U.S. patent application number 09/874939 was filed with the patent office on 2002-12-05 for free standing substrates by laser-induced decoherency and regrowth.
Invention is credited to Miller, David J., Solomon, Glenn S..
Application Number | 20020182889 09/874939 |
Document ID | / |
Family ID | 32993259 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020182889 |
Kind Code |
A1 |
Solomon, Glenn S. ; et
al. |
December 5, 2002 |
FREE STANDING SUBSTRATES BY LASER-INDUCED DECOHERENCY AND
REGROWTH
Abstract
A method for the production of crack-free Group III-Nitride
layers is disclosed. The method proceeds by growing a crack-free
first layer of Group III-Nitride on a starting substrate. A partial
to complete loss of coherency is then achieved between a lattice of
the first layer and a lattice of the starting substrate. A second
layer is grown to form a composite layer that includes the first
layer and the second layer such that the first layer is between the
second layer and the substrate. The starting substrate may then be
completely separated from the composite layer to produce the
freestanding crack-free Group III-Nitride layer.
Inventors: |
Solomon, Glenn S.; (Redwood
City, CA) ; Miller, David J.; (San Francisco,
CA) |
Correspondence
Address: |
JOSHUA D. ISENBERG
204 CASTRO LANE
FREMONT
CA
94539
US
|
Family ID: |
32993259 |
Appl. No.: |
09/874939 |
Filed: |
June 4, 2001 |
Current U.S.
Class: |
438/778 ;
257/E21.113; 257/E21.347 |
Current CPC
Class: |
H01L 21/0242 20130101;
H01L 21/0254 20130101; H01L 21/02694 20130101; C30B 25/02 20130101;
C30B 29/403 20130101; C30B 29/406 20130101; H01L 21/02664 20130101;
H01L 21/268 20130101; C30B 29/64 20130101; H01L 21/02458 20130101;
H01L 21/0237 20130101; C30B 29/60 20130101; C30B 23/02
20130101 |
Class at
Publication: |
438/778 |
International
Class: |
H01L 021/469 |
Claims
What is claimed is:
1) A method for the production of a crack-free Group III-Nitride
layer comprising the following steps: a) growing a crack-free first
layer of Group 111-Nitride on a starting substrate; b) achieving a
partial to complete loss of coherency between a lattice of the
first layer and a lattice of the starting substrate; c) growing a
second layer to form a composite layer without separating the first
layer from the starting substrate, wherein the composite layer
includes the first layer and the second layer, and wherein the
first layer is between the second layer and the starting substrate.
A method according to claim 1, wherein one or more of the first and
second layers includes GaN, AlN, InN or any alloy combination of
these materials.
2) A method according to claim 1, wherein the starting substrate
includes a material selected from the group consisting of one or
more of the following: sapphire, a transparent III-V substrate,
silicon carbide, zinc oxide, magnesium oxide, a silicon oxide,
lithium aluminate, lithium gallate, and/or lithium aluminum
gallate.
3) A method according to claim 1, wherein the first layer is grown
using vapor phase epitaxy (VPE), or chemical vapor deposition
(CVD), or metal-organic chemical vapor deposition (MOCVD), or
hydride vapor phase epitaxy (HVPE), or molecular beam epitaxy
(MBE), or sputtering, or pulsed laser deposition.
4) A method according to claim 1, wherein the second layer is grown
using vapor phase epitaxy (VPE), or chemical vapor deposition
(CVD), or metal-organic chemical vapor deposition (MOCVD), or
hydride vapor phase epitaxy (HVPE), or molecular beam epitaxy
(MBE), or sputtering, or pulsed laser deposition.
5) A method according to claim 1, wherein the first layer may be a
plurality of layers.
6) A method according to claim 1, wherein the second layer may be a
plurality of layers.
7) A method according to claim 1, wherein the partial to complete
loss of coherency in step b) is achieved by absorption of photons
at the first layer.
8) A method according to claim 7 wherein the photons impinge upon
the back side of the starting substrate, and whereby the photons
are largely able to pass unabsorbed through the starting
substrate.
9) A method according to claim 7 wherein the photons are generated
by a laser.
10) A method according to claim 9 wherein the laser emits photons
that are largely unabsorbed by the starting substrate but are
strongly absorbed by the first layer.
11) A method according to claim 10 wherein the partial to complete
loss of coherency is modulated by varying a profile, size, shape,
energy fluence, intensity, pulse duration, pulse frequency, and/or
step pitch of a beam from the laser.
12) A method according to claim 10, wherein the laser is scanned in
a pattern across the starting substrate to minimize the effects of
non-uniform stress induced by the partial to complete loss of
coherency between the first layer and the starting substrate.
13) A method according to claim 12 wherein the pattern is a spiral
starting from the outside edge of the starting substrate, working
inward.
14) A method according to claim 13 wherein the laser emits light
with an energy of between about 3.45 electron volts (eV) and 9.9
eV.
15) A method according to claim 1, further comprising the step of
depositing a third layer between the starting substrate and the
first layer.
16) A method according to claim 15, wherein the third layer
includes silicon, silicon oxide, silicon nitride, or silicon
oxynitride.
17) A method according to claim 15, wherein the third layer is
patterned.
18) A method according to claim 1, further comprising the step of
depositing a fourth layer between the first layer and the second
layer.
19) A method according to claim 18, wherein the fourth layer
includes silicon, silicon oxide, silicon nitride, or silicon
oxynitride.
20) A method according to claim 18, wherein the fourth layer is
patterned.
21) A method according to claim 1, whereby the partial to complete
loss of coherency is achieved at an elevated temperature (between
30.degree. C. and 1200.degree. C.) to minimize damage due to
thermal mismatch stresses.
22) A method according to claim 1, further comprising: during step
b) exposing the first layer and/or starting substrate to a
non-inert nitrogen-bearing gas to prevent the loss of nitrogen from
the first layer.
23) A method according to claim 22, wherein the non-inert
nitrogen-bearing gas is ammonia.
24) A method according to claim 1, whereby the partial to complete
loss of coherency is achieved in-situ, in a growth system.
25) A method, according to claim 24, whereby the partial to
complete loss of coherency is achieved at an elevated temperature
(between 30.degree. C. and 1200.degree. C.) to minimize damage due
to thermal mismatch stresses.
26) A method according to claim 24 further comprising: exposing the
first layer and/or starting substrate to a non-inert
nitrogen-bearing gas to prevent the loss of nitrogen from the first
layer.
27) A method according to claim 26, wherein the non-inert
nitrogen-bearing gas is ammonia.
28) A method according to claim 1 wherein steps b) and c) are
performed substantially simultaneously.
29) A method for the production of a freestanding crack-free Group
III-Nitride layer comprising the following steps: a) growing a
crack-free first layer of Group III-Nitride on a starting
substrate; b) achieving a partial to complete loss of coherency
between a lattice of the first layer and a lattice of the starting
substrate; c) growing a second layer to form a composite layer
without separating the first layer from the starting substrate,
wherein the composite layer includes the first layer and the second
layer, and wherein the first layer is between the second layer and
the starting substrate; d) separating the starting substrate from
the composite layer.
30) A method according to claim 29, wherein one or more of the
first and second layers includes GaN, AlN, InN or any alloy
combination of these materials.
31) A method according to claim 29, wherein the starting substrate
includes a material selected from the group consisting of one or
more of the following: sapphire, a transparent III-V substrate,
silicon carbide, zinc oxide, magnesium oxide, a silicon oxide,
lithium aluminate, lithium gallate, and/or lithium aluminum
gallate.
32) A method according to claim 29, wherein the first layer is
grown using vapor phase epitaxy (VPE), or chemical vapor deposition
(CVD), or metal-organic chemical vapor deposition (MOCVD), or
hydride vapor phase epitaxy (HVPE), or molecular beam epitaxy
(MBE), or sputtering, or pulsed laser deposition.
33) A method according to claim 29, wherein the second layer is
grown using vapor phase epitaxy (VPE), or chemical vapor deposition
(CVD), or metal-organic chemical vapor deposition (MOCVD), or
hydride vapor phase epitaxy (HVPE), or molecular beam epitaxy
(MBE), or sputtering, or pulsed laser deposition.
34) A method according to claim 29, wherein the first layer may be
a plurality of layers.
35) A method according to claim 29, wherein the second layer may be
a plurality of layers.
36) A method according to claim 29, wherein the partial to complete
loss of coherency in step b) is achieved by absorption of photons
at the first layer.
37) A method according to claim 36 wherein the photons impinge upon
the back side of the starting substrate, and whereby the photons
are largely able to pass unabsorbed through the starting
substrate.
38) A method according to claim 36 wherein the photons are
generated by a laser.
39) A method according to claim 38 wherein the laser emits photons
that are largely unabsorbed by the starting substrate but are
strongly absorbed by the first layer.
40) A method according to claim 39 wherein the partial to complete
loss of coherency is modulated by varying a profile, size, shape,
energy fluence, intensity, pulse duration, pulse frequency, and/or
step pitch of a beam from the laser.
41) A method according to claim 39, wherein the laser is scanned in
a pattern across the starting substrate to minimize the effects of
non-uniform stress induced by the partial to complete loss of
coherency between the first layer and the starting substrate.
42) A method according to claim 41 wherein the pattern is a spiral
starting from the outside edge of the starting substrate, working
inward.
43) A method according to claim 39 wherein the laser emits light
with an energy of between about 3.45 electron volts (eV) and 9.9
eV.
44) A method according to claim 29, further comprising the step of
depositing a third layer between the starting substrate and the
first layer.
45) A method according to claim 44, wherein the third layer
includes silicon, silicon oxide, silicon nitride, or silicon
oxynitride.
46) A method according to claim 44, wherein the third layer is
patterned.
47) A method according to claim 29, further comprising the step of
depositing a fourth layer between the first layer and the second
layer.
48) A method according to claim 47, wherein the fourth layer
includes silicon, silicon oxide, silicon nitride, or silicon
oxynitride.
49) A method according to claim 47, wherein the fourth layer is
patterned.
50) A method according to claim 29, whereby the partial to complete
loss of coherency is achieved at an elevated temperature (between
30.degree. C. and 1200.degree. C.) to minimize damage due to
thermal mismatch stresses.
51) A method according to claim 29, further comprising: during step
b) exposing the first layer and/or starting substrate to a
non-inert nitrogen-bearing gas to prevent the loss of nitrogen from
the first layer.
52) A method according to claim 51, wherein the non-inert
nitrogen-bearing gas is ammonia.
53) A method according to claim 29, whereby the partial to complete
loss of coherency is achieved in-situ, in a growth system.
54) A method, according to claim 53, whereby the partial to
complete loss of coherency is achieved at an elevated temperature
(between 30.degree. C. and 1200.degree. C.) to minimize damage due
to thermal mismatch stresses.
55) A method according to claim 53 further comprising: exposing the
first layer and/or starting substrate to a non-inert
nitrogen-bearing gas to prevent the loss of nitrogen from the first
layer.
56) A method according to claim 55, wherein the non-inert
nitrogen-bearing gas is ammonia.
57) A method according to claim 29 wherein steps b) and c) are
performed substantially simultaneously.
58) A method, according to claim 29, wherein step d) includes
cooling the wafer such that thermal mismatch stresses develop
sufficiently to separate or disconnect the composite layer from the
starting substrate.
59) A method, according to claim 29, wherein the complete
separation in step d) is accomplished by the absorption of photons
at the first layer.
60) A method, according to claim 59, wherein the photons are
generated by a laser.
61) A method, according to claim 29, wherein the separation step d)
may accomplished at an elevated temperature (between 30.degree. C.
and 1200.degree. C.) to minimize damage due to thermal mismatch
stresses.
62) A method, according to claim 29, wherein the separation step d)
may be accomplished in-situ, without necessitating the removal of
the starting substrate and composite layer from the growth
system.
63) A freestanding crack-free Group III-Nitride layer produced
according to the method of claim 30.
64) The freestanding crack-free Group III-Nitride layer of claim
63, wherein the Group III-Nitride includes a material chosen from
the group consisting of Gallium Nitride, Aluminum Nitride, Indium
Nitride or any alloy combination of these materials.
65) The freestanding crack-free Group III-Nitride layer of claim
63, wherein the first layer includes a plurality of layers.
66) The freestanding crack-free Group III-Nitride layer of claim
63, wherein the second layer includes a plurality of layers.
67) The freestanding crack-free Group III-Nitride layer of claim
63, wherein the first layer absorbs photons with an energy of
between about 3.45 electron volts (eV) and 9.9 eV.
68) The freestanding, crack-free Group III-Nitride layer of claim
63, further comprising a third layer disposed such that the first
layer is between the third layer and the second layer.
69) The freestanding, crack-free Group III-Nitride layer of claim
63, further comprising a fourth layer disposed between the first
layer and the second layer.
70) The freestanding, crack-free Group III-Nitride layer of claim
63 wherein the first layer has a thickness of between about 0.1
.mu.m and 100 .mu.m.
71) The freestanding, crack-free Group III-Nitride layer of claim
63 wherein the first layer has a thickness of between about 0.1
.mu.m and 10 .mu.m.
72) The freestanding, crack-free Group III-Nitride layer of claim
63 wherein the second layer has a thickness of between about 50
.mu.m and 500 m.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method for the production of a
high quality free-standing layer of Gallium Nitride or similar
material by heteroepitaxial deposition and subsequent removal from
a transparent substrate.
BACKGROUND
[0002] Gallium Nitride (GaN) has been recognized as having great
potential as a technological material. For example, GaN is used in
the manufacture of blue light emitting diodes, semiconductor
lasers, and other opto-electronic devices, as well as in the
fabrication of high-temperature electronics devices. One of the
greatest challenges for the large-scale production of GaN-based
devices is the lack of a suitable native GaN substrate. GaN is not
found in nature; it cannot be melted and pulled from a boule like
silicon, gallium arsenide, sapphire, etc., because at reasonable
pressures its theoretical melting temperature exceeds its
dissociation temperature. However, the fabrication of very high
crystal quality, thin layers of GaN, and its related alloys, for
use in electronic devices, requires that they be deposited
homoepitaxially onto an existing GaN surface. Such high quality
device layers cannot be directly grown heteroepitaxially, for
reasons that are outside the scope of this invention. The
techniques currently in use for the fabrication of high quality GaN
and related layers involve the heteroepitaxial deposition of a GaN
device layer onto a suitable but non-ideal substrate. Currently
such substrates include (but are not limited to) materials such as
sapphire, silicon, silicon carbide, gallium arsenide, lithium
gallate, lithium aluminate, and lithium aluminum gallate. All
heteroepitaxial substrates present challenges to the high-quality
deposition of GaN, in the form of lattice and thermal mismatch.
Lattice mismatch is caused by the difference in interatomic spacing
of atoms in dissimilar crystals. Thermal mismatch is caused by
differences in the coefficient of thermal expansion (CTE) between
joined dissimilar materials, as the temperature is raised or
lowered.
[0003] For the purpose of clarity, heteroepitaxial growth is
defined herein as a process whereby the atomic lattices of two
dissimilar materials are intimately joined together by atomic bonds
across their common interface. When the cross-linking bonds are
made in a regular and orderly array displaying long-range order,
the interface is said to be coherent. When the cross-linking bonds
are broken, bent, twisted, or otherwise distorted such that there
is no long-range order, the interface is said to have lost
coherency. Coherent interfaces are much stronger than incoherent
interfaces, due to the greater number of cross-linking bonds
between the materials. The loss of coherency may be partial; if
only a percentage of cross-linking bonds are broken or distorted in
an interface, the interface is partially coherent. The percentage
(by area) of broken or distorted bonds represents the level of
incoherency or loss of coherency for that interface.
[0004] The most commonly used heteroepitaxial substrate for GaN
deposition is sapphire (Al.sub.2O.sub.3), which has both a large
thermal mismatch and a large lattice mismatch compared to GaN. In
addition, the sapphire substrate is not electrically conductive,
and has poor thermal conductivity, limiting its heat sinking
capabilities, further reducing device performance and complicating
device processing. For reasons unrelated to the scope of this
invention, sapphire otherwise possesses superior properties as a
hetero-substrate. However, the large lattice mismatch results in
films that have very high defect densities, specifically in the
form of dislocations, which are especially undesirable from a
device fabrication point of view. (The formation of dislocations at
regular intervals along the interface does not affect its
coherency, as defined for the purposes of this application, for the
dislocations themselves exhibit a type of long-range order in their
distribution.) As with other epitaxial crystal growth processes, it
is necessary to grow a buffer layer of GaN on the sapphire surface
prior to the formation of device-quality layers. The buffer layer
will vary, depending on device tolerance to dislocations, whether
or not special growth techniques (such as growth through a mask
pattern, use of low temperature buffer layers, etc.) are employed,
as well as other factors. Typically, this GaN buffer thickness is
less than one micron to tens of microns thick. Defect densities,
however, predominantly in the form of dislocations, remain high
(.about.10.sup.10 cm.sup.2) resulting in diminished device quality.
In addition to the conventional buffer layer, a low temperature GaN
buffer layer is nearly always used. This layer is the first layer
deposited on the sapphire. The buffer layer is initially amorphous
and typically is 30-50 nm thick; it is recrystallized at the growth
temperature.
[0005] Besides dislocations and lattice mismatch problems, thermal
mismatch is also a consideration. Typically the GaN is deposited
onto sapphire at a temperature of between 1000-1100.degree. C.; as
the sample cools to room temperature, the difference in thermal
expansion (contraction) rates gives rise to high levels of stress
at the interface between the two materials. Sapphire has a higher
coefficient of thermal expansion (CTE) than does GaN. As the
sapphire substrate and GaN layer cool, the mismatch at the
interface puts the GaN under compression and the sapphire under
tension. Up to a point, the amount of stress is directly related to
the thickness of the deposited GaN, such that the thicker the film,
the greater the stress. Above a film thickness of approximately 10
microns, the stress levels exceed the fracture limits of the GaN,
and cracking and peeling of the film may result. Cracks in this
layer are much less desirable than high dislocation densities, and
should be avoided because of the risk of their catastrophic
propagation into the device layer during subsequent processing
steps.
[0006] One method to prevent such thermal stress-related problems
involves separating the sapphire substrate from the deposited film.
This may be done by physically removing the substrate (lapping and
polishing), or by focusing a very high-intensity light source (such
as from a laser) from the substrate side of the sample. The light
source emits photons having an emission energy that is not absorbed
by the sapphire. This second technique utilizes the difference in
absorption between the two materials: GaN has a room temperature
electron bandgap of approximately 3.45 eV, whereas sapphire has a
bandgap of 9.9 eV. Photons with an energy greater than
approximately 3.45 eV and less than 9.9 eV (corresponding to vacuum
wavelengths less than 359 nm but greater than 125 nm) are able to
pass through the back side of a sapphire wafer, where they are
absorbed in various amounts, depending on energy, by the GaN at the
interface. Once absorbed, the photons are converted to heat, which
locally disrupts the Ga--N bonds. If the incident radiation is
intense enough, large-scale local disruption results in a complete
loss of coherency between the lattice of the sapphire substrate and
the GaN. At lower radiation levels, the loss of coherency may only
be partial and incomplete, resulting in a film that is still
attached to the sapphire substrate, but is no longer completely
bonded to it.
[0007] Both aforementioned techniques have limitations. A
free-standing film must be sufficiently thick to have the required
mechanical strength necessary for subsequent device processing.
Typically, this requires a minimum thickness on the order of 50-100
microns. Deposition of a crack-free film with this thickness onto
sapphire is feasible if done carefully, however thermal stresses
will cause severe bowing in the wafer as it cools to room
temperature. Conventional lapping and polishing processes are not
effective at removing a concave substrate; alternatively, use of a
laser to remove the GaN from the sapphire can create unstable
localized regions of stress in the partially-removed film, leading
to layer fracture during the lift-off process.
[0008] Referring to the drawings, FIGS. 1(a)-1(b) schematically
illustrate the prior art when deposition of a thick layer of GaN
onto sapphire is desired. In FIG. 1(a), sapphire substrate 101 has
a thick (greater than 10 microns) film of GaN 102 deposited onto
it, at the growth temperature, which may be in the range of
1000-1100.degree. C. The actual method of deposition is not
relevant to this invention. Because the film of GaN nucleates onto
the substrate at this temperature, there is no thermal stress
present. FIG. 1(b) shows the effects of the large temperature
change as the sample cools to room temperature. In this figure,
sapphire substrate 101 is now under compressive stress and is bent
concave with respect to the deposited film. If the stresses are
great enough, cracks 103 may form in the substrate. The epitaxial
GaN 102 is under tensile stress, and is cracked, and may also peel
away from or otherwise degrade the interface with substrate
101.
[0009] FIG. 2 schematically represents a series of steps involved
in the conventional method for making a thick layer on a thermally
and/or lattice mismatched substrate. Step 201 calls for the
provision of a prepared substrate. This prepared substrate may be,
for example, plain sapphire, chemically cleaned prior to use. Step
202 is the setting of process parameters and growth conditions for
the growth of the thick, flat, high quality layer. Typically these
conditions are growth temperature, growth rate, flow rates for
precursor compounds, and relative ratios of gas flows in the
reactor. Step 203 calls for the deposition of the thick layer 102
onto the prepared substrate. The thickness of this layer is
preferentially in the range of 10-400 microns. In step 204, the
sample is cooled down to room temperature where it is removed,
intact, from the reactor. The wafer is bowed due to the residual
stress caused by the thermal mismatch between the epitaxial layer
and the substrate. This stress also leads to the formation of many
cracks 103 in the thick layer and the substrate.
[0010] FIGS. 3(a)-3(d) schematically illustrate the prior art
technique of using laser lift-off (LLO) to release a deposited GaN
film from the sapphire substrate. In FIG. 3(a), sapphire substrate
301 has had a film of GaN 302 deposited onto it, at the growth
temperature, and has subsequently been cooled to room temperature.
Film 302 is deposited in such a manner that cracks do not form
during the cooling down stage. In FIG. 3(b), laser beam 303
impinges upon the back side of the sapphire substrate. The laser is
of an energy such that its photons are strongly absorbed by the GaN
layer, while passing through the sapphire largely unabsorbed.
Typically the energy range of such photons is above 3.45 eV,
corresponding to a wavelength (in vacuum) of less than 359 nm but
greater than 125 nm. The source of these photons is typically a
pulsed ultraviolet laser, such as a tripled YAG or excimer laser;
however the characteristics that are important for this process are
not laser-specific. Any highly intense light source that can be
focused down to a spot will suffice. Because the beam impinges from
the sapphire side of the wafer, the GaN at the sapphire/GaN
interface 304 absorbs the photons very strongly, resulting in
localized heating. This localized heating is sufficient to disrupt
the Ga--N bonds, breaking the strained but coherent interface
between the lattice of the substrate 301 and the film 302.
Typically, the laser beam is swept across the backside of the wafer
to gradually release the epitaxial film from the substrate. If the
beam is sufficiently intense, all bonds will be broken, isolating
the two lattices. A less intense beam may be used to partially
disrupt the interface, breaking as few as 5% of the bonds, if such
an effect is desired. In FIG. 3(c) the process has continued. If
the laser beam is too intense or not swept properly, localized hot
areas can develop where the pressure from liberated nitrogen gas
beneath the epitaxial film can build up and cause a rupture in the
surface of the film, 305. Additionally, residual thermal stresses
in the as-yet unreleased areas can cause cracks 306 to develop in
the film, especially as the stress profile changes during the
debonding process. Both of these effects are undesirable and must
be avoided, typically by careful modulation of the impinging laser
power and scan rate, choosing a laser with a short pulse length,
and/or using a beam homogenizer to form an illuminated spot with
uniform intensity, among other techniques. Even with such
precautions, cracking of the released epitaxial film may still
occur, preventing the lift-off and removal of a whole layer to be
used as a free-standing substrate.
[0011] FIG. 4 schematically represents a series of steps involved
in the conventional method for laser lift-off of a GaN film from a
sapphire substrate. In step 401 a prepared substrate is provided.
This prepared substrate may be, for example, plain sapphire,
chemically cleaned prior to use. In step 402, the substrate has a
layer of GaN 302 deposited onto it at an elevated growth
temperature. In step 403 the substrate with GaN epitaxy is allowed
to cool to the ambient temperature and is unloaded from the growth
apparatus. In step 404 the grown wafer is placed into the LLO
apparatus, which typically consists of a laser, laser power
regulator, a wafer holder, and a beam steering mechanism to allow
the beam 303 to impinge over the entire backside surface of the
wafer. The beam then impinges over the backside of the wafer,
gradually debonding the epitaxial film from the sapphire. In step
405, the debonding is complete, the debonded epitaxial film is
removed from the sapphire by heating the wafer above 30.degree. C.
(the melting point of gallium metal) and the layers are gently
pulled apart. Often, the debonded layer is cleaned in an acid
solution to dissolve any remaining gallium from its backside
surface. Although free-standing epitaxial GaN films may be produced
by LLO, the high stress between the sapphire substrate and the GaN
layer often leads to cracking, fractures and other failures in the
GaN layer. Thus the yield of usable free-standing epitaxial GaN
films is often unacceptably low.
[0012] Because of the problems encountered with growing thick
layers of GaN on sapphire, and of the problems encountered in
attempting to remove GaN from the sapphire substrate using a
conventional LLO technique, a need exists for a method for the
laser lift-off and removal of GaN films from sapphire substrates
for the creation of high quality free-standing substrates.
SUMMARY OF THE INVENTION
[0013] The present invention provides a method for the production
of crack-free Group III-Nitride layers. The method proceeds by
growing a crack-free first layer of Group III-Nitride on a starting
substrate. A partial to complete loss of coherency is then achieved
between a lattice of the first layer and a lattice of the starting
substrate. A second layer is grown to form a composite layer that
includes the first layer and the second layer such that the first
layer is between the second layer and the substrate.
[0014] The present invention also provides a method for the
production of arbitrarily thick, crack-free, freestanding layers of
GaN or similar material for subsequent use as substrates. This
method proceeds by growing a crack-free first layer of Group
III-Nitride on a starting substrate. A partial to complete loss of
coherency between a lattice of the first layer and a lattice of the
starting substrate is then achieved. A second layer is grown to
form a composite layer that includes the first layer and the second
layer, and where the first layer is between the second layer and
the substrate. The starting substrate is then completely separated
from the composite layer to produce the freestanding substrate. In
both methods, an intense light source may be used to partially
disrupt the interface between this layer and the underlying
starting substrate, making said interface partially incoherent.
[0015] In both methods a crack-free second layer may be grown on
top of a crack-free first layer that has a partially incoherent
interface with respect to the underlying starting substrate.
[0016] Furthermore, a crack-free second layer may be grown on top
of a crack-free first layer (which has a partially incoherent
interface with respect to the underlying starting substrate),
in-situ, without necessitating a further cooling-down step.
[0017] These and other objects, advantages, and features of the
invention will be set forth in part in the description which
follows, and in part will become apparent to those having ordinary
skill in the art upon examination of the following, or may be
realized and attained as particularly pointed out in the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIGS. 1a and 1b are cross sectional schematic views showing
conventional (prior art) heteroepitaxial growth of thick GaN on
sapphire.
[0019] FIG. 2 schematically shows the process steps for the
conventional thick heteroepitaxial growth of GaN.
[0020] FIGS. 3a-c are cross-sectional schematic views showing a
conventional (prior art) technique for the laser lift-off of a GaN
film from a sapphire substrate.
[0021] FIG. 4 schematically shows the process steps for the laser
lift-off of a GaN film from a sapphire substrate, using the prior
art.
[0022] FIGS. 5a-e are cross-sectional schematic views illustrating
the process for the production of a freestanding GaN substrate
according to the first embodiment of the invention
[0023] FIG. 6 schematically shows the process steps for the
fabrication of a freestanding GaN substrate according to the first
embodiment of the invention
[0024] FIGS. 7a-e are cross-sectional schematic views illustrating
the process for the production of a freestanding GaN substrate
according to the second embodiment of the invention
[0025] FIG. 8 schematically shows the process steps for the
fabrication of freestanding GaN substrate according to the second
embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] For purposes of illustration, the present invention will be
described primarily in relation to the fabrication of a thick
freestanding layer of GaN grown on and subsequently removed from a
sapphire substrate, using a suitable growth technique such as
hydride vapor phase epitaxy (HVPE). It should be understood,
however, that the present invention is applicable to the deposition
of other materials including GaN, AlN, InN and/or their alloys,
and/or onto substrates other than sapphire, and/or using other
deposition techniques (such as metal organic chemical vapor
deposition (MOCVD), molecular beam epitaxy (MBE), sputtering,
evaporation, etc.). For the purpose of providing an example, the
following embodiments are described with respect to fabrication of
GaN substrates. The invention is not limited to just GaN
substrates, in fact, it is intended to be utilized with other III-V
materials. Those skilled in the art will recognize that the process
is equally applicable to producing substrates of other group III
nitrides and other III-V compounds.
[0027] First Embodiment of the Invention
[0028] FIGS. 5a-e schematically depict the method for fabricating a
crack-free freestanding GaN layer according to the first embodiment
of the invention. In FIG. 5a) a starting substrate 501 has a
crack-free first layer of GaN 502 deposited by means of HVPE or
other suitable method (such as MOCVD, MBE, etc.) The starting
substrate 501 may be sapphire, but it may also be of any other
material that is transparent to the region of the ultraviolet
spectrum where the energy exceeds the bandgap of the desired III-V
freestanding substrate material. The starting substrate may also be
specially prepared prior to the deposition of the first layer. Such
preparation may include special cleaning procedures or surface
treatments, and/or the application of a low temperature buffer
layer or layers, and/or the use of a patterned growth mask that
allows growth only on selected areas of the starting substrate.
[0029] First layer 502 is deposited in such a way as to avoid crack
formation. This may be accomplished by depositing a sufficiently
thin layer (preferably between 0.1 and 10 .mu.m) such that
accumulated thermal stresses on cooling down will not exceed the
physical limits of GaN. Alternatively, a thicker layer (up to 100
.mu.m) may be deposited if the resulting film has lower potential
for thermal stress accumulation, due to its higher defect density
or increased surface roughness. It is primarily important, however,
that first layer 502 cannot crack during deposition or during
cooling down to a subsequent processing temperature. First layer
502 may also be composed of a plurality of layers of varying
thickness and composition, as needed.
[0030] In FIG. 5b), the starting substrate 501 and first layer 502
have been removed from the growth reactor. First layer 502 is under
some thermal stress due to the thermal mismatch between the
materials, but cracks have not formed in the film. A laser beam 503
impinges upon the backside of starting substrate 501. Starting
substrate 501 does not absorb these photons, whereas the GaN at the
interface 504 does. Typically the photons are generated by a pulsed
ultraviolet laser, two examples of which are a XeCl excimer laser
(wavelength 345 nm) or a tripled YAG laser (wavelength 355 nm).
Both wavelengths are strongly absorbed by GaN, which has a room
temperature absorption edge of 359 nm, corresponding to an electron
bandgap of 3.45 eV.
[0031] To avoid thermal mismatch effects, it is sometimes helpful
to heat the starting substrate 501 and first layer 502 to an
elevated temperature prior to application of the laser beard. This
elevated temperature may be as high as, or higher than, the actual
growth temperature used during the deposition process (typically
1000-1100.degree. C.). The heating effectively reduces the
magnitude of the thermal strain, reducing the risk of crack
formation caused by non-uniform stress fields induced during the
laser process. In such cases of heating the starting substrate 501
and first layer 502 above 600.degree. C., it may be necessary to
supply a non-inert nitrogen bearing atmosphere (such as ammonia,
NH.sub.3) to prevent the surface of first layer 502 from suffering
the effects of thermal decomposition.
[0032] Photons 503 are strongly absorbed at the interface 504,
where they disrupt the Ga--N bonds, leading to a loss of coherency
505 between the lattice of the starting substrate 501 and the first
layer 502. Depending on factors such as laser pulse energy, peak
power, pulse duration, spot size, beam scan rate, etc. the desired
loss of coherency can be adjusted from partial (fewer than 5% of
bonds broken) to complete (100% of bonds broken.) The loss of
coherency between the lattice of the starting substrate 501 and the
lattice of the first layer 502 relieves the stress between the
starting substrate 501 and the first layer 502. Although the
coherency may be lost between the two lattices, the first layer 502
and starting substrate 501 are not yet physically separated.
[0033] In FIG. 5c), the starting substrate 501 with the first layer
502 is loaded again into the growth system for the deposition of
the second layer 506. The thickness of the second layer 506 layer
may be set arbitrarily; for use as a substrate typically the
thickness of layer 506 is between 50 and 500 .mu.m. The deposition
technique and conditions for depositing the second layer 506 may be
the same as, or different from the conditions used for the
deposition of layer 502. Together, layers 502 and 506 merge to form
a composite layer 507.
[0034] In FIG. 5d) the starting substrate 501 and composite layer
507 is again cooled and unloaded from the growth system. Although
the thickness of composite layer 507 is sufficiently large to
induce catastrophic thermal stress cracks, the partially disrupted
interface 505 effectively limits or eliminates the transmission of
stress between the dissimilar materials. Thus, cracks do not form.
Laser beam 503 is again applied to the backside of the starting
substrate 501 to effect the complete disruption of the Ga--N bonds
at the interface, allowing the release of the composite GaN layer
507 from the starting substrate. Alternatively, as shown in FIG.
5e), if the initial level of Ga--N bond disruption caused during
the first laser step was sufficiently high (>99%), the
accumulated strain during the cooling process will concentrate the
stresses on the remaining bonds, causing the composite layer 507 to
spontaneously shear away from the starting substrate 501. In such a
case, no second laser step will be necessary as the GaN substrate
will spring free of the starting substrate of its own accord. Also
within the scope of this invention are alternative techniques for
removing the substrate from the composite layer, such as lapping
and polishing, etc.
[0035] FIG. 6 schematically shows the series of steps involved in
the method for producing a thick freestanding layer from GaN on a
sapphire substrate, according to the first embodiment of the
invention. Step 601 calls for the provision of a prepared
substrate. In (optional) step 602, the prepared substrate has a
mask pattern applied to its surface. The mask is intended to
prevent growth except in the opened areas of the mask, in order to
improve crystal quality or aid in the later separation. The mask
may be of any material which inhibits growth on its surface and is
compatible with the growth process; typically such masks are made
of silicon oxide, silicon nitride, or silicon oxynitride.
[0036] The starting substrate is loaded into the growth system in
step 603. In step 604, an optional low temperature buffer layer is
set down, prior to step 605, where the initial layer of GaN is
deposited onto the sapphire. This layer may consist of a single
layer deposited at one temperature, or of a plurality of layers of
different compositions, deposited at different temperatures. In
step 606, the wafer, i.e. the starting substrate with the initial
GaN layer, is cooled to ambient temperature and unloaded. In
optional step 607, the wafer may be patterned with a mask, similar
to that which may have been applied in optional step 602, or
consisting of a different type of pattern, if desired. The purpose
of the mask layer is to improve the crystal quality of material
grown through and over it and/or to aid in the later removal of the
film from the substrate.
[0037] In step 608, the wafer is affixed to the LLO apparatus for
the partial to complete disruption of the Ga--N bonds linking the
starting substrate 501 to the first layer 502. Typically, this can
be done using different methods, as described herein.
[0038] In one alternative step 608-A the laser pulse intensity,
pulse width, and scan rate may be modulated such that each spot
induces a uniform but incomplete loss of coherency between the
lattice of the GaN layer and the lattice of the sapphire substrate.
The entire wafer may be uniformly illuminated, and experiences a
uniform loss of coherency of between 5% and greater than 99%
between the two lattices.
[0039] Alternatively, in step 608-B, the pulse intensity, width,
spot size, etc. may be set to cause total disruption of the
coherency at the interface of the lattice of the GaN and the
lattice of the sapphire. Each illuminated spot has total loss of
coherency associated with it; however the beam is swept in such a
manner that the entire surface is not illuminated uniformly. Some
areas of the substrate are not exposed, and have total coherency
maintained, whereas others are made completely incoherent by
exposure. By choosing parameters such as spot pitch distance, the
ratio of area made incoherent to the total area of the wafer can be
adjusted from 5% up to greater than 99%.
[0040] Alternatively, in step 608-C, the laser spot may be rastered
across the backside of the wafer in a pattern, such as a spiral,
square, diagonal, etc. The effect is to disrupt the coherency
between the lattice of the GaN and the sapphire in a systematic
fashion, reducing or eliminating the thermal stresses in a
geometrically controlled way to avoid cracking.
[0041] Regardless of which approach is followed for step 608, the
laser that is used is typically a tripled YAG or excimer laser,
with a spot size of 50 .mu.m to 500 .mu.m, a pulse width of 3 to 50
nanoseconds, and a total fluence of between 300 mJ and 500 mJ per
pulse. As the first layer is under thermal stress, it is often
advantageous to use an auxiliary heating mechanism such as a hot
plate to keep the wafer at an intermediate to high temperature
during this process. For example, if the layer is grown at
1000.degree. C., heating the wafer to 500.degree. C. during the
laser process reduces the thermal stress approximately by half,
reducing the film's tendency to crack.
[0042] The wafer is loaded into the growth system in step 609, and
the growth of the second GaN layer 506 is done in step 610.
Thickness of this layer is preferably between 50 and 500 microns,
more preferably 300 microns. The growth conditions for this layer
may be the same as those used for the first layer, or they may
differ in terms of growth rate, gas flows, partial pressures of
precursor gases, composition of material deposited, temperature,
etc. Layers 502 and 506 merge to form composite layer 507. In step
611 the wafer is cooled once again and removed from the growth
system. Although the total combined thickness of composite layer
507 on the starting wafer is considerable, the
partially-to-completely isolated lattices of the starting substrate
501 and first layer 502 do not transmit stresses effectively,
preventing crack formation.
[0043] In step 612 the composite layer 507 is removed from the
starting substrate 501. There are different methods by which this
may be accomplished, as described herein.
[0044] In a first alternative step 612-A, the wafer may be affixed
again into the LLO apparatus. This time, the laser is used to
completely disrupt 100% of the bonds at the interface, allowing for
the straightforward physical removal of the composite layer by
sliding it off the sapphire wafer.
[0045] Alternatively, in step 612-B, the few remaining bonds that
were left from the first laser step 608 may serve to concentrate
the now-intensified thermal stress induced by the thicker second
layer 506. As the wafer is cooled to the ambient temperature, the
concentrated stress exceeds the physical limits of the GaN at the
interface, causing the composite layer to spontaneously shear away
from the sapphire substrate.
[0046] Or, alternatively, in step 612-C, the composite layer 507 is
separated from the sapphire starting substrate 501 by methods such
as lapping or polishing the backside of the sapphire away. As the
coherency of the interface was already significantly reduced in the
first laser step 608, the wafer does not experience the severe
bowing that otherwise would be evident on such a wafer with a thick
layer deposited onto it.
[0047] Regardless of which alternative method is used in step 612,
the end result is a freestanding, crack-free GaN substrate 508
including the composite layer 507 [it is done, but renamed 508 to
keep with the flow of numbered items.]
[0048] Second Embodiment of the Invention
[0049] FIGS. 7a-e schematically depict a method for fabricating a
crack-free freestanding GaN layer according to the second
embodiment of the invention. In FIG. 7a) a starting substrate 701
is loaded into a growth system 702. The growth system 702 may be,
for instance, a HVPE system. Substrate 701 is placed onto a
susceptor 703, which holds the substrate in position during the
growth process. Susceptor 703 may be fashioned with a slit or
window 704 on its underside, which is designed to allow for the
free transmission of a laser beam through the susceptor onto the
underside of substrate 701.
[0050] In the growth system 702, substrate 701 has a crack-free
first layer of GaN 705 deposited by means of HVPE or other suitable
method (such as MOCVD, MBE, etc.) The starting substrate 701 may be
sapphire, but it may also be of any other material that is
transparent to the region of the ultraviolet spectrum where the
energy exceeds the bandgap of the desired III-V freestanding
substrate material. The starting substrate may also be specially
prepared prior to the deposition of the first layer. Such
preparation may include special cleaning procedures or surface
treatments, and/or the application of a low temperature buffer
layer or layers, and/or the use of a patterned growth mask that
allows growth only on selected areas of the starting substrate.
[0051] First layer 705 is deposited in such a way as to avoid crack
formation. This may be accomplished by depositing a sufficiently
thin layer (preferably between 0.1 and 10 .mu.m) such that
accumulated thermal stresses on cooling down will not exceed the
physical limits of GaN. Alternatively, a thicker layer (up to 100
.mu.m) may be deposited if the resulting film has lower potential
for thermal stress accumulation, due to its higher defect density
or increased surface roughness. It is primarily important, however,
that first layer 705 cannot crack during deposition or during
cooling down to a subsequent processing temperature. First layer
705 may also be composed of a plurality of layers of varying
thickness and composition, as needed.
[0052] In FIG. 7b), a laser beam 706 impinges in-situ upon the
backside of starting wafer 701, coming through the slit or window
704 in susceptor 703. Starting wafer 701 does not absorb the laser
light, whereas the GaN at the interface 707 between the starting
substrate and the first layer, does. Laser light 706 is strongly
absorbed at the interface 707, where it disrupts the Ga--N bonds,
leading to a loss of coherency 708 between the lattice of the
starting substrate 701 and the first layer 705. Depending on
factors such as laser pulse energy, peak power, pulse duration,
spot size, beam scan rate, etc. the desired loss of coherency can
be adjusted from partial (fewer than 5% of bonds broken) to
complete (100% of bonds broken.)
[0053] This laser process is performed in-situ in the growth
reactor, the starting substrate 701 is not unloaded during the
procedure. The wafer may be kept at or above the growth temperature
(typically between 1000-1100.degree. C. for GaN, lower for indium
gallium nitride-based alloys) to eliminate thermal mismatch effects
during the process. Alternatively, the wafer may be cooled to an
intermediate temperature below the growth temperature for the
procedure. In the case that the procedure occurs at a temperature
above 600.degree. C., it may be necessary to supply a non-inert
nitrogen bearing atmosphere (such as ammonia NH.sub.3) to prevent
the surface of first layer 705 from thermally-induced
decomposition. In other cases, however, it may be desirable to
perform the laser procedure at lower temperatures.
[0054] In FIG. 7c), the second layer 709 is grown on top of the
first layer 705. The thickness of this layer may be set
arbitrarily; for use as a substrate, the thickness of layer 709 is
typically between 50 and 500 .mu.m. The deposition conditions for
depositing layer 709 may be the same as, or different from the
conditions used for the deposition of layer 705. Second layer 709
may also be grown substantially simultaneously with the laser
process, without interruption between the steps. Together, layers
705 and 709 merge to form a composite layer 710.
[0055] In FIG. 7d) the starting substrate 701 and composite layer
710 are subjected to an in-situ laser process. A laser beam 706' is
applied to the backside of the starting substrate 701 to effect the
complete disruption of the Ga--N bonds at the interface, allowing
the release of the composite layer 710 from the starting substrate.
[Dave: The laser beam in this step may be different from the laser
beam in the earlier laser step. Therefore, they should have
different numbers.] If the initial level of Ga--(or In--or Al--)N
bond disruption caused during the first laser step was sufficiently
high (>99%), this step will be unnecessary, as the accumulated
strain during the cooling process will concentrate the stresses on
the remaining bonds, causing the composite layer to spontaneously
shear away from the starting substrate. Alternatively, it is
possible to perform the second laser process ex-situ, out of the
reactor, if it is so desired, or to use an alternative method to
separate the substrate from the composite layer, such as a lapping
and polishing technique. Regardless of the method, the net result,
as shown in FIG. 7e) is the complete crack-free separation of the
composite layer 710 from the starting substrate 701 to form a
freestanding, crack-free GaN substrate 711.
[0056] FIG. 8 schematically shows the series of steps involved in
the method for producing a thick freestanding layer of GaN on a
sapphire substrate, according to the second embodiment of the
invention. Step 801 calls for the provision of a prepared
substrate. In (optional) step 802, the prepared substrate has a
mask pattern applied to its surface. The mask is intended to
prevent growth except in the opened areas of the mask, in order to
improve crystal quality or aid in the later separation. The mask
may be of any material which inhibits growth on its surface and is
compatible with the growth process; typically such masks are made
of silicon oxide, silicon nitride, or silicon oxynitride.
[0057] The substrate is loaded into the growth system in step 803.
In step 804, an optional low temperature buffer layer is set down,
prior to step 805, where the initial layer of GaN is deposited onto
the sapphire. This layer may consist of a single layer deposited at
one temperature, or of a plurality of layers of different
compositions, deposited at different temperatures.
[0058] In step 806 the backside of the substrate is illuminated
with the laser beam 706, in situ, through the slit or window 704 in
susceptor 703 for the partial to complete disruption of the Ga--N
bonds linking starting substrate 701 to the first layer 705.
Typically, this can be done using different methods, as described
herein.
[0059] In one alternative step 806-A the laser pulse intensity,
pulse width, and/or scan rate are modulated such that each spot
induces a uniform but incomplete loss of coherency between the
lattice of the GaN layer and the lattice of the sapphire substrate.
The entire wafer is uniformly illuminated, and experiences a
uniform loss of coherency of between 5% and greater than 99%
between the two lattices.
[0060] In method 806-B, the pulse intensity, width, spot size, etc.
are set to cause total disruption of the coherency at the interface
of the lattice of the GaN and the lattice of the sapphire starting
substrate 701. Each illuminated spot has total loss of coherency
associated with it; however the beam is swept in such a manner that
the entire surface is not illuminated uniformly. Some areas of the
substrate are not exposed, and have total coherency maintained,
whereas others are made completely incoherent by exposure. By
choosing parameters such as spot pitch distance, the ratio of area
made incoherent to the total area of the wafer can be adjusted from
5% up to greater than 99%.
[0061] Alternatively, in step 806-C, the laser spot may be rastered
across the backside of the starting substrate in a pattern, such as
a spiral, square, diagonal, etc. The effect is to disrupt the
coherency between the lattice of the GaN and the sapphire starting
substrate in a systematic fashion, reducing or eliminating the
thermal stresses in a geometrically controlled way to avoid
cracking.
[0062] Regardless of which method is used in step 806, laser beam
706 is typically from a tripled YAG or excimer laser, with a spot
size of 50 .mu.m to 500 .mu.m, and a total fluence of between 300
mJ and 500 mJ per pulse. As this process is performed in-situ, it
is possible to keep the wafer at an elevated temperature (up to or
above the growth temperature, typically 1000-1100.degree. C.) to
eliminate the effects of thermal mismatch. If this is done at a
temperature above 600.degree. C., it will be necessary to have a
non-inert nitrogen-bearing atmosphere (such as ammonia, NH.sub.3)
present to prevent the surface of the first layer from suffering
the effects of thermal decomposition.
[0063] In step 807, the second layer 709 is deposited on top of the
first layer 705. The thickness of the second layer 709 is
preferably between 50 and 500 microns, more preferably 300 microns.
The growth conditions for this layer may be the same as those used
for the first layer 705, or they may differ in terms of growth
rate, gas flows, partial pressures of precursor gases, temperature,
etc. It is also within the scope of this invention to have step 807
occur concurrently with step 806, i.e. the interface disruption may
occur at the same time as layer 709 is being deposited. Together,
layers 705 and 709 merge to form a composite layer 710.
[0064] In (optional) step 808, the laser is again applied, in-situ,
to the backside of the starting substrate 701. This time the laser
is used to completely disrupt the bonds, allowing the composite
layer to be removed from the starting substrate. Under certain
circumstances, described herein, this step may be omitted in lieu
of other steps 810-B, 810-C, or 810-D, below.
[0065] In step 809 the wafer is cooled once again and removed from
the growth system. Although the total combined thickness of GaN on
the starting wafer is considerable, the partially-to-completely
isolated lattices of the starting substrate 701 and first layer 705
do not transmit stresses effectively, preventing crack
formation.
[0066] In step 810 the composite layer 710 is removed from the
starting substrate 701. There are different methods by which this
may be accomplished, as described herein.
[0067] In method 810-A, which assumes that optional laser step 808
was done, the composite layer can be physically lifted or dragged
free of the sapphire substrate.
[0068] In one alternative step 810-B, the cooled wafer may be
affixed into an ex-situ LLO apparatus. This time, the laser is used
to completely disrupt 100% of the bonds at the interface, allowing
for the straightforward physical removal of the composite layer by
sliding it off the sapphire wafer.
[0069] Alternatively, in step 810-C, the few remaining bonds that
were left from the first laser step 806 may serve to concentrate
the now-intensified thermal stress induced by the thicker composite
layer 710. As the starting substrate 701 and composite layer 710
cool to ambient temperature, the concentrated stress exceeds the
physical limits of the GaN at the interface, causing the composite
layer to spontaneously shear away from the sapphire substrate.
[0070] In method 810-D, the composite layer 710 is separated from
the sapphire by methods such as lapping or polishing the backside
of the sapphire away. As the coherency of the interface 707 was
already significantly reduced in the first laser step 806, the
wafer does not experience the severe bowing that otherwise would be
evident on such a wafer with El thick layer deposited onto it.
[0071] Regardless of which method is used in step 810, the end
result is a freestanding, crack-free GaN substrate 711[it is
labeled, but I changed the # to 711, to go with the flow of number
labels]
[0072] The foregoing embodiments are set forth for the purpose of
example, and should not be construed as limiting the present
invention. The present teaching may be applied to other types of
apparatuses and methods. The description of the present invention
is intended to be illustrative and not limiting the scope of the
appended claims. Alternatives, modifications, and variations on
this method will be apparent to those skilled in the art.
* * * * *