SAW resonator with reduced spurious modes, electroacoustic filter and multiplexer

Abstract

SAW resonator with reduced spurious modes, comprising: - a carrier substrate, - an electrode structure above the carrier substrate, - a piezoelectric layer between the carrier substrate and the electrode structure, wherein - the carrier substrate has a crystal orientation with the Euler angles (-45°±10°; -54°±10°; 60°±30°), - the piezoelectric layer comprises LiTaO3 and has a crystal orientation with the Euler angles (0°; 56°±8°; 0°).

Description

[0001] The present application relates to electroacoustic resonators that can be used in carrier aggregation (CA) applications and that have reduced spurious modes.

[0002] The ongoing trend toward increased functionality, higher data rates, and smaller physical dimensions requires improved mobile communication devices and improved components for such devices. The development of next-generation mobile communication systems requires devices with outstanding performance.

[0003] In electroacoustic resonators, an electrode structure combined with a piezoelectric material converts between electromagnetic and acoustic RF signals due to the piezoelectric effect. However, in real devices, unwanted spurious modes can be excited in addition to desired acoustic modes. These unwanted acoustic spurious modes degrade the performance of the corresponding filter, making it difficult or impossible for conventional resonators to meet current or future specifications.

[0004] Layered constructions for electroacoustic resonators are known from US 9,190,981 B2 and US 9,413,334 B2.

[0005] From US 2015 / 0102705 A1, electroacoustic resonators are known that comprise dielectric functional layers.

[0006] From DE 102017111448 A1 the use of a silicon material as a carrier substrate is known.

[0007] From WO 2018 163842 A1, an acoustic wave device is known comprising a material layer having Euler angles and an elastic constant at the Euler angles, a piezoelectric body having a first and a second main surface opposite each other, which is directly or indirectly laminated to the material layer and has Euler angles, and an IDT electrode on at least one of the first and second main surfaces of the piezoelectric body, and in which a wavelength determined by an electrode finger pitch is λ.

[0008] However, conventional electroacoustic resonators, e.g., SAW (surface acoustic wave) resonators, include additional functional layers, e.g., for temperature compensation, passivation, and the like, which facilitate the generation of unwanted spurious modes.

[0009] Accordingly, an electroacoustic resonator is desired that enables RF filters, e.g., for mobile communication applications, that have outstanding performance, have a well-matched (not too high and not too low) electroacoustic coupling coefficient κ2, have a low temperature coefficient of frequency (TCF), are compatible with carrier aggregation applications in which other performance parameters are not degraded, meet strict specifications, can be used with different frequency ranges, and have a reduced strength of unwanted spurious modes, such as higher-order or bulk modes.

[0010] For this purpose, a SAW resonator is provided according to independent claim 1. Dependent claims provide preferred embodiments.

[0011] The SAW resonator comprises a support substrate, an electrode structure above the support substrate, and a piezoelectric layer between the support substrate and the electrode structure. The support substrate has a crystal orientation with Euler angles (-45°±10°; -54°±10°; 60°±30°). The piezoelectric layer comprises lithium tantalate (LiTaO3) and has a crystal orientation with Euler angles (0°; 56°±8°; 0°).

[0012] In SAW resonators, a desired acoustic mode is excited by the electrode structure in combination with the piezoelectric material, utilizing the piezoelectric effect. The electrode structure typically comprises interdigitated comb-like electrodes, each with two busbars and electrode fingers electrically connected to one of the two busbars. The desired acoustic mode typically propagates in the longitudinal direction x, a direction perpendicular to the extension of the electrode fingers (which extend along the transverse direction y), at the surface of the piezoelectric material.

[0013] Additional acoustic modes (spurious modes) can be excited due to, for example, nonlinear effects of the piezoelectric material or reflection effects within the corresponding waveguiding structure for the desired acoustic mode. In particular, resonator components that may be necessary to meet specific requirements, e.g., TCF layers and the like, can provide another source of excitation for unwanted modes. Accordingly, spurious modes in conventional resonators must be accepted as unavoidable side effects.

[0014] The separation of a spurious mode from a desired acoustic mode by the SAW resonator, as described above, removes the spurious mode at least partially, so that harmful effects are reduced and the performance of the resonator and the corresponding filter is improved.

[0015] Spurious modes can occur at frequency ranges sufficiently far removed from the resonator's operating frequencies. However, when carrier aggregation systems are involved, such spurious modes can then disrupt the operation of another frequency band. Although such spurious modes may have been tolerated in systems without carrier aggregation, such modes can no longer be accepted in new systems that support carrier aggregation, and the separation described above allows the present resonators to be used in carrier aggregation systems.

[0016] The decoupling of wanted from unwanted modes keeps the wanted acoustic energy in the acoustic track, while the (reduced) energy of unwanted acoustic modes can dissipate into the bulk material.

[0017] Furthermore, the materials and the material system and the layer arrangement can be selected such that the excitation of desired modes is enhanced while the excitation of unwanted modes is reduced.

[0018] The material parameters of the corresponding layered resonator structure are chosen to achieve the above effects. For example, the effects described above can be achieved by choosing the stiffness constants, the lattice constants, and the lattice orientation of the layers.

[0019] By providing the Euler angles, the orientation of the crystallographic axes relative to the top surface of the support substrate is also clearly defined. In addition, the propagation direction of the acoustic wave mode relative to the crystallographic axes is clearly defined by the Euler angles.

[0020] In this case, the Euler angles (A', µ, θ) are defined as follows: First, a set of axes x, y, z is taken as a basis, which are the crystallographic axes of the piezoelectric material.

[0021] The first angle, λ', specifies the amount by which the x-axis and y-axis are rotated around the z-axis, with the x-axis rotating in the direction of the y-axis. A new set of axes x', y', and z' is created accordingly, where z = z'.

[0022] In a further rotation, the z'-axis and the y'-axis are rotated around the x'-axis by an angle µ. In this case, the y'-axis is rotated in the direction of the z'-axis. A new set of axes x'', y'', z'' is created accordingly, where x' = x''.

[0023] In a third rotation, the x''-axis and the y''-axis are rotated around the z''-axis by the angle θ. In this case, the x''-axis is rotated in the direction of the y''-axis. A third set of axes x''', y''', z''' is created accordingly, where z'' = z'''.

[0024] In this case, the x'''-axis and the y'''-axis are parallel to the substrate surface. The z'''-axis is the normal to the substrate surface. The x'''-axis specifies the propagation direction of the acoustic waves.

[0025] The definition is consistent with the international standard IEC 62276, 2005-05, Annex A1.

[0026] In particular, it is possible that the carrier substrate has the Euler angles (-45°±5°; -54°±5°; 60°±10°) or (-45°±2°; -54°±5°; 60°±5°) or (-45°; -54°; 60°).

[0027] In addition, it is particularly possible that the piezoelectric layer has a crystal orientation with the Euler angles (0°; 56°±4°; 0°), (0°; 56°±2°; 0°) or (0°; 56°; 0°).

[0028] It is possible for the SAW resonator to additionally comprise a first intermediate layer. The first intermediate layer can be arranged between the carrier substrate and the piezoelectric layer. The first intermediate layer can have a thickness t IL1 where 0.05 λ ≤ t IL1 ≤ λ holds. In the context of the present SAW resonator, λ is the wavelength of the main acoustic mode propagating at the surface of the piezoelectric material along the longitudinal direction x.

[0029] It is possible that the first intermediate layer comprises or consists of a material selected from polycrystalline silicon (Si), an amorphous material, a dielectric material.

[0030] Furthermore, it is possible for the SAW resonator to comprise a second intermediate layer. The second intermediate layer can be arranged between the carrier substrate and the piezoelectric layer. The second intermediate layer can have a thickness t IL2 where 0.05 λ ≤ t IL2 ≤ 0.25 λ applies.

[0031] It is possible that the second intermediate layer comprises or consists of a material selected from silicon oxide and silicon dioxide (SiO2).

[0032] It is possible that the SAW resonator includes the first intermediate layer but not the second intermediate layer.

[0033] However, it is also possible that the SAW resonator includes the second intermediate layer but not the first intermediate layer.

[0034] It is also possible that the SAW resonator comprises the first intermediate layer and the second intermediate layer.

[0035] If the SAW resonator comprises both the first and second intermediate layers, the first intermediate layer may be disposed between the second intermediate layer and the piezoelectric layer. However, it may be preferable for the first intermediate layer to be disposed between the support substrate and the second intermediate layer.

[0036] The first, the second and the combination of both intermediate layers can provide the above-mentioned waveguiding structure, which helps to prevent unwanted modes from being excited, to suppress unwanted modes and / or to separate unwanted modes from the desired main mode.

[0037] It is possible that the piezoelectric layer has a thickness t PIL where 0.1 λ ≤ t PIL ≤ 0.3 applies.

[0038] It is possible that the electrode structure comprises or consists of aluminum (Al) and has a thickness t EL where 0.05 λ ≤ t EL≤ 0.2 λ applies.

[0039] It is possible for the SAW resonator to further comprise a passivation layer. The passivation layer may be arranged on or above the electrode structure. The passivation layer may comprise silicon nitride (Si3N4) and have a thickness t PAL where 0.0025 λ ≤ t PAL ≤ 0.05 λ applies.

[0040] It is also possible for the SAW resonator to be a resonator in an electroacoustic filter. Accordingly, an electroacoustic filter may comprise a SAW resonator as described above.

[0041] In particular, it is possible that two or more or each SAW resonator of the filter is of a type as described above.

[0042] The filter may have a branch-type circuit topology with series resonators electrically connected in series in a signal path and with parallel resonators arranged in parallel paths coupling the signal path to a ground potential.

[0043] Furthermore, it is possible for the filter to have a cross-element type circuit topology with at least one resonator that electrically couples a first connection of a first port to a second connection of a second port.

[0044] Such filters meet the CA requirements because even unwanted modes that would not be harmful in filters for non-CA applications because their characteristic frequencies are sufficiently far from characteristic frequencies of the filter can be suppressed or eliminated.

[0045] Accordingly, it is possible for such a filter to be arranged in a multiplexer, e.g., for CA applications. Accordingly, a multiplexer, e.g., for CA applications, may comprise a filter as described above.

[0046] In particular, it is possible that two or more or each filter is of a type as described above.

[0047] The special properties of the resonator as described above exhibit an intrinsic compensation of negative effects of conventional means for suppressing unwanted modes, so that the need for additional circuit components, e.g. for impedance matching, is significantly reduced.

[0048] In particular, an excessive increase in the electroacoustic coupling coefficient is prevented.

[0049] In particular, this positive effect can be achieved through the special angles, materials and layer thicknesses.

[0050] The reduced need for additional matching elements is also reflected in an improved (i.e. increased) slope of corresponding bandpass filters or bandstop filters.

[0051] Central aspects of the SAW resonator and details of preferred embodiments are shown in the accompanying schematic figures.

[0052] In the figures: Fig. 1 shows a cross-section through a possible layered construction; Fig. 2 shows a layered construction comprising a first separation layer; Fig. 3 shows a layered construction with a first and a second separation layer; Fig. Figure 4 shows, in plan view, a basic layout of the electrode structure; Fig. 5 illustrates possible circuit topologies of a duplexer; Fig. 6 gives the definition of Euler angles; Fig. Figure 7 shows the real paths of admittance curves for different Euler angles of the carrier substrate (frequency dependent); Fig. 8 shows the corresponding amount values; Fig. 9 shows the dependence of the Euler angle µ on the electroacoustic coupling coefficient ĸ 2 for LiTaO3; Fig. Figure 10 shows the dependence of the Euler angle µ on the temperature-induced frequency drift ΔTCF for LiTaO3; Fig. Figure 11 shows the real parts of the frequency-dependent admittances of a conventional resonator and a resonator as described above. Fig. Figure 12 shows the magnitudes of the frequency-dependent admittances of the conventional resonator and the resonator as described.

[0053] Fig. Figure 1 illustrates a cross-sectional view through the x'''-z' plane of a possible layered construction of the SAW resonator SAWR. The layered construction comprises a carrier substrate CS, on which further layer elements are arranged. In particular, the electrode structure ES is arranged above the carrier substrate CS. The piezoelectric layer PIL, which comprises or consists of a piezoelectric material, is arranged between the carrier substrate CS and the electrode structure ES. The specially selected Euler angles in combination with the two materials create an interface that can act as a wave mode separator. The layered construction has its layers arranged on top of one another in the vertical (z') direction. The electrode structure ES has electrode fingers whose cross-section is in Fig. 1. The extension of the electrode fingers is orthogonal to the x'''-z'-plane, which Fig. 1 shown cross-section, and along the transverse direction y'''.

[0054] Furthermore, Fig. 2 a possible layer construction including the second intermediate layer IL2.

[0055] Fig. Figure 3 illustrates the possibility of arranging both the first IL1 and the second IL2 intermediate layers. The first intermediate layer IL1 can be arranged between the second intermediate layer IL2 and the carrier substrate. However, it is also possible for the order of the first and second intermediate layers to be reversed.

[0056] Fig. Figure 4 illustrates a basic configuration of a SAW resonator in a plan view. The surface of the SAW resonator is parallel to the x'''-y''' plane. The propagation direction of the acoustic waves is parallel to the longitudinal (x''') direction. The electrode fingers EFI extend along the y''' direction. The busbars BB extend along the longitudinal direction x'''. The electrode fingers EFI are electrically connected to one of the two busbars BB and form finger-like interdigitated structures IDS. The finger-like interdigitated structures IDS form the electrode structure ES and are arranged between acoustic reflectors R to confine acoustic energy to the acoustic track. The electrode structure ES is arranged together with the reflectors R on the piezoelectric material PM.

[0057] Fig. Figure 5 illustrates a possible circuit topology of a duplexer as an example of a multiplexer. The duplexer includes a transmit filter TXF and a receive filter RXF. Each of the two filters has electroacoustic resonators, e.g., SAW resonators. The resonators can be series resonators SR electrically connected in series in a signal path. Parallel resonators PR electrically connect the signal path to ground. A common port CP can be electrically connected to an antenna AN. An impedance matching circuit IMC can be provided between the transmit filter TXF and the receive filter RXF to match the input and output impedances of the filters according to the corresponding frequencies.

[0058] Fig. Figure 6 illustrates the definition of Euler angles. The resulting axes x''', y''', and z''' correspond to the axes labeled x, y, and z in the figures above.

[0059] Fig. Figure 7 illustrates a comparison of the real paths of admittance curves of two resonators. Curve 1 corresponds to a resonator in which the silicon substrate has Euler angles (0°, 0°, 0°). Curve 2 corresponds to a resonator in which the carrier substrate has Euler angles (0°, 0°, 45°). It can be clearly seen that the orientation of the crystallographic axes of the carrier substrate significantly determines the performance of the resonator.

[0060] Accordingly, Fig. 8 the frequency-dependent amounts of the admittance curves of the resonators, the Fig. 7 correspond.

[0061] Fig. 9 shows the dependence of the Euler angle µ on the electroacoustic coupling coefficient ĸ 2In particular, the coefficient increases as the Euler angle µ decreases. The reduced angle µ—compared to conventional resonators—would lead to an undesirably high coupling coefficient. In conventional resonators, the excessively high coupling coefficient would be compensated for by additional circuit components, e.g., for impedance matching. However, with the material and layer composition described above, an intrinsic compensation of the excessively high coupling coefficient can be achieved, reducing or even eliminating the need for additional circuit components.

[0062] Likewise, Fig. Figure 10 shows the dependence of the Euler angle µ on the temperature-induced frequency drift ΔTCF. ΔTCF represents the difference in the TCF between the resonant and antiresonant frequencies of a resonator. This corresponds to the difference in the TCF between the left and right edges of a filter.

[0063] Fig. Figure 11 shows the real parts of the frequency-dependent admittances of a conventional resonator (curve 1), which shows several peaks associated with unwanted modes, and of a resonator as described above (curve 1) with a much smoother curve due to the reduction of unwanted modes.

[0064] Likewise, Fig. 12 the magnitudes of the frequency-dependent admittances of the conventional resonator (curve 1) and the resonator as described above (curve 1) with a much smoother course.

[0065] The resonator is not limited to the details and configurations shown above. Additional elements, such as TCF layers, passivation layers, waveguiding elements, and similar elements, can be present. Despite the possibility of multiple additional layers—which would lead to potential sources of unwanted spurious modes—spurious modes are reduced and performance is improved. List of reference symbols AN antenna BB busbar CP shared port CS carrier substrate EFI electrode finger ES electrode structure IDS finger-like interlocking structure IMC impedance matching circuit PAL passivation layer PIL piezoelectric layer PM piezoelectric material PR parallel resonator R acoustic reflector RXF receive filter SAWR SAW resonator IL1, IL2 first, second intermediate layer SR series resonator TXF transmission filter

Claims

[1] SAW reduced noise mode resonator comprising the following: - a carrier substrate, - an electrode structure above the support substrate, - a piezoelectric layer between the support substrate and the electrode structure, wherein - the support substrate has a crystal orientation with the Euler angles (-45°±10°; -54°±10°; 60°±30°), - the piezoelectric layer comprises LiTaO3 and has a crystal orientation with the Euler angles (0°; 56°±8°; 0°). [2] SAW resonator according to the preceding claim, wherein - the support substrate has a crystal orientation with the Euler angles (-45°±5°; -54°±5°; 60°±10°) or (-45°±2°; -54°±5°; 60°±5°) or (-45°; -54°; 60°) and / or - the piezoelectric layer has a crystal orientation with the Euler angles (0°; 56°±4°; 0°), (0°; 56°±2°; 0°) or (0°; 56°; 0°). [3] SAW resonator according to any of the preceding claims comprising a first intermediate layer - is located between the substrate and the piezoelectric layer, - a thickness t IL1 exhibits, where 0.05 λ ≤ t IL1 ≤ λ applies and where λ is the wavelength of the acoustic principal mode. [4] SAW resonator according to the preceding claim, wherein the first intermediate layer comprises or consists of a material selected from polycrystalline Si, an amorphous material, or a dielectric material. [5] SAW resonator according to any of the preceding claims, comprising a second intermediate layer - is located between the substrate and the piezoelectric layer, - a thickness t IL2 exhibits, where 0.05 λ ≤ t IL2 ≤ 0.25 λ applies and where λ is the wavelength of the acoustic principal mode. [6] SAW resonator according to the preceding claim, wherein the second intermediate layer comprises or consists of a material selected from silicon dioxide and SiO2. [7] SAW resonator according to one of the preceding claims, wherein the piezoelectric layer has a thickness t PIL exhibits, where 0.1 λ ≤ t PIL ≤ 0.3 λ applies and where λ is the wavelength of the acoustic principal mode. [8] SAW resonator according to any of the preceding claims, wherein the electrode structure - Al includes or consists of - a thickness t EL exhibits, where 0.05 λ ≤ t EL ≤ 0.2 λ applies and where λ is the wavelength of the acoustic principal mode. [9] SAW resonator according to any of the preceding claims, further comprising a passivation layer which - is located on or above the electrode structure and comprises Si3N4, and - a thickness t PAL exhibits, where 0.0025 λ ≤ tPAL ≤ 0.05 λ applies and where λ is the wavelength of the acoustic principal mode. [10] Electroacoustic filter comprising a SAW resonator according to any of the preceding claims. [11] Multiplexer for CA applications comprising the filter according to the preceding claim.

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