Lithographic Processes q Pattern generation and transfer Circuit

Lithographic Processes q. Pattern generation and transfer Circuit design Pattern data Master mask set Working mask set Pattern on wafers q. Increasing device density reducing minimum feature size q Through-put consideration

Wafer with IC Chips

Patterning by lithography and wet etc Mask Etching of Al film Cr patterned film transparent glass photoresist Si Al film Si. O 2 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 Sensitivity Adhesive Etch resistance Resolution

q 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 solventinduced swelling effect, hard to remove after using q 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 q UV Sources:

Pathways for pattern transfer

Design pattern Optical or ebeam writing Reticle Projection masks printing, step× 5 -20 and-repeat Working masks × 1 E-beam pattern generation No diffraction limitation, minimum feature size ~ 0. 15 m Reducing the back -scattering effects (proximity effects) by reducing beam energy

Pattern transfer to wafer: Printing q Contact printer: highest resolution (minimum feature size ~ 0. 15 m), but damages to masks and/or wafer limit mask lifetime Mask UV photoresist Si Si. O 2 film Si q Proximity gap printer: 2. 5 -25 m gap, compromising resolution (r d), minimum feature size 1 m q Projection: flexible, no damage, limited resolution in single projection q Step-and-repeat projection: high resolution in

A complete lithographic process Wafer with mask film (e. g. Si. O 2, Al) Removal of exposed photoresist Etching of mask film Photoresist coating (spin coating) Postba ke Removal of unexposed resist Prebake (softbak e) Develo p-ment Next process (e. g. implantation, deposition) Mask alignment Exposu re

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

Lift-off Process Ø Positive resist patterning Ø Metal deposit Ø Removal of resist and metal film above q Capable of forming thick and narrow metal lines q little damage to oxide surfaces

Move to EUV Source Mercury lamp Na Wavelengt m h (nm) e G- 436 Application feature size (nm) 500 l i n e H- 405 l i n e I- 365 l i 350 to 250

Multilayer Resists R 1, R 2 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