Title: Large dimension, flexible piezoelectric ceramic tapes
Abstract: A flexible detection/test tape includes a first flexible conductive layer, and a second flexible conductive layer positioned opposite the first conductive layer. A plurality of at least one of sensors, actuators or transducers are positioned between and are bonded to the first flexible conductive layer and the second flexible conductive layer. An insulative material is inserted around the plurality of at least one of the sensors, actuators or transducers. An electrical contact network connects to the first flexible conductive layer and the second flexible conductive layer, whereby power and control signals are provided to the flexible detection/test tape.
Patent Number: 6,964,201 Issued on 11/15/2005 to Xu, et al.
| Inventors:
|
Xu; Baomin (Cupertino, CA);
Buhler; Steven A. (Sunnyvale, CA);
Wong; William S. (San Carlos, CA);
Weisberg; Michael C. (Woodside, CA);
Solberg; Scott E. (Mountain View, CA);
Littau; Karl A. (Palo Alto, CA);
Elrod; Scott A. (La Honda, CA)
|
| Assignee:
|
Palo Alto Research Center Incorporated (Palo Alto, CA)
|
| Appl. No.:
|
376527 |
| Filed:
|
February 25, 2003 |
| Current U.S. Class: |
73/794 |
| Intern'l Class: |
G01N 003/00 |
| Field of Search: |
73/794,777,391,471,778,862.046
|
References Cited [Referenced By]
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| 5486494 | Jan., 1996 | Hotchkiss et al.
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| 5585136 | Dec., 1996 | Barrow et al.
| |
| 6071795 | Jun., 2000 | Cheung et al.
| |
| 6262516 | Jul., 2001 | Fukuda et al.
| |
| 6335263 | Jan., 2002 | Cheung et al.
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| 6370964 | Apr., 2002 | Chang et al.
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| 6408513 | Jun., 2002 | Kitahara et al.
| |
| 6771007 | Aug., 2004 | Tanielian.
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| 2002/0149296 | Oct., 2002 | Fujii et al.
| |
| Foreign Patent Documents |
| 63150979 | Jun., 1988 | JP.
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| 02005325 | Jan., 1990 | JP.
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| 04023370 | Jan., 1990 | JP.
| |
| 02162782 | Jun., 1990 | JP.
| |
Other References
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Letters, Jan. 10, 2000, vol. 76, No. 2, pp. 227-229.
Tsakalakos, L. et al., Excimer Laser Liftoff of Epitaxial Pb(Zr, Ti)O3
Thin Films and Heterostructures; Mat. Res. Soc. Symp. Proc., vol. 596, 2000
Materials Research Society, pp. 549-557; Ferroelectric Thin Films VIII, Nov. 29-Dec.
2, 1999.
Lukacs, M. et al., Novel PZT Films for Ultrasound Biomicroscopy; 1996
IEEE Ultrasonics Symposium, pp. 901-904.
Zou, L. et al., Sol-Gel Fabricated Thick Piezoelectric Ultrasonic Transducers
for Potential Applications in Industrial Material Processes; 1997 IEEE Ultrasonics
Symposium, pp. 1007-1011.
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Using a New Sol Gel Based Process; J. Appl. Phys. 81 (2), Jan. 15, 1997 American
Institute of Physics, pp. 876-881.
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Materials by Pd-In Metal Bonding and Laser Lift-Off; Journal of Electronic
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Composites; IEEE Transactions on Components, Packaging, and Manufacturing Technology—Part
A., vol. 20, No. 1, Mar. 1997, pp. 46-51.
Lee, Chin C., et al., Au-In Bonding Below the Eutectic Temperature; IEEE
Transactions on Components, Hybrids, and Manufacturing Technology, vol. 16, No.
3, May 1998, pp. 311-316.
Chen, Yi-Chia, Indium-Copper Multilayer Composites for Fluxless Oxidation-Free
Bonding; Thin Solid Films 283 (1996), pp. 243-246; 1996 Elsevier Science S.A.
Mathelin, D., et al., Improved Microcontact Technology, The Compete Network
presents Immico, (BE-8225), Up-dated: Spring 1998, pp. 1-9.
Sayer, M., et al., Macroscopic Actuators Using Thick Piezoelectric Coatings;
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Lin, Mark, et al., The Manufacture of Composite Structures with a Built-in
Network of Piezoceramics; Composites Science and Technology, 62 (2002), pp. 919-939.
|
Primary Examiner: Noori; Max
Attorney, Agent or Firm: Fay, Sharpe, Fagan, Minnich & McKee, LLP
Claims
1. A flexible detection/test tape comprising:
a first conductive layer, at least partially flexible;
a second conductive layer, at least partially flexible, positioned opposite the
first conductive layer;
a plurality of at least one of sensors, actuators or transducers positioned between
and directly bonded to the first flexible conductive layer and the second flexible
conductive layer; and
an electrical contact network connecting to the first flexible conductive layer
and the second flexible conductive layer, wherein power and control signals are
provided to the flexible detection/test tape.
2. The tape according to claim 1, wherein the plurality of sensors, actuators
or transducers are individually controlled.
3. The tape according to claim 1, wherein the plurality of sensors, actuators
or transducers are divided into and controlled as groups.
4. The tape according to claim 1, wherein an insulative material is inserted
between the conductive layers and around the plurality of the at least one of the
sensors, actuators, or transducers to electrically isolate the first conductive
layer and the second conductive layer.
5. The tape according to claim 1, wherein the at least one of sensors, actuators
or transducers are formed in a variety of different geometric shapes for use in
a single tape.
6. The tape according to claim 1, wherein the at least one of sensors, actuators
or transducers are designed as a long narrow strip, and wherein the tape acts as
an active fiber composite, having flexibility in only one direction.
7. The tape according to claim 1, wherein the at least one of sensors, actuators
or transducers is a plurality of sensors, actuators or transducers arranged on
the tape such that a density of the sensors, actuators or transducers is nonuniform.
8. The tape according to claim 1, wherein at least one of the first flexible
conductive layer and the second conductive layer is a polymer tape with a patterned
metallization layer, corresponding to shapes and distribution of the at least one
of sensors, actuators or transducers.
9. The tape of claim 1, wherein the at least one of sensors, actuators or transducers
are produced from a piezoelectric material.
10. The tape of claim 1, wherein the at least one of sensors, actuators or transducers
are produced from an antiferroelectric material, an electrostrictive material,
magnetostrictive material or other functional ceramic material.
11. The tape of claim 1, wherein on a single tape, some of the at least one of
sensors, actuators or transducers are made from one kind of piezoelectric material,
and others are made from another one or more kind of piezoelectric material.
12. The tape of claim 1, wherein on a single tape, some of the at least one of
sensors, actuators or transducers are made from one functional ceramic material,
including piezoelectric material, and others are made from one or more other functional
ceramic materials.
13. The tape of claim 1, wherein some of the at least one of sensors, actuators
or transducers have one thickness, and others have another one or more different thicknesses.
14. The tape of claim 1, wherein the at least one of the sensors, actuators or
transducers are positioned in a neutral plane of the tape.
15. The tape according to claim 1 further including,
a third conductive layer, at least partially flexible, positioned opposite the
first conductive layer;
a second plurality of at least one of sensors, actuators or transducers positioned
between and bonded to the first flexible conductive layer and the third flexible
conductive layer; and
a second electrical contact network connecting to the first flexible conductive
layer and the third flexible conductive layer, wherein power and control signals
are provided to the flexible detection/test tape.
16. The tape according to claim 15, wherein an insulative material is inserted
between and around the second plurality of at least one of sensors, actuators or
transducers to electrically isolate the first conductive layer and the third conductive layer.
17. The tape according to claim 15, wherein the first flexible layer is a non-conductive
layer with two metallized surfaces.
18. The tape according to claim 15, wherein the metallized surfaces are patterned
and the patterned configurations can be either the same or different from each other.
19. The tape according to claim 15, wherein the materials, shapes, and thickness,
of the second plurality of at least one of sensors, actuators or transducers are
either the same or different from the materials, shapes, and thickness of the first
plurality of at least one of the sensors, actuators or transducers.
20. The tape according to claim 1, wherein the plurality of actuators, sensors
or transducers are a plurality of piezoelectric element structures formed by a
deposition process, with a first electrode deposited on a first surface of each
of the piezoelectric element structures and a second electrode deposited on a second
surface of each of the piezoelectric element structures.
21. The tape according to claim 1, wherein the plurality of actuators, sensors
or transducers are a plurality of other functional ceramic materials including
antiferroelectric, electrostrictive or magnetostrictive element structures formed
by a deposition process, with a first electrode deposited on a first surface of
each of the antiferroelectric or other functional ceramic element structures and
a second electrode deposited on a second surface of each of the antiferroelectric
or other functional ceramic element structures.
22. The tape according to claim 1, wherein the at least one of sensors, actuators
or transducers are directly bonded at least one of the first flexible conductive
layer or the second flexible conductive layer with a non-conductive epoxy.
23. The tape according to claim 1, wherein the at least one of sensors, actuators
or transducers are directly bonded to at least one of the first flexible conductive
layer or the second flexible conductive layer with a non-conductive epoxy containing
conductive particles.
24. The tape according to claim 1, wherein the at least one of sensors, actuators
or transducers are directly bonded to at least one of the first flexible conductive
layer or the second flexible conductive layer with a thin film metal bonding.
Description
BACKGROUND OF THE INVENTION
Piezoelectric ceramics are commonly being used as sensors, actuators
and transducers because of their strong electromechanical coupling effect.
A detection/test system, which combines such sensors, actuators, transducers
with
feedback or feed-forward control circuitry, is an important technology for many
industry and military applications. One particular application is the active control
of vibrations. For example, active control of the vibration inside the body of
an airplane can greatly reduce the noise in the passenger cabin. Active control
of the vibration of the wings can greatly reduce the damping by airflow and thus
increase the efficiency of the airplane. Relatedly, active control of the vibration
of a submarine can greatly reduce the acoustic noise it generates and thus greatly
reduce its chance of being detected. Another application of detection/test systems
is real-time structural health monitoring. For example, embedded sensors and transducers
in a structure can produce in-site detection of cracks in the structures and thus
predict and assist in avoiding critical failure of the structure.
A significant drawback of piezoelectric ceramics is that it is difficult to make
a thin, large sheet (at many inches to several feet scale), due to the brittle
nature of the material. Due to this limitation, it cannot be mounted to a curved
surface or embedded in a structure which needs to be flexible. Unfortunately, many
real world applications require detecting and testing of curved surfaces and/or
flexible structure, thus the mentioned brittleness greatly limits the applications
of piezoelectric ceramic materials in detection/test systems.
An alternative is to use piezoelectric polymers which are flexible and can be
manufactured in large scale. Unfortunately, the piezoelectric effect of piezoelectric
polymers is weak—about one-tenth of piezoelectric ceramics—and the
materials are very soft.
One path taken to develop a detector/test system is represented by research at
Stanford University and which is coined as the Stanford Multi-Actuator-Receiver
Transduction Layer (SMART layer). Particularly, a manufacturing method has been
proposed for integrating a network of distributed piezoceramic actuators/sensors
onto laminated carbon/epoxy composite structures. The network of built-in actuators/sensors
is used to monitor the health of the host composite structure by acquiring information
about the condition of the structure throughout its life. The manufacturing method
applies a printed circuit board technique to fabricate a thin flexible layer with
a network of piezoceramics. It is used as an extra ply that is either inserted
into or bonded onto the surface of a composite laminate to give it actuating and
sensing capabilities. More particularly, the system implements the use of a flexible
printed circuit, commonly referred to as "Flex." The proposed concept used the
Flex technique to make a large, thin flexible layer that contains a network of
distributed piezoceramics connected by printed circuits.
However, the fabrication techniques for the SMART layer are labor intensive
and restrictive in design choices. Particularly, the disclosed fabrication process
for the SMART layer do not lend itself to obtaining of a flexible tape with high
density elements and a variety of geometric shapes for those elements, which in
turn permits more versatile functional capabilities. It also does not consider
use of elements within a thickness range of about 10 μm, or greater, formed
by a direct marking technology.
SUMMARY OF THE INVENTION
A flexible detection/test tape includes a first flexible conductive layer, and
a second flexible conductive layer positioned opposite the first conductive layer.
A plurality of at least one of sensors, actuators or transducers are positioned
between and are bonded to the first flexible conductive layer and the second flexible
conductive layer. An insulative material is inserted around the plurality of at
least one of the sensors, actuators or transducers. An electrical contact network
connects to the first flexible conductive layer and the second flexible conductive
layer, whereby power and control signals are provided to the flexible detection
test tape.
In an alternative embodiment, a method for producing a detection/test tape includes
depositing a material onto a surface of a first substrate to form a plurality of
element structures. Electrodes are deposited on a surface of each of the plurality
of element structures, and the element structures are bonded to a second substrate,
where the second substrate is conductive or has a conductive layer, and the second
substrate is carried on a carrier plate. The first substrate is removed from the
element structures and second side electrodes are deposited on a second surface
of each of the plurality of element structures. An insulative material is inserted
around the element structures to electrically isolate the two substrates used to
bond the element structures. A second side of the element structures is then bonded
to another substrate, where the other substrate is conductive or has a conductive
layer. Thereafter, the carrier plate carrying the second substrate is removed.
SUMMARY OF THE DRAWINGS
The invention may take form in various components and arrangements of components,
and in various steps and arrangements of steps. The drawings are only for purposes
of illustrating preferred embodiments and are not to be construed as limiting the invention.
FIG. 1 is a high level process flow for piezoelectric detection/test tape production;
FIG. 2 is a high level process flow for piezoelectric detection/test tape production
including attachment of the piezoelectric elements to a transfer substrate prior
to completion of the tape production process;
FIG. 3 illustrates a piezoelectric element array on a top surface of a carrier substrate;
FIGS. 4A and 4B show alternative embodiments of a piezoelectric element array
deposited with electrodes and other thin film metals for bonding, the piezoelectric
element array is on a top surface of a carrier substrate;
FIG. 5A illustrates an embodiment of a bonding of piezoelectric elements to
a conductive final target using a thin, nonconductive epoxy bonding containing
sub-μm (micrometer) conductive balls;
FIG. 5B shows a thin nonconductive epoxy bonding process;
FIG. 5C is an enlarged view of a section of FIG. 5B;
FIG. 5D illustrates a bonding of piezoelectric elements to a conductive final
target substrate using thin film intermetallic transient liquid phase bonding;
FIG. 6A depicts a bonding to a conductive transfer substrate using removable
conductive tape bonding;
FIG. 6B illustrates a bonding of the piezoelectric elements to the transfer
substrate which is an Indium-Tin-Oxide (ITO)-coated glass using thin, nonconductive
epoxy bonding containing sub-μm conductive balls;
FIG. 7A illustrates radiation of a beam through the carrier substrate during
a liftoff process;
FIG. 7B depicts a heat transfer for the liftoff process;
FIGS. 8A and 8B are alternative designs for bonding the elements array to a
final target substrate or a transfer substrate;
FIG. 9A illustrates bonding the piezoelectric elements array to a final target
substrate using thin, nonconductive epoxy bonding containing sub-μm conductive
balls, where the piezoelectric elements array is bonded to the transfer substrate
using removable conductive tape bonding;
FIG. 9B is a bonding of the piezoelectric elements array to the final target
substrate using thin film intermetallic transient liquid phase bonding, where the
piezoelectric elements array is bonded to the transfer substrate using removable
conductive tape bonding;
FIG. 9C is a bonding of the piezoelectric elements array to the final target
substrate using thin, nonconductive epoxy bonding containing sub-μm conductive
balls, where the piezoelectric elements array is bonded to an ITO-coated glass
using the thin, nonconductive epoxy bonding containing sub-μm conductive balls;
FIG. 9D depicts bonding the piezoelectric elements array to the final target
substrate using thin film intermetallic transient liquid phase bonding, where the
piezoelectric elements array is bonded to the ITO-coated glass using the thin,
nonconductive epoxy bonding containing sub-μm conductive balls;
FIG. 9E depicts bonding the two elements arrays to a final target substrate
using thin, nonconductive epoxy bonding containing sub-μm conductive balls,
where the elements array is bonded to the transfer substrate using removable conductive
tape bonding; the two elements arrays are deposited on two substrates and then
transferred to two transfer substrates;
FIG. 9F depicts bonding the two elements arrays to a final target substrate
using thin, nonconductive epoxy bonding containing sub-μm conductive balls,
where the elements array is bonded to the transfer substrate using removable conductive
tape bonding; the two elements arrays, with two different thicknesses for the elements
from one array to the other, are deposited on two substrates and then transferred
to two transfer substrate;
FIGS. 10A and 10B depict alternative embodiments of a partially constructed
system, wherein filler is inserted;
FIG. 11 is chart depicting transmission wavelength of a laser used in a process
of the present application;
FIG. 12 depicts one embodiment wherein the second final target substrate is
bonded to the piezoelectric elements;
FIG. 13 is a cross section view for one embodiment of a completed piezoelectric
tape according to the present application;
FIG. 14 is a sectional view along section line A—A of FIG. 13;
FIG. 15 is a sectional view at lines A—A, for another embodiment of a
piezoelectric ceramic tape.
FIG. 16 is a further A—A sectional view of a further embodiment of a piezoelectric
ceramic tape according to the present application;
FIG. 17 depicts a polymer tape with a patterned metallization layer which may
be implemented as a metal surface in accordance with the concepts of the present application;
FIG. 18A is the A—A sectional view of the fourth embodiment of a piezoelectric
ceramic tape;
FIG. 18B depicts a polymer tape with a patterned metallization layer which could
be used as the second final target substrate for the fourth embodiment;
FIG. 19 is yet a further cross section view for one embodiment of a completed
piezoelectric tape according to the present application;
FIG. 20 is a two-layer piezoelectric tape which may be accomplished in accordance
with the concepts of the present application.
DETAILED DESCRIPTION OF THE INVENTION
The present application provides for flexible detection/test tape and processes
to make such a tape. In one design, a plurality of piezoelectric ceramic elements
are sandwiched between two conductive layers, such as two metallized polymer films
or tapes, two metal foils, or one metallized polymer tape and one metal foil. The
configuration provides the assemblied piezoelectric tape with flexibility and a
potential dimension of several feet or more in scale. As will be described in greater
detail, the metallization layer in the polymer film can be patterned in such ways
that the piezoelectric elements can be connected to external circuitry as individual
elements, as several groups, of elements, or as a single group. Thus the piezoelectric
tape can work simultaneously as sensors, actuators or transducers. The area density
and the shape of the piezoelectric elements can be varied locally to meet the application
requirements. Also, since the disclosed manufacturing process permits for a high
density of elements, the operational functionality of the tape will not be significantly
less than a sheet of piezoelectric elements. The piezoelectric ceramic tapes can
be made by a process which combines screen printing or other direct marking method,
high temperature sintering, tape polishing, laser or other radiation liftoff, a
thin layer bonding which can remain electric contact between the bonded parts.
Specifics of the process will now be described.
FIG. 1 illustrates a high level process flow
10 for a first embodiment
of a manufacturing process according to the concepts of the present application.
While the following discussion focuses on producing piezoelectric thick film elements
(with thickness between 10 and 100 μm), it is to be appreciated the disclosed
processes may be used with other materials and may also be used for production
of thin-film elements (with thickness less than 10 μm) and elements with
thicknesses greater than 100 μm to millimeter in scale. Also, the following
techniques are intended to be applicable to the generation of individual elements
and arrays of elements.
Initially, piezoelectric ceramic thick film elements are fabricated by
depositing the piezoelectric material onto an appropriate substrate by use of a
direct marking technology
12. In the deposition techniques employed, ceramic
type powders are used in a preferred embodiment. The fabrication process includes
sintering the material preferably at a temperature of approximately 1100 to 1350°
C. for densification, although other temperature ranges may also be used in appropriate
circumstances. Following the fabrication process the surface of the formed structures
of piezoelectric elements are polished
14, preferably using a dry tape polishing
technique. Once the piezoelectric elements have been polished and cleaned, electrodes
are deposited on the surface of the piezoelectric elements
16. Next, the
piezoelectric elements are permanently bonded to a final target substrate
18.
The final target substrate is flexible and conductive or has a surface conductive
layer, such as a metal foil or a metallized polymer tape. In order to easily carry
during the fabrication process, the flexible target substrate can be put on another
rigid carrier plate. Typically, the composition of the piezoelectric ceramic elements
is doped or undoped PZT (lead zirconate titanate), but any other piezoelectric
materials, such as lead titanate, lead zirconate, lead magnesium titanate and its
solid solutions with lead titanate, lithium niobate, lithium tantanate, and others
may be used.
At this point, the substrate on which the piezoelectric elements were deposited
is removed through a liftoff process using radiation energy such as from a laser
or other appropriate device
20. The releasing process involves exposure
of the piezoelectric elements to a radiation source through the substrate, to break
an attachment interface between the substrate and the piezoelectric elements. Additional
heating is implemented, if necessary, to complete removal of the substrate. Once
the liftoff process has been completed, a second electrode is deposited on a second
surface of the piezoelectric material
22. Thereafter, poling of the elements
under high voltage obtains piezoelectric properties in the material
24.
The electric property, for example, a dielectric property, of each element is then
measured
26 to identify if the elements meet required criteria. An insulative
filler is inserted between the piezoelectric elements
28, whereafter the
piezoelectric elements are bonded to the second final target substrate
30.
Again the second final target substrate is flexible, such as a metal foil or metallized
polymer tape. The assembled arrangement can then be removed from the carrier plate
32.
Turning to FIG. 2, illustrated is a second high-level process flow
40
for a second embodiment of the present application. This process differs from FIG.
1 in that the bonding is to a transfer substrate rather than to a final target
substrate. Thus, the fabrication step
42, the tape polishing step
44
and the electrode depositing step
46 are performed in the same manner as
steps
12,
14 and
16 of FIG.
1. At bonding step
48,
the bonding is to a transfer substrate, as this connection is not intended to be
permanent. Thereafter, the liftoff step
50, the second electrode deposition
step
52, the poling step
54 and electric property test step
56,
which correlate to steps
20,
22,
24 and
26 of FIG.
1, are performed.
The piezoelectric elements are then bonded to a final target substrate
58,
in a procedure similar in design to step
18 of FIG.
1. Following
bonding step
58, the transfer substrate is removed
60. Thereafter,
the steps of inserting an insulative filler
62, bonding to the second final
target substrate
64 and removal of the carrier plate
66, are performed
similar to steps
28,
30 and
32 of FIG.
1. When bonding
to a final target substrate, a thin high strength bonding layer is used to minimize
or avoid undesirable mechanical damping or absorption of the bonding layer. This
bonding will, however, also permit maintaining of electrical contact between the
metal electrodes on the piezoelectric elements and the final target substrates
or a conductive surface of the final target substrates.
Employing the process of FIG. 2, only fully tested thick film elements
and arrays will be bonded to final target substrates, thus avoiding yield loss
of the piezoelectric tape.
The processes of FIGS. 1 and 2 are appropriate for the production of a flexible
piezoelectric ceramic tape in high volume, high usable yields, i.e. greater than
60 percent and more preferably over 90 percent, and still yet more preferably greater
than 98 percent.
With attention to FIG. 3, which illustrates steps
12 and
42 in
greater detail, piezoelectric ceramic elements
72 are deposited on an appropriate
substrate
74, and then sintered at 1100 to 1350° C. for densification.
The depositing step may be achieved by a number of direct marking processes including
screen printing, jet printing, ballistic aerosol marking (BAM) or acoustic ejection,
among others. Using these techniques permits flexibility as to the type of piezoelectric
element configurations and thicknesses. For example, when the piezoelectric elements
are made by screen printing, the screen printing mask (mesh) can be designed to
have various shapes or openings resulting in a variety of shapes for the piezoelectric
elements, such as rectangular, square, circular, ring, among others. Using single
or multiple printing processes, the thickness of the piezoelectric elements can
be from 10 μm to millimeter scale. Use of these direct marking techniques
also permits generation of very fine patterns and high density elements.
The substrate used in the processes of this application will have certain characteristics,
due to the high temperatures involved and—as will be discussed in greater
detail—the fact that the substrate is to be transparent for the liftoff process.
Specifically, the substrate is to be transparent at the wavelengths of radiation
beam emitted from the radiation source, and is to be inert at the sintering temperatures
so as not to contaminate the piezoelectric materials. A particularly appropriate
substrate is sapphire. Other potential substrate materials include transparent
alumina ceramics, aluminum nitride, magnesium oxide, strontium titanate, among
others. In one embodiment of the process, the substrate selected is transparent
for an excimer laser operating at a wavelength of 308 nm, and does not have any
requirement on its crystallographic orientation. It is preferable that the selected
substrate material be reusable, which will provide an economic benefit to the process.
After fabrication of the elements has been completed, the process moves to
step
14 (or
44), where the top surface of the piezoelectric elements
are polished through a tape polishing process to remove any possible surface damage
layer, such as due to lead deficiency. This step ensures the quality of the piezoelectric
elements and homogenizes the thickness of piezoelectric elements. By having a homogenized
thickness, each of the piezoelectric elements of an array will bond to the final
target substrate or the transfer substrate even when a very thin epoxy bonding
layer or a thin film intermetallic transient liquid phase bonding layer is used.
In one preferred embodiment, the tape polishing step is a dry tape polishing
process
that provides a planar flat polish out to the edge of the surfaces of the piezoelectric
elements, which avoids a crowning effect on the individual elements. Compared to
wet polishing processes, the dry tape polishing does not cause wearing of the edges
of the piezoelectric elements, making it possible to fabricate high-quality, thickness
and shape-identical piezoelectric elements. Once polishing has been completed,
the surface is cleaned, in one instance by application of a cleaning substance.
After polishing and cleaning, the process moves to step
16 (or
46)
where, as shown in FIG. 4A, metal electrodes
76 such as Cr/Ni or other appropriate
materials, are deposited on the surface of the piezoelectric elements by techniques
such as sputtering or evaporation with a shadow mask. The electrodes can also be
deposited by one of the direct marking methods, such as screen printing, and sintered
at suitable temperatures. Alternatively, when using a thin film intermetallic transient
liquid phase bonding process, certain low/high melting-point metal thin film layers
maybe used as the electrodes for the piezoelectric elements, thus in some cases
it is not necessary to deposit the extra electrode layer such as Cr/Ni. However,
preferably the thin film intermetallic transient liquid phase bonding process is
undertaken after metal electrode deposition, such as Cr/Ni deposition. While this
process will be discussed in greater detail below, generally a thin film layer
of high melting-point metal
78 (such as silver (Ag), gold (Au), Copper (Cu),
Palladium (Pd)) and a thin film layer of low melting-point metal
79 (such
as Indium (In), Tin (Sn)) may be deposited on the piezoelectric elements (or the
substrate) and a thin layer of high melting-point metal (such as Ag, Au, Cu, Pd)
may be deposited on the substrate (or the piezoelectric elements). These materials
are then used to form a bond. Also a multilayer structure with alternating low
melting-point metal/high melting-point metal thin film layers can be used.
For some uses, such as when the final target substrate or system is not expensive,
the piezoelectric elements are directly bonded to the final target substrate (step
18 of FIG.
1). For example, as depicted in FIG. 5A, the final target
substrate
82 is a flexible and conductive material, such as a metal foil
(thus it can also be used as common electrode). The final target substrate
82
could also be carried on a carrier plate
80 during the process. The placement
of final target substrate
82 to carrier plate
80 may be an action
where no bonding material is used between the two components. In alternative embodiments
some type of removable adhesive may be used to ensure placement of the metal foil.
The bonding to piezoelectric elements
72 is accomplished by using a nonconductive
epoxy layer
84 which can be as thin as less than 1 μm. The thin epoxy
contains sub-μm conductive particles, which in one embodiment maybe conductive
balls (such as Au balls)
85 so the epoxy is conductive in the Z direction
(the direction perpendicular to the surface of metal foil). Thus it can keep the
electric contact between the surface electrode of the piezoelectric elements and
the metal foil. The concentration of the conductive balls can be controlled in
such a range that the cured thin epoxy is conductive in the Z direction but not
conductive in the lateral directions, as done for the anisotropic conductive films.
The shrinkage of the epoxy maintains contact between the surfaces and the balls
in the Z direction.
In an alternative embodiment shown in FIGS. 5B and 5C, conductive balls
85
are removed, and bonding is accomplished using the nonconductive epoxy layer
84
alone. As shown in more detail by FIG. 5C, with controlled suitable surface roughness
or asperity of the piezoelectric elements and/or the final target substrate, electrical
contact is maintained via electrical contact points
86, formed when the
surface of the electrode
84 and metal foil
82 are moved into contact.
In a further embodiment, bonding to the final target maybe accomplished by using
the previously mentioned thin film intermetallic transient liquid phase metal bonding,
employing in one embodiment a high melting-point metal (such as Ag, Cu, Pd, Au,
etc.)-low melting-point metal (such as In, Sn) intermetallic compound bonding layer
or alloy
88, FIG.
5D.
More particularly, for thin film intermetallic transient liquid phase metal
bonding, a high melting-point metal thin layer such as a Pd thin layer is deposited
on the target substrate. Next the piezoelectric elements are moved into contact
with the Pd thin layer and heated under pressure above the melting point of the
low melting-point metal (In), e.g., about 200° C. By this operation the high
melting-point metal/low melting-point metal/high melting-point metal combination
such as the Pd/In/Pd layer (a high melting-point metal/low melting-point metal
such as Pd/In layer was previously deposited on the piezoelectric elements as shown
in FIG. 4B) will form the high melting-point metal-low melting-point metal intermetallic
compound bonding layer or alloy
88. This compound or alloy may be a PdIn
3
alloy layer which is about 1 μm-thick, which acts to bond piezoelectric elements
72 and target substrate
82. Functionally, the low melting-point metal
diffuses into the high melting-point metal to form the compound/alloy.
As the melting point of the formed intermetallic compound phase can be much higher
than that of the low melting-point metal, the working temperature of the bonding
layer can be much higher than the temperature used to form the bonding. For example,
when Indium (In) is used as the low melting-point metal and Palladium (Pd) is used
as the high melting-point metal, the bonding can be finished below or at 200°
C. as the melting point of In is about 156° C. However, the working temperature
of the formed intermetallic compound bonding layer, PdIn
3, can be well
above 200° C. because the melting point of PdIn
3 is about 664°
C. The thickness of the bonding layer could be from 1 to 10 μm, but a thinner
bonding layer (e.g., about 1 μm) is expected for this purpose. Further, the
amount of high and low melting-point metals can be controlled so they will be totally
consumed to form the intermetallic bonding layer.
Alternatively, when the final target substrate is expensive, or the
final target substrate is so large (to fabricate a very large piezoelectric tape)
that the piezoelectric elements have to be fabricated on more than one substrate,
bonding of the piezoelectric elements to the final target substrate is delayed.
Incorporation of the steps in FIG. 2 minimizes yield loss of the final target substrate
or the large area piezoelectric tape, which might otherwise occur due to piezoelectric
elements fabrication failures. Therefore, the process of FIG. 2 temporarily bonds
the piezoelectric elements to a transfer substrate in step
48, and then
finishes piezoelectric elements production and testing. Only a fully tested piezoelectric
thick film array of elements is then permanently bonded to the target substrate.
The temporary bonding process step
48 of FIG. 2, is illustrated by FIGS.
6A and 6B. In FIG. 6A, the bonding operation uses a removable conductive bonding
epoxy, such as a removable conductive tape
90, including 9712, 9713 and
9719 conductive tape from 3M Corporation. The transfer substrate
92 can
be a metallized glass with surface conductive layer
94, such as a metallization
layer. In an alternative embodiment depicted in FIG. 6B, the bonding operation
uses thin nonconductive epoxy
84 containing sub-μm conductive balls
85, to bond to a transfer substrate
98 such as a glass having an
ITO coating
100.
Once the piezoelectric elements have been either permanently bonded to a final
target substrate (step
18 of FIG. 1) or temporarily bonded to a transfer
substrate (step
48 of FIG.
2), the next step is to release the piezoelectric
elements
72 from substrate
74. The releasing of substrate
74
is accomplished by a liftoff operation as depicted in FIGS. 7A and 7B. The following
description is based on the arrangement of FIG.
5A. However, it is applicable
to all provided alternatives. Substrate
74 is first exposed to a radiation
beam (such as a laser beam) from a radiation source (such as an excimer laser source)
102, having a wavelength at which the substrate
74 is substantially
transparent. In this way a high percentage of the radiation beam passes through
the substrate
74 to the interface of the substrate and elements
72
at the surface of the substrate. The energy at the interface acts to break down
the physical attachment between these components. Following operation of the radiation
exposure, and as shown in FIG. 7B, heat is applied by a heater
104. While
the temperature provided by the heater will vary depending on the situation, in
one embodiment a temperature of between 40 to 50° C. is sufficient to provide
easy detachment of any remaining contacts to fully release the piezoelectric elements
72 from substrate
74. Desirably, the substrate is of a material that
allows it to be re-used after a cleaning of its surface.
In one experiment performed by the inventors, the radiation source is an excimer
laser source and the laser energy required to achieve separation by the present
procedure has been measured at about one-half what is mentioned as needed in the
Cheung et al. patent. This is considered in part due to the wavelength used in
the experiment (e.g., 308 nm), and also that the piezoelectric elements were printed
on substrates, therefore more weakly bound to the substrate compared to the epitaxially
grown single crystal films used in the previous work by Cheung et al.
Exposure to the radiation source does raise the potential of damage to the
surface of the piezoelectric elements, this potential damage should however be
no more than to a thickness of about 0.1 μm. Since the thickness of the piezoelectric
elements, in most embodiments, will be larger than 10 μm, the effect of the
surface damage layer can be ignored. However, if otherwise necessary or when piezoelectric
elements of less than 10 μm are formed by these processes, any surface damage
layer can be removed by appropriate processes including ion milling or tape polishing.
It is to be appreciated FIGS. 7A and 7B are simply used as examples, and the described
liftoff process may take place using alternatively described arrangements. Also,
for convenience FIGS. 7A and 7B correspond to the structure of FIG.
5A.
However, the same types of procedures may be applied to FIGS. 5B,
5D,
6A,
6B or other relevant arrangements in accord with the present teachings.
Next, as depicted in FIGS. 8A and 8B, second side surface electrodes
106,
such as Cr/Ni, are deposited on the released surfaces of elements
72 with
a shadow mask or by other appropriate method in accordance with step
22
of FIG. 1 or step
52 of FIG.
2. After second electrode deposition,
the processes move to steps
24 and
54, respectively, where the piezoelectric
elements
72 are poled under a voltage
108 sufficient, as known in
the art, to obtain piezoelectric properties. After poling, the electric property,
for example, the dielectric property, of the elements are measured (step
26
of FIG. 1; step
56 of FIG. 2) to identify if the piezoelectric elements
meet expected quality criteria. FIG. 8A corresponds to the arrangements shown in
FIG. 5A, and FIG. 8B corresponds to the arrangement of FIG. 6A, following release
of the substrates.
For the case where the piezoelectric thick film array of elements is temporally
bonded to a transfer substrate such as by the process of FIG. 2, steps
58
and
60 are undertaken. In the following these steps are implemented using
selected ones of the alternative arrangements previously described. It is to be
understood the present discussion is applicable for all disclosed alternative designs.
By use of temporary bonding, it is only after electric property measurement is
made that the piezoelectric array is bonded to the final target substrate.
Step
58 of FIG. 2 may be accomplished in the same manner as bonding step
18 of FIG.
1. FIGS. 9A-9D, show alternative bonding methods, including
a thin nonconductive epoxy bonding containing sub-μm conductive balls (FIG.
5A) and a thin film intermetallic transient liquid phase bonding (FIG.
5D).
Still further, the process could employ the thin nonconductive epoxy bonding of
FIGS. 5B and 5C. When this process is used, the surface roughness of the piezoelectric
elements and/or the substrate is preferably in a range of about 0.5 to 5 μm,
depending on the film thickness, the nature of the substrate, as well as the intended
use. The second surface of the piezoelectric elements could be very smooth due
to the smooth nature of the substrate surface. This means that, after liftoff,
rough tape polishing, sandblasting or other methods may be needed to increase the
surface roughness. It is to be understood the surface roughness will be a small
fraction of the overall thickness of the piezoelectric element and/or substrate.
The specific roughness being selected in accordance with a particular implementation.
If the thin film intermetallic transient liquid phase bonding is used, similar
to previous steps, a high melting-point metal/low melting-point metal such as Pd/In
is deposited on the second surface of the piezoelectric elements and a thin high
melting-point metal such as Pd layer is deposited on the surface of the final target
substrate. Deposition of the high melting-point/low melting-point metal layers
on the piezoelectric elements can be done either after the poling and electric
property test or before the poling and electric property test but after the electrode deposition.
It is to be appreciated that to make the flexible piezoelectric tape the final
target substrate needs to be flexible and the final target substrate or the surface
of the final target substrate needs to be conductive. Typically, the final target
substrate could be a metal foil or a polymer tape with metallized surface layer.
If appropriate, the final target substrate may also be put on rigid carrier plate
80, as shown in FIG. 5A, for easy carrying during the fabrication process.
FIGS. 9A-9D are related to the process of FIG. 2, where the first bonding step
is to a temporary connection, and the final target substrate
110 has a surface
conductive layer
116.
With more particular attention to FIG. 9A, to bond the piezoelectric elements
72 to final target substrate
110, nonconductive epoxy
84 containing
sub-μm conductive balls
85 is interposed between a surface of the
conductive layer
116 of the final target substrate
110 and piezoelectric
elements
72 with electrodes
106. The opposite side surfaces of the
piezoelectric elements
72 (i.e., having electrodes
76) are already
temporarily bonded to the transfer substrate
92 (via conductor
94)
through the use of a removable conductive tape
90.
FIG. 9B illustrates an alternative bonding of the piezoelectric elements
72
to final target substrate
110 using thin film intermetallic transient liquid
phase bonding
88, where the piezoelectric elements
72 are bonded
to the transfer substrate
92 using removable conductive tape
90.
The alternative bonding of FIG. 9C, shows the elements
72 bonded to the
final target substrate
110 using thin nonconductive epoxy bonding
84
containing sub-μm conductive balls
85. In this design, elements
72
are bonded to an ITO coated
100 glass substrate
98 using the thin
nonconductive epoxy
84 containing sub-μm conductive balls
85.
Depicted in FIG. 9D is an arrangement where the elements
72 are bonded
to the final target substrate
110 using thin film intermetallic transient
liquid phase bonding
88, where the piezoelectric elements
72 are
bonded to ITO coated
100 glass
98 using the thin nonconductive epoxy
84 containing sub-μm conductive balls
85.
In some instances when fabricating a large piezoelectric tape, the final target
substrate may be larger than the substrate available to deposit the piezoelectric
elements. Alternatively, for economic reasons a relatively small substrate may
be preferred to deposit the piezoelectric elements. In these situations, step
42
of FIG. 2 (or Step
12 of FIG. 1) may be accomplished by depositing the piezoelectric
elements on several substrates. Thereafter processing steps
44 and
46
are performed. Since the piezoelectric elements will be on several substrates,
step
48 will include bonding the piezoelectric elements to several transfer
substrates. Then, following processing steps
50-
56, in step
58
the several transfer substrates will be bonded to the same final target substrate.
The foregoing process not only permit formation of large piezoelectric tapes and
the use of small substrates, it also permits the attachment of different piezoelectric
materials, such as soft PZT and hard PZT, or other functional ceramic materials,
such as antiferroelectric materials, electrostrictive materials and magnetostrictive
materials, on the same final target substrate. This means that the tape can contain
different piezoelectric materials and/or other functional ceramic materials. For
fabricating antiferroelectric elements and electrostrictive elements, the poling
step (step
54) is not necessary.
Additionally, when bonded to the same final target substrate, if the
distances between elements on one transfer substrate and another transfer substrate
are sufficient, the thicknesses of the elements may be different from one transfer
substrate to another, and a second flexible substrate (explained in details later)
can still be bonded to the surface of all the elements. This means that the tape
can contain elements with different thicknesses.
To illustrate the above concepts, FIG. 9E depicts two transfer substrates
92,
and
93. Transfer substrate
92 has piezoelectric elements
72
bonded on it using removable conductive tape
90, and transfer substrate
93 has elements
73 (which may be another kind of piezoelectric material
or other functional ceramic materials) bonded on it using removable conductive
tape
91. The elements
72 and
73 are bonded to the same final
target substrate
110 using the thin nonconductive epoxy bonding
84
containing sub-μm conductive balls
85. FIG. 9F depicts transfer substrates
92 and
93, where transfer substrate
92 has elements
72
bonded on it using removable conductive tape
90, and transfer substrate
93 has elements
71, which have thicknesses different from elements
72, bonded using removable conductive tape
91. Elements
72
and
71 are bonded to the same final target substrate using the thin nonconductive
epoxy bonding
84 containing sub-μm conductive balls
85. The
distance between elements
71 and
72 is large enough so the second
flexible substrate can be bonded to all elements.
Once the final target substrate has been bonded to the elements, the process
proceeds to step
60 and the transfer substrates (such as
92,
93)
are removed, as shown in FIGS. 10A and 10B. For the case where the piezoelectric
elements are bonded to the transfer substrate using removable conductive epoxy,
such as tape, after permanent bonding to the final target is achieved, the tape
and the transfer substrate can be easily peeled off from the piezoelectric elements.
The present process makes it easy to take off the conductive tape. This is because
the conductive tape uses filled acrylic, such as the 3M 9712, 9713 and 9719 conductive
tapes, which lose most of their adhesion after being heated at a temperature of
between 150 and 200° C. The time needed for application of the heat will depend
upon the specific application. In some applications this level of heat may be applied
during the process to bond the piezoelectric elements
72 to the final target substrate.
For the case where the piezoelectric elements
72 are bonded to the ITO
coated glass using the thin nonconductive epoxy, the piezoelectric elements can
be released from the ITO coated glass by using the liftoff operation in a manner
similar as in steps
20 or
50 where the radiation source is a laser.
This is possible as the epoxy will also absorb the laser light, thus the laser
exposure will burn off the epoxy and release the piezoelectric elements from the
glass substrate. As the melting point of epoxy is much lower than that of the metal
and ITO electrodes, the laser exposure intensity may be controlled so it will only
burn off the epoxy and not cause any damage on the metal and ITO electrodes.
It should be noted that when using laser liftoff techniques to release the piezoelectric
elements from ITO-coated glass, in one embodiment an excimer laser with relatively
longer wavelength, such as Nd:YAG laser (λ=355 nm) and XeF (λ=351 nm)
is to be used. This is because, as shown in FIG. 11, the transmission of light
through ITO on glass will drop sharply around λ=300 nm, but around λ=350
nm the transmission can be about 80%. With such high transmission, the laser exposure
can be controlled so that only the epoxy is destroyed and damage to the ITO and
metal electrodes does not occur.
After removing the transfer carrier, solvent such as acetone or other appropriate
substance maybe used to clean off the residual of the conductive tape or the epoxy.
Thereafter in step
28 (or
62), and as illustrated in FIGS. 10A and
10B, a filler material
114 is inserted between the piezoelectric elements
72. The filler
114 may be any appropriate insulative material including
a punched polymer tape with openings slightly larger than the dimension of the
piezoelectric elements
72.
Once the filler has been inserted, the process moves to step
30 (or
62)
where, as depicted in FIG. 12, the second final target substrate
118 is
bonded to the top of a second surface of the piezoelectric elements. Again the
second final target substrate is flexible and the final target substrate or the
surface of the final target substrate is conductive. Typically, the final target
substrate could be a metal foil or a polymer tape with metallized surface layer.
It is to be appreciated that FIG. 12 corresponds to the configuration of FIG. 10A,
and the second final target substrate
118 has a surface conductive layer
119. However, the concept is also applicable to FIG. 10B, and other configurations
which may be constructed according to the present application. In this embodiment,
bonding is accomplished by thin nonconductive epoxy bonding
84 containing
sub-μm conductive balls
85. However, it is to be appreciated other
ones of the previously mentioned bonding techniques may also be used.
Lastly, the carrier plate
80 is removed (step
32, FIG. 1 or
step
66, FIG.
2). It should be noticed that, while the carrier plate
is not shown in FIGS. 9A-D, FIGS. 10A-B and FIG. 12, a rigid carrier plate (e.g.,
see FIGS. 5A-5B and
8A) may be located under the final target substrate
to support the final target substrate and for carrying the final target substrate
during the fabrication process.
FIG. 13 shows one embodiment of a flexible tape
120 manufactured in accordance
with the present application. FIG. 14 provides a A—A section view
120
of FIG.
13. In this configuration, a plurality of elements
72, such
as piezoelectric elements, are sandwiched between final target substrate
82
and the second final target substrate
118. Substrates
82 and
118
are flexible and conductive or have a surface conductive layer. The procedure to
make this tape is the same as the procedure to make the embodiment shown in FIG.
12 (therefore, final target substrate
110 with conductive surface
116
could just as easily have been used instead of substrates
82 or
118),
but in this embodiment the final target substrate
82 is a conductive material
or conductive layer, such as a metal foil, thus it does not have another conductive
surface layer, and the second final target substrate
118 is an insulative
material with a surface conducting layer
119, such as a metallized polymer tape.
For this design, the piezoelectric elements
72 are homogeneously distributed.
It is to be appreciated that layers
82 and
118 are used as illustrative
examples o
