Lithographic Processes

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Lithographic Processes

• Pattern generation and transfer
• Circuit design ? Pattern data ? Master mask
  set ?
• Working mask set ? Pattern on
  wafers
• Increasing device density ? reducing minimum
  feature size
• Through-put consideration

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Wafer with IC Chips
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Patterning by lithography and wet etching
Cr patterned film
Etching of Al film
Mask
transparent glass
photoresist
Si
Al film
SiO2 film
Pattern transfer to photoresist
Si
UV exposure
Develop solution
Si
Si
Si
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Photoresists
Chemical/texture change upon exposure to light
(UV), X-ray, e beam
Sensitivity Adhesive Etch resistance Resolution
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• Negative resists long-chain organic polymers,
  cross-linked upon UV exposure
• Kodak Microneg 747 polyisoprene rubber
  photoactive agent
• Thickness 0.3 1 ?m, feature size ? 2 ?m due
  to solvent-induced swelling effect, hard to
  remove after using
• Positive resist a mixture of alkali-soluble
  resin, photoactive
• dissolution inhibitor, and solvent
• PMMA (polymethylmethacrylate)
• Thickness 1 - 3 ?m, no solvent-induced
  swelling effect, feature
• size ? 2 ?m, easy to remove after using
• UV Sources
• Hg-Xe lamp, ? 250-290 nm
• Excimer lasers, deep UV, ? ? 200 nm
  (e.g. ArF, ? 193 nm )

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Pathways for pattern transfer
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E-beam pattern generation
Optical or e-beam writing
Projection printing, step-and-repeat
Reticle masks 5-20
Working masks 1
Design pattern
No diffraction limitation, minimum feature size
0.15 ?m Reducing the back-scattering effects
(proximity effects) by reducing beam
energy Raster scan mode Vector scan mode
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Pattern transfer to wafer Printing

• Contact printer highest resolution (minimum
  feature size 0.15 ?m), but damages to masks
  and/or wafer limit mask lifetime
• Proximity gap printer 2.5-25 ?m gap,
  compromising resolution
• (r ? ?d), minimum feature size ? 1 ?m
• Projection flexible, no damage, limited
  resolution in single projection
• Step-and-repeat projection high resolution in
  reduced area, acceptable throughput due to short
  exposure time of each frame

UV
Mask
photoresist
SiO2 film
Si
Si
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A complete lithographic process
Wafer with mask film (e.g. SiO2, Al)
Photoresist coating (spin coating)
Prebake (softbake)
Mask alignment
Exposure
Removal of exposed photoresist
Develop-ment
Postbake
Removal of unexposed resist
Next process (e.g. implantation, deposition)
Etching of mask film
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Contact to a diode

• Lithography
• Metallization
• (c),(d) lithography

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Lift-off Process

• Positive resist
• patterning
• Metal deposit
• Removal of
• resist and metal
• film above

• Capable of forming thick and narrow metal lines
• little damage to oxide surfaces

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Move to EUV
Source Name Wavelength (nm) Application feature size (nm)
Mercury lamp G-line 436 500
Mercury lamp H-line 405 500
Mercury lamp I-line 365 350 to 250
Excimer Laser XeF 351
Excimer Laser XeCl 308
Excimer Laser KrF 248 (DUV) 250 to 130
Excimer Laser ArF 193 150 to 70
Fluorine laser F2 157 lt 100
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Contrast enhancement
Multilayer Resists
R1, R2 sensitive to ?1, ?2
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Phase-Shifting Masks
Resolution improvement 2-4 times,
pattern-dependent
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Electron Projection Printing System
Direct e-beam writing ? 0.15?m, sequential,
only for the smallest features
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X-ray printing system
Difficulties photoresist and optical systems for
X-ray