Electroacoustic resonator

Abstract

An electroacoustic resonator, comprising: - a substrate (3) with a piezoelectric material, - an interdigital electrode structure on a top surface (33) of the substrate (3), wherein - the electrode structure comprises a first electrode (1) and a second electrode (2), each with a busbar (20) and a plurality of fingers (10), - the fingers (10) of both electrodes (1, 2) interlock, - the area of ​​the top (33) between the two busbars (20) is divided into two barrier areas (113), two trap areas (112) and a track area (111), wherein the trap areas (112) are arranged between the two barrier areas (113) and the track area (111) is arranged between the two trap areas (112), - at least some fingers (10) each comprise a barrier section (13), two drop sections (12) and a track section (13), wherein the barrier section (13) is assigned to the barrier area (113) that is closest to the busbar (30) assigned to the finger (10), the drop sections (12) are each assigned to one of the drop areas (112) and the track section (11) is assigned to the track area (111), - the fingers (11) are configured such that the speed of a principal mode of acoustic surface waves in the trap areas (112) is smaller than in the track area (111), - each electrode (1, 2) comprises a plurality of stubby fingers (30) that are shorter than the fingers (10), - each stub finger (30) is only assigned to the barrier area (113) that is closest to the busbar (20) assigned to the stub finger (30), - the electrodes (1, 2) are configured such that the velocity of the main mode in the barrier regions (113) is greater than in the track region (111), - wherein the stub fingers (30) and / or the barrier sections (13) of the fingers (10) are thinner and / or narrower than the track sections (11) of the fingers (10).

Description

[0001] An electroacoustic resonator is specified.

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

[0003] The electroacoustic resonator comprises a substrate containing a piezoelectric material. The substrate can be made entirely of the piezoelectric material or consist of a layer, for example, a thin film, of the piezoelectric material. Examples of piezoelectric materials include lithium tantalate (LiTaO3), lithium niobate (LiNbO3), aluminum nitrate (AlN), and quartz.

[0004] The electroacoustic resonator comprises an interdigital electrode structure on a top surface of the substrate. The interdigital electrode structure is preferably a metal structure. For example, the electrode structure consists of a metal such as Cu, Al, Pt, Ag, Au, Ti, or Cr, or of a compound or metal alloy thereof. The top surface of the substrate is preferably formed from the piezoelectric material. The interdigital electrode structure can be covered with additional layers of dielectric material, e.g., SiO2 or Si3N4, to achieve temperature compensation, passivation, or other additional functionalities.

[0005] The electrode structure comprises a first electrode and a second electrode, each with a busbar and a plurality of fingers. The fingers of each electrode are electrically connected via the electrode's busbar. The electrodes can each consist of a single material or have a layered structure. The fingers are preferably integral with the associated busbar. The electrodes are preferably in direct contact with the piezoelectric material or are separated from the piezoelectric material by a thin layer of insulating material. For example, each electrode comprises at least ten, at least 50, or at least 100 fingers.

[0006] The busbars of both electrodes run parallel or substantially parallel to each other. The main extension direction of the busbars preferably runs parallel to the longitudinal direction. A direction parallel to the top surface and perpendicular to the longitudinal direction is defined as the transverse direction. The electrode fingers extend transversely to the busbar, e.g., with a main extension direction of each finger that runs parallel to the transverse direction. Preferably, all fingers run parallel or substantially parallel to each other.

[0007] The fingers of both electrodes interlock. However, the fingers of different electrodes are electrically isolated from each other. In other words, the electrodes are interdigital. Together, the two electrodes form an interdigital transducer of the electroacoustic resonator. For example, the electroacoustic resonator is a SAW resonator.

[0008] The area of ​​the upper surface between the two busbars is divided into two barrier areas, two trap areas, and a track area. The trap areas are located between the two barrier areas, and the track area is located between the two trap areas. The areas are preferably each a continuous, strip-shaped region with a main extension direction parallel to the longitudinal direction. Along the transverse direction, the areas are arranged sequentially.

[0009] At least some fingers, preferably all fingers, each comprise a barrier section, two trap sections, and a track section. The barrier section is assigned to the barrier area that is closest to the busbar assigned to the finger. The trap sections are each assigned to one of the trap areas. The track section is assigned to the track area. Here, a section is assigned to an area if, in a top view of the substrate's upper surface, the section overlaps with the area, preferably completely. The assignment between a section and an area is particularly one-to-one. A busbar is assigned to a finger if the busbar belongs to the same electrode as the finger.

[0010] In other words, starting from the associated busbar, at least some fingers each comprise a barrier section, followed by a first trap section, followed by a track section, followed by a second trap section. The second trap section preferably forms the end of the finger furthest from the busbar. Preferably, the fingers each consist of only these four parts. Particularly preferably, the width of the fingers, measured as the longitudinal extent of the finger, is constant in each section. "Constant" means constant within the manufacturing tolerance. However, the width of the fingers may differ in different sections. Preferably, the height of the fingers, measured as the extent perpendicular to the top surface, is also constant in each section, but may differ in different sections.

[0011] The fingers are configured such that the velocity of a primary mode of surface acoustic waves is lower in the trap regions than in the track region. Acoustic surface waves propagate along the top surface of the substrate. The primary mode is an acoustic surface wave that propagates along the longitudinal direction and exhibits some polarization. The primary mode is, for example, a Rayleigh surface acoustic wave. Preferably, the velocity of the primary mode in the trap regions is at most 99%, 98%, or 97% of the velocity of the primary mode in the track region. Additionally or alternatively, the velocity of the primary mode in the trap regions is at least 93%, 94%, or 95% of the velocity in the track region.

[0012] During normal operation, the electroacoustic resonator generates the main mode, which is the desired acoustic surface wave. The resonator is configured so that, during operation, the main mode is generated and contained within the track area.

[0013] Each electrode comprises a multitude of fingertips. The fingertips are shorter than the fingers; that is, they are shorter than the fingers. The length of a finger / fingertip or finger segment is its transverse dimension. Each electrode can contain at least ten, at least 50, or at least 100 fingertips.

[0014] The stub fingers extend from the same side of the associated busbar as the fingers. This means that the stub fingers and the fingers of one electrode extend from the associated busbar to the busbar of the other electrode. The stub fingers can all have the same shape within the limits of the manufacturing tolerance. Likewise, the fingers that encompass the different sections can all have the same shape within the limits of the manufacturing tolerance.

[0015] Each stub finger is assigned only to the barrier area that is closest to the busbar assigned to it. In particular, the stub fingers do not overlap with the trap areas or the track area. The width of the barrier areas, measured perpendicular to the longitudinal direction, is preferably defined by the length of the assigned stub fingers.

[0016] The electrodes are configured such that the main mode velocity in the barrier regions is higher than in the track region. For example, the main mode velocity in the barrier regions is at least 101%, 102%, 104%, or 106% of the velocity in the track region. Additionally or alternatively, the main mode velocity in the barrier regions is at most 110%, 109%, 107%, or 105% of the velocity in the track region.

[0017] The velocity in the barrier regions is reduced compared to a resonator with a piston-shaped finger design but without the stubby fingers. This reduction in velocity in the barrier regions allows for an increase in velocity in the trap regions, further facilitating the suppression of unwanted modes.

[0018] In particular, the stub fingers and / or the barrier sections of the fingers are thinner and / or narrower than the track sections of the fingers. For example, the height and / or width of the stub fingers and / or the barrier sections is at most 90%, or at most 80%, or at most 70% of the height and / or width of the track sections. Additionally or alternatively, the height and / or width of the stub fingers and / or the barrier sections is at least 20%, or at least 40%, or at least 50% of the height and / or width of the track sections.

[0019] By reducing the width and / or height of the stub fingers and / or the barrier sections of the fingers, the mass load in the barrier areas can be reduced relative to the track area, thereby increasing the speed of the main mode relative to the track area.

[0020] In electroacoustic resonators, several unwanted (interfering) modes are present in addition to the desired main mode. These unwanted modes are modes that do not propagate along the main mode's direction of propagation (longitudinal direction) and can exhibit all possible polarizations, as well as modes with the same direction of propagation as the main mode but a different polarization. Generally, it is not possible to find an electrode structure design that allows the suppression of all unwanted modes. This is especially true for material systems with high electroacoustic coupling and / or a small separation between the main mode and differently polarized unwanted modes. In SAW filters consisting of such resonators, the remaining unwanted modes can lead to dips in the passband and at the filter skirts, group delay waves, trimming problems, and / or reduced power handling.

[0021] To suppress unwanted transverse modes (i.e., modes with non-zero propagation along the transverse direction perpendicular to the longitudinal direction), the electrode fingers can be formed using a so-called piston-mode design, in which the fingers are divided into a barrier section, two trap sections, and a track section. Such designs are known, for example, from US 2013 / 0051588 A1, DE 10 2010 046 087 A1, and US 2016 / 0072475 A1. The trap sections can be designed such that the velocity of the main mode is lower in the associated trap regions than in the track region. This can be achieved, for example, by using metal points in the trap sections to increase their mass compared to the track section.However, this can be problematic for high-frequency resonators, where the center-to-center spacing of adjacent fingers must be small. Manufacturing processes are often not precise enough to place metal dots on very small structures.

[0022] The inventors of the present invention conceived the idea of ​​additionally incorporating stubby fingers into the electrodes. These stubby fingers make it possible to reduce the velocity of the acoustic surface waves propagating along the longitudinal direction in the barrier regions flanking the trap sections. Consequently, the velocity in the trap sections does not need to be as low as it would be without the stubby fingers, potentially eliminating the need for metal points in the trap sections altogether. This approach saves costs and avoids the manufacturing limitations imposed by point alignment accuracy and minimum point size.

[0023] Another advantage of stubby fingers is that they provide an additional degree of freedom when optimizing the resonator for suppressing unwanted modes. By adjusting the width, length, and / or height of the stubby fingers, the velocity of the acoustic surface waves in the barrier regions can be controlled, thus optimizing the suppression of modes with transverse propagation directions. Furthermore, by optimizing the design of the stubby fingers, modes of unwanted polarizations can also be suppressed further.

[0024] According to at least one embodiment, the width and / or height of the fingers in the trap areas is greater than in the track area. Increasing the width and / or height increases the mass load on the fingers in the trap areas compared to the track area, thereby reducing the velocity of the main mode in the trap areas compared to the track area. For example, the width and / or height in the trap areas for each finger is at least 105%, 110%, 130%, 130%, or 150% of the width and / or height in the track area. Additionally or alternatively, the width and / or height for each finger in the trap areas is at most 250%, 200%, or 150% of the width and / or height in the track area.

[0025] According to at least one embodiment, the stubby fingers are shorter than the barrier sections. For example, the length of the stubby fingers is at most 80%, 90%, or 95% of the length of the barrier sections of the fingers. Additionally or alternatively, the length of the stubby fingers is at least 70% or 80% of the length of the barrier sections.

[0026] According to at least one embodiment, the width and / or height of the stub fingers is equal to the width and / or height of the barrier sections of the fingers. Here and in the following, two elements with the same height and / or width and / or length mean that the elements have the same height and / or width and / or length within the limits of the manufacturing tolerance. For example, deviations of a maximum of 10% or a maximum of 5% occur.

[0027] According to at least one embodiment, the height of the fingers in the trap sections is the same as in the track sections. In this case, the width of the fingers in the trap sections is preferably greater than in the track sections. Preferably, the height of the stub fingers and the fingers in the barrier sections is also the same as in the track sections. In this case, the width of the stub fingers and / or the barrier sections is preferably smaller than in the track sections.

[0028] Sufficiently deep trap regions, i.e., trap regions with low main mode velocity, can generally be achieved by using trap sections with increased height, e.g., by using metal dots. However, small feature sizes are required for high-frequency resonators. The application of metal dots can therefore be difficult. In the present invention, however, the trap sections can be chosen to have the same height as the track sections, since the trap depth does not need to be as deep due to the reduced main mode velocity in the barrier regions.

[0029] According to at least one embodiment, the fingers and the stub fingers are arranged alternately in each electrode. In particular, in each electrode, a stub finger is located between each pair of fingers and a finger is located between each pair of stub fingers.

[0030] According to at least one embodiment, each stub finger is located at the same height with respect to the propagation direction of the main mode, i.e., the longitudinal direction, as a finger of the other electrode. This means that a centerline passing through one stub finger, extending along the transverse direction, also passes through a finger of the other electrode. Preferably, the centerline of one stub finger is also the centerline of the finger of the other electrode at the same height. Preferably, the distance of each stub finger to the finger 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 of each stub finger to the finger of the other electrode at the same height is at most 0.5λ, at most 0.4λ, or at most 0.3λ, where λ is the wavelength of the main mode in the track region.The distance between two objects is defined here as the length of the shortest connection between the two objects.

[0031] 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, for example, a mobile phone. The RF filter can be a bandpass filter. A resonant 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.

[0032] Further preferred embodiments and developments of the electroacoustic resonator are described below in conjunction with the figures. Identical or similar elements, as well as elements with the same function, are identified in the figures by the same reference numerals. The figures and the proportions of the elements depicted in the figures are not to be considered to scale. Rather, individual elements, especially layers, may be exaggerated to improve clarity and / or comprehensibility.

[0033] They show: The Fig. 1 to 4 Examples of the electroacoustic resonator in different views and the Fig. 5 and Fig. 6 characteristic properties of the electroacoustic resonator based on graphene.

[0034] Fig. Figure 1 shows a first embodiment of the electroacoustic resonator in a top view. Fig. Figure 2 shows the electroacoustic resonator of Fig. Figure 1 shows a cross-sectional view through the section plane AA'. The electroacoustic resonator comprises a substrate 3 with a piezoelectric material, for example, LiNbO3. The substrate 3 includes a top surface 33 made of the piezoelectric material. For example, the substrate 3 comprises a thin film of the piezoelectric material. The cut angles of the piezoelectric material are, for example, (0°, 38°, 0°). The cut angles (λ', µ, θ) are the Euler angles that define the orientation of a top surface of the piezoelectric material with respect to the crystallographic axes of the piezoelectric material. The definition conforms to the international standard IEC 62276:2016.

[0035] An electrode structure is applied to the top surface 33 of the substrate. The electrode structure is made of a metal, for example, copper. The electrode structure comprises a first electrode 1 and a second electrode 2. Both electrodes 1 and 2 comprise a busbar 20 and a plurality of fingers 10. The busbars 20 both extend along a longitudinal direction L. The fingers 10 extend perpendicular to the busbars 20 along a transverse direction T, which is perpendicular to the longitudinal direction L.

[0036] Between the busbars 20, the upper surface 33 of the substrate 3 is divided into two barrier areas 113, two trap areas 112, and one track area 111. The areas 111, 112, and 113 are all strip-shaped with a main extension along the longitudinal direction L. The areas 111, 112, and 113 are arranged sequentially along the transverse direction T. The track area 111 is located between the two trap areas 112. The two trap areas 112 and the track area 111 are located between the two barrier areas 113. The barrier areas 113 are each adjacent to a busbar 20.

[0037] Each finger 10 comprises a barrier section 13 adjacent to the busbar 20 of the assigned electrode. Moving away from the busbar 20, a first trap section 12, a track section 11, and a second trap section 12 are arranged downstream of the barrier section 13 in this order. The barrier section 13 is assigned to or overlaps with the barrier area 113 adjacent to the assigned busbar 20. The trap sections 12 are assigned to or overlap with the trap areas 112, and the track section 11 is assigned to or overlaps with the track area 111.

[0038] In addition to the fingers 10, each electrode 1, 2 comprises stub fingers 30 extending away from the busbars 20. In each electrode 1, 2, a stub finger 30 is arranged between each pair of fingers 10. The stub fingers 30 are only assigned to, or overlap with, the barrier area 113 adjacent to the associated busbar 20. With respect to the longitudinal direction L, each stub finger 30 lies at the same height as a finger 10 of the other electrode.

[0039] During the operation of the in the Fig. 1 and Fig. In the electroacoustic resonator shown in Figure 2, a main mode of surface waves is generated, propagating longitudinally with a specific polarization. The main mode is essentially confined within the track region 111. The trap sections 12 of the fingers 10 are selected such that the velocity of the main mode is reduced in the trap regions 112 compared to the track region 111. This is achieved by making the trap sections 12 wider than the track sections 11, resulting in an increased mass load in the trap regions 112 compared to the track region 111. However, the height of the fingers 10 in the track sections 12 is the same as in the track section 11 (see Figure 2). Fig. 2), which is advantageous with regard to the production of fingers 10.

[0040] The stub fingers 30 are designed with a reduced width compared to the track sections 11. The barrier sections 13 of the fingers 10 have the same width as the track sections 11. The height of the stub fingers 30 and the barrier sections 13 is the same as in track section 11. Due to the reduced width of the stub fingers 30, the speed of the main mode in the barrier sections 113 is higher than in track section 111, but lower than if no stub fingers 30 were used.

[0041] The resulting velocity profile is shown in the diagram of Fig. Figure 5 shows the velocity profile. The x-axis represents the transverse direction T. The y-axis represents the velocity of the main mode. During operation of the resonator, the velocity profile leads to the one shown in the diagram of Fig. Figure 6 shows the profile of the main mode amplitude. Here, too, the x-axis 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 amplitude of the main mode has an almost rectangular shape with a nearly flat shape in the track region 111 and steep flanks in the trap regions 112. With such a profile for the main mode, unwanted modes propagating in the transverse direction are almost completely suppressed. In the Fig. 5 and Fig. 6 The small gap areas between the track areas 112 and the barrier areas 113, visible in the preceding figures, are not shown.

[0042] Fig. Figure 3 shows a second embodiment of the electroacoustic resonator. The resonator is shown only in cross-sectional view. A top view would be, for example, the same as in Fig. 1. In contrast to the electroacoustic resonator of Fig. 2 has the electroacoustic resonator of Fig. Three fingers 10 with a greater height in the trap sections 12 than in the track sections 11. This helps to further reduce the speed of the main mode in the track sections 112. Instead of a greater width, a greater height 12 can be used. In addition, the height of the fingers 10 in the barrier sections 13 can be less than in the track sections 11. Likewise, the height of the stub fingers 30 can be less than the height of the fingers 10 in the track sections 11.

[0043] Fig. Figure 4 shows a third embodiment of the electroacoustic resonator, again in a top view of the upper surface 33 of the substrate 3. The electroacoustic resonator of Fig. 4 differs from the electroacoustic resonator of Fig. 1 by the fact that the width of the fingers 10 in the barrier sections 13 is now also reduced compared to the width in the track sections 11.

[0044] In this way, the speed for the main mode in barrier regions 113 is increased compared to the case of Fig. 1 increased.

[0045] The electroacoustic resonators of Fig. 3 and Fig. 4 exhibit a similar characteristic velocity profile and a similar amplitude profile of the main mode as in the Fig. 5 and Fig. 6 shown.

[0046] The invention described here is not limited to the exemplary embodiments described. Rather, the invention encompasses every new feature and every combination of features, which in particular includes every combination of features in the claims, even if these features or this combination itself is not explicitly stated in the claims or exemplary embodiments. Reference character list: 1 first electrodes 2 second electrodes 3 Substrat 10 fingers 11 track section 12 Trap section 13 Barrier section 20 busbar 30 stubby fingers 33 Top 111 track area 112 Trap area 113 Barrier area T transverse direction L Longitudinal direction

Claims

[1] An electroacoustic resonator comprising: - a substrate (3) with a piezoelectric material, - an interdigital electrode structure on a top surface (33) of the substrate (3), wherein - the electrode structure comprises a first electrode (1) and a second electrode (2), each with a busbar (20) and a plurality of fingers (10), - the fingers (10) of both electrodes (1, 2) interlock, - the area of ​​the top (33) between the two busbars (20) is divided into two barrier areas (113), two trap areas (112) and a track area (111), wherein the trap areas (112) are arranged between the two barrier areas (113) and the track area (111) is arranged between the two trap areas (112), - at least some fingers (10) each comprise a barrier section (13), two drop sections (12) and a track section (13), wherein the barrier section (13) is assigned to the barrier area (113) that is closest to the busbar (30) assigned to the finger (10), the drop sections (12) are each assigned to one of the drop areas (112) and the track section (11) is assigned to the track area (111), - the fingers (11) are configured such that the speed of a principal mode of acoustic surface waves in the trap areas (112) is smaller than in the track area (111), - each electrode (1, 2) comprises a plurality of stubby fingers (30) that are shorter than the fingers (10), - each stub finger (30) is only assigned to the barrier area (113) that is closest to the busbar (20) assigned to the stub finger (30), - the electrodes (1, 2) are configured such that the velocity of the main mode in the barrier regions (113) is greater than in the track region (111), - wherein the stub fingers (30) and / or the barrier sections (13) of the fingers (10) are thinner and / or narrower than the track sections (11) of the fingers (10). [2] Electroacoustic resonator according to claim 1, wherein the speed of the main mode in the trap regions (112) is at most 99% and at least 93% of the speed of the main mode in the track region (111). [3] Electroacoustic resonator according to claim 1 or 2, wherein the speed of the main mode in the barrier regions (113) is at most 110% and at least 101% of the speed of the main mode in the track region (111). [4] Electroacoustic resonator according to one of the preceding claims, wherein the width and / or height of the fingers (10) in the trap areas (112) is greater than in the track area (111). [5] Electroacoustic resonator according to one of the preceding claims, wherein the stub fingers (30) are shorter than the barrier sections (13) of the fingers (10). [6] Electroacoustic resonator according to one of the preceding claims, wherein the width and / or height of the stub fingers (30) is equal to the width and / or height of the barrier sections (13) of the fingers (10). [7] Electroacoustic resonator according to one of the preceding claims, wherein the height of the fingers (10) in the trap sections (12) is the same as in the track sections (11). [8] Electroacoustic resonator according to one of the preceding claims, wherein in each electrode (1, 2) the fingers (10) and the stub fingers (30) are arranged alternately. [9] Electroacoustic resonator according to one of the preceding claims, wherein each stub finger (30) is at the same height with respect to the propagation direction of the main mode as a finger (10) of the other electrode (1, 2).

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