Ion Beam Lithogrpaphy Seminar New

7/30/2019 Ion Beam Lithogrpaphy Seminar New

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Presented bySudama Chaurasiya

Rugmani M

Pratheep P

Shashank Chetty S

Guided byDr. K. Suresh Babu


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MAGIC OF ION BEAM LITHOGRAPHY

LITHOGRAPHY PROCESS

NEXT GENERATION LITHOGRAPHY

INTRODUCTION

HISTORY OF ION BEAM TECHNOLOGY

ION BEAM LITHOGRAPHY APPARATUS

ION INTERACTIONS

IONS FOR LITHOGRAPHY ???

PHYSICAL PROPERTIES OF ION BEAMS

REASON FOR POSITIVE IONS ???

COMPARISON WITH OTHER


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Replication of a master pattern onto a substrate

Coat the substrate with a radiation-sensitive polymer film (a resist)

Expose specific area of film to radiation, which alters properties

(solubility) of film.

Strip

Resist

Develop

Pattern

Positive

NegativeSubstrate

Resist

Ion BeamRadiation


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- Air environment

- Complex maskfabrication ($4k-$12k)- Resists have lowsensivity- High cost X-raysources

-vacuum environment- direct write systems

(software masks)-slow writting over largeareas- very high system cost


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The ion-beam lithographic exposure tool is similar tothat of electron-beam exposure systems.

The major differences being in the source anddeflection system.

The key advantages of ion-beam lithography are highresolution and the fact that it can be used with resists

with poor sensitivities. This implies that minimum energy is delivered to the

substrate, in contrast to high energy electrons used inelectron-beam lithography, which penetrate relativelydeeply into the substrate and lose only a small fraction

of their energy in exposing the resist. Thus, resolution in ion-beam lithography is primarily

limited by the range of the secondary electronsproduced as the ion loses energy in the resist.


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o 1974: Seliger and Fleming recognized potential utilityof focused ion beam for maskless implantation of

dopants into Si.

minimum spot diameter was 3.5mm, beam energy:

60 keV

1975: Krohn and Ringo tested the feasibility of Ce, Ga,Hg as liquid sources

most suitable: liquid Ga ("15nm spot size, effective source

diameter "200nm

1979: Seliger et al. reported a scanning ion probe

system with a beam of gallium ions focused down to adiameter of 100 nm, current density 1.5A/cm2 (high

brightness)


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1982: Anazawa et al. constructed a 35 kV Ga-source SIM(Scanning ion microscope).

surface-sensitive SIM images

1985: Orloff and Sudraud proposed focused ion beamsystem for lithography and implantation with 10nm source

1985: Kato et al. pointed out advantages of the FIBtechnology for the fabrication of sub-micron structures and

other maskless processes, including

scanning ion microscopy

maskless ion implantation

maskless etching and deposition

resist exposure (lithography)


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Ion Beam Source

Ion Optics

VacuumChamber

Ion energies : 20 eV- 200 KeV

Beam Current : up to500 A/cm2

Ion Specimens : H,He, Ar, Hf, Ga, Si, Au,

Co, Pr, P+, BF2+,

etc


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ION-RESISTINTERACTION,

SHOWING

*SPUTTERINGOF

NEUTRALATOMS,

*EMISSIONOF

ELECTRONS, *LATTICEDAMAGE, *HEAT

GENERATION,*IMPLANTATION.

In addition, the

beam can

generate

secondaryelectrons that

participate in bond

breaking reactions

in resist

molecules.


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Ions have two principal features that make their use in

Lithography valuable to considerresolution and sensitivity

The resolution is intrinsically higher than for electrons

because electrons suffer from the so-called "proximity"effect ( developed pattern, is wider than the scanned pattern).

The lateral scattering of electrons and the creation of

energetic secondary electrons both serve to broaden theeffective size of a focused beam as it penetrates a resist.

In addition, backscattering of electrons from the resist orfrom the underlying substrate (If the resist is thin) provides abroad background about the incident beam.

As a result one feature tend to smear into a neighboringfeature with a loss of contrast.


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Ions scatter much less and produce secondaryelectrons of only very low energy, thus reducing the

spreading of exposure features in a resist to ~


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Forslow heavy ions (e.g.,30 keV Ga+), the interaction

process is that

altering the surfacestructure of a material,

modifying surfacechemistry, and

removing atoms from thesurface through sputtering.

The primary interaction

offast lighter ions(e.g., 100 keV3 MeV

protons) is that

deep penetration into

the material, with aminimal amount ofsurface disruption.


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The diameter of an ion is much greater, of order 10-10 m,

about the size of an atom. Generally, the diameter of a negative ion is somewhat

greater than that of a positive ion.

The radius of almost all negative ions is around (1.32.5)

10-10

m, while the radius of positive ions is about (0.11.7) 10-10 m.

For example, the radius of a potassium ion, K+, is 0.133nm, while the radius of Cl- is 0.181 nm.


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THREE BASIC OPERATING MODES

MAJOR COMPONENTS

TYPES OF ION-BEAM LITHOGRAPHY

FOCUSED ION BEAM TECHNOLOGY

PROBLEMS IN USING FOCUSED ION BEAM


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Emission of secondary ions and electrons

FIB imaging, low ion current Sputtering of substrate atoms

FIB milling, high ion current

Chemical interactions (gas assisted)

FIB deposition.


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IMAGING

ETCHING DEPOSITION


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Upper (condenser) lens : ions are focused and intoparallel beams.

Mass separator: filter out unwanted ion species Drift tube: eliminates the ions that are not directed

vertically.

Lower (objective) lens: helps in reducing the spot size

and in improving the focus.

Electrostatic beam deflector: controls the finaltrajectory or landing location of the ions on the substrate.

Nozzle: provide low energy non-focused electrons to

neutralize the charged substrate.

Multi-channel plate (MCP): recording the secondaryelectron emission and thereby, helps in viewing the

substrate


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All the components are usually placed in a low-pressure

chamber evacuated to the 10

-7

Torr. To prevent interference of the focused ions with particles

in the chamber.


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(a)Focused ion beam lithography,

(b)Proximity ion-beam lithography,

(c)Ion projection lithography.


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Developed in the late 1970s. Utilizes slow focused heavy ion beam. (e.g., 30 keV

Ga+ ions).

The beam itself is powerful enough to etch away

material. Therefore no resist and developing is needed. Used to sputter atoms or structurally modify the

surface of a material to produce topographically orstructurally modified surface patterns.

High-speed fabrication. Imaging resolution with Ga ions is limited to ~5 nm.


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A point beam

can be focusedto a fine spot(10 nm) from abright point

source and

deflected on the

surface, in an

arrangementcalled focused

ion beam

(FIB),to expose

the resistdirectlywithout amask.


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DEFLECTION OF BEAMThat is the time required for beam to find a new location in

response to a new deflection signal more than for electron.

STATISTICS AND MASK WRITING TIME.The writing time increases inversely as the square of the

focused beam diameter.

Both are related to writing speed


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FIB APPLICATION IN TEM

ADVANTAGES

PROXIMITY ION-BEAM LITHOGRAPHY

ION PROJECTION LITHOGRAPHY

ION SOURCES

GAS FIELD IONIZATION SOURCE

LIQUID METAL ION SOURCES

ADVANTAGES OF LMIS

GALLIUM: LIQUID METAL

PAIR OF LIMITATIONS: SOURCE

TWO MAJOR IMPROVEMENTS TO THE

SOURCE


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1. Pt Deposition

2. Coarse milling3. Fine milling

4. Pre-thinning

5. Lift out

6. Transfer of specimen to TEM

grid


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The imaging and high resolution milling capabilitiesof the FIB have made it possible to preselect a region

and mill a TEM specimen with submicrometerprecision.

Because thinning is uniform across the samplesurface are made available for TEM observation.

TEM specimen can be prepared for analysis in 3 to 5hours.

Minimizes artifact.


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The second way to employ ion beams in lithography is to

place a stencil mask in close proximity to a surface andirradiate the mask with a collimated beam of ions.

This is referred to as proximity ion-beam lithography, and it

is a 1:1 shadow mask printing.


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The third way to employion beams in lithography

is to combine the firsttwo techniques anduse an ion opticalcolumn to project theimage of the pattern of

a stencil mask onto awafer.

This is called ionprojection lithography(IPL), which was

pioneered at IonMicrofabrication Systems(IMS) of Austria in thelate 1980s


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In any finely focused Ion beam system the ion source is thekey element - brightness must be high, since it limits themaximum beam intensity that can be brought into a focused

spot. Energy spread of ions from the source must be small

because of chromatic aberration in focusing lenses.

Metals that have relatively low melting temperatures andlow reactivity.


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The ion sources that are currently available include Al,As, Au, Be, Bi, Cs, Cu, Ga, Ge, Er, Fe, In, Li, Ni, Pb, Pd,Pr, Pt, U, and Zn.

Among these, Ga is the most popular ion species usedin IBL.

In orderto lower the melting point and to control thereactivity, alloy sources, such as PdAs, PdAsB, AuSi,and AuSiBe, are frequently used to deliver thedopants for semiconductors.


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The source utilizes tips comparable to those used in fieldion microscopy.

A sharp tip is held at a potential a few kilovolts positivewith respect to an aperture.

The electric field at the tip is ~V/A hydrogen moleculefrom the gas that enters into the region of high field isboth attracted to the tip by polarization forces andionized.

when it is very close to the tip (~A) and then repelled inthe repulsive field.


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The source depends upon a liquid metalwhich wets but does not react with asolid needle.

In this case, a tungsten needle welded to atungsten wire hairpin (for heating) holds amolten gold ball at the crossing.

The needle is sharpened to a radius of afew microns (a rather blunt pointcompared with the gas field ionization tip).

Liquid gold wets the needle. When an appropriate voltage is applied

between the needle and an aperture theelectric field forces on the liquid can exceedthe surface tension force and the liquidmetal is pulled into a cone shape.

At the very tip of the cone the field thefield is high enough to produce fieldevaporation and ions of the liquid arepulled off at currents ~1-100A


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LMIS has high emission current density (highbrightness) and extremely small size of emissionarea.

These are favorable characteristics for achieving finefocus of ion beam.

Stable emission process


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Liquid metal sources have been made to yield ions of

many different species in Al, As, Au, Be, Bi, Cs, Cu, Ga,

Ge, Er, Fe, In, Li, Ni, Pb, Pd, Pr, Pt, U, and Zn. provide a source ofions of ~ 5 nm in diameter

its low melting point ( 29.8 C)

Its low volatility at the melting point conserves thesupply of metal and yields a long source life.

its low vapor pressure

it has excellent mechanical, electrical, and vacuumproperties.

its emission characteristics enable small energy spread.


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The majority of the ions generated are blocked by thesides of the aperture, as their velocity vectors are not

pointed along the direction of the beam.

The ion emission current is strongly dependent on thetip radius and on the tip surface condition.

The sharper the tip is, the higher the field; and the higher

the field, the stronger the ion emission.


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Hanson and Siegel have introduced Major Improvement

(Cold field emission guns). First, by running the tip at 4 K and introducing H2 at ~10 -2 Torr,

H2 condenses on the tungsten needle.

The H2 moves along the needle to the tip where it is field

desorbed as a molecular ion.

This flow along the tungsten needle greatly increases the

available H2 at the tip and hence the current.

The system is differentially pumped to prevent Ion molecule

collusions after ionization and acceleration

Secondly Its very important that that the ion beams areproduced from very end of the tip ( which has leastatom).

This greatly enhances the axial ion density.


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MASK AND RESIST

ADVANTAGES OF ION BEAM LITHOGRAPHY

ION CHANNELING

EFFECT ON MATERIAL REMOVAL RATE

EFFECT OF ANGULAR SPREAD

DRAWBACK OF ION BEAM LITHOGRAPHY

CHALLENGING INTEREST

CONCLUSION


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The mask needs to be made from an ion-transparent

substrate with a pattern made up of an absorbersurface (Vive versa).

The silicon acting as the absorber.

Polymethyl methacrylate (PMMA) is the most popular

type ofIBL resist. They have the better resolution, sensitivity, and etch

resistivity.

RESIST IN ION BEAM LITHOGRAPHY


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RESIST IN ION BEAM LITHOGRAPHY

Typical process flow for resist-based IBL

The pattern definition is performed by the physical modification of the

resist irradiated by ions.The key elements in the process are thus the employed resist and its

interaction with the beam employed for exposure.

Resist material behaves under ion beam irradiation largely depends on the

form of energy deposition.

MASK PREPARATION IN ION BEAM LITHOGRAPHY


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The main property required for

membrane Preparation is Coefficient

of Thermal expansion and Youngmodulus:

For this most preferred is Silica,

Alumina and Gold.

Alumina exhibit very low etch rate with

favorable Coefficient of thermal

expansion and young modulus.

MASK PREPARATION IN ION BEAM LITHOGRAPHY


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Comparison of limiting resolutions

Line Width [m]

0.01 0.1 1

con

tras

t

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

multilayer resistsingle layer resist

ION

X-RAY

OPTICS

E-BEAM


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When ions are implanted along the principal crystallineaxis, they may collide with lattice atoms in a similarway.

At each collision, the ion suffers only small-anglescattering and can penetrate the solid to deeperdistances. This phenomenon is called CHANNELING.

since the diameter of a positive ion is 0.02 0.34 nm, if a

channel presents an opening greater than the iondiameter then the ion can be implanted in the opencrystalline direction to a greater depth.


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Ion dose:A threshold valueexists.

48

3 x 1016 cm-3

Threshold

More (=>swelling)

Less (=>net MRR)


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Material removal rate m

Particle velocity V (known)

Particle diameter d (known)

Particle indentation depth w (known)

Semi contact radius of the indented surface a

Angle subtended by the arc at the center of the particle2

Area ploughed by the particle in the direction A.


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Distance from slit center [m]

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

numberofeventsa

twafer(=protondo

se)

0

5000

10000

15000

20000

25000

slit width = 1m

0.8

0.60.4

0.2

0.1

0.05

0.02

slit patternmembrane

wafer

400keV proton

10m

develop untilthis dose region

TRIMsimulation


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Ion-beam lithography is not without its drawbacks,

especially when compared to electron-beam lithography.

Shot noise is a major issue for ion-beam lithography, justas it is for electron-beam lithography.

Since energetic ions carry enough energy to displace

crystal lattice atoms, they can also cause considerabledamage to the substrate when subjected to verythinresists.

Due to interaction of ions with each other within the ion

optical column leads to resolution limit issues.


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New technology with no commercial instruments

available as yet

Difficulties in focusing MeV ions to sub-100nmdimensions

With the advent of compact magnetic quadrupole lenssystems, these difficulties have recently been overcome

first prototype p-beam writer has recently been designedand constructed at Centre for Ion Beam Applications

(CIBA), Singapore.


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Ion beam lithography is not yet a technology that isneeded at the feature sizes ofintegrated circuits currently

being produced.Its used in various applications like doping,photomasksrepair, material addition, (re)deposition, etc.

It is very low on the learning curve in comparison to

other advanced techniques such as e-beam or X-raylithography.

Its major advantage of high spatial resolution becauseof the absence of a proximity effect will become

important when feature sizes drop.


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Nanofabrication: Principles, Capabilities and Limits by

Zheng Cui (2008).

Micro-Nanofabrication Technologies and Application by

Zheng Cui (2005).

Principles of Lithography by Harry J. Levinson (2010).

Focused ion beam system: Basics and Applications by

Nan yao (2007).


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6 microns

8 microns

line width in the letters of the word Washington is 100 nm(close to the resolution limit)

Ion Beam Lithogrpaphy Seminar New

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