section 2: lithography jaeger chapter 2 ee143 ali javey
DESCRIPTION
The lithographic process EE143 – Ali JaveyTRANSCRIPT
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EE143 – Ali Javey Slide 5-1
Section 2: Lithography
Jaeger Chapter 2
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EE143 – Ali Javey Slide 5-2
The lithographic process
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EE143 – Ali Javey Slide 5-3
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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EE143 – Ali Javey Slide 5-4
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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EE143 – Ali Javey Slide 5-5
Photo Masks
• Example of 10X reticle for the metal mask - this particular mask is ten times final size (10 m minimum feature size - huge!)
• Used in step-and-repeat operation
• One mask for each lithography level in process
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EE143 – Ali Javey Slide 5-6
Lithographic Process
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EE143 – Ali Javey Slide 5-7
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, eliminating the need for the reduction step presented earlier
Contact printing
Proximity printing
Projection printing
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EE143 – Ali Javey Slide 5-8
Contact Printing
wafer
hv
photoresist
Resolution R < 0.5m
mask plate is easily damagedor accumulates defects
PhotoMaskPlate
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EE143 – Ali Javey Slide 5-9
Proximity Printing
wafer
hv
g~20m
exposed
Photoresist
R is proportional to ( g ) 1/2
~ 1m for visible photons,much smaller for X-ray lithography
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EE143 – Ali Javey Slide 5-10
Projection Printing
hv
lens
wafer
P.R.focal plane
~0.2 m resolution (deep UV photons)tradeoff: optics complicated and expensive
De-Magnification: nX
10X stepper4X stepper1X stepper
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EE143 – Ali Javey Slide 5-11
Aerial Images formed by Contact Printing, Proximity Printing and Projection Printing
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EE143 – Ali Javey Slide 5-12
Optical Stepper
wafer
scribe line
1 2
Imagefield
field size increaseswith future ICs
field size increaseswith future ICs
Translationalmotion
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EE143 – Ali Javey Slide 5-13
Light is in pulses of 20 ns duration at a repetition rate of a few kHz.
About 50 pulses are used.
Excimer Laser Stepper
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EE143 – Ali Javey Slide 5-14
Photon Sources
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EE143 – Ali Javey Slide 5-15
Resolution limits in projection printing
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EE143 – Ali Javey Slide 5-16
Resolution limits: Bragg condition
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EE143 – Ali Javey Slide 5-17
Image Degradation by lens
waferplane
parallelopticalbeam
grating withspatial frequency 1/P
P P=2L
...,2,1,0sin
nnP
-1
-2
+1
+2Mask lens
L L
0
sin = NA of lens
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EE143 – Ali Javey Slide 5-18
The /NA limit
x
x
Image on wafer optical system
Mask Intensity
Imax
O
The lens has to collect at least the n=1 diffracted beamsto show any spatially varying intensity on wafer.Therefore Pm ( = 2 lm) / NA
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EE143 – Ali Javey Slide 5-19
n=0
n=0 + n=1
n=0 + n=1 + n=3
n=0 + n=1 + n=3 + n=5
Resolution and the need for higher order terms
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EE143 – Ali Javey Slide 5-20
point
best
off
Depth of Focus (DOF)
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EE143 – Ali Javey Slide 5-21
FieldOxide
Photo mask
Different photo images
Example of DOF problem
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EE143 – Ali Javey Slide 5-22
( ) .
( )
1 0 6
22 2
lNA
want small l
DOFNA
want large DOF
m m
lm
(1) and (2) require a compromise between and NA !
Tradeoffs in projection lithography
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EE143 – Ali Javey Slide 5-23
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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EE143 – Ali Javey Slide 5-24
•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 = 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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EE143 – Ali Javey Slide 5-25
Image Quality Metric: Contrast
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EE143 – Ali Javey Slide 5-26
* simulated aerial image of an isolated line
Image Quality metric: Slope of image
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EE143 – Ali Javey Slide 5-27
resistresist
substrate
resist
substrate
resist
Position x
Finitecontrast
Infinitecontrast
Optical image
The need for high contrast
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EE143 – Ali Javey Slide 5-28
Resists for Lithography
• Resists– Positive– Negative
• Exposure Sources– Light– Electron beams– Xray sensitive
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EE143 – Ali Javey Slide 5-29
Two Resist Types• Negative Resist
– Polymer (Molecular Weight (MW) ~65000)– Light Sensitive Additive Promotes Crosslinking– Volatile Solvents– Light breaks N-N => Crosslink Chains– Sensitive, hard, Swelling during Develop
• Positive Resist– Polymer (MW~5000)– Photoactive Inhibitor (20%)– Volatile Solvents– Inhibitor Looses N2 => Alkali Soluble Acid– Develops by “etching” - No Swelling.
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EE143 – Ali Javey Slide 5-30
Positive P.R. Mechanism
Photons deactivatesensitizer
polymer +photosensitizer
less cross-linking
dissolvein developersolution
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EE143 – Ali Javey Slide 5-31
=E resist sensitivity
Resist contrast
T
log
Positive Resisthv
mask
P.R.
100%
E1 ETexposurephotonenergy(log scale)
resist thickness remaining
(linearscale)
exposed part is removed
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EE143 – Ali Javey Slide 5-32
Negative P.R. Mechanism
1
1log EET
hv => cross-linking => insoluble in developer solution.
hv
E1ET
remaining
photonenergy
after development
%
mask
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EE143 – Ali Javey Slide 5-33
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
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EE143 – Ali Javey Slide 5-34
Overlay Errors
+
+
+
+
Alignmentmarksfrom previousmaskinglevel
wafer
alignmentmask
photomaskplate
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EE143 – Ali Javey Slide 5-35
sisimm TTrR
T Tsi change of mask and wafer tempcoefficient of thermal expansion of
mask & Si
m
m si
, .,
run-out error
waferradius
Thermal Run-in/Run-out errors
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EE143 – Ali Javey Slide 5-36
(2) Translational Error
referrer
image
n+
Al
p
Rotational / Translational Errors
(3) Rotational Error
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EE143 – Ali Javey Slide 5-37
Overlay implications: Contacts
n+
SiO2 SiO2
Al“ideal”
p-Si
SiO2 SiO2
Al
p-Sin+ “short”, ohmic contact
Alignment error
SiO2
n+
SiO2
Al
Solution: Design n+ region larger than contact hole
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EE143 – Ali Javey Slide 5-38
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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EE143 – Ali Javey Slide 5-39
Total Overlay Tolerance
2 2total i
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
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EE143 – Ali Javey Slide 5-40
Standing Waves
substrate
PositivePhotoresist
hv
substrate
After developmentPositivePhotoresist.After developmentPositivePhotoresist.
Higher Intensity
Lower Intensity
Faster Development rate
Slower Development rate
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EE143 – Ali Javey Slide 5-41
P.R.
Intensity = minimum whenn
mdx2
x d
m = 0, 1, 2,...
Intensity = maximum whenn
mdx4
m = 1, 3, 5,...
n = refractive index of resist
SiO2/Si substrate
Standing waves in photoresists
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EE143 – Ali Javey Slide 5-42
Proximity Scattering
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EE143 – Ali Javey Slide 5-43
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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EE143 – Ali Javey Slide 5-44
Electron-Beam Lithography
V312.
Angstroms
for V in Volts
Example: 30 kV e-beam => = 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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EE143 – Ali Javey Slide 5-45
Types of Ebeam Systems
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EE143 – Ali Javey Slide 5-46
Resolution limits in e-beam lithography
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EE143 – Ali Javey Slide 5-47
The Proximity Effect