Method for controlling the quality of a front face of a polycrystalline substrate, in particular prior to it being assembled in order to manufacture a composite structure

Abstract

The invention relates to a method for controlling the quality of a substrate made of polycrystalline material, comprising the following steps: 1) carrying out full-wafer inspection of a front face of the substrate using a reflected dark-field microscopy technique implementing a laser scan of the front face while the substrate performs a rotational movement about an axis normal to said face, so as to obtain a map of a diffuse background-noise signal, the diffuse background-noise signal being referred to as the haze; 2) defining at least two regions of the front face, said regions being concentric and circular; 3) determining, for one of the -at least two- regions, referred to as the first region, a median haze value, referred to as the first median haze value, and for the other of the -at least two- regions, referred to as the second region, a median haze value, referred to as the second median haze value ; 4) scoring the substrate based on a comparison between the first median haze value and a first predetermined threshold, and on a comparison between the second median haze value and a second predetermined threshold.

Description

Method for inspecting the front face of a polycrystalline substrate, particularly prior to its assembly for the manufacture of a composite structure FIELD OF INVENTION

[0001] The present invention relates to the field of semiconductor materials, particularly composite structures comprising a thin film (derived from a donor substrate, for example, monocrystalline silicon carbide) transferred onto a polycrystalline support substrate (for example, SiC). It specifically relates to a method for characterizing and grading a polycrystalline support substrate prior to bonding to a donor substrate. The method provides information on the ability of the front face of the support substrate to form a good quality bonding interface, based on the "haze" parameter.

[0002] TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Silicon carbide and gallium nitride are particularly interesting materials for the manufacture of power devices, radio frequencies or devices operating at very high temperatures.

[0004] Although rapidly developing, high-quality monocrystalline SiC or GaN substrates remain expensive and difficult to source in large quantities. Therefore, layer transfer solutions offer an advantage for creating composite structures comprising a thin monocrystalline SiC or GaN layer (from the high-quality donor substrate) on a lower-cost support substrate, such as polycrystalline SiC (p-SiC), which can also offer advantages in terms of electrical or thermal conductivity.

[0005] A well-known thin-film transfer solution is the Smart Cut process. TMThis method is based on the implantation of light ions and the direct bonding of a donor substrate to a support substrate at a bonding interface. The implantation creates a fragile plane embedded in the donor substrate, along which a separation occurs, leading to the transfer of a thin layer onto the support substrate to form the composite structure. This process also allows for the recovery and recycling of the remaining donor substrate, potentially enabling one or more further layer transfers. Epitaxy can then be performed on the thin layer of the composite structure, followed by the fabrication of the electronic devices.

[0006] For the implementation of a layer transfer process to be economically viable, it is important to know how to control the quality of a support substrate, before the assembly step on the donor substrate, as it can greatly influence the quality of the final composite structure.

[0007] The reference techniques for characterizing the surface state of a polycrystalline substrate are atomic force microscopy (AFM) and white light interferometry (WLI or SWLI, for scanning white light interferometry, for example with Wyko® or Contour GTX equipment). TM ).

[0008] These measurement techniques can provide statistical parameters related to surface condition, such as RMS (root mean square), Ra (arithmetic mean), and PV (peak-to-pit) roughness, over an area dependent on the configuration and magnification used. For example, for AFM, scan sizes typically range from 5 x 5 µm. 2 at 30x30µm 2 For WLI, the typical scan size range is 100µm to 2mm and the lateral resolution depends on the lens used (usually from 100X to 2.5X).

[0009] The main limitations of the techniques mentioned are as follows:

[0010] - Each tool / configuration combination can probe a very specific spatial frequency range, depending on the scan size and lateral resolution. Since bonding performance is influenced by a wide range of spatial frequencies, a complete characterization requires multiple measurements (e.g., 5x5µm AFM). 2 , AFM 30x30µm 2 and WLI 50X). The specification of a good surface will correspond to several different values ​​(e.g., RMS or PV) related to the metrology used. This leads to a complex grading procedure, especially in a production environment;

[0011] - the metrology methods considered can only probe a very limited part of the surface of the supporting substrate (generally a few µm 2 or mm 2of a substrate typically having a diameter of 150mm or 200mm) and this can mask localized failure modes;

[0012] - AFM and WLI / SWLI tools are generally slow and not optimized for high-volume manufacturing.

[0013] SUBJECT OF THE INVENTION

[0014] The present invention addresses the stated problem. The invention relates to a method for testing a support substrate made of polycrystalline material, particularly silicon carbide, especially suitable for evaluating the ability of said substrate to form a high-quality bonding interface during assembly onto a donor substrate.

[0015] BRIEF DESCRIPTION OF THE INVENTION

[0016] The invention relates to a method for controlling a polycrystalline substrate comprising the following steps:

[0017] 1) full plate inspection of a front face of the substrate by a dark-field reflection microscopy technique employing a laser scan of the front face while the substrate performs a rotational movement around an axis normal to said face, so as to obtain a map of a diffuse background noise signal, said diffuse background noise signal being named haze;

[0018] 2) the definition of at least two regions of the front face, said regions being concentric and circular;

[0019] 3) the determination, for one of the -at least two- regions, called first region, of a median haze value, called first median haze value, and for the other of the -at least two- regions, called second region, of a median haze value, called second median haze value;

[0020] 4) the substrate gradation based on the comparison between the first median haze value and a first predetermined threshold, and on the comparison between the second median haze value and a second predetermined threshold.

[0021] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: step 2) includes the definition of a third region of the front face, step 3) includes the determination, for this third region, of a median haze value called the third median haze value, and in step 4), the gradation is also based on the comparison between the third median haze value and a third predetermined threshold; the first, second and third predetermined thresholds are different from each other; in step 2), three regions are defined and dimensioned as follows: a central region, having a radius less than or equal to 15% of the radius of the front face of the substrate, an intermediate annular region, having an external radius between 50% and 80% of the radius of the front face of the substrate, and a peripheral annular region extending from the external radius towards the edge of the substrate;the first region corresponds to the central region and the second region corresponds to the peripheral annular region; the control method includes a step 3') of determining a full-plate average haze value, representative of the front face of the substrate, and, in step 4), the grading is also based on the comparison between the full-plate average haze value and a predetermined full-plate threshold; the control method includes, before step 4), a step 3'') of identifying a local defect in the haze map, the local defect fulfilling a circularity criterion and resulting in an abrupt variation of the haze (in particular of more than 10%) compared to the haze of an area surrounding said defect; in step 4), the grading also takes into account the presence or absence of a local defect;at step 4): (i) if the first median haze value is greater than or equal to the first predetermined threshold, and / or if the second median haze value is greater than or equal to the second predetermined threshold, the substrate is assigned a low grade and optionally is sent to a reconditioning step; (ii) if the first median haze value is less than the first predetermined threshold, and if the second median haze value is less than the second predetermined threshold, the substrate is assigned a high grade and optionally is used in a composite structure manufacturing process; if the average haze value is greater than or equal to the predetermined full-plate threshold, a substrate that had obtained a high grade based on the median haze values ​​is downgraded to a low grade;If a local defect is present, a substrate that had obtained a high grade based on the full-plate mean and median haze values ​​is downgraded to a low grade; the grading of step 4) is used to qualify a grinding process and / or equipment implemented for the surface preparation of the substrate, prior to step 1): in case (i), the grinding equipment is not qualified; in case (ii), the grinding equipment is qualified.

[0022] The invention also relates to a method for manufacturing a composite structure, comprising the following steps:

[0023] a) the supply of a support substrate made of polycrystalline material,

[0024] b) the application of the control method as described above to the supporting substrate, and if the supporting substrate obtains a high grade,

[0025] c) the assembly of a donor substrate onto the support substrate,

[0026] d) the transfer of a thin layer from the donor substrate onto the support substrate, to form the composite structure.

[0027] Preferably, the manufacturing process includes, if the support substrate obtains a low grade in step b), the following step:

[0028] a1) the reconditioning of the support substrate including at least one step of mechanical or mechano-chemical grinding or polishing, and the reintroduction of the reconditioned substrate as a new support substrate in step a).

[0029] Preferably, the polycrystalline material of the support substrate is silicon carbide. BRIEF DESCRIPTION OF THE FIGURES

[0030] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0031] Presents a substrate made of polycrystalline material and a composite structure including said substrate as a support substrate and a thin layer;

[0032] This presents a map of a diffuse background noise (haze) signal representative of a front face of a substrate made of polycrystalline material (in particular SiC), obtained in step 1) of a control method according to the invention;

[0033] Presents a haze map on which three concentric and circular regions were defined in step 2) of a control method according to the invention;

[0034] This presents a haze map on which a local defect is visible and can be taken into account in step 4) of grading a control method according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] The invention relates to a method for inspecting a polycrystalline substrate 20. As is common in the semiconductor industry, the donor substrate 20 is preferably in the form of a circular wafer with a diameter of 100 mm, 150 mm, 200 mm, or even larger. The thickness of the substrate 20 extends along the z-axis in the figures and can range from a few hundred micrometers to approximately 1 mm. The front face 20a and back face 20b of the substrate 20 extend along a principal plane (x,y). The front face 20a is intended to be bonded to a thin film 10 to form a composite structure 100; it is this front face 20a that is inspected by the method according to the present invention, prior to its bonding.

[0036] Advantageously, the polycrystalline material of substrate 20 is silicon carbide, aluminum nitride or silicon.

[0037] The method comprises a first step 1), corresponding to the full-plate inspection of the front face 20a of the substrate 20 by a dark-field reflection microscopy technique employing a laser scan of the front face 20a, while the substrate 20 undergoes a rotational movement about a z-axis normal to said face 20a. Step 1) leads to obtaining a map of a diffuse background noise signal (known as "haze" in English terminology). In the following description, this diffuse background noise signal is referred to as haze.

[0038] Metrology equipment, well-known in the field of microelectronics, allows for such full-plate inspection: for example, the SP1 TM , the SPA2 TM or the SC1 TMfrom the company KLA. These devices are typically used to detect particles on the surface of a substrate. They also detect haze, which is light scattered isotropically or anisotropically by microscopic irregularities on the substrate's surface. In practice, the laser beam typically has a diameter of less than 100 µm; it scans the surface, and detectors capture the scattered light from multiple angles. The signal is then digitized, with pixels ranging from 100 to 500 µm on a side, to create the haze map.

[0039] For a given metrology instrument, haze is correlated with surface roughness across a range of multiple spatial frequencies. Thus, haze can be used as an indicator of surface behavior, combining several frequencies, similar to power spectral density (PSD), which the applicant was able to correlate with the surface's ability to produce good-quality bonding.

[0040] Within the scope of the present invention, it is also important that the metrology equipment used be capable of rotating the substrate 20 around an axis normal to the inspected front face 20a. Indeed, the polycrystalline substrate 20 generally undergoes one or more mechanical (or mechano-chemical) treatments, particularly on its front face 20a, to achieve a level of quality sufficient for assembly. These mechanical treatments (in particular, mechanical grinding) generate a surface finish with rotational symmetry, and rotation of the substrate during measurement is required to prevent the masking of certain areas of the mapping by interference patterns. Typically, during the full-plate inspection in step 1), the substrate 20 rotates at a speed exceeding 1000 rpm.

[0041] At the end of step 1), a map such as the one shown can be obtained. In this example, substrate 20 is made of polycrystalline silicon carbide and has a diameter of 150 mm; it has undergone surface preparation by mechanical grinding with a fine mesh grinding wheel, typically #8000. The concentric symmetry on the map is characteristic of this type of mechanical preparation. The metrology equipment used for the inspection in step 1) is an SPA2 that employs a deep UV laser.

[0042] Haze values ​​are expressed in ppm. They can obviously vary from one piece of equipment to another (depending on the settings) and should therefore not be considered absolute.

[0043] The control method then includes a step 2) corresponding to the definition of at least two concentric and circular regions of the front face 20a, covering the entire effective surface of said front face 20a. By effective surface, we mean the entire surface of the front face 20a or the entire surface less a peripheral exclusion zone (including the edge drop and the chamfer of the substrate 20), generally less than 5mm, or even less than or equal to 3mm.

[0044] In the example shown, three regions were defined on the map:

[0045] - a central RC region,

[0046] - an intermediate annular region RAI,

[0047] - a peripheral annular region RAP.

[0048] In another example, only two regions could have been defined: a central RC region and a peripheral RAP annular region. In yet another example, four regions could have been defined: a central RC region, two concentric intermediate annular regions, and a peripheral RAP annular region.

[0049] Advantageously, three regions RC, RAI, RAP are defined and dimensioned as follows:

[0050] - the central RC region has a radius less than or equal to 15% of the radius of the front face 20a of the substrate 20,

[0051] - the intermediate annular region RAI has an external radius between 50% and 80% of the radius of the front face 20a of the substrate 20,

[0052] - the peripheral annular region RAP occupies the remaining annular surface, up to the edge of the substrate 20 or up to a peripheral exclusion zone.

[0053] For example, for a 20-inch substrate with a diameter of 150 mm, the central RC region can extend over a radius of 0 to 9 mm, the intermediate RAI region over a radius of 9 to 50 mm, and the peripheral RAP region over a radius of 50 to 75 mm (or 72 mm considering a standard exclusion zone of 3 mm). For a 20-inch substrate with a diameter of 200 mm, the central RC region can extend over a radius of 0 to 15 mm, the intermediate RAI region over a radius of 15 to 75 mm, and the peripheral RAP region over a radius of 75 to 100 mm (or 97 mm considering a standard exclusion zone of 3 mm).

[0054] The control method then includes a step 3) corresponding to the determination: for one of the -at least two- RC, RAI, RAP regions defined in step 2), called the first region, of a median haze value, called the first median haze value, and for the other of the -at least two- RC, RAI, RAP regions defined in step 2), called the second region, of a median haze value, called the second median haze value.

[0055] The median haze value is a statistical indicator that represents the central level of the distribution of haze values ​​measured in the first region. It corresponds to the haze value such that 50% of the measured points have a lower value and 50% have a higher value.

[0056] When three regions are defined, the central region RC, the intermediate annular region RAI, or the peripheral annular region RAP can be chosen as the first and second regions, according to three possible combinations. We will see in the next step of the process that the first and second median haze values ​​(determined for the first and second regions respectively) are used to assign a grade to substrate 20: it may therefore be advantageous to choose specific regions as the first and second regions, particularly depending on the specific characteristics and potential signatures of the substrate 20 preparation process.

[0057] According to an advantageous variant, the central RC region is chosen as the first region, because the surface condition at the center of the substrate is critical to the suitability and quality of bonding, and the central region 20 can be rapidly impacted by a drift in the preparation (rectification) process of the substrate 20. And the peripheral annular RAP region is chosen as the second region, because the surface condition on the annular edge of the substrate is likely to impact the quality of the bonding interface.

[0058] Other combinations such as RC, RAI or RAI, RAP are of course also possible.

[0059] In the example shown, the median haze values ​​in the three regions are as follows: central region (RC): 66.7 ppm intermediate ring region (IRR): 58.6 ppm peripheral ring region (PRR): 84.7 ppm.

[0060] The control method according to the invention finally includes a step 4) of grading the substrate 20 based on the comparison between the first median value of the haze and a first predetermined threshold, and on the comparison between the second median value of the haze and a second predetermined threshold.

[0061] The grading reflects the "gluable" character of the substrate 20, namely its compatibility with a good quality assembly, for the purpose of manufacturing a composite structure 100. It is based solely on haze as a metric, which is an important advantage in terms of industrialization and mass production because metrology equipment such as the SP1, SC1 or SPA2 offer high measurement rates.

[0062] At least two haze values ​​are used to grade substrate 20: the first median value and the second median value, calculated respectively for the first and second regions (which, as previously discussed, are chosen from the at least two RC, RAI, and RAP regions defined on the front face 20a of substrate 20). The applicant observed that these two values ​​effectively graded substrates 20, ensuring reproducible and reliable results in terms of bonding and thin-film transfer.

[0063] The first and second thresholds can be determined empirically, by observing the bonding quality of the substrates 20 and the quality of the associated thin film transfer, for different haze values ​​in the first and second defined regions.

[0064] According to an advantageous implementation method, in step 3), a median haze value, referred to as the third median haze value, can also be determined in a third RC, RAI, or RAP region when three or more regions are defined. In step 4), the grading is further based on the comparison between the third median haze value and a third predetermined threshold. This threshold, like the others, can be determined empirically.

[0065] Preferably, the first, second, and third predetermined haze thresholds are different from each other, because the successful bonding of substrate 20 does not necessarily imply a homogeneous surface finish across the entire front face 20a. The predetermined haze threshold can therefore vary across different regions of the front face 20a, as the roughness frequencies themselves may differ. And the ability to form a good-quality bonding interface depends on the roughness frequencies involved in the different regions.

[0066] Preferably, the predetermined thresholds in the three aforementioned regions RC, RAI, RAP are such that the threshold applied to the central region RC is greater than the threshold applied to the peripheral annular region RAP, itself being greater than the threshold applied to the intermediate annular region RAI.

[0067] With reference to the example described in Figures 2 and 3, the thresholds may be defined according to the following first embodiment, providing a demanding gradation: Threshold applied to the central region RC: between 160 and 180 ppm; this threshold may be the first, second or third predetermined threshold, depending on whether the central region RC is defined as the first, second or third region; Threshold applied to the intermediate annular region: between 85 and 110 ppm; this threshold may be the first, second or third predetermined threshold, depending on whether the intermediate annular region RAI is defined as the first, second or third region; Threshold applied to the peripheral annular region: between 95 and 120 ppm; this threshold may be the first, second or third predetermined threshold, depending on whether the peripheral annular region RAP is defined as the first, second or third region.

[0068] If the first median haze value is below the first predetermined threshold and the second median haze value is below the second predetermined threshold, substrate 20 is assigned a high+ grade. If the first median haze value is greater than or equal to the first predetermined threshold, and / or the second median haze value is greater than or equal to the second predetermined threshold, the substrate is assigned a low grade.

[0069] With reference to the example described in Figures 2 and 3, the thresholds may be defined according to the following second embodiment, providing a less demanding gradation: Threshold applied to the central RC region: between 155 and 180 ppm; this threshold may be the first, second or third predetermined threshold, depending on whether the central RC region is defined as the first, second or third region; Threshold applied to the intermediate annular region: between 110 and 140 ppm; this threshold may be the first, second or third predetermined threshold, depending on whether the intermediate RAI annular region is defined as the first, second or third region; Threshold applied to the peripheral annular region: between 120 and 150 ppm; this threshold may be the first, second or third predetermined threshold, depending on whether the peripheral RAP annular region is defined as the first, second or third region.

[0070] If the first median haze value is below the first predetermined threshold and the second median haze value is below the second predetermined threshold, substrate 20 is assigned a high+ grade. If the first median haze value is greater than or equal to the first predetermined threshold, and / or the second median haze value is greater than or equal to the second predetermined threshold, the substrate is assigned a low grade.

[0071] Although the three thresholds have been described in the first and second embodiments, we recall that step 4) of gradation according to the invention only requires two haze values ​​(the first and second median values), and therefore only the two associated thresholds, to be operated.

[0072] According to a third embodiment, the control method may include a step 3') corresponding to the determination of a full plate average value of haze, representative of the entire effective surface of the front face 20a of the substrate 20. This average value is the arithmetic mean of the haze values ​​obtained over the entire said effective surface.

[0073] Considering the example of the, the average full-plate haze value is 71.9 ppm. In the SPA2 configuration used, the average full-plate haze value, for a 20 substrate prepared by fine mechanical grinding, can typically vary between a few ppm and a few hundred ppm, more specifically between 10 ppm and 200 ppm.

[0074] In the third embodiment, step 4) of gradation is further based on the comparison between the average full-plate haze value and a predetermined full-plate threshold.

[0075] The third embodiment may take up the thresholds defined according to the first embodiment previously stated, providing a demanding gradation, and consider a full plate threshold, for example, between 65 and 90 ppm.

[0076] Alternatively, the third embodiment may use the thresholds defined according to the second embodiment, providing a less demanding gradation and consider a full plate threshold, for example, between 90 and 115 ppm.

[0077] According to the third embodiment, if the average haze value is greater than or equal to the predetermined full plate threshold, a substrate 20 that had obtained a high (or high+) grade on the basis of -at least two- median haze values ​​(first and second median values) is downgraded to a low grade.

[0078] According to a fourth embodiment (which can be combined with the first, second, or third embodiments mentioned above), the inspection method includes, prior to step 4), a step 3'') for identifying a local defect DL in the haze map. The local defect DL sought to be identified in this step fulfills a circularity criterion and results in an abrupt variation of the haze relative to the haze of a surrounding area. Indeed, the applicant observed that a rounded local defect DL, characterized by a strong haze variation, corresponded with a high probability to the signature of a particle present on the back face 20b of the substrate 20 during the mechanical grinding process, which induces local over-removal of material on the front face 20a. This type of defect generates a non-bonded zone when the substrate 20 is assembled with a donor substrate 1.

[0079] The roundness criterion can be evaluated using various calculations. Specifically, the ISO definition states that the roundness criterion is the ratio of the radius of the circle inscribed within the local defect (DL) to the radius of the circle circumscribed about the defect. For example, the roundness criterion can be set to greater than 40% to identify circular or near-circular defects.

[0080] Abrupt haze variation can correspond to a variation in haze value of more than 10% between neighboring pixels or between neighboring areas (groups of pixels). For example, on the map, a local defect is visible in the upper right, in the peripheral annular region (RAP). When the haze varies by more than 15 ppm (or even more than 13.5 ppm) between the surrounding area (in the example, haze of approximately 50 ppm) and the local defect DL (in the example, haze of approximately 90 ppm), this constitutes an abrupt variation. Furthermore, the DL defect exhibits a high circularity criterion (>40%).

[0081] SPA2-type equipment enables the identification of local defects (called APDs for "abnormal phase defects") on a haze map, such as those sought in the context of the invention, and the qualification of the circularity criterion for these defects. APD detection can be based on several approaches, including the analysis of local haze variations; for example, any point where the haze value exceeds a certain threshold relative to the global median can be marked as suspect, or gradient analysis can be used to identify abrupt transitions between normal and affected areas.

[0082] Advantageously, the identification of a local defect DL in step 3'') is applied in the peripheral annular region RAP of the front face 20a of the substrate 20, said RAP region having an internal radius between 50% and 80% of the radius of the front face 20a and an external radius less than or equal to the radius of said front face 20a.

[0083] In the fourth embodiment, the grading in step 4) takes into account the presence or absence of a local DL defect on the haze map obtained in step 1). Preferably, the presence of any local DL defect results in the assignment of a low grade to the inspected substrate 20. Thus, even if the other criteria used in step 4) (median haze value(s), optionally full-plate mean haze value) correspond to a high (or high+) grade for substrate 20, the presence of a local DL defect may lead to the substrate being downgraded to a low grade.

[0084] The control method can be particularly useful for qualifying a process and / or equipment, especially for grinding, implemented for the surface preparation of the substrate 20, prior to step 1): - in the case where the substrate 20 obtains a low grade at the end of step 4), the process and / or equipment is not qualified and therefore requires new settings and a new test; - in the case where a high (or high+) grade is assigned to the substrate 20 at the end of step 4), the process and / or equipment is qualified and can operate the serial treatment of a plurality of substrates 20.

[0085] The invention also relates to a method for manufacturing a composite structure 100, comprising the following steps:

[0086] a) the supply of a support substrate 20 made of polycrystalline material,

[0087] b) the application of the control method described above, to the support substrate 20, and if the support substrate 20 obtains a high (or high+) grade,

[0088] c) the assembly of a donor substrate 1 onto the support substrate 20,

[0089] d) the transfer of a thin layer 10 from the donor substrate 1 onto the support substrate 20, to form the composite structure 100.

[0090] The assembly is preferably carried out by direct bonding, by molecular adhesion, according to a known process. The transfer of the thin film 10 is preferably performed by the Smart Cut process TM , also well known, which we will not describe in detail.

[0091] Applying the control method in step b) ensures the compatibility of the support substrate 20 with high-quality assembly and transfer. Composite structures 100 of the monocrystalline SiC type (thin film 10) on polycrystalline SiC (support substrate 20), meeting the expected specifications for the manufacture of power components (in particular), can thus be obtained.

[0092] The manufacturing process may include, if the support substrate 20 obtains a low grade in step b), a step a1) of reconditioning the support substrate 20 including at least one step of mechanical or mechano-chemical grinding or polishing, and the reintroduction of the reconditioned substrate as new support substrate 20 in step a).

[0093] Of course, the invention is not limited to the embodiments and examples described, and alternative embodiments may be made without departing from the scope of the invention as defined by the claims.

Claims

A method for inspecting a polycrystalline substrate (20) comprising the following steps: 1) full-plate inspection of a front face (20a) of the substrate (20) by a dark-field reflection microscopy technique employing a laser scan of the front face (20a) while the substrate (20) is rotated about an axis normal (z) to said face (20a), so as to obtain a map of a diffuse background noise signal, said diffuse background noise signal being called haze; 2) definition of at least two regions (RC, RAI, RAP) of the front face (20a), said regions (RC, RAI, RAP) being concentric and circular; 3) determination, for one of the at least two regions (RC, RAI, RAP), called the first region, of a median haze value, called the first median haze value, and for the other of the at least two regions (RC, RAI, RAP), called second region, of a median haze value, called second median haze value;4) the substrate gradation (20) based on the comparison between the first median haze value and a first predetermined threshold, and on the comparison between the second median haze value and a second predetermined threshold.; Method of controlling a substrate (20) made of polycrystalline material according to claim 1, wherein: - step 2) includes the definition of a third region (RC, RAI, RAP) of the front face (20a), - step 3) includes the determination, for this third region, of a median haze value called the third median haze value, - in step 4), the grading is also based on the comparison between the third median haze value and a third predetermined threshold. Method of controlling a substrate (20) made of polycrystalline material according to the preceding claim, wherein the first, second and third predetermined thresholds are different from each other. Method of controlling a substrate (20) made of polycrystalline material according to any one of the preceding claims, wherein, in step 2), three regions are defined and dimensioned as follows: - a central region (RC), having a radius less than or equal to 15% of the radius of the front face (20a) of the substrate (20), - an intermediate annular region (RAI), having an external radius between 50% and 80% of the radius of the front face (20a) of the substrate (20), - a peripheral annular region (RAP) extending from the external radius towards the edge of the substrate (20). Method of controlling a substrate (20) made of polycrystalline material according to the preceding claim, wherein the first region corresponds to the central region (RC) and the second region corresponds to the peripheral annular region (RAP). Method of controlling a substrate (20) made of polycrystalline material according to any one of the preceding claims, - comprising a step 3') of determining a full plate average value of the haze, representative of the front face (20a) of the substrate (20), - and wherein, in step 4), the grading is also based on the comparison between the full plate average value of the haze and a predetermined full plate threshold. Method of checking a substrate (20) made of polycrystalline material according to any one of the preceding claims, comprising, before step 4), a step 3'') of identifying a local defect (DL) in the haze map, the local defect (DL) fulfilling a circularity criterion and resulting in an abrupt variation of the haze of more than 10% compared to the haze of an area surrounding said defect (DL). Method of checking a substrate (20) made of polycrystalline material according to the preceding claim, wherein in step 4), the grading also takes into account the presence or absence of a local defect (DL). A method for testing a substrate (20) made of polycrystalline material according to any one of the preceding claims, wherein, in step 4): i) if the first median haze value is greater than or equal to the first predetermined threshold, and / or if the second median haze value is greater than or equal to the second predetermined threshold, the substrate (20) is assigned a low grade and optionally is sent to a reconditioning step; ii) if the first median haze value is less than the first predetermined threshold, and if the second median haze value is less than the second predetermined threshold, the substrate (20) is assigned a high grade and optionally is used in a process for manufacturing a composite structure (100). Method of controlling a substrate (20) made of polycrystalline material according to the preceding claim, wherein, if the average haze value is greater than or equal to the predetermined full plate threshold, a substrate (20) which had obtained a high grade on the basis of the median haze values ​​is downgraded to a low grade. Method of checking a substrate (20) made of polycrystalline material according to the preceding claim together with claim 8, wherein, if a local defect (DL) is present, a substrate (20) which had obtained a high grade on the basis of the full plate mean and median haze values ​​is downgraded to a low grade. A method for testing a substrate (20) made of polycrystalline material according to one of the three preceding claims, wherein the gradation of step 4) is used to qualify a grinding process and / or equipment implemented for the surface preparation of the substrate (20), prior to step 1): - in case (i), the grinding equipment is not qualified; - in case (ii), the grinding equipment is qualified. A method for manufacturing a composite structure (100), comprising the following steps: a) supplying a support substrate (20) of polycrystalline material, b) applying the control method according to any one of claims 9 to 12, to the support substrate (20), and if the support substrate (20) obtains a high grade, c) assembling a donor substrate (1) onto the support substrate (20), d) transferring a thin layer (10) from the donor substrate (1) onto the support substrate (20), to form the composite structure (100). Method of manufacturing a composite structure (100) according to the preceding claim, comprising, if the support substrate (20) obtains a low grade in step b), the following step: a1) the reconditioning of the support substrate (20) including at least one step of mechanical or mechano-chemical grinding or polishing, and the reintroduction of the reconditioned substrate as a new support substrate (20) in step a). Method of manufacturing a composite structure (100) according to one of the two preceding claims, wherein the polycrystalline material is silicon carbide.

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