Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
  • H03H9/02—Details
  • H03H9/02228—Guided bulk acoustic wave devices or Lamb wave devices having interdigital transducers situated in parallel planes on either side of a piezoelectric layer
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
  • H03H9/02—Details
  • H03H9/02007—Details of bulk acoustic wave devices
  • H03H9/02015—Characteristics of piezoelectric layers, e.g. cutting angles
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
  • H03H9/02—Details
  • H03H9/02007—Details of bulk acoustic wave devices
  • H03H9/02015—Characteristics of piezoelectric layers, e.g. cutting angles
  • H03H9/02039—Characteristics of piezoelectric layers, e.g. cutting angles consisting of a material from the crystal group 32, e.g. langasite, langatate, langanite
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
  • H03H9/02—Details
  • H03H9/02007—Details of bulk acoustic wave devices
  • H03H9/02062—Details relating to the vibration mode
  • H03H9/02078—Details relating to the vibration mode the vibration mode being overmoded

Definitions

• the present invention generally relates to acoustic wave devices and more specifically to an acoustic wave device based on plate acoustic waves.
• Acoustic wave devices in particular surface acoustic wave devices, are used in a variety of applications: filters, resonators oscillators, etc. in electronics/telecom, as sensors in liquid media, e.g. bio-chemical sensor, or temperature, flow and pressure sensor...
• An acoustic wave device typically employs a piezoelectric plate member with one or more electrodes configured to convert electrical signals to acoustic waves and vice versa, by exploiting the piezoelectric effect.
• AW acoustic waves
• BAW bulk acoustic waves
• SAW surface acoustic waves
• PAW plate acoustic waves
• BAW-based devices with in-plane components polarization such as QCM[REF 2], lateral field excitation BAW [REF 3], and shear mode TFBAR [REF 4], for example, are suitable for a sensor in liquid media by having close-to-zero out-of-plane displacement component at the surface, thus preventing the conversion of acoustic energy into longitudinal pressure wave in the liquid.
• QCM[REF 2], lateral field excitation BAW [REF 3], and shear mode TFBAR [REF 4] are suitable for a sensor in liquid media by having close-to-zero out-of-plane displacement component at the surface, thus preventing the conversion of acoustic energy into longitudinal pressure wave in the liquid.
• SAW are suitable for a sensor in liquid media, such as SH-SAW [REF 5] and Love mode (LM) [REF 6] and HVPSAW [REF 7],
• the radiation of acoustic energy into the liquid can also be prevented by using the AW with phase velocity, v P h, lower than the compressional sound velocity of liquid media (1500 m/s in water, for instance).
• this condition can be achieved by using the fundamental antisymmetry Lamb wave (Ao) mode [REF 8], However, it is limited only at a low plate thickness to wavelength ratio (h/A), typically at an h/A ratio lower than 0.1. Consequently, making the device very fragile or, if a thicker plate is used, a larger wavelength is necessary, thus lowering the operating frequency.
• PAW based devices have an advantage over SAW for sensor application in liquid media as they have two free surfaces that are both sensitive to surface perturbations.
• IDT interdigital transducer
• the opposite surface can be used as the sensing layer.
• the electrical connection is separated from liquid contact without requiring an additional thin dielectric layer covering the IDT.
• PAW have in-plane polarization, such as SH-APM [REF 10] and symmetric Lamb modes [REF 11 ],
• So fundamental symmetry Lamb wave
• the insertion loss will further increase when the sensor has a higher operating frequency, limiting the maximum operating frequency of the AW-based sensor application in liquid media.
• [REF 17] relates to simulation and experiments using S5 and S6 quasi-linear lamb wave modes in liquid media based on a two-port delamy line device on thick GaAs substrate. As reported, these modes are close to the v P h of longitudinal bulk acoustic wave LBAW and exhibit a longitudinal displacement component that is dominant across the plate.
• AW acoustic wave
• the present invention is based on the present inventors’ research, who have identified a novel type of higher-order plate acoustic waves with close-to-zero particle displacement at the surface of the piezoelectric plate, thus reducing the wave-liquid interaction length and the insertion loss.
• an acoustic wave device comprises a piezoelectric member with at least one electrode arrangement on a first face thereof, the piezoelectric member having a thickness between the first and second face.
• the excitation electrode arrangement is provided on a first face of the piezoelectric member to generate in the piezoelectric member Lamb waves having a predetermined wavelength X.
• the electrode configuration conventionally determines the wavelength X.
• the acoustic wave device is configured such that application of an exciting electrical signal (typically alternating voltage creating electric field) to the electrode arrangement excites high-order mode Lamb waves with quasi-zero surface components, the piezoelectric member being configured with a height, h, greater than the wavelength of the generated Lamb waves, and a ratio h/ being such that a phase velocity v P h of high order modes is below the phase velocity of fast shear bulk acoustic waves.
• an exciting electrical signal typically alternating voltage creating electric field
• the present invention thus proposes an acoustic wave device designed to generate and propagate plate acoustic waves (PAW) with close-to-zero surface displacement relative to the maximum displacement inside the solid-state substrate.
• PAW plate acoustic waves
• the acoustic energy of the QZ-PAW propagates and is concentrated in the center of the substrate. Since the QZ-PAW travel at the center of the piezoelectric member, the radiation of acoustic energy to the surrounding media will be minimized, and the insertion loss reduced.
• phase velocity of high order LW modes converge into v P h of (fast) shear bulk acoustic waves (SBAW), whereby elliptical ly polarized LW convert into QZ-PAW.
• SBAW shear bulk acoustic waves
• the QZ-PAW have very low, namely quasi-zero, surface displacement components, which means low total displacements (ui+ U2+ U3) at the surface.
• QZ-PAW modes have, by definition, less than 10%, preferably less than 5%, total surface displacement over maximum total displacements inside the piezoelectric member.
• the QZ-PAW with the lowest insertion loss has a dominantly longitudinal displacement component (ui) at the surface, with (ui/us) ratio higher than 1 .
• the piezoelectric member used in the invention is rather thick (h/ ratio > 1 ), and its excitation by the electrodes will also generate other conventional LW modes.
• the present AW device may thus be considered as a multi-mode AW device with QZ-PAW.
• the QZ-PAW modes are not known from [REF 16] or [REF 17],
• [REF 17] investigated LW modes with a v P h near LBAW.
• these articles do not report high order modes where the surface displacement components are close to zero, i.e. where ui , U2 and us are close to zero.
• the piezoelectric member may have any appropriate shape and properties to meet the requirements for exciting and propagating QZ-PAWs.
• the piezoelectric member normally has two opposite surfaces or faces, and may thus generally have a plate-like shape. The opposite faces are normally parallel and flat.
• the piezoelectric member includes a plate-like substrate having two parallel opposite faces.
• the plate-like substrate is a crystal that forms the main element of the piezoelectric member, in which the waves generated by means of the input electrode arrangement will propagate.
• This plate-like substrate may have a rectangular/parallelepiped shape, or disk shape, or other.
• the piezoelectric member includes or consists of a plate substrate of piezoelectric material (normally a monolithic crystal plate). If desirable, a piezoelectric layer can be deposited on the piezoelectric substrate to enhance the piezoelectric effect (e.g. a layer of ZnO on a GaAs plate).
• the piezoelectric member may include a non-piezoelectric plate substrate, having at least one face covered by a layer (thin film) of piezoelectric material.
• the piezoelectric member can comprise a non- piezoelectric substrate having two opposite parallel faces, where one of these faces supports a (ultra) thin layer of piezoelectric material, or both faces.
• the design of such layer/thin film is such that it shall permit acting as transducer to excite the non-piezo substrate, however without significantly affecting the type of the modes and the displacement profile of the modes therein .
• the piezoelectric member may include additional piezo layers, or other layers as may be suitable for various functions, as known in the art.
• additional piezo layers or other layers as may be suitable for various functions, as known in the art.
• Multilayered structures may also be used for temperature compensation, for example.
• the structure of the piezoelectric member it is mounted, in practice, to have two opposite free faces/surfaces and the thickness h is the distance between these two opposite free surfaces.
• the piezoelectric member respectively plate substrate, may have a thickness of up to 1 mm (for example between 1 and 1000 pm).
• the AW devices include one or more electrode arrangements, depending on the intended application.
• the electrodes are generally designed as transducers configured to convert an electrical signal into acoustic waves and inversely. Conventionally, the design of the electrode arrangement that receives the exciting voltage will determine the wavelength X of the excited Lamb waves.
• the electrode arrangement may be conveniently realized as a pattern of interdigitated transducer (IDT), where two comb-shapes electrodes have fingers meshing with each other. The distance/pitch between two neighboring fingers of one comb-shaped electrode corresponds to the wavelength X.
• Electrodes may e.g. be made from aluminium or gold.
• the piezoelectric member may use any plate-like substrate (monolithic crystal plate) capable of supporting QZ-PAW.
• the plate-like substrate is advantageously cut out at an angle allowing excitation of the QZ-PAW.
• the plate substrate has an Euler crystal cut angle of 0, 0, a, where a is between 0 and 90°.
• the plate substrate is conventionally prepared such that wave propagation occurs in a known direction.
• Crystal orientation is known from the crystal growth technology.
• the wave propagation direction is (110) and two electrode arrangements (input I output) are spaced from each other, aligned along this wave propagation direction.
• the direction is advantageously determined to obtain the maximum electromechanical coupling coefficient, and also in this case to obtain the lowest surface displacement.
• piezoelectric members configured as thin film piezoelectric layer(s) on a non- piezo substrate
• the piezoelectric layers may e.g. comprise or consist of ZnO, AIN, AIScN, PZT, BN, PMN-PT, LiNbOs or BaTiOs.
• Such piezoelectric layers may be deposited by any appropriate technique, e.g. Chemical Vapour Deposition (CVD), Physical Vapour Deposition (PVD), Atomic Layer Deposition (ALD), Pulsed laser deposition (PLD), Molecular Beam Epitaxy (MBE) and Chemical Solution Deposition (CSD).
• CVD Chemical Vapour Deposition
• PVD Physical Vapour Deposition
• ALD Atomic Layer Deposition
• PLD Pulsed laser deposition
• MBE Molecular Beam Epitaxy
• CSD Chemical Solution Deposition
• the present AW device can be manufactured by any appropriate technique. It may be noted here that the inventive AW device can be manufactured using existing techniques in the fields.
• the manufacturing process may be simpler (more flexible) than for conventional AW devices, since a relatively thicker plate is used, i.e. less fragile. Also, since the QZ-PAW travel inside the plate below the surface, they are not affected by the surface roughness of the piezoelectric member/substrate. Accordingly, the free faces of the piezoelectric member/substrate may have a roughness in the order of 1 nm to 10 pm, or 5 nm to 5 pm.
• the present AW devices constitutes a micro-electro-mechanical system (MEMS) that may find application in a variety of domains, in particular electronics I telecommunications for Ultra-high frequency applications, or in the field of biosensors.
• MEMS micro-electro-mechanical system
• the present AW device is built such that the piezoelectric member is mounted on a support so that its two opposite faces are free, e.g. fixed at its edges.
• the present AW device may be integrated in a system with control unit configured to apply an electric input signal to the electrode arrangement and measure an output signal in order to evaluate at least one parameter of a high-order mode Lamb wave with quasi-zero surface components, QZ-PAW.
• the control unit may be configured to evaluate changes of at least one of a phase shift, frequency shift, time delay and phase velocity shift of the output signal relating to a high-order mode Lamb wave with quasi-zero surface components.
• the present AW device may be configured to operate as resonator, filter, oscillator, delay line, in particular as one port resonator, reflective delay line one port resonator or two port delay line.
• the present AW device may also be used in a sensor system, in order to measure non-surface sensitive parameters, such as e.g. temperature, pressure or strain/torque. Indeed, changes of temperature, pressure and strain will have an effect on the crystal structure of the piezoelectric member/substrate and hence affect the resonance of QZ-PAW modes.
• non-surface sensitive parameters such as e.g. temperature, pressure or strain/torque. Indeed, changes of temperature, pressure and strain will have an effect on the crystal structure of the piezoelectric member/substrate and hence affect the resonance of QZ-PAW modes.
• the surface effects can be measured by means of the other Lamb wave modes with surface components, e.g. in-plane polarized modes.
• the QZ-PAW can be advantageously used as a reference signal: where a change of the measuring mode (e.g. in-plane polarized) occurs together with a change of the reference mode (QZ-PAW), this will indicate that the change of measuring mode is not only due to surface effects, but also e.g. to a change in temperature.
• the design of the AW device may be done with the help of simulation (finite element method simulation), which is one of the numerical methods to study the acoustic wave propagation and to obtain the velocity and/or the resonance frequency of Lamb waves. It is also possible to obtain the solution of wave propagation analytically or numerically using programming software such as Matlab.
• Lamb wave dispersion is well known (Lamb waves velocity is dependent on wave vector, frequency and thickness/wavelength ratio - see e.g. [REF 13-16] for Lamb wave characteristic equations or determination of dispersion curves with COMSOL simulation).
• the dispersion curves of Lamb waves in a "thick plate” show that the fundamental modes (Ao and So) will approach the Rayleigh wave velocity and the higher order mode will approach the shear wave (bulk acoustic wave-BAW) velocity, i.e. the fast SBAW.
• the skilled person will thus be able to select the appropriate design of piezoelectric member, namely the appropriate h/ ratio, to that the QZ-PAW may exist. That is the piezoelectric member is designed such that a solution of Lamb wave may exist, with velocity around the fast-SBAW, where the displacement profile across the plate is quasi-zero at the surface of the plate, and the maximum displacement is inside the plate.
• the ratio h/X parameters that may be adapted are material choice, crystal cut, and direction of propagation.
• the invention also relates to an acoustic wave system as claimed in claims 11 and 12.
• the invention also relates to an acoustic wave system as claimed in claim 13.
• the invention also relates to a method of operating the present acoustic wave device.
• Figure 1 is a principle perspective view of an embodiment of the present acoustic wave device
• Figure 2 are graphs (a) and (b) of Lamb wave dispersion curves in a piezoelectric plate (b); (c) are principle views representing the QZ-PAW displacements of the first three higher-order modes in a substrate;
• Figure 3 are graphs illustrating (a) the ratio between total displacement at the surface of the plate and the maximum displacement inside the plate (usurf/umax) and (b) ratio of longitudinal and shear vertical displacements (ui/us) at the surface of the plate;
• Figure 4 is a graph (simulation) of the S21 scattering parameter vs. frequency for the present acoustic wave device
• Figure 5 is a principle perspective view of an embodiment of acoustic wave device in contact with liquid media
• Figure 6 is a diagram illustrating displacement profile for three different modes (out of plane polarized, in plane polarized and QZ-PAW) in contact with liquid;
• Figure 7 is an experimental graph of the S21 scattering parameter vs. frequency for the present acoustic wave device
• Figures 8 and 9 are principle diagrams of alternative embodiments of the present AW device.
• a first embodiment of the present AW device 10 comprises a piezoelectric member, generally indicated 11 , comprised of a plate- like piezoelectric substrate 12 with two planar opposite faces: an upper, first face 14 and a lower, second face 16. Electrodes are arranged on the upper face 14, a first electrode 18 serving as input electrode whereas the second electrode 20 serves as output electrode. As will be recognized, the electrodes 18, 20 are formed as interdigitated electrodes (or Inter-digitated transducers IDTs) and act as transducers to convert the electrical energy into acoustic waves and vice versa in one of the surfaces.
• IDTs Inter-digitated transducers
• Reference sign h in Fig.1 indicates the thickness of the piezoelectric plate, i.e. the distance between the two faces 14 and 16. Also to be noted is the distance X between two neighboring fingers of the same comb-shaped element of electrode 18, which will generate in the plate 12 acoustic waves of corresponding wavelength X, when an electric signal is applied.
• the thickness h of the piezoelectric plate 12 is greater than the inter-digit distance X, i.e. the ratio h to X is greater than 1 .
• the present device 10 is configured to generate higher-order plate acoustic waves with close-to-zero particle displacement at the surface, QZ-PAW.
• various modes of Lamb waves are generated, in addition to the desired acoustic waves with close-to-zero particle displacement at the surface.
• the inventive device can be referred to as multi-mode plate acoustic wave device with a zero-surface displacement plate acoustic mode.
• the exciting electrode arrangement 18 is connected to a voltage source 19, whereby an alternating voltage is applied to the input electrode 18.
• a control unit (not shown) is connected to the second electrode arrangement 20, which generates an output signal based on the received acoustic waves.
• the control unit is typically configured to acquire, evaluate and process the output signal, in particular to analyze its components.
• the control unit may in particular be configured to monitor the resonance frequency of predetermined modes, inclusive the QZ-PAW, and compare it to reference values.
• the device of Fig.1 may typically form a delay line, whereby the distance between the two electrodes 18, 20 is typically a multiple of the wavelength X. Operation, namely signal analysis, of such delay line is known in the art and will not be detailed herein.
• the piezoelectric member 11 simply consists of a plate-like substrate 12 made of piezoelectric material.
• This substrate is normally a monocrystal having two opposite faces 14 and 16, and the thickness h is measured between these free faces 14,16.
• the piezoelectric member may comprise a non-piezo substrate having a layer of piezoelectric material at least on one face thereof, as will be explained with reference to Figs. 8 and 9, showing two alternative embodiments of AW device 10.1 and 10.2.
• the piezoelectric member 1 T includes a plate-like substrate 30 of nonpiezoelectric material having two opposite face 32 and 34.
• Substrate 30 may be a cubic centered monocrystalline material such as, e.g., silicon or germanium.
• the first (upper) face 32 is covered with a thin layer of piezoelectric material 36.
• the piezo layer 36 is deposited by any appropriate method.
• the piezo layer 36 may e.g. be a layer of ZnO, AIN, AIScN or PZT.
• piezo layer 36 has two opposite faces 36.1 and 36.2, where lower face 36.2 is in contact with upper face 32 of substrate 30.
• the upper face 36.1 of piezo layer 36 is a free surface, which supports the electrode arrangements 18, 20.
• the AW device 10.2 includes a piezoelectric member 1 T comprised of a substrate 30 and layer of piezoelectric material layer 36 similar to device 10.1 of Fig.8.
• a further thin piezoelectric layer 40 is covering the second face 34 of substrate 30. That is, piezo layer 40 has a face 40.1 in contact with bottom face 34 and an opposite free face 40.2.
• the thickness h of the piezoelectric member 1 T and 11 ” of Figs. 8 and 9 for computing the ratio h/ corresponds to the sum of the thicknesses of substrate 30 and piezoelectric layer(s) 36, 40. That is, distance between free faces 34 and 36.1 for AW device 10.1 and free faces 40.2 and 36.1 for AW device 10.2. Observations on the inventive principle
• U2 non-zero shear horizontal component (U2) us: shear vertical (out-of-plane.
• the total displacement is the sum of ui, U2, and us displacement components.
• the total displacement at the surface may then be referred to as Usurf, being the sum (ui + U2 + us) at the surface.
• the maximum total displacement inside the plate is noted Umax and corresponds to the maximum of (ui + U2 + us) within the plate.
• Lamb waves are a subset of surface acoustic waves (SAW) that propagate in elastic solid plates with two free surfaces (as opposed to SAW propagation on a free surface of semi-infinite substrate).
• SAW surface acoustic waves
• RW Rayleigh waves
• LW Lamb wave
• LW dispersion curves with two sets of sinusoidal waves (symmetric Sn and antisymmetric An modes), with two component displacement: longitudinal (in-plane, ui) and shear vertical (out-of-plane, us) displacement, with maximum displacement on the surfaces.
• LW might have three component displacements with non-zero shear horizontal (out-of-plane, us).
• LW is dispersive thus the phase velocity (v P h) is dependent on the frequency and the plate thickness to wavelength (h/ ) ratio. While LW are generally elliptically polarized (with both in-plane and out-of-plane particle displacement), under specific conditions such as LW fundamental symmetric mode at very low h/ ratio and higher order symmetric modes at the phase velocity close to longitudinal BAW velocity, the solution of LW modes have only ui displacement component with in-plane polarization with close-to-zero us displacement component (quasilongitudinal, QL-LW).
• in-plane polarized AW-devices could minimize the acoustic energy radiation, however, the loss of acoustic energy still occurs due to the penetration of in-plane particle displacement at the solid/fluid surfaces. The loss may reduce the acoustic waves signal significantly for AW-devices working in highly viscous media or ultra-high operating frequency.
• the present invention proposes a device exploiting plate acoustic waves (PAW) with close-to-zero surface displacement (quasi-zero, QZ-PAW), relative to the maximum displacement inside the solid plate.
• PAW plate acoustic waves
• QZ-PAW close-to-zero surface displacement
• Figure 2a shows the dispersion curves of LW propagating in a piezoelectric plate.
• the simulation is based on a GaAs plate with a (100) cut and where PAW travel in direction ⁇ 110>.
• the v P h fundamental modes are converging into the v P h of RW, while the v P h of higher order modes are converging into v P h of shear BAW at higher h/A ratio.
• the conversion of elliptically polarized LW into QZ-PAW is shown in figure 2b.
• the surface displacements are vanishing when the phase velocity is crossing the v P h of shear BAW.
• Figure 2(b) shows the inset where the higher-order modes are converging into the vph of SBAw-fast. As shown in figure 1 (b), at a higher h/A ratio, the modes are crossing the vph of SBAw-fast and then stagnating with a slight fluctuation slightly below the SBAw-fast v P h.
• the first three QZ-PAW modes: QZ-Ai, QZ-Si and QZ-A2 are shown in figure 2c. As can be seen, the top and bottom regions are dark grey, which corresponds to zero or quasi-zero total displacements, whereas light grey areas towards the inside of the plate indicate the presence of displacements. The particle displacement and the acoustic energy are concentrated inside the plate with almost zero total displacement values at the surfaces.
• Figure 3a shows the total displacement on the plate surface normalised by the maximum total displacement inside the plate (usurf/umax) vs the h/A ratio for the first four higher-order modes. As can be seen, the total surface displacements of the modes are reducing to the value of around 5% of the maximum total displacement. The surface total displacements are the lowest at h/A ratio of 2.3 to 3.1 , 3.5 to 4.8, and 3.8 to 4.7 for Ai Si and A2 modes respectively.
• Figure 3b shows the U1/U3 ratio at the surface of the plate. As shown, the modes are dominantly longitudinal at the surface at h/A ratio of 2.7, 3.9 and 4.3 for A1, Si and A2 modes respectively.
• the particle displacement components ui, U2, and U3 at the surface have been further investigated at h/A ratio of 2.7, 3.9 and 4.3 for A1 , S1 and A2 modes respectively. It appears that the us of the QZ-PAW modes are vanishing to zero at the surface of the plate, while the ui of the QZ-PAW modes are around 5% relative to the maximum displacement inside the plate. Thus, the quasi-zero surface displacement is contributed mainly by in-plane particle displacement.
• the U2 of the modes are constantly zero across the plate due to the cubic crystal symmetry of GaAs substrate.
• the QZ-PAW are high-order Lamb Waves with the ratio Usurf/umax being less than 10% or preferably less than 5%.
• the AW device is designed as two port delay line with a piezoelectric substrate as shown in Fig.1.
• the piezoelectric plate/substrate 12 of Fig.1 is a piezoelectric (100)-cut GaAs substrate with a thickness h of 625 pm.
• the waves are propagating at ⁇ 110> direction (Euler angle ZXZ -> 0 0 45).
• the IDTs have 50 finger pairs with an aperture of 50A.
• the length of the delay line is 200A so that the centre to centre distance between the transmitter and receiver IDTs (16, 18) is 250A or equal to 42 mm.
• 100 nm of aluminium is used as an electrode with 10 nm Cr as an adhesion layer.
• the present AW device will generate multiple LW modes with different polarization at the surface of the plate: out-of-plane polarization, in-plane polarization, and zero-surface displacement (QZ-PAW) as shown in figure 2.
• the wavelength A of the waves of the AW device of figure 1 is defined by design and the periodicity of the input electrode arrangement 18. All modes are excited by the same electrode arrangement 18 thus having the same wavelength (A). The modes have different resonance frequencies where the resonance frequency is equal to the phase velocity divided by the wavelength.
• the present AW device has at least one mode with zero surface displacement.
• Fig. 4 shows the modelized transmission characteristic of the AW device.
• the first peak just before 20 MHz corresponds to QZ-PAW modes.
• the following 3 peaks correspond to three out of plane polarized modes.
• the last peak corresponds to in-plane polarized modes.
• the peak of the QZ-PAW mode actually corresponds to the resonance of the three QZ-PAW modes A1 , S1 and A2.
• Fig. 5 schematically represents an application of the present AW device in the context of liquid media.
• the second face 16 of the AW device 10 opposite the face 14 bearing the electrodes 18, 20, is in contact with a viscous liquid medium 26.
• Figure 6 shows the radiation of acoustic energy in the solid/fluid interface.
• the acoustic energy of out-of-plane modes are fully radiated into the liquid.
• QZ-PAW modes remain essentially in the solid plate.
• in-plane polarized modes Due to the “penetration depth” of in-plane polarized modes particle displacement at the solid/fluid interface, some part of acoustic energy of in-plane modes is radiated into the liquid causing some loss in the transmitted signals (noted S21 signals).
• the in-plane polarized modes are sensitives to surface perturbation such as the change in the liquid viscosity, liquid mass density, and the mass loading at the surface of the plate.
• the in-plane polarized modes can be used as a sensor where the change in the signal is due to perturbations of the plate surfaces.
• the zero-surface displacement modes are the least sensitive to surface perturbation, with close to zero acoustic energy radiation to the liquid, thus they have the least losses when the surface is in contact with viscous liquid media.
• Figure 7 shows the experimental frequency response of multi-mode PAW devices when one of the plate surfaces is in contact with air (dry), line 70, or with highly viscous liquid (100% glycerol), line 72.
• dry dry
• highly viscous liquid 100% glycerol
• the present AW device with QZ-PQW modes can thus be used for any type of AW device application such as e.g. one port resonator, two port resonator and two port delay line.
• the present AW device can in particular be used in an acoustic wave sensor, for sensing temperature, pressure and torque/strain.
• the thickness of the plate substrate must be reduced to keep the same ratio between the thickness and the wavelength.
• Several processes can be used to thin down the plate such as substrate grinding and polishing or by bulk micromachining with physical or chemical etching. When the etching process is used to thin down the plate (such as a SAW grade wafer), the surface of the plate will have some roughness which, as is known, may affect the propagation of the acoustic waves.
• substrates such as wafers are available in the different grades, from ‘SAW grade’ ( ⁇ 1 nm) to rougher profiles.
• the substrate 12 thickness may have been adjusted by a wet etching process from the second face 16, which may thus have a surface roughness (Rz) in the nm range up to the pm range
• the signal strength of zero-surface displacement mode is also not affected by the media surrounding the device.
• the present AW device thus allows more flexibility in terms of manufacturing, namely less stringent manufacturing processes, since it can operate with rougher surfaces than conventional AW devices. Also, contrary to conventional UHF telecom devices such as delay lines etc, vacuum packaging is not required.
• the "expected" resonance frequency of QZ-PAW may be experimentally verified by liquid contact, as it is expected that the QZ- PAW modes will have minimum insertion loss.

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• 2021
  • 2021-10-01 EP EP21306380.3A patent/EP4160917A1/de not_active Withdrawn
• 2022
  • 2022-09-28 EP EP22790535.3A patent/EP4409744A1/de not_active Withdrawn
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