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
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
transparent glass
Cr patterned filmMask
Si
photoresist
SiO2 film
Al film
Si
UV exposure
Si
Develop solutionSi
Pattern transferto photoresist
Si
Etching of Al film
Patterning by lithography and wet etching
PhotoresistsChemical/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
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 modeVector scan mode
Designpattern
Reticle masks× 5-20
Working masks
× 1
Optical or e-beam writing
Projection printing, step-and-repeat
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
Mask
Si
photoresist
SiO2 film
Si
UV
A complete lithographic process
Wafer with mask film (e.g. SiO2, Al)
Photoresist coating (spin coating)
Prebake(softbake)
Mask alignment
ExposureDevelop-mentPostbake
Removal of exposed photoresist
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 EUVSource 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
Contrast enhancementMultilayer Resists
R1, R2 sensitive to 1, 2
Phase-Shifting Masks
Resolution improvement ~ 2-4 times, pattern-dependent
ElectronProjectionPrintingSystem
Direct e-beam writing: ~ 0.15m, sequential, only for the smallest features
X-ray printing system
Difficulties: photoresist and optical systems for X-ray