Process for transferring a useful layer onto a carrier substrate - Patent application

Abstract

The invention relates to a process for transferring a useful layer to a carrier substrate, comprising the steps of: a) providing a donor substrate including an embedded weakened plane, the useful layer being bounded by the front surface of the donor substrate and the embedded weakened plane; b) providing a carrier substrate; c) bonding the donor substrate using its front surface to the carrier substrate along a bonding interface to form a bonded structure; and d) annealing the bonded structure, wherein in the annealing step d) a predetermined pressure is applied to the embedded weakened plane for a period of time, the predetermined pressure being selected to initiate a splitting wave once a given level of weakening is reached, and at the end of the period, the given level of weakening is reached and the predetermined pressure causes the initiation and self-sustaining propagation of the splitting wave along the embedded weakened plane, and the useful layer is transferred to the carrier substrate.

Description

[Technical field]

[0001] The present invention relates to the field of microelectronics, in particular to a process for transferring a useful layer onto a carrier substrate. [Background technology]

[0002] A process for transferring a useful layer 3 onto a carrier substrate 4, shown in FIG. 1, is known from the prior art and is described in particular in documents WO 2005043615 and WO 2005043616, which comprises the following steps: - forming a buried weakened plane 2 by implanting light element species into a donor substrate 1 so as to form a useful layer 3 between the buried weakened plane and the surface of the donor substrate; Next, bonding the donor substrate 1 to a carrier substrate 4 to form a laminated structure 5; applying a heat treatment to the bonded structure 5 in order to weaken the embedded weakened plane; and finally, inducing a splitting wave by means of an energy pulse applied at the level of the buried weakened plane 2, with a self-sustained propagation of the splitting wave in the donor substrate 1 along said buried weakened plane 2. Includes.

[0003] In this process, elemental species implanted at the level of the buried weakened plane 2 induce the development of microcavities. The weakening heat treatment has the effect of promoting the growth and compression of these microcavities. By applying an additional external force (energy pulse) after the heat treatment, splitting waves are induced in the buried weakened plane 2, said waves propagate in a self-sustaining manner, resulting in the transfer of the useful layer 3 through the separation at the level of the buried weakened plane 2. Such a process makes it possible in particular to reduce the roughness of the surface after transfer.

[0004] This process may be used to fabricate silicon-on-insulator (SOI) substrates. In this case, the donor substrate 1 and the carrier substrate 4 are each formed from a silicon wafer, with the standard diameter of the substrate being typically 200 mm, 300 mm, or for later generations, 450 mm. Either the donor substrate 1 or the carrier substrate 4, or both, are surface oxidized.

[0005] SOI substrates must comply with very strict specifications, especially with regard to the average thickness and thickness uniformity of the useful layer 3. Compliance with these specifications is necessary for the semiconductor devices that will be formed in and on this useful layer 3 to function properly.

[0006] In some cases, the architecture of these semiconductor devices requires the placement of SOI substrates that exhibit a very small average thickness of the useful layer 3, for example less than 50 nm, and that exhibit a very high uniformity of the useful layer 3. The expected uniformity of the thickness can be at most about 5%, corresponding to a typical variation of + / - 0.3 nm to + / - 1 nm over the entire surface of the useful layer 3. Even if additional finishing steps, such as an etch or a surface smoothing heat treatment, are performed after the useful layer 3 has been transferred to the carrier substrate 4, it is important for the morphological surface properties to be as good as possible after the transfer to ensure that the final specifications are met.

[0007] The applicant has observed that useful layers 3 transferred according to the above process, obtained from bonded structures prepared under similar conditions and subjected to the same weakening heat treatment, do not exhibit reproducible morphological surface characteristics (roughness, thickness uniformity) from one wafer to the next. The non-reproducibility of morphological surface characteristics after transfer can affect the manufacturing yield, since the finishing steps are not always sufficient to bring all the roughness and thickness uniformity of the useful layer up to the required specification level. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention relates to a process for transferring a useful layer onto a carrier substrate, the process aiming at obtaining a low degree of surface roughness and a high degree of thickness uniformity for the transferred useful layer, and at improving the wafer-to-wafer reproducibility of the morphological surface properties of the transferred useful layer. [Means for solving the problem]

[0009] The invention relates to a process for transferring a useful layer onto a carrier substrate, comprising the following steps: a) providing a donor substrate including a buried weakened plane, said useful layer being bounded by a front surface of said donor substrate and said buried weakened plane; b) providing a carrier substrate; c) bonding the donor substrate with a front surface thereof to the carrier substrate along a bonding interface to form a laminated structure; d) annealing the bonded structure to increase the level of weakening of the buried weakened plane; Includes.

[0010] The transfer process includes: a predetermined pressure is applied to the buried weakened plane in an annealing step d) for a period of time, the predetermined pressure being selected to induce a splitting wave once a given level of weakening is reached; At the end of the period, the given level of weakening is reached and the predetermined pressure causes the initiation and self-sustaining propagation of the splitting wave along the embedded weakened plane, resulting in the useful layer being transferred to the carrier substrate. It is worth noting that:

[0011] According to other advantageous, non-limiting features of the invention, taken alone or in any technically feasible combination, the period of time is between 1 minute and 5 hours; said period being a fraction of the duration of said anneal between 1% and 100%, the transfer process is applied to a batch of a plurality of laminated structures, and the predetermined pressure is applied to the embedded weakened plane of each of the laminated structures to induce the split wave once the given level of weakening is reached for each laminated structure; The annealing in step d) is performed in a horizontally or vertically configured thermal processing apparatus suitable for batch processing of a plurality of bonded structures; the predetermined pressure is applied locally to the embedded weakened plane of the bonded structure using a wedge placed at the bonding interface and exerting a pressing force against the chamfered edges of the donor substrate and the carrier substrate of the bonded structure to generate a tensile strain in the embedded weakened plane; The pressing force is between 0.5N and 50N, said given level of weakening being determined by the area occupied by the microcavities in said buried weakened plane and being selected to be between 1% and 90%, preferably between 5% and 40%; d) annealing reaches a maximum temperature between 300° C. and 600° C.; The predetermined pressure is applied from the start of the annealing in step d), The donor substrate and the carrier substrate are made of monocrystalline silicon, and the buried weakened plane is formed by ion implantation of light element species into the donor substrate, the light element species being selected from hydrogen and helium or a combination of hydrogen and helium. To be utilized.

[0012] Other features and advantages of the invention will become apparent from the following detailed description of the invention, which is given with reference to the accompanying figures. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram of a process for transferring a thin layer according to the prior art. [Diagram 2] FIG. 1 is a diagram of a transfer process according to the invention. [Diagram 3] FIG. 13 is an example of batch processing of multiple structures in a transfer process according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In this description, the same reference signs in the figures will be used for the same types of elements. The figures are schematic representations that are not to scale for ease of reading. In particular, the thicknesses of the layers along the z-axis are not to scale relative to the lateral dimensions along the x- and y-axes, and the relative thicknesses of the layers with respect to each other do not necessarily have to be respected in the figures. It should be noted that the coordinate system (x,y,z) of FIG. 1 also applies to FIG. 2.

[0015] The invention relates to a process for transferring a useful layer 3 onto a carrier substrate 4. The useful layer 3 is so named because it is intended for use in the manufacture of components in the field of microelectronics or microsystems. The useful layer and the carrier substrate may vary substantially depending on the type of component intended and the intended application. The useful layer and the carrier substrate may in particular be made of monocrystalline silicon, but are naturally not limited to this material, since silicon is currently the most commonly used semiconductor material.

[0016] The transfer process according to the invention first comprises a step a) of preparing a donor substrate 1 from which a useful layer 3 will be taken. The donor substrate 1 comprises a buried weakened plane 2 (FIG. 2-a)). The buried weakened plane is advantageously formed by ion implantation of light elemental species into the donor substrate 1 at a defined depth. The light elemental species are preferably chosen from hydrogen and helium, or a combination of hydrogen and helium, since these elemental species promote the formation of microcavities near the defined implantation depth, resulting in the buried weakened plane 2.

[0017] The useful layer 3 is delimited by the front surface 1 a of the donor substrate 1 and by the buried weakened plane 2 .

[0018] The donor substrate 1 may be made of any of a variety of materials, including silicon, germanium, silicon carbide, IV-IV, III-V or II-VI semiconductor compounds, and piezo materials (e.g., LiNbO 3 , LiTaO 3 , etc. The donor substrate may further comprise one or more surface layers arranged on its front side 1 a and / or back side 1 b, which may be of any nature, e.g. dielectric.

[0019] The transfer process also includes the step b) of providing a carrier substrate 4 (FIG. 2-b).

[0020] The carrier substrate may be formed from at least one material selected from, for example, silicon, silicon carbide, glass, sapphire, aluminum nitride or any other material possibly available in the form of a substrate. The carrier substrate may also include one or more surface layers of any nature, for example a dielectric.

[0021] As mentioned above, one advantageous application of the transfer process according to the invention 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 either or both of said substrates comprise a surface layer of silicon oxide 6 on the front side of the substrate.

[0022] The transfer process then comprises a step c) of bonding the donor substrate 1 with its front side 1a to a carrier substrate 4 along a bonding interface 7 so as to form a laminated structure 5 (FIG. 2-c)).

[0023] The attachment operation may be carried out using any known method, in particular by direct bonding by molecular adhesion, by thermocompression or by electrostatic bonding. These well-known prior art techniques will not be described in detail here. However, prior to the bonding step, the donor substrate 1 and the carrier substrate 4 may undergo a surface activation and / or cleaning sequence to ensure the quality of the bonding interface 7 with regard to defects and bonding energy.

[0024] In the transfer process according to the invention, a step d) of annealing the bonded structure 5 is then performed in order to increase the level of weakening of the buried weakened plane 2. The range of temperatures at which the annealing may be performed for this operation of weakening the buried plane 2 depends mainly on the type of bonded structure 5 (homostructure or heterostructure) and on the nature of the donor substrate 1.

[0025] By way of example, in the case where the donor substrate 1 and the carrier substrate 4 are made of silicon, the annealing in step d) reaches a maximum temperature that is typically between 200°C and 600°C, advantageously between 300°C and 500°C, and even more advantageously between 350°C and 450°C.

[0026] The anneal may include a temperature ramp-up (typically between 200° C. and a maximum temperature) and a hold at the maximum temperature. Typically, the duration of such an anneal will be between tens of minutes and hours, depending on the maximum temperature of the anneal. The time / temperature pair determines the thermal budget applied to the bonded structure 5 during the anneal.

[0027] The level of weakening of the buried weakened plane 2 is determined by the area occupied by the microcavities present in the buried weakened plane 2. In the case of a silicon donor substrate 1, this area occupied by the microcavities may be characterized by infrared microscopy.

[0028] The level of weakening can range from low levels (<1%, below the detection threshold of the characterization instrument) to more than 80%, depending on the thermal budget applied to the laminate 5 during annealing. The weakening thermal budget is always kept below the splitting thermal budget at which spontaneous initiation of splitting waves in the buried weakening plane 2 is obtained during annealing.

[0029] It is recalled that in the prior art transfer process mentioned in the introduction, the laminated structure 5 is removed after an annealing step when the embedded weakened plane 2 shows a certain level of weakening. An energy pulse is then applied to the embedded weakened plane 2 to induce a splitting wave, the propagation of which leads to the transfer of the useful layer 3 to the carrier substrate 4. As mentioned above, the applicant has identified the problem of reproducibility of the morphological surface properties of the useful layer 3 after transfer, even if the steps in the process are carried out under identical conditions.

[0030] This lack of reproducibility is due in particular to variability in the steps of implanting light element species to form the buried weakened layer and the annealing step. This variability can result from non-uniformity in the implanted dose or energy, or from non-uniformity in temperature across or between structures. For one and the same thermal budget, the variation in the level of weakening via annealing can therefore be different for similar bonded structures annealed in batches or separately.

[0031] To overcome this problem, the transfer process according to the invention provides that in the annealing step d) a predetermined pressure is applied to the embedded weakened layer 2 for a certain period of time (FIG. 2-d). What is understood by "predetermined pressure" is a pressure of a defined and constant magnitude. The predetermined pressure may in particular be applied by applying a controlled mechanical load to the laminated structure 5, as will be explained in greater detail below.

[0032] The predefined pressure is selected such that once a given level of weakening is reached in the embedded weakened plane 2, a splitting wave is initiated which exhibits self-sustained propagation. Self-sustained propagation means that once initiated, the splitting wave propagates by itself, without the application of additional pressure, over the entire extent of the embedded weakened plane 2, so that the useful layer 3 is completely detached from the donor substrate 1 and transferred to the carrier substrate 4.

[0033] The period during which the predetermined pressure is applied to the embedded weakened plane 2 is typically longer than 1 minute. In particular, the period is between 1 minute and 5 hours. In other words, the period is a fraction of the duration of the anneal between 1% and 100%.

[0034] At the end of the period, a given level of weakening has been reached and the predefined pressure then causes the initiation and self-sustaining propagation of a splitting wave along the embedded weakened plane 2, resulting in the transfer of the useful layer 3 to the carrier substrate 4 (FIG. 2-e)). In the transfer process according to the invention, the initiation of a splitting wave in the embedded weakened plane 2 does not occur simultaneously with the application of the predefined pressure to said plane 2. The splitting wave is initiated by the predefined pressure only once the embedded weakened plane has reached a given level of weakening.

[0035] Applying a predefined pressure to the laminated structures 5 in this annealing step d) for a certain period of time allows splitting waves to be initiated in the embedded weakened plane 2 once a given, constant and reproducible level of weakening has been reached. Splitting waves are therefore no longer initiated with a constant thermal budget (as in the prior art transfer process described above), but with a constant level of weakening of the embedded weakened plane 2. Even in the case of variations in the implantation or annealing conditions of a batch and between laminated structures 5 processed in the same batch or in different batches, the same predefined pressure applied to the embedded weakened plane 2 of each structure 5 will initiate splitting waves to one and the same given level of weakening at the end of a period specific to each structure 5. This makes it possible to ensure a high level of reproducibility of the morphological surface properties of the useful layer 3, which depend greatly on the level of weakening of the embedded weakened plane 2 at the time of initiation and propagation of the splitting waves.

[0036] According to a first variant, a predetermined pressure is applied to the embedded weakened plane 2 from the start of the annealing step d), and a period (during which a predetermined pressure is applied to the embedded weakened plane 2) elapses from the start of the annealing (or possibly before) until a given level of weakening has been reached, when the splitting wave is thus initiated.

[0037] According to a second variant, a predefined pressure is applied after a predefined annealing duration, without interrupting said annealing. This variant can promote strengthening of the bonding interface 7 of the laminated structure 5 at the beginning of the annealing, prior to the application of the predefined pressure to the embedded weakening plane 2. In this case, a period of time elapses from the middle of the annealing until a predefined level of weakening is reached, when the splitting wave is initiated.

[0038] According to the process according to the invention, the given pressure is selected according to the level of weakening desired for the propagation of the splitting waves: a high pressure will allow the splitting waves to be initiated for a low level of weakening in the buried weakening plane 2, a low pressure will allow the splitting waves to be initiated for a high level of weakening in the buried weakening plane 2. The given level of weakening is determined by the area occupied by the microcavities in the buried weakening plane 2 and may be selected to be between 1% and 90%, preferably between 5% and 40%. A relatively low level of weakening, for example lower than 25%, promotes a reduction in the surface roughness after transfer and a high degree of thickness uniformity for the transferred useful layer 3.

[0039] The transfer process is advantageously applied for batch processing of a plurality of bonded structures 5, in which a predefined pressure is applied to the embedded weakened plane 2 of each of the bonded structures 5, such that a splitting wave is initiated once a given level of weakening has been reached for each bonded structure 5. In this case, the annealing in step d) may be performed in a horizontally or vertically configured thermal processing apparatus suitable for batch processing of a plurality of bonded structures 5.

[0040] Taking into account variations in the implantation or annealing conditions between the bonded structures 5, the period during which a given pressure is applied to the embedded weakened plane 2 and at the end of which a splitting wave will be initiated may be longer or shorter for each of the bonded structures 5, in particular the embedded weakened planes 2 may not all reach the given level of weakening that the applied given pressure would initiate at the same time. The duration of the annealing is determined to take these variations into account and to allow the initiation and self-sustaining propagation in the embedded weakened plane 2 for all of the bonded structures 5. Each bonded structure 5 will then undergo splitting of the embedded weakened plane 5 for a given level of weakening, i.e. at a constant and reproducible level.

[0041] The transfer process according to the invention allows the level of weakening through which the splitting wave will propagate to be selected and ensures that said wave is induced with a constant level of weakening for all of the laminated structures 5, which allows advantageous morphological surface properties (low roughness, high uniformity and wafer-to-wafer reproducibility) to be obtained for the transferred useful layer 3.

[0042] According to one advantageous embodiment, a predefined pressure is applied locally to the embedded weakened plane 2 by applying a mechanical point load to the laminated structure 5 with a wedge 10. The wedge 10 is placed at the bonding interface 7 and exerts a pressing force against the chamfered edges of the donor substrate 1 and the carrier substrate 4 of the laminated structure 5. This results in a tensile strain being generated in the embedded weakened plane 2. The magnitude of the pressing force is predetermined and constant. By way of example, the pressing force may be between 0.5 N and 50 N.

[0043] Example Applications The transfer process according to the invention may be used for the manufacture of SOI substrates, the useful layer 3 of which is very thin, in particular between a few nm and 50 nm. The example used is one in which the donor substrate 1 and the carrier substrate 4 are made of monocrystalline silicon, each in the form of a 300 mm diameter wafer. The donor substrate 1 is covered with a layer of silicon oxide having a thickness of 50 nm. A buried weakened plane 2 is formed in the donor substrate 1 by co-implantation of hydrogen and helium ions under the following conditions: H: implantation energy 38 keV, dose 1E16 H / cm 2 , He: implantation energy 25 keV, dose 1E16 He / cm 2 .

[0044] The buried weakened layer 2 is located at a depth of about 290 nm from the surface of the donor substrate 1. Together with the oxide layer 6, it defines a useful layer 3 of about 240 nm.

[0045] The donor substrate 1 is bonded to the carrier substrate 4 by direct bonding through molecular adhesion to form a laminated structure 5. Prior to bonding, the donor substrate 1 and the carrier substrate 4 may undergo known surface activation and / or cleaning sequences to ensure the quality of the bonding interface 7 in terms of defects and bonding energy.

[0046] A horizontally configured furnace 20 is used to perform batch annealing of a plurality of bonded structures 5 as described above. This type of heat treatment apparatus 20 includes a charge shovel 21 that carries a cassette 22 in which the bonded structures 5 are placed (FIG. 3). The charge shovel 21 moves between an insertion position where the bonded structures 5 are inside the furnace 20 and an exit position where the bonded structures 5 are outside the furnace 20.

[0047] A system of wedges 10 is mounted in each cassette 22 below or above the laminated structures 5 so as to exert a constant point pressure on the chamfered edges of the joined substrates of each laminated structure 5 .

[0048] It should be noted that in the case where wedges 10 are placed under the laminated structures 5, the weight of each laminated structure will constitute said pressing force. Alternatively, additional devices 11 may be provided to apply additional pressing forces locally at the ends and on top of the laminated structures 5.

[0049] This mechanical load, applied by the wedge system 10 to the laminated structure 5 (with or without the additional device 11), generates a predefined local tensile strain in the embedded weakened plane 2. The mechanical load can be applied from the start of the anneal or after a defined duration, which is always much shorter than the duration required for a given pressure to reach a given level of weakening that would result in the initiation of splitting waves.

[0050] For annealing at a maximum temperature of 350° C., spontaneous splitting occurs on average after 200 minutes.

[0051] When a weight of 500 g (corresponding to a pressing force of about 5 N) is applied to each of the laminated structures 5 via the additional device 11, the initiation of splitting waves occurs on average after 160 minutes for a level of weakening of about 16%.

[0052] When a weight of 1500 g (corresponding to a pressing force of about 15 N) is applied to each of the laminated structures 5 via the additional device 11, the initiation of splitting waves occurs on average after 110 minutes for a level of weakening of about 12%.

[0053] Following self-sustained propagation of the split wave in each of the bonded structures 5, what is obtained, after transfer, is the SOI substrate (transferred assembly 5a) and the remainder of the donor substrate 5b.

[0054] For the two above mentioned examples which induce separation waves at a certain degree of maturity, a transferred useful layer 3 is obtained which has very good morphological surface properties (low roughness, high uniformity) and is reproducible from one wafer to the next.

[0055] Finishing steps applied to the transferred assembly 5a include chemical cleaning operations and at least one high temperature smoothing heat treatment. At the completion of these steps, the SOI substrate contains a useful layer 3 having a thickness of 50 nm, a non-uniformity of less than 2% and a surface roughness of less than 0.3 nm RMS.

[0056] Of course, the invention is not limited to the implementations and examples described, and modified embodiments may be introduced into this specification without departing from the scope of the invention as defined by the claims.

Claims

1. A process for transferring a useful layer (3) onto a carrier substrate (4), comprising the steps of: a) providing a donor substrate (1) comprising a buried weakened plane (2), said useful layer (3) being bounded by a front surface (1a) of said donor substrate (1) and by said buried weakened plane (2); b) providing a carrier substrate (4); c) bonding the donor substrate (1) with its front side (1a) to the carrier substrate (4) along a bonding interface (7) to form a laminated structure (5); d) annealing the laminated structure (5) to increase the level of weakening of the buried weakened plane (2); In a process for transferring a useful layer (3), the process comprising: The process comprises: In the annealing step d), a predetermined pressure is applied to the buried weakened plane (2) for a period of time, said predetermined pressure being selected such that a splitting wave is induced once a given level of weakening has been reached; at the end of said period, said given level of weakening has been reached and said predetermined pressure causes the initiation and self-sustaining propagation of said splitting wave along said embedded weakened plane (2) resulting in the transfer of said useful layer (3) to said carrier substrate (4), the predetermined pressure is applied after a predetermined anneal duration without interrupting the anneal; the predetermined pressure is applied locally to the embedded weakened plane (2) of the bonded structure (5) by means of a wedge (10) placed at the bonding interface (7), the wedge (10) exerting a pressing force against the chamfered edges of the donor substrate (1) and the carrier substrate (4) of the bonded structure (5) so as to generate a tensile strain in the embedded weakened plane (2), 1. A process for transferring, applied to a batch processing of a plurality of laminated structures (5), characterized in that the predetermined pressure is applied to the embedded weakened plane (2) of each of the laminated structures (5) such that the splitting wave is initiated once a given level of weakening is reached for each laminated structure (5).

2. 2. The process for transferring of claim 1, wherein the period of time is between 1 minute and 5 hours.

3. 2. The process for transferring of claim 1, wherein said period is a fraction of the duration of said anneal between 1% and 100%.

4. 2. The process for transferring according to claim 1, wherein the annealing of step d) is carried out in a horizontally or vertically configured thermal processing apparatus (20) suitable for batch processing a plurality of bonded structures (5).

5. 2. The process for transferring according to claim 1, wherein the pressing force is between 0.5N and 50N.

6. A process for transferring according to any one of claims 1 to 5, wherein the given level of weakening is determined by the area occupied by microcavities in the buried weakened plane (2) and is chosen to be between 1% and 90%.

7. The process for transferring according to any one of claims 1 to 6, wherein said annealing in step d) reaches a maximum temperature between 300°C and 600°C.

8. 8. The process for transferring according to any one of claims 1 to 7, wherein the donor substrate (1) and the carrier substrate (4) are made of monocrystalline silicon and the buried weakened plane (2) is formed by ion implantation of light element species into the donor substrate, the light element species being selected from hydrogen and helium or a combination of hydrogen and helium.

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