Nanotechnology for Engineers : J. Brugger (LMIS-1) & P. Hoffmann (IOA)

Focused Ion Beam / Focused Electron Beam

Focused Ion BeamNanofabrication

NT II - 2007

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Nova 600 NANOLAB (FEI)

Dual-Beam Instrument

• Dual Beam: FIB and FEB in one instrument

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FIB – a Jack of all TradeMilling

Imaging

DepositionLithography

Doping

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Table of Contents

IntroductionIon SourceIon OpticsIon-Solid InteractionMillingImagingApplications

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Introduction

• Field emission reported the first time by R W Wood in 1897 (electrons)

• Theory based on quantum mechanical tunnelling (Fowler and Nordheim1928)

• Field Ion Microscope (FIM) introduced in the 50’s. For the first time atomic resolution has been achieved. (Müller 1951)

• Field ionisation based FIB were first developed in early 70’s.

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IntroductionPrinciple

I+

e-N0

e-e-I+

SampleA

Sampleholder

Surface modification

• Surface modification due to Interaction of impinging ions with the surface

• Elastic interaction⇒ displacement, sputtering, defects, ion-

implantation

• Inelastic interaction⇒ secondary e-, secondary ions, X-ray,

photons γ

Moving the beam ⇒ Surface patterning

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Instrumentation

• Ion source (GFIS, LMIS)

• Suppressor: Improves the distribution of extracted ions

• Extractor: High tension used for ion extraction

• Spray aperture: First refinement

• First lens: Parallelise the beam

• Upper octopole: Stigmator

• Variable aperture: Defines current

• Blanking deflector and aperture: Beam blanking

• Lower octopole: Raster scanning

• Second lens: Beam focusing

• MCP (Multichannel plat): Collecting secondary electrons used for imaging

Reyntjens S: J. Micromech. and Microeng. 11 (2001) 287-300

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Ion Sourcea) Gas Field Ionisation Source (GFIS)

• atoms (molecules) are trapped by polarizations forces

• Trapped atoms hop on the surface until they are ionisedIonisation: tunneling process withprobability D:

I : Ionisation potentialΦ : Work function of emitterV : El. Potentialc : constant

• Ions are ejected from the surface

-c(I- )VD eαΦ

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Ion Sourcea) Gas Field Ionisation Source (GFIS)

• Cooling the tip ⇒ higher residence time τr leads higher ionisation rate

• Ions: H+, He+, Ne+, etc

• low current -1 a)dI = 1 sr d

AμΩ

a) largest reported value (J. Orloff: High Resolution Focused Ion Beams, Kluwer Academic, 2003)

dΩ = sinϑ dϑ dϕ

L = 1

n

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Ion Sourceb) Liquid Metal Field Ionization Source (LMIS)

• High electrical fields at the apex of a rod leads to detachment of Ions• Liquid metal film is drawn into conical shape of the rod (W or Rh)• Wide variety of ion species including Al, As, Au, B, Be, Cs, Cu, Ga, Ge, Fe,

In, Li, Pb, Si, Sn, U, and Zn

Reservoir

Solid substrate

(W)Capillary flow

U

Counter electrode

Ga+ source from FEITaylor cone

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Ion Sourceb) Liquid Metal Field Ionization Source (LMIS)

• Surface force inward force

• Coulomb force outward force

• Maximum charge may beplaced on the surface

⇒ Rayleigh limit:

ε0 = 8.85 10-12 C2/J m dielectric constant

• Formation of Taylor Cone

Liquid droplet

charges

SF = 2 , : surface tensionrγ γ

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C 20

E qF , E = 2 4 r

επ ε

FS

FC

3Rh 0q = 8 rπ ε γ

r

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Ion Sourceb) Liquid Metal Field Ionization Source (LMIS)

Properties of metals used in LMIS

Promotes flow of liquid and wetting of substrateLow surface free energy

3

Dissolution of substrate alters the alloy composition

Low solubility in substrate

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Conserves supply of metal; promotes long source life

Low volatility at melting point

2

Minimise reaction between liquid and substrateLow melting point1

ReasonProperties

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Ion Sourceb) Liquid Metal Field Ionization Source (LMIS)

1180≈10-429821336Au

877< 10-82364429In

1070< 10-82952505Sn

423

961

672

T at whichp = 10-6 mbar

[K]

< 10008861090As

< 10-82510310Ga

< 10-81832544Bi

Vapor pressure p at Tm

[Torr]

Boiling point TB

[K]

Melting point Tm

[K]

Orloff J, M. Utlaut, L. Swanson: High Resolution Ion Beams, Kluwer Academic (2003)

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Ion SourceLMIS or GFIS

unlimited≈ 1500Lifetime [h]

50 b)5 a)Resolution [nm]

YesnoCryogenic operation

120Current

GFISLMIS

dI A d sr

μ⎡ ⎤⎢ ⎥Ω ⎣ ⎦

• Current and operation near ambient temperature are in favour for using LMIS

• Melting temperature Tm = 310 K and low vapour pressure favour Ga source for LMISa) Orloff J, M. Utlaut, L. Swanson: High Resolution Ion Beams, Kluwer Academic (2003)b) Escovitz W., T. Fox and R. Levi-Setti: Scanning Transmission Ion Microscopy with a Field Ionisation Sourc, Proc. Nat. Acxad. Sci. USA 72 (1975) 1826.

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Ion OpticsIntroduction

Intensity:

Brightness β:

dI , Current per steradiandΩ

2d I = , current per steradian per unit area per voltd dA V

βΩ

lension source

target source

xsxt

αsαt

Brightness is conserved over the system and independent of magnification:

2 2

s t

d I d I = = = d dA V d dA Vs t

s t

β βΩ Ω

βs βt

Typical values for β ~ 10 A cm-2 sr-1

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Ion OpticsElectrostatic lens

• Charged particles are accelerated in electrical field E

i

i

qEa = , a E !m

V

A

B⇒ Net acceleration towards

the center

⇒ V ~ 0.5 VA

(VA : Acceleration Voltage)

r r

l l

a ( ) > a ( )andv ( ) < v ( )

A B

A BIon

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Ion OpticsBeam properties

•Current I follows Gaussian distributionσ : standard deviationI0 : total currentr : radial coordinate,

beam centre r = 0

•Diameter of the beam is defined:(FWHM : full width half maximum)

2

-20II(r, ) = e

2

rσσ

σ π

⎛ ⎞⎜ ⎟⎝ ⎠

db

b

0

dI( , ) 12 = I 2

σ

00

I ( ) = I(r, ) drσ σ∞

∫

Total current I0

33300

23100

1950

1630

1210

71

db [nm]I0 [pA]

Typical currents and beam diameters

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Ion OpticsAberrations

• Astigmatism:

• Spherical aberration

• Chromatic aberration: Not all particles have exactly the same energy

• Space charge effects: more important for ions than for electrons

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Ion-Solid interaction

•• sputteringsputtering

•• implantationimplantation

•• damage damage

•• electron emissionelectron emission

•• thermal energythermal energy

Courtesy John Courtesy John MelngailisMelngailis

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Ion-Solid interactionsputtering Example

• Cross section of a tip deposited by FEB

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Ion-Solid interactionSputtering

• Physical sputtering: removal of material by elastic collisions between ions and target atoms

• Sputtering occurs at energies E > hundred eV• Typical ion-energy E: E > 5keV• Sputtering occurs via collision cascades• Most ejected atoms origin from the top few atomic layers

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Ion-Solid interactionSputtering Rates Rs

Courtesy John Courtesy John MelngailisMelngailis

es

i

NR = =N

ejected atoms= incoming ions

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Ion-Solid interactionVolume per Dose VD

DV = V I t

V: Volume

I: Current

t: Time

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Ion-Solid interactionSputtering Yield

• Sputtering yielddepends on incident angle φ

• Higher probability of collision cascades near the surface at higher φ• Sputtering yield has maximum for φ = 75°

φ

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Redeposition

redeposition

Scan speed

sample

• Sputtering yield can not be used to determine material removal

• Redeposition needs to be considered for precise structuring

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Gas-Assisted Etching

• Enhanced milling rate• Redeposition is reduced due to volatile reaction products• Typical gases: Cl2, I2, H2O, XeF2

• Etch enhancement:Sample

Ga+

gas

gasgas

gas inlet

7-10

None

W

7-10none7-12XeF2

none7-107-10Cl2

SiO2AlSi

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• Yield of chemical etching is linear to the surface coverage

Gas-Assisted EtchingModel

0 0

atoms N(t) N(t)Yield Y = = s , : surface coverage, s: maximum yieldion N N

0 0 desdesorption

N N NN = Fg 1- - msJ(t) - N N

reactionadsorption

τ

• ⎛ ⎞⎜ ⎟⎝ ⎠

F: gas flow

g: sticking coefficient

J: ion flux

τdes: desorption constant

m: number of molecules participating in reaction

ND: density of adsorbed molecules at the beginning of dwell period

NR: density of adsorbed molecules at the end of dwell period

• Solution for uniform beam:

Replenish:

Deplete:0 0

gF + Jms gF + Jms- t - tN N

D 0FgN(t) = N e + N 1 - e

Fg + Jms

⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠

0

Fg- tN

RN(t) = N e

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Gas-Assisted EtchingModel

• Removal by physical sputtering AS and chemical etching AR

D

removed atoms AR + ASY = = ions Jt

D = t

0 t = 0

JsAR = N(t) dtN

t

∫

• AS depends on the ion energy and how the the energy from ion impact is dissipated in the presence of a reactive precurser

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Gas-Assisted Etching

Interdigitated electrodes milled without gas-assisted etching

Interdigitated electrodes milled using gas-assisted etching

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Imaging

• Ions and secondary electrons may be used for imaging

• Interaction of ions with solids leads to generation of secondary electrons

Eion

e-

φw

Potential emission

(Auger neutralization)

EF

Eion > 2φwa)

Kinetic emission

• Inelastic collisions may result in excitation or ionisation of atoms

a) Bajales N. et al.: Surface Science 579, L97-L102 (2005)

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Imaging

• Yield of secondary electrons depends on material

• Material depending contrast

• Yield decreases with atomic number Z

• Low penetration depth zp

(10 nm < zp < 100 nm at 30kV)

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ImagingFIB and Electron Microscopy - a Comparison

Resolution:FIB and SEM are comparable; FIBs: up to 5nm, SEMs: up to 3nm

Sample handling:Both FIB and SEM comparable

Voltage contrast imaging:FIB performs better than low-voltage SEM (low intrinsic depth of ions)

Material analysis:SEM allows EDX, FIB doesn't (excication energy !). FIB would allow micro-

SIMS (some systems are installed)

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ApplicationsTEM-lamellas and Lift-out

TEM grid, 3mm diameter““LiftLift--outout””

15um15um

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Applicationscross-section

SIM image of Co tip deposited using FEB

SEM image of Co tip deposited using FEB

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ApplicationsAbsolute pressure sensor

Reference pressure

p = 10-6 mbar

Sealing

Deposition process

Finished encapsulation deposition

Reyntjens, S. and Puers, R.: A review of focused ion beam applications in microsystem technology.

J Micromech. Microeng. 11 (2001) 287-300.

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ApplicationsOptical Filter

Au

SiO2

Ti layer

Pt deposition

Cross-section

Zoom of sub-wavelength coaxial structure

Array of 20x20 coaxial structures

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ApplicationsChip Modification

Insertion of electrical connection: 1) Removal of isolating layer (milling)

2) Pt deposition (FIB deposition)