Transducer structure for single-port resonator
The transducer structure with mode suppression layers under the gaps between electrodes addresses the challenge of parasitic transverse modes in single-port resonators, achieving effective suppression and improved spectral purity through a simpler manufacturing process.
Patent Information
- Application Number
- FR2023010007
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-09-21
- Publication Date
- 2025-09-05
- Estimated Expiration
- 2039-12-30
AI Technical Summary
Existing transducer structures for single-port resonators struggle with parasitic transverse modes that degrade spectral purity and require complex, costly manufacturing processes to suppress.
A transducer structure with transverse mode suppression layers under the gaps between interdigitated comb electrodes, altering the phase velocity to reduce unwanted transverse modes without damaging the interdigitated electrodes, using simpler manufacturing methods.
Simplifies manufacturing while effectively suppressing transverse modes, preserving the intrinsic quality of the device and substrate surface, and enhancing spectral purity.
Smart Images
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Abstract
Description
Title of the invention: Transducer structure for single-port resonator
[0001] The invention relates to a transducer structure having transverse mode suppression means, in particular for a single-port resonator, and a method of manufacturing such a transducer.
[0002] The invention relates to the field of surface acoustic wave (SAW) devices made on piezoelectric single crystals or on composite substrates using a piezoelectric film, called POI, for piezoelectric-on-insulator. These composite wafers are used as waveguides for the excitation of true modes using interdigital transducers (IDT) for radio frequency (RF) filter fabrication. The transducer structure is a key part of so-called single-port resonators which are used as the impedance element of the mentioned filters. More precisely, so-called ladder filters exploit said resonators to provide the filter function.Similarly, single-port resonators, as used for sensor applications, and more specifically for wireless sensing, are affected by such effects, with means often being required to suppress them, for example by aperture narrowing (see e.g. EP 2 091 146 B1), a solution known to degrade the electromechanical coupling of the device. Single-port resonators comprise a transducer structure mounted in the center of the resonator with reflectors positioned on both sides of the transducer structure, while dual-port SAW resonators consist of two transducer structures (input / output) mounted in the center with reflectors arranged on both outer sides of the device.
[0003] Other types of SAW devices may benefit from the invention, such as longitudinally coupled resonator filters (LCRFs), dual mode SAW filters (DMS) or dual port SAW resonators, or delay lines used for example in oscillator stabilization for frequency source applications or in any other application (gravimetric sensing, for example).
[0004] Single-port resonators on POIs exhibit parasitic contributions that are due to lateral energy capture, producing so-called transverse modes that pollute the spectral signature of the device. It has therefore been proposed to provide means for suppressing these contributions in order to improve the spectral purity of the resonator response, and to enable the fabrication of a filter compatible with a specification, and more generally to manufacture a SAW device.
[0005] A difficulty in the state of the art is to manufacture structures capable of suppressing transverse modes.
[0006] It has been proposed to reload the electrode edge with metallic or dielectric coatings, as described in US 2012 / 0161577 A1 or in US 2013 / 0249647 A1.
[0007] In US 2012 / 0161577 Al, a metal is redeposited on top of the electrode edge, requiring significant pattern alignment precision, thus requiring expensive processing and control tools.
[0008] In US 2013 / 0249647 A1, the entire interdigital transducer (IDT) structure is covered with a dielectric layer that is thicker at the edge of the transducer than in the middle. In US 2015 / 0123746 A1, a dielectric film covers at least a portion of the piezoelectric substrate and the IDT electrodes, more specifically covers an area in which the electrode fingers are intermingled together, the acoustic velocity being greater in the central portion of the IDT finger than at the edge, to prevent the occurrence of said transverse modes. In most applications using a dielectric layer, the need to pattern the latter using acids or reactive ion etching or ion milling leads to degradation of the electrode quality when no etch stop solution is applied to prevent such an effect, or even leads to degradation of the substrate surface if it is not properly protected.
[0009] Other approaches are based on enlarging the electrode dimensions, as described in US 2018 / 0097508 A1 or US 2018 / 0375491 A1. These structures generate difficulties in ensuring the insulation of one electrode from another.
[0010] However, in all the approaches cited above, for these transducer structures, transverse modes, although reduced, are still possible.
[0011] The aim of the invention is thus to provide a transducer structure for single-access resonators with transverse mode suppression using a simpler manufacturing process compared to prior art devices while preserving the intrinsic quality of the device and the substrate surface.
[0012] The object of the invention is achieved by means of a transducer structure having transverse mode suppression means, in particular for a single-port resonator, comprising a piezoelectric substrate, in particular a piezoelectric composite substrate comprising a piezoelectric layer on a base substrate, at least one pair of interdigitated comb electrodes formed on the piezoelectric substrate, in particular a piezoelectric composite substrate comprising a piezoelectric layer on a base substrate, wherein the first comb electrode comprises a first bus bar and a plurality of electrode fingers alternating and shorter dummy electrode fingers, all extending from the bus bar, wherein the second comb electrode comprises a second bus bar and a plurality of electrode fingers extending from the second bus bar, the dummy electrodes of the first bus bar face the electrode fingers of the second bus bar and are separated from the electrode fingers of the second bus bar by first gaps, characterized in that it further comprises a transverse mode suppression layer provided partially under the first gap and selected such that the phase velocity of a guided wave is lower in the area of the transverse mode suppression layer than the phase velocity of the guided wave in the piezoelectric substrate in the central area under the alternating electrode fingers of the first and second comb electrodes.The presence of the transverse mode suppression layer leads to a slowing down of the wave velocity in space, which leads to the suppression or reduction of the unwanted transverse mode in the transducer structure. Thus, in the transducer structure according to the invention, the contribution of the transverse modes can be reduced. From an acoustic point of view, the transducer structure according to the invention makes it possible to transfer energy from the IDT finger to the dummy finger without reflection, or with reduced reflection, at the edge of said IDT finger.
[0013] In an alternative embodiment, the transverse mode suppression layer may extend beneath each of the first gaps. The transverse mode suppression layer allows for slowing down the wave velocity in the gap, which leads to the suppression or reduction of the unwanted transverse mode in the transducer structure. From an acoustic point of view, the transducer structure according to the invention allows for transferring energy from the IDT finger to the dummy finger without reflection, or with reduced reflection, at the edge of said IDT finger.
[0014] In an alternative embodiment, the second comb electrode may have shorter dummy electrode fingers alternating with the electrode straights, the dummy electrode fingers of the second bus bar face the electrode fingers of the first bus bar and are separated from the electrode fingers by second gaps, and further comprising a second transverse mode suppression layer extending at least partially under the second gaps and selected such that the phase velocity of a guided wave is lower in the area of the second transverse mode suppression layer than the phase velocity of the guided wave in the central area under the alternating electrode fingers of the first and second comb electrodes, between the first and second transverse mode suppression layers.The transverse mode suppression layer helps to slow down the wave velocity in space, which leads to the suppression or reduction of the unwanted transverse mode in the transducer structure. Thus, in . the transducer structure according to the invention, the contribution of transverse modes can be further reduced. From an acoustic point of view, the transducer structure according to the invention makes it possible to transfer energy from the IDT finger to the dummy finger without reflection, or with reduced reflection, at the edge of said IDT finger.
[0015] In an alternative embodiment, the transverse mode suppression layer may extend beneath all of the second gaps. The transverse mode suppression layer allows the wave velocity in the gap to be slowed down, which leads to the suppression or reduction of the unwanted transverse mode in the transducer structure. From an acoustic point of view, the transducer structure according to the invention allows energy to be transferred from the IDT finger to the dummy finger without reflection, or with reduced reflection, at the edge of said IDT finger.
[0016] In an alternative embodiment, the first transverse mode suppression layer may extend at least partially to the first bus bar and / or the second transverse mode suppression layer may extend at least partially to the second bus bar.
[0017] In an alternative embodiment, the first transverse mode suppression layer may extend at least partially or at least completely under the first bus bar and / or the second transverse mode suppression layer may extend at least partially or at least completely under the second bus bar.
[0018] In a variant of the embodiment, the first and / or second transverse mode suppression layer may comprise a different doping dose compared to the rest of the piezoelectric substrate in the central area under the interdigitated comb electrodes to modulate the phase velocity, in particular the different doping dose is a dose of Ti. The doping here corresponds to an intentional introduction of impurities into the piezoelectric substrate. The difference in doping of the at least one transverse mode suppression layer results in a reduction of the phase velocity of the mode in the area close to the gap separating the IDT electrodes compared to the dummy electrodes without the need to provide any lateral pattern.
[0019] In a variant of the embodiment, the first and / or second transverse mode suppression layer of the piezoelectric layer may comprise a passivation layer, in particular a dielectric passivation layer, more particularly a SiO2 layer.
[0020] In an alternative embodiment, the passivation layer may be at least partially integrated into the piezoelectric substrate.
[0021] In an alternative embodiment, the first transverse mode suppression layer and the second transverse mode suppression layer may be symmetrical about the centerline of the interdigitated comb electrodes.
[0022] In an alternative embodiment, the first transverse mode suppression layer and the second transverse mode suppression layer may have different shapes.
[0023] The object of the invention is also achieved by means of 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 on a base substrate, b) forming at least one pair of interdigitated comb electrodes on the piezoelectric substrate, wherein the first comb electrode comprises a first bus bar and a plurality of alternating electrode fingers and shorter dummy electrode fingers, all extending from the first bus bar, the second comb electrode comprises a second bus bar and a plurality of alternating electrode fingers extending from the second bus bar,the dummy electrodes of the first bus bar face the electrode fingers of the second bus bar and are separated from the electrode fingers of the second bus bar by first gaps, a step c) before step b) of providing a transverse mode suppression layer such that after step c), the transverse mode suppression layer is partially under the first gap, wherein the transverse mode suppression layer is chosen such that the phase velocity of a guided wave is lower in the area of the transverse mode suppression layer than the phase velocity of the guided wave in the piezoelectric substrate in the central area under the alternating electrode fingers of the first and second electrodes. Such a method allows the transverse mode suppression layer to be formed before the interdigitated electrodes are formed. Thus, using this method,the interdigitated electrodes cannot be damaged by the step of forming the suppression layer and, as a result, the intrinsic quality of the device and the substrate surface is preserved. This method allows the usual initial SAW manufacturing process to be kept unchanged. Unlike prior art devices in which the interdigitated electrodes are formed on the piezoelectric substrate, and then a transverse mode suppression layer is formed on the interdigitated electrodes. In addition, the method according to the invention results in a simpler manufacturing process compared to prior art devices.
[0024] In an alternative embodiment, the second comb electrode may further comprise shorter dummy electrode fingers alternating with the electrode fingers, and the dummy electrode fingers of the second bus bar face the electrode fingers of the first bus bar and are separated from the electrode fingers of the first bus bar by second gaps, and step c) of providing a transverse mode suppression layer may comprise providing a first lateral mode suppression layer and a second transverse mode suppression layer extending at least partially under the second gaps and chosen such that the phase velocity of a guided wave is lower in the area of the second transverse mode suppression layer than the phase velocity of the guided wave in the central area under the alternating electrode fingers of the first and second comb electrodes, between the first and second transverse mode suppression layers. Such a method allows the transverse mode suppression layers to be formed before the interdigitated electrodes are formed.Thus, using this method, the interdigitated electrodes cannot be damaged by the step of forming the suppression layer and, as a result, the intrinsic quality of the device and the substrate surface is preserved. This method allows the usual initial SAW manufacturing process to be kept unchanged. Unlike prior art devices, where the interdigitated electrodes are formed on the piezoelectric substrate, and then a transverse mode suppression layer is formed on the interdigitated electrodes. In addition, the method according to the invention results in a simpler manufacturing process compared to prior art devices.
[0025] In a variant of the invention, the step of providing a transverse mode suppression layer may comprise a step of modifying the doping dose of the piezoelectric substrate, in particular with an implantation step or a diffusion step of atomic species and / or a proton exchange technique. Again, the doping here corresponds to an intentional introduction of impurities into the piezoelectric substrate. A simple technique may be used to form the transverse mode suppression layer compared to the technique used in the manufacturing process of the devices of the prior art.
[0026] In an alternative embodiment, the step of providing a transverse mode suppression layer may comprise providing a passivation layer, in particular a dielectric passivation layer, more particularly a SiO2 layer. A material layer deposition technique may be used to form the transverse mode suppression layer, which is a simpler technique compared to the technique used in the manufacturing process of prior art devices. In an alternative embodiment, the step of providing a transverse mode suppression layer may comprise a step of at least partially integrating the passivation layer into the piezoelectric substrate.
[0027] In a variant of the invention, the passivation layer may be patterned by a lift-off or detachment process, such that the sides of the passivation layer are inclined in such a way that the deposited comb electrodes cover these sides without breakage, providing a smooth and continuous reduction of the phase velocity in the corresponding area. This thus steadily reduces the possibilities for the transverse mode to develop since the latter preferably needs an acute variation of the velocity at the edge of the electrodes acting as a wave reflector. In this case, no etching is necessary, thus keeping the substrate in good condition.In case the substrate needs to be preserved against organic contamination, a so-called wet etching process based on isotropic etching of the silicon dioxide layer can be implemented, also producing regular edges of the patterned passivation layer. SiO2 is therefore deposited homogeneously on top of the wafer as a first step, then an in situ etch mask of photoresist is formed on top of the layer, allowing local etching of the SiO2 layer. Alignment masks, however, have to be deposited in both cases to prepare the surface according to the usual technological processes for this purpose.
[0028] The object of the invention is also achieved using a SAW device, in particular a single-port resonator, comprising at least one transducer structure as described above. Such a SAW device can be manufactured using a simpler manufacturing process compared to the state of the art while preserving a reduced contribution of transverse modes, resulting in improved spectral purity of the resonator response.
[0029] The invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which reference numerals identify features of the invention.
[0030] [Fig. 1a] is a top view of a transducer structure according to a first embodiment of the invention.
[0031] [Fig.lb] is a side view of a transducer structure according to the first embodiment of the invention.
[0032] [Fig.2a] is a side view of a transducer structure according to a variant of the first embodiment of the invention.
[0033] [Fig.2b] is a side view of a transducer structure according to another variant of the first embodiment of the invention.
[0034] [Fig.2c] is a side view of a transducer structure according to another variant of the first embodiment of the invention.
[0035] [Fig.2d] is a side view of a transducer structure according to another variant of the first embodiment of the invention.
[0036] [Fig.3a] is a top view of a transducer structure according to a variant of the first embodiment of the invention.
[0037] [Fig.3b] is a top view of a transducer structure according to another variant of the first embodiment of the invention.
[0038] [Fig.3c] is a top view of a transducer structure according to another variant of the first embodiment of the invention.
[0039] [Fig.3d] is a top view of a transducer structure according to another variant of the first embodiment of the invention.
[0040] [Fig.3e] is a top view of a transducer structure according to another variant of the first embodiment of the invention.
[0041] [Fig.3f] is a top view of a transducer structure according to another variant of the first embodiment of the invention.
[0042] [Fig.4] is a schematic diagram of a manufacturing process for a structure of transducer according to the first embodiment of the invention and its variants.
[0043] [Fig.5] is a schematic diagram of a manufacturing process for a structure of transducer according to the fourth variant of the first embodiment of the invention.
[0044] The invention will now be described in more detail using advantageous embodiments by way of example and with reference to the drawings. The described embodiments are merely possible configurations, and it should be borne in mind that individual features as described above may be provided independently of each other or may be omitted altogether during the implementation of the present invention.
[0045] [Fig. 1a] is a top view of a transducer structure according to a first embodiment of the invention.
[0046] The transducer structure 100 includes a pair of interdigitated comb electrodes 102 and 112 provided on a piezoelectric substrate 120. The piezoelectric substrate 120 may be a bulk piezoelectric substrate or a composite substrate having a piezoelectric layer on a base substrate, which will be further explained below with respect to [Fig.lb].
[0047] The first interdigitated comb electrode 102 includes a plurality of electrode fingers 104 and a plurality of dummy electrode fingers 106 extending from a first bus bar 108. Similarly, the second interdigitated comb electrode 112 includes a plurality of electrode fingers 114 and a plurality of dummy electrode fingers 116 extending from a second bus bar 118. The dummy electrode fingers 106 of the first bus bar 108 face the electrode fingers 114 of the second bus bar 118, and are separated electrode fingers 114 of the second bus bar 118 by first spaces 110a. The dummy electrode fingers 116 of the second bus bar 118 face the electrode fingers 104 of the first bus bar 108, and are separated from the electrode fingers 104 of the first bus bar 108 by second spaces 110b. Here, the first and second spaces 110a and 110b are identical.
[0048] According to a variant of the invention, the second comb electrode 112 does not have a dummy electrode finger 116 extending from the second bus bar 118, only a plurality of electrode fingers 116. In this case, the second spaces 110b are defined by the distance between the second bus bar 118 and the facing electrodes 104 of the first comb electrode 102.
[0049] According to a variant of the invention, not all of the electrode fingers 104, 114 of the first 108 and / or second 118 bus bar have a dummy electrode finger 106, 116 of the corresponding bus bar 108, 118 facing them, and vice versa.
[0050] In a variant, the first and second spaces 110a, 110b may be different throughout the transducer structure 100, in particular the first and second spaces 110a, 110b may randomly be different in terms of length or position in the transducer structure 100.
[0051] The interdigitated comb electrodes 102, 112 are formed of any suitable conductive metal, for example aluminum or aluminum alloys, such as Al-Cu, Al-Ti or Al-Si.
[0052] The two interdigitated comb electrodes 102, 112 are connected to opposite potentials +V / -V. The transducer structure 100 has an electrode pitch p, being defined by the Bragg condition, and thus p is equal to  / 2, A being 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 center of the electrode finger 104 of the electrode 102 and the center of the adjacent electrode finger 114 of the electrode 112. The transducer structure 100 is synchronous, that is, the interdigitated electrode fingers 104, 114 have the same width, period and shape.
[0053] The electrode fingers 104, 114 and the dummy electrode fingers 106, 116 have a width a. According to the art, the metallic aspect ratio is defined by a / p.
[0054] The electrode fingers 104, 114 of the interdigitated electrodes 102, 112 are respectively identical, with the same width a and the same length 1;. In a variant, the interdigitated electrodes could have different electrode fingers 102, 112.
[0055] Similarly, the dummy electrode fingers 106, 116 of the interdigitated electrodes 102, 112 are respectively identical, with the same width a and the same length l}. Alternatively, the interdigitated electrodes 102, 112 could have different dummy electrode fingers 106, 116.
[0056] In [Fig.1a], the interdigitated electrodes 102, 112 are identical, namely they have the same electrode fingers, the same pitch p and the same first and second spaces 110a, 110b. Alternatively, they could be different, with a different pitch p, different spaces and different electrode fingers 102, 112 and dummy electrode fingers 106, 116.
[0057] According to the invention, two transverse mode suppression layers 122 and 132 are provided between the substrate 120 and the interdigitated comb electrodes 102, 112. The first transverse mode suppression layer 122 is positioned under the first gaps 110a, and the second transverse mode suppression layer 132 is positioned under the second gaps 110b. Their physical properties are chosen such that the guided ground phase velocity of the transducer structure is lower in the area of the transverse mode suppression layers 122, 132 compared to the guided ground phase velocity of the transducer structure in the piezoelectric substrate 120 in the remaining area 134 which is the central area 136 under the alternating electrode fingers of the first and second electrodes.The guided wave can be any guided wave or true mode of the surface, including Rayleigh-type waves and shear waves and pure or quasi-pure shear or longitudinal waves.
[0058] In an alternative embodiment, a single transverse mode suppression layer is present in the transducer structure.
[0059] In this embodiment, the first and second transverse mode suppression layers 122 and 132 are rectangular, and have identical dimensions and shape. The areas 122 and 132 extend along the y direction, under all of the electrode fingers 104, 114 and all of the dummy electrode fingers 106, 116 of the transducer structure 100, thus, along the entire length of the transducer structure 100. They are symmetrical with respect to the centerline y of the interdigitated electrodes 102, 112 of the transducer structure 100.
[0060] The first and second transverse mode suppression layers 122 and 132 extend along the x direction, which is the propagation direction of an unwanted transverse acoustic wave in the transducer structure 100, over a length d. In [Fig. 1a], the transverse mode suppression layer 122 extends over the first gap 110a and over a portion of the dummy electrode fingers 106 and a portion of the electrode fingers 104. Similarly, the transverse mode suppression layer 132 extends over the second gap 110b and over a portion of the dummy electrode fingers 116 and a portion of the electrode fingers 114.
[0061] In [Fig.1a], the first and second transverse mode suppression layers 122 and 132 are located at the same distance 5 from the bus bar 108, 118 in the X direction. Alternatively, they may have a different distance from the bus bars.
[0062] In [Fig.lb], a side view cut away along a line AA of [Fig.la] of a transducer structure 100 according to the first embodiment of the invention is shown.
[0063] The piezoelectric substrate 120 is a composite piezoelectric substrate, comprising a piezoelectric layer 140 on a base substrate 144. A thin dielectric layer 142 is deposited between the piezoelectric layer 140 and the base substrate 144. Alternatively, the piezoelectric layer 140 may be deposited directly on top of the base substrate 144.
[0064] The thickness of the piezoelectric layer 140 is of the order of the wavelength, or less.
[0065] In this embodiment, the first and second transverse mode suppression layers 122 and 132 are a passivation layer deposited on the piezoelectric layer 140, before the interdigitated electrodes 102, 112 have been deposited on the substrate 120, and thus on the piezoelectric layer 140. The passivation layer is in direct contact with the piezoelectric layer 140.
[0066] The passivation layer is a dielectric layer, for example a layer of SiO2 or Ta2O5 or HfO2.
[0067] The transverse mode suppression layer 122, also referred to as the first transverse mode suppression layer, located on the right side of [Fig.lb], is completely covered by the electrode finger 104, which is connected to its bus bar 108.
[0068] The second transverse mode suppression layer 132 located on the left side of [Fig.lb] extends under the end 116a of the dummy electrode finger 116, the dummy electrode finger 116 being connected to its bus bar 118, the second gap 110b and the end 104a of the electrode finger 104, which is connected to the bus bar 108 at the connection 134.
[0069] In a variant of the invention, the passivation layers have inclined sides or wall edges, so that when the comb electrodes completely cover the passivation layers, no breakage of the deposited comb electrodes occurs at the wall edge of the passivation layer, ensuring regular and continuous covering of the electrode by the passivation layer.
[0070] In a variant, instead of using an additional passivation layer, the first and second transverse mode suppression layers 122, 132 may be formed within the piezoelectric layer 140, for example by having a different doping dose compared to the central area 136 of the piezoelectric layer 140, and the space between the transverse mode suppression layers 122, 132 and the bus bars having a width 5. The doping here corresponds to an intentional introduction of impurities into the piezoelectric substrate. The difference in doping of the at least one transverse mode suppression layer results in a reduction in the phase velocity of the mode near the space separating the IDT electrodes compared to the dummy electrodes without the need to provide any lateral pattern.
[0071] The transducer structure 100 according to the first embodiment operates as follows. The transducer structure according to the invention is used to excite and detect shear waves. The shear displacement direction alternates from one electrode to another when the transducer is excited by a +V / -V electrical polarization structure, because the device operates under Bragg conditions.
[0072] The presence of the first and second transverse mode suppression layers 122, 132 generates locations where SAW propagation is modified, such that a guided wave propagating in the area with the wave suppression layers 122, 132 has a lower phase velocity than a guided wave propagating in the central area 1326 of the substrate 120 that is unmodified. The first and second transverse mode suppression layers 122, 132 change the mode velocity near the first and second gaps 110-110b between the active electrode finger 104 and its associated dummy electrode finger 116. A slowing of the wave velocity in the gap 110a and 110b leads to the suppression or reduction of the unwanted transverse mode.
[0073] Thus, in the transducer structure 100 according to the invention, the contribution of transverse modes can be reduced. At the same time, the manufacturing process is simplified compared to transducer structures of the state of the art.
[0074] According to the invention, although the local modification of the substrate 120 in or near the first and second spaces 110a, 110b requires an additional manufacturing step, the usual initial SAW manufacturing process remains unchanged, simplifying the manufacturing of transverse mode-free devices. Furthermore, compared to other solutions requiring precise alignment between consecutive layers, here a rough mask alignment is sufficient to complete the manufacturing of a device.
[0075] As will be indicated below, variations of the embodiment will have various shapes and positions of the first and second transverse mode suppression layers 122 and 132 relative to the transducer structure 100.
[0076] Numerical references already used in the description of the first embodiment will not be repeated again, but reference will be made to their description.
[0077] [Fig.2a] is a side view of a transducer structure 200a according to a first variant of the invention.
[0078] Compared to the first embodiment, the passivation layer 246, 248 forming the first and second transverse mode suppression layers 122, 132 extend under the bus bars 108 and 118. The passivation layer 248 extends completely under the bus bar 118, while the other passivation layer 246 extends only partially under the bus bar 108. In this variant, the first and second transverse mode suppression layers 122 and 132 are not symmetrical.
[0079] In another variation, the first and second transverse mode suppression layers 122 and 132 may be integrated completely or partially into the piezoelectric layer 140.
[0080] [Fig.2a] is a side view of a transducer structure according to a second variant of the invention.
[0081] As mentioned in the description of the first embodiment, the first and second transverse mode suppression layers 222 and 232 of the piezoelectric layer 140, in this variant, are a locally modified structural piezoelectric layer 140. For example, the structural modification of the piezoelectric layer 140 corresponds to a different doping dose in the transverse mode suppression layer 222, 232 compared to the rest 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 variant of the second variant, the first and second transverse mode suppression layers 222, 232 extend only partially under the bus bars 108, 118. The first and second transverse mode suppression layers 222 and 232 are symmetrical.
[0084] [Fig.2d] is a side view of a transducer structure 200d according to a fourth variant of the invention.
[0085] Unlike the first three variants, while the first transverse mode suppression layer 232 extends partially under the bus bar 118, the second transverse mode suppression layer 222 does not extend at all under its corresponding bus bar 108. Again, the locally modified areas 222 and 232 are not identical here.
[0086] Figures 3a to 3f illustrate additional variations of the transverse mode suppression layers in a top view of the transducer structure. They all have different geometric shapes of the transverse mode suppression layer.
[0087] [Fig. 3a] is a top view of a transducer structure according to a fifth variant of the first embodiment of the invention. Compared to the embodiment shown in [Fig. 1a], here, the transverse mode suppression layer 222 extends at least partially to the bus bar 108 on a first side and possibly even under the bus bar 108, while on the other side, the transverse mode suppression layer 232 is located at a distance 5 from the bus bar 108, as in the first embodiment. As a result, the transverse mode suppression layers 222 and 232 are different in terms of 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] is a top view of a transducer structure, in particular according to the second variant shown in [Fig.2b], where the two transverse mode suppression layers 222, 232 extend under the bus bars 208, 218 and are symmetrical in terms of shape and dimensions.
[0090] In the seventh variant, illustrated in [Fig. 3d], compared to the variant shown in [Fig. 3c], the two transverse mode suppression layers 222, 232 extend even further outward than the bus bars 208, 218 in the X direction. In one variant, only one of the transverse mode suppression layers 222, 232 extends further outward than the bus bars 208, 218.
[0091] In the eighth variant shown in [Fig. 3e], compared to the embodiment shown in [Fig. la], the transverse mode suppression layers 222, 232 have a convex or concave shape in order to improve mode confinement when less energy is radiated at the edge of the interdigital transducer.
[0092] In the ninth variant, as shown in [Fig.3f], different shapes can be used for the transverse mode suppression layers 222, 232. In [Fig.3f], the shape of the second transverse mode suppression layer 232 is rounded, while the shape of the first transverse mode suppression layer 222 is pointed. The rounded shape of the transverse mode suppression layers 232 is rounded with angles at different values to break any possible synchronism effect. This is also the case for pointed patterns, the angles being chosen in such a way that no coherence occurs from one electrode edge to another.
[0093] [Fig.4] is a schematic diagram of the steps of the method of manufacturing a transducer structure according to the first embodiment of the invention and any of its variants.
[0094] According to step a), a piezoelectric substrate 420 is provided. In [Fig.4], the piezoelectric substrate 420 is a composite substrate, comprising a piezoelectric layer 440 on a base substrate 444. In this embodiment, a SiO2 layer is also present between the base substrate 444 and the piezoelectric layer 440.
[0095] According to a first variant 1), two transverse mode suppression layers 422 and 432 are provided by doping the piezoelectric layer 440 with a different dose compared to the rest of the piezoelectric substrate 420 in at least one area of the piezoelectric substrate 420.
[0096] This step comprises an implantation or diffusion of atomic species, in particular Ti, to modify the concentration of the atomic species in the piezoelectric substrate of the transverse mode suppression layer.
[0097] According to a further variant, the step of providing the transverse mode suppression layer may be a proton exchange process.
[0098] The proton exchange process includes basic proton exchange from an organic proton source and an annealing post-treatment, which involves only heating the sample to redistribute the lithium and hydrogen ions.
[0099] The proton exchange technique is described by Chung et al, “Proton-Exchanged 36° YX LiTaOï Waveguides for Surface Acoustic Wave”, IEEE transactions on UFFC, vol. 53, no. 2, 2006. The use of PE allows for local modification of the piezoelectric layer. The proton exchange technique is an interesting technique for an industrial approach to obtain the expected effect as it will not affect the surface roughness and therefore conforms perfectly to a planetary technology treatment as used in the SAW industry.
[0100] According to a second variant 2) of the method, the step of providing the transverse mode suppression layer is a step of depositing a passivation layer. The deposited layer may be a dielectric passivation layer, more particularly a SiO2 layer.
[0101] The passivation layer 446 is deposited on the piezoelectric substrate 424, in order to form the first and second transverse mode suppression layers 422 and 432.
[0102] According to a third variant 3d), the passivation layer can be at least partially integrated into the piezoelectric substrate 440. This variant comprises an etching step, in order to remove a part of the piezoelectric substrate 440 in the zones 422, 432, or any other process which makes it possible to reduce the thickness t of the piezoelectric substrate 440 in the zones 422, 432. The passivation layer is then deposited locally or completely on the substrate, and a chemical-mechanical polishing (CMP) step is applied to recover a perfectly flat surface in accordance with the usual SAW manufacturing requirement.
[0103] Next, a pair of interdigitated comb electrodes are formed on the piezoelectric substrate, using a combination of layer deposition and patterning steps, as shown in step c) of [Fig.4].
[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, such that the sides of the passivation layer are inclined in such a way that the deposited comb electrodes cover these sides without breakage, providing a smooth and continuous reduction of the phase velocity in the corresponding area. In this case, no etching is necessary, thus keeping the substrate in good condition. Alignment masks must, however, be deposited to prepare the surface according to the usual technological processes for this purpose.
[0105] As a first step a), the POI substrate 420 is cleaned with an O2 plasma process. Then, a deposition of a photoresist film 448, in particular by spin coating, is carried out (step b)). A UV lithography step is carried out through a UV mask 450 on the photoresist film 448 as step c), so as to obtain a patterned photoresist film having a plurality of structures 452 with inclined wall edges, in step d). In step e), the passivation layer 446 is deposited on the POI substrate 420 and on the patterned photoresist film 450, as well as between the structures 452 of the patterned photoresist film 450 to form the transverse mode suppression layers 422, 432.Due to the inclined wall edges of the structures 452 of the patterned photoresist film 450, the deposition of the passivation layer 446 on the POI substrates 420 results in the passivation layer 446 being deposited between the structures 452 of the patterned photoresist film 448 and also comprising inclined wall edges, as shown in step e). Finally, a step f) is performed to remove the structures 452 of the patterned photoresist film 450 with the passivation layer 446 on top, and only the passivation layer 446 present directly on the POI substrate 420 between the structures 452 of the patterned photoresist film 450 is left on the surface of the POI substrate 420.
[0106] According to the method of the invention, the interdigitated electrodes are formed above the transverse mode suppression layer and the piezoelectric substrate. The transverse mode suppression layer is formed before the interdigitated electrodes are formed. Unlike the devices of the prior art, where the interdigitated electrodes are formed on the piezoelectric substrate, and then a transverse mode suppression layer is formed on the interdigitated electrodes.
[0107] According to a variant of the invention, the interdigitated electrodes are formed above the first and / or second transverse mode suppression layer and the piezoelectric substrate. The transverse mode suppression layers are thus formed before the interdigitated electrodes are formed. Unlike the devices of the prior art, where the interdigitated electrodes are formed on the piezoelectric substrate, and then a transverse mode suppression layer is formed on the interdigitated electrodes.
[0108] Various embodiments of the invention have been described. It should nevertheless be understood that various modifications and improvements may be made.
Claims
1. Claims Transducer structure having transverse mode suppression means, in particular for a single-port resonator, comprising: a piezoelectric substrate (120, 140), in particular a piezoelectric composite substrate comprising a piezoelectric layer (140, 440) on a base substrate (144, 444); at least one pair of interdigitated comb electrodes (102, 112) formed on the piezoelectric substrate (120, 140), wherein the first comb electrode (102) comprises a first bus bar (108) and a plurality of electrode fingers (104) alternating with shorter dummy electrode fingers (106), all extending from the first bus bar (108), wherein the second comb electrode (112) comprises a second bus bar (118) and a plurality of electrode fingers (114) extending from the second bus bar (118), wherein the dummy electrode fingers (106) of the first bus bar (108) face the electrode fingers (114) of the second bus bar (118) and are separated from the electrode fingers (114) by first gaps (110a), the transducer structure further comprising a transverse mode suppression layer (122, 132, 222, 232, 422, 432) provided partially under the first gaps (110a) and selected such that the phase velocity of a guided wave is lower in the area of the transverse mode suppression layer (122, 132, 222, 232, 422, 432) than the phase velocity of the guided wave in the central area (136) under the alternating electrode fingers (104, 114) of the first and second comb electrodes (102, 112); and wherein the second comb electrode (112) further comprises shorter dummy electrode fingers (116) alternating with the electrode fingers (114), the dummy electrode fingers (116) of the second bus bar (118) face the electrode fingers (104) of the first bus bar (108) and are separated from the electrode fingers (104) by second gaps (110b), and further comprising a second transverse mode suppression layer (122, 132, 222, 232, 422, 432) extending at least partially under the second spaces (110b) and chosen such that the phase velocity of a guided wave is lower in the area of the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) than the phase velocity of the guided wave in the central area (136) under the alternating electrode fingers (104, 114) of the first and second comb electrodes (102, 112), between the first and second transverse mode suppression layers (122, 132, 222, 232, 422, 432); and wherein 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 transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends under each of the first spaces (110a).
3. A transducer structure according to claim 1 or 2, wherein the transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends under all of the second spaces (110b).
4. A transducer structure according to any one of claims 1 to 3, wherein the first transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends at least partially to the first bus bar (108) and / or the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends at least partially to the second bus bar (118).
5. A transducer structure according to any one of claims 1 to 4, wherein the first transverse mode suppression layer (122, 132, 222, 232, 422, 432) further extends at least partially or at least completely under the first bus bar (108) and / or the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) extends at least partially or at least completely under the second bus bar (118).
6. A transducer structure according to any one of claims 1 to 5, wherein the first and / or second transverse mode suppression layer (122, 132, 222, 232, 422, 432) is formed within the piezoelectric layer (140) having a different doping dose compared to the rest of the piezoelectric substrate (120, 420) in the central area (136) under the interdigitated comb electrodes (102, 112) to modulate the phase velocity of the wave guided transducer structure, especially the different doping dose is a Ti dose.
7. A transducer structure according to any one of claims 1 to 5, wherein the first and / or second transverse mode suppression layer (122, 132, 222, 232, 422, 432) comprises a passivation layer (246, 248, 446), in particular a dielectric passivation layer, more particularly a SiO2 layer.
8. The transducer structure of claim 7, wherein the passivation layer (246) is at least partially embedded in the piezoelectric substrate (120, 420).
9. A method of manufacturing a transducer structure comprising the steps of: a) providing a piezoelectric substrate, in particular a piezoelectric composite substrate comprising a piezoelectric layer on a base substrate; b) forming at least one pair of interdigitated comb electrodes on the piezoelectric substrate, wherein the first comb electrode comprises a first bus bar and a plurality of alternating electrode fingers and shorter dummy electrode fingers all extending from the first bus bar, the second comb electrode comprises a second bus bar and a plurality of alternating electrode fingers extending from the second bus bar, the dummy electrodes of the first bus bar face the electrode fingers of the second bus bar and are separated from the electrode fingers by first gaps,and a step c) before step b) of providing a transverse mode suppression layer such that after step c), the transverse mode suppression layer is partially under the first gaps, wherein the transverse mode suppression layer is chosen such that the phase velocity of a guided wave is lower in the area of the transverse mode suppression layer than the phase velocity of the guided wave in the piezoelectric substrate in the central area under the alternating electrode fingers of the first and second comb electrodes, and the second comb electrode further has shorter dummy electrode fingers alternating with the electrode fingers, and the dummy electrode fingers of the second bus bar face the electrode fingers of the first bus bar and are separated, electrode fingers of the first bus bar by second gaps, and wherein step c) of providing a transverse mode suppression layer comprises the step of providing a first transverse mode suppression layer and a second transverse mode suppression layer extending at least partially under the second gaps and selected such that the first transverse mode suppression layer and the second transverse mode suppression layer (122, 132, 222, 232, 422, 432) have different shapes and the phase velocity of a guided wave is lower in the area of the second transverse mode suppression layer than the phase velocity of the guided wave in the central area under the alternating electrode fingers of the first and second comb electrodes, between the first and second transverse mode suppression layers.
10. A method according to claim 9, wherein step c) of providing the transverse mode suppression layer comprises a step of modifying the doping dose of the piezoelectric substrate to modulate the phase velocity of the guided wave of the transducer structure, in particular with an implantation step or a diffusion step of atomic species and / or a proton exchange technique.
11. The method of claim 9, wherein step c) of providing the transverse mode suppression layer comprises providing a passivation layer, in particular a dielectric passivation layer, more particularly a SiO2 layer.
12. The method of claim 11, wherein step c) comprises a step of at least partially integrating the passivation layer into the piezoelectric substrate.
13. A method according to any preceding claim 11 or 12, wherein the passivation layer is patterned by a lift-off process, such that the sides of the edge walls of the passivation layer are inclined.
14. SAW device, in particular single-port resonator, comprising a transducer structure according to any one of the preceding claims 1 to 8.