lecture 9 biomedical microtechnology and nanotechnology · the plates of a micro sized capacitor...

Lectu

re 9

Bio

medic

al

Mic

rote

chnolo

gy

and

Nanote

chnolo

gy

Microfabricationand Nanofabrication

applied to Biomedical Instrumentation

Why Micro/Nano?

SCALING OF PARAMETERS

The values of various parameters depends on the dimensions of the system

and this proves to be helpful in a number of cases.

Exam

ple

: C

antile

ver

bendin

g(M

echanic

al Para

mete

rs)

Force F

Deflection d

Density of Material = 3.5 x 103kg/m

3

Young’s Modulus = 1012N/m

2

Material properties such as Young’s modulus remain approximately the

same in the micro and macro versions.

However, they are relatively more different in case of nanodimensions

because nanodimensions come closer to molecular level.

Why Micro/Nano?


Mass

Mass = Density x Volume = Constant x S

3

Consider that the dimensions of the cantilever are reduced 10000times, i.e

the length, breadth and thickness change from 100 cm, 10cm and 1cm to

100microns, 10 microns and 1 micron respectively.

If S represents any dimension in general then,

Therefore mass goes down (104)3or is reduced 1012times as the original beam

Str

ength

to M

ass R

atio

Total strength scales with its cross-sectional area. Hence, total strength scales

as S

2. Hence, total strength to mass ratio scales as S

-1.

As a result, the micro cantilever is 104times stronger than the macro model.

Why Micro/Nano?


Deflection

333

4

3Ebt

Fl

EI

Fl

d=

=

Moment of Inertia

Young’s Modulus

Force

thickness

breadthlength

Force will vary with cross-sectional area if the stress is to be kept constant

Therefore, deflection is proportional to S

1. Therefore, the same stress is

generated in the two models if the deflection in the microcantileveris 10-4

times the deflection in the macro model, thus maintaining the bending shape.

A much smaller force can be sensed (10-8times) with the micro cantilever.

Why M

icro

/Nano

?


Fre

quency

Frequency scales as the square root of the ratio of the stiffness and

mass. Thus, frequency scales as S

-1. Hence, micro and nano

applications can be high frequency applications.

Huang e

t al h

ave a

chie

ved a

nanom

echanic

alsilic

on c

arb

ide

resonato

r fo

r ultra

hig

h fre

quency

applications.

Resonant fr

equencie

s h

ave b

een a

s h

igh

as 6

32 M

Hz.

Refe

rence :

htt

p://w

ww

.bu.e

du/n

em

s/S

iC%

20hig

h%

20fr

equency%

20H

enry

.pdf

Why M

icro

/Nano

?


Exam

ple

: C

apacitor(E

lectr

ical P

ara

mete

rs)

Voltage

Voltage = E x gap

Thus, voltage will scale as the gap between the plates.Therefore a

much smaller voltage will be required in the micro case to produce

the same effect.

If the same electric field E = 108V/m needs to be mainitainedbetween

the plates of a micro sized capacitor and a macro sized capacitor, then

gap

+ V

gnd

Why M

icro

/Nano

?


Ele

ctr

osta

tic F

orc

e

Electrostatic force = Area x (electrostatic field)2

Thus, electrostatic force scales as S

2if electrostatic field is maintained same.

However, if voltage is to be maintained same, electrostatic force would be

independent of scaling.However, its effect in the micro case would be more

pronounced because relatively, inertial forces are very low.

Electromagnetic force scales as S

4 if magnetic field is to be constant and thus,

the world of memsrelies on electrostatic motors as opposed to electromagnetic

motors .

Capacitance

Capacitance scales as S

1.

Why M

icro

/Nano

?

An electrostatic micromotor


An electrostatic comb drive actuator

Advantages of Micro/NanoFluidics

for Biomedical Applications

•D

evic

e siz

e fo

r hand-h

eld

in

str

um

enta

tion and poin

t-of-care

te

sting is m

inim

al.

•Pro

vid

es f

or

effic

ient

use o

f expensiv

e c

hem

ical

reagents

and

low

pro

duction

costs

per

devic

e

allow

ing

dis

posable

m

icro

fluid

icsyste

ms.

•Pre

cis

e v

olu

metr

ic c

ontr

ol of sam

ple

s a

nd r

eagents

is p

ossib

le,

whic

h leads to h

igher sensitiv

itie

s.

•H

igh-thro

ughput

bio

logic

al

scre

enin

g

is

made

possib

le

by

faste

r sam

pling tim

es thro

ugh p

ara

llel pro

cessin

g o

f sam

ple

s.

•In

-situ p

roduction o

f unsta

ble

com

pounds for

bio

logic

al assays

is a

lso p

ossib

le.

•R

atio o

f surf

ace a

rea to v

olu

me is h

igh a

nd thus, th

e s

ensin

g is

more

effective in c

ase o

f ele

ctr

ochem

ical sensors

etc

.

Disadvantages of Micro/Nano

Fluidics for Biomedical Applications

•B

ubble

s b

lock e

xits.

This

could

be c

ontr

olled b

y e

ither prim

ing a

t hig

h p

ressure

s o

r by u

sin

g d

iffe

rent prim

ing a

gents

such a

s e

thanol or carb

on

dio

xid

e.

•U

nw

ante

d p

art

icle

s

Fin

e filte

ring o

f solu

tions b

ecom

es im

port

ant.

•Surf

ace tensio

n p

lays funny.

Mic

roscale

modeling n

eeds to b

e d

one a

nd m

echanic

s is n

ot

part

icula

rly intu

itiv

e. H

ow

ever, s

urf

ace tensio

n forc

es c

ould

be

explo

ited a

s w

ell !

•In

terf

acin

g w

ith the m

acro

scale

equip

ment is

not easy.

Micro/Nanoapplied to BME

Taken from : http://mems.colorado.edu/c1.res.ppt/ppt/g.tutorial/ppt.htm


Taken from : www.heartcenteronline.com


Balloon Angioplasty

and

StentProcedure

Ste

nt

Pro

cedure

htt

p:/

/ww

w.m

dm

ercy

.co

m/v

ascu

lar/

dis

cover

i

es/b

allo

on

_st

ent_

gif

_b

ig.h

tml

htt

p:/

/ww

w.m

ed.u

mic

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du

/1li

br/

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atio

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art.

htm

Bal

loon A

ngio

pla

sty


Micromachinedsilicon neural probe arrays

Taken from

http://www.ee.ucla.edu/~jjudy/publications/conference/msc_2000_judy.pdf

Michigan Probe


Drug Delivery Probes


An implantable blood

pressure sensor developed

by CardioMEMS

Surgical microgripperactuated by SMA

Taken from


Micro/Nano

Fabrication Techniques

Generalized Microfabrication


Photolithography

Cle

an w

afe

r: to remove particles on the surface as well as any

traces of organic, ionic, and metallic impurities

Dehydra

tion b

ake: to drive off the absorbed water on the surface

to promote the adhesion of PR

Coating :

a) Coat wafer with adhesion promoting film

(e.g., HMDS) (optional)

b) Coat with photoresist

Soft

bake: to drive off excess solvent and to promote adhesion

Exposure

Post exposure

bake(optional): to suppress standing wave-effect

Develo

p

Cle

an, D

ry

Hard

bake: to harden the PR and improve adhesion to the

substrate

Photolithography

Taken from :http://www2.ece.jhu.edu/faculty/andreou/495/2003/LectureNotes/Handout3a_PhotolithographyI.pdf

Add

itiv

e P

roce

sses

Oxid

ation

Thermal Oxidation of Silicon is done in a furnace in wet or dry conditions

Add

itiv

e P

roce

sses

Dopin

gPurp

ose o

f D

opin

g in M

EM

S

-Make P++ etch stop

-Change restivityof the film

(e.g. make piezoresistor,connectingwire)

Dopants: N type (Phosphorous, Arsenic), P type (Boron)

Dopin

g M

eth

ods

1.Diffusion

Dopantsare diffused thermally into the

substrate in furnace at 950 –1280 0C.

It is governed by Fick’sLaws of Diffusion.

Dopantions bombarded into targeting

substrate by high energy.

Ion implantation are able to place any ion at

any depth in sample.

2. Ion Implantation

Add

itiv

e P

roce

sses

Physic

al V

apor

Depositio

n (P

VD

)

1. E

vapora

tion

Thermal Evaporator

Deposition is achieved by evaporation

or sublim

ation of heated metal onto

substrate.

This can be done either by resistance

heating or by e-beam bombardment.

Additive Processes

Physic

al V

apor

Depositio

n (P

VD

)

2. Sputtering

Sputtering is achieved by accelerated

inert ion (Ar+) by DC or RF drive in

plasma through potential gradient to

bombard metallic target.

Then the targeting material is

sputtered away and deposited onto

substrate placed on anode.

Additive Processes

Physic

al V

apor

Depositio

n (P

VD

)

Additive Processes

Chem

ical V

apor D

epositio

n (C

VD

)

Mate

rials

deposited

Polysilicon, silicon nitride (Si3N4), silicon oxide (SiOx), silicon carbide (SiC) etc.

How

does C

VD

Work

?

EGaseous reactants are introduced into chamber at elevated temperatures.

EReactant reacts and deposits onto substrate

Types o

f C

VD

LPCVD (Low Pressure CVD), PECVD (Plasma Enhanced CVD)

Salient Featu

res

ECVD results depend on pressure, gas, and temperature

ECan be diffusion or reaction limited

EVaries from film

composition, crystallization, deposition rate and electrical and

mechanical properties

Subtractive Processes

Dry Etching

1. D

ry C

hem

ical Etc

hin

g

HF E

tchin

g

HF is a powerful etchantand hence, highly dangerous.

XeF

2Etc

hin

g

2XeF2+Si→2Xe+SiF4

EIsotropic etching (typically 1-3µm/min)

EDoes not attack aluminum, silicon dioxide, and silicon nitride


Reaction M

echanis

m

Produce reactive species in gas-phase Reactive species diffuse to the solid

Adsorption, and diffuse over the surface Reaction Desorption

Diffusion

Dry

Etc

hin

g

Pla

sm

a E

tchin

g


Dry Etching

3. D

eep R

eactive Ion E

tchin

g (D

RIE

)

A very high-aspect-ratio silicon etch method

(usually > 30:1)

BO

SC

H P

rocess

EEtch rate is 1.5 –4 µm/min

ESF6 to etch silicon

EApprox. 10nm flourcarbonpolymer (similar

is plasma deposited using C

4H8

EEnergetic ions (SF6+) remove protective

polymer at the bottom trench


DRIE Etched Pillars


Wet Etching

Isotropic etchantsetch in all directions

at nearly the same rate.

Commonly use chemical for Silicon is

HNA (HF/HNO3/Acetic Acid)

This results in a finite amount of

undercutting

Isotr

opic

Wet Etc

hin

g

Subtr

active P

rocesses

Wet Etching

Anisotropic etchantsetch much faster

in one direction than in another.

Etchantsare generally Alkali

Hydroxides (KOH, NaOH, CeOH, ..)

KO

H o

n s

ilic

on

ESlower etch rate on (111) planes

EHigher etch rate on (100) and (110)

planes (400 times more faster than

the (111) plane)

ETypical concentration of KOH

is around 40 wt%

Reaction:

Silicon (s) + Water + Hydroxide Ions →

Silicates + Hydrogen

Anis

otr

opic

Wet Etc

hin

g

Metal Patterning

Surface Micromachining

Example

An insulin pump fabricated by classic MEMS technology

(Surface Micromachining)

1. Pumping membrane

2. Pumping chamber

3. Inlet

4. Outlet

5. Large mesa

6. Upper glass plate

7. Bottom glass plate

8. patterned thin layer (for improved fluidics)

MEMS Packaging

Fabrication of

MicrofluidicChannels

Materials

•Silicon / Sicompounds

-Classical MEMS approach

-Etching involved

•Polymers / Plastics

-Newer methods

-primary die yet needed

-easy fabrication of subsequent

components

Etching Methods

Step 1 : Etching of Si

-Isotropic / Anisotropic

-HNA for isotropic

-KOH/EDP/TMAH for

anisotropic

-RIE can also be used

for high aspect ratios

Etching Methods

Step 2 : Closure of channel

a)Bonding another

substrate

b)LPCVD coating

c)Ground Plate Supported

Insulating Channels

Etching Methods

Step 2 : Closure

d) Closing Holes in the

mask material

-channel is defined

by a sequence of

holes.

-channel formed by

underetching

Etching Methods

Step 2 : Closure

e) Burying channels

beneath surface

-Trench made using

RIE.

-KOH etching to form

microchannels

-Oxide fills trench

A Comparative study

Using Polymers/Plastics

•Imprinting and Hot Embossing

•Injection Molding

•Laser Photoablation

•Soft Lithography

•X ray Lithography (LIGA)

Imprinting/Embossing

•Stamp made in Sior

metal

•Stamp pressed on

Plastic to form

microfluidic channels

•Many common

plastics successfully

imprinted

Soft Lithography

•Elastomericpolymer

cast in a Sistamp and

cured

•Polymer is peeled off

•Channel architecture

thus transferred to the

polymer

•PDMS technology is

becoming popular

Laser Photoablation

•High aspect ratio

channels achievable

•Laser pulses in the

UV region used

•Sealing by thermal

lamination with a

PET/PE film

at 1250C

•Depth controllable

References

http://www.kuos.org/archives/MEMS%20Short%20Course.p

df

http://mems.colorado.edu/c1.res.ppt/ppt/g.tutorial/ppt.htm

http://mems.cwru.edu/shortcourse/

http://www2.ece.jhu.edu/faculty/andreou/495/2003/LectureNotes/Handout3a_Ph

otolithographyI.pdf

http://www.memsnet.org

Lectu

re 9

Bio

medic

al

Mic

rote

chnolo

gy

and

Nanote

chnolo

gy

Microfabricationand Nanofabrication

applied to Biomedical Instrumentation

Why Micro/Nano?


The values of various parameters depends on the dimensions of the system

and this proves to be helpful in a number of cases.

Exam

ple

: C

antile

ver

bendin

g(M

echanic

al Para

mete

rs)

Force F

Deflection d

Density of Material = 3.5 x 103kg/m

3

Young’s Modulus = 1012N/m

2

Material properties such as Young’s modulus remain approximately the

same in the micro and macro versions.

However, they are relatively more different in case of nanodimensions

because nanodimensions come closer to molecular level.