U.S. patent application number 13/520371 was filed with the patent office on 2013-03-14 for illumination systems and methods for photoluminescence imaging of photovoltaic cells and wafers.
This patent application is currently assigned to BT Imaging Pty. Ltd.. The applicant listed for this patent is Robert A. Bardos, Ian A. Maxwell, Wayne McMillan, Thorsten Trupke, Juergen Weber. Invention is credited to Robert A. Bardos, Ian A. Maxwell, Wayne McMillan, Thorsten Trupke, Juergen Weber.
Application Number | 20130062536 13/520371 |
Document ID | / |
Family ID | 44226061 |
Filed Date | 2013-03-14 |
United States Patent
Application |
20130062536 |
Kind Code |
A1 |
Bardos; Robert A. ; et
al. |
March 14, 2013 |
Illumination Systems and Methods for Photoluminescence Imaging of
Photovoltaic Cells and Wafers
Abstract
Methods are presented for analysing semiconductor materials (8),
and silicon photovoltaic cells and cell precursors in particular,
using imaging of photoluminescence (12) generated with high
intensity illumination (16). The high photoluminescence signal
levels (16) obtained with such illumination (30) enable the
acquisition of images from moving samples with minimal blurring.
Certain material defects of interest to semiconductor device
manufacturers, especially cracks, appear sharper under high
intensity illumination. In certain embodiments images of
photoluminescence generated with high and low intensity
illumination are compared to highlight selected material properties
or defects.
Inventors: |
Bardos; Robert A.; (Surry
Hills, AU) ; Weber; Juergen; (Coogee, AU) ;
Trupke; Thorsten; (Coogee, AU) ; Maxwell; Ian A.;
(Five Dock, AU) ; McMillan; Wayne; (Surry Hills,
AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bardos; Robert A.
Weber; Juergen
Trupke; Thorsten
Maxwell; Ian A.
McMillan; Wayne |
Surry Hills
Coogee
Coogee
Five Dock
Surry Hills |
|
AU
AU
AU
AU
AU |
|
|
Assignee: |
BT Imaging Pty. Ltd.
Surry Hills
AU
|
Family ID: |
44226061 |
Appl. No.: |
13/520371 |
Filed: |
January 4, 2011 |
PCT Filed: |
January 4, 2011 |
PCT NO: |
PCT/AU2011/000004 |
371 Date: |
October 30, 2012 |
Current U.S.
Class: |
250/459.1 ;
250/208.1; 250/458.1 |
Current CPC
Class: |
G01N 21/6489 20130101;
G01N 2201/062 20130101; G01N 2201/08 20130101; G01N 2201/06113
20130101; G01N 21/9501 20130101 |
Class at
Publication: |
250/459.1 ;
250/458.1; 250/208.1 |
International
Class: |
G01N 21/956 20060101
G01N021/956; 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
1-24. (canceled)
25. A method of identifying defects or features in a semiconductor
material, said method comprising the steps of: obtaining in 1
second or less an image of a photoluminescence response from a
substantial portion of said semiconductor material generated with
illumination having an intensity of at least 6 Suns; and processing
said image to obtain information regarding defects or features in
said semiconductor material.
26. A method as claimed in claim 25 wherein illumination is applied
at an intensity of at least 8 Suns.
27. A method as claimed in claim 25 wherein illumination is applied
at an intensity of at least 10 Suns.
28. A method as claimed in claim 25, wherein said defects or
features are selected from the group consisting of cracks,
dislocations, impurities, shunts, selective emitter structures and
emitter layers.
29. A method as claimed in claim 25, wherein said information
comprises distinguishing between different types of defects or
features in said semiconductor material.
30. A method as claimed in claim 25, wherein the photoluminescence
response is captured using a Time Delay Integration camera, or
using frame averaging.
31. A method as claimed in claim 30 wherein an illumination
intensity of between 8 and 16 Suns is applied to the semiconductor
material.
32-34. (canceled)
35. A method as claimed in claim 25, wherein said semiconductor
material is at an elevated temperature while said photoluminescence
is being generated.
36. A method as claimed in claim 35 wherein said semiconductor
material is at a temperature of about 200.degree. C.
37. A method as claimed in claim 35 wherein said semiconductor
material is at a temperature of above 200.degree. C.
38. An apparatus adapted to carry out the method of claim 25.
39. 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:
an illumination system for applying illumination with an intensity
of at least 6 Suns to generate a photoluminescence response from
said semiconductor material; an image capture device for obtaining
in 1 second or less an image of photoluminescence emanating from a
substantial portion of said illuminated semiconductor material; and
a processor for processing said image to obtain information
regarding defects or features in said semiconductor material.
40. A production line as claimed in claim 39 wherein said
illumination system is adapted to apply illumination with an
intensity of at least 8 Suns.
41. A production line as claimed in claim 39 wherein said
illumination system is adapted to apply illumination with an
intensity of at least 10 Suns.
42. A production line as claimed in claim 39, wherein said at least
one analysis device is eye-safe when in operation.
43. A production line as claimed in claim 42, wherein said
illumination system is eye-safe when in operation.
44. A production line as claimed in claim 39, wherein said
illumination system comprises a flash lamp, an LED, a laser system
or a pulsed light source.
45. A production line for the production of a photovoltaic device
from a semiconductor material, wherein analysis devices adapted to
carry out the method of claim 25 are provided on either side of a
respective process step in said production line to analyse the
effect of said process step on said semiconductor material.
46-50. (canceled)
51. An article of manufacture comprising a computer useable medium
having a computer readable program code configured to conduct the
method defined in claim 25.
52. An image of a photoluminescence effect generated from a
semiconductor material by illumination when produced by the method
according to claim 25.
53. (canceled)
54. An article of manufacture comprising a computer useable medium
having a computer readable program code configured to operate the
production line defined in claim 39.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to illumination systems, and
methods using these systems, for the characterisation of
semiconductor materials using photoluminescence imaging. The
illumination systems have particular application to the
characterisation of silicon-based photovoltaic cells and cell
precursors.
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] The semiconductor industry has for decades used
photoluminescence (PL), the generation of luminescence with above
band gap excitation, as a non-destructive method for investigating
direct band gap semiconductor materials, especially for the
presence of defects. Generally, the PL has been generated using a
laser focused onto a small area of the sample, and to investigate a
large area the laser beam or sample is raster scanned to generate a
map of the PL emission. Focused beam excitation has traditionally
been used because of a desire for high spatial resolution, e.g.
when mapping defect distributions, and because the associated high
intensity generates a stronger PL response. AT&T Bell Labs have
considered the possibility of PL imaging of direct band gap
semiconductors using broad area illumination (G. Livescu et al,
Journal of Electronic Materials 19(9) 937-942 (1990)), but despite
the advantage of rapid measurement apparently concluded it was
inferior to scanning systems because of problems with non-uniform
illumination and low sensitivity (G. E. Carver, Semiconductor
Science and Technology 7, A53-A58 (1992)). In any event PL has
usually been used to investigate high value semiconductor samples
during or as part of the manufacture of computer chips and the
like, where rapid measurement is not essential.
[0005] To a lesser extent, PL mapping with focused beam excitation
has also been used to characterise indirect band gap semiconductor
samples, as disclosed for example in published PCT patent
application No WO 98/11425 A1. As with direct band gap
semiconductors, the choice of focused beam illumination has been
motivated for reasons of higher spatial resolution and greater PL
response, with the latter being particularly significant for
indirect band gap materials because of their much lower radiative
quantum efficiency.
[0006] In the photovoltaic (PV) cell industry, dominated by
silicon-based cells, the throughput is currently of order 1-2
seconds per wafer, so that measurement speed is critical for
in-line inspection. Even in off-line sampling the sheer number of
wafers handled by PV cell production facilities necessitates fast
measurement, making broad area PL imaging far more attractive than
PL scanning/mapping. In 2005 the founders of the present applicant
demonstrated that it was in fact possible to inspect silicon-based
PV cells and wafers with broad area PL imaging despite the low
radiative quantum efficiency, and disclosed suitable methods and
systems in published PCT patent application No WO 07/041,758 A1
entitled `Method and system for inspecting indirect bandgap
semiconductor structure`, the contents of which are incorporated
herein by reference. Most if not all commercially available PL
imaging systems for silicon-based photovoltaics utilise laser
illumination because they provide the required illumination
intensity over a narrow wavelength band, enabling rejection of
stray illumination at the camera with commercially available
filters, and the laser beam can be readily expanded and homogenised
for the purposes of illumination uniformity across an entire cell,
typically 15.6.times.15.6 cm.sup.2.
[0007] There are however a number of drawbacks with the current
laser-based PL imaging systems. Firstly there are significant light
safety issues, especially if the excitation light is in the 750 to
1000 nm region (as is typical for silicon samples) which can be
focused onto the retina with no protective blink response.
Consequently the PL measurement chamber must be optically isolated,
requiring shutters, doors or equivalent mechanisms, adding
complexity and cost to the sample transfer mechanisms into and out
of the chamber.
[0008] Secondly, current PL imaging systems require the sample to
be stationary to prevent blurring of the PL image. This is because
with broad area excitation the PL emitted from many silicon
samples, and raw or unpassivated silicon samples in particular, can
be of such low intensity that even the most sensitive silicon-based
CCD cameras require an exposure time of order at least 1 second to
acquire a sufficient PL signal. Although this `stationary sample`
requirement is acceptable for off-line applications, it is a
complication for in-line applications and it would be advantageous,
especially with fragile wafers or on large or fast production
lines, to avoid any stop/starting of individual wafers.
[0009] Current systems are also limited in terms of their ability
to differentiate between various material properties or defects,
e.g. shunts, dislocations and cracks, in a semiconductor material
or a PV cell produced from semiconductor material, or to identify
the presence of certain defects amongst other features.
[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 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 with reduced light safety requirements. It is another
object of a preferred form of the present invention to provide a
method for classifying features and defects in a semiconductor
device by comparing photoluminescence images acquired with widely
differing illumination intensities. It is another object of a
preferred form of the present invention to provide a method for
classifying features and defects in a semiconductor device with
photoluminescence images acquired with high intensity
illumination.
SUMMARY OF THE INVENTION
[0011] In accordance with the first aspect, the present invention
provides a method of analysing a semiconductor material comprising
applying illumination from a high intensity illumination source to
the semiconductor material for a sufficient time and at sufficient
intensity to induce photoluminescence, obtaining an image of said
photoluminescence and analysing said image to determine the
location and/or nature of features and defects in the semiconductor
material.
[0012] In a preferred embodiment, the illumination source is an
eye-safe light source, more preferably a non-laser light source. In
another embodiment, the illumination is provided by a pulsed light
source, LED or laser system. In a particularly preferred
embodiment, the light source is a flash lamp. In another
embodiment, the light source comprises one or more light emitting
diodes (LED). In another embodiment a laser-based illumination
system is developed, the overall output of which is eye-safe. As
mentioned above, the laser-based PL imaging systems of the prior
art have significant light safety issues. The potential hazard of
laser light sources, and of illumination systems incorporating
them, 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 near IR light from non-laser
sources, e.g. from high power LEDs, and the regulatory requirements
are less onerous, 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 light sources. Rather, the key
factors are the intensity of light entering the pupil and the image
size on the retina. With this in mind, it is preferred for a PL
imaging system to use relatively low brightness, eye-safe
illumination systems. Importantly, we will show that it is possible
to provide the high intensity illumination required for silicon and
other indirect bandgap materials with eye-safe illumination
systems. Furthermore even if an illumination system itself is not
eye-safe, it is possible for the PL imaging system as a whole to be
eye-safe without resorting to stringent laser safety measures such
as safety shutters and interlocks.
[0013] The use of high intensity illumination for generating
photoluminescence provides significant advantages in terms of time
and cost. For example, flash lamp equipment is generally cheaper
than laser light sources and if applied correctly, can provide
sufficient energy to produce a photoluminescence effect within a
very short space of time, up to 100 milliseconds preferably up to
10 and most preferably a few milliseconds as compared with
conventional laser based PL imaging systems.
[0014] The term `high intensity` refers to an intensity of
illumination which is high relative to that which would be used to
obtain a conventional photoluminescence image with conventional
sensor and laser technology. To explain, using a conventional
silicon CCD camera, a photoluminescence (PL) image can be obtained
with laser illumination at 1 or 2 Suns, where 1 Sun is considered
to be equivalent to 100 mW/cm.sup.2. The applicant has found that
by applying illumination with flash lamps, pulsed lasers, LEDs etc
at levels (e.g. 50 Suns or greater) that are one or two orders of
magnitude higher, relative to the conventional intensities, the PL
response of the wafer is quite different and the resulting PL image
is quite different from a conventional PL image. This allows
operators to determine other attributes of the semiconductor
material which would not be determined by conventional
techniques.
[0015] With other systems e.g. using a different camera, `high
intensity` may mean illumination at the level of 6-8 Suns or
greater. For instance, as will be disclosed below, for a PL imaging
system with a hybrid flash illumination-Time Delay Integration
(TDI) arrangement utilising silicon CCD cameras, relatively high
intensity illumination of around 8-16 Suns is used compared with
only 2 Suns using a silicon CCD camera in a standard `stop sample
and expose` image mode. Again, using alternative sensors such as a
MOSIR (photocathode CCD based) camera the absolute intensity
applied to the wafer may be lower than 50 Suns but it would still
be regarded as a `high intensity` when viewed in combination with
the sensor. Accordingly the term `high intensity` should not be
regarded as an absolute value but rather regarded as `high`
relative to a conventional system where samples are stopped for
measurement of the PL response. It will be clear to the person
skilled in the art that this is affected by the measurement
technique and sensor used.
[0016] In some respect the `high intensity` can be considered as a
ratio of the required illumination intensity relative to camera and
sample sensitivity. For example, in a previous system 1 Sun
illumination was used with a 1 second capture time for raw wafers
with CCD cameras, whereas in the new system only a 1 millisecond
capture time is required for about 10 Suns or more illumination for
pre-diffusion wafers and with an InGaAs camera. Similarly, a 1
millisecond capture time can be used for a 100 Sun illumination for
pre-diffusion wafers and with a Si CCD camera.
[0017] When the relative sensitivity to the silicon PL emission is
improved, for example by using an InGaAs camera, the required
illumination intensity drops accordingly by a factor of 10 or more.
This has an additional benefit of allowing a drop in the required
time for the same intensity level. Preferably the associated optics
are modified to remove stray light, incident light etc, i.e.
optimised for each camera and light source combination. In other
words, the illumination intensity is a function of the relative
camera sensitivity to PL emission, preferably with optimised
optical filtering.
[0018] By `flash` illumination we refer to short duration intense
illumination preferably provided by a flash lamp, pulsed LED etc.
Typically this is between 50 and 1,000 Suns (5-100 Watts/cm.sup.2)
preferably 75 to 200 Suns and most preferably around 100 Suns of
illumination applied to the semiconductor material. This
illumination is typically provided within fractions of a second,
e.g. 100 milliseconds, preferably up to 10 milliseconds and most
preferably within a few milliseconds. In some embodiments such
illumination can be provided in 1 millisecond or less.
[0019] Illumination can also be provided by pulsed lasers which
still provide high intensity illumination. In the interests of eye
safety, such pulsed lasers can illuminate the sample with very
short and therefore low energy pulses, e.g. less than 1
microsecond. Alternatively, longer pulses or even cw laser emission
can be provided in a less hazardous form by mechanically agitating
one of the optical components, e.g. mirrors in the optical system,
to smear out any image formed on the retina, reducing the effective
intensity at the retina, and therefore the thermal effects.
[0020] In a second aspect, the present invention provides a method
for producing a photoluminescence image of a semiconductor
material, said method comprising the steps of: applying to said
semiconductor material at least one pulse of illumination with an
intensity of between 50 and 1,000 Suns-within 100 milliseconds; and
capturing the photoluminescence response as an image.
[0021] In a preferred embodiment, the high intensity illumination
produced by a flash lamp is applied to a moving semiconductor
material. The rapid energy application and image capture system
provides the opportunity for obtaining a PL image of the
semiconductor material while still moving. Due to the length of
time required to produce photoluminescence and capture the image,
in a conventional system using a silicon based camera it is almost
always necessary to stop the semiconductor material to avoid
blurring of the image. Using the aforementioned flash system allows
inducement of photoluminescence and capture of the
photoluminescence effect as an image within fractions of a second,
e.g. 10 or preferably 5 and most preferably up to a few
milliseconds. We note that it may take considerably longer to read
out the image, depending on the camera technology, but this does
not affect the ability to acquire images with minimal blurring.
[0022] In a third aspect, the present invention provides a method
for producing an image of a photoluminescence response in a
semiconductor material, said method comprising the steps of:
applying sufficient illumination to the semiconductor material to
produce a photoluminescence response; and capturing an image of the
photoluminescence response within 100 milliseconds with a silicon
camera.
[0023] In a fourth aspect, the present invention provides a method
for producing an image of a photoluminescence response in a silicon
sample, said method comprising the steps of: applying sufficient
illumination to the silicon sample to produce a photoluminescence
response; and capturing an image of the photoluminescence response
within 10 milliseconds with a camera that captures substantially
all of the photoluminescence response of silicon.
[0024] Preferably the photoluminescence response is captured with a
camera that captures most or all of the PL emission spectrum which,
subject to the type of sample, could be for example a silicon
sensor based camera, a compound semiconductor sensor based camera
or a compound semiconductor photocathode based camera.
[0025] Preferably the semiconductor material is an indirect band
gap semiconductor material such as silicon. The inventive method
can be applied to such a semiconductor material in the form of an
ingot, a block, a wafer or a complete or partially completed
photovoltaic device.
[0026] As mentioned above, the high intensity illumination source
can be selected from laser sources or incoherent sources such as
flash lamps, LEDs etc. Irrespective of the nature of the source,
the overall brightness of the illumination system is preferably
sufficiently low or the exposure sufficiently short, e.g. up to a
few milliseconds, to mitigate the need for light safety
shielding.
[0027] To some extent the upper limit of the time for the above
mentioned process depends upon belt speed. Current state of the art
belt speeds are between 100 mm per second and 200 mm per second on
a cell line and approximately twice that on a wafer line. To reduce
or avoid 1-pixel blurring it is necessary either to stop or slow
down the sample or, preferably, to expose the sample and capture
the PL response as an image in less than a few milliseconds,
preferably less than 1 millisecond, for an on-sample pixel size of
160 .mu.m and a belt speed of 150 mm per second. The preferred
exposure time depends on line speed, pixel size and the acceptable
level of blurring.
[0028] In a fifth aspect, the present invention provides a method
of identifying defects or features in a semiconductor material,
said method comprising the steps of: obtaining a first image of a
photoluminescence response from said semiconductor material
generated with a first, higher illumination intensity; obtaining a
second image of a photoluminescence response from said
semiconductor material generated with a second, lower illumination
intensity; and comparing said first and second images.
[0029] In a sixth aspect, the present invention provides a method
of differentiating defects or features in a semiconductor material,
said method comprising the steps of: applying to the semiconductor
material a predetermined level of high intensity illumination
adapted to obtain a photoluminescence response characteristic of a
predetermined defect or feature; capturing the resultant
photoluminescence response; and analysing said response to
determine the presence and/or location of such defects or
features.
[0030] In a seventh aspect, the present invention provides a method
of identifying defects or features in a semiconductor material,
said method comprising the steps of: obtaining an image of a
photoluminescence response from said semiconductor material
generated with illumination having an intensity of at least 6 Suns;
and processing said image to obtain useful information.
[0031] The applicant has determined a significant and surprising
advantage of using high intensity illumination over conventional
low intensity PL imaging systems. It has found that the PL response
of certain defects in a semiconductor material or PV cell differs
with the illumination intensity. That is, certain defects appear
differently in PL images generated with high or low intensity
light. Such a differential effect may improve the ability of
operators to classify defects in a semiconductor material than
would otherwise be the case using conventional low intensity PL
imaging alone.
[0032] The applicant has also determined that an image of PL
generated by high intensity illumination can, in itself, be more
accurate in indicating the precise location of certain defects. For
example cracks show up more clearly and sharply in an image of PL
generated with high intensity illumination. This is significant and
quite surprising and, as will be appreciated, provides significant
advantages over the prior art.
[0033] The aforementioned high intensity photoluminescence imaging
systems can be used throughout the production line of a
photovoltaic device. They can be used by themselves or, preferably,
in combination with other imaging and testing equipment.
[0034] In this regard, in an eighth aspect, the present invention
provides a production line for the production of a photovoltaic
device 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 high
intensity illumination source for applying illumination with
intensity of at least 6 Suns to a semiconductor material, and an
image capture device for obtaining an image of photoluminescence
emanating from said illuminated semiconductor material.
[0035] The aforementioned photoluminescence imaging systems can be
used throughout the production line of a photovoltaic device. They
can be used by themselves or, preferably, in combination with other
imaging and testing equipment.
[0036] In a ninth aspect, the present invention provides a method
of analysing a semiconductor material, said method comprising the
steps of: inducing photoluminescence from said material by applying
a high intensity illumination for a sufficient time and at an
intensity of at least 6 Suns; and obtaining an image of said
photoluminescence, wherein said illumination is provided by an
eye-safe illumination system.
[0037] As mentioned above, flash, LED or pulsed lasers provide a
high intensity light source suitable for PL imaging. PL imaging
systems containing high intensity laser-based illumination systems
which are not eye-safe, as discussed above, generally require
expensive and complex safety devices and optics. The use of
eye-safe yet high intensity illuminations systems including sources
such as flash lamps, LEDs, low pulse energy lasers etc not only
produce good PL images but substantially reduce the costs
associated with ensuring containment of illumination.
[0038] The aforementioned high intensity photoluminescence imaging
systems can be used by themselves or, preferably, in combination
with a low intensity photoluminescence imaging system, and
optionally with other measurement techniques. In this regard, in a
tenth aspect, the present invention provides an apparatus for
determining the quality of a finished photovoltaic cell comprising
(a) a high intensity photoluminescence system, and at least one of
(b) a low intensity photoluminescence system, and (c) components
for determining the Series Resistance of the cell.
[0039] The aforementioned photoluminescence imaging systems can be
used as separate measurement tools, or they can be integrated into
a process tool. In this regard, in yet a further aspect, the
present invention provides a production tool comprising at least
one PL measurement system and one process production system.
[0040] The present invention also provides an article of
manufacture comprising a computer readable medium having a computer
readable program code configured to carry out the aforementioned
method and/or operate the production line or apparatus.
[0041] The present invention also provides in yet a further aspect,
an image of a photoluminescence effect from a semiconductor
material illuminated flash illuminated by high intensity
illumination as hereinbefore described.
[0042] The present invention also provides substantial advantages
when applying additional stimuli to a semiconductor material. One
such stimulus can be the application of thermal energy to the
material, since it is known that the electronic properties of
different features have different temperature dependence. For
instance, in some applications a photoluminescence image may be
obtained of a multicrystalline silicon cell at an elevated
temperature, e.g. 200.degree. C., where lifetime related variations
in the image are strongly suppressed while cracks, for example,
remain clearly visible.
[0043] Above about 200.degree. C. however, thermal emission from
the cell (and heating element) becomes the dominant signal and
saturates the sensor, making conventional or low intensity PL
measurement impossible since for an image capture time of around 1
second the thermal emission simply saturates a CCD camera.
[0044] The present application, on the other hand, provides for
high intensity illumination of the cell which does not require long
image exposure or capture times. For instance, where a flash lamp
is used to illuminate the semiconductor material, a PL response can
be captured in around 1 millisecond. Such a short response or
capture time means that the thermal emission signal from a heated
wafer is 1000 times lower than for conventional systems, i.e. 1
millisecond compared to 1 second.
[0045] This is a particularly useful process when using a silicon
CCD sensor. Using an InGaAs camera also allows a shorter
measurement time, but it measures longer wavelengths than a Si CCD
camera and is therefore relatively more sensitive to thermal
emissions from the wafer or cell.
[0046] This embodiment of the present inventive process is
particularly important for wafers at high temperatures at two
critical stages of production, namely diffusion and firing. For
instance, a PL imaging system can be integrated after diffusion or
after firing. Providing such an imaging system after the firing
step would be preferable since generally wafers pass through the
firing tool linearly, i.e. one by one, whereas diffusion is either
a batch process or several wafers are processed in parallel.
[0047] The resultant high intensity images, preferably obtained by
flash lamp illumination, could be extremely useful. Also the
samples need not necessarily be held at a constant temperature
during the process. Indeed, a high intensity (e.g. flash) image
could be taken during the cooling steps if it was desired. In such
a way it could be determined whether the firing/diffusion step
and/or cooling produce any effects on the wafer, detrimental or
otherwise.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] 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:
[0049] FIG. 1 illustrates in side view a system suitable for
in-line PL imaging of semiconductor samples;
[0050] FIG. 2 illustrates in side view a PL imaging system
according to an embodiment of the present invention;
[0051] FIGS. 3(a) and 3(b) show in plan view and side view an
arrangement of a flash lamp and camera according to a preferred
embodiment of the invention;
[0052] FIGS. 4(a) and 4(b) show PL images of a silicon-based PV
cell precursor acquired with illumination intensities of .about.1
Sun and .about.100 Suns respectively;
[0053] FIGS. 5(a), 5(b) and 5(c) illustrate how a finger shunt in a
PV cell appears differently in images of PL generated with
different illumination intensities;
[0054] FIGS. 6(a), 6(b) and 6(c) illustrate the different responses
of shunts and dislocations to varying illumination intensity;
[0055] FIG. 7 illustrates schematically the incorporation of
flash-based PL imaging systems into a semiconductor device
production line;
[0056] FIGS. 8 and 9 illustrate in side view PL imaging systems
with illumination from an LED bar array; and
[0057] FIGS. 10(a), 10(b) and 10(c) are diagrammatic views of a PL
imaging system using time delay integration with high intensity
illumination.
DETAILED DESCRIPTION
[0058] Preferred embodiments of the invention will now be
described, by way of example only, with reference to the
accompanying drawings.
[0059] Photoluminescence (PL) imaging is known to be a rapid and
convenient technique for characterising silicon ingots, blocks,
wafers, as well as silicon-based photovoltaic (PV) cells both
during and after manufacture, using systems and methods described
in the abovementioned published PCT patent application No WO
07/041,758 A1. The PL emission from silicon arises primarily from
band-to-band recombination in the wavelength range 900 to 1300 nm,
and can provide information on many material, mechanical and
electrical parameters of relevance to PV cell performance including
minority carrier diffusion length and minority carrier lifetime,
and the impact of certain materials impurities and defects on the
these properties.
[0060] FIG. 1 shows a PL imaging system 2 suitable for acquiring PL
images of semiconductor devices such as silicon PV cells during
their production process without removing them from the production
line. This system includes two outer transport belts 4 to interface
with a continuous on-belt production line and an inner transport
belt 6 to bring a sample 8 to a stop in a measurement chamber 10.
Inside the measurement chamber, photoluminescence 12 generated from
the sample with broad area photo-excitation from a coherent laser
source 14 of above-band gap light 16 is directed onto an imaging
detector 18 such as a silicon CCD camera via collection optics 20,
with the system preferably including homogenisation optics 22 to
improve the uniformity of the broad area excitation and a long-pass
filter 24 in front of the detector or the collection optics to
block excitation light. The system may also include one or more
filters 26 to select the wavelength range of the photo-excitation,
and light-tight shutters 28 that open to allow samples in and out
of the measurement chamber 10 to satisfy light safety requirements
if necessary, i.e. if the system would not otherwise be eye-safe.
In stand-alone systems with manual sample handling, e.g. for
off-line inspection of silicon wafers or PV cells, the transport
belts are not required, but any light safety issues remain would
remain.
[0061] As mentioned above however, it would be advantageous to
avoid having to bring the samples to a stop for measurement. 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, that could be placed anywhere in a PV cell
production line, for example above a transport belt bearing samples
along the line, without requiring special modifications e.g. to
satisfy stringent light safety requirements. The invention will be
described with reference to various systems and methods for
acquiring PL images of silicon-based PV cells and cell precursors,
but the systems and methods are also applicable to PV cells and
cell precursors or other devices based on other indirect or direct
band gap semiconductors.
[0062] In a first embodiment, referred to hereinafter as a `high
intensity-based PL` system and illustrated in FIG. 2, a substantial
area (preferably at least 1 cm.times.1 cm, more preferably the
entire area) of a silicon-based PV cell or cell precursor 8 moving
on a transport belt 36 is illuminated with excitation light 16 from
a high intensity light source 30 such as a xenon flash lamp, pulsed
LED, or other non-laser optical illumination source, or a laser
light source with a short pulse period (say less than 1 ms), and
the resulting PL emission 12 acquired with a silicon CCD camera 18
(or any other camera that can capture part or all of the
photoluminescence band between 900 nm and 1300 nm). Other
components that will generally be present include an excitation
filter 26, PL collection optics 20 and a long pass filter 24 as in
the FIG. 1 system, and a reflector 32 may be present for directing
a greater amount of the excitation light onto the sample. If the
excitation light is from a broad band source such as a flash lamp,
the excitation filter 26 is a more critical component than for
laser excitation because of the necessity to prevent excitation
light in the PL emission band from reaching the camera. The entire
high intensity PL system 27 is preferably enclosed in a box 29, to
be discussed later.
[0063] The image acquisition time in the FIG. 2 system will be
determined by the overlap of the illumination pulse and the camera
shutter time, and it is generally advantageous for both to be
short. The illumination time should be short to reduce power
consumption and avoid excessive heating of the excitation filter
and the sample, bearing in mind that high illumination intensity is
generally required to generate sufficient PL signal within a short
acquisition time. For preference, the camera shutter is
substantially synchronised with the pulsed excitation source,
subject to the limitations of shutter speed; for example the
activation times for commercially available mechanical shutters are
typically in the millisecond range. 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 several 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. 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 or two rows of pixels in the imaging camera or
less. 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 PV cell line throughput of 1 wafer per
second (i.e. line speed of order 15 cm/s) and a 1 megapixel camera
(1024.times.1024 pixels), this guideline would suggest an image
acquisition time of duration 1 ms or less, compared to the .about.1
s acquisition time permitted for in-line inspection of samples
brought to a stop on a 1 wafer per second line.
[0064] This great reduction in permitted acquisition time obviously
presents challenges for measuring a sufficient PL signal, but can
to a large extent be compensated by the high intensity of
commercially available flash lamps for example. We have
demonstrated illumination intensities of up to 1000 Suns (100
W/cm.sup.2) across a standard silicon PV cell (15.6 cm.times.15.6
cm) with a flash lamp, and although the PL signal does not
necessarily scale linearly with illumination intensity, it is
generally true that more intense illumination generates a greater
PL signal. To explain further, the dependence is essentially linear
at low injection levels (i.e. low illumination conditions) because
the PL signal is proportional to the minority carrier
concentration, whereas at high injection levels the dependence
becomes quadratic, convoluted with loss mechanisms such as Auger
recombination. In any event, it will be seen that we have been able
to acquire PL images of silicon PV cell precursors with
illumination exposures on the ms timescale, which is encouraging
for in-line inspection. The time required to process an image is of
course longer than the illumination/exposure time, since it
includes factors such as read out time which may for example be
around 700 to 1000 milliseconds for Si CCD cameras. However with
most cameras it is certainly possible to keep up with a line speed
of order one wafer per second.
[0065] As mentioned above, pulsed excitation is also possible. PL
intensity is proportional to the product of the electron and hole
concentrations, i.e. I.sub.PL.varies.n*p. With increasing
illumination intensity the PL response to illumination thus changes
from being linear in the excess carrier density .DELTA.n (at low
injection conditions) to being proportional to the square of
.DELTA.n (at high injection conditions) subject to various loss
mechanisms. In many cases it is desirable to achieve a specific PL
intensity with minimal exposure time. Using high illumination
intensity can be beneficial in cases where the intensity is
sufficiently high to reach high injection conditions, due to the
quadratic PL response to excess carriers.
[0066] Flash illumination is one potential approach to get high
illumination intensity. Another is to use a pulsed light source
which, compared to a continuous wave (cw) light source with the
same average optical power, reaches much higher instantaneous
intensities. Higher PL intensities can therefore be achieved with
the same average optical power. Since quantitative analysis of PL
is more difficult at high injection, because the PL response no
longer depends linearly on excess carrier density, the above
approaches are therefore of particular interest where one only
needs to analyse spatial features (patterns) rather then absolute
PL intensities, for example for characterisation of as-cut
multicrystalline wafers.
[0067] One specific preferred embodiment with flash lamp
illumination, illustrated in FIG. 3(a), includes on the
illumination side a Broncola ring flash C 30 producing a 1
millisecond pulse that, after passing through an excitation filter
26 comprising a 6 mm thick KG1 Schott glass short pass filter,
illuminates a silicon sample 8 with an intensity of 10-100
W/cm.sup.2 (100 to 1000 Suns). On the imaging side this system
comprises collection optics 20, a long pass filter 24 and a 1
Megapixel silicon CCD camera 18 for acquiring an image. The system
may also include a cylindrical reflector 37 if greater illumination
intensity on the sample is required (e.g. of order 1000 Suns), and
a shroud 39 to prevent excitation light entering the camera. As
shown schematically in plan view in FIG. 3(b) the ring-shaped flash
lamp 30 allows the camera 18 to be 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 FIGS. 1 and 2 where one or both of the
illumination source 14 and camera 18 is angled with respect to the
surface of the sample 8. This arrangement 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 imaging systems.
[0068] Flash lamps are preferable over laser illuminators,
especially near IR laser illuminators, because of reduced light
safety considerations. Although flash lamps are high intensity
sources, their extended size means that under typical viewing
conditions a much larger image is produced on the retina than with
prior art laser sources, with consequently reduced thermal hazard
to the retina. The thermal hazard is further reduced by the short
pulse lengths of typical flash lamps. Flash lamp illuminators may
also offer advantages of reduced cost and footprint.
[0069] As mentioned above in relation to FIG. 2, the excitation
filter 26 is an important component with broad band flash lamp
illuminators or other high intensity light sources because of the
necessity to remove excitation light in the PL emission band, and
there are a number of factors to be considered. 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 or a combination of absorption and dielectric
filters. A KG1 glass filter from Schott is an example of a short
pass absorption filter suitable for PL imaging of silicon samples.
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.
[0070] 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 PV
cells/precursors where the flash lamp may need to be activated at a
frequency of order 1 Hz or higher. 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 by 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.
[0071] FIG. 4(a) shows a PL image of a commercial passivated
monocrystalline silicon wafer 8 with an emitter layer, acquired
with a conventional PL imaging system where the wafer was
illuminated at .about.1 Sun (100 mW/cm.sup.2) with a near IR diode
laser and the image acquired with a silicon CCD camera with an
exposure time of two seconds. The image reveals four deliberately
introduced cracks 40A to 40D and several dark patches 42 indicative
of low carrier lifetime material. FIG. 4(b) shows a PL image of the
same wafer acquired with a flash-based high intensity PL imaging
system, where the wafer was illuminated through a 650 nm short pass
absorption filter with a 1 ms pulse having an estimated intensity
of 100 Suns from a xenon flash lamp. Comparison of the two images
immediately shows that all features are sharper and the low
lifetime regions less dark in the image acquired with flash lamp
illumination. The large, slightly brighter feature 43 appearing in
the FIG. 4(b) image is a support post visible through the wafer,
which has substantial transparency at the imaging wavelengths.
[0072] The blurring in the FIG. 4(a) image arises from lateral
currents flowing from high lifetime regions into adjacent
defect-rich (i.e. low lifetime) regions to equilibrate the charge
carrier distribution. This current flow occurs primarily in the
emitter layer which for efficient PV operation is designed with a
sheet resistance that enables low loss carrier transport under
.about.1 Sun illumination. Higher illumination intensities generate
more charge carriers, resulting in larger lateral current flows, in
which case the emitter sheet resistance causes greater transport
losses, effectively isolating defect-rich regions from surrounding
high lifetime areas. As shown in FIG. 4(b), at 100 Suns
illumination the lateral carrier transport is reduced to such an
extent that the cracks 40A to 40D stand out much more clearly,
lessening the chance of a crack detection algorithm missing cracks
or reporting false positives. For example the crack 40A at the top
right in FIG. 4(b) is more likely be missed in the FIG. 4(a) image
because of the proximity of other low lifetime features.
[0073] Shunting is another cause of decreased efficiency in PV
cells, and can cause more severe problems later on with hot spots
and module failure. The most common cause of shunting is
wrap-around of the emitter layer, but shunts are also caused by
material inclusions or metal fire-through. Severely shunted cells
are currently detected at the end-of-line IV tester, but earlier
detection, additionally with spatial precision, would be
beneficial. For example the ability to locate shunts, rather than
just detecting their presence from their global effect on cell
efficiency as done in IV testing, is important for remedial actions
such as laser isolation currently performed by some PV cell
manufacturers.
[0074] It is known that the location of a shunt can be determined
with thermal (mid IR) cameras, but we believe that PL imaging
offers a cheaper and quicker alternative. The applicant has
observed that shunts in PV cells appear quite differently in PL
images acquired with different illumination intensities, again
because of limited lateral carrier transport at high illumination.
At lower illumination shunts appear as blurred dark patches because
they draw charge carriers in from surrounding regions, whereas they
become more localised at higher illumination. This differential
visibility effect is particularly marked for shunts because the
current flow into them depends logarithmically on illumination
intensity, compared to a linear dependence for most other
recombination active defects including dislocations, cracks and
impurities, meaning that comparison of high/low illumination
intensity PL images may be particularly valuable for discriminating
shunts from other defects. To demonstrate, FIGS. 5(a), 5(b) and
5(c) show PL images of a silicon PV cell with a shunted finger,
acquired with on-sample illumination intensities of order 1 Sun,
100 Suns and 1000 Suns respectively. In the low intensity image
(FIG. 5(a)) the finger shunt is clearly identified by the large
blurred dark area 60, but its precise location is difficult to
determine and the blurred dark area obscures other features such as
dislocation clusters 62 that may be of interest. As the
illumination intensity is increased (FIGS. 5(b) and 5(c)) the
finger shunt 64 appears progressively more localised, in clear
distinction from the dislocation clusters which appear more or less
the same in each image.
[0075] Similarly, FIGS. 6(a), 6(b) and 6(c) show PL images of a
silicon PV cell with a group of shunts 66 within a large
dislocation cluster 62, acquired with illumination intensities of
order 1 Sun, 100 Suns and 1000 Suns respectively. As in the
previous example, the shunts appear more localised as the
illumination intensity increases while the dislocation clusters
appear essentially unchanged. In FIG. 6(c) (highest illumination
intensity) the locations of two individual shunts 66 within the
large dislocation cluster 62 are indicated. Clearly the combination
of images enables an operator or image analysis routines to
distinguish between these two defect types.
[0076] The present inventive technique is also useful for finding
defects and other features in selective emitter structures. This
aspect utilises the reduced lateral blurring that occurs at high
illumination intensity in samples with an emitter. Selective
emitter structures use various processing methods to create highly
doped regions (usually lines) within an otherwise homogeneous
lightly doped surface region, which are subsequently metalised
using either screen printing or plating for example. Shunts and
other defects can be introduced locally into the sample during
either the doping or the metalisation processes, or both. PL
imaging with high illumination intensity such as with a flash, or a
comparison between high and low illumination intensity PL images,
allows the exact position and shape of such defects and shunts to
be identified much more accurately than, for instance, conventional
low-intensity PL imaging, for example a one-Sun image.
[0077] The high intensity illumination of the above described
process may also be used to monitor the diffusion and post
diffusion processes in cell production as will now be
explained.
[0078] Recombination in the emitter is one of the loss mechanisms
of a solar cell. Emitter recombination is a process that reduces
the effective excess carrier lifetime and thus the PL signal,
particularly at high excess carrier densities. Apart from the
excess carrier density, the magnitude of the emitter recombination
is dependent on the background doping of the base, the emitter
doping profile and the surface passivation.
[0079] Since emitter recombination increases relative to bulk
recombination (and may become dominant) at higher excess carrier
density, a PL image taken at high illumination intensity is
affected more strongly by the emitter recombination then a PL image
taken at lower illumination intensity. Qualitative or quantitative
information about the emitter quality can therefore be gained
either from a single PL image taken with high illumination
intensity or from a comparison of two PL images, one taken with
high illumination intensity, the other one with low illumination
intensity. When this is carried out after the diffusion step, it
provides a monitor of the diffusion process. When carried out after
a post-diffusion step, it provides a monitor of the cumulative
effect of the processes between that step and the diffusion step
inclusive. When carried out before and after a post-diffusion step,
the comparison of the emitter quality determined before and after
the step provides a monitor of that process step.
[0080] Turning now to industrial implementations of high intensity
PL imaging, we believe that high intensity-based systems are
potentially well-suited to in-line inspection of devices on a PV
cell production line. As shown schematically in FIG. 7, high
intensity-based PL imaging systems 27 can be mounted in simple
boxes 29 over a transport belt 36 carrying semiconductor devices 8
along a production line, and located before or after all or
selected process stations 38. In a production line for
silicon-based PV cells, the processes in individual stations
typically include saw damage etch, emitter diffusion, silicon
nitride deposition, screen printing of metal contacts, thermal
treatment, edge isolation and IV testing. All or a substantial
fraction of devices entering or leaving selected process stations
can be inspected in a contact-less fashion and without interrupting
their motion along the production line, offering powerful means for
quality control of the devices and process control of the various
stations. The high intensity-based PL imaging systems are
preferably mounted in some form of enclosure as shown in FIG. 7, to
provide a significant distance between the light source and an
operator, and at the least to prevent direct viewing of the light
source (i.e. the light must bounce off the sample or another
object). However this simple enclosure is to be distinguished from
the complex systems of light-tight shutters that could be required
under laser safety regulations; these will be unnecessary if the PL
imaging system as a whole is eye-safe, simplifying the automation
and integration into a production line. In certain embodiments the
PL imaging system as a whole is eye-safe because the illumination
system is eye-safe. In other embodiments the PL imaging system as a
whole is eye-safe even if the illumination system is not, for
example via measures such as the prevention of direct viewing of
the illumination system output, or the presence of production line
guarding that provides some minimum distance between an operator
and the PL imaging system. We note that more sensitive cameras
(e.g. InGaAs cameras as noted below) may allow use of the
configuration as shown in FIG. 7 with lower intensity light sources
or for even shorter illumination periods.
[0081] 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.
[0082] 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`.
[0083] 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.
[0084] 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.
[0085] 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..gtoreq..alpha..sub.max (100 mrad),
the Class 1 AEL is 42 mW (i.e. 65.times. higher) through a 7 mm
diameter limiting aperture.
[0086] 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.
[0087] 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 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.
[0088] Approach (i) decreases the intensity of light at the eye,
while approach (ii) increases the angular subtense .alpha. 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 a 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. (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.
[0089] 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 HI 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.
[0090] 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.
[0091] Some PV cell process steps, including IV testing, laser edge
isolation and the laser-based selective emitter processing used for
some high performance cell designs, require the PV cell to be
brought to a stop, in which case a high intensity-based PL imaging
system could be incorporated within the particular station to
inspect the cell before, during or after that process step. Because
high intensity-based PL images can be acquired in a matter of
milliseconds, this will have essentially no effect on the process
flow.
[0092] We note that high intensity illumination is applicable at
all stages of PV block, wafer and cell production, but requires
adjustments to the wavelength range of the illumination. In one
particular example, the surface region of as-cut silicon wafers is
damaged to such an extent as to be essentially useless for PL
imaging purposes since there is extremely high non-radiative
recombination in this region, so that the shorter wavelength
(visible) portion of a flash lamp spectrum, being absorbed close to
the surface, generates little PL response. Excitation light closer
to the 1000 nm band edge of silicon penetrates deeper and generates
a measurable PL response, but careful filtering of the illumination
is required to pass light in this region and yet reject wavelengths
overlapping with the detectable PL emission band, which is
approximately 900 nm to 1160 nm for silicon CCD cameras. The
options for achieving this include absorption filters and
dielectric filters.
[0093] For silicon-based PV cells, reliable crack detection is a
highly desirable capability. While some types of defects in PV
cells can be remedied, e.g. laser isolation of shunts, or at least
do not become worse over time, cracks have the potential to grow
from tiny and difficult to detect micro-cracks, ultimately causing
catastrophic failure of a PV cell. Furthermore one defunct cell can
compromise an entire module. FIG. 4(b) clearly shows the value of
flash-based PL imaging for crack detection, and the differences in
contrast between high and low lifetime regions (FIGS. 4(a) and
4(b)) and between shunts and dislocations (FIGS. 5(a) to 6(c))
suggests that comparison (e.g. image subtraction) between PL images
acquired with high and low illumination intensities could also be
of value. In particular, image subtraction or the like could be
used to highlight defects such as shunts that appear differently
under different illumination intensities.
[0094] While a qualitative comparison of PL images such as those
shown in FIGS. 4(a) and 4(b) is relatively straightforward,
quantitative image comparison (i.e. subtraction or the like) is
more difficult because the results are sensitive to any grey level
change. There are many imperfections in imaging systems that can
interfere with defect detection algorithms, and quantitative image
comparison requires that these imperfections match. It is also
desirable to align the two images and for the gain and offset
calibrations and flat field corrections to match. Therefore in one
embodiment it is desirable to capture the two images without moving
the sample to simplify the image comparison, and this is more
easily achieved if the total image capture and read out time is
less than 1 second or so, so as to keep up with line speeds.
[0095] Clearly there are benefits in acquiring PL images under
different illumination conditions, and we now turn to consideration
of suitable PL imaging configurations for this purpose. In one
embodiment, one or more reflectors such as the cylindrical
reflector 37 shown in FIG. 3(a) can be used to enhance the
illumination intensity, while in another embodiment neutral density
filters can be used to attenuate a flash lamp emission as required
for lower intensity excitation. Alternatively the drive power of
the flash lamp can be adjusted as required; if the signal-to-noise
ratio is too low at reduced flash intensities, the PL response of
several low-intensity flashes can be accumulated into one image by
reading out and averaging the individual images, or by firing a
rapid sequence of low intensity flashes during one image exposure
cycle. In other embodiments, a PL imaging system contains a
separate low intensity illumination source, such as one or more
LEDs or even a laser system. A wide range of LEDs are available
with different powers and emission wavelengths, and LEDs are in
general inexpensive light sources and, like flash lamps, are
extended sources so have lesser light safety concerns than do
lasers. Notwithstanding light safety issues, we also envisage
situations where laser sources are also of value, either in
combination with flash lamps or LEDs, or indeed by themselves. For
example lasers, with their narrow emission bands, may be better
suited for samples such as as-cut silicon wafers where the
excitation wavelength needs to be close to the PL emission band to
avoid being absorbed in the surface damage layer. Additionally, for
certain choices of camera, e.g. an InGaAs camera, a larger portion
of the PL spectrum is sampled, so the illumination pulse can be
many times shorter for the same response. Preferably the pulse is
sufficiently short for the energy per pulse to be low enough for
eye safety to be of much lower concern, so that laser safety
mechanical systems may not be required.
[0096] LEDs may be used to generate extremely bright ms pulses like
flash lamps, and in all the descriptions above the use of the term
`flash` should be used interchangeably with pulsed LED.
[0097] Even lower intensity LED light can still be adapted for
in-line inspection of semiconductor devices without interrupting
their motion. For example in the configuration shown in side view
in FIG. 8, excitation light 16 from an LED bar array 44 passes
through an excitation filter 26 and is focused with a cylindrical
lens 46 onto a semiconductor device 8 moving on a transport belt
36, and the PL emission 12 from the illuminated region 47 imaged by
a system of collection optics 20, that may for example include an
optical fibre bundle or one or more cylindrical lenses or double
Gaussian lenses, through a long pass filter 24 onto a line camera
48. LEDs can also be pulsed for short periods, at approximately
10.times. their rated power.
[0098] In an alternative configuration shown in FIG. 9, the LED bar
array 44 and line camera 48 are on opposite sides of the sample 8,
with a gap 50 in the transport belt 36 allowing the sample to be
inspected across its entire width. The `back side illumination`
configuration shown in FIG. 9 has more complicated engineering, but
the sample acts as a long pass filter (being largely transparent in
the PL emission band) so a separate long pass filter 24 may not be
required. As the price of LEDs continues to fall, we also envisage
cost-effective PL imaging systems where arrays of high power LEDs
generate enough power to produce intensities of order 100 Suns
across a broad area, as required for the high power source 30 in
the configuration shown in FIG. 2. LED arrays are also eminently
suitable as less optically hazardous replacements for lasers in
broad area PL imaging systems where the sample is brought to a stop
for measurement. Again, the LED array and the camera can be on the
same side of the sample or on opposite sides. LEDs are commonly
packaged with an in-built lens, so that the emission is confined to
a relatively narrow range of angles. Consequently, it should be
possible to use a sharp cut-off dielectric filter as the excitation
filter 26.
[0099] While it would be advantageous to have PL imaging systems
located at several points along a PV cell production line for early
detection of process problems or defective wafers, end of line
testing is particularly important. At present finished cells are
primarily subjected to an IV test where their electrical
characteristics are measured under simulated solar illumination,
but there is clearly much additional useful information that could
be gained besides a global measure of performance. In one example,
high intensity-based PL imaging can be integrated into an IV test
unit, with the resulting spatially resolved defect information used
to determine why particular cells show poor performance in the IV
test instead of just sorting them into quality bins. The existing
.about.1 Sun source of an IV test unit could be used to generate
low illumination intensity PL images, for comparison with high
illumination intensity PL images as discussed previously, e.g. for
improved crack and shunt detection. End of line PL imaging, in
combination with PL imaging at earlier stages, could also be used
as part of a difference or manufacturing execution system (MES)
style image system. Another advantage of combining a PL imaging
system with an IV test unit is that the camera can be used to
acquire electroluminescence (EL) images, where luminescence is
generated by electrical excitation, while the subject cell is
electrically contacted for the IV test. It is advantageous to
gather as much information as possible at this stage, because there
is a risk of breakage of fragile silicon PV cells every time they
are contacted.
[0100] Series resistance (Rs) imaging is a topic of great interest
in the PV cell industry at present, because of its ability to
locate electrical problems such as broken fingers and excessive
contact resistance, as well as cracks that disrupt current flow.
Luminescence systems can be used to determine quantitative series
resistance with EL, PL and various hybrid means, with most of these
methods requiring the capture of at least two images under
different conditions. Incorporation of a flash-based PL imaging
system with its associated camera into an IV test station would
enable PL and/or EL-based Rs imaging techniques, such as those
described in published PCT patent applications WO 07/128,060 A1 and
WO 09/129,575 A1, to be performed. This would be a significant
improvement over the current situation where IV testing can only
measure an average Rs value across a cell.
[0101] However there is a limit as to how many additional tests can
be combined with IV testing without slowing down the entire
production line, recalling that in a sequential process the flow
rate is limited by the slowest step. In the alternative, Rs imaging
can be performed in a separate stage before or after IV testing,
which may require cells to be contacted again.
[0102] However the preferred end of line test device combines the
following attributes: (a) a high intensity PL source (>50 Suns);
(b) a low intensity PL source (<10 Suns); and (c) one or more
spatially inhomogeneous optical filters to illuminate selected
portions of a sample as described in published PCT patent
application No WO 2010/130013 A1. Together with an appropriate
camera this enables the non-contact module to measure: (a) shunts
and other features from the difference of the high and low Sun
images; (b) quantitative series resistance using the inhomogeneous
filters; (c) defects (e.g. impurities) that impact cell performance
and that are more discernible in low Sun measurements; and (d)
features relating to the emitter discernible in high Sun PL images.
This non-contact module would be placed before or after the IV
tester, and preferably used in conjunction with the IV tester to
identify specific faults, causes of specific faults, poor materials
and process errors, as well as provide data that can be used to
improve the manufacturing process via MES systems and the like.
[0103] PL imaging can also be used to inspect individual PV cells
and groups of cells during moduling, where a number of cells are
connected into a single module for installation in the field. For
example a module manufacturer may use PL imaging for quality
control of incoming PV cells, looking for problems such as cracks,
shunts and series resistance issues, or to monitor the cells during
the moduling process, looking for cracks or potential hot spots for
example. As before, the advantages of reduced light safety
requirements with using a non-coherent or short pulse illuminator
apply. Flash lamp illumination may also be useful at the moduling
stage because of its high power; for example instead of
illuminating a single cell with an intensity of .about.100 Suns, a
flash lamp could be used to illuminate a large number of cells with
an intensity of .about.1 Sun.
[0104] The above description is generally written with silicon CCD
or CMOS cameras in mind However we note that other cameras, e.g.
InGaAs cameras, can be used. As described in PCT patent application
No AU2010/001045 entitled `Photoluminescence imaging systems for
silicon photovoltaic cell manufacturing`, incorporated herein by
reference, InGaAs cameras and the like have advantages and
disadvantages compared to silicon-based cameras. The primary
benefit is that such cameras are sensitive throughout the silicon
PL emission spectrum, so that significantly more of the PL signal
can be captured, potentially up to 30.times. more. By capturing
more of the PL signal, the time requirements on the light source
are dramatically reduced for the same measurement time result. For
example a laser pulse can be many times shorter, and even down to 1
ms if the compound semiconductor camera pixel sensor size is much
larger than the equivalent silicon camera. At 1 ms a pulsed laser
may not require laser safety shutters because the total energy in
the pulse may be sufficiently small not to constitute a hazard. In
principle, the safety hazard for any pulsed laser can be reduced by
reducing the pulse length and therefore the total energy per pulse.
The main drawback of such cameras is the image smearing caused by
the fact that the longer wavelength PL light that these cameras
detect may move an excessive distance within the sample prior to
escaping from the surface, due to silicon transparency at these
wavelengths.
[0105] One of the challenges for PL imaging systems is as-cut
wafers or silicon blocks. With surface damage and low quality
silicon at least in the surface region, these types of samples,
especially as-cut wafers, have poor PL response. In applications
such as incoming wafer sorting, there is a need to acquire PL
images rapidly without stopping the samples, preferably with an
illumination system that requires no light safety shutters, and at
reasonable cost. The present invention therefore provides several
options, including short pulse broad area illumination with a
suitably filtered flash lamp and an InGaAs or Si CCD camera, and
laser/LED (1-2 Suns) illumination with an InGaAs camera, e.g. with
640.times.512 pixels, using a number of full frame 1 ms images in a
frame averaging type approach.
[0106] If any of these systems requires too long an exposure for PL
response, again the present invention provides additional
embodiments to allow non-stop measurement, including (a) tracking
of the sample with a mirror during measurement, or (b) frame
averaging a number (say up to 10) 1 millisecond images to reduce
the signal to noise to an acceptable level.
[0107] Another key benefit of fast measurement in a non-contact
system is potential integration into a process tool. One example is
in laser processing of partially processed solar cells, where rapid
PL measurements taken before and after processing would reveal all
positive and negative changes on the sample due to processing by
simple subtraction of the before and after images, thus allowing
quality control and process control. Other process steps where this
technique could be used include screen printing, IV testing,
diffusion and passivation. Integration into the process line is
aided by the simplicity of the PL measurement system--a light
source with optics and a camera that can be arranged in many
configurations. Images can be taken before or after processing, or
both, and processing can be continuous or stop-start.
[0108] The flexibility, accuracy and speed provided by the present
invention finds utility in a number of environments. Yet another
approach to rapid analysis of a semiconductor such as a silicon
wafer is to combine the use of high intensity illumination with a
Time Delay Integration (TDI) based PL imaging system to produce a
combined mechanism/process that is suitable for non-stop in-line
use. In this case, the `high intensity` illumination is high
relative to the intensity used in the prior art for stationary PL
imaging of silicon wafers.
[0109] TDI typically uses a full frame CCD with up to several
hundred lines (rows) of pixels. For the following discussion we
will consider the use of a full frame TDI-CCD with 1024 columns and
128 rows, the performance of which is otherwise similar to CCDs
used in the prior art for stationary PL imaging of silicon wafers,
for PL imaging of p-type raw wafers of approximately 1 Ohm.cm
resistivity. In this case, a continuous illumination source with
intensity approximately in the range of 8 to 16 Suns is required
over an area of approximately 13% of the wafer to measure 3600
wafers per hour with 1 Megapixel resolution.
[0110] FIGS. 10(a)-10(c) show a top view of wafers 52A and 52B and
their movement relative to a TDI sensor 54. Essentially the shaded
areas 56A and 56B within the sensor show the pixels that are
capturing wafer images, with the relative CCD height (number of
rows) exaggerated compared to the sample being measured.
[0111] In FIG. 10(a) the image of the leading edge of a wafer 52A
has integrated on the TDI sensor 54 for half the total time
available, assuming constant wafer velocity, as indicated by the
shaded area 56A extending halfway across the TDI sensor. In FIG.
10(b) the leading edge of the wafer 52A has finished integration
and its PL signal read out. In FIG. 10(c) imaging of the first
wafer 52A is almost complete and imaging of the next wafer 52B has
started, as indicated by the shaded areas 56A and 56B.
[0112] In a further embodiment, a standard CCD camera (i.e. not
optimised for TDI) as used in the prior art for stationary PL
imaging of silicon wafers may be used for the combined high
intensity-TDI PL imaging of silicon wafers. For a throughput of
about 1 wafer per second for instance, this embodiment requires a
readout speed of about 1 MHz to acquire a 1 Megapixel image. This
is well within the capability of standard CCD cameras used in the
prior art for stationary PL imaging of silicon wafers. In this
embodiment, no signal to noise compromise is required to apply the
TDI technique to PL imaging of silicon wafers.
[0113] The limit of TDI gain (number of rows over which an image
can be synchronously built up without significant blurring) is
affected by a number of parameters including: [0114] Lens
distortion; [0115] Belt position synchronisation accuracy or
velocity measurement accuracy; [0116] Alignment of the CCD columns
with the belt direction.
[0117] The abovementioned TDI method may be less desirable for
diffused wafers since the emitter will bias up parts the wafer that
are not being imaged, potentially reducing the photoluminescence
response from the strip being imaged at any given moment. This can
be overcome by over-illuminating the camera field of view, i.e.
providing substantial additional illumination to the target area to
ensure a sufficient PL response. Of course there may be some of
additional equipment cost and increased power consumption.
[0118] The aforementioned TDI method is also advantageous for raw
wafers, since the illumination of the camera field of view does not
have to be uniform in the `vertical` direction (i.e. the CCD column
direction), which corresponds to the belt motion direction, since
images are integrated over this direction. This increases optical
design freedom to address laser safety issues or the design of
illumination systems with horizontal uniformity (i.e. in the CCD
row direction). In a further embodiment, applied particularly to
the case of raw wafers, there may be no need at all for uniformity
of illumination in the horizontal direction. Rather, the
illumination may, for example, consist of staggered rows of
discrete light spots, for example staggered arrays of illuminated
squares, enabling simple projection of LED array light sources onto
the wafer.
[0119] As mentioned above, in one embodiment of the present
invention flash lamp illumination is used to produce a PL response.
This flash method of in-line PL imaging relies on the high
intensity light pulse being short enough to eliminate blurring
caused by sample movement. On the other hand for quantitative
analysis of PL data, and/or for measurement of certain features or
parameters of importance to PV devices, such as shunts, it may be
preferable to use lower intensity illumination. However these
illumination levels require a sample to be stationary to prevent
blurring. One approach that largely overcomes the limitations of
both methods separately is a hybrid of flash and TDI based PL
imaging, referred to hereinafter as `FTDI imaging`.
[0120] A standard square CCD array may be used in this approach.
Since in a normal TDI operation a gain of about 10 would be used,
the following embodiment will be discussed in similar terms. In
this case the height of the image on the CCD plane must be less
than the height of the CCD active area by at least 10 rows; for
example for a 1024.times.1024 CCD, when the top of the image is
aligned with the first row, the bottom of the image must not extend
below row 1014.
[0121] In practice, a margin of about 55 rows is allowed for sample
alignment--this may be reduced or the true number of rows in the
wafer image may be reduced, or both. In this embodiment, the wafer
image moves from top to bottom of the CCD and the alignment margin
is taken to be zero. The flash is triggered when the leading edge
of the wafer reaches a position on the belt where the top of the
wafer image reaches or passes Row 1. The CCD vertical clock
(row-clock) is synchronised to the motion of the wafer until the
image has moved down 10 rows. After that time, the flash lamp
source is off and the CCD may be read out in normal frame-readout
mode (typically using a faster vertical clock than during the FTDI
mode for a 15 cm per second belt speed).
[0122] An FTDI-PL system could also be used in conventional flash
mode. For example this may be used where high light intensity is
advantageous and a short (non-blurring) pulse is adequate (i.e. for
emitter quality measurements on diffused wafers). The FTDI-PL mode
could be used at lower peak intensity (10 or 20 times lower for
example) when bulk lifetime is being measured, or it could be used
at the full intensity for raw wafers where the full pulse energy
(i.e. long pulse) capability of the flash system is desired to
achieve adequate signal to noise. Depending on CCD details, an
FTDI-PL system may be usable in standard TDI mode, with the number
of rows being the maximum possible TDI gain; in typical practice
about 100 rows is the practical maximum. The large pixel size (on
the sample) used in PL imaging in solar cell inspection offers some
scope to increase the typical maximum TDI gain since the row
synchronisation is not as demanding as for smaller sample pixels on
faster moving belts in other industries.
[0123] Although the description above refers to flash TDI, we note
that the embodiment is not restricted to flash but can be any high
intensity short light pulse as described above, e.g. pulsed laser,
LED etc.
[0124] 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.
* * * * *