Thin film bulk acoustic resonator with small lateral excitation for enhanced q-factor

Abstract

An acoustic resonator device includes a conductor pattern formed on a surface of a piezoelectric plate. The conductor pattern includes a first bus line, a second bus line, and n interleaved parallel fingers of an interdigital transducer (IDT), where n is a positive integer. The fingers alternate from the first bus line and the second bus line. A first finger and ann'th finger are disposed at opposite ends of the IDT. The conductor pattern further includes a first reflector element proximate and parallel to the first finger, and a second reflector element proximate and parallel to then'th finger. When an RF signal is applied between the first bus line and the second bus line, the first reflector element is substantially at the same electrical potential as the first finger, and the second reflector element is substantially at the same electrical potential as then'th finger.

Description

[0001] Copyright and Trademark Notice

[0002] A portion of the disclosure of this patent document contains material that is subject to copyright protection. This patent document can show and / or describe matters which can appear as or can be patentably similar to the owner's trademarks. The copyright and trademark owner has no objection to facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office files or records, but otherwise reserves all copyright and trademark rights whatsoever.

[0003] Cross Reference to Related Applications

[0004] This patent claims priority to provisional patent application entitled “SAMLL HIGH Q XBAR RESONATORS” filed on April 20, 2020, application number 63 / 012,849; provisional patent application entitled “SAMLL FEFLECTORTS TO IMPROVE XBAR LOSS” filed on August 17, 2020, application number 63 / 066,520; and provisional patent application entitled “SMALL REFLECTORS TO IMPROVE PERFORMANCE OF TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS AT A SPECIFIED FREQUENCY” filed on September 4, 2020, application number 63 / 074,991. All of these applications are incorporated herein by reference. TECHNICAL FIELD

[0005] The present disclosure relates to radio frequency filters using acoustic wave resonators, and in particular to filters for use in communication devices. BACKGROUND

[0006] A radio frequency (RF) filter is a two-terminal device configured to pass some frequencies and stop others, where “pass” means to transmit with relatively low signal loss, and “stop” means to block or substantially attenuate. The range of frequencies passed by a filter is called the “passband” of the filter. The range of frequencies stopped by such a filter is called the “stopband” of the filter. A typical RF filter has at least one passband and at least one stopband. The specific requirements for a passband or stopband depend on the specific application. For example, a “passband” can be defined as a range of frequencies in which the insertion loss of the filter is less than a defined value, such as 1 dB, 2 dB, or 3 dB. A “stopband” can be defined as a range of frequencies in which the rejection of the filter is greater than a defined value, such as 20 dB, 30 dB, 40 dB, or more, depending on the specific application.

[0007] RF filters are used in communication systems that transmit information over wireless links. For example, RF filters can be found in the 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, point-to-point radio links, and other communication systems. RF filters are also used in radar and electronic and information warfare systems.

[0008] RF filters often require a number of design trade-offs to achieve the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost for each particular application. Particular design and fabrication methods and enhancements can benefit one or several of these requirements at the same time.

[0009] Enhancements in the performance of RF filters in wireless systems can have a wide-ranging impact on system performance. System performance can be improved by improving RF filters, such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be achieved individually or in combination at various levels of the wireless system, such as at the RF module, RF transceiver, mobile or fixed sub-system, or network level.

[0010] High performance RF filters for current communication systems typically incorporate acoustic resonators, including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic resonators (FBARs), and other types of acoustic resonators. However, these prior art technologies are not suitable for use at higher frequencies, which are needed for future communication networks.

[0011] To achieve wider communication channel bandwidths, higher frequency communication bands will need to be used. The 3GPP (Third Generation Partnership Project) has standardized radio access technologies for mobile phone networks. The 5G NR (New Radio) standard defines radio access technologies for fifth generation mobile networks. The 5G NR standard defines several new communication bands. Two of these new communication bands are n77 and n79, where n77 uses a frequency range of 3300 MHz to 4200 MHz and n79 uses a frequency range of 4400 MHz to 5000 MHz. Both band n77 and band n79 use time division duplexing (TDD), so a communication device operating in band n77 and / or band n79 uses the same frequency for uplink and downlink transmissions. Bandpass filters for the n77 and n79 bands must be able to handle the transmit power of the communication device. The 5G NR standard also defines millimeter wave communication bands with frequencies between 24.25 GHz and 40 GHz.

[0012] A laterally excited thin film bulk acoustic resonator (XBAR) is an acoustic resonator structure used for microwave filters. Such XBARs are described in U.S. Patent 10,491,291 entitled "LATERALLY EXCITED THIN FILM BULK ACOUSTIC RESONATOR." An XBAR resonator includes an interdigital transducer (IDT) formed on a thin free layer or diaphragm of single crystal piezoelectric material. The IDT includes a first set of parallel fingers extending from a first bus bar and a second set of parallel fingers extending from a second bus bar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric film. The XBAR resonator provides very high electromechanical coupling and high frequency capability. XBAR resonators can be used in a variety of RF filters, including band reject filters, band pass filters, duplexers, and multiplexers. XBARs are well suited for use in filters for communication bands having frequencies above 3 GHz. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 includes a schematic plan view of a laterally excited thin film bulk acoustic resonator (XBAR), two schematic cross-sectional views, and a detailed view.

[0014] Figure 2 is a schematic block diagram of a band pass filter using an acoustic resonator.

[0015] Figure 3 is a plot of the Q factor of an XBAR versus the number of fingers in an interdigital transducer (IDT) of the XBAR.

[0016] Figure 4 is a schematic plan view of an IDT having reflector elements.

[0017] Figure 5 is a schematic plan view of another IDT having reflector elements.

[0018] Figure 6 is a plot comparing the normalized Q factor of an XBAR having reflector elements and an XBAR not having reflector elements at a resonant frequency.

[0019] Figure 7 is a plot comparing the normalized Q factor of an XBAR having reflector elements and an XBAR not having reflector elements at an anti-resonant frequency.

[0020] Figure 8 is a plot showing the relative Q factor for a representative XBAR as a function of reflector element spacing and label at a frequency of 5150 MHz.

[0021] Figure 9 is a plot showing the relative Q factor for an XBAR having two reflector elements at each end as a function of frequency and reflector element label.

[0022] Figure 10 is a plot showing the relative Q-factor of an XBAR with one reflector element at each end as a function of frequency and reflector element label.

[0023] Figure 11 is a plot showing the relative Q-factor of an XBAR with five reflector elements at each end as a function of frequency and reflector element label.

[0024] Figure 12 is a plot comparing the performance of two bandpass filters using XBARs with and without reflector elements.

[0025] Figure 13 is a flowchart of a method for fabricating an XBAR or filter using an XBAR.

[0026] Throughout the specification, elements appearing in the drawings are assigned three or four digit reference numbers, where the two least significant digits are unique to that element, and the most significant digit or digits are the figure number in which the element is first shown. It can be assumed that elements described without being bound to the drawings have the same characteristics and functions as previously described elements having the same reference number. DETAILED DESCRIPTION

[0027] Description of the Device

[0028] Figure 1 A simplified schematic top view and orthogonal cross-sectional views of an XBAR 100 are shown. XBAR resonators such as resonator 100 can be used in a variety of RF filters, including bandstop filters, bandpass filters, duplexers, and multiplexers.

[0029] XBAR 100 is composed of thin film conductor patterns formed on the surface of a piezoelectric plate 110, which has parallel front and back faces 112, 114, respectively. The piezoelectric plate is a thin single crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut so that the directions of the X, Y, and Z crystal axes are known and consistent with respect to the front and back faces. The piezoelectric plate can be Z-cut, that is, the Z axis is perpendicular to the front and back faces 112, 114. The piezoelectric plate can be a rotated Z-cut or a rotated YX-cut. XBARs can be fabricated on piezoelectric plates having other crystal orientations.

[0030] The back side 114 of the piezoelectric plate 110 is attached to the surface of the substrate 120, but a portion of the piezoelectric plate 110 is not attached to the surface of the substrate 120. This portion of the piezoelectric plate 110 forms a diaphragm 115, which spans a cavity 140 formed in the substrate. The portion of the piezoelectric plate spanning the cavity is referred to herein as the "diaphragm" 115 because this portion is physically similar to the diaphragm of a microphone. Figure 1 As shown, the diaphragm 115 is adjacent to the remainder of the piezoelectric plate 110 around the entire periphery 145 of the cavity 140. In this case, "adjacent" means "continuous connection without any other items in between". In other configurations, the diaphragm 115 may be adjacent to the piezoelectric plate for at least 50% of the periphery 145 of the cavity 140.

[0031] Substrate 120 provides mechanical support for piezoelectric plate 110. Substrate 120 can be, for example, silicon, sapphire, quartz, or some other material, or a combination of these materials. The back side 114 of piezoelectric plate 110 can be bonded to substrate 120 using wafer bonding processes. Alternatively, piezoelectric plate 110 can be grown on substrate 120, or attached to substrate in some other way. Piezoelectric plate 110 can be directly attached to substrate, or it can be attached via one or more intermediate material layers (…). Figure 1 (Not shown) is attached to substrate 120.

[0032] The conventional meaning of "cavity" is "empty space within a solid". Cavity 140 can be a hole that passes completely through substrate 120 (as shown in cross sections AA and BB), or it can be a groove in substrate 120 below diaphragm 115. For example, cavity 140 can be formed by selectively etching substrate 120 before or after attaching piezoelectric plate 110 to substrate 120.

[0033] The conductor pattern of XBAR 100 includes an interdigital transducer (IDT) 130. IDT 130 includes a first plurality of parallel fingers, such as fingers 136, and a second plurality of fingers, wherein the first plurality of parallel fingers extend from a first busbar 132, and the second plurality of fingers extend from a second busbar 134. The term "busbar" refers to a conductor from which the fingers of the IDT extend. The first and second plurality of parallel fingers are staggered. The staggered fingers overlap by a distance AP, which is commonly referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of IDT 130 is the "length" of the IDT.

[0034] The first and second buses 132 and 134 serve as terminals of the XBAR 100. An radio frequency or microwave signal applied between the two buses 132 and 134 of the IDT 130 excites the dominant acoustic mode within the piezoelectric plate 110. The dominant acoustic mode is a volume shear mode, in which acoustic energy 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. Therefore, the XBAR is considered a transversely excited thin-film volume wave resonator.

[0035] IDT 130 is placed on piezoelectric plate 110 such that at least the fingers of IDT 130 are positioned on diaphragm 115, which spans or hangs over cavity 140. Figure 1 As shown, the cavity 140 is rectangular, and the size of the rectangle is greater than the aperture AP and the length L of IDT 130. The cavity of the XBAR can have different shapes, such as regular or irregular polygons. The cavity of the XBAR can have more or fewer four sides, which can be straight or curved.

[0036] To facilitate Figure 1 As shown, the geometric pitch and width of the IDT fingers are significantly enlarged relative to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR in an IDT 130 has more than ten parallel fingers. A single XBAR in an IDT 130 may have hundreds, possibly thousands, of parallel fingers. Similarly, the thickness of the IDT fingers and piezoelectric plate is significantly enlarged in the cross-sectional view.

[0037] Referring now to a detailed schematic cross-sectional view, a front dielectric layer 150 may optionally be formed on the front side of the piezoelectric plate 110. By definition, the “front side” of XBAR refers to the side facing away from the substrate. The front dielectric layer 150 may be formed only between the IDT fingers (e.g., IDT fingers 138b), or it may be deposited as a capping layer such that the dielectric layer is formed between and on the IDT fingers (e.g., IDT fingers 138a). The front dielectric layer 150 may be a non-piezoelectric dielectric material, such as silicon dioxide, aluminum oxide, or silicon nitride. The thickness of the front dielectric layer 150 is typically less than about one-third of the thickness tp of the piezoelectric plate 110. The front dielectric layer 150 may be formed from multiple layers of two or more materials. In some applications, a back dielectric layer (not shown) may be formed on the back side of the piezoelectric plate 110.

[0038] IDT fingers 138a and 138b may be one or more layers of aluminum, aluminum alloy, copper, copper alloy, beryllium, gold, tungsten, molybdenum, chromium, titanium, or certain other conductive materials. If the IDT fingers are made of aluminum or an alloy containing at least 50% aluminum, the IDT fingers are considered "substantially aluminum." If the IDT fingers are made of copper or an alloy containing at least 50% copper, the IDT fingers are considered "substantially copper." A thin layer (relative to the total thickness of the conductor) of other metals (e.g., chromium or titanium) or other metal layers may be formed below and / or above the fingers as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or passivate or encapsulate the fingers and / or improve power handling. The busbar of the IDT ( Figure 1 132 and 134 in the text can be made of the same or different material as the finger.

[0039] Dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which can be referred to as the pitch of the IDT and / or the pitch of the XBAR. Dimension m is the width or "mark" of the IDT fingers. The geometry of the IDT in an XBAR is significantly different from that used in a surface acoustic wave (SAW) resonator. In a SAW resonator, the pitch of the IDT is half the wavelength of the sound wave at the resonant frequency. Additionally, the mark pitch ratio of an SAW resonator IDT is typically close to 0.5 (i.e., the width of the mark or finger is approximately one-quarter of the wavelength of the sound wave at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the finger width w. The pitch p is typically 3.3 to 5 times the finger width w. Furthermore, the pitch p of the IDT is typically 2 to 20 times the thickness of the piezoelectric plate 110. The pitch p of the IDT is typically 5 to 12.5 times the thickness of the piezoelectric plate 210. The width *m* of the IDT fingers 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 fingers can be 500 nm or greater, thus allowing the IDT to be easily fabricated using photolithography. The thickness of the IDT fingers can range from 100 nm to approximately equal to the width *m*. The thickness of the IDT's busbars (132, 134) can be equal to or greater than the thickness *tm* of the IDT fingers.

[0040] Figure 2 This is a schematic circuit diagram and layout of a high-frequency bandpass filter 200 using XBAR. The filter 200 has a conventional trapezoidal filter architecture, comprising three series resonators 210A, 210B, and 210C and two parallel resonators 220A and 220B. The three series resonators 210A, 210B, and 210C are connected in series between the first and second ports (hence the term "series resonators"). Figure 2In the diagram, the first and second ports are labeled "In" and "Out," respectively. However, filter 200 is bidirectional, and either port can be used as the filter's input or output. Two parallel resonators 220A and 220B are connected to ground from the node between the series resonators. The filter may include additional reactive components, such as capacitors and / or inductors (not shown in the diagram). Figure 2 (As shown in the diagram). All parallel and series resonators are XBARs. An example is three series resonators and two parallel resonators. A filter may have more or fewer than five total resonators, more or fewer than three series resonators, and more or fewer than two parallel resonators. Typically, all series resonators are connected in series between the filter's input and output. Typically, all parallel resonators are connected between ground and the node between the input, output, or two series resonators.

[0041] In the exemplary filter 200, three series resonators 210A, B, C and two parallel resonators 220A, B are formed on a single plate 230 of piezoelectric material bonded to a silicon substrate (not visible). In some filters, the series and parallel resonators may be formed on different plates of piezoelectric material. Each resonator includes a respective IDT (not shown), wherein at least the fingers of the IDT are disposed above a cavity in the substrate. In this and similar context, the term "respective" means "to associate things one-to-one," that is, to have a one-to-one correspondence. Figure 2 In the diagram, the cavity is schematically shown as a dashed rectangle (e.g., rectangle 235). In this example, each IDT is positioned above its respective cavity. In other filters, the IDTs of two or more resonators can be positioned on a single cavity.

[0042] Each resonator 210A, 210B, 210C, 220A, 220B in filter 200 resonates with a very high admittance and exhibits anti-resonance with a very low admittance. Resonance and anti-resonance occur at the resonant frequency and anti-resonant frequency, respectively, and these frequencies may be the same or different for each resonator in filter 200. In oversimplified terms, each resonator can be considered short-circuited at its resonant frequency and open-circuited at its anti-resonant frequency. The input-output transfer function will approach zero at the resonant frequency of the parallel resonators and the anti-resonant frequency of the series resonators. In a typical filter, the resonant frequency of the parallel resonators lies below the lower edge of the filter passband, while the anti-resonant frequency of the series resonators lies above the upper edge of the passband. In some filters, a front dielectric layer (also called a "frequency setting layer"), represented by a dashed rectangle 270, can be formed on the parallel resonators to set their resonant frequency to be lower than that of the series resonators.

[0043] The Q factor of an acoustic resonator is typically defined as the peak energy stored over one cycle of an applied RF signal divided by the total energy dissipated or lost over that cycle. The Q factor of an XBAR is a complex function of many parameters, including the length or number of fingers in the XBAR's IDT.

[0044] Possible loss mechanisms in acoustic resonators include resistive losses in the IDT and other conductors; viscous or acoustic losses in the piezoelectric plate, IDT fingers, and other materials; and acoustic energy leakage from the resonator structure. The peak energy stored in the resonator is proportional to the resonator's capacitance. In an XBAR resonator, capacitance is proportional to the number of IDT fingers. Resistive and viscous losses are also proportional to the number of IDT fingers. Acoustic energy leaking from the resonator in the transverse direction (i.e., parallel to the IDT fingers) is proportional to the resonator's length and therefore also proportional to the number of IDT fingers. In contrast, the energy lost from the ends of the IDT in the longitudinal direction (i.e., perpendicular to the IDT fingers) is roughly constant and independent of the number of IDT fingers. As the number of IDT fingers and the peak energy stored in the XBAR decrease, the acoustic energy lost in the longitudinal direction becomes part of the ever-increasing stored peak energy.

[0045] Figure 3 This is a graph showing the normalized Q-factor of a representative XBAR as a function of the number of fingers in the XBAR's IDT. The "normalized Q-factor" is the Q-factor of an XBAR with a finite number of IDT fingers divided by the Q-factor of a hypothetical XBAR with the same structure and an infinite number of IDT fingers. Figure 3 In this model, the normalized Q-factor is quantified as a percentage of the Q-factor of the XBAR with an infinite number of IDT fingers. Specifically, solid line 310 is the curve of the normalized Q-factor at the resonant frequency, while dashed line 320 is the curve of the normalized Q-factor at the anti-resonant frequency. This was obtained through simulation using the finite element method. Figure 3 The data in the middle.

[0046] Figure 3 The normalized Q-factor of an XBAR with a finite number of IDT fingers is shown to be less than 100%, meaning the Q-factor of an XBAR with a finite number of IDT fingers is less than the Q-factor of a similar XBAR with an infinite number of IDT fingers. Although Figure 3Not shown, but for a very large number of IDT fingers, the normalized Q factor of XBAR can asymptotically approach 100%. As expected, the normalized Q factor depends on the number of IDT fingers. In particular, for XBARs with fewer than approximately 20 IDT fingers, the normalized Q factor drops sharply due to the increasing importance of acoustic energy loss in the longitudinal direction.

[0047] Figure 4 This is a plan view of an exemplary conductor pattern 400 that reduces acoustic energy leakage at the ends of an XBAR in the longitudinal direction. Conductor pattern 400 includes an IDT 430 and four reflector elements 462, 464, 466, and 468. IDT 430 includes a first busbar 432, a second busbar 434, and a plurality of n staggered IDT fingers extending alternately from the first and second busesbars. In this example, n, the number of IDT fingers, is equal to 24. In other XBARs, n can be 20 to 100 or more IDT fingers. IDT finger 436 is the first finger, and IDT finger 438 is the n'th finger. The IDT fingers are numbered from left to right (e.g., ...). Figure 4 (as shown) is arbitrary, and the first finger and the n'th finger can be reversed.

[0048] like Figure 4 As shown, odd-numbered IDT fingers extend from the first busbar 432, while even-numbered IDT fingers extend from the second busbar 434. IDT 430 has an even number of IDT fingers, such that the first and n'th IDT fingers 436 and 438 extend from different busbars. In some cases, an IDT may have an odd number of IDT fingers, such that the first and n'th IDT fingers, as well as all reflector elements, extend from the same busbar.

[0049] A total of four reflector elements are disposed outside the periphery of IDT 430. A first reflector element 462 is located near and parallel to the first IDT finger 436 at the left end of IDT 430. A second reflector element 466 is located near and parallel to the n'th IDT finger 438 at the right end of IDT 430. An optional third reflector element 464 is parallel to the first reflector element 462. An optional fourth reflector element 468 is parallel to the second reflector element 466.

[0050] The first and third reflector elements 462 and 464 extend from the first busbar 432 and therefore have the same potential as the first IDT finger 436. Similarly, the second and fourth reflector elements 466 and 468 extend from the second busbar 430 and therefore have the same potential as the n'th IDT finger 438.

[0051] Reflector elements 462, 464, 466, and 468 are configured to confine acoustic energy within the region of IDT 430, thereby reducing acoustic energy loss in the longitudinal direction. For this purpose, the spacing pr between adjacent reflector elements, between reflector elements 462 and 466, and between adjacent first and n' IDT fingers is typically greater than the spacing p of the IDT fingers. The width or marking mr of reflector elements 462, 464, 466, and 468 need not be equal to the marking m of the IDT fingers. As described later, the marking mr of the reflector elements can be selected to optimize the Q factor at a specific frequency or frequency range.

[0052] Figure 5 This is a plan view of another conductor pattern 500, which reduces acoustic energy leakage at the ends of the XBAR in the longitudinal direction. Conductor pattern 500 includes an IDT 530 and four reflector elements 562, 564, 566, and 568. The IDT 530 includes a first busbar 532, a second busbar 534, and a plurality of interlaced IDT fingers extending alternately from the first and second busbars as described above. IDT fingers 536 and 538 are located at the left and right ends of the IDT 530 (e.g., ...). Figure 5 The first and n'th IDT finger (as shown).

[0053] A total of four reflector elements are disposed on the outer periphery of IDT 530. The first and third reflector elements 562 and 564 are located close to and parallel to the first IDT finger 536 at the left end of IDT 530. The first and third reflector elements 562 and 564 are connected to each other but not to buses 532 and 534. The first and third reflector elements 562 and 564 are capacitively coupled to the first IDT finger 536 and therefore have substantially the same potential as the first IDT finger 536. If, when an RF signal is applied between buses 532 and 534, the potential between a reflector element and the first IDT finger is smaller than the potential between adjacent IDT fingers, the reflector elements are considered to have substantially the same potential.

[0054] Similarly, the second reflector element 566 and the fourth reflector element 568 are located close to and parallel to the n'th IDT finger 538 at the right end of the IDT 530. The second reflector element 566 and the fourth reflector element 568 may be connected to each other or not connected to either of the buses 532 and 534. The second and fourth reflector elements 566 and 568 are capacitively coupled to each other and capacitively coupled to the n'th IDT finger 538, and therefore are at almost the same potential as the n'th IDT finger 538.

[0055] Reflector elements 562, 564, 566, and 568 are configured to confine acoustic energy within the region of IDT 530, thereby reducing acoustic energy loss in the longitudinal direction. For this purpose, the pitch pr between adjacent reflector elements and between reflector elements 562 and 566 and adjacent terminal IDT fingers is typically greater than the pitch p of the IDT fingers. The width or marking mr of reflector elements 562, 564, 566, and 568 does not necessarily have to be equal to the marking m of the IDT fingers. The marking mr of the reflector elements can be selected to optimize the Q factor for specific frequencies within the frequency range.

[0056] Figure 6 The normalized Q factor varies with and without the α-value. Figure 4 The graph shows the variation in the number of IDT fingers for another XBAR with a similar reflector element. Specifically, solid line 610 is a graph of the normalized Q factor at its resonant frequency for an XBAR without a reflector element. Dashed line 620 is a graph of the normalized Q factor at the resonant frequency for a similar XBAR with two reflector elements on each side of the IDT. In both cases, the piezoelectric plate is 400 nm thick lithium niobate, the IDT fingers are 500 nm thick aluminum, the IDT spacing p = 4 μm, and the IDT finger marking m = 1 μm. For the XBAR with reflector elements, pr = 4.2 μm and mr = 0.735 μm. XBARs with reflector elements and at least 10 fingers can have a normalization factor of up to 80%.

[0057] Figure 7 The normalized Q factor varies with and without the α-value. Figure 4 The graph shows the variation in the number of IDT fingers for another XBAR with a similar reflector element. Specifically, solid line 710 is a graph of the normalized Q factor of an XBAR without a reflector element at its anti-resonant frequency. Dashed line 720 is a graph of the normalized Q factor at the anti-resonant frequency for a similar XBAR with two reflector elements on each side of the IDT. In both cases, the piezoelectric plate is 400 nm thick lithium niobate, the IDT fingers are 500 nm thick aluminum, the IDT spacing p = 4 μm, and the IDT finger marking m = 1 μm. For the XBAR with reflector elements, pr = 8 μm and mr = 0.80 μm. XBARs with reflector elements and at least 14 fingers can have a normalization factor of up to 80%.

[0058] Figure 8Figure 800 illustrates the relationship between the pitch pr and marking mr of the reflector elements in an exemplary XBAR device at a fixed frequency of 5150 MHz. The exemplary XBAR device has a 400 nm thick lithium niobate piezoelectric plate and an aluminum IDT, as well as 500 nm thick reflector elements. The pitch and marking of the IDT fingers are 4 μm and 1 μm, respectively. Each end of the IDT has two reflector elements. The lighter shaded regions 810A, 810B, 810C, and 810D identify combinations of pr and mr with a normalized Q factor greater than or equal to 85%. The darker shaded regions 820A, 820B, 820C, and 820D identify combinations of pr and mr with a normalized Q factor greater than or equal to 90%. For comparison, without reflector elements, the normalized Q factor of this XBAR at 5150 MHz is 74%. Although in Figure 8 Not marked in the text, but Figure 8 There are still combinations of pr and mr, where the normalized Q factor is less than 75%, indicating that improper configuration of reflector elements can reduce the XBAR Q factor.

[0059] There are multiple combinations of pr and mr that improve the normalized Q factor to 85% or 90%. To obtain a normalized Q factor greater than or equal to 90%, pr must be greater than or equal to 1.2 times the spacing p of the IDT fingers. For pr = 6 micrometers (1.5p), there are at least four mr values ​​that improve the normalized Q factor to over 90%.

[0060] Figure 9 Figure 900 illustrates the relationship between the markings *mr* and frequency for the reflector elements of an exemplary XBAR device, where there are two reflector elements on each side of the IDT, and *pr* = 5.2 micrometers. As in the previous example, the exemplary XBAR device has a 400 nm thick lithium niobate piezoelectric plate and an aluminum IDT, along with reflector elements that are 500 nm thick. The spacing and markings of the IDT fingers are 4 micrometers and 1 micrometer, respectively. Lighter shaded areas (e.g., area 910) identify combinations of frequencies with a normalized Q factor greater than or equal to 85% and *mr*. Darker shaded areas (e.g., area 920) identify combinations of frequencies with a normalized Q factor greater than or equal to 90% and *mr*. For comparison, this XBAR has a normalized Q factor of 74% at 5150 MHz without reflector elements.

[0061] Figure 900 shows that, for a given reflector element spacing pr, the frequency at which the Q factor of the XBAR needs to be improved must be considered when selecting the reflector element marking mr. For example, selecting mr = 0.95 μm provides a normalized Q factor greater than 90% in the frequency range from approximately 4980 MHz to greater than 5200 MHz. Selecting mr = 1.7 μm provides a normalized Q factor greater than 90% in the frequency range from less than 4700 MHz to approximately 4950 MHz. However, compared to an XBAR without reflector elements, selecting mr = 1.7 μm can actually reduce the Q factor at 5200 MHz.

[0062] Figure 10 Figure 11000 illustrates the relationship between the marker mr of the reflector elements and the frequency of an exemplary XBAR device, where there is one reflector element on each side of the IDT, and pr = 5.2 micrometers. The exemplary XBAR device is identical to the previous example. Figure 9 As shown, lighter shaded areas (e.g., area 1010) identify combinations of frequencies with a normalized Q factor greater than or equal to 85% and mr. Darker shaded areas (e.g., area 1020) identify combinations of frequencies with a normalized Q factor greater than or equal to 90% and mr.

[0063] Figure 9 and Figure 10 The comparison shows that a single reflector element is generally less effective than two reflector elements in improving the normalized Q factor. However, in some applications, it is sufficient to place one reflector element at each end of the IDT. In this example, a single reflector element with mr = 0.75 micrometers (at each end of the IDT) can significantly improve the normalized Q factor in the frequency range of approximately 4770 MHz to 4970 MHz.

[0064] Figure 11 Figure 1100 illustrates the relationship between the markings mr and frequency of the reflector elements in an exemplary XBAR device, where there are five reflector elements on each side of the IDT, and pr = 5.2 micrometers. The exemplary XBAR device is identical to the previous example. Figure 9 As shown, lighter shaded areas (e.g., area 1110) identify combinations of frequencies with a normalized Q factor greater than or equal to 85% and mr. Darker shaded areas (e.g., area 1120) identify combinations of frequencies with a normalized Q factor greater than or equal to 90% and mr.

[0065] Figure 9 and Figure 11 The comparison shows that five reflector elements do not provide any significant improvement compared to two reflector elements.

[0066] Figure 12 This is a graph showing the performance of exemplary XBAR bandpass filters with and without reflector elements. Specifically, solid line 1210 represents the S-band N77 filter with two reflector elements at each end of the XBAR in the filter. 21 A graph showing the magnitude of the input-output transfer function. The reflector element of the parallel resonator is optimized for a frequency of 3.35 GHz, while the reflector element of the series resonator is optimized for a frequency of 4.2 GHz. These frequencies are located in or near the N77 band, where it is typically most difficult to achieve the lowest Si. 21 The requirements. Dashed line 1220 represents the S-value for the same filter without reflector elements on the XBAR. 21 The size of the image is shown in the diagram. All data was developed using simulations of the filter using the finite element method.

[0067] Including reflector elements at 3.35 GHz, S 21 It improved by 0.2 dB, and by 0.4 dB at 4.2 GHz. However, note that the inclusion of a mirror element will cause S at other frequencies. 21 A reduction of up to 0.25 dB illustrates the trade-offs that must be made during XBAR filter design. Figure 12 In the exemplary bandpass filter, when selecting the reflector elements for all parallel resonators, the maximum Q-factor at the same frequency (3.35 GHz) is considered, while when selecting the reflector elements for all series resonators, the maximum Q-factor at the same frequency (4.2 GHz) is considered. If the reflector elements for each resonator are optimized independently, it is possible to further improve the filter transfer function.

[0068] Method Description

[0069] Figure 13 This is a simplified flowchart outlining a method 1300 for manufacturing a filter device incorporating XBARs. Specifically, method 1300 is used to manufacture a filter device comprising multiple XBARs, some of which may include a frequency-setting dielectric layer. Method 1300 begins at 1305, where a device substrate and a piezoelectric material sheet disposed on a sacrificial substrate are shown. Method 1300 ends at 1395, yielding a complete filter device. Figure 13 The flowchart only includes the main processing steps. (It can be found in...) Figure 13 Various routine process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) are performed before, during, and after the steps shown.

[0070] although Figure 13The process for manufacturing a single filter device is described in general; however, multiple filter devices can be manufactured simultaneously on a common wafer (consisting of piezoelectric plates bonded to a substrate). In this case, each step of process 1300 can be performed simultaneously on all filter devices on the wafer.

[0071] Figure 13 The flowchart captures three variations of process 1300 for manufacturing an XBAR, differing in when and how the cavity is formed in the device substrate. The cavity may be formed at steps 1310A, 1310B, or 1310C. In each of the three variations of method 1300, only one of these steps is performed.

[0072] The piezoelectric plate can be, for example, lithium niobate or lithium tantalate, either of which can be Z-cut, rotary Z-cut, or rotary YX-cut. The piezoelectric plate can be other materials and / or other cuts. The device substrate can preferably be silicon. The device substrate can be other materials that allow deep cavities to be formed by etching or other processes.

[0073] In a variation of method 1300, one or more cavities are formed in the device substrate at 1310A, and then a piezoelectric plate is bonded to the substrate at 1315. A separate cavity can be formed for each resonator in the filter device. The one or more cavities can be formed using conventional photolithography and etching techniques. Typically, the cavity formed at 1310A will not penetrate the device substrate.

[0074] At position 1315, the piezoelectric plate is bonded to the device substrate. The piezoelectric plate and device substrate can be bonded using a wafer bonding process. Typically, the mating surfaces of the device substrate and piezoelectric plate are highly polished. One or more layers of intermediate material, such as oxide or metal, can be formed or deposited on the mating surface of one or both of the piezoelectric plate and device substrate. One or both mating surfaces can 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 device substrate or the intermediate material layer.

[0075] At 1320, the sacrificial substrate can be removed. For example, the piezoelectric plate and the sacrificial substrate can be a wafer of piezoelectric material that has been ion-implanted to create defects in the crystal structure along the plane defining the boundary between the piezoelectric plate and the sacrificial substrate. At 1320, the sacrificial substrate can be removed, for example by thermal shock, leaving the piezoelectric plate bonded to the device substrate, thereby separating the wafer along the defect plane. After removing the sacrificial substrate, the exposed surface of the piezoelectric plate can be polished or machined in some way.

[0076] Thin sheets of single-crystal piezoelectric material laminated onto a non-piezoelectric substrate are commercially available. In this application, lithium niobate and lithium tantalate sheets can be bonded to various substrates, including silicon, quartz, and fused silica. Thin sheets of other piezoelectric materials may be available now or in the future. The thickness of the piezoelectric sheet can be between 300 nm and 1000 nm. When the substrate is silicon, a SiO2 layer can be disposed between the piezoelectric sheet and the substrate. When using a commercially available piezoelectric sheet / device substrate laminate, steps 1310A, 1315, and 1320 of method 1300 are not performed.

[0077] At position 1345, by depositing and constructing on the front side of the piezoelectric plate Figure 1 One or more conductor layers are used to form a first conductor pattern comprising the IDT and each XBAR reflector element. The conductor layers can be, for example, aluminum, aluminum alloys, copper, copper alloys, or some other conductive metal. Optionally, one or more layers of other materials can be disposed below the conductor layers (i.e., between the conductor layers and the piezoelectric plate) and / or on top of the conductor layers. For example, a thin film of titanium, chromium, or other metals can be used to improve adhesion between the conductor layers and the piezoelectric plate. A second conductive pattern of gold, aluminum, copper, or other metals with higher conductivity can be formed above portions of the first conductor pattern (e.g., the IDT busbar and the interconnection between the IDTs).

[0078] A conductor pattern can be formed at 1345 by sequentially depositing a conductive layer and optionally one or more other metal layers on the surface of the piezoelectric plate. Excess metal can then be removed by etching through the patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

[0079] Alternatively, a stripping process can be used at 1345 to form each conductor pattern. Photoresist can be deposited on the piezoelectric plate and patterned to define the conductor pattern. Conductor layers, along with optional one or more other layers, can be sequentially deposited on the surface of the piezoelectric plate. The photoresist can then be removed, removing excess material and leaving the conductor pattern.

[0080] At 1350°, one or more frequency-setting dielectric layers can be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. For example, a dielectric layer can be formed over a parallel resonator to reduce the frequency of the parallel resonator relative to the frequency of the series resonator. One or more dielectric layers can be deposited using conventional deposition techniques such as physical vapor deposition, atomic layer deposition, chemical vapor deposition, or some other method. The deposition of the dielectric layer can be confined to selected areas of the piezoelectric plate using one or more photolithography processes (using a photomask). For example, a mask can be used to confine the dielectric layer to cover only the parallel resonator.

[0081] At 1355, a passivation / tuning dielectric layer is deposited over the piezoelectric plate and conductor pattern. The passivation / tuning dielectric layer can cover the entire surface of the filter, except for the pads used for electrical connections to circuitry outside the filter. In some instances of method 1300, the passivation / tuning dielectric layer can be formed after etching cavities in the device substrate at 1310B or 1310C.

[0082] In a second variation of method 1300, one or more cavities are formed in the back side of the device substrate at 1310B. A separate cavity can be formed for each resonator in the filter device. One or more cavities can be formed using anisotropic or orientation-dependent dry or wet etching to create an opening from the back side of the device substrate all the way to the piezoelectric plate. In this case, the resulting resonator device will have, as Figure 1 The cross-section shown.

[0083] In a third variation of method 1300, one or more cavities in the form of recesses in the device substrate can be formed in 1310C by etching the substrate with an etchant introduced through an opening in the piezoelectric plate. A separate cavity can be formed for each resonator in the filter device. The one or more cavities formed at 1310C will not penetrate the device substrate.

[0084] Ideally, after forming cavities at 1310B or 1310C, most or all filter devices on the wafer will meet a set of performance requirements. However, normal process tolerances will cause parameter variations, such as variations in the thickness of the dielectric layer formed at 1350 and 1355, variations in the thickness and linewidth of the conductor and IDT fingers formed at 1345, and variations in the thickness of the piezoelectric plate. These variations will cause the filter device's performance to deviate from this set of performance requirements.

[0085] To improve the yield of filter devices that meet performance requirements, frequency tuning can be performed by selectively adjusting the thickness of the passivation / tuning layer deposited on the resonator at 1355. The passband frequency of the filter device can be reduced by adding material to the passivation / tuning layer, and increased by removing material from the passivation / tuning layer. Typically, method 1300 is biased to produce a filter device with a passband that is initially below the desired frequency range but can be tuned to the desired frequency range by removing material from the surface of the passivation / tuning layer.

[0086] At 1360, a probe card or other device can be electrically connected to the filter to allow for radio frequency (RF) testing and measurement of filter characteristics (such as the input-output transfer function). Typically, RF measurements are performed on all or most filter devices fabricated simultaneously on the same piezoelectric plate and substrate.

[0087] At 1365, global frequency tuning can be performed by removing the surface material of the passivation / tuning layer using a selective material removal tool (e.g., a scanning ion mill as described above). 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 passband of each filter device to the desired frequency range. Test results from 1360 can be processed to generate a global profile indicating the amount of material removed based on the two-dimensional location on the wafer. Material is then removed using a selective material removal tool based on the profile.

[0088] At 1370, local frequency tuning can be performed, either in addition to or without the global frequency tuning performed at 1365. This "local" frequency tuning is performed with a spatial resolution smaller than that of a single filter device. Test results from 1360 can be processed to generate a map representing the amount of material to be removed from each filter device. Local frequency tuning may require the use of masks to limit the size of the region from which material is removed. For example, a first mask can be used to limit tuning to parallel resonators only, and a second mask can subsequently be used to limit tuning to series resonators only (and vice versa). This allows for independent tuning of the lower frequency band edges (by tuning parallel resonators) and the upper frequency band edges (by tuning series resonators) of the filter device.

[0089] After frequency tuning at 1365 and / or 1370, the filter assembly is completed at 1375. Actions that may occur at 1375 include forming pads or solder bumps or other means for establishing connections between the assembly and external circuitry (if such pads are not formed at 1345); cutting individual filter assemblies from a wafer containing multiple filter assemblies; other packaging steps; and other tests. The process terminates at 1395 after each filter assembly is completed.

[0090] Conclusion

[0091] Throughout this specification, the embodiments and examples shown should be considered as examples and not as limitations on the disclosed or claimed devices and processes. While many of the examples provided herein relate to specific combinations of method actions or system elements, it should be understood that those actions and elements can be combined in other ways to achieve the same objective. Regarding the flowcharts, additional and fewer steps may be taken, and the steps shown may be combined or further refined to implement the methods described herein. Actions, elements, and features discussed in connection with only one embodiment are not intended to exclude their similarity in other embodiments.

[0092] As used herein, “multiple” means two or more. As used herein, a “group” of items may include one or more such items. As used herein, whether in the written description or in the claims, the terms “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” etc., shall be understood as open-ended, i.e., referring to including but not limited to. Only the transitional phrases “consisting of…” and “substantially consisting of…” are closed or semi-closed transitional phrases relative to the claims. Ordinal numbers used in the claims, such as “first,” “second,” “third,” etc., are used to modify claim elements. This does not in itself indicate the priority, order, or sequence of action of one claim element over another, but is merely used to distinguish one claim element with the same name from another element with the same name (but with ordinal numbers), thereby differentiating 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. An acoustic resonator device, comprising: A piezoelectric layer; and A conductor pattern, located on the surface of the piezoelectric layer, comprising: An interdigital transducer (IDT) includes a first bus, a second bus, and n staggered fingers, where n is a positive integer, wherein the fingers extend alternately from the first bus and the second bus, and the staggered fingers include a first finger and an n'th finger at opposite ends of the IDT. A first reflector element, close to and parallel to the first finger; and A second reflector element, close to and parallel to the n'th finger, Wherein the center-to-center distance pr is at least one of the distance between the first reflector element and the first finger and the distance between the second reflector element and the n'th finger. The distance pr is greater than the pitch p, where the pitch p is the center-to-center spacing of a pair of adjacent fingers of the IDT. The first reflector element and the first finger are connected to the same busbar of the first busbar and the second busbar, and the second reflector element and the n'th finger are connected to the same busbar of the first busbar and the second busbar.

2. The apparatus according to claim 1, characterized in that, The first reflector element and the first finger are connected to the first busbar, and The second reflector element and the n'th finger are connected to the second busbar, and When a radio frequency signal is applied between the first bus and the second bus, the potential of the first reflector element is substantially the same as that of the first finger, while the potential of the second reflector element is substantially the same as that of the n'th finger.

3. The apparatus according to claim 1, characterized in that, The distance is greater than or equal to 1.2 times the pitch p.

4. The apparatus according to claim 1, characterized in that, The mark mr of at least one of the first reflector element and the second reflector element is not equal to the mark m of any of the interlaced fingers.

5. The apparatus according to claim 1, characterized in that, The device is a parallel resonator in a trapezoidal bandpass filter circuit with a passband, and Select the marker mr to improve the Q factor of the device at the lower edge of the passband.

6. The apparatus according to claim 1, characterized in that, The device is a series resonator in a trapezoidal bandpass filter circuit with a passband, and Select the marker mr to improve the Q factor of the device at the upper edge of the passband.

7. The apparatus according to claim 1, characterized in that, The conductor pattern also includes: A third reflector element, close to the first reflector element, the third reflector element being arranged such that the first reflector element is centrally located between the third reflector element and the first finger; and A fourth reflector element is located near the second reflector element, the fourth reflector element being arranged such that the second reflector element is centrally located between the fourth reflector element and the n'th finger.

8. The apparatus according to claim 7, characterized in that, The first reflector element, the third reflector element, and the first finger are connected to the first busbar, and The second reflector element, the fourth reflector element, and the n'th finger are connected to the second busbar.

9. The apparatus according to claim 7, characterized in that, The distance pr is the distance between the first reflector element and the first finger, between the first reflector element and the third reflector element, between the second reflector element and the fourth reflector element, and between the second reflector element and the n'th finger, and the distance pr is greater than or equal to 1.2 times the pitch p.

10. The apparatus according to claim 7, characterized in that, The marker mr of the first reflector element, the second reflector element, the third reflector element, and the fourth reflector element is configured to improve the Q factor of the device at a predetermined frequency.

11. The apparatus according to claim 10, characterized in that, The device is a parallel resonator in a trapezoidal bandpass filter circuit with a passband, and mr is selected to improve the Q factor of the device at the lower edge of the passband.

12. The apparatus according to claim 10, characterized in that, The device is a series resonator in a trapezoidal bandpass filter circuit with a passband, and mr is selected to improve the Q factor of the device at the upper edge of the passband.

13. A bandpass filter, comprising: Three or more individual acoustic resonators are connected in a trapezoidal filter circuit, where Each bulk acoustic resonator includes an interdigital transducer (IDT) and at least one reflector element adjacent to the end of the IDT. Wherein the distance pr is at least the distance between the at least one reflector element and the end of the IDT. Where pr is greater than the pitch p The pitch is the center-to-center spacing between a pair of adjacent fingers of the IDT. The at least one reflector element and the adjacent finger of the IDT are connected to the same busbar.

14. The bandpass filter according to claim 13, characterized in that, The at least one reflector element comprises a plurality of reflector fingers, and for each of the three or more individual acoustic resonators: The distance pr is at least one of the distance between the first reflector finger and the first finger of the adjacent IDT, the distance between the first reflector finger and the second reflector finger adjacent to the first reflector finger, or any combination thereof, and wherein pr is greater than or equal to 1.2 times the pitch p.

15. The bandpass filter according to claim 13, characterized in that, The three or more individual acoustic resonators include at least one parallel resonator, and The pitch and marking of the reflector element of the parallel resonator are selected to improve the Q factor of the parallel resonator at the lower edge of the passband of the bandpass filter, wherein the marking is the width of at least one of the adjacent fingers of the IDT.

16. The bandpass filter according to claim 13, characterized in that, The three or more individual acoustic resonators include at least one series resonator, and The pitch and marking of the reflector element of the series resonator are selected to improve the Q factor of the series resonator at the upper edge of the passband of the bandpass filter, wherein the marking is the width of at least one of the adjacent fingers of the IDT.

17. An acoustic resonator device, comprising: One substrate; A piezoelectric layer, the piezoelectric layer comprising a portion forming a diaphragm, the piezoelectric layer being directly attached to the substrate or attached to the substrate via one or more intermediate layers; and A conductor pattern, located on the surface of the piezoelectric layer, comprising: An interdigital transducer (IDT) includes a first bus, a second bus, and n staggered fingers, where n is a positive integer, wherein the staggered fingers extend alternately from the first bus and the second bus, and the staggered fingers include a first finger and an n'th finger at opposite ends of the IDT. A first reflector element is located near the first finger; and A second reflector element, located near the n'th finger. Wherein, distance pr is at least one of the distance between the first reflector element and the first finger and the distance between the second reflector element and the n'th finger. The distance pr is greater than the pitch p. The pitch p is the center-to-center spacing between a pair of adjacent fingers of the IDT, and The radio frequency or microwave signal applied to the IDT excites a shear mode, in which acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric layer and perpendicular to the direction of the electric field generated by the interlaced fingers.

18. The apparatus according to claim 17, characterized in that, The distance pr is greater than or equal to 1.2 times the pitch p.

19. The apparatus according to claim 17, characterized in that, The distance pr is greater than or equal to 1.2 times the pitch p and less than 1.5 times the pitch p.

20. The apparatus according to claim 17, characterized in that, The mark mr of at least one of the first reflector element and the second reflector element is not equal to the mark m of any of the interlaced fingers.

21. The apparatus according to claim 17, characterized in that, The conductor pattern also includes: A third reflector element, close to the first reflector element, the third reflector element being arranged such that the first reflector element is centrally located between the third reflector element and the first finger; and A fourth reflector element is located near the second reflector element, the fourth reflector element being arranged such that the second reflector element is centrally located between the fourth reflector element and the n'th finger.

22. The apparatus according to claim 17, characterized in that, The first reflector element and the first finger are connected to the first busbar, and The second reflector element and the n'th finger are connected to the second busbar, and When a radio frequency signal is applied between the first bus and the second bus, the potential of the first reflector element is substantially the same as that of the first finger, while the potential of the second reflector element is substantially the same as that of the n'th finger.

23. The apparatus according to claim 17, characterized in that, When a radio frequency signal is applied to the conductor pattern, the potential between the first reflector element and the first finger is different from the potential between the interlaced fingers.

24. The apparatus of claim 17, further comprising a dielectric layer on the piezoelectric layer.

25. The apparatus according to claim 24, characterized in that, The device is one of multiple parallel acoustic resonators in a trapezoidal filter circuit.

26. The apparatus according to claim 24, characterized in that, The first reflector element and the second reflector element are not connected to either the first bus or the second bus.

27. The apparatus according to claim 17, characterized in that, n is at least 14.

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