Strong base-assisted direct bonding method

Abstract

To provide a direct bonding method between two substrates and to provide an assembly that can be obtained with such a method.SOLUTION: A method includes the steps of: providing a first substrate 1 and a second substrate 2 respectively including a first hydrophilic bonding surface 3 and a second hydrophilic bonding surface 4; depositing on the first and / or on the second hydrophilic bonding surface a basic solution consisting of strong base molecules and deionized water; drying the first and / or the second hydrophilic bonding surface until a concentration between approximately 109 atom / cm2 and 1015 atom / cm2 of cations resulting from the strong base molecules on the first and / or on the second hydrophilic bonding surface is obtained; and contacting the first and the second hydrophilic bonding surface so as to obtain a spontaneous direct bonding and an assembly of the first substrate 1 and the second substrate 2 including a direct bonding interface 6.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to the field of direct joining. In particular, the present invention relates to a method for directly joining two substrates. According to a second aspect, the present invention relates to an assembly that can be obtained by such a method. [Background technology]

[0002] Direct bonding is a well-known technique and is used in industrial applications, such as the production of SOI by SOITEC or STMicroelectronics for the manufacture of imaging devices. As understood herein, direct bonding is the spontaneous bonding of two surfaces without adding any material to the interface between the surfaces to be bonded, and in particular without a thick layer of liquid, i.e., without a layer having a thickness greater than 100 nm. Nevertheless, it is possible for these surfaces to have several monolayers of water adsorbed on the surface, with a thickness of 0.25–1.25 nm, so that they appear dry to the naked eye.

[0003] A key characteristic of direct bonding lies in its adhesive energy, or the energy available to spontaneously initiate the bonding process. This is the energy that allows the two surfaces to deform so that they come into contact at an atomic scale, thus enabling van der Waals forces. For example, for two surfaces of silicon or hydrophilic silicon oxide, the adhesive energy is typically 30–100 mJ / m². 2 This bonding energy partially refers to the propagation time of the bonding wave, for example, the bonding wave conventionally propagates in 9 seconds for the direct bonding of two 200 mm silicon substrates.

[0004] Another important characteristic of direct bonding is its bonding energy, or, in other words, bonding energy. This is the energy required to separate the two bonding surfaces. In the context of bonding two silicon substrates covered with a thermal oxide approximately 145 nm thick, this bonding energy is typically 0.14–6 J / m 2 That is the case.

[0005] To achieve spontaneous bonding and produce a multilayer structure that remains stable over time, the surface is generally cleaned of organic and particulate contaminants that are detrimental to direct bonding. For example, the surfaces to be bonded are pre-cleaned using a caro-acid-based solution obtained with a mixture of 96% sulfuric acid and 30% hydrogen peroxide (3:1) at 180°C, and SC1 (a mixture of 30% ammonia, 30% hydrogen peroxide, and deionized water (1:1:5)) at 70°C. Alternatively, other highly oxidizing cleaning solutions, such as aqueous solutions containing ozone or others, may be used after treatment with UV light in the presence of gaseous ozone. The adhesion energy of an SiO2-SiO2 bond (e.g., two silicon substrates covered with approximately 145 nm of thermal oxide) chemically cleaned with caro and SC1 at 70°C in a cleanroom atmosphere is approximately 140 mJ / m immediately after bonding, without heat treatment. 2 That is the case.

[0006] The bonding energy also increases depending on the heat treatment applied after bonding at room temperature. The bonding energy increases as a function of the heat treatment temperature. For example, the bonding energy of SiO2-SiO2 is 3 J / m at 500°C, although this also depends on the surface treatment. 2 It increases slowly until it reaches a certain temperature, and then does not increase further until it reaches 800°C.

[0007] To further increase the bonding energy, another solution involves plasma treatment before contact. In nitrogen (N2) plasma for oxide-oxide bonding, the bonding energy is approximately 5 J / m at a treatment temperature of 300°C. 2 It increases rapidly. [Overview of the project]

[0008] However, the use of plasma may not be applicable to certain substrates, and / or its use may make industrialization more difficult due to the increased time and / or cost of the method. Plasma treatment also modifies surfaces over a thickness of several nanometers (1-10 nm). This modification can affect future devices. For example, on silicon sheets, plasma can create oxide layers that are difficult to control in terms of thickness and quality. On silicon oxide surfaces, certain plasmas, such as N2 plasma, can cause interfacial charge problems that could interfere with the electrical operation of future devices.

[0009] Furthermore, as described in reference FR1312269, it is also possible to add a small amount of molecules having basic functional groups such as alcohol functional groups and amino alcohols to the hydrophilic surface before bonding, thereby directly increasing the bonding energy without requiring plasma treatment.

[0010] One of the objectives of the present invention is to improve upon the shortcomings of the prior art. To this end, the present invention proposes a method for direct bonding between two substrates, the method comprising the following steps: a) A step of providing a first substrate and a second substrate, each including a first hydrophilic bonding surface and a second hydrophilic bonding surface, b) A step of depositing a basic solution containing a strong base molecule and deionized water onto the first and / or second hydrophilic bonding surface, c) In the first and / or second hydrophilic bonding surfaces, the cation generated from the strong base molecule is approximately 10 9 ~10 15 atoms / cm 2 The first and / or second hydrophilic bonding surfaces are dried to the concentration of the following: d) Bringing the first and second hydrophilic bonding surfaces into contact in order to obtain an assembly composed of the first and second substrates, which includes a direct bonding interface.

[0011] The direct bonding method thus made is a spontaneous and immediate direct bonding. In fact, the very low concentration of basic molecules on the hydrophilic bonding surface does not change the adhesion energy and it is possible to deform the surface to make contact at the atomic scale. It requires a lower bonding annealing temperature to achieve the same bonding energy as when there are no molecules present. There is no need to perform plasma treatment. Plasma treatment is still possible and does not reduce the bonding energy obtained for a given annealing temperature. The assembly thus obtained is durable and can be immediately handled by conventional microelectronics tools without the need to keep the first substrate in contact with the second substrate in a specific manner, apply any pressure, or even place the assembly in a specific bonding energy enhancing environment. Furthermore, step b) of depositing the basic solution and the drying in step c) do not significantly increase the cycle time. These are simple steps to implement and are inexpensive. The strongly basic molecules used in very small amounts are inexpensive.

[0012] As used herein, the expression "strong base" means a base with a pKa of 14 or more in water. A strong base dissociates completely in H2O, giving OH - ions and cations in the solution.

[0013] The term "hydrophilic surface" means that at least one monolayer of water is adsorbed on the surface at ambient pressure (i.e., in the range of 1 bar) in air with a relative humidity of at least 1%. The surface is said to be hydrophilic if the angle of a water droplet is less than 90°, preferably less than 50°, and even more preferably less than 5°.

[0014] According to one possibility, step c) of drying is performed such that the first hydrophilic bonding surface and the second hydrophilic bonding surface are covered with 1 to 5 atomic monolayers (thickness 0.25 to 1.25 nm) of H2O. Thus, these surfaces are dry on a macroscopic scale and any atomic monolayers of residual water on the surface are obtained conventionally after drying the hydrophilic surface.

[0015] According to one configuration, the strong base molecule is composed of an inorganic molecule, particularly an alkali metal hydroxide, an alkaline earth metal hydroxide, or a mixture of these molecules.

[0016] According to one possibility, the strong base molecule is selected from the bases of LiOH, NaOH, KOH, RbOH, CsOH, Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2, and mixtures of these bases.

[0017] According to a specific embodiment, the basic solution is deposited on the first and / or second hydrophilic bonding surfaces by spin coating. This spin coating enables the homogeneous spreading of the basic solution by means of an adjustable rotation speed of the plate, for example a rotation speed of 0 to 500 revolutions per minute. This method ensures that the entire surface under consideration is sufficiently covered.

[0018] Specifically, the basic solution contains strong base molecules at a concentration of 10 -8 ~10 -2 mol / l in deionized water.

[0019] Advantageously, step c) is carried out by centrifuging the first and / or second hydrophilic bonding surfaces (3, 4) at a rotation speed of, for example, about 2000 revolutions per minute, particularly for 45 seconds. The centrifugation is carried out, in particular, after the spin coating in step b), using the same apparatus and without moving the first and second substrates.

[0020] According to another configuration, step b) is carried out by immersing the first and / or second hydrophilic bonding surfaces in the basic solution.

[0021] According to another variant, step c) is carried out by Marangoni drying.

[0022] According to one possibility, the method further includes step e), in which the assembly obtained in step d) is heat-treated at a temperature of 50 to 500°C, particularly 50 to 350°C, and particularly 60 to 250°C. This bonding annealing heat treatment can further strengthen the bonding energy while remaining at a low temperature, i.e., below 500°C.

[0023] However, no particular drawbacks were found, and, apart from limitations such as the restriction on reaching high junction energies, it is possible to raise the temperature to much higher, such as 900°C, 1100°C, or 1200°C, if necessary.

[0024] According to a particular embodiment, the first and second substrates are made of silicon.

[0025] According to one possibility, the first hydrophilic bonding surface and / or the second hydrophilic bonding surface are at least partially formed by a hydrophilic film in a material selected from silicon oxide, silicon nitride, copper oxide, and mixtures thereof.

[0026] The hydrophilic silicon oxide film is selected from thermal oxides, deposited oxides, and native oxides.

[0027] In one modified example, the first substrate provided in step a) includes one or more first vignettes resulting from the vignetage of the first substrate, thereby achieving direct bonding of one or more first vignettes to the second substrate.

[0028] The first hydrophilic bonding surface is defined by the exposed surfaces of one or more first bonding layers.

[0029] In another modified form, the second substrate given in step a) obtains a direct bond between one or more first bonding layers and one or more second bonding layers by including one or more second bonding layers resulting from the bonding layer formation of the second substrate.

[0030] The second hydrophilic bonding surface is defined by one or more exposed surfaces of the second bonding layer.

[0031] Therefore, the present invention enables direct bonding of one or more first bonding layers to a 1 mm × 1 mm surface on, for example, a 300 mm second substrate, and enables direct bonding of, for example, one or more first bonding layers, each having an area of ​​approximately 1 mm × 1 mm, to one or more second bonding layers.

[0032] According to a second aspect, the present invention proposes an assembly comprising a first substrate and a second silicon substrate, each including a first hydrophilic bonding surface and / or a second hydrophilic bonding surface, wherein the first and second hydrophilic bonding surfaces are joined by direct bonding, and a strong base molecule is present at the direct bonding interface between the first and second hydrophilic bonding surfaces at approximately 10 9 ~10 15 atoms / cm 2 , especially about 10 10 ~10 14 atoms / cm 2 It is provided at this concentration.

[0033] These concentrations are measured by mass spectrometry methods such as ICPMS (Inductively Coupled Plasma Mass Spectrometry) or TXRF (Total Reflectance X-ray Fluorescence).

[0034] It is understood that this assembly possesses the bonding energy required to prevent the substrates from separating after contact.

[0035] The first and second substrates are each formed from materials selected from LTO, LNO, diamond, alumina, and semiconductor materials, particularly silicon, germanium, silicon oxide, germanium oxide, silicon nitride, SiOC, SiC, InP, GaAs, GaN, and combinations thereof.

[0036] The first and / or second substrate is covered with a layer of silicon oxide, silicon nitride, copper oxide, or a mixture of these compounds.

[0037] According to an exemplary embodiment, the first and second substrates given in step a) are silicon substrates having a diameter of 100 to 300 mm, particularly 200 mm, and the first hydrophilic bonding surface and the second hydrophilic bonding surface are each formed by a hydrophilic film of silicon oxide.

[0038] According to other features, the direct joining method according to the present invention includes one or more of the following features, which may be considered individually or in combination:

[0039] - The first and second bonding surfaces given in step a) are prepared by treatment with Kalo, SC1, and ozonated water to remove particulate matter and organic contaminants and to make their surfaces hydrophilic.

[0040] - In step d), the first and second joining surfaces are brought into contact at room temperature.

[0041] - The deposition of the basic solution in step b) includes forming a basic preparation obtained by first mixing strong base molecules in deionized water.

[0042] - According to one configuration, the basic solution is a deionized water with a mass concentration of 10 -8 ~10 -4 g·cm -3 In particular, the mass concentration is about 10 -5 g·cm -3 It contains NaOH.

[0043] - The bonding annealing heat treatment in step e) is performed for approximately 2 hours.

[0044] - The contact in step c) of this method is performed under atmospheric pressure or vacuum.

[0045] - The heat treatment in step d) of this method is carried out under atmospheric pressure.

[0046] - In this direct bonding method, no external pressure is applied to the first and / or second substrates in or after step d).

[0047] - The first and / or second hydrophilic bonding surfaces have, prior to step d), a smaller amount of strong base molecules than is required to form an atomic monolayer or to fill all of the bonding sites of the first and / or second hydrophilic bonding surfaces.

[0048] - The first and / or second hydrophilic bonding surfaces are entirely formed by a continuous hydrophilic film in a material selected from silicon oxide, silicon nitride, copper oxide, or a mixture thereof.

[0049] - The first and / or second hydrophilic bonding surfaces have a roughness of less than 0.5 nm RMS (root mean square).

[0050] - The first and / or second hydrophilic bonding surfaces are made of silicon having native oxides.

[0051] - The first and / or second hydrophilic bonding surfaces are flat.

[0052] - The first and second substrates are each independently selected from substrates having a diameter of 2 to 12 inches, particularly 200 to 300 mm.

[0053] - One or more first bonding layers and / or one or more second bonding layers have a transverse dimension or diameter of 100 μm to 5 mm, for example, the first and / or second bonding layers have a substantially square main surface with a transverse dimension of 100 μm × 100 μm.

[0054] - The first and / or second substrate each includes at least one electronic component, such as a chip, which is covered with a hydrophilic film prior to step b) of depositing the basic solution.

[0055] - The first and second substrates are each independently selected from a solid substrate (bulk) or a composite substrate (several material layers).

[0056] - The first and second base materials may have the same properties or different properties.

[0057] Other aspects, purposes, and advantages of the present invention will be better revealed by the following description of two embodiments, given as non-limiting examples and with reference to the accompanying drawings. In the following description, for the sake of simplification, identical, similar, or equivalent elements in different embodiments are given the same reference numerals. The drawings are not necessarily strictly to scale of all elements shown in order to improve their readability. [Brief explanation of the drawing]

[0058] [Figure 1] Figure 1 is a schematic diagram showing the deposition and drying of a basic solution onto the hydrophilic surface of a first substrate according to steps b) and c) of a method according to one embodiment of the present invention. [Figure 2] Figure 2 is a schematic diagram showing the direct joining of the first and second substrates forming the assembly, according to step d) of the method according to the embodiment of Figure 1. [Figure 3] Figure 3 is a schematic diagram showing the heat treatment applied to the assembly of the first and second substrates according to step e) of the method according to the embodiment of Figure 1. [Figure 4] Figure 4 is a graph showing an example of comparing the junction energies obtained according to the embodiment of Figure 1, as well as a control example. [Figure 5] Figure 5 is a schematic diagram showing the direct bonding of the first bonding layer to the second substrate according to an alternative embodiment of the present invention. [Modes for carrying out the invention]

[0059] As shown in Figures 1-3, the method according to the present invention includes providing first and second substrates 1 and 2 to hydrophilic bonding surfaces 3 and 4 (step a), depositing a basic solution B on the surfaces 3 and 4 (step b), drying it (step c), bringing them into direct contact for spontaneous direct bonding by creating a direct bonding interface 6 (step d), and performing possible bonding annealing on the thus formed assembly 7 (step e). In particular, the first and second substrates 1 and 2 are silicon wafers having a thickness of 725 μm and a diameter of 200 mm. They each have bonding surfaces 3 and 4 formed by a hydrophilic film 5 of thermal silicon oxide having a thickness of approximately 145 nm. These bonding surfaces 3 and 4 are cleaned by a Kalo treatment and an SC1 treatment to allow the removal of organic and particulate contaminants, and as a result the hydrophilic surfaces 3 and 4 are ready for bonding.

[0060] Then, a basic solution containing strong base molecules in deionized water is deposited on the hydrophilic bonding surface 3 of the first substrate 1 by spin coating at a rotational speed of 400 rpm according to step b) of this method (Figure 1). Prior to this, a strong base consisting of NaOH is dissolved in deionized water, and the concentration of NaOH is approximately 10 -5 g·cm -3 The basic solution was obtained. According to a modified example not shown, the strong base was selected from LiOH, KOH, RbOH, CsOH, Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2, and mixtures of these bases.

[0061] According to step c) of this method, the hydrophilic bonding surface 3 contains a monolayer of 1 to 5 atoms of water, and has approximately 10 strong base molecules. 13 atoms / cm 2 The solution is dried by centrifugation until it reaches the desired concentration. The drying centrifugation is performed at a speed of approximately 2000 revolutions per minute for approximately 45 seconds.

[0062] Before the first substrate 1 and the second substrate 2 are brought into contact to generate a direct bonding interface 6 through the spontaneous direct bonding of their hydrophilic bonding surfaces 3 and 4, the second substrate 2 is subjected to the same steps b) and c) (not shown) as the first substrate 1. The propagation of the bonding wave across 200 mm of substrates 1 and 2 takes approximately 9 seconds. This is equivalent to the propagation time of the bonding wave when no strong base molecules are present at interface 6. From this, it can be inferred that the bonding energy of the two hydrophilic bonding surfaces 3 and 4 does not change as a result of steps b) and c) of this method. The bonding energy of the assembly 7 obtained at the end of step d) is 140 mJ / m, the bonding energy obtained when no strong base is present at bonding interface 6. 2 The junction energy is greater than that, and according to this embodiment of the present invention, it is particularly about 200 mJ / m 2 That is the case.

[0063] In a modified version not shown, steps b) and c) are performed on only one of the first and second substrates 1, 2. In another modified version, the first hydrophilic surface 3 and the second hydrophilic surface 4 are formed not by a silicon oxide film 5, but by the hydrophilic silicon of the substrates 1, 2 (i.e., native oxides on the silicon surface).

[0064] According to one possibility shown in Figure 3, the bonding annealing heat treatment in step e) is performed on assembly 7 at atmospheric pressure. The result is shown by the dotted curve in Figure 4, with bonding energy on the vertical axis and annealing temperature in Celsius on the horizontal axis. The bonding energy is measured by a double lever method with forced displacement in an anhydrous atmosphere, as described, for example, in the paper F. Fournel, L. Continni, C. Morales, J. Da Fonseca, H. Moriceau, F. Rieutord, A. Barthelemy, and I. Radu, Journal of Applied Physics 111, 104907 (2012). The temperature is applied to assembly 7 for 2 hours, and after returning to ambient temperature, the measurement is performed on assembly 7. In the control example, direct bonding was performed simultaneously in the same manner as in the present invention, except that strong base molecules were not present on the bonding surface (solid curve). As shown in Figure 4, direct bonding assisted by basic molecules was evaluated at temperatures of 100°C, 200°C, 300°C, and 500°C, and its bonding energy was significantly larger than that of unassisted direct bonding from 100°C onwards. The difference was particularly large at 4.5 J / m for the assisted direct bonding according to the present invention. 2 While reaching this level, the joint energy in an unassisted joint is 0.5 J / m 2 It is less than 200°C, and increases after annealing at 200°C.

[0065] After annealing at 300°C and 500°C, the substrate inevitably separated during measurement, and the silicon substrate was damaged, making it impossible to measure the bonding energy (indicated by an "x" in the graph in Figure 4). Therefore, the direct bonding energy is 5 J / m, which is the fracture energy of silicon. 2 It is greater than that. Therefore, the bonding energy exceeds this value from 300°C. After bonding annealing at 500°C, the unassisted direct bonding is 2.2 J / m 2 It only reached that point.

[0066] According to another modification shown in Figure 5, the first substrate 1 given in step a) is formed with several first bonding layers 8, the exposed surface of which is a first hydrophilic bonding surface 3. The first bonding layers 8 are bonded to the second full wafer substrate 2 according to the method described above, in accordance with chip-to-wafer bonding. According to yet another modification not shown, the second substrate 2 is also formed with several second bonding layers, and the method according to the present invention allows for direct bonding of the first bonding layers 8 and the second bonding layers.

[0067] In an alternative configuration not shown, the first hydrophilic bonding surface 3 and the second hydrophilic bonding surface 4 are formed by a hydrophilic film 5 of copper oxide, at least partially. Specifically, first and second silicon substrates 1 and 2, each 300 mm in diameter and 775 μm thick, are prepared to have a directly bondable hybrid copper oxide surface. These first and second hydrophilic bonding surfaces 3 and 4 are typically composed of copper pads having 2.5 μm sides separated by 2.5 μm of SiO2. It is a hybrid surface with a 5 μm "pitch". Immediately after mechanical and chemical polishing to make these bonding surfaces 3 and 4 perfectly flat and bondable, and immediately before bonding, they are subjected to deionization in water, 10 -5 g / cm 3 A solution of NaOH is deposited by spin coating according to step b). After this spin coating, the two hydrophilic surfaces 3 and 4 are dried by centrifugation at 2000 rpm for 45 seconds according to step c). The two surfaces 3 and 4 are then brought into contact. The junction wave takes approximately 12 seconds to cross 300 mm, which is equivalent to a junction without this basic molecule. The assembly 7 is annealed at 400°C for 2 hours. No particular defects are observed under an acoustic microscope.

[0068] Therefore, the solution introduced by the present invention can be applied to substrates having temperature-sensitive components, and it is possible to significantly increase the direct bonding energy even when using large-diameter substrates, with or without heat treatment performed at low temperatures. The method is easy to implement, and the steps of depositing the basic solution and drying the surface are quick. The basic solution used is inexpensive, and the amount of base is very small, which does not change the bonding energy of substrates 1 and 2 and enables spontaneous direct bonding.

Claims

1. A method for direct bonding between two substrates (1, 2), comprising the steps of: a) providing a first substrate (1) and a second substrate (2) comprising a first hydrophilic bonding surface (3) and / or a second hydrophilic bonding surface (4), respectively; b) depositing a basic solution (B) comprising strong base molecules and deionized water on the first hydrophilic mating surface (3) and / or the second hydrophilic mating surface (4); c) In the first hydrophilic interface (3) and / or the second hydrophilic interface (4), the cations generated from the strong base molecules are about 10 ９ ~10 １５ atoms / cm ２ drying the first hydrophilic bonding surface (3) and / or the second hydrophilic bonding surface (4) to a concentration of d) bringing said first hydrophilic joining surface (3) and said second hydrophilic joining surface (4) into contact to obtain a spontaneous direct bond and an assembly (7) composed of said first substrate (1) and said second substrate (2) comprising a direct bond interface (6); A direct bonding method comprising:

2. The drying step of c) is carried out by drying the first hydrophilic bonding surface (3) and the second hydrophilic bonding surface (4). ２ 2. The direct bonding method according to claim 1, wherein the bonding is carried out so as to be covered with a monolayer of 1 to 5 atoms of O.

3. The strong base molecules include LiOH, NaOH, KOH, RbOH, CsOH, and Mg(OH). ２ , Ca(OH) ２ , Sr(OH) ２ , Ba(OH) ２ 3. The direct bonding method according to claim 1, wherein the base is selected from the group consisting of the bases listed below and mixtures of these bases.

4. The basic solution (B) is a solution of 10% ammonium hydroxide in the deionized water. －８ ~10 －２ 3. The direct bonding method according to claim 1, wherein the strong base molecule is present in an amount of mol / l.

5. 3. The direct bonding method according to claim 1 or 2, wherein step c) is carried out by centrifuging the first hydrophilic bonding surface (3) and / or the second hydrophilic bonding surface (4), for example at a rotation speed of about 2000 rpm, in particular for 45 seconds.

6. 3. The direct joining method according to claim 1 or 2, further comprising a step e) of subjecting the assembly (7) obtained in step d) to a heat treatment at a temperature of 50 to 500°C, particularly 50 to 350°C, particularly 60 to 250°C.

7. 3. The direct bonding method according to claim 1, wherein the first substrate (1) and the second substrate (2) are made of silicon.

8. 3. The direct bonding method according to claim 1 or 2, wherein the first hydrophilic bonding surface (3) and / or the second hydrophilic bonding surface (4) are at least partially formed by a hydrophilic film (5) made of a material selected from silicon oxide, silicon nitride, copper oxide and mixtures of these materials.

9. 3. The direct bonding method according to claim 1 or 2, wherein the first substrate (1) provided in step a) comprises one or more first bonding layers (8) resulting from bonding layer formation of the first substrate (1), thereby obtaining direct bonding of one or more first bonding layers to the second substrate (2).

10. An assembly (7) comprising a first substrate (1) and a second substrate (2) made of silicon, each substrate comprising a first hydrophilic bonding surface (3) and a second hydrophilic bonding surface (4), The first hydrophilic bonding surface (3) and the second hydrophilic bonding surface (4) are bonded by direct bonding, and strong base molecules are present at a direct bonding interface (6) between the first hydrophilic bonding surface (3) and the second hydrophilic bonding surface (4) at a concentration of about 10 ９ ~10 １５ atoms / cm ２ , especially about 10 １０ ~10 １４ atoms / cm ２ The assembly (7) is provided with a concentration of

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