U.S. patent application number 15/149190 was filed with the patent office on 2017-11-23 for methods and structures for preparing single crystal silicon wafers for use as substrates for epitaxial growth of crack-free gallium nitride films and devices.
The applicant listed for this patent is Ananda H. Kumar. Invention is credited to Ananda H. Kumar.
Application Number | 20170338110 15/149190 |
Document ID | / |
Family ID | 50441390 |
Filed Date | 2017-11-23 |
United States Patent
Application |
20170338110 |
Kind Code |
A9 |
Kumar; Ananda H. |
November 23, 2017 |
Methods and structures for preparing single crystal silicon wafers
for use as substrates for epitaxial growth of crack-free gallium
nitride films and devices
Abstract
This document describes the fabrication and use of ceramic
stabilizing layer fabricated right on the product silicon wafer to
facilitate its use as a substrate for fabrication of gallium
nitride films. A ceramic layer is formed and then attached to a
single crystal silicon substrate to form a composite silicon
substrate that has coefficient of thermal expansion comparable with
GaN. The composite silicon substrates prepared by this invention
are uniquely suited for use as growth substrates for crack-free
gallium nitride films, benefitting from compressive stresses
produced by choosing a ceramic having a desired higher coefficient
thermal expansion than those of silicon and gallium nitride.
Inventors: |
Kumar; Ananda H.; (Fremont,
CA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Kumar; Ananda H. |
Fremont |
CA |
US |
|
|
Prior
Publication: |
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Document Identifier |
Publication Date |
|
US 20160254146 A1 |
September 1, 2016 |
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Family ID: |
50441390 |
Appl. No.: |
15/149190 |
Filed: |
May 9, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14251634 |
Apr 13, 2014 |
9337024 |
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15149190 |
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13337045 |
Dec 23, 2011 |
8697541 |
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14251634 |
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61427142 |
Dec 24, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/045 20130101;
H01L 29/2003 20130101; H01L 21/02488 20130101; H01L 21/02516
20130101; H01L 33/007 20130101; H01L 21/02381 20130101; H01L
21/02433 20130101; H01L 21/02282 20130101; H01L 21/02002 20130101;
H01L 21/0254 20130101; H01L 21/02658 20130101 |
International
Class: |
H01L 21/02 20060101
H01L021/02; H01L 29/04 20060101 H01L029/04; H01L 29/20 20060101
H01L029/20 |
Claims
1. A method comprising providing a single crystal silicon
substrate, wherein the silicon substrate comprises a first side and
a second side; applying a paste on the first side of the silicon
substrate; sintering the paste to solidify the paste, wherein the
solidified paste has coefficient of thermal expansion (CTE) higher
than that of silicon.
2. A method as in claim 1 wherein the second side of the silicon
substrate comprises a (111) crystallographic surface.
3. A method as in claim 1 wherein the thickness of the silicon
substrate is less than 50 microns.
4. A method as in claim 1 further comprising applying an adhesive
to the first side of the silicon substrate before applying the
paste.
5. A method as in claim 1 the paste comprises a refractory metal,
ceramic, a powder of ceramic, glass, metal, or a mixture
thereof.
6. A method as in claim 1 wherein the paste comprises an adhesive
additive.
7. A method as in claim 1 wherein the sintering process is between
850 and 950 C.
8. A method as in claim 1 wherein the sintering process is between
800 and 1200 C.
9. A method as in claim 1 wherein the silicon substrate and the
solidified paste form a composite substrate, and wherein the
effective CTE of the composite substrate is higher than that of
GaN.
10. A method comprising providing a single crystal silicon
substrate, wherein the silicon substrate comprises a first side and
a second side; placing the silicon substrate in a deposition
chamber; depositing a layer on the first side of the silicon
substrate in vacuum, wherein the layer has coefficient of thermal
expansion (CTE) higher than that of silicon.
11. A method as in claim 10 the layer comprises a refractory metal,
ceramic, glass, metal, or a mixture thereof.
12. A method as in claim 10 wherein the silicon substrate and the
deposited layer form a composite substrate, and wherein the
effective CTE of the composite substrate is higher than that of
GaN.
13. A method comprising providing a single crystal silicon
substrate, wherein the silicon substrate comprises a first side and
a second side; applying a slurry on the first side of the silicon
substrate; sintering the slurry to solidify the slurry, wherein the
solidified slurry has coefficient of thermal expansion (CTE) higher
than that of silicon.
14. A method as in claim 13 wherein the thickness of the silicon
substrate is less than 50 microns.
15. A method as in claim 13 further comprising applying an adhesive
to the first side of the silicon substrate before applying the
slurry.
16. A method as in claim 13 the paste comprises a refractory metal,
ceramic, a powder of ceramic, glass, metal, or a mixture
thereof.
17. A method as in claim 13 wherein the slurry comprises an
adhesive additive.
18. A method as in claim 13 wherein the sintering process is
between 850 and 950 C.
19. A method as in claim 13 wherein the sintering process is
between 800 and 1200 C.
20. A method as in claim 13 wherein the silicon substrate and the
solidified slurry form a composite substrate, and wherein the
effective CTE of the composite substrate is higher than that of
GaN.
Description
[0001] This application is a continuation of and claims priority
from U.S. patent application Ser. No. 14/251,634, filed on Apr. 13,
2014, entitled "Methods and structures for preparing single crystal
silicon wafers for use as substrates for epitaxial growth of
crack-free gallium nitride films and devices"; which is
incorporated herein by reference.
BACKGROUND
[0002] Single crystal gallium nitride is a technologically
important material finding increasing use in high frequency RF
devices, and Light Emitting Diodes (LEDs). In the absence of
methods to form single crystals of this and similar materials from
melt, they are invariably grown by hetero-epitaxy by metal-organic
chemical vapor deposition, M-O-CVD, or by atomic layer deposition,
ALD, on single crystal substrates of sapphire (Al2O3), or silicon
carbide (SiC), because of their refractory nature, purity,
inertness, and reasonably close lattice structure match to gallium
nitride. Both sapphire and silicon carbide are in themselves
extremely hard to grow as single crystals, the larger the diameter,
the harder to make them. Until recently, nearly 90% of gallium
nitride crystals were grown on 2-inch diameter. Only in 2009 this
percentage dropped below 50%, and now most new LED fabricators are
using 4'' substrates, and some even venturing into 6'' diameter
sapphire wafers. Growing GaN on single crystal silicon carbide is
somewhat easier because of closer lattice matching, but silicon
carbide wafers are stuck at 2'' diameter. GaN growth, on the small
diameter sapphire wafers entails an enormous loss of productivity.
This is a great impediment to them affordable for replacing the
incandescent lighting. It is for this reason that there has been a
continuing effort to use silicon wafers as substrates for GaN
Epitaxy.
[0003] If silicon wafers can be used easily for growing gallium
nitride, the advantages of larger wafer sizes, wide availability,
atomically smooth growth surfaces, would quickly lead to their wide
adoption. Why is this not the case? Growing GaN epitaxially on
silicon (111) would face both a larger lattice mismatch (17%), and
a larger thermal expansion mismatch (about 50%). Researchers have
been able to bridge the lattice mismatch the same way as is done in
cases of sapphire and silicon carbide, here using buffer layers of
AlGaN to grow low defect GaN films on silicon. This greatly reduces
the lattice strain in GaN layer and, as a result, reduces the
dislocation density to reasonable levels. However, the sign and
magnitude of thermal contraction mismatch between GaN and silicon,
are such to give rise to extensive cracking of the latter upon
cooling. In practical terms, this limits the thickness and size of
useful devices, and the yield of such devices.
[0004] Some ingenious methods for growing GaN on silicon have been
developed to enable the use of silicon substrates for GaN growth.
Almost all these methods are based on modifying the growth surface
with a) use of multiple or varied buffer layers, b) limiting the
size of crystals growing and of crack prorogation by scoring the
silicon wafer surface, c) limiting growth surface with in-situ
silicon nitride masking, and allowing for lateral growth over the
masked areas to fill the surface, and d) to change the growth
morphology to nano rods. Even with these difficulties, after years
of development, limited commercial production of GaN on silicon
substrates has just begun.
[0005] The one case where a silicon wafer was modified on the
non-growth side missed the mark. They attached very thin silicon
111 wafer, or very thin single crystal silicon carbide wafer, to
polycrystalline silicon carbide wafers, apparently to reduce cost
of the growth wafers. They missed the mark in the sense, that the
support wafer bonded to the growth wafer, either had the same or
similar coefficient of thermal expansion to silicon, in one case,
and silicon carbide, in another, to make any difference in the
cracking behavior. Even then, the researchers reported growing good
quality GaN epitaxial layers on 2'' substrates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIGS. 1A-1B illustrate exemplary composite substrates
according to some embodiments of the present invention.
[0007] FIGS. 2A-2B illustrate an exemplary process flow for forming
a composite substrate according to some embodiments of the present
invention.
[0008] FIGS. 3A-3B illustrate an exemplary process flow for forming
a composite GaN substrate according to some embodiments of the
present invention.
[0009] FIG. 4 illustrates an exemplary flowchart for forming a
composite substrate according to some embodiments of the present
invention.
[0010] FIG. 5 illustrates another exemplary flowchart for forming a
composite substrate according to some embodiments of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] In some embodiments, this invention relates to preparing
single crystal silicon wafer for epitaxially growing gallium
nitride, which can prevent the cracking that generally occurs in
the deposited gallium nitride film, upon cooling from the elevated
growth temperatures. More particularly the invention teaches a new
approach that involves modifying the silicon (111) wafer substrate,
on the side opposite to the gallium nitride growth side, by coating
of a suitable material having a thermal expansion higher than that
of GaN. This layer, hereinafter referred to as a `stabilizing
layer`, induces compressive stresses in both the silicon and the
GaN layers, upon cooling from room temperature. Since the induced
compressive stress permeates into the GaN layer, though the silicon
wafer, it in effect neutralizes the tensile stress induced in the
GaN by silicon and, thereby, stabilizes it.
[0012] In some embodiments, the silicon substrate is first prepared
by integrally attaching, or forming the stabilizing layer, in a
separate step prior to using it for epitaxial deposition of gallium
nitride. At the GaN growth step, the epitaxially deposited GaAlN or
similar buffer layers, commonly known in the art, are first
deposited on the (111) Si surface to bridge the lattice mismatch to
GaN, followed by the deposition of GaN layer. Upon cooling to
ambient temperature, the GaN layer will remain intact without
cracking. The composite wafer can be processed for device making
and forming interconnections, after which it can be diced, and
separated from silicon layer by etching off the latter.
[0013] In some embodiments, the stabilizing layer can be a
refractory material, such as a thin film of metal or ceramic, thick
film metal or ceramic, or bulk substrates of metal, ceramic or
glass integrally bonded to the silicon wafer surface. It is
preferred that the material of the stabilizing layer have a
coefficient of thermal expansion, CTE, higher than that of GaN. The
silicon wafer can be in pre-stressed condition after the
application of the stabilizing layer, which generally requires a
higher temperature deposition for the layer. The thickness of the
stabilizing layer can be such as to induce compressive stress in
the silicon upon cooling from the layer application temperature. In
fact, the compressive stress can be sufficiently high also to
induce compressive stress in the gallium nitride after its
deposition on the silicon wafer, and cooling to ambient
temperature. The stress level is a function of the nature of the
coating material; the process used for its application, in
particular the application temperature, its CTE, and thickness.
Thinner silicon wafers are preferred, because thinner stabilizing
layers can be used to obtain the desired effect on the deposited
gallium nitride. These parameters can be modeled from known
thermal, mechanical and physical properties of silicon, the
stabilizing layer, and gallium nitride film. It is much more easily
determined, at the outset, from well designed experimentation.
[0014] FIGS. 1A-1B illustrate exemplary composite substrates
according to some embodiments of the present invention. In FIG. 1A,
a composite substrate 100 is shown, comprising a silicon layer 110
disposed on a support layer 120. In some embodiments, the silicon
layer 110 comprises a single crystal silicon layer. The silicon
layer can be less than 500 micron thick, preferably less than 100
microns, and more preferably is between 10 and 50 microns. In some
embodiments, the silicon layer is preferably have a (111)
crystallographic surface. In some embodiment, an adhesive layer 115
is disposed between the silicon layer 110 and the support layer
120, for example, to strengthen the bond between the two layers. In
some embodiments, the support layer has coefficient of thermal
expansion (CTE) higher than that of GaN. Also, the effective CTE of
the composite substrate 100 is higher than that of GaN so that a
subsequently deposited GaN layer on the silicon substrate 110 does
not crack at room temperature.
[0015] In some embodiments, the thickness of the support layer is
higher than that of the silicon layer. For example, the thickness
of the support layer can be thicker than 100 microns, preferably
thicker than 500 microns, and can be thicker than 1 mm.
[0016] In some embodiments, the support layer and the silicon layer
are strongly bonded together, and thus acting as a composite
substrate with respect to thermal expansion. The effective thermal
expansion of the composite substrate is a result of a balance
between the high thermal expansion of the support layer and the low
thermal expansion of the silicon. The effective coefficient of
thermal expansion can be calculated from the thermal expansion of
the composite substrate. In general, the coefficient of thermal
expansion is related to the individual coefficients of thermal
expansion and thicknesses of the support layer and the silicon
substrate. For example, a much thicker support layer will provide
an effective coefficient of thermal expansion similar to that of
the support layer, since the effect of the silicon layer is
smaller.
[0017] In FIG. 1B, a composite substrate 150 is shown, comprising a
GaN layer 130 disposed on a silicon layer 110 disposed on a support
layer 120. The substrate 150 can be formed by depositing a GaN
layer on a substrate 110. In some embodiments, the thickness of the
GaN is higher than 2 microns, and can be higher than 5 microns. The
effective CTE of the composite substrate 150 is higher than that of
GaN so that the GaN layer does not crack after cooled to room
temperature. In some embodiments, the GaN layer comprises a buffer
layer under a GaN layer for lattice matching with the silicon layer
110.
[0018] While the present description utilizes single crystal
silicon substrates, other substrates can be used, such as
silicon-containing substrates (e.g., SiGe substrates, composite
substrates having a silicon layer on a support substrate,
etc.).
[0019] Without limiting the scope of this invention the types of
stabilizing layer can be categorized into several categories, viz.
(i) vacuum deposited thin films of metal or ceramic, (ii) thick
films (of metal, ceramic, or glass), (iii) bulk substrates (of
metal, glass, or ceramic) with an attachment layer of glass or
metal, (iv) in-situ formed glass-ceramic coatings.
[0020] (i) Depositing Stabilizing Layers:
[0021] In some embodiments, highly stressed films can be deposited,
for example, at the backside of the silicon substrate. If the
stress of the composite substrates (e.g., silicon substrates having
the deposited films) is sufficiently high, it can compensate for
the stress induced by the GaN when cooling, and the stress of the
GaN would be compressive. In terms of thermal expansion, if the
thermal expansion of the composite substrates is higher than that
of the GaN, this would prevent cracking in the GaN upon cooling,
with the stress of the GaN compressive.
[0022] In some embodiments, sputtered or evaporated thin films of
refractory metals such as molybdenum and tungsten, if deposited at
elevated temperatures, can induce large compressive stress in
silicon. This process would be a natural choice in the
semiconductor processing culture, if the selected metals are not
detrimental to the subsequent processes, e.g., do not introduce
contamination to the GaN layer (or to the devices that form on the
silicon substrate), for example, reacting with the process gases,
the silicon substrate, and gallium nitride. The economics of this
approach depends on the thickness of films required to achieve the
desired level of beneficial compressive stress in the gallium
nitride layer later grown on the silicon wafer. The advantages of
this method are clean processing, ease of thickness control, good
thermal conductivity of metal films, and compatibility with
downstream device processing.
[0023] In addition to metals, other material, such as ceramic films
deposited by a sputtered process, can also be used to induce the
desired stress in the silicon substrate. The thickness of such
sputtered ceramic films would need to be optimized, since thin
films with inadequate thickness might not be able to withstand the
balancing tensile stress and the subsequently deposited gallium
nitride would crack upon cooling.
[0024] (ii) Coating Stabilizing Layers:
[0025] In some embodiments, alternating to deposition processes,
such as vacuum thin film deposition processes, other coating
processes can be used to form a composite substrate that can
sustain the device fabrication processes, for example, the high
temperature deposition of GaN.
[0026] In some embodiments, thick films are formed by applying to
substrate powders of ceramics, glass, or metals, in the form of a
spray-able slurry, or printable paste (such as metal inks) with
suitable organic binders and solvents, followed by heating to expel
the organics, and sinter the powder to produce bulk coatings on the
substrate. To improve the adhesion of the consolidated ceramic or
metal powders to the underlying substrates, an adhesive can be
used, such as mixing a glass powder with the ceramic or metal
powder. The glass powder would melt and help consolidate the
ceramic powder and also bond to the oxidized silicon surface. The
adhesive would need to be optimized to prevent interfering with the
subsequent device processing. For example, certain glass powder
exhibits temperature softening in the glass phase, and thus
limiting the refractoriness in the formation of the stabilizing
layer, which can be a serious limitation for using thick film
stabilizing layers for typical gallium nitride growth
temperatures.
[0027] Other criterions would also need to be considered in the
material and process selections for forming the thick film coating,
such as metal contamination, softening, and compatibility with
downstream device processing.
[0028] (iii) Bulk Substrates for Stabilizing Layer:
[0029] In some embodiments, bulk metal, such as stainless steel,
molybdenum, tungsten, and bulk ceramic substrates of
polycrystalline aluminum oxide, aluminum nitride, zirconia, can be
used as the support layer. Adhesion additives can be used, since
metal substrates and ceramic substrates might be separated at high
temperature. Exemplary adhesion materials include a glass or metal
bonding layer at the interface. The same is true for glass
substrates such as those made of Pyrex or similar refractory glass.
Limitations in size and cost for these substrates are a further
factor to be considered.
[0030] (iv) In-Situ Formed Glass-Ceramic Coating or Substrates for
Stabilizing Layer:
[0031] In some embodiments, powders of certain glass compositions,
when heated to temperatures in the range of the softening point of
the corresponding bulk glass, crystallize and densify to yield
essentially a ceramic body more refractory than the parent glass.
If these glass powders are suitably disposed on a suitable
substrate, such as a silicon wafer, they would also adhere well to
the substrate during such consolidation. This provides a convenient
method for forming self-adhering, refractory stabilizing layer of
this invention on the silicon wafer, provided the CTE of the
resulting glass-ceramic is higher than the CTE of gallium nitride.
A convenient method to dispose the glass powder is to first form a
green tape of the glass powder by mixing it with suitable polymeric
binders and plasticizers and solvents. The green tape technology is
already well developed for fabricating so called Low Temperature
Co-fired Ceramic, or simply LTCC substrates. Details of the LTCC
substrates have been disclosed in co-pending patent application
Ser. Nos. 12/558,486 and 12/558,490, hereby incorporated by
reference.
[0032] A wide choice of glass compositions are known in the
literature with the desired sintering and crystallizing
characteristics. These glass compositions have been developed for
fabricating low temperature co-fired ceramic, or simply LTCC,
substrates. In our preferred approach, glasses in the
MgO--Al.sub.2O.sub.3--SiO.sub.2 system, having MgO in the range of
15-28% by weight, Al.sub.2O.sub.3 in the range of 9-15% by weight,
the remainder made of silica, except for less than 2% of other
ingredients such as TiO.sub.2, ZrO.sub.2, P.sub.2O.sub.5, or
B.sub.2O.sub.3. The glass powders of these compositions fully
densify in the temperature range of 850.degree. C. to 950.degree.
C., and yielding dense, strong glass-ceramics having thermal
expansion coefficients in the range of 4-6.5 ppm/.degree. C.,
higher that of both silicon and gallium nitride, as desired.
[0033] In some embodiments, to form the stabilizing layer of this
invention, the green ceramic tape of the suitably chosen glass
powder is applied to the single crystal silicon wafer, typically of
(111) orientation, on the side opposite to the polished side
reserved for later gallium nitride deposition. The green tape is
placed on the silicon surface and heated to temperatures of
50-100.degree. C., and at pressures of 500-1000 psi designed to
soften and stick it securely to the wafer surface. The wafer-green
tape assembly is then cured at the required high temperature.
During this consolidation the glass sinters to a strong and dense,
glass-ceramic body, strongly bonded to the oxidized silicon
surface. On cooling from the consolidation temperature, the
differences in the thermal coefficients expansion, CTEs, of silicon
and the resulting glass-ceramic will induce significant compressive
stress in the silicon and corresponding tensile stress in the
glass-ceramic stabilizing layer. The glass-ceramic layer should be
sufficiently strong to resist cracking, and sufficiently thick to
avoid excessive wafer bow.
[0034] This pre-stressed silicon is then cleaned and prepared for
gallium nitride epitaxial deposition. At the deposition
temperature, the wafer composite will be essentially stress-free
and flat. As the wafer is cooled to ambient temperature,
compressive stresses develop in both silicon and the gallium
nitride layers, preventing cracking that would have otherwise
occurred in the absence of the glass-ceramic stabilizing layer. The
composite wafer is processed as needed to form discrete gallium
nitride layers. The wafer is then diced, and devices are released
by etching off the silicon.
[0035] Ceramic coatings for silicon wafers of this invention are
fabricated from ceramic precursors suitably disposed on one of the
major surfaces of the silicon wafer and heating in air to
temperatures in the range of 800-1200 C, and more preferably to
between 900-1100 C, expel the organic binders and consolidate the
ceramic powder into bulk ceramic coatings, whose CTE will be in the
range of 6-10.times.10 ppm/C. of GaN measured from its growth
temperature of around 1000 C. Examples of suitable ceramic
precursors include refractory ceramic powders, such as aluminum
oxide, zirconium oxide, mullite, mixed with suitable glass powders
that fuse during the first heating step to bind the ceramic powders
to form a bulk ceramic coating having CTE equal to or greater that
of gallium nitride
[0036] In some embodiments, the glass powders can comprise glass
compositions in systems in MgO--Al.sub.2O.sub.3--SiO.sub.2,
CaO--Al.sub.2O.sub.3--SiO.sub.2, BaO--Al.sub.2O.sub.3--SiO.sub.2,
or mixtures thereof, which when heated to temperatures in the
preferred temperature range of 900-1100 C, fuse and crystallize to
form a bulk coating having CTE>CTE of GaN.
[0037] A composite structure consisting of a continuous
(crack-free) single crystal gallium nitride layer greater than 2
micron thickness, single crystal silicon and polycrystalline
ceramic coating having CTE higher than that of gallium nitride as
measured from its (GaN's) growth temperature, in that order.
[0038] In some embodiments, gallium nitride layer may comprise
gallium nitride layer may consist of suitable buffer layers aimed
at bridging the mismatch in lattice between that of silicon and
gallium nitride. Gallium nitride layer may consist of suitable
gallium nitride alloy layers for electronic device fabrication. In
some embodiments, electronic devices can be fabricated on the GaN
layer, such as light emitting diode structures.
[0039] FIGS. 2A-2B illustrate an exemplary process flow for forming
a composite substrate according to some embodiments of the present
invention. In FIG. 2A, a slurry of glass powder 220 is disposed on
a silicon substrate 210. The slurry can be sprayed, or pasted on
the silicon substrate. The slurry can comprise glass powder,
polymeric binders, plasticizers, and solvents.
[0040] In FIG. 2B, the silicon substrate with the slurry layer is
heated solidify the slurry, forming a composite substrate
comprising a silicon layer 210 disposed on a ceramic layer 225.
[0041] FIGS. 3A-3B illustrate an exemplary process flow for forming
a composite GaN substrate according to some embodiments of the
present invention. In FIG. 3A, a composite substrate comprising a
silicon layer 210 disposed on a ceramic layer 225 is provided. The
composite substrate can be prepared by attaching a ceramic layer on
a silicon layer, such as in a process described above. In FIG. 3B,
an optional buffer layer 260 is deposited on the composite
substrate, followed by a GaN layer 270. The buffer layer 260 can
serve as a lattice matching layer, to match the lattice of GaN with
that of the silicon.
[0042] In some embodiments, the present invention discloses methods
for forming a continuous (or crack-free) gallium nitride layer on
silicon substrates. An exemplary method of forming a single crystal
gallium nitride layer of thickness exceeding 2 microns on a single
crystal silicon substrate can comprise the steps of
[0043] a. Forming a polycrystalline ceramic layer having CTE
greater than that of gallium nitride on one of the planar sides of
a single crystal silicon substrate in a first heating step
[0044] b. Cooling the ceramic-coated silicon substrate to ambient
temperature
[0045] c. Growing single crystal gallium nitride layer of thickness
exceeding 2 microns in thickness on the silicon surface opposite to
the ceramic coating on the silicon substrate in a second heating
step
[0046] d. Cooling the composite of single crystal gallium nitride
layer
[0047] e. Fabricating GaN nitride on the composite gallium nitride
layer (optional)
[0048] In some embodiments, a ceramic precursor is disposed on the
silicon substrate before the first heating step. After the first
heating, the ceramic precursor solidifies and bonds with the
silicon substrate to form a composite substrate. In some
embodiments, forming step of polycrystalline ceramic layer having
CTE equal to or greater that of gallium nitride including forming
from ceramic powder precursors in the first heating step. The first
heating step can be carried out to temperatures in the range of
800-1200, and most preferably to temperatures in the range of
900-1100 C.
[0049] In some embodiments, the ceramic precursors include
refractory ceramic powders, such as aluminum oxide, zirconium
oxide, mullite, mixed with suitable glass powders that fuse during
the first heating step to bind the ceramic powders to form a bulk
ceramic coating having CTE equal to or greater that of gallium
nitride. The ceramic precursors can include certain glass powders,
such as from glass compositions in systems in
MgO--Al.sub.2O.sub.3--SiO.sub.2, CaO--Al.sub.2O.sub.3--SiO.sub.2,
BaO--Al.sub.2O.sub.3--SiO.sub.2, or mixtures thereof, which when
heated to temperatures in the preferred temperature range fuse and
crystallize to form a bulk coating having CTE>CTE of GaN. The
single crystal silicon can be of (111) orientation. In some
embodiments, the single crystal silicon surface is from (100)
orientation, and which is then completely covered with pyramids of
having (111) facets produced by anisotropic etching.
[0050] In some embodiments, growing gallium nitride step includes
epitaxially depositing single crystal gallium nitride from
pyrolitic decomposition of certain gallium metal organic gaseous
precursors in MOCVD reactor at high temperatures. In some
embodiments, growing gallium nitride step includes first depositing
buffer layers aimed at bridging the lattices of silicon and of
gallium nitride to be grown thereon. The buffer layers can include
aluminum nitride, aluminum gallium nitride, zirconium fluoride, and
others known in the art. Gallium nitride layer can include gallium
nitride alloy layers required to form suitable device structures.
For example, the alloy layers can include magnesium doped gallium
nitride layers, or silicon doped gallium nitride layers. In some
embodiments, gallium nitride devices including light emitting
diodes (LED).
[0051] FIG. 4 illustrates an exemplary flowchart for forming a
composite substrate according to some embodiments of the present
invention. Operation 400 provides a single crystal silicon
substrate, wherein the silicon substrate comprises a first side and
a second side. Operation 410 attaches a layer to the first side of
the silicon substrate to form a composite substrate, wherein the
layer has coefficient of thermal expansion (CTE) higher than that
of GaN, wherein the effective CTE of the composite substrate is
higher than that of GaN so that a subsequently deposited GaN layer
on the second side of the silicon substrate does not crack at room
temperature. In some embodiments, the method further comprises
applying an adhesive to the first side of the silicon substrate
before attaching the layer.
[0052] In some embodiments, the second side of the silicon
substrate comprises a (111) crystallographic surface. The thickness
of the silicon substrate can be less than 50 microns. The attaching
the layer can comprise depositing the layer in vacuum. The layer
can comprise a refractory metal or ceramic. The attaching the layer
can comprise bonding a bulk metal layer or a bulk ceramic layer to
the silicon substrate through an adhesion layer. The attaching the
layer can comprise spraying a slurry on the silicon substrate and
sintering to bond the slurry with the silicon substrate, wherein
the slurry comprises a mixture of a powder of ceramic, glass,
metal, or a combination thereof. The slurry can comprise an
adhesive additive. The attaching the layer can comprise pasting a
paste on the silicon substrate sintering to bond the paste with the
silicon substrate, wherein the slurry comprises a mixture of a
powder of ceramic, glass, metal, or a combination thereof. The
paste can comprise an adhesive additive. The attaching the layer
can comprise disposing a glass powder to the silicon substrate and
sintering the glass powder to form the composite substrate.
[0053] FIG. 5 illustrates another exemplary flowchart for forming a
composite substrate according to some embodiments of the present
invention. Operation 500 provides a single crystal silicon
substrate, wherein the silicon substrate comprises a first side and
a second side. Operation 510 mixes glass powder with polymeric
binders to form a ceramic material, wherein the glass powder has
coefficient of thermal expansion (CTE) higher than that of GaN.
Operation 520 disposes the ceramic material to the first side of
the silicon substrate to form a composite substrate. Operation 530
sinters the composite substrate to bond the ceramic material with
the silicon substrate. Operation 540 deposits a layer of GaN on the
second side of the silicon substrate of the sintered composited
substrate, wherein the effective CTE of the sintered composite
substrate is higher than that of GaN so that the deposited GaN
layer does not crack when cooled to room temperature. In some
embodiments, a buffer layer is deposited before depositing the GaN
layer. In some embodiments, the ceramic material comprises a
magnesium oxide-aluminum oxide-silicon oxide composition. The
sintering comprises annealing at temperature 850 to 950 C.
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