Method of transferring a useful layer to a carrier substrate

Abstract

The invention relates to a method for transferring a useful layer onto a carrier substrate, comprising the following steps: a) providing a donor substrate comprising a buried-weakness plane, the useful layer being delimited by the front face of the donor substrate and the buried-weakness plane; b) providing a carrier substrate; 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; d) annealing the bonded structure to increase the degree of weakening of the buried-weakness plane, the transfer method being characterized in that: - during the annealing step d), a predetermined stress is applied to the buried-weakness plane for a period of time, the predetermined stress being chosen to induce a splitting wave when a given degree of weakening is reached; - at the end of this period of time, the given degree of weakening having been reached, the predetermined stress causes a splitting wave to be induced and to propagate self-sustained along the buried-weakness plane, resulting in the transfer of the useful layer to the carrier substrate.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of microelectronics. In particular, the present invention relates to a method for transferring a useful layer onto a carrier substrate. BACKGROUND

[0002] Figure 1 The method for transferring a useful layer 3 onto a carrier substrate 4 is known from the prior art; it is described in particular in documents WO 2005043615 and WO 2005043616, which comprises the following steps:

[0003] • forming a buried weakening plane 2 in the donor substrate 1 by implanting a light substance in the donor substrate 1, so as to form 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] • subjecting the bonded structure 5 to a thermal treatment 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 plane 2, the splitting wave propagating in a self-sustained manner along said buried weakening plane 2 in the donor substrate 1.

[0007] In this method, the substance implanted at the level of the buried weakening layer 2 initiates the formation of microcavities. The weakening thermal treatment has the effect of promoting the growth and pressurization of these microcavities. By applying an additional external force (energy pulse) after the thermal treatment, a splitting wave is initiated in the buried weakening plane 2, which 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 makes it possible in particular to reduce the surface roughness after transfer.

[0008] This method can be used to manufacture a silicon-on-insulator (SOI) substrate. In this case, the donor substrate 1 and the carrier substrate 4 are each formed from a silicon wafer, the standard diameter of which is generally 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] The SOI substrate must comply with very strict specifications. This is particularly true for the average thickness and thickness uniformity of the useful layer 3. Compliance with these specifications is necessary for the correct operation of the semiconductor devices formed in and on this useful layer 3.

[0010] In some cases, the structure of these semiconductor devices requires the provision of an SOI substrate provided with a useful layer 3 having a very low average thickness (for example lower than 50 nm) and a very high thickness uniformity. The thickness uniformity desired can be at most about 5%, corresponding to a variation typically from + / - 0.3 nm to + / - 1 nm over the entire surface of the useful layer 3. Even if additional finishing steps, such as etching or surface-smoothing thermal treatments, are carried out after the transfer of the useful layer 3 to the carrier substrate 4, it is important that the topographical surface properties after the transfer are as favourable as possible to ensure that the final specifications are met.

[0011] The Applicant has observed that the useful layer 3 transferred according to the aforementioned method, produced from a bonded structure prepared under similar conditions and subjected to the same weakening thermal treatment, does not show topographical surface properties (roughness, thickness uniformity) that are reproducible from wafer to wafer. The non-reproducibility of the topographical surface properties of the useful layer after the transfer can affect the production yield, since the finishing steps are not always successful in bringing the roughness and the thickness uniformity of all the useful layers to the required levels of specification.

[0012] In order to limit the amplitude of the regular pattern of thin layer thickness variations after the splitting, the document EP2933828 proposes bringing the assembly to be split into contact with an absorbing element to dissipate the acoustic vibrations emitted during the splitting wave initiation and self-sustained propagation.

[0013] Object of the invention

[0014] The present invention relates to a method for transferring a useful layer onto a carrier substrate. The aim of the method is to obtain, after the transfer, a low degree of surface roughness and a high degree of thickness uniformity of the useful layer, and to improve the wafer-to-wafer reproducibility of the topographical surface properties of the transferred useful layer.

[0015] Summary of the invention

[0016] The present invention relates to a method for transferring a useful layer onto a carrier substrate, said method comprising the following steps:

[0017] a) providing a donor substrate comprising a buried-weakened face, the useful layer being delimited by the front face of the donor substrate and by the buried-weakened face;

[0018] b) providing a carrier substrate;

[0019] 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;

[0020] d) annealing the bonded structure to increase the degree of weakening of the buried-weakened face.

[0021] The transfer method is characterised in that:

[0022] - in the annealing step d), a predetermined stress is applied to the buried-weakened face for a time period, the predetermined stress being chosen to induce a splitting wave when a given weakening degree is reached;

[0023] - at the end of the time period, the given weakening degree has been reached, the predetermined stress causing the splitting wave to be induced and to propagate self-sustained along the buried-weakened face, resulting in the transfer of the useful layer to the carrier substrate.

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

[0025] - the time period is between one minute and five hours;

[0026] - the time period is between 1% and 100% of the annealing duration;

[0027] - the transfer method is applied to a batch of bonded structures, wherein the predetermined stress is applied to the buried-weakened face of each bonded structure so that a splitting wave is induced when a given weakening degree is reached for each bonded structure;

[0028] - the annealing in step d) is performed in a heat treatment device in horizontal or vertical configuration, the heat treatment device being adapted to batch process a plurality of bonded structures;

[0029] - the predetermined stress is locally applied to the buried-weakened face of the bonded structure by a wedge positioned at the bonding interface and exerting a pressing force on the bevelled edges of the donor substrate and the carrier substrate of the bonded structure so that a tensile strain is generated in the buried-weakened face;

[0030] - the pressing force is between 0.5 N and 50 N;

[0031] - the given weakening degree is defined by the area occupied by the microcavities in the buried-weakened face and is chosen to be between 1% and 90%, preferably between 5% and 40%;

[0032] - the annealing in step d) reaches a maximum temperature between 300°C and 600°C;

[0033] - the predetermined stress is applied from the beginning of the annealing in step d);

[0034] - the donor substrate and the carrier substrate are made of monocrystalline silicon, and wherein the buried-weakened face is formed by implanting light mass ions into the donor substrate, the light mass being chosen from the group consisting of hydrogen and helium, or a combination of hydrogen and helium. BRIEF DESCRIPTION OF DRAWINGS

[0035] Other features and advantages of the application will appear from the following detailed description of the application, given with reference to the attached drawings, in which:

[0036] Figure 1Method for transferring a thin film according to the prior art is shown;

[0037] Figure 2 A transfer method according to the application is shown;

[0038] Figure 3 An example of batch processing of multiple structures in a transfer method according to the application is shown. DETAILED DESCRIPTION

[0039] In the description, the same reference signs can be used in the drawings for the same type of elements. The drawings are schematic representations for clarity and are not drawn to scale. In particular, the thickness of the layers along the z-axis is not to scale with respect to the lateral dimensions along the x-axis and y-axis; the relative thickness of the layers with respect to each other is not necessarily relative in the drawings. It should be noted that, Figure 1 The coordinate system (x, y, z) of Figure 2 .

[0040] The present application 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 of the carrier substrate can vary according to the type of component targeted and the target application. Since silicon is the most commonly used semiconductor material at present, the useful layer and the carrier substrate can in particular be made of monocrystalline silicon, but are of course not limited to this material.

[0041] The transfer method according to the application first comprises a step a) of providing a donor substrate 1 from which the useful layer 3 will be taken. The donor substrate 1 comprises a buried-weakness plane 2 Figure 2 of a)). The buried-weakness plane 2 is advantageously formed by implanting light-matter ions into the donor substrate 1 at a defined depth. The light-matter is preferably chosen from among hydrogen and helium, or a combination of hydrogen and helium, since these substances promote the formation of microcavities around a defined implantation depth, resulting in the buried-weakness plane 2. The useful layer 3 is delimited by the front face la of the donor substrate 1 and the buried-weakness plane 2.

[0042] The donor substrate 1 can be formed of at least one material chosen from among silicon, germanium, silicon carbide, IV-IV, III-V or II-VI semiconductor compounds and piezoelectric materials (e.g. LiNb03, LiTa03, etc.). It can also comprise one or more surface layers, which can have arbitrary properties, such as a dielectric, disposed on its front face la and / or back face lb.

[0043] The transfer method also comprises a step b) of providing a carrier substrate 4 Figure 2 of b)).

[0044] The carrier substrate can be formed, for example, from at least one material chosen from silicon, silicon carbide, glass, sapphire, aluminum nitride or any other material that can be obtained in the form of a substrate. It can also comprise one or more surface layers having any properties, for example dielectric.

[0045] As mentioned above, one advantageous application of the transfer method according to the application is the production of SOI substrates. In this particular case, the donor substrate 1 and the carrier substrate 4 are made of monocrystalline silicon and one or both of the substrates comprise a silicon oxide surface layer 6 on their front face.

[0046] The transfer method next comprises a step c) of bonding the donor substrate 1 to the carrier substrate 4 along the bonding interface 7 from the front face la of the donor substrate 1 to form a bonded structure 5 Figure 2 of c)).

[0047] This attachment operation can be performed using any known method, in particular by direct bonding by molecular adhesion, by hot pressing or by electrostatic bonding. These known prior art will not be described in detail herein. However, it is recalled that, prior to bonding, the donor substrate 1 and the carrier substrate 4 will undergo a surface activation and / or cleaning treatment to ensure the quality of the bonding interface 7 in terms of defects and bonding energy.

[0048] In the transfer method according to the application, a step d) of annealing the bonded structure 5 is then performed so that the degree of weakening of the buried weakened face 2 is increased. The temperature range in which the annealing can be performed for the operation of weakening the buried face 2 mainly depends on the type of the bonded structure 5 (homostructure or heterostructure) and the properties of the donor substrate 1.

[0049] By way of example, in the case of a donor substrate 1 and a carrier substrate 4 made of silicon, the annealing in step d) reaches a maximum temperature generally between 200°C and 600°C, advantageously between 300°C and 500°C and even more advantageously between 350°C and 450°C.

[0050] The annealing can comprise a temperature rise (generally between 200°C and the maximum temperature) and a holding at the maximum temperature. Generally, the duration of such an annealing will be between a few tens of minutes and a few hours, depending on the maximum temperature of the annealing. The pair time / temperature determines the thermal budget applied to the bonded structure 5 during the annealing. The degree of weakening of the buried weakened face 2 is defined by the area occupied by the microcavities present in the buried weakened layer 2. In the case where the donor substrate 1 is made of silicon, the area occupied by the microcavities can be characterized by infrared microscopy.

[0051] Depending on the thermal budget applied to the joint structure 5 during annealing, the degree of weakening can range from low (<1%, below the detection threshold of the characterization instrument) to greater than 80%. The weakening thermal budget is always kept below the splitting thermal budget that allows for the spontaneous generation of splitting waves in the embedded weakened surface 2 during annealing.

[0052] Recall that in the prior art transfer method mentioned in the introduction, when the buried weakened surface 2 exhibits a certain degree of weakening, the bonding structure 5 is removed after the annealing step. An energy pulse is then applied to the buried weakened surface 2 to induce a splitting wave: the propagation of the splitting wave causes the useful layer 3 to be transferred to the carrier substrate 4. As stated above, the applicant has recognized that even if the steps in this method are performed under the same conditions, there are problems with the reproducibility of the morphological surface properties of the useful layer 3 after transfer.

[0053] This lack of reproducibility is particularly due to the variability of the steps involved in injecting the light material to form the embedded weakening layer and the annealing process. This variability may stem from inhomogeneities in the injection dosage or energy, or from temperature inhomogeneities across the structure or multiple structures. Therefore, for the same thermal budget, variations in the degree of weakening during annealing can differ for similar bonded structures annealed in batches or individually.

[0054] To overcome this problem, the transfer method according to the present invention envisions applying a predetermined stress to the embedded weakened layer 2 for a certain period of time during the annealing step d). Figure 2 (d) "Predetermined stress" should be understood as a stress with a defined and constant magnitude. The predetermined stress can be applied, in particular, by applying a controlled mechanical load to the joint structure 5, which will be described in more detail below.

[0055] The predetermined stress is selected such that when a given degree of weakening is achieved in the buried weakened surface 2, a splitting wave exhibiting self-sustaining propagation is induced. Self-sustaining propagation means that once induced, the splitting wave propagates spontaneously throughout the entire range of the buried weakened surface 2 without the application of additional stress, causing the useful layer 3 to completely separate from the donor substrate 1 and transfer to the carrier substrate 4.

[0056] The time period during which the predetermined stress is applied to the embedded weakened surface 2 is typically longer than one minute. In particular, it ranges from one minute to five hours. In other words, this time period is a fraction of the annealing duration, ranging from 1% to 100%.

[0057] At the end of this time period, a given weakening level is reached: then, a predetermined stress causes a splitting wave to propagate self-sustainingly along the buried weakened surface 2, resulting in the transfer of the useful layer 3 to the carrier substrate 4. Figure 2e)). In the transfer method according to the application, the initiation of the splitting wave in the buried-weakened plane 2 does not occur simultaneously with the application of the predetermined stress to said plane 2. The splitting wave is initiated by the predetermined stress only when the buried-weakened plane reaches a given weakening degree.

[0058] The application of the predetermined stress to the bonded structure 5 in this way for a certain period of time in the annealing step d) allows the initiation of the splitting wave in the buried-weakened plane 2 when a given, constant and reproducible weakening degree is reached. Thus, the splitting wave is no longer initiated with a constant thermal budget (as in the transfer method of the prior art described above) but with a constant weakening degree of the buried-weakened plane 2. Even in the presence of variations in the implantation or annealing conditions between the bonded structures 5 processed in the same batch or in different batches, the same predetermined stress applied to the buried-weakened plane 2 of each structure 5 will initiate the splitting wave for one and the same given weakening degree at the end of the specific period of time for each structure 5. This makes it possible to ensure a high reproducibility of the morphological surface properties of the useful layer 3, which are very dependent on the weakening degree of the buried-weakened plane 2 at the initiation and propagation of the splitting wave.

[0059] According to a first variant, the predetermined stress is applied to the buried-weakened plane 2 from the start of the annealing step d) : thus, the period of time (during which the predetermined stress is applied to the buried-weakened plane 2) lasts from the start of the annealing (or possibly before the annealing) until the given weakening degree is reached, at which the splitting wave is initiated.

[0060] According to a second variant, the predetermined stress is applied after a predetermined annealing duration, without interrupting said annealing. This variant can promote the consolidation of the bonding interface 7 of the bonded structure 5 at the start of the annealing, before the predetermined stress is applied to the buried-weakened plane 2. In this case, the period of time lasts from the middle of the annealing period until the given weakening degree is reached, at which the splitting wave is initiated.

[0061] According to the method of the application, the predetermined stress is chosen according to the weakening degree required for the propagation of the splitting wave. A large stress will allow a low degree of weakening of the buried-weakened plane 2, initiating the splitting wave; a smaller stress will allow a higher degree of weakening of the buried-weakened plane 2, initiating the splitting wave. The given weakening degree is defined by the area occupied by the microcavities in the buried-weakened plane 2 and can be chosen to be between 1% and 90%, preferably between 5% and 40%. A relatively low degree of weakening, for example lower than 25%, promotes a reduction in the surface roughness and an increase in the thickness uniformity of the transferred useful layer 3.

[0062] Advantageously, the transfer method is applied to the batch processing of a plurality of bonded structures 5, in which a predetermined stress is applied to the buried-weakened face 2 of each bonded structure 5, so that the splitting wave is triggered when a given weakening degree has been reached for each bonded structure 5. In this case, the annealing in step d) can be performed in a heat treatment device in horizontal or vertical configuration, adapted to batch process a plurality of bonded structures 5.

[0063] Taking into account the variations of implantation or annealing conditions between the bonded structures 5, the period of time during which the predetermined stress is applied to the buried-weakened face 2 and the splitting wave is triggered at the end for each bonded structure 5 can be longer or shorter: in particular, the buried-weakened faces 2 will not all reach the given weakening degree for which the applied predetermined stress would simultaneously cause the triggering. The duration of the annealing is defined so as to take these variations into account and allow the triggering and self-sustained propagation of the splitting wave in the buried-weakened face 2 for all the bonded structures 5. Then, for a given weakening degree, i.e. at a constant and reproducible degree, each bonded structure 5 will undergo the splitting of its buried-weakened face 5.

[0064] The transfer method according to the application allows selecting the weakening degree at which the splitting wave will propagate and ensures that said wave is triggered at a constant weakening degree for all the bonded structures 5: this allows obtaining advantageous morphological surface properties (low roughness \ high uniformity and wafer-to-wafer reproducibility) for the useful layer 3 of the transfer.

[0065] According to one advantageous embodiment, the predetermined stress is locally applied to the buried-weakened face 2 by means of a point mechanical load applied to the bonded structure 5 by means of a wedge 10. The wedge 10 is positioned at the bonding interface 7 and exerts a pressing force on the bevelled edges of the donor substrate 1 and of the carrier substrate 4 of the bonded structure 5. This results in the creation of a tensile strain in the buried-weakened face 2. The magnitude of said pressing force is predetermined and constant. For example, the pressure can be between 0.5 N and 50 N.

[0066] Exemplary Applications:

[0067] The transfer method according to the application can be used to produce SOI substrates whose useful layer 3 is very thin, in particular between a few nanometres and 50 nanometres.

[0068] The examples used are donor substrates 1 and carrier substrates 4 made of monocrystalline silicon, each in the form of a 300 mm diameter wafer. The donor substrate 1 is covered with a silicon oxide layer having a thickness of 50 nm. The buried-weakened face 2 is formed in the donor substrate 1 by co-implantation of hydrogen and helium ions under the following conditions:

[0069] • H: implantation energy 38 keV, dose 1 E16 H / cm 2 ;

[0070] • He: implantation energy 25 keV, dose 1 E16 He / cm 2 .

[0071] The buried weakening layer 2 is located at a depth of about 290 nm from the surface of the donor substrate 1. It defines, with the oxide layer 6, a useful layer 3 of about 240 nm.

[0072] The donor substrate 1 is bonded directly to the carrier substrate 4 by molecular adhesion to form a bonded structure 5. Prior to 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.

[0073] A batch annealing of a plurality of bonded structures 5 as described above is performed using an oven 20 of horizontal configuration. This type of thermal treatment device 20 comprises a charging shovel 21 carrying containers 22 in which the bonded structures 5 are arranged Figure 3 ). The charging shovel 21 moves between an entry position, in which the bonded structures 5 are inside the oven 20, and an exit position, in which the bonded structures 5 are outside the oven 20.

[0074] A wedge system 10 is located on each container 22, below or above the bonded structure 5, to exert a constant point pressing force on the bevelled edge of the bonded substrates of each bonded structure 5.

[0075] It should be noted that, in the case where the wedge 10 is located below the bonded structure 5, the weight of each bonded structure can constitute said pressure. Alternatively, an additional device 11 can be provided to locally exert an additional pressure on the edge and above the bonded structure 5.

[0076] The mechanical load exerted by the wedge system 10 (with or without additional device 11) on the bonded structure 5 generates a predetermined local tensile strain in the buried weakening plane 2. The mechanical load can be applied from the start of the annealing or after a determined duration. This determined duration is always much shorter than the duration required to reach a given weakening degree at which the predetermined stress will cause the initiation of a splitting wave.

[0077] For an annealing with a maximum temperature of 350°C, the spontaneous splitting occurs on average after 200 minutes.

[0078] When a weight of 500 g (corresponding to a pressing force of about 5 N) is applied to each bonded structure 5 by the additional device 11, the initiation of a splitting wave occurs on average after 160 minutes for a weakening degree of about 16%.

[0079] When a weight of 1500 g (corresponding to an extrusion force of about 15 N) is applied to each joining structure 5 by the additional device 11, the initiation of a splitting wave occurs, on average, after 110 minutes for a weakening degree of about 12%.

[0080] After the self-sustained propagation of the splitting wave in each joining structure 5, the remaining portion 5b of the donor substrate and the SOI substrate (the transferred assembly 5a) are obtained after transfer.

[0081] For the two examples described above, which initiate a splitting wave at constant maturity, the morphological surface properties of the transferred useful layer 3 are obtained, which are very advantageous (low roughness, high uniformity) and are reproducible from wafer to wafer.

[0082] The finishing steps applied to the transferred assembly 5a include a chemical cleaning operation and at least one high-temperature smoothing heat treatment. After completion of these steps, the SOI substrate comprises a useful layer 3 having a thickness of 50 nm, a non-uniformity lower than 2% and a surface roughness lower than 0.3 nm RMS.

[0083] Of course, the present application is not limited to the described implementations and examples, and variant embodiments can be introduced therein without departing from the scope of the present application as 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) Provide a donor substrate (1) including a buried weakening surface (2), wherein the useful layer (3) is defined by the front side (1a) of the donor substrate (1) and the buried weakening surface (2); b) Provide a carrier substrate (4); c) The donor substrate (1) is bonded to the carrier substrate (4) along the bonding interface (7) by the front side (1a) of the donor substrate (1) to form a bonding structure (5); as well as d) Anneal the joint structure (5) to increase the degree of weakening of the embedded weakened surface (2); The transfer method is characterized by: - In the annealing step d), a predetermined stress is applied to the embedded weakened surface (2) for a certain period of time, wherein the predetermined stress is selected to induce a splitting wave when a given degree of weakening is reached; - At the end of the time period, the given weakening level has been reached, and the predetermined stress causes the splitting wave to be triggered and self-sustainingly propagate along the buried weakening surface, resulting in the transfer of the useful layer (3) to the carrier substrate (4). The given degree of weakening is defined by the area occupied by the microcavity in the embedded weakening surface (2). The transfer method is applied to the batch processing of multiple joint structures (5), wherein the predetermined stress is applied to the embedded weakening surface (2) of each of the multiple joint structures (5) such that the splitting wave is triggered when the given weakening degree is reached for each joint structure (5).

2. The transfer method according to claim 1, wherein, The time period is between one minute and five hours.

3. The transfer method according to claim 1, wherein, The time period is a portion of the annealing duration that is between 1% and 100%.

4. The transfer method according to claim 1, wherein, The annealing in step d) is carried out in a heat treatment apparatus (20) with a horizontal or vertical structure, which is suitable for batch processing of multiple joint structures (5).

5. The transfer method according to claim 1, wherein, The predetermined stress is locally applied to the embedded weakened surface (2) of the joint structure (5) by a wedge (10) positioned at the joint interface (7) and applying pressure to the chamfered edges of the donor substrate (1) and the carrier substrate (4) of the joint structure (5), so that tensile strain is generated in the embedded weakened surface (2).

6. The transfer method according to claim 5, wherein, The extrusion pressure is between 0.5N and 50N.

7. The transfer method according to claim 1, wherein, The given degree of weakening was selected to be between 1% and 90%.

8. The transfer method according to claim 1, wherein, The given degree of weakening was selected to be between 5% and 40%.

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

10. The transfer method according to claim 1, wherein, The predetermined stress is applied from the start of annealing in step d).

11. The transfer method according to claim 1, wherein, The donor substrate (1) and the carrier substrate (4) are made of monocrystalline silicon, and wherein the buried weakening surface (2) is formed by implanting light material ions into the donor substrate, the light material being selected from hydrogen and helium, or a combination of hydrogen and helium.

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