WO2020038667A1

WO2020038667A1 - Tf-saw transducer with improved suppression of unwanted modes

Info

Publication number: WO2020038667A1
Application number: PCT/EP2019/069421
Authority: WO
Inventors: Jiman YOON; Christian Huck
Original assignee: RF360 Europe GmbH
Priority date: 2018-08-20
Filing date: 2019-07-18
Publication date: 2020-02-27
Also published as: DE102018120223A1

Abstract

A TF-SAW transducer with improved suppression of unwanted modes is provided. The transducer has a transversal velocity profile with a periodic structure.

Description

TF-SAW transducer with improved suppression of unwanted modes

The present invention refers to TF-SAW transducer with improved profiles of the acoustical wave mode.

TF-SAW (TF = Thin Film; SAW = Surface Acoustic Wave)

transducers are electroacoustic transducers that may be used in RF filters working with acoustical waves. Such transducers have an electrode structure arranged above or directly on a piezoelectric material. The dielectric material is provided as a thin film via thin film processing techniques, e.g.

smart cut, mechanical polishing or material deposition. An according filter can comprise one or more electroacoustic resonators in one or more acoustic tracks. The resonators comprise transducers with IDT structures (IDT = Inter

Digitated Transducer) such as interdigitating electrode fingers, each of which is connected to one of two bus bars of the transducer. Utilizing the piezoelectric effect the transducer converts an electromagnetic RF signal into

acoustic waves and vice versa.

If unwanted wave modes are not suppressed in resonators the electrical properties of respective RF filters are deterio rated .

From WO 2011/088904 Al, WO 2015/007319 A1 and WO 2016/095976 and Al SAW transducers with reduced spurious modes are known.

What is wanted is a transducer with a strong confinement of acoustic energy in the piezoelectric thin film. Further, such a transducer should have a good suppression of unwanted modes and minimized losses. In particular, a transducer should have minimized transversal losses and suppressed transversal modes. Also, such a transducer should be producible with a reduced number of process steps, e.g. to improve its

reliability. Further, such a transducer should allow RF filters with good filter performance, e.g. a low insertion loss .

It is, thus, an object to provide a TF-SAW transducer that allows improved electrical properties of respective filters.

In particular, it is an object to provide a TF-SAW transducer in which transversal modes are sufficiently suppressed.

For this purpose, a TF-SAW transducer according to the independent claim is provided. Dependent claims provide preferred embodiments.

The TF-SAW transducer comprises a longitudinal direction and a transversal direction orthogonal to the longitudinal direction. The transducer further comprises an IDT electrode structure comprising electrode fingers arranged on or above a piezoelectric thin film. The longitudinal direction defines the main propagation direction of the acoustic waves. The transversal direction mainly defines the direction of extension of the electrode fingers. The transducer further comprises a transversal velocity profile of acoustic waves propagating in the transducer and an acoustically active region with interdigitating electrode fingers. The

acoustically active region is mainly defined as the overlap region of electrode fingers of opposite polarity, i.e. as the area in which acoustic waves propagating in the longitudinal direction are excited when an RF signal is applied to the transducer. The transversal velocity profile has a periodic structure in this active region. The periodic structure has a plurality of minimal values and a plurality of maximal values larger than the minimal values. Further, the periodic

structure is flanked on both sides by an edge structure of the transversal velocity profile. The velocity in the edge structure is lower than the maximal values of the periodic structure .

It is possible that the periodic structure has two outermost sections with a maximum velocity.

I.e. there are two stripes of a lower velocity per unit cell arranged next to the periodic structure within the active re gion. Here, the unit cell denotes a segment of the acoustic track with a length in the longitudinal direction of the acoustic wavelength l.

The transducer further comprises a transversal velocity pro file of acoustic waves propagating in the transducer and an acoustically active region. The acoustically active region is mainly defined as the overlap region of electrode fingers of opposite polarity, i.e. as the area in which acoustic waves propagating in the longitudinal direction are excited when an RF signal is applied to the transducer.

The transversal velocity profile has a periodic structure in this active region. The periodic structure has a plurality of minimal values and a plurality of maximal values larger than the minimal values. Further, the periodic structure is flanked on both sides by an edge structure of the transversal velocity profile. The velocity in the edge structure is higher than the minimal values of the periodic structure. It is possible that the periodic structure has two outermost sections with a minimum velocity.

I.e. there are two stripes of a higher velocity in each unit cell arranged next to the periodic structure within the ac tive region.

It is possible that the transducer piezoelectric thin film comprises or consists of LiNbCy, LiTaCy and/or quartz. Other piezoelectric materials such as langasites, langanites, langatates, AIN, ZnO, KNP0O3, NaNb0₃, GaP0₄ and Li₂B407 are also possible .

Possible orientations for LiNb0₃ are the conventional LN RY-X cuts .

Possible crystal cuts for LiTa0₃ are: LT 36..56 RY-X.

It is also possible that the piezoelectric material comprises other composites of the respective crystallographic families or other cut angles.

Depending on the piezoelectric material and the cut angle it might be beneficial to use dummy fingers.

The two stripes (in the edge structure) per unit cell can extend over the length of the transducer resulting in a total number of two stripes per transducer. Each of the two stripes can be arranged directly next to a respective side of the periodic structure. The velocity profile can be a Dn/n waveguide. I. e. the peri odic structure can be a part of Dn/n waveguide.

The periodic structure and the edge structure establish a re gion of stripes of chosen velocity values extending in the longitudinal direction.

The wording "periodic structure" denotes the shape of the ve locity profile in the transversal direction. In the periodic structure the velocity profile, thus, comprises identical sections of higher and lower velocity being arranged next to one another and extending in the transversal direction.

The periodic structure can consist of a sinusoidal structure, a saw tooth structure, a square-wave structure. However, the periodic structure can be built-up of a combination of these structures .

It is possible that the periodic structure has a periodicity in the periodic length but the amplitude of minimum and maximum velocity values follow a profile, e. g. a parabola, sine function or a cosine function.

It was found that the combination of the periodic structure and the edge structure define a velocity profile in the ac tive region of a transducer in which not only unwanted trans versal modes are effectively suppressed or even eliminated.

In one embodiment the edge structure comprises two stripes per unit cell being arranged directly next to a respective side of the periodic structure. Thus, the edge structure di rectly flanks the periodic structure with no other section in between . In principle, the length of the edge structure is not limited to the periodic length of the periodic structure. The length can be larger than the periodic length or smaller than the periodic length.

The phrase "length" when referred to the transducer itself means the extension in the longitudinal direction. The phrase "width" when referred to the transducer itself means the ex tension in the transversal direction.

The phrase "length" when referred to an electrode finger or to the velocity profile means the extension in the transver sal direction. The phrase "width" when referred to an elec trode finger or to the velocity profile means the extension in the longitudinal direction.

In one embodiment the transducer further comprises one stripe of a gap structure per unit cell flanking the edge structure. The number of electrode fingers per unit cell in the

acoustically active region is twice the number of the

electrode fingers in the gap region. Thus, only one stripe of the gap structure exists in each unit cell. In the gap structure the velocity is larger than the maximal value of the periodic structure. The active region is arranged between the longitudinal sections of the gap structure, i.e. the gap structure is not a part of the active region.

It is possible that the gap structure corresponds to an area of piezoelectric material of the transducer where the ends of electrode fingers of one polarity oppose elements, e.g. the bus bar itself or dummy fingers connected to the bus bar, of the respective other electrode. In one embodiment the gap structure's stripes have a length from 0.5 l to 10 l or, especially, from 1 l to 3 l. Here, the phrase "length" denotes the extension along the transversal direction .

Here, l denotes the wavelength of the main mode, i.e. the wanted acoustic waves propagating in the longitudinal direction. The wavelength l is mainly defined by the periodic length of the finger structure, e.g. of the average periodic length, of the transducer.

The gap structure can be flanked by structures of reduced ve locity. The reduction of the velocity may be caused by an in crease of the finger width or by the mass loading which may be achieved by an additional metal layer.

In one embodiment the gap structure has a metallization ratio h between 0.1 and 0.9, e.g. between 0.2 and 0.8. The

metallization ratio h is defined as

h = ( Wl +W2 +...+Wn) /l where w_± denotes the width of the i-th electrode finger of n electrode fingers within a distance of length l along the longitudinal direction. In a conventional transducer in the active region n equals 2. In a splitfinger transducer n may equal 4. In the region of the acoustic track corresponding to the gap structure only electrode fingers of one polarity may be present. Thus, n may be equal to 1.

In one embodiment the transducer comprises a piezoelectric material, two bus bars arranged on the piezoelectric thin film and aligned parallel to the longitudinal direction and interdigitated electrode fingers. The fingers are arranged on the thin film, connected to one of the bus bars, and aligned parallel to the transversal direction.

The overlap of fingers of opposite polarity defines the ac tive region.

The presence of the electrode fingers on the piezoelectric thin film establishes a convenient way to shape the velocity profile: With the mass of the fingers, the fingers acoustic impedance and electric resistivity details of the wave propagation, especially the wave velocity can be manipulated. Increasing the mass loading of the transducer locally - e.g. via material of the electrode structure of the bus bars and electrode fingers - mainly leads to a decrease of the wave velocity. Increasing the stiffness parameters of the acoustic track - e.g. via a material with a high Young's Modulus - mainly leads to an increase of the velocity.

In one embodiment, accordingly, the transversal velocity pro file is adjusted via one or more measures selected from:

- a reduced velocity by increased mass loading by increased finger width,

- a reduced velocity by increased mass loading by increased finger thickness,

- a reduced velocity by increased mass loading by additional material deposited on the electrode fingers,

- a reduced velocity in the gap region by increased mass loading by dummy patches in the gap region,

- a reduced velocity by increased mass loading by material deposited in stripes on the electrode fingers,

- an increased velocity by reduced mass loading by reduced finger width, - an increased velocity by reduced mass loading by reduced finger thickness,

- an increased velocity by reduced mass loading by material removed from the electrode fingers,

- an increased velocity in the gap structure by decreased mass loading by removing material, e.g. thickness or width of electrode fingers, in a gap region corresponding to the gap structure .

The metallization ratio h in regions of a lower velocity can be in the range from 0.2 to 0.8. Values between 0.3 and 0.7 may be preferred.

The metallization ratio h in regions of a higher velocity can be in the range from 0.1 to 0.7. Values between 0.2 and 0.6 may be preferred.

The periodic length in the periodic structure can be in the range from 0.1 to 3 l, l being the acoustic wavelength (in the longitudinal direction) .

The ratio between the length of the higher velocity divided by the periodic length can be in the range from 0.2 to 0.8. A ratio between 0.4 and 0.6 may be preferred.

For TF-SAW transducers the following is true: The length of the sections of the edge structure may be in the range from 0.1 l to 3 l. Lengths between 0.2 l and 2.5 l may be pre ferred .

It is possible that the transversal velocity profile com prises further periodic or aperiodic or symmetric or asymmet- ric structure. However, it is also possible that the trans versal velocity profile in the acoustic track consists of the above mentioned structures.

The periodic structure of the transversal velocity profile can - exclusively or additionally - be obtained via a

dielectric material having a periodic shape in the

transversal direction. A corresponding periodic dielectric material can have a periodic structure in the active region along the transversal direction (y) according to the

transversal velocity profile. The periodic dielectric

material contributes to the formation of the shape of the periodic structure of the velocity profile.

The use of a dielectric material as a material for setting the local wave velocity has less effects on the electrical properties of the transducer. In particular, stray capacities are reduced compared to a conducting material like a metal. However, the wide range of densities of metals (e.g. up to the density of Gold) cannot be obtained with a dielectric material .

It is possible that the shape of the periodic dielectric material coincides with the periodic structure of the

velocity profile. This may be the case if a segment of the periodic dielectric material has an additional mass and locally reduces the acoustic velocity or if a section of the periodic dielectric material having higher stiffness

parameters increases the acoustic velocity. Then, the segment of the periodic dielectric material and the segment with reduced/increased velocity share the same place of the transducer . The relation between mass/density (p) , stiffness (c) and velocity (v) is: v=sqrt (c/p).

It is possible that the periodic dielectric material is the only reason for the periodic velocity profile. However, other means such as a locally increased finger thickness /

metallization ratio h or sections of the electrode fingers with different stiffness or density and the presence of the periodic dielectric material can work together to form the shape of the velocity profile.

It is possible that the periodic dielectric material is structured from a passivation layer, a structured material from a TCF-compensation layer (TCF: Temperature Coefficients of Frequency) or an additional structured material with the only purpose of forming the shape of the velocity profile.

Thus, it is possible that the periodic dielectric material is arranged directly on the electrode fingers, in or on a passivation layer deposited above the electrode fingers, in or on a TCF-compensation layer deposited above the electrode fingers or on the top side of the transducer.

It is possible that the periodic dielectric material

comprises strips arranged on the electrode fingers.

It is possible that the periodic dielectric material

comprises strips arranged above the electrode fingers.

The stripes can have a quadratic or a rectangular shape.

However, circles and an ellipsoid shape is also possible. It is possible that the periodic dielectric material

comprises strips arranged between the electrode fingers or elevated over center positions between electrode fingers.

It is possible that the periodic dielectric material

comprises strips extending along the longitudinal direction. The length of the strips may equal the length of the acoustic track or of the transducer.

It is possible that the periodic dielectric material has a density different from a density of dielectric material surrounding the periodic dielectric material. It is also possible that the periodic dielectric material has a

stiffness different from a stiffness of dielectric material or metal surrounding the periodic dielectric material.

The stiffness parameters and the density of the dielectric material having the periodic structure are quantities that can be used to affect a wave's velocity. Thus, by choosing an appropriate material at a specific location the velocity profile can be adjusted to match a profile optimized for its wave guiding properties.

The periodic structure of the velocity profile may have a difference in velocity of approx. 30 m/s to 300 m/s between the lowest velocity and the highest velocity. A velocity difference between 70 m/s and 200 m/s may be preferred. High velocity differences may result in the need for narrow edge regions between the transducer's electrodes.

The passivation layer may comprise silicon dioxide, AI₂O₃, AIN, Si_{3 4} or similar dielectric materials. The periodic dielectric material can comprise Ta20s, Nb20s,

HfO, AI₂O₃, AIN, Si3 4, GeCy, SiCt or similar dielectric materials .

A TCF compensation layer can comprise SiCt and doped SiCy.

The SiCy can be doped by F (fluorine) , B (Boron) , Ti

(Titanium) .

It is possible that the periodic structure has two outermost sections with a minimum velocity.

I.e. there are two stripes of a higher velocity in each unit cell arranged next to the periodic structure within the active region.

The two stripes per unit cell can extend over the length of the transducer resulting in a total number of two stripes per transducer .

In principle, the length of the edge structure is not limited to the periodic length of the periodic structure. The length can be larger than the periodic length or smaller than the periodic length.

With a transducer as described above the normalized overlap integral <F | Y_h> for the fundamental mode n = 1 can be in the range of 0.95 or above. As the overlap integral describes the match between the (normalized) excitation function F and the (normalized) wave mode shape Y_h and as the different modes Y_h are orthogonal a value just below 1 prevents higher modes to be excited. Brief description of the Figures

The transducer is explained in greater detail on the basis of exemplary and not limiting embodiments and associated Figures below .

Fig. 1 shows the orientation of the longitudinal direction x with respect to the transversal direction y.

Fig. 2 shows the transversal velocity profile v along the transversal direction y.

Fig. 3 shows the orientation of the transducer with re

spect to the longitudinal direction x and the transversal direction y.

Fig. 4 illustrates a possible causal connection between the velocity profile and a physical realization of the transducer's electrode structure.

Fig. 5 shows electrode structures the material of which is partially removed to obtain the respective velocity profile .

Fig. 6 shows an embodiment of electrode structures with additional material deposited on locally etched electrode structures.

Fig. 7 shows the use of cut outs to reduce the local mass loading to increase the local wave velocity. Fig. 8 shows stripes of a periodic dielectric material ar ranged on interdigitated electrode fingers and be tween the fingers and extending along the longitu dinal direction.

Fig. 9 shows stripes of a periodic dielectric material ar ranged on interdigitated electrode fingers.

Fig. 10 shows a cross section of a transducer with stripes according to Fig. 8.

Fig. 11 shows a cross section of a transducer with a TCF- compensation layer and a passivation layer.

Fig. 12 shows a cross section of a transducer with a struc- tured cover layer.

Fig. 13 shows a cross section of a transducer with a struc- tured layer covered by a passivation layer with a flat surface.

Fig. 14 shows a cross section of a transducer with a struc tured layer covered by a passivation layer following the structure of the structured layer.

Fig. 15 shows a cross section of a transducer with an

intermediate layer covered by a structured pas sivation layer.

Fig. 16 shows a simulation of the real part of the

admittance of a resonator comprising a TF-SAW transducer as described above (2) compared to that of a conventional transducer (1) . Fig . 17 shows a simulation of the imaginary part of the admittance of a resonator comprising the TF-SAW transducer as described above (2) compared to that of a conventional transducer (1) .

Fig. 18 shows a simulation of the magnitude of the

admittance of a resonator comprising a TF-SAW transducer as described above (2) compared to that of a conventional transducer (1) .

Fig. 1 shows a substrate SU which may carry the piezoelectric material (PM) such as lithium niobate (LiNbCy) or lithium tantalate (LiTaCy) . x denotes the longitudinal direction, y denotes the transversal direction. Interdigital transducers are arranged in such a way that the main direction of

propagation is parallel to the x-direction. A crystal

orientation of the piezoelectric material is chosen to obtain a high coupling coefficient.

Fig. 2 illustrates the velocity profile VP of acoustic waves propagating in the thin film shown in Fig. 1. The velocity profile has a periodic structure PS which is flanked by edge structures ES in such a way that the periodic structure PS is arranged between the edge structures ES in the transversal direction y. The periodic structure PS comprises areas with a relative high velocity v and areas with a relative low veloc ity. The areas of high and low velocity alternate in such a way that a periodic velocity profile in the periodic struc ture is obtained. The velocity in the edge structure is lower than the maximum velocity in the periodic structure PS. The velocity in the edge structure may be equal to the lowest velocity in the periodic structure. However, the velocity in the edge structure ES may differ from the lowest velocity in the periodic structure. Also, the length of the edge struc ture is not limited. However, it may be preferred that the length of the respective stripe of the edge structure ES is larger than half of the periodic length of the periodic structure PS. Here the phrase "length" denotes the extension of the edge structure in the transversal direction.

Fig. 3 shows the orientation of the transducer TD comprising bus bars BB and electrode fingers EF with respect to the lon gitudinal direction x and the transversal direction y. The area in which the electrode fingers of opposite electrodes overlap is called the acoustically active region AAR. The bus bars are oriented parallel to the longitudinal direction x. The electrode fingers EF are oriented parallel to the trans versal direction y.

When an RF signal is applied to the bus bars and the bus bars have opposite polarities, an acoustic wave is excited in the piezoelectric material (PM) .

Fig. 4 shows the connection of the electrode structure and the velocity profile v. The velocity of acoustic waves at the surface or an interface of a piezoelectric material depends on the mass loading at the interface. A higher mass loading and/or a higher reflection decreases the velocity. However, higher elastic constants of a material deposited on the pie zoelectric material increase the velocity. Thus, the shape of the velocity profile v along the transversal direction y can directly depend on geometric structures of the material arranged on the piezoelectric substrate, e.g. the electrode structures comprising the bus bars BB and the electrode fin gers EF. To obtain the periodic structure of the velocity profile VP, the electrode fingers EF may have a shape with a corresponding periodic symmetry. To obtain minima in the ve locity profile, the local finger width can be increased com pared to sections where the velocity should be maximal and the according finger width is, thus, reduced. Further, in or der to obtain the edge structure with a reduced velocity com pared to the maximal velocity within the periodic structure, the finger width can be larger in the area corresponding to the edge structure compared to the area corresponding to the highest velocities in the periodic structure.

Fig. 5 shows an important aspect in forming the velocity profile: the height of the metallization establishing the electrode fingers can be varied along the transversal direction to adjust the velocity profile. A periodic profile can be etched into the electrode finger to obtain the periodic structure PS. The thicknesses in the regions corresponding to the edge structures and the gap structures can be adjusted accordingly.

Fig. 6 shows a further embodiment of the transducer shown in Fig. 5 where further to the etching to obtain different thicknesses in the electrode structure, a dielectric, e.g. a silicon dioxide, is arranged on the electrode structures. Further, another material is deposited in stripes in the gap regions to adjust the velocity profile.

Fig. 7 shows the use of cut outs to reduce the local mass loading to increase the local wave velocity. Fig. 8 shows a possible arrangement of periodic stripes con sisting of the periodic dielectric material PDM. The right portion of Fig. 8 illustrates the effect of the dielectric material PDM on the transversal velocity profile. At segments of the electrode fingers EF with the material PDM and at seg ments between the fingers the mass loading is increased and the velocity is accordingly reduced.

Fig. 9 shows a possible arrangement of periodic stripes consisting of the periodic dielectric material PDM. Again, the right portion of Fig. 9 illustrates the effect of the dielectric material PDM on the transversal velocity profile. At segments of the electrode fingers EF with the material PDM the mass loading is increased and the velocity is accordingly reduced. The area between the fingers is free from the peri odic dielectric material.

Fig. 10 shows a cross section of the transducer of Fig. 8.

The mass loading is obtained by arranging the stripes of the dielectric material PDM having a higher density than its sur rounding, in particular the material deposited on the elec trode fingers which may be a material of a TCF-compensation layer TCF compensating different temperature coefficients or of a passivation layer PL.

Fig. 11 shows a cross section of a transducer with a TCF- compensation layer TCF and a passivation layer PL. The piezoelectric material establishes a carrier material similar to a substrate SU on which or above which material of the electrode structure is deposited, e.g. a carrier substrate below the thin film piezoelectric material and the TCF compensation layer. The presence of the piezoelectric material in the form of a thin film adds additional possible locations for a TCF- compensation layer. In addition to components where the material of the TCF-compensation layer is arranged on or above the electrode structure or on or above the

piezoelectric material it is possible to arrange a TCF- compensation layer under, e.g. directly under, the

piezoelectric material. The strongly reduced thickness of the piezoelectric material renders a TCF-compensation layer efficient even if the TCF-compensation layer' s material is arranged at the respective "other" side of the piezoelectric material .

Thus, for all embodiments, the TCF-compensation layer - or any acoustically and/or thermally active layer can be above or below the piezoelectric material.

Fig. 12 shows a cross section of a transducer with a struc tured layer. A periodic transversal pattern is structured into the layer to support the formation of the velocity profile's periodic shape.

Fig. 13 shows a cross section of a transducer with a struc tured layer covered by a passivation layer PL with a flat surface. Although the transducer does not reveal a periodic transversal pattern at its surface, the periodic pattern structured in the structured layer and - as a negative pattern - at the bottom side of the passivation layer PL helps forming the velocity profile VP if the material of the structured layer and of the passivation layer have different stiffness or density values. Fig. 14 shows a cross section of a transducer with a struc tured layer covered by a passivation layer PL. The

passivation layer PL mainly has a constant thickness. Thus, its surface follows the structure of the structured layer.

Fig. 15 shows a cross section of a transducer with an

intermediate layer INL covered by a structured passivation layer PL comprising the periodic pattern needed to support the formation of the velocity profile. The intermediate layer can comprise a dielectric layer. This allows to use a

material for the passivation layer that does not need to fulfill too strict requirements concerning its resistivity. The material of the passivation layer can be chosen according to its chemically inert properties, only, and independent from its electrical properties.

Figs. 16, 17 and 18 show simulations of the real part (Fig. 16), of the imaginary part (Fig. 17) and of the magnitude (Fig. 18) of the admittance of a resonator comprising a TF- SAW transducer as described above compared to that of a conventional transducer. In the Figures curves (2) refer to the improved transducer while curves (1) refer to a

conventional reference transducer. It is clearly visible that unwanted excitation of modes is efficiently suppressed in the above described resonator.

The resonator is not limited to the details described above or shown in the Figures. Resonators comprising additional structures, e.g. reflectors or additional layers above or below the thin film piezoelectric material, are also

comprised . List of Reference Signs:

AAR: acoustically active region

BB: bus bar

DM: dielectric material

DS : dielectric stripe

EF: electrode finger

ES : edge structure

GS : gap structure

INL: intermediate layer

MS : metallic stripe

PS : periodic structure

SU: substrate

TD: transducer

v : velocity

VP: velocity profile

x : longitudinal direction y: transversal direction

G: gap

PDM : periodic dielectric material PM: piezoelectric material

PL: passivation layer

TCF : TCF compensation layer v : velocity

Claims

1. A TF-SAW transducer (TD) , comprising

- a longitudinal direction (x) and a transversal direction (y) orthogonal to the longitudinal direction (x) ,

- an IDT electrode structure comprising electrode fingers arranged on or above a piezoelectric thin film,

- a transversal velocity profile (VP) of acoustic waves propagating in the transducer (TD) ,

- an acoustically active region (AAR) with interdigitating electrode fingers (EF) ,

wherein

- the transversal velocity profile (VP) has a periodic structure (PS) in the active region (AAR) ,

- the periodic structure (PS) has a plurality of minimal values and a plurality of maximal values larger than the minimal values,

- the periodic structure (PS) is flanked on both sides by an edge structure (ES) of the transversal velocity profile (VP) .

2. The transducer (TD) of the previous claim, wherein the thin film comprises or consists of LiNbCy, LiTaCy or quartz .

3. The transducer (TD) of one of the previous claims

comprising dummy fingers.

4. The transducer (TD) of one of the previous claims, where the edge structure (ES) comprises two stripes per unit cell, each of the two stripes being arranged directly next to a respective side of the periodic structure (PS) .

5. The transducer (TD) of the previous claim, where the stripes of the edge structure (ES) have a length 1 being larger than 50 % of a periodic length of the periodic structure (PS) .

6. The transducer of claim 4, where the stripes of the edge structure have a length 1 being smaller than 50 % of a periodic length of the periodic structure (PS) .

7. The transducer (TD) of one of the previous claims,

comprising

- two stripes of a gap structure (GS)

where

- in the gap structure (GS) the velocity (v) is larger than the maximal value of the velocity (v) of the periodic structure (PS) ,

- the active region (AAR) is arranged between the two stripes of the gap structure (GS) .

8. The transducer (TD) of the previous claim, where the gap structure's (GS) stripes have a length from 0.5 l to 10 l or from 1 l to 3 l.

9. The transducer (TD) of one the two previous claims, where the gap structure (GS) has a metallization ratio h from 0.1 to 0.9.

10. The transducer (TD) of one of the previous claims,

comprising

- two bus bars (BB) arranged on the piezoelectric thin film (TF) and aligned parallel to the longitudinal direction (x) , - interdigitated electrode fingers (EF) , each arranged on the piezoelectric thin film (TF) , connected to one of the bus bars (BB) , and aligned parallel to the transversal

direction (y) .

11. The transducer (TD) of one of the previous claims, where the transversal velocity profile (VP) is adjusted via one or more measures selected from:

- a reduced velocity (v) by increased mass loading by

increased finger (EF) width,

- a reduced velocity (v) by increased mass loading by

increased finger (EF) thickness,

- a reduced velocity (v) by increased mass loading by

additional material deposited on the electrode fingers (EF) ,

- a reduced velocity (v) in the gap structure (GS) by

increased mass loading by dummy patches in a gap region corresponding to the gap structure (GS) ,

- a reduced velocity (v) by increased mass loading by

material deposited in stripes on the electrode fingers (EF) ,

- an increased velocity (v) by reduced mass loading by

reduced finger (EF) width,

- an increased velocity (v) by reduced mass loading by

reduced finger (EF) thickness,

- an increased velocity (v) by reduced mass loading by

material removed from the electrode fingers (EF) ,

- an increased velocity (v) in the gap structure (GS) by decreased mass loading by removing material in a gap region corresponding to the gap structure (GS) .

12. The transducer (TD) of one of the previous claims,

comprising - a periodic dielectric material (PDM) having a periodic structure in the active region along the transversal direction (y) according to the transversal velocity profile,

wherein

- the periodic dielectric material contributes to the

formation of the shape of the periodic structure of the velocity profile.

13. The transducer (TD) of the previous claim, where the periodic dielectric material (PDM) comprises

- structured material from a passivation layer,

- structured material from a TCF-compensation layer or

- an additional structured material.

14. The transducer (TD) of one of the two previous claims, where the periodic dielectric material (PDM) is arranged

- directly on the electrode fingers (EF) ,

- in or on a passivation layer deposited above the electrode fingers (EF) ,

- in or on a TCF-compensation layer deposited above the electrode fingers (EF) or

- on the top side of the transducer.

15. The transducer (TD) of one of the previous claims, where the periodic dielectric material (PDM) comprises strips arranged on the electrode fingers.

16. The transducer (TD) of one of the previous claims, where the periodic dielectric material (PDM) comprises strips arranged above the electrode fingers (EF) .

17. The transducer (TD) of one of the previous claims, where the periodic dielectric material (PDM) comprises strips arranged between the electrode fingers or elevated over center positions between electrode fingers.

18. The transducer (TD) of one of the previous claims, where the periodic dielectric material (PDM) comprises strips extending along the longitudinal direction (x) and having the length of the transducer.

19. The transducer (TD) of one of the previous claims, where the periodic dielectric material (PDM) has

- a density different from a density of dielectric material surrounding the periodic dielectric material (PDM) or

- a stiffness different from a stiffness of dielectric material surrounding the periodic dielectric material

(PDM) .

20. The transducer (TD) of one of the previous claims, where the edge structure (ES) comprises two stripes per unit cell, each of the two stripes being arranged directly next to a respective side of the periodic structure (PS) .