APPARATUS COMPRISING SAID SURFACE ACOUSTIC WAVE RESONATOR

ELEMENT

TECHNICAL FIELD

The disclosure relates to a surface acoustic wave resonator element comprising a transducer arrangement.

BACKGROUND

A SAW (surface acoustic wave) device is a device using the propagation of elastic waves on a surface of a material or at an interface between several materials, and are commonly used in micro-mechanical resonators and filters. SAW devices use so called interdigitated transducers (IDTs) to transform radio-frequency (RF) signals into acoustic waves or acoustic waves into RF signals.

SAW devices ususally comprise two IDTs which are spaced from and aligned with one another on a substrate surface for the propagation of surface acoustic waves therebetween. The IDTs comprise electrode fingers which, in the longitudinal direction, i.e., in the direction in which the acoustic waves propagate, are arranged alongside one another. The fingers are usually connected alternately to a first and a second busbar. The acoustic track is the region of the substrate in which surface acoustic waves propagate during the operation of the device.

One problem associated with SAW devices is the generation of transverse acoustic modes which is due to the elastic wave velocity in the IDT region being slower than the velocity in the busbar regions. Transverse modes disturb the transmission characteristic of the device and constitute a loss mechanism.

Hence, there is a need for solutions that are capable of suppressing undesired acoustic waves in SAW devices and thereby improve their performance. SUMMARY

It is an object to provide an improved SAW device. The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description, and the figures.

According to a first aspect, there is provided a surface acoustic wave resonator element comprising a transducer arrangement, the transducer arrangement comprising an active area; two conductive areas; a first interface area and a second interface area, each interface area extending between the active area and one of the conductive areas. The active area comprises a plurality of longitudinally extending interdigitating electrodes, each interface area comprising a plurality of longitudinally extending dummy fingers, and each dummy finger sharing a longitudinal axis with one electrode. Adjacent interdigitating electrodes and adjacent dummy fingers are separated by longitudinally extending gaps, each gap extending from the first interface area to the second interface area. The transducer arrangement further comprises at least one continuous transverse mode suppression structure, the continuous transverse mode suppression structure extending within the first interface area, the active area, and the second interface area; or being superimposed with the first interface area or the second interface area.

Such a solution facilitates achieving a near near piston mode of operation by tuning the in-plane surface acoustic wave dispersion. The piston mode of operation is a prerequisite for successfully suppressing the spurious transverse modes of multi-layer surface acoustic wave resonator elements. The piston mode is achieved by means of the high velocity facilitated by the continuous transverse mode suppression structure arranged between the electrodes, or dummy fingers, which also may allow tuning multiple resonant frequencies while keeping the surface acoustic wave resonator element thickness and metallization the same.

In a possible implementation form of the first aspect, the continuous transverse mode suppression structure extends coplanarly with the electrodes and the dummy fingers in a first plane, tuning the in-plane wave dispersion to be near vertical. This makes the use of electrode slanting unnecessary, alleviating the associated loss effects, while allowing use of a single lithography step.

In a further possible implementation form of the first aspect, the surface acoustic wave resonator element comprises a plurality of continuous transverse mode suppression structures, each continuous transverse mode suppression structure being configured to fill one of the longitudinally extending gaps, allowing the thickness of the surface acoustic wave resonator element to be kept at a minimum.

In a further possible implementation form of the first aspect, a dimension of the continuous transverse mode suppression structures is smaller than a corresponding dimension of the electrodes, the dimensions extending in a direction perpendicular to the first plane, the difference in dimensions allowing tuning of multiple resonant frequencies.

In a further possible implementation form of the first aspect, the dimension of the electrode is larger than 0.05 x electrode pitch and smaller than 0.25 x the electrode pitch, the electrode pitch being a distance between center axes of two adjacent electrodes, allowing a desired tuning to be achieved.

In a further possible implementation form of the first aspect, the continuous transverse mode suppression structure extends at least partially in a plane parallel with a plane comprising the electrodes and the dummy fingers. This improves waveguiding while suppressing the undesired transverse mode effects. Furthermore, it will result in an improved resonance Q factor.

In a further possible implementation form of the first aspect, the surface acoustic wave resonator element comprises two continuous transverse mode suppression structures, each continuous transverse mode suppression structure being superimposed with one of the first interface area and the second interface area. This improves waveguiding while providing a means for spurious mode suppression through piston-load structures in the vicinity of the interface between the interface areas and the active area. Furthermore, it will result in an improved resonance Q factor.

In a further possible implementation form of the first aspect, the continuous transverse mode suppression structure is arranged above or below the dummy fingers, as seen in a direction perpendicular to the second plane. This facilitates a simple and flexible manufacturing process.

In a further possible implementation form of the first aspect, a dimension of the continuous transverse mode suppression structure is between 0.02 x electrode pitch and 0.2 x electrode pitch, the electrode pitch being a distance between center axes of two adjacent electrodes. This allows tuning of the SAW velocity and the in-plane velocity dispersion in the respective regions. As a result, a desired velocity profile or in-plane dispersion can be achieved within the SAW device.

In a further possible implementation form of the first aspect, the transverse mode suppression structure is configured to not extend into the active area, facilitatning matching of velocity profiles of different areas of the surface acoustic wave resonator element.

In a further possible implementation form of the first aspect, the transverse mode suppression structure is configured to extend at least partially across at least one of the conductive areas, facilitating manufacture of the surface acoustic wave resonator element.

In a further possible implementation form of the first aspect, the transverse mode suppression structure is configured to extend only partially across the first interface area and/or the second interface area, providing maximum flexibility to the solution.

In a further possible implementation form of the first aspect, one border of the continuous transverse mode suppression structure extends within the first interface area or the second interface area, the border extending transversely to the longitudinal gaps, providing maximum flexibility to the solution.

In a further possible implementation form of the first aspect, one border of the continuous transverse mode suppression structure extends within the conductive area adjacent the first interface area or the second interface area, providing maximum flexibility to the solution.

In a further possible implementation form of the first aspect, two borders of the continuous transverse mode suppression structure extend within the first interface area or the second interface area, the borders extending transversely to the longitudinal gaps, providing maximum flexibility to the solution.

In a further possible implementation form of the first aspect, the electrodes are configured to allow propagation of a surface acoustic wave with a first velocity Vi and the transverse mode suppression structure is configured to allow propagation of a corresponding surface acoustic wave with a second velocity V2 in the first interface area and/or the second interface area, the second velocity V2 being higher than the first velocity Vi. This facilitates achieving the piston mode of operation, which is required to successfully suppressing the spurious transverse modes.

In a further possible implementation form of the first aspect, Vi<V2< 1.5 x Vi, facilitating achieving the piston mode of operation.

In a further possible implementation form of the first aspect, the continuous transverse mode suppression structure comprises SislS , AI2O3, AIN, BeO, and/or Be, materials facilitating the desired velocity profile.

In a further possible implementation form of the first aspect, the surface acoustic wave resonator element further comprises a discontinuity between each pair of electrode and dummy finger sharing the longitudinal axis, the discontinuity extending at a border between the active area and the first interface area or the second interface area. This ensures the dummy fingers do not affect the suppression of the spurious transverse modes negatively.

In a further possible implementation form of the first aspect, the continuous transverse mode suppression structure is neither arranged within nor superimposed with the discontinuity. This ensures the velocity does not increase in the discontinuity region, and prevents shortcuts in embodiments comprising conductive materials.

In a further possible implementation form of the first aspect, the surface acoustic wave resonator element further comprises a piezoelectric substrate structure, the transducer arrangement being carried by the piezoelectric substrate structure, the piezoelectric substrate structure comprising at least two layers superimposed onto each other, the two layers comprising: a piezoelectric layer extending adjacent the transducer arrangement and comprising a piezoelectric material being LiTaCh or LiNbCh, and a substrate layer comprising one of Si, sapphire, SiC, quartz, and YAG, materials facilitating the desired velocity profile as well as the desired performance of the surface acoustic wave resonator element.

In a further possible implementation form of the first aspect, the surface acoustic wave resonator element further comprises at least one functional layer arranged between the piezoelectric layer and the substrate layer, the functional layer(s) comprising at least one of SiCh, SisNs, AIN, and AI2O3, increasing the flexibility of the surface acoustic wave resonator element. According to a second aspect, there is provided an electronic apparatus comprising the surface acoustic wave resonator element according to the above. This allows for an electronic apparatus comprising a surface acoustic wave resonator element successfully suppressing spurious transverse modes.

These and other aspects will be apparent from the embodiments described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the aspects, embodiments, and implementations will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

Fig. la shows a schematic top view of a transducer arrangement in accordance with an example of the embodiments of the disclosure;

Fig. lb shows a schematic side view of a surface acoustic wave resonator element comprising the transducer arrangement of Fig. la;

Fig. 2a shows a schematic top view of transducer arrangements in accordance with further examples of the embodiments of the disclosure;

Fig. 2b shows a schematic side view of a surface acoustic wave resonator element comprising one example of a transducer arrangement illustrated by Fig. 2a;

Fig. 2c shows a schematic side view of a surface acoustic wave resonator element comprising a further example of a transducer arrangement illustrated by Fig. 2a.

DETAILED DESCRIPTION

The present invention relates to an electronic apparatus comprising at least one surface acoustic wave resonator element 1, the surface acoustic wave resonator element 1 being described in more detail below.

The present invention furthermore relates to a surface acoustic wave resonator element 1 comprising a transducer arrangement 2, the transducer arrangement 2 comprising an active area 3, two conductive areas 4, a first interface area 5 and a second interface area 6, each interface area extending between the active area 3 and one of the conductive areas 4, the active area 3 comprising a plurality of longitudinally extending interdigitating electrodes 7, each interface area comprising a plurality of longitudinally extending dummy fingers 8, each dummy finger 8 sharing a longitudinal axis Al with one electrode 7, adjacent interdigitating electrodes 7 and adjacent dummy fingers 8 being separated by longitudinally extending gaps 9, each gap 9 extending from the first interface area 5 to the second interface area 6, the transducer arrangement 2 further comprising at least one continuous transverse mode suppression structure 10, the continuous transverse mode suppression structure 10 extending within the first interface area 5, the active area 3, and the second interface area 6 or being superimposed with the first interface area 5 or the second interface area 6.

Figs, lb, 2b, and 2c show examples of surface acoustic wave resonator elements 1 comprising a transducer arrangement 2.

The transducer arrangement 2, shown also in Figs, la and 2a, comprises an active area 3, two conductive areas 4, a first interface area 5, and a second interface area 6. Each interface area extends between the active area 3 and one of the conductive areas 4. In other words, the active area 3 forms the center area of the transducer arrangement 2, the conductive areas 4 form opposite edge areas of the transducer arrangement 2, and the first interface area 5 and the second interface area 6 form intermediate areas connecting the center area to the edge areas.

The active area 3 comprises a plurality of longitudinally extending interdigitating electrodes 7, also referred to as fingers. By interdigitating is meant that the active area 3 comprisse two sets of fingers which are arranged such that the fingers of the first set of fingers alternate with the fingers of the second set of fingers, each first finger being separated from a neighboring first finger by a second finger and, correspondingly, each second finger being separated from a neighboring second finger by a first finger.

Each conductive area 4 may be a busbar. Furthermore, the electrodes 7 and/or the conductive areas 4 may be slanted (not shown).

The first interface area 5 and the second interface area 6 both comprise a plurality of longitudinally extending dummy fingers 8. Each dummy finger 8 shares a longitudinal axis Al with one electrode 7, as illustrated in Figs, la and 2a. The length of the dummy finger 8, along the longitudinal axis Al, may be between 1 x electrode pitch D3 and 5 x electrode pitch D3. Adjacent interdigitating electrodes 7 and adjacent dummy fingers 8 are separated by longitudinally extending gaps 9, each gap 9 extending from the first interface area 5 to the second interface area 6. In other words, the each first finger is separated from a neighboring second finger by one gap 9 and, correspondingly, each second finger is separated from a neighboring first finger by one gap 9.

The transducer arrangement 2 further comprises at least one continuous transverse mode suppression structure 10. The continuous transverse mode suppression structure 10 either extends within the first interface area 5, the active area 3, and the second interface area 6, i.e. extends from the first interface area 5, across the active area 3, to the second interface area 6 as shown in Fig. la, or the continuous transverse mode suppression structure 10 is superimposed with the first interface area 5 or the second interface area 6. Fig. 2a shows an embodiment comprising two continuous transverse mode suppression structures 10, one being superimposed with the first interface area 5 and the other being superimposed with the second interface area

6.

Figs, la and lb show an embodiment wherein the continuous transverse mode suppression structure 10 extends coplanarly with the electrodes 7 and the dummy fingers 8 in a first plane Pl. The surface acoustic wave resonator element 1 may comprise a plurality of continuous transverse mode suppression structures 10, each continuous transverse mode suppression structures 10 being configured to fill one of the longitudinally extending gaps 9 as shown best in Fig. lb. By continuous is meant that the continuous transverse mode suppression structure 10 extends uninterruptedly and forms one solid mass within the gap 9. The continuous transverse mode suppression structure 10 may, preferably, be a substantially flat element.

The dimension DI of the continuous transverse mode suppression structures 10 may be smaller than a corresponding dimension D2 of the electrodes 7, the dimensions DI, D2 extending in a direction perpendicular to the first plane Pl, i.e., corresponding to a thickness or height of the electrodes 7 and continuous transverse mode suppression structures 10. The dimension D2 of the electrode 7 may be larger than 0.05 x electrode pitch D3 and smaller than 0.25 x the electrode pitch D3. The acoustic impedance for the shear wave of the continuous transverse mode suppression structures 10 may be at least 25 % larger than the impedance for the electrode

7. The continuous transverse mode suppression structure 10 may also be arranged such that it extends at least partially in a plane P2 parallel with a plane Pl comprising the electrodes 7 and the dummy fingers 8. The continuous transverse mode suppression structure 10 may, preferably, be a corrugated sheet element as shown in Fig. 2a or a substantially flat sheet element as shown in Fig. 2b.

As mentioned above, the surface acoustic wave resonator element 1 may comprise two continuous transverse mode suppression structures 10, each continuous transverse mode suppression structure 10 being superimposed with one of the first interface area 5 and the second interface area 6. In such an embodiment, the continuous transverse mode suppression structure 10 may be arranged above or below the dummy fingers 8, as seen in a direction perpendicular to the second plane P2. Fig. 2a shows an embodiment wherein the continuous transverse mode suppression structure 10 is arranged partially above the dummy fingers 8 and partially between the dummy fingers, forming one continuous transverse mode suppression structure 10 in a direction perpendicular to the longitudinal axes Al. Fig. 2b shows an embodiment wherein the continuous transverse mode suppression structure 10 is arranged below the dummy fingers 8, also forming one continuous transverse mode suppression structure 10 in a direction perpendicular to the longitudinal axes Al.

The dimension DI of the continuous transverse mode suppression structure 10 may be between 0.02 x electrode pitch D3 and 0.2 x electrode pitch D3.

The transverse mode suppression structure 10 may be configured to not extend into the active area 3, i.e. it extends only above or below the first interface area 5 or the second interface area 6. The transverse mode suppression structure 10 may extend completely across the first interface area 5 or the second interface area 6, as shown in Fig. 2a. However, transverse mode suppression structure 10 may also extend only partially across the first interface area 5 and/or the second interface area 6. In other words, one border 10a of the continuous transverse mode suppression structure 10 may extend within the first interface area 5 or the second interface area 6, the border 10a extending transversely to the longitudinal gaps 9. The opposite border 10a may also extend within the first interface area 5 or the second interface area 6, or at the edge of the first interface area 5 or the second interface area 6. In other words, two borders 10a of the continuous transverse mode suppression structure 10 may extend within the first interface area 5 or the second interface area 6, the borders 10a extending transversely to the longitudinal gaps. The transverse mode suppression structure 10 may also be configured to extend at least partially across at least one of the conductive areas 4, as shown in Fig. 2a. The transverse mode suppression structure 10 may extend completely or partially above or below the adjacent conductive area 4. In other words, one border 10a of the continuous transverse mode suppression structure 10 may extend within the conductive area 4 adjacent the first interface area 5 or the second interface area 6. The opposite border 10a may also extend within the first interface area 5 or the second interface area 6, or at the edge of the first interface area 5 or the second interface area 6.

The electrodes 7 may be configured to allow propagation of a surface acoustic wave with a first velocity Vi and the transverse mode suppression structure 10 may be configured to allow propagation of a corresponding surface acoustic wave with a second velocity V2 in the first interface area 5 and/or the second interface area 6, the second velocity V2 being higher than the first velocity Vi. In one emnbodiment, Vi<V2< 1.5 x Vi.

The continuous transverse mode suppression structure 10 may comprise either a dielectric material or a conductive material. The continuous transverse mode suppression structure 10 may comprise SisN4, AI2O3, AIN, BeO, and/or Be.

The surface acoustic wave resonator element 1 may further comprise a discontinuity 11 between each pair of electrode 7 and dummy finger 8 sharing the longitudinal axis 1, the discontinuity 11 extending at a border 12 between the active area 3 and the first interface area 5 or the second interface area 6 as shown in Figs, la and 2a. The discontinuity 11 may be a gap. The continuous transverse mode suppression structure 10 may be arranged neither within the discontinuity 11 nor superimposed with the discontinuity 11. In other words, the continuous transverse mode suppression structure 10 may extend adjacent as side of, but not into, the discontinuity 11 as shown in Fig. la. Furthermore, the continuous transverse mode suppression structure 10 may extend adjacent an top or bottom of, but not into, the discontinuity 11 as shown in Fig. 2a.

The end section of the electrode 7, adjacent the dummy finger 8, may comprises a mass deposited on the electrode 7 (not shown) and/or a widening of the electrode 7, see Fig. 2a, in a direction perpendicular to the longitudinal axis Al. The surface acoustic wave resonator element 1 may further comprise a piezoelectric substrate structure 13, the transducer arrangement 2 being carried by the piezoelectric substrate structure 13.

The piezoelectric substrate structure 13 comprises at least two layers superimposed onto each other, as shown in Figs, lb, 2b, and 2c. The two layers comprise a piezoelectric layer 13a extending adjacent the transducer arrangement 2 and comprising a piezoelectric material being LiTaCh or LiNbCh, and a substrate layer 13c comprising one of Si, sapphire, SiC, quartz, and YAG. For example, for LiTaO3 the euler angles may be (0, -70 to -25, 0), and for LiNbO3 the euler angles may be (0, -75 to +40, 0).

The surface acoustic wave resonator element 1 may further comprising at least one functional layer 13b arranged between the piezoelectric layer 13a and the substrate layer 13c, the functional layers comprising at least one of SiCh, SisNs, AIN, and AI2O3. The functional layer may, in other words, be a composite of different materials.

The various aspects and implementations have been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

The reference signs used in the claims shall not be construed as limiting the scope. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this disclosure. As used in the description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate.