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Section 2: Lithography
Jaeger Chapter 2Litho Reader
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The lithographic process
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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
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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
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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
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Lithographic Process
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Printing Techniques
• 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
Contact printing
Proximity printing
Projection printing
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Contact Printing
wafer
hv
photoresist
Resolution R < 0.5μm
mask plate is easily damagedor accumulates defects
PhotoMaskPlate
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Proximity Printing
wafer
hv
g~20μm
exposed
Photoresist
R is proportional to ( λ g ) 1/2
~ 1μm for visible photons,much smaller for X-ray lithography
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Projection Printing
hv
lens
wafer
P.R.focal plane
~0.2 μm resolution (deep UV photons)tradeoff: optics complicated and expensive
De-Magnification: nX
10X stepper4X stepper1X stepper
Diffraction
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EE143 – Ali Javey
Aerial Images formed by Contact Printing, Proximity Printing and Projection Printing
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• 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
Photon Sources
Optical Projection Printing ModulesOptical System:
illumination and lens
Mask: transmission and diffraction
Resist: exposure, post-exposure bake and dissolution
Wafer Topography: scattering
Alignment:
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EE143 – Ali Javey
Optical Stepper
wafer
scribe line
1 2
Imagefield
field size increaseswith future ICs
field size increaseswith future ICs
Translationalmotion
Resolution in Projection Printing
Minimum separation of a star to be visible.
f = focal distanced = lens diameter
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EE143 – Ali Javey
Resolution limits in projection printing
=n.sinϴ, where n is the index of refraction
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point
Depth of Focus (DOF)
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EE143 – Ali Javey
ΔFieldOxide
Photo mask
Different photo images
Example of DOF problem
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( )
( ) .
( )
1 0 6
22 2
lNA
want small l
DOFNA
want large DOF
m m≅
= ±
λ
λ
(1) and (2) require a compromise between λ and NA !(1) and (2) require a compromise between λ and NA !
Tradeoffs in projection lithography
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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
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•A liquid with index of refraction n>1 is introduced between theimaging 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.
Immersion Lithography
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Image Quality Metric: Contrast
Contrast is also sometimes referred as the Modulation Transfer Function (MTF)
Questions:
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How does contrast change as a function of feature size?
How does contrast change for coherent vs. partially coherent light?
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* simulated aerial image of an isolated line
Image Quality metric: Slope of image
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resistresist
substrate
resist
substrate
resist
Position x
Finitecontrast
Infinitecontrast
Optical image
The need for high contrast
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Resists for Lithography
• Resists– Positive– Negative
• Exposure Sources– Light– Electron beams– Xray sensitive
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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.
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Positive P.R. Mechanism
Photons deactivatesensitizer
polymer +photosensitizer
dissolvein developersolution
⇒
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Resist contrast Qf≡
⎛⎝⎜
⎞⎠⎟
1
100
log
Positive Resisthv
mask
P.R.
100%
E1 ETexposurephotonenergy(log scale)
resist thickness remaining
(linearscale)
exposed part is removed
Q
Qf
Q0
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Negative P.R. Mechanism
hv => cross-linking => insoluble in developer solution.
hv
E1ET
remaining
photonenergy
after development
%
mask
Q0
Qf
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Positive vs. Negative Photoresists
• Positive P.R.:higher resolutionaqueous-based solventsless sensitive
• Negative P.R.:more sensitive => higher exposure throughputrelatively tolerant of developing conditionsbetter chemical resistance => better mask materialless expensivelower resolutionorganic-based solvents
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Overlay Errors
+
+
+
+
Alignmentmarksfrom previousmaskinglevel
wafer
alignmentmask
photomaskplate
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( )sisimm TTrR αα ⋅Δ−⋅Δ⋅=
Δ ΔT Tsi change of mask and wafer tempcoefficient of thermal expansion of
mask & Si
m
m si
, .,
==α α
run-outerror
waferradius
(1) Thermal Run-in/Run-out errors
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(2) Translational Error
referrer
image
n+
Al
p
Rotational / Translational Errors
(3) Rotational Error
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Overlay implications: Contacts
n+
SiO2 SiO2
Al“ideal”
p-Si
SiO2 SiO2
Al
p-Sin+ “short”, ohmic contact
Alignment error Δ
SiO2
n+
SiO2
Al
Solution: Design n+ region larger than contact hole
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Overlay implications: Gate edgeFox“Ideal”
poly-gate
S/D implantn+ Electrical
short
“With alignment error”
Solution: Make poly gate longer to overlap the FOX
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Total Overlay Tolerance
σ σ2 2total i
i=∑
σi = std. deviation of overlay error for ith masking stepσtotal = std. deviation for total overlay error
Layout design-rule specification should be > σtotal
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Standing Waves
substrate
PositivePhotoresist
hv
substrate
After developmentPositivePhotoresist.After developmentPositivePhotoresist.
Higher Intensity
Lower Intensity
Faster Development rate
Slower Development rate
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P.R.
Intensity = minimum whenn
mdx2λ
−=
x d
m = 0, 1, 2,...
Intensity = maximum whenn
mdx4λ
−= m = 1, 3, 5,...
n = refractive index of resist
SiO2/Si substrate
Standing waves in photoresists
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Proximity Scattering
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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 imaging2: thin develop-stop layer, used for pattern transfer to 33: thick layer of hardened resist
(imaging layer)
(etch stop)(planarization layer)
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Electron-Beam Lithography
V312.
=λAngstroms
for V in Volts
Example: 30 kV e-beam => λ = 0.07 Angstroms
NA = 0.002 – 0.005Resolution < 1 nm
But beam current needs to be 10’s of mA for a throughput of more than 10 wafers an hour.
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Types of Ebeam Systems
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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
Resolution limits in e-beam lithography
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The Proximity Effect
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Richard Feynman
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EE143 – Ali Javey
EE143 – Ali Javey
Dip Pen Nanolithography
Dip-Pen Nanolithography: Transport of molecules to the surface via water meniscus.
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Dip-pen Lithography, Chad Mirkin, NWU
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EE143 – Ali Javey
Patterning of individual Xe atoms on Ni, by Eigler (IBM)
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