Carrier comprising a layer for trapping electrical charges for a composite substrate

Abstract

The invention relates to a method for preparing a carrier (1) for a composite substrate (S), which method comprises forming a superficial porous layer (P) on a first face (1c) of the carrier (1), and dispensing a viscous solution comprising a solvent and a precursor of a filler material on the first face (1c) of the carrier (1) so as to absorb at least some of the viscous solution in open pores of the superficial porous layer (P). In a fourth step, the carrier (1) is heat-treated to transform the viscous solution present in the open pores in order to fill the open pores with the filler material.

Description

SUPPORT COMPRISING AN ELECTRICAL CHARGE TRAPPING LAYER FOR A COMPOSITE SUBSTRATE. FIELD OF THE INVENTION

[0001] The invention relates to a method for preparing a support having an electric charge trapping layer, the support being intended to receive a thin crystalline layer by a layer transfer technique. A composite substrate formed from such a support finds its application in the field of integrated electronic components, in particular radiofrequency (RF) components processing signals whose frequency may typically be between 20 kHz and 300 GHz, or more. The thin layer of the composite substrate may consist 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 support as such, the invention also relates to the method for manufacturing the composite substrate comprising this support and on which the thin layer has been transferred. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] The state of the art is full of solutions aimed at forming a support provided with an electric charge trapping layer (more simply “trapping layer” in the remainder of this description). Such a support is intended to form the base substrate of a composite substrate, for example of the silicon-on-insulator type. The trapping layer makes it possible to limit the electromagnetic coupling which occurs between an RF component formed in or on the thin layer of the composite substrate and this support. The electric traps of the trapping layer limit the mobility of the charge carriers and therefore the interactions with the electromagnetic fields resulting from the high-frequency signals produced by the RF components propagating in the depth of the composite substrate. This preserves the quality of the useful signals by limiting their non-linear distortion, insertion losses and possible influences between components.

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

[0004] Document US2017062284 proposes forming this trapping layer in porous silicon. However, such a layer is chemically and mechanically fragile. It is likely to degrade during the dragging applied to the support, in particular the transfer treatments of the monocrystalline thin layer during the manufacture of the composite substrate. These treatments can mechanically stress the support, for example during the transfer of the thin layer by fracturing a donor substrate that has been previously assembled to it. 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 transfer of the thin layer.

[0005] Document US2018047614 proposes filling the pores of the porous layer, for example using amorphous or polycrystalline silicon, by physical vapor deposition or by chemical vapor deposition of the filling material. Document US2022359272 proposes forming a mesoporous layer having hollow pores whose internal walls are mainly lined with oxide by annealing in an oxidizing atmosphere. SUBJECT OF THE INVENTION

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

[0007] In order to achieve this aim, the subject of the invention proposes a method for preparing a support for a composite substrate, the preparation method comprising: a first porosification step aimed at forming a surface porous layer on a first face (1c) of the support; a second step of providing 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 in open pores of the surface porous layer; a fourth step of heat treatment 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 punching layer has good mechanical and chemical resistance.

[0009] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: the support has a resistivity of less than 10 ohm centimeters, preferably between 1 and 2 ohm centimeters; 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; the first porosification step is carried out electrochemically or photoelectrochemically; the surface porous layer has a thickness of between 100 nm 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 filling material is a hydrogenated silsesquioxane resin. the method comprises, before the fourth heat treatment step, a preliminary step of evaporation of the solvent.the fourth heat treatment step comprises exposing the support to a temperature between 300°C and 1100°C; a covering layer comprising the filling material is also formed on and in contact with the surface porous layer; Preparation method according to the preceding claim in which the covering layer is removed.

[0010] According to another aspect, the subject of the invention provides a method of manufacturing a composite substrate comprising the provision of a support prepared as presented above and the transfer of a thin monocrystalline layer onto the first face of the support.

[0011] The monocrystalline thin layer can be made of silicon, silicon carbide or a piezoelectric material. BRIEF DESCRIPTION OF THE FIGURES

[0012] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

[0013]

[0014] Represents a support in accordance with the invention;

[0015]

[0016] The represents a composite substrate using a support in accordance with the invention;

[0017]

[0018]

[0019]

[0020]

[0021] Figures 3a, 3b, 3c and 3d illustrate a method of preparing a support in accordance with the invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] La represents a support 1 in accordance with the invention and intended to receive, by a layer transfer technique, a thin crystalline layer to form a composite substrate S, represented on the. In a very general manner, the support 1 comprises, on the side of a first face 1c, a trapping layer 1a formed from a porous layer of which at least some of the pores are entirely 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 in particular comprise or consist of a dielectric such as a silicon oxide. Due to its constitution, the trapping layer 1a has a high resistivity, typically greater than 10 kohm.cm over its entire thickness and good mechanical and chemical resistance.

[0023] The morphology of the pores may be arbitrary and the porous layer, at least some of the pores of which have been filled with the filling material, may be macroporous (pore diameter greater than 50nm), mesoporous (pore diameter between 2nm and 50nm) and / or nanoporous (pore diameter less than 2nm) in nature. However, a mesoporous material will preferably be chosen. The porosity rate is typically between 40% and 60%, but this does not form an essential characteristic of the invention.

[0024] The person skilled in the art will be able to choose, depending on the nature of the filling material, the porosity rate and the nature of the pores which ensure a good balance between the mechanical properties and the desired electrical properties of the trapping layer 1a.

[0025] The trapping layer 1a 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 better electromagnetic insulation of the components of the support. The part 1b of the support 1 located under the trapping layer 1a 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 the, with a dielectric layer 2, on and in contact with the trapping layer 1a, but this dielectric layer 2 is perfectly optional. When it is present, it can be made of or comprise silicon oxide, silicon nitride and / or a silicon oxynitride.

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

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

[0029] To form the trapping layer 1a, and with reference to figures 3a to 3d which illustrate a method for preparing a support in accordance with the invention, a first porosification step is applied to the starting support aimed at forming a surface porous layer P on at least one of its faces 1c (designated “first face” in the remainder of this description).

[0030] This step can be carried out in several ways. According to a first approach, 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. Reference may be made to the document by Starostina et al “Porous-Silicon Formation in HF–HNO3–H2O Etchants”, RussianMicroelectronics 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 surface porous layer P whose thickness is relatively reduced, of 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 can lead to choosing this support 1 so that it has low resistance, of the order of 10 Ohm.cm or less. It is possible to consider treating a support 1 so that it has a low resistance thickness on the surface, the rest of the support 1b then being able to have a more freely chosen resistivity. 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 a phenomenon of anodic dissolution in an acid medium, by electrolysis. In practice, the support 1 is placed in an electrochemical cell, its first face 1c 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 called IPA or ethanol). The support 1 is in contact with an anode, for example in contact with the face opposite the first face 1c, a cathode being placed opposite the first face 1c to allow the circulation of a current through the electrolyte and the support 1. The current density circulating in the cell (typically between 1 and 50mA / cm 2) leads to the formation of the superficial porous layer on the side of the first face 1c of the support 1, i.e. the one exposed to the electrolyte. This approach makes it possible to form a relatively thick porous layer P, of the order of 10 micrometers, or even 20 micrometers, by extending the duration of this step.

[0033] It is noted that there are other electrochemical cell configurations for forming the surface porous layer P. In particular, so-called "double cell" configurations are known (see for example document US11049724), in which the two opposite faces of the starting 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 the manner in which this first porosification step was carried out, at the end of this step there is a superficial porous layer P on a first face 1c of the support 1. The pores constituting this superficial porous layer P may, for some of them, be open and open (directly or indirectly) onto the first face 1c of the support. The parameters of this step can be controlled to obtain a determined porosity rate and nature of the pores (their average dimensions in particular), as mentioned in a previous passage of this description. The porosification duration 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 preparation of the support 1, a viscous solution is prepared comprising a solvent and a precursor of a filling material. This solution is intended to fill the pores of the porous layer P, it must therefore 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 includes a filler precursor. "Filler precursor" means any material that, directly or after processing, leads to the formation of 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. Hydrogenated silsesquioxane is an inorganic compound of formula [HSiO 3 / 2] n which, after heat treatment leading to the evacuation of at least part of the hydrogen it contains, forms a silicon-rich oxide SiO chemically resistant to the etching product such as tetramethylammonium hydroxide (TMAH or TMAOH). It can be combined with an organic solvent, for example methyl isobutyl ketone (often noted MIBC or MIBK in English), to form the viscous solution.

[0038] However, the invention is in no way limited to a silicon oxide filling material and this precursor. It is possible to envisage a filling material based on silicon nitride, silicon oxynitide or an alloy comprising silicon and carbon 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 filler material precursor polymers is for example documented in US4312970A or US4756977A.

[0040] In a third step of the method for preparing the support substrate 1, the viscous solution is dispensed onto the first face 1c 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 1c of the support uniformly to allow the open pores to be filled over the entire extent of the first face 1c. This step, illustrated in the, can in particular be implemented by centrifugation (“spin coating” according to the English terminology of the field), by immersion (“dip coating”), by spray coating or by simple flow coating.

[0041] 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 do not communicate, directly or indirectly, with the first face 1c of the support cannot receive, by flow, the viscous solution. However, this does not pose a clear problem since these non-opening pores are not in the majority in the surface porous layer.

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

[0043] Optionally, at this stage of the process, a solvent evaporation step can be provided. This evaporation step can be obtained 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 this evaporation step has been applied or not, a method for preparing a support 1 in accordance with the invention provides, after the third step of dispensing the viscous solution, a fourth heat treatment step. This step, shown in the, aims to transform the viscous solution present in the open pores of the porous layer so that they are filled with the filling material. If the prior step of evaporation of the solvent has not been applied, the solvent is removed from the solution during this heat treatment. This also leads to the removal of the volatile species present in the precursor of the filling material, so that, at the end of this heat treatment, the pores are actually filled with this material.For example, when the precursor of the viscous solution is hydrogenated silsesquioxane, the heat treatment leads to the evacuation of at least part of the hydrogen from the precursor (in this case forming the volatile species) to fill the pores with an oxide rich in silicon and depleted in hydrogen.

[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 is hydrogenated silsesquioxane, the annealing can be between 300°C and 1100°C.

[0046] When a covering layer 4 is present on the support, the heat treatment step also leads to the transformation of the viscous material making up the covering layer. It is possible to eliminate this layer, if it is not desirable for the use of the support, for example by a polishing step. Alternatively, it can be preserved and contribute to forming the dielectric layer 3 of the support 1, if the filling material is actually dielectric in nature.

[0047] Whether this covering layer 4 is preserved or not, a preparation method in accordance with the invention may also comprise a step of forming the dielectric layer 2 on the support, by a conventional deposition method.

[0048] La 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 was transformed by the trapping layer 2 formed of silicon and pores filled with a filling material.

[0049] As already mentioned, the support substrate 1 is intended to receive, by transfer, a thin layer 3 and thus form a composite substrate S, shown for illustration purposes in the. The thin layer 3 is generally of a crystalline nature, and advantageously monocrystalline. The support 1 has suitable properties (in terms of surface roughness and deformation in particular) or has been previously treated (for example by polishing) to receive such a thin layer 3. The composite substrate S comprises, in contact and interposed between the support 1 and the thin layer 3, a dielectric layer 2. This dielectric layer has a thickness chosen according to the nature of the components which will be formed in and on the thin layer 3 and therefore 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 per se, the transfer of the thin layer 3 onto the support 1 is usually carried out by assembling a free face of a donor substrate to the first face 1c of the support 1, preferably by molecular adhesion. It is planned to provide at least one of these faces with a surface dielectric layer, these layers forming in combination 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 layer 3. It may therefore be a substrate formed from a monocrystalline semiconductor, for example silicon, or a substrate formed from a monocrystalline piezoelectric material or comprising a surface layer of such a monocrystalline 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 transfer aiming to place these components on the support 1 to take advantage of its radiofrequency properties.

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

[0054] After this assembly step, the donor substrate is reduced in thickness to form the thin layer 4. This reduction step can be carried out by mechanical or chemical thinning, in particular when the donor substrate comprises components that it is desired to place on the support 1. The reduction in thickness of the donor substrate can preferably be carried out by fracture at a weakening plane previously introduced into the donor substrate, for example by implantation of light species such as hydrogen and / or helium. This weakening plane defines, with the free surface of the donor substrate, the thin layer 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 thin monocrystalline layer transferred onto the support 1.

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

Claims

A method of preparing a support (1) for a composite substrate (S), the preparation method comprising:a first porosification step aimed at forming a surface porous layer (P) on a first face (1c) of the support (1);a second step of providing 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 (1c) of the support (1) so as to absorb at least part of the viscous solution into open pores of the surface 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 filling material. Preparation process 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. Preparation process according to one of the preceding claims in which 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. Preparation process according to one of claims 1 to 3 in which the first porosification step is carried out electrochemically or photoelectrochemically. Preparation process according to one of the preceding claims in which the surface porous layer (P) has a thickness of between 100 nm and 20 micrometers. Preparation process according to one of the preceding claims in which the third dispensing step is carried out by centrifugation. Preparation process according to one of the preceding claims in which the filling material is a dielectric. Preparation process according to one of the preceding claims in which the filling material is silicon oxide. Preparation process according to one of the two preceding claims in which the precursor of the filling material is a hydrogenated silsesquioxane resin. Preparation process according to one of the preceding claims in which the process comprises, before the fourth heat treatment step, a preliminary step of evaporation of the solvent. Preparation process according to one of the preceding claims in which the fourth heat treatment step comprises exposing the support (1) to a temperature between 300°C and 1100°C. Preparation method according to one of the preceding claims, in which a covering layer (4) comprising the filling material is also formed on and in contact with the surface porous layer (P). Preparation method according to the preceding claim, in which the covering layer (4) is removed. Method for manufacturing a composite substrate (S) comprising providing a support (1) prepared according to one of the preceding claims and transferring a thin monocrystalline layer (4) onto the first face of the support. Manufacturing method according to the preceding claim in which the monocrystalline thin layer (4) is made of silicon, silicon carbide or a piezoelectric material.

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