U.S. patent application number 13/489031 was filed with the patent office on 2012-09-27 for integrated shadow mask/carrier for pattern ion implantation.
This patent application is currently assigned to VARIAN SEMICONDUCTOR EQUIPMENT ASSOCIATES, INC.. Invention is credited to Steven Anella, Nicholas Bateman, Kevin Daniels, Atul Gupta, Russell Low, Robert Mitchell, Benjamin Riordon.
Application Number | 20120244692 13/489031 |
Document ID | / |
Family ID | 44800245 |
Filed Date | 2012-09-27 |
United States Patent
Application |
20120244692 |
Kind Code |
A1 |
Bateman; Nicholas ; et
al. |
September 27, 2012 |
INTEGRATED SHADOW MASK/CARRIER FOR PATTERN ION IMPLANTATION
Abstract
An improved, lower cost method of processing substrates, such as
to create solar cells is disclosed. In addition, a modified
substrate carrier is disclosed. The carriers typically used to
carry the substrates are modified so as to serve as shadow masks
for a patterned implant. In some embodiments, various patterns can
be created using the carriers such that different process steps can
be performed on the substrate by changing the carrier or the
position with the carrier. In addition, since the alignment of the
substrate to the carrier is critical, the carrier may contain
alignment features to insure that the substrate is positioned
properly on the carrier. In some embodiments, gravity is used to
hold the substrate on the carrier, and therefore, the ions are
directed so that the ion beam travels upward toward the bottom side
of the carrier.
Inventors: |
Bateman; Nicholas; (Reading,
MA) ; Daniels; Kevin; (Lynnfield, MA) ; Gupta;
Atul; (Beverly, MA) ; Low; Russell; (Rowley,
MA) ; Riordon; Benjamin; (Newburyport, MA) ;
Mitchell; Robert; (Winchester, MA) ; Anella;
Steven; (West Newbury, MA) |
Assignee: |
VARIAN SEMICONDUCTOR EQUIPMENT
ASSOCIATES, INC.
Gloucester
MA
|
Family ID: |
44800245 |
Appl. No.: |
13/489031 |
Filed: |
June 5, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12895927 |
Oct 1, 2010 |
8216923 |
|
|
13489031 |
|
|
|
|
Current U.S.
Class: |
438/531 ;
257/E21.346 |
Current CPC
Class: |
H01L 21/677 20130101;
H01L 21/68735 20130101; H01J 2237/31711 20130101; Y02P 70/521
20151101; Y02P 70/50 20151101; H01L 31/1804 20130101; Y02E 10/547
20130101; H01J 37/3171 20130101 |
Class at
Publication: |
438/531 ;
257/E21.346 |
International
Class: |
H01L 21/266 20060101
H01L021/266 |
Claims
1. A method of implanting ions into a substrate comprising: placing
a first substrate on a carrier, wherein said carrier has a surface
defining at least one aperture that extends through said carrier
and wherein said substrate is held on said carrier using gravity;
implanting said first substrate through said aperture; removing
said first substrate from said carrier; placing a second substrate
on said carrier after said removing, wherein said second substrate
is held on said carrier using gravity; implanting said second
substrate through said aperture; and removing said second substrate
from said carrier,
2. The method of claim 1, wherein said implanting comprises using
ion beam projected substantially in a direction opposite of
gravity.
3. The method of claim 1, further comprising aligning said first
substrate on said carrier with respect to said aperture and
aligning said second, substrate on said carrier with respect to
said aperture.
4. The method of claim 1, wherein said carrier further comprises a
second aperture,
5. The method of claim 4, further comprising placing a third
substrate on said carrier with said first substrate and implanting
said third substrate through said second aperture.
6. The method of claim 4, further comprising placing a fourth
substrate on said carrier with said second substrate and implanting
said fourth substrate through said second aperture.
7. The method of claim 1, wherein said carrier is part of a
conveyor belt.
8. A method of implanting ions into a substrate comprising: placing
a plurality of substrates on a carrier, wherein said carrier has a
surface defining a plurality of apertures that extend through said
carrier and wherein said substrates are held on said carrier using
gravity such that each of said substrates is disposed over one of
said apertures; implanting said substrates through, said, apertures
with an ion beam; and removing said substrates from said
carrier.
9. The method of claim 8, wherein said ion beam is projected
substantially in a direction opposite of gravity.
10. The method of claim 8, wherein each of said substrates is
implanted with the same pattern using said apertures.
11. The method of claim 8, wherein each of said apertures has equal
dimensions.
12. The method of claim 8, further comprising aligning each of said
substrates on said carrier with respect to said apertures.
13. The method of claim 8, wherein said carrier is part of a
conveyor belt.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation application of U.S. application Ser.
No. 12/895,927, filed Oct. 1, 2010 and entitled "Integrated Shadow
Mask/Carrier for Patterned Ion Implantation," the disclosure of
which is hereby incorporated by reference.
BACKGROUND
[0002] Solar cells are typically manufactured using the same
processes used for other semiconductor devices, often using silicon
as the substrate material. A semiconductor solar cell is a device
having an in-built electric field that separates the charge
carriers generated through the absorption of photons in the
semiconductor material. This electric-field is typically created
through the formation of a p-n junction (diode) which is created by
differential doping of the semiconductor material. Doping a part of
the semiconductor substrate (e.g. surface region) with impurities
of opposite polarity forms a p-n junction that may be used as a
photovoltaic device converting light into electricity.
[0003] FIG. 1 shows a cross section of a representative solar cell
100. Photons 101 enter the solar cell 100 through the top surface
105, as signified by the arrows. These photons pass through an
anti-reflective coating 110, designed to maximize the number of
photons that penetrate the solar cell 100 and minimize those that
are reflected away from the solar cell 100.
[0004] Internally, the solar cell 100 is formed so as to have a p-n
junction 120. This junction is shown as being substantially
parallel to the top surface 105 of the solar cell 100 although
there are other implementations where the junction may not be
parallel to the surface. The solar cell is fabricated such that the
photons enter the substrate through the n-doped region, also known
as the emitter 130. While this disclosure describes p-type bases
and n-type emitters, n-type bases and p-type emitters can also be
used to produce solar cells and are within the scope of the
disclosure. The photons with sufficient energy (above the bandgap
of the semiconductor) are able to promote an electron within the
semiconductor material's valence band to the conduction band.
Associated with this free electron is a corresponding positively
charged hole in the valence band. In order to generate a
photocurrent that can drive an external load, these electron hole
(e-h) pairs need to be separated. This is done through the built-in
electric field at the p-n junction. Thus any e-h pairs that are
generated in the depletion region of the p-n junction get
separated, as are any other minority carriers that diffuse to the
depletion region of the device. Since a majority of the incident
photons are absorbed in near surface regions of the device, the
minority carriers generated in the emitter need to diffuse across
the depth of the emitter 130 to reach the depletion region and get
swept across to the other side. Thus to maximize the collection of
photo-generated current and minimize the chances of carrier
recombination in the emitter 130, it is preferable to have the
emitter 130 be very shallow.
[0005] Some photons 101 pass through the emitter region 130 and
enter the base 140. These photons 101 can then excite electrons
within the base 140, which are free to move into the emitter 130,
while the associated holes remain in the base 140. As a result of
the charge separation caused by the presence of this p-n junction
120, the extra carriers (electrons and holes) generated by the
photons 101 can then be used to drive an external load to complete
the circuit.
[0006] By externally connecting the emitter 130 to the base 140
through an external load, it is possible to conduct current and
therefore provide power. To achieve this, contacts 150a and 150b,
typically metallic, are placed on the outer surface of the emitter
130 and the base 140. Since the base 140 does not receive the
photons 101 directly, typically its contact 150b is placed along
the entire outer surface of the base 140. In contrast, the outer
surface of the emitter 130 receives photons 101 and therefore
cannot be completely covered with contacts 150a. However, if the
electrons have to travel great distances to the contact, the series
resistance of the cell increases, which lowers the power output. In
an attempt to balance these two considerations; the distance that
the free electrons must travel to the contact 150a or 150b, and the
amount of exposed emitter surface 160 illustrated in FIG. 2; most
applications use contacts 150a that are in the form of fingers.
FIG. 2 shows a top view of the solar cell of FIG. 1. The contacts
150a are typically formed so as to be relatively thin, while
extending the width of the solar cell 100. In this way, free
electrons need not travel great distances, but much of the outer
surface of the emitter is exposed to the photons. Typical contacts
150a on the front side of the substrate are 0.1 mm wide, with an
accuracy of approximately +/-0.1mm. These contacts 150a are
typically spaced between 1-5 mm apart from one another. While these
dimensions are typical, other dimensions are possible and
contemplated herein.
[0007] A further enhancement to solar cells is the addition of
heavily doped substrate contact regions. FIG. 3 shows a cross
section of this enhanced solar cell. The solar cell 100 is as
described above in connection with FIG. 1, but includes heavily
n-doped contact regions 170. These heavily doped contact regions
170 correspond to the areas where the contacts 150a will be affixed
to the solar cell 100. The introduction of these heavily doped
contact regions 170 allows much better contact between the solar
cell 100 and the contacts 150a and significantly lowers the series
resistance of the solar cell 100. This pattern of including heavily
doped regions on the surface of the substrate is commonly referred
to as selective emitter (SE) design. These heavily doped regions
may be created by implanting ions in these regions. Thus, the terms
"implanted region" and "doped region" may be used interchangeably
throughout this disclosure.
[0008] A selective emitter design for a solar cell also has the
advantage of higher efficiency cells due to reduced minority
carrier losses through recombination due to lower dopant/impurity
dose in the exposed regions of the emitter layer. The higher doping
under the contact regions provides a field that collects the
majority carriers generated in the emitter and repels the excess
minority carriers back toward the p-n junction.
[0009] Such structures are typically made using traditional
lithography (or hard masks) and thermal diffusion. An alternative
is to use implantation in conjunction with a traditional
lithographic mask, which can then be removed easily before dopant
activation. Yet another alternative is to use a shadow mask or
stencil mask in the implanter to define the highly doped areas for
the contacts. All of these techniques utilize a fixed masking layer
(either directly on the substrate or upstream in the beamline).
[0010] All of these alternatives have drawbacks. For example, the
processes enumerated above all contain multiple process steps. This
causes the cost of the manufacturing process to be prohibitive and
may increase substrate breakage rates. These options also suffer
from the limitations associated with the special handling of solar
cells, such as aligning the mask with the substrate and the cross
contamination with materials that are dispersed from the mask
during ion implantation.
[0011] Consequently, efforts have been made to reduce the cost and
effort required to dope a pattern onto a substrate. While some of
these efforts may be successful in reducing cost and processing
time, often these modifications come at the price of reduced
accuracy. Typically, in semiconductor processes, masks are very
accurately aligned. Subsequent process steps rely on this accuracy.
For example, referring to FIG. 4, after the heavily doped regions
170a-c have been implanted, contacts 150a are pasted to the
substrate. Each of these processes is usually performed relative to
some reference mark or fiducial. This mark may be an edge or corner
of the substrate, or a specific mark or feature on the substrate.
Since each of these process steps is typically referenced to a
specific point, it is imperative that a high degree of accuracy be
maintained. These efforts to reduce cost and processing steps
degrade this accuracy, thereby potentially impacting the
performance and yields of the devices made using these methods.
[0012] Therefore, there exists a need to produce solar cells where
the number and complexity of the process steps is reduced, while
maintaining adequate accuracy so that subsequent process steps are
correctly positioned. While applicable to solar cells, the
techniques described herein are applicable to other doping
applications.
SUMMARY OF THE INVENTION
[0013] An improved, lower cost method of processing substrates,
such as to create solar cells is disclosed. In addition, a modified
substrate carrier is disclosed. The carriers typically used to
carry the substrates are modified so as to serve as shadow masks
for a patterned implant. In some embodiments, various patterns can
be created using the carriers such that different process steps can
be performed on the substrate by changing the carrier or the
position with the carrier. In addition, since the alignment of the
substrate to the carrier is critical, the carrier may contain
alignment features to insure that the substrate is positioned
properly on the carrier. In some embodiments, gravity is used to
hold the substrate on the carrier, and therefore, the ions are
directed so that the ion beam travels upward toward the bottom side
of the carrier.
BRIEF DESCRIPTION OF THE FIGURES
[0014] FIG. 1 shows a cross section of a solar cell of the prior
art;
[0015] FIG. 2 shows a top view of the solar cell of FIG. 1;
[0016] FIG. 3 shows a cross section of a solar cell using selective
emitter design;
[0017] FIG. 4 shows a top view of the solar cell of FIG. 3;
[0018] FIG. 5 shows a representative coordinate system;
[0019] FIG. 6 is a representative illustration of an ion implanter
used in accordance with an embodiment;
[0020] FIG. 7 shows a shadow mask being used to form the doped
regions shown in FIG. 4;
[0021] FIGS. 8A-C show several embodiments in accordance with the
present disclosure;
[0022] FIG. 9 shows the implant process according to one
embodiment;
[0023] FIGS. 10A-B are embodiments of a carrier having two
different patterns;
[0024] FIG. 11 shows another embodiment of a carrier having two
different patterns;
[0025] FIG. 12 is a table showing the results of shifting
substrates in a carrier having a plurality of different
patterns;
[0026] FIG. 13 shows another embodiment of a carrier having two
different patterns; and
[0027] FIG. 14 shows another embodiment of a carrier having two
different patterns suitable for an IBC solar cell.
DETAILED DESCRIPTION
[0028] FIG. 4 shows a top view of the solar cell manufactured using
the methods of the present disclosure. The solar cell is formed on
a semiconductor substrate 100. The substrate can be any convenient
size, including but not limited to circular, rectangular, or
square. Although not a requirement, it is preferable that the width
of the solar cell 100 be less than the width of the ion beam used
to implant ions in the solar cell 100. However, no such limitation
exists with respect to the orthogonal direction of the solar cell.
In other words, a solar cell 100 can be arbitrarily long, and can
be in the shape of a roll of solar cell material. Typically, the
substrates for solar cells 100 are very thin, often on the order of
300 microns thick or less.
[0029] As described above, the solar cell has an n-doped emitter
region and a p-doped base. The substrate is typically p-doped and
forms the base, while ion implantation is used to create the
emitter region. A block diagram of a representative ion implanter
600 is shown in FIG. 6. Of course, one skilled in the art will
recognize that numerous other ion implanter designs exist and may
be used. An ion source 610 generates ions of a desired species,
such as phosphorus or boron. A set of electrodes (not shown) is
typically used to attract the ions from the ion source 610. By
using an electrical potential of opposite polarity to the ions of
interest, the electrodes draw the ions from the ion source 610, and
the ions accelerate through the electrode. These attracted ions are
then formed into an ion beam 650, which then passes through a
source filter 620. The source filter 620 is preferably located near
the ion source 610. The ions within the ion beam 650 are
accelerated/decelerated in column 630 to the desired energy level.
A mass analyzer magnet 640, having an aperture 645, is used to
remove unwanted components from the ion beam 650, resulting in an
ion beam 650 having the desired energy and mass characteristics
passing through resolving aperture 645.
[0030] In certain embodiments, the ion beam 650 is a spot beam. In
this scenario, the ion beam 650 passes through a scanner 660,
preferably an electrostatic scanner, which deflects the ion beam
650 to produce a scanned beam 655 wherein the individual beamlets
657 have trajectories which diverge from scanner 660. In certain
embodiments, the scanner 660 comprises separated scan plates in
communication with a scan generator. The scan generator creates a
scan voltage waveform, such, as a sine, sawtooth or triangle
waveform having amplitude and frequency components, which is
applied to the scan plates. In a preferred embodiment, the scanning
waveform is typically very close to being a triangle wave (constant
slope), so as to uniformly expose the scanned beam 655 at every
position of the substrate for nearly the same amount of time.
Deviations from the triangle are used to make the beam uniform. The
resultant electric field causes the ion beam to diverge as shown in
FIG. 6.
[0031] An angle corrector 670 is adapted to deflect the divergent
ion beamlets 657 into a set of ion beamlets 657 having
substantially parallel trajectories. Preferably, the angle
corrector 670 comprises a magnet coil and magnetic pole pieces that
are spaced apart to form a gap, through which the ion beamlets 657
pass. The coil is energized so as to create a magnetic field within
the gap, which deflects the ion beamlets 657 in accordance with the
strength and direction of the applied magnetic field. The magnetic
field is adjusted by varying the current through the magnet coil.
Alternatively, other structures, such as parallelizing lenses, can
also be utilized to perform this function.
[0032] Following the angle corrector 670, the scanned beam 655 is
targeted toward the substrate, such as the solar cell to be
processed. The scanned beam typically has a height (Y dimension)
that is much smaller than its width (X dimension). This height is
much smaller than the substrate, thus at any particular time, only
a portion of the substrate is exposed to the ion beam. To expose
the entire substrate to the scanned beam 655, the substrate may be
moved relative to the beam location.
[0033] The substrate, such as a solar cell, is attached to a
substrate holder 675. The substrate holder 675 may provide a
plurality of degrees of movement. For example, the substrate holder
675 can be moved in the direction orthogonal to the scanned beam
655. A sample coordinate system in shown in FIG. 5. Assume the beam
is in the XZ plane. This beam can be a ribbon beam, or a scanned
spot beam. The substrate holder can move in the Y direction. By
doing so, the entire surface of the solar cell 100 can be exposed
to the ion beam, assuming that the solar cell 100 has a smaller
width than the ion beam (in the X dimension).
[0034] Substrates are moved into and from the process chamber
through the use of carriers. In some embodiments, the carriers are
rectangular, such as box shaped, and are capable of holding a
plurality of substrates. In other embodiments, a separate,
typically flat, carrier is used for each substrate. In one
embodiment, the substrate is removed from the carrier and placed on
the substrate holder 675 in preparation of processing, such as by a
robotic arm. One reason to remove the substrate from the carrier
may be to minimize cross-contamination for multi-species processes.
After processing has been completed, the robotic arm returns the
substrate to the carrier. The substrate, contained within the
carrier, can now be transported outside the chamber. In another
embodiment, the substrate remains in the carrier during the implant
process. This allows the carrier to serve as an alignment reference
for the substrate. This also allows the carrier to have a pattern
on it which will serve as a mask in the presence of an ion
beam.
[0035] There may be additional reasons to utilize a carrier. For
example, a carrier supports the substrate in multiple axis during
transport. If the substrate, for example, resides in a pocket of
the carrier, the tool can move the substrate/carrier combination
faster than if it relied exclusively on friction to the substrate
(e.g., backside pads). Thus, with a fragile substrate constrained
in the carrier, the handling system may be passively (e.g., pins)
or actively (e.g., grabbers) held for more secure transport.
[0036] In addition, in some embodiments, a carrier can be made of
conductive materials to form an electrical ground path to the
substrate. In some embodiments, a carrier can be used to apply an
electrical voltage to the substrate, such as a pulsed voltage for a
plasma tool.
[0037] A carrier can easily be either adapted or replaced in an
implanter to enable the handling of alternate substrate sizes or
shapes.
[0038] Finally, once a substrate is rigidly constrained within a
carrier, reliable positional references can be made to the carrier
alone. In other words, locating to a kinematic pin feature in the
carrier can be done repeatedly very accurately, and without risking
substrate breakage.
[0039] In addition to beam line ion implanters, plasma doping
systems can also be used. A plasma doping system forms a plasma
containing the dopant using an electron cyclotron resonance plasma
source, a helicon plasma source, a capacitively coupled plasma
source, an inductively coupled plasma source, a DC glow discharge,
a microwave source, or an RF source, as examples. The substrate,
which is located within a chamber containing this plasma, is then
biased using either a pulse or DC voltage and ions are accelerated
into the surface of the substrate. Other ion implanters, including
those with or without mass analysis, may be used.
[0040] There are a number of methods that can be used to create the
doping pattern shown in FIG. 4. In some embodiment, the pattern is
created by traditional implantation techniques. For example, an ion
beam can be used to implant the surface of the solar cell 100 which
is exposed to the beam. In some embodiments, the emitter 160 is
doped using an ion implantation across the entire surface, also
known as a blanket implant. The more heavily doped region 170 is
then created using a mask. FIG. 7 shows a mask 12 disposed between
the source of ions and the solar cell 100. The mask 12 includes one
or more apertures 14 that allow the passage of ions 13. The mask 12
will block ions 13 that do not pass through the apertures 14. Those
areas which are exposed to the ion beam become implanted or doped
regions 170.
[0041] However, the use of a traditional shadow mask requires
precise alignment processes. In some embodiments, the shadow mask
is placed between the ion beam and the substrate holder 675, while
the substrate is clamped to the substrate holder. In this
embodiment, there is an alignment process that must be completed to
properly orient the shadow mask to the clamped substrate. In some
embodiments, the substrate is moved while the shadow mask is held
stationary. In other embodiments, the substrate is held stationary
while the shadow mask is moved to perform the alignment.
[0042] To eliminate these alignment processes, the present
disclosure uses the substrate carrier as the shadow mask. In one
embodiment, the substrates are placed flat on the substrate
carrier. In some embodiments, alignment features are used to insure
that the substrate is properly positioned on the carrier. In
addition, one surface of the carrier, typically the bottom surface,
has apertures or openings in the shape of the desired doping
pattern. FIG. 8A shows a substrate carrier 800, which supports a
single substrate. The substrate carrier 800 includes a plurality of
slots or apertures 805, through which ions may pass, thereby
allowing the exposed regions of the substrate resting on the
carrier to be implanted. In some embodiments, the slots or
apertures are the result of the removal of the material used to
construct the carrier. In other embodiments, the slots are the
result of the use of a material which allows the transmission of
ions through it. In another embodiment, the bottom surface of the
carrier 800 may be constructed by combining stacks of thin material
and spacers, which form the desired pattern. In another embodiment,
the solid parts of the bottom surface of the carrier may be wires
tensioned across the substrate. In yet another embodiment, the
carrier 800 may have a bottom surface that is substantially open.
The carrier 800 could support or be independently aligned to a
separate mask, which is positioned between the bottom surface of
the carrier 800 and the substrate. In this embodiment, the carrier
800 supports the substrate, and the carrier 800 and/or mask are
registered to a mask within the implanter. This "dual-registration"
approach may allow the option of repeatably registering multiple
masks to one substrate.
[0043] The carrier 800 may be constructed of any material capable
of withstanding the ion implantation process, such as graphite,
Silicon carbide or silicon. In some embodiments, the slots 805 are
between 50 .mu.m and 800 .mu.m in width, and are used to create the
highly doped selective emitter regions 170, as described in
connection with FIG. 3. While FIG. 8A shows a substrate carrier 800
suitable for holding a single substrate, other embodiments are
possible. FIG. 8B shows a substrate carrier 810 which is able to
accommodate four substrates, located in positions 811-814. FIG. 8C
shows a substrate 820 carrier configured to hold as many as 20
substrates. The size of the carrier and the number of substrates
that can be supported is not limited by the present disclosure.
Similarly, while FIGS. 8A-C show the pattern as a series of slots,
other patterns may also be used and are within the scope of the
disclosure.
[0044] As described above, one or more alignment features may be
included in the carrier to properly position the substrate relative
to the pattern. These alignment features may be on the side
opposite that impacted by the ion beam. In one embodiment, two
points are used to align to the edge of the substrate to reference
the location of the pattern. The substrate could be referenced in
two dimensions as well for two dimensional patterning. The
referencing of the substrate to the carrier may be done by tipping
the carrier and allowing the substrate to slide against the
alignment feature due to the force of gravity. In another
embodiment, alignment features are not used and can be replaced by
an optical recognition system to align the doped lines to the metal
lines at the metallization step. The alignment features may be in
the carrier, in the mask or within the implanter. While gravity may
be used to slide the substrates gently against an alignment
feature, an active device, such as a robot, could be used as
well.
[0045] In operation, the substrate is placed on a substrate
carrier. The carrier may hold any number of substrates, as shown in
FIGS. 8A-8C, although it is preferable that each substrate is
positioned over a corresponding pattern. As described above, the
pattern may be a series of apertures or slots that allow ions to be
implanted into the substrate. The pattern of apertures or slots
matches that which is to be implanted in the substrate. For
example, the pattern of FIG. 8A can be used to implant the heavily
n-doped contact regions 170 of FIG. 3. The substrate is placed on
the carrier and may be aligned using alignment features located on
the carrier, such as on the top surface of the carrier. The
substrates may be held in position by, for example, gravity. In
other words, the patterns of FIG. 8A-C are created on the bottom
surface of the carrier. The populated carrier is then placed in the
process chamber.
[0046] In some embodiments, as shown in FIG. 9, the carrier 850
remains horizontally oriented, such that its bottom surface 851 is
parallel to ground. The ion beam 870 is then incident on the bottom
surface 851 of the carrier, such that the dopant ions are implanted
into the substrates 860 through the pattern of apertures on the
bottom surface 851. In some embodiments, the carrier 850 is scanned
through the ion beam 870, such that it moves while the beam is held
stationary. In one particular instance, this carrier 850 is part of
a conveyor belt. In other embodiments, the carrier 850 is held
stationary and the ion beam 870 scans across the carrier. In the
case of a pulsed plasma implant, the carrier 850 and substrate are
stationary relative to the beam.
[0047] The ion beam 870 can be at any angle relative to the bottom
surface 851 of the carrier 850, although in some embodiments, an
ion beam 870 normal to the bottom surface 851 may be desirable. The
angle of incidence can be modified by either changing the direction
of the ion beam 870, tilting the carrier 850, or a combination of
the two actions. In embodiments where gravity is used to hold the
substrate 860 in place, the maximum angle of tilt may be
limited.
[0048] In some embodiments, as described above, gravity is used to
hold the substrate 860 in place in the carrier 850. In other
embodiments, the substrate 860 is held in place, such as by an
electrostatic or mechanical clamp, so that the carrier 850 can be
tilted to a greater extent, such as completely vertically. For
example, the pattern of the mask may serve as the active clamping
surface.
[0049] FIGS. 8B-C show a plurality of locations in which the
substrates may be placed. In these figures, each location has an
identical pattern. However, other embodiments are also possible.
For example, FIG. 10A shows a carrier 830, which is configured to
hold two substrates. The first location 831 has a pattern similar
to that shown in FIGS. 8A-C, which may be used to implant the
heavily n-doped contact regions 170 of FIG. 3. The second location
832 has almost all of the material removed, such the almost the
entire surface of the substrate located in second location 832 is
implanted by the ion beam. Small tabs 833 may be used to support
the substrate when in this position. A border or other edge also
may be used. This second location 832 may be used to perform a
blanket implant on the substrate.
[0050] In some embodiments, the carrier 830 may be loaded with two
substrates, such that the first is positioned in first location 831
so that it is pattern implanted while the second is positioned in
second location 832 and is blanket implanted. After the implant is
completed, the positions of the substrates in the carrier 830 may
be switched, such that the first substrate is now in second
location 832 and is blanket implanted, while the second is pattern
implanted in first location 831. Such an arrangement allows two
separate process steps (blanket and pattern implantation) to be
performed on two substrates using a single tool. The substrates may
be changed using, for example, a substrate handling robot.
[0051] In another embodiment, a single substrate is loaded into
carrier 830, such as in first location 831. The carrier 830 is then
placed so as to be impacted by the ion beam. After the blanket
implant is completed, the carrier 830 is shifted from first
location 831 to second location 832. In some embodiments, the
carrier is tilted such that the substrate slides from first
location 831 to second location 832. In other embodiment, a
substrate handling robot is used. At this point, the carrier 830 is
again moved so as to be impacted by the ion beam. The substrate is
now pattern implanted. In this way, a single substrate can have two
implants performed on, it using a single tool. This allows a
substrate to receive both a blanket implant to create an emitter
region 130 and a pattern implant to create heavily n-doped contact
regions 170.
[0052] To improve, alignment, the carrier 830 may be tilted toward
first location 831 so that the substrate slides to the end of the
carrier. After the first implant, the carrier 830 may be tilted
toward second position 832 so that the substrate slides to the
opposite end of the carrier 830. This method insures that the
substrate is aligned, with the patterns on the bottom surface of
the carrier 830.
[0053] To improve yield, each location 831, 832 may be extended to
form rows so as to hold a plurality of substrates, each substrate
adjacent to the other, as shown in FIG. 10B. In this embodiment,
the carrier 835 has two rows 836, 837, each capable of holding five
substrates. The tilting process described above may be used to
align all substrates located in row 836 first. After the implant is
completed, the carrier 835 may be tilted slide the substrates into
row 837. Note also that other methods may be user to shift the
substrates from one row to the second row.
[0054] FIG. 11 shows a larger carrier 840, having four rows 841-844
having two different patterns that can be used to simultaneously
implant 20 substrates. The ten substrates in rows 841, 843 may be
pattern implanted. The ten substrates in rows 842, 844 may be
blanket implanted. After this implant is completed, the substrates
may be moved to another row to allow each substrate to receive both
types or patterns of implants.
[0055] In another embodiment, the lowest row 844 is left vacant,
such that substrates are only loaded into the top three rows
841-843 of the carrier 840. The carrier 840 is then moved so as to
be impacted by the ion beam. After the implant is completed, the
substrates are caused to shift downward by one row. In other words,
the substrates in top row 841 are shifted to second row 842.
Similarly, the substrates in rows 842, 832 and shifted to rows 843,
844 respectively. The carrier 840 is then moved so as to be
impacted by the ion beam. In this way, the substrates each now
receive a second implant, of a different type than the first
implant. In other words, those which were blanket implanted in row
842 during the first implant are now pattern implanted in row 843.
Those that were pattern implanted in row 841, 843 are now blanket
implanted in rows 842, 844.
[0056] In some embodiments, the substrates are shifted from one row
to an adjacent row by tilting the carrier. In this embodiment, the
substrates slide until touching against substrates in an adjacent
row or an alignment feature. In other embodiments, the substrates
may be mechanically pushed from one row to another. Alternatively,
the substrates may be held stationery, such as by electrostatic
clamping, while the carrier is advanced.
[0057] The above description shows two patterns, where one is a
series of slots and the second is for a blanket implant. However,
the disclosure is not limited to these patterns. For example, two
different patterns, each having a series of slots (perhaps oriented
in different directions) may be used. Similarly, other types of
patterns may be used for the various rows.
[0058] For example, it is possible to create two separate pattern
features by referencing two different alignment features within the
carrier. This technique would duplicate the same implant pattern on
two different positions of the substrate. An advantage of this
technique is that the irregularities contained within the pattern
will always be matched by the duplicate. This technique may help
with tolerancing between patterns.
[0059] In addition, the disclosure is not limited to only two
patterns. Three or more different patterns can be used with a
single carrier. Furthermore, if desired, each substrate may receive
patterned implants using each of the patterns on the carrier. For
example, a carrier may utilize three different patterns, A, B and
C. These patterns may be arranged in adjacent rows, such as A, B,
C, A, B. If substrates are placed in the first three rows, after
two shifts and three implants, all substrates would have been
implanted with patterns A, B and C. If desired, the substrates may
be shifted fewer times, thereby creating substrates with different
doping patterns on them. FIG. 12 shows the various doping patterns
that can be achieved using adjacent rows having three different
patterns, A, B and C. As can be seen, the doping patterns implanted
in the substrates after 0 shifts or 1 shift are unique to the row
in which the substrate was originally placed. Thus, using fewer
than 2 shifts, it is possible to create substrates of different
patterns. However, if the substrates are shifted 2 times, then all
substrates are ultimately implanted with all three patterns. This
concept can be expanded to an arbitrary number of patterns if
desired, for example, if N patterns are used, N-1 shifts can be
used to produce identical doping patterns on all substrate. A fewer
number of shifts will create unique substrates. To allow N-1
shifts, it is necessary that at least this number of rows were not
populated with substrates, thus allowing the placed substrates to
be able to shift to unpopulated or vacant rows.
[0060] In addition, although FIGS. 10 and 11 show that all patterns
in a particular row are identical, this is not a limitations of the
present disclosure. For example, in FIG. 13, the carrier 900 has
two rows 901, 902. Each row has five positions of columns 910-914.
The patterns of row 901 are arranged from left to right as A, B, A,
B, A. To achieve uniform doping of all substrates, it may be
desirable to have the opposite alternating set of patterns in row
902. For example, row 902 may have patterns B,A,B,A,B, such that
each column 910-914 has both patterns in it. The patterns do not
have to be arranged in alternating fashion as shown in FIG. 13. To
achieve uniform doping of all substrates, it is only important that
each column 910-914 has each type of pattern in it.
[0061] In addition, any of the above embodiments can be used for
applications where there is a need to have successive implants of
different species. For example, one pattern may be implanted with a
first species, such as phosphorus, and another pattern may be
implanted with a second species, such as boron. The carrier 840,
such as those shown in FIGS. 10A-B and 11, may be used for each
implant with the position of the substrates shifting in the carrier
between implant steps. The successive implants may be performed in
different implanters, or they may be performed in two implanters
that have been clustered into a single vacuum system, or they may
be performed in the same process chamber from separate ion sources,
or from a single ion source that can switch quickly between
species.
[0062] FIG. 14 shows a carrier 990, with a first row 991 and a
second row 992. The pattern in the first row 991 differs from that
in the second row 992. In this embodiment, the patterns are created
so as to implant an interdigitated back contact (IBC) for a solar
cell. In one embodiment, the substrates are populated in first row
991. The substrates are then exposed to an ion beam, containing one
species of dopant, such as for example n-type dopants. The
substrates are then moved to the second row 992, using any of the
methods described above and aligned. The substrates are then
exposed to an ion beam, containing a second species of dopant, such
as for example p-type dopants. The two patterns are created so as
to create non-overlapping interdigitated regions of highly doped
material. Such a doping pattern may be used on the back side of an
IBC solar cell.
[0063] The terms and expressions which have been employed herein
are used as terms of description and riot of limitation, and there
is no intention in the use of such terms and expressions of
excluding any equivalents of the features shown and described (or
portions thereof). It is also recognized that various modifications
are possible within the scope of the claims. Other modifications,
variations, and alternatives are also possible. Accordingly, the
foregoing description is by way of example only and is not intended
as limiting.
* * * * *