PROCESS OF ASSEMBLY OF TWO SUBSTRATES BY MOLECULAR ADHESION

Abstract

The invention relates to a method for transferring a thin film (7) onto a final substrate (11), the thin film (7) and the final substrate (11) having different coefficients of thermal expansion. The method comprises transferring the thin film (7) onto an intermediate substrate (5) at a first adhesion interface (IA1) and forming a dielectric surface layer (10) on the exposed face of the thin film (7). The method also comprises activating the dielectric surface layer (10) by exposing it to a plasma having a radiofrequency power density strictly greater than 0.8 W / cm², then assembling the thin film (7) via the dielectric surface layer (10) to the final substrate (11), thus defining a second adhesion interface (IA2).Finally, the process includes mechanically stressing the final support (11) and / or the intermediate support (5) to detach the intermediate support (5) from the thin film (7) at the first adhesion interface (IA1). Figure to be published with the abbreviation: Fig. 4g.

Description

Title of the invention: METHOD FOR ASSEMBLING TWO SUBSTRATES BY MOLECULAR ADHESION FIELD OF INVENTION

[0001] The invention relates to a method for assembling two substrates by molecular adhesion. Its applications include, in particular, microelectronics, microsystems, and photonics. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Molecular adhesion is a process for joining two bodies in which the principal faces or surfaces of these two bodies, perfectly clean, flat, and smooth, are brought into intimate contact with each other to promote the development of molecular bonds, for example, van der Waals or covalent bonds. The two bodies are then joined without the use of an adhesive. These bonds can be strengthened by applying a heat treatment.

[0003] This process is used in particular for the manufacture of substrates which find application in the fields of microelectronics, microsystems, photonics... In these fields, the substrates generally have the form of circular wafers of materials which can be single-crystal, polycrystalline or amorphous, conductive, semiconductive or insulating.

[0004] In these fields, and to facilitate substrate assembly, it is common practice to form a dielectric surface layer, typically of silicon oxide, on one or both of these two substrates, by deposition, possibly followed by a polishing step, or by simple oxidation when the substrate is formed or contains silicon. It is also common practice to prepare the main faces to make them clean (particularly in terms of particles and contaminants) and smooth (to provide a surface roughness typically below 0.5 nm in root mean square value measured by atomic force microscopy over an observation field of 5 microns by 5 microns).

[0005] After their preparation, the two substrates are placed one on top of the other, at the level of their principal faces. The application of pressure on one or the other of the two substrates causes the principal faces to come into intimate and point contact, and the propagation of this intimate contact over the entire extent of the principal faces in the form of a wave, called a "bonding wave".

[0006] It is also sometimes considered to make at least one of these faces reactive by applying a plasma, typically of oxygen or nitrogen. This surface preparation aims to increase the bonding energy between the two assembled substrates. Reference may be made in this regard to the publication by Paul Lindner, "Plasma activated wafer bonding for thin Silicon on insulator substrate fabrication," Proceedings of the Electrochemical Society, 2004. Figure 1 represents equipment 1 for applying such a plasma treatment to a substrate 2 for activation prior to bonding. This equipment comprises an activation chamber 3, generally placed at subatmospheric pressure, into which a controlled flow of oxygen or nitrogen is introduced. The chamber contains two electrodes 4a, 4b between which the plasma is formed by ionization of the oxygen or nitrogen molecules. This plasma is initiated by applying, using a generator G, radio frequency (RF) power (typically 13.56 MHz) to one of the electrodes 4a, while the other electrode is grounded 4b.

[0007] In order to improve the quality of the assembly of two substrates, document WO2023202917A1 proposes to exploit an activation step of the dielectric surface layer which covers at least one of the substrates to introduce, in this layer, a dose of sulfur greater than 3.0 E13 at / cmA2. SUBJECT OF THE INVENTION

[0008] One object of the invention is to improve the approach proposed by this document and to exploit it in the field of layer transfer. BRIEF DESCRIPTION OF THE INVENTION

[0009] To achieve this goal, the object of the invention proposes a method for transferring a thin film onto a final substrate, the thin film and the final substrate having different coefficients of thermal expansion. According to the invention, the method comprises: a. form an intermediate structure comprising the thin film assembled to an intermediate support at a first adhesion interface of the thin film, the thin film and the intermediate support having identical coefficients of thermal expansion; b. form a dielectric surface layer on the exposed face of the thin layer of the intermediate structure; c. activate the dielectric surface layer by exposing it to a plasma formed between two electrodes subjected to a radiofrequency power density strictly greater than 0.8W / cmA2 of an activation chamber into which a controlled flow of a gas including sulfur is introduced; d. to assemble the thin film via the dielectric surface layer to the final support and thus define a second adhesion interface for the thin film, the second adhesion interface having, due to from the activation stage of the surface layer, an adhesion energy greater than that of the first interface; e. mechanically stress the final support and / or the intermediate support to dismantle the intermediate support of the thin film at the level of the first interface, and thus complete the transfer of the thin film onto the final support.

[0010] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: - the formation of the intermediate structure includes the assembly of the thin layer to the intermediate support by molecular adhesion; - the thin layer is taken from a donor substrate; - the formation of the intermediate structure includes the assembly of the donor substrate to the intermediate support; - the formation of the intermediate structure includes a heat treatment of the assembly formed by the donor substrate and the intermediate support; - the transfer process further includes the formation of another dielectric surface layer on the exposed face of the final support; - the transfer process includes an activation of the other dielectric surface layer before the step of assembling the thin film to the final support, by exposing it to a plasma; - the activation of the other dielectric layer includes its exposure to a plasma formed between two electrodes subjected to a radiofrequency power density strictly greater than 0.8W / cmA2 of an activation chamber into which a controlled flow of a gas including sulfur is introduced; - the mechanical stressing step includes the insertion of a blade between the intermediate support and the final support and the application by said blade of a spreading force; - the thin layer and the intermediate support are made of the same piezoelectric material; - the piezoelectric material is lithium tantalate or lithium niobate; - the final support comprises a base substrate on which resides a layer for trapping electrical charges; - the dielectric surface layer is a silicon oxide or a silicon oxynitride. BRIEF DESCRIPTION OF THE FIGURES

[0011] 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:

[0012] [Fig.1]

[0013] Fig. 1 represents a plasma activation device capable of implementing an activation step of a process according to the invention;

[0014] [Fig.2]

[0015] Fig. 2 represents a graph relating the thickness of a surface layer of deposited silicon oxide to its roughness, as measured by X-ray reflectivity, before and after the application of a plasma activation comprising a fluorinated species;

[0016] [Fig.3]

[0017] Fig. 3 presents graphs illustrating the smoothing effect of the dielectric layer according to the value of certain parameters of a plasma activation (pressure prevailing in the chamber, radio frequency power, and SF6 flow rate);

[0018] [Fig.4a] [Fig.4b] [Fig.4c] [Fig.4d] [Fig.4e] [Fig.4f] [Fig.4g]

[0019] Figures 4a, 4b, 4c, 4d, 4e, 4f and 4g illustrate an application of an assembly method according to the invention to form a composite substrate;

[0020] [Fig.5]

[0021] Fig. 5 represents a composite substrate that can be formed by applying an assembly process according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] The applicant continued the studies reported in document WO2023202917A1 concerning a method for assembling two substrates, each having a principal face, by molecular adhesion. At least one of the two substrates is provided with a dielectric surface layer on the side of its principal face. This method comprises the steps of bringing the principal faces of the two substrates into contact, and then initiating and propagating a bonding wave between the principal faces of the two substrates to assemble them together.

[0023] According to this document, the process includes, prior to the contacting step, a preparation step of the dielectric surface layer aimed at introducing, into this layer, a dose of sulfur greater than 3.0 E13 at / cmA2. This preparation step leads to a significant increase in the bonding energy of the two assembled substrates. When the sulfur dose exceeds the aforementioned dose, the bonding wave propagates at a reduced speed, which leads to the elimination, or at least a significant reduction, of peripheral bonding defects (often referred to as "pits") that develop at the bonding interface, at the locations where the bonding wave encounters the edge of the substrates.

[0024] In a particular embodiment, the preparation step includes activating the dielectric surface layer with a nitrogen or oxygen plasma containing sulfur hexafluoride (SF6). This activation is carried out for a duration of between 15 seconds and 2 minutes, during which the substrate is exposed to the nitrogen or oxygen plasma. A controlled amount of sulfur hexafluoride is mixed with the oxygen or nitrogen for a predetermined period within the activation time, which is preceded and followed by periods during which the dielectric surface layer is exposed to the oxygen or nitrogen plasma.

[0025] More specifically, under so-called "reference" application conditions, the activation of the dielectric surface layer comprises the introduction of oxygen (O2) into the activation chamber at a flow rate of 75 sccm (standard cubic centimeters per minute) for a duration of 30 seconds. Within this 30-second period, approximately 11 seconds after its onset, a flow rate of 5 sccm of SF6 is introduced for a period of 5 seconds. The activation chamber is maintained at 50 mTor (6.66 Pa) during this treatment, and a radiofrequency power of 150 W is applied to one of the chamber's electrodes.

[0026] Surprisingly, the applicant observed that the activation of a dielectric layer under these reference conditions had the effect of reducing the roughness of this dielectric layer, whereas this is not the case for the activation of a surface dielectric layer by a plasma consisting of oxygen only.

[0027] Roughness is usually measured by atomic force microscopy. According to this technique, a nanometer-sized tip carried by a flexible arm is scanned across a measurement field of the surface to be characterized, this field having dimensions on the order of one to a few micrometers on each side. The surface topography causes the arm to flex during this scanning. Measuring this flexing allows for the determination of the surface roughness over the entire measurement field. The measurement has a resolution of less than 1 nm vertically, but on the order of 5 to 10 nm laterally, primarily due to the size of the measurement tip.

[0028] The experiments reported in the aforementioned document WO2023202917A1 were carried out in particular on lithium tantalate wafers on which a 30 nm thick surface layer of silicon oxide had been previously formed by PECVD (Plasma Enhanced Chemical Vapor Deposition). Due to the electrically insulating nature of this wafer, the silicon oxide layer tends to become electrically charged, which makes it difficult to measure roughness by atomic force microscopy, the tip of The measurement tends to be retained electrostatically on the wafer surface, which disrupts the measurement. Furthermore, a 30nm thick deposited silicon oxide layer generally exhibits relatively low roughness, below the limit allowing the wafer to bond by molecular adhesion, so the issue of roughness is not addressed by document WO2023202917A1. Experiments

[0029] To analyze in greater detail the effect of activating the dielectric surface layer, and in particular its roughness, the applicant carried out roughness measurements by X-ray reflectivity (a technique referred to as XRR in the relevant scientific literature). The basic principle of this technique consists of projecting an X-ray beam onto the surface to be characterized and measuring the intensity of the reflected X-rays. The measurement field is typically on the order of mmA². Using a simulation of the reflectivity model, a very precise measurement of the layer thickness, surface roughness, and interface roughness can be obtained. The short wavelength, on the order of 0.1 nm, of the X-rays makes it possible to achieve atomic resolution, although the measurement is in reciprocal space, which differs from the measurements provided in real space by AFM.This measurement is not affected by any electrical charge present in the surface layer.

[0030] The XRR roughness measurement observations are shown on the graph in [Fig. 2]. This graph represents the measured roughness R of a dielectric layer (on the x-axis in arbitrary units) as a function of the layer thickness T (on the y-axis in nm). The measurements shown are of a silicon dioxide layer deposited on a silicon substrate by PECVD. Curve A represents the generally linear relationship between the thickness T and the roughness R of the layer immediately after deposition, and curve B represents the XRR measurements after the activation step under the reference application conditions described above. The roughness line C of the deposited layer (curve A) is shown on this graph. Above this line, the surface conditions do not allow, even after the smoothing achieved by the activation step, for sufficiently high-quality molecular adhesion.

[0031] To make the best use of this observation, and in a new series of experiments, the applicant studied the effect of the parameters of the activation equipment, in particular the radio frequency power applied to one of the electrodes and the vacuum level of the chamber, on the roughness of a dielectric surface layer.

[0032] These experiments were carried out on silicon wafers 150 mm in diameter on which a 50 nm layer of silicon oxide was deposited by PECVD. Each wafer in a pair of these wafers underwent an activation step before the platelets of the pair are bonded together by molecular adhesion. The silicon oxide layers underwent no other treatment to reduce their roughness; in particular, they were not polished. The bonding energy was measured after a strengthening heat treatment at 180°C for 60 min, using a double cantilever beam method in an anhydrous environment.

[0033] A summary of the results obtained is presented in the table below, in which the pressure in the activation chamber, the radiofrequency power applied to an electrode of the plasma equipment, and the flow rate of SF6 introduced into the chamber over a period of 5 seconds were varied relative to the reference conditions. The table also includes the measured XRR roughness and the average bonding energy measured after a 100°C-1h heat treatment applied to the assembly. For completeness, it should be noted that the frequency of the radiofrequency signal applied to one of the electrodes was 13 MHz. Reference Pressure (mT) Power (W) SF6 Flow Rate (sccm) XR R Roughness Pre-Activation (in A) XR R Roughness Post-Activation (in A) Anhydrous Bonding Energy (mJ / mA2) 1 150 100 4 17.3 15.1 2 90 200 5 18 8.5 3 30 200 6 17.5 6.2 2780 4 90 150 6 17.6 10.3 5 30 200 4 17.6 5.9 2785 6 150 200 4 17.2 9.4 2510 7 90 150 6 17.2 9.6 2495 8 30 150 4 17.6 7.6 2735 9 90 150 5 17.8 10.4 2612 10 30 191.5 7 17.7 6.9 2765 11 150 200 7 17.8 9.7 12 150 200 5 17.9 9.8 2510 13 50 150 5 17.7 9.1 2675 14 30 300 4 17.7 4.6 2835

[0034] These data are summarized on the graphs of [Fig.3] which model the effect of the variation of a parameter (pressure Pr, power Pa or SF6 flow rate Db) on the smoothing of the dielectric layer (variation of the roughness AR before / after activation).

[0035] It can be observed in this [Fig.3] that the smoothing effect is very sensitive to the radiofrequency power of the plasma, this smoothing effect being maximized in the window studied for the maximum power explored of 300W. It is therefore generally desirable that the activation be carried out at a relatively high power, strictly greater than 150W (i.e. a power density strictly greater than 0.8 W / cmA2 when referring the power to the surface of the wafer), and preferably between 250W and 300W (a power density between 1.4 W / cmA2 and 1.7 W / cmA2).

[0036] The smoothing effect is also sensitive, to a lesser extent, to the pressure prevailing in the chamber, a relatively low pressure leading to a more favorable smoothing effect. Therefore, and advantageously, this pressure is chosen to be set below 50 millitorr (6.66 Pa), preferably below 30 millitorr (4 Pa).

[0037] Finally, we observe that the smoothing effect appears to be very insensitive to the flow rate of SF6, which can therefore be freely chosen, for example in a relatively low flow rate of 4 sccm.

[0038] It is noted that the bonding energy, for all the bonds that were analyzed, is particularly high, in accordance with the findings of document WO2023202917A1. This is notably the case for the bonding of the plates in row 14 of the table, which received the highest activation power of 300W. Assembly process

[0039] The results just presented can be exploited in a sequence consisting of the steps of bringing the main faces of the two substrates into contact, and of initiating and propagating a bonding wave, which were described previously. To this end, prior to the contact step, an activation step is provided for the dielectric surface layer, comprising exposing this layer to a plasma formed between two electrodes of an activation chamber. The dielectric surface layer may have been formed on the substrate by deposition, for example by PECVD deposition. The thickness can be chosen relatively freely, for example less than 90 nm or between 30 nm and 90 nm, because the surface roughness, which can be relatively significant for thicker deposits, will be rectified by the activation step due to its smoothing effect.

[0040] The pressure in the activation chamber can be controlled to be less than 50 millitorr (6.66 Pa), preferably less than 30 millitorr (4 Pa). A controlled flow of oxygen or nitrogen and a controlled flow of a sulfur-containing gas are introduced into the chamber. This controlled flow of oxygen or nitrogen may be equal to, or on the order of, 75 sccm. The controlled flow of gas containing sulfur can be between 3 sccm and 7 sccm of SF6.

[0041] The exposure is conducted for an activation period of between 15 seconds and 2 minutes, during which a radiofrequency power strictly greater than 150W is applied to one of the electrodes. Preferably, the radiofrequency power is between 250W and 300W.

[0042] The sulfur-containing gas, for example SF6, may be introduced at a controlled flow rate for the entire duration, or for only a portion of this duration, to mix with oxygen or nitrogen for a predetermined period of the activation time. Preferably, this predetermined period does not include the plasma ignition time, as this ignition can lead to the introduction of sulfur species from the plasma into the surface layer in a non-repeatable manner, which can be detrimental to proper process control. Advantageously, therefore, the predetermined period during which the dielectric surface layer is exposed to the plasma formed from a sulfur-containing gas is respectively preceded and followed by periods during which the dielectric surface layer is exposed to the plasma formed solely from an oxygen or nitrogen activation gas.

[0043] The activation step of the dielectric surface layer is followed by the step of bringing the main faces of the two substrates into contact, then by the step of initiating and propagating a bonding wave between the main faces of the two substrates to assemble them together.

[0044] A cleaning step for the main face of the substrate can be provided between the activation step and the contacting step. This cleaning step may consist of, or include, brushing this face while pouring a liquid onto it, such as demineralized water or a solution of SCI (acronym for "standard clean 1"). Advantageously, the cleaning step is applied to the main faces of both substrates directly before they are assembled. Application example

[0045] The assembly process just described can find multiple applications. In particular, it can be used in a double transfer process for a single layer, a detailed description of which can be found in document EP703609 or document WO0237556A1. As detailed in these documents, such a double transfer makes it possible to circumvent the problem related to differential expansion that arises during the direct transfer of a layer, taken from a so-called donor substrate, onto a final support, when this donor substrate and the final support have different coefficients of thermal expansion. In very general terms, this double transfer process aims to first transfer the thin film onto an intermediate support having a coefficient of thermal expansion equal to or close to that of the donor substrate. of the thin film. The thin film is bonded to the intermediate support at a first adhesion interface by molecular bonding, exhibiting a first adhesion energy. The free face of the thin film is then bonded to the final support at a second adhesion interface, also by molecular bonding, this second interface exhibiting a second adhesion energy, higher than the first. In a subsequent step, the final support and / or the intermediate support, between which the thin film is sandwiched, are mechanically stressed to detach the intermediate support from the thin film at the first interface, which has the lowest adhesion energy, thus completing the transfer of the thin film onto the final support.

[0046] By way of illustration, we wish to form a composite structure S, shown in [Fig. 5], to form radio frequency components. The composite structure S is formed of a piezoelectric thin layer 7, for example of lithium tantalate, on a final support 11 of silicon. Silicon has a coefficient of thermal expansion estimated at 2.6 x 10⁶ K. Lithium tantalate, on the other hand, is anisotropic and has a coefficient of thermal expansion of 16.0 x 10⁶ K along the a11 axis and 4.0 x 10⁶ K along the a33 axis. Regardless of the crystallographic axis considered, these two materials have very different coefficients of thermal expansion, which makes the fabrication of this structure by a "direct" approach problematic.

[0047] To avoid any ambiguity, the expression "coefficient of thermal expansion" used in this description with respect to a layer or substrate refers to the coefficient of expansion along a defined direction in the principal plane defining that layer or substrate. If the material is anisotropic, the value of the coefficient used will be the one with the largest magnitude. The value of the coefficient is that measured at room temperature. When it is stated that these coefficients are "different," it is meant that their difference exceeds 10% of the smaller coefficient. Conversely, when it is stated that these coefficients are identical, it is meant that their difference is less than or equal to 10% of the smaller coefficient.

[0048] In this radio frequency application in which the thin film 7 is made of piezoelectric material, the final support 11 may advantageously comprise a base substrate 1la of monocrystalline silicon, which may have a high resistivity, greater than 1000 ohms. The base substrate 1la may be provided with an electrical charge trapping layer 11b, for example, a layer of polycrystalline silicon. In such a case, the trapping layer 11b is disposed in the composite substrate S between the piezoelectric thin film 7 and the base substrate 1la. The composite substrate may also comprise a buried dielectric layer 12 disposed between the piezoelectric thin film 7 and the final support 11, in contact between This thin layer 7 and the charge-trapping layer 11b, when present, are also present. Advantageously, the buried dielectric layer 12 is made of silicon dioxide or silicon oxynitride. The latter material, in particular, forms a diffusion barrier for certain species contained in the thin layer 7 towards the charge-trapping layer 11b (hydrogen, lithium, for example), which can degrade its electrical trapping properties.

[0049] To manufacture such a composite substrate S, a process of transferring the thin layer 7 onto the final support 11 is implemented, using a double transfer of this layer, which takes advantage of the molecular adhesion assembly process which was the subject of the first section of this description.

[0050] More specifically, and with reference to Figures 4a to 4g, an intermediate structure Si is first formed comprising the thin layer 7 bonded to an intermediate support 5 at a first adhesion interface IA1. The thin layer 7 and the intermediate support 5 have identical coefficients of thermal expansion. For example, the thin layer can be taken from a lithium tantalate donor substrate and transferred onto an intermediate support 5 made of a bulk lithium tantalate substrate.

[0051] The transfer can be implemented using Smart Cut® technology, as illustrated in Figures 4a to 4c. This technology, well known to those skilled in the art, involves transferring the thin film by assembling the donor substrate 6 and the intermediate support 5. A dielectric layer 8, typically a silicon oxide, is pre-formed on the donor substrate 6 and / or on the intermediate support 5 and is intercalated, in the intermediate Si structure, between the transferred thin film 7 and the support 5. The thin film is defined in the donor substrate by the introduction of light species, usually by implanting hydrogen and / or helium ions, these species tending to form a weak zone 9 delimiting the thin film.The detachment of the thin layer 7 from the rest of the donor substrate 6, and its transfer onto the support, is obtained by mechanically or thermally stressing the assembly formed by the donor substrate and the support ([Fig.4c]), which causes the fracture of the donor substrate at the level of the fragile zone 9. .

[0052] As an alternative to implementing Smart Cut® technology, the donor substrate can be thinned mechanically and / or chemically after assembly with the intermediate support to define the thin film. This approach also incorporates a dielectric layer on the donor substrate and / or the intermediate support to facilitate assembly.

[0053] Whether the thin layer is formed by detachment at a fragile area of ​​the donor substrate or by thinning of the donor substrate, its transfer requires that the donor substrate 6 be made bonded to the intermediate support 5 at a The first Al adhesion interface exhibits sufficient energy. To this end, at least one face of the donor substrate 6 and the intermediate support 5, which are intended to be joined, can be activated by plasma. Moderate annealing of the intermediate Si structure can also be performed before the detachment of the thin film 7 to enhance the adhesion energy and facilitate the transfer of the thin film.

[0054] It is noted that, since the donor substrate 6 from which the thin film 7 is taken and the intermediate support have the same coefficients of thermal expansion, this preparation step of the intermediate support 5 can include heat treatment steps without generating stresses likely to damage it. In the case of a lithium tantalate donor substrate and intermediate support, the assembly formed by the donor substrate 6 and the intermediate support 7 can be heated to a temperature of 200°C to 300°C, sufficient to cause the fracture of the donor substrate and the transfer of the thin film 7 in a transfer process implemented according to Smart Cut® technology. This temperature is higher than that which could have been applied to an assembly formed by the donor substrate 5 and the final silicon substrate 11, due to the large difference in coefficient of thermal expansion.

[0055] It is possible to plan, at the end of the thin film transfer step 7 ([Fig.4c]), to apply a finishing step, in particular when it has been transferred according to Smart Cut® technology, to restore the crystallographic and surface quality of the thin film 7. This may involve annealing this layer under a neutral or oxidizing atmosphere and / or thinning it, for example by mechanochemical polishing or by ion etching.

[0056] Continuing the description of the transfer process, which is the subject of this part of the description, it then involves ([Fig. 4d]) forming a surface dielectric layer 10 on the exposed face of the thin layer 7 of the intermediate structure Si. This surface dielectric layer 10 is intended to form, at least in part, the buried dielectric layer 12 of the composite substrate S. It also allows for the deployment of the molecular activation and adhesion step that was presented in the previous section of the description. Furthermore, this surface dielectric layer 10 can be made of silicon oxide or silicon oxynitride. It is advantageously deposited, for example, using a PECVD deposition technique. It can have any thickness, preferably between 1 nm and 50 nm, and even more preferably between 30 nm and 90 nm.In such a range, the surface dielectric layer has sufficient thickness to compensate for the thinning that may occur during the subsequent activation step and does not exhibit excessive roughness, which could not be sufficiently reduced during this activation step.

[0057] Preferably, and when the surface dielectric layer 10 is chosen within the aforementioned thickness range, the transfer process is free of any treatment, other than the activation step, to reduce its roughness. In this case, it is notably free of any mechano-chemical polishing step aimed at reducing the roughness of the surface dielectric layer 11, which is highly advantageous.

[0058] In a subsequent step illustrated in [Fig. 4e], the dielectric surface layer 10 is activated by exposing it to a plasma formed between two electrodes subjected to a radiofrequency power density strictly greater than 0.8 W / cm² in an activation chamber into which a controlled flow of a sulfur-containing gas is introduced. The frequency of the radiofrequency signal can be chosen at 13 MHz or on the order of 13 MHz, within 25%. As presented in the preceding section of this description, such activation makes it possible to prepare the surface, in particular by reducing roughness, and to produce a particularly high adhesion energy during the subsequent molecular adhesion step.

[0059] All the variants described in the first section of this description can be implemented within the framework of this application. In particular, plasma exposure can be conducted for an activation period of between 15 seconds and 2 minutes, during which, in addition to the sulfur-containing gas, a controlled flow of oxygen or nitrogen is introduced into the activation chamber. The controlled flow of a sulfur-containing gas, which may be sulfur hexafluoride, is introduced into the activation chamber for a predetermined period of the activation time, for example, a period of less than 10 seconds. This period can be placed in the middle of the activation time and can be preceded and followed by periods during which only the flow of oxygen or nitrogen is introduced into the activation chamber.It is also possible to control the vacuum level present in the activation chamber during the activation period, for example to place it at a pressure below 50 milli-Torr (6.66 Pa), preferably at 30 milli-Torr (4 Pa).

[0060] Regardless of the precise conditions under which the activation step of the surface dielectric layer 10 is carried out, this step is followed by the assembly of the thin film 7, via the surface dielectric layer 10, to the final support 11 ([Fig. 4f]). A second adhesion interface IA2 of the thin film 7 is thus defined; this second adhesion interface IA2, due to the activation step of the surface dielectric layer 10, exhibits an adhesion energy greater than that of the first interface IA1 which retains the thin film 7 to the intermediate support 5.

[0061] In the illustrative example, the final support 11 is composed of a basic substrate 1la made of monocrystalline silicon provided with an electrical charge trapping layer 11b. It can be assumed that this trapping layer 11b has been previously provided itself with a dielectric layer, for example by oxidation or by deposition. This dielectric layer, when present, combines with the surface dielectric layer 10 formed on the thin layer 7 to constitute the buried dielectric layer 12 of the composite substrate S.

[0062] This assembly implements a molecular adhesion technique, i.e. the contacting of the final support 11 and the surface dielectric layer 10, then the initiation and propagation of a bonding wave between the main faces brought into contact to assemble them together.

[0063] The final support 11 can be activated by plasma before its assembly by molecular adhesion to the surface dielectric layer 10. This plasma activation can be identical to that applied to the surface dielectric layer 10 or different. For example, it could involve exposing this final support 11 to a plasma composed of oxygen or nitrogen, without introducing a controlled quantity of a sulfur-containing gas into the activation chamber.

[0064] In all cases, at the end of this assembly step shown in [Fig. 4f], the thin film 7 is sandwiched between, on the one hand, the intermediate support 5, at the level of a first adhesion interface IA1, and on the other hand, the final support 11, at the level of a second adhesion interface IA2. Due to the particular activation step of the surface layer, the adhesion energy of the second interface is greater than that of the first interface.

[0065] To increase the adhesion energy of the second interface IA2, in this example, the structure comprising the temporary support and the final support can be exposed to a temperature not exceeding 100°C.

[0066] A final step of the transfer process, shown in [Fig. 4g], aims to dismantle the intermediate support from the sandwich structure in which the thin film is placed, thereby completing the transfer of the thin film 7 onto the final support 11. To this end, the final support and / or the intermediate support are mechanically stressed. This mechanical stress tends to cause the adhesion interface with the lowest energy, i.e., the first interface IA1, to break down, thus freeing the thin film 7 from the intermediate support 5.

[0067] The mechanical stressing step may include the insertion of a blade, or more generally a tool, between the intermediate support 5 and the final support 11 and the application by the blade or by the tool of a force separating the two supports 5,11.

[0068] We can then eliminate the dielectric layer 8, or residues of this layer, which can cover the thin layer 7, to finalize the preparation of the composite substrate S as illustrated in [Fig.5].

[0069] Of course the invention is not limited to the modes of implementation described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

[0070] Thus, the application just described is by no means limited to a thin film 7 made of lithium tantalate. It applies more generally to a thin film 7 made of any material. When this material is chosen to exhibit piezoelectric properties, as is the case in the example described, it can also be lithium niobate.

[0071] Although sulfur hexafluoride has been taken here as an example of a gas containing sulfur (and fluorine), the invention is by no means limited to the use of this gas. In particular, any sulfur-containing gas such as sulfuryl fluoride (SO2F2) or thionyl tetrafluoride (SO2F4) may be used as a replacement.

[0072] The activation step according to the invention can be applied to a single dielectric surface layer formed on one of the two substrates to be assembled, even if both substrates have such a dielectric surface layer. The activation step can also be applied to each of the substrates, in which case each is provided with a dielectric surface layer.

[0073] This surface dielectric layer can be made of SiO2, as given as an example in the present description. But it can more generally be a dielectric layer of any kind, for example, SiON or silicon nitride.

Claims

Demands

1. A method for transferring a thin film (7) onto a final substrate (11), the thin film (7) and the final substrate (11) having different coefficients of thermal expansion, and the method comprising: a. form an intermediate structure (Si) comprising the thin layer (7) assembled to an intermediate support (5) at the level of a first adhesion interface (IA1) of the thin layer, the thin layer (7) and the intermediate support (5) having identical coefficients of thermal expansion; b. form a dielectric surface layer (10) on the exposed face of the thin layer (7) of the intermediate structure (Si); c. activate the dielectric surface layer (10) by exposing it to a plasma formed between two electrodes subjected to a radiofrequency power density strictly greater than 0.8 W / cmA2 of an activation chamber into which a controlled flow of a gas comprising sulfur is introduced; d. assemble the thin film (7) via the dielectric surface layer (10) to the final support (11) and thus define a second adhesion interface (IA2) of the thin film, the second adhesion interface (IA2) having, due to the activation step of the surface layer (10), an adhesion energy greater than that of the first adhesion interface (IA1); e. mechanically stress the final support (11) and / or the intermediate support (5) to dismantle the intermediate support (5) from the thin film (7) at the level of the first adhesion interface (IA1), and thus complete the transfer of the thin film (7) onto the final support (11).

2. Transfer method according to the preceding claim wherein the formation of the intermediate structure (Si) comprises the assembly of the thin layer (7) to the intermediate support (5) by molecular adhesion.

3. Transfer method according to the preceding claim wherein the thin layer (7) is taken from a donor substrate (6) and the formation of the intermediate structure (Si) comprises the assembly of the donor substrate (6) to the intermediate support (5).

4. Transfer method according to the preceding claim wherein the formation of the intermediate structure (Si) comprises a heat treatment of the assembly formed from the donor substrate (6) and the intermediate support (5).

5. Transfer method according to any one of the preceding claims further comprising the formation of another dielectric surface layer on the exposed face of the final support (11).

6. Transfer method according to the preceding claim comprising an activation of the other dielectric surface layer before the step of assembling the thin film (7) to the final support (11), by exposing it to a plasma.

7. A transfer method according to the preceding claim in which the activation of the other dielectric layer comprises its exposure to a plasma formed between two electrodes subjected to a radiofrequency power density strictly greater than 0.8W / cmA2 of an activation chamber into which a controlled flow of a gas comprising sulfur is introduced.

8. A transfer method according to any one of the preceding claims, wherein the mechanical stressing step comprises the insertion of a blade between the intermediate support (5) and the final support (11) and the application by said blade of a spreading force.

9. A transfer method according to any one of the preceding claims wherein the thin layer (7) and the intermediate support (5) are made of the same piezoelectric material.

10. A transfer method according to the preceding claim wherein the piezoelectric material is lithium tantalate or lithium niobate.

11. A transfer method according to any one of the preceding claims wherein the final support (11) comprises a base substrate (1a) on which resides an electrical charge trapping layer (11b).

12. A transfer method according to any one of the preceding claims wherein the dielectric surface layer (10) is a silicon oxide or a silicon oxynitride.

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