Surface acoustic wave device with phase adjustment

Abstract

Various embodiments relate to a surface acoustic wave, SAW, device. A SAW device (102) may comprise a piezoelectric substrate (210); and at least one interdigital transducer, IDT (200), on the piezoelectric substrate (210), the IDT (200) comprising a plurality of electrode fingers (202) arranged parallel to each other in a propagation direction of the surface acoustic wave and spaced apart with a first electrode finger pitch (218); first areas (212) located by opposing edges of the IDT (200) in the propagation direction of the surface acoustic wave and containing at least one of the plurality of electrode fingers (202); a second area (214) located between the first areas (212) and containing at least one of the plurality of electrode fingers (202); and wherein the first areas (212) are configured to have a higher acoustic velocity than the second area (214), setting the conditions for a desirable phase adjustment within the SAW device (102). Devices and a method are disclosed.

Description

[0001] SURFACE ACOUSTIC WAVE DEVICE WITH PHASE ADJUSTMENT

[0002] TECHNICAL FIELD

[0003] The present disclosure generally relates to the field of surface acoustic wave (SAW) devices. Some embodiments of the disclosure relate to SAW devices, such as SAW resonators and filters, with a velocity profile structure for phase adjustment which can give performance advantages such as suppressing longitudinal mode ripples.

[0004] BACKGROUND

[0005] Surface acoustic waves are mechanical waves that propagate along a surface of a material, typically piezoelectric substrates. A SAW device uses piezoelectric characteristics of a piezoelectric material, and input and output transducer to convert an input signal of an electric wave into mechanical energy.

[0006] SUMMARY

[0007] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0008] It is an objective of the present disclosure to provide a SAW device with phase adjustment and a method for manufacturing said SAW device. The embodiments of the disclosure may provide an improved structure for a SAW device which enables to suppress longitudinal mode ripples.

[0009] The foregoing and other objectives may be achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description, and the drawings.

[0010] According to a first aspect, a surface acoustic wave device is provided. The device may comprise a piezoelectric substrate; and at least one interdigital transducer, IDT, on the piezoelectric substrate, the IDT comprising a plurality of electrode fingers arranged parallel to each other in a propagation direction of the surface acoustic wave and spaced apart with a first electrode finger pitch; first areas located by opposing edges of the IDT in the propagation direction of the surface acoustic wave and containing at least one of the plurality of electrode fingers; a second area located between the first areas and containing at least one of the plurality of electrode fingers; and wherein the first areas are configured to have a higher acoustic velocity than the second area. This provides a velocity profile which enables a desirable phase adjustment to suppress longitudinal mode ripples with the advantage of not consuming extra die area and not requiring higher lithography resolution for narrower pitch fabrication. This is enabled by increasing the acoustic velocity of a specified area in the SAW device.

[0011] According to an implementation form of the first aspect, the SAW device may further comprise two reflectors arranged on the piezoelectric substrate on both sides of the at least one IDT, the reflectors comprising: a plurality of electrode fingers arranged parallel to each other in the propagation direction of the surface acoustic wave and spaced apart with a second electrode finger pitch; a first area of the reflector located by an edge facing a border between the at least one IDT and the respective reflector and containing at least one of the plurality of electrode fingers of the respective reflector; and a second area of the reflector located by the opposite edge of the reflector and containing at least one of the plurality of electrode fingers of the respective reflector; wherein the first areas of the reflectors are configured to have a higher acoustic velocity than the second areas of the reflectors. A desirable phase adjustment in the SAW device is set by increasing the acoustic velocity of a specified area in the SAW device at both the IDT and the reflectors, which gives the performance advantage of suppressing the longitudinal ripples. According to an implementation form of the first aspect, the first areas of the at least one IDT and the first areas of the reflectors have approximately the same acoustic velocity. This enables suppressing the longitudinal mode ripples.

[0012] According to an implementation form of the first aspect, at least one of the first areas comprise a layer of material having a higher acoustic velocity than velocity of the surface acoustic wave in the SAW device. Hence, the higher acoustic velocity can be implemented by placing a relatively thin layer of high acoustic velocity material in contact with electrode fingers of the respective first area.

[0013] According to an implementation form of the first aspect, the first areas of the IDT cover an area of at least two first electrode finger pitches. Hence, the number of electrode fingers within the first area may be implementation specific.

[0014] According to an implementation form of the first aspect, the first areas of the reflectors cover an area of at least two second electrode finger pitches. Hence, the size of the first area may be implementation specific, and wherein at least two electrode finger pitches may be covered by the first area.

[0015] According to an implementation form of the first aspect, the layer of material having a higher acoustic velocity is arranged above, in-between or beneath at least one of the electrode fingers within the at least one first area. Hence, there are a plurality of options for arranging the layer of high acoustic velocity material within the first area.

[0016] According to an implementation form of the first aspect, a thickness of the layer of material having a higher acoustic velocity is less than 20 % of the first electrode finger pitch. Hence, a relatively thin layer of the material may be sufficient.

[0017] According to an implementation form of the first aspect, the material with higher acoustic velocity comprises silicon nitride, SiN, aluminum nitride, AIN, aluminum oxide, AI2O3, or diamond. Hence, a plurality of different materials may be used to implement the layer.

[0018] According to an implementation form of the first aspect, the electrode fingers within at least one of the first areas have a lower thickness compared to a thickness of the electrode fingers outside the at least one first area. Hence, the higher acoustic velocity may be implemented by reducing thickness of the specific electrode fingers instead of using the layer of material having higher acoustic velocity.

[0019] According to an implementation form of the first aspect, the thickness of the electrode fingers within the at least one first area is up to 50 % lower than the thickness of the electrode fingers outside the at least one first area. This enables to increase the acoustic velocity within the respective first area.

[0020] According to an implementation form of the first aspect, the acoustic velocity within the first areas is in a range of up to 10 % higher than the acoustic velocity within the associated second area. This enables to suppress longitudinal mode ripples of the surface acoustic wave.

[0021] According to an implementation form of the first aspect, the piezoelectric substrate comprises at least one of a single piezoelectric layer or a multi-layer structure with at least one piezoelectric layer on top. Hence, the piezoelectric layer can be implemented in different forms.

[0022] According to an implementation form of the first aspect, the SAW device further comprises a dielectric layer as a cover layer above the electrode fingers. This provides one design option for the SAW device, which can give benefit in temperature compensation, frequency shift control and etc. The dielectric layer may be, for example, a topmost layer of the SAW device, or the SAW device can comprise one or more additional layers (e.g., a passivation layer) above the dielectric layer.

[0023] According to an implementation form of the first aspect, the dielectric layer covers the plurality of electrode fingers of the reflectors and at least one IDT for the whole area of the SAW device. This provides one design option for the SAW device, which can give benefit in temperature compensation, frequency shift control and etc.

[0024] According to an implementation form of the first aspect, the dielectric layer comprises silicon dioxide, SiCh. This can give benefit in temperature compensation, frequency shift control, etc. According to an implementation form of the first aspect, the SAW device comprises a plurality of IDTs positioned between the reflectors. Hence, the SAW device may be implemented as a SAW resonator, which a single IDT positioned between the reflectors, or with multiple parallel IDTs positioned between the reflectors.

[0025] According to a second aspect, a method for manufacturing a surface wave acoustic device is provided. The method comprises providing a piezoelectric substrate; and forming at least one interdigital transducer, IDT, on the piezoelectric substrate, the IDT comprising: a plurality of electrode fingers arranged parallel to each other in a propagation direction of the surface acoustic wave and spaced apart with a first electrode finger pitch; first areas located by opposing edges of the IDT in the propagation direction of the surface acoustic wave and containing at least one of the plurality of electrode fingers; a second area located between the first areas and containing at least one of the plurality of electrode fingers; and wherein the first areas are configured to have a higher acoustic velocity than the second area. This enables manufacturing a SAW device capable of suppressing longitudinal mode ripples with the advantage of not consuming extra die area and not requiring higher lithography resolution for narrower pitch fabrication. This is enabled by forming a specified area in the SAW device with increased acoustic velocity.

[0026] According to an implementation form of the second aspect, the method further comprises forming two reflectors on the piezoelectric substrate on both sides of the at least one IDT, the reflectors comprising: a plurality of electrode fingers arranged parallel to each other in the propagation direction of the surface acoustic wave and spaced apart with a second electrode finger pitch; a first area of the reflector located by an edge facing a border between the at least one IDT and the respective reflector and containing at least one of the plurality of electrode fingers of the respective reflector; and a second area of the reflector located by the opposite edge of the reflector and containing at least one of the plurality of electrode fingers of the respective reflector; wherein the first areas of the reflectors are configured to have a higher acoustic velocity than the second areas of the reflectors. This enables that the longitudinal model ripples can be suppressed by forming specified areas in the SAW device at both the IDT and the reflectors with increased acoustic velocity.

[0027] According to an implementation form of the second aspect, the first areas of the at least one IDT and the first areas of the reflectors are configured to have approximately the same acoustic velocity. This enables that the specified areas providing higher acoustic velocity can be implemented identically, which may simplify the manufacturing process. However, it is still possible to implement the specified areas differently.

[0028] According to an implementation form of the second aspect, at least one of the first areas comprise a layer of material having a higher acoustic velocity than velocity of the surface acoustic wave in the SAW device. Hence, the higher acoustic velocity can be implemented by placing a relatively thin layer of high acoustic velocity material in contact with electrode fingers of the respective first area.

[0029] According to an implementation form of the second aspect, the first areas of the IDT cover an area of at least two first electrode finger pitches. Hence, the number of electrode fingers within the first area may be implementation specific, and wherein at least two electrode fingers may be included in the first area for easier manufacturing process.

[0030] According to an implementation form of the second aspect, the first areas of the reflectors cover an area of at least two second electrode finger pitches. Hence, the size of the first area may be implementation specific, and wherein at least two electrode finger pitches may be covered by the first area for easier manufacturing process.

[0031] According to an implementation form of the second aspect, the layer of material having a higher acoustic velocity is arranged above, in-between or beneath at least one of the electrode fingers within the at least one first area. Hence, there are a plurality of options for arranging the layer of high acoustic velocity material within the first area.

[0032] According to an implementation form of the second aspect, a thickness of the layer of material having a higher acoustic velocity is less than 20 % of the first electrode finger pitch. Hence, a relatively thin layer of the material may be sufficient. According to an implementation form of the second aspect, the material with higher acoustic velocity comprises silicon nitride, SiN, aluminum nitride, AIN, aluminum oxide, AI2O3, or diamond. Hence, a plurality of different materials may be used to implement the layer.

[0033] According to an implementation form of the second aspect, the electrode fingers within at least one of the first areas have a lower thickness compared to a thickness of the electrode fingers outside the at least one first area. Hence, the higher acoustic velocity may be implemented by reducing thickness of the specific electrode fingers instead of using the layer of material having higher acoustic velocity.

[0034] According to an implementation form of the second aspect, the thickness of the electrode fingers within the at least one first area is up to 50 % lower than the thickness of the electrode fingers outside the at least one first area. This enables to increase the acoustic velocity within the respective first area.

[0035] According to an implementation form of the second aspect, the acoustic velocity within the first areas is in a range of up to 10 % higher than the acoustic velocity within the associated second area. This enables to provide a structure for suppressing longitudinal mode ripples of the surface acoustic wave.

[0036] According to an implementation form of the second aspect, the piezoelectric substrate comprises at least one of a single piezoelectric layer or a multi-layer structure with at least one piezoelectric layer on top. Hence, the piezoelectric layer can be implemented in different forms.

[0037] According to an implementation form of the second aspect, the method further comprises forming a dielectric layer as a layer above the mentioned structure. This provides one design option for the SAW device, which can give benefit in temperature compensation, frequency shift control, etc.

[0038] According to an implementation form of the second aspect, the dielectric layer covers the plurality of electrode fingers of at least one of the reflectors and the IDT for the whole area of the SAW device. This provides one design option for the SAW device, which can give benefit in temperature compensation, frequency shift control and etc.

[0039] According to an implementation form of the second aspect, the dielectric layer comprises silicon dioxide, SiCh.

[0040] According to an implementation form of the second aspect, the method may comprise positioning a plurality of the formed IDTs in parallel between the reflectors. Hence, the SAW device may be implemented as a SAW resonator, which a single IDT positioned between the reflectors, or with multiple parallel IDTs positioned between the reflectors.

[0041] According to a third aspect, a surface acoustic wave filter is provided. The SAW filter comprises one or more SAW devices according to the first aspect.

[0042] Implementation forms of the present disclosure can thus provide SAW devices, and methods for manufacturing said devices. These and other aspects of the present disclosure will be apparent from the example embodiment(s) described below.

[0043] DESCRIPTION OF THE DRAWINGS

[0044] The accompanying drawings, which are included to provide a further understanding of the example embodiments and constitute a part of this specification, illustrate example embodiments and, together with the description, help to explain the example embodiments. In the drawings:

[0045] FIG. 1 illustrates an example of a SAW filter comprising a plurality of SAW devices, according to an embodiment of the disclosure;

[0046] FIG. 2 illustrates an example of a partial side view of a SAW device with a high velocity layer positioned on a first area of an IDT according to an embodiment of the disclosure;

[0047] FIG. 3 illustrates an example of a top view of the SAW device of FIG. 2 according to an embodiment of the disclosure; FIG. 4 illustrates an example of a plot showing a difference in longitudinal mode ripples between a comparative SAW device and a SAW device according to an embodiment of the disclosure;

[0048] FIG. 5 illustrates an example of a partial side view of a SAW device with a high velocity layer positioned on a first area of an IDT and a first area of a reflector according to an embodiment of the disclosure;

[0049] FIG. 6 illustrates an example of a top view of the SAW device of FIG. 5 according to an embodiment of the disclosure;

[0050] FIG. 7 illustrates an example of a plot showing a difference in longitudinal mode ripples between a comparative SAW resonator and a SAW resonator according to an embodiment of the disclosure;

[0051] FIG. 8 illustrates an example of a SAW device with a dielectric layer and a high velocity layer positioned on a first area of an IDT according to an embodiment of the disclosure;

[0052] FIG. 9 illustrates an example of a top view of the SAW device of FIG. 8 according to an embodiment of the disclosure;

[0053] FIG. 10 illustrates an of a partial side view of a SAW device with a dielectric layer and a high velocity layer positioned on a first area of an IDT and a first area of a reflector according to an embodiment of the disclosure;

[0054] FIG. 11 illustrates an example of a top view of the SAW device of FIG. 10 according to an embodiment of the disclosure;

[0055] FIG. 12 illustrates an example of partial side view of a SAW device with a reduced thickness of electrode fingers on a first area of an IDT according to an embodiment of the disclosure;

[0056] FIG. 13 illustrates an example of a top view of the SAW device of FIG. 12 according to an embodiment of the disclosure;

[0057] FIG. 14 illustrates an example of a partial side view of a SAW device with a reduced thickness of electrode fingers on first areas of the SAW device according to an embodiment of the disclosure;

[0058] FIG. 15 illustrates an example of a top view of the SAW device of FIG. 14 according to an embodiment of the disclosure;

[0059] FIG. 16 illustrates an example of a partial side view of a SAW device with a dielectric layer and a reduced thickness of electrode fingers on a first area of an IDT according to an embodiment of the disclosure;

[0060] FIG. 17 illustrates an example of a top view of the SAW device of FIG. 16 according to an embodiment of the disclosure;

[0061] FIG. 18 illustrates an example of a partial side view of a SAW device with a dielectric layer and a reduced thickness of electrode fingers on first areas of the SAW device according to an embodiment of the disclosure;

[0062] FIG. 19 illustrates an example of a top view of the SAW device of FIG. 18 according to an embodiment of the disclosure; and

[0063] FIG. 20 illustrates an example of a method for manufacturing a SAW device according to an embodiment of the disclosure.

[0064] Like references are used to designate like parts in the accompanying drawings.

[0065] DETAILED DESCRIPTION

[0066] Reference will now be made in detail to example embodiments, examples of which are illustrated in the accompanying drawings. The detailed description provided below in connection with the appended drawings is intended as a description of the present embodiments and is not intended to represent the only forms in which the present examples may be constructed or utilized. The description sets forth the functions of the examples and the sequence of operations for constructing and operating the examples. However, the same or equivalent functions and sequences may be accomplished by different examples.

[0067] For a SAW device, longitudinal mode ripples can be observed in its conductance plot. Such longitudinal mode ripples may degrade the passband of a SAW filter, which is composed of SAW devices. Therefore, the longitudinal mode ripple suppression is important for improving the performance in SAW filter design. On the other hand, due to high miniature requirement for SAW filters, the structures which can suppress the longitudinal mode ripples are expected not to consume extra space on die.

[0068] An objective of an example embodiment is to provide a SAW device with a velocity profile structure for desirable phase adjustment to suppress longitudinal mode ripples. According to an embodiment, a SAW device comprises at least one IDT on the piezoelectric substrate, the IDT comprising a plurality of electrode fingers arranged parallel to each other in a propagation direction of the surface acoustic wave and spaced apart with a first electrode finger pitch; first areas of the IDT located by opposing edges of the IDT in the propagation direction of the surface acoustic wave and containing at least one of the plurality of electrode fingers; a second area of the IDT located between the first areas and containing at least one of the plurality of electrode fingers; and wherein the first areas are configured to have a higher acoustic velocity than the second area. Hence, different electrode finger areas of the IDT are configured to have different acoustic velocities which enable suitable phase adjustment to suppress the longitudinal mode ripples, and further, to improve a passband of the SAW device.

[0069] FIG. 1 illustrates an example of a circuit diagram of a SAW filter 100 comprising a plurality of SAW devices 102, according to an embodiment of the disclosure. The SAW devices 102 may comprise at least one IDT positioned between reflectors on a piezoelectric substrate. The SAW devices 102 may be arranged between an input port 104 and an output port 106 of the SAW filter 100. Arrangement of the SAW devices 102 depend on configuration of the SAW filter 100. Performance of the SAW filter 100 may be improved with design of the SAW devices 102. In an embodiment, one or more of the SAW devices 102 are configured to have an area having a higher acoustic velocity than other areas of the SAW device 102. The area of higher acoustic velocity may be divided to be located on opposing edges of the IDT. More specifically, the areas of higher acoustic velocity may be located by edges of the reflectors facing the IDT. The areas of higher acoustic velocity may be implemented with at least one of a layer of higher acoustic velocity material or with a reduced thickness of electrode fingers located at the area of higher acoustic velocity. Next, examples for implementing such SAW devices 102 are provided.

[0070] FIG. 2 illustrates an example of a partial side view of a SAW device, such as the SAW device 102, with a high acoustic velocity layer 204 positioned on a first area 212 according to an embodiment of the disclosure.

[0071] The SAW device may comprise an IDT 200 having a plurality of electrode fingers 202. Electrode fingers may refer to metal strips positioned on a substrate. The electrode fingers 202 may be positioned on a piezoelectric substrate 210 of the SAW device 102. The piezoelectric substrate 210 may comprise one piezoelectric layer. Alternatively, the piezoelectric substrate 210 may be multi-layer substrate with at least one layer of piezoelectric material on top. The electrode fingers 202 are positioned above the piezoelectric material (204 can be below or above 202). The electrode fingers 202 may be equally spaced with a first electrode finger pitch 218. An electrode finger pitch may refer to a distance between centers of adjacent electrode fingers.

[0072] The SAW device 102 may further comprise two reflectors 206. The reflectors 206 may be positioned on each side of IDT 200 on the piezoelectric substrate 210, i.e., parallel with the IDT 200. The reflectors 206 may comprise a plurality of electrode fingers 208. The electrode fingers 206 may be equally spaced with a second electrode finger pitch 220. FIG. 2 shows a partial view of the structure of the SAW device, wherein a section of the structure around a border 216 of the IDT 200 and one of the reflectors 206 is shown.

[0073] The first electrode finger pitch 218 and the second electrode finger pitch 220 may be equal. Alternatively, the first electrode finger pitch 218 and the second electrode finger pitch 220 may have different values. For example, the first electrode finger pitch 218 may be smaller than the second electrode finger pitch 220. The electrode fingers 202, 208 may be arranged in parallel in a propagation direction of the surface acoustic wave (i.e., in a direction from a first reflector towards a second reflector). Each of the pitches may be kept constant between the respective electrode fingers. If the electrode finger pitch value(s) were varied at the edge of IDT and reflectors to suppress the longitudinal mode ripples, such ‘chirped’ electrode finger design would require higher lithography resolution than normal electrode fingers due to a decrease of the pitch value. Hence, fabrication in high frequency bands would be challenging. To avoid the drawback, the implementation of the higher acoustic velocity material on at least the first area 212 of the IDT 200 can suppress the longitudinal mode ripples without a decrease of any electrode finger pitch in device 102. The IDT 200 may comprise two first areas 212, each first area 212 located by an edge of the IDT 200 facing a respective reflector 206.

[0074] The first areas 212 are located by opposing edges of the IDT 200 in the propagation direction of the surface acoustic wave. The first areas 212 are configured to contain at least one of the plurality of electrode fingers 202. The first area 212 may have a width of at least two first electrode finger pitches 218. Hence, in an embodiment the first area 212 of the IDT 200 may comprise at least two electrode fingers 202. The second area 214 is located between the first areas 212. The second area 214 may be configured to contain at least one of the plurality of electrode fingers 202. In an embodiment, the second area 214 may have a width of at least two first electrode pitches and comprise at least two of the plurality of electrode fingers 202.

[0075] The first areas 212 may be configured to comprise a layer 204 of material having a higher acoustic velocity than velocity of the surface acoustic wave in the SAW device. The layer 204 of material having a higher acoustic velocity can be arranged above at least one of the electrode fingers 202 within the respective first area 212. In addition, or alternatively, the layer 204 can be positioned in-between at least one of the electrode fingers 202 within the respective first area 212. In addition, or alternatively, the layer 204 can be positioned beneath at least one of the electrode fingers 202 within the respective first area 212. The material with higher acoustic velocity can comprise, for example, Silicon Nitride, SiN, Aluminum Nitride, AIN, Aluminum Oxide, A12O3, or diamond. A thickness of the layer 204 may be less than 20 % of the first electrode finger pitch 218. The deposition of the high acoustic velocity layer 204 is configured to change the acoustic velocity of the specified electrode finger area, which suppresses the longitudinal mode ripples.

[0076] FIG. 3 illustrates an example of a top view of the SAW device of FIG. 2. The first areas 212 of the IDT 200 are located by the two edges of IDT 200 facing the border 216 between the IDT 200 and the respective reflector 206 and covered with a relatively thin layer 204 of the high acoustic velocity material. The electrode fingers 202 within the second area 214 of the IDT 200 and the electrode fingers 208 within the reflectors 206 are not covered by the layer 204, resulting different acoustic velocities at different electrode finger areas. In an embodiment, the first areas 212 may be located within 2-20 first electrode finger pitch 218 from the edge 216. FIG. 3 further illustrates how the acoustic velocity is increased at the first areas 212 compared to the other areas of the SAW device 102, i.e., the second area 214 of the IDT 200 and the areas of the reflectors 206. The acoustic velocity within the first areas 212 can be in a range of up to 10 % higher than the acoustic velocity within the associated second area 214.

[0077] FIG. 4 illustrates an example of a plot showing a difference in longitudinal mode ripples between a comparative SAW resonator (curve 400) and a SAW resonator (curve 402) according to an embodiment of the disclosure. The SAW resonator may comprise the SAW device as illustrated in FIG. 3. The comparative SAW resonator is designed without the first areas having a high acoustic velocity layer. The plot shows that the longitudinal mode ripples above -60dB are effectively suppressed by the structure of the SAW device of FIG. 3.

[0078] FIG. 5 illustrates an example of a partial side view of a SAW device, such as the SAW device 102, with a high acoustic velocity layer 204 positioned on first areas 212, 500 of electrode fingers 202, 208 of both the IDT 200 and the reflectors 206 according to an embodiment of the disclosure. The IDT 200 in FIG. 5 may be designed similar as in FIG. 2 and FIG. 3, wherein the first areas 212 are configured to have a higher acoustic velocity than the second area 214 by deposition of the layer 204 above, in-between or below at least one electrode finger 202 within the first area 212. Here, also the reflectors 206 may be configured with such first areas having the higher acoustic velocity. FIG. 5 shows the partial side view of the SAW device 102 such that only the IDT 200 and one of the reflectors 206 positioned on a piezoelectric substrate 210 are partially shown.

[0079] The first area 500 of the reflector 206 is located by an edge of the reflector 206 facing a border 216 between the IDT 200 and the respective reflector 206. The first area 500 covers at least one of the plurality of electrode fingers 208 of the respective reflector 206. The reflectors 206 further comprise a second area 502 located by the opposite edge of the reflector 206 compared to the first area 500. The second area 502 covers at least one of the plurality of electrode fingers 208 of the respective reflector 206. The first areas 500 of the reflectors 206 are configured to have a higher acoustic velocity than the second areas 502 of the reflectors 206. The first areas 212 of the IDT 200 and the first areas 500 of the reflectors 206 can have approximately the same acoustic velocity. The layer 204 of material having a higher acoustic velocity can be arranged above at least one of the electrode fingers 208 within the respective first area 500. In addition, or alternatively, the layer 204 can be positioned in-between at least one of the electrode fingers 208 within the respective first area 500. In addition, or alternatively, the layer 204 can be positioned beneath at least one of the electrode fingers 208 within the respective first area 500. The layer 204 arranged on the first areas 212 can have same or different thicknesses and properties as with the layer 204 arranged on the first areas 500. The material with higher acoustic velocity can comprise, for example, Silicon Nitride, SiN, Aluminum Nitride, AIN, Aluminum Oxide, A12O3, or diamond. A thickness of the layer 204 may be less than 20 % of the first electrode finger pitch 218.

[0080] FIG. 6 illustrates an example of a top view of the SAW device of FIG. 5 according to an embodiment of the disclosure. The electrode fingers 202 within the second area 214 of the IDT 200 and the electrode fingers 208 within the second areas 502 of the reflectors 206 are not covered by the layer 204, resulting different acoustic velocities at different electrode finger areas. In an embodiment, the first areas 212 may be located within 2-20 electrode finger pitch of the IDT 200 from the border 216. Further, the first areas 500 may be located within 2-20 electrode finger pitch of the reflectors 206 from the edge 216. For example, the IDT 200 can have the same or a different pitch in the first area 212 and the second area 214. For the reflector 206, the first area 500 can have either the same pitch as the IDT 200 and a same thickness of the layer 204, or a different pitch with a different thickness of the layer 204, as long as the acoustic velocity of the first area 500 is the same as with the first area 212. FIG. 3 further illustrates how the acoustic velocity is increased at the first areas 212 and the first areas 500 compared to the other areas of the SAW device 102, i.e., the second area 214 of the IDT 200 and the second areas 502 of the reflectors 206. The acoustic velocity within the first areas 212, 500 can be in a range of up to 10 % higher than the acoustic velocity within the associated second areas 214 in IDT 200.

[0081] FIG. 7 illustrates an example of a plot showing a difference in longitudinal mode ripples between a comparative SAW resonator (curve 700) and a SAW resonator (curve 702) according to an embodiment of the disclosure. The SAW resonator may comprise the SAW device as illustrated in FIG. 6. The comparative SAW resonator is designed without the first areas having a high acoustic velocity layer. The plot shows that the longitudinal mode ripples above -60dB are effectively suppressed by the structure of the SAW device of FIG. 6.

[0082] FIG. 8 illustrates an example of a partial side view of a SAW device with a dielectric layer 800 and a high acoustic velocity layer 204 positioned on a first area 212 of electrode fingers 202 of an IDT 200 according to an embodiment of the disclosure. FIG. 8 shows a partial view of the structure of the SAW device, wherein a section of the structure around a border 216 of the IDT 200 and one of the reflectors 206 is shown. The SAW device of FIG. 8 may be like the SAW device of FIG. 2 but comprises also the dielectric layer 800 as an upper layer (located on an opposite side of the SAW device 102 compared to the piezoelectric substrate 210) above the structure illustrated in FIG. 2. Therefore, details of the SAW device described in FIG. 2 are not repeated herein. The dielectric layer 800 can comprise, for example, silicon dioxide (SiO2). The dielectric layer may be located above each of the electrode fingers 202 of the IDT 200 and the electrode fingers 208 of the reflectors 206, as well as fill in gaps between the electrode fingers 202, 208 and at the border 216 of the IDT 200 and the reflectors 206. Inside the first area 212, the layer 204 may be located between the electrode fingers 202 and the dielectric layer 800. The dielectric layer 800 may be also referred to as a dielectric functional layer. The dielectric layer 800 may be a cover layer located above the electrode fingers 202, 208 (i.e., on top of at least one of the electrode fingers or the high acoustic velocity layer 204). The SAW device 102 may further comprise one or more additional layers above the dielectric layer, such as a passivation layer.

[0083] FIG. 9 illustrates an example of a top view of the SAW device 102 of FIG. 8, showing how the dielectric layer 800 can be arranged to cover the whole area of the SAW device. The layer 204 of high acoustic velocity material is located by both edges of the IDT 200 facing the reflectors 206, and a center section of the IDT 200, as well as both reflectors 206, are implemented without the layer 204. The dielectric layer 800 is located on top of each of the electrode fingers 202, 208 of the SAW device such that the layer 204 is beneath the dielectric layer 800.

[0084] FIG. 10 illustrates an example of a partial side view of a SAW device 102 with a dielectric layer 800 and a high acoustic velocity layer 204 positioned within first areas 212 of the IDT 200 and within first areas 500 of the reflectors 206 according to an embodiment of the disclosure. FIG. 10 shows a partial view of the structure of the SAW device, wherein a section of the structure around a border 216 of the IDT 200 and one of the reflectors 206 is shown. The SAW device 102 may otherwise have the same design as the SAW device of FIG. 5 but comprises also the dielectric layer 800 covering the whole device in FIG 5 as an above layer, at on opposite side of the SAW device compared to the piezoelectric substrate 210. Hence, details of the structure of the SAW device as described in FIG. 5 are not repeated herein. The dielectric layer 800 can be configured to cover the plurality of electrode fingers 208 of the reflectors 206 and the plurality of electrode fingers 202 of the IDT 200 for the whole area 212, 214, 500, 502 of the SAW device 102. The dielectric layer 800 can comprise, for example, silicon dioxide (SiO2).

[0085] FIG. 11 illustrates an example of a top view of the SAW device of FIG. 10, showing how each of the electrode fingers 202, 208 of the IDT 200 and the reflectors 206 are covered by the dielectric layer 800. Further, a subset of the electrode fingers 202, 208, which are located within the first areas by edges of the IDT 200 and the reflectors 206 facing each other, are in contact with the layer 204. That is, also the layer 204 is located below the dielectric layer.

[0086] FIG. 12 illustrates an example of partial side view of a SAW device with a reduced thickness of electrode fingers 202 within a first area 212 of an IDT 200 according to an embodiment of the disclosure. FIG. 12 shows a partial view of the structure of the SAW device, wherein a section of the structure around a border 216 of the IDT 200 and one of the reflectors 206 is shown.

[0087] Instead of using the layer of high acoustic velocity material, the first areas 212 may be configured to have the higher acoustic velocity by reducing a thickness of one or more electrode fingers 202 within the first areas 212. The thickness 1202 of the electrode fingers 202 within the first area 212 may be configured to be smaller compared to a thickness 1200 of the electrode fingers 202 outside the first area, such as the second area 214 of the IDT 200. The thickness 1202 may be also smaller compared to the thickness of the electrode fingers 208 of the reflectors 206 located on the same piezoelectric substrate 210. For example, the thickness 1202 may be up to 50 % lower than the thickness of the electrode fingers of the SAW device outside the first area 212. The thickness may be measured perpendicular from a top surface of the piezoelectric substrate 210. Outside the first area 212, the structure of the SAW device may correspond to the structure of the SAW device of FIG. 2, and is not therefore repeated herein. Reduction of the thickness may be selected such that the acoustic velocity within the first area 212 is in a range of up to 10 % higher than the acoustic velocity within the associated second area 214.

[0088] FIG. 13 illustrates an example of a top view of the SAW device of FIG. 12 according to an embodiment of the disclosure.

[0089] In an embodiment, the IDT 200 may comprise the first area 212 with reduced thickness of electrode fingers 202 on both sides of the IDT 200 facing the reflectors 206. The electrode fingers 208 of the reflectors 206 and the electrode fingers 202 of the IDT located between the first areas 212 may have greater thickness compared to the electrode fingers 202 located within the first areas 212. In an embodiment, the first areas 212 may be located within 2-20 electrode finger pitch of the IDT 200 measured from an edge facing the neighboring reflector 206.

[0090] In an embodiment, the higher acoustic velocity of the first areas 212 may be implemented with a reduced thickness of the electrode fingers 202 within one of the first areas 212 of the IDT 200. Hence, the IDT 200 may be manufactured such that the thickness of the electrode fingers 202 is reduced at one or more of the first areas 212.

[0091] FIG. 14 illustrates an example of a partial side view of a SAW device with a reduced thickness of electrode fingers on first areas 212, 500 of both an IDT 200 and reflectors 206 according to an embodiment of the disclosure. Design of the SAW device in FIG. 14 is similar to the SAW device in FIG. 12, but here also the electrode fingers 208 of the reflectors 206 within the first area 500 are formed on the piezoelectric substrate 210 with a reduced thickness. FIG. 13 shows a partial view of the structure of the SAW device, wherein a section of the structure around a border 216 of the IDT 200 and one of the reflectors 206 is shown.

[0092] The thickness 1202 of the electrode fingers 202, 208 within the first areas 212, 500 may be configured to be smaller compared to a thickness 1200 of the electrode fingers 202, 208 outside the first areas 212, 500, such as the second area 214 of the IDT 200 and the second area of the reflector 502. For example, the thickness 1202 may be up to 50 % lower than the thickness 1200 of the electrode fingers 202, 208 of the SAW device outside the first areas 212, 500. For example, reduction of the thickness may be selected such that the acoustic velocity within the first area 500 is in a range of up to 10 % higher than the acoustic velocity within the associated second area 502. The first areas 212, 500 may be located by both sides of a border 216 between the IDT 200 and the respective reflector 206. In an embodiment, the first area 212, 500 may be located within 2-20 electrode finger pitch of the IDT 200 or the reflector 206, respectively, from the border 216.

[0093] FIG. 15 illustrates an example of a top view of the SAW device of FIG. 14 according to an embodiment of the disclosure. FIG. 15 shows how the first areas 212 of the IDT 200 are located by both sides of the IDT 200 facing the reflectors 206. Further, there are at least two electrode fingers 202 between the first areas 212 of the IDT 200 without reduced thickness, i.e., the IDT 200 has electrode fingers 202 having a larger thickness in the middle of the IDT 200 compared to the first areas 212, which are on the two opposite sides of the IDT 200 in a propagation direction of surface acoustic waves. Further, both of the reflectors 206 have the first areas 500 of electrode fingers 208 with reduced thickness facing the first areas 212 of the IDT 200 such that there are at least two electrode fingers 208 with a larger thickness compared to the electrode fingers 208 of the first areas 500 at an opposing side of the reflector 206.

[0094] In an embodiment, the higher acoustic velocity of the first areas 212, 500 may be implemented with different methods. For example, one or more of the first areas 212, 500 may be fabricated to comprise one or more electrode fingers having the reduced thickness and one or more of the first areas 212, 500 (having a “normal” or “reduced” thickness of electrode fingers) may be fabricated to be covered with the layer of material having a higher acoustic velocity compared to a velocity of the surface acoustic wave in the SAW device.

[0095] FIG. 16 illustrates an example of a partial side view of a SAW device with a dielectric layer 800 and a reduced thickness of electrode fingers on a first area 212 of an IDT 200 according to an embodiment of the disclosure. FIG. 16 shows the structure of the SAW device partially, wherein a section of the structure around a border 216 of the IDT 200 and one of the reflectors 206 is shown.

[0096] The SAW device may be like the SAW device illustrated in FIG. 12, but wherein the dielectric layer 800 is added on top. The dielectric layer 800 is located on an opposite side of the SAW device 102 compared to the piezoelectric substrate 210. The dielectric layer 800 can comprise, for example, silicon dioxide (SiO2). The dielectric layer 800 may cover each of the electrode fingers 202, 208 as well as gaps between the electrode fingers 202, 208 and at a border 216 between the IDT 200 and the reflector 206.

[0097] FIG. 17 illustrates an example of a top view of the SAW device of FIG. 16, showing how the dielectric layer 800 can be implemented to cover the plurality of electrode fingers 208 of the reflectors 206 and the plurality of electrode fingers 202 of the IDT 200 for the whole area of the SAW device. The dielectric layer 800 may be deposited to cover a whole area of the SAW device. The whole area may match the area of the piezoelectric substrate.

[0098] FIG. 18 illustrates an example of a partial side view of a SAW device with a dielectric layer 800 and a reduced thickness of electrode fingers 202, 208 on first areas 212, 500 according to an embodiment of the disclosure. The SAW device of FIG. 18 has similar design to the SAW device of FIG. 14, but also comprises the dielectric layer 800 as an additional layer. The dielectric layer 800 may cover each of the areas 212, 214, 500, 502. The area covered by the dielectric layer 800 from the SAW device 102 area can extend to the whole piezoelectric substrate 210 area, i.e., covering the whole top portion of the die. FIG. 19 illustrates an example of a top view of the SAW device of FIG. 19 according to an embodiment of the disclosure, showing how the dielectric layer 800 is covering the area of each of the electrode fingers 202 of the IDT 200 as well as the electrode fingers 208 of both of the reflectors 206.

[0099] In an embodiment, the SAW device may comprise a plurality of IDTs. The plurality of IDTs may be positioned in parallel to each other between the two reflectors. Each of the plurality of IDTs may be implemented to comprise the areas of higher acoustic velocity, as described herein. The areas of higher acoustic velocity (compared to other areas of the SAW device) may be implemented, for example, by depositing the layer of high acoustic velocity material above, in between, or below one or more electrode fingers of the specified area. Alternatively, or in addition, the areas of higher acoustic velocity may be implemented with the one or more electrode fingers having a lower thickness in the specified area compared to other electrode fingers of the IDTs.

[0100] FIG. 20 illustrates an example of a method 2000 for manufacturing a SAW device according to an embodiment of the disclosure. The SAW device may comprise, for example, a SAW resonator, a SAW CRF (coupled resonator filter) structure, or the like.

[0101] At 2002, the method may comprise providing a piezoelectric substrate. The piezoelectric substrate may comprise one piezoelectric layer. Alternatively, the piezoelectric substrate may be multi-layer substrate with at least one layer of piezoelectric material on top.

[0102] At 2004, the method may comprise forming an interdigital transducer on the piezoelectric substrate, the IDT comprising a plurality of electrode fingers arranged parallel to each other in a propagation direction of the surface acoustic wave and spaced apart with a first electrode finger pitch. The IDT is further formed to comprise first areas located by opposing edges of the IDT in the propagation direction of the surface acoustic wave and containing at least one of the plurality of electrode fingers, and a second area located between the first areas and containing at least one of the plurality of electrode fingers, wherein the first areas are configured to have a higher acoustic velocity than the second area.

[0103] The method may further comprise forming two reflectors on the piezoelectric substrate on both sides of the at least one IDT. The reflectors may be formed to comprise a plurality of electrode fingers arranged parallel to each other in the propagation direction of the surface acoustic wave and spaced apart with a second electrode finger pitch; a first area of the reflector located by an edge facing a border between the at least one IDT and the respective reflector and containing at least one of the plurality of electrode fingers of the respective reflector; and a second area of the reflector located by the opposite edge of the reflector and containing at least one of the plurality of electrode fingers of the respective reflector; wherein the first areas of the reflectors are configured to have a higher acoustic velocity than the second areas of the reflectors.

[0104] Further features of the methods directly result from the functionalities and parameters of the methods and devices, for example the SAW device 102, as described in the appended claims and throughout the specification and are therefore not repeated here.

[0105] A device or a system may be configured to perform or cause performance of any aspect of the method(s) described herein. Further, a computer program may comprise program code configured to cause performance of an aspect of the method(s) described herein, when the computer program is executed on a computer. Further, the computer program product may comprise a computer readable storage medium storing program code thereon, the program code comprising instruction for performing any aspect of the method(s) described herein. Further, a device may comprise means for performing any aspect of the method(s) described herein. According to an example embodiment, the means comprises at least one processor, and at least one memory including program code, the at least one processor, and program code configured to, when executed by the at least one processor, cause performance of any aspect of the method(s).

[0106] Any range or device value given herein may be extended or altered without losing the effect sought. Also, any embodiment may be combined with another embodiment unless explicitly disallowed.

[0107] Although the subject matter has been described in language specific to structural features and / or acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as examples of implementing the claims and other equivalent features and acts are intended to be within the scope of the claims.

[0108] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item may refer to one or more of those items. Furthermore, references to ‘at least one’ item or ‘one or more’ items may refer to one or a plurality of those items.

[0109] The operations of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the scope of the subject matter described herein. Aspects of any of the embodiments described above may be combined with aspects of any of the other embodiments described to form further embodiments without losing the effect sought.

[0110] The term 'comprising' is used herein to mean including the method, blocks, or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or device may contain additional blocks or elements.

[0111] It will be understood that the above description is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from scope of this specification.

Claims

CLAIMS1. A surface acoustic wave, SAW, device (102), comprising: a piezoelectric substrate (210); and at least one interdigital transducer, IDT (200), on the piezoelectric substrate (210), the IDT (200) comprising: a plurality of electrode fingers (202) arranged parallel to each other in a propagation direction of the surface acoustic wave and spaced apart with a first electrode finger pitch (218); first areas (212) located by opposing edges of the IDT (200) in the propagation direction of the surface acoustic wave and containing at least one of the plurality of electrode fingers (202); a second area (214) located between the first areas (212) and containing at least one of the plurality of electrode fingers (202); and wherein the first areas (212) are configured to have a higher acoustic velocity than the second area (214).

2. The SAW device (102) of claim 1, further comprising: two reflectors (206) arranged on the piezoelectric substrate (210) on both sides of the at least one IDT (200), the reflectors (206) comprising: a plurality of electrode fingers (208) arranged parallel to each other in the propagation direction of the surface acoustic wave and spaced apart with a second electrode finger pitch (220); a first area (500) of the reflector (206) located by an edge facing a border between the at least one IDT (200) and the respective reflector (206) and containing at least one of the plurality of electrode fingers of the respective reflector (206); and a second area (502) of the reflector (206) located by the opposite edge of the reflector (206) and containing at least one of the plurality of electrode fingers (208) of the respective reflector (206); wherein the first areas (500) of the reflectors (206) are configured to have a higher acoustic velocity than the second areas (502) of the reflectors (206).

3. The SAW device (102) of claim 2, wherein the first areas (212) of the at least one IDT (200) and the first areas (500) of the reflectors (206) have approximately the same acoustic velocity.

4. The SAW device (102) of any preceding claim, wherein at least one of the first areas (212, 500) comprise a layer (204) of material having a higher acoustic velocity than velocity of the surface acoustic wave in the SAW device (102).

5. The SAW device (102) of any preceding claim, wherein the first areas (212) of the IDT (200) cover an area of at least two first electrode finger pitches (218).

6. The SAW device (102) of any of claims 2 to 5, wherein the first areas (500) of the reflectors (206) cover an area of at least two second electrode finger pitches.

7. The SAW device (102) of any of claims 4 to 6, wherein the layer (204) of material having a higher acoustic velocity is arranged above, in-between or beneath at least one of the electrode fingers (202, 208) within the at least one first area (212, 500).

8. The SAW device (102) of any of claims 4 to 7, wherein a thickness of the layer (204) of material having a higher acoustic velocity is less than 20 % of the first electrode finger pitch (218).

9. The SAW device (102) of any of claims 4 to 8, wherein the material with higher acoustic velocity comprises Silicon Nitride, SiN, Aluminum Nitride, AIN, Aluminum Oxide, AI2O3, or diamond.

10. The SAW device (102) of any preceding claim, wherein the electrode fingers (202, 208) within at least one of the first areas (212, 500) have a lower thickness (1202) compared to a thickness (1200) of the electrode fingers (202, 208) outside the at least one first area (212, 500).

11. The SAW device (102) of claim 10, wherein the thickness (1202) of the electrode fingers (202, 208) within the at least one first area (212, 500) is up to 50 % lower than the thickness (1200) of the electrode fingers (202, 208) outside the at least one first area (212, 500).

12. The SAW device (102) of any preceding claim, wherein the acoustic velocity within the first areas (212, 500) is in a range of up to 10 % higher than the acoustic velocity within the associated second area (214, 502).

13. The SAW device (102) of any preceding claim, wherein the piezoelectric substrate (210) comprises at least one of a single piezoelectric layer or a multi-layer structure with at least one piezoelectric layer on top.

14. The SAW device (102) of any preceding claim, further comprising a dielectric layer (800) as a cover layer above the electrode fingers (202, 208).

15. The SAW device of claim 14, wherein the dielectric layer (800) covers the plurality of electrode fingers (208, 202) of the reflectors (206) and at least one IDT (200) for the whole area (212, 214, 500, 502) of the SAW device (102).

16. The SAW device (102) of claim 14 or 15, wherein the dielectric layer (800) comprises silicon dioxide, SiCh.

17. The SAW device (102) of any of claims 2 to 16, wherein the SAW device (102) comprises a plurality of IDTs (200) positioned between the reflectors (206).

18. A method (1900) for manufacturing a surface wave acoustic device, comprising: providing (1902) a piezoelectric substrate (210); and forming (1904) at least one interdigital transducer, IDT (200), on the piezoelectric substrate (210), the IDT (200) comprising: a plurality of electrode fingers (202) arranged parallel to each other in a propagation direction of the surface acoustic wave and spaced apart with a first electrode finger pitch (218); first areas (212) located by opposing edges of the IDT (200) in the propagation direction of the surface acoustic wave and containing at least one of the plurality of electrode fingers (202); a second area (214) located between the first areas (212) and containing at least one of the plurality of electrode fingers (202); and wherein the first areas (212) are configured to have a higher acoustic velocity than the second area (214).

19. The method (1900) of claim 18, further comprising: forming two reflectors (206) on the piezoelectric substrate (210) on both sides of the at least one IDT (200), the reflectors (206) comprising: a plurality of electrode fingers (208) arranged parallel to each other in the propagation direction of the surface acoustic wave and spaced apart with a second electrode finger pitch (220);a first area (500) of the reflector (206) located by an edge facing a border between the at least one IDT (200) and the respective reflector (206) and containing at least one of the plurality of electrode fingers of the respective reflector (206); and a second area (502) of the reflector (206) located by the opposite edge of the reflector (206) and containing at least one of the plurality of electrode fingers (208) of the respective reflector (206); wherein the first areas (500) of the reflectors (206) are configured to have a higher acoustic velocity than the second areas (502) of the reflectors (206).

20. The method (1900) of claim 19, wherein the first areas (212) of the at least one IDT (200) and the first areas (500) of the reflectors (206) are configured to have approximately the same acoustic velocity.

21. The method (1900) of any of claims 18 to 20, wherein at least one of the first areas (212, 500) comprise a layer (204) of material having a higher acoustic velocity than velocity of the surface acoustic wave in the SAW device (102).

22. The method (1900) of any of claims 18 to 21, wherein the first areas (212) of the IDT (200) cover an area of at least two first electrode finger pitches (218).

23. The method (1900) of any of claims 19 to 22, wherein the first areas (500) of the reflectors (206) cover an area of at least two second electrode finger pitches.

24. The method (1900) of any of claims 21 to 23, wherein the layer (204) of material having a higher acoustic velocity is arranged above, in-between or beneath at least one of the electrode fingers (202, 208) within the at least one first area (212, 500).

25. The method (1900) of any of claims 21 to 24, wherein athickness of the layer (204) of material having a higher acoustic velocity is less than 20 % of the first electrode finger pitch (218).

26. The method (1900) of any of claims 21 to 25, wherein the material with higher acoustic velocity comprises silicon nitride, SiN, aluminum nitride, AIN, aluminum oxide, AI2O3, or diamond.

27. The method (1900) of any of claims 18 to 26, wherein the electrode fingers (202, 208) within at least one of the first areas (212, 500) have a lower thickness (1202) compared to a thickness (1200) of the electrode fingers (202, 208) outside the at least one first area (212, 500).

28. The method (1900) of claim 27, wherein the thickness (1202) of the electrode fingers (202, 208) within the at least one first area (212, 500) is up to 50 % lower than the thickness (1200) of the electrode fingers (202, 208) outside the at least one first area (212, 500).

29. The method (1900) of any of claims 18 to 28, wherein the acoustic velocity within the first areas (212, 500) is in a range of up to 10 % higher than the acoustic velocity within the associated second area (214, 502).

30. The method (1900) of any of claims 18 to 29, wherein the piezoelectric substrate (210) comprises at least one of a single piezoelectric layer or a multi-layer structure with at least one piezoelectric layer on top.

31. The method (1900) of any of claims 18 to 30, further comprising: forming a dielectric layer (800) as a cover layer above the electrode fingers (202, 208).

32. The method (1900) of claim 31, wherein the dielectric layer (800) covers the plurality of electrode fingers (208, 202) of at least one of the reflectors (206) and the IDT (200) for the whole area (212, 214, 500, 502) of the SAW device (102).

33. The method (1900) of claim 31 or 32, wherein the dielectric layer (800) comprises silicon dioxide, SiCh.

34. A surface acoustic wave filter (100), comprising one or more surface acoustic wave devices (102) according to any of claims 1 to 17.

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