U.S. patent application number 14/684943 was filed with the patent office on 2015-08-06 for in-line photoluminescence imaging of semiconductor devices.
The applicant listed for this patent is BT IMAGING PTY LTD. Invention is credited to Kenneth Edmund ARNETT, Robert Andrew BARDOS, Ian Andrew MAXWELL, Thorsten TRUPKE.
Application Number | 20150219560 14/684943 |
Document ID | / |
Family ID | 44226061 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150219560 |
Kind Code |
A1 |
MAXWELL; Ian Andrew ; et
al. |
August 6, 2015 |
IN-LINE PHOTOLUMINESCENCE IMAGING OF SEMICONDUCTOR DEVICES
Abstract
Methods and systems are presented for acquiring
photoluminescence images (2) of silicon solar cells and wafers (4)
as they progress along a manufacturing line (36). In preferred
embodiments the images are acquired while maintaining motion of the
samples. In certain embodiments photoluminescence is generated with
short pulse, high intensity excitation, (8) for instance by a flash
lamp (50) while in other embodiments images are acquired in line
scanning fashion. The photoluminescence images can be analysed to
obtain information on average or spatially resolved values of one
or more sample properties such as minority carrier diffusion
length, minority carrier lifetime, dislocation defects, impurities
and shunts, or information on the incidence or growth of cracks in
a sample.
Inventors: |
MAXWELL; Ian Andrew; (Five
Dock, AU) ; TRUPKE; Thorsten; (Coogee, AU) ;
BARDOS; Robert Andrew; (Surry Hills, AU) ; ARNETT;
Kenneth Edmund; (Boulder, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BT IMAGING PTY LTD |
Redfern |
|
AU |
|
|
Family ID: |
44226061 |
Appl. No.: |
14/684943 |
Filed: |
April 13, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13520375 |
Oct 26, 2012 |
9035267 |
|
|
PCT/AU2011/000005 |
Jan 4, 2011 |
|
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14684943 |
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Current U.S.
Class: |
250/459.1 ;
250/208.1; 250/458.1 |
Current CPC
Class: |
G01N 2201/062 20130101;
G01N 2201/06113 20130101; G01N 2201/08 20130101; G01N 21/9501
20130101; G01N 21/6489 20130101 |
International
Class: |
G01N 21/64 20060101
G01N021/64 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 4, 2010 |
AU |
2010900018 |
Jul 9, 2010 |
AU |
2010903050 |
Sep 3, 2010 |
AU |
2010903975 |
Claims
1. A method for analysing a sample of a semiconductor material,
said method comprising the steps of: passing said sample to a
measurement zone; applying an illumination to said sample to
produce a photoluminescence response from said semiconductor
material; and conducting at least a photoluminescence analysis of
said sample in said measurement zone while maintaining motion of
said sample, wherein said photoluminescence analysis comprises
providing a multi-pixel image capture device for acquiring an image
of the photoluminescence emitted from said sample.
2. A method according to claim 1, wherein said method is performed
with a system that meets Class 1 accessible emission limits (AELs)
without said measurement zone being enclosed.
3. A method according to claim 1, wherein the illumination source,
the image capture device or an optical element associated with the
illumination source or the image capture device moves or pivots to
follow the motion of said sample.
4. A method according to claim 1, wherein said analysis is
performed in less than 1 second.
5. A method according to claim 1, wherein said illumination is
applied to an area of said sample and said image is acquired with
an area image capture device in an acquisition time t, wherein said
sample is moving at a speed v relative to said area image capture
device, and wherein the product of the acquisition time t (s) and
the speed v (ms.sup.-1) is less than a distance on said sample
corresponding to one row of pixels in said area image capture
device.
6. A method according to claim 5, wherein said method is applied to
a sample of a semiconductor material comprising silicon, and said
photoluminescence is generated with an illumination intensity
between about 10 and 100 Wcm.sup.-2.
7. A method according to claim 5, wherein said illumination is
applied to the entire area of a surface of said sample, and said
area image capture device acquires an image of the
photoluminescence emitted from said entire area.
8. A method according to claim 1, wherein said illumination is
provided by a source positioned so as to illuminate a first portion
of said sample and said image capture device is positioned so as to
detect photoluminescence emitted from a second portion of said
sample, wherein said first portion and said second portion are at
least partially overlapping, and wherein said method further
comprises the steps of: moving said sample relative to said source
and to said image capture device such that said second portion is
scanned across a substantial area of said sample; and interrogating
said image capture device repeatedly to acquire an image of the
photoluminescence emitted from said area.
9. A method according to claim 8, wherein said image of the
photoluminescence is acquired with a line camera or a time-delay
integration camera.
10. A method according to claim 8, wherein said first portion is
wholly within said second portion.
11. A method according to claim 8, wherein said source provides
broad area illumination of said sample.
12. A method according to claim 8, wherein said first and second
portions extend across a substantial fraction of a dimension of
said sample, said dimension being substantially perpendicular to
the direction of movement of said sample.
13. A system for conducting an analysis of a sample of a
semiconductor material, said apparatus comprising: a transport
mechanism for transporting said sample to a measurement zone;
analysis equipment for conducting at least a photoluminescence
analysis of said sample within said measurement zone, said analysis
equipment comprising a source of predetermined illumination
suitable for generating photoluminescence from said semiconductor
material, and a multi-pixel image capture device for acquiring an
image of the photoluminescence emitted from said sample; and motion
apparatus to maintain motion of said sample within said measurement
zone during said analysis.
14. A system according to claim 13, wherein said system is adapted
such that, in use, said system meets Class 1 accessible emission
limits (AELs) without said measurement zone being enclosed.
15. A system according to claim 13, wherein said source, said image
capture device or an optical element associated with said source or
said image capture device is capable of moving or pivoting to
follow the motion of said sample.
16. A system according to claim 13, wherein said source of
predetermined illumination is adapted to illuminate an area of said
sample, and said image capture device is an area image capture
device adapted to capture an image of said photoluminescence in an
image acquisition time t, wherein said motion apparatus moves said
sample at a speed v relative to said area image capture device such
that the product of the image acquisition time t (s) and the speed
v (ms.sup.-1) is less than a distance on said sample corresponding
to one row of pixels in said area image capture device.
17. A system according to claim 16, wherein said system is
configured to conduct an analysis of a sample of a semiconductor
material comprising silicon, and said source is configured to
generate photoluminescence with an illumination intensity between
about 10 and 100 Wcm.sup.-2.
18. A system according to claim 16, wherein said source is adapted
to illuminate the entire area of a surface of said sample, and said
area image capture device is adapted to acquire an image of the
photoluminescence emitted from said entire area.
19. A system according to claim 13, wherein said source is adapted
to illuminate a first portion of said sample, and said image
capture device is adapted to detect photoluminescence emitted from
a second portion of said sample, wherein said first portion and
said second portion are at least partially overlapping, and wherein
said analysis equipment further comprises an interrogation module
for interrogating said image capture device repeatedly while said
motion apparatus moves said sample such that said second portion is
scanned across a substantial area of said sample, to acquire an
image of the photoluminescence emitted from said substantial
area.
20. A system according to claim 19, wherein said image capture
device comprises a line camera or a time-delay integration
camera.
21. A system according to claim 19, wherein said first portion is
wholly within said second portion.
22. A system according to claim 19, wherein said source is adapted
to provide broad area illumination of said sample.
23. A system according to claim 19, wherein said first and second
portions extend across a substantial fraction of a dimension of
said sample, said dimension being substantially perpendicular to
the direction of movement of said sample.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to methods and systems for
performing photoluminescence analyses of semiconductor devices, and
of silicon solar cells in particular, during or after their
production process.
RELATED APPLICATIONS
[0002] The present application claims priority from Australian
provisional patent application Nos 2010900018, 2010903050 and
2010903975, the contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0003] Any discussion of the prior art throughout this
specification should in no way be considered as an admission that
such prior art is widely known or forms part of the common general
knowledge in the field.
[0004] Photoluminescence (PL) imaging, performed for example using
apparatus and methods disclosed in PCT Patent Application
Publication No WO 2007/041758 A1 entitled `Method and System for
Inspecting Indirect Bandgap Semiconductor Structure` and
incorporated herein by reference, has been shown to be of value for
the rapid characterisation of silicon materials and devices, and
silicon wafer-based solar cells in particular. As shown
schematically in FIG. 1, luminescence 2 generated from a
semiconductor sample 4 with broad area photo-excitation from a
source 6 of above-bandgap light 8 can be imaged with a camera or
CCD array 10 via collection optics 11, with the system preferably
including homogenisation optics 12 to improve the uniformity of the
broad area excitation and a long-pass filter 14 in front of the
camera to block excitation light. The system may also include one
or more filters 15 to select the wavelength range of the
photo-excitation. With relatively thin samples it is also possible
to have the excitation source 6 and camera 10 on opposite sides of
the sample 4 as shown in FIG. 2, in which case the sample itself
can serve as a long-pass filter. However a long-pass filter 14 may
still be required if a significant amount of stray excitation
light, reflected for example off other components, is reaching the
camera. Either way, the acquired PL image can be analysed with a
computer 16, using techniques disclosed for example in published
PCT patent application Nos WO 2008/014537 A1, WO 2009/026661 A1 and
WO 2009/121133 A1, to obtain information on average or spatially
resolved values of a number of sample properties including minority
carrier diffusion length, minority carrier lifetime, dislocation
defects, impurities and shunts, amongst others, or on the incidence
or growth of cracks. In principle the entire process can be
performed in a matter of seconds or fractions of a second depending
on factors such as the quality of silicon material and the readout
speed of the camera, which is a timescale generally compatible with
current solar block, cell and wafer production lines where, for
example, the throughput for wafer and cell lines is of order one
cell per second or two, and for block production where 30 seconds
is typically available for the measurement of a full block
face.
[0005] However the current PL imaging system as described above
suffers from a number of disadvantages.
[0006] One disadvantage is that currently available PL imaging
systems require samples to be removed from production lines and
taken to the PL imaging tool, for example using robotic or manual
pick and place handling. Manual pick and place handling is a labour
intensive and slow process that often adds cost as well as being
slow, and although robotic pick and place handling systems using
platens or suction cups or similar are somewhat faster, they also
add cost. Either way, the limited speed means only a small sample
of the product in process can be tested. It would be beneficial to
be able to measure all or most of the work product.
[0007] A further disadvantage is that in current PL imaging systems
the sample has to be stationary during the measurement to prevent
blurring of the image. A blurred image can prevent or compromise
the capture of spatially resolved characterisation data,
complicating the design and/or incorporation of a PL imaging system
into production lines that are increasingly operating in continuous
mode without stopping. To explain, with broad area 1 Sun excitation
the photoluminescence emitted from many silicon samples, and raw or
unpassivated silicon samples in particular, can be of such low
intensity that even the most sensitive commercially available
silicon-based CCD cameras require an exposure time of order at
least 0.5 second to acquire a sufficient PL signal.
[0008] Yet another disadvantage with current PL imaging systems is
the common reliance on laser excitation sources, typically in the
near IR region of the spectrum. To explain, obtaining a measurable
PL signal from low photoluminescence quantum efficiency samples
such as raw or unpassivated silicon wafers and blocks (with quantum
efficiency of order 10.sup.-6) often requires illumination
intensities of 0.1 Watts/cm.sup.2 (.about.1 Sun) or greater. A
total optical power of tens of Watts is therefore required to
illuminate silicon solar cell wafers that may typically be
15.6.times.15.6 cm.sup.2 in area, and laser excitation sources are
usually considered to be essential to provide the required spectral
purity and beam shaping. Furthermore for silicon samples the
excitation light is typically in the near IR region (750 to 1000
nm), which is potentially very harmful because the eye focuses
near-infrared light onto the retina but its protective `blink
reflex` response is triggered only by visible light. The potential
hazard of laser light sources arises from the fact that they may be
much brighter than other light sources, where the brightness (in
units of power per unit area per unit solid angle) may be defined
for example as the optical power passing through an aperture (e.g.
a laser output aperture) divided by the aperture area divided by
the solid angle subtended by the optical beam in the far field.
When an extremely bright light source is viewed with the eye,
either directly or via intermediate optics such as a collimating
lens, the image formed on the retina can be extremely intense,
resulting in virtually instantaneous and permanent damage. However
although there is less likelihood of this occurring with incoherent
near IR light, e.g. from high power LEDs, it needs to be understood
that because brightness is a key parameter, light safety issues
cannot simply be ignored just because a system uses non-laser
(incoherent) light sources.
[0009] Current PL imaging systems are therefore further complicated
by light safety issues, since the PL measurement chamber generally
must be optically isolated to avoid the risk of operators being
exposed to high brightness IR light that could cause eye damage.
This usually requires shutters, doors or equivalent mechanisms,
adding complexity and cost to the sample transfer mechanisms into
and out of the PL measurement chamber. Because of these
complications, the basic PL imaging apparatus shown in FIG. 1 or 2
requires several modifications if it is to be used safely and cost
effectively to characterise silicon solar cells on a production
line.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to overcome or
ameliorate at least one of the disadvantages of the prior art, or
to provide a useful alternative. It is an object of a preferred
form of the present invention to provide methods and systems for
acquiring photoluminescence images of semiconductor devices during
their production process without removing them from the production
line. It is another object of a preferred form of the present
invention to provide methods and systems for acquiring
photoluminescence images of semiconductor devices during their
production process without interrupting the motion of the devices
through the production line. It is another object of a preferred
form of the present invention to provide methods and systems for
acquiring photoluminescence images of semiconductor devices using
imaging systems that are eye-safe without requiring light safety
shutters. It is another object of a preferred form of the present
invention to provide methods and systems for acquiring
photoluminescence images of silicon wafers or cells with a total
measurement time between 0.1 and 1 second per wafer or cell. It is
another object of a preferred form of the present invention to
provide a photoluminescence imaging system that does not have or
require an integrated sample handling stage.
[0011] According to a first aspect the invention provides a method
for analysing a sample of a semiconductor material, said method
comprising the steps of:
[0012] passing said sample to a measurement zone;
[0013] applying an illumination to said semiconductor material to
produce a photoluminescence response; and
[0014] conducting at least a photoluminescence analysis of said
sample in said measurement zone while maintaining motion of said
sample.
[0015] According to a second aspect of the invention provides a
method of conducting a photoluminescence analysis of a sample of a
semiconductor material moving through a measurement zone, said
method comprising the steps of: applying an illumination to the
semiconductor material for a sufficient time and intensity to
produce a photoluminescence response; and capturing an image of the
photoluminescence emanating from said semiconductor material,
wherein said image capture is obtained within a distance of up to 1
or 2 pixels of the imaging camera.
[0016] Preferably, said photoluminescence analysis comprises the
steps of:
[0017] illuminating an area of said sample with a predetermined
illumination to generate photoluminescence from said sample in
response to said illumination; and
[0018] acquiring an image of said photoluminescence with an area
image capture device in an acquisition time t,
[0019] wherein said sample is moving at a speed v relative to said
area image capture device, and wherein the product of the
acquisition time t (s) and the speed v (ms.sup.-1) is less than a
distance on said sample corresponding to one row of pixels in said
image capture device.
[0020] Preferably said illumination comprises incoherent light.
Said illumination may comprise a pulse of light.
[0021] Preferably, said photoluminescence analysis comprises the
steps of:
[0022] providing a source of predetermined illumination suitable
for generating photoluminescence from said sample, said source
being positioned so as to illuminate a first portion of said
sample;
[0023] providing an image capture device for detecting the
photoluminescence emitted from a second portion of said sample,
wherein said first portion and said second portion are at least
partially overlapping;
[0024] moving said sample relative to said source and to said image
capture device such that said second portion is scanned across a
substantial area of said sample; and
[0025] interrogating said image capture device repeatedly to
acquire an image of the photoluminescence emitted from said
area.
[0026] Preferably said illumination comprises incoherent light.
Preferably the image capture device is a line camera.
[0027] Preferably said first portion is from one to five times
wider than said second portion in the direction of movement of said
sample.
[0028] Alternatively, the image capture device is a time-delayed
integration camera.
[0029] In one embodiment said first portion is substantially
coterminous with said second portion. In another embodiment, said
first portion is wholly or partially within said second portion. In
yet another embodiment, said second portion is wholly or partially
within said first portion.
[0030] Preferably, said first and second portions extend across a
substantial fraction of a dimension of said sample, said dimension
being substantially perpendicular to the direction of movement of
said sample.
[0031] The semiconductor material may be raw or unpassivated
silicon. In that case, it is preferred if said photoluminescence is
generated with an illumination intensity between about 1 and 40
Wcm.sup.-2.
[0032] Alternatively, said semiconductor material may be passivated
silicon. In that case, it is preferred if said photoluminescence is
generated with an illumination intensity between about 0.1 and 10
Wcm.sup.-2.
[0033] Preferably the illumination source and/or an optical element
associated therewith moves within the measurement zone. In that
case, it is preferred that motion of the illumination source and/or
an optical element associated therewith is controlled to maintain a
predetermined alignment with a sample. The predetermined alignment
may be to avoid blurring of illumination of the sample.
[0034] Preferably, the image capture device and/or an optical
element associated therewith moves within the measurement zone. In
that case, it is preferred that motion of the image capture device
and/or an optical element associated therewith is controlled to
maintain a predetermined alignment with a sample. The predetermined
alignment may be to avoid blurring of image capture of the
sample.
[0035] Preferably, illumination is introduced into the imaging
optical system using a dichroic mirror. In that case, it is
preferred if photoluminescence data captured passes through said
dichroic mirror.
[0036] Preferably said photoluminescence analysis provides
information on average or spatially resolved values of one or more
properties of said sample, said properties being selected from the
group consisting of minority carrier diffusion length, minority
carrier lifetime, dislocation defects, impurities and shunts.
[0037] Preferably said sample is a silicon wafer.
[0038] Preferably said analysis is performed in less than 1
second.
[0039] In one embodiment, the configuration of the measurement zone
remains constant before, during and after data acquisition. The
measurement zone may be a shuttered or enclosed chamber.
Alternatively, said measurement zone is unenclosed.
[0040] According to a third aspect the invention provides a method
for analysing a sample of a semiconductor material, said method
comprising the steps of:
[0041] passing said sample to a measurement zone; and
[0042] conducting at least a photoluminescence analysis of said
sample in said measurement zone, wherein said measurement zone is
eye-safe without being enclosed.
[0043] According to a fourth aspect the invention provides a method
for analysing a sample of a semiconductor material, said method
comprising the steps of:
[0044] passing said sample to a measurement zone without using a
pick and place sample handling system; and
[0045] conducting at least a photoluminescence analysis of said
sample in said measurement zone.
[0046] According to a fifth aspect the invention provides a system
for conducting an analysis of a sample of a semiconductor material,
said apparatus comprising:
[0047] a transport mechanism for transporting said sample to a
measurement zone;
[0048] analysis equipment for conducting at least a
photoluminescence analysis of said sample within said measurement
zone; and
[0049] motion apparatus to maintain motion of said sample within
said measurement zone during said analysis.
[0050] Preferably said analysis equipment comprises:
[0051] an optical source for illuminating an area of said sample
with a predetermined illumination to generate photoluminescence
from said sample in response to said illumination; and
[0052] an area image capture device for capturing an image of said
photoluminescence in an image acquisition time t, wherein said
motion apparatus moves said sample at a speed v relative to said
area image capture device such that the product of the image
acquisition time t (s) and the speed v (ms.sup.-1) is less than a
distance on said sample corresponding to one row of pixels in said
area image capture device.
[0053] Preferably said analysis equipment comprises:
[0054] a source of predetermined illumination suitable for
generating photoluminescence from said sample, said source being
positioned so as to illuminate a first portion of said sample;
[0055] an image capture device for detecting the photoluminescence
generated from a second portion of said sample, wherein said first
portion and said second portion are at least partially overlapping;
and
[0056] means for interrogating said image capture device repeatedly
while said motion means moves said sample such that said second
portion is scanned across a substantial area of said sample, to
acquire an image of the photoluminescence emitted from said
area.
[0057] Preferably the illumination source and/or an optical element
associated therewith moves within the measurement zone. In that
case, it is preferred that motion of the illumination source and/or
an optical element associated therewith is controlled to maintain a
predetermined alignment with a sample. The predetermined alignment
may be to avoid blurring of illumination of the sample.
[0058] Preferably, the image capture device and/or an optical
element associated therewith moves within the measurement zone. In
that case, it is preferred that motion of the image capture device
and/or an optical element associated therewith is controlled to
maintain a predetermined alignment with a sample. The predetermined
alignment may be to avoid blurring of image capture of the
sample.
[0059] Preferably, illumination is introduced into the imaging
optical system using a dichroic mirror. In that case, it is
preferred if the photoluminescence passes through said dichroic
mirror prior to image capture.
[0060] Preferably the configuration of the measurement zone remains
constant before, during and after data acquisition.
[0061] In one embodiment, said measurement zone is a shuttered or
enclosed chamber. In an alternative embodiment said measurement
zone is unenclosed.
[0062] According to a sixth aspect the invention provides a system
for conducting an analysis of a sample of a semiconductor material,
said system comprising:
[0063] a transport mechanism for transporting said sample to a
measurement zone;
[0064] a light source for illuminating an area of said sample to
generate photoluminescence from said sample; and
[0065] analysis equipment for conducting at least a
photoluminescence analysis of said sample within said measurement
zone, wherein said measurement zone is eye-safe without being
enclosed.
[0066] According to a seventh aspect the invention provides a
system for conducting an analysis of a sample of a semiconductor
material, said system comprising:
[0067] a transport mechanism for transporting said sample to a
measurement zone without using a pick and place sample handling
system; and
[0068] analysis equipment for conducting at least a
photoluminescence analysis of said sample in said measurement
zone.
[0069] According to a eighth aspect the invention provides a method
of analysing a sample of semiconductor material from within a
series of samples of semiconductor material, said method comprising
the steps of:
[0070] passing said sample to a measurement zone; and
[0071] acquiring photoluminescence data from said sample in said
measurement zone while enabling motion of other samples in said
series of samples.
[0072] In one embodiment said sample is in motion during
acquisition of photoluminescence data.
[0073] In an alternative embodiment said sample is stationary
during acquisition of photoluminescence data.
[0074] The other samples in said series of samples may be in motion
during acquisition of photoluminescence data, or alternatively, the
other samples in said series of samples are not in motion during
acquisition of photoluminescence data.
[0075] According to a ninth aspect the invention provides a method
of analysing a sample of semiconductor material, said method
comprising the steps of:
conveying said sample to a point adjacent to a measurement zone by
a delivery transport means; conveying said sample into, through and
out of said measurement zone by a measurement zone transport means;
acquiring photoluminescence data from said sample in said
measurement zone; and conveying said sample from a point adjacent
the measurement zone by a removal transport means, wherein the
measurement zone transport means is controllable independently of
said delivery transport means or said removal transport means.
[0076] In one embodiment, the measurement zone transport means
and/or sample is motionless during the acquisition of
photoluminescence data.
[0077] In an alternative embodiment, the measurement zone transport
means and/or sample is in motion during the acquisition of
photoluminescence data.
[0078] Preferably said photoluminescence is generated by
illumination with incoherent light.
[0079] Preferably said photoluminescence is generated by
illumination comprising a pulse of light.
[0080] In one embodiment, analysing a sample of semiconductor
material takes place in a shuttered or enclosed analysis
chamber.
[0081] In an alternative embodiment, analysing a sample of
semiconductor material takes place in an unshuttered or at least
partially open analysis chamber.
[0082] Preferably the sample is illuminated by illumination
introduced using a dichroic mirror.
[0083] Preferably the acquired photoluminescence data is analysed
to obtain information on average or spatially resolved values of a
sample property selected from the group consisting of minority
carrier diffusion length, minority carrier lifetime, dislocation
defects, impurities and shunts.
[0084] Preferably said sample is a silicon wafer.
[0085] Preferably said analysis is performed in less than 1
second.
[0086] According to a tenth aspect the invention provides a method
for analysing a sample of a semiconductor material in a measurement
zone, said method comprising a transport mechanism for moving and
supporting said sample, wherein said transport mechanism contacts
no more than 10% of said sample, thereby leaving 90% of said sample
exposed at all times for analysis.
[0087] According to a eleventh aspect the invention provides a
method for analysing a sample of a semiconductor material in a
measurement zone, said method comprising a transport mechanism for
movement and support of said sample during analysis wherein during
said analysis at least a portion of said sample is left unsupported
across its entire width to provide an unobstructed region for said
analysis whereby, as a result of continued movement, the entire
sample is progressively unobstructed.
[0088] According to a twelfth aspect the invention provides a
system for analysing a sample of semiconductor material, said
system comprising:
a delivery transport means to convey said sample to a point
adjacent to a measurement zone; a measurement zone transport means
for conveying said sample into, through and out of said measurement
zone; an illumination means for generating photoluminescence from
said sample in said measurement zone; a detector for detecting said
photoluminescence; and a removal transport means for conveying said
sample from a point adjacent the measurement zone, wherein the
measurement zone transport means is controllable independently of
said delivery transport means or said removal transport means.
[0089] Preferably said illumination means emits incoherent
light.
[0090] Preferably said illumination means emits a pulse of
light.
[0091] Preferably the delivery transport means, measurement zone
transport means and removal transport means are independently
selected from the group consisting of a belt, a series of rollers,
a series of platens, a plurality of aligned belts and a vacuum
chuck.
[0092] Preferably the measurement zone transport means is a
plurality of aligned belts configured to support opposed sides of a
semiconductor sample, the area between said belts defining an
unobstructed central portion of the semiconductor sample.
[0093] Preferably light from the illumination means is introduced
to the sample using a dichroic mirror.
[0094] In one embodiment, analysing a sample of semiconductor
material takes place in a shuttered or enclosed analysis
chamber.
[0095] In an alternative embodiment, analysing a sample of
semiconductor material takes place in an unshuttered or at least
partially open analysis chamber.
[0096] Preferably the delivery transport means, the measurement
zone transport means and the removal transport means do not use a
pick and place sample handling system.
[0097] Preferably an acquired photoluminescence image is analysed
to obtain information on average or spatially resolved values of a
sample property selected from the group consisting of minority
carrier diffusion length, minority carrier lifetime, dislocation
defects, impurities and shunts.
[0098] Preferably said sample is a silicon wafer.
[0099] Preferably said analysis is performed in less than 1
second.
[0100] According to a thirteenth aspect the invention provides a
system for analysing a sample of a semiconductor material in a
measurement zone, said system comprising a transport mechanism for
moving and supporting said sample, wherein said transport mechanism
contacts no more than 10% of said sample, thereby leaving 90% of
said sample exposed at all times for analysis.
[0101] According to a fourteenth aspect the invention provides a
system for analysing a sample of a semiconductor material in a
measurement zone, said system comprising a transport mechanism for
movement and support of said sample during analysis wherein during
said analysis at least a portion of said sample is left unsupported
across its entire width to provide an unobstructed region for said
analysis whereby, as a result of continued movement, the entire
sample is progressively unobstructed.
[0102] According to a fifteenth aspect, the present invention
provides a production line for the production of a photovoltaic
device, said production line comprising a plurality of process
steps to convert a semiconductor material to said photovoltaic
device, said production line including at least one analysis device
comprising a illumination source for application to a semiconductor
material, and a non-stop image capture device for obtaining an
image of photoluminescence emanating from said illuminated
semiconductor material without stopping the semiconductor
material.
[0103] According to a sixteenth aspect, the present invention
provides a production line for the production of photovoltaic
device, said production line comprising a plurality of process
steps to convert a semiconductor material to said photovoltaic
device, said production line including at least one eye-safe
analysis device having a high intensity illumination system for
applying illumination with an intensity of greater than 10 Suns to
the semiconductor material, and an image capture device for
obtaining an image of photoluminescence emanating from said
illuminated semiconductor material, said analysis device being
adapted to illuminate said semiconductor material and capture an
image of photoluminescence emanating from said illuminated
semiconductor material without having to stop said semiconductor
material as it moves through said production line.
BRIEF DESCRIPTION OF THE DRAWINGS
[0104] Benefits and advantages of the present invention will become
apparent to those skilled in the art to which this invention
relates from the subsequent description of exemplary embodiments
and the appended claims, taken in conjunction with the accompanying
drawings, in which:
[0105] FIG. 1 illustrates a prior art system for PL imaging of a
semiconductor sample;
[0106] FIG. 2 illustrates another prior art system for PL imaging
of a semiconductor sample;
[0107] FIG. 3 illustrates a system suitable for in-line PL imaging
of semiconductor samples;
[0108] FIG. 4 shows in plan view the positioning of a semiconductor
sample on a transport belt;
[0109] FIG. 5 illustrates in side view a PL imaging system
according to an embodiment of the invention;
[0110] FIG. 6 shows in plan view an arrangement of a flash lamp and
camera according to a preferred embodiment of the invention;
[0111] FIG. 7 shows in side view a system for PL imaging of a
semiconductor sample;
[0112] FIGS. 8A and 8B show in plan view and side view respectively
a line camera system for acquiring PL images from a continuously
moving sample;
[0113] FIG. 9 shows in side view another line camera system for
acquiring PL images from a continuously moving sample;
[0114] FIGS. 10A and 10B show in side view and plan view
respectively an illuminator for a line camera PL system;
[0115] FIGS. 11A and 11B show in side view and plan view
respectively a system of collection optics for a line camera PL
system;
[0116] FIG. 12 shows another system of collection optics for a line
camera PL system; and
[0117] FIG. 13 shows in side view a TDI camera system for acquiring
PL images from a continuously moving sample.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0118] Preferred embodiments of the invention will now be
described, by way of example only, with reference to the
accompanying drawings.
[0119] FIG. 3 shows a PL imaging system according to one embodiment
of the present invention, suitable for acquiring photoluminescence
images of semiconductor devices during their production process
without removing them from the production line. This system,
hereinafter referred to as a `three-belt system`, includes two
outer transport belts 18 to interface with a continuous on-belt
production line or pick and place handling robots or other handling
mechanism common to a production line and an inner transport belt
20 to bring the sample 4 to a stop for measurement, and optionally
light-tight shutters 22 that open to allow the samples in and out
of the measurement chamber 24 to satisfy light safety requirements
if required, i.e. if the system would not otherwise be eye-safe. In
alternative embodiments the sample is moved with some other
transport mechanism such as rollers, platens or vacuum chucks
rather than transport belts.
[0120] Although the apparatus shown in FIG. 3 has the excitation
source 6 and camera 10 on the same side of the sample as in FIG. 1,
the FIG. 2 arrangement is also possible because, as shown in plan
view in FIG. 4, the inner transport belt 20 at least can be in
split form, allowing a substantial portion 26 of the sample 4 to be
illuminated and/or imaged. By way of concrete example, for a 15.6
cm.times.15.6 cm wafer supported on a pair of 5 mm wide transport
belts, up to 93% of the wafer surface area can be available for
illumination or imaging.
[0121] This three-belt system is a major advancement over the
current PL imaging sampling systems, since it enables measurement
of all or nearly all samples in a production process, subject to
both line and tool measurement speed. In this context, it
alleviates one of the major disadvantages of current PL imaging
systems.
[0122] However it would be advantageous, especially with fragile
wafers, or on large or fast production lines, to avoid any
stop/starting of individual wafers, especially since the rate of
stopping and starting becomes throughput limiting as overly rapid
acceleration or de-acceleration of samples on a belt may lead to
breakages or sample slip. It would be particularly advantageous to
have a system that can not only measure all or most of the samples
in a production process, but that does not require the sample to
stop for measurement, and requires few or no light safety measures.
In addition the absence of an expensive sample handling apparatus,
for example using robotic pick and place sample handling utilising
platens, suction cups or the like, integrated into the PL measuring
system would be of economic benefit.
[0123] Specifically, it would be especially advantageous to have a
PL imaging system that just included a camera, a light source and
optics as the major hardware components. Such a system could be
placed anywhere in a production line without requiring special
modifications, for example above a transport belt bearing samples
along the line. The invention is now described with reference to
various systems and methods for acquiring PL images of
semiconductor devices, such as fully or partially manufactured
silicon solar cells, during their production process without
interrupting their motion through the production line. The
invention is described with reference to configurations where the
imaging device and illuminator are stationary with the samples
moving past them, for example, on a system of transport belts or
robot grippers, however it should be noted that other
configurations are also within the scope of the present invention.
For example blurring can be avoided during long exposures by moving
or pivoting the imaging device and/or its associated optics to
follow the sample movement. In some embodiments the excitation
source and/or its associated optics can also be moved or pivoted to
follow the sample movement.
[0124] Before describing further embodiments of PL imaging systems,
and in particular those preferred embodiments with reduced or no
light safety requirements, it will be useful to include some
discussion of current laser safety standards and some strategies
for producing PL imaging systems with reduced light safety
requirements. As mentioned previously the brightness of a source,
which can be defined as the optical power passing through an
aperture divided by the aperture area divided by the solid angle
subtended by the optical beam in the far field, is a key parameter,
and light safety issues cannot simply be ignored just because a
system uses non-laser (incoherent) light sources.
[0125] In Australia and New Zealand, the standards for laser
classification and safety requirements are provided by AS/NZS
2211.1:2004 and its associated guidelines (AS/NZS 2211.10:2004),
based on the international standard IEC 60825-1:2001. An important
concept in laser safety is the `Maximum Permissible Exposure` (MPE)
level, which is defined in the standard as `that level of laser
radiation to which, under normal circumstances, persons may be
exposed without suffering adverse effects`. The definition further
states that `MPE levels represent the maximum level to which the
eye or skin can be exposed without consequential injury
immediately, or after a long time, and are related to the
wavelength of the radiation, the pulse duration or exposure time,
the tissue at risk and, for visible and near infra-red radiation in
the range of 400 nm to 1 400 nm, the size of the retinal
image`.
[0126] Since the wavelengths of light suitable for generating PL
from silicon are within this 400 to 1400 nm range, it follows that
retinal image size is a key factor for light safety in PL imaging
systems. Within certain limits, the MPE level increases with
increased image size on the retina, although there is no decrease
in the MPE below a certain minimum image size and no increase above
a certain maximum image size. For quantitative purposes the
standard uses an angular measure of the retinal image size, the
angle subtended by the source at the eye, .alpha.. This is
generally referred to as the `angular subtense` and is given
approximately by the source size divided by the distance between
the source and the eye. The angular subtense representing the image
size below which there is no further decrease in the MPE is
referred to as `.alpha..sub.min` (1.5 mrad), and exposure
conditions below this are referred to as `point source viewing`.
`Extended source viewing` conditions apply at angular subtenses
above .alpha..sub.min, and as the angular subtense increases from
.alpha..sub.min the MPE level increases until it reaches a maximum
at .alpha.=.alpha..sub.max (100 mrad), beyond which the MPE is
constant. It is important to note that if the source radiation is
modified by illumination optics, as shown in FIGS. 1-3 for example,
the `apparent source` for MPE purposes is the image, real or
virtual, that produces the smallest retinal image. For the purposes
of this specification, the term `illuminator` will be used to refer
to the portion of a PL imaging system that provides optical
excitation to a sample. An illuminator will include one or more
optical sources, possibly in combination with a number of other
components including filters and focusing optics.
[0127] In the standards, laser products are classified in a system
ranging from Class 1, `safe under reasonably foreseeable conditions
of operation`, to Class 4, `generally powerful enough to burn skin
and cause fires`, using limits known as `accessible emission
limits` (AELs). AELs are derived from MPEs using limiting apertures
and may be expressed as a power limit, an energy limit, an
irradiance limit, a radiant exposure limit, or a combination
thereof. The limiting aperture is usually taken to be 7 mm,
representing a dilated pupil as a `worst case scenario`. Although
meeting Class 1 AELs is necessary but not sufficient for making a
laser product Class 1, there being other constraints, for the
purposes of this specification a PL imaging system as a whole will
be considered to be `eye-safe` if it meets Class 1 AELs. Similarly,
the illuminator portion of an imaging system will be considered to
be `eye-safe` if it meets Class 1 AELs.
[0128] Relatively high brightness sources, typically required for
acquiring PL images of silicon PV samples on a timescale suitable
for in-line applications, are potentially hazardous because they
can result in a relatively high intensity at the eye, even at a
distance, or a relatively small retinal image (and correspondingly
low MPE level). However to determine the actual hazard, it is
necessary to consider brightness in combination with the viewing
conditions, in particular the angular subtense. The importance of
viewing conditions is demonstrated by the following specific
example. According to the calculation methodology prescribed in IEC
60825-1:2001, an 808 nm cw laser product can only be classified as
Class 1 (i.e. does not exceed the Class 1 AEL) if its emission
under point source viewing conditions (i.e. angular subtense
.alpha.<.alpha..sub.min) does not exceed 0.64 mW through a 7 mm
diameter limiting aperture. In contrast, for extended source
viewing conditions where .alpha.>.alpha..sub.max (100 mrad), the
Class 1 AEL is 42 mW (i.e. 65.times. higher) through a 7 mm
diameter limiting aperture.
[0129] The brightest light sources in common use are laser sources,
which have high temporal coherence (or equivalently, coherence
length) compared to non-laser (i.e. thermal) sources. Since
coherence is an inherent aspect of the lasing process, higher
coherence than thermal sources may be considered a necessary
condition for achieving the highest brightness practical light
sources. However coherence does not imply brightness, as it is not
a sufficient condition. In general, coherence length varies widely
(over orders of magnitude) between different laser types, but this
does not necessarily correlate with brightness. For example the
coherence length of a laser source can be increased by using a high
quality factor (Q) resonator at the expense of output power,
meaning that while the beam collimation (the `per unit solid angle`
part of the brightness definition) may be increased, the reduced
output power reduces the `power per unit area` part of the
brightness definition, counteracting the potential increase in
brightness.
[0130] Optics can be added to a light source to reduce the
brightness without altering the coherence, a trivial example being
an absorbing filter which may be used to reduce the brightness
arbitrarily without altering the coherence. Of significant
practical relevance for PL imaging systems of the present invention
are illuminator designs which have reduced brightness without
significantly reducing the intensity on the sample, typically all
or part of a wafer or PV cell. In certain embodiments this is
achieved in a second illuminator (`system 2`) compared to an
unimproved, prior art illuminator (`system 1`) by one or both
of:
(i) Increasing the solid angle filled by the light output from
system 2 relative to that of system 1. This may be expressed as
decreasing the `f number` or increasing the Numerical Aperture of
the illuminator, and essentially the excitation light is made to
diverge more rapidly so that its intensity at a distance is
reduced. (ii) Increasing the size of the source (real or apparent,
as discussed above in the context of illumination optics) in system
2, for example by dividing a single beam in system 1 into one or
more beams, or an array of beamlets, in system 2, or by
mechanically agitating a component of the illumination system (e.g.
a mirror). If system 1 already uses a number of beamlets, their
number may be significantly increased in system 2.
[0131] Approach (i) decreases the intensity of light at the eye,
while approach (ii) increases the angular subtense a which, subject
to the limits described above, may increase the MPE level as
follows:
(a) If .alpha. for system 1 was greater than .alpha..sub.min and
less than .alpha..sub.max, then the MPE level for system 2 is
greater than for system 1. (b) If .alpha. for system 1 was less
than .alpha..sub.min and .alpha. for system 2 is greater than
.alpha..sub.min, then the MPE level for system 2 is also greater
than for system 1. [0132] (c) If .alpha. for system 1 was less than
.alpha..sub.min and .alpha. for system 2 is also less than
.alpha..sub.min, then the MPE level for system 2 is the same as for
system 1.
[0133] By means of one or both of these measures, it is possible
for an illuminator to meet Class 1 AELs (i.e. be eye-safe) even
when the source itself is rated as high as Class 4. If the
illuminator does not meet Class 1 AELs, with or without these
measures, it is still possible for a PL imaging system as a whole,
or such system integrated into a production line or other
wafer/cell handling system, to meet Class 1 AELs without resorting
to stringent laser safety measures such as safety shutters and
interlocks. This represents a significant simplification for the
system integration; for example the configuration shown in FIG. 3
would be simplified considerably if the light-tight shutters 22
were not required and the measurement chamber 24 did not have to
enclose the imaging system on all sides. Instead, the PL system
itself or the production line guarding may provide some minimum
human access distance from the illuminator, and the PL system can
prevent direct viewing of the illuminator output, i.e. viewing will
be limited to reflections from a wafer or solar cell or some object
in the PL system or production line. Reflections off sample edges
are of particular concern, since broken wafers may present
mirror-like edge surfaces at unpredictable angles. Reducing the
illuminator brightness by increasing the divergence angle of the
excitation light (approach (i) described above) is particularly
useful in combination with measures that provide a minimum human
access distance. All these details need to be considered in
determining if a PL imaging system meets Class 1 AELs.
[0134] To summarise, it is preferred for a PL imaging system as a
whole, or such system integrated into a production line or other
wafer/cell handling system, to meet Class 1 AELs without resorting
to stringent laser safety measures such as safety shutters and
interlocks. More preferably, the illuminator meets Class 1 AELs.
With these light safety considerations in mind, we now turn to the
description of certain preferred embodiments of PL imaging systems
for in-line inspection of silicon solar cell samples. For both area
illumination schemes and line illumination schemes, the above
described approaches can be applied to reduce light safety
requirements.
[0135] In a first embodiment, referred to hereinafter as the `flash
lamp` approach and illustrated schematically in FIG. 5, a
substantial area (preferably at least 1 cm.times.1 cm, more
preferably the entirety) of a sample 4 moving through a measurement
zone on a transport belt 36 is illuminated with a short pulse of
excitation light 8 from one or more high intensity sources 50 such
as a xenon flash lamp or a pulsed LED, and the resulting PL
emission 2 from that area acquired with an area camera 10. In one
specific example, a Broncola ring flash C produces a 1 millisecond
pulse that, after passing through an excitation filter 15 (a 6 mm
thick KG1 Schott glass short pass filter), illuminates a silicon
sample with an intensity of 10-100 W/cm.sup.2 (100 to 1000 Suns),
and an image of the PL emission acquired with a 1 Megapixel silicon
CCD camera. The system may be surrounded by a cylindrical reflector
52 if greater illumination intensity on the sample is required. The
system can also include collection optics 11 and a long pass filter
14 as in the FIG. 1 system, and a shroud 54 to prevent excitation
light entering the camera. We note that although the overall speed
of the system may be limited by the camera readout time, depending
on the camera technology, this does not affect its ability to
acquire PL images of moving samples with minimal blurring. In the
context of the present invention this is the primary advantage of
high intensity, short pulse illumination, high intensity
illumination, for example up to 1000 Suns (100 W/cm.sup.2) also
provides surprising benefits for PL image clarity and for
identifying certain defects.
[0136] In the embodiment illustrated in FIG. 5, the flash lamp 50
is ring-shaped with the camera 10 centrally mounted, enabling both
to be pointed orthogonally to the surface of a sample for greater
illumination and imaging uniformity compared to configurations such
as those shown in FIG. 1 where one or both of the illumination
source 6 and camera 10 is angled with respect to the surface of the
sample 4. This arrangement, shown schematically in plan view in
FIG. 6, also has the benefit of allowing an overall more compact
system and, more importantly, the camera and flash lamp can both be
closer to the sample without obstructing the field of view or
casting a shadow. Having the flash lamp and camera closer to the
sample will generally improve the efficiency of both the
illumination and PL collection systems.
[0137] In an alternative embodiment illustrated in FIG. 7, a
dichroic mirror 43 is used to introduce the illumination 8 from a
flash lamp 50 (or any other source of illumination suitable for
generating PL) into the imaging optical system, so that the working
distance can be optimised entirely for the imaging system. To
mitigate the possibility of any PL generated from the dichroic
mirror itself reaching the camera 10, it is preferable for the
dichroic mirror to be placed well away from the sample so that the
collection optics 11 do not effectively focus any such PL emission
into the camera. If the system of collection optics has multiple
elements as shown in FIG. 7, an advantageous position for the
dichroic mirror is close to the first optical element 11a.
[0138] If the excitation light is from a broad band source such as
a flash lamp, the excitation filter 15 becomes an important
component because of the necessity to prevent longer wavelength
excitation light (overlapping the PL emission band) from reaching
the camera. Although dielectric filters have sharper transitions
from high to low transmission than absorption filters, which is
especially important for indirect band gap materials where the PL
emission is orders of magnitude weaker than the illumination, their
transmission has a strong angular dependence causing the
cut-on/cut-off wavelength to vary with incidence angle. The
coherent, directional emission from lasers is readily collimated
for efficient filtering with dielectric filters, but this is much
more difficult to achieve with the incoherent and essentially
isotropic emission from flash lamps or LEDs, favouring absorption
filters (such as the KG1 Schott glass filter mentioned above) or a
combination of absorption and dielectric filters. We note that
lamps that emit over a narrow wavelength range, such as low
pressure sodium lamps that emit an extremely narrow doublet around
590 nm, may be advantageous in that the illumination can be easily
separated from the silicon PL emission.
[0139] Apart from having less abrupt transitions from high to low
transmission, absorption filters may also suffer from a heating
problem, especially for the in-line inspection of solar
cells/precursors where the flash lamp may need to be activated at a
frequency of order 1 Hz or higher to inspect every sample. There
are several possible ways for dealing with such a heating problem,
including efficient air or liquid cooling of a solid absorption
filter, and using liquid filters where an absorbing liquid is
re-circulated through a flow cell, composed of glass for example,
and if necessary through a heat exchanger. Solutions of organic
dyes, for example a combination of the IRA 955 and IRA 1034
infrared absorbers from Exciton, Inc, may be suitable for removing
excitation light in the PL emission band. UV stability of organics
may be an issue when filtering flash lamp emission, but most UV
light can be blocked with a judicious choice of glass flow cell
material or addition of UV absorbing material in the filter or in
the cooling liquid if used, and in any event the optimal solution
for a given system of flash lamp, sample material and camera
technology may well involve a combination of filters and cooling
techniques.
[0140] In systems with flash lamp or other short pulse excitation,
the image acquisition time will be determined by the overlap of the
illumination time and the camera shutter time, and to minimise
blurring it is generally advantageous for both to be short. In
addition the illumination time (pulse duration) should be short to
reduce power consumption and avoid excessive sample heating,
bearing in mind that high illumination intensity is generally
required to generate sufficient PL signal within a short
acquisition time. Leaving the camera shutter open too long may
cause image blurring if the radiative lifetime is sufficiently long
for the sample to move a significant distance (e.g. by a distance
corresponding to one or two camera pixels) before the PL emission
has decayed, although this is only likely to be a problem for very
high carrier lifetime samples such as passivated monocrystalline
silicon where the lifetime can exceed several milliseconds. This
effect is expected to be negligible for typical multicrystalline
silicon wafers where the carrier lifetime is of order hundreds of
microseconds at most.
[0141] For preference, the illumination will be provided by a
pulsed excitation source with which the camera shutter is
substantially synchronised. The excitation source may for example
be a xenon flash lamp, a halogen flash lamp, a photographic flash,
an LED or a laser, singly or in an array, with a wavelength range
suitable for exciting band-to-band PL from the sample. Preferably
the illuminator should be eye-safe to minimise light safety
requirements. More preferably the illumination should be
incoherent, i.e. the illumination source should not be a laser,
although as mentioned previously incoherent illumination is not
necessarily eye-safe. We note that flash lamps (and to a lesser
extent LEDs) are advantageous in this regard because they are
extended sources, implying that their emission cannot be focused to
a point on the retina or, equivalently, limiting the minimum
retinal image size. In preferred embodiments the image acquisition
time is sufficiently short that the sample moves by a distance of
no more than that corresponding approximately to one row of pixels
in the imaging camera. This guideline depends on the speed of
movement of the samples and on the number of pixel rows in the
camera, but by way of example only, for a process line throughput
of 1 wafer per second and a 1 megapixel camera (1024.times.1024
pixels), this guideline would suggest an image acquisition time of
duration 1 millisecond (ms) or less, which is of order a thousand
times less than the time permissible if the wafer were to be
stopped for measurement. This thousand-fold decrease in acquisition
time needs to be compensated by a thousand-fold increase in
measurement speed, measurement speed being defined as the
luminescence signal, quantified for example as counts per pixel,
detected per second. It will be seen that the requisite increase in
measurement speed can be provided by some combination of increased
illumination intensity, improved PL collection efficiency and
different camera technologies and operation.
[0142] In a second embodiment, referred to hereinafter as the `line
scan` approach, a line camera (e.g. silicon or indium gallium
arsenide) is used instead of an area camera, and a 2-dimensional PL
image acquired line-by-line as the sample passes through the
measurement zone before the line camera. The illumination can be
broad area but for efficiency purposes it is preferable to
illuminate only the linear portion corresponding to the line
camera's view, preferably with some degree of `over-filling` of the
illumination so that the illuminated and imaged areas don't have to
be precisely aligned. A suitable system is shown schematically in
FIGS. 8A (plan view) and 8B (side view), where the excitation
source 6, line camera 28 and associated optics have been omitted
from the plan view for clarity. The system includes focusing optics
30 to focus the excitation light 8 onto the sample 4 such that the
illuminated portion 32 is coterminous with or slightly wider than
the imaged portion 34, and collection optics 11 to image the PL
emission 2 onto the line camera, as well as various other
components (homogenisation optics 12, long pass filter 14,
excitation filter 15 and computer 16) if required, similar to the
FIG. 1 system. In this particular example the width 33 of the
illuminated portion 32 is approximately three times larger than the
width 35 of the imaged portion 34, corresponding to a one pixel
margin either side of the imaged portion. However if the available
excitation power is a limiting factor, the illuminated and imaged
portions can be substantially coterminous. The sample is moved
through the measurement zone on transport belts 36, from left to
right in this case as indicated by the arrow 38, so that the
illuminated and imaged portions are effectively scanned across the
sample. The sample illumination will generally be continuous rather
than pulsed, and the excitation source may for example be an array
of LEDs or lasers, although for preference it is an incoherent
source, i.e. not laser-based, and the illuminator as a whole is
eye-safe. We note that from a light safety perspective line
illumination systems can be advantageous because the eye cannot
focus both axes simultaneously, limiting the minimum retinal image
size. The illuminated and imaged portions need not be oriented
perpendicularly to the direction of motion 38 as shown in FIG. 8A,
so long as they extend across the full width of the sample or at
least the full width of the sample area that needs to be measured.
However a substantially perpendicular orientation minimises the
area to be illuminated, thereby minimising the power requirement
for the excitation source, and is therefore to be preferred.
[0143] Another suitable system is shown schematically in side view
in FIG. 9. In this case the transport belts 36 have a gap or
unobstructed region 39 to allow the excitation source 6 and line
camera 28 to be on opposite sides of the sample 4. In this
particular embodiment the sample is illuminated through the gap,
but in an alternative embodiment the PL emission 2 could be imaged
through the gap. It will be appreciated that as a result of
continued movement across the gap, the entire sample will be
progressively unobstructed for illumination or imaging. In yet
another alternative embodiment, suitable for samples that need not
be analysed across their entire width (similar to the situation
shown in FIG. 4), the gap in the transport belts is omitted.
[0144] To maximise the spatial resolution of the `line scan`
approach, the width of the imaged portion 34 should correspond to
one row of pixels in the line camera 28. To achieve the same
spatial resolution as would be obtained from a 1 Megapixel area
camera, it is necessary to compensate for a thousand-fold decrease
in PL signal acquisition time with a thousand-fold increase in
measurement speed. This is essentially equivalent to the
measurement speed increase for the `flash lamp` approach described
above.
[0145] We turn now to a more detailed discussion of the
illumination and imaging parts of a line scan PL imaging system. As
shown in FIGS. 10A and 10B (side view and plan view respectively),
a line illuminator can comprise an optical fibre coupled laser or
LED array 56 and focusing optics 30 comprising a pair of
cylindrical lenses 58, 60 with dimensions and focal lengths chosen
according to the requirements of the specific system. As shown in
FIG. 10A, excitation light 8 emerging from the optical fibre 62 is
collimated in one direction by the first cylindrical lens 58, then
focused to a line 64 by the second cylindrical lens 60. As shown in
FIG. 10B the excitation light 8 continues to diverge in the
orthogonal direction, to cover the full width of the sample. This
arrangement tends to produce an approximately Gaussian intensity
distribution along the line 64, determined largely by the optical
fibre output, which is acceptable for PL imaging since it can be
corrected with a calibration procedure, so long as the illumination
intensity in the outer regions is sufficient to produce a
measurable PL response. Line illuminators that produce more uniform
intensity distributions are also known in the art.
[0146] Turning now to consideration of collection optics, one
possible system, comprising an arrangement of four cylindrical
lenses with dimensions and focal lengths chosen according to
specific apparatus requirements, is illustrated in FIGS. 11A (side
view) and 11B (plan view). PL emission 2 from the illuminated line
64 is collimated in one direction with a first cylindrical lens 72,
demagnified with a pair of cylindrical lenses 74, 76 in a `beam
expander` configuration, and focused onto the line camera 28 with a
fourth cylindrical lens 78. A number of other possible systems of
collection optics with varying degrees of complexity will occur to
those skilled in the art, using components such as lenses, mirrors
and optical waveguides. For example FIG. 12 illustrates
schematically a system where an array 68 of widely spaced optical
fibres collects PL emission 2 from an illumination line 64,
optionally with the aid of lenses, and guides it to a line camera
28. Generally speaking the choice will be informed by factors such
as cost (with off-the-shelf optical components preferred over
custom-made components) and the required collection efficiency; for
example if the PL signal is relatively strong, or if a high
sensitivity camera is used, a standard camera lens may suffice.
[0147] An important difference between `line scan` imaging
(illustrated in FIGS. 8A, 8B and 9) and the more conventional area
imaging (illustrated in FIGS. 1 and 5 for example) is that the
light gathering portion of the collection optics (lenses 72 and 74
in FIGS. 11A and 11B) can be located quite close to the sample
without obscuring the illumination, greatly enhancing the
collection efficiency (which may be defined as the number (or rate)
of luminescence photons that are detected divided by the number (or
rate) of luminescence photons emitted by the sample into a
hemisphere). Depending on the design details, we estimate that the
collection optics shown in FIGS. 11A and 11B are two or three
orders of magnitude more efficient than the typical collection
optics of a conventional `area imaging` (FIG. 1) PL system,
quantified below in the `baseline example`.
[0148] Similarly, the illuminator can be located quite close to the
sample without obscuring the PL collection optics, which can be
advantageous for light safety. To explain with reference to FIG.
10B, the closer the illuminator is to the sample, the greater the
divergence of the excitation light 8 impinging on the sample,
reducing the brightness of the illuminator.
[0149] Under quasi steady state conditions, a reasonable
approximation even for millisecond-level illumination times as may
be used with a flash-lamp for example since the minority carrier
lifetime in silicon as used in the solar industry is typically of
order 10 to 100 .mu.s, the minority carrier concentration An (which
affects the PL intensity) and the generation rate G (determined by
the illumination intensity among other things) are related by the
equation .DELTA.n=G*.tau. where .tau. is the minority carrier
lifetime. From this it follows that, for a given illumination
intensity, a stronger PL signal will be obtained from a sample with
a longer carrier lifetime, e.g. monocrystalline silicon compared to
multicrystalline silicon, or a passivated silicon wafer (in later
stages of a solar cell line) compared to a raw silicon wafer.
Although the minority carrier lifetime .tau. can only be considered
to be constant at low injection levels (i.e. low .DELTA.n), in
general a stronger PL signal will also result from more intense
illumination, i.e. larger generation rate G, which is an important
aspect of the `flash lamp` approach described previously.
[0150] As described in PCT patent application No AU2010/001045
entitled `Photoluminescence imaging systems for silicon
photovoltaic cell manufacturing`, incorporated herein by reference,
it is sometimes preferable when performing PL imaging of silicon
samples to use a camera technology such as indium gallium arsenide
(InGaAs) that, unlike silicon-based cameras, is sensitive across
the entire silicon PL emission spectrum. All other things being
equal, we estimate that replacing a silicon camera with an InGaAs
camera improves measurement speed by some 20.times.. Measurement
speed can also be improved by using cameras with larger pixels or,
at the expense of spatial resolution, binning pixels.
[0151] A third embodiment, somewhat similar to the `line scan`
approach, uses a time delay integration (TDI) camera. A TDI camera
can be thought of as an integrated array of line cameras, e.g. 96
or 128 lines of 1024 pixels on a single chip, typically using the
same silicon CCD technology as in conventional line or area
cameras. TDI cameras are well suited for acquiring images of a
moving sample, with the direction of movement perpendicular to the
pixel lines: as the sample is moved, the charge from the detected
signal is transferred to the next pixel line and accumulated, with
synchronisation of the transport speed and the charge transfer.
Consequently, a TDI camera with N pixel lines measures the signal
from a given portion of a sample N times, improving the
signal-to-noise ratio by a factor of N compared to a line camera
for the same total measurement time. A suitable system for this
`TDI camera` approach is shown schematically in side view in FIG.
13, and it will be seen that the configuration is quite similar to
the line scan system shown in FIG. 8B but with a TDI camera 40 in
place of the line camera. FIG. 13 also shows a transport belt drive
unit 42 controlled by the computer 16, in accordance with the need
to synchronise the motion of the sample 4 with the TDI camera
operation. A significant but less obvious difference with the line
scan system is that for a TDI camera with N pixel lines, the imaged
portion 34 needs to be N times wider than for the line scan system.
With a one pixel margin either side of the imaged portion as for
the `line scan` configuration shown in FIGS. 8A and 8B, the
illuminated portion 32 will be a factor of (N+2)/3 wider than in
the line scan configuration; alternatively the illuminated portion
will be N times wider than in the line scan configuration if the
illuminated and imaged portions are coterminous. Either way, for a
given excitation source 6 the illumination intensity on the sample
will be reduced by the same factor, and when designing a TDI system
this needs to be considered against the N times longer signal
acquisition time advantageously provided by the N pixel lines.
Finally, it will be appreciated that the variant configurations
discussed above for line camera systems, e.g. the configuration
shown in FIG. 9, are also applicable to TDI systems.
EXAMPLES
[0152] In this section a `baseline example` of an optical system is
provided, such as may be found in a prior art PL imaging system
with full field imaging onto an area camera, illustrated
schematically for example in FIG. 1 or 2, with which all subsequent
examples are to be compared. It is assumed that measurement noise
is dominated by statistical noise, i.e. the signal to noise is
given as the square root of the total number of counts, and define
2000 counts per pixel as the target for PL measurements that have
sufficient signal to noise. Several measurement
parameter/sample/hardware combinations will be outlined that yield
this target, and it will be observed that there are general rules
that can be used to derive alternative combinations. In each case
the sample will be a 156 mm.times.156 mm, 200 .mu.m thick 1
.OMEGA.cm p-type silicon wafer, but the specific material quality
will vary between three possibilities: [0153] As-cut unpassivated
multicrystalline silicon, with an effective carrier lifetime of 0.5
to 2 .mu.s (worst case') [0154] Passivated or diffused
multicrystalline silicon, with an effective carrier lifetime of
around 10 .mu.s [0155] High lifetime passivated monocrystalline
silicon, with an effective carrier lifetime of around 1 ms (best
case')
Baseline Example
[0156] Referring to FIG. 1, a 156.times.156 mm.sup.2 sample 4 is
illuminated with 750 nm light at an on-sample intensity of 100
mW/cm.sup.2 (1 Sun), and the photoluminescence 2 focused onto a Si
CCD-based 1 Megapixel (1024.times.1024 pixels) camera 10 having
5.times.5 .mu.m.sup.2 pixels using collection optics 11 comprising
an F#=2.8 lens with a focal length f=25 mm. To estimate the
collection efficiency, it is noted that to focus the entire sample
area onto the camera chip, the collection optics needs to have a
magnification M=1024.times.5e-3/156=0.033, requiring an object
distance (i.e. the distance between the lens and the sample) of
O=(1+1/M)*f=787 mm. Noting that the aperture of the lens is given
by D=f/#=25 mm/2.8=8.9 mm, the acceptance area of the lens will be
.pi.D.sup.2/4=63 mm.sup.2. Comparing this with the surface area of
a hemisphere of radius O (2.pi.O.sup.2), a collection efficiency of
0.0016% is calculated.
[0157] With this system, it is estimated that an unpassivated
multicrystalline silicon sample will yield between 2 and 6 counts
per second per pixel, implying a total measurement time of between
330 and 1000 s to achieve 2000 counts. In comparison, because of
the longer carrier lifetime an estimated 2000 counts could be
achieved in about 60 s or 600 ms for passivated multicrystalline
silicon or high lifetime passivated silicon samples
respectively.
Example 1
2D Area Imaging Geometry in Three-Belt System of FIG. 3 (Stationary
Sample)
[0158] To incorporate a three-belt system into a solar cell line
with a throughput of one cell per second, the measurement time
clearly cannot exceed one second. To guarantee this for
unpassivated samples it is necessary to achieve a thousand-fold
increase in measurement speed. This could be done for example with
a combination of: [0159] 1) Si-CCD camera with 20.times.20
.mu.m.sup.2 pixels (16.times. gain compared to the baseline
example) [0160] 2) F#=2 lens (2.times. gain) [0161] 3) Illumination
intensity of 1 W/cm.sup.2 i.e. 10 Suns (10.times. gain) [0162] 4)
2.times.2 pixel binning (4.times. gain) It will be appreciated that
many other combinations are possible, e.g. 40 Suns illumination
without pixel binning
Example 2
Line Scan System with Line Illumination (165 .mu.m Width and 156 mm
Length) and Detection on Unpassivated Wafer
[0163] In this case a 1 ms measurement time per line is needed,
requiring a 10.sup.6 times increase in measurement speed compared
to the Baseline Example. One possible combination is: [0164] 1)
InGaAs camera (20.times. gain) [0165] 2) 25.times.25 .mu.m.sup.2
pixels (25.times. gain) [0166] 3) Illumination intensity of 1
W/cm.sup.2 i.e. 10 Suns (10.times. gain) [0167] 4) Improved
collection efficiency (200.times. gain) It should be noted that
because the illuminated area is much smaller than in the Baseline
Example, it would be relatively straightforward to use much higher
illumination intensities (hundreds of Suns) if collection
efficiency improvements are more limited.
Example 3
Line Scan System with Line (165 .mu.m Width and 156 mm Length)
Illumination and Detection on Passivated Multicrystalline Wafer
[0168] As for Example 2 a 1 ms measurement time per line is needed,
which in this case requires a 60,000 times increase in measurement
speed compared to the Baseline Example. One possible combination
is: [0169] 1) Si-CCD camera with 20.times.20 .mu.m.sup.2 pixels
(16.times. gain) [0170] 2) Illumination intensity of 2 W/cm.sup.2
i.e. 20 Suns (20.times. gain) [0171] 3) Improved collection
efficiency (200.times. gain) Alternatively an InGaAs camera would
give the required measurement speed if collection efficiency
improvements are more limited, or if the PL intensity increases
sub-linearly with illumination intensity.
Example 4
Line Scan System with Line (165 .mu.m Width and 156 mm Length)
Illumination and Detection on High Lifetime Passivated Wafer
[0172] In this case a 600 times increase in measurement speed is
required to achieve a 1 ms measurement time. One possible
combination is: [0173] 1) Si-CCD camera with 20.times.20
.mu.m.sup.2 pixels (16.times. gain) [0174] 2) Illumination
intensity of 0.2 W/cm.sup.2 i.e. 2 Suns (2.times. gain) [0175] 3)
Improved collection efficiency (20.times. gain) The increase in
measurement speed could alternatively be achieved with the baseline
geometry, i.e. without any modification of the imaging system to
improve the collection efficiency, if an illumination intensity of
40 Suns were used.
Example 5
Flash Based System with Broad Illumination Spectrum (500-800 nm) on
Unpassivated Wafer
[0176] A 1 ms measurement time is needed, i.e. a 10.sup.6 times
improvement compared to the Baseline Example. Assuming that the
response of an unpassivated wafer to that spectrum is on average
the same as for 750 nm excitation, one possible combination is:
[0177] 1) InGaAs Camera (20.times. Gain) [0178] 2) 25.times.25
.mu.m.sup.2 pixels (25.times. gain) [0179] 3) Illumination
intensity of 50 W/cm.sup.2 i.e. 500 Suns (500.times. gain) [0180]
4) F#=1.4 lens or 2.times.2 pixel binning (4.times. gain) It will
be noted that an extremely high illumination intensity is used.
Example 6
Flash Based System with Broad Illumination Spectrum (500-800 nm) on
Passivated Multicrystalline Wafer
[0181] A 1 ms measurement time is needed, i.e. a 60,000 times
improvement compared to the Baseline Example. One possible
combination is: [0182] 1) Si-CCD camera with 20.times.20
.mu.m.sup.2 pixels (16.times. gain) [0183] 2) Illumination
intensity of 10 W/cm.sup.2 i.e. 100 Suns (100.times. gain) [0184]
3) F#=1.4 lens (4.times. gain) [0185] 4) 3.times.3 pixel binning
(9.times. gain) Using an InGaAs camera or improved collection
efficiency would allow lower illumination intensities; these
options would also be useful if the PL intensity increases
sub-linearly with illumination intensity.
Example 7
Flash Based System with Broad Illumination Spectrum (500-800 nm) on
High Lifetime Passivated Wafer
[0186] A 1 ms measurement time is needed, i.e. a 600 times
improvement compared to the Baseline Example. One possible
combination is: [0187] 1) Si-CCD camera with 20.times.20
.mu.m.sup.2 pixels (16.times. gain) [0188] 2) Illumination
intensity of 2 W/cm.sup.2 i.e. 20 Suns (20.times. gain) [0189] 3)
F#=2 lens (2.times. gain) This can be achieved with the baseline
geometry, i.e. without any modification of the imaging system to
improve the collection efficiency.
Example 8
TDI Camera System with 128 Lines on Unpassivated Wafer
[0190] 2000 counts per pixel is needed with 128 ms exposure time
for each part of the wafer, implying an 8000 times improvement in
measurement speed compared to the Baseline Example. One possible
combination is:
[0191] 1) Si-CCD camera with 20.times.20 .mu.m.sup.2 pixels
(16.times. gain) [0192] 2) Illumination intensity of 1 W/cm.sup.2
i.e. 10 Suns (10.times. gain) [0193] 3) Improved collection
efficiency (50.times. gain) Alternatively an InGaAs camera can
provide the required measurement speed if collection efficiency
improvements are more limited.
Example 9
TDI Camera System with 128 Lines on Passivated Multicrystalline
Wafer
[0194] 2000 counts per pixel are needed with 128 ms exposure time
for each part of the wafer, implying a 400 times improvement
compared to the Baseline Example. One possible combination is:
[0195] 1) Si-CCD camera with 20.times.20 .mu.m.sup.2 pixels
(16.times. gain) [0196] 2) Illumination intensity of 1 W/cm.sup.2
i.e. 10 Suns (10.times. gain) [0197] 3) 2.times.2 pixel binning
(4.times. gain)
Example 10
TDI Camera System with 128 Lines on High Lifetime Passivated
Wafer
[0198] 2000 counts per pixel are needed with 128 ms exposure time
for each part of the wafer, implying a 4 times improvement compared
to the Baseline Example. This can be achieved simply by using 4
times the illumination intensity or 2.times.2 pixel binning
[0199] In the above-described preferred embodiments, semiconductor
samples have been subjected to a photoluminescence analysis in the
form of PL imaging, i.e. the acquisition of a 2-dimensional image
of the photoluminescence generated from a substantial area of each
sample, with a view to deriving spatially resolved information on
one or more material properties. However other forms of
photoluminescence analysis are also within the scope of the present
invention. For example the PL emission from the illuminated area
could be fed into a spectrometer to analyse the spectral content,
say to look for a PL band indicative of an impurity. In another
example the total PL emission signal could be measured to yield
information on the average value of a sample property, with the
averaging being performed line-by-line or across the entire
illuminated area, or to provide a rapid method for identifying
defective (e.g. shunted) solar cells during or after
manufacture.
[0200] Although the present invention has been described with
particular reference to certain preferred embodiments thereof,
variations and modifications of the present invention can be
effected within the spirit and scope of the following claims.
* * * * *