WO2022184232A1

WO2022184232A1 - Spurious mode suppression in a multi-layer saw device

Info

Publication number: WO2022184232A1
Application number: PCT/EP2021/055087
Authority: WO
Inventors: Andreja ERBES; Xinyi Li; Xudong QIN
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2021-03-02
Filing date: 2021-03-02
Publication date: 2022-09-09
Also published as: CN116964933A

Abstract

The present disclosure relates to Surface Acoustic Wave (SAW) devices. The disclosure presents, in particular, a SAW device with a multi-layer structure, wherein spurious modes are suppressed in the SAW device. The SAW device (100) comprises a (111) silicon layer (101), and a set of interleave layers (102) provided on the silicon layer. Further, the SAW device comprises a rotated YX-cut lithium tantalate (LT) layer (103) provided on the set of interleave layers, and two or more electrodes (104) arranged on the LT layer. The electrodes are configured to convert an electrical signal to a SAW propagating in the LT layer.

Description

SPURIOUS MODE SUPPRESSION IN A MULTI-LAYER SAW DEVICE

TECHNICAL FIELD

The present disclosure relates to acoustic wave devices, in particular, to Surface Acoustic Wave (SAW) devices. The disclosure aims at improving the out-of-band spurious mode response of a SAW-based Radio Frequency (RF) filter. To this end, the disclosure presents a SAW device with a multi-layer structure, wherein off-band spurious modes are suppressed in the SAW device. Further, the disclosure also presents a method for fabricating the SAW device.

BACKGROUND

Acoustic wave devices are key components used in modem electronic circuits. The desired high frequency selectivity of such circuits, while maintaining a low electronic insertion loss, requires high quality factor mechanical resonators coupled in a filter topology. SAW devices allow designing SAW-based RF filters that could meet these requirements.

A SAW device is configured to couple an electrical, time varying signal, to a mechanical wave that travels on the surface of a piezoelectric material (e.g., a piezoelectric layer). To this end, alternating electrodes, for instance, metal interdigital transducer (IDT) electrodes, are provided on the surface of the piezoelectric layer. While the excited waves trigger the required modes of vibration, a conventional SAW device also suffers from the generation of spurious modes. These spurious modes can impact the quality and performance of the SAW device, and accordingly that of a SAW-based RF filter.

Multi-layer acoustic devices can be used to improve the energy confinement in a piezoelectric layer, which may lower the acoustic losses into a supporting substrate beneath the piezoelectric layer, and may improve the overall response of a filter designed based on an acoustic device, e.g., a SAW-based filter. However, such additional layers may at the same time increase the number of allowable propagating acoustic waves and mechanical modes. These waves can build up at frequencies located away from the normal operating frequency of the SAW-based filter (i.e., off-band). In modern radio communication standards, this is typically not allowed, since front end circuits rely on stringent off-band channel attenuation levels, in order to prevent interaction between different communications channels and /or communication standards. Thus, it is important to develop a technologically sound method that allows lowering the off- band spurious mode content of a SAW-based filter. In order to do so, a thorough understanding of the source of the buildup of the spurious modes is required.

To date, some of these issues have been addressed in the following ways. In particular, out-of- band attenuation specifications are conventionally resolved using off-chip passive components (e.g., combinations of capacitors, inductors, transformers, etc.), in order to generate the required poles or zeroes in the transmission characteristics of the overall RF filter. However, a disadvantage is that a compatible technology to produce the off-chip passive components is required. Further, compared to the high-Q of a SAW device, the RF passive components suffer from electrical losses, which increases the overall loss of the RF filter and degrade its performance. In addition, a certain size and complex packaging are required to integrate these passive components. There is also the risk of impacting the in-band filter response, due to non ideal passive components.

SUMMARY

In view of the above-mentioned problems and disadvantages, embodiments of the present invention aim to improve the current solutions. An objective is to reduce out-of-band spurious modes in a SAW device. The disclosure particularly aims at targeting the primary source of the spurious modes and wave propagation and build-up directly at the resonator level of the SAW device. The intention of the disclosure is further to lower the off-band spurious mode content of a SAW-based filter based on the SAW device. A specific goal is to create wide-band spurious-free responses. Furthermore, no external components should be required to achieve the required out-of-band attenuation specifications.

The objective is achieved by the embodiments of the invention as described in the enclosed independent claims. Advantageous implementations of the embodiments of the invention are further defined in the dependent claims.

Embodiments of the present disclosure rely on the use of specific anisotropic substrate orientations in multi-layer material stacks of the SAW device. The specific substrate orientations ensure that the wanted modes are effectively guided, while the energy guiding of other spurious modes is lowered for a wide frequency range. A first aspect of this disclosure provides a SAW device comprising: a (111) silicon layer; a set of interleave layers provided on the silicon layer; a rotated YX-cut lithium tantalate, LT, layer provided on the set of interleave layers; and two or more electrodes arranged on the LT layer, the electrodes being configured to convert an electrical signal to a SAW propagating in the LT layer.

The YX-cut LT (LiTa03) layer may cut perpendicularly to a Y-axis rotated by a certain rotation angle around a crystal X-axis, and the SAW may propagate along an X-axis direction. The LT layer is employed as representative piezoelectric layer of the SAW device, and the electrodes may be used to couple the time varying, electrical signal, to the SAW, which travels on the surface of the LT layer. The surface of the (111) silicon layer, on which the LT layer is provided, is along the silicon (111) plane. The use of this specific substrate crystal orientation, which is provided for the LT layer by the (111) silicon layer, enables lowering the number of available propagating waves which exist at the different material interfaces, therefore, breaking the boundary condition required for mode guiding/growth of spurious modes. Accordingly, the primary source of the spurious modes and wave propagation/build-up is targeted directly at the resonator level of the SAW device. As a consequence, out-of-band spurious modes in the SAW device are significantly reduced. Based on the SAW device of the first aspect, a SAW-based filter may be designed, which has a low off-band spurious mode content.

In an implementation form of the first aspect, a rotation angle of the LT layer is in a range of 18° to 55°.

That is, the LT layer is a q° YX-cut LT layer, wherein q° = 18°-55°. In particular, this means an X-propagating Y-cut LT layer within the 18°-55° range. The LT layer may be cut perpendicularly to the Y-axis rotated by the rotation angle of q° = 18°-55° around the crystal X-axis.

In an implementation form of the first aspect, the LT layer is a 26° YX-cut LT layer, or a 36° YX-cut LT layer, or a 42° YX-cut LT layer.

Accordingly, three specific LT layers with q° = 26°, 36° or 42° are envisaged. The 26° LT layer provides the highest piezoelectric coupling coefficient, while 36° and 42° allow using widely available piezoelectric materials. In an implementation form of the first aspect, an angle between the [110] -direction of the silicon layer and the X-direction of the LT layer is in a range of 0° to 30° or in a range of 90° to 120°.

Within these ranges, the suppression of spurious modes in the SAW device is the strongest.

In an implementation form of the first aspect, the two or more electrodes are periodically arranged on the LT layer with a pitch along the X-direction of the LT layer, wherein each electrode extends orthogonal to the X-direction.

This arrangement of electrodes, defining the propagation direction of the SAW in the LT layer, leads to the best spurious modes suppression.

In an implementation form of the first aspect, the LT layer is defined by a first set of Euler angles (li, mi, qi), wherein li is in a range of -3° to +3°, mi is in a range of -72° to -35°, and qi is in a range of -3° to +3°.

In an implementation form of the first aspect, the silicon layer is defined by a second set of Euler angles (h, m2, Q2), wherein l = 45°, m = 54,735°, and Q2 is in a range of 0° to 30° or in a range of 90° to 120°.

In an implementation form of the first aspect, the LT layer has a thickness in a range of 0.1 - 0.4 times a wavelength of the SAW at an operating frequency of the SAW device.

In particular, the LT layer may have a thickness in a range of 0.2 - 0.3 times a wavelength of the SAW at an operating frequency of the SAW device.

In an implementation form of the first aspect, the set of interleave layers has a thickness in a range of 0.0 - 0.4 times a wavelength of the SAW at an operating frequency of the SAW device.

In particular, the set of interleave layers may have a thickness in a range of 0.1 - 0.2 times a wavelength of the SAW at an operating frequency of the SAW device. In an implementation form of the first aspect, the two or more electrodes comprise aluminum electrodes or electrodes made from an aluminum-copper-alloy.

In an implementation form of the first aspect, the two or more electrodes have a thickness in a range of 0.02 - 0.12 times a wavelength of the SAW at an operating frequency of the SAW device.

The LT thickness, interleave layer thickness, and electrode thickness in the above implementation forms allows optimizing the multilayer stack of the SAW device with aluminum based electrodes for out-of-band spurious mode suppression.

In particular, the set of interleave layers may have a thickness in a range of 0.1 - 0.3 times the wavelength.

In an implementation form of the first aspect, the two or more electrodes comprise copper electrodes or electrodes made from a copper-aluminum-alloy.

In an implementation form of the first aspect, the two or more electrodes have a thickness in a range of 0.02 - 0.08 times a wavelength of the SAW at an operating frequency of the SAW device.

The LT thickness, interleave layer thickness, and electrode thickness in the above implementation forms allows optimizing the multilayer stack of the SAW device with copper based electrodes for out-of-band spurious mode suppression. In an implementation form of the first aspect, the set of interleave layers comprises a silicon oxide layer provided on the silicon layer, and the LT layer is provided on the silicon oxide layer.

The silicon oxide may be used as RF isolation, and/or as a wafer bonding layer between the LT layer and the silicon layer.

In an implementation form of the first aspect, the set of interleave layers comprises a polycrystalline silicon layer provided on the silicon layer, and a silicon oxide layer provided on the polycrystalline silicon layer, and the LT layer is provided on the silicon oxide layer.

The polycrystalline silicon layer may further support the spurious mode suppression. In an implementation form of the first aspect, the polycrystalline silicon layer is doped with a rare- earth element.

In an implementation form of the first aspect, the SAW device comprises a first part including the silicon layer and a second part including the LT layer, wherein the first part is bonded to the second part by the set of interleave layers arranged between the silicon layer and the LT layer.

Accordingly, the SAW device can be fabricated by producing separately the first part and the second part, and then bonding these parts together.

A second aspect of this disclosure provides a method for fabricating a SAW device, comprising: providing a (111) silicon layer; providing a rotated YX-cut lithium tantalate, LT, layer; bonding the silicon layer and the LT layer, wherein a set of interleave layers is arranged between the silicon layer and the LT layer; and forming two or more electrodes on the LT layer, the electrodes being configured to convert an electrical signal to a SAW propagating in the LT layer.

The method of the second aspect can also be used to fabricate the SAW device according to the implementation forms of the first aspect. The method of the second aspect provides the same advantages as described above for the SAW device for the first aspect.

The benefits provided by the SAW device of the first aspect and the method of the second aspect, respectively, include: • Passive methodology for out of band spurious mode suppression and transverse mode suppression

• No need of external passive components.

• Robust methodology, which is less prone to packaging or filter architecture issues.

• Can be implemented in a wide variety or range of (RF) filter products and frequency bands.

• Useful in other MEMS-based PLL timing circuits by reducing the number of available spurious modes on which an oscillator can lock-in.

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

FIG. 1 shows a SAW device according to an embodiment of the invention.

FIG. 2 shows an Euler angles convention for a rotated crystal axis.

FIG. 3 shows the symmetry of a silicon crystal.

FIG. 4 illustrates a (111) silicon wafer.

FIG. 5 shows a typical admittance response in a 42°LT/Si02/poly-Si/Si multilayer stack for different orientations of the silicon layer.

FIG. 6 shows the effect of a slowness surface in the sagittal plane of the silicon layer on the spurious modes in a 42°LT/ Si mulitlayer stack.

FIG. 7 shows a relation between the frequencies of the spurious modes and slowness surfaces of the LT layer and the silicon layer, exemplarily in a 26°LT/Si(l 11) multilayer stack.

FIG. 8 shows an optimization of the LT layer orientation for obtaining higher piezoelectric coupling coefficients. FIG. 9 shows an optimization of a propagation direction on the (111) plane of the silicon layer for improving suppression of the spurious modes.

FIG. 10 shows an influence of the propagation direction in the (111)- Si plane on the spurious modes in a LT/Si02/poly-Si/Si multilayer stack.

FIG. 11 shows an optimization of a multilayer stack with a 26° YX-cut LT layer for a maximum coupling coefficient of the main mode.

FIG. 12 shows an optimization of a multilayer stack with a 26° YX-cut LT layer for out- of-band spurious mode suppression.

FIG. 13 shows an optimization of a multilayer stack with a 26°YX-cut LT layer for out- of-band spurious mode suppression.

FIG. 14 shows an optimization of a multilayer stack with a 36° YX-cut LT layer for a maximum coupling coefficient of the main mode.

FIG. 15 shows an optimization of a multilayer stack with a 36° YX-cut LT layer for out- of-band spurious mode suppression.

FIG. 16 shows an optimization of a multilayer stack with a 36° YX-cut LT layer for out- of-band spurious mode suppression.

FIG. 17 shows an optimization of a multilayer stack with a 42°YX-cut LT layer for a maximum coupling coefficient of the main mode.

FIG. 18 shows an optimization of a multilayer stack with a 42°YX-cut LT layer for out- of-band spurious mode suppression.

FIG. 19 shows an optimization of a multilayer stack with a 42°YX-cut LT layer for out- of-band spurious mode suppression. FIG. 20 shows an influence of a polycrystalline silicon layer on the spurious mode suppression.

FIG. 21 shows an influence of an aluminum electrode thickness on the spurious mode suppression.

FIG. 22 shows an influence of a copper electrode thickness on the spurious mode suppression.

FIG. 23 shows an influence of a copper electrode thickness on the spurious mode suppression.

FIG. 24 shows a method of fabricating a SAW device according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a SAW device 100 according to an embodiment of the invention. In particular, FIG. 1 shows a unit-cell lOOu of the SAW device 100, wherein the shown unit-cell lOOu may be included one or more times in the SAW device 100, i.e., it may be repeated multiple times. The unit-cell lOOu comprises at least three materials: a piezoelectric layer 103, which comprises rotated YX-cut Lithium-Tantalate (LT) and is thus referred to as LT layer 103, a (11 l)-oriented silicon (substrate) layer 101, and a set of interleave layers 102 (e.g., the set including at least a silicon oxide layer) that may be used as RF isolation and/or as a wafer bonding layer between the piezoelectric LT layer 103 and the (111) silicon layer 101.

In other words, the SAW device 100 comprises the (111) silicon layer 101, the set of interleave layers 102 provided on the silicon layer 101, and the rotated YX-cut LT layer 103 provided on the set of interleave layers 102. The set of interleave layers 102 may comprise only one interleave layer (e.g., a silicon oxide layer), or may comprise two interleave layers (as exemplarily illustrated, e.g., a polycrystalline silicon layer 102a on the substrate and a silicon oxide layer 102b on the polycrystalline silicon layer 102a), or may comprise even more than two interleave layers. A rotation angle of the YX-cut LT layer 103 may be in a range of Q = 18° to 55°, i.e., the LT layer 103 may be a (18°-55°) YX-cut LT layer. For example, the LT layer 103 may be a 26° YX-cut LT layer, or a 36° YX-cut LT layer, or a 42° YX-cut LT layer.

Further, the SAW device 100 comprises two or more electrodes 104 (e.g., each electrode 104 being made of aluminum or copper), which are arranged on the LT layer 103. The two or more electrodes 104 are configured to convert an electrical signal to a SAW propagating in the LT layer 103. The two or more electrodes 104 may be periodically arranged on the LT layer 103 with a pitch p, for instance, they may be arranged along an X-direction of the LT layer 103. Thereby, each electrode 104 may extend orthogonal to the X-direction of the LT layer 103. The two or more electrodes 104 may optionally be surrounded and/or may be covered by a passivation layer 105. The two or more electrodes 104 may be two or more IDT electrodes.

The rotated YX-cut LT layer 103 and the silicon layer 101 may, respectively, be defined by a set of Euler angles. In this context, FIG. 2 shows an Euler angles convention for a rotated crystal axis. In particular, FIG. 2 illustrates a set of standard Euler angles (l, m, Q), which may be used for defining a crystal cut orientation of a certain layer. The crystal cut orientation may be chosen such that the orthogonal basis vectors of the layer (see FIG. 2(b)) are aligned to the required crystal orientations, as defined by three successive rotations (see FIG. 2(a)) of the basis vectors. The thickness of the layer is then defined in the z⁽³⁾ direction.

In particular, in the present case of the SAW device 100, the LT layer 103 may be defined by a first set of Euler angles (li, mi, qi), wherein li is in a range of -3° to +3°, mi is in a range of - 72° to -35°, and qi is in a range of -3° to +3. The silicon layer 101 may be defined by a second set of Euler angles (fa, ya, Q2), wherein l = 45°, m = 54,735°, and Q2 is in a range of 0° to 30° or in a range of 90° to 120°.

FIG. 3 shows the well-known symmetry of a silicon crystal. In particular, FIG. 3 shows a stereographic projection of symmetry elements of the silicon crystal (point symmetry class m3m). The (111) plane of the silicon crystal, which in the present case of the SAW device 100 defines the orientation of silicon layer 101, is normal to a 3-fold symmetry axis.

FIG. 4 shows an illustration of a (11 l)-oriented silicon wafer, which may be used to form the (111) silicon layer 101 of the SAW device 100. That is, the (111) silicon layer 101 may be a (11 l)-oriented silicon wafer, and the set of interleave layers 102 may be provide on this wafer. The (111) silicon layer may also be formed based on said wafer, e.g., based on separating or scribing the wafer. The flat on the (111) plane of the silicon wafer, which is shown in FIG. 4 is notably normal to the [1-10] direction.

FIG. 5 demonstrates the influence of the orientation of the (111) silicon layer 101 on the spurious modes, particularly their suppression. FIG. 5 specifically shows typical admittance responses in a multilayer stack including a silicon layer 101, a set of interleave layers 102 (comprising a polycrystalline silicon layer 102a provided on the silicon layer and a silicon oxide layer 102b provided on the polycrystalline silicon layer 102a), and a 42° YX-cut LT layer 103 provided on the silicon oxide layer 102b. Notably, in the present disclosure, such a multilayer stack may be abbreviated by “42°LT/Si0₂/poly-Si/Si”, and other, similar multilayer stacks may be abbreviated in a similar manner). The admittance shown in FIG. 5(a) and FIG. 5(b), respectively, include an imaginary part (Im(Y); also referred to as susceptance) and a real part (Re(Y); also referred to as conductance). The silicon layer 101 has different orientations for FIG. 5(a) and FIG. 5(b), respectively. In particular, for FIG. 5(a) the silicon layer 101 is a (001) silicon layer, and for FIG. 5(b) the silicon layer 101 is the (111) silicon layer as used in the SAW device 100 according to an embodiment of the invention described above.

As indicated in FIG. 5, the cut-off velocities of the slow shear (BAW1), the fast shear (BAW2) and the longitudinal (BAW3) bulk waves in the silicon layer 101 can be estimated for the different orientations of the silicon layer 101 from the BAW slowness surfaces in the sagittal planes (the planes perpendicular to electrodes of IDTs used for generation of SAW). It can be derived that the spurious modes - visible as resonances in the admittance response between around 2.4-2.6 GHz in FIG. 5(a) - are suppressed in FIG. 5(b), i.e., for the (11 l)-oriented silicon layer 101

FIG. 6 shows the effect of the slowness surface in the sagittal plane of the silicon layer 101 on the spurious modes in the 42°LT/Si0₂/poly-Si/Si multilayer stack. The spurious modes are determined mostly by the orientations of the LT layer 103 and the silicon layer 101, respectively, while the set of interleave layers 102 (e.g. SiCk and poly-Si) has only a minor influence on the spurious modes. In FIG. 6(b) a (111) silicon layer 101 is used, while in FIG. 6(a) a (001) silicon layer 101 is used. As in FIG. 5, the spurious mode suppression for the (111)- oreiented silicon layer 101 is derivable. Notbaly, in a ZX cut of silicon, the propagation direction (X) is an acoustic axis parallel to the 4-fold symmetry axis, due to the symmetry of silicon. Any acoustic axis is a singularity point on the slowness surface. The characteristics of BAWs (polarizations, Poynting vectors) change rapidly around acoustic axis and some of these BAWs become strongly coupled with wave motions in the LT layer 103. In addition, the concavity of the slowness surface existing around the X-axis of the silicon layer 101 means that additional modes can propagate with low attenuation and deteriorate the resonator performance. This is confirmed by spurious modes observed at the frequencies 2.4-2.7 GHz in 42LT/ZX-SL This frequency interval can be transformed into the interval of tangential slownesses (between vertical lines on the slowness surface). For the (111) silicon layer 101 there is no concavity and no acoustic axis along the propagation direction. Hence, the frequency response is spurious-free in a wide range. Accordingly, the use of a (11 l)-oriented silicon layer 101 suppresses the spurious modes compared to, for example, the use of a (OOl)-oriented silicon layer 101.

FIG. 7 shows a relationship between the frequencies of the spurious modes and the slowness surfaces of the LT layer 103 and the silicon layer 101, respectively, exemplarily in a 26°LT/ (111)-Si multilayer stack. In particualr, FIG. 7 shows schematically the partial modes in the LT layer 101 and the silicon layer 101, respectively, which are involved in the SAW device 100 at a fixed frequency (tangential slowness).

FIG. 8 shows that peizoelectric coupling coefficient k² changes in dependence of the rotation angle of the YX-cut LT layer 103 in a 9° LT/SiCk/poly-Si/SiQ l l) multilayer stack. Accordingly, the LT layer 103 can be optimized for a high k². In particualr, a rotation angle variation range of Q = 18-42° is shown in FIG. 8. However, the LT layer 101 may have a rotation angle Q in a range of 18° to 55°, i.e., the LT layer 103 may be an Q = 18°-55° YX-cut LT layer, as described above. As derivable from FIG. 8, the maximum coupling (highest k²) occurs in the LT layer 103 when the rotation angle is between about Q = 26° - 30°, wherein the exaxt location of the maximum coupling may depend on the LT layer thickness. In the present examople, this thickness was 0.2 times a wavelength of the SAW at the operting frequencys of the SAW device 100. Thus, for instance, a 26° YX-cut LT layer 103 may be selected for the SAW device 100. Further, the LT layer 103 of the SAW device may also be a 36° YX-cut LT layer or may be a 42° YX-cut LT layer, as commercially available and widely used. FIG. 9 shows an optimization of a propagation direction of a SAW in the LT layer 103 with respect to the (111) plane of the silicon layer 101 for best suppression of the spurious modes. In particular, an angle b may be defined between the [110] -direction of the silicon layer 101 and the X-direction of the LT layer 103, and this angle may be selected for best spurious mode suppression. As can be derived from FIG. 9, the angle b may particularly be selected to be in a range of 0°-30° or in a range of 90°-120° in the SAW device 100. FIG. 9 shows the variation of the angle b between 0-120°, exemplarily for a 36° YX-cut LT layer 103 and the (111) silicon layer 101, and shows that in these ranges the spurious modes are suppressed the most, while between 30° and 90° a ripple in the imaginary part of the admittance curve, indicating spurious modes, is more pronounced.

FIG. 10 shows an influence of the propagation direction of the SAW in the LT layer 103 with respect to the (111) silicon plane on the spurious modes, in exemplarily a 26°LT/Si0₂/poly- Si/Si(l 11) multilayer stack. In particular, Fig. 10(a) shows the admittance response for an angle b=0, and FIG. 10(b) shows the admittance response for an angle b =180°, wherein the shown admittance responses each include a magnitude of the admittance (upper curve) and real part of the admittance (lower curve). It can be seen that the spurious modes observed at the frequencies 2.4-2.7 GHz are suppressed in FIG. 10(a) compared to FIG. 10(b).

FIG. 11 shows an optimization of a multilayer stack with a 26° YX-cut LT layer 103 for maximizing the coupling coefficient k² of the main mode of the SAW propagating in the LT layer 103. In particular, FIG. 11 shows a dependence of the coupling coefficient k² on the thickness of the LT layer 103 (here referred to as “LT thickness”) and on the thickness of the set of interleave layers 102 (here referred to as “oxide thickness”). Both the LT thickness and the oxide thickness are shown for a range of about 0.1-0.5 times the wavelength of the SAW at an operating frequency of the SAW device 100.

FIG. 12 and FIG. 13 show an optimization of a multilayer stack with a 26° YX-cut LT-layer for out-of-band spurious mode suppression. In particular, they show that the thickness of the set of interleave layers 102 (h_ox) may be selected to be 0.1 (FIG. 12) or 0.25 (FIG. 13) times a wavelength of the SAW at the operating frequency, while the thickness of the LT layer 103 (h r) may be varied between 0.1-0.45 times the wavelength of the SAW at the operating frequency. For each combination, the resulting coupling coefficient is indicated and the amount of spurious mode suppression can be derived from the respective admittance response. The shown admittance responses each include a magnitude of the admittance (upper curve) and real part of the admittance (lower curve).

FIG. 14 shows an optimization of a multilayer stack with a 36° YX-cut LT layer 103 for maximizing the coupling coefficient k² of the main mode of the SAW propagating in the LT layer 103. In particular, FIG. 11 shows a dependence of the coupling coefficient k² on the thickness of the LT layer 103 (here referred to as “LT thickness”) and on the thickness of the set of interleave layers 102 (here referred to as “oxide thickness”). Both the LT thickness and the oxide thickness are shown for a range of about 0.1-0.5 times the wavelength of the SAW at an operating frequency of the SAW device 100.

FIG. 15 and FIG. 16 show an optimization of a multilayer stack with a 36° YX-cut LT-layer for out-of-band spurious mode suppression. In particular, they show that a thickness of the set of interleave layers 102 (h_ox) may be selected to be 0.1 (FIG. 15) or 0.25 (FIG. 16) times a wavelength of the SAW at the operating frequency, while the thickness of the LT layer 103 (h r) may be varied between 0.1-0.45 times the wavelength of the SAW at the operating frequency. For each combination, the coupling coefficient is indicated and the amount of spurious mode suppression can be derived from the respective admittance response. The shown admittance responses each include a magnitude of the admittance (upper curve) and real part of the admittance (lower curve).

FIG. 17 shows an optimization of a multilayer stack with a 42° YX-cut LT layer 103 for maximizing the coupling coefficient k² of the main mode of the SAW propagating in the LT layer 103. In particular, FIG. 11 shows a dependence of the coupling coefficient k² on the thickness of the LT layer 103 (here referred to as “LT thickness”) and on the thickness of the set of interleave layers 102 (here referred to as “oxide thickness”). Both the LT thickness and the oxide thickness are shown for a range of about 0.1-0.5 times the wavelength of the SAW at an operating frequency of the SAW device 100.

FIG. 18 and FIG. 19 show an optimization of a multilayer stack with a 42° YX-cut LT-layer for out-of-band spurious mode suppression. In particular, they show that a thickness of the set of interleave layers 102 (hox) is selected to be 0.1 (FIG. 18) or 0.25 (FIG. 19) times a wavelength of the SAW at the operating frequency, while the thickness of the LT layer 103 (hur) is varied between 0.1-0.45 times the wavelength of the SAW at the operating frequency. For each combination, the coupling coefficient is indicated and the amount of spurious mode suppression can be derived from the respective admittance response. The shown admittance responses each include a magnitude of the admittance (upper curve) and real part of the admittance (lower curve).

FIG. 20 shows an influence of whether a polycrystalline silicone layer 102a (short “poly-Si”) is present or not present in the set of interleave layers 102. The respective admittance responses shown in FIG. 20(a), (b) and (c), each include a magnitude of the admittance (upper curve) and real part of the admittance (lower curve). In FIG. 20(a) the set of interleave layers 102 comprises a silicon oxide layer 102b provided on the silicon layer 101, and the LT layer 103 is provided on the silicon oxide layer 102b. In FIG. 20(b) and (c), the set of interleave layers 102 comprises a polycrystalline silicon layer 102a provided on the silicon layer 101, and a silicon oxide layer 102b provided on the polycrystalline silicon layer 102a, and the LT layer 103 is provided on the silicon oxide layer 102b. In FIG. 20(b) the thickness of the polycrystalline silicon layer 102a is 1000 nm, in FIG. 20(c) it is assumed infinite. Notably, the poly crystalline silicon layer 102a may be doped with a rare-earth element. It can be derived that a polycrystalline silicon layer 102a of a certain thickness (e.g., in a range of 1 pm) can further improve the spurious modes suppression (see FIG. 20(b)) compared to no polycrystalline silicon layer 102a (FIG. 20(a)). However, the polycrystalline silicon layer 102a should not be too thick, as this may again increase the spurious modes (FIG. 20(c) simulated for an infinite polycrystalline layer 102a)

FIG. 21 shows an influence of a thickness of an aluminum electrode 104, which is provided on the LT layer 103, on the spurious mode suppression. As an example, a multilayer stack including a 42° YX-cut LT-layer 103 is used (particularly, 42°LT/Si0₂/poly-Si/Si). The thickness of the LT layer 103 (h r) is 0.3 times the wavelength of the SAW at the operating frequency. In this example, the thickness of the silicon oxide layer 102b (h_ox) is 0.2 times the wavelength of the SAW at the operating frequency. The thickness of the aluminum electrodes 104 (h_Ai) is varied between 0.002 - 0.12 times the wavelength of the SAW at the operating frequency. The coupling coefficient k² is indicated and the amount of spurious mode suppression can be derived from the respective admittance responses shown in FIG. 21. The shown admittance responses each include a magnitude of the admittance (upper curve) and real part of the admittance (lower curve). Generally, if the two or more electrodes 104 comprise aluminum electrodes or electrodes made from an aluminum-copper-alloy, the LT layer 103 may have a thickness in a range of 0.1 - 0.4 times a wavelength of the SAW at the operating frequency of the SAW device 100. Further, the set of interleave layers 102 may have a thickness in a range of 0.0 - 0.4 times a wavelength of the SAW at the operating frequency of the SAW device 100. The two or more electrodes 104 in this case may have a thickness in a range of 0.02 - 0.12 times a wavelength of the SAW at the operating frequency.

FIG. 22 shows an influence of a thickness of a copper electrode 104, which is provided on the LT layer 103, on the spurious mode suppression. As an example, a multilayer stack including a 42° YX-cut LT-layer 103 is used (particularly, 42°LT/Si0₂/poly-Si/Si). In this example, the thickness of the LT layer 103 (h r) is 0.1 times the wavelength of the SAW at the operating frequency. The thickness of the silicon oxide layer 102b (h_ox) is 0.1 times the wavelength of the SAW at the operating frequency. The thickness of the copper electrodes 104 (hc_u) is varied between 0.02 - 0.12 times the wavelength of the SAW at the operating frequency. The coupling coefficient k² is indicated and the amount of spurious mode suppression can be derived from the respective admittance responses shown in FIG. 22. The shown admittance responses each include a magnitude of the admittance (upper curve) and real part of the admittance (lower curve).

FIG. 23 shows an influence of a thickness of a copper electrode 104, which is provided on the LT layer 103, on the spurious mode suppression. As an example, a multilayer stack including a 42° YX-cut LT-layer 103 is used (particularly, 42°LT/Si0₂/poly-Si/Si). In this example, the thickness of the LT layer 103 (hur) is 0.3 times the wavelength of the SAW at the operating frequency. The thickness of the silicon oxide layer 102b (h_ox) is 0.1 times the wavelength of the SAW at the operating frequency. The thickness of the copper electrodes 104 (hc_u) is varied between 0.02 - 0.12 times the wavelength of the SAW at the operating frequency. . The coupling coefficient k² is indicated and the amount of spurious mode suppression can be derived from the respective admittance responses shown in FIG. 23. The shown admittance responses each include a magnitude of the admittance (upper curve) and real part of the admittance (lower curve).

If the two or more electrodes 104 comprise copper electrodes or electrodes made from a copper- aluminum-alloy, the LT layer 103 may have a thickness in a range of 0.1 - 0.4 times a wavelength of the SAW at an operating frequency of the SAW device 100. Further, the set of interleave layers 102 may have a thickness in a range of 0.0 - 0.4 times the wavelength of the SAW at an operating frequency of the SAW device 100. The two or more electrodes 104 in this case may have a thickness in a range of 0.02 - 0.08 times the wavelength of the SAW at the operating frequency.

FIG. 24 shows a method 200 of fabricating a SAW device according to an embodiment of the invention. The method 200 may lead to the SAW device 100 shown in FIG. 1. The method 200 comprises the following steps: a step 201 of providing a (111) silicon layer 101; a step 202 of providing a rotated YX-cut LT layer 103; a step 203 of bonding the silicon layer 101 and the LT layer 103, wherein a set of interleave layers 102 is arranged between the silicon layer 101 and the LT layer 103; and a step 204 of forming two or more electrodes 104 on the LT layer 103, the electrodes 104 being configured to convert an electrical signal to a SAW propagating in the LT layer 103.

The SAW device 100 may comprise a first part including the silicon layer 101 and a second part including the LT layer 103, wherein the first part may be bonded to the second part by the set of interleave layers 102 arranged between the silicon layer 101 and the LT layer 103. Accordingly, the method 200 may comprise a step of bonding the first part including the silicon layer 101 and the second part including the LT layer 103 using the set of interleave layers 102. The method 200 may also comprise providing the set of interleave layers 102 onto at least one of the first part and the second part, and then bonding the first part to the second part. For instance, wafer to wafer bonding may be used.

The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed subject matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

1. A surface acoustic wave, SAW, device (100), comprising: a (111) silicon layer (101); a set of interleave layers (102) provided on the silicon layer (101); a rotated YX-cut lithium tantalate, LT, layer (103) provided on the set of interleave layers (102); and two or more electrodes (104) arranged on the LT layer (102), the electrodes (104) being configured to convert an electrical signal to a SAW propagating in the LT layer (103).

2. The SAW device (100) according to claim 1, wherein: a rotation angle of the LT layer (103) is in a range of 18° to 55°.

3. The SAW device (100) according to claim 1 or 2, wherein: the LT layer (103) is a 26° YX-cut LT layer, or a 36° YX-cut LT layer, or a 42° YX-cut LT layer.

4. The SAW device (100) according to one of the claims 1 to 3, wherein: an angle between the [110] -direction of the silicon layer (101) and the X-direction of the LT layer (103) is in a range of 0° to 30° or in a range of 90° to 120°.

5. The SAW device (100) according to one of the claims 1 to 4, wherein: the two or more electrodes (104) are periodically arranged on the LT layer (103) with a pitch (p) along the X-direction of the LT layer (103), wherein each electrode (104) extends orthogonal to the X-direction.

6. The SAW device (100) according to one of the claims 1 to 5, wherein: the LT layer (103) is defined by a first set of Euler angles (li, mi, qi), wherein li is in a range of -3° to +3°, mi is in a range of -72° to -35°, and qi is in a range of -3° to +3°.

7. The SAW device (100) according to one of the claims 1 to 6, wherein: the silicon layer (101) is defined by a second set of Euler angles (h, m2, Q2), wherein l = 45°, m = 54,735°, and Q2 is in a range of 0° to 30° or in a range of 90° to 120°.

8. The SAW device (100) according to one of the claims 1 to 7, wherein: the LT layer (103) has a thickness in a range of 0.1 - 0.4 times a wavelength of the SAW at an operating frequency of the SAW device (100).

9. The SAW device (100) according to one of the claims 1 to 8, wherein: the set of interleave layers (102) has a thickness in a range of 0.0 - 0.4 times a wavelength of the SAW at an operating frequency of the SAW device (100).

10. The SAW device according to claim 8 or 9, wherein: the two or more electrodes (104) comprise aluminum electrodes or electrodes made from an aluminum-copper-alloy.

11. The SAW device (100) according to claim 10, wherein: the two or more electrodes (104) have a thickness in a range of 0.02 - 0.12 times a wavelength of the SAW at an operating frequency of the SAW device (100).

12. The SAW device (100) according to one of the claims 1 to 7, wherein: the LT layer (103) has a thickness in a range of 0.1 - 0.4 times a wavelength of the SAW at an operating frequency of the SAW device (100).

13. The SAW device (100) according to one of the claims 1 to 7 or 12, wherein: the set of interleave layers (102) has a thickness in a range of 0.0 - 0.4 times a wavelength of the SAW at an operating frequency of the SAW device (100).

14. The SAW device (100) according to claim 12 or 13, wherein: the two or more electrodes (104) comprise copper electrodes or electrodes made from a copper-aluminum-alloy.

15. The SAW device (100) according to claim 14, wherein: the two or more electrodes (104) have a thickness in a range of 0.02 - 0.08 times a wavelength of the SAW at an operating frequency of the SAW device (100).

16. The SAW device (100) according to one of the claims 1 to 15, wherein: the set of interleave layers (102) comprises a silicon oxide layer (102b) provided on the silicon layer (101), and the LT layer (103) is provided on the silicon oxide layer (102b).

17. The SAW device according to one of the claims 1 to 15, wherein: the set of interleave layers (102) comprises a polycrystalline silicon layer (102a) provided on the silicon layer (101), and a silicon oxide layer (102b) provided on the polycrystalline silicon layer (102a), and the LT layer (103) is provided on the silicon oxide layer (102b).

18. The SAW device (100) according to claim 17, wherein: the polycrystalline silicon layer (102a) is doped with a rare-earth element.

19. The SAW device (100) according to one of the claims 1 to 18, comprising: a first part including the silicon layer (101) and a second part including the LT layer (103), wherein the first part is bonded to the second part by the set of intermediate layers (102) arranged between the silicon layer (101) and the LT layer (103).

20. A method (200) for fabricating a surface acoustic wave, SAW, device, (100) comprising: providing (201) a (111) silicon layer (101); providing (202) a rotated YX-cut lithium tantalate, LT, layer (103); bonding (203) the silicon layer (101) and the LT layer (103), wherein a set of interleave layers (102) is arranged between the silicon layer (101) and the LT layer (103); and forming (204) two or more electrodes (104) on the LT layer (103), the electrodes (104) being configured to convert an electrical signal to a SAW propagating in the LT layer (103).