EE143 F2010
Lecture 4
Photolithography
Key Topics:
Photo =
Litho =
Graphy =
• Minimum Feature
Resolution
• Depth of Focus
• Overlay Errors
• Photoresist Response
• E-beam and EUV lithography
Professor N. Cheung, U.C. Berkeley
s = (through) light
s = stone
= writing
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Lecture 4
•Slow
• several nm resolution
Professor N. Cheung, U.C. Berkeley
•High throughput
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Professor N. Cheung, U.C. Berkeley
Lecture 4
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Lecture 4
Contact Printing
hv
Photo
Mask
Plate
photoresist
wafer
Resolution R < 0.5m
mask plate is easily damaged
or accumulates defects
Professor N. Cheung, U.C. Berkeley
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Lecture 4
Proximity Printing
hv
Photoresist
g~20m
wafer
exposed
R = k (  g ) 1/2
~ 1 m for visible photons,
much smaller for X-ray lithography
Professor N. Cheung, U.C. Berkeley
x
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Lecture 4
Projection Printing
hv
De-Magnification: nX
10X stepper
4X stepper
1X stepper
lens
focal plane
P.R.
wafer
~0.2 m resolution (deep UV photons)
tradeoff: optics complicated and expensive
Professor N. Cheung, U.C. Berkeley
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Lecture 4
Photon sources
 Hg Arc lam ps 436(G -line), 405(H -line), 365(I-line) nm
 Excim er lasers: KrF (248nm ) and ArF (193nm )
 Laser pulsed plasm a (13nm , EUV)
Source M onitoring
 Filters can be used to lim it exposure wavelengths
 Intensity uniform ity has to be better than several % over the collection area
 Needs spectral exposure m eter for routine calibration due to aging
Professor N. Cheung, U.C. Berkeley
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Lecture 4
Excimer Laser Stepper
Professor N. Cheung, U.C. Berkeley
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Lecture 4
Optical Stepper
field size increases
with future ICs
scribe line
1
2
wafer
Image
field
Professor N. Cheung, U.C. Berkeley
Translational
motion
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Aerial Images formed by Contact Printing, Proximity
Printing and Projection Printing
Professor N. Cheung, U.C. Berkeley
Lecture 4
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Professor N. Cheung, U.C. Berkeley
Lecture 4
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Lecture 4
Line Patterns to illustrate principle of Projection Printing
Professor N. Cheung, U.C. Berkeley
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Professor N. Cheung, U.C. Berkeley
Lecture 4
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Lecture 4
Qualitative Explanation
of image degradation by lens
+
Mask
2

lens
wafer
plane
+1
0
-1
parallel
optical
beam
2

Line grating with
spatial frequency 1/P
L
S
P
Professor N. Cheung, U.C. Berkeley
P sin   n
n  0, 1, 2,...
sin  = NA of lens
P=L+S
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Lecture 4
Mask Intensity
Imax
O
Image on wafer
x
After optical system
Imax
O
Professor N. Cheung, U.C. Berkeley
x
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Lecture 4
Effect of Fourier Components on aerial image ofn=0
a
rectangular waveform
n=0
n=0 + n=1
n=0 + n=1 + n=3
n=0 + n=1 + n=3 + n=5
Source: Chapter 8 , iSheats and Smith, Microlithography
Professor N. Cheung, U.C. Berkeley
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Why
lm 

NA
Lecture 4
?
The lens has to collect at least the n =1 diffracted beams
to show any spatially varying intensity on wafer.
Therefore printable Pminimum =  /sin  = /NA
For lm  (L+S)/2 = Pm/2   /NA
Professor N. Cheung, U.C. Berkeley
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Lecture 4
Depth of Focus (DOF)
off
point
best
Professor N. Cheung, U.C. Berkeley
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Lecture 4
Simulated aerial images with various degree of defocus
No defocus
Note degradation of
image contrast
and image slope
Defocus increases
Professor N. Cheung, U.C. Berkeley
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Lecture 4
Example of DOF problem
Photo mask

Field
Oxide
Step height > DOF
Different optical images
Professor N. Cheung, U.C. Berkeley
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Lecture 4
Focus versus Extreme Defocus (an illustration)
Large P features
Small P features
For Reference only
Best focus
Extreme Defocus
Professor N. Cheung, U.C. Berkeley
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Lecture 4
First-Order Projection Printing Considerations
1) Minimum feature resolution lm = k1 (/NA)
2) Depth of Focus DOF = k2 / (NA)2
where k1 and k2 are the technology factors
•NOTE: NA has contradictory effects on lm and DOF
Professor N. Cheung, U.C. Berkeley
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Lecture 4
Image Quality Metric: (a) Image Contrast
Prefer high Contrast
Professor N. Cheung, U.C. Berkeley
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Image Quality metric: (b) Slope of Image Intensity
Prefer large Slope
* simulated aerial image of an isolated line
Professor N. Cheung, U.C. Berkeley
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Photomask Layout Pattern is 2D; Needs 2D Fourier Transform
Photomask Pattern
Aerial Image Intensity on wafer
Professor N. Cheung, U.C. Berkeley
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