TRANSFER METHOD OF A LAYER WITH LOCALIZED REDUCTION OF THE ABILITY TO TRIGGER A BREAK

Description

TECHNICAL FIELD

[0001] The field of the invention is that of methods for transferring a layer from a donor substrate to a receiving substrate that employ fracturing along a weakening plane formed in the donor substrate. The invention relates more particularly to the control of the fracturing process. PREVIOUS TECHNIQUE

[0002] Smart Cut™ technology is a well-known technique for transferring thin films of materials, such as semiconductors. According to this technique, ionic species like hydrogen or helium are implanted into a donor substrate to create a plane of embrittlement. The donor substrate is then brought into contact with a recipient substrate, for example, by direct bonding. This technique then relies on the development of defects generated during implantation to induce fracturing. This development requires an energy input, generally achieved through heat treatment, to allow the formation of a confined layer of cavities and micro-cracks within which a fracture wave will initiate and propagate.This wave will gradually extend along the entire weakening plane, leading to the separation of the thin surface layer delimited by this weakening plane and the rest of the substrate.

[0003] Document US2016 / 0233125 discloses a method for transferring a useful layer.

[0004] Document WO 2018 / 029419 A1, which discloses the features of the preamble to claim 1, states that the interaction between the propagation of the fracture wave and acoustic vibrations emitted during the initiation and / or propagation of the fracture wave leads to the formation of periodic patterns of variations in thickness and / or roughness of the transferred layer, which extend over the entire surface of this layer. In other words, the fracture wave is deflected vertically from its plane of propagation according to the state of the instantaneous stresses of the material through which it passes, this stress state being influenced by the acoustic wave.

[0005] The lack of precise control over the location of fracture wave initiation leads to variability between transferred thin layers, particularly in terms of roughness and / or thickness. This variability necessitates the implementation of more advanced post-fracture controls and / or specific post-fracture treatments. To limit this variability, the aim is to initiate the fracture wave in a standardized manner. The previously cited document WO 2018 / 029419 A1 describes the creation of zones on the wafer where fracture initiation is preferential. Such creation may involve, in particular: to achieve a localized overdosing in a target area of ​​the species implanted during the formation of the weakening plane in the donor substrate; or to achieve a localized thermal budget surplus in a target area of ​​the species implanted during the formation of the weakening plane in order to grow more rapidly the defects in this area of ​​the weakening plane.

[0006] The document WO2018149906 A1 describes a process comprising a first implantation step in the central part of the donor substrate, followed by a second implantation, at a lower dose, at the periphery of the donor substrate, thus creating a localized reduction in the ability of the embrittlement plane to initiate the fracture wave. DESCRIPTION OF THE INVENTION

[0007] The invention proposes a complementary or alternative technique to the aforementioned techniques for confining the fracture wave initiation location to a target zone within the embrittlement plane. More particularly, it relates to a method for transferring a thin layer from a donor substrate to a recipient substrate, the donor substrate having an embrittlement plane delimiting the thin layer and a massive portion of the donor substrate. The method comprises the following steps: implantation of species carried out uniformly over the entire donor substrate to form a weakening plane which delimits the thin layer and a massive part of the donor substrate, bringing the donor substrate and the recipient substrate into contact, initiation and propagation of a fracture wave along the weakening plane.

[0008] This process also includes, prior to contact, a localized reduction step of the capacity of the weakening plane to initiate the fracture wave.

[0009] Some preferred, but not exhaustive, aspects of this process are as follows: During the localized reduction step of the weakening plane's capacity to initiate the fracture wave, a central area of ​​the weakening plane is not subject to the reduction of the capacity to initiate the fracture wave; during the localized reduction step of the weakening plane's capacity to initiate the fracture wave, a peripheral ring of the weakening plane is subject to the reduction of the capacity to initiate the fracture wave; during the localized reduction step of the weakening plane's capacity to initiate the fracture wave, a peripheral ring of the weakening plane, with the exception of a sector of the peripheral ring, is subject to the reduction of the capacity to initiate the fracture wave; the initiation of the fracture wave includes an energy input localized to the sector of the peripheral ring not subject to the reduction of the capacity to initiate the fracture wave;The peripheral crown is located at the edge of the embrittlement plane; the localized reduction step of the embrittlement plane's ability to initiate the fracture wave includes localized laser annealing of the donor substrate; the localized laser annealing of the donor substrate includes localized irradiation of a free face of the thin film by one or more laser pulses; the localized reduction step of the embrittlement plane's ability to initiate the fracture wave includes the formation of a localized amorphous zone in the thin film; the amorphous zone is located less than 100 nm, preferably less than 75 nm, and even more preferably less than 50 nm from the embrittlement plane. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Other aspects, objectives, advantages, and features of the invention will become clearer upon reading 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: There figure 1 represents the diffuse light emitted by the surface of a silicon substrate as a function of the energy fluence of a laser pulse irradiating it. figure 2 represents microscopic observations of the surface of regions of a substrate subjected to different energy levels of laser irradiation. figure 3 represents microscopic observations of the surface of regions of a substrate subjected to different numbers of laser pulses. figure 4 This illustrates a possible initial implementation of the localized reduction of the weakening plane's capacity to initiate fracture. figure 5This illustrates a second possible manifestation of the localized reduction in the capacity of the weakening plane to initiate fracture. figure 6 This illustrates a third possible manifestation of the localized reduction in the capacity of the weakening plane to initiate fracture. figure 7 represents a thickness of silicon amorphized by a silicon implantation, depending on different implantation conditions. DETAILED DESCRIPTION

[0011] The invention relates to a method for transferring a thin film from a donor substrate to a recipient substrate. The donor substrate may be a silicon substrate, or any other semiconductor or non-semiconductor material. By way of example, it may be silicon-germanium, germanium, or a III-V material.

[0012] The process includes a step of forming a weakening plane within the donor substrate, delimiting the thin layer and a bulk portion of the donor substrate. This weakening plane is formed by implanting species, typically hydrogen and / or helium, uniformly (particularly in terms of dose) across the entire donor substrate. Thus, the dose of implanted species is uniform across the entire extent of the weakening plane, and the weakening plane exhibits the capacity to initiate a uniform fracture wave throughout its extent.

[0013] The process involves bringing the donor substrate and the recipient substrate into contact to form a unit to be fractured. This formation can be achieved by direct assembly, for example, by molecular adhesion. Following contact, the process includes the initiation and propagation of the fracture wave along the embrittlement plane, leading to the detachment of the thin layer of the donor substrate and its transfer to the recipient substrate. This initiation and propagation of the fracture wave includes a heat treatment, known as embrittlement heat treatment, applied to the unit to be fractured. This heat treatment embrittles the donor substrate at its embrittlement plane and provides sufficient energy for the fracture wave, once initiated, to propagate in a self-sustaining manner.

[0014] In the first implementation variant, heat treatment alone is sufficient to initiate the fracture wave. In the second implementation variant, the process includes a localized energy input, either after or during the embrittling heat treatment, to initiate the fracture wave. This energy can be of mechanical, thermal, or any other origin. For example, it could be localized heating by a laser or energy input via ultrasound.

[0015] According to the invention, fracture wave initiation is spatially controlled by reducing, or even inhibiting, the ability of one or more target areas of the embrittlement plane to initiate the fracture wave. To achieve this, the method comprises, before contacting the donor and recipient substrates, applying a treatment to the donor substrate that locally reduces the embrittlement plane's ability to initiate the fracture wave. This spatially selective reduction results in fracture wave initiation being localized in a region of the embrittlement plane distinct from the target area(s) whose probability of fracture wave initiation is reduced by said treatment.And as will be detailed later, in one embodiment, the localized reduction of the ability to initiate the fracture wave is carried out in such a way as to reduce the probability of initiation of the fracture wave by a target area without that target area obstructing the propagation of the fracture wave.

[0016] The following describes a methodology for determining operating conditions for a treatment of the donor substrate that reduces or even inhibits the ability of a target area of ​​the embrittlement plane to initiate the fracture wave.

[0017] During heat treatment applied to an implanted substrate not assembled with a stiffener, implantation defects evolve into cavities that can lead to blistering or even exfoliation of the implanted layer. In the field of this invention, it is generally considered that operating parameters leading to such blistering / exfoliation of an unassembled substrate will also enable the transfer of a thin film when the substrate is assembled with a stiffener.

[0018] In one possible embodiment of the invention, the localized reduction of the embrittlement plane's ability to initiate a fracture wave may involve localized laser annealing of the donor substrate. The inventors have indeed verified that by locally irradiating an implanted substrate with a laser beam, it is possible to reduce or even inhibit blistering / exfoliation phenomena in the irradiated areas, whereas these phenomena are commonly observed in the non-irradiated areas. The absence of blistering / exfoliation in the irradiated areas means that these areas cannot initiate a fracture wave.

[0019] The operating conditions for such localized laser annealing can be determined as described below, based on an example of an embodiment of the process according to the invention, which uses a silicon donor substrate having a embrittlement plane formed by hydrogen and helium implantation with energies in the 30-40 keV range and a total dose (H + He) on the order of 2 x 10¹⁶ ions / cm². The laser annealing is a nanosecond laser annealing (NSLA) type, initially exploiting a single irradiation pulse with a wavelength of 380 nm and a duration (full width at half maximum) of 160 ns.Different regions of the donor substrate are irradiated with an energy fluence ranging from 0.4 J / cm² to 3.125 J / cm² in steps of 0.025 J / cm² and the quality of the surface of the donor substrate is evaluated by means of a diffuse background measurement ("haze" according to the commonly used Anglo-Saxon terminology) corresponding to the intensity of the light scattered by the surface of the layer, using the Surfscan ®< SP2 inspection tool from KLA-Tencor.

[0020] There figure 1This represents the diffuse background noise H emitted by a region of the donor substrate (in arbitrary units) as a function of the energy fluence ED (in J / cm²). A peak in noise intensity H is observed for fluences generally between 1.6 and 1.9 J / cm². This peak is linked to an increase in surface roughness due to the formation of surface structures whose presence, observed by scanning electron microscopy (SEM), can be associated with the onset of surface melting. Then, for fluences greater than 2 J / cm², SEM again shows that good surface quality is achieved, which can be linked to recrystallization following total surface melting during irradiation. For fluences greater than 3 J / cm 2< , a new peak in noise intensity H is observed and a surface deformation phenomenon is observed by SEM, creating craters.

[0021] The irradiated donor substrate is subjected to heat treatment at 500°C for two minutes to check for the presence or absence of blistering in the different irradiated regions, and therefore the possibility of initiating fracturing. figure 2 represents microscopic observations of the surface of different regions of the substrate, and it is observed that the higher the ED fluence of the irradiation, the more the blistering / exfoliation phenomenon is reduced, even to the point of being inhibited. For example, the inventors were able to observe, using SEM, on the one hand, surface exfoliation and the presence of microcracks at the weakening plane after irradiation at 2.15 J / cm² (which is consistent with fracture initiation), and, on the other hand, the presence of microbubbles at the weakening plane after irradiation at 2.75 J / cm² (which, as shown in the image...). figure 2does not cause blistering / exfoliation and is therefore not compatible with fracture initiation).

[0022] From the above, it follows that in the context of single-pulse irradiation, it is preferable to avoid fluences between 1.6 and 1.9 J / cm² or fluences above 3 J / cm² to prevent surface degradation, and instead favor fluences between 1.9 and 3 J / cm². Within this latter range, the higher the fluence, the more the blistering phenomenon is inhibited, and therefore the lower the fracture wave initiation capacity.

[0023] We have represented on the figure 3Microscopic observations of regions irradiated by 2, 10, or 100 NSLA pulses after they have undergone the heat treatment to verify blistering. It is observed that increasing the number of pulses increases the inhibition effect. In particular, for the example of 100 pulses, the inhibition effect is pronounced even at a fluence of 1.55 J / cm².

[0024] It follows from the above that it is possible to reduce or even inhibit blistering using laser annealing, and that it is possible to adjust the fluence and number of pulses to achieve the desired inhibition effect. Returning to the description of the method according to the invention, this method can therefore comprise localized irradiation of the free face of the thin film (i.e., the face intended to be in contact with the receiving substrate) by one or more laser pulses. It is, of course, possible to irradiate several target areas, and even to apply different irradiation parameters (in particular, fluence and number of pulses) between the target areas.

[0025] In one possible embodiment, during the localized reduction step of the fracture wave initiation capacity, a peripheral rim of the weakening plane P, with the exception of a sector of the peripheral rim, is subjected to said reduction. The thickness of the peripheral rim is, for example, 100 µm to 10 mm, typically 500 µm to 2 mm. The sector occupies an area between 200 x 200 µm² and 20 x 20 mm² (corresponding to an angular opening of between a few tenths of a degree and 10 degrees). To achieve this, and as illustrated by the figure 4A laser annealing process can be applied to a peripheral target region Rc of the donor substrate surface corresponding to said peripheral ring, excluding a portion Ez of this target region corresponding to said sector. In this embodiment, the initiation of the fracture wave can include a localized energy input (e.g., localized heating) at the sector of the peripheral ring not subjected to the localized reduction to initiate the fracture wave.

[0026] In another embodiment, during the localized reduction step of the fracture wave initiation capacity, the entire embrittlement plane P is subjected to said reduction except for a zone (for example, a central zone that can occupy an area between 100 x 100 µm² and 30 x 30 mm²) of the embrittlement plane. To achieve this, and as illustrated by the figure 5Laser annealing can be applied to the entire surface of the donor substrate except for a region Rz corresponding to said zone. In this example, the localized reduction of the fracture wave initiation capacity is preferably achieved in such a way as not to impede fracture wave propagation. To this end, the laser annealing can be performed at a fluence that does not completely inhibit microcrack formation and does not exclusively produce microbubbles, as seen in the previous example.

[0027] In another possible embodiment, during the localized reduction of fracture wave initiation capacity step, a peripheral ring of the embrittlement plane P present at the edge of the donor substrate is subjected to said reduction of fracture wave initiation capacity. To achieve this, and as illustrated in Figure 8, laser annealing can be applied to the surface of the donor substrate on a peripheral target region Rcb corresponding to said peripheral ring.

[0028] The peripheral target region Rcb preferably covers the rim at the edge of the plate, which, due to a chamfer, is not bonded to the receiving substrate and is not transferred to it. Indeed, without implementation of the invention, this unbonded rim is subject to blistering / exfoliation phenomena during the embrittling heat treatment. Defects are then generated, which must be eliminated by a specific post-treatment. With implementation of the invention, it is possible to reduce or even eliminate these blistering / exfoliation phenomena at the level of the unbonded rim and consequently avoid the need for specific post-treatment of defects that would be generated by these phenomena.

[0029] In another possible embodiment of the invention, the localized reduction of the embrittlement plane's ability to initiate the fracture wave may include, before or after the embrittlement plane's formation, the formation of a localized amorphous zone in the thin layer directly above the area where the fracture wave initiation capacity is to be reduced. By locating an amorphous zone in the vicinity of the embrittlement plane, for example, within 100 nm and advantageously within 75 nm or 50 nm of the embrittlement plane, it is possible to limit or even inhibit bubbling in the area of ​​the embrittlement plane located directly above this amorphous zone. The amorphous zone is preferably formed after the embrittlement plane has formed so as not to interfere with this formation.The amorphous zone can be obtained by ion implantation based in particular on one or more of the following species: silicon, germanium, phosphorus, arsenic, nitrogen or argon with a dose typically between 1014 and 1016 at / cm2. The. figure 7 represents the thickness of a silicon EaSi zone made amorphous by means of a silicon implantation as a function of the implantation energy E for different implantation doses (D 0.5: 5.10 14< at / cm 2< ; D 1: 10 15< at / cm 2< ; D 2: 2.10 15< at / cm 2< ; D 5: 5.10 15< at / cm 2< ).

Claims

1. A method for transferring a thin layer from a donor substrate to a receiver substrate, the method including the steps of: - implanting species in a uniform manner on the whole of the donor substrate to form therein an embrittlement plane (P) which delimits the thin layer and a bulk part of the donor substrate, - placing in contact the donor substrate and the receiver substrate, - initiating and propagating a fracture wave along the embrittlement plane, the method being characterised in that it comprises, before the placing in contact, a step of locally reducing a capacity of the embrittlement plane to initiate the fracture wave.

2. The method according to claim 1, in which, during the step of locally reducing the capacity of the embrittlement plane to initiate the fracture wave, a central zone (Rz) of the embrittlement plane is not subjected to the reduction of the capacity to initiate the fracture wave.

3. The method according to one of claims 1 and 2, in which, during the step of locally reducing the capacity of the embrittlement plane to initiate the fracture wave, a peripheral crown (Rcb) of the embrittlement plane is subjected to the reduction of the capacity to initiate the fracture wave.

4. The method according to one of claims 1 and 2, in which, during the step of locally reducing the capacity of the embrittlement plane to initiate the fracture wave, a peripheral crown (Rc) of the embrittlement plane, with the exception of a sector (Ez) of the peripheral crown, is subjected to the reduction of the capacity to initiate the fracture wave.

5. The method according to claim 4, in which initiating the fracture wave includes locally inputting energy at the level of the sector of the peripheral crown not subjected to the reduction of the capacity to initiate the fracture wave.

6. The method according to one of claims 3 to 5, in which the peripheral crown is localised at the edge of the embrittlement plane.

7. The method according to one of claims 1 to 6, in which the step of locally reducing the capacity of the embrittlement plane to initiate the fracture wave includes carrying out a localised laser annealing of the donor substrate.

8. The method according to claim 7, in which carrying out the localised laser annealing of the donor substrate includes locally irradiating a free face of the thin layer by one or more laser pulses.

9. The method according to one of claims 1 to 6, in which the step of locally reducing the capacity of the embrittlement plane to initiate the fracture wave includes forming a localised amorphous zone in the thin layer.

10. The method according to claim 9, in which the amorphous zone is localised at less than 100 nm, preferably at less than 75 nm, and even more preferably at less than 50 nm from the embrittlement plane.