15-398 Introduction to Nanotechnology Pushing The Limits of Photolithography • Reduce wavelength (λ) Nanoscale Lithography • Use Reducing Lens • Increase Numerical Aperture (NA) of Lens Seth Copen Goldstein – E.g., Immersion optics [email protected] •Hacks (k1) CMU – PSM, OPC, RET, off-axis illumination • Rayleigh eqn: min-feature-size=k1λ/NA lecture 4 © 2004-5 Seth Copen Goldstein 1 lecture 4 © 2004-5 Seth Copen Goldstein 2 Trends in λ, NA, k1 NA & Immersion ICKnowledge.com • NA = n sinα = d/(2f) – N is index of refraction of medium – α is angle of acceptance •Air, n=1 • Water, n≅1.47 • Result -> improve min linewidth 30%! CUHG, Chap 5. lecture 4 © 2004-5 Seth Copen Goldstein 3 lecture 4 © 2004-5 Seth Copen Goldstein 4 E-Beam? FIB? Example of E-beam patterning • Use high-energy electrons to alter/ablate a resist P = 40 nm P = 45 nm • Issues: – Secondary electrons – Scattering in resist (or off substrate) – Serial process P = 50 nm P = 60 nm – alignment Handbook of Microlithography, Micromachining and lecture 4 © 2004-5 Seth Copen Goldstein Microfabrication, SPIE (1997)5 lectureUofA, 4 Nanolithography © 2004-5 Seth Copen Goldstein 6 Microelectronics isn’t everything Today • Reactive organics • Soft Lithography/Nanoimprint • 3-D structures • Scanning Probe Lithography • Edge Lithography • Top-down fabrication to create nanoscale features using a physical mold or tip for patterning. lecture 4 © 2004-5 Seth Copen Goldstein 7 lecture 4 © 2004-5 Seth Copen Goldstein 8 Soft Litho Basic Example • Replica Molding transfer features from master to replica by curing a liquid • Embossing transfer of features from master to replica by pressing • Microcontact Printing transfer of material on master to replica by stamping Unless otherwise notes, pictures from Unconventional Nanofabrication, Gates etal, ARMR 2004 lecture 4 © 2004-5 Seth Copen Goldstein 9 lecture 4 © 2004-5 Seth Copen Goldstein 10 The Mold Causes of Distortion • Some important features: • Differences in thermal expansion – Master should be reusable many times between master and replica (self-cleaning) • Shrinkage during curing – Create fine features –Flexible – x-linking – Stable –Evaporation – Optically transparant (for some • Adhesion at time of separation processes) • Collapse at separation – Thermally stable –Inert – Low adhesive forces lecture 4 © 2004-5 Seth Copen Goldstein 11 lecture 4 © 2004-5 Seth Copen Goldstein 12 PDMS Replica Molding • Poly(dimethylsiloxane) Rigid Mold Replica molding silicone rubber • Very common mold material. A resist Soft Mold • Inert to materials being patterned • Low surface energy (eases release) • Optically transparant Microtransfer Micromolding molding • Thermally stable in capillaries •Tough •Flexible • No solvent evaporation •Low-temp curing lecture 4 © 2004-5 Seth Copen Goldstein 13 lecture 4 © 2004-5 Seth Copen Goldstein 14 Rigid Molds Soft Molds • E.g., Step-and-flash • Can pattern non-planar and soft – Add Liquid pre-polymer surfaces –Press master • Photo- or thermal curing –Expose to UV large range of materials can be • 30nm features molded • High-aspect ratio of lines • Potential for large surface area molds •Rapid cycle-time • Alignment of master and replica • Replica needs to be planar • Multi-layer structures hard lecture 4 © 2004-5 Seth Copen Goldstein 15 lecture 4 © 2004-5 Seth Copen Goldstein 16 Examples Embossing • Press mold into replica to create structures • Generally, thermally assisted • Used at microlevel commercially (e.g., DVD) • Nanoscale features <50nm . , Advanced Materials, 16, 1369 1369 . , Advanced Materials, 16, et al MIMIC (2004) S. Jeon S. Jeon E. Kim et al, Nature, 376, 581 (1995) Microcontact transfer lecture 4 © 2004-5 Seth Copen Goldstein 17 lecture 4 © 2004-5 Seth Copen Goldstein 18 Embossing – rigid molds Embossing – soft molds • Nanoimprint lithography (NIL) • SAMIM, solvent-assisted micromolding • Press mold, heat polymer, cool, • Reduces pressure and temp needed due release to solvent • Features 10nm, high-aspect ratio • Eliminates trapped air pockets • Long exposure times (5 – 10 min) • 60nm features, low aspect ratio • Hard for large • Non-planar surfaces areas • Large areas lecture 4 © 2004-5 Seth Copen Goldstein 19 lecture 4 © 2004-5 Seth Copen Goldstein 20 Mircocontact Printing Microcontact printing • Transfers “ink” from master to • Planar and curved surfaces replica • Many different kinds of “ink” • Ink is a self-assembled monolayer • Min resolution affected by diffusion (SAM) of molecules • 50nm • Organics supported features •Multilayersok lecture 4 © 2004-5 Seth Copen Goldstein 21 lecture 4 © 2004-5 Seth Copen Goldstein 22 Soft Lithography Soft Lithography • Pluses and minues? •Advantages – Avoids complexity of photolithography – Inexpensive – Some 3-D possible –Organics can be patterned • Disadvantages – Heat, pressure can be harmful – Still experimental (low yield) – Limits due to lift-off? lecture 4 © 2004-5 Seth Copen Goldstein 23 lecture 4 © 2004-5 Seth Copen Goldstein 24 SPL DPN • Scanning Probe Lithography • AFM tip is “inked” with material to be –STM tip deposited –AFM tip • Material is adsorbed on target –Dip-Pen lithography • <15nm features • Arrays of DPN in production http://www.chem.northwestern.edu/~mkngrp/dpn.htm lecture 4 © 2004-5 Seth Copen Goldstein 25 lecture 4 © 2004-5 Seth Copen Goldstein 26 Schematic of DPN Arrays •ar www.nanoink.net lecture 4 © 2004-5 Seth Copen Goldstein 27 lecture 4 © 2004-5 Seth Copen Goldstein 28 Array Example Edge Litho • Using Edges produced in another process to double pitch • Using Edges in material as a stamp (more like NIL) lecture 4 © 2004-5 Seth Copen Goldstein 29 lecture 4 © 2004-5 Seth Copen Goldstein 30 SNAP SNAP V. Other • Create a stamp by using the cleaved • EBL can make 20nm patterns, but … edges of a layered material formed – Lift-off for small wires at low pitch or through MBE high-pitch limits final result • Then, e.g., transfer SAMS – Result is min pitch much higher (~60nm?) – Pitch is everything? –EBL serial • Soft-litho master – how created? 8nm Pt nanowire array lecture 4 © 2004-5 Seth Copen Goldstein 31 lecture 4 © 2004-5 Seth Copen Goldstein 32 Summary • Top-down litho can get to fine features and fine pitches. • What about complexity of the pattern? • Keep in mind: – Master manufacturing – Alignment, imprinting, releasing – resist/solvent/material chemistry lecture 4 © 2004-5 Seth Copen Goldstein 33.