Fabrication of High Frequency Spherically Shaped Ceramic

23 I
IEEE TRANSACTIONS ON ULTRASO‘VICS. FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 41, NO. 2 , MARCH 1994
Fabrication of High Frequency
Spherically Shaped Ceramic Transducers
Geoffrey R. Lockwood, Daniel H. Turnbull, Member, IEEE, and F. Stuart Foster, Member, IEEE
Abstract-Difficultyinobtainingwellfocusedefficientultrasoundtransducershaslimitedthedevelopment
ofnew
high
frequency applications of B-mode imaging. This paper describes
a methodforfabricatinghighfrequency
(53 MHz)spherically
focused lead zirconate titanate (PZT) transducers. A transducer
is fabricated by bonding a malleable backing layer onto a thin
plate of PZT and then pressing the plate into
a spherically shaped
well.Thebackinglayerevenlydistributesstressesacrossthe
materialwhen it ispressedintothewell.Localconcentrations
of stress which lead to fracture are avoided and the material can
be deformed without macroscopic cracking.
The characteristics
of a 2 mm diameter 53 MHz PZT transducer with a 4 mm focal
length are described. A lateral beam width of 68 p n and a 12 dB
depth of field of 1.5 mm were obtained. The minimum two-way
insertion loss of the system was -25 dB and the 6 dB bandwidth
of thepulseechoresponsewas
30%. An image of a resolution
phantom and anin vivo skin image illustrate the excellent imaging
characteristics of the transducer.
Focussing the beam using a phased array is also not practical
dueto the prohibitively small array elementspacing that is
required at high frequencies (A/2 = 15 pm at 50 MHz). In
light of this problem, most imaging systems which employ
high frequency ceramic transducers either leave the beam
unfocused or weakly focused using a spherical mirror 1121.
This paper describes a new process for fabricating spherically shaped high frequency (> 20 MHz) ceramic transducers.
The design and construction of a 2 mmdiameter S3 MHz
transducer with a 4 mm focal length aredescribed.The
transducer’s lateral and axial beam width, pulse-echo response?
bandwidth and insertion loss are given. Images of a resolution
phantom and normal human skin demonstrate the potential of
these transducers in a realistic clinical application.
11. TRANSDUCER
FABRICATION
I. INTRODUCTION
R
ECENTLY. a number of high frequency (20-80 MHz) Bmode ultrasound imaging systems have been developed
for visualization of the eye, the skin, endoluminal structures
and intravascular structures [l]-[7]. Central tothedevelopment of these systems has been the development of high frequencybroad-bandwidthtransducers [X]. Spherically shaped
polymer transducers in the frequency range from 5&100 MHz
have been reported by Sherar er al. [9]. These transducers have
excellent beam properties, broad-bandwidth and the flexibility
of the polymer material make them relatively easy to fabricate.
Unfortunately they are also characterized by high losses and
a low electromechanical coupling coefficient. The two-way
insertion loss of a polymer transducer is typically as poor
as -40 dB [9]. Dielectric constantsforpolymers
are very
low ( F ~ / C O = 6.5 for PVDF at 50 MHz [91) making these
materials unsuitable forapplications, such as intravascular
imaging, where the size of the transducer is very small.
Low insertion loss (- 17 dB) high frequency (20-80 MHz)
ceramic transducers have been reported by Foster er al. [lOl.
Although the sensitivity of high frequency ceramic transducers
is a great improvement over polymer transducers, it has not
been possible to spherically shape these transducers. The
required thickness of the material ( < 100 /Lm) is too thin to
allow accurate machining from a bulk sample and too thick
for fabrication using Sol Gel or sputtering techniques [ l l ] .
Manuscript received July 1.1. 1993; revised September 15, 1993: accepted
September 20. 1991.
Theauthorsare
with Sunnybrook Health ScienceCenter,
Riechmann
Research Building, Toronto, Ont. M4N 3MS. Canada.
IEEE Log Number 9214981.
Lead zirconate titanate ceramics (PZT)are ideally suited
for the development of miniature high frequency transducers
due to their high dielectric constant (t-’/c0 = 1296) and high
sensitivity ( k t = 0.38 at 53 MHz) [ 101. Although, the required
thickness of this material (-40 pm at 53 MHz) is too small to
allow accurate machining of spherically curved radiators, flats
plates as thin as 25 //,m can be obtained by lapping. These
plates are extremely fragile and will fracture unless handled
very carefully. We have found that this characteristic of the
ceramic can be modified by bonding a malleable material
onto the back or front (orboth) surfaces of theceramic.
Provided that an appropriate material is used and the ceramic
is sufficiently thin (< 100 pm), the composite(ceramic
backingmaterial) can be deformed without fracture.This
allows the ceramic to be spherically shaped by simply pressing
the composite into a spherically shaped well.
The steps involved in the fabrication of a S3 MHzPZT
transducer with a conductiveepoxy backing are illustrated
in sequential stepsfrom the top to the bottom of Fig. 1.
A bulk sample of high density PZT (D3203, Motorola,Albuquerque,NM) is lapped to a thickness corresponding the
desired resonant frequency (-40 //,m). A 100 Irm backing layer
of conductive epoxy (Ablebond 16-1. Ablestick Laboratories,
Gardena. CA) is cast onto the back surface of the ceramic.
Once the epoxy has cured, the composite is cut to the finished
dimensions (2 mm diameter),gently heated (65°C) and pressed
into a spherically shaped well using a ball bearing (X mm
diameter). Heatisusedto
make the epoxy slightly flexible
while the composite is being shaped. The composite is then
cooled to room temperature and removed from the well. An
1 mm) is cast onto the
additional layer of conductive epoxy
+
(W
IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 41, NO. 2. MARCH 1994
232
I
thickness
-2mm
connector
desired
Cut
Stabllldng
layer
Ball bearing
SMA barrel
Lap bulk mT sampleto
transducertofinished
size
Centerpin
COndLJCtNe epoxy
backing
n-
mastic insert
Heat and pressIntowell
2 mm dia.spherlcalty
shaped PZT element
Front eiectcde
-.,
Removetransducerfromwell.
applybackinmaterial
and
eaectrcde
front
Stablildnglayer
Backingmaterial
Fig. 1 . Diagram showing the steps (from top to bottom) for fabricating a
spherically shaped ceramic transducer.
back surface to provide additional support for the transducer
and to form an acoustically lossy but electrically conductive
backinglayer.
An exampleof a spherically shaped 53 MHz PZT transducer
mounted in an SMA barrel connector is shown in Fig. 2 . The
transducer is 2 mm in diameter with focal distance of 4 mm.
Electrical contactto the transducer is made using the center
pin
of the connector and by evaporating a chrome-gold electrode
across the front faceof the transducer and the SMA connector.
111. TRANSDUCER
CHARACTERISTICS
The characteristics of the transducer were measured using
3. Thepulseris
theexperimentalsystemshown
inFig.
connected to the transducer through a protection circuit
which
consists of an expander(modelDEX-3,Matec,Hopkinton,
( d g = 5 cm, d2 = 25 cm)
MA)andtwotransmissionlines
joined at a T junction. A thirdtransmissionline ( d l = 5 2
cm) joins the T junction to a limiter (model
1850, Wiltron,
MorganHill,CA)and
the 50 0 input of a 300 MHz digitizing oscilloscope (model 54 201D, Hewlett Packard, Palo
Alto,CA). Thetransducerwastunedusingatwoelement
transmission line matching circuit consisting
of a parallel open
circuit transmission line stub(45 cm) and a series transmission
line (145 cm). A more detailed discussion of the matching and
protection circuit are given in a companion paper [ 131.
The pulse echo response of the system was measured by
recording the reflection from a quartz flat placed at the focal
region of the transducer. The focal region of the transducer
was found by adjusting the separation between the quartz flat
and the transducer to maximize the amplitude of the returned
signal.Thetransducer
was excited with a 17 V monocycle
pulse produced by an Avtech pulse generator (Avtech, Ottawa,
ONT, model AVB2-CO). The pulse-echo response and pulse
(b)
Fig. 2. Cross-section diagram (a) and photograph (b) of a spherically shaped
53 MHz ceramic transducer mounted in an SMA barrel connector.
spectrum are given in Fig. 4(a) and (b). The pulse is centered
at 53 MHz with a 6 dB bandwidth of 30% and an amplitude
of approximately 100 mV.
of the system wascalculated
Thetwo-wayinsertionloss
from the ratio o f the received pulse spectrum to the source
spectrum (at the output of the pulser). Corrections were made
for the attenuation in water (2 x 10-'f2 dB/mm, f in MHz)
and the reflection coefficient of the quartz flat (0.82). Fig. 5
shows that a minimum insertion loss of -25 dB was obtained.
The effective axial beam profile, shown in Fig. 6, was measured by recording the amplitude of the pulse echo response
as a function of the separation between the transducer and the
quartz flat. The - 12 dB width of the axial beam profile is
1.5 mm. The maximum value of the beam profile occurs at a
close to the expected geometric
distance of 4.2 mm which is
focal distance of 4.0 mm.
LOCKWOOD er al.: HIGH-FREQUENCYCERAMICTRANSDUCERS
233
0.05
I
,
pulser
0.03
digital
oscilloscope
0.02
r-7
E -0.01
a
l
r
a
l
matching
circuit
limiter
I
Tl
water bath
Fig. 3. Schematicdiagram of theexperimentalsystem
for measuringthe
pulse echo response and beam properties of the transducer.
An estimate of the lateral beam width of the transducer was
measured by scanning the transducer, in the lateral direction,
placed atthefocal
acrossa 20 pm diameterglassfiber
region of the transducer. The transducer was scanned using
amicrometer with aresolution of k 5 km. Theamplitude
of thereceived signal asafunction
of position is plotted
width at half
in Fig. 7. The width of theresponse(full
width
maximum (FWHM)) is 68 pm.Thetheoreticalbeam
X(Focal Distance)/(Diameter) for a 2 mm diameter, 53 MHz
(X = 28 pm) transducer with a4.2 mm focallength is 59
pm. There is reasonable agreement between the experimental
line response and the theoretical beam width considering the
finite size of the glass fiber (20 pm) and the resolution of the
micrometer ( 5 pm).
i
-0.02
-0.03
'I
-0.04
-0.05
0.00
I
0.05
0.1 0
0.1 5
Time (us)
0.20
0.25
0.30
60
70
80
(a)
1 .o
20
30
40
50
Frequency (MHz)
(b)
Fig. 4. (a) Pulse-echo response and (b) pulse spectrum. The transducer was
of
excited with a 17 V monocycle pulse. The pulse has a center frequency
53 MHz and a 6 dB bandwidth of 308.
- 6 0 " " ' " ' ' ' " ' " '
30
35
40
45
50
55
60
Frequency (MHz)
IV. IMAGING EXAMPLES
The 53 MHz focused PZT transducer has been incorporated
into a B-mode ultrasound backscatter microscope this is being
developed for skin imaging [14]. The transducer is mounted
on the shaft of a linearpositionencoder (DG 810-L probe
and MD20A detector, Sony Magnescale, Tokyo, Japan) which
followsarotating
cam to producealinearmotion
ofthe
transducer overan 8 mm path. The linear encoder emitspulses
every 4 pm, which are used to trigger the pulse generator and
subsequent data acquisition. For a 2 mm x 2 mm image size,
every encoder pulse is used, while for larger image formats, a
subset of encoder pulses are used to keep the total number
65
70
75
Fig. 5. Two-wayinsertion loss. Theinsertion loss wascalculatedfromthe
ratio of the received pulse spectrum to the source spectrum (both measured
across 50 Q). A minimum insertion loss of -25 dB was obtained.
of lines (512) in theimageconstant.Thereceivedsignals
areprocessedthroughalogarithmicamplifier,demodulated
and then transferred to the scan converter which produces the
resulting video image. The scan converter had been designed
to provide a variety of imagesizesfrom a full 8 mm x 8
mm imagedown to a 2 mm x 2 mm imagewhich can be
positioned anywhere along the 8 mm path of the transducer.
x 2 mmimage of a resolution
Fig. 8 showsa2mm
phantom,consisting of seven20 pm diameterglass fibers
IEEE TRANSAmIONS ON ULTRASONICS,FERROELECTRICS.ANDFREQUENCYCONTROL,VOL.
234
41, NO. 2 , MARCH 1994
0
-3
8
-6
-9
-12
-15
-1 a
-21
2
3.0
2.5 2.0
3.5
4.5
4.0
6.0
5.5
5.0
6.5
Distance from Transducer (mm)
Fig. 6. Axial beam profile. The 12 dB depth of field is 1.5 mm. The profile
W& measured by plotting the amplitude of the signal returned from a quart
flat as a function of the separation between the transducer and the quartz flat.
-36
(d
-125
1
,
-1 00
.
I
-75
.
I
-50
,
I
-25
I
I
0
,
,
25
.
1
,
50
,
75
1
I
100
Fig. 8. 2 mm x 2 mm image of a resolution phantom consisting of 70 I'm
diameter glass fibers with spacing between fibers rangingfrom SO to 400 I'm.
The calipers (bright dots) are SO {rm apart.
,\,
125
Lateral Position (urn)
Fig. 7. Lateral line response. The width of the response (FWHM) is 68 m.
The laterallineresponsewasmeasured
by scanningthetransducerinthe
lateral direction across a 20 p m diameter glass fiber.
having center-to-center spacings of 500, 400, 300, 200, 100,
and 50 pm from right to left. It is evident from this image
that the lateral resolution of the skin UBM system, with the
53 MHz focussed PZT transducer is close to the 50 pm lower
limit of the resolution phantom.Fig. 9 shows an 8 mm X 8 mm
image of normal forearm skin, demonstrating the superficial
epidermis (e), the dermis (d) and subcutaneous tissues (S), as
well as anumber of blood vessels (v). Thequality of this
imagedemonstrates the excellentpotential of the focussed
PZT transducer for high frequency ultrasound imaging, and
in
particular for imaging small skin lesions and measuring skin
thickness with a resolution close to 50 pm.
v.
SUMMARYANDCONCLUSION
The development ofnew high frequency (> 20 MHz) Bmode imaging systems has been restricted
by the availabilityof
suitable transducers. Imaging systems which employ polymer
transducers can produce well focused ultrasound beams but
suffer from poor insertion loss. Alternatively, systems which
employ planar ceramic transducers have excellent sensitivity
but relatively poor beam properties.
Fig. 9. 8 mm x 8 mm image of normal forearm skin showing the main skin
layers: epidermis (e), dermis (d), and subcutaneous tissue (S,. A hair (h) and
a number of blood vessels (v) are also evident.
In this paper we describedamethod of fabricating high
frequency spherically shaped ceramic transducers. The transducers are fabricatedby lapping a bulk sample of ceramic to a
thickness correspondingto the desired resonant frequencyand
then casting a thin conductive epoxy layer onto the backof
the sample. The backing material stabilizes the grain structure
of the ceramic and allows the transducer to be pressed into a
spherical shape without fracturing the ceramic. This procedure
was illustrated by describing the design and construction of a
2 mm diameter 53 MHz PZT transducer with a focal distance
of 4 mm. A beam width (FWHM) of 68 m and a 12 dB depth
of field of 1.5 mmwere measured. The system had a 6 dB
bandwidth of 30 % and a minimum two-way insertion loss of
-25 dB. This insertion loss is significantly better than can be
obtained using polymer devices but the bandwidth is poorer.
Polymer transducers typically have an insertion loss of -40
dB and a 6 dB bandwidth of SO%> [S]. A further improvement
in bandwidth and insertion loss could be achieved through the
A phantom image
use of one or more acoustic matching layers.
TRANSDUCERS
LOCKWOOD
CERAMIC
et al.: HIGH-FREQUENCY
and an in vivo skin image demonstrate the excellent sensitivity
and beam properties of this transducer.
The development of high frequency spherically shaped
ceramic transducers should improvetheperformance
of existing high frequency imaging systems and encourage the
development of new imaging systems.
REFERENCES
C. J. Pavlin,M.D.Sherar,
and F. S. Foster,"Subsurfaceultrasound
Ophthalmology, vol. 97, pp.
microscopicimagingoftheintacteye,"
24&250, 1990.
F. L. Lizzi, M. C. Rorke, Solkil-Melgar, A. Kalisz, and J. Driller, "Interfacing very-high frequency transducers to digital-aquisition scanning
systems," Proc. Soc. Photo-opt. Inst. Engineering (SPIE),vol. 1733, pp.
313-321, 1992.
M.Berson, F. Vaillant,F. Patat, and L. Pourcelot, "High-resolution realUltrasound in Med.and Biol., vol.18, pp.
timeultrasonicscanner,"
471478, 1992.
T. Yano, H. Fukukita, S. Uneo,and A. Fukumoto, "40 MHzultrasound diagnostic system for dermitologic examination",
in Proc. IEEE
Ultrason. Symp., 1987,pp.875-878.
R. W. Martin, F.E.
Silverstein, and M. B. Kinney, "A 20 MHz
ultrasound system for imaging the intestinal wall," Ultrasound in Med.
and B i d . , vol. 15. pp.273-280,1989.
C. R.Meyer, E. H. Chiang, K. P.Fechner, D. W. Fitting, and A. J.
Buda,"Feasibility of high-resolutionintravascularultrasonicimaging
catheters," Radiology, vol.168, pp.113-116, 1988.
C. J. Hartley, M. P. Sartori, and P. D. Henry, "Intravascular imaging
with ultrasound," SPIE, Microsensors and catheter imaging technology,
vol.904, pp.103-106, 1988.
F. S. Foster, C. J. Pavlin, G.R. Lockwood, L. K. Ryan, K. Harasiewicz,
L. Berube, and A. M. Rauth, "Principles and applications of ultrasound
backscatter microscopy," IEEE Trans. Ultrason. Ferroelec. Freq. Contr.
in press.
of high
M. S. Sherar and F. S. Foster, "The designandfabrication
UltrasonicImaging,
frequencypoly(viny1idenefluoride)transducers,''
vol. 1 1 , pp. 75-94, 1989.
F. S. Foster, L. K. Ryan, and D. H. Turnbull, "Characterization of lead
zirconate titanate ceramics for use in miniature high frequency (20-80
MHz) transducers," IEEE Trans. Ulrrason. Ferroelec. Freq. Contr.,vol.
38, pp. 446-453, 1991.
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[l21 G.R Lockwood, L. K. Ryan, and F. S. Foster. "A 45 to 55 MHL needlebased ultrasound system for invasive imaging," Ultrasonic Imugiqq, v01
15, pp. 1-13, 1993.
[l31 G . R. Lockwood,andF.
S. Foster,"Modeling and optimization ofhigh
frequency transducers," IEEE Trans. Ultrason. Ferrorlec Freq. Corn.,
issue, this
pp. 225-230.
[l41 D. H. Turnbull,B. G. Starkoski, K. A. Harasiewicz,andF. S. Foster,
"A 40-80MHzB-scanultrasoundbackscattermicroscope
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Geoffrey R. Lockwood, foraphotograph
230.
and biography.seethisissue,p.
Daniel H. Turnbull ("92)
was borninNiagara
Falls, Ont., Canada, in 1955. He recelved the B S c .
degree in mathematics in 1978fromBrockUniM . S . degree in
versity,St.Catharines.Ont.,the
appliedmathematics in 1981 fromtheCalifornia
Institute of Technology. Pasadena. He also received
theM.A.Sc.degree
in mechanicalengineering in
1983 and the Ph.D. degree in medical biophysics in
1991 from the University of Toronto, Toronto, Ont.
He wasaSystem,Engineer
at Imatron,Inc.,a
medical imaging company in South San Francisco,
CA, from1983to1986.
Heis currentlywiththeDepartment
of Medical
Physics at theToronto-BayviewRegionalCancerCentre
and an assistant
Professor with the Department of Medical Biophysics, University of Toronto.
His current research interests include transducer materials, ultrasonic phasedarray systems, and applications of very high-frequency ultrasonics to medical
imaging.
F. Stuart Foster
230.
("91).
for a photograph and biography, see this issue,
p.