U.S. patent application number 13/889602 was filed with the patent office on 2014-05-29 for quartz boat method and apparatus for thin film thermal treatment.
This patent application is currently assigned to Stion Corporation. The applicant listed for this patent is Stion Corporation. Invention is credited to Paul Alexander, Jurg Schmitzberger, Ashish Tandon, Robert D. Wieting.
Application Number | 20140147800 13/889602 |
Document ID | / |
Family ID | 45493965 |
Filed Date | 2014-05-29 |
United States Patent
Application |
20140147800 |
Kind Code |
A1 |
Alexander; Paul ; et
al. |
May 29, 2014 |
QUARTZ BOAT METHOD AND APPARATUS FOR THIN FILM THERMAL
TREATMENT
Abstract
A method of supporting a plurality of planar substrates in a
tube shaped furnace for conducting a thermal treatment process is
disclosed. The method uses a boat fixture having a base frame
including two length portions and a first width portion, a second
width portion, and one or more middle members connected between the
two length portions. Additionally, the method includes mounting a
removable first grooved rod respectively on the first width
portion, the second width portion, and each of the one or more
middle members, each first grooved rod having a first plurality of
grooves characterized by a first spatial configuration. The method
further includes inserting one or two substrates of a plurality of
planar substrates into each groove in the boat fixture separated by
a distance.
Inventors: |
Alexander; Paul; (San Jose,
CA) ; Schmitzberger; Jurg; (San Jose, CA) ;
Tandon; Ashish; (Sunnyvale, CA) ; Wieting; Robert
D.; (Simi Valley, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Stion Corporation; |
|
|
US |
|
|
Assignee: |
Stion Corporation
San Jose
CA
|
Family ID: |
45493965 |
Appl. No.: |
13/889602 |
Filed: |
May 8, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13171089 |
Jun 28, 2011 |
8461061 |
|
|
13889602 |
|
|
|
|
61367211 |
Jul 23, 2010 |
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Current U.S.
Class: |
432/253 |
Current CPC
Class: |
H01L 21/67109 20130101;
H01L 21/67313 20130101; H01L 21/6734 20130101; H01L 31/18 20130101;
H01L 21/67316 20130101 |
Class at
Publication: |
432/253 |
International
Class: |
H01L 21/673 20060101
H01L021/673 |
Claims
1. An apparatus for holding substrates for thermal treatment
comprising: a frame fixture having a substantially rectangular
prism shape, the frame fixture including a base frame, a top frame,
side connection bars coupling the base frame and the top frame, the
base frame having two width members and middle joint members
connected between the two length members; a first grooved rod
removably mounted on each of the two width members and each of the
middle joint members, each first grooved rod including a first
plurality of grooves for supporting planar substrates; a first
grooved bar removably mounted on each of two width members of the
top frame, each first grooved bar including a second plurality of
grooves aligned with the first plurality of grooves for guiding the
plurality of planar substrates into the apparatus; and a rack
structure configured to be a mechanical support of the frame
fixture in a loading position inside a furnace for subjecting the
plurality of planar substrates in the first configuration to one or
more reactive thermal treatment processes.
2. The apparatus of claim 1 wherein the frame fixture and the first
grooved rod and the first grooved bar comprise quartz.
3. The apparatus of claim 1 wherein each of the plurality of planar
substrates comprises glass having at least one surface overlaid by
a photovoltaic precursor layer which includes at least copper,
indium, and gallium species.
4. The apparatus of claim 1 wherein each of the plurality of planar
substrates comprises a piece of soda lime glass having a form
factor selected at least from a 65 cm.times.165 cm rectangle, a 20
cm.times.50 cm rectangle, a 20 cm.times.20 cm square.
5. The apparatus of claim 1 wherein the first configuration
comprises an arrangement of the plurality of planar substrates in a
substantially vertical orientation and parallel to a neighboring
planar substrate at a distance equal to or greater than a
predetermined value associated with at least the furnace and the
one or more thermal treatment processes.
6. The apparatus of claim 5 wherein the predetermined value
comprises a minimum spacing for maintaining a sufficient convection
flow so that a temperature difference across each planar substrate
is smaller than 15 degrees Celsius at least during a dwell stage of
the thermal treatment processes.
7. The apparatus of claim 6 further comprising a second grooved rod
for replacing the first grooved rod to mount on each of the two
width members and each of the one or more middle joint members,
each second grooved rod including a second plurality of grooves
respectively configured to support a plurality of planar substrates
in a second configuration, each groove supporting at least one
planar substrate.
8. The apparatus of claim 7 wherein the second configuration
comprises an arrangement of the plurality of planar substrates
substantially parallel to each other with a front/back surface of
one planar substrate facing another front/back surface of a
neighboring planar substrate at a first/second distance away, the
first distance being equal to or greater than the predetermined
value, the second distance being substantially smaller than the
first distance.
9. The apparatus of claim 8 wherein the front surface of each
planar substrate comprises a precursor layer being subjected to the
one or more thermal treatment processes.
10. The apparatus of claim 8 wherein the second distance is
substantially zero and each groove is configured to support two
planar substrates in back-to-back configuration.
11. The apparatus of claim 1 wherein the furnace is made of quartz
material for enclosing a volume configured to form a gaseous
environment comprising at least selenium species or sulfur species
during the one or more thermal treatment processes.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 13/171,089, filed Jun. 28, 2011, which claims priority to
U.S. Provisional Application No. 61/367,211, filed Jul. 23, 2010,
commonly assigned, and hereby incorporated by reference in its
entirety for all purposes.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to photovoltaic
materials and manufacturing method. More particularly, the present
invention provides a method and apparatus for thin film thermal
treatment. Embodiments of the invention include a method and
apparatus for holding a plurality of extra large substrates for
achieving substantially uniform substrate temperature during a
thermal process to form a photovoltaic absorber material, but it
would be recognized that the invention may be applied for other
thin-film treatment applications.
[0003] From the beginning of time, mankind has been challenged to
find ways of harnessing energy. Energy comes in forms such as
petrochemical, hydroelectric, nuclear, wind, biomass, solar, and
more primitive forms such as wood and coal. More recently,
environmentally clean and renewable source energy has been desired.
Clean and renewable sources of energy also include wind, waves,
biomass, and the like. Still other types of clean energy include
solar energy.
[0004] Solar energy technology generally converts electromagnetic
radiation from the sun to other forms of energy. These other forms
of energy include thermal energy and electrical power. For
electrical power applications, solar cells are often used. Although
solar energy is environmentally clean and has been successful to a
point, many limitations remain before it becomes widely used
throughout the world. As an example, one type of solar cell uses
crystalline materials, which are derived from semiconductor
material ingots. These crystalline materials can be used to
fabricate optoelectronic devices that include photovoltaic and
photodiode devices that convert electromagnetic radiation to
electrical power. However, crystalline materials are often costly
and difficult to make on a large scale. Additionally, devices made
from such crystalline materials often have low energy conversion
efficiencies. Other types of solar cells use "thin film" technology
to form a thin film of photosensitive material to be used to
convert electromagnetic radiation into electrical power. Similar
limitations exist with the use of thin film technology in making
solar cells. That is, efficiencies are often poor. Additionally,
film reliability is often poor and cannot be used for extensive
periods of time in conventional environmental applications. Often,
thin films are difficult to mechanically integrate with each other.
These and other limitations of these conventional technologies can
be found throughout the present specification and more particularly
below.
[0005] As an effort to improve thin film solar cell technology,
processes of manufacturing an advanced CIS and/or CIGS based
photovoltaic film stack on sized substrates with planar, tubular,
cylindrical, circular or other shapes have been introduced. There
are various manufacturing challenges in forming the photovoltaic
film stack, such as maintaining structure integrity of substrate
materials, controlling chemical compositions of the ingredients in
one or more precursor layers, carrying out proper reactive thermal
treatment of the one or more precursor layers within a desired
gaseous environment, ensuring uniformity and granularity of the
thin film material during reactive thermal treatment, etc.
Especially, when manufacturing the thin film based solar cell on
large sized substrate, temperature uniformity across whole
substrate surface is desired. While conventional techniques in the
past have addressed some of these issues, they are often inadequate
in various situations. Therefore, it is desirable to have improved
system and method for processing thin film photovoltaic devices on
planar or non-planar shaped, fixed or flexible substrates.
BRIEF SUMMARY OF THE INVENTION
[0006] This invention provides a method and apparatus for holding
large substrates with precursor material for thermal treatment. The
method and apparatus provide an improved loading configuration for
a plurality of substrates during thermal processes for manufacture
of thin film based photovoltaic devices.
[0007] This invention provides a method of holding a plurality of
planar substrates for thermal treatment in a tube shaped furnace
having a first end and a second end. The tube shaped furnace is
surrounded by heaters for conducting a thermal treatment process.
The first end has a door and the second end is insulated. The
method further includes providing a boat fixture having a base
frame coupled to a top frame. The base frame includes two length
portions and a first width portion, a second width portion, and one
or more middle members connected between the two length portions. A
grooved rod is mounted respectively on the first width portion, the
second width portion, and each of the one or more middle members.
The substrates are placed in the boat fixture so that the grooves
support spaced-apart planar substrates by a desired distance. The
boat fixture is loaded via a rack fixture into the tube shaped
furnace for the thermal treatment process.
[0008] The method includes a substrate arrangement where each
substrate is inserted alone in one groove without a front/back
phase configuration, and with a minimum spacing between any
neighboring substrates at least equal to a desired value, e.g.
about 1 inch.
[0009] In another embodiment, the loading configuration includes a
substrate arrangement where each substrate is inserted alone in one
groove with a front/back surface of any substrate directly faces
another front/back surface of a neighboring substrate inserted in
another groove at a desired spacing.
[0010] In yet another embodiment, the loading configuration
includes replacing each first grooved rod on the base frame by a
second grooved rod mounted on the first width portion, the second
width portion, and each of the one or more middle members. Each
second grooved rod includes a second plurality of grooves in a
second spatial configuration.
[0011] In an alternative embodiment, the present invention provides
an apparatus for holding one or more substrates for thermal
treatment. The apparatus includes a frame fixture having a
substantially rectangular prism shape including a base frame, a top
frame, one or more side connection bars coupled the base frame and
the top frame. The base frame has two width members and one or more
middle joint members connected between two length members. The
apparatus further includes a first grooved rod removably mounted on
each of the two width members and each of the one or more middle
joint members. Each first grooved rod includes a first plurality of
grooves respectively configured to support a plurality of planar
substrates in a first configuration. Additionally, the apparatus
includes a first grooved bar removably mounted on each of two width
members of the top frame. Each first grooved bar includes a second
plurality of grooves respectively aligned with the first plurality
of grooves for guiding the plurality of planar substrates.
Furthermore, the apparatus includes a rack structure configured to
be a mechanical support of the frame fixture in a loading position
inside a furnace for subjecting the plurality of planar substrates
in the first configuration to one or more reactive thermal
treatment processes.
[0012] In yet another alternative embodiment, the present invention
provides a method for processing substrates through thermal
treatments with each of their temperature difference being
controlled to within 15 degrees Celsius at least during a dwell
stage between ramping up or down stages. Moreover, the furnace
provides an enclosed volume for subjecting a precursor film
containing at least copper and indium species overlying one surface
of each substrate to a gaseous selenium or sulfur species to
produce a photovoltaic absorber for solar cell.
[0013] The invention provide an apparatus for holding a plurality
of planar substrates overlaid with photovoltaic precursor layers in
a furnace tube. The furnace tube has heaters to supply heat energy
in a controlled manner. The furnace can be filled with desired
gaseous species including selenium and/or sulfur for a reactive
thermal treatment of the precursor layer for manufacture of
photovoltaic cells. In particular, the apparatus utilizes a quartz
boat fixture having a rack fixture configured with a plurality of
grooves for supporting large planar (rectangular or square in most
cases) glass substrates. Some embodiments of the invention provide
loading configurations of a plurality of planar substrates in the
boat fixture by aligning them vertically, arranging the plurality
of grooves with a proper size for each one, and disposing them with
an optimum spacing between each other and in periodic groups, and
others. In some embodiments, the loading configurations of the
substrates in the quartz boat fixture allow an effective convection
flow in between the planar substrates to transfer heat energy from
hotter regions to cooler regions of the each substrate during the
reactive thermal treatment process.
[0014] In a specific embodiment, the temperature across each
substrate is maintained substantially uniform or at least the
temperature difference is controlled to be smaller than a set
value. In another specific embodiment, an alternative loading
configuration includes two substrates being inserted in each groove
in a back-to-back manner with respective precursor layer exposed at
front surface. The precursor layer of one of the two planar
substrates loaded in a first groove faces the precursor layer of
one of two planar substrates loaded in a neighboring groove at a
predetermined distance away from one side, while the precursor
layer of another one of the two planar substrates in the first
groove faces the precursor layer of one of two planar substrates
loaded in a neighboring groove at the predetermined distance away
from an opposite side. Therefore, the number of planar substrates
loaded in the quartz boat fixture is increased compared to loading
one substrate only to one groove without reducing the spacing
between the subjected surfaces of neighboring substrates. In a
specific embodiment, the predetermined distance between the
neighboring substrates is associated with the furnace tube
configuration, the dimension of the substrates, the gap distance
between the loaded substrates and inner wall of the furnace tube,
as well as the one or more thermal treatment processes. For
example, the furnace is made as tube shape for facilitating gaseous
convection flow therein. The furnace tube is made of quartz
material which is semi-transparent to thermal radiation from
several zoned heaters around tube with temperature control for
different regions. Coolers also can be added for easily ramping
furnace temperature down as desired in certain stages of the
thermal treatment process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a side view along an axis of a furnace tube
including a quartz boat fixture with loaded planar substrates for
manufacture of photovoltaic cells;
[0016] FIG. 2 is a perspective view of the quartz boat fixture for
loading a plurality of planar substrates;
[0017] FIG. 3 is a side view from the closed end of the furnace
tube loaded with a quartz boat fixture holding a plurality of
planar substrates;
[0018] FIG. 4 is a side view of the furnace tube loaded with a
quartz boat fixture holding a plurality of planar substrates;
[0019] FIG. 5 is a side view of the furnace tube loaded with a
quartz boat fixture holding a plurality of planar substrates;
[0020] FIG. 6 is a cross-section view of convection flows according
to an embodiment of the present invention.
[0021] FIG. 7 is a cross-section view of convection flows according
to another embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] FIG. 1 is a side view along an axis of a furnace tube
including a quartz boat fixture with loaded planar substrates for
manufacture of photovoltaic cells according to an embodiment of the
present invention. As shown, the furnace tube 100 is characterized
by a length of tubular shaped container or chamber enclosing a
volume of space. In an embodiment, the furnace tube 100 has a first
end 112 that is configured to be engaged, covered, and sealed by a
cover member (or a door) 120 and a second end 114 on the opposite
side. The inner wall of the furnace tube 100 is a smooth surface
for facilitating internal gas convection during thermal process. In
another embodiment, the furnace tube 100 is set up in an
orientation with its tube axis 101 along horizontal (or floor)
direction. The second end 114 can be also opened and usually is
thermally insulated. A gas supply device and/or a gas monitor (not
shown) can be coupled to the furnace tube 100 for providing a
controllable gaseous environment required for conducting one or
more reactive thermal treatments of any work samples loaded in the
furnace tube 100. A plurality of heaters 160 are wrapped around the
furnace tube for providing radiation heat required for the thermal
treatments. The volume of space 170 of the furnace tube 100,
depending on its diameter and length, allows the boat fixture 140
with inserted substrates to be loaded into the furnace tube 100 and
is supported by a rack fixture 142.
[0023] The boat fixture 140 can hold a plurality of planar
substrates 150 for thermal treatment within the furnace tube 100.
For example, 40 or more numbers of planar rectangular shaped
substrates can be held in the boat fixture 140 at one time. Each
planar substrate can have a dimension of a few tens of centimeters
in width and more than 100 centimeters in length. Glass substrates
with 65 cm.times.165 cm rectangular shape are used for manufacture
of thin-film photovoltaic cells. In certain applications, smaller
sized substrates in dimensions of about 20 cm.times.20 cm, or 20
cm.times.50 cm, or other form factors, can be loaded with a slight
modification of the loading configuration of the boat fixture.
During the thermal treatment processes, a temperature uniformity of
the substrate usually is a key process parameter. As a simple
indicator of the temperature uniformity, a temperature difference
across the substrate, i.e., .DELTA.T=T1-T2 can be monitored, where
T1 represents a temperature near bottom region of the substrate and
T2 represents a temperature near top region of the substrate. Of
course, more complete temperature mapping over a whole surface may
be needed.
[0024] In an alternative embodiment as illustrated in FIG. 1, the
invention also provides a method for forming a flexible loading
configuration of a plurality of planar substrates in a furnace. As
shown, the boat fixture 140, which is removably supported onto the
rack fixture 142, is configured to load a plurality of planar
substrates 150 substantially in a vertical orientation along the
gravity direction. In a specific embodiment, the boat fixture 140
is made of quartz or other material being both a good thermal
conductor and an electrical insulator. Throughout this
specification, it is also directly referred to be a quartz boat,
although it does not intend to limit the choice of materials for
the boat fixture 140. In another specific embodiment, the boat
fixture 140 can be loaded in and out through the first end 112. A
rack member 141 can be used to couple the boat fixture 140 to a
loader (not shown) when the cover member 120 is opened and load the
boat fixture from the loader or unload the boat fixture back to the
loader.
[0025] In a specific embodiment, the boat fixture 140 has a base
frame 145 and a top frame 146, although only side view is shown in
FIG. 1. The boat fixture 140 includes several grooved rods 148
removably mounted on the base frame 145 and two grooved bars 149
removably mounted on the top frame 146. Each grooved rod 148
includes a plurality of grooves or slots spatially aligned for
respectively supporting a plurality of planar substrates 150. The
two grooved bars 149 also have the corresponding plurality of
grooves for providing guide to the loaded substrates 150. In an
embodiment, each groove is disposed an equal spacing away from its
neighboring groove and each groove has a size fit for support at
least one substrate. In another embodiment, each grooved rods 148
or each grooved bars 149 can be replaced by another set of grooved
rods or grooved bars which contain grooves in an alternative
configuration with either a different groove spacing or groove
spacing and groove size combination. For example, in a
configuration a groove may have a first spacing from its left
neighboring groove while a second spacing from its right
neighboring groove. Further, each groove may have a bigger groove
size for holding two planar substrates in a back-to-back
configuration. In an alternative embodiment, each of the plurality
of planar substrates 150 in substantially vertical orientation and
parallel to each other as the quartz boat fixture 140 is loaded
into the furnace tube 100, provided that the tube axis 101 is set
along a horizontal direction. Of course, there are many other
variations, modifications, and alternatives.
[0026] Additionally, FIG. 1 also shows one or more baffles 130 and
131 being placed between the first end 120 (with a door) and the
loaded quartz boat fixture 140. The baffles are used for
controlling the convection flow and keeping the heated gases being
substantially circulated around the spatial region occupied by the
loaded substrates instead being lost to relative cold door 130.
Further a crescent shaped baffle 131 is disposed to a lower portion
position of the furnace tube 100 for blocking the colder gases
return to the substrate spatial regions. The crescent shaped baffle
131 is made of quartz and has a curved body with a height and a
curvature substantially similar to that of the inner wall. In an
embodiment, the crescent shaped baffle can have an arc length about
one half of an inner perimeter of the furnace tube or less. In
another embodiment, the crescent shaped baffle can have an arc
length about of an inner perimeter of the furnace tube or less. In
another embodiment, the crescent shaped baffle can have an arc
length ranging from one half to about 2/3 of an inner perimeter of
the furnace tube 100. In an embodiment, the height and the arc
length are parameters adjustable for achieving desired adjustment
to the convection flows or currents passed by.
[0027] As used herein, the "crescent" means a "shape produced when
a circular disk has a segment of another circle removed from its
edge, so that what remains is a shape enclosed by two circular arcs
of different diameters which intersect at two points," although
there can be variations, modifications, and alternatives, according
to one or more embodiments. For example, some descriptions or
definitions can be found in public information website such as
http://en.wikipedia.org/wiki/Crescent. As an example, the term can
include one or more crescent shaped members, although there can
also be partial crescent shaped members.
[0028] As used herein, "top", "bottom", "open", close", have their
plain meanings for illustrating the configuration used in the
exemplary figures in the specification and should not be treated as
a claimed limitation. Similarly, the terms "lower" and/or "upper"
are to be interpreted by ordinary meaning, and do not have any
specific reference regarding to the direction of gravity according
to one or more embodiments. In some embodiments, the terms lower
and/or upper can be reference to gravity, although there can be
some general variations, modifications, and alternatives.
[0029] Referring to FIG. 1, utilizing the loading configuration
according to the embodiments of the present invention a plurality
of planar substrates 150 can be loaded into the furnace tube 100.
In an embodiment, each of the plurality of planar substrates is a
panel of glass. In an example, the substrate can be a 20
cm.times.20 cm square shaped glass panel or a 20 cm.times.50 cm
rectangular shaped glass panel. In another example, the substrate
can be a 65 cm.times.165 cm rectangular shaped glass panel, though
any other form factors are not excluded. Specifically, the
substrate is a soda lime glass which is widely used as a
transparent substrate panel for thin-film solar module. In other
embodiments, the substrate can be made of other transparent
materials including fused silica, quartz, and others. In some
embodiments, the substrates with other shapes including planar
rectangular, square, and disk, and non-planar cylindrical rod,
tube, semi-cylindrical tile, or even flexible foil can be used
depending on applications.
[0030] In an embodiment, on each of the plurality of planar
substrates one or more over layers has formed on a front surface
through some thin-film processes. For example, a precursor layer
including copper species, indium species, and/or indium-gallium
species may be formed on the front surface of each substrate using
a sputtering technique. The substrates are then loaded into the
furnace tube for subjecting the precursor layer to subsequent
thermal treatment. In an embodiment, the precursor layer can be
reactively treated in a gaseous environment within the furnace tube
containing selenide species, or sulfide species, and nitrogen
species, etc. When the furnace tube temperature is ramped up the
substrates are heated and so do the working gases within the
furnace tube. The heated gaseous selenium species, which may flow
around following internal convection flow, react with the
copper-indium-gallium species in the precursor layer overlying the
substrates. The thermal treatment process may include several steps
for temperature ramping up, dwelling, and ramping down. As a result
of the reactive process, the precursor layer is transformed to a
photovoltaic film stack containing copper indium (gallium)
diselenide (CIGS) compound, which can be served as an absorber
layer of a thin-film photovoltaic cell. More detail descriptions
about the thermal treatment process for forming the CIGS
photovoltaic film stack of thin film solar cells can be found in
U.S. Patent Application No. 61/178,459 titled "Method and System
for Selenization in Fabricating CIGS/CIS Solar Cells" filed on May
14, 2009 by Robert Wieting, commonly assigned to Stion Corporation
of San Jose and hereby incorporated by reference.
[0031] In another embodiment, each of the plurality of planar
substrates is disposed into the quartz boat fixture 140 in a
loading configurations. The loading configurations can be
customized using a few grooved rods or bars mounted on a frame
structure of the quartz boat fixture. FIG. 2 is a perspective view
of the quartz boat fixture for loading a plurality of planar
substrates according to an embodiment of the present invention. As
shown, the boat fixture 140 is a frame fixture having a
substantially rectangular prism shape, including a base frame 145
and a top frame 146, the base frame 145 and the top frame 146 being
fixedly coupled by several side connection bars. The base frame 145
has a first width portion 1451, a second width portion 1452, and
one or more middle members 1453, 1454 connected between two length
portions 1455 and 1456. Each grooved rod 148 is installed on each
of the first width portion 1451, the second width portion 1452, and
one or more middle members 1453, 1454 of the base frame 145. As
shown, each grooved rod 148 includes a plurality of grooves,
presumably facing upward, disposed in a certain configuration along
the length of rod. In particular, each groove is configured to
support one or more pieces of glass substrates. The four grooved
rods 148 are aligned so that one groove from each grooved rod is
just in position to insert at least one planar substrate. In the
implementation, each loaded planar substrate sits in the four
grooves of the respective grooved rods substantially in a vertical
orientation in parallel to the gravity direction and all loaded
planar substrates are substantially in parallel to each other. In
addition, two grooved bars 149 are installed respectively at each
width member of the top frame 146. Each grooved bar 149 also has a
plurality of grooves with substantially the same configuration as
those on the grooved rods 148 and is aligned with corresponding
groove of the grooved rods 148. The grooved bars have their facing
mainly inward to provide guides to the loaded planar substrates.
For example, a quartz boat fixture designed to load a plurality of
planar substrates substantially in a horizontal orientation
perpendicular to the gravity direction can be an alternative
option, using grooved rods or grooved bars mounted on the side
connection bars as substrate support and guide.
[0032] FIG. 3 is a simplified side view of the furnace tube loaded
with a quartz boat fixture holding a plurality of planar substrates
according to an embodiment of the present invention. As shown, a
plurality of planar substrates 211, 212, etc. is inserted
respectively in a plurality of grooves 251, 252, etc. in a specific
configuration within a quartz boat fixture 240 according to an
embodiment of the present invention. In particular, the plurality
of grooves is disposed with an equal spacing for each neighboring
groove. Although each groove has a size for supporting at least one
substrate, only every other groove is used for one substrate to be
loaded in a vertical orientation while leaving one groove in
between empty. For example, groove 251 supports a substrate 211 and
the groove 252 is empty, while next groove supports a next
substrate 212 and so on. In certain embodiments, number of empty
grooves may be greater than one. As the plurality of planar
substrates 211, 212 etc are inserted into the quartz boat fixture
240 in the loading configuration described above, a spacing 215 is
provided for every neighboring substrates. The quartz boat fixture
240 is then loaded into the tube shaped furnace 200.
[0033] In an embodiment, the present invention provides a method
for loading planar substrates in a predetermined configuration.
Firstly, the method provides a plurality of equally spaced grooves.
Secondly, the method provides inserting the planar substrates into
the grooves with an increased spacing 215 between two neighboring
planar substrates beyond that of two neighboring grooves by
intentionally skipping one or more grooves. When the plurality of
planar substrates in this loading configuration is subjected to a
reactive thermal treatment within a gaseous environment inside the
tube shaped furnace 200, the advantage of having a relative wide
spacing 215 between two neighboring substrates is to have
sufficient spatial room for vertical convection flow. In a specific
embodiment with the substrates in vertical orientation, the hotter
work gas within the furnace tube 200 is carried by the convection
flow upwards while colder gas flows down along the substrate
surfaces. Wider spacing between the planar substrates can
facilitate the interflowing which helps to improve temperature
uniformity of each loaded substrate, especially during quick
temperature ramping up or down processes. For example, when the
substrates are heated from relative low temperatures which are
increased in a ramping-up process via the plurality of heaters (not
shown) associated with the furnace tube 200, convection in between
the substrates plays more important role in heat transfer than
radiation. At higher temperatures radiation will dominate
convection and major heat transfer within the substrate is not
highly dependent on substrate spacing. In another example, the
larger substrate spacing naturally reduces the total number of
substrates loadable with a fixed dimension of the furnace tube 200.
Therefore, substrate spacing may need to be optimized and
pre-determined. Once a desired spacing between substrates is
determined the corresponding grooves pattern can be built into the
grooved bar or grooved rod just for installing onto the boat
fixture for loading substrates in a desired configuration.
[0034] In alternative embodiments, other parameters affecting the
effectiveness of heat transfer or subsequently the substrate
temperature uniformity include time (or heater/cooler ramping rate)
and thermal mass or total number of the loaded substrates. These
parameters are also interrelated. For example, when total number of
substrates is increased for seeking higher production yield, it
actually demands a wider substrate spacing 215 for effective
convection flow which ends up reducing the total number of
substrates loadable. For the substrates with dimensions as large as
65.times.165 cm, high temperature difference (.DELTA.T) can lead to
warping and breaking of the substrate. A uniform temperature is
desired for uniformity of thin-film process, precursor reactions,
and of course, better performance of the devices. For example, a
.DELTA.T less than 15 degrees Celsius may be desired during dwell
stage of the thermal treatment process while the .DELTA.T may
become as large as 100 degree Celsius during ramping up/down stage
if the ramping rate is high. Therefore, one or more embodiments of
the present invention include using a simulation model to determine
a trend of changing the loading configuration and how it affect the
temperature uniformity. In an embodiment, the model is scaled down
from a real system by using a smaller substrate and furnace size in
order to reduce the computation time. The substrates in the model
are 20 cm.times.50 cm instead of the full size of 65 cm.times.165
cm. The substrate thickness and spacing are also correspondingly
scaled, and totally only 16 substrates are fit in the model with
their substrate spacing set at a range from 0.5 inches to several
inches. The time scale is substantially reduced so that the ramp
rates can be faster than the real system. The behavior trends of
this model closely track those of the full size system, although
absolute values of temperatures across the substrates may be off
the true values from a real system. For example, the scaled system
is simulated for the tube shaped furnace 200 loaded with a quartz
boat 240 including the grooved rods 248 and grooved bars 249 having
a specific groove configuration with a selected spacing. This
simulation model can be used as a guide for the real system
implementation and finally help to determine an optimized loading
configuration for a specific number of substrates. In a specific
implementation, a "Computational Fluid Dynamics (CFD)" software is
used for analyzing the convection fluid flow and computing heat
transfer response of a simulation model system based on the scaled
structure parameters. In particular, a commercially available
program named CFdesign.TM. by Blue Ridge Numerics, Inc is applied
by adjusting input parameters for system, materials, boundary, and
initial conditions. Of course, other general purpose computational
fluid dynamics simulation software may be used.
[0035] FIG. 4 is a simplified side view of the furnace tube loaded
with a quartz boat fixture holding a plurality of planar substrates
according to an alternative embodiment of the present invention.
This diagram is merely an example and should not unduly limit the
claims herein. One skilled in the art would recognize other
variations, modifications, and alternatives. As shown, a plurality
of planar substrates 311, 312, etc are inserted respectively in a
plurality of grooves 351, 352 etc in a specific configuration
within a quartz boat fixture 340 according to an embodiment of the
present invention. In an implementation, the quartz boat fixture
340 loaded in the furnace tube 300 is substantially the same as the
quartz boat fixture 240 loaded in furnace tube 200 except that the
grooved rods 348 for supporting the substrates and grooved bars 349
for guiding the substrates are replaced from grooved rods 248 and
grooved bars 249. Embodiment illustrated in FIG. 4 also provides a
method for loading a plurality of planar substrates in an
alternative configuration. In particular, this loading
configuration can provide improved production yield by increasing
total number of the substrates while still retaining sufficient
substrate spacing for facilitating internal convection without any
structural changes to the dimension of the furnace tube 300 and
rack fixture 345, 346 of the quartz boat 340.
[0036] In a specific embodiment, the plurality of grooves is
arranged in a periodic group in this alternative loading
configuration. Each group includes at least a first groove and a
second groove being disposed at a predetermined spacing away from
the first groove. The spacing in between can be smaller, equal to,
or greater than a size of a single groove. Each group is disposed
to be physically next to its neighboring group so that the first
groove of the group is next to a second groove of its neighboring
group on one side and the second groove of the group is next to a
first groove of its neighboring group on another side. Each groove
is configured to let one substrate being inserted. For example, a
substrate 311 is inserted in the first groove 351 of the first
group and substrate 312 is inserted in the second groove 352 of the
group with a spacing 315 away from the substrate 311. The spacing
315 is substantially determined by the spacing between the first
groove 351 and second groove 352. In addition, the first groove 353
of a very next group is disposed physically next to the second
groove of the first group. Arranged in periodic fashion, the second
groove 354 of the very next group is also disposed at the spacing
away from the first groove 353. Correspondingly, each groove of the
very next group can be inserted in a substrate 313 (in groove 353),
or a substrate 314 (in groove 354).
[0037] In another specific embodiment, the loading configuration
described in FIG. 4 includes a specific front/back phase
configuration for each planar substrate inserted in corresponding
the first and the second groove of each of the periodic groups of
grooves. In an implementation, each substrate is a planar glass
substrate that has been worked through one or more thin-film
processes. The planar glass substrate has a front surface overlaid
by a thin-film composite material while its corresponding back
surface can be a bare glass. In an example, the thin-film composite
material is a photovoltaic precursor film containing at least
cooper species, indium species, and/or gallium species, or silver
species. Therefore, an embodiment of the present invention includes
inserting two such planar substrates respectively into each group
with their photovoltaic precursor film on each substrate directly
facing each other. For example, surface 1A of substrate 311
contains a photovoltaic precursor film which faces a photovoltaic
precursor film formed on surface 2A of substrate 312. The back
surface of substrate 312 faces a back surface of another substrate
313 inserted in a first groove 353 of a neighboring group. Because
chemical reaction between the precursor film and ambient work gas
inside the furnace 300 will occur only on the front surface of each
substrate, only the convection flow between those front surfaces of
two planar substrates matters while any convection flow between the
back sides is less important. Embodiment of the present invention
associated with this loading configuration includes further
determining a minimum front surface spacing between two substrates
inserted in each group. This also can be accomplished using a
simulation model based on a system and substrates with scaled-down
dimensions to determine a temperature trend of change following the
spacing change. The simulation model is executed using a class of
software program named CFdesign.TM. based on the scaled system
structure and material parameters. Via the simulation, a range of
desired spacing values can be determined so that the corresponding
groove patterns can be built into each of the grooved rods 348 and
grooved bars 349. Then these grooved rods and bars can be mounted
to customize a standard quartz boat fixture 340 for loading a
plurality of substrates in this new loading configuration. One
advantage of this loading configuration is the total number of
substrates loadable for a fixed quartz boat size is increased
comparing to inserting substrates with a simple equal spacing but
without front/back phase configuration. Of course, there can be
many variations, alternatives, and modifications.
[0038] FIG. 5 is a side view of the furnace tube loaded with a
quartz boat fixture holding a plurality of planar substrates
according to another alternative embodiment of the present
invention. As shown, a plurality of planar substrates 411, 412, etc
are inserted in a plurality of grooves 451 in a pairwise
back-to-back configuration within a quartz boat fixture 440
according to an embodiment of the present invention. In an
implementation, the quartz boat fixture 440 loaded in the furnace
tube 400 is substantially the same as the quartz boat fixture 240
loaded in furnace tube 200 except that the grooved rods 448 for
supporting the substrates and grooved bars 449 for guiding the
substrates are replaced from grooved rods 248 and grooved bars 249.
Embodiment illustrated in FIG. 5 also provides a method for loading
a plurality of planar substrates in an alternative configuration.
In particular, this loading configuration can provide further
improved production yield by increasing total number of the
substrates while still retaining sufficient substrate spacing for
facilitating internal convection without any structural changes to
the dimension of the furnace tube 400 and frames 445, 446 of the
quartz boat fixture 440.
[0039] In a specific embodiment, each of the plurality of grooves
451 is configured to be able to fit in a pair of planar substrates
in a back-to-back loading configuration. In a specific embodiment,
the spacing is a single constant for all neighboring grooves,
although it can be varied along the grooved rod or bar from its end
to its middle. Again, in this configuration the substrate loading
is performed with its front/back phase configured. Depending on
embodiments, each substrate 411 can be a glass panel for
manufacturing photovoltaic cells. In one implementation, the glass
substrate has a front surface being covered by several films of
materials including a precursor layer formed on top while leaving
its back surface a bare glass. A plurality of these substrates is
loaded in the furnace tube fixture 400 for subjecting the precursor
layers to one or more reactive thermal treatment processes and
forming a photovoltaic absorber. Therefore, only front surface of
each planar substrate with the precursor layer on top needs to be
exposed to ambient reactive gas within the furnace tube and the
back side of the same substrate can be physically touched with
another back side of another substrate, and both these two
substrates can be inserted into a single groove (with a enlarged
size). For example, substrate 411 and substrate 412 are inserted
together in a back-to-back configuration into a first groove. A
second groove is disposed a distance away from the first groove.
The front side 1A of substrate 412 would be facing front side 2A of
another one of two substrates that are inserted back-to-back
together in the second groove. The groove-to-groove spacing is
configured to be sufficiently large so that the substrate spacing
415 (i.e., 1A-2A spacing) is equal to or greater than a
predetermined value. Because of the thermal process involves
chemical reaction between solid phase film material (the precursor
layer) on the substrates and gaseous phase material filled in the
furnace tube, larger spacing between those vertically disposed
substrates can facilitate convection flow in between during the
reactive thermal treatment processes. This specific configuration
minimizes the back-back spacing (substantially equal to zero) so
that more spatial room is provided for increasing front-front
spacing 415 between two substrates disposed in neighboring grooves
and for loading more substrates in such a configuration.
[0040] In another specific embodiment, the loading configuration
described in FIG. 5 includes a specific front/back phase
configuration for each substrate and an additional specific size
configuration for each groove to fit with just two substrates.
Furthermore, embodiment of the present invention associated with
this loading configuration includes further determining a minimum
front-front spacing between two substrates respectively inserted in
two neighboring grooves. This still can be accomplished using a
simulation model based on a system and substrates with scaled-down
dimensions to determine a temperature uniformity trend of change
following the change of the front-front spacing. The simulation
model is executed using a computational fluid dynamics software,
for example, a program named CFdesign.TM. provided by Blue Ridge
Numerics, Inc, based on the scaled system parameters. Through the
simulation, a range of desired spacing values can be determined so
that the corresponding groove patterns can be built into each of
the grooved rods 448 and grooved bars 449. Then these grooved rods
448 and bars 449 can be mounted to customize a standard quartz boat
440 for loading a plurality of planar substrates 411 and 412 in the
corresponding configuration. One advantage of minimizing the
front-front spacing is to maximize total number of substrates
loadable for a fixed quartz boat size. An example is to set the
substrate spacing as one single value for all the inserted
substrates, although the spacing can be varied from one side of the
quartz boat to another side. Of course, there can be many
variations, alternatives, and modifications.
[0041] FIG. 6 is an exemplary cross-section view of simulated
convection flows according to an embodiment of the present
invention. In a specific embodiment, the present invention provides
a method of using a simulation model for determining at least a
trend of change in temperature distribution associated with
substrate loading configuration inside a tube shaped furnace. As
shown, the top portion (A) of FIG. 6 is a cross-section view along
an axis of a modeled tube shaped furnace 600. The modeled furnace
600 is substantially similar to real-life furnace tube 100, but is
reduced in dimensions for obtaining results with a manageable
computation time. In an implementation, a class of computational
fluid dynamics software is used in these simulations. For example,
program named CFdesign.TM. provided by Blue Ridge Numerics, Inc is
applied with a model based on scaled down system dimensions and
material parameters and initial conditions substantially close to a
real system developed according to embodiments of the present
invention. Additionally shown in FIG. 6A, one of plurality of
rectangular shaped planar substrates 610, also in reduced
dimensions, is loaded in a central position within the model
furnace 600. During one or more reactive thermal treatment
processes, heated gaseous species tend to move upward and colder
gaseous species flow downward to form convection flows within the
volume of the model furnace 600. Based on the results from
simulation, FIG. 6A shows at least part of the convection flow 650
along the axial cross section plane moving in a spatial region
between the furnace inner wall and the edges of the substrate
610.
[0042] FIG. 6B provides another cross-section view of the same
system setup used in the simulation model. As shown, in a
configuration according to an embodiment of the present invention
16 substrates 610 are loaded one by one separately with an equal
spacing 615 in vertical orientation. Although it is not explicitly
included in the simplified simulation model, these substrates can
be supported by a sample holder. For example, a quartz boat 140
described in FIG. 1 of the specification including several grooved
rods mounted on its bottom rack fixture and aligned to let at least
one substrate inserted in each groove. FIG. 6B shows at least part
of the convection flow 650 moving along a cross section plane
perpendicular to the tube axis. Simulation results based on the
CFdesign.TM. program indicates that the heated gaseous species flow
upward along mainly the spacing between the furnace inner wall and
the edges of the substrate 610 then flow downward along the spacing
between the substrates 610.
[0043] The simulation results also include effects of geometric
form factors of the system on the temperature distribution for each
substrate 610. Although the absolute values may be off the real
system, trends of changes associated with at least some system
parameters including the furnace dimension/shape, substrate
dimension, total number of loaded substrates (thermal mass),
spacing between neighboring substrates, etc. can be determined. In
a specific embodiment, a trend of change associated with the
substrate spacing for a particular loading configuration shown in
FIG. 6 can be obtained. For example in FIG. 6B a loading
configuration associated with the tube shaped furnace 600 has total
number of 16 substrates 610 each being disposed alone and separated
from its neighbor by a spacing 615. A trend of change based on the
simulation indicates that reducing the spacing 615 for loading
substrates 610 more than 16 may limit the up-down convection flow
resulting in poor temperature uniformity. In addition, the trend of
change based on the simulation indicates that increasing the
spacing 615 may further improve the substrate temperature
uniformity while reducing the total number of loaded substrates
610, which affects production yield. To certain degrees, further
increasing substrate spacing may not substantially improve
temperature uniformity (while other system parameters may become
major factors). Therefore, an optimal loading configuration for a
specific system setup can be practically determined and recommended
for real system. Of course, there are many other alternatives,
variations, and modifications.
[0044] FIG. 7 is an exemplary cross-section view of convection
flows according to another embodiment of the present invention. As
shown, in another embodiment of the present invention, a simulated
convection flow pattern 750 is plotted for a modeled system with a
plurality of planar substrates 710 being disposed in a back-to-back
paired configuration within a tube shaped furnace 700. FIG. 7A
shows at least a portion of convection flow 750 along an axial
plane and between inner wall of the furnace 700 and edges of the
substrates 710. FIG. 7B shows at least another portion of
convection flow 750 along a cross-section plane perpendicular to
the axis of the furnace tube 700. The furnace 700 is substantially
the same as model furnace 600, based on real tube shaped furnace
100. FIG. 7B shows that by pairing the substrates the substrate
spacing 715 between each pair is doubled while total number of
substrates 710 can be kept the same.
[0045] This loading configuration takes advantage of the substrate
samples with only single-side surface subjected to the reactive
thermal treatment. According to embodiments of the present
invention, the substrates 710 are glass substrates with front
surface pre-coated by several thin-film materials including bottom
electrode film and an exposed precursor film overlying the
electrode film and the opposite back surface may be simply a bare
glass. During reactive thermal treatment processes, only the
precursor film needs to be exposed to work gases filled in the
volume of the furnace 700. Therefore, placing the back surface of a
planar substrate against another back surface of another planar
substrate certainly will not affect the expected treatment of the
precursor film on the front surface while allowing more room for
adjusting the spacing between the front surfaces containing
precursor films. In another embodiment, this loading configuration
allows increasing the total number of loaded substrates without
reducing the spacing between the precursor films on the front
surfaces from a neighboring pair of substrates. As a result, the
loading configuration helps to enhance system production yield
because of more substrates can be loaded for treatment in one
setting but still allows sufficient room for gaseous species to
flow between the subjected precursor film surfaces and induce an
effective thermal convection good for substrate temperature
uniformity. Again this is modeled using a system with reduced
dimension or total number of loaded substrates. Although the
absolute values regarding the loading configuration and resulted
temperature difference across each substrates may not match real
system, a trend of change can be determined and provide guidance
for optimizing processing parameters of actual system depending on
one or more embodiments. Of course, there are many variations,
alternatives, and modifications.
[0046] While the present invention has been described using
specific embodiments, it should be understood that various changes,
modifications, and variations to the method utilized in the present
invention may be effected without departing from the spirit and
scope of the present invention as defined in the appended claims.
For example, the tubular shaped furnace is illustrated as an
example. In addition to use an optimized loading configuration for
the plurality of substrates, a carefully optimized heating/cooling
supply system and heater/cooler configuration can also
significantly improve temperature uniformity across the substrates
as large as about 2 feet by 5 feet or greater for thin-film
photovoltaic devices. Additionally, although the above embodiments
described have been applied to reactive thermal treatment for
forming CIS and/or CIGS photovoltaic film stack on the substrate,
other thin-film based reactive thermal treatment processes
certainly can also be benefited from the embodiments, without
departing from the invention described by the claims herein.
Depending on the embodiment, the present method can also be applied
to silicon based photovoltaic devices.
* * * * *
References