Laterally excited thin-film bulk acoustic resonator with improved coupling and reduced energy leakage

Abstract

A transversely excited thin-film bulk acoustic resonator with improved coupling and reduced energy leakage. Acoustic resonators, acoustic filter devices, and methods of manufacturing the same. An acoustic resonator device includes a piezoelectric plate having a front and a back side, an interdigital transducer (IDT) located on the front side, the interdigital transducer including staggered fingers extending alternately from opposing first and second busbars, the overlap distance of the staggered fingers defining an aperture of the acoustic resonator, and a dielectric layer formed on and between the staggered fingers, wherein the dielectric layer extends from one edge of the aperture between the first busbar and an opposite edge of the aperture between the second busbar.

Description

Technical Field

[0001] This disclosure relates to radio frequency filters using acoustic resonators, and more particularly to filters used in communication devices. Background Technology

[0002] Radio frequency (RF) filters are two-ended devices configured to allow some frequencies to pass while blocking others. "Passing" means transmitting with relatively low signal loss, while "blocking" means blocking or essentially attenuating the signal. The range of frequencies a filter can pass through is called its "passband." The range of frequencies blocked by such a filter is called its "stopband." A typical RF filter has at least one passband and at least one stopband. The specific requirements for the passband or stopband depend on the application. For example, a "passband" can be defined as a frequency range where the filter's insertion loss is better than defined values ​​such as 1 dB, 2 dB, or 3 dB. A "stopband" can be defined as a frequency range where the filter's rejection is greater than defined values, such as 20 dB, 30 dB, 40 dB, or greater, depending on the application.

[0003] RF filters are used in communication systems that transmit information over wireless links. For example, RF filters can be found in cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, Internet of Things (IoT) devices, laptops and tablets, fixed-point radio links, and the RF front end of other communication systems. RF filters are also used in radar and electronic and information warfare systems.

[0004] RF filters typically require numerous design trade-offs to achieve the optimal balance between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost for each specific application. Specific design and manufacturing approaches and enhancements can simultaneously benefit one or more of these requirements.

[0005] Enhancements to the performance of RF filters in wireless systems can have a wide-ranging impact on system performance. Improvements to RF filters can lead to improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, and higher reliability. These improvements can 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.

[0006] High-performance RF filters used in current communication systems typically incorporate acoustic resonators, including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, thin-film bulk acoustic wave (FBAR) resonators, and other types of acoustic resonators. However, these existing technologies are not suitable for use at higher frequencies and bandwidths, which future communication networks require.

[0007] To obtain wider communication channel bandwidth, higher frequency communication bands are necessary. 3GPP (3rd 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. Among these new 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 bands n77 and n79 use Time Division Duplex (TDD), therefore, communication devices operating in bands n77 and / or n79 will use the same frequencies for uplink and downlink transmissions. Bandpass filters in the n77 and n79 bands must be able to handle the transmit power of the communication devices. High frequencies and wide bandwidths are also required in the 5GHz and 6GHz radio bands. The 5G NR standard also defines millimeter wave communication bands between 24.25 GHz and 40 GHz.

[0008] The transversely excited film bulk acoustic resonator (XBAR) is an acoustic resonator structure used in microwave filters. Such an XBAR is described in U.S. Patent 10,491,291, entitled "TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR". The XBAR resonator includes an interdigital transducer (IDT) formed on a thin floating layer or diaphragm of a single-crystal piezoelectric material. The IDT includes a first set of parallel fingers extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a sheared master acoustic wave in the piezoelectric diaphragm. The XBAR resonator provides high electromechanical coupling and high-frequency capability. XBAR resonators can be used in a variety of RF filters, including band-stop filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly well-suited for use in filters in communication bands above 3 GHz. Summary of the Invention

[0009] An acoustic resonator device includes: a piezoelectric plate having a front side and a back side; an interdigital transducer (IDT) located on the front side, including staggered fingers extending alternately from opposing first and second busbars, the overlap distance of the staggered fingers defining an aperture of the acoustic resonator device; and a dielectric layer formed on and between the staggered fingers, wherein the dielectric layer extends from between one edge of the aperture and the first busbar to between an opposing edge of the aperture and the second busbar.

[0010] A filter device includes: a piezoelectric plate having a front side and a back side; a conductor pattern located on the front side, the conductor pattern including a plurality of interdigital transducers (IDTs) of corresponding plurality of resonators, each of the plurality of IDTs including staggered fingers extending alternately from opposing first and second busbars, the overlap distance of the staggered fingers defining the aperture of a corresponding resonator of the plurality of resonators, wherein at least one of the plurality of resonators includes a dielectric layer formed on and between the staggered fingers, wherein the dielectric layer extends from between one edge of the aperture and the first busbar to between an opposing edge of the aperture and the second busbar.

[0011] A method of manufacturing an acoustic resonator device includes: forming an interdigital transducer (IDT) on the front side of a piezoelectric layer, the IDT including staggered fingers extending alternately from opposing first and second busbars, the overlap distance of the staggered fingers defining an aperture of the acoustic resonator device; and forming a dielectric layer on and between the staggered fingers, wherein the dielectric layer extends from between an edge of the aperture and the first busbar to between an opposing edge of the aperture and the second busbar. Attached Figure Description

[0012] Figure 1 Includes a schematic plan view, two schematic cross-sectional views, and a detailed view of a transversely excited membrane acoustic resonator (XBAR).

[0013] Figure 2 This is a schematic block diagram of a bandpass filter that uses an acoustic resonator.

[0014] Figure 3 This is a schematic plan view of the XBAR with improved coupling and reduced acoustic energy leakage.

[0015] Figure 4 It is a graph showing the absolute value of the admittance varying with the frequency of XBAR with and without coupling improvements and reduced acoustic energy leakage.

[0016] Figure 5 It is a conductivity diagram that varies depending on the frequency of the XBAR with and without coupling improvement and reduced acoustic energy leakage.

[0017] Figure 6 This is a flowchart of a method for manufacturing filters with improved coupling and reduced acoustic energy leakage of XBAR.

[0018] Throughout the specification, elements appearing in the accompanying drawings are assigned three- or four-digit reference numerals, where the two least significant digits are unique to that element, and one or two most significant digits are the drawing number in which the element is first shown. Elements not described in conjunction with the accompanying drawings may be assumed to have the same characteristics and functions as previously described elements with the same reference numerals. Detailed Implementation

[0019] Component Description

[0020] Figure 1 A simplified schematic top view and orthogonal cross-sectional view of the XBAR 100 are shown. XBAR resonators, such as the resonator 100, can be used in a variety of RF filters, including band-stop filters, band-pass filters, duplexers, and multiplexers.

[0021] XBAR 100 consists of a thin-film conductor pattern formed on the surface of a piezoelectric plate 110, which has a front side 112 and a back side 114 that are parallel to each other. The piezoelectric plate is a thin single-crystal layer made of a piezoelectric material, such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientations of the X, Y, and Z crystal axes relative to the front and back sides are consistent and uniform. The piezoelectric plate can be Z-cut, that is, the Z-axis is perpendicular to the front and back sides 112, 114. The piezoelectric plate can be ZY-cut, rotated Y-cut, rotated Z-cut, or rotated YX-cut. XBARs can be fabricated on piezoelectric plates with other crystal orientations.

[0022] The back surface 114 of the piezoelectric plate 110 is attached to the surface of the substrate 120, except for a portion of the piezoelectric plate 110 that 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 110 that spans 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 around at least 50% of the periphery 145 of the cavity 140.

[0023] 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 surface 114 of piezoelectric plate 110 can be attached to substrate 120 using wafer bonding processes. Alternatively, piezoelectric plate 110 can be grown on substrate 120 or attached to the substrate in some other way. Piezoelectric plate 110 can be directly attached to the substrate, or it can be attached via one or more intermediate material layers (not on the substrate). Figure 1 (As shown in the figure) is attached to substrate 120.

[0024] 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.

[0025] The conductor pattern of the XBAR100 includes an interdigital transducer (IDT) 130. The 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 extending from the fingers of the IDT. 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 "orifice" of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the "length" of the IDT.

[0026] 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 primary acoustic mode within the piezoelectric plate 110. The primary acoustic mode is a bulk 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 bulk resonator.

[0027] 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 length L of the orifice AP and 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.

[0028] To facilitate Figure 1As shown, the geometric spacing and width of the IDT fingers are significantly enlarged relative to the length (dimension L) and orifice (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.

[0029] Referring now to a detailed schematic cross-sectional view (detail C), a front dielectric layer 150 (or coating) 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 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.

[0030] 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.

[0031] Dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which may be referred to as the IDT pitch and / or XBAR pitch. Dimension m is the width or "mark" of the IDT fingers. The geometry of the IDT in an XBAR differs significantly from that used in a surface acoustic wave (SAW) resonator. In a SAW resonator, the IDT pitch 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 IDT pitch p is typically 2 to 20 times the finger width m. Pitch p is typically 3.3 to 5 times the finger width m. Furthermore, the IDT pitch p is typically 2 to 20 times the thickness of the piezoelectric plate 210. The IDT pitch p is typically 5 to 12.5 times the thickness of the piezoelectric plate 210. The width m of the IDT fingers in an XBAR is not limited to one-quarter of the wavelength of the sound wave at resonance. For example, the width of the XBAR IDT fingers can be 500 nm or greater, making it easy to fabricate the IDT using photolithography. The thickness of the IDT fingers can range from 100 nm to approximately equal to the width in nm. The thickness of the IDT's busbars (132, 134) can be equal to or greater than the thickness of the IDT fingers.

[0032] 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 resonator"). Figure 2 In 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. The inclusion of three series resonators and two parallel resonators is merely illustrative. Filters can 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 following node, which is one of the nodes between the input, output, or two series resonators.

[0033] 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 piezoelectric material plates. 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.

[0034] 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 overly simplified 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 front and / or back of the parallel resonators to set the resonant frequency of the parallel resonators below that of the series resonators. In other filters, the diaphragm of a series resonator may be thinner than that of a parallel resonator. In some filters, series and parallel resonators can be fabricated on separate chips with different piezoelectric plate thicknesses.

[0035] Three-dimensional simulations of the XBAR device show that some acoustic energy may leak or be lost at the ends of the IDT fingers, i.e., in the gap between the electrode ends and the busbar. A well-guided waveguide exhibits higher-order waveguide modes formed along the aperture. Transverse modes of the acoustic wave couple to the gap and can create energy confinement within it. This manifests either as a loss or as a strong stimulus. Coupling to various transverse modes is possible due to the non-orthogonality between the uniform electric field along the aperture and the transverse mode pattern. These transverse modes should be electrically decoupled via the IDT to suppress parasitic modes.

[0036] Resonators with low loss and low spurious content can be designed by improving the waveguide (e.g., suppressing wave radiation between the IDT terminals and the bus) while electrically decoupling higher-order transverse modes. Decoupling is typically achieved through a "piston" design, where the fundamental transverse mode is uniformly distributed along the orifice during resonance. Therefore, the fundamental transverse mode pattern is consistent with the external electric field pattern. In waveguide theory, waveguide modes are functionally orthogonal, so all higher-order modes are orthogonal to the fundamental mode pattern, which is identical to the external electric field in the piston mode. Thus, higher-order transverse modes are electrically decoupled from the IDT.

[0037] Figure 3 This is a schematic plan view of the XBAR 300 with improved coupling and reduced acoustic leakage. Similar to... Figure 1 XBAR 100 and XBAR 300 include a piezoelectric plate 310 situated on a substrate having a cavity (not shown), and an IDT 330 having staggered fingers 336 extending alternately from busbars 332 and 334 on the piezoelectric plate 310. A front dielectric layer 350 is situated on the IDT. For example, the dielectric layer 350 may be formed of SiO2, SiN3, Al2O3, Ta2O5, titanium oxide, titanium nitride, AlN (a non-piezoelectric phase), diamond, and / or other non-piezoelectric dielectric materials and combinations thereof. The staggered fingers overlap by a distance AP, defining the “orifice” of the IDT. As mentioned above, the number and relative dimensions of the individual elements, such as the number of fingers, the spacing between fingers, the width of fingers, the orifice length, etc., are not necessarily depicted accurately and are exaggerated for ease of presentation.

[0038] Undesirable parasitic modes can be significantly reduced if there is no dielectric layer in the region between the ends of the fingers 336 and the busbars 332, 334, or if the dielectric layer is significantly reduced. However, for certain configurations (e.g., 120-yx lithium niobate piezoelectric plates), the coupling of the resonator can be reduced if the dielectric layer exactly overlaps with the vias and does not extend into the region between the ends of the fingers 336 and the busbars 332, 334. Figure 3 In the XBAR 300, the dielectric layer 350 extends beyond the edge of the via by a distance do, but does not extend all the way to the buses 332 and 334. The edge of the via is a line that passes along the end of the IDT finger at the gap between the end of the IDT finger and the bus. The via has two opposing edges, a first edge adjacent to one of the buses, and a second opposing edge adjacent to the other of the buses. Coupling is improved when the dielectric layer extends beyond the edge of the via but does not extend all the way to the bus. The extension (or overhang) is chosen to optimize coupling without introducing additional losses or parasitic modes.

[0039] The resonant frequency of the primary shear acoustic mode excited by the IDT 330 in the piezoelectric plate 310 between the dielectric layer 350 and the busbars 332 and 334 is higher than the resonant frequency of the primary shear acoustic mode excited by the IDT 330 in the piezoelectric plate 310 covered by the dielectric layer 350. The reason for the higher frequency is that the frequency is approximately inversely proportional to the device thickness. This results in reduced energy leakage from the ends of the fingers because the waveguide is improved.

[0040] For example, the length do can be less than or equal to 20tp. In one example, the length do is approximately 2 µm.

[0041] Figure 4 Is it following Figure 3 Figure 400 shows the absolute admittance values ​​of XBARs with and without a dielectric layer, based on frequency variations. In this example, the dielectric layer is formed of SiO2. Solid curve 410 is a graph of the absolute admittance values ​​of XBARs with a dielectric layer only at the orifice of the IDT. Dashed curve 420 is a graph of the absolute admittance values ​​of XBARs with a dielectric layer, where the dielectric layer extends 2 μm beyond the orifice edge. Curve 420 shows an improvement in admittance compared to curve 410.

[0042] Figure 5 Is it following Figure 3 Figure 500 shows the absolute values ​​of the admittance for the frequency variation of XBARs with and without a dielectric layer. Solid curve 510 is a graph of the absolute admittance of an XBAR with a dielectric layer only at the orifice of the IDT. Dashed curve 520 is a graph of the conductance of an XBAR with a dielectric layer, where the dielectric layer extends 2 μm beyond the orifice edge. Compared to curve 510, curve 520 shows a reduction in conductance. For filters using this XBAR as a parallel resonator, the reduced conductance can lead to improved insertion loss.

[0043] Method Description

[0044] Figure 6 This is a simplified flowchart outlining process 600 for manufacturing a filter device incorporating XBARs with improved coupling and reduced acoustic leakage. Specifically, process 600 is used to manufacture a filter device comprising multiple XBARs. Process 600 begins at 605, where a device substrate and a piezoelectric material sheet are disposed on a sacrificial substrate. Process 600 ends at 695, at which point the filter device is complete. The flowchart in Figure 6 only includes the main process steps. Further details can be found at... Figure 6 Various routine process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) are performed before, during, after, and in the steps shown.

[0045] Figure 6The process for fabricating a single filter device is described in general, allowing multiple filter devices to be fabricated simultaneously on a common wafer (consisting of piezoelectric plates bonded to a substrate). In this case, each step of process 600 can be performed simultaneously on all filter devices on the wafer.

[0046] Figure 6 The flowchart captures three variations of process 600 used to fabricate XBARs, differing in when and how cavities are formed in the device substrate. Cavities can be formed at steps 610A, 610B, or 610C. In each of the three variations of process 600, only one of these steps is performed.

[0047] The piezoelectric plate is typically a rotatable ZY-cut or YX-cut lithium niobate. The piezoelectric plate can be some other material and / or some other cut. The device substrate is preferably silicon. The device substrate can be some other material that allows for the formation of deep cavities through etching or other processes.

[0048] In a variation of process 600, one or more cavities are formed in the device substrate at 610A before the piezoelectric plate is bonded to the substrate at 615. A separate cavity can be formed for each resonator in the filter device. Furthermore, the cavities can be shaped and formed such that two or more resonators can be located on a diaphragm above a cavity. Conventional photolithography and etching techniques can be used to form one or more cavities. Typically, the cavities formed at 610A will not penetrate the device substrate.

[0049] At position 615, the piezoelectric plate is bonded to the device substrate. The piezoelectric plate and device substrate can be bonded using wafer bonding processes. Typically, the mating surfaces of the device substrate and piezoelectric plate are highly polished. One or more layers of intermediate material, such as oxides or metals, can be formed or deposited on the mating surfaces of the piezoelectric plate and / or device substrate. One and / or two mating surfaces can be activated using, for example, plasma processing. The mating surfaces can then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and device substrate or intermediate material layers.

[0050] At 620, 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 a plane that defines the boundary between the piezoelectric plate and the sacrificial substrate. At 620, the wafer can be split along the defect plane, for example by thermal shock splitting, separating the sacrificial substrate and leaving the piezoelectric plate bonded to the device substrate. After the sacrificial substrate is removed, the exposed surface of the piezoelectric plate can be polished or treated in some way.

[0051] The first conductor pattern, comprising the IDT (including busbars and interlaced fingers) and reflector elements of each XBAR, is formed at 645 by depositing and patterning one or more conductor layers on the front side of the piezoelectric plate. 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 above 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 conductor pattern, made of gold, aluminum, copper, or other metals with higher conductivity, can be formed on portions of the first conductor pattern (e.g., the IDT busbars and the interconnections between the IDTs).

[0052] Each conductor pattern can be formed at 645 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 layers can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques.

[0053] Alternatively, a stripping process can be used at 645° to form each conductor pattern. Photoresist can be deposited on the piezoelectric plate and patterned to define the conductor pattern. Conductor layers, and optionally, 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.

[0054] At 650°, one or more frequency-setting dielectric layers can be formed by depositing one or more dielectric materials on the front side of the piezoelectric plate. For example, a dielectric layer can be formed on a parallel resonator, thereby reducing the frequency of the parallel resonator relative to 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 other methods. The dielectric layer deposition can be confined to selected areas of the piezoelectric plate using one or more photolithography processes (using photomasks). For example, a mask can also be used to confine the dielectric layer to cover only the parallel resonator.

[0055] Also at 650, when the dielectric layer is formed, a portion of the area between the busbar and the ends of the interleaved fingers can be masked to prevent the formation of the dielectric layer in that area. For example, the dielectric layer may extend only a certain distance (e.g., 2 μm) beyond the edge of the via into the area between the ends of the busbar and the interleaved fingers, as per [reference to...]. Figure 3 Furthermore, the dielectric layer in this region can be formed to be thinner than the rest of the dielectric layer. Alternatively, the dielectric layer in this region can be removed after the dielectric has been formed.

[0056] At 655, a passivation / tuning dielectric layer is deposited on 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 external circuitry. In some instances of process 600, the passivation / tuning dielectric layer can be formed after a cavity in the device substrate is etched at 610B or 610C.

[0057] In a second variation of process 600, at 610B, one or more cavities are formed in the back side of the device substrate. A separate cavity can be formed for each resonator in the filter device. Furthermore, the cavities can be shaped and formed such that two or more resonators can be mounted on a diaphragm above one cavity. 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, for example, […]. Figure 1 The cross-section shown.

[0058] In a third variation of process 600, one or more cavities in the form of recesses can be formed in the device substrate at 610C by etching the substrate with etchant introduced through an opening in the piezoelectric plate. A separate cavity can be formed for each resonator in the filter device. Furthermore, the cavities can be shaped and formed such that two or more resonators can reside on a diaphragm above a cavity. The one or more cavities formed at 610C do not penetrate the device substrate.

[0059] Ideally, after forming the cavity in 610B or 610C, most or all of the filter devices on the wafer will meet a set of performance requirements. However, normal process tolerances can lead to variations in parameters, such as variations in the thickness of the dielectric layer formed at 650 and 655, variations in the thickness and linewidth of the conductor and IDT fingers formed at 645, and variations in the piezoelectric plate thickness. These variations cause the filter device performance to deviate from the performance requirements.

[0060] To increase 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 655. 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, process 600 is biased to produce filter devices 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.

[0061] At 660, a probe card or other device can be used to electrically connect to the filter to allow radio frequency (RF) testing and measurement of filter characteristics such as the input-output transfer function. Typically, RF measurements are performed on some or most of the filter devices, which are fabricated simultaneously on a common piezoelectric plate and substrate.

[0062] At 665, global frequency tuning can be performed by removing material from the surface of the passivation / tuning layer using a selective material removal tool, such as the scanning ion milling machine described earlier. “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 660 can be processed to generate a global contour map indicating the amount of material removed based on a two-dimensional location on the wafer. The material is then removed according to the contour map using a selective material removal tool.

[0063] At 670, local frequency tuning can be performed in addition to or instead of the global frequency tuning performed at 665. This "local" frequency tuning is performed with a spatial resolution smaller than that of individual filter devices. Test results from 660 can be processed to generate a map indicating the amount of material to be removed at each filter device. Local frequency tuning may require the use of masks to limit the size of the material removal region. For example, a first mask can be used to limit tuning to parallel resonators only, while a second mask can subsequently be used to limit tuning to series resonators only (or vice versa). This will allow independent tuning of the lower band edge (by tuning parallel resonators) and the upper band edge (by tuning series resonators) of the filter devices.

[0064] After frequency tuning is completed at 665 and / or 670, the filter devices are finished at 675. Actions that may occur at 675 include forming bonding pads or solder bumps or other means for establishing connections between the device and external circuitry (if such pads are not formed at 645); cutting individual filter devices from a wafer containing multiple filter devices; other packaging steps; and additional testing. The process ends at 695 after each filter device is completed.

[0065] Conclusion

[0066] 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 or 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.

[0067] 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 plate with a front and a back; An interdigital transducer (IDT), located on the front side, includes staggered fingers extending alternately from opposing first and second busbars, the overlap distance of which defines the aperture of the acoustic resonator device; and A dielectric layer is formed on and between the interlaced fingers, wherein a portion of the piezoelectric plate forms a diaphragm spanning a cavity beneath the piezoelectric plate, and the interlaced fingers are on the diaphragm. The dielectric layer covers the aperture and extends beyond the edge of the aperture and the opposite edge of the aperture by a distance "do", which "do" is less than or equal to 20tp, where "tp" is the thickness of the piezoelectric plate. The dielectric layer terminates before the first busbar and before the second busbar, such that a first trench without a dielectric layer is between the edge of the orifice and the first busbar, and a second trench without a dielectric layer is between the opposite edge of the orifice and the second busbar.

2. The device according to claim 1, characterized in that, The dielectric layer is shaped to improve performance relative to acoustic resonators with a dielectric layer extending throughout the entire IDT.

3. The device according to claim 1, characterized in that, The dielectric layer extends beyond the edge of the aperture and the opposite edge of the aperture by 2 μm.

4. The device according to claim 1, characterized in that, The interlaced fingers of the IDT have a spacing that is 2 to 20 times the thickness tp of the piezoelectric plate.

5. A filter device, comprising: A piezoelectric layer having a front and a back side; A conductor pattern is located on the front side, the conductor pattern comprising multiple interdigital transducers (IDTs) of corresponding plurality of resonators, each of the plurality of IDTs comprising staggered fingers extending alternately from opposing first and second busbars, the overlap distance of the staggered fingers defining the aperture of the corresponding resonator of the plurality of resonators. In this configuration, a portion of the piezoelectric layer forms a diaphragm spanning corresponding cavities in a plurality of cavities, with staggered fingers of each of the plurality of IDTs on the corresponding diaphragm. At least one of the plurality of resonators includes a dielectric layer formed on and between the interlaced fingers. The dielectric layer covers the aperture and extends beyond the edge of the aperture and the opposite edge of the aperture by a distance do, which do is less than or equal to 20tp, where tp is the thickness of the piezoelectric layer. The dielectric layer terminates before the first busbar and before the second busbar, such that a first trench without a dielectric layer exists between the edge of the orifice and the first busbar, and a second trench without a dielectric layer exists between the opposite edge of the orifice and the second busbar. The dielectric layer is shaped to improve the performance of the at least one resonator relative to a resonator having a dielectric layer extending over the entire IDT of the resonator, thereby improving coupling without introducing additional losses or parasitic modes and reducing energy leakage from the ends of the interlaced fingers.

6. The device according to claim 5, characterized in that, The dielectric layer extends beyond the edge of the aperture and the opposite edge of the aperture by 2 μm.

7. A method for manufacturing an acoustic resonator device, comprising: An interdigitated transducer (IDT) is formed on the front side of the piezoelectric layer. The IDT includes staggered fingers extending alternately from opposing first and second busbars. The overlap distance of the staggered fingers defines the aperture of the acoustic resonator device. A cavity is formed in the substrate, and a diaphragm of the piezoelectric layer is formed across the cavity, such that the interlaced fingers are on the diaphragm; and A dielectric layer is formed on and between the interlaced fingers. The dielectric layer covers the aperture and extends beyond the edge of the aperture and the opposite edge of the aperture by a distance do, which do is less than or equal to 20tp, where tp is the thickness of the piezoelectric layer. The dielectric layer terminates before the first busbar and before the second busbar, such that a first trench without a dielectric layer is between the edge of the orifice and the first busbar, and a second trench without a dielectric layer is between the opposite edge of the orifice and the second busbar.

8. The method according to claim 7, characterized in that, The dielectric layer is shaped to improve performance relative to acoustic resonators with a dielectric layer extending throughout the entire IDT.

9. The method according to claim 7, characterized in that, The dielectric layer extends beyond the edge of the aperture and the opposite edge of the aperture by 2 μm.

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