Section 2: Lithography
Jaeger Chapter 2
Litho Reader
EE143 – Ali Javey
Slide 5-1
The lithographic process
EE143 – Ali Javey
Slide 5-2
Photolithographic Process
(a)
(b)
(c)
(d)
(e)
(f)
(g)
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Substrate covered with silicon
dioxide barrier layer
Positive photoresist applied to
wafer surface
Mask in close proximity to
surface
Substrate following resist
exposure and development
Substrate after etching of
oxide layer
Oxide barrier on surface after
resist removal
View of substrate with silicon
dioxide pattern on the surface
Slide 5-3
Photomasks - CAD Layout
• Composite drawing of the masks
for a simple integrated circuit
using a four-mask process
• Drawn with computer layout
system
• Complex state-of-the-art CMOS
processes may use 25 masks or
more
EE143 – Ali Javey
Slide 5-4
Photo Masks
• Example of 10X reticle for the metal
mask - this particular mask is ten
times final size (10 mm minimum
feature size - huge!)
• Used in step-and-repeat operation
• One mask for each lithography level
in process
EE143 – Ali Javey
Slide 5-5
Lithographic Process
EE143 – Ali Javey
Slide 5-6
Printing Techniques
Contact
printing
Proximity
printing
Projection
printing
• Contact printing damages the mask
and the wafer and limits the number
of times the mask can be used
• Proximity printing eliminates
damage
• Projection printing can operate in
reduction mode with direct step-onwafer
EE143 – Ali Javey
Slide 5-7
Contact Printing
hv
Photo
Mask
Plate
photoresist
wafer
Resolution R < 0.5mm
mask plate is easily damaged
or accumulates defects
EE143 – Ali Javey
Slide 5-8
Proximity Printing
hv
g~20mm
Photoresist
wafer
exposed
R is proportional to (  g ) 1/2
~ 1mm for visible photons,
much smaller for X-ray lithography
EE143 – Ali Javey
Slide 5-9
Projection Printing
hv
De-Magnification: nX
lens
10X stepper
4X stepper
1X stepper
focal plane
P.R.
wafer
~0.2 mm resolution (deep UV photons)
tradeoff: optics complicated and expensive
EE143 – Ali Javey
Slide 5-10
Diffraction
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Slide 5-11
Aerial Images
formed by Contact Printing, Proximity Printing and Projection Printing
EE143 – Ali Javey
Slide 5-12
Photon sources
Photon Sources
 Hg Arc lamps 436(G-line), 405(H-line), 365(I-line) nm
 Excimer lasers: KrF (248nm) and ArF (193nm)
 Laser pulsed plasma (13nm, EUV)
Source Monitoring
 Filters can be used to limit exposure wavelengths
 Intensity uniformity has to be better than several % over the collection area
 Needs spectral exposure meter for routine calibration due to aging
EE143 – Ali Javey
Slide 5-13
Optical Projection Printing Modules
Optical System:
illumination and lens
Resist: exposure, post-exposure
bake and dissolution
Mask: transmission and
diffraction
Wafer Topography:
scattering
Alignment:
14
Optical Stepper
field size increases
with future ICs
scribe line
1
2
wafer
Image
field
Translational
motion
EE143 – Ali Javey
Slide 5-15
Resolution in Projection Printing
f = focal distance
d = lens diameter
Minimum separation of a
star to be visible.
16
Resolution limits in projection printing
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Slide 5-17
Depth of Focus (DOF)
point
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Slide 5-18
EE143 – Ali Javey
Slide 5-19
Example of DOF problem
Photo mask

Field
Oxide
Different photo images
EE143 – Ali Javey
Slide 5-20
Tradeoffs in projection lithography

(1) lm  0.6
NA
( 2) DOF  
want small lm

2 NA
2
want large DOF
(1) and (2) require a compromise between  and NA !
EE143 – Ali Javey
Slide 5-21
Sub-resolution exposure: Phase Shifting Masks
Pattern transfer of two closely
spaced lines
(a) Conventional mask
technology - lines not
resolved
(b) Lines can be resolved
with phase-shift
technology
EE143 – Ali Javey
Slide 5-22
Immersion Lithography
•A liquid with index of refraction n>1 is introduced between the
imaging optics and the wafer.
Advantages
1) Resolution is improved
proportionately to n. For
water, the index of
refraction at  = 193 nm
is 1.44, improving the
resolution significantly,
from 90 to 64 nm.
2) Increased depth of focus at
larger features, even those
that are printable with dry
lithography.
EE143 – Ali Javey
Slide 5-23
Image Quality Metric: Contrast
Contrast is also sometimes referred as the Modulation Transfer Function (MTF)
EE143 – Ali Javey
Slide 5-24
Questions:
How does contrast change as a function of feature size?
How does contrast change for coherent vs. partially coherent light?
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Slide 5-25
Image Quality metric: Slope of image
* simulated aerial image of an isolated line
EE143 – Ali Javey
Slide 5-26
The need for high contrast
Optical image
Infinite
contrast
resist
resist
Finite
contrast
resist
substrate
resist
substrate
Position x
EE143 – Ali Javey
Slide 5-27
Resists for Lithography
• Resists
– Positive
– Negative
• Exposure Sources
– Light
– Electron beams
– Xray sensitive
EE143 – Ali Javey
Slide 5-28
Two Resist Types
• Negative Resist
– Composition:
• Polymer (Molecular Weight (MW) ~65000)
• Light Sensitive Additive: Promotes Crosslinking
• Volatile Solvents
– Light breaks N-N in light sensitive additive => Crosslink Chains
– Sensitive, hard, Swelling during Develop
• Positive Resist
– Composition
• Polymer (MW~5000)
• Photoactive Dissolution Inhibitor (20%)
• Volatile Solvents
– Inhibitor Looses N2 => Alkali Soluble Acid
– Develops by “etching” - No Swelling.
EE143 – Ali Javey
Slide 5-29
Positive P.R. Mechanism
Photons deactivate
sensitizer
 dissolve
in developer
solution
polymer +
photosensitizer
EE143 – Ali Javey
Slide 5-30
Positive Resist
hv
mask
exposed
part is
removed
100%
(linear
scale)
P.R.
Resist contrast 
resist thickness remaining
Q
E
10
Q
E
fT
exposure
photon
energy
(log scale)
1
 Qf 
log10  Q 
 0
EE143 – Ali Javey
Slide 5-31
Negative P.R. Mechanism
hv
% remaining
mask
after
development
QET
f
Q
E1
photon
energy
0
hv => cross-linking => insoluble in
developer solution.
EE143 – Ali Javey
Slide 5-32
Positive vs. Negative Photoresists
• Positive P.R.:
 higher resolution
 aqueous-based solvents
 less sensitive
• Negative P.R.:
 more sensitive => higher exposure throughput
 relatively tolerant of developing conditions
 better chemical resistance => better mask material
 less expensive
 lower resolution
 organic-based solvents
EE143 – Ali Javey
Slide 5-33
Overlay Errors
+
+
alignment
mask
wafer
+
+
photomask
plate
Alignment
marks
from
previous
masking
level
EE143 – Ali Javey
Slide 5-34
(1) Thermal Run-in/Run-out errors
R  r  Tm  m  Tsi  si 
run-out wafer
error radius
Tm , Tsi  change of mask and wafer temp.
 m ,  si  coefficient of thermal expansion of
mask & Si
EE143 – Ali Javey
Slide 5-35
Rotational / Translational Errors
(2) Translational Error
image
Al
n+
p
(3) Rotational Error
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referrer
Slide 5-36
Overlay implications: Contacts
Al
SiO2
“ideal”
SiO2
n+
p-Si
Al
SiO2
Alignment error
SiO2
n+

“short”, ohmic contact
p-Si
Solution: Design n+ region larger than contact hole
Al
SiO2
SiO2
n+
EE143 – Ali Javey
Slide 5-37
Overlay implications: Gate edge
“Ideal”
Fox
S/D implant
Electrical
n+
short
“With
alignment
error”
poly-gate
Solution: Make poly gate longer to overlap the FOX
EE143 – Ali Javey
Slide 5-38
Total Overlay Tolerance

2
total
  i
2
i
i = std. deviation of overlay error for ith masking step
total = std. deviation for total overlay error
Layout design-rule specification should be > total
EE143 – Ali Javey
Slide 5-39
Standing Waves
hv
Higher Intensity
Faster Development rate
Lower Intensity
Slower Development rate
Positive
Photoresist
substrate
Positive
Photoresist.
After development
substrate
EE143 – Ali Javey
Slide 5-40
Standing waves in photoresists
x
d
P.R.
SiO2/Si substrate

x d m
2n
m = 0, 1, 2,...

Intensity = maximum when x  d  m
4n
n = refractive index of resist
m = 1, 3, 5,...
Intensity = minimum when
EE143 – Ali Javey
Slide 5-41
Proximity Scattering
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Slide 5-42
Approaches for Reducing Substrate Effects
• Use absorption dyes in photoresist
• Use anti-reflection coating (ARC)
• Use multi-layer resist process
1: thin planar layer for high-resolution imaging (imaging layer)
2: thin develop-stop layer, used for pattern transfer to 3 (etch stop)
3: thick layer of hardened resist (planarization layer)
EE143 – Ali Javey
Slide 5-43
Electron-Beam Lithography
12.3

V
Angstroms
for V in Volts
Example: 30 kV e-beam
=>  = 0.07 Angstroms
NA = 0.002 – 0.005
Resolution < 1 nm
But beam current needs
to be 10’s of mA for a
throughput of more
than 10 wafers an hour.
EE143 – Ali Javey
Slide 5-44
Types of Ebeam Systems
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Slide 5-45
Resolution limits in e-beam lithography
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Slide 5-46
The Proximity Effect
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Slide 5-47
Richard Feynman
Dip Pen Nanolithography
Dip-Pen Nanolithography: Transport of molecules to the surface via water meniscus.
Dip-pen Lithography, Chad Mirkin, NWU
Patterning of individual Xe atoms on Ni, by Eigler (IBM)