SURFACE ELASTIC WAVE DEVICE WITH ELECTRODES EMBEDDED IN A PIEZOELECTRIC LAYER, DESIGN AND MANUFACTURE THEREOF

Abstract

A surface elastic wave device (100) comprises a piezoelectric layer (120), electrodes (150A, 150B) embedded in the piezoelectric layer, and a substrate (130) supporting the piezoelectric layer and the electrodes, the substrate satisfying the following two conditions: an attenuation of an electrode elastic mode in the piezoelectric layer is less than 0.1 dB / λ; and a velocity ratio between a bulk elastic mode of shearing of the substrate and a fundamental elastic mode of shearing of the piezoelectric layer is smaller than a predetermined velocity ratio value, the substrate being selected from gallium arsenide and a glass having a Young's modulus of between 60 GPa and 180 GPa, a Poisson's ratio of between 0.15 and 0.35, and a density of between 2000 kg / m3 and 6000 kg / m3. Figure to be published with the abstract: Fig. 2

Description

Title of the invention: SURFACE ELASTIC WAVE DEVICE WITH ELECTRODES EMBEDDED IN A PIEZOELECTRIC LAYER, DESIGN AND MANUFACTURE THEREOF Technical field

[0001] The field of the invention is that of surface elastic wave devices (SAW devices for Surface Acoustic Wave in English terminology) with composite structures integrating electrodes embedded in a thin layer of piezoelectric material. Prior art

[0002] Surface elastic wave devices, or SAW devices, are used in many applications, and in particular in electronic applications where they form the central element of filters, oscillators, delay lines or even transformers.

[0003] Piezoelectric materials generate an electrical voltage when they are deformed under the action of mechanical stress, and, conversely, deform when an electrical voltage is applied to them.

[0004] Therefore, when an alternating electrical signal is applied to one or more electrodes in contact with a piezoelectric layer, a mechanical signal (i.e. an oscillation or a vibration) is generated at the level of this piezoelectric material: the electrical signal is transformed into a mechanical signal.

[0005] The mechanical signal propagating in the piezoelectric material exhibits a frequency dependence on the alternating electrical signal, which dependence is a function of the characteristics of the electrode(s), the properties of the piezoelectric material, and other factors such as the shape of the elastic wave device and other structures constituting the device.

[0006] Elastic wave devices exploit this frequency dependence to provide one or more functions by means of surface acoustic wave (SAW) resonators or SAW transducers, which are increasingly used to form, for example, so-called "SAW filters" implemented in the transmission and reception of RF signals for applications in telecommunications.

[0007] A SAW filter comprises at least one surface elastic wave transducer, potentially surrounded by reflecting mirrors made up of electrodes arranged eg periodically and satisfying the so-called Bragg condition, forming a resonator by reflection of the waves emitted by the transducer in phase towards the latter. The filter can advantageously be composed of a combination of such resonators, it can also have at least one so-called input transducer and at least one so-called output transducer. It can exploit electrical or elastic couplings between resonators or transducers. Its time response can be finite (case of conventional transverse filters) or infinite (case of resonator filters). In all cases, the person skilled in the art indistinctly designates these structures as SAW filters.

[0008] The geometry and dimensions of the transducers, the types and shapes of the materials used condition the characteristics of the SAW filter such as the coupling and reflection factors, the quality factor Q, the bandwidth, the parasitic responses, the suppression of resonances of higher orders than the resonance used, or even the temperature dependence of the mode used.

[0009] Patent document WO 2021 / 053401 discloses a SAW transducer comprising electrodes forming interdigitated combs and having the particularity of being embedded in the piezoelectric layer, as illustrated by [Fig.l].

[0010] The acoustic impedance of these electrodes is lower than that of the piezoelectric layer, so as to contain the propagation of a shear mode essentially in the volume of the electrodes by reflection against the walls of the electrodes at a frequency higher than that of the fundamental shear mode of the piezoelectric layer, this mode being designated by "electrode mode", or "electrode mode" in English terminology.

[0011] The interdigitated comb configuration of the electrodes and the excitation by means of opposite polarities of two adjacent fingers of the combs, however, allows the electrode mode to generate, in the piezoelectric layer, coherent propagating shear waves leading to a resonance phenomenon for an operational elastic wavelength X.

[0012] The operational elastic wavelength A of the transducer is linked to the resonance frequency fr of this transducer by the relation / . = vlk,v representing the propagation speed in the piezoelectric layer.

[0013] A SAW transducer of patent document WO 2021 / 053401 makes it possible to use higher frequencies than those used in conventional SAW transducers, usually based on thin-film electrodes located on the surface of the piezoelectric layer and for which the elastic waves have frequencies arising from the natural modes of the latter.

[0014] Indeed, the resonance frequency / , of the electrode mode is defined by the resonance of the elastic waves in the volumes of the electrodes, which occurs at a higher frequency than those of the natural modes of the piezoelectric layer of the conventional devices.

[0015] However, other characteristics than the resonance frequency fr are important for practical applications, in particular the purity of the frequency response, and those skilled in the art are seeking optimization and / or alternatives as to the choice of materials suitable for forming the substrate of a SAW transducer to employ an electrode mode. Statement of the invention

[0016] A first aim of the invention is to provide surface elastic wave devices capable of employing an electrode mode and of exhibiting acceptable performance for practical applications.

[0017] A second aim of the invention is to provide a method for choosing a substrate suitable for a SAW device comprising electrodes embedded in a piezoelectric layer and intended to operate by excitation of an elastic mode specific to these electrodes.

[0018] A third aim of the invention is to provide a manufacturing method integrating the choice of a suitable substrate in the manufacturing of a SAW device.

[0019] To these ends, the invention relates to a surface elastic wave device comprising a piezoelectric layer, electrodes embedded in the piezoelectric layer and a substrate supporting the piezoelectric layer and the electrodes, the substrate satisfying the following two conditions: - an attenuation of an elastic electrode mode in the piezoelectric layer is less than 0.1 dB / X; and - a speed ratio between a speed of a volume elastic mode of shearing of the substrate and a speed of a fundamental elastic mode of shearing of the piezoelectric layer is smaller than a predetermined speed ratio value,

[0020] the substrate being chosen from gallium arsenide and a glass having a Young's modulus of between 60 GPa and 180 GPa, a Poisson's ratio of between 0.15 and 0.35, and a density of between 2000 kg / m3 and 6000 kg / m3.

[0021] Such a SAW device with embedded electrodes and intended to operate by excitation of a specific elastic mode of these electrodes will be able to operate satisfactorily if it uses a substrate respecting the two conditions on attenuation and the speed ratio defined above, that is to say that the electrode mode which propagates is preponderant compared to the fundamental shear mode of the piezoelectric layer.

[0022] Compliance with the condition on attenuation, or propagation losses, ensures that the substrate is capable of accessing the elastic electrode mode, i.e. that the mode elastic electrode can effectively be excited and propagate properly in the piezoelectric layer, and be used in SAW devices as described above, comprising an electrode embedded in the piezoelectric layer.

[0023] An advantage of this elastic electrode mode is that it allows access to relatively high working frequencies (for example greater than 3 GHz), and this for devices not requiring manufacturing methods located at the limits of what is currently technically feasible, and whose reliability and robustness are assured.

[0024] According to other non-limiting characteristics of the invention, taken alone, or in any technically feasible combination:

[0025] - the substrate is formed from glass having a Young's modulus of between 60 GPa and 80 GPa, a Poisson's ratio between 0.15 and 0.25, and a density between 2100 kg / m3 and 2400 kg / m3;

[0026] - the substrate is formed from gallium arsenide.

[0027] The invention extends to a method for determining the adaptation of a substrate to an elastic surface wave device comprising a piezoelectric layer, electrodes embedded in the piezoelectric layer and said substrate, the method being implemented by means of a computer system and comprising the step of emitting a signal representative of an adaptation of the substrate to the elastic surface wave device, when: - an attenuation of an elastic electrode mode in the piezoelectric layer is less than 0.1 dB / X; and - a speed ratio between a speed of a bulk elastic mode of shearing of the substrate and a speed of a fundamental elastic mode of shearing of the piezoelectric layer is smaller than a predetermined speed ratio value.

[0028] According to other non-limiting characteristics of the invention, taken alone, or in any technically feasible combination: - the computer system comprises a computing computer unit and a computer memory functionally in communication with the computing computer unit and storing a database storing parameters of geometry and nature of the electrodes, of the piezoelectric layer, and of the substrate, the computing computer unit implementing the following steps: - retrieving from the database of the computer memory data including parameters of geometry and nature of the electrodes, of the piezoelectric layer, and of the substrate; - calculate an attenuation of an elastic electrode mode in the piezoelectric layer, from the recovered data;

[0029]

[0030]

[0031]

[0032]

[0033] - compare the attenuation of the electrode elastic mode in the piezoelectric layer to a predefined attenuation value of 0.1 dB / X; - calculate a speed ratio between a volume elastic mode of shearing of the substrate and a fundamental elastic mode of shearing of the piezoelectric layer, from the recovered data; - compare the speed ratio to a predetermined speed ratio value; - emit the signal representative of an adaptation of the substrate to the surface elastic wave device, when: - the attenuation of the electrode elastic mode in the piezoelectric layer is smaller than the preset attenuation value; and - the speed ratio is smaller than the predetermined speed ratio value. According to other non-limiting characteristics of the invention, applying to the device as well as to the method, taken alone or in any technically feasible combination: - the predetermined speed ratio value is equal to 1; - the predetermined value of the speed ratio is equal to 0.9; - the attenuation Attn is defined by the formula . u. bi \ where, for an excitation of the electrodes Attn= -Wn^^logy) 'p generating a propagating mode of an elastic shear wave, A is the acoustic reflection coefficient per electrode, the log function represents the decimal logarithm such that log(e)~0.434 and fh and fb are frequencies associated with a resonance of the elastic shear wave at which an electrical susceptance of the elastic wave device reaches a maximum and a minimum respectively. The invention also extends to a method of manufacturing the SAW device with electrodes embedded in the piezoelectric layer, method comprising said determining method. BRIEF DESCRIPTION OF THE FIGURES Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures, in which: [Fig.l] [Fig.l] schematically illustrates in plan view a SAW transducer with electrodes embedded in a piezoelectric layer; [Fig.2] [Fig.2] schematically illustrates the SAW transducer of [Fig.l] according to a sectional view of this transducer along the plane passing through the segment indicated by Y-Y' on the planar view and perpendicular to it;

[0034] [Fig.3] [Fig.3] illustrates a possible geometry of the electrodes of the SAW transducer of [Fig.l], used for the computer modeling of this transducer;

[0035] [Fig.4] [Fig.4] shows a simulation graph of the electrical admittance of a SAW resonator centered on the resonance of an electrode mode;

[0036] [Fig.5] [Fig.5] shows a simulation graph of the electrical admittance of a SAW transducer showing the effects of a bulk elastic mode of substrate shear and a fundamental elastic mode of piezoelectric layer shear;

[0037] [Fig.6] [Fig.6] illustrates a ladder SAW filter;

[0038] [Fig.7] [Fig.7] shows a theoretical transfer function of a first example ladder filter;

[0039] [Fig.8] [Fig.8] shows a theoretical transfer function of a second example of a ladder filter;

[0040] [Fig.9] [Fig.9] shows an evaluation diagram of a substrate for a SAW device;

[0041] [Fig. 10] [Fig. 10] illustrates a device for implementing a method corresponding to the diagram of [Fig.9]; and

[0042] [Fig. 11] [Fig. 11] illustrates a method of manufacturing a SAW device. DETAILED DESCRIPTION OF THE INVENTION

[0043] The working frequency of surface elastic wave devices (SAW devices) is defined by the synchronism frequency of the transducers.

[0044] In order to increase the operating frequency of surface elastic wave devices, different approaches can be considered.

[0045] A first approach consists of reducing the wavelength of the elastic waves used in the devices, the frequency being inversely proportional to this wavelength.

[0046] However, this method is limited by the current technical boundaries of UV lithography in use in the SAW filter industry which do not allow the manufacture of electrodes with a width less than a few hundred nanometers, the wavelength mentioned above being equal to twice the 2p period of the interdigitated electrodes of the SAW devices.

[0047] In addition, problems of structural stability of the electrodes may also arise: the power handling of these devices is insufficient to meet the current requirements due to the small size of the electrodes and the resulting power density.

[0048] A second approach to increasing the operating frequency of SAW devices is to increase the speed of the elastic waves in the device, the frequency being proportional to this speed.

[0049] However, the speed of elastic waves in SAW devices is limited by the properties of the substrate materials.

[0050] In order to circumvent this limitation, one can envisage, for example, the use of waves guided by means of composite substrates.

[0051] The solution envisaged within the framework of the present document is based on the integration of the electrodes in the piezoelectric layer in order to increase the speed of the working mode, that is to say the speed of the elastic wave used in the SAW device considered.

[0052] It is understood in this document that a piezoelectric layer is a layer consisting of one or more piezoelectric materials, that is to say a layer of one or more materials having piezoelectric characteristics.

[0053] We consider a pair of inter-digitated electrodes (IDTs for Inter-Digitated Transducer in English terminology) of general shapes similar to those used in a conventional SAW transducer with electrodes resting on the piezoelectric layer, with the difference that the electrodes are here embedded in the piezoelectric layer.

[0054] By integrating the IDTs into the piezoelectric layer, an elastic shear mode that is predominantly confined in each of the electrodes (called "electrode mode") can be excited. The acoustic impedance of these electrodes is lower than that of the piezoelectric layer.

[0055] This wave is polarized parallel to the substrate and perpendicular to the side walls of the electrodes and produces a shear displacement in the direction orthogonal to the direction of propagation, with significant advantages compared to conventionally used surface waves, particularly in terms of achievable frequency for a propagating elastic mode for a given geometry and dimensions of a SAW transducer, as detailed in patent document WO21053401.

[0056] One problem to be addressed is that there is no standard or table of material characteristics to refer to in order to determine the materials suitable for the manufacture of such a transducer.

[0057] The choice of a substrate is particularly critical, since it conditions the very possibility of using the transducer according to the electrode mode and, beyond that, the performance of the device.

[0058] In this context, the inventors of the present invention tested numerous materials as candidates for use as a substrate, and investigated what charac- characteristics could serve as selection criteria.

[0059] Thus, the inventors were able to determine characteristics proving predictive of the suitability of a substrate material for a SAW device intended to implement an electrode mode.

[0060] Two features have been isolated, allowing a substrate to be selected in such a way that a SAW device comprising the selected substrate will actually be capable of properly implementing an electrode mode.

[0061] "Suitably" means that the electrode mode propagates predominantly relative to the fundamental shear mode of the piezoelectric layer, i.e. the ratio of the attenuation coefficient of the fundamental mode of the piezoelectric layer to the attenuation coefficient of the electrode mode is preferably greater than 10, more preferably greater than 50, even more preferably greater than 100.

[0062] As illustrated by Figures 1 and 2, the SAW devices 100 considered as an example consist of a SAW transducer consisting of a piezoelectric layer 120 of thickness h, of a first and a second interdigitated electrodes 150A and 150B, embedded in the piezoelectric layer 120 and of height corresponding here to the thickness of the piezoelectric layer, and of a substrate 130 supporting the piezoelectric layer and the electrodes.

[0063] Although the electrodes are preferably of the same thickness as the piezoelectric layer and do not exceed the latter in height, other geometries are acceptable, as long as the electrode mode can excite the piezoelectric layer: it is sufficient that the electrodes are partially through-passing with respect to the piezoelectric layer and could also exceed the piezoelectric layer in height.

[0064] The present embodiment presents the situation of direct contact between the substrate 130 and each of the piezoelectric layer 120 and the electrodes 150A and 150B, it is however entirely possible to have an intermediate layer covering the substrate and interposed between the latter and each of the piezoelectric layer 120 and the electrodes 150A and 150B.

[0065] This intermediate layer may be a bonding layer used to fix the piezoelectric layer to the substrate, such as a silicon dioxide layer having a thickness of 10 nm to 400 nm, preferably between 20 nm and 150 nm, for example 30 nm, without this being restrictive.

[0066] It should be noted that all of the calculations and conclusions mentioned in this description also apply to situations where this intermediate layer is present and for the different electrode geometries mentioned above, the inventors having obtained identical conclusions for the different scenarios.

[0067] The pair of electrodes comprises a first electrode 150A and a second electrode 150B each with a lower face in direct contact or not with an upper face 130up of the substrate and side faces 150iat in direct contact or not with the piezoelectric layer 120.

[0068] In the same way as for the intermediate layer interposed between the substrate and the piezoelectric layer, it is here possible to envisage having a layer interposed between the electrodes and the piezoelectric layer as well as between the lower face of the electrodes and the substrate or the intermediate layer, without this influencing the authors' conclusions on the choice of a substrate.

[0069] The electrodes 150A and 150B respectively comprise fingers 152A and 152B extending in the same direction D, so as to form a periodic structure of period 2p in a direction perpendicular to the direction D, in which the fingers of the two electrodes are placed alternately, so as to form a pair of interdigitated electrodes or IDTs.

[0070] The elastic wavelength X of the mode excited by the transducer is equal to the period 2p, the transducer operating under Bragg conditions for this particular wavelength which corresponds to the operational elastic wavelength mentioned above.

[0071] This period 2p is also understood as the distance separating the central axes of extension of two adjacent fingers of the same electrode, that is to say axes each forming an axis of symmetry of the corresponding finger in planar view, this axis being parallel to the direction D of extension of the fingers.

[0072] The structure described by [Fig. 1] was modeled by computer, for example by a finite element calculation method, as illustrated by [Fig. 3], which shows a period of the model in the x direction parallel to the surface of the substrate and perpendicular to the D direction, only a small part of the substrate being illustrated in the y direction perpendicular to the surface of the substrate, the latter being considered as semi-infinite.

[0073] This modeling was used by the inventors to determine by computer simulation the electromechanical characteristics of the SAW devices as a function of the materials used, and thus test the relevance of numerous characteristics in the selection of the material constituting the substrate.

[0074] The inventors have found that two characteristics make it possible to predict the suitability of a substrate of a given nature for the manufacture of a SAW device intended to employ an electrode mode, on the one hand the attenuation of the elastic electrode mode in the substrate, and on the other hand the attenuation of the fundamental shear mode in the piezoelectric layer.

[0075] First condition - attenuation of the electrode mode

[0076] A substrate located under the piezoelectric layer improves confinement and pro electrode mode pagation, but not all materials are suitable for forming a substrate allowing the structure illustrated by [Fig.l] and modeled as illustrated by [Fig.3] to properly exploit an electrode mode, for example for the design of filters.

[0077] Thus, the electrode mode propagates in the piezoelectric layer and is attenuated there by radiation in the substrate, hence the importance of the latter in the characteristics of the device.

[0078] Among the characteristics considered and used by the inventors to simulate the behavior of SAW devices, it has been found that those whose substrate is such that the attenuation of the electrode mode in the piezoelectric layer is less than the attenuation limit value of 0.1 dB / X are the substrates allowing the device to properly exploit an electrode mode. This attenuation limit value has been chosen in such a way that a SAW device which comprises a substrate satisfying this criterion has performances at least equivalent to the typical performances of a conventional SAW device based on the exploitation of a mode of the piezoelectric layer by means of surface electrodes, with in addition the possibility of operating at higher frequencies thanks to the electrode mode.

[0079] Thus, substrates leading to an attenuation of the electrode mode in the piezoelectric layer less than or equal to the attenuation limit value of 0.1 dB / X are retained as suitable for the excitation of the electrode mode.

[0080] Conversely, substrates leading to an attenuation of the electrode mode in the piezoelectric layer greater than the attenuation limit value of 0.1 dB / X are rejected, considered unsuitable for the excitation and exploitation of an electrode mode in a SAW device.

[0081] A method of calculating attenuation is detailed in the doctoral thesis of Y. Fusero “Theoretical and experimental study of high-speed, high-coupling surface wave devices: application to high-frequency telecom filters”, University of Franche-Comté, pp. 74-75, 2001.

[0082] [Fig.4] shows a simulation graph of the electrical admittance of a SAW transducer in linear scales with, as a function of the frequency indicated on the abscissa and extending from 2.36 to 2.37 GHz, a conductance G and the associated susceptance B indicated in Siemens per meter on the ordinate and extending between -IxlO10 S / m and 2xl010 S / m.

[0083] The resonance peak of the conductance G is associated with a minimum and a maximum of the susceptance B at the respective frequencies / / , and / / , located on either side of the peak.

[0084] The attenuation of the electrode mode in the piezoelectric layer, designated by Attn hereinafter, can be expressed by the following equation Eq. 1:

[0085] A.. tt fkfh , Eq. 1 LJ Attn- -4()7r^y--T-lo0e 1 Jk+fh

[0086] in which A is the reflection coefficient per electrode,^ and / 6 are respectively the frequencies of the maximum and minimum of the susceptance (imaginary part of the admittance) associated with the SAW device considered, as illustrated by [Fig.4],

[0087] Second condition - attenuation of the fundamental shear mode

[0088] Fulfilling the first condition ensures that an electrode mode can be effectively excited in a SAW device and propagate under conditions similar to or close to those of an elastic wave excited by a conventional SAW device.

[0089] However, this criterion alone does not guarantee the practical exploitability of this mode.

[0090] Indeed, the general response of the device as well as the purity of the response in frequency conditions its performance for practical applications such as telecommunication signal filtering operations.

[0091] In order to obtain a frequency response of acceptable purity, it is in particular necessary to suppress the fundamental shear mode of the piezoelectric layer.

[0092] This suppression can be done by attenuation of this mode in the substrate, and can be evaluated by means of a formula identical to equation Eq. 1 above, this time applied to the fundamental shear mode of the piezoelectric layer.

[0093] It was determined following numerous simulations that the attenuation of the fundamental shear mode of the piezoelectric layer by the substrate must be greater than or equal to 1 dB / X for the SAW device to have an acceptable frequency response of purity.

[0094] This condition reflects the fact that the substrate allows an attenuation of the mode of the piezoelectric layer sufficiently strong so that this mode is absorbed by the substrate and does not propagate in the piezoelectric layer, which would have unacceptable effects on the purity of the frequency response, in particular the presence of a second conductance peak with high coupling at a frequency approximately 2.5 times lower than that of the electrode mode.

[0095] The value of 1 dB / X represents a limit value at which there can still be a superposition of modes capable of producing parasitic responses.

[0096] As already mentioned, this attenuation can be calculated using equation 1 in the same way as the attenuation of the electrode mode.

[0097] Alternatively, compliance with this condition, which relates to an attenuation criterion, can be verified by means of a criterion relating to the ratio of the speed Cvoi.sub of a volume elastic mode of shearing of the substrate to the speed Cfond.piezo of the mode fundamental elastic shear of the piezoelectric layer.

[0098] The elastic volume shear mode is the slow speed shear mode called SSBW (Surface Skimming Bulk Wave) in which the elastic waves are no longer confined to the upper layers of the structure but penetrate the volume of the substrate.

[0099] Thus, with a substrate in which this ratio Cvoi.sub / Cfond.pieZO is greater than or equal to 1 (speed of a volume elastic mode of shearing of the substrate greater than or equal to the speed of a fundamental elastic mode of shearing of the piezoelectric layer), the fundamental elastic mode of shearing of the piezoelectric layer will tend to be guided in the piezoelectric layer rather than being absorbed by the substrate, and will therefore tend to deteriorate the frequency response of the device considered, in particular its spectral purity.

[0100] Such a substrate will be rejected as unsuitable for ensuring satisfactory operation of the device in question.

[0101] Conversely, a substrate in which this ratio Cvoi.SUb / Cfond.piezo is less than 1 (speed of a volume elastic shear mode of the substrate less than the speed of the fundamental elastic shear mode of the piezoelectric layer) guides the fundamental elastic shear mode of the piezoelectric layer in the volume of the substrate and the latter absorbs the mode, preventing it from deteriorating the frequency response of the device considered.

[0102] Such a substrate may be chosen for the manufacture and implementation of the device in question.

[0103] The smaller the speed ratio described above, the stronger the radiation in the substrate of the mode of the piezoelectric layer will be and the better the spectral purity of the frequency response of the device will be.

[0104] Even if a ratio of 1 can be considered acceptable for certain applications, it is preferable to retain a ratio smaller than 1, a ratio of 0.9 already allowing a clear improvement in the frequency response and therefore in the performance of the device.

[0105] The mode speeds can be determined by measurements on one or more test structures on the substrate.

[0106] Alternatively, the celerities can be determined by a finite element calculation method, and more specifically a FEM-BEM method, Finite Element Method - Boundary Element Method in English terminology, modeling in two dimensions a period of a given SAW transducer taking into account the radiation effects in the substrate, of geometry such as illustrated by [Fig.3].

[0107] A finite element calculation method makes it possible to take into account the embedding of the electrodes in the piezoelectric layer as well as the mass effect. electrodes.

[0108] Since the frequency response of the given SAW transducer has been calculated according to the above method, the velocity of a volume elastic mode of shearing of the substrate and the velocity of a fundamental elastic mode of shearing of the piezoelectric layer can be obtained from the formula Eq. 2 below:

[0109] c = 2x / Eq. 2

[0110] where c represents the speed of a wave considered, 2 its wavelength and / its frequency.

[0111] The wavelength 2 is determined by the geometry of the electrodes of the given SAW transducer and the frequency of the mode considered, this frequency being able to be extracted from the frequency response of this transducer, as illustrated by [Fig.5].

[0112] [Fig. 5] shows, for a given SAW transducer illustrated by [Fig. 1], a simulation graph of the electrical impedance of a SAW transducer in linear scale on the abscissa and in logarithmic scale on the ordinate with, as a function of the frequency indicated on the abscissa and extending from 2.2 to 2.6 GHz, a conductance G and the associated susceptance B indicated in Siemens per meter on the ordinate, the ordinates visible on the graph extending between 100 S / m and IxlO11 S / m.

[0113] The particular signatures of the bulk elastic shear mode of the substrate and of a fundamental elastic shear mode of the piezoelectric layer are indicated respectively by BS and FS on the graph.

[0114] From this graph, the frequency of the "Bulk Shear" mode at about 2.55 MHz and the frequency of the "Fundamental Shear" mode at about 2.36 MHz can be derived, which allow the speeds of these modes to be calculated from equation Eq. 2 in this given SAW device. Method for evaluating a substrate

[0115] Based on the substrate acceptance conditions detailed above, a method 900 for evaluating a material for use as a substrate in a SAW device with electrodes embedded in a piezoelectric layer can be detailed, a method illustrated by the diagram of [Fig.9].

[0116] Such a method is preferably implemented by means of a computer system 1000 illustrated by [Fig. 10], comprising a computing computer unit 1010 and a computer memory 1020 operatively in communication with the computing computer unit and storing a database storing parameters of geometry and nature of the electrodes, of the piezoelectric layer, and of the substrate.

[0117] In a step 910, the computer unit 1010 retrieves from the database 920 data including parameters of geometry and nature of the electrodes, of the piezoelectric layer, and of the substrate.

[0118] These data include in particular the characteristics of the material considered for the substrate: its nature and, where applicable, its crystallographic orientation.

[0119] In a step 920, the computing unit calculates an attenuation of an electrode elastic mode in the piezoelectric layer from the data recovered in step 910.

[0120] In a step 925, the computing unit compares the attenuation calculated in step 920 to a predefined attenuation value.

[0121] If the attenuation of the elastic electrode mode is greater than or equal to the predefined attenuation value, then the computer unit emits a substrate rejection signal RI, the latter not being capable of allowing the SAW device to implement an electrode mode by acousto-electric excitation.

[0122] If the attenuation of the elastic electrode mode is smaller than the predefined attenuation value (situation indicated by “Y”), then the computer unit emits a signal Al of partial acceptance of the substrate, the latter respecting at least one of the conditions necessary for its acceptance, which is here that of its ability to allow the SAW device to implement an electrode mode by acousto-electric excitation.

[0123] This predefined attenuation value can be 1 dB / X, for the reasons explained above.

[0124] At a step 930, the computer unit calculates a speed ratio between a volume elastic mode of shearing of the substrate and a fundamental elastic mode of shearing of the piezoelectric layer, from the recovered data.

[0125] In a step 935, the computer unit compares the speed ratio calculated in step 930 to a predetermined speed ratio value.

[0126] If the speed ratio is smaller than the predetermined speed ratio value (situation indicated by "Y"), then the computer unit emits a signal A2 of partial acceptance of the substrate, the latter respecting at least one of the conditions necessary for its acceptance, which is here that of its ability to sufficiently attenuate the fundamental shear mode of the piezoelectric layer so that the SAW device considered has acceptable performances in terms of purity of its frequency response.

[0127] If the speed ratio is greater than or equal to the predetermined speed ratio value, then the computer unit emits a signal R2 rejecting the substrate, the latter not being capable of allowing the SAW device to operate in an acceptable manner.

[0128] The predetermined speed ratio value may be equal to 1, preferably equal to 0.9 as explained above.

[0129] At a step 940, the computer unit checks whether the two signals A1 and A2 have actually been emitted and, in this case, emits a signal A representative of an acceptance of the substrate in the sense that the latter fulfills the two conditions necessary to be considered as capable of forming a SAW device intended to operate by implementing implements an electrode mode.

[0130] This method of evaluating a substrate makes it possible to choose a substrate for a SAW device intended to implement an electrode mode, without having to go through a time-consuming and costly phase of manufacturing test samples and characterizing the latter from an electro-acoustic point of view, or at least makes it possible to target the type of samples to be produced.

[0131] It should be noted that these conditions for accepting a substrate apply particularly to cases where the electrodes are made of a relatively light metal, such as aluminum or an aluminum alloy (for example, an aluminum alloy with 2% copper A1Cu2%), and where the piezoelectric layer is formed of lithium tantalate LiTaO3 or lithium niobate LiNbO3, according to the geometry described in Figures 1 and 2.

[0132] However, these conditions remain valid for other electrode and piezoelectric layer materials, as well as for geometries not limited to those of Figures 1 and 2. Manufacturing process of a SAW device

[0133] The method for evaluating a substrate described above can be integrated into a method 1100 for manufacturing a SAW device illustrated by the diagram of [Fig. 11], and the first step 1110 of which consists of selecting the substrate for a device with characteristics already partially defined, i.e. for a geometry and materials of piezoelectric layers and electrodes already fixed.

[0134] This first step 1110 may consist of the implementation of the method 900 illustrated by [Fig.9].

[0135] In a step 1120, a piezoelectric layer is formed directly on a substrate retained as being acceptable at the end of step 1110.

[0136] Alternatively, in step 1120, the piezoelectric layer can be obtained separately, then attached to the substrate by molecular bonding.

[0137] In a step 1130, the piezoelectric layer is prepared to accommodate the electrodes, for example by etching a predetermined pattern in the thickness of the piezoelectric layer.

[0138] In a step 1140, the electrodes are formed so as to be embedded in the piezoelectric layer according to conventional methods.

[0139] By proceeding according to this method, one can expect to obtain a SAW device capable of exploiting an electro-acoustic excitation electrode mode. Examples of substrates

[0140] Compliance with the first and second characteristics defined above was verified for a set of transducers with the geometry illustrated in Figures 1 and 2.

[0141] In a first series of transducers, a quartz glass substrate, or "fused quartz" in English terminology, was considered in association with electrodes of an aluminum alloy with 2% copper A1Cu2% and a piezoelectric layer made of lithium tantalate LiTaO3 or lithium niobate LiNbO3, for SAW transducers designed to operate at 4.8 GHz or 2.4 GHz.

[0142] The results of the simulations are shown in Table 1 below.

[0143] [Tables 1] Piezoelectric material and working frequency LiTaO3 4.8 GHz LiTaO3 2.4 GHz LiNbO3 4.8 GHz LiNbO3 2.4 GHz C(Subst) in m / s 3766 C(Piezo) in m / s 4.658 4.620 3.876 3.797 C(Subst) / C(Piezo) 0.81 0.82 0.97 0.99 (OK) (OK) (OK) (OK) Attenuation (Elec) in dB / X 3.18.103 2.54.102 9.31.102 7.72.102 (OK) (OK) (OK) (OK) Attenuation (Piezo) in dB / X 3.61 6.12 3.06 4.09 (OK) (OK) (OK) (OK) Substrate status OK OK OK OK

[0144] C(Subst) and C(Piezo) indicate respectively the speed of a volume elastic mode of shearing of the substrate and the speed of a fundamental elastic mode of shearing of the piezoelectric layer, C(Subst) / C(Piezo) indicates the ratio of these speeds.

[0145] Attenuation (Elec) and Attenuation (Piezo) indicate the attenuations of an electrode elastic mode in the piezoelectric layer and the fundamental mode of the piezoelectric layer in the substrate, respectively.

[0146] Comments (OK) indicate that the substrate meets the condition concerned by the corresponding box in the table, i.e. one of the attenuation or speed ratio criteria detailed above.

[0147] We see here that it is possible to use quartz glass as a substrate for each of the SAW transducers considered, which the table indicates by a substrate status of "OK".

[0148] In a second series of transducers, a commercial Planoptik Eagle XG glass with a Young's modulus of 64 GPa, a Poisson's ratio of 0.2 and a density of 2200 kg / m3 is considered instead of the quartz glass of the first series, the other parameters remaining the same.

[0149] The results of the simulations are shown in Table 2 below.

[0150] [Tables2] Piezoelectric material and working frequency LiTaO3 4.8 GHz LiTaO3 2.4 GHz LiNbO3 4.8 GHz LiNbO3 2.4 GHz C(Subst) in m / s 3531 C(Piezo) in m / s 4.702 4.704 3.966 3.772 C(Subst) / C(Piezo) 0.75 0.75 0.89 0.94 (OK) (OK) (OK) (OK) Attenuation (Elec) in dB / X 3.77.103 1.41.102 9.94.102 4.95.102 (OK) (OK) (OK) (OK) Attenuation (Piezo) in dB / X 3.66 5.40 5.12 7.05 (OK) (OK) (OK) (OK) Substrate status OK OK OK OK

[0151] We find that it is possible to use this glass as a substrate for each of the SAW transducers considered.

[0152] In a third series of transducers, we consider a quartz substrate of AT cut (IEEE notation (YXl) / 36°) instead of the quartz glass of the first series, the other parameters remaining the same.

[0153] The results of the simulations are shown in Table 3 below.

[0154] [Tables3] Piezoelectric material and working frequency LiTaO3 4.8 GHz LiTaO3 2.4 GHz LiNbO3 4.8 GHz LiNbO3 2.4 GHz C(Subst) in m / s 5103 C(Piezo) in m / s 4.728 4.930 4.242, 4.594 C(Subst) / C(Piezo) 1.08 (NOK) 1.04 (NOK) 1.20 (NOK) 1.11 (NOK) Attenuation (Elec) in dB / X 2.63.103 (OK) 1.08.102 (NOK) 7.43.102 (OK) 2.13.102 (NOK) Attenuation (Piezo) in dB / X 4.39.103 (NOK) 6.64. 105 (NOK) 1.17. 103 (NOK) 6.18. 103 (NOK) Substrate status NOK NOK NOK NOK

[0155] We find here that it is not possible to use AT-cut quartz as a substrate for any of the SAW transducers considered, the substrates being rejected and their statuses then being indicated as NOK.

[0156] Indeed, even if the condition on the attenuation of the electrode mode in the substrate is respected for operation at 4.8 GHz for the two types of piezoelectric layer (LiTaO3 and LiNbO3), the condition on the attenuation of the eigenmode of the piezoelectric layer is not respected, as indicated by the comments (NOK).

[0157] The table thus indicates by (NOK) the violation of the condition on the attenuation of the fundamental mode of the piezoelectric layer in the substrate which must be greater than or equal to 1 dB / X to respect the condition.

[0158] The table also indicates by (NOK) the violation of the condition on the ratios of speeds of a volume elastic mode of shearing of the substrate on the speed of a fundamental elastic mode of shearing of the piezoelectric layer, which must be less than 1.

[0159] In a fourth series of transducers, a silicon substrate (100) is considered instead of the quartz glass of the first series, the other parameters remaining the same.

[0160] The results of the simulations are shown in Table 4 below.

[0161] [Tables4] Piezoelectric material and working frequency LiTaO3 4.8 GHz LiTaO3 2.4 GHz LiNbO3 4.8 GHz LiNbO3 2.4 GHz C(Subst) in m / s 5845 C(Piezo) in m / s 4.852 5.291 4.359 4.954 C(Subst) / C(Piezo) 1.20 (NOK) 1.10 (NOK) 1.34 (NOK) 1.18 (NOK) Attenuation (Elec) in dB / X 2.27.103 (OK) 1.51.101 (NOK) 7.27.102 (OK) 2.47.101 (NOK) Attenuation (Piezo) in dB / X 2.25.104 (NOK) 2.06. 105 (NOK) 1.25. 105 (NOK) 2.20. 105 (NOK) Substrate status NOK NOK NOK NOK

[0162] As for the AT-cut quartz substrate, we find that it is not possible to use silicon (100) as a substrate for any of the SAW transducers considered, the substrates being rejected and their statuses then being indicated as NOK.

[0163] Results of additional calculations similar to those presented above validated the use of the following substrates: a gallium arsenide substrate and a commercial Planoptik Borofloat glass having a Young's modulus of 73 GPa, a Poisson's ratio of 0.23 and a density of 2380 kg / m3. Application example 1

[0164] In order to illustrate the influence of the fundamental shear mode of the piezoelectric layer in the case of a device resulting from an adequate choice of substrate, the theoretical transfer function (parameter S2i) of a conventional 600 ladder SAW filter comprising twelve identical 620 resonators mounted in a ladder (eight in series and four in parallel) between a 630 input and a 640 SAW output was calculated, as illustrated in [Fig.6].

[0165] [Fig.7] illustrates the theoretical transfer function of such a filter, formed of resonators designed to operate at 4.8 GHz and comprising a quartz glass substrate provided with a piezoelectric layer of LiTaO3 and electrodes made of A1Cu2% alloy.

[0166] As indicated by Table 1, quartz glass is a material considered acceptable for forming the substrate.

[0167] The signature of the fundamental shear mode of the piezoelectric layer appears at about 2.4 GHz and increases the rejection to -34 dB. Application example 2

[0168] In order to illustrate the influence of the fundamental shear mode of the piezoelectric layer in the case of a device resulting from an inappropriate choice of substrate, a second application example repeats application example 1, with the difference that the substrate of the resonators is formed from AT-cut quartz.

[0169] As seen in Table 3, AT-cut quartz is not considered acceptable for forming the substrate, as the attenuation of the fundamental shear mode of the piezoelectric layer in the substrate is insufficient.

[0170] [Fig.8] illustrates the theoretical transfer function of such a filter.

[0171] The signature of the fundamental shear mode of the piezoelectric layer appears at about 2.4 GHz and increases the rejection to almost 0 dB.

[0172] Comparison with application example 1 makes it clear that the attenuation of the mode of the piezoelectric layer is insufficient, significantly degraded compared to the previous application example, and disrupts the response of the filter, with a rejection rising to almost 0 dB.

[0173] Of course, the invention is not limited to the above description, and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Claims

1. A surface elastic wave device (100) comprising a piezoelectric layer (120), electrodes (150A, 150B) embedded in the piezoelectric layer, and a substrate (130) supporting the piezoelectric layer and the electrodes, the device being characterized in that the substrate satisfies the following two conditions: - an attenuation of an electrode elastic mode in the piezoelectric layer is less than 0.1 dB / X; and - a speed ratio between a volume elastic mode of shearing of the substrate and a fundamental elastic mode of shearing of the piezoelectric layer is smaller than a predetermined speed ratio value equal to 1, the substrate being chosen from gallium arsenide and a glass having a Young's modulus of between 60 GPa and 180 GPa, a Poisson's ratio of between 0.15 and 0.35, and a density of between 2000 kg / m3 and 6000 kg / m3.

2. A device according to claim 1, wherein the substrate is formed from glass having a Young's modulus of between 60 GPa and 80 GPa, a Poisson's ratio of between 0.15 and 0.25, and a density of between 2100 kg / m3 and 2400 kg / m3.

3. The device of claim 1, wherein the substrate is formed from gallium arsenide.

4. A device according to any one of claims 1 to 3, wherein the predetermined speed ratio value is equal to 0.

9.

5. Device according to any one of claims 1 to 3, in which the attenuation is defined by the formula Aîtn = - where for an excitation of the electrodes zr-A f^f, ■ h ■ h generating a propagative mode of an elastic shear wave, A is the coefficient of acoustic reflection by the electrodes, the function log represents the decimal logarithm such that log(e)~0.434, etfh and fh are frequencies associated with a resonance of the elastic shear wave at which a susceptance of the elastic wave device reaches a maximum and a minimum respectively.

6. A method (900) of determining the suitability of a substrate for a surface elastic wave device comprising a piezo-layer electric, electrodes embedded in the piezoelectric layer and said substrate, the method being implemented by means of a computer system and characterized in that it comprises the step of emitting (840) a signal representative of an adaptation of the substrate to the surface elastic wave device, when: - an attenuation of an elastic electrode mode in the piezoelectric layer is less than 0.1 dB / X; and - a ratio between a velocity of a bulk elastic shear mode of the substrate and a velocity of a fundamental elastic shear mode of the piezoelectric layer is smaller than a predetermined velocity ratio value equal to 1.

7. Method (900) for determining the adaptation of a substrate to a surface elastic wave device according to claim 6, characterized in that the computer system (900) comprises a computer computing unit (910) and a computer memory (920) operatively in communication with the computer computing unit and storing a database storing parameters of geometry and nature of the electrodes, of the piezoelectric layer, and of the substrate, the computer computing unit implementing the following steps: - retrieving (810) from the computer memory database (920) data including geometry and nature parameters of the electrode, the piezoelectric layer, and the substrate; - calculating (820) an attenuation of an elastic electrode mode in the piezoelectric layer, from the recovered data; - comparing (825) the attenuation of the electrode elastic mode in the piezoelectric layer to a predefined attenuation value of 0.1 dB / X; - calculating (830) a speed ratio between a volume elastic mode of shearing of the substrate and a fundamental elastic mode of shearing of the piezoelectric layer, from the recovered data; - compare (835) the speed ratio to a predetermined speed ratio value; - emitting (840) the signal representative of an adaptation of the substrate to the surface elastic wave device, when: - the attenuation of the electrode elastic mode in the substrate is smaller than the predetermined attenuation value; and - the speed ratio is smaller than the predetermined speed ratio value.

8. A method according to any one of claims 6 and 7, wherein the predetermined speed ratio value is equal to 0.

9.

9. A method according to any one of claims 6 to 8, wherein the attenuation is defined by the formula Atm = - -10~ 77 AÛ. log ( e) °Ù' for An excitation of the electrodes generating a propagating mode of an elastic shear wave, A is the acoustic reflection coefficient by the electrodes, the log function represents the decimal logarithm such that log(e)~0.434, etfh etfh are frequencies associated with a resonance of the elastic shear wave at which a susceptance of the elastic wave device reaches a maximum and a minimum respectively.

10. A method (1100) of manufacturing a surface elastic wave device comprising a piezoelectric layer, electrodes embedded in the piezoelectric layer and a substrate, characterized in that it comprises the method (900) of determining the suitability of a substrate material according to any one of claims 6 to 9.

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