How to join two hydrophilic surfaces

Abstract

1. A method for directly bonding two substrates (10, 20), comprising the steps of: providing a first substrate (10) and a second substrate (20), wherein the first substrate (10) is covered with a first hydrophilic surface (11) and the second substrate (20) is covered with a second hydrophilic surface (21); depositing specific molecules (30) on the first hydrophilic surface (11) and / or the second hydrophilic surface (21), wherein the specific molecules (30) comprise a hydrophilic functional group and a basic functional group separated by at least one atom; contacting the first hydrophilic surface (11) with the second hydrophilic surface (21), thereby bonding the two hydrophilic surfaces (11, 21) to each other and assembling the first substrate (10) and the second substrate (20); and optionally applying a bonding annealing heat treatment, preferably at a temperature of 500°C or less.

Description

[Technical field]

[0001] The present invention relates to a method for directly bonding two hydrophilic surfaces, as well as to the assemblies obtained by such a method.

[0002] The invention has applications in many industrial sectors, in particular in the manufacture of silicon-on-insulator (SOI) or, for example, in the manufacture of image sensors.

[0003] The invention is particularly interesting since it makes it possible to obtain an assembly having good mechanical resistance due to the low thermal annealing budget after bonding. [Background technology]

[0004] Direct bonding is a spontaneous bond between two surfaces without the addition of material, especially without the addition of a thick layer of liquid material or polymer. However, it is possible to have a few monolayers of water adsorbed on the surfaces to be bonded, especially if the surfaces to be bonded are macroscopically dry, although a few monolayers of water are hydrophilic. Direct bonding is conventionally performed at ambient temperature and pressure, although this is not obligatory. For SiO2-SiO2 bonding, the surfaces to be bonded are pre-cleaned with a solution containing Caro's acid base, usually obtained at 180 °C with 96% sulfuric acid and 30% oxygen-saturated water (3:1) and a mixture of SC1 (a mixture of 30% ammonia, 30% oxygen-saturated water and deionized water (1:1:5) at 70 °C). These washes allow to remove organic and particulate contaminations that are very harmful to direct bonding. Alternatively, it is possible to use other highly oxidizing cleaning solutions, for example with a treatment using aqueous solutions containing ozone or exposure to UV light in the presence of gaseous ozone.

[0005] Direct binding has the following characteristics: - Bond energy (also called adhesion energy): This is the energy required to separate two bonding surfaces, the bond energy changes depending on the thermal annealing applied after bonding. - Adhesion energy: this is the energy available to carry out spontaneous bonding. It is the energy that allows the deformation of two surfaces to bring them into contact on an atomic scale, with the help of van der Waals forces.

[0006] For example, for two hydrophilic silicon oxide or silicon surfaces, the adhesion energy is typically between 30 and 100 mJ / m 2 The adhesion energy of SiO2-SiO2 bonded by chemical cleaning using Caro and SC1 bases at 70 °C in a clean room environment is about 140 mJ / m immediately after bonding without annealing. 2 In the case of bonding, the adhesion energy increases with the temperature of thermal annealing. For example, the bond energy increases slowly, reaching 3 J / m at 500 °C. 2 and then plateaus at 800°C (Figure 1).

[0007] A solution to increase the bond energy is to use a plasma treatment. As shown in Figure 1, using nitrogen plasma (N2) the bond energy for oxide-oxide bonds is 5 J / m at an annealing temperature of 300 °C. 2 rises rapidly to

[0008] However, using plasma may not be compatible with certain substrates and / or it may extend the time and / or cost of the process, making them more difficult to industrialize. Plasma treatments also modify the surface over a thickness of a few nanometers (1 to 10 nm). This modification may interfere with subsequent devices. For example, in the case of silicon plates, plasma produces a layer of oxide that is difficult to control in terms of thickness and quality. On silicon oxide surfaces, certain plasmas, such as N2 plasma, may cause interface charge problems and interfere with the electrical operation of subsequent devices. Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to overcome the drawbacks of the prior art and to propose a bonding method that can obtain strong bonding energy at low temperatures (usually 500° C. or less, preferably 300° C. or less) without using plasma treatment. [Means for solving the problem]

[0010] For this purpose, the present invention provides providing a first substrate and a second substrate, the first substrate being covered with a first hydrophilic surface and the second substrate being covered with a second hydrophilic surface; depositing specific molecules on a first hydrophilic surface of a first substrate and / or a second hydrophilic surface of a second substrate, the specific molecules comprising hydrophilic and basic functional groups separated by at least one atom; contacting the first hydrophilic surface with a second hydrophilic surface, whereby the first substrate and the second substrate are assembled by bonding of the first hydrophilic surface and the second hydrophilic surface; Optionally, applying a bonding annealing heat treatment at a temperature preferably below 500° C., even more preferably below 300° C.; We propose a method for direct bonding of two substrates, including:

[0011] The present invention is fundamentally distinguished from the prior art by the use of a specific molecule that contains a hydrophilic functional group and a basic functional group. The hydrophilic functional group allows the specific molecule to be easily chemisorbed to the hydrophilic surface to be bonded. The basic functional group allows the pH of the adsorbed water monolayer to be raised. The presence of this molecule substantially increases the resulting binding energy.

[0012] By "hydrophilic surface" is meant that at least one monolayer of water is adsorbed on the surface at atmospheric pressure (i.e., about 1 bar) in air having a relative humidity of at least 1%. A surface is said to be hydrophilic if the water droplet angle is less than 90°, preferably less than 50°, and even more preferably less than 5°.

[0013] Preferably, the first and second hydrophilic surfaces are made of an oxide, which may be a thin layer of deposited oxide, a thin layer of oxide obtained by heat treatment (also called thermal oxide) and / or a thin layer of oxide obtained by chemical treatment (also called native oxide or chemical oxide).

[0014] According to a first advantageous alternative embodiment, the specific molecules completely cover the first hydrophilic surface of the first substrate and / or the second hydrophilic surface of the second substrate. Advantageously, there are between 1 and 5 layers of the specific molecules on the hydrophilic surface. Advantageously, there is only one monolayer.

[0015] According to a second advantageous alternative embodiment, the specific molecules cover between 0.05% and 10%, preferably between 0.05% and 1%, of the first hydrophilic surface of the first substrate and / or the second hydrophilic surface of the second substrate.

[0016] According to an alternative of the first advantageous embodiment, the specific molecules are deposited in liquid state.

[0017] According to an alternative of the second advantageous embodiment, the specific molecules are deposited in gaseous state.

[0018] Advantageously, the hydrophilic functional group of a particular molecule is an alcohol functional group and / or the basic functional group is a primary, secondary or tertiary amine.

[0019] Advantageously, the molar mass of the particular molecule is less than or equal to 500 g / mol, preferably less than or equal to 200 g / mol.

[0020] Advantageously, the first substrate and / or the second substrate are silicon substrates. The substrate or substrates are covered with a thin layer of silicon oxide. The oxide layer can be a deposited layer, a thermal oxide layer or a native oxide layer.

[0021] This method of combining has many advantages: - Surface treatment is carried out using specific molecules in liquid or gaseous state. The method is easy to implement and therefore can be easily industrialized. -A small amount of a specific molecule is sufficient to ensure strong binding. This method is applicable to many materials and is inexpensive. -The adhesion energy does not decrease when molecules are used. There is no bubble formation, delamination and / or defects at the bonding interface even after heat treatment. The bonding temperature is compatible with many applications and / or many substrates (whose nature and / or the presence of electronic and / or optoelectronic components requires a low thermal budget). - the method results in an assembled structure that has good mechanical resistance after bonding and is compatible with subsequent methods such as Smart Cut and / or any method that creates mechanical stress in the assembly, e.g. mechanical thinning.

[0022] The present invention also relates to an assembly comprising a first substrate and a second substrate, wherein the first substrate is covered with a first hydrophilic surface and the second substrate is covered with a second hydrophilic surface, and wherein a specific molecule is disposed between the first hydrophilic surface of the first substrate and the second hydrophilic surface of the second substrate, the specific molecule comprising a hydrophilic functional group and a basic functional group, and wherein the two functional groups of the specific molecule are separated by at least one atom.

[0023] Advantageously, the hydrophilic functional group of a particular molecule is an alcohol functional group and / or the basic functional group is a primary, secondary or tertiary amine.

[0024] Other features and advantages of the present invention will become apparent from the following supplemental description.

[0025] Needless to say, this supplemental description is provided only for the purpose of illustrating the scope of the present invention and should not be construed as limiting this scope in any way.

[0026] The invention should be better understood when reading the description of embodiments, given for informational purposes only and in a non-limiting manner, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0027] [Figure 1] FIG. 1 is a graph showing bonding energy versus thermal annealing temperature performed after direct bonding with and without plasma activation, each thermal anneal lasting approximately 2 hours according to the prior art. [Diagram 2] FIG. 2 shows diagrammatically in cross section two substrates assembled by direct bonding according to a particular embodiment of the invention, one of the substrates being covered with a particular molecule. [Diagram 3] FIG. 3 is a graph showing the bond energy with post-bonding thermal annealing when the surface of one oxide layer of a substrate is saturated with certain molecules according to certain embodiments of the present invention. [Figure 4] FIG. 4 is a graph showing the bond energy after thermal annealing when the surface of one oxide layer of a substrate is partially covered with certain molecules according to a specific embodiment of the present invention. [Diagram 5] FIG. 5 is a graph showing binding energies with post-binding thermal annealing for different volume dilutions of a particular molecular solution, according to certain embodiments of the present invention. [Figure 6] FIG. 6 shows images obtained by Scanning Acoustic Microscopy (SAM) of a specific molecule-implementing bond after thermal annealing at 800° C. according to a specific embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] For ease of reading the figures, the various parts shown in the figures are not necessarily drawn to uniform scale.

[0029] It should be understood that the various possibilities (alternatives and embodiments) are not mutually exclusive and can be combined together.

[0030] Additionally, in the following description, terms that depend on the orientation of the structures, such as "above" and "below," are used, taking into account that the structures are oriented as shown in the figures.

[0031] Although this is by no means a limitation, the invention has particular application in the field of SOI and imager manufacturing.

[0032] This method involves directly bonding two substrates 10, 20, more specifically, directly bonding two hydrophilic surfaces, for example using an oxide.

[0033] This method is (a) providing a first substrate 10 and a second substrate 20, the first substrate 10 being covered with a first hydrophilic surface 11 and the second substrate 20 being covered with a second hydrophilic surface 21; (b) attaching a specific molecule 30 to a first hydrophilic surface of the first substrate and / or a second hydrophilic surface of the second substrate, the specific molecule 30 comprising a hydrophilic functional group and a basic functional group, the two functional groups of the specific molecule being separated by at least one atom; (c) contacting the first hydrophilic surface 11 with the second hydrophilic surface 21, whereby the two hydrophilic surfaces 11, 21 are bonded to each other and the first substrate 10 and the second substrate 20 are assembled; Includes.

[0034] Each substrate 10, 20 provided in step (a) comprises two main faces parallel to each other, at least one of which is covered with a hydrophilic surface 11, 21 (Figure 2).

[0035] The first substrate 10 and / or the second substrate 20 may be made of silica, a metal, a metal alloy or a semiconductor material.

[0036] Preferably, the first substrate 10 and / or the second substrate 20 are made of a material selected from Si, Ge, InP, AsGa, Al2O3, SiO2, Si3N4, SiC, GaN, LNO, LTO, Cu, Ti, Ni. The first and second substrates may be of the same or different nature.

[0037] The first substrate 10 and the second substrate 20 are covered with hydrophilic surfaces 11, 21, ie they have at least one monolayer of water adsorbed on their surfaces.

[0038] Preferably, the first substrate 10 is covered with a first thin oxide layer to form a first hydrophilic surface, and the second substrate 20 is covered with a second thin oxide layer to form a second hydrophilic surface.

[0039] For example, the first substrate 10 and the second substrate 20 are covered by a layer of native oxide or a layer of thermal oxide.

[0040] According to a first alternative embodiment, one of the substrates may be covered by a layer of native oxide and the other by a layer of thermal oxide.

[0041] According to another alternative embodiment, the two substrates may be covered by a layer of thermal oxide.

[0042] According to another alternative embodiment, the two substrates may be covered by a layer of native oxide.

[0043] In all these alternatives, the layer of thermal oxide can be replaced with a deposited layer of oxide.

[0044] Advantageously, before step (b), one or more pretreatments can be carried out, for example mechanical and / or chemical pretreatments. For example, in the case of silicon and its oxides, a chemical cleaning is advantageously used, for example at a temperature of 70° C., with a base of Caro's acid obtained in a mixture of 96% sulfuric acid and 30% oxygen-saturated water (3:1) at 180° C. and SC1 (a mixture of 30% ammonia, 30% oxygen-saturated water, deionized water (1:1:5) at 70° C.). Other cleanings can be considered.

[0045] The substrates 10, 20 are preferably wafers.

[0046] During step (b), specific molecules 30 are deposited to improve the bonding.

[0047] Particular molecule 30 is an organic molecule. It includes a hydrophilic functional group and a basic functional group that are separated by at least one atom. In other words, the two functional groups are not on the same atom, e.g., not on the same carbon atom.

[0048] By "basic functional group" is meant that the particular molecule 30 contains a group that can be protonated. Preferably, the basic group is a primary amine, a secondary amine or a tertiary amine. It can also be the ionization of an acidic functional group, for example the ionization of a functional group derived from a sulfonic acid group SO3H, -SO3 - It is also possible to have an alcoholate group (RO-) for this basic function.

[0049] Hydrophilic functional groups are, for example, hydroxyl functions, ethers, amides, esters or ketones.

[0050] The hydrophilic functional group is preferably a hydroxyl functional group (also called an alcohol functional group), which may be a primary, secondary or tertiary alcohol.

[0051] Even more preferably, the hydrophilic functional group is a hydroxyl functional group and the basic functional group is a primary, secondary or tertiary amine.

[0052] A particular molecule 30 may contain some acidic functional groups and / or some basic functional groups.

[0053] Preferably, the molecule contains a basic functional group and at least one hydroxyl functional group, for example, it contains a basic functional group and one, two or three hydrophilic functional groups.

[0054] A particular molecule 30 can be cyclic or acyclic. Preferably, it is acyclic. If the molecule contains an open carbon chain, the carbon chain can be linear or branched. It preferably contains from 1 to 10 carbon atoms, even more preferably from 1 to 6 carbon atoms.

[0055] Advantageously, the molecular weight of the particular molecule 30 is less than or equal to 500 g / mol, preferably less than or equal to 200 g / mol. It is advantageously greater than 40 g / mol.

[0056] For illustrative purposes and in a non-limiting manner, the specific molecule 30 is selected from N,N-diethyl-2-amino-ethanol (CAS:100-37-8), dimethylaminoethanol or DMAE (CAS:108-01-0), diethylethanolamine or DEAE (CAS:100-37-8), monoethanolamine (CAS:141-43-5), N-methyldiethanolamine or MDEA (CAS:105-59-9), aminomethanol (CAS:3088-27-5), N-methylhydroxylamine (CAS:593-77-1), diethanolamine or DEA (CAS:111-42-2), dimethanolamine (CAS:7487-32-3), triethanolamine (CAS:102-71-6) and trimethanolamine (CAS:14002-32-5).

[0057] A specific molecule 30, on the hydrophilic surface 11 of the first substrate 10, or on the hydrophilic surface 12 of a second substrate 20, or on both the hydrophilic surface 11 of the first substrate 10 and the hydrophilic surface 21 of the second substrate 20, It can be deposited.

[0058] The specific molecules 30 can be deposited in a liquid and / or gas state.

[0059] A pure solution of the particular molecule or a dilute solution of the particular molecule can be used.

[0060] Advantageously, it is possible to add a base to the solution to raise the pH, for example KOH, NaOH, Na2CO3 or NH4OH can be used.

[0061] To deposit the specific molecules 30 in a liquid state, any liquid deposition technique can be used, such as coating, spraying, spin-coating deposition, preferably spin-coating deposition is used.

[0062] To deposit a particular molecule 30 in the gaseous state, the substrate is placed in a closed enclosure where a reservoir is arranged to contain a solution (pure or diluted) of the particular molecule. The solution is then left to evaporate within the closed reactor.

[0063] A particular molecule 30 may completely or partially cover the surface on which it is deposited.

[0064] The amount deposited may be such that a continuous monolayer is formed on the hydrophilic surface (saturation of the surface). It is also possible to place multiple monolayers on a surface, for example up to five monolayers on a surface.

[0065] "Partially" means in particular less than 50%, preferably less than 10%. Even more preferably, the specific molecule covers less than 1% and even less than 0.1% of the surface to which it is attached. Advantageously, it covers at least 0.05% of the surface to which it is attached. According to a particularly advantageous embodiment, it covers between 0.05% and 1%, even more preferably between 0.05% and 0.1% of the surface to which it is attached.

[0066] During step (c), the surfaces to be bonded (i.e. the hydrophilic surfaces 11, 21) are brought into contact, thereby obtaining an assembly comprising two substrates 10, 20 bonded to each other at their hydrophilic surfaces 11, 21. Step (c) is preferably carried out at ambient temperature (i.e. a temperature of about 20° C.).

[0067] The specific molecule 30 remains attached to the hydrophilic surfaces 11, 21 and is enclosed in the bond interface: it is trapped in the assembly and cannot evaporate.

[0068] The assembly thus obtained comprises two substrates 10, 20 each covered with a hydrophilic surface 11, 21, between which a specific molecule 30 as defined above is located.

[0069] The method may include a later step during which a thermal annealing is performed after bonding. The post-bonding thermal annealing may strengthen the direct bond obtained in step (c). The temperature of the thermal annealing may for example be in the range of 100°C to 1200°C. Preferably, the temperature is in the range of 100°C to 500°C, even more preferably in the range of 100°C to 300°C.

[0070] The bond energy of the assembly after annealing ranges from 600 mJ to 5000 mJ depending on the temperature of the thermal annealing. For example, for a temperature of 300° C., the bond energy ranges from 3500 mJ to 4000 mJ.

[0071] Illustrative and Non-Limiting Example of an Embodiment In the various embodiments described, the plates are silicon plates with a diameter of 200 mm and a thickness of 725 μm. They are thermally oxidized to obtain a silicon oxide film with a thickness of 145 nm on the surface. They are then washed with ozone water and SC1, followed by SC2 (a mixture of 30% hydrochloric acid, 30% saturated oxygen water and water 1:1:5) to make their gill surfaces hydrophilic.

[0072] A specific molecule of conjugation here is N,N-diethyl-2-amino-ethanol (CAS: 100-37-8).

[0073] (Example 1: Surface saturation) A beaker containing 10 mL of pure specific molecule (not diluted) is placed in a reactor of about 50 or 100 L capacity. The solution is left to evaporate for 1 hour and 30 minutes in a closed reactor. Two silicon plates are then placed in this atmosphere for 3 minutes, after which they bond. The bonding wave traverses 200 mm in about 8 seconds, which corresponds to bonding in the absence of this molecule.

[0074] The bonds are annealed and the attachment energies are measured at various temperatures. The bond energies obtained with the specific molecule are more substantial than without the specific molecule (Figure 3). On the other hand, since the surface saturates quickly, it is independent of the exposure time: the same results are obtained with exposure times of 30 seconds, 8 minutes or 15 minutes.

[0075] (Example 2: Partial saturation of the surface) In a reactor of about 50 or 100 L capacity, a beaker containing 1 mL of water and 0.02 mL of a specific molecule is placed. The solution is left to evaporate for 1 hour and 30 minutes in a closed reactor. Two silicon plates are then placed in this atmosphere for 1 minute, after which they bond. The bonding wave traverses 200 mm in about 8 seconds, which corresponds to bonding in the absence of this molecule.

[0076] The bond is annealed and the attachment energy is measured at various temperatures. As shown in Figure 4, when the surface is partially saturated, the resulting bond energy is greater than when the surface is fully saturated. The lower concentration, due to the lower saturation vapor pressure of the dilute solution, allows for better bond energies to be obtained over the entire temperature range.

[0077] FIG. 5 shows the binding energy with dilution of a solution of a particular molecule.

[0078] (Example 3: Partial saturation of the surface with solution) Spin-coating deposition is used to deposit a solution containing 1% by volume of a specific molecule on the surface of the plate: a flow of a mixture of water and product is generated on the surface and rotated at 30 rpm until the entire surface is covered. The product dispensing is then stopped and the plate is spun at 2000 rpm for 30 seconds to remove the excess. The two plates are then bonded. The bonding wave traverses 200 mm in about 8 seconds, which corresponds to bonding in the absence of this molecule.

[0079] The bonds are annealed and the adhesion energies are measured at various temperatures and the same results are obtained as for Example 2.

[0080] Observation with a scanning acoustic microscope (SAM) confirms that the joint is free of defects (Figure 6).

[0081] For comparison, the following tests were performed: - The plate was cleaned with isopropyl alcohol vapor. - The plates were cleaned with isopropyl alcohol vapor and then diluted ammonia to 2% was added before binding them.

[0082] No visible effect on the binding energy was observed. [Explanation of symbols]

[0083] 10 First substrate 20 Second substrate 11 First hydrophilic surface 21 Second hydrophilic surface 30 Specific Molecules

Claims

1. Providing a first substrate (10) and a second substrate (20), said first substrate (10) being covered with a first hydrophilic surface (11) and said second substrate (20) being covered with a second hydrophilic surface (21); depositing on said first hydrophilic surface (11) and / or said second hydrophilic surface (21) specific molecules (30) comprising a hydrophilic functional group and a basic functional group separated by at least one atom; bringing said first hydrophilic surface (11) into contact with said second hydrophilic surface (21), whereby said two hydrophilic surfaces (11, 21) are bonded to each other and said first substrate (10) and said second substrate (20) are assembled; A method for directly bonding two substrates (10, 20), comprising:

2. 2. The method of claim 1, characterized in that the specific molecules (30) completely cover the first hydrophilic surface (11) and / or the second hydrophilic surface (21).

3. 2. The method of claim 1, characterized in that the specific molecules (30) cover between 0.05% and 10% of the first hydrophilic surface (11) and / or the second hydrophilic surface (21).

4. 4. The bonding method according to claim 1, wherein the specific molecules (30) are deposited in liquid state.

5. 4. The bonding method according to claim 1, wherein the specific molecules (30) are deposited in the gaseous state.

6. 6. The method of claim 1, wherein the hydrophilic functional group of the specific molecule (30) is an alcohol functional group and / or the basic functional group is a primary, secondary or tertiary amine.

7. 7. The method according to claim 1, wherein the molar mass of the specific molecule (30) is less than or equal to 500 g / mol.

8. 8. The method according to any one of the preceding claims, characterized in that the first substrate (10) and / or the second substrate (20) are silicon substrates.

9. 9. The method according to any one of claims 1 to 8, characterized in that the first hydrophilic surface and / or the second hydrophilic surface is made of an oxide.

10. An assembly comprising a first substrate (10) and a second substrate (20), said first substrate (10) is covered with a first hydrophilic surface (11), said second substrate (20) is covered with a second hydrophilic surface (21), said first hydrophilic surface (11) is bonded to said second hydrophilic surface (21); An assembly, wherein a specific molecule (30) comprising a hydrophilic functional group and a basic functional group separated by at least one atom is deposited between the first hydrophilic surface (11) and the second hydrophilic surface (21).

11. 11. The assembly according to claim 10, characterized in that the hydrophilic functional groups of the particular molecules (30) are alcohol functional groups and / or the basic functional groups are primary, secondary or tertiary amines.

12. The bonding method described in claim 1, characterized in that after assembling the first substrate (10) and the second substrate (20), a bonding annealing heat treatment at a temperature below 500°C is applied.

13. The binding method described in claim 3, characterized in that the specific molecule (30) covers 0.05% to 1% of the first hydrophilic surface (11) and / or the second hydrophilic surface (21).

14. The method of claim 7, wherein the molar mass of the specific molecule (30) is 200 g / mol or less.

15. The method of claim 9, wherein the first hydrophilic surface and / or the second hydrophilic surface is a deposited oxide, a thermal oxide and / or a native oxide.

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