Microelectromechanical systems consisting of the form of a piezoelectric resonance thin film based on an α - quartz layer, and manufacturing methods thereof

Abstract

The present invention relates to the manufacture of a microelectromechanical system in the form of a piezoelectric resonant thin film consisting of a piezoelectric epitaxial pseudo-substrate based on an epitaxial α-quartz layer on a silicon wafer, and to a method for manufacturing such a microsystem.

Description

Technical Field

[0001] The present invention generally relates to the manufacture of a microelectromechanical system in the form of a piezoelectric resonant membrane composed of a piezoelectric epitaxial pseudo-substrate based on an epitaxial quartz-α layer on a silicon wafer, and to a method for manufacturing such a microsystem.

Background Art

[0002] Piezoelectric materials play a central role in many everyday applications due to their unique ability to generate electric charges under mechanical stress or induce mechanical deformation from an electrical input. Due to these properties, they are a major component in motion sensing devices and resonators present in many wireless network sensors, which are devices capable of collecting and transmitting environmental data on their own. Thus, within this framework, piezoelectric materials are found in a variety of military, medical, and environmental applications. Today, the monolithic integration of these materials in silicon technology and their micromachining to develop cost-effective alternative processes with excellent performance are one of the central points of current technology.

[0003] Generally, piezoelectric materials such as PZT (lead zirconate titanate PbZrTiO3), ZnO (zinc oxide), and AlN (aluminum nitride) are integrated in the form of thin layers on Si substrates for chip commercialization. On the other hand, quartz-based devices have hitherto been microfabricated (micromachined) from large crystals [1]. This has the drawback that it cannot be reduced in size to less than 10 μm in thickness, and in most of its applications, it is necessary to bond the quartz crystal to a silicon substrate [2]. These drawbacks mean a great burden for the microelectronics industry. This is because there is currently a high demand for thinner single-crystal quartz wafers in order to enable the use of devices at higher operating frequencies and to achieve lower detection levels with better sensitivity [3].

[0004] α-Quartz is a strategic material in Europe that is widely used as a piezoelectric material due to its excellent properties. In fact, α-quartz has excellent thermal and chemical stability, and high mechanical properties, and has become one of the best candidates for frequency control devices and acoustic and mass sensor technologies. The scientific publication Carretero-Genevrier, A. et al. ("Pathways to Epitaxial α-Quartz Thin Films with Soft-Chemistry-Based Tunable Textures") Science 340, 827-831 (2013) [4] describes the direct chemical integration (chemical integration) of epitaxial α-quartz on a silicon substrate (100), and the international application WO2014 / 016506 [5] is based on this. In particular, this application teaches how the structure of the α-quartz thin layer on the silicon substrate is adapted by chemical deposition in solution (dip-coating technology, generally indicated by the English term "dip-coating" by those skilled in the art), which enables control of the characteristics (texture), density, and thickness of the thin layer. Disclosure of the Invention Problems to be Solved by the Invention

[0005] However, the quartz-α thin layers obtained by this technique cannot be used in the manufacture of piezoelectric microelectromechanical systems (MEMS).

[0006] Therefore, there is great interest in the development of an industrializable technique for manufacturing a microelectromechanical system in the form of a piezoelectric resonant thin film, including a piezoelectric epitaxial pseudo-substrate based on an epitaxial α-quartz layer on a silicon wafer. Such a microelectromechanical system has micrometer dimensions and is intended, for example, for the manufacture of sensors.

Means for Solving the Problem

[0007] For this purpose, the applicant has developed a microelectromechanical system in the form of a piezoelectric resonant thin film, which consists of: - A piezoelectric epitaxial pseudo-substrate comprising a silicon wafer (100) having a back surface and a front surface, and an epitaxial α-quartz thin layer (100) on the front surface (21) of the wafer; - A stack of three successive layers of SiN, SiO2, and SiN deposited on the back surface of the wafer; - At least one opening penetrating from the front surface of the wafer to a depth p1 of a part of the stack and the silicon wafer (100), the opening defining (specifying) an unprotected area of silicon (100) in a plane parallel to the wafer at a depth p1 inside the wafer.

[0008] Regarding the "thin film", it should be understood that it is a coating whose thickness can vary from a few atomic layers to 10 micrometers, for example, from 2 angstroms to 20 μm, for example, from 100 nm to 2 μm. For example, this can be composed of thin layers as described in "Recent Advances in Thin Films: Materials Horizons from Nature to Nanomaterials" by S. KUMAR and Dr. D. K. ASWAL, Springer, Singapore. https: / / doi.org / 10.1007 / 978-981-15-6116-0_1 [6]. For example, when this coating is deposited on a substrate such as a silicon wafer, it can change the properties of the deposited substrate.

[0009] Regarding the "epitaxial α-quartz" on the silicon wafer, it should be understood as the growth (orientation) of α-quartz arranged with respect to a substrate such as a single-crystalline silicon wafer, and its crystal lattice has a common symmetry consisting of elements such as lattice matching or the number of lattice cells. As an example, it may be composed of α-quartz epitaxially grown with respect to a substrate such as a silicon wafer obtained by epitaxy such as hetero-epitaxy. For example, it may be composed of hetero-epitaxy in which α-quartz and silicon are crystals with different chemical properties.

[0010] Regarding the "epitaxial quartz-α thin film", it should be understood as an epitaxial α-quartz layer with a thickness ranging from 100 nm to 2 μm on the surface of a substrate such as a silicon wafer. For example, it may be composed of an epitaxial α-quartz layer obtained by the coherent growth of the α-quartz. Advantageously, the epitaxial quartz-α thin film may be lattice-matched to a substrate such as a silicon wafer.

[0011] An "epitaxial pseudo-substrate" should be understood as a pseudo-substrate that includes a coherent interface at the common surface between a substrate such as a silicon wafer and a thin layer of α-quartz formed thereon. The "epitaxial pseudo-substrate" should be understood as buffer layers of epitaxial crystals on substrates with different structures. Advantageously, these epitaxial layers and so-called metamorphic buffer layers can provide solutions to the deficiencies of native substrates for (i) the development of new microelectronics technologies, (ii) the expansion of the application scope of existing devices, or (iii) entirely new technologies by stabilizing material phases with inaccessible properties (see Ding, C. et al., Wafer-scale single crystals: crystal growth mechanisms, manufacturing methods, and functional applications. J Mater Chem C 9, 7829-7851 (2021)) [7].

[0012] Advantageously, the thickness of the wafer is preferably in the range of 100 microns, and its surface is preferably polished.

[0013] In other words, the thickness of the silicon wafer (100) may be 100 to 2000 microns, for example 100 microns.

[0014] In other words, at least one surface of the silicon wafer (100), for example, at least two surfaces, at least three surfaces, at least four surfaces, for example, all surfaces of the silicon wafer (100) may be polished. For example, the front and back surfaces of the silicon wafer may be polished.

[0015] Advantageously, each of the back and front surfaces of the wafer is at least 20 cm 2 , preferably 20 cm 2 ~82 cm 2 and may have a surface area.

[0016] Advantageously, the α-quartz thin layer (100) may have a homogeneous crystallization (crystal) with a mosaicity (mosaic degree) around the peak (100) of quartz of 6° to 1° and a thickness of 100 nm to 1 μm.

[0017] Preferably, the thickness of the α-quartz (100) thin layer ranges from 200 nm to 1 μm.

[0018] Preferably, the α-quartz thin layer may have a homogeneous crystallization (crystal) with a mosaicity (mosaic degree) of 2.5° to 1.4°.

[0019] A microelectromechanical system in the form of a piezoelectric resonant thin film may include a piezoelectric epitaxial pseudo-substrate including a silicon wafer (100) having a back surface and a front surface and an epitaxial α-quartz (100) thin layer on the front surface (21) of the wafer, and may have a thickness E corresponding to the sum of the thicknesses of three consecutive layer stacks of SiN, SiO2, and SiN deposited on the back surface of the wafer. The thickness E of the microelectromechanical system in the form of a piezoelectric resonant thin film may be 100 μm to 1 mm, for example, 200 μm to 300 μm, for example, 280 μm.

[0020] According to the present invention, a microelectromechanical system in the form of a piezoelectric resonant thin film may have a thickness E' (E' = E - p1) corresponding to the difference between the thickness E and the depth p1.

[0021] In other words, a microelectromechanical system in the form of a piezoelectric resonant thin film may have a thickness E' at an opening defining an unprotected region of silicon (100).

[0022] The thickness E' may be between 1 μm and 50 μm, for example, 2 μm to 13 μm, or may be equal to 2 μm, for example.

[0023] As an advantageous embodiment of the microelectromechanical system according to the present invention, the unprotected region of the silicon (100) of the opening has a square shape at a depth p1 and may have sides of, for example, 1 mm, 2.5 mm, 3 mm, 3.5 mm, and 4 mm.

[0024] In other words, the unprotected region of the silicon (100) of the opening at a depth p1 is 1 mm 2 to 16 mm 2 , for example 6.25 mm 2 9 mm 2 12.25 mm 2 and can have a surface area of.

[0025] Preferably, the microelectromechanical system according to this advantageous embodiment consists of four straight sidewalls starting from the front and being substantially perpendicular to the wafer inside the wafer and down to a predetermined depth p2 located below the depth p1. The sidewalls are extended by four trapezoidal inclined walls and form an angle α of 54.7° with respect to a plane parallel to the wafer located at the depth p2 so as to demarcate the contour of the hollow frustum of a pyramid in a concave shape.

[0026] Advantageously, the microelectromechanical system according to the present invention may further have a configuration including a first gold layer disposed on the α - quartz (100) thin layer and a second gold layer (6) disposed on the unprotected region of the silicon (100).

[0027] Advantageously, the microelectromechanical system according to the present invention may further have a configuration including an epitaxial thin layer based on ZnO or Al2O3 or HfO2 microcrystals having a crystalline orientation (110) on the α - quartz (100) thin layer (3).

[0028] As an example, the microelectromechanical system according to the present invention may include an α-quartz thin layer having an epitaxial back face on the front surface of the silicon wafer and a whole surface in contact with a gas and / or a liquid.

[0029] Here, the gas may be any gas known to those skilled in the art. For example, the gas may be composed of dioxygen (O2), carbon dioxide (CO2), nitrogen dioxide (NO2), methane (CH4), nitrous oxide (NO), or any mixture thereof. For example, it may consist of air containing 78.087% dinitrogen (N2), 20.95% dioxygen (O2), 0.93% argon (Ar), and 0.041% carbon dioxide (CO2).

[0030] Here, the liquid may be any liquid known to those skilled in the art. For example, the liquid may be composed of biological liquids such as blood, cerebrospinal fluid, urine, saliva, or any mixture thereof. For example, it may be composed of a cell culture medium. For example, it may be composed of a commercially available cell culture medium, such as a culture medium containing Dulbecco's Phosphate Buffered Saline (DPBS) with a pH ranging from 7.0 to 7.3, commercially available from Thermo Fisher Scientific. For example, it may be composed of a cell culture medium containing bovine serum albumin (BSA), such as a 1X saline buffer solution with a pH ranging from 5 to 7 and containing 1% BSA. For example, it may be composed of a cell culture medium containing 10% fetal bovine serum, a 1% penicillin-streptomycin solution, and HEPES, such as Iscove's Modified Dulbecco's Medium (IMDM), commercially available from Thermo Fisher Scientific. It may be composed of an aqueous solution, an oily solution, or an emulsion. For example, it may be composed of water, preferably ultrapure water, such as ultrapure water with a resistivity of 18 - 19 MΩ·cm, for example, 18.2 MΩ·cm.

[0031] Here, the front surface of the α-quartz thin layer may be partially or entirely in contact with a gas and / or a liquid. For example, when the α-quartz thin layer is entirely in contact with a gas and / or a liquid, the ratio of the surface area of the front surface of the α-quartz thin layer in contact with the gas and / or the liquid is 100%.

[0032] The surface of the front surface of the α-quartz thin layer in contact with the gas and / or the liquid may be arranged parallel to the unprotected region of the silicon (100) of the opening at a depth p1.

[0033] The surface area of the front surface of the α-quartz thin layer in contact with the gas and / or liquid may be the same as or different from the surface area of the unprotected area of the silicon (100) of the opening at depth p1. For example, the surface of the front surface of the α-quartz thin layer in contact with the gas and / or liquid is 1 mm 2 ~16 mm 2 , for example 6.25 mm 2 , 9 mm 2 , 12.25 mm 2 and may be.

[0034] The shape of the surface of the front surface of the α-quartz thin layer in contact with the gas and / or liquid may be square or circular.

[0035] Here, the gas and / or liquid may be accommodated in a tank provided with at least one opening arranged to contact the front surface of the α-quartz thin layer.

[0036] The tank may have a shape selected from a cube shape, a parallelepiped shape, and a dome shape. For example, the tank may be cube-shaped.

[0037] The tank may be deformable or fixed (non-deformable).

[0038] The tank may be composed of any suitable material known to those skilled in the art. For example, it may be composed of a material selected from the group including silicon and polydimethylsiloxane (PDMS). For example, the material may be silicon.

[0039] The tank has a volume consisting of 60 μL to 160 μL, for example 80 μL to 140 μL, of gas and / or liquid.

[0040] For example, the volume of the tank may be 352.8 mm 3 and may be.

[0041] The tank may be completely or partially filled with gas and / or liquid.

[0042] The tank further comprises at least one inlet opening for the gas and / or liquid of the tank.

[0043] The tank further comprises at least one outlet opening for the gas and / or liquid of the tank.

[0044] Another object of the present invention is a method for manufacturing a microelectromechanical system according to the present invention described above, the method comprising the following steps: A) Supplying or manufacturing a piezoelectric epitaxial pseudo-substrate comprising a silicon wafer (100) having a back surface and a front surface, as defined above, i.e., and an epitaxial α-quartz thin layer (100) on the front surface of the wafer; B) Depositing a stack of three consecutive layers of SiN, SiO2, SiN on the back surface of the wafer, C) Pre-etching the stack by a physical dry etching process to form at least one cavity therein; and D) A chemical etching step (e.g., acidic chemical etching using TMAH (tetramethylammonium hydroxide) or basic chemical etching using KOH) for forming at least one opening in the silicon wafer (100) from the at least one cavity, the opening being a non-protected region of silicon (100) in a plane parallel to the wafer located at a depth p1 inside the wafer.

[0045] As a first embodiment of the method according to the present invention, the step C) of pre-etching the stack may consist of laser etching, in which case the method according to this first embodiment may further comprise the following: - Between process C) and process D), etch the wafer by laser (for example, using a femtosecond laser) from depth p1 to depth p2 within the wafer located between the depth p1 and the stack, to excavate four straight sidewalls substantially perpendicular to the wafer from the cavity to a depth p2; and - Process C”) of protecting the α-quartz (100) thin layer; - From depth p2, extend chemical etching to depth p1, expand the straight sidewalls by four trapezoidal inclined walls, form an angle α of 54.7° with respect to a plane parallel to the wafer located at depth p2, and delimitate the contour of the hollow frustum of a pyramid in a concave shape in process D).

[0046] In this first embodiment, the laser is used at a frequency of 57 MHz with a diameter (beam diameter) of 15 μm and / or an output of 2 W.

[0047] As a second embodiment of the method according to the present invention, process C) of pre-etching the stack may be constituted by reactive ion etching (the English acronym RIE generally meaning “Reactive Ion Etching”), and the method then further includes the following process between process B) and process C): - Process B1) of protecting the back surface of the stack by depositing a negative resin layer, and subsequently - Process B2) of depositing a photolithography mask including at least one orifice on the negative resin layer, and then - Process B3) of exposing the set to ultraviolet radiation (for example, 37.5 mj.cm -2 for 5 seconds), and annealing treatment, and - Process B4) of immersing in a negative developer bath to form at least one cavity in the negative resin layer, consisting of Step C) related to etching by reactive-ion etching is intended to extend the etching of the cavity in the stack to form four straight sidewalls therein; and, Chemical etching step D) extends chemical etching from the cavity to a depth p1, and the straight sidewalls are extended by four trapezoidal inclined walls, forming an angle α of 54.7° with respect to the wafer.

[0048] Advantageously, in both of these two embodiments of the method according to the invention, a piezoelectric epitaxial pseudo-substrate that can be manufactured as follows is used: A1) A step of preparing a composition containing a solvent, at least one silica precursor and / or colloidal silica, and a catalyst selected from the above elements with an oxidation degree of +2 forming a group consisting of strontium, barium, calcium, magnesium and beryllium, or a catalyst selected from the above elements with an oxidation degree of +1 forming a group consisting of cesium, rubidium, lithium, sodium or potassium, wherein the catalyst has a catalyst:SiO2 molar ratio in the range of 0.0375 to 0.125, preferably in the range of 0.075 to 0.125, and more preferably in the range of 0.1; A2) A step of supplying a silicon wafer (100) having a back surface and a front surface. Wafer 2 is made of N-doped silicon, and the resistivity is in the range of 0.025 ohm / cm 2 The respective surface areas of the front and back surfaces are at least 20 cm 2 Preferably, it is in the range of 20 cm 2 to 82 cm 2 ; A3) A step of depositing at least one layer of the composition obtained at the end of step A1) by spin-coating, wherein the deposition is performed on at least a part of the back surface of the wafer; and A4) Heat-treat at a temperature of 400°C to 600°C to form a thin layer (3) of consolidated amorphous silica at the completion of step C’). Steps A3) and A4) can be repeated continuously one or more times, preferably 4 times. A5) Heat-treat the consolidated amorphous silica thin film at a temperature ranging from 800°C to 1200°C.

[0049] Advantageously, the composition prepared in step A1) may further contain a non-ionic surfactant such as polyoxyethylene cetyl ether.

[0050] Preferably, the composition prepared in step A1) can be composed of a precursor selected from methyltrimethoxysilane (MTMS), tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), dimethyldimethoxysilane, and mixtures thereof, preferably tetraethoxysilane (TEOS).

[0051] Advantageously, step A3) involving deposition by spin-coating may include the following steps: - A first phase of dynamically dispersing (distributing) the composition of step A1) by centrifuging at a speed of 100 to 500 rpm for 5 to 10 seconds; then; - A second phase of forming an α-quartz (100) thin film by centrifuging at a speed of 500 to 6,000 rpm for 10 to 40 seconds. The two dispersion (distribution) phases are separated by a waiting time of 0 to 15 seconds.

[0052] Preferably, the heat treatment step A5) can be carried out at 980°C for 5 hours in a tubular furnace with an air flow rate of 12 L / min.

[0053] In this second embodiment, step B) of the method according to the invention can preferably be carried out at 280 °C by plasma-enhanced chemical vapour deposition (generally denoted by the English acronym PECVD).

Brief Description of the Drawings

[0054] The following examples illustrate the invention in connection with the figures described below, but do not limit the scope of the invention: - Figure 1 schematically shows the various steps of a method for manufacturing a microelectromechanical system according to a first embodiment; - Figure 2 specifically shows step D) relating to chemical etching (using TMAH) using a protection system; - Figure 3 shows the influence of laser etching during the method for manufacturing a microelectromechanical system according to a first embodiment (so-called hybrid etching) combining laser etching and chemical etching: · Figure 3a specifically shows a comparison of the etching depth obtained as a function of the laser output and the number of passes thereof. The comparison is being carried out using a profilometer (surface profiler); · Figure 3b specifically shows a comparison of the etching depth obtained as a function of the laser output and the number of passes, and the comparison is being carried out by an optical microscope (this method has the same accuracy as the method used in Figure 3a); Figure 3c shows an image obtained using a profilometer by a laser etching optical microscope at a fixed output of 2 W while varying the number of laser passes: the acquired optical image is a 3D representation of various cavities. It is possible to obtain 100 μm for 6 passes, 115 μm for 7 passes, 132 μm for 8 passes, and 160 μm for 10 passes; Figure 3d shows a combination of laser etching and chemical etching with TMAH heated to 84 °C in a water bath: the etching kinetics for an attack (etching) of 3 hours 30 minutes is 0.8 μm / min; Figure 3e is an SEM image of the surface state before and after TMAH attack (etching) (it can be seen that the surface state of the etched area is improved by chemical attack (etching): upper part), and a depth profilometer obtained after laser etching combined with chemical etching by TMAH consisting of 3 hours and 30 minutes, and the finally obtained depth is 286 μm; - Figure 4 is a diagram that enables the calculation of the width d of the opening to obtain a membrane of a desired width. - Figure 5 schematically shows another step of the method for manufacturing a microelectromechanical system according to the second embodiment. - Figure 6 specifically shows the etching of silicon by chemical etching performed by TMAH heated to 84°C, and includes the following: · Figure 6a shows the change in the etching depth of silicon over time; · Figure 6b shows the change in the kinetics of Si(100) etching by TMAH over time; · Figure 6c is an optical profilometer of a 3D display of the cavity created by etching, showing that a depth of 90 μm has been created on a Si(100) substrate with a thickness of 100 μm. - Figure 7 shows the characteristics of the α - quartz layer carried out under "optimal" conditions on a 2 - inch diameter Si wafer: · Figure 7a is composed of an optical image showing the continuity of the α - quartz thin layer (right side) and an SEM image showing the thickness of the crystallized α - quartz layer (left side); · Figure 7b is an AFM image showing the texture and roughness of the α - quartz thin layer; · Figure 7c shows the results of θ - 2θ diffraction, and the inset includes a mapping of the mosaicity grade of the wafer around the peak (100); · Figure 7d is a pole figure showing the epitaxy between the Si wafer and the α - quartz layer. - Figure 8 shows a microelectromechanical system formed on a piezoelectric resonant thin film according to the present invention on a 100 μm substrate: · Figure 8a is an image of a 3-inch wafer before crystallization (left photo) and after crystallization (right photo); · Figure 8b shows a microscopic image consisting of an SEM image (left) and a 3D display of the etching performed during chemical etching with TMAH; · Figure 8c shows the analysis of a quartz membrane by X-ray diffraction (c1, upper right) performed at the completion of the film manufacturing method, the 2D diffractogram of the quartz layer of the film, and the rocking curve of the quartz adjacent to the film (lower), with the mosaicity being 1.67°; · Figure 8d shows a 3D view of the microelectromechanical system according to the present invention; - Figure 9 shows the characterization of a piezoelectric membrane made of α-quartz(100) / Si(100) by laser vibrometry: · Figure 9a is a laser vibrometry that reveals a resonance frequency of 54.25 kHz and a quality factor of 1,346 in air; · Figure 9b consists of the frequencies of different mechanical modes of a piezoelectric membrane with f = 54.25 kHz and the mapping of the amplitude motion (pm) of the piezoelectric membrane excited by current at different harmonics with f = 88.7 kHz, f = 128.6 kHz, and f = 198.1 kHz. - Figure 10 shows a graph (Figure 10A) representing the evolution of the resonance frequency in kHz (horizontal axis) and the maximum deformation of the piezoelectric membrane as a function of its thickness and surface area in mm 2 (vertical axis). Figure 10B shows photos of substrates including membranes of 2.5×2.5 mm, 3×3 mm, 3.5×3.5 mm, and 4×4 mm (left side). - Figure 11 has a front surface of 352.8 mm 3A photograph (FIG. 11A) of a microelectromechanical system including a thin layer of α-quartz in contact with a silicon tank is shown. FIG. 11B shows the change in the resonance frequency kHz (horizontal axis) of the piezoelectric film as a function of its surface area mm 2 (vertical axis).

[0055] FIGS. 1 to 6 are disclosed in the above-described explanatory part (detailed description of the figures) according to the present invention, and FIGS. 7 to 11 are described in more detail in the following examples for explaining the present invention without limiting the scope of the present invention. Detailed description of the drawings

[0056] FIG. 1 schematically shows another step of a method for manufacturing a microelectromechanical system according to a first embodiment. - Step A (FIG. 1a): The first step is to supply or manufacture a piezoelectric epitaxial pseudo-substrate 1 including a silicon (quartz) (100) wafer 2 having a back surface 20 and a front surface 21, and an epitaxial α-quartz (100) thin layer 3 on the front surface 21 of the wafer 2. Next, to remove impurities that may contaminate the material or change (tamper with) the microfabrication process, the quartz (100) / Si(100) substrate is thoroughly cleaned using acetone, ethanol, and isopropyl alcohol (IPA); - Step B (FIG. 1b): Thereafter, the deposition of a stack 4 consisting of three consecutive layers of SiN41, SiO242, and SiN43 is performed on the back surface 20 of the wafer 2; This deposition preferably consists of plasma-enhanced chemical vapour deposition (PECVD) at 280°C: The stack is composed of 400 nm of SiN at 280°C, 400 nm of SiO2 at 280°C, and 400 nm of SiN at 280°C on Si(100). It is very important to follow the deposition order, deposition temperature, and thickness of these layers; - Step C (FIG. 1c): This consists of the step of pre-etching the stack 4 with a laser to form at least one cavity 40 therein, which step draws the shape required for the future film and enables shortening of the final chemical attack (etching) time. The required etching depth is set according to the energy of the laser used; - Step C’ (Figure 1d): This consists of the step of laser-etching the wafer 2 up to the depth p2 of the wafer 2 located between the depth p1 and the stack 4, and digging four straight sidewalls 51A, 52A, 53A, 54A substantially perpendicular to the wafer 2 from the cavity 40 up to the depth p2; - Steps C” and D (Figures 1e and 2): This first consists of the step of protecting the back surface of the stack 4 by depositing a negative resin layer (for example, AZ2070) (in relation to the subsequent chemical etching with TMAH): Thereafter, the set is placed in a water bath (Figure 2). The expected etching rate ranges from 0.8 μm / min after 1 hour (see Figure 3). Once the required film thickness is obtained, the set needs to be carefully removed from the water bath; This consists of a chemical etching step of forming at least one opening 5) in the silicon (100) wafer 2 from the cavity 40, and this opening 5) becomes the unprotected region 50 of silicon (100) in a plane parallel to the wafer 2 at the depth p1 inside the wafer 2. - Deposition of a gold layer (Figure 1f): The device can be made to function by depositing gold layers on the front and back surfaces by cathode sputtering. Figure 3 shows an examination (study, analysis) of the influence of laser etching in the method of the present invention on the surface state of Si(100).

[0057]

[0058] ​Different tests were carried out according to the laser output (1W to 4W) and the number of passes of the laser in the etching area (1 to 10 times).

[0059] The laser is used with a frequency of 57 MHz and a diameter (beam diameter) of 15 μm. Figures 3a and 3b show the change in the etching depth obtained as a function of these various parameters. The higher the output and the greater the number of passes, the deeper the depth. An output of 2W was selected and various different etchings were performed according to the number of passes of the laser.

[0060] Figure 3c shows the change in the etching depth. At an output of 2W, etching of an average of 15 μm per pass of the laser is possible. The advantage of this process is that it can etch a fairly deep depth of Si(100) very quickly without using more complex and expensive technical means such as deep reactive ion etching (usually represented by the acronym DRIE in English which means "Deep Reactive Ion Etching") or more time-consuming technical means such as chemical etching with KOH or TMAH (the etching duration is in the range of a few seconds because the laser moves at a speed of 170 mm / second). However, after this laser etching, the surface state of the future film is very rough as shown in the 3D image of Figure 3c. To smooth this surface and complete the etching more gently to accurately obtain the desired depth, chemical etching with TMAH (heated to 84 °C in a water bath) is performed. The resulting kinetics is 0.8 μm / min (Figure 8d). TMAH smooths the surface state (Figure 8e), which is an essential feature for the accurate mechanical operation of the film. By a chemical attack (etching) of 3 hours and 30 minutes, Si(100) is etched by 171 μm (Figure 8f).

[0061] Therefore, the combination of laser etching and chemical etching enables improvement in the quality, speed, and control of etching while ensuring a good surface state of the film.

[0062] Figures 4 and 5 relate to a method of manufacturing a microelectromechanical system according to a second embodiment, and FIG. 5 schematically shows another step of the method of manufacturing the microelectromechanical system according to the first embodiment. - Step A (FIG. 5a): Similar to the method according to the first embodiment, the first step is a step of supplying or manufacturing a piezoelectric epitaxial pseudo-substrate 1 composed of a silicon (100) wafer 2 having a back surface 20 and a front surface 21, and an epitaxial α-quartz (100) thin layer 3 on the front surface 21 of the wafer 2. Next, a thorough cleaning of the quartz (100) / Si(100) substrate is performed using acetone, ethanol, and IPA in order to remove impurities that may contaminate the material or change (tamper with) the microfabrication process; - Step B (FIG. 5b): The same as the method according to the first embodiment (illustrated in FIG. 1b). Next, pre-dehumidification of the substrate 1 is performed at 115° C. for 5 minutes; - Step B1 (FIG. 5c): The back surface of the stack 4 is protected by depositing a negative resin layer 70 (for example, a negative resin sold under the brand name AZ2070), and this deposition can be carried out, for example, by centrifuging at 4,000 rpm for 30 seconds. Then, the set is placed on a hot plate at 115° C. for 1 minute. - Step B2 (FIG. 5d): A photolithography mask 71 including at least one orifice 72 is deposited on the negative resin layer 70. This photolithography mask needs to be manufactured upstream after drawing. This mask includes a single masking level consisting of different squares of various sizes corresponding to future films. The size of the square needs to be calculated according to not only the size of the required film but also the depth of the etching to be performed. In fact, since this chemical etching is performed on Si(100), an etching is created at an angle of 54.7° along the crystal plane of silicon. Depending on the thickness of the substrate, it is necessary to prepare a sufficiently large opening d through which the entire thickness b having this angle can pass. The required length can be found by rapid trigonometric calculations (see Figure 4). Subsequently, the photolithography mask is placed at a selected location on the resin. - Steps B3 and B4 (Figure 5e): Exposure to UV is carried out for 5 seconds at a dose of 37.5 mJ.cm -2 . Subsequently, an annealing treatment is carried out at 115 °C for 1 minute. Next, the set is immersed in a negative-type developer (e.g., negative developer MIF726) for 1 minute to form a cavity 40 corresponding to the square placed on the photolithography mask in the negative-type resin layer 70. Subsequently, the substrate 1 is placed on a hot plate at 125 °C for 5 minutes to vitrify the resin 70. - Step C (Figure 5f - 5h) To remove the protective layers at various locations exposed by the photolithography process (Figure 5f), proceed to a dry etching process C) composed of reactive-ion etching. The alternating layer of SiN / SiO2 / SiN is removed by a gas mixture of 60 sccm of CHF3, 20 sccm of O2, and 10 sccm of Ar ionized at 100 W. This process enables reaching Si(100) which will be etched later (Figure 5g). Finally, excess resin is removed by oxygen plasma treatment with 90 sccm of O2 for 10 minutes. Subsequently, thorough cleaning is performed with acetone, ethanol, and IPA (Figure 5h). - Step D (Figure 5i - 5k) Step D) proceeds as follows: · The pseudo-substrate is placed on a carrier adapted to its form by AMMT (Figure 5i and 2), with the quartz surface facing inward. · Thereafter, the set is placed in a water bath. The expected etching rate is 0.4 μm / min after 1 hour (see Fig. 5). Once the required film thickness is obtained (Fig. 5j), the set needs to be carefully removed from the water bath. Finally, a functional device (5k) is created by depositing a gold layer on the front and back surfaces by cathode sputtering.

Mode for Carrying Out the Invention

[0063] The properties of the products used in the manufacture of the microelectromechanical system according to the present invention based on the piezoelectric epitaxial pseudo-substrate, the method implemented for its manufacture and the optimization of its operating conditions, as well as the property evaluation method, will be described in detail below.

[0064] Products, Raw Materials: - Wafers of N-doped silicon (or "wafer"): Standard disk-shaped wafers consisting of 2, 3, 4 inches (i.e., 5.08 cm, 7.62 cm, 10.16 cm in diameter respectively) are used. - 98% tetraethoxysilane (TEOS), sold by Sigma-Aldrich. - Ethanol (EtOH). - Ultra-high purity H2O. - 37% hydrochloric acid (HCl), sold by Sigma-Aldrich. - Strontium chloride (SrCl2·6H2O), sold by Sigma-Aldrich. - Monohexadecyl ether, sold by Sigma-Aldrich under the brand name Brij-58 (registered trademark). - 25% TMAH solution. - Negative resin sold by Microchemicals under the brand name AZ2070. - Negative developer MIF72 sold by Microchemicals.

[0065] Apparatus for Manufacture and for Structural and Microstructural Property Evaluation -LPKF's Protoplaste U4 -For determining (measuring) the roughness and appearance of the α-quartz layer (100), an optical microscope and an atomic force microscope (AFM: the acronym for the English "atomic force microscope", sold by Veeco under the brand name MULTIMODE); -An optical microscope using an optical microscope with the brand name KEYENCE; -A field emission scanning electron microscope (SEM or SEM-FEG, meaning scanning Electron Microscopy-Field Emission in English), sold by Hitachi under the brand name SU90, to determine the thickness of the α-quartz layer (100); -A diffractometer of the Bruker setup sold under the brand name GADDS D8, with copper irradiation at 1.54056 Å, to determine epitaxy, mosaicity, and crystal homogeneity. -A profilometer (surface shape measuring instrument) sold by Veeco

[0066] <Example 1: Manufacture of an example of a piezoelectric epitaxial pseudo-substrate according to the present invention> According to step A) of the method according to the present invention, a solution of precursors having the following initial composition (in molar units) is prepared. TEOS: Brij-58: HCl: EtOH: SrCl2: 1: 0.3: 0.7: 25: 0.1

[0067] A 3-inch silicon wafer with a thickness of 100 μm and a conductivity of 0.025 Ω / cm is used.

[0068] Next, according to step C) of the method according to the present invention, the precursor composition prepared during step A) is deposited on one of the surfaces 20 of this wafer 2. The deposition is carried out by spin-coating at a temperature of 20°C and a relative humidity of 40% under the following conditions: i. Dynamically disperse (distribute, allocate) 1 mL of the solution at 300 rpm for 5 seconds; ii. Subsequently, perform the final rotation at 2,000 rpm for 30 seconds.

[0069] According to step C’) of the method according to the present invention, by heat treatment at 450 °C, the layer having such a deposited composition is hardened to obtain consolidated amorphous silica, which forms a precursor thin layer of the α-quartz thin layer (100).

[0070] Repeat the succession of these steps C) and C’) four times.

[0071] Next, the final heat treatment of step D) is performed, in which the silicon wafer thus coated with amorphous silica is heated at 980 °C for 5 hours in a tubular furnace with an air flow rate of 12 L / min. Then, turn off the power of the furnace and leave it to cool naturally to 25 °C.

[0072] When step D) of the method according to the present invention is completed, a silicon wafer (or “wafer”) (100) covered with an α-quartz layer is obtained. When a silicon wafer (or “wafer”) (100) covered with a quartz α layer is obtained, it is characterized as follows (see Figure 7): - Figure 7a (right side) shows that the α-quartz thin layer thus obtained is composed of crystalline domains of α-quartz penetrated by forming a homogeneous and continuous mattress; - Figure 7a (left side) shows a cross-section of the α-quartz layer with a thickness of 710 nm; - Figure 7b (AFM image) shows the surface texture and roughness of a layer with an average roughness of 10 nm measured over a 50×50 μm surface area; - Figure 7c shows that the crystallized layer is actually a single crystal layer of α-quartz. In the performed mapping, a mosaicity of 1.7° uniform over the entire Si wafer is shown with respect to the peak (100) of α-quartz; - Figure 7d shows the results of the epitaxy study by X-ray diffraction (XRD). In particular, it shows the epitaxial relationship of the quartz layer (100) on the silicon substrate (100) across the entire pole figure around the reflection (100) = 20.9°. Figure 7d also shows the existence of two quartz domains perpendicular to each other. These two domains with the same epitaxial relationship as the silicon substrate ([

[0210] α quartz (100) / /

[0100] Si(100)]) are made possible by the cubic symmetry of the silicon substrate. Finally, Figure 7d shows a 3D display model of the directions and relationships of the two crystal domains of the epitaxial quartz dense layer on silicon. - Figure 7d also shows the existence of two orthogonal quartz crystal domains having the same epitaxial relationship as silicon. The existence of these two crystal domains of the quartz layer is made possible by virtue of the cubic symmetry of the silicon substrate.

[0073] <Example 2: Fabrication of a piezoelectric microelectromechanical system from a piezoelectric epitaxial pseudo-substrate according to the present invention> The piezoelectric epitaxial pseudo-substrate according to the present invention is made according to the aforementioned example.

[0074] Next, from these piezoelectric epitaxial pseudo-substrates, 25 piezoelectric films are microfabricated by chemical etching using a 25% TMAH solution according to the second embodiment of the method according to the present invention. This etching is carried out for 3 hours and 45 minutes, which is the time required to etch approximately 95 μm of Si(100).

[0075] Figure 8 shows the completion of the entire process. Figure 8a shows a 3-inch wafer before crystallization (left) and after crystallization (right). The microscope images in Figure 8b are, respectively, the SEM image and 3D display of the etching performed during chemical attack (etching) by TMAH.

[0076] Figure 8c shows the structural characteristics of the piezoelectric film by the micro-diffraction technique. The mosaicity values of the film indicate that the microfabrication process has not impaired the crystal quality of the α layer.

[0077] Next, to determine the mechanical performance of the α-quartz(100) / Si(100) piezoelectric film, a characteristic evaluation was performed by laser vibrometry (see Figure 9). A frequency scan was executed to find the resonance frequency of the structure, i.e., the frequency at which the vibration amplitude of the piezoelectric film is the largest. For a 1.1-mm 2 film, a resonance frequency of 54.25 kHz was measured (Figure 9a). Also, the amplitude of the vibration is proportional to the amplitude of the injected current, which corroborates the piezoelectric nature of the film. The quality factor in ambient air was calculated to be 1,346. Mapping of the film at the resonance frequency (Figure 9b) made it possible to sense (observe) its mechanical operation corresponding to the intended operation. Next, the other three resonance modes were visualized as harmonics at f = 88.7 kHz, f = 128.7 kHz, and f = 196.4 kHz, respectively (Figure 13c).

[0078] These results are evidence of the consistent mechanical behavior of the piezoelectric film of epitaxial quartz on silicon with a very high quality factor, which makes this piezoelectric film very attractive for many applications (e.g., biomedical applications in gas sensors or the creation of very accurate mass balances).

[0079] <Example 3: Precise control of the resonance frequency by controlling the size and thickness of the piezoelectric film made of α-quartz> The resonant frequency and displacement of a piezoelectric film made of α-quartz depend on its surface (surface area) and thickness. By controlling these two morphological parameters, it becomes possible to accurately control the resonant frequency and adapt the film morphology to the frequency range of the targeted application.

[0080] According to Example 2, several piezoelectric films based on quartz of different dimensions were fabricated. These films are squares with sides of 2 mm, 2.5 mm, 3 mm, 3.5 mm, and 4 mm. Each of these films is fabricated in two series: a series with a thickness of 2 μm and a series with a thickness of 13 μm.

[0081] In other words, thicknesses of 2 μm and 13 μm correspond to the thickness E’ of a microelectromechanical system consisting of the morphology of a piezoelectric resonant thin film at the level of the opening (position value) defining the unprotected area of silicon (100).

[0082] This study (verification) demonstrates the control of the size (area) and thickness of the film. It emphasizes the influence of these parameters on the resonant frequency and maximum amplitude of the device. An increase in the surface area (surface) of the film decreases the value of the resonant frequency but increases the maximum displacement. When the thickness is 2 μm, the 4 mm 2 film resonates at a frequency of 10.66 kHz with a displacement of 1.5 nm, while the 16 mm 2 film resonates at 3.35 kHz with a displacement of 36.35 nm.

[0083] The results of the experimental data including the resonant frequency f 2 and the maximum displacement of each film with surface areas of 4, 6.25, 9, 12.25, 16 mm r and thicknesses of 2, 13 μm are shown in Table 1 below. Figure 10 also shows that thinner films resonate at lower frequencies than thicker films. The two films have a surface area of 9 mm 2 and the film with a thickness of 2 μm has a displacement D r of 16.58 nm at f maxOn the other hand, the film with a thickness of 13 μm resonates at fr = 26.55 kHz with a displacement D of 6.6 nm max and resonates. Furthermore, the film with a thickness of 13 μm scans a wider range of frequencies than the film with a thickness of 2 μm.

[0084] This example shows that precise control of the resonance frequency of a piezoelectric film made of α-quartz can be achieved by controlling its size and thickness.

[0085] The results clearly demonstrate that a piezoelectric film made of α-quartz with a thickness of 2 μm can be advantageously used for gas detection by the photoacoustic effect, as described, for example, in Trzpil, W. et al. Analytic Optimisation of Cantilevers for Photoacoustic Gas Sensor with Capacitive Transduction. Sensors 21, (2021) [8] In particular, the example clearly demonstrates that a piezoelectric film made of α-quartz with a thickness of 2 μm can have a resonance frequency in the range of 3 - 11 kHz that enables gas detection.

Table 1

[0086] <Example 4: Precise control of the resonance frequency of a piezoelectric film made of α-quartz as a function of the medium in contact with the openings present in the film> In this example, the films used correspond to the piezoelectric films based on quartz with different dimensions described in Example 2. They consist of square films with sides of 2.5 mm, 3 mm, 4 mm and a thickness of 2 μm.

[0087] These films were fabricated in two series: - A series where the back of the epitaxial α-quartz thin film is in contact with air, as described in Example 3; and - A series where the film contains a thin layer of quartz α and its front face has a volume of 352.8 mm 3Series in contact with the liquid in a silicon cubic tank. The tank has an opening facing the α-quartz thin layer, and the surface area of the opening is 70.56 mm 2 The volume of the liquid in the reservoir (tank) was 80 μL of water.

[0088] Research (verification) of the resonance frequency as a function of the surface area of the piezoelectric film was independently conducted for these two series.

[0089] The results of the experimental data including the resonance frequencies f 2 of each film with surface areas of 6.25, 9, and 16 mm r in gas, i.e., air containing 78.087% nitrogen (N2), 20.95% oxygen (O2), 0.93% argon (Ar), 0.041% carbon dioxide (CO2), or in liquid, i.e., water, are shown in Table 2 below. The two films have a surface area of 9 mm 2 The film in the presence of gas, i.e., air, resonates at f r = 8.16 kHz, while the film in the presence of liquid resonates at f r = 0.902 kHz.

[0090] The experimental data demonstrate that when the tank contains 80 μL of water, the resonance frequency decreases. In other words, the presence of liquid decreases the resonance frequency of the film.

[0091] Therefore, the results obtained clearly prove that the piezoelectric film according to the present invention can be in a vibrating state when the front surface of the α-quartz thin layer epitaxially grown from the piezoelectric film is in contact with the liquid.

[0092] Therefore, the results obtained clearly prove that the resonance frequency of the piezoelectric film according to the present invention can vary depending on whether the front surface of the α-quartz thin layer is in contact with gas and / or liquid.

[0093] Thus, this example shows that the membrane can be used for biological or other applications, such as the detection and / or quantification (numerical conversion) of cells that may be present in a liquid, even in the presence of the liquid. [Table 2]

[0094] List of References 1. J. S. Danel & G. Delapierre. Quartz: a material for microdevices. Journal of Micromechanics and Microengineering 1, 187 (1991). 2. B. Imbert et al. Thin film quartz layer reported on silicon. in 1-4 (2011). doi:10.1109 / FCS.2011.5977829. 3. Brinker, C. J. & Clem, P. G. Quartz on Silicon. Science 340, 818-819 (2013). 4. Carretero-Genevrier, A. et al. Soft-Chemistry-Based Routes to Epitaxial alpha-Quartz Thin Films with Tunable Textures. Science 340, 827-831 (2013). 5. C Boissiere, A Carretero-Genevrier, M Gich, D Grosso, C Sanchez: "Process for preparing an epitaxial alpha-quartz layer on a solid substrate, material obtained and uses", WO20140165 or EP2875172. 6.S. KUMAR, Dr. D. K. ASWAL, Recent Advances in Thin Films. Materials Horizons: From Nature to Nanomaterials. Springer, Singapore. https: / / doi.org / 10.1007 / 978-981-15-6116-0_1 7.Ding, C. et al. Wafer-scale single crystals: crystal growth mechanisms, fabrication methods, and functional applications. J Mater Chem C 9, 7829-7851 (2021) 8.Trzpil, W. et al. Analytic Optimisation of Cantilevers for Photoacoustic Gas Sensor with Capacitive Transduction. Sensors 21, (2021)

Claims

1. A microelectromechanical system (10) in the form of a piezoelectric resonant thin film, comprising: - a silicon (100) wafer (2) having a back surface (20) and a front surface (21); an epitaxial α-quartz (100) thin layer (3) on the front surface (21) of the wafer (2); a piezoelectric epitaxial pseudo-substrate (1) composed of; - Three consecutive layer stacks (4) of SiN(41), SiO ２ (42), and SiN(43) deposited on the back surface (20) of the wafer (2); - at least one opening (5) that penetrates the stack (4) and a part of the silicon (100) wafer (2) from the front surface (21) of the wafer (2) to a depth p1, the opening (5) defining (explicitly) a non-protected region (50) of silicon (100) in a plane parallel to the wafer (2) at a depth p1 inside the wafer (2).

2. The microelectromechanical system (10) according to claim 1, wherein the α-quartz thin layer (3) has a mosaicity (mosaic degree) of 6° to 1° around the peak (100) of quartz and a homogeneous crystallization (crystal) with a thickness of 100 nm to 1 μm.

3. The microelectromechanical system (10) according to claim 1 or claim 2, wherein the thickness of the α-quartz (100) thin layer (3) ranges from 200 nm to 1 μm.

4. The microelectromechanical system (10) according to any one of claims 1 to 3, wherein the α-quartz thin layer (3) has a homogeneous crystallization (crystal) with a mosaicity (mosaic degree) of 2.5° to 1.4°.

5. The microelectromechanical system (10) according to any one of claims 1 to 4, wherein the non-protected region (50) of silicon (100) has a square shape at a depth p1.

6. The opening (5) starts from the front surface (21), is inside the wafer (2), and consists of four straight sidewalls (51A, 52A, 53A, 54A) substantially perpendicular to the wafer (2) to a depth p2 located below the depth p1. The sidewalls (51A, 52A, 53A, 54A) are extended by four trapezoidal inclined walls (51B, 52B, 53B, 54B) and form an angle α of 54.7° with a plane parallel to the wafer (2) located at a depth p2. The microelectromechanical system (10) according to claim 5, characterized in that.

7. The microelectromechanical system (10) according to any one of claims 1 to 6, further comprising a first gold layer (6) disposed on the α-quartz (100) thin layer (3) and a second gold layer (6) disposed on the non-protected region (50) of the silicon (100).

8. On the α-quartz (100) thin layer (3), ZnO or Al having a crystal orientation (110) ２ O ３ or HfO ２ The microelectromechanical system (10) according to any one of claims 1 to 7, further comprising an epitaxial thin layer based on microcrystals.

9. In the method for manufacturing a microelectromechanical system (10) according to any one of claims 1 to 4, the method includes the following steps: A) Supplying or manufacturing a piezoelectric epitaxial pseudo-substrate (1) defined in any one of claims 1 to 4; B) A step of depositing a stack (4) of three consecutive layers of SiN(41), SiO ２ (42), and SiN(43) on the back surface (20) of the wafer (2); C) Pre-etching the stack (4) by a physical dry etching process to form at least one cavity (40) therein; and D) A chemical etching process for forming at least one opening (5) in the silicon (100) wafer (2) from the at least one cavity (40), wherein the opening (5) defines a non-protected region (50) of silicon (100) in a plane parallel to the wafer (2) located at a depth p1 inside the wafer (2).

10. The step C) of pre-etching the stack (4) may be composed of laser etching, and the method further includes the following: - Between step C) and step D), laser-etching the wafer (2) to a depth p2 in the wafer (2) located between the depth p1 and the stack (4) to excavate four linear sidewalls (51A, 52A, 53A, 54A) substantially perpendicular to the wafer (2) from the cavity (40) to a depth p2; and - Step C") of protecting the α-quartz (100) thin layer (3); - Extending the chemical etching from the depth p2 to the depth p1 to expand the linear sidewalls (51A, 52A, 53A, 54A) by four trapezoidal inclined walls (51B, 52B, 53B, 54B) to form an angle α of 54.7° with respect to a plane parallel to the wafer (2) located at the depth p2, step D). The method according to claim 9, characterized in that.

11. The method according to claim 10, characterized in that in step C'), the laser is used at a frequency of 57 MHz and a diameter (beam diameter) of 15 μm.

12. The method according to claim 10 or claim 11, characterized in that the output of the laser is 2 W.

13. The step C) of pre-etching the stack (4) is composed of reactive ion etching, and the method further includes the following steps between step B) and step C): - Step B1) of protecting the back surface of the stack (4) by depositing a negative resin layer (70); and subsequently - Step B2) of depositing a photolithography mask (71) including at least one orifice (72) on the negative resin layer (70), and then - Step B3) of exposing the set to ultraviolet radiation, and annealing treatment; - Step B4) of immersing in a negative developer to form at least one cavity (40) in the negative resin layer (70), which consists of The step C) related to etching by reactive ion etching is intended to extend the etching of the cavity (40) in the stack (4) to form four straight sidewalls (51A, 52A, 53A, 54A) therein; and The chemical etching step D) extends the chemical etching from the cavity (40) to a depth p1, and the straight sidewalls (51A, 52A, 53A, 54A) are extended by four trapezoidal inclined walls (51B, 52B, 53B, 54B) to form an angle α of 54.7° with respect to the wafer (2). The method according to claim 9, characterized in that.

14. Step A) is performed as follows: Step of preparing a composition containing a solvent, at least one silica precursor and / or colloidal silica, and a catalyst selected from the above elements with a +2 oxidation state forming a group consisting of strontium, barium, calcium, magnesium and beryllium, or a catalyst selected from the above elements with a +1 oxidation state forming a group consisting of cesium, rubidium, lithium, sodium or potassium, wherein the catalyst has a catalyst:SiO ２ molar ratio ranging from 0.0375 to 0.125; A2) A step of supplying a silicon (100) wafer (2) having a back surface (20) and a front surface (21); A3) A step of depositing at least one layer of the composition obtained at the completion of step A1) by spin coating, and the deposition is performed on at least a part of the back surface (20) of the wafer (2); and A4) A step of performing a heat pretreatment at a temperature of 400°C to 600°C to form a solidified amorphous silica thin layer (3) at the completion of step C'); A5) A step of heat-treating the solidified amorphous silica thin film (3) at a temperature ranging from 800°C to 1200°C The method according to any one of claims 9 to 12, characterized in that it consists of.

15. The composition prepared in step A1) comprises a precursor selected from methyltrimethoxysilane (MTMS), tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), dimethyldimethoxysilane, and mixtures thereof, preferably tetraethoxysilane (TEOS), and the method according to claim 14 is characterized in that.

16. Step A3) is: - A first phase of dynamically dispersing (distributing, allocating) the composition of step A1) by centrifuging at a speed of 100 to 500 rpm for 5 to 10 seconds; then; - A second phase of forming an α-quartz (100) thin film by centrifuging at a speed of 500 to 6,000 rpm for 10 to 40 seconds, The two dispersion (distribution, allocation) phases are separated by a waiting time of 0 to 15 seconds The method according to claim 14 or claim 15, characterized in that.

17. Steps A3) and A4) are repeated continuously one or more times, and the method according to any one of claims 14 to 16 is characterized in that.

18. On the back surface (20) of the wafer (2), step B) related to the deposition of layers of SiN (41), SiO ２ (42), and SiN (43) is preferably carried out at 280 ° C by plasma-enhanced chemical vapor deposition, the method according to any one of claims 13 to 16.

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