1. 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

2. Wafer with IC Chips

3. 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

4. Photoresists Sensitivity Adhesive Etch resistance Resolution
Chemical/texture change upon exposure to light (UV), X-ray, e beam
Sensitivity
Adhesive
Etch resistance
Resolution

5. Positive resist: a mixture of alkali-soluble resin, photoactive
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 m, no solvent-induced swelling effect, feature
size  2 m, easy to remove after using
UV Sources:
Hg-Xe lamp,  ~ nm
Excimer lasers, deep UV,   200 nm (e.g. ArF,  = 193 nm )

6. Pathways for pattern transfer

7. E-beam pattern generation
Optical or e-beam writing
Projection printing, step-and-repeat
Reticle masks
× 5-20
Working masks
× 1
Design
pattern
E-beam pattern generation
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

8. 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: 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
Mask UV
photoresist
SiO2 film Si Si

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

10. Contact to a diode
Lithography
Metallization
(c),(d) lithography

11. 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

12. Application feature size (nm)
Move to EUV
Source
Name
Wavelength (nm)
Application feature size (nm)
Mercury lamp
G-line
436
500
H-line
405
I-line
365
350 to 250
Excimer
Laser
XeF
351
XeCl
308
KrF
248 (DUV)
250 to 130
ArF
193
150 to 70
Fluorine laser F2
157
< 100

13. Multilayer Resists
Contrast enhancement
R1, R2 sensitive to 1, 2

14. Phase-Shifting Masks
Resolution improvement ~ 2-4 times, pattern-dependent

15. Electron Projection Printing System
Direct e-beam writing: ~ 0.15m, sequential, only for the smallest features

16. X-ray printing system
Difficulties: photoresist and optical systems for X-ray