U.S. patent application number 17/460131 was filed with the patent office on 2022-02-24 for transversely-excited film bulk acoustic resonator with tether-supported diaphragm.
The applicant listed for this patent is Resonant Inc.. Invention is credited to Ventsislav Yantchev.
Application Number | 20220060168 17/460131 |
Document ID | / |
Family ID | |
Filed Date | 2022-02-24 |
United States Patent
Application |
20220060168 |
Kind Code |
A1 |
Yantchev; Ventsislav |
February 24, 2022 |
TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH
TETHER-SUPPORTED DIAPHRAGM
Abstract
An acoustic resonator device includes a substrate and a
piezoelectric plate. A first portion of the piezoelectric plate is
on the substrate. A second portion of the piezoelectric forms a
diaphragm suspended over a cavity in the substrate. An interdigital
transducer (IDT) is on a surface of the piezoelectric plate, the
IDT including first and second busbars on the first portion and
interleaved IDT fingers on the diaphragm. A plurality of tethers
support the diaphragm over the cavity, each tether providing an
electrical connection between a corresponding one of the
interleaved IDT fingers and one of the first and second
busbars.
Inventors: |
Yantchev; Ventsislav;
(Sofia, BG) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Resonant Inc. |
Austin |
TX |
US |
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|
Appl. No.: |
17/460131 |
Filed: |
August 27, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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17108984 |
Dec 1, 2020 |
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17460131 |
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63067326 |
Aug 19, 2020 |
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International
Class: |
H03H 9/02 20060101
H03H009/02; H03H 9/205 20060101 H03H009/205; H03H 9/54 20060101
H03H009/54 |
Claims
1. An acoustic resonator device comprising: a substrate; a
piezoelectric plate, a first portion of the piezoelectric plate on
the substrate, and a second portion of the piezoelectric plate
forming a diaphragm suspended over a cavity in the substrate; an
interdigital transducer (IDT) on a surface of the piezoelectric
plate, the IDT comprising first and second busbars on the first
portion and interleaved IDT fingers on the diaphragm; and a
plurality of tethers supporting the diaphragm over the cavity, each
tether providing an electrical connection between a corresponding
one of the interleaved IDT fingers and one of the first and second
busbars.
2. The acoustic resonator device of claim 1, wherein the first and
second busbars are on opposite sides of the cavity.
3. The acoustic resonator device of claim 2, wherein the
interleaved IDT fingers are connected to the first and second
busbars alternately.
4. The acoustic resonator device of claim 1, wherein the
interleaved IDT fingers comprise a first conductor level, and each
tether of the plurality of tethers comprises the first conductor
level and a corresponding section of the piezoelectric plate
connecting the first portion and the diaphragm.
5. The acoustic resonator device of claim 4, wherein each of the
first and second busbars comprises the first conductor level and a
second conductor level.
6. The acoustic resonator device of claim 1, wherein a portion of
each tether of the plurality of tethers forms an oblique angle with
a long direction of the corresponding interleaved IDT finger.
7. The acoustic resonator device of claim 6, wherein the oblique
angle is greater than or equal to 30 degrees and less than or equal
to 60 degrees.
8. The acoustic resonator device of claim 1, wherein at least a
portion of each tether of the plurality of tethers is curved.
9. The acoustic resonator device of claim 1, wherein each tether of
the plurality of tethers is continuously curved.
10. The acoustic resonator device of claim 1, each tether of the
plurality of tethers comprising: a first segment extending from the
first or second busbar; a third segment extending from the
diaphragm colinear with the corresponding IDT finger; and a second
segment joining the first segment and the third segment, the second
segment forming an oblique angle with a long direction of the
corresponding IDT finger.
11. The acoustic resonator device of claim 1, wherein a width of
each tether of the plurality of tethers is greater than a width of
the corresponding IDT finger.
12. The acoustic resonator device of claim 11, wherein a width of
each tether of the plurality of tethers is one-half of a pitch of
the IDT fingers.
13. An acoustic resonator device comprising: a piezoelectric plate
on a substrate, a portion of the piezoelectric plate forming a
diaphragm suspended over a cavity in the substrate; an interdigital
transducer (IDT) on the piezoelectric plate, the IDT comprising
interleaved IDT fingers on the diaphragm and first and second
busbars not on the diaphragm; and a plurality of tethers supporting
the diaphragm over the cavity, each tether electrically connecting
a corresponding one of the interleaved IDT fingers and one of the
first and second busbars by spanning an open space between the
diaphragm and the one of the first and second busbars.
14. The acoustic resonator device of claim 13, wherein the first
and second busbars are on opposite sides of the cavity.
15. The acoustic resonator device of claim 14, wherein the
interleaved IDT fingers are connected to the first or second
busbars alternately.
16. The acoustic resonator device of claim 13, wherein the
interleaved IDT fingers comprise a first conductor level, and each
tether of the plurality of tethers comprises the first conductor
level and a corresponding portion of the piezoelectric plate.
17. The acoustic resonator device of claim 16, wherein each of the
first and second busbars comprises the first conductor level and a
second conductor level.
18. The acoustic resonator device of claim 13, wherein a section of
each tether of the plurality of tethers forms an oblique angle with
a long direction of the corresponding interleaved IDT finger.
19. The acoustic resonator device of claim 18, wherein the oblique
angle is greater than or equal to 30 degrees and less than or equal
to 60 degrees.
20. The acoustic resonator device of claim 18, each tether of the
plurality of tethers comprising: a first segment extending from the
first or second busbar; a third segment extending from the
diaphragm colinear with the corresponding IDT finger; and a second
segment joining the first segment and the third segment, the second
segment forming the oblique angle with the long direction of the
corresponding interleaved IDT finger.
21. The acoustic resonator device of claim 13, wherein at least a
portion of each tether of the plurality of tethers is curved.
22. The acoustic resonator device of claim 13, wherein each tether
of the plurality of tethers is continuously curved.
23. The acoustic resonator device of claim 13, wherein a width of
each tether of the plurality of tethers is greater than a width of
the corresponding interleaved IDT finger.
24. The acoustic resonator device of claim 23, wherein a width of
each tether of the plurality of tethers is one-half of a pitch of
the interleaved IDT fingers.
Description
RELATED APPLICATION INFORMATION
[0001] This patent is a continuation of application Ser. No.
17/108,984, filed Dec. 1, 2020, entitled TRANSVERSELY-EXCITED FILM
BULK ACOUSTIC RESONATOR WITH TETHER-SUPPORTED DIAPHRAGM, which
claims priority to provisional patent application 63/067,326, filed
Aug. 19, 2020, entitled XBAR WITH TETHER-SUPPORTED DIAPHRAGM, which
is incorporated herein by reference.
NOTICE OF COPYRIGHTS AND TRADE DRESS
[0002] 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
[0003] 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
[0004] 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 passband or stop-band depend on the 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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 (5G)
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.
[0010] The Transversely-Excited Film Bulk Acoustic Resonator (XBAR)
is an acoustic resonator structure for use in microwave filters.
The XBAR is described in patent 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
[0011] FIG. 1 includes a schematic plan view, two schematic
cross-sectional views, and a detail view of a transversely-excited
film bulk acoustic resonator (XBAR).
[0012] FIG. 2 is a plan view of an XBAR with a tether-supported
diaphragm.
[0013] FIG. 3A is a schematic cross-sectional view of the XBAR with
a tether-supported diaphragm at a section C-C defined in FIG.
2.
[0014] FIG. 3B is a schematic cross-sectional view of the XBAR with
a tether-supported diaphragm at a section D-D defined in FIG.
2.
[0015] FIG. 4A is a schematic cross-sectional view of the XBAR with
a tether-supported diaphragm at a section E-E defined in FIG.
2.
[0016] FIG. 4B is an alternative schematic cross-sectional view of
the XB AR with a tether-supported diaphragm at a section E-E
defined in FIG. 2.
[0017] FIG. 5 is a schematic plan view illustrating flexion of a
single tether.
[0018] FIG. 6 is a graph of the displacement of the diaphragm of a
conventional XBAR and a tether-supported diaphragm due to a
twenty-degree temperature change.
[0019] FIG. 7 is flow chart of a method for fabricating an XBAR
with a tether-supported diaphragm.
[0020] 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
[0021] Description of Apparatus
[0022] FIG. 1 shows a simplified schematic top view and orthogonal
cross-sectional views of an 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.
[0023] The XBAR 100 is made up of a thin film conductor pattern
formed on a surface of a piezoelectric plate 110 having parallel
front and back surfaces 112, 114, respectively. The piezoelectric
plate is a thin single-crystal layer of a piezoelectric material
such as lithium niobate, lithium tantalate, lanthanum gallium
silicate, gallium nitride, or aluminum nitride. The piezoelectric
plate is cut such that the orientation of the X, Y, and Z
crystalline axes with respect to the front and back surfaces is
known and consistent. The piezoelectric plate may be Z-cut, which
is to say the Z axis is normal to the front and back surfaces 112,
114. The piezoelectric plate may be rotated Z-cut or rotated
YX-cut. XBARs may be fabricated on piezoelectric plates with other
crystallographic orientations.
[0024] The back surface 114 of the piezoelectric plate 110 is
attached to a surface of a substrate 120 except for a portion of
the piezoelectric plate 110 that forms a diaphragm 115 spanning a
cavity 140 formed in the substrate. The portion of the
piezoelectric plate that spans the cavity is referred to herein as
the "diaphragm" 115 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". In
other configurations, the diaphragm 115 may be contiguous with the
piezoelectric plate around at least 50% of the perimeter 145 of the
cavity 140.
[0025] 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 bonded to the substrate 120 using a wafer bonding process.
Alternatively, the piezoelectric plate 110 may be grown on the
substrate 120 or 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 material layers (not shown in FIG. 1).
[0026] "Cavity" has its conventional meaning of "an empty space
within a solid body." 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 under the diaphragm 115 (as shown
in FIG. 4B). 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.
[0027] The conductor pattern of the XBAR 100 includes an
interdigital transducer (IDT) 130. The IDT 130 includes a first
plurality of parallel fingers, such as finger 136, extending from a
first busbar 132 and a second plurality of fingers extending from a
second busbar 134. The term "busbar" means a conductor from which
the fingers of an IDT extend. 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.
[0028] 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. 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.
[0029] 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 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.
[0030] 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. A
typical XBAR has more than ten parallel fingers in the IDT 130. An
XBAR may have hundreds, possibly thousands, of parallel fingers in
the IDT 130. Similarly, the thicknesses of the IDT fingers and the
piezoelectric plate in the cross-sectional views are greatly
exaggerated.
[0031] Referring now to the detailed schematic cross-sectional
view, a front-side dielectric layer 150 may optionally be formed on
the front side of the piezoelectric plate 110. The "front side" of
the XBAR is, by definition, the surface facing away from the
substrate. The front-side dielectric layer 150 may be formed only
between the IDT fingers (e.g. IDT finger 138b) or may be deposited
as a blanket layer such that the dielectric layer is formed both
between and over the IDT fingers (e.g. IDT finger 138a). The
front-side dielectric layer 150 may be a non-piezoelectric
dielectric material, such as silicon dioxide, alumina, or silicon
nitride. A thickness of the front side dielectric layer 150 is
typically less than about one-third of the thickness of the
piezoelectric plate 110. The front-side dielectric layer 150 may be
formed of multiple layers of two or more materials. In some
applications, a back-side dielectric layer (not shown) may be
formed on the back side of the piezoelectric plate 110.
[0032] The IDT fingers 138a, 138b may be one or more layers of
aluminum, an aluminum alloy, copper, a copper alloy, beryllium,
gold, tungsten, molybdenum, chromium, titanium or some other
conductive material. The IDT fingers are considered to be
"substantially aluminum" if they are formed from aluminum or an
alloy comprising at least 50% aluminum. The IDT fingers are
considered to be "substantially copper" if they are formed from
copper or an alloy comprising at least 50% copper. Thin (relative
to the total thickness of the conductors) layers of metals such as
chromium or titanium may be formed under and/or over and/or as
layers within the fingers to improve adhesion between the fingers
and the piezoelectric plate 110 and/or to passivate or encapsulate
the fingers and/or to improve power handling. The busbars (132, 134
in FIG. 1) of the IDT may be made of the same or different
materials as the fingers.
[0033] 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 "mark" of
the IDT fingers. The geometry of 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 w of the fingers. In
addition, the pitch p of the IDT is typically 2 to 20 times the
thickness of the piezoelectric plate 210. The width of the IDT
fingers in an XBAR is not constrained to be near 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
readily fabricated using optical lithography. The thickness of the
IDT fingers may be from 100 nm to about equal to the width w. The
thickness of the busbars (132, 134) of the IDT may be the same as,
or greater than, the thickness tm of the IDT fingers.
[0034] FIG. 2 is a plan view of an XBAR 200 with a tether-supported
diaphragm. Like the XBAR 100 of FIG. 1, the XBAR 200 has a
piezoelectric plate 210. A cavity 240 having a perimeter 245 is
formed in a substrate (not visible) beneath the piezoelectric plate
210. A first portion (i.e. the portion outside of the cavity
perimeter 245) of the piezoelectric plate 210 is attached to the
substrate. Another portion of the piezoelectric plate 210 forms a
diaphragm 215 suspended over the cavity 240. Unlike the diaphragm
115 of FIG. 1, the diaphragm 215 is not contiguous with the rest of
the piezoelectric plate 210. Rather, the diaphragm 215 is separated
from the piezoelectric plate by space open to the cavity 240. The
open space is bridged only by a plurality of tethers, of which
tethers 250 and 252 are identified in FIG. 2.
[0035] The tethers, such as tethers 250, 252, serve three purposes.
First, the tethers provide mechanical support to suspend the
diaphragm 215 over the cavity 240. Second, the tethers provide, by
flexing, a means to absorb stresses placed on the diaphragm by
changes in the device temperature. Third, the tethers provide
electrical and thermal connections from first and second IDT
busbars 232, 234 to the interleaved IDT fingers (of which only IDT
fingers 236, 238 are identified in FIG. 2) disposed on the
diaphragm 215. The first and second busbars 232, 234 are disposed
on the first portion (the portion attached to the substrate) of the
piezoelectric plate 210 on opposite sides of the cavity 240. Since
each IDT finger must be connected to one of the busbars 232, 234,
there is a one-to-one correspondence between IDT fingers and
tethers. Alternate IDT fingers are connected to either busbar 232
or busbar 234 via respective tethers.
[0036] FIG. 3A and FIG. 3B are cross-sectional views of the XBAR
200 at sections C-C and D-D, respectively. As identified in FIG. 2,
sections C-C and D-D are not planar. Section C-C follows a path
along the center of tether 250 and IDT finger 236. Section D-D
follows a path along the center of tether 252 and IDT finger
238.
[0037] Referring back to FIG. 3A and FIG. 3B, the piezoelectric
plate 210 is attached to a substrate 320. The piezoelectric plate
210 may be single-crystal lithium niobate, lithium tantalate, or
some other piezoelectric material. The orientation of the axes of
the piezoelectric plate 210 are known and consistent. The
piezoelectric plate 210 may be Z-cut, rotated Z-cut, rotated YX
cut, or some other orientation. The substrate 320 may be silicon or
some other material that can be anisotropically etched to form the
cavity 240.
[0038] The diaphragm 215 is a portion of the piezoelectric plate
210. The IDT fingers 236/238 are formed by a first conductor level
362. The first conductor level 362 may be one or more layers of
metal as previously described. The diaphragm 215 is suspended over
the cavity 240 and supported by the tethers 250, 252. The tethers
250,252 are formed by portions of the piezoelectric plate 210 and
the first conductor level 362. A second conductor level 364 may be
formed over all or portions of the busbars 232, 234 to improve
thermal and electric conductivity.
[0039] FIG. 4A and FIG. 4B are alternative cross-sectional views of
the XBAR 200 at section E-E defined in FIG. 2. These views of the
XBAR 200 are comparable to the cross-sectional view A-A of the XBAR
100 shown in FIG. 1.
[0040] Portions of the piezoelectric plate 210 are attached to and
supported by the substrate 320. A portion of the piezoelectric
plate 210 forms the diaphragm 215 suspended over a cavity 240, 240'
formed in the substrate 320. Unlike the XBAR 100 of FIG. 1, the
diaphragm 215 of the XBAR 200 is not contiguous with the supported
portions of the piezoelectric plate 210, but is separated from the
supported portions of the piezoelectric plate 210 by spaces 410,
415. Interleaved IDT fingers, such as fingers 236, 238, are
disposed on the diaphragm 215. The IDT fingers are formed of a
first conductor level 362.
[0041] In FIG. 4A, the cavity 240 penetrates completely though the
substrate 320. In FIG. 4B, the cavity 240' is a recess in the
substrate 320. In this case, the cavity 240' may be formed by
etching the substrate using an etchant introduced through the
spaces around the diaphragm 215 and spaces between the tethers (not
visible in FIG. 4A or 4B).
[0042] Although not shown in FIG. 3A, FIG. 3B, FIG. 4A, and FIG.
4B, the XBAR 200 may include one or more dielectric layers. For
example, the XBAR 200 may include a bonding layer disposed between
the substrate 320 and the piezoelectric plate 210. When the
substrate is silicon, the bonding layer may be, for example,
silicon dioxide. When a bonding layer is present between the
substrate 320 and the piezoelectric plate 210, the bonding layer
may remain or be removed from the back side (i.e. the side facing
the cavity 240) of the diaphragm 215.
[0043] When multiple XBARs are connected in a ladder filter
circuit, a dielectric frequency setting layer may be formed over
the diaphragms and IDT fingers of shunt resonators to lower the
resonance frequencies of the shunt resonators relative to the
resonance frequencies of series resonators. Further, a thin
passivation dielectric layer may be applied over most or all of the
XBAR 200 to passivate and seal the surface.
[0044] FIG. 5 is an expanded plan view of a single tether 550. The
line 515 is the edge of a diaphragm, and the line 532 is the edge
of an IDT busbar and cavity. The region between the edge of the
diaphragm 515 and the edge of the busbar 532 is a space open to a
cavity 540 beneath the diaphragm. A plurality of tethers, including
tether 550, span this space to hold the diaphragm 515 suspended
over the cavity 540. The tether 550 provides electric and thermal
connections between the busbar 532 and an IDT finger 536 disposed
on the surface of the diaphragm 515.
[0045] The tether 550 includes a first segment 552 extending from
the busbar 532, a third segment 556 extending from the diaphragm
515, and a second segment 554 connecting the first and third
sections 552, 556. The second segment 554 is configured to flex in
the plane of the diaphragm. To this end, the second segment 554
forms an oblique angle with respect to the side of the busbar 532
and the long direction of the IDT finger 536, which is to say the
second segment 554 is not parallel to or perpendicular to either
the side of the busbar 532 and the long direction of the IDT finger
536. For example, the angle .theta. between the side of the second
segment 554 and the long direction of the IDT finger 536 may be 30
to 60 degrees.
[0046] In the example of FIG. 5, the first segment 552 extends
perpendicularly from the edge of the busbar 532. The first segment
552 may extend from the side of the busbar 532 at some other angle.
The first segment 552 may not be present, in which case the second
segment 554 will extend from the side of the busbar 532 at an
oblique angle.
[0047] In the example of FIG. 5, the third segment 556 extends
perpendicularly from the edge of the diaphragm 515 and is colinear
with the corresponding IDT finger 536. The third segment 556 may
extend from the side of the diaphragm 515 at some other angle. The
third segment 556 may not be present, in which case the second
segment 554 will extend from the side of the diaphragm 515 at an
oblique angle.
[0048] The distance, at a nominal temperature, between the side of
the busbar 532 and the side of the diaphragm 515 is the dimension
d4, which is the total distance spanned by the tether 550. The
second segment 554 of the tether 550 spans a distance d2, which is
greater than or equal to 50% of d4. In other words,
d2.gtoreq.d1+d3, where d1 and d3 are that distances spanned by the
first and third tether segments 552, 556. Either or both of d1 and
d3 may be zero. All of d1, d2, d3, and d4 are measured
perpendicular to the side of the busbar 532.
[0049] The tether 550 may have some shape other than three straight
sections. For example, some or all of the corners where the
segments 552, 554, 556 intersect may be rounded. The tether 550 may
be continuously curved, in which case at least a potion of the
curved tether forms an oblique angle with respect to the side of
the busbar 532 and the long direction of the IDT finger 536.
[0050] When an XBAR is used in a filter, some power is dissipated
on the diaphragm due to resistive losses in the IDT fingers and
acoustic or viscose losses in the IDT fingers and the diaphragm
itself. The primary path for removing heat from the diaphragm is
conduction along the IDT fingers to the busbar and then to the
device substrate. The presence of tethers increases the length of
the heat flow path from the diaphragm to the substrate compared to
a conventional XBAR. To reduce the impact of the longer heat flow
path, the width (dimension wt in FIG. 5) of the tethers may be
larger than the width (dimension w in FIG. 5) of the IDT fingers.
The width of the tethers may be, for example, p/2, where p is the
pitch of the IDT.
[0051] All of the components of an XBAR have a respective
temperature coefficient of expansion (TCE). A preferred substrate
for XBAR devices is a silicon wafer, which has low cost and
well-developed processes for forming cavities. However, the TCE of
silicon is substantially lower than the TCE of lithium niobate or
lithium tantalate, which are the preferred materials for the
piezoelectric plate. The expansion or contraction of the diaphragm
of an XBAR in response to a change in temperature will be greater
in magnitude than the expansion or contraction of the surrounding
area (which is dominated by the low TCE of the silicon substrate).
The difference in expansion or contraction can cause bowing or
rippling of the diaphragm.
[0052] In FIG. 5, the dashed line 515' represents the position of
the edge of the diaphragm (relative to the edge of the busbar 532)
after a temperature increase of 25 degrees Celsius. The dimension
d4' is the new distance from the edge of the busbar 532 to the edge
of the diaphragm 515', which is less than the original distance d4.
The new position and shape of the tether 550' is shown in dashed
lines. The tether 550' has flexed to absorb the change in the
distance from the edge of the busbar to the edge of the
diaphragm.
[0053] FIG. 6 is a graph of the displacement of the diaphragm of a
conventional XBAR and a tether-supported diaphragm due to a
twenty-degree temperature change. Specifically, the solid line 610
is a plot of the displacement normal to the plane of the diaphragm
for a diaphragm that is fully contiguous with the rest of the
piezoelectric plate as shown in FIG. 1. A temperature change of 20
degrees causes the diaphragm to ripple with a peak-to-peak
amplitude of 850 nm. The displacement along the center of the
diaphragm was determined by simulation using a finite element
method. In this example, the aperture and length of the IDT are 50
microns and 325 microns, respectively.
[0054] The dashed line 620 is a plot of the displacement normal to
the plane of the diaphragm for a tether supported diaphragm as
shown in FIG. 2. A temperature change of 20 degrees causes the
diaphragm to curl up about 40 nm at the ends of the IDT. The
difference between the solid line 610 and the dashed line 620 is
evidence of the effectiveness of the tethers for absorbing
differences in the TCE of the diaphragm and the substrate and thus
reducing stress in the diaphragm.
[0055] Description of Methods
[0056] FIG. 7 is a simplified flow chart of a process 700 for
making an XBAR with a tether-supported diaphragm or a filter
incorporating such XBARs. The process 700 starts at 705 with a
substrate and a plate of piezoelectric material and ends at 795
with a completed XBAR or filter. The flow chart of FIG. 7 includes
only major process steps. Various conventional process steps (e.g.
surface preparation, cleaning, inspection, baking, annealing,
monitoring, testing, etc.) may be performed before, between, after,
and during the steps shown in FIG. 7.
[0057] The piezoelectric plate may be, for example, lithium niobate
or lithium tantalate. The piezoelectric plate may be Z-cut, rotated
Z-cut, or rotated YX-cut. The piezoelectric plate may be some other
material and/or some other cut. The substrate may be a silicon
wafer or a silicon on insulator wafer. The substrate may be a wafer
of some other material that allows formation of deep cavities by
etching or other processing.
[0058] The substrate and the piezoelectric plate will be bonded
together at 720. Prior to bonding, optional steps may be taken to
prepare the substrate. For example, at 710A, lateral and/or
vertical etch stops may be formed in the substrate. A lateral etch
stop is a structure to constrain the lateral extend of a
subsequently etched cavity. A vertical etch stop is a structure to
limit the depth of a subsequently etched cavity. Lateral and
vertical etch stops may be formed in the substrate as described in
pending patent application Ser. No. 16/913,417, titled
TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR WITH LATERAL ETCH
STOP, which is incorporated herein by reference. Alternatively, at
710B, the cavities may be formed in the substrate and then filled
with a sacrificial material that will be subsequently removed.
[0059] At 720, the piezoelectric plate is bonded to the substrate.
The piezoelectric plate and the substrate may be bonded by a wafer
bonding process. Typically, the mating surfaces of the substrate
and the piezoelectric plate are highly polished. One or more layers
of intermediate materials, such as an oxide or metal, may be formed
or deposited on the mating surface 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 layers.
[0060] A conductor pattern, including IDTs of each XBAR, is formed
at 730 by depositing and patterning one or more conductor layer on
the front side of the piezoelectric plate. The conductor layers may
be, for example, aluminum, an aluminum alloy, copper, a copper
alloy, titanium, chrome, tungsten, molybdenum, or some other
conductive metal. Optionally, one or more layers of other materials
may be disposed below (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, chrome, or other metal may be
used to improve the adhesion between a conductor layer and the
piezoelectric plate. The conductor pattern formed at 730 includes
the first metal level 362 of the IDT fingers, the tethers, and the
busbars. The conductor pattern formed at 730 may also include the
second conductor level 364 to improve the electrical and thermal
conductivity of portions of the conductor pattern (for example the
IDT busbars and interconnections between the IDTs).
[0061] The conductor pattern may be formed at 730 by depositing the
conductor layers and, optionally, one or more other metal layers in
sequence over the surface of the piezoelectric plate. The excess
metal may then be removed by etching through patterned photoresist.
The conductor layer can be etched, for example, by plasma etching,
reactive ion etching, wet chemical etching, and other etching
techniques.
[0062] Alternatively, the conductor pattern may be formed at 730
using a lift-off process. Photoresist may be deposited over the
piezoelectric plate and patterned to define the conductor pattern.
The conductor layers and, optionally, one or more other layers may
be deposited in sequence over the surface of the piezoelectric
plate. The photoresist may then be removed, which removes the
excess material, leaving the conductor pattern.
[0063] The two metal levels 362, 364 (shown in FIGS. 3A and 3B)
and/or layers within either metal level may be deposited and
patterned using different processes.
[0064] At 740, a front-side dielectric layer may be formed by
depositing one or more layers of dielectric material on the front
side of the piezoelectric plate. The one or more dielectric layers
may be deposited using a conventional deposition technique such as
sputtering, evaporation, or chemical vapor deposition. The 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 lithography processes (using photomasks)
may be used to limit the deposition of the dielectric layers to
selected areas of the piezoelectric plate, such as only between the
interleaved fingers of the IDTs. Masks may also be used to allow
deposition of different thicknesses of dielectric materials on
different portions of the piezoelectric plate.
[0065] After the conductor pattern and dielectric layers are
formed, the tethers may be defined at 745 by etching the
piezoelectric plate between the tethers and around the perimeter of
the diaphragm.
[0066] One or more cavities are then formed in the substrate at
750. A separate cavity may be formed for each resonator in a filter
device. For example, the one or more cavities may be formed by
etching the substrate using an etchant introduced through openings
in the piezoelectric plate formed at 745. A separate cavity may be
formed for each resonator in a filter device. The extent of the
cavities may be defined by lateral and/or vertical etch stops
previously formed in the substrate at 710A. Alternatively, the
cavities may be formed at 750 by etching or otherwise removing the
sacrificial material filling the cavities previously formed at
710B.
[0067] In all variation of the process 700, the filter device is
completed at 760. Actions that may occur at 760 include depositing
an encapsulation/passivation layer such as SiO.sub.2 or
Si.sub.3O.sub.4 over all or a portion of the device; forming
bonding pads or solder bumps or other means for making connection
between the device and external circuitry; excising individual
devices from a wafer containing multiple devices; other packaging
steps; and testing. Another action that may occur at 760 is to tune
the resonant frequencies of the resonators within the device by
adding or removing metal or dielectric material from the front side
of the device. After the filter device is completed, the process
ends at 795.
[0068] Closing Comments
[0069] 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.
[0070] 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.
* * * * *