DSpace at VNU: Electrothermal Microgripper With Large Jaw Displacement and Integrated Force Sensors

DSpace at VNU: Electrothermal Microgripper With Large Jaw Displacement and Integrated Force Sensors tài liệu, giáo án, b...

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

microgrip-per jaw displacement up to 32µm 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

sens-ing microgripper can potentially be used in automatic

manipula-tion systems in microassembly and microrobotics [2008-0064]

Index Terms—Electrothermal actuator, microgripper,

piezo-resistive sensor, polymeric actuator, sensing microgripper.

I INTRODUCTION

WHEN manipulating micro-objects, the dexterity,

accu-racy, 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

surgery, microrobotics, and microassembly

The manipulation of objects with traditional

micro-grippers without a built-in force sensor normally requires a

camera to obtain visual feedback This approach results in a

2-D image The depth perception of the contact between the

manipulating tool and the object being manipulated is lost,

making it difficult to identify the position of the tool [1]

Moreover, only displacements and not force can be detected

A microgripper with a built-in force sensor can address this

limitation and, thus, is suitable for holding objects firmly, while

avoiding any squeezing of delicate objects

Manuscript received March 12, 2008; revised July 22, 2008 Current version

published December 4, 2008 Subject Editor C.-J Kim.

T Chu Duc is with the Faculty of Electronics and Telecommunication,

College of Technology, Vietnam National University, Hanoi, Vietnam (e-mail:

trinhcd@vnu.edu.vn).

G.-K Lau is with the School of Mechanical and Aerospace

Engineer-ing, Nanyang Technological University, Singapore 639798 (e-mail: mgkLau@

ntu.edu.sg).

J F Creemer and P M Sarro are with the Electronic Components,

Tech-nology and Materials Laboratory, Delft Institute of Microsystems and

Nano-electronics, Delft University of Technology, 2628 CT Delft, The Netherlands

(e-mail: j.f.Creemer@tudelft.nl; p.m.Sarro@tudelft.nl).

Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2008.2007268

In recent years, several designs of microgrippers with force feedback have been demonstrated A force-sensing

microgrip-per for minimally invasive surgery application [2] employs

piezoelectric actuation with strain gauge sensors on the side wall of the structure It is capable of actuating at high fre-quency (hundreds of hertz) with very high driving voltage

In [3], a similar design device was presented It uses electro-magnetic actuation and piezoelectric force sensing It gener-ates large displacements at low voltage and a linear sensing output However, the main limitations of the aforementioned devices are the incompatibility with CMOS technology and rather large dimensions Electrothermal actuators with

built-in piezoresistive force sensors were presented built-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 100μm with a force

sensitivity of 4.41 kV/m and a corresponding 70-nN force-sensing 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 piezore-sistive force-sensing cantilever beams [8] The proposed sensing microgripper is capable of providing a large jaw displacement and output sensing voltage This device is ca-pable of monitoring the jaw displacement and resulting ap-plied force The device is made on silicon-on-insulator (SOI) wafers with a fabrication process compatible with CMOS technology

II DESIGN

In Fig 1, a schematic drawing of the sensing microgripper

is shown The structure is based on the combination of silicon-polymer electrothermal microactuators and piezoresistive lat-eral 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 micro-gripper arm [9]

1057-7157/$25.00 © 2008 IEEE

Fig 1 Schematic drawing of the sensing microgripper.

Fig 2 Front- and cross-side views of a sensing microgripper arm with

geometry symbols and parameters.

Fig 2 shows the front- and cross-side views of the sensing

microgripper design The geometrical parameters are given in

Table I

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

sil-icon 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 6μm wide,

75 μm long, and 30 μm thick The SU8 polymer layers are

3μm wide The length/width (Lcomb/HSU8) and height/width

(T/HSU8) ratios of the polymer layer are 25 and 10,

respec-tively (see Table I) These values, being greater or equal to

10, satisfy the prerequisite for the maximum constraint effect

TABLE I

G EOMETRY OF THE S ENSING M ICROGRIPPER

[10]–[12] When the heater is activated, the generated heat is efficiently transferred to the surrounding polymer through the deep silicon comb finger structure that has a large interface area with the polymer layer The polymer layers expand along the lateral direction causing a bending displacement of the actua-tor arm

As the polymer, we have selected SU8 2002 (Microchem Inc.) Its low viscosity (7.5 cSt) is specifically developed to produce thin layers (2–3 μm) [13] and is low enough for the

void-free filling of the 3-μm-wide trenches The main

prop-erties of the materials used are summarized in Table II This electrothermal microgripper can be actuated with a low driving voltage, power consumption, and operating temperature

B Thermomechanical Finite Element Modeling

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 “steady-state” 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 dissipa-tion through convecdissipa-tion and radiadissipa-tion 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]

TABLE II

P ROPERTIES OF S ILICON , A LUMINUM , AND SU8

Fig 3 Steady-state thermal profile on actuator and cantilever.

Fig 3 shows the simulated steady-state temperature profile

along the line through the middle point of all comb fingers

of the actuator and sensing cantilever when the microgripper

jaw displacement is 25μm at the applied voltage of 4.5 V The

maximum temperature change of 195◦C in the actuator occurs

approximately at 300μm from the anchor along its longitudinal

axis The temperature in the cantilever changes linearly from

ambient temperature at the anchor to 189 ◦C at its tip The

simulated temperature at the microgripper jaws is 190◦C

The average working temperature in the electrothermal

actu-ator is estimated from the aforementioned simulated

tempera-ture at the middle point of all comb fingers Fig 4 shows the

simulated microgripper jaw displacement versus the average

temperature change and also the maximum temperature change

The maximum displacement of the two microgripper jaws

djaws is 25μm at the average temperature change of 150 ◦C,

corresponding to a maximum temperature change of 195◦C

(see Fig 3) The displacement of the sensing cantileverdcan

is also simulated and shown in Fig 4 The maximum sensing

cantilever tip displacement is 9.3 μm when the microgripper

jaw displacement is 25μm (see Figs 1 and 4) The initial gap

between the two jaws of the microgripper is designed to be

40μm Therefore, this proposed sensing microgripper is

ex-pected to be capable of gripping micro-objects with a diameter

of 15–40μm.

The simulated static lateral stiffnessK l of the sensing

mi-crogripper arms is 1.8 kN/m This value is obtained using a

me-chanical 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 dis-placement 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 piezoresis-tors 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 resistorsR S1andR S2(see Fig 2) The

resis-tance change of the piezoresistors depends on the displacement

u of the tip of the cantilever beam and is given by [8]

ΔR

R = −π l K lcan

I l



L − L2s



(1)

where L is the length of the cantilever, L s is the length of the piezoresistors, z is the distance from the resistor to the

neutral plane of the cantilever, π l is the longitudinal piezore-sistive coefficient of the resistors (in this paper, we assume the values of room-temperature first-order piezoresistive coeffi-cients reported in [13]), and I l = (1/12)W3

canT is the lateral

momentum of inertia of the cantilever K lcan is the lateral

stiffness of the sensing cantilever given by [8], [27]

K lcan= ESiWcan3 T

whereESiis the Young’s modulus of the silicon crystal,Wcan

is the width of the cantilever, and T is the thickness of the

cantilever

The resistance change is estimated to be 12% when the tip of

the sensing cantilever is bent 9.3μm corresponding to a 25-μm

displacement of the microgripper jaws (see Fig 4)

The resistance of the piezoresistor also varies with the

tem-perature The length of the piezoresistors is 68μm (see Fig 2

and Table I) Considering the simulated temperature

distribu-tions 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 25μm The resistance change of the piezoresistor depends on

the temperature changeΔTres, and it is given by

ΔR T

where αSi= 1.3 × 10 −3 is the temperature coefficient of

re-sistance (TCR) of thep-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

R S1 andR S2 are R0+ ΔR T + ΔR and R0+ ΔR T − ΔR,

respectively, the output voltage of the Wheatstone bridge is

given by [8]

Vout= (2R 2VCCR0ΔR

0+ ΔR T)2− ΔR2  12ΔR

R0VCC (4) whereVCCis the bias voltage The output voltage is expected to

change by 1.7 mV when the displacement of the sensing

micro-gripper jaws is 1μm Combining (4) and the simulated lateral

stiffness K l, the sensitivity of this sensor is estimated to be

1.9 V/N For the large microgripper displacement of 25μm and

resulting average temperature change of 20◦C in the sensing

piezoresistors, the approximation of (4) is valid within 5.7%

Another second-order effect that should be considered is the

temperature sensitivity of the piezoresistive coefficient, which,

according to (1), directly influences ΔR Our piezoresistive

coefficient is dominated by the material coefficient π44 of

p-type silicon In the range of 25 ◦C–140◦C, it has a

tempera-ture coefficient of 300–500 ppm/◦C [28] For a temperature rise

of 20◦C on average, this yields a change in the output voltage

of 0.8% This is, in most cases, negligible

The thermal and1/f noises are two dominant noise sources

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]

V n= 2



4k B TresR(fmax− fmin) + αV B2

c i L s W s T s ln fmax

fmin

1/2

(5) where V B is the voltage across a resistor with a total number

of carriersN , α is a dimensionless parameter that is between

3.2 × 10 −6and5.7 × 10 −6 in single crystal silicon [30],c i is the charge carrier concentration,Tresis the temperature in the resistors, andL s,W s, andT sare the resistor length, width, and thickness, respectively (see Table I)

The minimum detectable displacement (MDD) and mini-mum 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/V n MDF= Fjaw

Vout/V n (6) where ujaw is the sensing microgripper jaw displacement and

Fjaw is the lateral force applied to the jaws of the sensing microgripper

III FABRICATION

The realized sensing microgripper is shown in Fig 5 The device is 490 μm long, 350 μm wide, 30 μm thick, and with

a 40-μm gap between the two jaws The piezoresistive

force-sensing cantilever is 390μm long and 10 μm wide with four

piezoresistors on the surface [see Fig 5(b)] Other parameters related to the geometry can be found in Table I The fabrication process (see Fig 6) is based on the Delft Institute of Microsys-tems and Nanoelectronics (DIMES) bipolar process [26], [31] and the silicon-polymer actuator process [7], [32]

SOI wafers with 527-μm-thick silicon (p-type, 100

orien-tation), 400-nm-thick silicon buried dioxide layer, 30-

μm-thick single-crystal silicon layer (p-type, 100 orientation),

and 1-μm-thick n-type epitaxial layer, with a resistivity of

0.5 Ω · cm, are used An additional 500-nm-thick p-type

epitaxial layer with a resistivity of 3.75 × 10 −2 Ω · cm is

grown to form the piezoresistors By using epitaxial growth,

a uniformly doped layer with an accurate thickness within 2%–3% of nominal value can be obtained, resulting in resistors

of well-defined sizes The piezoresistors are defined using reactive ion etching (RIE) of silicon as shown in Fig 6(b)

A 300-nm-thick low-pressure chemical-vapor-deposited sil-icon nitride layer is deposited as an electrical insulation layer

on the front side On the back side, it serves as the masking layer during etching in KOH solution Then, on the wafer front side, contact windows are opened, and a 600-nm-thick aluminum layer is deposited The piezoresistor connections and electrothermal heaters are defined by using RIE [see Fig 6(c)]

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

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)]

IV MEASUREMENTSETUPS

For the electrical characterization of the microgripper, dc voltages are applied by using an HP4155A semiconductor parameter analyzer (Agilent Technologies, Inc.) The voltage is ramped from 0 to 4.5 V The displacement is monitored by the charge-coupled device camera on the top of the probe station The static displacement of the microgripper at any actuating voltage is then obtained by enlarging the picture and comparing

it with the initial picture External mechanical vibrations cause

a blur on the static picture which determines the accuracy

of the measurement This inaccuracy is about ±1.5 μm At

the same time, a bias voltage VCC with an amplitude of

1 V is applied to the Wheatstone bridge The Wheatstone bridge output is also monitored by the semiconductor parameter analyzer

The thermal behavior of the microgripper is investigated

by using a Cascade probe station with a heated wafer chuck (Cascade Microtech, Inc.) The investigated temperature range

is from 20 ◦C to 200 ◦C (the highest temperature of this measurement system) with 10 ◦C stages and an accuracy of

±0.1 ◦C In order to get a stable temperature on the device, the

measurement is performed 5 min after the chuck temperature has reached the setting point to allow sufficient stabilization This externally supplied thermal energy causes expansion in the constrained polymer layer and the resulting actuation

A DSP lock-in amplifier SR850 (Stanford Research Systems, Inc.) is used to characterize the frequency behavior

of this sensing microgripper A sine signal with amplitude of

VPP= 4 V, offset of 2 V, and frequency in the range from 0.1

to 500 Hz is applied to the actuator The corresponding output signal of the Wheatstone bridge is recorded

Fig 7 Device operation: (a) Initial position of the sensing microgripper jaws,

(b) when 4.5 V is applied to both arms.

Fig 8 Simulated and measured sensing microgripper jaw displacement

ver-sus applied voltage The maximum measured displacement is 32µm at 4.5 V.

V MEASUREMENTRESULTS

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 40μm can be seen The

distance between the two jaws is close to 8μm 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

acti-vated 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

micro-object with a diameter from 8 to 40μm.

The power consumption is calculated by the applied voltage

and the corresponding current on the electrothermal

microactu-ators 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

displace-ment of the microgripper jaws

Fig 9 Sensing microgripper jaw displacement versus power consumption.

Fig 10 Sensing microgripper jaw displacement versus average working temperature.

The average increasing temperature in the electrothermal actuatorΔTavecan be estimated by monitoring the change of the resistance of the aluminum heater It is given by

ΔTave= Ract(ΔTave) − Ract(ΔT0)

Ract(T0)

1

whereαAlis the TCR of aluminum film (see Table II),Ract(T0)

is the resistance of the electrothermal actuator (205Ω at room temperature of −20 ◦C), andRact(ΔTres) is the resistance of the actuator when the average temperature on the actuator is changed byΔTavedegrees The maximum resistance change is 72% at the applied voltage of 4.5 V, resulting in a maximum average temperature change of 176 ◦C Fig 10 shows the jaw displacement versus the average working temperature The experimental values come within 7% of the simulated ones The results of the thermal characterization are also shown

in Fig 10 The values obtained with the external heat mode come within 7% and 5% of the electrical and simulated ones, respectively It indicates that the aluminum depositing process behaves as expected, and the average working temperature of

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 temperatureT gwhere 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] TheT g of SU8 is nearly the baking

temperature when it is below 220◦C for a baking time of 20 min

[17] However, theT gcan increase gradually up to the

steady-state 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

ranges can be distinguished The data points lay along straight

fitting lines, which intersect each other just above 120 ◦C

This is fairly close to the steady-state SU8 glass transition

temperature of 118◦C reported in [17] It indicates that the

proposed postbake process of this device is sufficient in this

context Furthermore, it explains the nonlinear characteristic of

the displacements due to the power consumption and also the

working temperature (see Figs 9 and 10)

B Force-Sensing Cantilever Beam Characteristics

Fig 11 shows the measured output signal of the Wheatstone

bridge versus the voltage applied on the electrothermal

mi-croactuator The zero-stress resistance value of the

piezoresis-tors at room temperature is 39 kΩ The bias voltage is 1 V dc

The maximum output voltage of the sensor bridge is 49 mV

when the voltage applied to the actuator is 4.5 V The relation

between the output voltage and the sensing microgripper jaw

displacement is also shown in Fig 11 The sensitivity of the

sensing microgripper derived from this curve is 1.5 kV/m

This curve is linear within 2% The experimental results come

within 10% of the calculated ones obtained from (4), indicating

that the epitaxial growth, etching process, and resistor contacts

behave as expected

Fig 11 also shows the output voltage of the piezoresistive force-sensing cantilever when the microgripper grips a 23-

μm-diameter object with the inset clamped object image The sens-ing 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 voltageV is then calculated as

FContact = K l ∗ (d  (V ) − d(V )) (8) where K l is the lateral stiffness of the sensing microgripper arm,d  (V ) is the displacement of microgripper jaws at applied

voltage V without the clamped object in between the two

jaws (dashed line in Fig 11), andd(V ) is the displacement of

microgripper jaws at applied voltageV with the clamped object

in between the two jaws (solid line in Fig 11)

Fig 12 shows the calculated contact force of this proposed 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 diam-eter 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 microassem-bly, 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 sens-ing 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 maximum displacement) and with a time period of 6 s/sweep for 24 h (14 400 cycles) The same reliability testing process is

TABLE III

P ERFORMANCE OF THE S ENSING M ICROGRIPPER

Fig 12 Contact force between microgripper jaws and the objects versus the

applied voltage.

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

Fig 14 Microgripper is bonded on the modified dual in-line package In this way, both mechanical manipulation and electrical connection of the sensing microgripper are possible.

modified socket chip (see Fig 14) Then, the chip is mounted on

anxyz micromanipulator that allows moving the microgripper

in three dimensions

As testing objects,∼30-μm-diameter glass balls placed on a

silicon wafer surface are used The glass balls are rearranged to form the letter “L” as shown in Fig 15 The microgripper tip is moved to approach the ball [see Fig 15(a)] The microgripper closes to grasp the object The chip is then moved to the target position using thexyz manipulator [see Fig 15(b) and (c)] The

microgripper finally opens to release the ball [see Fig 15(d)] When releasing the glass ball, we sometimes observe stiction between the microgripper jaw and the object However, we can get rid of this adhesion force by applying a small force between the glass ball and the silicon wafer surface before releasing the object

VI CONCLUSION

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 490μm long, 350 μm wide, and 30 μm thick A

microgripper jaw displacement up to 32μm at an applied

volt-age of 4.5 V is measured The microgripper can be used to grasp

an object with a diameter of 8–40μm The maximum average

Fig 15 Manipulating micro-glass balls to form letter L: (a) Initial position.

(b) Microgripper closes to grasp the ball (c) Microgripper is moved to the right

position (d) Microgripper opens to release the ball.

working temperature change is 176◦C at 4.5 V The output

volt-age of the piezoresistive sensing cantilever is up to 49 mV when

the jaw displacement is 32μm The force sensitivity is

mea-sured to be up to 1.7 nN/m, and the corresponding displacement

sensitivity is 1.5 kV/m The bandwidth frequency of this

pre-sented sensing microgripper is measured as 29 Hz The MDD

and MDF are estimated to be 1 nm and 770 nN, respectively

The fabrication process is based on conventional bulk

micro-machining and polymer filling, and it is CMOS compatible The

characteristics of this sensing microgripper will make the

ma-nipulation of small objects more efficient, more accurate, and

less tiring than with currently available grippers due to its large

jaw displacement and sensing sensitivity The presented

sens-ing microgripper could be used in automatic systems for

mi-croassembly and in microrobotics In addition, the microgripper

could be of use in living cell handling or in minimally invasive

surgery, provided the working temperature is lowered and the

electronics are properly isolated from the liquid environment

ACKNOWLEDGMENT

The authors would like to thank the DIMES-IC Processing

group for the technical support, P J F Swart of the Electronic

Components, Technology and Materials group for the help,

G de Graaff of the Electronics Instrumentation Laboratory for

the help with the electronic and mechanical measurements, and

J Wei, M Saadaoui, and H W van Zeijl of the Electronic

Com-ponents, Technology and Materials group and S L Paalvast and

W J Venstra of the Precision and Microsystems Engineering

Department for their suggestions and discussions

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Trinh Chu Duc received the B.S degree in

physics from Hanoi University of Science, Hanoi, Vietnam, in 1998, the M.Sc degree in electrical engineering from Vietnam National University, Hanoi, in 2002, and the Ph.D degree from Delft University of Technology, Delft, The Netherlands,

in 2007 His doctoral research concerned piezoresistive sensors, polymeric actuators, sensing microgrippers for microparticle handling, and microsystems technology.

He is currently an Assistant Professor with the Faculty of Electronics and Telecommunication, College of Technology,

Vietnam National University.

Gih-Keong Lau received the B.Eng (with first-class

honors) and M.Eng (by research) degrees in me-chanical engineering from Nanyang Technological University (NTU), Singapore, in 1998 and 2001, respectively, and the Ph.D degree from Delft Univer-sity of Technology, Delft, The Netherlands, in 2007, where his research topics were polymer microactua-tors and microfabrication.

From 2001 to 2003, he was a Research Associate with the Centre for Mechanics of Microsystems, NTU, where he worked on the topology optimization

of compliant mechanisms and piezoelectric actuators for hard disk drives and,

since 2008, has been an Assistant Professor with the School of Mechanical and

Aerospace Engineering His current research interests are electroactive polymer

actuators and their microfabrication.

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 elec-tronics 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 Sys-tematIC 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 mi-croelectromechanical 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 engineer-ing 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 Fel-low 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 December 2001, she became the A van Leeuwenhoek Professor, and, since

2004, has been the Head of the Electronic Components, Materials and Tech-nology Laboratory She has authored or coauthored more than 350 journal and conference papers.

Dr Sarro was the recipient of the EUROSENSORS Fellow Award in 2004 for her contribution to the field of sensor technology In April 2006, she became a member of the Dutch Royal Academy of Science, and in November

2006, she was elected an IEEE Fellow for her contributions to micromachined sensors, actuators, and microsystems She is a member of the technical program committees for several international conferences (IEEE MEMS, IEEE Sensors, EUROSENSORS, and Transducers), the Technical Program Cochair for the First IEEE Sensors 2002 Conference, and the Technical Program Chair for the Second and Third IEEE Sensors Conference (2003 and 2004) She is the General Cochair of IEEE MEMS 2009 She is also a member of the AdCom of the IEEE Sensors Council.