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Electron Beam Lithography in Nanofabrication

Lee Chow

Department of Physics

University of Central Florida4/7/2011 Lecture 8 2

Electron Solid Interaction

Electron Beam Lithography

Some Applications

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Electron solid interaction

Electron interactions with matter are complex. They can be classified into two categories:1. Elastic scattering

(a) Backscattering

2. Inelastic scattering(a) Cathodoluminescence(b) S.E. production(c) Auger electron(d) phonon excitation(e) X-ray production

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The energy of the electron and the substrate used are the two major factors that determine the penetration depth of the electrons. Depend on the interactions between the produced particles and substrate, we can sample different region of the substrate.

Electron solid interaction

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Electron solid interaction

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Comparison with high energy light ions

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Comparison with X-ray and Ga ions

Photon Electron Ga ion

Energy 10 keV 10 keV 10 keV

Material Si Si Si

Depth 130 m 1 m 30 nm

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Penetration depth of electron

Increases as energy of electron increasesDecreases as the Z value of substrate increases.

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Elastic and inelastic scattering 

E0 = accelerating voltage (of electrons emitted from gun); usually 5-25 keV

(a) Elastic Interactions

•Backscattering of electrons (BSE)

(b) Inelastic Interactions

•Plasmon excitation (in metals, loosely bound outer-shell electrons are excited)

•Phonon excitation (lattice oscillations, i.e., heating)

•Secondary electron excitation (SE)

•Inner-shell ionization (Auger electrons, X-rays)

•Bremsstrahlung (continuum) x-ray generation

•Cathodoluminescence radiation (non-metal valence shell phenomenon)

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Backscatter Electron Production

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Secondary Electron Production

SE imaging: the signal is from the top 5 nm in metals, and the top 50 nm in insulators. Thus, fine scale surface features are imaged.

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Energy spectrum of emitted electrons

(a) Regions I and II are backscattering electrons,Region III is secondary electron

(b) Dash line is measured and solid line is calculated.

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Cathodoluminescence of ZnO

Sn doped

Al doped

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Cathodoluminescence of ZnO

Sn doped ZnO nanorod

SEM image CL image

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X-ray Generation

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Characteristic X-ray emission

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Electron range in solids( 2 keV >E > 40 keV)

Please note that there are several versions of the above equation. A simple version is given next. 

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Range of electrons in Solids

68.1)( oEK

mR

K = 0.064 = density (g/cm3)Eo = Energy in (keV)

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Mean free path of electron in solid(Between 1 eV to 1000 eV)

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• At very low energies, the electrons do not have a very efficient way of losing energy to the crystal lattice, so the mean free path is long.

• At intermediate energies (on the order of 50 eV), the incident electrons can very easily lose energy by creating electronic excitations or ionization events in the solid, and the mean free path is very short.

• At very high energies, the electrons are moving so fast that they literally "zip by" the other atoms so fast that the electrons of the stationary atoms do not have time to respond.

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Electron Beam Lithography

• Types of EBL

1. Electron Beam Direct Write

2. Electron Projection Lithography

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oThroughput of direct writing is very low : research tool or low pattern density manufacturingoProjection stepper (EPL) is in development stage only (primarily by Nikon).oMask making is the biggest challenge for the projection methodoBack-scattering and secondary electron result in proximity effect –reduce resolution with dense patterns.oOperates in high vacuum (10-6 –10-10 torr) –slow and expensive

Issues needed to be resolved

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• The advantages of electron lithography are: (1) Generation of micron and submicron resist geometries (2) Highly automated and precisely controlled.(3) Greater depth of focus (4) Direct patterning without a mask

• The biggest disadvantage of electron lithography is its low throughput (approximately 5 wafers / hour at less than 0.1 µ resolution). Therefore, electron lithography is primarily used in the production of photomasksand in situations that require small number of custom circuits.

Advantages and disadvantages of EBL

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Electron Beam Direct Write• An electron gun or electron 

source that supplies the electrons. 

• An electron column that 'shapes' and focuses the electron beam. 

• A mechanical stage that positions the wafer under the electron beam. 

• A wafer handling system that automatically feeds wafers to the system and unloads them after processing. 

• A computer system that controls the equipment. 

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Specifications of a real Instrument

Raith150

• Beam size ≤ 2nm @ 20 keV

• Beam energy 100eV ‐ 30 keV

• Minimum line width 20 nm

• Import file format GDSII, DXF, CIF, ASCII, BMP 

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Comparison of three different systems

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Electron beam resists

1. Important parameters

2. Types of resist

3. Resist limitations

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EBL resists

Types of resist• Positive resist Polymethyl

methacrylate (PMMA)

• Negative resist

Important parameters Resolution (nm) Sensitivity (C/cm^2)

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PMMA is an excellent e-beam resist

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Common Resists for e‐beam

• Positive resists– ZEP520A

+ good etch resistance

+ fast

+ good resolution (~ 10nm)

- expensive ($3/mL)

– PMMA+ cheap ($1/mL)

+ good for liftoff

+ high resolution (< 10nm)

- poor etch resistance

- slow

• Negative resist– XR‐1541  (HSQ)

+ good etch resistance (HSQ is basically SiO2)

+ excellent resolution (6.5nm)

- slow

- expensive ($4/mL)

– ma‐N 2403 (Novolak)+ good etch resistance

+ optical DUV exposable

+ faster than HSQ

± moderately priced ($2/mL)

- poor adhesion to quartz

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Resist Comparison

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

no

rmal

ized

res

ist

thic

knes

s

100 1000 800 600 500 400 300 200 2000 3000

dose (uC/cm2)

HSQ

PMMA

ZEP

resist200 480 1280

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Metal Liftoff

evaporate metal ontopatterned resist

strip resist

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Forward Scattering (α)

• as electrons enter resist, they experience small angle scattering, effectively broadening the initial beam diameter

• forward scattering is minimized by using the thinnest possible resist and highest accelerating voltage

5.1)/(9.0 btf VRd

df = effective beam diameter (nm)Rt = resist thickness (nm)Vb = acceleration voltage (kV)

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Backscattering (β)

• as electrons pass thru resist and enter substrate, many will undergo large angle scattering events

• these electrons may return back into the resist at a significant distance from the incident beam, causing additional resist exposure → this is called the proximity effect

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Effect of electron energy

Source: SPIE Handbook of Microlithography, Section 2.3 Electron‐Solid Interactions

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Simulated Electron Energy Profile

Source: SPIE Handbook of Microlithography, Section 2.3 Electron‐Solid Interactions

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Proximity Effect Correction by Shape Modulation

original CAD pattern

simulated doseprofile

calculated shape modification to achieve desired

line

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Dose Dependencies

pattern size

pattern density

required dose

required dose

resist thickness required dose

acceleration voltage required dose

substrate AMU required dose

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Applications of EBL

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Gratings used in distributed Feedback Lasers.

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Micro-ring resonators

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Nanofluidic Channels

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Polygon/Array of Dots

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Multiple Pattern Arrays

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Fine Line Structure

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Electron Beam Lithography at UCF

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Electron Beam Lithography at UCF

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