U.S. patent application number 10/409261 was filed with the patent office on 2004-10-14 for maintenance of fluidic dielectrics in rf devices.
Invention is credited to Pike, Randy T..
Application Number | 20040200629 10/409261 |
Document ID | / |
Family ID | 33130575 |
Filed Date | 2004-10-14 |
United States Patent
Application |
20040200629 |
Kind Code |
A1 |
Pike, Randy T. |
October 14, 2004 |
MAINTENANCE OF FLUIDIC DIELECTRICS IN RF DEVICES
Abstract
Method for preventing degradation of a fluid dielectric (106) in
an RF device (100). The method can include the steps forming a
substrate (102) of the RF device (100) from a low temperature
co-fired ceramic (LTCC), positioning within a cavity structure
(104) of the substrate (102) at least one fluid dielectric (106),
and agitating the fluid dielectric (106) with a piezoelectric
device (112). According to one aspect of the invention, the
piezoelectric device (112) can be formed from lead zirconate
titanate.
Inventors: |
Pike, Randy T.; (Grant,
FL) |
Correspondence
Address: |
SACCO & ASSOCIATES, PA
P.O. BOX 30999
PALM BEACH GARDENS
FL
33420-0999
US
|
Family ID: |
33130575 |
Appl. No.: |
10/409261 |
Filed: |
April 8, 2003 |
Current U.S.
Class: |
174/11R |
Current CPC
Class: |
H01P 3/088 20130101 |
Class at
Publication: |
174/011.00R |
International
Class: |
H02G 015/28 |
Claims
We claim:
1. A method for preventing degradation of a fluid dielectric in an
RF device, comprising the steps of: forming a substrate of said RF
device from a low temperature co-fired ceramic (LTCC); positioning
within a cavity structure of said substrate at least one fluid
dielectric; and agitating said fluid dielectric with a
piezoelectric device.
2. The method according to claim 1 further comprising the step of
selecting a material for said piezoelectric device to include lead
zirconate titanate (PZT).
3. The method according to claim 2 further comprising the step of
bonding said PZT to said substrate.
4. The method according to claim 3 wherein said bonding step is
further comprised of positioning said PZT in contact with said
substrate and co-firing said substrate together with said PZT.
5. The method according to claim 2 further comprising the step of
doping said PZT to enhance bonding with said substrate.
6. The method according to claim 5 further comprising the step of
doping said PZT with a material selected from the group consisting
of calcium lead, zirconium, oxygen, and titanium.
7. The method according to claim 6 further comprising the step of
doping said PZT with a rare earth element.
8. The method according to claim 7 further comprising the step of
selecting said rare earth element from the group consisting of
Ruthenium, Osmium, Rhenium, Halfnium, Tantalum, and Germanium.
9. The method according to claim 5 further comprising the step of
selecting said doping level to be in the range from between about
0.5 to 18 percent weight.
10. The method according to claim 2 further comprising the step of
forming at least one electrical contact in said substrate coupled
to said PZT for applying an exciter voltage.
11. The method according to claim 1 wherein said piezoelectric
device is in direct contact with said fluid dielectric.
12. An RF device comprising: a substrate formed of a low
temperature co-fired ceramic (LTCC); a cavity structure formed
within said substrate; at least one fluid dielectric contained
within said cavity structure; and a piezoelectric device for
agitating said fluid dielectric.
13. The RF device according to claim 12 wherein said piezoelectric
device is comprised of lead zirconate titanate (PZT).
14. The RF device according to claim 13 wherein said PZT is bonded
to said substrate.
15. The RF device according to claim 14 wherein said PZT and said
substrate are co-fired.
16. The RF device according to claim 13 wherein said PZT is doped
to enhance embedded interstitial bonding with said substrate.
17. The RF device according to claim 16 wherein said PZT is doped
with a material selected from the group consisting of lead,
zirconium, oxygen, titanium and calcium.
18. The RF device according to claim 17 wherein said PZT is doped
with a rare earth element.
19. The RF device according to claim 18 wherein said rare earth
element is selected from the group consisting of Ruthenium, Osmium,
Rhenium, Halfnium, Tantalum, and Germanium.
20. The RF device according to claim 16 wherein a dopant material
comprises between about 0.5 to 18 percent weight of said PZT.
21. The RF device according to claim 13 further comprising at least
one electrical contact formed in said substrate and coupled to said
PZT for applying an exciter voltage.
22. The RF device according to claim 12 wherein said piezoelectric
device is in direct contact with said fluid dielectric.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Statement of the Technical Field
[0002] The inventive arrangements relate generally to RF devices
and more particularly to preventing degradation of fluid dielectric
that are used in RF devices.
[0003] 2. Description of the Related Art
[0004] Glass ceramic substrates calcined at 850-1,000 C are
commonly referred to as low-temperature co-fired ceramics (LTCC).
This class of materials have a number of advantages that make them
especially useful as substrates for RF systems. For example, low
temperature 951 co-fire Green Tape.TM. from Dupont.RTM. is Au and
Ag compatible, and it has a thermal coefficient of expansion (TCE)
and relative strength that are suitable for many applications. The
material is available in thicknesses ranging from 114 .mu.m to 254
.mu.m and is designed for use as an insulating layer in hybrid
circuits, multi-chip modules, single chip packages, and ceramic
printed wire boards, including RF circuit boards. Similar products
are available from other manufacturers.
[0005] LTCC substrate systems commonly combine many thin layers of
ceramic and conductors. The individual layers are typically formed
from a ceramic/glass frit that can be held together with a binder
and formed into a sheet. The sheet is usually delivered in a roll
in an unfired or "green" state. Hence, the common reference to such
material as "green tape". Conductors can be screened onto the
layers of tape to form RF circuit elements antenna elements and
transmission lines. Two or more layers of the same type of tape is
then fired in an oven. The firing process shrinks all of the
dimensions of the raw part. Accordingly, it is highly important
that the material layers all shrink in a precise, predetermined way
that will provide consistent results from one module to the
next.
[0006] Recent interest in fluid dielectric materials suggest the
use of LTCC as a substrate because of its known resistance to
chemical attack from a wide range of fluids. The material also has
superior properties of wetability and absorption as compared to
other types of solid dielectric material. These factors, plus
LTCC's proven suitability for manufacturing miniaturized RF
circuits, make it a natural choice for use in RF devices
incorporating fluid dielectrics.
[0007] Still, the use of fluid dielectrics raises new potential
problems. For example, fluid dielectrics can suffer degradation
from a variety of factors. For example, the degradation can occur
due to temperature variations, micro-gravity, phase separation,
particulate settling and orientation, ionic migration, dendritic
growth, and other intrinsic molecular separation phenomena. Some of
these problems are less likely to occur in dynamic systems.
However, even in the case of dynamic systems, fluids can separate
due to particle fallout, particle separation, sedimentation, eddy
effects and so on. These kinds of fluid degradations will effect
the overall electrical characteristics of the fluid dielectric,
regardless of whether the fluid is a dielectric suspension,
dielectric agglomerate, a dielectrically loaded fluid, or a polymer
blend.
SUMMARY OF THE INVENTION
[0008] The invention concerns a method for preventing degradation
of a fluid dielectric in an RF device. The method can include the
steps forming a substrate of the RF device from a low temperature
co-fired ceramic (LTCC), positioning within a cavity structure of
the substrate at least one fluid dielectric, and agitating the
fluid dielectric with a piezoelectric device. According to one
aspect of the invention, the piezoelectric device can be formed
from lead zirconate titanate (PZT) component. According to another
aspect of the invention, the piezoelectric device can be in direct
contact with the fluid dielectric. Also, at least one electrical
contact can be provided in the substrate and coupled to the PZT for
applying an exciter voltage.
[0009] The method can also include the step of bonding the PZT to
the substrate. The bonding step can be performed by positioning the
PZT in contact with the substrate and co-firing the substrate
together with the PZT. The PZT can be elementally doped to enhance
embedded interstitial bonding with the substrate. For example, the
PZT can be doped with calcium, lead, zirconium, oxygen, titanium,
or a rare earth element selected from the group consisting of
Ruthenium, Osmium, Rhenium, Halfnium, Tantalum, and Germanium. The
doping level can be advantageously selected to be in the range from
between about 0.5 to 18 percent weight of the PZT
[0010] The invention can also include an RF device that includes a
substrate formed of a low temperature co-fired ceramic (LTCC). A
cavity structure can be formed within the substrate and at least
one fluid dielectric can be contained within the cavity structure.
Further, a piezoelectric device can be provided for agitating the
fluid dielectric. The piezoelectric device can be in direct contact
with the fluid dielectric. The RF device can also include at least
one electrical contact formed in the substrate and coupled to the
PZT for applying an exciter voltage.
[0011] According to one aspect, the piezoelectric device can be
comprised of lead zirconate titanate (PZT) that is bonded to the
substrate. The PZT and the substrate can be advantageously co-fired
together. The PZT can also be doped to enhance embedded
interstitial bonding with the substrate. For example, the PZT can
be doped with calcium, lead, zirconium, oxygen, titanium, or a rare
earth element selected from the group consisting of Ruthenium,
Osmium, Rhenium, Halfnium, Tantalum, and Germanium. In any case,
the dopant material can comprise between about 0.5 to 18 percent
weight of the PZT.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a perspective view of an RF device that is useful
for understanding the present invention.
[0013] FIG. 2 is a cross-sectional view of the RF device in FIG. 1,
taken along line 2-2.
[0014] FIG. 3 is a perspective view of a ceramic material lay-up
that is useful for understanding a process for fabricating the
device in FIG. 1.
[0015] FIG. 4 is a perspective view of the ceramic material lay-up
in FIG. 3 after firing, and showing the addition of the PZT and top
layers.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0016] An RF device 100 that incorporates a fluid dielectric is
illustrated in FIG. 1. The RF device 100 can include any type of RF
circuit or component that advantageously makes use of at least one
type of fluid dielectric to enhance performance or aid in
controlling an operating parameter of the device. In FIG. 1, the RF
circuitry is illustrated as including an RF transmission line
component 108. However, the invention is not so limited. For
example, the RF componentry can include, without limitation,
antenna elements, matching sections, delay lines, beam steering
elements, tunable transmission lines, stubs and filters, variable
attenuators, cavity structures, and any other type of RF component
that can benefit from the use of fluid dielectrics.
[0017] The RF device 100 also includes one or more cavity
structures 104 formed in a substrate 102. The cavity structure 104
can be provided for constraining or transporting a fluid dielectric
106 within a defined region of the substrate 102 for advantageously
utilizing the fluid dielectric 106 in the RF device. For example,
the cavity structure 104 can define a fluid reservoir for storing
fluid dielectric 106 when it is not in use. Alternatively, the
cavity structure 104 can be a portion of a conduit used for
transporting the fluid dielectric 106 from one portion of the
substrate to another. Further, the cavity structure can be provided
for constraining the fluid dielectric 106 in a predetermined region
that is directly coupled to an RF element. For example, in FIG. 1,
the cavity structure 104 is positioned generally adjacent to the
transmission line 108 so that the electrical properties of the
fluid dielectric can directly influence the operational
characteristics of the transmission line element.
[0018] In some instances it can also be desirable to include a
conductive ground plane 110 on at least one side of the substrate
102. For example, the ground plane 110 can be used in those
instances where the RF circuitry includes microstrip circuit
elements such as transmission line 108. The conductive ground plane
110 can also be used for shielding components from exposure to RF
and for a wide variety of other purposes. The conductive metal
ground plane can be formed of a conductive metal that is compatible
with the substrate 102.
[0019] The substrate 102 can be formed of a ceramic material. Any
of a wide variety of ceramics can be used for this purpose.
However, according to a preferred embodiment, the substrate can be
formed of a glass ceramic material fired at 850.degree. C. to
1,000.degree. C. Such materials are commonly referred to as
low-temperature co-fired ceramics (LTCC).
[0020] Commercially available LTCC materials are commonly offered
in thin sheets or tapes that can be stacked in multiple layers to
create completed substrates. For example, low temperature 951
co-fire Green Tape.TM. from Dupont.RTM. may be used for this
purpose. The 951 co-fire Green Tape.TM. is Au and Ag compatible,
has acceptable mechanical properties with regard to thermal
coefficient of expansion (TCE), and relative strength. It is
available in thicknesses ranging from 114 .mu.m to 254 .mu.m. Other
similar types of systems include a material known as CT2000 from W.
C. Heraeus GmbH, and A6S type LTCC from Ferro Electronic Materials
of Vista, Calif. Any of these materials, as well as a variety of
other LTCC materials with varying electrical properties can be
used.
[0021] According to a preferred embodiment, at least one agitation
mechanism is provided for agitating the fluid dielectric 106. As
illustrated in FIGS. 1 and 2, the agitation mechanism in the
present invention is preferably a piezoelectric device 112. Use of
piezoelectric agitation for fluid dielectrics in RF devices
provides several distinct advantage as compared to other
micro-agitation techniques.
[0022] One important advantage of piezoelectric agitation is the
high degree of reliability of such devices due to the general
absence of moving parts. Further, fluid dielectrics present special
mixing problems that are not common to many other types of fluid
mixing. Agitation systems that use manifolds, impellers, actuators,
and certain other active systems can degrade fluid dielectric
systems by inducing first, second, or higher order shear forces on
the fluid. These shear forces can break inter- and intra-molecular
bonds within the fluid dielectric 106, thereby causing a
detrimental effect on the electrical performance of the fluid.
Piezoelectric agitation is more subtle, considerably reducing the
potential for damage to the fluid dielectric. Finally, RF circuit
devices for certain civilian, military and space-based,
applications must be capable of operating in extreme environmental
conditions. Piezoelectric systems can operate effectively over a
wide range of temperatures and in microgravity conditions that may
occur in these environments. For all these reasons, piezoelectric
agitation is particularly well suited for maintaining fluid
dielectric in RF devices as described herein.
[0023] Referring to FIGS. 1 and 2, the piezoelectric device 112 is
preferably positioned so that it forms a portion of the lining 114
of the cavity structure 104 in direct contact with the fluid
dielectric 106. However, the invention is not limited in this
regard and it is also possible to provide the piezoelectric device
112 disposed behind a membrane (not shown) so that it is not
directly exposed to the fluid dielectric 106. The membrane could be
formed of a thin layer of LTCC or some other material.
[0024] Electrical contacts 116 are preferably formed in the
substrate 102 and coupled to the piezoelectric device 112 for
applying an exciter voltage from a source 118. When the exciter
voltage is applied to the piezoelectric device 112, the
piezoelectric device will be induced to mechanically deform in the
conventional manner of piezoelectric materials.
[0025] Despite the advantages offered by making use of
piezoelectric agitation techniques, the integration of
piezoelectric materials in a LTCC stack of an RF device presents
certain problems. More particularly, a piezoelectric device for use
in such application should be formed of a material that can be
chemically bonded to the LTCC substrate and should have physical
properties that are compatible with the LTCC co-firing process.
[0026] LTCC is typically a composition of calcium, potassium,
titanium, magnesium and oxygen. The precise formulation depends
upon the commercial source. By comparison, most piezoelectric
materials are not compatible with LTCC because they are polymeric
based compositions that will thermally degrade during the co-firing
process.
[0027] Many crystalline materials exhibit piezoelectric behavior.
However, they do not generally exhibit the effect strongly enough
to be used in the present invention. Materials that do exhibit the
piezoelectric effect strongly include quartz, Rochelle salt, barium
titanate, and polyvinylidene flouride (a polymer film). However,
these materials are not chemically compatible with the LTCC in a
way that will facilitate interstitial bonding. They also have
physical properties that are not compatible with LTCC firing
processes. Accordingly, these materials are unsuitable for use in
LTCC based applications. According to a preferred embodiment, the
piezoelectric device 112 can be comprised of a known class of
piezoelectric materials comprised of lead zirconate titanate
(Pb(Zr.sub.0.5Ti.sub.0.5)O.sub.3, or PZT). These include, without
limitation, PZT-2, PZT-4, PZT-4D, PZT-5A, PZT-5H, PZT-5J, PZT-7A,
PZT-8. In general, PZT requires a driving voltage of 3 VDC, to
produce a deformation amplitude of 126 nmo-p at a resonance
frequency peak of 304.35 kHz. It has a maximum Q value of 705. The
chemical composition of PZT can also be expressed as
PbZr.sub.0.52Ti.sub.0.48O.sub.3.
[0028] In general, PZT has physical properties that are remarkably
well suited for integration in a substrate 102 formed of LTCC. This
factor is very important from an integration standpoint. For
example, the coefficient of thermal expansion (CTE) for PZT ranges
from -3.5 to 11.times.10.sup.-6/K depending on the element ratio.
This range is compatible with LTCC from a processing standpoint. In
this regard, those skilled in the art will readily appreciate that
it is important to closely match the CTE of the piezoelectric
material to that of the LTCC in order to prevent micro-cracking,
stress/strain, and warpage in the LTCC stack-up.
[0029] Ordinary PZT will not generally be chemically compatible
with the LTCC so as to form the desired ionic or covalent molecular
bonds in the co-firing process. Therefore, in order to make the PZT
more compatible with LTCC, it is preferable to performing a doping
step that includes doping the PZT with one or more elements
contained in the PZT. Since calcium is the most common element in
PZT it is preferred to dope with calcium PbZr
PbZr.sub.0.52Ti.sub.0.48O.sub.3 x Ca.sup.2+ (Calcium Doped).
[0030] However, other elements contained in the LTCC could also be
used as dopants including for example, lead, zirconium, oxygen, and
titanium. Also, certain rare earth materials could be used for this
purpose, as they are capable of having high oxidation states that
can induce additional molecular bonds. Examples of rare earth
element that might be selected could include Ruthenium, Osmium,
Rhenium, Halfnium, Tantalum, or Germanium. In any case, the dopant
material can comprise between about 0.5 to 18 percent weight of the
PZT. Excessive doping levels are preferably avoided as they can
potentially lead to problems relating to interstitial cracking and
the harmonic response of the PZT. A properly doped PZT form can be
positioned on a pre-fired LTCC substrate and the compositions can
be co-fired together to form a single unit.
[0031] As shown by the arrows in FIG. 2, the actuation force
(vector) generated by the PZT during agitation should be generated
and localized on the PZT form. This force is then projected into
the fluid dielectric with the force/harmonic force vector projected
at an angle of about 90.degree. relative to the lining 114 of the
cavity structure 104. A DC bias voltage for the PZT can be applied
at electrodes 116 as shown in FIG. 2. The bias voltage can drive
the PZT to a resonant frequency around 304.35 kHz. The electrodes
would preferably be on opposite ends of the slab as shown in FIG.
2.
[0032] Referring now to FIG. 3, a process for manufacturing an RF
device as described herein shall now be described in greater
detail. As shown in FIG. 3, the process can begin by forming an
LTCC stack 300 using conventional LTCC processing techniques. The
stack 300 can be comprised of a plurality of layers of Green
Tape.RTM., or any other similar type LTCC material, so as to define
a portion of the substrate 102. The stack 300 can also define at
least a portion of the cavity structure 104 and can include a void
304. Once again, it should be noted that the shape, size and
location of the cavity structure shown herein is merely by way of
example and the invention is not intended to be limited to a cavity
structure of any particular size, shape or location. Electrical
contacts 116 as described above can be positioned within the void
304. Thereafter, the LTCC stack 300 can be fired in the
conventional manner. LTCC initial firing temperature is typically
up to about 500.degree. C. to about 1110.degree. C. depending on
the particular design.
[0033] After firing, a slurry or putty-like mixture of pre-doped
PZT can be disposed in the void 304 as illustrated in FIG. 4. The
part can subsequently be co-fired at a temperature of between about
500.degree. C. to 800.degree. C. to form the piezoelectric device
112. The remaining processing steps for completing the part,
including the placement and firing of one or more ceramic layers
402, and the addition of RF circuit component(s) 108, can be
performed in accordance with conventional LTCC fabrication
techniques.
[0034] Finally, those skilled in the art will note that PZT
ceramics must be poled to exhibit the piezo effect. During
polarization the piece is heated (to allow alignment of the dipoles
in the PZT and an electric field is applied. Conversely, a poled
PZT will depole when heated above the maximum allowed operating
temperature. PI HVPZTs have a Curie temperature of 300.degree. C.
and can be operated up to 150.degree. C. (with P-702.10 high
temperature option). LVPZTs show a Curie temperature of 150.degree.
C. and can be operated up to 80.degree. C.
[0035] While the preferred embodiments of the invention have been
illustrated and described, it will be clear that the invention is
not so limited. Numerous modifications, changes, variations,
substitutions and equivalents will occur to those skilled in the
art without departing from the spirit and scope of the present
invention as described in the claims.
* * * * *