Lecture 5 Fundamentals of Multiscale Fabrication Multiscale fabrication IV: Unconventional nanolithography (top-down) Kahp-Yang Suh Associate Professor SNU MAE sky4u@snu.ac.kr

Micro/Nanofabrication Bulk Top-down approach Bottom-up approach Atom, molecule

Origin: movable metal characters The world’s first printed masterpiece called “Jick-Zi-Simkyung (직지심경)” was first invented in Korea in the early 11th century, which precedes that of Germany by more than 200 years! A review paper by Michel et al. from IBM (2001)

Unconventional nanofabrication Different physical and chemical principles are utilized Top-down approach (Lithography) - Nanoimprint lithography : mechanical pressing, plastic deformation … - Soft lithography : conformal contact, molecular diffusion … - Capillary force lithography : capillary force, viscosity, surface tension … - Dip-pen nanolithography : contact printing, surface diffusion … Bottom-up approach (Self assembly) - Self assembled monolayers : covalent bonding, crystallization … - Colloidal self assembly : capillary force, evaporation/condensation … - Dewetting/buckling : intermolecular forces, elastic stress, surface tension

Top-down approaches Photolithography Scanning beam lithography - Conventional methods - Unconventional methods Photolithography Scanning beam lithography - Electron beam lithography - Focused ion beam lithography - Ion projection lithography - Extreme UV lithography - X-ray lithography Nano imprint lithography Capillary force lithography (CFL) Rigiflex lithography Soft lithography - Microcontact printing (μCP) - Micromolding in capillaries (MIMIC) Adhesive force lithography

Unconventional nanolithography

Nanoimprint lithography (NIL) Nanoimprint lithography or hot embossing is a technique of imprinting nanostructures on a substrate (polymer) using a master mold (silicon tool) Apply embossing force on the substrate via the mold under vacuum Heat the substrate and mold to just above Tg of the substrate Cool the substrate and mold to just below Tg Master/mold from photo-lithography De-embossing of the mold and substrate

Examples of generated patterns

Imprinting + Etching

Current challenges of NIL Large-area pattering – Uniform pressure application Clean demolding – Mold surface treatment Removal of residual layer Air trap and defect generation – Use of vacuum Combination of large and small patterns – Non-uniform thickness distribution

UV-assisted NIL Step and Flash Imprinting Lithography (S-FILTM): Profs. C. G. Wilson S. V. Sreenivasan. (UT Austin)

Examples of generated patterns

Room Temperature Nanoimprint Lithography RT-NIL process has the unique features of enabling step-and-repeat and multiple imprinting, which is impossible in the conventional high-temperature imprint processes. D. Y. Khang, H. Yoon and H. H. Lee, “Room-Temperature Imprint Lithography”, Adv. Mater., 13, 749 (2001)

Lateral force AFM images Nanoimprint lithography set-up in NFTL UV imprint (under vacuum) Lateral force AFM images AFM images Thermal imprint 70 nm lines Large area patterning (4 inch) Uniform pressure Imprinting under vacuum (no air trap) Minimized residual layer 200 nm lines

Place the mold on the polymer surface Cooling and mold removal Capillary Force Lithography (CFL) PDMS mold Polymer Substrate Place the mold on the polymer surface 1. Heating (T > Tg) 2. Solvent-laden film 3. UV exposure Cooling and mold removal Meniscus Thick film Thin film K. Y. Suh, Y. S. Kim, and H. H. Lee, “Capillary force lithography”, Adv. Mater., 13, 1386 (2001)

Examples of generated patterns Thick and Thin polymer films Complex and Large-area patterning Film thickness: 1.5 m Polystyrene, 130ºC, 6 hrs Film thickness: 180 nm Styrene-Butadiene-Styrene copolymer 120C, 1 hr

Pattern transfer to a substrate Patterning + Etching Fine structures Pattern transfer to a substrate Etching condition - CH3 (40 sccm) - CF4 (10 sccm) 50 mTorr - SiO2 substrate

What is capillarity? R Young-Laplace equation  h glass Laplace pressure vs. Gravity Tube size ~ typically on the order of mm Capillary rise is relatively fast water glass mercury

Capillary kinetics Assumption: Poiseuille flow (neglect of inertial force) R  z 1. Without gravity (LWR equation) 1. With gravity R: hydraulic radius : viscosity z(t): capillary movement ze: equilibrium capillary rise : density g: gravity coefficient Diverges as z  ze

Examples Silicon oil in glass tube with r = 0.315 mm. Inertia-induced oscillations

Merits of capillarity for nanofabrication Familiar and physically well understood A natural, spontaneous phenomenon ~ no need to apply an external energy or stimulus One-step and three dimensional patterning (cf. photolithography) Versatile use ~ capillary rise or depression

First introduction of capillary force E. Kim, Y. Xia, and G. M. Whitesides, Nature, 376, 581 (1995) MIcroMolding In Capillary (MIMIC)

Examples of generated patterns Generation of structures It was found that the relation, z  t 1/2, is satisfied qualitatively!

Limitations of MIMIC Low resolution (> ~ 1 m) PDMS need to have a network structure inside Slow and incomplete patterning (use of vacuum?) No capillary action with hydrophilic bio fluids ( ~ 105º)

Rigiflex (rigid + flexible) lithography Rigidity High resolution - Sub 100 nm - Durability - Mold handling Rigiflex polymer mold Flexibility Conformal contact High throughout (Roll-to-Roll) - Large area

Mold properties - Polyurethane acrylate mold (PUA) 1. tunable mechanical rigidity (E: 20 MPa ~ 2 GPa) (cf. PDMS : 1.8MPa) 2. flexibility (50~100 um thickness) 3. small shrinkage (0.7 %) 4. light transmittance 5. low surface energy (20 ~ 50 mJ/m2) UV curable property High throughout - Curing time < ~ 10 s S. J. Choi et al, JACS 2004 D. Suh et al, Adv. Mater. 2005 High resolution - Sub-100 nm pattern Mechanical Hardness Flexibility Low surface energy - Conformal contact - Large area

Recent achievements Hierarchical structure 80nm nanohairs Langmuir (2006) 80nm nanohairs Nano Lett. (2006) 70nm ZnO nanogrooves Nanotechnology (2005, 2006) 50nm PEG nanostructure Adv. Mater. (2005) Multi-height nanostructure Appl. Phys. Lett. (2001) Al buckled nanostructure Adv. Mater. (2002)

Soft Lithography A class of techniques involving a soft elastomeric mold such as poly(dimethylsiloxane) (PDMS) Forms of Soft Lithography Microcontact Printing (μCP) Replica Molding Hot Embossing Microtransfer Molding (μTM) Micromolding in Capillaries (MIMIC) Near-Field Phase Shift Lithography Related techniques, e.g. film mask lithography

Soft lithography techniques

Fabrication Methods: Master and Replication In clean room: On lab bench: Mix and pour PDMS over master Clean Si wafer Spin coat photoresist Allow to set; peel from master Exposure to UV light through mask Microfluidics Contact Printing Micromolding Imprinting/Embossing Develop

PDMS Properties Low interfacial free energy (21.6dyn/cm) and good chemical stability; most molecules or polymers being patterned or molded do not adhere irreversibly to, or react with, the surface of PDMS Hydrophobic and does not swell with humidity High gas permeability (probably #1 to most gas species) Good thermal stability (up to 186℃ in air) Prepolymers being molded can be cured thermally Optically transparent down to 300nm; prepolymers being molded can also be cured by UV cross-linking Isotropic and homogeneous Stamps or molds made from this material can be deformed mechanically to manipulate the patterns and relief structures in their surfaces Durable when used a stamp (used >50 times over a period of several months without noticeable degradation in performance) Interfacial properties can be changed readily either by modifying the prepolymers or by treating the surface with plasma to form siloxane SAMs to give appropriate interfacial interactions with other materials with a wide range of interfacial free energies

Rapid prototyping

Resolution limit of PDMS W To prevent self-matting : Glassmaker et al. J. Roy. Soc. Interface (2004)

Microcontact printing (CP) An “ink” of alkanethiols is spread on a patterned PDMS stamp (an alkane is hydrocarbon which is entirely single bonded: CnH2n+2. A thiol is a sulfhydryl group: SH) The stamp is then brought into contact with the substrate, which can range from metals to oxide layers The thiol ink is transferred to the substrate where it forms a self-assembled monolayer that can act as a resist against etching Features as small as 300 nm have been made in this way.

Self-assembled monolayers (SAMs)

Microcontact printing (CP)

<Clean mold release> Adhesive force lithography Thin film & Interfacial energy 3 3 Mold Mold 2 2 Polymer Polymer 1 1 Substrate Substrate <Clean mold release> <Detachment> Wij : work of adhesion at the interface ij Aij : interfacial area

Adhesion Adhesion Physical property Chemical property Fabrication < cover paper> Physical property Nano pattern – van der Waals force Chemical property Material property – Sticky Fabrication Nature, 448, 338 (2007) Detachment lithography Nanoimprint lithography MEMS Control of adhesion Control of demolding Control of collapse Langmuir, 23, 12555 (2007) J. Vac. Sci. Technol. B, 26, 458 (2008) J. Adhesion Sci. Technol., 17, 519 (2003)

Measurement of Adhesion Thermodynamic method - contact angle 1 2 Work of adhesion W12 Physical method – equipment, tape and AFM (Biomaterial) (CNT) J. Adhesion Sci. Technol., 17, 519 (2003) Nature nanotechnology, 3, 261 (2008)

Spreading coefficient Work of adhesion A 1 1 W12 B 2 2 Spreading coefficient : S S>0 : Wetting S<0 : Dewetting Work of cohesion 1 1 ½ W11 Interfacial energy I am going to introduce spreading coefficient. first I show different between work of adhesion and work of cohesion. We calculate Work of adhesion between different materials. However we calculate work of adhesion between same material. We can know work of cohesion is double value of surface energy. Gama b is surface energy at substrate, gama a is surface energy at liquid on substrate. and gama AB is interfacial energy between liquid and substrate. we calculate interfacial energy using this simple equation. If spreading coefficient is positive, a liquid on substrate can wet spontaneously. However this value is negative, liquid is not wet spontaneously. If we use substrate with high surface energy such as cleaned glass and hydrocarbon with low surface energy. Hydrocarbon is spread on substrate. 1 1 1/2W11 = 40

How can we increase adhesion? Physical assistance: Demolding velocity Chemical assistance Loo et al. (Nano Lett. 2003) Meitl et al. (Nature Mater. 2006) Light (UV) assistance Physical assistance: High pressure JACS, 2002,124,13583 and JACS, 2006,128,858 Pease et al. (Nature nanotechnology 2007)

Detachment nanolithography Operability :Wmold/polymer > WPolymer/ Substrate Substrate Harmonic mean method Young´s Equation Mold NPB Young-Dupree Equation Substrate Geometric mean method

Examples of generated patterns (NPB on Au) Organic layer Gold grain 2 μm 500 nm ~800 nm 100 nm ~300 nm NPB layer

Control of interfacial energy Then, what happens if ? 1 um Nanodrawing Jeong et al., Nano Lett. (2006)