Focused Ion Beam Nanofabrication - LMIS1 | EPFL€¢ Theory based on quantum mechanical tunnelling (Fowler and Nordheim ... T. Fox and R. Levi-Setti: ... 2 7-12 none 7-10 Cl 2 7-10

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

Focused Ion BeamNanofabrication

Nanotechnologie ISemestre d’hiver 2004-05

Prof. J. Brugger

MER P. Hoffmann2

Focused Ion Beam / Focused Electron Beam

Nova 600 NANOLAB (FEI)

Dual-Beam Instrument

• Dual Beam: FIB and FEB in one instrument

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Prof. J. Brugger

MER P. Hoffmann3


FIB – a Jack of all TradeMilling

Imaging

DepositionLithography

Doping

Prof. J. Brugger

MER P. Hoffmann4


Table of Contents

IntroductionIon SourceIon OpticsIon-Solid InteractionMillingImagingApplications

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Prof. J. Brugger

MER P. Hoffmann5


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.

Prof. J. Brugger

MER P. Hoffmann6


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

Scanning the beam ⇒ Surface patterning

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MER P. Hoffmann7


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 plate): Collecting secondary electrons used for imaging

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

Prof. J. Brugger

MER P. Hoffmann8


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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MER P. Hoffmann9


Ion Sourcea) Gas Field Ionisation Source (GFIS)

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

• Ions: H+, He+, Ne+, etc

• Maximal 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

nr

Prof. J. Brugger

MER P. Hoffmann10


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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MER P. Hoffmann11



• 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

20

C 20

E qF = , E = 2 4 r

επ ε

FS

FC

3Rh 0q = 8 rπ ε γ

r

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MER P. Hoffmann12



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

4

Conserves supply of metal; promotes long source life, necessary for good vacuum conditions

Low volatility at melting point

2

Minimise reaction between liquid and substrateLow melting point1

ReasonProperties

Substrate

Liquid

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MER P. Hoffmann13



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)

Prof. J. Brugger

MER P. Hoffmann14


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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MER P. Hoffmann15


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

Prof. J. Brugger

MER P. Hoffmann16


Ion OpticsElectrostatic lens

• Charged particles are accelerated in electrical field E

i

i

qEa = , a E !m

rrr r

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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Prof. J. Brugger

MER P. Hoffmann17


Ion OpticsBeam properties

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

beam centre r = 0

•Diameter of the beam is defined:(FWFM : 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

Prof. J. Brugger

MER P. Hoffmann18


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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MER P. Hoffmann19


Ion-Solid interaction

•• sputteringsputtering

•• implantationimplantation

•• damage damage

•• electron emissionelectron emission

•• thermal energythermal energy

Courtesy John Courtesy John MelngailisMelngailis

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MER P. Hoffmann20


Ion-Solid interactionsputtering Example

• Cross section of a tip deposited by FEB

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MER P. Hoffmann21


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

Prof. J. Brugger

MER P. Hoffmann22


Ion-Solid interactionSputtering Rates Rs

Courtesy John Courtesy John MelngailisMelngailis

es

i

NR = =N

ejected atoms= incoming ions

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MER P. Hoffmann23


Ion-Solid interactionVolume per Dose VD

DV = V I t

V: Volume

I: Current

t: Time

Prof. J. Brugger

MER P. Hoffmann24


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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MER P. Hoffmann25


Redeposition

redeposition

Scan speed

sample

• Sputtering yield can not be used to determine material removal

• Redeposition needs to be considered for precise structuring

Prof. J. Brugger

MER P. Hoffmann26


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

7-10none7-12Xe2

none7-107-10Cl2

SiO2AlSi

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MER P. Hoffmann27


• 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 des

desorption

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

reactionadsorption

τ

• ⎛ ⎞⎜ ⎟⎝ ⎠ 1424314243

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

Prof. J. Brugger

MER P. Hoffmann28


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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MER P. Hoffmann29


Gas-Assisted Etching

Interdigitated electrodes milled without gas-assisted etching

Interdigitated electrodes milled using gas-assisted etching

Prof. J. Brugger

MER P. Hoffmann30


Imaging• Ions and secondary electrons may be used for imaging

• Positive or negative biased detector for collecting electrons of ions, respectivelly

• 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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Prof. J. Brugger

MER P. Hoffmann31


Imaging

• Yield of secondary electrons depends on material

• Material depending contrast

• Yield of e- decreases with atomic number Z

• Low penetration depth zp of the ions(10 nm < zp < 100 nm at 30kV)⇒ higher surface sensitivity

Prof. J. Brugger

MER P. Hoffmann32


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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MER P. Hoffmann33


ApplicationsTEM-lamellas and Lift-out

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

15um15um

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MER P. Hoffmann34


Applicationscross-section

SIM image of Co tip deposited using FEB

SEM image of Co tip deposited using FEB

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MER P. Hoffmann35


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.

Prof. J. Brugger

MER P. Hoffmann36


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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MER P. Hoffmann37


ApplicationsChip Modification

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

2) Pt deposition (FIB deposition)

Documents

Focused Ion Beam Nanofabrication - LMIS1 | EPFL€¢ Theory based on quantum mechanical tunnelling (Fowler and Nordheim ... T. Fox and R. Levi-Setti: ... 2 7-12 none 7-10 Cl 2 7-10