Decoupled lateral excitation film bulk acoustic resonator

Abstract

Acoustic wave resonator devices and filters are disclosed. An acoustic wave resonator includes a substrate and a piezoelectric plate having parallel front and back faces, the back face attached to the substrate. A decoupling dielectric layer is located on the front face of the piezoelectric plate. An interdigital transducer (IDT) is formed on the decoupling dielectric layer such that interleaved fingers of the IDT are located over a portion of the piezoelectric plate that is suspended over a cavity formed in the substrate.

Description

[0001] Copyright and Trademark Design Statement

[0002] This patent document contains material protected by copyright. The patent document may show and / or describe, or may become, trademark designs of the owner. While the copyright and trademark design owner has no objection to any fax copy of the patent disclosure because it appears in the Patent and Trademark Office documents or records, they retain copyright and trademark design rights.

[0003] Cross-references to related applications

[0004] This patent claims priority to the following provisional patent application filed on January 15, 2021, application number 63 / 137,736, entitled “XBAR WITH INSULATING LAYER BETWEEN ELECTRODE AND PIEZOELECTRICMEMBRANE TO REDUCE ACOUSTIC COUPLING”, which is incorporated herein by reference. Technical Field

[0005] This disclosure relates to radio frequency filters using acoustic resonators, and more particularly to filters used in communication devices. Background Technology

[0006] Radio frequency (RF) filters are two-ended devices configured to allow some frequencies to pass while blocking others. "Passing" means transmitting with relatively low insertion loss, while "blocking" means blocking or essentially attenuating. 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 less than a defined value 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 a defined value, such as 20 dB, 30 dB, 40 dB, or greater, depending on the application.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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 3300MHz to 4200MHz, and n79 uses a frequency range of 4400MHz to 5000MHz. 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. The bandpass filters for 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 5G bands. The 5G NR standard also defines millimeter wave communication frequency bands between 24.25 GHz and 40 GHz.

[0012] 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

[0013] The present invention discloses an acoustic resonator device, comprising: a substrate; a piezoelectric plate having a parallel front and a back side, the back side being attached to the substrate; a decoupling dielectric layer located on the front side of the piezoelectric plate; and an interdigital transducer (IDT) formed on the decoupling dielectric layer such that the interlaced fingers of the IDT are located above a portion of the piezoelectric plate, this portion of the piezoelectric plate being suspended over a cavity formed in the substrate.

[0014] The IDT is configured to excite shear acoustic waves in the piezoelectric plate in response to a radio frequency signal applied to the IDT.

[0015] The piezoelectric plate is a rotating Y-cut lithium niobate.

[0016] The decoupling dielectric layer comprises silicon dioxide.

[0017] The thickness between the front and back sides of the piezoelectric plate is greater than or equal to 200 nm and less than or equal to 1000 nm.

[0018] Wherein, the spacing between the fingers of the IDT is greater than or equal to twice the thickness of the piezoelectric plate and less than or equal to 25 times the thickness of the piezoelectric plate.

[0019] The IDT's fingers have a width, and the spacing is greater than or equal to twice the width and less than or equal to 25 times the width.

[0020] It also includes: a front dielectric layer formed on the front side of the piezoelectric plate between the fingers of the IDT, wherein the resonant frequency of the acoustic resonator device is partially determined by the thickness of the front dielectric layer.

[0021] The front dielectric layer includes at least one of silicon dioxide and silicon nitride.

[0022] The IDT includes one of aluminum, aluminum alloy, copper, copper alloy, beryllium, and gold.

[0023] The present invention also discloses a filter device comprising: a substrate; a piezoelectric plate having a parallel front side and a back side, the back side being attached to the substrate; a decoupling dielectric layer located on the front side of the piezoelectric plate; and a conductor pattern formed on the decoupling dielectric layer, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of corresponding plurality of resonators, wherein the staggered fingers of each of the plurality of IDTs are located above various portions of the piezoelectric plate, the various portions of the piezoelectric plate being suspended over one or more cavities formed in the substrate.

[0024] All of the plurality of IDTs are configured to excite shear acoustic waves in the piezoelectric plate in response to a corresponding radio frequency signal applied to each IDT.

[0025] The piezoelectric plate is a rotating Y-cut lithium niobate.

[0026] The decoupling dielectric layer comprises silicon dioxide.

[0027] The thickness between the front and back sides of the piezoelectric plate is greater than or equal to 200 nm and less than or equal to 1000 nm.

[0028] Each of the plurality of IDTs is located in a corresponding portion of the piezoelectric plate, and the corresponding portion of the piezoelectric plate is suspended on a corresponding cavity formed in the substrate.

[0029] The plurality of resonators includes a parallel resonator and a series resonator.

[0030] The thickness of the first dielectric layer deposited between the fingers of the IDT in the parallel resonator is greater than the thickness of the second dielectric layer deposited between the fingers of the IDT in the series resonator.

[0031] The present invention further discloses a method for manufacturing an acoustic resonator device on a piezoelectric plate having a parallel front and a back side, the back side being attached to a substrate, the method comprising: forming a cavity in the substrate such that a portion of the piezoelectric plate is suspended over the cavity; forming a decoupling dielectric layer on the front side of the piezoelectric plate; and forming an interdigitated transducer (IDT) over the decoupling dielectric layer such that the interlaced fingers of the IDT are positioned above the portion of the piezoelectric plate suspended over the cavity.

[0032] The piezoelectric plate is a rotated Y-cut lithium niobate and the decoupling dielectric layer includes silicon dioxide. Attached Figure Description

[0033] Figure 1 Includes a schematic plan view, two schematic cross-sectional views, and a detailed view of the transversely excited membrane acoustic resonator (XBAR).

[0034] Figure 2 This is a schematic block diagram of a bandpass filter that uses an acoustic resonator.

[0035] Figure 3 This is the admittance amplitude diagram of XBAR using YX-cut and Z-cut lithium niobate separators.

[0036] Figure 4 This is a schematic cross-sectional view of an XBAR with a decoupled dielectric layer between the IDT fingers and the piezoelectric diaphragm.

[0037] Figure 5 These are admittance amplitude diagrams of XBARs with decoupled dielectric layers of different thicknesses.

[0038] Figure 6 It is an electromechanical coupling diagram that varies with the thickness of the decoupling dielectric layer.

[0039] Figure 7 This is a graph of the input-output transfer function of the band N79 filter using decoupled XBAR.

[0040] Figure 8 This is a flowchart of a method for manufacturing decoupled XBARs or using filters with decoupled XBARs.

[0041] 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

[0042] Component Description

[0043] Figure 1 A simplified schematic top view and orthogonal cross-sectional view of the XBAR 100 are shown. XBAR-type 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.

[0044] 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 known and consistent. 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 a rotated Z-cut or a rotated YX-cut. XBARs can be fabricated on piezoelectric plates with other crystal orientations.

[0045] 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 122 of the substrate 120. This portion of the piezoelectric plate 110 forms a diaphragm 115, which spans a cavity 140 formed in the substrate. The portion of the piezoelectric plate spanning the cavity is referred to herein as the "diaphragm" 115 because this portion is physically similar to the diaphragm of a microphone. Figure 1 As shown, the diaphragm 115 is adjacent to the remainder of the piezoelectric plate 110 around the entire periphery 145 of the cavity 140. In this case, "adjacent" means "continuous connection without any other items in between". In other configurations, the diaphragm 115 may be adjacent to the piezoelectric plate around at least 50% of the periphery 145 of the cavity 140.

[0046] 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 otherwise attached to the substrate. Piezoelectric plate 110 can be directly attached to the substrate, or it can be attached via one or more intermediate material layers (…). Figure 1 (Not shown in the image) is attached to substrate 120.

[0047] 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.

[0048] 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 "aperture" of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the "length" of the IDT.

[0049] 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.

[0050] IDT 130 is placed on piezoelectric plate 110 such that at least the fingers of IDT 130 are positioned on diaphragm 115 of the piezoelectric plate, which spans or hangs over cavity 140. Figure 1 As shown, the cavity 140 is rectangular, and the size of the rectangle is greater than the aperture AP and the length L of IDT 130. The cavity of the XBAR can have different shapes, such as regular or irregular polygons. The cavity of the XBAR can have more or fewer four sides, which can be straight or curved.

[0051] To facilitate Figure 1 As shown, the geometric spacing and width of the IDT fingers are significantly enlarged relative to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR in an IDT 130 has more than ten parallel fingers. A single XBAR in an IDT 130 may have hundreds, possibly thousands, of parallel fingers. Similarly, in the cross-sectional view, the thickness of the IDT fingers and piezoelectric plate is significantly enlarged.

[0052] Referring now to a detailed schematic cross-sectional view (detail C), a front dielectric layer 150 may optionally be formed on the front side of the piezoelectric plate 110. By definition, the “front side” of XBAR refers to the side facing away from the substrate. The front dielectric layer 150 may be formed only between the IDT fingers (e.g., IDT fingers 138b), or it may be deposited as a capping layer such that the dielectric layer is formed between and on the IDT fingers (e.g., IDT fingers 138a). The front dielectric layer 150 may be a non-piezoelectric dielectric material, such as silicon dioxide, aluminum oxide, or silicon nitride. The thickness of the front dielectric layer 150 is typically less than about one-third of the thickness tp of the piezoelectric plate 110. The front dielectric layer 150 may be formed from multiple layers of two or more materials. In some applications, a back dielectric layer (not shown) may be formed on the back side of the piezoelectric plate 110.

[0053] 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.

[0054] Dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which can 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 meters. The thickness of the IDT's busbars (132, 134) can be equal to or greater than the thickness of the IDT fingers.

[0055] 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 node between the input, output, or two series resonators.

[0056] 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.

[0057] Each resonator 210A, 210B, 210C, 220A, 220B in filter 200 resonates when the resonator admittance is very high and anti-resonance when the resonator admittance is very low. Resonance and anti-resonance occur at the resonant frequency and anti-resonant frequency, respectively, and for each resonator in filter 200, the resonant frequency and anti-resonant frequency may be the same or different. In overly simplified terms, each resonator can be considered a short circuit at its resonant frequency and an open circuit 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 is below the lower edge of the filter passband, while the anti-resonant frequency of the series resonators is above the upper edge of the passband. In some filters, a front dielectric layer (also called a "frequency setting layer"), represented by a dashed rectangle 270, can be formed on the parallel resonators to set the resonant frequency of the parallel resonators to be lower than the resonant frequency 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.

[0058] Lithium niobate (LN) is the preferred piezoelectric material for XBARs. LN exhibits very high electromechanical coupling and can be used as a thin sheet attached to a non-piezoelectric substrate. While various crystal orientations can be used in XBARs, two commonly used orientations are Z-cut (Euler angles 0°, 0°, 90°) and rotated Y-cut (Euler angles 0°, β, 0° where 0° < β < 70°). Rotated Y-cut LNs with 30° < β < 38° exhibit higher electromechanical coupling than Z-cut LNs. Furthermore, while both Z-cut and rotated Y-cut LN XBARs are susceptible to lateral (parallel to the direction of the IDT fingers) acoustic energy leakage, this loss can be minimized in rotated Y-cut LN XBARs using relatively simple structures. Minimizing acoustic losses in Z-cut LN XBARs requires more complex structures, necessitating additional fabrication steps. XBARs using rotated Y-cut LNs may exhibit fewer and smaller parasitic modes than Z-cut LN XBARs.

[0059] Figure 3 It is the admittance amplitude diagram of two XBARs 300. Figure 3 The data shown and all subsequent examples were obtained by simulating XBARs using the finite element method. Solid curve 310 represents the admittance of an XBAR using a Y-cut LN piezoelectric plate with β = 30°. Dashed curve 320 represents the admittance of an XBAR using a Z-cut LN piezoelectric plate. In both cases, the piezoelectric plate thickness is 400 nm, the IDT electrodes are aluminum, the IDT spacing is 3 μm, and the IDT finger markings are 0.5 μm. The resonant frequency FR of both XBARs is approximately 4760 MHz, and the anti-resonant frequencies FA of the Y-cut and Z-cut XBARs are approximately 5550 MHz and 5350 MHz, respectively. The differences between the resonant and anti-resonant frequencies of the Y-cut and Z-cut XBARs are approximately 590 MHz and 790 MHz, respectively. Electromechanical coupling can be achieved through the parameter k. 2 eff Quantization, where k 2 eff =(FA) 2 –FR 2 ) / FA 2 . Figure 3 Rotational Y-cutting and Z-cutting XBAR k 2 eff They were 26.4% and 20.8% respectively.

[0060] The significant difference between the resonant and anti-resonant frequencies of a rotated Y-cut LN XBAR enables the design of filters with very wide bandwidths. However, for some filter applications, the difference between the resonant and anti-resonant frequencies can be too large. For example, the frequency range of the 5G NR band N79 is between 4400MHz and 5000MHz. Bandpass filters for the N79 band cannot be implemented using conventional rotated Y-cut LN XBARs. As mentioned earlier, the resonant frequency of the parallel resonator in a trapezoidal filter circuit is typically just below the lower edge of the filter's passband, and the anti-resonant frequency of the parallel resonator is within the passband. Conversely, the anti-resonant frequency of the series resonator is typically just above the upper edge of the filter's passband, while the resonant frequency of the series resonator is within the passband. To achieve these two requirements, the difference between the resonant and anti-resonant frequencies of the resonators needs to be less than or equal to the filter bandwidth. The difference between the resonant and anti-resonant frequencies of a rotated Y-cut LN XBAR is 790MHz, which is greater than the 600MHz bandwidth of the N79 band.

[0061] Figure 4 This is a detailed cross-sectional schematic diagram of a "decoupled" XBAR resonator (DXBAR) 400. The decoupled XBAR 400 includes a piezoelectric plate 410 with thickness tp and IDT fingers 438 with thickness tm, spacing p, and width m. The materials of the piezoelectric plate 410 and the IDT fingers 438 can be as described above.

[0062] Decoupled XBAR 400 and Figure 1 The difference between XBAR 100 shown in detail C is the presence of a dielectric layer 450 between the IDT fingers 438 and the diaphragm 410. The function of the dielectric layer 450 is to “decouple” XBAR 400, that is, to reduce the electromechanical coupling of XBAR 400. Dielectric layers such as dielectric layer 450 will be referred to herein as “decoupling dielectric layers.” The degree of decoupling depends in part on the thickness tdd of the decoupling dielectric layer 450.

[0063] The decoupling dielectric layer 450 can be made of, for example, silicon dioxide, silicon nitride, aluminum oxide, or some other suitable dielectric material. In some applications, silicon dioxide is a preferred material for the decoupling dielectric layer 450. Figure 1 Compared to XBAR 100, silica provides a significant secondary benefit by reducing the temperature coefficient of frequency (TCF) of XBAR 400.

[0064] Although Figure 4 Not shown, but one or more additional dielectric layers, for example Figure 1The dielectric layer 150 can be formed over the IDT fingers 438 and the decoupling dielectric layer 450. Additional dielectric layers may include frequency-setting layers, typically formed over the IDT of a parallel resonator in a ladder filter circuit to reduce its resonant frequency relative to the resonant frequency of the series resonator. Additional dielectric layers may also be or include passivation and tuning layers, which seal the surface of the device and provide sacrificial material that can be selectively removed to tune the resonant frequency.

[0065] Figure 5 Figure 500 shows the admittance magnitude of three decoupled XBAR devices as a function of frequency. Solid curve 510 shows the admittance magnitude of the decoupled XBAR with tdd (thickness of the decoupling dielectric layer) = 70 nm. Dashed curve 520 shows the admittance magnitude of the decoupled XBAR with tdd = 80 nm. Dotted curve 530 shows the admittance magnitude of the decoupled XBAR with tdd = 90 nm. All three XBARs use rotating Y-cut piezoelectric plates with Euler angles of 0°, 30°, and 0°, respectively.

[0066] Increasing the thickness of the decoupling dielectric layer increases the total thickness of the XBAR diaphragm, which leads to a corresponding decrease in the resonant frequency. Increasing the thickness of the decoupling dielectric layer reduces electromechanical coupling, thereby decreasing the difference between the resonant and anti-resonant frequencies. The k-values ​​of the three XBARs... 2 eff The values ​​are 21%, 20%, and 19%. The k-value of the XBAR at tdd = 80nm (dotted curve 520) 2 eff It is roughly the same as XBAR, which uses Z-cut piezoelectric plates.

[0067] The effect of decoupling the dielectric layer will be proportional to the thickness of the piezoelectric plate. Figure 6 It is the XBAR of lithium niobate cut using rotational Y-cutting with Euler angles of 0°, 127.5°, and 0°. 2 eff Figure 600 shows the variation of the ratio of tdd (thickness of the decoupling dielectric layer) to tp (thickness of the piezoelectric plate), with hollow circle 610 representing... Figure 3 LN XBAR, solid circle 620 represents Figure 5 The three XBARs. The dashed line 630 is a reasonable linear approximation of the data points within this range of tdd / tp.

[0068] The tdd / tp ratio will typically be greater than or equal to 0.05 to obtain a useful k. 2 eff The ratio of TDD to TP generally does not exceed 0.5.

[0069] Figure 7Figure 700 shows the performance of a preliminary bandpass filter design using decoupled XBARs for the N79 bandpass filter. Specifically, curve 710 is a graph showing the amplitude of the filter's S²,1 (input-output transfer function) versus frequency. The analog filter incorporates seven decoupled XBARs within a ladder-shaped filter circuit. The piezoelectric plates are rotated Y-cut lithium niobate. The thickness of the decoupling dielectric layer is approximately 22% of the piezoelectric plate thickness. A frequency-setting dielectric layer is formed above the parallel resonators, and a passivation dielectric layer is formed above all resonators.

[0070] The frequency of an XBAR or DXBAR is primarily determined by the thickness of its diaphragm, including the piezoelectric plate and any dielectric layer. The selection criteria for the IDT marking and spacing of the XBAR are to minimize the impact of spurious modes, specifically targeting spurious modes at frequencies removed from the filter's passband. The length and aperture of the XBAR or DXBAR are jointly determined by the capacitance required to match the desired filter input and output impedances, as well as the expected power dissipation in the device.

[0071] For a given IDT spacing and marking, the capacitance per unit area of ​​a DXBAR will be less than that of an XBAR. This reduction in capacitance is due to the presence of the decoupling dielectric layer, whose dielectric constant is significantly lower than that of the piezoelectric plate. However, the marking / spacing design space for DXBARs (for low spurious modes) tends to use smaller spacing values. Smaller spacing results in a larger capacitance per unit area, which offsets the reduction in capacitance due to the presence of the decoupling dielectric layer. Therefore, filters using DXBARs do not need to be larger than those using XBARs, and in some cases, may be smaller.

[0072] A minor but significant benefit of using silicon dioxide to decouple the dielectric layer is an improvement in the temperature coefficient of frequency (TCF). A DXBAR with a decoupled dielectric layer thickness of approximately 22% of the piezoelectric plate thickness has a TCF of 65 at the resonant frequency and 62 at the anti-resonant frequency. A similar XBAR using Z-cut lithium niobate has a TCF of 105 at the resonant frequency and 83 at the anti-resonant frequency.

[0073] Using a decoupled dielectric layer to reduce the electromechanical coupling of XBARs gives filter designers more freedom. Filter designers can customize the electromechanical coupling to meet the specific requirements of the filter without requiring the piezoelectric plate to have a unique cut angle.

[0074] Method Description

[0075] Figure 8This is a simplified flow chart summarizing process 800 for manufacturing a filter device incorporating DXBARs. Specifically, process 800 is used to manufacture a filter device comprising multiple DXBARs, some of which may include a frequency-setting dielectric layer. Process 800 begins at 805, where a device substrate and a piezoelectric material sheet are disposed on a sacrificial substrate. Process 800 ends at 895, completing the filter device. Figure 8 The flowchart only includes the main processing steps. (It can be found in...) Figure 8 Various routine process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) are performed before, during, and after the steps shown.

[0076] Figure 8 The 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 800 can be performed simultaneously on all filter devices on the wafer.

[0077] Figure 8 The flowchart captures three variations of process 800 used to fabricate XBARs, differing in when and how cavities are formed in the device substrate. Cavities can be formed at steps 810A, 810B, or 810C. In each of the three variations of process 800, only one of these steps is performed.

[0078] The piezoelectric plate is typically a rotated, Y-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.

[0079] In a variation of process 800, one or more cavities are formed in the device substrate at 810A before the piezoelectric plate is bonded to the substrate at 815. A separate cavity can be formed for each resonator in the filter device. The one or more cavities can be formed using conventional photolithography and etching techniques. Typically, the cavities formed at 810A will not penetrate the device substrate.

[0080] At 815°, 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 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.

[0081] At 820°, 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 820°, 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.

[0082] Thin sheets of single-crystal piezoelectric material laminated onto non-piezoelectric substrates are commercially available. At the time of this application, both lithium niobate and lithium tantalate sheets can be used with various substrates, including silicon, quartz, and fused silica. Other piezoelectric material sheets may be available now or in the future. The thickness of the piezoelectric sheet can be between 300 nm and 1000 nm. When the substrate is silicon, a layer of SiO2 can be disposed between the piezoelectric sheet and the substrate. When using a commercially available piezoelectric sheet / device substrate stack, steps 810A, 815, and 820 of process 800 are not performed.

[0083] At 825, a decoupling dielectric layer is formed by depositing a dielectric material on the front side of the piezoelectric plate. The decoupling dielectric layer is typically silicon dioxide, but can also be another dielectric material, such as silicon nitride or aluminum oxide. The decoupling dielectric layer can be a composite of two or more dielectric materials or a layer of two or more dielectric materials. The decoupling dielectric layer can be patterned such that it exists on some portions of the piezoelectric plate but not on others. The decoupling dielectric layer can be formed as two or more separately patterned layers, such that decoupling dielectric layers of different thicknesses exist on different portions of the piezoelectric plate.

[0084] A first conductor pattern, comprising the IDT and reflector elements of each XBAR, is formed at 845 by depositing and patterning one or more conductor layers on the front side of the piezoelectric plate. All or part of the first conductor pattern may be on top of the decoupling dielectric layer formed at 825. The conductor layer may be, for example, aluminum, aluminum alloy, copper, copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below the conductor layer (i.e., between the conductor layer and the piezoelectric plate) and / or above the conductor layer. For example, a thin film of titanium, chromium, or other metals may be used to improve adhesion between the conductor layer and the piezoelectric plate. A second conductor pattern of gold, aluminum, copper, or other metals with higher conductivity may be formed on a portion of the first conductor pattern (e.g., the interconnect between the IDT bus and the IDT).

[0085] Individual conductor patterns can be formed at 845 by sequentially depositing conductive layers 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, and other etching techniques.

[0086] Alternatively, a stripping process can be used at 845° to form each conductor pattern. Photoresist can be deposited on the piezoelectric plate and patterned to define the conductor patterns. Conductor layers, along with one or more optional additional 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 patterns.

[0087] At 850°, 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. One or more photolithography processes (using a photomask) can be used to confine the dielectric layer to a selected area of ​​the piezoelectric plate. For example, a mask can also confine the dielectric layer to cover only the parallel resonator.

[0088] At 855, 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 800, the passivation / tuning dielectric layer can be formed after etching cavities in the device substrate at 810B or 810C.

[0089] In a second variation of method 800, one or more cavities are formed in the back side of the device substrate at 810B. A separate cavity can be formed for each resonator in the filter device. One or more cavities can be formed using anisotropic or orientation-dependent dry or wet etching to create an opening from the back side of the device substrate all the way to the piezoelectric plate. In this case, the resulting resonator device will have, for example, […]. Figure 1 The cross-section shown.

[0090] In a third variation of method 800, one or more cavities in the form of grooves in the substrate can be formed at 810C by etching the substrate with an etchant introduced through an opening in the piezoelectric plate. A separate cavity can be formed for each resonator in the filter device. The one or more cavities formed at 810C do not penetrate the device substrate.

[0091] Ideally, after forming the cavity in the 810B or 810C, most or all of the filter devices on the wafer will meet a set of performance requirements. However, normal process tolerances can cause variations in parameters, such as variations in the thickness of the dielectric layer formed at 850 and 855, variations in the thickness and linewidth of the conductor and IDT fingers formed at 845, and variations in the thickness of the piezoelectric plate. These variations cause the filter device performance to deviate from a set of performance requirements.

[0092] 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 855. 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 800 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.

[0093] At 860, 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 all or most of the filter devices, which are fabricated simultaneously on a common piezoelectric plate and substrate.

[0094] At 865, 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 mill 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 860 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.

[0095] At 870, local frequency tuning can be performed in addition to or instead of the global frequency tuning performed at 865. This "local" frequency tuning is performed with a spatial resolution smaller than that of individual filter devices. Test results from 860 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.

[0096] After frequency tuning at 865 and / or 870, the filter devices are completed at 875. Actions that may occur at 875 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 845); cutting individual filter devices from a wafer containing multiple filter devices; other packaging steps; and additional testing. The process ends at 895 after each filter device is completed.

[0097] Conclusion

[0098] 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.

[0099] 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 substrate; A piezoelectric plate having a front side and a back side, wherein the thickness between the front side and the back side is greater than or equal to 200 nm and less than or equal to 1000 nm, and the back side is attached to the substrate; A decoupling dielectric layer is located on the front side of the piezoelectric plate; and An interdigitated transducer (IDT) is formed on the decoupled dielectric layer such that the interlaced fingers of the IDT are positioned above a portion of the piezoelectric plate, which is suspended over a cavity formed in the substrate.

2. The device of claim 1, wherein, The IDT is configured to excite shear acoustic waves in the piezoelectric plate in response to a radio frequency signal applied to the IDT.

3. The device of claim 1, wherein, The piezoelectric plate is a rotating Y-cut lithium niobate.

4. The device of claim 1, wherein, The decoupling dielectric layer comprises silicon dioxide.

5. The device of claim 1, wherein, The spacing between the fingers of the IDT is greater than or equal to twice the thickness of the piezoelectric plate and less than or equal to 25 times the thickness of the piezoelectric plate.

6. The device according to claim 5, characterized in that, The fingers of an IDT have a width, and The spacing is greater than or equal to twice the width and less than or equal to 25 times the width.

7. The device of claim 1, wherein, Also includes: A front dielectric layer is formed on the front side of the piezoelectric plate between the fingers of the IDT. The resonant frequency of the acoustic resonator device is partly determined by the thickness of the front dielectric layer.

8. The device of claim 7, wherein, The front dielectric layer includes at least one of silicon dioxide and silicon nitride.

9. The device of claim 1, wherein, The IDT includes one of aluminum, aluminum alloy, copper, copper alloy, beryllium, and gold.

10. A filter device, comprising: A substrate; A piezoelectric plate having a front side and a back side, the thickness between the front side and the back side being greater than or equal to 200 nm and less than or equal to 1000 nm, the back side being attached to the substrate; A decoupling dielectric layer is located on the front side of the piezoelectric plate; and A conductor pattern is formed on the decoupling dielectric layer, the conductor pattern comprising multiple interdigital transducers (IDTs) of corresponding multiple resonators, wherein Interlaced fingers of each of the plurality of IDTs are located above various portions of the piezoelectric plate, which are suspended over one or more cavities formed in the substrate.

11. The filter device of claim 10, wherein, All of the plurality of IDTs are configured to excite shear acoustic waves in the piezoelectric plate in response to a corresponding radio frequency signal applied to each IDT.

12. The filter device of claim 10, wherein, The piezoelectric plate is a rotating Y-cut lithium niobate.

13. The filter device of claim 10, wherein, The decoupling dielectric layer comprises silicon dioxide.

14. The filter device of claim 10, wherein, Each of the plurality of IDTs is located in a corresponding portion of the piezoelectric plate, and the corresponding portion of the piezoelectric plate is suspended on a corresponding cavity formed in the substrate.

15. The filter device of claim 10, wherein, The plurality of resonators includes a parallel resonator and a series resonator.

16. The filter device of claim 15, wherein, The thickness of the first dielectric layer deposited between the fingers of the IDT in the parallel resonator is greater than the thickness of the second dielectric layer deposited between the fingers of the IDT in the series resonator.

17. A method for manufacturing an acoustic resonator device, the method comprising: The back side of a piezoelectric plate is attached to a substrate, wherein the thickness of the piezoelectric plate is greater than or equal to 200 nm and less than or equal to 1000 nm. forming a cavity in the substrate such that a portion of the piezoelectric plate is suspended over the cavity; forming a decoupling dielectric layer on a front side of the piezoelectric plate; and forming an interdigital transducer (IDT) over the decoupling dielectric layer such that interleaved fingers of the IDT are positioned over the portion of the piezoelectric plate suspended over the cavity.

18. The method of claim 17, wherein, the piezoelectric plate is rotated Y-cut lithium niobate and the decoupling dielectric layer comprises silicon dioxide.

19. The method of claim 17, wherein, a pitch of the interleaved fingers of the IDT is greater than or equal to 2 times a thickness of the piezoelectric plate and less than or equal to 25 times the thickness of the piezoelectric plate.

20. The method of claim 19, wherein, the interleaved fingers of the IDT have a width, and the pitch is greater than or equal to 2 times the width and less than or equal to 25 times the width.

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