U.S. patent application number 10/802186 was filed with the patent office on 2005-01-13 for process for producing semiconductor article using graded epitaxial growth.
This patent application is currently assigned to Massachusetts Institute of Technology. Invention is credited to Antoniadis, Dimitri A., Cheng, Zhi-Yuan, Fitzgerald, Eugene A., Hoyt, Judy L..
Application Number | 20050009288 10/802186 |
Document ID | / |
Family ID | 22845751 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050009288 |
Kind Code |
A1 |
Cheng, Zhi-Yuan ; et
al. |
January 13, 2005 |
Process for producing semiconductor article using graded epitaxial
growth
Abstract
A process for producing monocrystalline semiconductor layers. In
an exemplary embodiment, a graded Si.sub.1-xGe.sub.x (x increases
from 0 to y) is deposited on a first silicon substrate, followed by
deposition of a relaxed Si.sub.1-yGe.sub.y layer, a thin strained
Si.sub.1-zGe.sub.z layer and another relaxed Si.sub.1-yGe.sub.y
layer. Hydrogen ions are then introduced into the strained
Si.sub.zGe.sub.z layer. The relaxed Si.sub.1-yGe.sub.y layer is
bonded to a second oxidized substrate. An annealing treatment
splits the bonded pair at the strained Si layer, such that the
second relaxed Si.sub.1-yGe.sub.y layer remains on the second
substrate. In another exemplary embodiment, a graded
Si.sub.xGe.sub.x is deposited on a first silicon substrate, where
the Ge concentration x is increased from 0 to 1. Then a relaxed
GaAs layer is deposited on the relaxed Ge buffer. As the lattice
constant of GaAs is close to that of Ge, GaAs has high quality with
limited dislocation defects. Hydrogen ions are introduced into the
relaxed GaAs layer at the selected depth. The relaxed GaAs layer is
bonded to a second oxidized substrate. An annealing treatment
splits the bonded pair at the hydrogen ion rich layer, such that
the upper portion of relaxed GaAs layer remains on the second
substrate.
Inventors: |
Cheng, Zhi-Yuan; (Cambridge,
MA) ; Fitzgerald, Eugene A.; (Windham, NH) ;
Antoniadis, Dimitri A.; (Newton, MA) ; Hoyt, Judy
L.; (Belmont, MA) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
|
Family ID: |
22845751 |
Appl. No.: |
10/802186 |
Filed: |
March 17, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10802186 |
Mar 17, 2004 |
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10379355 |
Mar 4, 2003 |
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6713326 |
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10379355 |
Mar 4, 2003 |
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09928126 |
Aug 10, 2001 |
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6573126 |
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60225666 |
Aug 16, 2000 |
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Current U.S.
Class: |
438/407 ;
257/E21.125; 257/E21.127; 257/E21.568; 438/455; 438/459 |
Current CPC
Class: |
H01L 21/0245 20130101;
H01L 21/02532 20130101; H01L 21/02381 20130101; H01L 21/76254
20130101; H01L 21/02521 20130101; H01L 21/02546 20130101; H01L
21/0251 20130101; Y10S 438/933 20130101; H01L 21/02502
20130101 |
Class at
Publication: |
438/407 ;
438/455; 438/459 |
International
Class: |
H01L 021/00; C30B
001/00; H01L 021/84; H01L 021/46 |
Claims
What is claimed is:
1-40. (cancelled)
41. A method for forming a semiconductor layer, the method
comprising: forming a first heterostructure by: forming a graded
Si.sub.1-xGe.sub.x buffer layer on a first substrate, the graded
Si.sub.1-xGe.sub.x buffer layer having a Ge concentration x
increasing from zero to a value y; forming a relaxed
Si.sub.1-yGe.sub.y layer on the graded Si.sub.1-xGe.sub.x buffer
layer; forming a separation layer on the relaxed Si.sub.1-yGe.sub.y
layer; and forming a second relaxed layer over the separation
layer; bonding the first heterostructure to a second substrate to
define a second heterostructure; and splitting the second
heterostructure along the separation layer, wherein the second
relaxed layer remains on the second substrate after the second
heterostructure is split.
42. The method of claim 56, wherein the strained layer comprises at
least one of Si.sub.1-zGe.sub.z with z.noteq.y and a III-V
material.
43. The method of claim 41, wherein at least one of the relaxed
layer and the separation layer comprises at least one material
selected from the group consisting of Si.sub.1-wGe.sub.w, Ge, GaAs,
AlAs, ZnSe and InGaP.
44. The method of claim 41, further comprising: forming at least
one of a device layer and a device, after the step of forming the
second relaxed layer.
45. The method of claim 57, further comprising: forming an
insulating layer before the step of introducing ions.,
46. The method of claim 57, further comprising: planarizing the
second relaxed layer before the step of introducing ions.
47. The method of claim 57, wherein the ions comprise at least one
of hydrogen H.sup.+ ions and H.sub.2.sup.+ ions.
48. The method of claim 41 further comprising: planarizing the
second relaxed layer before bonding the first heterostructure to
the second substrate.
49. The method of claim 41, further comprising: cleaning at least
one of the first heterostructure and the second substrate before
the step of bonding.
50. The method of claim 41, wherein splitting the second
heterostructure comprises annealing.
51. The method of claim 41, further comprising: removing at least
one of (i) a remaining portion of the separation layer, and (ii) a
top portion of the second relaxed layer from the second substrate
after the step of splitting.
52. The method of claim 41, further comprising: forming at least
one of a device layer and a device after the step of splitting.
53. The method of claim 41, further comprising: after splitting the
second heterostructure along the separation layer, planarizing a
portion of the first heterostructure split from the second
substrate; and forming new layers on the remaining first
heterostructure portion.
54-55. (canceled)
56. The method of claim 41 wherein the separation layer comprises a
strained layer.
57. The method of claim 41, further comprising: introducing ions
into the separation layer, prior to bonding the first
heterostructure to the second substrate.
58. The method of claim 41 wherein the separation layer comprises a
defect layer.
59. The method of claim 41 wherein the second substrate comprises
silicon.
60. The method of claim 41 wherein the second substrate comprises
an insulator layer.
61. The method of claim 41 wherein bonding the first
heterostructure to the second substrate comprises bonding to the
insulator layer.
Description
PRIORITY INFORMATION
[0001] This application claims priority from provisional
application Ser. No. 60/225,666 filed Aug. 16, 2000.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a production of a general
substrate of relaxed Si.sub.1-xGe.sub.x-on-insulator (SGOI) for
various electronics or optoelectronics applications, and the
production of monocrystalline III-V or II-VI material-on-insulator
substrate.
[0003] Relaxed Si.sub.1-xGe.sub.x-on-insulator (SGOI) is a very
promising technology as it combines the benefits of two advanced
technologies: the conventional SOI technology and the disruptive
SiGe technology. The SOI configuration offers various advantages
associated with the insulating substrate, namely reduced parasitic
capacitances, improved isolation, reduced short-channel-effect,
etc. High mobility strained-Si, strained-Si.sub.1-xGe.sub.x or
strained-Ge MOS devices can be made on SGOI substrates.
[0004] Other III-V optoelectronic devices can also be integrated
into the SGOI substrate by matching the lattice constants of III-V
materials and the relaxed Si.sub.1-xGe.sub.x. For example a GaAs
layer can be grown on Si.sub.1-xGe.sub.x-on-insulator where x is
equal or close to 1. SGOI may serve as an ultimate platform for
high speed, low power electronic and optoelectronic
applications.
[0005] SGOI has been fabricated by several methods in the prior
art. In one method, the separation by implantation of oxygen
(SIMOX) technology is used to produce SGOI. High dose oxygen
implant was used to bury high concentrations of oxygen in a
Si.sub.1-xGe.sub.x layer, which was then converted into a buried
oxide (BOX) layer upon annealing at high temperature (for example,
1350.degree. C.). See, for example, Mizuno et al. IEEE Electron
Device Letters, Vol. 21, No. 5, pp. 230-232, 2000 and Ishilawa et
al. Applied Physics Letters, Vol. 75, No. 7, pp. 983-985, 1999. One
of the main drawbacks is the quality of the resulting
Si.sub.1-xGe.sub.x film and BOX. In addition, Ge segregation during
high temperature anneal also limits the maximum Ge composition to a
low value.
[0006] U.S. Pat. Nos. 5,461,243 and 5,759,898 describe a second
method, in which a conventional silicon-on-insulator (SOI)
substrate was used as a compliant substrate. In the process, an
initially strained Si.sub.1-xGe.sub.x layer was deposited on a thin
SOI substrate. Upon an anneal treatment, the strain was transferred
to the thin silicon film underneath, resulting in relaxation of the
top Si.sub.1-xGe.sub.x film. The final structure is
relaxed-SiGe/strained-Si/insulator, which is not an ideal SGOI
structure. The silicon layer in the structure is unnecessary, and
may complicate or undermine the performance of devices built on it.
For example, it may form a parasitic back channel on this
strained-Si, or may confine unwanted electrons due to the band gap
offset between the strained-Si and SiGe layer.
[0007] U.S. Pat. Nos. 5,906,951 and 6,059,895 describe the
formation of a similar SGOI structure:
strained-layer(s)/relaxed-SiGe/Si/insulator structure. The
structure was produced by wafer bonding and etch back process using
a P.sup.++ layer as an etch stop. The presence of the silicon layer
in the above structure may be for the purpose of facilitating
Si-insulator wafer bonding, but is unnecessary for ideal SGOI
substrates. Again, the silicon layer may also complicate or
undermine the performance of devices built on it. For example, it
may form a parasitic back channel on this strained-Si, or may
confine unwanted electrons due to the band gap offset between the
strained-Si and SiGe layer. Moreover, the etch stop of Pa in the
above structure is not practical when the first graded
Si.sub.1-yGe.sub.y layer described in the patents has a y value of
larger than 0.2. Experiments from research shows Si.sub.1-yGe.sub.y
with y larger than 0.2 is a very good etch stop for both KOH and
TMAH, as described in a published PCT application WO 99/53539.
Therefore, the KOH will not be able to remove the first graded
SityGey layer and the second relaxed SiGe layer as described in the
patents.
[0008] Other attempts include re-crystallization of an amorphous
Si.sub.1-xGe.sub.x layer deposited on the top of SOI
(silicon-on-insulator) substrate, which is again not an ideal SGOI
substrate and the silicon layer is unnecessary, and may complicate
or undermine the performance of devices built on it. Note Yeo et
al. IEEE Electron Device Letters, Vol. 21, No. 4, pp. 161-163,
2000. The relaxation of the resultant SiGe film and quality of the
resulting structure are main concerns.
[0009] From the above, there is a need for a simple technique for
relaxed SGOI substrate production, a need for a technique for
production of high quality SGOI and other III-V
material-on-insulator, and a need for a technique for wide range of
material transfer.
SUMMARY OF THE INVENTION
[0010] According to the invention, there is provided an improved
technique for production of wide range of high quality material is
provided. In particular, the production of relaxed
Si.sub.1-xGe.sub.x-on-insulator (SGOI) substrate or relaxed III-V
or II-VI material-on-insulator, such as GaAs-on-insulator, is
described. High quality monocrystalline relaxed SiGe layer, relaxed
Ge layer, or other relaxed III-V material layer is grown on a
silicon substrate using a graded Si.sub.1-xGe.sub.x epitaxial
growth technique. A thin film of the layer is transferred into an
oxidized handle wafer by wafer bonding and wafer splitting using
hydrogen ion implantation. The invention makes use of the graded
Si.sub.1-xGe.sub.x buffer structure, resulting in a simplified and
improved process.
[0011] The invention also provides a method allowing a wide range
of device materials to be integrated into the inexpensive silicon
substrate. For example, it allows production of
Si.sub.1-xGe.sub.x-on-insulator with wide range of Ge
concentration, and allows production of many III-V or II-VI
materials on insulator like GaAs, AlAs, ZnSe and InGaP. The use of
graded Si.sub.1-xGe.sub.x buffer in the invention allows high
quality materials with limited dislocation defects to be produced
and transferred. In one example, SGOI is produced using a SiGe
structure in which a region in the graded buffer can act as a
natural etch stop.
[0012] The invention provides a process and method for producing
monocrystalline semiconductor layers. In an exemplary embodiment, a
graded Si.sub.1-xGe.sub.x (x increases from 0 to y) is deposited on
a first silicon substrate, followed by deposition of a relaxed
Si.sub.1-yGe.sub.y layer, a thin strained Si.sub.1-zGe.sub.z layer
and another relaxed Si.sub.1-yGe.sub.y layer. Hydrogen ions are
then introduced into the strained Si.sub.zGe.sub.z layer. The
relaxed Si.sub.1-yGe.sub.y layer is bonded to a second oxidized
substrate. An annealing treatment splits the bonded pair at the
strained Si layer, whereby the second relaxed Si.sub.1-yGe.sub.y
layer remains on said second substrate.
[0013] In another exemplary embodiment, a graded Si.sub.1-xGe.sub.x
is deposited on a first silicon substrate, where the Ge
concentration x is increased from 0 to 1. Then a relaxed GaAs layer
is deposited on the relaxed Ge buffer. As the lattice constant of
GaAs is close to that of Ge, GaAs has high quality with limited
dislocation defects. Hydrogen ions are introduced into the relaxed
GaAs layer at the selected depth. The relaxed GaAs layer is bonded
to a second oxidized substrate. An annealing treatment splits the
bonded pair at the hydrogen ion rich layer, whereby the upper
portion of relaxed GaAs layer remains on said second substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIGS. 1A-1C are block diagrams showing the process of
producing a SGOI substrate in accordance with the invention;
[0015] FIGS. 2A and 2B are infrared transmission images of an
as-bonded wafer pair and a final SGOI substrate after splitting,
respectively;
[0016] FIG. 3 is a TEM cross-section view of a SiGe layer that was
transferred onto the top of a buried oxide;
[0017] FIG. 4 is an AFM for a transferred SGOI substrate showing
surface roughness; and
[0018] FIGS. 5-8 are block diagrams of various exemplary
embodiments semiconductor structures in accordance with the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] An example of a process in which SGOI is created by layer
transfer is described. The experiment was performed in two stages.
In the first stage, heteroepitaxial SiGe layers are formed by a
graded epitaxial growth technology. Starting with a 4-inch Si (100)
donor wafer 100, a linearly stepwise compositionally graded
Si.sub.1-xGe.sub.x buffer 102 is deposited with CVD, by increasing
Ge concentration from zero to 25%. Then a 2.5 .mu.m relaxed
Si.sub.0.75Ge.sub.0.25 cap layer 104 is deposited with the final Ge
composition, as shown in FIG. 1A.
[0020] The relaxed SiGe cap layer has high quality with very low
dislocation defect density (less than 1E6/cm.sup.2), as the graded
buffer accommodates the lattice mismatch between Si and relaxed
SiGe. A thin layer of this high quality SiGe will be transferred
into the final SGOI structure. The surface of the as-grown relaxed
SiGe layer shows a high roughness around 11 nm to 15 nm due to the
underlying strain fields generated by misfit dislocations at the
graded layer interfaces and thus chemical-mechanical polishing
(CMP) is used to smooth the surface. In the second stage, the donor
wafer is implanted with hydrogen ion (100 keV,
5E16H.sup.+/cm.sup.2) to form a buried hydrogen-rich layer. After a
surface clean step in a modified RCA solution, it is bonded to an
oxidized 106 Si handle wafer 108 at room temperature as shown in
FIG. 1B.
[0021] The wafer bonding is one of the key steps, and the bonding
energy should be strong enough in order to sustain the subsequent
layer transfer in the next step. Good bonding requires a flat
surface and a highly hydrophilic surface before bonding. On the
other hand, the buried oxide in the final bonded structure is also
required to have good electrical properties as it will influence
the final device fabricated on it. In the conventional Si film
transfer, thermal oxide on the donor wafer is commonly used before
H.sup.+ implantation and wafer bonding, which becomes the buried
oxide in the resulting silicon-on-insulator structure.
[0022] The thermal oxide of the Si donor wafer meets all the
requirements, as it has good electrical properties, has flat
surface and bonds very well to the handle wafer. Unlike the Si,
however, the oxidation of SiGe film results in poor thermal oxide
quality, and the Ge segregation during oxidation also degrades the
SiGe film. Therefore the thermal oxide of SiGe is not suitable for
the SGOI fabrication. In one exemplary experiment the SiGe film
will be directly bonded to an oxidized Si handle wafer. The high
quality thermal oxide in the handle wafer will become the buried
oxide in the final SGOI structure. Having a flat surface after a
CMP step, the SiGe wafer went through a clean step.
[0023] Compared to Si, one difficulty of SiGe film is that, SiGe
surface becomes rougher during the standard RCA clean, as the
NH.sub.4OH in RCA1 solution etches Ge faster than Si. Rough surface
will lead to weak bonding as the contact area is reduced when
bonded to the handle wafer. In this exemplary embodiment,
H.sub.2SO.sub.4--H.sub.2O.sub.2 solution is used in the place of
RCA I, which also meets the clean process requirement for the
subsequent furnace annealing after bonding. The SiGe surface after
H.sub.2SO.sub.4--H.sub.2O.sub.2 clean shows better surface
roughness compared to RCA1.
[0024] After this modified clean procedure, the SiGe wafer is
dipped in the diluted HF solution to remove the old native oxide.
It is then rinsed in DI water thoroughly to make the surface
hydrophilic by forming a fresh new native oxide layer that is
highly active. After spinning dry, the SiGe wafer is bonded to an
oxidized handle wafer at room temperature, and then annealed at
600.degree. C. for 3 hours. During anneal the bonded pair split
into two sheets along the buried hydrogen-rich layer, and a thin
relaxed Si.sub.0.75Ge.sub.0.25 film 110 is transferred into the
handle wafer, resulting in a SGOI substrate 112, as shown in FIG.
1B. A final 850.degree. C. anneal improves the
Si.sub.0.75Ge.sub.0.25/SiO.sub.2 bond. Thereafter, device layers
114 can be processed on the SGOI substrate 112 as shown in FIG.
1C.
[0025] FIGS. 2A and 2B are infrared transmission images of the
as-bonded wafer pair and the final SGOI substrate after splitting,
respectively. To investigate the surface of the as-transferred SGOI
substrate, transmission electron microscopy (TEM) and atomic force
microscopy (AFM) were used. The TEM cross-section view in FIG. 3
shows a 640 nm SiGe layer was transferred onto the top of a 550 nm
buried oxide (BOX). Surface damage is also shown clearly at the
splitting surface with a damage depth of 100 nm.
[0026] FIG. 4 shows a surface roughness of 11.3 nm in an area of
5.times.5 .mu.m.sup.2 by AFM for the as-transferred SGOI. The data
is similar to those from as-transferred silicon film by smart-cut
process, and suggests that a top layer of about 100 nm should be
removed by a final CMP step. After SiGe film transferring, only a
thin relaxed SiGe film is removed and the donor wafer can be used
again for a donor wafer. Starting from this general SGOI substrate,
various device structures can be realized by growing one or more
device layers on the top, as shown in FIG. 2C.
[0027] Electrical evaluation is in progress by growing a strain Si
layer on the top of this SGOI substrate followed by fabrication of
strained Si channel devices. Bond strength is important to the
process of the invention. AFM measurements were conducted to
investigate the SiGe film surface roughness before bonding under
different conditions. One experiment is designed to investigate how
long the SiGe surface should be polished to have smooth surface and
good bond strength, since the surface of the as-grown relaxed SiGe
layer has a high roughness around 11 nm to 15 nm. Several identical
4-inch Si wafers with relaxed Si.sub.0.75Ge.sub.0.25 films were
CMPed with optimized polishing conditions for different times.
Using AFM, the measured surface mircoroughness RMS at an area of 10
.mu.m.times.10 .mu.m is 5.5 .ANG., 4.5 .ANG. and 3.8 .ANG., for
wafer CMPed for 2 min., 4 min. and 6 min. respectively. After
bonding to identical handle wafers, the tested bond strength
increases with decreasing RMS. A CMP time of 6 min. is necessary
for good strength. In another experiment, two identical 4-inch Si
wafers with relaxed Si.sub.0.75Ge.sub.0.25 films were CMPed for 8
min. After two cleaning steps in H.sub.2SO.sub.4:H.sub.2O.sub.2
solution and one step in diluted HF solution, one wafer was put in
a new H.sub.2SO.sub.4:H.sub.2O.- sub.2 (3:1) solution and another
in a new NH.sub.4OH:H.sub.2O.sub.2:H.sub.- 2O (1:1:5), i.e. the
conventional RCA1 solution, both for 15 min. The resultant wafers
were tested using AFM. The wafer after
H.sub.2SO.sub.4:H.sub.2O.sub.2 solution shows a surface roughness
RMS of 2 .ANG. at an area of 1 .mu.m.times.1 .mu.m, which after
NH.sub.4OH:H.sub.2O.sub.2:H.sub.2O shows 4.4 .ANG.. Clearly, the
conventional RCA clean roughens the SiGe surface significantly, and
H.sub.2SO.sub.4:H.sub.2O.sub.2 should be used for SiGe clean.
[0028] In yet another experiment, the clean procedure is optimized
before bonding. For direct SiGe wafer to oxidized handle wafer
bonding (SiGe-oxide bonding), several different clean procedures
were tested. It has been found that the
H.sub.2SO.sub.4:H.sub.2O.sub.2 (24:1) solution followed by DI water
rinse and spin dry gives good bond strength. Alternatively, one can
also deposit an oxide layer on the SiGe wafer and then CMP the
oxide layer. In this case. SiGe/oxide is bonded to an oxidized
handle wafer, i.e. oxide-oxide bonding. Among different clean
procedures, it was found that NH.sub.4OH:H.sub.2O.sub.2:H.sub.2O
clean and DI water rinse following by diluted HF, DI water rinse
and spin dry gives very good bond strength.
[0029] FIG. 5 is a block diagram of an exemplary embodiment of a
semiconductor structure 500 in accordance with the invention. A
graded Si.sub.1-xGe.sub.x buffer layer 504 is grown on a silicon
substrate 502, where the Ge concentration x is increased from zero
to a value y in a stepwise manner, and y has a selected value
between 0 and 1. A second relaxed Si.sub.1-yGe.sub.y layer 506 is
then deposited, and hydrogen ions are implanted into this layer
with a selected depth by adjusting implantation energy, forming a
buried hydrogen-rich layer 508. The wafer is cleaned and bonded to
an oxidized handle wafer 510. An anneal treatment at
500.about.600.degree. C. splits the bonded pair at the
hydrogen-rich layer 508. As a result, the upper portion of the
relaxed Si.sub.1-yGe.sub.y layer 506 remains on the oxidized handle
wafer, forming a SGOI substrate. The above description also
includes production of Ge-on-insuiator where y=1.
[0030] During the wafer clean step prior to bonding, the standard
RCA clean for the silicon surface is modified. Since the NH.sub.4OH
in standard RCA1 solution etches Ge faster than Si, the SiGe
surface will become rough, leading to a weak bond. A
H.sub.2SO.sub.4--H.sub.2O.sub.2 solution is used in the place of
RCA1, which also meets the clean process requirement for the
subsequent furnace annealing after bonding. The SiGe surface after
the H.sub.2SO.sub.4--H.sub.2O.sub.2 clean showed better surface
roughness compared to RCA1. After the modified RCA clean, the
wafers are then immersed in another fresh
H.sub.2SO.sub.4--H.sub.2O.sub.2 solution for 10 to 20 min.
H.sub.2SO.sub.4--H.sub.2O.sub.2 renders the SiGe surface
hydrophilic. After a rinse in DI wafer and spin drying, the SiGe
wafer is bonded to an oxidized handle wafer at room temperature
immediately, and then annealed at 500.about.600.degree. C. for
wafer splitting.
[0031] FIG. 6 is a block diagram of another exemplary embodiment of
a semiconductor structure 600. The structure 600 includes a graded
Si.sub.1-xGe.sub.x buffer layer 604 grown on a silicon substrate
602, where the Ge concentration x is increased from zero to 1. Then
a relaxed pure Ge layer 606 and a III-V material layer 608, such as
a GaAs layer, are epitaxially grown on the Ge layer. Hydrogen ions
are implanted into the GaAs layer 608 with a selected depth by
adjusting implantation energy, forming a buried hydrogen-rich layer
610. The wafer is cleaned and bonded to an oxidized handle wafer
612. An anneal treatment splits the bonded pair at the
hydrogen-rich layer 610. As a result, the upper portion of the GaAs
layer 608 remains on the oxidized handle wafer, forming a
GaAs-on-insulator substrate.
[0032] FIG. 7 is a block diagram of yet another exemplary
embodiment of a semiconductor structure 700. A graded
Si.sub.1-xGe.sub.x buffer layer 704 is grown on a silicon substrate
702, where the Ge concentration x is increased from zero to a
selected value y, where y is less than 0.2. A second relaxed
Si.sub.1-zGe.sub.z layer 706 is deposited, where z is between 0.2
to 0.25. Hydrogen ions are implanted into the graded
Si.sub.1-xGe.sub.x buffer layer 704 with a selected depth, forming
a buried hydrogen-rich layer 708 within layer 704. The wafer is
cleaned and bonded to an oxidized handle wafer 710. An anneal
treatment at 500.about.600.degree. C. splits the bonded pair at the
hydrogen-rich layer 708.
[0033] As a result, the upper portion of the graded
Si.sub.1-xGe.sub.x buffer layer 704 and the relaxed
Si.sub.1-zGe.sub.z layer 706 remains on the oxidized handle wafer
710. The remaining graded Si.sub.1-xGe.sub.x buffer layer 704 is
then selectively etched by either KOH or TMAH. KOH and TMAH etch
Si.sub.1-xGe.sub.x fast when x is less 0.2, but becomes very slow
when x is larger than 0.2. Thus, the graded Si.sub.1-xGe.sub.x
buffer layer 704 can be etched selectively, leaving the relaxed
Si.sub.1-zGe.sub.z layer 706 on the insulating substrate 710 and
forming a relaxed SGOI substrate. In this process, the thickness of
the relaxed Si.sub.1-zGe.sub.z film 706 on the final SGOI structure
is defined by film growth, which is desired in some
applications.
[0034] FIG. 8 is a block diagram of yet another exemplary
embodiment of a semiconductor structure 800. A graded
Si.sub.1-xGe.sub.x buffer layer 804 is grown on a silicon substrate
802, where the Ge concentration x is increased from zero to a
selected value y between 0 and 1. A second relaxed
Si.sub.1-yGe.sub.y layer 806 is deposited, followed by a strained
Si.sub.1-zGe.sub.z layer 808 and another relaxed Si.sub.1-yGe.sub.y
layer 810. The thickness of layers 806, 808, and 810, and the value
z are chosen such that the Si.sub.1-zGe.sub.z layer 808 is under
equilibrium strain state while the Si.sub.1-yGe.sub.y layers 806
and 810 remain relaxed. In one option, hydrogen ions may be
introduced into the strained Si.sub.1-zGe.sub.z layer 808, forming
a hydrogen-rich layer 812. The wafer is cleaned and bonded to an
oxidized handle wafer 814. The bonded pair is then separated along
the strained Si.sub.1-zGe.sub.z layer 808.
[0035] Since the strain makes the layer weaker, the crack
propagates along this layer during separation. The separation can
be accomplished by a variety of techniques, for example using a
mechanical force or an anneal treatment at 500.about.600.degree. C.
when the hydrogen is also introduced. See, for example, U.S. Pat.
Nos. 6,033,974 and 6,184,111, both of which are incorporated herein
by reference. As a result, the relaxed Si.sub.1-yGe.sub.y layer 810
remains on the oxidized handle wafer, forming a relaxed SGOI
substrate. The thickness of layers 806, 808, and 810, and the value
z may also be chosen such that there are a good amount of
dislocations present in the Si.sub.1-zGe.sub.z layer 808 while the
top Si.sub.1-yGe.sub.y layer 810 remains relaxed and having high
quality and limited dislocation defects.
[0036] These dislocation defects in the Si.sub.1-zGe.sub.z layer
808 can then act as hydrogen trap centers during the subsequent
step of introducing ions. The hydrogen ions may be introduced by
various ways, such as ion implantation or ion diffusion or drift by
means of electrolytic charging. The value of z may be chosen in
such a way that the remaining Si.sub.1-zGe.sub.z layer 808 can be
etched selectively by KOH or TMAH. The layers 806 and 810 may also
be some other materials, for example pure Ge, or some III-V
materials, under the condition that the Ge concentration x in the
graded Si.sub.1-xGe.sub.x buffer layer 804 is increased from zero
to 1.
[0037] After all the semiconductor-on-insulator substrate obtained
by the approaches described above, various device layers can be
further grown on the top. Before the regrowth, CMP maybe used to
polish the surface.
[0038] Although the present invention has been shown and described
with respect to several preferred embodiments thereof, various
changes, omissions and additions to the form and detail thereof,
may be made therein, without departing from the spirit and scope of
the invention.
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