Surface acoustic wave device having electrodes embedded in a piezoelectric layer, and its design and production

Abstract

The present invention relates to 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, the substrate satisfying the following conditions: the attenuation of the elastic mode having an electrode 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 fundamental elastic mode for cutting the piezoelectric layer is smaller than a predetermined velocity ratio having a value equal to 1, and the substrate is selected from gallium arsenide and glass having a Young's modulus of 60 GPa to 180 GPa, a Poisson's ratio of 0.15 to 0.35, and a density of 3 2000 kg / m 3 to 6000 kg / m.

Description

Technical Field

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

Background Art

[0002] Elastic surface acoustic wave devices, i.e., SAW devices, are used in a wide range of applications, particularly in electronic devices, and form the core elements of filters, oscillators, delay lines, and transformers.

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

[0004] As a result, when an alternating electrical signal is applied to one or more electrodes in contact with the piezoelectric layer, a mechanical signal (i.e., oscillation or vibration) is generated in this piezoelectric material, and the electrical signal is converted into a mechanical signal.

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

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

[0007] A SAW filter includes at least one surface acoustic wave transducer, which may be surrounded by a reflector composed of periodically arranged electrodes, satisfies the so-called Bragg condition, and forms a resonator by reflecting the waves emitted in phase by the transducer back towards the transducer. Advantageously, the filter can be composed of a combination of such resonators and can feature at least one input transducer and at least one output transducer. It can utilize electrical or elastic coupling between resonators or between transducers. Its time response can be finite (in the case of a conventional transverse filter) or infinite (in the case of a resonator filter). In any case, these structures are indiscriminately referred to as SAW filters by those skilled in the art.

[0008] The geometry and dimensions of the transducer, as well as the type and shape of the materials used, determine the characteristics of the SAW filter, such as the coupling coefficient and reflection coefficient, the quality factor Q, the bandwidth, the spurious response, the suppression of higher-order resonances than the resonance used, and the temperature dependence of the modes used.

[0009] In International Patent Publication No. WO 2021 / 053401, as shown in FIG. 1, a SAW transducer is disclosed that has the particularity of being provided with electrodes forming an alternating interlocking comb shape and being embedded in a piezoelectric layer.

[0010] The acoustic impedance of these electrodes is lower than that of the piezoelectric layer such that, at frequencies higher than the frequency of the fundamental mode for cutting the piezoelectric layer, which is referred to as the "electrode mode", it essentially includes the propagation of the cut mode within the electrode volume due to reflection at the electrode walls.

[0011] However, the alternating interlocking comb shape configuration of the electrodes and the excitation by the opposite polarities of two adjacent protrusions of the comb shape enable the electrode mode to generate a coherent shear wave within the piezoelectric layer, causing a resonance phenomenon with respect to the operating elastic wavelength λ.

[0012] The operating elastic wavelength λ of the transducer is related to the resonance frequency f of the transducer by the relationship f r = v / λ, where v represents the propagation velocity in the piezoelectric layer. r

[0013] The SAW transducer of International Publication No. WO 2021 / 053401 enables the use of a frequency higher than the frequency employed in a conventional SAW transducer, where the elastic wave has a frequency derived from the eigenmode of the layer and is typically based on a thin film electrode located on the surface of the piezoelectric layer. In the remainder of this document, references to conventional SAW devices or transducers refer to such transducers based on thin film electrodes located on the surface of the piezoelectric layer.

[0014] This is because the resonance frequency f r of the electrode mode occurs at a frequency higher than the eigenmode of the piezoelectric layer in a conventional device and is defined by the resonance of elastic waves within the electrode volume.

[0015] However, features other than the resonance frequency f r , particularly the purity of the frequency response, are important for practical applications, and those skilled in the art are seeking optimization and / or alternatives in the selection of materials suitable for forming the substrate of the SAW transducer for adopting the electrode mode. SUMMARY OF THE INVENTION

[0016] A first object of the present invention is to provide an elastic surface acoustic wave device that can adopt the electrode mode, can exhibit acceptable performance for practical applications, and in particular has spectral purity of the frequency response that is acceptable from the perspective of practical applications and is conditioned by the attenuation of the fundamental mode for cutting the piezoelectric layer by the substrate.

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

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

[0019] To achieve these objects, the present invention relates to an elastic surface 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 conditions: - the attenuation of the elastic mode having electrodes in the piezoelectric layer is less than 0.1 dB / λ, and - the velocity ratio between the velocity of the bulk elastic mode for cutting the substrate and the velocity of the fundamental elastic mode for cutting the piezoelectric layer is less than a predetermined velocity ratio having a value equal to 1, The substrate is selected from gallium arsenide and glass having a Young's modulus of 60 GPa to 180 GPa, a Poisson's ratio of 0.15 to 0.35, and a density of 2000 kg / m 3 ~6000 kg / m 3 of density.

[0020] Such a SAW device having embedded electrodes and intended to operate by excitation of the eigenelastic modes of these electrodes, in combination with the piezoelectric layer and the embedded electrodes, when employing a substrate selected such that the two conditions regarding attenuation and velocity ratio defined above are satisfied, i.e., the propagation electrode mode is dominant over the fundamental mode for cutting the piezoelectric layer, operates well.

[0021] Ensuring that, due to compatibility with the condition regarding attenuation or propagation loss, the substrate can access the elastic electrode mode, i.e., the elastic electrode mode can actually be excited and can propagate properly within the piezoelectric layer, and can be used in a SAW device as described above having electrodes embedded in the piezoelectric layer.

[0022] One advantage of this elastic electrode mode is that it allows access to relatively high (e.g., above 3 GHz) operating frequencies for devices that do not require production methods at the limits of what is currently technically feasible and for which reliability and robustness are guaranteed.

[0023] According to other non-limiting features of the present invention, individually or in any technically feasible combination, - The ratio of the attenuation of the fundamental elastic mode for cutting the piezoelectric layer to the attenuation of the elastic electrode mode is greater than 10, - The electrodes pass completely through the piezoelectric layer, - The substrate has a Young's modulus of 60 GPa to 80 GPa, a Poisson's ratio of 0.15 to 0.25, and a density of 2100 kg / m 3 ~2400 kg / m 3 and is formed from glass, - The substrate is formed from gallium arsenide.

[0024] The present invention is extended to a method for determining the alignment of a substrate for an elastic surface wave device comprising a piezoelectric layer, an electrode embedded in the piezoelectric layer, and a substrate, the method being implemented using a computer system, - The attenuation of the elastic mode having an electrode in the piezoelectric layer is less than 0.1 dB / λ, - When the speed ratio between the speed of the volumetric elastic mode for cutting the substrate and the speed of the fundamental elastic mode for cutting the piezoelectric layer is less than a predetermined speed ratio having a value equal to 1, including the step of emitting a signal representing the alignment of the substrate for the elastic surface wave device.

[0025] According to other non-limiting features of the present invention, individually or in any technically feasible combination, - The computer system comprises a computer calculation unit and a computer memory that functionally communicates with the computer calculation unit and stores a database storing parameters related to the geometry and properties of the electrodes, piezoelectric layer, and substrate, and the computer calculation unit performs the following steps: - Retrieving data including parameters related to the geometry and properties of the electrode, piezoelectric layer, and substrate from a computer memory database; - Calculating the attenuation of the elastic electrode mode in the piezoelectric layer based on the retrieved data; - Comparing the attenuation of the elastic electrode mode in the piezoelectric layer with a predefined attenuation value of 0.1 dB / λ; - Calculating 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; - Comparing the velocity ratio with a predetermined value of the velocity ratio; - When the attenuation of the electrode elastic mode in the piezoelectric layer is less than the predefined attenuation value and - When the velocity ratio is less than the predetermined value of the velocity ratio, emitting a signal representing the alignment of the substrate with respect to the surface acoustic wave device.

[0026] According to other non-limiting features of the present invention, applied to the device and method, either alone or in any technically feasible combination, - The predetermined value of the velocity ratio is equal to 0.9, - The attenuation Attn of the electrode elastic mode is given by the formula

[0027]

Number

[0028] wherein, for the electrode excitation that generates the propagation mode of the elastic cut-off wave, Δ is the acoustic reflection coefficient for each electrode, and the logarithmic function is

[0029]

Number

[0030] represents the decimal logarithm such that, for f h and f bis the frequency associated with the resonance of the elastic cut-off wave at which the electrical susceptance of the elastic wave device reaches its highest and lowest points, respectively.

[0031] The present invention is also extended to a method for producing a SAW device in which the electrodes are embedded in a piezoelectric layer, and this method includes a determination method.

Brief Description of the Drawings

[0032] Other features and advantages of the present invention will become apparent from the following detailed description of the present invention with reference to the accompanying drawings.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0033] The operating frequency of a surface acoustic wave device (SAW device) is defined by the synchronization frequency of the transducer.

[0034] To increase the operating frequency of a surface acoustic wave device, various approaches can be taken.

[0035] The first approach is to reduce the wavelength of the elastic wave used in the device, and the frequency is inversely proportional to this wavelength.

[0036] However, this method is limited by the current technological limitations of UV lithography used in the SAW filter industry, which cannot manufacture electrodes with a width of less than several hundred nanometers, and the wavelength mentioned above is equal to twice the period p of the interdigitated electrodes of the SAW device.

[0037] In addition, problems may occur in the structural stability of the electrodes, and 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.

[0038] The second approach to increasing the operating frequency of a SAW device is to increase the velocity of the elastic wave in the device, and the frequency is proportional to this velocity.

[0039] However, the velocity of the elastic wave in a SAW device is limited by the characteristics of the substrate material.

[0040] To overcome such limitations, for example, it is possible to use guided waves on a composite substrate.

[0041] The solution envisioned in this document is based on the integration of electrodes in the piezoelectric layer in order to increase the speed of the operating mode, i.e., the speed of the elastic waves utilized in the SAW device in question.

[0042] In this document, it is understood that the piezoelectric layer is a layer composed of one or more piezoelectric materials, i.e., a layer of one or more materials having piezoelectric properties.

[0043] Here, consider a pair of interdigitated electrodes (or an Inter-Digitated Transducer (IDT)) that have the same general shape as those used in a conventional SAW transducer with electrodes placed on the piezoelectric layer, but differ in that the electrodes are embedded in the piezoelectric layer.

[0044] As a result of the integration of the IDT in the piezoelectric layer, it is possible to excite the elastic shear modes (known as "electrode modes") mainly limited to each of the electrodes. The acoustic impedance of these electrodes is lower than that of the piezoelectric layer.

[0045] This wave is polarized parallel to the substrate and perpendicular to the sidewalls of the electrodes, creating a shear operation in a direction perpendicular to the propagation direction, and has significant advantages over the surface waves conventionally employed with respect to the achievable frequency of the elastic propagation mode for a given geometry and dimensions of the SAW transducer, as detailed in particular in International Patent Application No. 21053401.

[0046] One problem to be solved is the lack of a measure or table of material properties to refer to in order to determine the materials suitable for the manufacture of such transducers.

[0047] The selection of the substrate is particularly important because it affects the very possibility of using the transducer according to the electrode mode and, moreover, also affects the performance of the device.

[0048] In this context, the inventors of the present invention tested many materials as candidates for use as a substrate and investigated which properties could serve as selection criteria.

[0049] By doing so, the inventors were able to determine the properties that predict the suitability of a substrate material as a substrate for a SAW device designed to implement the electrode mode.

[0050] It is divided into two features, and it is possible to select a substrate so that a SAW device provided with the selected substrate can actually implement the electrode mode suitably.

[0051] "Suitably" means that the electrode mode propagates mainly with respect to the fundamental mode for cutting the piezoelectric layer, that is, 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 having an attenuation below the attenuation of the SAW mode in a conventional transducer.

[0052] As shown in FIGS. 1 and 2, the SAW device 100 considered in this example consists of a SAW transducer composed of a piezoelectric layer 120 with a thickness h, first and second alternating interdigitated electrodes 150A and 150B with a height h corresponding to the thickness of the piezoelectric layer and embedded in the piezoelectric layer 120, and a substrate 130 that supports the piezoelectric layer and the electrodes.

[0053] The electrodes are preferably electrodes that completely penetrate the piezoelectric layer and have the same thickness as the piezoelectric layer. That is, as shown in FIGS. 2 and 3, the electrodes are in direct contact with the substrate 130 or an intermediate layer interposed between the substrate and the piezoelectric layer, and do not protrude above the piezoelectric layer. However, as long as the electrode mode can excite the piezoelectric layer, other geometries are acceptable. The electrodes are sufficient to partially protrude with respect to the piezoelectric layer 120, and they can also protrude above the piezoelectric layer. In this way, they can penetrate the entire piezoelectric layer and exceed its height.

[0054] In the case of non-penetrating electrodes, charges that do not appear in the case of penetrating electrodes appear under the electrodes. This means that the overall contribution of charges from one electrode to the other in the periodic alternating interlocking network can be smaller than in the case of penetrating electrodes. As a result, a SAW device having penetrating electrodes will have a stronger, and thus better, electromechanical coupling than a SAW device having non-penetrating electrodes. Non-penetrating electrodes may also couple residual fundamental modes that can adversely affect the spectral purity of the frequency response.

[0055] In this embodiment, there is direct contact between the substrate 130 and each of the piezoelectric layer 120 and the electrodes 150A, 150B. However, as already mentioned, it is perfectly possible to have an intermediate layer that covers the substrate and is interposed between the substrate and each of the piezoelectric layer 120 and the electrodes 150A and 150B.

[0056] This intermediate layer can include, but is not limited to, a bonding layer used to attach the piezoelectric layer to the substrate, such as a silicon dioxide layer having a thickness of 10 nm to 400 nm, preferably a thickness of 20 nm to 150 nm, for example, a thickness of 30 nm.

[0057] All calculations and conclusions mentioned herein also apply to the situation where this intermediate layer is present and to the situation of the different electrode geometries mentioned above. It should be noted that the inventors have obtained the same conclusions for different drawings.

[0058] The pair of electrodes includes a first electrode 150A and a second electrode 150B, each having a lower surface that contacts the upper surface 130 of the substrate up either directly or indirectly, and a side surface 150 that contacts the piezoelectric layer 120 either directly or indirectly. lat It has.

[0059] Similar to the intermediate layer between the substrate and the piezoelectric layer, it is also possible to have a layer between the electrode and the piezoelectric layer, and under the electrode, between the substrate or the intermediate layer, without affecting the authors' conclusions regarding the selection of the substrate.

[0060] The electrodes 150A and 150B each include fingers 152A and 152B that extend in the same direction D so as to form a periodic structure with a period p in a direction perpendicular to the direction D, and the fingers of the two electrodes are alternately arranged so as to form a pair of alternately interdigitated electrodes or IDTs.

[0061] The elastic wavelength λ of the mode excited by the transducer is equal to twice the period p, i.e., 2p, and the transducer operates under the Bragg condition for this specific wavelength corresponding to the operating elastic wavelength mentioned above.

[0062] This wavelength can also be understood as the distance separating the central axes that are on the extension lines of two adjacent fingers of the same electrode, i.e., the axes that each form the symmetry axis of the corresponding finger in the plan view, and this axis is parallel to the direction D on the extension line of the finger.

[0063] The structure described in FIG. 1 has been computer modeled using, for example, the finite element method as shown in FIG. 3. FIG. 3 shows the period of the model in the direction x that is parallel to the substrate surface and perpendicular to the direction D. In the direction y perpendicular to the substrate surface, only a small part of the substrate is shown, and the substrate is considered to be semi-infinite.

[0064] This model was used by the inventors to computer simulate the electromechanical characteristics of SAW devices as a function of the materials used, and thus to test the relevance of many characteristics in the selection of the materials that make up the substrate.

[0065] The inventors have found that two characteristics can be used to predict the suitability of a given substrate as the manufacture of a SAW device designed to employ the electrode mode: firstly, the attenuation of the elastic electrode mode in the substrate, and secondly, the attenuation of the fundamental mode for cutting into the piezoelectric layer. These parameters are simulated as a function of the piezoelectric layer and the electrodes.

[0066] First condition - attenuation of the electrode mode The substrate under the piezoelectric layer can improve the confinement and propagation of the electrode mode if it is adapted to the characteristics (properties, geometry) of the buried electrodes and the piezoelectric layer, but not all materials are suitable for forming a substrate that enables the structure modeled as shown in FIG. 1 and FIG. 3 to utilize the electrode mode properly, i.e., the electrode mode has an attenuation below that of the mode utilized in a SAW device with surface electrodes, for example, for filter design.

[0067] In this way, the electrode mode propagates through the piezoelectric layer and is attenuated by radiation into the substrate, which is why this is very important for the characteristics of the device.

[0068] Among the characteristics studied and used by the inventors to simulate the behavior of SAW devices, a substrate in which the attenuation of the electrode mode in the piezoelectric layer is less than the attenuation limit value of 0.1 dB / λ, preferably 0.05 dB / λ, more preferably 0.01 dB / λ, has been found to be a substrate that enables the device to appropriately utilize the electrode mode. This attenuation limit value is designed to operate in the electrode mode and has a substrate that matches the characteristics of the electrodes and the piezoelectric layer. Therefore, a SAW device capable of meeting this criterion has at least the same performance as the typical performance of a conventional SAW device based on the operation of one mode of the piezoelectric layer by the surface electrodes, and is selected to have the additional possibility of operating at a higher frequency as a result of the operation based on the electrode mode. Here, it should be understood that the attenuation of the elastic electrode mode of the SAW device according to the present invention has an attenuation below the attenuation of the mode of the piezoelectric layer excited by the surface electrodes.

[0069] Therefore, in combination with a given electrode and piezoelectric layer, a substrate that results in an attenuation of the electrode mode in the piezoelectric layer below the attenuation limit value 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 being suitable for the excitation of the electrode mode in the sense that the expected performance with respect to electrode mode attenuation is at least equivalent to the performance of a conventional SAW device based on the implementation of one mode of the piezoelectric layer by the surface electrodes.

[0070] Conversely, a substrate that causes an attenuation of the electrode mode in the piezoelectric layer exceeding the attenuation limit value of 0.1 dB / λ in combination with the electrodes and a given piezoelectric layer is rejected as being unsuitable for the excitation and operation of the electrode mode in a SAW device comprising the electrodes and the piezoelectric layer. In such a configuration, the surface acoustic wave attenuates too much to be able to propagate properly within the electrode array. For example, using a device of this configuration to create a filter is impossible because the insertion loss is too high.

[0071] The method for calculating the attenuation is described in detail in the doctoral thesis "Etude theorique et experimentale de dispositifs a ondes de surface a haute vitesse et fort couplage:application aux filtres telecom haute frequence" by Y. Fusero, Universite de Franche-Comte, pp. 74 - 75, 2001.

[0072] Figure 4 shows a simulation plot of the electrical admittance of a SAW transducer on a linear scale, with the conductance G, in Siemens per meter, shown on the y-axis as a function of the frequency in the range 2.36 - 2.37 GHz shown on the x-axis, and the associated susceptance B, from -1×10 10 S / m to 2×10 10 S / m.

[0073] The resonance peak of the conductance G is associated with the minimum and maximum values of the susceptance B at the frequencies f h and f b respectively, on either side of the peak.

[0074] The attenuation of the electrode mode in the piezoelectric layer (hereinafter referred to as Attn) can be expressed by Equation 1 below.

[0075]

Equation

[0076] where Δ is the electrode reflection coefficient, and f h and f b are the frequencies of the maximum and minimum susceptances (the imaginary part of the admittance) associated with the SAW device under consideration, as shown in Figure 4 respectively.

[0077] The Second Condition - Attenuation of the Fundamental Cutoff Mode Due to the compatibility with the first condition, ensure that elastic waves can be effectively excited and propagated within the SAW device under conditions similar to or close to those of the elastic waves excited by a conventional SAW device, where the electrode mode is designed to operate in a piezoelectric layer mode excited by a surface electrode.

[0078] However, this criterion alone does not guarantee the practicality of this mode.

[0079] In practice, the overall response of the device and the purity of the frequency response determine its performance in actual applications such as electrical communication signal filtering.

[0080] In order to obtain an acceptable purity of the frequency response, it is particularly necessary to suppress the fundamental mode for cutting the piezoelectric layer.

[0081] Such suppression can be achieved by attenuating this mode in the substrate and can be evaluated using the same equation as Equation 1 above, which can now be applied to the fundamental mode for cutting the piezoelectric layer.

[0082] Through many simulations, it has been determined that for the SAW device to have an acceptable purity of the frequency response, the attenuation of the fundamental mode for cutting the piezoelectric layer by the substrate must be 1 dB / λ or more.

[0083] This condition reflects the fact that a substrate adapted to a given piezoelectric layer provides a sufficiently strong attenuation of the mode of this piezoelectric layer, so that this mode is absorbed by the substrate and does not propagate within the piezoelectric layer, which will have an unacceptable impact on the purity of the frequency response, especially the presence of a second conductance peak with high coupling.

[0084] The value 1 dB / λ represents a limit value where it is still possible to have an overlap of modes that are likely to generate spurious responses.

[0085] As already mentioned, this attenuation can be calculated using Equation 1 in the same way as the attenuation of the electrode mode.

[0086] Alternatively, the compliance with this condition regarding the attenuation criterion can be verified by a criterion related to the ratio of the velocity C(Subst) of the volume elastic mode for cutting the substrate to the velocity C(Piezo) of the fundamental elastic mode for cutting the piezoelectric layer.

[0087] Thus, for a substrate where this C(Subst) / C(Piezo) ratio is 1 or more (the velocity of the volume elastic mode for cutting the substrate is equal to or greater than the velocity of the fundamental elastic mode for cutting the piezoelectric layer), the fundamental elastic mode for cutting the piezoelectric layer tends to be guided into the piezoelectric layer rather than being absorbed by the substrate, and thus tends to degrade the frequency response of the device in question, particularly its spectral purity.

[0088] Such a substrate will be rejected as unsuitable for the proper operation of the device in question.

[0089] Conversely, a substrate where the C(Subst) / C(Piezo) 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) propagates the fundamental elastic mode for cutting the piezoelectric layer into the substrate volume, and the substrate absorbs that mode, preventing degradation of the frequency response of the device in question.

[0090] Such a substrate can be used to produce and implement the device in question.

[0091] The smaller the velocity ratio described above, the stronger the radiation of the piezoelectric layer mode in the substrate, and the better the spectral purity of the frequency response of the device.

[0092] Ratio 1 may be considered acceptable for a particular application, but since ratio 0.9 already provides a clear improvement in frequency response and thus device performance, it is preferred to choose a ratio lower than 1.

[0093] The mode speed can be determined by measuring one or more test structures on the substrate.

[0094] Alternatively, the speed 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 two-dimensionally taking into account the effect of radiation in the geometry of the substrate, as shown in Figure 3.

[0095] Use the finite element calculation method to take into account the embedding of the electrodes in the piezoelectric layer and the mass effect of the electrodes.

[0096] The frequency response of a given SAW transducer has been calculated according to the above method, and the velocity of the volume elastic mode for cutting the substrate and the velocity of the fundamental elastic mode for cutting the piezoelectric layer can be obtained from Equation 2 below, c = λ × f Equation 2 Where c represents the velocity of the wave under consideration, λ represents its wavelength, and f represents its frequency.

[0097] The wavelength λ is determined by the geometry of the electrodes of a given SAW transducer and the frequency of the mode under consideration, and this frequency can be extracted from the frequency response of this transducer, as shown in Figure 5.

[0098] Figure 5 shows, for a given SAW transducer shown in Figure 1, a simulation graph of the electrical impedance of the SAW transducer as a function of frequency from 2.2 to 2.6 GHz shown on the x-axis, and Siemens per meter shown on the y-axis. The y-axis shown on the graph is from 100 S / m to 1×1011 It is shown on a linear scale on the x-axis and a logarithmic scale on the y-axis, with a conductance G in S / m and an associated susceptance B.

[0099] Specific traces of the volume elastic mode for cutting the substrate and the fundamental elastic mode for cutting the piezoelectric layer are indicated on the graph by BS and FS, respectively.

[0100] The frequency of the bulk shear mode at approximately 2.55 MHz and the frequency of the fundamental shear mode at approximately 2.36 MHz can be derived from this graph and used to calculate the velocities of these modes from Equation 2 in this given SAW device.

[0101] Method for evaluating a substrate Based on the acceptance conditions of the substrate detailed above, a method 900 for evaluating a material for use as a substrate in a SAW device in which electrodes are embedded in a piezoelectric layer can be detailed by the figure of FIG. 9.

[0102] Such a method is preferably implemented by a computer system 1000 comprising a computer calculation unit 1010 shown in FIG. 10 and a computer memory 1020 that communicates functionally with the computer calculation unit and stores a database storing parameters related to the geometries and properties of the electrodes, piezoelectric layer, and substrate.

[0103] In step 910, the computer unit 1010 retrieves data including parameters related to the geometries and properties of the electrodes, piezoelectric layer, and substrate from the database 920.

[0104] These data specifically include the properties of the material considered for the substrate: its nature and crystal orientation (if applicable).

[0105] In step 920, the computer unit calculates the attenuation of the electrode elastic mode in the piezoelectric layer from the data retrieved in step 910.

[0106] In step 925, the computer unit compares the attenuation calculated in step 920 with a predefined attenuation value.

[0107] If the attenuation of the elastic electrode mode is greater than or equal to the predefined attenuation value, the computer unit issues a rejection signal R1 for the substrate, which means that the SAW device is not suitable for enabling the implementation of the electrode mode by acousto - electric excitation.

[0108] If the attenuation of the elastic electrode mode is less than the predefined attenuation value (the situation indicated by "Y"), the computer unit issues a partial acceptance signal Al for the substrate, which meets at least one of the conditions required for its acceptance. In this case, it is within the capabilities of the SAW device to enable the implementation of the electrode mode by acousto - electric excitation.

[0109] This predefined attenuation value can be 1 dB / λ for the reasons explained above.

[0110] In step 930, the computer unit calculates the speed ratio between the volume elastic mode for cutting the substrate and the fundamental elastic mode for cutting the piezoelectric layer from the retrieved data.

[0111] In step 935, the computer unit compares the speed ratio calculated in step 930 with a predefined speed ratio value.

[0112] When the speed ratio is less than the value of a predetermined speed ratio (the situation indicated by "Y"), the computer unit emits a signal A2 for partial acceptance of the substrate, which satisfies at least one of the conditions necessary for its acceptance, and in this case, the SAW device in question has the ability to attenuate the fundamental mode for cutting the piezoelectric layer sufficiently for it to exhibit acceptable performance with respect to the purity of its frequency response.

[0113] When the speed ratio is greater than or equal to the value of the predetermined speed ratio, the computer unit emits a rejection signal R2 for the substrate as being unsuitable for acceptable operation of the SAW device.

[0114] The value of the predetermined speed ratio can be equal to 1, and preferably, as explained above, can be equal to 0.9.

[0115] In step 940, the computer unit checks whether the two signals A1 and A2 have actually been emitted, and if they have, emits a signal A representing acceptance of the substrate in the sense that the substrate satisfies the two conditions necessary for it to be considered suitable for forming a SAW device intended to operate using the electrode mode.

[0116] This substrate evaluation method enables the selection of a substrate to be combined with a given electrode and piezoelectric layer without going through the stage of manufacturing time-consuming and costly test samples and without characterizing them from an electroacoustic point of view, to obtain a SAW device designed to implement the electrode mode, or at least to enable samples of that type to be the subject of production.

[0117] These substrate acceptance conditions are, in particular, when the electrode is made of a relatively light metal such as aluminum or an aluminum alloy (e.g., 2% copper aluminum alloy A1Cu 2% ) and when the piezoelectric layer is lithium tantalate LiTaO 3 or lithium niobate LiNbO 3It should be noted that it applies when formed from.

[0118] However, these conditions remain valid for other electrode and piezoelectric layer materials, as well as for geometric shapes not limited to those shown in FIGS. 1 and 2.

[0119] Method for producing a SAW device The substrate evaluation process described above can be incorporated into a method 1100 for producing a SAW device shown in the diagram of FIG. 11. The first step 1110 is to select a substrate for a device whose characteristics are already partially defined, i.e., for a fixed geometry and material of the piezoelectric layer and electrodes.

[0120] This first step 1110 may consist of implementing the method 900 shown in FIG. 9.

[0121] In step 1120, the piezoelectric layer is formed directly on the substrate selected as acceptable in step 1110.

[0122] Alternatively, in step 1120, the piezoelectric layer can be obtained separately and then attached to the substrate by molecular bonding.

[0123] 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.

[0124] In step 1140, the electrodes are formed on the piezoelectric layer by conventional methods.

[0125] By proceeding according to this method, a SAW device that can utilize the electroacoustic excitation electrode mode can be expected to be obtained.

[0126] Exemplary substrates The compatibility with the first and second characteristics defined above has been verified for a set of transducers having the geometries shown in FIGS. 1 and 2.

[0127] In a first series of transducers, for SAW transducers designed to operate at 4.8 GHz or 2.4 GHz, fused silica or "fused quartz" was considered in relation to an electrode consisting of a 2% copper-aluminum alloy A1Cu 2% and a piezoelectric layer consisting of lithium tantalate LiTaO 3 or lithium niobate LiNbO 3 was considered in relation to a piezoelectric layer consisting of lithium tantalate LiTaO or lithium niobate LiNbO.

[0128] The results of the simulation are shown in Table 1 below.

[0129] [Table 1]

[0130] C(Subst) and C(Piezo) respectively represent the velocity of the volume elastic mode for cutting the substrate and the velocity of the fundamental elastic mode for cutting the piezoelectric layer, and C(Subst) / C(Piezo) represents the ratio of these velocities.

[0131] Attenuation (Elec) and Attenuation (Piezo) respectively represent 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.

[0132] Comment (OK) indicates that the substrate meets the conditions related to the corresponding column in the table, i.e., one of the criteria for attenuation or velocity ratio detailed above.

[0133] Here, it is found that it is possible to use fused silica as the substrate for each of the SAW transducers under consideration, and the table indicates this by the "OK" substrate status.

[0134] In the second series of transducers, instead of the first series of fused silica, commercially available 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 / m 3 is used, and all other parameters remain the same.

[0135] The results of the simulation are shown in Table 2 below.

[0136]

Table 2

[0137] It was found that this glass can be used as the substrate for each of the SAW transducers under consideration.

[0138] In the third series of transducers, instead of the first series of fused silica, an AT-cut quartz substrate (IEEE notation (YXl) / 36°) is considered, and all other parameters remain the same.

[0139] The results of the simulation are shown in Table 3 below.

[0140]

Table 3

[0141] It should be noted here that it is not possible to use AT-cut quartz as the substrate for any of the SAW transducers under consideration, the substrate is rejected, and its status is shown as NOK.

[0142] In practice, the conditions regarding the attenuation of the electrode mode in the substrate are satisfied for both types of piezoelectric layers (LiTaO 3 and LiNbO 3 ) for operation at 4.8 GHz, but as indicated by the comment (NOK), the conditions regarding the attenuation of the eigenmode of the piezoelectric layer are not satisfied.

[0143] Therefore, this table indicates a violation of the condition regarding the attenuation of the fundamental mode of the piezoelectric layer in the substrate, which must be 1 dB / λ or more to meet the condition, with (NOK).

[0144] This table also indicates a violation of the condition regarding the ratio of the velocity of the volume elastic mode for cutting the substrate to the velocity of the fundamental elastic mode for cutting the piezoelectric layer, which must be less than 1, with (NOK).

[0145] In the fourth series of transducers, a silicon substrate (100) is used instead of the first series of quartz glass, and all other parameters remain unchanged.

[0146] The results of the simulation are shown in Table 4 below.

[0147] [Table 4]

[0148] Note that, similar to the AT-cut crystal substrate, it is not possible to use silicon (100) as a substrate for any of the SAW transducers under consideration. The substrate is rejected and its status is shown as NOK.

[0149] Additional calculation results similar to those presented above have demonstrated the validity of the use of the following substrates: gallium arsenide substrates, and commercially available Planoptik Borofloat glass with a Young's modulus of 73 GPa, a Poisson's ratio of 0.23, and a density of 2380 kg / m 3 ³.

[0150] Further simulations show that the selection of substrates for the electrodes and piezoelectric layers is never intuitive. Table 5 below lists glasses and their physical properties (density ρ, longitudinal sound propagation velocity V L and transverse sound propagation velocity V T , Young's modulus E, and Poisson's ratio v).

[0151]

Table 5

[0152] The following Tables 6 and 7 show the results of simulations for a device intended to operate at wavelength λ using electrodes of height h where a lithium tantalate piezoelectric layer of height h of 600 nm is mounted and h / λ = 15% and h / λ = 30% are satisfied respectively. More specifically, these tables show the volume elastic mode C(Subst) velocity for cutting the substrate with respect to the glass of Table 5 associated with Au gold, Mo molybdenum and AlCu 2% alloy electrodes, the C(Subst) / C(Piezo) ratio of the fundamental elastic mode C(Piezo) for cutting the 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.

[0153]

Table 6

[0154]

Table 7

[0155] The "NOK" comment indicates that the associated value prohibits the corresponding substrate from being used in an electrode mode-based SAW device according to the speed ratio, attenuation of the electrode elastic mode, attenuation of the piezoelectric layer fundamental shear mode, and attenuation ratio defined in this specification.

[0156] According to the criteria defined in this document, only substrates made of one of the following glasses: C, D, E, Planoptik Eagle XG, Planoptik Borofloat and Pyrex, with a 2% copper-aluminum alloy AlCu 2%When combined with the electrodes fabricated thereby, it provides an inductive electrode mode (propagation loss less than 0.1 dB / λ) to the spectral purity of the frequency response such that the fundamental cutoff mode is attenuated (propagation loss greater than 1 dB / λ). For each configuration that meets these criteria, the ratio between the attenuation of the fundamental cutoff mode and the attenuation of the electrode mode is greater than at least 10.

[0157] Furthermore, for the same type of glass, the fact that it is determined whether the properties of the electrodes meet the criteria regarding (i) the attenuation of the electrode elastic mode in the piezoelectric layer and (ii) the velocity ratio between the volume elastic mode for cutting the substrate and the fundamental elastic mode for cutting the piezoelectric layer indicates that the substrate, the electrodes, and the piezoelectric layer must be matched. This matching is included in meeting the two criteria (i) and (ii) described above.

[0158] These criteria enable the materials used for the substrate, electrodes, and piezoelectric layer of the surface acoustic wave device to be selected as functions of each other based on the excitation of the electrode mode so as to exhibit a spectral response pure enough to be suitable for the intended application. Also, note that the phenomenon used to attenuate the fundamental elastic mode for cutting the piezoelectric layer does not affect the electrode mode.

[0159] Exemplary Application 1 In the case of a device resulting from a suitable selection of the substrate, to show the influence of the fundamental elastic mode for cutting the piezoelectric layer, as shown in FIG. 6, the theoretical transfer function (parameter S 21 ) of a conventional ladder SAW filter 600 in which twelve identical resonators 620 are connected in a ladder configuration (8 in series and 4 in parallel) between a SAW input 630 and a SAW output 640 has been calculated.

[0160] FIG. 7 shows the theoretical transfer function of such a filter composed of resonators designed to operate at 4.8 GHz and including a quartz glass substrate having a LiTaO 3 piezoelectric layer and an A1Cu 2% alloy electrode.

[0161] As shown in Table 1, fused silica is considered an acceptable material for forming the substrate.

[0162] Traces of the fundamental elastic mode for cutting the piezoelectric layer appear at approximately 2.4 GHz, increasing the rejection ratio to -34 dB.

[0163] Exemplary Use 2 In the case of a device due to an inappropriate selection of the substrate, to show the effect of the fundamental mode for cutting the piezoelectric layer, the second use example incorporates Use Example 1, but differs in that the resonator substrate is formed from AT-cut quartz.

[0164] As seen in Table 3, AT-cut quartz is not considered acceptable for forming the substrate because the attenuation of the fundamental mode for cutting the piezoelectric layer in the substrate is insufficient.

[0165] Figure 8 shows the theoretical transfer function of such a filter.

[0166] Traces of the fundamental elastic mode for cutting the piezoelectric layer appear at approximately 2.4 GHz, increasing the rejection ratio to approximately 0 dB.

[0167] Compared to Use Example 1, it is clear that the attenuation of the piezoelectric layer mode is insufficient, significantly deteriorating compared to the previous use example, inhibiting the filter response, and the rejection ratio rising to approximately 0 dB.

[0168] Naturally, the present invention is not limited to the above description, and deformation embodiments can be added without departing from the scope of the present 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 has the following two conditions for the substrate: The attenuation of the elastic mode having an electrode 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 fundamental elastic mode for cutting the piezoelectric layer is less than a predetermined velocity ratio value equal to 1, and the substrate is gallium arsenide, and a Young's modulus of 60 GPa to 180 GPa, a Poisson's ratio of 0.15 to 0.35, and 2000 kg / m ３ to 6000 kg / m ３ of a glass having a density of, characterized in that, the device (100).

2. The device according to claim 1, wherein a ratio of attenuation of a fundamental elastic 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 electrodes pass completely through the piezoelectric layer.

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

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 value of the predetermined speed ratio is equal to 0.

9.

7. The attenuation of the electrode elastic mode is defined by the formula 【Number 1】 where Δ is the acoustic reflection coefficient for each electrode with respect to an electrode excitation that generates a propagation mode of an elastic cut-off wave, and the logarithmic function is 【Number 2】 represents a common logarithm such that f ｈ and f ｂ is a frequency associated with the resonance of the elastic cut-off wave at which the electrical susceptance of the elastic wave device reaches its highest and lowest points respectively, the device according to any one of claims 1 to 6.

8. A method (900) for determining alignment of a substrate for an elastic surface wave device comprising a piezoelectric layer, electrodes embedded in the piezoelectric layer, and a substrate, the method being implemented using a computer system, the method comprising the step (840) of emitting a signal representing alignment of the elastic surface wave device when attenuation of an electrode elastic mode in the piezoelectric layer is less than 0.1 dB / λ and a ratio between a velocity of a volume elastic mode for cutting the substrate and a velocity of a fundamental elastic mode for cutting the piezoelectric layer is less than a predetermined velocity ratio having a value equal to 1.

9. The computer system (900) comprises a computer calculation unit (910) and a computer memory (920) that functionally communicates with the computer calculation unit and stores a database storing parameters related to geometries and properties of the electrodes, the piezoelectric layer, and the substrate, and the computer calculation unit performs the following steps: The step (810) of retrieving data including parameters related to geometries and properties of the electrodes, the piezoelectric layer, and the substrate from the computer memory database (920); The step (820) of calculating attenuation of an elastic electrode mode in the piezoelectric layer based on the retrieved data; Step (825) of comparing the attenuation of the elastic electrode mode in the piezoelectric layer with a predefined attenuation value of 0.1 dB / λ; Step (830) of calculating the velocity ratio between the volume elastic mode for cutting the substrate and the fundamental elastic mode for cutting the piezoelectric layer from the extracted data; Step (835) of comparing the velocity ratio with a predetermined velocity ratio value equal to 1; The attenuation of the electrode elastic mode in the substrate is less than the predefined attenuation value, and Step (840) of emitting the signal representing the alignment of the substrate with respect to the surface acoustic wave device when the velocity ratio is less than the predetermined velocity ratio value. A method (900) for determining the suitability of a substrate for a surface acoustic wave device according to claim 8, characterized by performing the above steps.

10. The method according to claim 8 or 9, wherein the value of the predetermined velocity ratio is equal to 0.

9.

11. The attenuation is defined by the formula [Number 3] where, for an electrode excitation that generates a propagation mode of an elastic cut-off wave, Δ is the acoustic reflection coefficient for each electrode, and the logarithmic function is 【Number 4】 represents a common logarithm such that f ｈ and f ｂ is a method according to any one of claims 8 to 10, wherein the electrical susceptance of the elastic wave device is a frequency associated with the resonance of the elastic cut-off wave that reaches the highest point and the lowest point thereof, respectively.

12. A method (1100) for manufacturing a surface acoustic wave device comprising a piezoelectric layer, an electrode embedded in the piezoelectric layer, and a substrate, characterized by including the method (900) for determining the alignment of the substrate material according to any one of claims 8 to 11.

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