Section 2: Lithography
Jaeger Chapter 2
Litho Reader
EE143 – Ali Javey
The lithographic process
EE143 – Ali Javey
Photolithographic Process
(a) Substrate covered with silicon
dioxide barrier layer
(b) Positive photoresist applied to
wafer surface
(c) Mask in close proximity to
surface
(d) Substrate following resist
exposure and development
(e) Substrate after etching of
oxide layer
(f) Oxide barrier on surface after
resist removal
(g) View of substrate with silicon
dioxide pattern on the surface
EE143 – Ali Javey
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
Photo Masks
• Example of 10X reticle for the metal
mask - this particular mask is ten
times final size (10 μm minimum
feature size - huge!)
• Used in step-and-repeat operation
• One mask for each lithography level
in process
EE143 – Ali Javey
Lithographic Process
EE143 – Ali Javey
Printing Techniques
Contact Proximity Projection
printing printing 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-on-
wafer
EE143 – Ali Javey
Contact Printing
hv
Photo photoresist
Mask
Plate
wafer
Resolution R < 0.5μm
mask plate is easily damaged
or accumulates defects
EE143 – Ali Javey
Proximity Printing
hv
g~20μm Photoresist
wafer exposed
R is proportional to ( λ g ) 1/2
~ 1μm for visible photons,
much smaller for X-ray lithography
EE143 – Ali Javey
Projection Printing
hv De-Magnification: nX
10X stepper
4X stepper
lens 1X stepper
focal plane
P.R.
wafer
~0.2 μm resolution (deep UV photons)
tradeoff: optics complicated and expensive
EE143 – Ali Javey
Diffraction
EE143 – Ali Javey
Aerial Images
formed by Contact Printing, Proximity Printing and Projection Printing
EE143 – Ali Javey
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
Optical Projection Printing Modules
Optical System:
illumination and lens
Resist: exposure, post-exposure
bake and dissolution
Mask: transmission and
diffraction
Wafer Topography:
scattering
Alignment:
EE143 – Ali Javey
Optical Stepper
field size increases
with future ICs
scribe line
1 2
wafer
Image
field Translational
motion
EE143 – Ali Javey
Resolution in Projection Printing
f = focal distance
d = lens diameter
Minimum separation of a
star to be visible.
EE143 – Ali Javey
Resolution limits in projection printing
=n.sinϴ, where n is the index of refraction
EE143 – Ali Javey
Depth of Focus (DOF)
point
EE143 – Ali Javey
EE143 – Ali Javey
Example of DOF problem
Photo mask
Δ
Field
Oxide
Different photo images
EE143 – Ali Javey
Tradeoffs in projection lithography
λ
(1) lm ≅ 0.6 want small lm
NA
λ
( 2) DOF = ± want large DOF
2( NA)
2
(1)
(1)and
and(2)
(2)require
requireaacompromise between λλand
compromisebetween andNA
NA!!
EE143 – Ali Javey
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
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 λ = 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
Image Quality Metric: Contrast
Contrast is also sometimes referred as the Modulation Transfer Function (MTF)
EE143 – Ali Javey
Questions:
How does contrast change as a function of feature size?
How does contrast change for coherent vs. partially coherent light?
EE143 – Ali Javey
Image Quality metric: Slope of image
* simulated aerial image of an isolated line
EE143 – Ali Javey
The need for high contrast
Optical image
Infinite
contrast Finite
contrast
resist resist resist resist
substrate substrate
Position x
EE143 – Ali Javey
Resists for Lithography
• Resists
– Positive
– Negative
• Exposure Sources
– Light
– Electron beams
– Xray sensitive
EE143 – Ali Javey
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
Positive P.R. Mechanism
Photons deactivate
sensitizer
⇒ dissolve
in developer
solution
polymer +
photosensitizer
EE143 – Ali Javey
Positive Resist
hv
100%
mask (linear
scale)
resist thickness remaining
exposed
Q
E Q
E exposure
part is P.R. 10 fT photon
removed energy
(log scale)
1
Resist contrast ≡
⎛ Qf ⎞
log10 ⎜ Q ÷
⎝ 0⎠
EE143 – Ali Javey
Negative P.R. Mechanism
% remaining
hv
mask
after
development photon
QET Q energy
f E1 0
hv => cross-linking => insoluble in
developer solution.
EE143 – Ali Javey
Positive vs. Negative Photoresists
• Positive P.R.:
9 higher resolution
9 aqueous-based solvents
8 less sensitive
• Negative P.R.:
9 more sensitive => higher exposure throughput
9 relatively tolerant of developing conditions
9 better chemical resistance => better mask material
9 less expensive
8 lower resolution
8 organic-based solvents
EE143 – Ali Javey
Overlay Errors
alignment
+ + mask
wafer
+ +
photomask Alignment
plate marks
from
previous
masking
level
EE143 – Ali Javey
(1) Thermal Run-in/Run-out errors
R = r ⋅ (ΔTm ⋅αm −ΔTsi ⋅αsi )
run-out wafer
error radius
ΔTm , ΔTsi = change of mask and wafer temp.
α m , α si = coefficient of thermal expansion of
mask & Si
EE143 – Ali Javey
Rotational / Translational Errors
(2) Translational Error
Al image
n+
p referrer
(3) Rotational Error
EE143 – Ali Javey
Overlay implications: Contacts
Al
SiO2 SiO2 “ideal”
n+
p-Si
Al
SiO2 SiO2
n+ “short”, ohmic contact
Alignment error Δ
p-Si
Solution: Design n+ region larger than contact hole
Al
SiO2 SiO2
n+
EE143 – Ali Javey
Overlay implications: Gate edge
S/D implant
“Ideal” Fox n+ Electrical
short
“With
alignment
error”
poly-gate
Solution: Make poly gate longer to overlap the FOX
EE143 – Ali Javey
Total Overlay Tolerance
σ 2
total = ∑σi 2
σi = std. deviation of overlay error for ith masking step
σtotal = std. deviation for total overlay error
Layout design-rule specification should be > σtotal
EE143 – Ali Javey
Standing Waves
hv
Higher Intensity Faster Development rate
Lower Intensity Slower Development rate
Positive
Photoresist
substrate
Positive
After development Photoresist.
substrate
EE143 – Ali Javey
Standing waves in photoresists
x
P.R. d
SiO2/Si substrate
λ
Intensity = minimum when x = d −m m = 0, 1, 2,...
2n
λ
Intensity = maximum when x = d −m m = 1, 3, 5,...
n = refractive index of resist
4n
EE143 – Ali Javey
Proximity Scattering
EE143 – Ali Javey
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
Electron-Beam Lithography
12.3 Angstroms
λ= for V in Volts
V
Example: 30 kV e-beam
=> λ = 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
Types of Ebeam Systems
EE143 – Ali Javey
Resolution limits in e-beam lithography
resolution factors
• beam quality ( ~1 nm)
• secondary electrons ( lateral range: few nm)
performance records
organic resist PMMA ~ 7 nm
inorganic resist, b.v. AlF3 ~ 1-2 nm
EE143 – Ali Javey
The Proximity Effect
EE143 – Ali Javey
EE143 – Ali Javey
Richard Feynman
EE143 – Ali Javey
EE143 – Ali Javey
EE143 – Ali Javey
Dip Pen Nanolithography
Dip-Pen Nanolithography: Transport of molecules to the surface via water meniscus.
EE143 – Ali Javey
Dip-pen Lithography, Chad Mirkin, NWU
EE143 – Ali Javey
EE143 – Ali Javey
Patterning of individual Xe atoms on Ni, by Eigler (IBM)
EE143 – Ali Javey
