U.S. patent application number 17/342544 was filed with the patent office on 2022-02-10 for transversely-excited film bulk acoustic resonator comprising small cells.
The applicant listed for this patent is Resonant Inc.. Invention is credited to Greg Dyer, Neal Fenzi, Bryant Garcia, Ryo Wakabayashi.
Application Number | 20220045661 17/342544 |
Document ID | / |
Family ID | |
Filed Date | 2022-02-10 |
United States Patent
Application |
20220045661 |
Kind Code |
A1 |
Fenzi; Neal ; et
al. |
February 10, 2022 |
TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR COMPRISING SMALL
CELLS
Abstract
Acoustic resonator devices are disclosed. An acoustic resonator
device includes a plurality of cells electrically connected in
parallel. Each cell includes an interdigital transducer (IDT) on a
piezoelectric plate, the IDT having at least 15 and not more than
35 interleaved fingers.
Inventors: |
Fenzi; Neal; (Santa Barbara,
CA) ; Wakabayashi; Ryo; (Santa Clara, CA) ;
Garcia; Bryant; (Belmont, CA) ; Dyer; Greg;
(Santa Barbara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Resonant Inc. |
Austin |
TX |
US |
|
|
Appl. No.: |
17/342544 |
Filed: |
June 9, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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63062395 |
Aug 6, 2020 |
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63067330 |
Aug 19, 2020 |
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63078886 |
Sep 15, 2020 |
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International
Class: |
H03H 9/13 20060101
H03H009/13; H05K 7/20 20060101 H05K007/20; H03H 9/24 20060101
H03H009/24 |
Claims
1. An acoustic resonator device comprising: a plurality of cells
electrically connected in parallel, each cell comprising: an
interdigital transducer (IDT) on a portion of a piezoelectric
plate, the IDT having at least 15 and not more than 35 interleaved
fingers, wherein the interleaved fingers of the IDTs of two or more
of the plurality of cells are on a common diaphragm; and a heat
sink finger between adjacent cells on the common diaphragm.
2. The device of claim 1, wherein the piezoelectric plate and the
IDT of each of the plurality of cells are configured such that a
radio frequency signal applied to the IDT excites a shear primary
acoustic mode in the piezoelectric plate.
3. The device of claim 1, each cell further comprising: one or more
reflector elements proximate and parallel to each terminal finger
of the IDT.
4. The device of claim 1, each cell further comprising: two
reflector elements proximate and parallel to each terminal finger
of the IDT.
5. The device of claim 1, wherein the common diaphragm spans
respective cavities in a substrate.
6. The device of claim 5, wherein the interleaved fingers of the
IDT of each of the plurality of cells are on respective
diaphragms.
7. The device of claim 5, wherein the interleaved fingers of the
IDTs of two or more of the plurality of cells are on a common
diaphragm.
8. The device of claim 7, wherein the interleaved fingers of the
IDTs of all of the plurality of cells are on a single piezoelectric
diaphragm.
9. The device of claim 1, wherein reflector elements between
adjacent cells on the common diaphragm are shared by the adjacent
cells.
10. The device of claim 7, further comprising: a heat sink finger
between adjacent cells on the common diaphragm.
11. The device of claim 5, further comprising: one or more openings
through the common diaphragm between adjacent cells on the common
diaphragm.
12. The device of claim 1, further comprising: an acoustic Bragg
reflector between a back surface of the piezoelectric plate and a
substrate, wherein the IDTs of the plurality of cells are on a
front surface of the piezoelectric plate.
13. An acoustic resonator device comprising: a plurality of cells
electrically connected in parallel, each cell comprising: a portion
of a piezoelectric plate; and an interdigital transducer (IDT) on
the portion of the piezoelectric plate, the IDT comprising
interleaved fingers, wherein the portion of the piezoelectric plate
and the IDT of each of the plurality of cells are configured such
that a radio frequency signal applied to the IDT excites a shear
primary acoustic mode in the portion of the piezoelectric plate,
and wherein each of the plurality of cells are acoustically
isolated from the other cells.
14. The device of claim 13, wherein each of the plurality of cells
comprises a same number of interleaved fingers.
15. The device of claim 13, each cell further comprising: one or
more reflector elements proximate and parallel to each terminal
finger of the IDT.
16. The device of claim 14, wherein reflector elements between
adjacent cells are shared by the adjacent cells.
17. The device of claim 13, further comprising a heat sink finger
between adjacent cells.
18. The device of claim 13, wherein the portion of the
piezoelectric plate is a diaphragm.
19. The device of claim 13, wherein the portions of the
piezoelectric plate of each of the plurality of cells are portions
of the same piezoelectric plate, and the acoustic resonator device
further comprises one or more openings through the piezoelectric
plate between adjacent cells.
20. The device of claim 13, further comprising: an acoustic Bragg
reflector between a back surface of the piezoelectric plate and a
substrate, wherein the IDTs of the plurality of cells are on a
front surface of the piezoelectric plate.
21. An acoustic resonator device comprising: a plurality of cells
electrically connected in parallel, each cell comprising: an
interdigital transducer (IDT) on a portion of a piezoelectric
plate, the IDT having at least 15 and not more than 35 interleaved
fingers, wherein the interleaved fingers of the IDTs of two or more
of the plurality of cells are on a common diaphragm; and one or
more openings through the common diaphragm between adjacent cells
on the common diaphragm.
22. The device of claim 21 wherein the piezoelectric plate and the
IDT of each of the plurality of cells are configured such that a
radio frequency signal applied to the IDT excites a shear primary
acoustic mode in the piezoelectric plate.
23. The device of claim 21, each cell further comprising: one or
more reflector elements proximate and parallel to each terminal
finger of the IDT.
24. The device of claim 21, each cell further comprising: two
reflector elements proximate and parallel to each terminal finger
of the IDT.
25. The device of claim 21 wherein reflector elements between
adjacent cells on the common diaphragm are shared by the adjacent
cells.
26. The device of claim 21 further comprising: an acoustic Bragg
reflector between a back surface of the piezoelectric plate and a
substrate, wherein the IDTs of the plurality of cells are on a
front surface of the piezoelectric plate.
Description
RELATED APPLICATION INFORMATION
[0001] This patent claim priority to the following provisional
patent applications: application 63/062,395, filed Aug. 6, 2020,
entitled SMALL LOW SPURIOUS UNIT CELL TO CONSTRUCT XBAR RESONATORS;
application 63/067,330, filed Aug. 19, 2020, entitled SMALL LOW
SPURIOUS UNIT CELL TO CONSTRUCT XBAR RESONATORS; and application
63/078,886, filed Sep. 15, 2020, entitled XBAR USING SMALLEST GOOD
RESONATOR CELLS.
[0002] This patent is related to application Ser. No. 17/229,767,
filed Apr. 13, 2021, entitled SMALL TRANSVERSELY-EXCITED FILM BULK
ACOUSTIC RESONATORS WITH ENHANCED Q-FACTOR.
NOTICE OF COPYRIGHTS AND TRADE DRESS
[0003] 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 matter which is or may become
trade dress of the owner. The copyright and trade dress owner has
no objection to the facsimile reproduction by anyone of the patent
disclosure as it appears in the Patent and Trademark Office patent
files or records, but otherwise reserves all copyright and trade
dress rights whatsoever.
BACKGROUND
Field
[0004] This disclosure relates to radio frequency filters using
acoustic wave resonators, and specifically to filters for use in
communications equipment.
Description of the Related Art
[0005] A radio frequency (RF) filter is a two-port device
configured to pass some frequencies and to stop other frequencies,
where "pass" means transmit with relatively low signal loss and
"stop" means block or substantially attenuate. The range of
frequencies passed by a filter is referred to as the "pass-band" of
the filter. The range of frequencies stopped by such a filter is
referred to as the "stop-band" of the filter. A typical RF filter
has at least one pass-band and at least one stop-band. Specific
requirements on a pass-band or stop-band depend on the specific
application. For example, a "pass-band" may be defined as a
frequency range where the insertion loss of a filter is better than
a defined value such as 1 dB, 2 dB, or 3 dB. A "stop-band" may be
defined as a frequency range where the rejection of a filter is
greater than a defined value such as 20 dB, 30 dB, 40 dB, or
greater depending on application.
[0006] RF filters are used in communications systems where
information is transmitted over wireless links. For example, RF
filters may be found in the RF front-ends of cellular base
stations, mobile telephone and computing devices, satellite
transceivers and ground stations, IoT (Internet of Things) devices,
laptop computers and tablets, fixed point radio links, and other
communications systems. RF filters are also used in radar and
electronic and information warfare systems.
[0007] RF filters typically require many design trade-offs to
achieve, for each specific application, the best compromise between
performance parameters such as insertion loss, rejection,
isolation, power handling, linearity, size and cost. Specific
design and manufacturing methods and enhancements can benefit
simultaneously one or several of these requirements.
[0008] Performance enhancements to the RF filters in a wireless
system can have broad impact to system performance. Improvements in
RF filters can be leveraged to provide system performance
improvements such as larger cell size, longer battery life, higher
data rates, greater network capacity, lower cost, enhanced
security, higher reliability, etc. These improvements can be
realized at many levels of the wireless system both separately and
in combination, for example at the RF module, RF transceiver,
mobile or fixed sub-system, or network levels.
[0009] High performance RF filters for present communication
systems commonly incorporate acoustic wave resonators including
surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW)
resonators, film bulk acoustic wave resonators (FBAR), and other
types of acoustic resonators. However, these existing technologies
are not well-suited for use at the higher frequencies and
bandwidths proposed for future communications networks.
[0010] The desire for wider communication channel bandwidths will
inevitably lead to the use of higher frequency communications
bands. Radio access technology for mobile telephone networks has
been standardized by the 3GPP (3.sup.rd Generation Partnership
Project). Radio access technology for 5.sup.th generation mobile
networks is defined in the 5G NR (new radio) standard. The 5G NR
standard defines several new communications bands. Two of these new
communications bands are n77, which uses the frequency range from
3300 MHz to 4200 MHz, and n79, which uses the frequency range from
4400 MHz to 5000 MHz. Both band n77 and band n79 use time-division
duplexing (TDD), such that a communications device operating in
band n77 and/or band n79 use the same frequencies for both uplink
and downlink transmissions. Bandpass filters for bands n77 and n79
must be capable of handling the transmit power of the
communications device. WiFi bands at 5 GHz and 6 GHz also require
high frequency and wide bandwidth. The 5G NR standard also defines
millimeter wave communication bands with frequencies between 24.25
GHz and 40 GHz.
[0011] The Transversely-Excited Film Bulk Acoustic Resonator (XBAR)
is an acoustic resonator structure for use in microwave filters.
The XBAR is described in U.S. Pat. No. 10,491,291, titled
TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR. An XBAR
resonator comprises 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 shear primary acoustic wave in the piezoelectric
diaphragm. XBAR resonators provide very high electromechanical
coupling and high frequency capability. XBAR resonators may be used
in a variety of RF filters including band-reject filters, band-pass
filters, duplexers, and multiplexers. XBARs are well suited for use
in filters for communications bands with frequencies above 3
GHz.
DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is schematic plan view and two schematic
cross-sectional views of a transversely-excited film bulk acoustic
resonator (XBAR).
[0013] FIG. 2 is an expanded schematic cross-sectional view of a
portion of the XBAR of FIG. 1.
[0014] FIG. 3 is an alternative schematic cross-sectional view of
an XBAR.
[0015] FIG. 4 is a schematic block diagram of a filter using
XBARs.
[0016] FIG. 5A is a plan view of IDT electrodes for an XBAR with
200 IDT fingers.
[0017] FIG. 5B is a graph of the magnitude of admittance as a
function of frequency for an XBAR using the IDT electrodes of FIG.
5A.
[0018] FIG. 6A is a plan view of an XBAR composed of six cells,
each having 33 IDT fingers.
[0019] FIG. 6B is a plan view of another XBAR composed of six
cells, each having 33 IDT fingers.
[0020] FIG. 7 is a graph comparing the magnitude of admittance as a
function of frequency for three XBARs with different numbers of
cells.
[0021] FIG. 8 is a plan view of an IDT electrode pattern with
reflector elements.
[0022] FIG. 9 is a graph of the normalized Q-factor as a function
of the number of IDT fingers in a cell.
[0023] FIG. 10 is a plan view of an IDT electrode pattern with two
cells separated by reflector fingers.
[0024] FIG. 11 is a plan view of an IDT electrode pattern with two
cells separated by reflector fingers with enhanced heat
removal.
[0025] FIG. 12 is a plan view of an IDT electrode pattern with two
cells separated by reflector fingers and an opening through the
piezoelectric plate.
[0026] Throughout this description, elements appearing in figures
are assigned three-digit or four-digit reference designators, where
the two least significant digits are specific to the element and
the one or two most significant digit is the figure number where
the element is first introduced. An element that is not described
in conjunction with a figure may be presumed to have the same
characteristics and function as a previously-described element
having the same reference designator.
DETAILED DESCRIPTION
[0027] Description of Apparatus
[0028] FIG. 1 shows a simplified schematic top view and orthogonal
cross-sectional views of a transversely-excited film bulk acoustic
resonator (XBAR) 100. XBAR resonators such as the resonator 100 may
be used in a variety of RF filters including band-reject filters,
band-pass filters, duplexers, and multiplexers. XBARs are well
suited for use in filters for communications bands with frequencies
above 3 GHz.
[0029] The XBAR 100 is made up of a thin film conductor pattern
formed on a surface of a piezoelectric plate 110 having a front
surface 112 and a back surface 114. The front and back surfaces are
essentially parallel. "Essentially parallel" means parallel to the
extent possible within normal manufacturing tolerances. The
piezoelectric plate is a thin single-crystal layer of a
piezoelectric material such as lithium niobate, lithium tantalate,
lanthanum gallium silicate, gallium nitride, or aluminum nitride.
The piezoelectric plate is cut such that the orientation of the X,
Y, and Z crystalline axes with respect to the front and back
surfaces is known and consistent. The piezoelectric plate may be,
for example, Z-cut, rotated ZY-cut and rotated YX-cut.
[0030] The back surface 114 of the piezoelectric plate 110 is
attached to a surface of the substrate 120 except for a portion of
the piezoelectric plate 110 that forms a diaphragm 115 spanning a
cavity 140 formed in the substrate 120. The portion of the
piezoelectric plate that spans the cavity is referred to herein as
the "diaphragm" due to its physical resemblance to the diaphragm of
a microphone. As shown in FIG. 1, the diaphragm 115 is contiguous
with the rest of the piezoelectric plate 110 around all of a
perimeter 145 of the cavity 140. In this context, "contiguous"
means "continuously connected without any intervening item".
[0031] The substrate 120 provides mechanical support to the
piezoelectric plate 110. The substrate 120 may be, for example,
silicon, sapphire, quartz, or some other material or combination of
materials. The back surface 114 of the piezoelectric plate 110 may
be attached to the substrate 120 using a wafer bonding process.
Alternatively, the piezoelectric plate 110 may be grown on the
substrate 120 or otherwise attached to the substrate. The
piezoelectric plate 110 may be attached directly to the substrate
or may be attached to the substrate 120 via one or more
intermediate material layers.
[0032] The cavity 140 is an empty space within a solid body of the
resonator 100. The cavity 140 may be a hole completely through the
substrate 120 (as shown in Section A-A and Section B-B) or a recess
in the substrate 120. The cavity 140 may be formed, for example, by
selective etching of the substrate 120 before or after the
piezoelectric plate 110 and the substrate 120 are attached.
[0033] The conductor pattern of the XBAR 100 includes an
interdigital transducer (IDT) 130. An IDT is an electrode structure
for converting between electrical and acoustic energy in
piezoelectric devices. The IDT 130 includes a first plurality of
parallel elongated conductors, commonly called "fingers", such as
finger 136, extending from a first busbar 132. The IDT 130 includes
a second plurality of fingers extending from a second busbar 134.
The first and second pluralities of parallel fingers are
interleaved. The interleaved fingers overlap for a distance AP,
commonly referred to as the "aperture" of the IDT. The
center-to-center distance L between the outermost fingers of the
IDT 130 is the "length" of the IDT.
[0034] The term "busbar" refers to the conductors that interconnect
the first and second sets of fingers in an IDT. As shown in FIG. 1,
each busbar 132, 134 is an elongated rectangular conductor with a
long axis orthogonal to the interleaved fingers and having a length
approximately equal to the length L of the IDT. The busbars of an
IDT need not be rectangular or orthogonal to the interleaved
fingers and may have lengths longer than the length of the IDT.
[0035] The first and second busbars 132, 134 serve as the terminals
of the XBAR 100. A radio frequency or microwave signal applied
between the two busbars 132, 134 of the IDT 130 excites a primary
acoustic mode within the piezoelectric plate 110. As will be
discussed in further detail, the primary acoustic mode is a bulk
shear mode where acoustic energy propagates along a direction
substantially orthogonal to the surface of the piezoelectric plate
110, which is also normal, or transverse, to the direction of the
electric field created by the IDT fingers. Thus, the XBAR is
considered a transversely-excited film bulk wave resonator.
[0036] The IDT 130 is positioned on the piezoelectric plate 110
such that at least the fingers of the IDT 130 are disposed on the
diaphragm 115 of the piezoelectric plate that spans, or is
suspended over, the cavity 140. As shown in FIG. 1, the cavity 140
has a rectangular shape with an extent greater than the aperture AP
and length L of the IDT 130. A cavity of an XBAR may have a
different shape, such as a regular or irregular polygon. The cavity
of an XBAR may more or fewer than four sides, which may be straight
or curved.
[0037] For ease of presentation in FIG. 1, the geometric pitch and
width of the IDT fingers is greatly exaggerated with respect 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
the IDT 110. An XBAR may have dozens, possibly hundreds, of
parallel fingers in the IDT 110. Similarly, the thickness of the
piezoelectric plate and IDT fingers are greatly exaggerated in the
cross-sectional views.
[0038] FIG. 2 shows a detailed schematic cross-sectional view of
the XBAR 100. The piezoelectric plate 110 is a single-crystal layer
of piezoelectrical material having a thickness tp. tp may be, for
example, 100 nm to 1500 nm. When used in filters for communications
bands from 3.4 GHZ to 6 GHz, the thickness tp may be, for example,
200 nm to 1000 nm.
[0039] The IDT fingers 230, 235 may be one or more layers of
aluminum, a substantially aluminum alloy, copper, a substantially
copper alloy, beryllium, gold, molybdenum, or some other conductive
material. Thin (relative to the total thickness of the conductors)
layers of other metals, such as chromium or titanium, may be formed
under and/or over the fingers 230, 235 to improve adhesion between
the fingers and the piezoelectric plate 110 and/or to passivate or
encapsulate the fingers. The busbars (132, 134 in FIG. 1) of the
IDT may be made of the same or different materials as the
fingers.
[0040] 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 m is the width or "mark" of
the IDT fingers. The IDT of an XBAR differs substantially from the
IDTs used in surface acoustic wave (SAW) resonators. In a SAW
resonator, the pitch of the IDT is one-half of the acoustic
wavelength at the resonance frequency. Additionally, the
mark-to-pitch ratio of a SAW resonator IDT is typically close to
0.5 (i.e., the mark or finger width is about one-fourth of the
acoustic wavelength at resonance). In an XBAR, the pitch p of the
IDT is typically 2 to 20 times the width m of the fingers. In
addition, the pitch p of the IDT is typically 2 to 20 times the
thickness tp of the piezoelectric plate 110. The width of the IDT
fingers in an XBAR is not constrained to one-fourth of the acoustic
wavelength at resonance. For example, the width of XBAR IDT fingers
may be 500 nm or greater, such that the IDT can be fabricated using
optical lithography.
[0041] The thickness tm of the IDT fingers 230, 235 may be from 100
nm to about equal to the width m. The thickness of the busbars
(132, 134 in FIG. 1) of the IDT may be the same as, or greater
than, the thickness tm of the IDT fingers. For XBARs intended for
high power applications, the thickness tm may be greater than 0.85
times the thickness tp of the piezoelectric plate 110, as described
in U.S. Pat. No. 10,637,438.
[0042] As shown in FIG. 2, the IDT fingers 230, 235 have
rectangular cross-sections. The IDT fingers 230, 235 may have some
other cross-section, such as trapezoidal, T-shaped, or stepped. The
IDT fingers 230, 235 are shown as single layer structures which may
be aluminum or some other metal. IDT fingers may include multiple
layers of materials, which may be selected to have different
acoustic loss and/or different acoustic impedance. When multiple
material layers are used, the cross-sectional shapes of the layers
may be different. Further, a thin adhesion layer of another
material, such as titanium or chrome, may be disposed between the
IDT fingers 230, 235 and the piezoelectric plate 110. Although not
shown in FIG. 2, some or all IDT fingers may be disposed in grooves
or slots extending part way through the piezoelectric plate
110.
[0043] FIG. 3 shows a detailed schematic cross-sectional view of a
solidly mounted XBAR (SM XBAR) 300. SM XBARs are first described in
U.S. Pat. No. 10,601,392. The SM XBAR 300 includes a piezoelectric
plate 110 and an IDT (of which only fingers 330 and 335 are
visible). The piezoelectric layer 110 has parallel front and back
surfaces 112, 114. Dimension tp is the thickness of the
piezoelectric plate 110. The width of the IDT fingers 330, 335 is
dimension m, thickness of the IDT fingers is dimension tm, and the
IDT pitch is dimension p.
[0044] In contrast to the XBAR devices shown in FIG. 1 and FIG. 2,
the IDT of an SM XBAR is not formed on a diaphragm spanning a
cavity in a substrate (120 in FIG. 1). Instead, an acoustic Bragg
reflector 340 is sandwiched between a surface 322 of a substrate
320 and the back surface 114 of the piezoelectric plate 110. The
term "sandwiched" means the acoustic Bragg reflector 340 is both
disposed between and mechanically attached to a surface 322 of the
substrate 320 and the back surface 114 of the piezoelectric plate
110. In some circumstances, thin layers of additional materials may
be disposed between the acoustic Bragg reflector 340 and the
surface 322 of the substrate 320 and/or between the Bragg reflector
340 and the back surface 114 of the piezoelectric plate 110. Such
additional material layers may be present, for example, to
facilitate bonding the piezoelectric plate 110, the acoustic Bragg
reflector 340, and the substrate 320.
[0045] The acoustic Bragg reflector 340 includes multiple
dielectric layers that alternate between materials having high
acoustic impedance and materials have low acoustic impedance.
"High" and "low" are relative terms. For each layer, the standard
for comparison is the adjacent layers. Each "high" acoustic
impedance layer has an acoustic impedance higher than that of both
the adjacent low acoustic impedance layers. Each "low" acoustic
impedance layer has an acoustic impedance lower than that of both
the adjacent high acoustic impedance layers. As will be discussed
subsequently, the primary acoustic mode in the piezoelectric plate
of an XBAR is a shear bulk wave. Each of the layers of the acoustic
Bragg reflector 340 has a thickness equal to, or about, one-fourth
of the wavelength of a shear bulk wave having the same polarization
as the primary acoustic mode at or near a resonance frequency of
the SM XBAR 300. Dielectric materials having comparatively low
acoustic impedance include silicon dioxide, carbon-containing
silicon oxide, and certain plastics such as cross-linked
polyphenylene polymers. Materials having comparatively high
acoustic impedance include hafnium oxide, silicon nitride, aluminum
nitride, silicon carbide, and diamond. All of the high acoustic
impedance layers of the acoustic Bragg reflector 340 are not
necessarily the same material, and all of the low acoustic
impedance layers are not necessarily the same material. In the
example of FIG. 3, the acoustic Bragg reflector 340 has a total of
six layers. An acoustic Bragg reflector may have more than, or less
than, six layers.
[0046] As shown in FIG. 3, the IDT fingers 330, 335 have
rectangular cross-sections. The IDT fingers 330, 335 may have some
other cross-section, such as trapezoidal, T-shaped, or stepped. The
IDT fingers 330, 335 are shown as single layer structures which may
be aluminum or some other metal. IDT fingers may include multiple
layers of materials, which may be selected to have different
acoustic loss and/or different acoustic impedance. When multiple
material layers are used, the cross-sectional shapes of the layers
may be different. Further, a thin adhesion layer of another
material, such as titanium or chrome, may be formed between the IDT
fingers 330, 335 and the piezoelectric plate 110. Although not
shown in FIG. 3, some or all IDT fingers may be disposed in grooves
or slots extending partially or completely through the
piezoelectric plate 110.
[0047] FIG. 4 is a schematic circuit diagram for a high frequency
band-pass filter 400 using XBARs. The filter 400 has a conventional
ladder filter architecture including three series resonators 410A,
410B, 410C and two shunt resonators 420A, 420B. The three series
resonators 410A, 410B, and 410C are connected in series between a
first port and a second port (hence the term "series resonator").
In FIG. 4, the first and second ports are labeled "In" and "Out",
respectively. However, the filter 400 is bidirectional and either
port may serve as the input or output of the filter. The two shunt
resonators 420A, 420B are connected from nodes between the series
resonators to ground. A filter may contain additional reactive
components, such as inductors, not shown in FIG. 4. All the shunt
resonators and series resonators are XBARs. The inclusion of three
series and two shunt resonators is exemplary. A filter may have
more or fewer than five total resonators, more or fewer than three
series resonators, and more or fewer than two shunt resonators.
Typically, all of the series resonators are connected in series
between an input and an output of the filter. All of the shunt
resonators are typically connected between ground and the input,
the output, or a node between two series resonators.
[0048] In the exemplary filter 400, the three series resonators
410A, B, C and the two shunt resonators 420A, B of the filter 400
are formed on a single plate 430 of piezoelectric material bonded
to a silicon substrate (not visible). Each resonator includes a
respective IDT (not shown), with at least the fingers of the IDT
disposed over a cavity in the substrate. In this and similar
contexts, the term "respective" means "relating things each to
each", which is to say with a one-to-one correspondence. In FIG. 4,
the cavities are illustrated schematically as the dashed rectangles
(such as the rectangle 435). In this example, each IDT is disposed
over a respective cavity. In other filters, the IDTs of two or more
resonators may be disposed over a single cavity.
[0049] Each of the resonators 410A, 410B, 410C, 420A, 420B in the
filter 400 has resonance where the admittance of the resonator is
very high and an anti-resonance where the admittance of the
resonator is very low. The resonance and anti-resonance occur at a
resonance frequency and an anti-resonance frequency, respectively,
which may be the same or different for the various resonators in
the filter 400. In over-simplified terms, each resonator can be
considered a short-circuit at its resonance frequency and an open
circuit at its anti-resonance frequency. The input-output transfer
function will be near zero at the resonance frequencies of the
shunt resonators and at the anti-resonance frequencies of the
series resonators. In a typical filter, the resonance frequencies
of the shunt resonators are positioned below the lower edge of the
filter's passband and the anti-resonance frequencies of the series
resonators are position above the upper edge of the passband.
[0050] FIG. 5A is a plan view of an XBAR 500 including an IDT
electrode pattern 530 on a surface of a piezoelectric plate 510. A
portion of the piezoelectric plate forms a diaphragm spanning a
cavity in a substrate (not visible) that supports the piezoelectric
plate 510. The dashed line 545 represents the perimeter of the
cavity. In this example, the perimeter of the cavity is rectangular
but may have some other shape. The diaphragm is the area of the
piezoelectric plate within the perimeter 545.
[0051] The IDT electrode pattern 530 includes a first bus bar 532,
and second busbar 534, and a plurality of interleaved fingers (of
which only finger 536 is identified) that extend alternately from
the first and second busbars. The term "busbar" means a conductor
to which a plurality of other conductors is connected. The
interleaved fingers are disposed on the diaphragm portion of the
piezoelectric plate. In this example, IDT electrode pattern 530
includes 200 interleaved fingers.
[0052] FIG. 5B is a chart 550 showing the performance of the XBAR
500 of FIG. 5A. Specifically, the solid line 560 is a plot of the
magnitude of the admittance, derived from simulation of the XBAR
500 using finite element methods, of the XBAR 500 as a function of
frequency. The admittance curve 560 exhibits a resonance, where the
admittance is maximum, at about 4.7 GHz and an anti-resonance,
where the admittance in minimum, at about 5.3 GHz. The admittance
curve 560 exhibits several artifacts including the resonance and
anti-resonance of a spurious mode within the dashed box 565. This
spurious mode is believed to be a harmonic of an S0 plate wave.
[0053] FIG. 6A is a plan view of an XBAR 600 partitioned into six
smaller resonators 630-1 to 630-6 which will be referred to herein
as "cells". As used in this patent, a "cell" is a small (but not
necessarily the smallest) XBAR that meets a set of performance
requirements that may include resonance frequency, anti-resonance
frequency, and Q-factor. Each cell includes all of the normal
elements of an XBAR--a piezoelectric diaphragm and an IDT including
two busbars and interleaved fingers on the diaphragm. A cell may
include additional elements, such as the reflector elements to be
discussed subsequently, that are not necessarily found in an
XBAR.
[0054] Multiple cells can be connected in parallel to provide an
XBAR of any desired size. In FIG. 6A, each cell 630-1 to 630-6
includes a respective IDT electrode pattern on a surface of a
piezoelectric plate 610. Each IDT has respective first and second
busbars (of which only busbars 632-1 and 634-1 are identified) and
a set of interleaved fingers extending from the busbars. The
interleaved fingers of the IDT of each cell 630-1 to 630-6 are
disposed on a respective diaphragm spanning a respective cavity in
a substrate (not visible) that supports the piezoelectric plate
610. Perimeters of the cavities are indicated by dashed rectangles,
of which only cavity 640-1 is identified. In this example, the
perimeters of the cavities are rectangular but may have some other
shape.
[0055] Solidly-mounted XBARs may also be partitioned into multiple
cells, in which case there are no cavities under the piezoelectric
plate. A continuous acoustic Bragg reflector may be between the
piezoelectric plate and the substrate under all of the cells.
[0056] The cells 630-1 to 630-6 are electrically connected in
parallel by conductors, represented schematically by conductor 635.
In this example, the IDT of each cell 630-1 to 630-6 has 33
fingers, resulting in 198 total fingers across the six cells. The
cells comprising and XBAR may typically, but not necessarily, have
the same number of fingers. Although the cells 630-1 to 630-6 are
shown in FIG. 6A arranged in a row, arrangement of the cells in a
linear array is not necessary.
[0057] FIG. 6B is a plan view of another XBAR 650 partitioned into
six cells 670-1 to 670-6. Each cell 670-1 to 670-6 includes a
respective IDT electrode pattern on a surface of a piezoelectric
plate 660. The busbars of the six cells 670-1 to 670-6 are segments
of common first and second busbars 682 and 684. Each IDT includes a
set of interleaved fingers extending from the busbars 682, 684. The
interleaved fingers of the IDTs of each cell 630-1 to 630-6 are
disposed on respective diaphragms spanning respective cavities in a
substrate (not visible) that supports the piezoelectric plate 660.
Perimeters of the cavities are indicated by dashed rectangles, of
which only cavity 690-1 is identified. In this example, the
perimeters of the cavities are rectangular but may have some other
shape.
[0058] The cells 670-1 to 670-6 are electrically connected in
parallel by the common busbars 682, 684. In this example, the IDT
of each cell 670-1 to 670-6 has 33 fingers, resulting in 198 total
fingers across the six cells. Additional wide fingers (not
identified) may extend from one of the bus bars between the
cavities to remove heat from the area of the diaphragms.
[0059] Assuming XBAR 500 of FIG. 5A, XBAR 600 of FIG. 6A, and XBAR
650 of FIG. 6B have the same piezoelectric plate thickness,
material, and crystal orientation and the same IDT pitch and mark,
the XBARs 500, 600, and 650 will have the same, or nearly the same,
resonance frequency, anti-resonance frequency, and capacitance.
However, the amplitude of at least some spurious modes may be
substantially smaller for XBAR 600 and XBAR 650 compared to XBAR
500.
[0060] Although the coupling between an RF signal applied to an
XBAR and any spurious mode is very small, spurious modes that
propagate along the length of an XBAR may be reinforced by the
small, but finite, reflections that occur at each IDT finger,
resulting in a nonlinear relationship between the amplitude of a
spurious mode and the number of IDT fingers, as will be illustrated
in FIG. 7.
[0061] FIG. 7 is chart showing an expansion of the area 565
identified in FIG. 5B. Specifically, the solid curve 710 is a plot
of the admittance of the XBAR 500 of FIG. 5A, which has 200 IDT
fingers, as a function of frequency. The dashed line 720 is a plot
of the admittance of the XBAR 600 of FIG. 6A, which has 198 IDT
fingers divided into 6 cells with 33 fingers per cell. The dot-dash
line 730 is a plot of the admittance of another XBAR with 200 IDT
fingers divided into ten cells with 20 fingers per cell. The
frequencies of the resonance and anti-resonance of the three XBARs
are essentially the same. All data was determined by simulation of
the XBARs using finite element methods.
[0062] In the example of FIG. 7, the amplitude (peak-to-peak
admittance change) of the spurious mode is about 6.3 dB for the
XBAR with 200 fingers (solid line 710). Dividing the XBAR into six
cells with 33 fingers per cell reduces the amplitude to about 2 dB
(dashed line 720). Dividing the XBAR into ten cells with 20 fingers
per cell reduces the amplitude to less than 1 dB (dot-dash line
730).
[0063] Reducing the number of fingers in the IDT of an XBAR may
lower the Q-factor. Possible loss mechanisms in an acoustic
resonator include resistive losses in the IDT and other conductors;
viscous or acoustic losses in the piezoelectric plate, IDT fingers,
and other materials; and leakage of acoustic energy out of the
resonator structure. The peak energy stored in a resonator is
proportional the capacitance of the resonator. In an XBAR
resonator, the capacitance is approximately proportional to the
number of IDT fingers. Resistive losses and viscose losses are also
approximately proportional to the number of IDT fingers. Acoustic
energy that leaks from the resonator in the transverse direction
(i.e., the direction parallel to the IDT fingers) is proportional
to the length of the resonator and thus also approximately
proportional to the number of IDT fingers. In contrast, energy lost
from the ends of the IDT in the longitudinal direction (i.e., the
direction normal to the IDT fingers) is roughly constant,
independent of the number of IDT fingers. As the number of IDT
fingers and the peak energy stored in an XBAR is reduced, the
acoustic energy lost in the longitudinal direction, although small,
becomes an ever-increasing fraction of the peak energy stored.
[0064] FIG. 8 is a plan view of an exemplary conductor pattern 800
for a cell with reduced acoustic energy leakage in the longitudinal
direction at the ends of the cell. The conductor pattern 800
includes an IDT 830 and four reflector elements 862, 864, 866, 868
as described in co-pending patent application Ser. No. 17/229,767.
The IDT 830 includes a first busbar 832, a second busbar 834, and a
plurality of n interleaved IDT fingers extending alternately from
the first and second busbars. In this example, n, the number of IDT
fingers, is equal to 24. In other XBARs, n may be in a range from
20 to 100 or more IDT fingers. IDT finger 836 is the 1.sup.st
finger and IDT finger 838 is the last or n'th finger. Numbering the
IDT fingers from left to right (as shown in FIG. 8) is arbitrary
and the designations of the 1.sup.st and n'th fingers could be
reversed. The 1.sup.st and n'th fingers may be collectively
referred to as the "terminal" fingers.
[0065] As shown in FIG. 8, the odd numbered IDT fingers extend from
the first busbar 832 and the even numbered IDT fingers extend from
the second busbar 834. The IDT 830 has an even number of IDT
fingers such that the 1.sup.st and n'th IDT fingers 836, 838 extend
from different busbars. In some cases, an IDT may have an odd
number of IDT fingers such that the 1.sup.st and n'th IDT fingers
and all of the reflector elements extend from the same busbar.
[0066] A total of four reflector elements are provided outside of
periphery of the IDT 830. A first reflector element 862 is
proximate and parallel to 1st IDT finger 836 at the left end (as
seen in the figure) of the IDT 830. A second reflector element 866
is proximate and parallel to n'th IDT finger 838 at the right end
of the IDT 830. An optional third reflector element 864 is parallel
to the first reflector element 862. An optional fourth reflector
element 868 is parallel to the second reflector element 866.
[0067] First and third reflector elements 862, 864 extend from the
first busbar 832 and thus are at the same electrical potential as
the 1st IDT finger 836. Similarly, second and fourth reflector
elements 866 and 868 extend from the second busbar 830 and thus are
at the same electrical potential as the n'th IDT finger 838. In
other embodiments, reflector elements may be connected to the other
busbar (i.e. the busbar that the 1.sup.st and n'th IDT fingers are
not connected to) such that the reflector elements are at the
opposite electrical potential from the 1.sup.st and n'th IDT
fingers.
[0068] The reflector elements 862, 864, 866, 868 are configured to
confine acoustic energy to the area of the IDT 830 and thus reduce
acoustic energy losses in the longitudinal direction. To this end,
the pitch pr between adjacent reflector elements and between
reflector elements 862 and 866 and the adjacent first and n'th IDT
fingers, respectively, is typically greater than the pitch p of the
IDT fingers. The width or mark mr of the reflector elements 862,
864, 866, 868 is not necessarily equal to the mark m of the IDT
fingers. As described in application Ser. No. 17/229,767, the mark
mr of the reflector elements may be selected to optimize Q-factor
at a specific frequency or range of frequencies.
[0069] FIG. 9 is a graph 900 of the normalized Q-factor as a
function of number of IDT fingers for XBARs with two optimized
reflector elements at each side of the IDT, similar to the
reflector elements shown in FIG. 8. "Normalized Q-factor" is the
Q-factor of the XBAR with a finite number of IDT fingers divided by
the Q-factor of a hypothetical XBAR with the same structure and an
infinite number of IDT fingers. Specifically, the solid curve 910
is a plot of the normalized Q-factor of an XBAR with reflector
elements at its anti-resonance frequency. The dot-dash curve 915 is
a plot of the normalized Q-factor of a similar XBAR without
reflector elements at its anti-resonance frequency. The dashed
curve 920 is a plot of the normalized Q-factor of the XBAR with
reflector elements at its resonance frequency. The dot-dot-dash
curve 925 is a plot of the normalized Q-factor of a the XBAR
without reflector elements at its resonance frequency. All data is
determined by simulation of XBARs using finite element methods.
[0070] FIG. 9 shows that an IDT with 30 fingers and reflector
elements provides a normalized Q-factor of 90% at either the
resonance or anti-resonance frequencies. Note that the optimum
reflector elements are different for the resonance and
anti-resonance frequencies. Since a normalized Q-factor of 90% is
not necessarily required for all XBARs in all filters, cells may
have as few as 15 fingers and typically will not have more than 35
fingers.
[0071] FIG. 10 is a plan view of a conductor pattern 1000 for an
XBAR with two cells disposed on a common piezoelectric diaphragm
that spans a common cavity. The dashed line 1045 represents a
perimeter of the cavity, which is to say the intersection of the
cavity and back side of the piezoelectric plate. In this example
the perimeter of the cavity is rectangular but may have some other
shape.
[0072] The conductor pattern 1000 includes a first busbar 1020 and
a second busbar 1025 that are common to both cells. The first cell
1030-1 has 17 interleaved IDT fingers extending alternately from
the first and second busbars 1020, 1025. The number of IDT fingers
was chosen for ease of representation. A cell may commonly have
more than 17 fingers and may have less than 17 fingers. A cell may
typically have not more than 35 fingers. The second cell 1030-2
also has 17 interleaved IDT fingers extending alternately from the
first and second busbars 1020, 1025.
[0073] In this example, the IDT fingers of both the first cell
1030-1 and the second cell 1030-2 have the same pitch p and the
same mark m. However, the pitch and mark of the IDT elements of
different cells are not necessarily the same. A small difference in
pitch and/or mark between cells may further reduce the amplitude of
spurious modes. For example, the IDT fingers of the first cell
1030-1 may have pitch p1 and mark m1 and the IDT fingers of the
second cell 1030-2 may have pitch p2 and mark m2, where p1.noteq.p2
and/or m1.noteq.m2.
[0074] The 17 IDT fingers of the first cell 1030-1 are disposed
between a first pair of reflector elements 1050 and a second pair
of reflector elements 1052. The 17 IDT fingers of the second cell
1030-2 are disposed between the second pair of reflector elements
1052 and a third pair of reflector elements 1054. The first cell
1030-1 and the second cell 1030-2 effectively share the second pair
of reflector elements 1052.
[0075] The reflector elements 1050, 1052, 1054 are configured to
reduce leakage of acoustic energy from the cells. To this end, the
pitch pr between adjacent reflector elements and between reflector
elements and adjacent IDT fingers is typically greater than the
pitch p of the IDT fingers. The mark mr of the reflector elements
may less than, equal to, or greater than the mark m of the IDT
fingers.
[0076] In the example of FIG. 10, a pair of reflector elements
1050, 1052, 1054 are positioned at the ends of each of the cells
1030-1 and 1030-2. As described in co-pending patent application
Ser. No. 17/229,767, in some circumstances a single reflector
element may be used, and there is little benefit in having more
than two reflector elements at the ends of the IDT of an XBAR.
Since the reflector elements 1050, 1052, 1054 are configured to
reduce leakage of acoustic energy, the cells 1030-1, 1030-2 are
substantially acoustically isolated from each other.
[0077] FIG. 11 is a plan view of a conductor pattern 1100 for
another XBAR with two cells disposed on a common piezoelectric
diaphragm that spans a common cavity. The dashed line 1145
represents a perimeter of the cavity, which is to say the
intersection of the cavity and back side of the piezoelectric
plate. In this example the perimeter of the cavity is rectangular
but may have some other shape.
[0078] The conductor pattern 1100 includes a first busbar 1120 and
a second busbar 1125 that are common to both cells. The first cell
1130-1 has 17 interleaved IDT fingers extending alternately from
the first and second busbars 1120, 1125. As in the previous
example, the number of IDT fingers was chosen for ease of
representation. The second cell 1130-2 also has 17 interleaved IDT
fingers extending alternately from the first and second busbars
1120, 1125.
[0079] In this example, the IDT fingers of both the first cell
1130-1 and the second cell 1030-2 have the same pitch p and the
same mark m. As previously described, the pitch and mark of the IDT
elements of different cells are not necessarily the same.
[0080] The 17 IDT fingers of the first cell 1130-1 are disposed
between a first pair of reflector elements 1150 and a group 1152 of
three reflector elements. The 17 IDT fingers of the second cell
1130-2 are disposed between the group 1152 of three reflector
elements and a second pair of reflector elements 1154. The first
cell 1130-1 and the second cell 1130-2 effectively share the group
1152 of three reflector elements.
[0081] The reflector elements 1150, 1152, 1154 are configured to
reduce leakage of acoustic energy from the cells as previously
described. The middle element 1156 of the group 1152 of three
reflector elements may be a heat sink finger with a substantially
wider mark and/or substantially greater thickness than the other
two reflector elements of the group. The heat sink finger 1156 may
facilitate removing heat from diaphragm.
[0082] FIG. 12 is a plan view of a conductor pattern 1200 for
another XBAR with two cells disposed on a common piezoelectric
diaphragm that spans a common cavity. The dashed line 1245
represents a perimeter of the cavity, which is to say the
intersection of the cavity and back side of the piezoelectric
plate. In this example the perimeter of the cavity is rectangular
but may have some other shape.
[0083] The conductor pattern 1200 includes a first busbar 1220 and
a second busbar 1225 that are common to both cells. The first cell
1230-1 has 17 interleaved IDT fingers extending alternately from
the first and second busbars 1220, 1225. As in the previous
examples, the number of IDT fingers was chosen for ease of
representation. The second cell 1230-2 also has 17 interleaved IDT
fingers extending alternately from the first and second busbars
1220, 1225.
[0084] In this example, the IDT fingers of both the first cell
1230-1 and the second cell 1230-2 have the same pitch p and the
same mark m. As previously described, the pitch and mark of the IDT
elements of different cells are not necessarily the same.
[0085] The 17 IDT fingers of the first cell 1230-1 are disposed
between a first pair of reflector elements 1250 and a second pair
of two reflector elements 1252. The 17 IDT fingers of the second
cell 1230-2 are disposed between the second pair of reflector
elements 1252 and a third pair of reflector elements 1254. The
first cell 1230-1 and the second cell 1230-2 effectively share the
second pair of reflector elements 1252.
[0086] The reflector elements 1250, 1252, 1254 are configured to
reduce leakage of acoustic energy from the cells as previously
described. The second pair of reflector elements 1252 are separated
by an opening 1260 that passes thought the diaphragm. In this
example, the opening 1260 is a rectangular slot, but may have some
other shape, such as an array of circular or square holes. The
opening 1260 may provide a passage for an etchant used to form the
cavity behind the piezoelectric plate.
[0087] The examples 1000, 1100, and 1200 of FIG. 10, FIG. 11, and
FIG. 12 respectively, all show two cells on a common diaphragm that
spans a common cavity. However, there may be more or fewer than two
cells on a common diaphragm. For example, an XBAR partitioned into
six cells may comprise six diaphragms with one cell per diaphragm
(as shown in FIG. 6A and FIG. 6B), three diaphragms with two cells
per diaphragm (as shown in FIG. 10), two diaphragms with three
cells per diaphragm, or one diaphragm with all six cells. In all
cases, each diaphragm spans a respective cavity. An XBAR may be
partitioned into more or fewer than six cells. An XBAR may be
partitioned into n cells, where n is an integer greater than one,
which may be arranged on one to n diaphragms.
CLOSING COMMENTS
[0088] Throughout this description, the embodiments and examples
shown should be considered as exemplars, rather than limitations on
the apparatus and procedures disclosed or claimed. Although many of
the examples presented 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 flowcharts, additional and
fewer steps may be taken, and the steps as shown may be combined or
further refined to achieve 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.
[0089] As used herein, "plurality" means two or more. As used
herein, a "set" of items may include one or more of such items. As
used herein, whether in the written description or the claims, the
terms "comprising", "including", "carrying", "having",
"containing", "involving", and the like are to be understood to be
open-ended, i.e., to mean including but not limited to. Only the
transitional phrases "consisting of" and "consisting essentially
of", respectively, are closed or semi-closed transitional phrases
with respect to claims. Use of ordinal terms such as "first",
"second", "third", etc., in the claims to modify a claim element
does not by itself connote any priority, precedence, or order of
one claim element over another or the temporal order in which acts
of a method are performed, but are used merely as labels to
distinguish one claim element having a certain 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 the alternatives also
include any combination of the listed items.
* * * * *