732, and drain 734.
Heterojunction Bipolar Transistor
[0128] Referring to FIG. 36 as well as to FIG. 6, a heterojunction
bipolar transistor (HBT) may be formed on SSOI substrate 100,
including strained layer 18, dielectric layer 52, and substrate 54.
A collector 810 for the HBT is formed in a portion of strained
layer 18 by the introduction of dopants into the strained layer 18
portion. Collector 810 includes a low-doped region 811 and a
high-doped region 812. Low-doped region 811 is doped at a
relatively low level, for example at
5.times.10.sup.16-1.times.10.sup.18 atoms/cm.sup.3, and has a
thickness T.sub.20 of, for example, 100-1000 .ANG.. High-doped
region 812 is doped to a level not less than the doping level of
low-doped region 811, preferably to a relatively high level of,
e.g., 1.times.10.sup.19-1.times.10.sup.21 atoms/cm.sup.3. Low-doped
region 811 and high-doped region 812 are doped with the same type
of dopants, and both may be doped either n-type or p-type. In an
embodiment, both regions are doped n-type. Collector 810 may be
electrically isolated from other devices formed on the substrate
through the use of, for example, trench isolation (not shown).
[0129] A total thickness T.sub.21 of collector 810 may be increased
to improve performance by subsequent additional deposition of a
material that is lattice matched to the original strained layer 18
portion. The additional material may be, for example, SiGe
lattice-matched to strained layer 18.
[0130] Referring to FIG. 37, a masking layer is formed over
collector 810. The masking layer may include a dielectric material,
such as, e.g., SiO.sub.2 or Si.sub.3N.sub.4. Photoresist is
disposed over the masking layer and patterned to expose an area of
the masking layer. This area is removed by, e.g., wet etching or
RIE, to define a mask 910 disposed over strained layer 18. Mask 910
exposes a region 920 of collector 810.
[0131] Referring to FIG. 38, a base 1010 is formed over region 920
of collector 810. Base 1010 may be formed selectively by, e.g.,
selective deposition of a semiconductor material only over region
920 defined by mask 910. The selective deposition can be done by
CVD methods, such as by APCVD, LPCVD, UHVCVD, or by MBE. In an
embodiment, base 1010 may be deposited non-selectively. The
non-selectively grown material will thus also form on a top surface
1012 of mask 910, and may be removed by further photolithography
and etch steps. Base 1010 has a thickness T.sub.22 of, e.g., of
50-1000 .ANG.. In an embodiment, T.sub.22 may be, for example
300-500 .ANG.. Base 1010 includes a semiconductor material like Si
or SiGe. In some embodiments, base 1010 is relaxed or compressively
strained. The in-plane lattice constant of collector 810 (strained
layer 18) was defined by relaxed layer 16 (see FIG. 1A). Therefore,
in order that base 1010 be relaxed, the Ge content of base 1010
should be equal to the Ge content of relaxed layer 16 (see FIG.
1A). Similarly, in order that base 1010 be compressively strained,
the Ge content of base 1010 should be greater than the Ge content
of relaxed layer 16. This difference in Ge content also provides a
base 1010 with a bandgap no larger than that of collector 810,
which can be advantageous to device operation. In other
embodiments, base 1010 is tensilely strained. In order that base
1010 be tensilely strained, the Ge content of base 1010 should be
less than the Ge content of relaxed layer 16 (see FIG. 1A).
Alternatively, base 1010 may be formed from the same material as
collector 810, for example strained Si. Base 1010 is doped the
opposite doping type as the collector, i.e., base 1010 is p-type
doped for an n-type doped collector. Base 1010 may be doped during
the deposition process, but may also be doped after deposition by
ion implantation. Base 1010 may be doped to a level of
1.times.10.sup.18-1.times.10.sup.19 atoms/cm.sup.3.
[0132] In some embodiments, the base doping may be significantly
higher, e.g., .gtoreq.10.sup.20 atoms/cm.sup.3. In such
embodiments, the outdiffusion of dopants from base 1010 may be
deleterious to device performance, and therefore the p-type doping
of base 1010 may be reduced within base 1010 in regions adjacent to
an emitter 1110/base 1010 interface (see FIG. 39) and a base
1010/collector 810 interface 1014 . These regions with reduced
doping may have thicknesses of, e.g., 10 .ANG.-30 .ANG..
[0133] In an embodiment, base 1010 contains an element with a
concentration of 1.times.10.sup.18-1.times.10.sup.20 atoms/cm.sup.3
that suppresses the diffusion of dopants out of base 1010 during
subsequent high temperature processing steps. A suitable element
for diffusion suppression may be, for example, carbon. In another
embodiment, base 1010 may be formed of SiGe, with the Ge content of
base 1010 being not uniform across the thickness of base 1010. In
this case, the Ge content of base 1010 may be graded in
concentration, with higher Ge content at base-collector interface
1014 and lower Ge content at a base upper surface 1016. In other
embodiments, the Ge content of base 1010 can have a trapezoidal or
triangular profile.
[0134] Referring to FIG. 39, an emitter 1110 is formed on base
1010. Emitter 1110 may be formed by the deposition of a
semiconductor layer over base 1010 and mask 910. The semiconductor
layer may be subsequently patterned by photolithographic and etch
steps to define emitter 1110. Emitter 1110 may include a
semiconductor material such as Si or SiGe, and may have a Ge
content lower than the Ge content of base 1010. In an embodiment,
emitter 1110 has a Ge content equal to that of relaxed layer 16
(see FIG. 1A) that originally defined the in-plane lattice constant
of strained layer 18 (and hence collector 810). In another
embodiment, the Ge content of emitter 1110 may be lower than that
of relaxed layer 16, and, therefore, emitter 1110 is tensilely
strained. In another embodiment, emitter 1110 may include the same
material as strained layer 18/collector 810, such as, for example,
strained Si.
[0135] Emitter 1110 has two regions: an upper emitter region 1111
and a lower emitter region 1112. Lower emitter region 1112 has a
thickness T.sub.23 of 10-2000 .ANG. and is doped with a same doping
type as collector 810 (and hence the opposite doping type of base
1010). For example, lower emitter region 11 12 and collector 810
may be doped n-type and base 1010 may be doped p-type. Lower
emitter region 112 may be doped at a concentration of
1.times.10.sup.17-5.times.10.sup.18 atoms/cm.sup.3, for example
1.times.10.sup.18 atoms/cm.sup.3. Upper emitter region 1111 has a
thickness T.sub.24 of, for example, 100-4000 .ANG. and is doped the
same doping type as lower emitter region 1112. Upper emitter region
1111 may be doped at a concentration of
1.times.10.sup.19-1.times.10.sup.21 atoms/cm.sup.3, for example
1.times.10.sup.20-5.times.10.sup.20 atoms/cm.sup.3. An HBT 1200
includes collector 810, base 1010, and emitter 1110.
[0136] After formation of emitter 11 10, metal contacts (not shown)
may be made to each of collector 810, base 1010, and emitter 1110.
Mask 910 may be removed or further patterned during the formation
of metal contacts. HBT 1200 may be a standalone device or may be
interconnected to other devices fabricated on SSOI substrate 100,
such as, for example, transistor 200 (see FIG. 8A), finFET 655 (see
FIGS. 21A and 21B), or double-gate transistor 790 (see FIG.
33).
[0137] In an embodiment, HBT 1200 may be formed on SSOS substrate
420 (see FIG. 13) by the steps described above with reference to
FIGS. 36-39. In another embodiment, HBT 1200 may be formed on
relaxed portion 504 of strained layer 18 (see FIG. 14) by the steps
described above with reference to FIGS. 36-39. In this embodiment,
collector 810 is formed in relaxed portion 504.
[0138] In another embodiment, HBT 1200 may be formed on a region of
SSOI substrate 100 (see FIG. 6) in which portions of strained layer
18 and dielectric layer 52 have been removed by the steps described
with reference to FIGS. 36-39. In this embodiment, collector 810 is
formed in substrate 54 and may be increased in thickness by
deposition of another semiconductor layer as described above. This
configuration enables the interconnection of HBT 1200 formed
directly on semiconductor substrate 54 with devices formed on other
portions of SSOI substrate 100, for example transistor 200 of FIG.
8A.
Formation of SSOI Substrate By Use of a Porous Semiconductor
Substrate
[0139] Referring to FIGS. 40A-40E, SSOI structure 100 (see FIG. 6)
may be formed by the use of a porous semiconductor substrate.
Referring to FIG. 40A, substrate 12 may be formed of a
semiconductor, such as Si, Ge, or SiGe. A plurality of pores 1514,
i.e., microvoids, are formed to define a porous layer 1516 in a
portion of substrate 12. Pores 1514 may have a median diameter of
5-10 nm and a pitch of 10-50 nm. Porous layer 1516 may have a
porosity of 10-50% and may extend a depth of d.sub.15 into
substrate 12 of approximately 1-5 .mu.m.
[0140] Referring to FIG. 40B, pores 1514 may be formed by, for
example, submerging substrate 12 into a vessel 1517 containing an
electrolyte 1518, such as hydrofluoric acid (HF), possibly mixed
with ethanol, with a cathode 1520 and an anode 1522 disposed in the
electrolyte 1518. A back surface chucking holder 1519a with a
vacuum pad 1519b may hold substrate 12 while it is submerged in
vessel 1517. A current may be generated between cathode 1520 and
anode 1522, through substrate 12, resulting in the electrochemical
etching of substrate 12, thereby forming pores 1514 at a top
surface 1524 of substrate 12. In an embodiment, prior to the
formation of pores 1514, substrate 12 may be planarized, e.g., by
CMP.
[0141] Referring to FIG. 40C, after the formation of pores 1514, a
plurality of layers 10 may be formed over porous top surface 1524
of substrate 12, as described with reference to FIG. 1A. Layers 10
may include, for example, graded buffer layer 14, relaxed layer 16,
and strained layer 18. Pores 1514 define cleave plane 20 in porous
layer 1516 of substrate 12.
[0142] Referring to FIG. 40D, substrate 12 with layers 10 is bonded
to handle wafer 50, including semiconductor substrate 54 and
dielectric layer 52, as described with reference to FIG. 3. Prior
to bonding, a dielectric layer may be formed on a top surface of
layers 10 to facilitate the bonding process and to serve as an
insulator layer in the final substrate structure.
[0143] Referring to FIG. 40E as well as to FIG. 40D, a split is
induced at cleave plane 20 by, for example, cleaving porous layer
1516 by a water or an air jet. The split results in the formation
of two separate wafers 1570, 1572. One of these wafers (1572) has
graded layer 14 and relaxed layer 16 (see FIG. 40c) disposed over
strained layer 18, with a first portion 1580 of substrate 12
disposed over graded layer 14. First portion 1580 of substrate 12
may be just trace amounts of material surrounding pores 1514.
Strained layer 18 is in contact with dielectric layer 52 on
semiconductor substrate 54. The other of these wafers (1570)
includes a second portion 1582 of substrate 12, including the bulk
of substrate 12 with perhaps trace amounts of material surrounding
pores 1514.
[0144] Referring to FIG. 6 as well as to FIG. 40E, first portion
1580 of substrate 12 is removed from graded layer 14 by a wet
chemical cleaning process utilizing, for example a mixture of
hydrogen peroxide (H.sub.2O.sub.2) and HF. Graded layer 14 and
relaxed layer 16 are removed in any one of the methods described
for the removal of relaxed layer portion 80 with reference to FIGS.
4 and 5. Removal of graded and relaxed layers 14, 16 results in the
formation of SSOI substrate 100.
[0145] Referring to FIG. 41A, SSOI substrate 100 (see FIG. 6) may
also be formed by the use of porous intermediate layers. For
example, plurality of layers 10 may be formed over substrate 12,
layers 10 including graded layer 14, relaxed layer 16, and strained
layer 18 (see FIG. 1A). Prior to the formation of strained layer
18, a plurality of pores 1614 may be formed in a top portion of
relaxed layer 16, thereby defining a porous layer 1616 in a top
portion 1617 of relaxed layer 16. Pores 1614 may be formed by the
methods described above with reference to the formation of pores
1514 in FIG. 40B. Porous layer 1616 may have a thickness T.sub.16
of, e.g., 1-5 .mu.m. Strained layer 18 may then be formed directly
over porous layer 1616. Pores 1614 define cleave plane 20 in porous
layer 1616.
[0146] Referring to FIG. 41B, in an alternative embodiment, after
the formation of porous layer 1616 in a portion of relaxed layer
16, a second relaxed layer 1620 may be formed over relaxed layer 16
including porous layer 1616. Second relaxed layer 1620 may include
the same material from which relaxed layer 16 is formed, e.g.,
uniform Si.sub.1-xGe.sub.x having a Ge content of, for example,
10-80% (i.e., x=0.1-0.8) and having a thickness T.sub.17 of, e.g.,
5-100 nm. In some embodiments, Si.sub.1-xGe.sub.x may include
Si.sub.0.70Ge.sub.0.30 and T.sub.17 may be approximately 50 nm.
Second relaxed layer 1620 may be fully relaxed, as determined by
triple axis X-ray diffraction, and may have a threading dislocation
density of <1.times.10.sup.6/cm.sup.2, as determined by etch pit
density (EPD) analysis. Strained layer 18 may be formed over second
relaxed layer 1620. Pores 1614 define cleave plane 20 in porous
layer 1616.
[0147] Referring to FIG. 41C, substrate 12 with layers 10 is bonded
to handle wafer 50, including semiconductor substrate 54 and
dielectric layer 52, as described with reference to FIG. 3.
[0148] Referring to FIG. 41D as well as to FIG. 41C, a split is
induced at cleave plane 20 by, for example, cleaving porous layer
1616 by a water or an airjet. The split results in the formation of
two separate wafers 1670, 1672. One of these wafers (1670) has top
portion 1617 of relaxed layer 16 (see FIG. 41A) disposed over
strained layer 18. Strained layer 18 is in contact with dielectric
layer 52 on semiconductor substrate 54. The other of these wafers
(1672) includes the substrate 12, graded layer 14, and a bottom
portion 1674 of relaxed layer 16.
[0149] Referring to FIG. 6 as well as to FIG. 41D, top portion 1617
of relaxed layer 16 is removed in any one of the methods described
for the removal of relaxed layer portion 80 with reference to FIGS.
4 and 5. Removal of top portion 1617 of relaxed layer 16 results in
the formation of SSOI substrate 100.
[0150] The bonding of strained silicon layer 18 to dielectric layer
52 has been experimentally demonstrated. For example, strained
layer 18 having a thickness of 54 nanometers (nm) along with
.about.350 nm of Si.sub.0.70Ge.sub.0.30 have been transferred by
hydrogen exfoliation to Si handle wafer 50 having dielectric layer
52 formed from thermal SiO.sub.2 with a thickness of approximately
100 nm. The implant conditions were a dose of 4.times.10.sup.16
ions/cm.sup.3 of H.sub.2.sup.+ at 75 keV. The anneal procedure was
1 hour at 550.degree. C. to split the SiGe layer, followed by a 1
hour, 800.degree. C. strengthening anneal. The integrity of
strained Si layer 18 and good bonding to dielectric layer 52 after
layer transfer and anneal were confirmed with cross-sectional
transmission electron microscopy (XTEM). An SSOI structure 100 was
characterized by XTEM and analyzed via Raman spectroscopy to
determine the strain level of the transferred strained Si layer 18.
An XTEM image of the transferred intermediate SiGe/strained
Si/SiO.sub.2 structure showed transfer of the 54 nm strained Si
layer 18 and .about.350 nm of the Si.sub.0.70Ge.sub.0.30 relaxed
layer 16. Strained Si layer 18 had a good integrity and bonded well
to SiO.sub.2 54 layer after the annealing process.
[0151] XTEM micrographs confirmed the complete removal of relaxed
SiGe layer 16 after oxidation and HF etching. The final structure
includes strained Si layer 18 having a thickness of 49 nm on
dielectric layer 52 including SiO.sup.2 and having a thickness of
100 nm.
[0152] Raman spectroscopy data enabled a comparison of the bonded
and cleaved structure before and after SiGe layer 16 removal. Based
on peak positions the compostion of the relaxed SiGe layer and
strain in the Si layer may be calculated. See, for example, J. C.
Tsang, et al., J. Appl. Phys. 75 (12) p. 8098 (1994), incorporated
herein by reference. The fabricated SSOI structure 100 had a clear
strained Si peak visible at .about.511/cm. Thus, the SSOI structure
100 maintained greater than 1% tensile strain in the absence of the
relaxed SiGe layer 16. In addition, the absence of Ge--Ge, Si--Ge,
and Si--Si relaxed SiGe Raman peaks in the SSOI structure confirmed
the complete removal of SiGe layer 16.
[0153] In addition, the thermal stability of the strained Si layer
was evaluated after a 3 minute 1000.degree. C. rapid thermal anneal
(RTA) to simulate an aggregate thermal budget of a CMOS process. A
Raman spectroscopy comparision was made of SSOI structure 100 as
processed and after the RTA step. A scan of the as-bonded and
cleaved sample prior to SiGe layer removal was used for
comparision. Throughout the SSOI structure 100 fabrication processs
and subsequent anneal, the strained Si peak was visible and the
peak position did not shift. Thus, the strain in SSOI structure 100
was stable and was not diminished by thermal processing.
Furthermore, bubbles or flaking of the strained Si surface 18 were
not observed by Nomarski optical microscopy after the RTA,
indicating good mechanical stability of SSOI structure 100.
[0154] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The foregoing embodiments are therefore to be considered
in all respects illustrative rather than limiting on the invention
described herein. Scope of the invention is thus indicated by the
appended claims rather than by the foregoing description, and all
changes which come within the meaning and range of equivalency of
the claims are intended to be embraced therein.
* * * * *