Method for directly bonding two substrates

Abstract

The invention relates to a method for directly bonding two substrates, the method comprising the following steps: a) providing a first substrate and a second substrate comprising, respectively, a first bonding surface made of hydrophilic silicon oxide and a second bonding surface made of hydrophilic silicon oxide, b) adding fluoride ions to the first hydrophilic silicon oxide bonding surface; c) bringing the first hydrophilic silicon oxide bonding surface into contact with the second hydrophilic silicon oxide bonding surface, so that the first substrate is adhered to the second substrate, by way of the fluoride ions at the bonding interface.

Description

[0001] DESCRIPTION

[0002] TITLE: Direct bonding process between two substrates

[0003] TECHNICAL FIELD

[0004] The invention relates to a method of direct bonding between two substrates and a method of transferring a layer onto a support substrate comprising such a method of direct bonding between two substrates.

[0005] STATE OF THE ART

[0006] Direct bonding is a technique widely used in microelectronics to assemble substrates and form multilayer structures. It consists of directly bringing the two surfaces to be bonded into contact, which will spontaneously adhere to each other, without additional material, particularly without the addition of a polymer adhesive. It is possible to have a few monolayers of water adsorbed on the surfaces to be bonded, particularly if they are hydrophilic, but the surfaces to be bonded must be macroscopically dry.

[0007] Spontaneous adhesion between the two surfaces to be bonded occurs under the effect of various attractive forces between the materials, for example under the effect of Van der Waals forces, capillary forces and / or hydrogen bonds involving water molecules adsorbed on the surfaces to be bonded. Covalent bonds can also be established between the two surfaces to be bonded.

[0008] For example, in the case where the surfaces to be bonded are hydrophilic silicon surfaces or silicon oxide surfaces, the adhesion between the two surfaces results mainly from condensation reactions between the silanols of the first surface to be bonded on the one hand and the silanols of the second surface to be bonded on the other hand. More precisely, each condensation reaction generates a siloxane bond between the two surfaces to be bonded and releases a water molecule according to the following reaction (1):

[0009] Sii-OH + Si2-OH -> Sii-O-Si2+ H2O (I)

[0010] The attractive forces mentioned above only act at short range and the formation of covalent bonds requires that the reactants are close enough for their molecular orbitals to overlap. Direct bonding therefore requires that the surfaces to be bonded are clean and flat, and have very low roughness, for example a roughness less than 0.5 nm. Typically, the reaction (I) mentioned above can only take place at the contact points between the two surfaces to be bonded. Furthermore, such a reaction (I) is very slow and equilibrated (not complete) at room temperature. The quality of direct bonding can be assessed by a quantity called "bonding energy" which corresponds to the energy required to separate the two surfaces once bonded.Due to the difficulties mentioned above, the bonding energy between two surfaces bonded by direct bonding at room temperature is not sufficient to guarantee good mechanical strength of the resulting multilayer structure in subsequent applications of said multilayer structure, for example if the multilayer structure is used in a process inducing high mechanical stresses on the bonding interface. Indeed, the bonding energy between two surfaces following direct bonding at room temperature of the two surfaces rarely reaches 200 mJ / m. 2 when the two surfaces adhere under the effect of the attractive forces previously mentioned. The reaction (I) of condensation of silanols does not allow to exceed 0.5 J / m 2 , even after several days.

[0011] One solution to increase the bonding energy is to heat the bonding interface after the two surfaces to be bonded have been brought into contact. For example, contacting two hydrophilic silicon or silicon oxide bonding surfaces can be followed by heat treatment of the bonding interface at 200 °C for 2 hours.

[0012] However, such high temperatures can induce, at the bonding interface, the formation of gases and bubbles, as well as significant thermal stress, particularly when the two substrates to be bonded have very different thermal expansion coefficients. Indeed, in the latter case, the difference in behavior of the two substrates under the effect of temperature variations is such that it will induce the opening of the bonding interface before it has had time to strengthen.

[0013] Another solution is to treat at least one of the surfaces to be bonded with gaseous ammonia NH3 or with an aqueous solution of ammonia NH4OH. Ammonia reacts partially with water to generate ammonium ions NH4 + and hydroxide ions, said solution is also often called ammonium hydroxide solution. Ammonia is a weak base which will react with the water adsorbed on the surface to be bonded, and thus generate hydroxide ions OH- according to the following acid-base reaction (2):

[0014] NH3+ H2O= NH4 + + HO' (2)

[0015] The hydroxide ions generated by the acid-base reaction (2) act as catalysts for the condensation reaction (I) and therefore increase its kinetics. However, ammonia NH3 is extremely volatile: the evaporation of surface ammonia results in a shift in the equilibrium of the acid-base reaction (2) towards the regeneration of ammonia NH3 and therefore the consumption of catalytic hydroxide ions. In other words, it is difficult to maintain and control the quantity of ammonia actually present on the surface to be bonded even though this maintenance is necessary for the proper functioning of the basic catalytic effect of the hydroxide ions as previously described.

[0016] According to document US 7,109,093 B2, it is possible to obtain a bonding energy of the order of a few J / m 2following direct bonding carried out at room temperature between a first bonding layer of a first substrate and a second bonding layer of a second substrate when the first bonding layer and / or the second bonding layer comprises a fluorinated oxide layer. In order to generate said fluorinated oxide layer in a bonding layer, for example in a silicon oxide layer, US 7,109,093 B2 describes a fluorination process comprising exposing the silicon oxide layer to hydrofluoric acid followed by annealing the layer thus exposed. Under the effect of annealing, the fluorine diffuses from the surface towards the interior of the silicon oxide layer, which generates the fluorinated oxide layer.

[0017] The method described by US 7,109,092 B2 further comprises a step of terminating the surface of the silicon oxide layer comprising the fluorinated oxide layer prior to implementing the direct bonding as such. The termination step may be a washing with water or ammonium hydroxide and makes it possible to generate or regenerate the surface chemical functions necessary for the adhesion of the surfaces according to the condensation reactions previously mentioned. According to US 7,109,092 B2, the fluorine which diffuses during annealing inside the material breaks Si-O-Si bonds, so that the fluorinated silicon oxide layer is porous. When implementing direct bonding according to reaction (I) or according to reaction (II), such porosity allows water or dihydrogen generated by reactions (I) and (II) to diffuse inside said porous fluorinated silicon oxide layer and to make the condensation reaction complete at room temperature.

[0018] However, in many cases, the implementation of the annealing described by US 7,109,092 B2 for the formation of the fluorinated oxide layer is impossible. This is the case, for example, when the substrates to be bonded are multilayer structures comprising at least two layers having very different coefficients of thermal expansion. This is also the case when at least one of the two substrates to be bonded comprises temperature-sensitive circuits or components and / or a weakening zone formed by implantation of atomic or ionic species in the context of a SmartCut™ process. In addition, the method described by US 7,109,092 B2 comprises numerous steps, which makes said method expensive and complex to implement.

[0019] BRIEF DESCRIPTION OF THE INVENTION

[0020] An aim of the invention is to overcome the aforementioned drawbacks of the state of the art.

[0021] To this end, the invention proposes a method for direct bonding between two substrates comprising the following steps: a) providing a first substrate and a second substrate respectively comprising a first bonding surface made of hydrophilic silicon oxide and a second bonding surface made of hydrophilic silicon oxide, b) supplying fluoride ions to the first bonding surface made of hydrophilic silicon oxide; c) bringing the first bonding surface made of hydrophilic silicon oxide and the second bonding surface made of hydrophilic silicon oxide into contact, so as to obtain adhesion of the first substrate with the second substrate with the fluoride ions at the bonding interface.

[0022] For the purposes of the present invention, the term "hydrophilic silicon oxide bonding surface" refers to a silicon oxide surface having Si-OH silanol groups. To be hydrophilic, the silicon oxide surface does not include, in particular, any surface silicon hydride groups -Si-H, or the ratio between the number of silicon hydride groups -Si-H and the number of surface silanol groups is less than 20%. Preferably, the silicon oxide surface does not include surface silicon hydride groups -Si-H, for example by reaction with water in a basic medium.

[0023] The addition of fluoride ions generates a hydrophilic silicon oxide surface on which fluoride ions are physisorbed, for example due to electrostatic interactions. It should be noted that the physisorbed fluoride ions can add transiently to the silicon atoms of the surface silanol groups, so that the hydrophilic silicon oxide bonding surface considered at a given time can include some HO-Si-F covalent bonds resulting from a nucleophilic addition reaction as shown schematically in Figure 1. The hydrophilic silicon oxide bonding surface is therefore the result of a dynamic equilibrium between physisorbed fluoride ions and added fluoride ions.However, the pentacoordinated silicon atoms resulting from the addition of fluoride ions are very unstable and have a very short lifetime, so that the hydrophilic silicon oxide bonding surface onto which the fluoride ions have been added essentially comprises physisorbed fluoride ions. The method of the invention makes it possible to obtain bonding energies greater than 1 J / m. 2 with a low thermal budget. For example, the bonding energy can be 2500 mJ / m 2 after adding fluoride ions according to one embodiment of the invention and one hour of contact between the two surfaces at 170°C. For comparison, the bonding energy is only 250 mJ / m 2when there has been no said addition of fluoride ions, all parameters of the bonding process remaining otherwise unchanged. In other words, the addition of fluoride ions has made it possible to multiply the bonding energy by ten. For example, after one hour of contacting the two surfaces at 100°C, the bonding energy is multiplied by 5 if there has been an addition of fluoride ions according to one embodiment of the invention.

[0024] The inventors attribute the achievement of the previously mentioned high bonding energies at a low thermal budget to a nucleophilic catalytic effect of the surface fluoride ions which improves the kinetics of the condensation reaction (I), at the origin of the adhesion between the first hydrophilic silicon oxide bonding surface of the first substrate and the second hydrophilic silicon oxide bonding surface of the second substrate. The inventors assume that the supply of fluoride ions to the first hydrophilic silicon oxide bonding surface successively leads to the physisorption of said fluoride ions on the first hydrophilic bonding surface followed by the nucleophilic addition of at least a portion of said physisorbed fluoride ions on silicon atoms of the silanol groups of the first hydrophilic silicon oxide bonding surface, so as to generate the previously mentioned pentacoordinated surface silicon atoms.Said pentacoordinated silicon atoms are highly reactive reaction intermediates: the electrophilicity of the silicon atoms of the first hydrophilic silicon oxide bonding surface is enhanced by the addition of fluoride ions, so that the oxygen of the silanol groups of the second hydrophilic silicon oxide bonding surface then readily replace the fluoride ions.

[0025] The high bonding energies previously mentioned are obtained in a single treatment step, and without thermal annealing of the hydrophilic silicon oxide bonding surfaces on which the addition of fluoride ions has been carried out prior to bringing the first substrate and the second substrate into contact, which is particularly advantageous when one of these substrates comprises a weakening zone and / or electronic components and / or several materials having different thermal expansion coefficients.

[0026] The catalytic effect of fluoride ions is probably specific to direct bonding between silicon oxide surfaces. However, the addition of fluoride ions is easier to implement and requires fewer special precautions than the ammonia treatments mentioned above, whose beneficial effect on bonding energy is very difficult to observe in practice, particularly due to the volatility of said ammonia.

[0027] According to other optional features of the invention taken alone or in combination when technically possible:

[0028] - the method further comprises the supply of fluoride ions to the second hydrophilic silicon oxide bonding surface prior to bringing the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface into contact;

[0029] - the contacting of the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface is carried out at room temperature;

[0030] - during the entire period between the addition of fluoride ions and the contacting of the first hydrophilic silicon oxide surface and the second hydrophilic silicon oxide surface, each hydrophilic silicon oxide bonding surface on which the addition of fluoride ions has been implemented is maintained at room temperature and / or the addition of fluoride ions on the first hydrophilic silicon oxide bonding surface is implemented at room temperature;

[0031] - the method further comprises a step d) of consolidation annealing carried out after step c) of bringing the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface into contact, the consolidation annealing being preferably carried out at a temperature of between 50°C and 250°C, even more preferably at a temperature of between 50°C and 100°C, for a duration of preferably between 15 min and 4 hours, even more preferably for a duration of between 15 min and 2 hours;

[0032] - the supply of fluoride ions to the first hydrophilic silicon oxide bonding surface comprises the deposition of a liquid comprising fluoride ions and / or fluoride ion donor species on the first hydrophilic bonding surface;

[0033] - depositing the liquid on the first hydrophilic silicon oxide bonding surface comprises: depositing a volume of the liquid comprising the fluoride ions and / or the fluoride ion donor species in the center of the first hydrophilic silicon oxide bonding surface, spreading said volume of the liquid comprising the fluoride ions and / or the fluoride ion donor species by rotating the first hydrophilic silicon oxide bonding surface, said first hydrophilic silicon oxide bonding surface being kept rotating until ejection and evaporation of the volume of liquid comprising the fluoride ions and / or the fluoride ion donor species; - the liquid comprising the fluoride ions and / or the fluoride ion donor species does not comprise hydrofluoric acid or ammonium fluoride;

[0034] - the liquid comprising the fluoride ions and / or the fluoride ion donor species is obtained from the dissolution of a salt of the fluoride ion and a counterion which is a neutral or basic cation;

[0035] - the supply of fluoride ions to the first hydrophilic silicon oxide bonding surface comprises: exposing said first hydrophilic silicon oxide bonding surface to a fluorinated plasma, rinsing with water, or exposing to a humid atmosphere, the first hydrophilic silicon oxide bonding surface previously exposed to the fluorinated plasma;

[0036] - providing fluoride ions to the first hydrophilic silicon oxide bonding surface comprises exposing the first hydrophilic silicon oxide bonding surface to a gas containing fluorinated species;

[0037] - the gas containing fluorinated species does not include hydrofluoric acid;

[0038] - the gas comprises sulfur hexafluoride SFe and / or tetrafluoromethane CF4, mixed with ammonia NH3 OR nitrogen trifluoride NF3 in a basic medium;

[0039] - the supply of fluoride ions to the first hydrophilic silicon oxide bonding surface comprises: implanting fluorine using an implanter onto the first hydrophilic silicon oxide bonding surface, rinsing with water, or exposing to a humid atmosphere, the first previously implanted hydrophilic silicon oxide bonding surface;

[0040] - the first hydrophilic silicon oxide surface and / or the second hydrophilic silicon oxide surface are made of chemical oxide, deposited oxide, thermal oxide and / or native oxide;

[0041] - the first substrate is a multilayer structure comprising a first layer of a first material and a second layer of a second material, the difference in coefficient of thermal expansion between the first material and the second material being greater than or equal to 10% or 10' 6 K' 1 in absolute value.

[0042] - the first substrate comprises electronic component elements;

[0043] - the first substrate comprises a weakening zone formed by implantation of atomic or ionic species.

[0044] The invention extends to a method for transferring a layer onto a support substrate, said method comprising the following steps: a) providing a donor substrate comprising a first surface made of hydrophilic silicon oxide and a support substrate comprising a second surface made of hydrophilic silicon oxide, b) forming a weakening zone within the donor substrate so as to delimit a layer to be transferred between the first surface made of hydrophilic silicon oxide and the weakening zone, c) bonding between the donor substrate and the support substrate, the first surface made of hydrophilic silicon oxide and the second surface made of hydrophilic silicon oxide being at the bonding interface and the bonding between the donor substrate and the support substrate being implemented by a bonding method as previously described,d) detaching the donor substrate along the weakening zone so as to transfer the layer to be transferred onto the support substrate.,

[0045] The invention also extends to an assembly comprising a first substrate and a second substrate, the first substrate being covered by a first surface made of hydrophilic silicon oxide and the second substrate by a second surface made of hydrophilic silicon oxide, the first surface made of hydrophilic silicon oxide being bonded directly with the second surface made of hydrophilic silicon oxide, fluoride ions being arranged between the first bonding surface made of hydrophilic silicon oxide and the second bonding surface made of hydrophilic silicon oxide, in which the surface concentration of fluoride ions at the bonding interface between the first surface made of hydrophilic silicon oxide and the second surface made of hydrophilic silicon oxide is between 10 10 ions / cm 2and 10 14 ions / cm 2 , and wherein the fluoride ion concentration in the first substrate and in the second substrate at a distance greater than 1 nm from the bonding interface between the first substrate and the second substrate is less than 10 18 at / cm 3 .

[0046] Finally, the invention extends to a substrate having a first surface made of hydrophilic silicon oxide on which fluoride ions are physisorbed, the surface concentration of fluoride ions physisorbed on said first surface made of hydrophilic silicon oxide being between 10 10 ions / cm 2 and 10 14 ions / cm 2 , and wherein the fluoride ion concentration in said substrate at a distance greater than 1 nm from the first surface is less than 10 18 at / cm 3 .

[0047] BRIEF DESCRIPTION OF THE FIGURES

[0048] Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which:

[0049] - Figure 1 illustrates the nucleophilic role of the fluoride ion in the condensation reaction of silanols at the origin of the formation of a Si-O-Si siloxane bond between the two surfaces to be bonded;

[0050] - Figure 2 compares the bonding energy measured between two silicon oxide surfaces bonded using a direct bonding process, said direct bonding process successively comprising the deposition, centrifugation and evaporation of a cesium fluoride solution at a concentration of 10' 4M on the surfaces, bringing the two surfaces into contact and then carrying out a consolidation annealing at 220°C at the bonding energy measured between two silicon oxide surfaces bonded using a direct bonding process not including the deposition of the cesium fluoride solution, the bonding energy being represented as a function of the duration of the consolidation annealing at 220°C;

[0051] - Figure 3 compares the bonding energy measured between two silicon oxide surfaces bonded using a direct bonding method, said direct bonding method successively comprising the deposition, centrifugation and evaporation of a cesium fluoride solution on the surfaces, bringing the two surfaces into contact and then carrying out a consolidation annealing at 170°C with the bonding energy measured between two silicon oxide surfaces bonded using a direct bonding method not including the deposition of the cesium fluoride solution, the bonding energy being represented as a function of the duration of the consolidation annealing at 220°C;

[0052] - Figure 4 represents the bonding energy measured between two silicon oxide surfaces bonded using a direct bonding method, said direct bonding method successively comprising the deposition, centrifugation and evaporation of a cesium fluoride solution on the surfaces, bringing the two surfaces into contact and then carrying out consolidation annealing, depending on the temperature of the consolidation annealing, for different concentrations of the cesium fluoride solution;

[0053] - Figure 5 represents the bonding energy measured between two silicon oxide surfaces bonded using a direct bonding process, said direct bonding process comprising exposing the surfaces to a dilute hydrofluoric acid vapor, bringing the two surfaces into contact and then carrying out consolidation annealing, as a function of the consolidation annealing temperature (duration 2 h) for different exposure times to this vapor.

[0054] - Figure 6 represents the bonding energy measured between two silicon oxide surfaces bonded using a direct bonding process, said direct bonding process comprising immersing the plates in a very dilute hydrofluoric acid solution (0.05%) for 1 min, removing them followed by bringing the two surfaces into contact and then carrying out a consolidation annealing, depending on the consolidation annealing temperature. The reference is obtained by soaking in DI water for the same duration.

[0055] DETAILED DESCRIPTION OF EMBODIMENTS

[0056] Direct bonding process

[0057] The direct bonding method according to the invention comprises a first step of providing a first substrate comprising a first bonding surface made of hydrophilic silicon oxide and a second substrate comprising a second bonding surface made of hydrophilic silicon oxide.

[0058] Providing a first substrate comprising a first hydrophilic silicon oxide bonding surface and a second substrate comprising a second hydrophilic silicon oxide bonding surface

[0059] First substrate and second substrate

[0060] The first substrate and / or the second substrate may be, for example, made of a material chosen from silicon Si, germanium Ge, indium phosphide InP, gallium arsenide AsGa, alumina AI2O3, silicon oxide SiO2, silicon nitride SiaN4, silicon carbide SiC, gallium nitride GaN, copper, titanium, nickel, lithium niobate LiNbOa (known to those skilled in the art by the acronym LNO) and lithium tantalate LiTaOa (known to those skilled in the art by the acronym LTO).

[0061] Alternatively, the first substrate and / or the second substrate may be multilayer structures comprising at least two layers. For example, the first substrate - respectively the second substrate - comprises a first layer made of a first material and a second layer made of a second material. The first material and the second material are for example chosen from the materials mentioned above. According to this embodiment, the step of providing a first substrate comprising a first bonding surface made of hydrophilic silicon oxide and a second substrate comprising a second bonding surface made of hydrophilic silicon oxide may comprise one or more sub-steps of assembling said layers, so as to form the first substrate and / or the second substrate.

[0062] The direct bonding method according to the invention is particularly advantageous when the first substrate and / or the second substrate comprise layers made of materials having a large difference in coefficient of thermal expansion. A large difference in coefficient of thermal expansion is understood to mean a difference of 10% or more between the values ​​of said coefficients of thermal expansion.

[0063] Indeed, under the effect of a strong rise in temperature, and due to the differences in thermal expansion coefficients previously mentioned and the low elasticity of these structures, high stresses can appear. These stresses can locally exceed the breaking stress of the material or assembly and cause cracking or delamination. Such cracking or delaminations typically occur when the stresses are greater than 10 MPa, i.e. the product of the temperature variation by the difference between the thermal expansion coefficients between the first material and the second material is greater than 10' 4 . The bonding process makes it possible to avoid temperature variations greater than 100°C, the difference between the coefficients of thermal expansion of the first material and the second material may be, in absolute value, greater than 10' 6 K' 1or 10% in relative values. The annealing temperature is limited according to the differences in thermal expansion coefficients.

[0064] Furthermore, the first substrate and / or the second substrate may comprise one or more electronic component elements. For example, the first substrate and / or the second substrate comprise metal parts such as copper lines and silicon oxide insulating parts (hybrid bonding). Similarly, the step of providing a first substrate comprising a first hydrophilic silicon oxide bonding surface and a second substrate comprising a second hydrophilic silicon oxide bonding surface may in this case comprise one or more sub-steps of forming said electronic component elements.

[0065] The direct bonding method according to the invention is particularly advantageous when the first substrate and / or the second substrate comprise such electronic component elements because these materials are inherently inhomogeneous, since they contain, for example, metallic parts, insulating oxides, particular semiconductors. They therefore do not tolerate excessively large temperature variations, since said temperature variations are likely to cause the appearance of high stresses which can lead to the formation of defects (delamination for example). A temperature of 200°C thus appears as an upper limit which should not be exceeded for too long.

[0066] Finally, the first substrate and / or the second substrate may also comprise a weakening zone formed for example by implantation of atomic and / or ionic species.

[0067] The direct bonding method according to the invention is particularly advantageous when the first substrate and / or the second substrate comprise such a weakened zone because the material is then weakened locally to prepare for a future thermal fracture step and it is therefore desired to strengthen the entire remainder of the structure (therefore the bonding interface) without mechanically stressing the weakened implanted zone. Typically, the fracture temperatures by implantation are above 300°C but it is necessary to avoid exceeding 200°C so as not to damage the weakened structure.

[0068] The first substrate and the second substrate may be of the same nature or they may be different. The direct bonding method according to the invention is particularly advantageous when the first substrate (or a layer of said first substrate) and the second substrate (or a layer of the second substrate) are made of materials having a large difference in coefficient of thermal expansion for the same reasons as previously stated.

[0069] First bonding surface made of hydrophilic silicon oxide and second bonding surface made of hydrophilic silicon oxide

[0070] As previously mentioned, the first substrate and the second substrate respectively comprise a first hydrophilic silicon oxide bonding surface and a second hydrophilic silicon oxide bonding surface. A hydrophilic bonding surface is understood to mean a surface that has a strong affinity with water, such that at least a monolayer of water remains naturally adsorbed on the hydrophilic bonding surface. The contact angle of a hydrophilic surface is typically less than about ten degrees.

[0071] For example, the first hydrophilic silicon oxide bonding surface and / or the second hydrophilic silicon oxide bonding surface may be native silicon oxide surfaces. In other words, if the first substrate and / or the second substrate comprise a material naturally forming on its surface a silicon oxide layer - called a native silicon oxide layer - (typically silicon Si or silicon carbide SiC which naturally form a silicon oxide layer), the first hydrophilic silicon oxide bonding surface and / or the second hydrophilic silicon oxide bonding surface may be the surface of said native oxide layer.If the first substrate and / or the second substrate are made of silicon oxide or comprise a layer of silicon oxide, the first hydrophilic silicon oxide bonding surface and / or the second hydrophilic silicon oxide bonding surface may be directly the surface of said silicon oxide.

[0072] For example, the first hydrophilic silicon oxide bonding surface and / or the second hydrophilic silicon oxide bonding surface may be thermal silicon oxide surfaces. According to this embodiment, the step of providing a first substrate comprising a first hydrophilic silicon oxide bonding surface and a second substrate comprising a second hydrophilic silicon oxide bonding surface may comprise a sub-step of forming a first thermal silicon oxide layer on the first substrate and / or a sub-step of forming a second thermal silicon oxide layer on the second substrate, a surface of the first, respectively of the second, thermal silicon oxide layer forming the first, respectively the second, hydrophilic silicon oxide bonding surface.

[0073] Thermal oxide means an oxide formed by direct oxidation of a material by exposing said material, at high temperature, to oxygen (dry oxidation), water (wet oxidation) or possibly other oxidants (e.g. N2O).

[0074] A thermal silicon oxide layer may be formed on the surface of a silicon or silicon carbide substrate. For this purpose, the silicon or silicon carbide substrate is, for example, heated to a temperature preferably between 800°C and 1200°C and then a surface of said silicon or silicon carbide substrate is exposed to an atmosphere of dioxygen or water vapor. The oxidation temperature determines the growth rate of the thermal silicon oxide layer from the surface of the silicon or silicon carbide substrate exposed to the atmosphere of dioxygen or water vapor.In the case of wet oxidation, the formation of the thermal oxide layer may comprise the combustion of dihydrogen with dioxygen in a torch and the exposure of the silicon or silicon carbide substrate to the water vapor produced by said combustion, the heat generated by the combustion of the dihydrogen being further used to bring the silicon or silicon carbide substrate to the selected oxidation temperature.

[0075] The thickness of the thermal silicon oxide layer is, for example, between 1 nm and 1 pm.

[0076] For example, the first hydrophilic silicon oxide bonding surface and / or the second hydrophilic silicon oxide bonding surface may be chemical silicon oxide surfaces. According to this embodiment, the step of providing a first substrate comprising a first hydrophilic silicon oxide bonding surface and a second substrate comprising a second hydrophilic silicon oxide bonding surface may comprise a sub-step of forming a first chemical silicon oxide layer on the first substrate and / or a sub-step of forming a second chemical silicon oxide layer on the second substrate, a surface of the first, respectively of the second, chemical silicon oxide layer forming the first, respectively second, hydrophilic silicon oxide bonding surface.Chemical oxide means an oxide formed by oxidation of a material at low temperature (preferably at room temperature), in the presence of water or certain oxidants such as hydrogen peroxide H2O2 or ozone O3. A layer of chemical silicon oxide can be formed in this way on the surface of a silicon, silicon carbide or silicon nitride substrate, by exposing said surface at low temperature to water or to an oxidant as previously mentioned. The thickness of the layer of chemical silicon oxide on the surface of the silicon substrate is for example between 0.5 nm and 3 nm.

[0077] For example, the first hydrophilic silicon oxide bonding surface and / or the second hydrophilic silicon oxide bonding surface may be deposited silicon oxide surfaces. According to this embodiment, the step of providing a first substrate comprising a first hydrophilic bonding surface and a second substrate comprising a second hydrophilic bonding surface may comprise a sub-step of depositing a first layer of silicon oxide deposited on the first substrate and / or a sub-step of depositing a second layer of silicon oxide deposited on the second substrate, a surface of the first, respectively of the second, deposited oxide layer forming the first, respectively the second, hydrophilic silicon oxide bonding surface.

[0078] A deposited oxide is an oxide formed by chemically adding material, for example in the form of vapor or plasma. A layer of silicon oxide can be deposited on almost any type of material, including IV / 1 V, III / V and II / VI semiconductors as well as oxides such as LiTaCh or LiNbOa. The thickness of the deposited silicon oxide layer is, for example, between 1 nm and 10 pm.

[0079] For the deposition of a silicon oxide layer, the precursor providing the silicon is typically silane (SihL) while the precursor providing the oxygen is, for example, N2O or O2. For example, the substrate on which the silicon oxide layer is to be deposited is placed in a plasma chamber. Then, a carrier gas, for example dihydrogen, comprising the precursors of silicon and oxygen, for example SihL and N2O, is introduced into said plasma chamber. Under the effect of the plasma, the precursors decompose and react, forming a layer of silicon oxide on the surface of the substrate exposed to the plasma.

[0080] If the first substrate and / or the second substrate are made of silicon oxide or comprise a silicon oxide layer and / or if the first substrate and / or the second substrate are made of a material naturally forming a silicon oxide layer on the surface or comprise such a material, the provision of the first substrate comprising a first bonding surface of hydrophilic silicon oxide and of the second substrate comprising a second bonding surface of hydrophilic silicon oxide may still comprise the formation of a first thermally or chemically deposited silicon oxide layer on the first substrate and / or the formation of a second thermally or chemically deposited silicon oxide layer on the second substrate such that a surface of the first, respectively of the second, thermally or chemically deposited silicon oxide layer forms the first, respectively the second, hydrophilic silicon oxide bonding surface.

[0081] The first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface may be of the same nature or of different nature. For example, the first hydrophilic silicon oxide bonding surface may be a chemical silicon oxide surface and the second hydrophilic silicon oxide bonding surface a native silicon oxide surface. In this case, the step of providing a first substrate comprising a hydrophilic silicon oxide bonding surface and a second substrate comprising a second hydrophilic silicon oxide bonding surface may comprise a sub-step of forming a chemical silicon oxide layer on the first substrate. For further example, the first hydrophilic silicon oxide bonding surface may be a thermal silicon oxide surface and the second hydrophilic silicon oxide bonding surface a deposited silicon oxide surface.In this case, the step of providing a first substrate comprising a hydrophilic silicon oxide bonding surface and a second substrate comprising a second hydrophilic silicon oxide bonding surface may comprise a sub-step of forming a thermal silicon oxide layer on the first substrate and a sub-step of forming a silicon oxide layer deposited on the second substrate.

[0082] Such combinations are cited as examples and are in no way limiting of the scope of the invention. Any combination between native, chemical, thermal and deposited silicon oxide surfaces of the first substrate and the second substrate remains covered by the present invention.

[0083] Furthermore, depending on the nature of the first, respectively second, substrate that he wishes to obtain (monoblock substrate, or multilayer structure comprising materials having very different thermal expansion coefficients, possibly comprising electronic component elements and / or a weakening zone) and the application envisaged for the assembly resulting from the bonding between the first substrate and the second substrate (fabrication of SOI substrate for advanced microelectronics, substrate for RF, substrate for photonics) the person skilled in the art is able to choose the nature of the silicon oxide most suitable for the first, respectively second, substrate, as well as the order of sequence of the various sub-steps previously mentioned making it possible to provide the expected first, respectively second, substrate.

[0084] Supply of fluoride ions on at least the first hydrophilic bonding surface

[0085] The method further comprises a step of supplying fluoride ions to the first hydrophilic silicon oxide bonding surface, or to both the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface. As previously mentioned, following the supply of fluoride ions, the fluoride ions are essentially physisorbed onto the hydrophilic silicon oxide surface.

[0086] Liquid intake

[0087] According to a particular embodiment, the supply of fluoride ions comprises the deposition of a liquid comprising fluoride ions and / or fluoride ion donor species on the hydrophilic bonding surface.

[0088] For example, the liquid comprising fluoride ions is a solution of a fluoride salt, preferably an aqueous solution. The dissolved salt is for example chosen from ammonium fluoride, lithium fluoride, sodium fluoride, potassium fluoride and cesium fluoride, magnesium fluoride, calcium fluoride and barium fluoride. Cesium fluoride advantageously has better solubility in water than sodium fluoride or potassium fluoride. The concentration of the fluoride salt solution deposited on the first hydrophilic bonding surface is preferably between 10' 5 mol / L and 10' 3 mol / L to avoid the formation of crystals during evaporation.

[0089] For example, the liquid comprising fluoride ions and / or fluoride ion donor species is a dilute hydrofluoric acid solution and the fluoride ion donor species is hydrofluoric acid. The concentration of hydrofluoric acid is preferably chosen to be less than 0.5% so as to limit the etching phenomena of the oxide.

[0090] Preferably, however, the liquid comprising fluoride ions and / or fluoride ion donor species is not a hydrofluoric acid solution, an ammonium fluoride solution or any other liquid having the effects of hydrofluoric acid and ammonium fluoride.

[0091] Indeed, hydrofluoric acid and ammonium fluoride are acidic species, donors of H protons +which tend to oppose the desired catalytic effect. In addition, hydrofluoric acid and ammonium fluoride have the property of reacting with silicon oxide and thus etching the layer. Such etching damages the silicon oxide layer, particularly its dielectric properties, and is not desirable. The desired catalytic effect, on the contrary, requires maintaining the physisorbed fluoride ions on the surface at the bonding interface.

[0092] Thus, in a preferred embodiment, the liquid comprising the fluoride ions is a solution of a salt of fluoride ions and a counterion which is neutral or basic. The counterion may in particular be an alkali or alkaline-earth cation. For example, the liquid comprising fluoride ions is a solution of ammonium fluoride, lithium fluoride, sodium fluoride, potassium fluoride, cesium fluoride, magnesium fluoride, calcium fluoride and / or barium fluoride. More preferably, the counterion is furthermore non-metallic. The counterion may in particular be a polyatomic cation, for example a derivative of the ammonium ion NH4+ by substitution of one or more protons. For example, the liquid comprising fluoride ions is a solution of dimethylammonium fluoride.

[0093] The deposition of the liquid comprising fluoride ions and / or fluoride ion donor species can be carried out for example by coating, by spraying or by spin-coating (better known to those skilled in the art under the English term “spin-coating”).

[0094] Spraying means the dispensing of droplets of solution in aerosol form.

[0095] Coating means the act of dipping and removing a substrate from the solution.

[0096] Preferably, the deposition of the liquid comprising the fluoride ions and / or the fluoride ion donor species is carried out using a spinner. In other words, the deposition of the liquid comprising the fluoride ions and / or the fluoride ion donor species comprises rotating the first hydrophilic silicon oxide bonding surface and depositing a volume of the liquid comprising the fluoride ions and / or the fluoride ion donor species in the center of the rotating first hydrophilic silicon oxide bonding surface, for example using a pipette, the rotation of the first hydrophilic silicon oxide bonding surface making it possible to spread said volume of the liquid comprising the fluoride ions and / or the fluoride ion donor species over the entire first hydrophilic silicon oxide bonding surface.

[0097] The volume of the liquid comprising the fluoride ions and / or the fluoride ion donor species is preferably between 1 cm 3 and 100 cm 3 for a first substrate of 300mm diameter, while the rotation speed of the first bonding surface during deposition is preferably between 100 rpm and 3000 rpm.

[0098] Following the deposition of the volume of liquid comprising the fluoride ions and / or the fluoride ion donor species, the first hydrophilic silicon oxide bonding surface is then kept rotating until the volume of liquid comprising the fluoride ions and / or the fluoride ion donor species is ejected and evaporated.

[0099] Preferably, the first hydrophilic silicon oxide bonding surface is kept rotating at a rotation speed of between 100 rpm and 3000 rpm for a duration of between 1 s and 1 min.

[0100] The deposition of the liquid comprising the fluoride ions and / or the fluoride ion donor species by spin coating is very advantageous insofar as it can be implemented in machines conventionally used for bonding substrates in the microelectronics industry. Indeed, said machines already integrate, for the purposes of cleaning and preparing surfaces for their bonding, the equipment which allows the deposition of the volume of liquid comprising the fluoride ions and / or the fluoride ion donor species and the spreading of said volume as previously described. The person skilled in the art therefore does not need to invest in new equipment to implement the invention according to this embodiment.

[0101] Maintaining rotation of the first hydrophilic silicon oxide bonding surface until ejection and evaporation of the liquid volume comprising the fluoride ions and / or the fluoride ion donor species allows drying of the first hydrophilic silicon oxide bonding surface, while retaining a sufficient quantity of fluoride ions on the first hydrophilic silicon oxide bonding surface.

[0102] The choice of rotation speed allows, to a certain extent, to modulate the quantity of fluoride ions remaining on the surface at the time of bonding by modulating the share of the liquid volume comprising the fluoride ions and / or the fluoride ion donor species that is ejected relative to the share of said volume that is evaporated. A high speed allows excess solution to be removed more efficiently and to avoid the formation of drops, and increases the evaporation rate. However, if the rotation speed is higher than 3000 rpm, too large a share of the liquid volume comprising the fluoride ions and / or the fluoride ion donor species is ejected so that not enough fluoride ions remain on the surface. Conversely, if the speed is lower than 100 rpm, too large a share of the liquid volume comprising the fluoride ions and / or the fluoride ion donor species is evaporated, so that there are too many fluoride ions on the surface.

[0103] Furthermore, said amount of fluoride ions and / or fluoride ion donor species deposited retained on the surface can be adjusted using different parameters, such as the concentration of fluoride ions and / or fluoride ion donor species of the liquid comprising the fluoride ions and / or fluoride ion donor species and the pH of said liquid which influences the surface charge of the oxide.

[0104] According to this embodiment of the supply of fluoride ions, the method does not include any additional step of terminating the hydrophilic silicon oxide bonding surfaces, for example no RCA washing, between the supply of fluoride ions and the contacting of the two substrates to be bonded. Indeed, it is appropriate not to eliminate said fluoride ions physisorbed on the first, and possibly on the second, hydrophilic silicon oxide bonding surface so that said fluoride ions are found in close proximity to the bonding interface when the two substrates to be bonded are brought into contact and can play their role as catalyst.

[0105] As previously mentioned, the method according to the invention is particularly advantageous when the substrates to be bonded are substrates comprising a weakening zone formed by implantation of ions or atoms or comprising elements of electrical components and / or comprise several layers made of distinct materials, in particular materials having very different thermal expansion coefficients, for example thermal expansion coefficients differing by more than 10%.

[0106] Indeed, the addition of fluoride ions to each hydrophilic silicon oxide bonding surface to be treated can very advantageously be carried out at room temperature, typically at a temperature between 18°C ​​and 25°C. In addition, the method does not require annealing of each of the substrates on the surface of which the fluoride ions have been added. Annealing the hydrophilic silicon oxide bonding surfaces after addition of the fluoride ions would result in the diffusion of the fluoride ions from the first, possibly second, hydrophilic silicon oxide bonding surface towards the interior of the first, possibly second substrate, thus generating a porous layer in the subsurface.However, as previously mentioned, the desired effect is the catalysis, by the fluoride ions physisorbed on the first bonding surface made of hydrophilic silicon oxide and possibly on the second bonding surface made of hydrophilic silicon oxide, of the condensation reactions at the origin of the adhesion between the first bonding surface made of hydrophilic silicon oxide and the second bonding surface made of hydrophilic silicon oxide when said first and second bonding surfaces made of hydrophilic silicon oxide are brought into contact.Such an effect differs from that induced by the presence of a porous layer inside the first and possibly the second substrate which would be generated by the annealing of the substrates on which the fluoride ions have been added, namely shifting the equilibrium of said condensation reactions in the direction of adhesion between the first bonding surface made of hydrophilic silicon oxide and the second bonding surface made of hydrophilic silicon oxide by allowing the diffusion, far from the bonding interface, of the water produced in said reaction direction.

[0107] Thus, the addition of fluoride ions is preferably carried out at room temperature and each hydrophilic silicon oxide bonding surface on which the fluoride ion addition has been carried out is preferably maintained at room temperature for the entire duration between the addition of fluoride ions and the contacting of the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface. In this way, the method is also more economical, less energy-intensive and easier to implement.

[0108] According to a particular embodiment of the supply of fluoride ions in liquid form, the deposition of the liquid comprising the fluoride ions or the fluoride ion donor species may be preceded by an activation of the hydrophilic silicon oxide bonding surface using a plasma. Indeed, the activation of a silicon oxide bonding surface by plasma is known to those skilled in the art to have a beneficial effect on the bonding energy: the plasma in fact makes it possible to break numerous bonds, in particular Si-O-Si bonds and to generate highly reactive species. Upon exposure of the plasma-activated surface to water or a humid atmosphere, said reactive species will react with the water to form new surface Si-OH silanols.Thus, plasma activation ultimately makes it possible to increase the number of surface silanol groups which will then be able to react in the condensation reactions when the substrates are brought into contact for bonding. The inventors have demonstrated a beneficial effect of the combination of plasma activation and the supply of fluoride ions or fluoride ion donor species in liquid form since such a combination, all other parameters remaining unchanged, makes it possible to increase the bonding energy compared to simple plasma activation or the sole supply of fluoride ions or fluoride ion donor species in liquid form. Plasma is, for example, a plasma formed from a gas such as oxygen, nitrogen, helium or argon.Advantageously, the plasma does not comprise fluoride ion donor species and the supply of fluoride ions or fluoride ion donor species is carried out in a second separate step by means of the liquid comprising the fluoride ions or the fluoride ion donor species. Such an embodiment advantageously makes it possible to optimize the quantities of fluoride ions and / or fluoride ion donor species used and not to overfluorinate, and therefore modify, the silicon oxide layer.

[0109] Each hydrophilic silicon oxide bonding surface to be fluorinated can be exposed to said plasma for a period of between 1 s and 1 min.

[0110] Intake in gaseous form

[0111] Alternatively, the intake of fluoride ions may include exposure to a gas comprising fluoride ion donor species.

[0112] Upon contact with water adsorbed on the hydrophilic silicon oxide bonding surface, molecules of the fluoride ion donor species will adsorb on the hydrophilic silicon oxide bonding surface and dissociate to release fluoride ions.

[0113] For example, the fluoride ion donor species is gaseous hydrofluoric acid or gaseous boron trifluoride BF3.

[0114] Preferably, however, the fluoride ion donor species is not hydrofluoric acid or any other acidic gas or gas with an etching effect that could damage the silicon oxide layer. For example, boron trifluoride BF3 gas, when adsorbed on the surface, reacts with water and forms hydrofluoric acid. It is, moreover, a very toxic gas.

[0115] Thus, in preferred embodiments, the fluoride ion donor species is sulfur hexafluoride SFe and / or tetrafluoromethane CF4, mixed with ammonia NH3 to basify the surface and avoid producing hydrofluoric acid or the fluoride ion donor species is nitrogen trifluoride NF3 gas in a medium which, in a basic medium, gives nitrites and fluoride ions. The gas comprising the fluoride ion donor species is selected from argon or dinitrogen. Alternatively, the gas may be directly the vapors of the fluoride ion donor species.

[0116] The partial pressure of fluoride ion donor species is, for example, between 0.1 mbar and 1 bar.

[0117] Each hydrophilic silicon oxide bonding surface to be fluorinated is exposed to said gas for a period preferably between 1 s and 1 min.

[0118] According to this embodiment also, the supply of fluoride ions can very advantageously be implemented at low temperature. More precisely, the gas temperature can be between 18 °C and 25 °C. In addition, each hydrophilic silicon oxide bonding surface can be maintained, after supply of fluoride ions, at room temperature until the two substrates to be bonded come into contact.

[0119] According to a particular embodiment of the supply of fluoride ions in gaseous form, the exposure of the hydrophilic silicon oxide bonding surface to the gas comprising fluoride ion donor species may be preceded by an activation of the hydrophilic silicon oxide bonding surface using a plasma. Such plasma activation may be implemented as previously described for the supply in liquid form, and makes it possible to increase the bonding energy compared to simple plasma activation or to the sole supply of fluoride ions or fluoride ion donor species in gaseous form.

[0120] Advantageously, the plasma does not comprise fluoride ion donor species and the supply of fluoride ions or fluoride ion donor species is carried out in a second separate step using the gas comprising the fluoride ion donor species. Such an embodiment advantageously makes it possible to optimize the quantities of fluoride ion donor species used and not to fluorinate, and therefore to modify as little as possible, the silicon oxide layer.

[0121] On the other hand, when the supply of fluoride ions is carried out in gaseous form, the method also does not include an additional step of terminating the hydrophilic silicon oxide bonding surfaces, for example no washing, between the supply of fluoride ions in gaseous form and the contacting of the two substrates to be bonded so as not to eliminate the fluoride ions physisorbed on the hydrophilic silicon oxide bonding surface on which the supply of fluoride ions has been implemented. Supply in a plasma

[0122] Alternatively, the supply of fluoride ions can successively comprise:

[0123] - exposing the hydrophilic silicon oxide bonding surface to a fluorinated plasma, in other words to a plasma comprising fluoride ion donor species, so as to obtain a hydrophilic silicon oxide bonding surface activated by fluorinated plasma;

[0124] - rinsing the fluorinated plasma-activated hydrophilic silicon oxide bonding surface with water or exposing said surface to a humid atmosphere, for example air, so as to provide the fluoride ions within the meaning of the present invention.

[0125] Fluoride ion donor species included in plasma are typically selected from SFe, CF4 and CH2F2.

[0126] Fluoride plasma is, for example, a plasma formed from a gas such as oxygen, nitrogen, helium or argon into which the said fluoride ion donor species have been incorporated.

[0127] The concentration of fluoride ion donor species in plasma is preferably between 0.1% and 10% in order to optimize the interaction between the majority species in the plasma and the fluorinated molecules.

[0128] Each hydrophilic silicon oxide bonding surface to be fluorinated is preferably exposed to the fluorinated plasma for a period of between 1 s and 1 min.

[0129] Water rinsing of the fluorinated plasma-activated hydrophilic silicon oxide bonding surface is typically implemented by rinsing with deionized water.

[0130] Exposure of the hydrophilic silicon oxide bonding surface to fluorine plasma results in the breaking of many bonds and the formation of highly reactive species such as Si-F, Si-HF or Si-OF on the surface.

[0131] Upon rinsing or exposure to a humid atmosphere of the activated hydrophilic silicon oxide bonding surface, said highly reactive species will react with water to create numerous surface silanols and release mobile fluoride ions physisorbed near said surface silanols, thereby generating the supply of fluoride ions within the meaning of the present invention. Thus, as previously mentioned, exposure to a fluorinated plasma has the major drawback of physically modifying the silicon oxide layer and therefore its properties such as its dielectric constant. Supply in a plasma is therefore not the preferred supply method.

[0132] According to this embodiment also, the supply of fluoride ions can very advantageously be implemented at low temperature. More precisely, despite the fact that the plasma temperature (defined by the kinetic energy of the electrons) is very high (typically between 10 4 and 105 K), the substrate temperature remains between 18°C ​​and 30°C.

[0133] Finally, each hydrophilic silicon oxide bonding surface exposed to the fluorinated plasma and rinsed can then be kept at room temperature until the two substrates to be bonded come into contact.

[0134] Contribution by implantation

[0135] Alternatively, the supply of fluoride ions can successively comprise:

[0136] - the implantation of low-energy fluorine through the silicon oxide bonding surface, so as to obtain an implanted hydrophilic silicon oxide bonding surface,

[0137] - rinsing the implanted hydrophilic silicon oxide bonding surface with water or exposing said surface to a humid atmosphere, for example air, so as to provide the fluoride ions within the meaning of the present invention.

[0138] Low energy implantation is understood to mean implantation at implantation energies below 25 keV in SiFs or below 5 keV for fluorine ions, which corresponds to an implantation depth of the order of 5 nm to 10 nm.

[0139] Just like exposure to fluorine plasma, fluorine implantation results in the breaking of many bonds and the formation on the surface of highly reactive species such as Si-F, Si-HF or Si-OF.

[0140] Upon rinsing or exposure to a humid atmosphere of the implanted hydrophilic silicon oxide bonding surface, said highly reactive species will react with water to create numerous surface silanols and release mobile fluoride ions physisorbed near said surface silanols, thereby generating the supply of fluoride ions within the meaning of the present invention. Thus, as previously mentioned, implantation has the major drawback of physically modifying the silicon oxide layer and therefore its properties such as its dielectric constant. Supply by implantation is therefore not the preferred supply method.

[0141] According to this embodiment also, the supply of fluoride ions can very advantageously be implemented at low temperature. More precisely, the temperature of the substrate during implantation can be between 10 °C and 50 °C, with regulation of the temperature of the implanted substrate.

[0142] Finally, each hydrophilic silicon oxide bonding surface, after addition of fluoride ions by implantation then rinsing, can then be maintained at room temperature until the two substrates to be bonded come into contact.

[0143] Hydrophilic silicon oxide bonding surface after addition of fluoride ions

[0144] After the addition of fluoride ions according to any one of the embodiments previously described, the surface concentration of fluoride ions of each hydrophilic silicon oxide bonding surface is preferably between 10 10 ions / cm 2 and 10 14 ions / cm 2 Such a surface concentration remains sufficiently low not to disturb the interface properties while allowing efficient catalysis.

[0145] The measurement of the surface concentration of fluoride ions on the first hydrophilic silicon oxide bonding surface can be done by spectroscopy techniques such as XPS or mass spectroscopy.

[0146] Contacting the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface

[0147] In the case where only a supply of fluoride ions has been implemented on the first hydrophilic silicon oxide bonding surface, the direct bonding method further comprises a step of bringing the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface into contact, so as to obtain adhesion of the first substrate with the second substrate, the fluoride ions physisorbed on the first hydrophilic silicon oxide bonding surface being at the bonding interface. In other words, the fluoride ions have not been removed from the first hydrophilic silicon oxide bonding surface between the step of supplying the fluoride ions and the contacting step, for example due to an additional washing step.

[0148] In the case where a supply of fluoride ions has also been implemented on the second hydrophilic silicon oxide bonding surface, contact is made between the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface, so as to obtain adhesion of the first substrate with the second substrate, the fluoride ions physisorbed on the first hydrophilic silicon oxide bonding surface and on the second hydrophilic silicon oxide bonding surface being at the bonding interface.

[0149] In other words, the fluoride ions were not removed from the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface between the fluoride ion supply step and the contacting step, for example due to an additional washing step.

[0150] The supply of fluoride ions to both surfaces to be bonded can be advantageous to bring a greater quantity of fluoride ions to the interface, while using less concentrated solutions and thus reducing the risk of crystal formation during drying.

[0151] The contact between the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface is preferably carried out at room temperature.

[0152] The contacting may comprise the application of localized pressure, so as to generate a point of contact between the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface: the bonding wave propagates from the point of contact in a few seconds over the entire bonding interface. Alternatively, the contacting may be done without applying pressure. According to this embodiment, the first substrate and the second substrate may remain against each other for a few tens of seconds until the evacuation of the air between the two substrates causes the appearance of a point of contact and the start of the propagation of the bonding wave. The vapor pressure during the contacting of the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface is preferably between 1 Pa and 10 5 Pa. Tl

[0153] The addition of fluoride ions to at least one of the two hydrophilic silicon oxide bonding surfaces followed by bringing the two hydrophilic silicon oxide bonding surfaces into contact, the fluoride ions being at the bonding interface, and maintaining the assembly resulting from said contact at room temperature (after the end of the propagation of the bonding wave), makes it possible to achieve bonding energies of the order of 1 J / m 2 at 2J / m 2 after a few dozen hours. By comparison, such a bonding energy is only reached, without the addition of fluoride ions and all other experimental parameters remaining unchanged, after several days. Thus, the addition of fluoride ions makes it possible to increase the bonding kinetics.

[0154] To further increase said bonding kinetics, the direct bonding method may further comprise a subsequent step of consolidation annealing of the assembly obtained after bringing the first substrate and the second substrate into contact.

[0155] For example, annealing consists of applying a temperature between 50°C and 250°C, even more preferably between 50°C and 100°C, for a duration preferably between 15 min and 4 h, even more preferably between 15 min and 2 h. Consolidation annealing temperatures below 100°C would not allow bonding energies of the order of 1 J / m to be achieved. 2 or 2 J / m 2 in less than two hours without the addition of fluoride ions.

[0156] The annealing temperature is chosen according to the nature of the first substrate and the second substrate and their ability or not to withstand a given temperature.

[0157] Process for transferring a layer onto a support substrate

[0158] The invention extends to a method of transferring a layer onto a support substrate.

[0159] The method of transferring a layer onto a support substrate comprises a step of providing a donor substrate comprising a first surface of hydrophilic silicon oxide and a support substrate comprising a second surface of hydrophilic silicon oxide.

[0160] The donor substrate and the support substrate are such as the first substrate and the second substrate previously described.

[0161] For example, the donor substrate and / or the support substrate comprise silicon Si, germanium Ge, indium phosphide InP, gallium arsenide AsGa, alumina AI2O3, silicon oxide SiC>2, silicon nitride SiaN4, silicon carbide SiC, gallium nitride GaN, copper, titanium, nickel, LNO and / or LTO.

[0162] The donor substrate and / or the support substrate may be multilayer structures comprising at least two layers. For example, the donor substrate is a substrate referred to as a pseudo-donor substrate comprising a first layer of LTO and a second layer of silicon. For example, the support substrate is silicon.

[0163] In addition, the donor substrate and / or the support substrate may comprise one or more electronic component elements. For example, the donor substrate and / or the support substrate comprise copper connections insulated by silicon oxide portions.

[0164] Thus, the step of providing a donor substrate comprising a first hydrophilic silicon oxide surface and a support substrate comprising a second hydrophilic silicon oxide surface may comprise one or more sub-steps of arranging layers and forming electronic component elements as previously described.

[0165] As previously mentioned, the donor substrate and the support substrate respectively comprise a first hydrophilic silicon oxide surface and a second hydrophilic silicon oxide surface. The first hydrophilic surface and / or the second hydrophilic surface are typically native, thermal, chemical or deposited oxide surfaces as previously described. Thus, the step of providing a donor substrate comprising a first hydrophilic silicon oxide surface and a support substrate comprising a second hydrophilic silicon oxide surface may comprise forming said native, thermal, chemical or deposited silicon oxide surfaces according to any of the previously described embodiments.

[0166] The first hydrophilic silicon oxide surface and the second hydrophilic silicon oxide surface may be of the same nature (native, thermal, chemical or deposited). Alternatively, the first hydrophilic silicon oxide surface and the second hydrophilic silicon oxide surface may be of different natures.

[0167] The method for transferring a layer onto a support further comprises a step of forming a weakening zone within the donor substrate so as to delimit a layer to be transferred between the first hydrophilic surface and the weakening zone. According to a preferred embodiment, the weakening zone is formed by implanting atomic species into the donor substrate, the implantation being carried out through the first hydrophilic surface. The atomic species are implanted at a determined depth, this depth fixing the thickness of the layer to be transferred. The implanted atomic species are preferably hydrogen and / or helium.

[0168] Subsequently, the method of transferring a layer onto a support substrate further comprises bonding between the donor substrate and the support substrate, the first hydrophilic silicon oxide surface and the second hydrophilic silicon oxide surface being at the bonding interface and the bonding between the donor substrate and the support substrate being implemented by a direct bonding method according to any of the previously described embodiments. The supply of fluoride ions can be implemented on the first hydrophilic silicon oxide surface of the donor substrate, on the second hydrophilic silicon oxide surface of the support substrate or both on the first hydrophilic silicon oxide surface of the donor substrate and on the second hydrophilic silicon oxide surface of the support substrate.

[0169] The layer transfer method finally comprises detaching the donor substrate along the weakening zone so as to transfer the layer to be transferred onto the support substrate. The detachment along the weakening zone can be triggered by a mechanical action and / or a supply of thermal energy. According to a particular embodiment, the consolidation annealing and the detachment step along the weakening zone can be coupled within the same temperature rise ramp, for example by carrying out temperature steps.

[0170] Substrate

[0171] The invention also relates to a substrate having a first surface made of hydrophilic silicon oxide on which fluoride ions are physisorbed, the surface concentration of fluoride ions physisorbed on said first surface made of hydrophilic silicon oxide being between 10 10 ions / cm 2 and 10 14 ions / cm2 .

[0172] The first substrate and the first hydrophilic silicon oxide bonding surface may be according to any of the previously described embodiments.

[0173] As previously mentioned, the measurement of the surface concentration of fluoride ions on the first hydrophilic silicon oxide bonding surface can be done by spectroscopy techniques such as XPS or mass spectroscopy. In a preferred embodiment, said substrate does not comprise a buried fluorinated porous silicon oxide layer. In other words, the concentration of fluoride ions in the silicon oxide layer of the substrate, elsewhere than at the interface, is less than 10 18 at / cm 3Such a substrate can advantageously be obtained by avoiding the embodiments of the supply of fluoride ions and / or fluoride ion donor species which have an acidifying, etching or modifying effect on the silicon oxide layer as previously mentioned (for example plasma, implantation, exposure to liquid or vapor hydrofluoric acid and exposure to an ammonium fluoride solution).

[0174] Assembly comprising a first substrate and a second substrate

[0175] The invention finally relates to an assembly comprising a first substrate and a second substrate, the first substrate being covered by a first surface of hydrophilic silicon oxide and the second substrate being covered by a second surface of hydrophilic silicon oxide, the first surface of hydrophilic silicon oxide being bonded directly with the second surface of hydrophilic silicon oxide, fluoride ions being at the bonding interface.

[0176] The first substrate, the first hydrophilic silicon oxide bonding surface, the second substrate and the second hydrophilic silicon oxide bonding surface may be according to any of the previously described embodiments.

[0177] Preferably, the surface concentration of fluoride ions at the bonding interface between the first hydrophilic silicon oxide surface and the second hydrophilic silicon oxide surface is between 10 1 ° ions / cm 2 and 10 14 ions / cm 2 .

[0178] The measurement of the surface concentration of fluorine at the bonding interface between the first hydrophilic silicon oxide surface and the second hydrophilic silicon oxide surface can be done by high-resolution electron microscopy techniques, coupled with EELS (Electron Energy Loss Spectrocopy) or EDX (X-ray spectroscopy) type spectroscopies. Other direct dosage techniques can be used, by opening the interface.

[0179] In a preferred embodiment, said assembly does not comprise a buried fluorinated porous silicon oxide layer. In other words, the concentration of fluoride ions in each silicon oxide layer of the first substrate and the second substrate, from the first atomic layers (first nanometer below the surface for example) is less than 10 18 at / cm 3 The oxides are therefore not modified in volume and retain their properties (for example their dielectric constants and their refractive indices) up to their surface.

[0180] Such an assembly can advantageously be obtained by avoiding the embodiments of the supply of fluoride ions and / or fluoride ion donor species which have an acidifying, etching or modifying effect on the silicon oxide layer as previously mentioned (for example plasma, implantation, exposure to liquid or vapor hydrofluoric acid and exposure to an ammonium fluoride solution).

[0181] Examples

[0182] The following describes a series of examples of implementation of a method according to particular embodiments of the invention.

[0183] Example 1

[0184] 10mL of a cesium fluoride solution at a concentration of 10' is dispensed 4M on a first silicon substrate 200 mm in diameter having a first bonding surface made of hydrophilic silicon oxide of the thermal oxide type 100 nm thick. The system is rotated at a speed of 2000 rpm until the thickness of the film formed by the deposited solution is uniform. This uniformization can be conventionally observed by observing colored Newton fringes, which pass as the film thins under the combined effect of centrifugal flow and evaporation, until they disappear when the thickness of the film is below half a wavelength of visible radiation (i.e., typically less than 200 nm). The substrate is then kept rotating for about ten additional seconds, so as to evacuate the excess aqueous solution of cesium fluoride and to dry the first bonding surface made of hydrophilic silicon oxide.

[0185] A second silicon substrate of 200 mm diameter having a second bonding surface of hydrophilic silicon oxide of thermal oxide type of 100 nm thickness is provided, and the same treatment as previously described for the first silicon substrate is applied.

[0186] The first hydrophilic silicon oxide bonding surface is then brought into contact with the second hydrophilic silicon oxide bonding surface at room temperature so as to obtain adhesion by direct bonding between the first substrate and the second substrate. Optionally, a consolidation annealing of the newly formed structure comprising the first substrate and the second substrate is carried out at a temperature of 220°C for a period of between 10 minutes and 250 minutes.

[0187] Finally, the bonding energies obtained as a function of the duration of said consolidation annealing are measured by the classic blade insertion method, known as the “Maszara method”, in an anhydrous atmosphere. The results are represented in square symbols in Figure 2.

[0188] Comparative example 1

[0189] Exactly the same operation as in Example 1 is carried out, replacing the cesium fluoride solution with deionized water (DIW). The results are shown in round symbols in Figure 2.

[0190] This shows the effect of dispensing a solution containing fluoride ions.

[0191] Example 2

[0192] 10mL of a cesium fluoride solution at a concentration of 10' is dispensed 4M on a first silicon substrate 200 mm in diameter having a first bonding surface made of hydrophilic silicon oxide of thermal oxide type 100 nm thick. The system is rotated at a speed of 2000 rpm until the Newton fringes disappear. Then the substrate is kept rotating for about ten additional seconds, so as to evacuate the excess aqueous solution of cesium fluoride and to dry the first bonding surface made of hydrophilic silicon oxide.

[0193] A second silicon substrate of 200 mm diameter having a second bonding surface of hydrophilic silicon oxide of thermal oxide type of 100 nm thickness is provided and the same treatment as previously described for the first silicon substrate is applied.

[0194] The first hydrophilic silicon oxide bonding surface is then brought into contact with the second hydrophilic silicon oxide bonding surface at room temperature so as to obtain adhesion by direct bonding between the first substrate and the second substrate. Optionally, a consolidation annealing of the newly formed structure comprising the first substrate and the second substrate is carried out at a temperature of 170°C for a period of between 10 minutes and 250 minutes. Finally, the bonding energies obtained as a function of the duration of said consolidation annealing are measured by the conventional blade insertion method, known as the “Maszara method”, in an anhydrous atmosphere. The results are represented in square symbols in Figure 3.

[0195] Comparative example 2

[0196] Exactly the same operation as in Example 2 is carried out, replacing the cesium fluoride solution with deionized water (DIW). The results are shown in round symbols in Figure 3.

[0197] Example 3

[0198] 10mL of a cesium fluoride solution at a concentration of 10' is dispensed 4 M on a first silicon substrate 200 mm in diameter having a first bonding surface made of hydrophilic silicon oxide of thermal oxide type 100 nm thick. The system is rotated at a speed of 2000 rpm until the Newton fringes disappear. Then the substrate is kept rotating for about ten additional seconds, so as to evacuate the excess aqueous solution of cesium fluoride and to dry the first bonding surface made of hydrophilic silicon oxide.

[0199] A second silicon substrate of 200 mm diameter having a second bonding surface of hydrophilic silicon oxide of thermal oxide type of 100 nm thickness is provided and the same treatment as previously described for the first silicon substrate is applied.

[0200] The first hydrophilic silicon oxide bonding surface is then brought into contact with the second hydrophilic silicon oxide bonding surface at room temperature so as to obtain adhesion by direct bonding between the first substrate and the second substrate. Optionally, a consolidation annealing of the newly formed structure comprising the first substrate and the second substrate is carried out at a temperature between 100°C and 300°C for a period of 2 hours.

[0201] Finally, the bonding energies obtained are measured as a function of the consolidation annealing temperature and the concentration of the deposited cesium fluoride solution. The results are shown in Figure 4 in diamond symbols (concentration of the deposited cesium fluoride solution of 10' 3 mol / L), squares (10' concentration 4 mol / L) and triangles (10' concentration 5 mol / L). Comparative examples

[0202] Exactly the same operation as in Example 3 is carried out, replacing the cesium fluoride solution with deionized water (DIW). The results are shown in round symbols in Figure 4.

[0203] Example 4

[0204] A first silicon substrate of 200 mm diameter having a first bonding surface of hydrophilic silicon oxide of thermal oxide type of 100 nm thickness is provided. A second substrate identical to the first substrate is provided.

[0205] The first substrate and the second substrate are placed in an enclosure in which dilute vapor hydrofluoric acid is circulated, so as to expose the first hydrophilic silicon oxide bonding surface to the hydrofluoric acid vapor for a period of 10, 30 or 60 seconds.

[0206] The gas is then purged and the first hydrophilic silicon oxide bonding surface is brought into contact with the second hydrophilic silicon oxide bonding surface upon leaving the enclosure, at room temperature, so as to obtain adhesion by direct bonding between the first substrate and the second substrate. Optionally, consolidation annealing of the newly formed structure comprising the first substrate and the second substrate is carried out at a temperature between 100°C and 500°C for a period of 2 hours.

[0207] Finally, the bonding energies obtained are measured as a function of this consolidation annealing temperature. The results are shown in Figure 5 in triangle symbols (for 10s of exposure), squares (30s of exposure) or pentagon (60s of exposure). The reference corresponds to bonding carried out without introducing HF gas into the enclosure.

[0208] Example 5

[0209] Example 5 shows the effect of exposure to a 0.05% dilute hydrofluoric acid solution for 1 minute. The solution is dispensed by coating, i.e., by simply dipping the plates into a very dilute HF solution. The plates are then slowly removed from the solution. They appear "dry" upon leaving the solution. They are immediately bonded to each other. A 2-hour consolidation annealing is then carried out at different temperatures, from 100°C to 500°C. The curve in solid round symbols in Figure 6 represents the energies obtained after these different annealings. The reference curve corresponds to the same experimental protocol but where the hydrofluoric acid solution has been replaced by pure water (deionized water).

Claims

CLAIMS 1. A method of direct bonding between two substrates comprising the following steps: a) providing a first substrate and a second substrate respectively comprising a first bonding surface made of hydrophilic silicon oxide and a second bonding surface made of hydrophilic silicon oxide, b) supplying fluoride ions to the first bonding surface made of hydrophilic silicon oxide; c) bringing the first bonding surface made of hydrophilic silicon oxide and the second bonding surface made of hydrophilic silicon oxide into contact, so as to obtain adhesion of the first substrate with the second substrate with the fluoride ions at the bonding interface.

2. The method of claim 1, further comprising providing fluoride ions to the second hydrophilic silicon oxide bonding surface prior to contacting the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface.

3. Method according to one of claims 1 or 2, in which the contacting of the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface is carried out at room temperature.

4. Method according to one of claims 1 to 3, in which, during the entire time between the supply of fluoride ions and the contacting of the first hydrophilic silicon oxide surface and the second hydrophilic silicon oxide surface, each hydrophilic silicon oxide bonding surface on which the supply of fluoride ions has been carried out is maintained at room temperature and / or, in which, the supply of fluoride ions on the first hydrophilic silicon oxide bonding surface is carried out at room temperature.

5. Method according to one of claims 1 to 4, further comprising a step d) of consolidation annealing implemented after step c) of bringing the first hydrophilic silicon oxide bonding surface into contact with the second hydrophilic silicon oxide bonding surface, the consolidation annealing being preferably implemented at a temperature of between 50°C and 250°C, even more preferably at a temperature of between 50°C and 100°C, for a duration of preferably between 15 min and 4 hours, even more preferably for a duration of between 15 min and 2 hours.

6. Method according to one of claims 1 to 5, in which the supply of fluoride ions to the first hydrophilic silicon oxide bonding surface comprises the deposition of a liquid comprising fluoride ions and / or fluoride ion donor species on the first hydrophilic bonding surface.

7. The method of claim 6, wherein the deposition of the liquid on the first hydrophilic silicon oxide bonding surface comprises: - the deposition of a volume of the liquid comprising the fluoride ions and / or the fluoride ion donor species at the center of the first hydrophilic silicon oxide bonding surface, - spreading said volume of liquid comprising the fluoride ions and / or the fluoride ion donor species by rotation of the first hydrophilic silicon oxide bonding surface, said first hydrophilic silicon oxide bonding surface being kept in rotation until ejection and evaporation of the volume of liquid comprising the fluoride ions and / or the fluoride ion donor species.

8. Method according to one of claims 6 or 7, in the liquid comprising the fluoride ions and / or the fluoride ion donor species does not comprise hydrofluoric acid or ammonium fluoride.

9. Method according to one of claims 6 to 8, in which the liquid comprising the fluoride ions and / or the fluoride ion donor species comes from the dissolution of a salt of the fluoride ion and a counter-ion which is a neutral or basic cation.

10. Method according to one of claims 1 to 5, in which the supply of fluoride ions to the first hydrophilic silicon oxide bonding surface comprises: - exposing said first hydrophilic silicon oxide bonding surface to a fluorinated plasma, - rinsing with water, or exposing to a humid atmosphere, the first hydrophilic silicon oxide bonding surface previously exposed to the fluorinated plasma.

11. The method of any one of claims 1 to 5, wherein providing fluoride ions to the first hydrophilic silicon oxide bonding surface comprises exposing the first hydrophilic silicon oxide bonding surface to a gas containing fluorinated species.

12. The method of claim 11, wherein the gas containing fluorinated species does not comprise hydrofluoric acid.

13. Method according to one of claims 11 or 12, in which the gas comprises sulfur hexafluoride SFe and / or tetrafluoromethane CF4, mixed with ammonia NH3 or nitrogen trifluoride NF3 in a basic medium.

14. Method according to one of claims 1 to 5, in which the supply of fluoride ions to the first hydrophilic silicon oxide bonding surface comprises: - implantation of fluorine using an implanter on the first hydrophilic silicon oxide bonding surface, - rinsing with water, or exposing to a humid atmosphere, the first bonding surface made of previously implanted hydrophilic silicon oxide.

15. Method according to one of claims 1 to 14, in which the first hydrophilic silicon oxide surface and / or the second hydrophilic silicon oxide surface are made of chemical oxide, deposited oxide, thermal oxide and / or native oxide.

16. Method according to one of claims 1 to 15, in which the first substrate is a multilayer structure comprising a first layer of a first material and a second layer of a second material, the difference in coefficient of thermal expansion between the first material and the second material being greater than or equal to 10% or 10' 6 K -1 in absolute value.

17. Method according to one of claims 1 to 16, in which the first substrate comprises electronic component elements.

18. Method according to one of claims 1 to 17, in which the first substrate comprises a weakening zone formed by implantation of atomic or ionic species.

19. A method of transferring a layer onto a support substrate, said method comprising the following steps: a) providing a donor substrate comprising a first surface of hydrophilic silicon oxide and a support substrate comprising a second surface of hydrophilic silicon oxide, b) forming a weakening zone within the donor substrate so as to delimit a layer to be transferred between the first surface of hydrophilic silicon oxide and the weakening zone, c) bonding between the donor substrate and the support substrate, the first surface of hydrophilic silicon oxide and the second surface of hydrophilic silicon oxide being at the bonding interface and the bonding between the donor substrate and the support substrate being carried out by a bonding method according to any one of claims 1 to 17,d) detaching the donor substrate along the weakening zone so as to transfer the layer to be transferred onto the support substrate., 20. An assembly comprising a first substrate and a second substrate, the first substrate being covered by a first hydrophilic silicon oxide surface and the second substrate by a second hydrophilic silicon oxide surface, the first hydrophilic silicon oxide surface being bonded directly with the second hydrophilic silicon oxide surface, fluoride ions being arranged between the first hydrophilic silicon oxide bonding surface and the second hydrophilic silicon oxide bonding surface, wherein the surface concentration of fluoride ions at the bonding interface between the first hydrophilic silicon oxide surface and the second hydrophilic silicon oxide surface is between 1O 10 ions / cm 2 and 10 14 ions / cm2 , and wherein the fluoride ion concentration in the first substrate and in the second substrate at a distance greater than 1 nm from the bonding interface between the first substrate and the second substrate is less than 10 18 at / cm 3 .

21. Substrate having a first surface made of hydrophilic silicon oxide on which fluoride ions are physisorbed, the surface concentration of fluoride ions physisorbed on said first surface made of hydrophilic silicon oxide being between 10 10 ions / cm 2 and 10 14 ions / cm 2 , and wherein the fluoride ion concentration in said substrate at a distance greater than 1 nm from the first surface is less than 10 18 at / cm 3 .

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