SAW component

Abstract

SAW component, realized on a layer stack, comprising: - a substrate (SU) - a TCF compensation layer (CL) - a piezoelectric thin film (PL) and - an electrode structure (ES) wherein the piezoelectric thin film has a thickness and a cutting angle that favors excitation and propagation of a Rayleigh wave with wavelength λ as a principal mode, wherein the piezoelectric thin film is a lithium niobate film having a crystal cut with Euler angles of (0° / 125° ± 15° / 0°).

Description

[0001] The development of next-generation mobile communication systems requires filter components with outstanding performance and properties such as a high electromechanical coupling factor k. 2 and low temperature coefficient of frequency TCF.

[0002] SAW filter components in the form of a sandwich substrate system or a layer stack offer high coupling coefficients due to a sagittal waveguiding effect. One of the layers in a common multilayer substrate system is a SiO2 layer, which reduces the TCF (transient coefficient of frequency). Such a SiO2 layer is used as a TCF compensation layer, and its thickness can be adjusted to achieve a desired TCF reduction. However, due to differing impedances of the layers in sandwich systems, unwanted modes, such as bulk acoustic modes, can appear. These are undesirable because they generate unwanted resonances in adjacent frequency bands at higher frequencies.

[0003] A layer stack includes a support substrate such as silicon (Si) or sapphire (Al2O3), dielectric functional layers such as AlN and SiO2, and a piezoelectric material such as lithium tantalate (LiTaO3), onto which, for example, electrodes are applied.

[0004] Surface acoustic wave (SAW) filters in sandwich substrate systems exhibit inherently high electromechanical coupling because their suitably chosen layer sequence creates a sagittal waveguide. Furthermore, functional layers are applied to the electrodes or beneath the piezoelectric material to selectively enhance the component's properties, such as its temperature coefficient of frequency (TCF).

[0005] Some sandwich systems use a piezoelectric thin film made of lithium tantalate (LT). A cutting angle is chosen that primarily excites and propagates a leaking surface wave. This wave has a relatively high acoustic velocity and therefore requires an interdigital transducer (IDT), which has electrode fingers with a relatively large spacing (pitch) to excite a wave of the desired frequency compared to a wave with a lower acoustic velocity.

[0006] As a consequence of this limiting characteristic, the size of corresponding SAW components requires a minimum size to accommodate IDTs with the relatively large spacing. However, due to the ongoing demand for smaller components, a way is needed to further reduce the component size without compromising component performance.

[0007] Another disadvantage of currently used layered substrate systems results from the stacking of layers with different impedances. As a consequence, higher and bulk modes can propagate, which are detrimental in carrier aggregation applications.

[0008] Additionally, components using currently employed layered substrate systems exhibit a significant difference between the TCF of the left and right band boundaries, typically greater than 20 ppm / K. This is disadvantageous because the component's bandwidth changes significantly with increasing temperature, and the component's adaptation to temperature variations is no longer optimal.

[0009] Patent application DE 10 2017 111 448 A1 discloses the use of a specific material for the support substrate of a multilayer SAW device arranged on a support substrate. If a silicon material with a selected range of Euler angles is used as the support substrate material, improved suppression of interference signals is achieved.

[0010] Patent application US 2019 / 0253035 A1 describes an acoustic wave element and a manufacturing process for it. Patent application US 2018 / 0034439 A1 describes an acoustic wave generator.

[0011] Therefore, one goal is to provide an acoustic surface wave component that avoids at least one of the aforementioned disadvantages.

[0012] An acoustic surface wave (ASW) device is provided that utilizes a layered substrate system with a special material and a specific cut of a piezoelectric thin film. The appropriate choice of material and the cut of the piezoelectric thin film result in a low velocity of the excited wave mode, enabling the use of smaller components without compromising other performance parameters as specified.

[0013] The SAW component is realized through a layer stack that forms a sequence from bottom to top of - a substrate - a TCF compensation layer - a piezoelectric thin film and - an electrode structure comprising, wherein the piezoelectric thin film has a thickness and a cutting angle that favors the excitation and propagation of a Rayleigh mode as the main mode, wherein the piezoelectric thin film is a lithium niobate film having a crystal cut with Euler angles of (0° / 125° ± 15° / 0°).

[0014] In one embodiment, the Rayleigh wave is used as the main mode. In leaking surface wave (LSAW) systems, the Rayleigh wave is represented as a disturbance mode occurring below the leaking surface wave. With this new approach, the disturbance mode below the main mode is completely eliminated because, when the Rayleigh mode is used as the main mode, no other mode can propagate below the resonant frequency, as this has the lowest velocity in the system. Other disturbance modes, such as higher-order modes and bulk modes, are shifted to higher frequencies up to a distance above the main mode, where they no longer cause interference.

[0015] A further advantage arises from the lower acoustic velocity, which is reduced by approximately 20% compared to leaking surface waves. This enables the formation of IDTs and other electrode structures with a smaller pitch at the same frequency as the previous LSAW device. This allows for a reduction in the required device area.

[0016] In addition to reducing the propagation speed of the wave, the material and cut of the piezoelectric layer are chosen so that the other performance parameters of the SAW component, such as electromechanical coupling (k2) quality and TCF of the main mode, are not significantly degraded.

[0017] One surprising effect is that the difference in TCF between the left and right band boundaries has been significantly reduced to approximately 5 ppm / K. This results in a more constant bandwidth and a considerably reduced temperature shift across the entire passband.

[0018] A layer stack favoring the Rayleigh mode as the dominant mode can be achieved using a piezoelectric thin film composed of lithium niobate exhibiting a crystal cut with Euler angles of (0° / 125° ± 15° / 0°). This cut angle allows for a high coupling factor k. 2 This achieves a sufficiently wide bandwidth for the SAW component. The leaking surface wave is no longer excited (or only to a negligible extent) or cannot be found in the transmission curve.

[0019] The piezoelectric thin film can have a thickness x with 0.1 λ < x < 0.6 λ, where λ is the wavelength of the acoustic principal mode within the piezoelectric layer of a respective layer stack.

[0020] A suitable TCF compensation layer can be a SiO2 layer with a thickness y of 0.05 λ < y < 0.5 λ. Alternatively, the TCF compensation layer comprises doped SiO2 or GeO2.

[0021] In such a stack, the substrate comprises a high-resistance crystalline material. High resistivity is defined as a resistivity of at least 1000 Ωcm.

[0022] According to one embodiment, this substrate material is a high-resistance silicon with Euler angles of (-45°±10°, -54°±10°, 60°±20°) or (0°±10°, 0°±10°, 45°±20°). Alternatively, the substrate comprises quartz, sapphire, glass, spinel, and SiC.

[0023] A polycrystalline silicon layer with a thickness z, where 0.05 λ < z < 1.0 λ, can be placed between the substrate and the TCF compensation layer. This layer exhibits a relatively high acoustic velocity and improves the waveguiding of the SAW, while also reducing electrical losses by localizing charge carriers within it. Alternatively, the high acoustic velocity layer can be composed of AlN, Si3N4, diamond, or SiC.

[0024] Due to the good wave guidance of the layered system, leakage wave losses of the main mode can be prevented. This leaking wave is a shear wave with a relatively high propagation speed.

[0025] In an alternative embodiment, there is no additional layer between the substrate and the TCF compensation layer. Instead, the silicon substrate has an ion implant in an upper surface region, or it is an amorphous layer or a dielectric layer.

[0026] The electrode structure can have a layered structure with Al as a major component of the layered electrode structure. In this embodiment, the electrode structure has a preferred thickness z where 0.05 λ < z < 0.25 λ.

[0027] In alternative embodiments, the electrode structure can be a "heavy electrode" to further reduce the SAW speed. Useful examples include tungsten, molybdenum, copper-based electrode systems, titanium, and platinum.

[0028] The SAW component can be further enhanced by applying one or more dielectric passivation layers to the top surface of the electrode structure. The passivation layer can have a thickness w of 0.0025 λ < w < 0.2 λ. The dielectric material can be selected from Si3N4, SiO2, SiON, and Al2O3.

[0029] This will be explained in more detail below using specific embodiments and the associated illustrations. The illustrations are only schematic and may not show all elements, provided these omitted elements are known in the prior art and can be easily added by a skilled worker. Furthermore, the figures are not drawn to scale, and some details may be enlarged for clarity.

[0030] Fig. shows a layer stack according to one embodiment.

[0031] Fig. shows a conductor type structure of SAW resonators as an example of a filter circuit realized by the electrode structure.

[0032] Fig. shows an admittance of a one-port resonator of a currently typically used layer system compared to the admittance of a one-port resonator according to embodiments of the disclosure.

[0033] Fig. Figure 1 shows a layer stack according to an embodiment that implements a SAW component. The stack comprises a substrate SU, a TCF compensation layer CL, a piezoelectric thin film PL, and an electrode structure EL on a top surface. An optional additional layer AL can be arranged between the substrate and the TCF compensation layer. The electrode structure can implement a filter circuit, e.g., a conductor-type arrangement of resonators forming a bandpass filter or a bandstop filter (notch filter).

[0034] Fig. This is a schematic block diagram of a ladder-type arrangement of resonators as one embodiment of the SAW component. Acoustic SAW resonators are typically single-port resonators.

[0035] The ladder-type structure comprises a series of basic sections BS. Each basic section includes at least one series resonator R. S and also a parallel resonator R P Such basic sections BS can be connected in series in the number necessary to achieve a desired selectivity. Series resonators R S , belonging to adjacent basic sections, can form a common series resonator R S can be combined as well as parallel resonators R P They can also be combined if they are directly adjacent and belong to different basic sections BS.

[0036] Besides the example shown, just one basic section (BS) already constitutes a basic filter. Two, three, or more basic sections can provide sufficient selectivity.

[0037] For use with specific frequency bands, the frequency must be adjusted via the distance (pitch) of the electrode structure according to the formula f=v / A, where f represents the desired frequency of the final SAW component, v is the propagation speed of the acoustic wave, and A is 2 times the distance (pitch), which gives the wavelength λ that can be adjusted via the distance (pitch) of the IDT formed by the electrode structure.

[0038] By using the Rayleigh mode as the main mode of wave propagation, the speed of the acoustic wave can be reduced by about 20% from 3800m / s (LSAW) to 3100m / s (Rayleigh wave in a stack according to the embodiments described herein).

[0039] The Rayleigh mode can be set as the dominant wave mode by selecting the appropriate material, thickness, and crystal cut of the piezoelectric layer. Furthermore, the thickness and material of other layers within the overall stack can also be selected to support the desired wave mode.

[0040] As a result of using a Rayleigh wave, the spacing (pitch) of the electrode structure of the SAW component can also be reduced by 20% to achieve the same frequency response as the single-port resonator. Accordingly, the final SAW component, formed by connecting multiple single-port resonators, can be significantly reduced in size.

[0041] Fig. The figure shows the real part (upper diagram) and the absolute value (lower diagram) of the admittance of a single-port resonator implemented on an LSAW layer stack (curve 1 / black line) compared to that of a single-port resonator according to an embodiment (curve 2, red line). Both resonators are designed such that the major mode is at the same frequency.

[0042] Admittance curve 1, according to the stack using LSAW, shows interfering resonances of noise modes on both sides of the main mode. Below the main resonance at 1800 MHz, a small peak at about 1400 MHz results from a noise Rayleigh wave mode (see arrow SM in the upper diagram). Above the resonance frequency, higher modes and bulk modes begin to appear at a frequency of about 2300 MHz.

[0043] In contrast, curve 2, which corresponds to a SAW resonator formed on a layer stack of various configurations, no longer shows the interfering modes. By using the Rayleigh mode as the main mode, no other mode below the resonance frequency can propagate, since this mode (Rayleigh wave) has the lowest velocity in the system. Therefore, the peak of SM from curve 1 (see upper diagram) has disappeared. The frequency of the higher-order modes and bulk modes (compare peak of SM from curve 1 in the lower diagram) has also disappeared. Interfering modes do not occur below 2800 MHz, which is significantly further from the main resonance than in the resonator formed from the known layer stack. Therefore, the new SAW device is useful for forming filter components for carrier aggregation.

[0044] Another advantage of the proposed layer stack is the difference in TCF between the left and right band boundaries of a SAW filter component fabricated from this new stack. This difference is typically greater than 20 ppm / K in currently used systems and negatively impacts the bandwidth and matching of the component, as the band boundaries change significantly with temperature. This effect results in a bandwidth of a given SAW component that decreases with increasing temperature.

[0045] The proposed system reduces the difference in TCF to a value of less than 5 ppm / K. Furthermore, the bandwidth now remains more constant than that of a SAW component in a currently used stack.

[0046] Referring to the schematic layer sequence in Fig. The individual layers can be selected according to the following selection.

[0047] Substrate SU: Possible materials can be selected from - high-resistance Si with Euler angles (-45°±10°, -54°±10°, 60°±20°) - high-resistance Si with Euler angles (0°±10°, 0°±10°, 45°±20°) - one of quartz, glass, spinel and SiC

[0048] Additional AL layer (optional). Possible materials can be selected from: - Polycrystalline Si having a layer thickness x with 0.05 λ < x < 1 λ - AlN, Si3N4, diamond, SiC, which has a layer thickness x with 0 < x < 1 λ - not a layer, but a substrate that has an ion-implanted surface layer, an amorphous layer, or a dielectric layer on top of it

[0049] TCF compensation layer CL: Possible material - SiO2, which has a layer thickness of 0.05 λ < y < 0.5 λ - 0.5 λ of doped SiO2, GeO2

[0050] Piezoelectric thin film PL: Possible material - LiNbO3, which has a layer thickness x with 0.1 λ < x < 0.6 λ and Euler angle (0° / 125°± 15° / 0°) Electrode structure

[0051] Possible materials can be selected from - a layer sequence comprising Al as the main component, which has a layer thickness x with 0.05 λ < z < 0.25 λ - a copper-based electrode system with one or more layers - a single “heavy layer” selected from W, Mo, Ti, Ag, Au, Ta and Pt

[0052] Passivation layer (optional) - One or more layers having a thickness w of 0.0025 λ < w < 0.2 λ - Possible materials can be selected from: Si3N4, SiO2, SiON and Al2O3

[0053] In the following embodiment, the layers of a layer stack are specified in more detail. A single-port resonator is implemented on this layer stack. Based on the structure of this resonator, the admittance curve 2 was derived from Fig. simulated. The component is designed for a resonant frequency of 1800 MHz.

[0054] Substrate SU: high-resistance Si with Euler angles (-45°±10°, -54°±10°, 60°±20°) or (0°±10°, 0°±10°, 45°±20°).

[0055] Additional layer AL: Polycrystalline Si with a layer thickness x = 500nm; SiO2 with a thickness y = 550nm

[0056] Piezoelectric thin film PL: LiNbO3, which has a layer thickness x = 550 nm and Euler angle (0° / 125°± 15° / 0°).

[0057] Electrode structure EL: A layer sequence comprising Al as a main component, which has a layer thickness x = 150 nm.

[0058] Passivation layer PL: Si3N4, which has a thickness w = 10nm.

Claims

[1] SAW component, realized on a layer stack, comprising: - a substrate (SU) - a TCF compensation layer (CL) - a piezoelectric thin film (PL) and - an electrode structure (ES) wherein the piezoelectric thin film has a thickness and a cutting angle that favors excitation and propagation of a Rayleigh wave with wavelength λ as a principal mode, wherein the piezoelectric thin film is a lithium niobate film having a crystal cut with Euler angles of (0° / 125° ± 15° / 0°). [2] SAW component according to one of the preceding claims, wherein the piezoelectric thin film has a thickness x with 0.1 λ < x < 0.6 λ. [3] SAW component according to one of the preceding claims, wherein the TCF compensation layer is a SiO2 layer having a thickness y of 0.05 λ < y < 0.5 λ, wherein the TCF compensation layer comprises a layer of doped SiO2 or GeO2. [4] SAW component according to one of the preceding claims, wherein the substrate comprises a high-resistance crystalline material. [5] SAW component according to the preceding claim, wherein the substrate comprises a high-resistance silicon with Euler angles of (-45°±10°, -54°±10°, 60°±20°) or (0°±10°, 0°±10°, 45°±20°), or wherein the substrate comprises one of quartz, sapphire, glass, spinel and SiC. [6] SAW component according to one of the preceding claims, wherein a polycrystalline silicon is arranged as an additional layer (AL) between the substrate and the TCF compensation layer, having a thickness z, wherein 0.05 λ < z < 1.0 λ, the additional layer comprising one of AlN, Si3N4, diamond or SiC. [7] SAW component according to any of the preceding claims, wherein the electrode structure has a layered structure, wherein Al is the main component of the layered electrode structure, wherein the electrode structure has a thickness z with 0.05 λ < z < 0.25 λ, wherein the electrode comprises a copper-based electrode system with one or more layers, a single layer selected from W, Mo or Pt, Ta, Ag, Au or Ti. [8] SAW component according to one of the preceding claims, wherein a dielectric passivation layer is arranged on the electrode structure, wherein the passivation layer has a thickness w of 0.0025 λ < w < 0.2 λ, wherein the material of the dielectric passivation layer is selected from Si3N4, SiO2, SiON and Al2O3•

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