WO2012027788A1

WO2012027788A1 - Systems and methods for detecting crystal defects in monocrystalline semiconductors

Info

Publication number: WO2012027788A1
Application number: PCT/AU2011/001122
Authority: WO
Inventors: Wayne Mcmillan; Thorsten Trupke; Roger Kroeze
Original assignee: Bt Imaging Pty Ltd
Priority date: 2010-09-02
Filing date: 2011-09-01
Publication date: 2012-03-08

Abstract

Methods and systems are presented for detecting crystal defects such as slip lines in substantially monocrystalline semiconductor wafers and ingots using photoluminescence imaging. A sample of a substantially monocrystalline semiconductor such as Cz-grown or cast monocrystalline silicon is illuminated with light suitable for exciting band-to-band luminescence, one or more images of the luminescence acquired, and the images processed to obtain information about the prevalence or location of crystal defects in the sample. The methods are rapid and non-destructive, unlike existing chemical etching/optical imaging techniques, and the information derived can be used by manufacturers of substantially monocrystalline semiconductor ingots or wafers, or manufacturers of photovoltaic cells produced from such materials, to improve the quality of their products.

Description

Systems and Methods for Detecting Crystal Defects in Monocrystallinc

Semiconductors

Field of the Invention

The present invention relates to the characterisation of monocrystalline semiconductors, and in particular to the detection of crystal defects in silicon using photoluminescence imaging.

Related Applications

The present application claims priority from Australian provisional patent application Nos 2010903951 and 2010905464, the contents of which are incorporated herein by reference.

Background of the Invention

The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field. Commercial photovoltaic (PV) cells are most commonly manufactured from

multicrystalline (mc) silicon wafers that are cut from a cast mc silicon block. In a tradeoff of efficiency versus cost, PV cells can also be manufactured from monocrystall ine silicon (mono-Si), a higher quality material with much lower impurity levels than mc silicon. One method for producing mono-Si is the Czochralski (Cz) process, where cylindrical ingots typically 100 to 300 mm in diameter, and occasionally up to 450 mm in diameter, are pulled from a silicon melt, optionally doped with boron or phosphorus if p- type silicon or n-type silicon is required. The Cz process can be performed as a batch process from a fixed volume silicon melt, or as a continuous process from a replenished silicon melt, with the continuous version requiring more complex process control but producing ingots with a more uniform axial distribution of impurities. After the top and bottom taper sections have been cut off, an ingot 2 is cut into wafers 4 typically 1 0-200 μιτι thick as illustrated in Fig 1 A, for delivery to PV cell or integrated circuit

manufacturers. As shown in end view in Fig I B, cylindrical ingots 2 are often "shaped^", i.e. their sides sawn off, to produce a squared off ingot 6, prior to wafer cutting. Ingots up to 2 m long can currently be produced with the Cz process, although the ingots are typically cut into more manageable pieces, e.g. 25-30 cm long, for wafer cutting. The C/ process can also be used to obtain single crystals of other semiconductor materials such as germanium and gallium arsenide. Other crystal growing techniques suitable for growing monocrystalline semiconductors include the Bridgman-Stockbarger technique.

If an ingot is pulled from the melt too quickly, leading to excessively large temperature gradients, defects appear in the crystal structure. In particular slip lines, also know n as slip dislocations, occur along the crystal planes of the silicon, especially in the tail region of the ingot. Slip lines and other crystal defects such as dislocations and stacking faults can also be caused by poor temperature control, material quality issues, and the shock induced when an ingot is cut from the melt. In the context of PV cells, slip lines and other crystal defects represent a disruption to the periodic crystal structure and therefore introduce electronic states that can act as recombination centres. These recombination centres reduce the minority carrier lifetime and are therefore detrimental to the open circuit voltage, the short circuit current and the fill factor of PV cells, thereby degrading their overall efficiency. The detrimental effect of slip dislocations on these parameters in mono-Si PV cells has been demonstrated in K.L. Pauls, K. W. Mitchell and W. Chesarek 'The Effect of Dislocations on the Performance of Silicon Solar Cells', Proceedings of the 23^,d IF, HE Photovoltaic Specialists Conference (10- 14 May 1993), pp 209-2 13.

Furthermore this detrimental effect can be enhanced by the presence and/or precipitation of impurities which can be captured in the vicinity of slip lines and other crystal defects. Slip lines and their associated recombination centres are also detrimental in the microelectronics manufacturing industry. Cast monocrystalline silicon, also referred to as 'seeded casting^' silicon, has been proposed as a less expensive alternative to Cz-grown mono-Si for PV applications, see for example published US patent application No 2007/0169684 A I . Cast mono-Si ingots generally contain small sections of multicrystalline material so are not perfectly monocrystalline, however for the purposes of this specification the term 'substantially monocrystalline' is used to refer to cast mono-Si as well as conventional mono-Si formed by the Cz process or other known methods. Cast mono-Si ingots, like blocks of traditional multicrystalline Si, are cut into square shaped columns (typically 10x 10 cm^" up to 22 x 22 cm² in area), commonly known as 'bricks', before being cut into wafers.

Slip lines and certain other crystal defects in mono-Si can be detected with optical imaging (reflection or transmission) techniques once the sample has been etched, for example in a CrCVHiO-HF solution as described in .H. Yang 'An Htch for Delineation of Defects in Silicon', Journal of the Electrochemical Society vol 1 1 (5) pp. 1 140- 1 145 ( 1984). Fig 2 shows an optical image of the polished face of a chemically etched 2 mm thick slug 7 of Cz-grown silicon, where preferential etching has exposed slip lines 8 extending in from the edges of the slug in mutually perpendicular directions. I lowever this etching process is slow, uses dangerous chemicals (particularly HI^") and is destructive, and is therefore unsuitable for the rapid screening of semiconductor samples such as silicon wafers being fed into a PV cell or integrated circuit manufacturing line.

Wafer manufacturers currently discard the slip line-rich tail region of a Cz-grown silicon ingot in a laborious process where a 'slug' of silicon is cut from the tail end of the ingot at a location estimated by the operator, then etched and imaged optically. If slip lines are seen the process is repeated until no slip lines arc present, and the remainder of the ingot can then be sawn into wafers. Not only is this process time consuming, but it involves dangerous chemicals and entails the waste of at least some good quality silicon. Su in marv of the nvention

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. It is an object of the present invention in its preferred form to provide improved methods and systems for detecting crystal defects in bulk samples of monocrystailine semiconductors, and bulk samples of monocrystailine silicon in particular.

In accordance with a first aspect of the present invention there is provided a method for inspecting a substantially monocrystailine semiconductor material, said method comprising the steps of:

(a) illuminating an area of said material with excitation light suitable for generating photoluminescence from said material;

(b) acquiring at least one image of the photoluminescence emitted from said material; and

(c) processing said at least one image to obtain information on the prevalence or location of crystal defects in said material.

Preferably, the method is used to obtain information on the prevalence or location of slip lines, dislocations or stacking faults in the material. In preferred forms the information comprises the area sum or area average of the crystal defect density, or the relative distribution, spatial distribution, length distribution or total length of crystal defects in the material.

The area may be illuminated in a single illumination step, or in line-scanning fashion or in point-wise fashion. Each image may be acquired with an area camera, a line camera, a TDI camera or a photo-detector. Preferably, the processing step further comprises identifying low minority carrier lifetime regions in the material. In one preferred form the method is used for monitoring or controlling the quality of substantially monocrystalline semiconductor wafers entering a photovoltaic cell manufacturing line. In another preferred form the method is applied to an ingot or brick of substantially monocrystalline semiconductor material, and the method further comprises the step of: (d) utilising the information to guide the cutting of wafers from the ingol or brick. Alternatively the method is applied to a wafer, ingot or brick of substantially monocrystalline semiconductor material, and the method further comprises the step of: (c) utilising the information to apply a classification to the wafer, ingot or brick. Preferably, the classification is used to alter parameters associated with a series of processing steps in the formation of a photovoltaic cell from the material, so as to improve the quality of the photovoltaic cell. Alternatively, the classification comprises binning, rejecting or pricing of the material.

In one preferred form step (b) comprises acquiring at least two images in di fferent wavelength bands, and the method further comprises the step of: (0 calculating, from a comparison of two images in different wavelength bands, a measure of the bulk carrier lifetime across the area of the material.

Preferably, the method is applied to substantially monocrystalline silicon.

In accordance with a second aspect of the present invention there is provided a method for monitoring or controlling the quality of substantially monocrystalline semiconductor wafers entering a photovoltaic cell manufacturing line, said method comprising the steps of:

(a) illuminating an area of each of at least a selection of said wafers with excitation light suitable for generating photolumincscence from said wafers;

(b ) acquiring images of the photoluminescence emitted from said selection of wafers;

(c) processing said images to obtain information on the prevalence or location of crystal defects in said selection of wafers; and (d) applying an action in response to said information.

Preferably, the action comprises rejection of wafers, sorting of wafers into quality bins, or adjusting the parameters of one or more steps of the photovoltaic cell manufacturing line. I n one preferred form step (b) comprises acquiring at least two images of each selected wafer in different wavelength bands, and the method further comprises the step of: (e) calculating, from a comparison of two images in different wavelength bands, the bulk carrier lifetime across the area of each selected wafer. Preferably, the method is used to obtain information on the prevalence or location of slip lines, dislocations or stacking fau lts in the selection of wafers.

In accordance with a third aspect of the present invention there is provided a method for improving the quality of an ingot of a substantially monocrystalline semiconductor material being grown using a crystal growing process, said method comprising the steps of:

(a) illuminating said ingot from one or more sides with excitation light suitable for generating photoluminescence from said material;

(b) acquiring one or more images of the photoluminescence emitted from said ingot:

(c) processing said images to obtain information on the prevalence of crystal defects in said ingot; and

(d) adjusting one or more parameters of said crystal growing process i f the prevalence of said crystal defects is above a predetermined threshold.

Preferably, the method is applied repeatedly during the growth of the ingot. The method is preferably used to obtain information on the prevalence of slip l ines, dislocations or slacking faults in the ingot. In accordance with a fourth aspect of the present invention there is provided a method for improving the quality of wafers cut from an ingot or brick of a substantially

monocrystalline semiconductor material, said method comprising the steps of:

(a) illuminating said ingot or brick from one or more sides with excitation light suitable for generating photoluminescence from said material;

(b) acquiring one or more images of the photoluminescence emitted from said ingot or brick:

(c) processing said images to obtain information on the prevalence or location of crystal defects in said ingot or brick; and

(d) using said information to guide the cutting of wafers from said ingot or brick.

Preferably, the method is used to obtain information on the prevalence or location of slip lines, dislocations or stacking faults in the ingot or brick. In accordance with a fifth aspect of the present invention there is provided a method for inspecting a sample of a substantially monocrystalline semiconductor material, said method comprising the steps of:

(b) acquiring at least two images of photoluminescence emitted from said material, each image being of photoluminescence in a different wavelength band;

(c) calculating, from a comparison of at least two of said images, a measure of the bulk carrier lifetime of said material across said area; and

(d) processing at least one of said images to obtain information on the prevalence or location of crystal defects in said sample.

Preferably, the method is used to obtain information on the prevalence or location of slip lines, dislocations or stacking faults in the material. ln accordance with a sixth aspect of the present invention there is provided a system w hen used to implement the method according to any one of the first to fifth aspects of the present invention.

In accordance with a seventh aspect of the present invention there is provided a system for inspecting a substantially monocrystalline semiconductor material, said system

comprising:

(a) an excitation source for illuminating an area of said material with excitation light suitable for generating photoluminescence from said material;

(b) an image capture device for acquiring at least one image of the photoluminescence emitted from said material; and

(c) a processor adapted to process said at least one image to obtain information on the prevalence or location of crystal defects in said material.

Preferably, the processor is programmed to determine the area sum or area average of the crystal defect density, or the relative distribution, spatial distribution, length distribution or total length of crystal defects in the material. The image capture device preferably comprises an area camera, a line camera or a TD1 camera. In preferred forms the system further comprises a translation mechanism for effecting relative motion between the material and the excitation source or the image capture device. The translation mechanism is preferably adapted to move the material.

Preferably, the system further comprises one or more optical filters such that the image capture device can acquire two or more photoluminescence images in different wavelength bands. The processor is preferably programmed to calculate, from a comparison of the two or more photoluminescence images in different wavelength bands, a measure of the bulk carrier lifetime of the material across the area. ln preferred forms the system further comprises a rotator adapted to rotate the material so as to enable inspection of different faces of the material. Preferably, the excitation source and the image capture device are provided within a measurement chamber, and the system further comprises a transport mechanism for moving the material into or out of the measurement chamber. In preferred forms the processor is programmed to identify low minority carrier lifetime regions in the material.

In accordance with an eighth aspect of the present invention there is provided an article of manufacture comprising a computer usable medium having a computer readable program code configured to implement the method according to any one of the first five aspects, or to operate the system according to the sixth aspect or the seventh aspect.

Brief Description of the Drawings

Benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of exemplary embodiments and the appended claims, taken in conjunction with the accompanying drawings, in which:

Fig 1 A illustrates the cutting of wafers from a mono-Si ingot;

Fig 1 B illustrates the shaping of a mono-Si ingot;

Fig 2 shows an optical image of a slug of mono-Si where slip lines in the si licon have been exposed by chemical etching;

Fig 3 shows a photoluminescence image of the mono-Si slug shown in Fig 2, acquired before the sample was etched;

Fig 4 shows a photoluminescence image of a cast monocrystalline silicon wafer;

Fig 5 shows a photoluminescence image of a side face of an end section of a shaped mono-Si ingot; Fig 6 shows in schematic side view a bulk semiconductor inspection system according to one embodiment of the invention;

Fig 7 shows in schematic side view a bulk semiconductor inspection system according to another embodiment of the invention;

Fig 8 shows in schematic side view a bulk semiconductor inspection system according to yet another embodiment of the invention;

Fig 9 shows a perspective external view of a bulk semiconductor inspection system according to an embodiment of the invention;

Fig 1 shows in schematic side view the exterior of a bulk semiconductor inspection system according to yet another embodiment of the invention;

Figs 1 1 Λ and 1 1 B illustrate schematically various schemes for acquiring PL images along the length of a bulk semiconductor sample; and

Figs 1 2 A to 12D illustrate the operation of a rotator suitable for rotating a brick or ingot within a bulk semiconductor inspection system.

Detailed Description

Preferred embodiments of the invention will no be described, by way of example only, with reference to the accompanying drawings. Photoluminescence (PL) imaging is known to be a rapid and convenient technique for characterising silicon wafers, as well as silicon-based photovoltaic (PV) cells both during and after manufacture. As discussed in T. Trupke el al 'Progress with Luminescence Imaging for the Characterisation of Silicon Wafers and Solar Cells^*. 22^nd Huropean Photovoltaic Solar Energy Conference, Milan, September 2007, the PL emission from silicon samples can provide information on many material and electrical parameters of relevance to PV cell performance. The PL emission from silicon arises primarily from band-to-band recombination, in the wavelength range 900 to 1300 nm. although emission at longer wavelengths can also occur from defects such as dislocations. Suitable systems - 1 I - and methods for performing PL imaging of silicon and other semiconductor materials are described in published PCT patent application Nos WO 2007/041758 A 1. WO

2007/ 128060 A l , WO 201 1 /079353 A l and WO 201 1 /079354 A l , the contents of which are incorporated herein by reference. An important aspect of PL imaging is that images can be acquired and processed on a timescale of fractions of a second to 2 seconds, fast enough for in-line characterisation of samples on current PV cell manufacturing lines.

In certain embodiments a substantial area of a sample is illuminated in a single illumination step with light suitable for exciting band-to-band phololuminescence from the sample, and a PL image of the area acquired in a single exposure with an area camera such as a silicon CCD camera. Alternatively, a PL image of the area can be acquired line-byline with a line camera. In alternative embodiments a sample area can be il luminated in line-scanning fashion, i.e. line-by-line with a linear light source, say as samples move along a process line, and a PL image acquired with a line camera or an area camera. Note that since any linear light source has a finite width, an illuminated line is considered to be an illuminated area. A PL image can also be acquired in point-wise fashion with a small area excitation beam (e.g. a focused laser beam) scanned across the sample surface, in which case a simple photo-detector can be used to delect the PL emission from each point. Generally speaking, broad area illumination allows PL images to be acquired more rapidly, while small area (high intensity) illumination generates a stronger PL signal.

Fig 3 shows a PL image acquired from the mono-Si slug shown in Fig 2 before chemical etching. The image reveals several parallel linear dark (i.e. low PL intensity) features 9 that are invisible optically, but appear from comparison with Fig 2 to be indicative of slip lines in the mono-Si. The reduced PL intensity in the area around each slip line is caused by reduced minority carrier lifetime resulting from enhanced recombination. The dark features in the PL images are considerably wider than the actual physical extent o the slip lines, which are microscopic in width, because of di ffusion of carriers from the surrounding material into the slip lines. The PL image also reveals a region 10 of lo PL intensity at the periphery of the slug, presumably due to a higher concentration of impurities that reduce the minority carrier lifetime, and hence the PL intensity. Note that the actual edge 12 of the slug is difficult to discern in the image, because of the low PL signal at the periphery. It is known that enhanced concentrations of impurities (for example oxygen) at the periphery of Cz-grown silicon ingots can be caused by di ffusion 5 of molecules from the surrounding atmosphere into the silicon during crystal growth. A reduced PL intensity could also be caused by the background doping level being lower at the edge of the slab, but this is unlikely in Cz-grown mono-Si. The PL signal from the majority of the sample is essentially featureless. i O Because the dark features associated with the sl ip lines are straight and paral lel and stand out clearly from the background PL signal level, they can be relatively easily detected by edge detection algorithms such as Sobel-edge detection. Information on the prevalence or location of sl ip l ines in a sample, such as the area sum or area average of the density of slip lines, or the relative distribution, spatial distribution, length distribution or total length

1 5 of sl ip l ines, can be derived from a PL image and used for a number of purposes by

manufacturers of ingots, wafers or PV cells.

Fig 4 shows a PL image of a cast mono-Si wafer with no multicrystal l ine sections, reveal ing an extensive network of recombination active dislocations 14. Wh i le these

20 crystal defects are more complicated in shape than the mono-Si slip lines shown in Fig 3, it will still be relatively straightforward for an edge detection algorithm to identi fy and quanti fy them to provide information such as area sum or area average density of dislocations, or the relative distribution, spatial distribution, length distribution or total length of dislocations. Again, the information can be used for a number of purposes by

25 manufacturers of ingots, wafers or P V cells.

For example manufacturers of PV cells based on mono-Si or cast mono-Si could use the information on sl ip l ines or other crystal defects for quality control purposes, by acquiring PL images from all or a significant fraction/selection of the incoming wafers. Wafers with a density of slip lines or other crystal defects above a predetermined threshold could for example be rejected, and optionally returned to the wafer manufacturer. PV cell manufacturers could also use the information to classi fy the incoming wafers, for example to assign them to quality bins or to adjust the parameters of one or more steps of the PV cell manufacturing line, to improve the quality of the cells. For example a wafer could be rotated such that the bus bars avoid an area with sl ip l ines. Wafers from specific bins may be processed in specific processing lines that are optimised for the respective wafer bin. Wafer manufacturers could use PL imaging to check for slip l ines or other crystal defects in their wafers and price them accordingly, while manufacturers of mono-S i ingots could use the information to classify their ingots or to modify or optimise the crystal grow ing process.

The same considerations as above also apply for impurities detected in monocrystal line semiconductor samples. With PL offering a rapid means to detect slip lines or other crystal defects, impurity-rich regions and/or low li fetime regions, manufacturers of ingots, wafers and cells can use PL imaging for quality control, quality assurance, statistical process control, direct process control, forward-feeding process control, as part of a manufacturing execution system, quality pricing, manufacturing improvements, or R&D into improved products and processes.

Importantly for ingot and wafer manufacturers, we have also observed that sl ip l ines are visible in "side view' . For example Fig 5 shows a PL image of a 25 cm long squared off section 16 from one end of a mono-Si ingot, clearly showing a typical criss-cross pattern 18 associated with a high density of slip lines in the end region, before the good quality si licon 20 begins. Therefore instead of the laborious process of cutting and etch ing successive slugs of si licon, an operator could simply use PL imaging to locate the beginning of the good quality silicon. Depending on the design of the PL i maging system, an operator could either place al l or a substantial portion of the ingot in the measurement chamber, or cut one or more slugs in the area where they think the good silicon begins, as in the current etching-based process, and place them in the imaging chamber. I f a sample is too large to be imaged in a single frame, a number of PL images can be acquired and stitched together, on a time scale of the order of minutes. In one preferred embodiment, an entire ingot is moved through a PL imaging station, either stepwise for sequential area imaging or continuously for line-by-line imaging.

To refine the cutting decision, the ingot could be rotated and one or more other faces imaged, either along the entire ingot length or only in the slip line-rich sections identified from imaging the first face. It should be noted that although the ingot section 16 shown in Fig 5 was shaped (i.e. squared off) prior to PL imaging, this is not essential as PL imaging also reveals slip lines in cylindrical ingots. By identifying the transition from slip line-rich material to good material before shaping, a wafer manufacturer need only go to the trouble of shaping the good section of an ingot. We also note that because the PI . -based inspection method is non-destructive, a wafer manufacturer could still cut wafers from the slip line-rich sections, e.g. for applications less demanding of material quality.

PL-based inspection will not only show where the good quality silicon begins and ends, but it may also reveal defective regions, e.g. impurity-rich portions in the supposedly good quality section, that can be avoided in wafer cutting. More elaborate PL imaging procedures can also be employed in an inspection process, such as the 'two filter^' method described in published PCT patent application No WO 201 1 /00 1 59 A 1 . the contents of which are incorporated herein by reference, where two images of photoluminesccnce acquired in different wavelength bands are compared by calculating an intensity ratio image to determine a measure of the bulk carrier lifetime without having to calibrate each image to account for system-dependent factors. With further processing, this measure can measure of bulk lifetime can be converted to absolute bulk lifetime data. This is a much faster alternative to the μ-PCD method currently used to measure the bulk lifetime at the cut end of a mono-Si ingot. Alternatively, if the 'two filter' method is used routinely to obtain a measure of the bulk carrier lifetime, then the presence of slip lines or other crystal defects can be determined from either of the PL images. Because ingots don't need to be shaped for PL-based slip line inspection, ingot manufacturers could also use PL imaging from one or more sides of a mono-Si ingot to monitor the crystal growing process in situ, allowing one or more process parameters such as temperature, draw rate or rotation rate of the crystal relative to the crucible to be altered if, say, slip lines suddenly start appearing in the crystal. Repeated application of PL imaging would allow dynamic control of the crystal growing process, which may be particularly useful in a continuous Cz process. Lock-in detection techniques with a pulsed PL excitation source could be used to discriminate the PL emission from background thermal emission if necessary.

We turn now to description of suitable systems for inspecting bulk semiconductor samples. A number of variant systems are possible, depending for example on whether the samples are loaded into the measurement chamber manually or automatically, and on whether the PL images are acquired in area or line-scanning fashion. The systems wil l be described in the context of imaging one or more faces of a substantially rectangular bulk Si sample such as a shaped mono-Si ingot 6 (Fig I B) or a brick of cast mono-Si or multicrystalline Si, but it will be appreciated that the systems will also be suitable for imaging cylindrical ingots, with the possible addition of a stabilising mechanism to prevent unwanted movement of the ingot during measurement. These systems are not restricted to the inspection of substantially monocrystalline bulk silicon samples.

Fig 6 shows in schematic side view a bulk semiconductor inspection system 21 according to one embodiment of the present invention. This system includes a PL imaging apparatus inside a measurement chamber 22, a computer 23 and a transport mechanism in the form of a belt 24 (or similar mechanism such as rollers) onto which a sample 25 (shown in dotted outline) is placed for insertion into the chamber, optionally through a shutter 26 to satisfy light safety requirements if required, i.e. if the system would not otherwise be eye- safe. 1 f light safety in not a concern, the sample can simply be inserted through an opening in the measurement chamber. The computer has a control function to control the operation of the system and a processor function to perform image processing and computation, automatically or as directed by an operator. Typically, the computer will be programmed to look for and report on expected features of interest in the bulk samples, especially slip lines as revealed in the PL image shown in Fig 5. The PL imaging apparatus inside the measurement chamber comprises an excitation source 28 such as a laser or flash lamp for illuminating a substantial portion of a face 29 of a bulk sample 25, and an image capture device 30 in the form of an area camera such as a silicon CCD camera or an InGaAs camera for acquiring an image of the

photoluminescence 32 generated by the illumination 34. Other components such as a short pass filter 31 and col!imation optics 33 for the illumination, collection optics 35 for the photoluminescence, and a long pass filter 37 for preventing the illumination light from reaching the camera may also be present, as described for example in the abovementioned published PCT patent application No WO 2007/041758 A I . The system is also provided with a translation mechanism 41 in the form of a series of rollers 36 in a frame 39 for translating the sample to and fro within the measurement chamber, if the sample is too large to be imaged in one exposure. In an alternative embodiment the sample remains static within the measurement chamber, and the imaging apparatus is moved so as to inspect successive portions of the sample. In preferred embodiments the frame 39 can be moved vertically to adjust the height of the sample within the measurement chamber. Preferably, the system 21 also includes a set of one or more movable optical filters 38, e.g. on a filter wheel, to enable PL images to be acquired in different wavelength bands for spatial measurement of bulk lifetime as described in the abovementioned published PCT application No WO 201 1 /009159 A l . In one specific example, for measuring the bulk lifetime of silicon, a 'short wavelength^' image is acquired in the 950- 1000 nm region and a 'long wavelength' image in the region > 1050 nm, and an intensity ratio image calculated to obtain a spatially resolved measure related to bulk lifetime. Of course if the sample is too large to be imaged in one exposure, multiple 'short wavelength' and ' long

wavelength' images can be acquired and stitched together before the intensity ratio image is calculated. Alternatively if pairs of short and long wavelength images are acquired from specific portions of the sample, multiple intensity ratio images can be calculated and stitched together. In preferred embodiments the system 21 also includes a rotator for rotating the sample within the measurement chamber, so that two or more faces can be inspected; a suitable rotator will be discussed below with reference to Figs 12A to 121). Once the inspection process is completed, the sample exits the measurement chamber via 5 the rol lers 36 and transport belt 24.

Fig 7 shows in schematic side view a bulk semiconductor inspection system 40 according to another embodiment of the present invention, identical to the Fig 6 embodiment except for the provision of a second transport belt 24 (and shutter 26 i f required) on the other side i (⁾ of the measurement chamber 22, so that the sample can exit from that side.

Fig 8 shows in schematic side view a bulk semiconductor inspection system 42 according to yet another embodiment of the present invention, externally identical to the Fig 7 embodiment but with line scanning rather than area imaging PL components. In this

15 embodiment illumination 34 from a linear excitation source 44 such as a laser diode or LED array generates photoluminescence 32 from a linear portion 46 of a sample 25, and the photoluminescence detected with an image capture device 30 in the form of a line camera or a time delay integration (TD1) camera, e.g. with Si or InGasAs pixels. As with the 'area imaging' systems shown in Figs 6 and 7, the system 42 may also include a short 0 pass filter 31 and co!limation optics 33 for the illumination, collection optics 35 for the photoluminescence, and a long pass filter 37 for preventing the illumination light from reaching the camera. A translation mechanism 41 in the form of a series of rollers 36 in a frame 39 translates the sample 25 such that the illuminated portion 46 is effectively scanned along the sample. In an alternative embodiment the sample remains static within

25 the measurement chamber, and the imaging apparatus is scanned along the sample.

Preferably, the system 42 also includes a set of one or more movable optical filters 38. e.g. on a filter wheel, to enable PL images to be acquired in different wavelength bands for spatial measurement of bulk lifetime as described above with reference to the area imaging embodiment shown in Fig 6. In this case the sample is scanned as many times as

30 required to acquire the two or more images. In preferred embodiments the system 42 also includes a rotator for rotating the sample within the measurement chamber, described below with reference to Figs 12A to 12D, so that two or more faces can be inspected.

Fig 9 shows a perspective external view of the 'area imaging' inspection system 40 of Fig 7, or equivalently the ine scanning' inspection system 42 of Fig 8, with a bulk Si sample 25 on a transport belt 24 with guard rails 50, about to enter the measurement chamber 22 through an open shutter 26. Some of the rollers used to translate the sample w ithin the measurement chamber can be seen through the open shutter. Fig 10 shows in schematic side view the exterior of a bulk semiconductor inspection system 52 according to yet another embodiment of the present invention, where the measurement chamber 22 is equipped with a door 54 through which samples can be loaded either manually or robotically. Inside, the system 52 may include an area imaging PL apparatus or a line scanning PL apparatus as described above with reference to f igs 6 and 8 respectively, with system control and image processing performed by a computer 23. This inspection system 52 may also include a translation mechanism, rotator or optical filters for acquiring images in different wavelength bands as described above for other embodiments. In principle a measurement chamber (with openings or shutters as appropriate for light safety) can be made sufficiently large to accommodate an entire bulk sample for measurement. As shown in Fig 1 1 A this means that at least the lower portion of the measurement chamber 22 would need to be at least twice as long as the sample 25 if the sample is moved under a stationary PL imaging apparatus (i.e. illumination and imaging optics) represented by a dashed box 56, or somewhat shorter in the alternative

embodiment where the PL imaging apparatus is moved above a stationary sample. Either way, this minimum size requirement is not particularly onerous for multicrvstaliine or cast mono Si bricks that are typically 25 to 30 cm in length, or for mono-Si ingots that have been cut into similar length pieces in the usual fashion. However for whole mono-Si ingots that are typically up to 2 m long, it would be more convenient to run the sample ingot 25 through openings in the measurement chamber 22 as shown in Fig 1 1 B, with the proviso that this would not compromise light safety requirements. We turn now to description of a rotator that can be included as an option in a bulk semiconductor inspection system, to rotate a rectangular or substantially rectangular bulk sample such that any side face can be inspected. Fig I 2A illustrates a rotator 58 comprising a sample engagement portion in the form of a number of blades 60 with V- shaped sections lined with cushioning material 62, and a turning portion in the form of an axle 64 rotatably connected to a sliding frame 66. a shaft 70 hingedly connected to a flange 71 of one of the blades, and a first motor 72 adapted to drive the shaft back and forth in the Z direction as defined by the coordinate axes. The sliding frame is adapted to move along rails of a fixed frame 68 in the Z direction, driven by a second motor 74. In operation the first motor acts to rotate the axle and the blades, while the second motor acts to move the sliding frame such that a sample held in the V-shaped sections of the blades stays in approximately the same Z position as it rotates. ^* Initial', 'halfway^' and ' final' positions are shown in Figs 12A, 12B and I 2C respectively.

Fig 12D shows in perspective cutaway view a bulk semiconductor inspection system equipped with a rotator in the process of rotating a bulk silicon sample 25, with the blades fitting through gaps between the rollers. To perform a rotation operation the roller frame 39 is lowered on rails 76 so that the blades engage with the sample, and after the operation is completed the roller frame returns the sample to the inspection position while the rotator returns to the 'initial ' position ready for another rotation operation.

Apart from inspecting large bulk silicon samples such as bricks and ingots, an operator may also wish to look for slip lines or other crystal defects in smaller pieces of bulk silicon, such as the 'slugs' cut from an end of a mono-Si ingot for the purpose of determining where the good quality silicon begins. It will be appreciated that the bulk semiconductor inspection systems described above with reference to Figs 6 to 8 could also be used to inspect these slugs of silicon. In this situation PL images would generally be acquired from the cross-section area of the slug, as in the image shown in Fig 3. A bulk semiconductor inspection system could also be adapted to look for slip lines or other crystal defects in non-bulk samples such as wafers, although wafer inspection would general ly be performed on a system specifically designed for wafers.

Bulk semiconductor inspection systems utilising PL imaging may be provided in standalone fashion, or as modules for incorporation with other systems for characterising bulk samples, such as IR transmission systems for detecting inclusions or Eddy current systems for measuring conductivity (and therefore background doping concentration).

Not only are the above-described PL imaging-based systems and methods for inspecting bulk silicon samples faster and less chemically hazardous than the etching methods traditionally used to reveal slip lines in mono-Si, they are also non-destructive, having no lasting impact on the samples. Also they are sufficiently rapid to be performed on every sample in a PV cell manufacturing line, or alternatively on a selection of samples. PL imaging requires no direct contact with the sample and therefore does not risk damage to the sample such as cracking, chipping, warping or the introduction of chemical, atomic or physical features that lead to higher local recombination.

Although the present invention has been described with particular reference to certain preferred embodiments thereof, variations and modifications of the present invention can be effected within the spirit and scope of the following claims.

Claims

The claims defining the invention are as follows:

1 . A method for inspecting a substantially monocrystalline semiconductor material, said method comprising the steps of:

(a) illuminating an area of said material with excitation light suitable for generating photoiuminescence from said material;

(b) acquiring at least one image of the photoiuminescence emitted from said material; and

2. A method according to claim 1 , wherein said method is used to obtain information on the prevalence or location of slip lines, dislocations or stacking faults in said material.

3. A method according to claim 1 or claim 2. wherein said information comprises the area sum or area average of the crystal defect density, or the relative distribution, spatial distribution, length distribution or total length of crystal defects in said material.

4. A method according to any one of the preceding claims, wherein said area is illuminated in a single illumination step, or in line-scanning fashion, or in point-wise fashion.

5. A method according to any one of the preceding claims, wherein said at least one image is acquired with an area camera, a line camera, a TDI camera or a photo-detector.

6. A method according to any one of the preceding claims, wherein said processing step further comprises identifying low minority carrier lifetime regions in said material.

7. A method according to any one of the preceding claims, wherein said method is used for monitoring or controlling the quality of substantially monocrystalline

semiconductor wafers entering a photovoltaic cell manufacturing line.

8. Λ method according to any one of the preceding claims, wherein said method is applied to an ingot or brick of substantially monocrystalline semiconductor material, and said method further comprises the step of: (d) utilising said information to guide the cutting of wafers from said ingot or brick.

9. A method according to any one of claims 1 to 7, wherein said method is applied to a wafer, ingot or brick of substantially monocrystalline semiconductor material, and said method further comprises the step of: (e) utilising said information to apply a

classification to said wafer, ingot or brick.

10. A method according to claim 9, wherein said classification is used to alter parameters associated with a series of processing steps in the formation of a photovoltaic- cell from said material, so as to improve the quality of said photovoltaic cell.

1 1 . A method according to claim 9, wherein said classification comprises binning, rejecting or pricing of said material.

12. A method according to any one of the preceding claims, wherein step (b) comprises acquiring at least two images in different wavelength bands, and said method further comprises the step of: (1) calculating, from a comparison of two images in di fferent wavelength bands, a measure of the bulk carrier lifetime across said area of said material .

13. A method according to any one of the previous claims, wherein said method is applied to substantially monocrystalline silicon.

14. A method for monitoring or controlling the quality of substantially

monocrystalline semiconductor wafers entering a photovoltaic cell manufacturing l ine, said method comprising the steps of:

(a) illuminating an area of each of at least a selection of said wafers with excitation light suitable for generating photoluminescence from said wafers;

(b) acquiring images of the photoluminescence emitted from said selection of wafers:

(c) processing said images to obtain information on the prevalence or location of crystal defects in said selection of wafers; and

(d) applying an action in response to said information.

1 5. Λ method according to claim 14. wherein said action comprises rejection of wafers, sorting of wafers into quality bins, or adjusting the parameters of one or more steps of said photovoltaic cell manufacturing line.

16. Λ method according to claim 14 or claim 1 5, wherein step (b) comprises acquiring at least two images of each selected wafer in different wavelength bands, and said method further comprises the step of: (e) calculating, from a comparison of two images in different wavelength bands, the bulk carrier lifetime across said area of each selected wafer.

1 7. A method according to any one of claims 14 to 16. wherein said method is used to obtain information on the prevalence or location of slip l ines, dislocations or stacking faults in said selection of wafers.

1 8. A method for improving the quality of an ingot of a substantially monocrystall ine semiconductor material being grown using a crystal growing process, said method comprising the steps of: (a) illuminating said ingot from one or more sides with excitation light suitable for generating photoluminescence from said material;

(b) acquiring one or more images of the photoluminescence emitted from said ingot;

(c) processing said images to obtain information on the prevalence of crystal defects in 5 said ingot; and

(d) adjusting one or more parameters of said crystal growing process if the prevalence of said crystal defects is above a predetermined threshold.

1 . A method according to claim 18, wherein said method is applied repeatedly during 10 the growth of said ingot.

20. A method according to claim 18 or claim 19, w herein said method is used to obtain information on the prevalence of slip lines, dislocations or stacking faults in said ingot.

1 5 2 1 . A method for improving the quality of wafers cut from an ingot or brick of a

substantially monocrystalline semiconductor material, said method comprising the steps of:

0 (b) acquiring one or more images of the photoluminescence emitted from said ingot or brick;

25

22. A method according to claim 21 , wherein said method is used to obtain information on the prevalence or location of slip lines, dislocations or stacking faults in said ingot or brick.

23. A method for inspecting a substantially monocrystalline semiconductor material- said method comprising the steps of:

(d) processing at least one of said images to obtain information on the prevalence or location of crystal defects in said material.

24. A method according to claim 23, wherein said method is used to obtain information on the prevalence or location of slip lines, dislocations or stacking faults in said material.

25. A system when used to implement the method according to any one of claims 1 to 24.

26. A system for inspecting a substantially monocrystalline semiconductor material, said system comprising:

27. A system according to claim 26, wherein said processor is programmed to determine the area sum or area average of the crystal defect density, or the relative distribution, spatial distribution, length distribution or total length of crystal defects in said material.

28. A system according to claim 26 or claim 27, wherein said image capture device comprises an area camera, a line camera or a TDI camera.

29. A system according to any one of claims 26 to 28, further comprising a translation mechanism for effecting relative motion between said material and said excitation source or said image capture device.

30. A system according to claim 29, wherein said translation mechanism is adapted to move said material.

3 1 . A system according to any one of claims 26 to 30, further comprising one or more optical filters such that said image capture device can acquire two or more

photoluminescence images in different wavelength bands.

32. A system according to claim 31 , wherein said processor is programmed to calculate, from a comparison of said two or more photoluminescence images in different wavelength bands, a measure of the bulk carrier lifetime of said material across said area.

33. A system according to any one of claims 26 to 32, further comprising a rotator adapted to rotate said material so as to enable inspection of different faces of said material.

34. A system according to any one of claims 26 to 33, wherein said excitation source and said image capture device are provided within a measurement chamber, and wherein said system further comprises a transport mechanism for moving said material into or out of said measurement chamber.

35. A system according to any one of claims 26 to 34, wherein said processor is programmed to identify low minority carrier lifetime regions in said material.

36. An article of manufacture comprising a computer usable medium having a computer readable program code configured to implement the method according to any one of claims I to 24, or to operate the system according to any one of claims 25 to 35.