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