CN117177651A

CN117177651A - Transverse excitation film bulk acoustic resonator with multiple diaphragm thicknesses and manufacturing method

Info

Publication number: CN117177651A
Application number: CN202311211988.1A
Authority: CN
Inventors: 温切斯拉夫·扬捷切夫; 帕特里克·特纳; 罗伯特·B·哈蒙德
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 2019-08-28
Filing date: 2020-08-10
Publication date: 2023-12-05
Also published as: CN117177652A; WO2021041016A1; DE112020004113T5; JP2022537083A; CN114503295A; CN117177653A; CN114503295B; JP7459934B2

Abstract

A filter device and method are disclosed. The filter device includes a substrate having a surface. The back side of the single crystal piezoelectric plate is attached to the surface of the substrate, portions of the single crystal piezoelectric plate forming a plurality of diaphragms spanning respective cavities in the substrate. A conductor pattern is formed on the front surface of the piezoelectric plate, the conductor pattern including a plurality of interdigital transducers (IDTs) of a plurality of resonators. The interleaved fingers of at least one first IDT of the plurality of IDTs are disposed on a membrane having a first thickness, and the interleaved fingers of at least one second IDT of the plurality of IDTs are disposed on a membrane having a second thickness that is less than the first thickness.

Description

Transverse excitation film bulk acoustic resonator with multiple diaphragm thicknesses and manufacturing method

The application relates to a transverse excitation film bulk acoustic resonator with multi-diaphragm thickness and a manufacturing method thereof, which are divisional application of Chinese application patent application No. 202080070413.0, wherein the application date is 8/10/2020.

Technical Field

The present disclosure relates to radio frequency filters using acoustic wave resonators, and more particularly to filters for communication devices.

Background

A Radio Frequency (RF) filter is a two-terminal device configured to pass some frequencies while blocking other frequencies, where "pass" means transmitting with relatively low signal loss and "block" means blocking or substantially attenuating. The frequency range through which a filter passes is referred to as the "passband" of the filter. The frequency range blocked by such a filter is referred to as the "stop band" of the filter. A typical RF filter has at least one pass band and at least one stop band. The specific requirements of either the pass band or the stop band depend on the specific application. For example, a "passband" may be defined as a range of frequencies where the insertion loss of the filter is better than a defined value such as 1dB, 2dB, or 3 dB. A "stop band" may be defined as a range of frequencies where the rejection of the filter is greater than a defined value, such as 20dB, 30dB, 40dB or more, depending on the particular application.

RF filters are used in communication systems that transmit information over a wireless link. For example, RF filters are found in RF front ends of cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, internet of things (1 oT) devices, laptop and tablet computers, fixed point radio links, and other communication systems. RF filters are also used in radar and electronic and information combat systems.

RF filters typically require many design tradeoffs to achieve the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost for each particular application. Specific designs and manufacturing methods and enhancements may benefit from one or more of these requirements simultaneously.

The enhancement of the performance of RF filters in wireless systems can have a wide impact on system performance. System performance may be improved by improving RF filters, such as larger cell size, longer battery life, higher data rates, larger network capacity, lower cost, increased security, higher reliability, etc. These improvements may be achieved at various levels of the wireless system, either alone or in combination, such as at the RF module, RF transceiver, mobile or fixed subsystem, or network level.

To obtain a wider communication channel bandwidth, it is necessary to use a higher frequency communication band. Current LTE ^TM The (long term evolution) specification defines a frequency band between 3.3GHz and 5.9 GHz. These bands are not currently in use. Future proposals for wireless communications include millimeter wave communications bands with frequencies up to 28 GHz.

High performance RF filters for current communication systems typically incorporate acoustic wave resonators including Surface Acoustic Wave (SAW) resonators, bulk acoustic wave BAW resonators, film bulk acoustic wave resonators (FBAR) and other types of acoustic wave resonators. However, these prior art techniques are not suitable for use at higher frequencies, which are required for future communication networks.

Disclosure of Invention

The invention discloses a filter device, comprising: a substrate having a surface; a single crystal piezoelectric plate having a front side and a back side, the back side attached to a surface of the substrate, portions of the single crystal piezoelectric plate forming a plurality of diaphragms spanning respective cavities in the substrate; and a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a plurality of resonators, wherein interleaved fingers of at least one first IDT of the plurality of IDTs are disposed on a membrane having a first thickness, and interleaved fingers of at least one second IDT of the plurality of IDTs are disposed on a membrane having a second thickness, the second thickness being smaller than the first thickness.

Wherein the interleaved fingers of one or more of the plurality of IDTs are provided on respective membranes having other thicknesses between the first thickness and the second thickness.

Wherein the plurality of diaphragms comprises: at least one membrane having a first thickness; at least one membrane having a second thickness; and one or more diaphragms having one or more additional thicknesses intermediate the first thickness and the second thickness.

Wherein the single crystal piezoelectric plate and all IDTs are configured such that a respective radio frequency signal applied to each IDT excites a respective shear dominant acoustic mode within a respective membrane.

Wherein the direction of acoustic energy flow of all of said shear primary acoustic modes is substantially orthogonal to said front and said back faces of each diaphragm.

Wherein the single crystal piezoelectric plate is one of lithium niobate and lithium tantalate.

Wherein the second thickness is greater than or equal to 200nm and the first thickness is less than or equal to 1000nm.

Wherein each of the plurality of IDTs is provided on a respective membrane that spans a respective cavity.

Wherein the plurality of resonators includes at least one parallel resonator and at least one series resonator, an IDT finger of the at least one parallel resonator is provided on a diaphragm having the first thickness, and an IDT finger of the at least one series resonator is provided on a diaphragm having the second thickness.

Wherein the plurality of resonators includes a plurality of parallel resonators and a plurality of series resonators, IDT fingers of all parallel resonators are provided on a diaphragm having the first thickness, and IDT fingers of all series resonators are provided on a diaphragm having the second thickness.

The invention also discloses a method for manufacturing the filter device, which comprises the following steps: attaching a back side of a piezoelectric plate having opposite front and back sides and a first thickness to a surface of a substrate; selectively thinning a portion of the piezoelectric plate from the first thickness to a second thickness, the second thickness being less than the first thickness; forming cavities in the substrate such that portions of the single crystal piezoelectric plate form a plurality of diaphragms spanning the respective cavities; and forming a conductor pattern on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a plurality of resonators, wherein interleaved fingers of at least one first IDT of the plurality of IDTs are disposed on one or more diaphragms having the first thickness, and interleaved fingers of at least one second IDT of the plurality of IDTs are disposed on one or more diaphragms having the second thickness.

Wherein further comprising selectively thinning an additional portion of the piezoelectric plate to a third thickness intermediate the first thickness and the second thickness, wherein interleaved fingers of at least one third IDT of the plurality of IDTs are provided on a membrane having the third thickness.

Wherein, still include: optionally thinning the additional portion of the piezoelectric plate to one or more additional thicknesses intermediate the first thickness and the second thickness.

Wherein the single crystal piezoelectric plate and all IDTs are configured such that a respective radio frequency signal applied to each IDT excites a respective shear dominant acoustic mode within the respective membrane.

Wherein the direction of acoustic energy flow of all of said shear primary acoustic modes is substantially orthogonal to the front and back faces of each diaphragm.

Drawings

Fig. 1 includes a schematic plan view and two schematic cross-sectional views of a laterally excited thin film bulk acoustic resonator (XBAR).

Fig. 2 is a partially enlarged schematic cross-sectional view of the XBAR of fig. 1.

Fig. 3 is an alternative schematic cross-sectional view of the XBAR of fig. 1.

Fig. 4 is a diagram illustrating a shearing acoustic mode in XBAR.

Fig. 5 is a schematic block diagram of a bandpass filter containing seven XBARs.

Fig. 6A is a schematic cross-sectional view of a filter having a dielectric layer to set the frequency separation between the parallel resonator and the series resonator.

Fig. 6B is a schematic cross-sectional view of a filter with different piezoelectric diaphragm thicknesses to set the frequency separation between the parallel resonator and the series resonator.

Fig. 7 is a series of schematic cross-sectional views illustrating a process of controlling the thickness of a piezoelectric diaphragm.

Fig. 8 is a flow chart of a process for manufacturing a filter implemented with XBAR.

Figure 9 is a flow chart of another process for manufacturing a filter implemented with XBAR.

Figure 10 is a flow chart of another process for manufacturing a filter implemented with XBAR.

Figure 11 is a flow chart of another process for manufacturing a filter implemented with XBAR.

Throughout the specification, elements appearing in the figures are assigned three-digit or four-digit reference numbers in which the two least significant digits are specific for the element and one or both most significant digits are the figure number in which the element is first shown. Elements not described in conjunction with the figures may be assumed to have the same characteristics and functions as previously described elements having the same reference numerals.

Detailed Description

Device description

Fig. 1 shows a simplified schematic top view and orthogonal cross-sectional view of a laterally excited thin film bulk acoustic resonator (XBAR) 100. XBAR resonators such as resonator 100 may be used for various RF filters including band reject filters, band pass filters, diplexers, and multiplexers. XBAR is particularly suitable for filters in the communications band with frequencies above 3 GHz.

The XBAR 100 is composed of a thin film conductor pattern formed on the surface of a piezoelectric plate 110 having a front face 112 and a back face 114, which are parallel, respectively. The piezoelectric plate is a thin single crystal layer made of a piezoelectric material such as lithium niobate, lithium tantalate, langasite, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y and Z crystal axes relative to the front and back sides is always and consistent. In the example shown in this patent, the piezoelectric plate may be Z-cut, that is, the Z-axis is perpendicular to the back surface. However, XBAR can be fabricated on piezoelectric plates with other crystal orientations.

The back surface 114 of the piezoelectric plate 110 is attached to a substrate 120, the substrate 120 providing mechanical support for the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material. The piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process, or the piezoelectric plate 110 may be grown on the substrate 120, or the piezoelectric plate 110 may be attached to the substrate in some other manner. The piezoelectric plate may be attached directly to the substrate, or may be attached to the substrate via one or more intermediate material layers.

The conductor pattern of the XBAR 100 includes an interdigital transducer (1 DT) 130.IDT130 includes a first plurality of parallel fingers, such as fingers 136, extending from a first bus bar 132, and a second plurality of fingers extending from a second bus bar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap a distance AP, commonly referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of IDT130 is the IDT's "length".

The first and second bus bars 132, 134 serve as terminals of the XBAR 100. A radio frequency or microwave signal applied between the two bus bars 132, 134 of the IDT130 excites an acoustic wave within the piezoelectric plate 110. As will be discussed in detail below, the excitation acoustic wave is a bulk shear wave that propagates in a direction perpendicular to the surface of the piezoelectric plate 110, which is also perpendicular or transverse to the direction of the electric field generated by the IDT fingers. Thus, XBAR is considered a laterally excited thin film bulk wave resonator.

A cavity 140 is formed in the substrate 120 such that the portion 115 of the piezoelectric plate 110 containing the IDT 130 is suspended over the cavity 140 without contacting the substrate 120. The conventional meaning of "cavity" is "empty space within a solid". The cavity 140 may be a hole (as shown in cross-section AA and BB) completely through the substrate 120 or may be a recess in the substrate 120 (as shown subsequently in fig. 3). The cavity 140 may be formed before or after the piezoelectric plate 110 and the substrate 120 are attached together, for example, by selectively etching the substrate 120. As shown in fig. 1, the cavity 140 is rectangular in shape to an extent greater than the aperture AP and the length L of the IDT 130. The cavity of the XBAR may have different shapes, e.g. regular or irregular polygons. The cavity of the XBAR may be more or less than four sides, which may be straight or curved.

The portion 115 of the piezoelectric plate that is suspended over the cavity 140 will be referred to herein as a "diaphragm" (because there is no better term) because it is physically similar to the diaphragm of a microphone. The diaphragm may be continuously and seamlessly connected to the remainder of the piezoelectric plate 110 around all or substantially all of the perimeter of the cavity 140.

For ease of illustration in fig. 1, the geometric spacing and width of IDT fingers is greatly exaggerated relative to the length of XBAR (dimension L) and aperture (dimension AP). A typical XBAR has more than ten parallel fingers in IDT 110. One XBAR may have hundreds, possibly thousands, of parallel fingers in IDT 110. Similarly, the thickness of the fingers is greatly exaggerated in cross-section.

Figure 2 shows a detailed schematic cross-sectional view of the XBAR 100 of figure 1. The piezoelectric plate 110 is a single crystal layer made of piezoelectric material having a thickness ts, which may be, for example, 100nm to 1500nm. When used for LTE from 3.4GHZ to 6GHz ^TM In a filter of frequency bands (e.g. frequency bands 42, 43, 46), the thickness ts may be, for example, 200nm to 1000nm.

Alternatively, the front dielectric layer 214 may be formed on the front surface of the piezoelectric plate 110. By definition, the "front side" of an XBAR refers to the surface facing away from the substrate. The front side dielectric layer 214 has a thickness tfd. Front dielectric layer 214 is formed between IDT fingers 238. Although not shown in fig. 2, a front dielectric layer 214 may also be deposited over IDT fingers 238. A back dielectric layer 216 may optionally be formed on the back side of the piezoelectric plate 110. The back side dielectric layer 216 has a thickness tbd. The front side dielectric layer 214 and the back side dielectric layer 216 may be non-piezoelectric dielectric materials such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500nm. tfd and tbd are typically less than the thickness ts of the piezoelectric plate. tfd and tbd need not be equal and the front and back dielectric layers 214, 216 need not be the same material. The front side dielectric layer 214 and/or the back side dielectric layer 216 may be formed from multiple layers of two or more materials.

IDT finger 238 can be aluminum or a base aluminum alloy, copper or a base copper alloy, beryllium, gold, or some other conductive material. A thin (relative to the total thickness of the conductor) layer of other metal (e.g., chromium or titanium) may be formed under and/or over the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers. The bus bars (132, 134 in fig. 1) of the IDT can be made of the same or different materials as the fingers.

The dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. The dimension w is the width or "mark" of the IDT finger. The IDT of XBAR is significantly different from the IDT used in a Surface Acoustic Wave (SAW) resonator. In a SAW resonator, the IDT spacing is half the wavelength of the acoustic wave at the resonant frequency. In addition, the tag pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e., the width of the tag or finger is approximately one-quarter of the wavelength of the acoustic wave at resonance). In XBAR, the pitch p of IDTs is typically 2 to 20 times the finger width w. In addition, the pitch p of IDTs is typically 2 to 20 times the thickness ts of the piezoelectric plate 212. The width of the IDT finger in XBAR is not limited to one quarter of the wavelength of the acoustic wave at resonance. For example, the width of the XBAR IDT finger may be 500nm or more, so that the IDT can be manufactured using a photolithography technique. The thickness tm of the IDT finger can be from 100nm to about equal to the width w. The thickness of the bus bars (132, 134 in fig. 1) of the IDT may be equal to or greater than the thickness tm of the IDT finger.

Fig. 3 is an alternative cross-sectional view along section A-A defined in fig. 1. In fig. 3, a piezoelectric plate 310 is attached to a substrate 320. An optional dielectric layer 322 may be sandwiched between the piezoelectric plate 310 and the substrate 320. A cavity 340 that does not completely penetrate the substrate 320 is formed in the substrate below the portion of the piezoelectric plate 310 that contains the IDT of the XBAR. For example, the cavity 340 may be formed by etching the substrate 320 prior to attaching the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings 342 provided in the piezoelectric plate 310.

Since cavity 340 is etched from the front side of substrate 320 (either before or after attachment of piezoelectric plate 310), XBAR 300 shown in fig. 3 will be referred to herein as a "front side etched" configuration. Since cavity 140 is etched from the back side of substrate 120 after attachment of piezoelectric plate 110, XBAR 100 of fig. 1 will be referred to herein as a "back side etched" configuration.

Fig. 4 is an illustration of the dominant acoustic mode of interest in XBAR. Fig. 4 shows a small portion of an XBAR 400 that includes a piezoelectric plate 410 and three interleaved IDT fingers 430. An RF voltage is applied to the interleaved fingers 430. This voltage creates a time-varying electric field between the fingers. The direction of the electric field is transverse, or parallel, to the surface of the piezoelectric plate 410, as indicated by the arrow labeled "electric field". Due to the high dielectric constant of the piezoelectric plate, the electric field is highly concentrated in the plate with respect to air. The transverse electric field induces shear deformation in the piezoelectric plate 410 and thus strongly excites the shear mode acoustic modes. In this context, "shear deformation" is defined as a deformation in which parallel planes in a material remain parallel and at a constant distance during translation relative to each other. The "shear acoustic mode" is defined as an acoustic vibration mode in a medium that causes shear deformation of the medium. The shear deformation in XBAR 400 is represented by curve 460, with adjacent small arrows schematically indicating the direction and magnitude of atom motion. The degree of atomic motion and the thickness of the piezoelectric plate 410 are greatly exaggerated for ease of viewing. Although the atomic motion is primarily transverse (i.e., horizontal as shown in fig. 4), the direction of acoustic energy flow of the excited primary shear acoustic mode is substantially orthogonal to the surface of the piezoelectric plate as indicated by arrow 465.

As shown in fig. 4, there is substantially no electric field immediately below IDT finger 430, so acoustic modes are only minimally excited in region 470 below the finger. In these areas there may be evanescent acoustic movements. Since acoustic vibrations are not excited under the IDT fingers 430, acoustic energy coupled to the IDT fingers 430 is low (e.g., as compared to the fingers of the IDT in a SAW resonator), which minimizes viscous losses in the IDT fingers.

Acoustic wave resonators based on shear acoustic resonance can achieve better performance than current state-of-the-art Film Bulk Acoustic Resonators (FBAR) and solid-mount resonator bulk acoustic wave (SMR BAW) devices, where an electric field is applied in the thickness direction. In such devices, the acoustic mode is compressed in the direction of the flow of acoustic energy in the atomic motion and thickness direction. Furthermore, the piezoelectric coupling of shear wave XBAR resonance may be high (> 20%) compared to other acoustic wave resonators. Thus, high voltage electrical coupling enables the design and implementation of microwave and millimeter wave filters with considerable bandwidth.

Fig. 5 is a schematic circuit diagram of a high-band pass filter 500 using XBAR. The filter 500 has a conventional ladder filter architecture, including four series resonators 510A, 510B, 510C, 510D and three parallel resonators 520A, 520B, 520C. Four series resonators 510A, 510B, 510C, and 510D are connected in series between the first port and the second port. In fig. 5, the first and second ports are labeled "In" and "Out," respectively. However, filter 500 is symmetrical and either port is used as the input or output of the filter. The three parallel resonators 520A, 520B, 520C are connected to ground from a node between the series resonators. All parallel resonators and series resonators are XBAR. Although not shown in fig. 5, any and all resonators may be divided into a plurality of electrically connected parallel sub-resonators. Each sub-resonator may have a corresponding diaphragm.

The filter 500 may include a substrate having a surface, a single crystal piezoelectric plate having parallel front and back surfaces, and an acoustic bragg reflector sandwiched between the surface of the substrate and the back surface of the single crystal piezoelectric plate. The substrate, acoustic bragg reflector and piezoelectric plate are represented by rectangle 510 in fig. 5. The conductor pattern formed on the front surface of the single crystal piezoelectric plate includes an interdigital transducer (IDT) for each of four series resonators 510A, 510B, 510C, 510D and three parallel resonators 520A, 520B, 520C. All IDTs are configured to excite shear acoustic waves in the single crystal piezoelectric plate in response to a respective radio frequency signal applied to each IDT.

In a ladder filter such as filter 500, the resonant frequency of the parallel resonator is typically lower than the resonant frequency of the series resonator. The resonance frequency of the SM XBAR resonator is determined in part by the IDT spacing. IDT spacing can also affect other filter parameters, including impedance and power handling capability. For wideband filter applications, it may be impractical to use only the difference in IDT spacing to provide the required difference between the resonant frequencies of the parallel and series resonators.

As described in patent No. 10,601,392, a first dielectric layer (represented by dashed rectangle 525) having a first thickness t1 can be deposited over the IDTs of some or all of the parallel resonators 520A, 520B, 520C. A second dielectric layer (represented by the dashed rectangle 515) having a second thickness t2 less than t1 may be deposited over the IDTs of the series resonators 510A, 510B, 510C, 510D. A second dielectric layer may be deposited over the parallel resonator and the series resonator. The difference between the thickness t1 and the thickness t2 defines the frequency offset between the series resonator and the parallel resonator. By varying the spacing of the individual IDTs, the individual series or parallel resonators can be tuned to different frequencies. In some filters, more than two dielectric layers of different thickness may be used, as described in co-pending application 16/924,108.

Alternatively or additionally, the parallel resonators 510A, 510B, 510C, 510D may be formed on a piezoelectric plate having a thickness t3, and the series resonators may be fabricated on a piezoelectric plate having a thickness t4 less than t 3. The difference between the thicknesses t3 and t4 defines the frequency offset between the series resonator and the parallel resonator. By varying the spacing of the individual IDTs, the individual series or parallel resonators can be tuned to different frequencies. In some filters, three or more different piezoelectric plate thicknesses may be used to provide additional frequency tuning capabilities.

Fig. 6A is a schematic cross-sectional view of a parallel resonator and a series resonator of filter 600A, filter 600A using dielectric thickness to separate the frequencies of the parallel resonator and the series resonator. The piezoelectric plate 610A is attached to the substrate 620. Portions of the piezoelectric plate form a membrane that spans the cavity 640 in the substrate 620. Staggered IDT fingers, such as fingers 630, are formed on the membrane. A first dielectric layer 650 having a thickness t1 is formed over the IDT of the parallel resonator. A second dielectric layer 655 of thickness t2 is deposited over the parallel and series resonators. Alternatively, a single dielectric layer having a thickness t1+t2 may be deposited on the parallel resonator and the series resonator. The dielectric layer over the series resonators may then be thinned to a thickness t2 using a masked dry etch process. In either case, the difference between the total thickness of the dielectric layers (t1+t2) on the parallel resonator and the thickness t2 of the second dielectric layer defines the frequency offset between the series resonator and the parallel resonator. here

The second dielectric layer 655 may also be used to seal and passivate the surface of the filter 600A. The second dielectric layer may be made of the same material as the first dielectric layer or of a different material. The second dielectric layer may be a laminate of two or more sub-layers of different materials. Alternatively, an additional dielectric passivation layer (not shown in fig. 6A) may be formed over the surface of filter 600A. Furthermore, as will be described later, the thickness of the final dielectric layer (i.e., the second dielectric layer 655 or the additional dielectric layer) may be locally adjusted to fine tune the frequency of the filter 600A. Thus, the final dielectric layer may be referred to as a "passivation and tuning layer".

Fig. 6B is a schematic cross-sectional view of a parallel resonator and a series resonator of a filter 600B that uses piezoelectric plate thickness to separate the frequencies of the parallel resonator and the series resonator. The piezoelectric plate 610B is attached to the substrate 620. Portions of the piezoelectric plate form a membrane that spans the cavity 640 in the substrate 620. Staggered IDT fingers, such as fingers 630, are formed on the membrane. The thickness of the diaphragm of the parallel resonator is t3. The piezoelectric plate 610B is selectively thinned so that the diaphragm of the series resonator has a thickness t4 less than t3. the difference between t3 and t4 defines the frequency offset between the series resonator and the parallel resonator. Passivation and tuning layer 655 is deposited over the parallel and series resonators.

Description of the method

Fig. 7 is a series of schematic cross-sectional views illustrating a process of controlling the thickness of a piezoelectric diaphragm. View a shows a piezoelectric plate 710 having a non-uniform thickness bonded to a substrate 720. The piezoelectric plate 710 may be, for example, lithium niobate or lithium tantalate. The substrate 720 may be a silicon wafer or some other material as previously described. The thickness variation shown in the piezoelectric plate 710 is greatly exaggerated. The thickness variation should not exceed 10% of the thickness of the piezoelectric plate and may be a few percent or less.

View B of fig. 7 shows an optical measurement of the piezoelectric plate thickness using an optical thickness measurement tool 730 comprising a light source 732 and a detector 734. Optical thickness measurement tool 730 may be, for example, an ellipsometer/reflectometer. The optical thickness measuring tool 730 measures light reflected from the surface of the piezoelectric plate 710 and from the interface between the piezoelectric plate 710 and the substrate 720. Reflections from specific measurement points on the piezoelectric plate may be measured using a plurality of wavelengths of light, angles of incidence, and/or polarization states. The results of the multiple measurements are processed to determine the thickness of the piezoelectric plate at the measurement point.

The measurement process is repeated to determine the thickness of the piezoelectric plate at a plurality of measurement points on the surface of the piezoelectric plate. For example, the plurality of points may form a grid or matrix of measurement points on the surface of the plate. The measurement data may be processed and interpolated to provide a thickness map of the piezoelectric plate.

View C shows the removal of excess material on the piezoelectric plate using a material removal tool. In this context, "excess material" is defined as the portion of the piezoelectric plate that extends beyond the thickness of the target plate. In view C, the excess material to be removed is shown shaded. The material removal tool may be, for example, a scanning ion mill 740, a tool employing fluorine-based reactive ion etching, or some other tool. The scanned ion mill 740 scans the high energy ion beam 745 across the piezoelectric surface. The incidence of ion beam 745 on the piezoelectric plate removes material at the surface by sublimation or sputtering. The ion beam 745 may be scanned one or more times over the surface of the piezoelectric plate in a raster pattern. The ion current or dwell time of the ion beam 745 may be varied during the raster scan to control the depth of material removed from each point on the piezoelectric plate according to the thickness map of the piezoelectric plate. The result is a piezoelectric plate with significantly improved thickness uniformity as shown in view D. The thickness of any point on the piezoelectric plate may be substantially equal to the target plate thickness, where "substantially equal" refers to equal to the maximum equal range limited by the measurement accuracy and capability of the material removal tool.

View E illustrates selective removal of thin selected portions of the piezoelectric plate. Selected portions of the piezoelectric plate may be thinned, for example, to provide a diaphragm for the series resonator, as shown in fig. 6B. If the tool has sufficient spatial resolution to distinguish between areas of the piezoelectric plate to be thinned, a scanned ion mill or other scanned material removal tool may be used to thin selected portions of the piezoelectric plate. Alternatively, a scanning or non-scanning material removal tool 750 or an etching process may be used to remove material from portions of the piezoelectric plate surface defined by the mask 752. The result is a piezoelectric plate of reduced thickness area 760 that is suitable for the diaphragm of a series resonator, as shown in view F.

Fig. 8 is a simplified flow chart illustrating a process 800 for fabricating a filter device comprising XBAR. Specifically, process 800 is used to fabricate a filter device using a frequency setting dielectric layer over a parallel resonator as shown in fig. 6A. Process 800 begins at 805 with a device substrate and a thin sheet of piezoelectric material disposed on a sacrificial substrate. Process 800 ends at 895 when the filter device is completed. The flow chart of fig. 8 includes only the main process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, during, after, and during the steps shown in fig. 8.

Although fig. 8 generally describes a process for fabricating a single filter device, multiple filter devices may be fabricated simultaneously on a common wafer (composed of piezoelectric plates bonded to a substrate). In this case, each step of process 800 may be performed simultaneously on all of the filter devices on the wafer.

The flow chart of fig. 8 captures three variations of a process 800 for fabricating XBAR, differing in when and how cavities are formed in the device substrate. A cavity may be formed at step 810A, 810B, or 810C. In each of the three variations of process 800, only one of these steps is performed.

The piezoelectric plate may be, for example, lithium niobate or lithium tantalate, either of which may be Z-cut, rotary Z-cut, or rotary YX-cut. The piezoelectric plate may be some other material and/or some other cutout. The device substrate may preferably be silicon. The device substrate may be some other material that allows deep cavities to be formed by etching or other processes.

In one variation of process 800, one or more cavities are formed in the device substrate at 810A prior to bonding the piezoelectric plate to the substrate at 815. A separate cavity may be formed for each resonator in the filter device. Conventional photolithography and etching techniques may be used to form the one or more cavities. Typically, the cavity formed at 810A will not penetrate the device substrate, and the resulting resonator device will have a cross-section as shown in fig. 3.

At 815, the piezoelectric plate is bonded to the device substrate. The piezoelectric plate and the device substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the device substrate and the piezoelectric plate are highly polished. One or more layers of an intermediate material, such as an oxide or metal, may be formed or deposited on the mating surfaces of the piezoelectric plate and/or device substrate. One and/or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces can then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the device substrate or intermediate material layer.

At 820, the sacrificial substrate may be removed. For example, the piezoelectric plate and the sacrificial substrate may be wafers of piezoelectric material that have been ion implanted to create defects in the crystal structure along a plane defining a boundary between the piezoelectric plate and the sacrificial substrate. At 820, the wafer may be split along the defect plane, for example by thermal shock, separating the sacrificial substrate and leaving the piezoelectric plate bonded to the device substrate. After stripping the sacrificial substrate, the exposed surface of the piezoelectric plate may be polished or otherwise treated in some manner.

Single crystal piezoelectric material sheets laminated to non-piezoelectric substrates are commercially available. During this application, both lithium niobate and lithium tantalate plates may be bonded to a variety of substrates, including silicon, quartz, and fused silica. Other thin plates of piezoelectric material may be present now or in the future. The thickness of the piezoelectric plate may be between 300nm and 1000 nm. When the substrate is silicon, a layer of SiO can be arranged between the piezoelectric plate and the substrate ₂ . When using a commercially available piezoelectric plate/device substrate laminate, steps 810A, 815 and 820 of process 800 are not performed.

At 845, a first conductor pattern is formed by depositing and patterning one or more conductor layers on the front side of the piezoelectric plate, including IDTs for each XBAR. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Alternatively, one or more layers of other materials may be disposed below the conductor layer (i.e., between the conductor layer and the piezoelectric plate) and/or atop the conductor layer. For example, thin films of titanium, chromium, or other metals may be used to improve adhesion between the conductor layer and the piezoelectric plate. A second conductor pattern of gold, aluminum, copper, or other higher conductivity metal may be formed over portions of the first conductor pattern (e.g., IDT bus bars and interconnects between IDTs).

Each conductor pattern may be formed at 845 by sequentially depositing a conductive layer and optionally one or more other metal layers on the surface of the piezoelectric plate. Excess metal may then be removed by etching through the patterned photoresist. The conductor layer may be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques.

Alternatively, each conductor pattern may be formed at 845 using a lift-off process. A photoresist may be deposited on the piezoelectric plate and patterned to define the conductor pattern. A conductor layer, and optionally one or more other layers, may be deposited sequentially on the surface of the piezoelectric plate. The photoresist may then be removed, which removes excess material, leaving behind the conductor pattern.

At 850, one or more frequency setting dielectric layers may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. For example, a dielectric layer may be formed on the parallel resonator to reduce the frequency of the parallel resonator relative to the series resonator. The one or more dielectric layers may be deposited using conventional deposition techniques such as physical vapor deposition, atomic layer deposition, chemical vapor deposition, or some other method. One or more photolithographic processes (using a photomask) may be used to limit dielectric layer deposition to selected areas of the piezoelectric plate. For example, a mask may also be used to limit the dielectric layer to only cover the parallel resonators.

At 855, a passivation/tuning dielectric layer is deposited over the piezoelectric plate and the conductor pattern. The passivation/tuning dielectric layer may cover the entire surface of the filter except for pads for electrical connection with circuitry external to the filter. In some examples of process 800, the passivation/tuning dielectric layer may be formed after the cavity in the device substrate is etched at 810B or 810C.

In a second variation of process 800, at 810B one or more cavities are formed in a backside of a device substrate. A separate cavity may be formed for each resonator in the filter device. One or more cavities may be formed using anisotropic or orientation-dependent dry or wet etching to open holes from the back side of the device substrate all the way to the piezoelectric plate. In this case the resulting resonator device will have a cross section as shown in fig. 1.

In a third variation of process 800, one or more cavities in the form of grooves may be formed in the device substrate at 810C by etching the substrate using an etchant introduced through the openings in the piezoelectric plate. A separate cavity may be formed for each resonator in the filter device. The cavity or cavities formed at 810C will not penetrate the device substrate and the resulting resonator device will have a cross-section as shown in fig. 3.

Ideally, after 810B or 810C forms the cavity, most or all of the filter devices on the wafer will meet a set of performance requirements. However, normal process tolerances can result in parameter variations, such as variations in the thickness of the dielectric layer formed at 850 and 855, variations in the thickness and linewidth of the conductor and IDT fingers formed at 845, and variations in the thickness of the PZT sheet. These variations lead to deviations in the performance of the filter device from the performance requirements.

To improve the yield of filter devices meeting performance requirements, frequency tuning may be performed by selectively adjusting the thickness of passivation/tuning layers deposited on the resonator at 855. The frequency of the filter device passband may be reduced by adding material to the passivation tuning layer and increased by removing material from the passivation tuning layer. Typically, the process 800 is biased to produce a filter device having a passband that is initially below the desired frequency range but is tunable to the desired frequency range by removing material from the surface of the passivation/tuning layer.

At 860, a probe card or other device may be used to electrically connect with the filter to allow Radio Frequency (RF) testing and measurement of filter characteristics, such as input-output transfer functions. Typically, RF measurements are made on some or most of the filter devices, where the filter devices are fabricated on the same piezoelectric plate and substrate at the same time.

At 865, global frequency tuning may be performed by removing material from the surface of the passivation/tuning layer using a selective material removal tool, such as a scanned ion mill as previously described. The "global" tuning is performed at a spatial resolution equal to or greater than that of the individual filter devices. The "global" tuning is performed with a spatial resolution equal to or greater than that of a single filter device. The purpose of global tuning is to shift the pass band of each filter device toward the desired frequency range. The test results from 860 may be processed to generate a global contour map indicating the amount of material removed according to the two-dimensional location on the wafer. The material is then removed from the contour map using a selective material removal tool.

At 870, local frequency tuning may be performed in addition to or instead of the global frequency tuning performed at 865. The "local" frequency tuning is performed at a spatial resolution that is less than that of the individual filter devices. The test results from 860 may be processed to generate a map indicating the amount of material to be removed at each filter device. Local frequency tuning may require the use of a mask to limit the size of the region of material to be removed. For example, a first mask may be used to limit tuning to only parallel resonators, while a second mask may be subsequently used to limit tuning to only series resonators (or vice versa). This will allow the lower band edge (by tuning the parallel resonator) and the upper band edge (by tuning the series resonator) of the filter device to be tuned independently.

After frequency tuning is completed at 865 and/or 870, the filter device is completed at 875. Actions that may occur at 875 include forming bond pads or solder bumps or other means for establishing a connection between the device and external circuitry (if such pads are not formed at 845); dicing individual filter devices from a wafer containing a plurality of filter devices; other packaging steps; and additional testing. After each filter device is completed, the process ends at 895.

Figure 9 is a simplified flow diagram illustrating a process 900 for manufacturing a filter comprising XBAR. The process 900 begins at 905 with a substrate and a sheet of piezoelectric material and ends at 995, at which time the filter is completed. The flow chart of fig. 9 includes only the main process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, during, after, and during the steps shown in fig. 9.

The flow chart of fig. 9 captures two variations of a process 900 for manufacturing a filter that differ in when and how cavities are formed in a substrate. A cavity may be formed at step 810B or 810C. Only one of these steps is performed in each of the two variations of process 900.

Process steps having reference numerals 815 through 875 are substantially identical to corresponding steps of process 800 of fig. 8. A description of these steps will not be repeated. A significant difference between process 900 and process 800 is that RF test 960 and frequency tuning 965 are performed before the cavity is formed at 810B or 810C. When tuning is performed with the area of the resonator still attached to the substrate, the substrate provides mechanical support for the piezoelectric plate and acts as a heat sink for heat generated when material is removed from the passivation/tuning dielectric layer. This avoids the situation where the diaphragm may be damaged when tuning is completed after the cavity is formed in process 800.

Since tuning is performed while the area of the resonator is still attached to the substrate, the RF test at 960 cannot measure the actual performance parameters of the filter. Instead, the RF test at 960 measures other parameters that may be correlated to the filter performance after cavity formation. The RF test at 960 may measure the resonant frequency of other acoustic modes that may or may not remain after cavity formation. These modes may include Sezawa mode, rayleigh mode, and various body-sound modes. For example, the input/output transfer function of the filter device and/or the admittance of a single resonator may be measured across all or most of the filter devices fabricated simultaneously on the same piezoelectric plate and substrate.

The test results from 960 are processed to predict the performance of the filter device, which in turn is used to generate a contour map indicating the amount of material removed according to the two-dimensional location on the wafer. For example, the neutral network may be trained to convert the admittance of the resonator at frequencies from 0 to 1GHz to a prediction of the amount of material to be removed at a particular location on the contour plot.

At 965, the frequency of the filter device is selectively tuned by removing material from the surface of the passivation/tuning layer according to the contour map generated at 960. A selective material removal tool, such as a scanned ion mill as previously described, may be used to remove material. As previously described, global and/or local frequency tuning may be performed at 965. After the frequency tuning is completed, process 900 may be completed as previously described with respect to process 800.

Figure 10 is a simplified flow diagram illustrating another process 1000 for manufacturing a filter device comprising XBAR. In particular, process 1000 is used to fabricate filter devices having two or more different piezoelectric diaphragm thicknesses. For example, for series and parallel resonators, the devices may have different diaphragm thicknesses, as shown in fig. 6B. Process 1000 begins at 1005 with a substrate and a plate of piezoelectric material disposed on a sacrificial substrate and ends at 1095 with a completed filter device. The flow chart of fig. 10 includes only the main process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, during, after, and during the steps shown in fig. 10.

The flow chart of fig. 10 captures three variations of a process 1000 for fabricating an XBAR device that differ in when and how cavities are formed in a substrate. A cavity may be formed at step 810A, 810B, or 810C. Only one of these steps is performed in each of the three variations of process 1000.

Process steps having reference numerals from 815 to 875 are substantially identical to corresponding steps of the process 800 of fig. 8. A description of these steps will not be repeated. The process 1000 differs significantly from the process 800 in the addition of steps 1030 and 1035.

At 1030, selected areas of the piezoelectric plate are thinned. For example, the region of the piezoelectric plate that will become the diaphragm of the series resonator may be thinned, as shown in view E of fig. 7. Thinning may be performed using a scanning material tool such as an ion mill. Alternatively, the area to be thinned may be defined by a mask, and the material may be removed using an ion mill, a sputter etching tool, or a wet or dry etching process. In all cases, it is desirable to precisely control the depth of material removed from the wafer surface. After thinning, the piezoelectric plate will be divided into areas of two or more different thicknesses.

The surface remaining after the material is removed from the piezoelectric plate may be damaged, particularly if an ion mill or sputter etching tool is used at 1030. Some form of post-treatment, such as annealing or other heat treatment, may be performed at 1035 to repair the damaged surface.

After selectively thinning the piezoelectric plate at 1030 and repairing any surface damage at 1035, the remaining steps of process 1000 (shown in fig. 10) may be the same as the corresponding steps of process 800, with RF testing 860 and frequency tuning 865 occurring after the cavity is formed at 810A, 810B or 810C. Alternatively, the remaining steps of process 1000 (not shown in fig. 10) may be the same as the corresponding steps of process 900, with RF test 960 and frequency tuning 965 occurring before 810B or 810C forms the cavity. The formation of the frequency setting dielectric layer at 850 is not necessarily performed during the process 1000.

Fig. 11 is a simplified flow diagram illustrating another process 1100 for fabricating a filter device that includes XBAR. Specifically, process 1100 is used to fabricate a filter device with additional steps to improve thickness uniformity of the piezoelectric plate, as previously illustrated in fig. 7. The flow chart of fig. 11 includes only the main process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, during, after, and during the steps shown in fig. 11. Process steps having reference numerals 815 through 875 are substantially identical to corresponding steps of process 800 of fig. 8. Process steps 1030 and 1035 are substantially identical to corresponding steps of process 1000 of fig. 10. A description of these steps will not be repeated.

The flow chart of fig. 11 captures variations of a process 1100 for fabricating XBAR that differ in when and how cavities are formed in the substrate and how the frequency of the parallel resonator is shifted from the frequency of the series resonator. A cavity may be formed at step 810B or 810C. Only one of these steps is performed in any variation of process 1100. By forming a frequency setting dielectric layer over the parallel resonator at 850, the frequency of the parallel resonator may be shifted from the frequency of the series resonator. Alternatively, at 1030, the frequency of the parallel resonator may be shifted from the frequency of the series resonator by thinning the piezoelectric plate forming the diaphragm of the series resonator. One or both of these steps are performed in any variation of process 1100.

The main difference between process 1100 and the previously described process is the addition of steps 1120 and 1125. At 1120, an optical measurement of the piezoelectric plate thickness is made using an optical thickness measurement tool, such as an ellipsometer/reflectometer. The optical thickness measuring tool may measure light reflected from the surface of the piezoelectric plate and from the interface between the piezoelectric plate and the substrate. Multiple wavelengths of light, angles of incidence, and/or polarization states may be used to measure reflection from a particular measurement point on the piezoelectric plate. The results of the multiple measurements are processed to determine the thickness of the piezoelectric plate at the measurement point.

The measurement process is repeated to determine the thickness of the piezoelectric plate at a plurality of measurement points on the surface of the piezoelectric plate. The plurality of points may for example form a grid or matrix of measurement points on the surface of the plate. The measurement data may be processed and interpolated to provide a thickness map of the piezoelectric plate.

At 1125, excess material is removed from the piezoelectric plate using a material removal tool, as previously shown in view C of fig. 7. The material removal tool may be, for example, a scanning ion mill or other tool. The scanning ion mill scans a beam of energetic ions over the surface of the piezoelectric plate. Incidence of the ion beam on the piezoelectric plate removes material from the surface by sublimation or sputtering. The ion beam may be scanned one or more times over the surface of the piezoelectric plate in a raster pattern. The ion current or residence time of the ion beam may be varied during the raster scan to control the depth of material removed from each point on the piezoelectric plate according to the thickness map of the piezoelectric plate. The result is a piezoelectric plate with significantly improved thickness uniformity. As previously mentioned, the thickness of any point on the piezoelectric plate may be substantially equal to the target thickness.

Optionally, the portion of the piezoelectric plate that is destined to become the diaphragm of the series resonator may be thinned at 1030. Damage to the exposed surface of the piezoelectric plate that occurs at 1125 and/or 1030 may be removed by post-processing at 1035, as previously described.

The remaining steps of process 1100 (shown in fig. 11) may be the same as the corresponding steps of process 800, except that if the piezoelectric plate is selectively thinned at 1030, a frequency setting dielectric layer is formed at 850. In either case, the RF test 860 and the frequency tuning 865/870 may occur after the cavity is formed at 810B or 810C. Alternatively, the remaining steps of process 1100 (not shown in fig. 11) may be the same as the corresponding steps of process 900, with RF test 960 and frequency tuning 965 occurring before 810B or 810C forms the cavity.

End language

Throughout the specification, the illustrated embodiments and examples should be considered as examples, rather than limitations on the disclosed or claimed apparatus and processes. Although many of the examples provided herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to achieve the same objectives. With respect to the flowcharts, additional steps and fewer steps may be taken, and the illustrated steps may be combined or further refined to implement the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, "plurality" refers to two or more. As used herein, a "set" of items may include one or more of such items. As used herein, the terms "comprising," "including," "carrying," "having," "containing," "involving," and the like, whether in the written detailed description or in the claims, are to be construed to be open-ended, i.e., to mean including but not limited to. With respect to the claims, the transitional phrases "consisting of …" and "consisting essentially of …" are closed or semi-closed transitional phrases. Ordinal terms such as "first," "second," "third," and the like in the claims are used to modify a claim element by itself without the intention of indicating a priority or order of execution of a method action by one claim element over another claim element, but are merely used to distinguish one claim element having a same name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, "and/or" means that the listed items are alternatives, but alternatives also include any combination of the listed items.

Claims

1. A filter device comprising:

a first bulk acoustic wave resonator comprising:

a piezoelectric layer, a portion of the piezoelectric layer forming a first diaphragm having a first thickness over a respective cavity of the first bulk acoustic resonator; and

a first interdigital transducer IDT having interleaved fingers at the first diaphragm, wherein all of the interleaved fingers of the first IDT are only at one surface of the first diaphragm and not on an opposite surface of the first diaphragm; and

a second bulk acoustic wave resonator comprising:

a portion of the piezoelectric layer forming a second diaphragm having a second thickness different from the first thickness and above a respective cavity of the second bulk acoustic wave resonator; and

a second IDT having interleaved fingers at the second membrane, wherein all of the interleaved fingers of the second IDT are only at one surface of the second membrane and are not on an opposite surface of the second membrane.

2. The filter device of claim 1, further comprising interleaved fingers of one or more additional bulk acoustic wave resonators having respective diaphragms having one of the following thicknesses: the first thickness, the second thickness, and a third thickness different from the first thickness or the second thickness.

3. The filter device of claim 2, wherein each additional bulk acoustic wave resonator includes a respective diaphragm over a respective cavity.

4. The filter device of claim 1, wherein the piezoelectric layer and all IDTs are configured such that respective radio frequency signals applied to the first IDT and the second IDT excite mainly respective shear acoustic modes within the respective diaphragms.

5. The filter device of claim 4, wherein the direction of acoustic energy flow of each respective shear acoustic mode is substantially orthogonal to the opposing surfaces of the respective diaphragm.

6. The filter device of claim 1, wherein the piezoelectric layer is one of lithium niobate and lithium tantalate.

7. The filter device of claim 1, wherein the second thickness is less than the first thickness and greater than or equal to 100nm, and the first thickness is less than or equal to 1500nm.

8. The filter device of claim 1, further comprising interleaved fingers of one or more additional IDTs provided at respective additional membranes, the additional membranes having respective thicknesses different from the first thickness and the second thickness.

9. The filter device of claim 1, wherein the first bulk acoustic wave resonator is a parallel resonator in a ladder filter circuit and the second bulk acoustic wave resonator is a series resonator in the ladder filter circuit.

10. The filter device of claim 9, further comprising one or more additional parallel resonators and one or more additional series resonators, wherein respective first diaphragms of all parallel resonators have the first thickness and respective second diaphragms of all series resonators have the second thickness.

11. The filter device of claim 9, wherein a difference between the first thickness and the second thickness is associated with a frequency offset between the series resonator and the parallel resonator.

12. The filter device of claim 11, further comprising a respective dielectric layer over each of the series resonator and the parallel resonator, and wherein a frequency offset between the series resonator and the parallel resonator is further associated with different thicknesses of the respective dielectric layers of the series resonator and the parallel resonator.

13. The filter device of claim 1, wherein the following are both associated with a frequency offset between the first bulk acoustic resonator and the second bulk acoustic resonator: a difference between the first thickness and the second thickness; and a first pitch of the first bulk acoustic wave resonator that is different from a second pitch of the second bulk acoustic wave resonator.

14. The filter device of claim 1, further comprising a respective dielectric layer over each of the first bulk acoustic wave resonator and the second bulk acoustic wave resonator, and wherein the following is associated with a frequency offset between the first bulk acoustic wave resonator and the second bulk acoustic wave resonator: the difference between the first thickness and the second thickness, the spacing varying between each respective IDT, and the different thicknesses of the respective dielectric layers of the first bulk acoustic wave resonator and the second bulk acoustic wave resonator.