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Lecture 13
Basic PhotolithographyChapter 12 Wolf and Tauber
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AnnouncementsHomework:
• Homework 3 is due today, please hand them
in at the front.
• Will be returned one week from Thursday (16th
Nov).
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Announcements
• You are expected to produce a 4-5 page term paper on a
selected topic (from a list).
• Details / regulations are on the course website.
Term Paper:
• Term paper contributes 25% of course grade.
• You should have all been assigned your first-choice topic.
• The term paper should be handed in at the start of class on
Tuesday 21st November.
• The term paper will be returned to you in class on Thursday
30th November.
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Useful LinksBerkley:
• http://www-inst.eecs.berkeley.edu/~ee143/fa10/lectures/Lec_04.pdf
University of Michigan:
• http://web.eecs.umich.edu/~peicheng/teaching/EECS598_06_Winter/Lectu
re%2016%20-%20Mar%2009.pdf
MIT:
• http://www-
mtl.mit.edu/researchgroups/hackman/6152J/SP_2004/lectures/sp_2005_Le
cture09.pdf
KTH:
• https://www.kth.se/social/upload/4f3d0e38f276545a2b000003/Lecture%20
9%20Litho.pdf
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Lecture 13• Overview of Photolithography Process
• Mask Fabrication.
• Photoresists.
• Photoresist Deposition.
• Exposure.
• Development.
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Overview of
Photolithography Process
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Feature Sizes
Way of
quantifying
tolerance in
distance of
mask from
wafer
Deep ultra-
violet
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Patterning Wafers• Overall the process of patterning a wafer can be
broadly divided into 3 steps:
Mask Design Wafer ExposureMask Writing
• We are interested in the process of wafer exposure.
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The Photolithography Process• Apply photoresist.
PR
Substrate
Ap
ply
DevelopEtchStrip
Expose
Mask
• Expose photoresist through a patterned mask or reticle.
• Develop PR by immersing it in a solvent which preferentially
dissolves the PR of higher solubility.
Photoresist
• Process the exposed part of the wafer.
• Strip away the remaining photoresist.
• Inspect pattern.
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Photo room
Photo Process Flowchart
Clean
Exposure
Apply HMDS
Adhesion Promoter
Develop
prebakeSpin Coat
Plasma
“descum”postbake
Process
(Etch / Implant/ Lift-off)
Strip
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Mask Fabrication
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Mask Fabrication• Starting material for reticle
manufacturing is ~80 nm thick film of
chromium covered with resist and
anti-reflective coating (ARC).
• Chromium has very good adhesion
and opaque properties.
• Substrate: quartz glass plate.
• Patterned by direct writing using e-
beam or laser usually wet etching of
Cr after exposure.
• 4 or 5× magnification is normal for projection lithography.
• Pellicle used for dust protection of reticle.
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Optical Proximity Correction• As we will see later, the wave-nature of light means that we
cannot exactly recreate the features on a wafer:
Divergent
light sourceCollimating
lens
Aperture
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Optical Proximity Correction• Optical Proximity
Correction (OPC):
Clever mask
engineering based on
software algorithms
can compensate
some of this error.
• This requires
sophisticated
computer modeling.
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OPC Examples
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Photoresists
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Photoresists• Photoresist (PR): An organic compound (often a polymer) with
a photoactive component (PAC) whose solubility changes
upon exposure to radiation (light).
• Positive Photoresist:
• Irradiated regions become more soluble than non-
irradiated regions.
Expose
Mask
DevelopEtchStrip
More soluble
regions
hnN2
R
O
R
CO
+ N2
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Photoresists• Photoresist (PR): An organic compound (often a polymer) with
a photoactive component (PAC) whose solubility changes
upon exposure to radiation (light).
• Negative Photoresist:
• Irradiated regions become less soluble than non-irradiated
regions.
Expose
Mask
More soluble
regions
DevelopEtchStrip
hn
SU8
Cross-linked
polymer
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Comparison of Photoresists
Characteristic Positive Negative
Adhesion to Silicon Fair Excellent
Relative Cost More expensive Less expensive
Developer Base Aqueous Organic
Solubility in the
developer
Exposed region is
soluble
Exposed region is
insoluble
Minimum Feature 0.5 µm 2 µm
Step Coverage Better Lower
Wet Chemical
ResistanceFair Excellent
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Key Parameters of a Photoresist• Resolution: The smallest opening or island structure that can
be made under a given set of process conditions (related to
contrast).
• Registration: Overlay accuracy from layer to layer.
• Sensitivity: The number of photons it takes to cause the
chemical response in the PR. A resist is more sensitive if it
takes a lower dose to reproduce the mask geometry on the
wafer.
• Shelf life: The time you can reliably store a PR.
• Etch Resistance: The ability of the resist which remains on the
wafer after it is patterned to withstand the process
environment that the exposed wafer is subjected to.
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Photoresist Deposition
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HMDS: Adhesion Promoter• HMDS is the common name for
hexamethyldisilazane: ((CH3)3Si)2NH.
H H H H
Si Si O
Si Si OO
O Si
Si SiO
O Si
O O O O
Si Si O
Si Si OO
O Si
Si SiO
O Si
O O O O
SiH3C
CH
3
CH3 SiH3C
CH
3
CH3SiH3CC
H3
CH3 SiH3C
CH
3
CH3
• Common photo-resists do not wet the
surface of H-terminated Si/SiO2 very
well.
• HMDS is applied to the
surface to improve wetting
before the photoresist is
deposited.
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HMDS: Adhesion Promoter• HMDS is applied in vapor phase.
• But first hydroxyl groups must be removed
from the wafer surface.
H H H H
Si Si O
Si Si OO
O Si
Si SiO
O Si
O O O O
Si Si O
Si Si OO
O Si
Si SiO
O Si
• Cleaning or oxygen plasma can be used.
• Must be conducted in inert atmosphere.
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HMDS: Adhesion Promoter• HMDS is applied in vapor phase.
Industrially:
• HMDS is applied using a
bubbler.
In the lab:HMDS-rich
atmosphere
Cleaned
Wafer
Hotplate
(~100°C)
HMDS in
beaker
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Photoresist Deposition• Photoresist is deposited by spin-coating.
• Photoresist (in solution) is deposited onto center of wafer.
• Wafer is rotated and material is spread out by centrifugal
force.
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Photoresist Deposition1). Photoresist (in solvent) is
deposited in center of the
wafer. Wafer is held in
place with vacuum chuck.
2). Wafer is rotated slowly
(200 rpm) to distribute
material.
3). Accelerate the wafer to
final speed (~5000 rpm).
Spin the wafer at constant
speed for 30 – 60s. Forms
a uniform film and
evaporates solvent.
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Photoresist Deposition• Industrially this is done with robotic arms and automated
dispensers:
• Extremely uniform films can be deposited using spin coating
(rms roughness ~ Å’s)
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Film Thickness• It can be shown (we won’t) that the final film thickness
depends on the spinning speed via:
𝑡 ∝1
𝜔Film
thickness
Angular
velocity
• Actual thickness will also
depend on:
• Concentration.
• Solvent evaporation
rate
• Viscosity.
• Local temperature.
• Local humidity.
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Exposure
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Photoresist Exposure to UV• With our photoresist deposited, we now need to develop it.
• To do this we expose it to light (typically UV).
• Our photo-resist molecules
absorb these short wavelength
photons.
• Bonds are broken in the polymer.
• Either the molecules become
more soluble in the developer
(positive photoresist).
• Or the molecules cross-link an
become insoluble (negative
photoresist).
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Exposure Techniques• Three approaches are typically taken to exposure:
Mask
Contact
Printing
• Defects
• Bowing of mask
Mask
Proximity
Printing
space
(≈25 mm)
• 2 -4 μm resolution
2-5 X
reduction
Mask
lens
Projection
Printing
1:1 Printing
Printing System Magn. Resolution (μm) Use
Contact 1 × 0.1 – 1 Research
Proximity 1 × 2 – 4 Low Cost
Projection 2-5 × 0.1 - 1 Mainstream VLSI
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Light Sources• Traditionally, mercury vapor lamps were employed for
photolithography.
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Light Sources• Mercury lamp has four
main emission lines:
• E, G, H, I:
• For projection printing,
we ideally want
monochromatic light.
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Light Sources• Nowadays for projection
printing, and excimer laser is
employed:
LaserEmission
Wavelength (nm)Resolution (μm)
Max Energy (mJ /
Pulse)
Repetition Rate
(pulses / second)
KrF 248 0.18 - 0.25 300 – 1500 150
ArF 193 0.10 – 0.13 175 – 300 400
F2 157 < 0.1 6 10
• These are pulsed lasers.
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Diffraction• Modern lithography tools are limited by the spreading of light
(and not their optical elements)
• Light passing through an aperture of similar dimensions to
the wavelength of the incident light (λ~ 100’s nm), will result
in diffraction.
Divergent
light source Collimating
lens
Aperture
Diffraction pattern
(Airy disk)
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Diffraction• The type of diffraction observed depends on the mask-wafer
separation.
• Hard-contact: (almost) no diffraction.
• Proximity: Near field (Fresnel) diffraction.
• Projection: Far field or (Fraunhofer) diffraction.
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Diffraction• The difference between Fresnel (near field) and Fraunhofer (far
field) is defined by the dimensionless Fresnel number (𝐹):
𝐹 =𝑊2
𝐿𝜆
• Where:
• 𝑊 is size of the aperture.
• 𝐿 is the distance of the screen (wafer) from the aperture.
• 𝜆 is the wavelength of the incident light.
• We define:
• 𝐹 ≫ 1 as Fresnel diffraction.
• 𝐹 ≪ 1 as Fraunhofer diffraction.
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Fresnel Diffraction• Fresnel (near field) occurs in proximity printing.
𝐿
• Minimum resolvable feature size is:
𝑊𝑚𝑖𝑛 = 𝑘𝐿𝜆
• Where:
• 𝑘 is an experimental parameter associated with the
process conditions.
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Example• Determine the minimum feature size when exposing a wafer
to i-line irradiation, using a mask 25 μm from the surface of
the wafer. Assume for this example 𝑘 = 1.
• From before we know the wavelength of the i-line is 𝜆𝑖 = 365
nm.
𝑊𝑚𝑖𝑛 = 𝑘𝐿𝜆
• Work in microns: 𝜆𝑖 = 0.365 μm.
𝑊𝑚𝑖𝑛 = 1 × 25 × 0.365
𝑊𝑚𝑖𝑛 = 3 μm
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Fraunhofer Diffraction• Fraunhofer (far field) occurs in projection printing.
𝑓 = Focal
length
𝑑 = Lens
diameter
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Fraunhofer Diffraction• Diffraction pattern from a single circular opening:
𝑓 = Focal
length
𝑑 = Lens
diameter
𝜆 =
Wavelength
of incident
light
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Fraunhofer Diffraction• We can think of a mask as a diffraction grating:
Divergent
light source Collimating
lens Mask
(diffraction
grating)
• Each aperture in mask acts as a point source.
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Fraunhofer Diffraction• We can think of a mask as a diffraction grating:
Divergent
light source Collimating
lensMask
• Each aperture in mask acts as a point source.
Focusing
lens Photoresist
on wafer
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Fraunhofer Diffraction• The resolution in Fraunhofer diffraction is defined by the
Rayleigh criterion.
• Rayleigh Criterion: when the
peak of one projection lands on
the first zero of the other.
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Fraunhofer Diffraction• The resolution in Fraunhofer diffraction is defined by the
Rayleigh criterion.
• Rayleigh Criterion: when the peak of one projection lands on
the first zero of the other:
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Fraunhofer Diffraction• The resolution in Fraunhofer diffraction is quantified by the
Rayleigh Criterion (𝑅):𝑓 = Focal
length𝑑 = Lens
diameter𝑅 = 𝑘1𝜆𝑓
𝑑• 𝑘1 is an experimental parameter associated with the system
and resist (0.6 <𝑘1 < 0.8).
• The parameter Τ𝑑 𝑓 is sometimes called 𝑁𝐴 (numerical
aperture):
𝑁𝐴 =𝑑
𝑓= 𝑛sin𝛼
• 𝑛 = index of refraction (1 in air).
• 𝛼 = maximum half angle of
incident light:
𝑅 =𝑘1𝜆
𝑁𝐴
𝛼 = Maximum half-angle
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Modulation Transfer Function• The modulation transfer function (MTF) quantifies how much
modulation of light we achieve on the wafer:
Divergent
light sourceCollimating
lensMask Focusing
lens
Photoresist
on wafer
Intensity at Mask
Position
1
0
Intensity on wafer
Position
1
0
𝐼𝑀𝑎𝑥
𝐼𝑀𝑖𝑛
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Modulation Transfer FunctionIntensity at
Mask
Position
1
0
Intensity on wafer
Position
1
0
𝐼𝑀𝑎𝑥
𝐼𝑀𝑖𝑛
• We define MTF:
𝑀𝑇𝐹 =𝐼𝑚𝑎𝑥 − 𝐼𝑚𝑖𝑛
𝐼𝑚𝑎𝑥 + 𝐼𝑚𝑖𝑛
• MTF is defined between 0 (small features) and 1 (large
features).
• Generally, MTF needs to be > 0.5 for the resist to resolve
features.
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MTF vs Feature Size
𝑀𝑇𝐹 =𝐼𝑚𝑎𝑥 − 𝐼𝑚𝑖𝑛
𝐼𝑚𝑎𝑥 + 𝐼𝑚𝑖𝑛
𝑀𝑇𝐹
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Depth Focus: δ• Depth of focus defines distance along straight optical path
that wafer can be moved but keep the image in focus
Divergent
light sourceCollimating
lensMask Focusing
lens
Photoresist
on wafer
𝛿
• Basically, 𝛿 defines how accurate we need to be when
positioning the lens at a distance from the wafer.
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Depth Focus: δ• Recall the resolution (Raleigh Criteria) was defined as:
𝛿
• 𝑘2 is another experimental parameter associated with the
system and resist (𝑘2~0.5).
𝑅 = 𝑘1𝜆𝑓
𝑑=𝑘1𝜆
𝑁𝐴
Focusing
lens
Photoresist
on wafer
𝑓
𝑑• The depth of focus is defined
as:
𝛿 =𝑘2𝜆
𝑁𝐴 2= 𝑘2𝜆
𝑓
𝑑
2
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Example• Consider a setup where the focal length is 5× the diameter of
the lens.
Focusing
lens
Photoresist
on wafer
𝛿
𝑓 = 5𝑑
𝑑
• Assume 𝑘1 = 𝑘2 = 0.5.
• Use F2 laser: 𝜆 = 157 nm,
𝑁𝐴 =𝑑
5𝑑= 0.2
𝑅 =0.5 × 157
0.2= 392.5 nm
𝛿 =0.5 × 157
0.2 2= 1.96 μm
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Standing Waves• Standing waves a problem, in particular when exposing on
reflective layers such as metals.
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Standing Waves
• Suppressed by antireflective coating (ARC) prior to resist
spinning
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Development
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Development• We now wish to dissolve the regions of the photoresist which
have higher solubility.
• Exposed regions (positive photoresist).
• Masked regions (negative photoresist).
• Three strategies:
Immersion
Developing
develop solution
Wafers
pH and # of lots processed
are monitored
Spray Developing
Developer
fresh developer with each batch
Puddle Technique
fixed amount of developer
dispensed and rinsed
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Development• We now wish to dissolve the regions of the photoresist which
have higher solubility.
• Exposed regions (positive photoresist).
• Masked regions (negative photoresist).
• We need the following:
• Original thickness of positive resist should not be
measurably reduced.
• Development time should be short.
• Minimum pattern distortion (negative resists tend to
swell).
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Prebake & Postbake• Evaporates resist solvent.
Clean
Exposure Develop
Apply HMDS
Plasma
“descum”
StripProcess
Adhesion Promoter
(Etch / Implant/ Lift-off)
Photo room
Spin Coat prebake
postbake
• Improves adhesion.
• Anneals out stress in PR.
• Increases etch resistance
(postbake).
• Bake ovens:
• Convection/ hot air.
• Infrared (IR).
• Hot-plate.
• Temperature:
• Prebake: 90 - 100°C.
• Postbake: 120°C.
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Plasma “descum” and Strip• Not all the resist is removed
during development. Clean
Exposure Develop
Apply HMDS
Plasma
“descum”
StripProcess
Adhesion Promoter
(Etch / Implant/ Lift-off)
Photo room
Spin Coat prebake
postbake
• Plasma “descum” or ashing is
used to remove the residue.
• Plasmas are also used to strip
the PR.
• Barrel or downstream etchers are used.
• O2 gas is used.
• Since the photoresist is organic we make use of elemental
oxygen free radicals in the plasma.
C-H Photoresist(s) + O(g) → CO2(g) + H2O(g)
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Downstream Plasma• Plasma Ashing: Downstream Etcher
• Charged particles (ions & electrons) stay in plasma.
Electronics &
Power supply
(RF Microwave)
R
E
A
C
T
A
N
T
Plasma
Free
radicals
Wafer
Wafer is downstream of the plasma
G
A
S
• Free radicals flow
to wafer.
• Free radicals etch
the wafer
isotropically
• No surface
damage and
heating due to ion
bombardment.
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Resist Contrast• Resist Contrast quantifies the ability of the resist to distinguish
light/dark in the aerial image.
• To evaluate it we plot the developed thickness (i.e. thickness
remaining after development) as a function of dose (𝑄).
Dose (mJcm-2) = Intensity (mW/cm-2) × time (s)
𝑄 = 𝐼𝑡
𝑄0𝑄0
𝑄𝑓𝑄𝑓
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Resist Contrast• Quantitatively, the contrast is defined by 𝛾:
𝛾 =1
log10𝑄𝑓𝑄0
• Where:
• 𝑄0 is the onset of
exposure effect.
• 𝑄𝑓 is the dose at
which the exposure
is completeD
evelo
ped
Th
ickn
ess
Log10(Q)
𝑄0𝑄0
𝑄𝑓
• 𝛾 is typically 2 – 10.
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Lift Off• Not used in VLSI, but can be used in research.
• Avoid etching of
difficult materials
• Can produce a wider
range of structures.
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Next Time…• We will be talking about advanced techniques.
• Techniques for going down to sub-100nm resolution.
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