BONDING PROCESS BY DIRECT ADHESION

Description

Technical field

[0001] The invention relates to the technical field of bonding by direct adhesion of two substrates.

[0002] The invention finds its application in particular in the transfer of thin layers by Smart Cut ™ technology. State of the prior art

[0003] If we consider a bonding process by direct adhesion of a first substrate to a second substrate, resulting from the direct contact of two surfaces, it is known that such a process is likely to lead to an inhomogeneity of the adhesion energy (also called bonding energy, and expressed in J / m 2< ), that is to say the energy to separate the two surfaces after their bonding. In particular, this inhomogeneity of the adhesion energy results in a lower adhesion energy on the edges of the two surfaces than on a central part of the two surfaces. For example, the difference in adhesion energy between the edges (for example over a distance of 1 cm) and a central part of the two surfaces can amount to 700 mJ / m 2< for two silicon substrates, covered with an oxide layer, and each having a diameter of 200 mm.

[0004] Furthermore, in the case of thin film transfer using Smart Cut™ technology, a high inhomogeneity of the adhesion energy can lead to detachment of the two substrates during the fracturing step with the presence of non-transferred areas and defects.

[0005] The person skilled in the art is therefore looking for a direct adhesion bonding process which makes it possible to obtain the most homogeneous possible adhesion energy of the surfaces in contact.

[0006] US 2016 / 233125 A1, US 2017 / 133347 A1, US 2015 / 194337 A1 and US 2014 / 235031 A1 disclose examples of direct bonding processes using controlled humidity. Statement of the invention

[0007] The invention aims to remedy all or part of the aforementioned drawbacks. To this end, the invention relates to a method of bonding by direct adhesion of a first substrate to a second substrate, comprising the successive steps: a) providing the first and second substrates, each comprising a first surface and a second opposite surface; b) bonding the first substrate to the second substrate by direct adhesion between the first surfaces of the first and second substrates, step b) being carried out at a temperature between 20°C and under a first gaseous atmosphere having a first relative humidity level, denoted φ 1 ; c) applying a thermal annealing to the bonded first and second substrates, at a thermal annealing temperature between 100°C and 700°C, step c) being carried out under a second gaseous atmosphere having a second relative humidity level, denoted φ 2 , verifying φ 2 ≥ φ 1 .

[0008] Thus, such a method according to the invention allows, thanks to step c), a significant improvement in the homogeneity of the adhesion energy of the first surfaces of the first and second substrates. Indeed, the inventors have observed experimentally that the quantity of water vapor within the thermal annealing atmosphere had an impact on the homogeneity of the adhesion energy. More precisely, the fact of at least maintaining the relative humidity level after step b) of bonding makes it possible to significantly improve the homogeneity of the adhesion energy by preventing the water present at the bonding interface, at the edge of the substrate, from diffusing out of the bonding interface.In other words, the first and second bonded substrates are maintained after step b), including during the execution of step c), under an atmosphere having a humidity level greater than or equal to φ 1 , so as to homogenize the adhesion energy of the first surfaces of the first and second substrates. Definitions

[0009] The term "substrate" means a self-supporting physical support made of a basic material enabling the manufacture of an electronic device or an electronic component. A substrate is typically a wafer (" wafer» in English) cut from a single-crystal ingot of semiconductor material. By "direct adhesion" we mean a spontaneous bond resulting from the contact of two surfaces, that is to say in the absence of an additional element such as an adhesive, a wax or a solder. The adhesion comes mainly from the van der Waals forces resulting from the electronic interaction between the atoms or molecules of two surfaces, from hydrogen bonds due to surface preparations or from covalent bonds established between the two surfaces. We also speak of molecular adhesion bonding or direct bonding. Direct adhesion bonding is advantageously carried out at ambient temperature and pressure, and cannot be assimilated to thermocompression bonding, eutectic bonding, or anodic bonding. Direct adhesion bonding can be followed by thermal annealing to strengthen the bonding interface. By "thermal annealing" is meant a heat treatment comprising a heating cycle with: a first phase of gradual increase in temperature until reaching a plateau value, a second phase where the heating temperature maintains the plateau value, and a third cooling phase.

[0010] The thermal annealing temperature is the plateau value of the heating temperature. Relative humidity is defined as the ratio (expressed as a %) of the amount of water vapor contained in a given volume of gas to the maximum amount of water vapor permitted in the given volume of gas. Relative humidity depends on the gas temperature and gas pressure. Relative humidity can also be expressed as the ratio (expressed as a %) of the partial pressure of water vapor contained in a volume of gas to the saturated vapor pressure of water.

[0011] The method according to the invention may comprise one or more of the following characteristics.

[0012] According to a characteristic of the invention, the water has a saturated vapor pressure within the second gaseous atmosphere, denoted P sat, and step c) comprises the steps: c 1 ) gradually increasing a heating temperature of the second gaseous atmosphere until reaching the thermal annealing temperature; c 2 ) measuring progressive heating temperatures during step c 1 ), and determining P sat for the measured progressive heating temperatures; c 3 ) gradually increasing, during step c 1 ), the partial pressure of water vapor, denoted pv , within the second gaseous atmosphere so that pv ≥ P sat φ 1 .

[0013] Thus, it is possible to adjust φ 2 (equal to the ratio between pv and P sat ) throughout the temperature rise step c 1 ) so that the relationship φ 2 ≥ φ 1 can be verified.

[0014] According to a characteristic of the invention, φ 2 is equal to 100%.

[0015] Thus, one advantage provided is to simplify the process by saturating the second gaseous atmosphere with water vapor, in order to ensure that the relation φ 2 ≥ φ 1 is always verified.

[0016] According to a characteristic of the invention, step c) is carried out so that the second gaseous atmosphere has a constant pressure.

[0017] Thus, an advantage provided is to simplify the adjustment of φ 2 which also depends on the pressure of the second gaseous atmosphere.

[0018] According to a characteristic of the invention, the first and second gaseous atmospheres comprise air.

[0019] According to a characteristic of the invention, step c) is carried out in an oven injecting water vapor, produced by ultrasound, into the second gaseous atmosphere.

[0020] Thus, an advantage provided by the injection of water vapor produced by ultrasound is that it can be easily integrated into a furnace which, conventionally, only allows dry atmospheres for relatively low temperatures (i.e. below 700°C).

[0021] According to a characteristic of the invention, the thermal annealing temperature is between 100°C and 500°C, more preferably between 200°C and 400°C, even more preferably between 250°C and 350°C.

[0022] According to a characteristic of the invention, step a) comprises a step a 1 ) consisting of implanting gaseous species in the first substrate, through the first surface, so as to form a weakening zone, a useful layer being delimited by the weakening zone and by the first surface of the first substrate; and the thermal annealing is applied during step c) according to a thermal budget adapted to fracture the first substrate along the weakening zone so as to expose the useful layer.

[0023] Thus, an advantage provided is to be able to transfer the useful layer onto the second substrate by Smart Cut™.

[0024] According to a characteristic of the invention, step a) comprises a step a 1 ) consisting of implanting gaseous species in the first substrate, through the first surface, so as to form a weakening zone, a useful layer being delimited by the weakening zone and by the first surface of the first substrate; and step c) is followed by a step d) consisting of fracturing the first substrate along the weakening zone so as to expose the useful layer.

[0025] Thus, thermal annealing can be applied during step c) according to a thermal budget corresponding to a fraction (e.g. between 50% and 80%) of the thermal budget necessary to fracture the first substrate along the weakening zone. Such a thermal budget of step c) makes it possible to ensure strong bonding at the edge of the substrate so that step d) can be carried out in a less humid, or even dry, atmosphere. Step d) can be carried out by an additional heat treatment, adapted to obtain the fracture of the first substrate along the weakening zone.

[0026] According to a characteristic of the invention, the first surfaces of the first and second substrates are made of a material selected from Si, SiO 2 , Ge, Al 2 O 3 , SiC, AsGa, InP, GaN, LiNbO 3 , LiTaO 3 .

[0027] According to the invention, step b) is carried out at a temperature between 20°C and 30°C. Brief description of the drawings

[0028] Other features and advantages will become apparent in the detailed description of various embodiments of the invention, the description being accompanied by examples and references to the accompanying drawings. Figure 1 is a schematic sectional view along the normal to the first surfaces - and second surfaces - of the first and second substrates, illustrating step c) of a method according to the invention. Figure 2 is a graph representing the temperature (in °C) on the abscissa and the saturated vapor pressure of water (in bars, on a logarithmic scale) on the ordinate, obtained by the Clapeyron formula. Figures 3a to 3e are schematic sectional views along the normal to the first surfaces - and second surfaces - of the first and second substrates, illustrating steps of a method according to the invention in the case of a transfer of a useful layer by Smart Cut ™ technology. Figure 1 And Figures 3a to 3eare not shown to scale to simplify their understanding. Detailed description of the implementation methods

[0029] Identical elements or those providing the same function will bear the same references for the different embodiments, for the sake of simplification.

[0030] An object of the invention is a method of bonding by direct adhesion of a first substrate 1 to a second substrate 2, comprising the successive steps: a) providing the first and second substrates 1, 2, each comprising a first surface 10, 20 and an opposite second surface 11, 21; b) bonding the first substrate 1 to the second substrate 2 by direct adhesion between the first surfaces 10, 20 of the first and second substrates 1, 2, step b) being carried out at a temperature between 20°C and 30°C under a first gaseous atmosphere having a first relative humidity level, denoted φ 1 ; c) applying a thermal annealing to the bonded first and second substrates 1, 2, at a thermal annealing temperature between 100°C and 700°C, step c) being carried out under a second gaseous atmosphere 3 having a second relative humidity level, denoted φ 2 , verifying φ 2 ≥ φ 1 . First and second substrates

[0031] The first and / or second substrates 1, 2 may be made of a semiconductor material. The first and / or second substrates 1, 2 may comprise a surface layer 12, 22 such as an oxide layer, as illustrated in Figures 3a to 3e .

[0032] The first surfaces 10, 20 of the first and second substrates 1, 2 are advantageously made of a material selected from Si, SiO 2 , Ge, Al 2 O 3 , SiC, AsGa, InP, GaN, LiNbO 3 , LiTaO 3 . Step b) gluing

[0033] The first gaseous atmosphere may comprise air. Step b) is carried out at room temperature, i.e. a temperature between 20°C and 30°C. Step b) may be carried out at ambient pressure or under vacuum. Step c) thermal annealing

[0034] The second gaseous atmosphere 3 may comprise air. Step c) is advantageously carried out so that the second gaseous atmosphere 3 has a constant pressure. As illustrated in Figure 1 , step c) is advantageously carried out in a furnace 4 injecting, using an injector 40, water vapor into the second gaseous atmosphere 3. The water vapor is advantageously produced by ultrasound. The thermal annealing temperature is advantageously between 100°C and 500°C, more preferably between 200°C and 400°C, even more preferably between 250°C and 350°C. The thermal annealing temperature is between 100°C and 700°C.

[0035] According to a first mode of implementation, the water has a saturated vapor pressure within the second gaseous atmosphere 3, denoted P sat, and step c) comprises the steps: c 1 ) gradually increasing a heating temperature of the second gaseous atmosphere 3 until reaching the thermal annealing temperature; c 2 ) measuring progressive heating temperatures during step c 1 ), and determining P sat for the measured progressive heating temperatures; c 3 ) gradually increasing, during step c 1 ), the partial pressure of water vapor, denoted pv , within the second gaseous atmosphere so that pv ≥ P sat φ 1 .

[0036] Step c 1 ) is performed so that the heating temperature follows a temperature ramp, for example of the order of 10°C / min.

[0037] As shown in the Figure 2 , step c 2 ) may include a step consisting of determining P sat as a function of any temperature T, for example using the Clapeyron formula: P sat T = P 0 exp LM R 1 T 0 − 1 T Or : P 0 is the reference pressure, equal to 1013.25 hPa; L is the latent heat of vaporization of water (at 20°C), equal to 2470 kJ / kg; M is the molar mass of water, equal to 0.01801 kg / mol; R is the universal constant of ideal gases, equal to 8.314 462 1 J.mol -1< K -1< ; T 0 is the reference temperature, equal to 373.15 K (100°C).

[0038] P sat can therefore be determined by injecting the value of the measured heating temperature into the Clapeyron formula.

[0039] Step c 3 ) is performed by adjusting the flow rate of the injector 40.

[0040] According to a second embodiment, step c) is executed so as to saturate the second gaseous atmosphere 3 with water vapor so that φ 2 is equal to 100%. In other words, step c) is executed by injecting sufficient water vapor, using the injector 40, and continuously - i.e. for the entire duration of step c) -, into the second gaseous atmosphere 3 so that φ 2 is equal to 100%. Application to the transfer of thin layers

[0041] Step a) may comprise a step a 1 ) consisting of implanting gaseous species in the first substrate 1 (called donor substrate), through the first surface 10, so as to form a weakening zone ZS, a useful layer 5 to be transferred (i.e. to be transferred onto the second substrate 2, called support substrate) being delimited by the weakening zone ZS and by the first surface 10 of the first substrate 1. The gaseous species implanted during step a 1 ) preferably comprise ionized hydrogen atoms. It is also possible to carry out co-implantation with other gaseous species such as helium. As a non-limiting example, when the first substrate 1 is made of silicon, step a 1 ) may be carried out with ionized hydrogen atoms according to the following parameters: an energy of 85 keV, a dose of 5x10 16< at.cm -2< .

[0042] Thermal annealing is advantageously applied during step c) according to a thermal budget adapted to fracture the first substrate 1 along the weakening zone ZS so as to expose the useful layer 5. For this purpose, the thermal annealing temperature of step c) can be between 250°C and 600°C, or even between 350°C and 500°C.

[0043] According to a variant, the method comprises a step d) consisting of fracturing the first substrate 1 along the weakening zone ZS so as to expose the useful layer 5, step d) being carried out after step c). The thermal annealing can then be applied during step c) according to a thermal budget corresponding to a fraction (eg between 50% and 80%) of the thermal budget necessary to fracture the first substrate 1 along the weakening zone ZS. Such a thermal budget of step c) makes it possible to ensure strong bonding at the edge of the substrate so that the fracture step d) (" splitting» in English) can be carried out under a less humid, or even dry, atmosphere. Step d) can be carried out by an additional heat treatment, adapted to obtain the fracture of the first substrate 1 along the weakening zone ZS. Implementation Example #1

[0044] The first and second substrates 1, 2 are two silicon wafers <001> 300 mm in diameter and 775 µm thick. One of the two substrates 1, 2 has a surface layer 12, 22 of oxide obtained by thermal oxidation, and with a thickness of 145 nm. The first and second substrates 1, 2 are cleaned and hydrolyzed in baths of deionized water enriched with ozone in an APM solution (" Ammonia-Peroxide Mixture ”) at 70°C.

[0045] The first and second substrates 1, 2 are bonded during step b) by direct adhesion at room temperature and at ambient pressure. The first gaseous atmosphere (ambient air) has a first relative humidity level verifying φ 1 =45%.

[0046] Step c) is carried out in a furnace 4, the thermal annealing temperature being equal to 300°C. The gradual rise in temperature is carried out according to a temperature ramp of 10°C / minute. The thermal annealing is applied at the annealing temperature for 2 hours (plateau phase). Step c) is carried out according to the second mode of implementation, that is to say that step c) is carried out by injecting sufficient water vapor, using the injector 40, and continuously -i.e. for the entire duration of step c)-, into the second gaseous atmosphere 3 so that φ 2 is equal to 100%.

[0047] The adhesion energy is measured at the end of step c) using the double lever with imposed displacement (DCB) technique “Double Cantilever Beam under prescribed displacement” in English), involving a cut of two beams: a 1 cm wide beam passing through the edges of the two glued substrates 1, 2, a 1 cm wide beam passing through a central portion of the two glued substrates 1, 2.

[0048] An adhesion energy of 4.8 J / m 2< is measured on the edges, and an adhesion energy of 4.7 J / m 2< on the central portion, i.e. a difference of 100 mJ / m 2< , a value much lower than 700 mJ / m 2< of the state of the art, which is more for a substrate diameter of 300 mm compared to a diameter of 200 mm of the state of the art. Implementation example #2

[0049] The first and second substrates 1, 2 are two silicon wafers <001> 300 mm in diameter and 775 µm thick. One of the two substrates 1, 2 has a surface layer 12, 22 of oxide obtained by thermal oxidation, and with a thickness of 145 nm. The first and second substrates 1, 2 are cleaned and hydrolyzed in baths of deionized water enriched with ozone in an APM solution (" Ammonia-Peroxide Mixture ”) at 70°C.

[0050] The first and second substrates 1, 2 are bonded during step b) by direct adhesion at room temperature and at ambient pressure. The first gaseous atmosphere (ambient air) has a first relative humidity level verifying φ 1 =45%.

[0051] Step c) is carried out in a furnace 4, the thermal annealing temperature being equal to 300°C. The gradual rise in temperature is carried out according to a temperature ramp of 10°C / minute. The thermal annealing is applied at the annealing temperature for 2 hours (plateau phase). Step c) is carried out according to the first implementation mode, i.e. step c) is carried out by injecting: a quantity of water vapor in the second gaseous atmosphere 3, during the gradual rise in temperature up to 120°C, such that φ 2 is maintained equal to 50%; a quantity of water vapor in the second gaseous atmosphere 3, from 120°C to the plateau phase, such that φ 2 is equal to 100%.

[0052] The adhesion energy is measured at the end of step c) using the double lever with imposed displacement (DCB) technique. Double Cantilever Beam under prescribed displacement » in English), involving a cut of two beams: a 1 cm wide beam passing through the edges of the two glued substrates 1, 2, a 1 cm wide beam passing through a central portion of the two glued substrates 1, 2.

[0053] An adhesion energy of 4.8 J / m 2< is measured on the edges, and an adhesion energy of 4.5 J / m 2< on the central portion, i.e. a difference of 300 mJ / m 2< , a value much lower than 700 mJ / m 2< of the state of the art, which is more for a substrate diameter of 300 mm compared to a diameter of 200 mm of the state of the art.

[0054] The invention is not limited to the embodiments disclosed. Those skilled in the art are able to consider their technically effective combinations and to substitute equivalents for them.

Claims

1. Process for attaching a first substrate (1) to a second substrate (2) by direct bonding including the successive steps of: a) providing the first and second substrates (1, 2), each comprising a first surface (10, 20) and an opposite second surface (11, 21); b) bonding the first substrate (1) to the second substrate (2) by direct bonding between the first surfaces (10, 20) of the first and second substrates (1, 2), step b) being carried out at a temperature of between 20°C and 30°C under a first gaseous atmosphere having a first relative humidity level denoted by φ1; c) applying a thermal annealing treatment to the bonded first and second substrates (1, 2) at a thermal annealing temperature of between 100°C and 700°C, step c) being carried out under a second gaseous atmosphere (3) having a second humidity level denoted by φ2, characterized in that the bonded first and second substrates (1, 2) are maintained after step b), including during step c), under an atmosphere having a relative humidity level φ2, superior or equal to φ1, satisfying φ2 ≥ φ1, such that the adhesion energy of the first surfaces (10, 20) of the first and second substrates (1, 2) is homogenized.

2. Process according to Claim 1, wherein water exhibits a saturated vapour pressure within the second gaseous atmosphere (3) denoted by Psat, and wherein step c) includes the steps of: c1) gradually increasing a heating temperature of the second gaseous atmosphere (3) until reaching the thermal annealing temperature; c2) measuring progressive heating temperatures over step c1) and determining Psat for the measured progressive heating temperatures; c3) gradually increasing, over step c1), the partial water vapour pressure, denoted by pv, within the second gaseous atmosphere (3) such that pv ≥ Psat φ1.

3. Process according to Claim 1, wherein φ2 is equal to 100%.

4. Process according to one of Claims 1 to 3, wherein step c) is carried out such that the second gaseous atmosphere (3) exhibits a constant pressure.

5. Process according to one of Claims 1 to 4, wherein the first and second gaseous atmospheres (3) include air.

6. Process according to one of Claims 1 to 5, wherein step c) is carried out in an oven (4) injecting water vapour, produced by means of ultrasound, into the second gaseous atmosphere (3).

7. Process according to one of Claims 1 to 6, wherein the thermal annealing temperature is between 100°C and 500°C, more preferably between 200°C and 400°C, even more preferentially between 250°C and 350°C.

8. Process according to one of Claims 1 to 7, wherein: - step a) includes a step a1) consisting in implanting gaseous species into the first substrate (1), through the first surface (10), so as to form a weakened zone (ZS), a useful layer (5) being delimited by the weakened zone (ZS) and by the first surface (10) of the first substrate (1); and - the thermal annealing treatment is applied in step c) according to a thermal budget that is suitable for splitting the first substrate (1) along the weakened zone (ZS) so as to expose the useful layer (5).

9. Process according to one of Claims 1 to 7, wherein: - step a) includes a step a1) consisting in implanting gaseous species into the first substrate (1), through the first surface (10), so as to form a weakened zone (ZS), a useful layer (5) being delimited by the weakened zone (ZS) and by the first surface (10) of the first substrate (1); and - step c) is followed by a step d) consisting in splitting the first substrate (1) along the weakened zone (ZS) so as to expose the useful layer (5).

10. Process according to one of Claims 1 to 9, wherein the first surfaces (10, 20) of the first and second substrates (1, 2) are produced in a material selected from Si, SiO2, Ge, Al2O3, SiC, AsGa, InP, GaN, LiNbO3, LiTaO3.