THIN-FILM SAW DEVICE WITH MULTILAYER WAVEGUIDE

Abstract

In at least one embodiment, the SAW device comprises a carrier substrate (1), a piezoelectric thin-film (2) on the carrier substrate, an interdigital electrode structure (3) on the piezoelectric thin-film and a layer stack (4) of waveguide layers. The layer stack is arranged between the carrier substrate and the piezoelectric thin-film. The layer stack comprises a first waveguide layer (41) and second waveguide layer (42), wherein a sound velocity in the first waveguide layer is at least 1.5 times as great as in the second waveguide layer. The device may comprise a temperature compensating layer (5) and a trap rich layer (6) between the layer stack and the carrier substrate.

Claims

1. A SAW device comprising: a carrier substrate, a piezoelectric thin-film on the carrier substrate, an interdigital electrode structure on the piezoelectric thin-film, and a layer stack of waveguide layers, said layer stack being arranged between the carrier substrate and the piezoelectric thin-film, wherein the layer stack comprises a first waveguide layer and a second waveguide layer, a sound velocity in the first waveguide layer is at least 1.5 times as great as a sound velocity in the second waveguide layer.

2. The SAW device according to claim 1, wherein the layer stack comprises several of the first waveguide layers and several of the second waveguide layers, wherein the first waveguide layers and the second waveguide layers are arranged alternately.

3. The SAW device according to claim 1, wherein: the sound velocity in the first waveguide layer is greater than a sound velocity in the piezoelectric thin-film, and the sound velocity in the second waveguide layer is smaller than the sound velocity in the piezoelectric thin-film.

4. The SAW device according to claim 1, wherein a mean thickness of the first and/or second waveguide layer is at most λ/4, wherein λ is the wavelength of a main sound wave during operation of the SAW device.

5. The SAW device according to claim 4, wherein a mean thickness of the piezoelectric thin-film is at most 0.6.Math.λ.

6. The SAW device according to claim 1, wherein: the first waveguide layer comprises or consists of one or more of the following materials: AlN, SiC, Al.sub.2O.sub.3, diamond like carbon, TiN, and the second waveguide layer comprises or consist of one or more of the following materials: SiO.sub.2, Si.sub.3N.sub.4, doped SiO.sub.2, GeO.sub.2.

7. The SAW device according to claim 1, wherein a TCF compensating layer having a positive temperature coefficient of frequency is arranged between the layer stack and the carrier substrate.

8. The SAW device according to claim 1, wherein a dielectric ion blocking layer is arranged between the layer stack and the carrier substrate.

9. The SAW device according to claim 1, wherein the SAW device comprises a SAW resonator, the interdigital electrode structure on the piezoelectric thin-film forming an interdigital transducer of the SAW resonator.

10. The SAW device according to claim 9, wherein the SAW resonator has a resonant frequency of at least 2.5 GHz.

Description

[0039] Hereinafter, a SAW device described herein will be explained in more detail with reference to drawings on the basis of exemplary embodiments. Same reference signs indicate same elements in the individual figures. However, the size ratios involved are not to scale, individual elements may rather be illustrated with an exaggerated size for a better understanding.

[0040] FIG. 1 shows an exemplary embodiment of a SAW device in cross-sectional view.

[0041] FIGS. 2 and 3 show simulations of the admittance of an exemplary embodiment of a SAW resonator and of a SAW resonator without a layer stack of waveguide layers.

[0042] FIG. 1 shows an exemplary embodiment of the SAW device in a cross-sectional view. The SAW device comprises a carrier substrate 1, on which the piezoelectric thin-film 2 is applied. On top of the piezoelectric thin-film 1, an interdigital electrode structure 3 comprising two electrodes 31, 32, each with a plurality of interdigitating fingers, is applied. Between the piezoelectric thin-film 2 and the carrier substrate 1, a layer stack 4 of waveguide layers is located. The layer stack 4 consists of four waveguide layers 41, 42. Two of the waveguide layers are first waveguide layers 41 and the other two waveguide layers are second waveguide layers 42. The first 41 and second 42 waveguide layers are stacked in an alternating fashion. The first waveguide layers 41 are identical within the manufacturing tolerance. Likewise, the second waveguide layers 42 are identical within the manufacturing tolerance.

[0043] Between the layer stack 4 and the carrier substrate 1, a TCF compensating layer 5 with a positive temperature coefficient of frequency is arranged. Moreover, a dielectric ion blocking layer 6 is arranged between the TCF compensating layer 5 and the carrier substrate 1.

[0044] The SAW device of FIG. 1 may be a SAW resonator. Reflectors of the SAW resonator are not shown. A sound velocity in the first waveguide layers 41 is at least 1.5 times as great as a sound velocity in the second waveguide layers 42. For example, the sound velocities are each measured for sound waves propagating in the same direction as the main sound wave of the SAW resonator, which, in FIG. 1, can be a direction from the left to the right or vice versa.

[0045] In the exemplary embodiment of FIG. 1, the carrier substrate 1 is a Si substrate with cut angles of (0°, 0°, 45°) and having a thickness of at least 20 μm. The ion blocking layer 6 is made of polycrystalline Si with a mean thickness of 250 nm. The TCF compensating layer 5 is a SiO.sub.2 layer with a mean thickness of 200 nm. The first waveguide layers 41 are AlN layers each with a mean thickness of 190 nm. The second waveguide layers 42 are SiO.sub.2 layers each with a mean thickness of 105 nm. The piezoelectric thin-film 2 is a LiNbO.sub.3 thin-film with cut angle of (0°, 170°, 0°) and a mean thickness of 100 nm. The electrode structure 3, 31, 32 is based on Al and has a mean thickness of 80 nm. However, the specified materials, cut angles and thicknesses are only to be understood as an example. In fact, depending on the intended application of the SAW device, the materials, thicknesses and cut angles and also the number of waveguide layers may vary. Particularly preferably, the layer stack 4 comprises more than two first waveguide layers 41 and more than two second waveguide layers 42.

[0046] The cut angles (λ′, μ, θ) are the Euler angles defining the orientation of a top surface of a substrate or carrier or layer with respect to the crystallographic axes of the substrate or carrier or layer. The definition is in accordance with the International Standard IEC 62276:2016.

[0047] FIGS. 2 and 3 show a simulation of the admittance of a simulated SAW resonator according to the present invention (solid line) compared to a simulated SAW resonator which does not comprise the layer stack of waveguide layers (dotted line). For example, the simulated SAW resonator according to the present invention is the SAW resonator according to the exemplary embodiment of FIG. 1.

[0048] In FIG. 2 the real part of the admittance and in FIG. 3 the absolute value of the admittance are shown. The y-axes each show the magnitude in dB, whereas the x-axes each show the frequency in MHz.

[0049] The introduction of the layer stack of waveguide layers with different sound velocities significantly improves the waveguiding, which can be seen in the lower level of the solid line compared to the dotted line in FIG. 2. Additionally, the layer stack of waveguide layers significantly increases k.sup.2, which can be seen in FIG. 3. The improvements in k.sup.2 and losses due to the layer stack of waveguide layers makes the SAW device suitable for applications at 5 GHz which require high k.sup.2 and Q.

[0050] The invention described herein is not limited by the description in conjunction with the exemplary embodiments. Rather, the invention comprises any new feature as well as any combination of features, particularly including any combination of features in the patent claims, even if said feature or said combination per se is not explicitly stated in the patent claims or exemplary embodiments.

REFERENCE SIGN LIST

[0051] 1 carrier substrate [0052] 2 piezoelectric thin-film [0053] 3 interdigital electrode structure [0054] 4 layer stack of waveguide layers [0055] 5 TCF compensating layer [0056] 6 ion blocking layer [0057] 31 first electrode [0058] 32 second electrode [0059] 41 first waveguide layer [0060] 42 second waveguide layer