U.S. patent application number 14/039290 was filed with the patent office on 2014-01-23 for integrated acoustic bandgap devices for energy confinement and methods of fabricating same.
This patent application is currently assigned to CYMATICS LABORATORIES CORP.. The applicant listed for this patent is CYMATICS LABORATORIES CORP.. Invention is credited to Peter Ledel Gammel, Marco Mastrapasqua, Hugo Safar, Rajarishi Sinha.
Application Number | 20140022009 14/039290 |
Document ID | / |
Family ID | 40532622 |
Filed Date | 2014-01-23 |
United States Patent
Application |
20140022009 |
Kind Code |
A1 |
Sinha; Rajarishi ; et
al. |
January 23, 2014 |
INTEGRATED ACOUSTIC BANDGAP DEVICES FOR ENERGY CONFINEMENT AND
METHODS OF FABRICATING SAME
Abstract
The present invention is directed to monolithic integrated
circuits incorporating an oscillator element that are particularly
suited for use in timing applications. The oscillator element
includes a resonator element having a piezoelectric material
disposed between a pair of electrodes. The oscillator element also
includes an acoustic confinement structure that may be disposed on
either side of the resonator element. The acoustic confinement
element includes alternating sets of low and high acoustic
impedance materials. A temperature compensation layer may be
disposed between the piezoelectric material and at least one of the
electrodes. The oscillator element is monolithically integrated
with an integrated circuit element through an interconnection. The
oscillator element and the integrated circuit element may be
fabricated sequentially or concurrently.
Inventors: |
Sinha; Rajarishi;
(Pittsburgh, PA) ; Gammel; Peter Ledel; (Millburn,
NJ) ; Mastrapasqua; Marco; (Annandale, NJ) ;
Safar; Hugo; (Westfield, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CYMATICS LABORATORIES CORP. |
PITTSBURGH |
PA |
US |
|
|
Assignee: |
CYMATICS LABORATORIES CORP.
Pittsburgh
PA
|
Family ID: |
40532622 |
Appl. No.: |
14/039290 |
Filed: |
September 27, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13339505 |
Dec 29, 2011 |
8564174 |
|
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14039290 |
|
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|
12002524 |
Dec 17, 2007 |
8089195 |
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13339505 |
|
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Current U.S.
Class: |
327/564 |
Current CPC
Class: |
Y10T 29/42 20150115;
H03H 9/175 20130101; H03H 9/564 20130101; H01L 27/02 20130101; H03H
9/0557 20130101; H03H 9/589 20130101 |
Class at
Publication: |
327/564 |
International
Class: |
H01L 27/02 20060101
H01L027/02 |
Claims
1. An integrated circuit device comprising: a circuit element means
having a resonator means and an acoustic confinement means for
preventing the ultrasonic wave from propagating away from the
resonator means; and an integrated circuit means having a
interconnect means; wherein the circuit element means is
monolithically connected to the integrated circuit means in a
unitary structure.
2. The integrated circuit of claim 1 wherein the resonator means
has first and second electrodes and wherein one of the first and
second electrodes is electrically connected to the integrated
circuit interconnect means.
3. The integrated circuit device of claim 1 wherein the circuit
element means further comprises a temperature compensation
means.
4. The integrated circuit of claim 1 wherein the acoustic
confinement means provides the circuit element means with a bandgap
at a predetermined frequency f.sub.0.
5. The integrated circuit of claim 4 wherein, in operation, the
resonator means vibrates within the acoustic confinement means.
6. The integrated circuit of claim 1 wherein the resonator means
and the acoustic confinement means form an acoustic band gap
structure.
7. The integrated circuit of claim 4 wherein the acoustic
confinement means has a peak reflectivity at the predetermined
frequency f.sub.0.
8. The integrated circuit of claim 1 wherein the acoustic
confinement means is a package for the resonator means.
9. The integrated circuit of claim 1 wherein the circuit element
means is an oscillator means.
10. The integrated circuit of claim 1 wherein the circuit element
means is a filter means.
11. The integrated circuit of claim 9 wherein the circuit element
means is an oscillator means and a filter means.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/339,505, filed on Dec. 29, 2011, which is a
divisional of U.S. patent application Ser. No. 12/002,524, filed on
Dec. 17, 2007, which issued on Jan. 3, 2012, as U.S. Pat. No.
8,089,195, the disclosures of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] The present invention provides monolithic integrated
circuits that are particularly adapted for use in timing
applications, as well as methods for designing and fabricating
same.
[0003] Conventional electronic appliances include a timing unit to
provide a timing base that controls the internal functioning of the
appliance as well as communications with other appliances or
devices. Such timing units often comprise a resonator and a driving
circuit. In many cases the resonator and circuit are fabricated
using different technologies, and thus need wiring or other manner
of interconnection in order to function together.
[0004] Typically, the resonator is a piezoelectric device that
vibrates in free space. The most common resonator is a quartz
crystal oscillator that when driven at one of its mechanical
resonant frequencies has either a minimum or maximum in its
electrical impedance, which is used by the driving circuit to lock
itself at this frequency value. Finally, the circuit outputs a
signal, for example a sine wave of a well determined and stable
frequency that is used by the appliance as a time base.
[0005] Another typically used type of resonator is a "MEMS"
resonator. MEMS stands for micro-electro-mechanical system. MEMS
devices integrate mechanical and electronic elements on a common
substrate through microfabrication technology.
[0006] For such devices, the requirement of free space vibration
implies that the devices need to be carefully packaged. Otherwise
they will not work as intended. For instance, quartz crystal
oscillators are often packaged in hermetically sealed ceramic
packages that allow motion of the quartz part. Motion in this case
is driven and recorded by means of electrical connections that join
electrodes on the quartz resonator with leads inside the package
that in turn are connected though the walls of the ceramic package
to external leads or pads. MEMS oscillators may also be placed in
hermetically sealed packages. However, unlike quartz crystal
oscillators, since MEMS devices are often fabricated on silicon
wafers, the hermetic cavity is commonly produced on this wafer
rather than on the package.
[0007] Such oscillator packaging technology requires the creation
of a cavity with very well controlled conditions and is commonly
filled with inert gases or a vacuum. The creation of such cavities
often requires delicate and expensive assembly processes. Further,
cavities pose additional challenge for making the necessary
electrical connection to the oscillator devices.
[0008] Unfortunately, by their fragile nature the aforementioned
free space devices are sensitive to vibration that degrades their
performance or shock that can destroy them. Such free space devices
are subject to a force when experiencing acceleration. This force
increases as the magnitude of acceleration increases. It is
possible for the force to eventually reach a level sufficient to
perturb the natural motion of the oscillator and even break the
delicate component.
[0009] It is also important to note that, typically, the
acceleration and resulting force experienced by resonator devices
is not steady, but rather changes over time. For example, in the
case of a rocket launch, devices aboard the rocket experience
acceleration that initially grows rapidly, reaches a maximum, and
eventually returns to nearly zero as the rocket is in flight.
Another example is in the case of devices located in a moving
vehicle where vibrations in the vehicle translate to rapidly
changing accelerations. The frequency spectrum of many common
vibrations is in the range of 10 to 100 kHz. This range can be
close, and often includes, the range at which common oscillators
work. The fact that such devices are free to move makes them very
sensitive to the aforementioned effects.
[0010] Thus, it is desirable to develop resonating devices which do
not rely on free space and which can be fabricated in an integrated
manner with associated circuitry such as a driving circuit. It is
also desirable to optimize such resonating devices for timing
applications.
BRIEF SUMMARY OF THE INVENTION
[0011] In accordance with aspects of the present invention,
monolithic devices are provided which include a resonator element
that can vibrate, within a certain frequency range, inside an
engineered solid. By removing the need for free space vibration in
a hermetic enclosure, devices embodying the present invention are
effectively immune to shock, vibration and perturbation of the
conditions at the surface of the devices. As explained in detail
below, design and fabrication of circuit elements with such
resonator elements may be done using the same technology as is used
for fabricating the driving circuit, thus providing for efficient
fabrication.
[0012] In one embodiment the integrated circuit device has an
oscillator element that provides a monolithic timing solution. The
oscillator element has a resonator element with at least a first
bottom electrode and second top electrode and a piezoelectric
material interposed between. The resonator element is embedded in
an acoustic confinement structure. The integrated circuit component
is typically a semiconductor substrate with a plurality of
semiconductor device elements formed thereon and at least one metal
interconnect layer formed over the plurality of semiconductor
elements. The oscillator element is monolithically integrated with
the integrated circuit element through at least one metal via
electrically interconnecting one of the first or second electrodes
with at least one metal interconnect layer.
[0013] In a further embodiment the resonator element has a
temperature compensation layer. In yet a further embodiment the
temperature compensation layer is sandwiched between the
piezoelectric layer and the top electrode of the resonator
element.
[0014] In yet another embodiment, the circuit element of the
integrated circuit device is a filter element. The filter element
has a resonator element that is embedded in an acoustic confinement
structure. In yet another embodiment, the integrated circuit device
has both an oscillator element and a filter element.
[0015] As further described in the embodiments, the acoustic
confinement structure is a periodic structure with a plurality of
layers. In these embodiments, the structure has at least two layers
of high acoustic impedance alternating with at least two layers
having a low acoustic impedance. In further embodiments the
periodic structure has a first period having a first layer of high
acoustic impedance material and a first layer of low acoustic
impedance material. The first period is under the bottom electrode.
A second period has at least two layers of the periodic structure
one of which is a second layer of high acoustic impedance material
and the other of which is a second layer of low acoustic impedance
material. The first period is disposed between the bottom electrode
and the integrated circuit component. The second period is disposed
on the top electrode.
[0016] Examples of suitable low acoustic impedance materials are
silicon (Si), polysilicon, silicon dioxide (SiO2), silicon
oxy-carbide ("SiO.sub.xC.sub.y"), aluminum (Al) and, polymers and
polymethylmethacrylate ("PMM"). Examples of high acoustic impedance
material include gold (Au), molybdenum (Mo) tungsten (W), iridium
(Ir), platinum (Pt), tantalum pentoxide ("TaO.sub.5") and AlN. In
one embodiment the first layer of low acoustic impedance material
is interposed between the bottom electrode and the first layer of
high acoustic impedance and the second layer of low acoustic
impedance material is interposed between the top electrode and the
second layer of high acoustic impedance material.
[0017] In a preferred embodiment, the thickness of the high
acoustic impedance material layer is about ten percent less than a
quarter wavelength thickness.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The foregoing and other objects and advantages of the
invention will be appreciated more fully from the following further
description thereof, with reference to the accompanying drawings,
wherein:
[0019] FIGS. 1A-B illustrate an exemplary vibratory system in
accordance with aspects of the present invention.
[0020] FIGS. 2A-B illustrate an exemplary diatomic vibratory system
in accordance with aspects of the present invention.
[0021] FIGS. 3A-B illustrate an exemplary diatomic vibratory system
having an impurity in accordance with aspects of the present
invention.
[0022] FIG. 4 is an exemplary piezoelectric structure in accordance
with aspects of the present invention.
[0023] FIG. 5 is a schematic illustrating an exemplary oscillator
circuit in accordance with aspects of the present invention.
[0024] FIG. 6 illustrates an interconnect structure for a
monolithic bandgap device in accordance with aspects of the present
invention.
[0025] FIG. 7 illustrates an alternative interconnect structure for
a monolithic bandgap device in accordance with aspects of the
present invention.
[0026] FIGS. 8A and 8B are cross sections of two embodiments of a
differentially driven oscillator element according to aspects of
the present invention
[0027] FIG. 9 is a top down view of another embodiment having a
filter element and an oscillator element.
DETAILED DESCRIPTION
[0028] The invention is described in terms of several embodiments.
These embodiments are described in terms of an oscillator element
that contains a resonant structure referred to as a resonant
element.
[0029] As explained above, quartz crystal and MEMS oscillators may
be unsuitable in various applications due to their fragile
structures. In accordance with certain aspects of the present
invention, such deficiencies may be overcome by employing
monolithic integrated circuits configured for use in timing and
related applications. In particular, materials are employed so that
vibrations of a certain frequency or range of frequencies cannot
propagate across them. Such materials can be said to have a
band-gap at a given frequency or frequencies.
[0030] In order to understand how materials and devices of the
present invention function, it is useful to consider a number of
idealized cases. First, the exemplary structure 10 of FIG. 1A
presents a linear chain of particles 12 of the same mass that are
interconnected by identical springs 14. This is a well-defined
problem in physics, and it is known that an acoustic wave of any
frequency f has an associated wave vector k according to the
following equation:
k=2*.pi./.lamda.. (Equation 1)
[0031] The wave vector k is determined by the wavelength .lamda..
The relationship between frequency and the wave vector is
illustrated in the plot 20 of FIG. 1B.
[0032] In a more complicated situation presented in the structure
30 shown in FIG. 2A, some of the particles 12 may be replaced by
other particles 32 having a different mass. The result is a
diatomic chain. In this situation, the solution indicates that
there is a certain frequency range about a given frequency,
f.sub.0, for which waves do not propagate along the structure. This
frequency region is termed the "bandgap" and the frequency f.sub.0
is defined by the masses of each particle 12 and 32 and the
strength of the springs joining the particles. This is shown in the
plot 40 of FIG. 2B.
[0033] An even more complicated situation occurs when an impurity
is placed into a diatomic chain. FIG. 3A illustrates a structure 50
which includes an impurity or particle 52 in a chain having the
particles 12 and the particles 32 connected by the springs 14. As
shown in the plot 60 of FIG. 3B, when the impurity is driven by a
force at frequency f0, the amplitude of motion Ux decays sharply
along the chain, because this frequency is not an allowed solution
for the chain motion.
[0034] Given this understanding, it is possible to address the
problem of packaging a resonator element so that it may move at
certain frequencies while being very resistant to shock and
vibrations. For instance, in accordance with an aspect of the
invention, the aforementioned particles are instantiated by layers
of specially selected materials while a piezoelectric film acts as
the "impurity" 52 of FIG. 3A.
[0035] FIG. 4 illustrates a cutaway view of an oscillator element
100 in accordance with a preferred embodiment of the present
invention. The oscillator element 100 is preferably disposed on a
substrate 102. As shown, the oscillator element 100 includes a
number of material layers that are disposed on the substrate 102.
The layers include a number of first layers 104 and a number of
second layers 106. The oscillator element 100 also includes a
resonator element 108 which preferably includes a piezoelectric
material 110 interposed between a bottom electrode 112a and a top
electrode 112b.
[0036] The layers 104 and 106 form an acoustic confinement
structure 114. In a preferred arrangement, the first and second
layers 104 and 106 are disposed in an alternating arrangement below
and above the resonator element 108. However, the number of layers
and their composition are largely a matter of design choice. For
example, the embodiment illustrated in FIG. 6 has two pairs of
layers (104 and 106) plus one additional layer 104. The balance
struck in this embodiment is between protection of the resonator
(which favors more layers) and manufacturing simplicity (which
favors fewer layers). Furthermore, while the illustrated structure
has alternating first (104) and second (106) layers, precise
correspondence between the number of first and second layers is not
required. For example, in FIG. 4 there are illustrated two pairs of
first and second layers (102, 104) and one additional second layer
(104). Furthermore, there is no requirement that the individual
first layer 102 or the individual second layer 104 be the same
material as long as the materials have the desired high or low
acoustic impedance. Furthermore, the oscillator element 100 is
desirably symmetric about the line A-A of FIG. 4, although it is
not required.
[0037] The resonator element 108 is operable to vibrate within the
encapsulating acoustic confinement structure 114. The piezoelectric
material 110, such as a piezoelectric film, can be vibrated by
applying an alternating electric field to the electrodes 112a and
112b. The vibration, for instance with respect to amplitude and
frequency, functions as an ultrasonic wave. The acoustic
confinement structure 114 prevents the ultrasonic wave from
propagating away from the piezoelectric material 110. The overall
structure of the oscillator element 100 creates an acoustic band
gap structure.
[0038] By keeping acoustic energy away from exterior surfaces of
the oscillator element 100, such as surface 116, the performance of
the resonator element 108 is protected from the external
environment. In addition, the monolithic structure of the
oscillator element 100 results in a self-packaged device that is
effectively insensitive to shock and vibration. This, in turn,
makes any subsequent assembly and packaging of systems
incorporating the oscillator element 100 much easier.
[0039] When fabricating oscillator elements 100 having the general
structure described above, it is important to select the thickness
and material(s) of each film or layer carefully to achieve desired
results. For instance, the thickness of the piezoelectric material
110 and the electrodes 112a and 112b should be selected so that the
resonator element 108 has a resonant frequency at the desired
frequency of oscillation. The relationship between frequency and
thickness is determined by parameters including density and speed
of sound of each material layer.
[0040] In accordance with an example where a 1.75 GHz resonator is
desired, the electrodes 112a and 112b may each comprise a layer of
about 0.3 .mu.m thick molybdenum ("Mo"), and the piezoelectric
material 110 may comprise a layer of about 1.4 .mu.m thick
aluminum-nitride ("AlN"). This determination was made by finite
element analysis using known techniques.
[0041] With regard to the acoustic confinement structure 114, it
can be seen in FIG. 4 that the structure may be split into a pair
of acoustic confinement portions that sandwich about or otherwise
encapsulate the resonator element 108. The first and second layers
104 and 106 of the acoustic confinement structure 114 desirably
alternate in both portions of the structure. One of the layers 104
or 106 preferably has a low density ("D") and a low speed of sound
("Vs"), while the other one of the layers 106 or 104 preferably has
a high density D and a high speed of sound Vs. The acoustic
impedance of a given layer is determined according to the following
equation:
acoustic impedance=D*Vs (Equation 2)
[0042] It has been determined that pairing one layer of material
that has a very high acoustic impedance with another layer that has
a very low acoustic impedance results in enhanced confinement of
acoustic energy, and thus a more effective oscillator element 100.
By way of example only, tungsten ("W") has a high density and a
high speed of sound, while silicon dioxide ("SiO.sub.2") has a low
density and a low speed of sound. Alternating layers of W with
layers of SiO.sub.2 provides excellent acoustic confinement.
[0043] The thickness and material of the high and low acoustic
impedance layers are preferably chosen so that the layers have a
peak reflectivity at the desired oscillation frequency (f.sub.0) of
the resonator element 108.
[0044] For the 1.75 GHz resonator example above, finite element
analysis can be used to identify suitable thickness for the
confinement layers. According to such analysis, layers of W about
0.78 .mu.m thick and layers of SiO.sub.2 about 0.85 .mu.m thick are
suitable.
[0045] While certain materials have been identified in the examples
above, the invention is not limited to those materials. Various
materials and combinations of materials can be employed for the
piezoelectric material 110, the electrodes 112a and 112b, and the
layers of the acoustic confinement structure 114.
[0046] By way of example only, suitable materials for the
piezoelectric layer 110 include the aforementioned AlN as well as
zinc oxide ("ZnO"). Voltage activated materials such as strontium
titanate ("STO") or barium strontium titanate ("BST") may also be
employed. Such voltage activated materials have a piezoelectric
strength that depends on a static voltage that can be applied
between the electrodes 112a and 112b in addition to the alternating
voltage. Although piezoelectric layer 110 is illustrated as a
single layer, other embodiments contemplate a piezoelectric layer
that has a plurality of individual piezoelectric layers.
[0047] While different materials may be employed for the electrodes
112a and 112b, it is desirable to select a combination of good
electrical conduction and low acoustic loss to achieve better
resonator performance. Such materials include Mo as well as W,
aluminum ("Al"), platinum ("Pt"), and/or iridium ("Ir"). Non metal
conductive materials (e.g. doped amorphous silicon) are also
contemplated as suitable.
[0048] As explained above, the acoustic confinement structure 114
desirably includes a series of alternating layers 104 and 106
disposed on both sides or otherwise enclosing the resonator element
108. Preferable low acoustic impedance materials include SiO.sub.2,
silicon oxy-carbide ("SiO.sub.xC.sub.y") Si, polysilicon, organic
materials such as polymethyl methacrylate (PMM), metals such as Al
and polymers. Suitable high acoustic impedance materials include,
by way of example only, W, Mo, Ir, Pt, tantalum pentoxide
("TaO.sub.5"), gold ("Au"), doped amorphous silicon, and AlN. In
embodiments where SiO.sub.xC.sub.y is the low acoustic impedance
material, SiO.sub.2 is a suitable high acoustic impedance material.
Thus, when choosing pairs of high and low acoustic impedance
materials, it can be seen that the exemplary materials identified
above provide a wide variety of combinations. While any combination
among such high and low impedance materials may be employed, some
desired combinations include W and SiO.sub.2, Ir and SiO.sub.2, W
and SiOH, as well as Ir and SiO.sub.xC.sub.y.
[0049] The thickness of each layer should be selected for operation
at the target frequency. The thicknesses identified above in the
various examples are merely illustrative. It is not required that
the thickness of a given layer be exact. For instance, it has been
determined that layers in the acoustic confinement structure 114
may vary by approximately 10% without substantial degradation in
performance. And layers in the resonator element 108 may vary by up
to about 5% or more preferably up to about 3% while achieving
satisfactory performance.
[0050] The configuration of the oscillator element 100 makes it
well suited for integration with other devices and/or components in
an integrated circuit. In an example, an integrated circuit may be
fabricated on a substrate and the oscillator element may be
electrically coupled to the integrated circuit via a metal
interconnection layer. For instance, the resonator of the
oscillator element may be electrically coupled to a driving circuit
using one or more via connections, resulting in a monolithic
oscillator solution. Alternatively, the resonator could be
fabricated with exposed pads. In this case, the resonator could be
coupled to external circuitry using wire bonding or solder
bumps.
[0051] The above-identified oscillator element 100 may be used in
timing applications as well as filter applications. However, in
accordance with preferred aspects of the invention, the oscillator
element is configured to optimize performance for a clock used in
timing applications. Such optimization results in selecting
different materials and thicknesses of those materials than one
would select for a filter structure.
[0052] It is important to understand that regardless of its use,
the electrical impedance of a single resonator has two main
frequencies. One is called the series resonance frequency ("fs") or
"zero" at which the electrical impedance reaches a minimum. The
other is called the parallel resonance ("fp") or "pole" at which
the electrical impedance reaches a maximum. Either one of these
resonances may be used in an oscillator circuit.
[0053] Resonators used for clocks are desirably operated along a
very narrow range around one of the frequencies fs or fp. Most
preferably, the range is less than 1% about the frequency. In
contrast, filter devices typically include several interconnected
resonators, each of which may have a different set of series and
parallel frequencies. In this case, a filter device is expected to
perform along a range of frequencies below the lowest fs and above
the highest fp among the resonators in the filter. This may be a
wide range of frequencies about a mean frequency and typically with
a bandwidth on the order of 10% to 20%.
[0054] Not only do the operating ranges differ substantially
between clocks and filters, but the impact of quality factor ("Qs")
of the resonator is also different. Qs is a measure of how sharp
the resonance is. The designs for resonators in clock applications
seek to maximize Qs. This enables the clock to reach a stable time
base, which is measured as very low phase noise in the output of
the device. In contrast, bandwidth is a major concern for filter
designers and Qs is less critical. Thus, conventional designs for
filter resonators often achieve lower Qs in order to obtain a
greater bandwidth.
[0055] The very different goals for clocking applications and
filter applications can result in surprisingly different choices in
materials and thickness when preparing the overall design and when
seeking to optimize the design.
[0056] As the disclosed device is particularly configured for
clocking applications, it is very important to focus on the
resonator element. For instance, in a case of a resonator element
used in a clocking circuit at its series resonance frequency, it is
desirable to maximize Qs while minimizing energy loss at the
electrode. Mathematically, this may be expressed as:
1/Qs=1/Q.sub.(electrical).sup.E+1/Q.sub.(acoustic).sup.E+N
(Equation 3)
where 1/Q.sub.(acoustic).sup.E and 1/Q.sub.(acoustic).sup.E
represent the electrical and acoustic energy losses at the
electrode, respectively, and the term N represents non-electrode
related terms. The two Q terms may be expressed as:
Q.sub.(electrical).sup.E=A*t/.rho. (Equation 4)
Q.sub.(acoustic).sup.E=B/10.sup.(.alpha.*t) (Equation 5)
Here, t represents the thickness of the electrode material, .rho.
is the electrical resistivity of the electrode material, and
.alpha. is the acoustic loss factor of the electrode material.
Factors A and B depend on electrode lateral dimensions, where the
electrode connects to the circuit, and other parameters which will
be selected to provide a device with the desired Q.sub.s for the
particular application. It is important to note that as the
electrode thickness increases, Q.sup.E.sub.(electrical) increases
while Q.sup.E.sub.(acoustic) decreases. Therefore, the total
Q.sub.s reaches a maximum at a determinable electrode thickness.
Using testing and analysis, it has been determined that a maximized
Q.sub.s occurs within the range of 0.4 .mu.m-0.5 .mu.m for an
electrode formed of Mo.
[0057] In some device applications (e.g. filters), it is known to
provide layers of acoustic impedance material with a thickness
equal to one quarter wavelength of the acoustic material in
question, or, in other words, equal to 0.25(Vs/f.sub.0) with Vs
being the speed of sound of the material in question and f.sub.0
the center of the frequency band over which the filter is designed
to operate.
[0058] In contrast with these other device applications, according
to an aspect of the present invention, the acoustic confinement
structure of the oscillator element is designed to maximize
Q.sub.s. It has been determined that the low acoustic impedance
material layers should be made thicker than the high impedance
material layers to achieve this objective. In particular, it is
preferred that a given low acoustic impedance material layer should
be on the order of 10% thinner than a quarter of the acoustic
wavelength of the respective layer. It is preferred that a given
high acoustic impedance material layer should be thinner than the
layer of low acoustic impedance material.
[0059] There are any number of different ways to implement an
oscillator and its attendant circuitry. FIG. 5 is a block diagram
representing a preferred oscillator circuit 200. The circuit 200
includes an oscillator element 202 which may be fabricated in
accordance with any of the embodiments herein, including any
variations in materials, thicknesses and layering for the resonator
element and attendant acoustic confinement structure. The
oscillator element 202 is preferably coupled to a variable
capacitor 204 as well as to an oscillator driving circuit ("XO")
206. The driving circuit 206 is linked to a divider 208. A control
circuit 210 is operable to drive the variable capacitor 204.
[0060] A temperature sensor 212 may optionally be coupled to the
control circuit 210 so that the oscillator circuit 200 may handle
temperature variations. The temperature sensor 212 may have one or
more sensing elements disposed in or around the oscillator element
202 and/or other components of the oscillator circuit 200. The
control circuit 210 and the temperature sensor 212 may each have a
memory, such as respective non-volatile memories 214 and 216,
associated therewith.
[0061] As shown in the figure, the oscillator driving circuit 206
outputs a reference frequency f.sub.ref which is fed to the divider
208. In turn, the divider 208 is operable to emit a desired
frequency f.sub.0. Preferably, the oscillator driving circuit 206
includes at least one active device such as a transistor that acts
as an amplifier.
[0062] In operation, when power is first applied to the oscillator
circuit 200, random noise or other transient voltage is generated
within the active device of the oscillator driving circuit 206 and
is amplified. This may be fed back through the oscillator element
202, which is by design a frequency selective device. Thus, only a
selected frequency, f.sub.0, is again amplified in a closed loop
sequence.
[0063] Small variations in the variable capacitor 204 may tune the
frequency for stable operation. The temperature sensor 212, either
alone or in conjunction with the control circuit 210, may use
temperature feedback data and/or temperature-related data stored in
the memory 216 and/or the memory 214 to correct small temperature
variations which would otherwise affect the frequency of
oscillation of the components in circuit 200.
[0064] The aforementioned configuration is operable to produce a
sinusoidal voltage of frequency f.sub.ref, which is determined by
design of the oscillator element 202, including its attendant
acoustic confinement structure, as well as the value of the
variable capacitor 204. In many cases, a different, and typically
lower, frequency output f.sub.0 is desired. The divider 208 is
operable to convert f.sub.ref to f.sub.0.
[0065] As discussed above, oscillator elements, including the
resonator element and the acoustic confinement structure provided
in accordance with the present invention, may be fabricated with
other components as part of a monolithic device.
[0066] There are various ways in which a given oscillator element
may be integrated with the other components of the overall device.
FIGS. 6 and 7 illustrate two different interconnection approaches,
either of which may be employed with any of the embodiments
disclosed herein.
[0067] Specifically, FIG. 6 presents a cutaway view of an
integrated circuit device 300 in which an oscillator element 302 is
monolithically integrated with an integrated circuit component 304
through an interconnection layer 306 (with interconnects 333). The
integrated circuit component 304 (shown with devices 331 and 332
formed therein) is preferably disposed on a substrate 305. As in
the example shown in FIG. 4, the oscillator element 302 includes a
resonator element 308 and an acoustic confinement structure 310. As
shown in FIG. 6 the resonator element 308 preferably includes
piezoelectric material 312 interposed between a bottom electrode
314a and a top electrode 314b.
[0068] In the present embodiment, it is preferred to include a
temperature compensation layer 316 between the piezoelectric
material 312 and the top electrode 314b. The temperature
compensation layer 316 desirably includes SiO.sub.2 or a similar
material, and is used to make the frequency, e.g. the f.sub.0,
insensitive to temperature changes. Metals (e.g. Ni--Ti) that
contribute to the electrode function are also contemplated as
suitable temperature compensation layers. The thickness of the
temperature compensation layer is selected based upon factors such
as electrode thickness, resonator thickness, etc. For example, a
1.75 GHz resonator with an AlN thickness of 1.250 .mu.m and an
electrode thickness of 0.3 .mu.m yields a temperature compensation
layer with a thickness of 0.065 .mu.m.
[0069] As shown in the figure, the top electrode 314b is coupled to
the interconnection layer 306 through trace or lead 318. For trace
318, a primary objective is to reduce interconnect electrical
resistance to a minimum. This is in contrast to a primary objective
for the electrodes where there is a balance between electric and
acoustic losses. Thus, in order to optimize performance, it is
desirable for the trace 318 to have a different thickness and
material than what is used for the electrodes 314a and 314b. For
instance, the metal of trace 318 is preferably thicker than that of
the electrodes 314a and 314b. Here, the trace 318 may be formed of
Al with a thickness on the order of 1 .mu.m or greater. In a
preferred example, the trace thickness is at least twice that of
the electrode thickness.
[0070] The acoustic confinement structure 310 is preferably
disposed on either side of the resonator element 308. Here,
alternating layers 320 and 322 are akin to the layers 104 and 106
of FIG. 4. Preferably layers 320 comprise a high acoustic impedance
material while layers 322 comprise a low acoustic impedance
material. The layers 322 of low acoustic impedance material may be
fabricated as part of a region 324 such as a dielectric region that
encapsulates the trace 318 and other portions of the oscillator
element 302. Thus, the dielectric or other encapsulant provides
protection for the integrated circuit device 300. The high and low
impedance materials may be of any of the types described herein.
There is no requirement that the individual high impedance and low
impedance layers in an acoustic confinement structure be made of
the same material. For example, an acoustic confinement structure
might have W/SiO.sub.2 as a first stack of high impedance/low
impedance materials and W/Al as a second stack of high
impedance/low impedance materials in one structure.
[0071] FIG. 7 presents an alternative configuration of the
integrated circuit device 300. Specifically, FIG. 7 presents a
cutaway view of an integrated circuit device 300' in which the
oscillator element 302 is monolithically integrated with the
integrated circuit component 304 through the interconnection layer
306. As in the examples shown in FIG. 4 and FIG. 6, the oscillator
element 302 includes a resonator element 308 and an acoustic
confinement structure 310. As shown in FIG. 6 the resonator element
308 preferably includes piezoelectric material 312 interposed
between a bottom electrode 314a and a top electrode 314b. As with
the embodiment of FIG. 6, it is preferred to include the
temperature compensation layer 316 between the piezoelectric
material 312 and the top electrode 314b.
[0072] The primary difference between the integrated circuit device
300' and the integrated circuit device 300 is how the device 300'
is electrically coupled to the integrated circuit component 304
(shown with devices 331 and 332 formed therein) through the
interconnection layer 306 (with interconnects 333). In the present
embodiment, trace 318' couples the bottom electrode 314a to the
interconnection layer 306. Either trace configuration may be
employed depending on how the integrated circuit device 300 is
fabricated. This gives the circuit designer flexibility in the
configuration and layout of the various components and
interconnections, which may be highly beneficial when fabricating
the monolithic integrated circuit device.
[0073] It should be noted that in an alternative configuration, the
temperature compensation layer 316 is disposed between the
piezoelectric material 312 and the bottom electrode 314a. Also,
while the examples in FIGS. 6 and 7 illustrate that the oscillator
element may be fabricated after the attendant circuitry has been
disposed on the substrate, similar techniques may be employed to
fabricate the oscillator element before or during fabrication of
the attendant circuitry.
[0074] Monolithic fabrication may be done using known VLSI
fabrication technology and equipment. For instance, the oscillator
element, integrated circuit component and any necessary
interconnections may be formed by masking, depositing, growing,
annealing, etching, etc. of various materials on a substrate such
as a silicon wafer substrate.
[0075] In another embodiment, a differential oscillator is provided
on a single substrate and monolithically integrated with an
integrated circuit device. Referring to FIG. 8A, a cutaway view of
an integrated circuit device 400 illustrates first oscillator
element 402 and second oscillator element 422 which are
monolithically integrated with the integrated circuit component 404
through an interconnection structure (not shown). As in the
examples shown in FIG. 4 and FIG. 6, the oscillator elements 402
and 422 include a resonator element 408 and an acoustic confinement
structure 410. The resonator element 408 preferably includes
piezoelectric material 412 interposed between a bottom electrode
414a and a top electrode 414b. As with the embodiment of FIG. 6, it
is preferred to include the temperature compensation layer 416
between the piezoelectric material 412 and the top electrode 414b.
The second oscillator element 422 shares the same acoustic
confinement structure 410, temperature compensation layer 416, and
top electrode 414b. The second oscillator element has its own
piezoelectric material 423 interposed between a bottom electrode
424 and a top electrode 414b. The piezoelectric portions 412 and
423 are typically formed by depositing a piezoelectric layer and
patterning the layer to form both portions.
[0076] FIG. 8B is an alternate structure in which the first
oscillator element 402 and second oscillator element 422 also share
the same piezoelectric material 412 but have separate bottom
electrodes. That is, first oscillator element 402 has electrodes
414a and 414b and second oscillator element 422 has electrodes 424
and 414b.
[0077] FIG. 9 is a top down view of integrated circuit device 400
with the top layers of the acoustic confinement structure removed
therefrom. The integrated circuit device has an oscillator element
402 as previously described and a filter element 430. The
oscillator element 402 and filter element 430 are supported by an
integrated circuit device substrate with a lower portion of the
acoustic confinement structure (not shown) formed thereon on which
is formed the oscillator element 402 and filter element 430. The
surface on which the oscillator element 402 and filter element 430
are formed is shown as 401. The footprint of the oscillator element
and the filter element are observed as formed on the same layer,
but not connected. It is advantageous from a manufacturing
perspective for the devices to share the same acoustic confinement
structure.
[0078] The oscillator element 408 has the previously described
electrodes 414a and 414b and piezoelectric material 412. The
temperature compensation layer for the oscillator element 402 is
not illustrated in FIG. 9. The filter element 430 does not have a
temperature compensation layer.
[0079] The filter element 430 is illustrated as a T-filter by way
of example and not by way of limitation. Other filter
configurations are well known to one skilled in that art and are
contemplated as suitable. Other examples of suitable filter
structures include, for example, ladder filters, lattice filters
and mechanically coupled filters. The filter element 430 has three
patterned electrodes, 432, 434, and 436 over which is formed a
continuous piezoelectric layer 440. A single electrode 450 is
formed on the piezoelectric layer 440. Electrodes 432 and 434
define the series resonance portion of the filter element 430 and
electrode 436 defines the shunt portion of the filter. Electrodes
432 and 434 are connected to a voltage source (not shown) and
electrode 436 is connected to ground. Connections to the voltage
source is through the common interconnect structure of the
integrated circuit device 400, as previously described. Electrode
450 is not electrically interconnected but provides the filter
element 430 with a source for capacitance.
[0080] As previously noted, it is advantageous if the devices are
embedded in the same acoustic confinement structure. From a
manufacturing perspective it is also advantageous if the individual
components of the resonator elements of both devices are fabricated
simultaneously. Specifically, it is advantageous if each of the
bottom electrodes, piezoelectric layer and top electrodes of the
oscillator element 402 and the filter element 430 are patterned
from respective single layers for each component formed on the
substrate 400.
[0081] While the invention has been disclosed in connection with
the preferred embodiments shown and described in detail, it will be
understood that the invention is not to be limited to the
embodiments disclosed herein, but is to be understood from the
following claims, which are to be interpreted as broadly as allowed
under the law.
[0082] Although the invention herein has been described with
reference to particular embodiments, it is to be understood that
these embodiments are merely illustrative of the principles and
applications of the present invention. It is therefore to be
understood that numerous modifications may be made to the
illustrative embodiments and that other arrangements may be devised
without departing from the spirit and scope of the present
invention as defined by the appended claims.
* * * * *