Volume sound wave device and method for manufacturing such a device

Description

TECHNICAL FIELD

[0001] The present invention relates, in general, to a volume acoustic wave device, and more particularly, to a method of making the layer based on a piezoelectric material forming the core of this device. STATE OF THE ART

[0002] Resonators based on a bulk acoustic wave (BAW) structure are commonly used to create RF filters. The core of BAW resonators is composed of a piezoelectric material that influences the final properties of the filter.

[0003] Lithium niobate (LiNbO3) has been used for several years as a piezoelectric material. Its intrinsic properties, such as its piezoelectric coupling coefficient, could allow the filter to resonate at high frequencies, for example, above 3.5 GHz, with a bandwidth of up to 600 MHz. To obtain a BAW filter with a high coupling coefficient, typically up to 50%, the LiNbO3-based piezoelectric layer must, in particular, exhibit high crystalline quality and controlled stoichiometry.

[0004] The document "A. Reinhardt et al., Acoustic filters based on thin single crystal LiNbO3 films: Status and prospects Proc. - IEEE Ultrason. Symp. 2014, 773" discloses a solution based on a transfer process known as Smart-Cut™ for fabricating a BAW filter comprising a piezoelectric layer of LiNbO3 on a silicon substrate. A thin layer of LiNbO3 is taken from a donor substrate, typically a single-crystal LiNbO3 substrate, and then transferred to a recipient substrate, typically a silicon substrate. This solution involves numerous technical steps and is costly.

[0005] Another solution involves synthesizing the LiNbO3-based piezoelectric layer on the silicon substrate via a buffer layer. The paper "Growth of highly near-c-axis oriented ferroelectric LiNbO3 thin films on Si with a ZnO buffer layer, P. You et al., Appl. Phys. Lett. 2013, 102, 051914" proposes forming a zinc oxide buffer layer on the silicon substrate before forming the lithium niobate-based layer on top of this buffer layer. However, the synthesized LiNbO3-based piezoelectric layer exhibits a textured appearance. It is not single-crystal. Furthermore, the ZnO-based buffer layer is not entirely compatible with integration into a functional device, particularly with certain etching steps such as etching the bottom electrode or etching to create the cavity beneath the ZnO. Adhesion problems are also observed.

[0006] The document CN111769809A discloses a manufacturing process for a BAW FBAR filter that eliminates CMP planarization prior to device membrane formation.

[0007] There is therefore a need to produce an electro-acoustic device comprising a piezoelectric layer based on lithium niobate on a silicon-based substrate, which offers improved quality, while limiting production costs.

[0008] One objective of the present invention is to meet at least part of this need.

[0009] The other objects, features, and advantages of the present invention will become apparent from an examination of the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY

[0010] To achieve this objective, according to one embodiment, a method for manufacturing an electroacoustic device comprising a piezoelectric layer is provided, according to claim 1 of the present invention. The method notably comprises the following steps: Provide a silicon-based substrate, Form a first electrode on the substrate, Form the piezoelectric layer on the first electrode, Form a second electrode on the piezoelectric layer.

[0011] The formation of the piezoelectric layer is done by epitaxy, on said first electrode, of a material of the type ABO3, O being oxygen, A being at least one first chemical element taken from among sodium (Na), potassium (K), barium (Ba), lithium (Li), lead (Pb), and B being at least one second chemical element taken from among zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta).

[0012] The first electrode is chosen in an electrically conductive refractory material based on nitride, advantageously such as TiN, VN, ZrN, TaN, HfN, NbN, or an alloy of said materials.

[0013] Preferably, the ABO3 material of the piezoelectric layer is taken from lithium niobate LiNbO3, lithium tantalate LiTaO3, or a Li(Nb, Ta)O3 alloy.

[0014] In the context of the development of the present invention, certain conditions for the epitaxial growth of the piezoelectric layer were determined. The first electrode should preferably exhibit at least some of the following properties: Having a crystalline structure and / or lattice parameter compatible with a silicon-based substrate. This allows, for example, the first electrode to be epitaxially bonded to the substrate. Having a crystalline structure and / or lattice parameter close to or compatible with lithium niobate or lithium tantalate. This allows the piezoelectric layer to be epitaxially bonded to the first electrode, thus forming a nucleation layer. Having a coefficient of thermal expansion close to that of lithium niobate or lithium tantalate. This helps limit the occurrence of cracks due to temperature variations during the formation of the piezoelectric layer. Being electrically conductive. Being able to block the diffusion of Li atoms into Si. This prevents a loss of stoichiometry in a piezoelectric layer based on lithium niobate or lithium tantalate, known as an LN / LT layer.Confining an acoustic wave, particularly to guarantee high performance for electro-acoustic devices.

[0015] A technical prejudice of the state of the art is that the nucleation layer must necessarily be oxide-based in order to avoid a loss of stoichiometry of the LN / LT layer due to diffusion of oxygen from the LN / LT layer to the nucleation layer.

[0016] On the contrary, to meet these specifications, the first electrode forming the nucleation layer is chosen according to the present invention from an electrically conductive refractory material based on a nitride. This material is typically a transition metal refractory nitride, for example TiN, VN, ZrN, TaN, HfN, NbN, or their alloys. During the development of the present invention, it was observed that such a nucleation layer unexpectedly allows the epitaxy of the LN / LT layer under favorable conditions. Surprisingly, it was also observed that such a nucleation layer blocks both oxygen diffusion and lithium diffusion into the silicon-based substrate. As a result, the LN / LT layer epitaxially grown on such a nucleation layer maintains the required stoichiometry.

[0017] Furthermore, the various crystal structures and lattice parameters of refractory transition nitrides are perfectly compatible with those of LN / LT materials and silicon. Such a nucleation layer can therefore advantageously be epitaxially grown on a silicon-based substrate, and then allow the epitaxy of the LN / LT layer.

[0018] Furthermore, refractory transition metal nitrides are conductive and can therefore be used as the first electrode in a BAW filter. If these materials are epitaxially grown on silicon, they can exhibit resistivity values ​​on the order of 15 µOhm·cm, which is perfectly compatible with the fabrication of a BAW filter.

[0019] Furthermore, refractory transition nitrides are resistant to the growth conditions of LiNbO3, in particular the high temperature required for the growth of LiNbO3, which can be on the order of 1000°C.

[0020] Furthermore, refractory transition nitrides are sufficiently hard to protect the silicon surface during LiNbO3 growth. For example, they protect the silicon from plasma and ion impact used in the implementation of a PVD physical vapor deposition technique for LiNbO3 growth.

[0021] Transition refractory nitrides can also exhibit interesting stiffness and acoustic impedance values ​​to further confine acoustic waves in the piezoelectric layer.

[0022] The present invention thus proposes a solution for forming an electroacoustic device on a silicon-based substrate—without a transfer step—via a first electrode based on a refractory transition nitride, acting as a nucleation layer. Thanks to this nucleation layer made of an electrically conductive refractory nitride-based material, the LN / LT piezoelectric layer is stoichiometric and of high crystalline quality.

[0023] The piezoelectric layer thus exhibits the properties required for a high-frequency BAW filter type application.

[0024] According to another aspect of the invention, an electro-acoustic device according to claim 6 of the present invention is provided, in particular a volume acoustic wave (BAW) device, comprising in particular, stacked in a vertical direction: a silicon-based substrate, a first electrode on said substrate, a piezoelectric layer on said first electrode, said piezoelectric layer being based on a material of type ABO3, O being oxygen, A being at least a first chemical element taken from sodium (Na), potassium (K), barium (Ba), lithium (Li), lead (Pb), and B being at least a second chemical element taken from zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta), a second electrode disposed on the piezoelectric layer.

[0025] The first electrode of the BAW device is made of an electrically conductive refractory nitride-based material. Preferably, the piezoelectric layer is based on lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or a Li(Nb,Ta)O3 alloy. The advantages mentioned above apply. mutatis mutandis.Furthermore, such a device can be directly integrated into silicon technology. It can be easily co-integrated with other microelectronic or optoelectronic devices or with (opto-)electromechanical microsystems (MEMS or MOEMS, respectively acronyms for "Microelectromechanical systems" and "Microoptoelectromechanical systems"). BRIEF DESCRIPTION OF THE FIGURES

[0026] The aims, objects, features and advantages of the invention will become clearer from the detailed description of embodiments thereof, which are illustrated by the following accompanying drawings in which: There figure 1 illustrates in cross-section a BAW-type electro-acoustic device according to an example useful for understanding the present invention. figure 2 illustrates in cross-section an electro-acoustic device of the BAW FBAR type according to an example useful for understanding the present invention. figure 3Aillustrates in cross-section an electro-acoustic device of the BAW SMR type according to an embodiment of the present invention. figure 3B illustrates in cross-section an electro-acoustic device of the BAW SMR type according to another embodiment of the present invention. figures 4A to 4E illustrate steps in a manufacturing process for a BAW FBAR type electro-acoustic device according to an example useful for understanding the present invention.

[0027] The drawings are provided by way of example and are not intended to limit the scope of the invention. They are schematic representations of the principle intended to facilitate understanding of the invention and are not necessarily to scale with practical applications. In particular, in the schematic diagrams, the thicknesses of the various layers and portions, and the dimensions of the patterns, are not representative of reality. DETAILED DESCRIPTION

[0028] Before proceeding with a detailed review of embodiments of the invention, optional features that may be used in combination or alternatively are stated below: For example, the electrically conductive nitride-based refractory material of the first electrode is chosen to exhibit or form: Resistance to oxidation, a barrier to lithium diffusion.

[0029] This prevents oxygen and lithium from diffusing from the LN / LT piezoelectric layer to the first electrode. The stoichiometry of the LN / LT layer is thus preserved.

[0030] As an example, the electrically conductive nitride-based refractory material of the first electrode is chosen to have sufficiently high hardness or mechanical stiffness to confine an acoustic wave within the LN / LT piezoelectric layer. For example, the Young's modulus of this material is greater than 300 GPa. Its hardness or strength can be greater than 9000 MPa.

[0031] As an example, the electrically conductive nitride-based refractory material of the first electrode is chosen to have a crystallographic structure compatible with the substrate and the LN / LT piezoelectric layer, such as a hexagonal or face-centered cubic structure. This helps to limit the occurrence of structural defects during the formation of the LN / LT layer by epitaxy.

[0032] As an example, the formation of the first electrode is configured so that said first electrode has a thickness e2 between 10 nm and 1000 nm, for example 100 nm. This makes it possible to obtain the desired crystalline quality and / or to limit the occurrence of structural defects.

[0033] As an example, the formation of the LN / LT piezoelectric layer is configured such that, after epitaxy, the LN / LT layer has a thickness e3 ranging from several hundred nanometers to several microns. This thickness e3 allows the desired resonant frequency for the BAW filter to be obtained.

[0034] According to one example, the electrically conductive nitride-based refractory material is taken from among the transition metal-based refractory nitrides such as titanium nitride TiN, tantalum nitride TaN, niobium nitride NbN, zirconium nitride ZrN, hafnium nitride HfN, vanadium nitride VN, or their alloys.

[0035] According to one example, the electrically conductive nitride-based refractory material of the first electrode is chosen from a transition refractory nitride such as titanium nitride TiN, tantalum nitride TaN, niobium nitride NbN, zirconium nitride ZrN, hafnium nitride HfN, vanadium nitride VN, or their alloys.

[0036] As an example, the electro-acoustic device is a suspended membrane type volume acoustic wave filter. In another example, the process comprises, after epitaxial formation of the piezoelectric layer, etching the silicon-based substrate beneath the first electrode bearing the piezoelectric layer, so as to form a cavity.

[0037] According to the invention, the electro-acoustic device is a Bragg reflector-guided volume acoustic wave filter. According to the invention, the method comprises, prior to the formation of the first electrode, the formation of a stack of alternating layers of strong and weak acoustic impedances.

[0038] Low impedance layers are based on TiN or VN, and high impedance layers are based on HfN or TaN.

[0039] According to one example, the formation of the first electrode and / or the formation of the LN / LT piezoelectric layer are carried out by pulsed laser ablation.

[0040] As an example, the formation of the first electrode and the formation of the LN / LT piezoelectric layer are performed by pulsed laser ablation successively within the same reactor without any air re-entry between these formations. This eliminates the need for an intermediate surface cleaning step, reduces the overall process time, and limits costs. It also results in low surface roughness of the LN / LT layer.

[0041] According to one example, the substrate is silicon-based oriented along (111), the first electrode is titanium nitride TiN-based oriented along (111), and the piezoelectric layer is oriented along (0001).

[0042] In one example, the LN / LT piezoelectric layer is in direct contact with the first electrode. Specifically, there is no intervening oxide layer between the first electrode and the LN / LT layer.

[0043] According to one example, the electrically conductive nitride-based refractory material of the first electrode is a transition refractory nitride such as titanium nitride TiN, tantalum nitride TaN, niobium nitride NbN, zirconium nitride ZrN, hafnium nitride HfN, vanadium nitride VN, or their alloys.

[0044] As an example, the electro-acoustic device is a suspended membrane type volume acoustic wave filter, comprising a cavity under the first electrode carrying the piezoelectric layer.

[0045] According to the invention, the electro-acoustic device is a Bragg reflector-guided volume acoustic wave filter, comprising, between the substrate and the first electrode, a stack of alternating layers having strong and weak acoustic impedances.

[0046] In one example, the first and second electrodes are contacted at the front face of the device.

[0047] In one example, the first electrode is contacted at the rear face of the device, and the second electrode is contacted at the front face of the device.

[0048] As an example, the silicon-based substrate is formed from a material taken from: silicon, SiC, SiGe.

[0049] As an example, the silicon-based substrate is monocrystalline.

[0050] According to an alternative example, the silicon-based substrate is polycrystalline.

[0051] According to one example, the silicon-based substrate is oriented along (111).

[0052] Unless otherwise required, it is understood that all the optional features described above may be combined to form an embodiment that is not necessarily illustrated or described. Such an embodiment is obviously not excluded from the invention. The features of an aspect of the invention, for example, the device or the method, may be adapted mutatis mutandis to the other aspect of the invention.

[0053] It is specified that, within the framework of the present invention, the terms "on", "overcomes", "covers", "underlying", "opposite" and their equivalents do not necessarily mean "in contact with". Thus, for example, the deposition of a first layer on a second layer does not necessarily mean that the two layers are directly in contact with each other, but means that the first layer at least partially covers the second layer by being either directly in contact with it, or by being separated from it by at least one other layer or at least one other element.

[0054] A layer can also be composed of several sub-layers of the same material or of different materials.

[0055] A substrate, stack, or layer "based" on a material A is defined as a substrate, stack, or layer comprising only that material A, or that material A and possibly other materials, such as alloying elements and / or dopants. Thus, a silicon-based substrate is defined, for example, as a Si, doped Si, SiC, or SiGe substrate. A TiN-based layer is defined, for example, as a TiN layer, doped TiN, or TiN alloys.

[0056] Selective etching with respect to or etching exhibiting selectivity with respect to means an etching configured to remove a material A or a layer A with respect to a material B or a layer B, and exhibiting an etching speed of material A greater than the etching speed of material B. Selectivity is the ratio between the etching speed of material A and the etching speed of material B.

[0057] The term "electrical conductor" means a material with a resistivity value of less than 10000 µOhm.cm, preferably less than 300 µOhm.cm, and even more preferably less than 100 µOhm.cm, especially for an electrical conductor formed by a lower electrode.

[0058] Several embodiments of the invention implementing successive steps of the manufacturing process are described below. Unless explicitly stated, the adjective "successive" does not necessarily imply, although this is generally preferred, that the steps follow each other immediately; intermediate steps may separate them.

[0059] Furthermore, the term "step" refers to the completion of a part of the process, and can designate a set of sub-steps.

[0060] Furthermore, the term "step" does not necessarily imply that the actions carried out during a step are simultaneous or immediately successive. Some actions in a first step may be followed by actions related to a different step, and other actions from the first step may be repeated later. Thus, the term "step" does not necessarily refer to unitary actions that are inseparable in time and in the sequence of phases of the process.

[0061] An orthonormal coordinate system, preferably comprising the x, y, and z axes, is shown in the accompanying figures. When only one coordinate system is shown on a single sheet of figures, that system applies to all figures on that sheet.

[0062] In this patent application, the thickness of a layer is measured in a direction normal to the principal extension plane of the layer. Thus, a layer typically has a thickness along the z-axis. The relative terms "on," "overlies," "under," "substrate," and "intercalated" refer to positions measured along the z-direction.

[0063] The terms "vertical" and "vertically" refer to a direction along the z-axis. The terms "horizontally," "horizontally," "laterally," and "laterally" refer to a direction in the xy plane. Unless explicitly stated otherwise, thickness, height, and depth are measured along the z-axis.

[0064] An element located "in line with" or "directly above" another element means that these two elements are both located on the same line perpendicular to a plane in which extends mainly a lower or upper face of a substrate, that is to say on the same line oriented vertically in the figures.

[0065] In the context of the present invention, a refractory transition nitride is a nitride of a transition metal or transition element (an element whose atoms have an incomplete d electron subshell, or which can form cations with an incomplete d electron subshell). These elements are grouped within the d block of the periodic table of elements.

[0066] In the examples described below, the illustrated LN / LT layer is based on lithium niobate (LiNbO3). A lithium tantalate (LiTaO3) layer or a layer based on a Li(Nb,Ta)O3 alloy can be substituted for this lithium niobate (LiNbO3) layer within the scope of this invention. Thus, all the characteristics and technical effects mentioned with regard to LiNbO3 are perfectly applicable and combinable with a LiTaO3 or Li(Nb,Ta)O3 layer.

[0067] X-ray diffraction analyses, for example in 2θ configuration, or in rotation around φ and / or Ω (phi-scan and omega-scan), can be carried out to determine the crystalline quality of the LN / LT layers and the electrodes or nucleation layers, and their epitaxial relationship.

[0068] There figure 1Figure 1 illustrates in cross-section a BAW HBAR type filter device, according to an example useful for understanding the invention. This device 1 typically comprises a silicon-based substrate 10 on which a first titanium nitride (TiN) electrode 21 is formed. A lithium niobate (LiNbO3) layer 30 is in direct contact with the first electrode 21. A second electrode 22 is placed above the piezoelectric layer 30 (LN / LT).

[0069] The use of a first electrode based on a refractory, electrically conductive nitride, particularly TiN, advantageously enables the heteroepitaxy of LiNbO3 on silicon substrates. This first electrode can thus be considered a nucleation layer. This solution allows for the fabrication of high-performance BAW electroacoustic devices. Epitaxy makes it possible to obtain a stoichiometric LiNbO3 layer, preferentially oriented along a single crystal orientation in all directions and of high crystalline quality. Furthermore, the thickness of this epitaxially obtained LiNbO3 layer is perfectly controlled. The LiNbO3 layer can be formed directly on substrates of various sizes, without an intermediate transfer step. This advantageously reduces the manufacturing costs of such a stack of layers.Such a process can also be directly integrated into a production plant using CMOS technology (MOS transistors - Metal / Oxide / Semiconductor - complementary type N and P).

[0070] The substrate 10 can be a bulk silicon substrate. Alternatively, this substrate 10 can be a SOI (Silicon On Insulator) substrate. Other substrates 10 can be considered, for example, SiC-based substrates. Such substrates are fully compatible with silicon technologies for microelectronics. Silicon can also be thermally and / or electrically conductive. This allows, for example, the dissipation of heat generated in an operating BAW filter. It also allows the device to be contacted via the back side 100, particularly when the silicon is doped.

[0071] The first electrode or nucleation layer 21 is preferably based on a refractory transition metal nitride. It preferably has a hexagonal or face-centered cubic crystallographic structure. Such a structure allows, in particular, epitaxy of the nucleation layer onto silicon oriented along (001) and (111). This also allows epitaxy of an LN / LT material in various crystal orientations. Preferably, the silicon is oriented along (111). The nucleation layer 21 is, for example, based on titanium nitride (TiN). TiN is typically thermally and electrically conductive. TiN is also a rigid material that allows an acoustic wave to be confined within an upper LN / LT layer. TiN can be propagated or deposited on silicon in a known manner. Vanadium nitride, zirconium nitride, tantalum nitride, hafnium nitride, and niobium nitride have properties similar to those of titanium nitride.They can also be advantageously chosen as the first electrode / nucleation layer 21. In the following, due to its dual function as an electrode and nucleation layer, the first electrode 21 is referred to as the "nucleation electrode 21".

[0072] The nucleation electrode 21 can be formed by a physical or chemical deposition technique, for example and preferably by pulsed laser ablation (PLD). It can alternatively be formed by one of the following techniques: chemical vapor deposition (CVD), preferably metal-organic precursor chemical vapor deposition (MOCVD), PVD sputtering, plasma-enhanced atomic layer deposition (PEALD).

[0073] It is preferably epitaxially grown on substrate 10. This results in a single-crystal nucleation layer 21. In another embodiment, the nucleation electrode 21 is polycrystalline, for example with a preferred orientation. An orientation along a growth plane (111) can typically be chosen for a TiN nucleation electrode 21.

[0074] The nucleation electrode 21 formed on the substrate 10 is preferably stoichiometric. It has a thickness e2 between 10 and 1000 nanometers, for example on the order of 100 nm. The thickness e2 of the nucleation electrode 21 can be chosen according to the desired crystalline quality.

[0075] The piezoelectric layer 30 LN / LT is based on lithium niobate (LiNbO3) or lithium tantalate (LiTaO3), or an alloy of the type Li(Ta,Nb)O3. This layer 30 can be formed by a physical or chemical deposition technique, for example, and preferably by pulsed laser ablation (PLD). It can alternatively be formed by one of the following techniques: chemical vapor deposition (CVD), preferably metal-organic precursor chemical vapor deposition (MOCVD), PVD sputtering, or molecular beam epitaxy (MBE).

[0076] The 30 LN / LT piezoelectric layer is advantageously epitaxially grown on the nucleation electrode 21. It can be single-crystal or polycrystalline, with, for example, a preferred orientation. An orientation along a (0001) growth plane can typically be chosen. This maximizes the propagation speed of acoustic waves within the 30 LN / LT layer. Other orientations can be selected depending on the desired application.

[0077] The 30LN / LT layer formed on the nucleation electrode 21 is preferably stoichiometric, for example Li₁Nb₁O₃. The atomic percentage of lithium is ideally close to 50%. The 30LN / LT layer has a thickness e₃ between 50 nanometers and a few hundred nanometers or a few microns, for example, on the order of 500 nm. The thickness e₃ of the 30LN / LT layer can be chosen according to the desired resonance frequency.

[0078] According to one possibility, the nucleation electrode 21 and the LN / LT layer 30 are both made in situ by PLD in the same growth reactor. The growth of the 30 LN / LT layer can thus be carried out directly after the completion of the growth of the nucleation electrode 21. This avoids re-exposing the nucleation electrode 21 to air before epitaxy of the 30 LN / LT layer. The surface of the nucleation electrode 21 is therefore not contaminated, polluted, or oxidized. This eliminates an intermediate cleaning step. The process time is thus reduced. This also limits the development of roughness during the formation of the 30 LN / LT layer. The state of the interface between the 30 LN / LT layer and the nucleation electrode 21 is therefore not altered, and the surface area of ​​the 30 LN / LT layer is thus optimized.

[0079] The second electrode 22 is typically made of an electrically conductive electrode material. This electrode material may also exhibit high acoustic impedance. Platinum, aluminum, or molybdenum may be chosen as the electrode material. The second electrode 22 may be structured to exhibit electrode patterns. Such an electrode 22 is typically formed by lithography / etching from an electrically conductive layer deposited on the LN / LT layer 30. In the electroacoustic devices 1 according to the present invention, the piezoelectric layer 30 is sandwiched between the first and second electrodes 21, 22, along the vertical z-direction. The piezoelectric layer 30 is preferably in direct contact with the first and second electrodes 21, 22.

[0080] There figure 2illustrates another embodiment of device 1. In this example, device 1 is a BAW filter of the FBAR type. The characteristics of the various elements described above in the case of the BAW HBAR (substrate, nucleation electrode, piezoelectric layer, second electrode) are also valid in the following embodiments. mutatis mutandis. In addition to the elements described above, the BAW FBAR includes a cavity 40 formed in the substrate 10, below the piezoelectric layer 30 and the nucleation electrode 21.

[0081] This cavity 40 can be formed by etching the substrate 10 through an opening in the nucleation electrode 21. Advantageously, in the case of a silicon substrate and a TiN nucleation electrode 21, the etching selectivity SSi:STiN between Si and TiN is sufficiently high, typically greater than 5:1, so that the nucleation electrode 21 is not damaged during the silicon etching. The silicon etching can be carried out by wet etching using a dilute ammonia solution (NH4OH) or tetramethylammonium hydroxide (TMAH). The nucleation electrode 21 then forms, with the piezoelectric layer 30, a membrane suspended above the cavity 40.

[0082] The cavity 40 is preferably formed directly above the piezoelectric layer 30. It can extend on either side of the edges 31, 32 of the piezoelectric layer 30, in projection in the xy plane.

[0083] THE figures 3A, 3Billustrate different embodiments of a BAW-SMR type device 1 according to the invention. In these devices, in addition to the elements of the BAW HBAR, a stack 50 of layers 51, 52 forming an "acoustic" Bragg reflector is intercalated between the substrate 10 and the nucleation electrode 21.

[0084] This stack 50 is preferably epitaxially grown on the substrate 10. It preferably exhibits electrical conductivity through all layers 51, 52. In the embodiment illustrated in the figure 3A The electrical contacts 41, 42 are made on the same side of the device 1, typically on the front face 101, directly on the first and second electrodes 21, 22. In the embodiment illustrated in the figure 3BThe electrical contacts 41 and 42 are made on two opposite sides of the device 1, typically on the front face 101 of the electrode 22 and on the rear face 100 of the substrate 10. The first electrode 21 is then indirectly contacted via the substrate 10 and the stack 50. This avoids the need for re-establishing contact on the front face for the first electrode 2, which is located beneath the piezoelectric layer 30. The number of process steps is thus reduced. This advantageously increases the usable surface area of ​​the piezoelectric layer 30 on the front face, in the xy plane.

[0085] Layers 51 and 52 are, respectively, layers of high and low acoustic impedance relative to each other. Typically, a factor greater than 2, and preferably greater than 3, is chosen between the high and low acoustic impedance values ​​(Mrayl unit). Advantageously, layers 51 and 52 can be electrically conductive. The high acoustic impedance layers are based on HfN or TaN. The low acoustic impedance layers are based on TiN or VN. The last layer 52 of the stack 50, below the piezoelectric layer 30, for example made of TiN, can advantageously form the nucleation electrode 21. This further reduces the number of steps required to fabricate the BAW-SMR.

[0086] THE figures 4A to 4E illustrate steps in a manufacturing process for a BAW-FBAR, according to an example useful for understanding the invention.

[0087] As illustrated in the figure 4APrior to the formation of the nucleation electrode 21, the silicon substrate 10 can be structured to form a mesa structure of height e1, bordered by at least one material retraction 11. The height e1 of the mesa structure can be on the order of a few hundred nanometers to a few microns, for example on the order of 500 nm. This structuring is typically carried out by conventional lithography / etching.

[0088] As illustrated in the figure 4B The nucleation electrode 21 is formed, preferably by epitaxy, on and around the mesa structure. A planarization step, typically by chemical-mechanical polishing (CMP), can then be carried out.

[0089] As illustrated in the figure 4C , the piezoelectric layer 30 is then epitaxially deposited on the nucleation electrode 21, and then the second electrode 22 is formed, for example by PVD deposition, on the piezoelectric layer 30.

[0090] As illustrated in the figure 4D The stacking of layers 30 and 22 is structured by lithography / etching so as to have a lateral dimension l2 smaller than the lateral dimension l1 of the underlying mesa structure. Advantageously, the nucleation electrode 21 has sufficient etching resistance to ensure good control of the etching termination during this initial etching, on the surface of the nucleation electrode 21. In one embodiment, when the nucleation electrode 21 is made of TiN, it forms a termination layer for the etching.

[0091] As illustrated in the figure 4EA second etch can be performed to form at least one opening 41 in the nucleation electrode 21, for example, at the edge of the stack of layers 30, 22. Layers 30, 22 are typically protected by the resin mask encapsulating them during the opening of the nucleation electrode 21. A third etch then forms the cavity 40 beneath the stack of layers 30, 22, 21. In particular, the mesa structure of the silicon substrate is removed through the opening 41 during this third etch. It is not necessary to close the cavity 40. The opening 41 in layer 21 is not necessarily filled or sealed.

[0092] Preferably, the first etching allows the piezoelectric layer 30 to be etched selectively with respect to the nucleation electrode 21. The third etching allows the substrate 10 to be etched selectively with respect to the nucleation electrode 21.

[0093] A BAW device of the FBAR type is thus advantageously obtained. The present invention advantageously enables the formation of electroacoustic devices comprising a high-quality crystalline LN / LT piezoelectric layer on silicon-based substrates, via a nucleation electrode made of an electrically conductive refractory nitride-based material, typically based on transition nitride. These BAW electroacoustic devices are advantageously directly integrable or co-integrable in silicon technology. Other applications can be envisaged.

Claims

1. Method for producing an electroacoustic device (1) comprising a piezoelectric layer (30), said electroacoustic device (1) being a bulk acoustic wave filter guided on a Bragg reflector, said method comprising the following steps: • Providing a silicon-based substrate (10), • Forming a first electrode (21) on the substrate (10), • Forming the piezoelectric layer (30) on the first electrode (21), • Forming a second electrode (22) on the piezoelectric layer (30), wherein the first electrode (21) is chosen made of a nitride-based electrically conductive refractory material, and wherein the formation of the piezoelectric layer (30) is done by epitaxy, on said first electrode (21), of an ABO3-type material, O being oxygen, A being at least one first chemical element taken from among sodium (Na), potassium (K), barium (Ba), lithium (Li), lead (Pb), and B being at least one second chemical element taken from among zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta), said method further comprising, before formation of the first electrode (21), a formation of a stack (50) of layers (51, 52) alternatively respectively having high and low acoustic impedances, the low impedance layers (52) being TiN- or VN-based, and the high impedance layers (51) being HfN- or TaN-based, the stack (50) forming a Bragg reflector.

2. Method according to the preceding claim, wherein the nitride-based electrically conductive refractory material is taken from among transition refractory nitrides with a basis of a transition metal.

3. Method according to the preceding claim, wherein the nitride-based electrically conductive refractory material is taken from among TiN, VN, ZrN, TaN, HfN, NbN, or their alloys.

4. Method according to any one of the preceding claims, wherein the formation of the first electrode (21) and the formation of the piezoelectric layer (30) are achieved by pulsed laser deposition successively within one same reactor without venting with air between said formations.

5. Method according to any one of the preceding claims, wherein the substrate (10) is silicon-based, oriented along (111), the first electrode (21) is titanium nitride TiN-based oriented along (111), and the piezoelectric layer (30) is oriented along (0001).

6. Electroacoustic device (1) comprising, stacked in a so-called vertical direction (z), a silicon-based substrate (10), a first electrode (21) on said substrate (10), a piezoelectric layer (30) on said first electrode (21), said piezoelectric layer (30) being with the basis of an ABO3-type material, O being oxygen, A being at least one first chemical element taken from among sodium (Na), potassium (K), barium (Ba), lithium (Li), lead (Pb), and B being at least one second chemical element taken from among zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta), a second electrode (22) disposed on the piezoelectric layer (30), the device (1) being characterised in that the first electrode (21) is made of a nitride-based electrically conductive refractory material, said electroacoustic device being a bulk acoustic wave filter guided on a Bragg reflector, comprising, between the substrate (10) and the first electrode (21), a stack (50) of layers (51, 52) alternatively having high and low acoustic impedances, the low impedance layers (52) being TiN- or VN-based, and the high impedance layers (51) being HfN- or TaN-based, the stack (50) forming a Bragg reflector.

7. Device (1) according to the preceding claim, wherein the nitride-based electrically conductive refractory material is taken from among transition refractory nitrides with the basis of a transition metal, for example TiN, VN, ZrN, TaN, HfN, NbN, or their alloys.

8. Device (1) according to any one of claims 6 to 7, wherein the first and second electrodes (21, 22) are in contact with a front face (101) of the device (1).

9. Device (1) according to any one of claims 6 to 7, wherein the first electrode (21) is contacted at a rear face (100) of the device (1), and wherein the second electrode (22) is in contact with a front face (101) of the device (1).