Method for assembling two substrates by molecular adhesion and structure obtained thereby

Abstract

The invention relates to a method for assembling two substrates by molecular adhesion, each substrate having a main surface, at least one of the two substrates bearing a dielectric surface layer on its main surface. The method includes (a) contacting the major surfaces of the two substrates, and then (b) initiating and propagating a bonding wave between the major surfaces of the two substrates to assemble them together with each other. According to the invention, before the contacting step, the method comprises a step of preparing a dielectric surface layer with the aim of introducing sulfur into the layer at a dose greater than 3.0 E13at / cm2. The invention also relates to a structure obtained by this method.

Description

Technical Field

[0001] The present invention is particularly suitable for the fields of microelectronics, microsystems, photonics, etc. Background Art

[0002] Molecular adhesion is a method for joining two objects in which the completely clean, flat and smooth main surfaces or surfaces of the two objects are brought into close contact with each other, thereby promoting the formation of molecular bonds (such as van der Waals bonds or covalent bonds). Therefore, the joining of the two objects can be achieved without the use of adhesives. These bonds can be strengthened by heat treating the joined parts.

[0003] This method is particularly used to produce substrates used in the fields of microelectronics, microsystems, photonics, etc. In these fields, the substrates usually take the form of round wafers of material that can be single-crystalline, polycrystalline or amorphous, conductors, semiconductors or insulators.

[0004] In these fields, in order to facilitate the joining of the substrates, a dielectric layer, typically a silicon oxide dielectric layer, is usually formed on one and / or the other of the two substrates, either by deposition (optionally followed by a polishing step) or by simple oxidation when the substrate is formed of or contains silicon. It is also usually necessary to prepare the main surfaces so as to make them clean (especially with respect to particles and contaminants) and smooth (so as to provide a surface roughness with a root mean square value typically lower than 0.5 nm over a field of view of 5 micrometers by 5 micrometers). It is sometimes also envisaged to render at least one of these surfaces reactive by applying a plasma (typically oxygen or nitrogen). The purpose of this surface preparation is to obtain a maximum bonding energy immediately after the joining step and before any thermal annealing.

[0005] After the preparation is completed, the main surfaces of the two substrates are arranged to overlap each other. Pressure is applied to one or the other of the two substrates to produce close point contact between the main surfaces, and this close contact is propagated across the main surfaces in the form of a wave called a "bonding wave".

[0006] The patent literature, technical literature and scientific literature include a large number of documents outlining the principles and uses of this bonding method. Reference may be made in particular to the publication by Rieutord et al. "Dynamics of a bonding front", Physical Review Letters, Vol. 94, p. 236101, 2005.

[0007] A problem often encountered when using molecular adhesion methods is that peripheral defects (commonly called "bumps") appear at the bonding interface, that is, at the point where the bonding wave meets the edge of the substrate. When the wave is initiated at the edge of the substrate, these peripheral defects are generally located in the peripheral area of ​​the edge completely opposite to the initiation point of the wave. When the wave is initiated in the central area of ​​one of the substrates, these peripheral defects may be located over the entire profile. These defects cause gas and / or water to be trapped at the bonding interface between the two substrates. When such defects occur, the two substrates are not in contact and the local adhesion energy is very low or even non-existent. Even if these defects exist, they are not necessarily visible immediately after the propagation of the bonding wave, but may appear during the thermal annealing of the bonded piece formed by bonding the two substrates together.

[0008] The speed of the bonding wave is considered to be a factor affecting the number of defects, and reducing this speed generally leads to a reduction in the number of defects or even a complete elimination. To this end, various techniques for controlling this speed have been proposed, in particular by treating the surface of one and / or another substrate (in particular by heating) (WO2007060145). Another method proposed to reduce the speed of the bonding wave is to direct a jet of gas (optionally hot) towards the initiation point of the bonding wave during bonding. However, if this treatment can effectively reduce the bonding speed, it is also easy to reduce the adhesion energy, which may be disadvantageous for certain applications. On the other hand, document WO2013160841 envisions that during the propagation of the wave, the substrate is maintained in a gas atmosphere having a negative Joule-Thomson coefficient at the temperature and pressure of the atmosphere, such as a helium atmosphere, a neon atmosphere or a hydrogen atmosphere.

[0009] Whichever prior art solution is chosen, and no matter how effective it may be, it constitutes an important constraint.

[0010] Subject matter of the invention

[0011] The object of the present invention is to provide a method for bonding by molecular adhesion which, unlike the prior art, prevents or at least limits the occurrence of "pimple" type defects. More specifically, the object of the present invention is to provide a method for bonding by molecular adhesion which provides high bonding energies while the number of "pimple" type defects is much lower than in the methods of the prior art and which is not limited by the prior art. Summary of the invention

[0012] To achieve this objective, the subject of the present invention provides a method for joining two substrates by molecular adhesion, each substrate having a main surface, at least one of the two substrates being provided with a dielectric surface layer on one side of its main surface. The method comprises the following steps:

[0013] a) contacting the main surfaces of the two substrates, and then;

[0014] b) Initiating and propagating a bonding wave between the major surfaces of the two substrates, thereby bonding the two surfaces together.

[0015] It is worth noting that, before the contacting step, the method comprises a step of preparing the dielectric surface layer with the aim of introducing into this layer a dose of sulphur greater than 3.0E13 at / cm^2.

[0016] The introduction of sulfur into the surface layer in such a dose surprisingly makes it possible to prevent or at least limit the occurrence of pimple-type defects even when two substrates are joined with high bonding energy.

[0017] Other advantages and non-limiting features according to the invention (which can be taken alone or in any technically feasible combination):

[0018] - The dose of sulfur introduced is 3.5E13 at / cm^2;

[0019] - the step of preparing a dielectric surface layer is also intended to introduce into this layer a dose of fluorine greater than 4.0E14 at / cm^2;

[0020] - the preparation step comprises activating the dielectric surface layer by means of a plasma of a sulfur-containing gas;

[0021] - the preparation step comprises activating the dielectric surface layer by means of a plasma containing sulfur hexafluoride;

[0022] - the plasma also contains oxygen or nitrogen;

[0023] - activation of the surface layer for an activation time between 15 seconds and 2 minutes;

[0024] - the surface layer is exposed to a plasma of oxygen or nitrogen throughout the duration of the activation and, for a specific period of the activation time, a controlled amount of sulfur hexafluoride is mixed with the oxygen or nitrogen;

[0025] - before and after the specific time period, respectively, a time period during which the dielectric surface layer is exposed to a plasma formed of an activated gas of oxygen or nitrogen;

[0026] - the preparation step comprises preparing a chamber of the plasma activation device with a plasma containing a sulfur gas, and then, after preparing the chamber, treating a dielectric surface layer in the chamber with a plasma containing oxygen or nitrogen;

[0027] - the preparation step comprises preparing a chamber of the plasma activation device with a plasma containing sulfur hexafluoride, and then, after preparing the chamber, treating a dielectric surface layer in the chamber with a plasma containing oxygen or nitrogen;

[0028] - the bonding method comprises heat treating the two substrates bonded together;

[0029] - the two substrates are provided, on the sides of their main surfaces, with a dielectric surface layer of silicon oxide or silicon nitride, on each of which a preparatory step is carried out;

[0030] - The dielectric surface layer consists of silicon oxide;

[0031] - the silicon oxide in the dielectric surface layer is doped with nitrogen, the nitrogen / oxygen ratio preferably being less than 0.5;

[0032] - the dielectric surface layer consists of silicon nitride;

[0033] Between the preparation step and the contacting step, the bonding method comprises a step of cleaning at least one main surface of the two substrates.

[0034] According to another aspect, the invention provides a structure comprising a support substrate, a buried dielectric layer arranged on and in contact with the support substrate, and a layer called a "working layer" located on and in contact with the buried layer.

[0035] According to the present invention, the buried dielectric layer contains sulfur in a dosage greater than 3.0E13 at / cm^2.

[0036] Other advantages and non-limiting features according to the invention (which can be taken alone or in any technically feasible combination):

[0037] - Sulfur dosage greater than 3.5E13 at / cm^2;

[0038] - The buried dielectric layer contains fluorine at a dose greater than 4.0E14 at / cm^2;

[0039] - the buried dielectric layer consists of silicon oxide;

[0040] - the silicon oxide in the buried dielectric layer is doped with nitrogen, the nitrogen / oxygen ratio preferably being less than 0.5;

[0041] - The buried dielectric layer consists of silicon nitride. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Other features and advantages of the present invention will become apparent from the following detailed description of the invention, which is given with reference to the accompanying drawings, in which:

[0043] [ Figure 1a ]

[0044] [ Figure 1b ]

[0045] Figure 1a and Figure 1bTwo examples of the joining method according to the invention are shown;

[0046] [ Figure 2 ]

[0047] Figure 2 A structure obtainable by the method according to the invention is shown. DETAILED DESCRIPTION

[0048] The present invention relates to a method for joining two substrates by molecular adhesion.

[0049] The two substrates can be of any kind, insulating (such as quartz, glass or piezoelectric material), conductive (metal) or semiconducting (such as silicon, gallium nitride, silicon carbide), in any available form: single crystal, polycrystalline or amorphous. The two substrates can each combine different kinds of materials. For example, one of the two substrates can be formed by a piezoelectric layer transferred directly to a silicon support or transferred to a silicon support through an intermediate layer (such as an adhesive). The two substrates can be of the same kind or of different kinds. They can in particular have different thermal expansion coefficients from each other, which limits the temperature to which the joining piece can be exposed, in particular in order to increase the bonding energy between the two substrates. The substrate can take the form of a circular wafer with a standardized diameter of 150, 200 or 300 mm, which is a general situation in the field of microelectronics, but the invention is by no means limited to joining two circular wafers or two wafers of standardized size.

[0050] Whatever the nature and shape of the two substrates, the substrates are intended to be joined together via their "major" surfaces.

[0051] In order to facilitate this bonding, at least one of the two substrates is provided with a dielectric layer, such as a silicon oxide layer or a silicon nitride layer, on one side of its main surface. Therefore, this dielectric surface layer constitutes the bonding surface. It can have any thickness, usually between a few nanometers and a few micrometers, for example between 1 nanometer and 5 micrometers. In some cases, the two substrates can be provided with such a dielectric surface layer.

[0052] The term "silicon oxide layer" refers to a thin layer consisting mainly of silicon oxide. Other chemical elements may also be introduced into it in trace amounts or in higher concentrations, as will be explained later in this specification. In particular, the layer may be doped with nitrogen (thus forming a SiON layer), for example, at a nitrogen / oxygen ratio of less than 0.5. The proportion of nitrogen in the layer can be characterized by measuring its refractive index, which ranges (at a wavelength of 620 nm) from 1.44 for SiO2 to 2.35 for Si3N4. Thus, the dielectric surface layer may consist of SiO 1-x N x Formed, wherein the proportion x of nitrogen in the layer can be between 0 and 1.

[0053] The dielectric surface layer can be formed by any suitable technique. When the substrate comprises silicon, a dielectric surface layer of silicon oxide can be formed by dry oxidation or wet oxidation. More generally, the dielectric surface layer can be formed by deposition. For example, this can include LPCVD (low pressure chemical vapor deposition) or PECVD (plasma enhanced chemical vapor deposition) deposition techniques. It can also include a centrifugal deposition technique known as "spin-on glass". The deposited dielectric layer can be polished to reduce its roughness (usually a root mean square value within a 5 micron × 5 micron field of view below a value of 0.7nm) and to make it compatible with the bonding method.

[0054] More generally, the exposed surfaces of the main surfaces are prepared in a preliminary step of preparing the two substrates so as to make them clean (especially with respect to particles and contaminants) and smooth (so as to provide a surface roughness below the above-mentioned values). To this end, the method according to the invention may comprise a prior step of cleaning and / or polishing of the substrates, which is known per se, in order to condition the two substrates for the subsequent bonding step. The cleaning step may in particular comprise brushing the main surfaces while pouring a liquid, such as deionized water, on these surfaces.

[0055] As mentioned in the introduction of this application, these bonding steps include:

[0056] a) contacting two substrates via their major surfaces, and then;

[0057] b) Initiate and propagate a bonding wave between the two substrates, thereby bonding the two surfaces together.

[0058] By way of illustration only, these bonding steps can be performed in an automatic bonding device. In this case, the first substrate is manipulated by the arm of the equipment handling system so that it is arranged on a flat support with its main surface exposed. Then, the second substrate is manipulated by the arm of the handling system so that it contacts the first substrate on one side of its main surface. The mobile device actuator applies a moderate pressure of a few Newtons to the exposed surface of the second substrate, so that close contact is generated between the first substrate and the second substrate and a bonding wave is initiated and propagated. This pressure can be applied anywhere, especially at the peripheral part or the central part of the exposed surface of the second substrate.

[0059] In all cases, no matter how the wave is initiated, the bonding wave propagates from the initiation site to the peripheral portion of the bonding member, thereby bringing the two substrates into close contact over the entire extent of their major surfaces. The propagation of the wave is not instantaneous, and this propagation may last for several seconds, depending on the size of the substrate.

[0060] The following steps are also generally provided: a dielectric surface layer is prepared (or two dielectric surface layers are prepared when both substrates are provided with such layers) by plasma activation, usually using oxygen or nitrogen plasma. The preparation step may include placing the substrate in a chamber of a plasma activation device, for example on a flat support arranged in the chamber, so that the surface layer is exposed to the plasma prepared by the source. An oxygen or nitrogen stream is introduced into the plasma source of the device at a controllable flow rate, and a plasma of the gas is generated, for example by means of a variable magnetic field or electric field. The plasma material is directed to the dielectric surface layer to activate the surface of the substrate. These operations can be performed at atmospheric pressure or below atmospheric pressure. This activation generally makes it possible to increase the bonding energy between the two substrates immediately after bonding or after applying an enhanced heat treatment.

[0061] Therefore, the applicant has carried out preliminary experiments in an attempt to examine the effect of plasma activation of the silicon oxide surface layer on the quality of the bonded parts obtained by molecular adhesion, as described in detail above. More specifically, these preliminary experiments have attempted to evaluate the benefits of plasma activation when the plasma is doped with sulfur hexafluoride (SF6). A summary of the results obtained is shown in the following table.

[0062]

[0063]

[0064] In these preliminary experiments, the substrates consisted of pairs of 150 mm lithium tantalate and silicon wafers, each provided with a 30 nm and 650 nm silicon oxide (nitrogen-free) surface layer, respectively, deposited by PECVD. Both surface layers were plasma activated at a power of 150 W for 30 seconds and then bonded according to the method described above for bonding by molecular adhesion. The bonding energy was measured by the double cantilever method after an intensive heat treatment at 180° C. for 60 min. The experiments reported in the table were carried out in a plasma apparatus of the CCP type (CCP = capacitively coupled plasma), with the exception of experiment 10, which was carried out in an ICP type (ICP = Inductively Coupled Plasma) apparatus.

[0065] Reference experiment

[0066] Experiment 1 in the table constitutes a reference experiment corresponding to the activation of the surface layer with oxygen plasma alone. In this activation (and in all the experiments reported in this table), the oxygen flow rate was set to 75 SCCM. The measured bonding energy was 1905 mJ / m^2, the bonding wave propagation time was 6 seconds, and the bonded part showed obvious pimple-type defects. Analysis of the silicon oxide surface layer by SIMS and TXRF showed, respectively, that the dose of introduced fluorine was 3.0E+13at / cm^2 and the dose of sulfur was 2.4E+12at / cm^2, corresponding to trace amounts of these elements.

[0067] Experiments 2 to 6

[0068] In these experiments, a controlled amount of SF6 was introduced during the activation of an oxygen-based plasma, which controlled amount was constantly increased for experiments 2 to 6. The controlled amount of SF6 was obtained by adjusting the flow rate and the duration of this flow. It was introduced at the beginning of the plasma activation or in the middle of this activation, the latter option being preferred since the source plasma was more stable and the phenomena better controlled. It was observed that during the activation carried out during experiments 2 to 6, large doses of sulfur and fluorine had been incorporated into the surface layer. In experiment 5, a controlled amount of SF6 was introduced during two sequences, the first being the introduction of 3 SCCM of SF6 for 3 seconds, followed by a sequence of the introduction of 5 SCCM of SF6 for 5 seconds, after pausing the flow.

[0069] To review, the measurement unit SCCM (standard cubic centimeters per minute) is a physical unit of mass flow of a gas at a density defined by standard temperature and pressure conditions, expressed in cm. 3 / min.

[0070] Experiments 7 to 10

[0071] In these experiments, a gas mixture containing about 20% SF6 (10 SCCM) and 80% oxygen (50 SCCM) was supplied to the source of a plasma device activated at 2000 W in a preliminary step without placing any substrate of the experiment in the device. The purpose of this preliminary step is to prepare the chamber of the plasma activation device, with the purpose of coating the inner surfaces of the chamber with sulfur-containing substances and fluorine-containing substances. Then, experiments were carried out by treating the silicon oxide surface layer of the substrate using conventional oxygen-based plasma activation alone in the chamber thus prepared. During the oxygen plasma activation process, the sulfur-containing substances and / or fluorine-containing substances coated on the inner surfaces of the chamber peel off from these surfaces and merge into the plasma throughout the activation process.

[0072] It was observed that all other experiments performed resulted in a significant increase in the measured bond energy compared to reference experiment 1. This effect of fluorine has been reported elsewhere and appears to result from the incorporation of fluorine into the surface layer at doses greater than 4.0E14 at / cm^2.

[0073] Even more unexpectedly, these results show that the speed of the bonding wave is affected by the dose of sulfur introduced into the dielectric surface layer: the greater the dose of sulfur, the more the speed of the bonding wave decreases. This effect is obtained without significantly affecting the bonding energy, and in all cases the bonding energy remains greater than that in the reference experiment. It therefore seems that the speed of this wave can be controlled by adjusting the dose of sulfur introduced into the dielectric surface layer.

[0074] If the sulfur dose is sufficient—greater than 3.0E13 at / cm^2, preferably greater than 3.5E13 at / cm^2—the velocity of the wave is low enough that the pimple-type defects are no longer noticeable, as shown in Experiments 4, 5, 6 and 10.

[0075] In addition to the silicon oxide (SiO2) layer reported in the table, other experiments were able to reproduce these results for dielectric surface layers. In particular, similar results were obtained when the dielectric surface layer was formed of silicon oxide with a nitrogen / oxygen ratio strictly greater than 0, and when the dielectric surface layer was formed of silicon nitride.

[0076] The applicant has used these experiments and these observations to propose a method for bonding by molecular adhesion that can prevent or at least limit the appearance of pimple-type bonding defects. Moreover, this benefit can be obtained without affecting the bonding energy of the bonded parts, and can even provide significantly improved bonding energy.

[0077] Thus, in the sequence consisting of the steps of contacting the main surfaces of the two substrates and initiating and propagating a bonding wave between the two substrates, the method according to the invention provides, before the contacting step, a step of preparing a dielectric surface layer, with the aim of introducing into this layer a dose of sulfur greater than 3.0E13 at / cm^2. It is advantageous when this dose is greater than 3.5E13 at / cm^2. This dose can be measured by TXRF measurements or SIMS measurements, as was the case in the reported experiments.

[0078] Advantageously, the dose corresponds to a dose which is present in a thickness of not more than 10 nm, or even not more than 7.5 nm, directly below the main surface of the substrate, since it is likely that the bonding properties can be influenced only in this thickness range.

[0079] In order to benefit from improved bonding while limiting the appearance of nodular defects, it is also possible to seek to introduce fluorine in a dose of 4.0E14 at / cm^2 in the dielectric surface layer. It is also advantageous here that this dose corresponds to a dose present in a thickness of no more than 10 nm, or even no more than 7.5 nm, directly below the main surface of the substrate, since it is likely that the bonding properties can be influenced only within this thickness range. This dose can be measured by TXRF measurements or SIMS measurements.

[0080] A first method that allows the introduction of sulfur at a targeted dose may consist in plasma activation of the dielectric surface layer by a sulfur-containing gas, such as sulfur hexafluoride. This method is Figure 1a theme.

[0081] A sulfur-containing gas (eg sulfur hexafluoride) can be mixed with an activation gas consisting, for example, of oxygen or nitrogen. The dosage of this mixture is such that the target dosage of sulfur (and optionally fluorine) is reached or exceeded at the end of the plasma activation step.

[0082] The duration of such plasma activation is usually between 15 seconds and 2 minutes. More precisely, the duration of activation is the time period from the moment the plasma is ignited in the device source to its extinguishing. During this time period, the dielectric surface layer is exposed to the plasma formed by the activation gas of oxygen or nitrogen.

[0083] The sulfur-containing gas may be introduced in a controlled flow rate throughout this time period, or only in a portion of this time period, so as to be mixed with oxygen or nitrogen during the specific time period of activation. This specific time period preferably does not include the plasma ignition moment, because such ignition may cause the sulfur-containing species and fluorine-containing species present in the plasma to be introduced into the surface layer in an unrepeatable manner, thereby failing to achieve perfect control of the introduced doses and possibly being detrimental to good process control, especially when the target doses of sulfur and fluorine used are close to the limiting doses. It is therefore advantageous that the specific time period during which the dielectric surface layer is exposed to the plasma formed by the sulfur-containing gas is preceded and followed by a time period during which the dielectric surface layer is exposed to the plasma formed by the activation gas of oxygen or nitrogen.

[0084] To complete Figure 1a According to the description, the step of preparing a dielectric surface layer (with the aim of introducing sulfur into this layer at a dose greater than 3.0E13 at / cm^2) is followed by a step of contacting the main surfaces of the two substrates, and then a step of initiating and propagating a bonding wave between the main surfaces of the two substrates to bond the two surfaces together.

[0085] As described in the previous section, a step of cleaning the major surfaces of the substrates may be provided between the preparation step and the contacting step. The cleaning step may correspond to or include brushing the surface while pouring a liquid (e.g., demineralized water) on it. Advantageously, the cleaning step is applied to the major surfaces of the two substrates and then immediately joined.

[0086] according to Figure 1b Another method is described in which the preparing step includes preparing a chamber of a plasma activation apparatus by a plasma containing a sulfur gas and then, after preparing the chamber, treating a dielectric surface layer of at least one substrate in the chamber with a plasma containing oxygen or nitrogen.

[0087] As has been shown, when substrate to be prepared is not present in the chamber, plasma activation is carried out by the plasma of sulfur-containing gas.Sulfur-containing gas can be a mixture of sulfur hexafluoride and an activated gas such as oxygen or nitrogen.The purpose of this substep is to coat sulfur-containing substances and optional fluorine-containing substances on the inner surface of the chamber, and then during the plasma activation of the dielectric surface layer provided in the substep after the preparation step, these substances are peeled off from the inner surface.Adjustment parameters (especially the flow of sulfur-containing gas, the duration of plasma and its power) make enough substances be produced and introduced into the surface layer in the substep after the plasma activation.

[0088] To complete Figure 1b According to the description of the method, the step of preparing the dielectric surface layer is followed by the step of contacting the main surfaces of the two substrates, and then the step of initiating and propagating a bonding wave between the main surfaces of the two substrates to bond the two surfaces together. The cleaning step described in the description of the previous method can also be performed.

[0089] The dielectric surface layer is advantageously a silicon oxide layer (in which nitrogen can be introduced in any desired proportions) or a silicon nitride layer.

[0090] Of course, before, during or at the end of the steps of the bonding method described above, further steps may also be applied to the substrates, in particular those method steps which result in a structure obtained by bonding two substrates by molecular adhesion.

[0091] In particular, it is conceivable to subject the joined part to an intensive heat treatment and to take steps to reduce the thickness of at least one of the two substrates, thereby forming a layer. This reduction in thickness can be achieved by thinning the substrate (for example grinding) or by fracturing. In this case, it is conceivable to introduce a lightweight material into the substrate to be fractured, usually before the contacting step of the joining method, in order to form a fragile fracture surface where the substrate needs to be reduced in thickness.

[0092] In this application, the bonding method is part of a method for transferring a layer and at the end of the method (see Figure 2 )What appears is a structure 1, which includes a supporting substrate 2, a dielectric buried layer 3 (advantageously a silicon oxide layer or a silicon nitride layer) located on and in contact with the supporting substrate 2, and a layer called a "working layer" 4 located on and in contact with the buried layer 3.

[0093] Depending on whether the method is to arrange a single dielectric surface layer on one substrate or the other or to arrange two dielectric surface layers on each main surface of the substrate, the bonding interface of the buried layer corresponds to its interface with the supporting substrate 2, its interface with the working layer 4 or is incorporated into its thickness.

[0094] The buried dielectric layer 3 contains sulfur in a dose greater than 3.0E13 at / cm^2 (advantageously greater than 3.5E13 at / cm^2). It may have fluorine in a dose greater than 4.0E14 at / cm^2. As mentioned above, when the buried dielectric layer consists of silicon oxide, other substances may be introduced, in particular nitrogen. Advantageously, these doses correspond to the doses present in a thickness of no more than 10 nm on one side and / or the other side of the bonding interface.

[0095] Of course, the invention is not limited to the embodiments described and variants may be implemented thereto without departing from the scope of the invention as defined in the claims.

[0096] Although sulfur hexafluoride is used here as an example of a sulfur (and fluorine) containing gas, the invention is by no means limited to the use of this gas. In particular, it is conceivable to use any sulfur containing gas, such as sulfuryl fluoride (SO2F2) or tetrafluorosulfuryl (SO2F4) instead.

[0097] The preparatory steps according to the invention can be applied to a single dielectric surface layer formed on one of the substrates, even if both substrates comprise such a dielectric surface layer. When each substrate has a dielectric surface layer, the preparatory steps can likewise be applied to each substrate.

[0098] Finally, in an alternative form not described in detail, the sulfur-containing species and the fluorine-containing species (if present) may be introduced into the dielectric surface layer during its formation, for example during oxidation or nitridation of a silicon-containing substrate, or during deposition of the dielectric surface layer.

Claims

1. A method for joining two substrates by molecular adhesion, each substrate having a main surface, at least one of the two substrates being provided with a dielectric surface layer on one side of its main surface, the method comprising the following steps: (a) bringing the main surfaces of two substrates into contact, and then; (b) initiating and propagating a bonding wave between the major surfaces of the two substrates, thereby bonding the two surfaces together; The method is characterized in that, before the contacting step, it also comprises a step of preparing the dielectric surface layer with the aim of introducing into this layer a dose of sulfur greater than 3.0E13 at / cm^2.

2. The joining method according to the preceding claim, wherein: The sulfur dose is greater than 3.5E13 at / cm^2.

3. The joining method according to any one of the preceding claims, wherein: The step of preparing a dielectric surface layer is also intended to introduce into this layer a dose of fluorine greater than 4.0E14 at / cm^2.

4. The bonding method according to any one of claims 1 and 2, wherein: The preparation step comprises activating the dielectric surface layer by means of a plasma of a sulfur-containing gas, for example a plasma containing sulfur hexafluoride.

5. The joining method according to the preceding claim, wherein: The plasma also contains oxygen or nitrogen.

6. The joining method according to the preceding claim, wherein: The activation of the dielectric surface layer is carried out for an activation time between 15 seconds and 2 minutes.

7. The joining method according to the preceding claim, wherein: The surface layer is exposed to a plasma of oxygen or nitrogen throughout the duration of the activation, and for a specific period of the activation time, a controlled amount of sulfur hexafluoride is mixed with the oxygen or nitrogen.

8. The joining method according to the preceding claim, wherein: Before and after the specific time period are time periods in which the dielectric surface layer is exposed to plasma formed of an activated gas of oxygen or nitrogen, respectively.

9. The bonding method according to any one of claims 1 and 2, wherein: The preparation step includes preparing a chamber of the plasma activation apparatus with a plasma containing sulfur gas, and then treating a dielectric surface layer in the chamber with a plasma containing oxygen or nitrogen after preparing the chamber.

10. A method of bonding according to any one of the preceding claims, comprising heat treating the two substrates bonded together.

11. The joining method according to any one of the preceding claims, wherein: Both substrates are provided on the sides of their main surfaces with dielectric surface layers of silicon oxide or silicon nitride, on each of which a preparatory step is carried out.

12. The joining method according to any one of the preceding claims, wherein: The dielectric surface layer consists of silicon oxide.

13. The bonding method according to any of the preceding claims, comprising, between the preparation step and the contacting step, a step of cleaning at least one main surface of the two substrates.

14. Structure (1), comprising a supporting substrate (2), a dielectric buried layer (3) arranged on and in contact with the supporting substrate (2), and a layer called a "working layer" (4) located on and in contact with the buried layer (3), wherein the structure is characterized in that the dielectric buried layer (3) contains sulfur at a dose greater than 3.0E13at / cm^2.

15. Structure (1) according to the preceding claim, wherein: The sulfur dose is greater than 3.5E13 at / cm^2.

16. Structure (1) according to the preceding claim, wherein The dielectric buried layer (3) contains fluorine at a dosage greater than 4.0E14at / cm^2.

17. The structure (1) according to any one of claims 14 to 16, wherein: The buried dielectric layer is composed of silicon oxide.

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