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
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