WO2019202067A1

WO2019202067A1 - Method for temporary or permanent wafer bonding

Info

Publication number: WO2019202067A1
Application number: PCT/EP2019/060061
Authority: WO
Inventors: Vincent Mevellec; Dominique Suhr; Céline DOUSSOT
Original assignee: Aveni
Priority date: 2018-04-20
Filing date: 2019-04-18
Publication date: 2019-10-24
Also published as: TW202004995A

Abstract

The present invention relates to a method for temporary or permanent bonding of two solid substrates, one of the two substrates having a conducting or semiconducting surface. This method comprises electrografting of a very thin polymer layer on said conducting or semiconducting surface, and assembling the two substrates through the electrografted polymer layer at a processing temperature higher than 150°C. For the first time the present invention makes it possible to provide a temporary bonding process and a permanent bonding process using the same and single adhesive polymer.

Description

METHOD FOR TEMPORARY OR PERMANENT WAFER BONDING

The present invention relates to a method for temporary or permanent bonding of two solid substrates, one of the two substrates having a conducting

5 or semiconducting surface. This method comprises electrografting of a very thin polymer layer on said conducting or semiconducting surface, and assembling the two substrates through the electrografted polymer layer at a processing temperature higher than 150°C.

Smaller, more powerful and more complex devices like smartphones,0 smartwatches or computers demand miniaturization and packaging of chips, MEMS, NEMS and other module types. Smaller transistors, thinner interconnects, use of through silicon vias and thinning silicon wafers are some ways to reach this goal.

Thinning wafers has the advantage of packaging and stacking chips,5 power devices and smart cards, but face the drawback not to be trivial to handle since they tend to easily warp, fold and break.

For instance, thin silicon wafers (< 100 pm), such as those used in making through silicon vias (TSV) in 3D integration technologies, are fragile and difficult to handle due to their high flexibility.

0 In order to improve thin wafers handling it has been suggested to attach (in a temporary manner) the thin wafer to a carrier wafer (in such a way that the two wafers may be bonded together in a detachable manner). One of the methods used for attaching two wafers is by applying a thin layer of an adhesive polymer on a surface of the carrier wafer and bonding the carrier wafer thus5 treated with the thin wafer to be handled.

In order to be efficient, the proposed solution must guarantee the ability of the wafer to resist to backside grinding, photolithography, chemical processes such as etching and plating, and must allow wafer handling.

Bonding two substrates or wafers together is an important process in0 the fabrication of many of the microelectronic modules cited before. It enables the fabrication of unique types of substrates and allows fabrication and packaging of complex three-dimensional 3D micro-components. In parallel, thinned wafers enable improved heat dissipation, three-dimensional stacking and reduced electrical resistance.

5 A wide variety of wafer bonding techniques exist including direct bonding, anodic bonding, solder bonding, eutectic bonding, thermo-compression bonding, direct metal to metal bonding, ultrasonic bonding, low-temperature melting glass bonding, and adhesive bonding. The latter technique comprises applying a polymer adhesive layer on a surface of one or both wafers to be bonded and attaching the first wafer to the second wafer in a face-to-face relationship through the polymer adhesive layer. After the two substrates are put into contact, pressure and heat are applied to force into intimate contact and bonding. The polymer adhesive can be converted from a liquid or viscoelastic state into a solid state or can also directly stick to the other surface by different type of bonds, like Van der Walls bindings.

Adhesive wafer bonding shows several advantages.

Adhesive polymer bonding can be performed at relatively low temperatures depending on the polymer material, typically between room temperature and 250°C. Other advantages of this technique are insensitivity to topology of the wafer surfaces, compatibility with standard complementary metal-oxide semiconductor CMOS wafers, and ability to join practically any type of wafer materials. Indeed, adhesive wafer bonding does not require special wafer surface treatments, such as planarization and excessive cleaning.

Known polymers that are used for adhesive bonding may include benzocyclobutene (BCB), epoxy composition and polylactide. Many of them need thermal or photo-induced curing.

While adhesive wafer bonding is a comparably simple, robust, and low- cost process, issues, such as thickness and uniformity of the polymer layer, limited temperature stability, limited data about long-term stability or lack of hermetic sealed bonds towards gasses and moisture are parameters that need to be considered.

For some applications, such as 3D IC's application or in smart cut application, very thin adhesive layer(s) are required or preferred. Direct bonding and adhesive bonding through ultra-thin intermediate layers have scarcely been reported so far for small bonding areas, and even more scarcely for 100 mm or higher wafer scale.

Thus, there is a clear need for reliable large wafer-scale adhesive bonding processes featuring ultra-thin intermediate polymer adhesive layers, having adequate bond strength between several types of materials, a resistance to high temperature and backside grinding.

The invention provides a polymer adhesive and methods of use that comply with these requirements, and that are adapted for temporary or permanent bonding. For the first time the present invention makes it possible to provide a temporary bonding process and a permanent bonding process using the same and single adhesive polymer.

The process of the invention also has the advantage to be adapted to a

5 very large number of substrate types.

The present invention generally provides a method for coating a surface of a substrate, in particular metallic or semiconductor substrate, in protic media with a polymer adhesive layer capable of adhering in a permanent or temporary manner to various other substrates such as semiconductors, metals, silicon0 oxides, polymer or isolator.

The polymer layer obtainable by a first embodiment of the process of the disclosure can be thinner than 300 nm, and its thickness can be easily tuned and controlled by an electrical process (electrografting). The polymer layer thus obtained can resist to high temperatures, in particular as high as 400°C. The5 present inventors have surprisingly found that a thickness as low as 100 nm give a bonding force value over 0.6 J/m² that is compatible with temporary bonding.

It has also been shown that the polymer layer obtainable by a second embodiment of the process of the disclosure can bond in a permanent manner two substrates. More specifically such a polymer layer having a thickness as low0 as 300 nm may provide a bonding force value of about 5 to 6 J/m² that is compatible with permanent bonding. The polymer layer thus obtained can also resist to high temperatures, in particular as high as 350°C.

It has been discovered, and this constitutes the foundation of the present invention, that it is possible to obtain the aforementioned results in a satisfactory5 manner at the industrial scale by employing:

- on the one hand, an electrografting solution that is protic in nature, and in particular an aqueous solution, comprising a film precursor that is chosen from monomers that are soluble in this solvent; and

- on the other hand, an electrografting protocol in pulsed mode making0 possible formation of a continuous and uniform film at a growth speed that is compatible with industrial constraints.

Thus, according to a first aspect, the present invention relates to a method for bonding a first solid substrate to a second solid substrate, said first substrate comprising one free surface which is a conducting or semiconducting5 surface, preferably a silicon surface, and said second substrate comprising one free surface, said method comprising:

- a step A) of electrografting a polymer layer on the conducting or semiconducting surface of the first substrate, said step A) comprising

- a step Aa) of bringing the conducting or semiconducting surface of the

5 first substrate into contact with a liquid solution comprising:

- a protic solvent;

- at least one diazonium salt;

- at least one monomer that is chain-polymerizable and soluble in said protic solvent;

0 - at least one acid in a sufficient quantity to stabilize said diazonium salt by adjusting the pH of said solution to a value less than 7, preferably less than 2.5; said step Aa) being followed by

- a step Ab) of polarizing the conducting or semiconducting surface of the first substrate with a potentio- or galvano-pulsed mode for a duration sufficient5 to form thereupon a polymer layer having a thickness of at least 60 nanometres, preferably between 60 nanometres and 500 nanometres, more preferably between 100 nanometres and 300 nanometres; said polymer layer having a free surface,

- a step B) of adhesive bonding comprising Ba) contacting the free surface of the0 polymer layer with the free surface of the second substrate, Bb) obtaining an assembly, Be) compressing said assembly,

- at least one step C) of heat treating the assembly at a temperature T of from 150°C to 400°C, preferably from 250°C to 300°C.

According to a particular feature, the thickness of the polymer layer is5 preferably between 100 and 300 nanometres.

According to a particular feature, said heating step C is carried out for a period of time of 5 to 60 minutes

PERMANENT BONDING

For permanent bonding, this method preferably consists essentially or0 exclusively of steps A, B and C as defined above.

According to a particular feature of this method, the free surface of the second substrate is a conducting or semi-conducting surface, preferably a silicon surface, or a polymer or isolator.

According to a another particular feature of the method, the second5 substrate comprises a conducting or semiconducting layer, and said method comprises, prior to step B, a step A2) of electrografting a polymer layer on the conducting or semiconducting surface of the second substrate; said electrografting step A2) comprising a step A2a) and a step A2b) respectively identical to step Aa) and step Ab) as described before. The polymer layer thus obtained forms said free surface of the said second substrate.

Figure 1 depicts a flow chart of a processing method embodiment in accordance with this first embodiment of the present invention.

As shown by this flowchart a permanent bonding of two substrates may be obtained by one of the following subsequent steps:

- A, B, C or

- A, A2, B, C.

If needed, a process may be performed on the free surface of one or both of the two wafers thus bonded, such process being for example back- grinding, chemical-mechanical polishing, etching, metallization, patterning, annealing and combinations thereof, at a temperature of from room temperature to about 350°C.

TEMPORARY BONDING

For temporary bonding, the method is characterized in that it further comprises a step Al), prior to step B, comprising heat treating the polymer layer of the first substrate obtained at the end of step A, at a temperature T1 of from 120°C to 300°C, preferably for a period of 5 to 30 minutes, the bonding between said first and second substrates thus obtained being temporary.

For example, heat treatment Al) may be performed at 250°C for 10 minutes, preferably under nitrogen atmosphere.

Generally, the first substrate thus treated is left to cool down at room temperature before the following step B of the process is performed.

According to this method, a step C2) subsequent to step C) of heat treating the assembly obtained at the end of step B), at a temperature T3 greater than the temperature T, preferably by more than at least 50°C, and a step D) of separating (or debonding) the first and second substrates may be provided in addition to previously mentioned steps.

According to a particular feature, said heating step C2 is carried out for a period of time of 2 to 30 minutes

As mentioned in relation to permanent bonding, the method can further comprise a step Cl) of heat treatment that is subsequent to step C) but prior to step C2) at a temperature T2 which is lower than temperature T3, preferably by at least 50°C. Typically, this heating step treatment Cl, may be applied for a period of time of 1 to 60 minutes.

This heating step Cl) may be part of a process performed on the second surface of said second substrate for example in case of backside grinding or processing.

Figure 2 depicts a flow chart of a processing method embodiment in accordance with this second embodiment of the present invention.

PERMANENT OR TEMPORARY BONDING

For permanent as well as temporary bonding, the compressing step allowing the adhesive bonding of the two substrates can be performed at a pressure of from 1 to 20 kN, preferably of from 6 kN, preferably for a period of time of 10 minutes.

ELECTROGRAFTING STEP A

Advantageously, the protic solvent used in the aforementioned STEP A for electrografting a polymer layer upon the first surface of the first substrate is chosen from the group consisting of water, preferably deionized or distilled water; the hydroxylated solvents, in particular alcohols having 1 to 4 carbon atoms; carboxylic acids having 2 to 4 carbon atoms, in particular formic acid and acetic acid, and mixtures thereof.

Water constitutes the protic solvent currently preferred in the context of the invention.

Generally speaking, many diazonium salts are capable of being used for the implementation of step A according to method of the invention, and in particular the diazonium salts mentioned in the document WO 2007/099218.

Thus, according to a particular characteristic, the diazonium salt is an aryldiazonium salt chosen from the compounds of the following formula (I):

R-N₂ ⁺A (I) in which:

- A represents a monovalent anion,

- R represents an aryl group.

As an example of an aryl group R, it is possible in particular to mention the unsubstituted, mono- or polysubstituted aromatic or heteroaromatic carbon structures, consisting of one or more aromatic or heteroaromatic rings, each comprising 3 to 8 atoms, the heteroatom(s) being chosen from N, O, S, or P; the optional substituent(s) preferably being chosen from electron-attracting groups such as I\I0₂, COH, ketones, CN, C0₂H, NH_2i esters and the halogens.

The particularly preferred groups R are the nitrophenyl and phenyl groups.

Among the compounds of formula (I) above, A may especially be chosen from inorganic anions such as halides like G, Br and Cl , haloboranes such as tetrafluoroborane, and organic anions such as alcoholates, carboxylates, perchlorates and sulphates.

According to a preferred embodiment of the method, the diazonium salt of the aforementioned formula (I) is chosen from phenyldiazonium tetrafluoroborate, 4-nitrophenyldiazonium tetrafluoroborate,

4-bromophenyldiazonium tetrafluoroborate, 2-methyl-4-chlorophenyldiazonium chloride, 4-benzoylbenzenediazonium tetrafluoroborate, 4-cyanophenyldiazonium tetrafluoroborate, 4-carboxyphenyldiazonium tetrafluoroborate, 4- acetamidophenyldiazonium tetrafluoroborate, 4-phenylacetic acid diazonium tetrafluoroborate, 2-methyl-4-[(2-methylphenyl)-diazenyl]benzenediazonium sulphate, 9,10-dioxo-9,10-dihydro-l-anthracenediazonium chloride, 4- nitrophthalenediazonium tetrafluoroborate, and napthalenediazonium tetrafluoroborate, 4-aminophenyldiazonium chloride.

Preferably, the diazonium salt will be chosen from phenyldiazonium tetrafluoroborate and 4-nitrophenyldiazonium tetrafluoroborate.

The diazonium salt is generally present within the liquid electrografting solution in a quantity between 10^'3 and lO^M, preferably between 5xl0^'3 and 3X 10 ²M.

Generally speaking, the electrografting solution contains at least one monomer that is chain-polymerizable and soluble in the protic solvent.

The choice of a monomer soluble in the protic solvent constitutes an essential and original feature of the invention.

"Soluble in a protic solvent" is here understood to denote any monomer or mix of monomers whose solubility in the protic solvent is at least 0.5M.

Those skilled in the art will have no difficulty in choosing the monomers capable of being used in the context of the present invention.

These monomers will advantageously be chosen from vinyl monomers soluble in the protic solvent and satisfying the following general formula (II):

in which the, identical or different, groups

to FU represent a monovalent non- metal atom such as a halogen atom or a hydrogen atom, or a saturated or 5 unsaturated chemical group such as a Ci-C₆ alkyl or aryl, a -COOR₅ group in which R_s represents a hydrogen atom or a -Ce alkyl, nitrile, carbonyl, amine or amide group.

Preferably, water-soluble monomers will be used. Such monomers will advantageously be chosen from ethylenic monomers comprising pyridine groups0 such as 4-vinylpyridine or 2-vinylpyridine, or from ethylenic monomers comprising carboxylic groups such as acrylic add, methacrylic acid, itaconic acid, maleic acid, fumaric acid and their sodium, potassium, ammonium or amine salts, amides of these carboxylic acids and in particular acrylamide and methacrylamide along with their N-substituted derivatives, their esters such as 2-hydroxyethyl5 methacrylate, glycidyl methacrylate, dimethylamino- or diethylamino (ethyl or propyl) (meth)acrylate and their salts, quaternized derivatives of these cationic esters such as, for example, acryloxyethyl trimethylammonium chloride, 2- acrylamido-2-methylpropane sulphonic acid (AMPS), vinylsulphonic acid, vinylphosphoric acid, vinyllactic acid and their salts, acrylonitrile, N-0 vinylpyrrolidone, vinyl acetate, N-vinylimidazoline and its derivatives,

N- vinylimidazole and derivatives of the diallylammonium type such as dimethyldiallylammonium chloride, dimethyldiallylammonium bromide and diethyldiallylammonium chloride.

The quantitative composition of the liquid electrografting solution may5 vary within broad limits.

Generally speaking, this solution comprises:

- at least 0.3M of polymerizable monomer(s),

- at least 5xlO ³M of diazonium salt(s),

the molar ratio of the polymerizable monomer(s) to the diazonium salt(s) being0 between 10 and 300.

As previously mentioned, the use of an electrografting protocol in pulsed mode constitutes a preferred embodiment for carrying out step A, to the extent that this particular protocol makes it possible, completely unexpectedly and in contrast to a cyclic voltammetry electrografting protocol, to obtain a continuous and uniform thin film or layer with a growth kinetics compatible with industrial constraints.

Generally speaking, the polarization of the surface to be covered by the film is produced in a pulsed mode, each cycle of which is characterized by:

5 - a total period P of between 0.01 and 2 seconds (0.12 seconds in the example);

- a polarization time T_on of between 0.01 and 1 s (0.1 seconds in the example) during which a potential difference of 20 V to 120 V is applied to the surface of the substrate (cathode potential of -80 V in the example); and

- a reversed polarization time T_rev of between 0.01 and 1 s (0.100 seconds in the example) during which a potential difference of 0 V to 20 V is applied to the surface of the substrate (anodic potential of 5 V in the example).

The method of preparing an adhesive polymer film which has just been described is especially useful in the preparation of wafer bonding for temporary5 bonding.

The substrate is preferably solid, and may be rigid or flexible depending on the materials the substrate is made of.

According to a second aspect, the disclosure concerns a method of manufacturing microelectronic devices comprising a method for temporary or0 permanent bonding as described according to the first aspect of the invention.

According to a particular embodiment, the assembly be cut in order to obtain microchips. In that case, the first substrate is a wafer or a chip, and the second substrate is a wafer or a chip.

In a particular technical application, the first substrate is a MOEMS or5 MENS wafer, and in that the second substrate is a CMOS integrated circuit.

In the case of wafer handling, the second substrate is a silicon wafer having a thickness of 500 to 1.000 microns, and the method can comprise a step of chemical-mechanical polishing of a free surface of the second substrate to reduce the thickness of the second substrate to a value of less than 300 microns.0 The invention is illustrated in the Example below.

Example 1 - Preparation of a polv-4-vinylpyridine fP4VP) film on a planar p-doped silicon substrate. 5 Substrate:

The substrate was a 200 mm-large p-doped silicon wafer having a 750 mhti thickness and a 20 W.sh resistivity.

Solution:

The electrografting solution was an aqueous solution prepared by mixing: -2 litres of a solution A containing 8.2 g/i (0.035 mol/l) of 4-nitrophenyldiazonium tetrafluoroborate and 150 ml/I (1.81 mol/l) of HCI (37% in mass); and

-0.6 litre of al solution B containing 81,25 ml/l (0.75 mol/l) of pure 4- vinylpyridine and and 125 ml/I (1.50 mol/l) of HCI (37% in mass);

in a glass reactor.

Since the solution obtained was stored in the freezer, a standing time was necessary to reach the room temperature.

The solution was degassed for 10 minutes, prior to use, by an argon or nitrogen flow, in order to enhance its durability.

Protocol:

To carry out electrografting on the silicon substrate, the following system has been used, which was composed of:

- a wafer holder equipped with means for making an electrical contact at the edge of the wafer, and shaped to support the substrate, the assembly thus constituted being intended to serve as the work electrode;

- a carbon sheet intended to serve as the counter-electrode;

- a stabilized electrical power supply and electrical connection device.

- a light source (halogen lamp, 150 W) placed in front of the sample of p- doped silicon so as to obtain a measured light intensity on the surface of the sample. The lamp was placed at a distance of around 1 m from the surface of the sample. Light filters could be placed between the light and the glass reactor in order to have uniform 600 lumens on all the wafer surface that was placed inside the glass reactor, before introducing the reactive solution. The sample was illuminated throughout the duration of the experiment.

The electrografting of the poly-4-vinylpyridine (P4VP) onto the surface of the silicon substrate has been carried out by applying to the substrate a "reversed pulsed voltage" electrochemical protocol for a predetermined duration of around 30 minutes to 1 hour (30 minutes in the example), comprising: - a total period P of between 0.01 and 2 seconds (0.12 seconds in the example);

The duration of this electrografting step depends, as will be understood, on the desired thickness of the polymer adhesive layer. This duration can easily be determined by those skilled in the art, as the growth of the layer is a function of the time.

In the aforementioned conditions, a polymer (P4VP) layer having a thickness of 140 nanometres has been obtained.

Once the electrografting has been finished, the wafer has been washed with water and dried in a dedicated automatic wafer cleaner (spin-coater) All these steps are made in an ISO 5 environment.

Characterizations:

The sample thus prepared has been analyzed by infrared spectroscopy and by Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS).

As the infrared spectrum obtained shows the presence of the adsorption bands at 1580 cm ¹ (P4VP), 1520 cm ¹ (diazonium) and 1350 cm ¹ (diazonium) confirms the presence of the polymer (P4VP) electrog rafted onto the surface of the silicon.

The characteristic P4VP peaks observed during the ToF-SIMS analysis in positive ions are reported in Table 1.

Table 1: Characteristic peaks of P4VP deposit on n-doped Si observed by Time-of- Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) in positive ions

A scanning electron microscope (SEM) analysis or a dedicated ellipsometer for 200 mm wafers has enabled to check the thickness and uniformity of the polymer layer.

A crucial step for distinguishing permanent or temporary bonding appears here. If an anneal (heat treatment at a temperature) of between 120°C and 300°C, preferably 250°C, is performed after polymer grafting, the following steps of bonding will lead to temporary bonding.

A temporary bonding was thus obtained in this example by an anneal step at 120°C to 300°C, preferably 250°C, for 5 to 30 minutes, preferably 10 minutes, under nitrogen or forming gas.

In a second step, the assembly of formerly obtained wafer with the polymer layer has been glued to a 200 mm silicon wafer using a EVG 520 (EV

Group) machine.

The two wafers have been brought into contact, compressed at 6 kN for about 10 minutes and heated at 300°C.

The resulting adhesion level (1.3 J/m²) allowed to back grind the glued silicon wafer at a 550 micron material thickness. The stack resisted to this treatment, and no crack or peeling have been noticed at this stage.

After grinding and acoustic analysis, the stack was mounted on a frame, heated up at 350°C for 10 minutes and the non-treated silicon wafer (second substrate) was easily separated by sliding it off and had a final 200 microns thickness. Since the polymer was electrografted on the wafer carrier (first substrate), where the chemical bonds allow very high adhesions between the substrate and the polymer, no traces of polymer were found on the thinned silicon wafer.

Claims

1. A method for bonding a first solid substrate to a second solid substrate, said first substrate comprising one free surface which is a conducting or

5 semiconducting surface, preferably a silicon surface, and said second substrate comprising one free surface,

said method comprising:

0 - a step Aa) of bringing the conducting or semiconducting surface of the first substrate into contact with a liquid solution comprising:

- a protic solvent;

- at least one diazoniu salt;

- at least one monomer that is chain-polymerizable and soluble in said5 protic solvent;

- at least one acid in a sufficient quantity to stabilize said diazonium salt by adjusting the pH of said solution to a value less than 7, preferably less than 2.5; said step Aa) being followed by

- a step Ab) of polarizing the conducting or semiconducting surface of the0 first substrate with a potentio- or galvano-pulsed mode for a duration sufficient to form thereupon a polymer layer having a thickness of at least 60 nanometres, preferably between 60 nanometres and 500 nanometres, more preferably between 100 nanometres and 300 nanometres; said polymer layer having a free surface,

5 - a step B) of adhesive bonding comprising Ba) contacting the free surface of the polymer layer with the free surface of the second substrate, Bb) obtaining an assembly, Be) compressing said assembly,

0

2. The method according to claim 1, characterized in that the free surface of the second substrate is a conducting or semi-conducting surface, preferably a silicon surface, or a polymer or isolator. 5

3. The method according to claim 1 or 2, characterized in that it further comprises a step Cl) of heat treatment that is subsequent to step C) at a temperature of from room temperature to 350°C.

5 4. The method according to claim 1, characterized in that the second substrate comprises a conducting or semiconducting layer, and in that said method comprises a step A2) of electrografting a polymer layer on the conducting or semiconducting surface of the second substrate; said electrografting step A2) comprising a step A2a) and a step A2b) respectively identical to step Aa) and0 step Ab) as described in Claim 1, said polymer layer thus obtained forming said free surface of the said second substrate.

5. The method according to claim 1, characterized in that said compressing step Be) is performed at a pressure of from 1 to 20 kN.

5

6. The method according to claim 1, characterized in that it further comprises a step Al), prior to step B, comprising heat treating the polymer layer of the first substrate obtained at the end of step A, at a temperature T1 of from 120°C to 300°C, preferably for a period of 5 to 30 minutes, the bonding between said first0 and second substrates thus obtained being temporary.

7. The method according to claim 6, characterized in that it further comprises a step C2) subsequent to step C) of heat treating the assembly obtained at the end of step B), at a temperature T3 greater than the temperature T, preferably by5 more than at least 50°C, and in that it comprises a step D) of debonding the first and second substrate.

8. The method according to claim 6 or 7, characterized in that it further comprises a step Cl) of heat treatment that is subsequent to step C) but prior to0 step C2), at a temperature T2 which is lower than temperature T3, preferably by at least 50°C.

9. A method of manufacturing microelectronic devices comprising a step of bonding two substrates, preferably two wafers, said bonding step being5 performed by the method as described in any of claims 1 to 8.

10. The method according to claim 9, characterized in that it comprises a step of cutting the assembly to obtain microchips.

11. The method of claim 9, characterized in that the first substrate is a wafer or 5 a chip.

12. The method of claim 11, characterized in that the second substrate is a wafer or a chip. 0

13. The method of claim 9, characterized in that the first substrate is a MOEMS or MEMS wafer, and in that the second substrate is a CMOS integrated circuit.

14. The method of claim 9, characterized in that the second substrate is a silicon wafer having a thickness of 500 to 1,000 microns, and chemical-mechanical5 polishing of a free surface of the second substrate is carried out to reduce the thickness of the second substrate to a value less than 300 microns.