Nano fabrication by e-beam lithographie - uni-jena.de...Introduction to Nanooptics, Summer Term...

Introduction to nanooptics, Summer Term 2012, Abbe School of Photonics, FSU Jena, Prof. Thomas Pertsch

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Lecture 14

Nano fabrication by e-beam lithographie

Introduction to Nanooptics, Summer Term 2012, Abbe School of Photonics, Jena, Prof. Thomas Pertsch

• Introduction • Motivation • General EBL process • Fundamentals

• Basics and Physics of EBL • Resist Technology • Charging • EBL Writing Strategies • Interaction between E-beam and Substrate • Proximity Effect • Proximity Function • Periodic Structures • Resolution Limits in EBL

• Some Special Exposure Techniques • Cell Projection Exposure • Variable Dose Exposure • Mask Preparation for Lift-Off • Overlay Exposure

Electron Beam Lithography - EBL


Example: Diffraction grating on 9“ fused silica mask blank

Electron Beam Lithography


Def Lithography: “Method for printing on a smooth surfcace” Photolith., UV lith., Interference lith., Scanning probe lith., X-Ray lith., EBL,…

Why EBL? • far-field imaging technique à diffraction resolution limit (Abbe)

• sub-100nm-features: lde Broglie ~100nm à Uelectron ~12 eV

• high technological standard of electron beam control and manipulation

• capable to structure arbitrary (non-periodic, non-symmetric) patterns

Applications of EBL • mask fabrication (e.g. chromium on glass)

• direct writing (rapid prototyping)

• nano devices in research and development (R&D)

Requirements of EBL • Extremly complex technological background, clean room

• Neccessatiy of experienced staff to operate an EBL system

Motivation


Positive resist Negative resist

Functional layer (optional)

Substrate

Resist Exposure

Development

Dry Etching Deposition of functional layer

Removal of Resist

Lift-Off

Final Element

Subtractive method Additive method

Final Element

...

General EBL Process


1) Sample preparation

Silica substrate

Resist Thin TCO layer

2) E-Beam Lithography

3) Resist development

4) Layer deposition

Layer stack (Au, MgO, Au)

5) Covered resist structure

6) Wet-chemical lift-off

Process Development: Lift-Off


Lift-Off: Critical Steps Achieve an undercut resist profile

Top-Resist (low sensitivity)

E-Beam

(Thick) Resist & • Low beam energy • Overexposure • Overdevelopment

E-Beam

Bottom-resist (high sensitivity)

à Rule of thumb: Resist thickness ~ 2-3x thickness of layer(s) to be lifted

Thick Resist: J Clean Lift-Off, elevated Lift-Layer L Lateral resolution diminished due

to enhanced electron scattering

Thin Resist: J Higher lateral resolution

L Less undercut, Lift-Off unstable

Limits of aspect ratio


Lift-Off: Critical Steps Highly Directional Deposition

Flawless Mask Lift-Off • Long soaking of sample in solvent (>10 hours) • Support Lift-Off by elevated temperature and Mega- / Ultrasonic

Sputtering:

Thermal Evaporation:

• Evaporation is preferable • Maximize distance between deposition source and target (>0,5 m) • Clear / Activate Sample Surface à Ar-Plasma, Heating,...


Etching: Critical Steps

• Atomic lattices of different materials do not „fit“ perfectly • Resist coating and tempering (180° C) leads to stress induced bubbles and cracks • Not suitable for EBL L

• No reactive dry etching for gold available • Physical layer removal by Ion Beam Etching (IBE) • Very low selectivity, thick resist mask needed, no etch stop L

Dry Etching

Resist Coating & Tempering

• IBE on metals can lead to redepostion and metal-resist compounds • These turn out to be chemically very stable and hardly soluble • Pattern often spoiled by „garden fences“ L

Resist Removal


electromagnetic electrostatic

F = q · (E + v x B)

Either magnetic or electrostatic fields can be used to focus electrons

just as glass lenses are used to focus rays of light.

à Electron Optics by an electro-magnetical lens system

à Analogy to Beam-Optics

E-Beam Deflection and Focus

Fundamentals


variable angular apertures

cross over

shaped beam

Variable shaped beam

circular aperture

cross over

Gaussian spot

Gaussian beam

·

electron optics

·

resolution: >5nm >50nm

writing speed: low fast

E-Beam Imaging Systems

electron optics

Fundamentals


Fundamentals

Variable shaped beam Gaussian beam E-Beam Imaging Systems

E-beam writer LION LV1 masks max. 5” x 5” wafer max. 5” beam 2nm overlay 50nm incr. 2.5 (0.1) nm

E-beam writer Vistec SB 350 masks max. 9” x 9” wafer max. 9” resolution 50nm overlay 14nm incr. 1.0nm


Fragmentation of PMMA (Polymethylmethacrylate) during exposure

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 O O

O C

C H

H

H

C

O

H

H H

o

Result: PMMA-chains split, enhanced solubility in MIBK (Methylisobutylketone)

Fundamentals


Change of resist solubility a liquid developer

Positive Resist: Average molecular weight reduced by exposure → exposed area is solved much faster in developer and thus removed

Negative Resist: Average molecular weight increased by exposure (cross-linking of molecules) → unexposed area is removed in developer

e.g. PMMA: MIBK : Isopropanol 1:1, t=30...60 sec, Stop in Isopropanol, N2-Drying

Fundamentals


• Introduction • Motivation • General EBL process • Fundamentals

• Basics and Physics of EBL • Resist Technology • Charging • EBL Writing Strategies • Interaction between E-beam and Substrate • Proximity Effect • Proximity Function • Periodic Structures • Resolution Limits in EBL

• Some Special Exposure Techniques • Cell Projection Exposure • Variable Dose Exposure • Mask Preparation for Lift-Off • Overlay Exposure

Electron Beam Lithography - EBL


Resist Technology

Sample Preparation: Coating & Tempering

1. Spin Coating 2. Tempering (Pre-Exposure-Bake)

Hotplate (or oven)

Typical tempering figures: T= 90...210 °C t = 5...30 min


Dose D = Deposited electrical charge per area unit [µC/cm2]

I – current, t – exposure time, F – area, j – current density

Resist properties: Sensitivity & Contrast

Contrast curve measurement: Expose uniform square areas (e.g. 100µm) with increasing dose

Increase of Dose Value

Resist Technology

tjF

tID ×=×

=


Contrast curve measurement



Real (non-binary) resist

0 %

100 %

Resist hight after developm

ent

Ideal (binary) resist

Sensitivity D0 = Threshold dose, for which a (large) area of a

given resist is completely removed (clearing dose)

D0

D0


Resist Technology


∞

D1 D0


10 0.0

0.2

0.4

0.6

0.8

1.0

Ideal resist g = Real resist (Zep520) =18

Dose [µC/cm2]

St

anda

rdiz

ed R

esis

t Hig

ht

g

Quantitative measure for binary behaviour of a resist

(slope in contrast kurve)

Resist Technology

1

1

0log-

úúû

ù

êêë

é÷÷ø

öççè

æ=

DD

gContrast

Nano fabrication by e-beam lithographie - uni-jena.de...Introduction to Nanooptics, Summer Term...

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