U.S. patent application number 12/789357 was filed with the patent office on 2010-12-02 for plasma spraying and recrystallization of thick film layer.
This patent application is currently assigned to Integrated Photovoltic, Inc.. Invention is credited to Tatyana Dulkin, Lawrence Hendler, Raanan Zehavi, Sharone Zehavi.
Application Number | 20100304035 12/789357 |
Document ID | / |
Family ID | 43220539 |
Filed Date | 2010-12-02 |
United States Patent
Application |
20100304035 |
Kind Code |
A1 |
Zehavi; Raanan ; et
al. |
December 2, 2010 |
Plasma Spraying and Recrystallization of Thick Film Layer
Abstract
A linear process tool comprising at least two deposition modules
each comprising one or more plasma spray guns operable to move in a
direction approximately orthogonal to the direction of a substrate
carrier is configured to deposit at least a first and second layer,
in direct contact with each other, wherein a first layer is of
first composition and the second layer is of second composition
different than the first composition.
Inventors: |
Zehavi; Raanan; (Sunnyvale,
CA) ; Zehavi; Sharone; (Sunnyvale, CA) ;
Hendler; Lawrence; (Sunnyvale, CA) ; Dulkin;
Tatyana; (Sunnyvale, CA) |
Correspondence
Address: |
FERNANDEZ & ASSOCIATES, LLP
P.O. BOX D
MENLO PARK
CA
94026
US
|
Assignee: |
Integrated Photovoltic,
Inc.
Sunnyvale
CA
|
Family ID: |
43220539 |
Appl. No.: |
12/789357 |
Filed: |
May 27, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61181496 |
May 27, 2009 |
|
|
|
61296799 |
Jan 20, 2010 |
|
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Current U.S.
Class: |
427/450 ;
118/723R; 427/446; 427/452; 427/454; 427/455 |
Current CPC
Class: |
C23C 4/02 20130101; C23C
4/134 20160101 |
Class at
Publication: |
427/450 ;
118/723.R; 427/446; 427/452; 427/454; 427/455 |
International
Class: |
C23C 4/10 20060101
C23C004/10; C23C 4/12 20060101 C23C004/12; C23C 4/06 20060101
C23C004/06; C23C 4/08 20060101 C23C004/08; C23C 4/02 20060101
C23C004/02 |
Claims
1. A linear process tool comprising; first and second deposition
modules each comprising one or more plasma spray guns operable to
move in a direction approximately orthogonal to the direction of
motion of a substrate carrier; wherein the first deposition module
is configured to deposit a first layer on a substrate on the
substrate carrier and the second deposition module is configured to
deposit a second layer in direct contact with the first layer on
the substrate on the substrate carrier such that the first layer is
of first composition and the second layer is of second composition
different than the first composition; wherein the plasma spray guns
comprise components exposed to the plasma stream comprising at
least one constituent of the primary source material in the plasma
stream; wherein the primary source of materials for the plasma
spray guns for the deposited layers is a first and second powder of
first and second compositions produced by jet mills wherein the
substrate carrier conveys the substrate from an entrance of the
linear process tool to an exit at a predetermined speed.
2. The linear process tool of claim 1 further comprising a zone
melt recrystallization module comprising a source of radiation; and
optical components operable to convert the source of radiation into
a linear line of radiation ranging from about 0.5 mm to 3 mm wide
extending across the width of the substrate placed on the substrate
carrier such that a portion of the deposited material layer on the
substrate is irradiated in the linear line of radiation and heated
at least to the melting point of the deposited material for a
predetermined time; such that as the portion of the substrate with
a deposited material layer passes out from underneath the linear
line of radiation the deposited material layer portion now outside
the radiated zone cools below its melting point and recrystallizes
into a preferred orientation based on its composition.
3. The linear process tool of claim 1 wherein the recrystallized
deposited material layer exhibits a minority carrier diffusion
length greater than 40 microns and a grain size larger than the
deposited material layer thickness.
4. The linear process tool of claim 1 further comprising means for
ultrasonic vibration applied to the portion of the deposited
material layer on the substrate being irradiated in the linear line
of radiation and heated at least to the melting point.
5. The linear process tool of claim 1 wherein the substrate is
flexible.
6. The linear process tool of claim 5 wherein the flexible
substrate is configured such that it enters the linear process tool
from a roll and exits the linear process tool on to a roll.
7. The linear process tool of claim 1 wherein the first and second
compositions are chosen substantially from a group consisting of
silicon, silicon-germanium alloys, Group IV elements and/or alloys,
Group IV oxides, nitrides, carbides and mixtures thereof, metal
oxides, nitrides, carbides and mixtures thereof.
8. A method for making a structure comprising first layer of first
composition and second layer of second composition in contact
wherein the first composition is different from the second
composition comprising the steps: depositing the first layer on a
transported substrate with a first plasma spray gun operable to
move in a direction approximately orthogonal to the transported
direction; and depositing the second layer on the transported
substrate with a second plasma spray gun operable to move in a
direction approximately orthogonal to the transported direction;
wherein the first plasma spray gun comprises components exposed to
the plasma stream comprising at least one constituent of a first
primary source material in the plasma stream; wherein the first
primary source material for the first plasma spray gun for the
first layer is a first powder of the first composition produced by
jet mills; wherein the second plasma spray gun comprises components
exposed to the plasma stream comprising at least one constituent of
a second primary source material in the plasma stream; wherein the
second primary source material for the second plasma spray gun for
the second layer is a second powder of the second composition
produced by jet mills.
9. The method for making a structure of claim 8 further comprising
the step of: heating a portion of the transported substrate to
initiate zone melt recrystallization in a linear line ranging from
about 0.5 mm to 3 mm wide extending across the width of the
transported substrate to the melting point of the deposited
material for a predetermined time; and cooling and recrystallizing
the previously heated portion of the transported substrate into a
preferred orientation based on its composition.
10. The method for making a structure of claim 9 wherein the
recrystallized portion of the transported substrate exhibits a
minority carrier diffusion length greater than 40 microns and a
grain size larger than the deposited material layer thickness.
11. The method for making a structure of claim 9 further comprising
the step: applying ultrasonic vibration to the portion of the
transported substrate being heated to initiate zone melt
recrystallization.
12. The method for making a structure of claim 9 wherein the
transported substrate is flexible.
13. The method for making a structure of claim 8 wherein the first
and second compositions are chosen substantially from a group
consisting of silicon, silicon-germanium alloys, Group IV elements
and/or alloys, Group IV oxides, nitrides, carbides and mixtures
thereof, metal oxides, nitrides, carbides and mixtures thereof.
Description
PRIORITY
[0001] This application claims benefit of provisional applications
61/181,496, filed May 27, 2009 and 61/296,799, filed Jan. 20,
2010.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] This application is related to U.S. application Ser. Nos.
11/881,501, 11/782,201, 12/074,651, 12/720,153, 12/749,160,
61/181,496, 61/305,796, 61/235,610, 61/239,739, 61/263,282,
61/296,799, 61/300,804; all owned by the same assignee and all
incorporated by reference herein in their entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The invention relates generally to plasma spraying of
silicon and other materials. In particular, the invention relates
to the combination of plasma spraying and recrystallization of
deposited material to form devices with advantageous
structures.
[0005] 2. Description of Related Art Including Information
Disclosed Under 37 CFR 1.97 and 1.98
[0006] Plasma spraying has been suggested over many years for
forming silicon semiconducting devices including silicon solar
cells. Such efforts have not found ready commercialization, because
of the low quality of the sprayed silicon. Interest in plasma
sprayed semiconducting silicon has been rekindled recently in the
hope of providing a low cost manufacturing method for silicon solar
cells. Zehavi et al. have disclosed a distinctive plasma spray gun
for plasma spraying high-quality silicon in U.S.2008/0220558.
[0007] Sprayed silicon to date has suffered from the nature of
plasma spraying in which the silicon arrives at a substrate 10,
illustrated in the cross-sectional view of FIG. 1, as liquid drops
12 intermittently striking the surface and immediately cooling to
form separate lamellae 14 in the form of overlapping plates or
platelets, as is well known. Voids 16 may form between the
lamellae. If the spraying is done at some ambient pressure, voids
16, containing oxygen or an ambient gas, are trapped within the
silicon layer being formed. The oxygen degrades the quality of the
silicon layer and its ability to function as a semiconductor. Even
if the silicon is sprayed in an argon ambient, argon bubbles exist
and provide scattering centers in the silicon, degrading at least
its carrier mobility. For solar cells, the carrier diffusion length
should be longer than the absorption length of the incident
radiation.
[0008] Voids 16 contribute to the pervasive porosity of plasma
spray material. Processing conditions can be adjusted to reduce the
porosity, but it is believed that the voids 16 are inherently
formed in plasma spraying. It thus appears necessary to
recrystallize plasma sprayed silicon, that is, to heat the sprayed
layer to a sufficiently high temperature to fuse the silicon
lamellae into larger, void-free volume of crystalline silicon even
if only polycrystalline silicon is needed. However,
recrystallization has not been completely effective and it needs to
be adapted to the production environment required for large,
low-cost solar cells. The instant invention discloses an enhanced
recrystallization method to be combined with low-pressure plasma
spraying so that voids 16 are eliminated or at least minimized and
preferably are eliminated from the deposited material by the
recrystallization step.
[0009] Equipment is available to plasma spray within a vacuum
chamber. However, this equipment is not readily adaptable for
plasma spraying silicon solar cells. Typically, the plasma gun is
mounted on a multi-jointed robot arm. Such a robot requires a large
vacuum chamber; the arm is ill suited for coating large panels with
a uniformly thick silicon layer needed for commercial solar cells;
conventional vacuum chambers are not suitable for large scale
production.
[0010] de Souza, et al. teach a technique for Grain Reorientation
Annealing in U.S.2010/0112792; in a preferred embodiment a silicon
layer is heated to less than 1350.degree. C. in order to
recrystallize, or more precisely, reorient, in the same orientation
as a substrate on which the layer has been deposited. In
U.S.2010/0075060, Narwankar discloses a process tool with
micro-plasma spray guns for carbon nanotube growth on silicon
substrates. In U.S.2008/0057212 Dorier, et al., disclose a plasma
spraying device for spraying materials onto a substrate. In U.S.
2003/0113481 Huang discloses a method for depositing a coating onto
a solid substrate employing a plasma source for fabricating a solar
cell. In U.S. Pat. No. 4,379,020 Glaeser et al., disclose
recrystallizing amorphous films into large polycrystalline films.
None of the prior art teaches or suggests the limitations and
advantages of the disclosed invention.
BRIEF SUMMARY OF THE INVENTION
[0011] In one embodiment of the instant invention a linear process
tool comprising at least two deposition modules each comprising one
or more plasma spray guns operable to move in a direction
approximately orthogonal to the direction of a substrate carrier is
configured to deposit at least a first and second layer, in direct
contact with each other, wherein a first layer is of first
composition and the second layer is of second composition different
than the first composition. The plasma spray guns are of a type
described in U.S. Ser. No. 12/074,651 wherein the plasma spray gun
comprises components exposed to the plasma stream comprising at
least one constituent of the primary source of material exposed to
the plasma stream; the primary source of material for the spray gun
for the deposited layer is a powder produced by a jet mill as
described in U.S. Ser. No. 11/782,201. The substrate carrier
conveys a substrate from an entrance to the linear process tool to
an exit at a predetermined speed wherein the speed varies between
about zero and 20 cm/min.
[0012] In one embodiment of the instant invention a linear process
tool comprising at least two deposition modules also comprises at
least one zone melt recrystallization, ZMR, module wherein a source
of radiation is optically engineered into a narrow, linear line of
radiation extending across the width of a substrate placed on the
substrate carrier such that deposited material on the substrate is
irradiated in the linear line of radiation and heated at least to
the melting point of the deposited material; as a portion of a
substrate with a deposited material layer is moved underneath the
narrow, linear line of radiation by the substrate carrier the
deposited material layer portion exposed to the radiation meets or
exceeds its melting point; as the portion of the substrate with a
deposited material layer passes out from underneath the narrow,
linear line of radiation by the substrate carrier the deposited
material layer portion now outside the radiated zone cools below
its melting point and recrystallizes into a preferred orientation
based on its composition. In some embodiments the recrystallized,
deposited material layer exhibits a minority carrier diffusion
length greater than 40 microns and a grain size larger than the
layer thickness; in some embodiments a minority carrier diffusion
length is greater than 20 microns.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0013] FIG. 1 shows schematically as deposited lamellae resulting
from a plasma spray.
[0014] FIG. 2 shows schematically a linear process tool.
[0015] FIG. 3 shows schematically a 3D version of a linear process
tool.
[0016] FIG. 4 shows schematically an optically engineered radiation
source.
[0017] FIG. 5 shows schematically an ultrasonic generator assisting
in ZMR.
[0018] FIG. 6 shows reflection and absorption versus wavelength for
silicon.
[0019] FIG. 7 shows schematically an optical system for creating a
linear line of radiation.
[0020] FIG. 8 shows schematically an optical system for "beam
homogenization".
DETAILED DESCRIPTION OF THE INVENTION
[0021] According to one aspect of the invention, as schematically
illustrated in FIG. 2, plasma spraying of silicon is performed in a
vacuum deposition chamber 20 inside of which one or more plasma
spray guns 22 having spray nozzles located within the chamber 20
spray silicon onto substrates 24 transported under the guns 22 by
conveyor 26 or a series of moving stages. In this embodiment, the
source of silicon is silicon powder injected into the plasma flow
of argon and a small quantity of hydrogen to increase the thermal
conductivity and enthalpy of the plasma. Improving the thermal
conductivity and enthalpy improves the transfer of heat to the
silicon. Other gases, such as helium can also be used for the same
purpose. Unprocessed substrates 24 are loaded into a deposition
chamber 20 from an entry load lock 28 through an isolation valve
30, for example a slit valve. Similarly, processed substrates are
removed from the deposition chamber 20 to an exit load lock 32
through another isolation valve 34.
[0022] Plasma spraying commences when the deposition chamber has
been pumped down to a desired pressure, for example, less than 500
milli-Torr, preferably with an oxygen fraction less than a critical
value such as 10 ppm. One or more unprocessed substrates 24 are
loaded by means well known in integrated circuit processing into
the entry load lock 28 through a vacuum loading door, not shown,
while entry isolation valve 30 to the deposition chamber is
closed.
[0023] After the unprocessed substrates 24 have been loaded, the
loading door is closed and the entry load lock 28 is vacuum pumped
to a pressure substantially equal to that of the deposition chamber
20. A cassette can be used to load and store multiple substrates in
the load lock 28 so as to reduce loading time and minimize pump
down time. The unprocessed substrates 24 are sequentially loaded
through the opened entry isolation valve 30 into the deposition
chamber 20 for plasma spraying of silicon onto the substrates 24.
It is appreciated that plasma spraying entails the use of a carrier
gas, which is typically argon; optionally other gases may be used,
including helium, hydrogen, etc. However, the deposition chamber 20
is continuously pumped to keep the argon pressure low. On the
completion of processing, the processed substrate 24 is transferred
through the opened exit isolation valve 34 into the exit load lock
32, which previously has been pumped down to deposition pressures.
Again, a cassette may be in the exit load lock 32 to store multiple
processed substrates 24. When the cassette is full or it is desired
to remove the processed substrates 24, the exit isolation valve 34
is closed, a vacuum door on the exit load lock 32 is opened, and
the cassette or individual substrates are removed from the exit
load lock 32 into the ambient.
[0024] The illustrated system is an in-line processing system of
the type well known in integrated circuit and display panel
processing. It is understood that the deposition chamber need not
include a separate exit load lock but use a single load lock for
loading and unloading. It is also understood that the in-line
system could include additional processing chambers, perhaps
isolated from the plasma spraying chamber. A more detailed
embodiment of the invention illustrated in the orthographic view of
FIG. 3 includes an in-line configuration of multiple chambers
mounted on a table 40 for processing substrates 42, for example, of
graphite. Substrates may be a variety of shapes and materials
depending upon the application; exemplary substrates are silicon,
metallurgical grade silicon, graphite, carbon, silicon carbide
coated graphite, flexible graphite and others known to one
knowledgeable in the art. In some embodiments a flexible substrate
comprises carbon and is between about 0.5 mm to about 5 mm thick;
optionally a flexible substrate may have a silicon carbide outer
coating; optionally a flexible substrate may be configured into a
long ribbon more than 100 mm wide and more than 1,000 mm long;
optionally a flexible substrate may be more than 10 meters long;
optionally, long enough for roll-to-roll processing.
[0025] Multiple substrates 42 are loaded into a cassette 43. Two
separately operated entry load locks 44, 46 can accommodate the
cassette 43. The loading doors of both load locks are shown open
although in typical operation they are opened only one at a time.
The cassette 43 can be loaded outside of the load lock 44, 46 and
then placed in the opened one of the load locks 44, 46 or the
cassette 43 can be loaded with unprocessed substrates 42 while
remaining in the opened one of the load locks 44, 46. The
replication of entry load locks 44, 46 allows one of them to be
loaded while the other is at vacuum and transferring substrates to
the next stage.
[0026] Each load lock 44, 46 contains a robot 48 or other substrate
handling mechanism to sequentially transfer unprocessed substrates
42 from the cassette 43 through an opened, unillustrated, isolation
valve to the next stage. The isolation valve of the other load lock
44, 46 being loaded with fresh substrates needs to be closed during
the loading.
[0027] In one embodiment, the first processing stage includes a
first plasma spraying chamber having an overhead plasma gun 52
partially exposed outside the chamber 50 including connections 54,
56 for the argon spraying gas and cooling water and a vacuum-gated
hopper 58 for the silicon powder and required dopant to be sprayed
but having a spraying nozzle inside the chamber 50 which is
maintained at a desired low pressure in the range of less than 200
Torr including the argon and hydrogen used in plasma spraying.
Substrate support and moving means are included within the chamber
50 to support the substrate 42 transferred to it by the robot 48 in
one of the load locks 44, 46. For example, the means may be an X-Y
stage 60 which can move the supported substrate 42 under the plasma
gun 52 in two-orthogonal directions in the plane perpendicular to
the spraying axis to deposit a generally uniformly thick layer of
silicon on the supported substrate 42. Alternatively, the means
particularly for a circular substrate 42 may be a rotatable
pedestal and the plasma gun 52 may include a vacuum-sealed linear
slide in the roof of the chamber 50 so that the gun nozzle can move
its spray plume along a radius of the rotating pedestal.
[0028] The illustrated plasma spraying chamber 50 can have a volume
considerably smaller than conventional chambers described in the
prior art. Chamber 50 is also configured to process a continuous
stream of substrates while pumped down and not exposed to
contaminating ambient. Accordingly, it is both economical to build
and efficient to operate. Optionally, chamber 50 may be configured
to process a "continuous substrate" such as a flexible substrate in
a long continuous sheet or roll. Prior art systems are found in
U.S. Pat. No. 6,186,090, U.S. Pat. No. 6,367,411, U.S. Pat. No.
6,488,995, U.S. Pat. No. 7,629,206; all incorporated by reference
herein in their entirety.
[0029] In this embodiment, the next stage is a first transfer
chamber 64 preferably but not necessarily having isolation valves,
unillustrated, on its entry and exit sides and having a robot
disposed therein which is capable of removing a partially processed
substrate from the first plasma spraying chamber 50 through the
opened entry isolation valve and transferring it to a second plasma
spraying chamber 66 similar through the opened exit isolation
valve. The second plasma spraying chamber 66 is similar to the
first one 50 and also includes a plasma gun 68 and ancillary
equipment inside and outside of the chamber 66. The replication of
plasma spraying chambers is useful to increase the throughput, to
produce a graded doped layer, for example p-type, and to isolate
the spraying of n-type and p-type silicon if both are being
sprayed. To provide proper isolation between the two plasma
spraying chambers 50, 66, the entry and exit isolation valves on
the first transfer chamber 64 should be opened alternately; that
is, both should not be opened at the same time. One vacuum pumping
system 70 may pump both the entry load/locks 44, 46 and the first
transfer chamber 64 with separate valving to each of the pumped
chambers. Another vacuum pumping system 72 may pump the two plasma
spraying chambers 50, 66.
[0030] The stage or module following is a second transfer station
76 including its own robot and entry and exit isolation valves.
When the substrate 42 has completed spray deposition in the second
plasma spraying chamber 66, the robot in the second transfer
chamber 76 moves it from the second plasma spraying chamber 66 to a
recrystallization chamber 78 through alternately opened entry and
exit isolation valves.
[0031] A recrystallization chamber or module 78 may assume various
forms. The illustrated embodiment is configured for zone melt
recrystallization (ZMR) and includes, optionally, a conveyor belt
82 moving a series of the substrates 42 through an RF coil 84. The
RF coil 84, for example supplied with 13.56 MHz electrical power,
couples sufficient RF energy into a limited area of the passing
substrates 42 that the deposited film, for example silicon, melts
or at least becomes mobile such that the lamellae 14 of FIG. 1
merge and the voids 16 are functionally eliminated. In ZMR, the
molten zone progresses from one lateral side of the substrate 42 to
the other so as to orient or regularize the crystallography in a
single direction, for instance [100]. Vacuum transfer of substrates
between the plasma spraying chambers 50, 66 and the
recrystallization chamber 78 prevents the low-pressure voids 16 in
the generally porous plasma sprayed silicon from filling with
higher pressure gas during transfer.
[0032] Recrystallized substrates 42 cool as the conveyor belt 82
moves them towards two exit load locks 90, 92, here both shown in
their opened state. Separate, unillustrated, isolation valves
selectively isolate each of the exit load locks 90, 92 from
recrystallization chamber 78. The exit load locks 90, 92 are
configured and operate similarly to the entry load locks 44, 46 and
allow the removal of fully processed substrates 42 from processing
chambers held at low pressure during processing of a large number
of substrates. A third vacuum pumping system 94 pumps the
recrystallization chamber 78 and a fourth vacuum pumping system 96
pumps the second transfer station 76 and the two exit load locks
90, 92.
[0033] Other types energy may be used for of recrystallizing. For
example, as illustrated in the side cross-sectional view of FIG. 4,
an incandescent light source 100 linearly extending parallel to a
surface of the substrate 42 emits radiation upon a reflector 102
positioned in back of the light source 100. The reflector 102
extends parallel to the linear light source 100 out of the plane of
the illustration and is shaped in the illustrated plane to focus
reflected radiation 104 as a narrow linear beam 106 at the surface
of the substrate of sufficient intensity to locally heat the
surface of the substrate to a desired recrystallization
temperature. As the substrate 42 is moved in the illustrated
horizontal direction, linear beam 106 moves across the surface of
substrate 42 to effect ZMR. Additional optics, as shown in FIG. 7,
which may be refractive, diffractive, reflective, or a combination
of any or all types, performing the same or complementary functions
as the reflector 102 may be positioned between the light source 100
and the substrate 42. Linear laser diode array 705 is an exemplary
radiation source; first beam shaping lens 710 collects radiation
and focuses into fly's eye lens 715, transmitting to collimator
720; second beam shaping lens 725 transmits to focusing lens 730
which focuses radiation into a narrow line incident at ZMR plane
750; ZMR plane defines a substrate surface 42 and location of the
"melt line". FIG. 7, collectively, comprises exemplary "optical
components" sufficient to transform a radiation source, optionally
a linear laser diode array 705, into a line source incident on a
surface of moving substrate 42 at ZMR plane 750. Linear laser diode
array 705 comprises a stack of line arrays of diodes. Along the
line of the line array, divergence is low and is referred to as the
slow axis. Along the stack of diodes divergence is larger and this
is referred to as the fast axis. First beam shaping lens 710 is a
lens or group of lenses to reduce aberrations in the slow axis.
Fly's eye lens 715 comprises an array of small imaging lenses whose
purpose is to homogenize the beam. Each array of the stack is
imaged onto the same area after the fly's eye lens. Collimator 720
is a lens whose purpose is to bring both the fast and slow axes
into minimum divergence. Second beam shaping lens 725 is a lens or
group of lenses to correct for the uniformity of power intensity
and create a desired beam profile in the ZMR plane. Focusing lens
730 focuses the collimated beam onto the substrate 42 surface at
plane 750.
[0034] FIG. 8 shows schematically an optical system for "beam
homogenization" based on the disclosure of Tanaka in U.S. Pat. No.
6,393,042. The fly's eye lens 715 comprises an array of lenslets
which reproduce the linear diode array 705 once each lenslet. The
end result is that each entire line is reimaged over every other
line to create a homogeneous image focused at ZMR plane 750
producing a "melt line" on the surface of substrate 42. A more
detailed discussion is found on the Suss MicroOptics web site,
www.suss-microoptics.com/downloads/SMO_catalog.pdf [May 26, 2010],
incorporated by reference herein.
[0035] Laser modules emitting directed beams and associated optics
to produce similar scanned linear beams are described in U.S. Pat.
No. 6,987,240. Alternatively, plasma guns similar to the plasma
spray guns used for spraying silicon may be used to locally heat a
substrate to a required recrystallization temperatures.
[0036] Eliminating voids and resulting densification of plasma
sprayed material, optionally silicon, can be promoted during a
recrystallization process, as illustrated in the schematic side
cross-sectional view of FIG. 5, by supporting substrate 42 on a
stiff support 110 coupled to ultrasonic generator 112, optionally,
a piezo-electric transducer driven to produce kHz or MHz
vibrations. Ultrasonic waves are coupled through substrate 42 to
the surface irradiated by RF or radiant energy from linear beam
106. The ultrasonic waves promote the diffusion of the gaseous
content of the voids 16 out of the molten or nearly molten silicon
to collapse the voids 16 and densify the silicon.
[0037] In some embodiments linear processing as illustrated in FIG.
3 is adapted to a roll-to-roll system including one or more
serially arranged plasma spraying chambers, or modules and one or
more recrystallization chambers or modules. A supply roll of
substrate material in flexible ribbon form is loaded into a
location corresponding to an entry load lock. The substrate
material may be stainless steel ribbon, graphite foil, molybdenum
foil, ceramic foil or other suitable material in flexible ribbon
form. An end of the ribbon is threaded through the serially
arranged chambers and attached to a take up roll at a position
corresponding to an exit load lock. Once the substrate ribbon is
loaded into the system, the system is closed and pumped down to
processing pressure. In some embodiments no transfer chambers or
robots are needed between the chambers. If vacuum isolation is
required, the isolation valves can be replaced by a pair of rollers
closely accommodating the ribbon. Substrate ribbon is advanced
through a deposition system and serially processed through various
modules for deposition and ZMR, exiting onto a take up roll. At the
completion of processing, a substrate ribbon is removed from a
system; additional processing steps may be necessary for an
intended purpose such as a processed solar cell or ceramic
membrane. Other take up means may be substituted for the take up
roll.
[0038] In some embodiments a plasma spray gun as described in U.S.
Ser. No. 12/074,651 and silicon powder as described in U.S. Ser.
No. 11/782,201 are used; in some embodiments different plasma spray
guns and different materials are used.
[0039] In some embodiments a method of forming solar cells,
comprises the steps of pumping at least one plasma spraying
chamber; sequentially plasma spraying at a processing pressure of
between 0.1 to 200 Torr either a roll or a plurality of substrates
with silicon to form at least one silicon layer thereon; and
recrystallizing the at least one silicon layer in a chamber held at
a processing pressure of less than 200 Torr; optionally the roll or
the substrates are not exposed to a pressure of more than 200 Torr
between the plasma spraying and the recrystallizing steps;
optionally, subjecting the at least one silicon layer to ultrasonic
vibration during the recrystallizing step; optionally, loading a
plurality of the substrates into a load lock chamber isolated from
a spraying chamber use d for the plasma spraying; pumping the load
lock chamber to the processing pressure; and transferring the
substrates from the load lock chamber to the spraying chamber which
has already been pumped to the processing pressure.
[0040] In some embodiments a processing system, comprises an entry
load lock chamber capable of being loaded with a plurality of
substrates and is pumped to a pressure of less than 200 Torr; one
or more plasma spraying chambers configured to spray silicon
coupled to the load lock chamber through an isolation valve and
being capable of being pumped to a pressure of less than 200 Torr;
optionally, a recrystallization chamber coupled to the one or more
plasma spraying chambers at a pressure of less than 200 Torr and
capable of recrystallizing silicon layers formed on substrates
received from the one or more plasma spraying chambers; optionally,
further comprising an exit load lock chamber capable of receiving a
plurality of the substrates from the recrystallization chamber
through an isolation valve and of being pumped to a pressure of
less than 200 Torr.
[0041] In some embodiments a processing system, comprises a vacuum
chamber capable of being pumped to less than 200 Torr; a supply
roll of a ribbon substrate disposed in the chamber; a take up roll
disposed in the chamber for taking up the ribbon substrate; at
least one plasma spraying stations configured to spray silicon onto
the ribbon substrate disposed in the chamber between the supply and
take up rolls for spraying silicon on the ribbon substrate;
optionally a recrystallization station disposed in the chamber
between the at least one plasma spraying station and the take up
roll and configured to recrystallize silicon sprayed on the ribbon
substrate.
[0042] Zone melt recrystallization (ZMR) has been discussed and
implemented in many applications requiring the formation of a high
quality, low fault, crystal lattice after a material has been
produced with substandard crystalline properties. Examples of this
application are thin film depositions in solar cell fabrication or
flat panel display devices. In both these cases, if the deposition
is amorphous, there is a need to recrystallize the surface to
achieve the required electrical properties of the device.
[0043] In bulk materials, float zone technology is very similar in
method to achieve a similar result in which a narrow region of a
crystal is molten, and this molten zone is moved along the crystal
(in practice, the crystal is pulled through the heater). By
controlling the speed of the bulk material through the molten area,
crystal defects can repair themselves, or, impurities can be
removed from the bulk material by being "pushed" forward by the
melt zone.
[0044] The basic requirement of ZMR is to generate enough localized
heat in order to melt a portion of the deposited material and to
continue melting fresh material entering the zone as material
leaving the zone solidifies and recrystallizes according to the
crystalline structure of the material behind the melt zone, which
acts as a seed. Common methods, well documented in the literature,
used in solar applications use either a high power halogen lamp
focused on the surface undergoing ZMR or a carbon strip heating
element, heated by passing a high current through the strip,
relying on the resistance of the carbon to generate heat. Both of
these applications are capable of ZMR, but require significant
control, and are not easily implemented in a manufacturing
environment. Most systems in use are custom made by the end user,
and each method has specific shortcomings. The halogen lamp systems
are relatively unstable and difficult to control due to the natural
fluctuations of the lamp filament and their relatively short
lifetime. Additionally halogen lamp and carbon strip heating
elements require significant base heaters to raise the overall
temperature of the devices being processed to around
1000-1200.degree. C. at which point, the ZMR is able to effectively
recrystallize a layer of a few microns thickness, typically between
about 2 to 5 microns. In some embodiments with a laser or LED diode
array system with the disclosed optics a ZMR module is able to
effectively recrystallize a layer of from a few microns thickness
to more than 40 microns, typically between about 1 to 50 micron
deposited layer thickness.
[0045] Another common application is based on excimer lasers and is
in use for thin film transistor (TFT) flat panel displays (FPDs).
The deposition for TFT FPDs puts down a layer of amorphous silicon
typically measured in nanometers as compared to an optimum layer
thickness of approximately 30 microns in solar applications. In
other words, the layers deposited in TFTs are less than one-tenth
the thickness of layers deposited in solar applications. Excimer
laser recrystallization, as performed for TFT applications result
in crystal domains of approximately 0.1 micron. The crystal domains
needed in solar applications in order to achieve the necessary
electronic properties are on the order of tens of microns, a
difference of more than two orders of magnitude. Excimer lasers are
used in TFT applications because the energy is absorbed in the
surface and does not propagate into the bulk of the material. For
solar applications of ZMR the energy must propagate into the
silicon layer. In other words ZMR implementation in solar
applications is a volume process, significantly differentiating it
from existing excimer laser based ZMR for TFT panels.
[0046] Some embodiments of the instant invention comprise a linear
array of diode lasers working at about 805 nm wavelength. A laser
of this type may be a Coherent 4000L diode laser. By working at 805
nm, we see close to 70% of the incident light is absorbed by
silicon (at 600 microns thickness), the remaining 30% is reflected,
as shown in FIG. 6.
[0047] In some embodiments a linear array of lasers is imaged
across the length of the surface being processed, which is
typically 156 mm.times.156 mm for standard pseudo square solar
cells. This creates a narrow line, approximately 0.5 to 4 mm wide,
along the length of the solar cell. This line melts the surface
silicon which has been deposited on the substrate and capped by an
oxide layer to prevent agglomeration of the melted silicon into
balls. The line output of the laser array is then scanned across
the surface of the wafer, either using a slowly rotating mirror, a
slow galvo controlled mirror, a robotic arm moving the entire laser
head, or a motion control system moving the wafer underneath the
line.
[0048] By moving the beam relative to the surface at a rate of
approximately 1 mm/sec the beam continues to melt all unmelted
surface area entering the line scan, while the surface exiting the
line scan solidifies and recrystallizes in alignment with the
crystal lattice of the material behind the melt zone. Because the
preferred recrystallization is to the [100] plane, no seed crystal
is required in the disclosed implementation of ZMR.
[0049] In another embodiment, a focused spot of radiation is
scanned linearly across the surface being processed to create a
"line" of melted material in the deposited thick film. This "melt
line" is generated by a rotating minor or a galvo controlled minor.
An optical system is implemented to keep the radiation beam in
focus at all points of the "melt line". The energy of the beam is
adjusted to result in a continuous melt of the surface layer in the
area of the beam. As in the previous embodiment disclosed, a "melt
line" is moved relative to the surface at a rate of approximately 1
mm/sec; the beam continues to melt all unmelted surface area
entering the "melt line" scan, while the surface exiting the "melt
line" scan solidifies and recrystallizes in alignment with the
crystal lattice of the material behind the melt zone.
[0050] In some embodiments a "melt line" is generated by
cylindrical optics in conjunction with a radiation source; another
embodiment generates a "melt line" by using diffractive optics.
Another embodiment uses a high temperature source, optionally a hot
plate, so that the surface being processed is elevated to a
temperature close to the melting point of the material undergoing
ZMR. This has the advantage of reducing the power requirements of a
laser performing the ZMR, and, in some instances, reducing the
thermal stresses generated by high temperature gradients in
substrates. Use of a hot plate or other heat source to supplement a
radiation source may result in less material losses due to stress
related breakage.
[0051] In some embodiments a method of zone melt recrystallization
of a layer of material comprises the steps of: scanning the surface
of the layer with a laser beam such that the laser beam creates a
thin line approximately 0.5 to 3 mm wide of molten phase material,
and the line then progresses slowly across the surface of the layer
in such a fashion that the material entering into the heated zone
is melted and the material leaving the heated zone solidifies
according to the crystal structure of the solid material behind the
zone; optionally a liquid zone is created by rapidly scanning a
spot of light such that the surface illuminated by the spot is a
line that is in a continuously liquid phase; optionally, a liquid
zone is created by imaging a linear array of light onto the
surface, such that the surface illuminated by the line is in a
continuously liquid phase; optionally, a liquid zone is created by
imaging a spot of light onto a line image on the surface such that
the surface illuminated by the line is in a liquid phase;
optionally, a line scan is generated by a rapidly rotating mirror
and appropriate optical system to maintain uniformity of beam size
and energy density; optionally, a line scan is generated by a
vibrating, galvonometrically controlled minor and appropriate
optical system to maintain uniformity of beam size and energy
density; optionally, a melt line is imaged using cylindrical
optics; optionally, a melt line is imaged using diffractive optics;
optionally, a melt line is slowly scanned across the surface of a
layer by using a slowly rotated minor and appropriate optical
system to maintain uniformity of beam size and energy density;
optionally, a melt line is slowly scanned across the surface of the
layer by using a galvonometrically controlled minor and appropriate
optical system to maintain uniformity of beam size and energy
density; optionally, a melt line is slowly scanned across the
surface of a layer by moving the beam with a robotic arm;
optionally, a melt line is slowly scanned across the surface of a
layer by slowly moving the layer under the line; optionally,
material being recrystallized is silicon; optionally, material
being recrystallized is chosen from a group consisting of silicon,
silicon-germanium alloys, Group IV elements and/or alloys, Group IV
oxides, nitrides, carbides and mixtures thereof, metal oxides,
nitrides, carbides and mixtures thereof, transition metal oxides,
nitrides, carbides and mixtures thereof, rare earth metal oxides,
nitrides, carbides and mixtures thereof; optionally, a layer being
recrystallized is a layer of a solar cell; optionally, an
additional heat source is used to increase the base temperature of
a substrate in order to reduce the physical and thermal stress on
the layer material in order to prevent breakage of the layer during
zone melt recrystallization or after zone melt recrystallization;
optionally, a base heater reaches a temperature just below the
melting point of the layer being recrystallized; optionally, a
system is enclosed in an environmentally controlled chamber to
prevent chemical changes in the materials being processed due to
interactions with a natural environment at elevated
temperatures.
[0052] In some embodiments a linear process tool comprises first
and second deposition modules each comprising one or more plasma
spray guns operable to move in a direction approximately orthogonal
to the direction of motion of a substrate carrier; wherein the
first deposition module is configured to deposit a first layer on a
substrate on the substrate carrier and the second deposition module
is configured to deposit a second layer in direct contact with the
first layer on the substrate on the substrate carrier such that the
first layer is of first composition and the second layer is of
second composition different than the first composition; wherein
the plasma spray guns comprise components exposed to the plasma
stream comprising at least one constituent of the primary source
material in the plasma stream; wherein the primary source of
materials for the plasma spray guns for the deposited layers is a
first and second powder of first and second compositions produced
by jet mills wherein the substrate carrier conveys the substrate
from an entrance of the linear process tool to an exit at a
predetermined speed; optionally a linear process tool further
comprises a zone melt recrystallization module comprising a source
of radiation; and optical components operable to convert the source
of radiation into a linear line of radiation ranging from about 0.5
mm to 3 mm wide extending across the width of the substrate placed
on the substrate carrier such that a portion of the deposited
material layer on the substrate is irradiated in the linear line of
radiation and heated at least to the melting point of the deposited
material for a predetermined time; such that as the portion of the
substrate with a deposited material layer passes out from
underneath the linear line of radiation the deposited material
layer portion now outside the radiated zone cools below its melting
point and recrystallizes into a preferred orientation based on its
composition; optionally, the recrystallized deposited material
layer exhibits a minority carrier diffusion length greater than 40
microns and a grain size larger than the deposited material layer
thickness; optionally, a linear process tool further comprises a
means for ultrasonic vibration applied to the portion of the
deposited material layer on the substrate being irradiated in the
linear line of radiation and heated at least to the melting point;
optionally, a substrate processed by the linear process tool is
flexible; optionally, a the flexible substrate is configured such
that it enters the linear process tool from a roll and exits the
linear process tool on to a roll; optionally, the first and second
compositions are chosen substantially from a group consisting of
silicon, silicon-germanium alloys, Group IV elements and/or alloys,
Group IV oxides, nitrides, carbides and mixtures thereof, metal
oxides, nitrides, carbides and mixtures thereof.
[0053] In some embodiments a method for making a structure
comprising first layer of first composition and second layer of
second composition in contact wherein the first composition is
different from the second composition comprising the steps:
depositing the first layer on a transported substrate with a first
plasma spray gun operable to move in a direction approximately
orthogonal to the transported direction; and depositing the second
layer on the transported substrate with a second plasma spray gun
operable to move in a direction approximately orthogonal to the
transported direction; wherein the first plasma spray gun comprises
components exposed to the plasma stream comprising at least one
constituent of a first primary source material in the plasma
stream; wherein the first primary source material for the first
plasma spray gun for the first layer is a first powder of the first
composition produced by jet mills; wherein the second plasma spray
gun comprises components exposed to the plasma stream comprising at
least one constituent of a second primary source material in the
plasma stream; wherein the second primary source material for the
second plasma spray gun for the second layer is a second powder of
the second composition produced by jet mills; optionally, the
method for making a structure of claim 8 further comprises the step
of heating a portion of the transported substrate to initiate zone
melt recrystallization in a linear line ranging from about 0.5 mm
to 3 mm wide extending across the width of the transported
substrate to the melting point of the deposited material for a
predetermined time; and cooling and recrystallizing the previously
heated portion of the transported substrate into a preferred
orientation based on its composition; optionally, a recrystallized
portion of the transported substrate exhibits a minority carrier
diffusion length greater than 40 microns and a grain size larger
than the deposited material layer thickness; optionally, a method
for making a structure further comprises the step: applying
ultrasonic vibration to the portion of the transported substrate
being heated to initiate zone melt recrystallization; optionally, a
transported substrate is flexible; optionally, the first and second
compositions are chosen substantially from a group consisting of
silicon, silicon-germanium alloys, Group IV elements and/or alloys,
Group IV oxides, nitrides, carbides and mixtures thereof, metal
oxides, nitrides, carbides and mixtures thereof.
[0054] The foregoing described embodiments of the invention are
provided as illustrations and descriptions. They are not intended
to limit the invention to a precise form as described. In
particular, it is contemplated that functional implementation of
invention described herein may be implemented equivalently in
various combinations or other functional components or building
blocks. Other variations and embodiments are possible in light of
above teachings to one knowledgeable in the art of semiconductors,
thin film deposition techniques, and materials; it is thus intended
that the scope of invention not be limited by this Detailed
Description, but rather by Claims following. All patents, patent
applications, and other documents referenced herein are
incorporated by reference herein in their entirety for all
purposes.
[0055] In the preceding description, numerous specific details are
set forth, such as particular structures, components, materials,
dimensions, processing steps and techniques, in order to provide a
thorough understanding of the present invention. However, it will
be appreciated by one of ordinary skill in the art that the
invention may be practiced without these specific details. In other
instances, well-known structures or processing steps have not been
described in detail in order to avoid obscuring the invention.
[0056] It will be understood that when an element as a layer,
region or substrate is referred to as being "on" or "over" another
element, it can be directly on the other element or intervening
elements may also be present. In contrast, when an element is
referred to as being "directly on" or "directly over" another
element, there are no intervening elements present. It will also be
understood that when an element is referred to as being "connected"
or "coupled" to another element, it can be directly connected or
coupled to the other element or intervening elements may be
present. In contrast, when an element is referred to as being
"directly connected" or "directly coupled" to another element,
there are no intervening elements present.
* * * * *
References