U.S. patent application number 13/484243 was filed with the patent office on 2012-11-08 for crystal growth apparatus and method.
This patent application is currently assigned to AXT. Inc.. Invention is credited to A. Grant Elliot, Weiguo Liu.
Application Number | 20120282133 13/484243 |
Document ID | / |
Family ID | 41132086 |
Filed Date | 2012-11-08 |
United States Patent
Application |
20120282133 |
Kind Code |
A1 |
Liu; Weiguo ; et
al. |
November 8, 2012 |
CRYSTAL GROWTH APPARATUS AND METHOD
Abstract
Systems and methods are disclosed for crystal growth using VGF
and VB growth processes to reduce body lineage. In one exemplary
embodiment, there is provided a method of inserting an ampoule with
raw material into a furnace having a heating source, growing a
crystal using a vertical gradient freeze process wherein the
crystallizing temperature gradient is moved relative to the crystal
and/or furnace to melt the raw material and reform it as a
monocrystalline compound, and growing the crystal using a vertical
Bridgman process on the wherein the ampoule/heating source are
moved relative each other to continue to melt the raw material and
reform it as a monocrystalline compound.
Inventors: |
Liu; Weiguo; (San Leandro,
CA) ; Elliot; A. Grant; (Palo Alto, CA) |
Assignee: |
AXT. Inc.
|
Family ID: |
41132086 |
Appl. No.: |
13/484243 |
Filed: |
May 30, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12104443 |
Apr 17, 2008 |
8231727 |
|
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13484243 |
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Current U.S.
Class: |
420/579 ;
117/83 |
Current CPC
Class: |
C30B 29/42 20130101;
Y10T 117/1056 20150115; C30B 35/00 20130101; Y10T 117/1068
20150115; C30B 11/003 20130101; C30B 11/002 20130101; C30B 11/007
20130101; Y10T 117/1032 20150115 |
Class at
Publication: |
420/579 ;
117/83 |
International
Class: |
C30B 11/02 20060101
C30B011/02; C22C 28/00 20060101 C22C028/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2008 |
CN |
200810089545.9 |
Claims
1.-17. (canceled)
18. A method for crystal growth, comprising: inserting an ampoule
with a crucible having a seed and raw material into a furnace
having a heating source; growing a crystal using a vertical
gradient freeze process wherein the crystallizing temperature
gradient within a heating source is moved relative to the crucible
which is stationary to melt the raw material and reform it as a
monocrystalline compound; and growing, at a predetermined crystal
growth length, the crystal using a vertical Bridgman process on the
ampoule in the furnace wherein the ampoule is moved relative to the
heating source which is stationary to continue to melt the raw
material and reform it as a monocrystalline compound.
19. The method of claim 18 wherein the heating source is a
stationary heating source.
20. The method of claim 18, wherein the furnace has a tapered
crystal growth region and wherein the predetermined crystal growth
length is about 0.25 to about 50 mm above the tapered crystal
growth region.
21. The method of claim 18, wherein the furnace further comprises a
vertical freeze gradient furnace.
22. The method of claim 20 further comprising producing a crystal
ingot having no body lineage.
23. The method of claim 22, wherein the crystal ingot is gallium
arsenide.
24. The method of claim 18, wherein growing the crystal using the
vertical gradient freeze process further comprising growing the
crystal at a cooling rate of about 0.1 to about 10.0.degree.
C./hour and at a temperature gradient of between about 0.5 and
about 10.0.degree. C./cm.
25. The method of claim 24, wherein growing the crystal using the
vertical Bridgman process further comprises growing the crystal at
a cooling rate of about 0.1 to about 10.0.degree. C./hour and a
temperature gradient from about 0.5 to about 10.0.degree.
C./cm.
26. The method of claim 18 further comprises loading, using a
loading crucible, a loading charge of raw Gallium Arsenide material
into the crucible to provide a larger amount of raw Gallium
Arsenide material to the crucible.
27. A method for crystal growth, comprising: inserting an ampoule
with a crucible having a seed and raw material into a furnace
having a heating source; growing a crystal using a vertical
gradient freeze process wherein the crystallizing temperature
gradient within the stationary heating source is moved relative to
the crucible which is stationary to melt the raw material and
reform it as a monocrystalline compound; and growing, at a
predetermined crystal growth length, the crystal using a vertical
Bridgman-Stockbarger process on the ampoule in the furnace wherein
the heating source containing the crystallizing temperature
gradient is moved relative to the ampoule which is stationary to
continue to melt the raw material and reform it as a
monocrystalline compound.
28. The method of claim 27, wherein the furnace has a tapered
crystal growth region and wherein the predetermined crystal growth
length is about 0.25 to about 50 mm above the tapered crystal
growth region.
29. The method of claim 27 wherein the furnace further comprises a
vertical freeze gradient furnace.
30. The method of claim 28 further comprising producing a crystal
ingot having no body lineage.
31. The method of claim 30, wherein the crystal ingot is gallium
arsenide.
32. The method of claim 27, wherein growing the crystal using the
vertical gradient freeze process further comprising growing the
crystal at a cooling rate of about 0.1 to about 10.0.degree.
C./hour and at a temperature gradient of between about 0.5 to about
10.0.degree. C./cm.
33. The method of claim 32, wherein growing the crystal using the
vertical Bridgman-Stockbarger process further comprises growing the
crystal at a cooling rate of about 0.1 to about 10.0.degree.
C./hour and a temperature gradient from about 0.5 to about
10.0.degree. C./cm.
34. The method of claim 27 further comprises loading, using a
loading crucible, a loading charge of raw Gallium Arsenide material
into the crucible to provide a larger amount of raw Gallium
Arsenide material to the crucible.
35. A product comprising a crystal produced by a process for
crystal growth, the process comprising: inserting an ampoule with a
crucible having a seed and raw material into a furnace having a
heating source; growing a crystal using a vertical gradient freeze
process wherein the crystallizing temperature gradient within a
heating source is moved relative to the crucible which is
stationary to melt the raw material and reform it as a
monocrystalline compound; and growing, at a predetermined crystal
growth length, the crystal using a vertical Bridgman process on the
ampoule in the furnace wherein the ampoule is moved relative to the
heating source which is stationary to continue to melt the raw
material and reform it as a monocrystalline compound.
Description
CROSS-REFERENCE TO RELATED APPLICATION INFORMATION
[0001] This is a continuation of application Ser. No. 12/104,443,
filed Apr. 17, 2008, published as US2009/0249994A1, now U.S. Pat.
No. ______, all of which are incorporated herein by reference in
entirety.
BACKGROUND
[0002] 1. Field
[0003] Systems and methods herein relate generally to the growth of
Group III-V, II-VI and related monocrystalline compounds and in
particular to a method and apparatus for growing such compounds
with reduced body lineage.
[0004] 2. Description of Related Information
[0005] Electronic and opto-electronic device manufacturers
routinely require commercially grown, large and uniform single
semiconductor crystals which, when sliced and polished, provide
substrates for microelectronic device production. The growth of a
semiconductor crystal involves heating polycrystalline raw material
to its melting point (typically in excess of 1,200.degree. C.) to
create a polycrystalline raw material melt, bringing the melt into
contact with a high quality seed crystal, and allowing the
crystallization of the melt when in contact with the seed crystal.
The crystallization of the melt forms an essentially cylindrical
crystal (an ingot) along a vertical axis with the seed crystal
below the polycrystalline raw materials. The equipment necessary to
form the semiconductor crystal includes a crystal growth furnace,
an ampoule, a crucible, and a crucible support. The crucible has a
lower, narrow portion, called a seed well.
[0006] Drawbacks exist with the conventional crystal growth process
and crystal growth equipment. For example, known crystal growth
process creates a crystal that has body lineage defect which
reduces the useful overall length of the crystal grown using the
conventional crystal growth process. The reduction in the overall
length of the grown crystal results in a lower yield. Accordingly,
there is a need for a crystal growth apparatus and method that
overcomes drawbacks such as these in known systems.
SUMMARY
[0007] Systems and methods consistent with the invention are
directed to growth of moncrystalline compounds.
[0008] In one exemplary embodiment, there is provided a method of
bringing an ampoule with raw material within a furnace having a
heating source, growing a crystal using a vertical gradient freeze
process wherein the crystallizing temperature gradient is moved
relative to the crystal or furnace to melt the raw material and
reform it as a monocrystalline compound, and growing the crystal
using a vertical Bridgman process on the wherein the
ampoule/heating source are moved relative each other to continue to
melt the raw material and reform it as a monocrystalline
compound.
[0009] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not restrictive of the invention, as
described. Further features and/or variations may be provided in
addition to those set forth herein. For example, the present
invention may be directed to various combinations and
subcombinations of the disclosed features and/or combinations and
subcombinations of several further features disclosed below in the
detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The accompanying drawings, which constitute a part of this
specification, illustrate various implementations and aspects of
the present invention and, together with the description, explain
the principles of the invention. In the drawings:
[0011] FIG. 1 is a cross sectional view of an examplary crystal
growth apparatus 20 consistent with certain aspects related to the
innovations herein;
[0012] FIG. 2 illustrates an exemplary crystal ingot with body
lineage consistent with certain aspects related to the innovations
herein;
[0013] FIG. 3 illustrates an exemplary method for crystal growth
using Vertical Gradient Freeze (VGF) and Vertical Bridgman (VB)
process steps consistent with certain aspects related to the
innovations herein; and
[0014] FIG. 4 illustrates an exemplary method for loading the
crystal growth furnace shown in FIG. 1 consistent with certain
aspects related to the innovations herein.
DETAILED DESCRIPTION OF ONE OR MORE EMBODIMENTS
[0015] Reference will now be made in detail to the invention,
examples of which are illustrated in the accompanying drawings. The
implementations set forth in the following description do not
represent all implementations consistent with the claimed
invention. Instead, they are merely some examples consistent with
certain aspects related to the invention. Wherever possible, the
same reference numbers will be used throughout the drawings to
refer to the same or like parts.
[0016] The apparatus and method are particularly applicable to an
apparatus and method for gallium arsenide (GaAs) crystal growth and
it is in this context that the apparatus and method are described.
It will be appreciated, however, that the apparatus and method has
greater utility since the apparatus and method can be used to
produce other Group III-V, Group II-VI and related monocrystalline
compounds.
[0017] FIG. 1 is a cross sectional view of an example of a crystal
growth apparatus 20. The apparatus may include a crucible support
22 in a furnace 24, such as a furnace that establishes a
crystallizing temperature gradient which may be used in a vertical
gradient freeze (VGF) or vertical Bridgman (VB) crystal growing
and/or, if the furnace is moveable, a vertical Bridgman-Stockbarger
process. The crucible support 22 provides physical support for and
allows for thermal gradient control to an ampoule 26 (that in one
implementation is made of quartz) that contains a crucible 27. The
crucible support 22, when the furnace is in operation, can be moved
during the crystal growth process. Alternatively, the crucible
support is fixed and the furnace, when in operation, can be moved
during the crystal growth process. The crucible 27 may contain a
seed crystal 28, a grown monocrystalline crystal/compound 30 formed
on top of the seed crystal and raw melt material 32. In one
embodiment, the crucible 27 may be a pyrolitic boron nitride (pBN)
material with a cylindrical crystal growth portion 34, a smaller
diameter seed well cylinder 36 and a tapered transition portion 44.
The crystal growth portion 34 has a diameter equal to the desired
diameter of the crystal product. The current industry standard
crystal diameters are 2 inch, 3 inch, 4 inch, 5 inch, 6 inch and 8
inch ingots that can be cut into wafers. At the bottom of the
crucible 27, the seed well cylinder 36, in one implementation, may
have a closed bottom and a diameter slightly larger than that of a
high quality seed crystal 28, e.g., about 6-25 mm, and a length on
the order of 30-100 mm. The cylindrical crystal growth portion 34
and the seed well cylinder 36 may have straight walls or may taper
outwardly on the order of one to a few degrees to facilitate the
removal of the crystal from the crucible 27. The tapered transition
portion 38 between the growth portion 34 and the seed well cylinder
36 has an angled side wall pitched at, for example approximately
45-60 degrees, with a larger diameter equal to and connected to the
growth zone wall and a narrower diameter equal to and connected to
the seed well wall. The angled side wall may also be at other
angles that are more steep or less steep than 45-60 degrees.
[0018] Before insertion in the crystal growth furnace 24, the
crucible 27 is loaded with raw materials and inserted into the
ampoule 26. The ampoule 26 may be made of quartz. The ampoule 26
has a shape similar to that of the crucible 27. The crucible is
cylindrical in a crystal growth region 40, cylindrical with a
narrower diameter in its seed well region 42 and has a tapered
transition region 44 between the two regions. The crucible 27 fits
inside the ampoule 26 with a narrow margin between them. The
ampoule 26 is closed at the bottom of its seed well region 42 and,
like the crucible, sealed on top after the crucible and raw
materials are loaded. The bottom of the ampoule 26 has the same
funnel shape as the crucible 27.
[0019] Since the ampoule-crucible combination has a funnel shape,
the crucible support 22 is required to accommodate this funnel
shape and hold the ampoule 26 stable and upright inside the furnace
24. In other implementations, the ampoule-crucible combination may
retain different shapes, and the basic structure of the crucible
support 22 would be changed accordingly to fit the specific
different shape. According to an embodiment, the stability and
strength to the ampoule and its contents are provided through a
solid, thin-walled cylinder 50 of the crucible support 22. The
solid, thin-walled cylinder 50 accommodates the funnel end of the
ampoule structure 26. In one embodiment, the crucible support
cylinder 50 is made of a heat conducting material, preferably
quartz. In other embodiments, silicon carbide and ceramic may also
be utilized to form the crucible support cylinder 50. The cylinder
50 makes a circle of contact with ampoule 26, where the upper rim
of the cylinder 50 meets the shoulder of the ampoule's tapered
region 38. Such configuration leads to minimal solid-to-solid
contact which ensures that little or no undesirable, relatively
uncontrollable heat conduction occurs. As a result, heating is able
to be generated by other, more controllable processes.
[0020] A low density insulating material, such as ceramic fiber,
fills the majority of the inside of the support cylinder 50 with
only a hollow axial core 52 in approximately the center of the
insulating material left empty to receive the seed well 42 of the
ampoule 26. In other embodiments, the low-density insulating
material may also comprise alumina fiber (1,800.degree. C.),
alumina-silica fiber (1,426.degree. C.), and/or zirconia fiber
(2,200.degree. C.). The insulating material is carefully placed in
the crucible support 22. The weight of the ampoule 26, as it sits
on top of the cylinder 50, pushes the insulating material down and
forms the slanted insulating material edge 54. Filling the majority
of the cylinder interior with a low-density insulator reduces the
flow of air, which ensures that little or no undesirable,
relatively uncontrollable convection flow will take place. Like
conduction, convection is an uncontrollable heat transfer method
that works to the detriment of the VGF and other crystal growth
process.
[0021] The hollow core 52, with a diameter approximately equal to
the ampoule seed well 42, extends downward to a small distance
below the bottom of the ampoule seed well 42. In another
embodiment, the hollow core 52 extends through the crucible support
from the bottom of the seed well to the bottom of the furnace
apparatus 24. The hollow core 52 provides a cooling path from the
center of the crystal. It contributes to cooling in the seed well
and in the center of the growing crystal. With this construction,
heat energy can escape down through the center of the solid crystal
and seed, down through this hollow core 52 in the insulating
material within the crystal support 22. Without the hollow core 52,
the temperature of the center of the cooling ingot would naturally
be higher than the crystal material nearer to the outer surface. In
this case, the center of the ingot in any horizontal cross section
would crystallize later after its perimeter had solidified.
Crystals with uniform electrical properties cannot be made under
these conditions. With the creation of a hollow core 52 included in
the crystal support method, heat energy is conducted down through
the bottom of the ampoule 26 and the hollow core 52 from where it
radiates back out of radiation channels 56. It is important to
reduce heat energy from the center of the growing crystal so that
the isothermal layers are kept flat across the crystal diameter.
Maintaining a flat crystal-melt interface allows the production of
crystals with uniform electrical and physical properties.
[0022] The low-density insulating material within the cylinder 50
obstructs the flow of heat radiation from a set of furnace heat
elements 60 to the ampoule 26 in the seed well region 42, so this
method requires the creation of a plurality of horizontal radiation
channels/openings/tunnels 56 through the insulation material. The
radiation channels 56 penetrate the insulating material to provide
heat radiation outlets to controllably transfer heat from the
furnace heating elements 60 to the ampoule seed well 42. The
number, shape and diameter of the radiation channels 56 varies
depending on specific conditions. The radiation channels may also
be slanted, bent or wave-like. The radiation channels also do not
necessary have to be continuous, as they may extend only partially
through the insulating material. This helps minimize convection
currents. In one embodiment, the diameter of these channels is
small, on the order of a pencil width, so that convection airflow
is insignificant. Larger holes with cross-sectional area on the
order of a square inch or more can also be used according to other
embodiments of the invention. The radiation channels 56 through the
insulating material also work in conjunction with the hollow core
52 in the center of the insulating material to radiate heat energy
drawn from the center of the crystal, and cool the crystal with
planar isothermal temperature gradient layers. The radiation
channels 56 enable temperature control and directly relate to
crystal growth yield.
[0023] The furnace 24 as shown in FIG. 1 is an example of a furnace
that may be used for both Vertical Gradient Freeze (VGF) and
Vertical Bridgman (VB) or Vertical Bridgman-Stockbarger (VBS)
crystal growth processes. Other furnaces may also be used. In the
VGF crystal growth process the crystallizing temperature gradient
within a heat source, which may itself be stationary, is being
moved while the crystal is held stationary. In the VB crystal
growth process, the heat source and its fixed crystallizing
temperature gradient are kept stationary while the crystal is
moved. In the VBS crystal growth process the heat source and its
fixed crystallizing temperature gradient are moved while the
crystal is kept stationary.
[0024] FIG. 2 illustrates a crystal ingot 70 with body lineage 72.
As shown in FIG. 2, the body lineage is typically formed when
crystal growth occurs in more than one different growth plane. When
the body lineage occurs, the crystal at, and above, the body
lineage is unusable and must be recycled. Thus, the body lineage
reduces the yield of the crystal growth process and it is desirable
to reduce body lineage. Some furnaces and processes change the
angle of the tapered portion of the furnace, but this does not
solve the body lineage problems. A furnace and crystal growth
process that overcomes this body lineage problem results in, for a
given furnace, a longer length crystal which thus results in a
larger yield.
[0025] FIG. 3 illustrates a method 80 for crystal growth using
Vertical Gradient Freeze (VGF) and Vertical Bridgman (VB) process
steps that reduce the body lineage resulting in longer crystals and
higher yield. In the crystal growth process, the furnace is
prepared for crystal growth (82) as described above. For the
initial crystal growth from the seed, the VGF process (84) is used.
At some point in the crystal growth process, the VB process (86) or
the VBS process is used to complete the crystal growth. When the VB
or VBS process is used, the melt/solid line is held at a level and
then the process is continued with fixed conditions since the
process changes typically required for VGF process as the volume
decreases are not needed. In one implementation of the process, the
VB process may be used at approximately 12-15 mm (1/2 inch) above
the tapered region 38 as shown in FIG. 1. The combination of the
VGF and VB processes results in longer crystals with fewer body
lineage. The above method may be used with the furnace shown in
FIG. 1, but may also be used with any other crystal growth furnace.
The method may be used to grow crystals from 2 inch-6 inch, or
larger, in diameter.
[0026] As shown in FIG. 4, a loading crucible 90 may be located
above the crucible 27 and allows the crucible 27 to be loaded with
more raw material. In particular, the raw gallium arsenide material
92 is solid and therefore cannot be tightly packed into the
crucible 27 to be melted. Thus, the loading crucible is used to
hold extra raw material that can be melted and then drain down into
the crucible which results in a larger Gallium Arsenide charge in
the crucible 27 which in turn results in a larger length Gallium
Arsenide crystal. For example, about 65% of the raw material may be
initially loaded into the loading crucible 90 and 35% of the raw
material is loaded directly into the crucible 27. As a non-limiting
example, the above method for loading the crystal growth furnace, a
15 kg charge may be loaded into the furnace that produces a 115 mm
ingot with no lineage that results in 115 wafers.
[0027] Now, an example of the growth of a 6'' (150 mm) diameter
Gallium Arsenide grown using the above crystal growth furnace and
method (VGF and VB combined) is described in more detail. To grow
one exemplary crystal, the dimensions of the crucible were 150 mm
diameter and 170 mm length crystal growth region 40. The diameter
of the crucible in the seed well region 42 was 7 mm. In the
example, 14 kg of GaAs polycrystalline material was loaded for
un-doped GaAs ingot growth use. In operation, at first, the GaAs
seed crystal is inserted in the bottom portion of the pBN crucible
27. Next, 14 kg of GaAs polycrystalline material, 100 g of boron
trioxide as the liquid sealant are added therein. Then, the charge
loaded pBN crucible was inserted in a quartz ampoule. The quartz
ampoule was sealed under reduced pressure with a quartz cap. The
quartz ampoule is then loaded in the furnace and placed on the
crucible support.
[0028] Once the ampoule is loaded into the furnace, the quartz
ampoule may be heated at the rate of approximately 270.degree.
C./hour. In one exemplary process, when the temperature reaches
27-28.degree. C. over the melting point of Gallium Arsenide
(1238.degree. C.), the temperature point may be held until all of
the polycrystalline Gallium Arsenide material melts (approximately
10 hours). Once the polycrystalline Gallium Arsenide material melt,
a VGF method was first used for crystal growth. The temperature may
then be reduced in the lower heating zone slowly to let crystal
growth starting at the seed part begin and continue through the
transition region until the crystal growth region cools at the
cooling rate 0.3-0.47.degree. C./hour while maintaining the
temperature gradient from 1.2 to 1.8.degree. C./cm.
[0029] According to this exemplary process, when the crystal has
grown approximately one inch high in the crystal growth region, the
VB process may be started. In the VB process, the crucible down
speed is precisely controlled so as to get a cooling rate of
0.29.degree. C./hour and a temperature gradient of from 1.8 to
5.2.degree. C./cm. A resulting crystal of 81 mm length and high
quality may be achieved via such a process, from a 105 mm long
ingot, which is a crystal yield of 77%. The single crystal
substrate from starting growth portion to end of growth portion may
have a carrier concentration of 9.02E6/cm.sup.3 to 5.30E6/cm.sup.3
and a resistivity of 1.33E8 .OMEGA..cm to 1.64E8 .OMEGA..cm.
Further, the dislocation density may be 3000/cm.sup.2 at the
starting portion and 5000/cm.sup.2 at the end of growth portion. It
is well known in the art that one can, by making suitable changes
to the various system parameters, operate the process in regimes
beyond those demonstrated explicitly in the embodiments.
[0030] While the foregoing has been with reference to a particular
embodiment of the invention, it will be appreciated by those
skilled in the art that changes in this embodiment may be made
without departing from the principles and spirit of the invention,
the scope of which is defined by the appended claims.
* * * * *