U.S. patent application number 10/264935 was filed with the patent office on 2003-12-11 for strained-semiconductor-on-insulator device structures.
Invention is credited to Currie, Matthew T., Fitzgerald, Eugene A., Hammond, Richard, Langdo, Thomas A., Lochtefeld, Anthony J..
Application Number | 20030227057 10/264935 |
Document ID | / |
Family ID | 29716052 |
Filed Date | 2003-12-11 |
United States Patent
Application |
20030227057 |
Kind Code |
A1 |
Lochtefeld, Anthony J. ; et
al. |
December 11, 2003 |
Strained-semiconductor-on-insulator device structures
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; (Cambridge, MA) ;
Currie, Matthew T.; (Windham, NH) ; Fitzgerald,
Eugene A.; (Windham, NH) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Family ID: |
29716052 |
Appl. No.: |
10/264935 |
Filed: |
October 4, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60386968 |
Jun 7, 2002 |
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60404058 |
Aug 15, 2002 |
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Current U.S.
Class: |
257/347 ;
257/E21.121; 257/E21.127; 257/E21.129; 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 |
Current CPC
Class: |
H01L 21/76264 20130101;
H01L 21/02381 20130101; H01L 29/78684 20130101; H01L 21/0251
20130101; H01L 21/02532 20130101; H01L 27/1203 20130101; H01L 21/84
20130101; H01L 21/76275 20130101; H01L 21/0245 20130101; H01L
29/7842 20130101; H01L 29/78603 20130101; H01L 21/02521 20130101;
H01L 21/02502 20130101; H01L 21/02505 20130101; H01L 21/76254
20130101; H01L 29/66772 20130101; H01L 29/66916 20130101; H01L
29/78687 20130101; H01L 21/76259 20130101 |
Class at
Publication: |
257/347 |
International
Class: |
H01L 027/01 |
Claims
What is claimed is:
1. A structure comprising: a first substrate having a dielectric
layer disposed thereon; and a first strained semiconductor layer
disposed in contact with the dielectric layer.
2. The structure of claim 1 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.
3. The structure of claim 2 wherein the strained semiconductor
layer comprises silicon.
4. The structure of claim 3 wherein the strained semiconductor
layer is substantially free of germanium, and any other layer
disposed in contact with the strained semiconductor layer is
substantially free of germanium.
5. The structure of claim 2 wherein the strained semiconductor
layer comprises germanium.
6. The structure of claim 2 wherein the strained semiconductor
layer comprises silicon germanium.
7. The structure of claim 2 wherein the strained semiconductor
layer comprises gallium arsenide.
8. The structure of claim 2 wherein the strained semiconductor
layer comprises indium phosphide.
9. The structure of claim 2 wherein the strained semiconductor
layer comprises zinc selenide.
10. The structure of claim 1 wherein the strained semiconductor
layer is tensilely strained.
11. The structure of claim 1 wherein the strained semiconductor
layer is compressively strained.
12. The structure of claim 1 wherein the strained semiconductor
layer comprises a strained portion and a relaxed portion.
13. The structure of claim 1, further comprising: a second strained
semiconductor layer in contact with the first strained
semiconductor layer.
14. The structure of claim 13 wherein the first strained
semiconductor layer is compressively strained and the second
strained semiconductor layer is tensilely strained.
15. The structure of claim 13 wherein the first strained
semiconductor layer is tensilely strained and the second strained
semiconductor layer is compressively strained.
16. The structure of claim 1, further comprising: a transistor
including a source region and a drain region disposed in a portion
of the strained semiconductor layer; a gate disposed above the
strained semiconductor layer and between the source and drain
regions; and a gate dielectric layer disposed between the gate and
the strained semiconductor layer.
17. The structure of claim 1 wherein the strained semiconductor
layer has been formed on a second substrate, has been disposed in
contact with the dielectric layer by bonding, and has a lower
dislocation density than an initial dislocation density of the
strained semiconductor layer as formed.
18. The structure of claim 17 wherein the initial dislocation
density has been lowered by etching.
19. The structure of claim 1 wherein the strained semiconductor
layer has been grown with an initial dislocation density and has a
dislocation density less than the initial dislocation density.
20. The structure of claim 1 wherein the strained semiconductor
layer has been formed by epitaxy.
21. The structure of claim 1 wherein the strained semiconductor
layer has a thickness uniformity of better than approximately
.+-.5%.
22. The structure of claim 1 wherein the strained layer has a
thickness selected from a range of approximately 20 angstroms-1000
angstroms.
23. The structure of claim 1 wherein the strained layer has a
surface roughness of less than approximately 20 angstroms.
24. The structure of claim 1 wherein the substrate comprises
silicon.
25. The structure of claim 1 wherein the substrate comprises
germanium.
26. The structure of claim 1 wherein the substrate comprises
silicon germanium.
27. A structure comprising: a relaxed substrate comprising a bulk
material; and a strained layer disposed in contact with the relaxed
substrate, wherein the strain of the strained layer is not induced
by the underlying substrate and the strain is independent of a
lattice mismatch between the strained layer and the relaxed
substrate.
28. The structure of claim 27 wherein the bulk material comprises a
first semiconductor material.
29. The structure of claim 27 wherein the strained layer comprises
a second semiconductor material.
30. The structure of claim 29 wherein the bulk material comprises a
first semiconductor material.
31. The structure of claim 30 wherein the first semiconductor
material is essentially the same as the second semiconductor
material.
32. The structure of claim 31 wherein the first semiconductor
material and the second semiconductor material comprise
silicon.
33. The structure of claim 27 wherein a lattice constant of the
relaxed substrate is equal to a lattice constant of the strained
layer in the absence of said strain.
34. The structure of claim 27 wherein the strain of the strained
layer is greater than approximately 1.times.10.sup.-3.
35. The structure of claim 27 wherein the strained layer has been
formed by epitaxy.
36. The structure of claim 27 wherein the strained layer has a
thickness uniformity of better than approximately .+-.5%.
37. The structure of claim 27 wherein the strained layer has a
thickness selected from a range of approximately 20 angstroms-1000
angstroms.
38. The structure of claim 27 wherein the strained layer has a
surface roughness of less than approximately 20 angstroms.
39. The structure of claim 27, further comprising: a transistor
including a source region and a drain region disposed in a portion
of the strained semiconductor layer; a gate contact disposed above
the strained semiconductor layer and between the source and drain
regions; and a gate dielectric layer disposed between the gate
contact and the strained semiconductor layer.
40. A structure comprising: a substrate comprising a dielectric
material; and a strained semiconductor layer disposed in contact
with the dielectric material.
41. The structure of claim 40 wherein the dielectric material
comprises sapphire.
42. The structure of claim 40 wherein the semiconductor layer has
been formed on a second substrate, has been disposed in contact
with the dielectric material by bonding, and has a lower
dislocation density than an initial dislocation density of the
semiconductor layer as formed.
43. The structure of claim 42 wherein the initial dislocation
density has been lowered by etching.
44. The structure of claim 40 wherein the semiconductor layer has
been formed by epitaxy.
45. A method for forming a structure, the method comprising:
providing a first substrate having a first strained semiconductor
layer formed thereon; bonding the first strained semiconductor
layer to an insulator layer disposed on a second substrate; and
removing the first substrate from the first strained semiconductor
layer, the strained semiconductor layer remaining bonded to the
insulator layer.
46. The method of claim 45 wherein the strained semiconductor layer
is tensilely strained.
47. The method of claim 45 wherein the strained semiconductor layer
is compressively strained.
48. The method of claim 45 wherein the strained semiconductor layer
comprises a surface layer after the removal of the first
substrate.
49. The method of claim 45 wherein the strained semiconductor layer
comprises a buried layer after the removal of the first
substrate.
50. The method of claim 45 wherein removing the first substrate
from the strained semiconductor layer comprises cleaving.
51. The method of claim 50 wherein cleaving comprises implantation
of an exfoliation species through the strained semiconductor layer
to initiate cleaving.
52. The method of claim 51 wherein the exfoliation species
comprises at least one of hydrogen and helium.
53. The method of claim 50 wherein providing the first substrate
comprises providing the first substrate having a second strained
layer disposed between the substrate and the first strained layer,
the second strained layer acting as a cleave plane during
cleaving.
54. The method of claim 53 wherein the second strained layer
comprises a compressively strained layer.
55. The method of claim 54 wherein the compressively strained layer
comprises Si.sub.1-xGe.sub.x.
56. The method of claim 45 wherein providing the first substrate
comprises providing the first substrate having a relaxed layer
disposed between the substrate and the first strained layer.
57. The method of claim 56, further comprising: planarizing the
relaxed layer prior to forming the first strained semiconductor
layer.
58. The method of claim 57, further comprising: after planarizing
the relaxed layer, forming a relaxed semiconductor regrowth layer
thereon.
59. The method of claim 45, further comprising: forming a
dielectric layer over the first strained semiconductor layer prior
to bonding the first strained semiconductor layer to an insulator
layer.
60. The method of claim 45 wherein removing the first substrate
from the strained semiconductor layer comprises mechanical
grinding.
61. The method of claim 45 wherein bonding comprises achieving a
high bond strength at a low temperature.
62. The method of claim 61 wherein the bond strength is greater
than or equal to about 1000 milliJoules/meter squared
(mJ/m.sup.2).
63. The method of claim 61 wherein the temperature is less than
approximately 600.degree. C.
64. The method of claim 61 wherein bonding comprises plasma
activation of a surface of the first semiconductor layer prior to
bonding the first semiconductor layer.
65. The method of claim 64 wherein plasma activation comprises use
of at least one of an ammonia (NH.sub.3), an oxygen (O.sub.2), an
argon (Ar), and a nitrogen (N.sub.2) source gas.
66. The method of claim 61 wherein bonding comprises planarizing a
surface of the first semiconductor layer prior to bonding the first
semiconductor layer.
67. The method of claim 66 wherein planarizing comprises
chemical-mechanical polishing.
68. The method of claim 45, further comprising: relaxing a portion
of the first strained semiconductor layer.
69. The method of claim 68 wherein the portion of the first
strained semiconductor layer is relaxed by introducing a plurality
of ions into the portion of the first strained semiconductor
layer.
70. The method of claim of claim 45, further comprising: forming a
transistor 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.
71. A method for forming a structure, the method comprising:
providing a substrate having a relaxed layer disposed over a first
strained layer, the relaxed layer inducing strain in the first
strained layer; and removing at least a portion of the relaxed
layer selectively with respect to the first strained layer.
72. The method of claim 71 wherein providing the substrate
comprises bonding the first strained layer to the substrate.
73. The method of claim 72 wherein the first strained layer is
bonded to an insulator layer disposed on the substrate.
74. The method of claim 71, further comprising: before providing
the substrate, forming the first strained layer over the relaxed
layer on another substrate.
75. The method of claim 71 wherein the portion of the relaxed layer
is removed by oxidation.
76. The method of claim 71 wherein the portion of the relaxed layer
is removed by a wet chemical etch.
77. The method of claim 71 wherein the portion of the relaxed layer
is removed by a dry etch.
78. The method of claim 71 wherein the portion of the relaxed layer
is removed by chemical-mechanical polishing.
79. The method of claim 71, further comprising: after removal of at
least a portion of the relaxed layer, planarizing the strained
layer.
80. The method of claim 79 wherein planarizing the strained layer
comprises chemical-mechanical polishing.
81. The method of claim 79 wherein planarizing the strained layer
comprises an anneal.
82. The method of claim 81 wherein the anneal is performed at a
temperature greater than 800.degree. C.
83. The method of claim 71 wherein providing the substrate
comprises providing the substrate having an etch stop layer
disposed between the relaxed layer and the strained layer.
84. The method of claim 83 wherein the etch stop layer is
compressively strained.
85. The method of claim 83 wherein the strained layer comprises
silicon, the relaxed layer comprises silicon germanium, and the
etch stop layer comprises silicon germanium carbon.
86. The method of claim 83 wherein the relaxed layer comprises
Si.sub.1-yGe.sub.y, the etch stop layer comprises
Si.sub.1-xGe.sub.x, and x is greater than y.
87. The method of claim 86 wherein x is approximately 0.5 and y is
approximately 0.2.
88. The method of claim 83 wherein the etch stop layer enables an
etch selectivity to the relaxed layer of greater than 10:1.
89. The method of claim 88 wherein the etch stop layer enables an
etch selectivity to the relaxed layer of greater than 100:1.
90. The method of claim 83 wherein the etch stop layer has a
thickness selected from a range of about 20 angstroms to about 1000
angstroms.
91. The method of claim 71 wherein providing the substrate
comprises forming the relaxed layer over a graded layer.
92. A method for forming a structure, the method comprising:
providing a first substrate having a dielectric layer disposed
thereon; forming a semiconductor layer on a second substrate, the
semiconductor layer having an initial misfit dislocation density;
bonding the semiconductor layer to the dielectric layer; removing
the second substrate, the semiconductor layer remaining bonded to
the dielectric layer; and reducing the misfit dislocation density
in the semiconductor layer.
93. The method of claim 92 wherein the misfit dislocation density
is reduced by removing a portion of the semiconductor layer.
94. The method of claim 93 wherein the portion of the semiconductor
layer is removed by etching.
95. The method of claim 93, further comprising: after removing a
portion of the semiconductor layer to reduce misfit dislocation
density, forming a regrowth layer over the semiconductor layer
without increasing misfit dislocation density.
96. The method of claim 95 wherein the regrowth layer is formed by
epitaxy.
97. A method for forming a structure, the method comprising:
providing a first substrate having a dielectric layer disposed
thereon; forming a semiconductor layer on a second substrate, the
semiconductor layer having an initial misfit dislocation density;
bonding the semiconductor layer to the dielectric layer; removing
the second substrate, the semiconductor layer remaining bonded to
the dielectric layer; and growing a regrowth layer over the
semiconductor layer.
98. The method of claim 97 wherein the semiconductor layer and the
regrowth layer comprise the same semiconductor material.
99. The method of claim 97 wherein the semiconductor layer and the
regrowth layer together have a misfit dislocation density not
greater than the initial misfit dislocation density.
100. A method for forming a structure, the method comprising:
providing a first substrate having a strained layer disposed
thereon, the strained layer including a first semiconductor
material; bonding the strained layer to a second substrate, the
second substrate comprising a bulk material; and removing the first
substrate from the strained layer, the strained layer remaining
bonded to the bulk semiconductor material, wherein 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.
101. The method of claim 100 wherein the bulk material comprises a
second semiconductor material.
102. The method of claim 101 wherein the first semiconductor
material is substantially the same as the second semiconductor
material.
103. The method of claim 100 wherein the second substrate comprises
silicon.
104. The method of claim 100 wherein the strained semiconductor
layer comprises silicon.
105. A method for forming a structure, the method comprising:
providing a first substrate having a semiconductor layer disposed
over a strained layer; bonding the semiconductor layer to an
insulator layer disposed on a second substrate; and removing the
first substrate from the strained layer, the semiconductor layer
remaining bonded to the insulator layer.
106. The method of claim 105 wherein the semiconductor layer is
substantially relaxed.
107. The method of claim 105 wherein the semiconductor layer
comprises at least one of a group II, a group III, a group IV, a
group V, and a group VI element.
108. The method of claim 105 wherein the strained layer comprises
at least one of a group II, a group III, a group IV, a group V, and
a group VI element.
109. The method of claim 107 wherein the semiconductor layer
comprises germanium and the strained layer comprises silicon.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/386,968 filed Jun. 7, 2002 and U.S. Provisional
Application No. 60/404,058 filed Aug. 15, 2002; the entire
disclosures of both 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.sup.-3. 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.
SUMMARY
[0005] The present invention includes a
strained-semiconductor-on-insulato- r (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. 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.
[0006] This approach enables the fabrication of well-controlled,
epitaxially-defined, thin strained semiconductor layers directly on
an insulator layer. Tensile strain levels of .about.1% 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.
[0007] In an aspect, the invention features a structure that
includes a first substrate having a dielectric layer disposed
thereon, and a first strained semiconductor layer disposed in
contact with the dielectric layer.
[0008] 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 element,
such as silicon, germanium, silicon germanium, gallium arsenide,
indium phosphide, or zinc selenide. The strained semiconductor
layer may be substantially free of germanium, and any other layer
disposed in contact with the strained semiconductor layer may be
substantially free of germanium. The strained semiconductor layer
may be tensilely strained or compressively strained. The strained
semiconductor layer may have a strained portion and a relaxed
portion.
[0009] A second strained semiconductor layer may be in contact with
the first strained semiconductor layer. The first strained
semiconductor layer may be compressively strained and the second
strained semiconductor layer may be tensilely strained, or vice
versa.
[0010] The structure may include a transistor having a source
region and a drain region disposed in a portion of the strained
semiconductor layer, a gate disposed above the strained
semiconductor layer and between the source and drain regions, and a
gate dielectric layer disposed between the gate and the strained
semiconductor layer.
[0011] The strained semiconductor layer may have been formed on a
second substrate, may have been disposed in contact with the
dielectric layer by bonding, and may have a lower dislocation
density than an initial dislocation density of the strained
semiconductor layer as formed. The initial dislocation density may
have been lowered by etching. The strained semiconductor layer may
have been grown with an initial dislocation density and may have a
dislocation density less than the initial dislocation density. The
strained semiconductor layer may have been formed by epitaxy. The
strained semiconductor layer may have a thickness uniformity of
better than approximately .+-.5%. The strained layer has a
thickness selected from a range of approximately 20 angstroms-1000
angstroms. The strained layer has a surface roughness of less than
approximately 20 angstroms. The substrate may include silicon
and/or germanium.
[0012] In another aspect, the invention features a structure
including a relaxed substrate including a bulk material, and a
strained layer disposed in contact with the relaxed substrate. The
strain of the strained layer is not induced by the underlying
substrate, and the strain is independent of a lattice mismatch
between the strained layer and the relaxed substrate. The bulk
material may include a first semiconductor material. The strained
layer may include a second semiconductor material. The first
semiconductor material may be essentially the same as the second
semiconductor material. The first and second semiconductor material
may include silicon. A lattice constant of the relaxed substrate
may be equal to a lattice constant of the strained layer in the
absence of strain. The strain of the strained layer may be greater
than approximately 1.times.10.sup.-3. The strained layer may have
been formed by epitaxy. The strained layer may have a thickness
uniformity of better than approximately .+-.5%. The strained layer
may have a thickness selected from a range of approximately 20
angstroms-1000 angstroms. The strained layer may have a surface
roughness of less than approximately 20 angstroms.
[0013] The structure may include a transistor having a source
region and a drain region disposed in a portion of the strained
semiconductor layer, a gate contact disposed above the strained
semiconductor layer and between the source and drain regions, and a
gate dielectric layer disposed between the gate contact and the
strained semiconductor layer.
[0014] In another aspect, the invention features a structure
including a substrate including a dielectric material, and a
strained semiconductor layer disposed in contact with the
dielectric material.
[0015] One or more of the following features may be included. The
dielectric material may include sapphire. The semiconductor layer
may have been formed on a second substrate, have been disposed in
contact with the dielectric material by bonding, and have a lower
dislocation density than an initial dislocation density of the
semiconductor layer as formed. The initial dislocation density may
have been lowered by etching. The semiconductor layer may have been
formed by epitaxy.
[0016] In another aspect, the invention features a method for
forming a structure, the method including providing a first
substrate having a first strained semiconductor layer formed
thereon, bonding the first strained semiconductor layer to an
insulator layer disposed on a second substrate and, removing the
first substrate from the first strained semiconductor layer, the
strained semiconductor layer remaining bonded to the insulator
layer.
[0017] One or more of the following features may be included. The
strained semiconductor layer may be tensilely or compressively
strained. The strained semiconductor layer may include a surface
layer or a buried layer after the removal of the first
substrate.
[0018] Removing the first substrate from the strained semiconductor
layer may include cleaving. Cleaving may include implantation of an
exfoliation species through the strained semiconductor layer to
initiate cleaving. The exfoliation species may include at least one
of hydrogen and helium. Providing the first substrate may include
providing the first substrate having a second strained layer
disposed between the substrate and the first strained layer, the
second strained layer acting as a cleave plane during cleaving. The
second strained layer may include a compressively strained layer.
The compressively strained layer may include S.sub.1-xGe.sub.x. The
first substrate may have a relaxed layer disposed between the
substrate and the first strained layer.
[0019] The relaxed layer may be planarized prior to forming the
first strained semiconductor layer. After the relaxed layer is
planarized, a relaxed semiconductor regrowth layer may be formed
thereon. A dielectric layer may be formed over the first strained
semiconductor layer prior to bonding the first strained
semiconductor layer to an insulator layer. Removing the first
substrate from the strained semiconductor layer may include
mechanical grinding. Bonding may include achieving a high bond
strength, e.g., greater than or equal to about 1000
milliJoules/meter squared (mJ/m.sup.2), at a low temperature, e.g.,
less than approximately 600.degree. C.
[0020] Bonding may include plasma activation of a surface of the
first semiconductor layer prior to bonding the first semiconductor
layer. Plasma activation may include use of at least one of an
ammonia (NH.sub.3), an oxygen (O.sub.2), an argon (Ar), and a
nitrogen (N.sub.2) source gas. Bonding may include planarizing a
surface of the first semiconductor layer prior to bonding the first
semiconductor layer by, e.g., chemical-mechanical polishing. A
portion of the first strained semiconductor layer may be relaxed
such as by, e.g., introducing a plurality of ions into the portion
of the first strained semiconductor layer.
[0021] A transistor may be formed 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.
[0022] In another aspect, the invention features a method for
forming a structure, the method including providing a substrate
having a relaxed layer disposed over a first strained layer, the
relaxed layer inducing strain in the first strained layer, and
removing at least a portion of the relaxed layer selectively with
respect to the first strained layer.
[0023] One or more of the following features may be included. The
first strained layer may be bonded to the substrate, including,
e.g., to an insulator layer disposed on the substrate. The first
strained layer may be formed over the relaxed layer on another
substrate. The portion of the relaxed layer may be removed by,
e.g., oxidation, a wet chemical etch, a dry etch, and/or
chemical-mechanical polishing. After removal of at least a portion
of the relaxed layer, the strained layer may be planarized by,
e.g., chemical-mechanical polishing and/or an anneal. The anneal
may be performed at a temperature greater than 800.degree. C.
[0024] The substrate may have an etch stop layer disposed between
the relaxed layer and the strained layer. The etch stop layer may
be compressively strained. The strained layer may include silicon,
the relaxed layer may include silicon germanium, and the etch stop
layer may include silicon germanium carbon. The relaxed layer may
include Si.sub.1-yGe.sub.y, the etch stop layer may include
Si.sub.1-xGe.sub.x, and x may be greater than y, e.g., x may be
approximately 0.5 and y may be approximately 0.2. The etch stop
layer enables an etch selectivity to the relaxed layer of greater
than 10:1, e.g., greater than 100:1. The etch stop layer may have a
thickness selected from a range of about 20 angstroms to about 1000
angstroms. The relaxed layer may be formed over a graded layer.
[0025] In another aspect, the invention features a method for
forming a structure, the method including providing a first
substrate having a dielectric layer disposed thereon, and forming a
semiconductor layer on a second substrate, the semiconductor layer
having an initial misfit dislocation density. The semiconductor
layer is bonded to the dielectric layer, and the second substrate
is removed, the semiconductor layer remaining bonded to the
dielectric layer. The misfit dislocation density in the
semiconductor layer is reduced.
[0026] One or more of the following features may be included. The
misfit dislocation density may be reduced by removing a portion of
the semiconductor layer, such as, e.g., by etching. After removing
a portion of the semiconductor layer to reduce misfit dislocation
density, a regrowth layer may be formed over the semiconductor
layer without increasing misfit dislocation density. The regrowth
layer may be formed by epitaxy.
[0027] In another aspect, the invention features a method for
forming a structure, the method including providing a first
substrate having a dielectric layer disposed thereon, forming a
semiconductor layer on a second substrate, the semiconductor layer
having an initial misfit dislocation density. The semiconductor
layer is bonded to the dielectric layer. The second substrate is
removed, the semiconductor layer remaining bonded to the dielectric
layer, and a regrowth layer is grown over the semiconductor
layer.
[0028] One or more of the following features may be included. The
semiconductor layer and the regrowth layer may include the same
semiconductor material. The semiconductor layer and the regrowth
layer together may have a misfit dislocation density not greater
than the initial misfit dislocation density.
[0029] In another aspect, the invention features a 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, and bonding the
strained layer to a second substrate, the second substrate
including a bulk material. The first substrate is removed from the
strained layer, the strained layer remaining bonded to the bulk
semiconductor material. 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.
[0030] One or more of the following features may be included. The
bulk material may include a second semiconductor material. The
first semiconductor material may be substantially the same as the
second semiconductor material. The second substrate and/or the
strained semiconductor layer may include silicon.
[0031] In another aspect, the invention features a method for
forming a structure, the method including providing a first
substrate having a semiconductor layer disposed over a strained
layer. The semiconductor layer is bonded to an insulator layer
disposed on a second substrate, and the first substrate is removed
from the strained layer, the semiconductor layer remaining bonded
to the insulator layer.
[0032] One or more of the following features may be included. The
semiconductor layer may be substantially relaxed. The semiconductor
layer and/or the strained layer may include at least one of a group
II, a group III, a group IV, a group V, and a group VI element. The
semiconductor layer may include germanium and the strained layer
may include silicon.
BRIEF DESCRIPTION OF DRAWINGS
[0033] FIGS. 1A-6 are schematic cross-sectional views of substrates
illustrating a method for fabricating an SSOI substrate;
[0034] FIG. 7 is a schematic cross-sectional view illustrating an
alternative method for fabricating the SSOI substrate illustrated
in FIG. 6;
[0035] FIG. 8 is a schematic cross-sectional view of a transistor
formed on the SSOI substrate illustrated in FIG. 6;
[0036] FIGS. 9-10 are schematic cross-sectional views of
substrate(s) illustrating a method for fabricating an alternative
SSOI substrate;
[0037] FIG. 11 is a schematic cross-sectional view of a substrate
having several layers formed thereon;
[0038] FIGS. 12-13 are schematic cross-sectional views of
substrates illustrating a method for fabricating an alternative
strained semiconductor substrate; and
[0039] FIG. 14 is a schematic cross-sectional view of the SSOI
substrate illustrated in FIG. 6 after additional processing.
[0040] Like-referenced features represent common features in
corresponding drawings.
DETAILED DESCRIPTION
[0041] 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.
[0042] 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., 20-70%
(i.e., y=0.2-0.7) and a thickness T.sub.1 of, for example, 2-7
micrometers (.mu.m). 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, 20-70%
(i.e., x=0.2-0.7), 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 cm.sup.-2, as determined by etch pit density
(EPD) analysis.
[0043] 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.
[0044] 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 III, 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). 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.. 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), or
by molecular beam epitaxy (MBE). 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. After formation, strained layer 18 has an initial misfit
dislocation density, of, for example, 0-10.sup.5 cm.sup.-1. In one
embodiment, strained layer 18 is tensilely strained. In another
embodiment, strained layer 18 is compressively strained.
[0045] 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.
[0046] 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.
[0047] 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 3-5.times.10.sup.16/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/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.
[0048] In some embodiments, strained layer 18 may be planarized by,
e.g., CMP, to improve the quality of the subsequent bond. 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 can be a 10 minute anneal at 800.degree. C.
in a nitrogen 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.
[0049] 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, silicon
nitride (Si.sub.3N.sub.4), aluminum oxide, etc. In other
embodiments, handle wafer 50 may comprise 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.
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, and
nitrogen. 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.
[0050] 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 silicon substrate 12,
graded layer 14, and a remaining portion 82 of relaxed layer 16. 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.
[0051] Referring to FIG. 4 as well as to FIG. 5, relaxed layer
portion 80 is removed from strained layer 18. Relaxed layer portion
80, including, e.g., SiGe, is oxidized 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.
[0052] 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.2-
O.sub.2) at a ratio of 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.
[0053] In an embodiment, after cleaving and prior to removal of
relaxed layer portion 80 by, e.g., wet oxidation, a CMP step may be
performed to remove part of relaxed layer portion 80 as well as to
increase the smoothness of its surface. A smoother surface will
improve the uniformity of subsequent complete removal by, e.g., wet
oxidation.
[0054] 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 or an anneal at a temperature greater than,
for example, 800.degree. C.
[0055] 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, 20-1000 .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, 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. 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, e.g., dielectric layer 52, is also substantially free of
Ge.
[0056] Referring to FIG. 7, in an alternative embodiment, relaxed
layer portion 80 may be removed by a selective wet etch which 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 mixture of hydrofluoric
acid, hydrogen peroxide, and acetic acid
(HF:H.sub.2O.sub.2:CH.sub.3COOH), at a ratio of 1:2:3.
Alternatively, relaxed layer portion 80 may be removed by a dry
etch which 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.
[0057] Strained semiconductor-on-insulator substrate 100 may be
further processed by CMOS SOI MOSFET fabrication methods. For
example, referring to FIG. 8, 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 hafnium
oxide (HfO.sub.2)or hafnium silicate (HfSiON, 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, or a metal. 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.2O.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.
[0064] 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 .+-.5%. Surface 92 of strained layer 18
may have a surface roughness of less than 20 .ANG..
[0065] 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.
[0066] 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 4.times.10.sup.16/cm.sup.3
H.sub.2.sup.+ dose 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.
[0067] 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.sub.2 and having a thickness of
100 nm.
[0068] Raman spectroscopy data enabled a comparison of the bonded
and cleaved structure before and after SiGe layer 16 removal. Based
on peak positions the composition 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.sup.-1. 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, SiGe, and Si--Si relaxed SiGe Raman peaks in the SSOI
structure confirmed the complete removal of SiGe layer 16.
[0069] 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.
[0070] 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.
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