The development of a piezo-resistive pressure transducer is discussed suitable for recording pressures typically encountered in biomedical applications. The pressure transducer consists of a thin silicon diaphragm containing four strain-sensitive resistors, and is fabricated using silicon monolithic integrated-circuit technology. The pressure transducers can be as small as 0.7 mm outer diameter, and are, as a result, suitable for mounting at the tip of a catheter. Pressure-induced stress in the diaphragm is sensed by the resistors, which are interconnected to form a Wheatstone bridge

Angell, J. B.

English

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FINAL REPORT ON
RESEARCH ON PRESSURE SENSORS
FOR BIOMEDICAL INSTRUMENTS
PERFORMED ON
NASA-AMES GRANT NGL 05-020-401
FEBRUARY 19, 1975
ABSTRACT
A four-year research program which began in October 1969 led to the
development of a piezo-resistive pressure transducer suitable for recording
pressures typically encountered in biomedical applications. The pressure
transducer consists of a thin silicon diaphragm containing four strain-sen-
sitive resistors, and is fabricated using silicon monolithic integrated-
circuit technology. 'Pressure-induced stress in the diaphragm is sensed by the
resistors, which are interconnected to form a Wheatstone bridge.
The pressure transducers can be as small as 0.7 mm outer diameter, and
are, as a result, suitable for mounting at the tip of a catheter. Because
of the work achieved on the grant, additional programs to further develop the
pressure transducer have been established, and a licensing agreement has
been signed with Cordis Laboratories Inc. of Miami, Florida to apply the
principles of the sensor in a commerically available catheter.
(NASA-CR-142351) RESEARCH ON PRESSURE N75-1963 4
SENSORS FOR BIOMEDICAL INSTRUMENTS Final
Report (Stanford Univ.)
Unclas
G3/35 14640
REPRODUCED BY
NATIONAL TECHNICAL
INFORMATION SERVICE
U. S. DEPARTMENT OF COMMERCE
SPRINGFIELD, VA. 22161 .
https://ntrs.nasa.gov/search.jsp?R=19750011562 2020-03-22T23:44:52+00:00Z
FINAL REPORT NASA-Ames Grant NGL 05-020-401
RESEARCH ON PRESSURE SENSORS FOR BIOMEDICAL INSTRUMENTS
I. Introduction. This report covers the research on the development of
a silicon-diaphragm pressure transducer which was begun on October 1,, 1969
and concluded on September 30, 1973. The program resulted in the first
version of a very successful series .of pressure transducers, on which develop-
ment has continued for a variety of biomedical applications. The principal
work conducted on the grant resulted in a publication which anpeared in the
March, 1973 issue of the IEEE Transactions on biomedical engineering. Because
this publication thoroughly describes thework on this phase of the grant,
it is incorporated into this report as I. This paper was based on
the Ph.D. dissertation of one of the authors, Samaun, which was accepted by
Stanford in August 1971.
The other phases of the project were less intensive then the work of
Samaun and are reported below. First was an attemot to develoo a two-lead
oressure transducer which would reduce from four to two the number of connections
which must be made to the small pressure transducer. Second was the attempt
to use ion-implantation to form the strain-sensitive resistors, in order to
provide much higher resistance values, so that short circuit currents which
might result from rupture of the diaphragm could be kept low enough to avoid
any threat of lethality or damage to the biological environment in which the
sensor was used.
II. The Successful Pressure Sensor. A thin-diaphragm piezoresistive
pressure sensor for biomedical instrumentation has been developed using mono-
lithic integrated-circuit (IC) techniques. The piezoresistive effect has been
chosen for this device because it provides an observable resistance change that
is a linear function of pressure and is observable at low stress levels, A
diaphragm is used as a stress magnifying device; its magnification is proportional
to the square of the ratio of the diaphragm diameter to its thickness. The
pressure-induced stresses in the diaphragm are sensed by properly oriented
piezoresistors interconnected to form a bridge.
An anisotropic etching technique is used for the formation of the dia-
phragms; this technique makes possible a novel thickness monitoring scheme that
also acts as a chip separation etch. Sensors with diaphragm diameters of 0.5 mm
and thickness of only 5 jm, surrounded by a 0.15 mm wide ring of thick silicon,
have been batch fabricated using this technique. An intrinsic sensitivity of
14 wV/V supply/mmHg has been achieved.
Temperature drift in these sensors is dominated by the temoerature
dependence of the piezoresistive coefficient. A temperature-compensation
circuit has been devised for these sensors by deriving a temoerature-deoendent
signal that is pressure independent for the compensation of the temoerature-
dependent part of the bridge unbalance voltage.
These sensors, after being mounted on the tip of a small catheter, can be
inserted into the biological system through the inner bore of a larger catheter
Page 2
February 19, 1975
that was formerly occupied by a guide wire. The sensors have been utilized
for acute measurements of blood oressure in dogs with satisfactory results.
Details of this structure have been published. The article is incor-
porated in this paper as Appendix I. Prior to the oublication of this paper,
an abridged version as presented at the 1971 IEEE Internation Solid-state
Circuits Conference, and is included in the digest of technical papers of
that conference.
III. A Two-Lead Pressure Transducer. The pressure transducer described in
Appendix I requires four leads for making connection to the four corners of the
Wheatstone bridge in the transducer. Because it is difficult to make dependable
connections to such a small structure, it was deemed desirable to attempt to
develop a two-lead version, as shown schematically in Figure 1. In this
version, a bridge consisting of two diodes and two resistors is driven by a
ac source through a capacitor. If both resistors have exactly the same value,
no dc voltage is developed on the capacitor. However, if the resistors are
unequal (for example, if they were changed by pressure induced stress in the
diaphragm, then more current flows during one-half of the ac waveform then
during the second half.) Thus, a direct voltage will be developed on the
capacitor. This direct voltage can be measured by a low-frequency detector
which does not respond to the a-c sensing signal. As a result, only two leads
(such as a coaxial cable or a highly flexible twisted pair) are needed to
connect to the transducer.
Initial fabrication attempts indicate that a pressure transducer can
be fabricated using the above principle to reduce the required number of leads.
However, this work could not be carried far enough, during the conduct of this
research work, to result in a usable design.
IV. A Completely Safe Transducer. The pressure transducer fabricated as
described in Appendix I using diffused p-resistors as the strain-sensitive
elements. With thermal diffusion as the means of introducing the doping, it
was found that a sheet resistivity of the order of 100 ohms/square is optimum,
As a result, these resistors run in the range of 2-5 kilohms.
Because sensing voltages must be of the order of one to ten volts applied
to the bridge, in order to obtain good signal-to-noise ratio at the output of
the bridge, short-circuit currents with a diffused-resistor bridge are of the
order of 0.1-1.0 ma, which is high enough to be of some concern in certain
biomedical applications. On the other hand, a short-circuit current of 10 pa
is considered completely safe in all circumstances, Thus, if the resistors'
values can be increased by a factor of 30 to 100, a completely safe pressure
transducer will result. Because ion implantation can be used to provide care-
fully controlled layers of impurity dopants with much smaller total concentration
then thermal diffusion, attempts were made to fabricate four-resistor dia-
phragms with sheet resistivities of 5,000 ohms/square. The two such implant-
ations that were tried were successful in that the desired resistance range
was achieved. However, the yields in processing the structures so formed into
thin diaphragms, together with subsequent mounting, were low enough that
no successfully mounted structures could be evaluated in vivo.
Page 3
February'l9, 1975
V. Continuation Work. Although the work supported by this grant was
ended in 1973, work on such pressure transducers has continued with support
from the National Institutes of Health. The major development in this
continuing work has been to perfect an absolute pressure transducer, in which
a glass cavity is hermetically sealed to one side of the pressure sensitive
diaphragm to provide a reference pressure. The current model consists of a
glass cap that has a well etched in it and a silicon diaphragm containing four
diffused piezoresistors connected to form a Wheatstone bridge. A cross section
of this structure is shown in Figure 2.
SILICON DIAPHRAGM HERMETICALLY SEALED CHAMBER
THICK SILICON RING
GLASS PLATE
WELL L--HEATER
FIGURE 2 -- ABSOLUTE-PRESSURE TRANSDUCER STRUCTURE
The diaphragm is surrounded by a thick rim of silicon for support and
protection. To construct the absolute pressure transducer, the glass cap
with the well positioned directly over the diaphragm is anodically bonded to
the silicon rim. The anodic bond is accomplished by heating the glass and
silicon to 4000C and then applying a potential of 600 V to the silicon with
the glass cap grounded. The bond is hermetic, and thus the well and diaphragm
now form a hermetically sealed chamber with a predetermined pressure trapped
therein.
The diaphragm contains integrated-circuit resistors that form elements
of a Wheatstone bridge. Their values change as the diaphragm flexes. A
patent application has been signed to protect various aspects of this structure.
Cordis Laboratories of Miami, Florida has signed a license agreement with
Stanford University to use this technique for making pressure transducers in
the fabrication of pressure cells to be incorporated in the wall of a catheter.
Several of these absolute pressure transducers have been developed and
tested. The overall dimensions are 1.5 x 2.0 x 0.2 mm- They have an intrinsic
sensitivity of 30 uV/mnmHg/V supply and a temperature drift equivalent to
Page 4
February 19, 1975
-2 nmmHg/oC. The output voltage is linear with pressure for changes up to
300 mmHg. Our current work is aimed at investigating different techniques for
reducing the temperature drift.
PRESSURE
INDICATOR
D-C
AMPLIFIER RADIAL
-TANGENTIAL
LOW-PASS RESISTOR
FILTER
AC
SOURCE
ELECTRONICS--- TWO-LEAD SILICON DIAPHRAGM
CABLE WITH
THICK SUPPORTING RIM
FIGURE 1 -- TWO-LEAD PRESSURE TRANSDUCER K2
REFERENCES
1) Samaun, Kensall D. Wise, and James B. Angell, "An IC Piezoresistive
Pressure Sensor for Biomedical Instrumentation", IEEE Transactions
On Biomedical Engineering, Volume BME-20, No. 2, pp. 101-109, March 1973.
2) Samaun, K.D. Wise, E.D. Nielsen, and James B. Angell, "An IC Piezoresistive
Pressure Sensor for Biomedical Instrumentation", Digest of Technical
Papers of the 1971 IEEE International Solid-state Circuit Conference,
pp. 104-105, February 1971.
3) Most of the conduct work on this development has been supported by NIH
Contract 70-2557 and NOl1-HD3-2774 entitled "Physiology of the Oviduct,"
with support for certain phases being provided by Integrated Electronics
for Medical Applications" NIH Grant 5 PO1 GM17940-05


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