Resonant Cavity Surface Acoustic Wave (SAW) Filters

Abstract

To provide a new coupled cavity filter structure.SOLUTION: The invention relates to a coupled cavity filter structure, using a surface acoustic wave, comprising an acoustic wave propagating substrate 102, input and output transducer structures 112 and 114 provided over the substrate, and one reflecting structure 116. Each of the input and output transducer structures comprises inter-digitated electrodes 124 and 126 located alternately. The reflecting structure comprises at least one or more metal strips 114 located between the input transducer structure and the output transducer structure at a certain distance from the input transducer structure and the output transducer structure in a propagation direction of an acoustic wave. The acoustic wave propagating substrate 102 is a composite substrate comprising a base substrate 106 and a piezoelectric layer 104.SELECTED DRAWING: Figure 5a

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 technology]

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

[0003] The synthesis of SAW filters requires different kinds of tools and allows different kinds of structures to be realized, however, the use of conventional filter structures in SAW devices faces various problems, such as device compactness and performance.

[0004] SAW filter devices typically use wafers made from monolithic quartz, LiNbO3 or LiTaO3 crystals as the piezoelectric material. However, the use of piezoelectric substrates results in high sensitivity to temperature or weak electromechanical coupling, depending on the piezoelectric material used. This results in poor filter passband characteristics.

[0005] Filter performance is specified using several parameters, such as bandwidth, in-band insertion loss, and stop and transition bandwidths that separate the passband and stopband.

[0006] Moreover, 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 located that result in poles or zeros depending on the way in which they are connected to each other (in series or in parallel). The synthesis of such filters is based on the combination of these poles and zeros that achieves less ripple in the band and improved out-of-band rejection, given a given coupling factor between the signal source and the filter structure. In either case, the filter consists only in a series of cavities connected to each other or located along the waveguide and accessible by electrical connectors or directly through the edges of the waveguide. To provide an electromagnetic filtered signal for the SAW device, the conversion of electromagnetic waves to acoustic waves and vice versa must be achieved. Between the electrode structures are combined in such a way that the resonances can be combined electrically or acoustically to produce the filtering effect.

[0007] At present, the solutions currently developed for SAW filters mainly use three different architectures to realize the filter function: impedance element grids (so-called SAW ladders), or longitudinally coupled resonator filters (LCRFs), or double-mode SAW (DMS) filters, all based on coupling IDTs with diffraction gratings operating close to the Bragg condition. However, these approaches generally allow more than one pole to be placed in the passband, thus resulting in devices with non-optimized performance and requiring a relatively large occupation area. Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to overcome the aforementioned drawbacks by providing a surface acoustic wave (SAW) filter device deposited on a composite substrate with an improved design for compactness, simplicity, and versatility, as well as good performance. [Means for solving the problem]

[0009] The object of the present invention is achieved by a coupled cavity filter structure using surface acoustic waves, in particular guided surface acoustic waves, comprising an acoustic wave propagation substrate, at least one input transducer structure and one output transducer structure provided on the substrate, each with staggered comb electrodes, and one reflecting structure, the reflecting structure comprising at least one or more metal strips located at a distance d from the input transducer structure and the output transducer structure in the propagation direction of the acoustic waves and between the input transducer structure and the output transducer structure, characterized in that the acoustic wave propagation substrate is a composite substrate comprising a base substrate and a piezoelectric layer. By including such a filter structure, the passband can be adjusted and at the same time a filter structure occupying a smaller area can be realized compared to the filter structures described above in the art.

[0010] According to a variant of the invention, the coupled cavity filter structure can be configured such that the surface acoustic waves are shear or longitudinal waves in the piezoelectric layer. Whereas in the prior art Rayleigh waves were used, the use of a piezoelectric layer instead of a bulk substrate opens the way to using different types of acoustic waves, thus providing further optimization parameters. Guided shear waves can achieve the highest electromechanical coupling achievable using composite substrates. They allow reaching higher wave speeds than elliptically polarized waves, with more opportunities for thermal compensation compared to conventional Rayleigh-like waves. Furthermore, the use of a given combination of substrate and inducing conditions allows to induce longitudinally polarized guided waves, achieving higher speeds than other wave types such as shear waves and Rayleigh waves, with a coupling of more than 5%.

[0011] According to a variant of the invention, the staggered interdigital electrodes of at least one input and one output transducer structure may be defined by the 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 makes it possible to achieve optimal dimensions and induction conditions or coupling conditions for a given frequency, bandwidth and energy confinement.

[0012] According to a variant of the invention, the coupled cavity filter structure may further comprise at least one Bragg mirror located away from the input and / or output transducer structures, opposite to the side on which the one reflecting structure is located in the propagation direction of the acoustic waves. The presence of a Bragg mirror located close to the transducer structures makes it possible to reduce losses in the structure.

[0013] According to a variant of the invention, the coupled cavity filter structure may comprise a number of reflecting structures spaced apart from each other by a gap g in the propagation direction of the acoustic waves and at a distance d relative to the input and output transducer structures, each gap g between the reflecting structures and each gap d between the transducer structure and a reflecting structure adjacent to the transducer structure forming an acoustic cavity. The inclusion of more than one reflecting structure in the structure provides the structure with a number of acoustic cavities. More acoustic cavities allow the transition band to be narrowed.

[0014] According to a variant of the invention, the dimensions of each acoustic cavity of the cavity filter structure can be smaller than λ / 4, in particular so that the phase velocity in the cavity exceeds the phase velocity in the reflecting structure, which can improve the resonance conditions and thus the performance of the filter.

[0015] According to a variant of the invention, the distance between adjacent reflecting structures of the plurality of reflecting structures and / or the distance between the reflecting structures and adjacent transducer structures can be the same or different. By adapting the size of the gap and therefore the size of the cavity, the filter parameters can be improved.

[0016] According to a variant of the invention, the reflecting structure or structures are arranged to reduce the coupling coefficient k of the composite substrate and of the electrodes of the transducer structure. s 2 Larger unitary metal strip reflection coefficient, especially the coupling coefficient k s 2 The unitary metal strip reflection coefficient may be at least 1.5 times greater than the coupling coefficient, which is a higher ratio of the reflection coefficient divided by the coupling coefficient. For a given bandwidth, this results in a substantially flat in-band transfer function and a sharp transition band with less in-band ripple effects than an unsatisfactory filter.

[0017] According to a variant of the invention, each reflective structure may comprise at least one or more metal strips, the pitch of which is the same as or different from the electrode pitch of the transducer structure. The use of highly reflective structures may give the mirror a higher tolerance to manufacturing variations, but it also makes it possible to shift the zero point of the reflection function in order to improve the out-of-band rejection.

[0018] According to a variant of the invention, the metal strips of each reflecting structure can be electrically connected to each other, so that a constant value of electric potential can be realized across each reflecting structure, thus improving the reflection coefficient of the reflecting structures in the Bragg condition.

[0019] According to a variant of the invention, the number of metal strips of each of the reflecting structures of the plurality of reflecting structures may be less than 30, preferably less than 20, so that the reflection coefficient of the plurality of reflecting structures is greater than 0.5, in particular greater than 0.8. Therefore, the cavity confinement of acoustic energy may be improved and mode coupling conditions may be created in the structure.

[0020] According to a variant of the invention, the difference between the acoustic impedance of the material from the piezoelectric layer and the material from the metal strip of each of the reflecting structures of the plurality of reflecting structures can be such that the reflection coefficient of the plurality of reflecting structures is greater than 0.5, in particular greater than 0.8. A high reflection coefficient allows the dimensions of the filter structures to be reduced.

[0021] The object of the present invention is further achieved by a coupled cavity filter structure using surface acoustic waves, in particular guided surface acoustic waves, comprising an acoustic wave propagation substrate, at least one input transducer structure and one output transducer structure provided on the substrate, each with an electrode, and one reflecting structure, the reflecting structure comprising a groove located at a distance L from the input transducer structure and the output transducer structure in the propagation direction of the acoustic waves and between the input transducer structure and the output transducer structure, characterized in that the acoustic wave propagation substrate is a composite substrate comprising a base substrate and a piezoelectric layer. By including such a filter structure, the passband can be adjusted while a filter structure occupying less area can be realized compared to the filter structures described above in the art.

[0022] According to a variant of the invention, the coupled cavity filter structure can be configured such that the surface acoustic waves are shear or longitudinal waves in the piezoelectric layer. Whereas in the prior art Rayleigh waves were used, the use of a piezoelectric layer instead of a bulk substrate opens the way to the use of different types of acoustic waves, thus providing further optimization parameters. Guided shear waves can achieve the highest electromechanical coupling achievable using composite substrates. They allow reaching wave speeds higher than elliptically polarized waves, with more opportunities for thermal compensation compared to conventional Rayleigh-like waves. Furthermore, the use of a given combination of substrate and inducing conditions allows to induce longitudinally polarized guided waves, achieving higher speeds than other wave types such as shear waves and Rayleigh waves, with a coupling of more than 5%.

[0023] According to a variant of the invention, the coupled cavity filter structure may further comprise at least one further groove located away from the input transducer structure and / or the output transducer structure, opposite the side on which the one reflecting structure is located in the propagation direction of the acoustic waves. The presence of a groove located close to the transducer structure makes it possible to reduce losses in the structure and may for example be configured to achieve total reflection of the acoustic waves propagating at the transducer.

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

[0025] According to a variant, the coupled cavity filter structure may further comprise at least one further groove located away from the input transducer structure and / or the output transducer structure, on the side opposite to the side on which the one reflecting structure is located in the propagation direction of the acoustic waves.

[0026] According to a variant, the depth D3 of at least one further groove located opposite the side on which one reflecting structure is located in the propagation direction of the acoustic wave, away from the input transducer structure and / or the output transducer structure, may be of the order of λ or more (approximately λ or more).

[0027] According to a variant, the coupled cavity filter may comprise a number of reflecting structures spaced apart from each other by a gap g and located in the direction of propagation of the acoustic wave at a distance L from and between the input and output transducer structures, each gap g between the reflecting structures forming an acoustic cavity. According to a variant, the distance L2 between the edge of the groove and the position A, B corresponding to the end of the pitch of the transducer structures (812, 814) on the side on which the groove is located in the direction of propagation of the acoustic wave may form an acoustic cavity. The inclusion of more than one reflecting structure provides the structure with a number of acoustic cavities. More acoustic cavities allow the transition band to be narrowed.

[0028] According to a variant, the distance between the edge of the groove (822, 1022, 1322, 1422, 1522, 1722) and the edge of the at least one further groove (932, 934) may be of the order of nλ. Such a dimension may improve the resonance conditions and thus the performance of the filter.

[0029] According to a variant, the clearance angle of the groove of the reflecting structure, when the clearance angle is the angle between the transverse axis X and the edge wall of the groove, may be of the order of 70° or more, in particular of the order of 90°.

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

[0031] According to a variant, the at least one further groove is for example configured to achieve total internal 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 reflecting structures may be located on the surface of the substrate which is also the surface of the substrate on which the transducer is located.

[0033] According to a variant, the acoustic cavity formed between at least two grooves of the plurality of reflecting structures may be located at a depth encompassed between the surface of the substrate and the bottom surface 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, in particular the number of electrode fingers of each transducer structure, so that the filter structure is more versatile and the transducer structure can be modified to optimize the coupling efficiency of the modes in the reflecting structure in order to achieve low insertion losses.

[0035] According to a variant of the invention, the acoustic cavity can be divided into sub-cavities that can be separated from each other by further layers, making it possible to generate evanescent coupling from one cavity to another. The sub-cavities can therefore favour the energy confinement in the structure and lead to an improvement in the compactness of the device.

[0036] According to a variant of the invention, the cavity filter structure comprises at least three or more transducer structures in the direction of propagation of the acoustic waves. The source density can be increased and an improved rejection can be achieved. Furthermore, the filter bandwidth can be flatter than for the same filter including only two transducers.

[0037] According to a variant of the invention, in particular by adapting the electrode geometry of the transducer structure, for example the thickness, width and / or length and / or number and / or shape of the electrodes, as well as the thickness of the piezoelectric layer, which must be 5% or more of the wavelength λ, the electromechanical coupling coefficient k of the shear wave, preferably of the guided wave or of the guided longitudinal wave in the piezoelectric layer, k s 2 The properties of the piezoelectric layer and of the electrodes of the transducer structure may be selected such that is greater than 5%, in particular greater than 7%.

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

[0039] According to a variant of the invention, the cavity filter structure may further comprise a dielectric layer, in particular a SiO2 layer, sandwiched between the base substrate and the piezoelectric layer. The dielectric or passivation layer may improve the attachment of the piezoelectric layer onto the base substrate, while still maintaining the temperature stability of the surface acoustic wave device, further improving the electromechanical coupling. The dielectric layer preferably has a thickness of less than 1 μm, in particular 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, as well as piezoelectric relaxors, such as PMN-PT and related materials, gallium nitride (GaN), lithium tantalate LiTaO3 or lithium niobate LiNbO3, where the lithium tantalate LiTaO3 or lithium niobate LiNbO3 has a crystal orientation relative to the lithium tantalate LiTaO3 or lithium niobate LiNbO3 defined as (YXl) / θ according to the standard IEEE 1949 Std-176, and the crystal orientation angle θ is comprised between 0° and 60° or between 90° and 150°.

[0041] According to a variant of the invention, the base substrate of the composite substrate can be one of silicon, in particular a high resistivity silicon substrate including a trap rich layer, carbon diamond, sapphire or silicon carbide. By high resistivity, an electrical resistivity higher than 1000 Ohm·cm can be understood. To transfer the piezoelectric layer to silicon, a mass production method such as SmartCut™ can be used, using ion implantation into the piezoelectric source substrate to define the layer to be transferred, mounting the source substrate to the silicon substrate and transferring the layer by thermal or mechanical treatment. A simpler approach based on bonding the piezoelectric substrate to the base substrate and subsequent thinning (via CMP, grinding, polishing) of the piezoelectric substrate may also be used for the invention, being particularly suitable for thick piezoelectric layers envisaged with a final thickness of the order of 5 μm to 20 μm. Both approaches, layer transfer via SmartCut™ or layer transfer via joining / thinning, result in a high quality, single crystal piezoelectric layer formed on the base substrate.

[0042] According to a variant of the invention, the base substrate may comprise a Bragg mirror below the piezoelectric layer. The Bragg mirror consists of a stack of layers with periodically alternating acoustic impedance deposited or fabricated on a plate of any inorganic material. Provided that the thickness of each layer is approximately a quarter of an acoustic wavelength, the stack of layers behaves like a mirror for the waves induced in the upper piezoelectric layer. The mirror therefore reflects waves with a component pointing towards the substrate depth and confines the waves in the piezoelectric layer.

[0043] According to a variant of the invention, the coupled cavity surface acoustic wave filter structure provides a filter bandpass comprised between 0.5% and 10%. By varying the parameters of the cavity filter structure, it is possible to vary the bandpass of the filter device, so that the device can be adapted to the user's specifications to achieve the required filter bandpass.

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

[0045] The object of the present invention is further achieved by using a SAW ladder filter device comprising at least two of the above-mentioned coupled cavity filters, and the at least two coupled cavity filter devices can be located on a single line. Since the coupled cavity filters according to the present invention can be located on a single line, the arrangement and connection of multiple cavity filters does not require as much space as the conventional SAW ladder device. The SAW ladder filter device according to the present invention is a smaller device than the conventional SAW ladder filter device.

[0046] The invention can be understood by reference to the following description considered in conjunction with the accompanying drawings, in which reference characters identify features of the invention. [Brief description of the drawings]

[0047] [Figure 1a] FIG. 1 shows a coupled-cavity surface acoustic wave filter structure according to a first embodiment of the present invention. [Figure 1b] 1A and 1B are diagrams showing modified examples of the coupled-cavity surface acoustic wave filter structure according to the first embodiment of the present invention. [Figure 2a] FIG. 2 shows a coupled cavity surface acoustic wave filter structure according to a second embodiment of the present invention. [Figure 2b] FIG. 11 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to a second embodiment of the present invention. [Figure 2c] FIG. 11 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to a second embodiment of the present invention. [Figure 2d]FIG. 11 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to a second embodiment of the present invention. [Figure 2e] FIG. 11 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to a second embodiment of the present invention. [Figure 3a] FIG. 2b shows the performance of a coupled cavity surface acoustic wave filter structure including a composite substrate according to the present invention, comprising a 500 nm SiO2 layer between a 6 μm LiTaO3(YXl) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate. [Figure 3b] FIG. 2b shows the performance of a coupled cavity surface acoustic wave filter structure including a composite substrate according to the present invention, comprising a 500 nm SiO2 layer between a 6 μm LiTaO3(YXl) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate. [Figure 3c] FIG. 2b shows the performance of a coupled cavity surface acoustic wave filter structure including a composite substrate according to the present invention, comprising a 500 nm SiO2 layer between a 6 μm LiTaO3(YXl) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate. [Figure 3d] FIG. 2b shows the performance of a coupled cavity surface acoustic wave filter structure including a composite substrate according to the present invention, comprising a 500 nm SiO2 layer between a 6 μm LiTaO3(YXl) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate. [Figure 4] FIG. 2c shows a table listing the properties of the coupled cavity surface acoustic wave filter structure shown in FIG. 2b in accordance with the present invention. [Figure 5a] FIG. 13 shows a coupled-cavity surface acoustic wave filter structure according to a third embodiment of the present invention. [Figure 5b] FIG. 13 shows a coupled-cavity surface acoustic wave filter structure according to a third embodiment of the present invention. [Figure 6] FIG. 13 shows a coupled-cavity surface acoustic wave filter structure according to a fourth embodiment of the present invention. [Figure 7]FIG. 13 shows a coupled-cavity surface acoustic wave filter structure according to a fifth embodiment of the present invention. [Figure 8a] FIG. 13 shows a coupled-cavity surface acoustic wave filter structure according to a sixth embodiment of the present invention. [Figure 8b] FIG. 13 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to the sixth embodiment of the present invention. [Figure 8c] FIG. 13 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to the sixth embodiment of the present invention. [Figure 8d] FIG. 13 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to the sixth embodiment of the present invention. [Figure 8e] FIG. 13 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to the sixth embodiment of the present invention. [Figure 8f] FIG. 13 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to the sixth embodiment of the present invention. [Figure 8g] FIG. 13 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to the sixth embodiment of the present invention. [Figure 8h] FIG. 13 is a diagram showing a modified example of a coupled-cavity surface acoustic wave filter structure according to the sixth embodiment of the present invention. [Figure 9] FIG. 13 shows a device according to a third embodiment of the invention used for a practical example of simulation. [Figure 10a] FIG. 10 illustrates simulated performance of the coupled-cavity surface acoustic wave filter structure shown in FIG. 9 according to the third embodiment of the present invention. [Figure 10b] FIG. 10 illustrates simulated performance of the coupled-cavity surface acoustic wave filter structure shown in FIG. 9 according to the third embodiment of the present invention. [Figure 11a] FIG. 10 illustrates the effect of device parameters on the simulated performance of the coupled-cavity surface acoustic wave filter structure shown in FIG. 9 according to a third embodiment of the present invention. [Figure 11b]FIG. 10 illustrates the effect of device parameters on the simulated performance of the coupled-cavity surface acoustic wave filter structure shown in FIG. 9 according to a third embodiment of the present invention. [Figure 11c] FIG. 10 illustrates the effect of device parameters on the simulated performance of the coupled-cavity surface acoustic wave filter structure shown in FIG. 9 according to a third embodiment of the present invention. [Figure 11d] FIG. 10 illustrates the effect of device parameters on the simulated performance of the coupled-cavity surface acoustic wave filter structure shown in FIG. 9 according to a third embodiment of the present invention. [Figure 12a] FIG. 12a shows an example of a prior art SAW ladder filter device. [Figure 12b] FIG. 12b shows an example of a SAW ladder filter device according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] Figure 1 shows a coupled cavity surface acoustic wave filter structure according to a first embodiment of the invention. In Figure 1a, a coupled cavity surface acoustic wave filter structure 100 is realized in a composite substrate 102. The composite substrate 102 comprises a layer of piezoelectric material 104 with crystallographic axes X, Y and Z formed on a base substrate 106.

[0049] The piezoelectric layer 104 in this embodiment is in particular LiTaO3 or LiNbO3 with a cut orientation defined by (YXl) / θ according to standard IEEE 1949Std-176, where the crystal orientation angle θ is comprised between 0° and 60° or between 90° and 150°, potassium niobate KNbO3 and similar material compositions, such as KTN, as well as other piezoelectric layers using sputtered or epitaxial films, such as aluminum nitride AlN, zinc oxide ZnO, PZT, GaN or any composition of AlN and GaN.

[0050] The thickness of the piezoelectric layer 104 formed on the base substrate 106 is of the order of magnitude of one wavelength λ or less, in particular approximately 20 μm or less. The thickness t of the base substrate 106 is greater than the thickness of the piezoelectric layer 104. A preferred situation corresponds to a base substrate thickness that is at least 10 times greater than the thickness of the piezoelectric layer 104, in particular 50 to 100 times greater, which corresponds to a base substrate thickness equal to 250 μm to 500 μm.

[0051] The base substrate 106 used in the first embodiment of the present invention is a silicon substrate, particularly a high resistivity silicon substrate. The orientation of the silicon substrate is preferably (100) because of its higher acoustic wave propagation velocity compared to other crystal orientations, such as (110), (111) or (001), for example, but other crystal orientations, such as (110), (111) or (001), can also be used. Instead of silicon, other substrate materials can be selected that have acoustic wave propagation velocity higher than one of the piezoelectric layers, for example, carbon diamond, sapphire, or silicon carbide can be used.

[0052] In a variant of the invention, the base substrate 106 may further comprise a so-called trap rich layer close to the upper layer of piezoelectric material, which improves the insulating performance of the base substrate and may be formed of at least one of polycrystalline, amorphous or porous material, such as, for example, polycrystalline silicon, amorphous silicon or porous silicon, although the invention is not limited to such materials.

[0053] In a variant of the invention, the base substrate 106 may further comprise a Bragg mirror below the piezoelectric layer 104. A Bragg mirror consists of a stack of layers with periodically alternating acoustic impedance deposited or fabricated on a plate of any inorganic material. The acoustic impedance is the product of the wave speed multiplied by the material density and is expressed by Rayleigh, M Rayleigh, i.e. 10 6Preferably, the piezoelectric layer is deposited or fabricated on the layer stack for the stimulation and detection of acoustic waves. The stack can be advantageously composed of tungsten and silica, or Si3N4 and SiO2, or Mo and Al alternating, and generally any combination of materials with an acoustic impedance ratio greater than 2. The inorganic subplate can be advantageously standard silicon, or high resistivity silicon or glass, and generally any material with a thermal expansion coefficient (TCE) less than 6 ppm / K. A trap rich layer can further be incorporated to improve electrical insulation. The first layer of the stack can be advantageously SiO2, or generally any material that can be used to bond the piezoelectric layer to the above-mentioned composite substrate.

[0054] 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 attachment of the piezoelectric material layer 104 to the base substrate 106. The SiO2 layer 110 is 200 nm thick, but in variants the thickness of the SiO2 layer 110 may vary. It may be larger or smaller than 200 nm thick, in particular it may vary between 10 nm and 6 μm.

[0055] The coupled cavity filter structure 100 further comprises two transducer structures 112, 114 and one reflecting structure 116 located between the two transducer structures 112, 114 at a certain distance d from the transducer structures 112, 114 in the propagation direction X shown in Fig. 1. The area located between the reflecting structure 116 and the one transducer structure 112, 114, for example an area 118 with its width defined by the distance d, corresponds to an acoustic cavity 120. In this case, the electrodes are centered within the pitch p of the transducers 112, 114. Thus, in the following, the ends of the pitch p of the transducer structures 112, 114 are located at a certain distance from the electrodes 128. In one example, if the ratio a / p of the transducer structures 112, 114 is 50%, the end of the pitch p is a distance equal to λ / 8 from the first electrode 128 of the transducer structures 112, 114.

[0056] As a result, the acoustic cavity extends between the reflecting structure 116 and the end of the pitch p of the transducer structures 112, 114 on the side on which the reflecting structure 116 is located. Thus, in the coupled cavity surface acoustic wave filter structure 100, various acoustic cavities exist in the direction of propagation of the acoustic waves, and in the coupled cavity filter structure shown in Figure 1a, there are two acoustic cavities 120.

[0057] The reflective structure 116 typically comprises one or more metal strips 122 and is defined by a pitch (not shown) of the metal strips 122 corresponding to the distance between the metal strips 122 in the reflective structure 116. As with the transducer structures 112, 114, the pitch in the reflective structure 116 is defined by including metal strips centered within the pitch.

[0058] The transducer structures 112 and 114 correspond to an input transducer structure 112 and an output transducer structure 114, but the positions of the input transducer structure 112 and the output transducer structure 114 may be swapped such that in the propagation direction of the acoustic waves 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, whereas the S symbol represents the output acoustic signal of the transducer structure.

[0059] Each transducer structure 112, 114 comprises two staggered interdigital electrodes 124, 126, each comprising a plurality of electrode means 128, 130, respectively. In this embodiment, the electrode means 128, 130 have the shape of electrode fingers. The interdigital electrodes 124, 126 and the electrode fingers 128, 130 of each of the interdigital electrodes 124, 126 are made of an aluminum-based material, for example pure aluminum, or an aluminum alloy, for example Al doped with Cu, Si, or Ti. Nevertheless, other materials may be used which provide a stronger reflection coefficient due to a smaller relative electrode thickness. In that regard, preferred electrode materials are copper (Cu), molybdenum (Mo), nickel (Ni), platinum (Pt) or gold (Au) with adhesion layers such as titanium (Ti) or tantalum (Ta) or chromium (Cr), zirconium (Zr), palladium (Pd), iridium (Ir), tungsten (W), and the like.

[0060] The transducer structures 112, 114 are further defined by an electrode pitch p (not shown) corresponding to the edge-to-edge electrode finger distance between two adjacent electrode fingers 128, 130 from the opposing interdigital electrodes 124 and 126. In a variation of the invention, the electrode pitch p is defined by the Bragg condition given by p=λ / 2, where λ is the operating acoustic wavelength of the transducer structures 112, 114. By operating acoustic wavelength λ, it can be understood that λ is the acoustic wavelength according to λ=V / f, where f is the desired center frequency of the filter structure and V is the phase velocity of the mode being used. Such transducer structures are also referred to as staggered transducers (IDTs) with two fingers per wavelength.

[0061] In variations of the invention, staggered transducers may operate outside the Bragg condition, for example using induced structures with 3 or 4 fingers per wavelength, or transducers with 5 fingers per 2 wavelengths, or 7 or 8 fingers per 3 wavelengths.

[0062] The transducer structures 112 and 114 may be symmetrical, i.e., the transducer structures 112 and 114 include the same number of electrode fingers 128, 130 with the same characteristics. However, in variations of the invention, the transducer structures 112 and 114 may be different, and in particular, the transducer structures 112 and 114 may include different numbers of electrode fingers 128, 130.

[0063] 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 an embodiment variant, the electrode fingers 132, 134 may have different lengths l, widths w, and thicknesses h. The dimensions are determined based on the desired coupling coefficient k s or to take advantage of other features, such as rejection of transverse modes, modulation of IDT impedance, reduction of emission of undesired modes, etc.

[0064] In a variant of the invention, the transducer structures 112, 114 may be chirped, meaning that the electrode pitch p in the transducer structures can be varied continuously, either linearly or hyperbolically, which makes it possible to widen the operating frequency band of the transducer and can provide some robustness against temperature.

[0065] The pitch of the metal strips 122 of the reflecting structure 116 may be the same as the electrode pitch p of the transducer structures 112, 114. In a variant, the pitch of the metal strips 122 of the reflecting structure 116 may be different from the electrode pitch p of the transducer structures 112, 114.

[0066] In a variation of the invention, the reflecting structure 116 may be chirped as well to broaden the operating band of the filter and to increase the efficiency of resonance of the acoustic cavity 118 located between the transducer structures 112,114.

[0067] In a variant, the coupled cavity filter structure 100 further comprises two Bragg mirrors 132, 134. This variant is shown in Fig. 1b, where each Bragg mirror 132, 134 is located outside the coupled cavity filter structure 200, close to a transducer structure 112, 114, meaning on the other side of the acoustic wave propagation direction from the side on which the reflecting structure 116 is located. Each Bragg mirror 132, 134 is located at a distance s from its respective transducer structure 112, 114. Each Bragg mirror 132, 134 comprises one or more metal strips 136, defined by a pitch (not shown) of the metal strips 136 corresponding to the distance between the metal strips 136 in the Bragg mirror 132, 134.

[0068] In a variation of the invention, the reflective structure 116 and the Bragg mirrors 132, 134 may be constructed by etching grooves instead of 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.

[0069] In a variant, a passivation layer (not shown) may be formed on the transducer structure 112, 114 and on the at least one reflecting structure 116. The passivation layer may have the same or different predetermined thickness on the transducer structure 112, 114 and / or on the at least one reflecting structure 116. The passivation layer may also be formed on the Bragg mirror 132, 134. In this variant, the substrate may be a monolithic piezoelectric wafer, for example a lithium tantalate or lithium niobate bulk wafer, and the passivation layer may advantageously be a silica SiO2 layer or a tantalum pentoxide Ta2O5 layer. In this embodiment, the passivation layer has a positive thermal expansion coefficient (TCE) while the substrate has a negative thermal expansion coefficient (TCE) and the layer thickness is set to reduce the temperature coefficient of frequency (TCF) of the SAW device.

[0070] Figures 2a-2e show a combined cavity filter structure according to a second embodiment of the present invention and its variants. For all of Figures 2a-2e, the combined cavity filter structure is shown in a 2D plan view, and the substrate on which the combined cavity filter structure is located is no longer shown. However, the substrate is the same as the substrate 102 in Figures 1a, 1b. The same reference numerals as shown in Figures 1a and 1b are used to denote the same features and will not be described in detail again.

[0071] 2a, the coupled cavity filter structure 300 comprises two transducer structures 112, 114, each including two Bragg mirrors 132, 134 located adjacent to one transducer structure, similar to the coupled cavity filter structure 200. The difference from the coupled cavity filter 200 is that a plurality of reflecting structures, i.e., four reflecting structures 202, 204, 206, 208, are present between the transducer structures 112, 114. Each of the reflecting structures 202, 204, 206, 208 of the plurality of reflecting structures comprises at least one or more metal strips 210, and is defined by a pitch (not shown) of the metal strips 210 corresponding to the distance between the metal strips 210 in each reflecting structure 202, 204, 206, 208. In this example, each reflecting structure 204, 206, 208, 210 has a total of four metal strips 210, although there may be more or fewer metal strips. The reflecting structures 204, 206, 208, 210 of the plurality of reflecting structures may include the same number of metal strips 210, although in variations, each of the reflecting structures may include a different number of metal strips 210. For example, the number of metal strips 210 in the reflecting structures 204, 206, 208, 210 may increase and then decrease across the reflecting structures 204, 206, 208, 210 between the transducer structures 112, 114 to enhance resonance at the actual center of the structure.

[0072] These reflecting structures 202, 204, 206, 208 are separated from each other by a gap g. The area located between two adjacent reflecting structures, e.g. 202 and 204, with its width defined by the gap g, corresponds to an acoustic cavity 212. As in the coupled cavity filter structures 100 and 200, the area located between a reflecting structure and an adjacent transducer structure further corresponds to an acoustic cavity 214, but with a width defined by the distance d between the reflecting structure and the adjacent transducer structure. As in 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 located between the reflecting structure 116 and the end of the pitch p of the transducer structures 112, 114 on the side on which the reflecting structure 116 is located.

[0073] Thus, in a coupled cavity surface acoustic wave filter structure, various cavities are present in the propagation direction of the acoustic wave separated by reflecting structures or, alternatively, the cavities are surrounded by two reflecting structures between the transducers. Thus, in the coupled cavity filter structure 300 shown in FIG. 2a, a total of five acoustic cavities 212, 214 are present in the propagation direction of the mode. For a given number of reflecting structures between the transducer structures and in a condition where the transducer structures are operating in the Bragg condition, the number of acoustic cavities is equal to the number of reflecting structures plus one.

[0074] In a variation of the invention, the reflecting structures 202, 204, 206, 208 may be chirped as well to broaden the operating band of the filter and to increase the efficiency of resonance of the acoustic cavities 212, 214 located between the transducer structures 212, 214.

[0075] In a variation of the invention, the reflective structures 202, 204, 206, 208 and Bragg mirrors 132, 134 may be constructed by etching grooves instead of 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.

[0076] In a variant of the invention, the metal strips 136, 210 of the reflecting structures 202, 204, 206, 208 and / or of the Bragg mirrors 132, 134 may be electrically connected to each other. A variant in which the reflecting strips 136, 210 of both the reflecting structures 202, 204, 206, 208 and the Bragg mirrors 132, 134 are connected to each other is shown in Fig. 2b for a coupled cavity filter structure 400. This results in an improvement of the reflection coefficients of the reflecting structures 202, 204, 206, 208 and of the Bragg mirrors 132, 134 in the Bragg condition. All of the reflective structures 202, 204, 206, 208 and / or Bragg mirrors 132, 134 operate in a so-called short-circuit state, which means that all of the metal strips 136, 210 of a given reflective structure 202, 204, 206, 208 / Bragg mirrors 132, 134 are connected together, resulting in a constant value of electrical potential across the diffraction grating structure.

[0077] In a variant of the invention, the combined cavity filter structure may comprise three or even more transducer structures. In Fig. 2c a variant is shown in which three transducer structures 402, 404, 406 are present in the combined cavity filter structure 400. Furthermore, the combined cavity filter structure 400 differs from the combined cavity filter structure 300 shown in Fig. 2a, since the plurality of reflecting structures comprises a total of six reflecting structures 302, 304, 306, 308, 310, 312. Similarly, the same reference numerals are used to describe the same features as those already described in Fig. 2a for the combined cavity filter structure 300.

[0078] Two transducer structures 112, 114 are located outside the reflecting structures 302, 304, 306, 308, 310, 312, as in the structure of Figure 2a, and a third transducer structure 314 is located in the center of the reflecting structures 302, 304, 306, 308, 310, 312116, so that there are three reflecting structures 302, 304, 306 and 308, 310, 312 on each side of the third transducer structure 314. Furthermore, the transducer structure 314 is separated from the two adjacent reflecting structures by a distance d, which corresponds to the same distance between the transducer structure 112, 114 and the adjacent reflecting structure of each of the transducer structures 112, 114 (in this example 302 for the transducer structure 112 and 312 for the transducer structure 114). Thus, the coupled cavity filter 500 comprises a total of eight acoustic cavities 316. Such a cavity filter structure 500 is symmetrical and provides a stronger confinement of energy in the cavities 316 compared to the cavity filter structure 300 shown in FIG. 2a, which includes only two transducer structures 112, 114.

[0079] In a variation of the invention, the coupled cavity filter structure is not symmetrical due to the third transducer 314 not being located in the center of the coupled cavity filter structure.

[0080] In a variant of the invention, the acoustic cavities may be divided into sub-cavities. This variant is shown in Fig. 2d, where the sub-cavities are separated from each other by the presence of an additional layer. Similarly, the combined cavity filter structure 600 also differs from the combined cavity filter structure 300 shown in Fig. 2a, since the multiple reflecting structures comprise three reflecting structures 402, 404, 406 in total. Similarly, the same reference numerals are used to describe the same features as those already described in Fig. 2a for the combined cavity filter structure 300. In the cavity filter structure 600, the acoustic cavity 408 located between the reflecting structures 402, 404, 406 and between the transducer structures 112, 114 and the adjacent reflecting structures 402 and 406 of the transducer structures 112, 114, respectively, is divided into two sections, resulting in the presence of eight dark regions 410 and four white regions 412 between the two transducer structures 112, 114. The dark regions 410 represent locations where the wave speed is less than the wave speed in the white regions 412, thus resulting in better energy confinement than that in the coupled cavity filter structure 300 shown in FIG. 2a. Thus, the coupled cavity filter structure 600 provides additional poles, leading to an improved compactness of the coupled cavity filter structure 600.

[0081] In a variation of the invention, the input and output transducer structures are not symmetrical or identical, and this variation is shown in Fig. 2e. Similarly, the coupled cavity filter structure 700 also differs from the coupled cavity filter structure 300 shown in Fig. 2a, since the plurality of reflecting structures comprises a total of two reflecting structures 502, 504. Similarly, the same reference numerals are used to describe the same features as those already described in Fig. 2a for the coupled cavity filter structure 300. In Fig. 2, the number of electrode fingers of each transducer structure is different. In the coupled cavity filter structure 700, the transducer structure 114 comprises staggered comb electrodes 124, 126, each including five electrode fingers 128, 130, with a constant electrode pitch p throughout the transducer structure 114. The transducer structure 506 comprises staggered comb electrodes 508, 510, each of which includes eight electrode fingers 128, 130, and furthermore, there is a gap in the transducer structure 506 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 in close proximity to the transducer structures 114, 506, which results in greater losses and ripples than the coupled cavity filter structure 300 of Figure 2a. However, this coupled cavity filter structure 700 provides a more compact and simpler design than the structure shown in Figure 2a.

[0082] Coupled cavity surface acoustic wave filter structures function in the following manner: an input staggered transducer (IDT) launches acoustic energy towards a reflecting structure, inducing resonance in the reflecting structure. The reflecting structure is coupled to another reflecting structure, thus creating a coupling condition that results in the transfer of energy from one reflecting structure to another. Multiple such reflecting structures may be coupled to each other, but there is at least one output transducer structure that collects the transmitted energy.

[0083] Therefore, the present invention proposes to use a coupled cavity filter structure utilizing waveguiding of the top piezoelectric layer from a composite substrate and using an acoustically resonant cavity to couple energy from one reflecting structure to another.

[0084] In the case of coupled cavity filter structures, the resonance of the transducer structure occurs in the low frequency transition band of the filter, and the anti-resonance occurs approximately in the center of the filter band. Therefore, since a given mode must have a coupling factor proportional to the band pass achieved, the conditions for the electromechanical coupling coefficient are similar to those required for impedance filters, i.e. the coupling coefficient must be 1.5 to 2 times larger than the band pass achieved, allowing to reduce the insertion loss within this band. However, a reflection coefficient larger than the coupling coefficient, ideally 1.5 times larger than the coupling coefficient, is required to realize the filter band.

[0085] According to the present invention, the relationship of the reflection coefficient to the coupling coefficient can be achieved using a composite substrate even when the coupling coefficient of the transducer structure is 5% or more. This is especially true when using guided shear or longitudinal waves in the piezoelectric layer of the composite substrate.

[0086] Due to the thickness of the piezoelectric layer being smaller than the wavelength, the shear or longitudinal modes are guided in the piezoelectric layer. Furthermore, the energy losses in the composite substrate can be reduced. The thickness of the piezoelectric layer should be equal to or greater than 5% of the wavelength λ. In the case of a thick piezoelectric layer, the shear modes of the composite substrate are no longer fully guided, but contain lossy bulk components that reflect at the interface to the base substrate, resulting in parasitic modes or rattle effects. However, in the case of a thin piezoelectric layer, i.e., a thickness of the wavelength or less, the shear modes are fully guided without lossy bulk modes.

[0087] The figure of merit of a filter device is the filter transmittance which indicates the filter bandpass as a function of frequency, expressed as the level of loss in dB in the bandpass. The filter bandpass depends on various factors: the coupling coefficient, the number of cavities and the reflection coefficient.

[0088] Depending on the dimensions of the piezoelectric layer, the dimensions of the transducer structure, the length dimensions of the reflecting structures, the number of coupled reflecting structures and the coupling coefficients of the modes, through the design and selection of the transducer structures, it is possible to synthesize multi-pole and zero filters with very low insertion losses, i.e. better than 2 dB, in particular less than 1 dB, and with rejection of 15 dB to 20 dB or even greater.

[0089] Regarding the dimensions of the acoustic cavity, according to the prior art, the acoustic cavity should ideally be a quarter wavelength long or an odd number of quarter wavelengths to meet the optimal resonance conditions. In the present invention, the length of the acoustic cavity can be shorter than a quarter wavelength. This is due to the strong velocity change from the free surface to the grating area, which results in a much larger acoustic impedance mismatch than is achievable using standard true SAW solutions.

[0090] Regarding the coupling coefficient, for the composite substrate and for the metal strip parameters (material, dimensions), the coupling coefficient is directly related to the bandpass value by a factor of 0.7, therefore the required bandpass of the filter device can be realized by selecting the material and dimensions of the cavity filter structure.

[0091] Regarding the number of metal strips in the reflecting structure, the number is selected to provide a global reflection coefficient greater than 0.5, in particular greater than 0.8, in order to enable confinement of the acoustic energy in the cavity and thus provide a mode coupling condition.

[0092] As already mentioned, the magnitude of the reflection coefficient is preferably greater than the coupling coefficient, ideally at least 1.5 times higher than the coupling coefficient parameter. The higher the reflection coefficient, the fewer the number of metal strips and therefore the wider the filter bandwidth. For example, a reflection coefficient of greater than 15% allows the number of metal strips constituting the reflecting structure to be reduced, which directly affects the bandwidth of the filter, i.e. the fewer the number of metal strips in the reflecting structure, the wider the bandwidth, provided that the reflection coefficient of the structure is greater than or equal to 50%. Given a reflection coefficient of greater than 15%, a filter with a bandwidth wider than 5% can be realized.

[0093] A specific example of a filter device operating at 2.6 GHz with a bandwidth wider than 7% and an in-band ripple less than 0.6 dB is given with a 30 nm thick tantalum (Ta) electrode on a composite substrate comprising a 300 nm thick (YX1) / 52° LiNbO3 layer on a 1 μm thick SiO2 layer on a (100) silicon substrate. In this example, the reflection coefficient reaches 20% and the coupling factor reaches about 18%.

[0094] Figures 3a-d show the characteristics of a surface acoustic wave filter device with a cavity filter structure as shown in Figure 2b, which includes a composite substrate according to the invention, including a 500 nm SiO2 layer between a 6 μm LiTaO3(YXl) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate. This configuration is suitable for operation in the frequency range from 50 MHz to 250 MHz.

[0095] In such composite substrates, true modes of pure shear can be excited and propagate.

[0096] For this SAW filter device, a coupled cavity filter structure as shown in Figure 2b is used, i.e., an input transducer structure and an output transducer structure, each with adjacent Bragg mirrors and four reflecting structures, resulting in five acoustic cavities existing between the transducer structures. The metal strips of both the reflecting structures and the Bragg mirrors are connected together for short circuit operation.

[0097] The staggered transducer structure includes a number of electrode finger pairs set to 15, with an electrode pitch set to 9.95 μm and a ratio a / p set to 0.3. Furthermore, with 30 electrodes, the mirror grating period is set to 10 μm and a / p is set to 0.4. The gap between the mirror grating and the staggered transducer structure is set to 9 μm, corresponding to approximately half a wavelength. Two internal reflection structures of 14 electrodes each are separated by a gap g of 4.8 μm, corresponding to approximately a quarter wavelength. The aperture is therefore 3.1 mm.

[0098] Figure 3a shows the overall filter transfer function and the group delay of the filter as a function of frequency (MHz) from 170 MHz to 250 MHz. Group delay is a measure of phase linearity. Figure 3b shows a zoomed-in view of Figure 3a near the center frequency, thus focusing on the bandpass and showing very low transmission loss. Figure 3c shows the reflection coefficient, as well as the signs of the poles of the filter as a function of frequency (MHz) (minimum |S 11 | peak). Figure 3d shows the so-called reflection coefficient Smith abacus diagram currently used by those skilled in the art to evaluate the electrical impedance matching of both ports of the filter. The reflection coefficient should be centered approximately around the 50 ohm condition, i.e. the abacus centers are aligned.

[0099] The transfer function in Figure 3a shows the performance of the device with a filter bandpass between 210 MHz and 218 MHz, and in Figure 3b it can be seen that the filter bandpass is flat at about 0.5 dB with low in-band ripple.

[0100] Figure 4 shows a table listing the properties of the coupled cavity surface acoustic wave filter structures shown in Figure 2b according to the present invention. For all of the coupled cavity filter structures, the composite substrate used was the same as in Figure 3, i.e., a 6 μm LiTaO3(YXl) / 42° piezoelectric layer with a 500 nm SiO2 layer between the piezoelectric layer and the semi-infinite (100) silicon base substrate.

[0101] The coupled cavity filter structure corresponds to Fig. 2b, i.e. an input transducer structure and an output transducer structure, each with adjacent Bragg mirrors and four reflecting structures, resulting in five acoustic cavities present between the transducer structures. The metal strips of both the reflecting structures and the Bragg mirrors are connected together for short circuit operation.

[0102] All of the coupled cavity filter structures illustrated in Figure 4 are compatible with the propagation of pure shear wave modes and exhibit performances, i.e., filter bandpasses varying from 0.5% to 10%, low insertion losses less than 2 dB, rejection losses between 15 dB and 20 dB, coupling factors higher than 5%, and temperature coefficients of frequency (TCF) less than 20 ppm / K. Bandpasses as large as 10% can be achieved using thin LNO layers with, for example, Pt or W or Ta or Mo based electrodes with thicknesses compatible with current deposition techniques, i.e., h / λ is equal to less than 5%, where h is the pure thickness of the metal strip.

[0103] Therefore, it is possible to select materials to adjust the coupling coefficient for the required performance of the coupled cavity SAW filter device, and to adjust the number and size of the reflecting structures so as not to exceed the size limits of the coupled cavity filter structure.

[0104] The coupled cavity SAW filter device of the present invention uses shear waves in a composite substrate to achieve narrow filter bandpasses encompassed between 0.5% and 10%, low insertion loss of less than 2 dB, while providing smaller dimensions due to improved miniaturization, and a rejection loss of 15 dB to 20 dB and low ripple within the filter bandpass.

[0105] 5a and 5b show a coupled cavity surface acoustic wave filter structure according to a third embodiment of the present invention.

[0106] In the third embodiment, as in the first embodiment shown in Figures 1a and 1b, a coupled cavity surface acoustic wave filter structure 800 is realized in a composite substrate 102 that is the same as the substrate 102 in Figures 1a and 1b.

[0107] The same reference numerals as shown in Figures 1a and 1b are used to denote the same features and will not be described in detail again.

[0108] In this embodiment, a thin SiO 2 layer 108 is provided at the interface 110 between the piezoelectric layer 104 and the base substrate 106 to improve attachment of the piezoelectric material layer 104 to the base substrate 106, the thin SiO 2 layer 108 being 500 nm thick.

[0109] The coupled cavity filter structure 800 comprises two transducer structures 812, 814 and one reflecting structure 816 located between the two transducer structures 812, 814 at a certain distance L from the transducer structures 812, 814 in the propagation direction X shown in FIG. 5a.

[0110] Each transducer structure 812, 814 comprises two electrodes 824, 826 (not shown), each of which comprises a number of electrode means 828, 830, respectively. The interdigitated electrodes are connected to +V / -V potentials in an alternating manner, with electrode 824 connected to +V and electrode 826 connected to -V, or vice versa. In a variant, the electrodes 824, 826 can be interdigitated electrodes, in particular staggered electrodes.

[0111] The transducer structures 812, 814 are defined by an electrode pitch p (not shown) corresponding to the edge-to-edge electrode finger distance between two adjacent electrode fingers 828, 830 from opposing interdigital electrodes 824 and 826.

[0112] In this embodiment, the electrode pitch p of the transducer structures 812,814 is defined by a number of wavelengths nλ, where λ is the operating acoustic wavelength of the transducer structures 812,814.

[0113] The metallicity ratio a / p of a transducer structure is defined as the ratio of the electrode width a to the electrode pitch p.

[0114] In this embodiment of the invention, the reflecting structure 816 is realized by a 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 by a total depth D of the groove 822. The total depth D of the groove 822 is defined from the surface of the piezoelectric layer 104 on which the transducers 812, 814 are located to a bottom surface 822c of the groove 822. The depth D of the groove 822 is of the order of λ or more, in particular of the order of 10λ or more, λ being the wavelength of the surface acoustic wave.

[0115] Additionally, the groove 822 is further defined by an etch clearance angle θ 340 that defines the position of the groove's edge walls 822a, 822b relative to the horizontal axis X and the bottom surface of the groove 822c. The clearance angle θ 840 may be on the order of 70° or more, and more particularly on the order of 90°. Figure 5a shows a groove 822 having vertical edge walls corresponding to a clearance angle θ 340 of 90°.

[0116] The area located between the reflecting structure 816 and the transducer structures 812, 814, for example the area 818 with its width 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 located on the surface of the piezoelectric layer on which the transducers 812, 814 are located. The points A, B are located at the ends of the pitch of the transducer structures 812, 814, on the side on which the groove 822 is located, as shown in FIG. 5a. 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, since the electrodes are centered within the pitch. For example, if the ratio a / p is 50%, the ends of the pitch are located at a distance equal to λ / 8 of the first electrodes 830 of the transducer structures 812, 814. FIG. 5b corresponds to FIG. 5a, in which the transducer structure 812, 814 includes only two electrodes 828 and 830 to more clearly show the area 818 of the acoustic cavity 820.

[0117] In the coupled cavity surface acoustic wave filter structure 800, two acoustic cavities 820 exist in the direction of propagation of the acoustic wave in the coupled cavity filter structure shown in Figures 5a and 5b.

[0118] In this embodiment of the invention, the reflective structure 816 is realised by providing, for example by etching, grooves 822 instead of depositing metal strips as in the first and second embodiments.

[0119] The grooves 822 are provided in the piezoelectric layer 104 of the composite substrate 102 and in the SiO2 layer down into the base substrate 106 to a total depth D. D1 corresponds to the portion of the depth D that is realized only in the base substrate 106.

[0120] In a variant, the grooves 822 can be etched only through the piezoelectric layer 104 and through the SiO2 layer 108 down 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 is equal to 0.

[0121] In a fourth embodiment, based on the third embodiment, the coupled cavity filter structure 900 further comprises two Bragg mirrors 832, 834. This embodiment is shown in Figure 6, in which each Bragg mirror 832, 834 is located adjacent to a transducer structure 812, 814 on the other side of the acoustic wave propagation direction X from where the reflecting structure 816 is located.

[0122] Each Bragg mirror 832, 834 is located a distance s from its respective transducer structure 812, 814. Each Bragg mirror 832, 834 comprises one or more metal strips 836 and is defined by a pitch of the metal strips 836 corresponding to the distance between the metal strips 836 in the Bragg mirror 832, 834. As with the transducers, the pitch in the Bragg mirrors 832, 834 is defined by having a metal strip 840 centered within the pitch.

[0123] In this variant, the pitch of the Bragg mirrors 832, 834 is furthermore equal to n times the wavelength λ, and thus nλ.

[0124] In this case, on the side where the Bragg mirrors 832, 834 are located the wave is reflected with a phase change, whereas on the side of the groove the type of reflection depends on the width and depth of the groove.

[0125] In a fifth embodiment, based on the third embodiment, the coupled cavity filter structure 1000 comprises two further grooves 932, 934, each of which is located adjacent to a transducer structure 812, 814 on the other side of the acoustic wave propagation direction from the side on which the reflecting structure 816 is located. This embodiment is shown in FIG.

[0126] Each of the further grooves 932, 834 is located at a distance s from the respective transducer structure 812, 814 of the further groove 932, 834. Each of the further grooves 932, 934 is defined by a width L3 of the further groove 932, 934 and a total depth D3 of the further groove 932, 934. The total depth D3 of the further groove 932, 934 is defined from the surface of the piezoelectric layer 104 on which the transducer 812, 814 is located to the bottom surface 932c, 934c of the further groove 932, 934. A depth D2 is defined as the depth of the further groove 932, 934 from the bottom surface 822c of the groove 822 to the bottom surface 932c, 934c of the groove 932, 934. The total depth D3 is therefore defined as D plus D2, where D is the total depth of the grooves 822 of the reflecting structure 816. The depth D3 of the at least one further groove (932, 934) is on the order of λ or greater.

[0127] In this embodiment of the invention, the reflective structure 816 and the further grooves 932, 934 are realised by providing grooves, for example by etching, instead of by depositing metal strips as in the first and second embodiments.

[0128] Each of the further grooves 932, 934 is configured to provide total internal reflection of a propagating wave along the propagation direction.

[0129] In a variant, the coupled cavity filter structure may comprise a Bragg mirror and a groove each located adjacent to the transducer structures 812, 814 on one side of the input transducer and adjacent to the transducer structures 812, 814 on the other side of the acoustic wave propagation direction X on which the reflecting structure 816 is located, and a groove on the side of the output transducer.

[0130] A coupled cavity filter device according to one of the third to fifth embodiments operates in the same manner as the coupled cavity filter device according to the first embodiment, but the structural features of the coupled cavity filter device, i.e. the pitch of the transducers and mirrors, the dimensions of the cavity, are configured such that the conditions for the device to exhibit similar functionality to the first embodiment are met.

[0131] This is because in the case of reflection at the edge, the definition of the reflection position is geometrically defined. Therefore, whatever the origin of the phase in the transducer, the phase construction occurs only for an integer number of wavelengths. The reflection center equivalent to a Bragg mirror is more difficult to define, since 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 magnitude of the reflection coefficient at a single electrode of the grating adjusts the width of the spectral band corresponding to the reflection action of the grating. However, it can be seen that the phase variation of the reflection function between the beginning and the end of the mirror stop band is always within the range of π to 2 × π, and the magnitude of the reflection coefficient also affects the phase variation of the reflection coefficient with respect to the frequency.

[0132] Thus, replacing the reflective structures comprising metal strips in the first and second embodiments with reflective structures comprising grooves as shown in the third, fourth, fifth and sixth embodiments results in conditions involving multiples of λ for the coupled cavity filter device to function.

[0133] All of the variants of the combined cavity filter devices according to the first and second embodiments may also be applied to the combined cavity filter devices according to the third, fourth, fifth and sixth embodiments.

[0134] 8a to 8h show a coupled-cavity surface acoustic wave filter structure according to a sixth embodiment of the present invention and its modifications.

[0135] The substrate shown in Figures 8a and 8b is the same as substrate 102 in Figures 5a, 5b, 6 and 7. The same reference numerals as in Figure 7 are used to denote the same features and will not be described in detail again.

[0136] 8a, the coupled cavity filter structure 1100 comprises two transducer structures 812, 814 with two grooves 932, 934 each located adjacent to one transducer structure, similar to the coupled cavity filter structure 1000. The difference with the coupled cavity filter 900 is that there are multiple reflective structures, namely two reflective structures 1006 and 1016, between the transducer structures 812, 814. Each reflective structure 1006, 1016 of the multiple reflective structures corresponds to a groove 1022 and is defined by a width L1 and a total depth D of the groove 1022.

[0137] These reflecting structures 1006, 1016 are separated from each other by a gap g in the propagation direction X. The area 1008 located between two adjacent reflecting structures 1006, 1016, with its width defined by the gap g, corresponds to an acoustic cavity 1010. The acoustic cavity 1010 may be considered as a central cavity, whereas the cavity 1020 may be called a side cavity.

[0138] As in the case of the coupled cavity filter structures 800 and 900, the region 1018 located between the reflecting structures 1006, 1016 and the adjacent transducer structures 812, 814 further corresponds to an acoustic cavity 1020 having a width defined by the distance L2 between the edges of the reflecting structures 1006, 1016 and points A, B located on the surface of the piezoelectric layer 104.

[0139] Region 1008 actually comprises a piezoelectric layer 104 and a SiO 2 layer 108 on a base substrate 106 .

[0140] Thus, in the coupled cavity surface acoustic wave filter structure 1100, three cavities exist in the direction of acoustic wave propagation separated by reflecting structures, or alternatively the cavities are surrounded by two reflecting structures between the transducers.

[0141] In another variation of the sixth embodiment, the coupled cavity surface acoustic wave filter structure 1200 includes a region 1008 between the grooves 1006 and 1016 that does not include the piezoelectric layer 104 and does not include the SiO2 layer 108, as shown in Figure 8b. In the region 1008, only the base substrate 106 is present. According to a further variation, the surface of the base substrate may be etched such that the thickness of the base substrate in the region 1008 is smaller than the thickness in the region 1018.

[0142] For all of Figures 8c-8h which show further variations including sub-cavities, the coupled cavity filter structures 1300-1800 are shown in 2D plan view from above.

[0143] In the coupled cavity filter structure 1300 shown in Fig. 6c, two of the reflecting structures 1206, 1216 are separated from each other in the propagation direction Z by a gap w. The region 1208 may or may not be etched, similar to the region 1008 in Fig. 8a. In this case, the acoustic cavity 1020 is divided into two sub-cavities 1020a, 1020b.

[0144] In the coupled cavity filter structure 1400, the reflective structures 1316 are separated from one another by a gap g in the propagation direction X, as in the cavity filter device 1100, and the reflective structures 1316 are further separated from one another by a gap w in the propagation direction Z, as in the cavity filter device 1200. The region 1308 between the reflective structures may or may not be etched, similar to region 1008 in Figure 8a and region 1210 in Figure 8c.

[0145] In this variant, the acoustic cavity 1310, 1320 is divided into a number of sub-cavities 1310a, 1310b, 1320a, 1320b, similar to the device 600 according to the second embodiment of the invention.

[0146] In the coupled cavity filter structure 1500, as shown in Fig. 8e, multiple reflecting structures 1506, 1516 are symmetrically separated by a central region 1510 having a width w1 in the direction Z over the total distance between the two transducer structures 812, 814 where no reflecting structures 1506, 1516 are present. Furthermore, instead of being elongated etched grooves in the Z direction as in the previous variant, the grooves 1522 in this example are square shaped. For the sake of clarity, only one reflecting structure 1506, 1516 and one groove 1522 are shown in Fig. 8e.

[0147] In the coupled cavity filter structure 1600 shown in FIG. 8f, the dimensions of the region 1610 that does not include the reflective structures 1506, 1516 between the transducer structures 812, 814 are different from the region 1510 in FIG. 8e. The region 1610 is actually thinner, with three reflective structures removed in a row in the Z direction. The region 1610 may be the same as a break in the symmetry of the reflective structures. For clarity, only one reflective structure 1506, 1516 and one groove 1522 are shown in FIG. 8f.

[0148] In another variation, device 1700 shown in FIG. 8g corresponds to device 1500 of FIG. 8e and includes two further grooves 1532, 1534 with a plurality of reflective structures 1506, 1516 and thus a plurality of grooves 1522.

[0149] In a variation of the invention, the coupled cavity filter structure 1800 may comprise three or even more transducer structures. In Figure 8h, a variation is shown in which three transducer structures 1210, 1212, 1214 are present in the coupled cavity filter structure 1200.

[0150] Two of the transducer structures 1210, 1214 are located outside the reflecting structures 1706, 1716, as in the structure of Fig. 2c, while the third transducer structure 1212 is located in the middle of the reflecting structures 1706, 1716, so that there is one reflecting structure 1706, 1716 on each side of the third transducer structure 1212, and therefore a groove 1722. The transducer structure 1212 is further separated from the two adjacent reflecting structures 1706, 1716 by a distance L, which corresponds to the same distance between the transducer structure 1210, 1214 and the adjacent reflecting structure of each of the transducer structures 1210, 1214 (in this example 1706 for transducer structure 1210 and 1716 for transducer structure 1214). Such a cavity filter structure 1800 is symmetrical and provides stronger confinement of energy in the cavity 1720 compared to the cavity filter structure 1300 shown in Figure 8c, which includes only two transducer structures 812, 814. Furthermore, in this example, the reflecting structures 1706, 1716 are also separated from each other by a distance w1, as in the device 1300 shown in Figure 8c.

[0151] In a variation of the invention, the third transducer 1210 is not located in the center of the coupled cavity filter structure so that the coupled cavity filter structure is not symmetrical.

[0152] FIG. 9 shows a device according to a third embodiment of the invention used for the simulations.

[0153] For this simulation, the device used corresponds to the device 1000 shown in FIG. 7, but in this example the transducer structures 812 and 814 each comprise only two electrode fingers 828, 830.

[0154] Furthermore, the following parameters were used for the structure: A composite substrate was used for the simulation, comprising a 200 nm SiO2 layer between a 500 nm LiTaO3(YXl) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate. The transducer electrode pitch p is equal to 800 nm, realizing a wavelength λ equal to 1.6 μm for a frequency operation around 2.5 GHz. The metallization ratio a / p is equal to 0.5, which means that the electrode width of a is equal to 400 nm. The electrode fingers are Al-Cu with a thickness set to 100 nm.

[0155] 10a and 10b show simulated characteristics of the coupled cavity surface acoustic wave filter structure shown in FIG. 9 according to a third example embodiment of the present invention.

[0156] In this practical example, the simulation was carried out using a mesh structure (not shown) with pitch p equal to 2 μm, depth D1 equal to pitch p, depth D2 equal to 6 × p, width of groove L1 equal to 6.4 μm, and width L2 equal to 2 × p. A composite substrate was used, comprising a 600 nm thick LiTaO3 and a 500 nm thick SiO2 on a (100) Si-based substrate.

[0157] Figure 10a shows a graph of admittance as a function of frequency in GHz on the X-axis, with the real part on the left Y-axis and the imaginary part on the right-axis. Figure 10a shows two peaks at about 61800 GHz and 62000 GHz, with the peak at about 61750 GHz showing a stronger admittance than the other peak at about 62000 GHz.

[0158] Figure 10b shows a graph of the mutual admittance with the real part on the left Y-axis and the imaginary part on the right-hand axis as a function of frequency in GHz on the X-axis. Figure 10b shows the same two peaks at about 61800 GHz and 62000 GHz, but in this case with a more balanced contribution and the two peaks are equal in amplitude.

[0159] The graph shows two coupled modes, with a well-balanced contribution between the two allowing for effective definition of the filter bandpass, regardless of the actual amplitude level.

[0160] 11a to 11d show calculations of the admittance of a filter base structure for various dimensions of a reflecting structure according to the invention.

[0161] A composite substrate containing a 500 nm SiO2 layer between a 600 nm LiTaO3(YXl) / 42° piezoelectric layer and a semi-infinite (100) silicon substrate was used for the simulation.

[0162] 11a-11b show graphs of admittance as a function of frequency in GHz on the X-axis, with the real part on the left Y-axis and the imaginary part on the right-axis, for various trench depths D1. The frequency range shown is between 61000 GHz and 63000 GHz. All graphs show the same two peaks at about 61800 GHz and 62500 GHz as in FIG. 10b, and the graphs can be considered as double-peaked in that the first peak at about 61800 GHz has a higher admittance value than the second peak at about 63000 GHz. In FIG. 10d, the double peak indeed has a thinner bandwidth than in FIG. 11a, FIG. 11b, or FIG. 10c.

[0163] 11a to 11c, D1 varies between 1 μm and 0.2 μm, whereas in FIG. 11d, D1 varies between 1 μm and 2 μm.

[0164] In Figures 11a to 11d the same behavior of the peaks can be seen. As D1 decreases from 1 μm to 0.2 μm, the double peak morphology moves towards higher frequencies, the first peak becomes smaller in terms of admittance and the second peak becomes larger in terms of admittance. In Figures 11b, 11c and 11d the second peak becomes larger in terms of admittance so that it actually reaches a higher value than the first peak, whereas in Figure 11a both peaks are comparable for D1 equal to 0.2 μm.

[0165] 12a and 12b show an example of a SAW ladder filter device according to the prior art in FIG. 12a and an example of a SAW ladder filter device according to the present invention in FIG. 12b.

[0166] In Fig. 12a, each of the four transducer structures 1612, 1614, 1616, and 1618 is sandwiched between two reflecting structures 1632 and 1634. The staggered electrodes of the transducers 1612, 1614, 1616, and 1618 are connected to each other by metal lines 1640. As can be seen, such a design is bulky since the transducers and their metal connections require a lot of space. The transducers cannot be aligned on a single line, either vertically or horizontally, since they would need to be shifted from one to another to allow for connections between the transducers.

[0167] FIG. 12b shows a SAW ladder filter device 2000 in which two cavity-coupled filter devices 1000 according to the invention are located adjacent to each other on a single line. Each cavity-coupled filter device 1000 comprises two transducer structures 1712, 1714 and 1716, 1718, respectively, and their respective reflecting structures 1732, 1734 and 1716. Reflecting structure 1716 is located between the two transducer structures, e.g., 1716 is located between 1712 and 1714, and reflecting structures 1732 and 1734 are each located on one side of the transducer structure, opposite the side on which reflecting structure 1716 is located. In FIG. 12b, reflecting structures 1716, 1734 and 1732 are represented as grooves, as in FIG. 8. As in FIG. 12a, the electrodes of the transducers are connected to each other by metal lines 1740.

[0168] Although a SAW ladder filter device 2000 including two cavity coupled filter devices 1000 is shown in FIG. 12b, any variation of the cavity coupled filter device 1000 according to the present invention may be used for the SAW ladder filter device.

[0169] The SAW ladder filter device 2000 according to the present invention is more compact and does not require as much space as the SAW ladder filter device shown in FIG. 12a.

[0170] A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications and improvements may be made without departing from the scope of the claims that follow. [Explanation of symbols]

[0171] 102...acoustic wave propagation substrate, 104...piezoelectric layer, 106...base substrate, 112...input transducer structure, 114...output transducer structure, 116...reflection structure, 124, 126...comb electrodes.

Claims

1. An acoustic wave propagation substrate; at least one input transducer structure and one output transducer structure provided on the acoustic wave propagation substrate, each of the input transducer structure and the output transducer structure comprising an electrode; a reflecting structure, the reflecting structure comprising a groove located between the input transducer structure and the output transducer structure in the propagation direction of the acoustic wave; A surface acoustic wave coupled cavity filter structure comprising: the acoustic wave propagation substrate is a composite substrate including a base substrate and a piezoelectric layer, the coupled cavity filter structure further comprises a first further groove located away from the input transducer structure on the side opposite to the one reflecting structure of the input transducer structure in the propagation direction of the acoustic wave, and / or a second further groove located away from the output transducer structure on the side opposite to the one reflecting structure of the output transducer structure in the propagation direction of the acoustic wave, The depth D of the groove of the reflective structure is equal to or greater than λ, and the depth D of the first further groove and / or the second further groove ３ is greater than D, and a distance L 2 between an edge of the groove of the reflecting structure and positions A and B corresponding to the ends of the pitch of the input transducer structure and the output transducer structure on the side where the groove of the reflecting structure is located in the propagation direction of the acoustic wave forms an acoustic cavity; A coupled cavity filter structure, wherein the thickness of the piezoelectric layer is less than or equal to one wavelength λ, where λ is the wavelength of the surface acoustic wave.

2. The coupled cavity filter structure of claim 1 , wherein the surface acoustic wave is a guided acoustic wave.

3. The coupled cavity filter structure of claim 1 , wherein the surface acoustic waves are configured to be shear or longitudinal waves in the piezoelectric layer.

4. The depth D ３ The coupled cavity filter structure of claim 1 , wherein is greater than or equal to λ.

5. 2. The coupled cavity filter structure of claim 1, comprising a plurality of reflecting structures spaced apart from one another by a gap g in the propagation direction of the acoustic wave and positioned between the input transducer structure and the output transducer structure, each of the gaps g between the reflecting structures forming an acoustic cavity.

6. 2. A coupled cavity filter structure as claimed in claim 1, wherein the distance between an edge of the groove of the reflecting structure and an edge of the first further groove and / or the second further groove is n times λ, where λ is the wavelength of the surface acoustic wave.

7. 2. The coupled cavity filter structure of claim 1, wherein the groove depth D of the reflecting structure is greater than or equal to 10λ, where λ is the wavelength of the surface acoustic wave.

8. 2. A coupled cavity filter structure according to claim 1, wherein the first further groove and / or the second further groove are configured to provide total internal reflection of a propagating wave along the propagation direction.

9. 6. The coupled cavity filter structure of claim 5, wherein the acoustic cavity formed between at least two of the grooves of a plurality of the reflecting structures is located on a surface of the acoustic wave propagating substrate that is also the surface of the acoustic wave propagating substrate on which the input transducer structure and the output transducer structure are located.

10. 6. The coupled cavity filter structure of claim 5, wherein the acoustic cavity formed between at least two of the grooves of the plurality of reflecting structures is located at a depth encompassed between a surface of the acoustic wave propagation substrate and a bottom surface of at least two of the grooves of the plurality of reflecting structures located at a depth D.

11. 2. The coupled cavity filter structure of claim 1 , wherein the groove, the first further groove, and the second further groove of the reflecting structure are etched in the piezoelectric layer of the composite substrate down to the base substrate.

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