U.S. patent application number 12/350859 was filed with the patent office on 2009-04-30 for contour-mode piezoelectric micromechanical resonators.
This patent application is currently assigned to THE REGENTS OF THE UNIVERSITY OF CALIFORNIA. Invention is credited to Gianluca Piazza, Albert P. Pisano, Philip J. Stephanou.
Application Number | 20090108959 12/350859 |
Document ID | / |
Family ID | 37114551 |
Filed Date | 2009-04-30 |
United States Patent
Application |
20090108959 |
Kind Code |
A1 |
Piazza; Gianluca ; et
al. |
April 30, 2009 |
Contour-Mode Piezoelectric Micromechanical Resonators
Abstract
A contour mode micromechanical piezoelectric resonator. The
resonator has a bottom electrode; a top electrode; and a
piezoelectric layer disposed between the bottom electrode and the
top electrode. The piezoelectric resonator has a planar surface
with a cantilevered periphery, dimensioned to undergo in-plane
lateral displacement at the periphery. The resonator also includes
means for applying an alternating electric field across the
thickness of the piezoelectric resonator. The electric field is
configured to cause the resonator to have a contour mode in-plane
lateral displacement that is substantially in the plane of the
planar surface of the resonator, wherein the fundamental frequency
for the displacement of the piezoelectric resonator is set in part
lithographically by the planar dimension of the bottom electrode,
the top electrode or the piezoelectric layer.
Inventors: |
Piazza; Gianluca; (Lodi,
IT) ; Stephanou; Philip J.; (Berkeley, CA) ;
Pisano; Albert P.; (Danville, CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
THE REGENTS OF THE UNIVERSITY OF
CALIFORNIA
Oakland
CA
|
Family ID: |
37114551 |
Appl. No.: |
12/350859 |
Filed: |
January 8, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11145552 |
Jun 2, 2005 |
7492241 |
|
|
12350859 |
|
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Current U.S.
Class: |
333/189 |
Current CPC
Class: |
H03H 9/542 20130101;
Y10T 29/49005 20150115; H03H 3/02 20130101; H03H 2009/02196
20130101; H03H 9/172 20130101; H03H 9/54 20130101; H03H 2009/241
20130101; H03H 2009/02354 20130101; H03H 9/171 20130101; H03H 9/132
20130101; H03H 9/205 20130101; Y10T 29/42 20150115; H03H 2003/027
20130101 |
Class at
Publication: |
333/189 |
International
Class: |
H03H 9/15 20060101
H03H009/15 |
Goverment Interests
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED
RESEARCH OR DEVELOPMENT
[0001] This invention was made with Government Support under Grant
(Contract) No. NBCH1020005 awarded by the Department of Interior.
The Government has certain rights in the invention.
Claims
1-46. (canceled)
47. A micromechanical in-plane oscillating piezoelectric resonator,
comprising: a bottom electrode; a top electrode; a thin film
piezoelectric layer disposed between said bottom electrode and said
top electrode, said piezoelectric resonator having a planar surface
having a cantilevered periphery, dimensioned to undergo in-plane
lateral displacement; at least one micro via connected with the
bottom electrode, said micro via configured to electrically connect
the bottom electrode with a source of an alternating electric
field; and means for applying the alternating electric field across
the thickness of said piezoelectric resonator, said electric field
configured to cause said piezoelectric layer to have an in-plane
lateral displacement that is substantially in the plane of said
planar surface, wherein a center frequency of said piezoelectric
resonator is set in part lithographically by the planar dimensions
of at least one of said bottom electrode, said top electrode and
said piezoelectric layer.
48. The resonator of claim 47, further comprising means for
applying a DC voltage across the thickness of the piezoelectric
resonator, said DC voltage configured for tuning the center
frequency of the resonator.
49. The resonator of claim 47, wherein said center frequency of the
resonator is set at least in part by a pattern on the top or bottom
electrodes.
50. The resonator of claim 47, wherein a bandwidth of the resonator
is set at least in part in part lithographically.
51. The resonator of claim 47, wherein an impedance of the
resonator is set at least in part in part lithographically.
52. The resonator of claim 47, further comprising: a substantially
planar substrate for holding the resonators, and at least one
micromechanical piezoelectric resonator disposed on the planar
substrate, wherein the resonators are interconnected either in
parallel or in series.
53. The resonator of claim 52, wherein the planar substrate is made
of silicon.
54. The resonator of claim 52, wherein the planar substrate is made
of glass.
55. The resonator of claim 47, further comprising at least one
tether anchoring the resonator to a substrate or other
resonator.
56. The resonator of claim 55, wherein said tether comprises at
least one routing trace for routing electrical signals to the
electrodes.
57. The resonator of claim 47, wherein said means for applying an
alternating electric field comprise electrical interconnects to the
top electrode and the bottom electrode.
58. The resonator of claim 47, wherein said bottom electrode is
lithographically patterned to excite a specific region or regions
of the piezoelectric resonator.
59. The resonator of claim 47, wherein said top electrode is
lithographically patterned to excite a specific region or regions
of the piezoelectric resonator.
60. The resonator of claim 47, wherein said piezoelectric layer is
a layer selected from the group consisting of aluminum nitride,
gallium nitride, aluminum gallium arsenide, zinc oxide, quartz, and
combinations thereof.
61. The resonator of claim 47, wherein said bottom or said top
electrode is made from a material selected from the group
consisting of aluminum, platinum, tungsten, molybdenum, ruthenium,
chrome, gold, titanium, doped polycrystalline silicon, and
combinations thereof.
62. The resonator of claim 47 being a part of an array of
resonators formed by mechanically coupling the resonators in said
array to one another using said tethers.
63. The resonator of claim 47, wherein said one or more tethers are
configured for use as electrical connectors for exchanging
electrical signals with the piezoelectric resonator.
64. The resonator of claim 63 being one of more than one resonator
and wherein said more than one resonators are electrically cascaded
to form a network of said resonators.
65. The resonator of claim 64, wherein said network is a Pi, T, L,
shunt, or series network.
66. The resonator of claim 65 being a part of a band pass filter
circuit formed using a series of either of said Pi, T, L, series,
or shunt networks.
67. The resonator of claim 65, comprising 2 or more resonators that
are electrically connected.
68. The resonator of claim 65, comprising electrical connectors
used to route the signal from one resonator to the next.
69. The resonator of claim 47 being one of more than one resonator
and wherein said more than one resonator are connected and arranged
as a band-pass filter circuit.
70. The resonator of claim 47, wherein said piezoelectric resonator
is a part of a frequency reference in an oscillator circuit.
71. The resonator of claim 47, wherein said piezoelectric resonator
is a part of a band pass filter circuit.
72. The resonator of claim 47, wherein said piezoelectric layer has
a shape selected from the group consisting of a rectangle, a
circle, a polygon, a circular annulus, a rectangular annulus, a
polygonal annulus and combinations thereof.
Description
BACKGROUND OF THE INVENTION
[0002] The present invention relates to micromechanical resonators.
In particular, the present invention is related to a new class of
contour-mode piezoelectric micromechanical resonators that can be
employed as building blocks in wireless communication components
such as filters and oscillators.
[0003] Recent demand in wireless communication for miniaturized,
low-power, low-cost, on-chip and high-Q resonators to be employed
in front-end RF filters or as frequency references has focused
research efforts towards the development of new vibrating
micromechanical structures, capable of substituting existing
off-chip, bulky resonator technologies. Some promising alternatives
to currently adopted solutions (SAW or ceramic devices) have been
demonstrated (e.g., see, Li et al., IEEE MEMS, 821-824 (2004); and
Wang et al, IEEE MEMS, 641-644 (2004)) using in-plane,
electrostatically-transduced, micromechanical resonators made of
polysilicon or polydiamond. Although high quality factors have been
reported at ultra high frequency range (UHF), the exhibited
impedance values are too high for these resonators to be directly
coupled to antennas in RF systems. Also, the high temperature
fabrication steps involved with the deposition of the structural
layers ultimately complicate the integration of these devices with
state-of-the-art microelectronic components.
[0004] Film Bulk Acoustic Resonator (FBAR) technology (e.g., see,
Aigner et al., Transducers, 891-894 (2003); and Ruby et al., IEEE
International Solid-State Circuits Conference, 121-122) has proven
itself as a valid solution to replace conventional RF filters,
demonstrating relatively high quality factors (Q.about.2,500), and
small (several .OMEGA.) impedances. The fundamental frequency of
these devices is set by the film thickness. This constitutes a
major challenge to the manufacturing of FBARs. On one hand, in
order to obtain reasonable yields, a thickness tolerance of 0.1% is
needed. On the other hand, multiple frequency selective arrays of
resonators cannot readily be fabricated on a single chip, due to
fact that the frequency of vibration for the devices is set by the
film thickness.
[0005] There is therefore a need for an improved resonator that
does not suffer from the design disadvantages of currently
available resonators.
BRIEF SUMMARY OF THE INVENTION
[0006] The present invention is directed towards a new class of
contour-mode piezoelectric micromechanical resonators that can be
employed as building blocks in wireless communication components
such as filters and oscillators, and a method of making the same.
The piezoelectric materials used for the fabrication of the
micromechanical resonators includes, for example, aluminum nitride
(AlN), zinc oxide (ZnO), aluminum gallium arsenide (AlGaAs),
Gallium Nitride (GaN), quartz and other piezoelectric materials.
The use of contour modes, as opposed to FBAR technology which
employs a thickness mode, enables the fabrication of arrays of
microresonators with different frequencies on a single chip. In
addition, the contour mode micromechanical resonators in accordance
with the embodiments of the present invention can be operated in
air without significant performance degradation thereby reducing
related packaging costs. Low motional resistance and high quality
factor are thus provided on the same chip while spanning a
frequency range from MHz to GHz. The contour-mode piezoelectric
micromechanical resonators in accordance with the embodiments of
the present invention also enable the fine and coarse tuning of
their center frequencies directly on-chip without the need for
additional post-processing steps.
[0007] In one embodiment, the present invention provides a contour
mode micromechanical piezoelectric resonator. The resonator has a
bottom electrode; a top electrode; and a piezoelectric layer
disposed between the bottom electrode and the top electrode. The
piezoelectric resonator has a planar surface with a cantilevered
periphery, dimensioned to undergo in-plane lateral displacement at
the periphery. The resonator also includes means for applying an
alternating electric field across the thickness of the
piezoelectric resonator. The electric field is configured to cause
the resonator to have a contour mode in-plane lateral displacement
that is substantially in the plane of the planar surface of the
resonator, wherein the fundamental frequency for the displacement
of the piezoelectric resonator is set in part lithographically by
the planar dimension of the bottom electrode, the top electrode or
the piezoelectric layer.
[0008] In another aspect, the present invention provides a method
of fabricating a contour mode micromechanical piezoelectric
resonator body on a substrate. The method includes forming a
patterned bottom electrode above the substrate; forming a
piezoelectric layer above the bottom electrode; forming a patterned
top electrode on top of the piezoelectric layer; forming an opening
through the piezoelectric layer to the bottom electrode; and
etching the resonator body away from the substrate.
[0009] For a further understanding of the nature and advantages of
the invention, reference should be made to the following
description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIGS. 1(a)-(c) are exemplary diagrams of two
rectangular-shaped and one disk-shaped micromechanical resonator
configured for vibrating in a contour-mode, in accordance with one
embodiment of the present invention.
[0011] FIGS. 2(a)-(c) are exemplary diagrams showing in-plane
contour mode shapes for a rectangular plate resonator, in
accordance with one embodiment of the present invention.
[0012] FIGS. 3(a)-(d) are exemplary diagrams of one embodiment of
the fabrication process for the contour mode resonators in
accordance with the embodiments of the present invention.
[0013] FIGS. 4(a)-(c) are SEM images showing two rectangular-shaped
and one disk-shaped micromechanical resonator produced in
accordance with the fabrication processes of FIGS. 3(a)-(d).
[0014] FIG. 5 is an exemplary diagram of another rectangular-shaped
micromechanical resonator configured for vibrating in a
contour-mode, in accordance with an alternate embodiment of the
present invention.
[0015] FIGS. 6(a)-(b) are exemplary diagrams of a circular
ring-shaped (a) and a square-shaped ring (b) micromechanical
resonator configured for vibrating in a contour-mode, in accordance
with one embodiment of the present invention.
[0016] FIGS. 7(a)-(d) are exemplary diagrams of an alternate
embodiment of the fabrication process for the contour mode
resonators in accordance with the embodiments of the present
invention.
[0017] FIGS. 8(a)-(d) are SEM images showing a circular ring-shaped
and a square-shaped ring micromechanical resonator produced in
accordance with the fabrication processes of FIGS. 7(a)-(d).
[0018] FIG. 9(a)-(b) show a ladder filter configuration
incorporating the contour-mode micromechanical resonators in
accordance with the embodiments of the present invention (a); and a
schematic representation of the electrical response of individual
series and shunt resonators and a ladder filter made of the same
(b).
[0019] FIG. 10(a)-(b) are SEM images showing a ladder filter made
using ring-shaped micromechanical resonators (a) in accordance with
the embodiments of the present invention; and a ladder filter made
using rectangular-shaped micromechanical resonators (b) in
accordance with the embodiments of the present invention.
[0020] FIG. 11 is an exemplary oscillator circuit diagram that may
be configured to include a contour-mode micromechanical resonator
in accordance with the embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Overview
[0021] The embodiments of the present invention are directed
towards a new class of contour-mode piezoelectric micromechanical
resonators that can be employed as building blocks in wireless
communication components such as filters and oscillators, and
methods for making the same. The micromechanical resonator includes
a piezoelectric layer sandwiched between top and bottom electrodes,
suspended in air and anchored in one or more locations. Exemplary
devices were demonstrated using AlN as the piezoelectric layer. In
addition, other piezoelectric materials, such as AlGaAs, ZnO, GaN
and quartz can alternatively be used. The top or bottom electrodes
may be made of various materials including platinum (Pt), aluminum
(Al), molybdenum (Mo), tungsten (W), titanium (Ti), niobium (Nb),
ruthenium (Ru), chromium (Cr), doped polycrystalline silicon, or
doped AlGaAs compounds. An alternating current electric field
applied across the top and bottom electrode induces mechanical
deformations in the plane of the piezoelectric layer (contour mode)
via the d.sub.31 coefficient. At the device resonance frequency the
electrical signal across the device is reinforced and the device
behaves as an equivalent electronic resonant circuit (LC tank). The
advantage of using a mechanical resonator is the high quality
factor of such a structure and the consequent drastic reduction in
its power dissipation. In addition, the devices in accordance with
the embodiments of the present invention are extremely small (in
general 100.mu..times.200 .mu.m) and can be used as a frequency
reference in oscillator circuits and in band pass filters for
miniature radios, wristwatch, and cellular phones.
[0022] Contour-mode piezoelectric micromechanical resonators in
accordance with the embodiments of the present invention have been
demonstrated using AlN technology. In one example, a thin AlN layer
(e-g, 0.5 to 2 .mu.m thick) is sputter deposited on a patterned Pt
bottom electrode. A low stress nitride layer is used as an
insulating material between the bottom electrode and the silicon
wafer. A top aluminum electrode is patterned on top of the AlN in
order to sandwich the piezoelectric material at the location where
actuation is desired. The structures are released by dry etching of
silicon in xenon difluoride (XeF.sub.2), thereby eliminating
stiction forces and significantly increasing yield. The fabrication
steps occur at low temperature and offer the possibility to
integrate the resonant devices with state-of-the-art
microelectronic components. The patterning of the bottom electrode
enables the on-chip coarse frequency tuning of contour-mode
resonators. The acoustic impedance of Pt (significantly larger than
the one of AlN and Al) loads the resonator and shifts its center
frequency. This frequency change can be defined directly at the
mask level without significant impact on the resonator performance.
For example, two circular ring resonators having different
electrode areas (coverage changes from 90% to 67.5% of the total
resonator area) show a frequency shift of 6.8% from 232 MHz to 249
MHz. A fine-tuning mechanism of the resonator center frequency has
also been demonstrated by applying a DC bias voltage across the
piezoelectric film. The center-frequency can be shifted either up
or down by purely piezoelectric means. A tuning range of .about.6
kHz was obtained for a 23 MHz rectangular plate using a 30 V power
supply. The combined use of on-chip metal loading and DC biasing
enables the efficient tuning of the resonant frequency of the
piezoelectric devices without the need for post-processing step and
ultimately reducing the fabrication cost of such devices.
[0023] Rectangular, diamond-shaped and circular plates, as well as
circular and rectangular rings were successfully fabricated and
operated. Contour-mode piezoelectric resonators were demonstrated
to span a frequency range from 20 MHz to 1.15 GHz and can be
extended up to 4 GHz. Quality factors of about 5,000 were obtained
for rectangular plates excited in dilation-type contour modes at 23
MHz with motional resistance of 100.OMEGA.. Ring-shaped resonators,
excited in width-extensional mode shapes, exhibited quality factors
as high as 2,900 at a frequency of 473 MHz and a motional
resistance of 80.OMEGA..
Contour Modes in Plates
[0024] FIGS. 1(a) and 1(b) show schematic views of AlN rectangular
plates vibrating in dilation-type contour modes. FIG. 1(a) shows a
two-port, AlN rectangular plate resonator 102a with electrodes
103a, 105a placed parallel to the plate length, L, and having an
input port 107a and an output port 109a. FIG. 1(b) shows a
two-port, AlN rectangular plate resonator 102b with electrodes
103b, 105b placed parallel to the plate width, W, and having an
input port 107b and an output port 109b. The resonators are shown
to have a Pt bottom electrode and Al top electrodes. Disposed
between the top and bottom electrodes is the AlN piezoelectric
layer. A vertical electric field applied across the film thickness
induces extensional mechanical stress in the plane of the film
(through the d.sub.31 piezoelectric coefficient) and excites the
structures in a dilation-type contour mode. The two-port
configurations were implemented by patterning the top electrodes
symmetrically with respect to the length or the width of the plate.
In order to minimize anchor losses, the resonators were suspended
by two quarter-wavelength tethers 108a, and 108b.
[0025] Several in-plane contour mode shapes can be excited in these
resonators (e.g., see, Holland, IEEE Transaction on Sonics and
Ultrasonics, Su-15(2): 97-105 (1968)), but either electrode
configurations or energy loss mechanisms limit the detectable mode
shapes to the ones shown in FIG. 2. FIG. 2 shows dilation-type
contour modes (obtained via a finite elements analysis simulation)
excited in a two-port AlN 200.times.50 .mu.m resonator. FIG. 2(a)
shows a fundamental length-extensional mode shape, also referred to
as a first mode herein; FIG. 2(b) shows a first width-extensional
mode shape, also referred to as second mode hereon; and FIG. 2(c)
shows a second width-extensional mode showing effective
quarter-wavelength tethers. As is shown in FIGS. 2(a)-(c), the
application of an alternating electric field across the thickness
of the piezoelectric resonator causes the resonator to have a
contour mode in-plane lateral displacement that is substantially in
the plane of the planar surface of the resonator.
[0026] The resonator can most effectively be excited in
length-extensional (FIG. 2(a)) (e.g., see, Antkowiak et al.,
Transducers, 841-846 (2003)) and width-extensional (FIGS. 2(b)-(c))
dilation-type contour modes. A parameter that characterizes the
relative strength of each of the resonances and sets the value of
the equivalent motional resistance is the electromechanical
coupling, .eta..sub.em. Electromechanical coupling can either be
defined as the ratio of the output force to the input voltage, or
the ratio of the output charge to the mechanical displacement. For
a symmetric electrode topology such as the one used in FIGS. 1a-b,
.eta..sub.in .eta..sub.out are the same, specifically:
.eta. em = .intg. .intg. A ( d 31 T 31 + d 31 T 2 ) A Z max = 2 E 1
- .sigma. d 31 .intg. .intg. A ( .differential. u .differential. x
+ .differential. v .differential. y ) A Z max ( 1 )
##EQU00001##
where T.sub.1 and T.sub.2 represent normal stresses in the x and y
directions respectively, d.sub.31 is the piezoelectric coefficient,
u and v are the displacements in x and y directions (e.g., see,
Johnson, Mechanical Filters in Electronics, New York, N.Y. Wiley
(1983)), E is the Young's modulus, and .sigma. the Poisson ratio
for AlN, A is the electrode area, and Z.sub.max is the maximum
displacement for the structure. Taking this into account the
electrodes were placed so that .eta..sub.em is maximized.
Wineglass Contour Modes in Disks
[0027] FIG. 1(c) shows a schematic view of an AlN micromechanical
disk configured to be excited in a wineglass contour mode. Due to
the plane symmetry of AlN films, radial and compound mode shapes
(e.g., see, Onoe, The Journal of the Acoustical Society of America,
28(6):1158-1162 (1954)) can be effectively excited in thin circular
disks through the d.sub.31 piezoelectric coefficient. In order to
minimize anchor losses, the disk was pinned at two of its
quasi-nodal points. FIG. 1(c) shows a two-port, AlN disk-shaped
resonator 102c with electrodes 103c, 105c placed as diagonally
opposing quadrants, and having an input port 107c and an output
port 109c.
[0028] A two-port configuration is implemented by having each of
the two top electrodes 103c and 105c cover a quadrant of the disk.
By placing the electrode in such a fashion, maximum
electromechanical coupling is achieved. In this specific case
.eta..sub.em is given by:
.eta. em = 2 E ( 1 - .sigma. 2 ) d 31 .intg. .intg. A [ (
.differential. u .differential. v + .sigma. u r + 1 r
.differential. v .differential. r ) ] A Z max ( 2 )
##EQU00002##
where E, .sigma., A and Z.sub.max are the same as defined in
equation (1), u and v represent the radial and tangential
displacements (e.g., see, Onoe, The Journal of the Acoustical
Society of America, 28(6): 1158-1162 (1954)), respectively, and r
is the radial coordinate for the circular plate. Due to the
isovoluminal nature of the mode shape and the electrode covering
just half of the plate top surface, the electromechanical coupling
for the disk is relatively smaller (.about.8.times.) than that for
AlN plates of comparable frequency. The resonator of FIG. 1(c) has
been shown to be excited in a wineglass contour mode shape.
Fabrication Process
[0029] FIGS. 3(a)-(d) are exemplary diagrams of one embodiment of
the fabrication process 300 for the contour mode resonators in
accordance with the present invention. Briefly, in step (a), a thin
layer of AlN, a layer of Pt, and AlN film are deposited on a Si
substrate. In step (b), trenches are opened through the AlN layers
and the Pt layer by a chlorine-based dry etching and ion milling,
respectively. In step (c), openings to the bottom electrode are wet
etched through the AlN layer in a hot (e.g., .about.160.degree. C.)
phosphoric bath. In step (d), the Al top electrodes are evaporated
and patterned by lift-off. Further details of the fabrication
process are provided below. A three-mask, low-temperature (e.g.,
T.sub.max<350.degree. C.), post-CMOS compatible process was used
to fabricate rectangular and circular AlN plates. A thin (.about.50
nm) AlN buffer layer was deposited on a Si wafer to provide
electrical isolation. A 1.5 .mu.m AlN film was sandwiched between a
bottom platinum electrode and a top aluminum electrode. AlN is
reactively deposited using an Advanced Modular Sputtering Inc., AMS
2003 sputtering tool. C.sub.1-2-based dry etching was used to
pattern AlN and yielded fairly straight (within 16.degree. of
vertical) sidewalls with an etch rate of .about.80 nm/min. Oxide
deposited by electron cyclotron resonance was used as a hard mask
during the AlN etching step and the ion milling of the Pt layer.
Openings to contact the bottom electrode were wet etched through
AlN in a hot (e.g., .about.160.degree. C.) phosphoric bath. The top
Al electrode is evaporated and patterned so that maximum
electromechanical coupling is obtained. The structures are released
by dry etching of silicon in XeF.sub.2. This novel dry release step
for fully AlN resonators eliminates stiction forces and
significantly increases yield compared to other processes that use
wet release techniques (e.g., see Humad et al., IEDM, 957-960
(2001)). The deposition steps occur at low temperature
(T.sub.max.about.350.degree. C. when sputtering AlN) and thus can
be integrated with state-of-the-art microelectronic components.
FIG. 4 shows rectangular and circular plate resonators that were
fabricated using the aforementioned process. FIG. 4(a) shows an
80.times.20 .mu.m rectangular plate with electrodes parallel to the
resonator length. FIG. 4b shows a 50 .mu.m radius AlN circular
plate configured for excitation in a wineglass mode shape, and FIG.
4(c) shows an 80.times.20 .mu.m rectangular plate with electrodes
parallel to the resonator width.
Characterization--Plate and Disk Resonators
[0030] The fabricated micromechanical resonators, such as those
shown in FIG. 4 above, were tested in a Janis micro-manipulated RF
vacuum probe system in 5 mTorr vacuum and at atmospheric pressure.
Ground-Signal-Ground (GSG) probes from Picoprobe were used. The
bottom Pt electrode was grounded, whereas each of the two top
electrodes was used for either sensing or driving the device under
test. No interface circuitry between the resonator and the network
analyzer was needed. Given the low impedance values, it was
possible to directly measure the frequency response of the
resonators using an Agilent 4195A network analyzer.
[0031] A. Frequency Response of AlN Rectangular Plates
[0032] Various rectangular plates with length to width ratios of 2
and 4 were fabricated and tested. The resonators varied in length
from 80 to 200 .mu.m. All resonators, despite their size and
electrode configuration, exhibited the fundamental
length-extensional mode. The response in vacuum of a 200.times.50
.mu.m.sup.2 plate, showed a Q of 3,280 and a motional resistance,
R.sub.x, of only .about.150.OMEGA. when vibrating in its 1.sup.st
mode (FIG. 2(a)). Higher order modes in plates were excited with
electrodes placed parallel to the plate length and having a length
to width ratio of 4. Q as high as 4,470 were obtained from the
2.sup.nd mode (FIG. 2(b)) of a 200.times.500 .mu.m.sup.2 plate at
80.57 MHz in vacuum. An 80.times.20 .mu.m.sup.2 plate achieved a
measured frequency of 224.6 MHz with Q of 2,580 in vacuum, when
vibrating in its 3rd mode (FIG. 2(c)). The use of
quarter-wavelength tethers minimized the energy lost through the
anchors and enabled achieving quality factors higher than the one
recorded for the 1st mode. It should be noted that the resonators
in accordance with the embodiments of the present invention were
actuated in air without significant Q degradation. Recorded Q
reduction was about 20-30% for all resonators. For example, Q of
2,000 in air and Q of 2,580 in vacuum were recorded for an
80.times.20 .mu.m.sup.2 plate vibrating in its 3rd mode. Other
experimental measurements for dilation-type contour modes in AlN
rectangular plates are summarized in a publication by the inventors
herein (e.g., see, Piazza et al., "Dry-Released Post-CMOS
Compatible Contour-Mode AlN Micromechanical Resonators for VHF
Applications," Hilton Head Workshop, 38-41 (2004)). The recorded
frequencies agreed to within 3% of those obtained by a finite
element analysis.
[0033] A 200.times.50 .mu.m.sup.2 resonant plate exhibited a linear
TCF over a temperature range of 28-100.degree. C. TCF values of
.about.-26 ppm/.degree. C., .about.-25 ppm/.degree. C., and
.about.-22 ppm/.degree. C. were recorded for the same plate
vibrating in its 1.sup.st, 2.sup.nd and 3.sup.rd mode,
respectively. Furthermore, the center-frequency of a plate was
tuned both up and down by purely piezoelectric means. A constant
strain was induced in the resonator by superimposing a DC voltage
to the ac signal on the two top electrodes. This tuning mechanism
resulted in a .+-.3 kHz linear tuning range for a 22.97 MHz
rectangular plate using a 30 V power supply (slope 4.4 ppm/V). Such
a tuning scheme can be employed, for example, to implement
low-power active frequency compensation for temperature variations
of +101C.
[0034] B. Frequency Response of AlN Disks
[0035] 50 .mu.m radius AlN disks vibrating in a wineglass contour
mode shape exhibited Q values as high as 5,830 at a frequency of
43.26 MHz in vacuum. A high quality factor of 3,700 was recorded in
air for the same type of resonators. Using fairly small tethers
(e.g., 5 .mu.m wide) and anchoring the disk at its quasi-nodal
points--since tangential displacement is non-zero at these
locations--resulted in the highest Q for contour modes in AlN
plates. Despite its high Q disk resonators show a motional
resistance (.about.73 kW) much higher than the one recorded for
rectangular plates. Although, having the electrodes over the whole
resonator surface could decrease the motional resistance, the
equivoluminal nature of the mode shape makes its excitation
difficult in AlN films. The same resonator exhibited a linear TCF
of .about.-14 ppm/.degree. C. for a temperature range of
28-100.degree. C. While not being limited to a particular theory,
the smaller value of TCF compared to the one recorded for
rectangular plates may be a consequence of the isovoluminal mode
shape.
[0036] FIG. 5 shows an alternative embodiment of a
rectangular-shaped micromechanical resonator 500 configured for
vibration in its contour-modes, in accordance with the embodiments
of the present invention. The MEMS resonator 500 is shown to have a
three-layer structure, where a layer of piezoelectric material 504
is located between a bottom electrode 502 and a top electrode 506.
FIG. 5 also shows that a pad region on the bottom electrode 503 and
a pad region on the top electrode 507 allow for the application of
an alternating electric field across the thickness of the
piezoelectric resonator 500, where the electric field is configured
to cause the resonator to have a contour mode in-plane lateral
displacement in the planar surface of the resonator. FIG. 5 shows
the pad region 503 to be one two pads 503 that are located one on
either side of the top pad 507. In the embodiment of FIG. 5, the
pads 503, 507, that are used to apply an electric field across the
thickness of the resonator 500, are located on the same side as of
the resonator body. However, it should be noted that pads could be
placed on different sides of the resonator body. The geometry of
the piezoelectric layer 504, the geometry of the bottom electrode
502 and that of the top electrode 506 each in part contribute to
the fundamental frequency of the excitation of the piezoelectric
resonator. The geometries of the planar dimensions of the bottom
electrode, the top electrode and the piezoelectric layer are
patterned and set lithographically.
[0037] FIG. 5 shows that for one geometric configuration, the
bottom electrode 502 surrounds the resonator's three-layer
structure. The three-layer structure is connected with and
supported by the bottom electrode 502 via tether 509. Other than at
the tether 509, the three-layer stack has a free perimeter or
periphery that enables the three layer stack to be excited in its
contour-mode or lateral/extensional modes. The embodiment shown in
FIG. 5 is one of several geometric configurations within the scope
of the embodiments of the present invention, including the
configurations of FIGS. 1(a)-(c), FIGS. 4(a)-(c), as well various
other alternate embodiments described below, and variants and
combinations thereof. It should be noted that the various
alternative geometric configurations described herein are exemplary
embodiments that show various possible configurations. The scope of
the embodiments of the present invention is not limited solely to
these exemplary configurations.
Ring-Shaped Resonators
[0038] FIGS. 6(a)-(b) are exemplary diagrams of a circular
ring-shaped (a) 602a and a square-shaped ring (b) 602b
piezoelectric micromechanical resonator configured for vibrating in
a contour-mode, in accordance with an alternative embodiment of the
present invention. The resonator 602a has a three layer stack for a
circular-ring 604 shaped resonator body. A layer of piezoelectric
material 606 is located between a bottom electrode 610 and a top
electrode 608. The bottom electrode includes pad regions 612 and
the top electrode include pad region 614 that act as ground and
input electrodes respectively. Also shown in FIG. 6(a) is that the
three-layer ring-shaped structure 604 is connected with and
supported by the electrodes via tether 616. Other than at the
tether 616, the three-layer stack has a free perimeter or periphery
that enables the three layer stack to be excited in its
contour-mode or lateral/extensional modes. FIG. 6 shows a structure
where a circular-shaped ring resonator (602a) is configured to be
excited in a radial-extensional contour mode shape. FIG. 6 also
shows an alternate structure where a square-shaped ring resonator
(602b) is configured to be excited in a width-extensional contour
mode shape. Other than a circular or a square-shaped-ring
structure, other shapes may also be used, including rectangle, a
polygon, a circular annulus, a rectangular annulus, a polygonal
annulus and combinations thereof.
[0039] FIG. 6 shows a schematic view of exemplary circular and
square ring-shaped resonators that were excited in radial and
width-extensional contour mode shapes, respectively. For one
example, the resonator body was made out of a layer of AlN
sandwiched between bottom Pt and top Al electrodes. By applying an
AC electric field across the film thickness, the active AlN
piezoelectric layer undergoes (through the d.sub.31 piezoelectric
coefficient) an in-plane lateral displacement that is maximized at
resonance. It is known that circular rings and, similarly, square
rings vibrate in contour mode shapes whose fundamental frequency is
set primarily by the width of the ring (e.g., see, Li. et al., IEEE
MEMS, 821-824 (2004); Bircumshaw et al., Transducers, 875-878
(2003)).
[0040] Finite element analysis has shown that Al and especially Pt
electrodes affect the resonator center frequency. Pt has a large
mass density (6.5 times that of AlN); its mass loads the resonator
and decreases its resonant frequency. Finite element modeling shows
that this phenomenon can be exploited to lithographically vary the
center frequency of the resonator without substantially altering
its performance. This feature, namely the dimensions and the
geometry of the electrodes, which is unique to contour-mode
resonators, may be used to tune the center frequency of resonators,
for example, when employed in ladder filter structures.
[0041] The asymmetry in the composition of the resonator's layers
affects its purely in-plane motion by introducing some warping.
While not being limited to any theory, one could argue that this
phenomenon can ultimately degrade the quality factor and the
transduction efficiency of the resonator by causing loss of the
input energy in unwanted bending and charge cancellation.
Accordingly, a symmetric design may in general be preferable.
[0042] Another unique feature of the ring-shaped resonator designs
(e.g., see, Bircumshaw et al., Transducers, 875-878 (2003)) is the
possibility to arbitrarily select the value of motional resistance
via the choice of the lateral area of the ring. In analogy with
length-extensional rectangular plates (e.g., see, Piazza et al.,
MEMS, 20-23 (2005)) the motional resistance, R.sub.x of the rings
can be approximately expressed by (for R.sub.ave>>W):
R x .apprxeq. .pi. 2 8 T 2 .pi. R ave .rho. QE P 3 / 2 d 31 2 ( 1 )
##EQU00003##
[0043] where R.sub.ave, T and W are the ring average radius,
thickness and width; .rho. and E.sub..rho. are the mass density and
modulus of elasticity of AlN. The same equation holds for a square
ring where 2.pi.R.sub.ave is substituted by the average perimeter
of the ring. Therefore, this design permits changes in R.sub.x by
varying R or L (length of square ring) without affecting the
resonator center frequency, and provides an extra degree of freedom
to the circuit designer (e.g., in oscillators and pass-band filters
design).
[0044] In order to minimize energy loss to the substrate
quarter-wave supports were designed (e.g., see, Li et al., IEEE
MEMS, 821-824 (2004)). Also in order to investigate losses due to
anchoring, the same type of resonators were anchored by two
quarter-wave, diametrically opposed, notched supports. Notching
allows the anchors to be placed directly at the nodal line of the
ring and therefore reduces the interference between the supports
and the natural mode shape of the resonator (e.g., see, Li et al.,
IEEE MEMS, 821-824 (2004)).
[0045] FIGS. 7(a)-(d) are exemplary diagrams of an alternate
embodiment of the fabrication process 700 for the contour mode
resonators in accordance with the present invention. Briefly, in
step (a) low stress nitride is deposited by a low pressure chemical
vapor deposition process (LPCVD) onto a silicon substrate, followed
by a bottom electrode (e.g., Pt) patterning by lift-off and the
sputter deposition of the AlN piezoelectric layer. In step (b), the
top electrode (e.g., Al) is deposited and patterned by dry etching
in a chlorine environment (Cl.sub.2). In step (c), via access to
the bottom electrode is opened through the AlN layer, by wet
etching using a hot (e.g., 160.degree. C.) phosphoric acid
(H.sub.3PO.sub.4) bath. In step (d), the resonant device is dry
etched in Cl.sub.2 and the structure is dry released in XeF.sub.2.
Further details of the fabrication process are provided below.
[0046] For some examples, a four-mask, post-CMOS compatible process
(i.e., T.sub.max<400.degree. C.), such as the one described
above was used to fabricate the devices shown in FIG. 6 above. A 2
.mu.m AlN film was sandwiched between a bottom platinum electrode
and a top aluminum electrode. AlN films were sputter-deposited
using a single-module AMS PVD sputtering tool. Highly crystalline
films with rocking curve values below 20 were obtained. The
fabrication includes the patterning of the bottom Pt electrode on a
low stress nitride buffer layer in order to reduce parasitic
capacitance and electrical losses. The top Al electrode was
patterned by dry etching. Cl.sub.2-based dry etching was used to
pattern AlN and obtain fairly straight sidewalls (e.g., 16.degree.
from the vertical). During the dry etch process the AlN film was
masked by oxide deposited either by electron cyclotron resonance or
by low temperature chemical vapor deposition techniques
(T.sub.max<400.degree. C.). If LTO was used, the top Al
electrode was protected by a thin (<30 nm) layer of Nb that was
removed with the remaining oxide during a CF.sub.4-based dry etch
step. The structures were released by dry etching of silicon in
XeF.sub.2, thereby eliminating stiction forces and significantly
increasing yield. As described above, the patterning of the bottom
electrode enables the on-chip tuning of contour-mode
resonators.
[0047] FIGS. 8(a)-(d) are SEM images showing circular ring-shaped
and square-shaped ring micromechanical resonators produced in
accordance with the fabrication processes of FIGS. 7(a)-(d). FIG.
8(a) shows a SEM image of a single-supported circular-shaped
contour-mode 20 .mu.m wide ring resonator. FIG. 8(b) shows a SEM
image of a single-supported square-shaped contour-mode 20 .mu.m
wide ring resonator. FIG. 8(c) shows a SEM image of a two-support
circular-shaped contour-mode 10 .mu.m wide resonator and a detail
view of notched supports; and FIG. 8(d) shows a SEM image of a
two-support square-shaped contour mode 10 .mu.m wide resonator and
a detailed view of notched support. FIGS. 8(c) and 8(d) show
ring-shaped contour-mode resonators with notched supports. Notched
supports were introduced in order to reduce anchor losses. Although
the top Al electrodes of these notched resonators were patterned in
such a way to permit a two-port configuration, these devices were
successfully tested in a one-port configuration, for which fewer
spurious modes were observed.
Experimental Results--Ring-Shaped Resonators
[0048] The fabricated micromechanical resonators, similar to those
shown in FIGS. 8(a)-(d), were tested in a Janis micro-manipulated
RF probe station at atmospheric pressure. Ground-Signal-Ground
(GSG) probes from Picoprobe were used. S.sub.11 parameters for each
of the devices were extracted and converted into equivalent
admittances using an Agilent E8358A network analyzer. One-port
S.sub.11 parameter calibration (SOL) was performed using short and
open reference structures directly fabricated on the die under
test, whereas a 50.OMEGA. resistor on a ceramic substrate was used
as a load reference.
Frequency Response--Ring-Shaped Resonators
[0049] A typical response of a circular ring microresonator with a
single un-notched support showed that it had a motional resistance
as low as 56.OMEGA. with Q of 2,400 in air for a 223.9 MHz
resonator.
[0050] A comparison of identical resonators showed that notched
supports did not substantially improve the Q of the resonator, but
they did reduce the interference between the anchors and the
resonator motion, especially for resonators with smaller ring
widths. Less mechanical interference translates into a cleaner
electrical signal, with fewer spurious resonances. Other
experimental results showed that a Q of 2,900 was obtained for a
circular ring resonator at 472.7 MHz with notched supports, a width
of 10 .mu.m and inner radius of 90 .mu.m. The motional resistance
of this resonator was .about.84.OMEGA..
[0051] To further prove the less intrusive nature of notched
supports, a 20 .mu.m wide ring with 90 .mu.m inner radius was
excited in its 2.sup.nd overtone reaching a frequency of 656.2 MHz
with Q of 1,400, and R.sub.x.about.170.OMEGA..
[0052] The value of the motional resistance of the resonators is
controllable by changing the size of the inner radius. A 20 .mu.m
wide circular ring with an inner radius of 40 .mu.m exhibited a
motional resistance of 204.OMEGA. at 227.4 MHz. This reduction in
size affects the quality factor of the resonators such that the Q
value was reduced to 1,100 from 1,400.
[0053] By changing the width of top and bottom electrodes (without
changing the width of the AlN ring) the center frequency of these
resonators may be tuned lithographically. For example, two circular
ring resonators (20 .mu.m wide and with 140 .mu.m inner radius)
having different electrode area (coverage changes from 65% to 95%
of the total resonator area) show a frequency shift of 6.8% from
232 MHz to 249 MHz. The frequency shift does not affect the
strength of the signal because the majority of the strain is
located close to the nodal lines; therefore, the strain is
efficiently transduced. A TCF of approximately -25 ppm/.degree. C.
was recorded for the circular ring resonators.
[0054] Square-shaped micromechanical ring resonators were excited
in width-extensional mode shapes. The typical response for a 10
.mu.m wide resonator with inner ring length of 180 .mu.m is shows
that the resonators were able to reach a frequency as high as 475.3
MHz with respectable Q of 1,600 and R.sub.x.about.130.OMEGA.. A TCF
of approximately -25 ppm/.degree. C. was recorded for the
square-shaped micromechanical ring resonators. Additional
experimental results are provided in Piazza et al., "Low Motional
Resistance Ring-Shaped Contour-Mode AlN Piezoelectric
Micromechanical Resonators for UHF Applications," IEEE MEMS 20-23
(2005).
Examples--Anchor Losses in Circular Rings
[0055] In order to study the effect of anchor losses on circular
rings, un-notched tethers with three different widths (6, 10 and 20
.mu.m) were fabricated. The size of the supports slightly affects
the resonator center frequency, because of the increased stiffness,
but no net change in Q was recorded. Resonators with two notched
supports achieve Q factors as high as those obtained for devices
with just a single un-notched support. The inventors herein believe
that devices with a single notched support could provide a higher
Q.
Band Pass Filter Circuits
[0056] The contour-mode rectangular plate and ring-shaped
piezoelectric resonators described above may be used as the
building blocks for the various circuit configurations such as, for
example, multiple-frequency band pass filter and oscillator
circuits. The embodiments of the present invention enable the
formation of multiple-frequency, band pass filters on the same
chip. FIG. 9(a) shows a ladder filter (e.g., L-network)
configuration incorporating the contour-mode micromechanical
resonators in accordance with the embodiments of the present
invention (a). FIG. 9(b) shows a schematic representation of the
electrical response of individual series and shunt resonators and a
ladder filter of FIG. 9(a). The basic ladder filter configuration
is composed of series and shunt resonators (FIG. 9(a)) to form an L
network. Then these networks can be cascaded to form more
complicated multi-pole filters. These networks have been analyzed
using an ABCD matrix approach (e.g., see, Lakin et. al., "Thin Film
Bulk Acoustic Wave Filters for GPS," Ultrasonic Symposium, 471-476
(1992)). For an initial proof of concept, high order filters (3rd
and 4th) were built by simply cascading L networks.
[0057] When designing band pass filters, it is desired to achieve a
symmetrical group delay, low insertion losses and large out-of-band
rejection. In order to provide a symmetrical group delay the filter
termination, R.sub.term, is chosen to be the geometric mean of the
series and shunt resonator reactances (e.g., see, Lakin et. al.,
"Thin Film Bulk Acoustic Wave Filters for GPS," Ultrasonic
Symposium, 471-476 (1992)), such that:
R term = 1 .omega. C 1 C P C S ( 1 ) ##EQU00004##
where .omega..sub.c is the filter center frequency, and C.sub.P and
C.sub.S are the parallel capacitance of the shunt and series
resonators, respectively. Maximum S.sub.21 is obtained when the
parallel resonance of the shunt branch, f.sub.PP, coincides with
the series resonance of the series branch, f.sub.SS. In this
condition, the insertion losses can be expressed as:
I . L . .apprxeq. 1 - n .omega. C C S R term k t 2 Q ( 2 )
##EQU00005##
where n is the number of L networks, k.sub.t.sup.2 is the effective
electromechanical coupling of AlN (e.g., see, Lakin et. al., "Thin
Film Bulk Acoustic Wave Filters for GPS," Ultrasonic Symposium,
471-476 (1992)) and Q is the quality factor of the individual
series resonators. In order to minimize losses, large k.sub.t.sup.2
and Q are desired. Out-of-band rejection is set by the capacitance
ratio between the series and shunt branch and by the number of L
stages, such that:
S 21 _ Out - of - Band .apprxeq. 1 ( 1 + C P / 2 C S ) n ( 3 )
##EQU00006##
[0058] The fractional bandwidth of the filter is set by the
distance between the two zeros (the parallel resonance of the
series branch, fps, and the series resonance of the shunt branch,
f.sub.SP, respectively) of the filter transfer function. Ultimately
this translates into:
f PS - f SP f c .apprxeq. 8 .pi. 2 k t 2 ( 3 ) ##EQU00007##
[0059] Therefore, the bandwidth of the filter depends on the
electromechanical coupling coefficient and is theoretically limited
to a maximum of .about.2.5%, unless other external elements are
used.
[0060] Amongst the principal parameters on which the designer can
act are the values of the parallel capacitance of the series and
shunt resonators as well as their ratio and the number of stages
required. In order to minimize insertion losses and at the same
time provide good out-of-band rejection, a maximum number of 3 or 4
stages are selected. The capacitance ratio is chosen so that good
out-of-band rejection can be obtained while maintaining high-Q
resonators (e.g., in Piazza et al., "Low Motional Resistance
Ring-Shaped Contour-Mode AlN Piezoelectric Micromechanical
Resonators for UHF Applications," IEEE MEMS, 20-23 (2005)). It was
shown that Q degrades when the inner radius of the ring is made
smaller. In the example filter circuits used herein capacitance
ratios of 1 and 0.67 were used.
[0061] A four-mask, low-temperature, post-CMOS compatible
(T.sub.max<400.degree. C.) process, such the process of FIGS.
7(a)-(d) was used to fabricate the filter devices. The process flow
is substantially the process of FIG. 7(a)-(d), and used
micromachining technique to manufacture high quality AlN resonators
with high yield. For example, a single resonator has a 2 .mu.m AlN
film sandwiched between a bottom platinum electrode (.about.100 nm
thick) and a top aluminum electrode (.about.175 nm thick). AlN
films were sputter-deposited using an AMS PVD tool and exhibit
rocking curve values as low as 1.3.degree. on Pt seed layers. The
difference with respect to the fabrication process described in
FIGS. 7(a)-(d) involves performing the wet etch of AlN to access
the bottom electrode before the deposition and patterning of the
top Al electrode. This step is used to implement a ladder filter
topology, for which contact between the top and bottom electrodes
is required.
[0062] As it was shown in FIG. 9(b), the parallel resonance of the
shunt brunch should coincide with the series resonance of the
series branch for the filter to work properly. The shift can be
defined directly during the processing step used for patterning the
Pt electrode. The Pt electrode has a very large mass density, about
6.5 times the one of AlN. By removing small amounts of Pt the
center frequency of the resonator may be raised. Being most of the
strain (and consequently charge) and concentrated in the middle of
structure, small amounts of Pt are lithographically removed from
the edges of the microstructures (FIG. 10) without affecting the
overall performance of the resonators. The loading mechanism was
experimentally verified and may be explained by an analytical model
based on vibration techniques (e.g., see, Graff, "Wave Motion in
Elastic Solids," Dover Publications, Inc., New York). The
experiments showed that for a 100 nm thick Pt the center frequency
of a 90 .mu.m inner radius ring structure shifts by .about.7000 ppm
for each .mu.m of Pt that is removed from the width of the
structure, rectangular plate shifts by 500 ppm for each .mu.m of Pt
that is removed from the length of the structure.
[0063] FIG. 10(a) shows SEM images showing a ladder filter made
using ring-shaped micromechanical resonators in accordance with the
embodiments of the present invention. FIG. 10(a) shows a SEM view
of a ladder filter made from eight electrically cascaded
ring-shaped resonators and a detailed view of the ring and the
lithographically defined variation in the bottom electrode width
used to tune the center frequency of the series resonators. FIG.
10(b) shows SEM images of a ladder filter made using
rectangular-shaped micromechanical resonators in accordance with
the embodiments of the present invention. FIG. 10(b) shows a SEM
view of a ladder filter made from eight electrically cascaded
rectangular plate resonators and a detailed view of the single
rectangular resonator and the lithographically defined variation in
the bottom electrode length used to tune the center frequency of
the series resonators.
[0064] An alternate method for achieving the frequency shift is by
changing the dimensions of the inner radius of the rings. By having
a ring in the shunt branch with a diameter larger than the one in
the series branch, the resonance frequency of the resonators in the
shunt branch can be lowered, while increasing the out-of-band
rejection.
Experimental Results--Filters
[0065] The fabricated micromechanical filters were tested in a RF
probe station at atmospheric pressure. Ground-Signal-Ground (GSG)
probes were used. Two-port S-parameter calibration (SOLT) was
performed using short, open and through reference structures
directly fabricated on the die under test, whereas a 50.OMEGA.
resistor on a ceramic substrate was used as a load reference. Full
S-parameter matrices were extracted for each filter using an
Agilent E5071B network analyzer. No external terminations were
connected to the device under test. The network analyzer is used to
automatically change the terminations and compute the filter
transmission spectrum.
[0066] Eight rings, all with an inner radius of 90 .mu.m and 20
.mu.m width, were electrically cascaded in a ladder structure. The
frequency of the series and shunt branches were shifted by
.about.0.3%. This filter showed fairly moderate insertion losses of
-7.9 dB at 236.2 MHz, an out-of-band rejection of 26 dB and a 20 dB
shape factor of 2.79. The filter does not suffer from any other
spurious resonance. As described above, the frequency shift was
also defined by changing the size of the inner radius of the rings
in the shunt branch (inner radius of 140 .mu.m was used in the
shunt branch and 90 .mu.m in the series branch). Up to six rings
were connected in this configuration.
[0067] Four, six and eight 200 .mu.m long and 50 .mu.m wide
rectangular plates were tested in a ladder configuration as well.
Again, the frequencies were shifted by about 0.3%. Insertion losses
as low as -4 dB were recorded at 93.2 MHz; out-of-band rejection of
27 dB were achieved. The results showed that a second band pass
filter exists due to the length-extensional mode shape present in
the plate. This mode may be pushed further down in frequency by
changing the aspect ratio of the microstructures. The results for
four and six rectangular resonant filters are summarized in Table
1.
TABLE-US-00001 TABLE I Response parameters of band pass filters
built using rectangular plate resonators. Number of f.sub.c
BW.sub.3 dB I.L. 20 dB R.sub.term Resonators [MHz] [kHz] [dB] S.F.
[k.OMEGA.] 4 93.5 456 -2 N.A. 1.5 6 93.1 332 -4.7 2.7 2
[0068] In addition to the L network arrangement for the ladder
filter formed by electrically cascading a network of resonators,
other network arrangements, including Pi, T or L network
configurations may be used. Furthermore, filter networks may be a
part of a band pass filter circuit formed using a series of either
of the Pi, T or L networks.
[0069] The examples of the filter circuits in accordance with the
embodiments of the present invention show that using a novel and
disruptive MEMS resonator technology based on the excitation of
contour mode shapes in AlN microstructures, band pass filters at 93
and 236 MHz were produced, by electrically cascading up to eight
resonators in a ladder structure. These filters showed a good
performance, being characterized by low insertion losses (4 dB at
93 MHz), large close-in and out-of-band rejection (a 40 dB and
.about.27 dB, respectively, for a 93 MHz filter) and fairly sharp
roll-off with a 20 dB shape factor of .about.2.2. The filters
produced in accordance with the embodiments of the present
invention are approximately 30 times smaller than existing SAW
technology, commonly used in the IF bands for cell phones. In
addition, with a temperature coefficient of -25 ppm/.degree. C.,
they have 40% lower temperature sensitivity than SAW filters.
[0070] The MEMS resonator technology based on the excitation of
contour mode shapes in piezoelectric microstructures in accordance
with the embodiments of the present invention may be used to form
filter circuits such as those described above. In addition, the
contour-mode piezoelectric resonators in accordance with the
embodiments of the present invention may also be used as part of
other circuits, such as, for example, an oscillator circuit (e.g.,
a Pi-network). FIG. 11 is an exemplary oscillator circuit 1100
diagram that may be configured to include a contour-mode
micromechanical resonator 1102 in accordance with the embodiments
of the present invention.
[0071] As will be understood by those skilled in the art, the
present invention may be embodied in other specific forms without
departing from the essential characteristics thereof. For example,
the geometry or materials for the top and the bottom electrodes may
be the same or different, and either one may be produced from
aluminum, platinum, tungsten, molybdenum, ruthenium, chrome, gold,
titanium doped polycrystalline silicon, and combinations thereof.
These other embodiments are intended to be included within the
scope of the present invention, which is set forth in the following
claims.
* * * * *