Abstract:
The invention relates to a process of treating a structure for electronics or optoelectronics, wherein the structure that has a substrate, a dielectric layer having a thermal conductivity substantially higher than thermal conductivity of an oxide layer made of an oxide of a semiconductor material, an oxide layer made of an oxide of the semiconductor material, and a thin semiconductor layer made of the semiconductor material. The process includes a heat treatment of the structure in an inert or reducing atmosphere with a temperature and a duration chosen for inciting an amount of oxygen of the second oxide layer to diffuse through the semiconductor layer so that the thickness of the second oxide layer decreases by a determined value. The invention also relates to a process of manufacturing a structure for electronics or optoelectronics applications through the use of this type of heat treatment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of International application no. PCT/IB2006/003957 filed Dec. 26, 2006, the entire content of which is expressly incorporated herein by reference thereto. 

   BACKGROUND 
   The invention relates to the manufacturing of Semiconductor-On-Insulator (SeOI) structures for electronics or optoelectronics applications, and in particular Silicon-On-Insulator or SOI structures having a high thermal conductivity. 
   A SeOI structure comprises a substrate, a dielectric layer and a top semiconductor layer, the dielectric layer electrically insulating the top layer from the substrate. SeOI structures are usually manufactured by wafer bonding via the dielectric layer which acts both as an electric insulator and as a bonding layer between the top layer and the substrate. 
   The SeOI structures that are highly thermal conductor are especially used for dissipating the heat released from components to be manufactured in the top layer of the SeOI. It is particularly useful for components able to release a large quantity of heat, like high power frequency components. 
   To this end, it is known to provide a substrate with material(s) having good thermal conductivity, like monocrystalline or polycrystalline SiC. 
   For these kinds of structures, it would be also appreciated having a dielectric layer that is a good conductor of thermal energy. For this purpose, it is known to provide a dielectric nitride layer, like Si 3 N 4  or Si x N y O z  between the substrate and the top layer. However, the manufacturing of these SeOI structures by wafer bonding is difficult due to the fact that nitride materials have bad bonding properties. SiO 2  has better bonding properties, but it has a low thermal conductivity. 
   Accordingly, there is a need for manufacturing SeOI structures with high thermal conductivity while implementing a bonding of good quality. 
   SUMMARY OF THE INVENTION 
   The present invention now satisfies the prior art need for such SeOI structures. In particular, the invention relates to a process for treating a structure for use in electronics or optoelectronics applications, which comprises heat treating a structure comprising, successively, a substrate, a dielectric layer having a thermal conductivity substantially higher than that of an oxide layer made of an oxide of a semiconductor material, an oxide layer made of an oxide of a semiconductor material, and a semiconductor layer made of a semiconductor material, in an inert or reducing atmosphere at a temperature and a time sufficient to diffuse an amount of oxygen of the oxide layer through the semiconductor layer so that the thickness of the second oxide layer decreases by a predetermined amount. Advantageously, the thickness of the semiconductor layer is between around 250 angstroms and around 5000 angstroms, the temperature is about 1200° C. and the time is between around 5 minutes and 5 hours. 
   The oxide layer can have a thickness between around 100 angstroms and around 500 angstroms. The heat treatment can be applied so that substantially the whole oxide layer is removed or so that a part of the oxide layer remains. The dielectric layer has a thickness sufficient for electrically insulating the semiconductor layer from the substrate, considering the components to be manufactured in the semiconductor layer. 
   In this process, the dielectric layer can have a thermal conductivity that is higher than 10 W.cm −1 .K −1  and a thickness in the range of 1,000 to 5,000 Å. The preferred materials for the dielectric layer include nitride, diamond, alumina (Al 2 O 3 ), aluminum nitride (AlN), sapphire, or preferably Si 3 N 4 . The substrate can also be made of a material having high thermal conductivity. 
   Another embodiment of the invention relates to a process of manufacturing a structure for use in electronics or optoelectronics applications, which comprises providing a semiconductor layer made of a semiconductor material and having a predetermined thickness; providing a receiving wafer that successively includes a substrate, a top dielectric layer made of a dielectric material having a thermal conductivity that is higher than that of an oxide layer made of an oxide of the semiconductor material; forming a bonding interface that includes as a bonding layer an oxide of the same semiconductor material as that of the semiconductor layer; bonding the semiconductor layer to the receiving wafer at the bonding interface such that the dielectric layer is sandwiched between the semiconductor layer and the substrate, thus forming a structure comprising successively the substrate, the dielectric layer, the oxide layer and the thin semiconductor layer; and heat treating the structure in an inert or reducing atmosphere at a temperature and a time sufficient to diffuse an amount of oxygen of the oxide layer through the semiconductor layer so that the thickness of the oxide layer decreases by a predetermined amount. 
   In this process, the oxide layer may be formed on the dielectric layer, on the semiconductor layer, or on both the dielectric layer and the semiconductor layer. In a preferred arrangement, the semiconductor layer is provided as part of a donor substrate and which further comprises reducing the thickness of the donor substrate so that only the semiconductor layer is bonded to the receiving substrate. The thickness of the donor wafer can be reduced by implanting atomic species in the donor substrate to form a zone of weakness beneath the semiconductor layer, and supplying energy for detaching the semiconductor layer from the donor structure at the zone of weakness. 
   The heat treating temperature can be firstly chosen according to a determined profile, and then the predetermined thickness is chosen for determining the duration or the duration is chosen for determining the predetermined thickness, these choices being made for reducing the thickness of the first oxide layer by a predetermined value. A temperature of between 1,100° C. and 1,250° C. is suitable. Also, the predetermined thickness and temperature are chosen for having a mean reduction rate of the first oxide layer of at least about 0.5 angstroms per minute. When the thickness of the semiconductor layer is between around 250 angstroms and around 5,000 angstroms, the heat treating temperature can be about 1,200° C. and the treatment time between around 5 minutes and 5 h 
   Accordingly, it is possible to manufacture a SeOI with a dielectric layer that has a very good thermal conductivity while ensuring a bonding of good quality, i.e. a bonding similar to the bonding via an oxide layer. Indeed, once the oxide layer was used for ensuring a bonding of good quality between the semiconductor layer and the substrate, it is dissolved during the heat treatment (step (d)), for leaving the dielectric layer as the sole dielectric layer of the SeOI. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     Other features and advantages of the invention will appear in the detailed description that follows and which is illustrated by the drawing figures, wherein: 
       FIG. 1  shows a schematic cross-section view of structure according to the invention. 
       FIGS. 2A to 2E  show the different steps of a process of manufacturing the structure. 
       FIGS. 3 and 4  are schematic cross-section views of the structure, illustrating the diffusion phenomena. 
       FIG. 5  is a graph showing distribution of oxygen inside the structure after a heat treatment according to the invention. 
       FIG. 6  shows difference of the BOX thickness of a heat-treated BOX in a SOI wafer after a heat treatment according to the invention, along the whole area of the BOX, measured by ellipsometry. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a structure  60  from which the treatment according to the invention will be processed, is shown. This structure  60  comprises a substrate  10 , a dielectric layer  30 , an oxide layer  40 , and a thin semiconductor layer  50 . The dielectric layer  30  is made of a material having a higher thermal conductivity than that of an oxide layer made of an oxide of the semiconductor material. This dielectric layer  30  may preferably be made of a nitride material or of diamond, alumina (Al 2 O 3 ), aluminum nitride (AlN), sapphire. 
   This structure  60  is aimed to be heat treated for dissolving the oxide layer  40 , and obtaining then a SeOI structure comprising the substrate  10 , the dielectric layer  30  and a semiconductor layer  50 ′. Preferably, the semiconductor layer  50 ′ comprises the de-oxidized oxide layer  40  and the thin semiconductor layer  50  (see  FIG. 2E ). Alternatively, the SeOI structure comprises the substrate  10 , the dielectric layer  30  and a semiconductor layer  50 ′ which comprises a very thin oxide layer coming from the partial dissolution of the oxide layer  40 , and the thin semiconductor layer  50 . 
   The substrate  10  stiffens the whole structure  60 . To this aim, it has a sufficient thickness, typically of hundreds of micrometers. The substrate  10  may be formed of a single bulk material, like Si, Ge, SiC, GaN, sapphire, glass, quartz, or other materials. Preferably, the substrate  10  is made of a material having good thermal conductivity, like monocrystalline or polycrystalline SiC. Alternatively, the substrate  10  is a composite structure formed of at least two materials, stacked one onto the other. 
   The semiconductor layer  50  is made of at least one semiconductor material. The semiconductor layer  50  may be of Si, SiC, Ge, SiGe, SiGeC, a Group III-V material, a Group II-VI material or another semiconductor material. The semiconductor layer  50  may alternatively be a combination of or a superposition of at least two of these materials and/or a superposition of several sub-layers. 
   The semiconductor material may be monocrystalline, polycrystalline or amorphous. It may be doped or non-doped, porous or non-porous. The semiconductor layer  50  is advantageously formed for receiving electronic or optoelectronic components. 
   According to the invention, the semiconductor layer  50  is advantageously thin. Its thickness is advantageously less than about 5,000 angstroms, and in particular less than 2,500 angstroms. For example, the semiconductor layer  50  may have a thickness between around 250 angstroms and 2,500 angstroms, or between around 250 angstroms and 1,200 angstroms. Especially, the thickness of the semiconductor layer  50  may be chosen between 500 and 1,000 angstroms, for accelerating oxygen diffusion. 
   The oxide layer  40  is buried in the structure  60 , located between the dielectric layer  30  and the semiconductor layer  50 . The oxide layer  40  is made of an oxide of the semiconductor material. If the semiconductor layer  50  is constituted of several semiconductor sub-layers, the oxide layer  40  can be made of an oxide of the semiconductor material of the adjacent sub-layer. For example, if the semiconductor layer  50  is of Si, the oxide layer  40  is of SiO 2 . 
   This oxide layer  40  is configured for having adhesive properties. It is to be noticed that this oxide layer  40  is not configured for having electrical insulating properties in order to electrically insulate the electronic or optoelectronic components to be formed in the semiconductor layer  50  from the substrate  10 . The oxide layer  40  may be thin. 
   Its thickness may be chosen less than 500 angstroms or less than this thickness. For example, this thickness may be between around 100 angstroms and around 500 angstroms or between around 200 angstroms and around 500 angstroms. A thickness of between 350 and 500 angstroms may be considered as optimum if the semiconductor layer  50  was initially transferred by bonding (via the oxide layer  40 ) by the SMART CUT® technology, and if a heat treatment is further implemented for densifying the oxide layer  40 . Indeed, this thickness may be chosen for both ensuring a SMART CUT® technology of good quality (i.e. so as to capture water at the interface) and for allowing a dissolution of the oxide layer  40  in a relatively short time. 
   The dielectric layer  30  is buried in the structure  60 , located between the substrate  10  and the oxide layer  40 . The dielectric layer  30  is typically made of a dielectric material having a high thermal conductivity, like a nitride of the semiconductor material, like Si 3 N 4 , Si x O y N z , diamond, alumina (Al 2 O 3 ), aluminum nitride (AlN), or sapphire. 
   A dielectric layer  30  is considered to have high thermal conductivity when its thermal conductivity is higher than that of the oxide layer  40 , or, more particularly, when its thermal conductivity is greater than 10 W.cm −1 .K −1  at room temperature. This dielectric layer  30  may be thin, or it can be configured for having electrical insulating properties in order to at least partly electrically insulate the electronic or optoelectronic components to be formed in the semiconductor layer  50  from the substrate  10 . The dielectric layer  30  is not specifically configured for providing adhesive properties. 
   Additionally, the dielectric layer  30  is configured for conducting a determined amount of heat. If the dielectric layer  30  is made of a nitride material (like Si 3 N 4 ), diamond, alumina (Al 2 O 3 ), aluminum nitride (AlN), sapphire, its thickness may be similar to or lower than 5,000 angstroms, and may be in the range of 1,000 to 5,000 angstroms. Also, this thickness may be between around 100 angstroms and around 1,000 angstroms or between around 200 angstroms and around 500 angstroms. Its thickness may also be of a few angstroms. Moreover, this dielectric layer  30  is preferably formed for having a uniform thickness. The obtained uniformity value may be of +/−3% or lower. 
   The manufacturing of this structure  60  may be accomplished by a wafer bonding technique, as illustrated on  FIGS. 2A to 2E , between a first wafer  70  and a second wafer  80 . Especially, with reference to  FIG. 2A , the manufacturing can be firstly implemented by providing a first wafer  70  with the substrate  10  and the dielectric layer  30 , the dielectric layer  30  being a top layer. In a preferred embodiment, the dielectric layer  30  is formed on the substrate  10 . The purpose of this dielectric formation is to provide a buried dielectric layer with a predetermined thickness for forming, after bonding, the insulator part of a SeOI structure highly conductive of thermal energy, the insulator part of this structure being the dielectric layer  30 . 
   The dielectric layer  30  may be a nitride layer formed by nitridation of the top of the substrate  10 . For example, if the substrate  10  has a superficial layer made of Si or SiGe, a Si 3 N 4  layer  20  may be formed at the surface by nitridation. Alternatively, the dielectric layer  30  may be formed by deposition (e.g. CVD) of aggregates made of the dielectric material. For example, Si 3 N 4  or Diamond aggregates may be deposed. 
   The parameters of the dielectric formation (like temperature, gas flows) are controlled such that the dielectric layer  30  is a dielectric barrier between the components to manufacture in the semiconductor layer  50  and the substrate  10 . Particularly, the material, the thickness, and eventually the intrinsic structure, of it are chosen to this end. It is to be noticed that this dielectric layer  30  is not aimed to be a bonding layer, like in the prior art. Accordingly, no defaults are trapped at a bonding interface, and its quality is better. 
   Additionally, the dielectric formation parameters can be chosen for improving the interface with the substrate  10 , lowering the defaults at the interface, and for having a good thickness homogeneity. The thickness of the dielectric layer  30  may then be lower than a standard thickness of a bonding layer. 
   Advantageously according to the invention, the dielectric layer  30  is thin. For example, the dielectric layer  30  has a thickness, after bonding, between around 1,000 and 5,000 angstroms, or between around 200 angstroms and around 500 angstroms, or between 350 and 500 angstroms. Of course, the dielectric layer  30  has also to be sufficiently thick for conducting the determined amount of thermal energy. 
   With reference to  FIG. 2B , a second step consists of providing the second wafer  80  with the semiconductor layer  50  within, the semiconductor layer  50  lying at the surface of the second wafer  80  defining a front layer. The second wafer  80  may be of a single bulk material, the semiconductor layer  50  being then in the bulk material or grown on it. 
   Alternatively, the second wafer  80  may be a composite wafer comprising a holder substrate and a multilayer structure (not shown). In particular, the second wafer  80  can include a buffer structure between the holder substrate and the semiconductor layer  50  arranged for adapting the lattice parameter between these two elements and/or for confining defaults. For example, the second wafer  80  comprises a Si holder substrate, a SiGe buffer layer with a Ge concentration continuously increasing in thickness from the holder, and a SiGe or Ge and/or a strained Si semiconductor layer  50  over it. Some Carbon can be added in these materials. 
   Advantageously, the semiconductor layer  50  has been epitaxially grown. Crystalline growth of the epitaxial layer may have been obtained using the known techniques of LPD (or more specifically LPCVD), CVD and MBE (respectively Liquid Phase Deposition, Chemical Vapor Deposition, and Molecular Beam Epitaxy). 
   With reference to  FIG. 2C , a third step consists of bonding the first wafer  70  to the second wafer  80  such that the semiconductor layer  50  faces the dielectric layer  30 . Advantageously, the bonding is firstly implemented by well-known bonding techniques (see, for example, “Semiconductor Wafer Bonding Science and Technology” by Q.-Y. Tong and U. Gösele—a Wiley Interscience publication, Johnson Wiley &amp; Sons, Inc—for more details). Thus, for example, molecular bonding of hydrophilic surfaces or surfaces rendered hydrophilic may be done. 
   Well-known cleaning steps may be implemented just before bonding. Optionally, a plasma treatment of one and/or the other of the two surfaces to be bonded, followed by conventional annealing or RTA treatment (rapid thermal annealing), is implemented. 
   With reference to  FIG. 2C , the oxide layer  40  was formed, before bonding, on the semiconductor layer  50  and/or on the dielectric layer  30 , for being buried at the bonding interface after bonding. This oxide layer  40  is formed by specific means on the semiconductor layer  50  and/or on the dielectric layer  30 . The oxide layer  40  may be formed by oxidation of the top part of the semiconductor layer  50 . For example, if the semiconductor layer  50  is of Si or SiGe, SiO 2  layer  40  may be formed at the surface by oxidation. 
   Alternatively, the oxide layer  40  may be formed by deposition of aggregates constituted of the oxide material on the semiconductor layer  50  and/or on the dielectric layer  30 . For example, SiO 2  aggregates may be deposited. 
   The parameters of the formation of the oxide are controlled such that the oxide layer  40  is a bonding layer sufficiently thick for ensuring a sufficient adhesivity between the first and second wafers  70 - 80 . Especially, if a SMART CUT® technology is planned to be processed in the first wafer  70 , the oxide layer  40  has to be sufficiently thick for avoiding problems associated with water and particles captured at the bonding interface that can generate some interfacial defaults and/or bubbles in the semiconductor layer  50  during a subsequent heat treatment. 
   On the other hand, it is preferable that this thickness is not too high for avoiding that the dissolution heat treatment lasts too much time. The oxide layer  40  may have a thickness below 600 angstroms, or below 500 angstroms, or between 200 and 500 angstroms. The preferred thickness is between 350 and 500 angstroms as previously explained. 
   With reference to  FIG. 2C , the second wafer  80  and the first wafer  70  are bonded together such that the oxide layer  40  is located at the interface, as previously explained. Optionally, at least one step of heating is additionally implemented for reinforcing the bonds at the interface. 
   Referring to  FIG. 2D , the structure  60  is obtained by reducing the second wafer  80  such that a rear portion is removed. Only the semiconductor layer  50  is kept. Any technique of wafer reduction may be used, such as chemical etching technique, lapping then polishing, SMART CUT® technology which is known per se to the skilled person (see, for example, G. Celler, Frontiers of Silicon-on-Insulator, Journal of Applied Physics, Vol. 93, no. 9, May 1, 2003, pages 4955-4978). 
   In particular, if using the SMART CUT® technology, the second wafer  80  is implanted prior to bonding, with atomic species (such as hydrogen, helium or a combination of them, and/or other atomic species) at energy and dose selected for producing within a zone of weakness at a depth close to the thickness of the semiconductor layer  50 . The implantation may be carried out before or after forming the oxide layer  40 . Finally, once the bonding has been carried out, SMART CUT® technology comprises supplying suitable energy (such as thermal and/or mechanical energy) for rupturing the zone of weakness, thus detaching the rear portion  60  from the semiconductor layer  50 . 
   An optional step of finishing (e.g., by polishing, CMP, cleaning, . . . ) may be implemented after the reduction step, in order to have a smooth and homogeneous semiconductor layer  50 . This finishing step may be implemented prior to or after the heat treatment described herein. Other steps may also be provided, with no limitation according to the invention. The obtained structure  60  comprises successively the substrate  10 , the dielectric layer  30 , the oxide layer  40 , and the thin semiconductor layer  50 . 
   A heat treatment according to the invention is then processed for reducing or removing the thickness of the oxide layer  40 . The heat treatment is implemented in an inert or reducing atmosphere, such as argon, hydrogen or a mixture of them. With reference to  FIG. 2E , the heat treatment is processed such that the oxide layer  40  reduces in thickness or is entirely dissolved, by oxygen diffusion through the semiconductor layer  50 . 
   The final structure  100  is a SeOI structure, with an insulator part formed by the dielectric layer  30  and eventually by a thin remaining part of the oxide layer  40 . The semiconductor part  50 ′ of the SeOI structure  100  is the semiconductor layer  50  and the de-oxidized part of the oxide layer  40 . During the heat treatment, it is to be noticed that a part of the semiconductor layer  50  may have been evaporated away by the inert gas treatment. 
   For illustrating the reduction of the oxide layer  40  due to oxygen diffusion,  FIGS. 3 and 4  show respectively a cross sectional view of the structure  60 , one during diffusion and the other after diffusion. The structure  60  contains two diffusion domains: 
   left side (top semiconductor layer  50 ) and 
   right side (substrate  10 —dielectric layer  30 ); 
   separated by the oxide layer  40  with a thickness d ox . 
   It is assumed that the diffusion of oxygen is in one dimension—the diffusion equation is then: 
               ∂     C   ⁡     (     x   ,   t     )           ∂   t       =       D   ⁡     (   T   )       ⁢         ∂   2     ⁢     C   ⁡     (     x   ,   t     )           ∂     x   2                 
wherein: the x-axis extends transversally to the layer planes, has its origin at the center of the oxide layer  40 , and is pointed to the positive value in the semiconductor layer  50 , and to the negative value in the bulk substrate  10 .
 
   C(x, t) is the oxygen concentration at time t and at x. 
   D(T) is the diffusion coefficient of the oxygen in the semiconductor (unit: cm 2 /s). 
     FIG. 5  schematically shows distribution of oxygen in the structure during a heat treatment. If the top semiconductor layer  50  is sufficiently thin, some oxygen of the oxide layer  40  diffuses through it and evaporates in the atmosphere at the surface of it. This diffusion is accelerated by the fact that the atmosphere is chosen inert, as it can be deduced from the boundary conditions. 
   In particular, the following reaction occurs at the surface of the semiconductor layer  50  if the inert atmosphere contains hydrogen and the layer is in silicon:
 
SiO 2 +H 2 →H 2 O+SiO↑ if atmosphere is H 2  
 
SiO 2 +Si→2SiO↑ if atmosphere is Ar
 
   For increasing the efficiency of this diffusion, a previous deoxidation of the surface of the semiconductor layer  50  may be done. 
   The dielectric layer  30  prevents from the diffusion through the substrate  10 . Then, after a determined time and if the thickness of the semiconductor layer  50  is small with respect to the oxygen diffusion length (D*t) 1/2 , it has been calculated that the diffusion time is acceptable. In this last case, the determined time is about 100 s, at about 1,200° C. 
   In such conditions the steady flux is defined as:
 
 F=D ( T )* C   0 ( T )/ d   se  
         where: d se  is the thickness of the semiconductor layer  50     where C 0 (T) is the equilibrium oxygen solubility in the semiconductor at annealing temperature.
 
Oxide dissolution time for decreasing the oxide layer  40  thickness d ox  by a predetermined value Δd ox , is:
       

   
     
       
         
           time 
           = 
           
             
               
                 
                   d 
                   Se 
                 
                 * 
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   d 
                   ox 
                 
               
               
                 
                   D 
                   ⁡ 
                   
                     ( 
                     T 
                     ) 
                   
                 
                 * 
                 
                   C 
                   ⁡ 
                   
                     ( 
                     T 
                     ) 
                   
                 
               
             
             * 
             N 
           
         
       
     
       
       
         
           where: N is the concentration of oxygen atoms in oxide. 
         
       
     
  
   For example, if the semiconductor layer  50  is of monocrystalline Si then N=4.22e22, and the oxide layer  40  is of SiO 2 , and if d se =1000 angstroms and Δd ox =20 angstroms: time=1.86e−12*exp(4.04 eV/kT). It has been found that the main parameter affecting the time is the anneal temperature and the thickness of the top semiconductor layer  50 . 
   For example, and based on numerical simulation, the minimum annealing conditions to dissolve 20 angstroms of interfacial SiO 2 , with 1000 angstroms of top Si layer, in a Ar or H 2  atmosphere, are:
         1,100° C. for 2 hr, or   1,200° C. for 10 min, or   1,250° C. for 4 min.
 
The temperature and the duration of the heat treatment are then chosen for inciting an amount of oxygen of the oxide layer  40  to diffuse through the semiconductor layer  50 . Then, the thickness of the oxide layer  40  decreases by a predetermined value.
       

   Additionally, the thickness of the semiconductor layer  50  may also have been chosen, when forming it, for inciting the diffusion. Particularly, the thickness of the semiconductor layer  50  and the temperature of the heat treatment determine the mean reduction rate of the oxide layer  40 . The greater the thickness, the less the rate: the greater the temperature, the greater the rate. For example, the thickness and temperature may be predetermined such that at least about 0.5 angstroms per minute of oxide layer  40  mean reduction rate is reached. To this purpose, for a temperature of about 1,200° C., a thickness of a (110) Si monocrystalline layer  10  is chosen less than 2,500 angstroms. 
   Only the duration of the heat treatment is then necessary to control for accurately reducing the thickness of the oxide layer  10  by a predetermined value. Alternatively, the thickness of the semiconductor layer  50  has been chosen for reducing the oxide layer  40  by a predetermined value by implementing the heat treatment with a predetermined duration and a predetermined temperature. The predetermined temperature may be chosen about 1,000° C. to 1,300° C., and especially around 1,100° C. to 1,200° C. The thickness of the semiconductor layer  50  may be between around 250 angstroms and around 1,000 angstroms, the predetermined temperature is about 1,200° C. and the predetermined duration is between around 5 minutes and 5 hours. 
   The heat treatment is processed for reducing the oxide layer  40  by a predetermined thickness. By adjusting precisely the parameters of the heat treatment, it is then possible to control precisely the reduction of material in the oxide layer  40 , for finally having an oxide layer  40  with a desired thickness. According to the invention, it is then possible to control precisely the thickness of the oxide layer  40  of SeOI. Particularly, it is possible to remove the whole oxide layer  40 . Alternatively, it is possible to leave a thin oxide layer (of about 10-100 angstroms) in order to improve the electrical properties at the interface (i.e. to decrease the Dit). 
   Additionally, the bonding between the semiconductor layer  50  and the substrate  10  can be done with an oxide layer  40  having a thickness greater than a limit thickness beyond which the deformation of the semiconductor layer  50  and bubbles are avoided. Furthermore, as risks of deterioration of the semiconductor layer  50  are decreased, the thickness of the latter can also be decreased, while still respecting manufacturing specifications. Thus, the components to be manufactured in the semiconductor layer  50  may be more miniaturized and have lower power consumption than the prior art. 
   A main advantage of the invention is that the dielectric layer  30  is maintained in its initial configuration, even if the diffusing heat treatment is implemented. Indeed, the dielectric layer  30  is not used for the bonding, and its initial dielectric and thermal properties can thus be maintained. The dielectric properties of the dielectric layer  30  can then be initially calibrated very precisely, without taking account of the next heat treatment.