A carrier for a composite substrate including a layer for trapping charges.

Abstract

The present invention relates to a method for preparing a carrier (1) for a composite substrate (S), the method comprising: forming a porous surface layer (P) on a first surface (1c) of the carrier (1); dispensing a viscous solution comprising a solvent and a precursor of a filling material onto the first surface (1c) of the carrier (1) so as to absorb at least a portion of the viscous solution into open pores of the porous surface layer (P). In a fourth step, the carrier (1) is heat-treated to transform the viscous solution present in the open pores so that the open pores are filled with the filling material.

Description

Technical Field

[0001] This invention relates to a method for preparing a carrier having a charge-trapping layer for receiving a thin crystalline layer via a layer transfer technique. Composite substrates formed from such carriers can be applied in the field of integrated electronic components, particularly in the field of radio frequency (RF) components that process signals, typically at frequencies between 20 kHz and 300 GHz or higher. The thin layer of the composite substrate can be composed of a semiconductor material (e.g., silicon) or an insulating material (e.g., a material having piezoelectric and / or ferroelectric properties). In addition to the method for preparing the carrier itself, this invention also relates to a method for manufacturing a composite substrate containing the carrier and on which a thin layer has been transferred. Background Technology

[0002] The prior art is rich with solutions aimed at forming carriers with charge-trapping layers (more simply referred to as "trapping layers" in the remainder of this specification). Such carriers are used as the base substrate for forming composite substrates, such as silicon-on-insulator type. The trapping layer allows for the limitation of electromagnetic coupling between the carrier and RF components formed in or on a thin layer of the composite substrate. The electron trapping of the trapping layer limits the mobility of charge carriers and thus limits their interaction with the electromagnetic fields generated by high-frequency signals propagating into the bulk of the composite substrate. Therefore, the quality of useful signals is maintained while limiting their nonlinear distortion, insertion loss, and possible crosstalk between components.

[0003] Ideally, a trapping layer is formed with a high density of structural defects, such as dislocations, grain boundaries, amorphous regions, interstitials, inclusions, and pores. These structural defects, for example, create traps of charge that easily flow through the material at sites of incomplete or dangling chemical bonds. Therefore, conductivity is prevented in the trapping layer, resulting in a high resistivity.

[0004] Document US2017062284 proposes forming such a barrier layer from porous silicon. However, this layer has proven to be chemically and mechanically brittle. It is prone to degradation during processes applied to the carrier, particularly during the transfer of thin single-crystal layers in the fabrication of composite substrates. These processes can mechanically stress the carrier, for example, during the transfer of the thin layer by separating the donor substrate that has been pre-bonded thereto. They can also chemically stress it, for example, during the cleaning or wet etching steps of a finishing process for finishing composite substrates, after the thin layer transfer.

[0005] Reference US2018047614 proposes, for example, using amorphous or polycrystalline silicon, to fill the pores of a porous layer through physical vapor deposition or chemical vapor deposition of a filling material. Reference US2022359272 itself proposes forming a mesoporous layer with hollow pores, the inner walls of which are mainly covered with oxides by annealing in an oxidizing atmosphere.

[0006] Purpose of the invention

[0007] The object of this invention is to provide an alternative method for forming a charge trapping layer composed of a porous layer that is mechanically and chemically robust. Summary of the Invention

[0008] To achieve this objective, the present invention provides a method for preparing a support for a composite substrate, the method comprising: - A first step of porosification aimed at forming a porous surface layer on the first surface (1c) of the carrier; - A second step of providing a viscous solution comprising a solvent and a precursor for a filler material; - A third step of dispensing the viscous solution onto the first surface of the carrier in order to absorb at least a portion of the viscous solution into the open pores of the porous surface layer; - The fourth step of heat treatment of the carrier is to transform the viscous solution present in the open pores, thereby filling the open pores with the filling material.

[0009] Because at least some of the pores are completely filled with the filling material, the trapping layer has good mechanical strength and chemical resistance.

[0010] Other advantageous and non-limiting features of the invention may be adopted individually or in any technically feasible combination: - The carrier has a resistivity of less than 10 ohm·cm, preferably between 1 and 2 ohm·cm; - The first step of the porosification is carried out by etching in an acid bath, which in particular includes a mixture of nitric acid and hydrofluoric acid; - The first step of the porosification is carried out electrochemically or photoelectrochemically; - The porous surface layer has a thickness between 100 nm and 20 micrometers; - The third step of the distribution is carried out by centrifugation; - The filling material is a dielectric; - The filler material is silicon dioxide; - The precursor of the filler material is a hydrogen silsesquioxane resin; - The method includes a preparatory step of evaporating the solvent before the fourth step of the heat treatment; - The fourth step of the heat treatment includes exposing the carrier to a temperature between 300°C and 1100°C; - A cover layer including the filling material is also formed on and in contact with the porous surface layer; - The preparation method according to the preceding claim, wherein the covering layer is removed.

[0011] According to another aspect, the subject matter of the present invention provides a method for manufacturing a composite substrate, the method comprising providing a carrier prepared as described above, and transferring a thin single-crystal layer onto a first surface of the carrier.

[0012] The thin single-crystal layer may be composed of silicon, silicon carbide, or piezoelectric materials. Attached Figure Description

[0013] Other features and advantages of the invention will become apparent from the following detailed description of the invention with reference to the accompanying drawings, in which: [ Figure 1 ] Figure 1 A carrier according to the present invention is shown; [ Figure 2 ] Figure 2 A composite substrate utilizing a carrier according to the present invention is shown; [ Figure 3a ] [ Figure 3b ] [ Figure 3c ] [ Figure 3d ] Figure 3a , 3b Figures 3c and 3d illustrate a method for preparing a carrier according to the present invention. Detailed Implementation

[0014] Figure 1 A carrier 1 according to the invention is shown, which is designed to receive a thin crystalline layer by a layer transfer technique in order to form Figure 2The composite substrate S is shown. Generally, the carrier 1 includes a trapping layer 1a formed of a porous layer on its first surface 1c, at least some of the pores of which are completely filled with a filling material. Preferably, a majority proportion of the pores are filled with the filling material, for example, more than 50%, more than 60%, or even more than 95%. This material may specifically include or be composed of a dielectric (e.g., silicon oxide). Due to its construction, the trapping layer 1a has a high resistivity, typically greater than 10 kΩ·cm over its entire thickness, and exhibits good mechanical strength and chemical resistance.

[0015] The pores can have any shape, and the porous layer in which at least some of the pores have been filled with a filling material can have macroporous properties (pore diameter greater than 50 nm), mesoporous properties (pore diameter between 2 nm and 50 nm), and / or nanoporous properties (pore diameter less than 2 nm). However, it is preferred to choose a material with mesoporous properties. The porosity is typically between 40% and 60%, but this is not a necessary feature of the invention.

[0016] Those skilled in the art will be able to select the porosity and pore properties based on the properties of the filler material, which ensures a good balance between the mechanical and electrical properties required for the trapping layer 1a.

[0017] The thickness of the trapping layer 1a is typically between 100 nm and 20 micrometers, but preferably greater than 1 micrometer or greater than 10 micrometers. The relatively thick trapping layer allows for better electromagnetic isolation of the carrier component. The portion 1b of the carrier 1 located below the trapping layer 1a itself has a thickness of several hundred micrometers to ensure the mechanical strength of the component.

[0018] like Figure 1 As shown by the dashed lines, a carrier 1 can be provided having a dielectric layer 2 on and in contact with the trapping layer 1a, but this dielectric layer 2 is entirely optional. When present, it can consist of or contain silicon oxide, silicon nitride, and / or silicon oxynitride.

[0019] Traditionally, in the field of substrates used for integrated devices, the carrier 1 can be in the form of a circular wafer with a diameter of 100, 150, 200, 300 or even 450 mm.

[0020] Preferably, the starting support in which the trapping layer 1a has been formed is made of monocrystalline silicon. It advantageously has a relatively low resistivity of less than 10 ohm·cm, preferably on the order of 1 to 2 ohm·cm, but this does not preclude the possibility that the substrate may have different resistivities, as will be disclosed in the remainder of this specification. It may be p-type or n-type.

[0021] To form the capture layer 1a, refer to Figures 3a to 3d , Figures 3a to 3dA method for preparing a carrier according to the present invention is shown, wherein a first step of porosimetry is applied to a starting carrier with the aim of forming a porous surface layer P on at least one surface 1c (referred to as "first surface" in the remainder of this specification) of the starting carrier.

[0022] This step can be performed in several ways. According to the first method, the first porosification step is carried out by etching in an acid bath, particularly a bath containing a mixture of nitric acid and hydrofluoric acid. The physical phenomena resulting from this chemical treatment leading to the formation of a porous surface layer can be described in the paper “Porous-Silicon formation in HF–HNO3–H2O Etchants” by Starostina et al., Russian Microelectronics 31, 88–96 (2002).

[0023] This method tends to form a porous surface layer P with a relatively thin thickness, on the order of 1 micrometer or less. The relatively low resistivity of the carrier 1 is conducive to the formation of the porous layer, which can lead to selecting the carrier 1 with a low resistivity on the order of 10 ohm·cm or less. It is conceivable to treat the carrier 1 to have a low resistivity thickness on its surface, and then the rest of the carrier 1b can have a more freely chosen resistivity. This can include introducing a dopant into this surface thickness or forming a doped surface layer by deposition.

[0024] According to another method, the first porosimetry step is carried out electrochemically or photoelectrochemically by electrolysis, utilizing the anodic dissolution phenomenon in an acidic medium. Specifically, a support 1 is placed in an electrochemical cell, its first surface 1c exposed to an electrolyte solution based on hydrofluoric acid (HF). This solution can have an HF concentration greater than 30% and may contain additives (e.g., isopropanol (IPA) or ethanol). The support 1 is in contact with the anode, for example, a surface contact on the opposite side of the first surface 1c, and the cathode is positioned facing the first surface 1c to allow current to flow through the electrolyte and the support 1. The current density flowing in the cell (typically between 1 and 50 mA / cm²) 2 This process (between the two sides) results in the formation of a porous surface layer on the first surface 1c of the carrier 1 (i.e., the surface exposed to the electrolyte). This method allows for the formation of relatively thick porous layers P on the order of 10 micrometers or even 20 micrometers by extending the duration of this step.

[0025] It should be noted that other electrochemical unit structures exist, making it possible to form a porous surface layer P. In particular, a "two-unit" structure is known (see, for example, US11049724), in which two opposite sides of the starting support are contacted with the same electrolyte solution or different solutions. In this case, one of the sides can be irradiated, causing that side to generate charge carriers during the electrochemical phase.

[0026] Regardless of how the first porosimetry step is performed, at the end of this step, a porous surface layer P is obtained on the first surface 1c of the carrier 1. The pores constituting the porous surface layer P may be open for some of them and appear (directly or indirectly) on the first surface 1c of the carrier. The parameters of this step can be controlled to obtain a given porosity and given properties of the pores (particularly their average size), as mentioned in the preceding paragraph of this specification. The porosimetry time (all others being equal) defines the thickness of the surface porous layer P, which is intended to form a trapping layer. Preferably, this thickness will be selected between 100 nm and 20 micrometers, as already explained.

[0027] In the second step of preparing carrier 1, a viscous solution comprising a solvent and a filler material precursor is prepared. This solution is intended to fill the pores of the porous layer P, therefore it must have a low viscosity to allow it to permeate and flow into the pores of layer P. The viscosity of this solution can be significantly controlled by adjusting the proportion of solvent in the solution.

[0028] In addition to the solvent, the solution also contains precursors to the filler material. “Precursors to the filler material” means any material that, once the solvent is removed from the viscous solution, directly or after processing, results in the acquisition or formation of the filler material.

[0029] In a preferred embodiment, the filler material is silicon dioxide, and its precursor is silsesquioxane. Sisesquioxane has the chemical formula [HSiO]. 3 / 2 ] n An inorganic compound, after heat treatment that removes at least a portion of the hydrogen contained therein, forms a silicon-rich oxide (SiO) that is chemically resistant to etchants such as tetramethylammonium hydroxide (TMAH or TMAOH). It can be combined with organic solvents such as methyl isobutyl ketone (commonly known as MIBK) to form a viscous solution.

[0030] However, the present invention is by no means limited to silicon oxide filler materials and the precursor. By using silicon and nitrogen-based precursors, such as silazane, disilazane, or polysilazane-type compounds, filler materials based on silicon nitride, silicon oxynitride, or alloys containing silicon and carbon can be envisioned.

[0031] The use of these filler precursor polymers is documented, for example, in US4312970A or US4756977A.

[0032] In the third step of the method for preparing the carrier substrate 1, a viscous solution is dispensed onto a first surface 1c of the carrier 1 so that at least a portion of the viscous solution is absorbed into the open pores of the porous surface layer P. This dispensing step is advantageously designed to uniformly coat the first surface 1c of the carrier so that these open pores can be filled over the entire area of ​​the first surface 1c. Figure 3b The step shown can be performed specifically by spin coating, dip coating, spray coating, or flow coating.

[0033] The low viscosity of the solution allows it to penetrate into the pores of the surface porous layer P to fill them. It should be noted that the pores of this layer, which are not directly or indirectly connected to the first surface 1c of the carrier, cannot receive the viscous solution by flow. However, since these blind pores are not the main pores in the surface porous layer, this does not pose any significant problem.

[0034] If an excess of the viscous solution is supplied, it tends to form a capping layer 4 on the porous surface layer P and come into contact with the porous surface layer P. Due to the viscous nature of the solution, especially when distributed by centrifugation, this capping layer can be several micrometers thick and very uniform.

[0035] Optionally, a solvent evaporation step may be provided at this stage of the method. This evaporation step can be achieved by simply drying the carrier or by raising its temperature in an oven, for example, to a moderate temperature between 50°C and 300°C.

[0036] Regardless of whether the evaporation step is applied, the method for preparing the carrier 1 according to the invention provides a fourth heat treatment step after the third step of distributing the viscous solution. Figure 3c As shown, the purpose of this step is to transform the viscous solution present in the pores of the porous layer, thereby allowing them to be filled by the filling material. If no preliminary solvent evaporation step is applied, the solvent is removed from the solution during this heat treatment. This also results in the removal of volatile substances present in the precursor of the filling material, so that the pores are effectively filled by the material at the end of the heat treatment. For example, when the precursor of the viscous solution is a hydrogen silsesquioxane, the heat treatment causes at least some of the hydrogen to be removed from the precursor (in this case, forming volatile substances) so that the pores can be filled with a silicon-rich and hydrogen-poor oxide.

[0037] This heat treatment is certainly suitable for the properties of viscous solutions, and more specifically for the properties of precursors present in such solutions. When the precursor is a hydrogen silsesquioxane, annealing can be performed between 300°C and 1100°C.

[0038] When the capping layer 4 is present on the carrier, the heat treatment step also causes the adhesive material constituting the capping layer to be transformed. If the carrier is not desired, this layer can be removed, for example, by a polishing step. Alternatively, if the filler material is essentially a dielectric, it can be retained and contribute to the formation of the dielectric layer 3 of the carrier 1.

[0039] Regardless of whether the capping layer 4 is retained, the preparation method according to the present invention may further include the step of forming a dielectric layer 2 on a carrier by conventional deposition methods.

[0040] Figure 3d The carrier substrate 1 obtained at the end of this process is shown (in the example shown, no dielectric layer 2 is formed). The initially formed porous surface layer P has been transformed by the trapping layer 2, which is formed by silicon and pores filled with filler material.

[0041] As already mentioned, the carrier substrate 1 is intended to receive the thin layer 3 by transfer, and thus form the composite substrate S. Figure 2 The thin layer 3 is illustrated in the figure. The thin layer 3 is typically crystalline in nature and advantageously single-crystal. The carrier 1 has suitable properties (particularly in terms of surface roughness and deformation) or has been pre-treated (e.g., by polishing) to receive such a thin layer 3. The composite substrate S includes a dielectric layer 2 that contacts the carrier 1 and the thin layer 3 and is interposed between them. This dielectric layer has a thickness selected based on the properties of the components to be formed in and on the thin layer 3, and therefore on the application targeted by the composite substrate S. For example, this thickness can be between 10 nm and 10 micrometers.

[0042] As is known per se, the transfer of the thin layer 3 onto the carrier 1 is typically achieved by bonding the free surface of the donor substrate to the first surface 1c of the carrier 1, preferably by molecular adhesion. This is accomplished by providing a surface dielectric layer to at least one of these surfaces, which collectively form the dielectric layer 2 of the composite substrate S.

[0043] The properties of the donor substrate are selected based on the desired properties of thin layer 3. Therefore, it can be a substrate formed of a single-crystal semiconductor (e.g., silicon), a substrate formed of a single-crystal piezoelectric material, or a substrate including a surface layer of such a single-crystal piezoelectric material. In this case, it can be lithium tantalate or lithium niobate.

[0044] The donor substrate may also have completed or semi-complete components, and the transfer is intended to place these components on carrier 1 in order to take advantage of its radio frequency characteristics.

[0045] Just like carrier 1, the donor substrate can take the form of a circular wafer, the size of which can correspond to the size of the carrier.

[0046] Following this bonding step, the thickness of the donor substrate is reduced to form a thin layer 4. This thickness reduction step can be performed by mechanical or chemical thinning, particularly when the donor substrate includes components that are intended to be placed on the carrier 1. The reduction in the thickness of the donor substrate can preferably be achieved by separation at a weakened plane previously introduced into the donor substrate, for example by implanting a light material such as hydrogen and / or helium. This weakened plane, together with the free surface of the donor substrate, defines the thin layer 3.

[0047] After the thinning or preferably separation step, steps for finishing the thin layer 3 can be applied, such as polishing, heat treatment in a reducing or inert atmosphere, sacrificial oxidation, etc.

[0048] At the end of these steps, a composite substrate S is obtained, which is formed by a thin single-crystal layer transferred onto the carrier 1.

[0049] Needless to say, the present invention is not limited to the described embodiments, and variations in its implementation are possible without departing from the scope of the invention as defined by the claims.

Claims

1. A method for preparing a support (1) for a composite substrate (S), the method comprising: - A first step of porosification aimed at forming a porous surface layer (P) on the first surface (1c) of the carrier (1); - A second step of providing a viscous solution comprising a solvent and a precursor for a filler material; - A third step of dispensing the viscous solution onto the first surface (1c) of the carrier (1) so as to absorb at least a portion of the viscous solution into the open pores of the porous surface layer (P); - The fourth step of heat treatment of the carrier (1) is to transform the viscous solution present in the open pores so that the open pores are filled by the filling material.

2. The preparation method according to claim 1, wherein, The carrier (1) has a resistivity of less than 10 ohm·cm, preferably between 1 ohm·cm and 2 ohm·cm.

3. The preparation method according to any one of claims 1 to 2, wherein, The first step of the porosification is carried out by etching in an acid bath, particularly a bath comprising a mixture of nitric acid and hydrofluoric acid.

4. The preparation method according to any one of claims 1 to 3, wherein, The first step of porosification is carried out electrochemically or photoelectrochemically.

5. The preparation method according to any one of claims 1 to 4, wherein, The porous surface layer (P) has a thickness between 100 nm and 20 micrometers.

6. The preparation method according to any one of claims 1 to 5, wherein, The third step of the distribution is performed by centrifugation.

7. The preparation method according to any one of claims 1 to 6, wherein, The filling material is a dielectric.

8. The preparation method according to any one of claims 1 to 7, wherein, The filler material is silicon dioxide.

9. The preparation method according to any one of claims 1 to 8, wherein, The precursor of the filler material is a hydrogen silsesquioxane resin.

10. The preparation method according to any one of claims 1 to 9, wherein, The method includes a preparatory step of evaporating the solvent before the fourth step of the heat treatment.

11. The preparation method according to any one of claims 1 to 10, wherein, The fourth step of the heat treatment includes exposing the carrier (1) to a temperature between 300°C and 1100°C.

12. The preparation method according to any one of claims 1 to 11, wherein, A cover layer (4) including the filling material is also formed on and in contact with the porous surface layer (P).

13. The preparation method according to any one of claims 1 to 12, wherein, The covering layer (4) is removed.

14. A method of manufacturing a composite substrate (S), the method comprising providing a carrier (1) prepared as described in any one of claims 1 to 13, and transferring a thin single-crystal layer (4) onto the first surface of the carrier.

15. The manufacturing method according to claim 14, wherein, The thin single-crystal layer (4) is composed of silicon, silicon carbide or piezoelectric material.

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