IEEE
TRANSACTIONS ON
ELECTRON
DEVICES,
VOL.
40.
NO.
3.
MARCH
1993
52
I
Pressure Transducer with Au-Ni Thin-Film Strain
Gauges
K.
Rajanna,
S.
Mohan, M. M. Nayak, N. Gunasekaran, and A.
E.
Muthunayagam
Abstract-The behavior of a pressure transducer with Au-Ni
(89:
11)
film as strain gauges have been studied. The effects of
post-deposition heat treatment on the resistance of the thin-film
strain gauges and hence the output performance of the pressure
transducer are discussed. The effect
of
a repeated number of
pressure cycles carried out over a period of eight months has
also been reported. The maximum nonlinearity and the hyster-
esis is improved from
0.92%
FSO
to
0.06%
FSO
after
1000
pressure cycles. The output behavior of the pressure trans-
ducer with temperature has also been studied.
I. INTRODUCTION
ESSURE transducers are basically electromechani-
p"
cal devices used for a variety of applications. The pri-
mary function of the pressure transducer is to sense fluid
pressure and provide an electrical output proportional to
the input pressure. A pressure transducer essentially con-
sists of a diaphragm which undergoes deformation due
to
applied pressure. This mechanical deformation of the dia-
phragm in converted into an electrical response by a strain
gauge bonded to it. It was more common to use foil-type
strain gauges for this purpose. However, in recent years,
the use of thin-film strain gauges as sensors in pressure
transducers has gained importance due to their numerous
merits. The distinct advantages are absence
of
adhesive
material, flexibility to tailor the properties
of
the sensing
film, etc. Although several investigations on the strain
sensitivity of different metal [1]-[16] and alloy films [17]-
[
191 have been reported, the publications on the utility of
these films as strain gauges in devices such as pressure
transducers are few.
In the present paper we report the study of a pressure
transducer with Au-Ni (89
:
11) alloy thin-film strain
gauges with a meandering path pattern
[15].
The starting material for fabrication was Au-Ni
(82
:
l8), a high-temperature brazing alloy with a unique
characteristic, namely, its liquid and solid temperatures
are the same (950°C). Although this alloy is being used
for brazing of stainless-steel foils, it is found to have very
Manuscript received March 13, 1992; revised June 3, 1992. This work
was supported by
ISRO,
Department
of
Space, through the
RESPOND
pro-
gram. The review
of
this paper was arranged by Associate Editor
S.
D.
Senturia.
K.
Rajanna and
S.
Mohan are with the Instrumentation and Services Unit,
Indian Institute
of
Science, Bangalore 560
012,
India.
M. M. Nayak,
N.
Gunasekaran, and A. E. Muthunayagam, are with the
Pressure Transducer and Fabrication Facility, Liquid Propulsion Systems
Centre, ISRO, Bangalore 560
008,
India.
IEEE Log Number 9206465.
good adhesion and corrosion resistance properties [20]. In
view of these and since no study of this alloy with respect
to strain gauges has been reported, an attempt to explore
its suitability and applicability for thin-film strain gauge
application has been made in the present work.
11.
EXPERIMENTATION
In the present work an integrated diaphragm assembly
has been employed.
A
special feature of this diaphragm
design
is
the incorporation of a strain-relieving cavity to
take care of the strain due to mechanical handling, mount-
ing, or assembly process. The material used for the fab-
rication of the pressure transducer is precipitately hard-
ened X17U4 steel. Further, details including the
dimensions of the diaphragm assembly and the informa-
tion on diaphragm surface preparation are given in our
earlier publication [21]. Before depositing the sensing film
(strain gauges), the insulating films consisting of alternate
layers of A1203 and SiOz were deposited on the diaphragm
surface using reactive electron beam evaporation tech-
nique. The diaphragm assembly was maintained at 200°C
during deposition. The need to deposit insulating oxide
layers, the deposition process details, and the necessity to
have multilayers instead of single-layer oxide films have
already been described by Rajanna
et
a1
[2 13.
After the deposition of the insulating oxide layers, cop-
per contact pad films and interconaecting copper pad films
of thickness greater than
3000
A
were deposited.
Sub-
sequently, Au-Ni film (strain gauge film) was deposited.
The Au-Ni (82
:
18) material in the
form
of wire (obtained
from Wilkinson Company,
USA)
was evaporated from a
molybdenum boat. It is believed that since gold and nickel
possess nearly the same vapor pressures at a given tem-
perature, the possibility
of
fractionization
of
this alloy
during evaporation is much smaller. In order to minimize
the possibility of fractionization, a shutter was placed
above the source during initial heating of the boat. The
shutter was removed only when the boat temperature was
sufficiently high to evaporate both the constituents of the
alloy. The boat temperature during the evaporation was
close to 1200°C. The analysis of the deposited film by
plasma emission spectrometry technique revealed the Au-
Ni composition as 89.2%
:
10.8%. This deviation in com-
position from starting material
is
attributed to the fact that
single-source evaporation of a composite containing two
or more components results in incongruent evaporation
0018-9383/93$03.00
0
1993 IEEE
522
IEEE TRANSACTIONS
ON
ELECTRON DEVICES, VOL.
40.
NO.
3,
MARCH
1993
C
T1
0
Bonding
pad
for attachirq
the electrical leads
-
Interconnecting pads
-
5ensing film
[AU-NI
film]
Fig.
1.
Thin-film strain gauge pattern deposited on the pressure transducer
diaphragm.
[22]. The thickness of the Au-Ni film (650
A)
during
deposition was measured and later compared using a mul-
tiple beam interferometer. Prior to deposition of copper
and Au-Ni, the substrate was subjected to ionic cleaning
for about
10
min. Precision mechanical masks were used
while depositing the above films of required pattern. These
masks were prepared using a CNC (Computer Numeri-
cally Controlled) spark erosion machine with a dimen-
sional tolerance
of
k
1 pm. The strain-gauge pattern
adopted (for the four gauges, as shown in Fig.
1)
and their
location on the diaphragm are the same as those already
reported in our earlier paper [23].
The four gauges were connected in Wheatstone bridge
configuration with all active gauges. The gauges experi-
encing the tensile strain (near the center) are connected in
one set of opposite arms and those experiencing the com-
pressive strain (at the diaphragm edge) in the other. It is
important to note that the tensile strain causes the resis-
tance of the gauge to increase and compressive strain re-
sults in decrease of resistance. The stabilization of the
film was carried out by subjecting the diaphragm assem-
bly to post-deposition heat treatment.
The output characteristics of the transducer were stud-
ied using a dead weight pressure calibrator (Model 5020
of Desgranges et Huot, France). The details of the pro-
cedure followed are given elsewhere [23].
111.
RESULTS
AND
DISCUSSION
A.
Effects
of
Post-Deposition Heat Treatment
on
the
Thin-Film Strain Gauge System
As can be seen in Table I, the post-deposition heat
treatment resulted in a decrease in gauge resistance. The
reduction in gauge resistance is attributed to annealing of
defects incorporated in the film structure during deposi-
tion [24], [25]. It is important
to
note here that whereas
the manganese film pressure transducer [23] had an av-
erage reduction
of
6.5%
in gauge resistance after heat
treatment at 120°C, the average reduction achieved in the
present Au-Ni film resistance was only 0.29%. There-
fore, in order to make sure of the almost complete an-
nealing of defects, the heat treatment process was re-
peated at 170°C. After the second heat treatment, the
average decrease in gauge resistance was 1.36%. How-
ever, further heat treatment did not result in any addi-
tional decrease in gauge resistance.
TABLE
I
PRESSURE TRANSDUCER DIAPHRAGM
RESISTANCE VALUES
OF
THIN-FILM STRAIN GAUGES DEPOSITED
ON
THE
Resistance Values
(51)
After Heat After Second
Details
of
the Treatment Round
of
Heat
Thin-Film Strain Before Heat at 120°C Treatment at
Gauges Treatment for
1
h 170°C for 2
h
Gauges located at the edge of the diaphragm
96.58
96.17 94.50
99.59 99.31 98.23
gauge
C,
gauge
CZ
Gauges located near the center
of
the diaphragm
105.79 105.45 104.68
gauge
Ti
gauge
T2
104.92
104.78
104.00
The insulation resistance of the oxide layers increased
from about 300 to 40
000
MQ at
10
V
dc due to post-
deposition heat treatment. The increase in insulation re-
sistance is believed to be due to enhanced oxidation of the
individual oxide layers and the formation of stronger in-
terfaces between layers.
B.
Individual Gauge Response
Figs.
2
and 3 show the variation of relative change in
resistance
AR/R
with pressure for the gauges
(TI
and
T2)
located near the center of the diaphragm and those at the
edge
(Cl
and
C2)
after the post-deposition heat treatment.
It is observed that for all the gauges, the variation in
AR/R
with pressure is more linear and repeatable (between
gauges) after the heat treatment process when compared
to the variation of the same before the heat treatment.
C.
Output Performance
of
the Thin-Film Pressure
Transducer
The variation in output voltage of the pressure trans-
ducer as a function of input pressure is quite linear. The
values of non-linearity and hysteresis (in percentage FSO;
full scale output) versus pressure (in bars) are shown in
Fig.
4.
The maximum nonlinearity and hysteresis ob-
served is 0.92% FSO. By considering the resistance of
the thin-film strain gauges, the film thickness, the power
dissipation factor, etc., the bridge excitation was opti-
mized to 3
V
dc. It has been observed that higher exci-
tation voltages showed instability above
1
%
of
FSO
in the
output of the pressure transducer under zero load condi-
tion. This instability at higher excitation voltages is con-
sidered to be due to excessive heating of the thin-film
strain gauges.
Further, it was found that the nonlinearity and hyster-
esis improved after
1000
pressure cycles of a nominal
pressure of 30 bar for 3-min duration carried out over a
period of
8
months. The typical data of maximum non-
linearity and hysteresis observed after
500
cycles is 0.39
%
FSO. Again the maximum nonlinearity and hysteresis
found after additional
500
cycles is
0.06%
FSO. This in-
dicates that films require about
1000
cycles to achieve
nonlinearity and hysteresis of the order of 0.06%, which
RAJANNA
et
al.:
PRESSURE TRANSDUCER WITH Au-Ni THIN-FILM STRAIN GAUGES
,
/t
30
AFTER
HEAT
TREATMENT
,/
I
0
GAUGE
TI
+
GAUGE
T,
:
.
;i
0
4
8
12
16
20
24 28
Fig. 2. Variation
of
relative change in resistance
AR/R
with pressure
for
gauges located near the center of the diaphragm-after heat treatment.
PRESSURE
(bar)
0
AFTER HEAT TREATMENT
0
GAUGE C1
t
-q
-20
+
GAUGE
C2
I
PRESSURE
(bar)
-30
04
Fig. 3. Variation
of
relative change in resistance.
AR/R
with pressure for
the gauges located at the edge
of
the diaphragm-after heat treatment.
EXCITATION
3V
K
0
ASCENDING
t
DESCENDING
-.,
0-
8
--v
-
$
-1.21
Fig. 4. Nonlinearity and hysteresis versus pressure
is thought to be due to stress-relieving effects. Similar ob-
servation has been reported in the resistance-strain char-
acteristics of metal films by Parker and Krinsky
[
1
].
It has been observed that the
FSO,
zero-offset plots were
linear and parallel with a negative slope of
0.11
1
mV/ “C.
Also the net output was found to be linear in the temper-
ature range
-20°C
to
+7O”C
with a maximum deviation
of less than
0.6%.
In comparison with pressure transducer made with Mn
523
film strain gauges
[23],
the present Au-Ni film strain
gauge transducer shows a stability of
0.6%
in a time span
of
4
months although the gauge
resistant?
is lower and
the film thickness is
of
the order of
650
A.
Also, it has
been observed that the Mn film strain gauge transducer
inevitably requires an overlayer to avoid atmospheric in-
fluence leading to change in resistance and hence output
drift with time, whereas, Au-Ni film pressure transducers
do not require any overlayer.
IV.
CONCLUSIONS
The output performance of a thin-film pressure trans-
ducer with Au-Ni film as the strain gauge has been stud-
ied with continuous excitation for at least
2
h before tak-
ing any output readings. It was found that the optimum
bridge excitation was
3
V
dc, since higher excitation volt-
ages showed instability above
1%
FSO
which is thought
to
be due to excessive self-heating. The maximum non-
linearity and hysteresis observed after
1000
repeated pres-
sure cycles over a period of
8
months was
0.06%
FSO.
The output behavior of the pressure transducer with tem-
perature was also studied after stabilizing at each temper-
ature for
2
h and it was found that the net output was
linear with a maximum deviation of less than
0.6%.
ACKNOWLEDGMENT
The authors wish to thank
M.
Ram for his involvement
in the fabrication of precision masks. Thanks are also due
to N. V. G. Nair and N.
K.
Ganesan for their assistance
in carrying out the experimental work. The active partic-
ipation of
S.
Srinivasulu during the discussion of the work
reported in this paper is gratefully acknowledged.
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K.
Rajanna graduated from the University
of
Mysore, India, with the B.Sc. degree in physics
and mathematics in 1973. He received the M.Sc.
degree in physics from the same university in
1976, and was awarded the M.Sc. (Engg.) degree
from the Indian Institute of Science, Bangalore,
India in 1988 for his research work on thin-film
strain gauges.
He started his career in the National Aeronau-
tical Laboratory and later spent some time in the
Reactor Research Centre (DeDartment of Atomic
.L
Energy), Kalpakkam, before joining the Indian Institute of Science in 1983.
At present he is a Senior Scientific Officer in the Instrumentation and
Ser-
vices Unit of the Indian Institute of Science. His current interest is in the
area of thin-film transducers. Presently he is completing the requirements
for the Ph.D. degree.
S.
Mohan received the M.Sc. degree in physics
from Sri Venkateswara University in 1967 and
continued to work for the Ph.D. degree at the same
university in the field
of
thin films.
He was a member of the teaching faculty
of
the
Physics Department at the Sri Venkateswara Uni-
versity during the period 1970-1977. He was
se-
lected as a National Associate by University
Grants Commission in 1977 and worked
for
3
months in the Thin Film Laboratory of the Indian
Institute of Technology, New Delhi- He joined the
Indian Institute of Science, Bangalore, in 1977
as
a Senior Scientific Officer
and since 1988 has been an Associate Professor in the Instrumentation and
Services Unit, in charge of the Vacuum and Thin Films group. In 1981 he
was honored by Andhra Pradesh Academy of Sciences with a Young Sci-
entist award. He was awarded the MRSI medal for 1992 by the Matenals
Research Society of India. His areas
of
interest include thin-film sensors,
superconducting films, optical devices, and tnbological coatings
He
was
the General Secretary of the Instrument Society of India during 1983-1989
and since 1989 has been the Convener, Thin Films group of the Materials
Research Society of India. He has been the Editor of
Journal
of
Instrument
Society
of
India
since 1991.
He
was a Visiting Professor for
3
months in
1990 at Forschung Zentrum, Julich, Germany. He has visited a number of
countries to give lectures in reputed laboratories.
He
has published about
80 scientific publications in refereed journals and 6 review articles.
M.
M.
Nayak received the B.E. degree in elec-
trical engineering from Bangalore University,
Bangalore, India, in 1982. Subsequently in 1984
he received the D.I.I.Sc. degree (a specialized
post-graduate diploma in electronics design tech-
nology) from the Indian Institute
of
Science, Ban-
galore.
He has undergone a one-year advanced training
program at S.E.P. (Societe Europeenne de Pro-
pulsion), France, on space-qualified high-preci-
sion pressure transducers. In 1982 he was honored
with an NRDC (National Resdarch and Development Corporation of India)
award for his developmental work on semiconductor pressure transducers.
At present he is an engineer in charge of the developmental cell at the
pressure transducer and fabrication facility, LPSC, of the Indian Space Re-
search organization. He is extensively involved in the development of thin-
film technologies for space applications which include pressure trans-
ducers, liquid level sensors and cryogenic sensors. He is working towards
the Ph.D. degree.
N. Gunasekaran received the degree in mechan-
ical engineering from Madras University, Mad-
ras, India, in 1964.
He has been with the Liquid Propulsion Sys-
tems Centre (LPSC) of ISRO for the last twenty
years. He heads various groups engaged in the
fabrication of hardware for rocket engines, liquid
engines, development and batch production of
transducers. At present he is
the
general manager
of
the Transducers and Light Alloy Fabrication
Facility (TLF), LPSC. In recognition of his de-
velopmental work on semiconductor pressure transducers, he was honored
with an NRDC award in 1982. His areas of interest include the design,
development, and fabrication of various components for space application
and precision transducers. Recently, he has also been involved in the de-
sign and development of cryogenic stages for GSLV (Geo-Stationary
Launch Vehicles).
A.
E.
Muthunayagam was born in India on Jan-
uary
11,
1939. He received the
B.E.
degree in
me-
chanical engineering in 1960 from the University
of Madras, Madras, India, and the M.E. degree
in power engineering from the Indian Institute of
Science, Bangalore, India. In 1965, he received
the Ph.D. degree from the School of Mechanical
Engineering, Purdue University, West Lafayette,
IN.
He joined the Indian Space Research Organi-
zation (ISRO) in 1966. As an outstanding propul-
sion engineer, he contributed significantly to the rocket propulsion tech-
nology development at ISRO. He is primarily responsible for the setting
up of the Liquid Propulsion Systems Centre (LPSC) and streamlining its
operation in building extensive test facilities
for
qualifying the space pro-
pulsion systems. His achievements also include development
of
second and
fourth liquid stages for the PSLV, reaction control systems, and SITVC
for launch vehicle projects such as SLV3, ASLV, and PSLV, monopro-
pellant reaction control systems for IRS satellites, and liquid propulsion
systems for INSAT-I1 project. He is the recipient of the Dr. V. M. Ghatage
Award for the year 1989 from the Aeronautical Society of India. He is
involved as a member
of
several Indian space programme projects/boards.
He is also a member of the Space Propulsion Committee of the Intema-
tional Astronautical Federation, Paris, France. He is the Director, Liquid
Propulsion Systems Centre (LPSC), ISRO and is also a member of ISRO
Council.