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

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

3. 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)

4. 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

5. 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

6. Unconventional nanolithography

7. 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

8. Examples of generated patterns

9. Imprinting + Etching

10. 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

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

12. Examples of generated patterns

13. 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)

14. 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

15. 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)

16. 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

17. 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

18. 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

19. 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

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

21. 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

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

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

24. 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)

25. 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

26. 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

27. 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)

28. 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

29. Soft lithography techniques

30. 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

31. 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

32. Rapid prototyping

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

34. 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.

35. Self-assembled monolayers (SAMs)

36. Microcontact printing (CP)

37. <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

38. 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, (2007)
J. Vac. Sci. Technol. B, 26, 458 (2008)
J. Adhesion Sci. Technol., 17, 519 (2003)

39. 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)

40. 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

41. How can we increase adhesion?
Physical assistance: Demolding velocity
Chemical assistance
Loo et al. (Nano Lett )
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)

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

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

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