FR3083467A1 - METHOD FOR SEALING PARTS BETWEEN THEM WITH AN EUTECTIC ALLOY BASED ON ALUMINUM - Google Patents

Abstract

The invention relates to the field of methods for sealing parts together with an aluminum-based eutectic alloy and in particular such a method comprising: - providing 110 two parts to be sealed together, - depositing 120 a first layer based on aluminum on the first part, - Oxyder 125 a free surface of the first layer, then - deposit 130 a second layer based on germanium on the first layer, and - deposit 140 a complementary layer based on germanium on the second part, then - contact and pressurize 150 the parts together, the second layer being directly in contact with the complementary layer, and - apply annealing 160 until reaching, or even exceeding, a melting temperature of said eutectic alloy, to form a solder based on said eutectic alloy between the parts.

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to the field of bonding processes by brazing parts together with a eutectic alloy. Such methods can also be qualified as eutectic sealing methods. The invention relates more particularly to a method of sealing parts together with an aluminum-based eutectic alloy. Such methods find for application the development of electromechanical microsystems.

STATE OF THE ART

Solder bonding with a eutectic alloy is a special type of metallic bonding. The intermediate bonding layer present between surfaces of parts bonded together is composed of an alloy of elements which can give rise, under conditions of temperature and composition, to an eutectic microstructure. In this particular type of metallic bonding, the constituents are deposited on at least one of two surfaces of the parts to be bonded together, respecting the composition of the eutectic. When the surfaces are kept in contact and the temperature is raised above the melting point of the eutectic alloy, a liquid with the composition of the eutectic is formed. Upon cooling, the liquid forms a particular structure consisting of alternating phases of each of the constituents and which it is hoped will mechanically close the interface to form an assembly by sealing the parts together.

In the literature, there are many examples of bonding by brazing parts together with a eutectic alloy. So, we know how to get a paste interface:

- Cu-Sn [Cf. Y.-H. Ko, M.-S. Kim, J. Bang, T.-S. Kim, C.-W. Lee, Journal of ELECTRONIC MATERIALS (2015)],

- Au-Si [Cf. R.F. Wolffenbuttel, K.D. Wise, Sensors and Actuators A, 43 (1994) 223-229],

- Au-Sn [Cf. A. Garnier, X. Baillin, F. Hodaj, J Mater Sci: Mater Electron (2013) 24: 5000-5013], and

- Al-Ge [Cf. S. Sood, R. Hergert, O. Treichel, Advancing Microelectronics 41 (5), pp. 30-37 (2014)].

More particularly, there are methods for eutectically sealing parts with each other using an aluminum-based eutectic alloy. Each of these methods implements layer configurations of the components of the eutectic alloy that are different from each other.

According to a first configuration, called symmetrical, illustrated in FIG. 1A, a bilayer 4, 5 of the two elements of the alloy is deposited on each of the surfaces 1a, 2a of the parts 1,2 to be assembled. After solidification, a seal is obtained between the two parts 1, 2 by a microstructure based on a eutectic alloy 3 comprising aluminum 3a and germanium 3b; there is a continuous crack 10 at the sealing interface between the two bilayers, as illustrated in FIG. 1B. It is assumed that the two bilayers melted separately, oxidizing on their surface. Oxide films were trapped at the interface and gave rise to the formation of this crack 10. This structure is therefore unsatisfactory since this defect is critical with regard to the hermeticity of the structures.

According to a second, asymmetrical configuration, illustrated in FIG. 2A, a bilayer 4, 5 of the two elements of the alloy is deposited only on a 1a of the two surfaces 1a, 2a of the parts 1, 2 to be assembled. After assembly, and as illustrated in FIG. 2B, a seal is obtained between the two parts 1, 2 by a microstructure based on a eutectic alloy 3 comprising aluminum 3a and germanium 3b; there is a low adhesion of the eutectic alloy 3 on the upper part 2 due to the appearance of numerous cavities 11. Although the adhesion of the eutectic alloy is better on the lower part 1 than on the upper plate 2 , this architecture is also not satisfactory.

According to a third architecture, illustrated in FIG. 3, the two elements of the eutectic alloy are deposited separately, each on one of the surfaces 1a, 2a of the parts 1,2 to be assembled. More particularly, a layer 4 based on the first element is deposited on the surface 1a of the first part 1 and a layer 5 based on the second element is deposited on the surface 2a of the second part 2. This thus ensures the adhesion of the soldering on the surfaces 1a, 2a and the complete fusion of the two elements. In addition, the rise in temperature, up to the eutectic temperature, taking place only when the surfaces to be bonded are brought into contact via the two elements of the alloy, a single liquid phase is present, which reduces the risk of crack formation. However, this configuration implies the presence of native oxides of the constituent elements of the eutectic alloy at the interface between these elements, which can block the fusion of the alloy. In the case of Al-Ge seals, the presence of native aluminum oxide blocks the seal. In the literature, it is then recommended to press very hard (one exerts a pressure higher than 10MPa for example) [Cf. X.-H. Huang et al., ECS Transactions, vol. 50, no.7 (2013)]. This pressure may be too great for certain devices which may be on the parts that one wishes to glue. It is therefore impossible to use an Al-Ge eutectic sealant to bond these surfaces. There are, in the literature, alternatives for making this bonding at low pressure. One can for example use a bombardment of atoms of argon on the surfaces of parts to be assembled [Cf. G. L. Chua, B. Chen, N. Singh, 17th Electronics Packaging and Technology Conference (EPTC), IEEE (2015)]. However, this alternative is not necessarily compatible either with the preservation of the integrity of certain devices possibly present on the parts to be assembled.

Furthermore, it is known, in the literature, a particular type of stack making it possible to perform an Au-Sn sealing by solideliquid interdiffusion (SLID) [T. A. Tollefsen, A. Larsson, O. M. Lovvik, K. Aasmundtveit, Metallurgical and Materials Transactions B, vol. 43B, pp. 397-405 (2012)]. This particular type of stack includes a thin layer of gold deposited on the tin without the latter being returned to the air, and therefore without the tin being able to oxidize. The role of the gold layer is to prevent the oxidation of tin.

An object of the present invention is to provide a method of sealing parts with each other with an aluminum-based eutectic alloy which at least partially overcomes the drawbacks of existing methods.

More particularly, an object of the present invention is to propose such a sealing method which makes it possible to preserve the integrity of microelectronic devices or elements formed on one and / or the other of the parts prior to their assembly.

The other objects, characteristics and advantages of the present invention will appear on examining the following description and the accompanying drawings. It is understood that other advantages can be incorporated.

SUMMARY OF THE INVENTION

To achieve this objective, according to a first aspect, the present invention provides a method of sealing parts with each other with an aluminum-based eutectic alloy. The process includes the following steps:

- Provide at least two parts, each part having an assembly surface by sealing to at least one other part among said at least two parts,

- Deposit at least a first layer based on aluminum at least on the assembly surface of a first part among said at least two parts,

- Oxidize at least one free surface of the first layer, then

- Deposit a second layer based on an element capable of forming, with aluminum, said eutectic alloy, at least on the first layer, and

- Deposit an additional layer based on said element capable of forming, with aluminum, said eutectic alloy at least on the assembly surface of a second part among said at least two parts, then

- Contact and pressurize the assembly surfaces of the first part and the second part together, the second layer being directly in contact with the complementary layer, and

- Apply annealing until reaching, or even exceeding, a melting temperature Tf specific to said eutectic alloy, so as to form, from the first and second layers and the additional layer, a solder based on said eutectic alloy between the first and second pieces.

Preferably, the first layer is deposited directly on the assembly surface of the first part, or directly on a previously deposited undercoat, potentially directly, on the assembly surface of the first part, this undercoat does not not being based on aluminum, or even being free of aluminum.

When the temperature is, according to the process introduced above, greater than or equal to the melting temperature of the eutectic alloy, the constituent element of the layer deposited on the oxidized aluminum surface migrates through the oxide of aluminum and forms, with aluminum, a liquid film of eutectic composition.

In a different field, which relates to the recrystallization of germanium at temperatures below the melting point of the eutectic, the prior art "K. Nakazawa, K. Toko, N. Usami, T. Suemasu, Japanese Journal of Applied Physics 53, 04EH01 (2014) ”mentions the possibility for germanium to migrate by atomic diffusion through native aluminum oxide under the effect of temperature.

In the context of the development of the present invention, it has been observed that such an atomic diffusion through the native aluminum oxide under the effect of the temperature in the solid phase could trigger the formation of a liquid and cause l 'set of elements of the eutectic alloy of the first and second layers and of the complementary layer in the formation of a liquid of eutectic composition. It is this phenomenon which is used here to obtain a sealing interface between the first and second parts, the interface taking the form of a solder based on the constituent elements of the eutectic alloy. The solder obtained is more particularly free from cracking and cavity.

The method according to the invention thus makes it possible to overcome the obstacle presented by the methods according to the prior art, this obstacle being linked to the formation of alumina on the surface of the aluminum-based layer. Indeed, thanks to the method according to the invention, it is no longer necessary to avoid this formation of alumina; the constraints associated with this need can therefore be relaxed, or even eliminated. In particular, the applicable temperature and pressure conditions leading to the formation of the solder according to the invention can be such that they make it possible to preserve the integrity of any devices or elements of microelectronic devices formed on the parts before their assembly. In addition, it should be noted that the solder thus obtained can make it possible to provide electrical conduction between the two substrates through the interface.

For example, when the element capable of forming, with aluminum, said eutectic alloy is based on germanium, the annealing step can be carried out at a temperature strictly below 450 ° C., while being higher than the temperature of the eutectic of aluminum and germanium, the latter being equal to 424 ° C.

Optionally, the method according to the invention may comprise at least any of the optional features below taken separately or in combination.

The contacting and pressurizing step is at least partly concomitant with the annealing application step. The pressurization and annealing steps apply at least to each of the first and second layers and the additional layer. They make it possible to transform, by heating them, these three layers into a liquid with the eutectic composition, then they make it possible to transform, by cooling, this liquid with the eutectic composition into an eutectic microstructure.

The step of oxidizing the free surface of the first layer comprises at least one of the following steps: enabling the oxidation and carrying out the oxidation. The oxidation step can result in the formation of a surface layer of aluminum oxide by oxidation of the first layer. The surface layer may for example have a thickness of between 1 and 10 nm. Preferably, the aluminum oxide layer is not produced voluntarily, by a positive action, but it is not necessary to oppose the spontaneous formation of this oxide layer. Indeed, the oxidation of aluminum is spontaneous in contact with air; an oxide layer of a few nanometers is instantly formed which is very stable and the thickness of which does not change further if the conditions do not change.

Preferably, the second layer is deposited directly on the first layer, and more particularly directly on the surface layer of aluminum oxide.

Preferably, the additional layer is deposited directly on the assembly surface of the second part, or directly on a previously deposited sublayer, potentially directly, on the assembly surface of the second part. This sub-layer is preferably not based on said element capable of forming, with aluminum, said eutectic alloy; potentially, this sublayer is free of said element capable of forming, with aluminum, said eutectic alloy.

According to a second aspect, the present invention relates to a microelectronic system comprising at least first and second parts and a solder based on an eutectic alloy comprising aluminum between the first and second parts, the solder being free from cracking and cavity. In addition, at least one of the parts of said system may comprise at least one microelectronic device or element, for example a weld bead, the functionality of which would be negatively impacted if it were exposed to a temperature above 500 ° C., or even higher than 450 ° C.

BRIEF DESCRIPTION OF THE FIGURES

The aims, objects, as well as the characteristics and advantages of the invention will emerge better from the detailed description of an embodiment of the latter which is illustrated by the following accompanying drawings in which:

- Figure 1A schematically shows a first configuration of sealing two parts together according to the prior art, before contacting and under pressure of one part against the other;

- Figure 1B shows a sectional view obtained by FIB-SEM (for "electron microscope and focused ion beam scanning") of a seal obtained via the first configuration illustrated in Figure 1A;

- Figure 2A schematically shows a second configuration of sealing two parts together according to the prior art, before contacting and under pressure of one part against the other;

- Figure 2B on the right represents a sectional view obtained by FIBSEM of a seal obtained via the second configuration illustrated in Figure 2A on the left;

- Figure 3 schematically shows a third sealing configuration of two parts together according to the prior art, before contacting and under pressure of one part against the other;

- Figure 4 shows graphs of time evolution of temperature and pressure during the implementation of the method according to an embodiment of the invention;

- Figure 5 shows a flow chart of process steps according to an embodiment of the invention;

- Figure 6A schematically shows a sealing configuration of two parts together according to an embodiment of the invention, before contacting and under pressure of one part against the other; and

- Figure 6B shows a sectional view obtained by FIB-SEM of a seal obtained via the configuration illustrated in Figure 6A.

The drawings are given as examples and are not limitative of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily at the scale of practical applications. In particular, the relative thicknesses of the different layers are not representative of reality.

In FIG. 5, certain steps are only optional and are not essential. This is for example the case of the process steps framed by broken lines.

DETAILED DESCRIPTION OF THE INVENTION

Before starting a detailed review of embodiments of the invention, there are set out below optional features of the first aspect of the invention which may possibly be used in combination or alternatively:

- the steps for depositing the first layer, the second layer and the additional layer can be configured so that the ratio between:

o a thickness of the first layer and o a cumulative thickness of the second layer and of the complementary layer, respects a eutectic composition of the eutectic alloy, to within +/- 10%, preferably to within +/- 5%. Thus, we make sure to form a quality solder by controlling the deposition parameters of the different layers;

- Preferably, the temperature reached during annealing does not exceed a predetermined threshold value above which any one of the first and second parts, or even among said at least two parts, would be functionally and negatively impacted. Thus, the method makes it possible to preserve the functional, even structural, integrity of the parts to be assembled together.

- during the supplying step, at least one part among said at least two parts can comprise at least one microelectronic device or element, for example a weld bead, and the temperature reached during annealing does not exceed a predetermined threshold value above which said at least one microelectronic device or element would be functionally and negatively impacted. Thus, the method makes it possible to preserve the functional, even structural, integrity of any microelectronic devices or elements carried by the parts to be sealed together;

- according to any one of the two preceding characteristics, said predetermined temperature threshold value is equal to 500 ° C, and preferably equal to 450 ° C;

- the step for depositing the second layer and the step for depositing the additional layer can be configured so that a cumulative thickness of the second layer and of the additional layer is greater than 10 nm. Thus, it is ensured in particular of a satisfactory mixing of the constituent elements of the eutectic alloy;

the step for depositing the second layer and the step for depositing the additional layer can also be configured so that a thickness of the additional layer is greater than a thickness of the second layer, preferably at least three times superior;

- the oxidation step can result in the formation of a surface layer of aluminum oxide by oxidation of the first layer, and the deposition step of the second layer can be configured so that a thickness of the second layer is greater than a thickness of the surface layer, preferably greater than twice the thickness of the surface layer. Thus, one ensures the formation of a liquid in the proportions of the eutectic, by the sole fact of the rise in temperature until reaching, or even exceeding, the melting temperature Tf, and potentially even before the parts are brought into contact and under pressure against each other. More particularly, the element capable of forming, with aluminum, the eutectic alloy, will migrate through (or in) the layer of aluminum oxide, forming a liquid with the non-oxidized aluminum of the first layer and the native aluminum oxide layer is broken up and dispersed in the liquid;

- at least one of the first and second parts, or even of said at least two parts, is based on at least one of the following materials: silicon, silica, glass, sapphire, alumina , germanium, indium phosphide, gallium nitride, gallium sulfide, gallium arsenide, gallium nitride and silicon carbide. The process makes it possible to seal pieces of all kinds between them. Indeed, the nature of the parts to be sealed together does not influence the characteristics of the solder obtained by implementing the method;

- the method can also comprise at least one of the following steps:

o Before the step of depositing the first layer, form a sublayer, preferably chemically inert, at least on the assembly surface of the first part and o Before the step of depositing the additional layer, form a sub- layer, preferably chemically inert, at least on the assembly surface of the second part.

Thus, the process makes it possible, if necessary, to ensure that the nature of the parts to be sealed together does not influence the characteristics of the solder obtained by implementing the process by providing an undercoat making it possible to isolate physically and chemically the solder and the assembly surfaces of the parts. In addition, when the element capable of forming, with aluminum, said eutectic alloy and the nature of the part on which the complementary layer is intended to be deposited are identical, the sub-layer thus makes it possible not to create a quasi-source. infinity of said element suitable for forming, with aluminum, said eutectic alloy, so as to be able to respect the composition of the eutectic;

the deposition step of the first layer can be configured so that the first layer has a thickness of between 50 nm and 10 μm, preferably between 500 nm and 5 μm. This ensures that an eutectic seal is made up of a quantity of material allowing, on the one hand, to overcome defects, in particular of topographic type, that the parts to be sealed with one another, on the other hand to minimize the size of the assembly. In addition, the aluminum oxide particles, each of nanometric size, thus have a smaller size, preferably less than an order of magnitude, compared to the thickness of the eutectic seal, which is preferably greater than 100. nm;

each of the first and second parts, or even of the said at least two parts, comprises a wafer based on a semiconductor material, each wafer having two opposite circular faces, for example with a diameter equal to 200 mm, and the assembly surfaces of the first and second parts may each be formed by one of said circular faces, and the step of bringing the assembly surfaces into contact and under pressure may comprise at least one of:

o the application of at least one pressure level from one of the two parts to the other in a direction perpendicular to their assembly surfaces, said pressure level being between 0.05 Mpa and 3 Mpa, preferably between 0.1 Mpa and 0.5 Mpa, and o the application of at least one level of compression force from one of the two parts to the other in a direction perpendicular to their assembly surfaces, said level force between 2 kN and 90 kN, preferably between 2 kN and 90 kN.

The method is thus perfectly suited to sealing wafers together, in particular for the production of microelectronic devices comprising several wafers or parts of wafers superimposed between them;

each of the two parts may comprise a wafer based on a semiconductor material, and the assembly surfaces of the first and second parts are each formed by one of said circular faces, and the steps of bringing into contact and under pressure, and application of annealing may comprise at least one of:

- For a first duration ti, preferably between 10 and 30 min, and for example equal to 20 min:

o the application of a first pressure level Pi, for example equal to 0.1 MPa, of one of the two parts on the other in a direction perpendicular to their assembly surfaces and o a rise in temperature from l 'ambient to a first threshold temperature Ti lower than the melting temperature Tf of said eutectic alloy, for example the first threshold temperature Ti being less than the melting temperature Tf by a value at most equal to 10%, preferably at more equal to 5%, of the value of said melting temperature Tf, then

- For a second duration t ₂ -ti, preferably between 5 min and 10 h, for example equal to 1 h:

the application of a second pressure level P ₂ greater than the first pressure level P-ι, for example a second pressure level P ₂ equal to 0.5 MPa, from one of the two parts to the other according to a direction perpendicular to their assembly surfaces and o maintaining the first threshold temperature Ti, then

- During a third duration t ₃ -t ₂ , preferably between 2 and 10 min, o maintenance of the second pressure level P ₂ , o a rise in temperature from the first threshold temperature Ti to a second threshold temperature T ₂ greater than or equal to the melting temperature Tf of said eutectic alloy, for example the second threshold temperature T ₂ being greater than the melting temperature Tf by a value at most equal to 10%, preferably at most equal to 5%, the value of said melting temperature,

- For a fourth duration t ₄ -t ₃ , preferably between 1 min and 1 h, maintaining the second pressure level P ₂ and the temperature T ₂ , then

- During a fifth duration t ₅ -t ₄ , preferably between 10 and 30 min:

o at least one of maintaining the second pressure level P ₂ and applying a pressure level lower than the pressure level P ₂ , if applicable applying the pressure level lower than the pressure level P ₂ succeeding the maintenance of the second pressure level P ₂ , for example the lower pressure level being equal to the first pressure level Pi, and o cooling to ambient;

- The process can also comprise, before the steps of bringing into contact and under pressure and of applying an annealing, a step of rinsing with deionized water, then a drying step, of at least one among at least one free surface of the second layer and at least one free surface of the complementary layer, these steps being parameterized so as to remove at least part of a native oxide from the element capable of forming, with aluminum, said eutectic alloy, the steps of bringing into contact and under pressure and of applying an annealing then being carried out preferably avoiding any prolonged exposure of the parts to oxygen;

- at least one of the parts is supplied so as to comprise at least one microelectronic device or element, for example a weld bead;

the steps of bringing into contact and under pressure and the step of applying an annealing are carried out under vacuum, at a pressure between 10 ′ ³ and W ⁷ mbar, for example equal to 5.10 ′ ⁵ mbar;

- the element capable of forming, with aluminum, said eutectic alloy can be chosen from: germanium, silicon, gold, copper; and

- At least one of the first and second pieces may comprise at least first and second pieces of piece and a solder based on an eutectic alloy comprising aluminum between the two pieces, said solder sealing between them the two pieces and having been obtained by implementing the method according to any one of the preceding claims with the first and second pieces in place of the first and second pieces.

By a layer based on a material A is meant a layer comprising this material A and possibly other materials, these other materials being for example in a proportion equivalent to that of material A or in a lesser proportion.

The term “composition” or “proportions” (for example eutectic (s)) of an alloy is intended to refer to the composition or to the proportions of this alloy as represented by the molar fraction of the elements which up. The elements considered here being atoms, we speak equivalently of atomic fraction and atomic percentage (noted% at). A variation going from -10% to + 10% around a given composition means that the atomic fraction of each of the elements of the alloy varies around said given value in a range of values ranging:

- i) from said given value minus 10% of this value, ii) to said given value plus 10% of this value.

A variation of the atomic fraction of an element of the alloy implies an opposite variation in the same proportions of the other elements of the alloy.

The term “eutectic” alloy generally means an alloy whose composition is such that its melting point is lower than the melting point of the pure elements constituting the alloy. The alloy of eutectic composition (binary, ternary, quaternary, quinary) melts and solidifies at a determined and constant temperature. This so-called melting temperature is lower than the melting temperature of any of the elements that compose it, or even of another eutectic alloy that could be obtained by mixing the same elements in different proportions.

Here, the term “eutectic” alloy is more particularly understood to mean an alloy whose composition is such that its melting temperature Tf is strictly lower than a threshold temperature which it is desired not to exceed. Thus, if it is recommended to ensure that the proportions of the eutectic are best respected, in order to benefit as much as possible from the physicochemical phenomenon exploited by the present invention (and described below) and in order to be able to lower as much as possible the value of the threshold temperature not to be exceeded, it should be noted that an eutectic alloy according to the invention does not only mean an alloy whose composition is such that its melting temperature is minimum, but s' extends to any alloy having a composition close to the strict eutectic composition. It can thus be estimated that a composition equal to the strict eutectic composition to within +/- 10%, preferably to within +/- 5%, characterizes an eutectic alloy according to the present invention. Thus, when reference is subsequently made to an "eutectic composition" or to "eutectic proportions", these terms benefit from the same broad interpretation as that associated with any mention of an eutectic alloy as defined in the scope of the present invention.

The term “bilayer” is understood to mean a multilayer comprising two (mono) layers which are different from each other, for example by the material (s) from which these (mono) layers are made, and superimposed, preferably directly, one to the other.

“Annealing” is understood to mean a heating cycle comprising in a step of gradual rise in temperature followed by controlled cooling.

Reference is made, when speaking of “microstructure”, to a set of geometric characteristics of the elements that make up the structure of a material, on a scale where they can only be observed using microscopes or specialized techniques. When used in the word "microstructure", the prefix "micro-" does not refer to specific measurements. Thus, the microstructure of a material can include elements of nanometric size. However, the dimensions of a microstructure are generally calculated in micrometers.

The term “chemically inert layer” means a layer based on a material defined in that it does not react chemically with at least one other material intended to be in contact, in particular direct, with the chemically inert layer. The “chemically inert” aspect of the layer is to be evaluated with regard to the function of said chemically inert layer. For example, the layer is chemically inert at least under conditions of deposition of said at least one other material on said chemically inert layer. For example, the layer is chemically inert under conditions of use of a microelectronic device comprising the material of the chemically inert layer and said at least one other material.

The terms "lower" and "upper" mean respectively "lower or equal" and "higher or equal". Equality is excluded by the use of the terms "strictly inferior" and "strictly superior".

By "at least an order of magnitude higher (or lower)" is meant at least 10 times higher (or lower).

The term “ambient temperature” means the temperature of the air at the location where the process is intended to be carried out.

It is specified that in the context of the present invention, the term "on", "overcomes", "covers" or "underlying" or their equivalents does not necessarily mean "in contact with". Thus for example, the deposition of a first layer on a second layer, does not necessarily mean that the two layers are directly in contact with each other but it means that the first layer at least partially covers the second layer in being either directly in contact with it, or being separated from it by at least one other layer or at least one other element.

By microelectronic device or element means any type of device or element made with microelectronic means. These devices include in particular in addition to devices with a purely electronic purpose, such as a weld bead, micromechanical or electromechanical devices (MEMS, NEMS ...) as well as optical or optoelectronic devices (MOEMS ...).

By microelectronic system is meant any type of functional system comprising at least one microelectronic device. These systems include, in addition to purely electronic systems, micromechanical or electromechanical systems (MEMS, NEMS ...) as well as optical or optoelectronic systems (MOEMS ...).

In this patent application, the thickness is taken in a direction perpendicular to the assembly surfaces of the parts to be assembled. In FIGS. 1A, 1B, 2A, 2B, 3, 6A and 6B, the thickness is taken vertically.

The values given below are given to within a tolerance margin. This tolerance margin can be defined as ranging from -10% to +10% of the given value, preferably from -5% to +5% of the given value.

The invention aims in particular to achieve eutectic sealing between two parts at a temperature strictly below a predetermined threshold value above which one of the parts to be assembled would be functionally and negatively impacted. For example, said predetermined threshold value is equal to 500 ° C., preferably to 450 ° C. By not exceeding this threshold temperature, the devices or elements of microelectronic devices that one of the parts could possibly include can advantageously be preserved in their integrity.

With reference to FIGS. 4, 5, 6A and 6B, the sealing method 100 according to an embodiment of the invention is described below.

The method first comprises a step consisting in supplying 110 at least two pieces 1,2. Each supplied part 110 has an assembly surface 1a, 2a by sealing to at least one other part. The assembly surfaces 1a and 2a of the parts 1, 2 are intended to allow the parts to be assembled together. They are preferably substantially complementary to each other.

Each piece can be made from any material. It is for example based on at least one of the following materials: silicon, silica, glass, sapphire, alumina, germanium, indium phosphide, gallium nitride, gallium, gallium arsenide, gallium nitride and silicon carbide. However, it can also be a metal part, for example steel, iron, or copper.

Each piece can more particularly comprise a wafer based on a semiconductor material (or "wafer" according to English terminology). Generally, each slice has two circular faces opposite one another. A circular face of each part is capable of playing the role of assembly surface to the circular face of another part. The slices generally have a diameter equal to 200 mm or 300 mm. If the configuration illustrated in FIG. 6A shows diagrammatically two sections 1 and 2 of the same diameter, it is conceivable to implement the sealing method according to the invention to seal between them two sections of different diameter.

Furthermore, at least one of the parts supplied 110 may comprise at least one microelectronic device or element, for example a weld bead.

It is envisaged to form 115, 116 a sublayer 6, preferably chemically inert, at least on the assembly surface of one of a first part 1 and a second part 2. In this event, the process as illustrated in FIG. 5 can more particularly comprise, before the step of depositing 120 of the first aluminum-based layer 4, a step of forming 115 of a chemically inert layer 6 at least on the assembly surface 1a of the first part 1 (surface 1a on which the first layer 4 based on aluminum is intended to be deposited 120). Likewise, the method may comprise, before the step of depositing 140 of the complementary layer 5a, a step 116 of forming a chemically inert layer 6 at least on the assembly surface 2a of the second part 2 (surface 2a on which the additional layer 5a is intended to be deposited).

When at least one of the first and second parts 1, 2 is based on silicon, the chemically inert layer 6 can be formed 115, 116 based on silicon oxide, by surface oxidation of the part or based titanium nitride (TiN) for example by physical vapor deposition.

Each chemically inert layer can have at least one or the other of the following two functions: i) prevent the material of the part from reacting, in particular chemically, with the material of the layer intended to be deposited on the part and ii ) prepare the surface condition of the part so that the layer material intended to be deposited on the part can be deposited there.

In the particular case where the material of the layer intended to be deposited on the part is identical to the material on the basis of which the part is made, it is still possible that the sub-layer layer 6 aims to isolate the part from the layer intended to be deposited on the part, so that the physico-chemical phenomenon exploited by the process according to the invention is not influenced. More particularly, the fact that the physicochemical phenomenon exploited is partly based on the respect of proportions of the different materials involved at the level of the sealing interface, a part made of a material identical to that of the layer intended to be deposited on the piece could appear as an almost infinite source and thereby influence the physico-chemical phenomenon that one wishes to exploit. Isolating the part from the layer via a chemically inert layer 6 makes it possible to avoid this type of influence. Such an influence is not necessarily to be avoided, but can result in less fine control of the physico-chemical phenomenon exploited.

Following the step of supplying 110 of the two parts 1,2, and optionally the step of forming 115 of a chemically inert layer 6, the method comprises a step consisting in depositing 120 at least a first layer 4 based on d aluminum at least on the assembly surface 1a of the first part 1. This deposition step 120 is preferably configured so that the first layer 4 has a thickness of between 50 nm and 10 μm, more particularly between 60 nm and 10 pm, preferably between 500 nm and 5 pm. One of the advantages of eutectic sealing is to be free from topographical defects of the surfaces to be assembled. In order for the liquid with the eutectic composition to "cover" these defects, it is preferable to have enough liquid. The thickness ranges indicated above allow to be sure. However, in addition to minimizing the size of the assembly, there is no advantage in respecting the upper limit of these ranges. Furthermore, since the thickness of the aluminum oxide layer is a few nanometers, the nanometric particles generated by the breaking up and dispersion of the aluminum oxide layer should preferably represent a small fraction, or even very small. , of the solder joint. Consequently, a total thickness of the solder joint greater than 100 nm seems reasonable. Therefore, the thickness of the first aluminum-based layer 4 should preferably not be less than 50 or 60 nm.

Many deposition techniques for the first layer 4 can be envisaged, including: physical vapor deposition techniques (PVD for “Physical Vapor Deposition”) such as, for example, electron beam evaporation or sputtering , chemical vapor deposition techniques (CVD for “Chemical Vapor Deposition”) such as, for example, CVD at sub-atmospheric pressure (SACVD for “Sub-Atmospheric CVD”), CVD assisted by plasma (PECVD for "Plasma Enhanced CVD"), etc. In its broadest sense, the method according to the invention is not limited to the use of a specific deposition technique.

The method according to the embodiment illustrated in FIG. 5 then comprises a step consisting in depositing 130 a second layer 5 based on germanium at least on the first layer 4. The step 130 of depositing the second layer 5 can then be configured so that the second layer has a thickness between 10 nm and 100 nm.

The presently described embodiment effectively provides that the second layer 5 is based on germanium as an element capable of forming, with aluminum, a eutectic alloy. However, other elements suitable for forming, with aluminum, another eutectic alloy are also envisaged, among which: silicon, gold, and copper. Zinc, magnesium and calcium are also elements capable of forming a eutectic alloy with aluminum, but they are extremely oxidizable and very volatile elements, which can make them unsuitable for use in the process according to the invention. . Any element capable of forming an eutectic alloy with aluminum may therefore be required to be more oxidizable and / or less volatile, at least under the eutectic sealing conditions, than each of the elements among the zinc. , magnesium and calcium. There are other elements suitable for forming an eutectic alloy with aluminum, but the melting temperatures of the associated eutectics exceed that of the Al-Si eutectic, namely 577 ° C.

Note here that the melting temperature of an Al-Ge alloy with the eutectic composition is equal to 424 ° C. In addition, the strict proportions of Al-Ge eutectics are 29.5% at Ge and 70.5% at AI. Thus, in order to respect the proportions of the Al-Ge eutectic, the first layer 4 based on aluminum is necessarily thicker than the second layer 5 based on germanium.

Each of the deposition techniques set out above with respect to the deposition of the first layer 4 can be implemented to deposit the second layer 5.

The deposition of a bilayer comprising an aluminum layer and a germanium layer on the aluminum layer can be carried out so that the aluminum is not brought back into contact with oxygen. However, this requires special equipment. And generally, the steps of depositing the aluminum-based layer and the germanium-based layer are not carried out in the same hermetic enclosure, which results in the contacting of the aluminum of the first layer. 4 with atmospheric oxygen, and therefore by surface oxidation of the first layer 4 based on aluminum, before the deposition of the second layer based on germanium. This is how the germanium layer is generally deposited on an aluminum layer which has been brought back into contact with dioxygen and therefore has a surface layer of aluminum oxide. It is notably this surface layer of aluminum oxide which explains that the configurations envisaged in the prior art are unsatisfactory. Indeed, as seen in the introduction, the oxidation of aluminum, even limited to the surface of an aluminum-based layer, does not promote the formation of a hermetic bonding interface between two parts to be assembled.

In absolute terms, the process according to the invention does not require, in order to be able to be implemented, and to allow the obtaining of a hermetic bonding interface, that a surface layer of aluminum oxide 5a be formed. However, it is in this context that the method according to the invention has the full extent of its interest.

Thus, as illustrated in FIG. 5, the sealing method 100 according to the invention can advantageously comprise a step consisting in oxidizing at least one free surface of the first layer 4.

The oxidation step of the free surface of the first layer 4 can be spontaneous or controlled. More particularly, it can consist either of allowing oxidation, or of carrying it out by a positive act. Allowing oxidation simply consists in leaving the first layer 4 exposed to oxygen from the atmosphere, while carrying out the oxidation can consist in exposing the first layer 4 to oxygen, for example by inserting it into a enclosure with controlled atmosphere comprising oxygen. Whether spontaneous or controlled, the oxidation step results in the formation of a surface layer of aluminum oxide 4a by oxidation on the surface of the first layer 4. The surface layer 4a generally has a thickness of between 1 and 10 nm.

The second layer 5 is thus deposited directly on the first layer 4, and more particularly directly on the surface layer 4a of aluminum oxide.

Preferably, the deposition step 130 of the second layer 5 and the deposition step 140 of the complementary layer 5a are configured so that a cumulative thickness of the second layer 5 and of the complementary layer 5a is greater than 10 nm. Thus, the material on the basis of which the second layer 5 and of the complementary layer 5a are made will migrate through (or in) the layer of native oxide, form a liquid with aluminum and the native aluminum oxide is fragmented and dispersed in this liquid. Note that the dissolution of the native aluminum oxide in the liquid is practically zero. In addition or as an alternative, it is envisaged that the deposition step 130 of the second layer 5 is configured so that the thickness of the second layer 5 is at least equal, preferably greater, than the thickness of the layer surface 4a of aluminum oxide. For example, the thickness of the second layer 5 is equal to or greater than twice the thickness of the surface layer 4a. However, the distribution of the cumulative thickness of the second layer 5 and of the complementary layer 5a between layers 5 and 5a is not limiting as long as there are two on both sides, with a minimum thickness of 10 nm for the finer of 2.

With reference to FIGS. 5 and 6A, the step 140 of depositing an additional layer 5a based on germanium at least on the assembly surface 2a of the second part 2 is described below with reference to FIGS. 5 and 6A .

This deposition step 140 does not in any way involve the first part 1. Consequently, the deposition steps 120 and 130 on the one hand and the deposition step 140 on the other hand can be implemented decorrelated in time and / or in space.

The same deposition techniques as those set out above with respect to the deposition of the first layer 4 can be envisaged for carrying out the deposition 140 of the complementary layer 5a.

The deposition step 140 of the complementary layer 5a is configured, relative to the deposition step 130 of the second layer 5, so that a thickness of the complementary layer 5a is greater, preferably at least three times greater, at a thickness of the second layer 5.

In addition or as an alternative, the deposition steps 130 and 140 of the second layer 5 and of the complementary layer 5a are configured, relative to the deposition step 120 of the first layer 4, so that the ratio between:

- a thickness of the first layer 4 and

- a cumulative thickness of the second layer 5 and of the complementary layer 5a, respects a eutectic composition of the Al-Ge alloy.

Such a ratio of the cumulative thickness of the second layer 5 and of the additional layer 5a based on germanium on the thickness of the first layer 4 based on aluminum makes it possible to respect relatively precisely the eutectic composition of the alloy Al -Ge when it is equal to 0.59. However, this constraint can be relaxed in particular by exceeding, during the steps of bringing into contact and under pressure 150 and applying an annealing 160 (which will be described below), the minimum melting temperature Tf of the eutectic alloy al-Ge. Indeed, on both sides of the strict eutectic composition, alloy compositions exist whose associated melting temperatures Tf remain below the threshold temperature which it is desired not to exceed. Thus, it is entirely conceivable, even when one of the parts 1, 2 to be assembled comprises at least one device or element of microelectronic device liable to be damaged at a temperature above said threshold temperature, that the strict eutectic composition of the alloy is only respected more or less 10%. Preferably, the strict eutectic composition will be respected to within ± 5%.

Referring to Figure 5, the sealing method 100 according to the illustrated embodiment may further comprise, before bringing the parts 1, 2 into contact and under pressure 150 with one another and applying annealing 160, a rinsing step 145 with deionized water, then a drying step 146, of at least one of the free surface of the second layer 5 is the free surface of the complementary layer 5a. These steps are configured to remove at least part of native germanium oxide. Such a native oxide is in fact capable of forming between on the one hand the deposit 130, 140 of each layer of germanium 5, 5a and on the other hand the stages of bringing into contact and under pressure140 and of applying a annealing 160. The latter are preferably carried out as soon as possible after the rinsing and drying steps, in order to avoid any prolonged contact of the substrates 1, 2 with oxygen.

With reference to FIGS. 5 and 6B, the method of sealing 100 of parts 1, 2 to each other via an eutectic alloy 3 based on aluminum 3a then comprises the following steps:

- Put in contact and under pressure 150 the assembly surfaces 1a, 2a of the first part 1 and of the second part 2 between them by means of the first and second layers 4, 5 and of the complementary layer 5a, the second layer 5 being in contact, or even directly in contact, with the complementary layer 5a, and

- Apply an annealing 160 until reaching, or even exceeding, a melting temperature Tf specific to said eutectic alloy 3,

Thus, from the first and second layers 4, 5 and the complementary layer 5a, a brazing 3ab, as illustrated in FIG. 6B, is formed, based on said eutectic alloy 3 between the first and second parts 1, 2. The contacting and pressurizing step 150 is at least partly concomitant with the step of applying an annealing 160. This has been illustrated in FIG. 5 by the arrangement of the frames referenced 150 and 160 in a frame. larger including them and by the absence of arrow between the frames 150 and 160. Furthermore, the steps of pressurizing 150 and annealing 160 apply at least on each of the first and second layers 4, 5 and the complementary layer 5a. They make it possible to transform, by heating them, these three layers 4, 5 and 5a into a liquid with the eutectic composition, then they make it possible to transform, by cooling, this liquid with the eutectic composition into a eutectic microstructure.

When each of the two parts comprises elements of microelectronic devices, it may be necessary to take care of the alignment of the parts with one another, when they are brought into contact and under pressure 140, so as to correctly place the elements of microelectronic devices one by one. compared to others.

In particular in this context, bringing the two parts into contact and under pressure 140 can be carried out, by a machine for handling at least one of the parts 1, 2, this machine being equipped for example with a laser marking to control the alignment of the parts with each other. More generally, the contacting 140 of the two parts 1, 2 can be carried out by an operator.

When each of the two parts 1, 2 comprises a wafer having a diameter equal to 200 mm, the pressurizing 150 and annealing 160 steps may include at least one of:

the application of at least one pressure level from one of the two parts 1,2 to the other in a direction perpendicular to their circular faces 1a, 2a brought into contact 150, the pressure being between 0.05 Mpa and 3 Mpa, preferably between 0.1 Mpa and 0.5 Mpa, and

the application of at least one level of compressive force from one of the two parts 1,2 to the other in a direction perpendicular to their circular faces 1a, 2a brought into contact 150, the force standard being understood between 2 kN and 90 kN.

When each of the two parts has relatively flat germanium-based layers 5 and 5a, and more particularly having a surface roughness of less than 100 nm PV (for “peak-to-valley” according to Anglo-Saxon terminology), the application of a pressure level as stated above may be equivalent to the application of the force level as stated above. On the contrary, when at least one of the two parts has germanium-based layers 5 and 5a having a greater surface roughness, for example due to the presence of elements of microelectronic devices, such as weld beads, the pressure exerted by a given compression force from one part to the other is not distributed homogeneously at the contact interface between the parts 1, 2. In this case, it may be preferable to consider the compression force level criterion, rather than the pressure level criterion. It should also be noted that, if the ranges of pressure level and / or level of compression force values given above are suitable for substrates in the form of plates with a diameter equal to 200 mm, they are deemed to be able to be adapted to other dimensions of plates at the cost of a reasonable effort of reflection, for example by carrying out routine tests.

The bringing into contact and under pressure 150 therefore comprises the application of pressure, and more particularly of a determined pressure change, from one of the parts 1, 2 to the other; and the application of an annealing 160 includes the submission of the parts and of the layers 4, 5 and 5a (and 6 if applicable) interposed at a determined temperature, and more particularly to a determined temperature evolution. More particularly, the pressure and the temperature applied can vary during the pressurization 150 and annealing 160 steps in the ranges of values defined above.

The pressurizing 150 and annealing 160 steps are also carried out under vacuum. Preferably, this vacuum is a high vacuum, corresponding to a pressure between 10 ' ³ and 10' ⁷ mbar, for example equal to 5.10 ' ⁵ mbar.

An exemplary embodiment of the pressurization 150 and annealing 160 steps in terms of pressure and temperature evolution applied is illustrated in FIG. 4. According to this example, applied to parts taking the form of wafers and to an alloy Al-Ge eutectic, the pressurizing 150 and annealing 160 steps can comprise at least one of the following phases:

- a first phase, preferably of a duration ti between 10 and 30 min, and for example equal to 20 min, during which:

o a first pressure level P-ι, for example equal to 0.1 MPa, of one of the two parts 1, 2 on the other is applied in a direction perpendicular to their circular faces 1a, 2a and o the temperature is increased from ambient to a first threshold temperature Ti, for example equal to 415 ° C;

a second phase, preferably with a duration t ₂ -ti of between 5 min and 10 h, for example equal to 1 h, during which:

o a second pressure level P ₂ , for example equal to 0.5 MPa, of one of the two parts 1, 2 on the other is always applied in the direction perpendicular to the circular faces 1a, 2a, and o the first temperature threshold Ti is maintained;

- a third phase, preferably with a duration t ₃ -t ₂ of between 10 and 30 min, during which:

o the second pressure level P ₂ is maintained and o the temperature is increased from the first threshold temperature Ti to a temperature T ₂ between 424 ° C and 450 ° C;

a fourth phase, preferably with a duration t ₄ -t ₃ of between 1 min and 1 hour, for example equal to 10 min, during which the second pressure level P ₂ and the temperature T ₂ are maintained;

- a fifth phase, preferably with a duration t ₅ -t ₄ of between 10 and 30 min, during which:

o the second pressure level P ₂ is maintained and o the temperature is lowered from the last temperature T ₂ to the temperature Ti, for example with a ramp of 1 ° C per minute;

- a sixth phase, preferably with a duration t ₆ -ts of between 10 and 30 min, during which:

where the first pressure level Pi is applied and where the temperature is lowered from the temperature Ti to ambient temperature, for example with a ramp of 20 ° C. per minute.

When the temperature at which the parts 1, 2 and the layers interposed between these parts are subjected to the temperature T ₂ of between 424 ° C and 450 ° C, the germanium deposited on the first layer 4 based on aluminum migrates through the layer of aluminum oxide 4a and forms with aluminum a liquid film of eutectic composition.

More particularly, when each of the two parts has a diameter equal to 200 mm, the first pressure level Pi is preferably equal to 0.1 MPa and the second pressure level P ₂ is between 0.5 MPa and 3 MPa. The diameter of the parts can also be equal to 300 mm, in which case the values of the first and second pressure levels may be identical to those set out above by increasing the force (in kN) applied to exert each of said pressure levels.

Once the temperature has returned to ambient, the pressure exerted by one of the two parts on the other is released and the assembly formed by the two parts 1, 2 sealed together by the eutectic microstructure 3ab based on aluminum and germanium can be recovered. This assembly can then possibly undergo other treatments and potentially ultimately lead to the manufacture of a microelectronic system.

By observing FIG. 6B and by comparing it with FIGS. 1B and 2B, it clearly appears that the solder 3ab obtained is free from cracking and from cavity.

The example given above of carrying out the pressurization 150 and annealing 160 steps illustrated in FIG. 4 is not limitative of the invention. More particularly, it is also possible to apply a temperature T ₂ higher than the melting temperature of the tactical alloy, even before the parts are brought into contact and under pressure 150 therebetween. In which case a liquid film is formed on the surface of the first layer 4 based on aluminum from a part of the first layer 4 based on aluminum and the second layer 5 based on germanium. Said part of the first layer 4 based on aluminum can extend over the entire surface of the first layer 5 and over a determined thickness of this layer. Said determined thickness is proportional to the thickness of the second germanium-based layer 5. Following the formation of this liquid film, the parts 1.2 are brought into contact and under pressure 150, for example a pressure level equal to 0.16 Mpa (for substrates of 200mm in diameter) for 10 min. During this time, the solid germanium and aluminum phases still present on both sides of the liquid film diffuse in the liquid film (possibly passing through what remains of the native oxide layers), and this until to the complete fusion of the Al / Ge stack. The assembly is then cooled and the solidification of the eutectic alloy Al-Ge seals the interface between the two parts 1,2.

EXAMPLE OF IMPLEMENTATION

According to an exemplary embodiment, an Al-Ge eutectic sealing was carried out using the method described above with reference to FIGS. 4, 5 and 6A.

According to this example, slices or plates 1, 2 of silicon 200 mm in diameter are first coated with 50 nm of TiN deposited by PVD, as an under-layer 6. Then, a first layer 4, of aluminum and with a thickness of 1000 nm, is deposited by evaporation on a first plate 1. The surface of the first layer 4 of aluminum is then returned to the air and a native oxide is formed on the aluminum. A second layer 4, of germanium and a thickness of 10 nm, is then deposited by evaporation on the first layer 4 of aluminum oxidized at the surface. On the second plate, an additional layer 5a, of germanium and a thickness of 590 nm, is deposited on the TiN by evaporation.

Just before sealing, the plates 1, 2 are rinsed 145 with deionized water and then dried 146 to remove the native germanium oxide.

The two plates 1, 2 are then placed in a vacuum sealing equipment, at 5.10 ' ⁵ mbar. The sealing equipment can be that developed and marketed by the company Süss® under the trade name CB8. The temperature is then raised to 450 ° C, for example with a ramp of 20 ° C / min. Then, the plates 1, 2 are brought into contact and a pressure of 0.2 MPa is applied to the rear circular faces of the plates. This pressure is kept constant until the end of the process. The temperature is kept constant for 10 min, then the cooling takes place up to 400 ° C, for example with a ramp of 1 ° C / min. Finally, the plates are cooled to room temperature, for example with a ramp of 20 ° C / min.

The invention is not limited to the embodiments previously described and extends to all the embodiments covered by the claims.

One can cite, among the embodiments covered, that according to which the sealing method 100 comprises the supply 110 of three or more plates intended to be brought into contact and under pressure 150 two by two in the same stack. The plates intended to have an intermediate position between two other plates can be treated (or "processed") so that:

a first layer 4 based on aluminum is deposited on one of their circular faces and that each first layer 4 based on aluminum is covered with a second layer 5 based on germanium, and

- An additional layer 5a is deposited on the other of their circular faces.

Such an embodiment therefore results in the formation of a plurality of sealing interfaces 3ab, each of these sealing interfaces 3 being located between two first substrates neighboring them in the stack.

According to another example, a part 1, 2 supplied 110 may comprise two pieces and a solder 3ab based on an eutectic alloy comprising aluminum between the two pieces. The solder 3ab seals the two pieces together and was obtained by implementing the method according to any one of the preceding claims in which the two pieces play the role of the first and second pieces 1,2.

Claims (15)

1. A method of sealing (100) parts (1, 2) to each other with an eutectic alloy (3) based on aluminum (3a), the method comprising the following steps: - Supply (110) at least two parts (1,2), each part having an assembly surface (1a, 2a) by sealing to at least one other part among said at least two parts, - Deposit (120) at least one first layer (4) based on aluminum at least on the assembly surface (1a) of a first part (1) among said at least two parts, - Oxidize at least one free surface of the first layer (4), then - deposit (130) a second layer (5) based on an element (3b) suitable for forming, with aluminum (3a), said eutectic alloy (3), at least on the first layer (4), and - Deposit (140) a complementary layer (5a) based on said element (3b) suitable for forming, with aluminum (3a), said eutectic alloy (3) at least on the assembly surface (2a) of a second part (2) among said at least two parts, then - Put in contact and under pressure (150) the assembly surfaces (1a, 2a) of the first part (1) and the second part (2) between them, the second layer (5) being directly in contact with the complementary layer (5a), and - Apply an annealing (160) until reaching, or even exceeding, a melting temperature Tf specific to said eutectic alloy (3), so as to form, from the first and second layers (4, 5) and from the complementary layer (5a), a solder (3ab) based on said eutectic alloy (3) between the first and second parts (1,2). 2. Method (100) according to the preceding claim, in which the oxidation step (125) results in the formation of a surface layer (4a) of aluminum oxide by oxidation of the first layer (4), the surface layer (4a) having for example a thickness between 1 and 10 nm. 3. Method (100) according to any one of the preceding claims, in which the steps of depositing (120, 130, 140) the first layer (4), the second layer (5) and the complementary layer (5a ) are set so that the ratio between: - a thickness of the first layer (4) and - a cumulative thickness of the second layer (5) and of the complementary layer (5a), respects an eutectic composition of the eutectic alloy (3), to within +/- 10%, preferably to within +/- 5% . 4. Method according to any one of the preceding claims, in which the temperature reached during annealing (160) does not exceed a predetermined threshold value above which any one of the first and second parts (1, 2) , or even among said at least two parts, would be functionally and negatively impacted. 5. Method according to any one of the preceding claims, in which, during the supplying step (110), at least one part (1, 2) among said at least two parts comprises at least one microelectronic device or element, for example a weld bead, and in which the temperature reached during annealing (160) does not exceed a predetermined threshold value above which said at least one microelectronic device or element would be functionally and negatively impacted. 6. Method according to any one of the two preceding claims, wherein said predetermined temperature threshold value is equal to 500 ° C, and preferably equal to 450 ° C. 7. Method (100) according to any one of the preceding claims, in which the step of depositing (130) the second layer (5) and the step of depositing (140) the complementary layer (5a) are configured. so that a cumulative thickness of the second layer (5) and of the complementary layer (5a) is greater than 10 nm. 8. Method (100) according to any one of the preceding claims, in which the oxidation step (125) results in the formation of a surface layer (4a) of aluminum oxide by oxidation of the first layer (4), and in which the step of depositing (130) the second layer (5) is configured so that a thickness of the second layer (5) is greater than a thickness of the surface layer (4a), preferably greater than twice the thickness of the surface layer (4a). 9. Method (100) according to any one of the preceding claims, in which at least one of the first and second parts (1, 2), or even among said at least two parts, is based on one at less among the following materials: silicon, silica, glass, sapphire, alumina, germanium, indium phosphide, gallium nitride, gallium sulfide, gallium arsenide, nitride of gallium and silicon carbide. 10. Method (100) according to any one of the preceding claims, further comprising at least one of the following steps: - Before the deposition step (120) of the first layer (4), form (115) a sublayer (6), preferably chemically inert, at least on the assembly surface (1a) of the first part (1) and - Before the deposition step (140) of the complementary layer (5a), form (116) a sublayer (6), preferably chemically inert, at least on the assembly surface (2a) of the second part (2). 11. Method (100) according to any one of the preceding claims, in which the step of depositing (120) the first layer (4) is configured so that the first layer (4) has a thickness of between 50 nm. and 10 pm, preferably between 500 nm and 5 pm. 12. Method (100) according to any one of the preceding claims, in which each of the first and second parts (1, 2), or even among the said at least two parts, comprises a wafer based on a semi-material conductor, each wafer having two opposite circular faces, for example of a diameter equal to 200 mm, and in which the assembly surfaces (1a, 2a) of the first and second parts (1, 2) are each formed by one of said circular faces, and the contacting and pressurizing step (150) of the assembly surfaces (1a, 2a) comprises at least one of: - applying at least one pressure level from one of the two parts (1,2) to the other in a direction perpendicular to their assembly surfaces (1a, 2a), said pressure level being between 0.05 Mpa and 3 Mpa, preferably between 0.1 Mpa and 0.5 Mpa, and - Applying at least one level of compression force from one of the two parts (1,2) to the other in a direction perpendicular to their assembly surfaces (1a, 2a), said level of force being between 2 kN and 90 kN, preferably between 2 kN and 90 kN. 13. Method (100) according to any one of the preceding claims, in which each of the two parts (1, 2) comprises a wafer based on a semiconductor material, and in which the assembly surfaces (1a, 2a) first and second parts (1, 2) are each formed by one of said circular faces, and the steps of bringing into contact and under pressure (150) and of applying an annealing (160) comprise: - For a first duration ti, preferably between 10 and 30 min, and for example equal to 20 min: o the application of a first pressure level P-ι, for example equal to 0.1 MPa, of one of the two parts (1, 2) on the other in a direction perpendicular to their assembly surfaces ( 1a, 2a) and o a rise in temperature from the ambient to a first threshold temperature Ti lower than the melting temperature Tf of said eutectic alloy, for example the first threshold temperature Ti being lower than the melting temperature Tf d ' a value at most equal to 10%, preferably at most equal to 5%, of the value of said melting temperature Tf, then - During a second duration t ₂ -ti, preferably between 5 min and 10 h, for example equal to 1 h: o the application of a second pressure level P ₂ higher than the first pressure level P-ι, for example a second pressure level P ₂ equal to 0.5 MPa, from one of the two parts (1, 2) on the other in a direction perpendicular to their assembly surfaces (1a, 2a) and o maintaining the first threshold temperature Ti, then - During a third duration t ₃ -t ₂ , preferably between 2 and 10 min, o maintenance of the second pressure level P ₂ , o a rise in temperature from the first threshold temperature Ti to a second threshold temperature T ₂ greater than or equal to the melting temperature Tf of said eutectic alloy, for example the second threshold temperature T ₂ being greater than the melting temperature Tf by a value at most equal to 10%, preferably at most equal to 5%, of the value of said melting temperature, then - For a fourth duration t ₄ -t ₃ , preferably between 1 min and 1 h, maintaining the second pressure level P ₂ and the temperature T ₂ , - During a fifth duration t ₅ -t ₄ , preferably between 10 and 30 min: o at least one of maintaining the second pressure level P ₂ and applying a pressure level lower than the pressure level P ₂ , if applicable applying the pressure level lower than the pressure level P ₂ following the maintenance of the second pressure level P ₂ , for example the lower pressure level being equal to the first pressure level Pi, and o cooling to ambient. 14. Method (100) according to any one of the preceding claims, in which the contacting and pressurizing steps (150) and the step of applying an annealing (160) are carried out under vacuum, at a pressure between 10 ' ³ and 10' ⁷ mbar, for example equal to 5.10 ' ⁵ mbar. 15. Method (100) according to any one of the preceding claims, in which the element (3b) suitable for forming, with aluminum (3a), said eutectic alloy (3) is chosen from: germanium, silicon, gold, copper.