TECHNICAL FIELD

The present disclosure relates to surface acoustic wave (SAW) devices, in particular, a thin- film SAW (TF-SAW) devices. The disclosure presents a SAW device with a multi-layer stack that includes YAG as a novel acoustic material. The disclosure aims at improving the mechanical quality factor (Q), the electro-mechanical coupling coefficient (fcj ), and the out-of- band spurious mode response of the SAW device. This is, for instance, beneficial for micromechanical filters fabricated based on the multi-layer SAW device.

BACKGROUND

Micro-acoustic devices, such as SAW devices, are key components for signal processing, frequency generation, filtering, and sensing applications. Mobile communication systems, like GSM, UMTS, 2G/3G/4G/LTE/5G, Bluetooth, and W-LAN, allow making economical use of frequency allocation, digital radio quality, and intercontinental roaming. The market demands for mobile communication systems pushed an immense development progress and technology advances in both fields of micro-acoustic.

Wireless communication devices heavily rely on high performance band-pass transmission filters, which are used to reject any unwanted incoming RF signals, and to keep only the wanted transmitted signal. In some wireless communication applications, the band-pass transmission filters are required to have an exceptionally good selectivity of the incoming signal, which means letting through only a very narrow strip of the incoming frequency spectrum.

Some key technological requirements of such filters can be summarized as:

• High frequency;

• High selectivity for the suppression of spurious signals;

• Large bandwidth for high data rates;

• Low insertion loss factor for low power consumption;

• Miniaturized packages or integrated die filter modules for small hand held devices. SAW-based frequency filters with these features are used in most of the existing mobile telephone systems. To reduce the inherently high insertion loss of conventional SAW devices, a variety of special low-loss techniques has been developed, each technique being optimized for a specific application. The simulation and the design, first of the SAW devices, and second of the complete frequency filter characteristics, demands high sophisticated software tools with highly developed algorithms and models for accurate predictions of the characteristics of the build-up of the devices and their physical behavior.

A core element of an RF-filter is a resonator, which is often electrically coupled in a ladder configuration to generate a required frequency response. In the ladder configuration, resonators are simply electrically connected together, and are cascaded in series and parallel to form PI / T networks. Thereby, a frequency of a shunt resonator is lower than the series resonators, in order to generate the pass-band behavior.

A core element of a SAW device is a piezoelectric layer, which is disposed over numerous distinct material layers (i.e., a multi-layer stack is formed). The combination of different materials in the stack, and appropriate choice of thicknesses and material properties, can significantly improve the acoustic energy confinement in the piezoelectric layer. This effect lowers the acoustic losses and the radiation into the supporting substrate, and improves the overall response of a RF-filter produced based on the SAW device (wherein such an RF-filter is composed of an electrical and mechanical cascading of unit-cell resonator structures).

Having multiple buried layers in the layer stack of the SAW device, also increases the number of allowable propagating acoustic waves and mechanical modes. These waves can build up at certain frequencies located away from the main operating frequency band of the SAW device / RF filter. In modem radio communication standards, the out-of-band response needs to be tightly controlled, since front end circuits rely on stringent off-band channel attenuation levels to prevent any interaction between different communications channels and/or standards.

Thus, it is important to develop a technologically sound method to lower the off-band spurious mode content of SAW-based RF filters. In order to do so, a thorough understanding of the source of the buildup of these spurious modes is required, and then passive solutions to lower or remove the spurious modes need to be provided. To date, some of these issues have been addressed in the following ways:

Out-of-band attenuation specifications are addressed using off-chip passive components (e.g. combinations of capacitors, inductors, transformers, etc.) to generate the required poles/zeroes in the transmission characteristics of the overall RF-filter. A disadvantage is that this requires a compatible technology to produce off-chip passive components. Further, compared to the high-Q of the SAW device, the RF passive components suffer from electrical losses, which increases the overall loss of the RF-filter and degrade its performance. Also the size and complex packaging required to integrate these passives is a drawback, and there is always a risk of impacting the in-band filter response due to non-ideal passive components.

SUMMARY

For the present disclosure and its embodiments, the inventors further considered that, generally, optimization of an exemplary SAW device could be based on:

• Using high k piezoelectric materials, e.g., different crystal cuts of lithium tantalate (LT), lithium niobate (LN), aluminum nitride (AIN), disposed on a multi-layer substrate.

• Using a substrate composed of a high acoustic velocity supporting material, e.g. silicon. The main disadvantage, however, of using only a high velocity substrate is the natural build-up of higher order and out-of-band spurious modes.

• Adding an additional layer, e.g. silicon dioxide, for temperature compensation requirements into the multi-layer stack of the SAW device. However, a silicon/silicon oxide interface is known to generate additional interface charges, which can significantly degrade the RF performance of the SAW device.

• Using a rough or poly-silicon thin layer (e.g., 1 pm thick) called a “trap-rich” (TR) layer, to prevent unnecessary RF coupling through the substrate. However, this lowers the performance of the RF-filter. Further, this significantly increases the manufacturing complexity, and thereby limits the choice of alternative deposition techniques or optimization of the other layers composing the stack. In view of the above, this disclosure aims to provide an alternative multi-layer stack for a SAW device, in order to improve the RF performance of the SAW device, while keeping its manufacturing complexity and cost low. In particular, an objective is to lower the amount of spurious modes out-of-band. To this end, another objective of this disclosure is to find alternative materials for the layer stack.

These and other objectives are achieved by the embodiments of this disclosure as described in the enclosed independent claims. Advantageous implementations of the embodiments are further defined in the dependent claims.

In particular, the disclosed embodiments provide a SAW device that includes an yttrium- aluminum-garnet (YAG) material (or a corresponding doped version of it).

A first aspect of this disclosure provides a SAW device comprising: an YAG layer; an interleave layer arranged on the YAG layer; a piezoelectric layer arranged on the interleave layer; and two or more electrodes arranged on or above the piezoelectric layer, the electrodes forming an interdigital transducer configured to convert an electrical signal to a SAW propagating in the piezoelectric layer.

The electrodes may include a first electrode and a second electrode, which are arranged on the piezoelectric layer to form a resonator region. The resonator region comprises a region of the piezoelectric layer located between the first electrode and the second electrode.

The YAG layer may be included into the layer stack as a low-acoustic loss substrate material. Alternatively, it may be incorporated into the layer stack as an additional layer, which may electrically and acoustically isolate the resonator region of the SAW device from the substrate (which can be silicon in this case, or any corresponding high velocity substrate such as quartz, sapphire, silicon carbide, etc.).

As a consequence, the amount of spurious modes out-of-band is lowered. This improves the RF performance of the SAW device. Nevertheless, the manufacturing complexity and cost of the SAW device are kept low. The interleave layer may be used as RF isolation. In particular, benefits of the SAW device, having with the YAG layer in the stack, include:

• YAG is inherently a low acoustic loss material.

• A high kf is achievable with the SAW device, due to fast longitudinal/ shear bulk waves.

• High Q factor resonators can be built based on the SAW device, due to wave-guiding effects.

• A mismatch in the coefficient of thermal-expansion (CTE) of the YAG relative to the piezoelectric material of the SAW device can improve the passive temperature compensation and temperature-induced frequency drift of the core resonator of the SAW device.

• There is no need of complex TR layers at the interface with the interleave layer.

In an implementation form of the first aspect, the SAW device comprises a substrate layer; wherein the YAG layer is arranged on the substrate layer or is provided by the substrate layer.

That is, either the substrate itself is a low loss YAG substrate, or the YAG layer prevents acoustic losses from the resonator region (i.e., form the piezoelectric layer) into the substrate. The interleave layer may be used as a wafer bonding layer between layers above the substrate and the substrate.

In an implementation form of the first aspect, the substrate layer is a (111) silicon layer or a (100) silicon layer, and the YAG layer is arranged on the (111) silicon layer or the (100) silicon layer.

The use of this specific substrate crystal orientations, which is provided for the YAG layer by the 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 may be further reduced.

In an implementation form of the first aspect, a phase velocity V _Piez o of a primary mode of the SAW in the piezoelectric layer is related to a principle shear bulk acoustic wave velocity V _su b in the substrate layer according to: 0.6*V _S ub < Vpiezo < 0.85*V _su b.

In an implementation form of the first aspect, an acoustic impedance Z _su b of the substrate layer is related to an acoustic impedance Z _Piez o of the piezoelectric layer according to:

0.75*Zpiezo < Zsub < 1 ,25*Zpiezo-

The acoustic impedance of the substrate layer is defined as a product between a material density of the substrate layer and the phase velocity of the propagating acoustic modes.

With the above implementation forms, spurious modes can be most effectively reduced.

In an implementation form of the first aspect, the piezoelectric layer comprises at least one of a LT layer, and a LN layer, and an AIN layer.

In an implementation form of the first aspect, the LN layer is a rotated YX-cut LN layer having a rotation angle in a range of 115° to 135°.

That is, the LN layer is a YX-cut LN layer, wherein <I> = 115°-135°. In particular, this means an X-propagating Y-cut LN layer within the 115°-135° range. The LN layer may be cut perpendicularly to the Y-axis rotated by the rotation angle of O = 115°-135° around the crystal X-axis.

In an implementation form of the first aspect, the LN layer is a 120° YX-cut LN layer, or a 128° YX-cut LN layer.

Accordingly, two specific LN layers with <I> = 120 or 128° are envisaged.

In an implementation form of the first aspect, the LT layer is a rotated YX-cut LT layer having a rotation angle in a range of 18° to 65°.

That is, the LT layer is a 0 YX-cut LT layer, wherein 9 = 18°-65°. In particular, this means an X-propagating Y-cut LT layer within the 18°-65° range. The LT layer may be cut perpendicularly to the Y-axis rotated by the rotation angle of 0 = 18°-65° around the crystal X- axis.

In an implementation form of the first aspect, the LT layer is a 25° YX-cut LT layer, or a 36° YX-cut LT layer, or a 42° YX-cut LT layer, or a 50° YX-cut LT layer.

Accordingly, four specific LT layers with 9° = 25°, 36°, 42° or 50° are envisaged. The 25° 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, the two or more electrodes are periodically arranged on or above the LT layer or the LN layer with a pitch along the X-direction of the LT layer or the LN layer, wherein each of the electrodes extends orthogonal to the X-direction.

This arrangement of electrodes, defining the propagation direction of the SAW in the LT or LN 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 ( i, pi, 0i), wherein i is in a range of -3° to +3°, pi is in a range defined by the rotation angle of the LT layer minus 90°±5°, and 0i is in a range of -3° to +3°; and/or the LN layer is defined by a second set of Euler angles ( 2, p2, 62), wherein Z.2 is in a range of -3° to +3°, p2 is in a range defined by the rotation angle of the LN layer minus 90°±5°, and 02 is in a range of - 3° to +3°.

In an implementation form of the first aspect, an angle between the [110]-direction of the (111) silicon layer and the X-direction of the LT layer or LN layer is in a range of -30° to 30° or in a range of 60° to 90°.

In an implementation form of the first aspect, the interleave layer comprises a silicon oxide layer, or a polycrystalline silicon layer and a silicon oxide layer arranged on the polycrystalline silicon 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 further comprises an additional interleave layer arranged on the piezoelectric layer, wherein the two or more electrodes are arranged on the additional interleave layer and above the piezoelectric layer.

In an implementation form of the first aspect, the additional interleave layer comprises a silicon nitride layer, or a silicon oxide layer, or a hafnium oxide layer.

In an implementation form of the first aspect, the additional interleave layer is a coherent layer arranged on the piezoelectric layer, or is separated into two or more layer parts arranged on the piezoelectric layer, each of the two or more layer parts being arranged between the piezoelectric layer and one of the two or more electrodes.

In an implementation form of the first aspect, a thickness of the piezoelectric layer is in a range of 0.1 - 0.4 times a wavelength of the SAW at an operating frequency of the SAW device, for example, in a range of 0.18 - 0.38 times the wavelength if the piezoelectric layer comprises an LT layer, or in a range of 0.19 - 0.33 times the wavelength if the piezoelectric layer comprises an LN layer.

For example, the thickness of the piezoelectric layer may in a range of 200 - 1000 nm, particularly, in a range of 400 - 800 nm.

In an implementation form of the first aspect, a thickness of the interleave layer is in a range of 0.0 - 0.6 times a wavelength of the SAW at an operating frequency of the SAW device, for example, in a range of 0.05 - 0.38 times the wavelength if the piezoelectric layer comprises an LT layer, or in a range of 0.43 - 0.6 times the wavelength if the piezoelectric layer comprises an LN layer.

For example, the thickness of the interleave layer may be in a range of 50 - 1500 nm, particularly, in a range of 100 - 1250 nm. In an implementation form of the first aspect, a thickness of the YAG layer is in a range of 0.8 - 5.0 times a wavelength of the SAW at an operating frequency of the SAW device, for example, in a range of 1.0 - 3.0 times the wavelength.

For example, the thickness of the YAG layer may be in a range of 1000 - 3000 nm or more.

The piezoelectric layer thickness, interleave layer thickness, and YAG layer thickness in the above implementation forms allows optimizing the multilayer stack of the SAW device for out- of-band spurious mode suppression.

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

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

For example, the thickness of the two or more electrodes is in a range of 50 - 200 nm, particularly, in a range of 100 - 150 nm.

The electrode thickness in the above implementation forms allows optimizing the multilayer stack of the SAW device for out-of-band spurious mode suppression.

In an implementation form of the first aspect, the SAW device further comprises a silicon nitride passivation layer, or a silicon oxide passivation layer, surrounding and covering the two or more electrodes.

A second aspect of this disclosure provides a method for fabricating a SAW device, the method comprising: providing a substrate layer; providing a piezoelectric layer; and bonding the substrate layer and the piezoelectric layer, wherein one or more interleave layers are arranged between the substrate layer and the piezoelectric layer, and wherein the substrate layer or the interleave layer arranged on the substrate layer comprises an yttrium-aluminum-garnet, YAG, layer; and forming two or more electrodes on the piezoelectric layer, the electrodes forming an interdigital transducer configured to convert an electrical signal to a SAW propagating in the piezoelectric layer.

The method of the second aspect may have implementation forms to fabricate the SAW device of the implementation forms of the first aspect. The method of the second aspect and its implementation forms thus achieve the same advantages as described above for the SAW device of the first aspect and its implementation forms.

The aspects and implementation forms (embodiments) of this disclosure accordingly target the primary source of spurious mode, and wave propagation and build-up, directly at the resonator level of the SAW device, in order to create wide-band spurious-free responses. In that case, no external components are necessary to achieve the required attenuation specifications out-of- band. The embodiments thereby make use of a specific high-Q acoustic material (YAG) for the substrate and/or for a layer close to the supporting substrate. This ensures that the wanted modes are effectively guided, while the energy guiding of other spurious modes is lowered, for a wide frequency range. Further, the use of appropriate substrate material enables to lower the number of available propagating waves which exist at the different material interfaces, therefore breaking the boundary condition required for mode guiding /growth of these spurious modes to exist.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof. 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 this disclosure.

FIG. 2 shows a SAW device according to an embodiment of this disclosure with a YAG layer provided on a substrate layer.

FIG. 3 a SAW device according to an embodiment of this disclosure with a YAG layer provided by a substrate layer, and with an additional interleave layer.

FIG. 4 shows a SAW device according to an embodiment of this disclosure with a YAG layer provide on a substrate layer, and with a (separated) additional interleave layer.

FIG. 5 shows the Euler angles convention for rotated crystal axis, for instance, of a LT or LN piezoelectric layer of a SAW device according to an embodiment of this disclosure.

FIG. 6 shows simulations for a multi-layer stack of a SAW device according to an embodiment of this disclosure, with a 25°YX LT piezoelectric layer.

FIG. 7 shows simulations for a multi-layer stack of a SAW device according to an embodiment of this disclosure, with a 36° YX LT piezoelectric layer.

FIG. 8 shows simulations for a multi-layer stack of a SAW device according to an embodiment of this disclosure, with a 42° YX LT piezoelectric layer.

FIG. 9 shows simulations for a multi-layer stack of a SAW device according to an embodiment of this disclosure, with a 50° YX LT piezoelectric layer.

FIG. 10 shows a typical admittance response in a 42° YX LT/SiO2/YAG layer stack of a SAW device according to an embodiment of this disclosure. FIG. 11 shows a typical admittance response in a 42° YX LT/SiO2/YAG/Si layer stack of a SAW device according to an embodiment of this disclosure, wherein substrate layer orientations are compared.

FIG. 12 shows a method according to an embodiment of this disclosure for fabricating a SAW device.

FIG. 13 shows exemplary parameters for building a multi-layer stack of a SAW device according to an embodiment of this disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Generally, the embodiments of this disclosure are based on a unit material stack, which comprises at least three material layers: a piezoelectric layer (for forming a resonator region or core region of a SAW device, e.g., an LT, LN or AIN layer), an interleave layer (e.g. a SiO2 layer), and the YAG layer.

FIG. 1 shows a SAW device 10 according to an embodiment. In particular, FIG. 1 shows a unitcell lOu of the SAW device 10, wherein the shown unit-cell lOu may be included one or more times in the SAW device 10, i.e., it may be repeated multiple times. The (unit cell lOu of the) SAW device 10 comprises the piezoelectric layer 13, the interleave layer 12, and the YAG layer 11. Thereby, the interleave layer 12 is arranged directly on the YAG layer 11, and the piezoelectric layer 13 is arranged directly on the interleave layer 12. The piezoelectric layer 13 allows to propagate a SAW. The YAG layer 13 reduces the spurious off-band-modes of the SAW device (i.e., reduces losses from the piezoelectric layer 13, for example, into a substrate of the SAW device 10). The interleave layer 12 further provides RF isolation between the piezoelectric layer 13 and the YAG layer 11 (and optionally a substrate of the SAW device 10).

The SAW device 10 further includes two or more electrodes 14, which are arranged directly on or above the piezoelectric layer 13 (i.e., it is possible that there is some further layer arranged between the piezoelectric layer 13 and the electrodes 13). The two or more electrodes 14 form an interdigital transducer (IDT) configured to convert an electrical signal to a SAW propagating in the piezoelectric layer 13. That is, the electrical signal provided via the IDT may excite the piezoelectric layer 13, resulting in the propagating SAW. For instance, as illustrated in FIG. 1, the two or more electrodes 14 may comprise a first electrode 14 and a second electrode 14. These two electrodes 14 may form a resonator region together with the region of the piezoelectric layer 13 located between the electrodes 14. The SAW may then propagate between the two electrodes in the piezoelectric layer 13, i.e. in the resonator region.

The SAW device 10 of FIG. 1 may, in particular, comprise at least one TF-SAW resonator region (if the piezoelectric layer 13 is a thin film layer) on a stack of other layers. In FIG. 1, there is shown only one interleave layer 12 between the piezoelectric layer 13 and the YAG layer 11. This interleave layer 12 can be a silicon oxide (SiCL) layer. As described below, multiple interleave sublayers may be possible as well. The YAG layer 11 may be a YAG substrate layer, or may be provided on a further substrate layer (not shown in FIG. 1).

FIG. 2 shows a SAW device 10 according to an embodiment, which builds on the embodiment shown in FIG. 1. Same elements in FIG. 1 and 2 are labelled with the same reference signs, and may be implemented likewise. Also the SAW device 10 of FIG. 2 comprises the YAG layer 11, the interleave layer 12, the piezoelectric layer 13, and the two or more electrodes 14 forming the IDT.

In the SAW device 10 of FIG. 2, the YAG layer 11 is specifically provided on a substrate layer 20 of the SAW device 10, wherein the substrate layer 20 may be a silicon layer. For example, the substrate layer 20 may be a (111) silicon layer or a (100) silicon layer. Further, the SAW device 10 comprises a passivation layer 21, which surrounds and covers the two or more electrodes 14. This may protect the electrodes 14, and may positively influence the boundary conditions of the piezoelectric layer 13. The passivation layer 21 may comprise silicon oxide (SiCh) or silicon nitride (SisNf). The passivation layer 21 can be deposited over the metal IDT electrodes 14. The piezoelectric layer 13 may be an LT and/or LN layer. The interleave layer 12 may be a silicon oxide layer.

It is also shown in FIG. 2, that the two or more electrodes 14 are periodically arranged on or above the piezoelectric layer 13 with a pitch (p). For instance, if the piezoelectric layer 13 is an LT or LN layer, this pitch may be along the X-direction of the LT layer or of the LN layer, respectively. Further, each of the at least two electrodes 104 may thereby extend orthogonal to the X-direction of the LT layer or the LN layer, respectively. Notably, the unit cell lOu may have a width of two times the pitch (2p), as illustrated. FIG. 3 shows a SAW device 10 according to an embodiment, which builds on the embodiment shown in FIG. 1 and 2. Same elements in FIG. 1, 2 and 3 are labelled with the same reference signs, and may be implemented likewise. Also the SAW device 10 of FIG. 3 comprises the YAG layer 11, the interleave layer 12, the piezoelectric layer 13, and the two or more electrodes 14 forming the IDT.

In the SAW device 10 of FIG. 3, the YAG layer 11 is provided by a substrate layer 20, i.e., it is a YAG substrate layer. The YAG substrate layer may be thicker than the YAG layer 11 of the SAW device 10 shown in FIG. 2. It is, however, possible also in this embodiment of FIG.

3, that the YAG layer 11 is provided on a substrate layer 20 (e.g., a silicon layer) as shown in FIG. 2.

The SAW device 10 of FIG. 3 also includes an additional interleave layer 31, which is arranged on the piezoelectric layer 13. In particular, the additional interleave layer 31 is arranged between the piezoelectric layer 13 and the two or more electrodes 14. For example, the two or more electrodes 14 may be arranged directly on the additional interleave layer 31 and thus above the piezoelectric layer 13. The additional interleave layer 31 may comprise a silicon nitride layer (SisN4), or a silicon oxide layer (SiCh), or a hafnium oxide (HfO) layer.

Further, in the SAW device 10 of FIG. 3, the interleave layer 12 comprises a first sublayer 12a and a second sublayer 12b. In particular, the interleave layer 12 may comprise a polycrystalline silicon layer 12a and a silicon oxide layer 12b arranged on the poly crystalline silicon layer 12a. The poly crystalline silicon layer 12a may reduce spurious modes, while the silicon oxide layer 12b provides RF isolation.

FIG. 4 shows a SAW device 10 according to an embodiment, which builds on the embodiment shown in FIG. 1-3. Same elements in FIG. 1-3 and FIG. 4 are labelled with the same reference signs, and may be implemented likewise. Also the SAW device 10 of FIG. 4 comprises the YAG layer 11, the interleave layer 12, the piezoelectric layer 13, and the two or more electrodes 14 forming the IDT. In the SAW device 10 of FIG. 4, the YAG layer 11 is again provided on the substrate layer 20 (e.g., a silicon or silicon-based substrate layer), but may be also provided by the substrate layer 20 as shown in FIG. 3.

Further, in the SAW device 10 of FIG. 4, the additional interleave layer 31 is an incoherent layer arranged on the piezoelectric layer 13, i.e., it is separated into two or more layer parts 31a, 31b, which are each arranged on the piezoelectric layer 13. Thereby, each of the two or more layer parts 3 la, 3 lb is arranged between the piezoelectric layer 13 and one of the two or more electrodes 14. This implementation of the incoherent layer is in contrast to the implementation shown in FIG. 3, where the additional interleave layer 31 may be a coherent layer arranged on the piezoelectric layer 13. At least, in FIG. 3, it is coherent between the first and the second electrode 14.

In all SAW devices 10 of FIG. 1-4, the piezoelectric layer 13 may be, or comprise, a LT layer, LN layer, or AIN layer. The LN layer may be a rotated YX-cut LN layer having a rotation angle in a range of 115° to 135°, for example a rotation angle of 120° or 128°. The LT layer may a rotated YX-cut LT layer having a rotation angle in a range of 18° to 65°, for example, a rotation angle of 25°, 36°, 42° or 50°.

The rotated YX-cut LT layer and LN layer may, respectively, be defined by a set of Euler angles. In this context, FIG. 5 shows an Euler angles convention for a rotated crystal axis. In particular, FIG. 5 illustrates a set of standard Euler angles (X, p, 9), 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. 5(b)) are aligned to the required crystal orientations, as defined by three successive rotations (see FIG. 5(a)) of the basis vectors. The thickness of the layer is then defined in the z ⁽³⁾ direction.

In particular, the LT layer of the SAW device 10 may be defined by a first set of Euler angles ( i, pi, 9i), wherein i is in a range of -3° to +3°, pi is in a range defined by the rotation angle of the LT layer minus 90°±5°, and 9i is in a range of -3° to +3°. The LN layer of the SAW device 10 may defined by a second set of Euler angles ( 2, p2, 92), wherein 2 is in a range of - 3° to +3°, p2 is in a range defined by the rotation angle of the LN layer minus 90°±5°, and 92 is in a range of -3° to +3°. FIG. 6 shows simulations for a multi-layer stack of a SAW device 10 according to an embodiment, wherein the piezoelectric layer 13 comprises a 25° YX-cut LT layer. In particular, FIG. 6 shows how a multilayer stack with the 25° YX-cut LT layer may be optimized for out- of-band spurious mode suppression (wherein the stack is exemplarily LT/SiCL/YAG). FIG. 6(a) shows an effective coupling coefficient k _t of the layer stack, while FIG. 6(b) shows a phase velocity at resonance (in m/s). Further, FIG. 6(a) and 6(b) show specifically a dependence of the effective coupling coefficient and the phase velocity at resonance, respectively, on the thickness of the interleave layer 12. Notably, a thickness of the interleave layer 12 (referred to as h _ox ) may be in a range of 0.0 - 0.6 times a wavelength (referred to as X) of the SAW at an operating frequency of the SAW device 10, for example, in a range of 0.05 - 0.38 times the wavelength, if the piezoelectric layer 13 comprises an LT layer. Or it may be in a range of 0.43 - 0.6 times the wavelength, if the piezoelectric layer 13 comprises an LN layer.

FIG. 7 shows simulations for a multi-layer stack of a SAW device 10 according to an embodiment, wherein the piezoelectric layer comprises a 36° YX-cut LT layer. Similar to FIG. 6, the FIG. 7 shows how a multilayer stack with the 36° YX-cut LT layer can be optimized for out-of-band spurious mode suppression (wherein the stack is exemplarily LT/SiO2/YAG).

FIG. 8 shows simulations for a multi-layer stack of a SAW device 10 according to an embodiment, wherein the piezoelectric layer 13 comprises a 42° YX-cut LT layer. Similar to FIG. 6, the FIG. 8 shows how a multilayer stack with the 42°YX-cut LT layer can be optimized for out-of-band spurious mode suppression (wherein the stack is exemplarily LT/SiCL/YAG).

FIG. 9 shows simulations for a multi-layer stack of a SAW device 10 according to an embodiment, wherein the piezoelectric layer 13 comprises a 50° YX-cut LT layer. Similar to FIG. 6, the FIG. 8 shows how a multilayer stack with the 50° YX-cut LT layer can be optimized for out-of-band spurious mode suppression (wherein the stack is exemplarily LT/SiCL/YAG).

Fig. 10 shows a typical admittance response in a 42° YX-cut LT/SiCL/YAG multilayer stack of a SAW device 10 according to an embodiment. That is, the YAG layer 11 is provided by a substrate layer 20, i.e., is a YAG substrate. The shown admittance responses include a real part of the admittance. For achieving such, or a similar, admittance response, a thickness of the LT layer of the SAW device 10 may be in a range of 0.1 - 0.4 times a wavelength of the SAW at an operating frequency of the SAW device 10, for example, in a range of 0.18 - 0.38 times the wavelength. Further, a thickness of the interleave layer 12 (silicon oxide) may be in a range of 0.0 - 0.6 times the wavelength, for example, in a range of 0.05 - 0.38 times the wavelength. The particular curve of the admittance response that is shown, is obtained for a thickness of the LT layer of 0.28 times the wavelength, and for a thickness of the interleave layer 12 of 0.23 times the wavelength.

FIG. 11 shows a typical admittance response in a 42°YX-cut LT/SiCL/YAG/Si stack of a SAW device 10 according to an embodiment. That is, the YAG layer 11 is provided on a silicon substrate layer 20. Different substrate orientations are compared in FIG. 11. That is, FIG. 11(a) is for a SAW device 10, wherein the substrate layer 20 is a (111) silicon layer, and FIG. 11(b) is for a SAW device 10, wherein the substrate layer 20 is a (100) silicon layer. The YAG layer 11 is arranged on the (111) silicon layer or the (100) silicon layer, respectively. Thereby, if the YAG layer 11 is arranged on the (111) silicon layer, an angle between the [110]-direction of the (111) silicon layer and the X-direction of the LT layer or LN layer, respectively, may be in a range of -30° to 30° or in a range of 60° to 90°.

For achieving such, or similar, admittance responses, a thickness of the piezoelectric layer 13 may be in a range of 0.1 - 0.3 times a wavelength of the SAW at an operating frequency of the SAW device 10, a thickness of the interleave layer 12 may be in a range of 0.1 - 0.3 times the wavelength, and a thickness of the YAG layer 11 may be in a range of 1.0 - 3.0 times the wavelength.

FIG. 12 shows a method 120 according to an embodiment of the disclosure. The method 120 is suitable for fabricating a SAW device 10 as shown in any of FIG. 1-4. The method 120 includes a step 121 of providing a substrate layer 20, for example, a silicon layer. Further, it includes a step 123 of providing a piezoelectric layer 13, for example, an LT layer or LN layer.

The method 120 also includes a step of bonding 124 the substrate layer 20 and the piezoelectric layer 13, wherein one or more interleave layers 12 are arranged between the substrate layer 20 and the piezoelectric layer 13. Thus, the method 120 also comprises a step 122 of providing the one or more interleave layers 12. The method 120 may comprise providing 122 the one or more interleave layers 12 onto at least one of the substrate layer 20 and the piezoelectric layer 13, and then bonding these together. For instance, wafer to wafer bonding may be used. The method 120 may also comprise a step of bonding the piezoelectric layer 13 to the substrate layer 20 by using the set of interleave layers 12, or wherein the interleave layers 12 are formed.

Either the substrate layer 20 or one of the interleave layers 12 comprises a YAG layer 11. That is the YAG layer 11 is provided by or on the substrate layer 12. If the YAG layer 11 is one of the interleave layers 12, then there are at least two interleave layers 12.

Further, the method 120 includes a step 125 of forming two or more electrodes 14 on the piezoelectric layer 13. The electrodes 14 form an IDT configured to convert an electrical signal to a SAW propagating in the piezoelectric layer 13.

FIG. 13 shows a table with parameters for obtaining optimized layer stacks for SAW devices 10 according to embodiments, in particular, for achieving zero temperature coefficient of frequency (TCF) at resonance (TCFA) and anti-resonance (TCFR). For each case, different examples of layer stacks - referred to as “Stack 1”, “Stack 2”, “Stack 3”, “Stack 4”, and “Stack 5” - are provided. Thereby, the table provides exemplarily parameter sets for the case of a 42° YX-cut LT/SiCh/YAG multilayer stack. That is, the piezoelectric layer 13 is the 42° LT layer, the interleave layer 12 is the silicon oxide layer, and the YAG layer 11 may be a YAG substrate. The table also indicates a pitch and mark-to-pitch for providing the two or more electrodes 14 forming the IDT.

In all of the above embodiments of the SAW device 10, the substrate layer 20 (if it is not the YAG layer 11) may be one of silicon, glass, ceramic, and the like, which can include silicon, a SOI technology substrate, gallium arsenide, gallium phosphide, gallium nitride, and/or indium phosphide or other example substrate, an alloy semiconductor including GaAsP, AlInAs, GalnAs, GalnP, or GalnAsP or combinations thereof.

The two or more electrodes 14 may each be formed by a metal and/or metal alloy layer such as copper, titanium, and the like, or may be a highly doped silicon layer. The two or more electrodes 14 may particularly comprise aluminum electrodes, or copper electrodes, or tungsten electrodes, or titanium electrodes, or electrodes made from an aluminum-copper-alloy, or electrodes made from a copper-aluminum-alloy. 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 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.