U.S. patent application number 11/004948 was filed with the patent office on 2005-04-21 for wafer bonded virtual substrate and method for forming the same.
This patent application is currently assigned to CALIFORNIA INSTITUTE OF TECHNOLOGY. Invention is credited to Atwater, Harry A. JR., Morral, Anna Fontcuberta I, Zahler, James M..
Application Number | 20050085049 11/004948 |
Document ID | / |
Family ID | 34886500 |
Filed Date | 2005-04-21 |
United States Patent
Application |
20050085049 |
Kind Code |
A1 |
Atwater, Harry A. JR. ; et
al. |
April 21, 2005 |
Wafer bonded virtual substrate and method for forming the same
Abstract
A method of forming a virtual substrate comprised of an
optoelectronic device substrate and handle substrate comprises the
steps of initiating bonding of the device substrate to the handle
substrate, improving or increasing the mechanical strength of the
device and handle substrates, and thinning the device substrate to
leave a single-crystal film on the virtual substrate such as by
exfoliation of a device film from the device substrate. The handle
substrate is typically Si or other inexpensive common substrate
material, while the optoelectronic device substrate is formed of
more expensive and specialized electro-optic material. Using the
methodology of the invention a wide variety of thin film
electro-optic materials of high quality can be bonded to
inexpensive substrates which, serve as the mechanical support for
an optoelectronic device layer fabricated in the thin film
electro-optic material.
Inventors: |
Atwater, Harry A. JR.; (So.
Pasadena, CA) ; Zahler, James M.; (Pasadena, CA)
; Morral, Anna Fontcuberta I; (Paris, FR) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
CALIFORNIA INSTITUTE OF
TECHNOLOGY
|
Family ID: |
34886500 |
Appl. No.: |
11/004948 |
Filed: |
December 7, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11004948 |
Dec 7, 2004 |
|
|
|
10761918 |
Jan 20, 2004 |
|
|
|
10761918 |
Jan 20, 2004 |
|
|
|
10125133 |
Apr 17, 2002 |
|
|
|
60284726 |
Apr 17, 2001 |
|
|
|
Current U.S.
Class: |
438/455 ;
438/458; 438/514; 438/57 |
Current CPC
Class: |
H01L 21/02096 20130101;
H01L 21/187 20130101; H01L 21/30625 20130101; H01L 31/03925
20130101; H01L 21/30604 20130101; Y02E 10/50 20130101; H01L
21/30612 20130101; H01L 31/18 20130101; Y10S 438/933 20130101; H01L
31/0392 20130101 |
Class at
Publication: |
438/455 ;
438/458; 438/514; 438/057 |
International
Class: |
H01L 021/76; H01L
021/30; H01L 021/46; H01L 021/00 |
Claims
We claim:
1. A virtual substrate comprising a non-silicon device film bonded
to a handle substrate, wherein: (1) the device film and the handle
substrate comprise different materials with different coefficients
of thermal expansion; and (2) strain states of the device film and
the handle substrate are such that there is a first temperature
that is greater than room temperature and less than 900 degrees
Kelvin, where the strains of the device film and handle substrate
are equal.
2. The virtual substrate of claim 1, wherein a magnitude of a bow
of the virtual substrate over a range of temperatures between 300
and 900 K is lower than a magnitude of a bow in the same virtual
substrate with a first temperature at or near room temperature.
3. The virtual substrate of claim 1, where the first temperature
ranges from 400 to 900 K.
4. The virtual substrate of claim 1, wherein the first temperature
equals to the temperature at which the handle substrate is bonded
to the device film.
5. The virtual substrate of claim 4, wherein the first temperature
ranges from 400 K to 700 K.
6. The virtual substrate of claim 1, wherein (1) a coefficient of
thermal expansion (CTE) difference between the handle substrate and
device film is greater than zero; (2) the device film is in
compressive strain at room temperature; and (3) the magnitude of
the strain in the virtual substrate at temperatures above room
temperature is below the magnitude of the strain for an identical
virtual substrate bonded at room temperature.
7. The virtual substrate of claim 1, wherein (1) the CTE difference
between the handle substrate and device film is less than zero; (2)
the device film is in tensile strain at room temperature; and (3)
the magnitude of the strain in the virtual substrate at elevated
temperatures above room temperature is below the magnitude of the
strain for an identical virtual substrate bonded at room
temperature.
8. The virtual substrate of claim 1, wherein (1) the device film is
germanium; (2) the handle substrate is silicon; and (3) the first
temperature is between 300 and 900 degrees Kelvin.
9. The virtual substrate of claim 1, wherein (1) the device film is
indium phosphide; (2) the handle substrate is silicon; and (3) the
first temperature is between 300 and 900 degrees Kelvin.
10. The virtual substrate of claim 1, wherein (1) the device film
is gallium arsenide; (2) the handle substrate is silicon; and (3)
the first temperature is between 300 and 900 degrees Kelvin.
11. The virtual substrate of claim 1, wherein the strain states of
the device film and handle substrate have been engineered to
control a bow of the virtual substrate at a temperature above room
temperature.
12. The virtual substrate of claim 1, wherein the strain states of
the device film and handle substrate have been engineered to
control at least one of a carrier mobility and a bandgap of the
device film at operating temperatures above room temperature.
13. The virtual substrate of claim 1, wherein the device film
comprises a semiconductor material suitable for fabrication of
optoelectronic devices.
14. The virtual substrate of claim 13, wherein the device film
comprises germanium or a compound semiconductor material and the
handle substrate comprises a silicon, GaAs, glass, quartz or
sapphire substrate.
15. The virtual substrate of claim 14, wherein the device film is
selected from Ge, GaN, GaAs and InP films and the handle substrate
comprises a silicon substrate.
16. The virtual substrate of claim 1, wherein the device film
comprises a ferro-electric oxide.
17. The virtual substrate of claim 1, further comprising a strain
compensation layer located on an opposite side of the handle
substrate from the device film.
18. The virtual substrate of claim 1, wherein the device film and
the handle substrate comprise semiconductor materials and an
interface between the device film and the handle substrate has a
resistance of 3.5 ohms cm.sup.2 or less.
19. A method for engineering a strain state of a virtual substrate,
comprising: (1) ion implantation of a device substrate; (2)
initiating bonding of the device substrate and a handle substrate
at a controlled temperature in order to control a resultant strain
state in the final virtual substrate; and (3) removing a portion of
the device substrate to leave a device film bonded to the handle
substrate, thereby forming the virtual substrate.
20. The method of claim 19, wherein the bonding is initiated at a
temperature above room temperature.
21. The method of claim 20, wherein the bonding is initiated at a
temperature between 400 and 900 K.
22. The method of claim 20, wherein the handle substrate and the
device substrate are maintained at different temperatures when the
bonding is initiated.
23. The method of claim 19, wherein (1) the device film is
germanium, gallium arsenide or indium phosphide; and (2) the handle
substrate is silicon or gallium arsenide.
24. The method of claim 19, wherein the strain states of the device
film and handle substrate have been engineered to control the bow
of the virtual substrate at a temperature above room
temperature.
25. The method of claim 19, wherein the strain states of the device
film and handle substrate have been engineered to control at least
one of a carrier mobility and a bandgap of the device film at
operating temperatures above room temperature.
26. The method of claim 19, wherein the device film comprises a
semiconductor material suitable for fabrication of optoelectronic
devices.
27. The method of claim 19, further comprising forming a strain
compensation layer on an opposite side of the handle substrate from
the device film.
28. The method of claim 19, wherein the device film and the handle
substrate comprise semiconductor materials and an interface between
the device film and the handle substrate has a resistance of 3.5
ohms cm.sup.2 or less.
29. A method for engineering a strain state of a virtual substrate,
comprising: (1) ion implantation of a device substrate; (2)
initiating bonding of a handle substrate and the device substrate
where the temperature of the handle substrate is different than the
temperature of the device substrate at a time of bond initiation;
and (3) removing a portion of the device substrate to leave a
device film bonded to the handle substrate, thereby forming the
virtual substrate.
30. The method of claim 29, wherein the bonding is initiated at a
temperature above room temperature.
31. The method of claim 30, wherein the bonding is initiated at a
temperature between 400 and 900 K.
32. The method of claim 29, wherein the temperature of the device
substrate is greater than the temperature of the handle substrate
at the time of bond initiation.
33. The method of claim 29, wherein (1) the device film is
germanium, gallium arsenide or indium phosphide; and (2) the handle
substrate is silicon or gallium arsenide.
34. The method of claim 29, wherein the strain states of the device
film and handle substrate have been engineered to control the bow
of the virtual substrate at a temperature above room
temperature.
35. The method of claim 29, wherein the strain states of the device
film and handle substrate have been engineered to control at least
one of a carrier mobility and a bandgap of the device film at
operating temperatures above room temperature.
36. The method of claim 29, wherein the device film comprises a
semiconductor material suitable for fabrication of optoelectronic
devices.
37. The method of claim 29, further comprising forming a strain
compensation layer on an opposite side of the handle substrate from
the device film.
38. The method of claim 29, wherein the device film and the handle
substrate comprise semiconductor materials and an interface between
the device film and the handle substrate has a resistance of 3.5
ohms cm.sup.2 or less.
Description
RELATED APPLICATIONS
[0001] The present application is a continuation-in-part
application related under 35 USC 120 to copending U.S. patent
application Ser. No. 10/125,133 filed on Apr. 17, 2002, which is
incorporated herein by reference, and which in turned claimed
priority to U.S. Provisional Patent Application, Ser. No.
60/284,726, filed on Apr. 17, 2001 which claimed priority pursuant
to 35 USC 119.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to the field of semiconductor
processing of films and in particular to processing nonsilicon
films on heterostructures.
[0004] 2. Description of the Prior Art
[0005] The optoelectronics, photovoltaics, telecommunications, and
LED industries have a need for a substrate technology that allows
them to use a low-cost readily available substrate like Si as a
mechanical support for a thin film of optoelectronic material on
which to fabricate a device. Some obvious advantages are improved
mechanical strength and superior thermal conductivity relative to a
bulk optoelectronic material.
[0006] Group III-V semiconductor layered structures grown on bulk
germanium substrates have been used in the prior art to create high
efficiency triple-junction solar cells with efficiencies greater
than 30%. However, these are prohibitively expensive for all but
space applications, because the Ge substrate constitutes a large
portion of this cost.
[0007] The optoelectronics, photovoltaics, telecommunications, and
LED industries would benefit from adopting a substrate technology
that allows them to use a low-cost readily available substrate like
Si as a mechanical support for a thin film of optoelectronic
nonsilicon material on which to fabricate a device. Some obvious
advantages are improved mechanical strength and superior thermal
conductivity relative to a bulk optoelectronic material.
BRIEF SUMMARY OF THE INVENTION
[0008] The invention is a method of forming a virtual substrate
comprised of an optoelectronic device substrate and handle
substrate comprising the steps of: initiating bonding of the device
substrate to the handle substrate; improving or increasing the
mechanical strength of the device and handle substrates; and
thinning the device substrate to leave a single-crystal film on the
virtual substrate such as by exfoliation of a device film from the
device substrate.
[0009] The method further comprises the step of providing a
pre-bonding treatment to allow the removal of a thin film.
[0010] The method still further comprises the step of cleaning
and/or passivating the device and/or handle substrates to
facilitate bonding.
[0011] The step of providing a pre-bonding treatment to allow the
removal of a thin film comprises the step of ion implanting the
device substrate to inject an amount of gas species into the device
substrate to form the internally passivated surfaces and to create
an internal pressure necessary to exfoliate a layer from the device
substrate upon annealing. In the illustrated embodiment ion
implanting the device substrate comprises implanting H.sup.+ or a
combination of H.sup.+ and He.sup.+.
[0012] The step of cleaning and/or passivating the device and
handle substrates to facilitate bonding comprises passivating the
surface of both the device and handle substrates to allow
hydrophobic wafer bonding. The step of passivating the surface of
both the device and handle substrates comprises the step of
enabling the formation of an intimate covalent bond between a
device film, exfoliated from the device substrate, and the handle
substrate in the virtual substrate to allow for the ohmic,
low-resistance interface electrical properties. The step of
cleaning and/or passivating the device and handle substrates to
facilitate bonding comprises the step of eliminating adsorbed water
on the surface of the device and handle substrates by means of a
low temperature bake in an inert atmosphere or in vacuum. The step
of eliminating adsorbed water on the surface of the device and
handle substrates by means of a low temperature bake comprising
baking at a temperature such that the vapor pressure of water is
below the partial pressure of water in the surrounding ambient.
[0013] The method further comprises the step of disposing a
selected material X on the device substrate to enable an
X-to-handle-substrate material bond with the handle substrate when
an exfoliated film from the device substrate is bonded with the
handle substrate. Material X is any material which is compatible
with the claimed methodology and can be determined empirically. The
method also further comprises the step disposing a selected
material X on the handle substrate to enable an
X-to-device-substrate material bond when an exfoliated film from
the device substrate is bonded with the handle substrate. The
method still further comprises the step of disposing a selected
material X on both the device and handle substrates to enable an
X-to-X material bond when an exfoliated film from the device
substrate is bonded with the handle substrate.
[0014] The step of cleaning and/or passivating the device and
handle substrates to facilitate bonding comprises the step of
removing residual particle contamination on the bonding surfaces of
the device and handle substrates. The step of removing residual
particle contamination comprises the step of impinging an inert gas
on the substrate at an elevated temperature to remove the particles
by combined physical impact and thermophoretic lifting effect. Or,
alternatively the particles can be removed by exposing the
substrates to ultra-sonic or mega-sonic vibrations in a liquid
bath.
[0015] The step of initiating bonding of the device substrate to
the handle substrate comprises the step of controlling the
temperature at which the device and handle substrates are brought
into contact with each other to select the strain state, whereby
substrate performance in high-temperature processes is improved, or
a device operation temperature strain selected to adjust a device
property such as bandgap or carrier mobility. The step of
initiating bonding of the device substrate to the handle substrate
comprises the step of holding the temperature of the device and the
temperature of handle substrates when brought into contact with
each other at different magnitudes to select the strain state.
[0016] In one embodiment, after initiating bonding of the device
substrate to the handle substrate, the mechanical strength of the
bond of the device and handle substrates is improved and the ion
implantation layer transfer process is thermally activated during
which uni-axial pressure is applied to the virtual substrate.
[0017] The mechanical strength of the device and handle substrates
is improved by using multiple pressure-temperature increments, or
continuously varying pressure-temperature combinations. In another
embodiment the mechanical strength of the bond of the device to the
handle substrate is improved by applying higher pressures to ensure
better substrate-substrate contact at lower temperatures prior to
exfoliation where the higher pressures would at higher temperatures
subdue exfoliation, and then reducing the pressure to a lower level
prior to annealing at higher temperatures so that exfoliation is
uninhibited.
[0018] The method further comprises the step of removing an upper
portion of the device film exfoliated from the device substrate,
whereby a smoother and less defect prone surface is provided for
subsequent optoelectronic device fabrication. The step of removing
an upper portion of the device film exfoliated from the device
substrate comprises chemically polishing the upper portion with a
damage selective etch, or mechanically polishing the upper portion
or both. In one embodiment the method further comprises the step of
performing homoepitaxy to leave a smooth defect-free surface. In
still a further embodiment the method further comprises the step of
processing the virtual substrate as a template for growth of an
optoelectronic device through hetero-epitaxy.
[0019] The invention also includes the virtual substrate which is
fabricated according to the forgoing methods.
[0020] While the apparatus and method has or will be described for
the sake of grammatical fluidity with functional explanations, it
is to be expressly understood that the claims, unless expressly
formulated under 35 USC 112, are not to be construed as necessarily
limited in any way by the construction of "means" or "steps"
limitations, but are to be accorded the full scope of the meaning
and equivalents of the definition provided by the claims under the
judicial doctrine of equivalents, and in the case where the claims
are expressly formulated under 35 USC 112 are to be accorded full
statutory equivalents under 35 USC 112. The invention can be better
visualized by turning now to the following drawings wherein like
elements are referenced by like numerals.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a block diagram illustrated two alternative
fabrication strategies for a virtual substrate.
[0022] FIG. 2 is a block diagram illustrated categories of film
materials which are used for a virtual substrate according to the
invention.
[0023] FIGS. 3a and 3b are diagrams illustrating respectively the
ion implantation and the resulting structure in the device
substrate.
[0024] FIGS. 4a and 4b are diagrams illustrating respectively the
device and handle substrate stack following ion implantation and
initial bonding and the wafer bonded virtual substrate following
the anneal and layer exfoliation.
[0025] FIGS. 5a and 5b are diagrams illustrating respectively the
post-layer transfer device substrate comprised of the near surface
ion implantation damage layer and the undamaged bulk, and the
removal of the damage by etching from the bulk device substrate
allowing the process to be repeated.
[0026] FIGS. 6a and 6b are diagrams illustrating respectively the
surface modification of the implanted device substrate with either
a film of the same chemical identity as the handle substrate, and a
wafer bonded substrate stack using this technique showing the
device substrate.
[0027] FIG. 7 is a graph of the strain state as a function of
temperature for a Ge/Si wafer bonded virtual substrate.
[0028] FIG. 8 is a graph of the strain state as a function of
temperature for an InP/Si wafer bonded virtual substrate.
[0029] FIG. 9 is a graph of the strain state as a function of
temperature for an GaAs/Si wafer bonded virtual substrate.
[0030] FIG. 10 is a graph of a wafer bonding temperature-pressure
curve as a function of time as used in the bond annealing
process.
[0031] FIGS. 11a and 11b are diagrams illustrating respectively the
wafer bonded virtual substrate following the anneal and layer
exfoliation and the wafer bonded virtual substrate following a
damage removal etch, polish or epitaxial growth on the device
film.
[0032] FIG. 12 is a graph of the rms surface roughness of a
transferred InP device film as a function of time for 1:2:1, 1:2:2,
1:2:4, and 1:2:5 dilutions of the
HCl:H.sub.3PO.sub.4:H.sub.2O.sub.2 etch chemistry
[0033] FIGS. 13a and 13b are diagrams illustrating respectively the
completed wafer bonded virtual substrate and a wafer bonded virtual
substrate with an epitaxially grown device fabricated on the device
thin film.
[0034] FIG. 14 is a diagram showing an optoelectronic structure
grown on a wafer bonded virtual substrate consisting of the device
film, the bonded interface, the handle substrate, and a strain
compensation layer deposited on the back surface of the
substrate.
[0035] The invention and its various embodiments can now be better
understood by turning to the following detailed description of the
preferred embodiments which are presented as illustrated examples
of the invention defined in the claims. It is expressly understood
that the invention as defined by the claims may be broader than the
illustrated embodiments described below.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] The fabrication 104 of virtual wafer bonded substrates could
take two possible approaches as diagrammatically illustrated in the
block diagram of FIG. 1. These approaches are the integration of a
thin device film with a bulk substrate prior to fabricating a
functional device indicated by block 100, or the device film can be
transferred to the handle substrate following the fabrication of a
functional logic device in the handle substrate and/or the
fabrication of a functional optoelectronic device in the
optoelectronic device substrate as depicted by block 102.
[0037] This specification summarizes a number of embodiments of the
fabrication of optoelectronic virtual substrates. We begin with a
summary of the technology generally employed and device structures
for which the virtual substrate product can be used. Next, more
material process steps are described in the order that they appear
in the fabrication process.
[0038] Process and Product Overview
[0039] For the purpose of the specification the term "device
substrate" 10 is the optoelectronic substrate from which a thin
film 12 will be removed. The term "handle substrate" 14 is defined
in the specification to refer to the substrate that is used as a
mechanical support for the device film 12, namely the remainder of
device substrate 10 after removal of film 12. The term "virtual
substrate" 16 is defined to be the completed structure of a thin
device film 12 on a handle substrate 14.
[0040] The materials of interest for device substrate 10 for the
discussion below can be considered all materials that are relevant
to wafer bonded virtual substrate device film materials for
optoelectronic, high-gain device fabrication as diagrammatically
illustrated diagrammatically in FIG. 2: III/V compound
semiconductors (i.e. GaAs, InP, GaN, etc.), II/VI semiconductors
(i.e. CdTe, etc.), group IV semiconductors (i.e. Ge for GaAs family
growth), and optically important Ferroelectric oxides (i.e.
LiNbO.sub.4, BaTiO.sub.4, etc.).
[0041] The handle substrate 14 will generally be Si, which is
abundantly available and has desirable electrical, mechanical, and
thermal properties. However, low-cost insulating substrates (i.e.
glass, sapphire, etc.) might also be employed as handle substrate
14.
[0042] A generic process for fabricating such virtual substrates 16
comprises the following steps:
[0043] 1) The device substrate 10 and handle substrate 14 may need
pre-bonding treatment to allow the removal of a thin film 12 (i.e.
ion-implantation as diagrammatically depicted in FIG. 3a).
[0044] 2) The device substrate 10 is cleaned and/or passivated to
facilitate bonding.
[0045] 3) Bonding is initiated as diagrammatically shown in FIG.
4a.
[0046] 4) The bond is strengthened to improve the mechanical
strength of the device substrate 10 and handle substrate 14.
[0047] 5) The device substrate 10 is thinned to leave a
single-crystal film 12 on the finished virtual substrate 16 as
shown in FIG. 4b for an ion-implanted substrate.
[0048] 6) In the case of ion implantation induced layer
exfoliation, the device substrate 10 from which the device film 12
was derived can be reprocessed by a means of surface polishing to
allow the reuse of the substrate 10 to transfer another device film
as illustrated in FIG. 4b.
[0049] Consider the concepts used for the fabrication of
optoelectronic virtual substrates 16. These will be listed in the
order in which they would appear in the generic process described
above.
[0050] Process Steps
[0051] 1) Ion Implantation
[0052] Prior to bonding, ion implantation of the device substrate
10 is performed to inject a necessary amount of gas species into
the substrate to form the internally passivated surfaces and
internal pressure necessary to exfoliate a layer from the substrate
upon annealing as diagrammatically depicted in FIG. 3a, which
illustrates pre-bonding ion implantation of the device substrate 10
with an ion beam 11, creating a modified structure as shown in FIG.
3b comprising a device thin film 12, an ion damaged layer for film
transfer 13, and the largely unaffected bulk of the device
substrate 10 which is now called the handle substrate 14.
[0053] This process is generally performed with H.sup.+ or a
combination of H.sup.+ and He.sup.+. However, other gas species may
be employed to produce an intra-substrate etch process to assist in
the exfoliation of the layer. For a given device substrate material
there is both a minimum implantation temperature to avoid
amorphization and a needed implantation temperature, namely a
minimum required dose relationship for this process.
[0054] a. H.sup.+ Implantation--A sufficient dose of H.sup.+ is
implanted to allow film exfoliation upon annealing. This dose is a
function of:
[0055] Implant energy
[0056] Implant temperature
[0057] Device substrate material
[0058] Film exfoliation anneal temperature
[0059] b. H.sup.+/He.sup.+ Co-implantation--A sufficient total dose
of H.sup.+ and He.sup.+ is implanted to allow film exfoliation upon
annealing. The concept of this approach being that the H plays a
chemical role of passivating internal surfaces, while the
chemically inert He efficiently migrates to internal surfaces to
provide pressure, and creates more damage per implanted ion than H
increasing the internal surface density. The necessary dose is a
function of:
[0060] Implant energy
[0061] Implant temperature
[0062] H/He ratio
[0063] Device substrate material
[0064] Film exfoliation anneal temperature
[0065] c. Etchant implantation--In addition to or replacement of
H.sup.+ implantation, chemical species known to etch a given
material can be implanted to create internally trapped chemical
species that are volatile and cause exfoliation upon annealing. The
chemical species chosen will be material specific according to
known etchant properties or empirical experience.
[0066] 2) Surface Passivation
[0067] Following implantation and prior to bonding, it is necessary
to passivate the surface of both the device and handle substrates
10, 14 to allow hydrophobic wafer bonding. The specific chemical
process necessary is device substrate specific. The purpose of this
step is to enable the formation of an intimate covalent bond
between the device film 12 and handle substrate 14 in the finished
virtual substrate 16 allowing for the possibility of ohmic,
low-resistance interface electrical properties. A necessary step in
enabling this finished device structure will be the elimination of
adsorbed water on the surface by means of a low temperature bake in
an inert atmosphere or in vacuum. The bake should reach a
temperature such that the vapor pressure of water at that
temperature is well below the partial pressure of water in the
surrounding ambient.
[0068] a. Group IV Passivation--The Group IV elemental
semiconductors, particularly Ge, are rendered hydrophobic by the
use of a dilute HF etching process. This leaves a predominantly
hydride terminated surface.
[0069] b. Group III/V Passivation--The Group III/V compound
semiconductors can be rendered hydrophobic by compound-specific
chemical treatments to leave a hydrophobically passivated surface
for bonding.
[0070] c. Group II/VI Passivation--The Group II/VI compound
semiconductors can be rendered hydrophobic by compound-specific
chemical treatments to leave a hydrophobically passivated surface
for bonding.
[0071] d. Ferroelelectric Oxides--Applications involving
ferroelectric oxides are fundamentally different than those for the
optoelectronic materials from the elemental and compound
semiconductors. For this reason handle substrate materials will be
chosen for their electrical and refractive properties, but there
will generally be no need to attempt hydrophobic wafer bonding to
the insulating ferroelectric thin films. Thus, surface passivation
will generally focus on forming a thin oxide on both the device and
handle substrates 10, 14.
[0072] 3) Surface Modification
[0073] Another enabling technology for extending this process to a
wide range of optoelectronic materials is the use of a deposited
surface modification layer 18 of arbitrary thickness to change the
nature of the physical interaction between the substrates 10, 14 as
depicted in FIGS. 6a and 6b. This can be done in one of three ways,
where X stands for any type of composition compatible with the
disclosed method:
[0074] a. Deposition of a layer 18 of material X on the device
substrate 10 to enable an X-handle material bond.
[0075] b. Deposition of a layer 18 of material X on the handle
substrate 14 to enable an X-device material bond.
[0076] c. Deposition of a layer 18 of material X on both substrates
to enable an X--X bond.
[0077] This technology enables the integration of a wide range of
optoelectronic materials by mastering bonding with a material which
is compatible or amendable to the disclosed process, which for the
moment is refernenced simply as material X. The generic process is
illustrated in FIGS. 6a and 6b. FIG. 6a illustrates the surface
modification of the implanted device substrate with either a
crystalline or amorphous film 40 of the same chemical identity as
the handle substrate 14. FIG. 6b illustrates a wafer bonded
substrate stack using this technique showing the device substrate
10, the ion implanted damage region 13, the device thin film 12,
the deposited bond mediating film 40, the bonded interface 42, and
the handle substrate 14.
[0078] More specific applications of this technique are:
[0079] d. Epitaxial Si bonding layer--This technique involves
epitaxially growing a strained thin film of Si on the device
substrate material. In this embodiment material X is strained thin
film Si. Through doing this an intimate and maximally strong bond
can be ensured between the device material and the Si epitaxial
layer. The device substrate 10 with the strained Si epitaxy is then
implanted through the epitaxial layer (not shown) in preparation
for wafer bonding and layer exfoliation. For material systems
utilizing Si handle substrates 14 this would allow direct Si--Si
wafer bonding using well established passivation techniques.
[0080] e. Amorphous Si bonding layer--This technique involves
depositing a thin layer of amorphous Si (not shown) at low
temperature on the device substrate 10 to enable the use of typical
Si surface preparation chemistries. In this embodiment material X
is amorphous Si. This process could be performed either prior to or
after ion implantation of the device substrate 10. For material
systems utilizing Si handle substrates 14 this would allow direct
Si--Si wafer bonding using well established passivation
techniques.
[0081] 4) Particle Removal
[0082] Following surface passivation, it may be necessary to remove
residual particle contamination on the bonding surfaces of the
device and handle substrates 10 and 14. This has been efficiently
done by performing a clean with a CO.sub.2 particle jet 20 as
depicted in FIGS. 4a and 4b. FIG. 4a is a diagram of a device
substrate 10 and handle substrate 14 stack following ion
implantation and initial bonding, showing the undamaged bulk device
substrate 10, the ion implanted damage layer 13, the device thin
film 12, the wafer bonded device/handle interface 42, and the
handle substrate 14. FIG. 4b is a diagram showing the wafer bonded
virtual substrate 16 following the anneal and layer exfoliation,
and showing the undamaged bulk device substrate 10 with its ion
implanted damaged surface region 13. Also shown is the wafer bonded
virtual substrate comprised of the ion implantation damaged surface
region 13 of the device film 12, the undamaged transferred device
film 12, the wafer bonded interface 42, and the handle substrate
14. The substrates 10 and/or 14 is held at an elevated temperature
and a throttled gas/particle jet of CO.sub.2 is impinged on the
surface of substrates 10, 14 removing particles by a combined
physical impact and thermophoretic lifting effect.
[0083] This has been demonstrated for Si, Ge, and InP by
maintaining the substrates at a temperature greater than 50.degree.
C. during application of the CO.sub.2.
[0084] 5) Elevated Temperature Bond Initiation
[0085] When bonding dissimilar materials there is generally a
coefficient of thermal expansion mismatch between the two materials
resulting in a temperature-dependent strain state of the device
thin film 12 in the virtual substrate 16 that is governed by the
equation 1 ( T ) = T 0 T ( T ' ) T '
[0086] where .DELTA..alpha.(T) is the difference in the thermal
expansion coefficients .alpha. between the two substrates as a
function of temperature, T, and where T.sub.0 is the temperature of
the zero strain condition, typically assumed to be the bond
initiation temperature. Thus, by controlling the temperature at
which two substrates are brought into contact, the strain state at
temperatures of interest can be engineered. This could be
advantageous for improving substrate performance in
high-temperature processes, or a device operation temperature
strain could be engineered to adjust a key device property such as
bandgap or carrier mobility. The following descriptions describe
the general types of strain temperature-strain dependences that
could be achieved as a function of the sign of
.DELTA..alpha.(T).
.DELTA..alpha.(T)=.alpha..sub.handle(T)-.alpha..sub.device(T)
[0087] Such that positive values of strain indicate a film in
tension and negative values of strain indicate a film in
compression.
[0088] a. .DELTA..alpha.(T)>0:
[0089] 1. Room Temperature Bonding--In this case the film 12 will
be in tension at elevated processing temperatures. This will lead
to changes in the lattice matching in heteroepitaxy on the virtual
substrate 16 and a tendency to concave substrate bowing.
[0090] 2. Elevated Temperature Bonding--In this case the film will
have a zero strain condition at the bonding temperature such that
tensile strain and concave wafer bowing will be reduced at higher
processing temperatures. Likewise, the film 12 will be in
compressive strain at room temperature and likely at device
operating temperatures leading to convex wafer bow. This could
change device operation and enable design of novel devices based on
strain control of materials parameters.
[0091] b. .DELTA..alpha.(T)<0:
[0092] 1. Room Temperature Bonding--In this case the film will be
in compression at elevated temperatures. This will lead to changes
in the lattice matching in heteroepitaxy on the virtual substrate
and a tendency to convex substrate bowing.
[0093] 2. Elevated Temperature Bonding--In this case the film 12
will have a zero strain condition at the bonding temperature such
that compressive strain and convex bowing will be reduced at higher
processing temperatures. Likewise, the film 12 will be in tensile
strain at room temperature and likely at device operating
temperatures leading to concave wafer bow. This could change device
operation and enable design of novel devices based on strain
control of materials parameters. Most materials of interest for the
wafer bonded virtual substrates 16 described belong to this
category.
[0094] a. Ge/Si Bonding--FIG. 7 is a graph which illustrates the
predicted strain in Ge/Si bonding as a function of temperature for
substrates bonded at various values of T.sub.0. The film
compression at high temperatures can be reduced by initiation
bonding at higher temperatures, T.sub.0.
[0095] b. InP/Si Bonding--FIG. 8 is a graph which illustrates the
predicted strain in InP/Si bonding as a function of temperature for
substrates bonded at various values of T.sub.0. Again, the film
compression at high temperatures can be reduced by initiation
bonding at higher temperatures, T.sub.0.
[0096] c. GaAs/Si Bonding--FIG. 9 is a graph which illustrates the
predicted strain in GaAs/Si bonding as a function of temperature
for substrates bonded at various values of T.sub.0. Once again, the
film compression at high temperatures can be reduced by initiation
bonding at higher temperatures, T.sub.0.
[0097] 6) Dissimilar Temperature Bond Initiation
[0098] For some desired engineered stain states, no single elevated
bond temperature will enable the fabrication of that device.
Likewise, for materials with very similar coefficients of thermal
expansion strain engineering will be difficult. To further enable
control of strain at a desired temperature, bonding could be
initiated between substrates held at different temperatures. In
this way, the thermo-mechanical strain state could be more freely
controlled or artificially built into the finished structure. In
this case the temperature dependent strain state is given by 2 ( T
) = T 0 T ( T ' ) T ' + 0
[0099] where the value y.sub.0 is the strain built into the bonded
structure upon bond initiation and is given by 3 0 = T 0 T d d ( T
' ) T ' + T h T 0 h ( T ' ) T '
[0100] In this expression, T.sub.d and T.sub.h are the temperatures
of the device substrate 10 and the handle substrate 14 respectively
at the instant of bond initiation. The temperature T.sub.0 is the
effective bond initiation temperature. The dissimilar temperatures
of the substrates at bond initiation make this term difficult to
determine. T.sub.0 must have a value between T.sub.d and T.sub.h
and will depend upon the experimental apparatus used in fabrication
of the virtual substrate 16 and can be determined experimentally.
The built-in strain approaches 4 0 = T h T d ( T ' ) T '
[0101] for device and handle substrates 10, 14 with very similar
coefficients of linear expansion. Bonding at different wafer
temperatures can be conducted in the following circumstances.
[0102] a. .DELTA..alpha.(T)>0:
[0103] 1. T.sub.d>T.sub.h--These conditions enable the addition
of a positive stress component causing the device film to be under
increased tension at elevated temperatures.
[0104] 2. T.sub.d<T.sub.h--These conditions enable the addition
of a negative stress component reducing the high temperature
tensile stress, but creating a greater low temperature compressive
stress.
[0105] b. .DELTA..alpha.(T)<0:
[0106] 1. T.sub.d>T.sub.h--These conditions enable the addition
of a positive stress component causing the device film to be under
a reduced degree of compressive stress at high temperatures.
[0107] 2. T.sub.d<T.sub.h--These conditions enable the addition
of a negative stress component increasing the high temperature
compressive stress, but creating a reduced low temperature tensile
stress.
[0108] c. .DELTA..alpha.(T)=0:
[0109] 1. T.sub.d>T.sub.h--A temperature independent tensile
stress can be imparted on the device film in this way.
[0110] 2. T.sub.d<T.sub.h--A temperature independent compressive
stress can be added to the device film.
[0111] 7) Annealing Under Pressure to Strengthen Bonding and
Exfoliate the Device Layer--
[0112] After the device and handle substrates 10, 14 have been
bonded, it is necessary to use a thermal cycle to improve bond
strength and to activate the ion implantation layer transfer
process. By performing this cycle under pressure, thermo-mechanical
strain can be accommodated in the bonded bulk substrate stack.
Additionally, bonding is strengthened by means of improved
substrate-substrate contact. A bonding process using multiple
pressure-temperature steps or even a continuously varying
pressure-temperature curve can be used to optimize the
effectiveness of pressure in the process.
[0113] Specifically, at lower temperatures prior to exfoliation,
higher pressures can be employed to ensure better
substrate-substrate contact, but these pressures would at higher
temperatures subdue exfoliation. By reducing the pressure to a
lower level prior to annealing to high temperatures, exfoliation is
uninhibited.
[0114] a. Variable Pressure Cycle--One possible embodiment is to
utilize independently varied pressure and temperature to optimize
the bonding process. At low temperatures, high pressures are
applied to strengthen the bond. At high temperatures the pressure
is then reduced to avoid the suppression of layer exfoliation in
the device substrate. A representative process is illustrated in
the graph of FIG. 10 where the wafer bonding temperature-pressure
curve is shown as a function of time in the bond annealing
process.
[0115] b. Single Pressure Cycle--The bonding process is also
improved by applying a uni-axial load to the bonded pair for the
duration of the anneal. In this process it is essential that the
load be small enough to avoid the suppression of blistering.
[0116] 8) Device Layer Modification
[0117] Following the transfer of the device layer 10 in the ion
implantation induced layer transfer process, the near surface
region of the device film 12 is both rough and defect rich. This
layer must be controllably removed to leave a surface that is
useful for subsequent processing to fabricate an optoelectronic
device as shown in FIGS. 11a and 11b. Depending on the device layer
10 this can be accomplished by:
[0118] a. Wet Chemical Polishing--This process uses a
device-film-dependent etch to controllably remove the ion
implantation induced damage layer 13 of the wafer bonded virtual
substrate 16, while simultaneously smoothing the surface of the
transferred layer. Etches for specific materials are given
below.
[0119] 1. Ge/Si:
[0120] i. HF:H.sub.2O.sub.2:H.sub.2O--This etch can be performed at
various dilution ratios x:y:z at various temperatures.
[0121] ii. HF:HNO.sub.3:C.sub.2H.sub.4O.sub.2:H.sub.2O--This etch
can be performed at various dilution ratios w:x:y:z at various
temperatures.
[0122] iii. H.sub.2O.sub.2:H.sub.2O--This etch can be performed at
various dilution ratios y:z at various temperatures.
[0123] 2. InP/Si:
[0124] i. HCl:H.sub.3PO.sub.4:H.sub.2O.sub.2--This etching solution
has been successfully used in ratios of 1:2:2 and 1:2:4. The
H.sub.2O.sub.2 acts as an oxidizing agent and the mixture of HCl
and H.sub.3PO.sub.4 etches the oxide. The combination of oxidation
followed by etching creates a smoothened surface and removes the
implantation damage. FIG. 12 is a graph which shows both the
surface roughness as a function of time for various etch
dilutions.
[0125] b. Chemical Mechanical Polishing--This utilizes both a
chemical and a mechanical slurry to etch through implantation
damage and leave a smooth surface.
[0126] 1. Ge/Si--A colloidal silica slurry in a KOH chemistry can
be used to polish the substrate.
[0127] 2. InP/Si--A colloidal silica slurry in a sodium
hypochlorite solution has been shown to smooth the surface of these
virtual substrate materials.
[0128] c. Homoepitaxial Smoothing--Even in circumstances when
chemical etching removes the implantation damage, but doesn't leave
an optimally smooth surface for subsequent heteroepitaxy,
homoepitaxy of the device film material on the etched sample has
been shown to reduce surface roughness. This can be performed as
the first step in the growth of a heteroepitaxial structure on the
wafer bonded virtual substrate.
[0129] 9) Epitaxial Heterostructure Growth
[0130] The finished virtual substrate 16 is meant to serve as a
template for growth of an optoelectronic device through
hetero-epitaxy. Through careful device layer modification, epitaxy
of a wide range of optoelectronic devices is made possible. A
representative image of such a structure is shown in FIGS. 13a and
13b. FIG. 13a is a diagram which shows the completed wafer bonded
virtual substrate 16 comprised of a thin device film 12, a wafer
bonded interface 42 and a handle substrate 14. FIG. 13b is a
diagram which shows a wafer bonded virtual substrate 16 with an
epitaxially grown device 40 fabricated on the device thin film
12.
[0131] 10) Strain Compensation Layer
[0132] One potential challenge in implementing wafer bonded virtual
substrates in the fabrication of devices in or on the transferred
layer by standard processing such as MOCVD, diffusion,
implantation, and lithography is the possibility of wafer bow due
to the presence of thermal expansion derived strain in the
transferred layer. A practical approach to minimizing this effect
would be to deposit a strain compensation layer on the back surface
of the handle substrate 14 as shown in FIG. 14. FIG. 14 is a
diagram which schematically shows an optoelectronic structure 40
grown on a wafer bonded virtual substrate 16 comprised of the
device film 12, the bonded interface 42, the handle substrate 14,
and a strain compensation layer 18 deposited on the back surface of
the substrate.
[0133] This concept would be implemented by depositing a thin film
18 on the back surface of the handle substrate 14 either prior to
or after the transfer of the device layer 10 to the handle
substrate 14. The strain compensation layer 18 must have the same
sign of .DELTA..alpha. relative to the handle substrate as the
device film 12. The zero bow condition is not a zero strain
condition, but rather a condition in which the strain energy of the
device film 12 and the strain compensation layer 18 are exactly
matched providing no driving force for substrate deformation. The
material and deposition technique can be chosen to minimize the
fabrication cost associated with this processing step. The strain
energy associated with a thin film 12 is increased with substrate
diameter, film strain, and film thickness. The strain compensation
layer material 18, deposition temperature, and thickness can be
chosen to tailor the zero bow process temperature. The following
are examples of how strain compensation could be performed for
several materials systems:
[0134] a. Ge/Si--The simplest case would be to deposit a film of Ge
on the back surface of the Si handle substrate.
[0135] b. InP/Si--Rather than deposit InP on the back of the
handle, a thin film of Ge could be utilized because of its ease of
deposition.
[0136] c. GaAs/Si--As in the InP/Si case, Ge would make a good
strain compensation layer.
[0137] d. Other Materials--For all of the systems mentioned above,
a low-cost material that is easily deposited is a suitable strain
compensation layer, provided that the sign of .DELTA..alpha. is
appropriate.
[0138] Many alterations and modifications may be made by those
having ordinary skill in the art without departing from the spirit
and scope of the invention. Therefore, it must be understood that
the illustrated embodiment has been set forth only for the purposes
of example and that it should not be taken as limiting the
invention as defined by the following claims. For example,
notwithstanding the fact that the elements of a claim are set forth
below in a certain combination, it must be expressly understood
that the invention includes other combinations of fewer, more or
different elements, which are disclosed in above even when not
initially claimed in such combinations.
[0139] The words used in this specification to describe the
invention and its various embodiments are to be understood not only
in the sense of their commonly defined meanings, but to include by
special definition in this specification structure, material or
acts beyond the scope of the commonly defined meanings. Thus if an
element can be understood in the context of this specification as
including more than one meaning, then its use in a claim must be
understood as being generic to all possible meanings supported by
the specification and by the word itself.
[0140] The definitions of the words or elements of the following
claims are, therefore, defined in this specification to include not
only the combination of elements which are literally set forth, but
all equivalent structure, material or acts for performing
substantially the same function in substantially the same way to
obtain substantially the same result. In this sense it is therefore
contemplated that an equivalent substitution of two or more
elements may be made for any one of the elements in the claims
below or that a single element may be substituted for two or more
elements in a claim. Although elements may be described above as
acting in certain combinations and even initially claimed as such,
it is to be expressly understood that one or more elements from a
claimed combination can in some cases be excised from the
combination and that the claimed combination may be directed to a
subcombination or variation of a subcombination.
[0141] Insubstantial changes from the claimed subject matter as
viewed by a person with ordinary skill in the art, now known or
later devised, are expressly contemplated as being equivalently
within the scope of the claims. Therefore, obvious substitutions
now or later known to one with ordinary skill in the art are
defined to be within the scope of the defined elements.
[0142] The claims are thus to be understood to include what is
specifically illustrated and described above, what is
conceptionally equivalent, what can be obviously substituted and
also what essentially incorporates the essential idea of the
invention.
* * * * *