Micri Greper

8/14/2019 Micri Greper

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Review by J. Cecil

Paper Title:

Design of a Micro-Gripper and anUltrasonic Manipulator for Handling

Micron Sized Objects

Authors: F . Beyeler , D. J. Bel l, B. J.Nelson, Y. Sun, A. Neild

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Focus

Discusses a MEMS (Microelectromechanical system)

gripper and an Ultrasonic manipulator.

The gripper forces were measured using an integrated

force sensor.The fabrication process was discussed.

Glass, polymer, cancer cells were manipulated.

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Introduction

The manipulation of the micro-scale objects are important applications in thebiomedical and biological research.

Similar importance for micro level applications are found in the areas like

MEMS (Microelectromechanical systems) and microelectronic devices.

The use and the end effects of the miniature grippers become vital for these

applications.

MEMS technology allows us to fabricate such devices.

Several gripper designs have been published in the recent years

But most have them give no force feed back

Some hybrid design were also developed using the theories of piezoelectricityand magnetism .

The above designs proved to be complicated and expensive.

This paper presents a novel design with electrostatic actuation and capacitive

force sensing.3


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Figure showing the solid model of the micro gripper with dimensions.

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Functioning of the gripper

The left arm of the gripper ispushed to the right side bythe actuator (see fig) untilthe gripper is closed

This is how the gripper picksup an object.

This generates a grippingforce that deflects theright arm

This deflection ismeasured by the combdrive for force sensing onthe right side (see fig)

Deflection isproportional to thegripping force

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Lateral comb drives are used to actuate thegripper

The comb finger electrodes are considered as

parallel platesThe driving force f efor a single finger pair is

given by

f e= ( t V2/d) Where is permittivity of air (8.85 x 10-12C2/(Nm2)

V is driving voltage, d is dist bet plates

t is thickness given by device layer of wafer6


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See fig

The right arm transmits the gripping force to

the movable capacitor plates of the

transverse comb drive

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The restoring force in the sensor (right arm)

is created by 4 flexures and

is given by

Where x is the deflection,

k is the spring constant,

E is the Youngs modulus,

t is the wafer thickness,

w is the width of the flexure,

la is the length of the flexures and lsis the length of the flexures of the

sensor.9


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Figure showing the Capacitive sensor with readout circuitry A balanced pair configuration of comb drive plates is used (see

above fig)

1 and 3 are stationary

2 are movable plates

A signal (Vout) is generated by the capacitive readout chip

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For gap distance d1


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The sensitivity, resolution and the range of the sensingsystem can be varied by changing the flexure length.

One of the important feature is that the force sensor can

also be used to measure the adhesive or the van der Waalforces.

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Step A - The process uses an insulated silicon layer with a device layerof 50 m, a handle layer of 400 m and a buried SiO2 layer of 2 m.

Step BUsing reactive ion etching (RIE) a 1.5 m layer of SiO2is

deposited on the wafer backside and then patterned.

Step CA 10 m thick layer of photo resist is patterned on thebackside. Using DRIE (deep reactive ion etching) the backside silicon is

etched. The 200 m, SiO2patterned in the step B is removed and the

remaining part is etched.

Step D250 nm of Aluminium is evaporated and patterned to create

pads for wire bonding by etching the metal.

Step EHeat conductive paste is used to glue the SOI wafer to a silicon

support. The rest of the parts of the device are also etched using DRIE

dry etching and the oxide plasma etch is used to remove the remaining

photo resist on top of the device.

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Figure showingmicroscope image of the

micro gripper fabricated

with the described SOI

process. The minimum

feature size of the

structures on device layer

is 5m, whichcorresponds to an aspect

ratio of 1: 10.

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Characterization

The gripper is wire bonded and glued as shown in thefigure below.

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The voltage ranges from 0 to 150 V for actuation.

This creates a deflection of 25 m and is multiplied by a

factor 4 to get the resultant deflection.

ACCULAB VI1mg was used to calibrate the forcesensor. The analog output was converted into digital

signal.

The performance of the sensor is summarized in table-1

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Experiment 1

Pick and Placemanipulations in air

using Glass spheres.

Gripping forces of 380N had been measured

at a driving force of

140 V.

Figure shows the pickingand releasing gripping

force profiles for a

35 m glass sphere. 19


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Experiment 2

Ultrasonic device was used to preposition copolymerspheres with a diameter of 74 m.

These particles were suspended in Ionized water.

Through the glass layer of the ultrasonic device thespheres were viewed from the top

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a) spheres are suspended in water inside the channel.b) spheres are aligned using the ultra-sonic force field.

After aligning the spheres the field is switched off.

c) the micro-gripper moves inside the channel filled

with water. A single sphere is gripped.

d) the gripper moves back and the sphere is brought

outside the channel.

e) the sphere is released on a glass plate next to the

ultrasonic device.

f) the next sphere is gripped.

g) the sphere is released on a glass plate.

h) process is repeated.

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Experiment 3

Same concept of experiment 2 was applied to biologicalcells.

HeLa cancer cells of approximate diameter 20 m were

used. Single cells have been successfully manipulated inside a

fluid medium.

Ultrasonic field was used to release the cells which

adhere to the gripper arms.

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Figure showing thesequence followed

during the experiment.

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a) HeLa cells are suspended in liquid.

b) The cells are aligned in three lines inside the channel.

The middle line can be used for picking up the cells

with the gripper. c) A smaller field of view is used at the position at the

end of the line highlighted by the white rectangle in

b). The gripper is inserted into the fluid channel. d) A single cell is picked up.

e) The cell is released at a different position using the

ultrasonic force field.

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Gripper in Use

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Conclusion

A system manipulator of micron sized particles of aMEMS gripper and a MEMS ultrasonic manipulator is

designed.

The gripper opens to a width of 100 m

Real time force feed back is provided involving high

sensitivity

Glass and polymer spheres have been manipulated

successfully.

The concept was also successfully demonstrated with

HeLa cells.27

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