U.S. patent application number 09/821426 was filed with the patent office on 2001-12-27 for method and apparatus for inspection of a substrate by use of a ring illuminator.
This patent application is currently assigned to TRW Inc.. Invention is credited to Arenberg, Jonathan W., Casement, L. Suzanne, Chodzko, Richard A., Chou, Mau-Song.
Application Number | 20010054693 09/821426 |
Document ID | / |
Family ID | 22481063 |
Filed Date | 2001-12-27 |
United States Patent
Application |
20010054693 |
Kind Code |
A1 |
Chou, Mau-Song ; et
al. |
December 27, 2001 |
Method and apparatus for inspection of a substrate by use of a ring
illuminator
Abstract
A method and apparatus for the inspection of a substrate
provides consistent detection of defects such as cracks,
differentiates between different types of defects, and, does not
excessively heat the substrate. An infrared radiating source
produces infrared energy which illuminates the substrate in a
uniform manner at an incident angle. An infrared camera collects a
portion of the infrared light which is reflected from the
substrate. An image is created from the collected light which
includes indicia of the defect. The image is examined as the
incident angle is varied for changes in the appearance of the
indicia.
Inventors: |
Chou, Mau-Song; (Rancho
Palos Verdes, CA) ; Chodzko, Richard A.; (Rancho
Palos Verdes, CA) ; Casement, L. Suzanne; (Fountain
Valley, CA) ; Arenberg, Jonathan W.; (Santa Monica,
CA) |
Correspondence
Address: |
Patent Counsel
TRW Inc.
S&EG Law Department, E2/6051
One Space Park
Redondo Beach
CA
90278
US
|
Assignee: |
TRW Inc.
|
Family ID: |
22481063 |
Appl. No.: |
09/821426 |
Filed: |
March 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09821426 |
Mar 29, 2001 |
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09138230 |
Aug 21, 1998 |
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6236044 |
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Current U.S.
Class: |
250/341.8 |
Current CPC
Class: |
G01N 25/72 20130101;
G01N 21/8806 20130101 |
Class at
Publication: |
250/341.8 |
International
Class: |
G01N 021/55 |
Claims
What is claimed is:
1. An apparatus for detecting a defect in a substrate comprising:
an infrared radiating source spaced from said substrate and
positioned to substantially uniformly illuminate said substrate
with infrared light, an infrared camera with a lens positioned to
collect reflected infrared light from said substrate; and, means
for creating an image from said reflected infrared light, said
image including indicia representative of said defect.
2. The apparatus as in claim 1, wherein said substrate comprises a
solar cell.
3. The apparatus as in claim 1, wherein said infrared radiating
source is in the shape of an annulus having a first opening, said
infrared radiating source positioned between said lens and said
substrate such that said substrate is visible to said camera
through said first opening.
4. The apparatus as in claim 3, wherein said infrared radiating
source is spirally coiled wire formed into an annular ring.
5. The apparatus as in claim 4, wherein said coiled wire is of
nickel/chromium material.
6. The apparatus as in claim 5, further comprising: an insulator
coupled to said spirally coiled for supporting said spirally coiled
wire.
7. The apparatus as in claim 6, wherein said insulator has a second
opening, said insulator positioned such that said substrate is
visible to said camera through said second opening.
8. The apparatus as in claim 7, wherein said insulator further
comprises a circular annular groove facing said substrate, and,
said spirally coiled wire positioned within said circular annular
groove.
9. The apparatus as in claim 6, wherein said insulator is a ceramic
material.
10. The apparatus as in claim 9, wherein said ceramic material is a
transite material.
11. The apparatus as in claim 1, wherein said camera has a first
spectral response and said radiating source has a second spectral
response, said first and second spectral responses being
approximately matched.
12. The apparatus as in claim 11, wherein said first and second
spectral responses are from a wavelength of 1 to 5 microns.
13. The apparatus as in claim 1, wherein said substrate includes a
cover glass, a long-pass filter being placed between said substrate
and said lens, said long-pass filter approximately blocking
radiation below a 2.0 micron wavelength.
14. A method for detecting a defect in a substrate comprising the
steps of: illuminating said substrate with an infrared light source
in an approximately uniform manner, collecting reflected infrared
light from said substrate, producing an image from said reflected
infrared light; and, examining said image for indicia
representative of said defect.
15. An apparatus for detecting defects in a substrate and for
determining a type of said defect: an infrared radiating source
spaced from said substrate and positioned to substantially
uniformly illuminate said substrate with infrared light at an
incident angle, an infrared camera with a lens positioned to
collect reflected infrared light from said substrate; and, means
for creating an image from said reflected infrared light, said
image including indicia representative of said defect; and, means
to change said incident angle.
16. The apparatus as in claim 15, wherein said means to change said
incident angle is provided by moving said infrared radiating source
closer to or further from said substrate.
17. The apparatus as in claim 15, wherein said indicia
representative of a substrate crack type defect is a dark image
which remains a dark image with said change of said incident
angle.
18. The apparatus as in claim 15, wherein said indicia
representative of a surface anomaly type defect is a dark image
which changes to a bright image as said incident angle is
increased.
19. The apparatus as in claim 15, wherein said indicia
representative of a delamination type defect is a bright image
which remains a bright image with said change of said incident
angle.
20. A method for detecting a defect in a substrate and for
determining at type of said defect comprising the steps of:
illuminating said substrate with an infrared light source in an
approximately uniform manner at an incident angle; collecting
reflected infrared light from said substrate; producing an image
from said reflected infrared light; examining said image for
indicia representative of said defect; and, varying said incident
angle while continuously monitoring said image for changes in the
appearance of the indicia.
21. The method of claim 20, wherein the step of varying said
incident angle further comprises the step of varying said incident
angle by moving said infrared light source closer to or further
away from said substrate.
22. The method of claim 21, wherein the step of varying said
incident angle while continuously monitoring said image further
comprises the step of changing said incident angle while
continuously monitoring said image, a crack type defect producing a
dark image at all said incident angles, and, a surface anomaly type
defect producing a dark image which changes to a bright image as
said incident angle is increased.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to inspection
devices, and more particularly to an device which can perform
inspections of a substrate by an infrared imaging technique.
[0002] Cracks or defects in a substrate can result in substrate
failures or failures in the system in which the substrate is
placed. When the substrate is used as a solar cell, the crack or
defect has the potential to severely limit the power output of the
solar panel which contains the defective solar cell. Once a crack
has begun, it is highly probable that it will propagate over time
to develop into a more significant crack; therefore, it is
important to detect not only large but also small cracks. Flight
program specifications typically require few or zero cracks in
solar panels many square meters in size.
[0003] A well-known method for inspecting solar cells involves
illuminating the solar cell from the side with a tungsten halogen
lamp and imaging the returned light with an infrared camera. Since
this technique requires illuminating the solar cell from one side,
it typically results in one side of the solar cell being too bright
and the other side being too dim such that the solar cell is not
uniformly illuminated. This lack of uniform illumination can mask
small cracks, thereby leaving them undetected.
[0004] This prior art method also typically requires operating the
tungsten halogen lamp at a relatively high intensity level in order
to provide sufficient illumination of the entire solar cell.
Because a tungsten halogen lamp has a quartz envelope, it strongly
absorbs energy in the 2 to 5 micron wavelength range of interest.
To compensate for such absorption losses, a tungsten halogen lamp
often operates at a relatively high temperature resulting in a
large fraction of the light output power having shorter
wavelengths. The shorter wavelength radiation, especially in the
1.0 to 2.0 micron range can result in glare due to strong
reflections of the solar cell cover glass in the 1.0 to 2.0 micron
wavelength range. Additionally, this high intensity level adds heat
to the solar cell, which can result in thermal expansion of the
cell, closing cracks temporarily during the inspection process such
that those cracks avoid detection. Excessive heating of the solar
cell can also result in ambiguous cell crack detection since the
cell re-radiates the heat which is then imaged by the camera.
Features in the image can appear as cracks even though no cracks
exist.
[0005] In addition, the current method cannot objectively and
definitively differentiate surface anomalies from substrate cracks.
Surface anomalies are mainly caused by the presence of
contaminants, such as solvent stains, on the surface of the
substrate. The contaminant is generally not harmful to solar cell
performance and typically does not result in a rejection of the
solar cell. For the current method, the image of a surface anomaly
is typically more diffuse that the image of a cell crack; however,
in many cases it is difficult to distinguish between a cell crack
and a surface anomaly based solely on the appearance of the image.
As a result, reinspection of the cell to distinguish between a
crack and a surface anomaly is typically required, resulting in
increased inspection time and cost. It is desirable to distinguish
surface anomalies from cell cracks to avoid any unnecessary
rejection of good cells or unnecessary rework of an otherwise good
cell.
[0006] What is needed therefore is an apparatus and method for
detecting defects in a substrate such as a solar cell which
provides consistent detection of defects such as cracks,
differentiates between types of defects, and, does not excessively
heat the substrate.
SUMMARY OF THE INVENTION
[0007] The preceding and other shortcomings of the prior art are
addressed and overcome by the present invention which provides an
apparatus for detecting a defect in a substrate which includes an
infrared radiating source spaced from the substrate and positioned
to substantially uniformly illuminate the substrate with infrared
light, an infrared camera with a lens positioned to collect the
reflected infrared light from the substrate and means for creating
an image from the reflected light, the image which including
indicia representative of the defect.
[0008] In a second aspect, the present invention provides a method
for detecting a defect in a substrate, including the steps of
illuminating the substrate with an infrared light source in an
substantially uniform manner, collecting reflected infrared light
from the substrate, producing an image from the reflected infrared
light and examining the image for indicia of a defect.
[0009] In still another aspect, the present invention provides an
apparatus for detecting defects in a substrate and determining the
type of each defect which includes an infrared radiating source
spaced from the substrate and positioned to substantially uniformly
illuminate the substrate with infrared light at an incident angle,
an infrared camera with a lens positioned to collect the reflected
infrared light from the substrate, means for creating an image from
the collected light which includes indicia representative of the
defect and means for varying the incident angle.
[0010] The foregoing and additional features and advantage of this
invention will become apparent from the detailed description and
accompanying drawing figures below. In the figures and the written
description, numerals indicate the various features of the
invention, like numerals referring to like features throughout for
both the drawing figures and the written description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Reference is now made to the detailed description of the
preferred embodiments illustrated in the accompanying drawings, in
which:
[0012] FIG. 1 is a side view of the inspection device in accordance
with the preferred embodiment of the present invention;
[0013] FIG. 2 is a detailed end view of the ring illuminator
illustrated in FIG. 1; and, FIG. 3 is a diagram showing an image of
a solar cell using the present invention, illustrating an image
indicia of defects.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Referring to FIG. 1, an inspection device 10 for inspecting
a substrate 12 and detecting defects such as cracks and surface
anomalies is illustrated. The inspection device 10 of the present
invention is preferably utilized to inspect solar cells 12 because
it provides consistent, accurate detection of cracks and accurate
differentiation between cracks, surface anomalies and delamination
type defects.
[0015] The present invention is not limited to detecting cracks,
surface anomalies and delamination type defects in a solar cells,
but may also be utilized to detect defects in other substrates such
as silicon, gallium arsinide, germanium and other substrates used
in integrated circuits or energy collecting devices.
[0016] As is illustrated in FIGS. 1 and 2, the inspection device 10
includes an infrared radiating source 14 and infrared camera 16.
The infrared radiating source 14 produces infrared energy 18 which
is utilized to illuminate the substrate 12. The source 14 can be
any radiating source 14 which provides uniform illumination of a
substrate 12, but it is preferred that the source 14 be a ring
illuminator 14 which is a radiating source in the shape of an
annulus or a ring. A ring configuration provides a symmetrical
radiation pattern such that the substrate 12 is uniformly
illuminated from a constant .theta. angle. As shown in FIG. 2, for
the preferred embodiment of the invention, the ring illuminator 14
is comprised of a small diameter heater wire of a material such as
nickel/chromium, which is spirally coiled and formed into an
annular ring of a first diameter 20. A 14 centimeter first diameter
20 is preferred for ease in handling and for providing good uniform
illumination at both a far and a near distance from the substrate
12 (FIG. 1). However, the present invention is not limited to a 14
centimeter first diameter 20 (FIG. 2) but may be practiced with
different size ring illuminators 14 having a variety of first
diameters 20.
[0017] Referring to FIGS. 1 & 2, for the preferred embodiment
of the invention, the ring illuminator 14 is supported by an
insulator 24. The insulator 24 can be formed from any material that
has low thermal conductivity and can withstand the temperatures
associated with the ring illuminator 14, however, it is preferred
that the insulator 24 be fabricated of a transite material. The
ring illuminator 14 can be fastened to the insulator 24 by use of
ceramic standoffs (not shown); however, it is preferred that the
insulator 24 contain a groove 25 in which the ring illuminator 14
is placed. The ring illuminator 14 can be secured into the groove
25 by several fasteners 26 which are preferably fabricated of a
ceramic material. An opening 28 is provided in the insulator 24
such that the camera 16 can view the substrate 12 through the
opening 28.
[0018] Infrared radiation 18 emitted from the ring illuminator 14,
is incident on the substrate 12 at an angle .theta., and
illuminates the substrate 12. The substrate 12 in turn reflects the
incident infrared radiation 18 in all directions. The lens 30 of an
infrared camera 16 views the substrate 12 through the opening 28 in
the insulator 24. A portion of the reflected radiation is incident
on the lens of the camera 16 and is captured and collected by the
camera 16. The camera 16 provides the means to create an image 32
from the collected reflected infrared light. The lens 30 of the
camera 16 is positioned along an axis 34 approximately normal to
the substrate 12 such that a substantial amount of surface area of
the substrate 12 can be focused within the field of view of the
camera 16. The camera 16 preferably has a spectral response that
matches the spectral outputs of the ring illuminator 14. One such
camera 16 is a platinum-silicide infrared camera 16 such as an
Infracam.TM. made by Inframatrics Inc., located in North Billerica,
Mass. The Infracam has a spectral response from a wavelength of 1
to 5 microns. The lens 30 is preferably a close-up lens 30 which
can be used to view a portion of the substrate 12 or the entire
substrate 12.
[0019] Many substrates 12 such as solar cells 12 include a cover
glass 36 with a coating that reflects strongly in the 1 to 2.0
micron wavelength range. For such solar cells 12, a long-pass
filter 38 may be added to the camera 16 to block the light
reflected from the cover glass 36 and reduce any glare from such
reflected light. Alternatively, the lens 30 can be made of a
germanium material which acts as a filter to block radiation below
a 2.0 micron wavelength.
[0020] For the preferred embodiment of the invention, a light
shield 40 is placed between the lens 30 of the camera 16 and the
insulator 24 to shield the camera 16 from any radiation from the
ring illuminator 14 and the insulator 24. The light shield 40 can
be formed integral with the insulator 24 or as a separate unit.
Preferably, the light shield 40 is a tube of polyvinyl chloride
material having an interior surface 42 which is non-reflective to
infrared light.
[0021] For the preferred embodiment of the invention, the camera 16
provides the means to create an image 32 from the collected
reflected infrared light. A separate processor, computer, or the
like can also provide the means to create and record an image 32.
The image 32 is preferably displayed on a monitor 44. If the
substrate 12 is a solar cell 12 which is free of cracks and surface
anomalies, the image 32 will be uniform in appearance except for
the grid structure 46 inherent in the solar cell structure. If the
substrate 12 includes defects, the image 32 will display indicia 48
and 50 of the defects.
[0022] When inspecting a solar cell 12, for a large distance d and
a small incident angle .theta., the indicia of both cell cracks 48
and surface anomalies 50 are displayed as dark images. To
distinguish between a surface anomaly 50 and a cell crack 48, the
ring illuminator 14 is repositioned closer to the solar cell 12
along the axis 34, thereby decreasing the distance d and increasing
the incident angle .theta.. The image 32 is monitored during the
movement of the ring illuminator 14. A cell crack 44 will appear
dark regardless of the distance and the incident angle .theta.. On
the other hand, a surface anomaly 50 will be displayed as a dark
image when the ring illuminator 14 is positioned at a significant
distance from the cell 12 such that the cell 12 is illuminated at a
small .theta. angle. The image 50 fades away as the ring
illuminator 14 is repositioned closer to the solar cell 12 and the
.theta. angle increases. The image 50 eventually reappears as a
bright image as the ring illuminator 14 is further repositioned
closer to the solar cell 12 illuminating the solar cell 12 at an
even greater .theta. angle. The image 32 of the surface anomaly 50
appears dark at a .theta. angle between approximately 20 and 40
degrees, the image fades away at about 45 to 46 degrees and becomes
distinctly brighter at about 58 degrees. The present invention thus
differentiates surface anomalies 50 from cell cracks 48 and
provides excellent, repeatable results when applied to solar cells
12 having textured surface structures, such as high efficiency
silicon cells, and can be used for inspection of bare solar cells
12 or solar cells 12 having a cover glass 36.
[0023] In addition, the present invention does not excessively heat
the substrate 12. A power input (not shown) into the coiled heater
wire of the ring illuminator 14 of about 35 Watts typically
provides a level of infrared radiation 18 which adequately
illuminates a solar cell 12 without a cover glass 36, and, a power
input of about 45 Watts typically provides a level of infrared
radiation 18 which adequately illuminates a solar cell 12 with a
cover glass 36. These power input levels translate to temperatures
of the ring illuminator 14 of approximately 325 to 375 degrees
Celsius respectively which is significantly lower than the prior
art tungsten halogen lamp such that heating of the solar cell 12 is
reduced. Therefore, a cell crack 48 can now be detected more
consistently with minimal excess heating.
[0024] The present invention also allows for a large surface area
of a substrate 12 to be illuminated substantially more uniformly
when compared to that achieved in the prior art because the present
invention provides symmetrical radiation at a constant incident
angle .theta. from all directions resulting in a uniformly
illuminated substrate 12.
[0025] In addition, the present invention provides a method to
differentiate cell cracks 48 from surface anomalies 50. However,
the present invention is not limited to detection of cell cracks 48
and surface anomalies 50 but can also detect other types of defects
in substrates 12 including delaminations between the substrate 12
and a second material, such as a metallized material, bonded to the
substrate 12. Delaminations are displayed as bright images and
remain bright with changes in illumination angle .theta. whereas
substrate cracks 48 are displayed as dark images and remain dark
with changes in illumination angle .theta., surface anomalies 50
are displayed as a dark image which changes to a bright image with
increases in illumination angle .theta..
[0026] It will be appreciated by persons skilled in the art that
the present invention is not limited to what has been shown and
described hereinabove, nor the dimensions of sizes of the physical
implementation described immediately above. The scope of invention
is limited solely by the claims which follows.
* * * * *