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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