FR3155632A1 - process for manufacturing a composite structure including a gradation step - Google Patents

Abstract

The invention relates to a method for manufacturing a composite structure comprising a thin layer of monocrystalline silicon carbide arranged on a support substrate of polycrystalline silicon carbide, the method comprising the following steps: 1) providing at least one donor substrate of monocrystalline silicon carbide, having a front face and a back face, the front face potentially having defects, called primary defects; 2) controlling the quality of the at least one donor substrate using a photoluminescence imaging technique so as to extract a map of the front face, called the first map, listing the primary defects identified as being of the microhole type, of the star stacking fault complex type, or of the point defect type; 3) transferring a thin layer, originating from a surface layer of the at least one donor substrate, onto a support substrate of polycrystalline silicon carbide, to obtain a composite structure and a residual donor substrate; 4) inspection of a free surface of the thin layer of the composite structure using a defect inspection technique involving the scattering of an ultraviolet laser beam, in order to extract a map of the free surface, called the second map, listing defects, called secondary defects; 5) grading of the composite structure, including a comparison of the first map and the second map. Figure to be published with the abstract: No figure

Description

process for manufacturing a composite structure including a gradation step FIELD OF THE INVENTION

The present invention relates to the field of semiconductor materials, in particular composite structures comprising a thin layer (from a monocrystalline silicon carbide donor substrate) transferred onto a polycrystalline silicon carbide support substrate. It relates in particular to a method for manufacturing such a composite structure, including a gradation step based on the control mapping of the donor substrate and on the control mapping of the thin layer of the composite structure.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

Silicon carbide is a particularly interesting material for the manufacture of power devices, radio frequencies or even devices operating at very high temperatures.

The quality of monocrystalline silicon carbide (c-SiC) substrates has improved significantly over the last ten years, accompanied by increasingly precise knowledge and detection of the different types of crystalline defects likely to be present in this material. The JEITA (“Japan Electronics and Information Technology Industries Association”) standard also includes four documents relating to the zoology of defects on/in a layer produced by homoepitaxy on a 4H-SiC substrate (EDR 4712/100) and to non-destructive procedures for optical inspection of these defects (EDR 4712/200, /300, /400). In particular, a procedure is indicated for evaluating and referencing defects by combining optical inspection and photoluminescence imaging.

Commercially available equipment, such as the SICA88 from Lasertec, allows the combination of visible light and photoluminescence confocal Nomarski prism microscopy techniques (as described by [1] T.Kimoto et al., “Fundamentals of Silicon Carbide Technology Growth Characterization Devices and Applications”, p.126, or [2] D. Baierhofer et al., Materials Science in Semiconductor processing 140 (2022) 106414) and is commonly used to inspect c-SiC substrates before or after epitaxy.

As an example, [3] Das et al (“Statistical analysis of killer and non-killer defects in SiC and the impact of device performance”, Material Science Forum, ISSN 1662-9752, Vol.1004, pp 458-463 (2020) Trans Tech Publications Ltd) proposes a statistical analysis of “killer” defects for devices grown on a homoepitaxial c-SiC layer, and shows optical and photoluminescence images of typical c-SiC defects.

Although in strong development, high-quality c-SiC substrates remain expensive and difficult to supply in large sizes. It is therefore advantageous to use layer transfer solutions to develop composite structures comprising a thin monocrystalline SiC layer (derived from the high-quality c-SiC donor substrate) on a lower-cost support substrate, for example polycrystalline SiC (p-SiC), which can also offer advantages in terms of electrical conductivity. A well-known thin-film transfer solution is the Smart Cut ^TM process, based on light ion implantation and direct bonding between a c-SiC donor substrate and a support substrate, at a bonding interface. The implantation creates a buried fragile plane along which a separation will take place, leading to the transfer of a thin c-SiC layer onto the support substrate, to form the composite structure, and allowing the recovery and recycling of the rest of the donor substrate to potentially perform one or more other layer transfers. Epitaxy can then be performed on the thin layer of the composite structure, followed by the development of the electronic devices.

For the implementation of a layer transfer process to be economically viable, it is important to know how to control the quality of the donor substrates, so as to avoid carrying out the transfer of a layer which would inevitably lead to a downgrading of the composite structure, or which would give rise to an epitaxial layer out of specification, in terms of “killer” defects. The zoology of c-SiC defects is relatively well known, and numerous studies (some of which are referenced above) tend to define the type and size of defects, present in an epitaxial layer of c-SiC, which would be killer for the components; nevertheless, the applicant has observed that the criteria for classifying defects in a donor substrate are not necessarily the same when epitaxy is carried out on said substrate and when a thin film transfer is carried out from said substrate.

It is therefore important to detect, but above all to precisely classify the crystalline defects (called primary defects) present on the donor substrates, in order to downgrade those, among these donor substrates, which will not allow the production of a thin layer of the required quality.

Furthermore, the detection and recognition of defects (so-called secondary defects) on and/or in the thin layer (from a donor substrate) of a composite structure whose support substrate is made of polycrystalline SiC (p-SiC) are complex because the p-SiC grains are visible under said thin layer, in particular with equipment combining confocal microscopy with Nomarski prism in visible light and photoluminescence imaging. However, reliable and efficient control (not requiring too much inspection time per structure) of the quality of the thin layer is required, to avoid continuing the epitaxy steps if secondary defects in the thin layer are likely to generate an out-of-specification density of killer defects in the homoepitaxial layer.

In view of the growing use of layer transfer techniques in the manufacturing chain of electronic devices on c-SiC, there is therefore a strong need to define control steps allowing reliable and rapid inspection, both of the donor substrates, at the start of the chain, and of the composite structures, in the middle of the chain, in order to assign a grade, as soon as possible in the overall manufacturing chain, to the donor substrates or composite structures and allow the achievement of the specifications and yields expected for the devices.

SUBJECT OF THE INVENTION

The present invention addresses the stated problem. The invention relates to a method for manufacturing a composite structure comprising a thin layer of monocrystalline silicon carbide transferred onto a support substrate of polycrystalline silicon carbide. The manufacturing method comprises a step of original, reliable and rapid gradation of the composite structure, combining an initial control of the donor substrate and a final control of said structure.

BRIEF DESCRIPTION OF THE INVENTION

The invention relates to a method for manufacturing a composite structure comprising a thin layer of monocrystalline silicon carbide arranged on a support substrate of polycrystalline silicon carbide, the method comprising the following steps:

1) providing at least one donor substrate made of monocrystalline silicon carbide, having a front face and a back face, the front face potentially having defects, called primary defects;

2) quality control of the – at least one – donor substrate by a photoluminescence imaging technique so as to extract a map of the front face, called first map, listing the primary defects identified as being of the micro-hole type, of the star stacking fault complex type or of the point defect type;

3) the transfer of a thin layer, originating from a surface layer of the – at least one – donor substrate, onto a polycrystalline silicon carbide support substrate, to obtain a composite structure and a residual donor substrate;

4) inspection of a free surface of the thin layer of the composite structure by a defect inspection technique by scattering an ultraviolet laser beam, so as to extract a map of the free surface, called a second map, listing defects, called secondary defects;

5) the gradation of the composite structure, including a comparison of the first mapping and the second mapping.

• l’étape 2) comprend les sous-étapes suivantes :
  i) l’inspection d’une face avant du substrat donneur pour détecter les défauts primaires, du fait de variations locales d’intensité d’un signal de photoluminescence émis par la face avant suite à une excitation par un faisceau incident, et pour former, pour chaque défaut primaire, une image par photoluminescence,
  ii) l’attribution à chaque défaut primaire détecté d’un type labellisé de défaut avec un certain niveau de similarité, grâce à un algorithme de reconnaissance d’images, entrainé sur différents types de défauts susceptibles d’être présents sur la face avant d’un substrat donneur en carbure de silicium monocristallin, tels que des micro-trous, des complexes de fautes d’empilement en étoile et des défauts ponctuels notamment liés à des inclusions, un niveau de contraste étant associé à l’image par photoluminescence de chaque défaut primaire détecté,
  iii) la classification de chaque défaut primaire par application des conditions suivantes :
  - si le type labellisé de défaut est micro-trou et si le niveau de similarité est supérieur à un premier niveau, le défaut primaire est classé comme défaut critique de type micro-trou,
  - si le type labellisé de défaut est complexe de fautes d’empilement en étoile et si le niveau de similarité est supérieur à un deuxième niveau, le défaut primaire est classé comme défaut critique de type complexe de fautes d’empilement en étoile,
  - si le type labellisé de défaut est défaut ponctuel, si le niveau de similarité est supérieur à un troisième niveau, et si le niveau de contraste est supérieur à un seuil prédéterminé, le défaut primaire est classé comme défaut critique de type défaut ponctuel,
  - dans les autres cas, le défaut primaire est classé comme défaut non critique.
• l’étape d’inspection i) est réalisée avec un faisceau incident de longueur d’onde 313nm et le signal de photoluminescence émis est collecté dans une plage de longueurs d’onde allant de 700nm à 1000nm ;
• le seuil prédéterminé est établi empiriquement pour un type de substrat donneur, sur la base d’une étude de corrélation entre la valeur de contraste de défauts primaires détectés sur une face avant d’un substrat donneur test et la présence de défauts secondaires sur une couche mince, issue du substrat donneur test, reportée sur un substrat support ;
• un substrat donneur présentant une densité de défauts primaires, classés comme défauts critiques quel que soit le type, supérieure à 1 défaut/cm², voire préférentiellement supérieure à 0,25 défaut/cm², est déclassé à l’issue de l’étape 2) et n’est pas utilisé pour l’étape 3) ;
• l’étape 5) comprend une réconciliation entre les défauts primaires classés comme défauts critiques et les défauts secondaires, chaque défaut primaire classé comme défaut critique et non associé à un défaut secondaire étant ajouté aux défauts de la deuxième cartographie pour être pris en compte dans une décision de gradation ;
• la décision de gradation correspond à un déclassement de la structure composite si cette dernière comporte :
  - une densité de défauts secondaires associés à des défauts primaires critiques et de défauts primaires classés comme défauts critiques ajoutés à la deuxième cartographie, supérieure à 0,25 défauts/cm², et/ou
  - une densité de défauts secondaires uniquement détectés sur la deuxième cartographie, non associés à des défauts primaires critiques, supérieure à 0,2 défauts/cm²;
• l’étape 5) comprend une réconciliation entre les défauts primaires classés comme défauts critiques et les défauts secondaires, et l’étape 5) comprend un contrôle complémentaire par une technique couplant microscopie optique en lumière visible et imagerie par photoluminescence, permettant de vérifier si des défauts sont présents dans la couche mince, aux emplacements desdits défauts primaires classés comme défauts critiques ;
• l’étape 3) comprend les sous-étapes suivantes :
  3a)l’implantation d’espèces légères dans un substrat donneur, pour former un plan fragile enterré délimitant, avec une face avant du substrat donneur, la couche superficielle à transférer ;
  3b)l’assemblage d’un substrat support en carbure de silicium polycristallin avec le substrat donneur implanté à l’étape 3a) ;
  3c)la séparation le long du plan fragile enterré pour former une structure composite intermédiaire comprenant la couche superficielle après transfert et le substrat support, d’une part, et le reste du substrat donneur, dit substrat donneur résiduel, d’autre part ;
  3d)l’application de traitements thermique(s), mécanique(s) et/ou chimique(s) à une surface libre de la couche superficielle, pour former la structure composite munie de la couche mince en carbure de silicium monocristallin ;
• le procédé de fabrication comprend une étape 6) de recyclage du substrat donneur résiduel pour en faire un substrat donneur recyclé, et dans lequel :
  - le substrat donneur recyclé est introduit directement à l’étape 3) en tant que substrat donneur, et
  - à l’étape 4) suivante, la gradation de la structure composite résultante utilise la première cartographie établie pour le substrat donneur initial dont est issu le substrat donneur recyclé.

According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination:

• step 2) includes the following sub-steps:
  (i) inspecting a front face of the donor substrate to detect primary defects, due to local variations in the intensity of a photoluminescence signal emitted by the front face following excitation by an incident beam, and to form, for each primary defect, a photoluminescence image,
  (ii) the attribution to each detected primary defect of a labeled type of defect with a certain level of similarity, using an image recognition algorithm, trained on different types of defects likely to be present on the front face of a monocrystalline silicon carbide donor substrate, such as micro-holes, star stacking fault complexes and point defects in particular linked to inclusions, a contrast level being associated with the photoluminescence image of each detected primary defect,
  (iii) the classification of each primary defect by applying the following conditions:
  - if the labeled defect type is micro-hole and if the similarity level is higher than a first level, the primary defect is classified as a critical defect of micro-hole type,
  - if the labeled defect type is star stacking fault complex and if the similarity level is higher than a second level, the primary defect is classified as a critical defect of star stacking fault complex type,
  - if the labeled defect type is point defect, if the similarity level is greater than a third level, and if the contrast level is greater than a predetermined threshold, the primary defect is classified as a critical defect of point defect type,
  - in other cases, the primary defect is classified as a non-critical defect.
• inspection step i) is carried out with an incident beam of wavelength 313nm and the emitted photoluminescence signal is collected in a wavelength range from 700nm to 1000nm;
• the predetermined threshold is established empirically for a type of donor substrate, on the basis of a correlation study between the contrast value of primary defects detected on a front face of a test donor substrate and the presence of secondary defects on a thin layer, originating from the test donor substrate, transferred to a support substrate;
• a donor substrate having a density of primary defects, classified as critical defects regardless of the type, greater than 1 defect/cm ² , or even preferably greater than 0.25 defect/cm ² , is downgraded at the end of step 2) and is not used for step 3);
• step 5) includes a reconciliation between the primary defects classified as critical defects and the secondary defects, each primary defect classified as critical and not associated with a secondary defect being added to the defects of the second mapping to be taken into account in a grading decision;
• the gradation decision corresponds to a downgrading of the composite structure if the latter includes:
  - a density of secondary defects associated with critical primary defects and of primary defects classified as critical defects added to the second mapping, greater than 0.25 defects/cm ² , and/or
  - a density of secondary defects only detected on the second mapping, not associated with critical primary defects, greater than 0.2 defects/ ^cm2 ;
• step 5) comprises a reconciliation between the primary defects classified as critical defects and the secondary defects, and step 5) comprises a complementary control by a technique coupling visible light optical microscopy and photoluminescence imaging, making it possible to verify whether defects are present in the thin layer, at the locations of said primary defects classified as critical defects;
• step 3) includes the following sub-steps:
  3a) the implantation of light species in a donor substrate, to form a buried fragile plane delimiting, with a front face of the donor substrate, the surface layer to be transferred;
  3b) assembling a polycrystalline silicon carbide support substrate with the donor substrate implanted in step 3a);
  3c) separation along the buried fragile plane to form an intermediate composite structure comprising the surface layer after transfer and the support substrate, on the one hand, and the remainder of the donor substrate, called residual donor substrate, on the other hand;
  3d) the application of thermal, mechanical and/or chemical treatments to a free surface of the surface layer, to form the composite structure provided with the thin layer of monocrystalline silicon carbide;
• the manufacturing process comprises a step 6) of recycling the residual donor substrate to make it a recycled donor substrate, and in which:
  - the recycled donor substrate is introduced directly into step 3) as a donor substrate, and
  - in the next step 4), the gradation of the resulting composite structure uses the first mapping established for the initial donor substrate from which the recycled donor substrate originates.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

FIG. 1 There FIG. 1 has a donor substrate, a composite structure without and with epitaxial layer;

FIG. 2

FIG. 2

FIG. 2

FIG. 2 There FIG. 2 , there FIG. 2 , there FIG. 2 and the FIG. 2 present examples of a correlation study, carried out on SICA88 type equipment, between primary defects present on a front face of a donor substrate and associated secondary defects, present on the free face of a thin layer in a composite structure; the primary defects are imaged by photoluminescence microscopy; they give rise to secondary defects when they have a contrast value greater than a predetermined threshold; the secondary defects are imaged by confocal optical microscopy with a Nomarski prism;

FIG. 3

FIG. 3

FIG. 3

FIG. 3

FIG. 3

FIG. 3 Figures 3a, 3b, 3c, 3d, 3e and 3f show steps of a method of manufacturing a composite structure according to the present invention;

FIG. 4 There FIG. 4 presents a map (first map) listing the primary defects of a donor substrate, in step 2) of the method according to the invention;

FIG. 5

FIG. 5 There FIG. 5 and the FIG. 5 present two examples of mapping (second mapping) listing the secondary defects of a composite structure (the thin layer coming from the donor substrate whose first mapping is also illustrated in the figures), in step 4) of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a method for manufacturing a composite structure 100 comprising a thin layer 10 of monocrystalline silicon carbide arranged on a support substrate 20 of polycrystalline silicon carbide. It will of course be understood that, even if the description only mentions the manufacturing of one composite structure 100 for reasons of readability, the method applies to the manufacturing of a plurality of composite structures 100.

In the description to follow, we will conventionally call “primary defects” the defects present on and/or in a donor substrate 1 from which the thin layer 10 originates, “secondary defects” the defects present on and/or in the thin layer of a composite structure 100, and “tertiary defects” the defects present on and/or in the raw epitaxial layer on the thin layer 10.

In a main plane (x,y), the donor substrate 1 and the composite structure 100 are preferably in the form of circular wafers with a diameter of 100mm, 150mm, 200mm, or even more. They could nevertheless be in any other form allowing their subsequent processing for the manufacture of components. The thickness of the substrates and structures extends along the z axis on the FIG. 1 . It is the front faces 1a, 10a, 150a of the substrate 1 and of the structure 100 which are inspected and likely to contain the aforementioned defects.

The manufacturing method comprises a first step 1) of providing a donor substrate 1 made of monocrystalline silicon carbide. The monocrystalline SiC can be of polytype 4H, 6H or 3C. The donor substrate 1 is preferably in the form of a wafer with a diameter identical to or very close to that of the support substrate 20 with which it will subsequently be assembled, and with a thickness typically between 300 μm and 800 μm. It has a front face 1a and a rear face 1b ( FIG. 3 ). The surface roughness of the front face 1a is advantageously chosen to be less than 1nm RMS, or even less than 0.5nm RMS, measured by atomic force microscopy (AFM) on a 20 μm x 20 μm scan. The doping type and the resistivity of the donor substrate 1 are defined according to the application and the devices targeted. Preferably, the front face 1a is a “carbon” face [000-1], to provide, after the transfer, a thin layer 10 with a front face 10a of “silicon” type [0001] in the composite structure 100.

Even if it is stated in step 1) the provision of a donor substrate 1, it can of course be provided at this step a plurality of donor substrates 1, in an industrial framework.

The second step 2) of the method comprises controlling the quality of the – at least one – donor substrate 1 by a photoluminescence imaging technique so as to extract a map of the front face 1a, called the first map, listing the primary defects identified as being of the micro-hole type, of the complex star stacking fault type and of the point defect type (for example, a defect linked to a species inclusion). Indeed, the applicant has identified that these types of defects could constitute critical defects which generate problematic secondary defects in the thin layer 10 of the composite structure 100.

Primary defects of a donor substrate 1 are made visible in photoluminescence imaging due to local variations in the intensity of a photoluminescence (PL) signal emitted by the front face 1a, following excitation by an incident beam. Local variations in the intensity of the PL signal reflect modified properties of the material (stresses, roughness, flatness, etc.), corresponding to the primary defect. The latter can appear on the generated PL image, in the form of white or black spot(s) depending on its characteristics.

This control step can be carried out on known equipment, for example SICA88 (Lasertec company) or “Photoluminescence scanner” (Intego company) or “MiPlato SiC” (EtaMax company). These devices traditionally use the combination of a visible light optical microscopy image and the photoluminescence image of the same defect to assign dimensional (size, surface) and intensity (contrast) criteria from the optical microscope signals and measured photoluminescence; they can also assign a typology criterion (predefined classes of defects) to each of the primary defects, based, for example, on a learning algorithm. The predefined classes typically correspond to defects of the micro-hole type (“micro-pipe”), star stacking fault complex, point defect (particularly linked to inclusions), scratch, particle, etc.

1. L’inspection de la face avant 1a du substrat donneur 1 :

Advantageously, step 2) comprises the three sub-steps described below:

1. Inspection of the front face 1a of the donor substrate 1:

1. L’attribution à chaque défaut primaire détecté d’un type labellisé de défaut :

This step aims to detect primary defects, due to local variations in the intensity of a photoluminescence signal emitted by the front face 1a following excitation by an incident beam, and to form, for each primary defect, a photoluminescence image. In SICA88 equipment, which will be favored in the remainder of this description, the incident excitation beam has a wavelength of 313nm and the emitted photoluminescence signal is collected in a wavelength range from 700nm to 1000nm.

1. The assignment to each detected primary defect of a labeled defect type:

1. La classification de chaque défaut primaire par application des conditions suivantes :

• si le type labellisé de défaut est micro-trou et si le niveau de similarité est supérieur à un premier niveau, le défaut primaire est classé comme défaut critique de type micro-trou,
• si le type labellisé de défaut est complexe de fautes d’empilement en étoile et si le niveau de similarité est supérieur à un deuxième niveau, le défaut primaire est classé comme défaut critique de type complexe de fautes d’empilement en étoile,
• si le type labellisé de défaut est défaut ponctuel, si le niveau de similarité est supérieur à un troisième niveau, et si le niveau de contraste est supérieur à un seuil prédéterminé, le défaut primaire est classé comme défaut critique de type défaut ponctuel,
• dans les autres cas, le défaut primaire est classé comme défaut non critique.

An image recognition algorithm, based for example on a SSD (Single Shot Detector) type model for object detection, is fed and trained on different types of defects likely to be present on the front face 1a of a donor substrate 1 made of monocrystalline silicon carbide, such as micro-holes, star stacking fault complexes and point defects in particular linked to inclusions (labeled defect types). The equipment can thus assign to each detected primary defect a labeled defect type, with a certain level of similarity. A contrast level is also associated with the photoluminescence image of each detected primary defect.

1. The classification of each primary defect by application of the following conditions:

• if the labeled defect type is micro-hole and if the similarity level is higher than a first level, the primary defect is classified as a critical defect of micro-hole type,
• if the labeled defect type is star stacking fault complex and the similarity level is higher than a second level, the primary defect is classified as a critical defect of star stacking fault complex type,
• if the labeled defect type is point defect, if the similarity level is greater than a third level, and if the contrast level is greater than a predetermined threshold, the primary defect is classified as a critical defect of point defect type,
• in other cases, the primary defect is classified as a non-critical defect.

The first, second and third levels of similarity can be different from each other, because they depend on the typology of the defect and the number of defects of each type used to train the image recognition algorithm.

The predetermined contrast threshold can be established empirically for a given type of donor substrate 1 (supplier, crystal structure, roughness of the front face, manufacturing technique, etc.), on the basis of a correlation study between the contrast value of primary defects detected on a front face 1a of one (or preferably several) test donor substrate(s) and the presence of secondary defects on one (or preferably a plurality of) thin layer(s) 10, originating from the test donor substrate(s), transferred to a support substrate 20.

The table of the FIG. 2 shows the correlation that can be observed between a critical primary defect ( FIG. 2 (a)) on a donor substrate 1 and a secondary defect ( FIG. 2 (b)) induced in the thin layer 10 coming from this substrate 1.

The tables in Figures 2b, 2c and 2d show that not all primary defects detected on a donor substrate 1 give rise to secondary defects in the transferred thin layer 10, and that it is therefore required to define criteria to correctly classify these primary defects into critical or non-critical defects. In addition to the similarity level applied to each primary defect associated with a labeled defect type, the contrast level of the photoluminescence image is an important criterion, in particular to judge the criticality of point defects, because these defects can present a wide range of contrasts, from low to high. This is the reason why a predetermined contrast threshold is used, to separate critical defects from non-critical defects. In the example of the FIG. 2 , the predetermined contrast threshold is defined at 600 au This threshold obviously depends on the recipe parameters, the characteristics of the support substrate 1 (roughness, doping, residual stress, production method, etc.) and a universal value cannot in any case be proposed.

For microhole or star-shaped stacking fault complexes, the similarity level is often sufficient because these defects have a very specific signature and associated high contrast. The photoluminescence image can also be combined with the optical microscopy image of these defects to further improve their classification.

Recall that the contrast on a photoluminescence image can be defined as the normalized difference between a photoluminescence signal intensity at the primary defect and a surrounding intensity in a field less than or equal to the microscope field size. Other calculations of the contrast value would give different numerical results and a predetermined contrast threshold, but with a similar meaning.

An example of a first mapping is given on the FIG. 4 : it lists the primary defects detected on a donor substrate 1 and classified as critical defects (notably of the star stacking fault complex type, micro-hole or point defect of the species inclusion type); another defect, classified as non-critical is illustrated at the top left of the FIG. 4 .

According to an advantageous embodiment, in step 2) of the method according to the invention, a donor substrate 1 having a density of primary defects, classified as critical defects regardless of the type, greater than 1 defect/cm ² , or even greater than 0.25 defect/cm ² , is downgraded at the end of step 2) and is not used for the subsequent step 3).

The manufacturing method then comprises a third step 3) of transferring a thin layer 10, originating from a surface layer 10' of the donor substrate 1, onto a support substrate 20 made of polycrystalline silicon carbide, to obtain the composite structure 100 and a residual donor substrate 1' devoid of the surface layer 10'.

Advantageously, the surface layer 10' is transferred by a thin layer transfer technique such as Smart Cut. A first sub-step 3a) comprises the implantation of light species in a donor substrate 1, to form a buried fragile plane 11 delimiting, with a front face 1a of the donor substrate 1, the surface layer to be transferred 10' ( FIG. 3 ). The light species are preferably hydrogen and/or helium, and are implanted in the donor substrate 1, at a depth consistent with the thickness of the targeted thin layer 10. These light species will form, around the determined depth, microcavities distributed in a thin layer parallel to the free surface 1a of the donor substrate 1, i.e. parallel to the (x,y) plane in the figures. This thin layer is called the buried fragile plane 11, for the sake of simplification. The implantation energy of the light species is chosen so as to reach the determined depth. For example, hydrogen ions will be implanted at an energy between 10 keV and 250 keV, and at a dose between 5 ^E 16/cm ² and 1 ^E 17/cm ² , to delimit a thin layer 10 having a thickness of the order of 100nm to 1500nm. Note that a protective layer may be deposited on the front face 1a of the donor substrate 1, prior to the ion implantation step. This protective layer may be composed of a material such as silicon oxide or silicon nitride for example. It may be removed prior to the following sub-step 3b). The latter corresponds to the assembly of a support substrate 20 made of polycrystalline silicon carbide, on the side of its front face 20a, with the implanted donor substrate 1, also on the side of its front face 1a ( FIG. 3 ). The support substrate 20 corresponds to the mechanical support of the future composite structure 100. The lateral dimensions in the main plane (x,y) of the support substrate 20 (its diameter in particular) are the same as those of the composite structure 100. The support substrate 20 has a thickness typically between approximately 50 μm and several hundred micrometers, for example between 50 μm and 650 μm, or between 100 μm and 450 μm, or between 200 μm and 350 μm.

The assembly is made by direct bonding, by molecular adhesion, along a bonding interface 40. Optionally, an intermediate layer can be formed on the front face 1a of the donor substrate 1, before or after the introduction of the light species, and in any case, before the assembly phase. This intermediate layer can be made of a dielectric, semiconductor or metallic material (such as for example silicon oxide, silicon, silicon carbide, tungsten, titanium, etc.). Optionally, an intermediate layer can also be deposited on the face 20a to be assembled of the support substrate 20, prior to the assembly; it can be chosen to be of the same nature or of a different nature from the intermediate layer mentioned for the donor substrate 1. An intermediate layer can optionally be deposited on either of the two substrates 1, 20 to be assembled. The objective of the intermediate layer(s) is essentially to promote the bonding energy (in particular in the temperature range below 1100°C), due to the formation of covalent bonds at lower temperatures than in the case of two SiC surfaces assembled directly; another advantage of this (these) intermediate layer(s) may be to improve the vertical electrical conduction of the bonding interface 40. The intermediate layer(s) is (are) intended to be buried in the bonded assembly 50 after assembly, and ultimately, in the composite structure 100.

Direct bonding by molecular adhesion does not require an adhesive material, as bonds are established at the atomic level between the assembled surfaces. Several types of molecular adhesion bonding exist, which differ in particular in their temperature, pressure, atmosphere or treatment conditions prior to bringing the surfaces into contact. Examples include room temperature bonding with or without prior plasma activation of the surfaces to be joined, atomic diffusion bonding (ADB), surface-activated bonding (SAB), etc.

Let us recall that the assembly step 3b) may comprise, prior to bringing the faces 1a, 20a to be assembled into contact, conventional sequences of cleaning by chemical means (for example, RCA cleaning), surface activation (for example, by oxygen or nitrogen plasma) or other surface preparations (such as cleaning by brushing), capable of promoting the quality of the bonding interface 40 (low defectivity, high adhesion energy).

The following sub-step 3c) corresponds to a separation along the buried fragile plane 11 to form an intermediate composite structure 100' comprising the surface layer 10' after transfer and the support substrate 20, on the one hand, and the remainder of the donor substrate 1', on the other hand ( FIG. 3 ). The separation along the buried fragile plane 11 is usually carried out by applying a heat treatment at a temperature between 800°C and 1200°C. Such a heat treatment induces the development of cavities and microcracks in the buried fragile plane 11, and their pressurization by the light species present in gaseous form, until the propagation of a fracture along said fragile plane 11. Alternatively or jointly, a mechanical stress can be applied to the bonded assembly 50 and in particular at the buried fragile plane 11, so as to propagate or help to mechanically propagate the fracture leading to the separation. At the end of this separation, on the one hand the intermediate composite structure 100' and the remainder 1' of the donor substrate are obtained. The free surface 10'a of the surface layer 10' is usually rough after separation: for example, it has a roughness between 5nm and 100nm RMS.

The following sub-step 3d) comprises the application of thermal, mechanical and/or chemical treatments to a free surface 10'a of the surface layer 10', to form the composite structure 100 provided with the thin layer 10 of monocrystalline silicon carbide having a surface roughness less than or equal to 0.5nm RMS, or even less than or equal to 0.1nm RMS (AFM scan 10x10μm ² or 20x20μm ² ) ( FIG. 3 ). In particular, this sub-step 3d) may comprise a mechanical-chemical smoothing treatment of the free surface 10'a of the surface layer 10'. A removal of between 50nm and 300nm makes it possible to effectively restore the surface state of said layer. It may also comprise at least one heat treatment at a temperature of between 1200°C and 1800°C. Such a heat treatment is applied to remove the residual light species from the surface layer 10' and to promote the rearrangement of its crystal lattice, thus forming the thin layer 10. It also makes it possible to reinforce the bonding interface 40.

At this stage of the process, the thin layer 10 of the composite structure 100 has a thickness typically between a few tens of nm and a few hundreds of nm, for example, between 50 nm and 800 nm. The types and doping levels of the thin layer 10 and the support substrate 20 are defined according to the intended applications and devices.

Returning to the general description of the manufacturing method according to the invention, step 3) of transferring the thin layer 10 onto the support substrate 20 is followed by a fourth step 4) of inspecting the free surface 10a of said thin layer 10 by a defect inspection technique based on dark field and bright field imaging associated with scanning the surface 10a with a DUV laser beam (for “deep UV”, typically with a wavelength of the order of 200-280nm); the laser beam is scattered by the defects proportionally to their size, which allows the location of said defects on the surface 10a and the estimation of their size. As an example, equipment such as KLA SPA2 can be used. The DUV laser beam penetrates very weakly into the thickness of the thin layer 10, it is therefore not polluted by the presence of the p-SiC grains of the support substrate 20. This technique makes it possible to detect the secondary defects present on the surface of the thin layer 10 and to associate them with known defects in the equipment database such as holes, bubbles, scratches, particles, other point or agglomerated defects, etc.

This inspection step is widely used in the semiconductor industry (particularly silicon), making it compatible with industrial speed standards and requirements.

Secondary defects may be induced by primary defects (crystalline defects present on the donor substrate 1 which gave rise to the thin layer 10), by particles or other surface contaminations which have not been completely eliminated, before the assembly of the donor substrate 1 and the support substrate 20, or by a specific problem at a stage of the manufacturing process (scratches, deposited particles, flaking, etc.). These may therefore be defects of crystalline origin, or defects linked to the transfer such as holes (local absence of thin layer 10), bubbles (defect at the bonding interface 40, at which the thin layer 10 is not bonded and forms a blister), particles, scratches, etc.

At the end of step 4), a map of the free surface 10a, called the second map, can be extracted, listing the secondary defects identified. An example of a second map listing the secondary defects detected on a composite structure 100 whose thin layer 10 comes from the donor substrate 1 (first map) is given on the FIG. 5 . Note that to compare the first and second mappings, it is required to apply a “mirror” flip of the first mapping, since the front face 1a donor substrate 1 is assembled on the support substrate 20, and therefore undergoes a flip. In this example, several primary defects classified as critical (visible on the first mapping) are also visible on the second mapping. Another example is given on the FIG. 5 : here, several primary defects classified as critical and visible on the first map, do not appear on the second map.

The advantage of this inspection step 4) is that it is rapid and therefore allows 100% control of composite structures in a production line. However, it is likely not to detect defects of crystalline origin present in the thin layer, because some of these defects do not induce diffusion of the UV laser beam and remain invisible to this inspection.

Conversely, a control technique combining visible light optical microscopy and photoluminescence imaging can make it possible to detect these defects of crystalline origin, but inspection may be made difficult by the presence of underlying p-SiC grains and, above all, requires a much longer processing time per structure, which is impactful in a high-volume production line.

Thus, the applicant has defined a fifth step 5) of grading the composite structure 100 reliably and efficiently, based on the use of the first mapping of the donor substrate 1 (step 2) from which the thin layer 10 of the composite structure 100 in question is derived, and of the second mapping of said structure 100 (step 4). This grading aims to separate the structures falling within the targeted specifications and the downgraded structures.

The second mapping is compared to the first mapping, having of course taken the precaution of applying a mirror reversal to the first mapping since the donor substrate 1, bonded to the support substrate 20, has its front face 1a (buried face of the thin layer 10) reversed. It is on the basis of this comparison that the gradation of the composite structure 100 is decided.

Advantageously, step 5) comprises a reconciliation between the primary defects classified as critical defects of the first mapping and the secondary defects of the second mapping. When a primary defect classified as critical defect cannot be associated with a secondary defect, because no defect is detected on the second mapping, at the location of said primary defect, an additional defect is added to the defects of the second mapping, to be taken into consideration in the grading decision. The primary defects identified as critical (i.e. generating defects in the transferred thin layer), are thus considered for the evaluation of the quality of the composite structures 100, without requiring a complex and time-consuming inspection in the end, on the composite structures 100.

According to a variant, step 5) comprises a reconciliation between the primary defects classified as critical defects and the secondary defects, and if a large number of primary defects classified as critical defects are not each associated with a secondary defect, step 5) comprises a complementary control of the composite structure 100 in question, by a technique coupling confocal microscopy in visible light and photoluminescence imaging, making it possible to verify whether defects are actually present in the thin layer 10 at the locations of said critical primary defects.

The grading step 5) of the manufacturing method according to the invention makes it possible to reliably and efficiently decide the grade to be assigned to the composite structures 100, by benefiting from precise control of the donor substrates 1 adapted to the field of layer transfer, by carrying out a rapid and standardized quality control in the end on all the composite structures 100, and by combining the teaching of these two controls.

• une densité de défauts secondaires associés à des défauts primaires critiques et de défauts primaires classés comme défauts critiques ajoutés à la deuxième cartographie, supérieure à 0,25 défauts/cm², et/ou
• une densité de défauts secondaires uniquement détectés sur la deuxième cartographie (c’est-à-dire non associés à des défauts primaires critiques) supérieure à 0,2 défauts/cm².

In particular, a composite structure 100 may be downgraded if it presents:

• a density of secondary defects associated with critical primary defects and of primary defects classified as critical defects added to the second mapping, greater than 0.25 defects/cm ² , and/or
• a density of secondary defects only detected on the second mapping (i.e. not associated with critical primary defects) greater than 0.2 defects/cm ² .

In a subsequent step, SiC epitaxial growth can be carried out on the thin layer 10 of the composite structures 100 having positively passed the gradation step 5), in order to increase the thickness of the latter and form the epitaxial layer 150 on and in which the devices will be developed ( FIG. 3 ). The epitaxial growth step may be carried out using techniques known in the state of the art.

The manufacturing method may also comprise a sixth step 6) of recycling the residual donor substrate 1' to make it a recycled donor substrate. Step 6) may comprise mechanical and/or chemical treatments, similar to those applied to the composite structure 100, implemented at the front face 1'a of the residual donor substrate 1'.

The recycled donor substrate is then directly introduced in step 3) as a new donor substrate 1. In step 5), the gradation of the resulting composite structure 100 uses the first mapping (step 2) established for the initial donor substrate 1 from which the recycled donor substrate originates.

The first mapping reflecting the quality of the initial donor substrate 1 is therefore used to grade each composite structure 100 produced from this substrate 1, in its initial state and after a plurality of recyclings. Although this control is time-consuming, it is industrially viable because it is amortized over a very large number of composite structures 100 manufactured from the donor substrate 1.

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

Claims (10)

Method for manufacturing a composite structure (100) comprising a thin layer (10) of monocrystalline silicon carbide arranged on a support substrate (20) of polycrystalline silicon carbide, the method comprising the following steps:
1) providing at least one donor substrate (1) made of monocrystalline silicon carbide, having a front face (1a) and a rear face (1b), the front face (1a) potentially having defects, called primary defects;
2) quality control of the – at least one – donor substrate (1) by a photoluminescence imaging technique so as to extract a map of the front face (1a), called first map, listing the primary defects identified as being of the micro-hole type, of the complex star stacking fault type or of the point defect type;
3) transferring a thin layer (10), originating from a surface layer (10') of the – at least one – donor substrate (1), onto a support substrate (20) made of polycrystalline silicon carbide, to obtain a composite structure (100) and a residual donor substrate (1');
4) inspecting a free surface (10a) of the thin layer (10) of the composite structure (100) using a defect inspection technique by scattering an ultraviolet laser beam, so as to extract a map of the free surface (10a), called a second map, listing defects, called secondary defects;
5) grading the composite structure (100), including a comparison of the first mapping and the second mapping. A method of manufacturing a composite structure (100) according to claim 1, wherein step 2) comprises the following sub-steps:
i) inspecting a front face (1a) of the donor substrate (1) to detect primary defects, due to local variations in the intensity of a photoluminescence signal emitted by the front face (1a) following excitation by an incident beam, and to form, for each primary defect, a photoluminescence image,
ii) the attribution to each detected primary defect of a labeled type of defect with a certain level of similarity, thanks to an image recognition algorithm, trained on different types of defects likely to be present on the front face (1a) of a donor substrate (1) made of monocrystalline silicon carbide, such as micro-holes, star stacking fault complexes and point defects in particular linked to inclusions, a contrast level being associated with the photoluminescence image of each detected primary defect,
(iii) the classification of each primary defect by applying the following conditions:
- if the labeled defect type is micro-hole and if the similarity level is higher than a first level, the primary defect is classified as a critical defect of micro-hole type,
- if the labeled defect type is star stacking fault complex and if the similarity level is higher than a second level, the primary defect is classified as a critical defect of star stacking fault complex type,
- if the labeled defect type is point defect, if the similarity level is greater than a third level, and if the contrast level is greater than a predetermined threshold, the primary defect is classified as a critical defect of point defect type,
- in other cases, the primary defect is classified as a non-critical defect. Method for manufacturing a composite structure (100) according to claim 2, wherein the inspection step i) is carried out with an incident beam of wavelength 313nm and the emitted photoluminescence signal is collected in a wavelength range from 700nm to 1000nm. Method for manufacturing a composite structure (100) according to one of claims 2 and 3, in which the predetermined threshold is established empirically for a type of donor substrate (1), on the basis of a correlation study between the contrast value of primary defects detected on a front face (1a) of a test donor substrate (1) and the presence of secondary defects on a thin layer (10), originating from the test donor substrate, transferred to a support substrate (20). Method for manufacturing a composite structure (100) according to one of claims 2 to 4, in which a donor substrate (1) having a density of primary defects, classified as critical defects regardless of the type, greater than 1 defect/cm ² , or even preferably greater than 0.25 defect/cm ² , is downgraded at the end of step 2) and is not used for step 3). Method for manufacturing a composite structure (100) according to one of claims 2 to 5, in which step 5) comprises a reconciliation between the primary defects classified as critical defects and the secondary defects, each primary defect classified as critical defect and not associated with a secondary defect being added to the defects of the second mapping to be taken into account in a grading decision. A method of manufacturing a composite structure (100) according to claim 6, wherein the grading decision corresponds to a downgrading of the composite structure if the latter comprises:
- a density of secondary defects associated with critical primary defects and of primary defects classified as critical defects added to the second mapping, greater than 0.25 defects/cm ² , and/or
- a density of secondary defects only detected on the second mapping, not associated with critical primary defects, greater than 0.2 defects/cm ² . Method for manufacturing a composite structure (100) according to one of claims 2 to 5, in which step 5) comprises a reconciliation between the primary defects classified as critical defects and the secondary defects, and step 5) comprises a complementary control by a technique coupling optical microscopy in visible light and photoluminescence imaging, making it possible to verify whether defects are present in the thin layer (10), at the locations of said primary defects classified as critical defects. Method for manufacturing a composite structure (100) according to one of the preceding claims, in which step 3) comprises the following sub-steps:
3a) the implantation of light species in a donor substrate (1), to form a buried fragile plane (11) delimiting, with a front face (1a) of the donor substrate (1), the surface layer (10') to be transferred;
3b) assembling a support substrate (20) made of polycrystalline silicon carbide with the donor substrate (1) implanted in step 3a);
3c) separation along the buried fragile plane (11) to form an intermediate composite structure (100') comprising the surface layer (10') after transfer and the support substrate (20), on the one hand, and the remainder (1') of the donor substrate, called residual donor substrate (1'), on the other hand;
3d) the application of thermal, mechanical and/or chemical treatments to a free surface (10'a) of the surface layer (10'), to form the composite structure (100) provided with the thin layer (10) of monocrystalline silicon carbide. Method for manufacturing a composite structure according to one of the preceding claims, comprising a step 6) of recycling the residual donor substrate (1') to make it a recycled donor substrate (1''), and in which:
- the recycled donor substrate (1'') is introduced directly into step 3) as donor substrate (1), and
- in the following step 4), the gradation of the resulting composite structure (100) uses the first mapping established for the initial donor substrate (10) from which the recycled donor substrate (1'') originates.