Transcript Lithographic Processes - National University of Singapore

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
Wafer with IC Chips
Patterning by lithography and wet etching
Etching of Al film
Cr patterned film
Mask
transparent glass
photoresist
Si
Al film
SiO2 film
Pattern transfer
to photoresist
Si
UV exposure
Develop solution
Si
Si
Si
Photoresists
Chemical/texture change upon exposure to light (UV), X-ray, e beam
Sensitivity
Adhesive
Etch resistance
Resolution
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 )
Pathways for pattern transfer
Design
pattern
Optical or ebeam writing
Reticle Projection printing, Working
masks step-and-repeat
masks
× 5-20
×1
E-beam pattern
generation
No diffraction
limitation, minimum
feature size ~ 0.15 m
Reducing the backscattering effects
(proximity effects) by
reducing beam energy
Raster scan mode
Vector scan mode
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
UV
Mask
photoresist
Si
SiO2 film
Si
 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
A complete lithographic process
Wafer with
mask film (e.g.
SiO2, Al)
Photoresist
coating (spin
coating)
Prebake
(softbake)
Mask
alignment
Removal of
exposed
photoresist
Postbake
Development
Exposure
Etching of
mask film
Removal of
unexposed
resist
Next process
(e.g. implantation,
deposition)
Contact to a diode
(a) Lithography
(b) Metallization
(c),(d) lithography
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
Move to EUV
Source
Mercury lamp
Excimer
Laser
Fluorine laser
Name
Wavelength (nm)
Application feature size (nm)
G-line
436
500
H-line
405
I-line
365
XeF
351
XeCl
308
KrF
248 (DUV)
250 to 130
ArF
193
150 to 70
F2
157
< 100
350 to 250
Multilayer Resists
R1, R2 sensitive to 1, 2
Contrast enhancement
Phase-Shifting Masks
Resolution improvement ~ 2-4 times, pattern-dependent
Electron
Projection
Printing
System
Direct e-beam
writing: ~ 0.15m,
sequential, only for the
smallest features
X-ray printing system
Difficulties: photoresist and optical systems for X-ray