A Review on Evolution, Current Trends and Future Scope of MEMS Piezoresistive Pressure Sensor

Abstract

Piezoresistive pressure sensors are the first MEMS devices to be commercialized. They work on the principle of change in resistivity of materials due to applied pressure. Literature reports lot of work and development in this area of sensing mechanisms. This paper provides a review on evolution of these sensors right from thin metal film based technology to the semiconductor technology. The paper presents the current trends in the design and use of piezoresistive pressure sensor. The paper also presents the future scope form these sensors which, will be around the extensive use of materials like SOI, SiC, DLC, CNT and Silicon Nanowires.

Full text

A Review on Evolution, Current Trends and
Future Scope of MEMS Piezoresistive Pressure
Sensor
Kirankumar B. Balavalad,
Dept. of Electronics & Communication Engieenring,
Basaveshwar Engineering College, Bagalkot,
karnataka, India.
(Affiliated to VTU, Belagavi)
kiranb4004@gmail.com
B. G. Sheeparmatti,
Dept. of Electronics & Communication Engieenring,
Basaveshwar Engineering College, Bagalkot,
karnataka, India.
(Affiliated to VTU, Belagavi)
sheepar@yahoo.com
Abstract— Piezoresistive pressure sensors are the first MEMS
devices to be commercialized. They work on the principle of change
in resistivity of materials due to applied pressure. Literature reports
lot of work and development in this area of sensing mechanisms.
This paper provides a review on evolution of these sensors right
from thin metal film based technology to the semiconductor
technology. The paper presents the current trends in the design and
use of piezoresistive pressure sensor. The paper also presents the
future scope form these sensors which, will be around the extensive
use of materials like SOI, SiC, DLC, CNT and Silicon Nanowires.
Keywords –MEMS, Piezoresistance, Pressure sensor, SOI, SiC,
DLC, CNT.
I. INTRODUCTION
Pressure sensors are used for control and monitoring in
thousands of everyday applications. Various types of pressure
sensors do exist, which can be classified based on the sensing
mechanism as, Force collector type of pressure sensors.
Capacitive type uses a diaphragm and pressure cavity to create
a variable capacitor to detect strain due to applied pressure,
capacitance increases as pressure deforms the diaphragm.
Common technologies use metal, ceramic, and silicon
diaphragms. Resonant type uses the changes in resonant
frequency in a sensing mechanism to measure stress, caused
by applied pressure. Thermal type uses the changes in thermal
conductivity of a gas due to density changes to measure
pressure. Ionization type measures the flow of charged gas
particles which varies due to density changes to measure
pressure. Common examples are the Hot and Cold Cathode
gauges. Piezoresistive strain gauge type uses
the piezoresistive effect of bonded or formed strain gauges to
detect strain due to applied pressure, resistance changes as
pressure deforms the material. Common technology types are
Silicon (single crystalline), Polysilicon, thin film, and bonded
metal foil. Piezoresistive pressure sensors are one of the very-
first products of MEMS technology. These sensors are widely
used in biomedical applications, automotive industry and
household appliances.
The piezoresistive pressure sensor have mainly been
studied and commercialized because of high yield and wide
dynamic range. In Piezoresistive pressure sensor,
the piezoresistive effect is a change in the electrical
resistivity of a semiconductor or metal when mechanical
strain is applied. Piezoresistive pressure sensors provide high
sensitivity enabling linear operation over wide range of
pressure, suitable for high pressure applications. Piezoresistive
pressure sensor technology is simple with highly standardized
fabrication process, whereas capacitive and other techniques
still face lack of process maturity in fabrication and nonlinear
performance. The silicon based pressure sensor is one of the
major applications of the piezoresistive sensor. The sensing
material in a piezoresistive pressure sensor is formed on a
silicon substrate, which bends with the applied pressure. A
deformation occurs in diaphragm, leading to a change in the
resistivity of the material. This change can be an increase or a
decrease according to the orientation of the resistors. This
paper aims at reviewing the evolution of piezoresistive
pressure sensors for sensing applications starting from thin
metal film to the complicated use of semiconducting materials
like Silicon, Pollysilicon, SOI, SIC etc. The paper provides the
current trends towards the design and development of
piezoresistive pressure sensors taking current applications into
consideration. Finally the paper tries to explore the future
direction of piezoresistive pressure sensors.
II. EVOLUTION OF PIEZORESISTIVE PRESSURE
SENSOR
The fundamental concept of piezoresistive effect is the change
in receptivity of a material resulting from an applied stress.
This effect in silicon material was first discovered by Smith in
1950’s [1]. Smith proposed the change in conductivity under
stress in bulk n-type material and designed an experiment to
measure the longitudinal as well as transverse piezoresistance
coefficients. In 1961 W.G. Pfann presented the shear
piezoresistive effect, designed several types of semiconductor
stress gauges to measure the longitudinal, transverse, shear
stress and torque, and employed a Wheatstone bridge type
gauge in mechanical signal measurement [2]. Lund and T.
Finstad [3] in 1999 studied the temperature dependence of
piezoresistance coefficient by four points bending experiment.
In 1989 P.J. French and A.G.R. Evans [4] presented the
piezoresistive effect in polysilicon and its applications to
strain gauges. Piezoresistive pressure sensor design is widely
studied at 1990's in MEMS and electronic packaging field by
Jaeger et al [5, 6]. They employed piezoresistive sensor made
on silicon chip to measure the stresses within electronic
packaging devices.
This trend continued into the 1970s, when
microsensor design began to move toward higher-volume,

lower cost applications, specifically the automotive industry.
Into the 1980s and the present, biomedical and automotive
applications are some of the most widely reported in the
literature. After 1980’s the sensor need and requirements
changed and there was a large demand for sensors which can
perform better in harsh environments (mechanically and
chemically aggressive). Soon after the sensors were begin to
expose to harsh environments amongst which high
temperature is one important parameter. In 1997 Y. Kanda
applied MEMS process to fabricate piezoresistive pressure
sensors on wafer for optimum design considerations. Recently
finite element method (FEM) is widely adopted for stress
prediction, thermal effect reduction, packaging design and
reliability enhancement of piezoresistive sensor [7].
Temperature has its effect on the resistivity of piezoresisitors
used in sensors. The performance of piezoresistive pressure
sensors started deteriorating with increasing levels of
temperature. Piezoresistive pressure sensor based on highly
doped silicon and porous silicon was proposed by T. toriyama,
Tanimoto, S.Sugiyama [8]. In 2008 the Shuwen Guo, Harald
et all, proposed that the maximum operating temperature of a
piezoresistive sensors can be raised when the silicon film is
sufficiently thin. The minority-carrier exclusion effect in ultra
thin film Smart-cut SOI enables resistance values to increase
monotonically with temperature up to 600oC [9].
J. H. Kim, et all, proposed the use of the silicon
nanowire in 2009, enabling the fabricated pressure sensor to
have the enhanced sensitivity and the reduced sensor size [10].
In 2012 Haisheng San, et all, reveled that the performance of
silicon piezoresistive pressure sensor suffers when they are
operated in extremely harsh environment, such as vibration,
shock and environment conditions with humidity, alkalescence
or acidity, electrostatic particles and so on, its requirement in
terms of reliability and stability is more rigorous than that of
many advanced applications [11] Hong Zhang, et all, in
2014 designed and fabricated a Si-Glass based MEMS
piezoresistive pressure sensor for harsh environment
applications. The sensor chips were fabricated using SOI
wafer-glass anodic bonding technology, which enables a
single boron-implemented piezoresistor to be on lower surface
of silicon diaphragm and be vacuum-sealed in glass cavity
[12]. Yangxi Zhang, at all, in 2014 reports a monolithic
integration multifunctional MEMS sensor for acceleration and
pressure measurement based on cavity SOI wafer. In 2015
G.D. Liu1, et all, proposed a SOI high temperature pressure
sensor using a thermostable electrode of TiSi2/Ti/TiN/Pt/Au.
Meanwhile piezoresistive ressure sensors using SIC (Silicon
carbide) were designed and fabricated and literature shows
few morks on SIC based MEMS pressure sensors. But the
development of SIC films for sensors development suffers a
lot with little process maturity. But efforts are being put by
researchers towards development of SIC based piezoresistive
pressure sensors.
III MEMS PIEZORESISTIVE PRESSURE SENSORS
Piezoresistive pressure sensor design consists of
piezoresistive element resting on top/bottom of diaphragm. A
contact is established with the electrodes, thus measuring the
resistance of the electrode. The application of pressure on the
sensor causes a deflection of the membrane and this causes a
change in resistance of the electrode. The change in resistivity
(R) can be measured as a change in voltage (V) or current (I)
as given by Ohm’s law: R = V/I [15, 16]. The best location to
place the piezoresistors would be the region of maximum
strain on the diaphragm. Piezoresistive sensors rely on the
piezoresistive effect which occurs when the electrical
resistance of a material changes in response to applied
mechanical strain. In metals, this effect is realized when the
change in geometry with applied mechanical strain results in a
small increase or decrease in the resistance of the metal. The
piezoresistive effect in semiconductors is primarily due to
changes at the atomic level and is better than in metals. As
stress is applied, the average effective mass of the carriers in
the semiconductors for example, silicon either increases or
decreases (depending on the direction of the stress, the
crystallographic orientation, and the direction of current flow).
This change alters the carrier mobility and hence its resistivity.
When piezoresistors are placed in a Wheatstone bridge
configuration and attached to a pressure-sensitive diaphragm,
a change in resistance is converted to a voltage output which is
proportional to the applied pressure.
Fig.1Conventional pressure sensor
Fig. 1 illustrates a conventional piezoresistive pressure sensor
model. Piezoresistors (orange colored blocks) are placed on
top of the diaphragm (light blue colored) supported by
substrate (green colored). the simulation set up of the
piezoresistive pressure sensor with square diaphragm. The
equations for maximum deflection and stress in a square
diaphragm are given by [15] equation 1.
3
2
12(1 )
Eh
Dv
=−
The change of electric resistance in silicon piezoresistance
gauge can thus be expressed as [16]
1R
GR
ε
Δ
⎞
⎛
=⎜⎟
⎝
⎠
Piezoresistors are connected in Wheatrstone’s bridge
format to reduce temperature effects a bit. In a typical silicon
pressure sensor made on (001) silicon wafer, membrane will
be formed by anisotropic etching in KOH an etch stops on
(111) planes. This etch creates the sides of the membrane
oriented along <110> directions. The diaphragm is sealed
from back side using anodic bonding in vacuum in order to
measure the absolute value of pressure. Then stress/pressure
applied can be measured by placing piezoresistors, often

connected in a Wheatstone bridge, on the membrane. And the
actual value of the pressure can be calculated from the output
voltage of the bridge. Figure 2. Shows a simple diagram of
four Piezoresistors connected in the form of Wheatstone’s
bridge.
Fig.2 Piezoresistors connected in the form of Wheatstone’s bridge
The output of a Wheatstone bridge can be given as [17]
The materials used for diaphragm in order to provide
mechanical support to sensor are typically silicon, aluminum,
PMMA, PDMS, Polycrystalline silicon dioxide (Sio2),
Parylene C etc. The insulator is placed between diaphragm
and piezoresistive elements, which acts as a substance in non-
conductive state that reduce heat transfer. The different types
of wafers used in design are silicon, SOI (silicon on insulator),
Double SOI and SIC (Silicon Carbide). And different types
interconnects used are metals such as Platinum, gold, copper).
Both bulk and surface micromachining are employed for the
fabrication of piezoresistive pressure sensor.
A. Performance Parameters
The sensitivity of a pressure sensor is defined as the relative
change in the output voltage per unit applied pressure [18]
11
OUT
in
V
R
SPV RR
ΔΔ
==
Δ
The deflection 0
win the linear region of operation can be
expressed as follows for a square diaphragm
4
041
w
P
a
hEhg
=
The maximum stress max
σ
, which occurs in the middle at the
edge of the square diaphragm, can be expressed by the
analytical expression for a square diaphragm as,
2
max a
ph
σ
⎛⎞
=⎜⎟
⎝⎠
B. Simulation tools
Recently FEM/FEA is widely used for pre-analysis of the
sensor for various parameters. The use of CAD tools can
improve work efficiency, lower cost, shorten development
cycle, and make the design more economic and products more
competitive. Before actual fabrication it is very important to
analyse the sensor for its performance. Using FEA/FEM tools
thorough optimization will be done. Later after repeated
analysis of the model it can be taken up for actual fabrication.
The stress distribution of pressure sensors is simulated using
tools like ANSYS, COVENTORWARE, COMSOL, MEMS+,
SUGAR and MATLAB software’s with the finite element
method (FEM). These software’s are very suitable for
simulation and optimization of piezoresistive pressure sensors.
Thickness and length-width ratio of the diaphragm and the
arrangement of the piezoresistors on its surface are determined
according to the simulation results.
C. Applications of piezoresistive pressure sensors
Piezoresistive pressure sensors are used in many applications
such as household appliances, automotive application like Oil
level, gas level, air pressure detection, barometric applications
(weather forecasting) and Medical field (Non-invasive and
invasive blood pressure monitors, Fetal heart rate monitors
Inhalers and ventilators, Wound management, Patient
monitoring systems, Spiro meter and respiratory therapy
devices, Dialysis systems, Drug delivery systems.). The
advantages of piezoresistive pressure are: Low-cost
fabrication opportunity, mature processing technology,
different pressure levels can be achieved according to the
application, various sensitivities can be obtained easily and
read-out circuitry can be either on-chip or discrete.
IV. CURRENT TRENDS IN MEMS PIEZORESISTIVE
PRESSURE SENSORS
Due to advent of technology and requirements, many
measurement and control applications require microsensors to
be used in the harsh environments. Generally these
environments include locations of high temperatures, intense
vibrations, erosive flows or corrosive media. Application
fields characterized by harsh environments include, for
example, aerospace, micropropulsion, automotive,
turbomachinery, oil well equipment, industrial process control,
nuclear power, and communication. Now sensors are being
exposed to such harsh environments amongst which high
temperature is one important parameter. Materials like SiC,
SOI and DLC are being extensively used for the design of
sensors capable of achieving high temperature and pressure
measurements. The current researchers are targeting high
temperature ranges >8000C. People are looking at materials
which can operate fine at elevated temperatures with basic
principles intact. Piezoresistive pressure sensors are now being
developed using materials which have better physical,
mechanical, chemical & electrical properties at high
temperatures. This has evolved the development of sensors
using materials like SiC, SOI & diamond, due to their
excellent properties at high temperatures. Custom fabrication
& packaging technologies are now being employed in the
development of these sensors.
out
RR
RR
V
⎜⎜⎝
+−+=4
3
21

Temperature compensation along with the sensing
element is being employed to help sensors to operate at high
temperatures. One such attempt is the use of two concentric
Wheatstone’s bridge on the diaphragm [19]. Paper presents the
design of silicon based temperature compensation were, eight
piezoresistors are designed on the polycrystalline silicon
membrane and constructed by two concentric Wheatstone-
bridge circuits to form two sets of sensors. The sensor in the
central circuit measures the pressure and temperature, while
the outer one measures only the deflection caused by the
working temperature. The other method employs the use of
SiC (Silicon Carbide). SiC has high melting point of 2730°C
and excellent thermal stability due to which SiC has become
good material for many MEMS high-temperature pressure
sensors. Silicon carbide also has many additional properties,
such as low density, high strength, low thermal expansion,
high thermal conductivity, high hardness, high elastic
modulus, excellent thermal shock resistance, superior
chemical inertness. Development of SiC is very difficult and
fabrication is not yet matured [20]. In recent years, great
progress in the growth of SiC bulk. Currently 6H-SiC, 4H-SiC
and 3C-SiC wafers are commercially available. Paper [21],
demonstrates 4H-SiC piezoresistive pressure sensor operating
at 8000C. SiC films are reported almost 15 times costlier than
the conventional silicon. As an alternate, in recent years SOI
has been extensively used in the fabrication of piezoresistive
pressure sensors. SOI has been reported to operate normally
till the maximum temperatures of 3000C. Paper [22],
demonstrates the novel high temperature pressure sensor,
which is based on the SOI construction, replacing traditional
pn junction insulation with SiO2 insulation. Even researchers
have focused their research towards the use of materials like
DLC (Diamond Like Carbon) to be used in design of
piezoresistive pressure sensor.
V. FUTURE OF MEMS PIEZORESISTIVE
PRESSUR SENSORS
The future of MEMS piezoresistive pressure sensors will be
focused around the extensive use of materials like SOI, SiC,
DLC and Carbon Nanotubes. People are targeting the use of
sensors in high temperatures hence researchers are trying new
methods and designs using SOI and SiC which are found good
for high temperature applications. The use of SOI and SiC has
been briefly explained in the section above. Diamond is a
superhard, wide bandgap, semiconductor material of high
mechanical strength and thermal stability and therefore an
ideal candidate for pressure sensors operating at elevated
temperatures. Using these properties in a diamond-on-Si
technology, a number of sensors and actuators have been
attempted. However, their implementation lags behind that of
silicon sensor technologies. Literature reports CVD diamond
films deposition on Si-substrates, micromachined into
structured membranes. New applications also need sensors to
be a small as possible. Sensors now are being scalled down to
nanoscale. In this regards people have demonstrated the use of
carbon nanotube based piezoresistive pressure sensors, where
the carbon nanotube is taken as piezoreisitor. The design and
performance of a piezoresistive surface micromachined
circular diaphragm based pressure sensor utilizing Single
walled Carbon nanotubes (SWNT) has been demonstrated in
[23]. A piezoresistive nano structure is placed on the
diaphragm. When the diaphragm deflects it results in change
in the resistivity of the nano structure according to applied
pressure. The paper shows, employing a nano structure results
in increased sensitivity of the sensor.
Figure 3. CNT based Piezoresistive Pressure Sensor model
(reproduced from [23])
In the above figure, pressure sensor design consists of
piezoresistive CNT element placed on top of Silicon/Si3N4
diaphragm. A contact is established with the SWNT utilizing
platinum electrodes to measure resistance of the nanostructure.
The application of pressure from back side of diaphragm
causes deflection of diaphragm and this leads to change in
resistance of the Carbon nanotube.
VI. CONCLUSION
The paper tries to present evolution of the piezoresistive
pressure sensors. It all started from thin metal films and now
onto semiconductors. The paper also presents briefly the
design and mechanism of piezoresistive pressure sensor.
Currently the research is focused onto designing sensors for
harsh environments involving high temperature and high
pressure. People are targeting the use of sensors at temperature
more than 8000C. And the future of MEMS piezoresistive
pressure sensors rely around the use of materials like SOI,
SiC, DLC, CNT and Silicon Nanowires.
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