1. JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 1 1 JULY 2004
Fabrication of focused poly„vinylidene fluoride-trifluoroethylene…
P„VDF-TrFE… copolymer 40–50 MHz ultrasound transducers
on curved surfaces
Michelle Robert, Gilbert Molingou, and Kevin Snook
Department of Bioengineering, Pennsylvania State University, University Park, Pennsylvania 16802
Jonathan Cannata and K. Kirk Shung
Department of Biomedical Engineering, University of Southern California, Los Angeles, California 90089
͑Received 13 November 2003; accepted 20 April 2004͒
Copolymer films such as poly͑vinylidene fluoride-trifluoroethylene͒ P͑VDF-TrFE͒ have lower
acoustic impedance compared to their ceramic counterparts, allowing for a better acoustic match to
tissues in the human body. Because of this, copolymer ultrasonic transducers are capable of yielding
the desirable characteristics of broad bandwidth and short pulse duration that allow better image
resolution to be achieved. In the past, such transducers in the frequency range from 40 to 80 MHz
have frequently been fabricated by spin coating the copolymer film onto a flat substrate and then
applying the film to a curved backing using an adhesive layer. The adhesive layer may cause
spurious signals at these frequencies, in addition to the film damage that may occur as a result of
such processing. In order to avoid these problems, a copolymer film can be directly spin coated onto
a curved substrate. The resulting devices had an operating frequency of over 40 MHz and
approximately a 75% bandwidth. The potential of several approaches that could be further explored
to increase the level of performance of such devices is also discussed. © 2004 American Institute
of Physics. ͓DOI: 10.1063/1.1760233͔
I. INTRODUCTION damage to the fragile piezoelectric film. Another problem is
that the adhesive layer can become a source of acoustic in-
Piezoelectric copolymers have been frequently used for
terference, which is a major concern at higher frequencies
biomedical ultrasound applications because their properties
where the thickness of the glue layer may approach the di-
are well suited for this purpose.1 Kawai discovered that poly-
mension of a wavelength of the acoustic signal. This paper
vinylidene fluoride ͑PVDF͒ had a slight residual polarization
will describe a process that spin coats the copolymer film
due to the CF2 dipoles aligning in a direction normal to the
directly onto a curved substrate for fabricating ultrasonic
surface prior to poling.2 PVDF must be stretched to properly
transducers in the frequency range of 30– 80 MHz for ultra-
align the dipoles and produce a piezoelectric form of the
sonic biomicroscopy,6 alleviating these problems when de-
material. The stretching creates a stress within the film that
signing and fabricating focused transducers with piezoelec-
can be eased with exposure to relatively low temperatures
tric polymers. The method used in this study is somewhat
͑around 65 °C͒, but not without a loss of the piezoelectric
similar to that reported by Kimura and Ohigash7 who depos-
properties obtained through the stretching.3 The problems re-
lated to the stretching of the film have been eliminated with ited P͑VDF-TrFE͒ thin films of up to 5 ␮m thick onto a
the development of a copolymer of PVDF and trifluoroeth- copper substrate for fabricating focused transducers at a reso-
ylene ͑TrFE͒, which was shown to demonstrate a higher nating frequency of up to 110 MHz. The major difference
level of piezoelectricity than its predecessor.4,5 Many com- between results reported by this paper and those by Kimura
mercially available copolymers consist of a 75/25 molar ratio and Ohigash7 lies in that the film thicknesses for transducers
of PVDF to TrFE. Because the P͑VDF-TrFE͒ film does not in the frequency range from 30 to 80 MHz are much thicker,
require stretching in order to exhibit piezoelectricity, it al- making it more difficult to prepare. The bandwidth achieved
lows for alternate fabrication methods. One such method that ͑75%͒ is at least at par with if not better than those of ultra-
has shown to be promising over the past few years is spin sonic transducers fabricated with other piezoelectric
coating the copolymer onto a substrate after dissolution in materials.8,9
methyl ethyl ketone ͑MEK͒.
II. DEVICE FABRICATION
The copolymer P͑VDF-TrFE͒ has recently been gaining
popularity for use in ultrasound transducers. The fabrication Many aspects of the established flat substrate spin coat-
of a focused transducer using the copolymer previously re- ing technique required modification in order to spin coat the
quired the film to be spin coated onto a flat substrate and copolymer solution onto a curved substrate. Very little infor-
transferred onto the rounded backing. However, this extra mation was given in previous studies7 as to how the copoly-
step required the use of an adhesive layer and a proper fit to mer was spin coated onto a curved substrate. The methods
the final substrate. Transferring the film from its original sub- that have been developed and will be reported herein are
strate to the curved backing of the transducer can result in therefore meant to serve as a basis for the further evolution
0021-8979/2004/96(1)/252/5/$22.00 252 © 2004 American Institute of Physics

2. J. Appl. Phys., Vol. 96, No. 1, 1 July 2004 Robert et al. 253
of the technology, and the results obtained are useful in dem-
onstrating that spin coating onto a curved substrate is a fea-
sible approach to transducer fabrication.
A. Solution preparation
P͑VDF-TrFE͒ powder of 3.75 g with a 75/25 molar ratio
͑MSI Inc., Valley Forge, PA͒ was disolved in 40 ml MEK in
a bottle with a screw-top lid. The lid was securely fastened
and the solution was spun in a mixing machine for 24 h to
promote homogeneity. The mixture was then heated in an
oven at 70 °C for 30 min to help increase the dissolution of
the solute. The solution could be kept and used for several
months after it had been made.
FIG. 1. Drawing of the screw clamp assembly ball bearing used to press
focus the aluminum substrate used as a transducer backing.
B. Substrate centering technique
An aluminum substrate was used to form the curved sur-
The curved aluminum substrate was first centered on top
face in this work. The preparation of the aluminum substrates
of the vacuum chuck of the spin coater ͑Chemat Technology
was crucial to the success of the spin coating process. The
Inc., Northridge, CA͒. One drop of copolymer was then ap-
design goal for the transducer was a 3 mm aperture with an f
plied to a curved substrate spinning at 2000 rpm and accel-
number of 3, and these design criteria mandated that the
erated to a speed of 3500 rpm for 30 s. The film was covered
substrates had a 3 mm diameter and a centered indentation
with the spin coater lid to slow the evaporation process and
with a spherical diameter of 18 mm. There were two major
was allowed to dry for at least 30 min before another layer
requirements of the substrate fabrication process.
was added in the same manner. A scanning electron micros-
͑1͒ The spherical indentation must be centered in order to copy ͑SEM͒ photograph of a cross section of the resulting
ensure that there are no ‘‘flat spots’’ around the edge of device is shown in Fig. 2.
the transducer surface. Such areas would negatively af- Several factors contribute to obtain a high-quality film of
fect the quality of the acoustic signal. uniform thickness and acceptable homogeneity. It was cru-
͑2͒ The surface of the indentation must not contain any cial to use a substrate with an extremely smooth finish, ide-
scratches with a depth greater than 0.5 ␮m to make it ally with no irregularities greater than 0.5 ␮m in depth. Mi-
conducive for the formation of a uniform film with ac- croscratches could alter the acoustic properties of the
ceptable piezoelectric properties. transducer by allowing variation in film thickness across the
surface. Another important factor to control was the rate of
The centering method that best fulfilled the aforemen- copolymer application. The viscosity of the copolymer solu-
tioned requirements consisted of several steps. An aluminum tion created drops that were approximately the size of the
rod of 3.18 mm diameter was machined to have a spherically aperture of the finished device. When more than one drop
shaped concave end using a 6.36 mm ball end mill. A 5 mm
length was parted off the original rod, and the back of the
disk was sanded until it was flat. A stainless steel ball bearing
with a 9 mm radius was used to create a substrate with the
desired f number of 3. The ball bearing was placed on the
prepared aluminum piece and was automatically centered
due to the spherically shaped impression in the substrate. The
assembly was pressed using a screw clamp jig in an arbor
press, as shown in Fig. 1. After the piece was spherically
molded, it was lapped using glycerine with 12.5 and 3.0 ␮m
aluminum oxide powder and 3M Finesse-It II finishing ma-
terial over another 9 mm radius ball bearing. The lapped
surface of the substrate prepared as described was extremely
smooth, and the spherical indentation was centered.
C. Spin coating procedure
Little detail on spin coating a piezoelectric copolymer
film directly onto a focused substrate for the purpose of
building a transducer can be found in the literature.7 At the
FIG. 2. A film with two spin coated layers of copolymer shows a thickness
time of this study no models or concrete experimental data
of Ϸ6 ␮m. The silver particles embedded in epoxy shown in the photograph
existed to provide a clue as to how the geometric shape of are not part of the finished device, but were needed for proper photographic
the substrate would alter the spin coating process. contrast.

3. 254 J. Appl. Phys., Vol. 96, No. 1, 1 July 2004 Robert et al.
was added to the substrate surface per film layer, the subse-
quent drops interfered with the natural fluid outflow within
the critical first few seconds of the spin coating process. This
interference created bubbles in the film during drying that
had subsequently popped and created craters within the film
during evaporation.
After analyzing these results, it became apparent that
both the substrate surface condition and the method by which
the copolymer was applied could impact the final outcome of
the thickness and quality of the film. Therefore the optimal
process required dropping a discrete amount of P͑VDF-
TrFE͒/MEK solution all at once onto a polished, rounded
surface. The specific volume applied was not critical to the
success of the process because the excess fluid was removed
from the aluminum substrate due to centripetal force. It was
crucial, however, that the amount of P͑VDF-TrFE͒/MEK so-
lution be added without interruption, either as a single drop
or as a finite steady stream. In these tests, each layer was
added as a single drop that was approximately the diameter
of the 3.18 mm spinning substrate. The final thickness of
each layer was Ϸ5– 6 ␮m after evaporation. All transducers
built for this study used two layers of copolymer film, but
additional layers could be added if a thicker copolymer film
was needed for a lower frequency application without a sig-
nificant loss of uniformity across the surface of the film. A
computer program was written to evaluate the uniformity of
the film based on SEM images as shown in Fig. 3, where
additional layers were added to create a thickness of over 80
␮m as can be seen in the graph of thickness versus position.
D. Curing and poling FIG. 3. ͑a͒ A program was written to measure uniformity by having a user
define the edges of the copolymer film by tracing along the edges of the film
After the spin-coated film had been given at least 30 min as shown on the computer screen. ͑b͒ The program displayed a plot of
to fully evaporate, it was cured in a 120 °C oven for at least thickness vs position to illustrate the uniformity of the film.
3 h. Curing or annealing the film helped to promote uniform
chemical and mechanical properties across the surface. The
substrate was properly masked by placing the substrate in the E. Transducer characterization
center of a brass ring of the same height and filling the gap Pulse-echo and insertion loss measurements were per-
with EPO-TEK 301 epoxy ͑Epoxy Tech., Billerica, MA͒. formed on the fabricated transducers to assess their perfor-
The addition of the epoxy ensured that the two sides of the mance. The transducers were excited with a Panametrics
film remained electrically isolated. The device was then sput- ͑Waltham, MA͒ 5900 PR pulser/receiver. The experimental
tered with Ϸ1000 Å of gold and chrome to create a top setting of the Panametrics unit used for the measurements are
electrode across the copolymer film. A hole was drilled into listed in Table I. A 50 ⍀ 30 cm cable was used to connect the
the bottom of the aluminum, and a wire was attached using a transducer to the pulser/receiver. The device was placed in a
conductive epoxy. The polymer was poled by applying a 20 degassed water bath with a quartz crystal with its flat surface
V/␮m voltage across the thickness of the film in a 90 °C placed perpendicular to the beam at the focus of the trans-
oven for 30 min to align the dipoles. The temperature was ducer that was 9 mm from the center of the transducer. The
dropped from 90 to 25 °C while maintaining a constant elec- received echo was displayed on a LeCroy ͑Chestnut Ridge,
tric field across the copolymer film. Once the device had NY͒ LC 534 Oscilloscope with 50 ⍀ coupling. Insertion loss
cooled and the electric potential was removed, the two sides was measured using an approach reported by Sherar and
of the film were shorted for at least 12 h to relax the excess Foster.8 More details of the experimental protocols can be
charge. The finished transducers were housed in a modified found in Snook et al.9
SMA connector ͑Fig. 4͒. Two devices of 3 mm diameter
were designed to have f number of 3 and built following the
III. RESULTS
fabrication procedure as outlined above. Device 1 had a cen-
ter frequency of 43 MHz and Device 2 had a center fre- The pulse-echo response and bandwidth of Device 2 are
quency of 41 MHz. shown in Fig. 5. All relevant characterization parameters

4. J. Appl. Phys., Vol. 96, No. 1, 1 July 2004 Robert et al. 255
FIG. 4. Design cross section of a spin-coated P͑VDF-TrFE͒ transducer. The
center conductor of the SMA connector was electrically connected to the
negative electrode of the P͑VDF-TrFE͒ film through the aluminum backing.
A sputtered layer of 1000 Å in thickness of chrome/gold was used to con-
nect the positive electrode of the P͑VDF-TrFE͒ to the brass housing and to
electrically shield the entire device.
were calculated for the two devices and are summarized in
Table II. Results on insertion loss measurements showed a
loss of 37 dB at 43 MHz after compensating for the attenu-
ation in water. Copolymer devices generally exhibit a higher
insertion loss and a lower sensitivity than lead titanate zir-
conate ͑PZT͒ devices due to their lower electromechanical
coupling coefficent.9
FIG. 5. Time-domain echo response ͑top͒ and normalized frequency spec-
IV. CONCLUSION trum for device 2.
Spin coating is a simple concept, but an extremely com-
plex process. Environmental concerns compound the diffi- The uniformity of a copolymer film coating is of para-
culty in achieving consistent results with spin coating be- mount importance in the success of the finished device be-
cause a diverse set of factors—from table vibrations to cause irregularities will impact negatively on the quality of
humidity levels—can alter the outcome. Such factors were the acoustic signal produced by the transducer. SEM showed
not controlled in this feasibility study, but a methodical that uniform coatings were possible on the curved aluminum
analysis of their impact could help to further understand the substrates. The thicknesses of these films were appropriate
fluid mechanics of the film formation. For all these reasons, for high frequency applications.
even spin coating on flat substrates can be a challenging task. One of the biggest challenges of the fabrication proce-
Spin coating on a curved substrate truly adds a new dimen- dure was to create a centered, spherical surface on the alu-
sion to the already complicated spin coating problem. The minum parts. This obstacle was overcome through the use of
fluid mechanics becomes much more complex as a result of a combination of techniques, including press focusing and
having new forces such as gravity that play a role in the final lapping over a curved surface.
shape and thickness of the film. The transducers produced by following the fabrication
procedure as described had center frequencies of over 40
MHz and average bandwidths of 75%. The performance of
TABLE I. The operational settings for the Panametrics 5900 PR pulser/ the devices are consistent with copolymer transducers pro-
receiver used for pulse-echo testing. duced using other fabrication methods, and it is expected that
Parameter Value
TABLE II. Measured transducer performance.
Pulse repitition
Frequency 1 kHz Parameter Device 1 Device 2
Input energy 1 ␮J
Damping 50 ⍀ Center frequency 43 MHz 41 MHz
Attenuation 5 dB Focal distance 8.54 mm 9.22 mm
Gain 40 dB F-Number 2.6 2.8
Low pass filter 200 MHz Ϫ6 dB bandwidth 67.25% 83%
High pass filter 2 MHz Signal amplitude 1.42 V 2.22 V

5. 256 J. Appl. Phys., Vol. 96, No. 1, 1 July 2004 Robert et al.
the devices would have a lower sensitivity due to the nature The materials used in the fabrication of this device were
of the active element material. However, the bandwidth selected for a variety of reasons. It is possible that the use of
achieved ͑75%͒ is at least in par with if not better than those other materials may lead to better performance characteris-
obtained with transducers fabricated from other materials. tics or to new applications. For example, the use of a differ-
There are many advantages to this approach over ones that ent backing material such as a polymer, silicon, or glass may
have been used in the past, the greatest of which is that the serve to improve both the sensitivity and the bandwidth of
problem of film handling is eased considerably and the film the device.10 Aluminum was chosen for this study due to its
is less likely to be damaged. Another advantage is that the conductive nature so that it could be used as an electrode for
processing of these devices is less complex in many regards, poling. Silicon backing is especially attractive because it of-
which may lead to a greater success rate in transducer pro- fers the possibility of the integration of imaging electronics
duction. with the ultrasonic sensor, which is necessary in the design
There are many applications for which P͑VDF-TrFE͒ de- of arrays in the frequency range from a few hundred MHz to
vices such as the ones reported in this study would be well GHz.
suited, including but not limited to, ultrasound backscatter
microscopy6 with clinical applications in imaging anterior
ACKNOWLEDGMENTS
segments of the eye and skin.
The authors would like to thank Eugene Gerber and Jay
V. FUTURE WORKS Williams for their technical advice and assistance. This work
The goal of this research was to demonstrate that it was has been supported by NIH Grant No. P41-EB2182.
possible to directly spin coat a piezoelectric copolymer layer
onto a curved substrate and produce an operational high fre- 1
L. F. Brown, R. L. Carlson, and J. M. Sempsrott, 1997 Proceedings of
quency transducer. Although there are many ways in which IEEE Ultrasonics Symposium ͑IEEE, New York, 1997͒, p. 1725.
the proposed process can be improved, it serves as a good 2
H. Kawai, Jpn. J. Appl. Phys. 8, L975 ͑1969͒.
3
starting point for future research in the area. L. F. Brown and A. M. Fowler, 1998 Proceedings of IEEE Ultrasonics
Symposium ͑IEEE, New York, 1998͒, p. 607.
Matching layers are almost always used with ceramic 4
T. Yamada, T. Ueda, and T. Kitayama, J. Appl. Phys. 52, 948 ͑1981͒.
active elements because of the large acoustic mismatch be- 5
H. Ohigashi and K. Koga, Jpn. J. Appl. Phys. 21, L455 ͑1982͒.
tween the ceramic material ͑around 34 MRayl͒ and the tis- 6
L. S. Foster, C. J. Pavlin, G. R. Lockwood, L. K. Ryan, K. A. Harasiewicz,
L. Berube, and A. M. Rauth, IEEE Trans. Ultrason. Ferroelectr. Freq.
sues of the human body ͑around 1.5 MRayl͒. They are not
Control 40, 608 ͑1993͒.
required in the design of a copolymer transducer because the 7
K. Kimura and H. Ohigash, J. Appl. Phys. 61, 4749 ͑1987͒.
acoustic impedance ͑Ϸ4.5 MRayl͒ is much closer to that of 8
M. D. Shearer and F. S. Foster, Ultrason. Imaging 11, 75 ͑1989͒.
9
the biological medium. Matching layers were not utilized in K. A. Snook, J. Z. Zhao, C. H. F. Alves, J. M. Cannata, W. H. Chen, R. J.
Meyer, T. A. Ritter, and K. K. Shung, IEEE Trans. Ultrason. Ferroelectr.
this work. However, use of a matching layer could serve to Freq. Control 49, 169 ͑2002͒.
further enhance the performance of the devices due to the 10
L. F. Brown, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 47, 1377
better coupling between the two media. ͑2000͒.