Micro / Nanosystems TechnologyWagner / Meyners 1
Micro/Nanosystems Technology
Dr. Dirk Meyners
Prof. Wagner
Micro / Nanosystems TechnologyWagner / Meyners 2
UV - Lithography
Outline
- Lithography – Overview
- UV-Lithography
- Resolution Enhancement Techniques
- Electron Beam Lithography
- Patterning with Focused Ion Beam
Micro / Nanosystems TechnologyWagner / Meyners 3
Lithography - Overview
*
Thin Films
Implant
Diffusion Etch
Test/Sort
Polish
LithoPatterned
wafer
Lithography is at the Center of the Wafer Fabrication Process
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Moore‘s Law
• Duplication of the complexity (number of circuits/transistors on a
chip) of integrated circuits every 2 years (exponentional growth)
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
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Moore‘s Law
https://ourworldindata.org/wp-content/uploads/2013/05/Transistor-Count-over-time.png
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Lithography - Overview
• Lithography is used to produce 2 1/2-D images using radiation
sensitive resist and controlled exposure to radiation.
• The most widely used form of lithography is photolithography using
light sensitive resist.
• UV-Lithography
• X-ray lithography
• Electron Beam Lithography
• Ion Beam Lithography
• Wave length λ of radiation limits achievable resolution. (resolution of
e- and ion-beam lithography limit by scattering)
De Broglie:mv
h
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Lithography - Overview
Ten Basic Steps of
Lithography
1. Surface Preparation
2. Resist Application
3. Soft Bake
4. Alignment & Exposure
5. Develop
6. Hard Bake
7. Inspection
8. Etch
9. Resist Removal
10.Final Inspection
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Lithography - Overview
Basic Steps of Lithography
Image source:
M. Madou, Lecture Notes, 2008
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Lithography - Overview
Basic Steps of Lithography
Coat with resist
Expose
Develop
Transfer pattern
Strip resist
Etch Doping Deposit film
Resist
Substrate
Radiation
Mask
LIFT-OFF
Image source: R. Zengerle, lecture notes, Mikrosystemtechnik
Micro / Nanosystems TechnologyWagner / Meyners 10
• Alternative scheme:
Hard masks
(1) Structure transfer to
a deposited layer on
the substrate
(2) Patterned layer is
used as hard mask in
a second etching step
[R. Zengerle, Mikrosystemtechnik, lecture notes]
Lithography - Overview
Basic Steps of Lithography
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Lithography - Overview
• A resist is a radiation sensitive polymer.
• patterned selectively
• resistant to following processing
Negative resist: Prints a pattern that is opposite of the pattern that is
on the mask.
Positive resist: Prints a pattern that is the same as the pattern on the
mask.
Image source:
R. Zengerle, lecture notes,
Mikrosystemtechnik
Micro / Nanosystems TechnologyWagner / Meyners 12
Lithography - Overview
• Negative lithography
Island
silicon substrate
oxide
photoresist
Window
Areas exposed to light become polymerized and resist the develop chemical.
Resulting pattern after the resist is developed.
photoresist
oxide
silicon substrate
Ultraviolet Light
Exposed area
of photoresist
Shadow on
photoresist
Chrome island
on glass mask
Image source:
M. Madou, Lecture Notes, 2008
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Lithography - Overview
• Positive lithography
silicon substrate
oxide
photoresist
Island
Window
Areas exposed to light become soluble.
Resulting pattern after the resist is developed.
Shadow on
photoresist
Exposed area
of photoresist
Chrome island
on glass mask
photoresist
silicon substrate
oxide
Ultraviolet Light
Image source:
M. Madou, Lecture Notes, 2008
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Lithography - Overview
• During exposure with UV-light the photo active compound
DiazoNaphtoQuinone- (DNQ-) sulfonate (left) separates a N2
molecule (middle), and converts into indene carboxylic acid (right)
requiring H2O. Compared to unexposed DNQsulfonate, the
carboxylic acid yields a resist development rate (alkaline solubility)
several orders of magnitude higher.
• quantum efficiency ≈ 20 .. 30 %
• backbone molecules (viscosity)
Example: (DNQ-) Reaction (positive)
λ λ
[www.microchemicals.eu]
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UV - Lithography
Outline
- Lithography – Overview
- UV-Lithography
- Resolution Enhancement Techniques
- Electron Beam Lithography
- Patterning with Focused Ion Beam
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1. Surface Preparation by Wafer Priming (HMDS)
• Adhesion of the resist is often insufficient on Si or SiO2
• HMDS as adhesion promoter
HMDS• Dehydration bake in enclosed
chamber with exhaust
• Clean and dry wafer surface
(hydrophobic)
• Hexamethyldisilazane (HMDS)
• Temp ~ 200 - 250°C
• Time ~ 60 sec
[M. Madou, Fundamentals of Microfabrication, Lecture Notes]
UV - Lithography
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• unpolar methyl groups form
hydrophobic surface with
corresponding resist
wettability and adhesion
HMDS
(hexamethyldisilazane)
[M. Madou, Fundamentals
of Microfabrication, Lecture
Notes]
UV - Lithography
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2. Resist Application
• Wafer or substrate are held
onto vacuum chuck
• Dispense few milliliters of
photoresist
• Slow spin ~ 500 rpm
• Ramp up to ~ 3000 - 5000
rpm
• Quality issues:
– thickness
– uniformity
– particles & defects
vacuum chuck
spindleto vacuum
pump
photoresist dispenser
UV - Lithography
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UV - Lithography
• Resist thickness T depends on:
– Spin speed
– Solution concentration
– Molecular weight (measured by intrinsic viscosity)
• In the equation for T, K is a calibration constant, C the polymer
concentration in grams per 100 ml solution, h the intrinsic viscosity,
and w the number of rotations per minute (rpm)
• The equation can be used to predict the thickness of the resist that
can be spun for various molecular weights and solution
concentrations of a given polymer and solvent system
w
hcKT
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Spin Coater
• Spin Coater OPTIspin ST22P
– Substrate size up to 8“
– Chucks for 4“, 6“, 8“ and
pieces
– spin speed up to 10,000 rpm
• Hot Plate
– HMDS Adhesion promoter
(C6H19NSi2)
– Temperature up to 200°
UV - Lithography
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UV - Lithography
• Spin Coating
• Spray Coating
Image source:
M. Madou, Lecture Notes, 2008
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UV - Lithography
• Partial evaporation of resist solvents
• Improves adhesion
• Improves uniformity
• Improves etch resistance
• Improves linewidth control
• Optimizes energy absorbance
characteristics of resist
3. Softbake
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UV - Lithography
• Transfers the mask image to the resist-coated wafer
• Activates photo-sensitive components of photoresist
• Quality issues: – linewidth resolution– overlay accuracy– particles & defects
UV Light Source
Mask
Resist
4. Alignment & Exposure
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Masks for optical Lithography:
Drawing with CAD
• Example for a mask
structure for microinductors
• Design the layout with
CAD-programms
UV - Lithography
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• Quartz glass plate with
structured chromium layer
(absorber pattern metal, Cr
typ. thickness: 100 nm)
• Typical costs:
– 30 €/cm2 for structures
> 5 µm
– 75 €/cm2 for structures
1 - 5 µm
– masks for 4“ wafer
approx. 400 - 2.500 €
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
Masks for optical Lithography:
UV - Lithography
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• deposit resist on chromium
coated quartz plate
• structure resist with
electron beam lithography
• develop resist
• wet etching of chromium
• resist removal
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
Fabrication of masks:
UV - Lithography
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UV - Lithography
[M. Madou, Fundamentals of Microfabrication, Lecture Notes]
Shadow Projection
Printing techniques:
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• Mask is pressed to the wafer/resist
→ in contact without gap
• (hard/soft) contact mode / vacuum
mode
+ structures in sub-μm range are
possible
+ minor failures at the pattern
transfer
– contamination of the mask
– defects by dirt particles
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Contact printing:
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• No direct contact between mask
and wafer/resist → proximity gap of
~ 30µm
+ less stress on the mask
+ higher lifetime of the mask
+ higher throughput
– lower resolution
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Proximity printing:
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• Downsizing of the mask structure by
objectives
• Step-by-step projection of the mask
structure on the wafer
• only one functional unit (e.g. chip) on the
wafer
+ enlarged masks easier in production:
• better to control
• cheaper production of masks (only
1 chip on the mask)
– equipment is expensive (lenses)
– low throughput
mirror
light source
filter
condenser lens
mask
objective
field of exposure
movable sample table
(step and repeat)
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Projection printing:
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• emission spectrum of a mask aligner with Hg light source contains
three lines
h-, i-line resist
g-, h-, i-line resist
Wavelength
Inte
nsity
[www.microchemicals.eu]
• the absorption
spectrum of the
photoresists is
matched to this Hg
emission spectrum.
• distinguish between
broadband sensitive
(g-, h-,and i-line)
photoresists, and
resists with a
absorption spectrum
more narrow
UV - Lithography
Ligth Source/Sensitivity of Resists:
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UV - LithographyResolution in contact and proximity
printing:
The theoretical resolution R is
equal to the minimum resolved
dimension with a grating mask
(bmin for a line or a space).
It is limited by diffraction to:
22
3min
zsbR
bmin : half grating period
s: gap between mask
and photoresist
surface
z: resist thickness
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UV - Lithography
Resolution in contact and proximity
printing:
Contact printing:
Typical values for a conventional system:
22
30
zRs
al)(theoretic 70
1
line)-(h 400
µm. R
µmz
nm
Proximity printing:
Typical values for a conventional system:
sRzs 2
3
al)(theoretic 4
20
line)-(h 400
µm R
µms
nm
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• Resolution is influenced by:
– wavelength (diffraction)
– gaps between resist and mask
UV - Lithography
• Reasons for a gap between resist and chromium layer of the mask:
– particles in the resist caused by either insufficient cleanroom
conditions, contaminated substrates, or expired photoresist
– bubbles in the resist film caused during dispensing, or an
insufficient delay time after refilling/diluting/moving the resist
– mask contamination by particles, or resist (previous exposures)
– rough, structured or curved (strained) substrates
– an edge bead, or a mask attached upsidedown
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Contributors to Non-Rectangular Wafer Pattern
• Lithography process variations:
– Lens aberration, misalignment, defocus, overexposure
• Sub-wavelength non-ideal optical effects due to
– Diffraction
• Those effects result in wafer pattern distortion:
– Line-end shortening, corner rounding, line-edge roughness
Micro / Nanosystems TechnologyWagner / Meyners 36
theore
tical m
ax.
resolu
tion (μ
m)
Light intensity distribution in a photo resist
film (cross section) with a - from top to
bottom- increasing gap between mask and
resist film.
[www.microchemicals.eu]
UV - Lithography
Resolution: gaps between resist and mask
resist thickness not considered
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• Resolution is influenced by:
– wavelength (diffraction)
– gaps between resist and mask
– defects in the mask
– bleaching of the photoresist
– contrast
– optical substrate properties
absorp
tion
coeff
(1/
μm
)
[www.microchemicals.eu]
UV - Lithography
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d/d
oof
the d
evelo
ped r
esis
t film
Log10 (exposure dose)
• The slope of the decay in the
contrast curve defines the
contrast
(remaining layer thickness d in
relation to the starting thickness d0)
UV - Lithography
Contrast:
high contrast
low contrast
DC (dose to clear) depends on
resist thickness, developer
concentration and development
time.
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• UV-reflecting substrates (e.g.
metallized) increase the
absorbed light dose in the resist
near the substrate
• UV-transmissive substrates
(e.g. quartz, glass, thick SiO2 on
Si, transparent polymers)
laterally guide light along the
substrate, cause reflections
from the chuck and reduce the
lateral resolution
[www.microchemicals.eu]
UV - Lithography
Resolution: optical substrate properties
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• wafer stage in a coventional mask aligner
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Alignment:
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• Pattern transfer using
several masks demands
the alignment with sub-µm
accuracy
– performance in a mask
aligner
– substrate is movable in
x, y and θ
– first mask aligned at the
flat
large scale production:
fully automatic alignment
and exposure
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Alignment:
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large scale production:
fully automatic alignment
and exposure
– subsequent masks are
aligned by means of
alignment markers
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Alignment:
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=
first mask second mask
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Alignment:
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mask 1 mask 2 mask 3
double exposure 1 + 2 multiple exposure 1 + 2 + 3
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Alignment:
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UV - Lithography
5 . Develop
Soluble areas of the resist are
dissolved by developer
chemical
• Visible patterns appear on wafer
– windows
– islands
• Quality issues:– line resolution– uniformity– particles & defects
vacuum chuck
spindle
developerdispenser
to vacuum pump
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UV - Lithography
Photoresist profiles:
– Undercut (LIFT-OFF)
– Vertical
– Overcut Dose : High
Developer: Low
Dose : Medium
Developer: Moderate
Dose : Low
Developer: Dominant
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substrate
resist
metal
Aim: Structuring metal without etch process
• structure the photoresist
• deposit metal (e.g. Pt) on the whole surface
• resolve the photoresist
– metal on resist is removed
– metal on substrate remains
• Inversion of the edge profile by image reversal technique enables the lift-off process
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Lift-Off process:
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• Double exposure of resist and chemical modification in between
→ positv resist behaves like high resolution negativ resist
solubleinsoluble
still
insoluble
solubleflood exposure
crosslinking
→ insoluble
1. Exposure 2. Temperature (~90°)
3. Exposure 4. Develop
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Image reversal:
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• Inversion of the edge profile
unexposed
exposed
Image Reversal
Standard Lithography
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
UV - Lithography
Image reversal:
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• example for different
resists for different
applications and
thickness
[www.microchemicals.eu]
UV - Lithography
Photoresists:
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• epoxy-based negativ resist
• layer thickness up to ~500µm (up to ~2mm
with several layers on top of each other)
• aspect ratio of 1:40 (high contrast of SU-8)
• high chemical resistance → application as
etching mask
[R. Zengerle, Mikrosystemtechnik, Lecture Notes]
–high volume shrinkage after postbake
–high stresses in large-scale structures
–handling more critical than for standard resists
UV - Lithography
SU-8 Technology:
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UV - Lithography
6. Hard bake
• Evaporate remaining resist
• Improve adhesion
• Higher temperature than soft
bake
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UV - Lithography
8. After inspection the pattern transfer is
performed, e.g. by etching
• Selective removal of upper layer of wafer through windows in resist:
subtractive• Two basic methods:
– wet acid etch
• Au with KI/I2• Si with HF
– dry plasma etch
• Reactive Ion Etching (RIE)
• Ion Beam Etching (IBE)• Quality issues:
– defects and particles– step height– selectivity
Alternatively• Adding materials (additive)• Two main techniques:
– Sputtering– evaporation
• Lift-Off
PlasmaPlasma
CF4CF4
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UV - Lithography
9. Resist removal (strip)
• No need for resist in the following
preparation step
• Two common methods:
– wet removal with solvent (e.g.
aceton or N-methyl-2-pyrrolidone
(NMP))
– dry plasma etching with O2-
plasma
• Followed by wet clean to remove
remaining resist and strip byproducts
O2O2
PlasmaPlasma
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UV - Lithography
10. Final inspection
• Resist has been completely removed
• Pattern on wafer matches mask pattern (positive resist)
• Quality issues:
– defects
– particles (cleanroom class)
– step height
– overlay accuracy
– critical dimensions
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