METHOD FOR PRODUCEING A STRUCTURE WITH A MULTIPLE OF MEMBRANES CONVEYING VOCASIONS

Description

FIELD OF INVENTION

[0001] The present invention relates to the field of microelectronics and microelectromechanical systems. In particular, the present invention relates to a method for collectively manufacturing a plurality of membranes overhanging and sealing cavities, within a structure comprising a support substrate and a thin film (the latter composing the membranes), the cavities being confined between the support substrate and a face of the thin film. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] MEMS (Microelectromechanical Systems) devices are widely used in the manufacture of various sensors for a multitude of applications: examples include pressure sensors, microphones, radio frequency switches, electroacoustic and ultrasonic transducers (e.g., pMUTs, or Piezoelectric Micromachined Ultrasonic Transducers), and more. Many of these MEMS devices are based on a flexible membrane positioned over a cavity. During operation, the membrane's deflection, linked to a physical parameter (e.g., the propagation of an acoustic wave for a pMUT), is converted into an electrical signal (or vice versa, depending on whether the device is in receiver or transmitter mode).

[0003] Several layer transfer processes exist for obtaining a structure comprising a plurality of membranes, each overlying a cavity. These processes are advantageously based on the direct bonding (i.e., without the addition of adhesive) of a donor substrate and a support substrate at their respective front faces. One or the other of the substrates has cavities opening onto its front face, usually the support substrate. These cavities are sealed during the molecular adhesion bonding step of the two substrates. A thinning step of the donor substrate results in the transfer of a thin film onto the support substrate. This thinning step may consist of mechanical, chemical, or mechanochemical thinning of the donor substrate, which may optionally include one or more buried stop layers to promote the formation of a uniform thin film.

[0004] It can also be based on the Smart Cut™ process, which employs a buried fragile plane, formed by implanting lightweight materials into the donor substrate. This plane, along with the front face of the donor substrate, delineates the thin layer to be transferred. As is well known, the growth of microcracks in the buried fragile plane, through thermal and / or mechanical activation, leads to separation along the plane, resulting in the transfer of the thin layer onto the supporting substrate. The remaining donor substrate can be reused for subsequent layer transfers.

[0005] The thinning step may also utilize a removable donor substrate, meaning one comprising the thin film to be transferred along with a temporary substrate via an interface or removable layer. Examples include a roughened bonding interface, a porous layer, or another interface / layer that allows for disassembly. After direct bonding of the donor substrate to the support substrate, thermal, chemical, and / or mechanical stress induces separation at the interface or removable layer, resulting in the transfer of the thin film onto the support substrate. The temporary substrate can potentially be reused for subsequent thin film transfers.

[0006] After the thin film is transferred to the substrate, it is usually observed that the membranes directly above the cavities exhibit a deformation, positive or negative (along an axis normal to the plane of the substrate's front face), which is not identical across all membranes. The variability of these deformations can complicate or render inhomogeneous subsequent technological steps required for the fabrication of MEMS devices; it can also generate variable electromechanical behaviors between the membranes of the structure, affecting device performance or manufacturing yield.

[0007] Document FR3100081 proposes a method for sealing a plurality of cavities with membranes that reduce the differential deformation of the latter.

[0008] Beyond reducing the variability of deformation, it appears interesting to aim for membrane deformation that is as low as possible, over the entire extent of the structure. SUBJECT OF THE INVENTION

[0009] The present invention proposes a method for collectively manufacturing a plurality of membranes overhanging and sealing cavities, within a structure comprising a support substrate and a thin layer (the latter composing the membranes), the cavities being delimited between a face of the support substrate and a face of the thin layer, and the membranes exhibiting minimal or even zero deformation over the entire extent of the structure. BRIEF DESCRIPTION OF THE INVENTION

[0010] The present invention defines a method for manufacturing a structure according to claim 1, and a method for manufacturing a structure according to claim 6. According to the present invention, these methods for manufacturing a structure comprising a plurality of membranes, each overhanging a cavity, comprise the following steps: a) a step of forming a plurality of cavities opening at the level of a front face of a support substrate, the cavities having a depth and an area in the plane of the front face, and being spaced by a spacing; b) a step of assembling by direct bonding of a donor substrate onto the support substrate, at the level of their respective front faces, so as to seal the cavities under vacuum, the direct bonding being hydrophilic and involving a given number of monolayers of water at a contact interface between the donor substrate and the support substrate; c) a step of transferring a thin layer from the donor substrate onto the support substrate, said thin layer composing the membranes above the cavities.

[0011] A specific area is around each cavity, in the plane of the contact interface, said specific area being expressed as a function of half the spacing. The manufacturing process is remarkable in that the area, the depth of each cavity, and the specific area are defined in step a) to satisfy the following relationship: S / A = (P atm xp) / (N x 10 15< xk B x T), with P atm the atmospheric pressure, N the number of water monolayers at the contact interface, k B the Boltzmann constant, and T the ambient temperature.

[0012] According to advantageous features of the invention, taken alone or in any feasible combination: the number of water monolayers is between 1 and 5, more particularly between 1 and 3; each cavity has a square shape, in the plane of the front face of the supporting substrate, and has a side length, the cavities are distributed in a matrix with a constant spacing between them, and, the spacing and the length of the cavities are related by the following relation: e / L = √[1 + [(P atm xp) / (N x 10 15< xk B x T)]] - 1; the support substrate is formed of at least one material selected from silicon, germanium, III-V semiconductor compounds, lithium tantalate, lithium niobate and glass, the thin film is formed of at least one material selected from silicon, germanium, III-V semiconductor compounds, lithium tantalate and lithium niobate, at least one of the support substrate and the thin film of the structure includes a material, along the contact interface, which oxidizes in the presence of water molecules;Step c) of transfer is based on the creation of a brittle plane buried in the donor substrate, prior to step b) of assembly, and on the application of a heat treatment during step c) to develop microcracks in the buried brittle plane and result in a separation along said plane.

[0013] According to the invention, the process of claim 6 comprises a preparatory sequence, prior to step a), in the case where the number of water monolayers involved in the direct bonding of step b) is not known, said preparatory sequence comprising: • The formation of a plurality of cavities opening onto a front face of a test support substrate, the cavities having the depth and lateral dimensions, and being spaced at a test spacing; • The assembly by direct bonding of a donor substrate onto the test support substrate, at their respective front faces, so as to seal the cavities under vacuum, the direct bonding being hydrophilic and involving a given number of water monolayers at a contact interface between the donor substrate and the test support substrate; • The transfer of a thin layer from the donor substrate onto the test support substrate, to form a test structure; • The measurement of an average deformation, along an axis normal to the plane of the front face of the test support substrate, of a plurality of membranes of the test structure;• Determining the spacing between the cavities to be applied in step a), as a function of the test spacing, the lateral dimensions of the cavities, the average deformation of the membranes, the depth of the cavities and the thickness of the thin layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the accompanying figures in which: [ Fig. 1 ] There figure 1 presents a structure developed according to the manufacturing process conforming to the present invention; [ Fig. 2a ] ] Fig. 2b ] ] Fig. 2c ] THE figures 2a, 2b , 2c present steps in the manufacturing process according to the present invention; the figure 2a offers a cross-sectional view and a top view; Fig. 3 ] There figure 3 presents a diagram of a deformed membrane above a cavity in a test structure implemented in the manufacturing process according to the invention; [ Fig. 4 ] There figure 4 presents a graph relating the spacing between the cavities and the average deflection obtained, for square cavities of fixed lateral dimension (25 µm) and depth (350nm), produced under similar bonding conditions (under vacuum 5.10 -3< mbar), with a silicon membrane of thickness 1.4µm.

[0015] Some figures are schematic representations which, for the sake of readability, are not to scale. In particular, the layer thicknesses along the z-axis are not to scale relative to the lateral dimensions along the x and y axes.

[0016] The same references on the figures can be used for elements of the same nature. DETAILED DESCRIPTION OF THE INVENTION

[0017] The invention relates to a method for manufacturing a structure 100 comprising a plurality of membranes 11, each overhanging a cavity 3 ( figure 1 ).

[0018] The manufacturing process includes firstly a step a) of forming a plurality of cavities 3 opening at a front face 2a of a support substrate 2 ( Figure 2a ).

[0019] The support substrate 2 advantageously has the shape of a wafer, with a diameter greater than 100 mm, for example 150 mm, 200 mm, or 300 mm, and has a front face 2a and a rear face 2b. Its thickness is typically between 200 and 900 microns. The support substrate 2 can be made of at least one material selected from silicon, germanium, III-V semiconductor compounds, lithium tantalate, lithium niobate, glass, or other material of interest for the intended application.

[0020] As is well known in itself, there are different methods for forming cavities 3 on the surface of a substrate; these may include lithography techniques (to define the distribution and shape of the cavities 3, in the (x,y) plane of the front face 2a) and etching techniques (to etch locally the support substrate 2, in the areas defined for the cavities 3, to a given depth).

[0021] Each cavity 3 has a depth p that typically varies between 100 nm and 100 µm. Its shape, in the (x,y) plane of the front face 2a, can be circular, square, rectangular, or polygonal. The characteristic dimension(s) (or lateral dimension(s)) of a cavity 3 in the (x,y) plane, namely its diameter (for a circular shape) or its side length L (for a square shape) or its width and length (for a rectangular shape), is / are typically between 1 µm and 500 µm. The area A (in the (x,y) plane) of a cavity 3 is therefore typically between 1 µm² and 0.25 mm².

[0022] The cavities 3 are spaced apart by a distance denoted e. This spacing can range from 1 µm to several hundred mm, for example 500 µm. Although not shown in the figures, it should be noted that the cavities 3 could be separated by a different spacing along the x-axis and along the y-axis.

[0023] A contour zone 4 is defined around each cavity 3, in the (x,y) plane of the front face 2a of the supporting substrate 2 (zones delimited by dashed lines on the figure 2a Each contour zone 4 surrounds a cavity 3 and is adjacent but disjoint from the contour zones 4 surrounding neighboring cavities 3. The set of contour zones 4 corresponds to the cavity-free surface 3 of the front face 2a of the supporting substrate 2.

[0024] The area of ​​a contour zone 4 is called the specific area S; it is expressed as a function of half the spacing e between neighboring cavities 3. In the example of the figure 2a (3 square-shaped cavities), the specific surface area is expressed as a function of the spacing e between the cavities 3 and the side length L of the cavities 3 as follows: S = e 2 + 2 L × e

[0025] This specific area S can of course be expressed differently depending on the shape of the cavities 3 and their distribution. In all cases, it can be expressed as a function of half the spacing e and the lateral dimensions of cavity 3.

[0026] The front face 2a of the support substrate 2 is intended to be assembled against a front face 1a of a donor substrate 1.

[0027] Like the support substrate 2, the donor substrate 1 advantageously has the shape of a wafer, with a diameter greater than 100 mm, for example 150 mm, 200 mm, or 300 mm, and has a front face 1a and a rear face 1b. Its thickness is typically between 200 and 900 microns. The donor substrate 1 can be made of at least one material selected from silicon, germanium, III-V semiconductor compounds, lithium tantalate, lithium niobate, or other material of interest for forming the membranes 11 above the cavities 3. Indeed, the donor substrate 1 is intended to provide a thin layer 10, which, transferred onto the support substrate 2, will form the plurality of membranes 11 overhanging the cavities 3.

[0028] The manufacturing process according to the invention then comprises a step b) of assembly by direct bonding of the donor substrate 1 onto the support substrate 2, at the level of their respective front faces 1a, 2a, so as to seal the cavities 3 and to form a bonded structure 150 ( figure 2b ).

[0029] The principle of direct bonding, well known in the state of the art, will not be described in further detail here. Because it is based on molecular adhesion between assembled faces, a very good surface condition (cleanliness, low roughness, etc.) of the substrates is required to obtain good bond quality.

[0030] Prior to assembly, the donor substrate (1) and support substrate (2) are typically prepared. For example, a standard sequence used in microelectronics, particularly for silicon-based substrates, includes ozone cleaning, SC1 cleaning ("Standard Clean 1"), and SC2 cleaning ("Standard Clean 2"), with intervening rinses. The surfaces to be joined may also be activated, for example by plasma, before contact to promote strong bonding energy between them.

[0031] Optionally, the donor substrate 1 and / or the support substrate 2 may include an intermediate layer on their respective front faces 1a, 2a to improve the bonding quality and energy of their interface. This intermediate layer may, in particular, be made of silicon oxide. In the specific case where both the donor substrate 1 and the support substrate 2 are made of silicon, the resulting structure 100 is a silicon-on-insulator (SOI) structure with buried cavities 3.

[0032] The direct bonding in step b) is carried out in a low-pressure chamber, so as to seal the cavities 3 under vacuum, typically with a pressure below 1 mbar. Advantageously, the pressure in the cavities 3 is on the order of 5 x 10⁻³ mbar.

[0033] In addition, the direct bonding of step b) is of the hydrophilic type, that is to say that it involves a given number (N) of water monolayers at a contact interface 5 between the donor substrate 1 and the support substrate 2. Depending on the surface preparations before assembly, the materials in contact (materials composing the donor substrate 1 and the support substrate 2) as well as the bonding atmosphere, the number of water monolayers at the contact interface 5 can vary, typically between 1 and 5.

[0034] For example, in the case of a silicon support substrate 2 assembled to a silicon donor substrate 1 equipped on its front face 1a with an intermediate layer of silicon oxide, via a direct hydrophilic bond, in an enclosure at 5.10 -3< mbar, the number of water monolayers at the contact interface 5 is between 1 and 3.

[0035] Note that the number of water monolayers on the surface of substrates 1,2 can be evaluated by X-ray reflection, for example in an ESRF type infrastructure.

[0036] As mentioned previously, the set of contour areas 4 corresponds to the cavity-free surface 3 of the front face 2a of the support substrate 2: the set of contour areas 4 is therefore assembled with the front face 1a of the donor substrate 1 to form the contact interface 5 of the bonded structure 150.

[0037] The manufacturing process according to the invention finally includes a step c) of transferring a thin layer 10, from the donor substrate 1, onto the support substrate 2: said thin layer 10 makes up the membranes directly above the cavities ( figure 2c ).

[0038] Step c) of transfer can be carried out by any known thin-film transfer technique (10). Preferably, step c) of transfer is based on the creation of a brittle plane buried in the donor substrate 1, prior to step b) of assembly. Such a buried brittle plane is typically formed by the implantation of light species (in particular hydrogen and / or helium, or other species), which induce microcavity-type defects (or "platelets" due to their usually lenticular shape), particularly near the peak of maximum concentration: this buried region of microcavities is called the buried brittle plane for simplicity. Step c) then comprises the application of a heat treatment to grow the microcavities and develop microcracks in the buried brittle plane, so as to result in separation along said plane.This technique refers to the Smart Cut™ process, well known for transferring a single-crystal thin film from a donor substrate 1 onto a support substrate 2, and will not be described in more detail here.

[0039] Note that finishing and / or smoothing steps (mechanical, mechano-chemical, chemical or thermal) of the free surface of the thin film 10 after transfer can be implemented to achieve the required crystalline and surface quality for the thin film 10.

[0040] The present invention applies to cases where at least one of the donor substrates 1 and support 2 comprises a material, at the contact interface 5, which oxidizes in the presence of water molecules (H₂O). Thus, during the various heat treatment(s) applied to the bonded structure 150, either to increase the bonding energy of the contact interface 5, or to perform step c) or a further step of the process, an oxidation reaction of said material occurs, resulting in the formation of gaseous dihydrogen molecules.

[0041] In the particular example of a contact interface 5 involving silicon, the oxidation reaction is written as follows: Si + 2H 2 O -> SiO 2 + 2H 2 .

[0042] The applicant has demonstrated that the H2 gas is produced at the contact interface 5 and is able to diffuse and fill the cavities 3. The presence of this gas therefore causes a pressure increase in the volume of the sealed cavity 3, which may induce a deformation of the membrane 11 when it is formed and subjected to atmospheric pressure P atm on its upper face.

[0043] As an example, the graph of the figure 4 Figure 11 shows the evolution of the average deflection of membrane 11 as a function of the spacing e between 3 square cavities with sides of 25 µm, with a membrane thickness (silicon) of 1.4 µm, and under given bonding conditions. Experimentally, it can be seen that there is a spacing condition for which the membrane deformation is zero, or at least very low.

[0044] The invention aims precisely to minimize this deformation, by establishing design rules linking the bonding conditions, the dimensions of the cavities 3 and their distribution on the support substrate 2.

[0045] For this purpose, the area A, the depth p of each cavity 3, and the specific area S of the contour zones 4 are defined in step a) of the process, to satisfy the following relationship: S A = P atm × p N × 10 15 × k B × T with P atm the atmospheric pressure (10 5< Pa), p the depth of each cavity, N the number of water monolayers at the contact interface 5, k B the Boltzmann constant (1.38.10 -23< J / K) and T the ambient temperature (300K).

[0046] Step b) of assembly by direct bonding defines the number N of water monolayers that will be present at the contact interface 5. It has been established by the applicant that the estimated quantity n H2 of H2 gas molecules produced at the contact interface 5 and diffusing into each cavity 3 is related to the specific area S and the number of monolayers N according to the following relationship: n H 2 = S × N × 10 15

[0047] By substituting this relation Equ.3 into the ideal gas law, we obtain the aforementioned relation Equ.2, which defines the design rules linking bonding conditions, dimensions and distribution of the cavities 3. In other words, the area A (lateral dimensions), the depth p of the cavities 3 and the spacing e between the cavities 3 can be defined from relation Equ.2, so as to minimize the deformation of the membrane 11, at the end of the process in step c).

[0048] In the particular case of square-shaped cavities 3 (in the (x,y) plane of the front face 2a of the supporting substrate 2), having a side length L and distributed in a matrix fashion with a constant spacing e between them, the following relationship can be established: e L = 1 + P atm × p N × 10 15 × k B × T − 1 with P atm the atmospheric pressure, p the depth of each cavity 3, N the number of water monolayers at the contact interface 5, k B the Boltzmann constant and T the ambient temperature.

[0049] This equation can be used to develop nomograms relating the spacing e and the side length L, for square cavities with different depths p and for a number of water monolayers that can vary between 1 and 5. These nomograms provide design rules, for given bonding conditions, allowing the deflection of the membrane 11 to be minimized above the cavities 3.

[0050] According to one variant, the manufacturing process includes a preparatory sequence, prior to step a), which applies in the case where the number N of water monolayers involved in the direct bonding of step b) is not known.

[0051] The preparatory sequence includes firstly the formation of a plurality of cavities opening at the level of a front face of a test support substrate; these cavities have a depth p and lateral dimensions (side length L, in the case of square cavities), and are spaced with a test spacing e 1.

[0052] The preparatory sequence then includes the assembly by direct bonding of a donor substrate 1 onto the test support substrate, at the level of their respective front faces, so as to seal the cavities under vacuum, the direct bonding being hydrophilic and involving a given number N of water monolayers at the contact interface between the donor substrate 1 and the test support substrate.

[0053] Then, a thin layer 10 from the donor substrate 1 is transferred to the test support substrate to form a test structure.

[0054] The preparatory sequence then involves measuring an average deformation h0, along a z-axis normal to the (x,y) plane of the front face of the test support substrate, of a plurality of membranes 11 of the test structure ( figure 3 From these measurements, the pressure difference between the inside and outside of cavity 3 is determined, which allows us to calculate the quantity of H2 gas produced under the bonding conditions used.

[0055] Considering that the membrane 11 is embedded on the edges of the cavity 3, the relationship between the maximum deflection h0, the pressure difference ΔP, and the lateral dimension L of the membrane 11 is written: h 0 = 3 64 1 − ν 2 Δ PL 4 E t 3

[0056] With t the thickness of the membrane 11 (and of the thin layer 10), E and v, respectively the Young's modulus and the Poisson's ratio of the thin layer 10.

[0057] Equation Equ.5 can also be written as: Δ P = E 64 t 3 h 0 3 1 − ν 2 L 4

[0058] Neglecting the change in volume associated with the deformation of membrane 11, we can write the following relationship between the quantity n' H2 of gas in the cavity and the pressure in the form: P atm + Δ P × A × p = n ′ H 2 × k B × T = N × 10 15 × e 1 2 + 2 L . e 1 × k B × T

[0059] With A the area of ​​the cavity, L its lateral dimension, p its depth, e 1 the test spacing between the cavities.

[0060] This same expression, for an undeformed membrane 11, with a spacing e between the cavities 3, gives: P atm × A × p = n H 2 × k B × T = N × 10 15 × e 2 + 2 L . e × k B × T

[0061] The relationship between the two expressions Equ.7 and Equ.6 leads to establishing an intermediate relationship Equ.8', resulting in a relationship Equ.8 between the targeted spacing e to minimize the deformation of the membranes 11, under the tested bonding conditions, with the dimensions (L, p) of the cavities 3 also tested and the characteristics of the transferred thin layer 10: e 2 + 2 L . e e 1 2 + 2 L . e 1 = P atm P atm + Δ P = 1 1 + E 64 t 3 h 0 P atm 3 1 − ν 2 L 4 e = L × − 1 + 1 + 1 1 + E 64 t 3 h 0 P atm 3 1 − ν 2 L 4 × e 1 2 L 2 + 2 e 1 L

[0062] The preparatory sequence therefore makes it possible to identify the spacing e between the cavities 3, to be applied in step a), which will allow a minimum deformation of the membranes 11 after transfer, under the bonding conditions and with the physical (thickness and nature of the thin layer 10) and dimensional (lateral size and depth of the cavities) characteristics tested.

[0063] The preparatory sequence has been described in the case of square-shaped cavities. Equations Equ.5 to Equ.8 could of course be adapted to take into account a different shape, which would in particular result in an adjusted expression for the area A of the cavities and the specific area S of the contour zones 4.

[0064] The present invention can be used for a wide range of MEMS or NEMS (“Nanoelectromechanical systems”) devices, or for any other application requiring the production of suspended membranes with very low deflection.

[0065] The invention is not limited to the embodiments described and alternative embodiments may be made without departing from the scope of the invention as defined by the claims.

Claims

1. Method for manufacturing a structure (100) comprising a plurality of membranes (11), each overhanging a cavity (3), the manufacturing method comprising the following steps: a) a step of forming a plurality of cavities (3) opening onto a front face (2a) of a support substrate (2), the cavities (3) having a depth (p) and an area (A) in the plane (x,y) of the front face (2a), and being spaced apart by a spacing (e), a contour zone (4) being defined around each cavity (3), in the plane (x,y) of the front face (2a), each contour zone (4) surrounding a cavity (3) and being adjacent, but separated from the contour zones (4) surrounding the neighbouring cavities (3), all of the contour zones (4) corresponding to the surface devoid of cavities (3) of the front face (2a) of the support substrate (2); b) a step of assembling by direct bonding of a donor substrate (1) on the support substrate (2), at their respective front faces (1a, 2a), so as to seal the cavities (3) under vacuum, the direct bonding being hydrophilic and involving a given number (N) of water monolayers at a contact interface (5) between the donor substrate (1) and the support substrate (2); c) a step of transferring a thin layer (10) from the donor substrate (1) on the support substrate (2), said thin layer (10) composing the membranes (11) in vertical alignment with the cavities (3); the manufacturing method being characterised in that: - being defined that each contour zone (4) has an area called specific area (S) around each cavity (3), in the plane of the contact interface (5), said specific area (S) being expressed as a function of half of the spacing (e), - the area (A), the depth (p) of each cavity, and the specific area (S) are defined in step a) to satisfy the following relationship: S / A = (Patm × p) / (N × 1015 × kB × T), with Patm the atmospheric pressure, N the number of water monolayers at the contact interface, kB the Boltzmann constant and T the ambient temperature.

2. Manufacturing method according to the preceding claim, wherein the number (N) of water monolayers is comprised between 1 and 5, more specifically, comprised between 1 and 3.

3. Manufacturing method according to any one of the preceding claims, wherein: - each cavity (3) has a square shape in the plane (x,y) of the front face (2a) of the support substrate (2), having a side length (L) and the cavities (3) are distributed in a matrix, with a constant spacing (e) between them, - the spacing (e) and the length (L) of the cavities are connected by the following relationship: e / L = root[1 +[(Patm × p) / (N × 1015 × kB × T)]] - 14. Manufacturing method according to any one of the preceding claims, wherein: - the support substrate (2) is formed of at least one material chosen from among silicon, germanium, III-V semiconductor compounds, lithium tantalum, lithium niobate and glass, - the thin layer (10) is formed of at least one material chosen from among silicon, germanium, III-V semiconductor compounds, lithium tantalum and lithium niobate, and - at least one from among the support substrate (2) and the thin layer (10) of the structure (100) comprises a material, along the contact interface (5), which is oxidised in the presence of water molecules.

5. Manufacturing method according to any one of the preceding claims, wherein the transfer step c) is based on: - the creation of a fragile plane buried in the donor substrate (1), prior to the assembly step b), and - the application of a heat treatment during step c) to develop microcracks in the buried fragile plane and to result in a separation along said plane.

6. Method for manufacturing a structure (100) comprising a plurality of membranes (11), each overhanging a cavity (3), the manufacturing method comprising the following steps: a) a step of forming a plurality of cavities (3) opening onto a front face (2a) of a support substrate (2), the cavities (3) having a depth (p) and an area (A) in the plane (x,y) of the front face (2a), and being spaced apart by a spacing (e); b) a step of assembling by direct bonding of a donor substrate (1) on the support substrate (2), at their respective front faces (1a, 2a), so as to seal the cavities (3) under vacuum, the direct bonding being hydrophilic and involving a given number (N) of water monolayers at a contact interface (5) between the donor substrate (1) and the support substrate (2); c) a step of transferring a thin layer (10) from the donor substrate (1) on the support substrate (2), said thin layer (10) composing the membranes (11) in vertical alignment with the cavities (3); the manufacturing method being characterised in that it comprises a preparatory sequence, prior to step a), in the case where the number of water monolayers (N) involved in the direct bonding of step b) is not known, said preparatory sequence comprising: - the formation of a plurality of cavities opening onto a front face of a test support substrate, the cavities having the depth (p) and lateral dimensions (L), and being spaced apart by a test spacing (e1); - the assembly by direct bonding of a donor substrate (1) on the test support substrate, at their respective front faces, so as to seal the cavities under vacuum, the direct bonding being hydrophilic and involving a given number (N) of water monolayers at a contact interface between the donor substrate (1) and the test support substrate; - the transfer of a thin layer (10) from the donor substrate (1) on the test support substrate, to form a test structure; - the measuring of an average deformation (h0), about an axis (z) normal to the plane (x,y) of the front face of the test support substrate, of a plurality of membranes (11) of the test structure; - the determining of the spacing (e) between the cavities to be applied in step a), as a function of the test spacing (e1), of the lateral dimensions (L) of the cavities, of the average deformation (h0) of the membranes (11), of the depth (p) of the cavities and of the thickness of the thin layer (10).