Soft Lithography

Soft Lithography: An inexpensive and
versatile way to make micro- and
nanostructures in any lab.
Ayo Olanrewaju
PhD Student, DJGroup
March 4, 2013

Outline
§ Definition and rationale for soft lithography
§ Elastomeric stamps: PDMS characteristics
§ Replica molding using elastomeric stamps
§ Microcontact printing of chemical, biological, and
electrical materials.
2

Soft Lithography
“A collection of techniques based on printing,
molding, and embossing with an elastomeric
stamp”
Qin, Xia, and Whitesides, Nature Protocols, 5(3), 2010
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Motivation for Soft Lithography
Conventional photolithography:
• Patterns accurate planar
structures.
• Many commercial applications
(e.g. microelectronics).
• Often inaccessible to nonspecialists
• Inherently expensive
• Limited range of materials
4

Soft lithography:
Motivation (contd.)
• Accessible to non-specialists
• Non-planar and 3-dimensional
substrates
• Patterning large areas (cm scale)
• New applications and materials
(e.g. biotechnology, flexible
electronics)
5

Outline
§ Definition and rationale for soft lithography
§ Elastomeric stamps: PDMS characteristics
§ Replica molding using elastomeric stamps
§ Microcontact printing of chemical, biological, and
electrical materials.
6

Elastomeric stamps: PDMS is the most common
PDMS = Polydimethylsiloxane
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How to make a PDMS stamp
1. Mix pre-polymer and curing agent and
pour onto master
2. Cure at 80˚C for 2.5 hours
3. Peel stamp and trim
Master – original structure used to make stamp.
Stamp – patterned polymer used for soft lithography.
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PDMS is soft and enables conformal contact
Soft and rubbery backbone of PDMS
Conformal contact
9

Modifying surface chemistry of PDMS
hydrophobic
Plasma Oxidation
hydrophilic
http://www.ims.ut.ee/~alar/microtech/Ch1_5/
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Properties of PDMS
Liquid pre-polymer
High thermal stability
High chemical stability
[ Easy molding
[ Use at high temperature
[ Use with acids and bases
Transparent for UV/VIS [ To visualize experiments
Solvents swell PDMS (exceptions: Methanol, Ethanol, Isopropanol…)
Slow thermal curing (a few hours) is undesirable when translating
from conceptual prototype to commercial product
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Other soft lithography materials: OSTE
OSTE: Off-Stoichiometric Thiolene polymers
Rapid UV curing (seconds)
Tunable surface chemistry
Adjustable mechanical properties
Carlborg, C. F. et al, Lab Chip 11, 3136 (2011).
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Other materials: OSTE (contd.)
Carlborg et al, Lab
Chip 11, 3136
(2011).
13

Outline
§ Definition and rationale for soft lithography
§ Elastomeric stamps: PDMS characteristics
§ Replica molding using elastomeric stamps
§ Microcontact printing of chemical, biological, and
electrical materials.
14

Replica Molding
Use PDMS stamp to make
copies of original structure
1 stamp can make > 50
replicas
Replicas can be made of
different polymers
Xia, Y. & Whitesides, G. M. Annu. Rev. Mater. Sci. 28, 153–184 (1998).
15

Replica Molding (contd.)
Good feature replication (conformal contact!)
Curing only shrinks replicas by ~1%
PU - Polyurethane
Xia, Y. & Whitesides, G. M. Annu. Rev. Mater. Sci. 28, 153–184 (1998).
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Limits on aspect ratio in PDMS replicas
Be mindful of aspect ratio limits of PDMS replicas to avoid defects in your devices
Qin, D., Xia, Y. & Whitesides, G.M., 2010. Nat. Protocols, 5(3), pp.491-502.
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Replica molding of microfluidic devices
Fabrication of peristaltic pumps by multilayer soft lithography
Unger et al, Science 2000 DOI:10.1126/science.288.5463.113
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Multilayer soft lithography for fluidic logic operations
Allows fast, programmable, and parallelized flow control in microfluidic systems
Naga Sai Gopi K. Devaraju and Marc A. Unger, Lab Chip, 2012, DOI:10.1039/C2CLC1155F
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Multilayer Soft Lithography: Microfluidic Shift Register
Naga Sai Gopi K. Devaraju and Marc A. Unger, Lab Chip, 2012, DOI:10.1039/C2CLC1155F
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Outline
§ Definition and rationale for soft lithography
§ Elastomeric stamps: PDMS characteristics
§ Replica molding using elastomeric stamps
§ Microcontact printing of chemical, biological, and
electrical materials.
21

Microcontact printing
Using an elastomeric stamp to transfer an “inked
material” onto a substrate
Wilbur, J. L., Kumar, A., Kim, E. & Whitesides, G. M. Adv. Mater. (1994).
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Microcontact Printing: Self Assembled Monolayers
Micropatterned
PDMS stamp
Inking
Solution of alkanethiols
in ethanol
Print self-assembled
monolayers as masks for
micro-patterning metal
substrates outside the
cleanroom
Drying,
printing
Removal of
the stamp
Inked stamp
Metal substrate
(Au, Ag, Cu, Pd...)
SAM
Selective
etch
SAM = Self-assembled monolayer
Micropatterned
substrate
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Self-Assembled Monolayers as Ink
Self-assembled monolayers of alkanethiols on coinage metals (Au, Ag, Pt, Cu, …)
Simple and spontaneous organization of alkanethiols
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Microcontact printing SAMs for wet etching metal substrates
SAMs spread on substrate,
restricting feature sizes to
>100nm.
SAM layer is thin (2-3 nm) so
only serves as mask for
isotropic wet etching
Wilbur, J. L., Kumar, A., Kim, E. & Whitesides, G. M. Adv. Mater. (1994).
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Microcontact Printing of Proteins
I
Ink Stamp
Protein
solution
II Blow dry
III Contact
IVSeparate
Structured
PDMS
Stamp
Inked
stamp
Hard
substrate
Patterned
surface
Transfer of proteins from
low to high surface energy
Proteins act as a “solid” ink
– no spreading
Greater than 99% protein
transfer in less than 1
second
Bernard, A., et al., Langmuir 1998, 14, 2225-2229.
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Printing adhesive proteins for cell co-cultures
50 µm
Could be used to test drug effects on the brain in vitro (i.e. without animal models)
S.G. Ricoult et al, J Neuroscience Methods, 208, 10-17 (2012)
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Nanocontact printing of protein gradients
Developed a low cost, lift-off
printing technique to pattern 100
nm protein spots.
Study cell migration - critical in
biological processes e.g. nervous
system wiring
Ricoult et al., Small, 2013, 10.1002/smll.201202915
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Microcontact printing of cells
Weibel D.B Langmuir, 2005, 21, 6436-6442.
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Flexible electronics
Printing conductive nanomaterials onto flexible substrates using elastomeric stamps
Viventi, J. et al., 2011 Nat Neurosci, 14(12), pp.1599-1605.
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Flexible electronics as neural implants
Flexible mesh electrodes enable conformal contact and high quality measurements
Viventi, J. et al., 2011 Nat Neurosci, 14(12), pp.1599-1605.
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Microcontact printing in this course
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Patterns for Microcontact Printing
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Conclusions
§ Soft lithography is a rapid, inexpensive, and accessible
fabrication technique.
§ PDMS replicas can be used to make microfluidic devices.
§ Microcontact printing allows patterning of chemical,
biological, and electrical materials with a wide range of
applications.
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Acknowledgements
DJGroup
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References
1.Wolfe, D. B., Qin, D. & Whitesides, G. M. Methods in Molecular Biology. 583, 81–107 (Humana
Press: Totowa, NJ, 2009)
2.Xia, Y. & Whitesides, G. M. Soft Lithography. Annu. Rev. Mater. Sci. 28, 153–184 (1998).
3.International Technology Roadmap For Semiconductors. www.itrs.net/reports
4.Weibel, D. B. et al. Bacterial Printing Press that Regenerates Its Ink: Contact-Printing Bacteria
Using Hydrogel Stamps. Langmuir 21, 6436–6442 (2005).
5.Bernard, A., Delamarche, E., Schmid, H. & Michel, B. Printing Patterns of Proteins - Langmuir
(ACS Publications). Langmuir 14, 2225–2229 (1998).
6.Xie, Y. & Jiang, X. Microcontact printing. Methods Mol. Biol. 671, 239–248 (2011).
7.Santhanam, V. & Andres, R. P. Microcontact Printing of Uniform Nanoparticle Arrays. Nano Lett.
4, 41–44 (2004).
8.Qin, D., Xia, Y. & Whitesides, G. M. Soft lithography for micro- and nanoscale patterning. Nat
Protoc 5, 491–502 (2010).
9.Chai, J., Wang, D., Fan, X. & Buriak, J. M. Assembly of aligned linear metallic patterns on silicon.
Nature Nanotech 2, 500–506 (2007).
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