System for optical inspection of substrates using same or different wavelengths

Abstract

The invention relates to a system (1) for optical inspection of a substrate (3). The system comprises an illumination device defining an inspection area (I) on the substrate (3), a support (2) for receiving the substrate (3), and an inspection device defining an inspection area (D) on the substrate (3). According to the invention, the examination region (I) is positioned in front of at least a part of the examination region (D) with respect to the scanning direction (SD).

Description

Technical Field

[0001] The present invention relates to a system for optically inspecting a substrate, for example a substrate exhibiting photoluminescent properties, to detect the possible presence of defects such as particles or crystal defects, or more generally to characterize the surface and subsurface conditions of the substrate. The substrate may be a wafer made of single-crystalline material used for manufacturing devices in the fields of microelectronics, optics, microsystems or optoelectronics, for example. Background Art

[0002] Substrates such as crystallized wafers for electronic, optical or optoelectronic applications need to be inspected to detect, identify and / or characterize possible defects present on or just below their surface. Defects may be particles, crystal defects, scratches or surface roughness.

[0003] Such inspection is usually intended to provide qualitative or quantitative information, such as, for example, the location, size and / or nature of defects. Information about the surface and subsurface conditions of a substrate can represent the quality of a substrate manufacturing process or a production step using the substrate. US20080059094A1 discloses a defect inspection device that can provide this type of information.

[0004] Some substrates may exhibit photoluminescent properties, which may be intrinsic to the material or may be due to crystal defects or doping present in the thickness of the substrate.

[0005] As is well known, photoluminescence is exhibited by certain materials, which, when exposed to radiation, emit radiation of different wavelengths in various directions. Examples of materials that exhibit this behavior are compound semiconductors, such as indium phosphide, gallium arsenide, silicon carbide, which find application in the semiconductor industry, for example in electric vehicles or other power-related applications, due to their ability to handle higher powers and / or higher frequencies than conventional silicon-based electronics.

[0006] Photoluminescence behavior can be advantageously used to characterize these materials. In this case, the material is illuminated by a light beam of wavelength short enough so that the energy is greater than the band gap energy of the material, and the resulting photoluminescence radiation is detected at various wavelengths, such as in the near UV, visible or infrared regions. In order to better characterize and classify defects or intrinsic properties of the material, photoluminescence can also be used in combination with known measurement techniques that use reflection or scattering of a light beam at the illumination wavelength, for example, as described in US7304310B1.

[0007] It is also known to use inelastic scattering methods to characterize substrates or materials. In this case, some of the scattered photons experience a change in wavelength during interaction with the material. Examples of inelastic scattering include Raman scattering and Brillouin scattering. In contrast, elastic scattering involves light scattered at the excitation wavelength.

[0008] In some applications, photoluminescent radiation or inelastic scattering may be considered detrimental, adding unnecessary noise to the scattering measurement.

[0009] Document JP5850447B1 describes a confocal inspection system for characterizing substrates made of SiC material. The inspection system generates photoluminescence in the SiC material of the substrate using an incident light beam with a wavelength between 313nm and 365nm. A plurality of confocal detectors are combined with filters to selectively detect light scattered by the substrate at the excitation wavelength, band edge photoluminescence emission of the crystalline material at 380nm, and photoluminescence emission in the visible and NIR range due to defects. The confocal inspection system enables spatial filtering of the photoluminescence emission to provide only the emission generated on the surface or the immediately adjacent sub-surface of the substrate to the detector.

[0010] In the semiconductor industry, the throughput or number of wafers that can be analyzed per unit time is critical. However, in applications of interest related to defect characterization, the photoluminescence phenomenon is inherently slow due to the time period between the absorption of the material and the emission of the photon, and the decay time of the induced luminescence, which can last several microseconds.

[0011] Prior art inspection systems must operate at slower speeds than, for example, scatter detection in order to collect sufficient photoluminescent radiation.

[0012] Purpose of the Invention

[0013] The object of the present invention is to at least partially solve the above mentioned problems and to propose a system for optical inspection of substrates, in particular substrates made of composite materials, which exhibit photoluminescence properties and / or inelastic scattering properties. Another object of the present invention is to provide an inspection system capable of efficiently measuring and distinguishing radiation emitted by elastic scattering, inelastic scattering and / or photoluminescence, respectively. Another object of the present invention is to provide an inspection system capable of measuring at least one of elastic scattering, inelastic scattering and photoluminescence radiation, for example in a dark field configuration mode, without these measurements being influenced or disturbed by the other measurements. Another object of the present invention is to provide an inspection system capable of measuring photoluminescence radiation with high sensitivity, high resolution and high speed. Another object of the present invention is to provide an inspection system capable of providing information about the substrate and its defects, allowing these defects to be identified and classified. Summary of the invention

[0014] To this end, the invention relates to a system for optical inspection according to the claims.

[0015] When the substrate moves under the illumination beam, the residence time of the defect in the detection region of the system according to the present invention is greater than and / or delayed relative to the residence time of the defect in the inspection region. The speed at which the incident light is scanned over the exposed surface of the substrate can be increased without sacrificing the collection of the photoluminescent signal and / or without any inelastic scattering effects, and the throughput of the inspection system is increased.

[0016] Additional non-limiting features according to this aspect of the invention, taken alone or in any technically feasible combination:

[0017] - the illumination device comprises a beam splitter to form two incident light beams that converge and intersect each other on the substrate to define an inspection area;

[0018] - the illumination device is configured so that the incident light beams intersect each other at the surface of the substrate, or at the bottom of the substrate, or within the substrate;

[0019] - the illumination device is configured to generate a coherent incident light beam to form an interference pattern in the examination area;

[0020] - the interference pattern consists of a single bright fringe;

[0021] at least one incident light beam is contained in an illumination plane which forms an angle with the normal to the surface of the support comprised between a grazing incidence angle and a normal incidence angle, or preferably an angle comprised between 30 and 80 degrees;

[0022] -The light source is a continuous or pulsed laser;

[0023] - the collector comprises a narrow-angle collecting channel for collecting radiation originating from the detection area in a solid angle around a normal z to the support;

[0024] - narrow angle collection channel including collection lens;

[0025] - the collector comprises a wide-angle collecting channel for collecting radiation emitted from the detection area in a solid angle deviating from a normal z of the support;

[0026] - the wide-angle collection channel comprises at least one collection mirror;

[0027] - the detector comprises at least one electro-optical transducer, such as a photodiode;

[0028] - the detector comprises a plurality of electro-optical transducers arranged in a matrix;

[0029] - the detector comprises at least one optical fiber optically associated with at least one electro-optical transducer, the free end of the at least one optical fiber forming the sensing surface;

[0030] - the system comprises a controller configured to move the support relative to the illumination device to scan a surface of the substrate having the inspection area;

[0031] - the detection device is configured such that the detection area is larger than the examination area along the scanning direction, preferably 1.2 or 1.3 times larger, and more preferably 2 or 10 times larger;

[0032] - the detection device is configured such that the detection area includes the examination area;

[0033] - The detection area and the inspection area are centered;

[0034] - the detection area is off-centered relative to the examination area in the scanning direction;

[0035] - the detection device is configured such that the detection area is composed of different detection sub-areas;

[0036] - the detection device is configured so that the detection area does not completely contain the examination area;

[0037] - the system comprises a processing unit (10) arranged to correlate measurement signals acquired in the detection region (D) with positions of the examination region (I) at the origin of said measurement signals;

[0038] - the detection device further comprises separation means for separating the radiation of at least one first wavelength and the radiation of at least one second wavelength from each other;

[0039] - the separation means comprises at least one filter and / or at least one dichroic beam splitter arranged in the system;

[0040] - Filters and / or dichroic beam splitters based on wavelength-dependent absorption, reflection or diffraction principles;

[0041] - the separation device further comprises at least one focusing lens and / or at least one collimating lens;

[0042] - the separation device is positioned between the collector and the sensing surface of the detector;

[0043] - a separation device is arranged between the optical fiber and the at least one electro-optical transducer;

[0044] - The system comprises a narrow angle collection channel and a wide angle collection channel, the narrow angle collection channel having a separation means for selectively detecting radiation at a first wavelength and the wide angle collection channel having a separation means for selectively detecting radiation at a second wavelength. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Numerous other features and advantages of the present invention will become apparent from reading the following detailed description when considered in conjunction with the accompanying drawings, in which:

[0046] - Figure 1 A lighting device of an inspection system according to an embodiment is shown;

[0047] - Figure 2 A detection device of an inspection system according to an embodiment is shown;

[0048] - Figure 3a represents a specific configuration of the lighting device enabling definition of an inspection volume intersecting the exposed surface of the substrate;

[0049] - Figure 3b to Figure 3c represents various possible configurations according to which the inspection region of the inspection system is positioned in front of at least a portion of the detection area with respect to the scanning direction;

[0050] - Figures 4a to 4c Various embodiments of separation devices representing inspection systems;

[0051] - Figures 5 to 10 Various embodiments of an inspection system provided with a separation device are shown. DETAILED DESCRIPTION

[0052] The present invention relates to a system 1 for optical inspection of a substrate 3, for example exhibiting photoluminescent properties. The system can be used to detect or characterize defects, such as crystal defects and / or particles, or to detect the intrinsic photoluminescence of the substrate 3. For simplicity of expression, the term "defect" will be used in this disclosure to denote a feature of the substrate 3 that induces a change in the emission of scattered or photoluminescent radiation or intrinsic photoluminescence, regardless of the nature of the feature.

[0053] The system 1 is particularly suitable for inspecting substrates exhibiting photoluminescent properties, such as SiC substrates of any polytype (e.g. 4H SiC or 6H SiC), or compound semiconductor substrates, including, for example, GaN, GaAs or InP. The substrates to be inspected are usually provided as wafers (i.e. disks of material of standardized diameter), but the system 1 can be used to inspect substrates of any shape and size. The substrate 3 can be made of a single material (bulk substrate) or can be a composite substrate (a surface film of semiconductor material provided on a support). In the latter case, and in the context of the present invention, at least the surface film exhibits photoluminescent properties.

[0054] System 1 comprises a lighting device. Figure 1 In the embodiment shown above, the illumination device comprises a light source S which generates an incident light beam exhibiting at least one first wavelength. The light source S may be, for example, a continuous or pulsed laser. The incident light beam is directed and guided onto the exposed surface of the substrate 3, and the projection of the incident light beam on the exposed surface of the substrate 3 defines the inspection area I. Defects present on or near the surface of the substrate induce emission of scattered and / or photoluminescent radiation, which may be collected and measured as will be explained in the latter part of this specification. The radiation emitted by the surface may therefore exhibit at least a first wavelength (e.g., in the case of elastically scattered light). The radiation may also exhibit at least one second wavelength different from the at least first wavelength (e.g., in the case of photoluminescence or in the case of any inelastic scattering effect).

[0055] The incident light beam is preferably monochromatic and presents an inspection wavelength that is short enough in the embodiment shown so that the energy is greater than the band gap energy of the material of the substrate 3. The inspection wavelength may be in the UV, visible or infrared range. For example, for a SiC 4H bulk substrate 3 (exhibiting a band gap of 3.265 eV), the light source S may be selected to generate an incident light beam presenting an inspection wavelength within 313 nm to 365 nm. This material generates photoluminescence radiation with a wavelength close to 380 nm. For an undoped GaAs bulk substrate 3 (exhibiting a band gap of 1.42 eV), the inspection wavelength may be selected to be below 500 nm, and the photoluminescence radiation generated by the substrate 3 presents a wavelength of 873 nm.

[0056] exist Figure 1In the embodiment shown, the light source S is coupled via a first optical fiber F to a beam splitter BS which propagates light in two separate optical fibers F1, F2. The ends of the two separate optical fibers F1, F2 are positioned in the inspection system to produce two converging incident light beams B1, B2 which cross each other to define an inspection volume intersecting the surface of the substrate 3. This particular configuration is achieved by two converging optical elements 8, 8' (such as focusing lenses) arranged in the propagation paths of the incident light beams B1, B2, respectively. More generally, the light beams may cross each other at the surface of the substrate or at the bottom of the substrate or within the substrate.

[0057] from Figure 3a As can be clearly seen in Figure 1, the advantage of this embodiment is that it enables an inspection volume bounded in the xy plane and in depth (i.e., z direction) to be defined by the intersection of two incident beams. In the intersection volume, the illumination power is at least twice that provided by a single beam, thus enhancing the generated scattered and photoluminescent radiation relative to radiation from elsewhere along the respective beam. To optimize this positioning, the two incident beams B1, B2 can be arranged to cross around their focal position or beam waist of the Gaussian beam.

[0058] In this configuration, the inspection area I is defined as the intersection of the inspection volume and the surface of the substrate 3. The inspection area I is generally elliptical in shape. Depending on the angle of incidence of the beams B1, B2 and their focusing, the main axis of the inspection area I may be between 20 μm and 150 μm.

[0059] The incident light beams B1 and B2 are contained in the illumination plane IP ( Figure 2 (as can be seen in Fig. 1 ), the illumination plane IP forms an angle with the normal z of the surface of the substrate 3, which angle can have any value from grazing incidence (>80 degrees) to vertical incidence (0 degrees). Typically, the illumination plane IP forms an angle between 30 degrees and 80 degrees with the normal z.

[0060] In certain embodiments, the illumination device may be configured to generate coherent incident light beams B1, B2 that interfere to form an interference pattern in the inspection region I. Figure 1 and Figure 3a In the embodiment shown, the intersecting incident light beams B1, B2 originate from the same light source S and have equal optical paths to the inspection region I. In this configuration, an interference pattern may be generated, for example, the interference pattern is formed by the two incident light beams B1, B2 in the direction defined by the plane ( Figure 3aThe arrangement is particularly advantageous because it increases the spatial selectivity of the illumination. In addition, the interference pattern formed by the parallel fringes provides a modulation frequency in the measurement signal associated with the scattered radiation, which makes it possible to improve the signal-to-noise ratio of the detection system 1. If the decay time of the photoluminescent radiation is relatively short (in the order of nanoseconds), the modulation frequency may also be present in the measurement signal associated with that radiation.

[0061] It may also be advantageous to have an interference pattern comprising a single bright fringe. This improves the spatial localization of the inspection area I and therefore the spatial resolution of the inspection system 1. This can be achieved by choosing a suitable angle between the incident beams B1, B2 and / or using a light source S with a short coherence length. With an interference pattern consisting of a single bright fringe, the illumination power can be configured to be four times greater than the illumination power obtained from only one of the incident beams.

[0062] The system 1 also includes a support 2 for positioning the substrate 3 relative to the lighting device. The support 2 and the lighting device can be moved relative to each other according to a scanning direction SD of the lighting device relative to the substrate. As is well known in the art, the support 2 and / or the lighting device can be associated with an actuator that can move relative to each other to achieve such relative movement.

[0063] exist Figure 1 In the embodiment shown, the support 2 can rotate about the axis r and can translate in the (x, y) plane where the substrate 3 is located. With this configuration, the relative movement of the substrate 3 disposed on the support 2 can be controlled in such a way that the inspection area I scans the entire exposed surface of the substrate 3, for example, in a spiral trajectory.

[0064] continue Figure 1 As described above, the inspection system 1 also includes a controller 6 for controlling the relative movement of the support 2 relative to the lighting device to scan the inspection area I on the exposed surface of the substrate along the desired measurement path. The controller 6 may include a microcontroller, a data storage unit, an input / output port connected to the actuator and other elements of the inspection system 1, and additional computing resources configured in hardware or software to accurately control the displacement of the inspection area I. When the substrate 3 is accurately positioned on the support 2, the controller 6 stores and controls the position of the inspection area I in a reference item linked to the substrate 3 (for example, in linear or polar coordinates). Therefore, the position of the inspection area I on the substrate 3 can be known at any time.

[0065] Reference now Figure 2, the inspection system 1 also includes a detection device, which includes a collector 4 for collecting at least a portion of the radiation at the first wavelength and / or at the second wavelength originating from a detection area D of the substrate 3. The radiation can be generated, for example, by a defect located in the detection area D. The portion of radiation collected by the collector is referred to as "collected radiation" in the present disclosure. The detection device also includes a detector 5 optically associated with the collector 4, which presents a sensing surface or incident pupil 5a for capturing the collected radiation. In the illustrated embodiment, the detection area D corresponds to the optical conjugate or projection of the collector 4 to the sensing surface 5a. More generally, the detection area D can be defined as a region of the substrate that contains all emitted radiation collected by the collector 4 and the sensing surface or incident pupil 5a. The detection device 5 provides a measurement signal S(t), which represents the intensity change of the collected radiation when the illumination device moves along the scanning direction SD relative to the substrate 3.

[0066] exist Figure 2 In the illustrated embodiment, the collector 4 comprises a narrow angle collection channel for collecting radiation originating from the detection region D in a solid angle around a normal z to the support 2. The solid angle may typically extend between 0 and 10 degrees around the normal z. The narrow angle collection channel is configured to direct the collected radiation to a sensing surface 5a of the detector 5. The narrow angle collection channel may comprise a collection lens 4a disposed in the system 1, such as Figure 2 The collecting lens 8 of this embodiment is centered about an axis passing through the detection area D and perpendicular to the substrate 3.

[0067] The collector 4 also includes a wide-angle collection channel for collecting radiation emitted from the detection area D in a solid angle that deviates from the normal z of the support 2. The solid angle can generally extend from 60 degrees to 89 degrees around the normal z. The wide-angle collection channel is also configured to guide the collected radiation to the sensing surface 5a of the detector 5. The wide-angle collection channel may include at least one collection mirror 4b, 4c arranged in the system 1. Figure 2 As shown in the embodiment of , the wide-angle collection channel can be further divided into two channels: a forward collection channel in the forward scattering direction of light; and a backward collection channel in the backward scattering direction. Each of the forward collection channel and the backward collection channel is provided with its collection mirror 4b, 4c.

[0068] In some embodiments, the inspection system may be based on a dark field configuration to ensure that specular reflections of the incident light beams B1 , B2 on the substrate 3 are not collected by the collector 4 , and in particular not by narrow-angle collection channels or wide-angle collection channels.

[0069] In a particular embodiment, the detector 5 may include at least one electro-optical transducer 5b, such as a photodiode or a photodiode array. The electro-optical transducer 5b may constitute the sensing surface 5a of the detector 5, and in this case, it is positioned in the focal region of the collector 4 in the system 1. However, preferably, the electro-optical transducer 5b is expelled and the collected radiation is guided from the focal region of the collector to the electro-optical transducer 5b, for example, via an optical fiber 5c or a bundle of optical fibers 5c (generally referred to as "collection optical fibers" in the following parts of this specification). In this configuration, the free end of the collection optical fiber 5c is positioned in the focal region of the collector 4, and the core of the optical fiber forms the sensing surface 5a of the detector 5 at the free end.

[0070] Instead of a photodiode or a photodiode array, the electro-optical transducer 5b may be formed by a photomultiplier tube, an avalanche photodiode, an image sensor such as a so-called CMOS image sensor or a CCD image sensor. The image sensor may also exhibit a time delay integration feature. The detector 5 may also include a spectrometer to analyze the spectrum of the collected radiation, in particular the photoluminescent radiation (i.e. exhibiting at least the second wavelength).

[0071] In the inspection system 1 of the present disclosure, the size and position of the detection area D can be adjusted independently of the inspection area I. As described above, the detection area D is an optical conjugate or projection of the entrance pupil of the detector 5 (i.e., the sensing surface 5a of the detector 5) through the corresponding collection channel of the collector 4. Therefore, by positioning or resizing the sensing surface 5a differently, or by adapting the magnification of the collector 4, the detection area D can be positioned or resized differently relative to the substrate 3.

[0072] For example, a fiber bundle with a plurality of optical fibers arranged according to a specific pattern (straight line, circle, ...) can be used. Each optical fiber can form an entrance pupil 5a of a specific detector and be conjugated with a possibly different detection area D through a specific channel (narrow angle, wide angle, front, back, ...) of the collector 4. The position of the fiber entrance 5a relative to the collector 4 then defines the position of the detection area D.

[0073] According to an important feature, the inspection system 1 is configured in such a way that the inspection area I and the detection area D do not correspond exactly to each other. More precisely, the inspection area I is positioned in front of or in front of at least a portion of the detection area D with respect to the scanning direction SD, while the inspection area I scans the exposed surface of the substrate 3. In other words, the controller 6 is configured to control the relative movement of the support 2 with respect to the lighting device in such a way that along the measurement path applied by the controller 6, the inspection area I is positioned in front of at least a portion of the detection area D with respect to the scanning direction SD.

[0074] This means that a portion of the substrate 3 that leaves the inspection area I due to such displacement will still be in the detection area D for some time. As a result, when the substrate 3 moves under the illumination beam, the residence time of the defect in the detection area D is greater than and / or delayed relative to the residence time of the defect in the inspection area I. The measurement time of the photoluminescent radiation emitted by the defect is increased, and this, for example, enables the collection of radiation emitted during the photoluminescence build-up and decay times. As a result, the speed at which the illumination beam is scanned over the exposed surface of the substrate 3 can be increased without losing the collection of the photoluminescent signal, and the throughput of the inspection system 1 is improved.

[0075] A possible configuration for achieving this feature for efficient collection of the photoluminescent radiation is to configure the inspection system 1 in such a way that the detection area D is larger than the inspection area I (along the scanning direction). Preferably, the detection area D is at least 1.2 or 1.3 times larger, preferably at least two times larger, or even up to 10 times larger, than the inspection area I along the scanning direction. In order to properly collect the scattered radiation, the inspection system 1 is configured in such a way that the detection area D at least partially covers the inspection area I. For example, with a detection area D having a length of 100 μm to 200 μm and a large part of it (such as 50% to 100%) lagging behind the inspection area I along the scanning direction SD, and a substrate 3 rotating at several thousand revolutions per minute, an integration time of a few microseconds for the photoluminescent radiation can be easily achieved.

[0076] It must be noted that when the defect density in / on the substrate 3 is low enough so that a defect appears once in the detection area D, the spatial resolution of the inspection system 1 is limited by the size of the inspection area I, which may be in the range of 20 μm to 150 μm, for example, even if the detection area D is enlarged. Therefore, the increase in throughput is not at the expense of a decrease in spatial resolution.

[0077] The measurement signal S(t) emitted from the detection device 5 and representing the intensity variation of the collected radiation is provided to a processing unit 10. Such a processing unit comprises a computer or a microcontroller having a data storage unit and an interface port. The processing unit 10 is also connected to a controller 6, which provides the position of the inspection area I and / or the detection area on the wafer at any time. The processing unit is configured to provide a signal diagram representing the measurement signal S(t) as a function of the position in a reference item linked to the substrate 3. Preferably, since the detection area D and the inspection area I are not consistent in position (i.e., not centered), the processing unit is also configured to provide a signal diagram that takes into account a possible offset between the center of the inspection area I and the center of the detection area D.

[0078] Figure 3b to Figure 3e1 and 2 represent various possible configurations according to which the inspection area I is positioned in front of at least a portion of the detection area D relative to the scanning direction SD. In these figures, and for the sake of clarity, the scanning direction SD represents the relative movement of the illumination device relative to the substrate 3. In the illustrated embodiment in which the support 2 moves while the illumination device remains fixed, the movement of the substrate is opposite to the scanning direction SD. As previously described with reference to Figure 2 As explained, the various detection zones D may be defined and positioned as optical conjugates of different optical fibers of the fiber bundle.

[0079] Figure 3b A first configuration is illustrated, in which the inspection region I and the detection region D are centered, and the inspection region I is smaller in size than the detection region D and is therefore contained in this region.

[0080] exist Figure 3c In the configuration of , the detection area D still covers the inspection area I, but is offset in the scanning direction so that Figure 3b The point of the substrate generating the photoluminescent radiation stays in the field of view of the detector 5 longer when compared to the concentric configuration.

[0081] Figure 3d Another configuration is illustrated, in which the detection region D is composed of different detection sub-regions D1, D2. Such a configuration can be realized by collecting radiation at different positions relative to the inspection region I using, for example, a collection optical fiber 5c made of a plurality of optical fibers of a bundle of optical fibers. In the example shown, a first detection sub-region D1 covers the inspection region I to collect scattered and photoluminescent radiation. A further detection sub-region D2 located behind the inspection region I in the scanning direction SD is provided for collecting only photoluminescent radiation with a time delay. For example, such a multi-region detection can allow the estimation of the fluorescence lifetime, which can be useful for defect classification.

[0082] Figure 3e A further configuration is illustrated, in which the examination region I is not contained in the detection region D, but is positioned in front of it in the scanning direction SD. In this configuration, no scattered radiation is collected, and only a delayed portion of the photoluminescent radiation is collected.

[0083] The disclosed inspection system 1 enables the collection of scattered radiation and photoluminescent radiation emitted by defects of the inspected substrate.In some applications it may be beneficial to separate the contribution of each type of radiation in separate measurement signals or even filter out one type of radiation.

[0084] In some cases, when performing scattered or dark field measurements of particles, defects, "haze" or surface roughness features of the substrate 3, the photoluminescent radiation (particularly when such radiation is intrinsic to the material of the substrate 3) can be considered an undesirable source of noise. This is the case, for example, when inspecting substrates made of composite materials such as GaAs, InP or SiC. In this application, it would be beneficial to equip the inspection system 1 with high-pass (in terms of frequency) or band-pass filters, at least for some of the collection channels, which only allow radiation at the inspection wavelength to pass (to collect the scattered radiation) and cut off all radiation at longer wavelengths (to filter out the photoluminescent radiation).

[0085] In other applications, it is beneficial to allow only photoluminescent radiation to pass and block all radiation at the inspection wavelength. This is the case for characterizing substrates including light-emitting active layers for LED, μLED and VCSEL devices. Analysis of the collected radiation wavelength FWHM intensity, which can be performed in the inspection device 5 or after the inspection device, can help to better characterize the light-emitting active layer.

[0086] In other applications, scattered radiation and photoluminescence radiation can be used to inspect the substrate. For example, it is known that various defects that may be present in the thickness of an epitaxial SiC layer, such as basal plane dislocations (BPDs), carrots, TEDs, micropipes, can be efficiently characterized by combining scattered and photoluminescence measurements. In this application, it would be beneficial to equip the inspection system 1 with filters in the narrow-angle collection channels that retain only the photoluminescence radiation in the visible and / or NIR, or only the scattered radiation, while the wide-angle collection channels are dedicated to scattered radiation detection or photoluminescence radiation. This enables the same number of detectors to be used.

[0087] The separation or filtering out of specific radiation types can be obtained by a specific configuration of the inspection area I and the detection area D, as described with respect to Figure 3e The description has already been presented. In other embodiments, the inspection system may be provided with at least one separation device 7 to separate the photoluminescent radiation and the scattered radiation from each other or to filter out one of them. This is possible due to the fact that, in general, the photoluminescent radiation and the scattered radiation exhibit different wavelengths.

[0088] The separation means 7 may comprise at least one filter 7b and / or at least one dichroic beam splitter 7d appropriately arranged in the system to filter one of the two types of radiation or to separate the two types of radiation.

[0089] For example, in Figure 2In the inspection system 1 shown, the separation device 7 can be placed between the electro-optical transducer 5b and the collection optical fiber 5c. In this configuration, the collection optical fiber 5c transmits all the collected scattered radiation and photoluminescent radiation. The advantage of this configuration is that it is very simple and versatile, because the spectral selection is outside the collection channel, in the remote unit.

[0090] Figure 4a and Figure 4b Different embodiments of the separation device 7 are illustrated. In each of these embodiments, the filter 7b may be:

[0091] - a bandpass or high-pass filter (in terms of frequency) which allows the inspection wavelength to pass and rejects longer photoluminescence wavelengths, in order to detect the scattered radiation without being affected by the photoluminescence radiation;

[0092] - a bandpass or lowpass filter (in terms of frequency) which allows at least the photoluminescence wavelengths in the spectral region (near UV / visible, NIR) to pass and rejects the examined wavelengths, in order to detect the photoluminescence radiation without being influenced by the scattered radiation.

[0093] exist Figure 4a In the embodiment, the filter 7b is simply positioned between the focusing lens 7a and the electro-optical transducer 5b, and the focusing lens 7a focuses the light emitted from the optical fiber 5c on the filter 7b.

[0094] It is usually preferred to position the filter 7b in a collimated beam to avoid the introduction of optical aberrations and to optimize the filter from the perspective of its spectral selectivity. Figure 4b exemplified in an embodiment of , in which the filter 7b is positioned in the collimated light beam between the focusing lens 7a and the collimating lens 7c, the two lenses forming an optical relay.

[0095] exist Figure 4a and Figure 4b In the embodiment of the separating device 7 shown, it is of course possible to replace the filter by mounting the filter on a filter wheel, for example.

[0096] Figure 4c A further embodiment of the separation device 7 is illustrated, this time comprising a dichroic beam splitter 7d which splits the radiation emitted from the collection optical fiber 5c between the two detection channels. According to the preferred embodiment shown in the figure, the dichroic beam splitter 7d is positioned in the collimated beam between the collimating lens 7a and the focusing lens 7c of the optical relay.

[0097] The dichroic beam splitter 7d may be configured, for example, as follows:

[0098] - directing scattered radiation at the detection wavelength (eg UV radiation) to one electro-optical transducer 5b and directing photoluminescent radiation (eg visible or NIR radiation) to another electro-optical transducer 5b;

[0099] - directing photoluminescent radiation in a spectral range (eg visible or NIR) to one electro-optical transducer 5b and directing photoluminescent radiation in another spectral range (eg visible or NIR) to another electro-optical transducer 5b.

[0100] In other embodiments, the filter 7b or the dichroic beam splitter 7d of the separation device 7 can be positioned at the level of the collecting channel.

[0101] exist Figure 5 In the embodiment, the filter 7b is positioned between the collector 4 and the sensing surface or entrance pupil 5a of the detector 5. As shown, a single filter 7b may be used for each of the collection channels of the collector, or a plurality of different filters 7b may be used for different channels.

[0102] Figure 6 Illustrated with Figure 5 Another embodiment differs from the embodiment of the invention in that the filter 7b is placed in an optical relay configuration in front of the sensing surface 5a of the detector 5. As mentioned before, the optical relay configuration has the advantage of positioning the filter in the collimated part of the light beam formed by the collected radiation.

[0103] Figure 7 Illustrated with Figure 6 Another embodiment different from the embodiment of the invention is that the collimated light beam of the narrow angle collection channel of the collector 4 is deflected by the steering prism 7e. A mirror with a central hole can also play the same role. This embodiment makes it easier to implement different filters for different detection channels.

[0104] Figure 8 Illustrated with Figure 7 Another embodiment is different from the embodiment of the invention, except that the filter 7b and the turning prism 7e are replaced by a dichroic beam splitter 7d, which separates the radiation collected from the narrow angle collection channel and the radiation collected from the wide angle collection channel.

[0105] Fig. 9 Illustrated with Figure 6 Another embodiment different from the embodiment of the present invention is that the collection channel of the collector 4 is configured to provide a parallel collimated light beam, which is focused by a focusing lens 7a on the sensing surface 5a (here the core of the collection optical fiber 5c) of the detector 5. Then, the dichroic filter 7b can be positioned in the collimated part of the light beam.

[0106] Fig.10 Another embodiment is illustrated, in which the collection channel is configured as Fig. 9 The collected radiation is provided as a parallel collimated beam and as Figure 6 Includes turning prism 7e for narrow angle collection channels.

[0107] It has been observed that defects present at the surface of the substrate 3, such as pits or triangular defects, scatter light mostly at narrow angles. Defects present in the thickness of the substrate 3, for example defects present in the thickness of the surface epitaxial layer, radiate photoluminescent radiation over a wide range of solid angles. Therefore, Figure 7 , Figure 8 and Fig.10 Embodiments of are particularly advantageous in this regard. By selectively filtering the radiation collected in each of the narrow-angle collection channel and the wide-angle collection channel, it is possible to distinguish between surface defects and subsurface defects. For example, it may be advantageous to retain only the scattered signal in the narrow-angle collection channel and only the photoluminescence signal in the wide-angle collection channel. In this case, for example Figure 7 , Figure 8 or Fig.10 The system 1 comprises a narrow-angle collecting channel provided with a separation device 7 for selectively detecting scattered radiation and a wide-angle collecting channel provided with a separation device 7 for selectively detecting photoluminescent radiation.

[0108] The above embodiments have been described in a configuration that enables management of elastically scattered radiation at a first excitation wavelength and radiation emitted by the photoluminescence effect at a second wavelength different from the first wavelength.

[0109] Of course, these embodiments may be provided with separation means 7 arranged to manage radiation of various wavelengths due to other physical effects. For example, they may be used to manage and distinguish radiation emitted by elastic scattering at a first wavelength and by inelastic scattering at a second wavelength. They may also be used to manage and distinguish radiation emitted from inelastic scattering and photoluminescence, respectively.

[0110] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. A system (1) for optical inspection of a substrate (3), the system comprising: an illumination device comprising a light source (S) for generating at least one incident light beam exhibiting at least a first wavelength, said incident light beam being directed to an exposed surface of said substrate (3) so as to define an inspection area (I) on said substrate (3), defects present on or near said surface of said substrate triggering the emission of radiation at said first wavelength and / or radiation exhibiting at least a second wavelength different from said at least first wavelength; a support (2) for receiving the substrate (3) and for positioning the substrate (3) relative to the lighting device, the support (2) and the light source (S) being movable relative to each other according to a scanning direction (SD); a detection device comprising a collector (4) and a detector (5) optically associated with the collector (4), the collector (4) being intended to collect at least a portion of the radiation at the first wavelength and / or at the second wavelength originating from a detection zone (D) of the substrate (3), the detector (5) presenting a sensing surface (5a) for capturing the collected radiation, the detection zone (D) corresponding to the optical conjugate of the collector (4) to the sensing surface (5a), the detection device providing a measurement signal representative of intensity variations of the collected radiation when the substrate (3) moves along the scanning direction (SD); The system is characterized in that the system is configured to position the inspection area (I) in front of at least a portion of the detection area (D) relative to the scanning direction (SD), while the inspection area (I) scans the exposed surface of the substrate (3) so that a portion of the substrate (3) that leaves the inspection area (I) due to the relative movement of the support member (2) and the light source (S) will still be in the detection area (D) for some time.

2. The system (1) according to claim 1, wherein: The illumination device comprises a beam splitter to form two incident light beams which converge and intersect each other on the substrate (3) to define the inspection area (I).

3. The system (1) according to claim 2, wherein: The illumination device is configured to generate a coherent incident light beam to form an interference pattern in the inspection area (I).

4. The system (1) according to claim 3, wherein: The interference pattern includes a single bright fringe.

5. The system (1) according to any one of claims 1 to 4, wherein: The collector (4) includes a narrow-angle collecting channel and / or a wide-angle collecting channel, wherein the narrow-angle collecting channel is used to collect radiation originating from the detection area (D) in a solid angle around the normal z of the support (2), and the wide-angle collecting channel is used to collect radiation emitted from the detection area (D) in a solid angle deviating from the normal z of the support (2).

6. The system (1) according to any one of claims 1 to 5, wherein: The detector (5) comprises at least one electro-optical transducer (5b), such as a photodiode.

7. The system (1) according to claim 6, wherein: The detector (5) comprises at least one optical fiber (5c) optically associated with the at least one electro-optical transducer (5b), the free end of the at least one optical fiber (5c) forming the sensing surface (5b).

8. The system (1) according to any one of claims 1 to 7, wherein: The detection device is configured such that the detection area (D) is larger than the inspection area (I) along the scanning direction, preferably 1.2 or 1.3 times larger, and more preferably 2 or 10 times larger.

9. The system (1) according to any one of claims 1 to 8, wherein: The detection device is configured such that the detection region (D) contains the examination region (I), the detection region (D) being preferably off-center relative to the examination region (I) in the scanning direction (SD).

10. The system (1) according to any one of claims 1 to 9, wherein: The detection device is configured such that the detection area (D) is composed of different detection sub-areas (D1, D2).

11. A system (1) according to any one of claims 1 to 10, further comprising a processing unit (10), which is arranged to correlate the measurement signal acquired in the detection area (D) with a position of the inspection area (I) at the origin of the measurement signal.

12. The system (1) according to any one of claims 1 to 11, wherein: The detection device further comprises separation means (7) for separating radiation at the first wavelength and radiation at the second wavelength from each other.

13. The system (1) according to claim 12, wherein: The separation device is positioned between the collector (4) and the sensing surface (5a) of the detector (5).

14. The system (1) according to claim 12 or 13 when combined with claim 7, wherein: The separation device (7) is arranged between the optical fiber (5c) and the at least one electro-optical transducer (5b).

15. A system (1) according to any one of claims 1 to 14, wherein the system further comprises a narrow-angle collection channel and a wide-angle collection channel, wherein the narrow-angle collection channel has a separation device for selectively detecting radiation at the first wavelength, and the wide-angle collection channel has a separation device for selectively detecting radiation at the second wavelength.

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