Wafer-to-wafer direct bonding method

Abstract

The invention relates to a method of directly bonding (200) a first microelectronic device (100) on a second microelectronic device, comprising: providing a first device having a first flat surface (110) and a second device having a second flat surface (210), at least the first and second surfaces are treated with a plasma gas comprising at least a first fluorine-containing gas (having an atomic percentage F of fluorine), the first and second devices are transferred to a bonding apparatus, the first and second surfaces are immersed in a bonding atmosphere (1), and the first and second surfaces are bonded under the bonding atmosphere. By cooperatively controlling the atomic percentage F of fluorine and the relative humidity RH, the bonding speed Vc is less than or equal to 15 mm / s, more particularly less than 10 mm / s. The reduced bonding speed significantly reduces the deformation in the bonding process, and can obtain sufficiently high bonding energy, thereby ensuring good bonding between the two surfaces.

Description

TECHNICAL FIELD

[0001] The present invention relates to the assembly of microelectronic devices, in particular for 3D integration, and more particularly to wafer-to-wafer direct bonding. BACKGROUND

[0002] To increase the integration density of transistors in microelectronic devices, 3D integration technology appears as a very promising solution. This technology enables to exploit the third perpendicular dimension without relying on the miniaturization of the components. In this context, one implementation consists in stacking a layer of transistors on top of another already existing layer of transistors; another implementation consists in powering the transistors from the backside of the active area, thus reducing the size of the unit cell. Both implementations can be achieved thanks to wafer-to-wafer direct bonding technology. To access the transistors from the backside of the active area, the device is usually bonded to a support in order to remove the substrate in which the transistors are formed, for example by means of a thinning process, and thus to access the transistors.

[0003] However, the wafer-to-wafer direct bonding step often induces a deformation which can complicate the alignment operations in the process steps, for example the step of forming the power network by means of photolithography, which requires a very high alignment accuracy. Moreover, the stresses left on the wafer by the previous process steps also bring additional challenges. While the original deformation is usually about 80 nm, it can be reduced to about 10 nm by means of the alignment of the wafer and the chip area performed by the equipment used in the photolithography process. Even so, these values are still above the stringent requirements for nodes below 2 nm, and thus need to be further optimized.

[0004] There is thus a need to reduce the deformation during the bonding step while ensuring a sufficiently high adhesion energy (French: énergie d'adhérence) to guarantee the integrity of the wafer before its backside reconnection in the photolithography process. The plasma-activated bonding method with fluorine is a technique for forming a strong bond during the bonding process, thus enabling a good adhesion, and is particularly suitable for Si / Si bonding. The document by Wang et al. (doi.org / 10.1016 / j.microrel.2011.09.005) describes a bonding method which can also be used for bonding other materials such as oxides (SiO2) under fluorine plasma. This document demonstrates that this method has a high bonding energy: about 1.4 J / m 2 2.8 J / m 2 for SiO2 / SiO2 bonding. However, this document does not address the deformation problem in the bonding step.

[0005] The present invention aims to solve at least part of the above problems. SUMMARY

[0006] To achieve this object, according to an embodiment, a method for direct bonding of a first microelectronic device on a second microelectronic device is provided, comprising the following steps: • providing a first microelectronic device having a first planar surface, and a second microelectronic device having a second planar surface; • treating at least one of the first and second surfaces with a plasma gas containing at least one first fluorine-containing gas, and having a certain atomic percentage of fluorine F; • transferring the first and second devices to a bonding apparatus; • immersing the first and second surfaces in a bonding atmosphere having a controlled relative humidity RH; • bonding the oppositely arranged first surface with the second surface using the bonding apparatus under the bonding atmosphere, so that the two surfaces partially bond and propagate in a bonding wave at a bonding velocity Vc, The method is characterized in that the atomic percentage of fluorine F in the treating step and the relative humidity RH in the bonding step are cooperatively regulated, so that the bonding velocity Vc is less than or equal to 15 mm / s. Preferably, the velocity is less than 10 mm / s.

[0007] Contrary to the conventional idea that the bonding energy is improved by fast bonding, the above method achieves the slowing down of the propagation velocity of the bonding wave by regulating the surface chemistry of the surfaces to be bonded and the relative humidity RH of the bonding atmosphere. By reducing the humidity of the bonding atmosphere and simultaneously increasing the fluorine concentration in the plasma gas, the bonding wave can be effectively slowed down. The slowing down of the bonding wave plays an important role in reducing the deformation caused by direct bonding. By increasing the atomic percentage of fluorine F in the plasma gas, the bonding velocity is effectively reduced while maintaining good bonding of the surfaces of the devices. By selecting a suitable combination of (F, RH) parameters, the method can achieve a bonding velocity of less than 15 mm / s, and more preferably less than 10 mm / s, which is a relatively low level compared to the conventional bonding velocity (about 20 to 50 mm / s). This reduced bonding velocity significantly reduces the deformation during bonding and can achieve a high enough bonding energy to ensure good bonding between the two surfaces. BRIEF DESCRIPTION OF DRAWINGS

[0008] The objects, goals, features and advantages of the present application will be fully apparent from the detailed description of specific embodiments thereof, which description is given in connection with the following drawings, in which: Figures 1 to 4 The various steps of the method for direct bonding of two microelectronic devices according to an embodiment example of the present application are schematically shown in cross-section along the xz plane.

[0009] Figure 5The method steps for direct bonding of two microelectronic devices according to an embodiment of the application are schematically illustrated in cross-section along the xz plane.

[0010] Figures 6 to 10 The method steps for direct bonding of two microelectronic devices according to an embodiment of the application are schematically illustrated in cross-section along the xz plane.

[0011] Figure 11A The distribution of the deformation resulting from the direct bonding of two wafers according to the method of the application is illustrated.

[0012] Figure 11B The distribution of the deformation resulting from the direct bonding of two wafers according to the method of the application is illustrated.

[0013] Figure 12 The graph of the variation of the bonding energy as a function of the bonding speed is illustrated for different cases of direct bonding of two wafers.

[0014] The attached drawings are merely illustrative and do not limit the application. These drawings are schematic representations for the purpose of understanding the application and the proportions are not necessarily in accordance with actual application proportions. DETAILED DESCRIPTION

[0015] Before the embodiments of the application are described in detail, optional features are described which can be used in combination or individually: According to an example, the first and second devices can be directly transferred to the bonding apparatus after the plasma gas treatment, without an intermediate cleaning step.

[0016] Directly transferring the devices to the bonding apparatus after the plasma treatment, without an intermediate cleaning step, makes it possible to further slow down the bonding speed without significantly reducing the bonding energy.

[0017] According to an optional solution, the plasma gas treatment is applied separately to the first surface and to the second surface. Preferably, the same plasma gas is used for the treatment of both surfaces; however, different plasma gases can also be used.

[0018] According to an example, the relative humidity RH is greater than or equal to 0% and less than 45%.

[0019] According to an example, the relative humidity RH is less than or equal to 2%, preferably less than or equal to 1%.

[0020] The bonding step is usually performed in a clean room with a relative humidity of about 45%. The application proposes performing the bonding in an environment that is drier than the clean room, thus making it possible to significantly reduce the bonding speed Vc. However, slowing down the bonding wave results in a reduction in the bonding energy. To compensate for the bonding energy, the atomic percentage of fluorine F can be increased during the plasma treatment.

[0021] According to an example, the atomic percentage of fluorine F can be greater than or equal to 0.4%.

[0022] According to an example, the atomic percentage of fluorine F is less than or equal to 4%.

[0023] The atomic percentage of fluorine F can be higher than the value of about 0.4% reported in the document by Wang et al. The best bond energy can be obtained within the framework of the method by Wang et al. when the atomic percentage of fluorine F = 0.4%. However, the bonding speed Vc is not affected at this value of F and remains at a relatively high speed of about 30 mm / s. When the relative humidity decreases and the atomic percentage of fluorine exceeds 0.4%, it is possible to reduce the bonding speed. Good bond energy can be ensured at the same time since there is fluorine when the bonding surface is treated by a fluorine-containing plasma.

[0024] According to an example, the bonding device comprises a bonding chamber in which the first device 100 and the second device 200 are inserted during the transfer step. The process of immersing the first and second surfaces in the bonding atmosphere comprises injecting into the bonding chamber a flow of a third gas called bonding gas, so that the bonding gas is confined within the bonding chamber, thereby forming the bonding atmosphere.

[0025] The implementation of the bonding step in a chamber makes it possible, on the one hand, to reduce the possible presence of contaminants in the bonding atmosphere and, on the other hand, to better control the relative humidity in a limited space.

[0026] According to an example, the process of immersing the first and second surfaces in the bonding atmosphere comprises injecting into the bonding chamber a flow of a third gas called bonding gas, so that the bonding gas fills at least one zone located between the first and second surfaces arranged opposite each other, thereby forming the bonding atmosphere.

[0027] According to an example, the bonding gas consists of at least one of the following gases, or is a mixture comprising at least one of the following gases: He, CO2, N2, O2, Ne, Ar, CF4, SF6, NF3 and H2. The use of a gas with a short mean free path, such as CO2, to form the bonding atmosphere makes it possible to slow down the bonding wave effectively.

[0028] According to an example, the first gas consists of at least one of the following gases, or is a mixture comprising at least one of the following gases: SF6, CF4, NF3 and F2.

[0029] According to an example, the plasma gas comprises a second gas consisting of at least one of the following gases, or is a mixture comprising at least one of the following gases: N2, O2, Ar and He.

[0030] According to an example, the method further comprises a thermal treatment of the first and second surfaces before bonding, the treatment being able to bring the temperature of the first and second surfaces to greater than or equal to 20°C and / or less than or equal to 150°C (preferably less than or equal to 50°C). Increasing the temperature of the surfaces before bonding makes it possible to slow down the bonding wave.

[0031] According to an example, the first device is a wafer comprising a first stack comprising at least one transistor and in contact with the first surface; the second device is a wafer comprising a second stack comprising at least one transistor and in contact with the second surface.

[0032] The method makes it possible to assemble by direct bonding of two transistor layers along a third perpendicular direction, with minimal deformation between the two layers, thus improving the overall alignment precision of the two surfaces and simplifying the manufacturing steps after bonding.

[0033] According to an example, the first device is a wafer comprising a first stack comprising at least one transistor and in contact with the first surface, the first stack being formed on a substrate that is to be removed after bonding; the second device is a wafer comprising at least one support layer in contact with the second surface.

[0034] The method also makes it possible to transfer the transistor layer formed on the substrate by direct bonding on the support layer, then to remove the substrate, and to form the transistor power network from the back of the stack. The advantage of this method is that this transfer can be implemented with a significant reduction in surface deformation, which makes it possible to simplify the lithography steps (in particular the mark alignment) after bonding.

[0035] According to an example, at least one of the first and second surfaces is based on a semiconductor material, an oxide or a metal, or the at least one surface further comprises at least one region based on an oxide and at least one region based on a metal / semiconductor.

[0036] The method makes it possible to implement not only silicon surface bonding but also oxide surface bonding with reduced deformation. In fact, for Si / Si hydrophobic bonding, the bonding speed is reduced, but the method makes it possible to implement oxide surface bonding, with a bonding speed that is even lower or comparable to that of Si / Si hydrophobic bonding.

[0037] Within the scope of the present invention, a transfer and bonding method applied to wafer-to-wafer bonding is described. This method can be extended to the bonding of one or more chips to a wafer, or to the bonding of chip-to-chip. The method is preferably used for industrial implementation to achieve wafer-to-wafer transfer and bonding. The method belongs to the field of direct bonding, and the direct bonding can be a hybrid bonding. "Hybrid" means that the bonding interface is composed of at least two materials. "Direct" means that after the final bonding, the bonding interface directly corresponds to the two bonding surfaces, and no bonding layer (such as a polymer glue) is inserted between the two bonding surfaces. Direct bonding is also a spontaneous bonding, so it can complete the propagation of the bonding wave by itself without external support. For example, thermal compression bonding does not belong to this category.

[0038] In the present application, "wafer" generally refers to a substrate containing or carrying a plurality of chips. The wafer can not contain any elements. "Chip" generally refers to an integrated circuit containing microelectronic or optoelectronic elements, and can also refer to a microelectromechanical system (MEMS). The alignment operation can be achieved by alignment marks, or can be completed only by relying on the accuracy of equipment movement, or by means of mechanical positioning devices (using substrate edge positioning).

[0039] It should be noted that, within the scope of the present invention, "on", "covering", "underlayer", "relative" and their synonyms do not necessarily mean "in contact with". For example, the first layer is deposited on the second layer, which does not necessarily require direct contact between the two layers, but means that the first layer at least partially covers the second layer, whether in direct contact or separated by at least one other material or at least one other element.

[0040] In addition, a layer can be composed of multiple sub-layers of the same material or different materials.

[0041] A substrate, layer or device "based on / with material M" means a substrate, layer or device containing only the material M, or containing the material M and optionally other materials (such as alloy elements, impurities or doping elements).

[0042] In a broad sense, "steps of a method" refers to the execution process of part of the operations in the method, and the process can be divided into several sub-steps as needed. Several embodiments of the present invention will be described below, and these embodiments all use continuous steps of the manufacturing method. It should be noted that, unless otherwise specified, the adjective "continuous" does not necessarily mean that the steps must be performed immediately (although this is usually the case), and intermediate steps can separate them.

[0043] Furthermore, the term "step" does not imply that the actions performed in a step are necessarily performed simultaneously or in immediate succession. In particular, after certain actions of a first step have been performed, actions related to another step can be performed, after which the remaining actions of the first step can be resumed. Thus, the term "step" does not necessarily refer to a single and indivisible action in time in the ordering of the phases of the method.

[0044] The dimension value refers to manufacturing and measurement tolerances.

[0045] The terms "substantially", "about", "approximately" when referring to a numerical value mean "±10% of the value" and when referring to an angular direction mean "±10° of the direction". Thus, a direction substantially perpendicular to a plane refers to a direction having an angle of 90±10° with the plane.

[0046] The drawings show a preferred orthogonal reference frame comprising x, y, z axes.

[0047] The thickness of a layer or substrate is measured in a direction perpendicular to its surface, which corresponds to the direction of maximum extension of the layer or substrate. Thus, the direction of measurement of the thickness should be perpendicular to the main plane of the layer or substrate (i.e. of the support of the layers). In particular, the thickness can be measured in the direction of the z axis.

[0048] Reference will now be made, by way of example, to the accompanying drawings in Figures 1 to 4 which two direct bonding assembly methods of microelectronic devices are described.

[0049] As shown in Figure 1 , the method comprises providing a first microelectronic device 100 and a second microelectronic device 200. The first device 100 can comprise a first substrate S1 extending into a horizontal plane xy defined by a direction x and a direction y perpendicular to the direction x. At least one layer of a first stack E1 is deposited on the first substrate S1 in a direction z perpendicular to the directions x and y. The first device 100 has a first planar surface 110, referred to as a first bonding surface, which corresponds to the exposed face of the first stack E1.

[0050] The second device 200 can comprise a second substrate S2 extending into the xy plane, on which at least one layer of a second stack E2 is deposited in the direction z. The second device 200 has a second planar surface 210, referred to as a second bonding surface, which corresponds to the exposed face of the second stack E2.

[0051] The first and second bonding surfaces 110 and 210 are designed to be bonded to each other by direct bonding to assemble the first and second devices 100 and 200 along the z-direction. Prior to the bonding step, the method includes treating the first bonding surface 110 and the second bonding surface 210 with a fluorine-containing plasma gas. This plasma gas contains at least one fluorine-containing first gas. The plasma gas may contain a second gas or a mixture of multiple gases with the fluorine-containing first gas. The atomic percentage of fluorine in the plasma gas is denoted as F. Treating the bonding surfaces with a fluorine-containing plasma gas reduces the binding energy of direct bonding, thereby reducing the velocity of the bonding wave. Conversely, surface treatment with fluorine-containing plasma increases the bonding energy of the bonding surfaces after consolidation annealing. This binding energy depends on the concentration of fluorine in the plasma or the atomic percentage F of fluorine in the plasma gas.

[0052] After treating the first bonding surface 110 and the second bonding surface 210 with fluorine-containing plasma, the method includes transferring the first device 100 and the second device 200 to a bonding apparatus 20. The bonding apparatus 20 enables the operation, mutual alignment, and assembly of the devices via direct bonding.

[0053] like Figure 2 As shown, the first and second surfaces 110 and 210 are arranged opposite each other relative to the bonding interface 2 (which is preferably parallel to the xy plane). The alignment of the two surfaces 110 and 210 can be performed in a plane parallel to the bonding interface 2 and located on both sides of the bonding interface. Before bonding, the two bonding surfaces 110 and 210 are separated from each other and placed opposite each other, thereby defining region 1'.

[0054] like Figure 3 As shown, prior to performing the bonding step, the method includes immersing the first bonding surface 110 and the second bonding surface 210 in a bonding atmosphere 1. This bonding atmosphere 1 can be formed by injecting a gas flow of a third gas 22, called a bonding gas, into region 1', such that the bonding surfaces 110, 210 are immersed in the gas 22. The bonding apparatus 20 may include at least one gas injector 23 connected to a bonding gas reservoir for injecting the bonding gas 22. The gas injector 23 may be equipped with a first sensor and a valve capable of monitoring and regulating the flow rate of the bonding gas 22.

[0055] Bonding atmosphere 1 has a relative humidity denoted as RH. The relative humidity RH of bonding atmosphere 1 is preferably controlled before and during the bonding step. In fact, the bonding gas 22 injected into region 1' displaces the air that is normally humid within region 1'. By displacing the air with the preferably dry bonding gas 22, the relative humidity RH of bonding atmosphere 1 will gradually decrease until a stable value is reached before bonding. This RH value is preferably kept constant until the bonding is completed.

[0056] The bonding apparatus 20 may be configured with, for example, a second sensor (such as a hygrometer) to measure the relative humidity (RH) of the bonding atmosphere 1. This second sensor may be connected to a control system to enable real-time adjustment of the relative humidity (RH). Specifically, this can be achieved by connecting the first sensor to the valve of the injector 23 within the control system, thus forming a feedback loop to adjust the flow rate of the bonding gas 22 based on the measured relative humidity (RH) value in the bonding atmosphere 1.

[0057] By controlling the relative humidity of bonding atmosphere 1, a drier bonding atmosphere than the ambient atmosphere or the cleanroom where conventional direct bonding operations take place can be formed.

[0058] like Figure 4 As shown, the method involves achieving surface bonding by bringing the first surface 110 and the second surface 210 close together at the bonding interface 2 (with a spacing of less than 500µm, more preferably less than 100µm or even 50µm). Alternatively, the upper surface can be allowed to fall naturally onto the lower surface. The bonding atmosphere film trapped between the two surfaces will automatically maintain the spacing between them after several seconds, typically less than about 100µm, or even less than 10µm. The bonding interface 2 forms a plane that simultaneously contains the first surface 110 and the second surface 210. The bonding of the two surfaces 110 and 210 is completed in the bonding atmosphere 1. When the two surfaces 110 and 210 are brought close together as described above, local bonding is initiated in at least one region within the plane of the bonding interface 2 (e.g., at the center of the bonding surfaces). This initiation is achieved by bringing the two surfaces into local physical contact, typically using a bonding tip (une pointe de collage). Subsequently, even after the bonding tip is removed, this localized bond propagates radially from the center of the bonding surface to the edge within the plane of the bonding interface 2 in the form of a so-called "bonding wave." Based on this characteristic, this type of bonding is called "spontaneous bonding" (French: collage spontané). The bond exhibits self-propagation properties. Spontaneous bonding can be achieved by gravity when the top surface is only released above the bottom surface and the separation is maintained by a gas film. However, this method cannot control the initiation point and may generate multiple initiation points. The bonding wave is characterized by a bonding velocity Vc, which directly affects the bonding quality. The faster the bonding wave velocity (i.e., the greater the bonding velocity Vc), the more deformation is generated by the bonding. Here, "deformation" refers to the mechanical deformation generated along the bonding interface. For example, in the process of performing photolithography by aligning markers, the deformation generated by direct bonding is usually random and difficult to compensate for using numerical models.

[0059] By reducing the bonding speed Vc, the deformation generated by the bonding process can be significantly reduced. The dry bonding atmosphere 1 enables a reduction of the bonding speed. However, this reduction of the bonding speed is usually accompanied by a decrease of the bonding surface energy, which is not desired. A good and high bonding strength of the bonding surface ensures a good mechanical stability of the assembly, which is particularly critical during the manufacturing steps after the bonding.

[0060] To compensate for the decrease of the bonding energy, the atomic percentage of fluorine F is reduced during the fluorine plasma treatment, while the relative humidity RH of the bonding atmosphere 1 is simultaneously reduced during the bonding step. By adjusting these two parameters, a set of (F, RH) parameter combinations can be selected, which enables a significant reduction of the bonding speed during the bonding process, while ensuring a good bonding of the bonding surfaces. Thus, by simultaneously adjusting the F and RH parameters, a bonding speed Vc < 15 mm / s (more preferably < 10 mm / s) can be achieved. This bonding speed enables an effective reduction of the deformation generated by the bonding. On the other hand, by simultaneously adjusting the F and RH parameters, a sufficiently high bonding energy can be achieved for the bonding, which meets the production requirements of the microelectronic device assembly.

[0061] The atomic percentage of fluorine F in the plasma gas can be greater than or equal to 0.4%. When the F value is close to 0.4%, the influence of the fluorine concentration in the plasma on the bonding speed can be neglected. In this case, the relative humidity RH of the bonding atmosphere must be significantly reduced. To effectively control (in particular, to reduce) the bonding speed Vc, it is advantageous to increase the atomic percentage of fluorine F to more than 0.4%. A significant increase of the F value can be combined with a moderate reduction of the RH value. Likewise, a significant reduction of the RH value can also be combined with a moderate increase of the F value. Preferably, the atomic percentage F is < 4%, because when the F value exceeds 4%, the bonding of the two surfaces 110, 210 can be affected.

[0062] During the bonding step, the relative humidity RH of the bonding atmosphere 1 should be strictly less than 45%. Ideally, the relative humidity RH of the bonding atmosphere 1 during the bonding step can be 0 or slightly higher than 0%. For example, the relative humidity RH can be controlled to be < 10%, preferably < 2%, more preferably < 1%.

[0063] As a complementary solution to the simultaneous adjustment of the F and RH parameters, a reduction of the bonding speed Vc can be achieved by omitting the cleaning step (in particular, the chemical cleaning) of the bonding surfaces 110, 210 after the fluorine plasma surface treatment. According to this embodiment example, the first and second devices 100, 200 are directly transferred to the bonding apparatus 20 after the plasma treatment. For example, the transfer can be performed in a clean room atmosphere.

[0064] As a Figure 3 and Figure 4As shown, according to a variant, the process of immersing the first surface 110 and the second surface 210 in the bonding atmosphere 1 can be carried out in a non-enclosed space, i.e., the space is not completely sealed like a bonding chamber. According to this variant, a gas injector 23 can be positioned at region 1' such that the injected bonding gas 22 at least fills region 1' to form the bonding atmosphere 1. The bonding atmosphere 1 or the bonding gas 22 can extend beyond region 1', since the bonding surfaces 110, 210 and region 1' are all immersed in the bonding gas. The bonding atmosphere can contain part or all of the first and second devices. According to this variant, the bonding gas 22 does not need to be confined to a specific space. Bonding can be carried out in the bonding atmosphere 1, which can be locally formed by the bonding gas, for example, in a cleanroom.

[0065] like Figure 5 As shown, according to another variant, the bonding apparatus 20 may include a bonding chamber 21. In this variant, after the bonding surfaces are plasma-treated, devices 100, 200 are placed into the bonding chamber 21, such that the chamber 21 completely accommodates devices 100, 200, thereby forming a closed space. An injector 23 for the bonding gas 22 is disposed inside the bonding chamber 21. This injector may be located at region 1' or at other locations within the bonding chamber 21. The operation of immersing the bonding surfaces 110, 210 in the bonding atmosphere 1 specifically includes injecting the bonding gas 22 into the bonding chamber 21 to form the bonding atmosphere 1. According to one example, before the bonding atmosphere 1 is formed, ambient air within the bonding chamber may be gradually vented (replaced by the bonding gas 22) during the injection of the bonding gas 22, or may be completely vented before the injection of the bonding gas 22. The bonding gas 22 will fill the entire bonding chamber 21, particularly filling region 1' between the bonding surfaces 110, 210. Using bonding chamber 21 reduces contaminant residue during the bonding process. This is particularly advantageous in bonding methods that do not require surface cleaning after plasma treatment.

[0066] like Figures 1 to 5 As shown, the above method can achieve, for example, the assembly of two wafers by direct bonding. The first device 100 may be a first wafer containing a first substrate S1, on which a first stack E1 is formed. The first stack E1 may contain a first support layer E11 based on silicon oxide (referred to as a "BOX layer," i.e., a buried oxide layer), on which a first active layer E12 based on silicon is covered (e.g., containing at least one, preferably multiple, transistors T). The first stack E1 may also contain a protective layer E13 based on SiO2, the exposed surface of which is the first bonding surface 110. Each transistor T contains a source Ts, a drain Td, and a gate Tg (all three may be integrated in the protective layer E13), and has a channel Tc integrated in the active layer E12.

[0067] The second device 200 may be a second wafer containing a second substrate S2, on which a second stack E2 is formed. The second stack E2 may further include a second support layer E21 (BOX layer) based on silicon oxide, on which a second active layer E22 based on silicon is covered (which may contain at least one, preferably multiple, transistors T). In addition, the second stack E2 may further include a silicon dioxide (SiO2) based protective layer E23, the exposed surface of which is the second bonding surface 210. Each transistor T includes a source Ts, a drain Td, and a gate Tg (all three may be integrated in the protective layer E23), and a channel Tc integrated in the active layer E22.

[0068] like Figure 4 As shown, the present invention assembles two wafers by direct bonding along the z-direction. This assembly method, by vertically stacking two active layers containing transistors, can increase transistor density. The advantage of this method is that it enables bonding with less deformation, ensuring more precise alignment between the two wafers, thus facilitating subsequent bonding processes and enabling the industrial mass production of microelectronic components.

[0069] Figures 6 to 10 Another application example of the method of the present invention is shown. According to one example, such as... Figure 6 As shown, the first device 100 may be the same first wafer as described above. The second device 200 may be a third so-called support (carrier) wafer, comprising a third substrate S3 and a SiO2-based third support layer 230 formed on the third substrate S3. The exposed surface of the third support layer 230 is the second bonding surface 210. The third support layer 230 may not contain electronic components or active sublayers and is only used to support the first wafer.

[0070] The first wafer can be transferred to the support wafer using the direct bonding method described in this invention.

[0071] like Figure 7 As shown, after the bonding surfaces are treated with fluorine-containing plasma, the first wafer is flipped and aligned with the support wafer, so that the first bonding surface 110 and the second bonding surface 210 are positioned opposite each other. Subsequently, the bonding surfaces 110 and 210 are immersed in bonding atmosphere 1, and brought into contact during the bonding step, as shown. Figure 8 As shown. This method enables wafer transfer containing transistors while minimizing deformation of the first bonding surface of the direct bonding pair.

[0072] By transferring the first wafer onto the support wafer, operations on the back side Eb of the stacked E1 can be performed. For this purpose, as follows... Figure 9As shown, after the two wafers are bonded, the substrate S1 can be removed, thereby exposing the back surface Eb of the stack E1. This facilitates direct access to the various layers of the stack E1 (in particular, the active layer E12), without having to go through the substrate S1, which can be hundreds of microns thick. The substrate can be removed by, for example, chemical etching and / or grinding.

[0073] As shown, after the substrate is removed, a power supply network 150 can be fabricated on the back surface Eb of the stack E1. This power supply network is referred to as a "backside power supply network" (BS-PDN) because it is formed on the back surface. For example, the power supply network 150 can include vias 151 that extend through the stack E1 to the source Ts, drain Td, and gate Tg of the transistor T. Forming the power supply network on the back surface of the stack saves lateral space (i.e., space in the xy-plane), thereby increasing the density of transistors within the stack. The reduced bonding distortion using the method of the present application can facilitate the fabrication of the power supply network 150, which can involve photolithography and steps that use pre-fabricated mark points in the stack E1 for alignment. Figure 10 In the above example, the material of both bonding surfaces 110, 210 is oxide-based, in particular, silicon dioxide-based. The method can enable bonding of oxide surfaces (e.g., SiO2 / SiO2) at a lower bonding speed Vc than the bonding speed of hydrophobic silicon (Si / Si) surfaces. The direct bonding of the method is not limited to SiO2-based surfaces 110, 210, but can also be used for bonding surfaces based on other oxides, nitrides, semiconductors, or metals. As an example, at least one of the bonding surfaces 110, 210 can be based on Si3N4, SiCN, Al2O3, TaN, TiN, Si, Ge, Ti, Ni, Cu, Al, Ta, etc. The bonding can be hybrid bonding, i.e., the bonding surface includes regions of different materials, such as oxide or nitride regions, and metal or semiconductor regions.

[0074] The first fluorine-containing gas present in the plasma can be SF6, CF4, NF3, or F2, another fluorine-containing gas, or a mixture of multiple fluorine-containing gases. The second gas in the plasma can be a non-fluorine-containing gas, such as N2, O2, Ar, He, etc. The second gas can also be a mixture of multiple gases. The first and second gases that make up the plasma are selected to suit the formation of the plasma.

[0075]

[0076] ​The bonding gas 22 forming the bonding atmosphere 1 can be selected from the following group of gases: He, CO2, N2, O2, Ne, Ar, CF4, SF6, F2 and H2. The bonding gas 22 can also be a mixture of several gases. Preferably, the bonding gas should have a short mean free path, which simultaneously reduces the speed of the bonding wave (e.g. CO2). The use of helium, neon or hydrogen as bonding gas reduces the formation of defects called "picots" during the bonding process, which are usually found at the edges of the final structure (usually in the form of circular patches).

[0077] According to an example, the method can further comprise a step of heat treating the bonding surfaces 110, 210 before bonding. The heat treatment can be performed before or after the bonding surfaces are transferred to the bonding apparatus 20. The heat treatment can be performed at a temperature > 20°C and / or < 150°C, preferably < 50°C. According to a preferred embodiment, the heat treatment is performed at a temperature ranging from 20°C to 50°C. If the heat treatment is performed before the transfer to the bonding apparatus, the time interval between the treatment and the bonding must ensure that the temperature of the surfaces at the time of bonding is < 150°C, preferably between 20°C and 50°C.

[0078] A specific and non-limiting example of application of the method is described below. Two wafers of 300 mm in diameter to be bonded are provided, the surfaces of which are covered with a 100 nm thick thermal oxide protective layer, so that the bonding surfaces are based on thermal oxide. The bonding surfaces are then cleaned during a preliminary cleaning step using ozone water, prepared from deionized water and ozone at a dissolved concentration of 14 ppm (14 mg / L), which is then rinsed off the bonding surfaces of the wafers with deionized water. The bonding surfaces are then treated with APM (ammonium hydroxide-hydrogen peroxide mixture), which comprises three main components: ammonium hydroxide, hydrogen peroxide and deionized water, in a ratio of 1 : 1 : 5, at a temperature of 70°C. A very light etching is then performed in 0.1% hydrogen fluoride (HF) for 30 seconds, which is then rinsed off with deionized water. Each of the preliminary cleaning sub-steps described above lasts approximately 10 minutes (except for the HF etching step).

[0079] After the preliminary cleaning of the bonding surfaces, the wafers are transferred to the EVG® 850 LT apparatus. The bonding surfaces are then cleaned a second time using the Megpie® apparatus: power 90 W, rotation speed 30 RPM (revolutions per minute) for one minute. This second cleaning is performed using a mixture of deionized water and 2% ammonia, which effectively removes the particulate contaminants from the bonding surfaces.

[0080] Subsequently, the bonding surfaces were treated with a fluorine-containing plasma comprising a first gas CF4 (atomic percentage of fluorine F = 0.4%) and a second gas oxygen. The fluorine-containing plasma treatment was performed at a frequency of 47 kHz and 347 kHz for a duration of about 15 seconds. The two wafers were then directly transferred through a clean room atmosphere to a bonding chamber (clean room atmosphere parameters: relative humidity 45%, ambient temperature 21 °C). A bonding atmosphere was created by injecting helium bonding gas until the relative humidity RH in the bonding chamber was below 2%. Bonding of the two surfaces was then performed in a dry bonding atmosphere. The bonding process was initiated by locally applying pressure, preferably acting on the center of the wafer, with a pressure value of about 3500 mN. The bonding wave was propagated from the center outwards with a bonding speed Vc of less than 15 mm / s, more preferably less than 10 mm / s.

[0081] Figure 11A and 11B The in-plane displacement (IPD) profiles of the assemblies obtained by direct bonding are shown. These profiles present the wafer deformation characteristics resulting from direct bonding. Figure 11A The IPD profiles of assemblies obtained by direct bonding of two wafers under standard conditions, in particular with a conventional bonding speed of about 30 mm / s, are shown. The x-axis 302 and the y-axis 303 represent the distance from the center of the wafer in the x-direction and in the y-direction, respectively (unit: mm). The color scale 303 represents the deformation of the wafer after bonding measured along the z-axis (unit: pm). Figure 11B The IPD profiles of assemblies obtained by direct bonding of two wafers according to the specific example described above, in particular at a reduced bonding speed, i.e. with a bonding speed of less than 15 mm / s, more preferably less than 10 mm / s, are shown. A comparison of the two profiles clearly shows that the deformation resulting from direct bonding is significantly reduced at a reduced bonding speed. This result confirms that the bonding of surfaces treated with a fluorine-containing plasma in a dry atmosphere has a significant effect on reducing the deformation.

[0082] Figure 12 In the graph shown, the x-axis 401 represents the bonding speed Vc (unit: mm / s) and the y-axis 402 represents the binding energy measured after annealing at 300 °C (unit: mJ / m 2). A plurality of data points 411-415 are marked on the graph, which correspond to the bonding energy values obtained by using different bonding speeds in different wafer direct bonding cases. Data points 414 and 415 below the graph correspond to direct bonding without fluorine plasma surface treatment and in a standard bonding atmosphere (relative humidity 45%-50%). Since the surface is not treated by fluorine plasma before bonding, the bonding energy obtained in both cases does not reach the ideal value. Data point 415 corresponds to the bonding of a hydrophobic surface. For a hydrophobic bonding surface, the bonding speed is relatively low (about 10 mm / s). Data points 411, 412 and 413 above the graph correspond to direct bonding of surfaces treated by fluorine plasma. Data point 411 corresponds to direct bonding of two wafers with parameters F 411 =0.4 and RH 411 ≈50%, which obtains an optimal bonding energy of about 5800 mJ / m 2 and a relatively high bonding speed of about 32 mm / s. Data point 412 corresponds to direct bonding with the same parameters as data point 411 (i.e. F 412 =0.4 and RH 412 ≈50%), except that the bonding surface is not cleaned after fluorine plasma treatment in this case. Omitting the cleaning step of the bonding surface after fluorine plasma treatment allows the bonding speed to be reduced to nearly 18 mm / s. However, the decrease in bonding speed is accompanied by a slight decrease in bonding energy. Finally, data point 413 corresponds to the bonding of the method described in the present application, in which the surface is treated by F 413 =0.4% plasma and the bonding is performed in a dry environment with a relative humidity RH 413 ≈0%. In this case, the bonding speed is reduced to nearly 9 mm / s, and the bonding energy is about 4200 mJ / m 2 , which is sufficient to obtain good bonding of the surface and fully verifies the effectiveness of the direct bonding method.

[0083] The following gives examples of parameters that can be used to implement the present application (preferably used in combination), which are only for illustration and do not constitute a limitation: Atomic concentration of fluorine in the plasma: 4% Relative humidity of the bonding atmosphere: 40% Bonding wave speed: 6 mm / s Bonding energy: 1900 mJ / m 2 at 100°C (this value performs excellently at this temperature).

[0084] The invention is not limited to the above described embodiments, but the scope of protection of the invention covers all embodiments which are within the scope of the invention as defined by the features of the invention. Different specific examples of the direct bonding method have been set out herein. Other implementation variants can be formed by combining the above described features without departing from the principles of the invention. Furthermore, a description of features in relation to one aspect of the invention can be used in combination with features in relation to other aspects of the invention.

Claims

1. A method of direct bonding of a first microelectronic device (100) on a second microelectronic device (200), the method comprising the following steps: • providing a first microelectronic device (100) having a flat first surface (110) and a second microelectronic device (200) having a flat second surface (210); • plasma gas treating at least one of said first surface (110) and said second surface (210) with a plasma gas comprising at least one first fluorine-containing gas and having a certain atomic percentage of fluorine F; • transferring said first microelectronic device (100) and said second microelectronic device (200) to a bonding apparatus (20); • immersing said first surface (110) and said second surface (210) in a bonding atmosphere (1) having a controlled relative humidity RH; • bonding relatively disposed said first surface (110) and said second surface (210) using said bonding apparatus (20) in said bonding atmosphere (1), wherein said first surface (110) and said second surface (210) locally bond and propagate at a bonding velocity Vc in the form of a bond wave, the method being characterized in that the atomic percentage of fluorine F in the treating step and the relative humidity RH in the bonding step are co-controlled so that the bonding velocity Vc is less than or equal to 15 mm / s.

2. The direct bonding method according to claim 1, wherein, The step of transferring said first microelectronic device (100) and said second microelectronic device (200) to said bonding apparatus (20) is performed directly after the plasma gas treatment, without an intermediate cleaning step.

3. The direct bonding method according to claim 1, wherein, The plasma gas treatment of each of said first surface (110) and said second surface (210) is performed using the same plasma gas.

4. The direct bonding method according to claim 1, wherein, The relative humidity RH is greater than or equal to 0% and less than 45%.

5. The direct bonding method according to claim 4, wherein, The relative humidity RH is less than or equal to 2%.

6. The direct bonding method according to claim 1, wherein the atomic percentage of fluorine F is greater than or equal to 0.4%.

7. The direct bonding method according to claim 1, wherein, The atomic percentage of fluorine F is less than or equal to 4%.

8. The direct bonding method according to claim 1, wherein: • said bonding apparatus (20) comprises a bonding chamber (21) into which said first microelectronic device (100) and said second microelectronic device (200) are inserted during the transferring step, and • the step of immersing said first surface (110) and second surface (210) in said bonding atmosphere (1) comprises injecting into said bonding chamber (21) a flow of a third gas (22) called bonding gas, so that said bonding gas (22) is confined within said bonding chamber (21), thereby forming said bonding atmosphere (1).

9. The direct bonding method according to claim 1, wherein, The step of immersing the first surface (110) and the second surface (210) in the bonding atmosphere (1) comprises injecting a flow of a third gas (22), called bonding gas, so that the bonding gas (22) fills at least one region (1') between the oppositely arranged first surface (110) and second surface (210), thereby forming the bonding atmosphere (1).

10. The direct bonding method according to claim 8, the bonding gas (22) consisting of or comprising at least one of the following gases: He, CO2, N2, O2, Ne, Ar, CF4, SF6, NF3, F2 and H2.

11. The direct bonding method according to claim 9, wherein, The bonding gas (22) consists of or comprises at least one of the following gases: He, CO2, N2, O2, Ne, Ar, CF4, SF6, NF3, F2 and H2.

12. The direct bonding method according to claim 1, wherein, The first fluorine-containing gas consists of or comprises at least one of the following gases: SF6, CF4, NF3 and F2.

13. The direct bonding method according to claim 1, wherein, The plasma gas comprises a second gas consisting of or comprising at least one of the following gases: N2, O2, Ar and He.

14. The direct bonding method according to claim 1, further comprising a heat treatment of the first surface (110) and the second surface (210) before bonding, which heat treatment is capable of bringing the temperature of the first surface (110) and the second surface (210) to a temperature greater than or equal to 20°C and / or less than or equal to 150°C.

15. The direct bonding method according to any one of claims 1 to 14, wherein: • the first microelectronic device (100) is a wafer comprising a first stack (El) comprising at least one transistor (T) and being in contact with the first surface (110); • the second microelectronic device (200) is a wafer comprising a second stack (E2) comprising at least one transistor (T) and being in contact with the second surface (210).

16. The direct bonding method according to any one of claims 1 to 14, wherein: • the first microelectronic device (100) is a wafer comprising a first stack (El) on a substrate (SI), the first stack (El) comprising at least one transistor (T) and being in contact with the first surface (110), the substrate (SI) being to be removed after bonding; • the second microelectronic device (200) is a wafer comprising at least one support layer (230) being in contact with the second surface (210).

17. The direct bonding method according to any one of claims 1 to 14, wherein, At least one of the first surface (110) and the second surface (210) is based on a semiconductor material, an oxide, a nitride or a metal, or further comprises at least one region based on an oxide or a nitride and a region based on a metal or a semiconductor.

Images