Method for forming a relaxed or pseudo-relaxed useful layer on a substrate

A method for forming a relaxed or pseudo-relaxed useful layer on a substrate is described. The method includes growing a strained semiconductor layer on a donor substrate, bonding a receiver substrate to the strained semiconductor layer by a vitreous layer of a material that becomes viscous above a certain viscosity temperature to form a first structure. The method further includes detaching the donor substrate from the first structure to form a second structure comprising the receiver substrate, the vitreous layer, and the strained layer, and then heat treating the second structure at a temperature and time sufficient to relax strains in the strained semiconductor layer and to form a relaxed or pseudo-relaxed useful layer on the receiver substrate.

BACKGROUND ART

The present invention relates to the formation of a relaxed or pseudo relaxed layer on a substrate, the relaxed layer being in a material selected from among semiconductor materials, in order to form a final structure intended for electronics, optics or optoelectronics. For example, a semiconductor-on-insulator structure may be formed. The present invention comprises in particular the formation of a layer strained on and by the relaxed layer.

A Si layer strained by a relaxed or pseudo-relaxed SiGe layer may achieve advantageous properties such as a charge carrier mobility that is about 100% more significant than that present within a relaxed Si layer. A layer is “relaxed” if its crystalline material has a mesh parameter approximately identical to its nominal mesh parameter, in other words, substantially identical to the mesh parameter of the material in equilibrium. Conversely, the term “strained” is applied to any layer of crystalline material whose crystalline structure is strained resiliently in tension or in compression during crystal growth, such as epitaxy, which forces its mesh parameter to be appreciably different from the nominal mesh parameter.

It is known to form a relaxed layer on a substrate, particularly by implementing a method that includes epitaxy of a thin layer of semiconductor material on a donor substrate, bonding a receiver substrate on the thin layer, and removing the donor substrate. A semiconductor-on-insulator (SOI) structure can be made in this way, in which the semiconductor thickness is formed at least partly by the thin relaxed layer. The insulating layer is usually formed between the epitaxial growth and bonding steps. Relaxation of the thin layer can occur during the epitaxial growth step, or may occur during a subsequent treatment.

In the first case, it is known to use a donor substrate that includes a carrier substrate and a buffer layer. The buffer layer contains plastic deformations so that the covering epitaxial thin layer is relaxed of all strain. Processes for producing such a layer are for example described in U.S. Pat. No. 6,573,126 and in International Publication No. WO 99/53539. However, a buffer layer often takes a relatively long time to fabricate, and is costly to obtain.

In the second case, the donor substrate does not include a buffer layer and then a thin layer is epitaxially grown so that it is strained by the donor substrate. In this way, for example, a SiGe layer will be grown directly on a Si substrate to have a thickness such that the SiGe layer is strained overall.

A first technique for relaxing such a SiGe layer is described in the document by B. Hollander and colleagues entitled “Strain relaxation of pseudomorphic Si1-xGex/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). This method includes relaxing the SiGe layer, prior to bonding a receiver substrate onto a thin layer, by implanting hydrogen or helium ions in the Si substrate at a preset depth. However, relaxation rates usually obtained by this technique remain quite low relative to other techniques.

Research into a second technique is 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 and colleagues (Journal of electronic materials, vol 29, No 7, 2000). After removing the donor substrate, thermal treatment is applied to relax or pseudo-relax a strained SiGe layer that has been bonded to a borophosphoro silicate glass (BPSG). During the thermal treatment, the strained layer appears to relax because the layer of glass becomes viscous at the treatment temperature. It would be advantageous to use a layer of BPSG with a fairly low viscosity temperature TG(of the order of 625° C.). However, due to the viscosity of the BPSG layer obtained at these temperatures, subsequent thermal treatments at temperatures above TG, may have undesired effects, particularly on the structure of the treated wafer. Thus, for example, if transistors are made in the relaxed or pseudo-relaxed SiGe layer or in a strained Si layer grown on the relaxed or pseudo-relaxed SiGe layer, at a temperature above TGsome of the strains may relax because the layer of glass becomes viscous, which removes strains from the Si layer and adds strains in the SiGe layer. Such a result is contrary to the goal of retaining a relaxed SiGe layer and straining the Si layer as far possible. Thus, improvements in such techniques are desirable and necessary, and these are provided by the present invention.

SUMMARY OF THE INVENTION

The present invention relates to a method for forming a relaxed or pseudo-relaxed useful layer on a substrate. This method includes growing a strained semiconductor layer on a donor substrate; bonding a receiver substrate to the strained semiconductor layer by a vitreous layer of a material that becomes viscous above a certain viscosity temperature to form a first structure; detaching the donor substrate from the intermediate composite to form a second structure comprising the receiver substrate, the vitreous layer, and the strained layer; and heat treating the second structure at a temperature and time sufficient to relax strains in the strained semiconductor layer and to form a relaxed or pseudo-relaxed useful layer on the receiver substrate. Preferably, the donor substrate that includes a buffer layer and at least one strained semiconductor layer is grown on the buffer layer. If desired, the one strained layer may include a first layer of strained silicon and a second later of strained silicon-germanium.

The vitreous layer is advantageously formed on the strained layer or receiver substrate prior to bonding. In one embodiment, the vitreous layer is provided by growing a semiconductor material layer on the strained layer and applying a controlled treatment to convert at least part of the semiconductor material layer into a material which is viscous above the certain viscosity temperature. Thus, the second structure is heat treated at a temperature that is at least about or above the certain viscosity temperature. For example, the viscosity temperature of the vitreous layer can be greater than about 900° C. so that the heat treating occurs at a temperature above about 900° C. to about 1500° C.

When the semiconductor material layer comprises silicon, the controlled treatment may be a controlled thermal oxidation treatment that converts at least part of the silicon layer into a silicon oxide vitreous layer. This forms an inserted layer between the vitreous layer and the strained layer, and the inserted layer can become at least a partially strained layer after the heat treatment. Also, a strained semiconductor layer can be grown on the useful layer. This allows optic, electronic or optoelectronic components to be fabricated in either the useful layer or the strained semiconductor layer.

If desired, a bonding layer of material can be applied onto at least one of the vitreous layer, the receiver substrate or the strained layer prior to the bonding step. A preferred bonding layer comprises silicon oxide.

Further structures can be obtained by providing a zone of weakness in the donor substrate or the strained layer so that the donor substrate can be detached along the zone of weakness. Thus, after detachment, a third structure comprising the receiver substrate, the vitreous layer, the strained layer and a layer of donor material is formed. If desired, the layer of donor material can be removed before heat treating the third structure.

In a preferred embodiment, the buffer layer includes a first layer of silicon-germanium of variable composition and a second layer of silicon-germanium of constant composition. Advantageously, the donor substrate comprises a bulk structure of single crystal silicon, the first layer has a silicon content that varies from 100% adjacent the donor substrate to 0% adjacent the second layer. To obtain this structure, the buffer layer may be formed by stacking layers of different composition on the donor substrate. The second layer of the buffer layer may be a relaxed silicon-germanium layer, and the strained layer is grown upon it.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Provided are techniques for forming a useful relaxed or pseudo-relaxed layer on a substrate. A “useful layer” is a layer that receives components during subsequent treatments for use in electronics, optics, or optoelectronics.

The technique also involves forming a useful layer of strained material on the relaxed or pseudo-relaxed layer. The present method also makes it possible to retain an at least relative relaxation strength of an initially strained layer, during high temperature thermal treatments. In particular, at least the relative relaxation strength of a layer of Si1-xGexadjacent to a vitreous layer is retained, up to a temperature of approximately between about 900° C. and about 1200° C., or even at a higher temperature.

Thermal treatments may be applied during epitaxial growth on the relaxed Si1-xGexlayer, or during other processes, such as for example when fabricating components in the Si1-xGexlayer and/or in an epitaxial over-layer, which could be a strained Si layer.

The method according to the invention includes growing a thin layer of semiconductor material on a donor substrate, bonding a receiver substrate onto the thin layer, and removing the donor substrate.FIGS. 1ato1iillustrate a preferred method.

FIG. 1ashows a source wafer10that includes a donor substrate1and a strained Si1-xGexlayer2. In a first configuration, the donor substrate1is made entirely of monocrystalline Si which has a first mesh parameter. Such a donor substrate1is advantageously obtained by Czochralski growth. A second configuration includes a donor substrate1that is a pseudo-substrate having an upper Si layer (not shown inFIG. 1) and an interface with the strained layer2that has a first mesh parameter. The first mesh parameter of the upper Si layer is advantageously the nominal mesh parameter of Si, so that it is in a relaxed state. The upper Si layer additionally has a sufficiently large thickness such that it can impose its mesh parameter on the overlying strained layer2, without the strained layer significantly influencing the crystalline structure of the upper layer of the donor substrate1. Whichever configuration is chosen for the donor substrate1, it has a crystalline structure with a low density of structural defects, such as dislocations.

Preferably, the strained layer2is made of a single thickness of Si1-xGex. The concentration of Ge in this strained layer2is preferably above 17%, i.e. a value of x above 0.17. Since Ge has a mesh parameter about 4.2% higher than Si, the material selected to form the strained layer2thus has a second nominal mesh parameter which is appreciably higher than the first mesh parameter. Thus, the strained layer2is strained resiliently in compression by the donor substrate1. That is, it is strained from having a mesh parameter that is appreciably lower than the second nominal mesh parameter of the material of which it is made, and therefore from having a mesh parameter close to the first mesh parameter. In addition, the strained layer2preferably has an approximately constant atomic element composition.

The strained layer2is advantageously formed on the donor substrate1by crystal growth, such as by epitaxial growth, using known techniques such as, for example, LPD, Chemical Vapor Deposition (CVD) and Molecular Beam Epitaxy (MBE). It is advantageous to choose crystalline materials made of the donor substrate1and the strained layer2(in the vicinity of its interface with the carrier substrate1) so that a sufficiently small difference exists between the first and the second respective nominal mesh parameters to obtain a strained layer2without too many crystallographic defects. Such a technique avoids, for example, point defects or extensive defects such as dislocations. The difference in mesh parameter is typically between about 0.5% and about 1.5%, but can also have more significant values. For example, Si1-xGexwith x=0.3 has a nominal mesh parameter about 1.15% higher than that of Si. But it is preferable for the strained layer2to be of approximately constant thickness, so that it presents approximately constant intrinsic properties and/or to facilitate the future bonding with the receiver substrate5(as shown inFIG. 1e).

The thickness of the strained layer2must additionally remain below a critical elastic strain thickness to avoid relaxation or an appearance of internal plastic deformations. The critical elastic strain thickness depends mainly on the material selected for the strained layer2and on the mesh parameter difference with the donor substrate1. But it also depends on growth parameters such as the temperature at which it has been formed, or on the nucleation sites from which it has been grown, or on the growth techniques employed (for example CVD or MBE).

Critical thickness values for Si1-xGexlayers can be found by reference to the document entitled “High mobility Si and Ge structures” by Friedrich Schaffler (“Semiconductor Science Technology” 12 (1997) 1515-1549). For other materials, a skilled person can refer to the prior art to find the critical elastic strain thickness values of the material selected for the strained layer2that has been formed on the donor substrate1. Thus, for a layer of Si1-xGexwhere x is between 0.1 and 0.3, a typical thickness of between about 200 Å and 2000 Å will be chosen, preferably between 200 Å and 500 Å with consideration given to the growth parameters. Once formed, the strained layer2therefore has a mesh parameter approximately in the vicinity of that of its growth substrate1, and has internal resilient strains under compression.

FIG. 1cillustrates a vitreous layer4formed on the strained layer2, and the vitreous layer could also be formed on the receiver substrate7. The material of the vitreous layer4becomes viscous above a viscosity temperature TG. In the framework of the present method, a material is chosen for the vitreous layer4which has a minimum high viscosity temperature TGof about 900° C. This high viscosity temperature makes it possible to conduct high temperature thermal treatments without causing the vitreous layer4to become viscous. Thus, with reference toFIGS. 1hand1i, the structures30or40resulting from performing the present method will not have a part of their crystallographic structure modified by a viscous vitreous layer4. A single thermal treatment at a temperature around or above TGwill however be applied during the process (with reference toFIG. 1h) to relax the strained layer2. It is particularly advantageous to be able to retain the at least relative relaxation strength (which is obtained during a step with reference toFIG. 1h) of the Si1-xGexlayer up to a temperature of somewhere around 900° C. or even at a higher temperature.

The material of the vitreous layer4is advantageously at least one of SiO2or SiOxNy. When a vitreous layer4of SiOxNyis formed, it is possible to vary the value of y in order to change the viscosity temperature TGwhich, for this material, depends substantially on the nitrogen component. Thus, when the y content of the composition increases it is possible for the temperature TGof the vitreous layer to be changed typically between a TGof roughly that of SiO2(which may vary around 1150° C.) and a TGof roughly that of Si3N4(which is above 1500° C.). By varying the y parameter, it is thus possible to cover a wide range of viscosity temperatures TG. When the viscosity temperature TGdepends on the material of the vitreous layer, it may also fluctuate depending upon the conditions under which it has been formed. In one advantageous scenario, it will thus be possible to control the formation conditions of the vitreous layer4so as to preselect a viscosity temperature TG. It is thus possible to vary deposition parameters, such as the temperature, the time, the proportion of materials, and the gaseous atmosphere potential, to affect the TGtemperature.

Doping elements may thus be added to the principal gaseous elements contained in the vitrification atmosphere, such as boron and phosphorus which may also operate to reduce the viscosity temperature TG. The vitreous layer4is preferably deposited before the germanium contained in the strained layer2can diffuse into the atmosphere, or onto the receiver substrate7, particularly when the assembly is subjected to high temperature thermal treatment, such as RTA annealing treatment, or a sacrificial oxidation treatment.

In a preferred method of forming the vitreous layer4, the following steps are implemented. A layer3of semiconductor material is grown on the strained layer2as shown inFIG. 1b. Next, a controlled treatment is applied to convert at least part of the layer3into a material which becomes viscous above the viscosity temperature, to form the vitreous layer4. The material chosen for the layer3is advantageously Si so as not to modify the strain in the strained layer2. The thickness of the formed layer3is typically between 5 Å and about 5000 Å, and may be more particularly between 100 Å and about 1000 Å.

For the same reasons as those set out above, the crystal growth of the layer3is preferentially implemented before diffusion of Ge, in other words shortly after:the formation of the strained layer2, if the temperature for forming the strained layer2is maintained;a rise in temperature subsequent to a drop to ambient temperature obtained immediately after the formation of the strained layer2.

A preferential method is to grow the layer3in situ directly after growth of the strained layer2. The growth technique used for layer3may be an epitaxy technique such as LPD, CVD, or MBE. The vitreous layer4may be obtained by thermal treatment in an atmosphere with a preset composition. Thus, when the Si layer3is converted to the vitreous layer4of SiO2, it may be subjected to a controlled thermal oxidation treatment in order to convert the layer3into a vitreous layer. During this final step, it is important to have the correct proportions of oxidizing treatment parameters (such as the temperature, duration, oxygen concentration, the other gases of the oxidizing atmosphere, etc.) in order to control the thickness of the oxide that is formed, and to stop oxidation in the vicinity of the interface between the layer2and the layer3. For such thermal oxidation, a dry oxygen or water vapor atmosphere can be used, at a pressure equal to or above 1 atm. It will then be preferable to vary the duration of oxidation in order to control the oxidation of the layer3. However, such control may be achieved by varying one or more other parameters, alone or in combination with the time parameter. Further details can be found in U.S. Pat. No. 6,352,942 regarding forming such a vitreous SiO2layer4on a SiGe layer.

Another embodiment for forming the vitreous layer4replaces the two steps shown byFIGS. 1band1crespectively. In particular, immediately after the growth of the strained layer2(in order to prevent the diffusion of the Ge) atomic species are deposited to form the vitreous material using atom species deposition means. Thus, it is possible for example to deposit molecules of SiO2on the strained layer2to form the vitreous SiO2layer4.

In another variation, the vitreous layer4may be formed by first depositing amorphous Si atomic species to form an amorphous Si layer on the strained SiO2layer, and then thermally oxidizing this amorphous Si layer to make a vitreous SiO2layer4.

FIGS. 1d,1eand1fillustrate the steps for removing the strained layer2and the vitreous layer4from the donor substrate1so as to transfer them to a receiver substrate7. The method is composed of two main sequential steps:bonding the receiver substrate7onto the vitreous layer4, which is part of an assembly that includes the donor substrate1, strained layer2, and vitreous layer4; andremoving the donor substrate1.

With reference toFIG. 1e, the receiver substrate7is bonded to the vitreous layer4. Prior to bonding, an optional step of forming a bonding layer on the surface of the receiver substrate7may be implemented. The bonding layer has binding properties, at an ambient temperature or at higher temperatures, with the material of the vitreous layer4. Thus, for example, forming a layer of SiO2on the receiver substrate7may improve the quality of bonding, particularly if the vitreous layer4is SiO2. The bonding layer of SiO2is then obtained to advantage by deposition of atomic species of SiO2or by thermal oxidation of the surface of the receiver substrate7if its surface is Si. A bonding surface preparation step is advantageously used, prior to bonding, to render these surfaces as smooth and as clean as possible.

Adapted chemical treatments for cleaning the bonding surfaces may be applied, such as light chemical etching, RCA treatment, ozone baths, flushing operations, and the like. Mechanical or mechano-chemical treatments may also be applied, such as polishing, abrasion, Chemical Mechanical Planarization (CMP) or atomic species bombardment.

Bonding as such is achieved by bringing the bonding surfaces into contact with each other. The bonds are preferably molecular in nature making use of the hydrophilic properties of the bonding surfaces. To improve the hydrophilic properties of the bonding surfaces, the two structures to be bonded may be subject to immersion operations in baths, for example, flushing in de-ionized water. The bonded assembly may additionally be annealed in order to reinforce the bonds, for example, by modifying the nature of the bonds, such as covalent bonds or other bonds. Thus, when the vitreous layer4is made of SiO2, annealing may enhance the bonds, particularly if a bonding layer of SiO2has been formed prior to bonding on the receiver substrate7.

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

Once the assembly is bonded, material is preferably removed by detaching the donor substrate1at the level of a weakened zone6present in the donor substrate1, for example, by using mechanical energy. With reference toFIGS. 1dand1e, this weakened zone6is a zone approximately parallel to the bonding surface and has weakened bonds between the lower part1A of the donor substrate1and the upper part1B of the donor substrate1. These brittle bonds can be broken by force, such as by using thermal or mechanical energy.

According to a first embodiment of the zone of weakness6, a technique called SMART-CUT® may be applied. This method includes implanting atomic species in the donor substrate1, at the level of the zone of weakness6. The implanted species may be hydrogen, helium, a mix of these two species or other light species. In an implementation, implantation preferably takes place just prior to bonding.

The implantation energy is selected so that the species, implanted through the surface of the vitreous layer4, pass through the thickness of the vitreous layer4, the thickness of the strained layer2and a set thickness of the upper part1B of the receiver substrate1. It is preferable to implant sufficiently deep in the donor substrate1such that the strained layer2will not to be subjected to damage during the step of detaching the donor substrate1. The depth of implantation in the donor substrate is thus typically about 1000 Å.

The brittleness of the bonds in the zone of weakness6is found principally by choosing the proportion of the implanted species, the proportions being typically between about 1016cm−2and 1017cm−2, and more exactly between about 2.1016cm−2and about 7.1016cm−2. Detachment at the level of the zone of weakness6is then usually effected by using mechanical and/or thermal energy.

For more detail about the SMART-CUT® method, reference may be made to the document entitled “Silicon-On-Insulator Technology: Materials to VLSI, 2ndEdition” by J.-P. Colinge published by “Kluwer Academic Publishers”, pages 50 and 51.

According to a second embodiment of the zone of weakness6, a technique is applied which is described in U.S. Pat. No. 6,100,166. The weakened zone6is obtained before the formation of the strained layer2, and at the time of formation of the donor substrate1. In this case, the weakened zone or layer is made by:forming a porous layer on a carrier substrate1A of Si;growing a Si layer1B on the porous layer.

The carrier substrate1A, porous layer, and Si layer1B assembly then forms the donor substrate1, and the porous layer is the weakened layer6of the donor substrate1. Input of thermal and/or mechanical energy at the level of the porous weakened zone6results in detachment of the carrier substrate1A from the layer1B.

The preferred techniques for removing material at the level of a weakened zone6, obtained according to one of the two non-restrictive embodiments above, makes it possible to rapidly remove a substantial part of the donor substrate1. It also allows the removed part1A of the donor substrate1to be reused in another operation, such as one according to the present method. Thus, a strained layer2and possibly another part of a donor substrate and/or other layers may be reformed on the removed part1A, preferably after the surface of the removed part1A has been polished.

With reference toFIG. 1f, after detachment of the part1A, finishing techniques such as polishing, abrasion, Chemical Mechanical Planarization (CMP), thermal RTA annealing, sacrificial oxidation, chemical etching, taken alone or in combination, can be applied to remove layer1B and to perfect the stack (reinforcing the bonding interface, eliminating roughness, curing defects, etc.). A selective chemical etching step, taken in combination or not with mechanical means, may advantageously be applied at least as a final step. Thus, solutions based on KOH, NH4OH (ammonium hydroxide), TMAH, EDP or HNO3or solutions currently being studied combining agents such as HNO3, HNO2H2O2, HF, H2SO4, H2SO2, CH3COOH, H2O2, and H2O (as explained in the document WO 99/53539, page 9) may be advantageously employed in order to etch the Si part1B selectively in respect of the strained Si1-xGexlayer2.

After the bonding step, another technique to remove material without detachment and without a weakened zone may be applied according to the invention in order to remove the donor substrate1. For example, chemical and/or mechano-chemical etching could be used.

The material or materials of the donor substrate1to be removed may for example be etched possibly selectively, according to an “etch-back” process. This technique consists in etching the donor substrate1“from behind”, in other words from the free surface of the donor substrate1. Wet etching operations implementing etching solutions adapted to the materials for removal may also be applied. Dry etching operations may also be applied to remove the material, such as plasma etching or sputter etching. The etching operation or operations may additionally be only chemical or electro-chemical or photo-electrochemical. The etching operation or operations may be preceded by or followed by a mechanical attack on the donor substrate1, such as lapping, polishing, mechanical etching or sputtering with atomic species. The etching or etching operations may be accompanied by a mechanical attack, such as polishing possibly combined with action by mechanical abrasives in a CMP process.

All the above-mentioned techniques for removing material from the donor substrate1are proposed by way of example, but do not in anyway constitute a restriction, since the present method extends to all types of techniques able to remove material from the donor substrate1.

With reference toFIG. 1g, after material removal a structure is obtained comprising the receiver substrate7, the vitreous layer4and the strained layer2. With reference toFIG. 1h, thermal treatment is then applied at a temperature close to or above the viscosity temperature TG. The main purpose of this thermal treatment is to relax the strains in the strained layer2. Thermal treatment at a temperature above or around the viscosity temperature of the vitreous layer4will cause the latter layer to turn viscous, which will allow the strained layer2to relax at its interface with the vitreous layer4, leading to decompression of at least some of its internal strains. Thus, when the vitreous layer4is of SiO2and was obtained by thermal oxidation, thermal treatment at about 1050° C. minimum, preferentially at about 1200° C. minimum, for a preset time will cause a relaxation or pseudo-relaxation of the strained layer2. The thermal treatment may last typically from a few seconds to several hours. The strained layer2therefore becomes a relaxed layer2′.

Other effects of the thermal treatment on the structure may be sought, apart from the relaxation of the strained layer2. For example, another objective of thermal treatment may additionally be to obtain a bonding reinforcement anneal between the receiver substrate7and the vitreous layer4. Indeed, since the temperature selected for the thermal treatment is above or around the viscosity temperature of the vitreous layer4, the vitreous layer temporarily turns viscous, which may create particular and stronger adhesion bonds with the receiver substrate7. The result, therefore, is a structure30composed of relaxed Si1-xGexlayer2′, a vitreous layer4, and a receiver substrate7. This structure30is a Silicon Germanium On Insulator (SGOI) structure where the vitreous layer4is electrically insulating, such as for example a vitreous layer4of SiO2. The layer of relaxed Si1-xGex2′ of this structure then presents a surface having a surface roughness compatible with growth of another crystalline material. A light surface treatment, such as polishing, adapted to Si1-xGexmay possibly be applied to improve the surface properties.

With reference toFIG. 1i, and in an optional step, a Si layer11is grown on the relaxed Si1-xGex2′ layer with a thickness appreciably less than the critical strain thickness of the material of which it is composed, and which is therefore strained by the relaxed Si1-xGex2′ layer. The result is a structure40composed of a strained Si, a relaxed Si1-xGex2′ layer, a vitreous layer4, and a receiver substrate7. This structure40is a Si/SGOI structure where the vitreous layer4is electrically insulating, such as for example, a vitreous layer4of SiO2.

An alternative to this method is presented with reference toFIGS. 2ato2i. With reference in particular toFIG. 2c, the method is the same on the whole as that described with reference toFIGS. 1ato1i, with the exception of the step of conversion of the layer3into a vitreous layer4. That step is implemented here in such a way that less than the whole layer3is converted. There thus remains a an inserted layer5between the vitreous layer4and the strained layer2. This inserted layer5has a thickness of around 10 nm, and in all cases a thickness that is very much less than that of the strained layer2, and typically by a factor of at least 5 to as much as 10 to 100 or 1000, as desired or necessary.

During thermal treatment to relax the strain of the strained layer2, the strained layer will seek to reduce its internal elastic strain energy by virtue of the viscous properties of the vitreous layer4. Given that the inserted layer5is of less thickness relative to the overlying strained layer2, the strained layer2will impose its relaxation requirement on the inserted layer5. The strained layer2thus forces the inserted layer5to at least partially strain. The strained layer2then becomes an at least partially relaxed layer2′, and the relaxed inserted layer5then becomes a strained inserted layer5′. A discussion concerning this last point can be found in U.S. Pat. No. 5,461,243, at column 3, lines 28 to 42.

With reference toFIG. 2h, the inserted strained layer5′ is retained after the thermal treatment to relax the strained layer2. The structure30that is formed includes a relaxed Si1-xGexlayer, a strained Si layer, a vitreous layer4, and a receiver substrate7. This structure30is a SG/SOI structure, where the vitreous layer4is electrically insulating, such as for example a vitreous layer4of SiO2.

It is then possible to remove, for example, by selective chemical etching based on HF:H2O2:CH3COOH (selectivity of about 1:1000), the relaxed Si1-xGexlayer2′, in order to result in a strained Si layer, a vitreous layer4, and a receiver substrate7structure. This structure is a strained SOI structure, where the vitreous layer4is electrically insulating, such as for example a vitreous layer4of SiO2.

Instead of implementing chemical etching, a Si layer may be grown, with reference toFIG. 2i, on the relaxed layer2′ so as to form a strained Si layer11, approximately identical to the one inFIG. 1i. The structure40that is formed includes a strained Si layer, a relaxed Si1-xGexlayer, a strained Si layer, a vitreous layer4, and a receiver substrate7. This structure40is a Si/SG/SOI structure, where the vitreous layer4is electrically insulating, such as for example a vitreous layer4of SiO2.

In one particular scenario where the thermal treatment to relax the strains of the strained layer2is carried out at a temperature and for a time period greater, respectively, than a temperature and a reference time period beyond which the Ge diffuses in the Si, the Ge contained in the strained layer2may diffuse into the inserted strained layer5′. This is why it is preferable to implement the relaxation of the strained SiGe layer2before growing the strained Si layer11. But, in some cases, this diffusion effect, if it is appropriately controlled, may be desirable. Thus, diffusion can be controlled in such a way that the Ge species are distributed uniformly in the two layers2and5, forming a single layer of Si1-xGexhaving a substantially uniform Ge concentration. A discussion of this latter point can be found in U.S. Pat. No. 5,461,243, at column 3, lines 48 to 58.

According to one or other of the two preferred methods described above, or according to an equivalent, additional steps can be followed for making components. Preparation steps for the making of components may be implemented during the method, and at a minimum temperature of about 900° C. without distorting the strain factor of the relaxed layer2′ and of the strained layer11. Such preparation steps may be implemented in the strained SiGe layer2of the SGOI structure referenced inFIG. 1g, or in the relaxed or pseudo-relaxed SiGe layer2′ of the SGOI structure referenced inFIG. 1h, or in the strained Si layer11of the Si/SGOI structure referenced inFIG. 1i. Local treatments may for example be undertaken to etch patterns in the layers, for example by lithography, by photolithography, by reactive ion etching or by any other etching with pattern masking. In one particular case, patterns such as islets can be etched into the strained SiGe layer2in order to contribute to the effective relaxation of the strained layer2when applying the thermal relaxation treatment.

One or more steps for making components, such as transistors, in the strained Si layer11(or in the relaxed SiGe layer2′, where the latter is not coated with a strained Si layer11) may particularly be implemented at a minimum temperature of about 900° C. without distorting the strain factor of the relaxed layer2′ and of the strained layer11. In a particular process according to the present method, component-making steps are implemented during, or in continuation of, the thermal treatment to relax the strained SiGe layer2. Further, the strained Si layer epitaxy step may be implemented during or in continuation of component making steps.

FIG. 3represents a source wafer10before the formation of the zone of weakness6and the formation of the vitreous layer4. This embodiment differs from the examples previously detailed referring toFIGS. 1ato1iand2ato2iby the way of choosing the materials constituting the donor substrate1and the strained layer2. Here, the donor substrate1is composed of a holding substrate1-1of Si and a buffer structure composed of a buffer layer1-2in SiGe and an upper layer1-3in Si1-zGez. The holding substrate1-1is preferably in a bulk structure of single-crystal. The buffer layer1-2can for example be constituted of a stacking of layers so that the whole composition of Ge inside the buffer layer1-2gradually evolves from 0% at the interface with the holding substrate1-2to 100 z % of Ge at the interface with the upper layer1-3of Si1-zGez.

Contrary to the buffer layer1-2, the upper layer1-3has a Ge composition constant in its thickness. The upper layer1-3has a thickness that is sufficiently great to assign its lattice parameter to the overlied layer. Furthermore, the upper layer1-3of Si1-zGezhas a relaxed structure.

Thus, the buffer structure (composed of the buffer layer1-2and the upper layer1-3) allows:

an adaptation of the lattice parameter between the holding substrate1-1of Si and the nominal lattice parameter of Si1-zGezof the upper layer1-3;

a confinement of crystal defaults, the surface of the upper layer1-3being then without defaults or only having a few defaults.

Over the donor substrate1, the strained layer2is grown by epitaxy techniques, such as CVD techniques (PECVD, MOCVD . . . ).

Firstly, a strained Si layer2-1is formed on the donor substrate1with a thickness no more than the critical thickness beyond which a such Si layer2-1starts to relax its elastic strains. A Si1-xGexstrained layer2-2is then formed on the last Si strained layer2-1, so as to have a thickness less than the critical thickness of Si1-xGexbeyond which the elastic strains start to relax. Knowledge of the respective critical thickness of Si or Si1-xGexcan be found for instance in “High mobility Si and Ge structures” from Friedrich Schäffler (“Semiconductor science technology” 12 (1997) 1515-1549). The x-composition of Ge in the Si1-xGexlayer2-2is greater than the z-composition of Ge in the upper layer1-3.

The strained layer2includes the Si strained layer2-1and the Si1-xGexlayer2-2. The donor substrate1comprises the holding substrate1-1, the buffer layer1-2, and the upper layer1-3of Si1-zGez, and the previous examples, (presented referring to theFIGS. 1 and 2) disclose various embodiments of manufacturing a semiconductor-on-insulator structure30or40, can then be easily transposed from the source wafer10of theFIG. 3, the zone of weakness6being formed in the upper layer1-3or in the buffer layer1-2. Thereafter, a selective chemical etching is processed in order to remove the remaining part of the donor substrate1, employing for instance an etch agent as HF:H2O2:CH3COOH (having a selectivity of about 1:1000 between SiGe and Si). The semiconductor-on-insulator structure finally obtained (not shown) comprises successively a receiving substrate7, a vitreous layer4, the Si1-xGexstrained layer2-2and the Si strained layer2-1.

Then, a thermal treatment with a temperature close to or greater than the viscosity temperature of the vitreous layer4previously formed, is processed. This thermal treatment then relaxes at least partly the Si1-xGexlayer2-2. The relaxed Si1-xGexlayer2-2imposes then elastic constraint to the top Si strained layer2-1. Elastic constraints in the Si strained layer2-1(previously strained by the upper layer1-3of Si1-zGez) 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 Si1-xGexmaterial in its bulk configuration, is obtained.

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 Si1-xGex, but from a buffer structure adapting the parameter to a Si1-zGezwith z<x. Now, a buffer structure which adapts a lattice parameter to a Si1-xGexis 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 Si1-zGez. 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 proposed by way of example in the present document, but do not in anyway constitute a restriction, since the invention extends to all types of techniques able to implement a method according to the invention.

One or more epitaxies of whatever kind may be applied to the final structure (structure30or40referenced inFIGS. 1h,1i,2h,2i). For example, 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 Si or strained SiC layers may alternately be applied so as to form a multilayer structure. Once a final structure is obtained, finishing treatments, that may include for example annealing, may be applied.

The present invention is not restricted to a strained SiGe layer2, but extends also to a composition of the strained layer2of other types of materials, such as the III-V or II-VI type, or to other semiconductor materials. Further, the techniques described are proposed by way of example and do not in anyway constitute a restriction, since the present invention extends to all types of techniques operable to implement the disclosed method.

It should also be understood that, in the layers of semiconductors discussed in this document, other constituents may be added. For example, carbon with a carbon concentration in the layer under consideration appreciably less than or equal to 50% may be added, and more particularly with a concentration of less than or equal to 5%.