Method of joining two semiconductor substrates

Abstract

The invention relates to a method for joining two semiconductor substrates by molecular adhesion, said method comprising: a step a) of bringing a first substrate and a second substrate (2) into close contact so as to form an assembly having a bonding interface (4); a step b) of reactive annealing of the bonding interface (4) at a first temperature higher than a first predetermined temperature, said step b) generating bubbles at the bonding interface; a step c) of at least partial delamination of the two substrates at the bonding interface (4) so as to eliminate said bubbles; and a step d) of again bringing the first substrate and the second substrate (2) into close contact at the bonding interface (4) so as to reform the assembly.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of semiconductor materials for microelectronic components. In particular, the present invention relates to a method for bonding two semiconductor substrates by molecular adhesion. BACKGROUND

[0002] Bonding substrates by molecular adhesion ("direct wafer bonding") is a well-known technique which is applied in the fields of microelectronics, optoelectronics, and microelectromechanical systems, for example, for the manufacture of silicon-on-insulator substrates, multi-junction photovoltaic cells, and the production of 3D structures.

[0003] According to this technique, two substrates are brought into intimate contact so that their surfaces are close enough to each other to form atomic and / or molecular bonds between them. In this way, an adhesion is created between the two contact surfaces without the use of an intermediate adhesive layer, such as a polymer or adhesive layer.

[0004] The resulting assembly is then subjected to a heat treatment, generally at a temperature which can vary between 50°C and 1200°C, depending on the nature of the substrates and the envisaged application, in order to strengthen the adhesion.

[0005] In some cases, bonding by molecular adhesion results in the appearance of defects at the interface of the bond, known as bonding defects. These can be "bubble"-type defects ("bonding voids"). Bonding defects can result from the capture and accumulation of gaseous substances between the surfaces of the substrates being bonded. These substances can correspond to substances which were adsorbed on the surfaces of the substrates during their preparation before bonding. They can in particular correspond to residues from chemical reactions, in particular from water, which occurred on bringing the substrates into intimate contact or during the annealing for strengthening the bond.

[0006] The presence of bonding defects at the interface of the bond is very detrimental to the quality of the structure produced. For example, when the bonding step is followed by a step of thinning one of the two substrates to form a layer, by milling or using the Smart Cut TM technology, the absence of adhesion at the location of the bonding defects between the two surfaces can result in a local tearing of the thin layer at this location.

[0007] In addition, when a composite structure, for example comprising a thin layer of single-crystal silicon carbide (SiC) bonded to a carrier substrate made of SiC, is intended for the production of vertical power devices, good thermal and electrical conduction is required between the thin layer and the carrier substrate.

[0008] In order to bond the substrates which will produce the composite structure, there are two main methods for performing the direct bond: the hydrophilic method and the hydrophobic method.

[0009] In the hydrophilic method, the surfaces of the two substrates are treated so as to make them hydrophilic, in particular by generating a native oxide layer. A water layer is present between the two substrates so as to promote the formation of atomic and / or molecular bonds responsible for the adhesion between the two substrates. However, the native oxide layer present at the bonding interface affects and deteriorates the electrical conduction between the two substrates.

[0010] In the hydrophobic method, the surfaces of the two substrates are treated so as to make them hydrophobic: the native oxide layer is removed and the presence of water between the substrates is limited. In order to further limit the presence of water, the joining between the two substrates can be performed under a controlled atmosphere, such as a water-free atmosphere or a vacuum. The two substrates joined together using this method will exhibit good vertical electrical and thermal conduction (Yushin et al., Applied Physics Letters, 84(20), 3993-3995, 2004). However, obtaining these conditions in an industrial environment can be complex. Moreover, the Applicant has observed that when two SiC substrates joined using this method are subjected to temperatures higher than 700°C, pressurized bubbles can form at the interface between the two substrates and adversely affect the quality of the bond. This situation is particularly problematic when the assembly formed by joining the two substrates must be subjected to a heat treatment at a temperature exceeding the temperature at which these bubbles appear, for example in order to perform a layer transfer using the Smart Cut® method. TM The present invention aims to overcome all or some of the aforementioned drawbacks. The present invention relates to a method for joining two semiconductor substrates by molecular adhesion, which allows good electrical and thermal conduction of the joining interface and which reduces the number of bonding defects or even completely prevents the appearance of such bonding defects.

[0011] There is also another method based on surface activation ("surface active bonding", or SAB), described for example by F. Mu et al. (4th IEEE International Workshop on Low Temperature Bonding for 3D Integration (LTB-3D), 15-16 July 2014). The surfaces of the two substrates to be joined are subjected to atomic bombardment so as to activate their surfaces before joining them together at low temperature. This method makes it possible to obtain very good adhesion between the two substrates, but an amorphous layer is formed at the interface between the two substrates. The presence of this amorphous layer generally deteriorates the conduction between the two substrates.

[0012] Subject of the invention

[0013] The present invention aims to overcome all or some of the aforementioned drawbacks. The present invention relates to a method for joining two semiconductor substrates by molecular adhesion, which allows good electrical and thermal conduction of the joining interface and which reduces the number of bonding defects or even completely prevents the appearance of such bonding defects. SUMMARY

[0014] To achieve this aim, the subject of the application provides a method for joining two semiconductor substrates by molecular adhesion, said method comprising:

[0015] - a step a) of bringing into close contact a first substrate and a second substrate so as to form an assembly presenting a bonding interface;

[0016] - a step b) of reactive annealing of the bonding interface at a first temperature higher than a predetermined first temperature, this step b) generating bubbles at the bonding interface;

[0017] The method is characterized in that it comprises:

[0018] - a step c) of at least partial décollement of the two substrates at the bonding interface so as to eliminate the bubbles;

[0019] - a step d) of bringing into close contact again the first substrate (1) and the second substrate at the bonding interface so as to reform the assembly.

[0020] According to other advantageous and non-limiting features of the application, taken alone or in any technically feasible combination:

[0021] - the décollement step c) is performed under a controlled atmosphere;

[0022] - the controlled atmosphere of the décollement step c) is an anhydrous atmosphere or a vacuum;

[0023] - the décollement step c) is performed entirely or partially at a second temperature higher than or equal to ambient temperature;

[0024] - the second temperature is lower than 700°C, preferably lower than 200°C, even more preferably lower than 100°C;

[0025] - the décollement step c) comprises a mechanical separation of the two substrates by insertion of a blade at the bonding interface between the two substrates;

[0026] - the joining method comprises, after step d), a step e) of thinning of the first substrate so as to form a thin layer;

[0027] - the first substrate comprises a main face and a buried weakening plane, the thin layer being defined between the main face and the buried weakening plane;

[0028] - step e) comprises a cleaving along the buried weakening plane so as to transfer the thin layer onto the second substrate;

[0029] - step e) comprises a heat treatment at a temperature higher than or equal to a predetermined second temperature, to allow spontaneous cleavage along the buried weakening plane (1 b); the first temperature of step b) and the second temperature of step c) are lower than the predetermined second temperature. BRIEF DESCRIPTION OF DRAWINGS

[0030] Further features and advantages of the present application will become apparent from the following detailed description of the application, taken in conjunction with the accompanying drawings, in which:

[0031] - Figure 1 A composite structure produced according to the bonding method of the present application is shown;

[0032] - Figures 2a to 2e Steps in the bonding method according to the present application are shown;

[0033] - Figure 3 and Figures 4a to 4e Alternative or optional steps in the bonding method according to the present application are shown. DETAILED DESCRIPTION

[0034] As mentioned above, the present application relates to a method for bonding a first substrate 1 to a second substrate 2 by molecular adhesion, each substrate being formed of a semiconductor material.

[0035] More particularly, the present application seeks to form a composite structure 3' comprising a monocrystalline thin layer 1'disposed on a carrier substrate 2 Figure 1 The first substrate 1 is intended for forming the thin layer 1'in order to produce an element, and is therefore preferably formed of a high-quality monocrystalline material. The second substrate 2 is intended for forming the carrier substrate 2 of the composite structure 3', and can therefore be formed of a lower-quality monocrystalline or polycrystalline material.

[0036] Advantageously, the thin layer 1'has a thickness less than 1 micron, compatible with Smart Cut type methods. Before bonding, the first substrate 1 and the second substrate 2 have a thickness of the order of a few hundred microns.

[0037] The two substrates 1, 2 can be formed of different or identical semiconductor materials chosen from silicon carbide (SiC) and indium phosphide (InP). More generally, these materials can be binary, ternary or quaternary compounds formed of elements from group IV, as well as groups III and V of the periodic table.

[0038] The substrates 1, 2 each comprise a "main" face 1 a, 2a, which corresponds to the face that will be in close contact in order to perform the bonding.

[0039] Before the close contact, in order to achieve the direct bonding by molecular adhesion, the main faces 1a, 2a are advantageously subjected to various treatments. The aim of these treatments is to clean the main faces 1a, 2a in order to remove contaminants (particulates, organic matter, etc.) and potentially activate the main faces 1a, 2a in order to promote chemical surface terminaisons chimiques de surface that are favourable to the propagation of the bonding wave and to the high strength of the bonding interface 4. The main faces 1a, 2a can also be subjected to chemical mechanical polishing in order to make the main faces 1a, 2a as smooth as possible. It is also conceivable to form an electrically conductive intermediate layer on one of the main faces 1a, 2a or on each of the main faces 1a, 2a, which can also be made smooth by chemical mechanical polishing.

[0040] Figures 2a to 2e The steps of the bonding method according to the application are illustrated.

[0041] The first step a) of the application Figure 2a , Figure 4a comprises bringing the main face 1a of the first substrate 1 into close contact with the main face 2a of the second substrate 2 in order to form an assembly 3 exhibiting a bonding interface 4. By "close" contact is meant that the main faces 1a, 2a are brought into direct contact without an adhesive layer in order to bond them by molecular adhesion along the bonding interface 4.

[0042] The close contact of the substrates 1, 2 can be performed under ambient atmosphere or under a controlled atmosphere, for example under an inert gas and / or under vacuum. It is conceivable to perform this close contact at ambient temperature or at a higher temperature, for example between 30°C and 500°C.

[0043] After completion of the step a), in a next step b) Figure 2b , Figure 4b the assembly 3 is subjected to a reactive annealing of the bonding interface at a temperature higher than a predetermined first temperature, hereinafter referred to as the first temperature. This predetermined first temperature corresponds to a temperature above which the species trapped at the bonding interface on the main faces 1a, 2a, for example, in the case of hydrophobic bonding, hydrogen, fluorine, residues of water monolayers, and / or the species adsorbed before the contact are all or partially reactive in order to form the bubbles 5 at the bonding interface 4. The duration of this annealing is generally 1 hour.

[0044] In the case where the two substrates 1, 2 are formed of SiC, this predetermined first temperature is approximately 200°C and the first temperature can for example be chosen equal to 700°C.

[0045] The gas trapped in these bubbles 5 can be, for example, dihydrogène, water vapor or carbon dioxide, or other gases resulting from the thermal activation reactions at the bonding interface 4. It has been observed that, in the case of substrates 1, 2 made of SiC, these bubbles 5 can remain stable over a wide temperature range, usually up to 1100°C or even higher. This is problematic, in particular when the later thinning step in the method must be performed without exceeding the aforementioned range in which the bubbles 5 remain stable: the bonding defects, corresponding to the bubbles, tend to compromise the integrity and quality of the thin layer 1' resulting from the thinning of the first substrate 1.

[0046] The next step c) of the application Figure 2c , Figure 4c comprises at least partially detaching the two substrates 1, 2 at the bonding interface 4. Separating the two substrates 1, 2 by opening the bonding interface 4 allows eliminating the bubbles 5 by releasing the gas trapped therein.

[0047] The detaching step c) can comprise mechanically separating the two substrates 1, 2 by inserting a blade 6 at the bonding interface 4 between the two substrates 1, 2. The material of the blade is chosen to avoid any contamination, so as to be compatible with microelectronic applications. The blade is preferably made of Teflon. When the blade 6 is inserted at the bonding interface 4, it generates a detachment wave that propagates along the bonding interface 4 and causes the separation of the two substrates 1, 2. The detachment wave must and is sufficient to reach each of the bubbles 5 present at the interface, in order to release the trapped gas and eliminate all the bubbles 5 present. Thus, the detachment does not necessarily take place over the entire bonding interface 4; the two substrates 1, 2 can thus remain joined over a portion of the bonding interface 4.

[0048] The detaching step c) Figure 2c , Figure 4c is advantageously performed under a controlled atmosphere. It can be performed in a clean room, or preferably under a dry atmosphere, such as under dry nitrogen or under vacuum, so as to prevent particles or water from depositing on the main faces 1a, 2a of the substrates 1, 2, which could create bonding defects in the next steps.

[0049] The step c) can be performed entirely or partially at a second temperature higher than or equal to the ambient temperature. This second temperature is generally lower than 700°C, preferably lower than 200°C, more preferably lower than 100°C.

[0050] The step c) can also comprise a step of ion beam etching of the main surfaces 1a, 2a of the two substrates 1, 2 so as to remove any oxide layer. This generally involves bombarding with argon ions at an energy of a few tens of eV to a few hundred eV for a few tens of seconds.

[0051] Then, in a step d) (Figure 2d , Figure 4d ) in which the two substrates 1, 2 are brought back into close contact at the bonding interface 4 in order to reform the assembly 3 and to finally join the main surfaces 1a, 2a together. This step of recontacting is preferably performed under a controlled atmosphere, the composition and temperature of which are similar to those of step c) in order to avoid the introduction of new impurities that could generate new bubbles.

[0052] After this step d), the assembly 3 can be subjected to a heat treatment without generating bonding defects that could adversely affect the quality of the substrate 1 or of the thin layer 1' obtained upon completion of the optional later thinning step e). The absence of such harmful bonding defects can be explained by the fact that, after final joining, there is now no or very little substance at the bonding interface 4 that is able to react with the temperature. In addition, the precautions taken in steps c) and d) to avoid the introduction of new impurities also contribute to preventing bonding defects from occurring.

[0053] After completion of step d), if the thin layer 1' is required for the target application and the initial thickness of the first substrate 1 is not suitable, a step e) of thinning the first substrate 1 can be performed. If the step e) of thinning the first substrate 1 includes a heat treatment, it is particularly important to be able to subject the assembly 3 to a heat treatment without generating bonding defects. Indeed, if the assembly 3 cannot be subjected to a heat treatment without generating bonding defects, there is a risk of adversely affecting the quality of the thin layer 1' resulting from step e).

[0054] The step e) of thinning the first substrate 1 ( Figure 2e , Figure 4e ) aims to form a thin layer 1' in which various electronic components can later be manufactured.

[0055] According to a first embodiment ( Figure 2e ), the thin layer 1' can be formed by thinning the back face 1c of the first substrate 1 by grinding, dry or wet chemical etching and / or chemical mechanical polishing alternated with a cleaning procedure. One (or more) heat treatment(s) can be applied to consolidate the bonding interface 4 and / or to improve the crystal and / or surface quality of the thin layer 1' without generating new bubbles at the bonding interface 4.

[0056] According to a second embodiment, the thin layer 1' can be formed by layer transfer using the Smart Cut TM method. In this case, the joining method according to the application comprises, before the step a) of close contact, a step of forming a buried weakening plane 1b in the first substrate 1 ( Figure 3), then the thin layer 1'is defined between the main face 1 a and the buried-weakened plane 1 b. Advantageously, the buried-weakened plane 1 b is formed by ion implantation of light species down to a given depth. The implanted light species is preferably hydrogen, helium or a co-implantation of these two species. These light species will form microcavities distributed in a thin layer parallel to the main surface 1 a of the first substrate 1, around a given depth. For the sake of simplicity, this thin layer is called the buried-weakened plane 1 b. The implantation energy of the light species is chosen so as to reach a given depth in the first substrate 1, said depth corresponding to the targeted thickness of the thin layer 1 '.

[0057] Figures 4a to 4d The steps a) to d) of the second embodiment illustrated are consistent with the general description above.

[0058] Step e) comprises a cleaving along the buried-weakened plane 1 b so as to separate the thin layer 1'from the remaining part 1 " of the first substrate 1 and thereby transfer the thin layer 1'onto the second substrate 2 Figure 4e ). This cleaving can be induced by applying a thermal treatment to the assembly 3 at a temperature higher than or equal to a predetermined second temperature. This predetermined second temperature corresponds to the temperature at which a spontaneous cleaving can occur along the buried-weakened plane 1 b. In the case of a first substrate 1 made of SiC, this predetermined second temperature is typically comprised between 750°C and 1000°C. The cleaving thermal treatment can have a duration ranging from a few minutes to a few hours. During said thermal treatment, the microcavities present in the buried-weakened plane 1 b follow a growth kinetics until a spontaneous onset of a fracture wave, which will propagate over the whole extent of the buried-weakened plane 1 b and lead to a separation between the thin layer 1'bonded to the second substrate 2 and the remaining part 1 " of the first substrate 1. Alternatively, the separation can be induced by applying a localized stress or by a combination of thermal and mechanical stresses.

[0059] Still according to the second embodiment, the material of the first substrate 1 and the characteristics of the buried-weakened plane 1 b (related to the implantation conditions of the light species) must allow to obtain a cleaving at a second predetermined temperature at least 50°C to 150°C higher than the first temperature of step b) so as not to induce a premature separation of the thin layer 1'from the remaining part 1 " of the first substrate 1. Similarly, the first temperature of step b) Figure 4b ) and the second temperature of step c) Figure 4c ) must not exceed this second predetermined temperature.

[0060] Of course, other techniques than the one presented above for forming the thin layer 1'in step e) can be envisaged.

[0061] When the preceding steps a) to e) are completed, a composite structure 3' is obtained which does not present detrimental bonding defects. The composite structure 3' thus obtained exhibits a very good adhesion between the thin layer 1'and the carrier substrate 2.

[0062] Such a composite structure 3' can thus be used to form additional layers thereon by epitaxy, for example, at 1700°C, the additional layers having a thickness of 10 microns, in which the devices will be formed, without risking damaging the structure 3'.

[0063] The composite structure 3' can also exhibit very good vertical electrical and thermal conduction between the thin layer 1'and the carrier substrate 2. This is due to the material choice of the thin layer 1'and the carrier substrate 2, and also due to the absence of an intermediate bonding layer for joining them.

[0064] Of course, the application is not limited to the implementation described, and various variants can be made to the application without departing from the scope of the application as defined by the claims.

Claims

1. A bonding method for joining two semiconductor substrates by molecular adhesion, the bonding method comprising: Step a) brings the first substrate (1) and the second substrate (2) into close contact to form an assembly (3) that presents a bonding interface (4); Step b) involves reacting the bonding interface at a first temperature higher than a predetermined first temperature, which generates bubbles at the bonding interface. The predetermined first temperature corresponds to a temperature above which all or part of the substances trapped at the bonding interface under hydrophobic bonding react to form bubbles at the bonding interface. The joining method is characterized by including: Step c), at least partially peeling the first substrate (1) and the second substrate (2) at the bonding interface (4) in order to eliminate the air bubbles; Step d) involves bringing the first substrate (1) and the second substrate (2) back into close contact at the bonding interface (4) to reform the assembly (3). In this step, stripping step c) is performed under a controlled atmosphere, which is either an anhydrous atmosphere or a vacuum.

2. The joining method according to claim 1, wherein, The stripping step c) is performed wholly or partially at a second temperature higher than or equal to the ambient temperature.

3. The joining method according to claim 2, wherein, The second temperature is below 700°C.

4. The joining method according to claim 2, wherein, The second temperature is below 200°C.

5. The joining method according to claim 2, wherein, The second temperature is below 100°C.

6. The joining method according to claim 1, wherein, The stripping step c) includes mechanically separating the first substrate (1) and the second substrate (2) by inserting a blade at the bonding interface (4) between the first substrate (1) and the second substrate (2).

7. The joining method according to claim 2, wherein the joining method comprises: Step e) is performed after step d), which thins the first substrate (1) to form a thin layer (1').

8. The joining method according to claim 7, wherein, The first substrate (1) includes a main surface (1a) and a buried weakening plane (1b), and the thin layer (1') is defined between the main surface (1a) and the buried weakening plane (1b).

9. The joining method according to claim 8, wherein, Step e) includes splitting along the buried weakening plane (1b) to transfer the thin layer (1') onto the second substrate (2).

10. The joining method according to claim 9, wherein, Step e) includes heat treatment at a temperature higher than or equal to a predetermined second temperature to allow spontaneous splitting along the embedded weakening plane (1b), the predetermined second temperature corresponding to the temperature at which spontaneous splitting can occur along the embedded weakening plane (1b). The first temperature in step b) and the second temperature in step c) are lower than the predetermined second temperature.

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