WO2023236332A1

WO2023236332A1 - Plate mode resonator device

Info

Publication number: WO2023236332A1
Application number: PCT/CN2022/109290
Authority: WO
Inventors: Viktor Plesski; Naiqing Zhang
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2022-06-07
Filing date: 2022-07-30
Publication date: 2023-12-14

Abstract

An acoustic resonator device (1) comprising a substrate (2) and a piezoelectric layer (3), such as LN membrane, sandwiched between a floating potential bottom electrode (6) and pairs of top electrodes (7) to which RF voltage is applied, wherein a plurality of pedestals (8) are arranged periodically between the piezoelectric layer (3) and the substrate (2) to provide mechanical stability and heat evacuation. The pedestals (8) may be arranged to align with nods (12) of acoustic amplitudes of the piezoelectric layer (3) located in regions between the top electrodes to minimize acoustic energy leaking through pedestals (8). The substrate (2) may comprise a first substrate layer (15) of high acoustic wave velocity and high thermal conductivity such as diamond, supported on a second substrate layer (16) of a wafer material.

Description

PLATE MODE RESONATOR DEVICE

TECHNICAL FIELD

The present disclosure relates to an acoustic resonator device, and specifically to a thin-film bulk acoustic resonator device

BACKGROUND

Devices exploiting crystalline membranes (often of Lithium Niobate, LN) membranes of sub-micron thickness, such as bulk acoustic resonators (XBARs or YBARs) have significant drawbacks. Fabrication process required for such devices is difficult as the bottom side of the membrane must be open. The bulk acoustic resonators also need to be suspended over cavity for acoustic isolation. The devices are fragile as the membranes are very thin and have poor power-handling properties due to the low thermal conductivity of the thin membrane.

Therefore, a technical solution is necessary for resonator devices similar to classic surface acoustic wave (SAW) resonators having a solid support and heat evacuation path below the piezoelectric membrane, which is suitable for operation at 5 GHz frequency range with comparably high piezo-coupling and a critical dimension of electrodes CD > 0.3 μm for optical lithography.

A thin-film bulk acoustic resonator (FBAR) is a bulk wave resonator. If such a resonator is placed on some substrate with complete mechanic contact, it will not work, because the acoustic energy will be radiated into the substrate. Therefore, no resonance (or with very low Q-factor) can be obtained. To prevent this, in FBARs the membrane is either suspended over cavity or solidly attached to a Bragg stack, which is a system of many λ/4 layers alternating high and low acoustic impedances. This multilayer Bragg stack serves as a strong reflector not allowing the bulk waves to be radiated into substrate.

In YBAR devices the same problems remain, namely that the membrane suspended over cavity and of sub-micron thickness is very fragile, and that the crystalline membrane (LN) has low thermal conductivity and thus the device has poor power handling. Furthermore, the fabrication process of a Bragg stack is very complicated.

SUMMARY

It is an object to provide an improved acoustic resonator device which overcomes or at least reduces the problems mentioned above.

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

According to a first aspect, an acoustic resonator device is proposed in which a piezoelectric layer (such as LN membrane) sandwiched between pairs of top electrodes and a floating bottom electrode is attached by pedestals to a substrate to provide a spacing between the substrate and the piezoelectric layer. The bottom electrode area is limited to overlapping of the top electrodes.

This arrangement offers a solid mechanical support for the piezoelectric layer, improves the power handling capability due to improved heat dissipation through the spacing, and provides a sufficiently high piezo-coupling. The device according to the disclosure solves the problem of fragility of existing YBAR resonators using a submicron-thick membranes by attaching the membrane, i.e. the piezoelectric layer, through pedestals to a solid thick substrate.

In a possible implementation form of the first aspect the bottom electrode is provided with a floating potential, and the piezoelectric layer comprises a plurality of top electrodes arranged on the top surface. The adjacent top electrodes of the plurality of top electrodes are arranged with opposite polarities and with a pitch p defined by a regular distance between the centers of the adjacent top electrodes.

This provides an acoustic resonator arrangement that is scalable in multiple directions and can be implemented in devices of different dimensions as needed. This arrangement also offers the possibility to change or tune resonance frequency by changing the pitch and the electrode geometry.

In a further possible implementation form of the first aspect the pitch p is dimensioned to satisfy the condition of p< V _diam/V _LN*d _LN, wherein V _diam corresponds to the transverse acoustic velocity in the substrate, V _LN corresponds to the transverse acoustic velocity in the piezoelectric layer, and d _LN corresponds to the thickness of the piezoelectric layer. This arrangement enables to operate with minimal loss at an operation frequency Fr of the resonator, due to the absence of the bulk wave radiation into the substrate, and hence high Q factor of the resonator.

In a further possible implementation form of the first aspect the plurality of top electrodes comprises a periodic system of interdigitated electrodes, IDEs, arranged on the top surface and alternately connected to busbars, which arrangement enables applying AC voltage to the electrodes.

In an embodiment the busbars are arranged on the top surface. In another embodiment the busbars are arranged directly on the substrate, the bottom electrode area being available only in front of the overlapping area of the top electrodes.

In an embodiment the busbars comprise two layers made of different metal material.

In an embodiment a dielectric layer is arranged between the piezoelectric layer and the busbars, the dielectric layer comprising SiO2, or SiN.

In a further possible implementation form of the first aspect the piezoelectric layer comprises grooves arranged in regions of the piezoelectric layer not covered by top electrodes, enabling the top electrodes to vibrate more freely, increasing the piezo-coupling.

In different embodiments the groove depth can be ranging from zero to the entire thickness of the piezoelectric layer for maximum coupling of the resonator. In an embodiment the groove depth is more than 1/3 of the thickness of the piezoelectric layer.

In a further possible implementation form of the first aspect the piezoelectric layer comprises piezoelectric material arranged intermittently sandwiched between the plurality of pairs of top electrodes and the bottom electrode, the grooves extending the entire width and depth of the piezoelectric layer between the top electrodes.

In a further possible implementation form of the first aspect the plurality of pedestals are arranged to align with nods of acoustic amplitudes of the piezoelectric layer to minimize acoustic energy leaking through pedestals, the nods being located in regions of the piezoelectric layer not covered by a top electrode.

In a further possible implementation form of the first aspect the plurality of pedestals are arranged periodically between the piezoelectric layer and the substrate to provide sufficient mechanical stability while enabling heat evacuation.

In a further possible implementation form of the first aspect the plurality of pedestals are integral with or made from the same material as the substrate, and are formed as ridges between grooves in the substrate, thus increasing structural integrity and also simplifying the manufacturing process.

In a further possible implementation form of the first aspect the plurality of pedestals are at least partially embedded into recesses arranged on the bottom surface of the piezoelectric layer, which further increases structural integrity of the acoustic resonator device.

In possible embodiments the piezoelectric layer comprises Lithium Niobate, LN, or Lithium Tantalate, LT, AlN, AlScN, or any other suitable piezoelectric material.

In a further possible implementation form of the first aspect the piezoelectric layer is arranged with a rotated Y-cut, X-cut 30° Y-propagation, X-cut, or other suitable crystal orientation providing large coupling of piezoelectric materials.

In possible embodiments at least one of the bottom electrodes and the top electrodes are made of Al, Cu, Mo, Pt, W, Au, or their alloys.

In possible embodiments the plurality of pedestals comprise a material of high thermal conductivity, such as Al, Cu, Au, Si, SiO2, SiC, BN, or Si ₃N ₄. Pedestals with such high thermal conductivity offer good heat evacuation performance.

In a further possible implementation form of the first aspect the piezoelectric layer is mechanically attached to the substrate at its distal ends, to make the structure of the acoustic resonator device more robust.

In a further possible implementation form of the first aspect the substrate comprises a first substrate layer comprising a material of high acoustic wave velocity and high thermal conductivity such as diamond, SiC, BN, or Si. High thermal conductivity of the substrate enables excellent power handling capabilities. The problem of overheating and low power handling of the resonator device is solved by the heat evacuation through the pedestal to substrate in direction perpendicular to the membrane, while in YBAR devices the heat is mainly evacuated along the electrodes. Bulk acoustic wave radiation is avoided and Q factor of the resonator is also improved by using a high acoustic velocity substrate.

In a further possible implementation form of the first aspect the first substrate layer is arranged with a thickness d _S1 larger than the thickness of the piezoelectric layer to avoid bulk acoustic radiation and improve Q-factor of the resonator device.

In an embodiment the thickness of the first substrate layer is 4 to 20 times that of the thickness of the piezoelectric layer. The first substrate layer being so thick in relation to the piezoelectric layer enables bulk waves to cancel each other due to destructive interference of waves with opposite phase.

In a further possible implementation form of the first aspect the substrate comprises a second substrate layer arranged on a side of the first substrate layer opposite of the pedestals for supporting the first substrate layer, the second substrate layer preferably being arranged with a thickness d _S2 larger than the thickness of the first substrate layer d _S1, the second substrate layer comprising a wafer material such as Si, SiC, sapphire, quartz, or glass.

According to a second aspect, there is provided a method for manufacturing an acoustic resonator device comprising the steps of:

forming a substrate comprising a first substrate layer comprising a material of high acoustic wave velocity and high thermal conductivity and a supporting second substrate layer;

forming a plurality of pedestals on the substrate;

forming a second substrate comprising a piezoelectric layer with a bottom surface and a top surface arranged on a support layer, preferably a Si wafer;

providing a bottom electrode on the bottom surface of the piezoelectric layer;

wafer-to-wafer bonding of the bottom electrode;

eliminating the support layer of the second substrate; and

providing a plurality of pairs of top electrodes on the top surface of the piezoelectric layer.

Forming the acoustic resonator device according to these steps provides a sufficiently robust acoustic resonator, with great power handling capability due to the heat dissipation through the spacing, and provides a sufficiently high piezo-coupling, while also enabling a relatively simple and scalable manufacturing process. The method further allows usual SAW technology to be used with 5 GHz frequency range, which may be manufactured using optical lithography.

These and other aspects will be apparent from and the embodiment (s) described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the aspects, embodiments and implementations will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

Fig. 1 shows a cross-section of an acoustic resonator device in accordance with an example of the embodiments of the disclosure;

Fig. 2 shows a cross-section of an acoustic resonator device in accordance with another example of the embodiments of the disclosure;

Fig. 3 illustrates resonance vibration mode (SH1) in direction Y of the acoustic resonator device in accordance with another example of the embodiments of the disclosure;

Fig. 4 shows a cross-section of an acoustic resonator device in accordance with another example of the embodiments of the disclosure;

Fig. 5 shows a cross-section of an acoustic resonator device in accordance with another example of the embodiments of the disclosure;

Figs. 6A and 6B illustrate the effect of different piezoelectric membrane crystal orientations on piezo-coupling of an acoustic resonator device in accordance with another example of the embodiments of the disclosure;

Fig. 7 illustrates the effect of groove depth on piezo-coupling in different embodiments of acoustic resonator devices in accordance with examples of the disclosure;

Fig. 8 illustrates the effect of pitch on resonance frequency in different embodiments of acoustic resonator devices in accordance with examples of the disclosure;

Figs. 9A and 9B show 2D and 3D simulation result of an optimized configuration of an acoustic resonator device for practical optical lithography in accordance with an example of the disclosure.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant disclosure. However, it should be apparent to those skilled in the art that the present disclosure may be practiced without such details. In other instances, well known methods, procedures, systems, components, and/or circuitry have been described at a relatively high-level, without detail, in order to avoid unnecessarily obscuring aspects of the present disclosure.

The proposed structure for an acoustic resonator device 1 is multilayered and periodic.

Figure 1 illustrates one period of the structure of the acoustic resonator device 1, which may be multiplied for a device 1, for example, to have hundreds of such periods of the structure, as illustrated in a partial section shown in Fig. 2.

The acoustic resonator device 1 comprises a substrate 2 and a resonator layer which is a piezoelectric layer 3 made from lithium niobate (LN) , Lithium tantalate (LT) , AlN, AlScN, or any other suitable piezoelectric material, and comprises a bottom surface 4 and a top surface 5. The piezoelectric layer 3 may be arranged with a rotated Y-cut, X-cut 30° Y-propagation, X-cut, or other suitable crystal orientation providing large coupling piezo module, k ₃₄ ², of piezoelectric materials.

A bottom electrode 6 of a floating potential is provided on the bottom surface 4, and at least one pair of top electrodes 7 are provided on the top surface 5, the bottom electrode 6 and/or the top electrodes 7 being made of Al, Cu, Mo, Pt, W, Au, or their alloys.

The bottom electrode preferably covers the area opposite top electrodes, where they overlap in aperture direction (y) . The bottom electrode does not extend under the busbars.

Between the bottom electrode 6 and the substrate 2 a plurality of pedestals 8 are further arranged to provide a spacing between the piezoelectric layer 3 and the substrate 2. The pedestals 8 may be made from metal with high thermal conductivity, such as Al, Cu, Au, or dielectrics Si, SiO2, SiC, BN, or Si ₃N ₄. The pedestals 8 are placed periodically between the bottom surface 4 of the piezoelectric layer 3 and an upper layer of the substrate 2, providing the mechanical stability and good heat evacuation from the piezoelectric layer 3 to the substrate 2.

The substrate 2 comprises a first substrate layer 15 from a material of high acoustic wave velocity and high thermal conductivity such as diamond, or other materials which are mechanically close to diamond (high velocity of all bulk acoustic waves) . Such materials may be, for example, SiC (silicone carbide) or BN (boron nitride) or Si. The first substrate layer 15 is supported on a second substrate layer 16 of a wafer material such as Si, SiC, sapphire, quartz, or glass.

The acoustic velocity of the substrate 2 may be, for example, twice the acoustic velocity of the piezoelectric layer 3.

As shown in Fig. 2, a plurality of pairs of top electrodes 7 can be provided on the top surface 5, the adjacent top electrodes 7 of the plurality of top electrodes 7 being arranged with opposite polarities and with a pitch p. The pitch is defines as a regular distance between the centers of the adjacent top electrodes 7.

The pitch p can be dimensioned as approximately p < (2.4-3.0) d _LN where d _LN corresponds to the thickness of the piezoelectric layer 3, if such high acoustic velocity material as diamond is used. These dimensions enable avoiding bulk acoustic wave radiation into the bottom substrate.

The piezoelectric layer 3 can be dimensioned with a thickness of approximately d _LN= λ _LN/2. The main resonance frequency of SH1 mode is based on d _LN and pitch p with f _r= ( (v _t1/2d _LN) ²+ (v _t2/2p) ²) ^1/2, where v _t is the transverse acoustic velocity in the piezoelectric layer 3, and, thus, the resonance frequency can be controlled to some extend by the pitch p.

The piezoelectric layer 3 in an embodiment is an LN membrane of Y-cut in which transverse (vertical electric field) excites shear acoustic waves with displacements perpendicular to the screen. The resonance frequency corresponds to the membrane thickness of the piezoelectric layer 3 equal to λ/2 –half of wavelength. The main resonance mode is similar to SH1 plate mode.

The plurality of top electrodes 7 may be arranged as a periodic system of interdigitated electrodes (IDEs) , arranged on the top surface 5 (Fig. 1) and alternately connected to busbars arranged for applying AC voltage. In an embodiment the busbars are arranged on the top surface 5. In another embodiment the busbars are arranged directly on the substrate 2. In an embodiment the busbars comprise two layers made of different metal material.

In an embodiment a dielectric layer is arranged between the piezoelectric layer 3 and the busbars, the dielectric layer comprising SiO2, or SiN.

The bottom electrode 6 is a metal layer that serves as floating potential for the YBAR structure, offering vertical electric field with the top electrodes 7. The metal may be Al, with relatively small thickness, for example 50 nm. Large thickness may trap more acoustic energy in passive metal thus reducing piezo-coupling. If many such resonators are used in a “ladder” filter the bottom metal electrodes 6 of each resonator must be isolated, not connected. The individual small resonators created by top electrodes 7 and the bottom electrode 6 are connected in series through bottom electrode 6. The top electrodes 7 can have alternating potential V, 0, V, 0, and so on. In this case the floating bottom electrode 6 is not connected to any external voltage source and will get potential close to V/2, being influenced by parasitic capacitances to busbars and other conductors.

The first substrate layer 15 can be a relatively thin layer, for example having a thickness d _S1 only 4-20 times thicker than the thickness d _LN of the piezoelectric layer 3. However, the first substrate layer 15 needs to be sufficiently thick in relation to the piezoelectric layer 3 such that bulk waves generated into the substrate by each section cancel each other due to destructive interference of waves with opposite phase. The second substrate layer 16 may be arranged with a thickness d _S2 larger than the thickness d _S1 of the first substrate layer 15 and serves only for mechanical support of the top layers and for the heat evacuation. In the layer 16 no acoustic waves propagate near resonance and antiresonance frequencies of the resonator.

The pitch p is dimensioned to provide minimal loss at an operation frequency Fr of the resonator by satisfying the condition of p< (V _diam/V _LN*d _LN) , wherein V _diam corresponds to the transverse acoustic velocity in the substrate 2, V _LN corresponds to the transverse acoustic velocity in the piezoelectric layer 3, and d _LN corresponds to the thickness of the piezoelectric layer 3.

The velocity of sound V _SH1 may be around 4000 m/sin LN, and as high as 9600 m/sin a high-sound-velocity substrate. Hence, in the case of diamond substrate the pitch needs to be around 2.4 times smaller than the piezoelectric layer 3 thickness d _LN. This allows to go to 5GHz with pitch p<1.2 μm suitable for manufacturing with optical lithography.

The relatively small pitch (for comparison, XBARs may have about 20 times larger pitch) results in low level of parasitic modes. The arrangement also provides possibilities to change or tune resonance frequency by changing the pitch and electrode geometry.

Fig. 3 shows the amplitude distribution of vibration displacements in the piezoelectric layer 3 and the

electrodes

6, 7 of the main resonance mode, SH1 mode, at resonance frequency. SH1 mode has main vibration displacement with direction Y (perpendicular to the screen in Fig. 3) due to the vertical electric field created by the top and bottom electrodes of the structure and the choice of the cut of piezoelectric layer 3 with high value of piezo-module e34. The pedestals 8 serve as solid support and are not involved (or minimally involved) in the resonance vibration. High-velocity substrate 2 avoids bulk wave radiation and energy leakage downwards. Main acoustic energy is trapped in the piezoelectric layer 3, providing high Q factor of the resonator.

As shown in Fig. 3, the plurality of pedestals 8 are arranged to align with nods 12 of acoustic amplitudes of the piezoelectric layer 3 to minimize acoustic energy leaking through pedestals 8, the nods 12 being located in regions of the piezoelectric layer 3 not covered by a top electrode.

As shown in Fig. 4, the piezoelectric layer 3 may be arranged with grooves 11 in regions of the piezoelectric layer 3 not covered by top electrodes 7, enabling the top electrodes 7 to vibrate more freely, increasing the piezo-coupling. In different embodiments the groove depth can be ranging from zero to the entire thickness of the piezoelectric layer 3 (as in Fig. 4) for achieving maximum piezo-coupling of the resonator. In a preferred embodiment the groove 11 depth is more than 1/3 of the thickness of the piezoelectric layer 3. In the illustrated example, the piezoelectric layer 3 comprises piezoelectric material arranged intermittently sandwiched between the plurality of pairs of top electrodes 7 and the bottom electrode 6, the grooves 11 extending the entire width and depth of the piezoelectric layer 3 between the top electrodes 7.

As shown in Fig. 5, the plurality of pedestals 8 may be integral with or made from the same material as the substrate 2, and may be formed as ridges between grooves 11 in the substrate 2. In some embodiments, as shown in Fig. 5 as well, the plurality of pedestals 8 are at least partially embedded into recesses 14 arranged on the bottom surface 4 of the piezoelectric layer 3. The sections of the bottom electrode must remain electrically connected in this case.

In an embodiment (not shown) , the piezoelectric layer 3 may also be directly connected to the substrate 2 at its distal ends.

Figures 6A and 6B illustrate admittance curves of the resonator piezoelectric layer’s 3 crystal orientation effect on piezo-coupling on a logarithmic scale (log10 (abs (Y) ) and log10 (real (Y) ) ) for one period (arbitrary shift on vertical axis) . X-cut 30° with respect to Y _crys propagation LN offers larger piezo-coupling based on the structure and resonance mode (SH1) .

The plot of 6A indicates that common Y-cut LN (Euler angles [0 90 0] ) provides resonance frequency at 6.03 GHz and anti-resonance frequency at 6.78 GHz with piezo-coupling of 24.52%; while the plot of 6B indicates that the optimal X-cut, 30°Ypropagation LN (Euler angles [90 90 30] ) cut provides resonance frequency at 5.94 GHz and anti-resonance frequency at 7.02 GHz with piezo-coupling of 32.89%.

Fig. 7 illustrates the effect of groove trench depth of the piezoelectric layer 3 on resonance piezo-coupling, with thickness d _LN=300 nm. Grooved trenches can be used for larger piezo-coupling of the structure, as depicted also in Fig. 4. The grooves enable the top electrodes 7 and piezoelectric layer 3 underneath to vibrate more freely, hence increasing amplitude of vibration and the piezo-coupling.

Fig. 7 also indicates that as long as the groove depth is more than 1/3 of the piezoelectric layer 3 thickness d _LN, it gives good piezo-coupling performance for above 40%. With the trend of larger groove depth for higher piezo-coupling, maximum piezo-coupling of 45%can be achieved.

Fig. 8 illustrates the effect of pitch on resonance frequency of the structure. The plot indicates that the larger the pitch, the lower the resonance and anti-resonance frequency. Although the resonance frequency is mainly based on the thickness d _LN of the piezoelectric layer 3, with λ ～ 2d _LN, it also changes based on pitch p. With constant thickness of the piezoelectric layer 3, the resonance frequency may be tuned by changing the pitch p or by changing geometry of the top electrodes 7, for example, by changing the width of the top electrodes 7 or by constructing the top electrodes 7 with trapezoidal cross-section shape.

As further shown in Figs. 9A and 9B, large coupling (41%) can be achieved parasitic-mode-free, in a high-frequency (4.5-6 GHz) configuration with critical dimension CD > 320 nm for practical optical lithography with optimized configuration parameters, confirmed by both 2D and 3D FEM-simulation. It implies the validity and correctness of the model to further practical fabrication.

The exemplary model parameters used for the simulations are:

- Pitch: 0.8um

- Piezoelectric layer (film) thickness: 325nm

- Pedestal height: 300nm

- Top electrode (Al IDT) thickness: 50nm

- Bottom electrode (Al) thickness: 40nm

- IDT metallization ratio (MR) : 0.4

- Pedestal metallization ratio: 0.4

- X-cut 30° Y-propagating LN

- Groove depth 275nm

An exemplary method for manufacturing an acoustic resonator device 1 as depicted in the figures above may comprise a first step of forming a substrate 2 with a first substrate layer 15 of a material of high acoustic wave velocity and high thermal conductivity such as diamond, on a supporting second substrate layer 16 (wafer) .

Following this, a plurality of pedestals 8 are formed on the substrate 2.

A second (temporary) substrate comprising a piezoelectric layer 3 with a bottom surface 4 and a top surface 5 is than arranged on a support layer, preferably a Si wafer, wherein the temporary substrate is attached to the top surface 5 of the piezoelectric layer 3.

A bottom electrode 6 is provided on the bottom surface 4 of the piezoelectric layer 3, followed by wafer-to-wafer bonding of the bottom electrode 6 to the electrode structure 8.

The support layer of the second substrate is then eliminated; followed by providing a plurality of pairs of top electrodes 7 on the top surface 5 of the piezoelectric layer 3.

It will be understood that when a unit, module or engine is referred to as being “on” , “connected to” or “coupled to” another unit, module, or engine, it may be directly on, connected or coupled to, or communicate with the other unit, module, or engine, or an intervening unit, module, or engine may be present, unless the context clearly indicates otherwise. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

The various aspects and implementations have been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

The reference signs used in the claims shall not be construed as limiting the scope.

Claims

An acoustic resonator device (1) comprising:

a substrate (2) ;

a piezoelectric layer (3) comprising a bottom surface (4) and a top surface (5) , a bottom electrode (6) provided on the bottom surface (4) and

at least one pair of top electrodes (7) provided on the top surface (5) ; and

a plurality of pedestals (8) arranged between the bottom electrode (6) and the substrate (2) to provide a spacing between the piezoelectric layer (3) and the substrate (2) .
The acoustic resonator device (1) according to claim 1, wherein the bottom electrode (6) is provided with a floating potential, and wherein the piezoelectric layer (3) comprises a plurality of top electrodes (7) arranged on the top surface (5) , the adjacent top electrodes (7) of the plurality of top electrodes (7) being arranged with opposite polarities and with a pitch p defined by a regular distance between the centers of the adjacent top electrodes (7) .
The acoustic resonator device (1) according to claim 2, wherein the pitch p is dimensioned to provide minimal loss at an operation frequency Fr of the resonator by satisfying the condition of p< V _diam/V _LN*d _LN, wherein V _diam corresponds to the transverse acoustic velocity in the substrate (2) , V _LN corresponds to the transverse acoustic velocity in the piezoelectric layer (3) , and d _LN corresponds to the thickness of the piezoelectric layer (3) .
The acoustic resonator device (1) according to any one of claims 2 or 3, wherein the plurality of top electrodes (7) comprises a periodic system of interdigitated electrodes, IDEs, arranged on the top surface (5) and alternately connected to busbars arranged for applying AC voltage.
The acoustic resonator device (1) according to any one of claims 2 to 4, wherein the piezoelectric layer (3) comprises grooves (11) arranged in regions of the piezoelectric layer (3) not covered by top electrodes (7) , enabling the top electrodes (7) to vibrate more freely, increasing the piezo-coupling.
The acoustic resonator device (1) according to claim 5, wherein the piezoelectric layer (3) comprises piezoelectric material arranged intermittently sandwiched between the plurality of pairs of top electrodes (7) and the bottom electrode (6) , the grooves (11) extending the entire width and depth of the piezoelectric layer (3) between the top electrodes (7) .
The acoustic resonator device (1) according to any preceding claim, wherein the plurality of pedestals (8) are arranged to align with nods (12) of acoustic amplitudes of the piezoelectric layer (3) to minimize acoustic energy leaking through pedestals (8) , the nods (12) being located in regions of the piezoelectric layer (3) not covered by a top electrode.
The acoustic resonator device (1) according to any preceding claim, wherein the plurality of pedestals (8) are arranged periodically between the piezoelectric layer (3) and the substrate (2) to provide mechanical stability and heat evacuation.
The acoustic resonator device (1) according to any preceding claim, wherein the plurality of pedestals (8) are integral with or made from the same material as the substrate (2) , and are formed as ridges between grooves (11) in the substrate (2) .
The acoustic resonator device (1) according to any preceding claim, wherein the plurality of pedestals (8) are at least partially embedded into recesses (14) arranged on the bottom surface (4) of the piezoelectric layer (3) .
The acoustic resonator device (1) according to any preceding claim, wherein the piezoelectric layer (3) comprises Lithium Niobate, LN, or Lithium Tantalate, LT, AlN, AlScN, or any other suitable piezoelectric material.
The acoustic resonator device (1) according to any preceding claim, wherein the piezoelectric layer (3) is arranged with a rotated Y-cut, X-cut 30° Y-propagation, X-cut, or other suitable crystal orientation providing large coupling (piezo module k ₃₄ ²) of piezoelectric materials.
The acoustic resonator device (1) according to any preceding claim, wherein at least one of the bottom electrode (6) and the top electrodes (7) are made of Al, Cu, Mo, Pt, W, Au, or their alloys.
The acoustic resonator device (1) according to any preceding claim, wherein the plurality of pedestals (8) comprise a material of high thermal conductivity, such as Al, Cu, Au, or dielectrics such as Si, SiO2, SiC, BN, or Si ₃N ₄.
The acoustic resonator device (1) according to claim 9, wherein the piezoelectric layer (3) is mechanically attached to the substrate (2) at its distal ends.
The acoustic resonator device (1) according to any preceding claim, wherein the substrate (2) comprises a first substrate layer (15) comprising a material of high acoustic wave velocity and high thermal conductivity such as diamond, SiC, or BN..
The acoustic resonator device (1) according to claim 16, wherein the first substrate layer (15) is arranged with a thickness d _S1 larger than the thickness of the piezoelectric layer (3) d _LN, preferably 4 to 20 times of the thickness of the piezoelectric layer (3) d _LN.
The acoustic resonator device (1) according to any one of claims 16 or 17, wherein the substrate (2) comprises a second substrate layer (16) arranged on a side of the first substrate layer (15) opposite of the pedestals (8) for supporting the first substrate layer (15) , the second substrate layer (16) preferably being arranged with a thickness d _S2 larger than the thickness of the first substrate layer (15) d _S1, the second substrate layer (16) comprising a wafer material such as Si, SiC, sapphire, quartz, or glass.
A method for manufacturing an acoustic resonator device (1) comprising the steps of:

forming a substrate (2) comprising a first substrate layer (15) comprising a material of high acoustic wave velocity and high thermal conductivity and a supporting second substrate layer (16) ;

forming a plurality of pedestals (8) on the substrate (2) ;

forming a second substrate comprising a piezoelectric layer (3) with a bottom surface (4) and a top surface (5) arranged on a support layer, preferably a Si wafer;

providing a bottom electrode (6) on the bottom surface (4) of the piezoelectric layer (3) ;

wafer-to-wafer bonding of the bottom electrode (6) ;

eliminating the support layer of the second substrate; and

providing a plurality of pairs of top electrodes (7) on the top surface (5) of the piezoelectric layer (3) .