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 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

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

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 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 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

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 UV photoresist SiO2 film Si Si

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

Contact to a diode Lithography 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

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

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

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