Polymer Films as Acoustic Matching Layers - Khuri-Yakub ...

ABSTRACT
We have investigated usin some polymer films
such as polyimide and Parylene ?or acoustic impedance
matching applications. Polyimide films were spin
coated and Parylene films were vapor deposited on
silicon and lass substrates, respectively. The
impedance, ve P ocity of sound and acoustic attenuation
of these films were measured in the 70-230 MHz
frequency range. The curing temperature of the
polyimide films was also varied to determine the
dependence of the material properties on processing
conditions. The impedance of these films were
measured to be in the 2.7 to 3.7 Mrayl range.
Therefore, these films can be used to obtain a good
acoustic matching between most liquids and most low
impedance solids. The measured Q values also
promise operation at frequencies up to a few hundred
MHz.
INTRODUCTION
Efficient coupling of acoustic fields from a liquid
into a solid require a quarter-wave matching layer due
to the large impedance mismatch between most
liquids and solids. For most applications the
impedance of the matching layer has to be in the 3 to
10 Mrayl range. Unfortunately, there are not a lot of
materials available with an impedance in this
frequency range. Typically at gi ahertz frequencies a
vacuum-deposited thin-film 07 borosilicate glass
(impedance, Z, is 11 Mrayls) or amorphous carbon
(Z=8 Mrayls) is used as matching layers.1 At low
megahertz frequencies thin sheets of solids such as
epoxy composites are laminated on the substrate as
anti-reflection coatings. When frequencies on the
order of a few hundred megahertz are used, however,
neither of these techniques can be applied. Typical
wavelengths in solids are on the order of a few tens of
microns at frequencies around 100 MHz. Vacuum
deposition of materials at these thicknesses is either
ver costly or impossible at all. Laminating materials
wit{ such thicknesses can not be done because
polishing materials down to such thicknesses is very
difficult.
We have investigated polymer films such as
polyimide and Parylene as acoustic matching layers at
frequencies in the 100 to 200 MHz range. The
*E. L. Ginzton Laboratory, Stanford University,
Stanford, CA. 94305
1051-0117/90/0000-1337 $1.00 0 1990 IEEE
POLYMER FILMS AS ACOUSTIC MATCHING LAYERS
B. Hadimioglu and B. T. Khuri-Yakub*
XEROX Palo Alto Research Center
3333 Coyote Hill Road
Palo Alto, CA. 94304
measurements indicate that these films promise good
transmission efficiencies between most liquids and
especially low impedance solids such as silicon, glass
and quartz.
EXPERIMENTAL PROCEDURE
The acoustic measurements were performed using
a pulse-echo detection system. Figure 1 shows a
diagram of the setup. Typically 200 nsec-wide pulses
of rf signal are applied into a LiNbO3 acoustic
transducer. The transducer is bonded on a double side
polished quartz crystal and it generates plane sound
waves in the quartz rod. The transducer has a center
frequency of 130 MHz although its broad bandwidth
allows measurements in the 70 to 230 MHz frequency
range. The amplitude of the signal applied into the
transducer is controlled by a precision step attenuator.
The signals received from the transducer are detected
using a low-noise superheterodyne receiver. The
attenuation value of the step attenuator is recorded as
the frequency is varied to determine the frequency
response of the device-under-test. The system has a
dynamic range of over 80 dBs with a measurement
accuracy of better than T.5 dB over the entire
frequency range.
Figure 2 shows a diagram of the transducer-
measured device configuration along with various
signals received from the transducer. In order to
RFmclllator FETSwltch Digital
Attenuator
Mixer
I’ I
‘a +h
Control
Power Splitter Transducer
Test
wafer
Figure 1. The rf-electronics of the measurement system
1990 ULTRASONICS SYMPOSIUM - 1337

Liquid
(Zl)
Matching
Laver
Ar (Reflection loss)= A,(mat) - A,(ref)
At (Transmission loss) = At(ref) - At(mat)
Figure 2. Measurement setup with the various rfpulses
received.
accurately determine the losses in the matching layer,
first the amplitude of the reflected and transmitted
signals Al(ref) and At(ref) were recorded without the
matching layer present in the system as shown in
Figure 2. Then the same echoes were measured with
the matching layer present on the wafer. The
difference between these signals represent the
amplitude of the reflection and transmission
coefficients of the liquid/matching layer/solid
configuration, respectively.
In order to estimate the mechanical properties of
the material under test, the measured reflected and
transmission coefficients were compared to the values
predicted by a theoretical calculation of the response
of the system. The theoretical reflection and
transmission coefficients of the matching layer are
given by
and
1338 - 1990 ULTRASONICS SYMPOSIUM
where
Aloss is given by
where
and Zj, is the input impedance of the matching
layer/solid combination looking from the liquid side
given by
The attenuation constant a of the matching layer is
assumed to increase as the frequency to the power y.
Z/, Z and Z, are the impedance of the liquid, matching
layer and substrate, respectively. The matching layer
has a thickness of t and sound velocity of v. An
alternative definition for the attenuation constant is
the Q of the layer where Q=nf/va and f is the
frequency.
It should be noted here that since the phase of the
reflection coefficient is not measured, there are two
impedances that will match the measured magnitude
of the reflection and transmission coefficients. These
impedances are located symmetrically around the
quarter-wave matching impedance Zv4= (Z/ZJW In
order to determine the impedance of the matching
layer unambiguously, measurements were done using
both water and isopropyl alcohol as the coupling
liquid. Since isopropyl has an impedance much less
than that of water, it is easy to determine the
impedance of the matching layer accurately.
The pol imide films2 were applied by spin coating
on one sde of 2.5 mm thick double side polished
silicon wafers. The wafers were spun at 1200
rpm for 60 seconds which yielded final film thicknesses
of about 5 pm. After a drying at 90°C for 10 minutes, a
(3-stage cure was done at 140OC for 30 minutes. Holes
were photolithographically etched in the polyimide to
expose the silicon surface. These holes allow the
measurement of the polyimide thickness as well as the
measurement of the reference data. A final high
temperature cure was then applied for 1 hour. In
order to determine the dependence of the mechanical
properties of the olyimide films on processing
conditions, the fina P cure temperature was varied
between 14OOC and 325OC.
The Parylene films were applied using vapor phase

deposition on pyrex glass plates. Two different types
of Parylene films were measured, Parylene-N and
Parylene-C. The thickness of all the films were
mea ured by an a-Step machine with an accuracy of
T 508.
RESULTS
Figures 3 and 4 show a typical set of data along
with theoretical curves fitted on the experimental
data. Figures 3(a) and (b) show the reflection and
transmission loss, respectively, with water as the
coupling fluid. Figures 4(a) and (b) are corresponding
plots for isopropyl alcohol: The fit between the
experimental and theoretical curves are excellent in
most cases. The uncertainty in determining the
impedance of the matchin layer was found to be
T.05 Mrayl. The accuracy o 3 the velocity estimation is
better than 50 m/sec. The-attenuation value of the
J
z
: 51,,
1
,I
, J - / = + , .
..* . .
'5 0 I00 150 200 250
FREQUENCY (MHz >
PMI-325 HRTER
Figure3. (a) Measured reflection loss and (b)
transmission loss for a polyimide film.
Coupling liquid is water. Solid line is the
theoretical curve fitted on the data.
..
PM1-325 15C
20 ,
E
!
I
'5 t I I 1
0,I. ~ ' 100 ~ " ~ 150 ~ ~ 200 ."""' " ~ c20 ~ ~ '
FREQUENCY (MHzI
Figure4. (a) Measured reflection loss and (b)
transmission loss for a olyimide film.
Couplin liquid is isopropy P alcohol. Solid
line is $e theoretical curve fitted on the
data.
films, with the exception of Parylene-C, were found to
be such that typical Q was greater than 100 at 100
MHz. The power dependence, y, of the attenuation
constant was assumed to be 2 in all calculations. Such
attenuation values are too low to perturb the
measured reflection and. transmission losses
significantly in the measurement frequenc
The accuracy of the Q measurement is, there/or'ea.%
and should be considered as a lower limit for the
actual Q values. In order to determine the acoustic
attenuation more accurately, higher frequency
measurements are needed.
Table 1 shows the results of the measurements. It
can be seen that the impedance and sound velocity of
the polyimide films are near 3.6 Mra I and 2.4 kmlsec,
respectively. The change in the mec t! anical properties
of the polyimide films with the cure temperature is
1990 ULTRASONICS SYMPOSIUM - 1339

Cure
Sample # Material Temperature 2 Q
(kmysec, (Mrayls) (at 100 MHrl
CO
PM1-325 Polyimide 325 2.43 36 KN)
PM1-240 Polyimide 240 2 55 36 1w
PM1-180 Polyimide 180 2 53 37 100
PM1-140 Polyimide 140 2 57 3 75 100
PARNl ParyleneN
- 2.1 2.85 100
Table 1. The results of the measurements.
very small with a slight increase of the impedance as
the cure temperature is decreased. The Parylene films
show a sound velocity of 2.2 km/sec with a lower
impedance of about 2.9 Mrayl.
Table 2 shows the predicted operation of
polyimide films cured at 325OC for matching sound
from water into various low impedance solids at
frequencies as high as 600 MHz. The Q is assumed to
be 200 at 100 MHz with an inverse frequency
dependence. It can be seen that these films offer a
significant improvement in the transmission and
reflection losses at these frequencies for the substrates
considered. The acoustic attenuation is relatively low
compared to the data reported by Selfridge3 for low
impedance materials such as plastics. Both polyimide
and Parylene also offer easy application with thickness
ranging from sub-micron to several microns. These
materials are also relatively inert and stable4 which
should make them ideal for matching layer
applications.
COUPLING SOUND FROM WATER INTO SILICON AND GLASS
Matching layer: Polyimide (325%). U4 thick
Table 2. Predicted response of polyimide films cured
at 325oC.
CONCLUSION
We have measured the acoustic impedance,
velocity of sound and mechanical Q of polymide and
Parylene films. The measured impedances show that
both of these materials should yield a significant
improvement in the efficiency of coupling of sound
1340 - 1990 ULTRASONICS SYMPOSIUM
from liquids to low impedance solids such as silicon,
lass and quartz. The acoustic attenuation for these
4. ilms are also low enough to suggest operation of
these films up to a few hundred megahertz.
ACKNOWLEDGEMENTS
The authors wish to thank Robert Allen and
Barbara Minott for supplying the polyimide wafers.
1.
2.
3.
4.
REFERENCES
D. Rugar, "Cryogenic Acoustic Microscopy," Ph.D.
Dissertation, Stanford University (1981).
PIX-1400, Hitachi Chemical Co. Ltd., Japan.
A. R. Selfridge, "Approximate Material Properties
of Isotropic Materials," IEEE Trans. Sonics and
Ultrason., SU-32, 381 (1985).
See, for example, Polymers for Electronic
Applications, edited by J. H. Lai (CRC, Florida,
1989), Chapters 2 and 3.