SUPPORT INCLUDING AN ELECTRICAL CHARGE TRAPPING LAYER FOR A COMPOSITE SUBSTRATE.

Abstract

The invention relates to a method for preparing a support (1) for a composite substrate (S) comprising forming a surface porous layer (P) on a first face (1c) of the support (1), and applying a viscous solution comprising a solvent and a precursor of a filler material to the first face (1c) of the support (1) so as to absorb at least a portion of the viscous solution into open pores of the surface porous layer (P). In a fourth step, the support (1) is heat-treated to transform the viscous solution present in the open pores so that they are filled with the filler material. Figure to be published with the abbreviation: Fig 1

Description

Title of the invention: SUPPORT COMPRISING AN ELECTRICAL CHARGE TRAPPING LAYER FOR A COMPOSITE SUBSTRATE. FIELD OF INVENTION

[0001] The invention relates to a method for preparing a substrate having an electrical charge-trapping layer, the substrate being intended to receive a crystalline thin film by a layer transfer technique. A composite substrate formed from such a support finds application in the field of integrated electronic components, in particular radio frequency (RF) components processing signals whose frequency can typically be between 20 kHz and 300 GHz, or higher. The thin layer of the composite substrate can be made of a semiconductor material such as silicon or an insulating material, or such as a material having piezoelectric and / or ferroelectric properties. In addition to the method for preparing the substrate itself, the invention also relates to the method for manufacturing the composite substrate comprising this support and onto which the thin film has been transferred. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] The prior art abounds with solutions for forming a substrate equipped with an electrical charge trapping layer (more simply, "trapping layer" in the remainder of this description). Such a substrate is intended to form the base substrate of a composite substrate, for example, of the silicon-on-insulator type. The trapping layer limits the electromagnetic coupling that occurs between an RF component formed in or on the thin layer of the composite substrate and this substrate. The electrical traps of the trapping layer limit the mobility of charge carriers and therefore the interactions with the electromagnetic fields from the high-frequency signals produced by the RF components propagating through the depth of the composite substrate. This preserves the quality of the useful signals by limiting their nonlinear distortion, insertion losses, and possible inter-component influences.

[0003] Generally speaking, the aim is to form a trapping layer with a high density of structural defects such as dislocations, grain boundaries, amorphous zones, interstices, inclusions, pores, etc. These structural defects form traps for charges that may circulate in the material, for example, at incomplete or dangling chemical bonds. This prevents conduction in the trapping layer, which exhibits ... resulting in high resistivity.

[0004] US patent 2017062284 proposes forming this trapping layer with porous silicon. However, such a layer proves to be chemically and mechanically fragile. It is susceptible to degradation during the processes applied to the substrate, particularly the transfer treatments of the single-crystal thin film during the fabrication of the composite substrate. These treatments can mechanically stress the substrate, for example, during the transfer of the thin film by fracturing a donor substrate to which it has been previously bonded. They can also chemically stress it, for example, during the cleaning or wet etching steps of a finishing sequence of the composite substrate, applied after the thin film transfer.

[0005] US patent 2018047614 proposes filling the pores of the porous layer, for example with amorphous or polycrystalline silicon, by physical vapor deposition or by chemical vapor deposition of the filling material. US patent 2022359272, on the other hand, proposes forming a mesoporous layer with hollow pores whose internal walls are mostly lined with oxide by annealing under an oxidizing atmosphere. SUBJECT OF THE INVENTION

[0006] An object of the invention is to provide an alternative way to constitute an electrical charge trapping layer formed from a porous layer, this trapping layer being mechanically and chemically robust. BRIEF DESCRIPTION OF THE INVENTION

[0007] To achieve this goal, the object of the invention proposes a method for preparing a support for a composite substrate, the preparation method comprising: - a first porification stage aimed at forming a superficial porous layer on a first face (the) of the support; - a second step of supplying a viscous solution comprising a solvent and a precursor of a filling material; - a third step of dispensing the viscous solution onto the first face of the support so as to absorb at least part of the viscous solution into open pores of the superficial porous layer; - a fourth heat treatment stage of the support aimed at transforming the viscous solution present in the open pores so that they are filled with the filling material.

[0008] Because at least some of the pores are completely filled with the filling material, the trapping layer exhibits good mechanical and chemical resistance.

[0009] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: the support has a resistivity of less than 10 ohms centimeters, preferably between 1 and 2 ohms centimeters; the first step of porosification is carried out by etching in an acid bath, in particular a bath comprising a mixture of nitric acid and hydrofluoric acid; the first stage of porosification is carried out by electrochemical or photo-electrochemical means; the superficial porous layer has a thickness between 100nm and 20 micrometers; the third dispensing step is carried out by centrifugation; the filling material is a dielectric; the filling material is silicon oxide; The precursor of the filler material is a hydrogenated silsesquioxane resin. The process includes, before the fourth heat treatment step, a preliminary solvent evaporation step. the fourth heat treatment stage includes exposing the substrate to a temperature between 300°C and 1100°C; a covering layer is also formed comprising the filling material on and in contact with the superficial porous layer; A preparation method according to the preceding claim, in which the covering layer is removed.

[0010] According to another aspect, the object of the invention proposes a method for manufacturing a composite substrate comprising the provision of a prepared support as presented above and the transfer of a thin single-crystal layer onto the first face of the support.

[0011] The single-crystal thin film may be made of silicon, silicon carbide or a piezoelectric material. BRIEF DESCRIPTION OF THE FIGURES

[0012] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0013]

[0014]

[0015] [Fig. 1] Fig. 1 represents a support according to the invention; [Fig.2]

[0016] Figure [Fig.2] represents a composite substrate employing a support according to the invention;

[0017] [Fig.3a]

[0018] [Fig.3b]

[0019] [Fig.3c]

[0020] [Fig.3d]

[0021] Figures 3a, 3b, 3c and 3d illustrate a method for preparing a conforming support to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] Figure 1 shows a support 1 according to the invention and intended to receive, by a layer transfer technique, a thin crystalline film to form a composite substrate S, shown in Figure 2. In general, the support 1 comprises, on a first face, a trapping layer formed from a porous layer, at least some of whose pores are completely filled with a filler material. Preferably, a majority of the pores are filled with the filler material, for example, more than 50%, more than 60%, or even more than 95%. This material may, in particular, comprise or be made of a dielectric such as silicon dioxide. Due to its composition, the trapping layer exhibits high resistivity, typically greater than 10 kΩ·cm throughout its thickness, and good mechanical and chemical resistance.

[0023] The morphology of the pores can be arbitrary, and the porous layer, at least some of whose pores have been filled by the filler material, can be macroporous (pore diameter greater than 50 nm), mesoporous (pore diameter between 2 nm and 50 nm), and / or nanoporous (pore diameter less than 2 nm). Preferably, however, a mesoporous material is chosen. The porosity level is typically between 40% and 60%, but this is not an essential feature of the invention.

[0024] A person skilled in the art will be able to choose, according to the nature of the filling material, the porosity rate and the nature of the pores which ensure a good balance between the mechanical and electrical properties sought for the trapping layer la.

[0025] The trapping layer has a thickness typically between 100 nm and 20 micrometers, but preferably greater than 1 micrometer, or 10 micrometers, a relatively thick trapping layer allowing for better electromagnetic isolation of the support components. Part 1b of the support 1 located beneath the trapping layer has a thickness of several hundred microns to ensure the mechanical strength of the assembly.

[0026] It is possible to provide the support 1, as shown in dotted lines in the illustration of [Fig.1], with a dielectric layer 2, on and in contact with the trapping layer la, but this dielectric layer 2 is perfectly optional. When present, it may be composed of or comprise silicon oxide, silicon nitride and / or silicon oxynitride.

[0027] Conventionally in the field of substrates for integrated devices, the support 1 can be in the form of a circular plate whose diameter can be 100, 150, 200, 300 or even 450mm.

[0028] Preferably, the starting support in which the trapping layer is formed is made of monocrystalline silicon. It advantageously exhibits a relatively low resistivity, less than 10 Ohms·cm, preferably on the order of 1 to 2 Ohm·cm, but without excluding the possibility that this substrate may exhibit a different resistivity, as will be explained later in this description. It may be of the p-type or n-type.

[0029] To form the trapping layer la, and with reference to figures 3a to 3d which illustrate a method of preparing a support according to the invention, a first porosification step is applied to the starting support aimed at forming a superficial porous layer P on at least one of its faces le (designated "first face" in the rest of this description).

[0030] This step can be carried out in several ways. According to a first approach, the first porosification step is performed by etching in an acid bath, in particular a bath comprising a mixture of nitric acid and hydrofluoric acid. Reference may be made to the document by Starostina et al., “Porous-Silicon Formation in HF-HNO3-H2O Etchants,” Russian Microelectronics 31, 88–96 (2002), for a description of the physical phenomena leading to the formation of the surface porous layer using such a chemical treatment.

[0031] This approach tends to form a superficial porous layer P with a relatively small thickness, on the order of 1 micrometer or less. The formation of the porous layer is favored by a relatively low resistivity of the support 1, which may lead to choosing this support 1 to have low resistivity, on the order of 10 Ohm.cm or less. It is possible to treat a support 1 so that its surface thickness is low resistivity, the remainder of the support 1b then being able to have a resistivity chosen more freely. This may involve introducing dopants into this surface thickness or forming a doped surface layer by deposition.

[0032] According to another approach, the first porosification step is carried out electrochemically or photoelectrochemically, implementing an anodic dissolution phenomenon in an acidic medium, by electrolysis. In practice, the support 1 is placed in an electrochemical cell, its first face being exposed to an electrolyte solution based on hydrofluoric acid (HF). This solution may have an HF concentration greater than 30% and may include an additive (for example, isopropyl alcohol, also known as IPA, or ethanol). The support 1 is in contact with a The anode, for example, is in contact with the face opposite the first face (l), while the cathode is placed opposite the first face (l) to allow current to flow through the electrolyte and the support (l). The current density flowing in the cell (typically between 1 and 50 mA / cm²) leads to the formation of the superficial porous layer on the side of the first face (l) of the support (l), that is, the side exposed to the electrolyte. This approach makes it possible to form a relatively thick porous layer (P), on the order of 10 micrometers, or even 20 micrometers, by extending the duration of this step.

[0033] It should be noted that other electrochemical cell configurations exist for forming the surface porous layer P. In particular, so-called "double cell" configurations are known (see, for example, US patent 11049724), in which the two opposite faces of the initial support are in contact either with the same electrolyte solution or with different solutions. In this case, one of the faces can be illuminated so that this face generates charge carriers during the electrochemical phase.

[0034] Regardless of how this first porosification step was carried out, a superficial porous layer P is obtained at the end of this step on a first face l of the support 1. Some of the pores constituting this superficial porous layer P may be open and lead (directly or indirectly) to the first face l of the support. The parameters of this step can be controlled to obtain a specific porosity level and pore type (in particular their average dimensions), as mentioned in a previous section of this description. The duration of porosification defines, all other things being equal, the thickness of the superficial porous layer P, which is intended to form the trapping layer. Preferably, this thickness will be chosen to be between 100 nm and 20 micrometers, as already specified.

[0035] In a second step of preparing the support 1, a viscous solution is prepared comprising a solvent and a precursor of a filler material. This solution is intended to fill the pores of the porous layer P; therefore, it must have a low viscosity to allow it to penetrate and flow into the pores of this layer P. The viscosity of the solution can, in particular, be controlled by adjusting the proportion of solvent in the solution.

[0036] In addition to the solvent, the solution comprises a precursor of the filler material. "Precursor of the filler material" means any material which, directly or after processing, leads to obtaining or forming the filler material, once the solvent has been removed from the viscous solution.

[0037] In a preferred embodiment, the filler material is silicon dioxide and its precursor is hydrogenated silsesquioxane. Silsesquioxane Hydrogenated silicon dioxide (SiO₂) is an inorganic compound with the formula [HSiO₃ / 2]n which, after heat treatment that removes at least some of its hydrogen, forms a silicon-rich oxide (SiO₂) that is chemically resistant to etching products such as tetramethylammonium hydroxide (TMAH or TMAOH). It can be combined with an organic solvent, for example methyl isobutyl ketone (often abbreviated MIBC or MIBK), to form a viscous solution.

[0038] But the invention is by no means limited to a silicon oxide filler material and this precursor. A filler material based on silicon nitride, silicon oxynitide or an alloy comprising silicon and carbon can be envisaged by exploiting a precursor based on silicon and nitrogen, for example a chemical compound of the silazane, disilazane or polysilazane type.

[0039] The use of these precursor polymers of filler material is for example documented in US4312970A or US4756977A.

[0040] In a third step of the substrate preparation process 1, the viscous solution is dispensed onto the first face le of the support 1 so as to absorb at least part of the viscous solution into the open pores of the surface porous layer P. This dispensing step advantageously aims to coat the first face le of the support uniformly to allow the open pores to be filled over the entire extent of the first face le. This step, illustrated in [Fig. 3b], can in particular be carried out by centrifugation ("spin coating" according to the Anglo-Saxon terminology of the field), by immersion ("dip coating"), by spraying ("spray coating") or by simple flow ("flow coating").

[0041] The low viscosity of the solution allows it to penetrate and fill the pores of the surface porous layer P. It should be noted that the pores of this layer that do not communicate, directly or indirectly, with the first face l of the support cannot receive the viscous solution by flow. However, this does not pose a significant problem since these non-opening pores are not predominant in the surface porous layer.

[0042] If the viscous solution is supplied in excess, it tends to form a coating layer 4 on and in contact with the superficial porous layer P. This coating layer can have a thickness of several micrometers and be very uniform, due to the viscous nature of the solution, particularly when the dispensing is carried out by centrifugation.

[0043] An optional solvent evaporation step can be provided at this stage of the process. This evaporation step can be achieved by simply drying the support or by raising its temperature in an oven, for example to a moderate temperature between 50°C and 300°C.

[0044] Whether or not this evaporation step has been applied, a preparation process A support 1 according to the invention provides, after the third step of dispensing the viscous solution, a fourth heat treatment step. This step, shown in [Fig. 3c], aims to transform the viscous solution present in the open pores of the porous layer so that they are filled with the filler material. If the prior solvent evaporation step has not been applied, the solvent is removed from the solution during this heat treatment. This also leads to the elimination of volatile species present in the precursor of the filler material, so that, at the end of this heat treatment, the pores are effectively filled with this material.For example, when the precursor of the viscous solution is hydrogenated silsesquioxane, the heat treatment leads to the removal of at least some of the hydrogen from the precursor (forming the volatile species in this case) to fill the pores with a silicon-rich, hydrogen-depleted oxide.

[0045] This heat treatment is, of course, adapted to the nature of the viscous solution and, more particularly, to the nature of the precursor present in this solution. When this precursor is hydrogenated silsesquioxane, the annealing temperature can be between 300°C and 1100°C.

[0046] When a coating layer 4 is present on the substrate, the heat treatment step also transforms the viscous material composing the coating layer. This layer can be removed, if it is undesirable for the use of the substrate, for example by a polishing step. Alternatively, it can be preserved and contribute to forming the dielectric layer 3 of the substrate 1, if the filler material is indeed dielectric in nature.

[0047] Whether this coating layer 4 is preserved or not, a preparation process according to the invention may also include a step of forming the dielectric layer 2 on the support, by a conventional deposition process.

[0048] Fig. 3d represents the support substrate 1 obtained at the end of this treatment (without having formed, in the example shown, a dielectric layer 2). The initially formed surface porous layer P has been transformed by the trapping layer 2 formed of silicon and pores filled with a filler material.

[0049] As already mentioned, the substrate 1 is intended to receive, by transfer, a thin layer 3 and thus form a composite substrate S, shown for illustrative purposes in [Fig. 2]. The thin layer 3 is generally crystalline, and advantageously single-crystal. The substrate 1 has suitable properties (in terms of surface roughness and deformation in particular) or has been pre-treated (for example by polishing) to receive such a thin layer 3. The composite substrate S comprises, in contact with and interposed between the substrate 1 and the thin layer 3, a dielectric layer 2. This dielectric layer has a thickness chosen according to the nature of the components that will be formed in and on the thin layer 3 and therefore of the application targeted by the composite substrate S. This thickness can, for example, be between 10 nm and 10 micrometers.

[0050] As is well known, the transfer of the thin film 3 onto the support 1 is usually achieved by assembling a free face of a donor substrate to the first face 1 of the support 1, preferably by molecular adhesion. It is envisaged that at least one of these faces will be provided with a surface dielectric layer, these layers together forming the dielectric layer 2 of the composite substrate S.

[0051] The nature of the donor substrate is chosen according to the desired nature of the thin film 3. It may therefore be a substrate made of a single-crystal semiconductor, for example silicon, or a substrate made of a single-crystal piezoelectric material or comprising a surface layer of such a single-crystal piezoelectric material. In this case, it may be lithium tantalate or lithium niobate.

[0052] It is also possible that the donor substrate has finished or semi-finished components, the aim being to place these components on the support 1 to take advantage of its radio frequency properties.

[0053] Just like the support 1, the donor substrate can take the form of a circular plate, the dimensions of which can correspond to those of the support.

[0054] After this assembly step, the donor substrate is reduced in thickness to form the thin film 4. This reduction step can be carried out by mechanical or chemical thinning, particularly when the donor substrate includes components that are to be placed on the support 1. The thickness reduction of the donor substrate can preferably be achieved by fracturing at a weakening plane previously introduced into the donor substrate, for example by implanting light species such as hydrogen and / or helium. This weakening plane, together with the free surface of the donor substrate, defines the thin film 3.

[0055] After this thinning or, preferably, fracturing step, finishing steps of the thin layer 3 can be applied, such as a polishing step, heat treatment under a reducing or neutral atmosphere, sacrificial oxidation, etc.

[0056] At the end of these steps, we have a composite substrate S formed from the single-crystal thin layer transferred onto the support 1.

[0057] Of course the invention is not limited to the modes of implementation described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Demands

1. A method for preparing a support (1) for a composite substrate (S), the preparation method comprising: - a first porosification step aimed at forming a superficial porous layer (P) on a first face (le) of the support (1); - a second step of supplying a viscous solution comprising a solvent and a precursor of a filler material; - a third step of dispensing the viscous solution onto the first face (le) of the support (1) so as to absorb at least part of the viscous solution into open pores of the superficial porous layer (P); - a fourth step of heat treatment of the support (1) aimed at transforming the viscous solution present in the open pores so that they are filled with the filler material.

2. A preparation method according to the preceding claim in which the support (1) has a resistivity of less than 10 ohm centimeters, preferably between 1 and 2 ohm centimeters.

3. A preparation method according to any one of the preceding claims, wherein the first porosification step is carried out by etching in an acid bath, in particular a bath comprising a mixture of nitric acid and hydrofluoric acid.

4. A preparation method according to any one of claims 1 to 3 wherein the first porosification step is carried out by electrochemical or photoelectrochemical means.

5. A preparation method according to any one of the preceding claims wherein the superficial porous layer (P) has a thickness between 100nm and 20 micrometers.

6. A preparation method according to any one of the preceding claims, wherein the third dispensing step is carried out by centrifugation.

7. A preparation method according to any one of the preceding claims wherein the filling material is a dielectric.

8. A preparation method according to any one of the preceding claims in

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15. in which the filling material is silicon oxide. A preparation method according to one of the two preceding claims, wherein the precursor of the filling material is a hydrogenated silsesquioxane resin. A preparation process according to any one of the preceding claims, wherein the process comprises, before the fourth heat treatment step, a preliminary solvent evaporation step. Preparation process according to any one of the preceding claims wherein the fourth heat treatment step comprises exposing the support (1) to a temperature between 300°C and 1100°C. Preparation method according to any one of the preceding claims wherein a covering layer (4) comprising the filling material is also formed on and in contact with the superficial porous layer (P). Preparation method according to the preceding claim in which the removal of the covering layer (4) is carried out. Method of manufacturing a composite substrate (S) comprising supplying a support (1) prepared according to one of the preceding claims and transferring a thin single-crystal layer (4) onto the first face of the support. A manufacturing method according to the preceding claim in which the single-crystal thin film (4) is made of silicon, silicon carbide or a piezoelectric material.