U.S. patent application number 09/484804 was filed with the patent office on 2002-05-30 for thin film resonator and method.
Invention is credited to Panasik, Carl M..
Application Number | 20020063497 09/484804 |
Document ID | / |
Family ID | 23925672 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020063497 |
Kind Code |
A1 |
Panasik, Carl M. |
May 30, 2002 |
Thin Film Resonator And Method
Abstract
A thin film resonator and method includes a first electrode
(110) and a second electrode (112) substantially parallel to the
first electrode (110). An intermediate layer (120) is disposed
between and coupled to the first and second electrode (110, 112).
The intermediate layer (120) includes a first piezoelectric layer
(122), a second piezoelectric layer (124), and a spacer layer (130)
disposed between the first and second piezoelectric layers (122,
124). The spacer layer (130) has an acoustic impedance
substantially the same as the first and second piezoelectric layers
(122, 124) and is formed of a disparate material.
Inventors: |
Panasik, Carl M.; (Garland,
TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
23925672 |
Appl. No.: |
09/484804 |
Filed: |
January 18, 2000 |
Current U.S.
Class: |
310/364 |
Current CPC
Class: |
H03H 3/02 20130101; H03H
9/175 20130101 |
Class at
Publication: |
310/364 |
International
Class: |
H01L 041/04 |
Claims
What is claimed is:
1. An acoustic resonator, comprising: a first electrode; a second
electrode substantially parallel to the first electrode; an
intermediate layer disposed between and coupled to the first and
second electrodes; the intermediate layer comprising a first
piezoelectric layer, a second piezoelectric layer, and a spacer
disposed between the first piezoelectric layer and the second
piezoelectric layer; and the spacer layer having an acoustic
impedance substantially the same as that of the first and second
piezoelectric layers and comprising a disparate material.
2. The acoustic resonator of claim 1, the resonator comprising a
thin film resonator.
3. The acoustic resonator of claim 1, the intermediate layer
comprising a thickness substantially an acoustic half-wavelength of
a target wavelength for the resonator.
4. The acoustic resonator of claim 1, the spacer layer comprising a
non-piezoelectric material.
5. The acoustic resonator of claim 1, the first and second
piezoelectric layers each comprising a thickness of about a half
micron or less.
6. The acoustic resonator of claim 1, the spacer further comprising
silicon dioxide (SiO.sub.2).
7. The acoustic resonator of claim 1, the spacer comprising a
coefficient of thermal expansion substantially the opposite as that
of the first and second piezoelectric layers.
8. The acoustic resonator of claim 1, the intermediate layer
further comprising a thickness of at least three microns.
9. The acoustic resonator of claim 1, the first piezoelectric layer
disposed adjacent to the first electrode and the second
piezoelectric layer disposed adjacent to the second electrode.
10. A method for fabricating an acoustic resonator, comprising:
disposing an intermediate layer between a first electrode and a
second electrode; including a spacer between a first piezoelectric
layer and a second piezoelectric layer of the intermediate layer;
and substantially matching an acoustic impedance of the spacer to
that of the first and second piezoelectric layers.
11. The method of claim 10, wherein the acoustic resonator is a
thin film resonator.
12. The method of claim 10, wherein the intermediate layer
comprises a thickness substantially an acoustic half-wavelength of
a target wavelength for the acoustic resonator.
13. The method of claim 10, wherein the spacer comprises a
non-piezoelectric material.
14. The method of claim 10, wherein the first and second
piezoelectric layer each comprising a thickness of about a half
micron or less.
15. The method of claim 10, wherein the spacer comprises silicon
dioxide (SiO.sub.2)
16. The method of claim 10, further comprising mismatching the
coefficient of thermal expansion of the spacer to that of the first
and second piezoelectric layers.
17. The method of claim 10, wherein the intermediate layer
comprises a thickness greater than three microns.
18. The method of claim 10, further comprising disposing the first
piezoelectric layer adjacent to the first electrode and disposing
the second piezoelectric layer adjacent to the second
electrode.
19. A thin film resonator, comprising: a first electrode; a second
electrode substantially parallel to and spaced apart from the first
electrode; an intermediate layer disposed between and coupled to
the first and second electrodes and having a thickness
substantially an acoustic half-wavelength of a target frequency for
the thin film resonator; the intermediate layer comprising a first
piezoelectric layer adjacent to the first electrode, a second
piezoelectric layer adjacent to the second electrode, and a spacer
layer disposed between the first and second piezoelectric layer;
and the spacer layer comprising a non-piezoelectric material and
having an acoustic impedance substantially the same as that of the
first and second piezoelectric layers.
20. The thin film resonator of claim 19, wherein the thin film
resonator is a 900 megahertz (MHz) resonator and the intermediate
layer has a thickness of approximately 3.5 microns.
21. The thin film resonator of claim 19, the first and second
piezoelectric layers comprising zinc oxide (ZnO) and the spacer
layer comprises silicon dioxide (SiO.sub.2).
22. A method for fabricating a thin film resonator, comprising:
forming a first electrode layer outwardly of an acoustic reflector;
forming a first piezoelectric layer outwardly of the first
electrode layer; forming a spacer layer outwardly of the first
piezoelectric layer, the spacer layer having acoustic impedance
substantially the same as that of the first piezoelectric layer and
a second piezoelectric layer; forming the second piezoelectric
layer outwardly of the spacer layer; forming a second electrode
layer outwardly of the second piezoelectric layer; and patterning
and etching one or more of the first electrode layer, first
piezoelectric layer, spacer layer, second piezoelectric layer, and
second electrode layer to form the thin film resonator outwardly of
the acoustic reflector.
23. The method of claim 22, wherein a coefficient of thermal
expansion of the spacer layer is opposite that of the first and
second piezoelectric layer.
24. The method of claim 22, wherein the first and second
piezoelectric layers comprise zinc oxide (ZnO) and the spacer layer
comprises silicon dioxide (SiO.sub.2).
25. An integrated circuit chip including a plurality of on-chip
filters, at least one of the filters comprising a thin film
resonator, the thin film resonator comprising: a first electrode; a
second electrode substantially parallel to the first electrode; an
intermediate layer disposed between and coupled to the first and
second electrode; the intermediate layer comprising a first
piezoelectric layer, a second piezoelectric layer, and a spacer
layer disposed between the first piezoelectric layer and the second
piezoelectric layer; and the spacer layer having an acoustic
impedance substantially the same as that of the first and second
piezoelectric layers and comprising a disparate material.
26. The integrated circuit chip of claim 25, the intermediate layer
comprising a thickness substantially a half-wavelength of a target
frequency for the thin film resistor.
27. The integrated circuit chip of claim 25, the spacer layer
comprising a non-piezoelectric material.
28. The integrated circuit chip of claim 25, the first and second
piezoelectric layers each comprising a thickness of about a half
micron or less and the intermediate layer comprising a thickness of
about three microns or more.
29. The integrated circuit chip of claim 25, the spacer layer
comprising a coefficient of thermal expansion substantially
opposite of that of the first and second piezoelectric layers.
30. The integrated circuit chip of claim 25, further comprising the
first piezoelectric layer disposed adjacent to the first electrode
and the second piezoelectric layer disposed adjacent to the second
electrode.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates generally to the field of
frequency selection elements, and more particularly to a thin film
resonator and method.
BACKGROUND OF THE INVENTION
[0002] Televisions and radios as well as cellular phones and other
wireless devices all transmit and/or receive radio frequency
signals. Televisions and radios, for example, receive programming
from a number of stations in the form of radio frequency signals
that are transmitted by the stations. Cellular phones and other
two-way wireless communication devices communicate with a base
station by both transmitting and receiving radio frequency signals.
The radio frequency signals include voice traffic for a wireless
telephone connection or data traffic for a wireless Internet or
other network connection.
[0003] Televisions, radios, cellular phones and other wireless
devices are each assigned to different radio frequencies to allow
simultaneous operation of the devices within an area. Television,
for example, receives signals within the 55 to 800 megahertz (MHz)
range while radio receives signals within the 530 to 1,700
kilohertz (kHz) range for AM and within the 88 to 108 megahertz
(MHz) range for FM. Cellular phones, in accordance with U.S.
standards, operate in the 900 and 1800 megahertz (MHz) range.
[0004] Televisions, radios, cellular phones, and other wireless
devices each use radio frequency filters to separate out unwanted
radio frequency traffic from a desired signal, or channel. In
particular, televisions and radios use a number of filters to form
a tuner that allows each of the received stations to be selectively
tuned. Cellular phones operate at a preset frequency range and
include filters dedicated to that frequency range. In each case,
the filters discriminate between signals based on frequency
diversity to provide a stable signal for use by the receiving
device.
[0005] Radio frequency filters based on resonators are constructed
from pairs of inductors and capacitors arranged in parallel, from
crystal resonators and from thin film resonators. The inductor and
capacitor configuration resonates in a broad range and therefore
provides low quality signal discrimination. Crystal and thin film
resonators, on the other hand, resonate in a narrow range and
therefore provide high quality signal discrimination.
[0006] Crystal resonators include a crystal positioned between a
pair of posts. Although crystal resonators provide high signal
discrimination, they are limited to applications below 500
megahertz (MHz) due to crystal thickness limitations. As a result,
crystal resonators are not suitable for cellular and other lower
ultra high frequency (UHF) applications in the 300 to 3000
megahertz (MHz) range.
[0007] Thin film resonators are formed on a substrate that includes
an acoustic reflector. The acoustic reflector may be formed by an
air-gap or a number of reflecting layers. The thin film resonator
includes a piezoelectric layer positioned between two electrodes.
The piezoelectric layer may comprise zinc oxide (ZnO). Zinc oxide
surface acoustic wave (SAW) devices have been developed as thin
films on an insulator. TV-IF filters are produced as zinc oxide on
glass.
[0008] The piezoelectric layer has a thickness that is equal to
half the target wavelength for the resonator in order to provide
the proper resonance in the resonator. At 900 megahertz (MHz), a
half-wavelength is 3.5 microns. Because the piezoelectric layer can
only be formed at a slow rate of about 5 microns per hour due to
processing limitations, thin film resonators are time consuming and
expensive to fabricate. In addition, the substantial thickness of
the piezoelectric layer for lower UHF applications causes internal
stress within the resonator which leads to warping, bubbling, and
cracking defects.
SUMMARY OF THE INVENTION
[0009] The present invention provides a thin film resonator and
method that substantially reduces or eliminates disadvantages and
problems associated with previously developed systems and methods.
In particular, the conventional piezoelectric layer is replaced
with a sandwich layer of piezoelectric and non-piezoelectric
material that can be quickly deposited to provide a low cost and
high performance thin film resonator.
[0010] In accordance with one embodiment of the present invention,
a thin film or other suitable acoustic resonator comprises a first
electrode and a second electrode substantially parallel to one
another. An intermediate layer is disposed between and coupled to
the first and second electrodes. The intermediate layer includes a
first piezoelectric layer, a second piezoelectric layer, and a
spacer layer disposed between the first piezoelectric layer and the
second piezoelectric layer. The spacer layer has an acoustic
impedance substantially the same as that of the first and second
piezoelectric layers and comprises a disparate material.
[0011] More specifically, in accordance with a particular
embodiment of the present invention, the spacer layer has a
coefficient of thermal expansion substantially the opposite of that
of the first and second piezoelectric layers. In addition, the
spacer layer may be designed to offset the thermal expansion of
acoustic reflectors supporting the resonator.
[0012] Technical advantages of the present invention include
providing an improved acoustic resonator and improved filters
employing the acoustic resonators. In particular, the acoustic
resonator includes a sandwich of piezoelectric and
non-piezoelectric material between the electrodes. The
non-piezoelectric material is substantially uniform in thickness
and readily formed during fabrication. As a result, thin film and
other suitable acoustic resonators may be produced at low-cost.
[0013] Another technical advantage of the present invention
includes providing an ultra high frequency (UHF) acoustic
resonator. In particular, the non-piezoelectric spacer has a
substantially uniform thickness over a wide range. As a result, the
thickness of the resonator may be substantially increased to
support cellular phone and other UHF applications.
[0014] Yet another technical advantage of the present invention
includes providing a stable acoustic resonator. In particular, the
spacer layer may be formed of a material having an opposite thermal
expansion characteristics of the piezoelectric layer and/or the
acoustic reflector. As a result, stability of the resonator is
increased and the device may be configured to be insensitive to
temperature within an operational range.
[0015] Still another technical advantage of the present invention
includes providing an improved on-chip filter. In particular, a
thin film resonator is provided that may be readily fabricated
directly onto a substrate. The resonator may be combined with other
resonators to form an on-chip filter and on-chip filters combined
to form a single-chip transceiver.
[0016] Other technical advantages of the present invention will be
readily apparent to one skilled in the art from the following
figures, description, and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] For a more complete understanding of the present invention
and its advantages, reference is now made to the following
description taken in conjunction with the accompanying drawings,
wherein like reference numerals represent like parts, and in
which:
[0018] FIG. 1 is a block diagram illustrating a front end
transceiver for a radio frequency device in accordance with one
embodiment of the present invention;
[0019] FIG. 2 is a block diagram illustrating details of the
filters of FIG. 1 in accordance with one embodiment of the present
invention;
[0020] FIG. 3 is a cross-sectional diagram illustrating details of
the thin film resonators of FIG. 2 in accordance with one
embodiment of the present invention; and
[0021] FIG. 4 is a flow diagram illustrating a method for
fabricating the thin film resonator of FIG. 3 in accordance with
one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] FIG. 1 illustrates a front end transceiver 10 for a wireless
device in accordance with one embodiment of the present invention.
In this embodiment, the wireless device is a cellular phone
operated in accordance with the American standard at the 900
megahertz (MHz) range. It will be understood that the acoustic
resonators and filters of the present invention may be used in
connection with other types of cellular phones, wireless devices,
and other suitable devices that receive, transmit and/or use radio
frequency signals.
[0023] Referring to FIG. 1, the front end transceiver 10 includes
an antenna 12, a diplexer 14 including a receive filter 16, a
transmit filter 18, and a power divider, a line amplifier 20, an
image rejection filter 22, a mixer 24, and a voltage controlled
oscillator (VCO) filter 26. As described in more detail below, one
or more of the filters 16, 18, 22 and 26 may comprise thin film
resonators having low insertion loss at radio frequencies. In this
embodiment, the filters 16, 18, 22 and/or 26 may be fabricated
on-chip, directly onto an underlying substrate to form a single
chip radio. Accordingly, signal degradation of bond wire
connections and bond pad capacitance associated with off-chip
filters are eliminated.
[0024] The antenna 12 sends and receives signals to and from the
power divider of the diplexer 14. The diplexer 14 sends an incoming
signal through the receive filter 16 which filters out television,
satellite communication and radio frequencies in the cellular phone
application. The transmit filter 18 receives and filters outbound
signals to be transmitted from the cellular phone to a base
station. For incoming signals, the receive filter 16 passes the
resulting band limited signal to the low noise amplifier 20. From
the low noise amplifier 20, the signal is passed to the image
rejection filter 22. The image rejection filter 22 suppresses
pager, police radio and other thermal noise at the mixer 24 and
local oscillator image frequency.
[0025] From the image rejection filter 22, the further bandwidth
limited signal is passed to the mixer 24. The mixer 24 also
receives a signal from the voltage controlled oscillator (VCO)
filter 26 that is coupled to the VCO and implemented to remove
synthesizer spurs. The resulting in-band signal is output to the
intermediate frequency (IF) chain 28 for use by the cellular
device.
[0026] For the 900 megahertz (MHz) cellular phone, the front end
transceiver 10 may employ the IS-95, IS-136, or GSM standards. For
the IS-95 and IS-136 standards, the receive filter 16 operates in
the 869-894 MHz range, the transmit filter 18 operates in the
824-849 MHz range, the image rejection filter 22 operates in the
869-894 MHz range, and the voltage controlled oscillator filter 26
operates in the 940-965 MHz range. For the GSM standard, the
receive filter 16 operates in the 935-960 MHz range, the transmit
filter 18 operates in the 890-915 MHz range, the image rejection
filter 22 operates in the 935-960 MHz range, and the voltage
controlled oscillator filter 26 operates in the 1,006-1,031 MHz
range. The receive and transmit filters 16 and 18 are each 25 MHz
wide.
[0027] FIG. 2 illustrates details of a filter 50 for the front end
transceiver 10 in accordance with one embodiment of the present
invention. In this embodiment, the filter 50 is a ladder filter
comprising a plurality of acoustic resonators 54 and 56. The ladder
filter may be used for the receive filter 16, transmit filter 18,
image rejection filter 22 and/or the voltage controlled oscillator
filter 26 of the front-end receiver 10. The ladder filter 50 may
also be used in television, radio, wireless, and other suitable
devices that use radio frequency signals.
[0028] Referring to FIG. 2, the ladder filter 50 includes a
plurality of serial resonators 54 and a plurality of parallel
resonators 56. The serial resonators 54 are connected in series
between an input terminal 60 and an output terminal 62 to define a
series arm. The parallel resonators 56 are each respectively
connected in parallel between the series arm and a ground potential
to define a parallel arm.
[0029] In the ladder filter 50, the resonant frequencies of the
serial resonators 54 are constructed to be coincidental with the
anti-resonant frequencies of the parallel resonators 56. Thus, the
ladder filter 50 comprises a pass band defined by the anti-resonant
frequency of the serial resonator 54 and the resonant frequencies
of the parallel resonators 56. The frequency and other properties
of the individual serial and parallel resonators 54 and 56 may be
varied according to the particular desired function for the filter
50.
[0030] FIG. 3 illustrates details of acoustic resonator 100 for use
in the ladder or other suitable filter in accordance with one
embodiment of the present invention. In this embodiment, the
acoustic resonator 100 is a thin film resonator formed on-chip,
directly on an acoustic reflector 102 that itself is formed
directly on an underlying substrate 104. In accordance with a
particular embodiment, the substrate 104 comprises silicon or other
suitable semiconductor material and the acoustic reflector 102
comprises alternating layers having low and high acoustic impedance
such as silicon dioxide (SiO.sub.2) and tungsten (W). Further
information regarding the structure and materials of the acoustic
reflector 102 is described in co-owned U.S. patent application Ser.
No. ______ , filed Jan. 18, 2000 entitled Multiple Frequency
Acoustic Reflector Array and Monolithic Cover for Resonators and
Method, which is hereby incorporated by reference. In this
embodiment, the resonator 100 is a solidly-mounted resonator. It
will be understood that the resonator 100 may be via-isolated,
air-gap isolated, or otherwise suitably supported by and
acoustically isolated from any necessary substrate.
[0031] Referring to FIG. 3, the thin film resonator 100 includes a
first electrode 110 disposed outwardly of the acoustic reflector
102. A second electrode 112 is substantially parallel to or
co-planer with, and spaced apart from the first electrode 110 to
provide the necessary quality factor. For a one gigahertz (GHz)
application in which the resonator 100 has a quality factor of
1,000, the distance between the first and second electrodes 110 and
112 vary by 50 angstroms or less over the area of the resonator.
The first and second electrodes 110 and 112 comprise metal or other
suitable conductive material conventionally deposited. In a
particular embodiment, the electrodes 110 and 112 each comprise
aluminum and are between 0.1 and 1 micron thick.
[0032] An intermediate, or resonating, layer 120 is disposed
between and coupled to the first and second electrodes 110 and 112.
The intermediate layer includes a first piezoelectric layer 122
coupled to the first electrode 110, a second piezoelectric layer
124 coupled to the second electrode 112 and a spacer, or
interstitial, layer 130 disposed between the first and second
piezoelectric layers 122 and 124. To provide resonance, the
intermediate layer 120 has a thickness that is a half wave length
of a target frequency for the resonator 100. Thus, for a 900 MHz
acoustic resonator, the intermediate layer 120 has a
half-wavelength thickness of approximately 3.5 microns.
[0033] The first and second piezoelectric layers 122 and 124 each
have a substantially uniform and shallow thickness to provide
stability and minimize warping, bubbling, cracking or other
defects. In one embodiment, the first and second piezoelectric
layers 122 and 124 are each about a half micron in thickness to
provide good resonator performance while minimizing piezoelectric
thickness, which using conventional sputter techniques, has a
deposition rate of about 5 microns per hour. The thickness of the
first and second piezoelectric layers 122 and 124 may be suitably
adjusted to control the quality factor for the resonator 100. This
independent control of the piezoelectric layer thickness provides
an additional design factor for optimizing resonator 100
performance. The intermediate or resonating layer material
comprises zinc oxide (ZnO), aluminum nitride (ALN), silicon nitride
(SiN), gallium arsenide (GaAs), tungsten (W), or other suitable
materials having acceptable electromechanical coupling
coefficients.
[0034] The spacer 130 is substantially uniform in thickness and may
comprise the bulk of the thickness of the intermediate layer 120
depending on the frequency of the resonator 100. The spacer layer
130 should be formed from a material that can be deposited at a
relatively high rate and at a substantially uniform thickness up to
three or more microns. In addition, the material of the spacer
layer 130 should have an acoustic impedance that matches or is
substantially the same as that of the piezoelectric layers 122 and
124. The acoustic impedance of the layers 122, 124, and 130 are
substantially the same when they are within twenty (20) percent of
one another. In addition, to provide stability for the resonator
and the filter 50, the thermal expansion characteristics of the
spacer 130 should be opposite the thermal expansion characteristics
of the piezoelectric layers 122 and 124. Aluminum nitride (ALN),
for example, has a thermal expansion coefficient of 28 ppm/.degree.
C. while zinc oxide (ZnO) has a thermal expansion coefficient of 60
ppm/.degree. C. Materials have an opposite thermal expansion
coefficient when layers made from the materials form a structure
that is largely insensitive to temperature at least at room
temperature. Preferably, the device is at most quadratic with
temperature and has a zero slope at room temperature. Materials for
the spacer layer 130 include silicon dioxide (SiO.sub.2) and
aluminum (Al).
[0035] FIG. 4 is a flow diagram illustrating a method for
fabricating the thin film resonator 100 in accordance with one
embodiment of the present invention. In this embodiment, the
resonator 100 is formed on-chip directly onto the acoustic
reflector 102. It will be understood that the acoustic resonator
may be formed on other suitable supports without departing from the
scope of the present invention.
[0036] Referring to FIG. 4, the method begins at step 150 in which
the substrate 104 is provided. The substrate 104 may comprise
silicon, other semiconductor, or other suitable support material.
Next, at step 152, the acoustic reflector 102 is formed outwardly
of the substrate 104. The acoustic reflector 102 may be formed as
described in U.S. patent application entitled "Multiple Frequency
Acoustic Reflector Array and Monolithic Cover for Resonators and
Method" previously incorporated by reference.
[0037] Proceeding to step 154, the first electrode 110 is
conventionally deposited outwardly of the acoustic reflector 102.
At step 156, the first piezoelectric layer 122 is deposited
outwardly of the first electrode 110 using a conventional sputter
or other suitable process. At step 158, the spacer layer 130 is
deposited outwardly of the first piezoelectric layer 122. The
spacer layer 130 is deposited by DC plasma magnetron reactive
sputtering system, electron cyclotron resonance-chemical vapor
deposition (ECR-CVD), and other suitable processes.
[0038] Next, at step 160, the second piezoelectric layer 124 is
deposited outwardly of the spacer layer 130. Together, the first
and second piezoelectric layers 122 and 124 along with the spacer
layer 130 form a substantially uniform intermediate layer 130 and
have a thickness of an acoustic half-wavelength of the desired
frequency for the resonator 100. At step 162, the second electrode
layer 112 is conventionally deposited outwardly of the second
piezoelectric layer 124.
[0039] Proceeding to step 164, the deposited layers are suitably
patterned and etched or otherwise suitably processed to remove
excess material and form the thin film resonator 100. In this way,
a low-cost and high-performance thin film resonator 100 is
fabricated for use in radio filters for televisions, radios,
cellular phones, wireless devices and other suitable devices that
transmit, receive, and/or use radio frequency signals.
[0040] Although the present invention has been described as several
embodiments, various changes and modifications may be suggested to
one skilled in the art. It is intended that the present invention
encompass such changes and modifications as fall within the scope
of the appended claims.
* * * * *