Method for transferring a useful layer to a carrier substrate

Abstract

The present invention relates to a method for transferring a useful layer onto a carrier substrate, comprising the following steps: a) providing a donor substrate including an embedded weakening plane, the useful layer being defined by the front side of the donor substrate and the embedded weakening plane; b) providing a carrier substrate; c) bonding the donor substrate to the carrier substrate along a bonding interface from the front side of the donor substrate to form a bonding structure; d) annealing the bonding structure to apply a weakening thermal budget to the bonding structure and to bring the embedded weakening plane to a defined degree of weakening; e) initiating a cleavage wave in the embedded weakening plane by applying a stress to the bonding structure, the cleavage wave propagating along the embedded weakening plane in a self-sustaining manner to cause the useful layer to be transferred onto the carrier substrate. The transfer method is characterized in that the cleavage wave is initiated when the bonding structure is subjected to a temperature between 150 °C and 250 °C.

Description

Technical Field

[0001] The present invention relates to the field of microelectronics. Specifically, the present invention relates to a method for transferring a useful layer onto a carrier substrate. Background Art

[0002] Figure 1 The method of transferring the useful layer 3 onto the carrier substrate 4 shown is known in the prior art; this method is specifically described in documents WO 2005043615 and WO 2005043616 and includes the following steps:

[0003] · Forming a buried weakening plane 2 by implanting light substances in the donor substrate 1, thereby forming a useful layer 3 between this plane and the surface of the donor substrate;

[0004] · Then, bonding the donor substrate 1 to the carrier substrate 4 to form a bonded structure 5;

[0005] · Heat-treating the bonded structure 5 to weaken the buried weakening plane;

[0006] · And finally, initiating a splitting wave by an energy pulse applied at the level of the buried weakening layer 2, and the splitting wave propagates self-sustainedly along the buried weakening plane 2 in the donor substrate 1.

[0007] In this method, the substances implanted at the level of the buried weakening layer 2 start to form microcavities. The weakening heat treatment has the effect of promoting the growth and pressurization of these microcavities. By applying an additional external force (energy pulse) after the heat treatment, a splitting wave is initiated in the buried weakening layer 2, and this wave propagates in a self-sustained manner, causing the useful layer 3 to be transferred by separation at the level of the buried weakening plane 2. This method can especially reduce the surface roughness after transfer.

[0008] This method can be used to manufacture silicon-on-insulator (SOI) substrates. In this case, both the donor substrate 1 and the carrier substrate 4 are formed of silicon wafers, and the standard diameter of the silicon wafers is usually 200 mm, 300 mm or 450 mm for the next generation. One or both of the donor substrate 1 and the carrier substrate 4 are surface oxidized.

[0009] SOI substrates must meet very strict specifications. This is especially true for the average thickness and thickness uniformity of the useful layer 3. Meeting these specifications is necessary for the correct operation of semiconductor devices to be formed in and on this useful layer 3.

[0010] In some cases, the structure of these semiconductor devices requires the useful layer 3 of the SOI substrate to have a very low average thickness (e.g., below 50 nm) and a very high thickness uniformity. The desired thickness uniformity can be up to about 5%, corresponding to a variation typically from + / -0.3 nm to + / -1 nm over the entire surface of the useful layer 3. Even after an additional trimming step, such as etching or surface smoothing heat treatment, is performed after the useful layer 3 has been transferred to the carrier substrate 4, it is important that the morphological surface characteristics (specifically, the uniformity of thickness and surface roughness) be as favorable as possible after the transfer to ensure compliance with the final specifications.

[0011] The applicant has observed that when a cleavage wave is induced after a heat treatment at ambient temperature by applying an energy pulse to the buried weakening plane 2, some useful layers 3 can include a marbled irregular pattern due to local thickness variations after the transfer, with a variation amplitude of approximately 1 nanometer or half a nanometer. This marbling can be distributed over the entire useful layer 3 or only over a portion thereof. This contributes to causing non-uniformity of the useful layer 3.

[0012] It is difficult to eliminate this non-uniformity of the thickness of the useful layer 3 using conventional trimming techniques (etching, sacrificial oxidation, smoothing heat treatment, etc.) because these techniques are ineffective in erasing irregular patterns of this amplitude.

[0013] To limit the amplitude of the regular pattern of thin layer thickness variations after cleavage, document EP2933828 proposes bringing the component to be cleaved into contact with an absorption element to dissipate the acoustic vibrations emitted during the initiation and self-sustained propagation of the cleavage wave.

[0014] Object of the Invention

[0015] The present invention relates to a method for transferring a useful layer onto a carrier substrate and specifically aims to improve the thickness uniformity of the useful layer after the transfer.

[0016] Summary of the Invention

[0017] The present invention relates to a method for transferring a useful layer onto a carrier substrate, comprising the following steps:

[0018] a) providing a donor substrate comprising a buried weakening plane, the useful layer being delimited by the front face of the donor substrate and the buried weakening plane;

[0019] b) providing a carrier substrate;

[0020] c) bonding the donor substrate to the carrier substrate along a bonding interface from the front face of the donor substrate to form a bonded structure;

[0021] d) annealing the bonded structure to apply a weakening heat budget to the bonded structure and bring the buried weakening plane to a defined degree of weakening;

[0022] e) By applying stress to the bonding structure, a splitting wave is induced in the buried weakening plane, and the splitting wave propagates along the buried weakening plane in a self-sustaining manner to cause the useful layer to be transferred to the carrier substrate.

[0023] The transfer method is characterized in that the splitting wave is induced when the bonding structure is subjected to a temperature between 150°C and 250°C at least in its hottest region.

[0024] According to other advantageous and non-limiting features of the present invention, the following items are considered individually or in any technically feasible combination:

[0025] · The maximum temperature is between 180°C and 220°C;

[0026] · The annealing in step d) reaches a maximum holding temperature between 300°C and 600°C;

[0027] · The weakening heat budget is between 40% and 95% of the splitting heat budget, and the splitting heat budget causes a splitting wave to be spontaneously induced in the buried weakening plane during annealing;

[0028] · The induction in step e) is directly carried out after the annealing in step d) and before the temperature in the hottest region of the bonding structure reaches a temperature below 150°C;

[0029] · The annealing in step d) is carried out in a heat treatment device in a horizontal or vertical configuration, the heat treatment device being suitable for batch processing of a plurality of bonding structures, and the induction in step e) is carried out when the bonding structure leaves the device;

[0030] · Applying stress to the buried weakening plane corresponds to applying a local mechanical load to the bonding structure, in particular to the periphery of the structure;

[0031] · The local mechanical load is applied by inserting a wedge between the beveled edges of the donor substrate and the carrier substrate of the bonding structure at the bonding interface of the bonding structure;

[0032] · In step e), when there is a temperature gradient on the bonding structure, a local mechanical load is applied in the region of the bonding structure that experiences a lower temperature;

[0033] · When the temperature gradient is about 80°C, a local mechanical load is applied to the bonding structure;

[0034] · The donor substrate and the carrier substrate are made of single crystal silicon, and wherein the buried weakening plane is formed by implanting light matter ions into the donor substrate, and the light matter is selected from hydrogen and helium, or a combination of hydrogen and helium. Description of the Drawings

[0035] Other features and advantages of the present invention will become apparent from the following detailed description of the invention given with reference to the accompanying drawings, in which:

[0036] Figure 1 shows a method for transferring a thin film according to the prior art;

[0037] Figure 2 shows a transfer method according to the present invention;

[0038] Figures 3a to 3c shows a haze map at the surface of the useful layer including thickness non-uniformity after transfer;

[0039] Figure 4 shows a haze map at the surface of the useful layer transferred using the transfer method according to the present invention;

[0040] Figure 5 shows the steps in the transfer method according to the present invention;

[0041] Figure 6 is a graph showing the temperature and thermal gradient seen by the bonding structure in the steps of the transfer method according to the present invention;

[0042] Figure 7 shows the steps in the transfer method according to the present invention. Detailed Description

[0043] In the description, the same reference numerals in the drawings may be used for the same type of elements. The drawings are schematic representations and are not drawn to scale for clarity. In particular, the thickness of the layers along the z-axis is not in proportion to the lateral dimensions along the x-axis and y-axis; the relative thicknesses of these layers with respect to each other are not necessarily relative in the drawings. It should be noted that Figure 1 the coordinate system (x, y, z) applies to Figure 2 .

[0044] The present invention relates to a method for transferring a useful layer 3 onto a carrier substrate 4. The useful layer 3 is so named because it is intended for the production of components in the field of microelectronics or microsystems. The nature of the useful layer and the carrier substrate may vary depending on the type of target component and the target application. Since silicon is currently the most commonly used semiconductor material, the useful layer and the carrier substrate may in particular be made of single crystal silicon, but are of course not limited to this material.

[0045] The transfer method according to the present invention first includes step a): providing a donor substrate 1 from which the useful layer 3 is to be obtained. The donor substrate 1 includes a buried weakening plane 2 ( Figure 2a)). The buried weakening plane 2 is advantageously formed by implanting light material ions into the donor substrate 1 at a defined depth. The light material is preferably selected from hydrogen and helium, or a combination of hydrogen and helium, as these materials promote the formation of microcavities around the defined implantation depth, resulting in the buried weakening plane 2.

[0046] The useful layer 3 is defined by the front face 1a of the donor substrate 1 and the buried weakening plane 2.

[0047] The donor substrate 1 can be formed of at least one material selected from silicon, germanium, silicon carbide, IV-IV, III-V or II-VI semiconductor compounds, and piezoelectric materials (such as LiNbO3, LiTaO3, etc.). It can also include one or more surface layers of any nature, such as dielectrics, provided on its front face 1a and / or back face 1b.

[0048] The transfer method further includes step b): providing a carrier substrate 4 ( Figure 2 b)).

[0049] The carrier substrate can be formed of, for example, at least one material selected from silicon, silicon carbide, glass, sapphire, aluminum nitride, or any other material that can be obtained in substrate form. It can also include one or more surface layers of any nature, such as dielectric.

[0050] As described above, an advantageous application of the transfer method according to the present invention is the production of SOI substrates. In this specific case, the donor substrate 1 and the carrier substrate 4 are made of single-crystalline silicon, and one or both of said substrates include a silicon oxide surface layer 6 located on their front faces.

[0051] The transfer method then includes step c): bonding the donor substrate 1 to the carrier substrate 4 along the bonding interface 7 from the front face 1a of the donor substrate 1 to form a bonded structure 5 ( Figure 2 c)).

[0052] This attachment operation can be carried out using any known method, in particular by direct bonding by molecular adhesion, by thermocompression, or by electrostatic bonding. These known prior arts will not be described in detail herein. However, it should be remembered that before bonding, the donor substrate 1 and the carrier substrate 4 will undergo surface activation and / or cleaning treatments to ensure the quality of the bonding interface 7 in terms of defects and bonding energy.

[0053] In the transfer method according to the present invention, step d) is then carried out: annealing the bonded structure 5 to apply a weakening thermal budget to said structure 5 and bring the buried weakening plane to a defined degree of weakening ( Figure 2 d)). The time / temperature pair applied during annealing determines the thermal budget to which the bonded structure 5 is subjected.

[0054] For the operation of weakening the buried plane 2, the temperature range at which annealing can be carried out mainly depends on the type of the bonding structure 5 (homogeneous structure or heterogeneous structure) and the properties of the donor substrate 1.

[0055] For example, in the case of the donor substrate 1 and the carrier substrate 4 made of silicon, the annealing in step d) reaches a maximum holding temperature usually between 200 °C and 600 °C, advantageously between 300 °C and 500 °C, and even more advantageously between 350 °C and 450 °C.

[0056] More generally, for materials other than silicon used for the donor substrate 1 and / or the carrier substrate 4, the maximum holding temperature can usually be between 200 °C and 800 °C.

[0057] The annealing can include a temperature increase (usually between 200 °C and the maximum holding temperature) and a hold at the maximum temperature. Generally, the duration of such annealing will be between several tens of minutes and several hours, which depends on the maximum holding temperature of the annealing.

[0058] The degree of weakening of the buried weakening plane 2 is defined by the area occupied by the microcavities present in the buried weakening layer 2.

[0059] In the case where the donor substrate 1 is made of silicon, the area occupied by the microcavities can be characterized by infrared microscopy.

[0060] Depending on the thermal budget applied to the bonding structure 5 during annealing, the degree of weakening can range from a low degree (<1%, below the detection threshold of the characterization instrument) to greater than 80%.

[0061] The weakening thermal budget is always kept below the splitting thermal budget such that a splitting wave is spontaneously initiated in the buried weakening plane 2 during annealing. Preferably, the weakening heat is between 40% and 95% of the splitting thermal budget.

[0062] In the transfer method according to the present invention, step e) is then performed: a splitting wave is initiated along the buried weakening plane 2 by applying a stress to the buried weakening plane 2 of the bonding structure 5 ( Figure 2 of e)). After initiation, the splitting wave propagates in a self-sustaining manner, causing the bonding structure 5 to separate at the buried weakening plane 2. Self-sustaining propagation means that once initiated, the splitting wave propagates on its own throughout the extent of the buried weakening plane 2 without the application of an external stress, such that the useful layer 3 is completely separated from the donor substrate 1 and transferred to the carrier substrate 4. Thereby, a transfer assembly 5a of the donor substrate 1 and a remainder 5b ( Figure 2 of f)) are obtained.

[0063] The external stress is advantageously local and can be caused mechanically or by any other means, such as local heating by means of a laser or energy transfer by means of ultrasound.

[0064] Recalling that, by applying the prior art transfer method mentioned in the background art that includes mechanically initiating a splitting wave at ambient temperature, the applicant has observed a marbled-like irregular pattern that adversely affects the thickness uniformity of the useful layer 3 after transfer. The applicant has recognized that these irregular patterns are related to the instability of the propagation of the splitting wave due to insufficient energy stored in the (bonding structure 5 / embedded weakening layer 2) system.

[0065] To overcome these problems and improve the thickness uniformity of the useful layer 3 after transfer, the transfer method according to the present invention contemplates that, in step e), when the bonding structure 5 is subjected to a temperature between 150 °C and 250 °C at least in its hottest region, a splitting wave is initiated by applying an external stress to the embedded weakening surface 2. In fact, there is usually a temperature gradient on the bonding structure 5, so the present invention contemplates that the region of the bonding structure 5 that experiences the highest temperature (its hottest region) experiences a temperature between 150 °C and 250 °C. In other words, splitting is initiated when the highest temperature that the bonding structure 5 is locally or uniformly subjected to in its hottest region is within this temperature range.

[0066] Advantageously, the aforementioned highest temperature is between 180 °C and 220 °C, preferably about 200 °C.

[0067] In order to initiate a splitting wave when at least the hottest region of the bonding structure 5 is at a temperature higher than 150 °C, the energy stored in the system, particularly the energy stored in the embedded weakening surface 2 due to the presence of a pressurized gaseous substance in the microcavity, is sufficient to ensure the stable and self-sustaining propagation of the splitting wave.

[0068] Triggering the initiation when the bonding structure 5 is subjected to a maximum temperature lower than 250 °C limits the energy stored in the system, such that the excess energy released during splitting (i.e., the energy not consumed in the material rupture) does not cause the formation of other types of patterns on the surface of the transferred useful layer 3. Specifically, the applicant has observed that when the excess energy released during splitting is too high, regular patterns with high amplitudes appear and adversely affect the thickness uniformity of the useful layer 3 after transfer. For example, this is the case when a splitting budget is applied to the bonding structure 5 at an annealing hold temperature (e.g., 400 °C) and a splitting wave is spontaneously initiated. Thus, too much energy stored and released during the propagation of the splitting wave is also problematic for the thickness uniformity of the useful layer 3 after transfer.

[0069] The energy stored in the system depends on the degree of weakening of the buried weakening plane 3 and the temperature at which the splitting wave is initiated and propagates. When the energy stored in the system is sufficient to ensure stable and self-sustaining propagation (thus limiting the appearance of irregular patterns) and not too high so as to limit the amplitude of the regular patterns (the amplitude of the regular patterns also negatively affects the thickness uniformity), the transfer method according to the invention allows the initiation of the splitting wave.

[0070] As an example, Figure 3a 、 Figure 3b and Figure 3c show the haze maps at the surface of the useful layer 3 after spontaneous splitting, mechanically induced splitting at ambient temperature, and mechanically induced splitting at a maximum temperature of 100 °C for the bonding structure, respectively. It should be noted that the above mechanical separation operations are initiated by applying a local mechanical load to the bonding structure 5 and generating strain in the buried weakening plane 2. In each case, regular ( Figure 3a ) or irregular ( Figure 3b and Figure 3c ) marbling patterns are observed, which unfavorably affect the thickness uniformity (amplitude between 0.5 nm and 1.5 nm) of the useful layer 3 after transfer. Using the Surfscan TM inspection tool of KLA-Tencor, these patterns have become evident by measuring the haze (which corresponds to the intensity of the light scattered by the surface of the useful layer 3).

[0071] Figure 4 shows the haze map at the surface of the useful 3 transferred using the transfer method according to the invention: namely, after splitting is initiated by applying a local mechanical load to the bonding structure 5 to generate strain in the buried weakening plane 2 when the bonding structure 5 is subjected to a temperature of 200 °C at least in its hottest region. There are no regular or irregular and marbling patterns, thus significantly improving the thickness uniformity of the useful layer 3.

[0072] According to a first advantageous variant, the initiation in step e) occurs directly after the annealing in step d) and before the temperature in the hottest region of the bonding structure 5 reaches a temperature below 150 °C. For example, in the case where the temperature during annealing in step d) rises to 400 °C and then decreases, the initiation occurs when the bonding structure 5 (at least in its hottest region) is subjected to a maximum temperature below or equal to 250 °C and before it reaches a maximum temperature of 150 °C.

[0073] In fact, when the bonding structure 5 leaves the heat treatment device 20 for annealing, the bonding structure 5 is subjected to a temperature higher than 150 °C and lower than or equal to 250 °C ( Figure 2In the controlled exit region 23 of the highest temperature of e)), the step e) of initiating the cleavage wave can specifically occur. In particular, the annealing in step d) can be carried out in a heat treatment apparatus 20 having a horizontal or vertical configuration suitable for batch processing of a plurality of bonding structures 5; then, when the bonding structure 5 exits the apparatus 20, the initiation in step e) occurs in the exit region 23, where the highest temperature experienced by the structure 5 is controlled within the range required to initiate the cleavage wave. As the bonding structure 5 passes through the controlled exit region 23, an external stress for triggering the initiation of the cleavage wave is advantageously continuously applied to the bonding structure 5.

[0074] According to the second variant, the initiation in step e) occurs after the hottest region of the bonding structure 5 has reached a temperature below 150 °C. In this case, between the end of the annealing in step d) and the time of initiating the cleavage wave, the bonding structure 5 is maintained in a controlled atmosphere, which will require the bonding structure 5 to reach a highest temperature between 150 °C and 250 °C. In this case, the "controlled atmosphere" means a dry atmosphere with a humidity less than 0.1%. For example, when the temperature of the annealing in step d) rises to 400 °C and then drops to 150 °C and then the bonding structure 5 is returned to the ambient temperature, the structure is maintained in a dry atmosphere until the cleavage wave is initiated in step e).

[0075] Advantageously, for any of the variants mentioned above, applying a stress to the buried weakening plane 2 corresponds to applying a local mechanical load to the bonding structure 5, in particular to the periphery of the structure, to initiate the cleavage wave. For example, a local mechanical load can be applied by inserting a wedge 10 between the beveled edges of the donor substrate 1 and the carrier substrate 4 of the bonding structure 5 at the bonding interface 7 of the bonding structure 5. This results in the generation of tensile strain in the buried weakening plane 2.

[0076] Preferably, in step e), when there is a temperature gradient on the bonding structure 5, a local mechanical load is applied in the region of the bonding structure 5 that experiences a lower temperature. This region is hereinafter referred to as the "cold region" compared to the hottest region of the bonding structure 5 described above.

[0077] Returning to the case of the first variant example above, when one or more bonding structures 5 exit the heat treatment apparatus 20 where annealing has been carried out, there is usually a thermal gradient on each bonding structure 5 ( Figure 5 and Figure 6) This gradient is generally due to the geometry of the oven and the presence of the system for holding the bonding structure 5, which affects heat dissipation. For example, in the case of an oven 20 in a horizontal configuration, where the bonding structure 5 is placed vertically in a container 22 carried by a loading shovel 21, it can be observed that the lower region B of the bonding structure 5 (i.e., the region closest to the loading shovel 21) is colder than the upper region H of the bonding structure 5. Then, a local mechanical load is preferably applied in the lower region B of the bonding structure 5.

[0078] Advantageously again, when the temperature gradient is about 80 °C, i.e., 80 °C ± 15 °C, a local mechanical load is applied to the bonding structure 5. Figure 6 The temperature gradients measured after the bonding structure 5 is removed from the oven are shown in the upper region H, the central region C, and the lower region B.

[0079] The applicant has observed that a temperature gradient of 80 °C ± 10 °C experienced by the bonding structure 5 when the splitting wave is initiated helps to improve the thickness uniformity of the useful layer 3 after transfer.

[0080] Exemplary applications:

[0081] The transfer method according to the present invention can be used to produce SOI substrates, the useful layer 3 of which is very thin, especially between a few nanometers and 50 nanometers.

[0082] The examples used are a donor substrate 1 and a carrier substrate 4 made of single crystal silicon, each in the form of a 300 mm diameter wafer. The donor substrate is covered with a silicon oxide layer 6 having a thickness of 50 nm. A buried weakening plane 2 is formed in the donor substrate 1 by co-injecting hydrogen and helium ions under the following conditions:

[0083] · H: injection energy 38 keV, dose 1E16 H / cm 2 ;

[0084] · He: injection energy 25 keV, dose 1E16 He / cm 2 .

[0085] The buried weakening layer 3 is located at a depth of about 290 nm from the surface 1a of the donor substrate 1. Together with the oxide layer 6, it defines a useful layer 3 of about 240 nm.

[0086] The donor substrate 1 is directly bonded to the carrier substrate 4 by molecular adhesion to form the bonding structure 5. Before bonding, the donor substrate 1 and the carrier substrate 4 will undergo known surface activation and / or cleaning treatments to ensure the quality of the bonding interface 7 in terms of defects and bonding energy.

[0087] An oven 20 with a horizontal configuration is used to perform batch annealing of the multiple bonding structures 5 described above. This type of heat treatment apparatus 20 includes a loading shovel 21 that carries a container 22 in which the bonding structure 5 is disposed ( Figure 7 ). The loading shovel 21 moves between an entry position and an exit position. At the entry position, the bonding structure 5 is inside the oven 20, and at the exit position, the bonding structure 5 is outside the oven 20.

[0088] The wedge system 10 can be positioned on each container 22 below the bonding structure 5. The loading shovel 21 moves to the entry position where annealing is to be performed. Annealing includes a temperature rise from 200 °C to 380 °C, holding at 380 °C for two minutes, and a temperature drop to 225 °C.

[0089] When annealing is completed, the loading shovel 21 moves to its exit position.

[0090] As Figure 6 illustrated, once each bonding structure 5 has left the oven 20, it undergoes a temperature decrease. Each bonding structure 5 will move to an exit area 23 where it will experience a maximum temperature (in its hottest area) between 150 °C and 250 °C. In this exit area 23, a pressing device 11 located above the bonding structure 5 will successively apply a pressing on each bonding structure 5 such that the wedge 10 below it will be inserted between the chamfered edges of the bonding substrate of the bonding structure 5 at the bonding interface 7 ( Figure 7 ). The insertion of the wedge-shaped member 10 creates local tensile strain at the location of the buried weakening surface 2, thereby causing a splitting wave to be successively initiated in each bonding structure 5 as it passes under the pressing device 11.

[0091] Of course, tools other than the assembly formed by the wedge system 10 and the pressing device 11 can also be used to initiate a splitting wave in the bonding structure 5 according to the present invention.

[0092] Thus, for each bonding structure 5, a splitting wave is initiated when the structure is subjected to a maximum temperature between 150 °C and 250 °C, preferably about 200 °C. In the Figure 6 example, the exit area 23 where the splitting wave is initiated corresponds to the area where each bonding structure 5 experiences a maximum temperature of about 200 °C in its hottest area (top area H), an intermediate temperature of about 180 °C in its central area C, and a temperature of about 130 °C in its bottom area B. In the Figure 7 illustrated example, since the wedge 10 is located at the bottom of each bonding structure 5, initiation further occurs in the cold area (bottom area B: which experiences the lowest temperature) of the bonding structure 5.

[0093] After the self-aligned continuous propagation of the split wave, after the transfer, the SOI substrate (transferred component 5a) and the remaining part 5b of the donor substrate 1 are obtained. For the useful layer 3 to be transferred, a very high thickness uniformity is obtained.

[0094] The trimming steps applied to the transferred component 5a include a chemical cleaning operation and at least one high-temperature smoothing heat treatment. After completion of these steps, the SOI substrate includes a useful layer 3 with a thickness of 50 nm, and the final non-uniformity of its thickness is about 0.45 nm. It should be noted that, by comparison, an SOI substrate having a useful layer 3 including regular or irregular patterns after splitting will exhibit a final thickness non-uniformity greater than or equal to 0.7 nm.

[0095] Of course, the present invention is not limited to the described implementations and examples, and variant embodiments can be introduced therein without departing from the scope of the present invention defined by the claims.

Claims

1. A transfer method for transferring a useful layer (3) onto a carrier substrate (4), the transfer method comprising the following steps: a) Providing a donor substrate (1) including an embedded weakening plane (2), the useful layer (3) being defined by a front face (1a) of the donor substrate (1) and the embedded weakening plane (2); b) Providing a carrier substrate (4); c) Bonding the donor substrate (1) to the carrier substrate (4) along a bonding interface (7) from the front face (1a) of the donor substrate (1) to form a bonding structure (5); d) Annealing the bonding structure (5) without inducing a cleavage wave in the embedded weakening plane (2) by applying a weakening thermal budget to the bonding structure (5) and bringing the embedded weakening plane (2) to a defined degree of weakening, the weakening thermal budget being below a cleavage thermal budget that would cause spontaneous induction of the cleavage wave in the embedded weakening plane (2) during the annealing; e) After annealing the bonding structure (5), inducing the cleavage wave in the embedded weakening plane (2) by applying a stress to the bonding structure (5), the cleavage wave propagating along the embedded weakening plane (2) in a self-sustaining manner to cause the useful layer (3) to be transferred to the carrier substrate (4); The transfer method is characterized in that: The cleavage wave is induced when the bonding structure (5) is subjected to a temperature between 150 °C and 250 °C at least in its hottest region.

2. The transfer method according to claim 1, wherein, The temperature is between 180 °C and 220 °C.

3. The transfer method according to claim 1, wherein, The annealing in step d) reaches a maximum holding temperature between 300 °C and 600 °C.

4. The transfer method according to claim 1, wherein, The weakening thermal budget is between 40% and 95% of the cleavage thermal budget, and the cleavage thermal budget causes spontaneous induction of the cleavage wave in the embedded weakening plane (2) during the annealing.

5. The transfer method according to claim 1, wherein The induction in step e) is carried out directly after the annealing in step d) and before the hottest region of the bonding structure (5) reaches a temperature below 150 °C.

6. The transfer method according to claim 1, wherein, The annealing in step d) is carried out in a heat treatment device (20) in a horizontal or vertical configuration, the heat treatment device (20) being suitable for batch processing of a plurality of bonding structures (5), and wherein the induction in step e) is carried out when the plurality of bonding structures (5) leave the device (20).

7. The transfer method according to claim 1, wherein, Applying a stress to the embedded weakening plane (2) corresponds to applying a local mechanical load to the bonding structure (5).

8. The transfer method according to claim 7, wherein Applying a stress to the embedded weakening plane (2) corresponds to applying a local mechanical load to the periphery of the bonding structure (5).

9. The transfer method according to claim 7, wherein The local mechanical load is applied by inserting a wedge (10) between the beveled edges of the donor substrate (1) and the carrier substrate (4) of the bonding structure (5) at the bonding interface (7) of the bonding structure (5).

10. The transfer method according to claim 7 or 9, wherein, In step e), when there is a temperature gradient on the bonding structure (5), the local mechanical load is applied in the region of the bonding structure (5) that experiences a lower temperature.

11. The transfer method according to claim 10, wherein, When the temperature gradient is 80 °C, the local mechanical load is applied to the bonding structure (5).

12. The transfer method according to claim 1, wherein, The donor substrate (1) and the carrier substrate (4) are made of single crystal silicon, and wherein the buried weakening plane (2) is formed by implanting light matter ions into the donor substrate (1), and the light matter is selected from hydrogen and helium, or a combination of hydrogen and helium.

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