WO2011017772A1 - Détection de discontinuités dans des matériaux semi-conducteurs - Google Patents

Détection de discontinuités dans des matériaux semi-conducteurs Download PDF

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Publication number
WO2011017772A1
WO2011017772A1 PCT/AU2010/001041 AU2010001041W WO2011017772A1 WO 2011017772 A1 WO2011017772 A1 WO 2011017772A1 AU 2010001041 W AU2010001041 W AU 2010001041W WO 2011017772 A1 WO2011017772 A1 WO 2011017772A1
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Prior art keywords
light
images
image
crack
discontinuity
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PCT/AU2010/001041
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English (en)
Inventor
Thorsten Trupke
Juergen Weber
Ian Andrew Maxwell
Robert Andrew Bardos
Graham Roy Atkins
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Bt Imaging Pty Ltd
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Publication date
Priority claimed from AU2009903824A external-priority patent/AU2009903824A0/en
Application filed by Bt Imaging Pty Ltd filed Critical Bt Imaging Pty Ltd
Priority to CN201080046719.9A priority Critical patent/CN102575993B/zh
Publication of WO2011017772A1 publication Critical patent/WO2011017772A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/95—Investigating the presence of flaws or contamination characterised by the material or shape of the object to be examined
    • G01N21/9501—Semiconductor wafers
    • G01N21/9505—Wafer internal defects, e.g. microcracks
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/64—Fluorescence; Phosphorescence
    • G01N21/6489—Photoluminescence of semiconductors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L22/00—Testing or measuring during manufacture or treatment; Reliability measurements, i.e. testing of parts without further processing to modify the parts as such; Structural arrangements therefor
    • H01L22/10—Measuring as part of the manufacturing process
    • H01L22/12—Measuring as part of the manufacturing process for structural parameters, e.g. thickness, line width, refractive index, temperature, warp, bond strength, defects, optical inspection, electrical measurement of structural dimensions, metallurgic measurement of diffusions
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B2210/00—Aspects not specifically covered by any group under G01B, e.g. of wheel alignment, caliper-like sensors
    • G01B2210/56—Measuring geometric parameters of semiconductor structures, e.g. profile, critical dimensions or trench depth
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/8806—Specially adapted optical and illumination features
    • G01N2021/8822—Dark field detection
    • G01N2021/8825—Separate detection of dark field and bright field
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/0001—Technical content checked by a classifier
    • H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00

Definitions

  • the present invention relates to methods for detecting discontinuities, and in particular cracks, in semiconductor materials.
  • the invention has been developed primarily for detecting cracks in semiconductor wafers or photovoltaic cells or modules either during or after manufacture, however it will be appreciated that the invention is not limited to this particular field of use. Background of the Invention
  • Micro-cracks or cracks in general, are a source of reduced yield in the production of semiconductor devices.
  • cracks are a significant risk because of the fragility of silicon wafers combined with the high throughput (of the order of one wafer per second) and the mechanical handling required at several stages, such as the screen printing of electrical contacts and the tabbing and stringing of individual cells during module manufacture. Severe cracks can cause cell breakage, with breakage rates in
  • photovoltaic cell production being on the order of several percent of total
  • IR infrared
  • electroluminescence infrared
  • the present invention provides a method for detecting a discontinuity in a semiconductor material, said method comprising the steps of:
  • the discontinuity is a crack or an inclusion.
  • more than one discontinuity is identified.
  • the discontinuity may be naturally occurring defects, e.g. inclusions, or those which are induced, e.g. cracks.
  • the light which is generated and guided through the material is externally generated light which is shone into and guided through the material, or is generated within the material (i.e. PL or EL) and then guided through the material.
  • the discontinuity is detected by determining a differential in light intensity in the image, or a contrast difference or change in the image.
  • the light intensity differential may be a bright linear or curvilinear feature in a relatively darker background of the image, or a dark linear or curvilinear feature in a relatively brighter background of the image.
  • the light intensity differential is an abrupt increase or decrease in the intensity of light scattered or transmitted out of the material.
  • step (b) comprises acquiring two or more images, wherein the light is generated and guided so as to propagate laterally through the substantially planar semiconductor material in two or more directions.
  • step (c) preferably comprises monitoring variations in said light intensity differential between the two or more images.
  • step (a) comprises coupling the light into the material at one or more locations.
  • the light is coupled into the material through a textured surface of the material.
  • step (a) comprises illuminating a surface of the material at one or more locations with above band gap radiation, to generate the light as
  • step (b) comprises acquiring, with one or more line cameras, images of said material when said material is in relative motion with respect to said one or more line cameras.
  • the one or more line cameras are adapted to capture light from areas that are substantially parallel to said locations.
  • step (b) comprises acquiring, with two - A - or more line cameras adapted to capture light from areas on either side of each of said locations, images of said material.
  • step (a) comprises applying a forward bias to said material, to generate said light as electroluminescence.
  • the method of the invention further comprises the steps of:
  • step (e) comparing said optical image with the image acquired in step (b).
  • the method of the invention further comprises the steps of:
  • the photoluminescence in the photoluminescence image is generated with an illumination intensity of order 100 Suns or higher.
  • the photoluminescence image is acquired when the material is at elevated temperature following a high temperature processing step.
  • the high temperature processing step is selected from the group consisting of emitter diffusion and metal contact firing.
  • the method of the invention further comprises the step of: (h) calculating, for a discontinuity detected in step (c), one or more parameters selected from the group consisting of length, width, position and shape.
  • the method of the invention is applied to a thin film, wafer, or photovoltaic cell comprised of the semiconductor material, or alternatively is applied to a wafer or photovoltaic cell comprising multicrystalline or monocrystalline silicon.
  • the present invention provides a system for detecting a discontinuity in a semiconductor material, said system comprising:
  • an optical source adapted to couple light into the material at one or more locations such that the light is guided within the material
  • an imaging device sensitive to the light, for acquiring one or more images of the material, whereby the discontinuity is detected on the basis of a light intensity differential in the one or more images.
  • the optical source is adapted to couple light into the material through a textured surface of the material.
  • the present invention provides a system for detecting a discontinuity in a semiconductor material, said system comprising:
  • a source of above band gap radiation adapted to illuminate a surface of the material to generate photoluminescence, at least a portion of which is trapped as light guided within the material;
  • an imaging device sensitive to the light, for acquiring one or more images of said material, whereby the discontinuity is detected on the basis of a light intensity differential in said one or more images.
  • the imaging device comprises a line camera for acquiring images of the material as the material is moved relative to the line camera.
  • the imaging device comprises two line cameras for acquiring images of said material, adapted to collect light from areas of said material on either side of a location where light is launched into or generated within said material.
  • the imaging device comprises a line camera adapted to collect light from an area of said material between two locations where light is launched into or generated within said material.
  • the present invention provides a system for detecting a discontinuity in a semiconductor material, said system comprising:
  • a power supply adapted to apply a forward bias to said material to generate electroluminescence, at least a portion of which is trapped as light guided within said material;
  • an imaging device sensitive to said light, for acquiring one or more images of said material, whereby said discontinuity is detected on the basis of a light intensity differential in said one or more images.
  • system of the invention further comprises:
  • a first camera for acquiring an optical image of said material.
  • system of the invention further comprises:
  • the source of above band gap radiation is adapted to illuminate the material with an intensity of the order of 100 Suns or higher.
  • the source of above band gap radiation is a flash lamp.
  • system of the invention further comprises a processor adapted to analyse the one or more images for a light intensity differential and to detect, based on said light intensity differential, the presence of the discontinuity in the material.
  • the processor is further adapted to analyse said one or more images for variations in said light intensity differential when said light is generated so as to propagate in different directions and to detect, based on said variations, the presence of said discontinuity in said material.
  • the discontinuity is a crack
  • said processor is further adapted to calculate, for said detected crack, one or more parameters selected from the group consisting of length, width, position and shape of said crack.
  • the semiconductor material is a thin film, wafer, or photovoltaic cell. More preferably, the semiconductor material is a wafer or photovoltaic cell comprising multicrystalline or monocrystalline silicon.
  • the present invention provides an article of manufacture comprising a computer usable medium having a computer readable program code configured to conduct the method according to the first aspect or operate the system according to the second, third or fourth aspects.
  • Fig 1 illustrates conventional transmission methods for identifying cracks in a
  • Fig 2 shows a plot of absorption length versus wavelength for silicon
  • Figs 3A and 3B illustrate the interruption by a crack of long wavelength light propagating inside a wafer with smooth surfaces, and how this interruption can be detected
  • Figs 4A to 4C illustrate the interruption by a crack of long wavelength light propagating inside a wafer with textured surfaces, and how this interruption can be detected
  • Fig 5 illustrates the coupling of light from an external source into a wafer having a textured surface
  • Figs 6A and 6B illustrate in plan view and side view a crack detection apparatus according to an embodiment of the invention
  • Fig 7 illustrates in side view an illumination unit suitable for use in the embodiment shown in Figs 6A and 6B;
  • Fig 8 illustrates in side view a crack detection apparatus according to another embodiment of the invention.
  • Figs 9A, 9B and 9C show respectively a photoluminescence image, a visible light image and a long wavelength scattered light image of a fragment of a multicrystalline silicon photovoltaic cell containing a crack;
  • Fig 10 illustrates in plan view a crack detection apparatus according to yet another embodiment of the invention.
  • Figs 1 IA and 1 IB illustrate how the intensity differential caused by a crack can vary with the direction of light propagation with the sample
  • Fig 12 illustrates image contrast effects of variations in grain texture in a multicrystalline sample
  • Figs 13 A and 13B show PL images of a monocrystalline silicon wafer containing several cracks, acquired with ⁇ 1 Sun and -100 Suns illumination respectively.
  • the present invention relates primarily to methods for detecting cracks in semiconductor materials, and in particular to methods for detecting cracks in photovoltaic cells and modules formed from semiconductor wafers.
  • the inventive methods will be described with respect to crack detection in silicon wafer-based photovoltaic cells, but are not limited to this particular field of use.
  • the methods may also be applicable to thin film semiconductor materials, particularly if the substrate has a lower refractive index than the semiconductor material to encourage lateral guidance of long wavelength light in the thin film.
  • the methods may also be applicable to the detection of other forms of discontinuities that disrupt the guidance of long wavelength light, such as large inclusions of silicon carbide in silicon wafers.
  • the methods are of course not limited to silicon materials, but can also be used to detect features of interest (e.g. cracks, inclusions, discontinuities, etc) in a wide variety of other compound and single element
  • semiconductor materials including for example germanium, gallium arsenide, indium gallium arsenide, gallium nitride, indium phosphide, cadmium telluride, zinc selenide and copper indium gallium selenide.
  • illumination 2 at wavelengths where the sample 4 is strongly absorbing will be transmitted by an open crack 6 but absorbed elsewhere in the wafer, so that open cracks will appear as bright features to a detector 8 such as a conventional silicon-based camera.
  • Partially open or closed cracks 9 however cannot be detected reliably.
  • a partially open wedge-shaped crack 10 on the other hand will scatter light differently from non-cracked regions, and can be detected by a contrast change in the transmission of moderately absorbed light 12 that has a penetration depth approximately equal to the wafer thickness.
  • Light that is strongly absorbed will not penetrate to the camera 14 at all, while light that is weakly if at all absorbed will provide insufficient contrast at the camera.
  • the methods of the present invention use light that is sufficiently weakly absorbed within a semiconductor material to allow it to travel distances of several millimetres (or even centimetres) within the sample before its intensity would be below the detection limit of a suitable detector.
  • a suitable wavelength range is 1150-1700 nm, which is very weakly absorbed at room temperature and within the sensitivity range of near- infrared cameras such as InGaAs cameras or silicon cameras in combination with an InGaAs photocathode or some other IR-enhancing component.
  • Fig 2 shows the absorption length of photons in silicon at room temperature showing that for wavelengths > 1180 nm the penetration depth is greater than 10 cm.
  • suitably weakly absorbed light as 'long wavelength light' irrespective of its actual wavelength range, which will of course depend on the sample material.
  • the inventive methods rely on this long wavelength light being guided laterally within the sample over distances of millimetres to centimetres before escaping and being detected, and on the disruption of this lateral guidance by a crack or some other discontinuity, such as a large inclusion, which creates an intensity differential in an image of light escaping from the sample.
  • the refractive index of silicon is ⁇ 3.5 in the near infrared region, creating a large index difference with respect to the air gap within a crack, which will be present even in 'closed' cracks that are difficult to detect with conventional optical reflection or transmission techniques. Even if cracks are filled with a dielectric material such as silicon nitride (n ⁇ 1.8-2.2 depending on composition), commonly used as a passivation and antireflection layer in silicon-based photovoltaic cells, the refractive index difference will still be sufficiently large to influence the light travelling laterally through the wafer.
  • silicon nitride n ⁇ 1.8-2.2 depending on composition
  • long wavelength light 16 is guided laterally in a silicon wafer 4 with smooth surfaces 18 until it encounters a crack 6 that reflects a portion of the light out of the wafer. Assuming the crack is oriented such that the reflected light 20 is in the field of view of a suitable camera 22, the crack will appear bright against a dark background as shown in the intensity versus position line scan 23 shown
  • Fig 3B This picture is, however, not representative for most silicon wafers used in photovoltaic applications.
  • silicon photovoltaic cell wafers 24 typically have textured surfaces 26 to trap incident solar radiation within the wafer, increasing the likelihood of it being absorbed rather than reflected or transmitted.
  • portions 28 of the guided long wavelength light 16 may be scattered out of the wafer at each encounter with a textured surface in a more or less random fashion. Lateral propagation of long wavelength light will therefore be a sequence of scattering events with portions 28 coupled out of the wafer at each bounce, causing a gradual loss of intensity.
  • an imaging camera 22 may see a gradually decaying lateral intensity profile 29 across the region 30 to the left of a crack 6, as in the intensity line scans 23 shown schematically in Figs 4B and 4C. Propagation of the long wavelength light is interrupted by the crack, so that the region 32 to the right of the crack will appear dark to the imaging camera, resulting in a sharp intensity differential 33. Similar to the situation shown in Fig 3A, light 20 reflected by an appropriately angled crack may reach the camera, in which case the crack will appear as a bright linear or curvilinear feature 31 in a darker background as shown in Fig 4B. In other circumstances, e.g.
  • a crack may appear as a dark linear or curvilinear feature in a brighter background. Intensity differentials may be enhanced if a large fraction 34 of the long wavelength light is reflected by the crack back into the wafer, resulting in higher photon density and higher intensity of light scattered out of the wafer in the region to the left of the crack. We note that even if a crack does not extend completely through a wafer, it will still disrupt the propagation of long wavelength light.
  • Figs 3A and 4A are planar silicon wafers with parallel surfaces, which are well-suited for lateral guidance of light. However it will be appreciated that lateral guidance can also occur in non-planar samples, e.g. bowed wafers or samples with monotonically decreasing thickness.
  • the long wavelength light can be made to propagate laterally inside the sample.
  • it is launched or coupled into the sample from an external light source, while in alternative embodiments it is generated as luminescence within the sample.
  • the first approach allows the wavelength range to be chosen according to available light sources, whereas the second approach is limited to the wavelength range of the luminescence that can be generated from the sample material.
  • this luminescence is photoluminescence, i.e. luminescence generated by external illumination with a source of above band gap light such as, for silicon, an 805 nm laser or LED array, or a flash lamp.
  • the luminescence is electroluminescence, i.e.
  • luminescence generated by applying a forward bias to the sample At room temperature, spontaneous emission via band-to-band recombination in crystalline silicon has its peak at about 1140 nm, and contains significant contributions up to about 1250 nm.
  • Luminescence measurements with a suitable long pass filter e.g. an 1180 nm LP filter in front of the detector are thus suitable to exploit the long wavelength scattering effects.
  • textured wafers With textured wafers however, advantage can be made of the textured surface to couple long wavelength light into the wafer. As shown in Fig 5, a significant amount of incident long wavelength light 35 will be scattered or refracted by a textured surface 26 and captured by the wafer 24, within which it can propagate laterally as represented by the arrows 16. A similar effect can occur at the rough surface of an as-cut silicon wafer.
  • the illumination is preferably non- homogeneous across the sample, i.e. localised, with the camera detecting light scattered out of the sample from surface texture, cracks or inclusions in non-illuminated regions.
  • illumination intensity profiles and image acquisition configurations can be used, such as a periodic checkerboard pattern or a line pattern in combination with line scan cameras or area scan cameras.
  • the common aspect is that one or more images of a sample surface will be generated with one or more non-homogeneous illumination patterns, with cracks identified by a contrast in the amount of light escaping from the sample at the site of a crack or in the vicinity of a crack.
  • the images are analysed manually, e.g.
  • the image analysis is automated, using image processing algorithms for pattern recognition performed on single images or on combinations of more than one image, e.g. image comparisons, ratios or differences. Image processing could also occur by fitting a curve/model to a lateral light decay profile and thereby detecting a crack on the basis of the camera image showing large deviations relative to the fitted curve/model. Automated image analysis is preferred for high speed applications such as the in-line inspection of wafers or photovoltaic cells on a photovoltaic cell or module process line.
  • the illumination and detection are from the same surface of the sample, which is advantageous since it allows the technique to be applied to finished or partially processed photovoltaic cells with a fully metallised rear surface.
  • the invention is not limited to this specific embodiment, as illumination and detection from opposite surfaces is also possible and may be applied to partially processed cells.
  • This geometry has the advantage that light reflecting off the incident surface cannot contribute to the signal. Special measures to be described below with reference to Fig 7 may be required to mitigate the influence of reflected light when detection and illumination are from the same surface.
  • a schematic of a crack detection system is shown in Figs 6A (plan view) and 6B (side view).
  • a textured silicon wafer 24 carried on transport belts 36 passes under an illumination unit 48 that illuminates a substantially linear portion 40 of the sample with light in the 1150-1700 nm spectral range.
  • the illumination unit comprises a linear light source 38, which may for example be an array of LEDs, a laser, an array of lasers, a laser diode, an array of laser diodes, arc lamps or flash lamps, in combination with filters to select the required wavelength range if necessary, and a housing (baffle) 42 that prevents light escaping from the top and sides.
  • the light source(s) could be located remotely, with the light guided to the housing by an optical fibre bundle.
  • One or more line cameras 44 (not shown in Fig 6A) generate images of the sample by repeatedly capturing line images of regions 46 as the wafer is moved along on the transport belts.
  • the imaged regions are preferably parallel to the illumination line 40 at a distance R.
  • R The precise magnitude of R is not particularly important, although it should be large enough to reduce reflected light issues (discussed further below) without exceeding the absorption length of the long wavelength light.
  • a reflective sheet may be located adjacent the rear surface of the wafer to reflect escaped light back into the wafer; this would not be required if the rear surface were metallised.
  • the system includes two line cameras 44 and one
  • illumination unit 48 as illustrated in Figs 6A and 6B, with the line cameras imaging regions 46 in front of and behind the illumination line 40 with respect to the direction of motion 54 so that the entire surface of the wafer 24 can be inspected for cracks.
  • Another preferred embodiment uses the reverse configuration, i.e. two illumination units illuminating lines either side of a region imaged by a single line camera. It will be appreciated that a system including only one camera and one illumination unit will be unable to detect cracks in a portion of the wafer extending a distance R from the leading or trailing edge.
  • the light detected by the line scan camera(s) 44 is largely light that has entered the wafer and been guided laterally within the wafer before escaping at the region(s) 46 imaged by the camera(s).
  • One possible structure of an illumination unit 48 is shown in side view in Fig 7, with a baffle 42 having light trapping sections 50 with highly absorbing surfaces positioned on either side of a linear light source 38 to trap 'first bounce' light, i.e. light reflected off the incident surface (line 52). Since the wafer is in motion as indicated by the arrow 54, there needs to be a gap between the illumination unit and the wafer.
  • This gap should be as small as possible to minimise the chance of light (line 56) reflecting off the textured surface 26 at a large angle to reach the camera, although realistically the proximity is limited by fluctuations in wafer thickness, the extent of wafer bowing, and the flatness or stability of the belt system or other wafer moving mechanism. Even if the wafer is stationary, there will preferably be a gap between the illumination unit and the wafer to prevent mechanical damage. The amount of reflected light escaping can be further reduced if necessary by attaching a smooth and flexible absorbing material 58 (such as black silk) to the underside edges of the illumination unit (shown schematically only for the left hand side). This material would glide smoothly over the surface as the wafer moves on the belt, without scratching or otherwise damaging the wafer.
  • a smooth and flexible absorbing material 58 such as black silk
  • the light from the source can be polarised either intrinsically or with a polariser.
  • the reflected light will remain largely polarised and can be blocked with a cross polariser placed in front of the camera, whereas guided and scattered light 62 will have its polarisation scrambled.
  • the illumination unit may also include a diffuser 60 to homogenise the intensity along the illuminated line. Note that the surfaces of the wafer 24 are textured not smooth, as indicated by the irregular reflection angles of each illustrated path including the desired light path 62.
  • a schematic of a crack detection system according to another embodiment is shown in side view in Fig 8.
  • This system comprises one or more long wavelength light sources 38, preferably housed within a baffle 42, positioned proximate to the textured surface 26 of a silicon wafer 24, and an area camera 22 imaging a substantial portion of the wafer surface.
  • a point source could be positioned close to a corner or the centre of the wafer, or a linear source could be positioned at an edge of the wafer. More than one image could be acquired, with the one or more light sources positioned proximate different locations on the wafer surface, to maximise the likelihood of detecting a crack.
  • Fig 9A shows a photoluminescence (PL) image of a fragment 76 of a multicrystalline silicon photovoltaic cell, where the luminescence is generated by illuminating the fragment with an intensity of ⁇ 1 Sun from a near IR laser and detected with a silicon CCD camera as described in detail in published PCT patent application No WO
  • the PL image shows a number of low carrier lifetime (and therefore low PL intensity) features, including dislocation-rich regions 78 and a curvilinear stripe 80 surrounding a crack. Note that the low intensity stripe is
  • the metallisation pattern also appears dark in the PL image, because the metal lines block both the illumination and the luminescence.
  • Fig 9B shows an image of visible light reflected from the cell fragment 76, acquired with a silicon CCD camera, in which the metallisation pattern appears bright. The reflection image also reveals some grain structure in the silicon, but does not reveal the crack. The shadow in the middle of the cell fragment is cast by a 1300 nm LED 82 sitting on the sample surface, but not activated.
  • Fig 9C shows an image of the cell fragment 76 acquired with an InGaAs camera when the 1300 nm LED is activated. No attempt was made to prevent reflected light from reaching the camera, as evidenced by the bright arc 84 surrounding the LED 82.
  • the basis for the methods of the present invention is that cracks intercept the path of light scattered laterally within the sample. It is possible then that linear cracks that happen to extend substantially parallel to the direction of lateral light propagation may cause little or no contrast in light scattered out of the wafer. This potential problem may in fact be relatively insignificant because many cracks, particularly 'mechanical' cracks, are non-linear (for example mechanically- induced cracks in monocrystalline silicon are generally cross-shaped because of the crystallographic axes), and because the in-coupled long wavelength light will generally propagate in a variety of directions. Nevertheless the sensitivity of the inventive method to cracks with various orientations can be enhanced by applying the previously described procedures two or more times with the wafer held at different orientations.
  • the wafer may be rotated by 90 degrees with respect to the light source and camera orientation and the procedure repeated with the same measurement system, or the wafer may be rotated en route between two measurement systems.
  • two measurement systems (each with one or more line sources and line cameras) at different orientations image the wafer sequentially.
  • Fig 10 shows a configuration with two measurement systems 64, each comprising an illumination unit 48 and two line cameras (not shown) imaging the regions 46, oriented at +45 degrees and -45 degrees relative to the movement of a wafer 24 on transport belts 36. This configuration avoids having to rotate the wafer, and increases the likelihood that cracks with different orientation will be revealed in images acquired from the regions 46 by one or other of the measurement systems.
  • a minor disadvantage compared to the
  • Fig 6A is that for a given number of pixels in each camera, the spatial resolution of the images will be reduced by ⁇ 2 because of the diagonal orientation of the cameras.
  • the configuration shown in Fig 10 is merely exemplary, with many other configurations possible. For example three or more sequential measurement systems with different orientations can further increase the likelihood of identifying cracks of all orientations.
  • Fig 8 For systems with area cameras instead of line cameras, as shown in Fig 8 for example, the problem of identifying cracks with all orientations can be addressed by comparing two or more images taken with different illumination patterns that will cause long wavelength light to propagate through the sample in different directions.
  • a crack 6 will intercept the path of long wavelength light 16 propagating inside an illuminated wafer 24, resulting in a distinct intensity differential in the image at the position of the crack, with higher intensity of out- coupled light 28 in the area 30 on the illumination side of the crack.
  • the reliability of crack detection can be increased by acquiring one or more images with light propagating laterally in different directions. For example two images could be acquired with cameras 44 on either side of an illumination source 38 as shown in Figs 6A and 10, or a single image could be acquired with long wavelength light injected from two illumination sources at opposite ends of a wafer.
  • the crack-induced contrast between bright and dark areas is expected to be reversed for light propagating in opposite directions.
  • the position of bright and dark areas 30 and 32 on either side of a crack 6 depends on the direction of long wavelength light propagation 16. This contrast reversal can be analysed by an operator or with specific image processing algorithms, and in short features that appear with opposite contrast in the two images can be identified as cracks with greater confidence.
  • the acquisition of images with long wavelength light propagating in different directions offers a means for determining the width of a crack.
  • the bright area 30 will end at the left hand edge of the crack 6
  • the bright area 30 will end at the right hand edge of the crack.
  • the leading edge of the bright feature may correspond to an edge of the crack.
  • Fig 12 shows an alkaline-textured multicrystalline wafer 66 with a weakly textured (and therefore smooth and shiny) grain 68 surrounded by a strongly textured (and therefore rough) grain 70.
  • Fig 9B shows the visible light image shown in Fig 9B, where different grains have differing reflectivities.
  • a smoothly surfaced grain will tend to confine the long wavelength light laterally with minimal scattering, so will appear dark in an image of the out-scattered light, and conversely a strongly textured grain will appear bright. Consequently a long wavelength image will show an intensity differential at the grain boundary 72 that may be mistaken for a crack.
  • a given grain will influence an image twice, firstly at the position of the light source, and secondly in the area imaging by the camera.
  • a smoothly textured grain will reduce the amount of light coupled both in and out of the wafer, causing the grain structure to influence the image twice in a convoluted manner. Since the same effect does not occur for cracks, suitable deconvolution techniques may assist in distinguishing cracks from boundaries between differently textured grains.
  • long wavelength light can be generated directly inside a sample by either photoluminescence or electroluminescence as mentioned briefly above.
  • This generation technique is essentially independent of the surface quality of the sample, and can be applied to samples with either smooth or textured surfaces.
  • Photoluminescence has the advantage of being contact- less and can therefore be applied to partially processed wafers, whereas electroluminescence requires electrical contacts.
  • Photoluminescence has the additional benefit that the lateral variation of the luminescence generation can be freely controlled by the lateral illumination profile, whereas in electroluminescence the lateral generation profile is largely determined by the positions of the metal contacts of the device/sample under test.
  • photoluminescence also has the advantage that the excitation light is at shorter wavelengths than the luminescence used for detection, and can therefore be filtered out at or before the camera/detector. This relaxes any requirement for an essentially light-tight housing/baffle for the illumination source as shown in Figs 6A, 6B and 7, but a higher sensitivity detector is required to measure the relatively weak luminescence signal, particularly for indirect band gap materials such as silicon.
  • the short wavelength excitation light is absorbed inside the sample within a length typically less than the sample thickness, and the laterally guided long wavelength luminescence detected with a suitably sensitive camera.
  • Area or line scan cameras can be used to detect cracks via light scattering in much the same way as described above for the in-coupling method.
  • the excitation source could be in the wavelength range 500-1000 nm, more preferably 700-900 nm, and the detection would be limited to the spectral range >1100 nm to provide a relatively long propagation length within the wafer, to enhance the sensitivity to cracks.
  • the above described crack detection methods can be performed on a timescale compatible with the throughput of current photovoltaic cell manufacturing lines, of order one sample every second or every few seconds.
  • they are performed in-line with the sample moving on a belt or other continuously moving support, and images captured with one or more line cameras.
  • images can be captured with one or more area scan cameras either with the sample temporarily stationary, or with the sample in motion and the illumination and/or detection optics moving synchronously with it, or with the illumination provided from a pulsed source such as a flash lamp to prevent blurring.
  • a pulsed source such as a flash lamp
  • the crack detection methods of the present invention can be performed off-line as an R&D application, or in-line on a large number of selected samples or even on every sample, by several industries including wafer manufacturers, cell manufacturers and module manufacturers.
  • Wafer manufacturers could use the methods to identify wafers with cracks, with the results used for wafer rejection or quality binning, and to provide feedback to the wafer production process.
  • Photovoltaic cell manufacturers could use the methods for incoming wafer inspection and subsequent binning or rejection, or for crack detection in partially processed wafers for wafer quality binning or rejection, for process monitoring and control, or for crack detection in finished cells resulting in quality binning or rejection.
  • Module manufacturers could use the methods for incoming cell inspection and subsequent rejection or quality binning, or to monitor cell handling and to identify cracks in cells that have been tabbed or integrated into strings or modules.
  • the information about cracks obtained from the inventive methods can be used for process monitoring and process control, e.g. repair or modification of unsuitable processing or handling equipment that introduces or expands cracks.
  • process monitoring and process control e.g. repair or modification of unsuitable processing or handling equipment that introduces or expands cracks.
  • Each industry is likely to be interested in different sets of information on cracks, but in general wafer manufacturers, cell manufacturers and module manufacturers will all be interested in parameters or factors such as crack length and width, the orientation, shape and position of a crack, and whether a given crack is 'natural' (i.e. thermally induced) or mechanically induced, with the latter factor giving guidance as to the origin of a crack.
  • Shape is perhaps the single best indicator of whether a given crack is natural or mechanically induced, since natural cracks tend to be linear while mechanical cracks tend to be three-armed in multicrystalline silicon and cross-shaped in monocrystalline silicon. Crack position is an important factor, recalling that cracks close to a wafer or cell edge are more likely to grow and cause breakage, while crack orientation can also be useful information since a crack extending predominantly parallel to a bus bar is more likely to isolate part of a cell electrically than a crack extending predominantly normal to a bus bar. Information on crack size, i.e.
  • Information on crack size may also be of value simply as an indicator of the seriousness of a crack; for example crack theory predicts that cracks under a certain size act simply as stress relief sites, whereas cracks over a certain size are likely to grow if the sample is subject to differential stress.
  • the long wavelength lateral scattering methods of the present invention are suitable for obtaining information on the length, shape, position and orientation of cracks, and perhaps also width information as described above with reference to Figs HA andllB, depending on the resolution of the imaging device.
  • Photoluminescence imaging is already known to be of use for crack detection, as evidenced by the PL image in Fig 9A, and may provide information on several crack parameters including crack width. It is important to note however that the stripe 80 in the Fig 9A image represents the carrier depletion region along each side of the crack and not the crack itself, and its width is therefore more indicative of carrier transport properties than crack width.
  • PL images acquired with much higher illumination intensities than the ⁇ 1 Sun used for the Fig 9 A image provide a better indicator of crack width, since lateral carrier transport is suppressed, particularly in emitter layers, at high injection level conditions (i.e. high carrier densities) produced by intense illumination.
  • Figs 13A and 13B show PL images of a monocrystalline wafer with several intentionally produced and characteristically cross-shaped cracks 6, where the PL was generated with ⁇ 1 Sun and -100 Suns illumination intensities from a near IR laser and a flash lamp respectively and captured with a silicon CCD camera.
  • the cracks are revealed much more sharply in the high illumination PL image, and the associated carrier depletion features are more likely to provide a measure indicative of the actual widths of the cracks themselves.
  • Illumination intensities greater than 100 Suns are expected to reveal cracks even more sharply.
  • Another method for enhancing the contrast of cracks in PL imaging is to hold the sample at elevated temperature.
  • carrier lifetime-related contrast features such as dislocations and grain boundaries were suppressed in a PL image of a multicrystalline wafer held at 220 0 C, such that cracks became much more clearly visible.
  • This effect which is also expected to occur with electroluminescence imaging, is believed to be due to the temperature dependence of the Shockley-Read-Hall recombination lifetime, which causes the bulk lifetime of defected areas to increase with increasing temperature.
  • the above described temperature effect could find practical implementation in a PV cell manufacturing line by acquiring PL images of wafers at positions in the line where they are already at elevated temperature, e.g. after emitter diffusion or after firing of metal contacts. Of these two possibilities it is probably simpler to acquire PL images after firing since the wafers go through the process tool linearly whereas diffusion is either a batch process or processes several wafers in parallel. Since the samples would be cooling, i.e. not held at constant temperature, it would be advantageous to acquire PL images rapidly, e.g. with flash lamp excitation, at one or more positions chosen for optimal or maximum temperature effect. The high illumination intensity of a flash lamp would, of course, further increase the crack contrast.
  • the lateral scattering methods of the present invention can detect cracks in semiconductor wafers and provide a significant amount of information about those cracks, the methods are likely to be more powerful when combined with other imaging techniques, such as optical imaging and PL imaging, and in particular PL imaging with high intensity illumination and/or elevated sample temperature.
  • PL imaging is more likely than the lateral scattering technique to provide complete information on crack shape, especially for multi-armed cracks where the long wavelength light may not reach all of the arms.
  • comparison between the optical reflection and long wavelength light images of Figs 9B and 9C is informative because of the disappearance of the area 90 in Fig 9C.

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Abstract

L’invention concerne des procédés et systèmes, par lesquels la lumière diffusée latéralement dans un échantillon de semi-conducteur est imagée pour détecter une discontinuité comme une craquelure. La lumière peut être introduite dans l’échantillon en utilisant une source externe de lumière, ou générée in situ comme la photoluminescence de grande longueur d’onde. Les procédés sont décrits par rapport à la détection d’une craquelure dans des tranches en silicium et des cellules photovoltaïques, mais ils sont applicables en principe, à n’importe quelle tranche de semi-conducteur ou matériau à film fin.
PCT/AU2010/001041 2009-08-14 2010-08-13 Détection de discontinuités dans des matériaux semi-conducteurs WO2011017772A1 (fr)

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CN102867861A (zh) * 2011-07-04 2013-01-09 张陆成 太阳电池及修复太阳电池中的裂纹的方法
WO2013187998A1 (fr) * 2012-06-12 2013-12-19 Dow Global Technologies Llc Procédé et appareil de détection de discontinuités dans un panneau solaire
EP3046141A4 (fr) * 2013-09-13 2016-09-28 Kobe Steel Ltd Dispositif d'évaluation pour film fin semi-conducteur oxyde
JP2017133997A (ja) * 2016-01-29 2017-08-03 株式会社東京精密 亀裂検出装置及び亀裂検出方法
WO2018098516A1 (fr) * 2016-12-01 2018-06-07 Bt Imaging Pty Ltd Détermination de l'état de modules photovoltaïques
US9998072B2 (en) 2012-06-12 2018-06-12 Dow Global Technologies Llc Apparatus and method for locating a discontinuity in a solar array
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EP3298390B1 (fr) * 2015-05-04 2021-07-07 Semilab SDI LLC Imagerie par microphotoluminescence avec filtrage optique
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WO2023172472A1 (fr) * 2022-03-07 2023-09-14 Meta Platforms, Inc. Conception d'arrêt de fissure nanophotonique
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WO2012027788A1 (fr) * 2010-09-02 2012-03-08 Bt Imaging Pty Ltd Systèmes et procédés de détection de défauts cristallins dans des semi-conducteurs monocristallins
CN102867861A (zh) * 2011-07-04 2013-01-09 张陆成 太阳电池及修复太阳电池中的裂纹的方法
WO2013187998A1 (fr) * 2012-06-12 2013-12-19 Dow Global Technologies Llc Procédé et appareil de détection de discontinuités dans un panneau solaire
US9998072B2 (en) 2012-06-12 2018-06-12 Dow Global Technologies Llc Apparatus and method for locating a discontinuity in a solar array
EP3046141A4 (fr) * 2013-09-13 2016-09-28 Kobe Steel Ltd Dispositif d'évaluation pour film fin semi-conducteur oxyde
EP3298390B1 (fr) * 2015-05-04 2021-07-07 Semilab SDI LLC Imagerie par microphotoluminescence avec filtrage optique
EP3957981A1 (fr) * 2015-05-04 2022-02-23 Semilab SDI LLC Procede et systeme permettant d'identifier les defauts dans une plaque de semi-conducteur sur la base de la photoluminescence detectee et de la lumiere d'excitation reflechissante
JP2017133997A (ja) * 2016-01-29 2017-08-03 株式会社東京精密 亀裂検出装置及び亀裂検出方法
WO2018098516A1 (fr) * 2016-12-01 2018-06-07 Bt Imaging Pty Ltd Détermination de l'état de modules photovoltaïques
US10724965B2 (en) 2018-02-09 2020-07-28 Massachusetts Institute Of Technology Systems and methods for crack detection
WO2019157271A3 (fr) * 2018-02-09 2019-11-21 Massachusetts Institute Of Technology Systèmes et procédés de détection de fissures
US10768121B2 (en) * 2018-05-22 2020-09-08 The Boeing Company Methods, apparatus, and systems for inspecting holes in transparent materials
EP3599190A1 (fr) * 2018-07-26 2020-01-29 ContiTech Transportbandsysteme GmbH Dispositif d'inspection et installation de courroie dotée d'un tel dispositif d'inspection
WO2020150481A1 (fr) * 2019-01-16 2020-07-23 Rudolph Technologies, Inc. Détection de fissure de tranche
US11703324B2 (en) 2020-11-10 2023-07-18 The Boeing Company Apparatus, systems, and methods for the laser inspection of holes in transparent materials
WO2023172472A1 (fr) * 2022-03-07 2023-09-14 Meta Platforms, Inc. Conception d'arrêt de fissure nanophotonique
WO2023219488A1 (fr) * 2022-05-10 2023-11-16 Tt Vision Technologies Sdn. Bhd. Système d'inspection visuelle de modules photovoltaïques préinstallés

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