PROCESS OF ASSEMBLY OF TWO SUBSTRATES BY MOLECULAR ADHESION

Abstract

The invention relates to a method for assembling two substrates by molecular adhesion, at least one of the two substrates being provided with a dielectric surface layer. The method comprises activating the dielectric surface layer by exposure to a plasma formed between two electrodes (4a, 4b) of an activation chamber (3), for an activation time of between 15 seconds and 2 minutes and during which radiofrequency power is applied to one of the electrodes. The method comprises introducing into the activation chamber (3) a controlled flow of oxygen or nitrogen and a controlled flow of a gas comprising sulfur. The method is remarkable in that the radiofrequency power has a density strictly greater than 0.8 W / cm^2. Figure to be published with the abstract: Fig. 3

Description

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

[0001] The invention relates to a method for assembling two substrates by molecular adhesion. It finds its application in particular in the fields of microelectronics, microsystems, photonics, etc. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Molecular adhesion is a process for assembling two bodies during which the faces or main 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 of the van der Waals or covalent type. The assembly of the two bodies is then obtained without the use of an adhesive. These bonds can be reinforced by the application of 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, etc. In these fields, the substrates generally have the form of circular plates of materials which can be monocrystalline, polycrystalline or amorphous, conductive, semiconductive or insulating.

[0004] In these fields, and to facilitate the assembly of the substrates, it is usual to form a dielectric surface layer, typically of silicon oxide, on one and / or the other 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 usual to prepare the main faces to make them clean (in particular in terms of particles and contaminants) and smooth (to provide a surface roughness typically below 0.5 nm in mean square value measured by atomic force microscopy over a field of observation of 5 microns by 5 microns).

[0005] After their preparation, the two substrates are placed on top of each other, at their main faces. The application of pressure to one or other of the two substrates causes the main faces to come into intimate and punctual contact, and the propagation of this intimate contact over the entire extent of the main 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 oxygen or nitrogen. This surface preparation aims to increase the bonding energy between the two assembled substrates. In this regard, reference may be made to the publication by Paul Lindner "Plasma activated wafer bonding for thin Silicon on insulator substrate fabrication." Proceedings of the Electro-chemical Society, 2004. [Fig.l] shows equipment 1 for applying such a plasma treatment to a substrate 2 for activation before bonding. This equipment comprises an activation chamber 3, generally placed at sub-atmospheric pressure and 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 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 a step of activation of the dielectric surface layer which covers at least one of the substrates to introduce, into this layer, a dose of sulfur greater than 3.0 E13 at / cmA2. SUBJECT OF THE INVENTION

[0008] An aim of the invention is to improve the approach proposed by this document. BRIEF DESCRIPTION OF THE INVENTION

[0009] With a view to achieving this aim, the subject of the invention proposes a method of assembling by molecular adhesion two substrates each having a main face, at least one of the two substrates being provided with a dielectric surface layer on the side of its main face.

[0010] According to the invention, the method comprises the following steps:

[0011] (a) activating the dielectric surface layer, the activation comprising exposing the dielectric surface layer to a plasma formed between two electrodes of an activation chamber, the exposure to the plasma being conducted for an activation time of between 15 seconds and 2 minutes during which radiofrequency power is applied to one of the electrodes and comprising introducing into the activation chamber a controlled flow of oxygen or nitrogen and a controlled flow of a gas comprising sulfur;

[0012] (b) bringing the main faces of the two substrates into contact, then;

[0013] (c) the initiation and propagation of a bonding wave between the main faces of the two substrates to join them together.

[0014] This process is remarkable in that the radiofrequency power has a density strictly greater than 0.8 W / cmA2.

[0015] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: - the radiofrequency power density is between 1.4 W / cmA2 and 1.7 W / cmA2 ; the activation chamber is placed at a pressure lower than 50 milli-Torr (6.66 Pa), preferably 30 milli-Torr (4 Pa); the controlled flow of oxygen or nitrogen is formed from an oxygen flow of 75 sccm; the gas containing sulfur is sulfur hexafluoride; the controlled flow of sulfur hexafluoride is formed from a flow lower than 7 sccm, for example 4 sccm; the controlled flow of sulfur hexafluoride is introduced into the activation chamber for a determined period of the activation time; the determined period has a duration of less than 10s; the determined period is respectively preceded and followed by periods during which only the flow of oxygen or nitrogen is introduced into the activation chamber; the assembly method comprises a step of depositing the dielectric surface layer on at least one of the two substrates; the deposition is carried out using a plasma-assisted chemical vapor deposition technique; the dielectric surface layer has a thickness of less than 90 nm; the activation step is the only step applied to the dielectric surface layer to reduce its roughness. BRIEF DESCRIPTION OF THE FIGURES

[0016] 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:

[0017] [Fig.l]

[0018] [Fig.l] represents plasma activation equipment capable of implementing an activation step of a method according to the invention;

[0019] [Fig.2]

[0020] [Fig.2] represents a graph relating the thickness of a deposited surface layer of silicon oxide to its roughness, as measured by X-ray reflectivity, before and after the application of a plasma activation comprising a fluorinated species;

[0021] [Fig.3]

[0022] [Fig.3] shows 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, radiofrequency power, and SF6 flow rate);

[0023] [Fig.4a] [Fig.4b] [Fig.4c] [Fig.4d] [Fig.4e] [Fig.4f] [Fig.4g]

[0024] Figures 4a, 4b, 4c, 4d, 4e, 4f and 4g illustrate an application of a method assembly according to the invention to form a composite substrate;

[0025] [Fig.5]

[0026] [Fig. 5] represents a composite substrate which can be formed by applying an assembly method according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] The applicant has continued the studies reported in document WO2023202917A1 relating to a method of assembly by molecular adhesion of two substrates each having a main face. At least one of the two substrates is provided with a dielectric surface layer on the side of its main face. This method comprises the steps of bringing the main faces of the two substrates into contact, then initiating and propagating a bonding wave between the main faces of the two substrates to assemble them to each other.

[0028] According to this document, the method comprises, before the contacting step, a step of preparing 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 significantly increasing the bonding energy of the two assembled substrates. When the dose of sulfur exceeds the aforementioned dose, the bonding wave propagates with a reduced speed which leads to eliminating, or at least significantly decreasing, the peripheral bonding defects (often referred to as "pins") which develop at the bonding interface, at the locations where the bonding wave meets the edge of the substrates.

[0029] In a particular embodiment, the preparation step comprises activating the dielectric surface layer with a nitrogen or oxygen plasma comprising sulfur hexafluoride (SF6). This activation is carried out for an activation time 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 determined period of the activation time which is preceded and followed by periods during which the dielectric surface layer is exposed to the plasma consisting of oxygen or nitrogen.

[0030] More specifically, under so-called "reference" application conditions, the activation of the dielectric surface layer comprises the introduction into the activation chamber of oxygen (02) at a flow rate of 75 sccm (standard cubic centimeter per minute) for a period of 30 seconds. Within this 30-second period, approximately 11 seconds after its start, a flow rate of 5 sccm of SF6 is introduced for a period of 5 seconds. The activation chamber is maintained during this treatment at 50 mTor (6.66 Pa) and a radiofrequency power of 150W is applied to one of the chamber electrodes.

[0031] 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.

[0032] Roughness is usually measured by atomic force microscopy. According to this technique, a nanometric-sized tip carried by a flexible arm is scanned over a measurement field of the surface to be characterized, this field having a dimension of the order of one micrometer to a few micrometers on each side. The surface topography causes the arm to flex during this scanning. The measurement of this flexion makes it possible to establish a measurement of surface roughness over the extent of the measurement field. The measurement has a resolution of less than 1 nm vertically, but of the order of 5 to 10 nm laterally, mainly linked to the dimension of the measuring tip.

[0033] The experiments reported in the aforementioned document WO2023202917A1 were carried out in particular on lithium tantalate wafers on which a 30nm thick surface layer of silicon oxide had previously been formed by PECVD (acronym for the English expression "Plasma Enhanced Chemical Vapor Deposition" or plasma-assisted chemical vapor deposition). Due to the electrically insulating nature of this wafer, the silicon oxide layer tends to become electrically charged, which does not easily allow roughness measurement to be carried out by atomic force microscopy, the measuring tip tending to be electrostatically retained on the surface of the wafer, which disrupts the measurement.Furthermore, a deposited layer of silicon oxide 30nm thick generally has a relatively low roughness, below the limit allowing the wafer to be bonded by molecular adhesion, so the question of roughness is not addressed by document WO2023202917A1. Experiments

[0034] To analyze more precisely the effect of the activation of the dielectric surface layer, and in particular on its roughness, the applicant carried out roughness measurements by X-ray reflectivity (technique designated XRR in the scientific literature of the field). The basic principle of this technique consists of projecting a beam of X-rays onto the surface to be characterized and measuring the intensity of the reflected X-rays. The measurement field is typically of the order of mmA2. Using a simulation of the reflectivity model, a very precise measurement of the thickness of a layer, surface and interface roughness can be obtained. The short wavelength, of the order of 0.1 nm, of the X-rays makes it possible to achieve atomic resolution, although the measurement is located in reciprocal space, which is different from measurements provided in real space by the AFM. This measurement is not affected by any possible electrical charge present in the surface layer.

[0035] The XRR roughness measurement observations are reported on the graph in [Fig.2]. This represents the measured roughness R of a dielectric layer (on the abscissa axis in arbitrary units) according to the thickness of this layer T (on the ordinate axis in nm). The reported measurements concern a layer of silicon dioxide deposited on a silicon substrate by PECVD. Curve A represents the globally linear relationship linking the thickness T and the roughness R of the layer just after its deposition and curve B represents the XRR measurements after the activation step presenting the reference application conditions presented above. The roughness line C of the layer as deposited (curve A) has been placed on this graph above which the surface conditions do not allow, even after the smoothing obtained by application of the activation step, to obtain a bonding by molecular adhesion of sufficient quality.

[0036] 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 radiofrequency power applied to one of the electrodes and the vacuum level of the chamber, on the roughness of a dielectric surface layer.

[0037] These experiments were carried out on 150 mm diameter silicon wafers on which a 50 nm layer of silicon oxide was deposited by PECVD. Each wafer of a pair of these wafers underwent an activation step before the wafers of the pair were assembled together, by molecular adhesion. The silicon oxide layers did not undergo any other treatment aimed at reducing their roughness, in particular they were not treated by polishing. The bonding energy was measured after a strengthening heat treatment at 180°C for 60 min, by a blade insertion method (or "double cantilever beam" according to the Anglo-Saxon expression sometimes used) in an anhydrous environment.

[0038] 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 for a period of 5 seconds were varied in relation to the reference conditions. The measured XRR roughness and the average bonding energy measured after a heat treatment of 100°C-1h applied to the assembly were also reported in the table. For completeness, it is specified that the frequency of the radiofrequency signal applied to one of the electrodes was 13 MHz. Ref Pressure Power Flow Roughness Roughness Energy of (mT) (W) SF6 (sccm) XRR pre activation (in A) XRR post activation (in A) anhydrous bonding (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

[0039] 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).

[0040] It is observed that in this [Fig.3] 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 is carried out at a relatively high power, strictly greater than 150W (i.e. a power density strictly greater than 0.8 W / cmA2 by bringing the power back 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).

[0041] The smoothing effect is also sensitive, to a lesser extent, to the pressure prevailing in the chamber, a relatively low pressure leading to favoring the smoothing effect. Also, and advantageously, this pressure is chosen to be placed at a pressure lower than 50 milli-Torr (6.66 Pa), preferably lower than 30 milli-Torr (4 Pa).

[0042] Finally, we observe that the smoothing effect appears to be very insensitive to the SF6 flow rate, which can therefore be chosen freely, for example in a relatively low flow rate of 4 sccm.

[0043] It is noted that the bonding energy, for all the bonds that were analyzed, is particularly high, in accordance with the teachings of document WO2023202917A1. This is particularly the case for the bonding of the wafers in line 14 of the table, having received activation with the highest power of 300W. Assembly method

[0044] Advantage may be taken of the results which have just been presented in a sequence formed of the steps of bringing the main faces of the two substrates into contact, of initiation and of propagation of a bonding wave which have been presented previously. For this, provision is made, before the step of bringing into contact, for a step of activating the dielectric surface layer comprising the exposure of 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 may be chosen relatively freely, for example less than 90 nm or between 30 nm and 90 nm, because the surface roughness which may be relatively significant for the thickest deposits will be rectified by the activation step, due to its smoothing effect.

[0045] The pressure prevailing in the activation chamber can be controlled to be less than 50 milli-Torr (6.66 Pa), preferably less than 30 milli-Torr (4 Pa). A controlled flow of oxygen or nitrogen and a controlled flow of a gas comprising sulfur are introduced into the chamber. This controlled flow of oxygen or nitrogen can be equal to, or of the order of, 75 sccm. The controlled flow of the gas comprising sulfur can be between 3 sccm and 7 sccm of SF6.

[0046] The exposure is carried out for an activation time 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.

[0047] The gas comprising sulfur, for example SF6, may be introduced with a controlled flow rate for this entire duration, or for only part of this duration, to mix with the oxygen or nitrogen for a determined period of the activation duration. Preferably, this determined period does not include the moment of ignition of the plasma, because this ignition can lead to the introduction of the sulfur species of the plasma into the surface layer in a non-repeatable manner, which can be detrimental to the proper control of the process. Advantageously therefore, the determined period during which the dielectric surface layer is exposed to the plasma formed from a gas containing sulfur 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.

[0048] The step of activating the dielectric surface layer is followed by the step of bringing the main faces of the two substrates into contact, then the step of initiating and propagating a bonding wave between the main faces of the two substrates to assemble them together.

[0049] A cleaning step of the main face of the substrate may be provided, between the activation step and the contacting step. This cleaning step may correspond to or include brushing this face while pouring a liquid thereon, such as demineralized water or an SCI solution (acronym for “standard clean 1” or standard cleaning 1). Advantageously, the cleaning step is applied to the main faces of the two substrates, directly before their assembly. Application example

[0050] The assembly method which has just been presented can find multiple applications. It can in particular be used in a method of double transfer of a 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 linked to the differential expansion which appears 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 a very general manner, this double transfer method aims to first transfer the thin layer onto an intermediate support having a coefficient of thermal expansion equal to or close to that of the thin layer.The thin layer is assembled to the intermediate support at a first adhesion interface, by molecular bonding, having a first adhesion energy. The free face of the thin layer is assembled to the final support at a second adhesion interface, also by molecular bonding, this second interface having a second adhesion energy, higher than the first. In a following step, the final support and / or the intermediate support, between which the thin layer is sandwiched, are mechanically stressed to dismantle the intermediate support from the thin layer at the first interface which has the lowest adhesion energy, and thus complete the transfer of the thin layer to the final support.

[0051] By way of illustration, it is desired to form a composite structure S, shown in [Fig. 5], to form radiofrequency components therein, the composite structure S being formed of a thin piezoelectric layer 7, for example made of lithium tantalate, on a final support 11 made of silicon. Silicon has a thermal expansion coefficient estimated at 2.6 x 10 6 K-1. Lithium tantalate is anisotropic and has a thermal expansion coefficient of 16.0 x 10 6 K 1 along the axis a1 1 and 4.0 x 10 6 K 1 along the axis a33. Whatever the crystallographic axis considered, these two materials have very different thermal expansion coefficients, which which makes the production of this structure by a "direct" approach problematic.

[0052] To avoid any ambiguity, the expression "coefficient of thermal expansion" used in the present description in relation to a layer or a substrate refers to the coefficient of expansion in a direction defined in the principal plane defining this layer or this substrate. If the material is anisotropic, the value of the coefficient retained will be that of greatest amplitude. The value of the coefficient is that measured at room temperature. When it is indicated that these coefficients are "different", it means that their difference exceeds 10% of the smallest coefficient. Conversely, when it is indicated that these coefficients are identical, it means that their difference is less than or equal to 10% of the smallest coefficient.

[0053] In this radiofrequency application in which the thin layer 7 is made of piezoelectric material, the final support 11 can advantageously comprise a base substrate 11a made of monocrystalline silicon, which can have a high resistivity, greater than 1000 ohm.com. The base substrate 11a can be provided with an electric charge trapping layer 11b, for example a polycrystalline silicon layer. In such a case, the trapping layer 11b is arranged in the composite substrate S between the piezoelectric thin layer 7 and the base substrate 11a. The composite substrate can also comprise a buried dielectric layer 12 arranged between the piezoelectric thin layer 7 and the final support 11, in contact between this thin layer 7 and the charge trapping layer 11b when this layer is present. Advantageously, the buried dielectric layer 12 is made of silicon oxide or silicon oxynitride.This latter material makes it possible in particular to form 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 qualities.

[0054] To manufacture such a composite substrate S, a method 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 method which was the subject of the first section of this description.

[0055] More precisely, and with reference to FIGS. 4a to 4g, an intermediate structure Si is first formed comprising the thin layer 7 assembled to an intermediate support 5 at a first adhesion interface IA1. The thin layer 7 and the intermediate support 5 have identical thermal expansion coefficients. For example, the thin layer can be taken from a donor substrate made of lithium tantalate and transferred to an intermediate support 5 made of a solid lithium tantalate substrate.

[0056] The postponement can be implemented via Smart Cut™ technology, as illustrated in Figures 4a to 4c. This technology, well known in the person skilled in the art, proposes to transfer the thin layer by assembling the donor substrate 6 and the intermediate support 5. A dielectric layer 8, typically a silicon oxide, is previously formed on the donor substrate 6 and / or on the intermediate support 5 and is intercalated, in the intermediate structure Si, between the transferred thin layer 7 and the support 5. The thin layer is defined in the donor substrate by introducing light species, usually by implantation of hydrogen and / or helium ions, these species tending to form a fragile zone 9 delimiting the thin layer. The detachment of the thin layer 7 from the rest of the donor substrate 6, and its transfer to 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 fragile zone 9.

[0057] Alternatively to the implementation of Smart Cut technology, it is possible to provide mechanically and / or chemically thinning the donor substrate, after its assembly to the intermediate support, to define the thin layer. In this approach also, a dielectric layer is provided on the donor substrate and / or on the intermediate support to facilitate assembly.

[0058] Whether the thin layer is formed by detachment at a fragile zone of the donor substrate or by thinning of this donor substrate, its transfer requires that the donor substrate 6 be made integral with the intermediate support 5 at a first adhesion interface Al which has sufficient energy. For this purpose, it is possible to provide for plasma activation of at least one of the faces of the donor substrate 6 and of the intermediate support 5 which are intended to be assembled. It is also possible to provide moderate annealing of the intermediate structure Si, before the detachment of the thin layer 7, in order to reinforce the adhesion energy and allow the transfer of the thin layer.

[0059] It is noted that, the donor substrate 6 from which the thin layer 7 is taken and the intermediate support having the same coefficients of thermal expansion, this step of preparing the intermediate support 5 can comprise heat treatment steps without generating stresses likely to damage it. In the case of a donor substrate and an intermediate support made of lithium tantalate, the assembly formed of the donor substrate 6 and the intermediate support 7 can be raised to a temperature of 200°C to 300°C, sufficient to cause the fracture of the donor substrate and the transfer of the thin layer 7 in the context of a transfer implemented according to the Smart Cut technology. This temperature is higher than that which could have been applied to an assembly formed of the donor substrate 5 and the final substrate 11 made of silicon, due to the large difference in coefficient of thermal expansion.

[0060] It is possible to provide at the end of the step of transferring the thin layer 7 ([Fig.4c]) to apply a finishing step, in particular when this has been transferred according to the Smart Cut technology, to restore the crystallographic and surface quality of the thin layer 7. This may involve annealing this layer under a neutral or oxidizing atmosphere and / or thinning it, for example by chemical-mechanical polishing or ion etching.

[0061] Continuing the description of the transfer method which is the subject of this part of the description, it then provides ([Fig.4d]) to form a dielectric surface layer 10 on the exposed face of the thin layer 7 of the intermediate structure Si. This dielectric surface layer 10 is intended to form, at least in part, the buried dielectric layer 12 of the composite substrate S. It also makes it possible to deploy the activation and molecular adhesion step which was presented in the previous section of the description. Also, this dielectric surface layer 10 can be made of silicon oxide or silicon oxynitride. It is advantageously deposited, for example according to a PECVD deposition technique. It can have any thickness, and preferably between 10 nm and 50 nm, and even more preferably between, strictly, 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 next activation step and does not have excessive roughness, which could not be sufficiently reduced during this activation step.

[0062] Preferably, and when the surface dielectric layer 10 is chosen in the aforementioned thickness range, the transfer method is devoid of any treatment, other than the activation step, to reduce the roughness thereof. In this case, it is in particular devoid of any chemical-mechanical polishing step aimed at reducing the roughness of the surface dielectric layer 11, which is very advantageous.

[0063] In a following 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 / cmA2 of an activation chamber into which a controlled flow of a gas comprising sulfur is introduced. The frequency of the radiofrequency signal can be chosen at 13 MHz or of the order of 13 MHz, to within 25%. As presented in the previous section of this description, such activation makes it possible to prepare the surface, in particular by reducing the roughness, and to produce a particularly high adhesion energy during the following molecular adhesion step.

[0064] All the variants described in the first section of this description can be implemented within the framework of the present application. The exposure to the plasma can in particular be carried out for an activation time of between 15 seconds and 2 minutes during which, in addition to the gas comprising sulfur, a controlled flow of oxygen or nitrogen is introduced into the activation chamber. The controlled flow of a gas comprising sulfur, this gas possibly being sulfur hexafluoride, is introduced into the activation chamber for a determined period of the activation time, for example a period of less than 10s. This period can be placed in the middle of the activation time, and 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 of less than 50 milli-Torr (6.66 Pa), preferably 30 milli-Torr (4 Pa).

[0065] Whatever the precise conditions under which the step of activating the surface dielectric layer 10 is carried out, this is followed by the assembly of the thin layer 7, via the surface dielectric layer 10, to the final support 11 ([Fig.4f]). A second adhesion interface IA2 of the thin layer 7 is thus defined, this second adhesion interface IA2 having, due to the step of activating the surface dielectric layer 10, an adhesion energy greater than that of the first interface IA1 which retains the thin layer 7 to the intermediate support 5.

[0066] In the context of the example illustrated, the final support 11 is composed of a base substrate 11a made of monocrystalline silicon provided with an electric charge trapping layer 11b. It can be provided that this trapping layer 11b has itself been previously provided 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.

[0067] This assembly uses a molecular adhesion technique, i.e. bringing the final support 11 and the surface dielectric layer 10 into contact, then initiating and propagating a bonding wave between the main faces brought into contact to assemble them to each other.

[0068] It is possible to provide for plasma activation of the final support 11 before its assembly by molecular adhesion to the surface dielectric layer 10. This plasma activation may be identical to that applied to the surface dielectric layer 10 or different. For example, it may involve exposing this final support 11 to a plasma consisting of oxygen or nitrogen, and without introducing into the activation chamber a controlled quantity of a gas comprising sulfur.

[0069] In all cases, at the end of this assembly step shown in [Fig.4f], the thin layer 7 is sandwiched between, on the one hand, the intermediate support 5, at a first adhesion interface IA1, and, on the other hand, the final support 11, at a second adhesion interface IA2. Due to the particular step of activating the surface layer, the adhesion energy of the second interface is higher than that of the first interface.

[0070] To strengthen the adhesion energy of the second interface IA2, it is possible, in this example, to expose the structure comprising the temporary support and the final support to a temperature not exceeding 100°C.

[0071] A final step of the method, shown in [Fig.4g], of transfer aims to dismantle the intermediate support from the sandwich structure in which the thin layer is arranged and thus complete the transfer of the thin layer 7 onto the final support 11. For this purpose, 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 release the thin layer 7 from the intermediate support 5.

[0072] The mechanical stressing step may comprise the insertion of a blade, or more generally of 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 to separate the two supports 5, 11.

[0073] The dielectric layer 8, or residues of this layer, which can cover the thin layer 7, can then be removed to finalize the preparation of the composite substrate S as illustrated in [Fig.5].

[0074] Of course, the invention is not limited to the embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

[0075] Thus, the application which has just been described is in no way limited to a thin layer 7 of lithium tantalate. It applies more generally to a thin layer 7 made of any material. When this material is chosen to have piezoelectric properties as is the case in the example described, it can also be lithium niobate.

[0076] Although sulfur hexafluoride has been taken here as an example as a gas comprising sulfur (and fluorine), the invention is in no way limited to the use of this gas. In particular, it is possible to use as a replacement any sulfur gas such as sulfuryl fluoride (SO2F2) or thionyl tetrafluoride (SO2F4).

[0077] 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 these two substrates comprise such a dielectric surface layer. The activation step can also be applied to each of the substrates, and in this case each is provided with a dielectric surface layer.

[0078] This surface dielectric layer can be formed from SiO2 as has been taken as an example in the present description. But it can more generally be a dielectric layer of any nature, for example SiON, or silicon nitride.

Claims

Claims

1. Method for assembling by molecular adhesion two substrates each having a main face, at least one of the two substrates being provided with a dielectric surface layer on the side of its main face, the method comprising the following steps: (a) activating the dielectric surface layer, the activation comprising exposing the dielectric surface layer to a plasma formed between two electrodes (4a, 4b) of an activation chamber (3), the exposure to the plasma being carried out for an activation time of between 15 seconds and 2 minutes during which radiofrequency power is applied to one of the electrodes and comprising introducing into the activation chamber (3) a controlled flow of oxygen or nitrogen and a controlled flow of a gas comprising sulfur; (b) bringing the main faces of the two substrates into contact, then;(b) the initiation and propagation of a bonding wave between the main faces of the two substrates to assemble them to each other; the method being characterized in that the radiofrequency power has a density strictly greater than 0.8 W / cmA2.;

2. Assembly method according to the preceding claim in which the radiofrequency power density is between 1.4 W / cmA2 and 1.7 W / cmA2.

3. Assembly method according to the preceding claim in which the activation chamber (3) is placed at a pressure lower than 50 milli-Torr (6.66 Pa), preferably 30 milli-Torr (4 Pa).

4. Assembly method according to one of the preceding claims in which the controlled flow of oxygen or nitrogen is formed from an oxygen flow of 75 sccm.

5. Assembly method according to one of the preceding claims in which the gas comprising sulfur is sulfur hexafluoride.

6. Assembly method according to the preceding claim in which the controlled flow of sulfur hexafluoride is formed from a flow of less than 7 sccm, for example 4 sccm.

7. Assembly method according to one of the preceding claims in which the controlled flow of sulfur hexafluoride is introduced into the activation chamber (3) for a determined period of the activation duration.

8. Assembly method according to the preceding claim in which the determined period has a duration of less than 10s.

9. Assembly method according to one of the two preceding claims in which the determined period is respectively preceded and followed by periods during which only the flow of oxygen or nitrogen is introduced into the activation chamber.

10. Assembly method according to one of the preceding claims comprising a step of depositing the dielectric surface layer on at least one of the two substrates.

11. Assembly method according to the preceding claim in which the deposition is carried out by a plasma-assisted chemical vapor deposition technique.

12. Assembly method according to one of the two preceding claims in which the dielectric surface layer has a thickness of less than 90nm.

13. Assembly method according to one of the preceding claims in which the activation step is the only step applied to the dielectric surface layer to reduce its roughness.

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