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Rochester Institute of Technology

RIT Scholar Works

Theses

Thesis/Dissertation Collections

2005

Meniscus Studies in Water Immersion Optical

Lithography at 193 nm

Kiran J. Sonawane

Follow this and additional works at:

http://scholarworks.rit.edu/theses

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Recommended Citation

(2)

MENISCUS STUDIES IN WATER IMMERSION OPTICAL

LITHOGRAPHY AT 193 Dm

By

Kiran J. Sonawane

A

Thesis Submitted in Partial

Fulfillment of the Requirement for the

MASTER OF SCIENCE

IN

COMPUTER INTEGRATED MANUFACTURING

Approved by:

Dr. Satish G. Kandlikar

Department of Mechanical Engineering

Prof. S. M. Ramkumar

Department of Manufacturing and Mechanical

Engineering Technology

Prof. Liz Paciorek

Department of Mechanical Engineering

DEPARTMENT OF MANUFACTURING AND

MECHANICAL ENGINEERING TECHNOLOGY,

ROCHESTER INSTITUTE OF TECHNOLOGY

February, 2005

(3)

Meniscus Studies in Water Immersion Optical Lithography at

193nm

Permission Granted

I, Kiran J. Sonawane, hereby grant permission to the Wallace Library of the

Rochester Institute of Technology to reproduce my thesis in whole or in part.

Any reproduction will not be for commercial use or profit.

Date: _ _ _ _ _ _ _ _ _ _ Signature of Author

Kiran

J.

Sonawane

(4)

Acknowledgments

First of all I would like to thank my advisor Dr. Kandlikar for his continuous

guidance, support, andencouragement forthis thesisproject. Ithasbeen a privilegetowork withhimoverthesepastfewyears.

Aword ofthanks goesto Prof. S. M. Ramkumar forproviding guidance and support

as my Graduate advisor.

My

appreciation extends also to the entire Department of

Manufacturing

andMechanical

Engineering

Technology

as well as Mechanical

Engineering

Department for the opportunity to study and work here. RIT has made available the best

resourcesand

help

Icouldhave found for completing mythesis.

John Borrelli has also been a valuable source of

help

with manufacturing related

issues,

machiningskills, and general supportthroughouttheproject.

The

funding

support provided

by

International Sematech for conducting thisproject in the Thermal Analysis and Microfluidics

Laboratory

in the Mechanical

Engineering

Department atRITisgratefullyacknowledged.

Dave

Hathway,

Steve Kosciol and William Finch have also offered their valuable time, skills, and enthusiasm to make my thesis successful. Thanks to all the people in the Thermal Analysis and Microfluidics

Laboratory,

especially Dr.

Mukherjee,

Prabhu

Balsubramanian,

andMark Steinke fortheir

help

andinsight.

Finally,

Iwanttogive specialthanks tomy

family

andtomy American

family;

Susan Coilfortheircontinued

love, faith,

encouragement,and sacrifice.
(5)

ABSTRACT

A new approach for optical patterning that interposes a liquid between an exposure tool's projection lens and a wafer to achieve better depth of focus and resolution over conventional projection

lithography

is known as "Immersion Lithography". As

industry

focus is getting shifted towards the liquid immersion

lithography

to manufacture the

commercialimmersiontools, studyoffluid flow anddistributionisgetting verycrucial. The focus ofthis experimental work was to capture fluid flow and distribution and meniscus

behavior on both bare and photoresist-coated glass wafers as well as silicon wafers and

address one ofthemajor challenges of

immersion;

presenceofbubbles intheliquid between

thelensandthewafer.

A liquid meniscus is created

by

supplying water through a nozzle onto bare and

photo-resist-coated silicon and glass wafer surfaces. This meniscus is formed in the gap

betweenthe nozzle end and

top

surface ofthe wafer. The meniscus behavior is studied

by

varying wafer speed, the gap between the nozzle and the wafer, and mass flow rate using

high-speedphotography.The characteristicsof circularnozzle, rectangular nozzle and nozzle

lens unit with integral supply and suction port were examined under a variety ofoperating

parameters. Nozzle geometry is of interest in Liquid Immersion Lithography. The use of

degassedwater is explored in an effort to eliminate any gas evolution. The results indicate

that themeniscus shape andthecontact angle dependonthewafer surface. Alsopresenceof water droplets on the

incoming

wafer surface may break meniscus and

trap

bubbles. This

workprovides important insight into thefield of meniscus behavior andbubble influence in

liquidimmersion lithography.

(6)

Table

of contents

1. INTRODUCTION 1

1.1. IntroductiontoImmersion Lithography 1

1.2. IntroductiontoFluid IssuesinLithography 5

1.3. Fluid Issuesfrom experimentalpointof view 7

2. LITERATURE REVIEW 8

2.1. Previouswork on related to meniscusstudies 8

2.2. Objectivesof the present study 17

3. EXPERIMENTAL SETUP 18

3.1. ExperimentalInvestigationoffluidflow andMeniscus Behavior 18 3.2. DesignguidelinesforRotatingwaferandliquid deliverySystem 18 3.3. DesignConsiderationsforCircular Nozzle Studies 20 3.4. DesignConsiderationsforRectangular NozzleStudies 23

3.5. DesignConsiderationsforNozzle LensStudies 26

3.6. EffectofDissolvedGasesand needforDegassedwater 28

3.7. High-speedCamera 29

3.8. Water Management System 29

4. MANUFACTURING 30

4.1. Design DetailsofWafer Mounting Fixture 30

4.2. Design DetailsofRectangular Nozzle Details 31

4.2.1. 45

Nozzle 31

4.2.2. 60Nozzle 32

5. EXPERIMENTAL PORCEDURE 33

5.1. ProcedureforMaking Degassed Water 33

5.2. Experimental SetupandExperimental Procedure 35

6. CALCULATIONS 37

6.1. Mass FlowforNozzle Design 37

6.2. Linear Velocity 39

7. RESULTS AND DISCUSSION 40

7.1. ResultsforCircularDesign 40

7.1.1. Meniscus Flow Pattern 40

7.1.2.

Advancing

and

Receding

ContactAngle 42

7.1.3. Meniscus

Stability

44

7.2. ResultsforRectangular Design 47

7.2.1. Contactangle on Glassandphotoresistwafer 47

7.2.2. Effect ofNozzleangle 49

7.2.3.

Instability

ofMeniscus 50

7.2.4. Effect ofnon-degassedwater 54

(7)

7.2.6. Meniscuscharacteristics 58

7.2.7. Flow Visualization 59

7.3. ResultsforNozzleLens DesignwithIntegralSupplyandSuction Ports... 60

7.3.1. MeniscusFlow Pattern 60

7.3.2.

Operating

Suction andDischarge Pressure Effect 62

8. CONCLUSIONS 63

9. REFERENCES 65

(8)

List

of

Figures

Figure 1-1: Immersion Lithography 3

Figure 1-2: PrincipleofImmersion Lithography 4

Figure 2-1:The Impactof n-heptane droplet onastainless steel surface at24

C

(abubble can be Seento form at the point of

contact)

9

Figure 2-2: Comparisonof

(a)

the resultsfromthe model ofHarlowandShannon

(1967) (b)

with theexperimentalobservations fromChandra and

Avedisianstudy of droplet shape during the impact ofadroplet ona

surfaceat24

C

10

Figure 2-3: Numericalfree surfaceconfigurationand velocity vector field of a

droplet impinging on a flat platewithpressuredistribution along the

SURFACE 11

Figure 2-4: Numericalfree surfaceconfigurationand velocity vector field of a

droplet impinging on a flat platewithRe=

1500,

We=62.5and

Fr=42.86 12

Figure 2-5: SmallwaterClusterson platinum surface 13

Figure2-6: Single Dropleton anInclined PlatinumSurface. m= 1.15x10-6

kg,T=

22C,

Ts=

22C;

A)

A=

28, B)

A=

36,

C)

A=

40, D)

A=

56,

E)

A=

67,

F)

A=

79,

G)

A=86 14

Figure 2-7:Contact AnglesverseSurface Inclination Angle 15

Figure 2-8: PlotofRecedingandAdvancing ContactanglesversesSurface

Velocity 16

Figure 3-1: Wafer MountandLevelingSystem Flange 19

Figure 3-2: Wafer RunoutElimination 20

Figure3-3: Circular Needle Nozzle 21

Figure3-4:DesignofExperimental ApparatusforcircularNozzle 22 Figure 3-5: Design SchematicforRectangular Nozzles 23

Figure3-6: DetailsofRectangularNozzle/LensUnit 24 Figure3-7: DesignofExperimental ApparatusforRectangular Nozzle 25

Figure 3-8: DetailsofNozzlewith suctionandsupply port 26

Figure3-9: DesignofExperimentalApparatusforNozzlewith suction andSupply

port 27

Figure3-10: Rotating WaferandRectangular

Nozzle Assembly 29

Figure4-1:DrawingSketchesofFixtureused to mount wafer on aluminumhub,

(Scale

1:2,

Machine Used: Milling

Machine)

30

Figure4-2: DrawingSketchesofRectangularNozzle(45

angle), (Scale

2:1,

Machine Used: MillingandGrinding

Machine)

31

Figure4-3:Drawing SketchesofRectangularNozzle(60

angle), (Scale

2:1,

Machine Used: MillingandGrinding

Machine)

32

Figure 5-1: Degassed Water DeliverySystem 33

Figure 5-2: Experimentalschematic diagram of meniscus setup 35

Figure7-1: ContinuousMeniscusonBareGlass(sideview),(gap=

0.7mm,

mass flow

rate=

12.3 mg/s,velocity=

0.4m/s)

40

Figure7-2: MeniscusonGlass (Bottom

View),

(gap=

0.85mm,

massflow rate=16.5

mg/s,velocity=0.25

m/s)

41
(9)

Figure7-3: Contact Angle (WateronBare

Glass),

(gap=

0.85mm,

mass flow rate =16.5mg/s,velocity=

0.25m/s)

42

Figure 7-4: Contact Angle(Wateron

Photoresist),

(gap=

0.85 mm,mass flow rate =

16.5 mg/s,velocity=0.25

m/s) 43

Figure 7-5: Breaking MeniscusonBareglass,(gap=0.7mm,mass flowrate=15.8

mg/s,velocity=1m/s) 44

Figure 7-6: Breaking MeniscusonPhotoresist-Coated

Wafer,

(gap=

0.7mm,

mass flow rate=

2 mg/s,velocity=0.2

m/s) 45

Figure 7-7: Trapped Bubblein

Meniscus,

(gap=

0.7mm,

mass flow rate=

12.3 mg/s,

velocity=0.4

m/s) 46

Figure 7-8: Meniscus ShapeandContact Angle(45

Nozzle)

47 Figure 7-9: Meniscus ShapeandContact Angle(60

Nozzle)

48

Figure 7-10: Side-By-SideNozzle Angle Comparison 49

Figure 7-11: EffectofResidual WateronWafer

Surface,

(gap=

0.3mm,

mass flow rate=1.1 g/s,velocity=0.25

m/s) 50

Figure 7-12: Meniscus

Instability,

(gap=0.3mm,massflowrate=

1.3 g/s,velocity

=1.0m/s) 52

Figure 7-13: Bubble AttachmentonLenssurface duringRefill(particlesin

solution),gap =0.5mm, mass flow rate=5g/s,velocity=1.0m/s) 54

Figure 7-14: Bubble StuckonNozzle Surface(DI Water-Not

Degassed),

(gap=1.0mm,

mass flow rate=0.5g/s,velocity=0.25m/s) 55

Figure 7-15: Bubble

Entrapment,

(gap=

0.3mm,

mass flow rate=

1.1 g/s,velocity = 0.25

m/s) 57

Figure 7-16: Breaking Meniscus,(gap=1.0mm,mass flow rate=1.1 g/s,velocity

=1.0m/s) 58

Figure 7-17: Flow

Visualization,

(gap=

0.5mm,

mass flowrate=5g/s,velocity

=1.0m/s) 59

Figure 7-18: Continuous MeniscusonGlass wafer, (gap=

0.4 mm,mass flowrate =

1.1 g/s,velocity=

0.5m/s) 60

Figure7-19: Breaking MeniscusonGlasswafer, (gap=

0.4 mm,mass flow rate=0.5

g/s,velocity=1

m/s)

61

Figure 7-20: SuctionofWaferdue toSurfaceTensionandPressure Forces 62

(10)

List

of

Tables

Table 1:Calculationof mass flow forneedledesign 37

Table 2: Calculationof mass flow for nozzledesign 38

Table 3: Speed CalculationforNeedle Design 39

(11)

Nomenclature

NA Numerical Aperture

DOF DepthofFocus R Resolution limit

ki

Constant

k2

Constant

Re Reynoldsnumber We Webernumber Fr Froudenumber

T Surfacetemperature,K

V Linearvelocity, m/s

h

Gap

betweenwafer surface and nozzleend,m L Nozzle

length,

m

W Nozzle

Width,

m

R Equilibriumbubbleradius

r Radius of uniform circle onwhich constant motiontakes place,m

GreekSymbols

a Angleofinclination (surface

inclination)

a surfacetension

co angularvelocity,rad

6 Maximumangle ofincidence

rj Refractiveindexof medium

(12)

1. INTRODUCTION

1.1.

Introduction

to

Immersion

Lithography

Sincelast 150 yearsimmersionliquids havebeenusedinopticallithography. Alotof

experiments were performed

by

using air and oil asanimmersionfluid.

Initially

experiments have beenperformedwithlow-index air(r|=

1.00029)

between lensandwafer. Abbe

(1880)

was the first one who used oil as an immersion fluid instead of air. While performing the experimentAbbe used oilbetween a microscope objective and glass. The refractiveindex of

oil matched with the refractive index of glass on either side. This resulted in avoiding the effects of refraction at the interfaces. This method became famous as a "Homogeneous Immersion". Zeiss also conducted some experiments

by

using oil as a medium in 1880s. Abbe and Zeiss

(1880)

developed oil immersion systems

by

using oil (which has same

refractiveindex as oftheGlass).

Numerical aperture

(NA),

Resolution limit ofthe optical exposure system

(R)

and depthoffocus

(DOF)

are threeprominent parameters inthe field ofimmersion lithography. The conventional system or

Dry

lithography

uses air or gas as a medium. In immersion

lithography

some kind ofliquid is used in the space between the objective lens and wafer.

Theformula for Numericalaperture

(NA)

ofprojectionopticsisgivenby:

NA=n Sin 9

(1)

where, 0 is the maximum angle ofincidence and r\ is the refractive index ofthe medium. Numerical Aperture forimmersion

lithography

ishigherthan

dry lithography

because liquid

isthe mediumin case ofimmersion

lithography

and refractive index ofliquid

(typically

1.3 to

1.4)

is generally higher than that of air

(1.00029)

or gas.

(Considering

the angle of

incidence0 issamein boththecases.)

Theformulae forresolutionlimitandDepthofFocusare givenbelow:

R=kiX/r|/Sin0

(2)

where,

ki

isconstant.

DOF=k2nl/NA

(3)

(13)

The EquationsindicatethatDOFis

directly

proportionalto r\ (refractive

index)

which

concludes thatDOFofimmersion

lithography

is increased

by

factorofr\ comparedwith

dry

lithography,

assuming NA is same in boththe cases.

Hence,

the principle of

increasing

the index ofrefraction ofthe space between lens and wafer

by

using a liquid which has high

refractive index than the refractive index of air comes into play for immersion

lithography

and several experiments have been performed in search ofthe ideal fluid for immersion lithography.

It is not easy to choose the wavelength for next generation

lithography

considering

the

feasibility

of

technology

aswell as thecost ofthe tools involved. A short wavelength of

the exposure light is preferred in optical lithography. In case ofwavelength of 157 nm,

additional resolution enhancementis amust to makethe

technology

more feasible and a lot

of associated risks are involved in this method. Also the challenges of shorter wavelength

methods become very

difficult,

as an alternative method of using 193-nm wavelength

technology

comesinto play forsub-65-nmdevicenodes.

PrincipleofImmersion

Lithography

Technology:

Use ofanimmersionfluid inthe spacebetweenthebottom surface oflens and wafer

changes the path of light significantly. It offers

following

benefits compared with

dry

lithography:

DOFisenhancedforthegiven numerical aperture.

Immersion

lithography

makes itpossibleforlensdesignswithNA>1. Itgives
(14)

DefinitionofImmersion Lithography:

AsperInternational

Sematech,

anewapproachforopticalpatterningthatinterposesa

liquidbetweenan exposuretool'sprojectionlensand awafertoachievebetterdepthoffocus

and resolution over conventional projection

lithography

is called as "Immersion

Lithography."

Liquid

recovery

Immersion

liquid

Liquid

supply

V::;: :' ;;;

(Scanning

motion)

Wafer

Figure 1-1: Immersion

Lithography

Fig. 1-1 from Owa S. etc al.

(2004)

andFig. 1-2 from Nellis G. etcal.

(2004)

helps in

understanding the principle behind the immersion lithography. Fig. 1-1 shows schematic

representation of the principle of immersion lithography. The gap between the bottom surface ofthe lens and wafer surface is filled

by

an immersion liquid. Liquid supply and liquid recoveryare critical parameteralongwiththedesignoflensprojection. Fig. 1-2 shows

the simulation study diagram for immersion lithography. Ithelps inunderstandingthe study

(15)

Wafer

Lens

: t

Gap

Figure 1-2: PrincipleofImmersion

Lithography

Idealfluid for Immersion Lithography:

Primarily

the choiceofimmersion fluids isbasedontheir

transparency

as well asthe

needtosatisfythe

following

requirements:

Refractiveindexshouldbegreaterthan 1

Compatible with Photo-resist as well as material oflens so that it does not

affect

lens,

wafersurface and exposureprocess

Non-contaminating

Low optical absorption at193 nm

The use ofwater as an immersion fluid in case of 193 nm exposure is the most

effective solution in current situation. Water satisfies all the necessary requirements as

follows:

Wateristransparent tobelow0.05 cm"1 at 193nm.

Therefractiveindexof wateris 1.437at 193 nm.

At 193 nmwater has minimum reaction with photo-resist material as well as

with lens material. This reaction can be further reduced

by

modifying the

resist material.

DI Water (standard and

degassed)

or Ultra pure water limits the critical

concerns ofwetting,cleaninganddrying.

(16)

Alot of

imaging

simulations results comes to the conclusionthat 193 nmimmersion

lithography

(NA=1.05 to

1.23)

has equivalent performance to F2

(157nm) dry

(NA=0.85 to

0.93)

lithography. Also recent

industry

symposia hosted

by

SEMATECH showed 193 nm

immersion

lithography

is feasibletechnology. The importanceofthis

technology

will spread

sooninthemanufacturingarena.

1.2.

Introduction to Fluid Issues in

Lithography

In spite of193 nm

being

morebeneficial than 157-nm

dry

lithography;

it faces some

problems which need to be encountered to makel93-nm immersion

lithography

as a

promisingtechnology. A lot of experimental and simulation studies helped in solving some

oftheproblems.

The way fluidgetsintroducedinthegap betweenthelensandthewaferisone

ofthe crucial factors for the

feasibility

ofimmersion lithography. There are

twomethods:

o Passive

Filling

Process: Liquid is dispensed on the

top

surface of

wafer as apuddle and this movement ofwafer and liquid takes place

togetherunderthelens.

o Active

Filling

Process: Liquid is introduced atthe edge ofthe lens in

theformofjetsandwafermovesunderthelens.

The directionof wafer: A lot of simulationstudies were done to findout the

effect ofwaferdirectiononthe

filling

process offluid. First movingthewafer

from right to left direction that is in the direction offill process, results are

obtained. Thenresults are obtained when waferismovingatthesamevelocity

but in opposite direction that is against the

filling

process. But there is very

littledifferencefound betweenthe two cases.

Bubbleprevention and elimination: Oneofthemajorhydrodynamicproblems

forimmersion

lithography

is production ofbubbles inwater

during

exposure.

There are various causes for the production of bubbles. The turbulence

between liquid and the resist surfaces

during

the scanning process and
(17)

bubbles. Bubble causes scattering-induced flare which impairs the aerial image. Alsoaerial image gets furtherdegraded becauseof superimposition of

some portion of scattered aerial imagewith original aerialimage. Bubble not onlycauses scattering butalso causedefects if

they

are attachedto thesurface of wafer.

Therefore,

Prevention and elimination ofbubbles needs immediate

attention. Lots of studies have been done to understand how the bubble formation occurs andithas been foundoutthat

degassing

isthemosteffective method for the elimination of bubbles. There are several methods of producing degassedwater. Eventhoughwe obtainedperfectly degassedwater, stilllifetime ofdegassedwateris amajorconcernbecausewater willdissolve airthrough theboundary. A lotsimulationstudy has been doneandithas been foundoutthatairdissolution is fasterwhenthewaterisnotstationary because

ofthe presence ofthe random flow inside water and air dissolution into the centerareaisprevented

by

fresh supplyof water

by

local fillmethod.

Supply

of water and recovery: There are three approaches for the liquid

delivery

and recovery.

o Stage immersion: To submergethe whole stage needs a pool ofwater and requires significant engineering. So this option is not practically possible.

o Wafer Immersion: It is not possible to give quick motion to wafer because itwillcreate unstable motionof water.

o Local fill method: This is the most feasible method and currently under consideration. In this method water is dispensed between lens and the wafer

by

using a nozzle and surface tension helps in maintainingthepuddle ofthewater. Thismethod is useful at least for the exposure at the center area of the wafer. There are still some problems (edge shot

issues)

exists for the exposure near the edges of thewafer.

Precise measurement of water parameter: Critical parameters of water from

(18)

International Sematech andNIST standards. The values which are necessary forthedesignofprojection opticsare reported

by

NISTasfollows:

o r](X)= [1.43664+

-0.00002 (21.5

C,

193.39 nm)]

o dri/dT=

o The absorption coefficient for 193-nmisreported as 3% absorption

by

lmmthicknessof water.

These readingshave beennoted

by

using air-saturated water. So measurements need

to be taken with degassed water. Also there are some issues like type of resist for water

immersion,

thermal aberration

by

exposure light and full fieldprojection optics needs to be

exploredindetailfrom

feasibility

point ofview.

1.3.

Fluid

Issues from

experimentalpoint of view

Importance of prototype tool is

increasing

in the liquid immersion

lithography

environment. Sonozzlegeometryisofinterestin liquid immersion lithography. Butthere are few obstacles inthepath of successful making ofthenozzle lens systemlike bubbles in the

liquid between the lens and wafer surface and meniscus

instability

and the interaction of immersion fluidonthewafer surface.

The focus ofthis experimental work was to capture fluid flow and distribution and

meniscus behavior on both bare and photoresist-coated glass wafers as well as silicon and address one ofthemajorchallenges of

immersion;

bubblesintheliquidbetweenthelensand

thewafer. The experiment is concentrated ongetting the information on meniscus stability,

advancing and receding contact angle

by

varying parameters such as wafer speed and gap betweennozzle andwafer, and massflow. Bothbareandphotoresist-coatedglass and silicon
(19)

2.

LITERATURE REVIEW

2.1.

Previous

workon relatedto meniscus studies

Previous studies on meniscus were mainly focused on theoretical and experimental

aspects of evaporating meniscus geometry. But very little information is available on the

meniscus study at room temperature.

During

the early part of the 20th century, study of motion ofliquid andtheeffect of evaporationfromthe edgesof

drop

in aDroplet

Impinging

process has been done. Since photography is the crucial factor for close observation of geometrical parameters of

droplet,

major progress in this field was carried out with the

development ofhigh-speedphotography.

ChandraandAvedisian

(1991)

studiedthecollision dynamicsofaliquid dropleton a

solid metallic surface

by

using aflashphotographicmethod. A lotofstudywas doneonthe

dynamics of droplet impact with impact energy as the main parameter. But surface

temperature was the primary parameter in the experimental study ofdeformation process which was conducted

by

Chandra and Avedisian.

During

the study,

they

have observed the

entrapment ofbubble in a single droplet as shown inthe Fig. 2-1. The bubble is originated whenthe dropletcontactsthe surface atthe point ofimpactatt=1 ms and thenitrose above

the surface into the droplet.

They

have given two possible mechanisms forthe formation of bubble within the droplet considering the temperature ofthe droplet lower than its normal

boiling

point.

1. Entrapmentof airattheliquid-solidinterface

during

impact:- Deformation

oftheliquid

during

theimpactprocesscan causetheairentrapmentwhichleadstoformation ofbubblewithinthedroplet.
(20)

Figure2-1: The Impactof n-heptanedropleton a stainless steelsurface at24 C

(A bubblecanbeseen toformatthe pointofcontact)

Asshown in Fig.

2-2,

theresults fromthemodel ofHarlowand Shannon

(1967)

are

compared with the experimental observations from the study ofevolution ofdroplet shape

during

the impact of a droplet on a surface at 24

C

conducted

by

Chandra and Avedisian

(1991). The differences

they

foundare summarized asbelow:

1. In reality the film stopped moving because

initially

the internal pressure

which isresponsibleforthemotiondissipates aswaterstartsspreading untilthepressure can

not overcome the resistance to the motion because ofthe viscous effects at solid -liquid interface. But inthemodelanalysis, thefilmispredictedto spread withoutbound.

(21)

(a)

Oms

(6)

0.2ms

0.5ms

0.8ms

1.1ms

Figure 2-2: Comparison of

(a)

the resultsfromthe model ofHarlow

andShannon

(1967) (b)

withtheexperimentalobservations

from ChandraandAvedisian studyofdropletshape

during

theimpactof adropleton a surface at24

C

(22)

Hatta etc al.(1995) have studied a) the deformation behavior of a liquid droplet

impinging

on aflat solidsurfaceat roomtemperature and

b)

the flowfield insidethedroplet

by

numerical simulation as well as

by

experimental study.

They

have considered water as a

liquid in the first case and n-heptane is used inthe second case.

They

have usedMAC-type

solution methodforthe simulation study. The surface tensionparameterwas also takeninto

consideration. t=0.4 t=0.8

i

Re=1500 We=62.5 Fr=42.86

l)9tei3!!

t=1.2

^W^mttftififlM

t=1.6

^BHgagjBEffl mh)nmm>m:

M

=2.0 'lii'tln'^ipffWOTnTHiinm'' t=2.4

m

WrTTr^T'' "!VnT77H7T-TTy:^j

m

1=3.2 ,

llll))K-t=4.0 .^^Mmm^ t=6.0 ---^fililJiiijj rfgllifc

-r. rtfllf

"TTriw

1.5 0.5 0.5 1.5

Figure 2-3: Numerical freesurface configuration andvelocity

vectorfieldof a droplet

impinging

on aflatplate with

pressuredistribution

alongthe surface

(23)

0.5

-t0.4

t-1.2

U1.6

t2.0 t=2.4 t3.2 -^*.

j=4

t*6.0 V*

-Re1500

U-^ -***---V"

1.5 0.5

0.5

1.5

Figure 2-4: Numerical freesurface configuration andvelocity

vectorfieldof adroplet

impinging

onaflatplate

withRe=

1500,

We=62.5andFr= 42.86

As shownin the Fig.

2-3,

evolution ofthe free surface and the velocity vector field

forthecase of water as aliquidwithRe =

1500,

We =

62.5,

Fr=42.86is

presented. Fig. 2-4

shows thepressure distribution along thesolid surface forthe same parameters. Inthis case

p= 0 represents the ambient pressure and negative value of pressure means thevalue lower

than the ambient pressure. Also at every time stage, the magnitude ofvelocity vector is

common. The pressure at the

impinging

region when the droplet hits the solid surface is

higherthanthepressureattheperipheryand itresultsinrapid expansionofwaterfilmonthe

solid surface. And this collision dynamics of a water dropleton a solid surface is explained

(24)

by

using the results from numerical and experimental observation study. The effects of

viscous stresses and surface tensionhave been taken into account and emphasis is given to

the analysis of whole deformationafter the collision with the surface. It has been foundout

that water film formed

by

droplet

impinging

on a solid surface began to recoil from the

peripheral regiontowards thecenterafter theradius hasreachedto itsmaximum value. Also

the calculateddeformationprocess was inagreement withthe experimental studyresults. So

the final conclusion was deformation process of a liquid on a solid surface was not only

dependon the Webernumber,Reynoldsnumber but also dependon other parameters such

as initial kinetic energyof adroplet , the interactionof physical properties of aliquidwith a

solid surface andtheaffinityatthesolid/liquidinterface.

mwmmtmm

wmmviimm

Figure 2-5: SmallwaterClusters on platinum surface

Kandlikar et al.

(2001)

have done experimental work as well as simulation work

study for waterliquiddroplet incontact with a platinum surface.

They

measured thecontact

angle ofwater on aplatinum surface under saturated conditions and in a vacuum container

using de-ionized and degassed water as a fluid in the experimental study. The advancing,

receding and equilibrium contact angles are measured on a horizontal as well as on an

inclined platinum surfaceto

20,

30 and 40 degrees. The measurements ofcontact angles are

conducted as afunctionof size ofdropletand surface orientation.

(25)

Configurations ofthe small cluster of waterontheplatinum surface are showninthe

Fig. 2-5. The orientation of each water molecule is different from the rest of the water

moleculesbecauseofthehydrogen

bonding

ofwatermolecules.

Fig. 2-6 shows the single dropletonatiltedplatinum surface with avarying angle of

inclination.

(G)

Figure2-6: Single Dropleton anInclined Platinum Surface, m=

1.15x10-6

kg,

T=

22C,

Ts=

22C;

A)

a=

28,

B)

a.=

36, C)

a=

40,

D)

<x=

56,

E)

a=

67,

F)

a=

79,

G)

a=86

Fig. 2-7 shows the effect of surface inclination upon the advancing and receding

contact angles. It shows that the advancing and receding contact angles vary between the

limiting

values.
(26)

"55T

XX

x-DTT

XAdvancingAngle

?RecedingAngle

?

? ?

60 a> a. 50 o -40 CD 30 O) < 20 o 3 10

3

o

0 10 20 30 40 50 60 70 80 90

SurfaceInclination,a

(

degree)

Figure 2-7: Contact AnglesverseSurface Inclination Angle

Surface cleanliness is also an important parameter which affects the contact angle.

The experiment has been done

by

cleaning the platinum surface

by

acetone, DI water and

bottled compressed air stream. It has been concluded that surface cleaning technique has a

significantinfluenceonthecontact anglebehavior.

Kuan and Kandlikar

(2003)

have studied the effect of motion of meniscus

by

conducting an experimental study on a smooth, rotating and heated copper surface. This

studyprovides quantitative information ofthe shape and size of stable meniscus as well as

moving meniscus. The contact angle

(advancing

as well as receding contact angle) ofthe

stationarymeniscuswas foundtobealmostindependentofheat fluxandflowrate.

(27)

140 120 v a> i_ O) a> Q a O) c < o re *-c o o 100 80 -60 40 -20 ana <fi

B

o a O

Receding

D

Advancing

0.1 0.2 0.3

Surface

Velocity (m/s)

[image:27.554.143.345.52.297.2]

0.4

Figure 2-8: Plotof

Receding

and

Advancing

Contact anglesverses

Surface

Velocity

Fig. 2-8 shows thegraph of contact angle

(advancing

and receding angle)vs. surface

velocity. Zero velocity point in the graph indicates the stationary meniscus. The surface

temperature parameteris considered while conducting the experimental observations. With

increaseinthesurfacevelocity, therecedingcontact angleof meniscusdecreases andreaches

up to alower value andthenremains constant forthe furthervalues of surface velocity. The

change in advancing contact angle withrespect to surface velocity is not distinctenough to

notice. Thevalueofadvancingcontact angleisseentobeconstant aroundthevalueof1 15

.

Liquidimmersion

lithography

provides 193-nm

lithography by

replacingthe airwith

water meniscusbetween the lens andthewafer. The water needs tomeet several criteriain

orderto functioneffectively:

1. Thetemperatureofthewatershouldbe preciselycontrolledtowithin0.01 C.

2. Thewatershouldbe freeof airbubbles.

3. Themeniscus shouldbestable

during

themotion ofthelensoverthewafer surface.
(28)

To provide satisfactory answers to the above requirements, an experimental study is

undertaken to studythe fluidmechanics ofwater meniscus trapped between the lens and a

moving meniscus. The use ofdegassed water is explored in an effort to eliminate anygas

evolution.

Specifically,

the

following

objectives are setforthepresent work.

2.2.

Objectives

ofthepresent

study

Objectivesofthepresentworkare as follows:

1.

Develop

an apparatus to investigate meniscus behavior and fluid flow on both bare

andphotoresist-coatedglass and siliconwafers.

2. Obtain information on meniscus stability, advancing and receding contact angle

by

varyingparameters such aswafer speed andgap betweennozzle andwafer, and mass

flow

by

recording imageswithahigh-speed camera.

3.

Study

thebubbleentrapment atthemeniscusfront.

4. Mechanismsofbubblegenerationin liquidimmersionlithography.

5. Effectofdissolvedgases andneedfor degassedwater.

Detailsoftheexperimentalsetup andresults are presentedinthenext sections.

(29)

3.

EXPERIMENTAL

SETUP

3.1.

Experimental Investigation

of

fluid flow

and

Meniscus Behavior

Experiments were conducted to capture fluid flow and meniscus behavior on both

bare and photoresist-coated glass and silicon wafers. The characteristics of circular and

rectangular nozzles were examinedunder avarietyofoperatingconditions. The rectangular

nozzle was incorporated into a lens blank for a seamless transition between the nozzle and

lens interface. Wafer speed, gap betweennozzle and wafer, andmass flow werevaried, and

imageswere recorded with ahigh-speed camera atframerates of125 framespersecond

(fps)

to 1000 fps. Flow visualization experiments were also performedusing water containing a

concentration of2 and 10 pmbeads.

3.2.

Design

guidelines

for

Rotating

wafer and

liquid

delivery

System

The experiment required a rotating wafer and a liquid

delivery

system to form the

meniscus. A rotating wafer system that is capable ofvariable speeds was developed. A

portion ofthebottomofthe waferneededto be accessible in orderto film bottom views of

themeniscus onglasswafers. Aprecision spindle andbeltandpulleysystemwas developed

to accomplishthis. A

leveling

system was incorporatedto allowfortheelimination oftherun

outof amounted wafertowithin0.0127mmasshownin Fig. 3-1.

Afixture was designed for

locating

andattaching an aluminumhubinthecenter of a

wafer surface. The hub is then screwed to the

leveling

system flange using three 10-32

screws at 120. The flange has three spring plungers at 120

that produce a force of

approximately 3 lbs each in the

fully

compressed position. The aluminum hub is screwed

down to the flange surface, and then the screws are backed off until there is a slight gap

between thehub and the flange surface. The wafer is slowlyrotated and each ofthe three

screwsis tightenedas thewafer run-outismonitored and eliminatedusingatest indicatoras

showninFig. 3-2.

(30)

Flange Surface

Spring

Plunger [image:30.554.79.445.60.274.2]

10-32 Screw

Figure 3-1: Wafer Mountand

Leveling

System Flange

The liquid

delivery

system is used to dispense water through the nozzle onto the

wafer surface. It consists of a circular or rectangularnozzle, a flowmeter, and anTVpouch.

Thenozzle ispositioned above thewafer surface as shownin Fig. 3-3. The gap betweenthe

wafer surface and the end ofthe nozzle was varied throughout the experiment. A circular

nozzle was made from a syringeneedle. The diameter ofthe hollowneedle was 1 mm, and

theneedlewasheldperpendicularto thewafer surface.

(31)

Aluminum

Hub

Test

Indicator

Photoresist-coatedGlass

Wafer

Figure3-2: Wafer Run outElimination

3.3.

Design

Considerations for Circular Nozzle

Studies

Theexperimentalparametersforthecircular nozzlestudywere:

Velocity

range: 0.16m/s- 1

.23m/s

Mass flowrange: 1.4mg/s- 16.5 mg/s

Four types of wafers were used in this experiment: bare and

photoresist-coatedglass and silicon

The gap betweenthe waferand needle nozzle was varied from 0.4 mm - 1.0

mm

Thefluid fortheexperiment wasDI Water(standardand

degassed)

The diameterofthecircularneedle nozzle was 1 mm

[image:31.554.61.493.59.340.2]
(32)

Aluminum

Hub

Needle Nozzle

Photo-resist

Glasswafer

Leveling

System

High-speed

[image:32.554.65.491.45.403.2]

Camera

Figure 3-3: Circular Needle Nozzle

(33)

Fig. 3-4shows adiagramoftheexperimentalapparatusforthecircular nozzle andthe

locationofthehigh-speedcamera,nozzleandrotatingwafer.

Circular

Nozzle

Rotating

wafer

High-speed

camera

Figure3-4: DesignofExperimentalApparatus forcircularNozzle

(34)

3.4.

Design

Considerations

for Rectangular Nozzle

Studies

After performing the experiments with the circular nozzle, rectangular nozzle/lens

units weredesignedand manufactured. Oneunit was designedwitha45o angle ofinclination

and anotherunit was designedwith a60o angleofinclination. The design schematic forthe

rectangularnozzles isshownin Fig. 3-5.

Nozzle Dimensions: 1 mmx 10mm

Lens

Dimensions,

LxW: 10mm x 10mm

Gap

Size,

h: 0.3mm,0.5 mm, 1.0mm

Nozzlesweredesignedwithtwoangles ofinclination: 45and60

Suction

Port

m

Lens

L= 10mm

Nozzle

1 mm

[image:34.554.61.498.154.431.2]

Wafer Motion

Figure 3-5: Design Schematic for Rectangular Nozzles

The Design details for the rectangular nozzles are given in Figs.

3-6(a,

b,

c). The

characteristics ofboth designs were studied under a variety ofoperatingparameters, which

aregivenbelow:

Velocity

range: 0.25 m/s- 1

.0m/s

Mass flowrange: 0.5 g/s- 5.0

g/s

Four types of wafers were used in this experiment: bare and

photoresist-coated glass and silicon.

The gap betweenthewafer and rectangular nozzle was varied from 0.3 mm

-1.0mm.

(35)

The fluid used for the meniscus experiment was DI Water (standard and

degassed).

Water containing a concentration of2 and 10 Dm beads was usedtoperform

theflowvisualization experiments.

(a)

Side View ofRectangular Nozzle/Lens Unit

(b)

RearViewofRectangular Nozzle

(c)

Bottom ViewofRectangularNozzle

Figure 3-6: DetailsofRectangularNozzle/Lens Unit

(36)

Fig. 3-7shows adiagramoftheexperimental apparatus fortherectangular nozzle and

[image:36.554.133.421.121.477.2]

thelocationofthehigh-speedcamera,nozzle androtatingwafer.

Figure 3-7: Design ofExperimentalApparatus for Rectangular Nozzle

(37)

3.5.

Design Considerations

for Nozzle Lens

Studies

After performing the experiments with the circular nozzle, rectangular nozzle/lens,

Nozzle lensunitswithintegral suction andsupplyport weredesignedand manufactured. The

design schematicfortherectangularnozzlesisshownin Fig.

3-8(a,

b).

Supply

Port

Suction Port

h"

Suction Line

(a)

Bottom ViewofNozzlewith suction andsupplyport

Supply

Line

Pin

(b)

Side ViewofNozzlewith suction andsupplyport

Figure3-8: Details ofNozzlewith suction andsupplyport

(38)

Design DetailsofNozzlewithsuctionandsupplyport:

Nozzlewasdesignedwithtwoports:

a. Suction Port (angle

180)

b.

Supply

Port: (angle

120)

c.Depthof suction slot: 15.25 mm d. Depthofsupplyslot: 15.25mm

Fig. 3-9 shows a diagram ofthe experimental apparatus fornozzle with suction and

supplyport andthelocationofthehigh-speedcamera,nozzle androtatingwafer.

Figure 3-9: Design ofExperimental Apparatus forNozzlewith suction and

Supply

port

The characteristics of this design were studied under a variety of operating

parameters,whichare givenbelow:

Velocity

range: 0.25m/s

-1.0m/s

Mass flowrange: 0.5g/s

-5.0 g/s

Typeofwafer usedinthisexperiment: Glasswafer

(39)

The gap between thewafer and rectangular nozzle was varied from 0.25 mm -0.5

mm.

The fluidusedforthemeniscus experiment wasDI Water(standardanddegassed).

3.6.

Effect

of

Dissolved

Gases

and need

for Degassed

water

Agas bubblepresent in a continuous liquid phaseis under equilibrium as aresultof

surface

tension,

pressure difference between gas and

liquid,

and

buoyancy

force due to

differing

densities ina gravitational field. The

buoyancy

force acts inthe vertical direction

and causes the bubble to distort from its spherical shape and rise upwards. For the small

bubbles

being

considered

here,

theshape ofthebubblecanbeconsideredtobe spherical,and

its diameter can be obtained from a force balance across a diametrical plane as shown in

equation 1.

At equilibrium, neglecting the gravitational

force,

the surface tension force is

balanced

by

theforcedueto excess pressureinsidethegasbubble.

27tRcT=

7rR2(pg-pL)

(4)

Thustheexcess pressureinsidethegasbubble is given

by

(pg~Pi)=-%-R

(5)

Whereaisthesurface tensionof water andRistheequilibriumbubbleradius. Figure 2

shows the variation ofthe excess pressure as a function ofthe radius ofthe bubble at a

temperatureof20 C.

(40)

3.7.

High-speed

Camera

A high-speed camera was usedto capture the side view andbottom view images of

themeniscus atrecordingrates of125 - 1000 fps. Microscopic

and zoom lenses (12-75 mm)

were usedto capture the images ofthe meniscus. The camerawas mounted on a tripod for

side-viewrecordingand anXYZtableforbottom-view recording.

3.8.

Water Management System

A water management system was necessary in this experiment in order to form a

continuous meniscus onthewafer surface. The combination of air-pressureline andvacuum

line wasveryeffectiveinremovingtheexcesswaterwithout anymeniscusbreak-up. Fig.

3-10showsthelocationoftheair-pressurelineandthevacuumlineintheexperimental setup.

Leveling

System

Vacuum

Line

Wafer

Figure 3-10:

Rotating

WaferandRectangular Nozzle

Assembly

(41)

4. MANUFACTURING

4.1.

Design Details

of

Wafer

Mounting

Fixture

The

drawing

sketches offixtureusedtomountwafer on an aluminumhubare shown

in Fig. 4-1.

Milling

machineisusedtoperform

drilling

andmillingoperation onthefixture.

Figure 4-1:

Drawing

SketchesofFixtureusedtomount waferon aluminum

hub,

(Scale

1:2,

Machine Used:

Milling

Machine)

(42)

4.2.

Design Details

of

Rectangular Nozzle

Details

4.2.1. 45

"Nozzle

The

drawing

sketches ofRectangular Nozzle(45 angle)are showninFig. 4-2.

Milling

machine is used to perform

drilling

and milling operation on the fixture.

Grinding

machineisusedtoperformgrindingoperation on plastic partof nozzlelens.

(0.687) <&n (1375>

1 X \

t

^ ^

1

o

d

I

(0.687)

[image:42.554.95.488.184.435.2]

(0.03) i

Figure 4-2:

Drawing

SketchesofRectangular Nozzle (45

angle),(Scale

2:1,

Machine Used:

Milling

and

Grinding

Machine)

(43)

4.2.2. 60

"Nozzle

The

drawing

sketchesofRectangular Nozzle (60

angle)are showninFig. 4-3.

Milling

machineisusedtoperform

drilling

andmillingoperation onthefixture.

Grinding

[image:43.554.70.487.157.428.2]

machineisusedtoperformgrindingoperation on plastic part of nozzlelens.

Figure 4-3:

Drawing

Sketches ofRectangular Nozzle(60angle),(Scale

2:1,

MachineUsed:

Milling

and

Grinding Machine)

(44)

5. EXPERIMENTAL PORCEDURE

5.1.

Procedure for

Making

Degassed

Water

Watercontains variouskindsofdissolvedgases at various levels

depending

uponthe

temperature, solubility and exposure history. Therequirement ofthe experimentis degassed

and deionizedwater. Thepresence ofdissolvedgas can cause theoccurrence ofbubble. The

schematic ofthe water

delivery

system to give degassed water is as shown in Fig. 5-1. It

includes pressure chamber, heatexchanger, throttle valve, flowmeter and condenser. Water

is collected afterpassingthrough thecondenser

by

openingtheknob offlowmeter. Pressure

cookerisused as avessel whichgetspressurized after

heating

waterinsidethecooker.

Heat

Exchanger

Pressure

Chamber

Condenser

Flow

[image:44.554.86.427.286.547.2]

7FFTTT

Figure 5-1: DegassedWater

Delivery

System
(45)

Theprocedurefor preparingthedegassedmethodis describedas follows:

The commercially available pressure cooker is filled with deionized water. This pressure

cooker is equipped with different deadweights for different pressure settings above the

atmospheric pressure. The pressure cooker is heated with the

heating

plate and once the

desired 15 psi pressure is reached, then the deadweight is suddenly removed and pressure

cooker is allowedtoblow downand returnto atmospheric pressure. The airdissolved in the

wateris forced out withthesteam.Thisprocedureis followedone moretime. Thenthewater

is passed through the condenser to reach at the room temperature. The degassed water is

collectedin a pouch

by

openingthevalve andcontrollingtheknob offlowmeter.
(46)

5.2.

Experimental

Setup

and

Experimental

Procedure

The experimental setup is designed for

delivering

the de-ionized anddegassedwater

throughthenozzle on awafer surface. The experimentprocedureis same for allthree kinds

ofnozzle which arelistedasbelow: Circular Nozzle

Rectangular Nozzle

Nozzle Lens Design

Water Reservoir

Flow Meter

High

Intensity

Light

Source

Wafer

High

Speed

[image:46.554.82.426.126.594.2]

Camera

Figure 5-2: Experimentalschematicdiagram of meniscussetup

(47)

Figure 5-2 shows the schematic ofthe water

delivery

system for meniscus study. It

includes

Nozzle,

Degassedwaterpouch, aflowmeterwith attached regulatorvalve. Thetest

sectionincludesthe wafer,mountingofwafer system.

Wafer is glued to a special aluminum hub with the

help

of super glue. Then it is

mounted on another aluminum hub

by

using a fixture specially designed for

locating

and

attaching analuminumhub inthecenter of awafer. Thehubisthen screwedto the

leveling

system flange andthe aluminumhubisscreweddownto the flange surface. Wafer mounting

system is seating on the shaft which is connected to the shaft ofthe motor

by

using pulley

andbelt system. Rotational speedof motoris controlled

by

supplying voltagetomotorwith

the

help

of

digitally

regulated power supply. The wafer surface is leveled with the

help

of

leveling

gauge indicatorso that thewafer surfaceis parallelto the end of nozzle. This helps

in maintainingthe gap betweenwafersurface and nozzle end at thepreset level. Theneedle

is alsopositionedat certain radial distance fromthecenter of wafer. This gives thenecessary

amount ofrelativevelocityto thewaterdeliveredon wafersurface. Thespeed ofthemotoris

adjusted

by

changingthevoltage supply. Mass flowof wateris controlled withtheknob of

flow meter. The reading is taken at theparticular velocity ofmotor (which is at respective

voltage) and gap betweenwafersurface and nozzle end andradial distanceofnozzlethese2

parameters are fixed for therespective set ofreadings. Meniscus images are captured with

the

help

ofhigh speedcamera and lens system. All videos and photo sequences are usedto

studythemeniscusbehavioranditscharacteristics.

(48)

6.

CALCULATIONS

Mass flow andlinear velocity of meniscus were importantparameters which needed

to becontrolledintheexperiment. Soitwas crucialtodo the accurate measurement ofthese

parameters. Mass flow was controlled

by

using the flowmeter andvelocity ofthemeniscus

was varied

by

changingthevoltage suppliedtomotor.

6.1.

Mass Flow for Nozzle Design

While performing the experiment, mass flow was controlled

by

using the flowmeter

and flowmeter reading was noted down for each observation. Table 1 shows the flowmeter

reading and its respective mass flow for the needle design. Table 2 shows the shows the

flowmeter reading andits respective mass flow for therectangularnozzle as well as forthe

nozzlewithsuction andsupplyportdesign.

Flow-Meter

Reading

Mass-flow

(mg

/ sec)

32 1.84

48.5 3.58

58 4.65

63 5.07

85 10.70

99 15.68

Table 1: Calculationof massflow for needledesign

(49)

Flow-Meter

Reading

Mass-flow

(g/sec)

Alltheway-closed 0.53

3 1.14

4 1.27

5 1.36

7 1.97

Max_with_Flowmeter 2.76

[image:49.554.68.490.352.634.2]

Max_without Flowmeter 5.00

Table 2: Calculationof massflow fornozzledesign

Graph 1 of flow meter setting vs. mass flow is plotted

by

using the values in the

table1 and table 2 foreach nozzle design. The equation obtained from this graph isused to

findoutthemass flowvalueatany flowmetersetting.

120 100 c 80 (1) If) <1> 60 V 5 % o 40 20

Flow Meter

Setting

vs. Mass Flow

(needle)

y=31.584Ln(x)+10.734 R2

=0.995

6 8 10 12

Mass Flow(mg/second)

14 16 18

Graph 1: Graph offlowmeter setting vs. massflow

(50)

6.2.

Linear

Velocity

Linear velocity of meniscus is one of the important parameter in the study of

meniscus because it affects the shape of meniscus distinctly. The speed of the wafer is

changed

by

controlling the voltage supply to motor which in turn rotates the wafer. The

readings ofthevoltages andthespeed ofthewaferarerecorded

during

theexperiment. Table

3 showsthereadings of wafer speedtakenat respectivevoltageofmotor. The equation

(6)

is

usedtocalculatethelinear velocityof meniscus isasshownbelow:

V=r.oo

(6)

where, V Linear

Velocity

(m/s)

rRadiusofUniformcircle on whichconstantmotiontakesplace

(m)

coAngular

Velocity

(rad/sec)

Equation

(7)

isusedtogetthefinalvalueoflinear velocityinm/secafterconverting

therpmintorad/sec.

V=(1

.6

*0.0254)

* (rpm

*2*IT)

/60

(7)

Voltage

v)

Speed

(RPM)

5v 6v 8v 8.2v 10.1 v 15.2v 15.3 v 17v 17.1 v

41.8 65.9 104.1 108.3 146.9 247.2 250.2 283.4 285.6

40 68.6 105 109.6 146.3 245.9 250 284.4 285.5

44.7 68.7 102.4 109 147 246.2 250.1 284.1 286.5

41.1 68.4 103.9 110.2 146.5 246.9 250.3 283.9 285.6

43.9 66.7 103.2 110.1 146.9 246.6 250.2 284 286

42.7 68.1 103.1 110.7 146.7 245.3 250 283.7 286.7

41.8 69.4 104.3 111.1 147.1 246.3 250.1 283.8 286.4

43.1 68.1 106.2 112.1 146.9 248 250.9 284.3 285.9

43.6 66.9 104.7 110.7 147.4 247.4 251.6 284 286.3

42.9 66.6 103.5 111.3 147.6 246 252.1 284.1 286.4

Avg. Speed

(RPM)

42.56 67.74 104 110.31 146.9 246.58 250.6 284 286.1

Table3: Speed Calculation forNeedleDesign

[image:50.554.90.474.387.601.2]
(51)

7.

RESULTS

AND DISCUSSION

Meniscus shape, advancing andreceding contact angle and meniscus stabilityresults

forthecircular as well as therectangular nozzle design are presented inthis section. All the

images presented in the photo-sequencepatterns in this section were obtained with a high

speedcamera.

7.1.

Results for Circular Design

7.1.1. Meniscus Flow Pattern

Circular

Needle

[image:51.554.80.492.246.581.2]

Meniscus

Figure 7-1: Continuous MeniscusonBare Glass (sideview),

(gap

=

0.7mm,

mass

flow rate= 12.3

mg/s,velocity=

0.4m/s)

Figure 7-1 showsthecontinuous meniscus on a glass wafer. The image isa sideview.

The water is supplied through theneedle onto thewafer and themeniscus is formed in the

gap betweenthewaferand needle end. Highermass-flow and medium wafervelocityare the

(52)

factors that cause the meniscus to be continuous. The advancing contact angle of the

meniscusisnotsharp forthebareglasswafer.

Needle Meniscus

Leading

Edge

Figure 7-2: Meniscuson Glass (Bottom

View),

(gap

=

0.85mm,

massflow

rate=16.5mg/s, velocity=

0.25 m/s)

Figure 7-2 shows a continuous meniscus on a bare glass wafer. The images are a

bottom view. The water is supplied through the needle onto the wafer and the meniscus is

formedinthe gapbetweenthewafer and needle end. Figs.

7-2(1)

to

7-2(4)

are ofequaltime [image:52.554.75.502.101.479.2]
(53)

interval (4 ms). Fig.

7-2(1)

shows the first image ofthe continuous meniscus sequence. The

continuouspatternofthemeniscusisshowninFigs. 7-2

(2, 3,

and4).

7.1.2.

Advancing

and

Receding

Contact Angle [image:53.554.52.479.157.489.2]

Advancing

Contact Edge of Meniscus

/

Figure 7-3: Contact Angle (WateronBare

Glass),

(gap

=

0.85mm,

massflow

rate =16.5 mg/s, velocity=

0.25m/s)

Figure 7-3 shows the sequence ofthe contact angle formation on a bareglass wafer.

Theimages are a sideview. The advancingcontact edge isnot sharp inthis case. Fig.

7-3(1)

shows the dropletcoming out oftheneedle. Fig.

7-3(2),

2 ms

later,

shows the

drop

of water

touching

the

top

surface ofthe wafer. Fig.

7-3(3)

(1 ms

later)

and Fig.

7-3(4)

(2 ms

later)

showsthe

developing

meniscus with clearadvancingcontact edgeforming.
(54)

Advancing

Contact

Edgeof

Meniscus

Figure 7-4: Contact Angle (Wateron

Photoresist),

(gap

=

0.85mm,massflow

rate=

16.5mg/s,velocity=

0.25 m/s)

Fig. 7-4 shows the contact angle on a photoresist-coated wafer. The image is a side

view. The advancingcontact edgeisvery sharpinthiscase.

(55)

7.1.3. Meniscus

Stability

Circular Needle Nozzle

[image:55.554.78.491.83.452.2]

Meniscus

Figure 7-5:

Breaking

Meniscus onBare

Glass,

(gap

=

0.7mm,

massflow rate=

15.8 mg/s, velocity=

lm/s)

Figure 7-5 showsthesequence of a

breaking

meniscus on abareglass wafer fromthe

side view. Meniscus breakage happened because ofthe high wafervelocity, low mass flow

andlarge gap betweenthewafer surface and nozzle end.Fig.

7-5(1)

showsthefullmeniscus.

Figures

7-5(1)

to

7-5(4)

are at equal time intervals of8 ms. Fig.

7-5(2)

shows the

starting point ofthe meniscus breakage. Fig.

7-5(3)

shows a further stage ofthe meniscus

breakage,

and Fig.

7-5(4)

shows the empty gap between the wafer and the needle end

becausethemeniscushas completelybrokenatthatpoint.

(56)
[image:56.554.103.476.55.464.2]

Needle Meniscus

Figure 7-6:

Breaking

Meniscus onPhotoresist-Coated

Wafer,

(gap

=0.7mm

massflow rate=

2 mg/s, velocity=

[image:56.554.301.475.72.238.2]

0.2 m/s)

Figure 7-6 shows the sequence of a

breaking

meniscus on a photoresist-coatedwafer

from the side view. Meniscusbreakage happened because ofthehighwafer velocity, photo

resist surface characteristics, low mass flow and large gap between the wafer surface and

nozzle end. Fig.

7-6(1)

shows the water

drop

coming out ofthe needle. Fig.

7-6(2),

16 ms

later,

shows the complete meniscus. Fig.

7-6(3),

144ms

later,

shows themeniscus breakage

inprogress.Fig.

7-6(4),

8 ms

later,

showsthefinal stage ofthe

breaking

meniscus.
(57)

Needle

Bubble

Meniscus

Figure 7-7: Trapped Bubble in

Meniscus,

(gap

=

0.7mm,

massflowrate=12.3

mg/s,velocity=

0.4 m/s)

Figure 7-7shows atrappedbubble inawatermeniscus. Thewateris suppliedthrough

theneedle onto awaferandthemeniscus is formed inthegap betweenthewafer and needle

end. Thebubble is generatedwithinthe meniscus and gets trapped inside themeniscus. The

bubble stuck to thesurface inthemeniscus and didnot move alongthemeniscus. The main

reason for the presence ofthis bubble is gas in the water. Due to this observation, it is

recommended that degassed waterbe used for meniscus experiments and liquid immersion

lithography.

(58)

7.2.

Results for Rectangular Design

7.2.1. Contact angleonGlass andphotoresistwafer

Rectangular Nozzle

Meniscus

Bare Glass Photoresist-Coated

a.

(gap

=

0.3mm,

mass flow rate=0.5g/s, velocity=1.0 m/s)

Bare Glass Photoresist-Coated

b.

(gap

=

0.5mm,

massflow rate=1.9g/s, velocity=1.0m/s)

Bare Glass Photoresist-Coated

c.

(gap

=

1.0mm,

mass flowrate =1.2g/s,velocity=

1.0 m/s)

Figure 7-8: Meniscus ShapeandContact Angle(45

Nozzle)

(59)

The images in Fig. 7-8 are from an experiment performed using the 45 nozzle

design. It also shows the difference between the meniscus shape on bare glass and the

meniscus shape on a photoresist coating. The difference between contact angles on both

surfacetypescanbeclearlyseen.

Bare Glass

Rectangular

Nozzle

Meniscus

Photoresist-Coated

a.

(gap

=

0.5mm,

mass flowrate =0.5

g/s,velocity=1.0 m/s)

BareGlass Photoresist-Coated

b.

(gap

=1.0mm, massflowrate=0.5 g/s,velocity=1.0m/s)

Figure 7-9: Meniscus Shape andContact Angle(60

Nozzle)

The images in Fig. 7-9 are from an experiment performed using the 60 nozzle

design. It also shows the difference between the meniscus shape on bare glass and the

meniscus shape on a photoresist coating. The difference between contact angles on both

surface types canbe clearly seen. A comparison oftheimages inFig. 7-8 andFig. 7-9 also

(60)

shows the difference inmeniscus shapes and contact angles forthe two different angles of

[image:60.554.61.456.113.511.2]

inclination. A side-by-sidecomparisonforthe two differentangles ofinclination isshownin

Fig. 7-10.

7.2.2. EffectofNozzleangle 45<>Nozzle

yr Rectangular

jf

*^WO Nozzle

60Nozzle

~-jf i

''

St*

L

1

Meniscus "*S3

m

a.(Glass Wafer: gap=1.0mm, mass flowrate=l.l g/s,velocity=1.0m/s)

450Nozzle 60Nozzle

b. (Photoresist-Coated Wafer: gap=

0.5mm,

massflow rate=1.9g/s,velocity=1.0m/s)

Figure 7-10: Side-By-SideNozzle Angle Comparison

Figure 7-10 shows a side-view comparison of meniscus shape and contact angle on

bareglass andphotoresist-coated wafersforthe twonozzle angledesigns.

(61)

7.2.3.

Instability

ofMeniscus [image:61.554.63.493.75.553.2]

Meniscus

Leading

Edge RectangularNozzle/Lens Unit

Figure 7-11: EffectofResidual WateronWafer

Surface,

(gap

=

0.3mm,

mass

flowrate=1.1 g/s,velocity=

0.25 m/s)

(62)

The meniscus becomesunstable and forms surface waves at the

leading

edge due to the effect of residual water as shownin Fig. 7-11. Fig.

7-11(1)

shows the

leading

edge of

the meniscus on the rectangular nozzle-lens unit. Figs.

7-11(1)

to

7-11(6)

are at equal time

intervals of2 ms. Figs.

7-11(2)

and

7-11(3)

show the onset of meniscus instability. Figs.

7-1

1(4)

and7-1

1(5)

showthewavinessofthemeniscus. Figs. 7-1

1(6),

7-1

1(7)

(4ms

later)

and

7-11(8)

(4 ms

later)

showthewaviness ofthemeniscusthat ledto the formationofbubbles

and created a complete disturbance ofthe meniscus. This will become more prominent for

larger slots (25 mm).

Therefore,

actual

testing

with the scale model of the nozzle is recommended.
(63)
[image:63.554.283.458.85.414.2]

RectangularNozzle/Lens Unit

Figure 7-12: Meniscus

Instability, (gap

=0.3mm,mass flow rate= 1.3 g/s,

velocity=1.0m/s)

(64)

Figure 7-12 shows the

instability

ofthe meniscus on a glass wafer from the bottom

view.Thewaterissuppliedthrough thenozzle onto a glass wafer andthemeniscusisformed

in the gap between glass wafer and nozzle end. Fig.

7-12(1)

show the complete meniscus

formed in the gap between the wafer surface and the rectangular nozzle/lens unit. Figs.

7-12(2)

(4 ms

later)

and

7-12(3)

(12 ms

later)

show the wavy edge ofthe meniscus leads to

instability. Fig.

7-12(4),

4ms

later,

showsthe same

instability

inthemeniscus shape. Fig.

7-12(5),

2ms

later,

showsthatmeniscusis

becoming

stableagain.
(65)

7.2.4. Effectof non-degassed water

Rectangular Nozzle Lensunit

. " '.vi

UBS ,175

i

,....,.:.

6

HH

7

Figure7-13: Bubble AttachmentonLenssurface

during

Refill(particlesin

solution),

(gap

=0.5mm, massflow rate =5g/s, velocity=1.0 m/s) [image:65.554.59.492.87.589.2]
(66)
[image:66.554.82.476.243.592.2]

Figure 7-13 shows a bubble stuck to the lens blank surface

during

refill. The

experimentwas performed on a glass wafer. Theimageswere capturedfromthebottomwith

ahigh-speedcamera. Fig.

7-13(1)

shows ameniscus ofwater containingaconcentration of2

and 10 pmbeadsthatis formed betweenthe lens blanksurface oftherectangular nozzle unit

andthewafersurface.Figs.

7-13(1)

to

7-13(7)

are atequaltimeintervalsof2ms.

Figures

7-13(2)

to

7-13(6)

showthephoto-sequence of waterdroplet gradually going

away fromthelenssurface. Fig.

7-13(7)

shows abubblestuck onthelens surface.

Rectangular Nozzle

Figure 7-14: Bubble StuckonNozzle Surface(DIWater-Not

Degassed),

(gap

=1.0mm,mass flow rate=0.5 g/s,velocity=0.25 m/s)

Figure

Figure 2-8: Plot of Receding and Advancing Contact angles verses
Figure 3-1: Wafer Mount and Leveling System Flange
Figure 3-2: Wafer Run out Elimination
Figure 3-3: Circular Needle Nozzle
+7

References

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