Process for transferring a thin layer to a carrier substrate

Abstract

The invention relates to a process for transferring a thin layer to a carrier substrate, comprising: - joining a donor substrate (1) and the carrier substrate (2) by direct bonding of their respective front sides (1a, 2a) via a bonding interface (3), to form a bonded assembly (100) having an unbonded local region (31) within this bonding interface (3), the donor substrate (1) further comprising a buried fragile plane (11), - splitting along the buried fragile plane (11), the splitting being initiated in the unbonded local region (31) after growth of microcracks in said plane (11) by thermal activation, and leading to the transfer of a thin layer (10) from the donor substrate (1) to the carrier substrate (2). The process is noteworthy in that the unbonded local region (31) is generated by at least one rough region (31a) produced by scanning a laser beam over at least one of the front sides (1a, 2a) of the donor substrate (1) and carrier substrate (2) before they are joined, the scan covering an area of at least 100 microns by 100 microns and of at most 500 microns by 500 microns.

Description

METHOD FOR TRANSFERRING A THIN LAYER TO A SUPPORT SUBSTRATE FIELD OF THE INVENTION

[0001] The present invention relates to the field of microelectronics and semiconductors. In particular, the invention relates to a method for transferring a thin layer onto a support substrate, based on Smart Cut technology. TM , the thin layer having improved roughness after separation. The transfer method can in particular be implemented for the manufacture of an SOI structure. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Smart Cut technology TMis well known for the fabrication of SOI (silicon on insulator) structures and more generally, for the transfer of thin layers. This technology is based on the formation of a fragile plane buried in a donor substrate, by implantation of so-called light species in said substrate; the buried fragile plane delimits, with a front face of the donor substrate, the thin layer to be transferred. An assembly then takes place between the donor substrate and a support substrate, at their respective front faces, to form a bonded assembly. The assembly is advantageously carried out by direct bonding, by molecular adhesion, that is to say without the involvement of adhesive material: a bonding interface is thus established between the two assembled substrates.The growth of microcracks in the buried brittle plane, by thermal activation, can lead to spontaneous separation, along said plane, giving rise to the transfer of the thin layer onto the support substrate (forming the stacked structure). The remainder of the donor substrate can be reused for subsequent layer transfer. After separation, it is usual to apply finishing treatments to the stacked structure, to restore the crystalline quality and surface roughness of the transferred thin layer. These finishing steps may in particular involve oxidation or smoothing heat treatments (in a neutral or reducing atmosphere), chemical cleaning and / or etching and / or chemical-mechanical polishing steps, as is known to those skilled in the art. Finally, different inspection tools of the final structure make it possible to control the entire surface of the thin layer.

[0003] When the separation in the buried fragile plane is spontaneous, significant variabilities are observed in terms of surface roughness of the transferred thin layer, both in the high frequencies (microroughness) and in the low frequencies (undulations, local areas of high roughness, mottling, etc.). These variabilities are visible and measurable in particular via the aforementioned inspection tools, when controlling the thin layer in the final structure.

[0004] Recall that the surface roughness of the thin layer after finishing can be imaged by a map obtained using a Surfscan™ inspection tool from KLA-Tencor (). The level of roughness as well as potential patterns (marbling (M), dense zones (ZD)...) are measured or made apparent by measuring the diffuse background noise ("haze" according to the commonly used English terminology) corresponding to the intensity of the light scattered by the surface of the thin layer. The "haze" signal varies linearly with the square of the RMS surface roughness (root mean square roughness) in the spatial frequency range of 0.1μm -1 at 10μm -1 . For more complete information on this technique of inspecting and evaluating roughness over a large surface, please refer to the article “Seeing through the haze” by F. Holsteyns (Yield Management Solution, Spring 2004, pp50-54).

[0005] The maps of show the surface roughness of two thin films transferred from two bonded sets and treated identically until finishing. In map (A), a peripheral zone of residual roughness, called the "dense zone (ZD)" is observed (in particular, at the top of the map); map (B) is completely devoid of it. More marked mottling (M) is also apparent in map (A). The average and maximum roughnesses (expressed in ppm of "haze") are also significantly different between the two maps (A) and (B). La illustrates the variabilities in the final quality and roughness of the thin films, which mainly originate from the variabilities in surface roughness (high and low frequencies) after separation.

[0006] To improve the quality of the thin layers in the final stacked structure, it therefore remains important to reduce the surface roughness (regardless of the spatial frequency) of these layers, after transfer.

[0007] It is known from document US2010 / 330779 to form a local unbonded zone, at the bonding interface, bordered by a bonded region, to constitute a trigger for separation and thus limit the roughness of a thin layer of silicon transferred onto a glass support substrate. The local unbonded zone is obtained by producing a topology corresponding to a cavity and / or a dome (or peak), on the face to be assembled of the donor substrate (silicon) or the support substrate (glass). Cavities and / or peaks of the order of 2 to 3 micrometers are produced on the glass substrate, to induce an unbonded zone of several tens of mm 2 . SUBJECT OF THE INVENTION

[0008] The present invention provides a transfer method using a particular fracture initiator point, allowing the achievement of improved surface roughness of the thin layer after separation, to achieve excellent surface quality after the finishing steps of the stacked structure. The method is particularly advantageous for the manufacture of SOI structures. BRIEF DESCRIPTION OF THE INVENTION

[0009] The present invention relates to a method for transferring a thin layer onto a support substrate, comprising:

[0010] - the assembly of a donor substrate and the support substrate, by direct bonding at their respective front faces and along a bonding interface, to form a bonded assembly having a local non-bonded zone within this bonding interface, the donor substrate further comprising a buried fragile plane, - the separation along the buried fragile plane, initiated at the local non-bonded zone after growth of microcracks in said plane by thermal activation, the separation leading to the transfer of a thin layer from the donor substrate onto the support substrate.

[0011] According to the invention, the local non-bonded zone is generated by at least one rough zone produced by scanning a laser beam on at least one of the front faces of the donor and support substrates before their assembly, the scanning extending over a surface of at least 100 micrometers by 100 micrometers and at most 500 micrometers by 500 micrometers.

[0012] According to advantageous characteristics of the invention, taken alone or in any feasible combination: the rough zone is produced by surface melting of a material composing the donor substrate and / or the support substrate, on the side of their respective front face; the scanning of the laser beam has a duration of between 20 microseconds and 100 microseconds during which the beam has an energy density of between 5 mJ / m^2 and 7 mJ / m^2; the laser beam has a wavelength of 355 nm. the laser beam has a size inscribed in a circle of 80 micrometers in diameter; the local non-bonded zone is generated by a plurality of elementary rough zones separated from each other, each being produced by scanning the laser beam over a surface of at least 100 micrometers by 100 micrometers and at most 200 micrometers by 200 micrometers;the rough zone is produced on the front face of the donor substrate, before the formation of the fragile plane buried in said donor substrate; the donor substrate is formed of a first material at its front face, and a preliminary rough zone is produced on the surface of said material, and thermal oxidation of the first material of the donor substrate is carried out after the production of the preliminary rough zone and before the formation of the buried fragile plane, to form an insulating layer which will be assembled on the support substrate in the bonded assembly, said insulating layer comprising, on its free face, the rough zone directly above the preliminary rough zone;the support substrate is formed from a first material at its front face, and a preliminary rough zone is produced on the surface of said material, and thermal oxidation of the first material of the support substrate is carried out after the production of the preliminary rough zone, to form an insulating layer which will be assembled on the donor substrate in the bonded assembly, said insulating layer comprising, on its free face, the rough zone directly above the preliminary rough zone; the local non-bonded zone has at least one lateral dimension, in the plane of the bonding interface, less than 500 micrometers; the donor substrate and the support substrate have a disc shape with a diameter of 300 mm; the local non-bonded zone is located more than 3 mm from the edge of the bonded assembly, in the plane of the bonding interface; the local non-bonded zone is located less than 10 mm from the edge of the bonded assembly, in the plane of the bonding interface;the transfer method further comprises a step of annealing the bonded assembly to cause separation along the buried fragile plane, this step being carried out in an oven by keeping the bonded assembly vertically and angularly oriented so as to place the rough zone in an upper portion of the oven; the thin layer, originating from the donor substrate, is made of monocrystalline silicon and the support substrate comprises monocrystalline silicon, to form a stacked structure of the SOI type.;

[0013] Other characteristics and advantages of the invention will emerge from the detailed description which follows with reference to the appended figures in which:

[0014] Presents two representative maps of the surface roughness of two transferred thin layers, from two sets glued and treated identically until finishing, using a conventional process; the two maps were obtained using a Surfscan inspection tool. TM ;

[0015] The present graph indicates the surface roughness of the thin layers, as a function of the fracture time, for a plurality of bonded assemblies (different from the bonded assemblies stated with reference to the) and treated in an identical manner until finishing, according to a conventional method;

[0016] This presents a glued assembly occurring at an intermediate stage of the transfer process in accordance with the invention;

[0017] The present invention provides an example of a donor substrate or support comprising a rough area purposely formed on its front face, in accordance with the transfer method of the invention, which rough area has a predetermined roughness;

[0018] The present invention comprises a stacked structure and the remainder of a donor substrate, obtained by a transfer method according to the invention;

[0019] Presents images of the rough area obtained according to a first example of implementation;

[0020] Presents images of the rough area obtained according to a second implementation example;

[0021] Presents images of preliminary rough areas obtained according to a third implementation example. DETAILED DESCRIPTION OF THE INVENTION

[0022] Some figures are schematic representations which, for readability purposes, are not to scale. In particular, the layer thicknesses along the z axis are not to scale with the lateral dimensions along the x and y axes.

[0023] The same references in the figures and in the description may be used for elements of the same nature.

[0024] The invention relates to a method for transferring a thin layer onto a support substrate, to form a stacked structure. As mentioned in the introduction, such a stacked structure may in particular be of the SOI type, and comprise a thin surface layer of silicon, an intermediate insulating layer of silicon oxide and a support substrate of silicon. The support substrate may optionally comprise other functional layers, such as a charge trapping layer (for example a surface layer of polycrystalline silicon), for example for SOI structures intended for radiofrequency (RF) applications. The transfer method according to the invention is nevertheless not limited to the manufacture of SOI and can be applied to a number of other stacked structures in the field of microelectronics, microsystems and semiconductors.

[0025] The transfer method according to the invention is based on Smart Cut technology TM. When the separation in the buried brittle plane is spontaneous, the fracture time (i.e. the time after which the separation occurs, during the thermal fracture annealing) may differ between a plurality of identically treated bonded assemblies, undergoing the same annealing, in the same furnace. The fracture time (TF) depends on a multitude of parameters, linked to the formation of the buried brittle plane, to the fracture annealing, to the nature of the bonded assembly, etc. The applicant has noted that, for bonded assemblies prepared in a similar manner and undergoing the same fracture annealing, the separations which occur in short fracture times (TFc) give rise to lower high-frequency surface roughnesses (microroughness) of thin layers in the final stacked structures (i.e. after transfer and finishing) than the separations occurring in longer fracture times (TFl), as can be seen in the.In addition, long fracture times induce a local zone of very high roughness (called dense zone ZD) at the edge of the thin layer after fracture, which is rarely or not the case when the fracture time is short. This dense zone degrades the quality and roughness of the thin layer, even after finishing, as can be seen in the map (A) of the.

[0026] The transfer method according to the invention therefore aims to initiate, in an anticipated (short fracture time) and repeatable manner (low dispersion of the fracture time between a plurality of similar bonded assemblies), the spontaneous separation in the buried fragile plane, so as to substantially improve the surface roughness of the transferred thin layer.

[0027] For this, the transfer method comprises, firstly, the assembly of a donor substrate 1 and the support substrate 2, by direct bonding at their respective front faces 1a, 2a and following a bonding interface 3, to form a bonded assembly 100 ().

[0028] The donor substrate 1 is preferably in the form of a wafer with a diameter of 100mm, 150mm, 200mm, 300mm or even 450mm and a thickness typically between 250μm and 1mm. It comprises a front face 1a and a rear face 1b. The surface roughness of the front face 1a is chosen to be less than 1.0nm RMS, or even preferably less than 0.5nm RMS (measured by atomic force microscopy (AFM), for example on a scan of 20μm x 20μm). The donor substrate 1 may be made of silicon or any other semiconductor or insulating material, for which a thin layer transfer may be of interest (for example, SiC, GaN, III-V compounds, piezoelectric materials, etc.). Let us also note that the donor substrate 1 may comprise one or more additional layers 12, at least on the side of its front face 1a, for example an insulating layer.As illustrated in the, this additional layer 12 becomes an intermediate layer buried in the bonded assembly 100, after assembly of the donor substrate 1 and the support substrate 2.

[0029] The donor substrate 1 comprises a buried fragile plane 11, which delimits a thin layer 10 to be transferred. As is well known with reference to Smart Cut technology TM, such a buried fragile plane 11 can be formed by implantation of so-called light species, such as hydrogen, helium or a combination of these two species. The light species are implanted at a determined depth in the donor substrate 1, consistent with the thickness of the targeted thin layer 10. These light species will form, around the determined depth, microcavities distributed in a thin layer substantially parallel to the front face 1a of the donor substrate 1, i.e. parallel to the plane (x,y) in the figures. This thin layer is called the buried fragile plane 11, for the sake of simplification.

[0030] The implantation energy of the light species is chosen to achieve the determined depth. For example, hydrogen ions will be implanted at an energy between 10 keV and 210 keV, and at a dose between 5E16 / cm 2 and 1E17 / cm 2, to delimit a thin layer 10 having a thickness of the order of 100nm to 1500nm. It should be remembered that an additional layer may be formed on the front face 1a of the donor substrate 1, prior to the ion implantation step. This additional layer may be composed of a material such as silicon oxide or silicon nitride for example. It may be kept for the next assembly step (and form all or part of the intermediate layer of the bonded assembly 100), or it may be removed.

[0031] The support substrate 2 is also preferably in the form of a wafer with a diameter of 100mm, 150mm, 200mm, 300mm or even 450mm and a thickness typically between 250μm and 1mm. It comprises a front face 2a and a rear face 2b. The surface roughness of the front face 2a is chosen to be less than 1.0nm RMS, or even preferably less than 0.5nm RMS (measured by AFM, for example on a scan of 20μm x 20μm). The support substrate 2 may be made of silicon or any other semiconductor or insulating material, on which a thin layer transfer may be of interest (for example, SiC, GaN, III-V compounds, piezoelectric materials, insulating materials, etc.). It is also noted that the support substrate 2 may comprise one or more additional layers, at least on the side of its front face 2a, for example an insulating layer and / or a charge trapping layer.This (or these) additional layer(s) are buried in the bonded assembly 100, after assembly of the donor substrate 1 and the support substrate 2.

[0032] The assembly between the donor 1 and support 2 substrates is based on direct bonding by molecular adhesion. As is well known in itself, such bonding does not require an adhesive material, since bonds are established at the atomic scale between the assembled surfaces, forming the bonding interface 3. Several types of bonding by molecular adhesion exist, which differ in particular by their temperature, pressure, atmosphere or treatment conditions, prior to bringing the surfaces into contact. Examples include bonding at room temperature with or without prior plasma activation of the surfaces to be assembled, bonding by atomic diffusion ("Atomic diffusion bonding" or ADB according to English terminology), bonding with surface activation ("Surface-activated bonding" or SAB), etc.

[0033] The assembly step may comprise, before bringing the front faces 1a, 2a to be assembled into contact, conventional sequences of chemical cleaning (for example, RCA cleaning), surface activation (for example, by oxygen or nitrogen plasma) or other surface preparations (such as cleaning by brushing), capable of promoting the quality of the bonding interface 3 (low defectivity, high adhesion energy).

[0034] The bonded assembly 100 according to the invention has the particularity of comprising a local non-bonded zone 31 within the bonding interface 3 (). In other words, this local non-bonded zone 31 is bordered by the bonding interface 3, which is closed, reflecting the molecular adhesion forces which unite the front faces 1a, 2a of the assembled substrates 1, 2.

[0035] The local non-bonded area 31 is generated solely by the presence of a rough area 31a, deliberately produced on at least one of the front faces 1a, 2a of the donor 1 and support 2 substrates, before their assembly (). The rough area 31a is free from any low-frequency topology or undulation, i.e., whose typical wavelength would be greater than 100nm. It has a predetermined high-frequency roughness, which is greater than the roughness of the front faces 1a, 2a around this rough area 31a. The roughness of the front faces 1a, 2a may typically vary depending on the nature of the materials of said faces 1a, 2a and depending on the type of direct bonding implemented, but always makes it possible to obtain a bonded (closed) interface, whereas the predetermined roughness in the rough area 31a prevents local bonding between the two faces 1a, 2a.

[0036] The amplitude of the predetermined roughness is strictly greater than 0.5nm RMS and strictly less than 60.0nm RMS, with typical wavelengths between 10nm and 100nm (corresponding to high-frequency microroughness). As is well known, the term RMS ("root mean square") corresponds to a quadratic mean value of roughness. The technique for measuring this microroughness is atomic force microscopy (AFM), on 10x10μm scans. 2 at 30x30μm 2. Recall that the rough zone 31a does not have a topology that would correspond to a low-frequency undulation, but only a microroughness in the range of spatial frequencies stated above. The maximum peak-to-valley or PV amplitude (for "peak to valley") in the rough zone 31a is typically between 5.0nm and 300.0nm, preferably between 5.0nm and 100.0nm. In particular, in the example of the, the rough zone 31a has a roughness of 2.3nm RMS and 25nm PV, measured by AFM on a 10μm x 10μm scan (image on the right in the).

[0037] According to a particular embodiment notably implemented for the manufacture of SOI structures, the front face 1a, 2a, on which the rough zone 31a is produced, is made of monocrystalline silicon, and the predetermined roughness preferably has an amplitude of between 5nm RMS and 15nm RMS, and even more preferably, this amplitude is between 9 nm RMS and 12 nm RMS.

[0038] However, the invention is in no way limited to the formation of the rough zone 31a on a front face made of monocrystalline silicon and, in an alternative, the face on which the rough zone 31a is made is made of polycrystalline silicon.

[0039] Note that, when it is the front face 1a of the donor substrate 1 which carries the rough zone 31a, the latter is preferably produced before the formation of the buried fragile plane 11 in said substrate 1, to limit any damage or early ripening of the microcavities composing it.

[0040] It is also possible to produce a preliminary rough zone on a first material forming the donor substrate 1, at its front face; for example, the first material may be silicon. Thermal oxidation of the first material of the donor substrate 1 is then carried out, to form an insulating layer (for example, silicon oxide). The thickness of the insulating layer may, for example, be less than or equal to 200 nm. Said insulating layer (corresponding to the additional layer 12 illustrated in the) comprises, on its free face (the front face 1a of the donor substrate 1), the rough zone 31a directly above the preliminary rough zone produced in the first material.Indeed, the oxidation preserves at least in part the high-frequency roughness that the first material has in the preliminary rough zone: the aim will be to form a micro-roughness in the preliminary rough zone capable of giving rise to a predetermined roughness of the rough zone 31a after oxidation, in the aforementioned amplitude range. After the formation of the buried fragile plane 11 in the donor substrate 1, the insulating layer 12 is assembled on the support substrate 2 in the bonded assembly 100 and the rough zone 31a generates the local non-bonded zone 31 within the bonding interface 3. In this case, the rough zone 31a on the front face 1a of the donor substrate 1 was not directly produced by “roughening” the insulating layer 12, but comes from a preliminary rough zone present in an underlying material.

[0041] For completeness, we note that even if the formation of this preliminary rough zone has been described with reference to donor substrate 1, it could of course be applied to support substrate 2.

[0042] The bonded assembly 100 being formed and comprising the non-bonded local zone 31 within its bonding interface 3, the transfer method according to the invention provides for applying a thermal annealing thereto which will give rise to a spontaneous separation along the buried fragile plane 11. The separation leads to the transfer of the thin layer 10 from the donor substrate 1 onto the support substrate 2, to form the stacked structure 110 (). A non-transferred zone 31b, at the level of which the thin layer 10 has not been transferred, is present directly above the location of the non-bonded local zone 31. This non-transferred zone 31b may be of a dimension equal to or greater than the dimension of the non-bonded local zone 31. The remainder of the donor substrate 1' is also obtained.

[0043] The local unbonded zone 31 acts as a fracture initiator point, in the buried fragile plane 11 and directly above or close to said zone 31, after growth of microcracks in the buried fragile plane 11 by thermal activation. This fracture initiation occurs in an anticipated manner compared to a bonded assembly 100 which would not include the local unbonded zone 31: this gives access to short fracture times, which provide low surface roughness, after transfer, at the face 10a of the thin layer 10. In addition, a clear improvement in the surface roughness after transfer is noted, due to short fracture times obtained thanks to the local unbonded zone 31 which acts as a fracture initiator point.

[0044] The unbonded local zone 31 according to the invention differs from the state of the art in that it is solely due to the presence of a rough zone 31a on the front face 1a, 2a of one or other of the donor 1 and support 2 substrates. No low-frequency topology is involved (bumps, holes, cavities, particles). The internal volume of the unbonded local zone 31 thus created is extremely small; in fact, the accumulation, in this internal volume, of different gases originating in particular from the vaporization of the monolayers of water present on the assembled faces 1a, 2a or from the exo-diffusion of light species by the front face 1a of the donor substrate 1, allows rapid pressurization favorable to the initiation of the fracture as soon as the level of ripening of the microcracks in the buried fragile plane 11 allows it.

[0045] The local non-bonded zone 31 may be located at different positions in the plane of the bonding interface 3: in particular, in a central region of the bonded assembly 100 or in a peripheral region or even in an intermediate region between these two extremes. In the peripheral region, it is preferable for the local non-bonded zone 31 to be located more than 3 mm from the edge of the bonded assembly 100.

[0046] Preferably, in particular when the bonded assembly 100 has a thick layer 12, of more than 100 nm, it is preferable for the local non-bonded zone 31 to be located less than 10 mm from the edge of the bonded assembly 100, in the plane of the bonding interface 3.

[0047] Advantageously, a step of annealing the bonded assembly 100 is applied to cause separation along the buried fragile plane 11. This step is conventionally carried out in a furnace by holding the bonded assembly 100 vertically therein. Advantageously, the bonded assembly 100 is oriented angularly in the furnace chamber so as to place the rough zone 31a in an upper portion of the furnace. This upper portion generally has a higher temperature, which promotes the triggering of the fracture at the level of the rough zone 31a.

[0048] To avoid penalizing the integrity of the transfer of the thin layer 10, the local unbonded zone 31 advantageously has at least one lateral dimension, in the plane (x,y), less than 500 micrometers.

[0049] According to the invention, the rough zone 31a is produced by scanning a laser beam on at least one of the front faces 1a, 2a of the donor 1 and support 2 substrates before their assembly. This scanning extends over a surface of at least 100 micrometers by 100 micrometers and at most over a surface of 500 micrometers by 500 micrometers. In other words, the scanned surface circumscribes a surface of 100 micrometers by 100 micrometers and is inscribed in a surface of 500 micrometers by 500 micrometers. This approach is advantageous in that it makes it possible to form this rough zone while limiting as much as possible the contact with the rest of the front face 1a, 2a concerned, so as to reduce the risks of contamination or degradation (scratches, etc.) of this face, intended to be assembled.The beam is scanned using moving optical parts (a scanner), for example a set of mirrors, which makes it possible to keep the substrate stationary while treating a surface area with a relatively large area, i.e. larger than the beam size. This can therefore be small, for example inscribed in a circle with a diameter of 80 micrometers. In one embodiment, the laser beam has a rectangular shape of 70 micrometers by 40 micrometers.

[0050] The laser beam parameters are chosen to generate only a superficial melting of the material of the front face 1a, 2a, and only in the area to be roughened. We are therefore in a regime at the limit of melting and not at all in ablation regimes which remove material or hollow it out. This regime corresponds to a "discontinuous" melting of the irradiated surface; this means that melted and unmelted zones are induced at this surface, favorable to the formation of the desired high-frequency roughness. Too low an energy density does not allow melting to be achieved and no roughness or too little is generated; too high an energy density causes a "homogeneous" melting of the entire irradiated surface, which recrystallizes uniformly with a roughness that is potentially too restricted.It is therefore important to be in a regime at the limit of fusion, to generate high frequency roughness of expected amplitude and not create any topology, neither cavity, nor bump, only high frequency roughness, in the aforementioned RMS and PV ranges.

[0051] Preferably, the laser scanning is therefore carried out for a duration of between 20 microseconds and 100 microseconds during which the beam has an energy density of between 5 mJ / m^2 and 7 mJ / m^2. The wavelength of the laser used may be chosen between 100nm and 550nm, preferably between 250nm and 400nm, such as for example 355 nm.

[0052] The rough area 31a generated, after scanning the laser beam, is significantly smaller than the dimension of the irradiated surface. This is notably due to the fact that the energy density is not constant over the entire projection surface of the beam, and that it tends to be lower on its peripheral contour. Also, when the beam scans a large surface, up to 500 micrometers by 500 micrometers, the rough area has a smaller dimension and is included in this scanned surface.

[0053] Note that in the case where the material of the front face 1a,2a is a silicon oxide and the underlying material is silicon, the laser scan passes through the silicon oxide layer and will generate a melting of the silicon, which will induce the formation of wrinkles on the surface of the oxide. These wrinkles form the expected high-frequency roughness, in the irradiated region, on the front face 1a,2a.

[0054] It may be provided that the local unbonded zone 31 is generated by a plurality of elementary rough zones 31a close to each other, but separated from each other, for example between 2 and 5 elementary rough zones 31a. Each elementary rough zone is produced by scanning the laser beam over a surface of at least 100 micrometers by 100 micrometers and at most 200 micrometers by 200 micrometers, so that the local unbonded zone 31 remains overall of reduced size.

[0055] The shapes of the rough area 31a and the local unbonded area 31 are not necessarily square. They may, for example, be rectangular or of any other shape provided that the laser beam is scanned over an area of ​​at least 100 micrometers by 100 micrometers and at most 500 micrometers by 500 micrometers. Examples of implementation:

[0056] According to a first example, the transfer method is implemented to develop an FDSOI (fully depleted SOI) structure, i.e. having a low thickness of superficial thin layer (for example in the range 10nm to 50nm) and a low thickness of buried insulating layer (for example in the range 15nm to 25nm).

[0057] The donor substrate 1 is a monocrystalline silicon wafer 300 mm in diameter, and comprises at its front face 1a, an insulating layer 12 of silicon oxide 35 nm thick. The buried fragile plane 11 is formed by co-implantation of helium and hydrogen ions at respective energies of 40 keV and 25 keV, and at respective doses between 0.8 E16 / cm 2 and 1.8 E16 / cm 2 .

[0058] The support substrate 2 is a monocrystalline silicon wafer with a diameter of 300 mm.

[0059] The laser beam scanning is carried out in the center of the support substrate 2, on the side of its front face 2a, so as to cause a melting / recrystallization of the surface only, in a regime at the melting limit. The laser conditions are as follows: wavelength 355nm, the laser irradiates the surface for 50µs and with an energy density 6.3 mJ / cm², the dimension of the irradiated surface is square with a side of 100µm. These conditions therefore lead to the formation of a rough zone included in the irradiated surface, with an RMS roughness of 11 nm + / -0.5nm, measured by AFM on a scan of 30 μm by 30 μm (see for example the SEM image on the left of the and the optical mode image on the right of the).

[0060] A classic surface preparation (cleaning, plasma activation) of the two substrates 1,2 is then carried out with a view to bonding by molecular adhesion.

[0061] The assembly, based on direct contact between the front faces 1a, 2a of the donor substrates 1 and support 2, makes it possible to obtain a bonded assembly 100. The bonded assembly 100 comprises a local non-bonded zone 31 directly above the rough zone 31a (in the center of the bonded assembly 100), within the bonding interface 3. The local non-bonded zone 31 has a dimension of 40 μm by 60 μm.

[0062] A fracture annealing, carried out in a horizontal furnace (capable of treating a plurality of bonded assemblies 100 collectively), is applied between 200°C and 400°C. The local unbonded zone 31 makes it possible to initiate the fracture along the buried fragile plane 11 in a short time, i.e. approximately 25% of the average fracture time in the absence of the local unbonded zone 31, in the case of isothermal annealing. By considering a plurality of bonded assemblies 100 treated collectively, the local unbonded zone 31 also makes it possible to obtain less dispersed fracture times.

[0063] The SOI 110 structure obtained after separation has a non-transferred area 31b in place of the local non-bonded area 31 of the bonded assembly 100, and whose size is of the order of 100 micrometers to 200 micrometers. The surface quality 10a of the transferred thin layer 10 is improved compared to an SOI 110' structure obtained from a conventional bonded assembly, without a local non-bonded area 31. This is particularly visible after the application of the finishing steps (mainly oxidation and smoothing heat treatments) to cure the transferred thin layer 10 and smooth its surface: the SOI 110 structures obtained by the method of the invention do not show dense roughness areas at the periphery of the thin layer 10, unlike certain SOI 110' structures obtained by a conventional method (without a local non-bonded area 31).

[0064] According to a second example, the transfer method is implemented to develop an RFSOI structure (SOI for radiofrequency applications), i.e. having a thin surface layer, an insulating layer and a charge trapping layer on the support substrate 2.

[0065] The donor substrate 1 is a monocrystalline silicon wafer 300 mm in diameter, and comprises at its front face 1a, an insulating layer 12 of silicon oxide 200 nm thick. The buried fragile plane 11 is formed by co-implantation of hydrogen and helium ions at respective energies of 35 keV and 50 keV, and at respective doses between 0.8 E16 / cm 2 and 1.8 E16 / cm 2 . The support substrate 2 is a monocrystalline silicon wafer with a diameter of 300 mm. A polycrystalline silicon charge trapping layer approximately 1 μm thick is arranged on the support substrate 2, on the side of its front face 2a.

[0066] The laser scanning is carried out in a peripheral region of the donor substrate 1, between 3 and 10 mm from the edge of the donor substrate 1, on the side of its front face 1a, so as to cause melting / recrystallization of the surface only. This treatment is carried out before the formation of the buried fragile plane 11.

[0067] The laser scanning conditions are as follows: wavelength 355nm, scanning time 30µs, energy density 6.3 mJ / cm², irradiated surface square with side of 200 micrometers. The laser scanning therefore leads to the formation of a rough zone, included in the irradiated surface, with an RMS roughness of 9 nm + / -0.5nm, measured by AFM on a scan of 30 μm by 30 μm. The rough zone is represented on the (left in SEM image and in optical mode on the right).

[0068] A conventional surface preparation (cleaning, plasma activation) of the two substrates 1, 2 is then carried out with a view to bonding by molecular adhesion. The assembly, based on direct contact between the front faces 1a, 2a of the donor substrates 1 and support 2, makes it possible to obtain a bonded assembly 100. The bonded assembly 100 comprises a local non-bonded zone 31, directly above the rough zone 31a (at the edge of the bonded assembly 100), within the bonding interface 3. The local non-bonded zone 31 has a dimension of between 140 μm and 170 μm.

[0069] Fracture annealing, carried out in a horizontal furnace (capable of treating a plurality of bonded assemblies 100 collectively arranged vertically in the furnace chamber) is applied between 200°C and 550°C. The bonded assemblies are angularly oriented in the furnace chamber so that the rough zone 31a is arranged in a high position of the furnace chamber.

[0070] The local unbonded zone 31 makes it possible to initiate the fracture along the buried fragile plane 11 in a short time (approximately 30% of the average fracture time without a local unbonded zone 31). Considering a plurality of bonded assemblies 100 treated collectively, the local unbonded zone 31 also makes it possible to obtain less dispersed fracture times. The SOI structure 110 obtained after separation has a non-transferred zone in place of the local unbonded zone 31 of the bonded assembly 100, and whose size is between 250 and 350μm.

[0071] The surface quality 10a of the transferred thin layer 10 is improved compared to an SOI 110' structure obtained from a conventional bonded assembly, without a local unbonded area 31. This is particularly visible after the application of the finishing steps (mainly oxidation and smoothing heat treatments) to cure the transferred thin layer 10 and smooth its surface: the mapping of the SOI 110 structure obtained by the method of the invention shows an absence of a dense roughness area at the periphery of the thin layer 10, unlike the SOI 110' structure obtained by a conventional method (without a local unbonded area 31); it also shows a lower overall level of roughness over the entire surface of the thin layer 10 of the SOI 110 structure.

[0072] According to a third example, the transfer method is implemented to develop an SOI structure, i.e. having a thin surface layer, an insulating layer and a support substrate 2. The donor substrate 1 is a monocrystalline silicon wafer with a diameter of 300 mm.

[0073] Three successive scans of the laser beam are carried out in a central or peripheral region of the donor substrate 1, on the side of its front face 1a (therefore on the silicon), so as to cause a melting / recrystallization of the surface only. This forms three rough zones, separated by a distance of 80 micrometers. The laser conditions are as follows: wavelength 355 nm, duration of each beam scan 30 µs, energy density 6.3 mJ / cm², each irradiated surface is square with a side of 100 µm. These scans lead to the formation of preliminary rough zones at the level of the irradiated surface, with an RMS roughness greater than 11 nm + / - 0.5 nm, measured by AFM on a scan of 30 μm by 30 μm. These preliminary rough areas are shown in the (left image in optical mode of the 3 scanned areas and, on the right, a zoom in SEM mode on one of the scans).

[0074] After applying a standard cleaning to the donor substrate 1, a thermal oxidation is carried out, typically in the temperature range 900°C – 1050°C. At the end of this oxidation, the donor substrate 1 comprises at its front face 1a, an insulating layer 12 made of silicon oxide with a thickness of 200nm. Directly above the 3 preliminary rough zones, 3 rough zones 31a are present on the free face 1a of the insulating layer 12. These rough zones 31a have an RMS roughness substantially identical to that of the preliminary rough zones.

[0075] The buried fragile plane 11 is formed by co-implantation of hydrogen and helium ions at respective energies of 35 keV and 50 keV, and at respective doses between 0.8 E16 / cm 2 and 1.8 E16 / cm 2 , through the insulating layer 12, into the donor substrate 1. The support substrate 2 is a monocrystalline silicon wafer with a diameter of 300 mm.

[0076] A conventional surface preparation (cleaning, plasma activation) of the two substrates 1, 2 is then carried out with a view to bonding by molecular adhesion. The assembly, based on direct contact between the front faces 1a, 2a of the donor substrates 1 and support 2, makes it possible to obtain a bonded assembly 100. The bonded assembly 100 comprises a local non-bonded zone 31, directly above the rough zone 31a, within the bonding interface 3. A fracture annealing, carried out in a horizontal furnace (capable of treating a plurality of bonded assemblies 100 collectively) is applied between 200°C and 550°C. The local non-bonded zone 31 makes it possible to initiate the fracture along the buried fragile plane 11 in a short time (approximately 30% of the average fracture time without a local non-bonded zone 31). By considering a plurality of bonded assemblies 100 treated collectively, the local non-bonded zone 31 makes it possible to obtain less dispersed fracture times.

[0077] The SOI 110 structure obtained after separation has a non-transferred area in place of the local non-bonded area 31 of the bonded assembly 100, and the size of which is typically less than 500μm.

[0078] The surface quality 10a of the transferred thin layer 10 is improved compared to an SOI structure 110' obtained from a conventional bonded assembly, without a local unbonded zone 31: it shows an absence of a dense zone ZD of roughness at the periphery of the thin layer 10, as well as a lower overall level of roughness over the entire surface of the thin layer 10.

[0079] Of course, the invention is not limited to the embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

A method of transferring a thin layer (10) onto a support substrate (2), comprising:- assembling a donor substrate (1) and the support substrate (2), by direct bonding at their respective front faces (1a, 2a) and along a bonding interface (3), to form a bonded assembly (100) having a local non-bonded zone (31) within this bonding interface (3), the donor substrate (1) further comprising a buried fragile plane (11),- separation along the buried fragile plane (11), initiated at the local non-bonded zone (31) after growth of microcracks in said plane (11) by thermal activation, the separation leading to the transfer of a thin layer (10) from the donor substrate (1) onto the support substrate (2), the method being characterized in that the local non-bonded zone (31) is generated by at least one rough zone (31a) produced by scanning a laser beam on at least one front faces (1a,2a) donor (1) and support (2) substrates before their assembly, the scanning extending over a surface of at least 100 micrometers by 100 micrometers and at most 500 micrometers by 500 micrometers., Transfer method according to the preceding claim, in which the rough zone is produced by surface melting of a material making up the donor substrate (1) and / or the support substrate (2), on the side of their respective front face (1a, 2a). Transfer method according to the preceding claim, in which the scanning of the laser beam has a duration of between 20 microseconds and 100 microseconds during which the beam has an energy density of between 5 mJ / m^2 and 7 mJ / m^2. Transfer method according to the preceding claim, in which the laser beam has a wavelength of 355 nm. Transfer method according to one of the preceding claims in which the laser beam has a size inscribed in a circle of 80 micrometers in diameter. Transfer method according to one of the preceding claims, in which the local non-bonded zone (31) is generated by a plurality of elementary rough zones (31a) separated from each other, each being produced by scanning the laser beam over a surface of at least 100 micrometers by 100 micrometers and at most 200 micrometers by 200 micrometers. Transfer method according to one of the preceding claims, in which the rough zone (31a) is produced on the front face (1a) of the donor substrate (1), before the formation of the buried fragile plane (11) in said donor substrate (1). Transfer method according to the preceding claim, in which: - the donor substrate (1) is formed from a first material at its front face (1a), and a preliminary rough zone is produced on the surface of said material, and - thermal oxidation of the first material of the donor substrate (1) is carried out after the production of the preliminary rough zone and before the formation of the buried fragile plane (11), to form an insulating layer which will be assembled on the support substrate (2) in the bonded assembly (100), said insulating layer comprising, on its free face, the rough zone (31a) directly above the preliminary rough zone. Transfer method according to one of claims 1 to 6, in which: - the support substrate (2) is formed from a first material at its front face (2a), and a preliminary rough zone is produced on the surface of said material, and - thermal oxidation of the first material of the support substrate (2) is carried out after the production of the preliminary rough zone, to form an insulating layer which will be assembled on the donor substrate (1) in the bonded assembly (100), said insulating layer comprising, on its free face, the rough zone (31a) directly above the preliminary rough zone. Transfer method according to one of the preceding claims, in which the local non-bonded zone (31) has at least one lateral dimension, in the plane of the bonding interface (3), less than 500 micrometers. Transfer method according to one of the preceding claims, in which the donor substrate (1) and the support substrate (2) have a disc shape with a diameter of 300 mm. Transfer method according to one of the preceding claims, in which the local non-bonded zone (31) is located more than 3 mm from the edge of the bonded assembly (100), in the plane of the bonding interface (3). Transfer method according to the preceding claim, in which the local non-bonded zone (31) is located less than 10 mm from the edge of the bonded assembly (100), in the plane of the bonding interface (3). Transfer method according to one of the two preceding claims further comprising a step of annealing the bonded assembly (100) to cause separation along the buried fragile plane (11), this step being carried out in a furnace by holding the bonded assembly (100) vertically and oriented angularly so as to place the rough zone (31a) in an upper portion of the furnace. Transfer method according to one of the preceding claims, in which the thin layer (10), originating from the donor substrate (1), is made of monocrystalline silicon and the support substrate (2) comprises monocrystalline silicon, to form a stacked structure (110) of SOI type.

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