Method for transferring a thin layer
The method controls fracture initiation in thin film transfer by localized ion implantation and heat treatment, ensuring uniformity and integrity of the transferred film, addressing cost and non-uniformity issues in existing processes.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- COMMISSARIAT A LENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES
- Filing Date
- 2025-12-16
- Publication Date
- 2026-06-25
AI Technical Summary
Existing thin film transfer processes are costly and lack control over fracture initiation, leading to non-uniformity and damage to the thin film, which is critical for microelectronic circuits.
A method involving localized ion implantation of first atomic elements to create a weakening zone, followed by a heat treatment to grow microcracks, and a second uniform ion implantation to establish a plane of embrittlement, ensuring controlled fracture initiation without bubble formation, thus preserving the thin film integrity.
The method achieves repeatable and cost-effective thin film transfer with reduced fracture time, maintaining uniform thickness and reduced roughness, suitable for microelectronic applications.
Smart Images
Figure EP2025087296_25062026_PF_FP_ABST
Abstract
Description
Description Title: Thin Film Transfer Process TECHNICAL FIELD [ooi] The field of the invention is that of microelectronics. In particular, the invention relates to a method for transferring a thin film onto a receiving substrate. The method can be implemented for transferring a thin film of silicon, or of another material such as, for example, lithium tantalate, lithium niobate, silicon carbide, gallium nitride or indium phosphide. PREVIOUS STATE OF THE ART
[0002] A well-known method for transferring a thin film onto the front face of a receiving substrate is the Smart Cut™ process. In this process, a weakened plane is created by implanting light species through the front face of a substrate, called the donor substrate. Examples of light species include H₂ ions. + and / or H2 + and / or He+Then, the donor substrate is bonded to another substrate, called the receiving substrate. The technique used is frequently a so-called direct bonding, meaning it does not involve the addition of a bonding layer. Next, a heat treatment induces and then propagates microcracks under pressure containing light species. These microcracks grow primarily through coalescence events, during which the donor substrate material located between neighboring microcracks suddenly breaks, thus creating a larger microcrack. At a certain point, depending in particular on process parameters that led to the formation of the embrittlement plane and the nature of the substrates, the two bonded substrates spontaneously detach, not at the interface where they were bonded, but along the plane of implantation of the donor substrate.The thin layer located between the surface and the implantation plane of the donor substrate is thus transferred to the recipient substrate. The time between the start of the heat treatment and the moment of separation is called the time to fracture (TTF). Separation occurs as a fracture wave propagating from a region with a specific configuration, capable of initiating the fracture wave at the time of fracture. Such a region is called a fracture initiator. Since the fracture wave is very fast (typically several km / s), the actual fracture event has a negligible duration compared to the time to fracture.
[0003] After the two substrates are separated, finishing treatments are applied to the transferred thin film. These treatments aim to restore the crystalline quality of the transferred material, reduce the roughness of the fractured surface, and thin the transferred layer to the desired thickness. These finishing treatments may include thermal oxidation, chemical etching, chemical polishing, and / or heat treatments under controlled environments, such as reducing or neutral atmospheres.
[0004] It is known that fracture time influences the roughness and roughness uniformity of the fractured surface of the thin film, with shorter fracture times resulting in lower roughness of the fractured surface after finishing treatments. It is also known that the location of the fracture initiator impacts the roughness amplitude, its uniformity, and the thickness uniformity of the transferred thin film. Indeed, when the fracture propagates at high speed, it interacts with its own acoustic emission. These interactions result in local roughness modulations that disrupt the uniformity of the transferred thin film. These variations are called fracture wave patterns. A description of the mechanisms involved, along with examples, can be found in the following two publications: Crack Front Interaction with Self-Emitted Acoustic Waves, D. Massy, et al., Phys. Rev. Lett.121, 195501 - Published on November 5, 2018 and Landru, Didier, et al. "Fracture wake patterns in brittle solids." Physical Review Applied 15.2 (2021): 024068. Depending on the different stacking configurations and manufacturing processes, it can sometimes be advantageous to initiate the fracture at the center of the substrate, and sometimes at the edge. It therefore appears important to control, in a repeatable manner, both the position of the fracture initiator and the fracture time.
[0005] Document WO2023186595 proposes a solution for this purpose, in which, in addition to conventional hydrogen / helium implantation across the entire wafer, a localized hydrogen implantation at a very high dose (at least 3 times greater than the full-wafer hydrogen dose) is performed. This implantation also targets the same embrittlement plane but is carried out on a restricted area of approximately 10 pm 2 and 2 cm 2The overdosed zone is preferably placed in the center to limit the formation of fracture wave patterns. Implantation of the overdosed zone is performed through a mask or by controlled scanning of the hydrogen ion beam. Experimentally, fracture initiation from the overdosed zone is indeed observed, but the additional implantation time required to achieve such local overdoses significantly increases the cost of the procedure.
[0006] Document WO2023151852 teaches an alternative method for reducing fracture time in a controlled and repeatable manner, without increasing the time For implantation, one of the two front surfaces of the donor and recipient substrates is locally irradiated before bonding with a pulsed ultraviolet laser to create a rough area approximately 200 µm in diameter. This rough area then creates a non-bonded zone at the time of bonding, capable of initiating the fracture wave. It therefore acts as a fracture initiator. For this method as well, a central placement of the fracture initiator is preferred to obtain reduced roughness of the fractured surface. However, this process has the drawback of creating a hole in the thin film at the level of the rough area. At this location, there is no transfer (this is called the non-transfer zone). It is therefore unusable for fabricating microelectronic circuits.
[0007] Therefore, there is a need for a cost-effective thin-layer transfer process that allows the fracture step to be controlled in a repeatable manner without damaging the thin layer. DESCRIPTION OF THE INVENTION
[0008] The invention aims to remedy at least in part the disadvantages of the prior art, and more particularly to propose a cost-effective method for transferring a thin film onto a receiving substrate, preserving the integrity of the thin film, allowing the fracture time to be reduced in a controlled and repeatable manner, and controlling the position of the fracture initiator.
[0009] To this end, the object of the invention is a method for transferring a thin film from a donor substrate to a recipient substrate, comprising a first localized ion implantation of first atomic elements through a portion of a front face of the donor substrate to create a zone of embrittlement, the first atomic elements being chosen from hydrogen, helium, a noble gas, or a mixture thereof. The method comprises a second ion implantation of second atomic elements through the front face of the donor substrate to create a plane of embrittlement delimiting the thin film within the donor substrate, the second atomic elements being chosen from hydrogen, helium, or a mixture thereof. [ooio] The process includes molecular adhesion of the front face of the donor substrate to the recipient substrate to obtain a bonded structure; detachment of the thin film including a second heat treatment of the bonded structure; and a first heat treatment of the donor substrate, performed between the first and second ion implantations, to grow microcracks in the embrittlement zone so as to increase the degree of embrittlement of the embrittlement zone. Conditions of the first ion implantation and the first heat treatment are such that the front face is free of bubbles during bonding and such that the weakening zone is a fracture initiator during detachment of the thin layer. [ooii] Some preferred but not limiting aspects of this process are the following. [ooi2] The first ion implantation and the second ion implantation can be performed at a first depth and a second depth, respectively. The difference between the first and second depths can be less than 300 nm in absolute value. [ooi3] The first heat treatment can be carried out at a temperature between 300 °C and 600 °C. The duration of the first heat treatment can be greater than or equal to 5 minutes.
[0014] The weakening zone can have all its dimensions, measured parallel to the front face of the donor substrate, ranging from 10 pm to 10 mm. [ooi5] The weakening zone may be directly above the center of the front face of the donor substrate.
[0016] The first atomic elements can be helium ions.
[0017] The donor substrate can be silicon, gallium nitride, lithium tantalate, or lithium niobate.
[0018] The embrittlement zone may contain microcracks with an average size greater than or equal to 100 nm after the first heat treatment.
[0019] The receiving substrate may include a charge-trapping layer.
[0020] The donor substrate may include a dielectric layer and the front face may be a face of the dielectric layer. BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Other aspects, purposes, advantages and features of the invention will become more apparent from the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which: Figures 1A to 1F are schematic cross-sectional views of an example of process steps according to the invention; Figure 2A is a haze map of a fractured surface using a prior art transfer method; Figure 2B is an image obtained by interference profilometry of an area of the fractured surface using the prior art transfer method; Figures 3A and 3B are haze maps of fractured surfaces using a first example of a transfer method according to the invention; Figure 3C is an optical microscope view of an epicenter region obtained using the first example of a transfer method; Figure 3D is an atomic force microscope measurement of a height of the fractured surface obtained using the first example of a transfer method, in the vicinity of the epicenter region; Figure 4A is a haze map of a fractured surface using a second example of a transfer method according to the invention;Figure 4B is a roughness map obtained with an atomic force microscope at the level of a surface including a boundary of the epicenter zone of the fractured surface with the second example of the transfer process. Figure 4C is an image obtained by interference profilometry of an area of the fractured surface with the second example of the transfer process; Figure 5A is a haze map of a fractured surface with a comparative example of the transfer process; Figure 5B is an image obtained by interference profilometry of an area of the fractured surface with the comparative example; Figure 6A is a graph showing the power spectral densities of the roughness of the fractured surfaces obtained with the second example, the comparative example and the prior art transfer process; Figure 6B is a detail of Figure 6A. DETAILED DESCRIPTION OF SPECIFIC METHODS OF IMPLEMENTATION
[0022] In the figures and throughout the description, the same reference numerals represent identical or similar elements. Furthermore, the various elements are not drawn to scale to ensure clarity in the figures. Moreover, the different embodiments and variants are not mutually exclusive and may be combined. Unless otherwise indicated, the terms "approximately," "about," and "in the order of" mean within 10%, and preferably within 5%. Furthermore, The terms "between ... and ..." and equivalents mean that the boundaries are included, unless otherwise stated.
[0023] The invention relates to a method for transferring a thin film onto a receiving substrate. The method comprises a first localized ion implantation of first atomic elements through a portion of the front face of a donor substrate to create a zone of embrittlement at a first depth. The first ion implantation is followed by a first heat treatment to grow microcracks in the zone of embrittlement containing the first atomic elements. A second uniform ion implantation of second atomic elements is then performed substantially through the entire front face of the donor substrate to create a plane of embrittlement at a second depth close to the first depth. The front face of the donor substrate is bonded by molecular bonding to a front face of a receiving substrate. A second heat treatment then detaches the thin film at the plane of embrittlement.
[0024] The dose of the first localized ion implantation is sufficiently low to avoid creating bubbles on the front face of the donor substrate before bonding and to control the roughness of the fractured surface. Therefore, the conditions of the first ion implantation (particularly the implanted dose) and the first annealing are not conditions that lead to layer transfer by Smart Cut™. The first heat treatment results in localized ripening of the embrittlement zone, which, surprisingly, has the same effect on fracture initiation as a much higher dose, such as one of the doses used in document WO2023186595. Fracture starts repeatably at the embrittlement zone; that is, the embrittlement zone is a fracture initiator for at least 90% of the thin films transferred using the process of the invention, and possibly at least 95% or even 99%.
[0025] The initial ion implantation, combined with the first heat treatment, shortens the time to fracture time (TTF), particularly compared to a transfer process that does not involve the prior formation of a local embrittlement zone. The first heat treatment thus promotes fracture initiation after the second ion implantation, during the second heat treatment.
[0026] The first ion implantation occurs at a depth close to that of the second ion implantation. The cumulative concentrations of atomic elements implanted during the first and second successive ion implantations can locally modify the thickness of the transferred thin film at the embrittlement zone. Consequently, it can be advantageous to adjust the energy of the first ion implantation relative to an energy of the second ion implantation, for example to standardize the thickness of the transferred thin layer and / or minimize the topology of the fractured surface.
[0027] By "layer" we mean an extent of a crystalline or non-crystalline material, whose thickness along a Z axis is less, for example ten times or even twenty times, than its longitudinal dimensions of width and length in a plane (X, Y) orthogonal to the Z axis. A layer may include several sublayers, each satisfying the definition of "layer".
[0028] Throughout this description, a substrate is considered to be made of a particular material if it essentially comprises that material and possibly one or more additional layers formed on top of it. If a substrate is made of a particular semiconductor material, it essentially comprises that semiconductor material, possibly doped. The semiconductor material may also contain impurities, such as oxygen precipitates.
[0029] An example of a process for transferring a thin film 150 from a donor substrate 100 onto a recipient substrate 200 according to the invention will now be described with reference to Figures 1A to 1F. In each of these figures, a situation is shown on the left such that the process parameters lead to the transfer of a thin film of substantially uniform thickness. On the right, the process parameters lead to the transfer of a thin film with an excess thickness in a central region.
[0030] The donor substrate 100 is here in crystalline silicon, preferably oriented along a crystalline (100) orientation, but the process can also be directly applied to other crystalline semiconductor materials such as, for example, lithium tantalate, lithium niobate, silicon carbide, gallium nitride or indium phosphide, after possible adjustments of parametric values within the reach of a person skilled in the art, such as adjustments of dose and / or implantation energy.
[0031] The transfer process first involves the provision of a donor substrate 100 and a recipient substrate 200. The donor substrate 100 is, for example, a disc-shaped plate with a diameter of 100 mm, 150 mm, 200 mm, or 300 mm.
[0032] The donor substrate 100 may include one or more additional layers (not shown in the figures), for example, a dielectric layer, such as native silicon dioxide, deposited or thermally bonded, or silicon nitride. The additional layer(s) may have a cumulative thickness of between 4 nm and 800 nm. The donor substrate 100 comprises a front face 101a and a rear face 101b parallel and opposite to the front face 101a.
[0033] Similarly, the receiving substrate 200 has a front face 201a and a rear face 201b parallel and opposite to the front face 201a. It can be a disc-shaped plate with a diameter greater than or equal to that of the donor substrate 100. Here, the receiving substrate 200 is such a plate with the same diameter as the donor substrate 100. The receiving substrate 200 is rigid, that is to say, it typically has a thickness of a few hundred microns, for example equal to 525 pm, 675 pm, 725 pm or 775 pm.
[0034] The receiving substrate 200 may also include, on the side of its front face 201a or its rear face 201b, one or more additional layers (not shown in the figures), such as a dielectric layer, and / or a charge trapping layer, for example in polycrystalline silicon.
[0035] The front face 101a, 101b, and the rear face 201a, 201b of each donor or recipient substrate 100, 200 are connected by a lateral flank. The lateral flank may include a rounded surface and / or a chamfer.
[0036] In Figure 1A, a first localized ion implantation of first atomic elements is performed through a portion of the front face 101a of the donor substrate 100 to create a weakening zone 105 located at a first depth. The weakening zone 105 locally defines a flat surface substantially parallel to the front face 101a of the donor substrate 100. The first depth is the distance between the weakening zone 105 and the front face 101a, measured perpendicularly to it. When the donor substrate 100 includes one or more additional layers on the side of the front face 101a, the first depth is strictly greater than the cumulative thickness of the additional layer(s).
[0037] The flat surface has dimensions strictly smaller than the dimensions of the front face 101a in all directions of the front face 101a. The dimensions of the flat surface are typically between 10 µm and 10 mm in all directions parallel to the front face 101a. It preferably has the shape of a filled ellipse or a disk, for example with a diameter of 100 µm. Preferably, the embrittlement zone 105 is directly above the center of the front face 101a of the donor substrate 100 or an external region of the front face 101a located at the periphery of the donor substrate 100, for example, at a distance of between 2 mm and 3 mm from an edge of the front face 101a.
[0038] The first atomic elements are chosen from light atomic species, such as hydrogen or helium ions, or from noble gas ions taken from group 18 of the periodic table of elements, such as neon, argon, krypton, xenon, or radon. The first atomic elements may consist of a mixture of these chemical elements. The first ion implantation may include several ion implantation substeps, possibly with ions of different natures.
[0039] The first atomic elements are implanted at an initial dose, with an initial energy. The first dose can be between 0.5 x 10 16 atoms / cm 2 and 3.10 16 atoms / cm 2 , for example between 0.5.10 16 atoms / cm 2 and 2.10 16 atoms / cm 2 , or even between 0.5.10 16 atoms / cm 2 and 1.7.10 16 atoms / cm 2The first energy is a parameter that allows the first depth to be reached. For example, when the first atomic elements are helium ions and the donor substrate 100 is silicon, a first depth of 432 nm, 472 nm or 548 nm can respectively be reached with a first implantation energy of 50 keV, 60 keV or 70 keV.
[0040] The first ion implantation can be performed through a mask with a through-hole. The mask can be a mechanical part or a structured layer formed on the front face 101a of the donor substrate 100 by a process including a photolithography step. In the latter case, the structured layer is removed after the first implantation. Alternatively, the embrittlement zone 105 is obtained by a controlled scanning of the ion beam composed of the first atomic elements.
[0041] Following the step shown in Figure 1A, the first atomic elements begin to precipitate in microcavities extending primarily in two dimensions. Most of these microcavities extend in a plane roughly parallel to the front face 101a.
[0042] In Figure 1B, the donor substrate 100 undergoes an initial heat treatment to grow microcracks in the embrittlement zone 105, originating from the microcavities, thereby increasing the degree of embrittlement of the embrittlement zone 105. This initial heat treatment can typically be carried out in a vertical or horizontal furnace capable of processing several donor substrates 100 simultaneously, or in a rapid thermal annular (RTA) furnace. It subjects the donor substrate 100 to a temperature between 300°C and 600°C, for example, 350°C, for a duration between 5 and 60 minutes, for example, 15 minutes. The thermal budget of the initial heat treatment remains below a threshold beyond which blisters appear on the front face 101a.
[0043] After the first heat treatment, the microcracks have an average size D mwithin the embrittlement zone 105, greater than or equal to a minimum average dimension D m ,min so as to initiate a fracture wave in a repeatable manner at the level of the embrittlement zone 105 during the second heat treatment. The average size of the microcracks can be measured using an infrared confocal microscope by inspecting an area of the embrittlement zone 105, for example, 100 µm x 100 µm, or 200 µm x 300 µm. It corresponds to the average of the largest dimension of several microcracks measured parallel to the front face 101a. It is preferably calculated over an area containing 30 or more microcracks.
[0044] The minimum average dimension D m,min depends in particular on the material of the donor substrate 100. As an example, it is on the order of 100 nm for silicon (Si), gallium nitride (GaN), lithium niobate (LiNbO3) and lithium tantalate (LiTaO3).
[0045] It is possible to adjust the average size of the microcracks by modifying the conditions of the initial implantation and / or the initial heat treatment. For example, increasing the temperature and / or duration of the initial heat treatment and / or the initial dose will increase the average size of the microcracks. The average size of the microcracks remains below a value beyond which bubbles could form on the front face 101a.
[0046] In Figure 1C, a second uniform ion implantation of second atomic elements is performed through the entire front face 101a of the donor substrate 100 to create a weakening plane 110. The weakening plane 110 is substantially parallel to the front face 101a and has an area greater than or equal to the area of the front face 101a. It is located at a second depth. The second depth is the distance separating the weakening plane 110 from the front face 101a, measured perpendicular to the latter. Preferably, the difference between the first and second depths is less than 300 nm in absolute value. Even more preferably, the first and second depths are substantially equal, as shown in the left-hand portion of Figure 1C.
[0047] The second atomic elements are chosen from hydrogen and / or helium ions. The second ion implantation may include several ion implantation substeps. It is performed at a second implantation energy to reach the second depth, and at a second dose per implanted atomic element of between 0.9 and 10⁻¹¹. 16 atoms / cm 2 and 3.10 16 atoms / cm 2 , for example equal to 10 16 atoms / cm 2 .
[0048] In Figure 1D, a bonded structure 300 is obtained by direct molecular adhesion of the front face 101a of the donor substrate 100 to the front face 201a of the recipient substrate 200. Several direct bonding methods known to those skilled in the art are applicable to the invention. These may include, for example, hydrophilic or hydrophobic bonding at room temperature, possibly after plasma activation or chemical bonding of one or both of the respective front faces. This can also be atomic diffusion bonding (ADB), or surface activated bonding (SAB).
[0049] Figures 1E and 1F are schematic views, respectively, of an intermediate step and the result of a second heat treatment applied to the bonded structure 300. The second heat treatment leads to the transfer of a thin layer 150 from the donor substrate 100 and delimited by the embrittlement plane 110 and the front face 101a of the donor substrate 100. The second heat treatment can typically be carried out in a horizontal furnace capable of treating several bonded structures 300 simultaneously, at a temperature between 200°C and 500°C in the case of a silicon donor substrate 100.
[0050] A fracture wave is initiated at a zone of the weakening plane 110 located opposite the weakening zone 105, the latter being able to be within the weakening plane 110, particularly in the situation shown in the left-hand portion of Figure 1D. Thus, the weakening zone 105 acts as a fracture initiator. The fracture wave separates the thin layer 150 from a remaining portion of the donor substrate 100 at a fractured surface 151 of the thin layer 150, corresponding approximately to the weakening plane 110 and the weakening zone 105. The remaining portion of the donor substrate 100 is then removed (Figure 1F) and can be reused for the transfer of a new thin layer.
[0051] The fracture wave passes near the weakened zone 105, at an epicenter 152 of the thin layer 150. The epicenter 152 extends deep into the thin layer 150 from the fractured surface 151. Along each direction in a plane parallel to the front face 101a, the epicenter 152 has a dimension correlated to a dimension of the weakened zone 105 measured along the same direction. In the advantageous case where the first and second depths are substantially equal, the thin layer 150 has substantially the same thickness at every point within it; it is substantially uniform.
[0052] In the case where the second heat treatment is carried out in a furnace capable of processing several bonded structures 300 simultaneously, the conditions of the first ion implantation and the first heat treatment are such that the embrittlement zones 105 are a fracture initiator for at least 90% of the bonded structures 300, or even at least 95% or 99%. For this reason, when the donor substrate 100 is silicon, gallium nitride, lithium niobate, or lithium tantalate, the average size of the microcracks in the embrittlement zone 105 can be greater than or equal to 100 nm after the first heat treatment.
[0053] After separation of the thin layer 150, a finishing step comprising one or more of the finishing treatments known in the prior art can be applied to the thin layer 150. The finishing step may, for example, include a thinning substep involving sacrificial oxidation, followed by smoothing of the fractured surface 151 by heat treatment under a neutral gas, for example, argon. Following such a finishing step, the thin layer 150 has substantially the same thickness and roughness at every point within it. Thus, the integrity of the thin layer 150 is preserved.
[0054] The roughness of fractured surface 151 can be measured or mapped using an atomic force microscope (AFM). It is also possible to obtain a roughness map of fractured surface 151 using an inspection tool such as the one provided by KLA-Tencor, marketed under the Surfscan™ brand. In such a tool, the roughness level of a surface is measured or made apparent by measuring the diffuse background noise when the surface is illuminated by a light beam. This roughness measurement or evaluation technique is commonly referred to as "haze" measurement, in the technical field of the invention, borrowing an English term. The light beam can scan the surface to be mapped. It diffuses across the surface as a scattered beam.The intensity of the scattered beam is generally proportional to the variance of the surface height variations within a range of spatial frequencies that depend on the wavelength of the light beam. For more complete information on this technique, see the article "Seeing through the haze" by F. Holsteyns (Yield Management Solution, Spring 2004, pp. 50-54).
[0055] In connection with figures 3A, 3B, 4 and 5, results obtained with a first example of a transfer process according to the invention will be described.
[0056] For this first example, the first atomic elements are helium ions (He). + The first dose is equal to 2E16 at / cm² 2The first energy is equal to 60 keV, for a first depth approximately equal to 480 nm. The first implantation is carried out through a mask comprising a substantially circular through-hole of radius equal to 5 mm, located at the center of the front face 101a of the donor substrate 100.
[0057] The second implantation consists of a first substep of helium ion implantation (He). + at a dose of 1 E16 at / cm 2 and an energy of 38 keV, and a second sub-step of hydrogen ion implantation H + at a dose of 1 E16 at / cm 2 and an energy of 24 keV. The second depth is approximately equal to 300 nm.
[0058] The first heat treatment is carried out at 350 °C for one hour. The temperature of the second heat treatment is 500 °C.
[0059] Figures 3A and 3B are haze maps of two different fractured surfaces obtained with this first example of the transfer process. It can be observed that the fracture wave was initiated at the center.
[0060] For comparison, Figure 2A shows a haze map of a surface fractured by a prior art transfer process in which the embrittlement plane was created by co-implantation of helium and hydrogen ions, without prior formation of a embrittlement zone and without initial heat treatment. In Figure 2A, the haze signal highlights an interaction between the fracture wave and an acoustic wave, resulting in variations in the roughness of the fractured surface. The haze map identifies a fracture wave initiation point located on the left side of the plate. It can be observed that the roughness variations are less pronounced in Figures 3A and 3B than in Figure 2A.
[0061] Figure 3C is an optical microscope view of the epicenter region 152 obtained with the first example of the transfer process. Its width is between 1 cm and 1.5 cm, measured parallel to the front face 101a. Its height can be determined from the height profile in Figure 3D, measured using AFM. In Figure 3D, the fractured surface 151 was scanned along a segment crossing a boundary of the epicenter region 152. The x-axis shows the position in meters (pm) along the segment. The y-axis shows the height of the fractured surface 151 in millimeters (nm). The epicenter region 152 corresponds to the right-hand side of the height profile. The epicenter region 152 is raised by 200 nm, a value approximately equal to the difference between the first and second depths.
[0062] Figure 4A is a haze map of a fractured surface 151 with a second example of the transfer process according to the invention. The second example is identical to the first example, except for the first energy, here equal to 38 keV, for a first depth approximately equal to 300 nm and approximately equal to the second depth. It can be observed that the fracture wave was initiated at the center. The roughness level of the fractured surface 151 is uniform and lower than that obtained with the process of Figure 2A.
[0063] Figure 4B is a height map of fractured surface 151 obtained by atomic force microscopy, at a surface including a boundary of the epicenter region 152, the latter being located on the rougher part to the left. It can be seen that fractured surface 151 is substantially planar on either side of the region epicenter 152. The thin layer 150 therefore has substantially the same thickness at the epicenter region 152 as everywhere else.
[0064] Results obtained with a comparative example of transfer processes will now be described. The comparative example is identical to the second example of a transfer process, except for the first dose, which is here equal to that of helium ions (He). + used during the second implantation, i.e. equal to 1 E16 at / cm 2 Therefore, as with the second example, the first and second depths are approximately equal. However, the first dose, combined with the thermal budget of the first heat treatment, leads to an average microcrack size below 100 nm in the 105 embrittlement zone, after the first heat treatment.
[0065] Figure 5A is a haze map of the fractured surface 151 thus obtained. This can be compared to the maps in Figures 2A, 3A, and 3B discussed above. The fracture initiation zone is indicated by an arrow. This zone is located in an intermediate region between the center, the site of the first implantation, and the edge. The conditions of the comparative example therefore do not allow for repeatable control of the fracture initiator's position.
[0066] Figure 5B is an image obtained by interference profilometry of a region of the fractured surface compared to the example. The abscissa and ordinate are given in pm. Thus, the region measures 120 pm by 90 pm. The gray levels represent the variations in height of the fractured surface. The scale is given in nm. A calculation determined that the roughness is equal to 9.57 nm RMS (RMS stands for Root Mean Square).
[0067] The image in Figure 5B can be directly compared with those in Figures 2B and 4C. These latter images were obtained by interference profilometry on areas of fractured surfaces using, respectively, the prior art transfer method and the second example of a transfer method. The areas are all the same size. The calculated roughness values are 6.85 nm RMS for Figure 2B and 6.41 nm RMS for Figure 4C. Thus, the second example of a method resulted in a fractured surface roughness lower than that obtained with the prior art method. In contrast, the application of the comparative example degrades it.
[0068] Figures 6A and 6B allow us to compare the spectral decomposition of the roughness obtained with the second example of the process (disks), the comparative example (triangles), and the prior art process (squares). In these figures, the y-axis gives the power spectral density in m3 variations in height of the corresponding fractured surfaces. The x-axis gives the spatial frequency in m -1 . There Figure 6B is a detail of Figure 6A surrounding the spatial frequency range between 10 6 rrr 1 and 10 7 rrr 1 (spatial periods from 125 nm to 1 pm). It is observed that the second example of the process makes it possible to improve all the spectral components of roughness, particularly in the 10 range 6 rrr 1 at 10 7 rrr 1 .
[0069] Specific embodiments have just been described. Different variations and modifications will be apparent to those skilled in the art.
Claims
DEMANDS 1. A method for transferring a thin film (150) from a donor substrate (100) onto a recipient substrate (200) comprising: o a first localized ion implantation of first atomic elements through a portion of a front face (101a) of the donor substrate (100) to create a weakened zone (105), the first atomic elements being selected from hydrogen, helium, a noble gas, or a mixture thereof; o a second ion implantation of second atomic elements through the front face (101a) of the donor substrate (100) to create a weakened plane (110) delimiting the thin film (150) within the donor substrate (100), the second atomic elements being selected from hydrogen, helium, or a mixture thereof; o molecular adhesion of the front face (101a) of the donor substrate (100) onto the receiving substrate (200) to obtain a bonded structure (300),o a detachment of the thin film (150) comprising a second heat treatment of the bonded structure (300), the process being characterized in that o a first heat treatment of the donor substrate (100) is carried out between the first ion implantation and the second ion implantation to grow microcracks in the embrittlement zone (105) so as to increase the degree of embrittlement of the embrittlement zone (105), o conditions of the first ion implantation and the first heat treatment are such that the front face (101a) is free of bubbles during bonding and such that the embrittlement zone (105) is a fracture initiator during the detachment of the thin film (150).
2. Transfer method according to claim 1, wherein the first ion implantation and the second ion implantation are respectively carried out at a first depth and a second depth, the difference between the first depth and the second depth being less than 300 nm in absolute value.
3. Transfer method according to claims 1 or 2, wherein the first heat treatment is carried out at a temperature between 300 °C and 600 °C, for a duration greater than or equal to 5 minutes.
4. Transfer method according to any one of the preceding claims, wherein the embrittlement zone (105) has all its dimensions, measured parallel to the front face (101a) of the donor substrate (100), between 10 pm and 10 mm.
5. Transfer method according to claim 4, wherein the embrittlement zone (105) is directly above the center of the front face (101a) of the donor substrate (100).
6. Transfer method according to any one of the preceding claims, wherein the first atomic elements are helium ions.
7. Transfer method according to any one of the preceding claims, wherein the donor substrate (100) is silicon, gallium nitride, lithium tantalate, or lithium niobate.
8. Transfer method according to claim 7, wherein the embrittlement zone (105) has microcracks with an average size greater than or equal to 100 nm after the first heat treatment.
9. Transfer method according to any one of the preceding claims, wherein the receiving substrate (200) comprises a charge-trapping layer.
10. Transfer method according to any one of the preceding claims, wherein the donor substrate (100) comprises a dielectric layer and the front face (101a) is a face of the dielectric layer.