WO2012176044A1

WO2012176044A1 - Method of fabrication of a semiconductor substrate comprising porous silicon

Info

Publication number: WO2012176044A1
Application number: PCT/IB2012/001220
Authority: WO
Inventors: Christophe Figuet; Oleg Kononchuk
Original assignee: Soitec SA
Current assignee: Soitec SA
Priority date: 2011-06-23
Filing date: 2012-06-11
Publication date: 2012-12-27
Anticipated expiration: 2013-12-23
Also published as: FR2977074A1; TW201301506A

Abstract

The invention concerns a method of fabrication of a semiconductor substrate (8), comprising procuring a semiconductor donor substrate (1), transforming the donor substrate (1) comprising a porous support layer (2) constituted of a semiconductor material,, an active layer (3), non-porous, treating said donor substrate (1) to deform in expansion or in contraction the porous support layer (2'), said expansion inducing straining of the active layer (3) resulting in a strained active layer (3'), transferring at least part of the strained active layer (3') from the donor substrate (1) to a receiver substrate (8), recycling the donor substrate (1) for the fabrication of a strained active layer (3') that has been subjected to additional straining.

Description

METHOD OF FABRICATION OF A SEMICONDUCTOR SUBSTRATE COMPRISING POROUS SILICON

GENERAL TECHNCIAL FIELD

The invention concerns a method of fabrication of a semiconductor substrate.

PRIOR ART

Silicon on insulator (SQI^ substrates are widely used in the microelectronics industry.

In particular, strained silicon on insulator (sSOl) substrates are of great benefit for the fabrication of electronic components, notably because sSOl substrates offer increased mobility of electrons and holes and thus higher performance.

Of course, the strained silicon must have the lowest possible density of defects.

A known prior art method of fabrication of strained silicon consists in epitaxial growth of a strained layer of silicon on a buffer layer, generally of varying composition within the thickness.

Another method of fabrication of strained .silicon consists in using a porous silicon layer in contact with a surface silicon layer and to strain the porous silicon layer to induce an expansion or contraction strain in the surface silicon layer.

However, known sSOl fabrication methods, and more generally strained active layers, have' numerous drawbacks, in that they are costly and lengthy.

It is therefore necessary to propose a solution able to satisfy the requirements of the microelectronics industry.

SUMMARY OF THE INVENTON

The invention proposes to alleviate the drawbacks referred to above. To this end, the invention proposes a method of fabrication of a semiconductor substrate, characterized in that it comprises:

- a first step consisting in procuring a semiconductor donor substrate,

- a second step consisting in transforming the donor substrate so that it comprises:

o a porous support layer constituted of a semiconductor material, and

o an active layer,

^■ non-porous, and

^■ constituted of a semiconductor material,

- a third step consisting in treating said donor substrate to deform in expansion or in contraction the porous support layer,

o said expansion inducing straining of the active layer resulting in a strained active layer,

- a fourth step consisting in transferring at least part of the strained active layer from the donor substrate to a receiver substrate,

. - .a >fifth step of recycling comprising:

o applying the third step to the donor substrate obtained after the fourth step, thus enabling further deformation of the support layer in expansion or in contraction,

o said deformation inducing additional straining of the strained active layer with a view to the fabrication of a new receiver substrate comprising at least part of said strained active layer that has been subjected to additional straining.

The invention is advantageously complemented by the following features, separately or. in any technically feasible combination:

- the fifth step further comprises the subsequent execution of the fourth step of fabrication of a new receiver substrate comprising:

- at least part of the strained active layer,

- said strained active layer having a straining higher than that of the strained active layer obtained before the fifth recycling step; - the support layer and the active layer are constituted of silicon;

- the active layer, obtained after transformation of the donor substrate during the second step, has a thickness in the range 10 nm to 100 nm;

- the first step comprises the procuring of a donor substrate that includes a confinement structure including a semiconductor confinement layer, said confinement layer having a chemical composition different from that of the active layer, and the fourth step consisting in transferring at least part of the strained active layer from the donor substrate to the receiver substrate comprises the steps of:

■ introducing ions into the donor substrate,

^■ bonding the donor substrate and the receiver substrate,

^■ subjecting the donor substrate and the receiver substrate to a heat treatment comprising an increase in temperature during which the confinement layer attracts the ions to concentrate them in said confinement layer, and

■ detaching the donor substrate from the receiver substrate by fracture at the level of said confinement layer;

- the introduction of the ions into the donor substrate is effected by immersion of the donor substrate in a plasma containing said ions;

- the step consisting in transferring at least part of the strained active layer from the donor substrate to the receiver substrate comprises the steps of:

- creating a weakened area in the donor substrate,

- bonding the donor substrate and the receiver substrate, and

- causing a fracture at the level of said weakened area to detach the donor substrate from the receiver substrate;

- the method consists in applying cyclically the second, third, fourth and fifth steps for the fabrication of a plurality of receiver substrates each including a non-porous strained active layer and constituted of at least one semiconductor material starting from a donor substrate procured for the first step.

The invention also concerns a semiconductor substrate fabrication method, characterized in that it comprises: - a first step consisting in procuring a semiconductor donor substrate,

o a porous support layer constituted of a . semiconductor material, and

o an active layer,

^■ non-porous, and

■ constituted of a semiconductor material,

- a third step consisting in treating said donor substrate to deform in expansion or in contraction the porous support layer, said expansion inducing straining of the active layer resulting in a strained active layer,

- a fifth step of selection of a recycling path chosen from:

o a first recycling path, including execution of the third step starting with the donor substrate obtained after the fourth step, thus enabling further deformation of the support layer of said porous material, said deformation inducing additional straining of the strained active layer with a view to the fabrication of a new receiver substrate including at least part of said strained active layer that has been subjected to additional straining, and

o a second recycling path, including polishing of the donor substrate obtained after the fourth step with a view to fabrication of a receiver substrate including at least part of the strained active layer, said strained active layer having a strain identical to that preceding said second recycling path._.

The method is advantageously complemented by the following features, separately or in any technically feasible combination:

- the first and second recycling paths further include execution of the fourth step of fabrication of a new receiver substrate comprising: - for the first recycling path, at least part of the strained active layer, said layer having a higher strain than before recycling,

- for the second recycling path, at least part of the strained active layer, said strained active layer having a strain identical to that preceding recycling;

- the first step comprises the procurement of a donor substrate that includes a confinement structure including a semiconductor confinement layer, said confinement layer having a chemical composition different from the strained layer constituted of the third material, and

- the fourth step consisting in transferring at least part of the strained active layer from the donor substrate to the receiver substrate comprises the steps of:

■ introducing ions into the donor substrate, _y

^■ bonding the donor substrate and the receiver substrate,

^■ detaching the donor substrate from the receiver substrate by fracture at the level of said confinement layer;

- creating a weakened area in the donor substrate,

- bonding the donor substrate and the receiver substrate, and

- the method consists in applying cyclically the second, third, fourth and fifth steps for the fabrication of a plurality of receiver substrates each including a non-porous strained active layer and constituted of at least one semiconductor material starting from a donor substrate procured the first step. ,

The invention has numerous advantages and notably makes it possible to reduce the fabrication time of strained layers of good crystalline quality and likewise the associated fabrication costs.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, objects and advantages of the invention will emerge from the following description, which is. purely illustrative and non-limiting, and which should be read with reference to the appended drawings, in which:

- Figure 1 is a diagrammatic representation of a first embodiment of .the method of the invention;

- Figure 2 is a diagrammatic representation of an electrochemical anodization method; ,

- Figure 3 is one embodiment of an active layer transfer step of the invention;

- Figure 4 is a diagram of the concentration of ions in the donor substrate in the case of implantation and diffusion of said ions; and

- Figure 5 is a diagrammatic representation of a second embodiment of the method of the invention.

DETAILED DESCRIPTON

There is shown in Figure 1 one embodiment of a semiconductor substrate fabrication method of the invention.

By semiconductor substrate is meant a substrate including at least one layer of semiconductor material.

The expression "strained layer" refers to any layer of a semiconductor material the crystal structure of which is strained in tension or in compression relative to the natural crystal structure of the material. For example, it is possible to obtain strained layers during crystal growth, such as epitaxial growth, which modifies the crystal lattice, in particular in the growth direction.

Conversely, the expression "relaxed layer" refers to any layer of a semiconductor material that has a crystal structure free of any applied external strain, i.e. that has a lattice parameter identical to the lattice parameter of a layer of this material in solid monocrystalline form.

The method includes a first step E1 consisting in procuring a semiconductor donor substrate 1.

Advantageously, although this is not limiting on the invention, the donor substrate 1 is constituted at least in part of silicon. It may equally advantageously be germanium or semiconductor materials of type lll-V (alloy composed of an element from the third column of the periodic table of the elements and an element from the fifth column of the periodic table of the elements).

In one embodiment, there is a solid donor substrate 1 formed of a semiconductor material advantageously chosen from the aforementioned materials.

The method includes a second step E2 consisting in transforming the donor substrate 1 so that it includes:

- a porous support layer 2, constituted of a semiconductor material, and

- an active layer 3 that is ,

o non-porous, and

o constituted of a semiconductor material.

This second step of transformation of the donor substrate 1 advantageously includes a step shown in Figure 2 of electrochemical anodization of the donor substrate 1.

Thus the support layer 2 is for example formed of a material such as those" referred to above for the donor substrate 1 and likewise the active layer 3. The layer 2 and the layer 3 may be constituted of a plurality of semiconductor materials (alloy or superposition of a plurality of materials). In one embodiment, the support layer 2 and the active layer 3 are constituted of the same material.

In this case, the donor substrate 1 is placed in an enclosure 10 containing an electrolyte 11.

The electrolyte 11 is for example a solution containing hydrofluoric acid (HF). ^y

An anode 12 and a cathode 13 immersed in the electrolyte 11 are connected to an electrical current source 14.

The donor substrate 1 is positioned so that the support layer 2 is turned toward the cathode 13 and so that the active layer 3 is turned toward the anode 12.

An electrical current is applied between the anode 12 and the cathode 13 by means of the electrical current source 14.

This electrical current is generally constant.

Anodization is stopped when the required thicknesses of the porous support layer 2 and the active layer 3 have been achieved.

At the end of anodization the donor substrate 1 is rinsed.

The support layer 2 is advantageously p-doped, which enables acceleration of anodization.

c The active layer 3, obtained after transformation of the donor substrate 1 constituted of said material, advantageously has a thickness in the range 10 nm to a few hundred nanometres, in particular the range 10 nm to 100 nm.

The method includes a third step E3 consisting in treating said donor substrate 1 to deform the porous _. support layer 2 in expansion or contraction. During this step, internal strains are generated in the porous support layer 2 that will enable deformation of the porous support layer 2' by expanding or contracting it.

Expansion corresponds to expanding the material, i.e. deforming the material in tensiqn, while contraction corresponds to contracting the material, i.e. deforming the material in compression. This deformation induces straining of the active layer 3, resulting in a strained active layer 3'. The porous support layer 2' deformed in this way will induce strains in the seed layer 3, causing straining of the seed layer 3.

The third step E3 may for example include a step of thermal oxidation of the donor substrate 1. ^

In this case, the donor substrate 1 is subjected to a heat treatment (for example at a temperature in the' range 200°C to 800°C) in an atmosphere that may be an oxidizing atmosphere (including O₂₎ N0₂, etc.).

This generally causes expansion of the porous support layer 2'.

In another embodiment, the first step E3 may include nitriding, which generally enables generation of strains in compression and thus deformation in contraction Of the support layer 2'.

The fabrication method includes a fourth step E4 consisting in transferring at least part of the strained active layer 3' from the donor substrate 1 to a receiver substrate 8.

In one embodiment, this fourth step comprises the steps of:

- creating a weakened area 20 in the donor substrate 1 ,

- bonding the donor substrate 1 and the receiver substrate 8, and

- causing a fracture at the level of said weakened area 20 to detach . the donor substrate 1 from the receiver substrate 8.

The weakened area 20 is created by implantation of ions, such as hydrogen or helium ions. This weakened area 20 is generally created in the donor substrate 1.

At least part of the strained active layer 3' may be transferred from the donor substrate 1 to the receiver substrate 8 after constituting in the donor substrate a weakened area at the level of which a fracture can be instigated to effect the transfer.

The weakened area may have been constituted by implantation in the donor substrate as described above.

In this case, the ions are accelerated in the direction of the donor substrate surface/ The mean depth of penetration of the atoms is generally in the range 100 A to 1 μηι; this depth may be determined as a function of . the species implanted and the implantation energy. In the case of implantation, this features an implantation peak in the donor substrate. The ions implanted have an energy selected to enable them to pass through the material of the donor substrate. The, implantation peak depends on the energy of the ions.

It may equally have been constituted differently, for example in the manner described hereinafter.

Thus an embodiment is described of the fourth step E4 of transferring the strained layer 3' from the donor substrate 1 to a receiver substrate 8.

This embodiment is shown in Figure 3.

In this embodiment, the donor substrate 1 further includes a confinement structure 23 comprising a confinement layer 25 formed of a semiconductor material.

The role of this confinement layer 25 is to attract the ions subsequently introduced into the donor substrate (for example by diffusion) during thermal annealing of the donor substrate after this introduction and during which the ions will migrate preferentially to the confinement layer 25.

The confinement structure 23 comprising the confinement layer 25 is generally produced by epitaxial growth and is formed during the first step E1 of formation of the donor substrate 1. The confinement structure may be disposed in the seed layer 3 or between the support layer 2 and the active layer 3.

The epitaxial process enables precise control of the thickness of the confinement layer 25 and enables small thicknesses thereof to be obtained.

Moreover, epitaxial growth enables preservation of the crystallinity of the active layer 3' to be transferred. . ■

The material of the confinement layer is advantageously chosen from doped or undoped SiGe, or doped silicon. Other materials notably include germanium doped with boron, SiC doped with boron, and doped or undoped InGaN, AIGaN, InGaAs or AIGaAs.

Doping with boron, arsenic or antimony may be used, for example. Other materials and other dopants may be. used. In all cases, the confinement layer is constituted of a material having a different chemical composition to the strained active layer 3' to be transferred, which includes a difference at least in the proportions of the' chemical elements (e.g. SiGe with a different proportion of germanium), or in the type of material (e.g. SiGe for the layer 25 and Si for the layer 3'), or in the fact that the confinement layer is more highly doped than the strained layer 3' to be transferred (e g. SiGe doped with boron for the layer 25 and undoped or less doped SiGe for the layer 3'), or a combination of these differences.

An advantageous embodiment consists in using a confinement layer constituted of a strongly p-doped semiconductor material.

To enable transfer of the strained active layer 3', the fourth transfer step E4 includes a step E41 of introduction of ions 24 into the donor substrate 1. These ions enable creation of a weakened area in the donor substrate 1 , at which level a fracture may be effected.

These ions are advantageously hydrogen and/or helium ions.

They may be introduced in various ways.

The ions 24 are advantageously introduced into the donor substrate 1 by diffusion of the ions 24 into the donor substrate 1 following immersion of the donor substrate 1 in a plasma containing said ions. This introduction of ions 24 into the donor substrate 1 may be effected by techniques other than diffusion, for example by implantation.

The donor substrate 1 immersed in the plasma is subjected to electrical pulses. Positive ions present in the plasma are then accelerated toward the surface of the substrate into which they are introduced. As the plasma surrounds the substrate, all of the surface receives ions at the same time.

Another advantage of this ion introduction is its capacity to be applied on an industrial scale, as well as the short implementation time.

Another advantage of this ion implantation is that the area of diffusion of the ions into the donor substrate is very concentrated, of the order of a few nanometres thick in the direction normal to the main faces of the substrate (for example in the range 10 nm to 200 nm).

Ion introduction by plasma diffusion thus enables good results to be obtained in the transfer step in that this technique notably enables enrichment of the donor substrate 1 with ions of low acceleration voltage

(about 10 to 50 kV) with a high dosage (up to 10⁺¹⁸ at/cm²) in a shallow region (with a depth from a few tens of nanometres to around

200 nanometres, as mentioned above), which is not always accessible with an implantation technique. This is advantageous for subsequently transferring thin layers 6f the active layer 3' to be transferred. As explained hereinafter, this is advantageous for reducing the defects and roughness present in the transferred layer.

Even if the region is accessible for implantation, the high energy of the ions in the implantation process leads to the introduction of crystal defects into the active layer 3' to be transferred, making its subsequent use more difficult.

Figure 4 shows the concentration profile of the ions 24 in the donor substrate 1 as a function of the depth in the donor substrate 1 , in the case of diffusion (curve 26) and in the case of ion implantation (curve 27).

The fourth transfer step further includes a step E42 consisting in bonding the donor substrate 1 and the receiver substrate 8.

This bonding is effected by bringing into contact free surfaces of the donor substrate and the receiver substrate. These surfaces have usually been cleaned beforehand to ensure molecular adhesion of said surfaces.

The fourth transfer step then includes a step E43 of heat treatment of the donor substrate and the receiver substrate consisting in subjecting them to an increase in temperature.

If a confinement layer has been constituted, that layer is produced in one or more materials adapted to attract the ions introduced into the substrate towards said confinement layer during this temperature increasing heat treatment. Typical heat treatment temperatures are in the range 200°C to 700°C. For example, if the material of the confinement layer is silicon doped with boron and the ions introduced into the donor substrate are hydrogen ions, chemical interactions between the boron and the hydrogen will notably enable attraction of the hydrogen ions into the confinement layer. Another ion attraction factor may result from the strain difference (in tension or compression).

Accordingly, during the heat treatment of the donor substrate and the receiver substrate, the confinement layer attracts the ions to concentrate them in said confinement layer.

Another function of this heat treatment may be to strengthen the bonding energy between the donor substrate and the receiver substrate.

Annealing is carried out so that different effects are produced:

- the bonding energy between the donor substrate and the receiver substrate is increased,

- the ions are concentrated in the confinement layer until a

_ critical concentration is reached,

- these ions create cavities, which will coalesce,

- the pressure in these cavities increases until it causes a fracture in the confinement layer, which enables separation of the donor substrate from the receiver substrate.

These four effects may be obtained during a single thermal annealing step or during separate individual thermal annealing steps.

Accordingly, the' next step of the heat treatment is a step E44 consisting in detaching the donor substrate 1 from the receive substrate 8 by fracture at the level of said confinement layer 25.

Thus the strained active layer 3' is transferred.

The receiver substrate 8 is then treated by cleaning and polishing (CMP or otherwise) in order to eliminate residues of unwanted layers. This means in particular the residual confinement layer that has been transferred with the strained semiconductor layer.

The donor substrate 1 is also treated in order to be recycled, where appropriate, in the context of transferring a new strained active layer 3'. An advantage of this transfer method using the constitution of a confinement layer is that the fracture is highly localized, and occurs virtually uniquely or even uniquely only at the level of the confinement layer.

The post-fracture AFM roughnesses obtained with no confinement layer are typically of the order of 3 to 6 nm, but the confinement layer enables this roughness to be reduced to values of the order of 0.5 to 1 nm. This prevents the propagation of defects towards the strained semiconductor layer to be transferred. In the case of a classic transfer by ion implantation and fracture at the level of a weakened area without using a confinement layer, it is common for defects to appear in the substrate after fracture. This is notably caused by the extensive presence of ions in the substrate, which induces a fracture that is not localized and therefore a higher roughness. ^

The strained semiconductor active layer 3' transferred by this method therefore has a reduced roughness. In the case of a donor structure 1 including a silicon active layer 3' to be transferred and a confinement layer 23 of silicon doped with boron, for example, a roughness of the transferred silicon layer of 5 Angstrom units may be obtained (RMS value).

Moreover, it is often necessary to transfer strained semiconductor active layers 3' having a small thickness (for example in the range 20 nm to 500 nm). It is known that there exists a compromise between the strain present in the layer and the thickness of said layer. For a given strain, there exists a thickness beyond which the strain is relaxed through the appearance of defects.

This embodiment of the sixth step thus notably enables transfer of strained semiconductor layers 3' having a thickness in the range 10 nm to 200 nm.

A confinement layer having a thickness in the range 2 nm to 20 nm is advantageously used. The thinner the confinement layer, the more localized the fracture. For example, a confinement layer approximately 4 nm thick will enable the fracture to be confined within this area. Given the small thickness of the confinement layer, this layer does not interfere niuch if at all with the lattice parameter of the donor substrate.

It is generally possible to use a confinement structure 23 comprising a confinement layer 25 as described above and two protection layers disposed in contact with and on either side of the confinement layer, each of these protection layers being formed of a semiconductor material with a different chemical composition to the material of the confinement layer. The expression "different chemical composition" means that the materials are different or have chemical _, element proportions and/or are doped with a different dopant.

The transfer is implemented with the confinement structure in a similar manner to what has been described for the confinement layer.

These protection layers further limit the propagation of defects resulting from the fracture. The latter notably act as shields protecting the strained semiconductor active layer 3' to be transferred and confine defects liable to propagate toward the strained active layer 3' after the fracture in the confinement layer.

Embodiments include for the protection layers, by way of nonlimiting example:

- material of protection layers: Si(i_-x)Ge_x, material of confinement layer:

Si(i_-y)Ge_y (advantageous, the difference between x and y is at least 3%, preferably greater than 5%, even 10%), SiGe doped with boron or silicon doped with boron. There may also be cited the situation where the protection layers are of SiGe and the confinement layer of boron-doped silicon, and likewise the situation in which the protection layers are of SiGe and the confinement layer is of Ge doped with boron;

- material of protection layers: silicon, material of confinement layer:

Si(i-y₎Ge_y, SiGe doped with boron or silicon doped with boron;

- material of protection layers: germanium, material of confinement layer: SiGe doped with boron or silicon doped with boron or germanium doped with boron or SiGe; - material of protection layers: SiGe, material of confinement layer: SiC doped with boron;

- material of protection layers: AIGaN, material of confinement layer:

InGaN doped (Si, Mg) or not;

- material of protection layers: AIGaAs, material of confinement layer InGaAs doped (Si, Zn, S, Sn) or not.

The materials of the protection layers are advantageously also adapted to attract the ions introduced into the donor substrate toward the confinement layer during the heat treatment that increases the temperature of said donor substrate, for example doped or undoped SiGe enabling j attraction of hydrogen ions.

Additionally, or alternatively, it is advantageous for at least one of the protection layers to be an etch stop layer, constituted of a material enabling selective chemical etching of the protection layer vis a vis the strained active layer 3' constituted. This is generally a protection layer in contact with the strained active layer 3'.

This enables implementation of a step consisting in selectively etching the protection layer present ^on the receiver substrate 8 after detachment of the donor substrate 1.

Additionally, or alternatively, one of the protection layers is a chemical etch stop layer, constituted of a material enabling selective chemical etching of the protection layer vis a vis the support layer 2'.

The method may include a step consisting in selectively etching the protection layer present on the donor substrate after fracture, which enables reuse of the donor substrate.

It is advantageous to use protection layers in SiGe with a confinement layer in silicon doped with boron and with a strained active layer 3' to be transferred in silicon.

Following the fourth step E4, there is obtained a receiver substrate including at least part of the strained active layer 3V

Whatever embodiment is chosen for the transfer, the fourth step advantageously includes a preliminary step of formation of an oxide layer 18 in contact with the strained active layer 3' of the donor substrate 1 , the bonding of the donor substrate 1 and the receiver substrate 8 being effected at the level of said oxide layer 18.

Alternatively, or additionally, the receiver substrate 8 itself includes an oxide layer at the level of which the bonding with the, donor substrate lis effected.

The receiver substrate 8 is then treated in the classic manner, according to the intended applications. Generally it undergoes a finishing treatment notably including polishing.

The active layer 3' is advantageously in silicon and, after the fourth step, the receiver substrate 8 is a strained silicon on insulator substrate.

After the fourth step, the method also includes a step of recycling the donor substrate 1.

In a first embodiment, illustrated by the arrow E5 in Figure 1 , this is a fifth step E5 including execution of the third step E3 starting with the donor substrate 1 resulting from the fourth step, thus enabling further deformation of the porous support layer 2', and this in expansion or in contraction, said deformation inducing additional straining of the constrained active layer 3', with a view to the fabrication of a new receiver substrate 8 including said strained layer 3' that has been subjected to additional straining.

This recycling step is therefore aimed at fabricating a strained active layer 3' having a higher strain than the strained active layer 3' of said non- porous material obtained after the fourth step that precedes recycling.

It is thus a matter of recycling enabling the strain of the active layer (strained hon-porous material layer), to be increased. . ^{■ ■■'}

The fifth step advantageously further includes a repeat of the fourth step, enabling fabrication of a new receiver substrate 8 including at least part of the strained active layer 3', said strained active layer 3' having a higher strain than the strained layer of said non-porous material obtained before the recycling step. ,

Accordingly, this repeating of the. fourth step consists in transferring at least part of the strained active layer 3' that has been subjected to additional straining to a new receiver substrate, by the methods described above.

The fifth recycling step may include other additional steps.

In one embodiment, the fifth step of recycling the donor substrate 1 includes a step of polishing the strained active layer 3' of the donor substrate 1. _

Following the transfer of part of the strained layer 3' to the receiver substrate 8, roughness may be present on the surface of this layer, which may be reduced or even eliminated by polishing during recycling.

Alternatively, or additionally, the fifth recycling step may include a step of epitaxial growth enabling the thickness of the strained active layer 3' to be increased. This step consists in causing the material of the active layer 3' to grow on that layer to increase its thickness. This step may prove useful in the case where the thickness of the strained active layer 3' present in the donor substrate 1 to be recycled is- not sufficient to envisage the creation of a new receiver substrate including part of this layer. Epitaxial growth of said material thus enables the thickness of this active layer 3' to be increased.

The fabrication method is advantageously applied cyclically, i.e. the second, third, fourth and fifth steps are repeated, for the fabrication of a plurality of receiver substrates 8 each including a strained semiconductor active layer 3, from a donor substrate 1 procured for the first step.

In a second embodiment of the fabrication method of the invention, the latter method includes, in addition to the first, second, third and fourth 'steps described above, a fifth step of selection of a recycling path.

It is to be noted that all the features described above, and relating to the first, second, third and fourth steps are applicable to this embodiment of the fabrication method. These features are therefore not described again.

This fifth step of selection of a recycling path enables a choice to be made between two recycling path: a first recycling path E5 and a second recycling path E6.

The first recycling path E5 corresponds tp the fifth recycling step described above in the first embodiment of the fabrication method. The first recycling path E5 thus includes execution of the third step starting with the donor substrate 1 obtained after the fourth step, thus enabling further deformation of the support layer 2' of said porous material, said deformation inducing additional straining of the strained active layer 3', with a view to the fabrication of a new receiver substrate including at least part of the strained active layer 3' that has been subjected to additional straining.

This recycling path has been described in detail in the first embodiment. All the features described above for this recycling path are applicable here.

The second recycling path E6 includes polishing the donor substrate 1 obtained after the fourth step with a view to the fabrication of a receiver substrate including at least part of the strained active layer 3', the strain in said layer being identical to the straining preceding said second recycling path.

Accordingly, in the second recycling path, the straining of the active layer 3' of the non-porous material is maintained constant, in contrast to the first recycling path. The second recycling path is shown in Figure 5. The second recycling path thus advantageously includes, in addition to the aforementioned polishing, the repeating of the fourth step to transfer again at least part of the strained active layer 3'.

Accordingly, thanks to the method of the invention, recycling is very flexible and enables either recycling of the donor substrate 1 with a view to fabricating an active semiconductor layer feature additional straining or to recycling the donor substrate 1 with a view to fabricating an active semiconductor layer with exactly the same straining as before recycling.

The first and second recycling paths advantageously further include execution of the fourth step of fabrication of a new receiver substrate 8.

For the first recycling path, the new receiver substrate then includes at least part of the strained active layer 3', said layer having a higher strain than before recycling. For the second recycling path, the new receiver substrate then includes at least part of the strained active layer 3', said layer having a strain identical! to that preceding recycling.

The active layer js advantageously in silicon and the receiver substrate 8 is advantageously a strained silicon on insulator substrate.

The first recycling path advantageously includes a step of polishing the strained active layer 3'.

The first recycling path and/or the second recycling path advantageously include(s) a step of epitaxial growth, enabling the thickness of the strained active layer 3' to be increased.

The method advantageously includes the sequence consisting in applying cyclically the second, third, fourth and fifth steps (the fifth step being the step of selection of the recycling path) for the fabrication of a plurality of. receiver substrates 8 each including a strained active semiconductor layer 3' from a donor substrate 1 procured for the first step.

It is of course possible to apply to one iteration of the cycle the first recycling path and to another iteration of the cycle the second recycling ^■ path.

In a third embodiment, the fabrication method includes, in addition to the first, second, third and fourth steps, only the second recycling path described above.

Thus it is seen that the invention offers the possibility of effecting a plurality of strained layer transfers from the same substrate including a layer of a porous semiconductor.

The invention has numerous advantages also in terms of cost, delay, flexibility.

The invention finds numerous applications for the fabrication of strained active layers in the microelectronics industry.

Claims

1. Method of fabrication of a semiconductor substrate (8), characterized in that it comprises:

- a first step (E1) consisting in procuring a semiconductor donor jbstrate (1),

second step (E2) consisting in transforming the donor substrate (1) 3 that it comprises:

o a porous support layer (2) constituted of a semiconductor material, and

o an active layer (3),

■ non-porous, and

■ constituted of a semiconductor material,

- a third step (E3) consisting in treating said donor substrate (1) to deform in expansion or in contraction the porous support layer (2'), o said expansion inducing straining of the active layer (3) resulting in a strained active layer (3'),

- a fourth step (E4) consisting, in transferring at least part of the strained active layer (3') from the donor substrate (1) to a receiver substrate (8),

- a fifth step (E5) of recycling comprising:

o applying the third step to the donor substrate (1) obtained after the fourth step, thus enabling further deformation of the support layer (2') in expansion or in contraction, d said deformation inducing additional straining of the strained active layer (3') with a view to the fabrication of a new receiver substrate (8) comprising at least part of said strained active layer (3') that has. been subjected to additional straining.

2. Method according to Claim 1 , wherein the fifth step (E5) further comprises the subsequent execution of the fourth step (E4) of fabrication of a new receiver substrate (8) comprising: - at least part of the strained active layer (3'),

- said strained active layer (3') having a straining higher than that of the strained active layer (3') obtained before the fifth recycling step (E5). 3. Method according to either of Claims 1 or 2, wherein the support layer (2,2') and the active layer (3,

3') are constituted of silicon.

4. Method according to any one of Claims 1 to 3, wherein the active layer (3), obtained after transformation of the donor substrate (1) during the second step (E2), has a thickness in the range 10 nm to 100 nm.

5. Method according to any one of Claims 1 to 4, wherein:

- the first step (E1) comprises the procuring of a donor substrate (1), that includes a confinement structure (23) including a semiconductor confinement layer (25), said confinement layer (25) having a chemical composition different from that of the active layer (3), and

- the fourth step (E4) consisting in transferring at least part of the strained active layer (3Vfrom the donor substrate (1) to the receiver substrate (8) comprises the steps of:

■ introducing (E41) ions (24) into the donor substrate (1),

^■ bonding (E42) the donor substrate (1) and the receiver substrate (8),

^■ subjecting (E43) the donor substrate (1) and the receiver substrate (8) to a heat treatment comprising an increase in temperature during which the confinement layer (25) attracts the ions (24) to concentrate them in said confinement layer (25), and

^■ detaching (E44) the donor substrate (1) from the receiver substrate (8) by fracture at the level of said confinement layer (25).

6. Method according to Claim 5, wherein the introduction of the ions (24) into the donor substrate (1) is effected by immersion of the donor substrate ( l) in a plasma containing said ions.

7. Method according to any one of Claims 1 to 4, wherein the step consisting in transferring at least part of the strained active layer (3') from the donor substrate (1) to the receiver substrate (8) comprises the steps of:

- creating a weakened area (20) in the donor substrate (1),

- bonding the donor substrate (1) and the receiver substrate (8), and - causing fracture at the level of said weakened area (20) to detach the donor substrate (1) from the receiver substrate (8).

8. Method according to any one of Claims 1 to 7, consisting in applying cyclically the second, third, fourth and fifth steps for the fabrication of a plurality of receiver substrates (8) each including a non-porous strained active layer (3') and constituted of at least one semiconductor material starting from a donor substrate (1) procured for the first step.

9. Semiconductor substrate (8) fabrication method, characterized in that it comprises:

- a first step (E1) consisting in procuring a semiconductor donor substrate (1),

- a second step (E2) consisting in transforming the donor substrate (1) so that it comprises: - o a porous support layer (2) constituted of a semiconductor material, and

o an active layer (3),

^■ non-porous, and

^■ constituted of a semiconductor material, - a third step (E3) consisting in treating said donor substrate (1) to deform in expansion or in contraction the porous support layer (2'), said expansion inducing straining of the active layer (3) resulting in a strained active layer (3'),

- a fourth step (E4) consisting in transferring at least part of the strained active layer (3') from the donor substrate (1) to a receiver substrate (8),

- a fifth step (E5, E6) of selection of a recycling path chosen from:

o a first recycling path (E5), including execution of the third step starting with i the donor substrate (1) obtained after the fourth step, thus enabling further deformation of the support layer (2') of said porous material, said deformation inducing additional straining of the strained active layer (3') with a view to the fabrication of a new receiver substrate (8) including at least part of said strained active layer (3') that has been subjected to additional straining, and

o a second recycling path (E6), including polishing of the donor substrate (1) obtained after the fourth step with a view to fabrication of a receiver substrate including at least part of the strained active layer (3'), said strained active layer (3') having a strain identical to that preceding said second recycling path.

10. Method according to Claim 9 wherein the first and second recycling paths further include execution of the fourth step of fabrication of a new receiver substrate (8) comprising:

- for the first recycling path, at least part of the strained active layer (3'), said layer having a higher strain than before recycling,

- for the second recycling path, at least part of the strained active layer (3'), said strained active layer (3') having a strain identical to that preceding recycling.

11. Method according to either of Claims 9 or 10, wherein the active layer (3), obtained after transformation of the donor substrate (1) during the second step (E2), has a thickness in the range 10 nm to 100 nm.

12. Method according to any one of Claims 9 to 11 , wherein:

- the first step (E ) comprises the procurement of a donor substrate (1) that includes a confinement structure (23) including a semiconductor confinement layer (25), said confinement layer (25) having a chemical composition different from the strained layer (5) constituted of the third material, and

- the fourth step (E4) consisting in transferring at least part of the strained active layer (3') from the donor substrate (1) to the receiver substrate (8) comprises the steps of:

■ introducing (E41) ions (24) into the donor substrate (1),

■ bonding (E42) the donor substrate (1) and the receiver substrate (8),

- detaching (E44) the donor substrate (1) from the receiver substrate (8) by fracture at the level of said confinement layer (25).

13. Method according to Claim 12, wherein the introduction of the ions (24) into the donor substrate (1) is effected by immersion of the donor substrate (1) in a plasma containing said ions.

14. Method according to any one of Claims 9 to 11 , wherein the step consisting in transferring at least part of the strained active layer (3') from the donor substrate (1) to the receiver substrate (8) comprises the steps of:

- creating a weakened area (20) in the donor substrate (1),

- bonding the donor substrate (1) and the receiver substrate (8), and

- causing a fracture at the level of said weakened area (20) to detach the donor substrate (1) from the receiver substrate (8).

15. Method according to any one of Claims 9 to 14, consisting in applying cyclically the second, third, fourth and fifth steps for the fabrication of a plurality of receiver substrates (8) each including a non-porous strained active layer (3') and constituted of at least one semiconductor material starting from a donor substrate (1) procured for the first step.