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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 17, NO. 6, DECEMBER 2008

Electrothermal Microgripper With Large Jaw
Displacement and Integrated Force Sensors
Trinh Chu Duc, Gih-Keong Lau, J. Fredrik Creemer, Member, IEEE, and Pasqualina M. Sarro, Fellow, IEEE

Abstract—The novel design of a sensing microgripper based
on silicon-polymer electrothermal actuators and piezoresistive
force-sensing cantilever beams is presented. The actuator consists
of a silicon comb structure with an aluminum heater on top and
filled polymer in between the comb fingers. The sensor consists of a
silicon cantilever with sensing piezoresistors on top. A microgripper jaw displacement up to 32 µm at a 4.5-V applied voltage is
measured. The maximum average temperature change is 176 ◦ C.
The output voltage of the piezoresistive sensing cantilever is up to
49 mV at the maximum jaw displacement. The measured force
sensitivity is up to 1.7 V/N with a corresponding displacement
sensitivity of 1.5 kV/m. Minimum detectable displacement of 1 nm
and minimum detectable force of 770 nN are estimated. This sensing microgripper can potentially be used in automatic manipulation systems in microassembly and microrobotics.
[2008-0064]
Index Terms—Electrothermal actuator, microgripper, piezoresistive sensor, polymeric actuator, sensing microgripper.

I. I NTRODUCTION

W

HEN manipulating micro-objects, the dexterity, accuracy, and speed are considerably improved when the
force on the objects can be sensed and controlled in real time
[1]. The development of such miniaturized manipulators is of
great interest for operating on living cells, minimally invasive

devices are the incompatibility with CMOS technology and
rather large dimensions. Electrothermal actuators with builtin piezoresistive force sensors were presented in [4] and [5].
The jaw displacements and output sensing voltages are rather
small, limiting their application. An electrostatic microgripper
with an integrated capacitive force sensor is presented in [6].
This device is capable of motion up to 100 μm with a force
sensitivity of 4.41 kV/m and a corresponding 70-nN forcesensing resolution. However, the limitations of this device are
its large size and complicated electronic circuit required by the
electrostatic method used.
This paper presents a novel sensing microgripper based
on silicon-polymer electrothermal actuators [7] and piezoresistive force-sensing cantilever beams [8]. The proposed
sensing microgripper is capable of providing a large jaw
displacement and output sensing voltage. This device is capable of monitoring the jaw displacement and resulting applied force. The device is made on silicon-on-insulator (SOI)
wafers with a fabrication process compatible with CMOS
technology.
II. D ESIGN
In Fig. 1, a schematic drawing of the sensing microgripper
is shown. The structure is based on the combination of siliconpolymer electrothermal microactuators and piezoresistive lateral force-sensing cantilever beams. When the electrothermal
actuator is activated, the microgripper’s arm and also the
sensing cantilever are bent. This causes a difference in the
longitudinal stress on the opposite sides of the cantilever. This
changes the resistance values of the sensing piezoresistors on
the cantilever. The displacement of the microgripper jaws can
be monitored by the output voltage of the Wheatstone bridge of
the piezoresistive sensing cantilever beam. The contact force
between the microgripper jaws and clamped object is then
determined from the displacement and stiffness of the microgripper arm [9].

1057-7157/$25.00 © 2008 IEEE


A. Silicon-Polymer Electrothermal Microactuator
The microgripper is designed in normally open operating
mode. Each actuator has a silicon comb finger structure with
the aluminum metal heater on top. A thin layer of silicon nitride
is employed as the electrical isolation between the aluminum
structure and the silicon substrate. The gaps between the silicon comb fingers are filled with SU8 polymer (see Fig. 1).
Each actuator consists of 41 silicon comb fingers with SU8
polymer layers in between. The silicon fingers are 6 μm wide,
75 μm long, and 30 μm thick. The SU8 polymer layers are
3 μm wide. The length/width (Lcomb /HSU8 ) and height/width
(T /HSU8 ) ratios of the polymer layer are 25 and 10, respectively (see Table I). These values, being greater or equal to
10, satisfy the prerequisite for the maximum constraint effect

To simulate the performance of the proposed sensing
microgripper, a finite element modeling software COMSOL
(Comsol Inc.) is used. The related material properties (see
Table II) are assumed to be temperature independent. The 3-D
thermomechanical model is used to determine the “steadystate” temperature distribution within the actuator and sensing
cantilever structures. The thermal expansion and resulting
actuator displacement are computed based on the temperature
results [12].
The actuator is assumed to be immersed in air. The silicon
comb structure acts as heat source and the rest of the gripper
arm as a heat sink. The substrate is assumed to be thermally
grounded, and therefore, the temperature of the device anchors
is fixed and equal to the ambient temperature. The heat dissipation through convection and radiation into the atmosphere can
be ignored in comparison to the heat loss due to conduction in
the actuator anchors when the working temperature is below
500 K [23]–[25]. More details on the simulations can be found
in [7] and [12].

between the two jaws of the microgripper is designed to be
40 μm. Therefore, this proposed sensing microgripper is expected to be capable of gripping micro-objects with a diameter
of 15–40 μm.
The simulated static lateral stiffness Kl of the sensing microgripper arms is 1.8 kN/m. This value is obtained using a mechanical model with an external lateral load at the microgripper
jaws. The maximum output force of this microgripper is calcu-

Fig. 4. Simulated microgripper jaw displacement and the cantilever tip displacement versus the average working temperature change and maximum
temperature change.

lated through the maximum displacement of the microgripper
arm and its stiffness of 22.5 mN.
C. Piezoresistive Force-Sensing Cantilever Beam
The force sensor design is based on the lateral force-sensing
piezoresistive cantilever beam [8], [26]. The four piezoresistors
are located on the cantilever beam structure and connected to
create a Wheatstone bridge (see Figs. 1 and 2). The piezoresistors are aligned along the [110] direction in the (001) crystal
plane of the silicon wafer. The resistor pair located on the
cantilever are stress-sensing resistors. When the electrothermal
actuator is activated, the cantilever beam is bent parallel to the
wafer surface. Therefore, the differential change of resistance
occurs on the two resistors RS1 and RS2 (see Fig. 2). The resistance change of the piezoresistors depends on the displacement
u of the tip of the cantilever beam and is given by [8]
−πl Klcan
ΔR
=
R
Il

L−


The resistance change is estimated to be 12% when the tip of
the sensing cantilever is bent 9.3 μm corresponding to a 25-μm
displacement of the microgripper jaws (see Fig. 4).
The resistance of the piezoresistor also varies with the temperature. The length of the piezoresistors is 68 μm (see Fig. 2
and Table I). Considering the simulated temperature distributions in the sensing cantilever (see Fig. 3), the temperature in
the sensing piezoresistors is changed from ambient temperature
at the anchor to 60 ◦ C at the tip of the resistors. Therefore, the
temperature is, on average, changed by 20 ◦ C over the entire
sensing piezoresistors when the microgripper jaw displacement
is 25 μm. The resistance change of the piezoresistor depends on
the temperature change ΔTres , and it is given by
ΔRT
= αSi ΔTres
R0

(3)

where αSi = 1.3 × 10−3 is the temperature coefficient of resistance (TCR) of the p-type silicon [14]. The resistance will
change by 2.6% when the average temperature change in the
sensing piezoresistors is 20 ◦ C (see Fig. 3).
The Wheatstone bridge reduces the temperature influence on
the output voltage from a first- to second-order effect, because
both sensing resistors on a beam undergo the same temperature
shift. The two additional resistors outside the sensing cantilever
are not subjected to stress. They form a matched reference pair
that makes the sensor signal more insensitive to common-mode
external error sources, such as variations of the environmental
temperature (see Fig. 2). Assuming that, when the actuator
is activated, the resistance values of the sensing resistors
RS1 and RS2 are R0 + ΔRT + ΔR and R0 + ΔRT − ΔR,

of the piezoresistive cantilever [8], [29], [30]. The noise voltage
of the Wheatstone bridge over the bandwidth of interest (fmin ,
fmax ) is given by [8]
Vn = 2 4kB Tres R(fmax − fmin ) +

αVB2
fmax
ln
ci Ls Ws Ts fmin

1/2

(5)

where VB is the voltage across a resistor with a total number
of carriers N , α is a dimensionless parameter that is between
3.2 × 10−6 and 5.7 × 10−6 in single crystal silicon [30], ci is
the charge carrier concentration, Tres is the temperature in the
resistors, and Ls , Ws , and Ts are the resistor length, width, and
thickness, respectively (see Table I).
The minimum detectable displacement (MDD) and minimum detectable force (MDF) of the force sensor depend on the
minimum detectable signal which is determined by the noise
of the cantilever. The MDD and MDF corresponding to the
calculated noise of the piezoresistors can be estimated by
MDD =

ujaw
Vout /Vn

MDF =

aluminum layer is deposited. The piezoresistor connections and
electrothermal heaters are defined by using RIE [see Fig. 6(c)].


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Fig. 5. SEM pictures of (a) sensing microgripper and close-ups of (b) piezoresistors, (c) jaws, and (d) section of the thermal actuator.

IV. M EASUREMENT S ETUPS

Fig. 6. Schematic view of the sensing microgripper fabrication process.

The top silicon layer is subsequently etched by deep RIE
to define the silicon frame until the buried oxide layer is
reached [Fig. 6(d)]. Negative photosensitive SU8 2002 polymer
is applied and patterned [see Fig. 6(e)]. A special prebake and
postbake procedure is followed to ensure the void-free filling
of the high aspect ratio structures. More details can be found in
[7]. Finally, the bulk silicon is etched from the back side in a
33-wt% KOH solution at 85 ◦ C until the buried silicon dioxide
layer is reached. The front side of the wafer is protected during
the etching in KOH by a vacuum holder. The last step is the
release of the structure by dry etching the buried silicon dioxide
layer from the back side [see Fig. 6(f)].

For the electrical characterization of the microgripper, dc
voltages are applied by using an HP4155A semiconductor
parameter analyzer (Agilent Technologies, Inc.). The voltage is


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Fig. 7. Device operation: (a) Initial position of the sensing microgripper jaws,
(b) when 4.5 V is applied to both arms.
Fig. 9. Sensing microgripper jaw displacement versus power consumption.

Fig. 8. Simulated and measured sensing microgripper jaw displacement versus applied voltage. The maximum measured displacement is 32 µm at 4.5 V.

V. M EASUREMENT R ESULTS
A. Electrothermal Actuator Characteristics
Fig. 7 shows the images of several typical positions of
the microgripper jaws. In Fig. 7(a), the initial position is where
the gap between the two jaws which is 40 μm can be seen. The
distance between the two jaws is close to 8 μm when applying
a voltage of 4.5 V to both arms [see Fig. 7(b)].
Fig. 8 shows the displacement response of the microgripper
jaws in air when a dc voltage is applied to the electrothermal
actuator. This measured movement is the total change between
the two microgripper jaw positions when both arms are activated. The measured results are within 7.5% of the simulated
value for all data points. A maximum movement of 32 μm
is measured at an applied voltage of 4.5 V. Therefore, this
presented microgripper is capable of manipulating a microobject with a diameter from 8 to 40 μm.
The power consumption is calculated by the applied voltage
and the corresponding current on the electrothermal microactuators. Fig. 9 shows the measured with linear fitted and simulated
values of the jaw displacement versus power consumption. On
average, the device needs around 5 mW for a 1-μm displacement of the microgripper jaws.

Fig. 10. Sensing microgripper jaw displacement versus average working
temperature.

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 17, NO. 6, DECEMBER 2008

Fig. 11 also shows the output voltage of the piezoresistive
force-sensing cantilever when the microgripper grips a 23-μmdiameter object with the inset clamped object image. The sensing microgripper jaws close gradually until it grips the object.
The contact force between the microgripper jaws and the
clamped object can be estimated by the jaw displacement in
Fig. 11, considering the simulated gripper arm stiffness of
1.8 kN/m (see Table III). The contact force between gripper
jaws and object at the applied voltage V is then calculated as
FContact = Kl ∗ (d (V ) − d(V ))

Fig. 11. Output voltage of the force-sensing cantilever versus the applied
voltage on the electrothermal microactuator. The inset shows the microgripper
jaws with the clamped object.

the actuator can be well estimated from the resistance change
of the aluminum heater.
However, the physical properties of a polymer material such
as the volume coefficient of expansion, Young’s modulus, and
so on are greatly changed in pseudosecond order at the glass
transition temperature Tg where the material properties change
from the glassy region to the rubbery plateau region [33]. The
glass transition temperature of a polymer varies widely with
parameters such as the fabrication process and the microscopic
structure [17], [33], [34]. The Tg of SU8 is nearly the baking
temperature when it is below 220 ◦ C for a baking time of 20 min
[17]. However, the Tg can increase gradually up to the steadystate temperature of 118 ◦ C when the material is baked for a
longer time (60 min) at a constant temperature of 95 ◦ C.
The effect of the glass transition temperature is apparent
in the measurements of Fig. 10, where two different working

microgripper. The contact force is zero until the two gripper
jaws reach the object at an applied voltage of about 3.75 V. The
contact force then increases up to 135 mN at the applied voltage
of 4.5 V. Combining the measured results in Fig. 11 and the
calculated force, the sensitivity of this built-in force sensing is
estimated to be 1.7 V/N.
This sensing microgripper is capable of detecting the diameter of the clamped object and also the contact force between
the microgripper jaws and the object. This function is highly
desirable for the closed-loop system needed in microassembly, microrobotics, minimally invasive surgery, and living cell
surgery.
C. Response Frequency of the Sensing Microgripper
Fig. 13 shows the measured voltage gain and phase shift as
a function of frequency of this sensing microgripper using the
lock-in amplifier. The large-signal cutoff frequency of this sensing microgripper is measured as 29 Hz. The transient response
of the full range displacement of this sensing microgripper is
also characterized. The rise and settling times are measured to
be 13 and 18 ms, respectively.
Combining (6) and the output signal from Fig. 11 with the
frequency bandwidth of the range 0.1–29 Hz, the MDD and
the corresponding MDF of the sensing cantilever beam can be
estimated to be about 1 nm and 770 nN, respectively.
D. Reliability
The main failure mechanism observed during the test of
the microgripper is the appearance of cracks in the aluminum
heater and the silicon comb structure when the applied voltage
is increased to about 5 V and the working temperature of
the actuator is too high. There is no indication of the loss of
adhesion between the SU8 and the silicon plates even at these
temperatures. To investigate the lifetime of the microgripper, it
is repeatedly actuated in air with a 4-V amplitude (90% of its

get rid of this adhesion force by applying a small force between
the glass ball and the silicon wafer surface before releasing
the object.
Fig. 13. Bode diagram of the sensing microgripper. The sweep input voltage
is applied to electrothermal actuator, and the output of the piezoresistive
Wheatstone bridge is monitored. The cutoff frequency is 29 Hz.

repeated after one week and then one month. No degradation in
performance is noticed.
E. Object Manipulation
The microparticle manipulating ability of this microgripper
developed is investigated. The microgripper is bonded on the

VI. C ONCLUSION
A novel design of a sensing microgripper based on
silicon-polymer electrothermal actuators and piezoresistive
force-sensing cantilever beams is presented. The sensing
microgripper is 490 μm long, 350 μm wide, and 30 μm thick. A
microgripper jaw displacement up to 32 μm at an applied voltage of 4.5 V is measured. The microgripper can be used to grasp
an object with a diameter of 8–40 μm. The maximum average


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Fig. 15. Manipulating micro-glass balls to form letter L: (a) Initial position.
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J. Fredrik Creemer (S’97–A’01–M’03) received
the M.Sc. degree in electrical engineering from Delft
University of Technology, Delft, The Netherlands,
in 1995, the Diplôme d’Études Approfondis in electronics from the Université Paris-Sud, Orsay, France,
in 1996, and the Ph.D. degree (cum laude) from
Delft University of Technology, in 2002. His doctoral
research explored the effect of mechanical stress on
bipolar transistor characteristics.
He was an Analog Chip Designer, with SystematIC Design from 2002 to 2003. In 2003, he
was with the Kavli Institute of Nanoscience, as a Postdoctoral Researcher.
In 2006, he was an Assistant Professor with the Laboratory for Electronic
Components, Technology and Materials, Delft Institute of Microsystems and
Nanoelectronics, Delft University of Technology. His research interests are microelectromechanical system microreactors, transmission electron microscopy,
and microsystems technology.
Dr. Creemer was the recipient of the Else Kooi Award 2002 for the research
described in his dissertation and, in 2006, a Veni Grant.

Pasqualina M. Sarro (M’84–SM’7–F’07) received
the Laurea degree (cum laude) in solid-states physics
from the University of Naples, Naples, Italy, in
1980, and the Ph.D. degree in electrical engineering from Delft University of Technology, Delft,
The Netherlands, in 1987, where her thesis dealt
with infrared sensors based on integrated silicon
thermopiles.
From 1981 to 1983, she was a Postdoctoral Fellow with the Photovoltaic Research Group, Division
of Engineering, Brown University, Providence, RI.
She then joined the Delft Institute of Microsystems and Nanoelectronics, Delft
University of Technology, where she is responsible for research on integrated
silicon sensors and microelectromechanical systems (MEMS) technology. In