Resonant Cavity Surface Acoustic Wave (SAW) Filters

Abstract

The present invention relates to a coupled cavity filter structure that utilizes surface acoustic waves, specifically guided surface acoustic waves, and comprises: an acoustic wave propagation substrate; at least one input transducer structure and an output transducer structure, each comprising an interdigitated comb electrode, disposed on the substrate; and a reflective structure comprising at least one or more metal strips positioned at a distance between the input transducer structure and the output transducer structure in the direction of acoustic wave propagation; the reflective structure being characterized in that the acoustic wave propagation substrate is a composite substrate comprising a base substrate and a piezoelectric layer. The present invention also relates to a coupled cavity filter structure, wherein one reflective structure comprises a slot. The present invention also relates to a SAW ladder filter device, comprising at least two coupled cavity filter structures as described above, wherein at least two coupled cavity filter structures are positioned on a single line.

Description

Technical Field

[0001] The present invention relates to surface acoustic wave devices for filter applications, and more particularly to composite substrates for surface acoustic wave filter devices. Background Art

[0002] In recent years, surface acoustic wave (SAW) devices have been increasingly used in practical applications such as filters, sensors, and delay lines.

[0003] The synthesis of SAW filters requires different types of tools and allows for the realization of different types of structures. However, the use of classical filter structures in SAW devices faces various problems (such as compactness and performance of the device).

[0004] SAW filter devices typically use a wafer made of a single piece of quartz, LiNbO3, or LiTaO3 crystal as the piezoelectric material. However, depending on the piezoelectric material used, the use of a piezoelectric substrate can result in high temperature sensitivity or weak electromechanical coupling. This can lead to poor filter passband performance.

[0005] Filter performance is defined by using several parameters such as bandwidth, in-band insertion loss, rejection and transition bandwidth, separating the passband and rejection band.

[0006] Furthermore, the use of cavities to generate poles and zeros in the filter transfer function is a well-known technique that is systematically used when developing microwave filters operating at several GHz. Such filters require a waveguide along which resonant elements are arranged that generate poles or zeros depending on how they are connected to each other (in series or in parallel). The synthesis of such filters is based on a combination of these poles and zeros, thereby providing reduced in-band ripple and improved out-of-band suppression assuming a given coupling factor between the source and the filter structure. In any case, the filter consists simply of a series of cavities that are connected to each other or placed along the waveguide and accessed via electrical connectors or directly via the edge of the waveguide. For SAW devices, a conversion from electromagnetic waves to acoustic waves, and vice versa, must be achieved to provide an electromagnetically filtered signal. In between, the electrode structures are combined in such a way that the resonances can be combined electrically or acoustically to produce a filtering effect.

[0007] To date, currently developed solutions for SAW filters use three architectures to implement the filter function, mainly impedance element lattices (so-called ladder SAW) or longitudinally coupled resonator filters (LCRF) or dual-mode SAW (DMS) filters, both of which are based on coupled IDTs using gratings operating under near-Bragg conditions. However, these approaches generally allow the placement of two or more poles in the passband, resulting in suboptimal performance of the device and requiring a relatively large footprint.

[0008] The object of the present invention is to overcome the previously mentioned drawbacks by providing a surface acoustic wave (SAW) filter device deposited on a composite substrate having an improved design to achieve compactness, simplicity and versatility as well as good performance. Summary of the Invention

[0009] The objectives of the present invention are achieved by a coupled cavity filter structure that utilizes surface acoustic waves, specifically guided surface acoustic waves, and includes: an acoustic wave propagation substrate; at least one input transducer structure and an output transducer structure, the at least one input transducer structure and the output transducer structure being disposed on the substrate, the at least one input transducer structure and the output transducer structure each comprising an interdigitated comb electrode; and a reflective structure comprising at least one or more metal strips positioned at a distance d between the input transducer structure and the output transducer structure in the direction of acoustic wave propagation. The structure is characterized in that the acoustic wave propagation substrate is a composite substrate comprising a base substrate and a piezoelectric layer. With this filter structure, a passband can be adjusted, and a filter structure with a reduced footprint can be obtained compared to the aforementioned filter structures in the art.

[0010] According to a variation of the present invention, the coupled cavity filter structure can be constructed so that the surface acoustic waves are shear waves or longitudinal waves within the piezoelectric layer. Although Rayleigh waves are used in the prior art, the use of a piezoelectric layer instead of a bulk substrate opens the way to the use of different types of acoustic waves, thereby providing additional optimization parameters. Guided shear waves can provide the highest electromechanical coupling that can be achieved using a composite substrate. Compared to classical Rayleigh-like waves, they allow for more thermal compensation opportunities to obtain higher wave speeds than elliptically polarized waves. In addition, the use of a given combination of substrate and excitation conditions can excite longitudinally polarized guided waves, resulting in higher speeds than other wave types (shear waves and Rayleigh waves), with a coupling degree exceeding 5%.

[0011] According to a variant of the present invention, the interdigitated comb electrodes of at least one input transducer structure and one output transducer structure can be defined by a Bragg condition given by p = λ / 2, where λ is the operating acoustic wavelength of the transducer structure and p is the electrode pitch of the transducer structure. This approach can produce optimal dimensions and excitation or coupling conditions for given frequency, bandwidth, and energy constraints.

[0012] According to a variation of the present invention, the coupled cavity filter structure may further include at least one Bragg mirror located separately from the input transducer structure and / or the output transducer structure on the opposite side of the reflective structure in the direction of acoustic wave propagation. The presence of the Bragg mirror adjacent to the transducer structure reduces losses in the structure.

[0013] According to a variation of the present invention, a coupled cavity filter structure may include multiple reflective structures separated from each other by gaps g, positioned at a distance d in the direction of acoustic wave propagation and positioned between the input transducer structure and the output transducer structure. The gaps g between the reflective structures and the gaps d between the transducer structure and its adjacent reflective structures form an acoustic cavity. The fact that the structure has more than one reflective structure provides multiple acoustic cavities within the structure. A greater number of acoustic cavities can narrow the transition band.

[0014] According to a variation of the present invention, the size of each acoustic cavity of the cavity filter structure can be smaller than λ / 4, specifically making the phase velocity in the cavity better than the phase velocity in the reflective structure. Such a size can improve the resonance conditions and thus improve the performance of the filter.

[0015] According to a variation of the present invention, the distances between adjacent reflective structures in the plurality of reflective structures and / or the distances between a reflective structure and an adjacent transducer structure can be the same or different. By adjusting the size of the gap and thus the size of the cavity, the filter parameters can be improved.

[0016] According to a variant of the present invention, the reflective structure or structures may have a unitary metal strip reflection coefficient that is superior to the coupling coefficient k of the composite substrate and the electrodes of the transducer structure. s 2 Specifically, the reflection coefficient of the single metal strip is better than the coupling coefficient k s 2 At least 1.5 times. For a given bandwidth, a higher ratio of reflection coefficient to coupling coefficient provides a substantially flat in-band transfer function and a sharp transition band, with reduced in-band ripple effects compared to filters that do not meet this condition.

[0017] According to a variation of the invention, each reflective structure may comprise at least one or more metal strips having a pitch that is the same as or different from the electrode pitch of the transducer structure. The use of highly reflective structures can make the mirror more tolerant to manufacturing variations, but also shift the zero point of the reflectance function to improve out-of-band rejection.

[0018] According to a variation of the present invention, the metal strips of each reflective structure can be electrically connected to each other, thereby achieving a constant potential value in each reflective structure, thereby improving the reflection coefficient of the reflective structure under Bragg conditions.

[0019] According to a variation of the present invention, the number of metal strips in each of the multiple reflective structures can be less than 30, preferably less than 20, so that the reflection coefficient of the multiple reflective structures is greater than 0.5, specifically greater than 0.8. This improves the confinement of acoustic energy by the cavity and provides mode coupling conditions in the structure.

[0020] According to a variation of the present invention, the difference between the acoustic impedance of the material of the piezoelectric layer and the acoustic impedance of the material of the metal strips of each of the multiple reflective structures can result in a reflection coefficient of the multiple reflective structures that is better than 0.5, specifically better than 0.8. By increasing the reflection coefficient, the size of the filter structure can be reduced.

[0021] The objectives of the present invention are also achieved by a coupled cavity filter structure that utilizes surface acoustic waves, specifically guided surface acoustic waves, and includes: an acoustic wave propagation substrate; at least one input transducer structure and an output transducer structure, the at least one input transducer structure and the one output transducer structure being disposed on the substrate, the at least one input transducer structure and the one output transducer structure each including an electrode; and a reflective structure including a groove, the reflective structure being positioned at a distance L between the input transducer structure and the output transducer structure in the direction of acoustic wave propagation. The acoustic wave propagation substrate is a composite substrate comprising a base substrate and a piezoelectric layer. With this filter structure, a passband can be adjusted, and a filter structure having a reduced footprint can be obtained compared to the aforementioned filter structures in the art.

[0022] According to a variation of the present invention, the coupled cavity filter structure can be constructed so that the surface acoustic waves are shear waves or longitudinal waves within the piezoelectric layer. Although Rayleigh waves are used in the prior art, the use of a piezoelectric layer instead of a bulk substrate opens the way to the use of different types of acoustic waves, thereby providing additional optimization parameters. Guided shear waves can provide the highest electromechanical coupling that can be achieved using a composite substrate. Compared to classical Rayleigh-like waves, they allow for more thermal compensation opportunities to obtain higher wave speeds than elliptically polarized waves. In addition, the use of a given combination of substrate and excitation conditions can excite longitudinally polarized guided waves, resulting in higher speeds than other wave types (shear waves and Rayleigh waves), with a coupling degree exceeding 5%.

[0023] According to a variation of the present invention, the coupled cavity filter structure may further include at least one additional slot located separately from the input transducer structure and / or the output transducer structure on a side opposite to the side where the reflective structure is located in the direction of acoustic wave propagation. The presence of the slot adjacent to the transducer structure reduces losses in the structure and allows for configuration to achieve total reflection of acoustic waves propagating through the transducer.

[0024] According to a variant, the electrodes of at least one input transducer structure and one output transducer structure may be defined by an electrode pitch p equal to nλ, λ being the operating acoustic wavelength of said transducer structures.

[0025] According to a variation, the coupled cavity filter structure may further comprise at least one additional slot located on the opposite side of the reflective structure in the direction of acoustic wave propagation, separated from the input transducer structure and / or the output transducer structure.

[0026] According to a variant, the depth D3 of at least one additional groove located on the opposite side of a reflective structure from the input transducer structure and / or the output transducer structure in the direction of propagation of the sound waves may be approximately λ or greater.

[0027] According to a variation, the coupled cavity filter may include a plurality of reflective structures separated from each other by gaps g, positioned at a distance L between the input transducer structure and the output transducer structure in the direction of acoustic wave propagation, with each gap g between the reflective structures forming an acoustic cavity. According to a variation, the acoustic cavity may be formed by a distance L2 between the edge of the slot and the following positions A and B, that is, the positions A and B correspond to the ends of the pitch of the transducer structure (812, 814) on the side of the slot in the direction of acoustic wave propagation. The fact that the structure has more than one reflective structure provides multiple acoustic cavities in the structure. A larger number of acoustic cavities can narrow the transition band.

[0028] According to a variant, the distance between the edge of the slot (822, 1022, 1322, 1422, 1522, 1722) and the edge of at least one additional slot (932, 934) can be approximately nλ. Such dimensions can improve the resonance conditions and thus the performance of the filter.

[0029] According to a variant, the relief angle of the grooves of the reflective structure may be approximately 70° or greater, in particular approximately 90°, this relief angle being the angle between the horizontal axis X and the edge wall of the groove.

[0030] According to a variant, the depth of the grooves of the reflective structure is approximately λ or more, in particular approximately 10λ or more, λ being the wavelength of the surface acoustic wave.

[0031] According to a variant, at least one additional slot is configured to obtain total reflection of the propagating wave along the propagation direction.

[0032] According to a variant, the acoustic cavity formed between at least two grooves of the plurality of reflective structures may be located at the surface of the substrate, which is also the surface of the substrate on which the transducer is provided.

[0033] According to a variation, the acoustic cavity formed between at least two grooves of the plurality of reflective structures may be located at a depth comprised between a surface of the substrate and bottom surfaces of the at least two grooves located at a depth D.

[0034] According to a variant, the input and output transducer structures can be different, specifically the number of electrode fingers of each transducer structure can be different. This makes the filter structure more versatile, and the transducer structure can be modified to optimize the coupling efficiency of the modes within the reflective structure, thereby achieving low insertion loss.

[0035] According to a variation of the present invention, the acoustic cavity can be divided into separate sub-cavities. The sub-cavities can be separated from each other by additional layers, thereby allowing for evanescent coupling from one cavity to another. Thus, the sub-cavities facilitate energy confinement within the structure and can lead to improved device compactness.

[0036] According to a variation of the present invention, the cavity filter structure includes at least three or more transducer structures in the direction of acoustic wave propagation. This increases the source density, resulting in improved suppression. Furthermore, the filter band can be flatter compared to an identical filter with only two transducers.

[0037] According to a variant of the invention, the properties of the piezoelectric layer and the properties of the electrodes of the transducer structure can be chosen such that the electromechanical coupling coefficient k of a shear wave (preferably a guided shear wave or a guided longitudinal wave) in the piezoelectric layer is s 2Better than 5%, specifically better than 7%, to obtain higher filter bandpass properties, specifically by adjusting the electrode geometry of the transducer structure (such as the thickness, width and / or length and / or number and / or shape of the electrodes) and the thickness of the piezoelectric layer (the thickness of the piezoelectric layer should be greater than or equal to 5% of the wavelength λ).

[0038] According to a variant of the invention, the thickness of the piezoelectric layer can be selected so that the electromechanical coupling coefficient k of shear waves (preferably guided shear waves or guided longitudinal waves) in the piezoelectric layer (104) is s 2 Better than 5%, in particular better than 7%.For greater thicknesses (greater than λ), the acoustic wave loses its guiding properties, resulting in multiple wave emissions in the layer and energy losses in the substrate.

[0039] According to a variation of the present invention, the cavity filter structure may further include a dielectric layer, specifically a SiO2 layer, interposed between the base substrate and the piezoelectric layer. The dielectric layer, or passivation layer, may improve adhesion of the piezoelectric layer to the base substrate, but may also improve electromechanical coupling while maintaining the temperature stability of the surface acoustic wave device. Preferably, the dielectric layer has a thickness of less than 1 μm, specifically a thickness in the range of 100 nm to 1 μm.

[0040] According to a variant of the invention, the piezoelectric layer of the composite substrate can be aluminum nitride (AlN), zinc oxide (ZnO), PZT, potassium niobate KNbO3 and similar materials (such as KTN, etc.) and piezoelectric relaxers (such as PMN-PT and related materials), gallium nitride (GaN), lithium tantalate LiTaO3 or lithium niobate LiNbO3, and the lithium tantalate LiTaO3 or lithium niobate LiNbO3 has a lithium tantalate LiTaO3 or lithium niobate LiNbO3 crystal orientation defined as (YXI) / θ according to standard IEEE 1949Std-176, where the crystal orientation angle θ is included between 0° and 60° or between 90° and 150°.

[0041] According to a variant of the present invention, the base substrate of the composite substrate can be one of silicon (particularly a high resistivity silicon substrate including a trap-rich layer), carbon diamond, sapphire or silicon carbide. By high resistivity, it is understood that the resistivity exceeds 1000 Ohm.cm. In order to transfer the piezoelectric layer to silicon, a mass production method such as SmartCut can be used. TM, which uses ion implantation in a piezoelectric source substrate to define the layer to be transferred, attaches the source substrate to a silicon substrate, and transfers the layer by thermal or mechanical treatment. Simpler methods based on bonding the piezoelectric substrate to a base substrate and subsequently thinning the piezoelectric substrate (via CMP, grinding, polishing) can also be used in the present invention, particularly for thick piezoelectric layers with a final thickness of approximately 5 μm to 20 μm. Both methods (via SmartCut TM or layer transfer via bonding / thinning) both result in the formation of a single-crystal piezoelectric layer on the base substrate and are therefore of high quality.

[0042] According to a variation of the present invention, the base substrate can include a Bragg mirror beneath the piezoelectric layer. This Bragg mirror consists of a stack of layers with periodically alternating acoustic impedances, deposited or fabricated on top of a slab of any inorganic material. If the thickness of each layer is approximately one-quarter of the acoustic wavelength, the stack behaves like a mirror for waves excited in the top piezoelectric layer. The mirror thus reflects the waves, with the component pointing deep into the substrate, thereby confining the waves to the piezoelectric layer.

[0043] According to a variant of the invention, the coupled cavity surface acoustic wave filter structure has a filter bandpass comprised between 0.5% and 10%. The bandpass of the filter device can be varied by changing the parameters of the cavity filter structure, so the device can be adapted to the user's specifications to obtain the desired filter bandpass.

[0044] According to a variation of the present invention, the coupled cavity filter structure may further comprise a passivation layer formed on the transducer structure and the at least one reflective structure, the passivation layer having a predetermined thickness, the predetermined thickness being the same or different on the transducer structure and / or the at least one reflective structure.

[0045] The objects of the present invention are also achieved by a SAW ladder filter device, comprising at least two coupled cavity filters as described above, wherein the at least two coupled cavity filter devices can be positioned on a single line. Because the coupled cavity filters according to the present invention can be positioned on a single line, the positioning and connection of multiple cavity filters does not require as much space as in prior art SAW ladder devices. Compared to existing SAW ladder filter devices, the SAW ladder filter device according to the present invention is present in a more compact device. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which reference numerals identify features of the invention.

[0047] Figure 1a and Figure 1b The structure of a coupled-cavity surface acoustic wave filter according to a first embodiment of the present invention and its modifications are illustrated.

[0048] Figures 2a to 2e The structure of a coupled-cavity surface acoustic wave filter according to a second embodiment of the present invention and its modifications are illustrated.

[0049] Figures 3a to 3d Shown as Figure 2b The performance of a coupled cavity surface acoustic wave filter structure is shown, which has a composite substrate according to the present invention, which has a 500nm SiO2 layer between a 6μm LiTaO3(YXI) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate.

[0050] Figure 4 The following are shown: Figure 2b Table showing the characteristics of the coupled-cavity SAW filter structure.

[0051] Figure 5a and Figure 5b A coupled-cavity surface acoustic wave filter structure according to a third embodiment of the present invention is illustrated.

[0052] Figure 6 A coupled-cavity surface acoustic wave filter structure according to a fourth embodiment of the present invention is illustrated.

[0053] Figure 7 A coupled-cavity surface acoustic wave filter structure according to a fifth embodiment of the present invention is illustrated.

[0054] Figures 8a to 8h The structure of a coupled-cavity surface acoustic wave filter according to a sixth embodiment of the present invention and its modifications are illustrated.

[0055] Figure 9 An apparatus according to a third embodiment of the present invention is shown for a practical example of a simulation.

[0056] Figure 10a and Figure 10b The third embodiment of the present invention is shown as follows Figure 9 Simulated characteristics of the coupled-cavity surface acoustic wave filter structure shown.

[0057] Figures 11a to 11d The device parameters according to the third embodiment of the present invention are shown. Figure 9 The influence of the coupled-cavity SAW filter structure on the simulated characteristics is shown.

[0058] Figure 12a and Figure 12b An example of a SAW ladder filter device according to the prior art is illustrated ( Figure 12a ) and an example of a SAW ladder filter device according to the present invention ( Figure 12b ). DETAILED DESCRIPTION

[0059] Figure 1a FIG. 1 shows a coupled cavity surface acoustic wave filter structure according to a first embodiment of the present invention. Figure 1a In the embodiment of the present invention, a coupled cavity surface acoustic wave filter structure 100 is implemented on a substrate 102 as a composite substrate. The composite substrate 102 includes a piezoelectric material layer 104 having crystal axes X, Y, and Z formed on a base substrate 106.

[0060] The piezoelectric layer 104 in this embodiment is LiTaO3 or LiNbO3 (in particular having a cutting orientation defined by (YXI) / θ according to standard IEEE1949Std-176, where the crystal orientation angle θ is included between 0° and 60° or between 90° and 150°), potassium niobate KNbO3 and similar material compositions (such as KTN) and other piezoelectric layers using sputtered films or epitaxial films (for example, aluminum nitride AlN, zinc oxide ZnO, PZT, GaN or any composition of AlN and GaN).

[0061] The thickness of the piezoelectric layer 104 formed on the base substrate 106 is approximately one wavelength λ or less, specifically approximately 20 μm or less. The thickness t of the base substrate 106 is greater than the thickness of the piezoelectric layer 104. Preferably, the thickness of the base substrate is at least ten times greater than the thickness of the piezoelectric layer 104, specifically 50 to 100 times greater, which corresponds to a base substrate thickness of 250 μm to 500 μm.

[0062] The base substrate 106 used in the first embodiment of the present invention is a silicon substrate, specifically a high-resistivity silicon substrate. The silicon substrate preferably has a (100) orientation because the acoustic wave propagation velocity is higher in (100) than in other crystal orientations such as (110), (111), or (001), however, other crystal orientations may be used. As an alternative to silicon, other substrate materials may be selected whose acoustic wave propagation velocity is greater than that in the piezoelectric layer, such as carbon diamond, sapphire, or silicon carbide.

[0063] In a variation of the invention, the base substrate 106 may further include a so-called trap-rich layer near the top piezoelectric material layer, which improves the isolation performance of the base substrate and may be formed of at least one of a polycrystalline material, an amorphous material, or a porous material (such as, for example, polycrystalline silicon, amorphous silicon, or porous silicon), but the invention is not limited to such materials.

[0064] In a variation of the present invention, the base substrate 106 may also include a Bragg mirror below the piezoelectric layer 104. The Bragg mirror consists of a stack of layers with periodically alternating acoustic impedances, deposited or fabricated on top of any inorganic material slab. Acoustic impedance is the product of the wave velocity times the material density and is expressed in Rayleighs, preferably M Rayleighs (i.e., 10 6 Rayleigh). A piezoelectric layer is deposited or fabricated on top of the stack to excite and detect acoustic waves. The stack may advantageously consist of alternations of tungsten and silicon dioxide, or Si3N4 and SiO2, or Mo and Al, and generally consists of any pair of materials exhibiting an acoustic impedance ratio greater than 2. The inorganic substrate may advantageously be standard silicon or high resistivity silicon or glass, and generally is any material exhibiting a thermal coefficient of expansion (TCE) less than 6 ppm / K. It may also contain a trap-rich layer to improve electrical isolation. Advantageously, the first layer of the stack may be SiO2 or generally any material that can be used to bond the piezoelectric layer to the composite substrate described above.

[0065] In this embodiment, a thin SiO2 layer 108 is provided at the interface 110 between the piezoelectric layer 104 and the base substrate 106 to improve the adhesion of the piezoelectric material layer 104 to the base substrate 106. The thickness of the SiO2 layer 110 is 200 nm, but in variations, the thickness of the SiO2 layer 110 may vary and may be greater than or less than 200 nm, specifically, may vary between 10 nm and 6 μm.

[0066] The coupled cavity filter structure 100 further includes two transducer structures 112, 114 and a reflective structure 116. Figure 1a 14 is positioned between the two transducer structures 112, 114 in the direction of propagation X shown. The region between the reflective structure 116 and one transducer structure 112, 114 (e.g., region 118 whose width is defined by the distance d) corresponds to the acoustic cavity 120. In this example, the electrode is centered within the pitch p of the transducers 112, 114. Therefore, hereinafter, the end of the pitch p of the transducer structures 112, 114 is located at a distance from the electrode 128. In the example, when the ratio a / p of the transducer structures 112, 114 is 50%, the distance between the end of the pitch p and the first electrode 128 of the transducer structures 112, 114 is equal to λ / 8.

[0067] As a result, the acoustic cavity extends between the reflective structure 116 and the ends of the transducer structures 112, 114 at a pitch p on the side of the reflective structure 116. Therefore, in the coupled cavity surface acoustic wave filter structure 100, various acoustic cavities exist in the direction of acoustic wave propagation. Figure 1a In the coupled cavity filter structure shown, there are two acoustic cavities 120 .

[0068] The reflective structure 116 generally includes one or more metal strips 122 and is defined by a pitch (not shown) of the metal strips 122, which corresponds to the distance between the metal strips 122 within the reflective structure 116. Similar to the transducer structures 112, 114, the pitch in the reflective structure 116 is defined by centering the metal strips within the pitch.

[0069] Transducer structures 112 and 114 correspond to input transducer structure 112 and output transducer structure 114, but their positions can also be reversed, so that in the direction of sound wave propagation, the input transducer structure is on the right side of the structure and the output transducer structure is on the left side of the structure. The E symbol represents the input acoustic signal of the transducer structure, and the S symbol represents the output acoustic signal of the transducer structure.

[0070] Each transducer structure 112, 114 includes two interdigitated comb electrodes 124, 126, each of which includes a plurality of electrode means 128, 130. In the present embodiment, the electrode means 128, 130 have the shape of electrode fingers. The comb electrodes 124, 126 and their corresponding electrode fingers 128, 130 are formed from an aluminum-based material, such as pure aluminum or an aluminum alloy (such Al is doped with Cu, Si or Ti). However, other materials that generate a stronger reflection coefficient for a smaller relative electrode thickness can be used. In this regard, preferred electrode materials are copper (Cu), molybdenum (Mo), nickel (Ni), platinum (Pt) or gold (Au) with an adhesion layer, such as titanium (Ti) or tantalum (Ta) or chromium (Cr), zirconium (Zr), palladium (Pd), iridium (Ir), tungsten (W), etc.

[0071] The transducer structures 112, 114 are also defined by an electrode pitch p (not shown), which corresponds to the edge-to-edge electrode finger distance between two adjacent electrode fingers 128, 130 from the opposing comb electrodes 124 and 126. In a variation of the present invention, the electrode pitch p is defined by the Bragg condition given by p = λ / 2, λ being the operating acoustic wavelength of the transducer structures 112, 114. With respect to the operating acoustic wavelength λ, it is understood that λ is the acoustic wavelength that follows λ = V / f, where f is the predetermined center frequency of the filter structure and V is the phase velocity of the operating mode. This transducer structure is also referred to as an interdigital transducer (IDT) with two fingers per wavelength.

[0072] In variations of the invention, the IDT may operate without Bragg conditions, for example using an excitation structure with 3 or 4 fingers per wavelength or a transducer with 5 fingers per two wavelengths or a transducer with 7 or 8 fingers per three wavelengths.

[0073] The transducer structures 112 and 114 may be symmetrical, i.e., they have the same number of electrode fingers 128, 130 of the same characteristics. However, in a variant of the invention, the transducer structures 112 and 114 may also be different; in particular, the transducer structures 112 and 114 may have different numbers of electrode fingers 128, 130.

[0074] The electrode fingers 132, 134 of the comb electrodes 128, 130 all have substantially the same length l, width w, and thickness h. According to a variation of this embodiment, the electrode fingers 132, 134 may have different lengths l, widths w, and thicknesses h. The dimensions are adjusted to obtain a desired coupling coefficient k s Or take advantage of other features (such as eliminating shear mode, modulating IDT impedance, reducing unwanted mode emission, etc.).

[0075] In a variation of the present invention, the transducer structures 112, 114 can be chirped, meaning that the electrode pitch p in the transducer structure can be continuously varied in a linear or hyperbolic manner. This can expand the operating frequency band of the transducer and can also produce a certain degree of robustness to temperature.

[0076] The pitch of the metal strips 122 of the reflective structure 116 may be the same as the electrode pitch p of the transducer structures 112 , 114 . In a variation, the pitch of the metal strips 122 of the reflective structure 116 may be different from the electrode pitch p of the transducer structures 112 , 114 .

[0077] In a variation of the present invention, the reflective structure 116 may also be chirped to increase the operating frequency band of the filter and the resonance efficiency of the acoustic cavity 118 located between the transducer structures 112 , 114 .

[0078] In a variation, the coupled cavity filter structure 100 further includes two Bragg mirrors 132, 134. Figure 1b As shown, each Bragg mirror 132, 134 is positioned outside the coupled cavity filter structure 200 near the transducer structures 112, 114 in the direction of acoustic wave propagation, which means it is positioned on the other side of the side where the reflective structure 116 is located. Each Bragg mirror 132, 134 is positioned at a distance s from its corresponding transducer structure 112, 114. Each Bragg mirror 132, 134 includes one or more metal strips 136 and is defined by a pitch (not shown) of the metal strips 136, which corresponds to the distance between the metal strips 136 within the Bragg mirror 132, 134.

[0079] In a variation of the invention, the reflective structure 116 and Bragg mirrors 132, 134 may be constructed by etching grooves rather than depositing metal strips 136, 210. The grooves may be etched in the piezoelectric layer 104 of the composite substrate 102 and even down to the base substrate 106.

[0080] In a variation, a passivation layer (not shown) may be formed on the transducer structures 112, 114 and the at least one reflective structure 116. The passivation layer has a predetermined thickness that is the same or different across the transducer structures 112, 114 and / or the at least one reflective structure 116. A passivation layer may also be formed on the Bragg mirrors 132, 134. In this variation, the substrate may be a monolithic piezoelectric wafer such as a lithium tantalate or lithium niobate bulk wafer, and the passivation layer may advantageously be a silicon dioxide SiO2 layer or a tantalum pentoxide Ta2O5 layer. In this embodiment, the passivation layer has a positive coefficient of thermal expansion (TCE), while the substrate has a negative coefficient of thermal expansion (TCE), and the layer thickness is set to reduce the temperature coefficient of frequency (TCF) of the SAW device.

[0081] Figures 2a to 2e The coupled cavity filter structure according to the second embodiment of the present invention and its variants are shown. Figures 2a to 2e , the coupled cavity filter structure is shown in a 2D plan view, and the substrate on which the coupled cavity filter structure is located is no longer shown. However, the substrate is Figure 1a 、 Figure 1b The same substrate 102 as in Figure 1a and Figure 1b The same reference numerals are used to refer to the same features and will not be explained again in detail.

[0082] exist Figure 2aIn FIG, similar to the coupled cavity filter structure 200, the coupled cavity filter structure 300 includes two transducer structures 112 and 114, wherein two Bragg mirrors 132 and 134 are each located adjacent to one transducer structure. Unlike the coupled cavity filter 200, a plurality of reflective structures, namely four reflective structures 202, 204, 206, and 208, are located between the transducer structures 112 and 114. Each reflective structure 202, 204, 206, and 208 in the plurality of reflective structures includes at least one or more metal strips 210 and is defined by a pitch (not shown) of the metal strips 210, which corresponds to the distance between the metal strips 210 within each reflective structure 202, 204, 206, and 208. Here, the total number of metal strips 210 in each reflective structure 204, 206, 208, and 210 is four, but the number may be more or less. The reflective structures 204, 206, 208, 210 in the plurality of reflective structures may also have the same number of metal strips 210, but in variations, they may each have a different number of metal strips 210. For example, the number of metal strips 210 in the reflective structures 204, 206, 208, 210 may increase and then decrease across the reflective structures 204, 206, 208, 210 between the transducer structures 112, 114 to enhance resonance at the actual center of the structure.

[0083] These reflective structures 202, 204, 206, 208 are separated from each other by a gap g. The area between two adjacent reflective structures (e.g., 202 and 204, whose width is defined by the gap g) corresponds to the acoustic cavity 212. Similar to the coupled cavity filter structures 100 and 200, the area between a reflective structure and an adjacent transducer structure also corresponds to the acoustic cavity 214, but the width of the area is defined by the distance d between the reflective structure and the adjacent transducer structure. Similar to the first embodiment, the electrodes of the transducer structures 112, 114 are centered within the pitch p of the transducers 112, 114, and the acoustic cavity is defined as the area between the reflective structure 116 and the end of the pitch p of the transducer structures 112, 114 on the side where the reflective structure 116 is located.

[0084] Therefore, in the coupled cavity surface acoustic wave filter structure, there are various cavities separated by the reflective structures in the direction of acoustic wave propagation, or in other words, the cavity is surrounded by two reflective structures between the transducers. Figure 2a In the coupled cavity filter structure 300 shown, there are a total of five acoustic cavities 212, 214 in the mode propagation direction. For a given number of reflection structures between the transducer structures, and the transducer structures operating under Bragg conditions, the number of acoustic cavities is equal to the number of reflection structures plus one.

[0085] In a variation of the present invention, the reflective structures 202 , 204 , 206 , 208 may also be chirped to increase the operating frequency band of the filter and the resonance efficiency of the acoustic cavity 212 , 214 located between the transducer structures 212 , 214 .

[0086] In a variation of the invention, the reflective structures 202, 204, 206, 208 and the Bragg mirrors 132, 134 can be constructed by etching grooves rather than depositing metal strips 136, 210. The grooves can be etched in the piezoelectric layer 104 of the composite substrate 102 and even down to the base substrate 106.

[0087] In a variant of the present invention, the metal strips 210 of the reflective structures 202, 204, 206, 208 and / or the metal strips 136 of the Bragg mirrors 132, 134 may be electrically connected to each other. Figure 2b 1 shows a variant in which the reflective strips 210 of the reflective structures 202, 204, 206, 208 and the reflective strips 136 of the Bragg mirrors 132, 134 are connected to one another for the coupled cavity filter structure 400. This results in an improvement in the reflection coefficients of the reflective structures 202, 204, 206, 208 and the reflection coefficients of the Bragg mirrors 132, 134 under Bragg conditions. All reflective structures 202, 204, 206, 208 and / or Bragg mirrors 132, 134 are operated under so-called short-circuit conditions, meaning that all metal strips 210 of a given reflective structure 202, 204, 206, 208 / all metal strips 136 of a Bragg mirror 132, 134 are connected to one another, resulting in a constant potential value throughout the grating structure.

[0088] In a variation of the invention, the coupled cavity filter structure may comprise three or even more transducer structures. Figure 2c FIG. 4 shows a variation in which there are three transducer structures 402, 404, and 406 in the coupled cavity filter structure 400. The coupled cavity filter structure 400 is also different from the embodiment shown in FIG. Figure 2a The coupled cavity filter structure 300 shown in FIG. 3 includes a total of six reflective structures 302, 304, 306, 308, 310, and 312. Similarly, the same reference numerals will be used to describe the multiple reflective structures. Figure 2a The same features as those already described for the coupled cavity filter structure 300 in FIG.

[0089] and Figure 2aSimilar to the structure in , two of the transducer structures 112, 114 are positioned outside the reflective structures 302, 304, 306, 308, 310, 312, while the third transducer structure 314 is positioned in the middle of the reflective structures 302, 304, 306, 308, 310, 312, so that on each side of the third transducer structure 314, there are three reflective structures 302, 304, 306 and 308, 310, 312. The transducer structure 314 is also separated from the two adjacent reflective structures by a distance d, which corresponds to the same distance between the transducer structures 112, 114 and their corresponding adjacent reflective structures (here 302 for the transducer structure 112 and 312 for the transducer structure 114). The coupled cavity filter 500 thus includes a total of eight acoustic cavities 316. Figure 2a This cavity filter structure 500 is symmetrical and results in stronger energy confinement in the cavity 316 compared to the cavity filter structure 300 shown in FIG. 3 which has only two transducer structures 112 , 114 .

[0090] In a variation of the present invention, the third transducer 314 is not located in the middle of the coupled cavity filter structure, making the coupled cavity filter structure asymmetric.

[0091] In a variation of the present invention, multiple acoustic cavities may be divided into sub-cavities. Figure 2d As shown, the subcavities are separated from each other by the presence of additional layers. Figure 2a The coupled cavity filter structure 300 shown in FIG. 3 includes a total of three reflective structures 402, 404, and 406. Similarly, the same reference numerals will be used to describe the multiple reflective structures. Figure 2a The same features as those described for the coupled cavity filter structure 300 in FIG. 6 are shown in FIG. 7 . In the cavity filter structure 600, the acoustic cavity 408 located between the reflective structures 402, 404, 406 and between the transducer structures 112, 114 and their adjacent reflective structures 402 and 406 is divided into two parts, resulting in eight dark areas 410 and four white areas 412 between the two transducer structures 112, 114. The dark areas 410 represent locations where the wave velocity is less than that of the white areas 412, resulting in a larger ratio. Figure 2a The coupled cavity filter structure 300 is shown to have better energy confinement. As a result, the coupled cavity filter structure 600 generates an additional pole, which results in an improvement in the compactness of the coupled cavity filter structure 600.

[0092] In a variation of the invention, the input transducer structure and the output transducer structure are not symmetrical or identical, and the variation is as follows Figure 2e Similarly, the coupled cavity filter structure 700 is also different from the Figure 2aThe coupled cavity filter structure 300 shown in FIG. 3 is a schematic diagram illustrating a plurality of reflective structures, each of which includes two reflective structures 502 and 504. Similarly, the same reference numerals will be used to describe the coupled cavity filter structure 300. Figure 2a The same features have been described for the coupled cavity filter structure 300 in FIG. Figures 2a to 2e , the number of electrode fingers of each transducer structure is different. In the coupled cavity filter structure 700, the transducer structure 114 includes interdigitated comb electrodes 124, 126 each having five electrode fingers 128, 130, with a constant electrode pitch p throughout the transducer structure 114. The transducer structure 506 includes interdigitated comb electrodes 508, 510, but each of them has eight electrode fingers 128, 130, and furthermore, in the transducer structure 506, there is a gap between the third electrode finger 128 of the comb electrode 508 and the fourth electrode finger 130 of the comb electrode 510. Furthermore, in this variation, there is no Bragg mirror next to the transducer structures 114, 506, which would produce a difference in the ratio Figure 2a The coupled cavity filter structure 300 has more loss and ripple. However, the coupled cavity filter structure 700 provides a better Figure 2a The structure shown is more compact and simpler in design.

[0093] The coupled-cavity surface acoustic wave filter structure operates as follows. An input interdigital transducer (IDT) transmits acoustic energy into a reflective structure, exciting it to resonate. The reflective structures couple to each other, creating a coupling condition that results in energy transfer from one reflective structure to another. Multiple such reflective structures can be coupled to each other, but there is at least one output transducer structure that collects the transmitted energy.

[0094] Therefore, the present invention proposes the use of a coupled cavity filter structure that utilizes wave guiding from the top piezoelectric layer of the composite substrate and uses an acoustic resonant cavity that couples energy from one reflective structure to another.

[0095] In the case of a coupled cavity filter structure, the resonance of the transducer structure occurs in the low-frequency transition band of the filter, while the antiresonance occurs almost in the middle of the filter band. Therefore, the conditions for the electromechanical coupling coefficient are similar to those required for an impedance filter, namely, the coupling coefficient must be 1.5 to 2 times larger than the frequency band to be achieved, because a given mode must exhibit a coupling factor proportional to the bandpass to be achieved, thereby allowing the insertion loss within that band to be reduced. However, a large reflection coefficient greater than the coupling coefficient (ideally 1.5 times the coupling coefficient or greater) is required to achieve the filter band.

[0096] According to the present invention, a reflection coefficient versus coupling coefficient relationship can be achieved using a composite substrate even when the coupling coefficient of the transducer structure is 5% or greater. This is particularly true when using shear waves or longitudinal waves guided in the piezoelectric layer of the composite substrate.

[0097] Since the thickness of the piezoelectric layer is less than the wavelength, shear wave modes or longitudinal modes are guided within the piezoelectric layer. In addition, energy losses in the composite substrate can be reduced. The thickness of the piezoelectric layer should be greater than or equal to 5% of the wavelength λ. For thick piezoelectric layers, the shear modes of the composite substrate are no longer fully guided, but have a lossy bulk component that is reflected at the interface with the base substrate and leads to parasitic modes or rattle effects. However, for thin piezoelectric layers (i.e., wavelength or sub-wavelength thickness), the shear modes are fully guided without lossy bulk modes.

[0098] The quality factor of a filter device is the transmittance of the filter, which shows the filter bandpass as a function of frequency, with the loss level in dB in the bandpass. The filter bandpass depends on various factors, namely the coupling coefficient, the number of cavities and the reflection coefficient.

[0099] Depending on the size of the piezoelectric layer, the size of the transducer structure, the size of the length of the reflective structure, the number of coupled reflective structures and the coupling coefficient of the mode, a multi-pole zero-point filter with extremely low insertion loss (i.e. better than 2dB, specifically less than 1dB) and a suppression of 15dB to 20dB or even higher can be synthesized based on the design and selectivity of the transducer structure.

[0100] Regarding the size of the acoustic cavity, it should ideally be a quarter wavelength long or an odd number of quarter wavelengths to achieve optimal resonance conditions according to prior art. In the present invention, the cavity length can be less than a quarter wavelength. This is due to the large velocity variation from the free surface to the grating region, resulting in a much larger acoustic impedance mismatch than can be achieved using standard true acoustic SAW solutions.

[0101] Regarding the coupling coefficient, for the composite substrate and metal strip parameters (material, size), the coupling coefficient is directly related to the bandpass value by a factor of 0.7, so that the desired bandpass of the filter device can be obtained by selecting the material and size of the cavity filter structure.

[0102] As regards the number of metal strips in the reflective structure, it is chosen so as to generate a global reflection coefficient greater than 0.5, in particular greater than 0.8, allowing to confine the acoustic energy in the cavity and thus provide mode coupling conditions.

[0103] As already mentioned, the magnitude of the reflection coefficient is preferably greater than the coupling coefficient, ideally 1.5 or more times higher. The greater the reflection coefficient, the fewer the number of metal strips, and therefore the greater the filter bandwidth. For example, a reflection coefficient greater than 15% allows for a reduction in the number of metal strips that make up the reflective structure, which directly impacts the filter bandwidth: if the structure's reflection coefficient is greater than or equal to 50%, the fewer the number of metal strips in the reflective structure, the greater the bandwidth. Considering a reflection coefficient greater than 15%, a filter with a bandwidth greater than 5% can be achieved.

[0104] A specific example of a filter device operating at 2.6 GHz with a bandwidth exceeding 7% and an in-band ripple less than 0.6 dB is given. The filter device has a 30 nm thick tantalum (Ta) electrode on a composite substrate comprising a 300 nm thick (YXI) / 50° LiNbO3 layer on a 1 μm thick SiO2 layer on a (100) silicon substrate. In this example, a reflection coefficient of 20% is achieved, and a coupling factor of approximately 18%.

[0105] Figures 3a to 3d The characteristics of the surface acoustic wave filter device are shown. The surface acoustic wave filter device includes Figure 2b The cavity filter structure shown has a composite substrate according to the present invention with a 500nm SiO2 layer between a 6μm LiTaO3(YXI) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate. The structure is suitable for operation in the frequency range of 50MHz to 250MHz.

[0106] On this composite substrate, pure shear true acoustic modes can be excited and propagated.

[0107] For this SAW filter device, a Figure 2b The coupled cavity filter structure shown includes an input transducer structure and an output transducer structure, each with adjacent Bragg mirrors, and four reflective structures, resulting in five acoustic cavities between the transducer structures. The metal strips of each reflective structure and Bragg mirror are connected to each other for short-circuit operation.

[0108] The electrode pitch of the IDT structure is set to 9.95 μm, the ratio a / p is set to 0.3, and the number of electrode finger pairs is set to 15. In addition, the mirror grating period is set to 10 μm, the a / p ratio is set to 0.4, and there are 30 electrodes. The gap between the mirror grating and the IDT structure is set to 9 μm, which corresponds to about half a wavelength. Two internal reflection structures, each with 14 electrodes, are separated by a gap g of 4.8 μm, which corresponds to about a quarter wavelength. The aperture is then 3.1 mm.

[0109] Figure 3a An overall view of the filter transfer function and filter group delay as a function of frequency (MHz) (from 170MHz to 250MHz) is shown. Group delay is a measure of phase linearity. Figure 3b Shown Figure 3a A zoomed in view near the center frequency, thus focusing on the passband, shows very low transmission losses. Figure 3c is the reflection coefficient which is also a function of frequency (MHz) and shows the signature of the filter pole (minimum |S 11 |peak). Figure 3d The Smith chart is a graph of the so-called reflection coefficient that is currently used by those skilled in the art to evaluate the electrical impedance matching of two ports of a filter. The reflection coefficient must be centered around the 50 Ohm condition, i.e., the center of the abacus to be matched.

[0110] Figure 3a The transfer function in represents the performance of the device, where the filter passband is between 210MHz and 218MHz. Figure 3b In the figure, it can be seen that the filter bandpass presents a flat curve around 0.5dB, with low in-band ripple.

[0111] Figure 4 The following are shown: Figure 2b The characteristics of the coupled cavity SAW filter structures are shown in the table. For all coupled cavity filter structures, the composite substrate used is Figures 3a to 3d The same, i.e., a 6 μm LiTaO 3 (YXI) / 42° piezoelectric layer, with a 500 nm SiO 2 layer between the piezoelectric layer and the semi-infinite (100) silicon base substrate.

[0112] The coupled cavity filter structure corresponds to Figure 2b , namely an input transducer structure and an output transducer structure, each with an adjacent Bragg mirror, and four reflection structures, resulting in five acoustic cavities between the transducer structures. The metal strips of each reflection structure and Bragg mirror are connected to each other for short-circuit operation.

[0113] Figure 4 All coupled-cavity filter structures described in the paper correspond to propagation in pure shear wave modes and show performances such as filter bandpass varying from 0.5% to 10%, low insertion loss below 2 dB, rejection loss between 15 dB and 20 dB, coupling factors greater than 5%, and temperature coefficient of frequency (TCF) below 20 ppm / K. Bandpasses as high as 10% can be achieved using thin LNO layers (e.g., with Pt- or W- or Ta- or Mo-based electrodes of thickness compatible with current deposition techniques, i.e., h / λ equal to less than 5%, where h is the absolute thickness of the metal strip).

[0114] For the desired performance of the coupled cavity SAW filter device, the materials can therefore be selected to adjust the coupling coefficient and to adjust the number and size of the reflective structures so as not to exceed the size limitations of the coupled cavity filter structure.

[0115] The coupled cavity SAW filter device according to the present invention uses shear waves of a composite substrate to obtain a narrow filter bandpass comprised between 0.5% and 10%, with a low insertion loss of less than 2dB, a suppression loss of 15dB to 20dB and low ripple within the filter bandpass, and a reduced size due to improved compactness.

[0116] Figure 5a and Figure 5b A coupled-cavity surface acoustic wave filter structure according to a third embodiment of the present invention is illustrated.

[0117] In a third embodiment, Figure 1a and Figure 1b Similar to the first embodiment shown in Figure 1a and Figure 1b The coupled cavity surface acoustic wave filter structure 800 is implemented on the same composite substrate 102 as the substrate 102 .

[0118] Will use Figure 1a and Figure 1b The same reference numerals are used to refer to the same features and will not be explained again in detail.

[0119] In this embodiment, the thickness of the thin SiO 2 layer 108 provided at the interface 110 between the piezoelectric layer 104 and the base substrate 106 to improve the adhesion of the piezoelectric material layer 104 to the base substrate 106 is 500 nm.

[0120] The coupled cavity filter structure 800 includes two transducer structures 812, 814 and a reflective structure 816. Figure 5a The transducer 812 is shown positioned between the two transducer structures 812 , 814 at a distance L from the transducer structures 812 , 814 in the direction of propagation X.

[0121] Each transducer structure 812, 814 includes two electrodes 824, 826 (not shown), each electrode 824, 826 including a plurality of electrode devices 828, 830. The comb electrodes are alternatively connected to the +V / -V potential, with electrode 824 at +V and electrode 826 at -V, or vice versa. In a variant, the electrodes 824, 826 can be comb electrodes, in particular interdigitated electrodes.

[0122] The transducer structures 812 , 814 are defined by an electrode pitch p (not shown), which corresponds to the edge-to-edge electrode finger distance between two adjacent electrode fingers 828 , 830 from opposing comb electrodes 824 and 826 .

[0123] In this embodiment, the electrode pitch p of the transducer structures 812 , 814 is defined by a multiple of the wavelength nλ, where λ is the operating acoustic wavelength of the transducer structures 812 , 814 .

[0124] The metal ratio a / p of the transducer structure is defined as the ratio of the electrode width a to the electrode pitch p.

[0125] In this embodiment of the present invention, the reflective structure 816 is implemented by the groove 822 and is defined by a dimension L1 corresponding to the distance between the two side edge walls 822a and 822b of the groove 822 and a total depth D of the groove 822. The total depth D of the groove 822 is defined between the surface of the piezoelectric layer 104 on which the transducers 812 and 814 are located and the bottom surface 822c of the groove 822. The depth D of the groove 822 is approximately λ or greater, specifically approximately 10λ or greater, where λ is the wavelength of the surface acoustic wave.

[0126] Furthermore, the groove 822 is further defined by an etch relief angle θ340, thereby defining the position of the groove edge walls 822a, 822b relative to the horizontal axis X and the bottom surface of the groove 822c. The relief angle θ840 may be approximately 70° or greater, specifically approximately 90°. Figure 5a A slot 822 is illustrated having vertical edge walls corresponding to a relief angle θ 340 of 90°.

[0127] The area between the reflective structure 816 and the transducer structures 812, 814 (e.g., the area 818 whose width is defined by the distance L2) corresponds to the acoustic cavity 820. The distance L2 is defined as the distance between one edge wall 822a, 822b of the groove 822 and points A, B on the surface of the piezoelectric layer where the transducers 812, 814 are located. Points A, B are located at the ends of the pitch of the transducer structures 812, 814 on the side where the groove 822 is located, as shown in FIG. Figure 5a When the electrodes are centered within the pitch, the ends of the pitch of the transducer structures 812, 814 do not correspond to the ends of the first electrodes 830 of the transducer structures 812, 814. For example, when the ratio a / p is 50%, the ends of the pitch are located at a distance equal to λ / 8 from the first electrodes 830 of the transducer structures 812, 814. Figure 5b Corresponding to Figure 5a , where the transducer structures 812 , 814 include only two electrodes 828 and 830 , in order to more clearly illustrate the region 818 of the acoustic cavity 820 .

[0128] In the coupled cavity surface acoustic wave filter structure 800, Figure 5a and Figure 5b In the coupled cavity filter structure shown, there are two acoustic cavities 820 in the direction of acoustic wave propagation.

[0129] In this embodiment of the present invention, the reflective structure 816 is realized by providing (eg, etching) grooves 822 rather than depositing metal strips as in the first and second embodiments.

[0130] Grooves 822 are provided in the piezoelectric layer 104 of the composite substrate 102 and extend in the SiO 2 layer down into the base substrate 106 to a total depth D. D1 corresponds to the portion of the depth D realized only in the base substrate 106 .

[0131] In a variation, the grooves 822 may be etched only through the piezoelectric layer 104 and down through the SiO2 layer 108 to the surface of the base substrate 106, which is the interface 810 between the SiO2 layer 108 and the base substrate 106. Thus, D1 will be equal to zero.

[0132] In the fourth embodiment, based on the third embodiment, the coupled cavity filter structure 900 further includes two Bragg mirrors 832 and 834. Figure 6 As shown, each Bragg mirror 832 , 834 is positioned on the other side of the reflective structure 816 in the direction X of acoustic wave propagation, close to the transducer structures 812 , 814 .

[0133] Each Bragg mirror 832, 834 is positioned at a distance s from its corresponding transducer structure 812, 814. Each Bragg mirror 832, 834 includes one or more metal strips 836 and is defined by a pitch of the metal strips 836, which corresponds to the distance between the metal strips 836 within the Bragg mirror 832, 834. As with the transducers, the pitch in the Bragg mirrors 832, 834 is defined by centering the metal strips 840 within the pitch.

[0134] In this variant, the pitch of the Bragg mirrors 832, 834 is also equal to n times the wavelength λ, and is therefore nλ.

[0135] In this case, on the side where the Bragg mirrors 832, 834 are located, the waves will be reflected according to the phase variation, while on the side of the groove the type of reflection will depend on the width and depth of the groove.

[0136] In the fifth embodiment, based on the third embodiment, the coupled cavity filter structure 1000 includes two additional slots 932, 934, each of which is located on the other side of the reflective structure 816 near the transducer structures 812, 814 in the direction of sound wave propagation. Figure 7 shown.

[0137] Each additional groove 932, 934 is positioned at a distance s from its corresponding transducer structure 812, 814. Each additional groove 932, 934 is defined by its width L3 and its total depth D3. The total depth D3 of the additional grooves 932, 934 is defined between the surface of the piezoelectric layer 104 on which the transducers 812, 814 are positioned and the bottom surfaces 932c, 934c of the additional grooves 932, 934. The depth D2 is defined as the depth of the additional grooves 932, 934 from the bottom surface 822c of the groove 822 to the bottom surfaces 932c, 934c of the grooves 932, 934. Therefore, the total depth D3 is defined as D plus D2, where D is the total depth of the grooves 822 of the reflective structure 816. The depth D3 of at least one additional groove (932, 934) is approximately λ or greater.

[0138] In this embodiment of the invention, the reflective structure 816 and the additional grooves 932, 934 are realized by providing (eg etching) grooves rather than depositing metal strips as in the first and second embodiments.

[0139] Each additional slot 932 , 934 is configured to have total reflection of the propagating wave along the propagation direction.

[0140] In a variation, the coupled cavity filter structure may include a Bragg mirror and a slot on one side of the input transducer (positioned on the other side of the reflective structure 816 near the transducer structures 812 and 814 in the direction of acoustic wave propagation X), and a slot on the side of the output transducer.

[0141] The coupled cavity filter device according to one of the third to fifth embodiments operates similarly to the coupled cavity filter device according to the first embodiment, but its structural features (i.e., the pitch of the transducer and the mirror, the size of the cavity) have been adjusted so that conditions are met so that the device exhibits similar functions to the first embodiment.

[0142] This is because in the case of reflection on an edge, the definition of the reflection position is geometrically defined. Therefore, regardless of the origin of the phase within the transducer, the phase construction will only occur for an integer number of wavelengths. The definition of the equivalent reflection center of a Bragg mirror is more difficult because it exhibits a phase variation that depends mainly on the reflection intensity. The reflection function of a Bragg mirror is defined as the ratio of the reflected wave to the incident wave defined at one edge of the mirror. It is known in the art that the size of the reflection coefficient on a single electrode of a grating regulates the width of the spectral band corresponding to the reflective operation of the grating. However, since the phase variation of the reflection function between the start and the end of the mirror stop band is always in the range pi to 2×pi, the size of the reflection coefficient also seems to affect the phase variation of the reflection coefficient with frequency.

[0143] Therefore, replacing the reflective structure comprising metal strips of the first and second embodiments with the reflective structure comprising slots in the third, fourth, fifth and sixth embodiments results in conditions having multiples of λ for the coupled cavity filter device to operate.

[0144] All variants of the coupled cavity filter arrangement according to the first and second embodiments can also be applied to the coupled cavity filter arrangement according to the third, fourth, fifth and sixth embodiments.

[0145] Figures 8a to 8h The structure of a coupled-cavity surface acoustic wave filter according to a sixth embodiment of the present invention and its modifications are illustrated.

[0146] Figure 8a and Figure 8b The substrate shown in Figure 5a 、 Figure 5b 、 Figure 6 and Figure 7 The same substrate 102 will be used. Figure 7 The same reference numerals are used to refer to the same features and will not be explained in detail.

[0147] exist Figure 8a In FIG, similar to coupled cavity filter structure 1000, coupled cavity filter structure 1100 includes two transducer structures 812 and 814, wherein two slots 932 and 934 are each positioned adjacent to one transducer structure. Unlike coupled cavity filter 900, multiple reflective structures, namely, two reflective structures 1006 and 1016, are located between transducer structures 812 and 814. Each of the multiple reflective structures 1006 and 1016 corresponds to slot 1022 and is defined by the width L1 and total depth D of slot 1022.

[0148] These reflective structures 1006, 1016 are separated from each other by a gap g in the propagation direction X. The region 1008 between two adjacent reflective structures 1006, 1016 (whose width is defined by the gap g) corresponds to the acoustic cavity 1010. The acoustic cavity 1010 can be considered as a central cavity, while the cavity 1020 can be referred to as a side cavity.

[0149] Similar to the coupled cavity filter structures 800 and 900, the area 1018 located between the reflective structures 1006, 1016 and the adjacent transducer structures 812, 814 also corresponds to an acoustic cavity 1020, the width of which is defined by the distance L2 between the edges of the reflective structures 1006, 1016 and points A and B located on the surface of the piezoelectric layer 104.

[0150] Region 1008 actually includes the piezoelectric layer 104 and the SiO 2 layer 108 on top of the base substrate 106 .

[0151] Therefore, in the coupled-cavity surface acoustic wave filter structure 1100 , there are three cavities in the direction of acoustic wave propagation, and the three cavities are separated by the reflection structures, or in other words, the cavities are surrounded by two reflection structures between the transducers.

[0152] In another variation of the sixth embodiment, as Figure 8b As shown, the coupled-cavity surface acoustic wave filter structure 1200 has a region 1008 between the groove 1006 and the groove 1016, which is free of the piezoelectric layer 104 and the SiO2 layer 108. In the region 1008, only the base substrate 106 is present. According to another variation, the surface of the base substrate can also be etched so that the thickness of the base substrate in the region 1008 is less than the thickness of the base substrate in the region 1018.

[0153] For all Figures 8c to 8h , variations with sub-cavities are also illustrated, with coupled cavity filter structures 1300 to 1800 shown from top to bottom in a 2D plan view.

[0154] In such Figure 8c In the coupled cavity filter structure 1300 shown, two reflective structures 1206 and 1216 of the plurality of reflective structures are separated from each other by a gap w but in the propagation direction Z. Figure 8a Similar to the region 1008, the region 1208 may be etched or not. In this case, the acoustic cavity 1020 is divided into two sub-cavities 1020a, 1020b.

[0155] In the coupled cavity filter structure 1400, similar to the cavity filter device 1100, the plurality of reflective structures 1316 are separated from each other by gaps g in the propagation direction X, and similar to the cavity filter device 1200, the plurality of reflective structures 1316 are also separated from each other by gaps w but in the propagation direction Z. Figure 8a Area 1008 and Figure 8c Similar to the region 1210 , the region 1308 between the plurality of reflective structures may be etched or not.

[0156] In this variation, similar to the device 600 according to the second embodiment of the present invention, the acoustic cavities 1310, 1320 are divided into a plurality of sub-cavities 1310a, 1310b, 1320a, 1320b.

[0157] like Figure 8eAs shown, in the coupled cavity filter structure 1500, along the entire distance between the two transducer structures 812, 814, a plurality of reflective structures 1506, 1516 are symmetrically separated by a central region 1510 having a width w1 in the Z direction, in which no reflective structures 1506, 1516 exist. In addition, unlike the etched grooves that were elongated in the Z direction in the previous variants, the grooves 1522 are now square. For clarity, Figure 8e Only one reflective structure 1506, 1516 and one groove 1522 are marked.

[0158] like Figure 8f As shown, in the coupled cavity filter structure 1600, the size of the area 1610 between the transducer structures 812, 814 without the reflective structures 1506, 1516 is different from Figure 8e Region 1510 is thinner in nature, with three reflective structures removed in a row in the Z direction. Region 1610 can be assimilated to a symmetry violation of multiple reflective structures. Figure 8f Only one reflective structure 1506, 1516 and one groove 1522 are marked.

[0159] In another variation, Figure 8g The device 1700 shown corresponds to Figure 8e The device 1500 has two additional slots 1532 , 1534 , includes multiple reflective structures 1506 , 1516 and therefore includes multiple slots 1522 .

[0160] In a variation of the present invention, the coupled cavity filter structure 1800 may include three or even more transducer structures. Figure 8h , a variation is shown in which three transducer structures 1210 , 1212 , 1214 are present in the coupled cavity filter structure 1200 .

[0161] Similar to Figure 2c In the structure of FIG, two of the transducer structures 1210, 1214 are positioned outside of the reflective structures 1706, 1716, while the third transducer structure 1212 is positioned in the middle of the reflective structures 1706, 1716, so that there is one reflective structure 1706, 1716 on each side of the third transducer structure 1212 and therefore a slot 1722. The transducer structure 1212 is also separated from the two adjacent reflective structures 1706, 1716 by a distance L, which corresponds to the same distance between the transducer structures 1210, 1214 and their respective adjacent reflective structures (here 1706 for the transducer structure 1210 and 1716 for the transducer structure 1214). This cavity filter structure 1800 is symmetrical and is similar to the cavity filter structure 1800. Figure 8cCompared to the cavity filter structure 1300 shown with only two transducer structures 812, 814, a stronger energy confinement is produced in the cavity 1720. Figure 8c Similar to the illustrated device 1300 , the reflective structures 1706 , 1716 are also separated from each other by a distance w1 .

[0162] In a variation of the present invention, the third transducer 1210 is not positioned in the middle of the coupled cavity filter structure, making the coupled cavity filter structure asymmetric.

[0163] Figure 9 An apparatus according to a third embodiment of the invention is shown for simulation.

[0164] For this simulation, the device used corresponds to Figure 7 The device 1000 is shown, but here the transducer structures 812 and 814 each include only two electrode fingers 828 , 830 .

[0165] Furthermore, the following structural parameters were used. For the simulations, a composite substrate was used, which had a 200 nm SiO2 layer between a 500 nm LiTaO3(YXI) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate. The transducer's electrode pitch p was 800 nm, resulting in a wavelength λ of 1.6 μm at frequencies around 2.5 GHz. The metallization ratio a / p was 0.5, which meant that the electrode width a was 400 nm. The electrode fingers were made of Al-Cu and had a thickness of 100 nm.

[0166] Figure 10a and Figure 10b An example of the third embodiment of the present invention is shown. Figure 9 Simulated characteristics of the coupled-cavity surface acoustic wave filter structure shown.

[0167] In this practical example, simulations were performed using a grid structure (not shown) with a pitch p of 2 μm, a depth D1 equal to the pitch p, a depth D2 equal to 6×p, a groove width L1 equal to 6.4 μm, and a width L2 equal to 2×p. A composite substrate with a 600 nm thick LiTaO3 layer and a 500 nm thick SiO2 layer on a (100) silicon substrate was used.

[0168] Figure 10a Shown is a graph of the admittance as a function of frequency (in GHz) on the X-axis, with the real part of the admittance on the left Y-axis and the imaginary part on the right axis. Figure 10a Two peaks near 61800 GHz and 62000 GHz are shown, and the peak near 61750 GHz shows a stronger admittance than the other peak near 62000 GHz.

[0169] Figure 10b Shown is a graph of the transadmittance as a function of frequency (in GHz) on the X-axis, with the real part of the admittance on the left Y-axis and the imaginary part on the right axis. Figure 10b The same two peaks around 61800 GHz and 62000 GHz are shown, but this time the contribution is more balanced, with both peaks having equal amplitudes.

[0170] These figures illustrate two coupling modes, with two balanced contributions allowing the filter bandpass to be effectively defined regardless of the actual amplitude level.

[0171] Figures 11a to 11d Admittance calculations of the filter basic structure according to the present invention are shown for various sizes of reflection structures.

[0172] A composite substrate with a 500 nm SiO2 layer between a 600 nm LiTaO3(YXI) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate was used for the simulation.

[0173] Figures 11a to 11b Graphs of the admittance as a function of frequency (in GHz) on the X-axis are shown for varying depths D1 of the grooves, with the real part of the admittance on the left Y-axis and the imaginary part on the right. The frequency range shown is between 61000 GHz and 63000 GHz. All graphs show the frequency of the grooves around 61800 GHz and 62500 GHz. Figure 10b The same two peaks can be regarded as a double peak formation, where the first peak near 61800 GHz has a higher admittance value than the second peak near 63000 GHz. Figure 10a In fact, the double peak is Figure 11a 、 Figure 11b or Figure 10b The bandwidth is narrower.

[0174] exist Figures 11a to 11c In the , D1 varies from 1μm to 0.2μm, and in Figure 11d In the figure, D1 varies from 1 μm to 2 μm.

[0175] exist Figures 11a to 11d In , the same behavior of the peaks can be seen. When D1 decreases from 1 μm to 0.2 μm, the double peak formation shifts to higher frequencies, while the admittance of the first peak decreases and the admittance of the second peak increases. Figure 11b 、 Figure 11c and Figure 11d In the case of the second peak, the admittance increases so that it actually reaches a higher value than the first peak, while in Figure 11a In FIG, for D1 equal to 0.2 μm, the two peaks are equal.

[0176] Figure 12a and Figure 12b An example of a SAW ladder filter device according to the prior art is illustrated ( Figure 12a ) and an example of a SAW ladder filter device according to the present invention ( Figure 12b ).

[0177] exist Figure 12a , four transducer structures 1612, 1614, 1616, and 1618 are each sandwiched between two reflective structures 1632 and 1634. The interdigitated electrodes of transducers 1612, 1614, 1616, and 1618 are connected to each other by metal wires 1640. As can be seen, this design is cumbersome because the transducers and their metal connections require a large amount of space. The transducers cannot be aligned in a single line, either vertically or horizontally, because they need to be moved relative to each other to allow for connection between the transducers.

[0178] Figure 12b A SAW ladder filter device 2000 is shown in which two cavity coupled filter devices 1000 according to the present invention are positioned close to each other on a single line. Each cavity coupled filter device 1000 includes two transducer structures 812, 814, respectively, with corresponding reflective structures 932, 934 and 816. The reflective structure 816 is located between the two transducer structures, for example, 816 is located between 812 and 814, and the reflective structures 932 and 934 are each located on the side of the transducer structure opposite to the side where the reflective structure 816 is located. Figure 12b In FIG. 8 , the reflective structures 816, 934, and 932 are represented as grooves, as shown in FIG. Figures 8a to 8h As shown. Figure 12a Similarly, the electrodes of the transducer are connected to each other by metal wires 1740 .

[0179] Figure 12b A SAW ladder filter device 2000 having two cavity coupled filter devices 1000 is shown, but any variation of the cavity coupled filter device 1000 according to the present invention may be used for a SAW ladder filter device.

[0180] The SAW ladder filter device 2000 according to the present invention is more compact and does not require Figure 12a The SAW ladder filter device shown in FIG.

[0181] Several embodiments of the present invention have been described. However, it will be appreciated that various modifications and enhancements may be made without departing from the scope of the appended claims.

Claims

1. A coupled cavity filter structure, wherein the coupled cavity filter structure uses guided surface acoustic waves, and the coupled cavity filter structure comprises: substrate for acoustic wave propagation; at least one input transducer structure and one output transducer structure, the at least one input transducer structure and the one output transducer structure being disposed on the acoustic wave propagation substrate, the at least one input transducer structure and the one output transducer structure each comprising an interdigitated comb electrode; a reflective structure, the reflective structure comprising at least one metal strip, the reflective structure being positioned at a distance between the at least one input transducer structure and the one output transducer structure in the direction of propagation of the acoustic wave d Department; It is characterized by: The acoustic wave propagation substrate is a composite substrate comprising a base substrate and a piezoelectric layer, wherein the thickness of the piezoelectric layer is less than one wavelength λ, λ being the wavelength of the guided surface acoustic wave, and wherein the coupled cavity filter structure is configured such that the guided surface acoustic wave is a guided shear wave or a guided longitudinal wave within the piezoelectric layer.

2. The coupled cavity filter structure according to claim 1, wherein: The interdigitated comb electrodes of the at least one input transducer structure and the one output transducer structure are formed by p = λ The Bragg condition given by / 2 is limited, p is the electrode pitch of the at least one input transducer structure to the one output transducer structure.

3. The coupled cavity filter structure according to claim 1, further comprising at least one Bragg mirror, wherein the at least one Bragg mirror is located on the opposite side of the side where the one reflection structure is located, separately from the at least one input transducer structure and / or the one output transducer structure in the direction of propagation of the sound wave.

4. The coupled cavity filter structure according to claim 1, wherein the coupled cavity filter structure comprises a plurality of reflective structures, wherein the plurality of reflective structures are separated by gaps. g Separated from each other, the distance between the at least one input transducer structure and the one output transducer structure in the direction of propagation of the sound wave is d At each gap between the multiple reflective structures g and the respective distances between the at least one input transducer structure and the one output transducer structure and its adjacent reflective structure d Forming a vocal cavity.

5. The coupled cavity filter structure according to claim 4, wherein: The size of each acoustic cavity is smaller than λ / 4, so that the phase velocity in the acoustic cavity is greater than the phase velocity in the one reflection structure or the multiple reflection structures.

6. The coupled cavity filter structure according to claim 4, wherein: gaps between adjacent reflective structures among the plurality of reflective structures g and / or the distance between the reflective structure and the adjacent transducer structure d Same or different.

7. The coupled cavity filter structure according to claim 4, wherein: The one reflective structure or the plurality of reflective structures have a reflection coefficient associated with a single metal strip, the reflection coefficient being greater than a coupling coefficient associated with the composite substrate and the interdigitated comb electrodes of the at least one input transducer structure and the one output transducer structure. k s 2 .

8. The coupled cavity filter structure according to claim 7, wherein: The reflection coefficient is greater than the coupling coefficient k s 2 1.5 times.

9. The coupled cavity filter structure according to claim 4, wherein: The one reflective structure or each reflective structure in the plurality of reflective structures comprises at least one metal strip, the pitch of the at least one metal strip and the electrode pitch of the at least one input transducer structure and the one output transducer structure are p Same or different.

10. A coupled cavity filter structure, the coupled cavity filter structure using guided surface acoustic waves, the coupled cavity filter structure comprising: substrate for acoustic wave propagation; at least one input transducer structure and one output transducer structure, the at least one input transducer structure and the one output transducer structure being disposed on the acoustic wave propagation substrate, the at least one input transducer structure and the one output transducer structure each comprising an electrode; a reflective structure, said reflective structure comprising a groove, said reflective structure being positioned at a distance between said at least one input transducer structure and said one output transducer structure in the direction of propagation of the acoustic wave L Department; It is characterized by: The acoustic wave propagation substrate is a composite substrate comprising a base substrate and a piezoelectric layer, wherein the thickness of the piezoelectric layer is less than one wavelength λ, λ being the wavelength of the guided surface acoustic wave, and wherein the coupled cavity filter structure is configured such that the guided surface acoustic wave is a guided shear wave or a guided longitudinal wave within the piezoelectric layer.

11. The coupled cavity filter structure according to claim 10, wherein: The electrodes of the at least one input transducer structure and the one output transducer structure are arranged such that the electrode pitch of the at least one input transducer structure and the one output transducer structure is equal to a multiple of the wavelength. p limited.

12. The coupled cavity filter structure according to claim 10, further comprising at least one additional slot, wherein the at least one additional slot is located on the opposite side of the side where the one reflection structure is located, separately from the at least one input transducer structure and / or the one output transducer structure in the direction of propagation of the sound wave.

13. The coupled cavity filter structure according to claim 12, wherein: The depth of the at least one additional groove located on the opposite side of the side where the one reflection structure is located, separated from the at least one input transducer structure and / or the one output transducer structure in the direction of propagation of the sound wave. D 3 is λ.

14. The coupled cavity filter structure according to claim 12, wherein: The depth of the at least one additional groove located on the opposite side of the side where the one reflection structure is located, separated from the at least one input transducer structure and / or the one output transducer structure in the direction of propagation of the sound wave. D 3 Greater than λ.

15. The coupled cavity filter structure according to claim 10, wherein the coupled cavity filter structure comprises a plurality of reflective structures, wherein the plurality of reflective structures are separated by gaps. g Separated from each other, positioned at a distance L between the at least one input transducer structure and the one output transducer structure in the direction of propagation of the sound wave, each gap between the plurality of reflective structures g Forming a vocal cavity.

16. The coupled cavity filter structure according to claim 10, further comprising an acoustic cavity, wherein the width of the acoustic cavity is determined by the distance between the edge of the slot and the following positions A and B: L 2 It is defined that the positions A, B correspond to the ends of the electrode pitches of the at least one input transducer structure and the one output transducer structure on the side where the slot is located in the direction of propagation of the acoustic wave.

17. The coupled cavity filter structure according to claim 12, wherein: The distance between the edge of the groove and the edge of the at least one additional groove is a multiple of the wavelength.

18. The coupled cavity filter structure according to claim 10, wherein: Depth of the groove of the reflective structure D is λ.

19. The coupled cavity filter structure according to claim 10, wherein: Depth of the groove of the reflective structure D Greater than λ.

20. The coupled cavity filter structure according to claim 19, wherein: Depth of the groove of the reflective structure D is 10λ.

21. The coupled cavity filter structure according to claim 19, wherein: Depth of the groove of the reflective structure D Greater than 10λ.

22. The coupled cavity filter structure according to claim 12, wherein: The at least one additional slot is configured to have total reflection of the propagating wave along the propagation direction.

23. The coupled cavity filter structure according to claim 10, wherein: The at least one input transducer structure is different from the one output transducer structure.

24. The coupled cavity filter structure according to claim 23, wherein: The at least one input transducer structure and the one output transducer structure have different numbers of electrode fingers.

25. The coupled cavity filter structure according to claim 10, wherein: The properties of the piezoelectric layer and the properties of the electrodes of the at least one input transducer structure and the one output transducer structure are selected such that the electromechanical coupling coefficient k of the guided shear wave in the piezoelectric layer is s 2 Greater than 5%.

26. The coupled cavity filter structure according to claim 25, wherein: The properties of the piezoelectric layer and the properties of the electrodes of the at least one input transducer structure and the one output transducer structure are selected such that the electromechanical coupling coefficient k of the guided shear wave in the piezoelectric layer is s 2 Greater than 7%.

27. The coupled cavity filter structure according to claim 10, wherein: The thickness of the piezoelectric layer is selected so that the electromechanical coupling coefficient k of the guided shear wave in the piezoelectric layer is s 2 Greater than 5%.

28. The coupled cavity filter structure according to claim 27, wherein: The thickness of the piezoelectric layer is selected so that the electromechanical coupling coefficient k of the guided shear wave in the piezoelectric layer is s 2 Greater than 7%.

29. The coupled cavity filter structure according to claim 15, wherein: The acoustic cavity formed between at least two grooves of the multiple reflection structures is located at the surface of the sound wave propagation substrate, and the surface is also the surface of the sound wave propagation substrate on which the at least one input transducer structure and the one output transducer structure are provided, or the acoustic cavity formed between at least two grooves of the multiple reflection structures is located at a depth included between the surface of the sound wave propagation substrate and the bottom surface of the at least two grooves located at a depth D.

30. A SAW ladder filter device, comprising at least two coupled cavity filter structures according to any one of claims 1 to 29, wherein: At least two coupled cavity filter structures are positioned on a single line.

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