WO2004008119A1

WO2004008119A1 - Detection method and apparatus

Info

Publication number: WO2004008119A1
Application number: PCT/GB2003/003045
Authority: WO
Inventors: Victor Higgs
Original assignee: Aoti Operating Company, Inc.
Priority date: 2002-07-17
Filing date: 2003-07-14
Publication date: 2004-01-22
Also published as: GB0216622D0; AU2003281077A1; TW200411167A

Abstract

A method of quality control of structures of semiconductors such as silicon prior to device fabrication thereupon comprises exposing the surface of a semiconductor structure under test to at least one high-intensity beam of light from a suitable light source, preferably a laser, and in particular a high-intensity laser, and collecting photoluminescence (PL) produced by excitation of the semiconductor structure by the light beam; numerically analysing the photoluminescence emitted across the area of the structure; comparing the result with a predetermined acceptable specification range of photoluminescence; making a quality classification of the semiconductor structure based thereon; and in particular rejecting or selecting for remedial action semiconductor structures exhibiting a photoluminescence response outside the said predetermined acceptable specification range. In a preferred embodiment the method is applied as part of a quality control metric prior to device fabrication. In a refinement of the method a spatially resolved PL map is also collected. An apparatus for performing the method is also described.

Description

DETECTIONMETHODANDAPPARATUS

The invention relates to a non-destructive method and apparatus for detecting surface layer metal contamination and other defects in the structure of semiconductors such as silicon, in particular for the detection of defects in supplied precursor materials such as semiconductor wafers prior to device fabrication thereupon. The invention in particular provides for an improved quality control metric to be applied to supplied semiconductor materials in advance of device fabrication.

Advances in silicon technology in the last 50 years have produced dramatic improvements in chip performance, and an explosive growth in the technology. The semiconductors device industry's ability to progress by reducing the cost per device function by approximately 30% per year has been a key factor in its explosive growth. To maintain this trend; device geometries are becoming smaller to a point where the number of transistors which can be included on a single chip has increased to five million, as' a consequence this is putting more demands on the material properties of the starting semiconductor materials.

Incoming precursor materials such as wafers (both polished and epitaxial wafers) may contain a range of defects. It is acknowledged that the control of these defects in the staring material is a critical factor in the achievement of high IC yields in the devices fabricated thereupon (International Semiconductor Roadmap for Semiconductor Materials 2001, SEMATECH, 3101 Industrial Terrace Suite 106, Austin TX 78758). Defects include both surface chemical residues, such as organics and transition metals, and grown- in micro defects. Structural defects, such as epitaxial stacking faults and other large area defects, must also be controlled on epitaxial wafers. The removal and prevention of surface defects is a current state-of-the-art challenge for silicon and other semiconductor wafer technology.

Silicon-on-insulator wafers are also now used in Si device fabrication; these wafers offer the potential for high speed and low-power applications. However, theses wafers need to be characterized to determine the SOI material properties and how this impacts subsequent device properties.

The production of larger 300 mm diameter wafers is increasing (7% predicted in 2002) as a further processing development necessary to achieve the required economy of scale for large volume IC manufacturing. There is a key metrology need to measure defects and metal contamination on large area wafers with higher sensitivity and also to develop detection methods which give information relating to the spatial distribution of defects and contamination.

The move to use 300 mm starting wafers is also creating a driving force to use 300 reclaim wafers (re-cycled and cleaned processed wafers), because using reclaim wafers will reduce the overall cost of the transition from 200 mm wafers to 300 mm. There is however a reluctance to use reclaim wafers for some applications because the wafers may contain high levels of surface metal contamination due to incomplete surface cleaning. Again, a method which could characterise defects accurately on all such wafers would be of great benefit.

The standard method for measuring surface metal contamination is total x-ray reflection fluorescence (TXRF). This technique can determine the type of contamination and the concentration. However, this technique has no practical wafer mapping capability - a typical measurement covers 1 cm² and takes approximately 1 hour. As a result in practice typically a single measurement is taken from the centre of a wafer. The technique cannot practically give information relating to the spatial distribution of contamination across the wafer area in a realistic timescale. Also the technique is usually based outside the clean room area and it takes time to get the results due to the intensive demand. For all these reasons the technique is often limited to representative batch sampling only, since timescales are impractical to allow test of every unit in a given batch.

It is an object of the present invention to provide a method and apparatus for quality control of semiconductors such as silicon prior to device fabrication by detection of surface layer metal contamination and other defects in the structure which mitigates some or all of the above disadvantages.

It is a particular object of the present invention to provide a method and apparatus which provides for a more rapid assessment of the quality of supplied precursor materials such as semiconductor wafers prior to device fabrication thereupon.

It is a particular object of the present invention to provide a method and apparatus of quality assessment which offers enhanced throughput rates, and in particular enables an improved quality control metric in which it becomes practical to test all incoming precursor materials prior to device fabrication.

Thus, in accordance with the present invention in its first aspect a method of quality control of structures of semiconductors such as silicon prior to device fabrication thereupon comprises the steps of: exposing the surface of a semiconductor structure under test to at least one high-intensity beam of light from a suitable light source, preferably a laser, and in particular a high-intensity laser, and collecting photoluminescence (PL) produced by excitation of the semiconductor structure by the light beam; making an analysis of the collected photoluminescence signal and using that analysis as the basis for a quality classification of the suitability of the semiconductor for device fabrication.

The quality classification step comprises performing a numerical analysis of the collected photoluminescence signal, comparing the result of that numerical analysis with a predetermined acceptable photoluminescence specification such as a predetermined range of photoluminescence known to be associated with satisfactory quality, and making a quality classification of the semiconductor structure based thereon.

In one simple alternative the method comprises determining an average photoluminescence intensity, comparing the average with a predetermined acceptable specification range of photoluminescence, and making a quality classification of the semiconductor structure as above based thereon.

The average may be a whole area average based on mean photoluminescence intensity emitted across the area of the structure, or local area average wherein the area of the structure is divided into a two dimensional array of subregions, a mean photoluminescence intensity is determined for each subregion, the mean for each subregion is compared with a predetermined acceptable photoluminescence specification, and a quality classification as above is based thereon. This can be advantageous since the response attributable to an isolated defect could be swamped in a whole area average even though that defect was sufficiently serious to justify a quality rejection. At an appropriate subregion size it is possible to ensure that such a response can still be detected.

Use of a predetermined photoluminescence specification based on the mean is an example only. In the alternative, especially if the subregion approach is followed, other numerical parameters could be applied to the analysis of the photoluminescence signal, such as standard deviation, local maxima and/or minima, deviation from a predetermined baseline, or other numerical analysis method to determine, either on a local or whole area basis, a deviation of the photoluminescence response from predetermined parameters known to be associated with a semiconductor structure of satisfactory quality. Where reference is made below to numerical analysis based on average luminescence it will be appreciated that this is for exemplification only and that the precise numerical parameters chosen for the comparison between observed and predetemiined acceptable response is not critical to the invention.

The photoluminescence technique produces a much more rapid response than prior art TXRF techniques. It samples across the whole wafer area and produces an average result based upon that whole wafer and is accordingly more representative of the condition of the whole wafer than the TXRF technique, where sampling is in effect concentrated on specific arbitrary sample areas. Its speed and accuracy make it a much more effective and practical quality control method than prior art techniques.

In particular it becomes reasonable to test all incoming semiconductor base structures prior to fabrication. This allows for much improved quality control, and for accurate screening of (and hence increases the practicality of the use of) reclaim wafer structures as well as new as-fabricated wafers.

In a preferred further aspect the invention thus comprises a quality control metric for processing of incoming structures of semiconductors such as silicon prior to device fabrication thereupon, to be incorporated as part of a device fabrication process, comprising the steps of sequentially testing a series of such incoming structures in accordance with the foregoing first aspect of the invention, passing a structure exhibiting a photoluminescence response within the predetermined acceptable specification range of photoluminescence on to a device fabrication stage, rejecting a structure exhibiting a photoluminescence response outside the predeteπriined acceptable specification range of photoluminescence from the device fabrication stage.

Preferably rejected structures are passed for other action such as discard or remedial treatment, for example by cleaning. This is conveniently followed by retest and accept/reject as above. Additional predetermined photoluminescence parameter ranges might be determined and used to make additional decisions about rejected structures. For example a range may be determined (in particular at photoluminescence levels closely above and/or below those of the acceptable specification range) in which remedial action is to be followed, with structures outside even this remedial range being discarded immediately.

In accordance with this preferred aspect all structures are tested prior to device fabrication. Potential rejects are identified early, prior to expensive fabrication processes. The number of rejects necessary at the end of fabrication should be reduced significantly since the method of the invention enables accurate diagnosis and consistent discarding or treating of poor quality precursor semiconductor structures prior to fabrication in a rapid and convenient manner.

The photoluminescence technique produces a spatially resolved PL map at a resolution determined by the characteristics of the high-intensity beam of light. This can be exploited by further preferred features of the present method, but for the fundamental objective of the invention as a simple and rapid quality test for a whole wafer during processing an average PL intensity result over the whole wafer area is obtained. This can be related to a predetermined acceptable specification range developed in association with studies using slower analysis methods (eg TXRF). It has been surprisingly found, as described in detail below, that a close correlation can be demonstrated between defect data obtained from the near-surface-based PL technique of the present invention and prior art methods conventionally used.

The light beam is so controlled, and in particular beam power and/or wavelength and/or spot size so controlled, as to identify defects at a selective depth in said semiconductor structure, so as to collect PL information from a suitable near-surface depth, for example from the upper 12 μm of the semiconductor structure. For certain materials and devices, smaller depths may be appropriate, down to for example 5 μm or even 1 μm.

The present invention is a defect-monitoring tool that can be used to monitor surface contamination and other surface structural defects such as stacking faults and edge slip. Because this technique measures the surface region it will detect near-surface defects and contamination accurately. These defects are most determinative in their impact on device quality and performance. This further enhances the accuracy and reliability of the technique.

In accordance with the invention, a predetermined acceptable specification range of average photoluminescence is first determined and then used as a reference for the results for any given wafer for quality control purposes. The predetermined specification range will include a minimum and/or maximum photoluminescence value. In particular, it is known that the photoluminescence signal can be affected in different ways depending upon the particular chemical species comprised in the impurity. Accordingly, the specification range will preferably comprise a minimum and a maximum photoluminescence value.

A quality control decision is taken depending upon whether the measured result lies within the predetermined specification range to accept structures for device fabrication when within the range, and to reject when outside the range. Rejected items may be discarded or subjected to remedial action such as additional cleaning etc. The predetermined acceptable PL range will vary in accordance with the particular material and process involved and will be determined initially from existing quality control specification ranges by relating the PL responses produced by the present invention with responses in accordance with existing prior art measuring techniques.

Once such a specification range has been established, the present invention provides very high throughput relative to prior art methods. For example, for a 12 inch (300 mm) wafer equivalent results can be obtained in around five minutes which would take around an hour with existing methods.

Photoluminescence spectroscopy is a very sensitive technique for investigating both intrinsic and extrinsic electronic transitions at impurities and defects in semiconductors. When silicon is excited at low temperatures with laser irradiation above the band-gap of the material, electron hole pairs are produced. These carriers can recombine in various different ways, some of which give rise to luminescence. The electron hole pairs formed at low temperature can be trapped at impurities in silicon and they emit photons characteristic of this interaction, thereby giving impurity specific information in the photoluminescence spectra. There are a significant number of applications of PL spectroscopy to silicon including characterisation of silicon after different processing steps, characteristic of device fabrication for example implantation, oxidation, plasma etching, the detection of point defect complexes and the presence of dislocations. One of the most important applications includes the non-destructive measurement of shallow donors and acceptors such as arsenic, boron and phosphorous. Notably, this technique enables the measurement of the concentration of these shallow donors and acceptors. However, in conventional applications in order to obtain this spectral information and unambiguous chemical identification of the optical centres, measurements need to be carried out at liquid helium temperatures. It is known throughout the industry that at room temperature the PL signal is significantly weakened and very little useful spectral information can be obtained.

A room temperature technique is accordingly preferred, such as in particular that described by International patent application W098/11425, which describes a non-destructive technique which makes practical the detection of electrically active defects in semi-conductor structures based on room temperature PL. The patent application discloses a PL technique which has industrial application in that it enables the image to be produced within minutes and which has a further added advantage in producing micro imaging of small individual defects particularly near to the surface of the wafer, where the device is fabricated.

The technique provides information concerning defects in a semiconductor or silicon structure at a rate appropriate to industrial use and in particular enables us to visualise defects in the upper regions of the semiconductor or silicon structure and in particular near to the surface of same. The technique exploits enhanced non-radiative recombination of electron hole pairs at defects in a semiconductor or silicon structure with a view to enhancing contrast in a PL image of said semiconductor or silicon structure so as to enhance the viewing of defects in same. The preferred PL technique for use in the present invention is therefore that in W098/11425, incorporated herein by reference.

The success of the room temperature PL method disclosed therein is, in part, due to the probing volume probed by the laser being small, spatial resolution preferably 0.1 to 20 μm, ideally 2 to 5 μm, and with a peak or average power density of between 10⁴ to 10⁹ watts/cm², so that localised defects have much greater effect on the measured PL intensity and is also believed, in part, because since the excitation is focused the injected carrier density is high. This greatly increases the probability of non-radiated recombination at the defect and hence physical location of the defect. The present invention in certain preferred embodiments described in more detail below exploits this by preparing a spatial map, and more preferably still a spatial image, of the defects of which the PL response is representative.

Reference herein to a high-intensity laser is meant to include, without limitation, a high power density laser i.e.. where regardless of the power of the laser the emittance is focused.

In a preferred method of the invention a pulsed laser excitation source is used and ideally luminescence data is measured and/or the luminescence images collected as a function of time. This means that both depth and spatial resolution are improved and can be used to obtain information on the carrier capture cross sections of the defects. Time resolved measurements can also be used to measure the effective carrier lifetime and obtain lifetime maps.

The PL technique of the present invention generates a spatially resolved PL map across the area of the wafer. In the primary method of the invention, this data map is then processed to provide an average PL level across the whole wafer, which is compared with the reference to make the quality control decision. If the method is to be used for a simple accept/ reject quality control decision as a test prior to passing structures on for manufacture then only the averaged PL level is of concern, and the resolution of the map produced by the method is immaterial. Resolutions of the order of 7 mm are adequate. At this level of resolution, processing times are reduced, and test throughput rates maximised. For example it can take just five minutes to obtain a satisfactory accept/ reject result from a 12 inch (300 mm) wafer. Nevertheless, it is a particular advantage of the preferred photoluminescence technique of the present invention that it can additionally be used to generate a spatially resolved map of PL signals across the surface of the semiconductor structure under test, and in particular to generate a spatially resolved image of those signals. Such a spatially resolved map is simply not practical with prior art TXRF techniques. Accordingly, in a preferred embodiment, the method further comprises the step of generating such a map and/or such an image. In these circumstances, it can be appropriate to work to mapping/imaging resolutions of 0.5 mm or less.

Conveniently, the method further comprises storing the spatially resolved PL map on suitable data storage means and/or transmitting digitised data derived from the spatially resolved map through suitable processing means for onward processing and/or displaying the spatially resolved image on suitable display means.

The basic technique identifies an averaged PL intensity on which to base the accept/ reject decision. Spatially resolved information is especially useful in relation to rejected structures. A preferred more developed quality control strategy might therefore be to process each unit using the more rapid, basic technique, and to generate spatially resolved data for rejected structures only. In one embodiment, taking full advantage of the ability of the technique to produce spatially resolved data on the PL response of a semiconductor structure under test, it might be appropriate, as indicated, to work at higher resolutions. Accordingly, throughput will be slower than where the technique is used as a basis for basic accept/ reject quality control decisions only. A preferred more developed quality control strategy might therefore be to process each unit using the more rapid, basic technique, and to reprocess rejected structures at higher resolution using the additional functionality offered by the collection of spatially resolved data.

The ability to generate a map or image allows defect location to be generally identified. This can be exploited in a preferred embodiment of the invention in that the method as hereinbefore described can be used to rapidly screen semiconductor structures (5 minutes for a 300 mm wafer) and identify specimens for a full chemical analysis. This embodiment of the method comprises subjecting a semiconductor structure to the test as hereinbefore described to generate a spatially resolved map of PL signals across the surface of the semiconductor structure and, at least in the case of rejected structures, using the spatially resolved map to identify the general location of contamination, and further analysing the semiconductor structure in the identified location using a specific analysis technique such as TXRF to identify the impurity.

The method is suitable for any basic semiconductor structure on which devices are fabricated by thermal processing in familiar manner. In particular, the method is suitable for structures based on wafers of silicon and silicon alloys. The devices may be fabricated from simple single layer wafers or from multilayer wafers, for example formed in an epitaxial layer on a basic silicon wafer.

In accordance with a further aspect of the invention an apparatus for quality control of structures of semiconductors such as silicon prior to device fabrication thereupon comprises a high intensity light source, preferably a laser, and in particular a high-intensity laser; means to focus a high intensity beam of light from the light source onto a surface of a semiconductor structure under test; collection means to collect photoluminescence data from across the surface of the semiconductor structure under test produced by excitation of the semiconductor structure by the light beam; analysis means to process and numerically analyse the collected data; a comparator to compare the results of the analysis with predetermined acceptable specification parameters, and a transfer means adapted to transfer the structure to a particular further processing stage determined by whether the photoluminescence signal falls within the predetermined acceptable specification range.

In particular the transfer means is adapted to transfer a structure exhibiting a photoluminescence response within a predetermined acceptable specification range of photoluminescence on to a device fabrication stage, and to transfer a structure exhibiting a photoluminescence response outside a predetermined acceptable specification range of photoluminescence otherwise than to the device fabrication stage; and for example to a discard location or remedial treatment stage. The apparatus may thus be incorporated into a suitable production apparatus for such devices at a point immediately prior to apparatus for device fabrication.

The apparatus to perform the basic method generates data based on a numerical analysis of the PL response, for example an average PL signal across the whole area or a set of local area data as described. This is compared with predetermined acceptable specification parameters.

However, to perform the refined alternatives of the method described above, the apparatus preferably further comprises means to resolve the collected PL data into a spatially resolved PL map across the area of the semiconductor structure, and optionally further comprises means to convert the resolved data into a PL image and/or image/data storage means to store the map/image, and in particular to store successive map/images for future comparison, and/or means to transmit the map/image to a suitable remote data processor and/or image display means such as a visual display screen to display an image and/or related data to a user.

In a further aspect of the invention, there is provided a computer program and/or a suitably programmed computer for performing some or all of the steps of the method as hereinbefore described, and in particular for performing data processing steps on collected PL data, for example to determine average PL across the wafer area and/or to spatially resolve a PL map from collected PL data and/or to compare the average with a predetermined acceptable specification range.

The invention will now be described by way of example only with reference to Figures 1 to 8 of the accompanying drawings in which:

Figure 1 is an illustration of a suitable apparatus for obtaining the PL data;

Figure 2 is a schematic illustration of how data is processed;

Figure 3 illustrates PL signals recorded in accordance with the invention for different batches of supplied wafer;

Figure 4 illustrates PL signals recorded in accordance with the invention for different wafers in a batch;

Figure 5 illustrates correlation of PL data to a basic quality control accept/ reject decision;

Figure 6 illustrates a spatially resolved PL image of a wafer; Figures 7 and 8 illustrate a possible numerical analysis technique in accordance with the invention.

The apparatus illustrated in Figure 1 essentially comprises a PL imaging microscope which: towards the right hand side, comprises a bank of lasers (3- 8); towards the bottom comprises a sample stage such as an X-Y table or R- table; towards the left hand side comprises a microprocessor (40) and a display screen (39) and in the centre of the figure there are shown various optical components for directing light through the system.

In the embodiment shown in Figure 1 , six lasers are provided with a view to probing different depths in the sample. However, it is within the scope of the invention to use only one laser, or indeed to use a greater number of lasers. In any event, at least one of the lasers is a high intensity laser and ideally has a spot size of between 0.1 mm and 0.5 micron and a power density of between 10⁴ to 10⁹ watts/cm². A laser selector (16) coupled with said bank of lasers is provided so as to select one or more lasers for use and further also to select the frequency and wavelength of the lasers.

Conventional optics, such as optical fibres (9) are used to direct light towards the collimator to (10) and laser beam expander (11). An apodization plate (12) is positioned between laser beam expander (11) and beam splitter (31). Beam splitter (31) directs a fraction of light from the aforementioned lasers towards sample (2) via objective (34).

An automatic focus controller (30) is provided and coupled to a piezo driven focusing stage (33). The microscope is equipped with a conventional rotating turret (36) which is provided with at least one high numerical aperture objective for micro examination and one low numerical aperture objective for macro examination (34, 35) respectively. In addition, also coupled to turret (36) there is provided an optical displacement measuring system (38).

Cabling is provided so as to connect the automatic focusing controller (30) to microprocessor (40) and also a microscope objective indexing arrangement (32) to microprocessor (40).

Downstream of beam splitter (31) there is provided as filter wheel (13) for laser notch filters, down stream thereof there is provided a swing-aside folding mirror (14) whose function will be described hereinafter. Aligned with said mirror (14) there is provided a filter wheel (27) for wavelength selection, and rearward thereof there is provided a zoom lenses attached to a suitable CCD 2- D array detector (29).

Infinity system compensating lens (37) is provided in the optical path foremost of cold mirror (17) which reflects light towards a further filter wheel (23) for wavelength selection and a focusing lenses (24) which is foremost of a detector (25) for UV and visible light. Detector (25) is coupled to lock-in amplifier (26). This is used to obtain a reflected image of the surfaces.

Rearmost of cold mirror (17) is provided a further filter wheel (18) again for wavelength selection, and rearmost thereof a focusing lens (22) and a further aperture wheel (19) for pinhole selection which is provided foremost of a detector (21) for detecting the luminescence.

Both the UV and visible region detector (25) and infrared detector (21) are coupled to lock-in amplifier (26).

Operation of the system is explained having regard to the following. A range of wavelengths to probe different planes in the sample is provided by several lasers (3-8). The lasers can be modulated by a frequency generator (16) so that the signal emitted from the sample (2) can be isolated from background radiation by means of the detectors being synchronised to the laser modulation frequency by the lock-in amplifier (26). In a further embodiment, the range of wavelengths could be produced by using a tuneable laser and/or an Optical Parametric Oscillator. Each laser is connected to, and aligned with, a Multi-branch optical fibre (9) so that any or all of the lasers can illuminate the sample (2). The common end of the Multi-branch optical fibre terminates in an optical system (10) which coUimates the emerging light. This optical system is aligned with a beam expander (11) which matches the laser beam's diameter to that required by the microscope objectives (34,35) above the sample (2). The expanded beam then passes through an apodization plate (12) which distributes the optical energy evenly over the beam area.

The expanded and apodized beam is reflected by a beamsplitter (31) and passes to the microscope objectives (34 and 35). The beam is focused by a microscope objective (34 or 35) on to the sample. In the micro mode this objective is selected to focus the beam to a diffraction limited spot size. A rotating turret (36), operated by an indexing mechanism (32), permits the objective to be changed for the macro mode where a larger area of the sample can be illuminated. In a further embodiment the apodization plate (12) can be removed so that the spot for the micro mode can be made smaller to allow higher injection levels.

An optical displacement sensor (38) measures the distance to the sample and, by means of a feedback loop through the auto focus controller (30), maintains the correct spacing by means of the piezo actuated focusing stage (33). The Photoluminescence signal from the sample is collected by the microscope objective (34) (in the micro mode) and transported back through the beamsplitter (31) and a notch filter in the filter wheel (13) which contains notch filters matched to the range of laser wavelengths. The notch filter removes any reflected laser light, passing only the Photoluminescence signal.

The folding mirror (14) is swung out of the beam allowing the signal to pass to the tube lens (37), which may be incorporated to compensate for any infinity microscope objectives which may be used, and on to the cold mirror (17). This component reflects those wavelengths below a selected cut off point (approximately 700 nm) to the focusing lens (24) which focuses the signal into the detector (25). A filter wheel (23) in front of the detector focusing lens (24) contains filters to isolate selected wavelength bands.

The portion of the Photoluminescence signal lying in the wavelength range above the cut-off point passes through the cold mirror (17) and is similarly focused by the lens (22) into the detector (21). This signal also passes through a filter wheel (18) containing filters to isolate selected wavelength bands.

A series of pinholes of different diameters are contained in an aperture wheel (19) positioned in front of the detector (21). This aperture wheel can be moved axially by the piezo actuator (20) so that the pinholes can be positioned confocally with the desired image plane. By this means, planes at different depths in the sample (2) can be imaged to provide accurate depths information.

The electrical signal from the detectors (21, 25) is fed to the lock-in amplifier

(26) where it is synchronised with the modulation frequency of the laser (3-8) by means of a reference signal from the frequency generator (15). The electric signal is then fed to the central processor (40) for analysis. The PL image is obtained by raster scanning the stage. Alternatively optical scanning using galvo mirrors may be employed.

In an alternative micro mode of operation, the folding mirror (14) is swung into the beam of the Photoluminescence signal. The diverted signal passes through a filter wheel (27), which contains filters to isolate selected wavelength bands, and into the zoom lens (28). The zoom lens allows different magnifications to be used in imaging the illuminated spot on the sample (2) on to the CCD two dimensional array (29). This allows the illuminated area of the sample (2) to be imaged at different resolutions. The electrical signal from the CCD array is fed to the central processor (40) for analysis.

The processing of data is illustrated schematically in Figure 2. A sample (101) is transferred by a handling arm (102) onto a sampling base for testing by the device of Figure 1 to generate a PL signal. This is collected by the collection apparatus of Figure 1 (shown in simplified schematic form as the device 105).

The figure further illustrates processing of data. In a first processing path, in accordance with the main aspect of the invention, the PL map data is passed to a processor (107) which processes the data to determine an average PL intensity across the whole area of the sample (101).

The resulting average is passed to a comparator (108) which relates the average PL intensity data to a predetermined stored specification range within the data store (109), and based on that comparison passes a quality control decision onto the control unit (110). In the embodiment the control unit (110) acts directly upon the handling arm (102) which then transfers the sample (101) on to a device fabrication processing line or to a reject line for remedial action as appropriate. In an alternative mode of operation, the control unit (110) could for example be a display means giving an indication to an operator, who could then operate the arm (102) by separate control means for example to make an acceptable/reject choice, to divert the sample under test for remedial processing etc.

A secondary processing route, shown by the broken line, reflects the optional second aspect of the invention. In this optional aspect, data corresponding to the PL intensity map across the surface of the sample (101) is also passed to a secondary processing unit (111) which is able to resolve the data into a digitised spatially resolved map of intensity across the surface of the sample (101). The resulting map is passed to a data store (112) and to a visual display screen (113). The resolved data may be used to identify defect locations. In this way the basic apparatus could be used to rapidly screen wafers. The location of contamination may be identified from the wafer map, and then the wafers could be further analysed using TXRF to identify the impurity.

An illustration that the technique can be used to monitor incoming wafer quality is provided in Figures 3 and 4. First, a selection of different wafers were measured from different wafer manufactures. The results are shown in Figure 3. There is a clear difference in the PL signal obtained from each supplier indicating variations in the surface defect density of surface contamination level. Further measurements were carried out on batches of wafers from each supplier. A typical example is shown in Figure 4. Wafers from the same supplier have the same average value and this was found to be a general characteristic. Therefore the Average PL can be used as a metric to determine the incoming wafer quality.

To carry out the basic method of the invention a PL map is collected and processed and the average PL value calculated and then compared against a pre-detennined value. This specified range is used to deteraiine the wafer quality and therefore an accept/ reject process control procedure created. This procedure is illustrated graphically in Figure 5.

By setting up the specified range it is possible then to associate this range to any colour or grey-scale pattern and prepare a corresponding image. An example is illustrated in Figure 6. In the example wafer map the dark areas show areas outside the specified range. This simple coding can be used to illustrate the variation in the wafer quality.

After the wafer quality has been investigated using the average PL signal level, further measurements can be recorded with higher resolution PL imaging and/or using other techniques on wafers that have failed the specification. This will allow one to determine the type of defect present, also to inspect the spatial variation in more detail, which may give a clearer indication of the source of the contamination (e.g. poor surface polishing or incomplete cleaning).

To identify the source of contamination on the front surface the back surface of the wafer can be recorded as well. By comparison of the wafer maps, it will be apparent if the contamination originated on the wafer back surface. Then further action can be taken to prevent cross contamination to other process equipment of metrology tools.

Thus in accordance with the invention the photoluminescence tool is used as a rapid process control tool to determine wafer quality. Wafer maps are obtained and the measured PL response numerically analysed to provide a quality metric, in that a deviation of the photoluminescence response from predetermined parameters known to be associated with a semiconductor structure of satisfactory quality is used as the basis of a quality control decision. In the basic example given this can simply be a comparison of mean response over the whole area. However in general contamination can be a very localized and the average PL signal (averaged over all measured pixels in the wafer map) may not a reliable indication of such contamination. If wafer map is sub-divided using a 2D grid, then analyzed using average PL, very localized signals could be detected.

After a wafer map has been recorded a virtual grid may be applied and displayed on top of the wafer map using suitable software. This can be used to perform the analysis. A local area analysis also allows better location of problem sites in the structure. For example the grid elements that have failed may be indicated, a micro scan can be launched at the same location as the grid element to allows the area of interest to be inspected in more detail, and a report of the failed elements in the wafer map analysis can be exported in suitable format.

The local grid method is not restricted to a numerical analysis based on mean values. Any suitable pre-defined parameters, including average intensity, PL min, Max, Standard deviation and baseline) can be used to determine regions of contamination. The PL signal baseline method can be a more useful parameter to use because the variation in signal across is not uniform wafer. This technique is explained below.

A typical wafer map is shown in figure 7a with an associated histogram of PL intensity in Figure 7b.

Contamination is detected in the wafer map by the deviation from the baseline value and limits can be set. However, the PL average value in this wafer map is modified by the contamination. Whereas the value to be used should represent the signal level for an uncontaminated wafer. The peak value shown in Figure 8 represents the true PL value of a non-contaminated wafer. Modifying the baseline function to have a peak value would allow the customer to accurately track the contamination and with more sensitivity.

A suitable algorithm involves the following steps:

1. To define peak value as maximum value in histogram and use this for baseline

2. Search data for peak value. 3. Then calculate ±70% of maximum value (user defined) then re-define the peak maximum as the center position of these points.

4. Then calculate precise value of peak maximum and then define PL level.

5. Also allow user to input typical baseline value form uncontaminated wafer, this will help if there are two peaks of equal intensity.

The baseline of the wafer is the PL value that corresponds to the maximum number of points.

The baseline variation is defined by the following relationship: Baseline variation = PL value - baseline

The PL value is the AVG PL of each element of the grid. The baseline variation must be measured for each element.

Claims

1. A method of quality control of structures of semiconductors such as silicon prior to device fabrication thereupon comprising the steps of: taking a semiconductor prior to device fabrication; exposing the surface of a semiconductor structure under test to at least one high-intensity beam of light from a suitable light source, and collecting photoluminescence (PL) produced by excitation of the semiconductor structure by the light beam; making an analysis of the collected photoluminescence signal and using that analysis as the basis for a quality classification of the suitability of the semiconductor for device fabrication.

2. A method in accordance with claim 1 wherein the quality classification step comprises: determining an average photoluminescence intensity emitted across the area of the structure or subregions thereof; comparing the average with a predetermined acceptable specification range of photoluminescence; making a quality classification of the semiconductor structure based thereon.

3. A method in accordance with claim 1 or 2 wherein the quality classification step comprises rejecting or selecting for remedial action semiconductor structures exhibiting a photoluminescence response outside a predetermined acceptable specification range.

4. A method in accordance with any preceding claim applied as a quality control metric for processing of incoming structures of semiconductors such as silicon prior to device fabrication thereupon, to be incorporated as part of a device fabrication process, comprising the steps of sequentially testing a series of such incoming structures in accordance with the method of any preceding claim, passing a structure exhibiting a photoluminescence response within the predetermined acceptable specification on to a device fabrication stage, rejecting a structure exhibiting a photoluminescence response outside the predetermined acceptable specification from the device fabrication stage.

5. A method in accordance with claim 4 wherein rejected structures are then passed for remedial treatment, for example by cleaning, followed by a retest and repeat of the accept/reject step of claim 3.

6. A method in accordance with any preceding claim wherein the beam power and/or wavelength and/or spot size of the light beam is so controlled as to collect near- surface PL information from the upper 12 μm of the semiconductor structure.

7. A method in accordance with claim 6 wherein the light beam is so controlled as to collect near-surface PL information from the upper 1 μm of the semiconductor structure.

8. A method in accordance with any preceding claim wherein the PL response is obtained at around room temperature.

9. A method in accordance with any preceding claim wherein the light source is a high-intensity laser.

10. A method in accordance with claim 9 wherein the laser has a small probing volume with spot size 0.1 to 20μm, ideally 2 to 5μm, and with a peak or average power density of between 10⁴ to 10⁹ watts/cm².

11. A method in accordance with claim 9 or 10 wherein a pulsed laser excitation source is used and luminescence data is measured and/or the luminescence images collected as a function of time.

12. A method in accordance with any preceding claim further comprising the step of using the collected PL signals to generate a spatially resolved map of PL signals across the surface of the semiconductor under test, and in particular to generate a spatially resolved image of those signals.

13. A method in accordance with claim 12 further comprising the step of storing the spatially resolved PL map on suitable data storage means and/or transmitting digitised data derived from the spatially resolved map through suitable processing means for onward processing.

14. A method in accordance with claim 12 or 13 further comprising the step of displaying any generated PL image on suitable display means.

15. A method in accordance with one of claims 12 to 14 further comprising the steps of using the spatially resolved map to identify the general location of contamination, and further analysing the semiconductor structure in the identified location using a specific analysis technique such as TXRF to identify the impurity.

16. A method to identify and/or characterise defects in a succession of structures of semiconductors such as silicon comprising performing the method of one of claims 1 to 11 on each of the succession of structures, performing the additional steps of one of claims 12 to 15 only on those structures rejected as exhibiting a photoluminescence response outside the said predetermined acceptable specification range to generate spatially resolved data for rejected structures only.

17. An apparatus for quality control of structures of semiconductors such as silicon prior to device fabrication thereupon comprises a high intensity light source; means to focus a high intensity beam of light from the light source onto a surface of a semiconductor structure under test; collection means to collect photoluminescence data from across the surface of the semiconductor structure under test produced by excitation of the semiconductor structure by the light beam; analysis means to process and numerically analyse the collected data; a comparator to compare the results of the analysis with predetermined acceptable specification parameters, and a transfer means adapted to transfer the structure to a particular further processing stage determined by whether the photoluminescence signal falls within the predetermined acceptable specification range.

18. An apparatus in accordance with claim 17 wherein the beam power and/or wavelength and/or spot size of the light beam is so controlled that the apparatus is adapted to generate and collect near-surface PL information from the upper 12 μm of the semiconductor structure.

19. An apparatus in accordance with claim 17 wherein the beam power and/or wavelength and/or spot size of the light beam is so controlled that the apparatus is adapted to generate and collect near-surface PL information from the upper 1 μm of the semiconductor structure.

20. An apparatus in accordance with one of claims 17 to 19 wherein the light source is a high-intensity laser.

21. An apparatus in accordance with claim 20 wherein the laser has a small probing volume with spot size 0.1 to 20 μm, ideally 2 to 5μm, and with a peak or average power density of between 10⁴ to 10⁹ watts/cm².

22. An apparatus in accordance with one of claims 17 to 21 to wherein the transfer means is adapted to transfer a structure exhibiting a photoluminescence response within the predetermined acceptable specification range of photoluminescence on to a device fabrication stage, and to transfer a structure exhibiting a photoluminescence response outside the predetermined acceptable specification range of photoluminescence otherwise than to the device fabrication stage, and for example to a discard location or remedial treatment stage.

23. An apparatus in accordance with one of claims 17 to 22 further comprising means to resolve the collected PL data into a spatially resolved PL map across the area of the semiconductor structure.

24. An apparatus in accordance with claim 23 further comprising means to convert the resolved data into a PL image and or image/data storage means to store the map/image, and in particular to store successive map/images for future comparison, and/or means to transmit the map/image to a suitable remote data processor and/or image display means such as a visual display screen to display an image and/or related data to a user.