WO2005093807A1

WO2005093807A1 - Oxidation process of a sige layer and applications thereof

Info

Publication number: WO2005093807A1
Application number: PCT/IB2004/000925
Authority: WO
Inventors: Olivier Rayssac; Nicolas Daval
Original assignee: S.O.I.Tec Silicon On Insulator Technologies
Priority date: 2004-03-01
Filing date: 2004-03-01
Publication date: 2005-10-06
Also published as: WO2005093807A8

Abstract

The invention concerns an oxidizing process for forming on the surface of a SiGe layer an oxidized region, said process comprising an oxidizing thermal treatment of the SiGe layer under an oxidizing atmosphere for oxidizing its surface, characterized in that said oxidizing thermal treatment is carried out at a low temperature in order to incorporate a maximum amount of Ge from said SiGe layer into the oxidized layer, said oxidized layer being thus made of SixGeyOz. The invention also concerns applications of such process, as well as SiGe layers and SGOI multilayer structures obtained by such process and applications.

Description

OXIDATION PROCESS OF A SiGe LAYER and APPLICATIONS THEREOF

The present invention concerns an oxidizing process for forming on the surface of a SiGe layer an oxidized region, said process comprising an oxidizing thermal treatment of the SiGe layer under an oxidizing atmosphere for oxidizing its surface. The invention also concerns applications of such process, as well as SiGe layers oxidizes by such process, and resulting SGOI multilayer structures. It is specified that the definition of a "SiGe layer" covers in particular in this text a SiGe virtual substrate, and more generally a SiGe layer in a multilayer structure. It is also reminded that a SiGe "virtual substrate" is a multilayer substrate comprising a support (made e.g. of bulk Silicon) covered by a buffer layer, said buffer layer being itself covered by a relaxed SiGe layer. A "SiGe layer" can also be e.g. the surface SiGe layer of a SGOI- type multilayer structure which is composed of said surface SiGe layer and a support layer (e.g. in silicon), with an insulating layer between the SiGe layer and the support layer. Oxidizing thermal treatment of the kind mentioned above are generally referred to as "direct oxidation" - as opposed to the formation on the substrate of a surface oxide layer which has been created from material not initially present in the substrate (case e.g. of the deposition of an oxide layer on the substrate). Direct oxidation is a well-known method for forming a superficial oxidized region in a silicon layer. However, the application of known direct oxidation methods to a SiGe layer can be associated to specific problems - and in particular to the generation of dislocations within the SiGe layer. Such dislocations within the SiGe layer are generated because of the formation, under the surface oxidized region, of a Ge-enriched region since Ge from the surface region of the SiGe layer is rejected and tends to pile up at the interface between the surface oxide and the underlying SiGe layer, thus forming a buried Ge-enriched region. This Ge-enriched region has a lattice parameter which is different from the lattice parameter of the underlying SiGe layer. Therefore, as the oxidation of the SiGe layer is carried out and as the Ge concentration and the thickness of the Ge-enriched region increase, this Ge concentration and thickness can reach values corresponding to the apparition of dislocations due to the lattice parameter mismatch between the Ge-enriched region and the underlying SiGe. This limitation has been exposed in an article by LeGoues et al. ("Oxidation studies of SiGe" - J. Applied Physics 65(4), 15 February 1989, 1724). This article discloses a solution for oxidizing a structure comprising a SiGe layer. More precisely, the process described in this article aims at building an oxidized region made of SiO2. SiO2 is indeed normally desired when creating an oxide, since SiO2 has good physical properties, in particular :

• Properties of thermal stability which allows SiO2 to undergo further thermal treatments while staying in a stable state,

• Good properties of bonding of the Si02 with another substrate. SiO2 differs from oxides such as SiχGeγOz, which do not have the same properties. However, the process described in this article does only allow the formation of oxide layers having a very limited thickness. Indeed, in such process the formation of the Ge-enriched region generates - as mentioned above - dislocations when the thickness of said region reaches a critical thickness. And the critical thickness of this Ge-enriched region corresponds to an oxide SiO2 layer which remains thin (typically under 300 Angstroms). Thus, one could not apply the process described in the article of LeGoues for forming an oxide layer having a thickness over 300 Angstroms, since the buried Ge-enriched region formed by rejected Germanium would generate dislocations. It therefore appears that there is a need for a solution allowing oxidation of a SiGe layer, without generating dislocations. It is an object of the invention to provide such a solution. A further object of the invention is to allow the formation on a SiGe layer of an oxidized region with a significant thickness - typically over 300 Angstroms. Indeed, it would be advantageous to be able to form such a thick oxidized region on the surface of a SiGe layer for some applications. In particular, the formation of such thick oxidized regions would be advantageous in the perspective of the application of a Smart-Cut^®- type process on the oxidized SiGe layer. Such oxidized SiGe layer could be implanted through the thick oxidized region for forming into the SiGe layer an embrittlement zone, so that the implanted SiGe layer can form the "top" substrate in a Smart- Cut^® process. In such application, the oxidized region would constitute an amorphous surface protecting the underlying SiGe layer from the undesirable effects of implantation. The oxidized region would then be removed so as to leave the surface of the implanted SiGe free, allowing the subsequent bonding of this implanted SiGe layer with a support "base" substrate, to form a SGOI multilayer structure through a Smart-Cut^®-type process^". Furthermore, as will be explained in this text the formation of a thick oxidized region on the SiGe layer to be implanted would allow the formation of a transferred SiGe layer which could be thinner than what could be obtained with the known oxidizing processes. It is reminded that a Smart-Cut^®-type process implies the following steps : • Implantation of at least one species in a top substrate, in order to form an embrittlement zone in said top substrate,

• Bonding the implanted top substrate with a base substrate,

• Splitting the implanted top substrate at its embrittlement zone, to form a multilayer structure. A description of this type of process can be found in « Silicon On

Insulator Technology : materials to VLSI, 2^nd edition » from Jean Pierre

Colinge (Kluwer Academic Publisher) - in particular pp. 50-51. In the perspective of forming a thick oxidized region on a SiGe layer, the application of known direct oxidation methods which are used for oxidizing layers made of in a semiconductor material such as Silicon is particularly difficult. Indeed, the known methods for performing a direct oxidation comprise a thermal treatment at high temperatures - typically over

900°C. And transposing these known methods to the oxidation of a SiGe layer would generate dislocations such as mentioned above. Moreover, when forming an oxidized region on the surface of a top substrate to be used in a Smart-Cut^®-type process, it is normally sought to obtain an oxidized layer made of SiO2. SiO2 indeed has good electrical properties. It furthermore allows an efficient bonding of the oxidized top substrate with a base substrate. Furthermore, as mentioned above SiO2 is particularly stable when exposed to further thermal treatments - which is also generally desired. Therefore, when considering the option of using a Smart-Cut^®- type process on a top substrate in SiGe, one would naturally consider trying to obtain a SiO2 layer over the SiGe. This would incite the man skilled in the art to consider an oxidation method at high temperatures, since such high temperatures are required to make SiO2 from SiGe in a direct oxidation process. However, as it will be explained in details further in this text, the Applicant has surprisingly observed that turning away from these natural incitations to adopt a high temperature thermal treatment for the direct oxidation of a SiGe layer (these incitations resulting from the classical approach used on Si substrates which implies a high temperature treatment, as well as from the tendency to look for a solution which would allow the formation of a SiO2 oxide layer), could lead to an advantageous solution. And this advantageous solution allows forming on a SiGe layer a thick oxidized region, if desired. According to another aspect, a further object of the invention is to provide an efficient method for performing a sacrificial oxidation on a SiGe layer. It is reminded that a sacrificial oxidation comprises an oxidation step followed by a deoxidation step. Such "sacrificial oxidation" allows eliminating a given thickness of the layer or substrate which undergoes the sacrificial oxidation. And for some applications, intermediate treatments are performed between the oxidation and the deoxidation (see e.g. US 6 403 450 which discloses a stabilization high temperature annealing between the oxidation and the deoxidation steps). In the specific perspective of oxidating and then deoxidating a

SiGe layer for performing a sacrificial oxidation, the very nature of SiGe is a source of difficulties. Indeed, as evoked above during the oxidation of SiGe Germanium is segregated from Si - Si tending to be attracted by Oxygen for forming oxide, while Ge is less subject to such attraction by Oxygen. This segregation of Ge generates an uneven distribution of Ge within the oxidized region formed during the oxidation. Under known oxidation conditions, it furthermore generates a Ge- enriched region buried immediately under the oxidized region, and resulting from the accumulation of Ge rejected by the segregation during oxidation. Thus, the oxidation of SiGe generates a non-uniform distribution of Ge (within the oxidized region, and under this oxidized region), which makes a further deoxidation difficult. It has been said above that the oxidation of a SiGe layer was in itself associated with problems. And we have furthermore seen that the deoxidation of an oxidized SiGe layer would certainly be associated with additional problems, due to the uneven distribution of Ge. As mentioned above, an object of the invention is to allow an efficient sacrificial oxidation of a SiGe layer - among others for allowing the reduction of the thickness of a SiGe layer which forms the top substrate in a Smart-Cut^®-type process, after said top substrate has been bonded to a base substrate and splitted. Finally, a further object of the invention is to allow the formation of an oxidized region on a SiGe layer, said oxidized region having a highly uniform thickness. Such thickness uniformity is desired for many applications - e.g. in the perspective of an implantation through the oxidized region, or in the perspective of a sacrificial oxidation. In particular, an implantation through an oxidized region having a non-uniform thickness shall generate a buried embrittlement zone which shall also be non-uniform. And a sacrificial oxidation during which the oxidation builds an irregular oxide layer shall produce a rough surface after deoxidation. Now in order to attain the objects exposed above, the invention proposes according to a first aspect an oxidizing process for forming on the surface of a SiGe layer an oxidized region, said process comprising an oxidizing thermal treatment of the SiGe layer under an oxidizing atmosphere for oxidizing its surface, characterized in that said oxidizing thermal treatment is carried out at a low temperature in order to incorporate a maximum amount of Ge from said SiGe layer into the oxidized layer, said oxidized layer being thus made of SiχGeγOz. Specific, but non-limiting aspects of this oxidizing process are the following :

• said low temperature of the oxidizing thermal treatment is between 600°C and 800°C,

• said low temperature of the oxidizing thermal treatment is kept under a critical temperature which is related to the Ge concentration within said SiGe layer,

• said SiGe layer to be oxidized has a Ge concentration of 20% and said critical temperature is 750°C,

• said SiGe layer to be oxidized has a Ge concentration of 20% and the oxidizing thermal treatment is carried out at a temperature of 650°C,

• said oxidizing thermal treatment is carried out under an atmosphere containing H2O vapour,

• the amount of Ge which is incorporated into the oxidized region allows to make the Ge concentration in the oxidized SiχGeγOz equal to the concentration of Ge in the remainder of said SiGe layer,

• at the beginning of the oxidizing thermal treatment, before carrying out a low temperature thermal treatment for maximizing the amount of Ge from said SiGe layer which is incorporated into the oxidized region, an initial oxidation step is carried out in order to build on the surface of said SiGe layer to be oxidized a very thin layer of SiO2,

• said very thin layer of SiO2 has a thickness between 20 and 50 Angstroms,

• said initial oxidation step is carried out in a dry atmosphere,

• said initial oxidation step is carried out at 900°C during 5 minutes, or at 800°C during 30 minutes. The invention further proposes the application of such an oxidizing process for forming an oxidized region on a SiGe layer characterized in that said oxidized region has a thickness of at least 300 Angstroms. Specific, but non-limiting aspects of such application are the following :

• after the formation of an oxidized region on it said SiGe layer is implanted with at least one species in order to generate an embrittlement zone within said SiGe layer, said implantation traversing said oxidized region, said SiGe layer forming the top substrate of a Smart-Cut^®-type process,

• the thickness of the oxidized region is adapted to protect the SiGe layer from the effects of implantation others than the creation of an embrittlement zone, • the thickness of the oxidized region is adapted to absorb a predetermined portion of the energy of the implanted species, so that the remaining energy of the implanted species corresponds to a desired implantation depth within said SiGe layer,

• said desired implantation depth within the SiGe layer is about 2000 Angstroms.. The invention also proposes a sacrificial oxidation process of a SiGe layer comprising an oxidation and a deoxidation characterized in that the oxidation of said sacrificial oxidation is carried out by an oxidizing process as mentioned above. Specific, but non-limiting aspects of this sacrificial oxidation process are the following :

• the deoxidation comprises an etching with an etching solution having an etching effect which increases with the Ge concentration,

• the deoxidation comprises two steps : > A first step of removing the oxidized region, > A second step for removing an underlying Ge-enriched region, with said solution having a etching effect which increases with the Ge concentration,

• said first step is carried out with a HF etching, • said second step is carried out with a SC1 solution,

• the SiGe layer is the SiGe layer of a SGOI-type structure, said sacrificial oxidation process allowing a reduction of the thickness of said SiGe layer. Finally, the invention also proposes a SiGe layer oxidized through an oxidizing process such as mentioned above, such SiGe layer having one or both of the following properties :

• the thickness of the oxidized SiχGeγOz region of the oxidized SiGe layer is very uniform, presenting a standard deviation of no more than 1 ,5 % of the thickness of the oxidized region, • the oxidized SiχGeγOz region of the oxidized SiGe layer has a thickness of at least 300 Angstroms. This region can even have a thickness of at least 1000 Angstroms, and the invention also proposes a SGOI multilayer structure comprising such a SiGe layer. Other aspects, goals and advantages of the invention shall be apparent from the following description of the invention, made in reference to the attached drawings on which :

• Figures 1a and 1 b are cross-sectional views illustrating the two main steps of an oxidizing process according to the state of the art, performed on a SiGe layer,

• Figures 2a and 2b illustrate the two main steps of the oxidizing process according to the invention, the whole set of figures 2a to 2f illustrating an application of the oxidizing process according to the invention, • Figures 3a to 3c illustrate another application of the oxidizing process according to the invention, • Figure 4 shows examples of thermal treatments of the oxidizing process according to the invention.. In reference to figures 1a and 1b, a process for oxidizing a SiGe layer 10 is illustrated. This process is part of the prior art - it is representative of the process disclosed in the LeGoues article mentioned above. This process is a direct oxidation process - i.e. it allows forming oxide from the material of the layer 10. The layer 10 can be the SiGe layer of a multilayer structure (the other layers of such multilayer structure being not represented in figures 1a and 1 b). The layer 10 could also be a self-supporting layer. Figure 1a shows the SiGe layer 10 before its oxidation by an oxidizing thermal treatment under an oxidizing atmosphere. This thermal treatment aims at forming in a surface region of the layer 10 an oxidized region 11 made of SiO2, in particular for the reasons exposed in the introduction of this text. For that purpose, the temperature of the thermal treatment is high. It is typically over 900°C, even if somewhat lower temperatures are also possible for forming SiO2 (temperatures as low as 800°C could be possible for a SiGe layer with 25% Ge). Figure 1 b shows that in addition to the formation of a SiO2 region 11 , a buried Ge-enriched region 12 has also been formed by this thermal treatment. And as explained above, this Ge-enriched region prevents the formation of a thick oxidized region 11. This classical oxidation process cannot produce an oxidized region 11 thicker than about 300 Angstroms without also generating dislocations - which is naturally undesired. Figures 2a and 2b illustrate the effect of the oxidizing process according to the invention. In this process, a SiGe layer 10 (which is identical to the SiGe layer illustrated on Figures 1a and 1 b, and has therefore the same reference) undergoes an oxidizing thermal treatment. An oxidized region 110 is thus created in the thickness of layer 10. But in the case of the invention, the thermal treatment is carried out at a low temperature in order to incorporate a maximum amount of Ge from the SiGe layer 10 into the oxidized layer, said oxidized layer being thus made of SiχGeγOz and not of SiO2. The incorporation of Ge into the oxidized region (such incorporation is avoided in the known processes, which precisely aim at producing a SiO2 oxide) prevents the formation of a buried Ge-enriched region - or at least slows significantly such formation (no such Ge- (enriched region is represented on the drawings related to the invention). The invention aims at minimizing (and possibly suppressing) the rejection of Germanium from the oxidized region of the SiGe layer. Therefore, the oxidizing process can be conducted much longer than the process illustrated in figures 1a and 1 b, and the oxidized region 110 which is obtained can be significantly thicker. Oxidized regions 110 thicker than 300 Angstroms - or even thicker than 1000 Angstroms - can thus easily be obtained. The temperature of the oxidizing thermal treatment is between 600°C and 800°C. This temperature of the oxidizing thermal treatment is kept under a critical temperature (which is related to the Ge concentration within the SiGe layer 10). For a SiGe layer having a Ge concentration of 20%, the critical temperature is around 750°C. The Applicant has determined that for a 20% concentration in Ge, in layer 10 an oxidizing thermal treatment performed at 650°C was well adapted. This oxidizing thermal treatment is carried out under a wet atmosphere - containing typically H2O vapour. The parameters of the oxidizing thermal treatment (and in particular its temperature) determine the amount of Germanium which shall be incorporated into the oxidized region 110 (examples of oxidizing thermal treatment according to the invention shall be detailed further in this text in reference to figure 4). And it is possible to define these parameters (i.e. the temperature, in first approach) so that practically no Ge is rejected from the oxidized region 110 in a buried region. In such case, the amount of Ge which is incorporated into the oxidized region allows to make the Ge concentration in the oxidized SiχGeγOz region 110 equal to the concentration of Ge in the remainder of the SiGe layer 11. As was said above, an object of the invention is to allow forming an oxidized region whose thickness is uniform. The process according to the invention provides such thickness uniformity. Indeed, the thickness of the oxidized SiχGeγOz region created by the process of the invention is very uniform, presenting a standard deviation of no more than 1 ,5 % of the thickness of the oxidized region And the thickness of the oxidized region 110 is even more uniform if at the beginning of the oxidizing thermal treatment, before carrying out the low temperature thermal treatment, an initial oxidation step is carried out at higher temperature in order to build on the surface of the SiGe layer to be oxidized a very thin layer of SiO2 (not represented). This very thin layer of SiO2 has a thickness between 20 and 50 Angstroms. The initial oxidation step is carried out in a dry atmosphere. It can be carried out e.g. at 900°C during 5 minutes, or at 800°C during 30 minutes. This initial high temperature treatment is therefore carried out during a short time only, and it does not generate a significant rejection of Ge in a buried Ge-enriched region. The formation of this initial thin SiO2 layer provides a good support for then forming a very uniform oxidized region 110. And the subsequent low temperature oxidizing thermal treatment shall still allow the incorporation of Ge from the SiGe layer 10 into the oxidized region 110. The oxidizing process of the invention can advantageously be used for oxidizing a SiGe layer so as to form a thick oxidized region 110 (i.e. at least 300 Angstroms thick). Such thick oxidized region on a SiGe layer can be used e.g. for forming with the SiGe layer the "top substrate" of a Smart-Cut^®-type process. In such case, after the formation of an oxidized region 110 on the

SiGe layer 10, the SiGe layer is implanted with at least one species in order to generate an embrittlement zone within its thickness. Such implantation I is illustrated in figure 2c. It creates an embrittlement zone 13. The implantation is performed through the oxidized region 110, which thus protects the underlying SiGe layer from the undesired effects of implantation (effects other than the creation of the embrittlement zone 13, such as the roughening of the surface which is directly exposed to implantation, and/or channelling effect). More precisely, it is possible to control the temperature and duration of the low temperature oxidizing thermal treatment for obtaining a desired thickness for the oxidized region. And this desired thickness can be selected in order to protect as efficiently as possible the SiGe layer 10 from the undesirable effects of implantation. This desired thickness T1 (see figure 2c) can also be selected to absorb a predetermined portion of the energy of the implanted species which traverse the oxidized region, (the thicker the region, the more energy delivered in it by the implanted species in order to traverse it). With such control of the energy implanted it is possible to precisely control the implantation depth of the species within the unoxidized region of the layer 10. This implantation depth indeed corresponds to (T2-T1), with T2 the total implantation depth within both oxidized and unoxidized regions of the layer 10 (see figure 2c). This implantation depth (T2-T1 ) corresponds to the thickness of the part 15 of the layer 10 which shall be transferred on a base substrate, in the subsequent Smart-Cut^®-type process. It should be noted that such fine control of the implantation depth within the unoxidized region of the layer 10 would not be possible if the oxidized region 110 was not thick enough. More precisely, covering the SiGe layer with an oxidized region allows to define by implantation within the SiGe layer a part 15 to be transferred which is thinner than what could be obtained when implanting the same SiGe layer uncovered. And the control of the thickness of the oxidized region (such control also allowing forming an oxidized region which is thick) allows to finely adjust the thickness of the part 15 to be transferred from the SiGe layer 10. In practice, when forming an embrittlement zone in a SiGe layer which is not covered by a layer such as the oxidized region 110, one must carry out the implantation with a minimum energy level (typically around 25 keV), corresponding to a minimum implantation depth (T2) in the implanted material. And this minimum implantation depth can be already greater than the thickness desired for the layer to be transferred by the Smart-Cut^® process. Thus, by controlling the thickness of the region 110, it is possible to control also the implantation depth within the unoxidized region of the SiGe layer 10 - even for obtaining very thin transferred layers 15 on the base substrate. Transferred layers 15 which are as thin as 2000 Angstroms can thus be obtained. Figures 2d-2f illustrate the following steps of the Smart-Cut^®-type process. Figure 2d illustrates the removal of the oxidized region 110. This region is indeed not well adapted for a bonding, since it is not made of SiO2 but of an SiχGeγOz oxide containing Germanium. This removal then corresponds to the deoxidation of a sacrificial oxidation. This deoxidation preferably comprises an etching with an etching solution having an etching effect which increases with the Ge concentration. More precisely, in an embodiment of the invention, the deoxidation comprises two steps :

• a first step of removing the oxidized region 110. This can be done with e.g. a HF solution,

• a second step for removing an underlying Ge-enriched region, with the solution having a etching effect which increases with the Ge concentration. This can be done with e.g. a SC1 solution. A deoxidation operation as described above can also be carried out on any SiGe layer oxidized - in particular a SiGe layer oxidized as described above : the application to a Smart-Cut^®-type process is of course not limitative. Figure 2e shows the bonding of what remains of the layer 10, with a base substrate 20. The layer 10 has been turned upside down for such bonding. The base substrate is provided with a surface layer 21 which allows an efficient bonding - e.g. a SiO2 layer. Finally, the structure obtained undergoes a splitting operation (through a thermical and/or mechanical treatment), for splitting the layer 10 at its embrittlement zone 13. Figure 2f shows the resulting structure 100, which is a SGOI with a surface layer of SiGe which can be as thin as desired. Figures 3a to 3c illustrate another application of the invention. Figure 3a shows a multilayer structure 30 comprising a surface SiGe layer 10', a support layer 31 and a buried oxide layer 32 between layers 10' and 31. The layer 31 can be e.g. in silicon. The layer 32 can be in SiO2. The structure 30 can have been made by any process - e.g. by bonding and/or deposition. It can also have been made by a transfer process (Smart-Cut^®-type or other). Figure 3b shows an oxidized region 110' which has been made like the region 110 of figure 2a. We have seen that this region could be practically as thick as desired. The thickness of the region 110' is therefore selected in order to leave in the layer 10' only a thin region 15', whose thickness is controlled through the control of the thickness of region 110'. As shown in figure 3c, the oxidized region 110' is then removed - e.g. by a deoxidation technique such as described above. A SGOI structure 100' is thus obtained - here again with a desired thickness for its SiGe layer 15'. Figure 4 shows examples of possible thermal treatments to be carried out in an oxidizing process according to the invention. This figure illustrates different thermal treatments, all carried out on SiGe layers with 20% of Germanium (20% SiGe) at 650°C. It comprises two curves 41 , 42 which illustrate the effects of low temperature thermal treatments on two different types of 20% SiGe layers. The two curves indicate the thickness of the oxidized region generated by the treatment, as a function of the duration of the treatment. On this figure, the curve 41 defined by the black triangles corresponds to a thermal treatment carried out on a 20% SiGe layer which has been grown (e.g. on an underlying buffer layer of a virtual substrate). The curve 41 shows a thickness of the oxidized region going from 100 nm (for a treatment of 60 minutes) to about 175 nm (for 120 minutes). The duration of the treatment can be adapted as a function of the desired thickness. The curve 42 defined by the white boxes corresponds to a thermal treatment carried out on a 20% SiGe layer which has been obtained by splitting an embrittlement zone (i.e. a SiGe layer such as layer 15 of the structure illustrated in figure 2f - but with no limitation on the layer thickness). This curve shows that the oxide thickness is larger, for the same duration of treatment (and at the same temperature). In the case of both curves, thick oxidized regions can be produced.

Claims

1. Oxidizing process for forming on the surface of a SiGe layer an oxidized region, said process comprising an oxidizing thermal treatment of the SiGe layer under an oxidizing atmosphere for oxidizing its surface, characterized in that said oxidizing thermal treatment is carried out at a low temperature in order to incorporate a maximum amount of Ge from said SiGe layer into the oxidized layer, said oxidized layer being thus made of SiχGeγOz.

2. Process according to the preceding claim characterized in that said low temperature of the oxidizing thermal treatment is between 600°C and 800°C.

3. Process according to one of the preceding claims characterized in that said low temperature of the oxidizing thermal treatment is kept under a critical temperature which is related to the Ge concentration within said SiGe layer.

4. Process according to the preceding claim characterized in that said SiGe layer to be oxidized has a Ge concentration of 20% and said critical temperature is 750°C.

5. Process according to the preceding claim characterized in that said SiGe layer to be oxidized has a Ge concentration of 20% and the oxidizing thermal treatment is carried out at a temperature of 650°C.

6. Process according to one of the preceding claims characterized in that said oxidizing thermal treatment is carried out under an atmosphere containing H2O vapour.

7. Process according to one of the preceding claims characterized in that the amount of Ge which is incorporated into the oxidized region allows making the Ge concentration in the oxidized SiχGeγOz equal to the concentration of Ge in the remainder of said SiGe layer.

8. Process according to one of the preceding claims characterized in that at the beginning of the oxidizing thermal treatment, before carrying out a low temperature thermal treatment for maximizing the amount of Ge from said SiGe layer which is incorporated into the oxidized region, an initial oxidation step is carried out in order to build on the surface of said SiGe layer to be oxidized a very thin layer of SiO2.

9. Process according to the preceding claim characterized in that said very thin layer of SiO2 has a thickness between 20 and 50 Angstroms.

10. Process according to one of the two preceding claims characterized in that said initial oxidation step is carried out in a dry atmosphere.

11. Process according to the preceding claim characterized in that said initial oxidation step is carried out at 900°C during 5 minutes, or at

800°C during 30 minutes.

12. Application of a process according to one of the preceding claims for forming an oxidized region on a SiGe layer characterized in that said oxidized region has a thickness of at least 300 Angstroms.

13. Application according to the preceding claim characterized in that after the formation of an oxidized region on it said SiGe layer is implanted with at least one species in order to generate an embrittlement zone within said SiGe layer, said implantation traversing said oxidized region, said SiGe layer forming the top substrate of a Smart-Cut^®-type process.

14. Application according to the preceding claim characterized in that the thickness of the oxidized region is adapted to protect the SiGe layer from the effects of implantation others than the creation of an embrittlement zone.

15. Application according to one of the two preceding claims characterized in that the thickness of the oxidized region is adapted to absorb a predetermined portion of the energy of the implanted species, so that the remaining energy of the implanted species corresponds to a desired implantation depth within said SiGe layer.

16. Application according to the preceding claim characterized in that said desired implantation depth within the SiGe layer is about 2000 Angstroms.

17. Sacrificial oxidation process of a SiGe layer comprising an oxidation and a deoxidation characterized in that the oxidation of said sacrificial oxidation is carried out by a process according to any of claims 1 to 11.

18. Process according to the preceding claim characterized in that the deoxidation comprises an etching with an etching solution having an etching effect which increases with the Ge concentration.

19. Process according to the preceding claim characterized in that the deoxidation comprises two steps : - A first step of removing the oxidized region, - A second step for removing an underlying Ge-enriched region, with said solution having a etching effect which increases with the Ge concentration.

20. Process according to the preceding claim characterized in that said first step is carried out with a HF etching.

21. Process according to one of the two preceding claims characterized in that said second step is carried out with a SC1 solution.

22. Process according to one of the five preceding claims characterized in that the SiGe layer is the SiGe layer of a SGOI-type structure, said sacrificial oxidation process allowing a reduction of the thickness of said SiGe layer.

23. SiGe layer oxidized through a process according to any of claims 1 to 15 characterized in that the thickness of the oxidized Si_xGe_YOz region is very uniform, presenting a standard deviation of no more than 1 ,5 % of the thickness of the oxidized region.

24. SiGe layer oxidized through a process according to any of claims 1 to 15 characterized in that the oxidized SiχGeγOz region has a thickness of at least 300 Angstroms.

25. SiGe layer according to the preceding claim characterized in that the oxidized SiχGeγOz region has a thickness of at least 1000 Angstroms.

26. SGOI multilayer structure having a SiGe layer according to any of the three preceding claims.