Electroacoustic resonator

Abstract

An electroacoustic resonator includes a substrate (3) having a piezoelectric material and an interdigital electrode structure on the top side (33) of the substrate. The electrode structure includes a first electrode (1) and a second electrode (2), each electrode having a bus bar (20) and a plurality of fingers (10). The fingers of the two electrodes cross each other. The region of the top side between the two bus bars is subdivided into two barrier regions (113), two trap regions (112), and one track region (111), the trap regions being located between the two barrier regions, and the track region being located between the two trap regions. At least some of the fingers each include a barrier portion (13), two trap portions (12), and one track portion (11), wherein the barrier portion is associated with the barrier region closest to the bus bar to which the finger is assigned, each trap portion is associated with one of the trap regions in the trap region, and the track portion is associated with the track region. The fingers are configured such that the velocity of the main mode of the surface acoustic wave is less in the trap region than in the track region. Each electrode includes a plurality of short fingers (30) that are shorter than the fingers. Each short finger is only associated with the barrier region closest to the bus bar to which the short finger is assigned. The electrodes are configured such that the velocity of the main mode is greater in the barrier region than in the track region.

Description

[0001] The present invention specifies an electroacoustic resonator.

[0002] One task to be solved is to specify an electroacoustic resonator with fewer spurious modes.

[0003] According to at least one embodiment, the electroacoustic resonator includes a substrate having a piezoelectric material. The substrate may be formed of the piezoelectric material or may include a layer of the piezoelectric material, such as a thin film. For example, the piezoelectric material is lithium tantalate (e.g., LiTaO3) or lithium niobate (e.g., LiNbO3) or AlN or quartz.

[0004] According to at least one embodiment, the electroacoustic resonator includes an interdigital electrode structure on the top side of the substrate. The interdigital electrode structure is preferably a metal structure. For example, the electrode structure is made of a metal, such as Cu or Al or Pt or Ag or Au or Ti or Cr, or a compound or metal alloy thereof. The top side of the substrate is preferably formed of the piezoelectric material. The interdigital electrode structure may be covered with an additional layer of a dielectric material (e.g., SiO2 or Si3N4) to provide temperature compensation, passivation, or other additional functions.

[0005] According to at least one embodiment, the electrode structure includes a first electrode and a second electrode, each electrode having a bus bar and a plurality of fingers. The fingers of each electrode are electrically connected through the bus bar of the electrode. The electrodes may each be composed of a single material or may have a layered structure. The fingers are preferably integrally formed with the assigned bus bar. The electrodes preferably directly contact the piezoelectric material, or they are separated from the piezoelectric material by a thin layer of an insulating material. For example, each electrode includes at least 10 or at least 50 or at least 100 fingers.

[0006] The bus bars of the two electrodes are parallel or substantially parallel to each other. The main extension direction of the bus bar is preferably parallel to the longitudinal direction. The direction parallel to the top side and perpendicular to the longitudinal direction is defined as the transverse direction. The fingers of the electrodes extend transversely to the bus bar, for example, the main extension direction of each finger is parallel to the transverse direction. Preferably, all the fingers are parallel or substantially parallel to each other.

[0007] According to at least one embodiment, the fingers of the two electrodes cross each other. However, the fingers of different electrodes are electrically isolated from each other. In other words, the electrodes are interdigital electrodes. The two electrodes together define an interdigital transducer of the electroacoustic resonator. For example, the electroacoustic resonator is a SAW resonator.

[0008] According to at least one embodiment, the region of the top side between two bus bars is subdivided into two barrier regions, two trap regions, and one track region. The trap regions are located between the two barrier regions, and the track region is located between the two trap regions. Preferably, these regions are all continuous, strip-shaped regions having a main extension direction parallel to the longitudinal direction. Along the transverse direction, these regions are arranged one after another.

[0009] According to at least one embodiment, at least some (preferably all) of the fingers each include a barrier portion, two trap portions, and one track portion. The barrier portion is associated with the barrier region closest to the bus bar assigned to the finger. Each trap portion is associated with one of the trap regions. The track portion is associated with the track region. Here, if in a plan view of the top side of the substrate, a portion overlaps with a region, preferably completely overlaps with the region, then the portion is associated with the region. In particular, the association between the portion and the region is one-to-one. If a bus bar and a finger belong to the same electrode, the bus bar is assigned to the finger.

[0010] In other words, starting from the assigned bus bar, at least some of the fingers each include a barrier portion, followed by a first trap portion, followed by a track portion, followed by a second trap portion. The second trap portion preferably forms the end of the finger remote from the bus bar. Preferably, the fingers are each composed of only these four portions. Particularly preferably, the width of the finger (measured as the extension of the finger along the longitudinal direction) is constant within each portion. "Constant" means constant within the manufacturing tolerances. However, the width of the finger can be different in different portions. Preferably, the height of the finger (measured as the extension perpendicular to the top side) is also constant within each portion, but can be different in different portions.

[0011] According to at least one embodiment, the fingers are configured such that the speed of the main mode of the surface acoustic wave is less in the trap region than in the track region. The surface acoustic wave propagates on the top side of the substrate. The main mode is a surface acoustic wave that propagates along the longitudinal direction and has a certain polarization. For example, the main mode is a Rayleigh surface acoustic wave. Preferably, the speed of the main mode in the trap region is at most 99% or at most 98% or at most 97% of the speed of the main mode in the track region. Additionally or alternatively, the speed of the main mode in the trap region is at least 93% or at least 94% or at least 95% of the speed in the track region.

[0012] During the intended operation, the electroacoustic resonator generates the main mode, i.e., the desired surface acoustic wave. The resonator is configured such that during operation, the main mode is generated within the track region and trapped within the track region.

[0013] According to at least one embodiment, each electrode includes a plurality of short fingers. The short fingers are shorter than the fingers, i.e., the short fingers have a length shorter than that of the fingers. The length of the finger / short finger or the length of a part of the finger is the extension along the transverse direction. Each electrode may include at least 10 or at least 50 or at least 100 short fingers.

[0014] The short fingers and the fingers extend from the same side of the assigned bus bar. This means that the short fingers and the fingers of the electrode extend from the assigned bus bar towards the bus bars of other electrodes. Within the manufacturing tolerances, the short fingers may all have the same shape. Similarly, within the manufacturing tolerances, the fingers including different parts may all have the same shape.

[0015] According to at least one embodiment, each short finger is only associated with the barrier region closest to the bus bar assigned to the short finger. In particular, the short finger does not overlap with the trap region and the track region. Preferably, the width of the barrier region (measured perpendicular to the longitudinal direction) is defined by the length of the assigned short finger.

[0016] According to at least one embodiment, the electrode is configured such that the speed of the main mode in the barrier region is greater than the speed in the track region. For example, the speed of the main mode in the barrier region is at least 101% or at least 102% or at least 104% or at least 106% of the speed in the track region. Additionally or alternatively, the speed of the main mode in the barrier region is at most 110% or at most 109% or at most 107% or at most 105% of the speed in the track region.

[0017] Compared with a resonator having a piston mode design with fingers but no short fingers, the speed in the barrier region is reduced. The reduction in the speed in the barrier region allows an increase in the speed in the trap region, which further allows suppression of unwanted modes.

[0018] In at least one embodiment, an electroacoustic resonator includes a substrate having a piezoelectric material and an interdigital electrode structure on a top side of the substrate. The electrode structure includes a first electrode and a second electrode, each electrode having a bus bar and a plurality of fingers. The fingers of the two electrodes cross each other. A region between the two bus bars on the top side is subdivided into two barrier regions, two trap regions, and one track region, with the trap regions located between the two barrier regions and the track region located between the two trap regions. At least some of the fingers each include a barrier portion, two trap portions, and one track portion, where the barrier portion is associated with the barrier region closest to the bus bar assigned to the finger, each trap portion is associated with one of the trap regions, and the track portion is associated with the track region. The fingers are configured such that the velocity of the main mode of a surface acoustic wave is less in the trap regions than in the track region. Each electrode includes a plurality of short fingers that are shorter than the fingers. Each short finger is associated only with the barrier region closest to the bus bar assigned to the short finger. The electrodes are configured such that the velocity of the main mode is greater in the barrier regions than in the track region.

[0019] In an electroacoustic resonator, in addition to the desired main mode, there are a number of unwanted (stray) modes. These unwanted modes are modes that do not propagate along the main mode propagation direction (= longitudinal direction) and may have all possible polarizations, as well as modes that have the same propagation direction as the main mode but have a different polarization from the main mode. Generally, it is not possible to find an electrode structure design that allows suppression of all unwanted modes. This is especially the case in material systems with high electroacoustic coupling and / or a small distance between the main mode and unwanted modes with different polarizations. In a SAW filter composed of such resonators, the remaining unwanted modes may cause dips in the passband and at the filter edges, group delay ripples, trimming problems, and / or reduced power persistence.

[0020] To suppress unwanted transverse modes (i.e., modes having non-zero propagation in a transverse direction perpendicular to the longitudinal direction), the fingers of the electrodes can be formed using a so-called piston mode design, where the fingers are subdivided into a barrier portion, two trap portions, and a track portion. The trap portions can be designed such that in the associated trap regions, the velocity of the main mode is less than in the track region. This can be achieved, for example, by using metal dots in the trap portions in order to increase the mass of the trap portions compared to the track portion. However, this can be problematic for high-frequency resonators, where the spacing between adjacent fingers must be small. Production methods are generally not sufficient to accurately place the metal dots on very small structures.

[0021] The idea of the inventors of the present invention is to additionally use short fingers in the electrodes. The short fingers allow reducing the speed of surface acoustic waves propagating in the longitudinal direction in the barrier region on the side of the trap region. As a result, the speed in the trap region does not have to be as small as without the short fingers, so that it is not necessary to use metal dots for the trap part at all. In this way, costs are saved and manufacturing limitations due to dot alignment accuracy and minimum dot size are avoided.

[0022] Another advantage of the short fingers is that they bring an additional degree of freedom when optimizing the resonator for the suppression of unwanted modes. In fact, by adjusting the width and / or length and / or height of the short fingers, the speed of the surface acoustic waves in the barrier region can be adjusted, and in this way the suppression of modes with a transverse propagation direction can be optimized. In addition, by optimizing the design of the short fingers, modes with unwanted polarizations can also be further suppressed.

[0023] According to at least one embodiment, the width and / or height of the fingers in the trap region is greater than the width and / or height in the track region. By increasing the width and / or height, the mass loading of the fingers in the trap region increases compared to the track region, so that the speed of the main mode in the trap region decreases compared to the track region. For example, for each finger, the width and / or height in the trap region is at least 105% or at least 110% or at least 130% or at least 150% of the width and / or height in the track region. Additionally or alternatively, for each finger, the width and / or height in the trap region is at most 250% or at most 200% or at most 150% of the width and / or height in the track region.

[0024] According to at least one embodiment, the short fingers and / or the barrier part of the fingers are thinner and / or narrower than the track part of the fingers. For example, the height and / or width of the short fingers and / or the barrier part is at most 90% or at most 80% or at most 70% of the height and / or width of the track part. Additionally or alternatively, the height and / or width of the short fingers and / or the barrier part is at least 20% or at least 40% or at least 50% of the height and / or width of the track part.

[0025] By reducing the width and / or height of the short fingers and / or the barrier part of the fingers, the mass loading in the barrier region can be reduced compared to the track region, and as a result, the speed of the main mode increases compared to the track region.

[0026] According to at least one embodiment, the short fingers are shorter than the barrier part. For example, the length of the short fingers is at most 80% or at most 90% or at most 95% of the length of the barrier part of the fingers. Additionally or alternatively, the length of the short fingers is at least 70% or at least 80% of the length of the barrier part.

[0027] According to at least one embodiment, the width and / or height of the short fingers is / are the same as the width and / or height of the barrier portion of the fingers. Here and hereinafter, two elements having the same height and / or width and / or length mean elements having the same height and / or width and / or length within the limits of manufacturing tolerances. For example, a deviation of at most 10% or at most 5% occurs.

[0028] According to at least one embodiment, the height of the fingers in the trap portion is the same as the height in the track portion. In this case, the width of the fingers in the trap portion is preferably greater than the width in the track portion. Preferably, the height of the short fingers and the fingers in the barrier portion is also the same as the height in the track portion. In this case, the width of the short fingers and / or the barrier portion is preferably less than the width in the track portion.

[0029] A trap region that is deep enough (i.e., a trap region with a low dominant mode velocity) can generally be achieved by using a trap portion with an increased height, for example by using metal dots. However, in high-frequency resonators, small structural dimensions are required. Therefore, the application of metal dots may be difficult. However, for the present invention, the trap portion can be selected to have the same height as the track portion because the trap depth does not have to be selected to be as deep due to the reduced velocity of the dominant mode in the barrier region.

[0030] According to at least one embodiment, in each electrode, the fingers and the short fingers are arranged in an alternating manner. In particular, in each electrode, there is a short finger between each pair of fingers, and there is a finger between each pair of short fingers.

[0031] According to at least one embodiment, with respect to the propagation direction of the dominant mode (i.e., the longitudinal direction), each short finger is located at the same height as the fingers of the corresponding other electrode. This means that the center line passing through the short finger (the center line extending in the transverse direction) also passes through the fingers of the other electrode. Preferably, the center line of the short finger is also the center line of the fingers of the other electrode at the same height. Preferably, the distance from each short finger to the fingers of the other electrode at the same height is at most 10% or at most 5% of the distance between the two busbars. Additionally or alternatively, the distance from each short finger to the fingers of the other electrode at the same height is at most 0.5·λ or at most 0.4·λ or at most 0.3·λ, where λ is the wavelength of the dominant mode in the track region. The distance between two objects is defined here as the length of the shortest connection between the two objects.

[0032] According to at least one embodiment, the electroacoustic resonator is part of an RF filter. The RF filter can be used in a communication device such as a mobile phone. The RF filter can be a band-pass filter. The resonance frequency of the electroacoustic resonator is, for example, at least 0.4 GHz or at least 2.5 GHz or at least 6 GHz or at least 8 GHz.

[0033] Further preferred embodiments and developments of the electroacoustic resonator are described below in conjunction with the drawings. Identical or similar elements and elements with the same functions are denoted by the same reference numerals in the drawings. The scales of the drawings and the elements shown therein are not considered to be shown to scale. Instead, for better presentation and / or better understanding, individual elements (especially layers) may be shown exaggerated in magnitude.

[0034] In the drawings:

[0035] Figures 1 to 4 Exemplary embodiments of the electroacoustic resonator are shown in different views, and

[0036] Figure 5 and Figure 6 Properties of the electroacoustic resonator are shown based on the illustration.

[0037] Figure 1 A first exemplary embodiment of the electroacoustic resonator is shown in a plan view. Figure 2 Shown in a cross-sectional view (when cut through the cutting plane AA’) is Figure 1 the electroacoustic resonator. The electroacoustic resonator includes a substrate 3 having a piezoelectric material (such as LiNbO3). The substrate 3 includes a top side 33 made of the piezoelectric material. For example, the substrate 3 includes a thin film of the piezoelectric material. The cut angle of the piezoelectric material is, for example, (0°, 38°, 0°). The cut angle (λ’, μ, θ) is the Euler angle that defines the orientation of the top surface of the piezoelectric material relative to the crystal axes of the piezoelectric material. This definition is according to the international standard IEC 62276:2016.

[0038] On the top side 33 of the substrate, an electrode structure is applied. The electrode structure is made of a metal such as Cu. The electrode structure includes a first electrode 1 and a second electrode 2. The two electrodes 1, 2 include busbars 20 and a plurality of fingers 10. The busbars 20 all extend along the longitudinal direction L. The fingers 10 extend perpendicular to the busbars 20 in a transverse direction T perpendicular to the longitudinal direction L.

[0039] Between the busbars 20, the top side 33 of the substrate 3 is subdivided into two barrier regions 113, two trap regions 112 and one track region 111. The regions 111, 112, 113 are all formed in strip shape, with the main extension direction along the longitudinal direction L. The regions 111, 112, 113 are arranged one after another along the transverse direction T. The track region 111 is located between the two trap regions 112. The two trap regions 112 and the track region 111 are located between the two barrier regions 113. The barrier regions 113 are each adjacent to the busbar 20.

[0040] The fingers 10 each include a barrier portion 13, and the barrier portion 13 is adjacent to the busbar 20 of the assigned electrode. In the direction away from the busbar 20, the first trap portion 12, the track portion 11 and the second trap portion 12 are arranged in this order downstream of the barrier portion 13 in each case. The barrier portion 13 is associated / overlaps with the barrier region 113 adjacent to the assigned busbar 20. The trap portion 12 is associated / overlaps with the trap region 112, and the track portion 11 is associated / overlaps with the track region 111.

[0041] In addition to the fingers 10, each electrode 1, 2 further includes a short finger 30 extending from the busbar 20. In each of the electrodes 1, 2, a short finger 30 is arranged between each pair of fingers 10. The short finger 30 is only associated and overlaps with the barrier region 113 adjacent to the assigned busbar 20. With respect to the longitudinal direction L, each short finger 30 is located at the same height as the fingers 10 of the corresponding other electrode.

[0042] During Figure 1 and Figure 2 the operation of the electroacoustic resonator shown, the main mode of the surface acoustic wave is generated, and the main mode propagates along the longitudinal direction with a specific polarization. The main mode is basically trapped in the track region 111. The trap portions 12 of the fingers 10 are selected such that the velocity of the main mode is reduced in the trap region 112 compared to the track region 111. This is achieved by forming the trap portion 12 wider than the track portion 11, which results in an increase in the mass loading in the trap region 112 compared to the track region 111. However, the height of the fingers 10 in the track portion 12 is the same as the height in the track portion 11 (see Figure 2 ), which is advantageous considering the production of the fingers 10.

[0043] The short finger 30 is designed to have a reduced width compared to the track portion 11. The barrier portion 13 of the finger 10 has the same width as the track portion 11. The height of the short finger 30 and the barrier portion 13 is the same as the height in the track portion 11. Due to the reduced width of the short finger 30, the speed of the main mode in the barrier region 113 is greater than that in the track region 111, but smaller compared to the case where the short finger 30 is not used.

[0044] The resulting speed profile is shown in the Figure 5 figure. The x-axis represents the transverse direction T. The y-axis represents the speed of the main mode. During the operation of the resonator, the speed profile generates the Figure 6 profile of the amplitude of the main mode as shown in the figure. Here, the x-axis also represents the transverse direction T. The y-axis represents the amplitude of the main mode. As can be seen, due to the design of the electrode structure, the main mode amplitude has an almost rectangular shape, with an almost flat shape in the track region 111 and steep sides in the trap region 112. In the case where the main mode has such a profile, the unwanted modes propagating in the transverse direction are almost completely suppressed. In Figure 5 and Figure 6 , the small gap region between the track region 112 and the barrier region 113 visible in the previous figure is not indicated.

[0045] Figure 3 shows a second exemplary embodiment of the electroacoustic resonator. The resonator is shown only in cross-section. For example, the plan view will be the same as in Figure 1 . Compared with the electroacoustic resonator of Figure 2 , the electroacoustic resonator of Figure 3 has a finger 10 whose height in the trap portion 12 is greater than that in the track portion 11. This helps to further reduce the speed of the main mode in the track region 112. The increased height 12 can be used instead of the increased width. In addition, the height of the finger 10 in the barrier portion 13 can be less than that in the track portion 11. Similarly, the height of the short finger 30 can be less than the height of the finger 10 in the track portion 11.

[0046] Figure 4 shows a third exemplary embodiment of the electroacoustic resonator, now again shown in a plan view on the top side 33 of the substrate 3. Figure 4 The electroacoustic resonator of Figure 1 is also different in that the width of the finger 10 in the barrier portion 13 is now reduced compared to the width in the track portion 11. In this way, compared with the case of Figure 1 , the speed of the main mode in the barrier region 113 is increased.

[0047] Figure 3 and Figure 4 The electroacoustic resonators have similar characteristic velocity profiles and similar main mode amplitude profiles, as shown in Figure 5 and Figure 6 as shown.

[0048] The invention described herein is not limited by the description of the exemplary embodiments. On the contrary, the invention includes any new feature and any combination of features, in particular any combination of the features in the patent claims, even if said features or said combination are not explicitly stated in the patent claims or the exemplary embodiments.

[0049] List of reference numerals:

[0050] 1 First electrode

[0051] 2 Second electrode

[0052] 3 Substrate

[0053] 10 Fingers

[0054] 11 Track portion

[0055] 12 Trap portion

[0056] 13 Barrier portion

[0057] 20 Bus bar

[0058] 30 Short fingers

[0059] 33 Top side

[0060] 111 Track region

[0061] 112 Trap region

[0062] 113 Barrier region

[0063] T Transverse direction

[0064] L Longitudinal direction

Claims

1. An electroacoustic resonator, comprising: a substrate having a piezoelectric material, and an interdigital electrode structure on the top side of the substrate, wherein: the electrode structure includes a first electrode and a second electrode, each electrode having a bus bar and a plurality of fingers, the fingers of the two electrodes cross each other, the region of the top side between the two bus bars is subdivided into two barrier regions, two trap regions, and one track region, the trap regions being located between the two barrier regions, and the track region being located between the two trap regions, at least some of the fingers each include a barrier portion, two trap portions, and one track portion, wherein the barrier portion is associated with the barrier region closest to the bus bar assigned to the finger, each of the trap portions is associated with one of the trap regions, and the track portion is associated with the track region, each electrode includes a plurality of short fingers shorter than the fingers, each short finger is only associated with the barrier region closest to the bus bar assigned to the short finger, each short finger and the one barrier portion among the at least some fingers have a first thickness; the one track portion among the at least some fingers has a second thickness greater than the first thickness; and the two trap portions of the at least some fingers have a third thickness greater than the second thickness.

2. The electroacoustic resonator according to claim 1, wherein the velocity of the main mode in the trap region is at most 99% and at least 93% of the velocity of the main mode in the track region.

3. The electroacoustic resonator according to claim 1 or 2, wherein the velocity of the main mode in the barrier region is at most 110% and at least 101% of the velocity of the main mode in the track region.

4. The electroacoustic resonator according to claim 1 or 2, wherein the width of the fingers in the trap region is greater than the width in the track region.

5. The electroacoustic resonator according to claim 1 or 2, wherein the short fingers and / or the barrier portions of the fingers are thinner and / or narrower than the track portions of the fingers.

6. The electroacoustic resonator according to claim 1 or 2, wherein the short fingers are shorter than the barrier portions of the fingers.

7. The electroacoustic resonator according to claim 1 or 2, wherein the width of the short fingers is the same as the width of the barrier portions of the fingers.

8. The electroacoustic resonator according to claim 1 or 2, wherein in each electrode, the fingers and the short fingers are arranged in an alternating manner.

9. The electroacoustic resonator according to claim 1 or 2, wherein each short finger is at the same height as the fingers of the corresponding other electrode with respect to the propagation direction of the main mode.

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