INTRODUZIONE ALLE NANOTECNOLOGIE

Massimo De Vittorio

Lab. Nanotecnologie NNL ISUFI - Università del Salento

Il Nanoworld Nel mondo microscopico, gli elettroni sfrecciano a velocità relativistiche e il mondo è quasi completamente vuoto.

Come nell’infinitamente grande, anche nell’infinitamente piccolo sembra esserci sempre qualcosa oltre.

L’atomo e’ formato da elettroni protoni e neutroni ma neanche queste particelle sono fondamentali e sono costituite dai quark (modello standard). C'è anche un'altra classe di particelle materiali fondamentali, i leptoni (un esempio è l'elettrone). Ci sono due generi di particelle: particelle che sono materia (come gli elettroni, i protoni, i neutroni, e i quark) e particelle che mediano le

interazioni (come i fotoni). Oltre a gravità e l'elettromagnetismo, ci sono altre due interazioni, che noi non possiamo notare dal momento che la loro sfera d'azione non è molto maggiore del nucleo atomico: l’interazione debole e l’interazione

forte

Nanoworld

Approcci tecnologici

“Top-down” –partendo da un componente “grande” o di volume (ad es un wafer) si rimuove localmente il materiale (come in una scultura).

In nanotecnologia : litografie ed attacchi chimici applicati ai circuiti integrati

“Bottom-up” – si assemblano componenti piccoli per costruire qualcosa di più complesso.

In nanotecnologia: auto-assemblaggio di atomi e molecole per costruire sistemi chimici e biologici

La “driving force”

L’architettura convenzionale

Miniaturizzazione estrema

Semiconduttori

1950

2000

Gordon Moore

• Nel 1965 predisse la famosa legge omonima secondo cui il numero di transistor per chip sarebbe raddoppiato ogni 2 anni.

• Questa legge è stata rispettata negli ultimi 40 anni e si pensa possa valere sino al 2010

• Dopo …

Technology roadmap

Top-down fabrication

• It makes use of depositing thin films, then “lithography” and plasma etching to make films into desired patterns on a silicon wafer.

Method used by integrated circuit industry to fabricate computer chips down to ~ 15 nm size

Intel’s transistors

Perchè rimpicciolire?

TREND of

MINIATURIZATION

L’approccio TOP-DOWN

Co

ati

ng

or

str

ipp

ing

ste

p

Remover

Lift-Off Etching

metal Remover

Pattern

Transfer

substrate

substrate resist

substrate

Spin coating

Lithography

substrate

Developing

Wafer

After x process steps

Top down approach

Typical Instrumentation

Process step substrate

substrate

substrate

Spin coating

Exposure

substrate

Developing

Cleaning

Spin coating

Film thickness measurement

Exposure

Developing

Inspection

Instrumentation

Film thickness measurement

Cleaning

Spin coating

Exposure

Developing

Inspection

• wet bench • (eye-) shower for accidents with acids • storage for chemicals • stove / hotplate • refrigerator • spin coater • stove / hotplate • Film Thickness Probe • Lithographic tool

• wet bench • storage for chemicals • optical microscope • sputtering machine • SEM

Hi-Res Lithographies

Mask transfer: •Optical Lithography DUV with Phase shifting masks (~50 nm) •X-Ray/EUV lithography (~20 nm) •Imprint Lithography (~10 nm)

Direct writing Electron Beam Lithography ( ~ 10 nm) Ion Beam Lithography ( ~ 10 nm) Scanning Proximal Probe Lithographies ( ~ 1 nm) Holographic Litography ( ~ 100 nm)

The minimum feature:

0.5 /NA=0.5 / nsen

Phase shift lithography

The ligth travelling in air for a distance a experiences a phase change:

aa

2=

Whereas through a trasparent material :

nan

2=

)1(2

= na

°= 180)1(2

=n

a

Immersion UV lithography It requires high refractive index liquids and fluids.

Water refractive index rapidly increases below 250 nm

(@ 193 nm is n=1.44, with a low absorption value of 0.05 cm-1).

By replacing the air in the image space with liquid it is possible to extend the DUV lithography @ 193 nm to the sub-45 nm node.

The minimum feature:

0.5 /NA=0.5 / nsen

Electron Beam Lithography

Source: SPIE Handbook of Microlithography

Technical setup of EBL tools

Beam deflection (E-static/magnetic)

Electro-magnetic Electro-static

F = q (E + v B)

Either magnetic or electrostatic fields can be used to focus electrons just as glass lenses are

used to focus rays of light.

Spherical aberration Chromatic aberration

Electron lenses have very poor performance (spherical and chromatic aberrations) compared to light lenses; thus electrons must be kept very close

to the axis - small

Beam deflection (E-static/magnetic)

Focus/Stigmator correction

stationary stage moving stage

fields stripes

versus

"write-on-the-fly“

Stage Movement Strategies

Raith Application Notes

raster scan vector scan

versus

round (Gaussian) beam shaped beam

versus

EBL Writing Strategies

Raith Application Notes

•dg : source demagnification dg=dv/M-1

•ds : spherical aberrations ds=1/2 Cs 3

•dc : chromatic aberration dc= Cc V/Vbeam

•ddiff : diffraction ddiff=1.2 /

diffraction is not so important (@ 15 keV , =0.1Å. = 10mrad):ddiff=1.2 / = 1 nm

2

d

2

c

2

s

2

g ddddd +++=

Resolution

• Positive resists usually work by polymer chain

scission; exposed polymer becomes more

soluble

• Negative resists usually work by cross linking

between polymer chains; exposed polymer

becomes insoluble

Resist interactions

(D. F. Kyser and N. S. Viswanathan, J. Vac. Sci. Technol. 12(6), 1305-1308 (1975))

C

C

C

C C

C

O

O

H

H

H

H H

H H

H H

H

H

H

H

H

H

H

C

C

C

C

O

O

n

C H

H

H

H

H

H

H

H

C

C

C

H

H

C C C

C

C

C

C C C

C

C

C

C

C C

C

C

O

O

H

H

H

H H

H

H

O

O O H

H

H

H

H

H

H

O

O

H

H H

H

H

H

H

O

H

H

H

C

H

C

C

C C

H

H H

H

H

H

H

m

C H

C

C

C

O

O

H H

H

H

H

C OO

OC

C H

H

H

C

O

H

H H

o

Induced chain scission (PMMA)

PATTERN TRANSFER: The Etching

Wet Etching

Reagenti

Diffusione Diffusione

Reazione

Substrato

Prodotti di reazione

Wet etching of GaAs/AlAs

= Ga

= As

<110> <111>

<111>

GaAs-AlGaAs <100> HCl:HF:H2O (10:1:1000) @ 278 K

(011)

(011)

GaAs-AlGaAs <100> H3PO4:H2O2:H2O (3:1:50) @ 300 K

Dry etching

Dry etching mechanisms

Physical etching (sputtering) Chemical etching

physical + chemical etching = RIE

Deep Reactive Ion Etching (DRIE)

(Electron Cyclotron Resonance-ECR RIE):

• charged particles are accelerated by a magnetic field created at microwaves

• dense plasma and low pressures

• separate control of the ion density and of their kinetic energy

• higher selectivity than RIE

(Inductively Coupled Plasma-ICP): Two separate area and RF power for the plasma and sample electrode

Deep Reactive Ion Etching (DRIE)

by ICP techno logy lower

etching times are required due

to the high density plasma at low pressure, which improves

anisotropy, e tch-rate and

selectivity

Microscopie

Atomic force Microscope

ab

dc

i r

Microscopio STM

I~ e-2dk

(Source: IBM)

NSOM – Near Field Scanning Optical Microscope

12 micron

Semiconduttori, Metalli, Ossidi, Litografie Fisica/Ingegneria

Elettronica ICT Automotive

Inorganici e organici, plastiche e polimeri Fisica/Chimica/Ingegneria

Nuova elettronica Packaging Materiali compositi Ambiente Energia Aerospace

Sistemi biologici Biocompatibilita’ Fisica/Chimica/Biologia/ Ingegneria

Materiali e protesi Diagnostica Nuovi medicinali Agroalimentare

Biomimesi Software biologico

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