U.D. ZeitnerFraunhofer Institut für Angewandte Optik und Feinmechanik

Jena

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

3.2 Lithography

“Printing on Stones”

Map of Munich

Stone Print

Wikipedia

Wafer

Mask

Shadow Printing

Photomask

Curtesy: R. Völkel, Suss Microoptics

Contact Printing

resist

substrate

light

mask

Mask Aligner

Mask Aligner

Mercury Emission Spectrum

e - lineghi

high pressure

Hg-vapor lamp

Proximity Printing

resist

substrate

light

mask

proximity gap

Pattern Generation by Photolithography

mask illumination

photomask

diffraction pattern

reduction of resolution with increasing z

z

Standard contact photolithography with a Mask Aligner:

geometric shadow printing

The inverse microscope

microscope lithography

microscope lens projection lens

imageobject

image object

light source

light source

Projection Lithography

resist

substrate

light

mask

projection

optics

𝑅 = 𝑘1𝜆

𝑁𝐴

Resolution:

R … minimum feature size

k1 … optics dependent factor

… wavelength

NA …numerical aperture of

imaging system

High-End Lithography Tool

DUV lithography stepper, =193nm

(ASML)

very low flexibility

EUV lithography stepper, =13.5nm

(ASML)

microelectronic chips

on Si-wafers

Photo Resistre

sis

t th

ickness a

fter

develo

pm

ent

exposure dose D

“hard“ resistD1

D2

1

1

210log

D

D

x

Dose

resist

Dth

UV-exposure:

Photoinitiator creates reactive

species

Chemical solubility in alkaline

media changes

Example: DNQ-based positive resist:

Positive Resist:

Printing Result: Hard Resist

Dose

Dth

x

resist

Resist pattern:

(almost) binary profile

Gradation Curve

resis

t th

ickness a

fter

develo

pm

ent

exposure dose D

“hard“ resist

“soft“ resist

suitable for

binary pattern

dose range for variable dose writing of

continuous surface reliefs

D1

D2

1

1

210log

D

D

Printing Result: Soft Resist

Dose

x

resist

Resist pattern:

continuous surface profile

(typically nonlinear wrt. exposure dose)

Technology for continuous profiles

variable dose exposure:

development:

resist

substrate

intensity modulated

exposure beam

t1 t2

dose dependent profile depth in

resist after development process

proportional transfer (RIE):

Ions (e.g. CF4)

element profile transferred

into substrate material

Photolithography Examples

ASML-Stepper

Zeiss SMT, WO 2003/075049

… for DUV-Lithography

Stepper Objective …

…aspheric lenses

Double Patterning

Pre-Compensation of Diffraction Effects

Optical Proximity Correction (OPC)

mask layout

image on wafer

without OPC with OPC

edge

rounding

line

shortening

serifs

Example:

Principle of

half tone masks

Principle of

gray tone masks

brightness in the

wafer plane

0

1

2

-1

-2grating period

or pitch >

0

1

2

-1

-2

0

1

2

-1

-2

0 0 0

grating period

ore pitch <

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

Example: resist structure

laser beam 1 laser beam 2

source:

Horiba Jobin Yvon

lithographic exposure with

an interference pattern

substrate

resist

Holography – Setup

Amplitude split by beam-splitter

(Ar+ laser)

(pinhole for spatial coherence)

Holography Examples

single exposure two crossed exposures

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 lamp

collimator

polarizer

interference filter

cold-light mirror

mask

substrate

Mask Aligner With Collimated Illumination

12

3 4

5

67

oblique incidence

normal incidence Suss MA6-NFH

h

L

-1st0th0-1

d b

Two beam interferenceSymmetric

diffraction angles

only 0th and -1st order

wavelength

dd

23

2

Littrow - mounting

angle of incidence

dL

2sin

Parameters:

• Wavelength / Pitch d

• Angle of incidence

• Groove depth h

Duty cycle f = b / d

rigorous calculations

duty cycle and

groove depth of the

mask grating

Equal intensities

Mask

ResistSubstrate

Principle of Pattern Transfer

Experimental Results

1 µm

1 µm

Mask

Copy

Phase mask Amplitude mask

1 µm

/2 < p < 3/2 /2 < p <

pmp

p p=pm/2

Incidence Angle

also usable for gratings with different

orientations (e.g. circular gratings)

Laser Lithography

Laser Lithography – Scanning Beam

scan

width

AOD

U~ deflection angle

substrate motion

AOM

U~ profile

mirror

focusing lens

DWL 400-FF Laser Writer

HIMT

basis system: DWL 400, Heidelberg Instruments

Laser: =405nm (laser diode)

max. writing field: 200mm x 200mm

min. spot size: 1µm

autofocus system: optical

writing mode: variable dose (max. 128 level)

spot positioning by stage movement and

beam deflection

lateral 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 axes

min. radius of curvature: 10mm

max. 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 shaper

depth: 1.7µmrefractive beam shaper

profile depth: 6µm

diffractive beam shaper

profile depth: 1.2µm

refractive lens array

profile depth: 35µm

diffractive lens array

profile depth: 1.5µm

Laserlithography – Example Structures

x/y-stage

electron gun

detector

beam on/of control

magnetic deflection system

and 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: 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

log

rela

tive e

nerg

y d

ensity

radius

r

Proximity Function

µmr 5,00

Lrµm 5,0

L ... total path length

of an electron

• exposure with high dose

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

no more statistical independency

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

PMMA 250 1560 1-3nm

ZEP 520 30 187 3nm

FEP 171 9.5 59 10(6)nm

Photoresists photons/(10nm x 10nm)

DUV 5,000 – 20,000 2nm

EUV 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 Beam

electron energy: 50keV

max. writing field: 300mm x 300mm

max. substrate thickness: 15mm

resolution (direct write): <50nm

number of dose levels: 128

address grid: 1nm

overlay accuracy: 12nm (mask to mean)

writing strategy: variable shaped beam / cell projection

vector scan

write-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

incident

beam

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

2µm

E-Beam Lithography: Example Structures

photonic crystal

effective medium grating

binary grating

400nm 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/µC

c = (53 3.1) nm

measured

fit

resis

t depth

[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

diff

ract

ion

effic

ien

cy [%

]

number of phase levels N

Expected Diffraction Efficiency

scalar theory:

N h

N

1sinc 2h

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 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

Effic

iency n

orm

aliz

ed t

o idea

l e

lem

ent

[%]

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

diffr

action e

ffic

iency h

Diffraction efficiency expected

(scalar theory)

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

Diffraction Efficiency in Reality

Surface tension generates small droplets with ideally

spherical surface shape

lens

Micro-Lenses in Nature

Water droplets

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

R

dC

hC

resist cylinder

substrate

Simplified lens design

2

3

3

2

2

1

L

LLC

d

hhh

The rim angle R of the lens must be

larger than the wetting angle W

W

dent

35° and n = 1.46 NAmin 0.35

WR

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