U.D. ZeitnerFraunhofer Institut für Angewandte Optik und Feinmechanik

Jena

Micro- and Nano-Technology...... for Optics

3.2 Lithography

Micro- and Nano-Technology...... for Optics

3.2 Lithography

“Printing on Stones”

Map of Munich

Stone Print

Contact Printing

resist

substrate

light

mask

Mask Aligner

Mask Aligner

Mercury Emission Spectrume - lineghi

Proximity Printing

resist

substrate

light

mask

proximity gap

Projection Lithography

resist

substrate

light

mask

projection optics

The inverse microscope

microscope lithography

microscope lens projection lens

imageobject

image object

light source

light source

Photolithography Examples

ASML-Stepper

Zeiss SMT, WO 2003/075049

… for DUV-Lithography

Stepper Objective …

…aspheric lenses

Double Patterning

Principle ofhalf tone masks

Principle ofgray tone masks

brightness in the

wafer plane

0

1

2

-1

-2grating period

or pitch > λ

0

1

2

-1

-2

0

1

2

-1

-2

0 0 0

grating period

ore pitch < λ

small medium highfilling factor:

blocking of higher orders by a lens

- Sub wavelength masks- HEBS glass masks- LDW glass masks

higher orders do not exist

Physics of Half-Tone- and Gray-Tone-Masks

half tone mask

objective

gray tone image

pulse densitypulse width

type of masks

+1-1

Courtesy of K. Reimer, ISIT/FhG

Also possible:

- combinations

- Error diffusion

Half-Tone Lithography

Holography Examples

12

3 4

5

67special features:

• adjustable angle of incidence: 0deg- 55deg ( ±1deg ) • low divergence: 0.1deg• interference filter: 313nm, 365nm, 435nm

1

2

3

4

5

6

7

mercury lampcollimator polarizerinterference filtercold-light mirrormasksubstrate

Mask Aligner With Collimated Illumination

12

3 4

5

67

oblique incidence

normal incidence Suss MA6-NFH

h

ϕL

-1st0th

ϕ0ϕ-1d b

Two beam interferenceSymmetric

diffraction angles

only 0th and -1st order→ wavelength

dd

23

2 << λ

Littrow - mounting→ angle of incidence

dL

2sin

λϕ =

Parameters:

• Wavelength λ / Pitch d

• Angle of incidence ϕ• Groove depth h

Duty cycle f = b / d

rigorous calculations→ duty cycle and

groove depth of themask grating

Equal intensities

Mask

ResistSubstrate

Principle of Pattern Transfer

Experimental Results

1 µm

1 µm

Mask

Copy

Phase mask Amplitude mask

1 µm

λ/2 < p < 3λ/2 λ/2 < p < λ

pmp

p p=pm/2

Incidence Angle

à also usable for gratings with different orientations (e.g. circular gratings)

Laser Lithography

Laser Lithography – Scanning Beam

scanwidth

AOD

U~ deflection angle

substrate motion

AOM

U~ profile

mirror

focusing lens

DWL 400-FF Laser Writer

HIMT

basis system: DWL 400, Heidelberg InstrumentsLaser: λ=405nm (laser diode)max. writing field: 200mm x 200mmmin. spot size: ∼1µmautofocus system: opticalwriting mode: variable dose (max. 128 level)

spot positioning by stage movement andbeam deflectionlateral scan (width up to 200µm at max. resolution)

writing speed: 10 – 20 mm²/min on planar substrates(depending on structure)

writing on curved substrates:

substrate table: cardanic mount, tilt in two orthogonal axesmin. radius of curvature: ∼10mmmax. surface tilt angle: <10°max. sag: 30mm

DWL 400-FF Laser Writer

variable dose exposure:

development:

resist

substrate

intensity modulated

exposure beam

t1 t2x

y

e-beam,

laser beam

writing pathsubstrate

movement

• dose dependent profile depth after development process• high flexibility for arbitrary surface profiles

Lithography with variable dose exposure

refractive beam shaperdepth: 1.7µm

refractive beam shaperprofile depth: 6µm

diffractive beam shaperprofile depth: 1.2µm

refractive lens arrayprofile depth: 35µm

diffractive lens arrayprofile depth: 1.5µmdiffractive lens arrayprofile depth: 1.5µm

Laserlithography – Example Structures

x/y-stage

electron gun

detector

beam on/of control

magnetic deflection systemand objective

aperture

stage positioning system

Laser interferometer (position feedback)

Electron Beam Column

Beam Diameter (Example)

here:about 6nm beam size

with proper systems 0.5nm beam size is achievable

scattering of electrons in

the material

distribution of

deposited dose

20keV

5-8µm

(material

dependent)

Photons Electrons

complex distributionexponential absorption(Lambert-Beer)

Dose

Material Interaction

electron beam

resist

substrate

primary electrons

direction changes in

statistical order

deceleration:à numerous material

dependent secondary effects:

� secondary electrons

� Auger-electrons

� characteristic x-ray radiation

� Bremsstrahlung radiation

Electron Deceleration

primary electrons

scattering volume

increasing beam energy

resist

substrate

Interaction Volume

electron beam

resist

substrate

Monte-Carlo Simulation of Electron Scattering

Proximity Function

region 1: primary electrons

region 2: back scattered electrons

region 3: x-ray radiation and

extensions of the beam

εlogre

lative e

nerg

y d

ensity

radius

r

Proximity Function

µmr 5,00 <≤

Lrµm <≤5,0

L ... total path lengthof an electron

• exposure with high doseà atoms are ionized and can be released from the crystal

• direct image of the beam

Direct Exposure of a NaCl-Crystal

pattern, realized by a fine electron beam on a NaCl crystal

desired structure

PMMA

250µC/cm²

without diffusion

with diffusion of molecules

Statistics of the Exposure Process

10nm

FEP 171

10µC/cm²

Statistics of the Exposure Process

desired structure

without diffusion

with diffusion of molecules

10nm

comparison of structures in

the resist

PMMA

250µC/cm²

FEP 171

10µC/cm²

Statistics of the Exposure Process

desired structure

10nm

High resist sensitivity in EBL àààà no more statistical independency

Resist exposure dose (µC/cm²) e- /(10nm x 10nm) LER (nm)

PMMA 250 1560 1-3nmZEP 520 30 187 3nmFEP 171 9.5 59 10(6)nm

Photoresists photons/(10nm x 10nm)

DUV 5,000 – 20,000 2nmEUV 200 - 500 ??

FEPZEP 520PMMADUV Photoresist

experiment

(resist pattern FEP 171)

modeling parameters● dose: 0.65 e-/nm² (10 µC/cm²)

● Gauss: 30 nm

● diffusion: 10 nm

● no quenching, no proximity effect …

schematic “modeling”

(polymer deprotection)

400nm

Roughness caused by statistic electron impact

The Vistec SB350 OS e-beam writer

basis system: SB350 OS (Optics Special), Vistec Electron Beamelectron energy: 50keVmax. writing field: 300mm x 300mmmax. substrate thickness: 15mmresolution (direct write): <50nmnumber of dose levels: 128address grid: 1nmoverlay accuracy: 12nm (mask to mean)writing strategy: variable shaped beam / cell projection

vector scanwrite-on-the-fly mode

500 nm

43nm

resist grating

100nm period

wafer

The Vistec SB350 OS e-beam writer

50keV electron column substrate loading station

E-beam writing strategies

aperture

incidentbeam

cross-section

Gaussian spot

Gaussian beam

•

electron optics

•

resolution: >1nm

writing speed: low

angular apertures

Variable shaped beam

>30nm

fast

lattice aperture

shaped beam

Cell-Projection

>30nm

extreme fast

2µm2µm

E-Beam Lithography: Example Structures

photonic crystal

effective medium grating

binary grating400nm period

0 5 10 15 20 25

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

fit model: h = a·Exp(b·D) + c

a = (-54.4 ± 0.74) nm

b = (0.00139 ± 7.9E-7) cm2/µCc = (53 ± 3.1) nm

measured

fit

resis

t de

pth

[nm

]

electron dose [µC/cm2]

3µm ARP 610

exposure: 0.5A/cm2, dose layer 1.0, 1.2, 1.5µC/cm2

development:60s ARP-developer + 15s Isopropanol

20s ARP-developer + 15s Isopropanol

blazed grating

diffractive element

E-Beam Lithography: Variable Dose Exposure

N masks/exposures and etching steps

mask 1

mask 2

mask 3

8 level profile

Principle: multiple executions of a binary structuring step

2N levels

Multilevel Profile Fabrication

0 5 10 15 20 25 30 350

10

20

30

40

50

60

70

80

90

100

diffr

actio

n e

ffic

iency [%

]

number of phase levels N

Expected Diffraction Efficiency

scalar theory:

N η

=N

1sinc2η

2 40.5%4 81.1%8 95.0%16 98.7%32 99.7%

2

4

816 32

(for a grating)

90% of the design efficiency 6% misalignment allowed

pixel size à misalignment allowed

500nm 30nm

250nm 15nm

-15 -10 -5 0 5 10 15 20 25 300

20

40

60

80

100

due to random alignment error

Eff

icie

ncy n

orm

aliz

ed

to

id

ea

l e

lem

en

t [%

]

Alignment error in x and y normalized to pixel size [%]

simulation 4-level

measurement

misalignment normalized to pixel size [%]

4-level element

Diffraction Efficiency reduced by overlay error

The real diffraction efficiency depends on:

- Overlay error

- line width error

- depth error

- edge angle

- design

- wavelength

- deflection angle

- number of diffraction orders

- ....

2 4 8 16 320Nnumber of phase levels

diff

raction e

ffic

iency ηη ηη

Diffraction efficiency expected

(scalar theory)

You will not get the best efficiency with the highest number of phase levels!!!!

Diffraction Efficiency in Reality

UV - light

photo mask

resist

substrate resist coating

photolithography

development

- thermal resist melting

- or reflow in solvent

atmosphere

modeling of the melting

Courtesy of A. Schilling, IMT

Resist melting technique for micro-lens fabrication

22

4

1LLLL drrh −−=

diameter resist cylinder = diameter lens

volume resist cylinder = volume lens

curvature radius of the lens: Lr

focal length: f

refraction index: n

)( airLL nnfr −=

Ideal:

dL

hL

αR

dC

hC

resist cylinder

substrate

Simplified lens design

2

3

3

2

2

1

L

LLC

d

hhh +=

The rim angle ααααR of the lens must be larger than the wetting angle ααααW

αW

dent

αααα ≈≈≈≈ 35°and n = 1.46 NAmin ≈≈≈≈ 0.35

αWαR

If not:

How to overcome this problem?

Typical wetting angle resist substrate ca. 25 deg

NA limitation by wetting angle

1) exposure

2) development

3) reflow solvent

atmosphere

substrate

resist

light

4) baking

Reflow process

• reflow technique reduces the wetting angle• edge of pedestal or passivation limits the spreading

Wetting angle < 1deg possible

pedestal