Method of eutectic sealing of two substrates

Abstract

1. A method comprising the steps of: a) providing a first substrate (111) covered with first beads (121) containing a first element; b) providing a second substrate (112) covered with second beads (122) containing a second element; c) placing the first bead (121) and the second bead (122) in contact with each other and carrying out a heat treatment for the formation of a eutectic phase in which the first element and the second element are alloyed, thereby forming a sealing bead 123 containing a eutectic alloy, and sealing the first substrate (111) to the second substrate (112), the formation of the eutectic phase being accompanied by the formation of an outflow portion (123'); A method in which at least one of the first substrate (111) and the second substrate (112) is locally covered with a wettable layer (130), whereby in step c) an outflow portion (123') of the eutectic alloy forms on the wettable layer (130).

Description

[Technical Field]

[0001] The present invention relates to the general field of microelectronics, and more particularly to the encapsulation (or packaging) of microelectronic devices such as microsystems of the MEMS (microelectromechanical systems), NEMS (nanoelectromechanical systems), MOEMS (micro-optoelectromechanical systems) or NOEMS (nano-optoelectromechanical systems) type, which involves encapsulating or enclosing said devices in a sealed cavity whose atmosphere is controlled.

[0002] The present invention relates to a method for hermetically sealing two substrates.

[0003] The invention also relates to a device obtained by such a method.

[0004] The invention is used in many industrial fields, for example in the fields of automobiles, mobile phones or video game consoles.

[0005] The invention is particularly interesting because it makes it possible to hermetically seal the substrate with a eutectic alloy while avoiding the risk of short circuits or mechanical disturbance of moving structures. [Background technology]

[0006] prior art Microelectronic devices of the MEMS, NEMS, MOEMS or NOEMS type are sensors or actuators with nanometer or micrometer dimensions. They are manufactured via microelectronic methods.

[0007] The encapsulation of these microelectronic devices makes it possible, on the one hand, to protect them from external elements (humidity, particulate contamination, reactive elements such as oxygen), and, on the other hand, to control the atmosphere prevailing in the cavity in which these devices are encapsulated (pressure, composition of the encapsulated gas, etc.). Depending on the intended use, the encapsulation pressure prevailing in the cavity is variable and typically ranges from 10 -3 Between mbar and 1 bar.

[0008] Various types of sealing can be performed for this encapsulation: thermocompression, eutectic sealing, anodic sealing, etc.

[0009] Currently, eutectic sealing is seen as the most promising option for the future.

[0010] Typically, in a eutectic sealing process, as shown in Figures 1A and 1B, alloy components are placed separately on the faces of the substrates 11, 12 to be assembled. A first bead of material 21 is formed on the first substrate 11. A second bead of material 22 is formed on the second substrate 12 (Figure 1A).

[0011] During sealing, the two substrates 11, 12 are placed in close contact and a temperature is applied to the entire system.

[0012] When the relative compositions of the two materials correspond to the eutectic concentrations, melting occurs at the eutectic temperature.

[0013] In the liquid phase, a homogeneous liquid composed of two materials exists between the two surfaces.

[0014] Solidification of the braze will then lead to the formation of a material consisting of a phase separation of the materials used to perform the eutectic melt within their respective solubility limits.

[0015] This material is then called a eutectic material, even though it is not formed by a single phase.

[0016] This eutectic material is then between the two substrates 11, 12, allowing for mechanical closure of the interface to form the final assembly.

[0017] The eutectic material thus passes through a liquid state before solidifying and forming a gas-tight bead 23 around the encapsulated chip (FIG. 1B).

[0018] However, the passage through the liquid phase can be accompanied by a flow of eutectic material from the region of the sealing beads (FIG. 2).

[0019] These solidifying outflows 23' can cause possible mechanical jamming of the moving structures and / or create short circuits between various regions of the device.

[0020] Therefore, there is a need to control leakage of the liquid braze.

[0021] For example, in the dissertation by V. Lumineau (2018 'Study of the bonding of MEMS by Al-Ge eutectic alloy'), it was proposed to form stops adjacent to the beads, which would limit the two plates being assembled from moving too close together and thus crushing the Al-Ge bilayer.

[0022] However, for this solution to work, solid stops must be placed at a certain distance on each side of the bead, but this solution cannot be applied to all devices, for example, due to miniaturization constraints of various structures. Summary of the Invention [Problem to be solved by the invention]

[0023] DISCLOSURE OF THE INVENTION One goal of the present invention is to overcome the drawbacks of the prior art and to propose a eutectic sealing method that allows for a hermetic sealing of two substrates while avoiding the phenomena of short circuits and mechanical interference. [Means for solving the problem]

[0024] For this purpose, the present invention proposes a method for eutectic sealing of two substrates, comprising the following steps: a) providing a first substrate having a first surface covered by first beads made of a first material, and optionally by one or more microelectronic devices, the first material including a first element; b) providing a second substrate having a first surface covered by second beads made of a second material and, optionally, by one or more microelectronic devices, the second material including a second element capable of forming a eutectic alloy with the first element; c) placing the first bead and the second bead in contact and performing a heat treatment to form a eutectic phase in which the first element of the first bead and the second element of the second bead are alloyed together, thereby forming a sealing bead containing a eutectic alloy and sealing the first substrate to the second substrate, wherein the formation of the eutectic phase is accompanied by the formation of an outflow portion of the eutectic alloy.

[0025] It is also possible to use first beads comprising a first material and a second material, for example in the form of a multilayer, and / or second beads comprising a first material and a second material, for example in the form of a multilayer.

[0026] It is therefore possible to create symmetrical configurations.

[0027] For example, it is also possible to form an asymmetric configuration by having a first bead including a first material and a second bead including a second material with a portion of the first material disposed thereon, in which case the first material is thinner on the first surface of the first substrate in order to preserve the eutectic concentration before melting.

[0028] According to another alternative embodiment, an asymmetric configuration is obtained by having a second bead comprising the second material and a first bead comprising the first material with a portion of the second material disposed thereon, in which case the second material on the first surface of the second substrate is thinner in order to preserve the eutectic concentration before melting.

[0029] The distribution of material on only one of the two faces or on both faces can be changed.

[0030] Advantageously, a configuration is chosen which leads to the formation of a flow of atoms through the bonding interface, i.e. an asymmetric configuration.

[0031] In this method, at least one of the first and second substrates is locally covered by a wettable layer, and the contact angle of the eutectic alloy droplet on the wettable layer is, on the one hand, at least 20°, preferably at least 40°, smaller than the contact angle of the eutectic alloy on the first surface of the first substrate, and, on the other hand, smaller than the contact angle of the eutectic alloy on the first surface of the second substrate, so that during step c), the outflow of the eutectic alloy preferably proceeds onto the wettable layer.

[0032] The present invention differs fundamentally from the prior art by the implementation of at least one zone of preferential wetting of the eutectic alloy with respect to the substrate: the eutectic material formed during step c) has better wettability on the wetting layer than on the faces of the first and second substrates.

[0033] Therefore, instead of placing a "physical" barrier (like a stop in the prior art), a so-called "energetic" barrier was used, with surfaces that have a high surface energy relative to the liquid metal, and a wetting zone with a low surface energy. The liquid metal wets this wetting zone as if it were dropping into a potential well. By skillfully integrating preferential wettability layers, the outflow of eutectic material is guided and kept within an area that is "non-critical" to the chip's function.

[0034] For example, in the case of the eutectic alloy Al-Ge, the wetting of this alloy is better on metallic surfaces than on dielectric-type surfaces. The placement of a metallic wettable layer on or close to the sealing beads makes it possible to include outflows in the case of substrates with surfaces made of dielectric material.

[0035] In particular, a substrate is selected on which the contact angle of a droplet of eutectic alloy is greater than or equal to 90°, preferably greater than or equal to 100°.

[0036] For example, a wettable layer is selected such that the contact angle of a droplet of eutectic alloy is 70° or less, preferably 60° or less, and even more preferably 40° or less.

[0037] By this sealing method, the molten metal alloy is deposited on the surface of the substrate to be assembled, and the outflow is contained by a wettable layer. The cavity thus sealed is airtight and the assembly has good mechanical strength.

[0038] Advantageously, the wettable layer (also called wetting layer) is a metal layer, preferably made of a metal chosen from W, Ti, Al, Au and Cu.

[0039] According to another advantageous alternative, the wettable layer is a layer of a metal nitride, such as TiN, WN, AlN, etc.

[0040] According to another advantageous alternative, the wettable layer is a layer of a semiconductor such as Si, Ge, SiC, AsGa, InP, etc.

[0041] Advantageously, the first side of the first substrate is made of a dielectric material, preferably SiO2 or Si3N4, a semiconductor material or a metal nitride such as TiN, WN, AlN (the wettable layer is then preferably made of a metal), and / or the first side of the second substrate is made of a dielectric material, preferably SiO2 or Si3N4, or a semiconductor material or a metal nitride such as TiN, WN, AlN (the wettable layer is then preferably made of a metal).

[0042] Advantageously, the eutectic alloy is chosen from Al-Ge, Au-In, Au-Sn, Au-Si, Bi-Sn and Au-Ge.

[0043] Advantageously, the first material is chosen from AlSi or AlCu for the eutectic alloy AlGe, Au for the eutectic alloys Au-In, Au-Sn, Au-Si or Au-Ge, Bi for the eutectic alloy Bi-Sn, and / or the second material is chosen from Ge for the eutectic alloy Al-Ge or Au-Ge, Sn for the eutectic alloy Au-Sn or Bi-Sn, Si for the eutectic alloy Au-Si, In for the eutectic alloy Au-In.

[0044] The first element has a volume V1 and the second element has a volume V2. Volumes V1 and V2 are selected to form a eutectic alloy with relative concentrations given by V1 and V2. The relative volumes and compositions of the two elements are selected to correspond to the eutectic concentrations.

[0045] According to a first advantageous embodiment, a first bead is arranged on a first wetting layer, the wetting layer protruding either on one side of the first bead or on both sides of the first bead, and / or a second bead is arranged on a second wetting layer, the wetting layer protruding either on one side of the second bead or on both sides of the second bead.

[0046] According to a second advantageous embodiment, the wettable layer is offset with respect to the first bead and / or with respect to the second bead, and the wettable layer is selectively connected to the first bead and / or to the second bead by cross-members, which are advantageously made of the same material as the wettable layer.

[0047] The invention also relates to a device thus obtained, which comprises a first substrate and a second substrate sealed to one another by sealing beads containing a eutectic alloy, at least one of the first and second substrates being locally covered by a wettable layer, the contact angle of a droplet of the eutectic alloy on the wettable layer being smaller, on the one hand, than the contact angle of the eutectic alloy on the first surface of the first substrate and, on the other hand, than the contact angle of the eutectic alloy on the first surface of the second substrate, and the effluent of the eutectic alloy being trapped on the wettable layer.

[0048] Advantageously, the wettable layer is a metal layer, preferably selected from W, Ti, Al, Au and Cu, the first side of the first substrate is made of a dielectric material, preferably SiO2, or a semiconductor material, and / or the first side of the second substrate is made of a dielectric material, preferably SiO2, or a semiconductor material.

[0049] Advantageously, the eutectic alloy is selected from Al-Ge, Au-In, Au-Sn, Au-Si, Bi-Sn and Au-Ge.

[0050] According to a first advantageous embodiment, the sealing beads are arranged on a wettable layer, the wettable layer either protruding above one side of the sealing beads or above both sides of the sealing beads.

[0051] According to a second advantageous embodiment, the wettable layer is locally offset with respect to the sealing beads, and the wettable layer is connected to the sealing beads by cross-members, which are advantageously made of the same material as the wettable layer.

[0052] Other features and advantages of the present invention will appear from the further description below.

[0053] Of course, this further description is given only as an example of the purpose of the present invention and should not be construed in any way as a limitation thereof.

[0054] The invention will be better understood on reading the description of exemplary embodiments given for purely informational and in no way limiting purposes, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0055] [Figure 1A] FIG. 1A shows a schematic representation of the various steps of the prior art eutectic sealing method described above. [Figure 1B] FIG. 1B shows a schematic representation of the various steps of a previously described prior art eutectic sealing method. [Figure 2] FIG. 2 is an image showing the eutectic alloy outflow obtained from the prior art method, as previously described and shown in FIGS. 1A and 1B. [Figure 3A] FIG. 3A illustrates schematically various steps of a eutectic sealing method according to certain embodiments of the present invention. [Figure 3B] FIG. 3B illustrates schematically various steps of a eutectic sealing method according to certain embodiments of the present invention. [Figure 4A] FIG. 4A shows schematically the various steps of a eutectic sealing method according to another particular embodiment of the present invention. [Figure 4B] FIG. 4B illustrates schematically the various steps of a eutectic sealing method according to another particular embodiment of the present invention. [Figure 5A] FIG. 5A illustrates schematically the various steps of a eutectic sealing method according to another particular embodiment of the present invention. [Figure 5B] FIG. 5B illustrates schematically the various steps of a eutectic sealing method according to another particular embodiment of the present invention. [Figure 6A] FIG. 6A illustrates schematically the various steps of a eutectic sealing method according to another particular embodiment of the present invention. [Figure 6B] FIG. 6B illustrates schematically the various steps of a eutectic sealing method according to another particular embodiment of the present invention. [Figure 7] FIG. 7 shows, both schematically and in a top view, beads disposed on a wettable layer according to certain embodiments of the present invention. [Figure 8A] FIG. 8A shows, in schematic form, various steps of a method of eutectic sealing according to another particular embodiment of the present invention, with the wettable layer and beads shown in cross section in FIG. 8A corresponding to those in FIG. 9, with the cross section defined by the dotted line. [Figure 8B] FIG. 8B shows, in schematic form, various steps of a method of eutectic sealing according to another particular embodiment of the present invention, with the wettable layer and beads shown in cross section in FIG. 8A corresponding to those in FIG. 9, with the cross section defined by the dotted line. [Figure 9] FIG. 9 shows, both schematically and in a top view, beads of material disposed on a wettable layer according to certain embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0056] In order to make the drawings more legible, the various parts shown in the drawings are not necessarily to uniform scale.

[0057] The various possibilities (alternatives and embodiments) are to be understood as not mutually exclusive but as being able to be combined together.

[0058] Additionally, in the following description, terms that depend on the orientation of a structure, such as "upper," "lower," etc., are applied while the structure is considered oriented as depicted in the drawings.

[0059] Detailed Disclosure of Specific Embodiments The method for eutectic sealing of two substrates will now be explained in more detail with reference to the accompanying Figures 3A and 3B, 4A and 4B, 5A and 5B, 6A and 6B, 7, 8A and 8B, 9.

[0060] The method for eutectic sealing of two substrates includes the following steps: a) providing a first substrate 111 having a first surface covered with first beads 121 made of a first material, and optionally one or more microelectronic devices 140, wherein the first material comprises a first element; b) providing a second substrate 112 having a first surface covered with second beads 122 made of a second material, and optionally one or more microelectronic devices 140, the second material including a second element capable of forming a eutectic alloy with the first element; c) placing a first bead 121 and a second bead 122 in contact; d) performing a heat treatment to form a eutectic phase in which the first element provided by the first bead and the second element provided by the second bead are alloyed, thereby forming a sealing bead 123 containing a eutectic alloy, and sealing the first substrate 111 to the second substrate 112 together with the sealing bead 123;

[0061] The resulting sealing bead 123 and the two substrates 121, 122 advantageously define an airtight cavity in which one or more microelectronic devices 140 are placed. Depending on the intended use, the encapsulation pressure prevailing in the cavity is variable and is typically 10 -3 Between mbar and 1 bar (or between 0.1 Pa and 100,000 Pa).

[0062] In this method, at least one of the first substrate 111 provided in step a) and the second substrate 112 provided in step b) is locally covered by a wettable layer 130, whereby during step c) an outflow portion 123' of the eutectic alloy is formed on the wettable layer 130 and is at least partially, and preferably completely, contained in this layer 130.

[0063] The substrates 111, 112 provided in step a) and in step b) respectively comprise two main surfaces parallel to each other, the first surface of the first substrate 111 being intended to be placed facing the first surface of the second substrate 112.

[0064] The substrates 111 and 112 can be made of mostly the same material, or the substrates 111 and 112 can be made of different materials, which can be selected from Si, Ge, InP, AsGa, Al2O3, SiC, GaN, LNO, and LTO. The first substrate 111 and / or the second substrate 112 may comprise an auxiliary substrate covered by a thin layer, which thus forms the first side of the first substrate or the first side of the second substrate.

[0065] The first surface of the first substrate 111 and / or the first surface of the second substrate 112 can be made of SiO2, Si3N4, a dielectric or a metal nitride such as TiN, WN or AlN.

[0066] Preferably, the first side of the first substrate 111 and / or the first side of the second substrate 112 is made of an oxide, such as SiO2, which can be a native oxide layer, a thermal oxide layer or a deposited layer of oxide.

[0067] It is therefore possible to have the first substrate 111 comprise an auxiliary substrate made of Si covered by a thin layer of SiO 2. The same applies to the second substrate 112.

[0068] The first side of the first substrate 111 and / or the first side of the second substrate 112 can be of the same nature or of different nature.

[0069] The first and second substrates each have a thickness of, for example, between 300 μm and 1,000 μm.

[0070] One or more microelectronic devices 140 can be disposed on the first side of the first substrate 111 and / or the first side of the second substrate 112. The microelectronic devices on the first substrate 111 can be the same as or different from those present on the second substrate 112.

[0071] Microelectronic devices refer to microelectronic components, such as microsystems of the MEMS (microelectromechanical systems), NEMS (nanoelectromechanical systems), MOEMS (micro-optoelectromechanical systems) or NOEMS (nano-optoelectromechanical systems) type, for example, it can be an infrared microdetector, a transistor, a microbattery, a capacitor, a supracapacitor, a photoelectric conversion component, an accelerometer, a pressure sensor, a microphone, an antenna, a rate gyro or gyroscope, or any other device considered necessary to achieve the final goal.

[0072] The first material of the first bead 121 can be chosen from AlSi or AlCu for the eutectic alloy AlGe, Au for the eutectic alloys Au-In, Au-Sn, Au-Si or Au-Ge, and Bi for the eutectic alloy Bi-Sn.

[0073] The second material of the second bead 122 can be selected from Ge for the eutectic alloy Al-Ge or Au-Ge, Sn for the eutectic alloy Au-Sn or Bi-Sn, Si for the eutectic alloy Au-Si, and In for the eutectic alloy Au-In.

[0074] Preferably, the first material comprises aluminum and the second material comprises germanium to form a eutectic alloy AlGe.

[0075] The first material can be composed of a first element and / or the second material can be composed of a second element.

[0076] For example, the first material can be aluminum and the second material can be germanium.

[0077] As mentioned above, it can have a symmetric or asymmetric configuration.

[0078] For example, it is possible to have a first bead in the form of a multilayer comprising a layer of aluminum covered by a thin layer of germanium (for example the thin layer has a thickness of 10 nm), and a second bead made of germanium.

[0079] Additional layers can be deposited on the first surface and first material of the first substrate and / or on the first surface and second material of the second substrate. For example, a thin layer of Ge can be deposited on aluminum itself, which is then deposited on a layer of SiO2. The wetting layer can then be removed. For example, a 10 nm layer of Ge can be deposited on aluminum. The presence of the Ge layer promotes the reaction between aluminum and germanium by facilitating the onset of the reaction on the aluminum, despite the presence of an aluminum oxide layer that may form. When the entire surface of the substrate is covered by this germanium layer, devices are formed on this layer. Portions of the Ge layer, and optionally, portions of the aluminum layer, can be removed if desired.

[0080] This layer is generally placed just before the bonding step.

[0081] Preferably, the wettable layer 130 is a metal layer. Advantageously, the wettable layer is made of W, Ti, Al, Au or Cu.

[0082] According to a highly advantageous embodiment, the wettable layer 130 is a metal layer, the first surface of the first substrate 111 is made of a dielectric, a semiconductor material or a metal nitride, and the first surface of the second substrate 112 is made of a dielectric, a semiconductor material or a metal nitride.

[0083] The wetting properties of the eutectic are better on the wettable layer 130 than on the first surface of the first substrate 111 or on the first surface of the second substrate 112. Therefore, the eutectic alloy remains on the wettable layer 130 rather than flowing out onto the substrate.

[0084] "Better eutectic wetting" means that the contact angle of a eutectic droplet on the first surface of the first substrate 111 or on the first surface of the second substrate 112 is at least 20°, and preferably at least 40°, greater than the contact angle of a eutectic droplet on the wettable layer 130.

[0085] For example, the contact angle of a droplet of AlGe on a surface made of SiO2 is greater than 110° (Dissertation V. Lumineau). Silicon oxide is not thus wetted by the eutectic alloy Al-Ge. Therefore, a wettable layer 130 is advantageously chosen such that the contact angle of a droplet of AlGe on this layer is less than or equal to 90°, preferably less than or equal to 70°.

[0086] The method used to determine the contact angle of a drop of eutectic alloy is described, for example, in the dissertation by V. Lumineau. The contact angle measurement is performed on a placed drop: the shape of the drop of eutectic alloy formed on a flat surface for temperatures above its melting temperature was observed. For this, the alloy was placed in a crucible made of alumina, with a capillary at its end with a diameter of 0.6 mm. After melting was performed and the experimental temperature was reached, a piston allowed the formation of a drop at the end of the capillary. The crucible was then lowered towards the surface of the solid to be investigated, and the drop was deposited. Observations were carried out in situ, at temperature, through a window using a camera (25 frames / s). Video recording of the droplet's spreading then allowed the measurement and calculation of the droplet's specific parameters via the software Drop Shape Analysis. The pumping system was 5.10 -7 mbar (or 5.10 -5 This makes it possible to obtain a high vacuum that can reach (up to 100 Pa).

[0087] The wettable layer 130 can be disposed according to several configurations.

[0088] The wettable layer 130 can be disposed between the first substrate 111 and the first beads 121 (FIGS. 4A and 4B) and / or between the second substrate 112 and the second beads 122 (FIGS. 3A and 3B).

[0089] The wetting layer 130 protrudes from the beads 121, 122, thereby having a free surface for containing the outflow portion 123'. The wetting layer 130 can protrude onto both sides of the beads 121, 122. Alternatively, it can protrude onto only one side of the beads 121, 122. The overflow area of ​​the wetting layer 130 can be between tens of nanometers and hundreds of micrometers.

[0090] The wettable layer 130 can protrude over both sides of the beads 121, 122 that it covers (FIGS. 3A and 3B, 4A and 4B).

[0091] Alternatively, the wettable layer may protrude only on one side of the bead (Figures 5A and 5B, 6A and 6B, 7). Advantageously, the wettable layer 130 protrudes only outside the cavity formed by the sealing bead 123 and the two substrates, directing and protecting the eutectic outflow 123' from the cavity containing the chip microelectronic device.

[0092] According to another alternative embodiment, the wettable layer 130 can be positioned next to the beads 121, 122 (FIGS. 8A and 8B, 9). It can be in contact with the beads 121, 122 or can be spaced apart from the beads 121, 122 (e.g., the wettable layer does not touch the beads). For example, the wettable layer 130 forms a belt around the beads. Cross-members (or "bridges") can facilitate evacuation of the outflow portion to the wettable layer. The cross-members can be made of the same or a different material as the wettable layer. Preferably, the wettable layer 130 is located only outside the sealing beads (e.g., outside the cavity) and is separated from the sealing beads.

[0093] A single wettable layer 130 has been described. It is also possible to have a wettable layer on each substrate 111, 112.

[0094] Alternatively, the first substrate 111 and the second substrate 112 are each locally covered by a wettable layer 130, whereby during step c) outflows 123' of the eutectic alloy form on the wettable layers 130 and are at least partially, and preferably entirely, contained in these layers.

[0095] The use of two wettable layers allows for better confinement of the eutectic material, thus stabilizing it. Such a configuration can also allow for a reduction in the dimensions of these layers relative to a single wettable layer, which is particularly advantageous for device miniaturization.

[0096] The two layers can be identically or differently disposed on each substrate.

[0097] The size of the wettable layer 130 is determined by the size of the beads, which is chosen to be large enough to contain the outflow.

[0098] During step c), two beads 121, 122 or even two substrates 111, 112 are placed in close proximity to one another, after which a heat treatment is carried out. The treatment can be carried out under a controlled atmosphere (vacuum, inert atmosphere) and / or with mechanical pressure on the entire surface. For example, a force of 2 kN or more, or 10 kN or more, or even 30 kN or more can be applied.

[0099] The temperature is selected depending on the eutectic alloy. To form a braze containing the eutectic alloy between the assembled substrates 111, 112, the temperature is selected to reach or even exceed the melting temperature of the eutectic alloy. Preferably, the applied temperature is lower than the melting temperature of the first bead 121 and the melting temperature of the second bead 122. For example, to produce a eutectic alloy Al-Ge, a temperature greater than 425°C is applied (the melting temperature of the eutectic alloy).

[0100] The sealing beads 123 are made of a eutectic alloy containing two elements, one from a first element in the first bead and the other from a second element in the second bead, also called a braze.

[0101] The formation of the beads 123 made of a eutectic alloy ensures high mechanical strength of the seal.

[0102] The sealing beads 123 have a width of, for example, between 30 μm and 200 μm, the width being chosen to be small enough to allow miniaturization, but large enough to allow good hermeticity.

[0103] The total thickness of the first and second materials is generally between 100 nm and 10 μm. Preferably, it is 1 μm. The use of a wettable layer can be combined with the use of mechanical stops as described in the prior art.

[0104] The above method makes it possible to produce assemblies whose thickness is less than 10 μm. In addition, the sealing is almost free of hole-type defects at the interface due to thermal compression, which is interesting for assemblies that require a controlled atmosphere. [Explanation of symbols]

[0105] 11 First board 12 Second board 21 First Bead 22 Second Bead 23 Airtight Beads 23' Outlet 111 First board 112 Second board 121 First Bead 122 Second Bead 123 Sealing beads 123' Outlet 130 layers 140 Microelectronic Devices

Claims

1. A method for eutectic sealing of two substrates (111, 112), comprising: a) providing a first substrate (111) having a first surface covered by first beads (121) made of a first material and, optionally, by one or more microelectronic devices (140), said first material comprising a first element; b) providing a second substrate (112) having a first surface covered by second beads (122) made of a second material and, optionally, by one or more microelectronic devices (140), said second material including a second element capable of forming a eutectic alloy with said first element; c) placing the first bead (121) and the second bead (122) in contact with each other and carrying out a heat treatment for the formation of a eutectic phase in which the first element of the first bead (121) and the second element of the second bead (122) are alloyed, thereby forming a sealing bead (123) containing the eutectic alloy and sealing the first substrate (111) to the second substrate (112), the formation of the eutectic phase being accompanied by the formation of an outflow (123') of the eutectic alloy, The method is characterized in that at least one of the first substrate (111) and the second substrate (112) is locally covered by a wetting layer (130), and the contact angle of the eutectic alloy droplet on the wetting layer (130) is, on the one hand, at least 20°, and preferably at least 40°, smaller than the contact angle of the eutectic alloy on the first surface of the first substrate (111) and, on the other hand, at least 20°, and preferably at least 40°, smaller than the contact angle of the eutectic alloy on the first surface of the second substrate (112), whereby in step c) the outflow portion (123') of eutectic alloy forms on the wetting layer (130).

2. 2. The method according to claim 1, characterized in that the first beads (121) comprise the first material and the second material, the first beads (121) being, for example, in the form of a multilayer, and / or the second beads (122) comprise the first material and the second material, the second beads (122) being, for example, in the form of a multilayer.

3. 3. The method according to claim 1 or 2, characterized in that the wettable layer (130) is a metal layer, preferably selected from W, Ti, Al, Au and Cu, or a metal nitride layer, for example a TiN, AlN or WN layer.

4. The first surface of the first substrate (111) is made of a dielectric material, preferably SiO ２ or made of Si ３ N ４ made of a semiconductor material or made of a metal nitride, and / or the first surface of the second substrate (112) is made of a dielectric material, preferably SiO ２ or made of Si ３ N ４ 4. The method according to claim 1, wherein the substrate is made of a semiconductor material or made of a metal nitride.

5. 5. The method according to claim 1, wherein the eutectic alloy is selected from Al-Ge, Au-In, Au-Sn, Au-Si, Bi-Sn and Au-Ge.

6. 6. The method according to claim 5, wherein the first material is selected from AlSi or AlCu for the AlGe eutectic alloy, Au for the Au-In, Au-Sn, or Au-Si eutectic alloy, and / or the second material is selected from Ge for the Al-Ge eutectic alloy, Sn for the Au-Sn or Bi-Sn eutectic alloy, and Si for the Au-Si eutectic alloy.

7. 7. The method according to claim 1, wherein the first bead (121) is disposed on the wetting layer (130) and the wetting layer (130) protrudes either on one side of the first bead (121) or on both sides of the first bead (121), or the second bead (122) is disposed on the wetting layer (130) and the wetting layer (130) protrudes either on one side of the second bead (122) or on both sides of the second bead (122).

8. 7. The method according to claim 1, wherein the wetting layer (130) is offset relative to the first bead (121) or relative to the second bead (122), and the wetting layer (130) is connected to the first bead (121) or to the second bead (122) by a cross member.

9. A device comprising a first substrate (111) and a second substrate (112) sealed to each other by sealing beads (123) containing a eutectic alloy, wherein at least one of the first substrate (111) and the second substrate (122) is locally covered by a wetting layer (130), the contact angle of a droplet of eutectic alloy on the wetting layer (130) being smaller, on the one hand, than the contact angle of the eutectic alloy on the first surface of the first substrate (111) and, on the other hand, than the contact angle of the eutectic alloy on the first surface of the second substrate (112), and an outflow portion (123') of the eutectic alloy being located on the wetting layer (130).

10. The wettable layer (130) is preferably a metal layer selected from W, Ti, Al, Au and Cu, or a layer made of a metal nitride, for example a layer of TiN, AlN or WN, and the first surface of the first substrate (111) is made of a dielectric material, preferably SiO ２ or made of Si ３ N ４ made of a semiconductor material or made of a metal nitride, and / or the first surface of the second substrate (112) is made of a dielectric material, preferably SiO ２ or made of Si ３ N ４ 10. The device according to claim 9, characterized in that it is made of a semiconductor material or made of a metal nitride.

11. 11. The device according to claim 9 or 10, wherein the eutectic alloy is selected from Al-Ge, Au-In, Au-Sn, Au-Si, Bi-Sn and Au-Ge.

12. The device according to any one of claims 9 to 11, characterized in that the sealing beads (123) are arranged on the wettable layer (130), and the wettable layer (130) protrudes either on one side of the sealing beads (123) or on both sides of the sealing beads (123).

13. A device according to any one of claims 9 to 11, characterized in that the wettable layer (130) is locally offset with respect to the sealing beads (123) and the wettable layer (130) is connected to the sealing beads (123) by cross members.

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