Transducer structures for single-port resonators with suppressed transverse modes.

Abstract

The present invention relates to a transducer structure with transverse mode suppression means, the transducer structure comprising a piezoelectric substrate (120, 170) and a pair of interdigitated interdigitated electrodes (102, 112) formed thereon. The first interdigital electrode (102) comprises a first bus bar (108) and a plurality of electrode fingers (104) interleaved with short dummy electrode fingers (106), the second interdigital electrode (112) comprises a second bus bar (118) and a plurality of electrode fingers (114), the dummy electrode fingers (106) of the first bus bar facing the electrode fingers (114) of the second bus bar, and further comprises a transverse mode suppression layer (122, 132, 222, 232, 422, 432) selected so that the phase velocity of the guided wave is smaller than the phase velocity of the guided wave in a central region (136) below the electrode fingers (104, 114).

Description

Technical Field and Background Art

[0001] The present invention relates to a transducer structure with transverse mode suppression means, in particular for a single-port resonator, and further to a method for manufacturing such a transducer.

[0002] The invention is in the field of surface acoustic wave (SAW) devices built on piezoelectric single crystals or on composite substrates using piezoelectric films, so-called POIs in the sense of Piezo-On-Insulator. Such composite wafers are used as waveguides for exciting true modes using interdigitated transducers (IDTs) for the manufacture of radio-frequency (RF) filters. The transducer structure is an important part in so-called single-port resonators used as impedance elements of the abovementioned filters. More precisely, so-called ladder filters make use of said resonators to realize the filter function. Single-port resonators used in sensor applications, and more precisely single-port resonators used for wireless sensing, are also subject to such effects, so that measures are often required to suppress them, for example by aperture tapers (see for example EP 2091146), which are known to reduce the electromechanical coupling of the device. A single-port resonator contains one transducer structure located in the center of the resonator and reflectors located on either side of the transducer structure, while a two-port SAW resonator consists of two transducer structures (input / output) located in the center and reflectors located on either side of the device.

[0003] Other types of SAW devices, such as longitudinally coupled resonator filters (LCRF), or double-mode-SAW (DMS) filters, or double-port SAW resonators, or delay lines used for oscillator stabilization, e.g. for frequency source applications, or any other application (e.g. gravimetric analysis), may also take advantage of the present invention.

[0004] In single-port resonators built on POIs, so-called transverse modes arise that exhibit spurious contributions due to lateral energy constraints and thus contaminate the spectral characteristics of the device. It is therefore proposed to provide a means to suppress such contributions and improve the spectral purity of the resonator response, allowing the fabrication of compliant filters, and more generally, compliant SAW devices.

[0005] With conventional techniques, it is difficult to manufacture a structure that can suppress transverse modes.

[0006] It has been proposed to reload the edges of the electrodes with a metallic or dielectric overlay, as disclosed in US Patent Application Publication No. 2012 / 0161577 or US Patent Application Publication No. 2013 / 0249647.

[0007] In US 2012 / 0161577, metal is redeposited on top of the electrode edges, resulting in high pattern registration accuracy but requiring expensive processing tools and controls.

[0008] In US2013 / 0249647, the entire interdigitated transducer (IDT) structure is covered by a dielectric layer that is thicker on the edges of the transducer than in the center. In US2015 / 0123746, a dielectric film covers the piezoelectric substrate and at least a part of the IDT electrodes, more precisely the area where the electrode fingers are interleaved with each other, and the acoustic velocity is higher in the center of the IDT fingers compared to the edges, thus avoiding the generation of transverse modes. Most applications using dielectric layers require the dielectric layers to be patterned using acid or reactive ion etching or ion milling, which, if an etch stop solution is not applied to prevent such effects, can degrade the electrode quality or even the substrate surface if not properly protected.

[0009] Other approaches are based on increasing the electrode dimensions, as described in US Patent Application Publication No. 2018 / 0097508 or US Patent Application Publication No. 2018 / 0375491. Such structures present difficulties in ensuring insulation from one electrode to another.

[0010] However, in all the approaches cited above, for such transducer structures, transverse modes are still possible, albeit reduced. Summary of the Invention

[0011] It is therefore an object of the present invention to provide a transducer structure for a single-port resonator in which lateral modes are suppressed, by a simpler manufacturing process compared to prior art devices, while preserving the inherent quality of the device and substrate surface.

[0012] The object of the invention is a transducer structure with transverse mode suppression means, in particular for a single port resonator, comprising a piezoelectric substrate and at least a pair of interdigitated comb electrodes formed on the piezoelectric substrate, in particular a piezoelectric composite substrate comprising a piezoelectric layer above a base substrate, the first comb electrode comprising a first busbar and a number of interdigitated electrode fingers and short dummy electrode fingers extending from the busbar, the second comb electrode comprising a second busbar and a number of electrode fingers extending from the second busbar, the dummy electrodes of the first busbar being interdigitated with the electrode fingers of the second busbar, and at least one pair of interdigitated interdigital electrodes facing the first busbar and spaced apart from the electrode fingers of the second busbar by a first gap, characterized in that the transducer structure further comprises a transverse mode suppression layer partially below the first gap, the transverse mode suppression layer being selected such that the phase velocity of the guided waves in the region of the transverse mode suppression layer is smaller compared to the phase velocity of the guided waves in the piezoelectric substrate in a central region below the interdigitated electrode fingers of the first and second interdigital electrodes. The presence of the transverse mode suppression layer slows down the wave velocity in the gap, thereby suppressing or reducing unwanted transverse modes in the transducer structure. Thus, the contribution of the transverse modes can be reduced in the transducer structure according to the invention. From an acoustic point of view, the transducer structure according to the invention allows the transfer of energy from the IDT fingers to the dummy fingers without or with reduced reflections at the edges of the IDT fingers.

[0013] In a variant of the embodiment, a lateral mode suppression layer can extend under each of the first gaps. The lateral mode suppression layer makes it possible to slow down the wave velocity in the gaps, which leads to the suppression or reduction of unwanted lateral modes in the transducer structure. From an acoustic point of view, the transducer structure according to the invention allows the transfer of energy from the IDT fingers to the dummy fingers with no or reduced reflections at the edges of the IDT fingers.

[0014] In a variant of the embodiment, the second comb electrode can comprise short dummy electrode fingers interleaved with the electrode fingers, the dummy electrode fingers of the second busbar facing the electrode fingers of the first busbar and spaced apart from the electrode fingers by the second gap, a second transverse mode suppression layer extending at least partially below the second gap, the second transverse mode suppression layer being selected such that the phase velocity of the guided waves in the region of the second transverse mode suppression layer is smaller compared to the phase velocity of the guided waves in the central region below the interleaved electrode fingers of the first and second comb electrodes and between the first and second transverse mode suppression layers. The transverse mode suppression layer allows to slow down the wave velocity in the gap, which leads to the suppression or reduction of unwanted transverse modes in the transducer structure. Thus, the contribution of the transverse modes can be further reduced in the transducer structure according to the invention. From an acoustic point of view, the transducer structure according to the invention allows energy to be transferred from an IDT finger to a dummy finger with no or reduced reflections at the edges of the IDT fingers.

[0015] In a variant of the embodiment, the second transverse mode suppression layer can extend under all the second gaps. The transverse mode suppression layer makes it possible to slow down the wave velocity in the gaps, which leads to the suppression or reduction of unwanted transverse modes in the transducer structure. From an acoustic point of view, the transducer structure according to the invention allows the transfer of energy from the IDT fingers to the dummy fingers with no or reduced reflections at the edges of the IDT fingers.

[0016] In variants of the embodiment, the first transverse mode suppression layer may extend at least partially to the first busbar and / or the second transverse mode suppression layer may extend at least partially to the second busbar.

[0017] In variants of the embodiment, the first transverse mode suppression layer may extend further, at least partially or at least entirely, beneath the first busbar, and / or the second transverse mode suppression layer may extend further, at least partially or at least entirely, beneath the second busbar.

[0018] In a variant of the embodiment, the first and / or second transverse mode suppression layer can have a different doping amount, in particular a Ti amount, compared to the rest of the piezoelectric substrate in the central region below the interdigitated electrodes in order to modulate the phase velocity. Doping here corresponds to the deliberate introduction of impurities into the piezoelectric substrate. The difference in doping at least in the transverse mode suppression layer reduces the phase velocity of the modes near the gap separating the IDT electrodes from the dummy electrodes, without the need for any lateral patterning.

[0019] In a variant embodiment, the first transverse mode suppression layer and / or the second transverse mode suppression layer of the piezoelectric layer may comprise a passivation layer, in particular a dielectric passivation layer, more in particular a SiO2 layer.

[0020] In an alternative embodiment, the passivation layer may be at least partially embedded into the piezoelectric substrate.

[0021] In a modified embodiment, the first transverse mode suppression layer and the second transverse mode suppression layer may be mirror symmetric about a centerline of the interdigitated comb electrodes.

[0022] In a variation of the embodiment, the first transverse mode suppression layer and the second transverse mode suppression layer can have different shapes.

[0023] In a variant of the embodiment, the piezoelectric substrate can be a piezoelectric composite substrate, which comprises a piezoelectric layer above a base substrate. Such a composite substrate not only allows for configurations that allow for frequency changes to be obtained at optimized temperatures, but also opens the possibility of using guided waves. The modes of the structure can exhibit different types of polarization, in particular elliptical polarization, but also mostly shear waves or pure or almost pure longitudinal compression waves, the electromechanical coupling of which is significantly greater than 7%, allowing the realization of filters with the bandwidth required in modern filter applications, in particular with more than 5%.

[0024] In a variant of the embodiment, the thickness of the piezoelectric layer can be smaller than the value of the wavelength λ of the acoustic transducer, in particular smaller than 0.7 times the value of the wavelength λ, and more particularly smaller than 0.5 times the value of the wavelength λ. For a selected operating frequency, the thickness of the piezoelectric layer is optimized in order to optimize the guiding function of the waves used. For a given thickness of the piezoelectric layer that allows a guiding function of the waves mainly at the surface, the invention is particularly interesting since parasitic transverse modes become more prominent due to the guiding of the waves and due to the possibility of in-phase reflections of such type of waves at the electrode lateral surfaces.

[0025] In a variant embodiment, the thickness of the suppression layer can be greater than 1 / 20 of the wavelength λ, in particular greater than 1 / 10 of the wavelength λ, and smaller than the thickness of the piezoelectric layer. The optimum thickness of the suppression layer found is evaluated in relation to the wavelength λ.

[0026] In a variant of the embodiment, the thickness of the suppression layer can correspond to at most ½ the value of the wavelength λ, in particular to at most ⅓ the value of the wavelength λ. Increasing the thickness over which the suppression layer extends can be detrimental to the desired suppression effect and can result in energy leakage due to diffraction of the energy of the main mode, reducing its quality factor.

[0027] In a variant of the embodiment, the thickness of the suppression layer can be of the order of magnitude of the thickness of the electrode, in particular ±50% of the nominal value, more particularly a thickness of an Al_Cu electrode with a Cu doping of 0.5-2% of 5-20% of the value of the wavelength λ. In this particular case, at least 90% of the transverse modes that would be present in the absence of a suppression layer are suppressed.

[0028] In variants of the invention, the guided waves can be elliptically polarized, shear waves, pure or almost pure longitudinal compression waves, with electromechanical coupling greater than 7%. The modes of the transducer structure can exhibit different types of polarization, allowing the realization of filters with the bandwidth required in modern filter applications, in particular with a bandwidth greater than 5%.

[0029] The object of the invention is also to provide a method for manufacturing a transducer structure, comprising the steps of: a) providing a piezoelectric substrate, in particular a piezoelectric composite substrate comprising a piezoelectric layer above a base substrate; and b) forming at least a pair of interdigitated comb electrodes on the piezoelectric substrate, the first comb electrode comprising a first busbar and a number of interdigitated electrode fingers and short dummy electrode fingers extending from the first busbar, the second comb electrode comprising a second busbar and a number of interdigitated electrode fingers extending from the second busbar, the dummy electrodes of the first busbar facing the electrode fingers of the second busbar. The present invention is realized by a method for manufacturing a transducer structure, comprising the steps of forming at least one pair of interdigitated interdigitated electrodes, the interdigitated electrodes being spaced apart from the electrode fingers of the second busbar by a first gap, and a step c) of providing a transverse mode suppression layer prior to step b), the transverse mode suppression layer being provided partially below the first gap after step c), the transverse mode suppression layer being selected such that the phase velocity of the guided waves in the transverse mode suppression layer is smaller compared to the phase velocity of the guided waves in the piezoelectric substrate in a central region below the interdigitated electrode fingers of the first and second electrodes. Such a method allows the formation of the transverse mode suppression layer before the formation of the interdigitated electrodes. Thus, using this method, the interdigitated electrodes are not damaged by the suppression layer formation step, thereby preserving the inherent quality of the device and the substrate surface. This method allows the normal SAW front-end manufacturing process to be maintained without modification, contrary to prior art devices in which the transverse mode suppression layer is formed on the interdigitated electrodes after they are formed on the piezoelectric substrate. Furthermore, the method according to the present invention results in a simpler manufacturing process compared to prior art devices.

[0030] In a variant of the invention, the step c) of providing a transverse mode suppression layer may comprise providing a first transverse mode suppression layer and a second transverse mode suppression layer, the second transverse mode suppression layer extending at least partially below the second gap and selected such that the phase velocity of the guided wave in the region of the second transverse mode suppression layer is smaller than the phase velocity of the guided wave in a central region below the interleaved electrode fingers of the first and second interdigital electrodes and between the first and second transverse mode suppression layers, the second interdigital electrodes further comprising short dummy electrode fingers interleaved with the electrode fingers, the dummy electrode fingers of the second busbar facing the electrode fingers of the first busbar and spaced apart from them by the second gap. In this way the transverse mode suppression layer can be formed before the interdigitated electrodes are formed. Thus, using this method, the interdigitated electrodes are not damaged by the suppression layer formation step, thereby preserving the inherent integrity of the device and substrate surface. This method allows the normal SAW front-end manufacturing process to be kept unchanged, as opposed to prior art devices where the transverse mode suppression layer is formed on the interdigitated electrodes after they are formed on the piezoelectric substrate. Furthermore, the method according to the invention results in a simpler manufacturing process compared to prior art devices.

[0031] In a variant of the invention, the step of providing a lateral mode suppression layer can include a modification step of the doping amount of the piezoelectric substrate, in particular with an implantation step or a diffusion step of atomic species and / or a proton exchange technique. Again, doping corresponds here to the deliberate introduction of impurities into the piezoelectric substrate. Simpler techniques can be used to form the lateral mode suppression layer compared to the techniques used in the manufacturing processes of devices of the prior art.

[0032] In a variant of the present invention, the step of providing a lateral mode suppression layer can include a step of providing a passivation layer, in particular a step of providing a dielectric passivation layer, more particularly a step of providing a SiO2 layer. In forming the lateral mode suppression layer, a material layer deposition technique can be used, which is a simpler technique compared to the techniques used in the manufacturing process of the prior art devices. In a variant of the present invention, the step of providing a lateral mode suppression layer can include a step of at least partially embedding the passivation layer into the piezoelectric substrate.

[0033] In a variant of the invention, the passivation layer can be patterned by a lift-off process, so that the lateral sides of the passivation layer can be inclined so that the deposited comb electrodes cover these lateral sides without breaking, which leads to a regular and continuous reduction of the phase velocity in the corresponding areas. This thus further reduces the possibility of transverse modes arising, since they preferably require a sharp velocity change at the electrode edges, which act as wave reflectors. In this case, no etching is necessary, so that the substrate remains in good condition. If the substrate has to be protected from organic contamination, a so-called wet etching process based on isotropic etching of the silicon dioxide layer can be implemented, which also allows smooth edges of the patterned passivation layer to be obtained. Thus, as a first step, SiO2 is deposited uniformly on the wafer, after which an in-situ etching mask of photoresist is patterned on the layer, which allows the SiO2 layer to be locally etched. However, registration marks must be formed to prepare the surface for this purpose according to the usual technological processes.

[0034] The objects of the invention are also realized by a SAW device, in particular a single-port resonator, comprising at least the transducer structure described above, which can be produced with a simpler manufacturing process compared to the prior art, while improving the spectral purity of the resonator response by reducing the contribution from transverse modes.

[0035] The invention can be understood by reference to the following description taken in conjunction with the accompanying drawings, in which reference characters identify features of the invention. [Brief description of the drawings]

[0036] [Figure 1a] 1 shows a plan view of a transducer structure according to a first embodiment of the present invention; [Figure 1b] 1 shows a side view of a transducer structure according to a first embodiment of the present invention; [Figure 2a] 4 shows a side view of a transducer structure according to a modified example of the first embodiment of the present invention. [Figure 2b] 13 shows a side view of a transducer structure according to another modified example of the first embodiment of the present invention. [Figure 2c] 13 shows a side view of a transducer structure according to another modified example of the first embodiment of the present invention. [Figure 2d] 13 shows a side view of a transducer structure according to another modified example of the first embodiment of the present invention. [Figure 3a] 4 shows a plan view of a transducer structure according to a modified example of the first embodiment of the present invention. [Figure 3b] 13 shows a plan view of a transducer structure according to another modified example of the first embodiment of the present invention. [Figure 3c] 13 shows a plan view of a transducer structure according to another modified example of the first embodiment of the present invention. [Figure 3d] 13 shows a plan view of a transducer structure according to another modified example of the first embodiment of the present invention. [Figure 3e]13 shows a plan view of a transducer structure according to another modified example of the first embodiment of the present invention. [Figure 3f] 13 shows a plan view of a transducer structure according to another modified example of the first embodiment of the present invention. [Figure 4] 1A to 1C are schematic diagrams illustrating a method for manufacturing a transducer structure according to a first embodiment of the present invention and its modified examples. [Diagram 5] 13A-13D show schematic diagrams of a method for manufacturing a transducer structure according to a fourth modified example of the first embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] In the following, the present invention will be described in more detail using advantageous embodiments of exemplary aspects with reference to the drawings. It should be noted that the described embodiments are merely possible configurations, and that when implementing the present invention, the individual features mentioned above can be provided independently of one another or can be omitted entirely.

[0038] FIG. 1a shows a plan view of a transducer structure according to a first embodiment of the present invention.

[0039] The transducer structure 100 includes a pair of interdigitated interdigitated electrodes 102 and 112 disposed on a piezoelectric substrate 120. The piezoelectric substrate 120 may be a bulk piezoelectric substrate or may be a composite substrate including a piezoelectric layer above a base substrate, where the composite substrate is described below with respect to FIG.

[0040] The first interdigitated electrode 102 includes a plurality of electrode fingers 104 and a plurality of dummy electrode fingers 106, both extending from a first busbar 108. Similarly, the second interdigitated electrode 112 includes a plurality of electrode fingers 114 and a plurality of dummy electrode fingers 116, both extending from a second busbar 118. The dummy electrode fingers 106 of the first busbar 108 face the electrode fingers 114 of the second busbar 118 and are spaced apart from the electrode fingers 114 of the second busbar 118 by a first gap 110a. The dummy electrode fingers 116 of the second busbar 118 face the electrode fingers 104 of the first busbar 108 and are spaced apart from the electrode fingers 104 of the first busbar 108 by a second gap 110b. Here, the first gap 110a and the second gap 110b are the same.

[0041] According to a variation of the present invention, the second comb electrode 112 does not have dummy electrode fingers 116 extending from the second bus bar 118, but only a plurality of electrode fingers 116. In this case, the second gap 110b is defined by the distance between the second bus bar 118 and the counter electrode 104 of the first comb electrode 102.

[0042] According to a variant of the invention, not all of the electrode fingers 104 of the first busbar 108 and / or the electrode fingers 114 of the second busbar 118 have opposing dummy electrode fingers 106, 116 of the corresponding busbars 108, 118, and vice versa.

[0043] In a variant, the first gap 110a and the second gap 110b may vary throughout the transducer structure 100, and in particular, the first gap 110a and the second gap 110b may vary randomly in length or position within the transducer structure 100.

[0044] The interdigitated electrodes 102, 112 may be formed from any suitable conductive metal, for example, aluminum or an aluminum alloy such as Al-Cu, Al-Ti, or Al-Si.

[0045] The two interdigitated interdigitated electrodes 102, 112 are connected to opposite potentials +V / -V. The transducer structure 100 has an electrode pitch p defined by the Bragg condition, so that p is equal to λ / 2, where λ is the operating wavelength of the transducer structure 100. The pitch p represents the distance between the centers of adjacent electrode fingers of the interdigitated electrodes. Thus, in this embodiment, p corresponds to the distance between the centers of the electrode fingers 104 of the electrode 102 and the centers of the adjacent electrode fingers 114 of the electrode 112. The transducer structure 100 is synchronous, i.e., the interdigitated electrode fingers 104, 114 exhibit the same width, the same period, and the same shape.

[0046] The electrode fingers 104, 114 and the dummy electrode fingers 106, 116 have a width a. According to the art, the metal aspect ratio is defined as a / p.

[0047] The electrode fingers 104, 114 of each of the interdigitated electrodes 102, 112 are identical and have the same width a and the same length l1. In a variant, the interdigitated electrodes can have different electrode fingers 102, 112.

[0048] Similarly, the dummy electrode fingers 106, 116 of the interdigitated electrodes 102, 112 are also identical and have the same width a and the same length l1. In a variant, the interdigitated electrodes 102, 112 can have different dummy electrode fingers 106, 116.

[0049] 1a, the interdigitated electrodes 102, 112 are the same, i.e., the interdigitated electrodes 102, 112 have the same electrode fingers, the same pitch p, the same first gap 110a, and the same second gap 110b. In a variant, the interdigitated electrodes 102, 112 can be different, having a different pitch p, different gaps, different electrode fingers 102, 112, and different dummy electrode fingers 106, 116.

[0050] According to the invention, two transverse mode suppression layers 122 and 132 are provided between the substrate 120 and the interdigitated interdigitated electrodes 102, 112. The first transverse mode suppression layer 122 is located below the first gap 110a and the second transverse mode suppression layer 132 is located below the second gap 110b. The physical properties of the transverse mode suppression layers are selected such that the phase velocity of the guided waves of the transducer structure is smaller in the region of the transverse mode suppression layers 122, 132 compared to the phase velocity of the guided waves of the transducer structure in the piezoelectric substrate 120 in the remaining region 134 forming the central region 136 below the interdigitated electrode fingers of the first and second electrodes. The guided waves can be any guided wave or can be a true mode of the surface, including Rayleigh-like waves, shear waves, pure or quasi-pure shear or longitudinal waves.

[0051] In a variation of the embodiment, there is only one transverse mode suppression layer in the transducer structure.

[0052] In this embodiment, the first transverse mode suppression layer 122 and the second transverse mode suppression layer 132 are rectangular and have the same size and shape. The regions 122 and 132 extend along the direction y under all of the electrode fingers 104, 114 and all of the dummy electrode fingers 106, 116 of the transducer structure 100, and thus along the entire length w of the transducer structure 100. The regions 122 and 132 are symmetrical about the centerline y of the interdigitated electrodes 102, 112 of the transducer structure 100.

[0053] The first transverse mode suppression layer 122 and the second transverse mode suppression layer 132 extend a length d along a direction x, which is the direction of propagation of unwanted transverse acoustic waves in the transducer structure 100. In Figure 1a, the transverse mode suppression layer 122 extends across the first gap 110a, across a portion of the dummy electrode finger 106, and across a portion of the electrode finger 104. Similarly, the transverse mode suppression layer 132 extends across the second gap 110b, across a portion of the dummy electrode finger 116, and across a portion of the electrode finger 114.

[0054] 1a, the first transverse mode suppression layer 122 and the second transverse mode suppression layer 132 are disposed at the same distance s in the X direction from the bus bars 108, 118. In a variant, the first transverse mode suppression layer 122 and the second transverse mode suppression layer 132 can have different distances to the bus bars.

[0055] FIG. 1b shows a cross-sectional side view along line AA in FIG. 1a illustrating a transducer structure 100 according to a first embodiment of the present invention.

[0056] In a particular embodiment, the support substrate 120 is a composite substrate including a piezoelectric layer 120 disposed on top of a base substrate 144. A thin dielectric layer 142 is deposited between the piezoelectric layer 140 and the base substrate 144. In a variant, the piezoelectric layer 140 can be deposited directly on top of the base substrate 144.

[0057] Such composite substrates not only allow constructions that allow frequency changes to be obtained with optimized temperature, but also open the possibility of using guided waves. The modes of the structure can exhibit different types of polarization, in particular elliptical polarization, but also predominantly shear waves or pure or almost pure longitudinal compression waves, the electromechanical coupling of which is significantly greater than 7%, allowing the realization of filters with the bandwidth required in modern filter applications, in particular with more than 5%.

[0058] In an advantageous embodiment, the thickness of the piezoelectric layer 140 (referred to as d_piezo) is on the order of or smaller than the wavelength λ, i.e., smaller than 0.7 times the value of the wavelength λ, or even smaller than 0.5 times the value of the wavelength λ, where λ is the operating wavelength of the transducer structure 100.

[0059] In practical cases, the thickness of the piezoelectric layer is smaller than 1 μm, in particular smaller than 700 nm. For a selected operating frequency, the thickness of the piezoelectric layer is optimized in order to optimize the guiding function of the waves used. For a given thickness of the piezoelectric layer that allows mainly guiding of waves at the surface, the invention is particularly interesting since parasitic transverse modes become more prominent due to the guiding of waves and due to the possibility of in-phase reflections in such type of waves at the electrode lateral sides. In this embodiment, the first transverse mode suppression layer 122 and the second transverse mode suppression layer 132 are passivation layers deposited on the piezoelectric layer 140 before the interdigitated electrodes 102, 112 are deposited on the substrate 120 and therefore before the interdigitated electrodes 102, 112 are deposited on the piezoelectric layer 140. The passivation layers are in direct contact with the piezoelectric layer 140.

[0060] The passivation layer is a dielectric layer, for example a SiO2 layer or a Ta2O5 layer or a HfO2 layer.

[0061] The transverse mode suppression layer 122, also called the first transverse mode suppression layer, is located on the right hand side in FIG. 1b and is completely covered by the electrode fingers 104 connected to the busbar .

[0062] The second transverse mode suppression layer 132, located on the left side in FIG. 1b, extends beneath the tips 116a of the dummy electrode fingers 116, which are connected to the bus bar 118, the second gap 110b and the tips 104a of the electrode fingers 104 which are connected to the bus bar 108 at connection portion 134.

[0063] In a variant of the present invention, the passivation layer has sloping sides or sloping wall edges, so that when the comb electrode completely covers the passivation layer, the deposited comb electrode does not break at the wall edges of the passivation layer, thereby ensuring that the electrode covers the passivation layer regularly and continuously.

[0064] In a variant, instead of using an additional passivation layer, the first lateral mode suppression layer 122 and the second lateral mode suppression layer 132 can be formed in the piezoelectric layer 140, for example by having a different doping amount compared to a central region 136 of the piezoelectric layer 140, and further by providing a space with a width s between the lateral mode suppression layers 122, 132 and the busbar. Doping corresponds here to the intentional introduction of impurities into the piezoelectric substrate. The different doping at least in the lateral mode suppression layers reduces the phase velocity of the modes near the gap separating the IDT electrodes from the dummy electrodes, without the need for any lateral patterning.

[0065] The thickness of the suppression layer(s) is at least 1 / 20 of the value of the wavelength λ, in particular at least 1 / 10 of the value of the wavelength λ.

[0066] The suppression layer(s) to be implemented advantageously extend to a certain depth (d_supp) in order to optimize the suppression of the transverse modes. Increasing the thickness to which the suppression layer extends may be detrimental to the desired suppression effect and may result in energy leakage due to diffraction of the energy of the main mode, reducing its quality factor. It is therefore advantageous in certain embodiments to limit the thickness of the suppression layer to a maximum of 1 / 2 the value of the wavelength λ, in particular to a maximum of 1 / 3 the value of the wavelength λ. The optimum thickness of the suppression layer found is evaluated in relation to the wavelength λ and defines the depth to which the piezoelectric layer is modified at the level of the suppression layer compared to its initial thickness.

[0067] In this way, an optimum-thickness window is obtained as follows.

[0068] The thickness d_supp of the suppression layer is configured between 1 / 20λ < 1 / 10λ < d_supp < 1 / 3λ < 1 / 2λ while considering d_supp < d_piezo < 0.7λ < 1λ.

[0069] In an actual case where the wavelength λ is around 4 μm, a relationship of 200 nm < 400 nm < d_supp < 1.3 μm < 2 μm and d_supp < d_piezo < 2.8 μm < 4 μm is obtained. In another actual case where the wavelength is around 1 μm, a relationship of 50 nm < 100 nm < d_supp < 300 nm < 500 nm and d_supp < d_piezo < 700 nm < 1 μm will be obtained.

[0070] The suppression of the transverse mode proposed by the present invention is also affected by the thickness of the comb-shaped electrode. A particularly interesting configuration is to select the thickness of the electrode, in particular the thickness of the Al_Cu electrode having 0.5 to 2% Cu doping, between 5 to 20% of the value of the wavelength λ. In that case, it is advantageous that the thickness of the (one or more) suppression layers proposed by the present invention is selected to be of the same order as the thickness of the comb-shaped electrode, in particular ±50% of the nominal value. In this thickness window, at least 90% of the transverse modes that exist when there is no suppression layer are suppressed by the suppression layer.

[0071] The transducer structure 100 according to the first embodiment functions in the following manner. The transducer structure according to the present invention is used to excite and detect shear waves. The shear displacement direction is alternating from one electrode to another when the transducer is excited by a +V / -V electric polarization structure because the device operates under Bragg conditions.

[0072] The presence of the first 122 and second 132 transverse mode suppression layers creates locations where SAW propagation is modified such that guided waves propagating into the regions with the wave suppression layers 122, 132 exhibit a smaller phase velocity compared to guided waves propagating through the central region 1326 of the unmodified substrate 120. The first 122 and second 132 transverse mode suppression layers modify the velocity of modes near the first 110a and second 110b gaps between the active electrode fingers 104 and their associated dummy electrode fingers 116. Slowing the wave velocity in the gaps 110a, 110b results in the suppression or reduction of unwanted transverse modes.

[0073] Thus, the contribution of transverse modes can be reduced in the transducer structure 100 according to the present invention, and at the same time, the manufacturing process is simplified compared to the transducer structures according to the prior art.

[0074] Although the local modification of the substrate 120 in or near the first and second gaps 110a and 110b according to the present invention requires additional manufacturing steps, the normal SAW front-end manufacturing process remains unchanged, simplifying the fabrication of the transverse mode releasing device. Moreover, compared to other solutions that require precise registration of subsequent layers, here a rough mask registration is sufficient to realize the fabrication of the device.

[0075] As shown below, various shapes and various positions of the first transverse mode suppression layer 122 and the second transverse mode suppression layer 132 with respect to the transducer structure 100 are shown in alternative embodiments.

[0076] For the reference symbols already used in the description of the first embodiment, reference may be made to that description, although the description will not be repeated.

[0077] FIG. 2a shows a side view of a transducer structure 200a according to a first variant of the invention.

[0078] Compared to the first embodiment, the passivation layers 246, 248 forming the first lateral mode suppression layer 122 and the second lateral mode suppression layer 132 extend below the busbars 108, 118. The passivation layer 248 extends completely below the busbar 118, whereas the other passivation layer 246 extends only partially below the busbar 108. In this variant, the first lateral mode suppression layer 122 and the second lateral mode suppression layer 132 are not symmetrical.

[0079] In another variation, the first transverse mode suppression layer 122 and the second transverse mode suppression layer 132 can be fully or partially embedded within the piezoelectric layer 140 .

[0080] FIG. 2b shows a side view of a transducer structure 200b according to a second variant of the invention.

[0081] As mentioned in the description of the first embodiment, the first lateral-mode suppression layer 222 and the second lateral-mode suppression layer 232 of the piezoelectric layer 140 in this modification are locally structurally modified piezoelectric layer 140. For example, the structural modification of the piezoelectric layer 140 corresponds to a different doping amount in the lateral-mode suppression layers 222, 232 compared to the remainder of the piezoelectric layer 140.

[0082] FIG. 2c shows a third variant of the transducer structure 200c of the first embodiment.

[0083] Compared to the second variant, the first transverse mode suppression layer 222 and the second transverse mode suppression layer 232 only extend partially below the busbars 108, 118. The first transverse mode suppression layer 222 and the second transverse mode suppression layer 232 are symmetrical.

[0084] FIG. 2d shows a side view of a transducer structure 200d according to a fourth variant of the invention.

[0085] Unlike the previous three variations, the first transverse mode suppression layer 232 extends partially under the busbar 118, while the second transverse mode suppression layer 222 does not extend at all under its corresponding busbar 108. Again, the locally modified regions 222 and 232 are not the same.

[0086] Figures 3a to 3f show further variants of the lateral mode suppression layer in plan view of the transducer structure, all of which show different geometric shapes of the lateral mode suppression layer.

[0087] Figure 3a shows a plan view of a transducer structure according to a fifth variant of the first embodiment of the invention. In comparison with the embodiment shown in Figure 1a, here the transverse mode suppression layer 222 extends on one side at least up to and even below the busbar 108, while on the other side the transverse mode suppression layer 232 is located at a distance s from the busbar 118, as in the first embodiment. As a result, the transverse mode suppression layers 222 and 232 have different dimensions and are not symmetrical.

[0088] In FIG. 3b a sixth variant is shown in which the transverse mode suppression layers 222 and 232 are tapered.

[0089] FIG. 3c shows a plan view of the transducer structure, in particular of the transducer structure according to the second variant shown in FIG. 2b, in which both lateral mode suppression layers 222, 232 extend below the busbars 208, 218 and are symmetrical in shape and size.

[0090] In a seventh variant shown in Figure 3d, compared to the variant shown in Figure 3c, both lateral mode suppression layers 222, 232 extend even further outward in the X direction than the busbars 208, 218. In the variant, only one of the lateral mode suppression layers 222, 232 extends further outward than the busbars 208, 218.

[0091] In an eighth variant shown in FIG. 3e, compared to the embodiment shown in FIG. 1a, the transverse mode suppression layers 222, 232 have a convex or concave shape, which can improve mode confinement due to less energy being radiated at the edges of the interdigitated transducers.

[0092] In a ninth variant, different shapes can be used for the transverse mode suppression layers 222, 232, as shown in Fig. 3f. In Fig. 3f, the shape of the second transverse mode suppression layer 232 is rounded, whereas the shape of the first transverse mode suppression layer 222 is sharp. The rounded shape of the transverse mode suppression layer 232 is rounded at different values ​​of angle to break any possible synchronization effect. This is also the case for sharp patterns, the angle being chosen so that no coherence occurs from one electrode edge to another.

[0093] FIG. 4 shows a schematic diagram of the different steps in a method for manufacturing a transducer structure according to a first embodiment of the invention or any of its variants.

[0094] In step a), a piezoelectric substrate 420 is prepared. In Fig. 4, the piezoelectric substrate 420 is a composite substrate, which comprises a piezoelectric layer 440 on a base substrate 444. In this embodiment, a thin SiO2 layer 442 is also present between the base substrate 444 and the piezoelectric layer 440. The piezoelectric substrate 420 is the same as the piezoelectric substrate 120 described in the first embodiment.

[0095] According to a first variant 1), in at least one region of the piezoelectric substrate, the piezoelectric layer 440 is doped differently compared to the remainder of the piezoelectric substrate, thereby providing two transverse mode suppression layers 422, 432.

[0096] This step involves implanting or diffusing atomic species, in particular Ti, in the piezoelectric substrate to modify the atomic species concentration of the transverse mode suppression layer.

[0097] According to a further variant, the step of providing a transverse mode suppression layer can be a proton exchange process.

[0098] The proton exchange process involves an elementary proton exchange from an organic proton source and a post-annealing treatment in which the sample is heated alone to redistribute the lithium and hydrogen ions.

[0099] The proton exchange technique is described in Chung et al., “Proton-Exchanged 36° YX LiTaO3 Waveguides for Surface Acoustic Wave”, IEEE transactions on UFFC, vol. 53, no. 2, 2006. By using PE, the piezoelectric layer can be locally modified. The proton exchange technique is interesting as an industrial approach to obtain the expected effects due to its complete compatibility with planar technology processes used in the SAW industry, as it does not affect the surface roughness.

[0100] According to a second variant 2) of the method, the step of providing a lateral mode suppression layer is a passivation layer deposition step. The deposition layer can be a dielectric passivation layer, more particularly a SiO2 layer.

[0101] A passivation layer 446 is deposited on the piezoelectric substrate 424 , thereby forming the first transverse mode suppression layer 422 and the second transverse mode suppression layer 432 .

[0102] According to a third variant 3), the passivation layer can be at least partially embedded in the piezoelectric substrate 440. This variant includes an etching step to remove parts of the piezoelectric substrate 440 in the regions 422, 432, or any other process that can reduce the thickness t of the piezoelectric substrate 440 in the regions 422, 432. A passivation is then deposited locally or over the entire substrate, and a chemical-mechanical polishing (CMP) step is applied to restore a perfectly flat surface compatible with the usual requirements in SAW manufacturing.

[0103] A combination of layer deposition and patterning steps is then used to form a pair of interdigitated interdigitated electrodes on the piezoelectric substrate, as shown in FIG. 4c.

[0104] According to a fourth variant of the invention, as shown in Fig. 5, the passivation layer can be patterned by a lift-off process, so that the lateral sides of the passivation layer can be inclined so that the deposited comb electrodes cover them without breaking, which leads to a regular and continuous reduction of the phase velocity in the corresponding areas. In this case, no etching is required, so that the substrate can be kept in good condition. However, registration marks must be formed to prepare the surface for this purpose according to the usual technological processes.

[0105] In the first step a), the POI substrate 420 is cleaned by O2 plasma process. Then, a photoresist film 448 is deposited (step b), in particular by spin coating. In step c), a UV lithography step is performed on the photoresist film 448 through a UV mask 450, thereby obtaining in step d) a patterned photoresist film consisting of a number of structures 452 with sloping wall edges. In step e), a passivation layer 446 is deposited on the POI substrate 420 and on the patterned photoresist film 450, thereby forming transverse mode suppression layers 422, 432 between the structures 452 of the patterned photoresist film 450. Due to the sloping wall edges of the structures 452 of the patterned photoresist film 450, the deposition of the passivation layer 446 on the POI substrate 420, as shown in step e), results in the passivation layer 446 being deposited between the structures 452 of the patterned photoresist film 448, which also have sloping wall edges. Finally, step f) is performed to remove the structures 452 of the patterned photoresist film 450 with the passivation layer 446 on their upper surface, thereby leaving only the passivation layer 446 on the surface of the POI substrate 420 between the structures 452 of the patterned photoresist film 450 that are directly on the POI substrate 420.

[0106] In accordance with the method of the present invention, interdigitated electrodes are formed on a transverse mode suppression layer and on a piezoelectric substrate, the transverse mode suppression layer being formed prior to the formation of the interdigitated electrodes, as opposed to prior art devices in which the transverse mode suppression layer is formed on the interdigitated electrodes after the interdigitated electrodes are formed on the piezoelectric substrate.

[0107] According to a variant of the invention, the interdigitated electrodes are formed on the first and / or second transverse mode suppression layer and on the piezoelectric substrate, so that the transverse mode suppression layer is formed before the interdigitated electrodes are formed, which is opposite to prior art devices in which the interdigitated electrodes are formed on the piezoelectric substrate and then the transverse mode suppression layer is formed on the interdigitated electrodes.

[0108] Various embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications and extensions can be made without departing from the scope of the following claims.

Claims

1. 1. A transducer structure with transverse mode suppression means, comprising: A piezoelectric substrate (120, 420); At least one pair of interdigitated comb electrodes (102, 112) formed on the piezoelectric substrate (120, 170), a first comb electrode (102) comprising a first bus bar (108) and a number of electrode fingers (104) extending from the first bus bar (108) interleaved with a number of short dummy electrode fingers (106) extending from the first bus bar (108), a second comb electrode (112) comprising a second bus bar (118) and a number of short dummy electrode fingers (106) extending from the first bus bar (108) interleaved with a number of short dummy electrode fingers (106) extending from the first bus bar (108), at least one pair of interdigitated comb electrodes (102, 112) comprising a plurality of electrode fingers (114) extending from a bus bar (118), the plurality of short dummy electrode fingers (106) of the first bus bar (108) facing the plurality of electrode fingers (114) of the second bus bar (118) and spaced apart from the plurality of electrode fingers (114) by a plurality of first gaps (110a); a second interdigitated electrode (112) further comprising a plurality of short dummy electrode fingers (116) interleaved with the plurality of electrode fingers (114), the plurality of short dummy electrode fingers (116) of the second busbar (118) facing the plurality of electrode fingers (104) of the first busbar (108) and spaced apart from the plurality of electrode fingers (104) by a plurality of second gaps (110b), a first transverse mode suppression layer (122, 132, 222, 232, 422, 432) partially disposed under the first plurality of gaps (110a), the first transverse mode suppression layer (122, 132, 222, 232, 422, 432) being selected such that a phase velocity of a guided wave in a region of the first transverse mode suppression layer (122, 132, 222, 232, 422, 432) is smaller than a phase velocity of the guided wave in a central region (136) below the interleaved electrode fingers (104, 114) of the first comb electrode and the second comb electrode (102, 112); a second transverse mode suppression layer (122, 132, 222, 232, 422, 432) extending at least partially beneath the second plurality of gaps (110b), the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) being selected such that a phase velocity of the guided wave in a region of the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) is smaller than a phase velocity of the guided wave in the central region (136) beneath the interleaved electrode fingers (104, 114) of the first and second comb electrodes (102, 112) and between the first and second transverse mode suppression layers (122, 132, 222, 232, 422, 432); Further equipped with A transducer structure, characterized in that the first transverse mode suppression layer and the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) have different geometric shapes.

2. The transducer structure of claim 1 , wherein the first transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends beneath each of the plurality of first gaps (110a).

3. The transducer structure of claim 1 , wherein the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends under all of the plurality of second gaps (110b).

4. 2. The transducer structure of claim 1, wherein the first transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends at least partially to the first busbar (108) and / or the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends at least partially to the second busbar (118).

5. 2. The transducer structure of claim 1, wherein the first transverse mode suppression layer (122, 132, 222, 232, 422, 432) further extends at least partially or at least entirely beneath the first busbar (108) and / or the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends at least partially or at least entirely beneath the second busbar (118).

6. 2. The transducer structure of claim 1, wherein the first and / or second transverse mode suppression layers (222, 232, 422, 432) are formed within a portion of the piezoelectric substrate (120, 420) to modulate the phase velocity of the guided waves of the transducer structure, and the first and / or second transverse mode suppression layers (222, 232, 422, 432) are formed by having a different doping amount compared to other remaining areas (134) of the piezoelectric substrate (120, 420) forming the central area (136) below the at least one pair of interdigitated comb electrodes (102, 112).

7. The transducer structure of claim 6, wherein the doping amount is a Ti amount.

8. 2. The transducer structure of claim 1, wherein the first transverse mode suppression layer (122, 132, 222, 232, 422, 432) and / or the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) comprises a passivation layer (246, 248).

9. The transducer structure of claim 8, wherein the passivation layer (246) is at least partially embedded into the piezoelectric substrate (120, 420).

10. The transducer structure of claim 1 , wherein the piezoelectric substrate (120, 420) is a composite substrate comprising a piezoelectric layer (140, 440) on a base substrate (144, 444).

11. 11. The transducer structure of claim 10, wherein the thickness of the piezoelectric layer (140, 440) is less than the value of the wavelength λ, where λ is the operating wavelength of the transducer structure (100).

12. The transducer structure of claim 11, wherein the thickness of the piezoelectric layer (140, 440) is less than 0.7 times the value of the wavelength λ.

13. 13. The transducer structure of claim 12, wherein the thickness of the piezoelectric layer (140, 440) is less than 0.5 times the value of the wavelength λ.

14. 12. The transducer structure of claim 11, wherein the thickness of the first and second transverse mode suppression layers (122, 132, 222, 232, 422, 432) is greater than 1 / 20 of the wavelength λ and less than the thickness of the piezoelectric layer (140, 440).

15. 15. The transducer structure of claim 14, wherein the thickness of the first and second transverse mode suppression layers (122, 132, 222, 232, 422, 432) is greater than 1 / 10 of the wavelength λ and less than the thickness of the piezoelectric layer (140, 440).

16. 16. A transducer structure according to claim 15, wherein the thickness of the first and second transverse mode suppression layers (122, 132, 222, 232, 422, 432) corresponds at most to half the value of the wavelength λ.

17. 17. A transducer structure according to claim 16, wherein the thickness of the first and second transverse mode suppression layers (122, 132, 222, 232, 422, 432) corresponds at most to 1 / 3 of the value of the wavelength λ.

18. 2. The transducer structure of claim 1, wherein the thickness of the first and second transverse mode suppression layers (122, 132, 222, 232, 422, 432) is ±50% of the nominal thickness of the interdigitated electrode (102, 112).

19. 20. The transducer structure of claim 18, wherein the interdigitated electrodes have a thickness of 5-20% of the value of the wavelength λ, where λ is the operating wavelength of the transducer structure (100), and are Al_Cu electrodes with Cu doping of 0.5-2%.

20. 2. The transducer structure of claim 1, wherein the guided wave is an elliptically polarized, shear wave, pure or nearly pure longitudinal compressional wave, and has electromechanical coupling greater than 7%.

21. 1. A method for manufacturing a transducer structure, comprising: a) providing a piezoelectric substrate; b) forming at least a pair of interdigitated comb electrodes on the piezoelectric substrate, a first comb electrode comprising a first bus bar and a plurality of interdigitated electrode fingers extending from the first bus bar and a plurality of short dummy electrode fingers extending from the first bus bar, and a second comb electrode comprising a second bus bar and a plurality of interdigitated electrode fingers extending from the second bus bar, the plurality of short dummy electrode fingers of the first bus bar facing the plurality of electrode fingers of the second bus bar and spaced apart from the plurality of electrode fingers by a plurality of first gaps; the second interdigitated electrode further comprising a plurality of short dummy electrode fingers interleaved with the plurality of electrode fingers, the plurality of short dummy electrode fingers of the second busbar opposing the plurality of electrode fingers of the first busbar and spaced apart from the plurality of electrode fingers of the first busbar by a plurality of second gaps, 2. The method of claim 1, further comprising: a step c) of providing first and second transverse mode suppression layers prior to step b), and after step c), the first transverse mode suppression layer is disposed partially beneath the first gap and the second transverse mode suppression layer extends at least partially beneath the second gaps, the first and second transverse mode suppression layers being selected such that a phase velocity of the guided wave in regions of the first and second transverse mode suppression layers is smaller compared to a phase velocity of the guided wave in the piezoelectric substrate in a central region beneath the interleaved electrode fingers of the first and second interdigital electrodes and between the first and second transverse mode suppression layers, and the first and second transverse mode suppression layers have different geometric shapes.

22. The method of claim 21 , wherein the piezoelectric substrate is a piezoelectric composite substrate comprising a piezoelectric layer above a base substrate.

23. 22. The method of claim 21, wherein the step c) of providing the first transverse mode suppression layer comprises modifying a doping amount of the piezoelectric substrate to modulate the phase velocity of the guided wave of the transducer structure.

24. 24. The method of claim 23, wherein the modifying the doping amount comprises an implantation or diffusion step of an atomic species and / or a proton exchange technique.

25. 22. The method of claim 21, wherein the step c) of providing the first transverse mode suppression layer comprises providing a passivation layer.

26. 26. The method of claim 25, wherein step c) comprises at least partially embedding the passivation layer into the piezoelectric substrate.

27. 26. The method of claim 25, wherein the passivation layer is patterned by a lift-off process, whereby edgewall profiles of the passivation layer are made sloped.

28. A SAW device comprising a transducer structure according to any one of claims 1 to 20.

29. A single-port resonator comprising a transducer structure according to any one of claims 1 to 20.

Images