CN113765495A

CN113765495A - Transverse-excited film bulk acoustic resonator using YX-cut lithium niobate for high power applications

Info

Publication number: CN113765495A
Application number: CN202110541519.0A
Authority: CN
Inventors: 布莱恩特·加西亚
Original assignee: Resonant Inc
Current assignee: Murata Manufacturing Co Ltd
Priority date: 2020-05-19
Filing date: 2021-05-18
Publication date: 2021-12-07
Also published as: DE102021112829A1

Abstract

Acoustic wave resonator devices, filters, and methods are disclosed. The acoustic wave resonator includes a substrate and a Lithium Niobate (LN) plate having front and back surfaces and a thickness ts. The back surface is attached to the surface of the substrate. A portion of the LN plate forms a membrane that spans the cavity in the substrate. An interdigital transducer (IDT) is formed on the front side of the LN plate, wherein interleaved fingers of the IDT are disposed on the membrane. The LN plate and the IDTs are configured such that a radio frequency signal applied to the IDTs excites shear dominant acoustic waves in the membrane. The euler angles of the LN panels are [0 °, β,0 ° ], with 0< β <60 °. The thickness of the interleaved fingers of the IDT is greater than or equal to 0.8ts and less than or equal to 2.0 ts.

Description

Transverse-excited film bulk acoustic resonator using YX-cut lithium niobate for high power applications

Copyright and trademark appearance statements

A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matters that are or may become the trademark appearances of the owners. Copyright and trademark appearance the owner of the copyright and trademark appearance rights are reserved although it is not a objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the patent and trademark office files or records.

Cross Reference to Related Applications

This patent claims priority from provisional patent application 63/026,824 entitled "IDT SIDEWALL ANGLE TO CONTROL spur models IN XBARS" filed on 19/5/2020.

Technical Field

The present disclosure relates to radio frequency filters using acoustic wave resonators, and more particularly to filters for use in communication devices.

Background

A Radio Frequency (RF) filter is a two-terminal device that is configured to pass some frequencies and block others, where "pass" means transmitting with relatively low signal loss and "block" means blocking or substantially attenuating. The range of frequencies passed by the filter is referred to as the "passband" of the filter. The range of frequencies blocked by such a filter is called the "stop band" of the filter. A typical RF filter has at least one pass band and at least one stop band. The specific requirements of the pass band or stop band depend on the specific application. For example, a "passband" may be defined as a range of frequencies in which the insertion loss of the filter is better than a defined value such as 1dB, 2dB, or 3 dB. A "stop band" may be defined as a frequency range in which the rejection of the filter is greater than a defined value, for example a value of 20dB, 30dB, 40dB or more, depending on the particular application.

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

RF filters typically require many design tradeoffs to achieve the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost for each particular application. Specific designs and manufacturing methods and enhancements may benefit from one or more of these requirements simultaneously.

The performance enhancement of RF filters in wireless systems can have a wide impact on system performance. System performance may be improved by improving the RF filter, such as larger cell size, longer battery life, higher data rate, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvement points may be implemented individually or in combination at various levels of the wireless system, for example at the RF module, RF transceiver, mobile or fixed subsystem or network level.

To obtain a wider communication channel bandwidth, it is necessary to use a higher frequency communication band. Current LTE^TMThe (long term evolution) specification defines a frequency band in the range of 3.3GHz to 5.9 GHz. These bands are not currently available. Proposals for wireless communication include millimeter wave communication bands at frequencies up to 28 GHz.

High performance RF filters for current communication systems typically incorporate acoustic wave resonators including Surface Acoustic Wave (SAW) resonators, bulk acoustic wave BAW) resonators, thin Film Bulk Acoustic Resonators (FBARs), and other types of acoustic wave resonators. However, these prior art techniques are not suitable for use at higher frequencies, which are required in future communication networks.

Disclosure of Invention

The invention discloses an acoustic wave resonator device, comprising: a substrate having a surface; a lithium niobate sheet having a front side and a back side, the back side being attached to a surface of the substrate, but a portion of the lithium niobate sheet forming a separator being unattached to the surface of the substrate, the separator spanning a cavity in the substrate; and an interdigital transducer (IDT) formed on the front side of the lithium niobate plate such that interleaved fingers of the IDT are disposed on the diaphragm, wherein the lithium niobate plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm, an Euler angle of the lithium niobate plate is [0 °, β,0 ° ], wherein β is greater than or equal to 0 ° and less than or equal to 60 °, and a thickness of the interleaved fingers of the IDT is greater than or equal to 0.8 times a thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.

Wherein β is greater than or equal to 26 ° and less than or equal to 34 °.

Wherein β is about 30 °.

Wherein further comprising a dielectric layer formed between the interleaved fingers of the IDT.

Wherein a thickness of the dielectric layer is less than or equal to 0.2 times a thickness of the lithium niobate plate, and a thickness of the interleaved fingers of the IDT is greater than or equal to 1.1 times and less than or equal to 2.0 times the thickness of the lithium niobate plate.

Wherein a thickness of the dielectric layer is greater than 0.2 times and less than or equal to 0.3 times a thickness of the lithium niobate plate, and a thickness of the interleaved fingers of the IDT is greater than or equal to 1.15 times and less than or equal to 1.8 times the thickness of the lithium niobate plate.

Wherein the thickness of the dielectric layer is less than or equal to 0.35 times the thickness of the lithium niobate plate.

Wherein the direction of acoustic energy flow of the primary acoustic mode is substantially orthogonal to the front and back faces of the diaphragm.

The present invention also discloses a filter device, comprising: a substrate having a surface; a lithium niobate sheet having a front side and a back side, the back side being attached to a surface of the substrate but portions of the lithium niobate sheet forming one or more membranes spanning respective cavities in the substrate; and a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic wave resonators, interleaved fingers of each of the plurality of IDTs disposed on a respective diaphragm of the one or more diaphragms, wherein the lithium niobate plate and all of the IDTs are configured such that a respective radio frequency signal applied to the IDTs excites a respective primary shear acoustic mode in the respective diaphragm, the lithium niobate plate having an Euler angle of [0 °, β,0 ° ], wherein β is greater than or equal to 0 ° and less than or equal to 60 °, and the interleaved fingers of all of the IDTs have a common thickness that is greater than or equal to 0.8 times the thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.

Wherein β is greater than or equal to 26 ° and less than or equal to 34 °.

Wherein β is about 30 °.

Wherein further comprising a frequency setting dielectric layer formed between the interleaved fingers of a subset of the plurality of IDTs.

Wherein a thickness of the frequency setting dielectric layer is less than or equal to 0.2 times a thickness of the lithium niobate plate, and a common thickness of the interleaved fingers of the IDT is greater than or equal to 1.1 times and less than or equal to 2.0 times the thickness of the lithium niobate plate.

Wherein a thickness of the frequency setting dielectric layer is greater than 0.2 times and less than or equal to 0.3 times a thickness of the lithium niobate plate, and a common thickness of the interleaved fingers of the IDT is greater than or equal to 1.15 times and less than or equal to 1.8 times the thickness of the lithium niobate plate.

Wherein the thickness of the frequency setting dielectric layer is less than or equal to 0.35 times the thickness of the lithium niobate plate.

Wherein the plurality of acoustic wave resonators includes one or more parallel resonators and one or more series resonators connected in a ladder filter circuit, and a subset of the plurality of IDTs is one or more parallel resonators.

Wherein the respective directions of acoustic energy flow of all the primary acoustic modes are substantially orthogonal to the front and back faces of the diaphragm.

Wherein the interleaved fingers of each IDT of the plurality of IDTs are disposed on the respective membrane spanning the respective cavity.

The invention also discloses a method for manufacturing the acoustic resonator device, which comprises the following steps: bonding a back side of a lithium niobate plate to a substrate such that a portion of the lithium niobate plate forms a diaphragm across a cavity in the substrate; and forming an interdigital transducer (IDT) on the front side of the lithium niobate plate such that interleaved fingers of the IDT are disposed on the diaphragm, wherein the lithium niobate plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a main shear acoustic mode in the diaphragm, the lithium niobate plate has an euler angle of [0 °, β,0 ° ], wherein β is greater than or equal to 0 ° and less than 60 °, and the interleaved fingers of the IDT have a thickness greater than or equal to 0.8 times the thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.

Wherein β is greater than or equal to 26 ° and less than or equal to 34 °.

Wherein β is about 30 °.

Drawings

Fig. 1 is a schematic plan view and two schematic cross-sectional views of a laterally excited film bulk acoustic resonator (XBAR).

Fig. 2 is a partially enlarged schematic cross-sectional view of the XBAR of fig. 1.

Fig. 3 is an alternative schematic cross-sectional view of the XBAR of fig. 1.

Fig. 4 is a diagram showing shear dominant acoustic modes in an XBAR.

Fig. 5 is a schematic block diagram of a filter using XBARs.

Fig. 6 is a schematic cross-sectional view of two XBARs showing frequency setting dielectric layers.

Fig. 7 is a graph of the piezoelectric coefficients of e14 and e15 for a lithium niobate plate with euler angle [0 °, β,0 ° ] as a function of β.

FIG. 8 is a graph comparing the admittance of XBAR formed on rotating Y-X lithium niobate and Z-cut lithium niobate.

Fig. 9 is a graph showing a preferred combination of IDT thickness and IDT pitch for an XBAR using rotating Y-X cut lithium niobate without a front side dielectric layer.

Fig. 10 is a graph showing a preferred combination of IDT thickness and IDT pitch for an XBAR using rotating Y-X cut lithium niobate with a front dielectric layer having a thickness 0.2 times the thickness of the piezoelectric diaphragm.

Fig. 11 is a graph showing a preferred combination of IDT thickness and IDT pitch for an XBAR using rotating Y-X cut lithium niobate with a front dielectric layer having a thickness 0.3 times the thickness of the piezoelectric diaphragm.

Fig. 12 is a graph showing a preferred combination of IDT thickness and IDT pitch for an XBAR using rotating Y-X cut lithium niobate with a front dielectric layer having a thickness 0.35 times the thickness of the piezoelectric diaphragm.

Figure 13 is a flow chart of a process for making an acoustic wave resonator or filter using rotating Y-X cut lithium niobate.

Throughout the specification, elements appearing in the drawings are assigned reference numerals of three or four digits, where the two least significant bits are specific to the element and one or two most significant bits are the figure number showing the element first. Elements not described in connection with the figures may be assumed to have the same characteristics and functions as previously described elements having the same reference numerals.

Detailed Description

Description of the devices

Figure 1 shows a simplified schematic top view and orthogonal cross-sectional view of a laterally excited thin film bulk acoustic resonator (XBAR) 100. XBAR resonators such as resonator 100 may be used in a variety of RF filters including band-stop filters, band-pass filters, duplexers, and multiplexers. XBAR is particularly suitable for use in filters for communications bands having frequencies above 3 GHz.

XBAR 100 is comprised of a thin film conductor pattern formed on the surface of a piezoelectric plate 110 having a front surface 112 and a back surface 114. The front and back surfaces are substantially parallel. "substantially parallel" means as parallel as possible within normal manufacturing tolerances. The piezoelectric plate 110 is a single crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, langasite, gallium nitride, or aluminum nitride. The piezoelectric plate 110 is cut so that the directions of X, Y and the Z-crystal axis are known and coincident with respect to the front and back surfaces. In the example proposed in this patent, the piezoelectric plate 110 is Z-cut, that is, the Z-axis is perpendicular to the front and

back surfaces

112, 114. However, XBARs can be fabricated on piezoelectric plates having other crystallographic orientations, including rotated Z-cuts and rotated YX-cuts.

The back surface 114 of the piezoelectric plate 110 is attached to the surface 122 of the substrate 120, except that a portion of the piezoelectric plate 110 is not attached to the surface 122 of the substrate 120, wherein the portion of the piezoelectric plate 110 forms a diaphragm 115, the diaphragm 115 spanning a cavity 140 formed in the substrate 120. The portion of the piezoelectric plate 110 spanning the cavity is referred to herein as the "diaphragm" because this portion is physically similar to the diaphragm of the microphone. As shown in fig. 1, the diaphragm 115 abuts the remainder of the piezoelectric plate 110 around the entire perimeter 145 of the cavity 140. In this case, "adjacent" means "continuously connected without any other article in between".

The substrate 120 provides mechanical support for the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material, or a combination of these materials. The back surface 114 of the piezoelectric plate 110 may be attached to the surface 122 of the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120, or the piezoelectric plate 110 attached to the substrate in some other manner. The piezoelectric plate 110 may be attached directly to the substrate, or may be attached to the substrate 120 via one or more intermediate layers of material.

The cavity 140 is an empty space within the XBAR 100 solid. The cavity 140 may be a hole that passes completely through the substrate 120 (as shown in cross-sections a-a and B-B) or may be a groove in the substrate 120 (as shown later in fig. 3). For example, the cavity 140 may be formed by selectively etching the substrate 120 before or after attaching the piezoelectric plate 110 to the substrate 120.

The conductor pattern of XBAR 100 includes an interdigital transducer (IDT) 130. IDTs are electrode structures for converting between electrical energy and acoustic energy in piezoelectric devices. The IDT130 includes a first plurality of parallel elongated conductors, commonly referred to as "fingers," such as fingers 136, extending from a first bus bar 132. The IDT130 includes a second plurality of fingers extending from a second bus bar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap a distance AP, which is commonly referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of the IDT130 is the "length" of the IDT.

The term "bus bar" refers to a conductor interconnecting the first and second sets of fingers in the IDT. As shown in FIG. 1, each

bus bar

132, 134 is an elongated rectangular conductor having a long axis orthogonal to the interleaved fingers and a length approximately equal to the length L of the IDT. The bus bars of the IDTs need not be rectangular or orthogonal to the interleaved fingers, and the length of the bus bars of the IDTs may be longer than the length of the IDTs.

First and second bus bars 132, 134 serve as terminals of XBAR 100. A radio frequency or microwave signal applied between the two

bus bars

132, 134 of the IDT130 excites a primary acoustic mode within the piezoelectric plate 110. As discussed in detail below, the primary acoustic mode is a bulk shear mode, wherein 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. XBAR is therefore considered to be a laterally excited thin film bulk wave resonator.

The IDT130 is placed on the piezoelectric plate 110 such that at least the fingers of the IDT130 are disposed on the diaphragm 115 of the piezoelectric plate, the diaphragm 115 straddling or suspended over the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular shape with a dimension that is greater than the length L of the aperture AP and IDT 130. The cavities of the XBAR may have different shapes, such as regular or irregular polygons. The cavities of the XBAR may have more or less than four sides, which may be straight or curved.

For ease of illustration in fig. 1, the geometrical spacing and width of the IDT fingers are greatly exaggerated relative to the length (dimension L) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device will have more than ten parallel fingers in IDT 110. XBARs may have hundreds or even thousands of parallel fingers in IDT 110. Similarly, the thickness of the fingers in the cross-sectional view is greatly exaggerated in this figure.

Fig. 2 shows a detailed schematic cross-sectional view of the XBAR 100. The piezoelectric plate 110 is a single crystalline layer of piezoelectric material having a thickness ts. ts may be, for example, 100nm to 1500 nm. When used for LTE from 3.4GHz to 6GHz^TMThe thickness ts may be, for example, 200nm to 1000nm, as in a filter for a band (e.g., bands 42, 43, 46).

A front dielectric layer 214 may be formed on the front surface of the piezoelectric plate 110. The "front side" of the XBAR is the side facing away from the substrate. The front dielectric layer 214 has a thickness tfd. The front dielectric layer 214 is formed between the IDT fingers 238. Although not shown in fig. 2, the front dielectric layer 214 can also be deposited on the IDT fingers 238. A backside dielectric layer 216 may be formed on the backside of the piezoelectric plate 110. The back dielectric layer 216 has a thickness tbd. The front and back

dielectric layers

214, 216 may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfd and tbd are typically smaller than the thickness ts of the piezoelectric plate. tfd and tbd need not be equal and the front side dielectric layer 214 and the back side dielectric layer 216 need not be the same material. The front side dielectric layer 214 and/or the back side dielectric layer 216 may be formed of multiple layers of two or more materials.

The IDT fingers 238 may be one or more layers of aluminum, a basic aluminum alloy, copper, a basic copper alloy, beryllium, gold, molybdenum, or some other electrically conductive material. Other thin (here thin relative to the total thickness of the conductor) layers of metal such as chromium or titanium may be formed under and/or over the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and/or to passivate or encapsulate the fingers. The bus bars (132, 134 in fig. 1) of the IDT can be made of the same or different material than the fingers. As shown in fig. 2, IDT finger 238 has a rectangular cross section. The IDT fingers can have some other cross-sectional shape, such as trapezoidal.

Dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension w is the width or "signature" of the IDT finger. The IDT of XBAR is substantially different from the IDT used in Surface Acoustic Wave (SAW) resonators. In the SAW resonator, the pitch of the IDT is half the wavelength of an acoustic wave at the resonance frequency. In addition, the tag pitch ratio of the SAW resonator IDT is typically close to 0.5 (i.e., the width of the tag or finger is about one-quarter of the wavelength of the acoustic wave at resonance). In XBAR, the pitch p of the IDT is typically 2 to 20 times the finger width w. In addition, the pitch p of the IDTs is typically 2 to 20 times the thickness ts of the piezoelectric plate 212. The width of the IDT finger in an XBAR is not limited to one quarter of the acoustic wavelength at resonance. For example, the XBAR IDT fingers can be 500nm or more in width, so that the IDT can be easily fabricated using photolithography techniques. The thickness tm of the IDT fingers can be from 100nm to about equal to the width w. The thickness of the bus bars (132, 134 in FIG. 1) of the IDT can be equal to or greater than the thickness tm of the IDT fingers.

Fig. 3 is an alternative cross-sectional view along section a-a defined in fig. 1. In fig. 3, a piezoelectric plate 310 is attached to a substrate 320. A portion of the piezoelectric plate 310 forms a diaphragm 315 spanning a cavity 340 in the substrate. The cavity 340 does not penetrate completely through the substrate 320. The fingers of the IDT are disposed on the membrane 315. For example, the cavity 340 may be formed by etching the substrate 320 before attaching the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings (not shown) provided in the piezoelectric plate 310. In this case, the diaphragm 315 may abut the remainder of the piezoelectric plate 310 around a greater portion of the perimeter 345 of the cavity 340. For example, the membrane 315 may abut the remainder of the piezoelectric plate 310 around at least 50% of the perimeter 345 of the cavity 340.

Fig. 4 is a graphical illustration of the primary acoustic modes of interest in an XBAR. Fig. 4 shows a small portion of an XBAR400 that includes a piezoelectric plate 410 and three interleaved IDT fingers 430. A Radio Frequency (RF) voltage is applied to interleaved fingers 430. This voltage creates a time-varying electric field between the fingers. As indicated by the arrows labeled "electric field," the direction of the electric field is primarily transverse, or parallel, to the surface of the piezoelectric plate 410. Since the dielectric constant of the piezoelectric plate is significantly higher than that of the surrounding air, the electric field is highly concentrated in the plate compared to air. The transverse electric field induces shear deformation in the piezoelectric plate 410, thereby strongly exciting a shear mode acoustic mode. Shear deformation refers to a deformation in a material in which parallel planes remain parallel and at a constant distance and are translated relative to each other. A "shear acoustic mode" is an acoustic vibration mode in a medium that causes shear deformation of the medium. Shear deformation in XBAR400 is represented by curve 460, with the adjacent small arrows schematically indicating the direction and magnitude of atomic motion. The extent of atomic motion and the thickness of the piezoelectric plate 410 are greatly exaggerated for clarity. Although the atomic motion is primarily transverse (i.e., horizontal as shown in FIG. 4), the direction of the acoustic energy flow of the excited primary shear acoustic mode is substantially perpendicular to the surface of the piezoelectric plate, as indicated by arrow 465.

The acoustic wave resonator based on shear acoustic wave resonance can achieve performance better than that of the current latest Film Bulk Acoustic Resonator (FBAR) and Solid Mount Resonator Bulk Acoustic Wave (SMRBAW) devices in which an electric field is applied in the thickness direction. In such a device, the acoustic mode is compressed in the direction of atomic motion and flow of acoustic energy in the thickness direction. Furthermore, the piezoelectric coupling for shear wave XBAR resonance can be higher (> 20%) than other acoustic wave resonators. High voltage electrical coupling makes it possible to design and implement microwave and millimeter wave filters with considerable bandwidth.

Fig. 5 is a schematic circuit diagram and layout of a high-frequency band-pass filter 500 using XBARs. The filter 500 has a conventional ladder filter architecture comprising three

series resonators

510A, 510B, 510C and two

parallel resonators

520A, 520B. The three

series resonators

510A, 510B, and 510C are connected in series between the first port and the second port (hence the term "series resonator"). In fig. 5, the first and second ports are labeled "In" and "Out", respectively. However, filter 500 is bi-directional and either port may be used as an input or an output of the filter. The two

parallel resonators

520A, 520B are grounded from the node between the series resonators. The filter may contain additional reactive components, such as inductors (not shown in fig. 5). All parallel resonators and series resonators are XBARs. The inclusion of three series resonators and two parallel resonators is merely an example. The filter may have more or less than five total resonators, more or less than three series resonators, and more or less than two parallel resonators. Typically, all series resonators are connected in series between the input and output of the filter. Typically, all parallel resonators are connected between ground and the input, output or node between two series resonators.

In the exemplary filter 500, the three series resonators 510A, B, C and the two parallel resonators 520A, B of the filter 500 are formed on a single plate 530 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a respective IDT (not shown), wherein at least the fingers of the IDT are disposed above the cavity in the substrate. In this and similar contexts, the term "respective" means "associating things one-to-one," that is, having a one-to-one correspondence. In fig. 5, the cavity is schematically shown as a dashed rectangle (e.g., rectangle 535). In this example, each IDT is disposed above a respective cavity. In other filters, IDTs of two or more resonators may be disposed on a single cavity.

Each

resonator

510A, 510B, 510C, 520A, 520B in the filter 500 has resonance if the admittance of the resonator is very high and has anti-resonance if the admittance of the resonator is very low. The resonance and antiresonance occur at a resonance frequency and antiresonance frequency, respectively, which may be the same or different for each resonator in filter 500. In overly simplified terms, each resonator may be considered a short circuit at its resonant frequency and an open circuit at its anti-resonant frequency. The input-output transfer function will be close to zero at the resonance frequency of the parallel resonator and the anti-resonance frequency of the series resonator. In a typical filter, the resonance frequency of the parallel resonators is located below the lower edge of the filter passband, while the anti-resonance frequency of the series resonators is located above the upper edge of the passband.

Fig. 6 is a schematic cross-sectional view of parallel resonators and series resonators of a filter 600, the filter 600 using a dielectric frequency setting layer to separate the resonance frequencies of the parallel resonators and the series resonators. The piezoelectric plate 610 is attached to a substrate 620. Portions of the piezoelectric plate 610 form a diaphragm that spans a cavity 640 in the substrate 620. Interleaved IDT fingers, such as fingers 630, are formed on the membrane. A first dielectric layer 650 having a thickness t1 is formed over the IDT of the parallel resonator. The first dielectric layer 650 is considered a "frequency setting layer," which is a dielectric layer applied to a first subset of the resonators in the filter to cancel the resonant frequencies of the first subset of the resonators relative to the resonant frequencies of the resonators, where the resonant frequencies of the resonators do not receive a dielectric frequencyA rate setting layer. The dielectric frequency setting layer is typically SiO₂But may also be silicon nitride, aluminum oxide, or some other dielectric material. The dielectric frequency setting layer may be a laminate or composite of two or more dielectric materials.

A second dielectric layer 655 of thickness t2 may be deposited over the parallel and series resonators. The second dielectric layer 655 serves to seal and passivate the surface of the filter 600. The second dielectric layer may be the same material as the first dielectric layer or a different material. The second dielectric layer may be a laminate or composite of two or more different dielectric materials. Furthermore, as described later, the thickness of the second dielectric layer may be locally adjusted to fine tune the frequency of the filter 600. Thus, the second dielectric layer may be referred to as a "passivation and tuning layer".

The resonant frequency of the XBAR is approximately proportional to the inverse of the total thickness of the diaphragm including the piezoelectric plate 610 and the

dielectric layers

650, 655. The diaphragm of the parallel resonator is thicker than the diaphragm of the series resonator by the thickness t1 of the dielectric frequency setting layer 650. Thus, the series resonator will have a lower resonance frequency than the parallel resonator. The difference in resonance frequency between the series resonator and the parallel resonator is determined by the thickness t 1.

This patent relates to XBAR devices on lithium niobate boards with euler angles [0 °, β,0 ° ]. For historical reasons, this plate configuration is often referred to as a "Y-cut," where the "cut angle" is the angle between the Y-axis and the plate normal. The "cutting angle" is equal to β +90 °. For example, a plate having an Euler angle [0 °, 30 °,0 ° ] is commonly referred to as a "120 ° rotated Y cut".

Fig. 7 is a graph 700 of two piezoelectric stress coefficients e15 and e16 for a lithium niobate plate having an euler angle [0 °, β,0 ° ]. The solid line 710 is a graph of the piezoelectric stress coefficient e15, which e15 relates the electric field along the x-axis to the shear stress or torque about the y-axis as a function of β. This shear stress excites the shear dominant acoustic modes shown in figure 4. The dashed line 720 is a graph of the piezoelectric stress coefficient e16, which e16 relates the electric field along the x-axis to the shear stress or torque about the z-axis as a function of β. This shear stress excites horizontal shear modes (e.g., SH0 plate modes) with atomic displacements perpendicular to the plane of fig. 4, which are unwanted parasitic modes in XBARs. Note that the two curves are identical and offset by 90 °.

Fig. 7 shows that the first piezoelectric stress coefficient is highest in the case where the euler angle β is about 30 °. The first piezoelectric stress coefficient is higher than about 3.8 (highest piezoelectric stress coefficient for non-rotated Z-cut lithium niobate) for 0 ° < β <60 °. In the case of an euler angle β of about 30 °, the second piezoelectric stress coefficient is zero, wherein the first piezoelectric stress coefficient is the largest. In this case, "about 30" means "within a reasonable manufacturing tolerance of 30". In the case of 26 ° < β < 34 °, the second piezoelectric stress coefficient is about 10% smaller than the first piezoelectric stress coefficient.

FIG. 8 is a graph 800 showing normalized amplitude (log scale) versus frequency for admittances of two XBAR devices simulated using a Finite Element Method (FEM) simulation technique. Dashed line 820 is a graph of the admittance of the XBAR on an X-cut lithium niobate plate. In this case, the Z crystal axis is orthogonal to the surface of the plate, and an electric field is applied along the Y crystal axis, and the euler angle of the piezoelectric plate is 0, 0, 90 °. Solid line 810 is a plot of the admittance of an XBAR on a 120Y cut lithium niobate plate. In this case, an electric field is applied along the X crystal axis, which lies in the plane of the surface of the lithium niobate plate. The YZ plane is perpendicular to the surface of the plate. The Z-axis is tilted by 30 ° with respect to the surface normal to the plate, and the euler angle of the piezoelectric plate is 0 °, 30 °,0 °. In both cases, the plate thickness was 400nm and the IDT fingers were 100nm thick aluminum. The substrate supporting the piezoelectric plate is silicon, and a cavity is formed under the IDT fingers.

The difference between the antiresonance and resonance frequency of the resonator on the rotating Y-cut plate (solid line 810) is about 200MHz greater than the difference between the antiresonance and resonance frequency of the resonator on the Z-cut plate (dashed line 820). The electromechanical coupling of the XBAR on the rotating Y-cut plate is about 0.32; the electromechanical coupling of the XBAR on the Z-cut board is about 0.24.

Us patent 10,637,438 describes XBAR resonators for high power applications. Us patent 10,637,438 also describes the use of a figure of merit (FOM) to define a design space (i.e., a combination of IDT conductor thickness, spacing and width) that provides an XBAR with acceptable performance for use in a filter. FOM is calculated by integrating the negative effects of spurious modes over a defined frequency range. For each combination of IDT conductor thickness and pitch, the FOM is calculated for the range of IDT finger widths. The minimum FOM value within the width of an IDT finger is considered to be the minimum FOM for that conductor thickness/pitch combination. The definition and frequency range of the FOM depends on the requirements of a particular filter. The frequency range typically includes the pass band of the filter and may include one or more stop bands. Spurious modes that occur between the resonant and anti-resonant frequencies of each hypothetical resonator may be weighted more heavily in the FOM than spurious modes that occur below the resonant or above the anti-resonant frequency. A hypothetical resonator with a minimum FOM below the threshold is considered likely to be "usable", that is, likely to have spurious modes low enough for use in a filter. A hypothetical resonator with a minimized cost function above the threshold is considered unusable.

Fig. 9 is a graph 900 illustrating the combination of IDT pitch p and IDT finger thickness tm that can provide a usable resonator. Both the IDT spacing and IDT finger thickness are normalized to the thickness ts of the piezoelectric plate. The graph is based on a two-dimensional simulation of an XBAR with a lithium niobate separator, an aluminum conductor, and no dielectric layer. XBARs with IDT spacing and thickness in

unshaded regions

910, 920, 930 may have sufficiently low spurious effects for use in filters. XBARs with IDT spacing and thickness in the

non-shaded regions

940, 950, 960 may have sufficiently low spurious effects for use in filters, but the IDT metal thickness is too low to be useful for high power applications. XBARs with IDT spacing and thickness in the middle shaded region have unacceptably high spurious modes for use in the target filter. In the absence of a dielectric layer, there is a usable resonator with an IDT finger thickness, wherein the IDT finger thickness is greater than or equal to 0.8 times and less than or equal to 2.0 times the piezoelectric plate thickness.

FIG. 10 is a graph 1000 illustrating combinations of IDT spacing and IDT finger thickness that may be useful resonancesThe front dielectric layer is provided with a thickness tfd equal to 0.2 times the thickness ts of the piezoelectric plate. The front dielectric layer may be a frequency setting dielectric layer deposited between IDT fingers of a subset of resonators in the filter circuit, here for example

parallel resonators

520A, 520B in the filter circuit of fig. 5. In graph 1000, both the IDT pitch and the IDT finger thickness are normalized to the thickness of the piezoelectric plate. The graph is based on a lithium niobate separator, an aluminum conductor and SiO₂Two-dimensional simulation of XBAR for the front side dielectric layer. XBARs with IDT spacing and thickness in the

unshaded regions

1010, 1020, and 1030 may have sufficiently low spurious effects for use in the filter. The XBAR with IDT spacing and thickness in the surrounding 1050 by the unshaded region 1040 may have sufficiently low spurious effects for use in a filter, but the IDT metal thickness is too low to be useful for high power applications. XBARs with IDT spacing and thickness in the middle shaded region have unacceptably high spurious modes for use in the target filter. In the case of the front dielectric layer (the thickness of the front dielectric layer is 0.2 times the thickness of the piezoelectric plate), there are available resonators that can be used for the thickness of the IDT finger, which is greater than or equal to 1.1 times and less than or equal to 2.0 times the thickness of the piezoelectric plate. In the case where the front-side dielectric layer thickness is less than or equal to 0.2 times the piezoelectric plate thickness, there are resonators available within the range of IDT finger thicknesses.

Fig. 11 is a combination of IDT spacing and IDT fingers showing that useful resonators can be provided with a front dielectric layer, which can be a frequency setting dielectric layer, having a thickness of 0.3 times the thickness of the piezoelectric plate. The IDT pitch and the IDT finger thickness are both normalized to the thickness of the piezoelectric plate. The graph is based on a two-dimensional simulation of an XBAR with a lithium niobate separator, an aluminum conductor, and a SiO2 front side dielectric layer. XBARs with IDT spacing and thickness in

non-shaded regions

1110, 1120, 1130 may have sufficiently low spurious effects to be used in the filter. XBARs with IDT spacing and thickness in the middle shaded region have unacceptably high spurious modes for the target filter. There are no available XBARs with thin IDT conductors. In the case of a front dielectric layer having a thickness equal to 0.3 times the thickness of the piezoelectric plate, there is a resonator available for an IDT finger thickness that is greater than or equal to 1.15 times and less than or equal to 1.8 times the thickness of the piezoelectric plate. In the range of IDT finger thickness where the front dielectric layer thickness is 02 times and less than or equal to 0.3 times the piezoelectric plate thickness, there is no resonator available.

Fig. 12 is a graph 1200 showing the combination of IDT spacing and IDT finger thickness that can provide useful resonators with a front dielectric layer having a thickness equal to 0.35 times the thickness of the piezoelectric plate. The IDT pitch and the IDT finger thickness are both normalized to the thickness of the piezoelectric plate. The graph is based on a lithium niobate separator, an aluminum conductor and SiO₂Two-dimensional simulation of XBAR for the front side dielectric layer. An XBAR with IDT spacing and thickness within a small unshaded area 1210 will have acceptably low spurious modes for use in the filter. XBARs with IDT spacing and thickness in the surrounding shadow region will have unacceptably high spurious modes for the target filter. There are no available XBARs with thin IDT conductors. 0.35 times the thickness of the piezoelectric plate is the upper limit of the thickness of the front dielectric layer. For thicker dielectric layers on the material, there is no useful XBAR.

Description of the method

Fig. 13 is a simplified flow diagram illustrating a process 1300 for fabricating an XBAR or filter incorporating an XBAR. The process 1300 begins 1305 with having a substrate and a plate of piezoelectric material, and ends 1395 with the completion of an XBAR or filter. The flow chart of fig. 13 includes only the main processing steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, during, after, and during the steps illustrated in fig. 13.

The flow chart of fig. 13 captures three variations of the process 1300 for fabricating XBARs that differ in when and how the cavities are formed in the substrate. A cavity may be formed at step 1310A, 1310B, or 1310C. In each of the three variations of process 1300, only one of these steps is performed.

The piezoelectric plate may be, for example, a rotating Y-cut lithium niobate. The euler angle of the piezoelectric plate is [0 °, β,0 °, where β is in the range of 0 ° to 60 °. Preferably, β may be in the range from 26 ° to 34 ° to minimize coupling to shear horizontal acoustic modes. β may be about 30 ° the substrate may preferably be silicon. The substrate may be of other materials that allow the formation of deep cavities by etching or other processes.

In one variation of process 1300, one or more cavities are formed in the substrate at 1320A, and then the piezoelectric plate is bonded to the substrate at 1320. A separate cavity may be formed for each resonator in the filter device. Conventional photolithography and etching techniques may be used to form the one or more cavities. Typically, the cavity formed at 1310A will not penetrate the substrate.

At 1320, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and the substrate may be bonded by a wafer bonding process. Generally, the mating surfaces of the substrate and the piezoelectric plate are highly polished. One or more layers of an intermediate material, such as an oxide or a metal, may be formed or deposited on the mating surfaces of one or both of the piezoelectric plate and the substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the substrate or intermediate material layer.

A conductor pattern including an IDT for each XBAR is formed at 1330 by: one or more conductor layers are deposited on the front side of the piezoelectric plate and patterned. The conductor pattern may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed underneath (i.e., between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chromium or other metal may be used to improve adhesion between the conductor layer and the piezoelectric plate. A conductive enhancement layer of gold, aluminum, copper, or other higher conductivity metal can be formed on portions of the conductor pattern (e.g., the interconnects between the IDT bus lines and the IDTs).

Conductor patterns may be formed at 1330 by sequentially depositing a conductive layer and optionally one or more other metal layers on the surface of the piezoelectric plate. Excess metal may then be removed by etching through the patterned photoresist. The conductor layer may be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

Alternatively, a conductor pattern may be formed using a lift-off process at 1330. A photoresist may be deposited on the piezoelectric plate and patterned to define a conductor pattern. A conductor layer and optionally one or more further layers may be deposited in sequence on the surface of the piezoelectric plate. The photoresist may then be removed, which removes excess material, leaving behind the conductor pattern.

At 1340, a front side dielectric layer can be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. The dielectric layer or layers may be deposited using conventional deposition techniques, such as sputtering, evaporation, or chemical vapor deposition. One or more dielectric layers may be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more photolithographic processes (using a photomask) may be used to limit the deposition of the dielectric layer on selected areas of the piezoelectric plate, such as between interleaved fingers of only the IDT. The mask may also be used to allow different thicknesses of the dielectric layer to be deposited on different portions of the piezoelectric plate.

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

In a second variation of process 1300, a back side dielectric layer may be formed at 1350. In the case where a cavity is formed at 1310B as a hole through the substrate, the back side dielectric layer may be deposited through the cavity using conventional deposition techniques (e.g., sputtering, evaporation, or chemical vapor deposition).

In a third variation of method 1300, one or more cavities in the form of grooves in the substrate may be formed at 1310C by etching the substrate using an etchant introduced through openings in the piezoelectric plate. A separate cavity may be formed for each resonator in the filter device.

In all variations of process 1300, the filter device is completed at 1360. Actions that may occur at 1360 include depositing a material such as SiO over all or a portion of the device₂Or Si₃O₄The encapsulation/passivation layer of (a); forming pads or solder bumps or other means for establishing a connection between the device and an external circuit; singulating a device from a wafer containing a plurality of devices; other packaging steps; and testing. Another action that may occur at 1360 is to adjust the resonant frequency of a resonator within the device by adding or removing metal or dielectric material to or from the front side of the device. After the filter device is completed, the process terminates at 1395.

Concluding sentence

Throughout the specification, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and processes disclosed or claimed. Although many of the examples provided herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flow diagrams, additional steps and fewer steps may be taken, and the illustrated steps may be combined or further refined to implement the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, "plurality" refers to two or more. As used herein, a "set" of items may include one or more of such items. As used herein, the terms "comprising," "including," "carrying," "having," "containing," "involving," and the like, whether in the written detailed description or in the claims, are to be construed as open-ended, i.e., to mean including, but not limited to. With respect to the claims, the transition phrases "consisting of …" and "consisting essentially of …" alone are closed or semi-closed transition phrases. Ordinal terms such as "first," "second," "third," etc., used in the claims are used to modify a claim element and do not by itself connote any priority, precedence, or order of one claim element over another or the order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a same name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, "and/or" means that the listed items are alternatives, but alternatives also include any combination of the listed items.

Claims

1. An acoustic wave resonator device comprising:

a substrate having a surface;

a lithium niobate sheet having a front side and a back side, the back side being attached to a surface of the substrate, but a portion of the lithium niobate sheet forming a separator being unattached to the surface of the substrate, the separator spanning a cavity in the substrate; and

an interdigital transducer (IDT) formed on the front surface of the lithium niobate plate such that interleaved fingers of the IDT are disposed on the diaphragm, wherein

The lithium niobate plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm,

the Euler angle of the lithium niobate plate is [0 °, beta, 0 ° ], wherein beta is greater than or equal to 0 ° and less than or equal to 60 °, and

the thickness of the interleaved fingers of the IDT is greater than or equal to 0.8 times the thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.

2. The device of claim 1, wherein β is greater than or equal to 26 ° and less than or equal to 34 °.

3. The device of claim 1, wherein β is about 30 °.

4. The device of claim 1, further comprising a dielectric layer formed between the interleaved fingers of the IDT.

5. The device of claim 4,

the dielectric layer has a thickness less than or equal to 0.2 times a thickness of the lithium niobate plate, and the interleaved fingers of the IDT have a thickness greater than or equal to 1.1 times and less than or equal to 2.0 times the thickness of the lithium niobate plate.

6. The device of claim 4,

the thickness of the dielectric layer is greater than 0.2 times and less than or equal to 0.3 times the thickness of the lithium niobate plate, and

the thickness of the interleaved fingers of the IDT is greater than or equal to 1.15 times the thickness of the lithium niobate plate and less than or equal to 1.8 times the thickness of the lithium niobate plate.

7. The device of claim 4,

the thickness of the dielectric layer is less than or equal to 0.35 times of the thickness of the lithium niobate plate.

8. The device of claim 1, wherein the direction of acoustic energy flow of the primary acoustic mode is substantially orthogonal to the front and back surfaces of the septum.

9. A filter device, comprising:

a substrate having a surface;

a lithium niobate sheet having a front side and a back side, the back side being attached to a surface of the substrate but portions of the lithium niobate sheet forming one or more membranes spanning respective cavities in the substrate; and

a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a corresponding plurality of acoustic wave resonators, interleaved fingers of each of the plurality of IDTs disposed on a corresponding membrane of the one or more membranes, wherein

The lithium niobate plate and all IDTs are configured such that a respective radio frequency signal applied to the IDT excites a respective main shear acoustic mode in the respective diaphragm,

the interleaved fingers of all of the IDTs have a common thickness that is greater than or equal to 0.8 times the thickness of the lithium niobate plate and less than or equal to 2.0 times the thickness of the lithium niobate plate.

10. The filter device of claim 9, wherein β is greater than or equal to 26 ° and less than or equal to 34 °.

11. The filter device of claim 9, wherein β is about 30 °.

12. The filter device of claim 9, further comprising a frequency setting dielectric layer formed between the interleaved fingers of the subset of the plurality of IDTs.

13. The filter device of claim 12,

the thickness of the frequency setting dielectric layer is less than or equal to 0.2 times the thickness of the lithium niobate plate, and the common thickness of the interleaved fingers of the IDT is greater than or equal to 1.1 times and less than or equal to 2.0 times the thickness of the lithium niobate plate.

14. The filter device of claim 12,

the thickness of the frequency setting dielectric layer is greater than 0.2 times and less than or equal to 0.3 times the thickness of the lithium niobate plate, and

the collective thickness of the interleaved fingers of the IDT is greater than or equal to 1.15 times the thickness of the lithium niobate plate and less than or equal to 1.8 times the thickness of the lithium niobate plate.

15. The filter device of claim 12,

the thickness of the frequency setting dielectric layer is less than or equal to 0.35 times the thickness of the lithium niobate plate.

16. The filter device of claim 12,

the plurality of acoustic wave resonators includes one or more parallel resonators and one or more series resonators connected in a ladder filter circuit, an

A subset of the plurality of IDTs is one or more parallel resonators.

17. The filter device of claim 9, wherein the respective directions of the acoustic energy flows of all the primary acoustic modes are substantially orthogonal to the front and back surfaces of the diaphragm.

18. The filter device of claim 9, wherein the interleaved fingers of each of the plurality of IDTs are disposed on a respective membrane spanning a respective cavity.

19. A method of manufacturing an acoustic resonator device, comprising:

bonding a back side of a lithium niobate plate to a substrate such that a portion of the lithium niobate plate forms a diaphragm across a cavity in the substrate; and

forming an interdigital transducer (IDT) on a front surface of the lithium niobate plate such that interleaved fingers of the IDT are disposed on the diaphragm, wherein,

the lithium niobate plate and IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm,

the Euler angle of the lithium niobate plate is [0 °, beta, 0 ° ], wherein beta is greater than or equal to 0 ° and less than 60 °, and

the thickness of the interleaved fingers of the IDT is greater than or equal to 0.8 times and less than or equal to 2.0 times the thickness of the lithium niobate plate.

20. The method of claim 19, wherein β is greater than or equal to 26 ° and less than or equal to 34 °.

21. The method of claim 19, wherein β is about 30 °.