Surface acoustic wave device with electrodes embedded in piezoelectric layer and design and production thereof

Abstract

The invention relates to a surface acoustic wave device (100) comprising a piezoelectric layer (120), an electrode (150A, 150B) embedded in the piezoelectric layer, and a substrate (130) carrying the piezoelectric layer and the electrode, the substrate satisfying the following two conditions: the attenuation of the elastic mode with the electrode in the piezoelectric layer is less than 0.1 dB / lambda; and a velocity ratio between a volumetric elastic mode for cutting the substrate and a basic elastic mode for cutting the piezoelectric layer is less than a predetermined velocity ratio having a value equal to 1, the substrate being selected from the group consisting of gallium arsenide and 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.

Description

Technical Field

[0001] The field of the invention is surface acoustic wave (SAW) devices having a composite structure incorporating electrodes embedded in a thin layer of piezoelectric material. Background Art

[0002] Surface acoustic wave devices or SAW devices are used in a wide range of applications, particularly in electronics where they form core elements of filters, oscillators, delay lines and transformers.

[0003] Piezoelectric materials generate voltage when deformed by mechanical stress, and deform conversely when voltage is applied.

[0004] Thus, when an alternating electrical signal is applied to one or more electrodes in contact with the piezoelectric layer, a mechanical signal (ie, oscillation or vibration) is generated at the piezoelectric material: the electrical signal is converted into a mechanical signal.

[0005] The mechanical signal propagating in the piezoelectric material exhibits a frequency dependence on the electrical alternating signal that is a function of the characteristics of the electrodes, the properties of the piezoelectric material, and other factors such as the shape of the elastic wave device and other structures that make up 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 so-called "SAW filters" used, for example, in the transmission and reception of RF signals in telecommunication applications.

[0007] The SAW filter comprises at least one elastic surface acoustic wave transducer, which is potentially surrounded by a reflector consisting of periodically arranged electrodes and satisfies the so-called Bragg condition, by reflecting the waves emitted by the transducer in phase toward the transducer to form a resonator. The filter can advantageously be composed of a combination of such resonators, and can also be characterized by at least one input transducer and at least one output transducer. It can utilize electrical or elastic coupling between resonators or transducers. Its time response can be finite (in the case of conventional transverse filters) or infinite (in the case of resonator filters). In all cases, those skilled in the art refer to these structures indiscriminately as SAW filters.

[0008] The geometry and dimensions of the transducer and the type and shape of the material used determine the characteristics of the SAW filter such as coupling and reflection factors, quality factor Q, bandwidth, spurious responses, suppression of resonances higher than the used resonance and temperature dependence of the used mode.

[0009] Patent document WO 2021 / 053401 discloses a SAW transducer, which includes electrodes formed into an interdigitated comb shape and has the particularity of being embedded in a piezoelectric layer, such as Figure 1 shown.

[0010] The acoustic impedance of these electrodes is lower than that of the piezoelectric layer, so that the propagation of the cutting mode, called "electrode mode", is substantially contained within the electrode volume by reflection on the electrode walls at frequencies higher than the frequency for the fundamental mode of cutting the piezoelectric layer.

[0011] However, the interdigitated comb configuration of the electrodes and the excitation of opposite polarities by means of two adjacent forks of the comb enable the electrode modes to generate coherently propagating shear waves in the piezoelectric layer, leading to a resonance phenomenon for the operating elastic wavelength λ.

[0012] The operating elastic wavelength λ of the transducer is given by the relation f r =v / λ and the resonant frequency f of the transducer r , where v represents the propagation velocity in the piezoelectric layer.

[0013] The SAW transducer of patent document WO 2021 / 053401 enables the use of higher frequencies than those employed in conventional SAW transducers, which are typically based on thin film electrodes located on the surface of a piezoelectric layer and for which the elastic waves have frequencies derived from the eigenmodes of the layer. In the remainder of this document, references to conventional SAW devices or transducers will refer to such transducers based on thin film electrodes located on the surface of a piezoelectric layer.

[0014] This is because the resonant frequency of the electrode mode f r It is defined by the resonance of elastic waves in the electrode volume, which occurs at higher frequencies than the natural modes of the piezoelectric layer in conventional devices.

[0015] However, in addition to the resonant frequency f r Properties other than these are important for practical applications, particularly the purity of the frequency response, and those skilled in the art are seeking optimization and / or alternatives when selecting a material suitable for forming a substrate for a SAW transducer employing an electrode pattern. Summary of the invention

[0016] The first object of the present invention is to provide an elastic surface acoustic wave device that can adopt an electrode mode and exhibits performance acceptable for practical applications, especially having a spectral purity of a frequency response that is acceptable from the perspective of practical applications and is adjusted by the attenuation of the substrate to the fundamental mode for cutting the piezoelectric layer.

[0017] A second object of the present invention is to provide a method for selecting a substrate suitable for a SAW device comprising electrodes embedded in a piezoelectric layer and designed to operate by excitation of an elastic mode peculiar to these electrodes.

[0018] A third object of the present invention is to provide a production method which integrates the selection of a suitable substrate in the production of SAW devices.

[0019] To this end, the present invention relates to an elastic surface wave device, which includes a piezoelectric layer, an electrode embedded in the piezoelectric layer, and a substrate carrying the piezoelectric layer and the electrode, and the substrate meets the following two conditions:

[0020] - the attenuation of the elastic mode with electrodes in the piezoelectric layer is less than 0.1 dB / λ; and

[0021] - a speed ratio between the speed for cutting the bulk elastic mode of the substrate and the speed for cutting the fundamental elastic mode of the piezoelectric layer is less than a predetermined speed ratio having a value equal to 1,

[0022] The substrate is selected from gallium arsenide and has a Young's modulus between 60 GPa and 180 GPa, a Poisson's ratio between 0.15 and 0.35, and a 2000 kg / m 3 Up to 6000kg / m 3 Between the density of glass.

[0023] Such a SAW device with embedded electrodes, intended to operate by excitation of the intrinsic elastic modes of these electrodes, will operate satisfactorily if a substrate is employed which is chosen in such a way that, in combination with the piezoelectric layer and the embedded electrodes, the two conditions regarding attenuation and velocity ratio defined above are satisfied, i.e., the propagating electrode mode dominates over the fundamental mode used to cut the piezoelectric layer.

[0024] Compliance with the conditions regarding attenuation or propagation losses ensures that the substrate is capable of acquiring elastic electrode modes, ie, elastic electrode modes can actually be excited and properly propagate in the piezoelectric layer and used in a SAW device comprising electrodes embedded in the piezoelectric layer as described above.

[0025] One advantage of this elastic electrode pattern is that it allows obtaining relatively high operating frequencies (eg, over 3 GHz) for the device without requiring production methods at the limits of what is currently technologically feasible, while ensuring its reliability and robustness.

[0026] Other non-limiting features according to the invention (alone or in any technically feasible combination):

[0027] - the ratio of the attenuation of the fundamental mode for cutting the piezoelectric layer to the attenuation of the elastic electrode mode is greater than 10;

[0028] -The electrode completely passes through the piezoelectric layer;

[0029] - The substrate has a Young's modulus between 60 GPa and 80 GPa, a Poisson's ratio between 0.15 and 0.25, and a strength of 2100 kg / m 3 Up to 2400kg / m 3 The density of glass formed between;

[0030] - The substrate is formed of gallium arsenide.

[0031] The present invention extends to a method of determining matching of a substrate with a surface elastic wave device, the surface elastic wave device comprising a piezoelectric layer, an electrode embedded in the piezoelectric layer and the substrate, the method being implemented with the aid of a computer system and comprising the step of issuing a signal indicating matching of the substrate with the surface elastic wave device in the following circumstances:

[0032] - the attenuation of the elastic mode with electrodes in the piezoelectric layer is less than 0.1 dB / λ; and

[0033] - a speed ratio between the speed for cutting the bulk elastic mode of the substrate and the speed for cutting the fundamental elastic mode of the piezoelectric layer is less than a predetermined speed ratio having a value equal to 1,

[0034] Other non-limiting features according to the invention (alone or in any technically feasible combination):

[0035] The computer system comprises a computer computing unit and a computer memory, the computer memory being in functional communication with the computer computing unit and storing a database storing parameters related to the geometry and properties of the electrodes, the piezoelectric layer and the substrate, the computer computing unit implementing the following steps:

[0036] - retrieving data from a computer memory database, including parameters related to the geometry and properties of the electrodes, piezoelectric layer and substrate;

[0037] - calculating the attenuation of the elastic electrode modes in the piezoelectric layer based on the retrieved data;

[0038] - comparing the attenuation of the elastic electrode mode in the piezoelectric layer with a predefined attenuation value of 0.1 dB / λ;

[0039] - calculating from the retrieved data the velocity ratio between the bulk elastic mode for cutting the substrate and the fundamental elastic mode for cutting the piezoelectric layer;

[0040] - comparing the speed ratio with a predetermined speed ratio value;

[0041] - A signal indicating matching of the substrate to the surface acoustic wave device is emitted in the following cases:

[0042] - the attenuation of the elastic mode of the electrode in the piezoelectric layer is less than a predefined attenuation value; and

[0043] - The speed ratio is less than a predetermined speed ratio value.

[0044] According to other non-limiting features of the invention, applied to the device and the method individually or in any technically feasible combination:

[0045] - the predetermined value of the speed ratio is equal to 0.9;

[0046] -The attenuation of the electrode elastic mode Attn is given by the formula Definition, where for the electrode excitation that produces the propagation mode of elastic shear waves, Δ is the acoustic reflection coefficient of each electrode, the function log represents the decimal logarithm, so that log(e) ≈ 0.434, and f h and f b is the frequency associated with the resonance of the elastic shear wave, at f h and f b The susceptance of the elastic wave device reaches its highest and lowest points respectively.

[0047] The invention also extends to a method for producing a SAW device having an electrode embedded in a piezoelectric layer, the method comprising the determination method. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Other features and advantages of the present invention will become apparent from the following detailed description of the invention with reference to the accompanying drawings, in which:

[0049] [ Figure 1 ] Figure 1 shows a schematic plan view of a SAW transducer having electrodes embedded in a piezoelectric layer;

[0050] [ Figure 2 ] Figure 2 Schematically shows Figure 1 A cross-sectional view of the SAW transducer along a plane passing through and perpendicular to a segment indicated by Y-Y' on the plan view;

[0051] [ Figure 3 ] Figure 3 Shows Figure 1 Possible electrode geometries for computer modeling of the SAW transducer shown in FIG.

[0052] [ Figure 4 ] Figure 4 shows a simulated diagram of the electrical admittance of a SAW resonator centered on the resonance of one electrode mode;

[0053] [ Figure 5 ] Figure 5shows a simulation of the electrical admittance of a SAW transducer showing the effect of a bulk elastic mode for cutting a substrate and a fundamental elastic mode for cutting a piezoelectric layer;

[0054] [ Figure 6 ] Figure 6 A step-mode SAW filter is shown;

[0055] [ Figure 7 ] Figure 7 The theoretical transfer function of a first example of a ladder filter is shown;

[0056] [ Figure 8 ] Figure 8 shows a theoretical transfer function of a second example of a ladder filter;

[0057] [ Fig. 9 ] Fig. 9 shows a substrate evaluation diagram for a SAW device;

[0058] [ Fig.10 ] Fig.10 shows the method for implementing the corresponding Fig. 9 A device of the method of the diagram in FIG. 1 ; and

[0059] [ Fig.11 ] Fig.11 A method for manufacturing a SAW device is shown. DETAILED DESCRIPTION

[0060] The operating frequency of a surface elastic wave device (SAW device) is limited by the synchronization frequency of the transducer.

[0061] Various approaches can be used to increase the operating frequency of a surface acoustic wave device.

[0062] The first approach is to reduce the wavelength of the elastic waves used in the device, with which the frequency is inversely proportional.

[0063] However, this approach is limited by the current state of the art of UV lithography used in the SAW filter industry, which does not allow the fabrication of electrodes with a width less than a few hundred nanometers, the wavelength mentioned above being equal to twice the period p of the interdigitated electrodes of the SAW device.

[0064] Additionally, problems may arise with regard to the structural stability of the electrodes: due to the small size of the electrodes and the resulting power density, the power handling of these devices is insufficient to meet current requirements.

[0065] A second method of increasing the operating frequency of a SAW device is to increase the velocity of the elastic waves in the device, to which the frequency is proportional.

[0066] However, the speed of elastic waves in SAW devices is limited by the properties of the substrate material.

[0067] To overcome this limitation, waveguides can be used, for example, on composite substrates.

[0068] The solution envisaged in this document is based on integrating electrodes in the piezoelectric layer in order to increase the speed of the operating mode, ie the speed of the elastic waves exploited in the SAW device in question.

[0069] In this document it is understood that a piezoelectric layer is a layer consisting of one or more piezoelectric materials, ie a layer of one or more materials having piezoelectric properties.

[0070] We consider a pair of interdigital electrodes (or IDTs for interdigital transducers) having a general shape similar to those used in conventional SAW transducers, where the electrodes rest on a piezoelectric layer, except that here the electrodes are embedded in the piezoelectric layer.

[0071] Due to the integration of the IDT in the piezoelectric layer, it is possible to excite elastic cutting modes (called "electrode modes") that are mainly confined to each electrode. The acoustic impedance of these electrodes is lower than that of the piezoelectric layer.

[0072] The wave is polarized parallel to the substrate and perpendicular to the electrode side walls, and produces a cutting motion in a direction orthogonal to the propagation direction, which has significant advantages over conventionally used surface waves, especially in terms of the achievable frequency of the elastic propagation mode of a SAW transducer of a given geometry and size, as described in detail in patent document WO21053401.

[0073] One problem to be solved is that there are no standards or tables of material properties to refer to in order to determine the materials suitable for making such transducers.

[0074] The choice of substrate is particularly critical since it affects the possibility to use the transducer in terms of electrode pattern and, among other things, the performance of the device.

[0075] Under such circumstances, the inventors of the present invention have tested many materials as candidate materials for use as a substrate, and investigated which properties can be used as selection criteria.

[0076] In doing so, the inventors have been able to identify properties that predict the suitability of a substrate material for a SAW device designed to implement an electrode mode.

[0077] The two features have been separated so that a substrate can be selected in such a way that a SAW device comprising the selected substrate can actually realize the electrode pattern appropriately.

[0078] "Suitably" means that the electrode mode propagates primarily relative to the fundamental mode used to cut the piezoelectric layer, i.e., the ratio of the attenuation (or attenuation coefficient) of the fundamental mode of the piezoelectric layer to the attenuation of the electrode mode is preferably greater than 10, more preferably greater than 50, and even more preferably greater than 100. Under these conditions, it is possible to operate an electrode mode with an attenuation less than or equal to the attenuation of a SAW mode in a conventional transducer.

[0079] like Figure 1 and Figure 2 As shown, the SAW device 100 considered in this example is composed of a SAW transducer, which is composed of a piezoelectric layer 120 with a thickness of h, a first interdigitated electrode 150A and a second interdigitated electrode 150B embedded in the piezoelectric layer 120 and with a height of h (here corresponding to the thickness of the piezoelectric layer), and a substrate 130 supporting the piezoelectric layer and the electrodes.

[0080] Although the electrodes are preferably electrodes that completely penetrate the piezoelectric layer and have the same thickness as the piezoelectric layer (i.e., they are in direct contact with the substrate 130), or have an intermediate layer interposed between them and the piezoelectric layer, and do not protrude above the piezoelectric layer 120, as shown in FIG. Figure 2 and Figure 3 As shown, other geometries are acceptable as long as the electrode pattern is able to excite the piezoelectric layer: it is sufficient for the electrodes to partially penetrate the piezoelectric layer; they can also protrude above the piezoelectric layer. In this way, they can penetrate the entire piezoelectric layer and exceed its height.

[0081] In the case of non-through electrodes, charges that do not appear in the case of through electrodes will appear under the electrode. This means that the total contribution of charge from one electrode to another in the periodic interdigital network can be less than in the case of through electrodes. As a result, a SAW device with through electrodes will have stronger electromechanical coupling than a SAW device with non-through electrodes, and therefore better electromechanical coupling. Non-through electrodes may also couple remnants of the fundamental mode, which may adversely affect the spectral purity of the frequency response.

[0082] In the present embodiment, there is direct contact between the substrate 130 and each of the piezoelectric layer 120 and the electrodes 150A, 150B. However, it is entirely possible to have an intermediate layer covering the substrate and interposed between the substrate and the piezoelectric layer 120 and each of the electrodes 150A, 150B, as already mentioned above.

[0083] The intermediate layer may comprise, for example, a bonding layer for attaching the piezoelectric layer to the substrate, such as a silicon dioxide layer having a thickness between 10 nm and 400 nm, preferably between 20 nm and 150 nm, for example 30 nm thick, but this is not limiting.

[0084] It should be noted that all calculations and conclusions mentioned in this specification are also applicable to the case where the intermediate layer exists, and to the different electrode geometries mentioned above, and the inventors obtained the same conclusions for different situations.

[0085] The pair of electrodes includes a first electrode 150A and a second electrode 150B, each having a contact with the upper surface 130 of the substrate. 上 The lower surface directly or indirectly in contact with the piezoelectric layer 120 and the side surface 150 directly or indirectly in contact with the piezoelectric layer 120 侧 .

[0086] As well as intermediate layers between substrate and piezoelectric layer, there can also be layers between electrode and piezoelectric layer and between the electrode underside and substrate or intermediate layer without affecting the authors' conclusions regarding the choice of substrate.

[0087] Electrode 150A and electrode 150B include fingers 152A and 152B, respectively, extending in the same direction D to form a periodic structure with a period p in a direction perpendicular to direction D, wherein the fingers of the two electrodes are alternately placed to form a pair of interdigitated electrodes or IDTs.

[0088] The elastic wavelength λ of the mode excited by the transducer is equal to twice the period p, ie 2p, and the transducer operates under Bragg conditions for this particular wavelength, which corresponds to the operational elastic wavelength mentioned above.

[0089] The wavelength is also understood to be the distance separating the central axes of the extension of two adjacent fingers of the same electrode (ie the axes each forming an axis of symmetry of the corresponding finger in plan view), this axis being parallel to the direction D of extension of the fingers.

[0090] For example, the finite element method has been used to Figure 1 The structure described in was computer modeled, as Figure 3 As shown, the figure shows the period of the model in the direction x parallel to the substrate surface and perpendicular to the direction D, and only a small part of the substrate is shown in the direction y perpendicular to the substrate surface, and the substrate is considered to be semi-infinite.

[0091] The inventors used this model to computer simulate the electromechanical properties of SAW devices as a function of the materials used, thereby testing the relevance of many properties in the choice of materials constituting the substrate.

[0092] The inventors have found that two properties can be used to predict the suitability of a given substrate for fabricating a SAW device designed to employ electrode modes: first, the attenuation of the elastic electrode modes in the substrate, and second, the attenuation of the fundamental mode used to cut into the piezoelectric layer. These parameters are modeled as a function of the piezoelectric layer and the electrodes.

[0093] First condition - attenuation of electrode mode

[0094] The substrate beneath the piezoelectric layer can improve the confinement and propagation of the electrode modes if adapted to the characteristics (properties, geometry) of the embedded electrodes and the piezoelectric layer, but not all materials are suitable for forming such a substrate: Figure 1 As shown Figure 3 The modeled structure shown is able to properly exploit electrode modes, ie electrode modes having an attenuation less than or equal to the attenuation of the modes exploited in SAW devices equipped with surface electrodes, for example for filter design.

[0095] In this way, the electrode mode propagates through the piezoelectric layer and is radiated into the substrate to be attenuated, which is why it is so important for the characteristics of the device.

[0096] Among the characteristics considered and used by the inventors in order to simulate the behavior of SAW devices, it was found that those substrates that make the attenuation of the electrode mode in the piezoelectric layer less than the attenuation limit of 0.1dB / λ, preferably 0.05dB / λ, and more preferably 0.01dB / λ are substrates that enable the device to properly utilize the electrode mode. This attenuation limit has been selected in such a way that a SAW device designed to operate in the electrode mode and including a substrate that matches the characteristics of the electrode and the piezoelectric layer so that this criterion can be met has a performance at least equivalent to the typical performance of a conventional SAW device based on the operation of one mode of the piezoelectric layer with the aid of a surface electrode, and has the additional possibility of operating at higher frequencies due to the operation based on the electrode mode. It should be understood here that the attenuation of the elastic electrode mode of the SAW device according to the present invention has an attenuation that is less than or equal to the attenuation of the piezoelectric layer mode excited by the surface electrode.

[0097] Therefore, a substrate that with a given electrode and piezoelectric layer combination results in an electrode mode attenuation in the piezoelectric layer less than or equal to an attenuation limit of 0.1 dB / λ, preferably 0.05 dB / λ, more preferably 0.01 dB / λ is retained in the context of the present invention as suitable for exciting the electrode mode, in the sense that the expected performance in terms of the electrode mode attenuation is at least equivalent to the performance of a conventional SAW device based on utilizing one mode of the piezoelectric layer with the aid of surface electrodes.

[0098] Conversely, substrates in which the combination of electrodes and a given piezoelectric layer results in an attenuation of the electrode mode in the piezoelectric layer exceeding the attenuation limit value of 0.1 dB / λ are rejected as unsuitable for excitation and operation of the electrode mode in a SAW device comprising said electrodes and piezoelectric layer. In such a configuration, the surface acoustic waves are excessively attenuated and cannot propagate properly in the electrode array. For example, it is impossible to produce a filter using a device of such a configuration because the insertion loss is too high.

[0099] The method for calculating the attenuation is described in detail in the doctoral thesis of Y. Fusero, “Etude theory and experiment of dissipation on surfaces at high vitesse and fort couple: application aux filters telecom high fréquence”, Université de Franche-Comté, pp. 74-75, 2001.

[0100] Figure 4 A simulated plot of the electrical admittance of a SAW transducer on a linear scale is shown as a function of frequency indicated on the x-axis and extending from 2.36 GHz to 2.37 GHz, with the conductance G and associated susceptance B indicated in siemens per meter on the y-axis and at −1×10 10 S / m and 2*10 10 Extends between S / m.

[0101] The resonant peaks of the conductance G are respectively related to the frequencies f on either side of the peak h and f b The minimum and maximum values ​​of the susceptance B at are associated.

[0102] The attenuation of the electrode mode in the piezoelectric layer (hereinafter referred to as Attn) can be expressed by the following equation Eq.1:

[0103]

[0104] Where Δ is the electrode reflection coefficient, f h and f b are the frequencies of the maximum and minimum susceptance (imaginary part of admittance) associated with the SAW device under consideration, respectively, as Figure 4 shown.

[0105] Second condition - attenuation of the fundamental cutting mode

[0106] Meeting the first condition ensures that the electrode mode can be efficiently excited in the SAW device and propagate under conditions similar to or close to those of elastic waves excited by a conventional SAW device designed to operate in a piezoelectric layer mode excited by a surface electrode.

[0107] However, this criterion alone cannot guarantee the actual usability of the model.

[0108] In fact, the overall response of a device, as well as the purity of its frequency response, determines its performance in real-world applications such as filtering of telecommunication signals.

[0109] In order to obtain a frequency response of acceptable purity, it is particularly necessary to suppress the fundamental mode used to cut the piezoelectric layer.

[0110] This suppression can be achieved by attenuating this mode in the substrate and can be evaluated using the same formula as Eq. 1 above, this time applied to the fundamental mode used to cut the piezoelectric layer.

[0111] It was determined through extensive simulations that the attenuation of the fundamental mode for the off-cut piezoelectric layer due to the substrate must be greater than or equal to 1 dB / λ in order for the SAW device to have an acceptably pure frequency response.

[0112] This condition reflects the fact that a substrate adapted to a given piezoelectric layer provides a sufficiently strong attenuation of a mode of this piezoelectric layer so that this mode is absorbed by the substrate and does not propagate into the piezoelectric layer, which would have an unacceptable impact on the purity of the frequency response, in particular the presence of a second conductance peak with high coupling.

[0113] The value of 1 dB / λ represents the limit at which one can still have superposition of modes which are likely to produce spurious responses.

[0114] As already mentioned, Equation 1 can be used to calculate this decay in the same way as the electrode mode decay.

[0115] Alternatively, compliance with this condition relating to the attenuation criterion can be verified by means of a criterion related to the ratio of the velocity C(substrate) for cutting the bulk elastic mode of the substrate to the velocity C(piezoelectric) for cutting the fundamental elastic mode of the piezoelectric layer.

[0116] Therefore, for a substrate in which the C(substrate) / C(piezoelectric) ratio is greater than or equal to 1 (the velocity of the volume elastic mode for cutting the substrate is greater than or equal to the velocity of the fundamental elastic mode for cutting the piezoelectric layer), the fundamental elastic mode for cutting the piezoelectric layer will tend to be guided into the piezoelectric layer rather than being absorbed by the substrate, and will therefore tend to degrade the frequency response of the device in question, in particular its spectral purity.

[0117] Such substrates would be rejected as being unsuitable for satisfactory operation of the device in question.

[0118] In contrast, a substrate in which the C(substrate) / C(piezoelectric) ratio is less than 1 (i.e., the velocity of the volume elastic mode for cutting the substrate is less than the velocity of the fundamental elastic mode for cutting the piezoelectric layer) will propagate the fundamental elastic mode for cutting the piezoelectric layer into the substrate volume, and the substrate absorbs this mode, preventing it from reducing the frequency response of the device in question.

[0119] Such a substrate can be used to produce and realize the device in question.

[0120] The smaller the above velocity ratio is, the stronger the radiation of the piezoelectric layer mode in the substrate is, and the better the spectral purity of the frequency response of the device is.

[0121] Although a ratio of 1 may be considered acceptable for certain applications, it is preferred to choose a ratio below 1, since a ratio of 0.9 already provides a significant improvement in the frequency response and therefore the performance of the device.

[0122] The pattern velocity may be determined by measuring one or more test structures on the substrate.

[0123] Alternatively, the velocity can be determined using the finite element method, more specifically FEM-BEM (Finite Element Method - Boundary Element Method), which models the period of a given SAW transducer in two dimensions, taking into account Figure 3 Radiation effects in the substrate geometry shown.

[0124] The finite element calculation method is used to consider the embedding of the electrodes in the piezoelectric layer and the mass effect of the electrodes.

[0125] Having calculated the frequency response of a given SAW transducer according to the above method, we can obtain the velocity of the bulk elastic mode for cutting the substrate and the velocity of the fundamental elastic mode for cutting the piezoelectric layer from the following formula Eq.2:

[0126] c=λ×f Eq.2

[0127] where c denotes the velocity of the wave under consideration, λ denotes its wavelength, and f denotes its frequency.

[0128] The wavelength λ is determined by the electrode geometry of a given SAW transducer and the frequency of the mode under consideration, which can be extracted from the frequency response of the transducer as Figure 5 shown.

[0129] against Figure 1 For the given SAW transducer shown in Figure 5 A simulated graph of the electrical impedance of a SAW transducer is shown, with the x-axis being a linear scale and the y-axis being a logarithmic scale, with the conductance G and the associated susceptance B indicated in Siemens per meter on the y-axis as a function of frequency indicated on the x-axis and extending from 2.2 GHz to 2.6 GHz, with the y-axis visible in the graph being between 100 S / m and 1×10 11 Extends between S / m.

[0130] The specific characteristics of the bulk elastic mode for the cut substrate and the fundamental elastic mode for the cut piezoelectric layer are indicated by BS and FS on the graph, respectively.

[0131] 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 from the graph, which can be used to calculate the velocities of these modes from Eq. 2 in this given SAW device.

[0132] Methods for evaluating substrates

[0133] Based on the substrate acceptance conditions detailed above, a method 900 for evaluating a material for use as a substrate in a SAW device having an electrode embedded in a piezoelectric layer may be detailed, such as Fig. 9 As shown in the diagram.

[0134] This method is preferably accomplished by Fig.10 The computer system 1000 shown is implemented, and the computer system includes a computer computing unit 1010 and a computer memory 1020, which is in functional communication with the computer computing unit and stores a database, which stores parameters related to the geometry and properties of the electrodes, piezoelectric layers and substrates.

[0135] In step 910, computer unit 1010 retrieves data from database 920, including parameters related to the geometry and properties of electrodes, piezoelectric layer and substrate.

[0136] In particular, these data include the characteristics of the material considered for the substrate: its nature and, if applicable, its crystallographic orientation.

[0137] In step 920 , the computer unit calculates the decay of the electrode elastic mode in the piezoelectric layer from the data retrieved in step 910 .

[0138] In step 925, the computer unit compares the attenuation calculated in step 920 with a predefined attenuation value.

[0139] If the attenuation of the elastic electrode mode is greater than or equal to the predefined attenuation value, the computer unit sends a signal R1 rejecting the substrate, which is not suitable for enabling the SAW device to realize the electrode mode by acoustoelectric excitation.

[0140] If the attenuation of the elastic electrode mode is less than a predefined attenuation value (the situation indicated by "Y"), the computer unit sends a signal A1 of a partial acceptance of the substrate, which satisfies at least one of the conditions necessary for its acceptance, in this case the ability of the substrate to enable the SAW device to realize the electrode mode by acoustic-electric excitation.

[0141] For the reasons explained above, the predefined attenuation value may be 1 dB / λ.

[0142] In step 930, the computer unit calculates the velocity ratio between the bulk elastic mode for cutting the substrate and the fundamental elastic mode for cutting the piezoelectric layer from the retrieved data.

[0143] In step 935, the computer unit compares the speed ratio calculated in step 930 with a predetermined speed ratio value.

[0144] If the speed ratio is less than a predetermined speed ratio value (the situation indicated by "Y"), the computer unit sends a signal A2 of partial acceptance of the substrate, which satisfies at least one of the conditions necessary for its acceptance, which in this case is the ability of the substrate to sufficiently attenuate the fundamental mode used to cut the piezoelectric layer so that the SAW device in question exhibits acceptable performance in terms of the purity of its frequency response.

[0145] If the speed ratio is greater than or equal to the predetermined speed ratio value, the computer unit issues a signal R2 rejecting the substrate as unsuitable for acceptable operation of the SAW device.

[0146] The predetermined speed ratio value may be equal to 1, preferably equal to 0.9, as explained above.

[0147] In step 940, the computer unit checks whether the two signals A1 and A2 have indeed been emitted, and if so, emits a signal A which represents an accepting substrate in the sense of satisfying the two conditions necessary to be considered suitable for forming a SAW device intended to operate using electrode mode.

[0148] This substrate evaluation method enables the selection of a substrate to be combined with a given electrode and piezoelectric layer to obtain a SAW device designed to realize the electrode mode without having to go through the time-consuming and expensive stage of manufacturing test samples and characterizing them from an electroacoustic point of view, or at least enables production to be tailored to the type of sample.

[0149] It should be noted that according to Figure 1 and Figure 2 These substrate acceptance conditions are particularly suitable for electrodes made of relatively light metals such as aluminum or aluminum alloys (e.g. 2% copper aluminum alloy AlCu 2％ )), and the case where the piezoelectric layer is formed of lithium tantalate LiTaO3 or lithium niobate LiNbO3.

[0150] However, these conditions remain valid for other electrode and piezoelectric layer materials and for Figure 1 and Figure 2 The geometry shown in remains valid.

[0151] Method for producing a SAW device

[0152] The above-described substrate evaluation process may be integrated into a method 1100 for producing a SAW device, such as Fig.11 As shown in the diagram in , a first step 1110 comprises selecting a substrate for a device with already partially defined properties, that is, for already fixed geometry and materials for the piezoelectric layer and electrodes.

[0153] The first step 1110 may include implementing Fig. 9 Method 900 shown in .

[0154] In step 1120 , a piezoelectric layer is formed directly on the substrate selected as acceptable in step 1110 .

[0155] Alternatively, in step 1120, the piezoelectric layer may be obtained separately and then attached to the substrate by molecular bonding.

[0156] In step 1130, the piezoelectric layer is prepared to accommodate the electrodes, for example by etching a predetermined pattern into the thickness of the piezoelectric layer.

[0157] In step 1140, electrodes are formed into the piezoelectric layer by conventional methods.

[0158] By proceeding according to this method, we can expect to obtain a SAW device capable of utilizing electroacoustically excited electrode modes.

[0159] Example substrate

[0160] For those with Figure 1 and Figure 2 A set of transducers of the geometry shown in has been verified to comply with the first and second characteristics defined above.

[0161] In the first series of transducers, for SAW transducers designed to operate at 4.8 GHz or 2.4 GHz, a 2% copper aluminum alloy AlCu 2％ A quartz glass substrate or "fused quartz" is associated with electrodes and a piezoelectric layer consisting of lithium tantalate LiTaO3 or lithium niobate LiNbO3.

[0162] The simulation results are shown in Table 1 below.

[0163] [Table 1]

[0164]

[0165] C(substrate) and C(piezoelectric) respectively indicate the velocity for cutting the bulk elastic mode of the substrate and the velocity for cutting the fundamental elastic mode of the piezoelectric layer, and C(substrate) / C(piezoelectric) represents the ratio of these velocities.

[0166] Attenuation(electrode) and Attenuation(piezoelectric) indicate the attenuation of the elastic electrode mode in the piezoelectric layer and the attenuation of the fundamental mode of the piezoelectric layer in the substrate, respectively.

[0167] The note (Pass) indicates that the substrate meets the condition referred to by the corresponding box in the table, ie, one of the attenuation or velocity ratio criteria detailed above.

[0168] Here we find that quartz glass can be used as substrate for each of the SAW transducers considered, which is indicated in the table by the substrate status "acceptable".

[0169] In the second series of transducers, a Young's modulus of 64 GPa, a Poisson's ratio of 0.2 and a load of 2200 kg / m 3 The commercially available Planoptik Eagle XG glass of density 1.340 W was substituted for the quartz glass of the first series, all other parameters remaining the same.

[0170] The simulation results are shown in Table 2 below.

[0171] [Table 2]

[0172]

[0173] We found that it is possible to use this glass as substrate for each of the SAW transducers considered.

[0174] In the third series of transducers, we considered an AT-cut quartz substrate (IEEE symbol (YX1) / 36°) instead of the quartz glass of the first series, all other parameters remaining the same.

[0175] The simulation results are shown in Table 3 below.

[0176] [Table 3]

[0177]

[0178] We note here that it was not possible to use AT-cut quartz as substrate for any of the SAW transducers considered, these substrates were rejected and their status indicated as failing.

[0179] Indeed, for both types of piezoelectric layers (LiTaO3 and LiNbO3), although the conditions on the attenuation of the electrode modes in the substrate are met for operation at 4.8 GHz, the conditions on the attenuation of the natural modes of the piezoelectric layer are not met, as indicated by the comment (Failed).

[0180] The table therefore indicates with (FAIL) that the condition regarding the attenuation of the fundamental mode of the piezoelectric layer in the substrate is violated, the attenuation having to be greater than or equal to 1 dB / λ to satisfy this condition.

[0181] The table also indicates with (Failed) that the condition regarding the ratio of the velocity for cutting the bulk elastic mode of the substrate to the velocity for cutting the fundamental elastic mode of the piezoelectric layer, which ratio must be less than 1, is violated.

[0182] In the fourth series of transducers, a silicon substrate (100) is used instead of the quartz glass of the first series, and all other parameters remain unchanged.

[0183] The simulation results are shown in Table 4 below.

[0184] [Table 4]

[0185]

[0186] As with AT-cut quartz substrates, we note that it is not possible to use silicon (100) as substrate for any of the SAW transducers considered, these substrates are rejected and their status is then indicated as failing.

[0187] Additional calculation results similar to those given above validate the use of a GaAs substrate with a Young's modulus of 73 GPa, a Poisson's ratio of 0.23, and a mass of 2380 kg / m 3 The density of commercially available PlanoptikBorofloat glass.

[0188] Further simulations show that the choice of substrate for electrodes and piezoelectric layers is by no means intuitive. Table 5 below lists the glasses and their physical properties (density p, longitudinal and transverse sound propagation velocities V L and V T , Young's modulus E and Poisson's ratio v).

[0189] [Table 5]

[0190] Glass <![CDATA[ρ[kg / m 3 ]]]> <![CDATA[v L [m / s]]]> <![CDATA[v T [m / s]]]> E[GPa] v Glass A 4360 4127 2414 63 0.24 Glass B 5000 6020 2900 113 0.349 Glass C 2470 5500 3100 60 0.267 Glass D 2440 5565 3408 68 0.2 Pyrex 2230 5710 3494 65 0.201 Glass E 2365 5620 3520 69 0.177 Planoptik Eagle XG 2380 - 3531 73 0.23 Planoptik Borofloat 2200 - 3481 64 0.2

[0191] Tables 6 and 7 below show simulation results for devices equipped with a lithium tantalum piezoelectric layer with a height h of 600 nm and intended to operate at a wavelength λ, where the height h of the electrode satisfies h / λ=15% and h / λ=30%, respectively. More specifically, these tables show the simulation results for devices with Au, Mo, and AlCu. 2％ The glasses in Table 5 associated with the alloy electrodes, the C(substrate) / C(piezoelectric) ratio of the C(substrate) velocity of the volume elastic mode of the cutting substrate and the C(piezoelectric) velocity of the fundamental elastic mode of the cutting piezoelectric layer, the attenuation Attn.Elec. of the elastic electrode mode in the substrate, the attenuation Attn.Cis. of the fundamental mode for cutting the piezoelectric layer, and the ratio Attn.Cis. / Attn.Elec. of the latter two.

[0192] [Table 6]

[0193]

[0194] [Table 7]

[0195]

[0196] The "Failed" annotation indicates that, according to the criteria defined in this article for velocity ratio, electrode elastic mode attenuation, piezoelectric layer fundamental shear mode attenuation, and attenuation ratio, the relevant values ​​prohibit the corresponding substrate from being used in electrode mode based SAW devices.

[0197] According to the guidelines defined in this document, substrates made of only one of the glasses C, D, E, Planoptik Eagle XG, Planoptik Borofloat and Pyrex are bonded to a 2% copper-aluminum alloy AlCu 2％ The electrode combinations are made to provide a spectral purity of the frequency response such that the fundamental cutting mode is attenuated (propagation loss greater than 1 dB / λ) with guided electrode modes (propagation loss less than 0.1 dB / λ). For each configuration that meets these criteria, the ratio between the attenuation of the fundamental cutting mode and the electrode modes is at least greater than 10.

[0198] Furthermore, for the same type of glass, the fact that the properties of the electrode determine whether the criteria related to (i) the attenuation of the electrode elastic mode in the piezoelectric layer and (ii) the velocity ratio between the bulk elastic mode for cutting the substrate and the fundamental elastic mode for cutting the piezoelectric layer are satisfied indicates that the substrate, the electrode and the piezoelectric layer must be matched. This matching is included in satisfying the above two criteria (i) and (ii).

[0199] These criteria make it possible to select the materials of the substrate, electrodes and piezoelectric layer for the SAW device as a function of each other, based on the excitation of the electrode modes, in such a way as to present a spectral response pure enough to satisfy the envisaged application. It should also be noted that the phenomena used to attenuate the fundamental mode used to cut the piezoelectric layer have no effect on the electrode modes.

[0200] Example Application 1

[0201] In order to show the influence of the fundamental modes for cutting the piezoelectric layer in the case of a device resulting from an appropriate choice of substrate, the Figure 6 The theoretical transfer function (parameter S) of the conventional ladder SAW filter 600 is shown as 21 ), the conventional ladder SAW filter has twelve identical resonators 620 (eight in series and four in parallel) connected in a ladder style between a SAW input 630 and a SAW output 640.

[0202] Figure 7The theoretical transfer function of such a filter is shown, which is composed of a resonator designed to operate at 4.8 GHz and includes a quartz glass substrate and a LiTaO3 piezoelectric layer and an AlCu 2％ Alloy electrode.

[0203] As shown in Table 1, quartz glass is considered an acceptable material for forming the substrate.

[0204] The feature for the fundamental mode of the cut piezoelectric layer appears at about 2.4 GHz and increases the suppression to -34 dB.

[0205] Example Application 2

[0206] In order to illustrate the influence of the fundamental mode for cutting the piezoelectric layer in the case of a device caused by an inappropriate substrate selection, the second application example follows the application example 1, except that the resonator substrate is formed of AT-cut quartz.

[0207] As can be seen in Table 3, AT-cut quartz was deemed unacceptable for forming the substrate because the attenuation of the fundamental mode in the substrate for the cut piezoelectric layer was insufficient.

[0208] Figure 8 The theoretical transfer function of such a filter is shown.

[0209] The feature for the fundamental mode of the cut piezoelectric layer appears at about 2.4 GHz and increases the suppression to nearly 0 dB.

[0210] A comparison with Application Example 1 clearly shows that the attenuation of the piezoelectric layer mode is insufficient, being greatly reduced compared to the previous Application Example, and interfering with the filter response, with the suppression rising to nearly 0 dB.

[0211] Of course, the invention is not limited to the above description and variants can be added without departing from the scope of the invention defined by the claims.

Claims

1. An elastic surface 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, wherein the device is characterized in that the substrate satisfies the following two conditions: The attenuation of the elastic mode with electrodes in the piezoelectric layer is less than 0.1 dB / λ; and the velocity ratio between the volume elastic mode for cutting the substrate and the basic elastic mode for cutting the piezoelectric layer is less than a predetermined velocity ratio, the predetermined velocity ratio being equal to 1, the substrate being selected from gallium arsenide and having a Young's modulus between 60 GPa and 180 GPa, a Poisson's ratio between 0.15 and 0.35, and a 2000 kg / m 3 Up to 6000kg / m 3 Between the density of glass.

2. The device according to claim 1, wherein The ratio of the attenuation of the fundamental mode for cutting the piezoelectric layer to the attenuation of the electrode elastic mode is greater than 10.

3. The device according to claim 1 or 2, wherein: The electrode completely penetrates the piezoelectric layer.

4. The device according to any one of claims 1 to 3, wherein The substrate has a Young's modulus between 60 GPa and 80 GPa, a Poisson's ratio between 0.15 and 0.25, and a strength of 2100 kg / m 3 Up to 2400kg / m 3 The density of glass formed between.

5. The device according to any one of claims 1 to 3, wherein The substrate is formed of gallium arsenide.

6. The device according to any one of claims 1 to 5, wherein: The predetermined speed ratio is equal to 0.

9.

7. The device according to any one of claims 1 to 6, wherein The attenuation of the electrode elastic mode is given by the formula Definition, where, for the electrode excitation that produces the propagation mode of elastic shear waves, Δ is the acoustic reflection coefficient of each electrode, the function log represents the decimal logarithm, so that log(e) ≈ 0.434, and f h and f b is the frequency associated with the resonance of the elastic shear wave, at f h and f b The susceptance of the elastic wave device reaches its highest point and lowest point respectively.

8. A method (900) for determining matching between a substrate and an elastic surface wave device, the elastic surface wave device comprising a piezoelectric layer, an electrode embedded in the piezoelectric layer, and the substrate, the method being implemented with the aid of a computer system and characterized in that: The method includes the step of issuing (840) a signal indicating matching of the substrate with the surface acoustic wave device when: The attenuation of the electrode elastic mode in the piezoelectric layer is less than 0.1 dB / λ; and the ratio between the speed for cutting the volume elastic mode of the substrate and the fundamental elastic mode for cutting the piezoelectric layer is less than a predetermined speed ratio, which has a value equal to 1.

9. The method (900) for determining suitability of a substrate for a surface acoustic wave device according to claim 8, characterized in that: The computer system (900) comprises a computer computing unit (910) and a computer memory (920), the computer memory being in functional communication with the computer computing unit and storing a database, the database storing parameters related to the geometry and properties of the electrodes, the piezoelectric layer and the substrate, the computer computing unit implementing the following steps: retrieving (810) data from the computer memory database (920), the data comprising parameters related to the geometry and properties of the electrodes, the piezoelectric layer, and the substrate; calculating (820) an attenuation of an elastic electrode mode in the piezoelectric layer based on the retrieved data; comparing (825) the attenuation of the elastic electrode mode in the piezoelectric layer to a predefined attenuation value of 0.1 dB / λ; calculating (830) from the retrieved data a velocity ratio between a bulk elastic mode for cutting the substrate and a fundamental elastic mode for cutting the piezoelectric layer; comparing the speed ratio to a predetermined speed ratio value (835), the predetermined speed ratio value being equal to 1; A signal indicating matching of the substrate with the surface acoustic wave device is issued (840) in the following cases: The attenuation of the electrode elastic mode in the substrate is less than the predefined attenuation value; and The speed ratio is less than the predetermined speed ratio value.

10. The method according to claim 8 or 9, wherein: The predetermined speed ratio is equal to 0.

9.

11. The method according to any one of claims 8 to 10, wherein: The attenuation is given by the formula Definition, where, for the electrode excitation that produces the propagation mode of elastic shear waves, Δ is the acoustic reflection coefficient of each electrode, the function log represents the decimal logarithm, so that log(e) ≈ 0.434, and f h and f b is the frequency associated with the resonance of the elastic shear wave, at f h and f b The susceptance of the elastic wave device reaches its highest point and lowest point respectively.

12. A method (1100) for manufacturing an elastic surface wave device, the elastic surface wave device comprising a piezoelectric layer, an electrode embedded in the piezoelectric layer, and a substrate, characterized in that: The method comprises a method (900) for determining a matching of a substrate material according to any one of claims 8 to 11.

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