Acta Mech Sin (2009) 25:1–12
DOI 10.1007/s10409-008-0222-6
REVIEW PAPER
Mechanical and electronic approaches to improve
the sensitivity of microcantilever sensors
Madhu Santosh Ku Mutyala ·
Deepika Bandhanadham · Liu Pan ·
Vijaya Rohini Pendyala · Hai-Feng Ji
Received: 27 September 2008 / Accepted: 8 October 2008 / Published online: 10 January 2009
© The Chinese Society of Theoretical and Applied Mechanics and Springer-Verlag GmbH 2009
Abstract Advances in the field of micro electro mechanical
systems and their uses now offer unique opportunities in
the design of ultrasensitive analytical tools. The analytical
community continues to search for cost-effective, reliable,
and even portable analytical techniques that can give reliable and fast response results for a variety of chemicals and
biomolecules. Microcantilevers (MCLs) have emerged as a
unique platform for label-free chem-sensor or bioassay. Several electronic designs, including piezoresistive, piezoelectric, and capacitive approaches, have been applied to measure
the bending or frequency change of the MCLs upon exposure
to chemicals. This review summarizes mechanical, fabrication, and electronics approaches to increase the sensitivity of
MCL sensors.
Keywords
Microcantilever · Sensor · MEMS · Sensitivity
1 Introduction
Micro electro mechanical systems (MEMS) have shown tremendous growth recently. MEMS devices now offer unique
opportunities in the design of ultrasensitive analytical methods. In 1994, it was realized that microcantilevers (MCLs)
can be made extremely sensitive to chemical and physical
changes [1,2]. To date, physical sensing has been demonM. S. K. Mutyala · D. Bandhanadham · L. Pan ·
V. R. Pendyala · H.-F. Ji
Institute for Micromanufacturing,
Louisiana Tech University,
Ruston, LA 71272, USA
H.-F. Ji (B)
Department of Chemistry, Drexel University,
Philadelphia, PA 19104, USA
e-mail: hj56@drexel.cedu
strated by detecting thermal energy, strain, magnetic field,
electric charge, viscosity, density, and infrared radiation.
Extremely sensitive chemical vapor sensors based on MCLs
have been demonstrated using selective coatings on the
MCLs. MCLs are a sort of transducers which convert physical quantity like force, strain, stress, etc. into measurable
quantity such as voltage or current. MCLs are typically made
in the form of rectangular or V shaped planks. An Electron
micrograph of a microcantilever (MCL) array is shown in
Fig. 1. MCLs act as a sensing platform when they are coated
with a molecular recognition thin film. Absorption or adsorption of corresponding chemicals in or on the coated film
results in the frequency change or bending of the MCL.
Resonant frequency The resonant frequency, f , of an oscillating MCL can be expressed as
K
1
,
(1)
f =
2π m ∗
where K is the spring constant of the lever and m ∗ is the
effective mass of the MCL. The effective mass can be related
to the mass of the beam, m b , through the relation: m ∗ = nm b ,
where n is a geometric parameter. It is clear that the resonant
frequency can change due to changes in mass and spring
constant. Resonance frequency of an MCL (dynamic mode)
can be used to detect chemical species in air. However, the
oscillation of the MCL is significantly dampened in an aqueous solution, which reduces the quality factor and subsequent
detection sensitivity of MCLs in aqueous solutions. It should
be noted that recent results showed that the second resonance
frequency of MCLs remains sharp in aqueous solutions [3]
and can be used for detecting biomolecules.
MCL bending One of the unique characteristics of MCLs
is that the device can be made to undergo bending due to
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2
Fig. 1 Electron micrographs of an MCL array
molecular adsorption by confining the adsorption to one side
of the MCL. This bending is due to the adsorption-induced
differential stress or film volume change on the MCL (static
mode).
For the surface stress induced MCL bending, from molecular point of view, the binding results in electrostatic
repulsion or attraction, intermolecular interactions, or a combination of these that alter the surface stresses on the MCL.
Using Stoney’s formula [4], the bending deflection of the
MCL due to adsorption can be written as:
3(1 − ν)L 2
δs,
(2)
Z =
ET 2
where Z is the observed deflection at the end of the MCL,
ν and E are Poisson’s ratio (0.2152) and Young’s modulus
(70 GPa for SiO2 , 156 GPa for silicon, 300 GPa for Si3 N4 )
for the substrate, respectively, T is the MCL thickness, L is
the length of the MCL, and δs is the differential stress on the
MCL.
Both frequency and bending approaches have been demonstrated to detect chemicals with sensitivity as high as partsper-trillion, and each approach has its own advantages and
disadvantages which can be used for specific applications.
MCL sensors hold a position as a cost-effective, sensitive
sensor platform for environmental, biomedical, homeland
security, and industrial applications. Compared to existing
technologies for chemical and biomolecular detection in general, the MCL sensor technology has three key advantages:
Low cost—Electronics for operation and control are relatively simple and inexpensive.
Low-power consumption—For static mode, since the MCL’s
bending signal is driven by molecular recognition, the only
power needed is for detection and display, allowing use of
light-weight battery power or photovoltaic cells.
Small size—The entire sensor could fit in an area with its
sides less than a few millimeters. The device can be readily
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M. S. K. Mutyala et al.
integrated into a robot system without much increase of
weight in order to protect the operator and to guarantee uniform detection strategies.
Other than those, MCL arrays could be used to simultaneously detect and identify various chemicals on a single chip; the analysis time is relatively shorter because the
assay does not require extensive sample preparation or signal
amplification; it could be used for in-situ detection.
By monitoring changes in bending response or frequency
change of an MCL, mass change and surface stress changes
induced by adsorption or molecular recognition can be
recorded precisely and accurately. Optical, piezoresistive,
piezoelectric, and capacitive methods have been used for the
detection of MCL’s deflection (Fig. 2).
Each sensing mechanism has its own advantages and drawbacks. Optical method is sensitive and can measure deflection
as small as 1 nm. However, the optical system is complicated
and not user friendly compared to the other methods. The
sensing systems based on piezoresistive, piezoelectric, and
capacitive methods are much simpler and hold the potential for implantable biosensing applications. These methods
do not require complex adjustment and alignment systems
as needed in optical readout scheme, and they could readily
be integrated with electrical circuit. However, the sensitivity
of these methods is generally lower than that of the optical method. Research is underway to either improve these
approaches or develop new approaches for MCL based
sensing. This review summarizes recent developments in
improving the sensitivity of MCL sensors based on the
piezoresistive, piezoelectric, and capacitive approaches.
2 Piezoresistive readout scheme
The piezoresistive phenomenon is the change in resistivity of
a material when subjected to stress. The piezoresistivity of a
material depends on its type; the change in its geometry upon
application of stress over it. Piezoresistive approach is one
of the most widely applied techniques for measuring stress
applied on an MCL. The advantage of this technique is that
the electrical output can be readily measured and the MCL
can be integrated on integrated circuit (IC) [5].
In a piezoresistive MCL the relationship between the stress
and the change in resistance is shown as the following equation:
R
= πl σl + πt σt ,
(3)
R
where R is initial resistance, R is change in resistance;
πl , πt are longitudinal and transverse piezoresistive coefficients, respectively; σl , σt are lateral and transverse stress,
respectively.
The change in the resistivity is typically measured by using
a Wheatstone bridge as shown in Fig. 3. A Wheatstone bridge
Approaches to improve the sensitivity of microcantilever sensors
3
Fig. 2 Illustration of four
methods for detecting MCL
motion with sub-nanometer
precision—(left) Laser-beam
deflection method; (middle)
Piezoresistive and piezoelectric
method; and (right) Capacitive
method
displacement and the R/R change increase with the
decrease in the thickness of piezoresistors; the highest sensitivity can be obtained when the piezoresistor’s length is
approximately 2/5 of the SiO2 MCL’s length (Fig. 4); an
increase of both Si and the leg width result in a decrease
in MCL’s deflection and its sensitivity; the sensitivity of the
MCLs with lower doping concentrations is more significant
than those with higher doping concentrations. Temperature
control is critical for thin piezoresistor in lowering the S/N
ratio and increasing the sensitivity.
Fig. 3 A Wheatstone bridge scheme[5]
2.2 Material selection
is made up of four piezoresistors: R1 , R2 , R3 , and R4 . One
of these resistors is replaced by an MCL. The differential
voltage (Vg ) from the Wheatstone bridge is proportional to
the change in the resistivity of the MCL [5].
The output voltage, V0 , from the Wheatstone bridge is
given as in Eq. (4).
Vin
R
,
(4)
V0 =
R
4
Most of the MCLs are made of silicon or silicon nitride
(Si3 N4 ) materials. Li et al. showed that piezoresistive MCLs
made of silicon dioxide were more sensitive than the siliconbased MCLs due to lower Young’s modulus of SiO2
(57–70 GPa) when compared to that of Si (170 GPa) and
Si3 N4 (290–380 GPa). The embedded piezoresistor is made
of single crystal silicon and is fully insulated by SiO2 (Fig. 5).
This reduces the electronic noise of the MCL since it is isolated from the surrounding environment [8].
where Vin is applied input voltage. The output voltage is
directly related to the change in the resistance R with
respect to the initial resistance R.
The first piezoresistive MCL showed the resistivity change
(R/R) of 1.2×10−7 when subjected to 1Å deflection [6].
It should be noted that this MCL was not originally designed
for sensors, but for atomic force microscopy. Since then, a
variety of strategies have been applied to increase the sensitivity.
2.1 Geometry optimization
The dimensions of the MCL and the piezoresistor play a critical role in the sensitivity of MCL sensors. By using Finite
Element Method (FEM) analysis, Chivukula et al. have
shown that optimizing the device dimensions is useful to
a greater extent in increasing the sensitivity of the device [7].
FEM analysis was done to investigate the optimized device
dimensions by varying the MCL and piezoresistor geometries
and the doping concentration of the piezoresistive device.
The simulation results showed that both MCL deflection
2.3 Stress concentration method
Concentrating the stress to a designated area is another
approach to enhance the sensitivity. Yu et al. reported a
method to concentrate the stress by introducing holes in the
MCLs. These holes caused the stress to be concentrated in
its vicinity. The size of the holes is 10µm × 5µm [5]. The
results showed the MCLs with holes were 1.3 times more
sensitive than those without holes in response to the same
MCL deflection (Figs. 6 and 7). This increase in the surface
stress is attributed to the discontinuity of the MCL structure
due to the holes.
Decreasing the thickness of the MCLs is a common strategy to increase the sensitivity. However, thin MCLs are more
fragile and harder to handle than the thicker ones. Naeli et al.
[9] reported with another stress-concentrating design that
made it possible to achieve both high sensitivity and toughness. In this design, they separated the stress in the piezoresistors from the stress on the beam structure. In fabrication,
the piezoresistive strings were laid on the wafer surface with
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M. S. K. Mutyala et al.
Fig. 4 Change is the resistance of the piezoresistor vs. the thickness and length of the piezoresistor when a 2 N/m surface stress is applied on the
surface of a SiO2 MCL [7]
Fig. 5 Schematic of a piezoresistive SiO2 MCL [8]
Fig. 7 Stress distribution vs. vertical displacements of MCLs with and
without holes [5]
2.4 Integration with piezoactuators
Fig. 6 The layout of a Wheatstone bridge design of piezoresistive
MCLs with stress concentrating holes [5]
a cavity underneath. Two thin piezoresistive clamped silicon
beams were released on the MCL surface. The new piezoresistive design showed an increase in the relative resistance
change by a factor of 5.2, as shown in Fig. 8, compared to an
MCL with the same thickness and conventional piezoresistor
design.
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Piezoresistive MCL can also detect mass change by integration with piezoactuators. Sone et al. [10] introduced an MCL
device that consists of a piezoresistive design, a Wheatstone
bridge circuit, a positive feedback controller, an exciting
piezoactuator, and a phase-locked loop (PLL) demodulator
(Fig. 9). In the design, R1 represents the piezoresistive MCLs
in the Wheatstone bridge. The oscillator includes a positive
feedback system, a preamplifier, and a piezoactuator driver.
The phase locked loop circuit is used for demodulating the
frequency signals from the circuit. Results showed that a
mass sensitivity of 190 fg/Hz and a mass resolution of about
500 fg were obtained in water. The mass sensitivity is 100
times greater than that of a quartz crystal oscillation system.
2.5 Minimizing the thermal effect
One concern in the field of piezoresistive MCL sensors is that
the sensor performance is readily influenced by the thermal
Approaches to improve the sensitivity of microcantilever sensors
5
Fig. 8 Left Comparison between conventional and stress-concentrating MCLs. Right SEM images of stress-concentrating beams isolated from
MCL base [9]
higher than that of the mechanical bandwidth of the MCL
is applied. The output current measured with respect to the
applied voltage gives the capacitance value which is a measure of the displacement of the MCL.
The capacitive current, IC (t), at the output [12] due to the
MCL displacement is given as:
∂ VAC
∂c(t)
∂
(C · V ) ∼
+ VDC
,
(5)
= Cp
∂t
∂t
∂t
where, C is the capacitance between the MCL beam and the
driver.
This capacitance consists of static component, Cp , and the
capacitance because of the MCL displacement, c(t). The first
term in the above equation, Cp ∂ V∂tAC , indicates the parasitic
current, and the second term, VDC ∂c(t)
∂t , indicates the current
due to the MCL oscillation.
The main advantages of capacitive microsensors are
low-power, simple structures, low temperature sensitivity and
low cost integration of the read out to the portable systems.
It outdraws peizoresistive systems because of its low power
consumption. The drawback is that the bending amplitude
is limited by the space between the MCL and the bottom
electrode.
IC (t) =
Fig. 9 A harmonic vibration mass biosensor system based on MCLs
[10]
stress during operation. Yang et al. developed a double-MCL
design to overcome the thermal stress effect which improves
the sensitivity as shown in Fig. 10 [11]. The double MCL is
composed of a top immobilized MCL and a bottom sensing
MCL. These two MCLs are connected in a way that the biaxial surface stress in the former can be converted to uniaxial
strain in the latter. The sensitivity of the double MCL could
be increased by increasing the length ratio or by decreasing
the thickness ratio of the two MCLs (Fig. 11). More than two
orders of sensitivity enhancement have been achieved and
the induced thermal effects are minimized.
3 Capacitive readout scheme
A capacitive readout scheme consists of an MCL over an
electrode which is ground biased as shown in Fig. 12. In
a capacitive sensing scheme, an AC voltage of frequency
3.1 Size, shape, and materials of the MCL
Li et al. [13] developed an MCL that is actuated to resonance
by in-plane electrodes as shown in Fig. 13. The resonant frequency is detected by the capacitive approach with on-chip
circuitry. For resonance frequency measurement, the sensitivity improvement can be realized by reducing the MCL
dimensions in order to increase stiffness and subsequent resonant frequency.
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6
M. S. K. Mutyala et al.
Fig. 10 a A conventional MCL
system with only one MCL;
b Double MCL design with one
MCL on the top and another at
the bottom [11]
Fig. 13 Circuit diagram of an electrostatically driven MCL to the
resonant frequency shift [13]
Fig. 11 Surface stress sensitivity of double and single MCL systems
with respect to the length ratio [11]
results suggest that the micro-bridge type of humidity sensor
provides a lower sensitivity, it is a more robust device than
the single beam MCL type.
3.2 Elimination of the parasitic effect
It is shown from Eq. 6 that the sensitivity of the capacitive MCL sensors improves as the parasitic capacitance is
decreased [12].
Cp
Cp
Vdc
Ic ∼
Vac + 0.39
A
=
ωCpa
Cp + Cpa
Cp + Cpa δ0
= Vp + Vd ,
Vg =
Fig. 12 Basic schematic of a capacitive MCL [12]
However, for bending mode, higher sensitivity will be
achieved with lower stiffness MCL and larger electrode area
[14]. In this work, Lee et al. compared the sensitivity of three
MCL shapes: single beam, double beam, and microbridge as
shown in Fig. 14.
The three sensors were tested in a closed chamber with
humidity level ranging from 45% Relative humidity (RH)
to 95% RH. Figure 15 shows the single beam MCL sensor
provides a high degree of humidity sensitivity. Although the
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(6)
where, Vp is the parasitic voltage, which is due to the applied
AC voltage, Vd is corresponding to the MCL displacement
and is proportional to both the DC voltage and the oscillation
amplitude, Vg is the resulting voltage.
Furthermore, reducing the dimensions typically results in
an increase of the mass responsibility and subsequent sensitivity. However, reducing the dimensions will also raise problems due to low-level current and the parasitic capacitance.
Electrostatic transduction in the nanometer-size regime
requires the minimization of the parasitic capacitance to
increase the sensitivity. Several CMOS circuitry [12,15,16]
were designed and placed very close to the transducer structure for such purpose to increase the sensitivity.
Approaches to improve the sensitivity of microcantilever sensors
7
Fig. 14 SEM images of the fabricated: a Single beam MCL; b Double beam MCL; c Microbridge [14]
4 Piezoelectric readout technique
Fig. 15 Temperature effect on sensitivity of the three sensors [14]
One CMOS circuitry to eliminate the parasitic capacitance
induced by the external bonding pads and wires [12] is shown
in Fig. 16. This CMOS circuitry is integrated with an MCL
by using a monolithic technology that combines a standards
CMOS processes and a nanofabrication method. The integrated system demonstrated a high sensitivity (10−18 g) and
a high spatial resolution (300 nm) and also had less noise
compared to resistive and transimpedance methods.
Ghatnekar-Nilson et al. [16] used electron beam lithography to define the resonator system comprising of an MCL, an
actuator, and a capacitive readout circuit on a CMOS circuit
(Fig. 17). This has improved the MCL sensitivity by reducing the effects caused by parasitic capacitance. An MCL of
420 nm in width, 20µm in length, and 600 nm thickness is
fabricated on the pre-processed CMOS chip. The mass sensitivity is 17 ag per Hz.
Davis et al. [17] shows, by reducing the size of MCL
to nanometer scale, the Q-factor has dramatically increased
from 50 to 100 in air and from 20 to 30,000 in vacuum.
This system has shown a reduction in parasitic capacitance
to approximately 3fF, which is sufficiently low for the capacitive readout system to function.
Another recently booming technique to measure the MCL
deflection or its resonant frequency change is the piezoelectric design. Piezoelectricity is the property of a material in
which a potential is developed across a material when it is
subjected to strain [18]. A piezoelectric MCL can be used
for actuation as well as sensing.
The piezoelectric materials, such as ZnO, are usually
deposited at the fixed ends of an MCL as shown in Fig. 18.
The deposition of the piezoelectric films onto the MCL is
carried out by several methods which include metal organic
chemical vapor deposition (MOCVD) [19], hydrothermal
method [20], sputtering [21], pulsed laser deposition [22],
and Sol-gel method [23]. Among these methods, sol-gel
deposition method is considered to be most cost-effective
and convenient. The formation of a highly homogenous piezoelectric layer and its composition can be readily controlled.
The factors that determine the sensitivity of the piezoelectric MCLs are discussed below.
4.1 Piezoelectric materials
Materials with higher piezoelectric constant normally result
in higher sensitivity. A wide variety of piezoelectric materials can be chosen for fabricating piezoelectric MCLs. The
most commonly used materials are ZnO and Lead Zirconium Titanate (LZT, also known as PZT) [18–24]. PZT has
a higher piezoelectric constant than ZnO, thus the sensitivity
of the MCL is improved when PZT is used as the piezoelectric material for the force sensor, instead of ZnO [25]. It is
also believed that this PZT sensor is easy to use in severe
environments such as low temperature and ultrahigh vacuum. The orientation of these layers also plays a pivotal role
in determining its sensitivity [26]. A PZT piezoelectric layer
with an orientation of <100> is used because its piezoelectric constant is 1.7 times of that of <111> orientation. The
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8
Fig. 16 Left Schematic drawing of the NEMS system based on a laterally vibrating MCL (s direction) electrostatically excited and with
capacitive readout. The structural layer for the MCL is one of the polysilicon layers used in CMOS technology. Right Optical images of the
nanomechanical resonator integrated monolithically with the CMOS
M. S. K. Mutyala et al.
voltage amplifier. The large image shows the CMOS circuit and the
area where the nanomechanical device is fabricated. The inset ia a zoom
image of it, where it can be better appreciated the MCL, the driver and
the integrated capacitor for proper circuit polarization [12]
Fig. 17 Optical images of
combined EBL and DWL [16]
thickness of the film and roughness of the surface also plays
a role in determining the piezoelectric constant [27] and the
subsequent sensitivity.
4.2 MCL geometry
Piezoelectric MCL dimensions play a role in determining
the piezoelectric constant and resonance frequency which in
turn determines the sensitivity of the MCL. Yi et al. investigated the effect of piezoelectric MCL’s length, width and
resonance mode on its sensitivity by determining the resonant frequency shift per mass change ( f /m) [18]. The
tip of the piezoelectric unimorph MCL (made up of PZT
and stainless steel layer) was loaded with point mass. The
resulting resonance frequency change was determined and
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compared experimentally and theoretically. Figure 19 shows
the f /m is larger when the MCL dimensions are smaller.
The influence of the MCL’s dimensions and the material’s
property on the resonance frequency shift, f , with respect
to the loaded mass, m is given by Eq. 7:
f n /m = (vn2 /4π )(1/L 3 w)(1/0.236 12ρ) E/ρ, (7)
where, the f /m is directly proportional to eigen value
υn2 and inversely proportional to length, L, and width, w;
E is the effective Young’s modulus and ρ is the effective
density of the unimorph MCL. By keeping the MCL thickness, Young’s modulus and density constant and decreasing
the MCL length and width by a factor of α, f /m can be
increased by a factor of α 4 .
Approaches to improve the sensitivity of microcantilever sensors
Fig. 18 Schematic of a typical piezoelectric MCL [18]
9
Fig. 21 Experimental and theoretical f with respect to m of the
MCL with 1.03 cm in length, 0.4 cm in width at mode 1 (squares) and
mode 2 (circles) [18]
the MCL is narrower. The influence of the resonance mode of
the MCL on its sensitivity (Fig. 21) shows that the frequency
shift is larger for the mode 2 operating MCL than the mode
1 with respect to the same mass loading.
4.3 Substrate effect on sensitivity
Fig. 19 f /m versus length for all the modes and widths [18]
Figure 20 shows that for the same MCL width the resonance frequency increases as the MCL length increases.
This was proved both theoretically (solid lines) and experimentally (symbols). The effect of the width on the frequency
shift (Fig. 20 right) shows the frequency shift is larger when
The electrical properties of one piezoelectric material depend
mainly on the crystallographic orientation of the material.
One factor affecting the sensitivity of the piezoelectric MCLs
is the substrate on which the piezoelectric layer is deposited.
Wang et al. [24] investigated the sensitivity of PZT thin films
on Pt/Ti and Ir/IrO2 substrates and the X-ray diffractometer
(XRD) results showed that the PZT film fabricated on Pt/Ti
substrates has a highly preferred orientation in the direction
of <100> and no preferred orientation on the Ir/IrO2 substrate. This is because Ir undergoes oxidation during annealing and the PZT deposition on the randomly oriented IrO2
does not produce the <100> orientation. The PZT film was
crystallized with columnar grains on the Pt/Ti substrate and
without columnar grains on Ir/IrO2 and the PZT layer on the
Pt/Ti surface was smoother and more uniform than that on the
Ir/IrO2 (Fig. 22). The PZT deposited on the Pt/Ti substrate
Fig. 20 Left Experimental and
theoretical f with respect to
m of the MCL with, Left
varying lengths at a fixed 0.4 cm
width, Right length 1.05 cm,
width 0.4 cm (squares) and
length 0.99 cm, width 0.2 cm
(circles) [18]
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M. S. K. Mutyala et al.
Fig. 22 SEM images of PZT
films deposited on (left) Pt/Ti
and (right) Ir/IrO2 [24]
Fig. 24 Schematic of an MCL sensor unit with two PZT layers [20]
Fig. 23 Charge change versus displacement before and after poling
[18]
has a higher piezoelectric constant than that on the Ir/IrO2
substrate.
Further studies showed that Pb diffused through the Pt/Ti
layer. On the other hand IrO2 has a very good barrier effect.
So, Pt/IrO2 is a suitable substrate to achieve higher piezoelectric constant and less defective microstructures.
4.4 Improving sensitivity by poling
Dong et al. developed [28] a new fabrication technique
wherein the PZT layer is subjected to poling. The PZT layer
is polarized at 100◦ C with an electric field of 125 kV/cm for
one hour. The MCL sensibility was 1.61 pC/mm before poling
and rose to 3.25pC/mm after poling (Fig. 23). This sensitivity increase was attributed to the increase in the piezoelectric
constant of the PZT layer upon poling.
4.5 Optimized positioning of the piezoelectric layer
Du et al. [20] fabricated a piezoelectric MCL sensor that
contains two PZT layers (Fig. 24). The PZT layers were synthesized by hydrothermal method and placed on a titanium
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Fig. 25 Optimized positioning of the PZT layer for maximum sensitivity. L is the MCL length, L p is the length of the sensing and actuation
layer, and a is the length between the fixed end of the MCL and the left
edge of the actuation/sensing layer [20]
substrate. The top layer acted as an actuation layer and the
bottom one acted as a sensing layer.
This work showed that the location of the actuation and
sensing layer played a role in improving the MCL sensitivity.
The MCL is most sensitive when the two ends of the actuation and sensing layers were placed at the points where the
strain is zero (Fig. 25).
4.6 Anti-cracking
The piezoelectric constant of the PZT film increases with
the thickness of the PZT layer but may result in cracks after
reaching certain limit due to the residual stress developed.
The cracks will result in lower sensitivity. In a conventional sol-gel process, the PZT films are patterned after the
Approaches to improve the sensitivity of microcantilever sensors
11
Fig. 26 Piezoelectric MCL
fabricated with (left)
conventional and (right)
modified method [29]
Fig. 27 SEM image of the
surface undergoing (left) three
annealing cycles; (right) four
annealing cycles [23]
completion of the annealing process. In a modified process
developed by Xie et al. [29], patterning is done before each
post-annealing process to remove unwanted PZT film.
Figure 26 compared the PZT surfaces fabricated by the
conventional method and the modified process.
Liu et al. also developed [23] a similar method to avoid
cracks of the PZT layer by modifying the layer coating process (Fig. 27).
5 Conclusion
MCLs provide an ideal platform for chemical and biomolecular detection. We can foresee the upcoming of more MCL
sensors. However, there is still a long way to go to convert the proof-of-concept results to robust, commerciallyavailable products. This review focuses on enhancing the
sensitivity of MCL sensors operating based on piezoresistive, capacitive, and piezoelectric principles by optimizing its
dimensions, choosing proper material, and adopting novel
fabrication techniques. It is seen that these factors play a
significant role in the MCL sensitivity.
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