CN119382655A

CN119382655A - A transversely excited film bulk acoustic resonator with multi-pitch interdigital transducers

Info

Publication number: CN119382655A
Application number: CN202411284363.2A
Authority: CN
Inventors: 布莱恩特·加西亚
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 2020-02-28
Filing date: 2021-02-22
Publication date: 2025-01-28
Also published as: DE102021104675A1; CN113328721A

Abstract

An acoustic resonator and a method of making the same are disclosed herein. The acoustic resonator comprises a single crystal piezoelectric plate having a front side and a back side, the back side being attached to a surface of a substrate, but a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate is not attached to the surface of the substrate. The conductor pattern on the front side comprises a multi-pitch interdigital transducer (IDT), the interlaced fingers of the IDT being disposed on the diaphragm.

Description

Transversely excited thin film bulk acoustic resonator with multi-section interdigital transducer

The application relates to a split application of a transverse excited film bulk acoustic resonator with a multi-section interdigital transducer, which is a Chinese application patent application No. 202110197006.2, the application of which is No. 2021, no. 2 and No. 22.

Technical Field

The present disclosure relates to radio frequency filters using acoustic wave resonators, and more particularly to bandpass filters with high power capability for use in 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 in which the insertion loss of the filter is less 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 (IoT) 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, enhanced security, higher reliability, etc. These improvements may be implemented individually or in combination at various levels of the wireless system, such as at the RF module, RF transceiver, mobile or fixed subsystem, or network level.

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. But these prior art techniques are not suitable for use at higher frequencies that are required for future communication networks.

To obtain a wider communication channel bandwidth, it is necessary to use a higher frequency communication band. The 3GPP (third generation partnership project) has standardized radio access technologies for mobile telephone networks. The radio access technology for the fifth generation mobile network is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communication bands. Two of these new communications bands are n77 and n79, where n77 uses a frequency range of 3300MHz to 4200MHz and n79 uses a frequency range of 4400MHz to 5000 MHz. Both frequency band n77 and frequency band n79 use Time Division Duplexing (TDD) so that communication devices operating in frequency band n77 and/or frequency band n79 use the same frequency for uplink and downlink transmissions. The bandpass filters of the n77 and n79 frequency bands must be able to handle the transmit power of the communication device. The 5G NR standard also defines millimeter wave communication bands having frequencies between 24.25GHz and 40 GHz.

Disclosure of Invention

An acoustic resonator includes a single crystal piezoelectric plate having a front side and a back side, the back side attached to a surface of a substrate, a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate, a conductor pattern formed on the front side, the conductor pattern including a multi-pitch interdigital transducer (IDT) having interleaved fingers of the IDT disposed on the diaphragm.

Wherein the piezoelectric plate and IDT are configured such that radio frequency signals applied to the IDT excite dominant shear acoustic modes in the diaphragm.

Wherein at any point along the length of the IDT, the pitch of the IDT is constant across the aperture of the IDT.

Wherein the mark of the IDT finger is constant over the whole IDT.

Wherein the multi-pitch IDT is divided into two or more sections along its length, each section having a respective pitch different from the pitches of the other sections.

Wherein the maximum pitch of the multi-pitch IDT of each of the two or more sections is p (1+delta), and the minimum pitch of the multi-pitch IDT of each of the two or more sections is p (1-delta), wherein p is a nominal pitch and delta is greater than 0 and less than or equal to 5.0%.

Wherein, delta is less than or equal to 1.0%.

Wherein the multi-pitch IDT is divided into three parts, and the pitches of the three parts are p (1-delta) and p (1+delta) respectively.

Wherein the pitch of the multi-pitch IDT continuously varies along the length of the IDT.

Wherein the pitch of the multisection IDT continuously varies between p (1-delta) and p (1+delta), where p is the nominal pitch of the IDT and delta is greater than 0 and less than or equal to 5.0%.

Wherein, delta is less than or equal to 1.0%.

A filter device is also disclosed that includes a single crystal piezoelectric plate having a front side and a back side, the back side attached to a surface of a substrate, portions of the piezoelectric plate forming a plurality of diaphragms that span respective cavities in the substrate, a conductor pattern formed on the front side, the conductor pattern including a plurality of interdigital transducers (IDTs) whose interleaved fingers are disposed on a respective one of the plurality of diaphragms, wherein a first IDT of the plurality of IDTs is a multi-pitch IDT.

Wherein the piezoelectric plate and the plurality of IDTs are configured such that each radio frequency signal applied to an IDT excites a dominant shear acoustic mode in the corresponding diaphragm.

Wherein all of the plurality of IDTs are multi-space IDTs.

Wherein the first IDT is divided into two or more sections along the length of the first IDT, and each section has a respective pitch different from the pitches of the other sections.

Wherein the maximum pitch of the first IDT of each of the plurality of sections is p (1+delta), and the minimum pitch of the first IDT of another of the plurality of sections is p (1-delta), wherein p is a nominal pitch, and delta is greater than 0 and less than or equal to 5.0%.

Wherein, delta is less than or equal to 1.0%.

Wherein the first IDT is divided into three portions, and pitches of the three portions are p (1-delta) and p (1+delta), respectively.

Wherein the pitch of the first IDT continuously varies along the length of the first IDT.

Wherein the pitch of the first IDT continuously varies between p (1-delta) and p (1+delta), where p is the nominal pitch of the IDT and delta is greater than 0 and less than or equal to 5.0%.

Wherein, delta is less than or equal to 1.0%.

Wherein a second IDT of the plurality of IDTs is a multi-pitch IDT, and a pitch change of the second IDT is different from a pitch change of the first IDT.

Wherein the second IDT is a part of a parallel resonator and the first IDT is a part of a series resonator.

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 enlarged schematic cross-sectional view of XBAR.

Fig. 4 is a diagram showing a shear-level acoustic mode in XBAR.

Fig. 5 is a graph of the admittance magnitude of an XBAR with a conventional interdigital transducer (IDT) as a function of frequency.

Fig. 6 is an expanded portion of the graph of fig. 5.

Fig. 7 is a plan view of a multi-pitch IDT.

Fig. 8 is a plan view of another multi-pitch IDT.

Fig. 9 is a graph of admittance magnitude of XBAR with multi-pitch IDT as a function of frequency.

Fig. 10 is an expanded portion of the graph of fig. 8.

Fig. 11 is a graph of input-output transfer functions (S2, 1) of a band-pass filter implemented with XBAR having a multi-pitch IDT.

Throughout the specification, elements appearing in the figures are assigned a three-digit or four-digit reference number 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

Description of the device

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 use in filters for communications bands having 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 parallel front and back faces 112, 114, 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 so that the orientation of the X, Y and Z crystal axes is known and consistent with respect to the direction of the front and back sides. In the example presented in this patent, the piezoelectric plate is Z-cut, that is, the Z-axis is perpendicular to the front and back faces 112, 114. However, XBAR can be fabricated on piezoelectric plates with other crystal orientations.

The back surface 114 of the piezoelectric plate 110 is attached to the surface of the substrate 120, but a portion of the piezoelectric plate 110 forming the diaphragm 115 across the cavity 140 formed in the substrate is not attached to the surface of the substrate 120. The portion of the piezoelectric plate that spans the cavity is referred to herein as the "diaphragm" 115 because this portion is physically similar to the diaphragm of the microphone. As shown in fig. 1, diaphragm 115 abuts the remainder of piezoelectric plate 110 around the entire perimeter 145 of chamber 140. In this case, "contiguous" means "continuously connected" without any other article in between. In other configurations, diaphragm 115 may abut the piezoelectric plate around at least 50% of perimeter 145 of chamber 140.

The substrate 120 provides mechanical support for the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material, or a combination of these materials. The back surface 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process. Or the piezoelectric plate 110 is grown on the substrate 120 or the piezoelectric plate 110 is attached to the substrate in some other way. The piezoelectric plate 110 may be directly attached to the substrate, or may be attached to the substrate 120 via one or more intermediate material layers (not shown in fig. 1).

The conventional meaning of "cavity" is "empty space within a solid". Cavity 140 may be a hole (as shown in cross-section AA and BB) completely through substrate 120, or may be a recess in substrate 120 below diaphragm 115. For example, the cavity 140 may be formed before or after attaching the piezoelectric plate 110 to the substrate 120, such as by selectively etching the substrate 120.

The conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 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. The radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites the dominant acoustic mode within the piezoelectric plate 110. As will be discussed in detail below, the primary acoustic mode is a bulk shear mode that propagates in a direction substantially 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 to be a laterally excited thin film bulk wave resonator.

The IDT 130 is placed on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on the membrane 115 of the piezoelectric plate, which spans or hangs over the cavity 140. As shown in fig. 1, the cavity 140 has a rectangular shape with a size larger 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 have more or less than four sides, which sides may be straight or curved.

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

Fig. 2 shows a detailed schematic cross-sectional view of the XBAR 100. The piezoelectric plate 110 is a single crystal layer of piezoelectric material having a thickness ts. ts may be, for example, 100nm to 1500nm. When used in filters for the LTE ^TM band (e.g., bands 42, 43, 46, n79, n 77) from 3.4GHZ to 6GHZ, 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. The front side dielectric layer 214 is formed only between the IDT fingers (e.g., IDT finger 238 b), or the front side dielectric layer 214 may be deposited as a cover layer such that a dielectric layer is formed between and over the IDT fingers (e.g., IDT finger 238 a). The front side dielectric layer 214 may be a non-piezoelectric dielectric material such as silicon dioxide or silicon nitride. tfd may be, for example, 0 to 500nm. tfd is typically less than the thickness ts of the piezoelectric plate. The front side dielectric layer 214 may be formed from multiple layers of two or more materials.

IDT fingers 238a and 238b can be aluminum, aluminum alloy, copper alloy, beryllium, gold, tungsten, molybdenum, or some other conductive material. An IDT finger is considered "substantially aluminum" if it is made of aluminum or an alloy containing at least 50% aluminum. An IDT finger is considered "substantially copper" if it is made of copper or an alloy containing at least 50% copper. Thin (relative to the total thickness of the conductor) layers of other metals (e.g., chromium or titanium) may be formed beneath and/or over the fingers and/or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers and/or to improve power handling. 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 geometry of the IDT of XBAR is significantly different from that used in Surface Acoustic Wave (SAW) resonators. In a SAW resonator, the pitch of the IDT is half the wavelength of the acoustic wave at the resonant frequency. In addition, the tag-to-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 IDT is typically 2 to 20 times the finger width w. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness ts of the piezoelectric plate 110. 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 easily 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 shows a detailed schematic cross-sectional view of a firmly mounted XBAR (SM XBAR) 300. SM XBAR is first described in application Ser. No. 16/381,141. SM XBAR 300 includes piezoelectric plate 110, idt (of which only fingers 336 and 338 are visible). The piezoelectric layer 110 has parallel front and back sides 112, 114. The dimension ts is the thickness of the piezoelectric plate 110. The width of the IDT fingers 336, 338 is the dimension w, the thickness of the IDT fingers is the dimension tm, and the IDT pitch is the dimension p.

Unlike the XBAR device shown in fig. 1 and 2, the IDTs of SM XBAR are formed on a membrane that spans a cavity in the substrate 120. Instead, the acoustic Bragg reflector 340 is sandwiched between the surface 222 of the substrate 220 and the back surface 114 of the piezoelectric plate 110. The term "sandwiched" means that the acoustic bragg reflector 340 is both disposed between the surface 222 of the substrate 220 and the back surface 114 of the piezoelectric plate 110 and mechanically attached to the surface 222 of the substrate 220 and the back surface 114 of the piezoelectric plate 110. In some cases, a thin additional layer of material may be disposed between the acoustic bragg reflector 340 and the surface 222 of the substrate 220 and/or between the bragg reflector 340 and the back surface 114 of the piezoelectric plate 110. For example, such additional layers of material may be present to facilitate bonding the piezoelectric plate 110, the acoustic Bragg reflector 340, and the substrate 220.

The acoustic bragg reflector 340 includes a plurality of dielectric layers alternating between a material having a high acoustic impedance and a material having a low acoustic impedance. "high" and "low" are relative terms. For each layer, the criteria for comparison are adjacent layers. The acoustic impedance of each "high" acoustic impedance layer is higher than the acoustic impedance of two adjacent low acoustic impedance layers. The acoustic impedance of each "low" acoustic impedance layer is lower than the acoustic impedance of two adjacent high acoustic impedance layers. As will be discussed later, the dominant acoustic mode in the XBAR piezoelectric plate is shear body waves. The thickness of each layer of acoustic bragg reflector 340 is equal to or about one quarter of the wavelength of a shear-body wave having the same polarization as the primary acoustic mode at or near the resonant frequency of SM XBAR 300. Dielectric materials having relatively low acoustic impedance include silicon dioxide, carbon-containing silicon oxide and certain plastics, such as crosslinked polyphenylene polymers. Materials with relatively high acoustic impedance include hafnium oxide, silicon nitride, aluminum nitride, silicon carbide. All of the high acoustic impedance layers of the acoustic bragg reflector 340 need not be the same material and all of the low acoustic impedance layers need not be the same material. In the example of fig. 3. The acoustic bragg reflector 340 has six total layers. The acoustic bragg reflector may have more or less than six layers.

Fig. 4 is a graphical representation of the dominant acoustic modes 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, the three interleaved IDT fingers 430 alternating between fingers and their electrical polarities. An RF voltage is applied to the interleaved fingers 430. This voltage creates a time-varying electric field between the fingers. As indicated by the arrow labeled "electric field," the direction of the electric field is primarily transverse or parallel to the surface of the piezoelectric plate 410. Due to the high dielectric constant of the piezoelectric plate, RF power is highly concentrated in the plate relative to air. The lateral electric field induces shear deformation in the piezoelectric plate 410 that (at the resonant frequency defined by the acoustic cavity formed by the volume between the two surfaces of the piezoelectric plate) is strongly connected to the shear dominant acoustic mode. In this context, "shear deformation" is defined as a deformation in a material in which parallel planes remain substantially parallel and remain constantly separated as they translate relative to one another. A "shear acoustic mode" is defined as an acoustic vibration mode in a medium that results in shear deformation of the medium. The shear deformation in XBAR 400 is represented by curve 460, with adjacent small arrows indicating the direction and relative magnitude of atom motion at the resonant frequency. The extent of atomic motion and the thickness of the piezoelectric plate 410 are greatly exaggerated for ease of viewing. Although the atomic motion is primarily lateral (i.e., horizontal as shown in fig. 4), the direction of acoustic energy flow of the primary acoustic mode of excitation is substantially perpendicular to the piezoelectric plate surface, as indicated by arrow 465.

As shown in fig. 4, there is substantially no electric field directly under IDT finger 430, so that acoustic modes are only minimally excited in region 470 under the finger. Short acoustic movements may occur in these areas. Since acoustic vibrations are not excited under IDT finger 430, acoustic energy coupled to IDT finger 430 is low (e.g., as compared to IDT fingers in SAW resonators), which minimizes viscous losses in IDT fingers.

Acoustic resonators based on shear acoustic resonance perform better than the current state-of-the-art Film Bulk Acoustic Resonators (FBAR) and firmly mounted resonator bulk acoustic wave (SMR BAW) devices, in which the electric field is applied in the thickness direction. In such devices, the acoustic modes are compressed in the direction of atomic motion and acoustic energy flow in the thickness direction. Furthermore, the piezoelectric coupling for shear wave XBAR resonance may be higher (> 20%) compared to other acoustic resonators. Thus, high voltage electrical coupling can design and implement microwave and millimeter wave filters with considerable bandwidth.

Fig. 5 is a graphical representation 500 of the admittance magnitude versus frequency of a first XBAR comprising a conventional (i.e., uniform pitch) IDT. Admittance is determined by modeling the first XBAR using a finite element method. Line 510 is a plot of admittance magnitude. The shear dominant acoustic mode of the first XBAR has the largest admittance at the resonant frequency FR and the smallest admittance at the antiresonant frequency FAR. Admittance diagram 510 also exhibits a plurality of spurious modes or secondary resonances, including substantial spurious modes at a frequency of about 1.825 GHz.

At least some of the spurious modes found in XBAR are propagating plate waves. The frequency of the propagating plate mode is proportional to the IDT finger pitch. In contrast, the XBAR resonance and antiresonance frequencies have only a small dependence on IDT pitch. For example, changing the IDT pitch from 7.5 times the thickness of the piezoelectric plate to 15 times the thickness of the piezoelectric plate (i.e., a 2:1 change) results in a change in the resonant frequency of the XBAR of about 3%.

Small changes in IDT pitch in XBAR can result in cancellation or destructive interference of spurious modes with negligible impact on the shearing principal mode. This effect is illustrated in fig. 6, and fig. 6 is an enlarged view of a portion of the graph of fig. 5, which contains the largest spurious mode. In fig. 6, a solid line 610 is a plot of admittance magnitude of an XBAR with a conventional IDT as a function of frequency, as previously shown in fig. 5. Dashed curve 620 is a plot of the admittance magnitude of XBAR versus frequency for an IDT pitch increase of 0.5%. Increasing the IDT pitch by this amount reduces the frequency of spurious modes by about 10MHz, thereby aligning the maximum admittance of curve 610 with the minimum admittance of curve 620. If two resonators with these admittance properties are placed in parallel, these two spurious modes will cancel each other at least to some extent. Increasing IDT pitch by 0.5% has negligible effect on the resonance and anti-resonance frequencies of the shear dominant acoustic mode of XBAR.

Fig. 7 is a plan view of an exemplary multi-pitch IDT 700. The "multi-space IDT" is an IDT in which the pitch between IDT fingers varies along the length of the IDT. At any given point along the length, the pitch does not change across the aperture of the IDT. Furthermore, the mark or finger width of a multi-pitch IDT typically remains unchanged across the IDT.

The multi-pitch IDT 700 includes a first busbar 732 and a second busbar 734, and a plurality of interleaved fingers, such as fingers 736. The interleaved fingers extend alternately from the first and second bus bars 732, 734. The multi-pitch IDT 700 is divided into three sections along the length L of the IDT, identified as section a, section B and section C, respectively. Each of sections a, B and C includes 20 fingers, for a total of 60 fingers in multi-pitch IDT 700. The use of three sections and 60 fingers is exemplary. The total number of fingers of the IDT may be more or less than 60. The IDT can be divided into two or more sections along its length, each section comprising a plurality of adjacent fingers. The total number of fingers may be substantially equally divided between two or more portions. In this case, "substantially" means "as close as possible". For example, an IDT having 100 fingers is substantially equally divided, wherein the 100 fingers are divided into 3 sections, and the 3 sections have 33, 34 and 33 fingers, respectively. The total number of fingers may be unevenly distributed between the two or more portions.

In this example, the pitch p of the B portion is the nominal pitch of the IDT. The pitch of the A part is p (1-delta), and the pitch of the C part is p (1+delta). Delta is greater than 0 and less than or equal to 5%. Delta is typically less than 1%. Delta may be selected during filter design to most effectively reduce spurious modes. At any point along the length L of IDT 700, the pitch is constant across aperture a. The mark or width of the IDT finger is constant and the same in all sections. When the IDT is divided into two or more portions, the maximum pitch may be p (1+δ), and the minimum pitch may be p (1+δ).

In the example multi-pitch IDT 700, the pitch monotonically increases from left (as shown) to right. This is not necessarily the case in all multi-pitch IDTs. Portions of the multi-pitch IDT can be arranged in other sequences. Further, in the multi-pitch IDT 700, the pitch variation between adjacent portions is constant. This is not necessarily the case in all multi-pitch IDTs. The pitch variation between adjacent sections may be the same or different.

Fig. 8 is a plan view of another multi-pitch IDT800 with a continuously-varying pitch. IDT800 includes a first bus bar 832 and a second bus bar 834, and a plurality of interleaved fingers, such as fingers 836. The interleaved fingers extend alternately from the first bus bar 832 and the second bus bar 834. IDT800 is not divided into sections, but continuously changes pitch along its length. IDT800 has 60 fingers, which is exemplary. The total number of fingers of the IDT may be more or less than 60.

As shown in fig. 8, the pitch of the left edge of IDT 800 is p (1- δ), and the pitch of the right edge of IDT 800 is p (1+δ). The pitch varies continuously between these two extremes. The change in pitch is typically, but not necessarily, a linear function of position along the length L of the IDT. Delta is greater than 0, less than or equal to 5%, typically less than 1%. Delta may be selected during filter design to most effectively reduce spurious modes. At any point along the length of IDT 800, the pitch is constant across aperture a. The mark or width of the IDT finger is constant across the IDT.

IDTs 700 and 800 can be incorporated into XBAR shown in fig. 1 and 2 or SMXBAR shown in fig. 3.

Fig. 9 is a graphical representation 900 of the admittance magnitude of a second XBAR comprising IDTs of varying pitch similar to IDT 700 of fig. 7 as a function of frequency. The IDT is divided into three sections along its length. The pitches of the three portions were 3.589, 3.6, and 3.611 μm (δ=0.3%), respectively. The second XBAR is identical to the first XBAR having the admittance characteristics previously shown in fig. 5 except for the IDT pitch. Admittance was determined by modeling the second XBAR using the finite element method. Line 910 is a plot of the magnitude of the admittance of the second XBAR. The shear dominant acoustic mode of the second XBAR has the largest admittance at the resonant frequency FR and the smallest admittance at the antiresonant frequency FAR. The resonance and antiresonance frequencies are the same as those of the XBAR with the IDT of uniform pitch. Admittance plot 910 also exhibits multiple spurious modes or secondary resonances. A comparison of fig. 5 and 9 shows that the size of all spurious modes is reduced in the second XBAR due to the use of IDTs with varying pitches.

Fig. 10 shows an expanded portion of the graph of fig. 9, which contains the largest spurious modes. In fig. 10, a solid line 1010 is a graph of admittance magnitude versus frequency for an XBAR that includes IDTs with varying pitches, as shown in fig. 9. Dashed line 1020 is a plot of the admittance magnitude of an XBAR with a conventional uniform pitch IDT as shown in fig. 6 as a function of frequency. Adding a multi-pitch IDT reduces the peak of spurious modes by 5dB.

Fig. 11 is a diagram of the size of S2,1 for two bandpass filters implemented with XBAR devices, S2,1 being the input/output transfer function. The S2,1 data is determined by simulating two filters using a finite element method. The solid line 1110 is a diagram of S2,1 using the first filter of XBAR with multi-pitch IDT. The first filter uses a ladder circuit with four series and four parallel resonators. As shown in fig. 7, each resonator includes an IDT divided into three equal sections along its length. The parameter delta is 0.3% for the series resonator and 0.4% for the parallel resonator.

Dashed line 1120 is a plot of S2,1 for a second band pass filter having a uniform pitch IDT, but otherwise identical to the first band pass filter. Comparison of curves 1110 and 1120 shows that the pass bands of the two filters are virtually identical. The spurious mode peak admittance of the first filter with the multi-pitch IDT is reduced by up to 8dB compared to the second filter.

The filters used to generate the data shown in fig. 11 are exemplary. The filter may have fewer or more than five resonators, as well as fewer or more than three series resonators and two parallel resonators. The multi-pitch IDT may be divided into two portions or more than three portions, or may be continuous. The number of portions may not be the same for all resonators in the filter, and the filter may include segmented and continuous multi-pitch IDTs. The value delta may be different for some or all of the resonators. The filter may comprise a combination of resonators having a uniform pitch and multi-pitch resonators.

All examples discussed above are directed to conventional XBAR as shown in fig. 1 and 2. As shown in fig. 3, multi-range IDTs can also be used to reduce spurious modes in firmly mounted XBARs. Similar reductions in spurious mode amplitude can be expected.

End language

Throughout the specification, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and processes disclosed or claimed. 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, only 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 bulk acoustic wave resonator comprising:

a piezoelectric layer attached to the substrate directly or through one or more intermediate material layers, a portion of the piezoelectric layer being located over the cavity below the piezoelectric layer;

a conductor pattern at the piezoelectric layer comprising an interdigital transducer, IDT, the IDT having interleaved fingers on the portion of the piezoelectric layer,

Wherein the center-to-center spacing between the interleaved fingers varies along the length of the IDT such that the pitch of one portion of the IDT is constant but varies relative to the constant pitch of the immediately adjacent portion of the IDT, the pitch being the center-to-center spacing between the interleaved fingers, and

Wherein the width of the interleaved fingers in the one portion is the same as the width of the interleaved fingers in the immediately adjacent portion.

2. The bulk acoustic wave resonator of claim 1, wherein the piezoelectric layer and the IDT are configured such that a radio frequency signal applied to the IDT excites shear acoustic modes in the piezoelectric layer.

3. The bulk acoustic wave resonator according to claim 1, wherein the piezoelectric layer and the IDT are configured to excite an acoustic wave having an electric field excited laterally in a thickness direction of the piezoelectric layer, wherein propagation of the acoustic wave is perpendicular to a direction of the electric field.

4. The bulk acoustic wave resonator of claim 1, wherein a pitch variation between the one portion and the immediately adjacent portion is less than or equal to 5%.

5. The bulk acoustic wave resonator according to claim 1, wherein the one portion of the IDT has a nominal pitch p, and the pitch of the immediately adjacent portions is increased relative to the nominal pitch.

6. The bulk acoustic wave resonator of claim 1, wherein the one portion of the IDT has a nominal pitch p, and the pitch of immediately adjacent IDT portions is reduced relative to the nominal pitch.

7. The bulk acoustic wave resonator of claim 1, wherein the IDT includes the one portion, the immediately adjacent portion, and additional portions, each portion including a plurality of adjacent fingers.

8. The bulk acoustic wave resonator of claim 1, wherein the width of the interleaved fingers is constant across a length L of the IDT, L being the distance between the outermost fingers of the IDT.

9. The bulk acoustic resonator of claim 1, wherein the conductor pattern further comprises a first busbar and a second busbar, and wherein the interleaved fingers extend alternately from the first busbar and the second busbar.

10. The bulk acoustic resonator of claim 9, wherein the first and second bus bars are made of a different material than the interleaved fingers.

11. The bulk acoustic wave resonator of claim 9, wherein the first and second bus bars are thicker than a thickness tm of the interleaved fingers.

12. The bulk acoustic wave resonator of claim 1 further comprising a dielectric layer over and between the interleaved fingers.

13. The bulk acoustic wave resonator according to claim 1, wherein a pitch of the one portion of the IDT is substantially constant, but varies with respect to a substantially constant pitch of the immediately adjacent portion of the IDT.

14. The bulk acoustic wave resonator of claim 1, wherein the width of the interleaved fingers in the one portion is substantially the same as the width of the interleaved fingers in the immediately adjacent portion.

15. A bulk acoustic wave resonator filter comprising:

a first bulk acoustic wave resonator in a ladder filter circuit, the first bulk acoustic wave resonator comprising:

Wherein the center-to-center spacing between the interleaved fingers varies along the length of the IDT such that the pitch of one portion of the IDT is constant but different from the constant pitch of the immediately adjacent portion of the IDT, the pitch being the center-to-center spacing between the interleaved fingers, and

A second bulk acoustic wave resonator in a ladder filter circuit, the second bulk acoustic wave resonator comprising:

Wherein the center-to-center spacing between the interleaved fingers varies along the length of the IDT such that the pitch of one portion of the IDT is constant but different than the constant pitch of the immediately adjacent portion of the IDT, the pitch being the center-to-center spacing between the interleaved fingers,

Wherein a change in pitch between the one portion of the first bulk acoustic wave resonator and the immediately adjacent portion of the first bulk acoustic wave resonator is different from a change in pitch between the one portion of the second bulk acoustic wave resonator and the immediately adjacent portion of the second bulk acoustic wave resonator.

16. The bulk acoustic wave resonator filter of claim 15 wherein the first bulk acoustic wave resonator is a series resonator in the ladder filter circuit.

17. The bulk acoustic wave resonator filter of claim 16 wherein the second bulk acoustic wave resonator is a parallel resonator in the ladder filter circuit.

18. The bulk acoustic wave resonator filter of claim 15 wherein the value δ is a change from portion to portion, and wherein δ differs between the first bulk acoustic wave resonator and the second bulk acoustic wave resonator.

19. The bulk acoustic wave resonator filter of claim 15 wherein the width of the interleaved fingers in the one portion of the first bulk acoustic wave resonator is the same as the width of the interleaved fingers in the immediately adjacent portion.

20. The bulk acoustic wave resonator filter of claim 15 wherein the width of the interleaved fingers in the one portion of the second bulk acoustic wave resonator is the same as the width of the interleaved fingers in the immediately adjacent portion.

21. A bulk acoustic wave resonator comprising:

a piezoelectric layer attached to the substrate either directly or through one or more intermediate material layers;

A conductor pattern at the piezoelectric layer comprising an interdigital transducer, IDT, having interleaved fingers on the piezoelectric layer,

22. The bulk acoustic wave resonator of claim 21 wherein the piezoelectric layer and the IDT are configured such that radio frequency signals applied to the IDT excite shear acoustic modes in the piezoelectric layer.

23. The bulk acoustic wave resonator of claim 21 wherein the piezoelectric layer and the IDT are configured to excite an acoustic wave having an electric field excited transversely in a thickness direction of the piezoelectric layer, wherein propagation of the acoustic wave is perpendicular to a direction of the electric field.

24. The bulk acoustic wave resonator of claim 21 wherein the pitch variation between the one portion and the immediately adjacent portion is less than or equal to 5%.

25. The bulk acoustic wave resonator of claim 21 wherein the one portion of the IDT has a nominal pitch p and the pitch of the immediately adjacent portions is increased relative to the nominal pitch.

26. The bulk acoustic wave resonator of claim 21 wherein the one portion of the IDT has a nominal pitch p and the pitch of the immediately adjacent portions is reduced relative to the nominal pitch.

27. The bulk acoustic wave resonator of claim 21, wherein the IDT includes the one portion, the immediately adjacent portion, and additional portions, each portion including a plurality of adjacent fingers.

28. The bulk acoustic wave resonator of claim 21 wherein the width of the interleaved fingers is constant across the length L of the IDT, L being the distance between the outermost fingers of the IDT.

29. The bulk acoustic resonator of claim 21, wherein the conductor pattern further comprises a first busbar and a second busbar, and wherein the interleaved fingers extend alternately from the first busbar and the second busbar.

30. The bulk acoustic resonator of claim 29, wherein the first and second bus bars are made of a different material than the interleaved fingers.

31. The bulk acoustic wave resonator of claim 29 wherein the first and second bus bars are thicker than a thickness tm of the interleaved fingers.

32. The bulk acoustic wave resonator of claim 21 further comprising a dielectric layer over and between the interleaved fingers.

33. The bulk acoustic wave resonator of claim 21 further comprising a cavity of the bulk acoustic wave resonator, wherein a portion of the piezoelectric layer and the interleaved fingers are located over the cavity.

34. The bulk acoustic wave resonator of claim 21 further comprising an acoustic reflector between the piezoelectric layer and the substrate.

35. The bulk acoustic resonator of claim 34 wherein the piezoelectric layer and the interleaved fingers are located above the acoustic reflector.

36. The bulk acoustic resonator of claim 34, wherein the acoustic reflector is an acoustic bragg reflector.

37. The bulk acoustic resonator of claim 34 wherein the acoustic reflector comprises a plurality of layers alternating between a layer of high acoustic impedance and a layer of low acoustic impedance.

38. A bulk acoustic wave resonator comprising:

Wherein a pitch is defined as a center-to-center spacing between the interleaved fingers, wherein the pitch varies along the length of the IDT such that a nominal pitch of a first portion of the IDT is constant but varies relative to a constant nominal pitch of an immediately adjacent portion of the IDT, and

Wherein the width of the interleaved fingers in the first portion is the same as the width of the interleaved fingers in the immediately adjacent portion.

39. The bulk acoustic wave resonator of claim 38 wherein the plurality of portions of the IDT include three or more consecutive interleaved fingers.

40. A bulk acoustic wave resonator comprising:

Wherein the center-to-center spacing between the interleaved fingers varies along the length of the IDT, an

Wherein the width of the interleaved fingers is constant throughout the IDT.