Method for obtaining a structure having a supporting substrate and an ultra-thin layer

0001The present invention relates to a process for obtaining a structure comprising at least one support substrate and an ultrathin layer of a material, in particular a semiconductor, and optionally an intermediate layer inserted between the two.

0002During the manufacture of composite substrates, in particular for applications in the fields of optics, electronics or optoelectronics, layer transfer methods are used which make it possible to transfer a layer from a source substrate, on a support substrate.

0003One of these layer transfer methods consists in implanting atomic species under the surface of a source substrate, so as to create therein a weakening zone which delimits a thin layer. Next, the free face of this thin layer is brought into intimate contact with the support substrate, then detachment of said thin layer from the rest of the source substrate and its transfer onto the support substrate.

0004This type of process is known under the trademark "Smart Cut".

0005The current evolution of technology consists in trying to transfer increasingly thin layers onto a support, so as to obtain composite substrates from which it will be possible to manufacture electronic components whose operating speed will be even faster than those of existing components.

0006On this subject, one can refer for example to the article of <nplcit id="ncit0001" npl-type="s"><text>Aaron Mand, "Value-Added Wafers Push Chips Ahead", Semiconductor International, November 2002</text></nplcit>.

0007However, attempts made so far have shown that when the thin layer to be transferred is very thin, that is to say less than about 100 nm, it has defects which do not appear in the case of the transfer of 'a thicker layer. Thus, it does not support the subsequent heat treatments carried out either to reinforce its bonding interface with the support, or to detach it from the source substrate. In fact, such heat treatments have the effect of increasing the pressure prevailing in the gas microbubbles present at the level of the embrittlement zone and of causing their degassing. The very thin transferred layer then has many defects, such as blisters or partial delamination.

0008Furthermore, it seems that when a very thin layer is transferred onto a support substrate, the quality of the bonding obtained is much more sensitive to the presence of particles or of hydrocarbon on the surface of the layers to be brought into contact than when the 'we transfer a thicker layer.

0009Likewise, when there is an intermediate layer between the thin layer to be transferred and the support substrate, it has been found that the thin transferred layer has all the more defects that this intermediate layer is thin.

0010The <figref idref="f0001">figure 1</figref> attached is a graph theoretically representing the total thickness <b>AND</b> of the transferred layer, i.e. the sum of the thicknesses of the thin layer and the interlayer, as a function of the number of defects <b>ND</b> on the finished structure, and this for a given thickness of interlayer.

0011As can be seen in this figure which is a schematic diagram, for a given thickness of intermediate layer (straight solid line), we arrive at a limit thickness <b>EL₁</b> the thin layer transferred below which one cannot descend; except to obtain a defective layer transfer. For an even smaller thickness of the intermediate layer (dotted straight line), the limit thickness<b>EL₂</b> of the transferred thin layer is even more important.

0012In the particular case of SOI substrates ("SOI" being the acronym of the English expression "Silicon On Insulator" which means "silicon on insulator") an implantation of atomic species, generally hydrogen ions, is carried out. the surface of a silicon substrate whose outer surface is oxidized, then a second silicon substrate is bonded to the assembly. After detachment and then annealing, the SOI substrate is obtained comprising a solid silicon layer covered with an oxide layer (SiO₂) and a layer of transferred silicon (see article "<nplcit id="ncit0002" npl-type="s"><text>Silicon on insulator material technology "by M.Bruel, Electron Letter, 31, 1201 (1995</text></nplcit>).

0013Below a certain thickness of the buried oxide layer, it is found that the transferred silicon layer has defects, the latter being all the more marked the more heat treatment is used at high temperature. On this subject, one can refer to the article of<nplcit id="ncit0003" npl-type="s"><text> Q.-y. Tong, G. Cha, R. Gafiteau and U. Gösele, "Low temperature wafer direct bonding", J. Microelectromech Syst., 3, 29, (1994</text></nplcit>).

0014One possible explanation for the appearance of these defects is the existence of a degassing at the bonding interface between the layer of silicon oxide and the second silicon substrate.

0015During the annealing heat treatment, called "stabilization treatment", occurring after the detachment step, a gas is formed at the bonding interface. In the case of a thick SOI substrate, the thickness of the transferred layer is important and plays the role of stiffener. Furthermore, it is assumed that the equally thick oxide layer acts as a sponge by absorbing the gases released at the bonding interface.

0016In the case of a thin SOI substrate in which the transferred layer and / or the oxide layer are fine, the abovementioned phenomena of absorption and stiffening effect do not take place and the degassing leads to poor sealing of the bonding interface.

0017Another possible explanation is that in fine SOI substrates, hydrogen is implanted at a shallow depth relative to the bonding interface. The transferred layer is therefore saturated with hydrogen which tends to migrate by diffusion towards the bonding interface.

0018SOI substrates obtained by current techniques include buried oxide layers (SiO₂) and superficial silicon whose thicknesses vary from 100 nm to 1.5 µm. To date, it has been difficult to obtain good quality SOI substrates in which these two layers are thinner.

0019The attempts made so far in this direction have all been aimed at strengthening the bonding between the layers, without however allowing satisfactory results to be obtained.

0020Finally, note that we know from the document <patcit id="pcit0001" dnum="WO0048278A"><text>WO-00/48278</text></patcit>, a method of forming a silicon layer for optical use comprising steps of layer transfer and thinning. This process aims to obtain a silicon layer for optical use, of a determined thickness called "optical" equal to<maths id="math0001"><math display="inline"><mrow><mfrac><mi>λ</mi><mrow><mn>4</mn><msub><mi>not</mi><mi>s</mi></msub></mrow></mfrac></mrow></math><img file="EP1631984B1_D0001.tif" /></maths> or <i>λ</i> is the wavelength of the light produced or received and n_s the refractive index of silicon.

0021However, this process is not intended to obtain an ultra thin layer and therefore does not in any way evoke the problems of appearance of blisters or defects during the transfer of an ultra thin layer on a support.

0022Finally, this document absolutely does not suggest using the intermediate stage of the transfer of a layer whose thickness is significantly greater than that of the ultrathin layer to be obtained.

0023The object of the present invention is to solve the aforementioned drawbacks and to provide a method for transferring onto a support substrate, a transferred layer and possibly an intermediate layer, the thicknesses of which are very small, that is to say less than about 50 nanometers (50 nm), or even less than 20 nm. We then speak of "ultra thin" layers.

0024The invention is particularly applicable to the manufacture of all substrates having a stack of layers, some of which are sensitive to degassing.

0025To this end, the invention relates to a process for obtaining a structure comprising at least one support substrate and an ultra thin layer derived from a source substrate, in particular of semiconductor material, for applications in the fields of electronics, optoelectronics or optics, this process comprising the steps of:<ul id="ul0001" list-style="dash" compact="compact"><li>a) bonding by molecular adhesion a support substrate on one of the faces, called "front face", of a source substrate which has internally a zone of embrittlement delimiting with said front face, a useful layer called "layer to be transferred" of which the thickness is significantly greater than that of said ultrathin layer,</li><li>b) detaching said support substrate and said useful layer to be transferred, from the rest of the source substrate, along said embrittlement zone, so as to obtain an intermediate structure comprising at least said transferred useful layer and said support substrate,</li><li>c) thinning said transferred useful layer until obtaining said ultra thin layer, this process being characterized in that the useful layer to be transferred from the intermediate structure has a thickness at least three times greater than or equal to that of said ultrathin layer obtained at the end of step c) of thinning and in that the thickness of said ultrathin layer is less than or equal to 100 nanometers and in that the thickness of the useful layer to be transferred from the intermediate structure is greater than or equal to 300 nanometers. According to other advantageous and non-limiting characteristics of the invention, taken alone or in combination:<ul id="ul0002" list-style="dash" compact="compact"><li>step a) of bonding by molecular adhesion of the support substrate is done on an intermediate layer present on the front face of said source substrate;</li><li>the thickness of the ultra thin layer is less than or equal to 50 nanometers;</li><li>the thickness of said intermediate layer is less than or equal to 50 nanometers;</li><li>said interlayer is a layer of insulating material;</li><li>the intermediate layer is a layer of a material chosen from silicon oxide, silicon nitride, insulating materials with high permittivity, diamond and combinations of these materials;</li><li>thinning is carried out by at least one of the following techniques, including mechanical-chemical polishing, annealing in an atmosphere containing hydrogen, argon or a mixture of the two, sacrificial oxidation and chemical etching ;</li><li>a thermal annealing treatment is carried out after step a) of bonding and before step c) of thinning;</li><li>the thermal annealing treatment is carried out during step b) of detachment;</li><li>the thermal annealing treatment is carried out before step b) of detachment;</li><li>the embrittlement zone is obtained by implantation of atomic species;</li><li>the embrittlement zone is a porous layer;</li><li>the detachment from step b) comprises the application of mechanical and / or thermal stresses;</li><li>the useful layer transferred is a layer comprising suspended patterns;</li><li>the source substrate is made of a material chosen from silicon, silicon carbide, germanium, silicon germanium, compounds IV-IV and compounds III-V or a combination of these materials;</li><li>the support substrate is made of a material chosen from silicon, silicon carbide, germanium, silicon germanium, compounds IV-IV and compounds III-V or a combination of these materials.</li></ul></li></ul>

0026Other characteristics and advantages of the invention will appear on reading the description which will now be made with reference to the accompanying drawings which show, by way of indication but not limitation, possible embodiments.

0027In these drawings:<ul id="ul0003" list-style="dash" compact="compact"><li>the <figref idref="f0001">figure 1</figref> is a graph representing the total thickness <b>AND</b> a transferred layer (thin layer and intermediate layer), depending on the number of defects <b>ND</b> observed on the finished structure, for two different thicknesses of said intermediate layer;</li><li>the <figref idref="f0001 f0002">Figures 2A to 2E</figref> and <figref idref="f0003 f0004">3A to 3E</figref> illustrate the successive stages of two variants of the process according to the invention.</li></ul>

0028The <figref idref="f0001 f0002">Figures 2A to 2E</figref> and <figref idref="f0003 f0004">3A to 3E</figref> are diagrams on which the different layers and their thicknesses are not shown to scale and some of them have been voluntarily enlarged for clarification purposes.

0029Referring to the <figref idref="f0001">figure 2A</figref>, we can see a source substrate 1 which internally has a weakening zone 12 delimiting two parts, namely a useful layer 13 to be transferred and a remainder 14 or rear part of this source substrate.

0030It has a face 10, called "front face" intended to come into contact with a support substrate 3 and located on the side of the useful layer 13 to be transferred and an opposite face 11, called "rear face".

0031The embrittlement zone 12 can be obtained by implantation of atomic species.

0032The expression "implantation of atomic species" means any bombardment of atomic, molecular or ionic species, capable of introducing these species into a material, with a maximum concentration of these species located at a depth determined relative to on the bombarded surface 10. The molecular or ionic atomic species are introduced into the material with an energy also distributed around a maximum.

0033The implantation of the atomic species in said source substrate 1 can be carried out for example, using an implanter by ion beam or an implanter by immersion in a plasma.

0034Preferably, this implantation is carried out by ion bombardment. Preferably, the implanted ionic species is hydrogen. Other ionic species can advantageously be used alone or in combination with hydrogen, such as rare gases (helium for example).

0035This implantation has the effect of creating in the volume of the source substrate 1 and at an average depth of penetration of the ions, the weakening zone 12, substantially parallel to the plane of the front face 10.

0036We can for example refer to the literature concerning the process known under the trademark "mart Cut".

0037The embrittlement zone 12 can also consist of a porous layer obtained, for example, by the method described in the document <patcit id="pcit0002" dnum="EP0849788A"><text>EP-0 849 788</text></patcit>.

0038The useful layer 13 to be transferred can also be a layer the front face 10 of which has cavities obtained by etching, such a layer being known to those skilled in the art under the name of "layer with suspended patterns". Such a layer is used in the production of microelectrical mechanical components known by the acronym "MEMS", from the English expression "Micro Electronical Mechanical Systems".

0039The support substrate 3 has a role of mechanical support and as such has a thickness of several hundred micrometers (typically around 300 μm).

0040It has a front face 30 intended to come into contact with the front face 10 of the source substrate 1 and an opposite face 31, called "rear face".

0041The source substrate 1 and the support substrate 3 can be made of any material, in particular those commonly used in applications in the fields of electronics, optoelectronics or optics, such as semiconductor materials.

0042As an illustrative example, it is possible to use, as constituent material of the support substrate 3 and of the source substrate 1, silicon, material frequently used in the abovementioned applications, silicon carbide germanium, silicon germanium, compounds IV-IV and III-V compounds or a combination of these materials, that is to say a multi-layer substrate comprising at least two layers of materials chosen from those mentioned above.

0043Compounds IV-IV are compounds, the two elements of which belong to column IVa of the periodic table. Compounds III-V are compounds of which one of the elements belongs to column IIIa of the periodic table and the other to column Va, such as for example, gallium nitride (GaN), gallium arsenide (AsGa) or indium phosphide (InP).

0044Whatever the method of obtaining the embrittlement zone 12, it will be formed so that the useful layer 13 to be transferred has a very large thickness compared to the thickness of the final layer which it is desired to obtain. on the support substrate 3.

0045The characteristics relating to the thickness of this layer to be transferred 13 will be described later.

0046As shown in the <figref idref="f0001">figure 2B</figref>, the front face 30 of the support substrate 3 is then bonded by molecular adhesion against the front face 10 of the source substrate 1.

0047Advantageously, a thermal annealing treatment of the stack is then carried out, the function of which is to reinforce the bonding interface between the front face 10 of the source substrate 1 and the front face 30 of the support substrate 3. This thermal treatment is carried out at an appropriate temperature depending on the chemical nature of the materials in contact.

0048As shown in the <figref idref="f0002">figure 2C</figref>, the next step is to detach the support substrate 3 and the transfer layer 13 from the rest 14 of the source substrate 1, along the embrittlement zone 12.

0049This detachment is carried out for example by application of stresses of mechanical origin which are generally tensile and / or bending and / or shear stresses.

0050These constraints can be applied for example, by a traction frame, by one or more blades introduced on the side of the aforementioned stack of layers, at the level of the embrittlement zone 12 or by a jet of fluid (liquid or gas), applied laterally at this same embrittlement zone.

0051The application of these mechanical stresses makes it possible to promote the propagation of a crack at the level of the embrittlement zone 12.

0052This detachment can also be carried out by applying constraints of thermal origin.

0053The mechanical and thermal constraints can also be combined.

0054An intermediate structure referenced 4 is thus obtained comprising the support substrate 3 and the useful layer 13 which is then called "transferred layer".

0055As illustrated in the <figref idref="f0002">2D figure</figref>, a step of thinning the transferred useful layer 13 is then carried out (arrow A), until an ultrathin layer, referenced 130 (see <figref idref="f0002">figure 2E</figref>).

0056The final structure obtained, referenced 5, comprises the support 3 and the ultra-thin layer 130.

0057Thinning can be carried out by one or other or more of the following techniques chosen, for example, from mechanical-chemical polishing (CMP), annealing in an atmosphere containing hydrogen and / or argon, sacrificial oxidation, dry or wet chemical etching.

0058Mechanical-chemical polishing is a technique combining mechanical polishing carried out by the passage of a rotary polishing head over the free surface 131 of the transferred layer 13 and a chemical polishing implemented by distributing an abrasive in pasty form or liquid, such as colloidal silica, between the surface of this polishing head and the surface 131 to be polished.

0059Annealing in an atmosphere containing hydrogen and / or argon consists in annealing the intermediate structure 4 in such an atmosphere, at a temperature of approximately 1050 ° C. to approximately 1350 ° C., for a few tens of seconds. a few tens of minutes. This annealing allows a rearrangement of the atoms of the surface 131 which move until reaching a stable energy level leading to a smoothing of the surface 131 and therefore to a reduction in the thickness (thinning) of the layer 13.

0060Sacrificial oxidation consists in heating the free surface 131 under an oxidizing atmosphere until forming an oxide layer which is eliminated and in repeating this operation several times until reaching the desired thickness for the ultrathin layer 130.

0061Finally, the chemical etching operations include wet etching carried out in a bath of chemicals or dry etching carried out in a gas plasma which etches the transferred layer 13. Dry etching can also be carried out by ion bombardment, for example of argon, which by a ballistic mechanical effect reduces the thickness of the layer 13, until the ultrathin layer 130 is obtained.

0062The final thickness of the ultrathin layer 130 obtained at the end of the thinning step is less than or equal to 100 nanometers (100 nm), or even less than or equal to 50 nanometers (50 nm).

0063The thermal annealing treatment described above and aimed at reinforcing the bonding interface between the front face 10 of the source substrate 1 and the front face 30 of the support 3 can also be carried out during the detachment step or after it, but always before the thinning step. These annealing heat treatments can also be repeated several times between bonding and thinning.

0064It will be noted that when the thermal annealing is carried out during the detachment step, it is possible to carry out a single heat treatment according to an increasing temperature curve.

0065Thus, for example, when the source substrate 1 is silicon, the detachment along the embrittlement zone 12 is carried out at approximately 500 ° C., then when a temperature of at least 1000 ° C. is reached that one holds for a few hours, the bonding interface is strengthened.

0066In accordance with the invention and contrary to what is currently achieved in the state of the art, the process for obtaining an ultrathin layer 130 consists of going through an intermediate step (obtaining an intermediate structure 4) during from which a useful layer 13 of a thickness significantly greater than that of the ultrathin layer 130 which one wishes to obtain at the end of the process is transferred.

0067The expression “appreciably larger” means that the layer to be transferred 13 has a thickness sufficient to withstand the various aforementioned thermal treatments for detaching or reinforcing its bonding interface with the support substrate 3, without this causing the formation of faults or blisters at this interface. The thickness of the layer to be transferred 13 is at least three times that of the ultrathin layer 130 of this layer 13 is greater than or equal to 300 nanometers whereas, by comparison, the ultrathin layer 130 is less than or equal to 100 nanometers, or even less than or equal to 50 nanometers.

0068The <figref idref="f0003 f0004">Figures 3A to 3E</figref> illustrate an alternative embodiment of the method described above together with the <figref idref="f0001 f0002">Figures 2A to 2E</figref>.

0069According to this variant, an intermediate layer 2 is interposed between the front face 10 of the source substrate 1 and the front face 30 of the support substrate 3 (see <figref idref="f0003">figure 3B</figref>).

0070As we can see on the <figref idref="f0003">figure 3A</figref>, this intermediate layer 2 is formed before the bonding step by molecular adhesion, preferably on the front face 10 of the source substrate 1. However, it could also be formed on the front face 30 of the support substrate 3 or on these two faces before 10 and 30.

0071This intermediate layer 2 can be obtained for example by a chemical vapor deposition technique known by the acronym CVD, from the English terminology "Chemical Vapor Deposition" or by epitaxy techniques, namely for example epitaxy in organo-metallic vapor phase ("MOCVD" from the English expression "Metal Organic Chemical Vapor Deposition"), molecular jet epitaxy ("MBE" from the English expression "Molecular Beam Epitaxy) or hydride vapor epitaxy (" HVPE "from the English expression" Hydride Vapor Phase Epitaxy ").

0072When this intermediate layer 2 is an oxide, it can also be obtained by oxidation of the solid substrate 1.

0073When the embrittlement zone 12 is obtained by implantation of atomic species, the deposition of the intermediate layer 2 is preferably carried out before implantation. The significant rise in temperature linked to the deposition process is in fact likely to be detrimental to the embrittlement layer 12.

0074On the other hand, when the weakening zone 12 is a porous layer, the intermediate layer 2 is deposited after the formation of this porous zone and of the useful layer 13 to be transferred, the latter being generally obtained by resumption of epitaxy.

0075This intermediate layer 2 can consist for example of a layer of an insulating material, in particular silicon oxide (SIO₂), silicon nitride (SI₃NOT₄), diamond, an insulating material with high permittivity or a combination of these materials, for example silicon oxynitride. This intermediate layer 2 can also be composed of several layers of the various aforementioned materials (multi-layer material).

0076The other steps of the process are identical to those which have just been described and will therefore not be repeated in detail. Note that after the detachment step shown on the<figref idref="f0003">figure 3C</figref>, an intermediate structure 4 ′ is obtained comprising the support 3, the intermediate layer 2 and the applied layer 13.

0077At the end of the thinning operation, a support substrate 3 is obtained, successively supporting the intermediate layer 2 and then the ultrathin layer 130 and forming a final structure referenced 5 '.

0078Preferably, the intermediate layer 2 is thin, that is to say less than or equal to 50 nm (50 nanometers).

0079Thanks to the method according to the invention, it is thus possible to obtain an SOI type substrate in which the intermediate layer of silicon oxide and the surface layer of silicon both have thicknesses less than 50 nm, which was not possible until 'now.

0080Two particular embodiments of the process according to the invention will now be described.

Example 1:

0081One proceeds to the thermal oxidation of a source substrate 1 of solid silicon so as to cover the latter with an intermediate layer 2 of silicon oxide (SiO2) 50 nanometers (50 nm) thick.

0082The embrittlement zone 12 is then formed by implantation of hydrogen ions H⁺ according to an implantation dose of 8.10¹⁶ H⁺/ cm² and an implantation energy of 210 keV, so as to obtain a transfer layer 13 with a thickness of 1.9 <i>µ</i>m (1.9 micrometers).

0083Bonding is then carried out by molecular adhesion of a support substrate 3 of solid silicon on said oxide layer 2.

0084The detachment of the remainder 14 from the source substrate 1 is then carried out by a heat treatment of less than 500 ° C. and the reinforcement of the bonding interface by a heat treatment at more than 1000 ° C.

0085Finally, thinning is carried out by polishing and sacrificial oxidation, until a final structure 5 ′ is obtained whose ultrathin silicon layer 130 is 50 nm thick, or even 20 nm if this thinning step is continued. .

Example 2:

0086The procedure is as in Example 1, except that the layer of silicon oxide 2 is 20 nanometers thick.

Example 3:

0087The procedure is as in Example 1 but with an implantation dose of 7. 10¹⁶ H⁺/ cm² and an implantation energy of 160 Kev, so that the transfer layer 13 has a thickness of 1.5 <i>µ</i>m (1.5 micrometers).

Example 4:

0088The procedure is as in Example 3 but with a layer of silicon oxide 2 with a thickness of 20 nanometers.