FR3076072A1 - METHOD OF BONDING BY THERMOCOMPRESSION OF TWO SUBSTRATES BY AN ALUMINUM INTERFACE - Google Patents

Abstract

The invention relates to the field of thermocompression bonding processes of metal surfaces to one another and proposes a thermocompression bonding process of two substrates 1, 2 by forming an aluminum bonding interface 3. The process comprises the following steps: depositing an aluminum-based layer on each of the two substrates; depositing a germanium-based layer on each aluminum-based layer, the depositing steps succeeding one another without bringing the the aluminum-based layer with oxygen, - bringing the substrates into contact with one another by the previously deposited germanium-based layers, then - applying a thermocompression of the contacted substrates so as to make migrating the germanium of each germanium layer into the aluminum of the aluminum-based layers to form the aluminum bonding interface 3.

Description

TECHNICAL FIELD OF THE INVENTION The invention relates to the field of bonding processes by thermocompression of metal surfaces therebetween. It finds for particularly advantageous application the field of the development of electromechanical microsystems.

Bonding by thermocompression of two substrates between them consists of hot pressing and vacuum two metal surfaces, previously transferred to the substrates, in order to obtain adhesion between the two metal surfaces.

STATE OF THE ART

In the literature, there are numerous examples of bonding by thermocompression of metal surfaces together. Thus, we know how to obtain a copper / copper bonding interface (Cf. KN Chen, A. Fan, R. Reif, Journal of Electronic Materials, Vol. 30, No. 4 (2001)), or / or ( Cf. CH Tsau, M A. Schmidt, SM Spearing, Materials Research Society Symposium Proceeding Vol. 605 (2000)), or titanium / titanium (Cf. J. Yu, Y. Wang, RL Moore, J. Lu, RJ Gutmann , Journal of the Electrochemical Society, Vol. 154, Issue 1, p. H20-H25 (2007)) for example.

There are also thermocompression bonding methods consisting in forming an aluminum / aluminum bonding interface (Cf. V. Dragoi, G. Mittendorfer, J. Burggraf, M. Wimplinger, ECS Transactions, 33 (4) 27-35 (2010 )). However, the latter are very difficult to produce due to the formation of aluminum oxide (or alumina) on the surface of the substrates before bonding them together by thermocompression.

To obtain an AI / AI bonding interface despite the presence of alumina, the literature recommends carrying out thermocompression under more severe conditions than usual. It is thus recommended to press the substrates with more force between them, and typically with a pressure higher than 34 MPa, and to heat the substrates to a temperature higher than 450 ° C (Cf. N. Malik, K. Schjolberg , E. Poppe, MM Visser Taklo, TG Finstad, Sensors and Actuators A 211 (2014)). Such thermocompression conditions, and in particular such a thermocompression temperature, do not make it possible to ensure the preservation of the integrity of certain devices or elements of microelectronic devices which would be formed on one and / or the other substrates prior to bonding by thermocompression.

There are also, in the literature, methods for forming an AI / AI bonding interface by thermocompression at a low temperature, typically at room temperature. We can cite for example the collage SAB (for “Surface Activated Bonding) AI / AI proposed by Akatsu et al (Cf. T. Akatsu, N. Hosoda, T. Suga, and M. Rühle, Journal of Materials Science 34, 4133 (1999)). However, this bonding requires working under ultra high vacuum, which is quite complex and expensive. In addition, this bonding uses a bombardment of argon atoms which is not necessarily compatible either with the devices or microelectronic elements possibly present on the substrates to be bonded.

An object of the present invention is to provide a method of bonding by thermocompression of two substrates by forming a bonding interface based on alternative or improved aluminum, or which even makes it possible to at least partially overcome the drawbacks of the existing methods.

More particularly, an object of the present invention is to propose such a method of bonding by thermocompression which makes it possible to preserve the integrity of micro-electronic devices or elements formed on one and / or the other of the substrates prior to their bonding. by thermocompression.

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 bonding by thermocompression of two substrates by forming a bonding interface based on aluminum. The process comprises the following stages: - Provide two substrates, - Deposit an aluminum-based layer on one face of each of the two substrates, - Deposit a germanium-based layer at least on each previously deposited aluminum-based layer , the deposition steps succeeding each other without bringing the aluminum-based layers back into contact with oxygen, - Put the substrates in contact with each other by the germanium-based layers previously deposited, then - Apply thermocompression of the substrates brought into contact so as to migrate the germanium from each germanium-based layer into the aluminum of the aluminum-based layers to form the aluminum-based bonding interface. The invention advantageously takes advantage of a phenomenon, already observed in another context and with a view to another application (cf. K. Nakazawa, K. Toko, N. Usami, T. Suemasu, Japanese Journal of Applied Physics 53, 04EH01 (2014)), migration of germanium in aluminum to obtain the bonding interface AI / AI. Germanium-based layers isolate aluminum-based layers from significant contact with oxygen before thermocompression is applied. Thus, the formation of alumina on the surface of aluminum-based layers is avoided. It is therefore no longer necessary to make the conditions, particularly in temperature, under which thermocompression must be made more severe. In particular, the applicable conditions leading to the formation of the aluminum-based bonding interface according to the method of the invention are such that they make it possible to preserve the integrity of any devices or elements of microelectronic devices formed on substrates prior to bonding by thermocompression. In addition, note that the bonding interface based on aluminum thus obtained can make it possible to ensure electrical conduction between the two substrates through the interface.

More particularly, thermocompression can be carried out at a temperature strictly below the temperature of the eutectic of aluminum and germanium (424 ° C), and preferably at a temperature below 420 ° C. This temperature condition is preferably maintained at least until the bonding interface based on aluminum is obtained. Following the formation of the bonding interface based on aluminum, the assembly can under certain conditions be brought to a temperature higher than the temperature of the eutectic, without affecting the function of the bonding interface based on d aluminum, but preferably this temperature remains below 450 ° C.

In order to significantly observe the migration phenomenon exploited by the process according to the invention, it is preferable that the thermocompression is also carried out at a temperature above 100 ° C. This temperature threshold makes it possible to guarantee that the step of applying thermocompression lasts a limited or even reasonable time for industrial applications.

Another object of the present invention relates to a microelectronic device comprising a bonding interface based on aluminum obtained by implementing the method according to the first aspect of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The objects, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment of the latter which is illustrated by the following accompanying drawings in which: - Figure 1 schematically shows the two substrates, before bringing them into contact, according to one embodiment of the invention; - Figure 2 schematically shows the two substrates brought into contact according to an embodiment of the invention; - Figure 3 shows schematically the two substrates linked by the aluminum-based bonding interface obtained following the thermocompression of the two substrates, previously brought into contact, according to one embodiment of the invention; - Figure 4 shows graphs of the temperature and pressure over time during the thermocompression step of the process according to an embodiment of the invention; and - Figure 5 shows a flow chart of process steps according to an embodiment of the invention.

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 on the scale of practical applications. In particular, the relative thicknesses of the different layers are not representative of reality.

In FIG. 5, the process steps framed by broken lines may only be optional.

DETAILED DESCRIPTION OF THE INVENTION

Before starting a detailed review of embodiments of the invention, the following are optional features of the first aspect of the invention which can optionally be used in combination or alternatively: - the thermocompression is carried out at a strictly temperature below the temperature of the eutectic of aluminum and germanium (424 ° C), and preferably at a temperature below 420 ° C, at least until the bonding interface based on aluminum is obtained ; and then the temperature can be increased by preferably remaining below 450 ° C; - thermocompression is carried out at a temperature above 100 ° C; each germanium-based layer is deposited on an aluminum-based layer so as to have a thickness of at least one order of magnitude less than that of the aluminum-based layer; each germanium-based layer is deposited so as to have a thickness of between 10 nm and 1 μm, preferably between 10 nm and 20 nm; each layer based on aluminum is deposited so as to have a thickness of between 100 nm and 10 μm, preferably of between 100 nm and 500 nm; the steps of depositing each germanium-based layer on an aluminum-based layer and of depositing the aluminum-based layer include depositing, under a controlled atmosphere at least in terms of pressure and in terms of rate d oxygen, of an Al / Ge bilayer by successive evaporations or sprays of at least part of an aluminum-based target and a germanium-based target. Preferably, the steps of depositing each layer based on germanium on a layer based on aluminum and depositing the layer based on aluminum are carried out in the same hermetic enclosure. - At least one step of depositing an aluminum-based layer is preceded by a step of forming a chemically inert layer with aluminum at least on the face of the substrate on which the aluminum-based layer is intended to be filed. The chemically inert layer is for example based on a material chosen from: silicon oxide and titanium nitride; each of the two substrates having a diameter equal to 200 mm, the thermocompression comprises at least one of: o the application of at least one pressure level of one of the two substrates to the other in a direction perpendicular to their faces brought into contact, the pressure 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 of one of the two substrates on the other in a direction perpendicular to their faces brought into contact, the force standard being between 2 kN and 90 kN; the thermocompression can comprise: o For a first duration t-ι, preferably between 10 and 30 min, and for example equal to 20 min: the application of a first pressure level Pi, for example equal to 0, 1 MPa, from one of the two substrates to the other in a direction perpendicular to their faces brought into contact and a rise in temperature from the ambient to a first threshold temperature Ti lower than the temperature of the eutectic of l aluminum and germanium, for example a first threshold temperature Ti equal to 415 ° C, then o For a second duration t2-ti, preferably between 5 min and 10 h, for example equal to 1 h: the application d a second pressure level P2 greater than the first pressure level Pi, for example a second pressure level P2 equal to 0.5 MPa, of one of the two substrates on the other in a direction perpendicular to their faces brought into contact and a maintie n of the first threshold temperature T-ι, then o For a third duration t5-t2, preferably between 10 and 30 min:

At least one of maintaining the second pressure level P2 and applying a pressure level lower than the pressure level P2, if applicable applying a pressure level lower than the pressure level P2 succeeding maintaining the second pressure level P2, for example the pressure level lower than the pressure level P2 being equal to the first pressure level Pi, and cooling to the ambient Ta.

In addition, during the third period t5-t2 and before cooling to the ambient Ta, the thermocompression can comprise an increase in the temperature above the temperature of the eutectic of aluminum and germanium, by example up to a temperature below 450 ° C, maintaining the second pressure level P2.

In addition or as an alternative, during the third period ts-t2, the cooling to ambient Ta can include, by maintaining the pressure P2, a first cooling sub-step with a ramp equal to 1 ° C. per minute from one from the first threshold temperature Ti and a temperature above the temperature of the eutectic of aluminum and germanium up to a respective one from a temperature below the first threshold temperature Ti, for example an equal temperature at 400 ° C, and a temperature substantially equal to the first threshold temperature Ti; then, by applying a pressure lower than the second pressure level, for example a pressure equal to the first pressure level P-i, a second sub-step of cooling down to ambient temperature Ta with a ramp equal to 20 ° C per minute; the thermocompression is configured so that, once the aluminum-based interface has been formed, the germanium has migrated into the aluminum so as to form, in and / or at the periphery of the aluminum-based interface, a spatial distribution of inclusions whose average characteristic size is preferably between 10 nm and 1 μm, and even more preferably between 10 nm and 20 nm; - at least one of the substrates is supplied so as to comprise at least one microelectronic device or element, for example a weld bead; - at least one of the substrates is based on silicon and forms a plate with a diameter equal to 200 mm; - The method can further comprise, before bringing the substrates into contact with each other, a step of rinsing with deionized water, then a drying step, these steps being configured so as to eliminate at least one part of native germanium oxide. The contacting of the substrates with each other is then preferably carried out so as to avoid any prolonged contact of the substrates with oxygen; and - the thermocompression is carried out under high vacuum, at a pressure between 10'3 and 10'7 mbar, for example equal to 5.10'5 mbar.

A film based on a material A is understood to mean a film comprising this material A and possibly other materials.

By “bilayer” is meant a multilayer comprising only two different monolayers, for example by their (s) material (s), superimposed.

The term “chemically inert layer with aluminum” means a layer based on a material defined in that it does not chemically react with aluminum at least under conditions of deposition of aluminum on said chemically inert layer, or even under conditions of use of a microelectronic device comprising the material of the chemically inert layer and aluminum.

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 purely electronic devices, micromechanical or electromechanical devices (MEMS, NEMS ...) as well as optical or optoelectronic devices (MOEMS ...).

In the present patent application, the thickness is taken in a direction perpendicular to the main faces of the substrate on which the different layers rest. In Figures 1 to 3, 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 -20% to + 20% of the given value, preferably from -10% to +10% of the given value. The invention aims in particular to achieve an AI / AI bonding between two substrates at a temperature strictly below 424 ° C. at least as long as the AI / AI interface is not formed, since it is possible then to increase the temperature , without however exceeding 450 ° C. By not exceeding this temperature, the devices or elements of microelectronic devices that would possibly include at least one of the two substrates would be preserved in their integrity.

A method of thermocompression bonding of two substrates by forming an aluminum-based bonding interface generally comprises the following steps: - providing two substrates, - depositing an aluminum-based layer on one face of each of the two substrates , - Put the substrates in contact with each other by the aluminum layers previously deposited, and - Apply thermocompression between the two substrates thus brought into contact to form an AI / AI bonding interface between the two substrates.

The steps of depositing aluminum-based layers and thermocompression are generally not carried out in the same hermetic enclosure, which results in the contacting of aluminum of aluminum-based layers deposited with oxygen. of the atmosphere, and therefore by surface oxidation of the aluminum-based layers, before thermocompression. As seen in the introduction, the oxidation of aluminum, even limited to the surface of the aluminum-based layers, does not promote the formation of the AI / AI bonding interface between the two substrates.

With reference to the figures, the bonding method 100 according to an embodiment of the present invention differs in particular from the bonding method as stated above in that it comprises, following the deposition 120 of each layer based on aluminum 4a, 4b, the deposition 130 of a germanium-based layer 5a, 5b at least on the aluminum-based layer 4a, 4b. The steps of depositing 120, 130 of each germanium layer 5a, 5b and of the underlying aluminum layer 4a, 4b are carried out so as to succeed one another without bringing the aluminum-based layers 4a, 4b back into contact with oxygen. To this end, the 120,130 deposition steps include, for example, the deposition, under a controlled atmosphere at least in terms of pressure and in terms of oxygen rate, of an Al / Ge bilayer by successive evaporations or sputterings of a portion at least one target based on aluminum and one target based on germanium. Preferably, the deposition steps 120, 130 are carried out in the same hermetic enclosure. It is however conceivable that the deposition 130 of a germanium-based layer can be carried out in another enclosure than the deposition 120 of the underlying aluminum-based layer, provided that the transfer from an enclosure to the other can be achieved without contacting aluminum with oxygen. Furthermore, numerous deposition techniques, whether aluminum layers 4a, 4b or germanium layers 5a, 5b, can be envisaged, including: physical vapor deposition techniques (PVD for “Physical Vapor Deposition”) ) such as, for example, electron beam evaporation or sputtering in particular, chemical vapor deposition techniques (CVD for “Chemical Vapor Deposition”) such as, for example, CVD at sub-pressure atmospheric (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 invention thus proposes to protect the aluminum-based layer 4a, 4b deposited on one face 1b, 2b of each of the two substrates 1, 2 by covering this layer with a layer of germa base nium 5a, 5b. The successive deposits of each aluminum-based layer 4a, 4b and of the germanium-based layer 5a, 5b intended to cover each aluminum-based layer 4a, 4b are carried out without contacting the aluminum-based layer 4a, 4b with oxygen. Thus, the aluminum of each aluminum-based layer 4a, 4b is protected by the germanium layer 5a, 5b which covers it with any significant oxidation between the time of its deposition 120 at least until contacting 140 substrates 1, 2 and thermocompression 150 of the substrates 1,2 together to form the bonding interface 3 based on aluminum. Each germanium-based layer 5a, 5b preferably completely covers the underlying aluminum-based layer, including its side edges. However, the method according to the present invention is not limited to such total recovery. In particular, at least part of the lateral edges of each aluminum-based layer 4a, 4b, or even part of the exposed face of each aluminum-based layer, may not be covered, and therefore not be protected from oxidation, if the latter is still sufficiently limited so as not to oppose the formation of the bonding layer 3 based on aluminum with a temperature applied during thermocompression 150 strictly below 424 ° C.

It has indeed been observed, surprisingly, that the presence of a germanium-based layer 5a, 5b on each aluminum-based layer 4a, 4b does not prevent the formation of a bonding interface 3 based on aluminum when the substrates 1, 2 are brought into contact 140 with each other by the germanium-based layers 5a, 5b, before a thermocompression 150 is applied to the substrates contacted. characterized in particular that it does not require a temperature rise exceeding 424 ° C.

If this result is surprising, he finds a scientific explanation in the fact that such a thermocompression 150 allows the germanium atoms to migrate between the aluminum atoms, in other words to diffuse through the atomic network of aluminum (and more particularly between the aluminum grains at the level of the so-called grain boundaries), in order to obtain a connection, in particular electrical, between the aluminum-based layers 4a, 4b initially isolated, in particular electrically, by the layers to germanium base 5a, 5b. Ultimately, these aluminum-based layers 4a, 4b no longer form a single layer which is none other than the bonding interface 3 based on aluminum. The thickness of the bonding interface 3 is substantially equal to the sum of the thicknesses of the aluminum-based layers 4a, 4b as deposited 120. With reference to FIG. 3, the germanium-based layers 5a, 5b are transformed, due to the migration of germanium into aluminum caused by thermocompression 150, into inclusions 7 of germanium within the bonding interface 3. The formation of a spatial distribution of such inclusions 7 occurs in a relatively short time short, typically in a time that can vary between 5 min and 10 h, once the thermocompression 150 step has started. Assuming that thermocompression is maintained for an almost infinite time, it is possible that all of the germanium in the germanium-based layers 5a, 5b migrates into the aluminum until it is trapped at the initial interface between each aluminum-based layer 4a, 4b and the face 1b, 2b of the substrate 1, 2 on which the aluminum-based layer 4a, 4b has been deposited. It can be seen that, including in this hypothesis, the method according to the present invention results in the formation of the bonding interface 3 based on aluminum. It is however advantageous that the thermocompression is not maintained for an almost infinite time, obviously from the point of view of the industrial applicability of the method according to the present invention, but also with a view to the implementation of the method according to the present invention for obtaining micro-electronic devices comprising a bonding interface 3 based on aluminum, in particular when the bonding interface is intended to play, in the micro-electronic devices obtained, a role of electrical conductor between the two substrates 1, 2, or between elements of microelectronic devices included by at least one of the two substrates. It is understood that under these conditions, the thermocompression 150 can be carried out at a temperature strictly lower than the temperature of the eutectic of aluminum and germanium (which is 424 ° C.), at least until obtaining the bonding interface 3 based on aluminum. It is indeed appropriate to take into account this eutectic temperature above which a liquid alloy of aluminum and germanium, noted below AIGe alloy, is formed. It should be noted that the formation of such an AIGe alloy is not entirely excluded by the method according to the present invention provided that it accompanies, without replacing it, the formation of the bonding interface 3 based on aluminum. Thus it is understood that, once the bonding interface 3 based on aluminum and the inclusions 7 of germanium formed, the treatment temperature can be increased, including beyond the temperature of the eutectic of l aluminum and germanium, without necessarily harming the bonding interface 3 and its functions. Such a rise in temperature can effectively lead to the formation of liquid inclusions (in fact nanodroplets) of AIGe alloy in or at the periphery of a bonding interface 3 which is still functional. If such a rise in temperature is not excluded, it is also not necessary, and therefore remains optional. These last considerations seem transparent to the quasi-semantic choices which consist in considering that the thermocompression 150 ends at the obtaining of the bonding interface 3 or, on the contrary, that the thermocompression 150 can be prolonged, in particular at temperatures above 424 ° C, temperature of the eutectic of aluminum and germanium, beyond obtaining the bonding interface 3.

In order to favor the formation of a spatial distribution of germanium inclusions 7 in the bonding interface 3 (in particular with respect to the formation of germanium-based layers at the initial interface between each aluminum-based layer 4a , 4b and the face 1b, 2b of the substrate 1,2 on which the aluminum-based layer 4a, 4b has been deposited), it is intended that each germanium-based layer 5a, 5b is deposited 130 on a layer based on aluminum 4a, 4b so as to have a thickness of at least one order of magnitude less than that of the layer based on aluminum 4a, 4b underlying. In particular, each germanium-based layer 5a, 5b can be deposited 130 so as to have a thickness of between 10 nm and 1 μm, preferably between 10 nm and 20 nm. Correlatively, each aluminum-based layer 4a, 4b can be deposited 120 so as to have a thickness of between 100 nm and 10 μm, preferably between 100 nm and 500 nm.

In addition, in order to ensure sufficient migration dynamics of germanium in the aluminum-based layers 4a, 4b, it is preferable that the thermocompression 150 is carried out at a temperature above 100 ° C. This ensures that the bonding interface 3 based on aluminum is obtained in a reasonable time, especially from an industrial point of view.

Thus, the temperature at which the thermocompression 150 is carried out is preferably between 100 ° C. and 424 ° C., even more preferably between 300 ° C. and 420 ° C.

According to a particular feature of the process as illustrated in FIGS. 1 to 3 and 5, each layer based on aluminum 4a, 4b can be deposited 120 on a chemically inert layer 6 with aluminum, such as for example a layer based on silicon oxide, based on titanium nitride, based on tin-indium oxide (ITO) or based on zinc-doped aluminum oxide (AZO). In this event, the process as illustrated in FIG. 5 comprises, before the step of depositing 120 of the aluminum, a step of forming 115 of the chemically inert layer 6 with the aluminum at least on the face 1b, 2b of the substrate 1, 2 on which the aluminum-based layer 4a, 4b is intended to be deposited 120. When at least one of the substrates 1, 2 is based on silicon, the chemically inert layer 6 can be produced , as a layer based on silicon oxide, by surface oxidation of the substrate, or as a layer based on titanium nitride for example by physical vapor deposition. As we will see in the example given below, at least one of the substrates 1, 2 forms a plate with a diameter equal to 200 mm.

Still with reference to FIG. 5, the method according to the illustrated embodiment may further comprise, before bringing the substrates 1,2 into contact with one another, a step of rinsing with deionized water, then a drying step. These steps are configured to remove at least part of native germanium oxide. Such a native oxide is in fact capable of forming between the deposit 130 of each layer of germanium 5a, 5b and the contacting 140 of the substrates 1, 2 with each other. This contacting 140 is 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.

In particular when each of the two substrates comprises elements of micro-electronic devices, it may be necessary to take care of the alignment of the substrates with one another, when they come into contact 140, so as to correctly place the elements of micro-electronic devices one to another. In particular in this context, the contacting 140 of the two substrates with one another can be carried out in the enclosure or the thermocompression 150 and intended to be produced, by a machine for handling at least one of the substrates 1, 2 equipped with example of a laser tracking system to control the alignment of substrates with each other. More generally, the contacting 140 of the two substrates 1, 2 can be carried out by an operator.

When each of the two substrates 1, 2 has a diameter equal to 200 mm, the thermocompression 150 may comprise at least one of: - the application of at least one pressure level of one of the two substrates 1,2 to the other in a direction perpendicular to their faces 1b, 2b brought into contact 140, 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 compression force from one of the two substrates 1,2 to the other in a direction perpendicular to their faces 1b, 2b brought into contact 140, the force standard being between 2 kN and 90 kN.

When each of the two substrates has relatively flat germanium-based layers 5a and 5b, 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 substrates has germanium-based layers 5a and 5b having a greater surface roughness, for example due to the presence of elements of microelectronic devices, such as welding, the pressure exerted by a given compression force from one substrate to the other is not homogeneously distributed at the contact interface between the substrates 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.

As its name suggests, thermocompression 150 therefore comprises the application of a pressure from one of the substrates 1, 2 to the other and the application of a determined temperature. More particularly, the pressure and the temperature applied can vary during thermocompression 150 within the ranges of values defined above, or even beyond these ranges of values once the bonding interface 3 is obtained.

The thermocompression 150 is also carried out under high vacuum, at a pressure between 10'3 and 10'7 mbar, for example equal to 5.10'5 mbar.

An exemplary embodiment of the thermocompression 150 in terms of evolution of pressure and temperature applied is illustrated in FIG. 4. According to this example, the thermocompression 150 comprises: - 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 of one of the two substrates 1, 2 on the other in a direction perpendicular to their faces 1b, 2b brought into contact 140 and o a rise in temperature from the ambient to a first threshold temperature T1, for example equal to 415 ° C., - For a second duration t2-ti, preferably between 5 min and 10 h, for example equal to 1 h : o the application of a second pressure level P2 from one of the two substrates 1,2 to the other, always in a direction perpendicular to their faces 1b, 2b brought into contact 140, the second pressure level P2 being preferably better lower than the first pressure level Pi, and o maintaining the first threshold temperature T-ι, then - Optionally for a third duration t3-t2, preferably between 10 and 30 min: o maintaining the second pressure level P2 and o an increase from the first threshold temperature Ti to a temperature T2, for example equal to 450 ° C. - for a fourth duration U-U, preferably between 10 and 30 min: o maintaining the second pressure level P2 and o cooling from the last temperature T2 to temperature T1 with a ramp of 1 ° C per minute. - for a fifth duration ts-t4, preferably between 10 and 30 min: o the application of the first pressure level Pi and o cooling to room temperature with a ramp of 20 ° C per minute.

More particularly, when each of the two substrates has a diameter equal to 200 mm, the first pressure level Pi is equal to 0.1 MPa and the second pressure level P2 is equal to 0.5 MPa to 3 MPa. The diameter of the substrates 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 substrates on the other is released and the assembly formed by the two substrates 1, 2 bonded together by the bonding interface 3 based aluminum can be recovered to possibly undergo other treatments and potentially ultimately lead to the manufacture of a microelectronic device.

EXAMPLES OF REALIZATION

According to a first embodiment, a sealing by AI / AI thermocompression was carried out using silicon plates 1, 2 of 200 mm in diameter covered with 500 nm of SiO 2 obtained by thermal oxidation of silicon. Then, an Al (1000 nm) / Ge (10 nm) bilayer is deposited by evaporation on each of the plates without bringing the aluminum layer back into contact with oxygen.

Just before applying thermocompression 150, the plates are preferably rinsed with deionized water, then dried to remove the native germanium oxide.

The two substrates 1, 2 are then quickly placed in contact 140 under vacuum at 5.10'5 mbar and at room temperature, then a very low pressure is applied to the exposed faces 1a, 2a of the substrates, typically a pressure equal to 0.1 MPa for 200 mm diameter substrates. Then the temperature rises to 415 ° C with a ramp of 20 ° C / min. Once this temperature is reached, the pressure on the exposed faces 1a, 2a of the substrates is increased to 0.5 MPa or 3 MPa for substrates with a diameter of 200 mm. The pressure and the temperature are maintained for 1 hour. This is the 150 thermocompression phase.

If the thermocompression 150 is carried out on substrates comprising elements of micro-electronic devices inducing differences in surface level of the germanium-based layers 5a, 5b, such as weld beads, the compression force applied on the rear face may be the same, but the pressure obtained at the bonding interface will be different and advantageously much higher.

During this annealing phase, the germanium migrates into, or even under, aluminum and an AI / AI contact is obtained between the two substrates, without the presence of alumina thanks to the oxidation protection constituted by the layers based on germanium 5a, 5b for aluminum-based layers 4a, 4b. In this sense, contacting 140 under vacuum before thermocompression 150 also plays an advantageous role.

According to a second exemplary embodiment, a sealing by AI / AI thermocompression was carried out using 1, 2 silicon plates 200 mm in diameter covered with 50 nm of TiN deposited by physical vapor deposition (PVD). Then, an Al (1000 nm) / Ge (10 nm) bilayer is deposited by evaporation on each of the plates without bringing the aluminum layer back into contact with oxygen.

Just before applying thermocompression 150, the plates are rinsed with deionized water and then dried to remove the native germanium oxide.

The two plates 1, 2 are then brought into contact 140 in a vacuum sealing equipment, at 5.10'5 mbar. A pressure of 0.1 MPa is then applied to the exposed faces 1a, 2a of the plates. The temperature is raised to 415 ° C with a ramp of 20 ° C / min. The pressure on the exposed faces 1a, 2a of the plates is high up to 0.5 MPa or 3 MPa for 30 min. The pressure is then kept constant while increasing the temperature up to 450 ° C for 10 min until the end of the thermocompression 150. The pressure is maintained and the cooling takes place up to 400 ° C with a ramp of 1 ° C / min. Finally, the pressure has dropped to 0.1 MPa and the plates 1, 2 are cooled to room temperature 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 bonding method 100 comprises the supply 110 of three substrates, or even more, intended to be brought into contact in the same stack. The substrates intended to have an intermediate position between the two substrates situated at the ends of the stack can be treated (or “processed”) so that an aluminum-based layer is deposited on each of their faces before each layer based on aluminum is not covered with a layer based on germanium. Such an embodiment therefore results in the formation of a plurality of bonding interfaces 3 based on aluminum, each of these bonding interfaces 3 being located between two first substrates neighboring them in the stack.

Claims (14)

1. A method of bonding (100) by thermocompression of two substrates (1, 2) by forming a bonding interface (3) based on aluminum, the method comprising the following steps: - Providing (110) two substrates ( 1,2), - Deposit (120) a layer based on aluminum (4a, 4b) on one face (1b, 2b) of each of the two substrates, - Deposit (130) a layer based on germanium (5a, 5b) at least on each previously deposited aluminum-based layer (4a, 4b) (120), the deposition steps (120, 130) succeeding each other without bringing the aluminum-based layers (4a, 4b) back into contact with oxygen, - Put in contact (140) the substrates (1, 2) with each other by the germanium-based layers (5a, 5b) previously deposited (130), then - Apply thermocompression (150) substrates (1, 2) brought into contact (140) so as to migrate the germanium from each germanium-based layer (5a, 5b) into the aluminum from the aluminum-based layers (4a, 4b) to form the bonding interface (3) based on aluminum. 2. Method (100) according to the preceding claim, wherein the thermocompression (150) is carried out at a temperature strictly lower than the temperature of the eutectic of aluminum and germanium, and preferably at a temperature lower than 420 ° C, at least until the bonding interface (3) based on aluminum is obtained. 3. Method (100) according to any one of the preceding claims, in which the thermocompression (150) is carried out at a temperature above 100 ° C. 4. Method (100) according to any one of the preceding claims, in which each germanium-based layer (5a, 5b) is deposited (130) on an aluminum-based layer (4a, 4b) so as to present a thickness of at least one order of magnitude less than that of the aluminum-based layer (4a, 4b). 5. Method (100) according to any one of the preceding claims, in which each germanium-based layer (5a, 5b) is deposited (130) so as to have a thickness of between 10 nm and 1 μm, preferably included between 10 nm and 20 nm. 6. Method (100) according to any one of the preceding claims, in which each aluminum-based layer (4a, 4b) is deposited (120) so as to have a thickness of between 100 nm and 10 pm, preferably between 100 nm and 500 nm. 7. Method (100) according to any one of the preceding claims, in which the steps of depositing (120) each germanium-based layer (5a, 5b) on an aluminum-based layer (4a, 4b) and deposition (130) of the aluminum-based layer (4a, 4b) comprise the deposition, under a controlled atmosphere at least in terms of pressure and in terms of oxygen rate, of a Al / Ge bilayer by evaporation or successive sprays of at least part of an aluminum-based target and a germanium-based target, preferably the steps of depositing (120) each germanium-based layer (5a, 5b) on a aluminum-based layer (4a, 4b) and deposition (130) of the aluminum-based layer (4a, 4b) being produced in the same hermetic enclosure. 8. Method (100) according to any one of the preceding claims, in which at least one step of depositing (120) an aluminum-based layer is preceded by a step of forming (115) a chemically inert layer. (6) with aluminum at least on the face (1b, 2b) of the substrate (1,2) on which the aluminum-based layer (4a, 4b) is intended to be deposited (120), the inert layer chemically (6) being for example based on a material chosen from: silicon oxide and titanium nitride. 9. Method (100) according to any one of the preceding claims, in which, each of the two substrates (1, 2) having a diameter equal to 200 mm, the thermocompression (150) comprises at least one of: application of at least one pressure level from one of the two substrates (1,2) to the other in a direction perpendicular to their faces (1b, 2b) brought into contact (140), 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 compression force of one of the two substrates (1,2) on the other in a direction perpendicular to their faces (1b, 2b) brought into contact (140), the force standard being between 2 kN and 90 kN. 10. Method (100) according to any one of the preceding claims, in which the thermocompression (150) comprises: - For a first duration t-ι, 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 substrates (1, 2) on the other in a direction perpendicular to their faces (1b, 2b) placed in contact (140) and o a rise in temperature from the ambient to a first threshold temperature Ti lower than the temperature of the eutectic of aluminum and germanium, for example a first threshold temperature Ti equal to 415 ° C, then - During a second duration t2-ti, preferably between 5 min and 10 h, for example equal to 1 h: o the application of a second pressure level P2 higher than the first pressure level Pi, by example a second pressure level P2 equal to 0.5 MPa, of one of the two substrates (1, 2) on the other in a direction perpendicular to their faces (1b, 2b) brought into contact (140) and o maintaining the first threshold temperature Ti, then - For a third duration t5- t2, preferably between 10 and 30 min: o at least one of maintaining the second pressure level P2 and applying a pressure level lower than the pressure level P2, if applicable applying d a pressure level lower than the pressure level P2 succeeding in maintaining the second pressure level P2, for example the pressure level lower than the pressure level P2 being equal to the first pressure level Pi, and o cooling down to l ambient. 11. Method (100) according to any one of the preceding claims, in which the thermocompression (150) is configured so that, once the aluminum-based interface (3) has been formed, the germanium has migrated into the aluminum so as to form, in and / or on the periphery of the aluminum-based interface (3), a spatial distribution of inclusions (7) whose average characteristic size is preferably between 10 nm and 1 μm. 12. Method (100) according to any one of the preceding claims, in which at least one of the substrates (1, 2) is provided (110) so as to comprise at least one microelectronic device or element, for example a cord Welding. 13. Method (100) according to any one of the preceding claims, further comprising, before bringing the substrates (1, 2) into contact with each other (140), a step of rinsing with water deionized and then a drying step, these steps being configured so as to eliminate at least a portion of native germanium oxide, the contact (140) of the substrates (1,2) with each other then being carried out preferably so as to avoid prolonged contact of the substrates (1,2) with oxygen. 14. Method (100) according to any one of the preceding claims, in which the thermocompression (150) is carried out under high vacuum, at a pressure between 10'3 and 10'7 mbar, for example equal to 5.10'5 mbar .