Method for manufacturing a photonic substrate

Abstract

The invention relates to a method for manufacturing a photonic substrate (1) comprising the preparation of a base substrate having a dielectric thickness (3') on and in contact with a trapping layer (4) and an interlayer made of an oxidizable material on and in contact with the dielectric thickness (3''). The method also comprises transferring an electro-optical layer (2) onto the interlayer to form an intermediate substrate. According to the invention, the intermediate substrate is thermally treated in an atmosphere containing oxygen at a temperature and for a duration sufficient to allow the diffusion of atmospheric oxygen through the electro-optical layer (2) and to allow the complete oxidation of the interlayer. Figure 1

Description

Title of the invention: Method for manufacturing a photonic substrate. FIELD OF THE INVENTION

[0001] The present invention relates to a photonic substrate, that is, a substrate that can be used to manufacture photonic components and to receive these components. This substrate finds a particular application in forming an optical modulation device. The invention relates more particularly to the manufacture of a photonic substrate comprising an embedded dielectric layer, this layer having a relatively large thickness. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Optical transmitters are devices that convert digital bit streams from the electrical domain to the optical domain, at rates ranging from a few tens of gigabits per second (Gbps) to hundreds of Gbps, typically between 200 Gbps and 400 Gbps for high-performance transmitters. Such a transmitter usually comprises a continuous-wave laser source and an optical modulator that encodes the binary stream into a light beam whose phase or intensity is modulated.

[0003] It is known in the prior art, for preparing such an optical modulator, to use a thin-film photonic substrate, consisting of a lithium niobate layer transferred onto a support via a dielectric layer. These modulators are sometimes referred to as thin-film modulators. Optical waveguides and electrodes are formed on and within the lithium niobate layer in order to guide the light radiation and modulate it by exploiting the Pockels effect.

[0004] For an introduction to thin-film photonic substrates, reference may be made to the document by Martin F. Volk et al, "Low loss ridge waveguides in lithium niobate thin films by optical grade diamond blade dicing," Opt. Express 24, 1386-1391 (2016).

[0005] For most of the applications envisaged, optical transmitters have strict performance requirements. The modulators must operate at very high speeds (typically an electro-optical bandwidth >67 GHz), be linear, and energy-efficient. This is recalled in the document by Prashanta Kharel, et al., "Breaking voltage-bandwidth limits in integrated lithium niobate modulators using micro-structured electrodes," Optica 8, 357-363 (2021).

[0006] These performance requirements impose strict characteristics on the photonic substrate used to form the modulators, both optically (low propagation loss, low device variability) and electrically (transmission losses over the targeted bandwidth). Transmission losses must be as low and frequency-equalized as possible for the modulator to be fast, linear, and energy-efficient.

[0007] The document by Gengxin Chen, et al., “High performance thin-film lithium niobate modulator on a Silicon substrate using periodic capacitively loaded traveling-wave electrode”, APL Photonics, 1 February 2022, proposes to reduce transmission losses in the thin-film photonic substrate by creating cavities in the silicon support, under the dielectric layer and the lithium niobate thin film. SUBJECT OF THE INVENTION

[0008] One object of the invention is to offer an alternative to prior art solutions. More specifically, one object of the invention is to provide a method for manufacturing a photonic substrate, this substrate enabling the formation of an optical modulation device capable of limiting transmission losses in this device. BRIEF DESCRIPTION OF THE INVENTION

[0009] With a view to achieving one of these goals, the object of the invention proposes a method for manufacturing a photonic substrate comprising the following steps: a. provide a support having a first face and a second face, opposite to the first; b. form a layer for trapping electrical charges on the first face of the support; c. form a dielectric thickness on and in contact with the electrical charge trapping layer; d. form an intercalary layer consisting of an oxidizable material on and in contact with the dielectric thickness; e. transfer an electro-optical layer made of a single-crystal material of optical quality, onto and in contact with the intercalated layer to form an intermediate substrate; f. heat-treat the intermediate substrate in an atmosphere comprising oxygen at a temperature and for a duration sufficient to allow diffusion of atmospheric oxygen through the electro-optical layer and to allow complete oxidation of the intercalated layer.

[0010] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: the electro-optical layer of the intermediate substrate has a surface directly exposed to the atmosphere including oxygen during heat treatment, the intermediate substrate being devoid of any surface layer capable of limiting or blocking the diffusion of oxygen through the electro-optical layer during this heat treatment; the manufacturing process includes a step of forming an amorphous layer on the first face of the support before the step of forming the electrical charge trapping layer; the step of forming the electrical charge trapping layer includes the formation of a complementary layer on the second face of the support; the dielectric thickness formation step includes the formation of a compensation layer on the side of the second face of the support; the manufacturing process further includes a finishing step of the electro-optical layer, before or after the heat treatment step of the intermediate substrate; the layer transfer step includes the assembly of a donor substrate with the interlayer layer; the donor substrate consists of a bulk substrate made of optical-grade single-crystal material; the donor substrate consists of a surface layer of optical-grade single-crystal material arranged on a manipulator substrate; the layer transfer step includes, before the assembly of the donor substrate with the intercalated layer, the formation of a brittle plane in the donor substrate by the introduction of light species of helium and / or hydrogen and, after the assembly of the donor substrate with the intercalated layer, the fracturing of the donor substrate at the level of the brittle plane; the optical-grade single-crystal material of the electro-optical layer is a ferromagnetic material such as lithium niobate; the heat treatment atmosphere is brought to a temperature between 1000°C and the Curie temperature of the ferromagnetic material; the formation of the intercalated layer includes the deposition of a layer of amorphous or polycrystalline silicon; the intercalated layer has a thickness between 0.5 micrometers and 10 micrometers; the dielectric thickness has a thickness less than or equal to 5 micrometers; - the dielectric thickness includes a second dielectric layer made of a material that forms a barrier to the diffusion of certain electrically conductive species present in the electro-optical layer. Brief description of the drawings

[0011] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the single attached figure in which:

[0012] [Fig.l] [Fig.2] [Fig.3] [Fig.5]

[0013] Figures 1, 2, 3 and 5 represent implementation methods of a photonic substrate;

[0014] [Fig.4]

[0015] Figure 4 represents a method, according to the invention, for manufacturing a photonic substrate. DETAILED DESCRIPTION OF THE INVENTION

[0016] For the sake of simplicity in the following description, the same references are used for identical elements or elements performing the same function in the different modes of implementation of the invention or in the presentation of the prior art.

[0017] Figures 1, 2 and 3 represent several embodiments of a photonic substrate 1 which is the subject of this description.

[0018] The photonic substrate 1 is advantageously in the form of a circular wafer, the diameter of which is standardized to allow handling by standard semiconductor industry equipment. This diameter is typically between 100 mm and 200 mm. However, the shape and dimensions of the substrate 1 are not a limitation, and these shapes and dimensions can be freely chosen.

[0019] Whatever the shape and size adopted by the photonic substrate 1, it generally comprises a support 5, an electric charge trapping layer 4 (more simply referred to as the "trapping layer" in the rest of this description) disposed on a first face of the support 5, a dielectric layer 3 on and in contact with the trapping layer 4 and an electrooptical layer 2 disposed on and in contact with the dielectric layer 3.

[0020] The electro-optical layer 2 is made of a material capable of producing electro-optical effects, that is, of varying at least one of its optical properties when subjected to a changing electrical quantity, for example, a changing electric field. This could be, for example, the Pockels effect. The material constituting the electro-optical layer 2 could be a ferroelectric material, such as lithium niobate or barium titanate, or even lithium tantalate. In all cases, it is made of a single-crystal material of optical quality and can have a thickness of between 300 nm and 600 nm to allow the definition of waveguides, for example those of an optical modulator.

[0021] For the purposes of this application, the "optical quality" of a layer means that the absorption coefficient of that layer is less than 10A-3 in the spectral band between 0.7 micrometers and 1.6 micrometers. Preferably, this absorption coefficient is less than 10A-2 in this spectral band.

[0022] Thus, when the electro-optical layer is made of single-crystal lithium niobate, it advantageously presents a crystal cut X, although it is not excluded that it may present another crystal cut, for example Z or Y.

[0023] To ensure proper propagation of the optical mode and limit excessive absorption of this mode, the electro-optical layer 2 has a free face (the face not in contact with the dielectric layer 3) with a roughness of less than 0.5 nm in root mean square value measured over a 10-micrometer by 10-micrometer area. It also advantageously has a thickness variability of less than 50 nm over its entire extent. Its interface with the dielectric layer 3 must be free, as far as possible, from defects such as particles, contaminants (particularly metals), and local delamination with the dielectric layer (known as "voids" in Anglo-Saxon terminology).

[0024] The dielectric layer 3 is also of optical quality and therefore has an absorption coefficient of less than 10A-3, preferably less than 10A-2, in the spectral band between 0.7 micron and 1.6 micron.

[0025] Advantageously, this is a silicon oxide which has a refractive index between 1.4 and 1.5 in the spectral range considered, which offers a significant contrast with the refractive index of the electro-optical layer 2 (between 2.18 and 2.25 when it is made of lithium niobate) allowing the optical mode to be confined in this layer.

[0026] This oxide can be formed, at least in part, using LPCVD (Low Pressure Chemical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition). This deposition can be completed by annealing to improve the absorption properties of this layer and ensure its optical quality, as will be detailed later in this description.

[0027] The dielectric layer 3 has sufficient thickness to distance the electro-optical layer 2 from the support 5, and to limit electromagnetic coupling. A significant thickness of the dielectric layer facilitates the injection of an optical mode in the electro-optical layer from an optical feed fiber, coupled by the edge to the photonic substrate. This thickness can thus be between 2 micrometers and 10 micrometers, preferably between 2 micrometers and 5 micrometers to facilitate the fabrication of the photonic substrate 1 by application of the conventional Smart Cut™ technology.

[0028] A particular embodiment, which will be described in a later section of this description, proposes an extension of this technology to particularly adapt it to a dielectric layer thickness 3 greater than 5 micrometers.

[0029] In certain embodiments, illustrated in Figures 2 and 3, a compensation layer 3' can be formed on the side of the second face of the support 5. It is known that when a relatively thick dielectric layer is formed on one face of a support, greater, for example, than 3 or 5 microns, this thickness can cause significant deformation of the support due to the stresses imposed by the dielectric layer. The compensation layer 3' can help maintain the flatness of the photonic substrate in such a situation of a relatively thick dielectric layer 3.The compensation layer 3' can be of a dielectric nature, for example of the same nature as that of the dielectric layer 3 or of a different dielectric nature, but this is not necessarily the case and in general this layer can be formed of any material bringing stresses to the support 5 aimed at compensating those brought by the dielectric layer 3. .

[0030] In the embodiment of [Fig. 1], the dielectric layer 3 consists of a single layer of silicon oxide with the properties just mentioned. This embodiment does not include a compensation layer 3'.

[0031] In the embodiment of [Fig. 3], the dielectric layer consists of a first dielectric layer 3a made of silicon oxide, having the same properties as the single dielectric layer 3 of the embodiment of [Fig. 1], and a second dielectric layer 3b. This second dielectric layer 3b forms a barrier to the diffusion of certain electrically conductive species present in the electro-optical layer 2, for example, lithium when this layer 2 is made of lithium niobate. This prevents these species from migrating to the trapping layer 4 and / or to the support 5, which would affect the electrical properties (particularly radio frequency) of this layer 4 and this support 5, and therefore of the photonic substrate 1 as a whole. This second dielectric layer 3b is particularly useful when the trapping layer 4 is made of polycrystalline silicon, which is susceptible to accommodating electrically conductive species. 。

[0032] The second dielectric layer 3b can thus comprise nitrogen and be formed, for example, of silicon nitride or silicon oxynitride. Preferably, it It has a relatively small thickness, less than that of the first dielectric layer 3a, in order to limit its optical impact. It is advantageously positioned between the charge-trapping layer 4 and the first dielectric layer 3a.

[0033] The photonic substrate 1 also includes a support 5 on which rests the trapping layer 4, which will be described in a later section of this description. The support 5 provides mechanical stability to the relatively thin layers forming the photonic substrate 1, and for this purpose has a thickness of several hundred microns.

[0034] Preferably, to contribute to the radio frequency performance of the photonic substrate 1, the support 5 has a high resistivity, greater than 750 ohms centimeters, and more preferably, greater than 2000 ohms centimeters. This limits the density of charges, holes or electrons, that are likely to move within it.

[0035] For reasons of availability and cost, the support 5 is preferably made of monocrystalline silicon. For example, and by way of illustration only, it may be a CZ silicon substrate with a low interstitial oxygen content of between 6 and 10 ppm, or an FZ silicon substrate which, in particular, has a naturally very low interstitial oxygen content. It may also be a CZ silicon substrate with a high interstitial oxygen content (referred to as "High Oi") exceeding 26 ppm.

[0036] Continuing the description of the embodiments illustrated in Figures 1, 2, and 3, and according to an important aspect, the photonic substrate 1, which is the subject of this description, comprises an electrical charge trapping layer 4, disposed between the dielectric layer 3 and the support 5, on the side of the first face of the support 5. This trapping layer 4 limits the movement of charges under the influence of the electromagnetic fields produced by the electrodes of the modulation device when the latter operates at very high frequencies. By limiting the electrical coupling between the modulator electrodes and the support 5, electrical losses and the linearity of the modulation device are thus reduced. This is particularly the case when the electrical signals have a frequency below 10 GHz or above 50 GHz.

[0037] The trapping layer 4 can be of a very diverse nature. Generally, it is a non-single-crystal layer exhibiting 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 this layer 4, which consequently exhibits high resistivity.

[0038] Generally, the trapping layer 4 can have a thickness between 10 nm and 30 micrometers, preferably between 50 nm and 5 micrometers.

[0039] According to one possible approach, and for reasons of simplicity of implementation, the trapping layer 4 is formed of a polycrystalline silicon layer. This layer can be formed by deposition on the first face of the support 5. In this case, and in order to preserve the polycrystalline quality of this layer during the thermal treatments that the photonic substrate 1 may undergo, an amorphous layer, of silicon dioxide for example, can advantageously be provided on the support 5 before the deposition of the trapping layer 4. In this case also, and as illustrated in [Fig. 3], the deposition of the trapping layer on the first face of the support can lead to the formation of a complementary polycrystalline layer p on the second face of the support 5.This complementary polycrystalline layer 4' has the same characteristics (or at least similar characteristics) as the trapping layer 4, particularly in terms of thickness, size and grain density.... .

[0040] Alternatively, the trapping layer 4 can be formed by implanting a relatively heavy substance, such as argon, into a surface layer of the support 5, on the side of its first face, in order to create the structural defects constituting the electrical traps. This trapping layer 4 can also be formed by porosifying a surface layer of the support 5, on the side of its first face, or by any other method suitable for creating structural defects in a surface layer of the support, these structural defects being capable of trapping electrical charges.

[0041] According to a particularly advantageous approach, the trapping layer 4 consists of a silicon-rich oxide deposited on the first face of the support 5. "Silicon-rich" refers to an oxide having an atomic concentration of silicon between 50% and 99.9%. Oxygen and, optionally, nitrogen are present in this layer 4 in substoichiometric atomic concentrations. Preferably, this oxide has an atomic concentration of silicon between 70% and 90% and an atomic concentration of oxygen between 10% and 30%. It may contain nitrogen in an atomic concentration between 8% and 45%.

[0042] A silicon-rich layer is usually formed from an amorphous matrix. This amorphous matrix may include crystalline silicon inclusions or crystalline silicon grains, the density and size of these inclusions and / or grains in the amorphous matrix depending on the relative proportions of oxygen, nitrogen, and silicon in the layer and the heat treatments it has undergone. In extreme cases, a silicon-rich layer may be entirely amorphous or entirely polycrystalline, the amorphous matrix in this latter case is reduced to amorphous inclusions present between the grains of the polycrystalline structure. Such a layer has the advantage of exhibiting a particularly high defect density, forming traps for electrical charges. It is therefore likely to exhibit very high resistivity, up to approximately 1OA12 ohm.cm. Further details on the formation of the trapping layer 4 in the form of a silicon-rich layer can be found in publications WO2024115410, WO2024115411, and WO2024115414.

[0043] The fabrication of the different modes of implementation of the photonic substrate 1 which have just been presented is the subject of the following section of this description.

[0044] The support 5 is first prepared to form, in successive steps, the trapping layer 4 (and optionally the complementary polycrystalline layer 4') and the dielectric layer 3. A compensation layer 3' may also be formed, and this formation can be simultaneous with the formation of the dielectric layer 3 if these two layers are of the same type. The resulting stack is referred to as the "base substrate" in the remainder of this description.

[0045] Regardless of the nature of the trapping layer 4, when it is formed by deposition on the support 5, this deposition can be carried out using a PECVD or LPCVD technique, or even by epitaxy. This deposition can occur on the first face of the support 5 or simultaneously on the two opposite faces of this support 5.

[0046] Thus, when the trapping layer 4 is a polycrystalline silicon layer, it can be formed by deposition at a moderate temperature, for example strictly between 600°C and 950°C. This can be a polycrystalline silicon layer deposited using an LPCVD technique. Alternatively, it can be formed at a relatively higher temperature, for example between 950°C and 1100°C, in an epitaxial reactor.

[0047] When the trapping layer 4 is a silicon-rich oxide, the deposition step includes introducing, into the deposition chamber, a first silicon precursor gas (for example, silane, with the formula SiH4) and at least one second oxygen precursor gas (nitrous oxide or dioxygen), and possibly nitrogen (for example, nitrous oxide, with the formula N2O). These precursor gases may be supplemented by a carrier gas, for example, nitrogen, argon, or helium. Separate precursor gases can, of course, be used for oxygen and nitrogen. As is well known, the precursor gases react in the deposition chamber, under controlled pressure and temperature conditions, to progressively form the trapping layer 3a. When this deposition is carried out using the PECVD technique, it is performed at a moderate temperature, typically between 150°C and 600°C, and at sub-atmospheric pressure. for example, a few Torr (i.e., a few hundred Pascals). When carried out using the LPCVD technique, this is performed at a temperature typically between 550°C and 750°C, also at a subatmospheric pressure of, for example, a few Torr (i.e., a few hundred Pascals).

[0048] In a subsequent step, the dielectric layer 3 can be formed on the trapping layer 3 to finalize the preparation of the base substrate. When a compensation layer 3', for example a dielectric layer, is planned on the side of the second face of the support 5, it can be produced first, for example by PECVD. The dielectric layer 3 can then be formed on the side of the first face of the support 5, on the trapping layer 4, for example by the same PECVD technique, in a thickness advantageously chosen between 2 microns and 10 microns.

[0049] Alternatively, the second dielectric layer 3b, for example a SiON layer, can be deposited onto the trapping layer 4 located on the first face of the support 5 to form a barrier, for example in LPCVD. Then, the first dielectric layer 3a, typically silicon oxide in PECVD, is deposited. This deposition can be carried out simultaneously on both faces of the support as already mentioned, and in this case, it is not necessary to have previously deposited the compensation layer 3' on the second face of the support 5.

[0050] Whether or not the second dielectric layer 2b is present, as already stated, the base substrate including the dielectric layer 3 can be annealed to improve its optical quality. Typically, this annealing can take place at a temperature higher than the deposition temperature of the dielectric layer 2b, under a dry or humid oxidizing atmosphere, to improve its stoichiometry. Annealing at low pressure is preferable. This pressure is typically less than 500 Torr (approximately 66 MPa), and preferably less than 100 Torr (approximately 13 MPa), to promote the desorption of chemical reaction byproducts trapped during deposition in the dielectric layer(s) 3, 3a, 3b.

[0051] Following these initial preparation steps of the base substrate, the photonic substrate 1 can be constituted by transferring an electro-optical layer 2. This transfer can be obtained by any suitable technique, for example by assembling a so-called "donor" substrate composed of, or comprising, the optical-quality single-crystal material intended to form the electro-optical layer with the base substrate.

[0052] The donor substrate, after being assembled to the base substrate, can be thinned or fractured to form the electro-optical layer 2. The thinning may include grinding and / or polishing steps aimed at progressively reducing the thickness of the donor substrate to provide the electro-optical layer 2 in the desired thickness and roughness, and thus complete the photonic substrate 1.

[0053] As an alternative to this thinning, a process conforming to the well-known Smart Cut™ technology can be deployed. This process involves introducing so-called "light" particles into the thickness of the donor substrate to create a weakening plane. This introduction is generally achieved by implanting the light species in ionic form. These species can be helium and / or hydrogen. After assembling the implanted donor substrate onto the base substrate, the donor substrate is fractured at the weakening plane by applying mechanical and / or thermal stresses.

[0054] A finishing sequence can then be applied, combining thinning steps (polishing, etching, etc.), annealing, and surface treatment to form the electro-optical layer 2 in the chosen thickness and roughness level. This approach also allows for precise control of thickness uniformity, which is mainly defined by the depth of introduction of the light species into the donor substrate, a depth that can be precisely chosen when this step is performed by implantation.

[0055] This completes the fabrication of the photonic substrate 1.

[0056] A manufacturing process based on Smart Cut technology is now presented, particularly suited to a photonic substrate 1 having a relatively thick dielectric layer, for example, greater than 5 micrometers. The process according to this embodiment is perfectly compatible with the general manufacturing process just described, and its description can be supplemented by all the details of the general process, not all of which are repeated here for the sake of brevity.

[0057] It is known that, according to Smart Cut™ technology, the detachment of the thin film from the donor substrate is achieved by the propagation, within this donor substrate, of a fracture wave. This wave propagates substantially within the plane of embrittlement, confined within this plane by the rigidity of the surrounding material.

[0058] It can be difficult to obtain a complete or localized fracture in a plane when the material is not sufficiently rigid. This is particularly the case when the dielectric layer 3, which is close to the embrittlement plane where the fracture occurs, has a relatively large thickness and, simultaneously, a relatively low rigidity.

[0059] For example, the presence of a silicon oxide dielectric layer (whose Young's modulus is on the order of 75 GPa) with a thickness greater than 5 micrometers can affect the proper propagation of the fracture wave.

[0060] This Young's modulus, relatively low, is to be compared to that of silicon (on the order of 130 GPa to 185 GPa) from which the support 5 can be formed or to that of lithium tantalate or lithium niobate (on the order of 170 GPa) from which the electro-optical layer can be formed.

[0061] In the approach proposed in this section, and with reference to [Fig. 4], a basic substrate 7 is first prepared, having all the characteristics presented in the previous section. Thus, in a first step SI, the electrical charge trapping layer 4 is formed on a first face of the support 5. This first step SI may include the simultaneous (or successive) formation of the complementary layer 4' on the second face of the support 5, this second face being opposite the first face.

[0062] As will be explained later, a manufacturing process according to this embodiment may lead to the basic substrate being exposed to a relatively high temperature, for example above 1000°C. This temperature may affect the quality of the electrical trapping layer, for example by recrystallizing it when it is formed from a polycrystalline silicon layer.

[0063] Also, and preferably, when the trapping layer 4 is made of a material capable of recrystallizing, such as polycrystalline silicon, an amorphous layer 5' is planned to be formed, during the first SI step, between the support 5 and the trapping layer 4. This amorphous layer 5' can be a silicon oxide and result from the oxidation of the support 5 when the latter is made of silicon. It can have a thickness ranging from a few nanometers to a few hundred nanometers, sufficient to crystallographically dissociate the trapping layer 4 from the support 5.

[0064] Alternatively, or in addition, a trapping layer 4 of sufficient thickness may be formed to avoid the risk of recrystallizing it entirely during the manufacture of the optical substrate 1. This thickness may, for example, be 2 micrometers or more.

[0065] It should be noted that a trapping layer 4 formed of a silicon-rich oxide, as previously described, is particularly temperature-stable and can withstand annealing at 1100°C for 2 hours without significant degradation. The recrystallization phenomenon observed in the case of a trapping layer 4 made of polycrystalline silicon is therefore less likely to occur, particularly by controlling the heat treatment so that it does not exceed the aforementioned thermal budget. Consequently, the amorphous layer 5' is not essential in such a case, and the thickness of the trapping layer 4 formed of such a silicon-rich oxide can be relatively thin, less than 1 micrometer or 500 nm.

[0066] Returning to the detailed description of the process shown in [Fig. 4], a dielectric thickness 3” is formed in a second step S2 on and in contact with the trapping layer 4. This dielectric thickness 3” is intended to form part of the dielectric layer 3 of the final substrate. This dielectric thickness 3” advantageously comprises silicon dioxide, but it can be foreseen that it may comprise other materials. It can thus be foreseen that the dielectric thickness 3” comprises a second dielectric layer 3b formed of a material that acts as a barrier to the diffusion of certain electrically conductive species present in the electro-optical layer, as previously explained with reference to the embodiment shown in [Fig. 3]. This can be a second dielectric layer 3b made of silicon nitride or silicon oxynitride.

[0067] The 3” dielectric thickness has a thickness less than or equal to 5 micrometers. Due to this limited thickness, it does not affect the propagation of the fracture wave during a subsequent layer transfer step on the base substrate, even if this 3” dielectric thickness is made of a material having a relatively low Young's modulus, for example silicon oxide.

[0068] It is of course possible to provide during this second step S2 to form the compensation layer 3' which is found in the implementation methods of the photonic substrate 1 of figures 2 and 3. It is also possible to provide for annealing of the substrate including the dielectric thickness 3”, in order to improve its optical quality, as has been described previously.

[0069] According to the process in accordance with the present embodiment, an interlayer 6 is then formed in a third step S3 on and in contact with the dielectric thickness 3'. This interlayer is formed of an oxidizable material, for example amorphous or polycrystalline silicon.

[0070] The aim is to form, during this third step S3, an interlayer 6 sufficiently rigid so as not to affect the propagation of the fracture wave during the subsequent layer transfer step. Preferably, therefore, this interlayer 6 has a Young's modulus greater than the Young's modulus of the material or materials forming the dielectric thickness 3”.

[0071] Even more advantageously, the Young's modulus of the interlayer 6 is closer to the Young's modulus of the electro-optical layer material 1 than it is to the Young's modulus of the material or materials forming the dielectric thickness 3”.

[0072] As will be made apparent in a later paragraph, the interlayer 6 is intended to combine with the dielectric thickness 3” to form the dielectric layer 3 of the photonic substrate 1. The interlayer typically has a thickness between 0.5 micrometers and 10 micrometers.

[0073] More generally, the thicknesses of these two layers 6, 3” will be chosen so that, after all the manufacturing process treatments, their cumulative thicknesses correspond to the desired thickness of the dielectric layer 3. In particular, it is possible to form such a relatively thick dielectric layer 3 in the final photonic substrate 1, with a thickness greater than 5 microns, while maintaining the relatively thin dielectric layer 3', less than 5 microns thick, in the intermediate substrate during its fabrication. Furthermore, the intercalated layer 6 and the dielectric layer 3” combined have a Young's modulus sufficient to allow the transfer of the electro-optical layer 1.

[0074] Following steps S1, S2, and S3, which have just been described, a basic substrate 7 is obtained, comprising the support 5, the trapping layer 4 disposed on the first face of the support 5, possibly via the amorphous layer 5, the dielectric thickness 3" disposed on the trapping layer 4, and the intercalated layer 6 disposed on the dielectric thickness. A complementary layer 4' may be provided on and in contact with the second face of the support 5, and / or the compensation layer 3' may be provided on the side of the second face of the support 5, in contact with this face or with the complementary layer 4' if the latter is present.

[0075] Figure 5 shows the variant in which a basic substrate 7 is planned to be formed comprising a complementary layer 4', an amorphous layer 5' and a compensation layer 3'.

[0076] It is noted that in its simplest version, the basic substrate 7 can consist of a support, a trapping layer 4, a dielectric thickness and the intercalated layer 6. The preparation of this basic substrate can therefore consist of providing a "silicon on insulator" type substrate with a trapping layer, the dielectric thickness of the basic substrate then being made up of the oxide buried on a silicon on insulator type substrate and the intercalated layer 6 of the surface layer in monocrystalline silicon of this substrate.

[0077] In a sequence of subsequent steps, the electro-optical layer 2 is transferred onto and in contact with the intercalated layer 6. The resulting assembly is designated "intermediate substrate" 1' in the remainder of this description.

[0078] This sequence provides, as previously described, for the supply of a donor substrate 8 consisting of a bulk substrate of optical-grade single-crystal material intended to form the electro-optical layer, or alternatively, a donor substrate consisting of a surface layer of optical-grade single-crystal material deposited on a manipulator substrate. This second approach is described in particular in US patent 11742817B2. In this case, and in accordance with the teachings of this patent, the coefficient of thermal expansion of the manipulator substrate is preferably chosen to be close to that of the base substrate.

[0079] During a embrittlement step S4, light hydrogen and / or helium particles are introduced into the thickness of the donor substrate 8 to form an embrittlement plane P. This introduction is generally achieved by ion implantation. The embrittlement plane P, together with the light species introduction face, defines the electro-optical layer 2, which will be transferred to the base substrate. The electro-optical layer typically has a thickness of between a few hundred nanometers and one micrometer, depending on the positioning of the embrittlement plane P within the depth of the donor substrate 8. Alternatively, a thin dielectric layer 8a, typically made of silicon dioxide, can be formed on the light species introduction face, either before or after this embrittlement step.

[0080] In a subsequent assembly step S5, the donor substrate 8 is joined with the base substrate 7 by bringing the thin layer 2 of the donor substrate (or the thin dielectric layer 8a if present) into contact with the interlayer layer 6 of the base substrate 7. This assembly can be carried out by any suitable technique, preferably one involving molecular adhesion of the two surfaces brought into contact. The surfaces to be joined may have been prepared, for example by polishing, to reduce their roughness and thus facilitate this bonding step.

[0081] In a subsequent detachment step S6, the donor substrate 8 is fractured at the level of the brittle plane P. This fracture can be achieved by applying thermal annealing to weaken the donor substrate 8 at the level of the brittle plane P, optionally assisted by the application of mechanical force to induce this fracture. A residue 8' of the donor substrate 8 from which the electro-optical layer 2 has been extracted can be reprocessed, in particular to extract a new layer.

[0082] The assembly steps S5 and detachment steps S6 are the conventional steps of Smart Cut™ technology, well known to those skilled in the art, so that it is not necessary to detail them further.

[0083] It is noted that the basic substrate 7, being equipped with the intercalated layer 6 and a relatively thin dielectric thickness 3', has sufficient rigidity to allow proper initiation and propagation of the fracture wave in the embrittlement plane or to confine this propagation close to this plane.

[0084] In a subsequent heat treatment step S7, the intermediate substrate 1' is heat-treated in an atmosphere containing oxygen. This heat treatment is carried out at a temperature and for a duration sufficient to allow the diffusion of atmospheric oxygen through the electro-optical layer 2 and allow the complete oxidation of the intercalated layer 6.

[0085] For example, when the electro-optical layer 2 is made of a ferromagnetic material, such as lithium niobate, the heat treatment temperature can range from 1000°C to the Curie temperature of the ferromagnetic material (around 1200°C for lithium niobate). Its duration naturally depends on the thickness of the interlayer 6 to be oxidized. Generally, it can range from 30 minutes to several hours.

[0086] A person skilled in the art will be able to determine very simply, using a test matrix, the temperature and duration of the heat treatment to be applied during this heat treatment step S7 to fully oxidize the intercalated layer 6, according to the particular characteristics of the intermediate substrate 1'.

[0087] The heat treatment step S7 can be carried out in a conventional oxidation furnace, and the atmosphere comprising oxygen can consist of a dry or humid oxidizing atmosphere.

[0088] Preferably, during the heat treatment step S7, the electro-optical layer 2 of the intermediate substrate 1' has a surface directly exposed to the oxidizing atmosphere. The intermediate substrate 1' is therefore free of any surface layer that could limit or block the diffusion of oxygen through the electro-optical layer 1 during the heat treatment.

[0089] In any event, at the end of this heat treatment step S7 the possible thin dielectric layer 8a, the oxidized intercalated layer 6 and the dielectric thickness 3” combine to form the dielectric layer 3 of the photonic substrate 1.

[0090] When the interlayer 6 is made of silicon, amorphous or polycrystalline, as previously proposed, its oxidation leads to the formation of an oxidized interlayer of silicon oxide. When the thin dielectric layer 8a (if present) and the dielectric thickness 3” are also made of silicon oxide, the resulting dielectric layer 3 is entirely made of silicon oxide.

[0091] To complete the fabrication of the photonic substrate 1, the finishing sequence for the transferred photonic layer can be specified, identical to that described in the detailed description of the general fabrication process. This finishing sequence can be carried out before or after the heat treatment step S7 of the intermediate substrate 1.

[0092] As stated in the introduction to this application, the photonic substrate can be used to form photonic components, and in particular thin-film optical modulation devices.

[0093] Thus, portions of the electro-optical layer 2 of the photonic substrate 1 can be selectively removed to define, in this layer 2, waveguides allowing the propagation of at least one optical mode in the modulator. Deposition steps, particularly of metals, can also be carried out to form the modulation electrodes allowing a variable electrical quantity (in particular an electric field) to be imparted to these waveguides, this variable electrical quantity being controlled to modify the optical properties of the material forming the waveguides and in order to modulate the optical mode(s) propagating therein.

[0094] 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 manufacturing a photonic substrate (1) comprising the Next steps: a. provide a support (5) having a first face and a second face, opposite to the first; b. form an electrical charge trapping layer (4) on the first face of the support (5); c. form a dielectric thickness (3') on and in contact with the electrical charge trapping layer (4); d. form an interlayer consisting of an oxidizable material on and in contact with the dielectric thickness (3”); e. transfer an electro-optical layer (2) formed of a single-crystal material of optical quality, onto and in contact with the intercalated layer to form an intermediate substrate; f. heat-treat the intermediate substrate in an atmosphere comprising oxygen at a temperature and for a duration sufficient to permit diffusion of atmospheric oxygen through the electro-optical layer (2) and permit complete oxidation of the intercalated layer.

2. A method for manufacturing a photonic substrate (1) according to claim 1 wherein the electro-optical layer (2) of the intermediate substrate has a surface directly exposed to the atmosphere comprising oxygen during the heat treatment, the intermediate substrate being devoid of any surface layer capable of limiting or blocking the diffusion of oxygen through the electro-optical layer during this heat treatment.

3. Method of manufacturing a photonic substrate (1) according to any one of the preceding claims comprising a step of forming an amorphous layer (5') on the first face of the support (5) before the step of forming the electrical charge trapping layer (4).

4. A method for manufacturing a photonic substrate (1) according to any one of the preceding claims, wherein the step of forming the electrical charge trapping layer (4) includes the formation of a complementary layer (4') on the second face of the support (5).

5. A method for manufacturing a photonic substrate (1) according to any one of the preceding claims wherein the step (S2) of forming the dielectric thickness (3”) includes the formation of a compensation layer (3') on the side of the second face of the support (5).

6. A method for manufacturing a photonic substrate (1) according to any one of the preceding claims further comprising a finishing step of the electro-optical layer (2), before or after the heat treatment step of the intermediate substrate.

7. A method for manufacturing a photonic substrate (1) according to any one of the preceding claims wherein the layer transfer step comprises the assembly of a donor substrate (8) with the interlayer layer (6).

8. A method for manufacturing a photonic substrate (1) according to the preceding claim in which the donor substrate (8) is made of a bulk substrate of optical-grade single-crystal material.

9. Method of manufacturing a photonic substrate (1) according to claim 7 wherein the donor substrate (8) consists of a surface layer of optical-grade single-crystal material disposed on a manipulator substrate.

10. A method for manufacturing a photonic substrate (1) according to any one of claims 7 to 9 wherein the layer transfer step comprises, before the assembly of the donor substrate (8) with the interlayer layer (6), the formation of a brittle plane (P) in the donor substrate (8) by introduction of light species of helium and / or hydrogen and, after the assembly of the donor substrate (8) with the interlayer layer (7), the fracturing of the donor substrate at the level of the brittle plane (P).

11. A method for manufacturing a photonic substrate (1) according to any one of the preceding claims wherein the optical-grade single-crystal material of the electro-optical layer is a ferromagnetic material such as lithium niobate.

12. A method for manufacturing a photonic substrate (1) according to the preceding claim in which the heat treatment atmosphere is brought to a temperature between 1000°C and the Curie temperature of the ferromagnetic material.

13. A method for manufacturing a photonic substrate (1) according to any one of the preceding claims wherein the formation of the interlayer (6) comprises the deposition of an amorphous or polycrystalline silicon layer.

14. A method for manufacturing a photonic substrate (1) according to any one of the preceding claims wherein the interlayer has a thickness of between 0.5 micrometers and 10 micrometers.

15. A method for manufacturing a photonic substrate (1) according to any one of the preceding claims wherein the dielectric thickness (3”) has a thickness less than or equal to 5 micrometers.

16. A method for manufacturing a photonic substrate (1) according to any one of the preceding claims in which the dielectric thickness (3”) comprises a second dielectric layer (3b) formed of a material forming a barrier to the diffusion of certain electrically conductive species present in the electro-optical layer (2).

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