1. Section 2: Lithography
Jaeger Chapter 2
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2. The lithographic process
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3. Photolithographic Process
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
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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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5. 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
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6. Lithographic Process
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7. 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-on-wafer, eliminating the need for the reduction step presented earlier
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8. Contact Printing hv photoresist wafer Resolution R < 0.5m
Mask
Plate
photoresist
wafer
Resolution R < 0.5m
mask plate is easily damaged
or accumulates defects
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9. R is proportional to (  g ) 1/2
Proximity Printing hv
g~20m
Photoresist
exposed
wafer
R is proportional to (  g ) 1/2
~ 1mm for visible photons,
much smaller for X-ray lithography
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10. Projection Printing hv De-Magnification: nX 10X stepper 4X stepper
lens
focal plane
P.R.
wafer
~0.2 mm resolution (deep UV photons)
tradeoff: optics complicated and expensive
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11. Aerial Images formed by Contact Printing, Proximity Printing and Projection Printing
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12. Optical Stepper
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13. Excimer Laser Stepper
Light is in pulses of 20 ns duration at a repetition rate of a few kHz. About 50 pulses are used.
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14. Photon Sources
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15. Resolution limits in projection printing
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16. Resolution limits: Bragg condition
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17. Image Degradation by lens
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18. The /NA limit
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19. Resolution and the need for higher order terms
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20. Depth of Focus (DOF)
off
point
best
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21. Example of DOF problem Photo mask Field Oxide Different photo images
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22. Tradeoffs in projection lithographylm
(1) and (2) require a compromise between  and NA !
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23. Sub-resolution exposure: Phase Shifting Masks
Pattern transfer of two closely spaced lines
Conventional mask technology - lines not resolved
Lines can be resolved with phase-shift technology
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24. Immersion Lithography
A liquid with index of refraction n>1 is introduced between the
imaging optics and the wafer.
Advantages
Resolution is improved proportionately to n. For water, the index of refraction at l = 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.
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25. Image Quality Metric: Contrast
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26. Image Quality metric: Slope of image
* simulated aerial image of an isolated line
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27. The need for high contrast
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28. Resists for Lithography
Positive
Negative
Exposure Sources
Light
Electron beams
Xray sensitive
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29. Two Resist Types Negative Resist Positive 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%)
Inhibitor Looses N2 => Alkali Soluble Acid
Develops by “etching” - No Swelling.
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30. Positive P.R. Mechanism Photons deactivate sensitizer less
cross-linking
dissolve
in developer
solution
polymer +
photosensitizer
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31. Positive Resist E = resist sensitivity 1 Resist contrast º æ E ö log ç÷ 10 è E ø 1
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32. Negative P.R. Mechanism
hv => cross-linking => insoluble in developer solution.
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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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34. Overlay Errors alignment mask + wafer photomask plate Alignment marks
from
previous
masking
level
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35. Thermal Run-in/Run-out errors
wafer
radius
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36. Rotational / Translational Errors
image Al n+
referrer p
(3) Rotational Error
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37. Overlay implications: Contacts
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38. Overlay implications: Gate edge
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39. Total Overlay Tolerance
si = std. deviation of overlay error for ith masking step
stotal = std. deviation for total overlay error
Layout design-rule specification should be > stotal
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40. Standing Waves
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41. Standing waves in photoresistsx
P.R. d
SiO2/Si substrate
Intensity = minimum when
m = 0, 1, 2,...
Intensity = maximum when
m = 1, 3, 5,...
n = refractive index of resist
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42. Proximity Scattering
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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 imaging
2: thin develop-stop layer, used for pattern transfer to 3
3: thick layer of hardened resist
(imaging layer)
(etch stop)
(planarization layer)
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44. Electron-Beam Lithography
Angstroms
for V in Volts
Example: 30 kV e-beam
=> l = 0.07 Angstroms
NA = – 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.
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45. Types of Ebeam Systems
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46. Resolution limits in e-beam lithography
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47. The Proximity Effect
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