Method for bonding two hydrophilic surfaces

Abstract

A method of direct bonding comprising the following steps: supplying a first substrate and a second substrate, the first substrate being covered by a first hydrophilic surface and the second substrate being covered by a second hydrophilic surface, deposition of a specific molecule on the first hydrophilic surface and/or on the second hydrophilic surface, the specific molecule comprising a hydrophilic functional group and a basic functional group, separated by at least one atom, contacting the first hydrophilic surface with the second hydrophilic surface, whereby the two hydrophilic surfaces are bonded one with the other, and the first substrate and the second substrate are assembled, optionally, application of a bonding annealing heat treatment, preferably at a temperature less than or equal to 500 C.

Claims

1. A method of direct bonding of two substrates comprising the following steps: supplying a first substrate and a second substrate, the first substrate being covered by a first hydrophilic surface and the second substrate being covered by a second hydrophilic surface, deposition of a specific molecule on the first hydrophilic surface or on the second hydrophilic surface, the specific molecule comprising a hydrophilic functional group and a basic functional group, separated by at least one atom, wherein the specific molecule is deposited in an amount configured to improve the bonding between the first hydrophilic surface and the second hydrophilic surface; and contacting the first hydrophilic surface with the second hydrophilic surface, on at least one of which the specific molecule is deposited, whereby the two hydrophilic surfaces are bonded one with the other, the specific molecule being disposed between the two hydrophilic surfaces at the amount configured to improve the bonding, and the first substrate and the second substrate are assembled with improved bonding via the specific molecule.

2. The method of bonding according to claim 1, wherein, after contacting the first hydrophilic surface with the second hydrophilic surface, the method comprises a step of application of a bonding annealing heat treatment.

3. The method of bonding according to claim 2, wherein the bonding annealing heat treatment is at a temperature less than or equal to 500 C.

4. The method of bonding according to claim 1, wherein the specific molecule is deposited on the first hydrophilic surface and on the second hydrophilic surface.

5. The method according to claim 1, wherein the specific molecule completely covers the first hydrophilic surface.

6. The method of bonding according to claim 1, wherein the specific molecule completely covers the second hydrophilic surface.

7. The method of bonding according to claim 1, wherein the specific molecule covers from 0.05% to 10% of the first hydrophilic surface or of the second hydrophilic surface.

8. The method of bonding according to claim 7, wherein the specific molecule covers from 0.05% to 1% of the first hydrophilic surface or of the second hydrophilic surface.

9. The method of bonding according to claim 1, wherein the specific molecule is deposited in liquid form.

10. The method of bonding according to claim 1, wherein the specific molecule is deposited in gaseous form.

11. The method of bonding according to claim 1, wherein the hydrophilic functional group of the specific molecule is an alcohol function.

12. The method of bonding according to claim 1, wherein the basic functional group is a primary, secondary or tertiary amine.

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

14. The method of bonding according to claim 1, wherein the molar mass of the specific molecule is less than or equal to 200 g/mol.

15. The method according to claim 1, wherein the first substrate or the second substrate is a silicon substrate.

16. The method according to claim 1, wherein the first hydrophilic surface or the second hydrophilic surface is made of oxide.

17. The method according to claim 16, wherein the first hydrophilic surface or the second hydrophilic surface is deposited oxide, thermal oxide or native oxide.

18. The method according to claim 1, wherein the specific molecule is chemisorbed on the first hydrophilic surface and/or the second hydrophilic surface.

19. The method of claim 1, wherein the the amount configured to improve bonding is sufficient to increase a bonding energy between the two hydrophilic surfaces at a temperature less than 500 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention shall be better understood when reading the description of embodiments given solely for the purposes of information and in a non-limiting manner, in reference to the accompanying drawings wherein;

(2) FIG. 1 described hereinabove, is a graph showing the bonding energy according to the thermal annealing temperature carried out after a direct bonding with or without plasma activation, each thermal annealing lasts about 2 h, according to the prior art;

(3) FIG. 2 diagrammatically shows, as a cross-section, two substrates to be assembled by direct bonding, with one of the substrates being covered by a specific molecule, according to a particular embodiment of the invention;

(4) FIG. 3 is a graph showing the bonding energy according to the thermal annealing carried out after bonding, when the surface of the layer of oxide of one of the substrates is saturated by a specific molecule, according to a particular embodiment of the invention;

(5) FIG. 4 is a graph showing the bonding energy according to the thermal annealing carried out after bonding, when the surface of the layer of oxide of one of the substrates is covered partially by a specific molecule, according to a particular embodiment of the invention;

(6) FIG. 5 is a graph showing the bonding energy according to the thermal annealing carried out after bonding, for different volume dilutions of a specific molecule solution, according to a particular embodiment of the invention;

(7) FIG. 6 shows an image, obtained by scanning acoustic microscopy (SAM), of a bonding implementing a specific molecule, after a thermal annealing at 800 C., according to a particular embodiment of the invention.

(8) The different portions shown in the figures are not necessarily shown according to a uniform scale, in order to make the figures more legible.

(9) The various possibilities (alternatives and embodiments) must be understood as not being exclusive from one another and can be combined together;

(10) Furthermore, in the description hereinafter, terms that depend on the orientation, such as on, under, etc. of a structure are applied by considering that the structure is oriented in the manner shown in the figures.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

(11) Although this is in no way a limitation, the invention has applications particularly in the field of manufacturing SOI and imager.

(12) The method is a method of direct bonding of two substrates 10, 20, and more particularly it is a method of direct bonding of two hydrophilic surfaces, for example with oxide.

(13) The method includes the following steps: a) supplying a first substrate 10 and a second substrate 20, the first substrate 10 being covered by a first hydrophilic surface 11, and the second substrate 20 being covered by a second hydrophilic surface 21, b) adding of a specific molecule 30 on the first hydrophilic surface of the first substrate and/or on the 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 one another, and the first substrate 10 and the second substrate 20 are assembled,

(14) Each substrate 10, 20 provided in step a) comprises two main faces parallel to one another. At least one of the main faces of each substrate is covered by a hydrophilic surface 11, 21 (FIG. 2).

(15) The first substrate 10 and/or the second substrate 20 can be made of silica, metal, metal alloy or a semi-conductor material.

(16) Preferably, the first substrate 10 and/or the second substrate 20 are made of a material chosen from Si, Ge, InP, AsGa, Al.sub.2O.sub.3, SiO.sub.2, Si.sub.3N.sub.4, SiC, GaN, LNO, LTO, Cu, Ti, Ni. The first substrate and the second can be of an identical nature or of different natures.

(17) The first substrate 10 and the second substrate 20 are covered by a hydrophilic surface 11, 21, i.e. there is at least one single-layer of water adsorbed on their surface.

(18) Preferably, the first substrate 10 is covered by a first thin layer of oxide in such a way as to form a first hydrophilic surface and the second substrate 20 is covered by a second thin layer of oxide in such a way as to form a second hydrophilic surface.

(19) For example, the first substrate 10 and the second substrate 20 are covered by a layer of native oxide or by a layer of thermal oxide.

(20) According to a first alternative embodiment, one of the substrates can be covered by a layer of native oxide and the other by a layer of thermal oxide.

(21) According to another alternative embodiment, the two substrates can be covered by a layer of thermal oxide.

(22) According to another alternative embodiment, the two substrates can be covered by a layer of native oxide.

(23) In all these alternatives, the layer of thermal oxide can be replaced with a deposited layer of oxide.

(24) Advantageously, before step b), one or more pre-treatments can be carried out, for example mechanical and/or chemical. For example, for the silicon and its oxide, advantageously, a chemical cleaning with a base of Caro's acid obtained with a mixture of sulphuric acid 96%, oxygenated water 30% (3:1) at 180 C. and of SC1 (mixture of ammonia 30%, oxygenated water 30% and deionised water (1:1:5)) will be used, for example, at a temperature of 70 C. Other cleanings can be considered.

(25) The substrates 10, 20 are, preferably, wafers.

(26) During step b), the specific molecule 30 is deposited in order to improve the bonding.

(27) The specific molecule 30 is an organic molecule. It includes a hydrophilic functional group and a basic functional group, separated by at least one atom. In other words, the two functional groups are not on the same atom, for example, they are not on the same carbon atom.

(28) Basic functional group means that the specific molecule 30 comprises a group that can protonate. Preferably, the basic group is a primary amine, a secondary amine or a tertiary amine. It could also be a group deriving from the ionisation of an acid functional group, for example from the ionisation of a functional group, for example from the sulphonic group SO.sub.3H, leading to SO.sub.3.sup.. It is also possible to have an alcoholate group (RO.sup.) for this basic function.

(29) The hydrophilic functional group is, for example, a hydroxyl functional group, ether, amide, ester or ketone.

(30) The hydrophilic functional group is, preferably, a hydroxyl functional group (also called alcohol function). This can be a primary, secondary or tertiary alcohol.

(31) Even more preferably, the hydrophilic functional group is a hydroxyl functional group and the basic functional group is a primary, secondary or tertiary amine.

(32) The specific molecule 30 can comprise several acid functional groups and/or several basic functional groups.

(33) Preferably, the molecule comprises a basic functional group and at least one hydroxyl functional group. For example, it comprises a basic functional group and one, two or three hydrophilic functional groups.

(34) The specific molecule 30 can be cyclic or acyclic. Preferably, it is acyclic. When the molecule includes an open carbon chain, the latter can be linear or branched. It includes, preferably, from 1 to 10 carbon atoms and even more preferably from 1 to 6 carbon atoms.

(35) Advantageously, the molecular weight of the specific molecule 30 is less than or equal to 500 g/mol and preferably less than or equal to 200 g/mol. It is, advantageously, greater than 40 g/mol.

(36) For the purpose of illustration and in a non-limiting manner, the specific molecule 30 is chosen 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).

(37) The specific molecule 30 can be deposited: on the hydrophilic surface 11 of the first substrate 10, or on the hydrophilic surface 12 of the second substrate 20, or both on the hydrophilic surface 11 of the first substrate 10 and on the hydrophilic surface 21 of the second substrate 20.

(38) The specific molecule 30 can be deposited in liquid form and/or in gaseous form.

(39) A pure solution of specific molecule or a diluted solution of specific molecule can be used.

(40) It is possible, advantageously, to add a base to the solution in order to increase the pH. For example, KOH, NaOH, Na.sub.2CO.sub.3, or NH.sub.4OH can be used.

(41) To deposit the specific molecule 30 in liquid form, any liquid deposition technique can be used, for example, by coating, by spraying, by spin-coating deposition.

(42) Preferably, spin-coating deposition is used.

(43) To deposit the specific molecule 30 in gaseous form, a substrate is placed in a closed enclosure, wherein a recipient is disposed containing a solution of the specific molecule (pure or diluted). Then the solution is left to evaporate in the closed reactor.

(44) The specific molecule 30 can entirely or partially cover the surface on which it is deposited.

(45) The quantity deposited can be such that a continuous single-layer is formed on the hydrophilic surface (saturation of the surface). It is also possible to have several single-layers on the surface, for example there can be up to 5 single-layers on the surface.

(46) Partially means in particular less than 50%, and preferably less than 10%. Even more preferably, the specific molecule covers less than 1%, even less than 0.1% of the surface to be bonded. Advantageously, it covers at least 0.05% of the surface to be bonded. According to a particularly advantageous embodiment, it covers from 0.05% to 1% of the surface to be bonded, and even more preferably from 0.05% to 0.1%.

(47) During step c), the surfaces to be bonded (i.e. the hydrophilic surfaces 11, 21) are contacted, whereby an assembly is obtained 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. at a temperature of about 20 C.).

(48) The specific molecule 30 remains attached on the hydrophilic surface or surfaces 11, 21 and is enclosed at the bonding interface. It is trapped in the assembly and cannot evaporate.

(49) The assembly thus obtained comprises two substrates 10, 20, each covered by a hydrophilic surface 11, 21, the specific molecule 30 such as defined hereinabove being disposed between the two hydrophilic surfaces.

(50) The method can comprise a later step during which a thermal annealing is carried out after bonding. The thermal annealing after bonding makes it possible to reinforce the direct bonding obtained in step c). The temperature of the thermal annealing ranges, for example, from 100 C. to 1200 C. Preferably, the temperature ranges from 100 C. to 500 C., and even more preferably from 100 C. to 300 C.

(51) The bonding energies of the assembly, after annealing, range from 600 mJ to 5000 mJ according to the temperature of the thermal annealing. For example, for a temperature of 300 C., the bonding energy ranges from 3500 mJ to 4000 mJ.

Illustrative and Non-Limiting Examples of an Embodiment

(52) In the various embodiments that shall be described, the plates are silicon plates 200 mm in diameter and 725 m thick. They were thermally oxidised in order to obtain on the surface a silicon oxide film 145 nm thick. They are then cleaned with ozonated water and with SC1 followed by a SC2 (mixture of hydrochloric acid 30%, oxygenated water 30% and water 1:1:5) to make their surfaces hydrophilic.

(53) The specific molecule for the bonding here is N,N-Diethyl-2-amino-ethanol (CAS: 100-37-8).

Example 1: Saturation of the Surface

(54) In a reactor having a volume of about 50 or 100 L, a beaker is placed containing 10 mL of the pure specific molecule (not diluted). The solution is left to evaporate in the closed reactor for 1 h30. Then, two silicon plates are placed in this atmosphere for 3 min and then they are bonded. The bonding wave is then about 8 s to pass through the 200 mm which is equivalent to the bonding without this molecule.

(55) The bonding is annealed and the adherence energies are measured at different temperatures. The bonding energy obtained with the specific molecule is more substantial than without the specific molecule (FIG. 3). On the other hand, as the surface is quickly saturated, there is no dependency with exposure time: the same results are obtained for durations of 30 s, 8 min or 15 min of exposure.

Example 2: Partial Saturation of the Surface

(56) In a reactor having a volume of about 50 or 100 L, a beaker containing 1 mL of water and 0.02 mL of the specific molecule is placed. The solution is left to evaporate in the closed reactor for 1 h30. Then the two silicon plates are placed in this atmosphere for 1 min and then they are bonded. The bonding wave is then about 8 s to pass through the 200 mm which is equivalent to the bonding without this molecule.

(57) The bonding is annealed and the adherence energies are measured at different temperatures. As shown in FIG. 4, the bonding energy obtained with a partial saturation of the surface is more substantial than when the surface is entirely saturated. A lower concentration due to a lower saturation vapour pressure of the diluted solution makes it possible to obtain a better bonding energy for the entire temperature range.

(58) FIG. 5 shows the bonding energy according to the dilution of the solution of specific molecule.

Example 3: Partial Saturation of the Surface from a Liquid Solution

(59) Spin-coating deposition is used to deposit a solution containing 1% by volume of the specific molecule on the surface of a plate: a stream of the water-product mixture is created on the surface that rotates at 30 rpm until the entire surface is covered. Then, the dispensing of the product is stopped and it is rotated at 2000 rpm for 30 s in order to remove the surplus. Then the two plates are bonded. The bonding wave is then about 8 s to pass through the 200 mm which is equivalent to the bonding without this molecule.

(60) The bonding is annealed and the adherence energies are measured at different temperatures, the same results are obtained as for example 2.

(61) Observations under the scanning acoustic microscope (SAM) confirm that the bondings are free of defects (FIG. 6).

(62) For the purpose of comparison, the following tests were conducted: plates were cleaned with isopropyl alcohol vapours, plates were cleaned with isopropyl alcohol vapours then ammonia diluted to 2% was added before bonding them.

(63) No visible effect was observed on the bonding energy.