System for optical inspection of substrates using the same or different wavelengths

Abstract

The present invention relates to a system (1) for optical inspection of a substrate (3), comprising an illumination device defining an inspection area (I) on the substrate (3), a support (2) for receiving the substrate (3), and a detection device defining a detection area (D) on the substrate (3). According to the invention, the inspection area (I) is positioned in front of at least a portion of the detection area (D) in relation to the scanning direction (SD).

Description

[Technical Field]

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

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

[0003] This inspection is generally intended to provide qualitative or quantitative information, such as the location, size, and / or nature of the defects. This information about the surface and subsurface condition of the substrate may be indicative of the quality of the substrate manufacturing process or production run in which the substrate is used. U.S. Patent Application Publication No. 200859094(A1) discloses a defect inspection apparatus capable of providing such type of information.

[0004] Some substrates may exhibit photoluminescent properties that may be inherent to the material or may result from doping or crystalline defects present within the thickness of the substrate.

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

[0006] Photoluminescence behavior can be advantageously utilized to characterize these materials. In this case, the material is illuminated with a light beam exhibiting a wavelength sufficiently short so that the energy is greater than the band gap energy of the material, and the resulting photoluminescence radiation is detected at various wavelengths, for example, in the near UV, visible, or infrared regions. To better characterize and classify defects or inherent properties of materials, photoluminescence can also be used in combination with known measurement techniques that use reflection or scattering of a light beam at the illumination wavelength, as presented, for example, in U.S. Patent No. 7,304,310 (B1).

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

[0008] In some applications, photoluminescence radiation or inelastic scattering may be considered a nuisance that adds unwanted noise to the scattering measurement.

[0009] Patent document No. 5850447 (B1) describes a confocal inspection system for characterizing substrates made of SiC material. The inspection system uses an incident light beam exhibiting a wavelength between 313 and 365 nm to generate photoluminescence within the SiC material of the substrate. Several confocal detectors are combined with optical filters to selectively detect light scattered by the substrate at the excitation wavelength, the band-edge photoluminescence emission of the crystalline material at 380 nm, and photoluminescence emission in the visible and NIR range due to defects. The confocal inspection system spatially filters the photoluminescence emission, allowing only that generated on or just below the surface of the substrate to be presented to the detector.

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

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

[0012] Object of the invention The present invention aims to at least partially solve the aforementioned problems and propose a system for optical inspection of substrates, in particular substrates made of composite materials and exhibiting photoluminescence 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 from 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 affected or disturbed by 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 substrates and their defects, and enabling the identification and classification of these defects. Summary of the Invention

[0013] To this effect, the invention relates to a system for optical inspection as claimed.

[0014] The dwell time of a defect in the detection region of a system according to the invention is longer than and / or delayed relative to the dwell time of the defect in the inspection region as the substrate is moved under the illumination beam. The speed at which the incident light is scanned across the exposed surface of the substrate can be increased without loss of photoluminescence signal collection and / or in the case of any inelastic scattering effects, improving the throughput of the inspection system.

[0015] According to further non-limiting features of this aspect of the present invention, alone or in any technically feasible combination: the illumination device includes a splitter that forms two incident light beams that converge and intersect on the substrate to define an inspection area; the illumination device is configured such 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; the illumination device is configured to generate a coherent incident light beam to form an interference pattern in the inspection region; - the interference pattern contains a single bright fringe, at least one incident beam is contained within an illumination plane, which forms an angle between grazing incidence and normal incidence or preferably an angle contained between 30 and 80 degrees relative to the normal to the surface of the support, the light source is a continuous or pulsed laser, the collector includes a narrow-angle collection channel for collecting radiation originating from the detection region in a solid angle around a normal z to the support, the narrow-angle collection channel includes a collection lens; the collector includes a wide-angle collection channel for collecting radiation emitted from the detection region at a solid angle away from the normal z of the support; the wide-angle collection channel includes at least one collection mirror; the detector comprises at least one electro-optical transducer, such as a photodiode; the detector comprises a plurality of electro-optical transducers arranged in a matrix; the detector comprises at least one optical fiber optically associated with at least one electro-optical transducer, a free end of said at least one optical fiber forming a sensing surface; the system includes a controller configured to move the support relative to the illumination device to scan a surface of the substrate having an inspection area; the detection device is configured such that the detection area is larger, preferably 1.2 or 1.3 times larger, more preferably 2 or 10 times larger, than the inspection area along the scanning direction; the detection device is configured such that the detection region comprises an inspection region; - the detection area and the inspection area are centrally located; the detection area is eccentric relative to the inspection area in the scanning direction; the detection device is configured such that the detection area consists of distinct detection sub-areas, the detection device is configured such that the detection area does not completely include the inspection area; the system comprises a processing unit (10) arranged to correlate the measurement signal acquired in the detection area (D) with the position of the examination area (I) at the origin of the measurement signal, the detection device further comprises a separation device for separating the radiation of at least one first wavelength and the radiation of at least one second wavelength from each other; the separation device includes at least one spectral filter and / or at least one dichroic beam splitter arranged in the system; spectral filters and / or dichroic beam splitters are based on wavelength-dependent absorption, reflection or diffraction principles, the separation device further comprises at least one focusing lens and / or at least one collimating lens, the separation device is positioned between the collector and the sensitive surface of the detector; the separation device is disposed between the optical fiber and the at least one electro-optical converter; The system includes a narrow-angle collection channel having a separation device for selectively detecting radiation at a first wavelength, and a wide-angle collection channel having a separation device for selectively detecting radiation at a second wavelength. [Brief explanation of the drawings]

[0016] Many other features and advantages of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings. [Figure 1] 1 illustrates an illumination device of an inspection system according to one embodiment. [Figure 2] 1 illustrates a detection device of an inspection system according to one embodiment. [Figure 3a] 1 depicts a particular configuration of an illumination device that allows for defining an inspection volume that intersects with the exposed surface of the substrate. [Figure 3b] 10A-10C illustrate various possible configurations in which the inspection region of the inspection system is positioned in front of at least a portion of the detection region relative to the scanning direction. [Figure 3c] 10A-10C illustrate various possible configurations in which the inspection region of the inspection system is positioned in front of at least a portion of the detection region relative to the scanning direction. [Figure 3d] [Figure 3e] [Figure 4a] 1 illustrates various embodiments of a separation device of an inspection system. [Figure 4b] 1 illustrates various embodiments of a separation device of an inspection system. [Figure 4c] 1 illustrates various embodiments of a separation device of an inspection system. [Figure 5] 1 illustrates various embodiments of an inspection system provided with a separation device; [Figure 6] 1 illustrates various embodiments of an inspection system provided with a separation device; [Figure 7] 1 illustrates various embodiments of an inspection system provided with a separation device; [Figure 8] 1 illustrates various embodiments of an inspection system provided with a separation device; [Figure 9] 1 illustrates various embodiments of an inspection system provided with a separation device; [Figure 10] 1 illustrates various embodiments of an inspection system provided with a separation device; DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention relates to a system 1 for the optical inspection of substrates 3, e.g. exhibiting photoluminescence properties. The system may be useful for detecting or characterizing defects such as crystalline imperfections and / or particles, or for detecting the intrinsic photoluminescence of the substrate 3. For ease of expression, the term "defect" is used in this disclosure to denote a feature of the substrate 3 that causes scattering or photoluminescence radiation, or a change in the intrinsic photoluminescence emission, whatever the nature of this feature.

[0018] System 1 is particularly adapted for inspecting substrates that exhibit photoluminescence 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 typically provided as wafers, i.e., disks of material with a normalized diameter, but system 1 may be utilized to inspect substrates of any shape and size. Substrate 3 may be a substrate made of a single material (bulk substrate) or a composite substrate (a surface thin film of semiconductor material disposed on a support). In the latter case, and in the context of the present invention, at least the surface thin film exhibits photoluminescence properties.

[0019] The system 1 includes an illumination device. In the embodiment depicted in FIG. 1, the illumination device includes a light source S that generates an incident light beam exhibiting at least one first wavelength. The light source S can be, for example, a continuous or pulsed laser. The incident light beam is guided and directed onto the exposed surface of the substrate 3, and the projection of the incident light beam onto the exposed surface of the substrate 3 defines an inspection area I. Defects present on or near the surface of the substrate cause the emission of scattered radiation and / or photoluminescent radiation, which can be collected and measured, as described later in this specification. Thus, the radiation emitted by the surface can 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 any inelastic scattering effect).

[0020] The incident light beam is preferably monochromatic and, in the illustrated embodiment, exhibits an inspection wavelength sufficiently short so that its energy is greater than the bandgap 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 bandgap of 3.265 eV), the light source S may be selected to generate an incident light beam exhibiting an inspection wavelength in the range of 313 nm to 365 nm. Such materials emit photoluminescence radiation at wavelengths close to 380 nm. For an undoped GaAs bulk substrate 3 (exhibiting a bandgap of 1.42 eV), the inspection wavelength may be selected to be less than 500 nm, and the photoluminescence radiation emitted by the substrate 3 will exhibit a wavelength of 873 nm.

[0021] In the embodiment depicted in FIG. 1 , a light source S is coupled via a first fiber F to a beam splitter BS, which propagates light in two separate optical fibers F1, F2. The tips of the two separate optical fibers F1, F2 are positioned within the inspection system to generate two converging incident light beams B1, B2, which intersect each other to define an inspection volume intercepting the surface of the substrate 3. This particular configuration is facilitated by two focusing optical elements 8, 8′, such as focusing lenses, respectively, disposed in the propagation paths of the incident light beams B1, B2. More generally, the light beams can intersect each other at the surface of the substrate, at the bottom of the substrate, or within the substrate.

[0022] As shown in Figure 3a, this embodiment is advantageous in that it allows for the definition of an examination volume bounded in the xy plane and in depth (i.e., in the z direction) by the intersection of the two incident beams. In the intersection volume, the illumination power is at least twice that provided by a single beam, thus enhancing the resulting scattered and photoluminescent radiation relative to radiation originating elsewhere along each beam. To optimize this localization, the two incident light beams B1, B2 can be positioned to intersect around their focal positions or, in the case of Gaussian beams, around their waists.

[0023] In such a 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 typically has an elliptical shape. Depending on the angles of incidence of the beams B1, B2 and their focusing conditions, the major axis of the inspection area I may be comprised between 20 μm and 150 μm.

[0024] The incident light beams B1, B2 are contained within an illumination plane IP (visible in FIG. 2) that forms an angle with respect to the normal z to the surface of the substrate 3 that can be any value between grazing incidence (>80 degrees) and normal incidence (0 degrees). Typically, the illumination plane IP presents an angle of 30 degrees to 80 degrees with respect to the normal z.

[0025] In certain embodiments, the illumination device can be configured to generate coherent incident light beams B1, B2 that interfere to form an interference pattern within the inspection area I. In the illustrated embodiment of FIGS. 1 and 3a, the intersecting incident light beams B1, B2 originate from the same light source S with equal optical paths to the inspection area I. In this configuration, it is possible to generate an interference pattern consisting of parallel fringes that modulate light in a direction defined by the plane of the two incident light beams B1, B2 (the x direction in FIG. 3a). This configuration is particularly advantageous because it improves the spatial selectivity of the illumination. Furthermore, the interference pattern consisting of parallel fringes provides a modulation frequency in the measurement signal associated with the scattered radiation, which allows for an improvement in the signal-to-noise ratio of the detection system 1. If the decay time of the photoluminescence radiation is relatively short (on the order of nanoseconds), the modulation frequency can also be present in the measurement signal associated with the photoluminescence radiation.

[0026] It may also be advantageous to have an interference pattern that includes 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 an appropriate angle between the incident light beams B1, B2 and / or by using a light source S with a short coherence length. In an interference pattern consisting of a single bright fringe, the illumination power can be up to four times greater than the illumination power that would result from only one of the incident light beams.

[0027] The system 1 also includes a support 2 for positioning the substrate 3 relative to the illumination device. The support 2 and the illumination device are movable relative to each other according to a scanning direction SD of the illumination device relative to the substrate. As is well known in the art, the support 2 and / or the illumination device may be associated with actuators capable of moving one relative to the other to achieve this relative movement.

[0028] 1, the support 2 may be rotated about axis r and translated in the (x,y) plane in which the substrate 3 resides. In such a configuration, the relative movement of the substrate 3 disposed on the support 2 may be controlled such that the inspection zone I scans the entire exposed surface of the substrate 3, for example in a spiral trajectory.

[0029] Continuing with FIG. 1 , the inspection system 1 also includes a controller 6 for controlling the relative movement of the support 2 with respect to the illumination device in order to scan the inspection area I over the exposed surface of the substrate along a desired measurement path. The controller 6 may include a microcontroller, data storage, input / output ports connected to actuators and other elements of the inspection system 1, and further computational resources configured in hardware or software to precisely control the displacement of the inspection area I. Once the substrate 3 is precisely positioned on the support 2, the controller 6 stores and controls the position (e.g., linear or polar coordinates) of the inspection area I in a reference linked to the substrate 3. As a result, the position of the inspection area I on the substrate 3 is known at any given time.

[0030] Referring now to FIG. 2 , the inspection system 1 also includes a detection device including a collector 4 for collecting at least a portion of the radiation at the first wavelength and / or the second wavelength emanating from a detection area D of the substrate 3. The radiation may be generated, for example, by a defect located within the detection area D. The portion of the radiation collected by the collector is designated in this disclosure as the “collected radiation.” The detection device also includes a detector 5 optically associated with the collector 4, the detector 5 presenting a sensing surface or entrance pupil 5a for capturing the collected radiation. In the illustrated embodiment, the detection area D corresponds to an optical conjugate or projection of the sensing surface 5a by the collector 4. More generally, the detection area D can be defined as the area of ​​the substrate encompassing all of the emitted radiation collected by the collector 4 and the sensing surface or entrance pupil 5a. The detection device 5 provides a measurement signal S(t) representative of the change in intensity of the collected radiation as the illumination device is moved along the scanning direction SD relative to the substrate 3.

[0031] In the embodiment depicted in Figure 2, collector 4 includes a narrow-angle collection channel for collecting radiation emanating from detection region D in a solid angle about normal z to support 2. The solid angle may typically extend from 0 to 10 degrees about normal z. The narrow-angle collection channel is configured to direct the collected radiation toward sensing surface 5a of detector 5. The narrow-angle collection channel may include collection lens 4a disposed within system 1, as shown in the embodiment of Figure 2. Collection lens 8 in this embodiment is centered on an axis that passes through detection region D and is perpendicular to substrate 3.

[0032] The collector 4 also includes a wide-angle collection channel for collecting radiation emitted from the detection region D at a solid angle away from the normal z of the support 2. This solid angle can typically extend from 60 to 89 degrees around the normal z. The wide-angle collection channel is also configured to direct the collected radiation toward the sensing surface 5a of the detector 5. The wide-angle collection channel can include at least one collection mirror 4b, 4c disposed within the system 1. As shown in the embodiment of FIG. 2, the wide-angle collection channel may be further divided into two channels: a forward collection channel in the direction of forward scattering of light and a rear collection channel in the direction of backscattering. Each of the front and rear collection channels is provided with a collection mirror 4b, 4c.

[0033] In some embodiments, the inspection system may be based on a dark field configuration, ensuring that specular reflections of the incident light beams B1, B2 on the substrate 3 are not collected by the collector 4, in particular not by the narrow-angle collection channel nor the wide-angle collection channel.

[0034] In certain embodiments, the detector 5 can include at least one electro-optical converter 5b, such as a photodiode or an array of photodiodes. The electro-optical converter 5b can constitute the sensing surface 5a of the detector 5; in such cases, the electro-optical converter 5b is positioned within the focal zone of the collector 4 in the system 1. Preferably, however, the electro-optical converter 5b is deported, and the collected radiation is guided from the focal zone of the collector to the electro-optical converter 5b by, for example, an optical fiber 5c or a bundle of optical fibers 5c (generally referred to hereinafter as the "collection fiber"). In such a configuration, the free end of the collection fiber 5c is positioned within the focal zone of the collector 4, and the core of the fiber at the free end forms the sensing surface 5a of the detector 5.

[0035] Instead of a photodiode or an array of photodiodes, the electro-optical converter 5b may be formed by an image sensor such as a photomultiplier tube, an avalanche photodiode, a so-called CMOS image sensor or a CCD image sensor. The image sensor may also exhibit time delay integration characteristics. The detector 5 may also include a spectrometer to analyze the optical spectrum of the collected radiation, in particular the photoluminescence radiation (i.e. exhibiting at least the second wavelength).

[0036] In the inspection system 1 of the present invention, the detection area D can be adjusted in size and position independently of the inspection area I. As mentioned above, the detection area D is the optical conjugate or projection of the entrance pupil of the detector 5, i.e., the sensing surface 5a of the detector 5, by the respective collection channel of the collector 4. Thus, by positioning or adjusting the size of the sensing surface 5a differently or by adapting the magnification of the collector 4, the detection area D can be positioned or sized differently relative to the substrate 3.

[0037] For example, a fiber bundle may be used in which several fibers are arranged according to a particular pattern (linear, circular, etc.). Each fiber may form the entrance pupil 5a of a particular detector and may be conjugated to a possibly distinct detection region D by a particular channel (narrow angle, wide angle, front, rear, etc.) of the collector 4. The position of the fiber inlet 5a relative to the collector 4 then defines the position of the detection region D.

[0038] According to an important feature, the inspection system 1 is configured such that the inspection zone I and the detection zone D do not correspond perfectly to one another. More precisely, the inspection zone I is positioned in front of or ahead of at least a portion of the detection zone D in relation to the scanning direction SD, while the inspection zone 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 illumination device, along a measurement path imposed by the controller 6, such that the inspection zone I is positioned in front of at least a portion of the detection zone D in relation to the scanning direction SD.

[0039] This means that the part of the substrate 3 that has left the inspection region I due to this displacement remains within the detection region D for some time. As a result, the dwell time of the defect in the detection region D is longer than and / or delayed relative to the dwell time of the defect in the inspection region I as the substrate 3 moves under the illumination beam. Although the measurement time of the photoluminescence radiation emitted by the defect is increased, this makes it possible, for example, to collect radiation emitted during the photoluminescence establishment and decay times. As a result, the speed at which the illumination beam is scanned across the exposed surface of the substrate 3 can be increased without losing collection of the photoluminescence signal, improving the throughput of the inspection system 1.

[0040] A feasible configuration for achieving the feature of efficient collection of photoluminescence radiation is to configure the inspection system 1 so that the detection region D is larger (along the scanning direction) than the inspection region I. Preferably, the detection region D is at least 1.2 or 1.3 times larger, preferably at least 2 times larger, or up to 10 times larger, along the scanning direction than the inspection region I. To adequately collect scattered radiation, the inspection system 1 is configured so that the detection region D at least partially covers the inspection region I. For example, if the detection region D has a length of 100-200 μm and a significant portion of it (such as 50%-100%) lags behind the inspection region I along the scanning direction SD, and the substrate 3 rotates at several thousand revolutions per minute, then integration times of the photoluminescence radiation of a few microseconds are easily achieved.

[0041] It should be noted that if the defect density in / on the substrate 3 is low enough that defects appear one at a time within the detection area D, the spatial resolution of the inspection system 1 is defined by the size of the inspection area I, which may be in the range of, for example, 20 μm to 150 μm, even if the detection area D is enlarged. Thus, increased throughput is not achieved at the expense of reduced spatial resolution.

[0042] A measurement signal S(t), representing the variation in intensity of the radiation emitted and collected by the detector device 5, is provided to a processing unit 10. Such a processing unit comprises a computer or microcontroller with a data storage device and an interface port. This 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 map representing the measurement signal S(t) as a function of the position in a reference linked to the substrate 3. Preferably, since the detection area D and the inspection area I are not coincident (i.e. not centered), the processing unit is further configured to provide a signal map taking into account a possible offset between the center of the inspection area I and the center of the detection area D.

[0043] 3b-3e depict various possible configurations in which the inspection region I is positioned in front of at least a portion of the detection region D relative to the scanning direction SD. In these figures, for clarity, the scanning direction SD represents the relative movement of the illumination device with respect to the substrate 3. In the illustrated embodiment, in which the illumination device remains fixed and the support 2 is moved, the movement of the substrate is in the opposite direction to the scanning direction SD. As explained above with reference to FIG. 2, the various detection regions D can be defined and positioned as optical conjugates of separate optical fibers of the fiber bundle.

[0044] FIG. 3b shows a first configuration in which the inspection area I and the detection area D are centered and the inspection area I has smaller dimensions than the detection area D and is therefore contained within it.

[0045] In the configuration of Figure 3c, the detection area D still covers the inspection area I, but is decentered in the scanning direction so that the point of the substrate generating the photoluminescence radiation remains longer within the field of view of the detector 5 compared to the concentric configuration of Figure 3b.

[0046] 3d shows another configuration in which the detection region D is made up of separate detection sub-regions D1, D2. This can be achieved by collecting radiation at different positions relative to the inspection region I, for example using a collection fiber 5c, which is a bundle of several optical fibers. In the example shown, a first detection sub-region DI covers the inspection region I and collects scattered radiation and photoluminescence radiation. A further detection sub-region D2 following the inspection region I in the scanning direction SD is provided to collect only photoluminescence radiation with a time delay. Such multi-zone detection may, for example, make it possible to estimate fluorescence lifetimes, which may be useful for defect classification.

[0047] Figure 3e shows a further configuration in which the examination region I is not contained within the detection region D, but is positioned in front of the detection region D in the scanning direction SD. In this configuration, no scattered radiation is collected, only the delayed part of the photoluminescence radiation is collected.

[0048] The inspection system 1 of the present disclosure allows for the collection of both scattered and photoluminescent radiation emitted by defects in the substrate being inspected. In some applications, it may be beneficial to separate the contribution of each type of radiation in a separate measurement signal, or even to filter out one type of radiation.

[0049] In some instances, photoluminescence radiation may be considered an unwanted noise source when performing scattered or dark-field measurements for characterization of particles, defects, "haze," or surface roughness of substrate 3, especially if this radiation is intrinsic to the material of substrate 3. This is the case, for example, when inspecting substrates made of compound materials such as GaAs, InP, or SiC. In that application, it would be beneficial to equip inspection system 1 for at least some of the collection channels with high-pass (in frequency) or band-pass filters that pass only radiation at the inspection wavelength (to collect scattered radiation) and cut out all radiation at longer wavelengths (to remove photoluminescence radiation).

[0050] In other applications, it may be beneficial to pass only photoluminescence radiation and block all radiation at the inspection wavelength. This is the case when characterizing substrates containing light-emitting active layers for LED, pLED, and VCSEL devices. Analysis of the collected radiation wavelength, FWHM, and intensity, which can be performed at or after the inspection device 5, can help to better characterize the light-emitting active layer.

[0051] In other applications, both scattered and photoluminescent radiation can be utilized for substrate inspection. For example, it is known that characterization of various defects, such as basal plane dislocations (BPDs), carrots, TEDs, and micropipes, that may be present within the thickness of an epitaxial SiC layer, can be performed efficiently by combining scattered and photoluminescent measurements. In such applications, it would be beneficial for the inspection system 1 to include filters in the narrow-angle collection channels that retain only photoluminescent radiation in the visible and / or NIR, or only scattered radiation, while the wide-angle collection channels are dedicated to scattered radiation detection or photoluminescent radiation. This allows the same number of detectors to be used.

[0052] As presented with respect to the description of Fig. 3e, separation or filtering of particular radiation types may be achieved by a particular configuration of the inspection region I and the detection region D. In other embodiments, the inspection system may comprise at least one separation device 7 for separating the photoluminescent radiation and the scattered radiation from each other or for filtering one of them. This is made possible by the fact that, generally speaking, the photoluminescent radiation and the scattered radiation exhibit different wavelengths.

[0053] The separation device 7 may include at least one spectral filter 7b and / or at least one dichroic beam splitter 7d suitably arranged in the system to filter one of the two types of radiation or to separate the two types of radiation.

[0054] For example, the separation device 7 may be placed between the electro-optical converter 5b and the collection fiber 5c in the inspection system 1 shown in Figure 2. In this configuration, the collection fiber 5c transmits all collected scattered and photoluminescent radiation. This configuration has the advantage of being very simple and versatile, as the spectral selection is outside the collection channel in the remote unit.

[0055] 4a and 4b show different embodiments of the separation device 7. In each of these embodiments, the spectral filter 7b comprises: a bandpass or highpass filter (in frequency) that passes the test wavelength and rejects longer photoluminescence wavelengths in order to detect the scattered radiation without being affected by the photoluminescence radiation; -It can be either a band-pass or low-pass filter (in frequency) that passes the photoluminescence wavelengths at least in the spectral region (near UV / visible, NIR) and rejects the inspection wavelengths, in order to detect the photoluminescence radiation without being affected by scattered radiation.

[0056] In the embodiment of FIG. 4a, the spectral filter 7b is simply positioned between the electro-optical converter 5b and a focusing lens 7a that focuses the light emitted from the optical fiber 5c onto the filter 7b.

[0057] It is usually preferable to position filter 7b in the collimated beam to avoid introducing optical aberrations and to optimize the filter in terms of its spectral selectivity. This is shown in the embodiment of Figure 4b, where filter 7b is positioned in the collimated beam between focusing lens 7a and collimating lens 7c, the two lenses forming an optical relay.

[0058] In the embodiment of the separation device 7 shown in Figures 4a and 4b, the filters may of course be replaceable, for example by being mounted on a filter wheel.

[0059] 4c shows a further embodiment of the separation device 7, which in this case comprises a dichroic beam splitter 7d which separates the radiation emitted from the collection 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 collimation lens 7a and the focusing lens 7c of the optical relay.

[0060] The dichroic beam splitter 7d may be, for example, - directing the scattered radiation of the inspection wavelength (e.g. UV radiation) to one electro-optical converter 5b and the photoluminescent radiation (e.g. visible or NIR radiation) to another electro-optical converter 5b; - It can be configured to direct photoluminescence radiation in one spectral range (e.g. visible or NIR) to one electro-optical converter 5b and to direct photoluminescence radiation in another spectral range (e.g. visible or NIR) to another electro-optical converter 5b.

[0061] In other embodiments, the spectral filter 7b or the dichroic beam splitter 7d of the separation device 7 may be positioned at the level of the collection channel.

[0062] In Figure 5, a spectral filter 7b is positioned between the collector 4 and the sensitive surface or entrance pupil 5a of the detector 5. A single filter 7b may be used for each of the collection channels of the collector, as shown, or several separate filters 7b may be used for the separate channels.

[0063] Figure 6 shows another embodiment which differs from that of Figure 5 in that the spectral filter 7b is positioned in an optical relay configuration in front of the sensitive surface 5a of the detector 5. As explained above, the optical relay configuration offers the advantage of positioning the filter in a collimated portion of the beam formed from the collected radiation.

[0064] Figure 7 shows another embodiment that differs from that of Figure 6 in that the collimated beam of the narrow-angle collection channel of the collector 4 is deflected by a rotating prism 7e. A mirror with a central hole can also perform the same function. This embodiment makes it easier to implement different spectral filters for different detection channels.

[0065] Figure 8 shows another embodiment that differs from that of Figure 7 in that the spectral filter 7b and rotating prism 7e are replaced by a dichroic beam splitter 7d that separates the collected radiation from the narrow and wide angle collection channels.

[0066] Figure 9 shows another embodiment that differs from that of Figure 6 in that the collection channel of the collector 4 is configured to provide a parallel collimated beam that is focused by a focusing lens 7a onto the sensitive surface 5a of the detector 5, here the core of the collection fiber 5c. In this case, a dichroic filter 7b can be positioned in the collimated portion of the beam.

[0067] FIG. 10 shows another embodiment in which the collection channel is configured to provide collected radiation as a parallel collimated beam as in FIG. 9, and includes a rotating prism 7e for a narrow angle collection channel as in FIG.

[0068] It has been observed that defects present on the surface of the substrate 3 (such as pits or triangular defects) scatter light primarily at narrow angles. Defects present within the thickness of the substrate 3 (e.g., defects present within the thickness of a surface epitaxial layer) emit photoluminescence radiation over a wide range of solid angles. Therefore, the embodiments of FIGS. 7, 8, and 10 are particularly advantageous in this regard. By selectively filtering the radiation collected in each of the narrow-angle and wide-angle collection channels, it is possible to distinguish between surface and subsurface defects. For example, it may be advantageous to retain only the scattering signal in the narrow-angle collection channel and only the photoluminescence signal in the wide-angle collection channel. In that case, for example, the system 1 of FIG. 7, 8, or 10 includes a narrow-angle collection channel provided with a separation device 7 for selectively detecting scattered radiation and a wide-angle collection channel provided with a separation device 7 for selectively detecting photoluminescence radiation.

[0069] The above-described embodiments are described in a configuration that allows managing elastically scattered radiation at a first excitation wavelength and radiation emitted from the photoluminescence effect at a second wavelength different from the first wavelength.

[0070] Of course, these embodiments may be configured with separation devices 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 from elastic scattering at a first wavelength and inelastic scattering at a second wavelength. They may also be used to manage and distinguish radiation emitted from inelastic scattering and photoluminescence, respectively.

[0071] 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), comprising: an illumination device for defining defects present on or near the surface of the substrate, comprising a light source (S) for generating at least one incident light beam exhibiting at least a first wavelength directed onto the exposed surface of the substrate (3) to cause emission of radiation at said first wavelength and / or radiation at at least a second wavelength different from said at least first wavelength onto an inspection area (I) of the substrate (3); a support (2) for receiving the substrate (3) and positioning it 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) for collecting at least a portion of the radiation of the first wavelength and / or the second wavelength emerging from a detection area (D) of the substrate (3) and a detector (5) optically associated with the collector (4), the detector (5) presenting a sensing surface (5 a) for capturing the collected radiation, the detection area (D) corresponding to an optical conjugation of the sensing surface (5 a) by the collector (4), the detection device providing a measurement signal representative of the change in intensity of the collected radiation as the substrate (3) moves along the scanning direction (SD), The system (1) is configured to position the inspection zone (I) in front of at least a portion of the detection area (D) in the scanning direction (SD) while the inspection zone (I) scans the exposed surface of the substrate (3), and characterized in that a portion of the substrate (3) that has moved out of the inspection area (I) due to the relative movement between the support (2) and the light source (S) remains within the detection area (D) for a while.

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

3. The system (1) according to claim 1, 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) of claim 1, wherein the interference pattern comprises a single bright fringe.

5. The system (1) according to any one of claims 1 to 4, wherein the collector (4) comprises a narrow-angle collection channel for collecting the radiation originating from the detection region (D) in a solid angle around the normal z of the support (2), and / or a wide-angle collection channel for collecting the radiation emanating from the detection region (D) in a solid angle away 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 converter (5b), such as a photodiode.

7. 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 converter (5b), the free end of the at least one optical fiber (5c) forming the sensing surface (5b).

8. 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, preferably 1.2 or 1.3 times larger, more preferably 2 or 10 times larger, than the inspection area (I) along the scanning direction.

9. The system (1) according to any one of claims 1 to 8, wherein the detection device is configured such that the detection area (D) includes the inspection area (I), and the detection area (D) is preferably eccentric relative to the inspection area (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 region (D) is made up of distinct detection sub-regions (D1, D2).

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

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

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

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

15. 15. The system (1) of any one of claims 1 to 14, further comprising a narrow-angle collection channel presenting a separation device for selectively detecting the radiation at the first wavelength, and a wide-angle collection channel having a separation device for selectively detecting the radiation at the second wavelength.

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