TECHNICAL FIELD The present invention relates to inspection apparatus for detecting defects in materials such as substrates. In particular but not exclusively embodiments of the present invention relate to apparatus for detecting defects in silicon solar cell substrates. In particular but not exclusively embodiments of the present invention relate to apparatus for detecting a crack breaking a surface of a material. Some embodiments of the invention may be useful for detecting microcracks breaking a surface of a material.

BACKGROUND

Currently over 80% of the global production of photovoltaic (PV) solar modules employ crystalline silicon cells. The modules are formed by interconnecting the silicon cells and undertaking a lamination process in which a protective coating is applied to encapsulate the solar cells.

The modules are typically designed to last for 25 years or more but the cells are fragile and the problem exists that the cells are frangible and a substantial number of cells suffer cracking or breakage during the module manufacturing process. Modules formed from cracked or broken cells are typically rendered inoperable.

In a large scale module manufacturing process there is a need for early detection of cracks so that the cracked cells can be discarded. However, the relatively rough surface of a silicon cell makes it very difficult to use optical methods for crack detection. Visual inspection is generally considered to be the only reliable method of crack detection at this time.

SUMMARY OF THE INVENTION

Embodiments of the invention may be understood with reference to the appended claims.

Aspects of the present invention provide an apparatus and a method. In one aspect of the present invention for which protection is sought there is provided inspection apparatus configured to detect a defect at an inspection surface of a body of material, the apparatus comprising a source of infra-red radiation and a detector, the apparatus being configured to irradiate the inspection surface of the body of material with a beam of radiation having one or more wavelengths in an inspection range of wavelengths, being the range from 5 to 15 micrometres, by means of the source and to capture by means of the detector an image of source radiation in the inspection range specularly reflected by the inspection surface, the apparatus being configured to detect a defect at the surface of the body of material in dependence on the captured image.

It is to be understood that the apparatus may be configured to detect one or more of a range of different defects including cracks breaking the inspection surface, particles of foreign matter on the inspection surface, pits or chips in the inspection surface and/or one or more other surface defects that result in a change in an amount of radiation from the source that is specularly reflected to the detector. This causes a change in contrast in the image captured by the detector. The detector may be a camera having a two-dimensional imaging element such as a CMOS (complementary metal oxide semiconductor) detector, CCD (charge coupled device) detector, microbolometer detector or any other suitable imaging element.

The apparatus may be configured to detect a crack breaking the inspection surface.

It is to be understood that the solar radiation-facing surface of silicon-based solar cell structures is typically a silicon or doped silicon surface that may be treated to optimise absorption of radiation over the photocurrent-inducing range of wavelengths, being typically defined as the range below 1 100 nm. The silicon surface may be arranged to be relatively rough so as to increase the amount of radiation absorbed. The surface may be textured, optionally having a periodic 3D surface structure. Some structures may be provided with a dielectric and/or metallic coating over the silicon surface to reduce reflection of radiation. Anti-reflection (AR) coatings can be useful in increasing the amount of radiation absorbed. The present applicant has recognised that since solar cell structures are designed to absorb radiation rather than to reflect, optical inspection of such structures is a non-trivial task. The present applicant has therefore devised the present invention to overcome the problem of inspection of highly optically absorbent materials, such as solar cell structures, in a highly convenient and cost effective manner. It is to be understood that radiation of any wavelength in the inspection range may be used such as 5um, 10um, 15um or any wavelength within the range from 5 to 15um. The radiation may comprise a plurality of wavelengths, e.g. comprise a plurality of wavelengths in the range from 5 to 15um such as wavelengths in the range 6 to 12um, 8 to 10um, 13 to 15um and/or any other suitable wavelengths in the range from 5 to 15um.

The radiation may be substantially monochromatic in some embodiments.

Advantageously, the source and detector may be arranged so that they are provided on the same side of the body of material as the inspection surface.

This feature has the advantage that the apparatus may be made more compact and may be easier to retrofit to a production environment. The apparatus may be configured to detect a defect at the surface of the material in dependence on a single captured image.

Thus, the apparatus may be configured to determine, from a single captured image, whether a defect exists at the surface. That is, reference to a plurality of captured images is not required in some embodiments.

The surface may be a free surface of the material, i.e. an exposed surface, not being a buried surface such as a surface coated with a further material resulting in the surface being 'buried' below the further material.

The apparatus may be configured to determine that a defect is present at the inspection surface of a substrate at least in part in dependence on the detection of a change in intensity of detected radiation as a function of position within the captured image. The apparatus may further comprise translation means operable to cause relative movement between the body of material and the source and detector such that an area of the inspection surface irradiated by the source is scanned over the inspection surface, wherein the source is configured to irradiate the body of material with a substantially elongate beam of radiation such that an area of the body of material irradiated by the beam is correspondingly elongate, the irradiated area being elongate in a direction normal to the direction in which the beam is scanned over the inspection surface. The translation means may be configured to translate the body of material relative to the source and detector or to translate the source and detector relative to the body of material. The apparatus may be thereby configured to cause the beam of radiation to be scanned over the inspection surface. As noted above, the source may be configured to irradiate the substrate with a substantially elongate beam of radiation, the beam being elongate in a direction normal to the direction of movement of the source and detector or the direction of movement of the body of material. The irradiated area may be considered to have a longitudinal axis in the plane of the inspection surface. The area of the body of material irradiated by the source may therefore be scanned over the body of material in a direction normal to the longitudinal axis of the area.

It is to be understood that the area of the inspection surface irradiated by the beam of radiation may be defined as an area over which an intensity of the beam exceeds a predetermined proportion of the maximum intensity of radiation falling on the substrate within the irradiated area. The predetermined proportion may be any suitable proportion such as 5%, 10%, 20%, 30%, 40%, 50% or any other suitable value. Thus, although in practice the source may irradiate substantially the whole of the surface of the body of material with at least some radiation, albeit relatively weakly in some areas, the elongate area imaged by the detector may be characterised as the area within which the intensity of radiation exceeds the predetermined proportion of the maximum intensity. The apparatus may be configured to apply a thresholding technique to an image captured by the apparatus such that only data corresponding to radiation having an intensity exceeding a threshold intensity is employed to detect a surface defect.

It is to be understood that in some arrangements contrast in images captured by the detector may be greater in longitudinal edge regions of images captured by the detector. Without committing to a particular theory as to why this is the case, one possible explanation is that saturation of the detector can occur due to the intensity of radiation specularly reflected by the surface in regions where the intensity of specularly reflected radiation detected by the detector is relatively high. However, regions of relatively high intensity of specularly reflected radiation in images captured by the detector are typically found to be in a central longitudinal region of the irradiated area, with the intensity of radiation decreasing towards the leading and trailing longitudinal edges of the area of the beam scanned over the body of material. In these regions saturation of the source tends not to occur due to the decreasing intensity of specularly reflected radiation and contrast between defects at the inspection surface such as cracks and the surrounding region of the inspection surface tends to be relatively high.

The present applicant has found that irradiating the body of material with a substantially elongate beam of radiation such that an area of the body of material irradiated by the beam is correspondingly elongate, the irradiated area being elongate in a direction normal to the direction in which the beam is scanned over the inspection surface, allows surface defects such as cracks breaking the inspection surface to be detected with a surprisingly high degree of reliability. The degree of reliability is such that inspection of the free surfaces of bodies of material such as photovoltaic materials deposited on substrates such as silicon substrates in an automated manner is made commercially viable for the first time by embodiments of the present invention.

The importance and significance of the present apparatus is substantial. Prior art techniques do not permit automated, reliable detection of cracks breaking the free surface of certain materials such as photovoltaic materials. Rather, the industry relies on inspection by the naked eye, which suffers the disadvantage that it is costly and relatively unreliable given the volumes of photovoltaic panels being produced today. The present apparatus enables inspection of substrates coated with photovoltaic materials before they are packaged into modules ready for installation in the field, increasing the yield of working modules and reducing the magnitude of losses suffered by manufacturers.

The translation means may be configured to convey a body of material having an inspection surface past the source and detector, which may remain in a substantially fixed position relative to one another, thereby to allow the apparatus to inspect the surface.

Alternatively or in addition the translation means may be configured to translate the source and detector, which may remain in a substantially fixed position relative to one another, past the body of material to be inspected thereby to allow the apparatus to inspect the body of material.

The apparatus may be configured to scan the area of the inspection surface irradiated by the source over the inspection surface and to capture by means of the detector an image of the inspection surface at a plurality of respective locations of the irradiated area over the inspection surface, the apparatus being configured to determine, by reference to each image in turn, whether a defect is present.

The apparatus may be configured to compared data in respect of defects detected in respective captured images thereby to increase a confidence with which a defect may be identified in a captured image.

Thus it is to be understood that in some embodiments the apparatus may be configured to correlate respective images in order to increase a confidence in a determination that a defect has been detected correctly in one image. For example, if a defect is detected in a predetermined number of images at similar locations of the image or at respective locations consistent with the presence of a defect such as a crack, the apparatus may be configured to provide an indication that a crack has been detected. The apparatus may be configured to generate a map of a surface indicative of the location of one or more defects on the surface by reference to the plurality of captured images.

The map may for example indicate the size and shape of defects such as cracks. The apparatus may be configured to generate an output signal indicative of the detection of a defect when the apparatus determines that a defect is present at the inspection surface.

The apparatus may be configured to generate an alarm when a defect is detected. Alternatively or in addition the apparatus may be configured to log data in respect of the detection of a defect so as to allow the body of material bearing the defect later to be identified. The data may be used to identify the defective body of material and isolate the body of material for processing separately from other bodies of material, for example by recycling of the body of material. Optionally the detector comprises a 2D array of detector elements, each detector element being operable to output a signal corresponding to an intensity of radiation incident thereon.

Optionally, the elongate area of the body of material irradiated by the beam spans substantially the whole of a width of the area of the body of material to be inspected, such that substantially the whole of the area of the body of material to be inspected may be inspected by substantially linear translation of the body of material with respect to the source and detector in a single pass of the body of material past the detector.

In some applications the body of material may be a substrate such as a silicon substrate bearing a coating such as a layer of photovoltaic (PV) material. The source may be configured such that the elongate area of the inspection surface of the layer of PV material irradiated by the source spans substantially the whole width or diameter of the substrate depending on the shape of the substrate. Accordingly, as the elongate area irradiated by the source is scanned over the inspection surface (by movement of the source or the substrate or both), substantially the whole of the area of the substrate may be irradiated in a single pass of the beam of radiation over the inspection surface.

The apparatus may be configured to store spatial intensity data corresponding to an intensity of detected radiation specularly reflected by an inspection surface as a function of relative position of a body of material and the source and detector.

Optionally, the inspection range of wavelengths is in the range of one selected from amongst substantially substantially 7 to substantially 14 micrometres, substantially 7.5 to substantially 14 micrometres and substantially 7.5 to substantially 13 micrometres.

Optionally, the inspection range of wavelengths is in the range from substantially 8 to substantially 12 micrometers.

Advantageously, the detector may be configured to detect radiation substantially only in the inspection range of wavelengths.

The apparatus may be arranged to capture an image of source radiation formed by specular reflection of radiation from the inspection surface at an angle of incidence to the inspection surface normal in the range from around 5 degrees to around 85 degrees, wherein the angle of incidence of radiation from the source is substantially equal to the angle of reflection of radiation towards the detector.

The apparatus may be arranged to capture an image of source radiation formed by specular reflection of radiation from the inspection surface at an angle of incidence to the inspection surface normal in the range from around 20 degrees to around 70 degrees, optionally from around 30 degrees to around 60 degrees, further optionally from around 40 degrees to around 50 degrees, and wherein the angle of incidence of radiation from the source is substantially equal to the angle of reflection of radiation towards the detector.

The apparatus may be arranged to capture an image of source radiation formed by specular reflection of radiation from the inspection surface at an angle of incidence to the inspection surface normal of substantially 45 degrees, wherein the angle of incidence of radiation from the source is substantially equal to the angle of reflection of radiation towards the detector.

The apparatus may be configured to capture an image of source radiation formed by specular reflection of radiation from the inspection surface in a direction substantially parallel to the inspection surface normal, wherein the angle of incidence of radiation from the source is substantially equal to the angle of reflection of radiation towards the detector.

Optionally, the source comprises a source emission element for emitting radiation in the inspection range, the apparatus being configured in use to maintain the source emission element at a substantially constant temperature that is selected to be a temperature in the range from around 20 Celsius to around 300 Celsius, optionally around 20 to around 200 Celsius, optionally 20 to around 100 Celsius, further optionally 20 to around 70 Celsius. It is to be understood that radiation within these ranges has been found by the present inventors to enable surprisingly high quality images of an inspection surface to be obtained, and enable highly reliable detection of surface defects such as cracks breaking the inspection surface. Optionally, the source comprises a source emission element for emitting radiation in the inspection range, the apparatus being configured in use to maintain the source emission element at a temperature in the range of one selected from amongst from substantially 30 Celsius to substantially 50 Celsius and from substantially 35 Celsius to substantially 45 Celsius.

The apparatus may be installed in-line in a solar cell processing facility and configured to detect cracks in solar cell structures conveyed past the apparatus.

In one aspect of the invention for which protection is sought there is provided a solar cell processing facility comprising apparatus according to another aspect of the invention. In a further aspect of the invention for which protection is sought there is provided a method of detecting a defect at an inspection surface of a body of material comprising:

directing infra-red radiation having one or more wavelengths in an inspection range of wavelengths from a source to an inspection surface of the body of material, the inspection range being the range from 5 to 15 micrometres, and

capturing by means of a detector an image of source radiation in the inspection range that is specularly reflected by the inspection surface, the method comprising detecting a defect at the inspection surface in dependence on the captured image. It is to be understood that embodiments of the present invention have been demonstrated to be effective at identifying defects in both substrates used for thin film photovoltaic applications and the thin film photovoltaic materials applied to these substrates. Some embodiments of the present invention may be useful in defect identification in flexible stainless steel used in the roll-to-roll production of flexible thin film photovoltaic modules for building-integrated applications and such inspection has been successfully demonstrated. Both the retrospective integration of apparatus according to embodiments of the present invention into the thin film module manufacturing environment and the 'day one' (non- retrospective) integration of the technology into production equipment are considered viable routes to exploit this process for manufacturing yield enhancement purposes.

In one anticipated application a tool embodying apparatus according to an embodiment of the present invention may be operated in a static mode, where the IR source is arranged to facilitate a specular reflection that is detected by an IR camera. The IR source irradiates substantially a full width of a taut stainless steel sheet as typically found in a high-speed roll- to-roll thin film coating plant and an image formed by the IR camera from the reflected radiation. Distortion of the reflection of source radiation from the IR source detected by the IR camera is useful in identifying the presence and location of a defect at the surface of the material. It is to be understood that, in the inspection of solar cell devices formed on silicon substrates the reflectivity of the inspection surface in respect of radiation incident thereon in the wavelength range 5-15um may be from a few %, such as 1 -2%, to 30 % or more. The present applicant has determined that for solar cell devices a "sweet spot" exists at a wavelength of around 12um where reflectivity of the inspection surface is around 20 for monocrystalline solar cell devices and around 15% for polycrystalline solar cell devices. It is to be understood that the present applicant has found that, at wavelengths below 5 microns, the reflectivity of such inspection surfaces is too low. At visible wavelengths solar cells are designed not to be reflective and may incorporate an antireflection coating as noted above. The relatively low amounts of light reflected at visible wavelengths explains at least in part why inspection of solar cell devices using the naked eye, the typical manner in which inspection of solar cell devices is performed, is prone to error and highly subjective.

Embodiments of the present invention may be used to detect cracks breaking the free surface of mono- and polycrystalline photovoltaic thin films formed on a range of substrates. Similarly, embodiments of the present invention may be used to detect cracks breaking the free surface of mono- and polycrystalline substrates.

The method may comprise providing the source and detector on the same side of the body of material as the inspection surface.

The method may comprise detecting a defect at the surface of the material in dependence on a single captured image.

The method may comprise determining that a defect is present at the inspection surface of a substrate at least in part in dependence on the detection of a change in intensity of detected radiation as a function of position within the captured image.

The method may comprise causing relative movement between the body of material and the source and detector such that radiation generated by the source is scanned over the inspection surface. The method may further comprise irradiating the body of material with a substantially elongate beam of radiation from the source such that an area of the body of material irradiated by the beam is correspondingly elongate, the irradiated area being elongate in a direction normal to the direction in which the beam is scanned over the inspection surface.

The method may comprise scanning the area of the inspection surface irradiated by the source over the inspection surface and capturing by means of the detector an image of the inspection surface at a plurality of respective locations of the irradiated area over the inspection surface, the method comprising determining, by reference to each image in turn, whether a defect is present. The method may comprise comparing data in respect of defects detected in respective captured images thereby to increase a confidence with which a defect may be identified in a captured image. The method may comprise generating a map of a surface indicative of the location of one or more defects on the surface by reference to the plurality of captured images.

In one aspect of the invention for which protection is sought there is provided inspection apparatus configured to detect a defect at an inspection surface of a body of material, the apparatus comprising a source of infra-red radiation and a detector, the source and detector being provided on the same side of the body of material as the inspection surface, the apparatus being configured to irradiate the inspection surface of the body of material with a beam of radiation in an inspection range of wavelengths, being the range from 5 to 15 micrometres, by means of the source and to capture by means of the detector an image of source radiation in the inspection range specularly reflected by the inspection surface, the detector being configured to detect radiation from the source that has been specularly reflected by the inspection surface, the apparatus being configured to detect a defect at the surface of the body of material in dependence on the captured image. It is to be understood that the apparatus may be configured to detect a range of defects including cracks breaking the inspection surface, particles of foreign matter on the inspection surface, pits or chips in the inspection and one/or one or more other surface defects that result in a change in an amount of radiation from the source that is specularly reflected to the detector. This causes a change in contrast in the image captured by the detector. The detector may be a camera having a two-dimensional imaging element such as a CMOS (complementary metal oxide semiconductor) detector, CCD (charge coupled device) detector, microbolometer device or any other suitable imaging element.

The apparatus may be configured to detect a crack breaking the inspection surface.

In one aspect of the invention for which protection is sought there is provided an inspection apparatus for detecting a crack in a substrate, the apparatus comprising a source of infrared radiation and a detector, the apparatus being operable to irradiate a substrate with radiation in an inspection range of wavelengths, being the range from 5 to 15 micrometres, by means of the source and to capture by means of the detector one or more images of the source reflected by an inspection surface of a substrate, the apparatus being operable to detect a crack in a substrate in dependence on the one or more captured images.

Embodiments of the present invention have the advantage that a relatively low cost, reliable method of crack detection may be provided capable of detecting cracks in silicon solar cell structures that are otherwise invisible to the naked eye. The cracks are detected by reference to the one or more captured images.

Embodiments of the present invention have the advantage that crack defects in substrates in the form of substantially planar silicon solar cell structures may be detected reliably. Embodiments of the present invention enable detection at relatively low cost and may be configured to be suitable for in-line inspection of solar cell structures. Apparatus according to some embodiments of the invention may be integrated into a production line and arranged to inspect solar cell structures conveyed therepast with no requirement for contact between the apparatus and the structures. Embodiments of the invention thus provide a non-invasive and non-destructive means for inspecting solar cell structures.

It is to be understood that the radiation may be radiation having one or more wavelengths in the inspection range. The radiation may be substantially monochromatic in some embodiments.

It is to be understood that the detector may be arranged to detect radiation in the whole range from substantially 5 micrometres to substantially 15 micrometres. Alternatively the detector may be arranged to detect radiation that has a wavelength in a range that is a subset of wavelengths or a sub-range of this range, such as from 7 to 15 micrometers. Other ranges are also useful. In some embodiments the detector may be arranged to detect radiation of substantially a single wavelength. Other arrangements are also useful.

The apparatus may be arranged to detect a crack in a substrate by detection of variations in contrast of an image of the source reflected by a substrate.

It is to be understood that the detector may be arranged to detect radiation from the source that has been specularly reflected at an inspection surface of a substrate, being an area of the substrate to be inspected, the detector being operable to detect radiation having a wavelength in an inspection range being a range in the range from substantially 5 to substantially 15 micrometres. It is to be understood that the process of determining automatically whether a crack is present may be performed by means of a computer executing computer program code. Embodiments of the present invention have the advantage that crack defects in substrates in the form of substantially planar silicon solar cell structures may be detected reliably. Embodiments of the present invention enable detection at relatively low cost and may be configured to be suitable for in-line inspection of solar cell structures. Apparatus according to embodiments of the invention may be integrated into a production line and arranged to inspect solar cell structures conveyed therepast with no requirement for contact with the structures. Embodiments of the invention thus provide a non-invasive and non-destructive means for inspecting solar cell structures.

It is to be understood that embodiments of the present invention take a completely new approach to crack detection by working well outside the visible spectrum in the long wavelength infrared region where substrates comprising silicon solar cell structures reflect at least some incident infra-red radiation. It is to be understood that the structures may also transmit some incident radiation. However a sufficient amount of radiation may be specularly reflected to allow apparatus to be provided with excellent crack detection capabilities.

Some embodiments of the present invention are arranged to detect an image of a heat source that is reflected by a silicon solar cell structure and to detect a change in the reflected image upon interaction of the radiation with a crack defect at a surface of the structure. In some embodiments apparatus is arranged to capture images of the heat source as the surface passes the apparatus such that the image of the heat source is effectively scanned across the substrate. In some embodiments a thermal imaging camera operating in the wavelength range from around 8 to around 12 micrometres (μηι) detects radiation that is specularly reflected by the surface such that an angle of incidence of the radiation is substantially equal to an angle of reflection of the radiation. The arrangement of a heat source and a thermal camera in a suitable geometry above a subject Si solar cell has been found to detect isolated crack defects in Si solar cells that are very difficult to see with the naked eye. In some embodiments the crack defects are found to be more readily identified when the heat source is translated relative to the subject cell or vice versa, where either the cell is linearly translated beneath a static thermal reflection cast by the heat source or the heat source is linearly translated above a static subject cell. The specular and diffuse thermal reflections imaged using the detector successfully reveal subtle crack defects across a Si solar cell structure. Images generated by the apparatus may be analysed by computer software, allowing operator-free and reliable identification of crack defects. Images are captured in real-time enabling rapid response whereby defective solar cell structures may be rejected at a production line.

Embodiments of the present invention permit reliable detection of crack defects to be performed before solar cell structures are packaged to form solar PV modules ready for installation in support structures at a point of use, such as on a roof of a building. The apparatus may be operable to determine that a crack is present in an inspection surface of a substrate at least in part in dependence on the detection of a change in intensity of radiation reflected by a substrate and detected by the detector.

The crack may be determined in dependence on this change in intensity exceeding a prescribed amount.

It is to be understood that the apparatus may be operable to determine that a crack is present in the inspection surface if the apparatus detects a change in intensity of radiation exceeding a prescribed amount over a prescribed distance over an inspection surface. The prescribed distance may be a distance corresponding to a prescribed number of pixels of an image captured by the apparatus, and therefore depend at least in part on an optical distance between the detector and the inspection surface. Thus the apparatus may determine that a crack is present at an inspection surface if the apparatus detects a change in intensity of radiation exceeding a prescribed amount over a prescribed number of pixels of an image captured by the apparatus. The image may be an image captured at substantially a single relative location of the apparatus and inspection surface, or an image formed by combining data captured at respective different relative locations of the apparatus and inspection surface. Other arrangements are also useful. Advantageously the detector comprises a 2D array of detector elements. The detector advantageously comprises an imaging device. The imaging device may be a camera device.

The apparatus may be operable repeatedly to determine whether a crack is present in an inspection surface in dependence on the detection of distortion of an image of the source reflected by an inspection surface relative to an uncracked surface. The detector may comprise a 2D array of detector elements, each detector element being operable to output a signal corresponding to an intensity of radiation incident thereon.

Each detector element may comprise a photodiode.

Alternatively the detector may comprise a bolometer device. Each detector element may comprise a bolometer element.

The apparatus may further comprise translation means operable to cause relative movement between the apparatus and a substrate.

The translation means may be operable to convey a substrate past the apparatus thereby to allow the apparatus to inspect a substrate. The translation means may for example comprise a conveyor such as a conveyor belt or other structure for moving the substrates past the apparatus.

The translation means may be operable to translate the apparatus past a substrate thereby to allow the apparatus to inspect a substrate.

The translation means may be operable to cause relative movement between the apparatus and a substrate in order to capture by means of the detector an image of a substrate at a plurality of relative positions of the substrate with respect to the apparatus. The apparatus may be operable such that images of the source are captured by reflection of radiation from substantially the whole of an inspection surface.

Since the apparatus may detect the presence of a crack by detection of distortion of an image of the source, the presence of a crack may be detected at substantially any location over an inspection surface.

It is to be understood that an image of a source is formed by radiation from the source that has been specularly reflected by a substrate to a detector. The apparatus may be operable to store spatial intensity data corresponding to an intensity of detected radiation specularly reflected by an inspection surface as a function of relative position of a substrate and apparatus. The inspection range may be in the range of one selected from amongst substantially 7 to substantially 14 micrometers and substantially 7.5 to substantially 13 micrometers.

The inspection range may be in the range from substantially 8 to substantially 12 micrometers.

The inspection range may be the range of wavelengths greater than 8 micrometers in some embodiments. The range may be in the range from 8.5 to 15 micrometers, or from 9 to 15 micrometers in some embodiments. Other ranges are also useful. The detector may be configured to detect radiation substantially only in the inspection range of wavelengths.

That is, the detector may be configured not to detect radiation that is outside of this range. This may be at least in part due to an inherent feature of the detector. Alternatively or in addition it may be at least in part due to the presence of a filter arranged to transmit radiation only in a predetermined range of wavelengths.

The apparatus may be arranged to detect an image of the source formed by reflection from a substrate at an angle of incidence in the range from around 20 degrees to around 70 degrees.

The apparatus may be arranged to detect an image of the source formed by reflection from a substrate at an angle of incidence of substantially 45 degrees. The source may comprise a source emission element for emitting radiation in the inspection range, the apparatus being arranged in use to maintain the source emission element at a substantially constant temperature that is selected to be a temperature in the range from around 20 Celsius to around 300 Celsius, optionally around 20 to around 200 Celsius, optionally 20 to around 100 Celsius, further optionally 20 to around 70 Celsius. The apparatus may be arranged in use to maintain the source emission element at a temperature in the range of one selected from amongst from substantially 30 Celsius to substantially 50 Celsius and from substantially 35 Celsius to substantially 45 Celsius. It is to be understood that choice of source temperature may be considered to be dictated at least in part by two key considerations in some embodiments. One is the contrast between an area of an inspection surface in a captured image from which an image of the source is reflected and an area from which an image of the source is not being reflected. In some embodiments it has been found advantageous if this contrast is greater than around 7 to 8 Celsius. Another is that if the temperature is too high, the detector signal may become saturated.

The apparatus may be configured to illuminate the substrate with IR radiation such that the image of the source captured by the detector spans substantially an entire width of a substrate to be inspected.

This feature has the advantage that if the substrate is scanned past the source (or the source scanned past the substrate) substantially the entire area of the substrate may be irradiated with IR radiation having a wavelength in the inspection range and an image of the source may be formed by reflection by the substrate over substantially the whole area of a substrate.

In some embodiments the substrates are around 154mm (6 inches) square. Other sizes are also useful.

The source may be arranged to be substantially elongate.

Alternatively or in addition the source may be curved in some embodiments. It is to be understood that a range of different shapes may be useful for the source. The source may be of any suitable shape and is not limited to being elongate and/or curved.

The source emission element may for example comprise a heated rod, wire, coil such as a helical coil or any other substantially elongate element that may be heated. However it is to be understood that a source of substantially any shape may be employed. The apparatus may be arranged to detect a difference in the reflected image of the source by the substrate in the chosen wavelength range relative to that which would be expected in the case of a substrate not having a crack. It is to be understood that some substrates may be at least partially bowed, resulting in a corresponding distortion of the reflected image. The apparatus may be configured to distinguish between distortion due to inherent features of an uncracked substrate such as bowing, from distortion due to the presence of a crack. For example the apparatus may be configured to detect relatively abrupt changes in the intensity of reflected light in an image that is characteristic of the presence of a crack and to ignore relatively gradual changes due for example to bowing.

The apparatus may be configured to detect changes in image intensity in an edge region of an image of the source reflected by a substrate.

In one aspect of the invention for which protection is sought there is provided apparatus according to a preceding aspect installed in-line in a solar cell processing facility and configured to detect cracks in solar cell structures conveyed past the apparatus.

In a further aspect of the invention for which protection is sought there is provided a method of detecting a crack in a substrate comprising:

directing infra-red radiation in an inspection range of wavelengths from a source to an inspection surface of the substrate, the inspection range being the range from 5 to 15 micrometres, and

capturing by means of a detector one or more images of the source reflected by the inspection surface, the method comprising detecting a crack in the substrate in dependence on the one or more captured images. In other words, the method comprises detecting a crack in the substrate by reference to the one or more captured images.

In an aspect of the invention for which protection is sought there is provided inspection apparatus for detecting a crack in a substantially planar substrate comprising a source of infra-red radiation and a detector arranged to detect radiation from the source that has been specularly reflected at an inspection surface of a substrate [being an area of the substrate to be inspected], the detector being operable to detect radiation having a wavelength in an inspection range being a range in the range from substantially 5 to substantially 15 micrometres. In one aspect of the invention for which protection is sought there is provided a method of inspecting an inspection surface, comprising:

directing infra-red radiation from a source at an inspection surface of a substrate; and

detecting by means of a detector radiation from the source that has been specularly reflected at the inspection surface,

whereby detecting radiation comprises detecting radiation having a wavelength in an inspection range of wavelengths in the range from substantially 5 to substantially 15 micrometres.

Optionally the range may be from substantially 7 to substantially 15 micrometres.

It is to be understood that by the term specular reflection is meant that an angle of incidence of detected radiation at the surface is substantially equal to the angle of reflection of the radiation from the surface.

The apparatus may be operable to illuminate an inspection surface with IR radiation from the source such that an angle of incidence of the radiation with respect to a reference substrate normal is substantially equal to a reference incidence angle over substantially an entire inspection surface area, the detector being arranged to detect radiation specularly reflected from an inspection surface wherein an angle of reflection with respect to the reference normal is substantially equal to the reference incidence angle.

The reference substrate normal may be a direction parallel to a support surface arranged to support the substrate during inspection, the support surface defining a reference plane of the substrate. In the case of an uncracked substrate, the reference plane may be the plane of the substrate. For example, a plane of a face of the substrate which rests on a support surface such as a platen or conveyor. In the case of a cracked substrate the reference plane may be the plane of a portion of the substrate that is substantially parallel to the support surface.

It is to be understood that in the case of a point source and a point detector, the area of an inspection surface for which the angle of incidence will be equal to the reference incidence angle will also be a point. However, real sources of IR radiation are typically of finite size and the source may be arranged to be of a size such that an area of the sample of suitable size may be illuminated with IR radiation substantially at the reference incidence angle for a given relative position of the source and substrate. The source may be arranged to be an elongate source in some embodiments. The apparatus may be arranged whereby an inspection surface is scanned laterally with respect to an elongate axis of the source, thereby to illuminate an area to be inspected with IR radiation at the reference incidence angle with respect to a substrate normal.

It is to be understood that local variations in the direction of a surface normal of the inspection surface may occur due for example to the presence of a grain having a surface facet with a surface normal not parallel to the reference substrate normal. Importantly, some embodiments of the present invention ensure that such all areas of the inspection surface, being the surface of interest to be inspected, are illuminated with radiation at the reference angle of incidence with respect to the reference substrate normal. Depending on the size of the angle between the reference substrate surface normal and local surface normal of the crystal grain, any specular reflection of radiation incident on such a crystal grain at the reference incidence angle with respect to the reference substrate normal may not be detected by the camera even if the substrate did not have any cracks, since the detector is arranged to detect radiation that is specularly reflected such that the angle of incidence and angle of reflection with respect to the reference substrate normal (and not a local surface normal) are each substantially equal to the reference incidence angle.

However, in the case that the substrate does have a crack, the applicants have found that portions of the inspection surface on opposite sides of a crack are typically oriented such that their local surface normals are non-parallel. Typically, the apparatus is arranged wherein when a substrate is supported on the inspection surface at least one portion of the substrate bears a portion of the inspection surface for which the surface normal is parallel to the reference substrate normal and therefore results in specular reflection that is detected by the detector. One or more other portions of the substrate with local surface normal not parallel to the reference substrate normal are found to cause specular reflection that is not detected by the detector if the local surface normal deviates sufficiently from the reference substrate normal.

The apparatus may be operable to detect radiation that has been specularly reflected from the inspection surface and to generate spatial intensity data corresponding to an intensity of detected radiation specularly reflected by the inspection surface as a function of position over at least a portion of the inspection surface. In a further aspect of the invention for which protection is sought there is provided a method of inspecting an inspection surface, comprising: directing infra-red radiation from a source at an inspection surface of a substrate; and detecting by means of a detector radiation from the source that has been specularly reflected at the inspection surface, whereby detecting radiation comprises detecting radiation having a wavelength in an inspection range of wavelengths in the range from substantially 5 to substantially 15 micrometres.

Some embodiments of the present invention provide inspection apparatus comprising a source of infra-red radiation and a detector arranged to detect radiation from the source that has been specularly reflected at an inspection surface of a substrate being an area of the substrate to be inspected, the detector being operable to detect radiation having a wavelength in an inspection range being a range in the range from substantially 7 to substantially 15 micrometres. In a further aspect of the invention for which protection is sought there is provided an inspection apparatus configured to detect a defect at an inspection surface of a body of material, the apparatus comprising a source of infra-red radiation and a detector, the apparatus being configured to irradiate the inspection surface of the body of material with a beam of radiation having one or more wavelengths in an inspection range of wavelengths by means of the source and to capture by means of the detector an image of source radiation in the inspection range specularly reflected by the inspection surface, the apparatus being configured to detect a defect at the surface of the body of material in dependence on the captured image. The inspection range of wavelengths may be any suitable range. The source may be configured to emit radiation of substantially a single wavelength in the inspection range, or radiation comprising radiation having a plurality of wavelengths in the inspection range.

For example, the source may be configured to emit radiation having a wavelength in the range from 5 micrometres and upwards, from 4 micrometres and upwards, from 6 micrometres and upwards, from 5.5 micrometres and upwards, from 4 to 20 micrometres, from 5 to 20 micrometres, from 5.5 to 20 micrometres, from 6 to 20 micrometres, from t to 15 micrometres, from 5.5 to 15 micrometres, from 6 to 15 micrometres or any other suitable range. Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination. For example features described in

connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

For the avoidance of doubt, it is to be understood that features described with respect to one aspect of the invention may be included within any other aspect of the invention, alone or in appropriate combination with one or more other features.

Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which:

FIGURE 1 is a schematic illustration of apparatus according to an embodiment of the present invention showing a source, a detector and a structure to be inspected;

FIGURE 2 is a schematic illustration of the apparatus of FIG. 1 showing ray traces illustrating specular reflection from a cracked surface;

FIGURE 3 shows (a) profilometry line scans of a cracked region (trace T1 ) and an uncracked region (trace T2) of a structure under inspection and (b) a corresponding optical micrograph of the structure;

FIGURE 4 shows (a) a plan view of a solar cell; (b) an image of the cell shown in (a) captured by means of the detector of the apparatus of FIG. 1 at one particular position of the cell with respect to the source and detector; and (c) a cross-sectional view of the structure of the cell; FIGURE 5 shows (a) a schematic showing a portion of a captured image showing a portion of an image of a source and (b) a line scan of image intensity being a plot of image intensity as a function of distance along a line that is fixed with respect to the image shown in (a);

FIGURE 6 shows images captured by means of a detector at source temperatures of (a) 43 Celsius, (b) 67 Celsius and (c) 94 Celsius and substrate temperatures of (i) 24-25 Celsius, (ii) 40 Celsius and (iii) 55-60 Celsius; and FIGURE 7 is a schematic illustration of apparatus according to a further embodiment of the present invention showing a source, a detector and a structure to be inspected.

DETAILED DESCRIPTION FIG. 1 is a schematic illustration of inspection apparatus 100 according to an embodiment of the present invention. The apparatus 100 has an infra-red (IR) source 1 10 and an IR detector in the form of a camera 120. The camera 120 is coupled to a computing device 105 in the form of a laptop computing device 105 operable to receive and store images generated by the camera 120.

The source 1 10 is arranged to generate IR radiation and to direct the IR radiation towards a conveyor 130 below the source 1 10. The conveyor 130 is arranged to convey substrates 135 in the form of solar cell structures past the source 1 10 and detector 120 to enable inspection of the substrates 135 in a solar cell production line. The conveyor 130 conveys the substrates 135 past the apparatus 100, below the source 1 10 and camera 120 in a substantially continuous manner. The source 1 10 and camera 120 are arranged in a reflection geometry (as opposed to a transmission geometry) whereby the camera 120 is arranged to detect IR radiation from the source 1 10 that has experienced specular reflection by a substrate 135.

It is to be understood that, because inspection takes place substantially wholly by reference to radiation reflected from a surface of a material facing the source 1 10 and detector 120, some embodiments may be employed in combination with existing manufacturing equipment relatively easily. Some embodiments may be incorporated into or retrofitted to existing manufacturing equipment with little or no modification of the manufacturing equipment. In contrast, inspection techniques requiring transmission of radiation through a material such as a substrate, with a detector located on an opposite side of the material, may not be compatible with existing manufacturing equipment, at least without substantial modification thereto. The camera 120 has a receiving aperture 120AP arranged to receive IR radiation. The camera 120 is arranged to capture an image of the source 1 10 reflected by a substrate 135 and to detect the presence of a crack by reference to contrast associated with the image of the source 1 10. By specular reflection is meant that an angle of incidence A in of radiation incident on the substrate 135 with respect to a substrate reference normal N is substantially equal to an angle of reflection A refl of the radiation with respect to the substrate reference normal N. Reference normal N is normal to a plane on which the substrate 135 rests In the arrangement shown in FIG. 1 the source 1 10 and camera 120 are arranged such that the camera 120 detects specularly reflected radiation for which A in and A refl are both substantially 45 degrees. Other angles are also useful. For example, in some embodiments A in and A refl are substantially equal and in the range from 30 to 60 degrees. Other values above and below this range are also useful. However it is to be understood that non- normal incidence, i.e. where A in and A refl are not zero, typically results in more reliable detection of crack defects in a given substrate in embodiments of the present invention.

FIG. 2 is a perspective view of the apparatus 100 of FIG. 1 . The IR source 1 10 has a linear source element in the form of a rod 1 10R that is arranged to be heated to a temperature of substantially 40-45 Celsius thereby to generate IR radiation of the desired wavelength. The camera 120 is in the form of a bolometer camera device and is operable to detect IR radiation in the range from around 8 micrometres to around 12 micrometres. The apparatus 100 is configured such that the camera 120 does not detect radiation that is outside of this range of wavelengths, although other arrangements are also useful. In some embodiments a filter may be provided to block radiation that is not within this range of wavelengths, such that such radiation is not detected by the camera 120.

In the embodiment shown, the source 1 10 is located a distance d1 of substantially 0.15m above an upper inspection surface of a substrate 135 to be inspected whilst the camera 120 is located a distance d4 of substantially 0.45m above the inspection surface. The optical path length for specular reflection in the manner described above is a distance d2 of 0.25m ]from the source 1 10 to the substrate 135 and a distance d3 of 0.6m from the substrate 135 to the camera 120, giving a total path length from the source 1 10 to the camera 120 of 0.85m. Other values of one or more of d1 , d2, d3 and d4 may be useful in some embodiments. FIG. 3(a) shows surface profile linescans obtained by means of a profilometer taken from cracked (trace T1 ) and uncracked (trace T2) regions, respectively, of a substrate 135. Trace T1 was recorded over a region where two cracks were present. The locations of the cracks along trace T1 are highlighted by circles C1 and C2. FIG. 3(b) is a corresponding optical micrograph of the cracked region showing one of the cracks. The location of the line at which the crack intersects the inspection surface of the substrate 135 is shown arrowed in the figure.

The change in gradient of trace T1 of FIG. 3(a) on opposite sides of each crack indicates that portions of the inspection surface of the substrate 135 on opposite sides of each crack are non-coplanar. Embodiments of the present invention exploit this feature to enable detection of a crack in a substrate 135 as described in more detail with respect to FIG. 2.

FIG. 2 shows the apparatus 100 in a process of inspecting a cracked substrate 135'. The crack is shown at 135C. Ray traces R1 , R2 and R3 for three substantially parallel, coplanar rays of IR radiation generated by the source 1 1 OR are shown. The rays are specularly reflected from the surface of the cracked substrate 135'. It can be seen that rays R1 and R2 are specularly reflected from the cracked substrate 135' on one side of the crack 135C whilst ray R3 is specularly reflected by the cracked substrate 135' on the opposite side of the crack 135C. Each of rays R1 , R2, R3 travel at an angle A in with respect to a substrate reference normal N.

The rays R1 , R2 are reflected such that A in and A refl are both substantially equal to 45 degrees and the rays R1 , R2 are detected by the camera 120. The rays remain substantially parallel to one another following reflection at the cracked substrate 135'.

However, portions of the substrate 135' on opposite respective sides of the crack 135C are non-coplanar due to the presence of the crack 135C. thus, for ray R3 the angle of incidence of ray R3 with respect to a local substrate normal N' is shown as angle A in'. The corresponding angle of reflection A refl' is also shown. Because N and N' are sufficiently different, ray R3 is not captured by camera 120 following reflection by the surface of the cracked substrate 135'. That is, ray R3 is no longer parallel to rays R1 and R2 following reflection. Thus, it can be understood that contrast will be present in an image generated by the camera 120 of the surface of the cracked substrate 135' due to the presence of the crack 135C. FIG. 4 shows (a) a photograph taken in the visible range of wavelengths of a cracked substrate 135' in plan view, (b) an image of the cracked substrate 135' recorded by the camera 120 and (c) a schematic illustration of the structure of the cracked substrate 135' in cross-section. From inspection of FIG. 4(a) it is apparent that the crack in the substrate is not revealed in the image.

The image of FIG. 4(b) was obtained at one particular location of the cracked substrate 135' with respect to the source 1 10 and camera 120 as the cracked substrate 135' was translated with respect to the source 1 10 and camera 120 in the direction of arrow X. As can be seen in FIG. 4(a), the cracked substrate 135' has two substantially parallel primary metallisation lines M1 , M2 thereon. The metallisation lines M1 , M2 were formed by depositing silver on the substrate 135' during fabrication thereof. The lines M1 , M2 produce two relatively bright, parallel linear features L1 , L2 in the IR image of FIG. 4(b). The substrate 135' also has multiple metallisation sub-lines M' running thereacross, substantially orthogonal to lines M1 , M2. The sub-lines M' are arranged to conduct to the primary metallisation lines M1 , M2 charge that is generated in the substrate 135' in regions away from the primary lines M1 , M2. These are also revealed in FIG. 3(a) as periodic spikes in the profilometer trace.

In addition to the linear features L1 , L2 in the image of FIG. 4(b) a substantially elongate region R1 of relatively high intensity IR radiation relative to the remainder of the image can be seen. This region represents an image of the source 1 10. It is formed by IR radiation specularly reflected from the cracked substrate 135'. It is to be understood that some diffusely scattered IR radiation may also contribute to the image, however it is understood that the contrast is primarily due to specularly reflected IR radiation. It can be seen that in a lower portion of region R1 relatively abrupt variations in image contrast are present. One particularly abrupt region is indicated by an arrow at C1 . This region was found to correspond to the location of a crack in the cracked substrate 135'. The crack is not visible in the photograph of FIG. 4(a). It is to be understood that the upper and lower regions of region R1 where image intensity falls relatively rapidly correspond to regions where image intensity is particularly sensitive to misorientation of a local inspection surface normal N' with respect to the reference substrate normal N. The size of this region is determined primarily by the size of the source 1 10.

It is to be understood that in the present embodiment the computing device 105 is operable to process images detected by the camera 120 in order to detect cracks 135C in the cracked substrate 135'. The images may be in the form of video footage captured by the device 105. The footage may be analysed in real time in some embodiments. The video footage may be stored at least temporarily in a database for later analysis and/or playback. The footage may be deleted automatically by the apparatus once it has been analysed.

In the present embodiment, the computing device 105 processes image data to detect substantially abrupt changes in image contrast that are indicative of the location of a crack. The computing device 105 is also configured to correlate data obtained in respective images of different areas of the cracked substrate 135' in order to increase a level of confidence in any determination that a crack 135C is present in a given region of a cracked substrate 135' based on analysis of a single image. In some embodiments, the computing device 105 is provided with a reference image of the source 1 10. The reference image may be obtained by reflection of radiation from the source 1 10 by an uncracked substrate 135 in some embodiments, or an uncracked region of a cracked substrate. Alternatively the reference image may be generated from data not derived from a captured image, for example by means of an algorithm or any other suitable method.

The computing device 105 may be configured to compare a captured image from a substrate with the reference image and to detect the presence of a crack in dependence on the comparison. The comparison may be made by applying a correlation function to the captured image with respect to the reference image, and detecting a crack in dependence on the value of one or more correlation coefficients between the images or portions of the images corresponding to an image of the source.

In some embodiments, distortion of the image of the source may be detected by analysis of a line scan across an image of the source 1 10 in a region where an image of the source is expected to be present. It is to be understood that the image of the source in a captured image may remain at substantially the same location within the image since the source and detector remain in substantially fixed positions with respect to one another, and the substrates 135 are of similar sizes and shapes. The computing device 105 may be configured to detect variations in intensity of an image along a line scan through an image, i.e. variations in intensity at image points defining a line, preferably a substantially straight line. The computing device 105 may determine that a crack is present if the image intensity varies by more than a prescribed amount over a prescribed number of pixels.

FIG. 5(a) is a schematic illustration of a portion of an image captured by the camera 120 in which a portion of an image R1 of the source 1 10 reflected by the cracked substrate 135' may be seen. The image can be seen to have a portion R1 S that is displaced or shifted with respect to a major portion of the image. This displaced portion R1 S is a consequence of the presence of a crack in the substrate 135'. It is to be understood that in the image shown, portions R1 and R1 S are of higher intensity than the remaining portion of the image since they correspond to infra-red radiation reflected by the cracked substrate 135' and detected by the camera 120. The computing device 105 is configured to perform a linescan across a substantially fixed region of each image captured by the camera 120. The position of the linescan is shown at LS in FIG. 5(a). An intensity of pixels along the linescan LS are plotted as a function of position in the graph of FIG. 5(b). When the intensity of a portion of the linescan LS exceeds a prescribed amount over a prescribed distance, the computing device 105 determines that a reflected image of a substrate 135' is present. The computing device 105 analyses the data in respect of the linescan LS in order to determine whether a crack is present. As noted above, in the present embodiment, the computing device 105 is configured to determine that a crack is present if a variation in linescan intensity over a prescribed number of pixels exceeds a prescribed amount. In some embodiments the prescribed number of pixels may be arranged to correspond to a distance of around 5mm to around 10mm with respect to the portion of the surface of the substrate along which the line scan is performed. In the embodiment of FIG. 1 the line scan LS is performed over a line that is arranged to be close to an edge of an image of a source 1 10, in order to obtain increased contrast due to the presence of a crack. The line scan LS is arranged to be substantially parallel to a longitudinal axis of the source 1 10 and is therefore substantially straight in the present embodiment. Other shapes of line scan LS are also useful, such as a curved shape or any other suitable shape. In the case that the source 1 10 is curved, the line scan may be curved in a corresponding manner.

FIG. 4(c) shows the structure of the substrate 135' of FIG. 4(a) in cross-section. The substrate 135' has a p-type doped portion 135p and an n-type doped portion 135n formed over the p-type doped portion. A rear side of the substrate 135' has a layer of silicon dioxide 135b formed thereover with a layer of aluminium 135a deposited over the silicon dioxide layer 135b, forming a 'back contact' layer of the substrate 135'. Linear openings 135bA are provided in the silicon dioxide layer 135b directly below metallisation sub-lines M' and filled with a conducting material to allow conduction of charge between the p-type doped layer 135p and the aluminium layer 135a.

Above the n-type doped layer 135n a second silicon dioxide layer 135c is provided. Above this layer 135c an anti-reflection (AR) coating 135d is provided. Metallisation sub-lines M' formed from silver are deposited on the AR coating 135d. An electrical connection is provided between each metallisation sub-line M' and n-type doped layer 135n through the AR coating 135d and silicon dioxide layer 135c.

FIG. 6 shows a series of images captured by the camera 120 of the same cracked substrate 135' in the same relative position with respect to the source 1 10 and camera 120 at various combinations of source temperature and substrate temperature. Images (i) - (iii) of sets (a), (b) and (c) were captured at substrate temperatures of (i) 24-25 Celsius, (ii) 40 Celsius and (iii) 55-60 Celsius respectively. The images of set (a) were recorded at a source temperature of substantially 43 Celsius. The images of set (b) were recorded at a source temperature of substantially 67 Celsius whilst the images of set (c) were recorded at a source temperature of substantially 94 Celsius.

It can be seen from the images that the lower the substrate temperature, the higher the image contrast.

It has been found that, whilst contrast may decrease with increasing substrate temperature, crack defects can nevertheless be identified reliably in substrates that are still at temperatures above room temperature following thermal processing as part of the manufacturing process of known solar cell structures. Accordingly, inspection of the substrates may be performed whilst the substrates 135 are cooling to a temperature suitable for further processing of the substrates 135. Accordingly, some embodiments of the present invention may be integrated into a solar cell manufacturing facility and employed to detect cracks in solar cell structures without increasing a length of time required to manufacture solar cell structures.

In an embodiment of the invention, a source temperature of substantially 43 Celsius is typically employed, providing excellent image contrast.

FIG. 7 is a schematic illustration of inspection apparatus 200 according to a further embodiment of the present invention. Like features of the apparatus 200 of FIG. 7 to those of the apparatus 100 of FIG. 1 are shown with like reference signs incremented by 100. As in the embodiment of FIG. 1 the apparatus 200 has an IR source 210 and an IR detector in the form of a camera 220. The camera 220 is coupled to a computing device 205 in the form of a laptop computing device 105 operable to receive and store images generated by the camera 220. In the embodiment of FIG. 7 the source 210 and camera 220 are arranged in a reflection geometry (as opposed to a transmission geometry) whereby the camera 220 is arranged to detect IR radiation from the source 210 that has experienced specular reflection by a substrate 235. The source 210 is arranged to direct a beam B of radiation at a reflector element 215 that is configured to specularly reflect the beam B towards the substrate 235 in a direction substantially parallel to the substrate normal N. The camera 220 has a receiving aperture 220AP arranged to receive IR radiation that has been reflected by the substrate 230 in a direction substantially normal to the substrate 230. As shown in FIG. 7, radiation specularly reflected by the substrate 230 passes back through the reflector element 215 to the camera 220.

It is to be understood that, because the receiving aperture 220AP has a finite width (or diameter) normal to the substrate surface normal, in practice radiation that has been specularly reflected by the substrate 230 within a finite range of angles to the substrate surface normal may be detected by the camera 220. The magnitude of the range of angles depends on the size of the aperture 220AP and the distance of the camera 220 from the substrate 230. Typical magnitudes of this range of angles are in the range from 0.01 degrees to 1 degree. Other values may be useful in some embodiments. The configuration of the apparatus 200 of the embodiment of FIG. 7, in which the angle of incidence of radiation from the source and the angle of specular reflection of radiation detected by the detector are substantially zero with respect to the surface normal, has the advantage that the source 210 and camera 220 may be provided in the same relatively compact package. It is to be understood that in some embodiments alignment of the source 210 and camera 220 may be made more robust and less prone to misalignment when provided in a single package rather than as separate components.

Embodiments of the present invention are useful in reducing costs and increasing a yield of working, packaged solar cell modules. This is because substrates with defective surfaces, such as surface having a crack breaking the surface, may be identified and rejected before module assembly.

Embodiments of the present invention are useful in identifying cracks in silicon solar cell substrates during in-line processing of silicon solar cell substrates and/or during lay-up of finished cells during module manufacture where the cells are integrated into modules. Other applications of embodiments of the present invention are also useful.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. blank page upon filling