WO2004077552A1

WO2004077552A1 - Relaxation of a thin layer after its transfer

Info

Publication number: WO2004077552A1
Application number: PCT/IB2004/000927
Authority: WO
Inventors: Bruno Ghyselen; Carlos Mazure; Emmanuel Arene
Original assignee: S.O.I.Tec Silicon On Insulator Technologies
Priority date: 2003-02-28
Filing date: 2004-03-01
Publication date: 2004-09-10
Also published as: FR2851847B1; JP4980049B2; EP1597758A1; FR2851847A1; JP2006519488A

Abstract

The invention relates to a method of forming a relaxed or pseudo-relaxed layer on a substrate, the relaxed layer (2’) being in a material selected from semiconductior materials, characterized in that the method comprises the following steps: (a) growing on a donor substrate (1) an elastically strained layer (2) consisting of at least a material selected among the semiconductor materials; (b) forming on the strained layer (2) or on a receiving substrate (7), a vitreous layer (4) made of a material viscous from a viscosity temperature; (c) bonding the receiving substrate (7), to the strained layer (2) via the vitreous layer (4); (d) removing a portion of the donor substrate (1), so as to form a structure comprising the receiving substrate (2), the vitreous layer (4), the strained layer (2) and the unremoved portion of the donor substrate (1) which thereby forms a surface layer (1B); (e) heat treating the structure at a temperature close to or higher than the viscosity temperature. The invention further relates to structures obtained during the method of forming a relaxed or pseudo-relaxed layer on a substrate.

Description

RELAXATION OF A THIN LAYER AFTER ITS TRANSFER

Field of the invention

The present invention relates to the formation of a relaxed or pseudo-relaxed layer on a substrate, the relaxed layer being made of a material selected from semiconductor materials, in order to form a final structure for electronics, optics or optoelectronics, such as for example a semiconductor-on-insulator structure.

The present invention comprises in particular the formation of a strained layer on and through the relaxed layer.

For example, a layer of Si strained by a relaxed or pseudo-relaxed SiGe layer may in this respect achieve interesting properties such as a charge carrier mobility of the order of 100%, larger than that present within a relaxed Si layer.

A layer is said to be "relaxed" herein if the crystalline material of which it consists, has a lattice parameter substantially identical with its nominal lattice parameter, i.e. with the lattice parameter of the material in its bulk equilibrium form.

Conversely, a layer is said to be "strained" herein if the crystalline material of which it consists is elastically strained in tension or in compression during crystal growth, such as epitaxy, which forces its lattice parameter to be substantially different from the nominal lattice parameter of this material.

Background of the invention

It is known how to form a relaxed layer on a substrate, notably by applying a method comprising the following steps:

(1 ) epitaxy of a thin layer of a semiconductor material on a donor substrate;

(2) bonding a receiver substrate at the thin layer;

(3) removal of a part of the donor substrate. A semiconductor-on-insulator structure may thereby be made, in which the semiconductor part comprises or consists of said relaxed thin layer at least partly and the insulator part is usually formed in an intermediate step between step (1 ) and step (2). The manufacture of the thin layer may be achieved:

• during the application of step (1 ); or

• during a subsequent treatment.

In the first case, it is known how to use a donor substrate consisting of a holding substrate and a buffer layer, the buffer layer confining plastic deformations so that the thin layer epitaxied thereon is relaxed from any strain. Such methods are for example described in documents

US 2002/0072130 and WO 99/53539.

However, a buffer layer is often long and costly to make. In the second case, the donor substrate does not comprise any buffer layer and step (1 ) then consists of growing the thin layer to be strained by the donor substrate.

Thus, for example, a SiGe layer will be grown directly on a Si substrate, on such a thickness that the SiGe layer is globally strained.

A first technique for relaxing the SiGe layer, notably as described in the document of B. Hollander et al. entitled "Strain relaxation of pseudomorphic Siι_-xGβ_x/Si(100) heterostructures after hydrogen or helium ion implantation for virtual substrate fabrication" (in Nuclear and Instruments and Methods in Physics Research B 175-177 (2001 ) 357- 367) consists of relaxing, before applying step (2), , the SiGe layer by implanting hydrogen or helium ions in the Si substrate at a determined depth.

However, relaxation rates usually obtained with this first technique remain rather low as compared with other techniques.

The study of a second technique is notably disclosed in the document entitled "Compliant Substrates: A comparative study of the relaxation mechanisms of strained films bonded to high and low viscosity" by Hobart et al. (Journal of Electronic Materials, vol.29, No.7, 2000).

After removing the donor substrate during step (3), a heat treatment is applied for relaxing or pseudo-relaxing a layer of strained SiGe, bonded to a BPSG glass during step (2). During the heat treatment, the strained layer thus seems to relax via the layer of glass which has become viscous at the treatment's temperature.

However, this latter technique involves relaxation of the SiGe thin layer when the latter is exposed. Now, exposure of such a SiGe layer (exposed) to a gas atmosphere during a heat treatment (such as a room RTA treatment, a sacrificial oxidization, or a recovery anneal) may prove to be disastrous for the quality of this layer, wherein Ge contained in the layer may notably diffuse outwards (which may cause decomposition of the layer) and the layer may be contaminated by external contaminants.

Furthermore, such a SiGe layer is on the surface and may therefore have to undergo treatments such as finishing treatments (polishing, smoothening, oxidization, cleanings, etc.).

Now, these treatments for SiGe are less well mastered than those for Si.

And this lack of control when working with SiGe, now causes further difficulties for making the desired structure.

Brief summary of the invention The present invention attempts to overcome these difficulties by providing a method of forming a relaxed or pseudo-relaxed layer on a substrate, the relaxed layer being in a material selected from semiconductor materials, the method comprising the following steps:

(a) growing an elastically strained layer consisting of a material selected from semiconductor materials, on a donor substrate;

(b) forming on the strained layer or on a receiving substrate, a vitreous layer made of a material viscous from a viscosity temperature;

(c) bonding the receiving substrate to the strained layer via the vitreous layer;

(d) removing a portion of the donor substrate so as to form a structure comprising the receiving substrate, the vitreous layer, the strained layer, and the unremoved portion of the donor substrate which thereby forms a surface layer;

(e) heat treating the structure at a temperature close to or higher than the viscosity temperature. Other features of the method for forming on a substrate a relaxed or pseudo-relaxed layer are: step (b) and step (c) are operated before any substantial diffusion of a least one species of the material of the strained layer, before contamination of the stressed layer, and before the surface of the stressed layer becomes uncontrollably reactive. after step (d), a controlled treatment is applied so as to transform at least a portion of the surface layer into a material viscous from the second viscosity temperature, thereby forming a second vitreous layer; - step (e) is operated during or in continuity after the formation of the second vitreous layer; after step (e), a step is applied for removing the second vitreous layer; the method further comprises a last step of crystal growth is applied on the structure of a material selected among semiconductor materials; the vitreous layer is formed on the receiving substrate, and the method further comprises before step (c), a step of forming on the strained layer a thin layer with a thickness less than the thickness of the strained layer;- step (b) comprises the following two successive operations: (b1) growing a semiconducting layer on the strained layer; (b2) completing a controlled treatment for transforming at least a portion of the layer formed in step (b1) into a viscous material, as from the viscosity temperature, thereby forming the vitreous layer; - before step (c), a step of forming a bonding layer on the receiving substrate; the bonding layer is in Si0₂; the removal of material of step (d) is substantially achieved by a detaching at an embrittlement area present in the donor substrate, at a depth close to the thickness of the surface layer, by supplying energy; before step (c), a step of forming the embrittlement area by implanting atomic species into the donor substrate; before step (a), a step of forming the donor substrate comprising the following operations: . • forming a porous layer on a crystalline backing substrate,

• growing a crystal layer on the porous layer: the whole [backing substrate / porous layer / crystal layer ] being the said donor substrate, the porous layer being the embrittlement area in the donor substrate. - the removal of material of step (d) comprises selective chemical etching; the vitreous layer formed in step (b) is electrically insulating; the vitreous layer formed in step (b) is in Si0₂; the donor substrate is in Si, and the strained layer is in S - _xGe_x; the donor substrate comprises a Si bulk holding substrate and a buffer structure adapting the lattice parameter of Si to Siι-_xGe_X) and the strained layer comprises a Si strained layer and a Siι__zGe_z strained layer with z > x; - the layer grown at step (b1) is in Si, and the controlled treatment applied during step (b2) is a controlled thermal oxidization treatment for transforming at least a portion of the Si of the layer formed during step (b1) into Si0₂, thereby forming the Si0₂ vitreous layer; said controlled treatment is a controlled thermal oxidization treatment for transforming at least a portion of Si in the surface layer into Si0₂, thereby forming a second Si0₂ vitreous layer; the method further comprises, after step (e), a step of removing the second vitreous layer by chemical treatment based on hydrofluoric acid; step (e) comprises a heat treatment; - the material used for growing on the structure after removal of the second vitreous layer is Si; the vitreous layer formed in step (b) is electrically insulating and the formed structure is a semiconductor-on-insulator structure, the semiconducting thickness of which comprises the strained layer which has been relaxed or pseudo-relaxed during step (e); the method further comprises steps of preparing for the manufacturing of components and/or steps of manufacturing components in the strained layer in a layer optionally epitaxied thereon.

Brief description of the drawings

Other aspects, objects and advantages of the present invention will become more apparent upon reading the following detailed description of the application of the preferred methods thereof, given as a non-limiting example, and made with reference to the appended drawings wherein: Figures 1a-1 i illustrate the different steps of a first method according to the invention.

Figures 2a-2i illustrate the different steps of a second method according to the invention.

Figures 3a-3i illustrate the different steps of a third method according to the invention.

Figures 4a-4i illustrate the different steps of a fourth method according to the invention.

Figures 5a-5i illustrate the different steps of a fifth method according to the invention.

Figures 6a-6i illustrate the different steps of a sixth method according to the invention.

Figure 7 illustrates an example of a source wafer, with layers to be transferred grown on a buffer structure in SiGe.

Detailed description of the invention A first object of the present invention consists of forming a relaxed or pseudo-relaxed useful layer on a substrate.

A second object of the present invention consists of forming on the relaxed or pseudo-relaxed layer, a useful layer of strained material.

A "useful layer" according to the invention, is a layer intended to receive components for electronics, optics, or optoelectronics during treatments subsequent to the application of the method according to the invention.

A third object of the invention is to protect, throughout the application of the method according to the invention and notably during heat treatments, the layer to be relaxed or pseudo-relaxed from the atmosphere surrounding the structure in which it is contained, in order to prevent at least one of the atomic species of the material of which it is made up, from being able to diffuse.

A fourth object of the invention is to be able to apply different surface finishing techniques onto the desired structure during its making without damaging the quality of the layer to be relaxed or pseudo-relaxed.

This fourth object should notably be achieved in the particular case when the layer to be relaxed or pseudo-relaxed is in Siι_-xGe_x, and when the use of different processing techniques on the structure, usually applied on Si structures or layers, is desired.

The method according to the invention comprises the three aforementioned main steps (1 ), (2) and (3).

A preferred method according to the invention is described with reference to Figures 1 a-1 i.

A source wafer 10 according to the invention is illustrated with reference to Figure 1a.

The wafer 10 consists of a donor substrate 1 and a strained Si-ι_ _xGe_x layer 2.

In a first configuration of the donor substrate 1 , the latter consists entirely of monocrystalline Si with the first lattice parameter. Advantageously this donor substrate 1 is now made by Czochralski growth.

In a second configuration of the donor substrate 1 , the latter is a pseudo-substrate comprising an upper Si layer (not illustrated in Figure 1 ), exhibiting an interface with the strained layer 2 and having a first lattice parameter at its interface with the strained layer 2.

Advantageously, the first lattice parameter of the upper layer is the nominal lattice parameter of Si, so that the latter is in a relaxed state.

The upper layer further has a sufficiently large thickness so as to be able to impose its lattice parameter to the overlying strained layer 2, without the latter substantially influencing the crystalline structure of the upper layer of the donor substrate 1.

Whichever the configuration selected for the donor substrate 1 , the latter has a crystalline structure with a low density of structural defects, such as dislocations. Preferably, the strained layer 2 only consists of a single thickness of

Siι_-xGe_x.

The Ge concentration in this strained layer 2 is preferably higher than 10%, i.e. an x value greater than 0.10.

As Ge has a larger lattice parameter than Si by about 4.2%, the selected material for forming this strained layer 2 thus has a second nominal lattice parameter which is substantially larger than the first lattice parameter.

The formed strained layer 2 is then elastically strained in compression by the donor substrate 1 , i.e. it is strained so as to have a lattice parameter substantially less than the second lattice parameter of the material of which it is made, and therefore have a lattice parameter close to the first lattice parameter.

Preferably, the strained layer 2 further has a substantially constant composition of atomic elements.

Advantageously, the strained layer 2 is formed on the donor substrate 1 by crystal growth, such as an epitaxy by using known techniques such as for example, LPD, CVD and MBE (respective abbreviations of Liquid Phase Deposition, Chemical Vapor Deposition, and Molecular Beam Epitaxy) techniques.

In order to obtain such a strained layer 2, without too many crystallographic defects, such as for example point defects, or extended defects such as dislocations, it is advantageous to select the crystalline materials forming the donor substrate 1 and the strained layer (in the vicinity of its interface with the holding substrate 1) so that they have a sufficiently small difference between their first and their second respective nominal lattice parameters.

For example, this lattice parameter difference is typically between about 0.5% and about 1.5%, but may also have larger values.

For example, Siι_-xGe_x with x=0.3 has a nominal lattice parameter larger than Si by about 1.15%. On the other hand, it is preferable that the strained layer 2 have a substantially constant thickness, so that it has substantially constant intrinsic properties and/or for facilitating the future bonding with the receiver substrate 5 (as illustrated in Figure 1 i).

In order to prevent relaxation of the strained layer 2 or occurrence of internal plastic type defaults, the thickness of the latter should further be less than a critical thickness for elastic strain. This critical elastic strain thickness mainly depends on the material selected for the strain layer 2 and on said lattice parameter difference with the donor substrate.

But it also depends on growth parameters such as the temperature at which it has been formed, on nucleation sites from which it was epitaxied, or on the growth techniques used (for example CVD or MBE).

As regards the critical thickness values for Si-ι_-xGe_x layers, reference may be made to the document entitled "High-mobility Si and Ge structures" by Friedrich Schaffler ("Semiconductor Science Technology" 12 (1997) 1515-1549).

As for the other materials, one skilled in the art shall refer to the state of the art in order to find out the value of the critical elastic strain thickness of the material which is selected for the strained layer 2 formed on the donor substrate 1. Thus, a Siι__xGβ_x layer with x between 0.10 and 0.30 has a typical thickness between 200 A and 2,000 A, preferably between 200 A and 500 A by notably adapting the growth parameters.

Once formed, the strained layer 2 therefore has a lattice parameter substantially close to that of its growth substrate 1 and exhibits internal elastic straines in compression.

With reference to Figure 1c, a vitreous layer 4 is formed on the strained layer 2 according to a first embodiment of the vitreous layer 4.

The material making up the vitreous layer 4 is such that it becomes viscous as from a viscosity temperature To- Advantageously, the material of the vitreous layer 4 is one of the following materials: BPSG, Si0₂, SiON.

When a SiO_xN_y vitreous layer 4 is formed, the value of y may advantageously be varied in order to change the viscosity temperature TQ which is substantially a function of the nitrogen composition for this material.

Thus, with a growing composition thereon, it is now possible to change the T_G of the vitreous layer 4 typically between a T_G of the order of that of Si0₂ (which may vary around 1 ,150°C) and a Tc of the order of that of Si₃N₄ (which is higher than 1 ,500°C).

A large TQ range may thereby be covered by varying y. The T_G values of the vitreous layer 4, if they essentially depend on the material of the vitreous layer, may also fluctuate according to the conditions under which it was formed.

In an advantageous scenario, the conditions for forming the vitreous layer 4 may thereby be adapted in a controllable in order to select a T_G "a la carte".

The deposition parameters may thus be varied such as temperature, duration, dosage and potential of the gas atmosphere, etc.

Doping elements may thereby be added to the main gaseous elements contained in the vitrification atmosphere, such as boron and phosphorus which may have the property of reducing TQ.

It is important that the strained layer 2 be covered with the vitreous layer 4 before: germanium contained in the strained layer 2 is able to diffuse into the atmosphere; and before - the strained layer 2 is contaminated significantly; and before the surface of the strained layer 2 becomes reactive in an uncontrollable way; notably when the whole undergoes a heat treatment at high temperature, such as an RTA type annealing treatment or a sacrificial oxidization treatment.

In a preferred embodiment of the vitreous layer 4, the following steps are applied onto the strained layer 2:

(b1) with reference to Figure 1b, growing a semiconductor material layer 3 on the strained layer 2; and then (b2) with reference to Figure 1c, applying a controlled treatment for transforming at least a portion of the layer formed in step (b1), into a viscous material as from the viscosity temperature, thereby forming the vitreous layer 4.

Advantageously, the material selected for layer 3 is Si in order not to change the strain in the strained layer 2. The thickness of the formed layer 3 is typically between about 5 A and about 5,000 A, more particularly between about 100 A and about 1 ,000 A.

For the same reasons as those explained above, crystal growing during step (b1 ) of the layer 3 is preferably applied before diffusion of Ge, i.e. shortly after:

• the formation of the strained layer 2 if the temperature for forming the strained layer 2 is maintained; or

• a rise in temperature subsequent to a fall in temperature down to room temperature having been caused immediately after forming the strained layer 2.

The preferred method for growing layer 3 is in situ growth directly in continuation of the growth of the strained layer 2.

The growth technique used during step (b1) may be an epitaxy

LPD, CVD or MBE technique. The vitreous layer 4 may be made by heat treatment under an atmosphere with a determined composition.

Thus, a Si layer 3 may undergo during step (b2) a controlled heat oxidization treatment in order to transform this layer 3 into a Si0₂ vitreous layer 4. During the latter step, it is important to accurately dose the parameters of the oxidization treatment (such as temperature, duration, oxygen concentration, the other gases of the oxidizing atmosphere, etc.) in order to control the formed oxide thickness and to stop oxidization in the vicinity of the interface between both layers 2 and 3. For such thermal oxidization, a dry oxygen or steam atmosphere will preferably be used at a pressure equal to or larger than 1 atm. Preferably, the oxidization duration will then be varied in order to control the oxidization of layer 3.

However, this control may be made by varying one or several other parameters, either combined or not with the time parameter. Reference may notably be made to document US 6,352,942 for more details concerning this embodiment of such a Si0₂ vitreous layer 4 on a SiGe layer.

According to a second embodiment of the vitreous layer 4, and as a replacement for said two steps (b1) and (b2) respectively referenced by Figures 1b and 1c, a deposition of atomic species is applied by means for depositing atomic species on the strained layer 2.

In a first case, atomic species consisting of the vitreous material will be deposited directly.

Thus, for example, Si0₂ molecules may be deposited in order to form the Si0₂ vitreous layer 4.

In a second case, the following operations will be applied:

• deposition of amorphous Si atomic species for forming an amorphous Si layer; and then: o thermal oxidization of this amorphous Si layer and thereby forming an Si0₂ vitreous layer 4.

Whichever one of these deposition cases is selected, the deposition of the atomic species should be made before diffusion of Ge, contamination and uncontrolled surface reactivation of the strained layer 2, and notably if the strained layer 2 remains at a high temperature in the meantime.

With reference to Figures 1d, 1e and 1f, steps are illustrated for taking up the strained layer 2 and the vitreous layer 4 from the donor substrate 1 in order to transfer them onto a receiving substrate 7.

For this purpose, the method according to the invention applies a technique consisting of two successive main steps:

• bonding the receiving substrate 7 with the vitreous layer 4; • removal of a portion of the donor substrate 1.

With reference to Figure 1i, said bonding is applied.

Before the bonding, an optional step for forming a bonding layer on at least one of the two surfaces to be bonded may be applied, this bonding layer having binding properties, at room temperature or at higher temperatures.

Thus, for example, forming a Si0₂ layer may improve the quality of the bond, notably if the other surface to be bonded is in Si0₂ or in Si.

This Si0₂ bonding layer is then advantageously made by depositing Si0₂ atomic species or by thermal oxidization of the surface to be bonded if the surface of the latter is in Si.

A step for preparing the surfaces to be bonded is advantageously applied before the bonding in order to make the surfaces as smooth and as clean as possible. Suitable chemical treatments for cleaning the surfaces to be bonded may be applied, such as weak chemical etchings, an RCA treatment, ozoned baths, rinses, etc.

Mechanical or mechanochemical treatments may also be applied such as polishing, abrasion, CMP (Chemical Mechanical Planarization) or atomic species bombardment.

The bonding operation as such is carried out by bringing the surfaces to be bonded into contact with one another.

The bonding linkages are preferably of a molecular nature by using the hydrophilic properties of the surfaces to be bonded. In order to impart or enhance hydrophilic properties of the surfaces to be bonded, preliminary dippings of both structures to be bonded in baths may be applied, such as for example rinsing with deionized water.

An anneal of the bonded whole may further be applied by reinforcing the bonding linkages, for example by changing the nature of the bonding linkages, such as covalent linkages or other linkages.

Thus, if the vitreous layer 4 is in Si0₂, an anneal may enhance the bonding linkages, notably if a Si0₂ bonding layer has been formed prior to the bonding to the receiving substrate 7.

For more details regarding the bonding techniques, reference may be made to the document entitled "Semiconductor Wafer Bonding" (Science and technology, Interscience Technology) by Q.Y.Tong, U.Gόsele and Wiley.

Once the whole is bonded, removal of material as preferred according to the invention is applied and it consists of separating a portion of the donor substrate 1 at an embrittlement area 6 present in the donor substrate 1 , by supplying energy.

With reference to Figures 1 d and 1 e, this embrittlement area 6 is an area substantially parallel to the bonding surface, and exhibits linkage brittleness between the lower portion 1a of the donor substrate 1 and the upper portion 1 b of the donor substrate 1 , whereby these brittle linkages may be broken when energy is supplied, such as thermal or mechanical energy.

According to a first embodiment of the embrittlement area 6, a technique called Smart-Cut^® is applied and firstly comprises implantation of atomic species into the donor substrate 1 , at the embrittlement area 6. The implanted species may be hydrogen, helium, a mixture of both of these species or other lightweight species.

Implantation preferably occurs just before bonding.

Implantation energy is selected so that the species implanted through the surface of the vitreous layer 4, cross the thickness of the vitreous layer 4, the thickness of the strained layer 2, and a determined thickness of the upper portion 1 b of the receiving substrate 1.

Implantation into the donor substrate 1 is preferably sufficiently deep so that the strained layer 2 does not undergo any damages during the detaching step from the donor substrate. The implant depth in the donor substrate is thus typically about

1 ,000 A. The brittleness of the linkages in the embrittlement area 6 is mainly found by the selection of the dosage of the implanted species, the dosage being thus typically between 10¹⁶ cm^"2 and 10¹⁷ cm^"2 and more specifically between about 2.10¹⁶ cm^"2 and about 7.10¹⁶ cm^"2. The detaching at the embrittlement area 6 is then usually carried out by supplying mechanical and/or thermal energy.

For more details concerning the Smart-Cut^® method, reference will be made to the document entitled "Silicon-On-lnsulator Technology: Materials to VLSI, 2^nd edition" by J.-P. Colinge, edited by Kluwer Academic Publishers, pp. 50 and 51.

According to a second embodiment of the embrittlement area 6, a technique notably described in document EP 0 849 788 is applied.

The embrittlement layer 6 is made here before forming the strained layer 2 and during the formation of the donor substrate 1. The making of the embrittlement layer comprises the following main operations:

• formation of a porous layer on a Si holding substrate 1 A; o growing a Si layer 1 B on the porous layer.

The whole /holding substrate 1 A porous layer/Si layer 1 B/ then forms the donor substrate 1 and the porous layer then forms the embrittlement area 6 of the donor substrate 1.

Supply of energy, such as supplying thermal and/or mechanical energy, at the porous embrittlement area 6, then leads to detaching of the holding substrate 1A from the layer 1 B. The preferred technique according to the invention for removing material at an embrittlement area 6, achieved according to one of the two non-limiting embodiments above, thus enables a large portion of the donor substrate 1 to be removed rapidly as a block.

It also provides the possibility of reusing the removed portion 1A of the donor substrate 1 in another method, like for example a method according to the invention. Thus, reforming a strained layer 2 on the removed portion 1A and another optional portion of a donor substrate and/or other layers may also be applied, preferably after polishing the surface of the removed portion 1A. With reference to Figure 1f, after separating the remaining portion

1 B from the removed portion 1A of the donor substrate 1 , removal of finishing material is applied enabling the remaining portion 1B to be removed.

Finishing techniques such as polishing, abrasion, CMP planarization, RTA thermal annealing, sacrificial oxidization, chemical etching, taken alone or in combination, may be applied for removing this portion 1 B and for perfecting the stacking (strengthening of the bonding interface, removal of bumps, curing defects, etc.).

Advantageously, at least at the end of a step, the removal of finishing material applies selective chemical etching, either combined or not with mechanical means.

Thus, solutions based on KOH, NH₄OH (ammonium hydroxide), TMAH, EDP or HN0₃, or solutions presently investigated which combine agents such as HN0₃, HN0₂, H₂0_2j HF, H₂S0 , H₂S0₂, CH₃COOH, H₂0₂ and H₂0 (as explained in document WO 99/53539, page 9) may advantageously be used for selectively etching the Si portion 1 B relatively to the strained Siι_-xGe_x layer 2.

After the bonding step, another technique for removing material without detaching and without any embrittlement area, according to the invention, may be applied for removing the portion of the donor substrate 1.

It consists of applying chemical and/or mechanochemical etching.

For example, optional selective etchings for the material(s) to be removed from the donor substrate 1 may be applied according an etch- back type method.

This technique consists in etching the donor substrate 1 from the back, i.e. from the free face of the donor substrate 1.

Wet etchings which apply etching solutions suitable for the materials to be removed, may be applied.

Dry etchings may also be applied for removing material, such as plasma or spray etchings.

Etching(s) may further only be chemical or electrochemical or photochemical.

Etching(s) may be preceded or followed by mechanical abrasion of the donor substrate 1 , such as a grinding, a polishing, a mechanical etching or spraying of atomic species.

The etching(s) may be accompanied by mechanical abrasion, such as polishing optionally combined with action of mechanical abrasives in a CMP method.

All the aforementioned techniques for removing material from the donor substrate 1 , are provided by way of example in the present document, but by no means are a limitation, the invention extending to all types of techniques capable of removing material from the donor substrate

1 , in accordance with the method according to the invention.

With reference to Figure 1f, a portion 1 B of the donor substrate 1 is preserved after removal.

This has the effect of leaving the strained layer 2 buried and thus of protecting it against the external atmosphere, thereby achieving said first object set for applying this method.

Regardless of the technique for removing material, selected from the techniques already discussed or from other known techniques, a surface finishing step for the remaining portion 1 B of the donor substrate 1 is advantageously applied, such as optionally selective chemical etching, CMP polishing, heat treatment, a bombardment with atomic species or any other smoothening technique. Thus, after applying a step for removing material of the Smart-Cut^® type, a smoothening treatment is preferably used such as one of the following treatments:

• polishing in order to obtain a thickness between about 200 A to about 800 A;

• Ar/H₂ RTA fast annealing followed by polishing in order to obtain a thickness between about 200 A to about 800 A;

• a single fast RTA annealing;

• Ar/H₂ oven annealing.

These finishing treatments are particularly performing within the framework of the invention as they are applied onto a Si surface (of the remaining portion 1 B of the donor substrate 1 ).

Indeed, if the SiGe strained layer 2 had been exposed, it would have been difficult to apply these techniques without deteriorating this strained layer 2, as these techniques are still poorly mastered for SiGe.

With the Si surface layer 1B, it is therefore possible to efficiently smooth the surface of the obtained structure after detaching.

With reference to Figure 1f, after removal of the material, a structure is obtained, comprising the receiving substrate 7, the vitreous layer 4, the strained layer 2 and a Si surface layer 1 B (which represents the remaining portion of the donor substrate 1). The strained layer 2 is thus protected substantially from the outside by the overlying surface layer 1 B and the underlying vitreous layer 4.

According to an alternative method, the surface layer 1 B is preserved as is.

However, with reference to Figure 1g, forming a second vitreous layer 8 at the surface of the structure consisting of a viscous material as from a second viscosity temperature is applied advantageously, thereby forming it.

The selected material for the second vitreous layer 8 may for example be one of the following materials: Si0₂, BPSG, SiO_xN_y. This second vitreous layer 8 is preferably formed by transforming the surface layer 1B into a vitreous layer 4, by means of a suitable controlled treatment.

Thus, the second vitreous layer 8 may be made by heat treatment under an atmosphere with a determined composition.

Thus, the Si surface layer 8 may undergo a controlled thermal oxidization treatment in order to transform this surface layer 8 into a Si0₂ vitreous layer 8.

During the latter step, it is important to accurately dose the parameters of the oxidizing treatment (such as the temperature, duration, oxygen concentration, the other gases of the oxidizing atmosphere, etc.) in order to control the formed oxide thickness, and to stop oxidization in the vicinity of the interface between both layers 2 and 1 B.

For such a thermal oxidization, a dry oxygen or steam atmosphere will preferably be used at a pressure equal to or larger than 1 atm, at a temperature between about 500°C and about 1 ,050°C. The oxidization duration will then preferably be varied for controlling the oxidization of the surface layer 8.

However, this control may be made by varying one or several other parameters, either combined or not with the time parameter.

Always with reference to Figure 1g, a heat treatment is then applied at a temperature close to or higher than the viscosity temperature.

This heat treatment has the main purpose of relaxing the strains in the strained layer 2.

Indeed, heat treatment at a temperature higher than or around the viscosity temperature TQ of the vitreous layer 4, will cause viscosity of the latter layer, which will allow the strained layer to relax at its interface with the vitreous layer 4, causing decompression of at least part of its internal strains.

Thus, if the vitreous layer 4 is in Si0₂ made by thermal oxidization, heat treatment at a minimum of about 1 ,050°C, preferably at about a minimum of 1 ,200°C for a determined duration, will cause relaxation or pseudo-relaxation of the strained layer 2. The heat treatment typically lasts between a few seconds and several hours.

This relaxation of the strained layer 2 is achieved without the strained layer 2 being in contact with the outside world, unlike the state of the art, notably by preventing diffusion of Ge.

The strained layer 2 therefore becomes a relaxed layer 2'.

Other effects of the heat treatment on the structure may be sought, in addition to the relaxation of the strained layer 2.

A second sought purpose when the heat treatment is applied, may further be the achievement of an anneal for strengthening the bonding between the receiving substrate 7 and the vitreous layer 4.

Indeed, as the temperature selected for the heat treatment is higher than or around the viscosity temperature of the vitreous layer 4, the latter having temporarily become viscous, may generate particular and stronger adhesion linkages with the receiving substrate 7.

Thus, again taking the example of a bonding between the Si0₂ vitreous layer 4 and the receiving substrate on which a Si0₂ bonding layer has been applied, viscosities of both contacted layers will generate particularly strong covalent linkages. A third sought purpose is to apply said heat treatment in order to form the second Si0₂ vitreous layer 8 by thermal oxidization.

Indeed, it may be desired that this vitreous layer 8 be formed during or in continuity of the same heat treatment than the one which relaxes the strained layer 2, by simultaneously injecting oxygen into the oven, or else one just follows the other or during a heat cycle.

Finally a structure 20 is obtained, consisting of the whole /vitreous layer 8/relaxed Siι_-xGe_x 27vitreous layer 4/receiving substrate 71.

The relaxed Si_1-xGe_x of layer 2' is thereby protected from the outside by both adjacent vitreous layers 4 and 8. In order to expose the relaxed Siι_-xGe_x layer 2'; it is then sufficient to remove the vitreous layer 8 for example by means of a suitable chemical treatment.

Thus, if the vitreous layer 8 is in Si0₂, the structure 20 will advantageously be treated with hydrofluoric acid HF in order to remove Si0₂ from the vitreous layer 8. With reference to Figure 1h, a structure 30 consisting of /relaxed

Siι._xGe_x 27vitreous layer 4/ receiving substrate 7/ is finally obtained.

This structure 30 is a SGOI structure (Silicon Germanium On Insulator) if the vitreous layer 4 is electrically insulating, such as for example a Si0₂ vitreous layer 4. The relaxed Siι_-xGe_x layer 2' of this structure then has a surface with a surface roughness compatible with growth of another crystalline material.

A slight surface treatment such as polishing, suitable for Siι_-xGe_x, may optionally be applied in order to improve surface properties. With reference to Figure 1i, and in an optional step of the invention, a growing Si layer 11 on the relaxed Siι_-xGe_x layer 2' is then applied with a thickness substantially less than the strain critical thickness of the material of which it consists, and it is therefore strained by the relaxed Siι_-xGe_x layer 2'. Finally a structure consisting of /strained Si/ relaxed Siι_-xGe_x

27vitrβous layer 4/ receiving substrate 7/ is then obtained.

This structure 40 is a Si/SGOI structure if the vitreous layer 4 is electrically insulating, such as for example a Si0₂ vitreous layer 4.

Alternatives of this method are presented with reference to Figures 2a-2i, Figures 3a-3i, and Figures 4a-4i.

With reference to Figures 2a-2i, and more particularly to Figure 2g, the method is globally the same one as described with reference to

Figures 1 a-1 i, except for the step for transforming the surface layer 1 B into a second vitreous layer 8 which is applied here so that the whole surface layer 1B is not transformed.

Thus, there remains a portion of the Si surface layer 1 B inserted between the second vitreous layer 8 and the strained layer 2, forming an intermediate layer 9.

With reference to Figure 2h, this intermediate layer 9 is preserved after the heat treatment for relaxing the strained layer 2. This intermediate layer 9 is advantageously preserved with a thickness less than the strain critical thickness so that it is strained subsequently by the relaxed layer 2'.

With reference to Figure 2i, growing a Si layer may be resumed on the intermediate layer 9 in order to form a strained Si layer 11 substantially identical with that of Figure 2i.

A smoothening step for the growth surface by means of one of the techniques already discussed in this document may be applied beforehand to the growth of silicon, in order to improve the quality of the crystal growing to be applied. In a particular scenario when the heat treatment for relaxing the strains of the strained layer 2 is carried out at a temperature and for a duration, respectively higher than a standard temperature and longer than a standard duration, as from which Ge diffuses into Si, Ge contained in the strained layer 2 may diffuse into the intermediate layer 9. This is why it is preferable to apply the relaxation of the strained

SiGe layer 2 before resuming epitaxy of the strained Si layer 11.

However, in certain other cases, this diffusion effect may be sought if it is suitably controlled.

Thus, diffusion may be controlled in such a way that the Ge species are uniformly distributed throughout both layers 2 and 9, forming a unique Si-ι_-xGe_x layer with a substantially uniformized Ge concentration.

A discussion of this latter point will notably be found in document US 5 461 243, column 3, lines 48-58.

With reference to Figures 3a-3i, and more particularly to Figure 3c, the method is globally the same as the one described with reference to

Figures 1a-1i, except for the step for transforming layer 3 into the vitreous layer 4 which is applied here so that the whole layer 3 is not transformed.

Thus, there remains a portion of the Si layer 3 inserted between the vitreous layer 4 and the strained layer 2, forming an inserted layer 5.

This inserted layer 5 is made so as to have a typical thickness around 10 nm, in any case much less than that of the strained layer 2.

During the heat treatment for relaxing the strain of the strained layer 2, the latter will want to reduce its internal elastic strain energy by utilizing the viscosity properties of the vitreous layer 4 which has become viscous, and, because the inserted layer 5 has a small thickness relatively to the overlying strained layer 2, the strained layer 2 will impose its relaxation requirement to the inserted layer 5.

The strained layer 2 thereby forces the inserted layer 5 to be under strain at least partially.

The strained layer 2 then becomes a relaxed layer 2' at least partially.

The relaxed inserted layer 5 then becomes a strained inserted layer 8'.

A discussion of the latter point will be found notably in document US 5 461 243, column 3, lines 28-42. With reference to Figure 3h, this strained inserted layer 5" is preserved after the heat treatment for relaxing the strained layer 2.

The formed structure is then a structure consisting of /relaxed Siι_ _xGe_x/strain Si/vitreous layer 4/ receiving substrate 71.

This structure 30 is a SG/SOI structure if the vitreous layer 4 is electrically insulating, such as for example a Si0₂ vitreous layer 4.

It is then possible to optionally remove, for example by selective chemical etching based on HF:H₂0₂:CH₃COOH (selectivity of about 1 :1 ,000), the relaxed Siι_-xGe_x layer 2', in order to finally obtain a structure consisting of /strained Si/vitreous layer 4/receiving substrate II. This structure is a strained SOI structure if the vitreous layer 4 is electrically insulating, such as for example a Si0₂ vitreous layer 4. Instead of applying this chemical etching, it is possible to grow a Si layer resumed on the relaxed layer 2' with reference to Figure 3i, so as to form a strained Si layer 11 , substantially identical with that of Figure 3i.

The formed structure is then a structure 40 consisting of /strained Si/relaxed Siι-_xGe_x/strained Si/ vitreous layer 4/receiving substrate 71.

This structure 40 is a Si/SG/SOI structure, if the vitreous layer 4 is electrically insulating, such as for example a Si0₂ vitreous layer 4.

In a particular scenario, when the heat treatment for relaxing the strains of the strained layer 2 is carried out at a temperature and for a duration, respectively higher than a standard temperature and longer than a standard duration, as from which Ge diffuses into Si, Ge contained in the strained layer 2 may diffuse into the strained inserted layer 5'.

This is why it is preferable to apply the relaxation of the strained SiGe layer 2 before resuming epitaxy of the strained Si layer 11. But, in certain other cases, this diffusion effect if it is suitably controlled may be sought.

Thus, diffusion may be controlled in such a way that the Ge species are distributed uniformly throughout both layers 2 and 5 forming a unique Siι_-xGe_x layer with a substantially uniformized Ge concentration. A discussion of the latter point will notably be found in document

US 461 ,243, column 3, lines 48-58.

With reference to Figures 4a-4i, and more particularly to Figures 4c and 4g, the method is globally the same as the one described with reference to Figures 1a-1 i, except for: • the step of transforming the layer 3 into a vitreous layer 4, which is applied here so that the whole layer 3 is not transformed;

• the step of transforming the surface layer 1 B into a second vitreous layer 8, which is applied here so that the surface layer 1 B is not transformed. In fact, this method comprises a step identical with the one described with reference to Figure 3c, forming an inserted layer 5 (see Figure 3c) and a step identical with the one described with reference to Figure 2g, forming an intermediate layer 9 (see Figure 2g).

The means for forming both of these layers 5 and 9, as well as the possibilities of development of their structure and their effect on the final structure, are therefore substantially the same as those described in the methods with reference to Figures 2a-2i and to Figures 3a-3i.

With reference to Figures 5a-5h, and more particularly to Figures 5b and 5d, the method is globally the same as the one described with reference to Figures 1 a-1 i, except that: - with reference to step 5b, the epitaxied Si layer 3 on the strained layer 2 is a very thin layer, the thickness of which is much less than that of the strained layer 2, typically from 100 to 300 A; with reference to Figure 5d, the vitreous layer 4 is formed on the receiving substrate 7. The Si layer 3 will thus enable: the overlying SiGe strained layer 2 to be protected from Ge diffusion, external contamination and uncontrolled potential reactivation of its surface; perfectly mastered surface finishing means to be applied for Si, whereas they are much less well mastered for SiGe, these finishing techniques (already detailed above in this document) notably providing good bonding with the receiving substrate 7.

With reference to Figure 5d, before the bonding, a vitreous layer 4 is formed on the receiving substrate 7 according to a first embodiment of the vitreous layer 4.

The material forming the vitreous layer 4 is such that it becomes viscous as from a viscosity temperature TQ.

Advantageously, the material of the vitreous layer 4 is one of the following materials: BPSG, Si0₂, SiON. This first embodiment for forming the vitreous layer 4 on the receiving substrate, is applied similarly to the first embodiment for forming the vitreous layer 4 on the strained layer 2 as described above in this document (with reference to Figure 1 c).

Thus, for example, oxidization of the Si surface of the receiving substrate 7 forms a Si0₂ vitreous layer 4. It is important that the formation of the vitreous layer 4 and the bonding of the vitreous layer 4 with the strained layer 2 be completed before Ge diffusion, contamination and uncontrolled reactivation of the surface of the strained layer 2, and notably if the strained layer 2 remains at a high temperature in the meantime. According to a second embodiment of the vitreous layer 4 on the receiving substrate, deposition of atomic species is applied by means for depositing atomic species on the receiving substrate 7.

In a first case, the atomic species consisting of vitreous material such as Si0₂ will be deposited directly. In a second case, the following operations will be applied:

• deposition of amorphous Si atomic species in order to form an amorphous Si layer; and then: o thermal oxidization of this amorphous Si layer and thereby making a Si0₂ vitreous layer 4. Whichever deposition case is selected, deposition of the atomic species should be achieved before Ge diffusion, contamination and uncontrolled reactivation of the surface of the strained layer 2, and notably if the strained layer 2 remains at a high temperature in the meantime.

With reference to Figures 5e, 5f, 5g and 5h, the same conditions and the same configurations as those discussed with reference to Figures 3f, 3f, 3h and 3i, are found, whereby the referenced layer 5 becomes the referenced layer 3 in the present method.

That is to say in particular, that during the heat treatment for relaxation: - the strained layer 2 then becomes at least a partially relaxed layer 2'; the inserted layer 3 then becomes a strained inserted layer 3'.

With reference to Figure 5g, the formed structure is then a structure consisting of /relaxed Siι__xGe_x/strained Si/vitreous layer 4/receiving substrate 71.

This structure 30 is a SG/SOI structure, if the vitreous layer 4 is electrically insulating, such as for example a Si0₂ vitreous layer 4.

The relaxed Si-i-_xGe_x layer 2' may then be optionally removed, for example by selective chemical etching based on HF:H₂0₂:CHsCOOH (selectivity of about 1 :1 ,000) in order to finally have a structure consisting of /strained Si/vitreous layer 4/receiving substrate II.

This structure is a strained SOI structure, if the vitreous layer 4 is electrically insulating, such as for example a Si0₂ vitreous layer 4.

Instead of applying this chemical etching, it is possible to grow a Si layer, with reference to Figure 5h, resumed on the relaxed layer 2' in order to form a strained Si layer 11 , substantially identical with that of Figure 5h. The formed structure is then a structure 40 consisting of /strained Si/relaxed Siι__xGe_x/strained Si/vitreous layer 4/receiving substrate 71.

In a particular scenario, when the heat treatment for relaxing the strains of the strained layer 2 is carried out at a temperature and for a duration respectively higher than a standard temperature and longer than a standard duration as from which Ge diffuses into Si, Ge contained in the strained layer 2 may diffuse into the strained inserted layer 3'.

This is why it is preferable to apply relaxation of the strained SiGe layer 2 before resuming epitaxy of the strained Si layer 11.

However, in certain other cases, this diffusion effect, if it is suitably controlled, may be sought. Thus, diffusion may be controlled in such a way that the Ge species are uniformly distributed throughout both layers 2 and 5, forming a unique Si_1-xGe_x layer with a substantially uniformized Ge concentration.

With reference to Figures 6a-6h, and more particularly to Figure 6f, the method is globally the same as the one described with reference to

Figures 5a-5h, except for the step for transforming the surface layer 1 B into a second vitreous layer 8, which is applied here so that the whole surface layer 1 B is not transformed.

With reference to Figure 6g, this intermediate layer 9 is preserved after the heat treatment for relaxing the strained layer 2.

Advantageously, this intermediate layer 9 is preserved with a thickness less than the strain critical thickness, so that it is strained subsequently by the relaxed layer 2'.

With reference to Figure 6h, it is possible to resume growth of a Si layer on the intermediate layer 9 so as to form a strained Si layer 11 , substantially identical with that of Figure 5h. A smoothening step for the growth surface by means of one of the techniques already discussed in this document may be applied prior to growing silicon, in order to enhance the quality of the crystal growth to be applied.

In a particular scenario when the heat treatment for relaxing the straines of the strained layer 2 is carried out at a temperature and for a duration respectively higher than a standard temperature and longer than a standard duration, as from which Ge diffuses into Si, Ge contained in the strained layer 2 may diffuse into the intermediate layer 9 or into the inserted layer 3. This is why it is preferable to apply the relaxation of the strained

SiGe layer 2 before resuming epitaxy of the strained Si layer 11. 3 θ ^"

However, in certain other cases, this diffusion effect if it is suitably controlled, may be sought.

Thus, diffusion may be controlled so that the Ge species are uniformly distributed throughout both layers 2, 3 and 9, forming a unique Siι-_xGe_x layer with a substantially uniformized Ge concentration.

According to any of the six preferred methods according to the invention above, or according to an equivalent thereof, steps for making components may be integrated or may follow this method according to the invention.

Thus, preparation steps for the making of components may be applied during the method at the strained SiGe layer 2 of the structure with reference to Figures 1g, 2g, 3g, 4g, 5f or 6f, at the relaxed or pseudo- relaxed SiGe layer 2' of the SGOI structure with reference to Figures 1h, 2h, 3h, 4h, 5g or 6g, or in the strained Si layer 11 of the Si/SGOI structure with reference to Figures 1i, 2i, 3i, 4i, 5h or 6h.

Preferably, these preparation steps will be achieved with the vitreous layer 8 always present in the structure, the latter protecting the underlying layers, and notably the strained layer 2 or the relaxed layer 2', both in SiGe.

For example, local treatments may be undertaken for etching through the vitreous layer 8, patterns in the layers, for example by lithography, photolithography, reactive ion etching, or any other etching technique with pattern masking.

In a particular case, patterns such as islands are thus etched into the SiGe strained layer 2 in order to contribute to proper relaxation of the strained layer 2 during the subsequent application of the relaxation heat treatment. One or several steps for making the components, such as transistors, in the strained Si layer 11 (or in the relaxed SiGe layer 2' if the latter is not covered with a strained Si layer 11 ) may notably be applied, preferably at a temperature less than TG (SO as not to change the strain ratio of the relaxed layer 2' and the strained layer 11).

In a particular method according to the invention, steps for making the components are applied during or in continuity of the heat treatment for relaxing the strained SiGe layer 2.

In a particular method according to the invention, the step for epitaxy of the strained Si layer is applied during or in continuity of the steps for making the components. Referring to figure 7 which represents a source wafer 10 before the formation of the embrittlement zone 6 and the formation of the vitreous layer 4, an embodiment of the invention is now presented, different from the various examples previously detailed referring to figure 1a to 1 i, 2a to 2i, 3a to 3i, 4a to 4i, 5a to 5h and 6a to 6h, by the way of choosing the materials constituting the donor substrate 1 and the strained layer 2.

Indeed, contrary to the different examples previously described, the donor substrate 1 here is composed of a holding substrate 1-1 of Si and a buffer structure composed of a buffer layer 1-2 in SiGe and an upper layer 1-3 in Siι_-2Ge_z. The holding substrate 1-1 is preferably in a bulk structure of single- crystal.

The buffer layer 1-2 can for example be constituted of a stacking of layers so that the whole composition of Ge inside the buffer layer 1-2 gradually evoluates from 0% at the interface with the holding substrate 1-2 to 100z% of Ge at the interface with the upper layer 1-3 of Siι_-zGe_z.

Contrary to the buffer layer 1-2, the upper layer 1-3 has a Ge composition constant in its thickness.

The upper layer 1-3 has a thickness sufficiently important to assign its lattice parameter to the overlied layer. Furthermore, the upper layer 1-3 of Siι_-2Ge_z has a relaxed structure. Thus, the buffer structure (composed of the buffer layer 1-2 and the upper layer 1-3) allows:

• an adaptation of the lattice parameter between the holding substrate 1-1 of Si and the nominal lattice parameter of Si-ι_-zGe_z of the upper layer 1 -3;

• a confinement of crystal defaults, the surface of the upper layer 1 -3 being then without or with a few defaults.

Over the donor substrate 1 , the strained layer 2 is grown by epitaxy techniques, such as CVD techniques (PECVD, MOCVD...). Firstly, a strained Si layer 2-1 is formed on the donor substrate 1 with a thickness no more than the critical thickness beyond which a such Si layer 2-1 starts to relax its elastic strains.

A Siι_-xGe_x strained layer 2-2 is then formed on the last Si strained layer 2-1 , so as to have a thickness less than the critical thickness of Siι_ _xGe_x beyond which the elastic strains start to relax.

Knowledge of the respective critical thickness of Si or Siι_-xGe_x can be found for instance in "High mobility Si and Ge structures" from Friedrich Schaffler ("Semiconductor science technology" 12 (1997) 1515-1549).

The x-composition of Ge in the Si<ι_-xGe_x layer 2-2 is greater than the z-composition of Ge in the upper layer 1 -3.

If we consider here that the strained layer 2 includes the Si strained layer 2-1 and the S __xGe_x layer 2-2, and that the donor substrate 1 comprises the holding substrate 1-1 , the buffer layer 1-2, and the upper layer 1-3 of Siι__zGe_z, the previous examples, (presented referring to the previous figures) of various embodiments of manufacturing a semiconductor-on-insulator structure 30 or 40, can then be easily transposed from the source wafer 10 of the figure 7, the embrittlement zone 6 being formed in the upper layer 1-3 or in the buffer layer 1-2.

After a step of surface finishing (using for instance polishing, chemical etching, oxidizing, annealing, or other means of finishing), the semiconductor-on-insulator structure then obtained (not shown) comprises successively a receiving substrate 7, a vitreous layer 4, the Siι_-xGe_x strained layer 2-2, the Si strained layer 2-1 and the remaining part of the Si_1-zGe_z upper layer 1-3.

Then, a thermal treatment with a temperature close to or greater than the viscosity temperature of the vitreous layer 4 previously formed, is processed.

This thermal treatment then relaxes at least partly the Siι_-xGe_x layer 2-2.

The relaxed Siι__xGe_x layer 2-2 imposes then elastic constraint to the top Si strained layer 2-1 and to the remaining part of the upper layer 1-3 of Siι__zGe_z.

Elastic constraints in the Si strained layer 2-1 (previously strained by the upper layer 1-3 of Siι_-zGe_z) are then increased by the fact that x- composition of Ge is more important than z-composition. Thus, a semiconductor-on-insulator structure with a lattice parameter, in its semiconductor part, close to or equal to those of the Si-i. _xGe_x material in its bulk configuration, is obtained.

An optional additional step of removal of the remaining part of the upper layer 1-3 of Siι_-zGe_z is processed, by means for example of a selective chemical etching employing for instance etch agent as HF:H₂0₂:CH₃COOH (selectivity about 1 :1000 between SiGe and Si).

Contrary to the prior art, this semiconductor-on-insulator structure is not obtained from a source wafer comprising a buffer structure adapting the parameter to a Siι_.-xGe_x, but from a buffer structure adapting the parameter to a Si-ι_-zGe_z with z < x.

Now, a buffer structure which adapts a lattice parameter to a Si-i. _xGe_x is thicker, comprises more stacking layers, and so is longer and more expansive to manufacture, than a buffer structure which adapts a lattice parameter to a Sι^'ι__zGe_z. This method according to the embodiment of the invention offers then technical and economical improvements comparing with the latter prior art.

The various techniques described in the invention are provided by way of example in the present document, but are by no means a limitation, the invention extending to all types of techniques able to apply a method according to the invention.

One or any epitaxies may be applied onto the final structure (structure 30 or 40 taken with reference to Figure 1h, 1i, 2h, 2i, 3h, 3i, 4h, 4i, 5g, 5h, 6g, 6h), such as an epitaxy of a SiGe or SiGeC layer, or an epitaxy of a strained Si or SiC layer, or successive epitaxies of SiGe or SiGeC layers and of alternately strained Si or SiC layers in order to form a multilayer structure.

Upon completion of the final structure, finishing treatments may optionally be applied, compris ing an anneal for example. The present invention is not limited either to a SiGe strained layer 2, but also extends to form ing the strained layer 2 in other types of materials of the lll-V or ll-VI type, or other semiconductor materials.

In the semiconductor layers discussed in this document, other constituents may be added thereto, such as carbon with a carbon concentration in the relevant layer substantially less than or equal to 50% or, more particularly with a concentration less than or equal to 5%.

Claims

1. A method of forming on a substrate a relaxed or pseudo-relaxed layer, the relaxed layer (2') being in a material selected from semiconductor materials, characterized in that the method comprises the following steps: (a) growing on a donor substrate (1) an elastically strained layer (2) consisting of at least a material selected among the semiconductor materials;

(b) forming on the strained layer (2) or on a receiving substrate (7), a vitreous layer (4) made of a material viscous from a viscosity temperature;

(c) bonding the receiving substrate (7) to the strained layer (2) via the vitreous layer (4);

(d) removing a portion of the donor substrate (1), so as to form a structure comprising the receiving substrate (7), the vitreous layer (4), the strained layer (2) and the unremoved portion of the donor substrate (1 ) which thereby forms a surface layer (1 B) ;

(e) heat treating the structure at a temperature close to or higher than the viscosity temperature.

2. The method according to the preceding claim, characterized in that step (b) and step (c) are operated before any substantial diffusion of at least one species of the material of the strained layer (2), before contamination of the strained layer, and before the surface of the strained layer (2) becomes uncontrollably reactive.

3. The method according to any of the preceding claims, characterized in that it further comprises, after step (d), a controlled treatment so as to transform at least a portion of the surface layer (1B) into a material viscous from a second viscosity temperature, and thereby forming a second vitreous layer (8).

4. The method according to the preceding claim, characterized in that step (e) is operated during or in continuity after the formation of the second vitreous layer (8).

5. The method according to the preceding claim, characterized in that further comprises, after step (e), a step of removing the second vitreous layer (8).

6. The method according to the preceding claim or claim 1 , characterized in that further comprises a last step of crystal growth on the structure of a material selected among semiconductor materials.

7. The method according to any of the preceding claims, characterized in that the vitreous layer (4) is formed on the receiving substrate (7), and in that the method further comprises before step (c), a step of forming on the strained layer (2) a thin layer with a thickness less than the thickness of the strained layer (2).

8. The method according to any of the preceding claims, characterized in that step (b) comprises the two following successive operations:

(bl) growing a semiconductor material layer on the strained layer; (b2) completing a controlled treatment for transforming at least a portion of the layer formed in step (b1) into a viscous material, as from the viscosity temperature, thereby forming the vitreous layer.

9. The method according to the any of the preceding claims, characterized in that further comprises, before step (c), a step of forming a bonding layer on the receiving substrate (7).

10. The method according to the preceding claim, characterized in that the bonding layer is in Si0₂.

11. The method according to any of the preceding claims, characterized in that the removal of material of step (d) is substantially achieved by a detaching at an embrittlement area present in the donor substrate (7), at a depth close to the thickness of the surface layer (1 B), by supplying energy.

12. The method according to the preceding claim, characterized in that further comprises, before step (c), a step of forming the embrittlement area by implanting atomic species into the donor substrate (7).

13. The method according to claim 11 , characterized in that further comprises, before step (a), a step of forming the donor substrate (7) comprising the following operations:

• forming a porous layer (6) on a crystalline holding substrate (1A);

• growing a crystal layer (1 B) on the porous layer (6); the whole [ holding substrate (1A) \ porous layer (6)\ crystall layer (1B)] being the said donor substrate (1), the porous layer (6) being the embrittlement area in the donor substrate (1).

14. The method according to any of the preceding claims, characterized in that the removal of material of step (d) comprises selective chemical etching.

15. The method according to any of the preceding claims, characterized in that the vitreous layer (4) formed in step (b) is electrically insulating.

16. The method according to the preceding claim, characterized in that the vitreous layer (4) formed in step (b) is in Si0₂.

17. The method according to any of the preceding claims, characterized in that :

• the donor substrate (1) is in Si ; and

• the strained layer (2) is in Siι__xGe_x.

18. The method according to the any of claims 1 to 16, characterized in that:

• the donor substrate (1) comprises a Si bulk holding substrate (1-1) and a buffer structure (1-2, 1-3) adapting the lattice parameter from Si to Siι__zGe_z; • the strained layer (2) comprises a Si strained layer (2-1) and a Siι-_xGe_x strained layer (2-2) with x > z.

19. The method according to claim 8, claim 16, and claim 17 or 18, characterized in that:

• the layer grown at step (b1 ) is in Si; • the controlled treatment applied during step (b2) is a controlled thermal oxidization treatment for transforming at least a portion of the Si of the layer formed during step (b1 ) into Si0_2j thereby forming the Si0₂ vitreous layer (4).

20. The method according to any of the three preceding claims combined with claim 3, characterized in that said controlled treatment is a controlled thermal oxidization treatment for transforming at least a portion of Si in the surface layer (1B) into Si0₂, thereby forming the second Si0₂ vitreous layer.

21. The method according to the preceding claim, characterized in that further comprises, after step (e), a step of removing the second vitreous layer (8) by chemical treatment based on hydrofluoric acid.

22. The method according to any of the four preceding claims, characterized in that step (e) comprises a heat treatment.

23. The method according to claim 6 combined with any of the five preceding claims, characterized in that the material used for growing on the structure after removal of the second vitreous layer (8) is Si.

24. The method according to any of the preceding claims, characterized in that the vitreous layer (4) formed in step (b) is electrically insulating and in that the formed structure is a semiconductor-όn-insulator structure, the semiconducting thickness of which comprises the strained layer (4) which has been relaxed or pseudo-relaxed during step (e).

25. The method according to any of the preceding claims, characterized in that further comprises steps of preparing for the manufacturing of components and/or steps of manufacturing components in the strained layer or in a layer (4) optionally epitaxied thereon (11 ). vitreousvitreousvitreousvitreousvitreous