FR2797347A1 - PROCESS FOR TRANSFERRING A THIN LAYER INCLUDING AN OVERFRAGILIZATION STEP - Google Patents

Abstract

The invention concerns a method for transferring a thin layer (18) from a source substrate (10) to a target support (30) comprising the following steps: a) implanting ions or gaseous species into the source substrate so as to form therein a zone (16) called cleavage zone, delimiting said thin layer (18) in the source substrate; b) transferring the substrate source onto the target support and making the thin layer integral with the target source; c) separating the thin layer (18) from the source substrate (10) along the cleavage zone. The invention is characterised in that the method comprises, prior to step b): a process for excess fragilization of the cleavage zone (16) produced by a heat treatment and/or by exerting mechanical stresses on the source substrate.

Description

 PROCESS <B> OF <B> TRANSFER <B> OF A THIN LAYER </ B> COMPRISING <B> A </ B> STEP <B> OF </ B> OF SURFRAGILIZATION <U> Technical field </ U> The present invention relates to a method of transferring a thin layer of a substrate, said source substrate, to a support called target support.

The invention finds applications in particular in the fields of microelectronics, micro-mechanics, integrated optics and integrated electronics.

It allows, for example to achieve structures in which the thin layer, which is a material selected for its physical properties, is carried on a support to form a multi-layer stack. Thus, the advantages of the materials of the thin layer and the support can be combined. The postponement of a layer makes it possible in particular to associate in the same structure parts which have a priori incompatibilities such as a significant difference in coefficients of thermal expansion.

 <U> state of the prior art </ U> The following text refers to a number of documents whose complete references are specified at the end of the description.

Among the processes generally used for the formation of thin layers, there may be mentioned in particular a cleavage method, well known under the name "Smart-cut", and illustrated by document (1). The "Smart-cut" process is essentially based on the implantation of hydrogen or another gas in neutral or ionic form in a substrate so as to form a weakened cleavage zone.

In the case of a planar substrate, the cleavage zone extends substantially parallel to its surface and is located in the substrate at a depth fixed by the implantation energy. The cleavage zone thus delimits in the substrate a thin surface layer which extends in thickness from the cleavage zone to the surface of the substrate.

A second step includes adhering the source substrate with a target medium such that the thin layer is integral with the target medium. The fixing of the thin layer on the target support can take place by means of an adhesive and / or via a bonding layer. It can also take place by direct molecular adhesion between the surface of the substrate and the surface of the target support.

In the latter case, it is however necessary that the faces that one wishes to adhere have certain properties such as good flatness and low roughness.

A final step of the process consists in fracturing the substrate according to the cleavage zone to separate the thin layer. This then remains attached to the target support.

In the case of the method described by document (1), the fracture (or cleavage) of the substrate is caused by energy input in the form of a heat treatment.

The implantation conditions define the cleavage zone and condition the separation of the thin layer from the substrate.

However, it has been observed that too much weakening of the cleavage zone after implantation, although favorable for the separation, causes deformations of the surface of the thin layer. The deformations are in the form of blisters and constitute an obstacle for fixing the thin layer on the target support.

This excessive embrittlement may be related to high dose ion implantation or lower dose ion implantation associated with annealing. The excessive embrittlement can therefore induce the appearance of blisters on the surface all the more easily as the weakened zone is close to the surface.

In some applications it is desired to obtain self-supporting thin films, that is to say which can be separated from the source substrate without being previously fixed on a support.

These thin films can then be reported later on different target media and, in particular, on media with which the source substrate can not be adhered before separation of the thin layer, for example, for reasons of compatibility of the coefficients. thermal expansion. In this regard, reference can be made to document (2) which proposes a method derived from that of document (1).

Document (2) proposes a method for obtaining the separation of its original substrate from a thin film which is self-supporting. For this, it is necessary that the gaseous species implanted are at a sufficient depth and / or that is deposited, after the implantation step, a layer of a material for stiffening the structure to obtain separation at the level of the implanted area without blisters.

The illustration of the technique of forming a thin layer by cleavage of a substrate may be completed by document (3) which suggests completing the fracture heat treatment (cleavage) of the substrate by the exercise of mechanical forces. bending, pulling and / or shearing.

Document (4), which also describes a method based on the principle established by document (1), shows that the thermal budget used to cause the fracture of the source substrate depends on the thermal budgets of all heat treatments imposed on the source substrate. from implantation to fracture.

Thermal budget means the heat treatment time / heat treatment temperature pair.

In a number of applications it is necessary to assemble a thin layer of a source substrate with a target carrier that has a thermal expansion coefficient different from that of the source substrate.

In these applications, it is generally difficult to subject the structure obtained after assembly of the source substrate and the target support, to heat treatment with a thermal budget sufficient to ensure separation of the thin layer from the source substrate.

One solution to this problem is then to modify the implantation conditions by performing an overdose of the implanted species. Such an overdose makes it possible to reduce the thermal budget of the separation fracture (cleavage).

By way of example, when the source substrate is a silicon wafer, a dose of implanted hydrogen ions of 1.101 w / cm 2 instead of 6.10 16 / cm 2 allows for a heat treatment duration of a few hours of lowering the temperature from 400 C to 280 C.

The solution consisting in overdosing the implantation is however not always satisfactory because of the difference in the coefficient of thermal expansion that can exist between the substrate and the support. Indeed, the thermal budget required for the separation can be such that it causes the release of the substrate and the support and / or the breaking of the substrate and / or support in their volume.

An alternative solution to avoid delamination between the thin layer and the target support under the effect of differential expansion, is to thin the source substrate before the fracture step (cleavage). This solution, suggested by document (5), however, has the disadvantage of an additional thinning operation and that of a significant material consumption.

The use of mechanical forces to separate the source substrate from the thin layer, as mentioned above with reference to document (3), also makes it possible to reduce the fracture thermal budget, particularly in the case where contact have different coefficients of expansion. The exercise of mechanical forces on the source substrate and / or the target support is however not always possible, especially when the materials used are fragile, or when the cleavage zone is not weakened enough by the ion implantation.

Finally, the separation and thin film transfer techniques described above involve a number of constraints and compromises. These constraints are imposed in particular by the type of materials used to constitute the source substrate, the thin layer and the target support. SUMMARY OF THE INVENTION The present invention aims to propose a method for transferring a thin layer, not having the difficulties and limitations of the processes indicated above.

One aim is in particular to propose such a process which implements a reduced or even zero thermal budget to obtain a separation fracture of the thin layer.

Another aim is to propose a method adapted to the transfer of a thin layer onto a target support, in which the materials of the thin layer and of the target support have different coefficients of thermal expansion.

Yet another object is to propose a transfer method in which an excellent surface state of the source substrate (without blisters) can be preserved so as to allow good adhesion with the target support, with or without the addition of a binder (glue), and allowing to have a very weakened cleavage zone.

Finally, an object of the invention is to provide a transfer method for obtaining, after transfer, a thin layer on the target support, which has a free surface with a low roughness.

To achieve these aims, the invention more precisely relates to a method for transferring a thin layer of a source substrate to a target medium comprising, in order, the following steps: a) ion implantation or gaseous species in the source substrate so as to form therein a so-called cleavage zone, which delimits said thin layer in the source substrate, b) the transfer of the source substrate onto the target support and the joining of the thin layer with the support target, c) the separation of the thin layer from the source substrate according to the cleavage zone. According to the invention, the process comprises, prior to step b), a formation of a thickness of material film between the cleavage zone and the surface of the substrate such that this thickness is greater than, equal to or close to a limiting thickness for which the film is self-supporting, and - an overfragmentation of the cleavage zone caused by a heat treatment and / or by the exercise of mechanical forces on the source substrate.

According to a first embodiment of the invention, the step of forming a film thickness consists in carrying out the implantation step a) so as to obtain the cleavage zone at said thickness, the film then being constituted by the thin layer.

According to a second embodiment of the invention, the step of forming a film thickness consists in carrying out a step of forming a so-called thickener layer on the thin layer, the thin layer and the layer of thickness then forming the film.

The limiting thickness of the film is the thickness which makes it possible to stiffen the structure in order to obtain separation of the film at the level of the cleavage zone without blister surface appearance. It is this limiting thickness that makes it possible to obtain self-supporting films. This thickness depends in particular on the mechanical properties of the materials, but also on the separation conditions of step c) such as, for example, the rise in temperature of the heat treatment. According to an advantageous mode, the invention also comprises the realization, before step b), of all or part of micro-electrical and / or micro mechanical and / or optoelectronic components.

The separation of the thin layer from the source substrate, carried out in step c), can take place under the effect of a heat treatment, under the effect of mechanical forces or under the effect of these actions combined.

However, thanks to the over-embrittling step, the thermal budget and / or the mechanical forces used during step c) for the separation fracture can be particularly reduced. This has the advantage of not causing adhesion failure between the thin film and the target support, even in case of a difference in the thermal expansion coefficients of the materials contacted.

Another advantage of the invention is to eliminate or reduce the mechanical forces exerted on the parts in contact and thus avoid damage. This facilitates the separation.

It should be noted that the over-embrittlement step does not induce differential expansion stresses insofar as this step is performed before the transfer of the source substrate to the target support (step b).

According to one advantageous aspect, the over-embrittlement comprises a heat treatment implemented with a thermal budget greater than or equal to 50%, and preferably greater than 60%, of an overall thermal budget allowing separation. The overall thermal budget considered here takes into account not only the heat treatments operated in the strict context of the process of the invention, but also includes possible heat treatments implemented, for example, for producing components or for depositing of materials on the thin layer between steps a) and b).

As mentioned above, steps c) and the step of over-embrittlement can involve an exercise of mechanical forces.

These mechanical forces include, for example, the application of forces in the form of mechanical pressure and / or mechanical tension and / or forces in the form of a gas pressure.

The separation heat treatment may also be chosen sufficient to cause in step c) a separation of the thin layer, by simple separation of the source substrate and the target support.

The implantation step a) makes it possible to form cavities in the source substrate located in the cleavage zone.

The cavities (or micro-cavities or platelets or microbubbles) can be in different forms. They may be spherical and / or flattened with a thickness of only a few inter-atomic distances. Moreover, the cavities may contain a free gas phase and / or gas atoms derived from the implanted ions, fixed on the atoms of the material forming the walls of the cavities. The heat treatments undergone by the source support, and in particular a super-heat treatment of the process, lead to the coalescence of all or part of the cavities. The coalescence thus causes the surfragilization of the substrate in the cleavage zone.

This phenomenon also makes it possible to obtain, after the transfer and the separation fracture, a free surface of the thin layer with a low roughness.

The thickener layer, for example of Si, SiO 2, Si 3 N 4 or SiC, covers all or part of the thin layer. The thickness of the thickener layer to obtain a film thickness is chosen for example in a range from 3 to 10 Dun for an SiO 2 thickener.

It should be noted that a layer used as a thickener layer may be a layer that also serves in whole or in part to the production of electronic, optoelectronic, or mechanical components on the surface of the thin layer.

The invention also relates to a method for transferring a thin layer of a source substrate comprising the following steps: a) an implantation of ions or gaseous species in the source substrate so as to form therein a so-called cleavage zone; , which delimits said thin layer in the source substrate, b) the separation of the thin layer from the source substrate according to the cleavage zone, according to the invention, the method further comprises, prior to step b) a formation of a thickness of film of material between the cleavage zone and the surface of the substrate such that this thickness is greater than, equal to or close to a limit thickness for the film to be self-supporting, and a superfragmentation of the zone cleavage caused by heat treatment and / or the exertion of mechanical forces on the source substrate.

Other features and advantages of the invention will become more apparent from the description which follows, with reference to the figures of the accompanying drawings. This description is given for purely illustrative and non-limiting purposes.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic sectional view of a source substrate and illustrates an ion implantation operation.

- Figure 2 is a schematic section of the source substrate of Figure 1 and illustrates the formation of a thick layer.

- Figure 3 is a schematic section of the source substrate of Figure 2 and illustrates a weakening step.

FIG. 4 is a diagrammatic section of a structure formed of the source substrate of FIG. 3, carried on a target support.

FIG. 5 is a diagrammatic section of the structure of FIG. 4 after separation fracture of the source substrate. <U> Detailed description of an embodiment of </ U> <U> the invention </ U> The following description relates to a transfer of a thin layer of silicon on a silica target support fondue (wrongly called quartz).

The invention can however be implemented for other solid materials, whether they are crystalline - or not. These materials can be dielectric, conductive, semi-insulating or semiconducting.

Likewise, the target support may be a final or intermediate support, such as a handle, a solid substrate or a multilayer substrate.

The method can in particular be used for the transfer of LiNbO3 layers or III-V semiconductors such as AsGa, InP on silicon or SiC.

Figure 1 shows an initial substrate 10 made of silicon. It undergoes a hydrogen ion implantation indicated with arrows 12. This implantation corresponds to step a1) of the method.

Implantation, performed for example with a dose of 6.1016 / cm 2 and an energy of 70 keV, makes it possible to form microcavities 14 in the substrate 10 at a depth of about 7000. This depth also corresponds to the thickness of a thin layer 18. This is delimited on the surface of the substrate by a zone 16, called cleavage zone, comprising the microcavities 14. Anterior or preferably subsequent to this implantation, the thin surface layer 18 may be subjected to other treatments, known per se, for the formation in this layer of electronic, optical, or mechanical components. These components are not shown in the figure for the sake of clarity.

Similarly, for a better readability of the figures, the different layers or characteristics shown, are not in a uniform scale. In particular, the very thin layers are represented with an exaggerated thickness.

FIG. 2, which corresponds to the step of using a thickness of the process, shows the deposition of a layer of silicon oxide 20 having a thickness of the order of or greater than 5 μm on the thin layer 18. The silicon oxide layer is deposited for example by a plasma-assisted chemical vapor deposition method at a temperature of 300.degree.

The silicon oxide layer 20 has a role of thickener of the thin layer 18. In other words, its function is to prevent deformation of the thin layer under the effect of subsequent heat treatments.

Figure 3 corresponds to the step of surfragilisation of the process. During this step, the substrate undergoes treatments aimed at further weakening the cleavage zone 16.

In the illustrated example, a heat treatment is carried out at a temperature of the order of 450 C for a duration of the order of 12 minutes.

This thermal budget is preferably greater than 60% of the thermal budget necessary to obtain a separation only by annealing. Such overfragmentation is possible with a sufficient film thickness.

It is observed that the heat treatment causes a partial coalescence of the microcavities 14 of the cleavage zone 16.

During this operation, the thick layer 20 which covers the thin layer 18 prevents its deformation and in particular prevents the formation of blisters.

In the absence of this layer, blisters would be likely to appear with a heat treatment at 450 C after a duration of the order of 2 minutes which corresponds only to a thermal treatment of 10% of the heat treatment required for separation .

The heat treatment may be followed by a step of polishing the free side of the thickener layer just to improve the surface roughness so as to prepare it for molecular bonding.

Figure 4 shows the carry of the source substrate 10 on a target medium 30 which in this case is a quartz plate.

The transfer is carried out so as to bring into contact a planar face of the target support with the free planar face of the thickener layer 20. Molecular adhesion forces exerted on the faces brought into contact ensure the securing (fixation ) between the source substrate and the target medium.

When such molecular adhesion is not possible for reasons of the nature of the materials or the quality of the surfaces, the transfer can take place via a binder or an adhesive.

Molecular adhesion forces may be enhanced for example by heat treatment and / or surface preparations. In the example illustrated, and because of the large differences in coefficients of thermal expansion between silicon and quartz, the heat treatment is carried out at a relatively low temperature, of the order of 200 C for a period of 20 hours.

This heat treatment can contribute to inducing contractes such that the fracture of the substrate can be obtained according to this zone.

Figure 5 illustrates step c) of the method which corresponds to a fracture of the source substrate. The fracture takes place according to the cleavage zone and separates the thin layer 18 from the remaining part of the substrate 10. This last part can then be reused for example for a new thin-layer transfer.

The thin layer 18 remains integral with the support 30 via the thickener layer 20. In another example, not shown, where the thickness of the thin layer is large enough to prevent deformation, the thickener layer can be omitted. The thin layer is then in direct contact with the target support.

By way of example, there may be mentioned the use of an SiC substrate, which is implanted at about 200 KeV so as to obtain a film without a thickener, of about 1.5 μm. In this example, the surfragilization can be obtained without thickener.

The fracture of the source substrate can be caused by the exercise of a mechanical force and / or by a heat treatment.

In the illustrated example, a razor blade (not shown) can be inserted by hand at the weakened zone.

The invention is particularly applicable to the production of a thin layer of silicon on split silica whose main interest is to have a transparent support with a semiconductor layer that may comprise components.

 DOCUMENTS <B> <U> CITES </ U> </ B> (1) FR-A-2,681,472 (US-A-5,374,564) (2) FR-A2,738,671 (US-A-5,714 395) (3) FR-A-2,748,851 <B> (4) </ B> FR-A-2,767,416 (5) FR-A-2,755,537

Claims (9)

A method of transferring a thin film (18) from a source substrate (10) to a target medium (30) comprising the steps of a) implanting ions or gaseous species into the source substrate so as to to form therein a zone (16), said cleavage, which delimits said thin layer (18) in the source substrate, b) the transfer of the source substrate on the target support and the joining of the thin layer with the target support, c ) the separation of the thin layer (18) from the source substrate (10) according to the cleavage zone, characterized in that it comprises prior to step b). the formation of a thickness of material film between the cleavage zone and the surface of the substrate such that this thickness is greater than, equal to or close to a limit thickness so that the film is self-supporting, and a superfragmentation of the zone cleavage (16) caused by heat treatment and / or mechanical stress on the source substrate. 2. The method of claim 1, wherein the separation of the thin layer (18) from the source substrate (10) is caused by a heat treatment and / or by the exercise of mechanical forces. 3. Method according to claim 1 or 2, wherein the surfragilization comprises a heat treatment implemented with a thermal budget for separation. 4. The method of claim 2, wherein the separation heat treatment is chosen to cause, in step c), a separation of the thin layer (18) by simply spacing the source substrate and the target medium. The method according to claim 1, wherein step c) and the step of over-embrittling comprise exerting forces in the form of mechanical pressure and / or mechanical tension and / or forces in the form of a gas pressure. A method of transferring a thin layer (18) of a source substrate (10) comprising the steps of: a) implanting ions or gaseous species into the source substrate to form a region (16); ), said cleavage, which delimits said thin layer (18) in the source substrate, b) the separation of the thin layer (18) from the source substrate (10) according to the cleavage zone, characterized in that it comprises before step b). a formation of a thickness of film of material between the cleavage zone and the surface of the substrate such that this thickness is greater than, equal to or close to a limit thickness for the film to be self-supporting, and a superfragmentation of the zone cleavage (16) caused by heat treatment and / or mechanical stress on the source substrate. 7. Process according to claim 1, characterized in that the step of forming a film thickness consists in carrying out the implantation step a) so as to obtain the cleavage zone at said thickness, the film then being constituted by the thin layer. The method of claim 1, wherein the step of forming a film thickness comprises forming a thickener layer (20) on the thin layer, the thin layer and the thickener layer forming then the movie. 9. The method of claim 1, further comprising producing, before step b), all or part of micro-electronic components and / or micro mechanical and / or optoelectronic.