Applications of High-Performance MEMS Pressure Sensors
Based on Dissolved Wafer Process
Srinivas Tadigadapa and Sonbol Massoud-Ansari
Integrated Sensing Systems (ISSYS) Inc.,
387 Airport Industrial Drive, Ypsilanti, MI 48198
ISSYS’ exclusive dissolved wafer process (DWP) technology has enabled the
development of a variety of the state-of-the-art, high performance pressure sensors for a
wide range of applications. DWP is a simple, single-sided wafer fabrication process, which
can very reproducibly and accurately fabricate both low and high aspect ratio single crystal
silicon structures at high densities. In this paper, we will discuss the performance and
applications of the pressure sensors fabricated by this wafer processing technique. These
applications include diverse areas such as ultra-sensitive, high vacuum measurements;
barometric pressure sensors; ultra-high sensitivity altimeters; miniaturized pressure sensors
for biomedical applications; and multiple sensors on a single chip for applications requiring
extended dynamic range and improved accuracy, as well as enhanced reliability.
Introduction
Miniaturization of sensors and
actuators requires the ability to create
accurate, three-dimensional microstructures,
which are capable of performing thermoelectro-mechanical functions.
Several
innovative modifications and adaptations in
the silicon microelectronics fabrication
processes have led to the possibility of
creating such accurate, three-dimensional,
free-standing structures from silicon.
One
such fabrication technique known as the
“Dissolved Wafer Process” (DWP) was
developed at the University of Michigan by
Professor Kensall Wise and his group.
ISSYS Inc. has exclusive rights to six
patents from the University of Michigan,
which comprehensively deal with all aspects
of the DWP fabrication technique and its
applications. After a brief presentation of
the DWP technology, we will demonstrate
the versatility of DWP by the performance
and the variety of applications of capacitive
pressure sensors manufactured on the
flexible platform of this technique.
Background
Integrated
micromachining
technologies can be divided into four major
categories: bulk micromachining (including
DWP), polysilicon surface micromachining,
electroplate micromachining (also known as
LIGA) and post-processed CMOS wafer
micromachining. Bulk micromachining is the
dominant
commercial
microsensor
technology in which the silicon wafer is
directly etched to create three-dimensional
devices, with thickness up to that of the
single wafer. Major disadvantages of bulk
micromachining are the relatively large
silicon real estate consumption and the lack
of complete compatibility with standard
CMOS processing. Surface micromachining
manipulates materials (mainly polysilicon)
that are deposited on the front surface of the
silicon wafer. The bulk silicon wafer is
primarily used as a substrate and acts as a
structural support for the micromachined
devices. Major advantages of surface
micromachining are the small silicon area
Industrial Pressure Sensor
used for the mechanical structures,
compatibility with standard microelectronics
processes and capability of fabricating
multilevel microstructures. The major
drawback of surface micromachining is the
use of polysilicon as the mechanical material
and several yield and mate rial reliability
issues associated with thin micromachined
structures. In electroplated micromachining,
microdevices with very high aspect ratio are
fabricated with deep X-ray lithography,
electro-forming and possible molding
processes. In post-process CMOS wafer
technology, standard CMOS processed
wafers are further post-processed to release
microstructures formed from the thin films
available in this process (typically polysilicon
and SiO2). A standard post process step
involves a single maskless etch stop.
Dissolved Wafer Process
DWP is a versatile fabrication
technique, which allows the creation of a
variety of three-dimensional, free-standing,
single-crystal, silicon-microstructures on
glass substrates. DWP is a simple, singlesided wafer fabrication process, which is
both reproducible and accurate. Using DWP,
it is possible to simultaneously create high
aspect ratio thick and/or thin microstructures
on the same chip and at high densities.
Single crystal silicon is an ideal
micromechanical material. It has a Young’s
modulus and hardness comparable to those
of stainless steel, with a density only a third
and yield strength three times that of
stainless steel. As a crystalline material, it is
devoid of the plastic deformation phase and
is practically fatigue and hysteresis free.
Figure 1 shows the simplified process flow in
fabricating a diaphragm using DWP. The
fabrication sequence consists of silicon
processing, glass processing, electrostatic
bonding, and wafer dissolution. Typical
silicon processing starts with a 550 µm thick,
p-type (100) oriented silicon wafer. A KOH
etch is first performed to define a recess that
will later provide the gap between the silicon
diaphragm and the glass substrate (Fig. 1a).
This is followed by a patterned deep-boron
diffusion step, which defines the supporting
rim for the diaphragm (Fig. 1b). Typical
thickness of deep-boron diffusion varies
between 10µm-15µm. The diaphragm itself
is formed by using shallow-boron diffusion
(2µm-5µm) (Fig. 1c). Glass processing
consists of metallization to form lead
transfers, etching and/or formation of holes
as required. Once the silicon and glass
processing are completed, the silicon and
glass wafers are electrostatically bonded
together. The silicon wafer is then etched
(dissolved) away in Ethylene Diamine
Pyrocatechol (EDP), leaving only heavily
boron-doped areas on the glass wafer (Fig.
1d).
It can be seen that DWP is a versatile
process in providing silicon microstructures
with a wide range of shapes and
dimensions. The fabrication process allows
for the implementation of many variations in
shapes and structures without increasing
either the number of mask steps or process
complexity. Therefore, DWP offers a very
flexible fabrication platform, which can be
used to manufacture a wide range of
micromachined sensors and actuators. In
Figure 1. Simplified process flow for Dissolved Wafer Process (DWP).
Industrial Pressure Sensor
addition, interfacing with circuit chips can be
achieved using a simple hybrid flip chip
approach.
sensors, the gap between the diaphragm
and the glass acts as the reference cavity
ISSYS has successfully fabricated
and tested a wide range of pressure sensor
prototypes using DWP. The basic structure
of the proposed pressure sensor is shown in
Figure 1d. The diaphragm and the bottom
electrode on the glass act as a parallel plate
capacitor. In response to pressure variations
the diaphragm deflects, causing a variation
in the capacitance. Figure 2 shows a multielement sensor with 9 different pressure
sensors. These elements measure full-scale
pressure in the range of 0-1 Torr to 0-2200
Torr.
Sensitivity (pF/Torr)
1000
Pressure Sensors manufactured using
DWP and their Applications
Theory
Experiment
100
10
1
0.1
0.01
0.001
0
2
4
6
8
Diaphragm Diameter (mm)
Figure 3 Sensitivity of pressure sensors as a
function of the diaphragm diameter.
and is sealed up at ultra-high vacuum.
ISSYS has patent pending technique by
which the electrical leads are transferred
from the the vacuum sealed cavity to the
outside. The multi-sensor chip also
incorporates temperature sensors with a
resolution of 2000 ppm/ºC. In addition to the
measurement of vacuum, the differential
pressure sensors along with the temperature
sensors can be used for the measurement
Figure 2 Sensor die containing nine
pressure sensor covering a pressure
measurement range from 0–2200 Torr to
0–1 Torr.
Figure 3 illustrates the theoretical and
experimentally measured sensitivity of these
capacitive pressure sensors as a function of
the diaphragm size. Capacitive pressure
sensors can be fabricated for the
measurement of absolute or differential
pressure. In case of absolute pressure
Figure 4 ISSYS’ most sensitive pressure sensor
with a sensitivity of 120 pF/Torr .
of fluid flow.
Industrial Pressure Sensor
Figure 5 Performance of the 0-250 mTorr
range. This pressure sensor is capable of
resolving sub-? Torr vacuum pressures.
possible to measure pressures in the sub? Torr range. Figure 5 shows the measured
performance
of
this
device.
High
performance vacuum pressure sensors find
applications in several fields such as
semiconductor equipment industry, vacuum
instrumentation, and flow measurement
applications.
Vacuum pressure sensors can also
be used as barometric pressure sensors.
However, the typical sensitivity of capacitive
pressure sensors increases exponentially
with the decrease in the full operating range.
ISSYS has developed a proprietary
technology,
which
overcomes
this
shortcoming and allows for the operating
range and the sensitivity to be independently
designed. For example, one of our sensor
designs, with an overall dimension of 8mm X
8mm, exhibits a resolution of better than 2
mTorr at 800 Torr pressure range (a
resolution of 1/400,000). If this sensor is
used as an altimeter, it will have an
outstanding resolution of better than one
inch at the sea level, and a resolution of
about 1-foot at 30,000 feet, as shown in
Figure 6. The simulated performance shown
in Figure 6, however, has not been
compensated for temperature variations.
Barometric pressure sensors can also be
used in wind tunnels; for very accurate
measurement of pressure and pressure
distribution on a variety of airfoils and
aircraft
models;
barometric
pressure
measurement for weather monitoring
applications; and in multi-sensor, microinstrumentation clusters requiring low-power
barometric pressure sensors.
ISSYS is currently developing ultraminiature pressure sensors for biomedical
applications. Typical size of these pressure
sensors is 300 ? m X 4 mm. This size is
perfectly suited for several biomedical
applications such as single/multi-point
catheters, measurement of intracranial
pressure, pacemaker applications and other
30
Resolution (inch)
In early 1998, ISSYS fabricated the
first prototype of one of the most sensitive
capacitance diaphragm vacuum pressure
sensors known. This sensor is shown in
Figure 4. The full-scale range of this sensor
is from 0 – 0.25 Torr and has a sensitivity of
over 100 pF/Torr. Using this sensor, it is
25
20
15
10
5
0
0
10000
20000
30000
40000
50000
Altitude (feet)
Figure 6 Simulated performance of an altimeter
using ISSYS’ proprietary barometric pressure
sensor.
implanted
coronary
pressure
measurements. Figure 7 shows a typical
biomedical pressure sensor prototype in the
eye of a needle. The performance of the
sensor is shown in Figure 8. The sensitivity
of this device was measured to be 3 fF/Torr
in the 500 – 1000 Torr pressure range. This
pressure range adequately covers the
very versatile micromachining technology
and offers a flexible manufacturing platform
for a variety of micromachined sensors and
actuators. In addition, ISSYS has several
patents
covering
all
aspects
of
manufacturing such silicon micromachined
pressure sensors. Current patent pending
areas include: sensor design, electrical lead
transfer from sealed cavities, packaging,
and corrosion resistant technology.
References
K.D. Wise and Samaun, “Methods for
forming regions of predetermined thickness
in silicon”, U.S. Patent 3,888,708, June 10,
1975.
Figure 7 Multi-site pressure sensing catheter
using ISSYS’ 300 um X 4mm pressure
sensor. The device is shown in the eye of a
needle to give a perspective of its size.
K.D. Wise and H.L. Chau, “Ultraminiature
pressure sensor and method of making
same”, U.S. Patent 4,881,410, November
21, 1989.
5.50
Sensitivity = 3.2 fF/Torr
Capacitance (pF)
5.00
4.50
K.D. Wise and H.L. Chau, “Method of
making an unltraminiature pressure sensor”,
U.S. Patent 5,013,396, May 7, 1991.
4.00
3.50
3.00
500
600
700
800
900
1000
Pressure (Torr)
Figure 8 Performance of the pressure sensor
designed for biomedical applications
pressure measurement requirements for
biomedical applications and provides a
measurement sensitivity of better than 1
Torr. ISSYS is currently working on the
biocompatible, corrosion-resistant packaging
of these sensors and their integration into
long-term implantable pressure monitoring
systems.
Conclusions
In conclusion, ISSYS Inc. has
developed capacitive pressure sensors for a
variety of applications. These pressure
sensors have been fabricated using ISSYS’
exclusive dissolved wafer process. DWP is a
L.J. Spangler and K.D. Wise, “Fullyintegrated single-crystal silicon on insulator
process, sensors, and circuits” U.S. Patent
5,343,064, August 30, 1994.
Y. Gianchandani and K. Najafi, A bulk silicon
dissolved
wafer
process
for
microelectromechanical
devices,
IEEE/ASME J. MEMS, Vol. 1, No. 2, pp. 7785, 1992.
K. Najafi, Integrated sensors in biological
environments, Invited Paper, Sensors and
Actuators, Vol. B1, 1990.
Y Zhang and K.D. Wise, An Ultra-sensitive
capacitive pressure sensor with bossed
dielectric diaphragm, Digest Solid-State
Sensor and Actuator Workshop, Hilton
Head, pp. 205-208, June 1994.
S.T. Cho and K.D. Wise, Technical Digest,
Intl. Conf. On Solid State Sensors and
Actuators, pp. 400, June 1991.
Y. Zhang and K.D. Wise, A high-accuracy
multi-element silicon barometric pressure
sensor, Digest Intl. Conf. On Solid State
Sensors and Actuators, Stockholm, pp. 608611, June 1995.