WO2023241828A1

WO2023241828A1 - Surface acoustic wave resonator element and electronic apparatus comprising said surface acoustic wave resonator element

Info

Publication number: WO2023241828A1
Application number: PCT/EP2023/050588
Authority: WO
Inventors: Ventsislav Yantchev
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2022-06-14
Filing date: 2023-01-12
Publication date: 2023-12-21
Also published as: WO2023241786A1

Abstract

A SAW resonator element (1) comprises conductive elements (2, 3), an active area (4) being arranged between said conductive elements (2, 3) and separated from each conductive element (2, 3) by an interface area (5, 6). Electrodes (9, 10) extend from each conductive element (2, 3) across one interface area (5, 6) and said active area (4) towards the other conductive element (2, 3). Dummy fingers (7, 8) extend from each conductive element (2, 3) towards said active area (4). The dummy fingers (7, 8) and electrodes (9, 10) are separated by gaps (12) along a longitudinal axis (A1). The dummy fingers (7, 8) and/or electrodes (9, 10) comprise a first portion (17) having a first width (W1) and a second portion (15) having a second width (W2)larger than said first width (W1). Each second portion (15) is arranged at least partially within one interface area (5, 6).

Description

SURFACE ACOUSTIC WAVE RESONATOR ELEMENT AND ELECTRONIC APPARATUS COMPRISING SAID SURFACE ACOUSTIC WAVE RESONATOR ELEMENT

The present application claims priority from International Patent Application No. PCT/EP2022/066189, filed on June 14, 2022, entitled "SURFACE ACOUSTIC WAVE RESONATOR ELEMENT AND ELECTRONIC APPARATUS COMPRISING SAID SURFACE ACOUSTIC WAVE RESONATOR ELEMENT", which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The disclosure relates to a surface acoustic wave resonator element comprising conductive elements, a plurality of electrodes, and a plurality of dummy fingers. The disclosure furthermore relates to an electronic apparatus comprising such a surface acoustic wave resonator element.

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 is 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 usually 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 improving 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 two conductive elements; an active area arranged between the conductive elements; the active area being separated from each conductive element by means of an interface area; a plurality of electrodes, each electrode extending from one of the conductive elements across one of the interface areas and across the active area towards the other of the conductive elements; a plurality of dummy fingers extending from each conductive element towards the active area, each dummy finger sharing a longitudinal axis with one electrode such that an end of the dummy finger is separated from an end of the electrode by a gap; the dummy fingers and/or the electrodes each comprising a first portion having a first width and a second portion having a second width, the second width > the first width, each second portion being arranged at least partially within one of the interface areas.

By providing wider portions, lower velocity regions are created that allow the matching of interface areas with the active area. This facilitates achieving a near piston mode of operation by properly tuning the in-plane surface acoustic wave dispersion in the different regions. 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 matching the interface areas with the active area using the wider portions. The wider portions provide improved waveguiding, which leads to improved energy confinement, i.e., lower energy losses.

In a possible implementation form of the first aspect, the second portion is arranged at the end of the dummy finger and/or the end of the electrodes. This allows the matching of the dummy finger with the electrode. In a further possible implementation form of the first aspect, a first part of the second portion is arranged on one of the dummy fingers and a second part of the second portion is arranged on one of the electrodes, the first part and the second part being separated by the gap. 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 second portion is arranged on the electrode, in a region extending within one of the interface areas. This facilitates a further configuration for matching the dummy finger with the electrode.

In a further possible implementation form of the first aspect, the surface acoustic wave resonator element comprises a first conductive element and a second conductive element; a first interface area extending between the first conductive element and the active area, and a second interface area extending between the second conductive element and the active area; a plurality of first electrodes extending from the first conductive element across the first interface area and the active area, and a plurality of second electrodes extending from the second conductive element across the second interface area and the active area; the first interface area comprising a plurality of first dummy fingers extending from the first conductive element towards the active area, each first dummy finger sharing a longitudinal axis with one of the second electrodes, and the second interface area comprising a plurality of second dummy fingers extending from the second conductive element towards the active area, each second dummy finger sharing a longitudinal axis with one of the first electrodes, facilitating an interdigital transducer.

In a further possible implementation form of the first aspect, the first width and the second width is measured along a transverse axis extending perpendicular the longitudinal axes.

In a further possible implementation form of the first aspect, a length of the second portion, measured along the longitudinal axis, is between 0.5 x a length of the first portion and 3 x the length of the first portion. This facilitates achieving the piston mode of operation, which is required to successfully suppress the spurious transverse modes.

In a further possible implementation form of the first aspect, the length of the second portion is equal to the length of the first portion, providing optimum conditions for achieving the piston mode of operation. In a further possible implementation form of the first aspect, the dummy fingers and the electrodes all have one uniform thickness, the thickness being is measured in a direction perpendicular to the transverse axis and to the longitudinal axes. This facilitates the manufacture of the surface acoustic wave resonator element.

In a further possible implementation form of the first aspect, the first portion of each dummy finger has a first thickness and the second portion of each dummy finger has a second thickness, and wherein each electrode has the second thickness in the active area, and wherein the first portion of each electrode has the first thickness and the second portion of each electrode has the second thickness in the first interface area and the second interface area, the first thickness < the second thickness. The thinner portions provide improved waveguiding, which leads to improved energy confinement, i.e., lower energy losses.

In a further possible implementation form of the first aspect, the first electrodes are arranged alternatingly with the second electrodes within a common plane, the first electrodes being separated from adjacent second electrodes and adjacent first dummy fingers by longitudinally extending spaces, and the second electrodes being separated from adjacent first electrodes and adjacent second dummy fingers by longitudinally extending spaces, each space extending from the first interface area to the second interface area, allowing a common IDT structure to be used as a starting point.

In a further possible implementation form of the first aspect, the first electrodes, the second electrodes, the first dummy fingers, and the second dummy fingers extend in the common plane, the first thickness and the second thickness being measured in a direction perpendicular to the common plane and to the longitudinal axes. This makes the use of electrode slanting unnecessary, alleviating the associated loss effects.

In a further possible implementation form of the first aspect, the second portions arranged within the first interface area have identical lengths and locations along the longitudinal axes, and wherein the second portions arranged within the second interface area have identical lengths and locations along the longitudinal axes. This facilitates achieving the piston mode of operation, which is required to successfully suppress the spurious transverse modes. In a further possible implementation form of the first aspect, the second portion(s) comprises a mass deposited thereon. By adding masses, lower velocity regions are created that allow matching of the interface areas with the active area, which in turn facilitates achieving the piston mode.

In a further possible implementation form of the first aspect, the surface acoustic wave resonator element comprises at least one dielectric layer extending along the transverse axis, the dielectric layer being partially superimposed with the active area and one of the interface areas. This suppresses 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 dielectric layer is a continuous layer covering the second portions and the gaps, suppressing the undesired transverse mode effects.

In a further possible implementation form of the first aspect, the continuous dielectric layer fills any section of space extending between one dummy finger and adjacent electrode(s), and any section of space extending between adjacent electrode(s), improving the performance of the surface acoustic wave resonator element.

According to a second aspect, there is provided an electronic apparatus comprising a surface acoustic wave resonator element according to any one of the previous claims. 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. 1 shows a schematic top view of a surface acoustic wave resonator element in accordance with an example of the embodiments of the disclosure; Fig. 2 shows a schematic and partial cross-sectional view of an apparatus comprising a surface acoustic wave resonator element in accordance with an example of the embodiments of the disclosure;

Figs. 3 to 6 show schematic top views of surface acoustic wave resonator elements in accordance with examples of the embodiments of the disclosure;

Figs. 7 to 10 show schematic top views of surface acoustic wave resonator elements in accordance with further examples of the embodiments of the disclosure.

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 electronic apparatus may be any kind of electronic apparatus such as a tablet or a smartphone.

The surface acoustic wave resonator element 1 may be arranged on a piezoelectric substrate structure comprising at least two layers superimposed onto each other, illustrated in Fig. 2 as hatched areas. The two layers may comprise a piezoelectric layer extending adjacent the surface acoustic wave resonator element 1 and comprising a piezoelectric material being LiTaCF or LiNbCh, and a substrate layer 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 comprise at least one functional layer arranged between the piezoelectric layer and the substrate layer, the functional layers comprising at least one of SiCh, SisNa, AIN, and AI2O3. The functional layer may, in other words, be a composite of different materials.

The surface acoustic wave resonator element 1 comprises two conductive elements 2, 3, also referred to as first conductive element 2 and second conductive element 3. The conductive elements 2, 3 may each be a busbar.

An active area 4 is arranged between the conductive elements 2, 3, as shown in Fig. 1.

The active area 4 is separated from each conductive element 2, 3 by means of an interface area 5, 6. A first interface area 5 may extend between the first conductive element 2 and the active area 4 and a second interface area 6 may extend between the second conductive element 2 and the active area 4. In other words, the active area 4 forms the center area of the surface acoustic wave resonator element 1, the conductive elements 2, 3 form opposite edge areas of the surface acoustic wave resonator element 1, and the interface areas 5, 6 form intermediate areas connecting the center area to the edge areas.

A plurality of electrodes are arranged such that each electrode (9, 10) extends from one of the conductive elements 2, 3 across one of the interface areas 5, 6 and across the active area 4 towards the other of the conductive elements 2, 3. A plurality of first electrodes 9 may extend from the first conductive element 2 across the first interface area 5 and across the active area 4. Correspondingly, a plurality of second electrodes 10 may extend from the second conductive element 3 across the second interface area 6 and across the active area 4. The plurality of first electrodes 9 interleave or interdigitate with the plurality of second electrodes 10, the electrodes also commonly being referred to as “fingers”. By “interdigitating” is meant that the active area 4 comprises two sets of fingers which are arranged such that the fingers of a first set of fingers, i.e. the first electrodes 9, alternate with the fingers of a second set of fingers, i.e. the second electrodes 10, each first finger or electrode 9 being separated from a neighboring first finger or electrode 9 by a second finger or electrode 10 and, correspondingly, each second finger or electrode 10 being separated from a neighboring second finger or electrode 10 by a first finger or electrode 9.

A plurality of dummy fingers 7, 8 extend from each conductive element 2, 3 towards the active area 4. Each dummy finger 7, 8 shares a longitudinal axis Al with one electrode 9, 10 such that an end 7a, 8a of said dummy finger 7, 8 is separated from an end 9a, 10a of said electrode 9, 10 by a gap 12, preferably an air gap. For example, the first interface area 5 may comprise a plurality of first dummy fingers 7 extending from the first conductive element 2 towards the active area 4. Correspondingly, the second interface area 6 may comprise a plurality of second dummy fingers 8 extending from the second conductive element 3 towards the active area 4. Each first dummy finger 7 shares a longitudinal axis Al with one of the second electrodes 10 and each second dummy finger 8 shares a longitudinal axis Al with one of the first electrodes 9. In other words, each gap 12 separates a first dummy finger 7 from an end area 10a of a second electrode 10 or separates a second dummy finger 8 from the end area 9a of a first electrode 9. The gaps 12 form discontinuities along the longitudinal axes Al. As shown in Fig. 1, the first electrodes 9 may be arranged altematingly with the second electrodes 10 within a common plane Pl, the first electrodes 9 being separated from adjacent second electrodes 10 and adjacent first dummy fingers 7 by longitudinally extending spaces 13, and the second electrodes 10 being separated from adjacent first electrodes 9 and adjacent second dummy fingers 8 by longitudinally extending spaces 13, each space 13 extending from the first interface area 5 to the second interface area 6. In other words, each first electrode 9 is separated from a neighboring second electrode 10 by one space 13 and, correspondingly, each second electrode 10 is separated from a neighboring first electrode 9 by one space 13.

The length of the dummy fingers 7, 8 along the longitudinal axes Al may be between 1 x the electrode pitch and 6 x the electrode pitch, the electrode pitch being the distance between the center axes of adjacent electrodes as illustrated by the double-headed arrow in Fig. 1.

The dummy fingers 7, 8 and/or the electrodes 9, 10 each comprise a first portion 17 having a first width W1 and a second portion 15 having a second width W2. The second width W2 > the first width Wl, i.e., the second width W2 is larger than the first width Wl. In other words, the second portion 15 means a local area that is wider than the remainder of the dummy finger 7, 8 or electrode 9, 10. The first width Wl and the second width W2 may be measured along a transverse axis A2 extending perpendicular to the longitudinal axes Al.

A length L2 of the second portion 15, measured along the longitudinal axis Al, may be between 0.5 x a length LI of the first portion 17 and 3 x the length LI of the first portion 17. The second portion 15 has a length which, in other words, is between half of that of the first portion 17 and three times that of the first portion 17. The length L2 of the second portion 15 may be equal to the length LI of the first portion 17.

All second portions 15 arranged within the first interface area 5 may have identical lengths L2 and locations along the longitudinal axes AL Correspondingly, all second portions 15 arranged within the second interface area 6 may have identical lengths L2 and locations along the longitudinal axes Al. Bu “locations” is meant that the second portions 15 all begin and end at the same coordinate along the longitudinal axes AL

Each second portion 15 is arranged at least partially within one of the interface areas 5, 6. The second portion 15 may be arranged at the end 7a, 8a of the dummy finger 7, 8, as shown in Figs. 3 and 7. The second portion 15 may also be arranged at the end 9a, 10a of the electrode 9, 10, as shown in the top parts of Figs. 4 and 8.

As illustrated in the top parts of Figs. 6 and 10, a first part 15a of the second portion 15 may be arranged on one of the dummy fingers 7, 8 and a second part 15b of the second portion 15 may be arranged on one of the electrodes 9, 10, the first part 15a and the second part 15b being separated by the gap 12.

As illustrated in the top parts of Figs. 4, 5, 8, and 9, a second portion 15 may be arranged on the electrode 9, 10, in a region extending within one of the interface areas 5, 6.

As illustrated in the bottom parts of Figs. 4, 5, 8, and 9, a second portion 15 may be arranged on the electrode 9, 10, in a region extending within one of the interface areas 5, 6.

As illustrated in the bottom parts of Figs. 3, 6, 7, and 10, additional second portions 15 may be arranged on the electrode 9, 10 in the active area 4.

As illustrated in Figs. 3 to 6, the dummy fingers 7, 8 and the electrodes 9, 10 may all have one uniform thickness, the thickness being is measured in a direction DI perpendicular to the transverse axis A2 and to the longitudinal axes Al.

As shown in Fig. 2 and in Figs. 7 to 10 by means of hatched areas, the first portion 17 of each dummy finger 7, 8 may have a first thickness T1 and the second portion 15 of each dummy finger 7, 8 may have a second thickness T2 Each electrode 9, 10 has a corresponding second thickness T2 in the active area, i.e., the parts of the electrodes 9, 10 that extend within the active area 4 have a uniform topology with a thickness equal to the second thickness T2.

Furthermore, as shown in Figs. 7 to 10 by means of hatched areas, the first portion 17 of each electrode 9, 10 may have the first thickness T1 and the second portion 15 of each electrode 9, 10 may have the second thickness T2 in the first interface area 5 and the second interface area 6. Each electrode 9, 10 has the second thickness T2 in the active area. The first thickness T1 < the second thickness T2, i.e., the first thickness T1 is less than the second thickness T2.

The second portion 15 may comprise a mass deposited thereon (not shown).

The surface acoustic wave resonator element 1 may further comprise at least one dielectric layer 16 extending along the above-mentioned transverse axis A2, the dielectric layer 16 being partially superimposed with the active area 4 and one of the interface areas 5, 6, i.e. with the first interface area 5 and the active area 4 or with the second interface area 6 and the active area 4. The dielectric layer 16, in other words, covers a border area between the first interface area 5 and the active area 4 or a border area between the second interface area 6 and the active area 4.

The dielectric layer 16 may be a continuous layer covering the second portions 15 and the gaps 12. By “continuous” is meant that the dielectric layer 16 extends uninterruptedly and forms one solid mass.

The continuous dielectric layer 16 may fill any section of space 13 that extends between one dummy finger 7, 8 and adjacent electrode(s) 9, 10, as well as any section of space 13 that extends between adjacent electrodes 9, 10. In other words, the continuous dielectric layer 16 may, to some extent, extend coplanarly with the electrodes 9,10 and the dummy fingers 7, 8 in the common plane Pl and to some extent in a further parallel plane on top of the electrodes 9, 10 and the dummy fingers 7, 8.

The length of the dielectric layer 16, measured along the longitudinal axes Al may be at least 2 x the length L2 of the second portion 15. The length may be equal to 2 x the length L2 plus the length of the gap 12 along the longitudinal axes Al. In other words, the length of the dielectric layer 16is at least double the length L2 of the second portion 15.

The surface acoustic wave resonator element 1 may be manufactured by means of electrode fabrication comprising: applying photoresist patterning on a layered substrate, deposition of e.g. a Al/Cr/Al composite to form an etch stop thin conductive layer, liftoff/removal of the photoresist pattern, applying further photoresist patterning, etching the top Al layer, and liftoff/removal of the further photoresist pattern. The Cr layer may be, e.g., Cu or Ag. A further method of manufacturing the surface acoustic wave resonator element 1 comprises: depositing an Al layer on a layered substrate, applying photoresist patterning, depositing a further Al layer, liftoff/removal of the photoresist pattern, applying further photoresist patterning, etching the top Al layer, and liftoff/removal of the further photoresist pattern.

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”, “rightwar dly”, “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.

Claims

1. A surface acoustic wave resonator element (1) comprising

-two conductive elements (2, 3);

-an active area (4) arranged between said conductive elements (2, 3); said active area (4) being separated from each conductive element (2, 3) by means of an interface area (5, 6);

-a plurality of electrodes (9, 10), each electrode (9, 10) extending from one of said conductive elements (2, 3) across one of said interface areas (5, 6) and across said active area (4) towards the other of said conductive elements (2, 3);

-a plurality of dummy fingers (7, 8) extending from each conductive element (2, 3) towards said active area (4), each dummy finger (7, 8) sharing a longitudinal axis (Al) with one electrode (9, 10) such that an end (7a, 8a) of said dummy finger (7, 8) is separated from an end (9a, 10a) of said electrode (9, 10) by a gap (12); said dummy fingers (7, 8) and/or said electrodes (9, 10) each comprising a first portion (17) having a first width (Wl) and a second portion (15) having a second width (W2), said second width (W2) > said first width (Wl), each second portion (15) being arranged at least partially within one of said interface areas (5, 6).

2. The surface acoustic wave resonator element (1) according to claim 1, wherein said second portion (15) is arranged at said end (7a, 8a) of said dummy finger (7, 8) and/or said end (9a, 10a) of said electrode (9, 10).

3. The surface acoustic wave resonator element (1) according to claim 2, wherein a first part (15a) of said second portion (15) is arranged on one of said dummy fingers (7, 8) and a second part (15b) of said second portion (15) is arranged on one of said electrodes (9, 10), said first part (15a) and said second part (15b) being separated by said gap (12).

4. The surface acoustic wave resonator element (1) according to any one of the previous claims, wherein said second portion (15) is arranged on said electrode (9, 10), in a region extending within one of said interface areas (5, 6).

5. The surface acoustic wave resonator element (1) according to any one of the previous claims, comprising

-a first conductive element (2) and a second conductive element (3);

-a first interface area (5) extending between said first conductive element (2) and said active area (4), and a second interface area (6) extending between said second conductive element (2) and said active area (4); a plurality of first electrodes (9) extending from said first conductive element (2) across said first interface area (5) and said active area (4), and a plurality of second electrodes (10) extending from said second conductive element (3) across said second interface area (6) and said active area (4); said first interface area (5) comprising a plurality of first dummy fingers (7) extending from said first conductive element (2) towards said active area (4), each first dummy finger (7) sharing a longitudinal axis (Al) with one of said second electrodes (10), and said second interface area (6) comprising a plurality of second dummy fingers (8) extending from said second conductive element (3) towards said active area (4), each second dummy finger (8) sharing a longitudinal axis (Al) with one of said first electrodes (9).

6. The surface acoustic wave resonator element (1) according to any one of the previous claims, wherein said first width (Wl) and said second width (W2) is measured along a transverse axis (A2) extending perpendicular said longitudinal axes (Al).

7. The surface acoustic wave resonator element (1) according to any one of the previous claims, wherein a length (L2) of said second portion (15), measured along said longitudinal axis (Al), is between 0.5 x a length (LI) of said first portion (17) and 3 x said length (LI) of said first portion (17).

8. The surface acoustic wave resonator element (1) according to claim 7, wherein said length (L2) of said second portion (15) is equal to said length (LI) of said first portion (17).

9. The surface acoustic wave resonator element (1) according to any one of the previous claims, wherein said dummy fingers (7, 8) and said electrodes (9, 10) all have one uniform thickness, said thickness being is measured in a direction perpendicular to said transverse axis (A2) and to said longitudinal axes (Al).

10. The surface acoustic wave resonator element (1) according to any one of claims 1 to 8, wherein said first portion (17) of each dummy finger (7, 8) has a first thickness (Tl) and said second portion (15) of each dummy finger (7, 8) has a second thickness (T2), and wherein each electrode (9, 10) has said second thickness (T2) in said active area, and wherein said first portion (17) of each electrode (9, 10) has said first thickness (Tl) and said second portion (15) of each electrode (9, 10) has said second thickness (T2) in said first interface area (5) and said second interface area (6), said first thickness (Tl) < said second thickness (T2).

11. The surface acoustic wave resonator element (1) according to any one of the previous claims, further comprising at least one dielectric layer (16) extending along said transverse axis (A2), said dielectric layer (16) being partially superimposed with said active area (4) and one of said interface areas (5, 6).

12. The surface acoustic wave resonator element (1) according to claim 11, wherein said dielectric layer (16) is a continuous layer covering said second portions (15) and said gaps (12).

13. The surface acoustic wave resonator element (1) according to claim 12, wherein said continuous dielectric layer (16) fills any section of space (13) extending between one dummy finger (7, 8) and adjacent electrode(s) (9, 10), and any section of space (13) extending between adjacent electrode(s) (9, 10).

14. An electronic apparatus comprising a surface acoustic wave resonator element (1) according to any one of the previous claims.