TRANSVERSALLY EXCITED ACOUSTIC FILM VOLUME RESONATORS WITH IMPROVED COUPLING AND REDUCED ENERGY LOSS

Abstract

Acoustic resonator device comprising: a piezoelectric plate (310) with a front and back surface; an interdigital transducer (330) (IDT) on the front surface comprising nested fingers (336) extending alternately from opposing first and second busbars (332, 334), wherein an overlap distance of the nested fingers (336) defines an aperture (AP) of the acoustic resonator device;and a dielectric layer (350) arranged above and between the nested fingers (336) of the IDT, wherein the dielectric layer (350) extends continuously from an edge of the aperture (AP) and the first busbar (332) to an opposite edge of the aperture (AP) and the second busbar (334), wherein the dielectric layer (350) is thinner in at least one region between the edge of the aperture (AP) and the first busbar (332) than in a region above the aperture (AP) of the IDT, and wherein the acoustic resonator device is a shunt resonator of a conductor filter.

Description

COPYRIGHT AND TRADE PRESENTATION NOTICE

[0001] Part of the disclosure in this patent specification contains material that is subject to copyright protection. This patent specification may show and / or describe items that are, or may become, the trade dress of the proprietor. The proprietor of the copyright and trade dress does not object to the facsimile reproduction by anyone of the patent disclosure as it appears in the patent files or records of the Patent and Trademark Office, but otherwise reserves all rights to the copyright and trade dress. INFORMATION ABOUT RELATED REGISTRATIONS

[0002] This patent claims priority over provisional patent application No. 63 / 140,030, filed on January 21, 2021, entitled XBAR MIT VERBESSERTEM PISTON-MODE (IMPROVED PISTON MODE XBAR), which is incorporated herein by reference. BACKGROUND area

[0003] This disclosure relates to high-frequency filters that use acoustic wave resonators, and in particular to filters for use in communication equipment. Description of the related prior art

[0004] A high-frequency filter (HF filter) is a two-port device configured to pass some frequencies and block others, where "pass" means transmission with relatively little signal loss and "block" means blocking or significant attenuation. The range of frequencies passed by a filter is called the filter's "passband." The range of frequencies blocked by such a filter is called the filter's "stopband." A typical RF filter has at least one passband and at least one stopband. Specific requirements for a passband or stopband depend on the application. For example, a "passband" might be defined as a frequency range in which the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB.A "stopband" can be defined as a frequency range in which the suppression of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB or more, depending on the application.

[0005] RF filters are used in communication systems where information is transmitted wirelessly. Examples include the RF front ends of cellular base stations, mobile phones and computers, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptops and tablets, fixed-point radio links, and other communication systems. RF filters are also used in radar and electronic warfare systems.

[0006] RF filters typically require many design compromises to achieve the best balance between performance parameters such as insertion loss, suppression, isolation, power handling, linearity, size, and cost for each specific application. Specific design and manufacturing methods and improvements can simultaneously benefit one or more of these requirements.

[0007] Improvements to the RF filters in a wireless system can have a broad impact on system performance. RF filter enhancements can be used to provide system performance improvements such as larger cells, longer battery life, higher data rates, greater network capacity, lower costs, improved security, and higher reliability. These improvements can be implemented at many levels of the wireless system, both individually and in combination, such as the RF module, RF transceiver, mobile or fixed subsystem, or network level.

[0008] High-performance RF filters for current communication systems typically incorporate acoustic wave resonators, including surface acoustic wave resonators (SAW resonators), bulk acoustic wave resonators (BAW resonators), film bulk acoustic wave resonators (FBAR resonators), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies and bandwidths proposed for future communication networks.

[0009] The desire for wider communication channel bandwidths inevitably leads to the use of higher frequency communication bands. Radio access technology for mobile networks has been standardized by the 3GPP (3rd Generation Partnership Project). Radio access technology for 5th generation (5G) mobile networks is defined in the 5G NR (New Radio) standard. The 5G NR standard defines several new communication bands. Two of these new communication bands are n77, which uses the frequency range from 3300 MHz to 4200 MHz, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Both the n77 and n79 bands use time-duplex duplexing (TDD), so a communication device operating in the n77 and / or n79 bands uses the same frequencies for uplink and downlink transmissions. The bandpass filters for bands n77 and n79 must be able to handle the transmit power of the communication device.WiFi bands at 5 GHz and 6 GHz also require a high frequency and large bandwidth. The 5G NR standard also defines millimeter-wave communication bands with frequencies between 24.25 GHz and 40 GHz.

[0010] The transversely excited acoustic film volume resonator (XBAR) is an acoustic resonator structure for use in microwave filters. The XBAR is described in US patent 10,491,291, entitled "Transversely Excited Film Bulk Acoustic Resonator." An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer or membrane of a single-crystal piezoelectric material. The IDT contains 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 nested within each other. A microwave signal applied to the IDT excites a primary acoustic shear wave in the piezoelectric membrane.XBAR resonators offer very high electromechanical coupling and high-frequency capability. XBAR resonators can be used in a variety of RF filters, including bandstop filters, bandpass filters, duplexers, and multiplexers. XBARs are well-suited for use in filters for communication bands with frequencies above 3 GHz. List of characters Fig. Figure 1 contains a schematic top view, two schematic cross-sectional views and a detail view of a transversely excited acoustic film volume resonator (XBAR). Fig. Figure 2 is a schematic block diagram of a bandpass filter with acoustic resonators. Fig. Figure 3 is a schematic top view of an XBAR with improved coupling and reduced acoustic energy loss. Fig. Figure 4 is a diagram of the absolute value of admittance as a function of frequency for XBAR with and without improved coupling and reduced acoustic energy loss. Fig. Figure 5 is a diagram of the conductance as a function of frequency for XBAR with and without improved coupling and reduced acoustic energy loss. Fig. Figure 6 is a flowchart of a process for manufacturing a filter with XBAR featuring improved coupling and reduced acoustic energy loss.

[0011] In this description, elements appearing in drawings are assigned three- or four-digit reference identifiers, where the two least significant digits are specific to the element and the one or two most significant digits are the drawing number in which the element is first introduced. For an element not described in connection with a drawing, it can be assumed to have the same characteristics and function as a previously described element with the same reference identifier. DETAILED DESCRIPTION Device description

[0012] Fig. Figure 1 shows a simplified schematic top view and orthogonal cross-sectional views of an XBAR 100. XBAR-type resonators such as the XBAR 100 can be used in a variety of RF filters, including bandstop filters, bandpass filters, duplexers, and multiplexers.

[0013] The XBAR 100 consists of a thin-film conductor structure formed on the surface of a piezoelectric plate 110 with parallel front and back surfaces 112 and 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 such that the orientation of the X, Y, and Z crystal axes with respect to the front and back surfaces is known and consistent. The piezoelectric plate can be Z-cut, meaning the Z-axis is perpendicular to the front and back surfaces. The piezoelectric plate can also be ZY-cut, rotated Y-cut, rotated Z-cut, or rotated YX-cut. XBARs can be fabricated on piezoelectric plates with other crystallographic orientations.

[0014] The rear surface 114 of the piezoelectric plate 110 is attached to a surface of a substrate 120, except for a region of the piezoelectric plate 110 that forms a membrane 115 spanning a cavity 140 formed in the substrate. The region of the piezoelectric plate spanning the cavity is referred to here as the "membrane" 115 due to its physical similarity to the membrane of a microphone. As in Fig. As shown in Figure 1, the membrane 115 borders the rest of the piezoelectric plate 110 around the entire circumference 145 of the cavity 140. In this context, "bordering" means "continuously connected without an intervening element." In other configurations, the membrane 115 can border the piezoelectric plate around at least 50% of the circumference 145 of the cavity 140.

[0015] The substrate 120 provides mechanical support for the piezoelectric plate 110. The substrate 120 can be made of, for example, silicon, sapphire, quartz, or another material or a combination of materials. The back surface 114 of the piezoelectric plate 110 can be attached to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 can be grown on the substrate 120 or attached to the substrate in another way. The piezoelectric plate 110 can be attached directly to the substrate or via one or more intermediate material layers (in Fig. (1 not shown) are attached to substrate 120.

[0016] “Cavity” has the conventional meaning of “an empty space within a solid body.” The cavity 140 can be a hole that passes completely through the substrate 120 (as shown in Section AA and Section BB), or a recess in the substrate 120 beneath the membrane 115. The cavity 140 can be formed, for example, by selectively etching the substrate 120 before or after attaching the piezoelectric plate 110 and the substrate 120.

[0017] The conductor structure of the XBAR 100 incorporates an interdigital converter (IDT) 130. The IDT 130 comprises a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132, and a second plurality of fingers extending from a second busbar 134. The term "busbar" refers to a conductor from which the fingers of an IDT extend. The first and second plurality of parallel fingers are nested within each other. The nested fingers overlap over a distance AP, commonly referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the "length" of the IDT.

[0018] The first and second busbars 132 and 134 serve as terminals for the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132 and 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. The primary acoustic mode is a volume shear mode in which acoustic energy propagates along a direction that is essentially orthogonal to the surface of the piezoelectric plate 110 and also perpendicular to the direction of the electric field generated by the IDT fingers. Therefore, the XBAR is considered a transversely excited film volume wave resonator.

[0019] The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are arranged on the membrane 115, which spans or is suspended above the cavity 140. As shown in Fig. As shown in Figure 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. An XBAR cavity can have a different shape, such as a regular or irregular polygon. The XBAR cavity can have more or fewer than four sides, which may be straight or curved.

[0020] To simplify the presentation in Fig. Figure 1 shows that the geometric spacing and width of the IDT fingers are greatly exaggerated in relation to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 130. An XBAR can have hundreds, possibly thousands, of parallel fingers in the IDT 130. Similarly, the thicknesses of the IDT fingers and the piezoelectric plate are greatly exaggerated in the cross-sectional views.

[0021] Referring to the detailed schematic cross-sectional view (Detail C), a front-facing dielectric layer 150 (or coating) can optionally be formed on the front of the piezoelectric plate 110. The "front" of the XBAR is, by definition, the surface facing away from the substrate. The front-facing dielectric layer 150 can be formed only between the IDT fingers (e.g., IDT finger 138b) or applied as a top layer, so that the dielectric layer is formed both between and over the IDT fingers (e.g., IDT finger 138a). The front-facing dielectric layer 150 can be a non-piezoelectric dielectric material, such as silicon dioxide, aluminum, or silicon nitride. The thickness of the front-facing piezoelectric layer 150 is typically less than one-third of the thickness tp of the piezoelectric plate 110.The front-side dielectric layer 150 can be formed from multiple layers of two or more materials. In some applications, a rear-side dielectric layer (not shown) can be formed on the back of the dielectric plate 110.

[0022] The IDT fingers 138a and 138b can consist of one or more layers of aluminum, an aluminum alloy, copper, a copper alloy, beryllium, gold, tungsten, molybdenum, chromium, titanium, or another conductive material. The IDT fingers are considered "essentially aluminum" if they consist of aluminum or an alloy containing at least 50% aluminum. The IDT fingers are considered "essentially copper" if they consist of copper or an alloy containing at least 50% copper. Thin (relative to the overall thickness of the conductors) layers of metals, such as chromium or titanium, can be formed under and / or over and / or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or to passivate or encapsulate the fingers and / or to improve power handling. The busbars (132, 134 in Fig. 1) The IDTs can be made of the same or different materials as the fingers.

[0023] Dimension p is the center-to-center distance or "pitch" of the IDT fingers, which can be referred to as the IDT spacing and / or the XBAR spacing. Dimension m is the width or "mark" of the IDT fingers. The geometry of an XBAR's IDT differs significantly from those used in surface acoustic wave resonators (SAW resonators). In a SAW resonator, the IDT spacing is half the acoustic wavelength at the resonant frequency. Furthermore, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e., the mark or finger width is about one-quarter of the acoustic wavelength at the resonant frequency). In an XBAR, the IDT spacing p can be 2 to 20 times the finger width m. The spacing p is typically 3.3 to 5 times the finger width m.Furthermore, the spacing p of the IDT can typically be 2 to 20 times the thickness of the piezoelectric plate 210. The spacing 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 an XBAR is not limited to approximately one-quarter of the acoustic wavelength at resonance. For example, the width of the XBAR IDT fingers can be 500 nm or more, so that the IDT can readily be fabricated by optical lithography. The thickness of the IDT fingers can range from 100 nm to approximately equal to the width m. The thickness of the busbars (132, 134) of the IDT can be equal to or greater than the thickness of the IDT fingers.

[0024] Fig. Figure 2 is a schematic circuit diagram and layout for a high-frequency bandpass filter 200 with XBAR. The filter 200 has a conventional ladder filter architecture with three series resonators 210A, 210B, 210C and two shunt resonators 220A, 220B. The three series resonators 210A, 210B and 210C are connected in series between a first terminal and a second terminal (hence the term "series resonator"). Fig. The first and second terminals are labeled "In" and "Out," respectively. However, filter 200 is bidirectional, and each terminal can serve as either an input or an output. The two shunt resonators 220A and 220B are connected to ground from the nodes between the series resonators. A filter can contain additional reactive components, such as capacitors and / or inductors, which are located in Fig. Figure 2 is not shown. All shunt resonators and series resonators are XBAR. The inclusion of three series and two shunt resonators is exemplary. A filter may have more or fewer than five total resonators, more or fewer than three series resonators, and more or fewer than two shunt resonators. Typically, all series resonators are connected in series between an input and an output of the filter. All shunt resonators are typically connected between ground and the input, the output, or a node between two series resonators.

[0025] In the exemplary filter 200, the three series resonators 210A, 210B, and 210C and the two shunt resonators 220A and 210B of filter 200 are formed on a single plate 230 of piezoelectric material bonded to a silicon substrate (not shown). In some filters, the series resonators and shunt resonators may be formed on separate plates of piezoelectric material. Each resonator contains a corresponding IDT (not shown), with at least the fingers of the IDT positioned over a cavity in the substrate. In this and similar contexts, the term "each" means "related to each other," i.e., with a one-to-one correspondence. Fig. Figure 2 shows the cavities schematically as dashed rectangles (e.g., rectangle 235). In this example, each IDT is arranged over a corresponding cavity. In other filters, the IDTs of two or more resonators may be arranged over a single cavity.

[0026] Each of the resonators 210A, 210B, 210C, 220A, and 220B in filter 200 exhibits a resonance, at which the resonator's admittance is very high, and an antiresonance, at which the resonator's admittance is very low. The resonance and antiresonance occur at a resonant frequency and an antiresonance frequency, respectively, which may be the same or different for the various resonators in filter 200. In a highly simplified manner, each resonator can be considered a short circuit at its resonant frequency and an open circuit at its antiresonance frequency. The input-output transfer function will be close to zero at the resonant frequencies of the shunt resonators and at the antiresonance frequencies of the series resonators. In a typical filter, the resonant frequencies of the shunt resonators lie below the lower edge of the filter's passband, and the antiresonant frequencies of the series resonators lie above the upper edge of the passband.In some filters, a dielectric layer (also called a "frequency-tuning layer"), represented by the dashed rectangle 270, can be formed on the front and / or back surface of the shunt resonators to lower the resonant frequencies of the shunt resonators relative to the resonant frequencies of the series resonators. In other filters, the diaphragms of the series resonators can be thinner than the diaphragms of the shunt resonators. In some filters, the series resonators and the shunt resonators can be manufactured on separate chips with different piezoelectric plate thicknesses.

[0027] Three-dimensional simulations of XBAR devices show that some acoustic energy can escape or be lost at the ends of the IDT fingers, i.e., in the gap between the electrode ends and the busbars. A well-guided wave exhibits high-order waveguide modes that form along the aperture. The transverse modes of the acoustic wave couple to the gap and can create an energy confinement within it. This manifests as either losses or strong disturbances. Coupling to different transverse modes is possible because the uniform electric field along the aperture and the structure of the transverse modes are not orthogonal. These transverse modes should be electrically decoupled by the IDT to suppress unwanted modes.

[0028] A low-loss, low-noise resonator can be designed by improving waveguiding (e.g., suppressing wave radiation between the IDT ends and the busbar) while simultaneously electrically decoupling the higher-order transverse modes. Decoupling is typically achieved through a piston design, where the fundamental transverse mode at resonance exhibits a uniform distribution along the aperture. Thus, the structure of the fundamental transverse mode coincides with the structure of the external electric field. In waveguide theory, the waveguided modes are functionally orthogonal; that is, all higher-order modes are orthogonal to the structure of the fundamental mode, which is identical to the external electric field in the piston mode. Consequently, the higher-order transverse modes are electrically decoupled from the IDT.

[0029] Fig. Figure 3 is a schematic top view of an XBAR 300 with improved coupling and reduced acoustic energy loss. Similar to the XBAR 100 from Fig. Figure 1 of the XBAR 300 comprises a piezoelectric plate 310 on a substrate with a cavity (not shown) and an IDT 330 with nested fingers 336 extending alternately from busbars 332, 334 on the piezoelectric plate 310. A front-facing dielectric layer 350 is located above the IDT. The dielectric layer 350 can consist, for example, of SiOz, SiN3, Al2O3, Ta2O5, titanium oxide, titanium nitride, AlN (non-piezoelectric phase), diamond, and / or other non-piezoelectric dielectric materials and combinations thereof. The nested fingers overlap by a distance AP, which defines an "aperture" of the IDT. As described above, the number and relative sizes of individual elements to each other, e.g., For example, the number of fingers, the distance between the fingers, the width of the fingers, the length of the aperture, etc., are not necessarily depicted accurately, but exaggerated to simplify the representation.

[0030] Undesired interference modes can be significantly reduced if no dielectric layer is present, or if the dielectric layer is significantly reduced, in the region between the ends of the fingers 336 and the busbars 332, 334. However, in certain configurations (e.g., 120 μx lithium niobite piezoelectric plates), the coupling of the resonator can be reduced if the dielectric layer precisely overlaps the aperture and does not extend into the region between the ends of the fingers 336 and the busbars 332, 334. In the XBAR 300 of Fig. 3 The dielectric layer 350 extends a distance do beyond an edge of the aperture, but not to the busbars 332, 334. An edge of the aperture is a line running along the ends of the IDT fingers at the gap between the ends of the IDT fingers and the busbars. The aperture has two opposite edges: a first edge adjacent to one of the busbars and an opposite second edge adjacent to the other busbar. Coupling is improved if the dielectric layer extends beyond the edge of the aperture but not to the busbars. The extension (or overhang) is chosen to optimize coupling without introducing additional losses or disturbance modes.

[0031] A resonant frequency of the primary acoustic shear mode excited by the IDT 330 in the piezoelectric plate 310 between the dielectric layer 350 and the busbars 332, 334 has a higher frequency than the primary acoustic shear mode excited by the IDT 330 in the piezoelectric plate 310 covered by the dielectric layer 350. The higher frequency is due to the fact that the frequency is approximately inversely proportional to the thickness of the device. This results in reduced energy loss at the finger ends, as improved wave conduction is achieved.

[0032] The length do can be, for example, less than or equal to 20tp. In one example, the length do is approximately 2 µm.

[0033] Fig. Figure 4 is a diagram of the absolute value of the admittance as a function of frequency for XBAR with and without a dielectric layer, as in Fig. 3 described. In this example, the dielectric layer is formed from SiO2. The solid curve 410 represents the absolute value of the admittance for an XBAR with a dielectric layer over a single aperture of an IDT. The dashed curve 420 represents the absolute value of the admittance for an XBAR with a dielectric layer extending a length do of 2 µm beyond the edge of the aperture. Curve 420 shows an improved admittance compared to curve 410.

[0034] Fig. Figure 5 is a diagram of the conductance as a function of frequency for XBAR with and without a dielectric layer, as in Fig. As described in section 3, the solid curve 510 represents the conductance for an XBAR with a dielectric layer only over one aperture of an IDT. The dashed curve 520 represents the conductance for an XBAR with a dielectric layer extending a length do of 2 µm beyond the edge of the aperture. Curve 520 shows a reduced conductance compared to curve 510. This reduced conductance can lead to improved insertion loss for a filter using this XBAR as a shunt resonator. Procedure description

[0035] Fig. Figure 6 is a simplified flowchart summarizing a process 600 for fabricating a filter device with XBAR featuring improved coupling and reduced acoustic energy loss. Specifically, the process 600 serves to fabricate a filter device with multiple XBAR. The process 600 begins at 605 with a device substrate and a thin plate of piezoelectric material arranged on a sacrificial substrate. The process 600 ends at 695 with a finished filter device. The flowchart in Fig. Section 6 contains only the most important process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, curing, annealing, monitoring, testing, etc.) can be performed before, between, after, and during the process described in the text. Fig. The 6 steps shown are carried out.

[0036] While Fig. While section 6 generally describes a method for fabricating a single filter device, multiple filter devices can be fabricated simultaneously on a common wafer (consisting of a piezoelectric plate bonded to a substrate). In this case, each step of procedure 600 can be performed simultaneously on all filter devices on the wafer.

[0037] The flowchart in Fig. Figure 6 shows three variants of process 600 for manufacturing an XBAR, which differ in when and how the cavities are formed in the substrate of the device. The cavities can be formed in steps 610A, 610B, or 610C. In each of the three variants of process 600, only one of these steps is performed.

[0038] The piezoelectric plate can typically consist of ZY-cut or YX-cut lithium niobate. The piezoelectric plate can also be made of a different material and / or with a different cut. The device substrate can preferably be made of silicon. The device substrate can also be made of a different material that allows the formation of deep cavities by etching or other methods.

[0039] In one variant of process 600, one or more cavities are formed in the substrate of the device at 610A before the piezoelectric plate is bonded to the substrate at 615. A separate cavity can be formed for each resonator in a filter device. The cavities can also be shaped and formed such that two or more resonators can be located on a membrane above a single cavity. The one or more cavities can be produced using conventional photolithography and etching techniques. Typically, the cavities formed at 610A do not penetrate the substrate of the device.

[0040] In step 615, the piezoelectric plate is bonded to the device substrate. The piezoelectric plate and the device substrate can be bonded using a wafer bonding process. Typically, the contact surfaces of the device substrate and the piezoelectric plate are highly polished. One or more layers of intermediate materials, such as an oxide or metal, can be formed or deposited on the contact surface of the piezoelectric plate or the device substrate. One or both contact surfaces can be activated, for example, by a plasma process. The contact surfaces can then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the device substrate or the intermediate material layers.

[0041] At 620, the sacrificial substrate can be removed. The piezoelectric plate and sacrificial substrate can, for example, consist of a wafer of piezoelectric material into which ions have been implanted to create defects in the crystal structure along a plane that forms a boundary between the future piezoelectric plate and the sacrificial substrate. At 620, the wafer can be cleaved along the defect plane, for example by thermal shock, detaching the sacrificial substrate and leaving the piezoelectric plate bonded to the device substrate. The exposed surface of the piezoelectric plate can be polished or otherwise machined after the sacrificial substrate has been detached.

[0042] A primary conductor structure, comprising the IDT (including the busbars and nested fingers) and the reflector elements of each XBAR, is formed at 645 by depositing and structuring one or more conductor layers on the front face of the piezoelectric plate. The conductor layer can be made of, for example, aluminum, an aluminum alloy, copper, a copper alloy, or another conductive metal. Optionally, one or more layers of other materials can be placed below (i.e., between the conductor layer and the piezoelectric plate) and / or above the conductor layer. For example, a thin film of titanium, chromium, or another metal can be used to improve adhesion between the conductor layer and the piezoelectric plate.A second conductor structure made of gold, aluminum, copper or another metal with higher conductivity can be formed over parts of the first conductor structure (e.g. the IDT busbars and the connections between the IDTs).

[0043] Any conductor structure can be formed at 645 by successively depositing the conductor layer and, if desired, one or more other metal layers onto the surface of the piezoelectric plate. The excess metal can then be removed by etching through structured photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques.

[0044] Alternatively, any conductor structure can be formed using a lift-off process at 645. Photoresist can be deposited onto the piezoelectric plate and structured to create the conductor structure. The conductor layer, and optionally one or more other layers, can be deposited successively onto the surface of the piezoelectric plate. The photoresist can then be removed, thus removing the excess material and leaving the conductor structure.

[0045] At 650, one or more frequency-tuning dielectric layers can be formed by depositing one or more layers of dielectric material on the front surface of the piezoelectric plate. For example, a dielectric layer can be formed over the shunt resonators to attenuate their frequencies relative to the frequencies of the series resonators. The one or more dielectric layers can be deposited using a conventional deposition method such as physical vapor deposition, atomic layer deposition, chemical vapor deposition, or another method. One or more lithographic techniques (using photomasks) can be employed to restrict the deposition of the dielectric layers to selected areas of the piezoelectric plate.For example, a mask can be used to limit a dielectric layer so that it only covers the shunt resonators.

[0046] Even at 650, the portion of the area between the busbars and the ends of the nested fingers can be masked when dielectric layers are formed, in order to prevent the formation of the dielectric layer in this area. For example, the dielectric layer may only extend a certain distance (e.g., 2 µm) beyond the edge of the aperture into the area between the busbars and the ends of the nested fingers, as described in relation to Fig. 3 described. Furthermore, the dielectric layer over this area can be thinner than the rest of the dielectric layer. Alternatively, the dielectric layer in this area can be removed after dielectric formation.

[0047] In process 655, a dielectric passivation / tuning layer is deposited over the piezoelectric plate and the conductor structures. The dielectric passivation / tuning layer can cover the entire surface of the filter, except for the pads for electrical connections to circuits outside the filter. In some embodiments of process 600, the dielectric passivation / tuning layer can be formed after the cavities in the device substrate have been etched in either process 610B or 610C.

[0048] In a second variant of method 600, one or more cavities are formed on the back side of the device substrate at 610B. A separate cavity can be formed for each resonator in a filter device. The cavities can also be formed such that two or more resonators can be positioned on a membrane above a single cavity. The one or more cavities can be formed by anisotropic or orientation-dependent dry or wet setting to open holes through the back side of the device substrate to the piezoelectric plate. In this case, the resulting resonator devices have a cross-section as shown in Fig. 1 shown.

[0049] In a third variant of method 600, one or more cavities in the form of depressions in the device substrate can be formed at 610C by etching the substrate with an etchant introduced through openings in the piezoelectric plate. A separate cavity can be formed for each resonator in a filter device. The cavities can also be formed such that two or more resonators can be located on a membrane above a cavity. The one or more cavities formed at 610C do not penetrate the substrate of the device.

[0050] Ideally, after the cavities are formed at 610B or 610C, most or all filter devices on a wafer meet a set of performance requirements. However, normal process tolerances lead to variations in parameters such as the thickness of the dielectric layer formed at 650 and 655, variations in the thickness and linewidths of conductors and IDT fingers formed at 645, and variations in the thickness of the piezoelectric plate. These variations contribute to the filter device's performance deviating from the performance requirements.

[0051] To improve the yield of filter devices that meet performance requirements, frequency tuning can be achieved by selectively adjusting the thickness of the passivation / tuning layer deposited over the resonators at process 655. The passband frequency of a filter device can be lowered by adding material to the passivation / tuning layer, and the passband frequency of the filter device can be raised by removing material from the passivation / tuning layer. Typically, process 600 is designed to produce filter devices with passbands that are initially lower than a required frequency range but can be tuned to the desired frequency range by removing material from the surface of the passivation / tuning layer.

[0052] At 660, a test card or other device for making electrical connections to the filter can be used to enable high-frequency (RF) testing and measurements of filter characteristics such as the input-output transfer function. Typically, RF measurements are performed on all or a large portion of the filter devices, which are fabricated simultaneously on a common piezoelectric plate and substrate.

[0053] 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 an ion mill, as previously described. This "global" tuning is performed with a spatial resolution equal to or greater than that of a single filter device. The goal of global tuning is to shift the passband of each filter device toward a desired frequency range. The test results from 660 can be processed to create a global contour map that specifies the amount of material to be removed as a function of its two-dimensional position on the wafer. The material is then removed according to the contour map using the selective material removal tool.

[0054] At 670, local frequency tuning can be performed in addition to, or instead of, the global frequency tuning at 665. The "local" frequency tuning is performed with a spatial resolution smaller than that of a single filter device. The test results from 660 can be processed to create a map indicating the amount of material that needs to be removed at each filter device. Local frequency tuning may require the use of a mask to limit the size of the areas from which material is removed. For example, a first mask can be used to restrict the tuning to shunt resonators only, and a second mask can then be used to restrict the tuning to series resonators only (or vice versa).This would allow independent tuning of the lower band limit (by tuning shunt resonators) and the upper band limit (by tuning series resonators) of the filter devices.

[0055] After frequency tuning at 665 and / or 670, the filter device is completed at 675. Actions that may be performed at 675 include the fabrication of bond pads or solder joints, or other means of connection between the device and external circuitry (if such pads were not fabricated at 645), the cutting of individual filter devices from a wafer containing multiple filter devices, other packaging steps, and additional testing. After each filter element is completed, the process ends at 695. Concluding remarks

[0056] Throughout this entire description, the embodiments and examples shown should be considered as models and not as limitations of the disclosed or claimed devices and procedures. Although many of the examples presented here involve specific combinations of process activities or system elements, it should be understood that these activities and elements can be combined in other ways to achieve the same objectives. With regard to flowcharts, additional or fewer steps can be taken, and the steps shown can be combined or further refined to achieve the procedures described herein. Activities, elements, and features discussed only in connection with one embodiment are not intended to exclude a similar role in other embodiments.

[0057] As used here, "multiple" means two or more. As used here, a "set" of elements may comprise one or more such elements. In the form used here, whether in the written description or in the claims, the terms "comprising," "including," "bearing," "having," "containing," "incorporating," and the like are to be understood as being unlimited, i.e., including but not limited to. Only the transitional phrases "consisting of" and "consisting substantially of" are closed or semi-closed transitional phrases with respect to claims. The use of ordinal expressions such as "first," "second," "third," etc.In the claims, the use of "and / or" to modify a claim element does not in itself imply a priority, precedence, or order of one claim element over another, nor does it indicate the temporal order in which the activities of a process are carried out. Rather, it is merely used as a distinguishing term to differentiate one claim element with a particular name from another element with the same name (but for the use of the ordinal expression to distinguish the claim elements). As used here, "and / or" means that the listed elements are alternatives, but the alternatives also include every combination of the listed elements. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 63 / 140030

[0002] US 10491291

[0010]

Claims

[1] Acoustic resonator device comprising: a piezoelectric plate with front and back surfaces; an interdigital transducer (IDT) on the front surface comprising interleaved fingers extending alternately from opposite first and second busbars, wherein an overlapping distance of the interleaved fingers defines an aperture of the acoustic resonator device; and a dielectric layer formed over and between the interleaved fingers, the dielectric layer extending from one edge of the aperture and the first busbar to an opposite edge of the aperture and the second busbar. [2] The device of claim 1, wherein a shape of the dielectric layer is configured to improve performance compared to an acoustic resonator device having a dielectric layer extending over the entire IDT. [3] The device of claim 1, wherein a portion of the piezoelectric plate forms a diaphragm spanning a cavity in a substrate. [4] The device of claim 3, wherein the interleaved fingers are located on the membrane. [5] The device of claim 1, further comprising a first trench without a dielectric layer between the edge of the aperture and the first bus bar and a second trench without a dielectric layer between the opposite edge of the aperture and the second bus bar. [6] The device of claim 1, wherein the dielectric layer extends across the edge of the aperture and the opposite edge of the aperture by a distance of less than or equal to 20 tp, where tp is a thickness of the piezoelectric plate. [7] The device of claim 1, wherein the dielectric layer extends about 2 µm beyond the edge of the aperture and the opposite edge of the aperture. [8] Filter device comprising: a piezoelectric plate with front and back surfaces; a conductor structure on the front surface, the conductor structure comprising a plurality of interdigital transducers (IDTs) of a respective plurality of resonators, each of the plurality of IDTs comprising interleaved fingers extending alternately from opposite first and second busbars, an overlapping distance of the interleaved fingers defining an aperture of a respective resonator of the plurality of resonators, wherein at least one resonator of the plurality of resonators comprises a dielectric layer formed over and between the interleaved fingers, the dielectric layer extending from one edge of the aperture and the first busbar to an opposite edge of the aperture and the second busbar. [9] The device of claim 8, wherein a shape of the dielectric layer is arranged to improve performance compared to a resonator having a dielectric layer extending over the entire IDT. [10] A device according to claim 8, wherein a portion of the piezoelectric plate forms a plurality of diaphragms extending across respective cavities in a substrate. [11] The device of claim 10, wherein the interleaved fingers are each located on one of the plurality of membranes. [12] The apparatus of claim 8, wherein the at least one resonator of the plurality of resonators further comprises a first trench without a dielectric layer between the edge of the aperture and the first busbar and a second trench without a dielectric layer between the opposite edge of the aperture and the second busbars. [13] The device of claim 8, wherein the dielectric layer extends across the edge of the aperture and the opposite edge of the aperture by a distance of less than or equal to 20 tp, where tp is a thickness of the piezoelectric plate. [14] The device of claim 8, wherein the dielectric layer extends about 2 µm beyond the edge of the aperture and the opposite edge of the aperture. [15] A method of manufacturing an acoustic resonator device, comprising: Forming an interdigital transducer (IDT) on a front surface of a piezoelectric layer, the IDT comprising interleaved fingers extending alternately from opposing first and second busbars, an overlapping distance of the interleaved fingers defining an aperture of an acoustic resonator device; and Forming a dielectric layer over and between the interleaved fingers, the dielectric layer extending from one edge of the aperture and the first busbar to an opposite edge of the aperture and the second busbar. [16] The method of claim 15, wherein a shape of the dielectric layer is configured to improve performance compared to an acoustic resonator device having a dielectric layer extending across the entire IDT. [17] The method of claim 15, wherein a portion of the piezoelectric plate forms a membrane spanning a cavity in a substrate. [18] The method of claim 17, wherein the interleaved fingers are located on the membrane. [19] The method of claim 15, wherein the dielectric layer extends across the aperture by a distance less than or equal to 20tp, where tp is a thickness of the piezoelectric plate. [20] The method of claim 15, wherein the dielectric layer extends about 2 µm beyond the edge of the aperture and the opposite edge of the aperture.