Method for characterising the edges of a wafer

Abstract

The invention relates to a characterisation method comprising the following steps: a) providing a blank wafer (10) which is made of a monocrystalline material and has a front face, the peripheral region (100) of which extends over a width less than or equal to 5 mm, the wafer (10) being a donor substrate reconditioned after a surface layer of the donor substrate has been transferred onto a carrier substrate; b) obtaining an image of the front face of the wafer, the image being derived from measurements by optical interferometry in grazing light; c) applying a grid defining a plurality of adjacent cells uniformly distributed over the image; d) computing a local flatness parameter, for each cell covering at least part of the peripheral region, based on the measurements taken in step b); e) defining a threshold for the local flatness parameter, such that, when the value of the local flatness parameter associated with a cell is greater than the threshold, a topological defect is assigned to the cell, the topological defect corresponding to an untransferred crown residue.

Description

Method for characterizing the edges of a wafer FIELD OF INVENTION

[0001] The present invention relates to the field of materials, particularly those in wafer form, for microelectronic applications. It concerns a method for characterizing the edges of a blank wafer and is particularly well-suited to the characterization of donor substrates after the transfer of a surface layer in the Smart Cut process. TM and after a reconditioning treatment for reuse. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] The Smart Cut process TMis well known for the fabrication of SOI (silicon-on-insulator) substrates and more generally for the production of very thin, high-quality stacks of single-crystal layers, even when the different stacked materials have structures incompatible with epitaxial growth. The buried brittle plane 2, formed by ion implantation of light species (e.g., hydrogen and / or helium) in a donor substrate 1, is the separation point between the surface layer 11 to be transferred and the remainder 1' of said donor substrate. It should be noted that ion implantation generates microcavity-type defects in the buried brittle plane 2, which can develop into microcracks under thermal activation and lead to fracture propagation along said brittle plane 2.The Smart Cut process is also based on an assembly (via a bonding interface 3) between the donor substrate 1 and a support substrate 4, onto which the surface layer 11 will be transferred. Each of the two substrates is usually in the form of a circular plate, the edge 1c of which is chamfered and a peripheral part of the front face 1a (assembled face) has an edge drop, representative of the surface treatments applied to prepare said substrates.

[0003] The edge drop and chamfer induce the presence of a peripheral unbonded zone (ZPNC) between the donor substrate 1 and the support substrate 4 when they are assembled ((ii)), which typically extends over a width of less than 5 mm from the wafer edge. The surface layer 11 is not transferred from the donor substrate 1 to the support substrate 4 at this peripheral ZPNC zone ((iii)). The ring of untransferred surface layer is called the C ring. After transfer, the C ring of the remaining 1' of the donor substrate forms a step extending around the entire circumference of the wafer, the height of which corresponds to the thickness of the transferred layer 11, typically less than 1 micrometer.

[0004] To allow a new use of the remainder 1' of the donor substrate ((iii)), it is required to recondition it by applying in particular polishing treatments to its front face 1a, so as to make this ring C disappear and to obtain a reconditioned donor substrate 1R whose front face 10a is compatible with a new high quality assembly ((iv)).

[0005] Materials such as silicon carbide, gallium nitride or lithium tantalate are expensive and difficult to polish: it is therefore particularly important to be able to effectively assess the quality of the reconditioning around such donor substrates, before reintroducing them into a transfer cycle.

[0006] Conventional full-plate metrology equipment (e.g., Lasertech's SICA, KLA Tencor's SPA2 or SP1) allows for the characterization of the front face of a substrate, excluding a peripheral area (known as the exclusion zone), typically between 1 mm and 5 mm wide. C-ring residues (topologies) in the peripheral region 100 of a reconditioned donor substrate 1R may be located within this exclusion zone and, consequently, go completely undetected with these types of metrology.

[0007] US7324917 document relates to a method for evaluating the characteristics of a surface of a sample having an edge. SUBJECT OF THE INVENTION

[0008] The present invention proposes a method for characterizing the edges of a wafer, in particular, a peripheral region of a donor substrate that has been reconditioned after a thin film transfer, so as to define whether said substrate is compatible with a new layer transfer cycle. BRIEF DESCRIPTION OF THE INVENTION

[0009] The invention relates to a method for characterizing a peripheral region of a platelet, comprising the following steps:

[0010] a) the supply of a blank wafer in a single-crystal material, having a front face whose peripheral region extends from the edge of the wafer over a width less than or equal to 5mm;

[0011] b) obtaining an image of the front face of the plate including all or part of the peripheral region, said image being obtained from grazing light optical interferometry measurements made on said front face;

[0012] c) the application of a grid defining a plurality of adjacent cells uniformly distributed over the image of the front face;

[0013] d) the calculation of a local flatness parameter, for each cell covering at least part of the peripheral region, from the measurements taken in step b);

[0014] e) the definition of a threshold for said local flatness parameter, such that, when the value of the local flatness parameter associated with a cell is greater than the threshold, a topological defect is assigned to said cell.

[0015] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: the wafer is a donor substrate reconditioned after transfer of a surface layer of said donor substrate onto a support substrate, and the topological defect corresponds to a residue of untransferred corona; the front face has a surface roughness less than or equal to 1nm RMS, outside the peripheral region, measured by atomic force microscopy on scans from 5x5 square micrometers to 30x30 square micrometers; for obtaining the image in step b), the wafer is held in an optical interferometry device in grazing light, on a platform, by suction, on the side of a rear face opposite to the front face of the wafer;the peripheral region includes a chamfer extending from the edge of the wafer over a width less than or equal to 2mm, and step b) includes obtaining a raw map imaging the entire front face of the wafer, without applying an exclusion zone, then applying a crop to the raw map, to exclude the chamfer and obtain the image; each cell of the grid applied in step c) extends over an area of ​​a few mm; 2 a few hundred mm 2;the cells of the grid applied in step c) have a square shape and a side dimension greater than or equal to 5mm and less than or equal to 30mm, advantageously equal to 10mm;the local flatness parameter calculated in step d) is the SFQR;the threshold defined for step e) is 1.3 micrometers, preferably 1 micrometer;the single-crystal material forming the wafer is chosen from silicon carbide, gallium nitride and lithium tantalate.

[0016] The invention also relates to a manufacturing process comprising:

[0017] - a step of transferring a surface layer from a donor substrate onto a support substrate, to obtain a structure comprising said surface layer assembled onto the support substrate on the one hand, and the remainder of the donor substrate on the other hand, the remainder of the donor substrate comprising a non-transferred outer ring,

[0018] - a reconditioning step of the remaining donor substrate, involving at least one mechanical grinding or mechanical or mechano-chemical polishing aimed at removing the peripheral ring, to form a reconditioned donor substrate called a wafer,

[0019] - a metrology step applying the characterization method as mentioned above.

[0020] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: the manufacturing process further comprises a sorting step, based on the result of step e) of the characterization method, during which it is decided whether the wafer is returned to a new reconditioning step or introduced into a new surface layer transfer step; if at least one cell covering a part of the peripheral region of the wafer is affected by a topological defect in step e), the wafer is returned to a new reconditioning step; if at least two neighboring cells, each covering a part of the peripheral region of the wafer, are affected by a topological defect in step e), the wafer is returned to a new reconditioning step;if fewer than five cells covering part of the peripheral region are considered to have a topological defect, the wafer is introduced into a new layer transfer cycle, without a new reconditioning step. BRIEF DESCRIPTION OF THE FIGURES

[0021] 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:

[0022] This presents classic steps in a process of transferring a surface layer from a donor substrate to a support substrate;

[0023] Presents a donor substrate after a surface layer has been transferred (iii), a reconditioned donor substrate (iv) and two examples of reconditioned donor substrate with corona residues in the peripheral region (v) visualized by confocal optical microscopy;

[0024]

[0025]

[0026] La, laet la represent steps of a characterization method according to the present invention;

[0027] This presents a schematic definition of SFQR and SBIR, known local flatness parameters;

[0028] Presents a graph relating the extent of topological defects of the crown residue type to the local flatness parameter SFQR;

[0029]

[0030] Figures 1 and 2 each present an example of characterization according to a method according to the invention; on the left, an image obtained in step b), on the right, a grid is applied (step c), a local flatness parameter is calculated (step d) (value displayed in the cell) and a threshold is defined (step e) to qualify the cells of the grid with (dark cell) or without (lighter cell) topological defect; it should be noted that the elements in white dotted lines on the figures (on the right) have been added to locate a cell and the peripheral region of the wafer, but are not part of the image (with grid and values ​​of the local flatness parameter) returned by the measuring equipment.

[0031] The same references in the figures can be used for elements of the same type. Some figures are schematic representations which, for the sake of clarity, are not drawn to scale. In particular, the layer thicknesses along the z-axis are not to scale with respect to the lateral dimensions along the x and y axes; and the relative thicknesses of the layers are not necessarily to scale in the figures. DETAILED DESCRIPTION OF THE INVENTION

[0032] The present invention relates to a method for characterizing a peripheral region 100 of a blank wafer 10. A wafer, particularly in the field of microelectronics, is usually a disk with a diameter between 50 mm and 300 mm, or even 450 mm. Its thickness is typically between 100 micrometers and 1000 micrometers, for example, approximately 400 μm, 600 μm, or 700 μm. The wafer 10 is said to be blank because it has no intentionally created structure or pattern; in other words, the wafer 10 does not contain any components at any stage of manufacture; it is made of a solid material, without any stacking of layers.

[0033] Step a) of the method provides a blank wafer 10, made of a single-crystal material, for example, silicon carbide, gallium nitride, lithium tantalate, silicon, etc. The wafer 10 has a front face 10a whose peripheral region 100 extends from the edge 10c of the wafer 10 over a width less than or equal to 5 mm. This peripheral region 100 usually includes a bevel which generally has a width, from the edge 10c of the wafer 10, less than or equal to 2 mm, less than or equal to 1 mm, or even less than or equal to 0.5 mm. The chamfer is an extreme peripheral area whose shape usually follows SEMI standards, and in which there is a strong variation in thickness and flatness of the insert 10. As a reminder, the chamfer is machined on the inserts to eliminate any sharp edge on the edge likely to weaken said inserts.

[0034] The peripheral region 100 encompasses the chamfer, but also an adjacent annular zone offset towards the center of the plate 10, up to about 5mm from the edge 10c, which may present problems of flatness, roughness and defect.

[0035] According to a preferred embodiment, the wafer 10 supplied in step a) is a reconditioned donor substrate 1R after the transfer of a surface layer 11 of said donor substrate 1 (before reconditioning) onto a support substrate 4, by a process based on the implantation of light species and assembly, such as the Smart Cut process. As mentioned in the introduction, to remove the corona C present in the peripheral region of the remainder 1' of the donor substrate, after transfer of the surface layer 11 ((iii) and (iii)), it is common to apply mechanical and / or mechano-chemical surface treatments, such as mechanical grinding, polishing or CMP ((iv)). Preferably, after these treatments, the front face 10a of the reconditioned donor substrate 1R,10 has a surface roughness less than or equal to 1nm RMS, outside the peripheral region 100, measured by atomic force microscopy on scans from 5x5 square micrometers to 30x30 square micrometers.

[0036] The step associated with the untransferred corona C remains difficult to erase, especially when the material constituting the donor substrate 1 has significant hardness; thus, it sometimes happens that residues of this corona C form topological defects, as illustrated in figure (v). In the image on the left, the corona residues (topological defects) extend in the peripheral region 100 for approximately 4.9 mm from the edge 10c of the wafer 10; in the image on the right, they extend for approximately 1.6 mm from the edge of the wafer 10. These topological defects have a height (along the z-axis in the figures) generally less than a few micrometers, typically less than or equal to 1 micrometer, or even a few hundred nanometers. They can be distributed with a spatial period on the order of 10 micrometers.

[0037] At least some of these topological defects can induce bonding defects or increasing peripheral unbonded zone (ZPNC) (and therefore C-ring) widths, if they remain on the reconditioned 1R donor substrates when introduced into a new layer transfer cycle.

[0038] The characterization method according to the invention then includes a step b) corresponding to obtaining an image of the front face 10a of the plate 10, including all or part of the peripheral region 100. The image is obtained from grazing light optical interferometry measurements made on said front face 10a ().

[0039] Equipment such as the Corning Tropel® Ultrasort2 is known for performing optical interferometry measurements of wafer flatness and thickness variation. Flatness defects on the front face 10a create interference fringes that form the basis of the measurements provided by this equipment. Flatness variations of + / -0.3 μm can be detected. Some optical interferometers employ a Fizeau lens that captures an image of the entire front face 10a of the wafer 10 using a CCD (charge-coupled device) detector with a spatial resolution on the order of 0.4 mm.

[0040] Returning to the preferred embodiment mentioned above, we note that the spatial resolution of these devices is much greater than the typical spatial period of topological defects corresponding to C-ring residues. These topological defects therefore cannot be directly imaged.

[0041] Advantageously, the wafer 10 is held in the grazing light optical interferometry equipment, on a platform, by suction, on the side of its rear face 10b (opposite to the front face 10a), before performing the interferometric measurements to form the image. This prevents the macroscopic shape or deformation of the wafer 10 (which can reach 100 μm) from affecting the detection of the wafer edge 1c and the measurements in the peripheral region 100.

[0042] Also advantageously, step b) first involves obtaining a raw map image of the entire front face 10a of the wafer 10, without applying an exclusion zone, i.e., including the entire peripheral region 100. Secondly, the raw map is cropped to exclude the extreme edge 1c of the wafer and the chamfer, in order to obtain the image that will be used in the subsequent steps of the method. This exclusion is made possible by the significant contrast caused by the chamfer (due to its considerable variation in height) on the raw map: the boundary corresponding to the start of the chamfer is easily identifiable by software, and digital cropping can be applied to the raw map to obtain the image of interest. This cropping functionality is, for example, available on the Tropel® Ultrasort2 equipment.

[0043] Alternatively, an exclusion zone (less than or equal to 2mm, less than or equal to 1mm, or even less than or equal to 0.5mm) can be defined according to the geometry of the chamfer (known or measured according to applicable SEMI standards) of plate 10, and applied to the raw mapping to obtain the image of interest.

[0044] In these cases (digital cropping or cropping by applying a determined exclusion zone), the image of the front face 10a of the plate 10 includes only a part of the peripheral region 100. In this case, it is the annular part of the peripheral region 100 where the crown residues can be the most troublesome for the purpose of reusing the plate 10.

[0045] Note that after one or more reconditioning steps, the chamfer width decreases and tends to become very small compared to the width of the peripheral region 100 in which topological defects are likely to exist.

[0046] The method then includes a step c) of applying a grid 200 defining a plurality of adjacent cells 20, uniformly distributed over the image of the front face ().

[0047] Advantageously, the 200 grid covers 100% of the peripheral region 100 shown in the image; however, the method applies as soon as more than 50% of the peripheral region 100, more than 60%, more than 75%, more than 80%, or even more than 90% of the peripheral region 100 is covered by 20 cells.

[0048] Preferably, each cell 20 of the grid 200 extends over an area of ​​a few mm 2 a few hundred mm 2The cells 20 may, in particular, have a square shape and a side dimension greater than or equal to 5 mm and less than or equal to 30 mm, advantageously equal to 10 mm. The dimensions of the cells 20 are chosen so that they encompass the peripheral region 100 to be characterized, in order to capture all the information on potential topological defects.

[0049] The next step (d) of the method according to the invention corresponds to the calculation of a local flatness parameter, at least at the level of each cell 20 covering (in whole or in part) the peripheral region 100. This calculation is performed from the measurements made in step (b) and on which the image of the front face 10a is based. It is usually carried out by the analysis system of the optical interferometry equipment used; a value of the chosen local flatness parameter is thus associated with each of the cells 20 of interest.

[0050] Of course, the calculation of the local flatness parameter can be applied to each of the 20 cells of the grid, without being limited to those located in the peripheral region 100.

[0051] Different local flatness parameters can be chosen and calculated for each cell 20, including (): SFQR (“Site Front Quotient Range”) or LTIR (“Localized Total Indicated Range”): this parameter translates the total variation of local topology relative to a defined reference plane, on the front face 10a of the plate 10, by the least squares method; SBIR (“Site Backside Ideal Range”): this parameter translates the total variation of local topology on the front face 10a relative to an average reference plane taken on the back face 10b.

[0052] Other known parameters such as SFQD (“Site frontside least squares focal plane deviation”), ESFQR (“Edge site front quotient range”), ESFQD (“Edge site frontside least squares focal plane deviation”), SBID (“Site backside ideal focal plane deviation”), SF3R (“Site frontside 3 points reference focal plane range”), SF3D (“Site frontside 3 points focal plane deviation”), SFLR (“Site frontside least squares focal plane range”), SFLD (“Site frontside least squares focal plane deviation”) can also be defined as a local flatness parameter calculated in step d) at the scale of each cell 20.

[0053] Advantageously, the local flatness parameter calculated in step d) is the SFQR, particularly in the preferred embodiment in which the wafer 10 is a reconditioned 1R donor substrate.

[0054] The characterization method also includes a step e) consisting of defining a threshold for the local flatness parameter. Thus, when the value of the local flatness parameter associated with a cell 20 is greater than the threshold, a topological defect is assigned to said cell 20; in other words, the area of ​​the front face 10a of the wafer 10 to which the cell 20 is superimposed contains (or at least is suspected of containing) a topological defect.

[0055] If the value of the local flatness parameter associated with a cell 20 is less than or equal to the threshold, said cell 20 is considered to be free of topological defects.

[0056] The threshold can, for example, be set at 1.5 micrometers, 1.3 micrometers, 1.2 micrometers, 1.1 micrometers, 1 micrometer, or even less. It is understood, of course, that the threshold can depend on the chosen local flatness parameter, but also on the type of insert 10 (material, nature of residual defects, etc.).

[0057] In particular, it presents the extent in the peripheral region 100 of the C crown residues (topological defects) on different reconditioned 1R,10 silicon carbide donor substrates, as a function of the SFQR. Each point corresponds to said extent and the local flatness parameter measured locally in a cell 20. The extent of the crown residues corresponds to the width, from the edge 10c of the 1R,10 substrate, in which topological defects are present and is recorded by confocal optical microscopy, as illustrated in la(v).

[0058] The applicant noted that there was a relationship between the density of corona residues (related to their extent in the peripheral region 100) and the SFQR, even though these residues have a spatial period much shorter than that of the defects for which the measurement of the SFQR parameter is planned.

[0059] The higher the SFQR, the greater the density of topological defects related to C-corona residues in the peripheral region 100.

[0060] The acceptable SFQR threshold for reconditioned donor substrates 1R,10 to be compatible with high-quality surface layer transfer in the example of laest is approximately 1 micrometer. When the SFQR of a cell 20 is below this threshold, the C corona residues in the area associated with this cell 20 are either completely absent from the peripheral region 100, or present only sporadically over an area typically on the order of or less than 0.5 mm, or even 0.2 mm: in both cases, the cell 20 can be considered free of topological defects.

[0061] The invention also relates to a manufacturing process comprising a step of transferring a surface layer 11 from a donor substrate 1, a step of reconditioning the remainder 1' of the donor substrate after transfer, and a metrology step involving the characterization method described above.

[0062] The transfer step includes the formation of a brittle plane embedded 2 in the support substrate 1, particularly through the implantation of light species such as hydrogen or helium, as is well known in the Smart Cut process. It also includes the assembly of the donor substrate 1 implanted onto a support substrate 4, via a bonding interface 3, with or without an intermediate layer. The assembly is preferably based on molecular adhesion, thus without the use of an adhesive. Heat treatment and / or mechanical stress applied to the bonded assembly allows a fracture wave to propagate along the brittle plane 2 and separate the surface layer 11 from the donor substrate 1, transferring it to the support substrate 4.

[0063] As mentioned in the introduction, the remainder 1' of the donor substrate includes at the periphery a non-transferred ring C due to the presence of a chamfer and an edge drop at the periphery of the donor substrates 1 and support 4 which induces a peripheral zone that is not glued or insufficiently glued.

[0064] The reconditioning step applies to the remaining portion of the donor substrate 1' and includes at least one mechanical grinding or mechanical or chemical polishing process aimed at removing the peripheral rim C, to form a reconditioned donor substrate 1R, referred to as wafer 10. This reconditioning step also aims to restore a good surface finish (low roughness, low defects) to the entire front face 10a of the wafer 10, since it is known that separation by fracture wave propagation leaves a surface roughness on the order of a few nm to a few tens of nm RMS. Known reconditioning techniques can be implemented; in practice, material removal on the front face 1a of the remaining portion 1' of the donor substrate, on the order of a few micrometers, typically between 1 μm and 10 μm, is performed during the reconditioning step. Conventional cleaning and surface treatments are also applied.

[0065] After the conditioning step, the front face 10a of the reconditioned donor substrate 1R,10 has a surface roughness less than or equal to 1nm RMS, outside the peripheral region 100, measured by atomic force microscopy on scans from 5x5 micrometers to 30x30 micrometers.

[0066] Finally, the manufacturing process includes a metrology step which consists of applying to the wafer 10 (reconditioned donor substrate 1R) the method of characterization of the peripheral region 100 previously described.

[0067] The manufacturing process may also include a sorting step based on the result of step e) of the characterization method, during which it is decided whether the wafer is returned to a new reconditioning step or whether it can be introduced into a new layer transfer step, as a donor substrate.

[0068] According to a first option, if at least one cell 20 covering part of the peripheral region 100 is considered to have a topological defect (i.e., if the value of the local flatness parameter is greater than the threshold defined in step e)), the wafer 10 is returned to a new reconditioning step.

[0069] According to a second option, if at least two neighboring cells 20, each covering a part of the peripheral region 100, are considered to have a topological defect, the wafer 10 is returned to a new reconditioning step.

[0070] According to a third option, if fewer than five cells 20 covering part of the peripheral region 100 are considered to have a topological defect, the wafer 10 is introduced into a new layer transfer cycle, without a new reconditioning step.

[0071] Two examples of application of the characterization method according to the invention are presented in figures 6 and 7.

[0072] The 10 wafers introduced in step a) are single-crystal SiC donor substrates that have undergone a reconditioning step according to the aforementioned manufacturing process.

[0073] Each wafer 10 is loaded onto the measurement platform and held in place by suction by its rear face 10b, in a Tropel Ultrasort 2 type instrument enabling grazing light optical interferometry measurements. An image of the front face 10a is obtained for a first (left) and a second (left) wafer 10 after step b). In these examples, the instrument automatically applies a digital cropping of the raw maps to obtain the images of interest, thus excluding the chamfer (associated with very high contrast on the raw maps) of the two wafers.

[0074] A grid 200 with 20 square cells of 10mmx10mm is applied to the image (step c), so that the peripheral region 100, defined 5mm from the edge 10c of the wafer 10, is at least 50% covered by the cells 20. The definition and application of this grid to the image is part of the functions of the measuring equipment.

[0075] The chosen local flatness parameter is SFQR, and a value of this parameter is calculated for each of the 20 cells of the 200 grid (step d); this functionality is also available on the measuring equipment.

[0076] As can be seen in figures 6 and 7 (right), an SFQR value (in micrometers) is recorded on each cell 20 of the grid 200.

[0077] In these examples, a threshold of 1µm is defined for the SFQR. Thus, all cells 20 for which the SFQR value is greater than this threshold (in particular the cells 20 in the peripheral region 100) are identified as having a topological defect; in figures 6 and 7 (right), it is the cells 20 that appear with a darker background than the others.

[0078] The first platelet ( ) has twenty-six cells 20 in the peripheral region 100, to which a topological defect is assigned. This is the signature of significant corona residues in the peripheral region 100.

[0079] According to the manufacturing process of the invention, the sorting step will return this first wafer to a new conditioning step, preventing it from being used as a donor substrate and inducing a poor quality transfer.

[0080] The second platelet () has four cells 20 in the peripheral region 100, not adjacent, to which a topological defect is assigned.

[0081] Depending on the second and third options of the sorting step in the manufacturing process, this wafer can be used for a new layer transfer step.

[0082] The first sorting step option is more restrictive; it may be chosen, for example, when the stacked structure targeted at the end of the layer transfer step must meet demanding specifications in terms of surface layer edges or exclusion zone width. According to this first option, the second wafer must undergo a further reconditioning step before being reintroduced as a donor substrate in a new transfer step.

[0083] Of course, the invention is not limited to the embodiments and examples described, and alternative embodiments can be made without departing from the scope of the invention.

Claims

A method for characterizing a peripheral region (100) of a wafer (10), comprising the following steps: a) providing a blank wafer (10) made of a single-crystal material, having a front face (10a) whose peripheral region (100) extends from the edge (10c) of the wafer (10) over a width less than or equal to 5 mm, said wafer (10) being a reconditioned donor substrate (1R) after transfer of a surface layer (11) of said donor substrate (1) onto a support substrate (4); b) obtaining an image of the front face (10a) of the wafer (10) including all or part of the peripheral region (100), said image being obtained from grazing light optical interferometry measurements made on said front face (10a); c) applying a grid (200) defining a plurality of adjacent cells (20) uniformly distributed over the image of the front face (10a);d) the calculation of a local flatness parameter, for each cell (20) covering at least a part of the peripheral region (100), from the measurements made in step b); e) the definition of a threshold for said local flatness parameter, such that, when the value of the local flatness parameter associated with a cell (20) is greater than the threshold, a topological defect is assigned to said cell, said topological defect corresponding to a non-transferred corona residue (C). Characterization method according to the preceding claim, wherein the front face (10a) has a surface roughness less than or equal to 1nm RMS, outside the peripheral region (100), measured by atomic force microscopy on scans from 5x5 micrometers to 30x30 micrometers. A characterization method according to any one of the preceding claims, wherein, for obtaining the image in step b), the wafer is held in a grazing light optical interferometry device, on a platform, by suction, on the side of a rear face (10b) opposite the front face (10a) of the wafer (10). A characterization method according to any one of the preceding claims, wherein: - the peripheral region (100) includes a chamfer extending from the edge (10c) of the wafer (10) over a width less than or equal to 2mm, and - step b) comprises: - obtaining a raw map imaging the entire front face (10a) of the wafer (10), without applying an exclusion zone, - applying a crop to the raw map, to exclude the chamfer and obtain the image. A characterization method according to any one of the preceding claims, wherein each cell (20) of the grid (200) applied in step c) extends over an area of ​​a few mm 2 a few hundred mm 2 . A characterization method according to any one of the preceding claims, wherein the cells (20) of the grid (200) applied in step c) have a square shape and a side dimension greater than or equal to 5mm and less than or equal to 30mm, advantageously equal to 10mm. Characterization method according to any one of the preceding claims, wherein the local flatness parameter calculated in step d) is the SFQR. Characterization method according to the preceding claim, wherein the threshold defined for step e) is 1.3 micrometer, preferably 1 micrometer. A characterization method according to any one of the preceding claims, wherein the single-crystal material forming the wafer (10) is selected from silicon carbide, gallium nitride and lithium tantalate. Manufacturing process comprising: - a step of transferring a surface layer (11) of a donor substrate (1) onto a support substrate (4), to obtain a structure comprising said surface layer (11) assembled on the support substrate (4) on the one hand, and the remainder of the donor substrate (1') on the other hand, the remainder of the donor substrate (1') comprising at the periphery a non-transferred ring (C), - a step of reconditioning the remainder of the donor substrate (1'), involving at least one mechanical grinding or mechanical or mechano-chemical polishing aimed at removing the peripheral ring (C), to form a reconditioned donor substrate (1R) called wafer (10), - a metrology step applying the characterization method according to one of the preceding claims to the wafer (10). Manufacturing method according to the preceding claim, further comprising a sorting step, based on the result of step e) of the characterization method, during which it is decided whether the wafer (10) is returned to a new reconditioning step or whether it is introduced into a new surface layer transfer step. Manufacturing method according to the preceding claim, wherein, if at least one cell (20) covering a part of the peripheral region (100) of the wafer (10) is affected by a topological defect in step e), the wafer (10) is returned to a new reconditioning step. Manufacturing method according to claim 11, wherein, if at least two neighboring cells (20), each covering a part of the peripheral region (100) of the wafer (10), are affected by a topological defect in step e), the wafer (10) is returned to a new reconditioning step. Manufacturing method according to claim 11, wherein, if fewer than five cells (20) covering a part of the peripheral region (100) are considered to have a topological defect, the wafer (10) is introduced into a new layer transfer cycle, without a new reconditioning step.

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