WO2020020643A1

WO2020020643A1 - Saw device with a slanted resonator

Info

Publication number: WO2020020643A1
Application number: PCT/EP2019/068578
Authority: WO
Inventors: Christian Huck; Gholamreza Dadgar Javid
Original assignee: RF360 Europe GmbH
Priority date: 2018-07-25
Filing date: 2019-07-10
Publication date: 2020-01-30
Also published as: DE102018118005A1

Abstract

A SAW device comprises on a piezoelectric material an acoustic track extending between two acoustic reflectors. Two or more IDT sections (IS1, IS2) are present in the acoustic track and form at least one IDT of the SAW device. Each IDT section has an extension along a respective slanting axis (SA1, SA2) that encloses an angle with the longitudinal axis (LA). At least two adjacent IDT sections possess different slanting angles.

Description

SAW device with a slanted resonator

The present invention refers to a SAW device that uses a slanted resonator design to suppress spurious transversal modes but has a more compact layout occupying less chip area than present designs. The SAW device may be an

electroacoustic filter or a multiplexer. It may be embodied as a ladder-type or DMS filter.

Electroacoustic filters, e.g. multiplexers, can be used in wireless communication systems. In such filters

electroacoustic resonators are arranged in a filter topology. Electroacoustic resonators employ the piezoelectric effect to convert between RF signals and acoustic waves. Typical electroacoustic resonators are SAW resonators (SAW = surface acoustic wave) including TCSAW (temperature compensated SAW) , TFSAW (thin film SAW) , and GBAW resonators (guided bulk acoustic wave) . In all SAW resonators comb-shaped electrode structures with interdigitating electrode fingers are

arranged on a piezoelectric material. Two interdigitated comb-shaped electrode structures can form an interdigital transducer IDT. Excited acoustic waves propagate along a surface of the piezoelectric material in a preferred

direction parallel to the longitudinal axis which is given by the crystallographic x-axis.

Slanted resonators extend along a slanting axis that encloses a slanting angle with the longitudinal axis. The electrode fingers of the comb-shaped electrode structures extend normal to the longitudinal axis but are continuously shifted to each other in transversal direction when going from a first finger to an adjacent second finger. Slanted SAW resonators can be used as an alternative to apodized resonators for suppression of spurious transversal modes, e.g. in TCSAW or TFSAW. Radio frequency filters based on such slanted resonators typically use the same and thus a unitary slanting angle of the

acoustic track along the entire resonator length at all resonators to achieve a compact layout. Moreover all acoustic tracks comprising slanted resonators use the same slanting angle. However, while this slanting approach works well for resonators used in Tx filters based on a ladder-type design, it is not applicable to Rx filters based on DMS structures due to reduced longitudinal acoustic coupling between

neighboring interdigital transducers - IDTs - connected in series .

Further, a slanted design has a larger dimension on the surface of the filter chip in two lateral dimensions and thus needs more surface area. As far as an acoustic track with slanted resonators and another acoustic track that is not slanted are arranged on the same chip this requires still more chip area than a design where all acoustic tracks are either non-slanted or slanted with the same angle.

Hence it is an object to provide a SAW device that avoids the above mentioned problems.

This and other objects are met by a SAW device according to the independent claim. More detailed features and beneficial embodiments are given by dependent claims.

A SAW device is proposed comprising a piezoelectric material. The piezoelectric material may be a bulk material that is useful for forming a substrate of the SAW device or a thin film layer on an arbitrary carrier substrate. An acoustic track formed on top of the piezoelectric material extends between two acoustic reflectors. Two or more IDT sections are arranged in the acoustic track and form at least one

interdigital electrode of an electroacoustic resonator. Each resonator is adapted to excite a SAW propagating along a longitudinal axis defined in the surface of the substrate:

All electrode fingers of the resonators are arranged normal to the longitudinal axis.

Each IDT section extends longitudinally along a respective slanting axis that follows the transversal center of the IDT section and encloses an angle a with the longitudinal axis of -30° < a £ 30°. A first angle al of a first IDT section is different from a second angle a2 of a second IDT section arranged directly adjacent to the first IDT section in the same acoustic track.

Each IDT section comprises two comb-shaped electrode

structures that are interdigitating and form an overlap region. Opposite to the fingertips of overlapping fingers of the interdigital electrode non-overlapping stub fingers may be present that are connected to the busbar that is opposite to that of the respective busbar the overlapping finger is connected to. The slanting axis of an IDT section extends parallel to the transversal center of the respective IDT section and thus, the slanting axis is parallel to the extension of the overlap region.

In the following a design comprising such a SAW resonator structure with two IDT sections is called a broken slanted resonator design. The busbars may be arranged parallel to each other and parallel to the slanting axis. In an IDT section that is slanted relative to the longitudinal axis the electrode fingers are still normal to the longitudinal axis but no more normal to the transversal center of the IDT section given by the slanting axis.

Alternatively the busbars are not parallel to the slanting axis and - together with the longitudinal axis - include an angle smaller than the angle between the slanting axis and the longitudinal axis.

In a preferred embodiment the busbars are parallel to the x- axis. Such an arrangement needs stub fingers of different lengths filling out a respective triangle of the non-overlap regions between the overlap region and a respective busbar.

Preferably, non-overlapping stub fingers are also present in those SAW devices where the busbars are extending parallel to the slanting axis.

At least one overlap region of an IDT section of the SAW device is slanted. Due to the slanted orientation the overlap between a first and an adjacent second electrode finger is shifted towards a transversal direction normal to the

longitudinal axis by a small amount relative to the overlap between the second and an adjacent third electrode finger.

The directly adjacent IDT section may be slanted too or may have a slanting angle a equal to zero.

Such a SAW device provides additional degrees of freedom when designing the SAW device that may be a filter. The angled arrangement of different IDT sections allows to optimally exploit the surface area without losing too much space and hence without wasting precious surface area while spurious transversal modes are still suppressed.

Preferably a first slanting angle ai of a first IDT section can be chosen to be between 0° and 30°, while a second slanting angle 0,2 of a second IDT section can be chosen to be between 0° and -30°. In this context and by definition an angle of a positive value has to be understood to denote an angle that is measured counterclockwise while a negative angle denotes an angle measured clockwise.

In a preferred embodiment a first slanting angle ai of a first IDT section has the same absolute value like the slanting angle 0,2 of an adjacent second IDT section but has an inverse sign. More preferably, the first slanting angle ai is set to 5° < (Xi < 15° and the second angle 0,2 is set to -5° > 02 > -15⁰.

By this angle combination the first IDT section is slanted towards the longitudinal axis while the second IDT section is slanted to extend away from the longitudinal axis or vice versa. In both cases the two adjacent IDT sections form a V- shaped arrangement that encloses an angle between 120°and 180° .

According to an embodiment the first and second IDT section belong to the same IDT and thus have two common busbars.

Hence, a bent or broken IDT is achieved.

According to a second embodiment a third and a fourth IDT section have different slanting angles and belong to different IDTs and are thus electrically isolated against each other. In these cases the longitudinal ends of the IDT sections are facing each other so that the apertures of the two adjacent IDTs/IDT sections provide a maximum transversal overlap and a maximum longitudinal acoustic coupling can take place. In either case the coupling between two IDT sections that enclose an angle there between is better than the coupling between two IDT sections that are slanted by the same angle and thus extend linearly over the full length of the two IDTs, respectively IDT sections. By setting two suitable angles the amount of coupling between neighbored IDTs/IDT sections can be adjusted within a given range.

A SAW device according to the second embodiment can be embodied as a longitudinally coupled dual mode SAW filter that is a DMS filter.

In the first or second embodiment a first slanting angle ai and a second slanting angle 0,2 may be equal in their absolute values | ai | = | a₂ I but have inverse signs a₂ = - ai .

Three or more IDT sections can form a zigzag topology that allows saving more surface area on the substrate that can be used for arranging there other structures or for reducing the size of the SAW device.

In a zigzag topology the angles of subsequent IDT sections are alternatingly higher and lower than the respective angle of the foregoing IDT section. This means a series of at least three subsequent slanting angles ai to a₃ with the condition ai > a₂ < a3 or ai < a₂ > a.3. In a specific embodiment the following may be true: (Xl = 0,3 = -0^,2.

In a zigzag topology or in an arrangement of just two

adjacent IDT sections the absolute values of a first angle ai and a second angle 0₂ may be equal but not equal to zero that I ai | = | a.₂1 and 0₂ = - ai . For more than two IDT sections this can be true for each pair of neighbored IDT sections. Then a symmetrical arrangement is achieved. A symmetric arrangement relative to the middle of the track is beneficial for a DMS filter and is hence an aimed design goal.

In a zigzag topology IDT sections may form a single elongated IDT. A zigzag topology can also be formed by some IDT

sections that are electrically isolated and hence form separate IDTs each.

In a beneficial embodiment two subsequent IDT sections form a V-shaped arrangement with two legs. The area between the two legs can then be used to place there a passive element that can be formed by a metallized structure on the surface of the substrate. The passive element may be electrically connected to at least one of the IDT sections. Such passive elements can be used as matching elements of the SAW device e.g. of a SAW filter device. These elements may be connected to one or more IDT sections in series or in parallel. The passive element may be e.g. a capacitance or an inductance. By arranging such a passive element between the two legs of a V- shaped arrangement of two subsequent IDT sections an

efficient space management can easily be achieved.

Such a passive element can be coupled to an inner busbar of the first leg of the V-shaped arrangement of IDT sections. However, it may be also advantageous to place the passive element between the two legs without any contacting of the element to any of the busbars of the V-shaped arrangement.

It is possible that two or more passive elements that may be of the same or of different kind are arranged on the surface of the substrate between the legs of a V-shaped arrangement of two adjacent IDT sections.

But alternatively, any other conducting element of the SAW device may also be coupled to the passive element and at least one IDT section.

Finally, a SAW device may comprise different forms of IDTs and IDT sections arranged in a multitude of acoustic tracks. One of these tracks comprises two or more slanted IDT

sections with different slanting angles a. Further, each of the other tracks independently comprises an arrangement of

- just one slanted IDT section where the slanting angle is not zero, or

- two or more IDTs with one or more IDT sections each

comprising one slanted IDT section where the slanting angle is not zero, or

- one IDT with one or more IDT sections that have a

slanting angle a of zero degrees.

In such a SAW device at least two of the tracks have

different arrangements. All possible embodiments as explained above can be realized in a single SAW device on one and the same substrate. Hence such a device adds a variety of

additional degrees of freedom for the designer of the SAW device e.g. the filter device.

The SAW device may have a substrate that is cut from a crystal ingot. Moreover the SAW device may be embodied as TCSAW device (= temperature compensated SAW) that comprises an additional layer of a dielectric material with a positive temperature coefficient of frequency. Alternatively the invention can be embodied in a TFSAW (= thin film SAW) formed from a wafer as mentioned above with subsequent thinning of wafer thickness. The piezoelectric wafer may be wafer-bonded to an arbitrary carrier substrate before thinning. A TFSAW may also be formed by epitaxial deposition of a piezoelectric material on a carrier substrate. Furthermore, one or more functional layers may be added to provide a desired

functionality to the TFSAW wafer.

The SAW device may have electrode fingers and opposing stub fingers with a transversal gap between the electrode finger and the facing tip of a respective stub finger that is minimized to be for example between 100 nm and 500 nm. It is advantageous to minimize the transversal gap thereby

respecting power durability (PD) and electrostatic discharge (ESD) obligations during fabrication.

According to another embodiment the SAW device may have an IDT section whose busbars are oriented parallel to the slanting axis and the length of the stub fingers is chosen from 0.5l to 5 l wherein l is the wavelength of the acoustic wave propagating in the acoustic track.

According to yet another embodiment the SAW device may have two adjacent IDT sections belonging to the same IDT but having different lengths and different slanting angles.

In a more specified embodiment the device is arranged on a substrate comprising a thin film of lithium tantalite LT with a crystal orientation y-cut rot 42XY or y-cut rot 50XY. The device has a slanted IDT section with a preferred

slanting angle a of |a| = 10° ± 2° and stub fingers of a preferred length from 2l ± 0.5 l.

In the following section the invention will be explained in more detail with reference to specific embodiments and the related figures. The figures are only schematic and may depict some details in enlarged form or may fail to show possible details that are described or shown at another location or figure.

Figure 1A shows in a schematic depiction two IDT sections enclosing different angles with the longitudinal axis,

Figure IB shows more details of the IDT section that is depicted only schematically in Figure 1A;

Figure 2 shows simulated admittance curves of a structure according to Figure 1A compared with a structure as shown in Figure IB;

Figure 3 shows another embodiment of two IDT sections enclosing different angles with the longitudinal axis;

Figure 4 shows four subsequent IDT sections that form a zigzag arrangement;

Figure 5A shows schematically two longitudinally acoustically coupled subsequent IDT sections that are slanted with the same slanting angle; Figure 5B shows schematically two longitudinally acoustically coupled subsequent IDT sections that are slanted with

different slanting angles;

Figures 6 shows two subsequent IDT sections forming a V- shaped arrangement with a passive element arranged between the two legs of the V;

Figure 7 shows a slanted IDT section with busbars oriented in parallel to the longitudinal axis and resulting stub fingers of different length in the non-overlap region;

Figure 8 shows an arrangement of two subsequent IDT sections slanted to the longitudinal axis with different angles and two busbars that are oriented in parallel to the longitudinal axis with stub fingers of varying length arranged in the non overlap region;

Figure 9 shows an embodiment where three IDTs are arranged in an acoustic track between two acoustic reflectors in a zigzag topology forming a longitudinally coupled dual mode SAW filter;

Figure 10 shows another embodiment of a DMS filter with three IDTs where each IDT comprises a number of different slanted IDT sections within the same IDT;

Figure 11 shows a SAW device comprising a number of different acoustic tracks with different arrangements of resonators, IDTs and IDT sections.

Figure 1A shows a simple embodiment of the invention

comprising two adjacent IDT sections IS1 and IS2 in a simplified depiction. A first IDT section RSI extends along a first slanting axis SA1 that includes an angle cxl to the longitudinal axis LA where the longitudinal axis is the longitudinal propagation direction of the acoustic wave. The directly adjacent second IDT section IS2 includes a slanting angle cx2 to the longitudinal axis LA where cxl is not equal to cx2. The second IDT section IS2 extends parallel to the second slanting axis SA2. For clarity reason each slanting axis SAi is depicted adjacent to the respective IDT section ISi. The slanting angles oq may have absolute values from zero up to 30 degrees. An optimized slanting angle is chosen in dependence on the actual SAW material system and the desired properties of the SAW device that the depicted arrangement is a part of. For simplicity reasons Figure 1A does neither show other IDT electrodes nor the acoustic reflectors at the end of each track nor further elements that are necessary for forming a SAW filter device.

Figure IB shows an exemplary IDT section IS depicting most important parts thereof. The IDT section IS comprises two busbars BB, BB' from which electrode fingers EF are extending to interdigitate alternatingly . The electrode fingers EF are oriented normal to the longitudinal axis LA and form an overlap region that extends parallel to a slanting axis SA. A slanting angle a is measured between the longitudinal axis and the slanting axis SA. The busbars BB may be oriented in parallel to the slanting axis or alternatively deviate from such a parallel orientation. Not shown are stub fingers that are present in a preferred IDT section design in the non overlap region that is arranged between the overlap region and a respective busbar. If the orientation of the busbar BB deviates from the orientation of the slanting axis SA a non overlap region is formed having a triangular shape (shown in Figure 7 or 8 for example) . Preferably the overlap between two adjacent electrode fingers EF is the same along the whole length of the IDT section IS and more preferably is the same in all IDT sections IS.

Figure 2 shows the simulation results of a one-port resonator with two slanted IDT sections with different slanting angles forming a V-shaped arrangement as depicted in Figure 8, for example. As a reference a SAW resonator comprising one slanted IDT section only is used, as shown in Figure 7 for example. Both resonators are comparable in their static capacitance due to the same active aperture given by the area of the overlap region OR.

Figure 2 shows the real part, the imaginary part and the absolute value of the admittance. From the figure it can be taken that the admittance curves of a slanted geometry as known from the art, and a broken slanted geometry according to the invention (with two IDT sections slanted differently to the longitudinal axis) have a comparable course. Within the stop band of the resonator nearly no difference can be recognized. It seems that a broken slanted design according to the invention makes the curve more even with smaller ripple in the upper stop band half. Beyond the two stop bands the admittance of the broken slanted design shows more ripple which seems to be due to the occurrence of longitudinal

Fabry-Perot resonances that occur more strongly at the interface of two adjacent IDT sections which have slanting angles with opposite signs.

Despite showing only the admittance of a one-port resonator these findings suggest that such positive results can also be achieved in a DMS filter using broken slanted resonator design .

Figure 3 shows another embodiment of how two adjacent IDT sections IS1, IS2 can be arranged relative to one another. In this example the first IDT section IS1 includes a slanting angle cxl to the longitudinal axis while the second IDT section IS2 extends parallel to the longitudinal axis such that the slanting angle of the second IDT section IS2 is 0. Further, the length of the depicted two IDT sections is different but may also be the same.

Figure 4 shows a zigzag arrangement of subsequent IDT

sections IS. Depicted are four IDT sections IS1 to IS4, but a zigzag arrangement can generally be achieved with three or more IDT sections. Each IDT section IS comprises a slanting angle that is enclosed between the slanting axis SA of the respective IDT section and the longitudinal axis LA. Each IDT section IS may have a different slanting angle. Each IDT section may have a length that may be equal for all IDT sections. Moreover, the length may be different for two adjacent IDT sections or may be different for all of the IDT sections. A single IDT may comprise one or more of these IDT sections IS.

Each of the IDT sections includes a slanting angle to the longitudinal axis LA where the slanting angles of two

subsequent IDT sections ISn, IS(n+l) are different, e.g. at least in sign. As shown in Figure 4 a zigzag arrangement of IDT sections may extend as a whole in parallel to the

longitudinal axis but it is also possible that the zigzag topology extends with an angle relative to the longitudinal axis. This means that not only IDT sections are slanted but also the total zigzag arrangement can be slanted against the longitudinal axis.

Moreover, despite a symmetric arrangement of IDT sections is preferred when realizing a DMS filter the arrangement may also have no symmetry element.

Figure 5 shows two adjacent IDT sections IS1, IS2 that may form part of a DMS filter. While the IDT sections of Figure 5A are both slanted with the same slanting angle such that they share the same slanting axes SA in Figure 5B the two IDT sections are arranged with different slanting angles in the broken slanted design according to the invention. The

depicted arrows symbolize the longitudinal acoustic coupling between two IDT sections. It becomes evident that the broken slanted design in Figure 5B allows a better longitudinal acoustic coupling with regard to the design of Figure 5A.

In all embodiments each two subsequent IDT sections IS1, IS2 with different slanting angles form a V-shaped arrangement. Thereby some free space between the inner legs of the V- shaped arrangement is spared allowing to arrange therein an element like a passive element PE.

Figure 6 shows a very general depiction of such an

arrangement that uses the free space between the two legs of the V-shaped arrangement. A passive element PE may be

interconnected to one or both IDT sections or to any other element of the SAW device or of the circuit the SAW device is arranged in. The passive element PE may be a capacitance or an inductance for example. It may be formed by a structured metallization on top of the free substrate surface.

Alternatively a discrete passive element can be arranged on the substrate between each two legs of a V. The passive element PE may connected to one leg, to two legs or is just arranged between the legs to only use the free space without being connected to a busbar of the V or of another IDT section. If connected to a resonator the passive element PE may be used as a matching element of the SAW device.

An arrangement where the free space between the two legs of the V-shaped arrangement is used by placing any element of the SAW device or a circuit there results in a better

exploitation of the available chip area. Then it is possible to reduce the area of the SAW device because the space for the additional element like the passive element PE is saved at another location on the surface of the substrate.

Figure 7 shows an IDT section IS comprising one interdigital transducer. The transducer comprises a first and a second busbar BB1, BB2. Electrode fingers EF are extending from each busbar to interdigitate in an overlap region OR. In the figure, the overlap region is surrounded and enclosed by a virtual frame to better characterize the area. Between the tip of an electrode finger EF and the busbar that is not connected to this electrode finger EF, a stub finger SF is arranged. Thereby the non-overlap region between the overlap region and a respective busbar BB is filled with stub fingers and the non-overlapping section of the electrode fingers EF.

A further feature of the depicted interdigital transducer is the orientation of the overlap region OR that is parallel to the slanting axis of this IDT section. Contrary to the formerly described arrangements, the busbars are not parallel to the slanting axis. Hence, the overlap region OR is

orientated along the slanting axis SA and, thus, slanted against the linearly extending busbars. This means that each non-overlap region of the IDT section has an area in the shape of a triangle. Then the stub fingers SF have

necessarily various lengths to completely fill the non overlap region NOR. However, one of the slanting axes SA may be oriented in parallel to the longitudinal axis such that besides the unavoidable transversal gap and optionally short stub fingers SF no varying non-overlap region NOR is formed adjacent to this IDT section IS.

Figure 8 shows the arrangement of two such IDT sections IS1, IS2, each having a different slanting angle a relative to the longitudinal axis. Both adjacent IDT sections share the same busbars BB1, BB2 and each busbar has a linear and straight extension that may be arranged parallel to the longitudinal axis but not parallel to the slanting axis of any of the two IDT sections. Here too, the schematically depicted non-overlap region NOR between the overlap region OR and the opposing busbar BB is filled with stub fingers SF.

According to a variant the non-overlap region NOR may be covered with a continuous metal layer that can be formed by structuring one or more busbars accordingly. Then, a

respective busbar section has triangular shape.

IDTs formed by at least one IDT section are arranged within an acoustic track between two reflectors RF. A DMS filter comprises two or more IDTs. Preferably an odd number of IDT electrodes is used for designing a DMS filter to allow a symmetric arrangement of the IDT electrodes relative to the longitudinal middle of the acoustic track. Figure 9 shows a schematic block diagram of a DMS filter comprising three interdigital transducers IDT1 to IDT3, each interdigital transducer IDT comprising an IDT section IS as described above such that the DMS filter has a broken slanted design. Each of the slanting angles of the IDT sections may be different. Slanting angles al and a2 may alternate according to the relation

al = (- a2)

to form a regular symmetric zigzag arrangement of IDT section IS. A reflector RF each is arranged at both lateral

(longitudinal) ends of the acoustic track of the DMS filter.

However, the interdigital transducers which form resonators of the DMS structure are not restricted to comprise only one IDT section each. Hence, each resonator may comprise two or more IDT sections that are slanted with a respective slanting angle where different IDT sections may have different

slanting angles.

A DMS filter may have more than three interdigital

transducers that are usually alternatingly connected to a first and a second terminal.

Figure 10 shows a further embodiment of a DMS filter

comprising at least three interdigital transducers IDT1, IDT2 and IDT3. The first interdigital transducer IDT1 comprises two IDT sections IS1, IS2 each having a respective slanting angle al, a2 relative to the longitudinal axis. In this embodiment the first slanting angle al is greater than 0 and greater than the second slanting angle a2 which may be zero as shown in the figure or not. The second interdigital transducer IDT2 comprises three IDT sections IS3 to IS5, each IDT section IS including a

respective slanting angle relative to the longitudinal axis. The third IDT section IS3 is arranged with a low slanting angle preferably of zero like the second IDT section IS2.

This allows maximum longitudinal acoustic coupling between second and third IDT section and hence maximum coupling between first and second interdigital transducers IDT1 and IDT2. The slanting angle 4 of the fourth IDT section IS4 which is the second IDT section of the second transducer IDT2 and which is arranged in the middle of the second

interdigital transducer IDT2, is greater than the slanting angle 3 of the third IDT section IS3 and greater than the slanting angle 5 of the fifth IDT section IS5.

The third interdigital transducer IDT3 on the right side of the figure comprises two IDT sections IS6 and IS7 each including a respective slanting angle b, 7 to the

longitudinal axis. The slanting angle 7 of the outermost right IDT section IS7 is greater than the slanting angle b of the sixth IDT section IS6.

As a consequence, the outermost IDT sections of each

interdigital transducer IDT that are facing each other may have a small slanting angle or a zero slanting angle. It is also possible that the slanting angles of each two outermost IDT sections that are directly adjacent to each other are equal but not zero. Hence, the two adjacent outermost IDT sections between first and second or second and third

interdigital transducer IDT extend in parallel or almost in parallel. In the figure, the slanting angles of outermost IDT sections IS2, IS3, IS5 and IS6 are depicted to be zero but this is not a necessary feature of the invention as explained above .

By this arrangement the longitudinal acoustic coupling between the adjacent interdigital transducers is at a maximum as indicated in the figure with the double-sided arrows.

If the two adjacent outermost IDT sections would be inclined relative to each other, the coupling would be reduced. Hence, the arrangement of the DMS filter depicted in Figure 10 combines the advantage of a slanted orientation for

transversal mode suppression with the advantage of a high longitudinal acoustic coupling between the outermost IDT sections of two adjacent resonators. In this embodiment each present slanting angle may be different from the other used slanting angles. But it is preferred to design a DMS filter with a high symmetry relative to a middle transducer or a middle IDT section. A symmetric arrangement of transducers may be achieved if IDT sections that have the same symmetric element are equal in their absolute values of slanting angle and equal in length.

The IDT sections of the DMS filter as shown in Figure 10, for example, may have different lengths. It is preferred that the outermost IDT sections with the lowest slanting angles have a smaller length than the other IDT sections but they need to be long enough to ensure optimum longitudinal acoustic coupling between adjacent IDTs. Further, it is possible to divide a resonator in more than the depicted two or three IDT sections such that an according interdigital transducer may comprise four or more IDT sections. Short IDTs may have only one IDT section. All possible variations can be used to increase the degrees of freedom when designing a specific DMS filter. The

optimization of the filter can be made towards better filter performance or towards better use of chip area. Usually a trade-off has to be made which can be optimized by the possible variations.

Further variations of the SAW filter are possible which are per se known from the art and can advantageously improve the SAW device. The mode that propagates in the acoustic track of the SAW filter can be formed as a pure piston mode by adding mode-forming features to the design of the electrode fingers. Such features may comprise additional mass load at the finger tips or a greater finger width at the tips thereof. Different gap lengths are possible to reduce unwanted transversal modes. It is preferred that the transversal gap is as small as possible. With the present available technology, a small gap of 100 nm to 500 nm can be achieved.

In a slanted IDT section the aperture that is defined by the transversal length of a finger overlap is shifted along the longitudinal axis from finger to finger in y-direction. But the shift is small enough that the apertures that have the greatest shift relative to the outermost aperture at the beginning or the end of the resonator still have a mutual overlap when looking parallel to the longitudinal axis. This means that the coupling between different ends of an IDT section is still high enough to allow suitable operation of the resonator.

Figure 11 exemplarily shows a SAW device comprising a number of resonators and filters that may be divided into slanted IDT sections as well as resonators that are not divided into IDT sections and resonators that are not slanted relative to the longitudinal axis. In the left part two V-shaped

arrangements of two IDT sections each arranged adjacent to each other so that the IDT sections of the two arrangements are cascaded in parallel to each other. In the right part of the depicted circuit a zigzag arrangement of IDT sections is shown providing a DMS structure according to the invention. The top resonator shown in the circuit and the bottom

resonator comprise one IDT section only that forms no

slanting angle to the longitudinal axis. By such an

arrangement of differently oriented resonators and IDT sections an optimal exploitation of the available space can be achieved without strong performance degradation. The invention has been explained with reference to a limited number of embodiments and figures but is not restricted to the shown embodiments. The broadest scope of the invention is defined by the combination of features given in claim 1.

List of used reference symbols

Claims

1. A SAW device comprising

- a piezoelectric substrate having at least a layer of a piezoelectric material

- an acoustic track on the piezoelectric material

extending between two acoustic reflectors (RF)

- two or more IDT sections (IS) forming at least one

interdigital transducer (IDT) of an electroacoustic resonator

wherein

- each IDT section is adapted to excite a SAW propagating parallel to an longitudinal x-axis defined in the surface of the substrate, all electrode fingers of the interdigital electrode being arranged normal to the x- axis

- each IDT section (ISn) has a slanting axis (SA) that encloses a slanting angle a_n with the longitudinal axis of -30° < <X_n £ 30°

- a first angle ai of a first IDT section (IS1) is set

different from a second angle 0,2 of a second IDT section (IS2) arranged directly adjacent to the first IDT section ( IS1 ) .

2. The SAW device of the foregoing claim,

wherein the first angle ai is set to 5° < ai < 15°

wherein the second angle 0,2 is set to -5° > op > -15°.

3. The SAW device of one the foregoing claims,

wherein the first and second IDT section (IS1,IS2) are directly adjacent to each other in the acoustic track, belong to the same interdigital transducer and, thus, have common busbars .

4. The SAW device of one the foregoing claims,

wherein the slanting angles s, ou of a third and a fourth IDT section (IS3,IS4) are different

wherein third and fourth IDT section are directly adjacent to each other but belong to different IDTs.

5. The SAW device of one the foregoing claims,

comprising three or more IDT sections (IS1,IS2,. . , IS_n) that are arranged in a zigzag topology such that the slanting angles a_n of subsequent IDT sections (IS_n) are alternatingly higher and lower than the slanting angle of the respective foregoing IDT section (IS_n-i).

6. The SAW device of one the foregoing claims,

wherein the absolute values of a first slanting angle ai and a second slanting angle 0,2 are equal in their absolute values I eg | = | a.21 and 0,2 = - ai .

7. The SAW device of one the foregoing claims,

embodied as a longitudinal coupled dual mode SAW filter comprising three or more IDTs arranged in the acoustic track between the two reflectors (RF)

each IDT comprising at least one IDT section (IS) .

8. The SAW device of one the foregoing claims,

wherein two subsequent IDT sections (IS) form a V-shaped arrangement with two legs - wherein the area between the two legs is occupied by a passive element formed by a metallization on the surface of the substrate

- wherein the passive element is or is not electrically connected to one of the IDTs.

9. The SAW device of the foregoing claim,

wherein the passive element is a capacitance or an inductance coupled to an inner busbar of a leg.

10. The SAW device of one the foregoing claims,

wherein the passive element is a capacitance or an inductance coupling the inner busbars of two IDT sections belonging to different IDTs.

11. The SAW device of one the foregoing claims,

- wherein each IDT section has electrode fingers

alternatingly connected to a first and a second busbar

- wherein electrode fingers are connected to first and

second busbar and mutually overlap in an overlap region

- wherein a non-overlapping region is arranged between a respective busbar and the overlap region

- wherein stub fingers are arranged in the non-overlapping region

- wherein the angle between the busbars and the

longitudinal axis is different from the angle between the slanting axis and the longitudinal axis.

12. The SAW device of one of the foregoing claims,

wherein the angle between the busbars and the longitudinal axis is zero and the absolute value of the angle between the slanting axis and the longitudinal axis is greater than zero.

13. The SAW device of one the foregoing claims,

comprising a multitude of acoustic tracks

wherein each track independently comprises an arrangement chosen from the group of

- just one slanted IDT section, or

- two or more slanted IDT sections with different slanting angles a and section widths, or

- two or more IDTs with one or more IDT sections, or

- one IDT with one or more IDT sections having a slanting angle a of zero degrees

wherein at least two of the tracks have different

arrangements chosen from the above group.

14. The SAW device of one the foregoing claims,

embodied as a thin film SAW - TF-SAW - device with or without temperature compensation means, a temperature compensated bulk SAW - TC-SAW - or a non-compensated bulk SAW device.

15. The SAW device of one the foregoing claims,

wherein a transversal gap between an electrode finger and the facing tip of a respective stub finger is minimized to be between 100 nm and 500 nm.

16. The SAW device of one the foregoing claims,

wherein the busbars of an IDT section are oriented parallel to the slanting axis and the length of the stub fingers is from 1.5 l to 2.5 l wherein l is the wavelength of the acoustic main mode propagating in the acoustic track.

17. The SAW device of one the foregoing claims,

wherein two adjacent IDT sections of the same IDT have different lengths and different slanting angles.

18. The SAW device of one the foregoing claims,

arranged on a substrate comprising a thin film or a bulk material of LT with a crystal orientation y-cut rot 42XY or y-cut rot 50XY

having a slanted IDT section with a slanting angle a of |a| = 10° ± 2°

wherein stub fingers of a length from 1.5 l to 2.5 l are present in a non-overlapping region

a transversal gap between the tips of an overlapping finger and a respective stub finger is set to about 350 nm or less.

19. The SAW device of one the foregoing claims,

comprising a first and a second IDT directly adjacent to each other

wherein the first IDT comprises two IDT sections, at least one of these IDT sections has a slanting angle that is not zero

wherein the slanting angles of the two outermost IDT sections of first and second IDT that are facing each other are equal .